Nonprecious anodic catalysts for low-molecular-hydrocarbon fuel cells: Theoretical consideration and current progress

Progress in Energy and Combustion Science 77 (2020) 100805

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Progress in Energy and Combustion Science journal homepage: www.elsevier.com/locate/pecs

Nonprecious anodic catalysts for low-molecular-hydrocarbon fuel cells: Theoretical consideration and current progress Mohammad Ali Abdelkareem a,b,c,∗, Enas Taha Sayed b,c, Hend Omar Mohamed d, M. Obaid c,e,f,∗, Hegazy Rezk g,h, Kyu-Jung Chae d,∗ a

Department of Sustainable and Renewable Energy Engineering, University of Sharjah, PO Box 27272, Sharjah, United Arab Emirates Center of Advanced Materials Research, University of Sharjah, P.O. Box 27272, Sharjah, United Arab Emirates Chemical Engineering Department, Faculty of Engineering, Minia University, AlMinya, Egypt d Department of Environmental Engineering, Korea Maritime and Ocean University, 727 Taejong-ro, Yeongdo-gu, Busan 49112, Republic of Korea e Global Desalination Research Center (GDRC), School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju, 61005, South Korea f King Abdullah University of Science and Technology (KAUST), Water Desalination and Reuse Center (WDRC), Division of Biological & Environmental Science & Engineering (BESE), Thuwal, 23955-6900, Saudi Arabia g College of Engineering at Wadi Addawaser, Prince Sattam Bin Abdulaziz University, Saudi Arabia h Electrical Engineering Department, Faculty of Engineering, Minia University, Egypt b c

a r t i c l e

i n f o

Article history: Received 26 April 2019 Accepted 20 October 2019

Keywords: Nonprecious catalyst Actual fuel cell operation Zeolite Metal–organic frameworks Ni-based catalyst Ni-free nonprecious catalyst

a b s t r a c t Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy with high efficiency. The high cost of platinum catalysts and sluggish reaction kinetics are the main challenges in the development of low-temperature fuel cells. Although significant efforts have been made to prepare effective non-precious-metal-based oxygen reduction reaction (ORR) catalysts, suitable anodic catalysts are still far from realization. The reported onset potential of a nonprecious anodic catalyst toward low-molecular-weight hydrocarbons, such as methanol, ethanol, and urea, in alkaline media is approximately 0.35 V (vs. Ag/AgCl), which is far from the theoretical potentials of −0.61, −0.54, and −0.55 V (vs. Ag/AgCl), respectively. Therefore, some researchers concluded that nonprecious anodic catalysts are not practical, taking into account the ORR potential of 0.2 V (vs. Ag/AgCl) in alkaline media. Recently, however, several reports demonstrated an open-circuit voltage (OCV) of more than 0.8 V using non-precious-metalbased anodic catalysts, which contradicts expectations. Therefore, to answer these conflicting claims, this review intensively discusses the possibility of using nonprecious metals, for example Ni-based catalysts, for actual electricity generation in direct (methanol, ethanol, and urea) fuel cells, and the different methods applied to achieve the highest values of OCV. Also, the progress done in the preparation of nonprecious anodic catalysts is reviewed. Finally, conclusions and recommendations to prepare durable and active fuel cells using non-precious-metal-based anodic catalysts are presented. © 2019 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nonprecious anodic catalyst, theoretical considerations, and current status . . 2.1. Difference between in-situ and ex-situ onset potentials . . . . . . . . . . . . . 2.2. Methods to increase cathode potential . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of urea and simple alcohol oxidation over Ni catalyst in alkaline 3.1. Urea oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Simple alcohols oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Corresponding authors. E-mail addresses: [email protected] (M.A. Abdelkareem), [email protected] (M. Obaid), [email protected] (K.-J. Chae).

https://doi.org/10.1016/j.pecs.2019.100805 0360-1285/© 2019 Elsevier Ltd. All rights reserved.

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M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

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Different methods for increasing the electrochemical oxidation activity of Ni . . . . . . . 4.1. Alloying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Transition metals/metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Nonmetals and metalloids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Non-oxide metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Metal carbides and nitrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Transition metal phosphides, selenides, sulfides, and borides . . . . . . . . 4.3. Other configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Increasing surface area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1. New morphologies of the catalyst support . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2. Ni-based catalysts with new morphologies . . . . . . . . . . . . . . . . . . . . . . . 5. Crystalline porous materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Zeolite preparation and modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Zeolite-modified electrodes in methanol, ethanol, and urea oxidations . 5.2. Metal-organic frameworks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. Design methods of MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Ni-free nonprecious catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. In-situ preparation of catalyst on the diffusion layer . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Challenges and recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Nowadays, energy scarcity and environmental pollution are the main challenges affecting human society and hindering communities’ development. Therefore, the exploration of clean energy resources and their related devices have attracted worldwide interest [1–4]. Recently, great attention has been paid to fuel cells (FCs) as a new technology for the direct conversion of the chemical energy stored in fuels into electricity with high efficiency and little to no environmental impact [5–7]. FCs have several advantages, including high theoretical efficiency, low emissions, and the ability to use biomass-derived fuels [8,9]. In particular, low-temperature FCs are considered as efficient, environmentally friendly, and silent power sources that can be used for both low-power-consumption devices (i.e., portable devices) and medium-power ones (i.e., vehicles). Low-temperature FCs can be directly fed with low-molecularweight hydrocarbons, such as methanol, ethanol, and urea, showing a high open-circuit voltage (OCV) similar to that of protonexchange membrane FCs [10,11]. Methanol and ethanol, the simplest alcohols, which are easily stored and transported in liquid form, have high energy densities of 6.1 and 8.2 kW h kg−1 , respectively [12], and can be produced from different biomass resources [13–18]. Moreover, urea is a nontoxic fuel with a high energy density of 16.9 MJ/l [19,20], which is industrially produced in large amounts as fertilizer and exists in industrial wastewater and natural urine [21–23]. Despite the promising features of low-temperature FCs that are directly fueled with methanol, ethanol, or urea, their commercialization is hindered by the high cost of the Pt and Pt-alloy catalysts that are used at both the anode and cathode [24–26]. Pt is not only expensive and limited in resources, but it is also easily poisoned by carbon monoxide (CO) and other intermediates that are formed during oxidation of the different fuels [27–31]. A good catalyst should be cheap, abundant, and able to adsorb the reactants easily on its surface with a large number of active sites. In addition, it should have high mechanical strength, long-term stability (i.e., anti-poisoning capability) along with excellent electron-, ion, and mass-transfer capabilities. Therefore, a tremendous amount of work has been devoted to replacing Pt catalysts with non-

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precious-metal-based ones at both the anode and cathode of lowtemperature FCs [32,33]. This extensive research succeeded in producing nonprecious cathodic catalysts that showed a high oxygen reduction reaction (ORR) activity, such as nitrogen-doped carbon materials [34–40], metal carbides and nitrides [41–45], and metal oxides [46–48]. Meanwhile, nonprecious anodic catalysts are still far from realization. To date, the best-reported catalyst for the oxidation of low-molecular-weight hydrocarbons (methanol, ethanol, and urea) in alkaline media is Ni, which is used in the form of Ni oxide or hydroxides [49]. Nevertheless, the reported Ni oxide or hydroxide catalysts still exhibited a low electrical conductivity, a high onset potential, few active sites, and a low stability [50,51]. Another critical point of the Ni and Ni-based catalysts is their high onset potential (around 0.35 V (vs. Ag/AgCl)), which indicates that their application in real FCs is still far from realization considering the low cathodic potential in basic solution (theoretical 0.2 V (vs. Ag/AgCl)) [52]. In this respect, different strategies have been developed to enhance the catalytic performance of Ni and Ni-based materials, such as: i) alloying Ni with other metallic and non-metallic elements [53–55], ii) increasing the surface area, iii) preparing non-metal-oxide Ni-based catalysts (e.g., metal phosphide [56–58], selenides, and sulfides [59,60]) which exhibit higher electrical conductivities than metal oxides and hydroxides, iv) preparing nickel-free nonprecious catalysts, and v) prepare the catalysts in-situ on the surface of an electrically conductive porous electrode. Various techniques have been applied to increase the surface area of the catalyst, including: i) using a high-surface-area catalyst support (e.g., carbon nanofibers (CNFs), carbon nanotubes (CNTs) [61], graphene, and carbon sponges [62,63]), ii) designing catalysts with new surface morphology and structure (e.g., Ni nanowires [64] and Ni nanoribbons [65]), and iii) using zeolites [66,67] and metal-organic frameworks (MOFs) [20,68–70]. Recently, a number of studies have summarized the progress in preparing Ni-based catalysts for urea oxidation [71–73]; however, to the best of our knowledge, no studies have reviewed the work toward preparing Ni-based catalysts in direct alcohol FCs (i.e., methanol and ethanol). Moreover, the advances made in preparing non-oxide Ni catalysts, Ni-free nonprecious catalysts, and zeolite-

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

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Table 1 Anodic, cathodic, and overall reaction of urea, methanol, and ethanol fuel cells in alkaline media.

Methanol FC [74,75]

At anode

At cathode

CH3 OH + 6 OH− → CO2 + 5H2 O + 6 e− E anode = −0.81 V vs SHE

3 O 2 2

(1)

+ 3H2 O + 6 e− → 6OH− E cathode = 0.402 V vs SHE

(4)

3O2 + 6H2 O + 12 e− → 12OH− E cathode = 0.402 V vs SHE

Ethanol FC [76]

C2 H5 OH + 12 OH− → 2CO2 + 9H2 O + 12 e− E anode = −0.74 V vs SHE

Urea FC [77]

CO(NH2 )2 + 6 OH− → CO2 + 5H2 O + N2 + 6 e− (7) E anode = −0.746 V vs SHE

and MOF-based materials have not been introduced yet, despite their importance. The main purpose of this review is to discuss the discrepancies between theoretical considerations and the current status of research on non-precious-metal catalysts based on the differences between reported in-situ and ex-situ data. Here, the various strategies applied to increase the activity of Ni-based catalysts toward methanol, ethanol, and urea oxidation, to improve FC performance, are summarized and evaluated. This review is organized as follows: Section 2 summarizes the theoretical considerations and current status of non-preciousmetal-based catalysts. Section 3 summarizes and describes the mechanisms of urea and simple-alcohol oxidation over Ni catalyst in alkaline media. Section 4 presents various methods and strategies to increase the electrochemical oxidation activity of Ni. Section 5 describes the progress in preparing crystalline porous materials, i.e., MOFs and zeolite. Section 6 summarizes the progress done in preparing Ni-free nonprecious catalyst. Section 7 highlights the progress made in preparing a standalone electrode. Finally, Section 8 discusses the remaining challenges, provides recommendations for future work or opportunities, and presents concluding remarks on the subject. 2. Nonprecious anodic catalyst, theoretical considerations, and current status The reactions of alkaline FCs using methanol (Eqs. (1)–(3)), ethanol (Eqs. (4)–(6)), and urea (Eqs. (7)–(9)), can be expressed by the following reactions at standard temperature and pressure, Table 1. Under alkaline conditions, Ni-based catalysts are known as the best active nonprecious catalysts toward methanol, ethanol, and urea oxidation [65,78–83]. The reported onset potentials for methanol, ethanol, and urea using a three-electrode cell structure are approximately 0.35 V (vs. Ag/AgCl) [84–86]. These are significantly higher than the theoretical value, as shown in the above

+ 3H2 O + 6 e− → 6OH− E cathode = 0.4 V vs SHE

Overall reaction (2)

CH3 OH + 32 O2 → CO2 + 2H2 O E cell = 1.21 V

(3)

(5)

C2 H5 OH + 3O2 → 2CO2 + 3H2 O E cell = 1.14 V

(6)

(8)

CO(NH2 )2 + 32 O2 → CO2 + 2H2 O + N2 (9) E cell = 1.146 V

3 O 2 2

equations, or even those obtained using a Pt catalyst [87–89]. For instance the onset potential of methanol oxidation (1 M) in 1 M KOH at 50 mV s−1 over Ni catalyst (deposited on the surface of reduced graphene oxide (rGO)) is approximately 0.35 V (vs. Ag/AgCl) (Fig. 1b), compared to −0.5 V (vs. Ag/AgCl) in the case of Pt/C under the same operating conditions (Fig. 1a) [90], indicating that Ni has a higher over-potential by 0.85 V compared to that of Pt/C. Using 0.2 V (vs. Ag/AgCl) (theoretical cathodic potential) for the oxygen reduction in an alkaline media where the anodic onset potentials are measured in a three-electrode cell structure, an OCV of 0.8 V is expected in the case of a Pt-anode catalyst, whereas a negative OCV is expected in the case of anodic Ni-based catalysts (i.e., nonspontaneous FC reactions). Therefore, some researchers considered that Ni-based catalysts are effective for the electrolysis of lowmolecular-weight hydrocarbons, whereas they are ineffective catalysts for FC applications [91,92]. Recently, however, Ni-based catalysts have been successfully used in actual FC configurations, showing high OCV values [93] that contradict previous expectations. This section highlights and discusses such contradictions and explains the difference between the onset potential measured using a three-electrode cell structure, i.e., ex-situ evaluations, and that measured using a twoelectrode cell structure, i.e., in-situ measurements. Also, various methods used for increasing cell voltage are presented and discussed. 2.1. Difference between in-situ and ex-situ onset potentials Using anodic Ni-based catalysts, the actual FC operation (using a two-electrode cell structure) demonstrated high OCV [94–100]. Direct urea fuel cell (DUFC) using Ni/C anode, and MnO2 cathode exhibited an OCV of 0.43 V at room temperature with a power output of 0.27 mW cm−2 using 1 M urea (Fig. 2a) [77]. When the cathode was humidified, the OCV and power increased to 0.65 V and 1.3 mW cm−2 , respectively (Fig. 2b) [101]. The significant increase in the performance with humidification was related to the

Fig. 1. Cyclic voltammograms of methanol (1 M) oxidation in 1 M KOH at 50 mV s−1 using: (a) Pt/C and (b) Ni/rGO [90].

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M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Fig. 2. Current-voltage/power density of direct urea fuel cell (DUFC) composed of a Ni/C anode and a MnO2 cathode using: (a) 1 and 3 M urea concentrations at room temperature without cathode humidification [77], and (b) 1 M urea at different cell temperatures and 100% humidified cathode conditions, reproduced with permission from [101].

Fig. 3. (a) Cyclic voltammograms of Ni deposited on different carbon supports using three-electrode cell structure (0.33 M urea in 1 M KOH at 50 mVs−1 ), (b) i–V and i–P curves at different Ni loadings using 1 M urea in 3 M KOH as the anolyte and 1.5 M H2 SO4 as the catholyte, and (c) in-situ electrode potentials during the i–V measurements shown in Fig. 3b. NI05, NI12, and NI20 represent Ni loadings of 5, 12, and 20 mg cm−2 , respectively, reproduced with permission from [107].

enhancement of the formation of OH− anions, which improved the cathode potential. Xu et al. studied the relation between the exsitu and in-situ onset potentials for electrodes of Ni-doped with different percentages of Co [94]. Their findings are summarized by the following points: i) the ex-situ onset potentials for urea oxidation of Ni/C using different cobalt percentages in 1 M KOH are typical to those reported in the literature in the range of 0.3–0.4 V vs. Hg/HgO, ii) a high OCV between 0.55 and 0.65 V is obtained for the different anodic catalysts, iii) there is a direct relationship between the ex-situ onset potential and the in-situ OCV; for instance, a decrease of 100 mV in the ex-situ onset potential (using 20% Co) resulted in an increase in the OCV by 100 mV, and iv) the ex-situ values of the anodic current density and the onset potential are useful indicators for the in-situ cell performance (two-electrode configuration). Despite the important findings reported by Xu et al. [94], they did not report the in-situ anodic and/or cathodic potentials. Fortunately, the in-situ electrode potentials of membrane-less direct urea microfluidic FCs using Ni/C of various morphologies and Pt/C as a cathode were reported by Zhang et al. [102] (Fig. 3). The following main points are concluded from this work: i) An ex-situ onset potential of 0.4 V vs. Hg/HgO has observed for Ni deposited on different carbon supports (Fig. 3a). ii) In-situ measurements showed an OCV of more than 0.9 V (Fig. 3b), iii) The in-situ anodic onset potentials are entirely different from the ex-situ values. For instance, the in-situ anode potentials are approximately −0.2 V (vs. Ag/AgCl) for the different catalysts (Fig. 3c), which are 0.6 V lower than those measured using the three-electrode cell structure, 0.4 V (Fig. 3a). Importantly, this study demonstrated a

significant difference between the ex-situ and in-situ onset potentials; therefore, the ex-situ onset potential should not be used for calculating the cell voltage. The difference between the in-situ and ex-situ onset potentials could be related to the difference in the operating conditions, i.e., direct contact between the electrodes and a solid membrane in the in-situ measurements, compared to immersing the electrode in liquid electrolyte in ex-situ measurements. However, the exact reason is not clear yet, and further work is required to clear the reason(s) for such a big difference between them. The above discussion demonstrates that nonprecious anode catalysts are successfully used in real FC applications, exhibiting a high power density of 27 mW cm−2 at room temperature (20 °C) [103], which is higher than those reported using high Pt–Ru loading of 8–12 mg cm−2 [104–106]. To further increase the cell voltage of direct low-hydrocarbon FCs using anodic nonprecious catalysts, researchers used different strategies, such as studying the effect of pH at the anode and cathode and replacing the oxygen by applying other oxidants at the cathode, which are summarized in the following section. 2.2. Methods to increase cathode potential Although using a basic solution significantly increases the catalytic activity toward the oxidation of various fuels at the anode side, the theoretical cathode potential is too low (0.4 V (vs. SHE) at pH = 14), compared to that under acidic conditions (1.23 V (vs. SHE)). Thus, researchers have developed various methods to increase the cathode potential, while maintaining high activity using a basic anodic solution. Such methods are summarized in Fig. 4a.

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

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Fig. 4. (a) Schematic diagram showing the effects of the anodic and cathodic acidities and the oxidant type on the electrode potential (vs. SHE) and cell voltage of a direct methanol fuel cell, and (b) Relationship between common reference electrodes.

As shown in Fig. 4a, altering the cathode operating condition from alkaline (using oxygen oxidant) to acidic (using H2 O2 ) resulted in an increase in the cell voltage by a factor of 2.14, from 1.21 to 2.59 V vs. SHE, when the anode was operated under alkaline conditions. Fig. 4b shows the relationship between different reference electrodes. It worth mentioning that when converting electrode potential to or from SHE, the pH value must be considered. For instance E(SHE) = E (SCE) + 0.241 + 0.0591 pH [107] is used for converting between SCE and SHE.

The direct urea fuel cell (DUFC) operated with Ni9 Co1 nanowires as an anode (0.33 M urea in 9 M KOH as an anolyte), Pd/C fibers as a cathode (2 M H2 O2 in 2 M H2 SO4 as a catholyte), and Nafion 115 membrane; a high cell voltage of 0.92 V and power density of 7.4 mWcm−2 at room temperature were obtained [97]. The high cell voltage is attributed to the high theoretical cathode potential of H2 O2 in acidic media (1.78 V (vs. SHE)), compared to 0.4 V (vs. SHE) in the case of oxygen in alkaline media. The electrode and overall cell reaction were described in Eqs. (10)–(12).

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M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

At anode:

CO(N H2 )2 + 8KOH → N2 + 6H2 O + K2C O3 + 6K + + 6e− Eanode = −0.75 V (vs. SHE )

(10)

At cathode:

3H2 O2 + 3H2 SO4 + 6e− → 6H2 O + 3SO4 2− Ecathode = 1.76 V (vs. SHE )

(11)

Overall cell reaction:

CO(N H2 )2 + 3H2 O2 + 3H2 SO4 + 8KOH → K2C O3 + 3K2 SO4 + N2 + 12H2 O Ecell = 2.51 V (vs. SHE ) (12) In addition to the high theoretical reduction potential of H2 O2 , hydrogen peroxide is easy to store and requires less pumping energy; which results in an easier heat management of the fuel cell [108]. Moreover, the kinetics of H2 O2 reduction is faster than that of O2 reduction [109,110], so H2 O2 has a great potential to be used as a cathode oxidant. However, despite of the several advantages of using H2 O2 , it has been reported that exposing of a Nafion membrane to this substance for prolonged times results in a loss of its conductivity due the destruction of the ion clusters in the membrane by H2 O2 [111]. This effect is significantly accelerated by the presence of traces of metal ions such as Fe2+ , Pt2+ , Ni+2 , Cr2+ , Cu2+ , and Co2+ in H2 O2 [112]. Introducing a peroxidedecomposition catalyst within the membrane is considered as an effective way to reduce its degradation by H2 O2 [113], for instance, a composite membrane consisting of Nafion and ZrO2 nanoparticles was found to improve the durability of the Nafion membrane. The ZrO2 nanoparticles embedded in the membrane decomposed the H2 O2 species that had permeated into it, thus lowering their concentration [114]. Also, cerium composite membranes demonstrated a high stability in the case of both Nafion and sulfonated poly (ether ether ketone) [115,116]. Metal ions are another type of non-oxygen-based oxidants that are effectively used at the cathode of low-temperature FCs. For instance, chromium ions (Cr IV) in wastewater were effectively used in direct urine/Cr(VI) [117], with a theoretical potential value of 1.33 V (vs. SHE) at the cathode according to the following equation:

C r2 O27− + 14H + + 6 e− → 2C r 3+ + 7H2 O E

cathode

= 1.33 V (vs. SHE )

(13)

A specially designed cell configuration can instantaneously treat the wastewaters and produce energy using the metal ion oxidants. Fig. 5a shows a cell consisting of an anode (Ni/C) in urine solution, a middle chamber with anion and cation exchange membranes separated by KCl solution, and a cathode (carbon cloth) in chromium wastewater solution. This produced a maximum power of 3.5 mWcm−2 with an OCV of 1.3 V (Fig. 5b) and simultaneously treated the wastewater at the anode (i.e., urine removal) and the cathode (i.e., heavy metal removal: Cr IV (Fig. 5c). 3. Mechanism of urea and simple alcohol oxidation over Ni catalyst in alkaline media Understanding the reaction mechanism at the molecular level is essential for developing active, non-precious-metal-based catalysts for urea and simple-alcohol oxidation. This section includes a complete description of the urea-oxidation process at the molecular level. Upon the exposure of Ni to air, it spontaneously oxidizes into NiO, which in turn forms Ni(OH)2 in water or even in humid air:

Ni(metal ) + H2 O → (NiOH )ad + H + + e−

(14)

Fig. 5. (a) Schematic diagram of urine/Cr(VI). (b) Power output using neat urine at the anode and different Cr IV ions in 0.25 M H2 SO4 as the catholyte at 20 °C. (c) Variation in Cr ion concentration during current discharge, reproduced with permission from [117].

(NiOH )ad + H2 O → (NiOH.H2 O )ad

(15)

(NiOH.H2 O )ad → Ni(OH )2 + H + + e−

(16)

The formation of Ni(OH)2 is significantly increased in alkaline media (Eq. (17)) [118].

Ni + 2OH − → Ni(OH )2 + 2e−

(17)

During cyclic voltammetry of Ni catalyst in a fuel-free alkali medium such as KOH, the hydroxyl groups adsorbed on the catalyst surface result in the growth of Ni(OH)2 . At low potentials, an amorphous and less stable form of Ni(OH)2 (i.e., α -Ni(OH)2 ) is

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

formed, which is converted into a more stable form (namely, β Ni(OH)2 ) at high potentials [119,120]. With a further increase in the potential, β -Ni(OH)2 is converted into β -NiOOH, which is partially converted into ϒ-NiOOH at 0.35 V (vs. Hg/HgO) and reduced back to β -Ni(OH)2 during the reverse scan [121]. The cyclic voltammograms of Ni in the fuel-free alkali demonstrated redox peaks at 387 mV (vs. Hg/HgO) in the forward scan and 260 mV (vs. Hg/HgO) in the backward scan, these correspond to Ni+2 (Ni(OH)2 )/Ni+3 (NiOOH) [122]:

N i(OH )2(s ) + OH − ↔ N iOOH(s ) + H2 O(l ) + e−

(18)

Upon the addition of urea or other fuels, a significant increase in the current density is noticed around 0.35 V (vs. Hg/HgO) in the forward scan, indicating that NiOOH (β -NiOOH) is the active site [123–125]. In the following section, the proposed mechanism for the oxidation of urea and other simple alcohols over the surface of non-precious catalysts (i.e., Ni) in alkaline media is explained.

Chemical reaction:

6N i(OH )2(s ) + 6OH − ↔ 6N iOOH(s ) + 6H2 O(l ) + 6e− H2 O(l ) → 6Ni(OH )2(s ) + N2(g) + C O2(g)

Several studies have been performed to illustrate the mechanism of urea oxidation over Ni catalysts in alkaline media [123,126–128], through indirect and direct reactions, as explained below. In the case of the indirect reaction, the process takes place in two steps: first, the electrooxidation of Ni(OH)2 to form NiOOH occurs, releasing electrons (Eq. (19)). Then, the chemical oxidation of urea to produce CO2 , N2, and water (Eq. (20)) [129,130]: Electrooxidation reaction:

6N i(OH )2(s ) + 6OH − ↔ 6N iOOH(s ) + 6H2 O(l ) + 6e−

(19)

(20)

Overall reaction:

CO(N H2 )2(aq) + 6OH − → N2(g) + 5H2 O(l ) + C O2(g) + 6e−

(21)

However, in case of the direct reaction, urea is directly oxidized over NiOOH [127,131,132] according to the following Eqs. (22)–(25):

[N iOOH.CO(N H2 )2 ]ads + 6OH − → [NiOOH.C O2 ]ads +N2 + 5H2 O + 6e−

(22)

N iOOH + OH − → [N iOOH.OH ]ads + e−

(23)

2− [NiOOH.C O2 ]ads + 2[NiOOH.OH ]ads → 3NiOOH + C O3 + H2 O



[NiOOH.C O2 ]ads + 2 OH 3.1. Urea oxidation

7

(24)

 − sol

→ NiOOH + C O3 2− + H2 O

(25)

Nevertheless, it should be noted that some researchers reported that both mechanisms could take place at the same time [131]. Daramola et al. studied the decomposition of urea into HCNO, NCO− , NH3 , CO2 , and N2 on the surface of the Ni catalyst by density functional theory (DFT) [128]. The authors proposed that urea initially adsorbs on the surface of NiOOH through the formation of a bridge between Ni and the N or O atoms in the urea molecule. Urea then dissociates through one of three different proposed reaction paths (see Table 2). In reaction paths 1 and 2, the H atoms removed from the two amine groups one by one, followed by bonding of the two nitrogen atoms in an intermediate step, whereas, the bonding of the nitrogen atoms in path 3 occurs before the complete removal of the hydrogen atoms from the two amine groups.

Table 2 Three different reaction paths for urea oxidation on NiOOH (M). Reactions

Description

Rate constant (L mol−1 s−1 )

Adsorption of urea molecules on the Ni catalyst Hydrogen removal from the first amine group by the OH− group. Removal of the second hydrogen from the first amine group by the OH− group. Hydrogen removal from the second amine group by the OH− group. Hydrogen removal from the second amine group by the OH− group.

–

Free energy (kJ mol−1 )

Reaction path 1 1

CO(NH2 )2 + M → [M.CO(NH2 )2 ]ads

(26)

2

[M.CO(NH2 )2 ]ads + OH− → [M.CONH2 .NH]ads + H2 O + e−

(27)

−

→ [M.CONH2 N]ads + H2 O + e

−

3

[M.CONH2 .NH]ads + OH

4

[M.CONH2 N]ads + OH− → [M.CONHN]ads + H2 O + e−

(29)

5

[M.CONHN]ads + OH− → [M.CO.N2 ]ads + H2 O + e−

(30)

6 7 8

A bond between the two nitrogen atoms is formed [M.CO.N2 ]ads + OH− → [M.CO.OH]ads + N2 + e− [M.CO.OH]ads + OH− → [M.CO2 ]ads + H2 O + e−

(31) (32)

9

[M.CO2 ]ads + OH− → M + CO2

(33)

(28)

Adsorption of OH− and release of N2 The new OH− removes hydrogen from the previously adsorbed OH− group. CO2 desorption from the catalyst surface

66.2

1.4 ∗ 10−17 ∗

2.3 10

−21

−28.9 −185.1

4.1 ∗ 107

75.4

8.8 ∗ 1015

−178.2

– 7.3 ∗ 108 1.6

– 392.7 −156.6

4.3 ∗ 10−65

1242.2

Reaction path 2 It is similar to that of path 1, and the only difference is the hydrogen removal from the second amine group. Therefore the rate constants of steps 2 to 9 are different. Reaction path 3 Step 1 and 2 are similar to those of path 1 3 A detachment of the amine group from the carbon and its attachment to the NH group. (34) Hydrogen removal from the amine group by the OH− group. 4 [M.CO.NH2 .NH]ads + OH− → [M.CONHNH]ads + H2 O + e− (35) Hydrogen removal by the OH− group. 5 [M.CONHNH]ads + OH− → [M.CONHN]ads + H2 O + e− (36) Further removal of the hydrogen by the OH− 6 [M.CONHN]ads + OH− → M.CO.N2 + H2 O + e− Steps 7, 8, and 9 are similar to those in paths 1 and 2.

8

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

4.1. Alloying

Fig. 6. Schematic diagram of ethanol oxidation over a Ni catalyst in alkaline media [135].

In all the reaction paths, the last reaction (i.e., CO2 removal) is the rate-determining step, as clearly supported from the highest free energy and the lowest reaction rate constant as confirmed by other researchers [123,128,133]. Moreover, CO2 is found to be the main reason for the degradation of urea oxidation compared to CO because it reacts with the Ni catalyst to form inactive nickel carbonate [134].

4.1.1. Transition metals/metal oxides It has been found that the alloying of Ni with transition metals such as Co [137], Cr [138], Sn [139,140], Zn [103], Mn [141], Mg [124], Cu [142], and Al [143] resulted in increasing its activity. Co is a well-known transition metal that significantly increased the oxidation activity of Ni toward methanol [144–148], ethanol [149], and urea [150]. Regardless of the fuel used, an increase in the Co content in the NiCo alloys resulted in a general decrease in the onset potential [149–151] due to the formation of CoOOH at lower potential than that of Ni. Notably, an onset potential of 0.11 V (vs. Ag/AgCl) is the lowest value reported for methanol oxidation in alkaline media using Ni0.5Co0.5/CNF [145], and −50 mV (vs. Ag/AgCl) was reported in the case of ethanol using Ni0.1Co0.9/CNF [149]. However, Co cannot act as a standalone catalyst, and thus, the increase in its content beyond a certain level resulted in decreasing current generation, due to the decrease of the Ni catalyst [149,152,153]. For instance, doping Ni with 10% Co (Ni0.9Co0.1/CNF) slightly decreased the onset potential from 320 to 310 mV (vs. Ag/AgCl), and a significant increase in the current density from 41 to 100 mA cm−2 was obtained at 0.7 V (vs. Ag/AgCl) using 3 M ethanol. Further, an increase in the Co content (Ni0.1Co0.9/CNF) resulted in a decrease in both the onset potential (less than −50 mV (vs. Ag/AgCl)) and the current density (less than 50 mA cm−2 ) [149]. Therefore, an optimum Co content is required to achieve the highest current density and the lowest onset potential. A hydrothermally prepared urchin-like NiCo2 O4 demonstrated high methanol oxidation activity [154], where both Ni+2 /Ni+3 and Co+2 /Co+3 are considered as active sites [155–158] as follows (assuming complete oxidation into CO2 and H2 O) [159,160]:

NiC o2 O4 + OH − + H2 O − 3e− → NiOOH + 2CoOOH

(41)

NiOOH + C H3 OH + 1.25O2 → Ni(OH )2 + C O2 + 1.5H2 O

(42)

3.2. Simple alcohols oxidation Although urea can be entirely oxidized into CO2 and N2 , as shown above, several researchers have reported that simple alcohols are partially oxidized to form acetate acid in the case of ethanol [85,135], and format in the case of methanol [136] according to Eqs. (37)–(40) and as shown in Fig. 6: However, still further work is needed to clear the detailed reaction mechanism.

NiOOH + C H3C H2 OH −→ Int ermediat e 1 + Ni(OH )2

(37)

NiOOH + Int ermediat e 1 −→ C H3CHO + Ni(OH )2

(38)

NiOOH + C H3CHO −→ Int ermediat e 2 + Ni(OH )2

(39)

NiOOH + Int ermediat e 2 −→ C H3COOH + Ni(OH )2

(40)

4. Different methods for increasing the electrochemical oxidation activity of Ni Although Ni is the best reported nonprecious catalyst for the oxidation of light hydrocarbons such as methanol, ethanol, and urea in alkaline media, Ni activity is still lower than that of Pt-based catalysts. Therefore, researchers have used different approaches for increasing the oxidation activity of Ni such as alloying, using different supports or catalysts with high surface area, preparing non-oxide Ni catalysts, and using zeolite and MOFs. These methods are discussed in detail in this section.

2C oOOH + 2C H3 OH + 2.5O2 → 2C o(OH )2 + 2C O2 + 3H2 O (43) The increase in the Co content in NiCo hydroxides yielded a decrease in the current density and the onset potential, whereas in the metal form, the lowest onset potential was obtained at 75% Co, and the highest current obtained at 54% Co [152]. The methanol oxidation activity of NiCo alloys not only depends on the oxide or hydroxide form [152], but also on the crystalline structure [161]. An acicular and rough surface of Co7 Ni5 (hcp structure) showed higher oxidation activity toward urea (with a lower onset potential by 57 mV and a higher anodic current density by a factor of two), compared to the case of an fcc structure (fine-grained morphology) [161]. The improved performance with Co alloying is ascribed to the following reasons: i) the role of Co as a CO tolerance material by providing OH groups and/or a ligand effect [162–166]; ii) the ability of Co to adsorb fuel (ethanol) molecules on its surface by the interaction of free electron pairs of the O-atom of the fuel (ethanol) to the partially vacant d-orbital of Co and its role as a CO anti-poisoning agent [165,166]; iii) Co effectively works as a cocatalyst for methanol oxidation [167], where it acts as a chemical modifier of structural and/or electronic effects [168,169], and iv) Co enhances the oxidation state of Ni, thereby promoting electron transfer [170,171]. Alloying of Ni with Mn also improved its oxidation activity [141,172–174]. Ni–Mn alloy (prepared using 90 wt% Ni acetate and 10 wt% Mn acetate) exhibited a low urea oxidation onset potential of −0.085 V (vs. Ag/AgCl). The authors related the high activity of the Ni–Mn alloys to their crystalline structure (similar to that of precious catalysts) [172], the high fuel absorbability on MnO2 [141],

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

and the role of Mn as a co-catalyst that enhanced the removal of CO, similar to Co [175]. In another study, through a template-free hydrothermal method followed by heat treatment in air at 800 °C for 2 h, Periyasamy et al. [176] prepared Ni2 MnO4 , Ni1.5 Mn1.5 O4 , and NiMn2 O4 and investigated their urea oxidation activities in comparison to NiO and MnO2 . The results revealed that MnO2 had no catalytic activity, whereas Ni and NiMn oxides had high oxidation activity. Ni1.5 Mn1.5 O4 exhibited the lowest onset potential of 0.29 V compared to 0.34 V (vs. Ag/AgCl) for Ni2 MnO4 and NiMn2 O4 , and the highest current density of 6.9 mA cm−2 compared to 1.81, 1.79, and 0.91 mA cm−2 for NiMn2 O4 , Ni2 MnO4 , and NiO, respectively, at 0.5 V (vs. Ag/AgCl). The high activity of Ni1.5 Mn1.5 O4 was related to the facile formation of NiOOH and the presence of three different phases: (Mn)tet (NiMn)2oct O4 , (Mn)tet (Ni)2oct O4 , and NiO. Mg is another transition metal that resulted in increasing the Ni oxidation activity toward methanol and ethanol [124]. The Ni– MgO prepared on the surface of carbon using electro-deposition exhibited high activity in terms of current and stability compared to those of Ni/C. The improved performance was related to the role of Mg in promoting the formation of β -NiOOH (active site for oxidation) at the expense of γ -NiOOH. Additionally, NiCu alloys showed high activity toward methanol [177,178], ethanol [179,180], and urea oxidation [125]. The alloying of Ni with Cu resulted in Cu electrons filling the Ni D-band vacancies, which inhibited the volume expansion of Ni ions during the oxidation of alcohols in alkaline media [181], and led to a synergetic effect in the case of NiCu [182–184]. The ternary NiCoCu alloy exhibited the highest activity toward methanol oxidation compared to that of any of the Ni–Cu or Ni–Co binary alloys [185]. The same finding was obtained when a ternary alloy of CuCoNi, deposited on the surface of glassy carbon (GC) modified by CNTs, was used toward methanol oxidation, whereas the standalone CuO and CoO showed poor methanol oxidation activities [186]. The improved activity was related to: i. the role of Co and Cu as co-catalysts, especially in the case of Co that had one of its CoOOH redox at a potential of 0.07 V vs. SCE [187], and ii. the synergism between NiO and the other two oxides, i.e., CoO and CuO, which resulted in more transformation of Ni II into Ni III [94,188]. NiCr alloy is one of the few nonprecious catalysts that showed relatively high activity toward hydrogen oxidation under actual FC operation [189], and thus, it was expected to show high oxidation activity for low-molecular-weight hydrocarbons. Although Cr has no activity toward urea oxidation, an alloy containing 40 Cr (Ni60 Cr40 /C) exhibited high urea oxidation reaction (UOR) activity compared with NiCr alloys containing different percentages of Cr [190]. Alloying Ni with Cr resulted in good dispersion of Ni with small nuclei over the carbon support, easier charge transfer confirmed from the small semicircle in electrochemical impedance spectroscopy (EIS) spectra, and higher kinetics of urea oxidation from the lower Tafel slop compared to that of Ni/C and/or other NiCr/C alloys. The improved performance was ascribed to the role of Cr in decreasing the density of states (DOS) of the D-band through weakening the Ni–O bond [189], and the enhancement in the formation of the NiIII /NiII redox couples [191], as reported in cases of methanol [138] and ethanol [192]. Other metals such as Fe, Sn, and Cd also augmented the oxidation activity of Ni catalysts. Fe2 O3 strongly enhanced the electrochemical oxidation activity of Ni [193], where Ni–Fe acted as a bifunctional electrocatalyst for urea oxidation [194,195]. Furthermore, adding Fe to Ni increased electrical conductivity, increasing the charge transfer rate. Sn alloying with Ni did not improve the activity of Ni toward methanol oxidation in terms of the onset potential and current density, but the NiSn nanoparticles (NPs) showed high stability compared to that of Ni [139]. The role of Sn in the stability enhancement of Ni toward methanol oxidation is attributed to the function of OH− adsorbed on the Sn surface

9

on oxidizing the adsorbed intermediates, as well as the incorporation of Sn in the Ni structure (Ni3 Sn2 ), which modifies the electronic structure of Ni. A NiCd alloy (7%Ni–93% Cd) prepared by electrodeposition on the surface of a graphite electrode showed high activity toward methanol oxidation in alkaline media, compared with that of Ni/graphite (anodic peak of 230 mA cm−2 compared with 107 mA cm−2 ), whereas pure Cd showed no activity toward methanol oxidation [196]. The high activity was ascribed to the high surface area and porosity of the NiCd catalyst, as well as the strong electronic effect of Cd. Table 3 summarizes the roles of the different elements in enhancing the activity of Ni catalyst. 4.1.2. Nonmetals and metalloids Recently, there has been much interest in metal phosphates, owing to their unique chemical and physical properties and simple preparation methods [198–202]. Through ionic bonds, Ni can combine with nonmetals such as phosphorus, for instance, reflux method was used to fabricate nickel phosphate NPs of size 70 nm and specific surface area 22.3 m2 g−1 [201]. The prepared materials demonstrated high oxidation activity and stability toward urea due to the high absorbability of PO4 −3 ions to small organic molecules, and the presence of the backbone structure of phosphate (high corrosion resistivity), respectively. A redox peak at 0.48 V was associated with Ni3 (PO4 )2 /Ni3 (OH)3 (PO4 )2 , which is considered an active site for urea oxidation [203,204]. Ammonium nickel phosphate ((NH4 )NiPO4 •6H2 O) was calcined at different temperatures (30 0, 60 0, and 90 0 °C), producing β -nickel pyrophosphate (β -Ni2 P2 O7 ), and their activity and properties were investigated by Meguerdichian et al. [205]. A relatively amorphous β -Ni2 P2 O7 was obtained at 300 °C with the highest electrochemical active surface area (ECSA) of 142 cm2 mg−1 compared to 115 cm2 mg−1 for the pristine sample ((NH4 ) NiPO4 •6H2 O). However, increasing the calcination temperature resulted in decreases in the ECSA to 130 and 44 cm2 mg−1 for β -Ni2 P2 O7 obtained at 60 0 and 90 0 °C, respectively, as well as reducing the porosity. As a result, the β -Ni2 P2 O7 obtained at 300 °C exhibited the highest urea oxidation activity, which was related to the exposure of high Ni+2 active sites with heat treatment [205]. In contrast, the nickel phosphate prepared by the heat treatment of ammonium nickel phosphate at 90 0 °C (NP-90 0) exhibited the highest ethanol oxidation activity of 62.36 μA peak anodic current, compared to 29.4 and 8.36 μA for NP-700 and NP-500, respectively, using 3 M ethanol in 0.5 M NaOH at 50 mV s−1 [206]. The high activity of NP-900 was attributed to the largest surface area of 21.7 m2 g−1 , the existence of the single highly crystalline structure of Ni3 (PO4 )2 at 900 °C compared with the mixture of Ni3 (PO4 )2 and NiP2 O7 for NP-700 (with ESCA of 6.6 m2 g−1 ), and the amorphous structure (with ESCA of 5.6 m2 g−1 ) for NP-500. Various morphologies of Ni and/or Co phosphides and/or phosphates have been successfully prepared using the hydrothermal method at different molar ratios of Co to Ni salts of 0.053 (NiCoP-1), 0.018 (NiCoP-2), 0.33 (NiCoP-3), and 1 (NiCoP-4) [207]. Interestingly, in methanol-free KOH (0.5 M), an increase in the Co/Ni ratio resulted in shifting the redox peaks from 0.35 V vs. SCE for NiCoPO-1 and NiCoPO-2 to 0.5 V vs. SCE for NiCoPO-3 and NiCoPO-4. The first redox peak (0.35 V) is related to the oxidation of Ni(OH)2 to NiOOH, and the second is ascribed to the oxidation of CoOOH to CoO2 and vice versa [208,209]. Moreover, NiCoPO-2 exhibited boarder redox peaks and larger enclosed CV areas, indicating the overlapped contribution of the Ni and Co redox peaks [208] and high electrochemical activity [210], respectively. In the presence of 1 M MeOH, NiCoPO-2 showed the highest current density and the lowest onset potential. These results highlighted the importance of optimizing the Co content, which acts as a chemical modifier to allow Ni to reach a higher oxidation state and facilitate the electron transfer, thereby augmenting the methanol oxidation [123–125,210]. The authors also reported the

10

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805 Table 3 Summary of the roles of each metal/nonmetal in increasing the oxidation activity of Ni. Element Co

Zn-Co Sn Nitrogen LaNiO3 Sm-P

Cr

Se Metallic Ni(OH)2 (S incorporation) Mn Mo ZnO (support) Core/shell structure of Cu/NiCu nanowires Ni-MgO Ni-Fe

Role - Reduces the onset potential, but at the expense of the current density; therefore, there is an optimum percentage. - Increases defects and film breakage, and thus increases surface area. - Raises Ni oxidation state. - Ni reaction reaches higher oxidation states and reduces onset potential while maintaining a high current density. - Its usage as support increases charges transfer and reduces poisoning as it promotes CO conversion to CO2 . - Increases electron conductivity and adsorption of the fuel. - Ni exists in Ni3+ , which has high oxidation activity. - Sm results in weakening the adsorption of the intermediates, and thus reducing poisoning. - P has a surplus of valence electrons, which results in modifying the electronic structure of Ni. - P also reduces corrosion. - Increases reversibility and amorphous structure enhances Ni dispersion, eliminates water oxidation, and decreases onset potential. Cr also decreases the density of states of the D-band through weakening the Ni-O bond. - Increases electrical conductivity, and reduces poisoning. - Increases number of active sites increases wettability, increases electron conductivity, and decreases the energy barrier for intermediate formation. - Renders the electronic structure similar to those of precious catalysts, and reduces the poisoning effect. - Increases ECSA and electrical conductivity. - Shifts down the D-band center of Ni, and results in uniform and good dispersion of the catalyst. - Owing to the fast charge in the Cu that acts as a source of electrons to the active NiCu alloy shell as well as the synergetic effect of NiCu. - Mg promotes the formation of β -NiOOH at the expense of γ -NiOOH, the former of which has high stability and activity. - Acts as a bifunctional electrocatalyst, and increases the rate of charge transfer.

possibility that electron tunneling between adjacent metal phosphates and phosphides would also enhance the electron transfer through the NiCoPO catalyst [211]. 4.2. Non-oxide metal Non-oxide metals such as metal phosphides, nitrides, carbides, sulfides, selenides, and others, are class of materials that showed superior electrocatalytic activities compared to those of the relevant metal oxides and thus showed excellent activity in many applications, such as electrochemistry, catalysis, and sensing technology [212,213]. Metal non-oxides (with low electronegativity compared to that of oxygen) have superior physicochemical properties compared with metal oxides, owing to the electron-rich bonds in their outer layer structure and the tunable interaction between the non-oxygen element and the metal [214,215]. 4.2.1. Metal carbides and nitrides Metal carbides and nitrides are considered as best candidates to replace Pt soon. Since Levy and Boudart discovered that tungsten carbides displayed Pt-like behavior in several catalytic reactions, extensive fundamental research has been carried out on metal carbides and nitrides [216,217]. These investigations demonstrated that carbides and nitrides are suitable catalysts for a wide variety of reactions that typically utilized group-VIII noble metals as catalysts. During the alloying of M- with non-oxygen atoms such as carbon and nitrogen, the space between the two atoms is enlarged and, therefore, the density of the unoccupied d-orbitals is decreased due to the band contraction. As a result, the DOS around the Fermi level and consequently, catalytic properties similar to those of noble metals are obtained [218–220]. Transition metal carbides can be formed when carbon atoms, produced by the decomposition of hydrocarbons or other carboncontaining molecules, are incorporated into interstitial metal sites [221]. The interstitial carbides and nitrides of early transition met-

als (groups IVB-VIB) have unique physical and chemical properties [222,223], which make them good candidates for various applications such as heterogeneous catalysis with higher activity, selectivity, and resistivity to poisoning compared with their parent metals [224–227]. Upon the formation of interstitial compounds, the metal lattice expands and the metal-metal distance increases, which causes a contraction of the metal D-band, resulting in a greater DOS near the Fermi level, as compared with the parent metal [228]. Owing to the outstanding properties of metal carbides and nitrides, they have demonstrated promising results in energy conversion/storage devices [229–232]. Tungsten carbide nanofibers (NFs) prepared by the carbothermal reduction of tungsten oxide NFs obtained from the calcination of polyvinyl pyridine (PVP) NFs containing ammonium metatungstate, at 700 °C [233], exhibited low methanol oxidation activity in acidic media. When Ni NPs were supported with tungsten carbide (WC) on carbon or CNT, the catalytic activity toward urea oxidation was improved, compared with that of Ni, despite the low activity of WC toward urea oxidation [234–237]. These results are ascribed to increase in the surface area of the Ni nanoclusters over the WC surface and the synergetic electron transfer between the Ni and WC, which facilitate the scission of the C–N bond and promoting the oxidation of the hydrocarbon into CO2 without forming CO and other poisoning intermediates. In contrast, a mixture of mesoporous carbon and mesoporous WC showed an activity toward methanol oxidation in acidic media with an over-potential of only 200 mV (Fig. 7) compared to that of Pt. This can be attributed to the importance of the mesoporous structure that increases the surface area of the WC and prevents carbon deposition over the surface of the WC [238]. Silicon carbide (NiO/SiC) supported by CNTs exhibited high catalytic activity toward methanol oxidation; however, the significant increase in the activity was related to the excellent dispersion of Ni over the Ni NPs/SiC and CNTs, the smooth electron transfer between the catalyst and the support, as well as the easy mass transfer in the three-dimensional network-like structure [239].

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

11

Fig. 7. (a and b) Cyclic voltammograms of (1) a catalyst-free glassy carbon electrode using 4 M methanol in 0.5 M H2 SO4 , (2) an ordered mesoporous carbon (OMC)/WC electrode in methanol-free H2 SO4 , (3) OMC/WC using 4 M methanol in 0.5 M H2 SO4 , and (4) a platinum disk electrode using 4 M methanol in 0.5 M H2 SO4 . All experiments were carried out at 50 °C and 10 mV s−1 , reproduced with permission from [242].

4.2.2. Transition metal phosphides, selenides, sulfides, and borides P is one of the nonmetals that has a surplus of valence electrons, and it can react with most of the elements in the periodic table. P regulates the electronic structure of Ni, where the electron density of Ni metal clusters is changed to provide a more significant number of active sites [240]. Moreover, P is a nonmetal with high corrosion resistivity [241]. Transition metal phosphides showed higher electrochemical activity compared to transition metal oxides or hydroxides, owing to their high electrical conductivity [50,242,243]. Furthermore, transition metal phosphides contain higher surface electron content compared to those of nitrides, carbides, and sulfides [244]. Nickel phosphides (NiP) demonstrated high activity toward methanol, ethanol, and urea oxidation [50]. However, combining phosphide with samarium (Sm, an abundant metal with catalytic activity owing to its incomplete 4f level) enhanced the Ni oxidation activity toward urea. The ternary alloy of Ni5 Sm-P/C demonstrated superior oxidation activity toward urea, with the highest anodic peak current density of 325 mA cm−2 g−1 at 0.45 V vs. Hg/HgO, compared with those of Ni–P/C, Ni5 Sm/C, and Ni/C [245]. Interestingly, adding P (Ni–P/C) or Sm–P (Ni5 Sm–P/C) decreases the onset potential of Ni/C, signifying that P and Sm promote Ni to reach a higher oxidation state (NiOOH), decrease the overpotential for urea oxidation, and facilitate the electron transfer for urea oxidation [246]. Additionally, Cu-incorporated NiP (coupling of Cu and P) amended the methanol oxidation activity, owing to the role of copper hydroxide/oxyhydroxide in methanol oxidation, as well as the stabilization of the formation of β -NiOOH and suppression of α NiOOH [247]. β -Ni(OH)2 is the active site for the electrochemical oxidation with low charge transfer resistivity [124,125,248]. NiSe [249] revealed high methanol oxidation activity (Fig. 8), which is higher than that of NiO and even that of Ni3 S2 . This high methanol oxidation activity is due to the faster electron transfer in NiSe [250] than those in NiO and Ni3 S2 , as confirmed from the EIS measurements. Boron (B) is another metalloid that resulted in increasing Ni activity toward ethanol oxidation [251–253] and methanol oxidation [254]. For instance, a Ni–B (5 wt% B) prepared on carbon electrode using electroless plating showed high activity toward methanol and ethanol, higher by a factor of two than that of NiP/C [251]. The improved activity of NiB is due to B facilitating the organic

oxidation and improve the catalyst stability thorough the removal of the intermediates from the catalyst surface [255]. Similar results were reported by Zhang et al. [256], who demonstrated that the activity of amorphous NPs of Ni–B alloy, prepared on the surface of nanoporous Cu film, had a lower ethanol oxidation onset potential (49 mV lower), and a significant increase in the anodic peak current density (43.36 times higher), compared with those obtained using bulk Ni electrodes. In a similar work, Muench et al. reported that the activity of unsupported Ni-B nanotubes (NT), prepared by electroless plating on the surface of an ion-track-etched polymer template [253], toward ethanol oxidation is higher than that of the Ni nanotubes (Ni NT) prepared by the same method. It showed 10 mV lower in the onset potential than that obtained for Ni NT, while the anodic peak current density was higher by 25% [253]. Importantly, changes in the morphology and chemical structure of the support of Ni-B showed enhancement in the activity toward methanol oxidation. Compared to Ti-supported Ni–B, the Ni–B deposited on TiO2 NT revealed significant improvement in the performance [254]. The significant increase in the performance was ascribed to the high surface area of the TiO2 NTs and its high methanol absorbability [257,258]. 4.3. Other configurations The perovskite LaNiO3 exhibited a higher urea oxidation activity compared to NiO because the Ni+3 form of Ni has a higher activity than Ni+2 (in the case of NiO) [134]. At the cathode of the FCs, nitrogen-doped carbon materials demonstrated an excellent ORR activity, comparable to that of Pt [259–261]. The high ORR activity of the nitrogen-doped materials was related to the role of nitrogen in enriching the carbon surface with π -electrons, thus increasing the surface energy and the concentration of n-type carriers, and reducing the work-function [262]. At the anode of the FCs, nitrogen-doped carbon-supported Ni catalysts (i.e., NiNC-1, NiNC-2, NiNC-3, and NiNC-4) exhibited a superior ethanol oxidation activity than carbon-supported ones. This finding can be explained as follows: Doping of the carbon support with nitrogen improved the catalytic activity [263–265], because of the n-type or metallic behavior of the nitrogen functional groups, which change the

12

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Fig. 8. (a) cyclic voltammograms (CVs) of NiSe/Ni compared to those of Ni3 S2 /Ni and NiO/Ni in 1 M KOH at 10 mV s−1 , (b) CVs of NiSe/Ni with and without methanol (0.5 M) at 10 mV s−1 , and (c) CVs of NiSe/Ni, Ni3 S2 /Ni, and NiO/Ni in 1 M KOH containing 0.5 M methanol [249].

nucleation, increase the number of active sites, enhance the conductivity (due to the lone electron pair of the nitrogen atom), and improve the wettability of the nitrogen-doped supports [266–271]. Additionally, nitrogen-doped CNF supports (N-CNF) were prepared by electrospinning a mixture of polyacrylonitrile (PAN), polyaniline (PAi), and graphene in mass ratios of 10:2:0.3 followed by stabilization and calcination at 800 °C under an inert atmosphere [272]. These revealed not only a higher current density (0.31 A cm−2 at 0.6 V vs. SCE compared to 0.22 A cm−2 for NiOs /CNF), but also a lower onset potential (300 mV vs. SCE compared to 380 mV vs. SCE for NiOs /CNF). This improved performance was again related to the role of nitrogen, as also confirmed by Thamer et al. [264], who investigated the effects of nitrogen doping on the activities of Ni/CNF [264] and Co/CNF [273]. Ni NPs of size 2 nm surrounded by a thin layer of nitrogendoped carbon were prepared by Shi et al. [274] via the heat treatment of glucose, urea, and nickel acetate. In addition to the small size of the Ni NPs (2 nm), the nitrogen-containing functional groups and the porous carbon coating that provides highconductivity surface area, gave the catalyst superior activity toward ethanol oxidation with a current density of 327 mA cm−2 at 0.63 V vs. SCE (using 1 M ethanol in 0.1 M NaOH). Adding MnOx to the carbon support (7.5 wt%) resulted in decreasing the size of the Ni NPs and increasing the charge transfer rate, thereby increasing the activity of Ni toward methanol oxidation [275]. Similarly, microwave-assisted NaBH4 reduction of Ni on MnOx /C was found to be a useful technique for preparing small particles that enhance the activity toward methanol oxidation [276]. Ni/TiO2 NTs exhibited better performance when illuminated compared to that without illumination, especially for long-term operation [277]. The improved activity was related to the simultaneous methanol electrooxidation, photo-electrooxidation under illumination [278,279], and the role of TiO2 NTs in adsorbing methanol [280]. The long-term stability under illumination was related to the oxidation of intermediates as well as methanol by the OH produced during the oxidation of H2 O and OH− by the holes [281,282]. The effect of alloying of Ni with different elements (metals and nonmetals) onset potential, anodic peak potential and current density is summarized in Table 4.

4.4. Increasing surface area The surface area of a catalyst directly affects its catalytic activity. The higher surface area of the catalyst not only increases the number of active sites, which contribute to the oxidation activity of the catalyst but it also usually enhances the mass transfer between the catalyst and the reactants and/or the electrolyte ions. Different strategies have been used for increasing the surface area, as discussed in the following section.

4.4.1. New morphologies of the catalyst support Compared to conventional zero-dimensional catalyst supports, i.e., carbon nanoparticles, one-, two-, and three-dimensional catalyst support morphologies have demonstrated better mass transfer, catalyst dispersion, electron transfer, and thus better performance. This section summarizes the recent progress in preparing catalyst supports with new morphologies. 4.4.1.1. One-dimensional support. Catalyst supports with onedimensional morphologies, such as NFs, NTs, and nanorods have demonstrated high performance in energy conversion/storage devices, owing to their high electrical conductivity (especially in case of metal oxides), improved mass transfer properties, and higher aspect ratio [286–288]. A NF morphology can be obtained using different methods, such as drawing [289–291], template [292–294], self-assembly [64,295,296], and electrospinning [297–306]. Among these methods, electrospinning is the most common, owing to its simplicity, cost-effectiveness, high productivity, room-temperature operation, and applicability for a wide range of materials. Carbon nanofiber. Metal-based catalysts supported on CNFs are usually prepared through the mixing of a transition metal(s) precursor (usually acetate) with a low-thermal-stability polymer such as polyvinyl-alcohol (PVA) or PVP, followed by electrospinning, stabilization, and calcination under an inert atmosphere such as N2 or Ar. During this process, the transition metal precursor is decomposed into metal atoms that are usually embedded in the CNF, and the other part presents on the surface. The metal surface is usually covered by a thin carbon layer that acts as a protective layer for the transition metal. Moreover, this carbon layer increases the adsorption of the fuel on its surface, improving the overall performance [180]. The doping of Ni/CNF alloyed with different transition metals such as Cu [180], Cd [307], Mn [174], Sn [308,309], Co [145,149], and Cr [310] resulted in high oxidation activity toward low-molecular-weight hydrocarbons. For instance, alloying of Ni/CNF with different percentages of Co exhibited an increase in activity toward methanol until 50% Co, and then the activity decreased with further increases in the Co content [145]. Compared to NP catalysts, NF catalysts have demonstrated better performance, owing to the improved electron and mass transfer through the NF structure, especially at a high applied voltage (high reaction rate). As discussed in the alloying section, Co itself showed no catalytic activity toward methanol oxidation. Therefore, the performance decreased with further increases in the Co concentration beyond 50%. The effects of different supports with different morphologies, i.e., graphene “Ni/Gr” (Fig. 9a), CNF “Ni/CNFs” (Fig. 9b), and carbon NPs “Ni/CNPs” (Fig. 9c), on the activity of Ni toward ethanol oxidation have been investigated [311]. It was found that changes in the support morphology resulted in changes in the optimum Ni loading on the surface, where the optimum Ni loadings were 6, 10, and 60% for CNPs, CNFs, and Gr supports, respectively. Such increases in

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

13

Table 4 Effect of doping of Ni with metal and nonmetal on the onset potential and current generation. Catalyst

Preparation method

Morphology

Onset potential [V]

Anodic peak potential [V]

Anodic peak current density [mAcm-2 ]

Fuel conc. & electrolyte

Scan rate [mVs-1 ]

Ni2 Cr1

Nanoparticles

0.465

0.765

16 at 0.765 V

[138]

Nanoparticles

0.209

0.479

~16

50

[193]

Ni-Al

Electrodeposition

Layered structure

0.425

0.645

1.4 mA at 0.645 V

50

[143]

Nickel–cobalt hydroxide Ni

Electrodeposition

Nanoparticles

0.149

0.299

20

10

[150]

Electrodeposition

Nanoparticles

0.329

~0.399

22

10

[153]

Electrodeposition followed by alkaline leashing Electrodeposition followed by alkaline leashing Electrodeposition

Rough nanoparticles

0.249

0.399

24

1 M MeOH, 0.25 M NaOH 0.33 M urea, 1 M KOH 0.5 M MeOH, 0.5 M KOH 0.33 urea, 0.5 M KOH 0.33 M urea, 0.5 M KOH 0.33 M urea, 5 M KOH

100

Ni3 Co2 /C

Thermal decomposition Chemical reduction

10

[153]

Rough nanoparticles

0.39 0.289

~0.54 0.439

67

0.33 M urea, 5 M KOH

10

[153]

Films

0.32

0.506

~1 (normalized)

50

[161]

Hydrothermal

Nanoparticles

−0.085

~0.58

~70

50

[172]

Hydrothermal followed by heat treatment at 800 °C in air for 2 h Hydrothermal followed by heat treatment at 800 °C in air for 2 h Hydrothermal

Nanoparticles

0.34

–

1.79 at 0.5 V

0.1 M urea, 0.5 M NaOH 0.5 M urea, 1 M KOH 0.33 M urea in 1 M KOH

10

[176]

Nanoparticles

0.34

–

1.81 at 0.5 V

0.33 M urea in 1 M KOH

10

[176]

0.29

0.5

6.9

10

[176]

0.14

–

0.33 M urea in 1 M KOH 0.5 M MeOH in 0.1 M NaOH 0.33 M urea in 1 M KOH 2 M EtOH in 1 M NaOH 0.33 M urea in 1 M KOH

100

[185]

10

[190]

50

[192]

10

[205]

10

[234]

10

[236]

10

[245]

10

[245]

10

[146]

50

[251]

Leached Ni-Zn-Co Leached Ni-Zn

Co7 Ni3 -hcp NiMn/C, 90 wt%Ni Ni2 MnO4

NiMn2 O4

Ni1.5 Mn1.5 O4 NiCoCu/graphite electrode Ni60 Cr40 /C

Electrodeposition

Hexagonal nanoparticles Nanoparticles

Chemical reduction

Nanoparticles

0.32

–

Ni-Cr2 O3

Electrodeposition

Nanoparticles

0.349

0.649

15 at 0.5 V, 20 at 0.6 V 2933 mA/mgNi at 0.55 V 330

β -Ni2 P2 O7

Precipitation [NH4 ]NiPO4 •6H2 O, followed by calcination in air at 300 °C Sequential impregnation Impregnation

Nanoparticles/GCE

0.345

–

40 at 0.545 V

Nanoclusters

0.319

0.439

~500 A/cm2 mg

Nanoparticles

0.339

~0.459

682.9 mA/cm2 mg

20%Ni-20%WC/C 20%Ni-20%WC/C

2

0.33 M urea in 1 M KOH 0.33 M urea in 1 M KOH 0.33 M urea in 5 M KOH 0.33 M urea in 5 M KOH 0.33 M urea in 5 M KOH 2 M MeOH in 1 M NaOH

Reference

Ni5 Sm-P/C

Chemical reduction

Nanoparticles

–

0.368

327.8 mA/cm g

Ni-P/C

Chemical reduction

Nanoparticles

–

0.358

213.2 mA/cm2 gmg

Ni5 Sm/C

Chemical reduction

Nanoparticles

–

0.407

15.5 mA/cm2 g

Ni-B/carbon electrode

Thin film

0.384

0.722

300 at 722 mV

Thin film

0.359

0.773

282 at 773 mV

2 M EtOH in 1 M NaOH

50

[251]

Thin film

0.495

–

30 at 665 mV

0.5 M MeOH in 1 M KOH

50

[254]

Nanoparticles

0.299

0.399

26

100

[134]

Nanoparticles

0.47

0.7

920 μA at 0.7 mV

in

20

[283]

Nanoparticles

~0.355

0.545

70.4

1

10

[284]

NiCr/C, 40%Cr

Chemical reduction

Nanoparticles

0.32

0.55

~97

0.33 M urea, M 0.1 M MeOH 0.1 M NaOH 0.33 M urea, M 0.33 M urea, M

5

Ni/SDSPOAP/CPEa Ni2 P-C

Electroless deposition for 60 min Electroless deposition for 60 min Electroless deposition on Ti sheet Reverse phase hydrolysis Electropolymerization Hydrothermal

1

10

[285]

Ni-B (5% B)/carbon electrode Ni-B/Ti

LaNiO3 /C

All potentials are versus Ag/AgCl saturated. a SDS is sodium doceyl sulfate, and POAP is Poly(o-aminophenol).

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M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Fig. 9. SEM images of Ni (6 wt%) decorated on different supports: (a) Ni/Gr, (b) Ni/CNF, and (c) Ni/CNP, reproduced with permission from [311].

the optimal Ni loading were related to the available support surface area. Interestingly, CNFs revealed a higher diffusion coefficient compared to CNPs and Gr. However, at 10 wt% Ni, the peak current density and peak potential were 53 mA cm−2 at 0.75 V (vs. Ag/AgCl), 28 mA cm−2 at 0.63 V (vs. Ag/AgCl), and 25 mA cm−2 at 0.6 V (vs. Ag/AgCl), for Ni/CNFs, Ni/Gr, and Ni/NPs, respectively, using 50 mV s−1 in 2 M ethanol in 1 M KOH solution. Importantly, using stable thermal polymers, such as PAN in an electrospinning process, remarkably affected the position of the synthesized metal atoms on the CNF, such that all the metal atoms were on the surface of the CNF and none were embedded in the CNF. However, the Ni NPs prepared on the surface of the CNF (NiO/CNF) revealed high methanol oxidation activity [263], and the activity was further increased by the doping of nitrogen into the CNF supports (NiOs/N–CNF) [264,272]. The incorporation of 20 wt% of CNFs into mesoporous Ni3 (PO4 )2 NPs increased the surface area (84.3 m2 g−1 vs. 19.0 m2 g−1 ), ECSA (3.0 vs. 1.15 cm2 ), and electrical conductivity, resulting in an increase in its activity toward ethanol oxidation (0.5 M NaOH containing 0.2 M EtOH), where the anodic peak potential increased 10 times, and the onset potential decreased from 0.43 to 0.4 (vs. Ag/AgCl) [312]. Table 5 summarizes the different studies using electrospinning in preparing nonprecious catalysts for direct alcohol FC applications. Carbon nanotubes. CNTs are another class of one-dimensional supports that have outstanding electrical and mechanical properties, and they are effectively used in various applications, including energy storage/conversion devices. CNTs are classified into two main categories: single-walled CNTs (SWCNTs) and multi-

walled CNTs (MWCNTs). CNTs are manufactured using several techniques, such as chemical vapor deposition (CVD) [317,318], arc discharge [319,320], and laser ablation [321,322]. Multiwalled carbon nanotubes (MWCNTs) have the merits of a highly graphitized wall, nanosized channel structure, and sp2-C-constructed surface [287,323], which result in improving the electron and mass transfer [324]. Owing to their outstanding properties, CNTs are extensively used in FCs, including the preparation of nonprecious anodic catalysts that can be used for methanol, ethanol, and urea oxidation. CNTs have high electrical conductivity, and their morphology facilitates the good dispersion of Ni NPs, resulting in the enhanced catalytic activity of Ni/MWCNT toward urea electrooxidation, compared to Ni/C. For example, the peak current of Ni/MWCNT was found to be two times higher than that of Ni/C using 0.33 M urea dissolved in 1 M KOH [102]. Among the different loadings of Ni over MWCNT support (Ni/MWCNT) prepared by the reduction method, Ni/MWCNT with 80 wt% Ni achieved the highest urea oxidation activity, owing to the good dispersion of the catalyst over the MWCNTs, which resulted in a high ECSA of the catalyst [324]. The incorporation of MWCNTs in nanoporous nickel phosphate resulted in increasing its activity toward ethanol oxidation [85]. The MWCNTs could increase the surface area, facilitate the electron transfer, and improve the mass transfer between the electrode and the anolyte [325–327]. The application of ZnO/MWCNTs as a support for the Ni–Cu alloy (Ni–Cu/ZnO@MWCNT) enhanced the dispersion of the catalyst Ni–Cu on the surface, and using

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

15

Table 5 Activity of nonprecious catalysts prepared by electrospinning toward methanol, ethanol, and urea oxidation. Catalyst

Onset potential

Anodic peak potential

Anodic current density

Conc. (M)

Scan rate (mV s−1 )

Ref.

NiO

0.35 V

–

1 M MeOH

50

[313]

Co-Cr/CNF Co/N-CNF Ni-Co/CNF Ni/N-CNF

4 M MeOH 0.1 M MeOHb 3 M MeOH 3 M MeOH

50 100 50 50

[314] [315] [145] [264]

Cox Cuy /CNF NiO/CNF NiO/N-CNF

0.32 V for CrCo(0.1/1)/CNF 0.4 V 0.11 V using Ni0.5 Co0.5/CNF 0.35 V for Ni/N–CNF (5 wt% urea) 0.3 V using Cu5% Co95% /CNF 0.425 V 0.345 V

2 M MeOH 1 M MeOH 1 M MeOH

50 50 50

[316] [263] [263]

Ni (NPs) Ni/Gr Ni/CNF Ni/CNP CoCu/CNF Ni0.9 Co0.1 /CNF Ni0.1 Co0.9 /CNF Ni-Co/CNF Ni5 Cd5 /CNF Ni0.9 Co0.1 /CNF NiMn/CNF Ni/CNF Ni/CNP Ni/N-CNFa

0.37 V 0.32 V (Ni/Gr 6 wt%)a 0.34 V (Ni/CNF 10 wt%)a 0.34 V 0.37 V 0.32 V < −0.05 V 0.30 V 0.35 V 0.32 V 0.29 V 0.32 V 0.39 V 0.36 V

0.5 M EtOH 4 M EtOH 3 M EtOH 1 M EtOH 1 M EtOH 5 M EtOH 3 M EtOH 3 M EtOH 1 M urea 5 M urea 2 M urea 4 M urea 1 M urea 0.5 M urea

50 50 50 50 50 50 50 50 50 50 50 50 50 50

[311] [311] [311] [311] [180] [149] [149] [183] [307] [149] [174] [174] [174] [265]

0.9 Va

15 mA cm−2 at 0.6 V for NF and 3.5 mA cm−2 for NP 81 mA cm−2

c

c

d

d

0.8 Va

176 mA cm−2

– 0.675 0.675

– 220 mA cm−2 (0.17 mg cm−2 ) 315 mA cm−2 (catalyst loading is 0.17 mg cm−2 ) 37.5 mA cm−2 103.5 mA cm−2 (Ni/Gr 60 wt%) 68.5 mA cm−2 46.5 mA cm−2 14 mA cm−2 142 mA cm−2 – 145 mA cm−2 68 mA cm−2 142 mA cm−2 290 mA cm−2 g−1 215 mA cm−2 g−1 140 mA cm−2 g−1 51.5 mA cm−2

0.57 1 V (Ni/Gr 60 wt%)a 0.85 Va 0.78 V 0.49 Vb 0.86 Va – 1 Va 0.67 V 0.86 Va 0.58 V 0.54 V 0.51 V 0.73 Va

All potentials are versus Ag/AgCl saturated. a Although peak appeared at a voltage higher than 0.7 V, methanol oxidation is far from this value; therefore, this catalyst has a high anti-poisoning property, and the current obtained would be related to the electrolysis of the fuel, not its oxidation. b For 0.4 to 0.8 V, 0.1 M is best, whereas, for higher voltages, 2 M is best; therefore, 0.1 M is taken as optimal. c No clear peak appeared, even at 1 M, with a very tiny slope at 0.6 V and a current density of 30 mA cm−2 at 0.1 M methanol. d Anodic peak potential did not appear, even at the potential of 1 V; therefore, this catalyst has a high anti-poisoning property.

this catalyst in a DUFC yielded a power density of 26.9 mW cm−2 at 20 °C, which increased to 44.36 mW cm−2 at 50 °C (Fig. 10) [125]. Jin et al. reported that Ni electrodeposited on the surface of MWCNTs, with and without the grafting of a 4-nitroaniline (NA) radical monolayer on the surface of the MWCNTs, yielded high activity toward ethanol oxidation in 0.1 M NaOH [328]. In other study, Deng et al. investigated the effect of the incorporation of MWCNTs (15, 25, 35, and 45 wt%) into NiCo/C–N (fabricated by the direct pyrolysis of a Ni–Co salt/polyaniline (PANI) composite at 800 °C under a N2 atmosphere), on methanol and ethanol oxidation activities [329]. The results showed that the oxidation activities toward methanol (1 M) and ethanol (1 M) are significantly increased with increasing MWCNT content to 35 wt%, whereas they decreased at

higher MWCNT percentages. The addition of the MWCNTs (up to 35 wt.%) augmented the charge transfer between the catalyst and solution, as well as improving the dispersion of the Ni catalyst, whereas a further increase in the CNTs (to 45 wt.%) decreased the active catalyst sites (Ni NPs), thereby reducing the catalytic activity [329]. The modification of GC with CNTs to be used as a support for a ternary alloy of CuCoNi (prepared by electrodeposition) improved the catalyst activity toward methanol oxidation [186]. The activities of different Ni-based catalysts using CNTs as support are summarized in Table 6.

4.4.1.2. Two-dimensional support. Graphene (Gr) is an exciting material, which consists of a single layer of carbon atoms (one atom

Fig. 10. (a) In-situ performance of a DUFC using a Ni-Cu/ZnO@MWCNT anode (anolyte: 0.70 M urea/3 M KOH solution and catholyte: humidified air). (b) Schematic showing the reaction on the surface of the Ni-Cu/ZnO@MWCNT anode catalyst, reproduced with permission from [125].

16

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Table 6 Effect of CNT support on current density and onset potential of Ni-based catalysts. Catalyst

Preparation method

Morphology

Onset potential [V]

Anodic peak potential [V]

Anodic peak current density [mA cm−2 ]

Fuel conc. and electrolyte

Scan rate, mV s−1

Ref.

CuCoNi@CNT

Electrodeposition

0.225

–

0.3

–

20

[125]

Nanoparticles/CNTs

0.309

0.459

46.6

10

[235]

Thin film over NTs

0.345

1 M MeOH in 1 M NaOH 0.07 M urea in 0.4 M KOH 0.33 M urea in 1 M KOH 0.5 M MeOH in 1 M KOH

[186]

Hydrothermal

4.2 mA at 0.645 V 30.02 at 0.38 V

100

NiCu/ZnO@MWCNT Ni-WC/MWCNT

Nanoparticles over CNT Nanoparticles

50

[254]

Monodisperse/CNTs

~0.299

0.499

~1700 (/mg)

0.33 M, 1 M

10

[324]

NiCo/C-N/CNT

Impregnation method Electroless deposition on TiO2 NTs Thermal decomposition Direct pyrolysis

Nanoparticles /CNTs

0.38

–

181 at 0.85 V

50

[329]

NiCo/C-N/CNT

Direct pyrolysis

Nanoparticles /CNTs

0.36

–

213 at 0.85 V

50

[329]

SNF–MWCNT using 5 ml ionic liquid

Microwave heating process

Hollow nanocubes/CNTs

0.295

0.395

9

1 M EtOH in 1 M NaOH 1 M MeOH in 1 M NaOH 0.33 M urea in 5 M KOH

20

[330]

Ni-B/TiO2 (NTs)

80%Ni/MWCNT

360 at 665 mV

All potentials are versus Ag/AgCl saturated.

thick) bonded together in a honeycomb crystal lattice (hexagonal lattice) [331,332]. Gr has shown promising properties, such as high crystal quality, excellent mechanical properties (Young’s modulus ~1.1 TPa and fracture strength of 0.125 TPa) [333], high thermal conductivity (~5 kW m−1 K−1 ) [334], good electrical conductivity (based on the preparation methods) [335], high intrinsic mobility (2 × 105 cm2 v−1 s−1 ), high optical transmittance (~97.7%), and a large theoretical specific surface area (2630 m2 g−1 ) [336–338]. The high surface area of Gr (2630 m2 g−1 ), its high conductivity, and ability to facilitate electron transfer through itself make the Gr and its derivatives promising electrode materials for electrocatalytic applications in energy devices such as FCs [339]. Gr is widely used as a support for precious catalysts in different lowtemperature FCs [340–343], or even as a standalone catalyst at the cathode [344–347]. This section focuses on the application of Gr as a support for the non-precious catalyst for methanol, ethanol, and urea. Pure Ni catalysts have the disadvantages of easy COpoisoning as well as low electroactive sites [348–350]. Therefore, three different forms of Ni (Ni, Ni alloys, and Ni hydroxide/oxides) were supported onto Gr to enhance its catalytic activity and its anti-poisoning property. For example, a one-pot electroreduction process of graphene oxide (GO) with Ni ions successfully produced a reduced graphene oxide (rGO)-supported Ni [351]. The fabricated rGO-Ni nanocomposites improved the electrochemical performance and enhanced the current density. Nickel–molybdenum incorporated into graphene (Ni–Mo/Gr) and used for alkaline urea electrooxidation [349]. However, by investigating the structure-activity relationships and electrocatalytic kinetics of Ni2 Mo1 /Gr, they found that the electrocatalytic activity and stability were affected by the structural and electronic effects. Ni supported on rGO (Ni/rGO), prepared by the electrochemical reduction of Ni on the surface of a GC electrode precoated with GO, demonstrated high urea oxidation activity compared to that of Ni NPs (with a two-fold higher current density at 0.6 V vs. Hg/HgO). The high activity was related to the large active surface areas of the Gr sheets, which can promote electron transfer of urea oxidation [351]. Ni/rGO prepared by refluxing a mixture of nickel acetate (NiAc) and GO at 120 °C for 10 h, followed by calcination under an inert atmosphere 850 °C, exhibited high urea oxidation activity compared to the Ni NPs. Moreover, the activity increased as the initial wt% of NiAc to GO increased to 100 wt%, but it decreased as the initial NiAc content increased further to 150 wt%. At low NiAc (25 wt.%), the available Ni NPs over the Gr sheet were few, and thus performance was low.

On the contrary, at high NiAc (150 wt%), Ni covered the whole surface of the Gr, which obstructed the adsorption of urea molecules, and thus the performance decreased [352]. Glass et al. [353] directly reduced nickel chloride in a GO aqueous solution followed by annealing at different temperatures (30 0–70 0 °C) for urea electrooxidation (Fig. 11). They found that the GO support affected the Ni NPs, where the reduction of Ni directly onto the GO yielded a smaller particle size and improved the urea oxidation, compared to the plain Ni catalyst. Ni/rGO annealed at 300 °C enhanced the urea oxidation current, and a more favorable onset potential was observed, compared to that of the bare Ni catalyst. These catalysts were examined in a micro direct urea/hydrogen peroxide FC. The OCV of the examined cells were 0.21, 0.26, 0.36, and 0.33 for Ni, Ni/rGO, Ni/rGO-300, and Ni/rGO-700, respectively (Fig. 11). The annealing of the catalyst was shown to increase the OCV compared to the nonannealed and bare nickel catalysts. However, the Ni/rGO-700 sample displayed a slightly lower OCV compared to the Ni/rGO-300, which was related to the partial oxidation of the Ni with the GO at high temperature [353]. Ni prepared on the surface of Gr, followed by calcination at 850 °C under an argon atmosphere (Fig. 12a), revealed high activity toward methanol oxidation compared to that of Ni NPs (Fig. 12b). The activity was further increased by incorporating Co (Fig. 12c). The high activity with Gr was related to the outstanding conductivity and adsorption capacity of the Gr support, as well as the effect of Co [354]. In addition to the superior performance of Gr, adding nitrogen to the Gr structure (N-rGO) improves its electrical properties, where N-rGO is an n-type dopant that acts as a source of electrons [355]. The N-rGO support, synthesized by an electrochemical method using urea choline chloride and urea as the reducing and doping agents, exhibited high activity toward ethanol oxidation, owing to the role of nitrogen as well as the high surface area and high electrical conductivity of graphene [356]. Ni NPs decorated on Gr, synthesized by a one-step process using commercial sugar and different concentrations of NiAc (0, 1, 2, 3, 4, and 5 wt%), exhibited higher activities toward urea oxidation compared with unsupported Ni NPs. Importantly, the use of very low or very high concentrations of the Ni precursor (NiAc) resulted in decreasing the active surface area: at lower NiAc content, there was a Ni deficiency, whereas, at higher percentages, the Ni particle size increased [357]. NiCo2 O4 -rGO, fabricated by growing hexagonal nanoplates of NiCo2 O4 on rGO using a two-step

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Fig. 11. Schematic representation of the fabrication of Ni/rGO catalyst and the performance of the obtained electrodes at different annealing temperatures towards urea oxidation, reproduced with permission from [353].

solution-phase method, achieved high activity toward methanol oxidation, where the rGO controlled the aggregation of the NiCo2 O4 nanocrystalline growth during the formation of the NiCo2 O4 nanoplates [358,359]. Overall, the enhanced electrocatalytic activity of Gr-supported catalysts for methanol, ethanol, and urea is attributed to the microstructure of Gr (defects and functional groups) and the interaction of single or multi-catalysts with Gr. Furthermore, it was suggested that the functional groups on the Gr surface may act as anchoring sites for the catalyst precursor, preventing the catalyst aggregation and resulting in an increase in the surface area and the catalyst distribution on the surface [360]. 4.4.1.3. Three-dimensional support. A highly porous carbon sponge, prepared by a template method, supporting Ni exhibited high urea oxidation activity (30 mV decrease in the onset potential, and eight times higher anodic current density), owing to the significant increase in the ECSA [62]. Through the hydrothermal reduction of GO and Ni ions, a magnetic, porous, and lightweight three-dimensional (3D) freestanding Ni NPs/Gr aerogel was successfully fabricated (Fig. 13(a–c)) [361]. The 3D composite architecture exhibited excellent electrocatalytic activity and high stability for ethanol electro-oxidation in an alkaline electrolyte compared to that in the case of Ni/MWCNTs (Fig. 13 (d and e)). The 3D Gr aerogel provided a good dispersion of the Ni NPs, in addition to the high conductivity of the Gr and the superior porosity of the Gr aerogel, thereby facilitating the mass transfer to the active sites [361]. However, the incorporation of PVA during the preparation of the Gr aerogels resulted in increasing the chemical and mechanical stabilities of the aerogels (Fig. 14 (a–d)) [193]. The addition of Fe ions increased the number of pores in the Gr aerogels, resulting in an increase in the ECSA (from 30 to 70.8 m2 g−1 ), in addition to the bifunctional catalyst activity of Ni and Fe. As a result, the addition of iron ions reduced the onset potential by 51 mV and improved the current density (170% at 0.5 V (vs. Ag/AgCl)) [193]. The improved activity was also related to the porous network of NiO–Fe2 O3 -induced gelation of the GO layers [197,362]. Three-dimensional NiCu/rGO foam is prepared by a selfpropagated combustion method [363]. The addition of rGO increases the porosity of the foam. However, the Cu60 Ni40 /rGO demonstrated high methanol oxidation activity (1 M methanol in

1 M KOH), compared to that prepared without rGO, i.e., Cu60 Ni40 . The existence of CuO (110) is important for the oxidation of methanol without forming CO intermediates [364], where it promoted the oxidation of methanol into formaldehyde without forming the CO-poisoning intermediates [365], the foam also showed high CO oxidation activity via the direct oxidation of CO without being adsorbed [366]. A 3D porous electrode of Ni/CNT supported on the surface of a porous sponge (Ni/CNT@Sponge) was investigated under actual FC operation showing, 3.9 mW cm−2 using 3 M urea [367]. Additionally, El-Deeb et al. reported that a nanoneedle structure of nickel cobaltite on the surface of carbon aerogel (hydrothermally prepared) exhibited high methanol oxidation activity compared to that of an unsupported one. The high activity was related to the synergetic effects between the pore geometry and porosity of the carbon aerogel [368]. Moreover, highly porous Ni-Co/MWCNTs aerogels demonstrated a superior urea oxidation activity owing to the synergetic effect of the porous support and the alloying effect [369]. The activities of different Ni-based catalysts using Gr as a support are summarized in Table 7. 4.4.2. Ni-based catalysts with new morphologies In the previous section, the Ni-based catalysts had a zerodimensional morphology, i.e., NPs, and the supports had different morphologies, i.e., one-, two-, and three-dimensional. This section aims to summarize the different morphologies of Ni-based catalysts, rather than the zero-dimensional ones, that are used to increase the catalyst activity. 4.4.2.1. One-dimensional catalyst. The preparation of NiO in the NF morphology using electrospinning significantly improved its activity toward methanol oxidation, where the current generated from methanol oxidation is 7.5 mA cm−2 , which is three times greater than that obtained in the case of Ni NPs at 1 M methanol and 0.5 V (vs. Ag/AgCl) (Fig. 15a) [313]. Although the NF morphology did not affect the onset potential (0.35 V), a higher optimum methanol of 1 M is obtained compared to 0.1 M for the Ni NPs (Fig. 15a), which was associated with the ease of mass transfer and electron transfer in the NF compared to the NPs (Fig. 15b). One-dimensional Ni nanowire arrays (Ni NWA) were successfully produced through electrodeposition on the surface of a polycarbonate template [64,372]. The Ni NWA significantly enhanced

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Fig. 12. (a) Schematic diagram showing the preparation of CoNi decorated on a graphene surface, (b) Cyclic volt.ammograms of Gr, Ni NPs, and Ni-Gr.(c) Cyclic voltammograms of NiCo with different compositions decorated on graphene; using 3 M methanol in 1 M KOH at 50 mV s−1 and 25 °C, reproduced with permission from [354].

Table 7 Effects of using graphene support on the current density and onset potential of Ni-based catalysts. Catalyst

Preparation method

Morphology

Onset potential [V]

Anodic peak potential [V]

Anodic peak current density [mA cm−2 ]

Fuel conc. and electrolyte

Scan rate, mV s−1

Ni@carbon sponge

Electrodeposition

3D carbon sponge

0.35

0.47

~290

15

NiOFe2 O3 /rGO/PVA

Aerogel

~0.36

0.51

44.6

Nanoparticles(rough texture nanoparticles)/Gr

0.38

5.84 + 0.48 at 0.65 V

Co0.2 Ni0.2 / graphene NiCo/N-rGO

Chemical reduction followed by freeze drying GO modified hummer methodNi/rGO was synthesized using aqueous-based reduction Modified hummer Electrochemical

0.1 M urea, 5 M NaOH 0.33 M urea in 1 M KOH

Nanoparticles /Gr

−0.04

215

Nanoparticles/Gr

0.2

NiCo2 O4 /rGO

Hydrothermal

Hexagonal NP/Gr

301

16.6

Graphene/NiCo2 O4

Hydrothermal

0.326

Cu60Ni40 /rGO

Self-propagated combustion Hydrothermal

Flower hollow nanostructures/Gr Porous foam

0.296

–

791 A/g at 0.406 V 280 at 0.944 V

Sheets

~0.38

~0.75

~135

Hydrothermal

Nanorods/Gr

0.351

Ni/rGO-300

Ni-loaded Gr, from 1:1 of NiAc:GO NiMoO4 /graphene

All potentials are versus Ag/AgCl saturated.

–

80 at 0.85 V

49

Ref.

20

[193]

0.33 M urea in 1 M KOH

20

[353]

3 M MeOH in 1 M KOH 1 M EtOH in 0.5 M KOH 0.5 M MeOH in 1 M KOH 0.5 M MeOH in 1 M KOH 1 M MeOH in 1 M KOH 0.5 M urea in 1 M KOH 2 M MeOH in 1 M KOH

100

[354]

50

[356]

50

[358]

50

[359]

100

[363]

50

[370]

50

[371]

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Fig. 13. Digital images of the as-prepared 3D Ni/Gr aerogel (a) floating on water, (b) after drying, (c) the aerogel can be lifted using a magnet, and the inset shows a schematic of the formation of the 3D Ni/graphene aerogel in the hydrothermal process. (d) Cyclic voltammograms of Ni NPs/Gr aerogels compared to that of Ni NPs/MWCNTs at 100 mV s−1 using 0.1 M ethanol in 1 M NaOH. (e) Chronoamperometry measurements on Ni/Gr at 0.66 V vs. Hg/HgO using different ethanol concentrations in 1 M NaOH [361].

the ECSA by 8.57 times compared with bare Ni sheet [372]. Compared to the Ni bare sheet, the NWA structure of Ni demonstrated higher and more stable activities toward urea, hydrogen peroxide, and urea peroxide oxidation. Apart from the high ECSA of the NWA, it also exhibited better mass and charge transfer [372]. Nanoporous anodic aluminum oxide (AAO) template is considered as an effective method for preparing one-, two- and threedimensional nanostructures [373]. Yan et al. [374] prepared Ni NWA by electrodeposition on AAO template, that demonstrated high urea oxidation activity due to their high surface area of prepared Ni NWA. The incorporation of Co in the Ni NWA, i.e., Ni– Co NWA significantly improved its activity toward urea oxidation, where using 10% Co (Ni 9) decreased the onset potential to 0.19 V

vs. SCE, and the current density increased to 380 mA cm−2 at 0.6 V vs. SCE. The NiCo NWA showed a considerably high power output (7.4 mW cm−2 at 25 °C using 0.33 M urea), and stability under actual DUFC operation. Additionally, mesoporous NiCo2 O4 fibers, prepared using an easily controlled template-free method, demonstrated high ethanol oxidation activity, compared with NiO and Co3 O4 nanopowders, in terms of current density and onset potential, owing to the high surface area of the porous NF structure [375]. A core/shell structure of Cu/NiCu NWs exhibited higher methanol oxidation activity compared with Cu NW or NiCu NPs (Fig. 16) [142]. This could be attributed to the fast charges in the Cu NW that act as a source of

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Fig. 14. (a–d) Graphical representation of the formation of the NiO-Fe2 O3 /rGO/PVA aerogel and optical images of the different prepared materials. (e) Effect of adding Fe2 O3 on the activity toward urea oxidation, reproduced with permission from [193].

electrons to the active NiCu alloy shell, as well as the synergetic of NiCu [182–184]. Owing to the hollow structure of nanotubes (NT), they had a higher surface area than that of NF and NW, and thus, some researchers preferred preparing the catalyst in the NT form. Muench et al. found that the activity of unsupported Ni NT, prepared by electroless plating on the surface of an ion-track-etched poly-

mer template (Fig. 17a) [253], toward ethanol oxidation is higher than that of Ni NW. The current density at 0.6 V (vs. Ag/AgCl) for Ni NT was 14.8 mA cm−2 compared to 2.18 mA cm−2 for Ni NW, using 0.5 M ethanol in 0.1 M NaOH at 50 mV s−1 (Fig. 17b). This improvement with the Ni NT was ascribed to its high surface area and the easy transfer of the electrolyte ions to the active sites [253]. Moreover, the doping with B resulted in a

Fig. 15. (a) Forward scan of Ni with different morphologies, i.e., nanofibers (NF) and nanoparticles (NP) in 1 M KOH containing different methanol concentrations, and. (b) Schematic diagram showing the electron transfer in the nanofibers and nanoparticles [313].

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Fig. 16. (a) Schematic diagram showing the preparation of Cu/NiCu core-shell structure by in-situ reduction of metal ions using nickel chloride and capping agent (D1821) in oleylamine medium. Images of the solution at different reaction temperatures of 85 °C (b) and 185 °C. (c) Images of Cu NWs suspension at 185 °C for 4 h. (e) Image of Cu/NiCu NWs-220 suspension at 220 °C for 1 h. TEM images (scale bar is 50 nm) of the Cu NWs (f), and Cu/NiCu NWs-220 (g). (h) Schematic diagram showing mechanism of methanol oxidation over Cu/NiCu NWs. (i) Cyclic voltammograms of Cu NW, Cu/NiCu core-shell NW, and NiCu NPs compared with that of a bare GCE using 1 M methanol in 1 M KOH at 50 mV s−1 , reproduced with permission from [142].

Fig. 17. (a) Schematic diagram showing the preparation of NiB NT by electroless plating on the surface of an ion-track-etched polymer template; (b) cyclic voltammograms of Ni NW, Ni NT, and NiB NT in 0.1 M NaOH at 50 mV s − 1 , reproduced with permission from [253].

further increase in the activity owing to the role of B as discussed above. Song et al. [376] prepared mesoporous nickel phosphate nanotubes (Meso NiPO NT), mesoporous nickel phosphate nanosheets (Meso NiPO NS), microporous VSB-5, and mesoporous nickel phosphate (Meso NiPO). All the samples showed high activity toward methanol oxidation, and both the Meso NiPO NT and NS samples revealed the highest activity. Although Meso NiPO NS demonstrated a higher anodic peak current density that obtained in the case of Meso NiPO NT, i.e., 44.97 mA cm−2 at 0.66 V and 40.83 mA cm−2 at 0.7 V, respectively, the latter exhibited higher stability and anti-poisoning performance, as confirmed from the ratio of the forward to backward anodic peak current densities.

NiO (aggregated nanoflakes), Co3 O4 (irregular spheres), and NiCo2 O4 (nanorods) were combined with β -Ni(OH)2 (NPs) and hybrid catalysts were investigated toward methanol oxidation. β Ni(OH)2 –NiCo2 O4 showed the highest methanol oxidation activity using 1 M methanol in 0.1 M NaOH [167]. Transmission electron microscopy (TEM) images show that NiCo2 O4 maintained its nanorod structure with β -Ni(OH)2 as small NPs adhered on its surface. This structure had the highest ECSA of 0.418 cm2 compared to 0.19, 0.085, 0.35, 0.08, 0.071, and 0.073 cm2 for β -Ni(OH)2 – NiO, β -Ni(OH)2 –Co3 O4 , NiCo2 O4 , NiO, Co3 O4 , and β -Ni(OH)2 , respectively. In another study, NiMnoO4 nanorods supported on the surface of carbon or Gr were fabricated using a hydrothermal synthesis method and used as a catalyst for methanol oxidation. The carbon-supported NiMnO4 showed higher electrooxidation activity

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Fig. 18. FESEM images of 3D urchin-like oxides of Co3 O4 (a), Ni0.25 Co2.75 O4 (b), Ni0.5 Co2.5 O4 (c), Ni0.75 Co2.25 O4 (d), Ni1 Co2 O4 (e), and Co0.33 Ni0.67O (f). Cyclic voltammograms of 3D urchin-like oxides without (g) and with methanol (h) at 50 mV s − 1 [385].

than the Gr-supported one, owing to the one-dimensional nanostructure of the NiMnO4 , which leads to the uniform dispersion of highly conductive carbon, good electrical conductivity, and better ion transfer [371]. 4.4.2.2. Two-dimensional catalyst. Two-dimensional (2D) nanosheets (NS) have a high surface area, a large number of active sites, easy accessibility of electrolyte ions to the active sites, and thus higher activity [377–380]. NiOOH NS were prepared via a hydrothermal method using PVP as a surfactant

template and shape-directing agent [381]. The NiOOH NS were subsequently transferred by adsorption on the surface of electrochemically pretreated glassy carbon electrode (EPGC). The CVs of the NiOOH/EPGC in 0.1 M NaOH exhibited the successful transfer of the NiOOH and the linear increase in the anodic and cathodic peak potential with increasing the scan rate, indicating that the electron transfer for NiOOH occurs through a surface-controlled mechanism [382]. The prepared electrode demonstrated the activity for methanol and glucose oxidation. The performance of NiOOH/EPGC was significantly higher than that in the case of

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Fig. 19. Effect of the treatment time on the morphology of NiCo2 O4 [386].

NiOOH NS deposited on untreated GC, because of the rougher surface and the strong interaction between C–O and the NiOOH NS. The activity of Ni(OH)2 NS was significantly increased by heating under H2 S atmosphere, owing to the incorporation of metallic sulfur in the Ni(OH)2 NS, which increased the wettability and active sites and decreased the charge transfer resistivity [383]. 4.4.2.3. Three-dimensional catalyst. Soliman et al. [384] demonstrated that the morphology and the activity of Ni NPs, prepared by electrodeposition, are affected by the acidity of the bath solution, i.e., using a pH of 0.8 or 5.5 for NiSO4 solution. A spherulitelike particle was obtained in the acidic condition (pH of 5.5), and in the strong acidic condition (pH 0.8), a smooth and bald island was formed. Such a spherulite-like structure of the catalyst granted a higher surface area and better mass transfer. Three-dimensional urchin-like spinels Co3 O4 , Ni0.25 Co2.75 O4 , Ni0.75 Co2.25 O4 , Ni1 Co2 O4 , and cubic Co0.33 Ni0.67 O of 4 μm diameter were prepared by the hydrothermal method (Fig. 18(a–f)). Although all the catalysts had the same urchin-like structure, the Ni1 Co2 O4 catalyst exhibited the highest surface area, a mesoporous structure, and narrow pore-size distribution. As a result, Ni1 Co2 O4 exhibited the best performance toward methanol oxidation, and the lowest activity was obtained for the Ni-free sample, i.e., Co3 O4 , (Fig. 18(g and h)) [385]. Similar results were reported by Prathap et al. [210]. By controlling the hydrothermal treatment time, three different morphologies of NiCo2 O4 microspheres were obtained (Fig. 19) [386]. Initially, hydrothermal treatment for 4 h resulted in a formation of flower-like hierarchical structures (NCO-flower-like HSs), which changed to nanoflake@nanoneedle-like multiple hierarchical structures (NCO-MHS) when the time increased to 6 h; a further increase in the time (8 h) altered the structures to urchin-like hierarchical structures (NCO-urchin-like HSs) [387,388]. The NCOMHS had the largest surface area of 69.7 m2 g−1 , compared to 59.4 m2 g−1 for NCO-flower-like HSs and 63.2 m2 g−1 for NCOurchin-like HSs. As a result, the NCO-MHS exhibited the best activity toward methanol oxidation with an onset poetical of 0.42 V vs. Hg/HgO. The activity of the NiCo2 O4 , especially NCO-MHS, is related to the high surface area, the presence of mixed-valence oxides that could produce two different active sites for methanol oxidation, and the mesoporous structure that acts as channels for ions and mass transfer to the reaction sites [158]. For the same reasons, hollow spheres and nanocups of Ni(OH)2 monolayers [389] and hollow sodium nickel fluoride (SNF) nanocubes deposited on the surface of MWCNTs showed high urea oxidation activity [330], and the mesoporous nanoflower NiO [390] and nanospheres of NiCo2 O4 exhibited high methanol oxidation [391]. Through controlling the P and/or Ni precursors, Song et al. prepared different morphologies and crystal structures of Ni phosphate (NiPO), i.e., nanotubes (NiPO-a), flower-like (NiPO-b, -c), and stacked nanocrystals (NiPO-d) (Fig. 20a) [392]. Among the different prepared materials, NiPO-c with flower-like structure displayed the

best activity toward urea oxidation and the best stability with only 5% loss of performance, whether after 96 h of current discharge or after storing in urea solution for 134 days (Fig. 20b and c). In another study, urchin-like Ni and/or Co phosphides and/or phosphates with different molar ratios of Co to Ni salts (0.053 (NiCoP-1), 0.018 (NiCoP-2), 0.33 (NiCoP-3), and 1 (NiCoP-4)) were prepared. NiCoP-2 showed the highest surface area with a mesopore volume of 540.5 m2 g−1 [207]. Additionally, urchin-like NiP microspheres, prepared by a solvothermal method, showed high urea oxidation activity, not only with a significant increase in the surface area of the urchin-like structure microspheres compared to Ni NPs (75% increase in ECSA), but also with an improved electron transfer with P, and the easy removal of the intermediates [240]. The effect of catalyst morphology on the activities of different Nibased catalysts are summarized in Table 8. 5. Crystalline porous materials Zeolites and metal–organic frameworks (MOFs) are porous crystalline materials that have demonstrated significant catalytic activities in different applications [403–405]. Zeolites are built of tetrahedral SiO2 and AlO4 structures while MOFs are composed of metallic atoms connected by an organic linker. While zeolites are considered microporous materials, that is, their pore sizes are below 2 nm (and often even below 1 nm), MOFs are considered mesoporous solid materials because they have pore sizes between 2 and 50 nm. The mesoporous structure of MOFs is preferred (compared to that of zeolites) because it offers advantages from the highsurface-area- and mass-transfer points of views. However, the organic linkers used in MOFs are not suitable for stable operation under harsh conditions [406]. 5.1. Zeolites Recently, considerable attention has been paid to zeolites as an alternative catalyst to accelerate the fuel oxidation in FCs [407– 409]. The term zeolite was coined in 1756 by a Swedish mineralogist, who observed the rapid evaporation rate of water, which was absorbed by this material under high temperature. Based on this, the material was named as zeolites, referring to the Greek (zeo), meaning "to boil" and (lithes), meaning "stone" [410,411]. Zeolite materials have a microporous crystalline structure of aluminosilicate (Al, Si) coupled with an oxygen bond O-linked tetrahedral structure (TO4 ), where each apical oxygen atom is shared with an adjacent tetrahedron. When tetrahedra containing Si4+ and Al3+ are connected to form a 3D zeolite framework, there is a negative charge associated with each Al3+ atom. The negative framework charge is balanced by an exchangeable cation, yielding electrical neutrality, as shown in Figure 21 [412–414]. The multitude of possibilities of these tetrahedra together supports the zeolite family with various structures of respect to the units, size, and

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Fig. 20. (a) Schematic diagram showing the preparation of NiPO materials with different morphologies. (b) Cyclic voltammograms before and after storing in 0.1 M urea in 1 M KOH for 134 days. The CV were done at 50 mV s − 1. (c) Current densities at different potentials before and after storing, reproduced with permission from [392].

dimensionality of the channel system [415]. The regular micropore dimension and uniform shape of zeolite structures allow them to act as molecular sieves to select the specific molecules passing through it, depending on their size and shape; this phenomenon is known as “shape selectivity” [416–418]. The advantage of using zeolites material as a support-selective catalyst is the ability to enhance the kinetic reaction on the surface of the electrode materials, especially in the case of a direct methanol FC (DMFC). In such a FC, Bronsted acidity along with the surface Si-OH groups, present in meso Zeolites, facilitate the adsorption of methanol and OH− , which in turn accelerates the overall kinetics of the methanol oxidation reaction. Overall, the excellent selectivity, long-term stability, large micropore volume and high specific surface area of zeolite materials make them particularly interesting as ionexchange resins, adsorbent materials, and selective catalysts of FCs [419–421]. 5.1.1. Zeolite preparation and modification There are different methods for zeolite preparation, such as hydrothermal, sol–gel, thermal decomposition, and ion-exchange methods [423–426]. The application of zeolites as an alternative catalyst for FCs is considered a promising solution to overcome the drawbacks of Pt, such as high cost and surface blocking by the adsorbed carbonaceous substances [67]. Another aspect that makes zeolites attractive catalyst materials for FCs is the short diffusion path, such that the active sites are more readily accessible [427,428]. Despite all the mentioned advantages of zeolite materials, their low electrical conductivity is still the major challenge

that reduces their catalytic activity for fuel oxidation [429]. Many efforts have been exerted to modify the catalytic activity and further properties of zeolite electrodes using different modification techniques [430–434], such as oxygen plasma treatment [435], acid treatment [434], and metal-ion exchange techniques [436]. Among those techniques, the ion exchange process is considered the most common, cost-effective, and simple method to modify the zeolite structure by metal-ion incorporation [431]. The zeolite is kept immersed in the solution of a metal salt for several hours to allow metal ions to replace zeolite cations [437]. Moreover, incorporation of metal ions into a zeolite framework can also be performed by the immersion method [438], stirring method [439], and sodium borohydride reduction method [426]. 5.1.2. Zeolite-modified electrodes in methanol, ethanol, and urea oxidations Generally, zeolite materials are known as heterogeneous catalysts that catalyze hydrocarbon-cracking, methanol conversion, or disproportionation and alkylation of aromatics. Moreover, many researchers have investigated the metal zeolite-modified electrodes (ZME) as an effective electrocatalyst for several applications [415,440–442]. Pang et al. investigated both ZME and ZME/ Pt as electrocatalysts of direct ethanol fuel cell, and the results showed that zeolite enhances the long-term stability of Pt catalyst during the electrooxidation process of alcohol [426]. Moreover, Samant et al. [443] studied the catalytic behavior of ZME/carbon, Pt/carbon, and Pt/ ZME/carbon as catalyst materials on the anodic electrooxidation of methanol in acidic conditions. The obtained re-

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25

Fig. 21. Reaction mechanism of cyclohexene aromatization over ZSM-5 zeolite electrocatalyst material [422].

sults showed that Pt/ZME/Carbon achieved the highest catalytic performance, followed by ZME/carbon and finally, the Pt/Carbon catalyst material. This enhancement in the electrocatalytic performance is due to the formation of specific CO clusters in the ZME sieve framework [443]. The ZME coupled with iron ions (Fe/ZME) showed efficient performance toward methanol oxidation in an alkaline medium [444], because of the enhancement in the electron transfer rate and decrease in the over-potential for methanol oxidation. The effect of the methodology used for the synthesis of ZME anodic electrocatalysts was reported for both methanol and ethanol FCs by Ramirez et al. [445]. Pt NPs was successfully supported with faujasite zeolite/carbon-modified electrode (FAU-C), prepared by a sol–gel (Pt-FAU-C SG) and/or hydrothermal (Pt-FAUC HT) process, and they were used for electrocatalyst applications in alcohol FCs. The Pt-FAU-C SG showed the highest current density for methanol and ethanol oxidation, compared to Pt- FAUC HT, and Pt/C. These results highlighted the importance of the ZME preparation method and its effect on the material structure, morphology, and electrocatalytic behavior [445]. Moreover, based on the physisorption analysis, the microporosity of Pt-FAU-C SG was lower than that exhibited in the case of Pt-FAU-C HT. Thus, the electrocatalyst Pt-FAU-C SG presents a higher load of Pt, and as a result, a higher electrocatalytic activity toward methanol and ethanol oxidation reaction, compared to a Pt/C electrode. The results obtained by the above studies confirmed the great ability of ZME toward methanol and ethanol oxidation, compared with carbon-based electrodes. This high performance is attributed to

the protonic entities on the ZME surface, which makes it more hydrophilic than carbon, resulting in a decrease in the ohmic resistance and enhanced anodic generated current, compared with carbon-based electrodes. Hassaninejad-Darzi et al. [85] coupled a nanoporous nickel phosphate molecular sieve (NP) with MWCNTs in a CPE electrode; however, the electrode showed activity in NaOH only after Ni deposition on it. Ni-MWCNT-NP/CPE showed higher activity than NiCPE in 0.1 M NaOH with and without 0.001 M EtOH. The superior Ni-MWCNT-NP/CPE performance was a result of MWCNT and NP addition. MWCNT enhanced the electrical conductivity, chemical stability, electron transfer rate, surface area, and electron transfer between the electrode and analyte, while NP introduced pores and channels, facilitating Ni2+ diffusion into it [85]. Table 9 summarizes the performances of zeolite-based nonprecious catalysts. 5.2. Metal-organic frameworks MOFs, also known as porous coordination polymers (PCPs), are microporous crystalline materials, which have become a very strong field of research over the last 15 years [449]. The main structure of MOFs consist of organic ligands, also known as secondary building units (SBUs), and metal nodes, which combine to form structures ranging from the microporous to mesoporous scale [450,451]. The combination of the metal clusters and the organic linkers possess many desirable characteristics for applications as heterogeneous catalysts to promote a wide range of oxidation reactions [452]. The interest in MOFs for catalysis derives

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M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Table 8 Effects of morphology on the activities of different Ni-based catalysts in terms of current density and onset potential. Catalyst

Preparation method

Morphology

Onset potential [V]

Anodic peak potential [V]

Anodic peak current density [mA cm−2 ]

Fuel conc. and electrolyte

Scan rate, mV s−1

Ref.

Ni2 P

Hydrothermal and thermal treatment Hydrothermal

Porous nanoflower

0.24

–

750 mA at 0.6 V

0.6 M urea in 5 M KOH

15

[50]

Nanoflower

0.27

–

450 mA at 0.6 V

15

[50]

nanowire

0.25

0.5 V

160 at 0.5 V

0.6 M urea in 5 M KOH 0.33 M urea in 5 M KOH

10

[64]

Nanowire

0.19

–

380 at 0.6 V

0.33 M u rea in 5 M KOH

10

[97]

Foam-like structure

0.58

0.7

62.36 μA

3 M EtOH in 0.5 M NaOH

50

[206]

Urchin-like structure Urchin-like composed of nanowires grown from the center

0.355

No clear peak

[207]

0.7

1 M MeOH, 0.5 M KOH 1 M MeOH in 0.1 M NaOH

50

0.37

800 A g−1 at 645 mV 0.633 mA at 0.7 V

50

[210]

Nanoflake forming micrometerscale ball-like morphology

0.62

0.8

0.229 mA at 0.8 V

1 M MeOH in 0.1 M NaOH

50

[210]

Microsphere with atypical urchin-like structures

0.346

~0.476

~40

0.33 M urea in 1 M KOH

10

[240]

Nanotubes

0.44

0.63

20.5 at 630 mV

0.5 M EtOH in 0.1 M NaOH

50

[253]

Nanotubes

0.42

0.62

15.5 at 620 mV

0.5 M EtOH in 0.1 M NaOH

50

[253]

Nanofiber

0.4

0.5

6 mA at 0.6 V

0.4 M EtOH 0.5 M NaOH

100

[312]

Nanospheres/ nanosheets Porous foam

0.39

0.53

128

10

[349]

0.396

–

150 at 0.976 V

0.33 M urea in 1 M KOH 1 M MeOH in 1 M KOH

100

[363]

Nanowires

~−0.1

0.1, 0.45

~240, ~550

10

[372]

Mesoporous nanofibers/GCE

0.365

–

1.3 mA at 0.545 V

1 M Urea peroxide in 6 M KOH 0.5 M EtOH in 1 M NaOH

20

[375]

Nanosheets

0.3

–

14 at 0.4 V

50

[385]

Flower-like

0.349

–

34 A g−1 at 0.499 V

0.5 M MeOH in 0.1 M KOH 0.5 M MeOH in 1 M KOH

10

[386]

Nanoneedles on the surface of nanoflakes

0.319

–

40 A g−1 at 0.595 V

0.5 M MeOH in 1 M KOH

10

[386]

Ni(OH)2 NS Ni

NiCo (9:1)

Ni3 (PO4 )2

NiCoP-2 NiCo2 O4

NiO

Ni-P

Ni-B

Ni

Ni3 (PO4 )2

Ni2 Mo1 /C Cu60 Ni40

Galvanostatic electrodeposition with aid of PCT Galvanostatic electrodeposition with aid of PCT Thermal decomposition at 900 °C of ammonium nickel phosphate Hydrothermal Hydrothermal at 120 °C for 6 h followed by drying at 80 °C for 24 h then calcination at 300 °C for 4 h Hydrothermal at 100 °C for 10 h followed by drying at 80 °C for 24 h then calcination at 300 °C for 4 h Solvothermal at 160 °C for 16 h followed by heat treatment at 100 °C for 12 h. Electroless deposition on polymer template Electroless deposition on polymer template Hydrothermal at 90 °C for 10 h Chemical reduction Selfpropagated combustion

Nickel NWA Electrodeposition Mesoporous NiO NiCo2 O4 NiCo2 O4

NiCo2 O4

Easily controlled template-free Hydrothermal Hydrothermal at 105 °C for 4h Hydrothermal at 105 °C for 6h

(continued on next page)

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

27

Table 8 (continued) Catalyst

Preparation method

Morphology

Onset potential [V]

Anodic peak potential [V]

Anodic peak current density [mA cm−2 ]

Fuel conc. and electrolyte

Scan rate, mV s−1

Ref.

NiCo2 O4

Hydrothermal at 105 °C for 4h

Flower-like

0.329

–

37 A g−1 at 0.595 V

0.5 M MeOH in 1 M KOH

10

[386]

Monolayer nanocup array Nanospheres

~0.415

~0.495

~90 (/mg)

10

[389]

0.269

–

14 A g

0.33 M urea, 1 M KOH 0.5 M MeOH in 1 M KOH

10

[391]

Nanosheet

0.33

0.65

43

0.1, 3

50

[392]

Open ended

0.345

–

0.33, 5 M KOH

10

[393]

0.33 M of urea in 5 M KOH 0.5 M urea in 1 M KOH

10

[394]

10

[395]

Ni(OH)2 NiCo2 O4

NiPO

α -Ni(OH)2

Electrodeposition Fast microwave followed by calcination at 300 °C for 2 h Chemical reduction

−1

Electrodeposition using ZnO nanorod template Hydrothermal

α Ni(OH)2

Nanosheets

0.249

0.349

Nanosheets

0.33

–

Nanosheets

0.33

–

130.5 at 0.6 V

0.5 M urea in 1 M KOH

10

[395]

Rose-like on carbon cloth

0.99

0.479

30.5 at 0.58 V

50 mM urea in 0.1 M KOH

20

[396]

Meso NiPO NT

Hydrothermal followed by heat treatment in N2 at 450 °C Hydrothermal followed by heat treatment in O2 at 450 °C Hydrothermal at 90 °C for 12 h Sol–gel

154 (mA cm−2 mg−1 ) 249.5 at 0.6 V

Nanotubes

0.39

0.7

40.83 at 0.7 V

50

[397]

Meso NiPO NS

Sol–gel

Nanosheets

0.39

0.66

44.97 at 0.66 V

50

[397]

NiPO

Sol–gel using CTAB surfactant Sol-gel using CTAB surfactant Solvothermal followed by polymerization Hydrothermal

Nanotubes

0.485

0.715

116 μA at 0.715 V

0.5 M MeOH in 0.5 M KOH 0.5 M MeOH in 0.5 M KOH 1 M MeOH in 0.1 M NaOH

50

[398]

Nanotubes

0.445

0.715

42 at 0.675 V

1 M MeOH in 0.1 M NaOH

50

[399]

Core/shell of nanosphere

0.34

–

111 at 0.5 V

1 M MeOH in 1 M KOH

50

[400]

Nanoribbons

~0.269

0.399

~8 (per mg)

0.33 M, 5 M

10

[401]

3D vertically grown nanowire arrays

0.26

0.506

485

0.33 M urea in 1 M KOH

10

[402]

Ni(OH)2 r-NiMo4 /NF

p-NiMo4 /NF

NiCo2 O4

Si-NiPO

Ppy/Co3 O4

Ni(OH)2 nanoribbons Nix Co3−x O4 with Ni2+ : Co2+ ratio of 1.

Hydrothermal

60 mA cm−2 mg−1 at 0.545 V

nanotubes

All potentials are versus Ag/AgCl saturated.

from several unique features of these materials, including high metal content, high surface area, porosity, and structural stability [453,454]. Additionally, MOFs are unique in that their surface properties can be easily modified and manipulated through the organic ligands; therefore, they are very flexible for synthesis [455,456]. The optical, electrical, and magnetic properties of MOFs extend their applications to different applications such as separation processes [457], gas storage [455], and catalyst materials for electrochemical devices, especially low-temperature FCs [68–70]. Among those fields, MOFs show promising catalytic activities for oxidation reactions in FCs with all fuel types, such as ethanol, methanol, and urea oxidation reactions, owing to the abovementioned unique characteristics, in addition to hydrogenation reactions, Knoevenagel condensations, ring opening of epoxides, and epoxidation of olefins [458]. The pore and channel of MOFs offer radius and volume exclusion for targets with different size, shape, polarity, and conformation [459].

5.2.1. Design methods of MOFs The electrochemical activity of MOFs can be achieved by introducing a redox-active part or combining the catalytic sites of metal particles [460]. The incorporation of redox-active organic ligands represents a strategy to prepare ra-MOFs by taking advantage of the ligand π –π ∗ bands [461–463]. The extension of the conjugated system of the ligand is a potential approach for the construction of ra-MOFs, such as the derivatives of benzene, pyridine, imidazole, and thiophene. The large dislocation area facilitates UV absorption, luminescent emission, and charge mobility, which are important information sources monitored by chemical sensors. The long-lived excited state is stabilized by charge delocalization via extensive π –π ∗ and/or π –n stacking interactions [461,462,464]. Another method for the preparation of the activity of MOFs for electrochemical applications is using redox-active metal complexes as active sites of MOFs. The use of metal complexes, made from stable ligands, provides MOFs with the properties of the metal ions and its electron atmosphere, polarization, and stereo specificity, as well as higher redox activity and catalytic

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M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Table 9 Current density and onset potential of zeolite-based nonprecious catalysts. Catalyst

Preparation method

Morphology

Onset potential [V]

Anodic peak potential [V]

Reference electrode

Anodic peak current density

Fuel conc. & electrolyte

Scan rate, mVs-1

Ref

Ni/ZMC

Hydrothermal

Cubic crystallites

0.42

0.68

Ag/AgCl

800 mA g−1

20

[446]

SnO2 /m-ZSM-5

Sol–gel

−0.45

−0.18

Hg/HgO

16 A g−1

50

[447]

CeO2 30%/Nano ZSM-5

Ion exchange

Anxiolytic with a diameter of approximately 350 nm Spheroid nanocrystal structure

0.48

0.695

Ag/AgCl

52.6 A g−1

50

[408]

Ni0.5 Cu0.5 Co2 O4 (40%)/Ni2 CuZSM

Ion exchange

Nanoflake morphology

0.4

0.63

SCE

20 mA cm−2

50

[66]

Nano-ZSM-5/Ni

Under potential deposition

Spheroid nanocrystal morphology

0.42

0.58

Ag/AgCl

0.004 mA cm−2

50

[407]

Ni/poly(Py-CoFA)/CuS/CFAZeolites

Hydrothermal

Spherical with bucky ball structure

0.2

1.1

SCE

25 mA cm−2

0.1 M MeOH in 0.1 M NaOH 0.8 M MeOH in 0.5 M NaOH 0.5 M MeOH in 0.5 M NaOH M.0.5 MeOH in 1.0 M NaOH 16 mM glucose in 0.1 M NaOH 0.5 M MeOH in 0.5 M NaOH

50

[448]

All potentials are versus Ag/AgCl saturated.

capacity [465]. Also, the pore and channel of MOFs offer radius and volume exclusion for targets with different size, shape, polarity, and conformation [459]. The host-guest interaction further enhances the selectivity by Lewis acidic or basic sites in ligands, open metal sites, hydrophobic interactions, and aromatic π groups in MOFs [466]. Moreover, the incorporation of electron-donating or electron-withdrawing groups in the ligands results in the formation of donor-acceptor structures in the MOFs, and thus tunes electron orbital energy level of electrochemical devices. The substrate specificity of the catalytic site improves the selectivity of electrochemical response [467]. Moreover, the active sites introduced by the active species provide an efficient energy and electron transfer between those sites and redox reactions for a fast electrochemical response [462]. Furthermore, the integration of NPs in MOFs has been applied for enhanced electrochemical applications. In this case, the MOFs would act as a support and template for the control of the size and shape of the nanoparticles [20]. To date, several examples of the incorporation of NPs into the cavities of MOF systems have been reported in this field [468,469]. The preparation of such materials was achieved through a broad range of synthetic methods, for example, solvent-free gas-phase loading of MOFs with volatile organo-metallic precursors, which are very commonly applied for CVD [468]. Thermal or photochemical decomposition of the infiltrated precursors under a reactive or inert gas atmosphere leads to the formation of the desired NPs. Other loading methods that have been applied include liquid and incipient wetness impregnation, solid grinding, or encapsulation of preformed nanoparticles. Owing to this interaction, an enhanced catalytic property can be achieved in addition to the formation of monodispersed metal-oxide nanocrystals, providing the possibility of developing a novel class heterogeneous catalysts. 5.2.2. Applications There are many different applications of MOFs in electrochemical FCs. The use of MOF-modified electrodes for the electrocatalytic reduction of CO2 has a great potential to decrease the adverse impacts of CO2 on the environment by converting it to useful organic molecules [470]. As a catalyst for ethanol electrooxida-

tion, the [(HOC2 H4 )2 Cu(II)] MOF was used, and higher tolerance to oxidation, as well as lower over-potential, was observed, compared to Pt-based catalysts [472]. Furthermore, MOF was successfully used in separation and purification processes [473]. Wang et al. [474] investigated a pomegranate Ni/C as an active redox site on a MOF template to be used as an electrocatalyst for urea oxidation. The results exhibited a significant increase in the produced current density, compared with commercial Pt/C as an anode catalyst under the same conditions [474]. They followed a method of introducing Zn into Ni/C-MOF to form a higher surface area, owing to the evaporation of Zn during calcination, which leaves behind huge pores and open channels. These properties make the produced catalyst achieve high activity toward oxidation reaction with a low over-potential of 40 mV and current density of 10 mA cm−2 , in addition to its high UOR activity at the onset potential of 1.33 V vs. RHE. Tran et al. [475] also prepared amorphous and highly mesoporous Ni-MOF microspheres using a hydrothermal method. An electrode with an active area of 0.25 cm2 was prepared out of the material and MWCNT on ITO glass. It showed superior activity toward urea electrooxidation compared with MWCNT and Ni-MOF in 0.1 M KOH with 13.9 mA/cm2 peak current vs. 5.9 and 1.4 mA/cm2 for Ni-MOF and MWCNT, respectively, in 10 mM urea. At 0.45 V, stability was achieved in only 10 s, and the current was 100% retained even after 30 days of storage [475]. Meanwhile, Zhu et al. studied a 2D MOF NS for the electrocatalytic oxidation of urea. The reason for the use of a 2D shape instead of the more common 3D morphology is that more metal active sites would be exposed by the ultrathin NSs for the catalytic reaction, and the open pores between flexible NSs would promote the mass transfer of the electrolyte and the diffusion of gas products [20]. The fabricated 2D Ni-MOF exhibited a current density of 10 mA cm−2 at a required potential of 1.36 V. Furthermore, Yang et al. have studied MOFs as an electrocatalyst for ethanol oxidation [472]. The ethanol sorption properties were tested first for [(HOC2 H4 )2 dtoaCu], and it was found that it absorbs 0.8 mol of ethanol per 1.0 mol of the target compound, showing very strong interaction with ethanol that would facilitate the electrooxidation. By CV observations, the addition of ethanol showed considerable enhancement in the oxida-

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

29

Fig. 22. Cyclic voltammograms of: (a) Cu5% Co95% /CNF using 1 M KOH during activation, and (b) Cu5% Co95% CNF and NiCNFs using 0.5 M methanol at 100 mV s − 1 [316].

tive current density, demonstrating that the ethanol electrooxidation on the tested MOF material is dependent on the concentration of ethanol [472]. Wu et al. studied a nickel–organic framework (NiOF), prepared by solvothermal synthesis using nickel chloride and benzene dicarboxylic acid precursors, as an anode catalyst for the electrooxidation of urea. The results showed the capability of enhancing the current density and efficiency in urea oxidation, because of the large number of redox active sites of Ni/NiOx NPs, with a unique macroporous conductive structure, which exhibited high electrocatalytic behavior and allowed for the rapid transport of electrons, electrolyte, and gaseous products [471]. 6. Ni-free nonprecious catalysts Ni-based catalysts showed high electrochemical oxidation activity toward low-molecular-weight hydrocarbons such as methanol, ethanol, and urea. However, few nonprecious elements, rather than Ni, exhibited activity toward them. This section summarizes the progress in the preparation of Ni-free nonprecious catalysts that are active toward the oxidation of low-molecular-weight hydrocarbons. Co is already used as an effective co-catalyst that significantly improves the oxidation activity of Ni toward different low-molecular-weight hydrocarbons such as methanol [385,386], ethanol [149,375], and urea [396,476]. However, Co showed poor electrochemical oxidation activity as a standalone catalyst. The activity of Co could be improved through alloying with other components and/or through preparing a catalyst with the high surface area. Whereas a standalone Co or Cu catalyst has no activity toward methanol oxidation, the doping of Co/CNF with different ratios of Cu [316] demonstrated high activity toward methanol oxidation. An alloy of Cu5% Co95% exhibited higher methanol oxidation activity than that of Ni/CNF (Fig. 22b). Moreover, this catalyst showed an interesting redox peak potential that appeared during activation around 0.05 V (vs. Ag/AgCl) (Fig. 22a), which is related to the formation of (Cu5% Co95% )OOH, similar to that of NiOOH. Although methanol oxidation typically started at 0.3 V (vs. Ag/AgCl) (Fig. 22b), the authors considered that (Cu5% Co95% )OOH is the active site for methanol oxidation [316]. Similarly, neither Co nor Cr could act as an active standalone catalyst for methanol oxidation. However, an alloy catalyst of the proper composition, Cr/Co demonstrated a considerable activity toward methanol [314] and ethanol [477] oxidation. The authors revealed that the metal oxy-hydroxide (MOOH) is not the active site for ethanol oxidation, and the alloy that formed between Co and

Cr is the active site. Notably, the optimum methanol and ethanol concentrations were 4 M, which is the highest reported optimum fuel concentrations for non-precious catalysts, as seen in Tables 4– 9. Moreover, the anodic peak potential increased as the ethanol concentration increased, and the peak disappeared at high ethanol concentrations. This behavior could be attributed to the increase in the activity with increasing adsorbed ethanol molecules on the catalyst surface [209]. It was also found that doping of Co/CNF with nitrogen resulted in increasing the activity, where the current density increased with increasing nitrogen content to 4% urea (initial content in the electrospinning solution) and then decreased with further increases in urea. The nitrogen increased the wettability of the Co/N-CNF catalyst surface and enhanced its electrical conductivity, resulting in improved methanol oxidation activity [315]. The alloying of Co with Sr, i.e., Co3 O4 –SrCO3 /CNF [478], did not improve the Co activity toward urea oxidation, where a high onset potential above 0.55 V (vs. Ag/AgCl) was recorded with a feeble oxidation activity up to 0.9 V (vs. Ag/AgCl) [478]. However, the doping of Co with Cd (in NP form, i.e., CoCd/CNP) improved its activity toward ethanol oxidation [479]. The preparation of nano-architectures is another method to increase electrical conductivity and the electrocatalytic activity of metal oxides [480]. Using the hydrothermal method, followed by heat treatment at 500 °C under an inert atmosphere for 3 h, and then in air at 350 °C for 2 h, Shenashen et al. [481] fabricated hierarchical nitrogen-doped Co3 O4 @C architectures (N-Co3 O4@C) with different morphologies of nanoneedles (NNs), nanorod pellets (NRPs), and crossed-X nanosheets (X-NSs) (Fig. 23). Contrary to the findings of the feeble activity of Co NPs toward methanol oxidation [482], the prepared catalysts showed high stability and activity toward methanol oxidation (Fig. 24) [481]. The different prepared catalysts demonstrated pairs of redox couples (Fig. 24a) that related to the Co2+ /Co3+ (a1/a4), and Co3+ /Co4+ (a2/a3) [483]. NRPs had the largest enclosed area, indicating high accessibility to the active sites. The results in Fig. 24b reveal high methanol oxidation activity. The variation of the activity among the different catalysts would be related to the difference in the electron conductivity and accessibility to the active sites. The different catalysts demonstrated acceptable stability, whether from the CV after 500 cycles (Fig. 24c), and the current discharge at 0.52 V vs. Hg/HgO (Fig. 24d). The excellent activity of the prepared catalysts could be ascribed to the following points: i. the mesoporous structure that facilitates the ion diffusion to the active sites, ii. low charge resistance due to the mesotubular veins, iii. easy mass transfer of reactants due to the presence of the ex-

30

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Fig. 23. Schematic diagram showing the preparation of mesoporous N-Co3 O4 @C catalysts with different morphologies of (a) nanoneedles (NNs), (b)crossed-X nanosheets (XNSs), and (c) nanorod pellets (NRPs). The different morphologies were obtained through hydrothermal treatment, using urea and cobalt chloride. Polydopamine (PDA) formed a layer around the different morphologies, i.e., NRPs, NNs, and X-NSs during the hydrothermal treatment. The N-Co3 O4 @C structures were obtained through calcination at 400 °C, where PDA dissociated into nitrogen-doped carbon which covered the hierarchical Co3 O4 architectures, reproduced with permission from [481].

terior groove surfaces and the interior mesospace frameworks, and iv. preferential growth along the high active [210] plane that contains more active sites of Co ions (Co3+ ) toward methanol oxidation [484]. Co3 O4 nanospheres, prepared by a hydrothermal process, also exhibited activity toward methanol oxidation and its activity significantly increased when it was covered by polypyrrole (ppy) [400]. The authors related the increase in the activity by the ppy shell to an increase in the conductivity and the formation of electroactive CoNx sites that can catalyze the methanol oxidation. Additionally, through using different cobalt salts of chloride, sulfate, and acetates in hydrothermal processes, different Co3 O4 morphologies of microflowers, microspheres, and nanograss, respectively, were prepared on Ni foam [485]. The nanosphere structure had the highest surface area of 100 m2 g−1 , which is two times that of the nanoflower and 150% that of the nanograss, and therefore demonstrated the best activity toward methanol oxidation. Using a liquid crystal template and surfactant, Al-Sharif et al., prepared a mesoporous crystalline Co phosphate (CoP) [486]. The mesoporous CoP demonstrated high methanol oxidation activity, compared with that of bulk CoP or even mesoporous Co(OH)2 , which can be attributed to the high surface area of the mesoporous CoP (ten times higher than bulk CoP) and the role of the phosphate in promoting methanol oxidation. A 3D hierarchical structure of Cu(OH)2 @CoCO3 (OH)2 .nH2 O (core-shell NWs) on Cu foam achieved a considerable activity toward methanol oxidation with high stability (CVs not affected by 500 cycles at 10 mV s−1 , or with current discharge at 0.48 V (vs.

SCE) for 80 0 0 s) [487]. The activity was related to the Co4+ /Co3+ redox in alkaline media [488]. Three-dimensional mesospheres of ZnCo2 O4 /Ni foam, prepared by a hydrothermal process followed by heat treatment in air at 400 °C, showed methanol oxidation activity with an onset potential of 0.5 V (vs. Ag/AgCl). It is important to highlight the fact that this is considered as a high onset potential, and thus, it may not act as an effective FC anode catalyst. The mechanism of methanol oxidation was related to the formation of both of ZnOOH and CoOOH, according to the following equations [159,489]

ZnC o2 O4 + OH − + H2 O − 3e− → ZnOOH + 2CoOOH

(44)

ZnOOH + C H3 OH + 1.25 O2 → Zn(OH )2 + C O2 + 1.5 H2 O

(45)

2C oOOH + 2 C H3 OH + 2.5 O2 → 2C o(OH )2 + 2C O2 + 3H2 O (46) Similarly, CuCo2 O4 with a nanograss structure prepared on the surface of Cu foam showed higher activity toward methanol oxidation [489]. The methanol oxidation is typically similar to that of ZnCo2 O4 , where CoOOH and CuOOH worked as the active sites. Three-dimensional MnCo2 O4 microspheres, prepared by hydrothermal treatment followed by annealing at 550 °C, demonstrated an activity toward methanol oxidation. Nevertheless, the reported onset potential was approximately 0.5 V (vs. Ag/AgCl), which was higher than the other reported value (approximately 0.35 V (vs. Ag/AgCl)) [490]. The authors referred the activity of 3D MnCo2 O4 to the synergetic effect between Co and Mn oxide

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

31

Fig. 24. Cyclic voltammograms of nitrogen-doped Co3 O4 @C with different architectures, i.e., nanoneedles (NNs), nanorod pellets (NRPs), and crossed-X nanosheets (X-NSs) in 0.5 M NaOH at 50 mV s−1 : (a) without methanol, (b) using 1.5 M methanol (first cycle), and (c) using 1.5 M methanol (500 cycle). (d) Chronoamperometric measurements of the different samples at 0.52 V (vs. Hg/HgO), reproduced with permission from [481].

(induced higher electronic conductivity), and an increase in the electron transfer at the interface between the electrode and electrolyte. Likewise, a 3D tertiary hierarchy structure composed of MnO2 NSs (10 nm), prepared by electrodeposition on the surface of MnCo2 O4 nanoflakes (50–100 nm), which were hydrothermally prepared on the surface of macropores of Ni foam (>500 nm), exhibited high urea oxidation activity and stability, compared with those of MnCo2 O4 /Ni foam, MnO2 /Ni foam, and Ni foam (Fig. 24(c and d)) [491]. The high activity of the prepared catalyst was attributed to: i. the diversity of the pore structure, which granted the ease of the transport of the electrolyte ions, ease of mass transfer, and thus highly accessible surface area and larger electrode/electrolyte interfaces [492]; ii. the perfect connection between the different layers granted the easy electron transfer between the different layers; iii. the ultrathin NS of the MnO2 with a graphene-like structure (Fig. 25(a and b)) had a large surface area and a large number of edge sides, which were expected to have activity [493,494]. XPS analysis revealed the existence of a surface rich with redox couples in the (Mn+2 /Mn+3 /Mn+4 and Co+2 /Co+3 ), providing a high density of electrochemically active sites [495]. Moreover, the multiple valence states of the metal cations in the core-shell structure of the MnO2 NS/MnCo2 O4 provide efficient donor–acceptor chemisorption sites that would enhance the urea oxidation [496,497]. Cu is another transition metal that demonstrated an oxidation activity toward methanol oxidation in alkaline media [498]. A leached CuZn/Cu electrode, prepared by the electrodeposition of Cu and Zn over a Cu electrode followed by etching in alkali,

exhibited higher methanol oxidation activity compared with the bare Cu electrode. The increase in the current was related to the increases in the surface area and porosity, as well as the improved tolerance of the leached CuZn/Cu electrode toward poisoning, as confirmed from the higher forward anodic peak current density (If )/reverse anodic peak current (Ir ) ratios [499]. Multilaminated NPs of Cu electrodeposited on the surface of the conductive substrate also exhibited high activity for methanol oxidation in alkaline media [500]. The activity of the laminated Cu was related to the large number of edges and corners of the laminated NPs. However, the incorporation of a polymer, such as poly(2amino-5-mercapto-1,3,4-thiadiazole)-PAMT, between the laminated Cu NPs significantly augmented the catalytic activity, owing to the enlarged surface area, and improved the electron transfer. Both Cu(OH)2 NWs, prepared on the surface of the Cu substrate using surface chemical oxidation in alkaline media, and CuO NW, obtained by heat treatment of Cu(OH)2 NWs in the air for 1 h at 300 °C, showed high methanol oxidation activity [501]. The high activity of the prepared NWs was related to the direct growth of the active sites on the surface of the substrate, which ensured high mechanical stability and fast electron transfer between the active sites and the current collector substrate. Both the chemical deposition of Cu, and electrochemical corrosion of Cu wire resulted in increasing the activity of Cu toward methanol oxidation (three to five times higher oxidation peak current density). The increased performance was related to the formation of Cu dendrites, which have a higher surface area, as well as the formation of Cu(III), respectively [502]. However, the tested concentration was very low, 40 mM methanol in 0.1 M NaOH. In

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M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Fig. 25. (a) High-resolution SEM image of MnO2 NS. (b) TEM image of the MnO2 NS. (c) Cyclic voltammograms of MnO2 NS/MnCo2 O4 nanoflakes/Ni foam (MMCN) at 5 mV s−1 . (d) Chronopotentiometric measurement of MMCN at 10 mA cm−2 using 0.5 M urea in 1 M NaOH [491].

another study, Cu and Cu2 O with nanorod morphology showed a relative activity toward methanol oxidation in 0.5 M H2 SO4 [503]. However, using Cu/CuO under acidic media is not practical where corrosion would immediately occur [504], and the reported current could even be associated with the corrosion of Cu rather than methanol oxidation. Further experiments must be carried out to clarify this point. The surface decoration of polypyrrole-coated nanoporous Cu (ppy/Cu) with tin resulted in a significant increase in the ppy/Cu toward methanol oxidation in acidic solution [505]. This was related to the role of tin in increasing the tendency toward methanol dehydrogenation, increasing the electrode surface area, and oxidation of the intermediates. By controlling the electrolytic current, time, and temperature, different morphologies of Fe2 (MoO4 )3 , i.e., nanorods, nanospheres, and nanotubes, were successfully prepared [506]. All the prepared catalysts showed a high methanol oxidation activity that is comparable to that obtained using Pt foil. The interesting point in this study that the onset potential is very low (−0.4 V vs. Hg/HgO), and it is close to that of Pt foil (Fig. 26). Moreover, the anodic peak current density in the case of the nanotube structure is better than that in the case of Pt foil. The authors related the activity of the alloy to the Mo atoms on the surface and the nanotube morphology, which increased the surface area and facilitated access of the fuel and electrolyte to the active sites. Other trials investigated the activity of Zn prepared in a flower-like morphology [507], (ZnO–CeO2 )/CNF [508], and composite materials of graphene and polyaniline [509]. Although (ZnO–CeO2 )/CNF showed a low onset potential of −50 mV toward methanol oxidation, the current was small [508], while ZnO with a flower-like morphology did not show remarkable activity toward urea oxidation, despite its high surface area [507].

Fig. 26. Cyclic voltammograms of Fe2 (MoO4 )3 (with different morphologies) and a Pt foil using 1 M methanol in 0.1 M KOH at 5 mV s−1 , reproduced with permission from [506].

Two-dimensional crystalline structures are expected to have outstanding properties in different applications, such those of graphene [510,511], that are strongly attributed to the structural features, such as lateral size [512], thickness, defects, and crystal planes [512,513]. Through tuning the zeta potential of aqueous solutions of metal precursors, selective sedimentation of large 2D crystals was attained, leaving the small counterparts in the solution. The small 2D crystals of MoS2 with a surplus of exposed edges are more electrocatalytically active compared to their

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33

Table 10 Current density and onset potential of Ni-free nonprecious catalysts. Catalyst

Preparation method

Morphology

Onset potential [V]

Anodic peak potential [V]

Anodic peak current density [mA cm−2 ]

Fuel conc. and electrolyte

Scan rate, mV s−1

Ref.

Mesoporous Co3 O4 S-MnO2

Easy-controlled template-free Redox reaction

0.445

–

[375]

–

0.5 M EtOH in 1 M NaOH 0.5 M u rea in 1 M KOH

20

0.34

0.4 mA at 0.545 V 260 at 0.6 V

10

[517]

MnO2 NS/MnCo2 O4 NW/Ni NF

Hydrothermal followed by electrodeposition Hydrothermal

Mesoporous nanofibers/GCE 2D crystals on graphene coated Ni foam Hierarchal NS/NW/NF

0.33

–

384 at 0.7 V

0.5 M urea in 1 m KOH

5

[491]

Nanorod pellets Nanowires

0.299

–

50 at 0.399 V

50

[481]

0.385

–

40 A g 0.595 V

1.5 M MeOH in 0.5 M NaOH 0.5 M MeOH in 1 M KOH

10

[501]

Nanowires

0.355

–

37 A g − 1 at 0.595 V

0.5 M MeOH in 1 M KOH

10

[501]

Nanoneedle array/Ti mesh

0.476

–

40 at 0.776 V

0.3 M urea in 1 M KOH

2

[518]

Core/shell of nanosphere

0.34

–

111 at 0.5 V

1 M MeOH in 1 M KOH

50

[400]

Laminated layer of Cu Nanotubes

0.545

0.925

188 at 665 mV

50

[500]

−0.501

- 0.001

3.27 at - 0.001 V

0.5 M MeOH in 0.5 M NaOH 1 M MeOH in 0.1 M KOH

5

[506]

N-Co3 O4 @C Cu(OH)2

CuO

CoS2 NA/Ti

Ppy/Co3 O4

Surface chemical oxidation Surface chemical oxidation followed by heat treatment at 300 °C in air for 1 h Hydrothermal 110 °C 8 h, followed by sulfonation at 400 °C 2 h. Solvothermal followed by polymerization

Cu/PAMT/C Electrodeposition Fe2 (MoO4 )3 Electrochemical

−1

at

All potentials are versus Ag/AgCl saturated.

large counterparts [494,514]. Through tuning of the zeta potential of an aqueous solution of MnO2 , prepared through redox reaction between KMnO4 and sodium dodecyl sulfate, a 2D MnO2 was successfully prepared [515,516]. Typically, a small MnO2 (S-MnO2 ) with lateral size of 170 nm and a thickness of 0.95 nm (that is the same as a single-layer MnO6 unit in MnO2 ) [515,516], as well as large MnO2 (L-MnO2 ) crystals of 500 nm to several micrometers, are successfully prepared [517]. The S-MnO2 showed high urea oxidation activity, which is related to the large number of active centers owing to the highly exposed edges and planer surfaces. The activities reported in the different works performed on Ni-free catalysts are summarized in Table 10.

porous layer. The catalyst layer is the most essential part of the electrode, which contains catalyst NPs that are the active sites. Binders such as Teflon [533] or Nafion [534] are commonly applied to fix the carbon and catalyst NPs to the surface of the diffusion and microporous layers, respectively. Such a layered structure of the electrodes resulted in decreasing the electrical con-

7. In-situ preparation of catalyst on the diffusion layer The electrode of a FC consists of three different layers, i.e., the diffusion layer, microporous layer, and catalyst layer [7,519], as shown in the schematic diagram (Fig. 27). On the surface of the electrode, a highly conductive current collector (gold-plated stainless steel), equipped with flow channels, is placed to collect electrons for an external circuit. The diffusion layer is usually made of carbon paper [520,521], or carbon cloth [104–106], and metallic materials such as Ni foam [522,523] and stainless steel wire cloth [524] have also recently been used. The diffusion layer is responsible for facilitating the mass transfer, and thus, it usually contains macrospores to perform this function. Over the surface of the diffusion layer, a microporous layer is usually used to smooth the surface of the diffusion layer [525]. Carbon NPs are the common materials that are used for the microporous layer. However, CNFs [526–528], CNTs [529,530], and Gr [531,532] have recently been used in the micro-

Fig. 27. Schematic diagram showing the electrode structure and the current collector on it.

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ductivity of the electrode, and the usage of the binder (with low or no electrical conductivity) not only resulted in decreasing the electrical conductivity, but it also blocked the pores of the electrode [535]. Moreover, the CNPs applied to the microporous layer block the macrospores of the diffusion layer, and catalyst NPs block part of the microporous layer. Furthermore, a considerable part of the catalyst is lost during the electrode preparation. Some researchers tend to add a conductive material to the prepared catalyst [536], such as polyaniline polymer [537], and graphene [538]. However, such additives increase the electrical conductivity at the expense of the active sites, and some of the active sites are further lost through chemical or physical bonding with the conductive material [539]. Another effective strategy is the direct preparation of the catalyst on the surface of a diffusion layer (such as carbon cloth and Ni foam), which would have numerous advantages as concluded here: (i) High mechanical stability of the catalyst layer due to the good contact with the diffusion layer [540–544]. (ii) Excellent electron transfer due to the binder-free process [545]. (iii) The catalyst was prepared directly on the surface, which results in complete exposure of the catalyst without loss in the pores of the microporous or in the diffusion layer. (iv) Easy mass transfer to the active sites due to the macropores of the diffusion layer and binder-free process. (v) In the case of a metallic diffusion layer, such as Ni foam, the extension of the metallic diffusion layer is used as a current collector, thereby further improving the electron transfer in the cell. The following section summarizes the progress achieved to fabricate standalone electrodes using direct preparation of the catalyst on the diffusion layer. For the abovementioned reasons, NiO nanowalls, hydrothermally prepared on Ni foam, exhibited high urea oxidation activity and stability [546]. A single-layered (SL) structure of Ni(OH)2 has a higher surface area and larger number of active sites compared to those of the multilayered (ML) structure of the nanowall Ni(OH)2 [539]. Restacking of the single layer usually occurs in liquid delamination during the solvent evaporation [547–549], whereas in reverse microemulsion, a mass of surfactant is used to restrict the growth of the Ni(OH)2 . However, these surfactants cover a considerable portion of the Ni(OH)2 active sites. Using a solvothermal method at 120 °C for 4 h, Lin et al. [539] successfully prepared a complete electrode structure composed of single layer Ni(OH)2 NSs (SL-α Ni(OH)2 -NS) on the surface of carbon cloth with a nanowall structure of 0.8 nm thickness. The authors reported that the solvent type plays a vital role in the preparation of the SL Ni(OH)2 structure. Methanol, with a lower surface tension than that of water, thoroughly wets the surface of the CC and promoted the growth of metastable SL α Ni(OH)2 NS, compared to stable ML β -Ni(OH)2 in the case of using water. The d spacing of the 001 planes in ˚ is found to be larger than that in case of β α Ni(OH)2 NS, 8 A, ˚ Such a larger d spacing is suitable for ion transNi(OH)2 , 4.8 A. fer between the sheets and affords better lattice strain, which provides stability during repeated electrochemical processes (during urea oxidation), and thus better performance of the α -Ni(OH)2 NS (436 mA cm−2 ) compared to that of β -Ni(OH)2 NS (12.4 mA cm−2 ), at 0.5 V (vs. Ag/AgCl) in 1 M KOH with 0.33 M urea [550–552]. The better performance of SL α Ni(OH)2 NS was related to the high specific surface area (208.8 m2 g−1 vs. 25.5 m2 g−1 ), the high ECSA (ECSA of the SL α -Ni(OH)2 NS is four times that of the ML β -Ni(OH)2 NS), the only adsorption of ions occurs in the non-Faradic region or electric double layer [553–556], and the low charge transfer resistance (better UOR kinetic) [64]. However, the

long-term stability showed a degradation in the current generated at 0.5 V (vs. Ag/AgCl) due to the transfer from the SL α Ni(OH)2 NS to ML β -Ni(OH)2 NS [557,558], and the appearance of the NiCO3 as a result for the strong absorption of the CO2 on the surface of the Ni active sites [128,133,134]. Ji et al. prepared open-ended nickel hydroxide nanotubes on the surface of Ni foam using electroless plating over a ZnO nanorod template, as shown schematically in Fig. 28a. The prepared layer showed a typical nanotube structure, as shown in the inset of Fig. 28c, compared with a NS structure obtained without using a ZnO template. The nanotube structure showed improved activity compared with that of the NSs with and without urea (Fig. 28(d and e)), where the current density reached 60 mA cm−2 g−1 at 0.5 V vs. SCE compared to 10 mA cm−2 g−1 in 1 M KOH containing 0.33 M urea in the case of Ni(OH)2 NSs [393]. Among different bimetal hydroxides of Ni–Cr, Ni–Mn, Ni–Fe, Ni– Zn, Ni–Co, and Ni–Cu, prepared on the surface of carbon cloth fibers using hydrothermal treatment at 120 °C for 14 h, Ni–Fe demonstrated the best activity and stability toward methanol oxidation (Fig. 29(g and h)) [559]. It was also found that the morphology depended on the metal combination (Fig. 29(a–f)). The combination of Fe with Ni increased the surface area by ten times compared to Ni(OH)2 , and raised the conductivity from 0.1– 0.2 mS cm−1 to 3.5–6.5 mS cm−1 [560]. EIS demonstrated smaller semicircles in the case of Ni–Fe hydroxides, indicating fast charge transfer and reaction kinetics [131]. The nanowall structure of cubic NiO, obtained by thermal treatment of hydrothermally grown α -Ni(OH)2 nanowalls on Ni foam (NF), achieved high urea oxidation activity, compared with that prepared on the st. st. or from NiO powder deposited on the st. st. sheet surface [561]. The nanowall structure had higher ECSA (10 times that of Ni(OH)2 NPs), high electron transfer between the NS and the Ni support, and facile mass transfer of the electrolyte and fuel [65,394]. Furthermore, the modification of the surface atoms through structural engineering [562,563], defect engineering [494,564], and surface/interface treatment [565,566], would further increase the activity. Tong et al. [395] prepared a porous NiMoO4 (p-NiMoO4 )NS and surface-oxygen-vacancy-rich (r-NiMoO4 )NS on Ni foam using a hydrothermal method followed by heat treatment at 450 °C under O2 or N2 atmospheres, respectively. The crystalline structures of Ni and Mn are similar in the two catalysts. However, r-NiMoO4 contains more significant oxygen defects than those in the case of p-NiMoO4 , especially those attached to Mo. The r-NiMoO4 catalyst demonstrated high catalytic activity toward urea oxidation (Fig. 30a) that is higher than other catalysts, i.e., pNiMoO4 , NiMo precursor/NF, and NF (Fig. 30b). The high activity of r-NiMoO4 was because of the oxygen vacancy that improved the kinetics of the urea oxidation, as is clear from the lower slope in the Tafel plot (Fig. 30c), the improved charge transfer, as shown by the smaller semicircle in the high-frequency EIS spectra (Fig. 30d), and the improved mass transfer, as shown by the independence of the current generation on the scan rate (Fig. 30f). Moreover, rNiMoO4 demonstrated high stability, as indicated by the negligible change in performance even after 10 0 0 cycles (Fig. 30e). The high activity of r-NiMoO4 was related to, i. the delocalized free electrons near the oxygen defect, which activated the nearest metal ions to be more active toward fuel (urea) adsorption, ii. the shift in the Fermi surface of r-NiMoO4 to the conduction band edge [567–570], and iii. the 3D structure of r-NiMoO4 , which facilitated the mass transfer of the electrolyte ions and urea to the active sites, as well as the facile removal of the reaction products. By controlling the time of the hydrothermal treatment of Ni foam in a mixture of Ni nitrate, ammonia fluoride, and urea, at 110 °C followed by heat treatment in air at 300 °C, Yue et al. [571] synthesized different morphologies of Ni compounds on Ni foam (Ni(OH)2 NS at 6 h, small NWs over the NSs at 8 h, and

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35

Fig. 28. (a) Schematic diagram showing the preparation of Ni(OH)2 nanotubes on the surface of Ni foam (Ni(OH)2 NT/NF)., SEM images of: (b) a bare Ni foam and (c) Ni(OH)2 NS/NF. The inset of (c) shows a high-resolution image of (Ni(OH)2 NS. (e, f) Cyclic voltammograms of Ni(OH)2 NT/NF and Ni(OH)2 NS/NF in 1 M KOH at 10 mV s−1 (d) without urea, and (e) using 0.33 M urea, reproduced with permission from [393].

multidirectional NWs on the NSs at 9 h). The 3D structure of the Ni(OH)2 –NW@NS/Ni obtained at 9 h showed higher activity than those prepared at 6 and 8 h (Fig. 31a), better activity than Ni foam, activated Ni foam (200 cycles of CV from 0 to 1 V in 1 M KOH using 5 mV s−1 ), and even better than Pt/C over Ni foam, using 0.33 M urea in 1 M KOH at 5 mV s−1 (Fig. 31b). The 3D Ni(OH)2 NW@NS/Ni foam also exhibited high electrochemical and mechanical stability, where the current was discharged continuously for 40 h, and the morphology was not affected, owing to the excellent contact between the nanoarrays and the Ni foam, which caused good mechanical stability high electrical conductivity, and facile electron transfer [545,572]. The significant increase in the surface area (33 times that of the geometrical surface area) [573,574], led to the superior behavior of the 3D Ni(OH)2 -NW@NS/Ni. Moreover, the prepared structure would work as a self-standing elec-

trode without need for deposition on another material or even using ionic conductors, such as Nafion, which have low electrical conductivity [535,575,576]. The prepared 3D Ni(OH)2 -NW@NS/Ni foam also exhibited high methanol oxidation activity [577], which could be assigned to: i. facile mass transfer due to the presence of the macropores in its structure [383,578] and the increased number of active sites [579]; ii. high conductivity [578], fast kinetics, and thus, lower onset potential [580], iii. high roughness factor [383,581]; and iv. perfect catalytic charge and mass transfer [517,582]. In another study, Wang et al. [50] prepared porous flower-like Ni2 P on Ni foam by phosphating the Ni(OH)2 NSs of a flowerlike structure prepared by a hydrothermal process, as shown schematically in Fig. 32a. The high-resolution TEM images revealed that Ni2 P had a porous nanoflower structure of 30–90 nm diam-

36

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Fig. 29. (a–f) SEM images of different Ni-M alloys: (a) Ni–Cr, (b) Ni–Mn, (c) Ni–Fe, (d) Ni–Co, (e) Ni–Cu, and (f) Ni–Zn. (g) Cyclic voltammograms of Ni(OH)2 and Ni-Fe over a Ni foam using 0.33 M urea in 1 M NaOH at 25 mV s−1 . (h) Chronoamperometry of the different Ni-alloys using 0.33 M urea in 1 M NaOH at 0.5 V (vs. Ag/AgCl) [559].

eter (Fig. 32c), compared to nonporous Ni(OH)2 NSs (Fig. 32b). Such a porous structure facilitates good contact between the electrolyte ions and the active sites, improving the interfacial reactions [392], and the ECSA (based on the charge required for the reduction in NiOOH in the cathodic scan [583]). The porous Ni2 P NS sample showed excellent activity toward urea oxidation with an onset potential and current density of 0.24 V (vs. Ag/AgCl) and 750 mA cm−2 , compared to 0.27 V (vs. Ag/AgCl) and 450 mA cm−2 for Ni(OH)2 NS, respectively, at 15 mV s−1 using 0.6 M urea in 5 M KOH (Fig. 32d). Through surface engineering, Yue et al. [60] prepared NiO NSs on the surface of Ni foam (NF), i.e., NiO/NF using the hydrothermal method followed by annealing at 300 °C for 2 h, and electrodeposited (NiO-Ni/NF). The hybrid NiO-Ni/NF catalyst revealed high activity in terms of high current density and decreased onset potential toward urea oxidation. When the scan rate was increased from 5 to 50 mV s−1 , no significant effect on the current density

was observed, demonstrating an efficient charge and mass transfer of the NiO-Ni/NF. The improvement in the performance can assigned to the binder-free electrode structure (providing high electrical conductivity, high mass transfer, and high ECSA), the synergetic effect between the excellent contact between the Ni, NiO, and Ni foam (improving both the ion and electron transfer), and the increased number of active sites. In another study using the hydrothermal method and electrodeposition, a porous Co3 O4 (core)/NiO (shell) is prepared and investigated for methanol oxidation [584]. The prepared core-shell structure showed superior methanol oxidation activity, owing to the synergetic effect between Co and Ni, good electrical conductivity, excellent mechanical stability, and the porous structure of the core-shell structure granting full access of the electrolyte to the active sites. Similarly, NiSe NWs, prepared by the solvothermal method on the surface of NF [249], and nickel cobaltite (NiCo2 O4 ) NSs [476], prepared using the hydrothermal process on the surface

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

37

Fig. 30. Cyclic voltammograms of: (a) an oxygen-vacancy-rich sample (r-NiMoO4 ) in 1 M KOH with and without 0.5 M urea, (b) different samples using 0.5 M urea in 1 M KOH. (c) Tafel plot of different samples in 1 M KOH containing 0.5 M urea. (d) EIS spectra of different samples. (e) Comparison between the 1st and 10 0 0th cycles in 1 M KOH containing 0.5 M urea. (f) Effect of the scan rate on the current density at 0.8 V, reproduced with permission from [395].

Fig. 31. Forward scan for: (a) Ni(OH)2 prepared over a Ni foam using different hydrothermal times, and (b) Ni(OH)2 NS@NW/Ni foam, bare Ni foam, activated Ni foam, and Pt/C/Ni foam in 1.0 M KOH with 0.33 M urea at 5 mV s−1 , reproduced with permission from [571].

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M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Fig. 32. (a) Schematic diagram showing the manufacturing steps of Ni2 P. High-resolution TEM images of Ni(OH)2 NS (b) and porous Ni2 P NS over Ni foam (c). (d) Cyclic voltammograms of the Ni foam, Ni(OH)2 NS, and Ni2 P NS using 0.6 M urea in 5 M KOH at 15 mV s−1 , reproduced with permission from [50].

of the st. st. gauze, exhibited superior activity toward methanol and urea oxidation, respectively. A 3D rose flower-like NiCo2 O4 prepared on carbon cloth, using hydrothermal treatment, showed one of the best-reported performances of a DUFC with a power density of 38 mW cm−2 with a slight loss in performance after 180 h [396]. The oxidation activities of different standalone electrodes are summarized in Table 11. 8. Challenges and recommendations (i) The differences between the in-situ and ex-situ onset potentials are not clear yet and detailed studies are needed to explore the difference. (ii) The different methods applied to increase the OCV are based on using a basic anolyte and an acidic catholyte on the two sides of the Nafion membrane (acidic character). The cell lifetime is expected to be short due to the corrosion of the transition metals by the acidic membrane. Preparing an active anodic non-precious-metal-based catalyst with high corrosion resistivity would eliminate the use of the basic anodic media (even it is preferable from a kinetics point of view), as it can work effectively in acidic media. Such a catalyst could be achieved in the following ways: a. Metal carbides and nitrides have a high corrosion resistivity and considerable oxidation activity. The preparation of nanosized metal carbides and nitrides is considered as an effective strategy to increase the activity and maintain a high corrosion resistivity. b. MXenes are a new class of 2D transition metal carbides, nitrides, or carbonitrides that have demonstrated outstanding properties [585–587]. MXenes have exhibited superior performance in batteries [588–590], supercapac-

itors [591–593], and solar energy applications [594,595], or as photocatalysts [596–598] and electrocatalysts [599–602]. Furthermore, MXenes are also prepared in different morphologies such as nanotubes and nanoribbons [603–605]. To date, no applications of MXenes as catalysts or catalyst supports for a non-precious-metal catalyst for the oxidation of low-molecular-weight hydrocarbons have been reported. It is expected that MXenes could be applied as an effective catalyst or catalyst support for Ni-based catalysts, or even for Ni-free nonprecious catalysts. c. Metal chalcogenides (phosphides, sulfides, and selenides) do not only have a high electrical conductivity but are also highly stability in acidic media [606–608].Thus they can be used in contact with a Nafion membrane and applied to hybrid-pH operating conditions. d. The coverage of the catalyst with a thin layer of a conducting polymer, such as ppy or polyaniline can protect the transition metal from corrosion by improving the catalyst activity through increasing its conductivity. Additionally, the transition metal-N sites can work as additional active sites for the oxidation process. Furthermore, the heat treatment of such materials will leave a thin layer of nitrogen-doped carbon that increases the activity and protects the catalyst from corrosion [400]. e. The preparation of metal/CNF materials by heat treatment of electrospun nanofibers composed of transitionmetal precursors and low-stability polymers, such as PVA, resulted in covering the transition metal with a thin layer of carbon that can protect it while retaining its catalytic activity [609,610]. A graphene layer (a few-nanometer thick) [611], a polymer layer [612], or a

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

39

Table 11 Activities of standalone electrodes in terms of current density and onset potential. Catalyst

Preparation method

Morphology

Onset potential [V]

Anodic peak potential [V]

Anodic peak current density [mA cm−2 ]

Fuel conc. and electrolyte

Scan rate, mV s−1

Ref.

NiO/NF

Hydrothermal at 100 °C for 10 h then annealing at 300 °C for 2 h Hydrothermal at 100 °C for 10 h then annealing at 300 °C for 2 h followed by electrodeposition of Ni Template-free growth Galvanostatic electrodeposition with aid of PCT Pyrolysis

Nanosheets/NF

0.335

–

280 at 0.545 V

0.33 M urea in 1 M KOH

50

[60]

Nanosheets/NF

0.326

–

335 at 0.545 V

0.33 M urea in 1 M KOH

50

[60]

Nanosheet/Ni foam Nanowire

0.21

0.56

559

10

[93]

0.19

–

380 at 0.6 V

0.6 M urea in 5 M KOH 0.33 M urea in 5 M KOH

10

[94]

Shell–core /Ni foam

0.345

~0.495

~50

0.33 M urea, 1 M KOH

5

[194]

Thin film

0.384

0.722

300 at 722 mV

2 M MeOH in 1 M NaOH

50

[251]

Thin film

0.359

0.773

282 at 773 mV

2 M EOH in 1 M NaOH

50

[251]

A thin layer of Ni-B over a porous layer of Cu Thin film

0.341

0.45

126.2 at 450 mV

0.3 M EtOH in 1 M KOH

10

[252]

0.495

–

30 at 665 mV

0.5 M MeOH in 1 M KOH

50

[254]

436 at 0.5 V

0.33 M urea, 1 M KOH

50

[539]

0.33 M urea in 172 mA cm−2 mg−1 1 M NaOH at 0.6 V 0.33 M urea in 217 mA cm−2 mg−1 1 M NaOH at 0.6 V

25

[559]

25

[559]

0.33 M urea in 1 M NaOH

25

[559]

0.33 M urea in 1 M NaOH

25

[559]

0.33 M urea in 1 M NaOH

25

[559]

0.33 M urea in 1 M NaOH

25

[559]

0.33 M urea in 1 M KOH

10

[561]

NiO–Ni/NF

Ni(OH)2 /NF NiCo (9:1)

Carbon encapsulated NiFe/ nickel foam Ni-B/carbon electrode Ni-B /carbon electrode NiB/nanoporous Cu Ni-B/Ti

SL α Ni(OH)NS

Electroless deposition for 60 min Electroless deposition for 60 min Ultrasoundassisted electroless Electroless deposition on Ti sheet Hydrothermal

NiCr

Hydrothermal at 120 °C 14 h

Ni-Fe

Hydrothermal at 120 °C 14 h

Ni-Mn

Hydrothermal at 120 °C 14 h

Ni-Zn

Ni-Cu

Ni-Co

NiO

NiO

Ni(OH)2 – NW@NS/NF

A 3D structure composed of a single layer of Ni(OH)2 nanosheets over carbon cloth Nanopowder on carbon cloth Nanopowder on carbon cloth

0.32

0.455

0.6

0.47

–

Nanosheets on carbon cloth

0.42

–

Hydrothermal at 120 °C 14 h

Nanosheets on carbon cloth

0.5

Hydrothermal at 120 °C 14 h

Nanowires on carbon cloth

0.47

Hydrothermal at 120 °C 14 h

Nanowires on carbon cloth

0.4

Hydrothermal at 95 °C for 8 h followed by heat treatment at 300 °C Hydrothermal at 95 °C for 8 h followed by heat treatment at 300 °C Hydrothermal

Nanowall on Ni foam

0.315

–

25 mA cm−2 mg−1 at 0.6 V 345 at 0.545 V

Nanowall on stst sheet

0.315

–

112 at 0.545 V

0.33 M urea in 1 M KOH

10

[561]

3 D NW@NS

0.335

–

10 at 0.385 V

0.33 M urea, 1 M KOH

5

[571]

98 mA cm−2 mg−1 at 0.6 V – 50 mA cm−2 mg−1 at 0.6 V – 40 mA cm−2 mg−1 at 0.6 V –

(continued on next page)

40

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

Table 11 (continued) Catalyst

Preparation method

Morphology

Onset potential [V]

Anodic peak potential [V]

Anodic peak current density [mA cm−2 ]

Fuel conc. and electrolyte

Scan rate, mV s−1

Ref.

Ni(OH)2 – NW@NS/NF Co3 O4 NF (core)/NiO NW(shell)/Ni foam NiCo2 O4 over stst gauze NiCo2 O4

Hydrothermal

3 D NW@NS

0.476

–

89 at 0.596 V

10

[577]

Hydrothermal and electrodeposition

Nano-array

0.574

–

112 at 0.324 V

0.5 M MeOH, 1 M KOH 0.5 M MeOH in 1 M KOH

25

[584]

Hydrothermal at 120 °C and 18 h Hydrothermal at 90 °C and 12 h Hydrothermal followed by electrodeposition

NS over stst gauze Rose-like on carbon cloth Hierarchal NS/NW/NF

0.279

0.549

[476]

0.479

20

[396]

0.33

–

384 at 0.7 V

0.33 M urea in 5 M KOH 50 mM urea in 0.1 M KOH 0.5 M urea in 1 m KOH

10

0.099

200 mAmg-1 at 0.549 V 30.5 at 0.479 V

5

[491]

MnO2 NS/MnCo2 O4 NW/Ni NF

All potentials are versus Ag/AgCl saturated.

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

porous oxide layer [613], can be used for the same purpose. Co is one of the elements that resulted in a significant decrease in the onset potential, but at the expense of the electrooxidation activity. Surface engineering can help in preparing a proper NiCo alloy hierarchical structure that would exhibit a high activity in terms of high current density and low onset potential. Nanoporous anodic aluminum oxide (AAO) template is a facile method for preparing one-, two-, and threedimensional nanostructures [373]. Little work has been done to prepare nonprecious catalysts for the anodes of low hydrocarbon fuel cells. It is expected that preparing two- and three- dimensional catalysts using the AAO template will lead to significant improvements in performance. Carbothermal treatment of ammonium metatungstate under Ar and H2 atmospheres resulted in the formation of a mixture of mesoporous carbon and mesoporous WC [238]. The obtained WC showed a superior activity toward methanol oxidation under acidic conditions with 200 mV overpotential compared to that of Pt, without catalyst poisoning (no CO or other harmful intermediates). Such material is expected to show a high performance in case it used as a support for Ni-based catalysts under basic conditions. Nitrogen significantly improved the catalytic performance, whether as a support or with a support including the embedded part of the catalyst. However, no complete data are available on the actual role of nitrogen. Future work is needed to clarify its role and optimize its composition. In all previous studies, Cd was incorporated by heat treatment at high temperatures, and the majority was lost during preparation. CNF prepared by electrospinning followed by the reduction of the metal salts of Cd and/or Ni salts could be an effective method to obtain a higher Cd content. Although much research has been dedicated to transition metals, mainly Ni and its alloys, there is only one report on tungsten carbide nanofibers prepared by electrospinning. It is well known that the use of a metal carbide support as a transition metal significantly improves the activity; therefore, this work must be expanded by using the metal carbide and/or nitride either as a support for a nonprecious catalyst or as a standalone catalyst. a. Doping phosphides with relatively cheap rare-earth metals such as Ce improved their electrochemical activity toward the hydrogen evolution reaction through modifying their physicochemical and electrochemical properties. This is expected to benefit the electrochemical oxidation

activity toward low-molecular-weight organics, such as methanol, ethanol, and urea. b. Nitrogen doping of phosphide improved its activity toward hydrogen evolution [614], which is expected to increase its oxidation activity toward low-molecular-weight hydrocarbons as well. c. It was demonstrated that TiN and WC NPs have a synergetic effect on the general activity of Ni [615,616]. Such a highly active porous structure might be beneficial in the application as a non-precious catalyst for electrochemical oxidation. d. A leaf-like conductive structure of a Co MOF nanoarray was successfully prepared on the surface of a carbon cloth exhibiting a high activity toward glucose oxidation with an onset potential of approximately 0.2 V (vs. Ag/AgCl) [617], which could be used for methanol, ethanol, and urea oxidation. e. Defect engineering such as enriching the surface with oxygen defects resulted in a significant increase in the activity; however, the authors only investigated one temperature (450 °C). Changing the temperature of the heat treatment is expected to change the activity, so there is probably an optimum heat treatment temperature that requires investigation. (ix) The in-situ preparation of MOFs over macroporus substrates such as Ni foam demonstrated a superior performance in energy storage devices [618,619], and water oxidation [620]. A similar performance is expected for the oxidation of urea and simple alcohols. 9. Conclusions In this review, we have discussed the feasibility of using nonprecious-metal-based catalysts in FCs based on the reported onset potential, as well as promising results for actual cell operation and increasing the cell voltage. Moreover, we have discussed the progress achieved in preparing active nonprecious electrooxidation catalysts for methanol, ethanol, and urea. The following conclusions can be drawn: (i) Despite the significant achievements in preparing nonprecious-metal-based catalysts with high electrochemical oxidation activities comparable to that of Pt in terms of the current generation, the onset potential of 0.3 V (vs. Ag/AgCl) (using a three-electrode cell arrangement) is far from those of Pt or Pt alloys (−0.5 V (vs. Ag/AgCl)). Even the actual onset potential of −0.2 V (vs. Ag/AgCl) (using a two-electrode

M.A. Abdelkareem, E.T. Sayed and H.O. Mohamed et al. / Progress in Energy and Combustion Science 77 (2020) 100805

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)

cell arrangement) is still high, and significant efforts must be done to decrease it. The onset potential measured ex-situ cell can be used as an indicator for the cell voltage, but cannot be used to calculate it. Hybrid-pH conditions at the anode (basic) and the cathode (acidic) effectively increase the oxidation activity at the anode while maintaining a high cell voltage. Moreover, applying an oxidant rather than O2 with a high reduction potential, such as hydrogen peroxide and metal ions, is an effective strategy to increase the cell voltage. CO2 desorption from the surface of the Ni catalyst is the rate-determining step in urea oxidation reactions. Nevertheless, CO2 is the major factor that results in the drastic degradation of the performance of the Ni catalyst due to the formation of the carbonate and/or bicarbonate. Alloying of Ni with metallic and nonmetallic elements significantly improved its oxidation activity, and the non-oxide form of Ni demonstrated a higher activity than the oxide form. Enriching the surface with oxygen defects resulted in significant increases in the activity; the high activity was related to: i) the presence of delocalized free electrons near the oxygen defects, which activated the nearest metal ions toward urea adsorption, ii) the shift in the Fermi surface of r-NiMoO4 to the conduction band edge [567–570], iii) the 3D structure of r-NiMoO4 facilitated the mass transfer of electrolyte ions and urea to the active sites as well as the easy removal of reaction products. Increasing the surface area of the catalyst by using new support morphologies or preparing the catalyst itself in new morphologies are effective and important strategies for increasing the surface area and improving the mass transfer, thus improving the performance of non-precious catalysts. The in-situ growth of a catalyst directly on the surface of the diffusion layer is a promising method to increase its activity and stability, showing the following advantages: a. High mechanical stability due to the intimate contact between the catalyst layer and the diffusion layer [540– 544]. b. An excellent electron transfer between the catalyst and the diffusion layer [545]. c. High exposure of the catalyst, and no catalyst loss in the diffusion or microporous layer. d. The open structure of the macropores of the diffusion layer and the absence of a microporous layer result in outstanding access of the reactants to the active sites, as well as the easy removal of the products. e. Metallic diffusion layer can act as a current collector, which would further improve the electron transfer. Although standalone transition metals, as opposed to Ni, have low or no oxidation activity, preparing them in nanoarchitecture structures, and/or alloying them with each other led to oxidation activities that were sometimes higher than those of Ni-based catalysts.

Declaration of Competing Interest The authors certify that there is no conflict of interest associated with this work. Acknowledgments This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP)

41

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[613] Zhang Q, Lee I, Ge J, Zaera F, Yin Y. Surface-Protected etching of mesoporous oxide shells for the stabilization of metal nanocatalysts. Adv Funct Mater 2010;20:2201–14. [614] Sun A, Shen Y, Wu Z, Wang D. N-doped MoP nanoparticles for improved hydrogen evolution. Int J Hydrog Energy 2017;42:14566–71. [615] Menny S, Valerio M, Davide E, Guylhaine C, Debora R, Cristina G, et al. Sponge-like nickel and nickel nitride structures for catalytic applications. Adv Mater 2014;26:1272–6. [616] Na J, Tao Z, Mingyuan Z, Aiqin W, Hui W, Xiaodong W, et al. Direct catalytic conversion of cellulose into ethylene glycol using nickel-promoted tungsten carbide catalysts. Angew Chem 2008;120:8638–41. [617] Wei Z, Zhu W, Li Y, Ma Y, Wang J, Hu N, et al. Conductive leaflike cobalt metal–organic framework nanoarray on carbon cloth as a flexible and versatile anode toward both electrocatalytic glucose and water oxidation. Inorg Chem 2018. [618] Bahaa A, Balamurugan J, Kim NH, Lee JH. Metal–organic framework derived hierarchical copper cobalt sulfide nanosheet arrays for highperformance solid-state asymmetric supercapacitors. J Mater Chem A 2019;7:8620–32. [619] Xu X, Shi W, Li P, Ye S, Ye C, Ye H, et al. Facile fabrication of three-dimensional graphene and metal–organic framework composites and their derivatives for flexible all-solid-state supercapacitors. Chem Mater 2017;29:6058–65. [620] Duan J, Chen S, Zhao C. Ultrathin metal-organic framework array for efficient electrocatalytic water splitting. Nat Commun 2017;8:15341. Mohammad Ali Abdelkareem spent 8 years in one of the pioneer labs in Japan in the field of electrochemical energy devices, i.e., fuel cells. During his Ph.D. study, Dr. Mohammad developed a novel electrode structure for Direct Methanol Fuel Cell (DMFC), thereby; the methanol concentration which can be used efficiently in DMFC has been increased from 7 to 100 wt%. After his Ph.D. study, he worked on a project for the fabrication of a complete passive fuel cell stack operated with high methanol concentration based on this novel electrode structure. As the high cost of the electrodes of the direct methanol fuel cells is one of the main obstacles facing their commercialization, Dr. Mohammad is working on the development of nano-sized material for the replacement of the Pt catalyst with non-precious catalysts. Dr. Mohammad secured international and national funds of more than 60 0,0 0 0 USD through them he built up two pioneer labs in the field of the Solid Oxide Fuel Cells and Microbial Fuel Cells. Right now Dr. Mohammad is working as an Associate Professor in the Sustainable and Renewable Energy Engineering Department, University Sharjah, UAE. He is working on developing energy conversion/storage devices such as fuel cells and supercapacitors. Dr. Mohammad published more than 60 SCI journal papers. Enas Taha Sayed got her Ph.D. from Japan in the field of bioelectrochemical energy devices, i.e., microbial fuel cells. During this period, she has studied the electrochemical behavior of S. cerevisiae, Baker’s yeast, in a mediatorless microbial fuel cell. Moreover, she has studied the effect of modifying carbon anodes on the cell performance. Then Dr. Enas participated different national and international projects for developing fuel cells with a total fund of more than 60 0,0 0 0 USD. Dr. Enas has strong experience in the different electrochemical techniques used for the evaluation of the different electrode materials for fuel cells in general and specifically in microbial fuel cells. Dr. Enas published more than 25 SCI journal papers. Hend Omar Mohamed earned her B.Sc. and M.Sc. in Chemical Engineering department, faculty of engineering, Elmina University, Egypt. While, she has received her Ph.D. from bio nano system engineering at Chonbuk national university, Jeonju, South Korea. She has started her career as post doctor researcher in National Institute of Environment Research center in Korean Maritime and Ocean University, Busan, (South Korea). She has strong knowledge in the fields of bio electrochemistry, waste water treatment and fabrication of the advanced materials. She has an extensive experiences, more than 8 years, at synthesis of nanomaterials such as, nanocatalysts and nanofibers electrodes for the different types of fuel cells, especially, microbial electrolysis cell, microbial fuel cell, ethanol, methanol and urea fuel cells. She participated in many interdisciplinary research projects as a princi-

pal researcher, to fabricate new advanced electrodes for fuel cells applications on large scale. Dr. Hend leads more than one academic researcher groups included undergraduate, master and Ph.D. students in the field of synthesis of the key components for fuel cells, such as, bio anodes, photocathodes and proton exchange membranes. She published more than 20 SCI research papers for highly ranked journals, included, water research, chemical engineering, bio resource technology, and power source journals. M. Obaid received his B.Sc. and M.Sc. from Chemical Engineering department, faculty of engineering, Minia University, Egypt. While, he has received his Ph.D. from Bionano System Engineering at Chonbuk National University, Jeonju, South Korea. He has started his career as a postdoctor researcher in Global Desalination Research Center (GDRC), School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju, South Korea. He has strong knowledge in the fields of Membrane fabrication, desalination, electrospinning, wastewater treatment and fabrication of the advanced materials. He has an experiences, more than 5 years, at synthesis of nanofibers and nanomaterials such as, electrospun nanofiber membranes, and nanofibers catalysts and electrodes for different applications. He published more than 35 SCI research papers in highly ranked journals, in addition to patents and international conferences. Hegazy Rezk received the B. Eng. and M. Eng. degrees in electrical engineering from Minia University, EGYPT in 2001 and 2006 respectively, and his Ph.D. from Moscow Power Engineering Institute, Moscow. He was a postdoctoral research fellow in Moscow State University of Mechanical Engineering, Russia for 6 months. Dr. Hegazy was a visiting Researcher at Kyushu University, Japan, for one year. Currently, Hegazy Rezk is Associate Professor in Electrical Engineering Department, Collage of Engineering at Wadi Addwaser, Prince Sattam University, Saudi Arabia. He authored more than 50 technical papers. His present research interests include renewable energy, smart grid, hybrid systems, power electronics, Optimization and artificial intelligence Kyu-Jung Chae received his Ph.D. from Gwangju Institute of Science and Technology (GIST), in Environmental Engineering, 2010. After obtaining a doctorate, he worked at Kolon Global Corp., a leading company in environmental sectors in South Korea, as a principal researcher and licensed Professional Engineer. Since 2014, he has been working as a professor at Korea Maritime and Ocean University. He has led more than 30 research projects as a principal investigator, published 82 papers for highly ranked peer-reviewed journals (including 15 domestic papers), 3 books, and achieved 40 patents. In addition, he developed many New Excellent Technologies certified by Korean government and successfully commercialized those technologies in the industrial fields. The strongest point which makes him unique researcher compared to other scientists is his extensive and balanced experiences in both academia and industry for over 19 years since 20 0 0, so that he knows well the real needs in environmental research in both fundamental and practical aspect. His areas of expertise are water-energy nexus technology, bioelectrochemical cells, energy self-sufficient wastewater treatment, membrane bioreactor, microbial fuel cells, environmental nano-materials, and advanced wastewater treatment.