Recent developments in nickel based electrocatalysts for ethanol electrooxidation

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Recent developments in nickel based electrocatalysts for ethanol electrooxidation Akshay N. Vyas a, Ganesh D. Saratale b, Shrikrishna D. Sartale a,* a

Thin Films and Nanomaterials Laboratory, Department of Physics, Savitribai Phule Pune University, Pune, 411 007, India b Department of Food Science and Biotechnology, Dongguk University-Seoul, Ilsandong-gu, Goyang-si, Gyeonggido, 10326, Republic of Korea

highlights

graphical abstract


Overview of nickel based electrocatalysts

for

ethanol

electrooxidation.
Partial

replacement

of

noble

metals by Ni reduces cost of the electrocatalyst.
Influence of support, composition and synthesis on electrocatalytic performance.

article info

abstract

Article history:

A survey is done to gain a general idea in the development of various nickel based anode

Received 28 February 2019

electrocatalysts for ethanol electrooxidation reaction. Platinum and other noble metal

Received in revised form

electrocatalysts are very well known but their cost and scarcity is a major issue hampering

14 August 2019

its use on a commercial level. Apart from cost, the poisoning of noble metal electro-

Accepted 19 August 2019

catalysts due to CO is also another issue. These issues can be tackled by partially or fully

Available online xxx

replacing the noble metal electrocatalysts by non-noble metal electrocatalysts. The use of electrocatalytically active non-noble metal like nickel provides an excellent alternative.

Keywords:

Hence, major thrust is laid upon the use of nickel in the form of either as a single or a

Direct ethanol fuel cell

complementary element in the electrocatalysts containing two (binary), three (ternary),

Ethanol electrooxidation

four (quaternary) or more noble and/or non-noble metals to improve the electrocatalytic

Non-noble metals

activity for ethanol electrooxidation reaction. The quality of an electrocatalyst is decided

Nickel

on a number of factors. Onset potential and current density are the two main parameters

Electrocatalysts

representing the activity of electrocatalysts. Complete oxidation of ethanol to give CO2 is a major requirement to extract maximum current. Literature survey shows that support, synthesis approaches and elemental compositions greatly contributes to enhance the

* Corresponding author. E-mail address: [email protected] (S.D. Sartale). https://doi.org/10.1016/j.ijhydene.2019.08.218 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Vyas AN et al., Recent developments in nickel based electrocatalysts for ethanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.218

2

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electrocatalytic performance of nickel based electrocatalysts towards ethanol electrooxidation reaction. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

3O2 þ 12Hþ þ 12e / 6H2O

Introduction A fuel cell is basically an electrochemical device which directly converts the chemical energy stored in an externally supplied fuel to electrical energy. It requires a reductant (supplied to anode) and an oxidant (supplied to cathode) to carry out the overall redox reaction producing electricity. Hydrogen is the best candidate as a fuel to be used for fuel cell application primarily due to its fast oxidation kinetics and high efficiency of a hydrogen/oxygen fuel cell. However, hydrogen is not freely available and needs to be produced from electrolysis of water, oil and coal gasification and natural gas reforming. Moreover, hydrogen in gaseous form is highly reactive and thus the storage of hydrogen is critical. Hence, the major issues concerned with the utilization of hydrogen as a fuel are its production and safe storage [1]. It is also not easy to handle and transport, making it difficult to be used for portable devices. A safe alternative to hydrogen fuel cells are direct alcohol fuel cells (DAFCs). Compared to hydrogen fuel cells, DAFCs stand apart with their distinct advantages such as ease in storage, handling and transportation of alcohols. Moreover, alcohols such as methanol (4.82 kW h/l), ethanol (6.28 kW h/l), 1 and 2-propanol (7.28 and 7.07 kW h/l) have higher energy density as compared to hydrogen gas (0.18 kW h/l; stored at 70 bar and 25  C). These advantages make it simpler to build portable energy devices with much higher efficiencies using DAFCs [2]. Much focus has been laid on development of direct methanol fuel cells (DMFCs). In DMFCs methanol as a fuel is oxidized at anode to give electrons (e), protons (Hþ) and carbon dioxide. The electrons and protons react with air (oxygen) at the cathode to give water. Thus, the by-products of a DMFC are carbon dioxide and water. The overall oxidation reaction of methanol is given below [3]. CH3OH þ H2O / CO2 þ 6Hþ þ 6e

(3)

The reaction (2) at anode electrode is balanced by the oxygen reduction reaction (ORR) [6] at the cathode given in reaction (3). Alike DMFC, in direct ethanol fuel cells (DEFCs) too CO2 is the expected end product. The CO2 emitted during the production and oxidation of ethanol process can be utilized by the biomass producing plants, thus completing the CO2 generation and consumption cycle and making DEFCs nonhazardous to environment. Hence, in the recent past focus has shifted from DMFCs towards DEFCs. A major problem for commercialization of DEFCs is that the complete oxidation of ethanol to give CO2 is not an easy process. It is very difficult to break the CeC bond and hence many intermediate products such as acetic acid and acetaldehyde are formed instead of the expected final product (CO2). Different products that can be obtained due to electrooxidation of ethanol are given in Fig. 1 [7]. The oxidation of ethanol to acetic acid or acetaldehyde yields 4 or 2 electrons, respectively [8]. The reactions (4 and 5) show production of acetic acid and acetaldehyde. C2H5OH þ H2O / CH3COOH þ 4Hþ þ 4e

(4)

C2H5OH / CH3CHO þ 2Hþ þ 2e

(5)

Thus, complete electrooxidation of ethanol to form CO2 by CeC bond scission is a difficult task. Another issue regarding the use of ethanol is the crossover of ethanol through a membrane like Nafion. But it has been found that the rate of ethanol crossover is less as compared to methanol [6]. The oxidation of ethanol can be carried out in both acidic as well as

(1)

However, methanol is toxic and is environmentally hazardous [4,5]. Apart from toxicity of methanol DMFCs also suffer from disadvantages like sluggish reaction kinetics and crossover of methanol through Nafion membrane towards cathode which results in the overall reduction of DMFC efficiency [6]. Ethanol in comparison to methanol is non-toxic, can be produced from biomass (agricultural waste products) and has higher energy density. Moreover, 12 electrons are generated during complete ethanol electrooxidation as compared to 6 electrons for methanol. The overall reaction for ethanol electrooxidation can be written as: C2H5OH þ 3H2O / 2CO2 þ 12Hþ þ 12e

(2)

Fig. 1 e The various products of ethanol after electrooxidation [7].

Please cite this article as: Vyas AN et al., Recent developments in nickel based electrocatalysts for ethanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.218

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alkaline electrolytes. However, the rate of ethanol electrooxidation reaction and oxygen reduction reaction were found to be faster in alkaline medium as compared to acidic medium [6]. The reaction kinetics of ethanol electrooxidation can be improved with the use of a better electrocatalyst. Platinum and platinum group metals widely considered as noble metals are the best known electrocatalysts but they are expensive and utilization of these metals as anode electrodes result in increment of the overall cost of fuel cells. They also suffer from slow reaction kinetics and CO poisoning. Keeping the shortcomings of noble metals in mind and also taking the advantage of use of alkaline electrolyte many research groups developed a number of electrocatalysts consisting of mono (single), binary (two), ternary (three) and quaternary (four) noble and/or non-noble metal nanoparticles to be used as anode electrodes for ethanol electrooxidation reaction. Partial replacement of noble metals by non-noble (Ni, Fe, Co, etc.) or semi-noble (Cu) metal nanoparticles has attracted a lot of interest in recent past basically because such partial replacement has helped not only to reduce the cost of anode electrode but also to improve the performance than noble metals in terms of DEFCs efficiency. Ni is one such element which stands tall amongst the various non-noble metals for various energy related applications. Nickel based electrocatalysts are exceptional material for applications such as supercapacitor [9] and hydrogen evolution [10]. Researchers have studied the effect of various parameters like support effect, temperature influence and morphological modifications as well as core-shell structures of Ni(OH)2 for supercapacitor application [11e14]. Other compounds containing Ni such as nickel oxide or nickel cobaltite also proved to be good materials for energy related applications [15,16]. Nickel is also recognized as a distinguished flexible support material where researchers have used Ni foam or foil as substrate for solid-state supercapacitors [17]. Oxides of nickel are also good for hydrogen evolution reaction [18].

3

Ni when alloyed with noble metals such as Pt, Pd, Ru and Rh shows enhanced electrocatalytic activity for ethanol electrooxidation [19e23]. Coupling Ni nanoparticles with Cu, Co or Fe also helps to improve its electrocatalytic activity [24e27]. Better efficiency of DEFCs due to partial replacement of noble metals by Ni nanoparticles is majorly dependent on the way these electrocatalysts are synthesized. The synthesis methods adopted and the support on which these electrocatalyst nanoparticles are synthesized are major factors defining their electrocatalytic activity. Different approaches have been successfully attempted by researchers all over the world to synthesize Ni and other noble/non-noble metal alloy/ composite nanoparticles, which were tested for ethanol electrooxidation reaction. In the present review we have discussed the various strategies by which researchers have tried to improve the electrocatalytic performance of anode for ethanol electrooxidation. A range of Ni based electrocatalysts prepared by using different synthesis methods on variety of supports with various compositions have discussed in detail and their impact on ethanol electrooxidation has highlighted.

Ethanol electrooxidation mechanism of Ni electrocatalyst The ethanol electrooxidation reaction by Ni nanoparticles is majorly dependent on the reversible redox transformation between Ni(II) 4 Ni(III). The virgin Ni nanoparticles when immersed in an alkaline electrolyte solution form hydroxide layer over the surface [30]. The potential cycling from lower (comparatively negative) to higher (comparatively positive) potential values also develops a layer of nickel hydroxide over the surface of Ni nanoparticles in the form of a-Ni(OH)2 at lower potentials and b-Ni(OH)2 at higher potentials. The aNi(OH)2 also slowly ages to form b-Ni(OH)2 [31]. This nickel hydroxide or Ni(II) is oxidized to nickel oxyhydroxide or Ni(III) at potentials between 0.4 and 0.5 V vs. SCE (Saturated calomel electrode) when a potential sweep is given anodically (from

Fig. 2 e Schematic of three-electrode experimental set-up for CV measurements to study ethanol electrooxidation performance.

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Fig. 3 e Typical CV of Ni nanoparticles grown on graphite substrate recorded in NaOH and NaOH with ethanol electrolyte solutions depicting onset potential and ethanol electrooxidation anodic peaks [32].

lower to higher potential) [32]. In an alkaline electrolyte solution containing ethanol the Ni(III) i.e. NiO(OH) undergoes reduction reaction to oxidize ethanol. The chemical reactions showing reversible redox transformation between Ni(II) and Ni(III) and ethanol electrooxidation can be shown as follows [28,33]: Ni(OH)2 þ OH 4 NiOOH þ H2O þ ee

(6)

NiO(OH) þ C2H5OH / Ni(OH)2 þ by-products

(7)

Ethanol electrooxidation on any electrocatalyst is studied by recording the cyclic voltammetry (CV) curve in an electrolyte solution containing ethanol. A three-electrode system consisting of a working electrode (electrocatalyst of interest coated on a conducting substrate), a counter electrode (usually a chemically stable conducting element such as platinum) and a reference electrode (an electrode for measuring potential having known and stable electrode potential) is used for this purpose. The schematic of a typical CV measurement experimental set-up is shown in Fig. 2. It can be seen from the reactions (2), (4) and (5) that the current density, which measures the number of electrons produced from ethanol electrooxidation, is one of the parameters deciding the efficiency of an electrocatalyst. CV denotes the graph of current density vs potential measured against a reference electrode. In the presence of ethanol in electrolyte solution the rise in current density is an indication of electrooxidation of ethanol [34]. With increase in potential one may also observe, parallel to the rising current density, an anodic peak in anodic (scanning form comparatively lower to higher potential) as well as cathodic (scanning form comparatively higher to lower potential) scans during CV indicating ethanol electrooxidation [32]. The electrooxidation of ethanol can form many products as seen in Fig. 1. Thus increment in current density and/or observation of anodic peak indicating ethanol electrooxidation are two important things reflecting

the activity of the electrocatalyst towards ethanol electrooxidation. The peak indicating ethanol electrooxidation was shown to be temperature dependent by Barbosa et al. [35]. The temperature at which ethanol electrooxidation reaction is carried out not only influences the current density but also decides the peak potential. Another factor which has its impact on the appearance of the peak indicating electrooxidation of the organic compound involved is the scan rate. El-Shafei showed that at high scan rate (200 mV/s) only the peak indicating oxidation of Ni(II) to Ni(III) was observed, whereas at low scan rate (2 mV/s) only the peak indicating oxidation of the organic compound mixed in the electrolyte solution was observed [36]. Fleischmann et al. proposed the scheme of electrooxidation of any organic compound as shown in reaction (7) [37]. The reaction (7) was based on the observation that the appearance of any peak indicating oxidation of the organic compound in question appeared overlapping the process of formation of Ni(III) [33]. Apart from current density, onset potential is an important parameter for ethanol electrooxidation which can be used to compare electrocatalytic activity of different electrocatalysts. The onset potential can be defined as a point from which the rise in current density due to ethanol electrooxidation is observed. It represents the electrode overpotential. The lesser is the overpotential; the better is the electrocatalyst and viceversa [24]. Hence, it is a general requirement that a good electrocatalyst should give high current density at low onset potential. This onset potential is described with respect to some reference electrode such as SCE, Silver/Silver-chloride (Ag/AgCl) electrode, reversible hydrogen electrode (RHE), etc. Typical CV curves recorded in NaOH and NaOH with ethanol electrolyte solutions for nickel nanoparticles grown on graphite substrate are shown in Fig. 3. The chronoamperometry (current vs. time graph) is often shown to denote the stability of the electrocatalyst. The prepared electrocatalyst needs to be good not just in terms of giving high current density and low onset potential but should also demonstrate high stability and durability against poisoning due to toxic carbon containing intermediate products. Typical chronoamperometric curves for a Pd, PdCu and PdCuNi electrocatalyst on carbon support are shown in Fig. 4 [85]. In the Figure PdCuNi/C shows higher current density and better

Fig. 4 e Chronoamperometric curves for Pd/C, PdCu/C and PdCuNi/C electrocatalysts [85] .

Please cite this article as: Vyas AN et al., Recent developments in nickel based electrocatalysts for ethanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.218

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stability than PdCu/C and Pd/C. This stability is often checked by recording large number of CV cycles too.

Support effect on electrocatalytic performance of Ni based electrocatalysts Increasing the surface area of the grown/deposited Ni based electrocatalysts either by decreasing their size or increasing the number of nanoparticles is known to enhance the ethanol electrooxidation reaction. A number of other factors such as dispersion of nanoparticles, efficient charge transfer and rejuvenation of active sites by continuously removing the adsorbed CO like toxic intermediates from the catalyst surface also help to improve the catalytic performance towards ethanol electrooxidation. Experimentally the support used for catalysts loading has on many occasions, as per literature, tended to promote the catalytic performance of Ni based electrocatalysts as compared to the conventional/pure noble metal electrocatalysts. Thus, the support on which nanoparticles of Ni based electrocatalysts are grown plays an important role. Alcantara et al. deposited Ni nanoparticles on anodized titanium. They observed that the anodization of Ti increased the adhesion as well as created a porous structure for the support of Ni nanoparticles. Nickel nanoparticles electrodeposited on optimized anodized titanium (anodized at 5 V for 1 h) showed better electrocatalytic activity for methanol electrooxidation and poorer for ethanol electrooxidation reaction [38]. Titanium foil as it is or SnO2 coated Ti foil has also served the purpose of providing a support for the electrocatalysts [39e41]. Rosario et al. prepared NiePd bimetallic electrocatalyst in a bilayer form over Ti foil for ethanol electrooxidation. They showed that NiPd/Ti (Ni over Pd, coated on Ti) could perform better than Pd/Ti (Pd coated on Ti) and far better than PdNi/Ti (Pd over Ni, coated on Ti). However, the observations showed that there was no contribution of Ti towards ethanol electrooxidation reaction. The enhancement in ethanol electrooxidation reaction for NiPd/Ti was explained on the basis of results as follows. Pd over Ni (PdNi/Ti) increased the electrical resistance because of presence of Ni oxides over Ni. Moreover, there was increased amount of PdO on the surface layer in case of PdNi/Ti. The active sites for ethanol electrooxidation on Pd were also on top having no contact with the inner Ni layer. Whereas, for Ni over Pd layer (NiPd/Ti) the electrolyte could access the active sites for ethanol electrooxidation on Pd layer through the crevices present in Ni layer. The lower Pd layer consisted more of metallic Pd as compared to PdO which was beneficial for ethanol electrooxidation reaction. The Ni layer over Pd layer also increased the active surface area on Pd by roughening its surface. The outer Ni layer acted as a protection for inner Pd layer against poisoning intermediates and also provided the Pd layer with hydroxyl species that could help to reactivate the Pd sites for ethanol electrooxidation. Optimizing the amount of Ni proved to be beneficial for fabricating an electrocatalytically active and stable electrocatalyst [39]. The scheme for enhancement in ethanol electrooxidation reaction using NiPd/Ti is illustrated in Fig. 5. Lo and Hwang used Ti metal as a substrate to coat NieP. Ti metal

5

was chosen as a substrate keeping in mind its mechanical properties. Ti was coated with a thin layer of SnO2 to prevent the oxidation of Ti substrate and to increase the adhesion between Ti and Ni deposits [40]. Tamasauskaite-Tamasiunaite et al. quoted literature in their introduction where Ti as a support helped to enhance the electrocatalytic property. Their investigation showed that when Ti was used as a support, nanoPt(Ni) displayed better electrocatalytic activity towards ethanol electrooxidation, exhibiting high oxidation current densities at lower potential values as compared to pure Pt [41]. Overall, researchers have given preference to Ti metal when choosing for a metal foil as a substrate for escalating performance towards ethanol electrooxidation. There are varied forms such as anodized Ti or Ti coated with SnO2 or simply Ti foil for supporting the electrocatalyst and the results prove that Ti is a competitive support material. Mild steel was used as a substrate for deposition of NieGd2O3 composite by Sivasakthi et al. [42]. It was observed that as the weight percent of Gd2O3 increased in the composite the formation of Ni(II) 4 Ni(III) redox couple increased. It was also observed that the current density increased with increment in Gd2O3 weight percent (wt%). It was observed that the presence of Gd2O3 reduced the particle size of Ni, increased the surface area and also contributed to increase the active sites for formation of NiO(OH) which eventually resulted in the enhancement of electrocatalytic activity towards ethanol electrooxidation. Here, the weight percentage of Gd2O3 played a much important role and mild steel was just used as a conducting support. The most impactful supports that researchers have explored till date are those made from Carbon. Various carbon nanomorphologies such as carbon nanoparticles, carbon nanofibres and graphene were used by Barakat et al. to support Ni nanoparticles for ethanol electrooxidation reaction [43]. Ni on graphene was prepared by mixing Ni salt in suspension containing graphene and reducing agent and sintered in argon at 850  C for 2 h. Ni on carbon nanofibres was prepared by electrospinning a sol-gel formed using nickel salt and poly-vinyl alcohol (PVA). Ni on carbon nanoparticles was prepared by drying the above sol-gel mixture in vacuum at

Fig. 5 e Scheme for enhancement in ethanol electrooxidation by using NiPd/Ti electrocatalyst [39].

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80  C and grinding it. For very low nickel content carbon nanoparticles were found to give better results [43]. The various carbon nanomorphologies used by Barakat et al. are shown in Fig. 6. The carbon support was preferred due to its adsorption tendency towards ethanol which eventually helped to transport the alcohol molecules to catalyst surface. The influence of various carbon nanomorphologies was explained based on CV results. It was observed that graphene as a support helped to increase the electrocatalytic performance towards ethanol electrooxidation, as compared to pristine Ni nanoparticles, by merely increasing the nickel content. The increment in Ni content favoured the electrocatalytic performance due to increment in total surface area and creation of new adsorption sites for ethanol. Carbon nanofibres are distinct and when they were used as support it resulted in equivalent performance to graphene and that too at low content of nickel. The nanofibre structures are easy to synthesize as compared to graphene. Carbon nanoparticles showed very efficient performance for ethanol electrooxidation but only for very low nickel content (2e4 wt%). At high Ni loading it was proposed that the nanofibre and nanoparticle structures fall short of providing enough space for ethanol adsorption and thus do not show enhanced efficiency. Thus, Ni content and related ethanol electrooxidation was shown to be strongly dependent on morphology of the support material. For loading the electrocatalysts graphene emerges as an attractive support material, because of its high conductivity and specific area providing easy electron transport and more adsorption capacity. A major challenge of using graphene as a support is that it may form agglomerates of graphene sheets which are stacked close to each other. Thus it may end up losing many electrochemically active sites. Taking this as a challenge a novel support in the form of graphene aerogel was used by Ren et al. [44] to prepare a three dimensional (3D) Ni nanoparticle/graphene aerogel for ethanol electrooxidation. The SEM image of 3D Ni/graphene is shown in Fig. 7. The SEM image shows finely dispersed Ni nanoparticles over the surface of graphene walls inside the aerogels. The nanoparticles of Ni over graphene aerogel showed no aggregates. The electrocatalytic performance of as prepared Ni/graphene aerogel towards ethanol electrooxidation was compared with that of Ni/MWCNT (MultieWalled Carbon Nano Tube) and it was found that the former showed a high current density and comparatively lower onset potential as compared to the latter. The superior performance of Ni/graphene aerogel was attributed to the highly porous network nature of graphene aerogel as well as the high electrical conductivity of graphene.

Fig. 7 e SEM image of 3D Ni/graphene aerogel [44]. Other form of graphene such as reduced graphene oxide (rGO) [45e48] is also widely used as an electrocatalyst support. rGO is widely considered as an excellent support material for catalysts due to its properties such as high electrical conductivity, high surface area, electrochemical stability and low cost. In addition the various oxygen functional groups present on the rGO surface provide extra active sites for the catalysts thereby helping to decrease the poisoning effect by eliminating carbonaceous intermediates [49]. Dutta and Ouyang emphasized on the properties of rGO such as high surface area and high electrical conductivity while citing it as an effective support material than carbon black [45]. They synthesized ternary NiAuPt nanoparticles by using a simple one pot synthesis by simultaneous chemical reduction of metal precursors and graphene oxide (GO). One distinct advantage of simultaneous reduction was that GO was completely reduced to rGO in presence of metal precursors than in their absence. Moreover, using GO as a precursor for the preparation of metal nanoparticles on rGO provided functional groups, attached on GO, which acted as anchoring sites for metal nanoparticles. The onset potential of the synthesized ternary NiAuPt showed considerably lower onset potential. The ethanol electrooxidation peak current density for ternary NiAuPt was 8, 4 and 2 times higher than pure Pt, bimetallic NiPt and AuPt grown on rGO nanosheets, respectively. The SEM and TEM images shown in Fig. 8 display the well dispersed nature and uniformity of NiAuPt nanoparticles on rGO nanosheets. Krishna et al. while preparing a novel catalyst namely palladium on nickel boride (Pd@NixB) supported with rGO nanosheets showed that in the absence of rGO nanosheets, the catalyst nanoparticles were highly agglomerated. In the presence of rGO nanosheets as support the catalyst nanoparticles were uniformly distributed and showed no signs of

Fig. 6 e Various carbon nano morphologies e (A) Graphene, (B) Carbon nanofibres and (C) Carbon nanoparticles supporting Ni nanoparticles [43]. Please cite this article as: Vyas AN et al., Recent developments in nickel based electrocatalysts for ethanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.218

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Fig. 8 e (A) SEM and (B) TEM images of NiAuPt ternary nanoparticles grown on rGO nansheets [45].

agglomeration thrusting the property of rGO to be able to prevent agglomeration of catalysts nanoparticles [46]. The SEM images of rGO supported and unsupported Pd@NixB nanoparticles are shown in Fig. 9 where the agglomeration of catalyst nanoparticles can be clearly observed when rGO nanosheets were not used as a support. Rezaee et al. used MnO2 modified rGO as a support for PdeNi nanoparticles electrocatalysts [47]. The rGO, MnO2 and Ni had significant roles to play in the enhanced electrocatalytic activity towards ethanol electrooxidation. The rGO helped in better dispersion of metal nanoparticles preventing agglomeration and giving smaller sized and uniform metal nanoparticles. The presence of Ni helped in weakening the adsorption of carbonaceous intermediates on Pd surface and increased its CO tolerance. Ni also provided oxygen containing species for oxidation of CO at lower potentials than Pd. MnO2 nanoparticles enhanced the formation of adsorbed OH species leading to elimination of poisonous intermediates thus contributing to the enhancement of overall ethanol electrooxidation activity. Zhang et al. used nitrogen doped graphene as a support for Ni core-Pd shell (Ni@Pd) nanoparticles and found that nitrogen doped on graphene helps not only in formation of small and highly dispersed Ni@Pd nanoparticles but also to remove intermediate poisoning species [50]. The nitrogen doped graphene also acted as a reductant thereby making the synthesis process devoid of surfactants and reducing agents and making the synthesis process greener. The TEM and HRTEM images of

Ni@Pd nanoparticles over nitrogen doped graphene are shown in Fig. 10. The Ni@Pd nanoparticles are evenly distributed over the nitrogen doped graphene. The reason for very fine dispersion of Ni@Pd nanoparticles was found to be the defects created by nitrogen doping which acted as anchoring sites for the metal nanoparticles. The HRTEM image showed the core made up of Ni (111) crystal planes surrounded by an outer shell made up of Pd (111) crystal planes. Maiyalagan et al. prepared PdeNi alloy nanoparticles and attributed the enhancement in catalytic activity and stability to the support provided by carbon nanofibres and the addition of Ni to Pd [51]. It was observed that the metal nanoparticles were uniformly distributed over the carbon nanofibre structure. Another carbon based material that is recommended as a support for electrocatalysts is carbon nanotube (CNT) [29,52e54]. CNT provides exceptional catalyst support due to properties such as nanometer size and high electrical conductivity. Deng et al. synthesized NiCo/CeN/CNT composite catalyst, with Ni to Co ratio being 3:1, by direct pyrolysis of nickel and cobalt salts and polyaniline at 800  C under N2 environment. Different mass percentages of CNTs (15, 25, 35 and 45%) were used for preparing the composites. It was observed that when an appropriate amount of CNT was added, the conductivity and dispersion of the NiCo/CeN catalyst improved. The experiments proved that CNT itself displayed very less electrocatalytic activity towards ethanol

Fig. 9 e SEM images of (A) rGO supported and (B) unsupported Pd@NixB nanoparticles [46]. Please cite this article as: Vyas AN et al., Recent developments in nickel based electrocatalysts for ethanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.218

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Fig. 10 e (A) TEM and (B) HRTEM images of Ni@Pd nanoparticles on N-doped graphene [50].

electrooxidation. On the basis of experiments it was proposed that addition of CNT to NiCo/CeN would enhance the electron transfer of alcohol oxidation between the catalyst interface and electrolyte solution. For 3:1 ratio of Ni to Co i.e. Ni3Co1/ CeN nanoparticles, aggregation was observed in absence or low content of CNT. On the other hand, high content of CNT resulted in decrement of metal catalyst percentage which led to the relative lowering of catalytically active sites. Out of the tested mass percentages 35% CNT showed excellent electrocatalytic activity towards ethanol electrooxidation reaction [53]. Singh et al. prepared composites of Pd, MWCNT and Ni. The additions of MWCNT (1e5%) seemed to greatly enhance the electrocatalytic activity of the Pd-MWCNT composite for ethanol electrooxidation. Addition of 1% MWCNT showed the best result. Further addition of 1% Ni improved the electrocatalytic activity even more (~50% enhancement). To prove their point Singh et al. calculated the electrochemical active surface area (EASA) and found that addition of 1% MWCNT greatly increases the porosity and thus the EASA. For higher percentage additions of MWCNT this enhancement in EASA was not so significant. The same trend as observed for MWCNT was followed for addition of Ni too [54]. Jayaseelan et al. prepared mesoporous NiCo2O4-MWCNT nanocomposite aerogels using sol-gel method [55]. They varied the MWCNT loadings by 0, 2, 5, 10 and 15 mg. The MWCNT support provided an interconnected fibrous network and allowed for uniform dispersion of the synthesized nanoparticles. The 15 mg MWCNT loading showed higher peak current density for ethanol electrooxidation. The NiCo2O4 nanoparticles were found to be interconnected with MWCNTs in the aerogel matrix thereby resulting in reduction of diffusion length of ethanol, helping charge transfer and improving the stability of NiCo2O4-MWCNT nanocomposites as compared to NiCo2O4 aerogel. The high electrical conductivity of 15 mg MWCNT loaded was cited as a reason for the observed higher current density and hence enhanced ethanol electrooxidation activity. Jin et al. modified the MWCNT surface with 4-aminobenzene [29]. They observed that the 4aminobenzene modified MWCNT can not only help to avoid the preferred nucleation process but also to control the Ni nanoparticle size. Similarly, Yi et al. used b-cyclodextrin modified MWCNTs as support for PdeNi alloy nanoparticles. b-cyclodextrin on MWCNT helped for excellent dispersion of

PdeNi alloy nanoparticles and thus helped to improve the electrocatalytic performance [56]. Hassaninejad-Darziand Gholami-Esfidvajan prepared a carbon paste electrode which was modified by nanoporous nickel phosphate molecular sieve and MWCNTs. Nickel phosphate on multi-walled carbon paste electrode helped to enhance the oxidation of ethanol by lowering the onset potential and overcoming the slow kinetics of reaction in alkaline medium [57]. Thus it can be seen that different research groups have used CNTs by either optimizing the loading content or by modifying its surface. Overall, the addition of CNTs contributes in the enhancement of electrocatalytic activity towards ethanol electrooxidation when used with optimum conditions pertaining to the catalyst materials. Another popular allotrope of carbon that is widely used as a support material for catalysts is the graphite. Graphite is quite stable chemically, robust in nature and has a rough surface thus providing larger surface area for deposition of catalyst material. Soliman et al. used graphite electrode as a support for electrodeposition of nickel nanoparticles, for ethanol electrooxidation [58]. In our recent work we have also used graphite as support for growth of nickel nanoparticles using successive ionic layer adsorption and reaction (SILAR) method [32]. Chelaghmia et al. activated the graphite support by subjecting it to mechanical treatment by various grades of polishing papers. This was further polarized at 1.8 V vs. Ag/ AgCl for 5 min followed by 50 CV scans within the cathodic and anodic limits in sulphuric acid solution. The activation of graphite enhanced the current density by accelerating the electronic exchange at the electrode interface but it itself had negligible contribution on the electrooxidation of ethanol [28]. Jafarian et al. modified graphite electrode by using poly-oaminophenol (POAP). POAP helped to form complexes between transition metals and amine sites of the polymer [59]. Cardoso et al. modified graphite electrode using nickeldimethylglyoxime complex [Ni(II)(DMG)2] for methanol and ethanol electrooxidation. They concluded that the ethanol electrooxidation reaction is a 4-electron process producing acetate ions. The peak current densities of ethanol electrooxidation increased with an increment in [Ni(II)DMG2] loading, OH and alcohol concentrations [60]. Fahim et al. discussed through their literature survey that presence of SnO2 is beneficial for ethanol electrooxidation

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reaction due to easier adsorption-dissociation of adsorbed OH over SnO2 surface which eventually helped to get rid of adsorbed CO from over the noble metal surface. Hence, they prepared a SnO2/graphene support for deposition of M@Pd (M¼Co, Ni or Cu) core shell nanostructures [61]. Stradiotto et al. used boron doped diamond as a support for Ni nanoparticles [62]. Boron doping provides a high conductivity along with retaining original properties of diamond such as hardness, durability, chemical inertness, etc. Apart from the above mentioned carbon supports many research groups have also preferred to use carbon black as a support for electrocatalyst [23,63e68]. Mohamed et al. deposited nanocomposite of Ni/Pd by electrospinning followed by calcination under Ar atmosphere on carbonized poly-vinyl alcohol (PVA) nanofibre. The synthesized PdeNi nanofibres showed improved electrooxidation of ethanol and urea in alkaline medium and formic acid in acidic medium. For ethanol electrooxidation a sharp rise in current density (5.9 times) than that in absence of ethanol was observed [69]. The various carbon supports provide high surface area, high electrical conductivity, active sites for adsorption of OH species and preventing agglomeration as well as helping in uniform distribution of catalyst nanoparticles, etc. The various carbon supports strongly affect the amount of catalyst to be loaded too. Hence, carbon based materials are widely preferred as support for ethanol electrooxidation. The above discussion also showcases that Ni based electrocatalysts perform better than conventional noble metal electrocatalysts for ethanol electrooxidation when carbon based supports are used. It is also evident that functionalizing/modifying the carbon based supports such as graphene, reduced graphene oxide, carbon nanotubes and graphite, etc. improves the electrocatalytic performance towards ethanol electrooxidation. Silicon based supports such as ZrO2/n-Si [27], 3D ordered silicon microchannel plates (Si MCP) [70] and Si-nanowires [71] are also explored by researchers in the quest for better electrocatalytic performance. The Si-MCP rendered a porous structure with high surface to volume ratio endowing higher mass loading of NiePd catalysts. Si nanowire support allowed a better dispersion of catalysts thus leading to enhanced catalytic activity for ethanol electrooxidation. In some instances nickel, or nickel based compounds were also used as support such as NieZn or NieZneP [72]. NieZn and NieZneP supported on Vulcan XC72 carbon black acts as a good support for spontaneous deposition of Pd nanoparticles. Pd-(NieZn) and Pd-(NieZneP) were found to be stable, active and better electrocatalysts than PdeC. NieFe grown on Cu substrate was also tested as support material for ethanol electrooxidation reaction by Lv et al. [73]. The experiment was focused in trying out a bimetallic support having large surface area. The current densities for Pd on NieFe/Cu were intensely high as compared to Pd on Cu. The ultrathin sheets of NieFe self-assembled on Cu substrate producing large surface area thus promoting the electrocatalytic activity. The NieFe and their corresponding oxides also helped in the adsorption of OH (OHads) species. These OHads species in turn helped to root out the toxic adsorbed CO containing intermediates thereby enhancing the

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electrocatalytic activity towards ethanol electrooxidation. Carbon containing elements such as nickel, nitrogen and sulphur (Ni-NSC) was used as a support for Pd nanoparticles and explored for ethanol electrooxidation by Yang et al. [74]. The synergistic effect between nickel, nitrogen, sulphur and palladium allowed the formation of free OH radicals on the surface of nickel, sulphur and nitrogen atoms. These OH radicals reacted with CH3CO intermediates on Pd surface to form CH3COO and rejuvenate the active sites on Pd. Another advantage of Ni-NSC was that the nitrogen and sulphur had bonding with Pd which eventually helped for dispersion of Pd nanoparticles and prevent their agglomeration. Pd (20 wt%) loaded over Ni-NSC support showed enhanced ethanol electrooxidation as compared to commercial Pd (20 wt%) loaded over carbon. In another case NiCo bimetallic nanoflakes were used as support for Au nanoparticles and tested for ethanol electrooxidation by Yu et al. [75]. Au on bimetallic NiCo showed higher current density and better electrocatalytic activity as compared to Au on Ni or Co. The enhanced performance was attributed to the synergistic effect between Au and NiCo. The inclusion of Co played an important role for bettering the tolerance against poisoning due to carbonaceous intermediates. Cai et al. prepared NieFe alloy nanoparticles over ZrO2/n-Si and used it for testing the photoelectrocatalytic activity for ethanol electrooxidation. The NiFe/ZrO2/n-Si photoanode showed improved activity for ethanol photo electrooxidation and enhanced stability as compared to Pt(Pd)/ZrO2/n-Si photoanodes, thus showing potential to replace Pt or Pd catalysts [27]. Fig. 11 shows the schematic of photoassisted ethanol electrooxidation for the NieFe alloy nanoparticles. Huang et al. deposited highly dispersed Pt atoms on the surface of RuNi nanoparticles and found enhanced electrocatalytic performance. The ability of Ru and Ni to generate oxygen containing species at lower potentials was found to be helpful for oxidation of intermediates formed at Pt sites. The amount of Pt, being small, can be reduced to a great extent. The RuNi were found to be helpful in distributing Pt nanoparticles thus boosting the ethanol electrooxidation reaction.

Fig. 11 e Schematic of photo assisted ethanol oxidation on NiFe/ZrO2/n-Si photoanode [27].

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The presence of Ru and Ni was also found to decrease the electronic density of Pt nanoparticles which in turn reduces the bonding strength between Pt and poisonous intermediates thus helping the ethanol electrooxidation [76]. Thus we see that the support plays a vital role in deciding the performance of an electrocatalyst. It can be seen that overall responsibility of a good support is to provide large surface area, better conductivity, molecular adsorption capacity, durability, excellent dispersion of catalyst nanoparticles, ability to prevent agglomeration and control particle size as well as have chemical inertness. Additionally it may also help to increase the stability of the catalyst material by removing the intermediate species which cause decrement in overall activity of the nickel based electrocatalysts. From the above discussion we see that carbon based supports such as graphene, modified graphene, CNTs and many other carbon based materials render good support for nickel based electrocatalysts towards ethanol electrooxidation.

Different synthesis approaches for Ni based electrocatalysts Literature shows that different synthesis approaches have been explored for synthesis of Ni based electrocatalysts. The type and conditions of synthesis methods decide the physicochemical properties like structure, composition, electrical conductivity, morphology, etc. of the electrocatalyst, which in turn affect the electrocatalytic activity towards ethanol electrooxidation reaction. Many research groups prepared electrocatalysts in the form of powder and then utilized them for electrocatalytic application by coating the powder over a suitable substrate such as glassy carbon or carbon paper with the help of a dispersant (e.g. ethanol, isopropanol, etc.) and a binder (e.g. PVA or Nafion) [77e79]. The problem of synthesizing material in the powder form and coating it with the help of a binder has been tackled by a few research groups by directly growing/coating the electrocatalyst nanostructures on a conducting substrate. Different synthesis approaches were used by various research groups to prepare electrocatalysts in the form of either powder or a coating/film to achieve improved results towards ethanol electrooxidation. Jin et al. synthesized Ni

nanoparticles using ultrasonic assisted technique. The ultrasonic assisted method provided a high catalytic surface area resulting in excellent electrocatalytic activity for ethanol oxidation and electrochemical ethanol sensing [80]. Electrodeposition is one of the finest and most widely used deposition method to deposit electrocatalysts on a conducting substrate. Electrodeposition can be done either using a two electrode or a three electrode assembly. It has been used to deposit nickel based electrocatalysts in either a single elemental form or alloy (binary/ternary) systems [28,29,38,47,56,58,62,81e84]. Co-electrodeposition was used by Rostami et al. to deposit different electrocatalysts containing Pd-Cu-Ni [85]. The method of co-electrodeposition allowed for varying the CuCl2:NiCl2 solution concentration ratio to obtain the optimized electrocatalyst for ethanol electrooxidation. All the three elements i.e. Pd, Cu and Ni were uniformly distributed on the electrode surface. It was proposed that the presence of Cu and Ni improved the electronic structure of Pd and also helped to remove the poisonous CO species from the surface of Pd thereby providing more number of active sites for ethanol electrooxidation. The observed electrocatalytic performance for the ternary PdeCueNi electrocatalyst was better than PdeCu and pure Pd catalysts. Another chemical deposition method used by our group for growth of nickel nanoparticles over graphite substrate was the cyclic process SILAR [32]. The procedure for growth of Ni nanoparticles using SILAR consisted of consecutive immersions of the graphite substrate in nickel chloride solution to physically adsorb the Ni2þ cations through van der Waals forces and reduction of these adsorbed cations using NaBH4 as the reducing agent to form Ni nanoparticles over graphite. The Ni precursor and reducing agents are kept in separate beakers and the substrate is rinsed using double distilled water in between successive immersions. The various parameters of SILAR such as precursor concentration, immersion times and number of SILAR cycles were shown to affect the ethanol electrooxidation performance. The schematic of SILAR method is shown in Fig. 12. Chen et al. used the Modified polyol method for preparing PdNi nanocatalysts over MWCNT. The presence of nickel hydroxide over the catalyst surface in large amounts facilitated the removal of toxic carbonaceous species from the catalyst surface and thus helped to improve its electrocatalytic

Fig. 12 e Schematic of SILAR deposition method (One complete cycle) for growing metal nanoparticles.

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Fig. 13 e Schematic of formation of hollow nanoporous NiPd electrocatalysts [20].

performance. The PdNi nanocatalyst performed better than pure Pd electrocatalyst [52]. The conventional polyol method uses stabilizing agents and is strictly carried out in an anhydrous organic solution, whereas the modified polyol method allows using an ethylene glycol/water mixed solution and is free from stabilizing agents. The modified polyol method is also able to control the particle size and its distribution [86]. All these merits worked in favour of enhancing the electrocatalytic activity of the synthesized PdNi nanocatalyst. A few other physical and chemical methods used for synthesizing Ni based electrocatalysts include DC sputtering [39], electroless plating [40,70,87], etc. To prepare binary, ternary or quaternary metallic systems or alloys, primarily in powder form, simple procedure of reduction of metal cations using a suitable reducing agent solution under appropriate conditions is also commonly used [19,20,23,67,77,79,88]. Obradovi’c et al. synthesized Pd and PdeNi nanoparticles using borohydride reduction and based on the CV results it was postulated that the potential cycling of PdeNi surface caused re-organization of the Pd and Ni active sites to improve the ethanol electrooxidation [63]. Synthesis methods are very important to get variation in morphologies. He et al. synthesized hollow nanoporous NiePd by in-situ depositing Pd nanoparticles over hollow Ni microspheres. The prepared hollow NiPd electrocatalysts showed improved electrochemical activity towards ethanol electrooxidation owing to its hollow cavity, nanoporous structure and increased EASA due to dendritic like surface structures. The fabrication method of hollow NiPd spheres included the following steps: 1) SiO2 microspheres were NH2functionalized by dispersing them in a solution containing ethanol, ultrapure water, ammonium hydroxide and (3Aminopropyl)trimethoxysilane (APTMS); 2) Au nanoparticles were synthesized and anchored over NH2 functionalized SiO2 precursor; 3) Growth of Ni nanoparticles over Au nanoparticles was done by reduction of Ni2þ ions using hydrazine as a reductant, in the process hollow Ni microspheres were produced; 4) Hollow nanoporous NiPd catalysts were formed by galvanic replacement between Ni and Pd2þ. Fig. 13 shows the schematic of synthesis procedure of NiPd hollow spheres [20]. Hollow flower like ternary NiPdPt electrocatalyst were synthesized by Hong et al. through galvanic replacement method between Ni and the noble metals Pt and Pd on MWCNT support [21].

Ni hollow spheres consisting of needle like nanoparticles of nickel were synthesized by Xu et al. by taking silica spheres as templates and gold nanoparticles as seed layer [89]. The procedure was similar to as shown in Fig. 13. The APTMS functionalized silica spheres were added in a colloidal solution of gold and were kept standing for 12 h. The nickel nanoparticles were prepared by reducing Ni2þ ions using hydrazine solution. Finally the silica core was dissolved using 2.0 M NaOH solution to get Ni hollow spheres [89]. The transmission electron microscopy (TEM) images of silica-nickel core-shell nanoparticles and Ni hollow spheres after dissolution of silica core are shown in Fig. 14. Galvanic displacement method was also used for deposition of nanoPt(Ni) nanoparticles. In it the electroless deposition of Ni onto Ti foil sheets was followed by a galvanic displacement method to prepare PteNi nanoparticles on Ti. The nanoPt(Ni)/Ti electrocatalyst showed enhanced ethanol electrooxidation activity as compared to pure Pt [41]. Qi et al. adopted the de-alloying method to prepare PdeNi nanoparticles. Basically ribbons of AlPdNi were prepared with compositions of 75:10:15 from pure Pd, Al and Ni ingots using a melt spinning technique. The dealloying of Al was further performed using a 20 wt% NaOH solution to form Pd40Ni60 alloy catalyst. The so formed alloy showed superior electrocatalytic activity for ethanol electrooxidation reaction. The enhanced ethanol electrocatalytic activity was attributed to the alloying effect of Ni. It was discussed that when Ni is alloyed with Pd the d-band centre of Pd shifts down while that of Ni shifts up allowing weak bonding of adsorbates (toxic species) on Pd and strong bonding on Ni. This in turn gives rise to better tolerance towards poisoning thus enhancing the overall ethanol electrooxidation activity. Moreover, although a poor electrocatalyst in its pure form, Ni is an oxophilic element which has the capacity to generate OHads at lower potentials thus facilitating the oxidative desorption of intermediate products [64]. Qiu et al. used the double-potential electrodeposition method to prepare PdeNi bimetallic thin films and observed improved electrocatalytic activity. The high electrocatalytic activity towards ethanol electrooxidation of bimetallic PdeNi was attributed to the electrocatalysis of Pd. Ni enhanced the catalytic activity by helping to oxidize the adsorbed CO-like species thus increasing its tolerance towards surface poisoning [90].

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Fig. 14 e (A) TEM image of silica-nickel core shell nanoparticles and (B) TEM image of Ni hollow spheres (The inset shows energy dispersive X-ray analysis (EDAX) of Ni hollow spheres) [89]. Wang et al. synthesized Pd nanoparticles adjacent to NiCoOx using polyol method followed by heat treatment. The current density of Pd-NiCoOx/C was more than of Pd/C and PdeNiO/C. The onset potential of Pd-NiCoOx for ethanol electrooxidation was negative than Pd/C, it also showed better stability. The presence of NiCoOx facilitated the adsorption of OH species which at lower potentials helped in removal of CO and other carbonaceous intermediates. The synergistic effect between NiO and CoOx facilitated the breakage of CeC bond. It was also observed that the strong metal support interaction between Pd and NiCoOx leads to enhanced EASA providing larger active sites than PdeNiO/C [68]. Many Ni-based binary electrocatalysts are synthesized in the form of core-shell nanoparticles. The core-shell structure basically affects the electronic properties of the electrocatalysts and thus helps to improve the overall ethanol electrooxidation activity. Fahim et al. synthesized core-shell nanoparticles of different metals by adopting various methods. Using SnO2/graphene as support the core metal nanoparticles (Co, Ni or Cu) were prepared by dissolving the salts of Co, Ni or Cu in double distilled water in which ethylene glycol and sodium citrate was used and pH was adjusted to 10 using KOH solution. After adding the SnO2/graphene to the above mixture the solution was exposed to microwave irradiation to form the core nanoparticles. The shell containing Pd nanoparticles was prepared by following the same steps [61]. The high resolution TEM images showing the lighter shell and darker core nanoparticles are displayed in Fig. 15. A similar process was used by Fetohi et al. to prepare Ni@Pt core-shell nanoparticles having different Pt:Ni atomic ratios [91]. The HRTEM image of Ni@Pt/C with Pt to Ni ratio 1:1 is shown in Fig. 16. Huang et al. prepared RuNi core nanoparticles on single walled carbon nanotubes by reducing the salts of Ru and Ni using NaBH4. The RuNi core was then covered finely by Pt nanoparticles using a galvanic displacement reaction method [76]. Pt ions can displace Ni atoms of the core to form a shell containing RuPt and thereby protect the inner Ni from being corroded. The process thus helped to form RuPt shell over RuNi core nanoparticles. The synthesis process of formation of finely dispersed Pt nanoparticles over RuNi core nanoparticles is shown in Fig. 17. Zhang et al. synthesized nickel

core-palladium shell nanoparticles by replacement method on nitrogen doped graphene [50]. The core-shell nanoparticles showed excellent activity towards ethanol electrooxidation. Zhang et al. used a novel solution based nanocapsule method to prepare PdeNi electrocatalysts [65]. The synthesis flowchart explaining nanocapsule method is shown in Fig. 18. The nanocapsule method helped not only to have a better control over the size distribution of PdeNi nanoparticles but also to facilitate the formation of more efficient contacts between Pd and Ni. Feng et al. synthesized PdNi bimetallic electrocatalyst through reduction of Pd and Ni oxides by calcining the metal dimethylglyoxime complex. The synthesized PdNi electrocatalysts showed improved EASA values and higher electrocatalytic activity and durability as compared to Pd and Pd/C catalyst. PdNi also showed smaller charge transfer resistance (Rct) indicating lesser resistance to electron transfer, as compared to Pd and Pd/C, during ethanol electrooxidation which was attributed to the addition of Ni to Pd [92]. Lee et al. used a one-pot synthesis method to prepare ultrafine (5 nm) PdeNi alloy nanoparticles. These nanoparticles were synthesized by chemical reduction of palladium and nickel acetylacetonate with tert-butylamine-borane complex in presence of oleic acid and oleylamine. The synthesis was done by simultaneously reducing the palladium acetylacetonate and nickel acetylacetonate using tert-butylamine-borane (TBAB) complex. Oleic acid acted as a stabilizer and oleylamine played the role of solvent, co-reductant and cosurfactant. TBAB helped to form monodispersed nanoparticles. The combination of both oleylamine and oleic acid helped as effective stabilizing agent and to form Monodispersed PdeNi nanoparticles. The synthesized PdeNi alloy nanoparticles catalysts exhibited significantly enhanced electrocatalytic activity and stability for ethanol oxidation as compared to commercial Pd(C) catalyst [93]. Liu et al. synthesized Ni catalysts on MWCNTs by chemical reduction using hydrazine in presence of cetyltrimethylammonium bromide (CTAB). The prepared Ni catalyst on MWCNTs showed improved electrocatalytic activity for ethanol electrooxidation reaction. The addition of CTAB gave relatively small Ni nanoparticles (20 nm) which helped to improve the electrocatalytic performance [94].

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Fig. 15 e HRTEM images of (A) Co@Pd/SnO2-graphene, (B) Ni@Pd/SnO2-graphene and (C) Cu@Pd/SnO2-graphene core shell nanoparticles [61]. Mao et al. designed ultrathin PteMoeNi nanowires (NWs) catalyst for ethanol electrooxidation. The synthesis was carried out using a H2-assisted solution route. The formation of relatively strong MoePt and MoeNi bonds instead of PteNi

Fig. 16 e HRTEM image of Ni@Pt/C with Pt:Ni ratio 1:1 [91].

bonds led to the stabilizing effect of Mo, on under coordinated sites of PteNi NWs. The PteMoeNi NWs with rich surface defects showed high density of low co-ordinated atoms which decreased amount of Pt per mass of catalyst and also facilitated the oxidation of ethanol. The incorporation of 3d transition metal (Ni) promoted the adsorption of hydroxyl species from water and removal of poisonous CO intermediates at a lower potential as compared to Pt/C and Pt black catalyst [95]. Shen et al. synthesized Pt, PtRh and PtRhNi nanoparticles on graphene nanosheets using one-pot polyol assisted method. The prepared ternary alloy nanoparticles showed better electrocatalytic performance as compared to binary or mono (Pt) catalyst. The increased activity was attributed to the bi-functional mechanism indicating the adsorption and activation of water molecules on metal/metal-oxide surfaces to form OH oxidizing species and helping to remove the toxic intermediates adsorbed on Pt surface. The presence of nickel

Fig. 17 e Depiction of formation of Pt nanoparticles over RuNi nanoparticles [76]. Please cite this article as: Vyas AN et al., Recent developments in nickel based electrocatalysts for ethanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.218

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Fig. 18 e Flowchart explaining the solution based nanocapsule method for preparation of PdNi electrocatalyst [65].

and nickel oxide/hydroxide was beneficial for the bifunctional mechanism. The enhanced activity was also a result of modified electronic structure of Pt induced by lattice strains and charge transfer, thus reducing the poisoning effects on Pt [22]. Ni and NieB nanotubes were synthesized by electroless plating. The electroless plating method provided a novel way to grow high surface area nanotubes of Ni and NieB thus enhancing the electrocatalytic activity for ethanol electrooxidation [96]. Nanotubes are highly anisotropic structures with large and easily accessible surface that helps transport of chemical species between the catalyst and its surrounding medium. The simple steps adapted for the synthesis of Ni nanotubes are given as follows: (1) Polycarbonate and Polyethylene terephthalate (PET) were irradiated with Au ions (Ion irradiation); (2) The PET surface was irradiated with UV light for better ethching. The polymer foils was ethched using NaOH solution (track etching). This step led to selective removal of ions track and formed cylindrical channels inside the foil; (3) Third was the activation step where a thin layer of Ag nanoparticles was coated by reducing Agþ ions, using a reducing solution of SnCl2 and trifluoroacetic acid in methanol and water, to initiate the metal deposition (Activation); (4) The electroless plating for pure Ni was carried out using a plating solution of nickel sulphate (Ni precursor), iminodiacetic acid (ligand), sodium hydroxide (for adjusting pH) and hydrazine (reductant). The NieB was deposited using a plating solution that contained solution of nickel sulphate and dimetylaminoborane (DMAB) as a reductant; (5) The last step was dissolution of template using an appropriate compound. The scheme of synthesis of Ni nanotubes is shown in Fig. 19. Ni/NiOeC electrocatalyst was prepared by Wang et al. by mixing glucose and nickel precursor and heating at high (800  C) temperature. Ni/NiOeC was obtained by

calcinations of NieC in air. Carbon coated Ni/NiO nanocomposites show superior electrocatalytic effect towards ethanol electrooxidation. The hollow Ni/NiOeC showed the best activity due to synergistic effect of crystal NiO, coating of carbon and hollow structure [97]. An in-situ microinjection method was applied by Zhang et al. to prepare NieB amorphous alloy nanoparticles [98]. Some other methods like pechini method and water-in-oil microemulsion method and mechanical alloying were also used by various

Fig. 19 e Schematic of electroless deposition method for Ni nanotubes [96].

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research groups to prepare Ni based electrocatalysts [78,99e101]. All the above discussed synthesis methods show that Ni based electrocatalysts promote the activity towards ethanol electrooxidation. Increasing the active surface area by developing different morphologies is a key factor in improving the electrocatalytic activities. Moreover placing Ni very near to noble metal catalysts ensures that the hydroxides formed over Ni adsorb OH species and eventually help to remove toxic carbonaceous species. Various parameters of different synthesis methods provide the liberty of obtaining the right structure for optimized ethanol electrooxidation. The partial replacement of conventional noble metal catalyst by Ni in various composition and/or morphology requires the synthesis methods to be modified accordingly. In this regard the various synthesis approaches help in engineering the catalytic surface thereby increasing the active sites, providing various morphologies and optimizing the size, composition and distribution of catalyst nanoparticles. Another important aspect of modifying different synthesis methods is reducing the cost of production of electrocatalysts. From the above discussion it is also clear that wet chemical methods prove advantageous in this respect. Wet chemical methods provide an easy pathway for induction of Ni into noble metal catalysts either in the form of alloy or multi-metallic compounds or composites, thereby improving their electrocatalytic activity.

Ni based electrocatalysts with different compositions A major problem that noble metals such as Pt and Pd face for the ethanol electrooxidation reaction is the poisoning of these metal catalysts due to carbon containing intermediates. This poisoning hinders the catalytic activity of these noble metal

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catalysts. Nickel helps to adsorb hydroxyl species over its surface and help in getting rid of the CO like toxic intermediates adsorbed over noble metals. This increases the catalytic activity and stability of the catalysts as well as tolerance towards CO during ethanol electrooxidation reaction. The cleavage of CeC bond in ethanol is a major hurdle in realizing the practical application of DEFCs. The synergistic effect between oxides of different non-noble metals helps to break the CeC bond of ethanol. Many researchers have tried various combinations using noble/non-noble metals and nonmetals to get the optimum results for ethanol electrooxidation reaction. The elemental compositions of the nickel based electrocatalysts significantly affect the current density and/or the onset potential. Barakat et al. used carbon nanofibres as support for NixCo1x alloy nanoparticles. They used electrospinning as deposition method. NixCo1-x alloy nanoparticles were incorporated (doped) in carbon nanofibres. Out of various compositions Ni0.9Co0.1 alloy nanoparticles showed highest current density of 142 mA/cm2, whereas, Ni0.1Co0.9 showed negative onset potential at e 0.050 V vs Ag/AgCl for 5.0 M ethanol concentration [102]. MWCNTs were used as reductants by Ding et al. to prepare PteNi bimetallic nanocatalysts. Different Pt:Ni molar ratios (1:1, 1:2, 1:3 and 1:4) were prepared by hydrothermal method. Out of the synthesized composites PtNi3/MWCNT was found to show better catalytic performance based on the lower onset potential for ethanol electrooxidation and higher peak current density [103]. A series of PdeNieP nanoparticles were prepared by Jiang et al. using wet chemical method. The Pd0.8Ni0.1P0.1 showed a comparatively lower onset potential, negative oxidation peak potential and high ethanol electrooxidation reaction activity as compared to Pd0.7Ni0.3. The ethanol electrooxidation on PdeNieP was found to undergo a 4-electron process giving

Fig. 20 e Scheme showing improved ethanol electrooxidation activity due to shortened PdeNi active sites [105]. Please cite this article as: Vyas AN et al., Recent developments in nickel based electrocatalysts for ethanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.218

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acetate as the end product. The amorphous structure provided more catalytically active sites [104]. Chen et al. tried to improve the ethanol electrooxidation activity by shortening the distance between Pd and Ni active sites. The electrocatalytic activity was found to enhance by decreasing the distance between Ni and Pd active sites. This shortening of distance between Ni and Pd active sites was achieved by transformation of Pd/NieP heterodimers into PdeNieP nanoparticles and tuning the Ni:Pd atomic ratio to 1:1 [105]. Fig. 20 shows the scheme as suggested by Chen et al. for improved ethanol electrooxidation activity by shortening the distance between Pd and Ni active sites. Sulaiman et al. synthesized PteNi octahedra for ethanol electrooxidation. The synthesis was done using a simple chemical reaction of Pt and Ni salts in presence of oleylamine, oleic acid and benzyl ether. The use of benzyl ether promoted the growth of octahedral PteNi exposing (111) facets which helped to convert ethanol to acetic acid. The poisoning effect due to CO was reduced in octahedral Pt2.3Ni. The octahedral Pt2.3Ni showed superior electrocatalytic effect than conventional Pt2Ni and commercial Pt in terms of anodic (forward) ethanol electrooxidation peak current [106]. Pt90Sn10, Pt90Ni10 and Pt80Sn10Ni10 were synthesized by co€ nnemann’s colloidal precursor method on reduction using Bo carbon support. The onset potential of ethanol electrooxidation lowered for carbon supported Pt90Sn10 and Pt80Sn10Ni10 alloys as compared to carbon supported Pt90Ni10 or Pt nanoparticles. It was observed that the presence of Ni in the Pt80Sn10Ni10 alloy promoted the CeC bond cleavage on Pt rich surface [107]. PtRhNi alloy nanoassemblies (ANAs) with varying Pt/Rh ratios were prepared by cyanogel reduction method. The PtRhNi-ANAs were strongly dependent on their composition ratios. Pt3Rh1Ni2-ANAs showed better electrocatalytic activity and durability for ethanol electrooxidation as compared to commercial Pt black catalyst [108]. Pt3Sn, Pt3Ni, Pt3SnNi were synthesized by Parreira et al. using polymeric precursor method. The Pt3SnNi showed better electrocatalytic activity as compared to the other binary catalysts for ethanol electrooxidation reaction. The high activity was attributed to the presence of Ni which modified the electronic structure of Pt and combined with Sn to facilitate the removal of adsorbed CO and to enhance the ethanol electrooxidation reaction [109].  et al. synthesized PtSn and PtSnNi by alcohol reducSpinace tion process [110]. Three different compositions of Pt:Sn were synthesized viz. 25:75, 50:50, 75:25. Out of which 50:50 showed the best catalytic activity. Ternary PtSnNi with a ratio of Pt:Sn:Ni of 50:40:10 was even better than Pt50:Sn50, in that it showed higher current densities in the whole potential range. The enhanced electrocatalytic activity of ternary Pt50Sn40Ni10 over binary Pt50Sn50 was attributed to the modification of the electronic properties of platinum due to nickel resulting in combination of electronic effect and bi-functional mechanism. Few research groups have also combined four or more elements to increase the current density or shift the onset potential towards more negative value for ethanol electrooxidation reaction. The inclusion of an extra element (a transition metal) ensures the modification of the electronic properties of Pt, thereby affecting the ethanol electrooxidation reaction. The compositions of the elements play a major role

in enhancing the catalytic activity towards ethanol electrooxidation reaction. Quaternary PtMnCuNi and PtMnMoNi electrocatalysts were prepared by reduction of metal salts by NaBH4. The molar ratios of the Pt:Mn:Cu:Ni and Pt:Mn:Mo:Ni as measured by inductive coupled plasma (ICP) optical emission spectroscopy was found to be 26:42:20:12 and 30:48:6:16, respectively. Shift of ethanol electrooxidation onset potentials was observed for both the quaternary alloys compared to the ternary alloys PtMnCu and PtMnMo [111]. Quaternary NiNbPtCu and quinary NiNbPtCuSn having compositions Ni59Nb40Pt0.6Cu0.4 and Ni59Nb40Pt0.6Cu0.2Sn0.2, respectively were prepared by mechanical alloying. Ni59Nb40Pt0.6Cu0.4 alloy provided higher current densities than Ni59Nb40Pt0.6Cu0.2Sn0.2. The alloy having high proportion of copper lead to better energetic efficiency for ethanol or bio-ethanol electrooxidation than for alloy having lower Cu content. The reason for this significant increase in catalytic activity was thought to be the modification of electronic cloud of platinum due to irregular electronic configuration of copper (3d104s1) [101]. Sekol et al. prepared Pd43Ni10Cu27P20 metallic glass nanowires by top-down nanomolding approach. The Pd43Ni10Cu27P20 metallic glass nanowires showed a lower onset potential for ethanol electrooxidation and for CO oxidation. The better durability and high catalytic activity were seen as a result of modified electronic structure and bi-functional method of the amorphous metal [112]. Carrareto et al. synthesized quaternary Pt50Sn20Ni25Ti5 electrocatalyst by thermal decomposition of polymeric precursors. The addition of Ni and Ti to PtSn/C was found to increase the electrocatalytic activity towards ethanol and glycerol oxidation. High performance liquid chromatography (HPLC) studies confirmed the presence of acetaldehyde as the main product after electrolysis [113]. The literature survey proves that by using an optimized composition of Ni with other noble/non-noble metals one can obtain enhanced electrocatalytic performance for ethanol electrooxidation. The research of combining various metal nanoparticles has flourished from bimetallic to ternary, quaternary and more. The results highlight that Ni can be effectively used as a partial replacement for the costly noble metal nanoparticles.

Conclusions Detailed investigations by research groups all over the world have constantly developed newer synthesis methods, found suitable supports and apt compositions for the growth/synthesis of various nickel based electrocatalysts for DEFCs. The focus has been on production of more number of nickel based electrocatalysts which could help not only to reduce the cost but also to enhance their activity. The issues concerning reaction kinetics, durability and catalytic activity of the anode can be improved by replacing conventional noble metal catalysts by nickel based electrocatalysts. The nickel based electrocatalysts synthesized using different synthesis methods and supports contribute immensely towards enhancement of the electrocatalytic activity towards ethanol electrooxidation reaction. This review summarizes the role of

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various supports, different synthesis approaches and compositions used for the development of suitable nickel based electrocatalysts for ethanol electrooxidation reaction. However, we do not claim that we have covered all the literature emphasizing on the utility of support, synthesis methods and different compositions. The review highlights recent developments where attempts have been made to improvise the electrocatalytic activity of nickel based electrocatalysts towards ethanol electrooxidation.

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Acknowledgement

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The authors are thankful to Savitribai Phule Pune University, Pune for financial support. One of the author GDS is thankful to Dongguk University, Seoul, South Korea under research fund 2018e2020.

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