Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1

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Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell Yang Zhao a, Xianhua Liu a,c,*, Xin Wang d, Pingping Zhang b,**, Jiafu Shi a a

Tianjin Key Lab. of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, Tianjin, 300354, PR China b College of Food Science and Engineering, Tianjin Agricultural University, Tianjin, 300384, PR China c School of Marine Science and Engineering, Tianjin University, Tianjin, 300072, PR China d College of Art and Science, Miami University, Oxford, OH, 45056, USA

article info

abstract

Article history:

Nickel foam modified by various electron mediators is an ideal non-noble metal anode for

Received 21 July 2017

direct glucose alkaline fuel cell. However, high cost, low stability, and toxicity of electron

Received in revised form

mediators largely hamper their practical application. Herein, we demonstrated a one-step

13 October 2017

electrodeposition method to produce high-performance nickel foam electrode decorated

Accepted 20 October 2017

with reduced graphene oxides and nickel oxides. The structure and morphology of the

Available online xxx

resulting 3D graphene-nickel oxide nanocomposites were characterized by UVeVis, Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy.

Keywords:

Furthermore, a composite anode was prepared by rolling an activated carbon layer on the

3D graphene

fabricated nickel foam electrode. At ambient temperature, the fuel cell equipped with the

Nickel oxides

composite anode exhibited excellent performance with a high peak power density of

Electron mediator

13.48 W m
2 under the condition of 1 M glucose, 3 M KOH, which was 39.30% higher than

Activated carbon

that of the bare activated carbon anode cell. In particular, electrochemical measurements

Glucose oxidation

demonstrated the high performance of the nanocomposite modified activated carbon anode was likely attributed to the synergistic effect of high conductivity of graphene and the catalytic activity of trivalent nickel towards glucose oxidation. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Fuel cells have been widely concerned in recent years because they represent a highly efficient and environmentally friendly alternative technology for energy production [1,2]. Glucose, as

the most abundant and important simple sugar in nature, has great potential for being used as high-density hydrogen carrier and sustainable energy source. It is cheap, easily available, non-explosive, nontoxic, and non-volatile [3,4]. Furthermore, glucose can yield 2870 kJ mol
1 energy under the condition of complete oxidation to CO2 via 24-electron transfer [5,6]. The

* Corresponding author. Tianjin Key Lab. of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, Tianjin, 300354, PR China. ** Corresponding author. E-mail addresses: [email protected] (X. Liu), [email protected] (P. Zhang). https://doi.org/10.1016/j.ijhydene.2017.10.120 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhao Y, et al., Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell, International Journal of Hydrogen Energy (2017), https://doi.org/ 10.1016/j.ijhydene.2017.10.120

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theoretical energy density of glucose is 4430 Wh kg
1. This value is comparable to that of the most common substrate for direct fuel cell like methanol (6100 Wh kg
1) [7,8]. Considering its advantages, glucose is more suitable for various portable applications, such as cell phones, power computers, and other portable low-power devices [1]. However, in these fields, glucose fuel cell is still at the prior stage compared with methanol and ethanol fed fuel cells [1,9e15]. Based on the catalyst material form, glucose fuel cells can be primarily classified into three kinds: enzymatic fuel cell, microbial fuel cell, and abiotic fuel cell [1,16]. The design of enzymatic glucose fuel cell is initially simple because the catalysts are selective, though enzymes are easy to be progressively deactivated for long-term application [17e19]. Microorganisms are more resistant to poisoning and loss of activity [1], but the most critical problems of microbial glucose fuel cell are their low performance and potential health risk [8,20]. The abiotic glucose fuel cells, by contrast, have longterm stability, affording them considerable durability. However, their prominent barriers are high cost, catalyst poisoning, and slow reaction kinetics of glucose oxidation [1,8]. Researchers have studied a variety of alloy catalysts with long lifetime and operational stability, Basu et al. [21] used NaBH4 reduction technique to add Pd and Au with Pt precursor to form PtePdeAu/C (metal ratio 1:1:1) catalyst that can efficiently electro-oxidize glucose in direct glucose fuel cell. In addition, there are many other alloy catalysts, such as: PteAu/ C [22], Pt/GNS [23], PdeAu/C [24], Pd/cMWCNT [25]. In general, Ni and other transition metals (e.g. Co3O4 [26], ZnOeAl2O3 [27], NiCoO2@CNT [28], CuO [29] and Mn3O4 [30]) are capable of catalyzing glucose oxidation in alkaline solutions even better than Pt [1]. Iwu et al. [31] fabricated a novel Ni nanofoam electrode applied for non-enzymatic glucose sensing by chemical bath deposition and thermal annealing. Kung et al. [32] covered a single layer of nickel hydroxide nanoparticles on the surface of a nickel foam, and found that the redox couple of Ni(OH)2/NiOOH formed on the electrode surface greatly enhanced the oxidation of glucose. Eshghi et al. [33] synthesized NickeleIron Double Hydroxide nanocomposites on graphene/glassy carbon electrodes by electrochemical method, and illustrated that graphene/NiFe LDH exhibits a high diffusion coefficient (1.80  10
4 cm2 s
1) as an electro catalyst for glucose electro oxidation. Although all these work have promoted the oxidation of glucose remarkably, the slow reaction kinetics still impedes the widespread use of glucose fuel cells [34]. Graphene is a one-atom-thick layer of carbon atoms bonded by sp2 bonds. This configuration provides this material with unusual properties, such as: large accessible surface area, excellent thermal and electrical conductivity [35]. Owing to these extraordinarily properties, graphene is an attractive carbon material to carry various catalysts [36]. Jafri et al. [37] prepared Pt loaded nitrogen doped graphene as electrocatalyst for proton exchange membrane fuel cell that showed excellent methanol oxidation activity. Chi et al. [38] fabricated a PdePt/rGO/NFP composite through one-step mild reduction process. By changing the proportion of catalyst components, the authors found that the anode with the Pd1Pt0.98/rGO/NFP catalyst exhibited a strong activity for

glucose oxidation. Yang et al. [34] reported the fabrication of the porous Co3O4@graphene microspheres for enzyme-free biosensor applications. Wang et al. prepared a threedimensional (3D) reduced graphene oxide-nickel foam as an anode for microbial fuel cell (MFC) [30]. Yang et al. [39] described a simple, cheap, and green method to produce porous graphene/nickel foam electrodes and its application in supercapacitors. Zhao et al. [40] developed a 3D graphene aerogel decorated with Pt nanoparticles as an efficient anode for MFC. K. Hoshi et al. [41] used a graphene-coated carbon fiber cloth electrode to absorb more enzymes on its surface. Although graphene is extensively explored as an electrode material for supercapacitors, sensors [42] and MFCs, there are few reports on its application in direct glucose alkaline fuel cells (DGAFCs). Electron mediators are normally used to facilitate the electron transfer and enhance the fuel cell performance [13]. In prior work, we reported the preparation of various highperformance activated carbon anodes modified by with different electron mediators, such as methyl viologen (MV) and 2-hydroxy-1, 4-naphthoquinone (NQ) [5,13,16]. However, most electron mediators are high-cost and have adverse impact to the environment [5]. The development of highperformance anode electrocatalyst for glucose oxidation is still in urgent need to avoid the use of additional electron mediators. In this work, a highly active 3D graphene-nickel oxide nanocomposite modified nickel foam electrode was prepared by a simple potentiostatic method. To the best of our knowledge, this is the first report of one-step electrodeposition of 3D graphene-nickel oxide nanocomposite on nickel foam electrode and application in DGAFCs. UVeVis, Raman, XPS and SEM were used to study the electrochemical reduction of graphite oxide and the morphology of the electrode. Furthermore, various electrochemical techniques, such as linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and Tafel tests, were employed to get further insight into the possible action mechanism. Finally, the performance of DGAFC equipped with the prepared anode was examined.

Methods Materials Graphite powder (purity: 99.95%, mesh: 325) was purchased from Jinrilai Graphite Co. Ltd (Qingdao, China). 2-hydroxy-1,4naphthoquinone (NQ) was purchased from J&K Scientific Co. Ltd (Beijing, China). 60 wt% PTFE solution was provided by Heshen Inc. (Shanghai, China). Activated Carbon (AC) powder (YEC-8A) was obtained from Yihuan Carbon Co. Ltd (Fuzhou, China). Nickel foam was purchased from Liyuan New Material Co. Ltd (Changde, China) (purity: 99.9%, number of pores per inch: 110, density: 380 g m
2 ± 20 g m
2, average pore size: 590 mm, thickness: 1.7 mm). Glucose, KMnO4, NaNO3, Na2SO4, NiSO4, HCl, H2SO4, H2O2 (30%), and KOH were all of analytical grade. Deionized water (Millipore, Milli Q, 18.3 MU) was used to prepare all the solutions.

Please cite this article in press as: Zhao Y, et al., Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell, International Journal of Hydrogen Energy (2017), https://doi.org/ 10.1016/j.ijhydene.2017.10.120

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Synthesis of graphene oxide solution (GO) Graphite oxide was prepared via the Hummer's method [43,44]. 3 g of graphite powder was mixed with concentrated H2SO4 (70 mL) in a beaker that was cooled in an ice bath for 10 min, 1.5 g of NaNO3 was added with continued stirring for 20 min. KMnO4 (9 g) was added slowly with stirring and cooling for 2.5 h (temperature was maintained at <20  C throughout the mixing). The mixture was further stirred at 35  C for 3.5 h after the addition of KMnO4, and then diluted with 150 mL of deionized water, causing an increase in temperature to 100  C and violent effervescence. After stirring for another 1.5 h, the reaction was stopped by the addition of 300 mL of deionized water and 20 mL of 30% H2O2 solution. The suspension was followed by centrifugation and careful washing by HCl solution (volume ratio 1:10) and deionized water at 3e5 times until near pH 7 [45]. The mixture was then diluted to 430 mL with deionized water. The concentration of the graphite oxide solution is approximately 7 mg ml
1. Finally, the graphite oxide solution was ultrasonicated for 4 h in an ultrasonic cleaner (53 HZ, 200 W), so that the stacked graphite oxide sheets were exfoliated under the ultrasonic action to obtain the graphene oxide solution [43].

Preparation of nickel foam electrode modified by reduced graphene oxide and nickel oxides (rGO-Ni) The nickel foam was dipped in the solution of 7 mg ml
1 GO, 40 mM NiSO4 and 0.1 M Na2SO4, and the mixture was ultrasonic for 15min. The GO modified nickel foam was then electro-reduced with the constant potential of
1.2 V (vs. Ag/ AgCl) for 800 s. Finally, the prepared rGO-Ni electrode was washed with deionized water and anhydrous ethanol.

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instrument. Raman spectroscopic characterization was performed on a laser micro Raman spectrometer (DXR, American) with a 532 nm laser. Scanning Electron Microscopy (SEM) images were recorded by a field-emission SEM (Hitachi S4800, Japan).

Electrochemical measurement Electrochemical test was performed on an electrochemical workstation (CHI 660E, CHI Instrument Co. Ltd, Shanghai, China). Conventional three-electrode system was employed including a platinum sheet as the counter electrode, an Hg/ HgO electrode as the reference electrode, and an anodes used as the working electrode [16]. The electrochemical behavior of rGO-Ni electrode was studied by cyclic voltammetry (CV) at a scan rate of 50 mV s
1 in 3 M KOH solution [39]. Linear sweep voltammetry (LSV) and Tafel curves of all the anodes were tested from open circuit potential (OCP) to 0 V with a scan rate of 1 mV s
1. Electrochemical impedance spectroscopy (EIS) of all the anodes were conducted over a frequency range of 100 kHze10 mHz at the OCP [47], and the data was analyzed using ZSimWin by fitting the EIS curve with an equivalent circuit. Besides CV curves, all the electrochemical tests were carried out in 1 M glucose and 3 M KOH solution. The polarization curve and power density measurement were obtained by varying the external resistance between the anode and the cathode from 9000 U to 4 U. The voltage was recorded when it approached a stable state (about 2.5 min). The power density was calculated according to P ¼ UI/A, where U is the voltage, I is the current and A is the surface area of the anode. The current density was calculated using the Ohm's law I ¼ U/Rex, where the Rex is the external resistance [5,49].

Fabrication of the electrodes for DGAFC

Results and discussion AC and NQ/AC anodes prepared by rolling method, according to previous studies [5,13,16,46]. The procedure was described in Supplemental Information and illustrated in Fig. S1. rGONi/AC anode was prepared with the same procedure as AC anode except that rGO-Ni modified nickel foam was used instead of pristine nickel foam. A rolling air-breathing oxygen reduction cathode was employed in this study which was prepared as previously described [47,48].

Fuel cell apparatus and assembly Single-chamber DGAFCs (40 mm long cylindrical chamber with a volume of 12 mL) were constructed as previously described [4]. The air-breathing oxygen reduction cathode and the anode had the same diameter of 38 mm (Fig. S2) [48]. All of our experiments were conducted under ambient conditions.

Material characterization UVeVis adsorption spectra of graphite, GO solution and rGONi electrode in the range of 200 nme800 nm were recorded by a UV-3000 spectrophotometer (Mapada, Shanghai, China). X-ray photoelectron spectroscopy (XPS) measurement was conducted on a Thermo Scientific Escalab 250Xi XPS

Characterization of rGO-Ni electrode The reaction process of electrochemical reduction of GO solution to rGO on the surface of nickel foam can be monitored using UVevis adsorption spectroscopy. Fig. 1A shows the UV absorption curve of graphite is approximately horizontal line. However, the peak of GO at around 230 nm and 300 nm can be ascribed to the p-p transition of aromatic C]C bonds and n-p transition of the C]O bonds [35,45], indicating that graphite was oxidized successfully by Hummer's method. Besides, the absorption peak increase gradually with the raise of GO solution concentration, suggesting that the GO has good dispersibility in water. Compared with GO, the absorption peak of rGO solution at around 300 nm disappeared and the peak at 230 nm redshifts to 270 nm after the electrochemical reduction (Fig. 1B), which indicating that the electronic conjugation within the GO was restored upon electrochemical reduction [50]. Furthermore, the change of color from light yellow to dark can also be seen as the reduction of GO solution to rGO, which has previously been suggested as an indicator of partial restoration of the carbon network [51]. From the UVeVis analysis, we can preliminarily determine the GO was successfully reduced to rGO.

Please cite this article in press as: Zhao Y, et al., Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell, International Journal of Hydrogen Energy (2017), https://doi.org/ 10.1016/j.ijhydene.2017.10.120

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Fig. 1 e UVevis absorption spectra collected from different concentrations of GO solution (A) and rGO (B).

Raman spectroscopy is a non-destructive approach to characterize graphitic materials. In carbonaceous material, the D band and G band correspond to sp3 and sp2 carbon stretching modes and their intensity ratio (ID/IG) is a measure of the amount of disorder present within the materials [2].

Fig. 2 e Raman spectra of rGO-Ni (A), GO (B) and graphite (C).

Fig. 2 illustrates the Raman spectrum of graphite, GO and rGONi. Graphite (Fig. 2C) has a weak peak at around 1338 cm
1 (D band) and a typical sharp peak at approximately 1577 cm
1 (G band). The D band arises from the out-of-plane vibrational modes and is indicative of sp3 carbon present, while the G band corresponds to the carbon sp2 vibrations of domains inplane, as in the graphite spectrum [35]. However, the Raman spectrum of GO (Fig. 2B) shows high intense deformation D band (1340 cm
1) and low intense graphitic G band (1588 cm
1) as compared to graphite, which can be mainly attributed to the incorporation of epoxy, hydroxyl, carboxyl and keto groups on the surface of graphitic planes [52]. After the electrochemical reduction process, both the D band and G band of GO shift towards those bands of graphite (from 1340 cm
1 to 1338 cm
1 & from 1588 cm
1 to 1576 cm
1) (Fig. 2A), suggesting the reduction of GO. Moreover, the intensity ratio of D/G bands for rGO is 1.69, higher than that of GO (1.44). This increase implies a decrease in average size of the in-plane sp2 domains upon the electrochemical reduction and partially disordered crystal structure of rGO, which can be ascribed to the formation of new conjugated domains [39,53]. Namely, the reduction process can remarkably increases the number of GO edges, which have been shown to contribute to the D-band intensity due to disruption of the aromatic network at these locations [35]. It is obvious that rGO-Ni has two additional Raman peaks at 500.42 cm
1 and 1067.40 cm
1. The Raman peaks at around 500 cm
1 belong to Ni2O3 according to the literature, so the main peak at 500.42 cm
1 shows the dominant composition is Ni2O3 [42,54e56]. The Raman could not confirm the further oxidation of Ni because of the strong sample absorption of the incident light results in insufficient detectable scattered Raman light. The sensitivity of the Raman technique in the case of Ni is apparently not sufficient to detect trace component of the material [54]. From the Raman spectrum analysis, we can determine that the graphene and nickel oxides were co-electrodeposited on the surface of nickel foam. In order to understand the chemical structure of graphene and nickel oxides, the surface composition of rGO-Ni was further confirmed by XPS measurement. The C 1s XPS spectrum of rGO-Ni in Fig. 3A shows four types of carbon with

Please cite this article in press as: Zhao Y, et al., Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell, International Journal of Hydrogen Energy (2017), https://doi.org/ 10.1016/j.ijhydene.2017.10.120

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Fig. 3 e (A) C 1s XPS spectrum of rGO-Ni. (B) Ni 2p XPS spectra of rGO-Ni. (C). Ni 2p XPS spectra of the nickel foam and rGO-Ni. (D) O 1s XPS spectrum of rGO-Ni.

different states are observed, which appear at 284.84 eV (CeC), 286.33 eV (CeO), 287.93 eV (C]O), 289.23 eV (OeC]O), respectively. In particular, the peak area of CeC band (284.84 eV) with the energy is much higher than that of oxygenated C rings, indicating most carbon atoms are arranged in a honeycomb [43]. In other words, most of the oxygen-containing functional groups were reduced successfully removed. The identification of Ni valence states by XPS depth profile measurements is difficult because of both a complicated peak structure and the fact that Ni oxide is very sensitive against ion beam damage [57]. Fig. 3C shows the different binding energies related to Ni 2p in nickel foam and the rGO-Ni electrode, which shows that the bonding state of nickel element has changed on the surface of nickel foam. In the Ni2p3/2 spectrum of the rGO-Ni electrode (Fig. 3B), the peaks observe at 855.9 eV and 861.35 eV prove the existence of Ni2O3 [42,57e59], and metallic Ni of Ni2p is detected at 852.67eV and 858.2eV, respectively [56,57]. The atomic ratio is calculated from the peak intensities of Ni3þ 2p and Ni 2p spectra. The intensity ratio of Ni2O3 peak and Ni peak is about 19.02. So the Ni3þ/Ni atomic ratio is about 19.02 in the measured surface

layer of the rGO-Ni electrode. Furthermore, the binding energies of O 1s (Fig. 3D) characteristic indicate the existence of Ni2O3 (531.48 eV) on the surface of the rGO-Ni electrode [57,58]. Correspondingly, the major component on the surface of the rGO-Ni electrode is Ni2O3, with trace of Ni. Fig. 4 present the SEM image of the untreated nickel foam (Fig. 4AeC) and rGO-Ni electrode (Fig. 4DeF). As shown in Fig. 4AeC, nickel foam exhibits a continuous porous 3D structure with a large specific surface area. Therefore, it can offers a highly conductive surface area for rGO and nickel oxides coating. Before loading rGO, the nickel foam has a smooth surface. After the deposition, the rGO sheets are uniformly deposited on nickel foam and maintain the 3D structure. Additionally, peony petal-like graphene structure can be observed in the high magnification SEM image (Fig. 4F). The multifold rGO sheets deposited on the nickel foam form an interconnected web that allows for sufficient electron pathways and a large surface area for nickel oxides loaded. All these observations indicate the successful reduction of GO and formation of nickel oxides (mainly Ni2O3) by the electrodeposition process.

Please cite this article in press as: Zhao Y, et al., Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell, International Journal of Hydrogen Energy (2017), https://doi.org/ 10.1016/j.ijhydene.2017.10.120

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Fig. 4 e SEM images of plain nickel foam (AeC) and rGO-Ni modified nickel foam (DeF); insets of A and D are the corresponding digital pictures. (G) Schematic of the rGO-Ni modified nickel foam. (H) Photograph of peony flower petals.

Electrochemical performance of rGO-Ni/AC anode in DGAFC The glucose oxidation mechanism on Ni(OH)2 had been explained by the role of the Ni(OH)2/NiOOH redox couple according to the following reactions [59,60]: Ni(OH)2 þ OH
/ NiOOH þ H2O þ e


[1]

NiOOH þ glucose / Ni(OH)2 þ glucolactone

[2]

Similarly, trivalent nickel in Ni2O3 may produce NiOOH in alkaline conditions so as to promote the oxidation of glucose. We inferred the reaction mechanism can be expressed as follows: Ni2O3 þ H2O / 2 NiOOH

[3]

NiOOH þ glucose / Ni(OH)2 þ glucolactone

[4]

Fig. 5 compares the CV curves of the rGO-Ni electrode deposited under different conditions (electrolyte, potential and deposition time) in the presence of 3 M KOH at the scan rate of 50 mV s
1. Fig. 5B shows that under the same deposition time of 500 s, the electrode with deposition voltage of
1.2 V exhibited the highest current density. Fig. 5C shows

that under the same deposition voltage of
1.2 V, the electrode with deposition time of 800 s exhibited the highest current density. Therefore, we inferred that the optimum condition for electrodeposition was
1.2 V, 800 s. The higher current density indicated better electrical conductivity and higher redox activity. As the deposition time increased from 400 s to 800 s, there were more amount of rGO deposited on the nickel foam, which enhanced the conductivity of the matrix. However, the further increase of deposition time to 1000 s led to depositing too much amount of rGO on the electrode surface, which might cover the active sites on the nickel foam and prevent the diffusion of substrates. The electrode was produced under the optimum deposition condition and used for DGAFC. To compare it with previous NQ/ AC anode, in which NQ was used as electron mediator and AC powder was used as a catalyst carrier, we also rolled AC layer for rGO-Ni electrode to maintain the consistency of the experimental conditions. Serving as the anode of DAGFC, the electrochemical performances of different anodes were measured in 1 M glucose and 3 M KOH solution. The LSV polarization curves of the AC, NQ/AC and rGO-Ni/ AC anodes are shown in Fig. 6A. The slopes of the polarization curves decrease in the order of bare AC (blank control group) > NQ/AC > rGO-Ni/AC. At the voltage of
0.4 V, the current density of the rGO-Ni/AC anode (28.56 mA cm
2) is 40.63% higher than that of the NQ/AC anode (20.31 mA cm
2)

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and 95.73% higher than that of the bare AC anode (14.59 mA cm
2), which apparently indicates that rGO-Ni promoted the glucose oxidation reactions on the anode. Besides, the Eonset of the AC anode was decreased when NQ or

Fig. 5 e Cyclic voltammetry (CV) curves of the rGO-Ni electrode under different electrodeposition conditions at the scan rate of 50 mV s¡1. (A) Different deposition electrolyte under the deposition voltage of ¡1.2 V and the deposition time of 500 s; (B) Different deposition voltage under the deposition time of 500 s; (C) Different deposition time under the deposition voltage of ¡1.2 V.

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rGO-Ni was added. The bare AC anode only had an Eonset of
0.729 V, while the Eonset values for the NQ/AC and rGO-Ni/ AC were
0.775 V, and
0.792 V, respectively, indicating that the better catalytic activity of rGO-Ni/AC than NQ/AC and bare AC anodes. Compared with LSV, EIS investigates electrode system by measuring impedance spectrum with wide frequency range, thus obtaining more information about electrode reaction kinetics and electrode interface structure. Fig. 6B shows the Nyquist plots of the different anodes. The appearance of the arc in the high frequency region is due to the resistance of the thin layer liquid film in the pore structure of the activated carbon layer to the transference of electrolyte ions and charge in the interior, while the low-frequency line segment is the typical characteristic of the semi-infinite diffusion impedance of the flat electrode surface. An equivalent circuit for porous electrodes was used to analyze the electrochemical properties of anodes as previously described [61], which composed of ohmic resistance (Ro), charge transfer resistance (Rct), diffusion resistance (Rd), double layer capacitance (Cdl) and pore adsorption capacitance (Cad) [47]. The total internal resistances (Rtotal) of the three anodes were composed of Ro, Rct and Rd. Table 1 shows the fitting data, indicating that Rtotal decreased in the order of bare AC > NQ/AC > rGO-Ni/AC. Rtotal of rGO-Ni/AC anode (1.4960 U) decrease by 30.66% and 10.87% compared to bare AC anode (2.1575 U) and NQ/AC anode (1.6784 U), respectively. It can be expected that the rGO-Ni/AC anode plays a vital role in increasing the maximum power density of the fuel cell. The Ro and Rct have the same trends with the Rtotal. Ro represents the impedance of the electrolyte between the reference electrode and the surface of anode and Rct represents the activation energy and the charge transfer process of the electrochemical reaction. In particularly, the Rct of the bare AC anode was 0.6075 U, which was 4 times as much as the NQ/AC anode (0.1593 U) and the rGO-Ni/AC anode (0.1576 U). Electron mediators, such as NQ, can enhance the electron transfer between the electrolyte solution and the current collector. Similar to NQ, rGO-Ni deposited on nickel foam can also facilitate the electron transfer process, leading to the reduction of Rct. That is the major reason why the performance of NQ/AC anode and rGO-Ni/AC anode are much better than bare AC anode. Fig. 7 shows the Tafel curves of the three anodes. Fig. 7A indicates all the three curves have the similar change trend with the increase of the voltage. Fig. 7B demonstrates linear regressions (R2 > 0.99) exist in the Tafel plots in the overpotential range of 0.3 and 0.4 V. Table 2 lists the linear fitting results. The exchange current densities decreased in the order of rGO-Ni/AC anode (12.24  10
3 A cm
2) > NQ/AC anode (10.52  10
3 A cm
2) > AC anode (7.14  10
3 A cm
2). A higher value i0 suggests a fast reaction with lower activation barrier. It furtherly demonstrate that the rGO-Ni modified AC anode can decrease the resistance and speed up the kinetics, and thus enhance the fuel cell performance. These data are consistent with the previous results of LSV and EIS. A series of power density measurements were performed in DGAFC equipped with a same cathode and different anodes to verify the performance of anodes. Fig. 8A compares the current density-voltage and power density plots of fuel cells

Please cite this article in press as: Zhao Y, et al., Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell, International Journal of Hydrogen Energy (2017), https://doi.org/ 10.1016/j.ijhydene.2017.10.120

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Fig. 6 e LSV polarization curves (A) and Nyquist plots of EIS (B) of different anodes. Test condition: 1 M glucose, 3 M KOH, and ambient temperature.

Table 1 e Fitting results of different anodes based on the equivalent circuit.

Table 2 e Exchange current density calculated from the Tafel plots.

Resistance

R0 (U)

Cad (F)

Rct (U)

Cdl (F)

Rd (U)

Anode

Bare AC NQ/AC rGO-Ni/AC

1.189 0.9801 0.7274

0.0002138 0.003018 0.001521

0.6075 0.1593 0.1576

21.81 32.37 42.2

0.361 0.539 0.611

equipped with different anodes. Significantly, the cell equipped with rGO-Ni/AC anode exhibits the maximum peak power density of 13.48 W m
2, which is 23.13% higher than that of the NQ/AC anode cell (10.94 W m
2) and 39.30% higher than that of the bare AC anode cell (9.67 W m
2). Moreover, the peak current densities decrease as follows: AC anode (41.85 A m
2)
rGO-Ni/AC NQ/AC Bare AC

Linear fitting equation

R2

10
3i0 (A$cm
2)

y ¼ 0.6521 
1.9122 y ¼ 0.6471 
1.9777 y ¼ 0.9423 
2.1460

0.99604 0.99160 0.99812

12.24 10.52 7.14

play a crucial role in improving cell performance. We can deduce there exist an efficient coordination and synergistic effect between nickel oxides and rGO sheets. On one hand, to make the nickel oxides work at full capacity as a catalyst material, rGO sheets are employed as a suitable substrate to anchor nickel oxides against agglomeration. On the other hand, to further facilitate mass transfer within rGO sheets, the nickel oxides are introduced among rGO sheets as the spacer to prevent the restacking. The rGO-Ni/AC anode prepared in this work can incorporate the efficient coordination and synergistic effect between nickel oxides and rGO sheets, enhanced the DGAFC performance remarkably without use of electron mediator and noble metals. Table 3 lists the recently published performance

Fig. 7 e Tafel plots of various anodes (A) and their enlarged scale graph for the over potential from 0.3 V to 0.4 V (B). Please cite this article in press as: Zhao Y, et al., Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell, International Journal of Hydrogen Energy (2017), https://doi.org/ 10.1016/j.ijhydene.2017.10.120

9

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1

Fig. 8 e Power density curves (A) and polarization curves (B) of DGAFC equipped with different anodes. Test condition: 1 M glucose, 3 M KOH, and ambient temperature.

Table 3 e Comparison of performance parameters for glucose oxidation on various electrodes. Ref This work [5] [38] [62] [8]

Anode catalyst

OCV/V

Pmax/W m
2

Type of test

[C6H12O6]/M

[KOH]/M

Membrane

T/ C

rGO-Ni/AC Ni foam (15 mM MV) Pd1Pt0.98/GA/NFP Au/MnO2eC Ag/Ni foam

0.792 0.75 0.78 0.86 0.592

13.48 5.20 12.5 11 20.3

14 mL Batch 28 mL Batch 10 mL Batch 25 mL Batch Continuous flow

1 1 0.5 0.3 1

3 3 3 1 2

No Yes Yes No Yes

25 25 25 30 80

parameters for glucose oxidation on various electrodes. It can be seen that the rGO-Ni/AC anode has a prominent comprehensive performance. Compared with other anodes listed in Table 3, the rGO-Ni/AC anode can get the highest power density without using noble metals and electron mediators under the room temperature. Furthermore, due to its simple and economic preparation method, the rGO-Ni/AC anode reported in this study is expected to be widely used in alkaline glucose fuel cells. It should be also mentioned that although the rGO-Ni/AC anode exhibited high performance, further researches about optimization of operational conditions and exploration of glucose oxidation pathway are necessary to be conducted to provide more insight on the rGO-Ni/AC electrode.

cost anode and its simple preparation method used in this work increase the feasibility of the large-scale application of DGAFC technology.

Acknowledgments This work was partially supported by the Natural Science Foundation of Tianjin City [No. 15JCYBJC21400].

Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2017.10.120.

Conclusion references 3D graphene-nickel oxides modified nickel foam electrode was produced using a simple potentiostatic method. Peony petal-like rGO sheets bound to the surface of the nickel foam inhibit restacking and allow the electrolyte to interact with a greater percentage of the graphene's surface area. The efficient coordination and synergistic effect between nickel oxides and rGO sheets were supposed to benefit the catalytic performance for glucose oxidation. The DGAFC equipped with rGO-Ni/AC anode achieves a higher power density and peak current density as compared to the device with bare AC anode and NQ/AC anode. Our results indicate that rGO-Ni is a good alternative catalyst to NQ and the composite structures may play a vital role in the electrocatalytic performance. The low-

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Please cite this article in press as: Zhao Y, et al., Peony petal-like 3D graphene-nickel oxide nanocomposite decorated nickel foam as high-performance electrocatalyst for direct glucose alkaline fuel cell, International Journal of Hydrogen Energy (2017), https://doi.org/ 10.1016/j.ijhydene.2017.10.120