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RGO catalyst with superaerophobic surface for high-performance hydrazine electrooxidation
Journal of Alloys and Compounds 788 (2019) 1240e1245
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A 3D porous Ni-Zn/RGO catalyst with superaerophobic surface for high-performance hydrazine electrooxidation Zhongbao Feng a, *, Dagang Li a, Lin Wang a, Qiang Sun a, Pai Lu a, Pengfei Xing a, Maozhong An b, ** a
School of Metallurgy, Northeastern University, Shenyang, Liaoning 110819, PR China State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2019 Received in revised form 20 February 2019 Accepted 1 March 2019 Available online 2 March 2019
It is of great importance to develop high-performance electrocatalysts in promoting hydrous hydrazine as a viable fuel. Herein, we report the synthesis of 3D porous superaerophobic Ni-Zn/RGO by bubble dynamic template method. The prepared porous Ni-Zn/RGO displays outstanding electrocatalytic activity with excellent stability towards hydrazine electrooxidation. For example, a current density of 469 mA cm2 at 0.30 V vs RHE, a retention rate of 92.6% after 5000 s and almost 100% selectivity towards the complete hydrazine oxidation can be achieved for Ni-Zn/RGO, which is at the top level among the reported electrocatalysts for hydrazine oxidation up to now. The mechanistic reason for the enhanced catalytic performance of Ni-Zn/RGO was discussed, which is primarily attributed to its active center of Ni electron richer, the large ESA, high electrical conductivity, and most importantly, the superaerophobic surface structure induced by the combining with RGO and its 3D porous architecture, hence enhancing the intrinsic activity and the number of active sites. It is believed that the prepared Ni-Zn/RGO catalyst has a potential application in hydrazine electrooxidation. © 2019 Elsevier B.V. All rights reserved.
Keywords: Direct hydrazine fuel cell Hydrazine oxidation Ni-Zn Electrocatalysis
1. Introduction Fuel cells powered by hydrogen are regarded as the next generation of ideal power source for mobile, portable and stationary power applications [1]. However, their application is severely hampered by the lack of safe and convenient means for hydrogen storage. Over the last decades, liquid fuels, in contrast to hydrogen, have attracted considerable attention, including ethanol [2], formic acid [3] sodium borohydride [4], and hydrazine [5] et al. Among various liquid fuel cells, direct hydrazine fuel cell (DHFC) stands out due to its highly theoretical cell voltage (1.56 V), high power density (5.4 KWh L1), moderate operation temperature and no CO2 emission [6e10].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Feng), [email protected] (M. An). https://doi.org/10.1016/j.jallcom.2019.03.007 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Anode reaction:
N2 H4 þ 4OH /N2 þ 4H2 O þ 4e
0
E ¼ 0:33 V vs: RHE Cathode reaction: 0
E ¼
O2 þ 2H2 O þ 4e /4OH
1:23 V vs: RHE
(1)
(2)
Cell reaction: N2 H4 þ O2 /N2 þ 2H2 O E0 ¼ 1:56 V vs: RHE (3) The full hydrazine oxidation in alkaline medium lead to the generation of eco-friendly N2 and H2O following Eqs (1)e(3). The key to promote hydrous hydrazine as a viable fuel is the development of electrocatalysts. A number of noble and non-noble transition metals and their alloys have been confirmed as active catalysts for hydrazine electrooxidation [11e18]. In a general view, non-noble metal catalysts exhibit higher catalytic activity but lower stability compared with noble metal catalysts. Ni is a potential lowcost, stable and highly active catalyst for hydrazine oxidation,
Z. Feng et al. / Journal of Alloys and Compounds 788 (2019) 1240e1245
however its durability is relatively low and pure Ni is hard to be used as catalyst owing to its easy run-off of Ni [19]. This can be improved by alloying with other metal or non-metal, including NiCo [20], Ni-Fe [21], Ni-Zn [22], Ni-B [23], Ni-P [24] and Ni-S [25]. Among these Ni-based catalysts, Ni-Zn alloy has attracted considerable attention. For example, Wang et al. synthesized nanosheet structured Ni-Zn alloy by electrodeposition and followed electrochemical leaching, and a current density of 300 mA cm2 at 0.40 V was achieved in 0.1 mol L1 hydrazine [26]. The electrocatalytic activity of Ni-Zn can be enhanced by combining with carbon. Atanassov et al. found that nanocrystalline Ni-Zn/Ketjenblack had twofold increase of activity in peak current density than nanocrystalline Ni-Zn [6]. Martinez et al. also found a significant enhanced electrocatalytic activity of nanocrystalline Ni-Zn/Ketjenblack with the addition of Ketjenblack [27]. Reduced graphene oxide (RGO) has large specific surface area and good electrical conductivity, which has been widely used as catalyst or carbon support for fuel cells [28]. However, up to now, few studies have focused on investigating the electrocatalytic performance of Ni-Zn/RGO for hydrazine oxidation. In contrast to the tremendous research in exploring various nanostructured catalysts, there is very limited attention on the management of the gas products on the microscopic electrode surface. This process is of great importance because the gas products accumulated on electrode surface may disturb the transport of hydrazine and result in a high pressure in the flow field, especially at high reaction rates [29,30]. It is reported that the adhesion behavior of gas bubbles can be flexibly tuned by structure and architecture of the surface. Sun et al. electrodeposited Cunanostructured film and 3D porous Ni-Cu alloy, and good catalytic hydrazine performance can be achieved due to their ‘superaerophobic’ effects [31,32]. The porous surface can afford discontinuous three phase (gas-liquid-solid) contact line, which can markedly reduce the adhesion force between the catalyst and gas products, thereby leading to the accelerated gas evolution behavior and significantly enhanced catalytic properties, especially at high reaction rates [33]. Notable developments have been made to enhance the catalytic performance of electrocatalysts towards hydrazine oxidation, however there is still substantial room for further improvement. In the present work, 3D porous superaerophobic Ni-Zn/RGO was synthesized by bubble dynamic template method during electrodeposition. The as-prepared Ni-Zn/RGO exhibits excellent activity, high stability and 100% selectivity towards the complete hydrazine oxidation. Interestingly, the mechanistic reason for the enhanced catalytic performance of Ni-Zn/RGO was discussed. 2. Experimental The graphene oxide (GO) was synthesized by a modified Hummers method [34]. Then, the RGO was electrodeposited on Ni foam by cyclic voltammogram (CV) in 0.5 g L1 GO solution using a conventional three-electrode cell (SEM image shown in Fig. S1). The Ni foam (Tianjin An Nuohe Chemical industry-Trade Co., LTD, 1.70 mm in thickness) with 1 cm2 was used as the working electrode, which has an open pore structure with an area density of about 420 g m2, and an average pore size of 0.20e0.60 mm. A platinum foil (99.99%, 1 cm2) and saturate calomel electrode (SCE) were employed as the counter electrode and reference electrode, respectively. The scan was from 1.5 V to 1.0 V at a scan rate of 50 mV s1 (40 segments). Ni-Zn/RGO was electrodeposited on RGO from the bath with 5,5-dimethylhydantoin 50 g L1, 1 1 Na4P2O7$10H2O 10 g L , ZnSO4$7H2O 2 g L , NiSO4$6H2O 10 g L1, K2CO3 20 g L1. The current density was 640 mA cm2 with the bath temperature of 323 K. The electroplating time was 2 min and the
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bath pH was 9e10. For comparison, Ni-Zn was electrodeposited on Ni foam from the same bath. Prior to electrodeposition, the Ni foam was cleaned by sonicating in ethanol to remove grease for 10 min, followed by immersing in 1.0 mol L1 HCl solution for 1 min to remove the surface oxide. The cathodic efficiency of electrodeposition (h) was calculated by Faraday's law according to equation (4), using deposit composition, charge passed and weight gained of the deposit.
h¼
ðm2 m1 ÞF X ci ni 100% I,t Mi
(4)
Where m1 (g) and m2 (g) are the weight before and after electroplating, I (A) is the total current passed in the plating time t (s), ci is the weight fraction of the element in the Ni-Zn alloy deposit, ni is the number of electrons transferred per atom of each metal, Mi is the molar mass of metal (g$mol1), and F is the Faraday's constant (96,485 C mol1). The phase structure of catalysts was identified by X-ray diffraction (XRD, D/max-gb) with Cu Ka radiation (l ¼ 0.1546 nm). Surface morphology of catalysts was observed by scanning electron microscope (SEM, Hitachi S4800) equipped with an energydispersive X-ray spectroscopy (EDX). The chemical states of the component of the catalysts were measured by X-ray photoelectron spectroscopy (XPS). The XPS spectra were taken with an ESCALAB250 system (Thermo VG, USA) at room temperature and the photoelectrons were detected with a hemispherical analyzer. The excitation source for the XPS measurements was Al Ka radiation (with photoelectron energy as 1486.6 eV). The adhesion force between the electrode interfaces and gas bubbles was determined by a high-sensitivity microelectromechanical balance system (Dataphysics DCAT11). A CHI760E electrochemical workstation was employed for the electrochemical measurements, which was carried out in a conventional three-electrode cell. The as-prepared Ni-Zn/RGO and NiZn with 1 cm2 were used as the working electrodes. A platinum foil (99.99%, 1 cm2) and Hg/HgO were employed as the counter electrode (CE) and reference electrode (RE), respectively. Linear sweep voltammetry (LSV), cyclic voltammograms (CV) and chronoamperometry (CA) were used to investigate the activity, electrochemical surface are (ESA) and stability of the catalysts. Electrochemical impedance spectroscopy (EIS) was performed in the frequency ranging from 100 KHz to 100 mHz with 5 mV amplitude. All potentials were reported with respective to the reversible hydrogen electrode (RHE) at pH 14. The volume of the anodic gas was measured by a classic water-displacement and then Faradic efficiency (FE) was calculated by comparing the experimental gas amount with theoretical value. 3. Results and discussion The bubble dynamic template method was used to produce the porous Ni-Zn and Ni-Zn/RGO on a gas-liquid-solid three phase interface during electrodeposition at large current density. Fig. 1aeb shows the surface morphologies of Ni-Zn and Ni-Zn/RGO catalysts, respectively. It is observed that uniform porous structure can be seen on Ni-Zn surface. A serious hydrogen evolution side reaction happens on the cathode owing to the large current density during electrodeposition. The insulating hydrogen bubbles will be adsorbed on the cathode surface due to the slow separation rate and will prevent the electrolyte from contacting the cathode. Therefore, the liquid-solid two-phase interface is changed to a gasliquid-solid three-phase interface. The electrodeposition of Ni-Zn can only proceed in the space between bubbles, which leads to the formation of a porous structure of Ni-Zn [35]. The porous Ni-Zn
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Fig. 1. a) SEM images of Ni-Zn catalyst, b) SEM images of Ni-Zn/RGO catalyst and c) corresponding XRD patterns of Ni-Zn and Ni-Zn/RGO catalysts.
catalyst has an average pore size of ~200 nm and a wall thickness of 20 nm (Fig. 1a). After combining with RGO, a thin layer of RGO can be observed on Ni foam surface and the porous structured Ni-Zn alloy is evenly distributed on RGO surface (Fig. 1b). This has been confirmed by the results of EDX-mapping (Fig. S2). Ni and Zn are found to be distributed uniformly on RGO surface. It is clear that RGO can significantly increase the surface active sites of Ni-Zn alloy due to its large specific surface area, which is beneficial to the full access of the electrolyte, thereby the significantly enhanced catalytic performance may be obtained. Moreover, the current efficiency of Ni-Zn and Ni-Zn/RGO is 15.4% and 18.1%, respectively. Fig. 1c displays the XRD patterns of Ni-Zn and Ni-Zn/RGO catalysts. Three diffraction peaks can be seen for both Ni-Zn and Ni-Zn/RGO, corresponding to (111), (200) and (220) planes (PDF # 70-0989), respectively. However, all peaks are shifted towards a lower angle due to the alloying with Zn. Only a-NiZn phase can be observed for both Ni-Zn and Ni-Zn/RGO catalysts. The peaks become narrow for Ni-Zn/RGO, indicating the increased particle size of Ni-Zn/RGO. TEM image of Ni-Zn/RGO (Fig. S3) shows that the interplanar spacing of the adjacent fringes was measured to be 0.178 nm and 0.207 nm, which matches well with the (200) and (111) planes of aNiZn phase, respectively. It shows an agreement with the XRD result. The electrocatalytic activities of Ni-Zn and Ni-Zn/RGO catalysts were investigated by LSV plots in 1 mol L1 NaOH solution with 0.1 mol L1 N2H4$H2O. As shown in Fig. 2a, Ni-Zn catalyst exhibits a high activity towards the hydrazine electrooxidation. The onset potential is ~ -0.098 V and the current density is 336 mA cm2 at 0.30 V vs RHE. For Ni-Zn/RGO, the onset potential negatively shifts to 0.103 V and the current density reaches to 469 mA cm2 at 0.30 V. The onset potential is reduced about 5 mV, and most importantly, the current density increases about 40% for Ni-Zn/RGO catalyst compared with Ni-Zn. The Tafel slopes (Fig. S4) are about
70 mV dec1 and 60 mV dec1 for Ni-Zn and Ni-Zn/RGO, respectively, suggesting more favorable catalytic kinetics of Ni-Zn/RGO for HzOR. It should be noted that the electrocatalytic performance of the reported Ni-Zn/RGO catalyst is at the top level among the reported electrocatalysts for the hydrazine oxidation up to date (Table 1). The RGO can accelerate the electron and mass transport, and the strongly coupled interface between RGO and Ni-Zn will lead to modified incorporation and manipulated electron structures. These effects of the RGO substrate can significantly improve the activity of the pristine active components [36e38]. N2 and H2O are the ideal products for hydrazine electrooxidation in practical DHFC development [20,39]. However, NH3 and H2 may also be generated based on Eqs. (5)e(7), leading to the decreased utilization efficiency of hydrazine. In order to investigate the possible reaction pathway of the hydrazine oxidation, and estimate the numbers of electron transfer processes during hydrazine electrooxidation, the gas generation rates of Ni-Zn and Ni-Zn/RGO at various current densities were measured. As shown in Fig. 2b, the fitting lines of generation rate vs. current density show a good consistent with the values calculated by the 4-electron-reacton for both Ni-Zn and Ni-Zn/RGO catalysts, suggesting that the Faraday efficiency of the current for hydrazine electrooxidation is almost 100% during the used range of current density for Ni-Zn and Ni-Zn/ RGO catalysts.
N2 H4 þ nOH /N2 þ nH2 O þ ð2 0:5nÞH2 þ ne
(5)
Where n is the electron number of the reaction (n ¼ 1, 2, 3 and 4).
N2 H4 /N2 þ 2H2 ðdecompositionÞ
(6)
N2 H4 /N2 þ 4NH3 ðdecompositionÞ
(7)
Fig. 2. a) LSV plots of Ni-Zn and Ni-Zn/RGO catalysts at 25 C in the electrolyte with 0.1 mol L1 N2H4$H2O and 1.0 mol L1 NaOH at a scan rate of 20 mV s1, and b) the variation of anodic gas generation rate as a function of current density for Ni-Zn and Ni-Zn/RGO catalysts.
Z. Feng et al. / Journal of Alloys and Compounds 788 (2019) 1240e1245
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Table 1 Catalytic properties of various catalysts towards hydrazine electrooxidation. Samples
NiZn/NF Ni-NSA/Ni CoNi-S/Ni Ni0.43Cu0.57/Cu Ni0.6Co0.4/Ni NiS2/TiM FeP/NF Ni-Zn/RGO
Electrolyte
Electrocatalytic performance
[N2H4] and [NaOH]/mol L1
j/mA cm2
Ea/V
O.P.b/V
1.0, 1.0, 2.0, 0.1, 3.0, 0.1, 0.5, 0.1,
320 228 118 300 292 150 125 469
0.60 0.25 1.00 0.47 0.22 0.30 0.50 0.30
0.14 0.04 0.00 0.02 0.08 0.05 0.00 0.10
1.0 3.0 0.1 3.0 0.5 1.0 1.0 1.0
Ref.
[7] [14] [25] [32] [40] [41] [42] This work
Note. a All potentials were relative to RHE at pH ¼ 14. b The onset potential is denoted as O.P..
Compared with Ni-Zn catalyst, an improved activity of hydrazine electrooxidation is observed for Ni-Zn/RGO. To gain insight onto the enhanced performance, we used the electrical doublelayer capacitance (CDL) that was measured by CV plots at various scan rates (Fig. S5) to determine the electrochemical active surface area (ESA) and measured the reaction resistance (Rct) of the catalysts by EIS plots. As shown in Fig. 3a, a markedly larger capacitance can be observed for Ni-Zn/RGO (15.49 mF cm2) than Ni-Zn catalyst (1.97 mF cm2), and the CDL of Ni-Zn/RGO is almost 8-fold higher compared with Ni-Zn catalyst. Since ESA is proportional to CDL, these results strongly confirm the above analysis that the active sites and surface area of Ni-Zn/RGO are significantly enhanced by combining with RGO. Fig. 3b shows the Nyquist plots and equivalent electrical circuit of Ni-Zn and Ni-Zn/RGO. In the equivalent electrical circuit, Rs and Rct represent the solution resistance and charge transfer resistance, respectively, CPE represents the catalyst/ solution interface. As displayed in Table S1, Ni-Zn/RGO catalyst exhibits a much lower Rct than Ni-Zn catalyst, indicating an enhanced electric conductivity. It is clear that the synergistic effects of the increased active surface area and enhanced electric conductivity are desirable for boosting the electrooxidation activity [43,44]. The catalytic stability of Ni-Zn and Ni-Zn/RGO is critical for evaluating the long-term catalytic ability of hydrazine electrooxidation. As seen in Fig. 4a, Ni-Zn/RGO can still maintain 91.4% of its initial activity after 5000 s. However, Ni-Zn only remains its initial activity of 83.6%. To gain insight into the enhanced stability, XPS analysis of the fresh and used Ni-Zn and Ni-Zn/RGO catalysts is displayed (Fig. S6). Compared with Ni-Zn/RGO, the signal of Ni (II)
for Ni-Zn is remarkably enhanced after the stability experiments, indicating that the surface oxidation of Ni-Zn/RGO can be inhibited during electrocatalysis after the combine with RGO. Thus, the superior stability of Ni-Zn/RGO catalyst may be stem from its inhibited surface oxidation during test. The CV cycles were also employed to evaluate the stability of Ni-Zn and Ni-Zn/RGO catalysts. As seen in Fig. 4b, Ni-Zn/RGO shows an excellent durability for the oxidation of hydrazine, with only 7.4% loss of its initial current density after 1000 cycles, superior to Ni-Zn (14.9%). In order to shed light on the enhanced catalytic activity of Ni-Zn/ RGO, XPS was employed to measure the chemical states of Ni-Zn and Ni-Zn/RGO. As shown in Fig. 5, two chemical states can be seen for both Ni and Zn elements in Ni-Zn and Ni-Zn/RGO. The Ni2p3/2 peak at 856.86 eV and Zn2p3/2 at 1022.26 eV can be safely assigned to Ni (II) and Zn (II) species, respectively, which is associated with the possible surface oxidation in the air. The other Ni2p3/2 peak and Zn2p3/2 peak at 852.46 eV and 1020.66 eV are assigned to Ni (0) and Zn (0), respectively. Ni-Zn/RGO catalyst shows a negative shift of 0.14 eV for the binding energy of Ni (0) and a slight positive shift of 0.28 eV for the binding energy of Zn (0) compared with Ni-Zn catalyst, resulting in easier electron transfer from Zn to Ni for Ni-Zn/RGO. This can be due to the fact that the existence of RGO is beneficial to the electron transfer from Zn to Ni. Hence, back electron donation from Ni to the antibonding orbital of N2H4 may be strengthened, leading to a more destabilization of the interaction of Ni-N2H4 and weaker bond of N-H, thereby higher electrocatalytic activity can be observed for Ni-Zn/RGO than Ni-Zn catalyst. It is noted that porous structure can reduce the contact area
Fig. 3. a) The capacitive current densities of Ni-Zn and Ni-Zn/RGO catalysts as a function of scan rate at open circuit potential, b) Nyquist curves of Ni-Zn and Ni-Zn/RGO catalysts at onset potentials.
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Fig. 4. a) CA plots of Ni-Zn and Ni-Zn/RGO catalysts at a constant-potential of 0.2 V vs RHE in the electrolyte with 3.0 mol L1 N2H4$H2O and 1.0 mol L1 NaOH, and b) cycling stability of Ni-Zn and Ni-Zn/RGO catalysts in the electrolyte with 3.0 mol L1 N2H4$H2O and 1.0 mol L1 NaOH.
Fig. 5. a) Ni 2p3/2, and b) Zn 2p3/2 XPS spectrum of Ni-Zn and Ni-Zn/RGO catalysts.
between the produced N2 and catalysts, resulting in fast removal speed of N2 gas bubbles. Therefore, the excellent electrocatalytic activity and high stability of the catalysts may be stem from the tight binding between the raw materials and substrate, and fast mass transfer for the release of the products on superaerophobic surface [45]. In order to verify the above hypothesis, the adhesionforce measurements of Ni-Zn and Ni-Zn/RGO were performed. As displayed in Fig. 6, both Ni-Zn and Ni-Zn/RGO have small adhesion force to the gas bubbles (20 mN and 10 mN, respectively). Moreover, Ni-Zn/RGO has a smaller value. The above results clearly indicate the superaerophobic nature of Ni-Zn/RGO. The lower adhesive performance of Ni-Zn/RGO can result in faster removal rate, thereby higher electrocatalytic performance can be observed for Ni-Zn/RGO compared with Ni-Zn. 4. Conclusions 3D porous superaerophobic Ni-Zn/RGO was successfully prepared on Ni foam by bubble dynamic template method during electrodeposition. The resulting porous Ni-Zn/RGO exhibits an extreme high electrocatalytic activity with excellent stability towards hydrazine electrooxidation compared to Ni-Zn catalyst. A current density of 469 mA cm2 at 0.30 V vs. RHE, a retention rate of 92.6% after 5000 s and almost 100% selectivity towards the complete hydrazine oxidation can be achieved for Ni-Zn/RGO, which is
Fig. 6. The adhesive force measurements of the gas bubbles on Ni-Zn and Ni-Zn/RGO.
at the top level among the reported electrocatalysts for hydrazine oxidation up to date. The mechanistic reason for the enhanced catalytic performance of Ni-Zn/RGO can be attributed to its active center of Ni electron richer, the large ESA, high electrical
Z. Feng et al. / Journal of Alloys and Compounds 788 (2019) 1240e1245
conductivity and most importantly, the superaerophobic surface induced by combining with RGO and its porous structure. This result may provide a good guideline for design and synthesis catalyst with high performance for hydrazine electrooxidation. Acknowledgments The authors are grateful for the supported by the Certificate of China Postdoctoral Science Foundation Grant (2018M631809), the Fundamental Research Funds for the Central Universities (N172503012), and State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2018DX03). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.03.007. References [1] L. Carrette, K.A. Friedrich, U. Stimming, Fuel cells: principles, types, fuels, and applications, ChemPhysChem 4 (2000) 162e193. [2] L. An, T.S. Zhao, Y.S. Li, Carbon-neutral sustainable energy technology: direct ethanol fuel cells, Renew. Sustain. Energy Rev. 50 (2015) 1462e1468. [3] X.L. Ji, K.T. Lee, R. Holden, L. Zhang, J.J. Zhang, G.A. Botton, M. Couillard, L.F. Nazar, Nanocrystalline intermetallics on mesoporous carbon for direct formic acid fuel cell anodes, Nat. Chem. 2 (2010) 286e293. [4] G. Rostamikia, M.J. Janik, Direct borohydride oxidation: mechanism determination and design of alloy catalysts guided by density functional theory, Energy Environ. Sci. 3 (2010) 1262e1274. [5] A. Serov, C. Kwak, Direct hydrazine fuel cells: a review, Appl. Catal. B Environ. 98 (2010) 1e9. [6] A. Serov, M. Padilla, A.J. Roy, P. Atanassov, T. Sakamoto, K. Asazawa, H. Tanaka, Anode catalysts for direct hydrazine fuel cells: from laboratory test to an electric vehicle, Angew. Chem. Int. Ed. 53 (2014) 10336e10339. [7] J.R. Varcoe, P. Atanassov, D.R. Dekel, A.M. Herring, M.A. Hickner, P.A. Kohl, A.R. Kucernak, W.E. Mustain, K. Nijmeijer, K. Scott, T. Xu, L. Zhuang, Anionexchange membranes in electrochemical energy systems, Energy Environ. Sci. 7 (2014) 3135e3191. [8] J.Y. Zhang, H. Wang, Y. Tian, Y. Yan, Q. Xue, T. He, H. Liu, C. Wang, Y. Chen, B.Y. Xia, Anodic hydrazine oxidation assists energy-efficient hydrogen evolution over a bifunctional cobalt perselenide nanosheet electrode, Angew. Chem. Int. Ed. 130 (2018) 7775e7779. [9] L. Zhang, D. Lu, Y. Chen, Y. Tang, T. Lu, Facile synthesis of Pd-Co-P ternary alloy network nanostructures and their enhanced electrocatalytic activity towards hydrazine oxidation, J. Mater. Chem. A 2 (2014) 1252e1256. [10] Y. Liang, Y. Zhou, J. Ma, J. Zhao, Y. Chen, Y. Tang, T. Lu, Preparation of highly dispersed and ultrafine Pd/C catalyst and its electrocatalytic performance for hydrazine electrooxidation, Appl. Catal. B Environ. 103 (2011) 388e396. [11] V. Rosca, M.T.M. Koper, Electrocatalytic oxidation of hydrazine on platinum electrodes in alkaline solutions, Electrochim. Acta 53 (2008) 5199e5205. [12] S.S. Narwade, B.B. Mulik, S.M. Mali, B.R. Sathe, Silver nanoparticles sensitized C60 (Ag@C60) as efficient electrocatalysts for hydrazine oxidation: implication for hydrogen generation reaction, Appl. Surf. Sci. 396 (2017) 939e944. [13] L. Zhang, W.X. Niu, W.Y. Gao, L.M. Qi, J.M. Zhao, M. Xu, G.B. Xu, Facetdependent electrocatalytic activities of Pd nanocrystals toward the electrooxidation of hydrazine, Electrochem. Commun. 37 (2013) 57e60. [14] Y. Kuang, G. Feng, P.S. Li, Y. Bi, Y.M. Li, X.M. Sun, Single-crystalline ultrathin nickel nanosheets array from in situ topotactic reduction for active and stable electrocatalysis, Angew. Chem. Int. Ed. 55 (2016) 693e697. [15] R. Liu, X. Jiang, F. Guo, N.N. Shi, J.L. Yin, G.L. Wang, D.X. Cao, Carbon fiber cloth supported micro-and nano-structured Co as the electrode for hydrazine oxidation in alkaline media, Electrochim. Acta 94 (2013) 214e218. [16] H.C. Gao, Y.X. Wang, F. Xiao, C.B. Ching, H.W. Duan, Growth of copper nanocubes on graphene paper as free-standing electrodes for direct hydrazine fuel cells, J. Phys. Chem. C 116 (2012) 7719e7725. [17] Q. Xue, Y. Ding, Y. Xue, F. Li, P. Chen, Y. Chen, 3D nitrogen-doped graphene aerogels as efficient electrocatalyst for the oxygen reduction reaction, Carbon 139 (2018) 137e144. [18] X. Xiao, T. Wang, J. Bai, F. Li, T. Ma, Y. Chen, Enhancing the selectivity of H2O2 electrogeneration by steric hindrance effect, ACS Appl. Mater. Interfaces 10 (2018) 42534e42541. [19] N.V. Rees, R.G. Compton, Carbon-free energy: a review of ammonia-and hydrazine-based electrochemical fuel cells, Energy Environ. Sci. 4 (2011) 1255e1260. [20] J. Sanabria-Chinchilla, K. Asazawa, T. Sakamoto, K. Yamada, H. Tanaka, P. Strasser, Noble metal-free hydrazine fuel cell catalysts: EPOC effect in competing chemical and electrochemical reaction pathways, J. Am. Chem. Soc.
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Ye, In situ visualization study of CO2 gas bubble behavior in DMFC anode flow fields, J. Power Sources 139 (2005) 79e90. [30] Z. Lu, W. Zhu, X. Yu, H. Zhang, Y. Li, X. Sun, X. Wang, H. Wang, J. Wang, J. Luo, X. Lei, L. Jiang, Ultrahigh hydrogen evolution performance of under-water “superaerophobic” MoS2 nanostructured electrodes, Adv. Mater. 26 (2014) 2683e2687. [31] Z. Lu, M. Sun, T. Xu, Y. Li, W. Xu, Z. Chang, L. Jiang, Superaerophobic electrodes for direct hydrazine fuel cells, Adv. Mater. 27 (2015) 2361e2366. [32] M. Sun, Z. Lu, L. Luo, Z. Chang, X. Sun, A 3D porous Ni-Cu alloy film for highperformance hydrazine electrooxidation, Nanoscale 8 (2016) 1479e1484. [33] D. Kong, H. Wang, Z. Lu, Y. Cui, CoSe2 nanoparticles grown on carbon fiber paper: an efficient and stable electrocatalyst for hydrogen evolution reaction, J. Am. Chem. Soc. 136 (2014) 4897e4900. [34] M. Srivastava, A.K. Das, P. Khanra, M.E. Uddin, N.H. Kim, J.H. Lee, Characterizations of in situ grown ceria nanoparticles on reduced graphene oxide as a catalyst for the electrooxidation of hydrazine, J. Mater. Chem. A 1 (2013) 9792e9801. [35] M. Wang, X. Yu, Z. Wang, X. Gong, Z. Guo, L. Dai, Hierarchically 3D porous films electrochemically constructed on gas-liquid-solid three-phase interface for energy application, J. Mater. Chem. A 5 (2017) 9488e9513. [36] D. Yu, L. Wei, W. Jiang, H. Wang, B. Sun, Q. Zhang, K. Goh, R. Si, Y. Chen, Nitrogen doped holey graphene as an efficient metal-free multifunctional electrochemical catalyst for hydrazine oxidation and oxygen reduction, Nanoscale 5 (2013) 3457e3464. [37] C. Tang, M.M. Titirici, Q. Zhang, A review of nanocarbons in energy electrocatalysis: multifunctional substrates and highly active sites, J. Energy Chem. 26 (2017) 1077e1093. [38] T. Yuan, A. Wang, K. Fang, Z. Wang, J. Feng, Hydrogen evolution-assisted onepot aqueous synthesis of hierarchical trimetallic PdNiRu nanochains for hydrazine oxidation reaction, J. Energy Chem. 26 (2017) 1231e1237. [39] D.A. Finkelstein, R. Imbeault, S. Garbarino, L. Roue, D. Guay, Trends in catalysis and catalyst cost effectiveness for N2H4 fuel cells and sensors: a rotating disk electrode (RDE) study, J. Phys. Chem. C 120 (2016) 4717e4738. [40] G. Feng, Y. Kuang, P.S. Li, N.N. Han, M. Sun, G.X. Zhang, X.M. Sun, Single crystalline ultrathin nickel-cobalt alloy nanosheets array for direct hydrazine fuel cells, Adv. Sci. 4 (2017), 1600179. [41] J.M. Wang, X. Ma, T.T. Liu, D.N. Liu, S. Hao, G. Du, R.M. Kong, A.M. Asiri, X.P. Sun, NiS2 nanosheet array: a high-active bifunctional electrocatalyst for hydrazine oxidation and water reduction toward energy-efficient hydrogen production, Mater. Today Energy 3 (2017) 9e14. [42] L. Zhang, D.N. Liu, S. Hao, L.S. Xie, F.L. Qu, G. Du, A.M. Asiri, X.P. Sun, Electrochemical hydrazine oxidation catalyzed by iron phosphide nanosheets array toward energy-efficient electrolytic hydrogen production from water, Chemistryselect 2 (2017) 3401e3407. [43] L.S. Wu, X.P. Wen, H. Wen, H.B. Dai, P. Wang, Palladium decorated porous nickel having enhanced electrocatalytic performance for hydrazine oxidation, J. Power Sources 412 (2019) 71e77. [44] A. Papaderakis, D. Tsiplakides, S. Balomenou, S. Sotiropoulos, Electrochemical impedance studies of IrO2 catalysts for oxygen evolution, J. Electroanal. Chem. 757 (2015) 216e224. [45] G. Feng, Y. Kuang, Y. Li, X. Sun, Three-dimensional porous superaerophobic nickel nanoflower electrodes for high-performance hydrazine oxidation, Nano Res. 8 (2015) 3365e3371.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A 3D porous Ni-Zn/RGO catalyst with superaerophobic surface for high-performance hydrazine electrooxidation Zhongbao Feng a, *, Dagang Li a, Lin Wang a, Qiang Sun a, Pai Lu a, Pengfei Xing a, Maozhong An b, ** a
School of Metallurgy, Northeastern University, Shenyang, Liaoning 110819, PR China State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2019 Received in revised form 20 February 2019 Accepted 1 March 2019 Available online 2 March 2019
It is of great importance to develop high-performance electrocatalysts in promoting hydrous hydrazine as a viable fuel. Herein, we report the synthesis of 3D porous superaerophobic Ni-Zn/RGO by bubble dynamic template method. The prepared porous Ni-Zn/RGO displays outstanding electrocatalytic activity with excellent stability towards hydrazine electrooxidation. For example, a current density of 469 mA cm2 at 0.30 V vs RHE, a retention rate of 92.6% after 5000 s and almost 100% selectivity towards the complete hydrazine oxidation can be achieved for Ni-Zn/RGO, which is at the top level among the reported electrocatalysts for hydrazine oxidation up to now. The mechanistic reason for the enhanced catalytic performance of Ni-Zn/RGO was discussed, which is primarily attributed to its active center of Ni electron richer, the large ESA, high electrical conductivity, and most importantly, the superaerophobic surface structure induced by the combining with RGO and its 3D porous architecture, hence enhancing the intrinsic activity and the number of active sites. It is believed that the prepared Ni-Zn/RGO catalyst has a potential application in hydrazine electrooxidation. © 2019 Elsevier B.V. All rights reserved.
Keywords: Direct hydrazine fuel cell Hydrazine oxidation Ni-Zn Electrocatalysis
1. Introduction Fuel cells powered by hydrogen are regarded as the next generation of ideal power source for mobile, portable and stationary power applications [1]. However, their application is severely hampered by the lack of safe and convenient means for hydrogen storage. Over the last decades, liquid fuels, in contrast to hydrogen, have attracted considerable attention, including ethanol [2], formic acid [3] sodium borohydride [4], and hydrazine [5] et al. Among various liquid fuel cells, direct hydrazine fuel cell (DHFC) stands out due to its highly theoretical cell voltage (1.56 V), high power density (5.4 KWh L1), moderate operation temperature and no CO2 emission [6e10].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Feng), [email protected] (M. An). https://doi.org/10.1016/j.jallcom.2019.03.007 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Anode reaction:
N2 H4 þ 4OH /N2 þ 4H2 O þ 4e
0
E ¼ 0:33 V vs: RHE Cathode reaction: 0
E ¼
O2 þ 2H2 O þ 4e /4OH
1:23 V vs: RHE
(1)
(2)
Cell reaction: N2 H4 þ O2 /N2 þ 2H2 O E0 ¼ 1:56 V vs: RHE (3) The full hydrazine oxidation in alkaline medium lead to the generation of eco-friendly N2 and H2O following Eqs (1)e(3). The key to promote hydrous hydrazine as a viable fuel is the development of electrocatalysts. A number of noble and non-noble transition metals and their alloys have been confirmed as active catalysts for hydrazine electrooxidation [11e18]. In a general view, non-noble metal catalysts exhibit higher catalytic activity but lower stability compared with noble metal catalysts. Ni is a potential lowcost, stable and highly active catalyst for hydrazine oxidation,
Z. Feng et al. / Journal of Alloys and Compounds 788 (2019) 1240e1245
however its durability is relatively low and pure Ni is hard to be used as catalyst owing to its easy run-off of Ni [19]. This can be improved by alloying with other metal or non-metal, including NiCo [20], Ni-Fe [21], Ni-Zn [22], Ni-B [23], Ni-P [24] and Ni-S [25]. Among these Ni-based catalysts, Ni-Zn alloy has attracted considerable attention. For example, Wang et al. synthesized nanosheet structured Ni-Zn alloy by electrodeposition and followed electrochemical leaching, and a current density of 300 mA cm2 at 0.40 V was achieved in 0.1 mol L1 hydrazine [26]. The electrocatalytic activity of Ni-Zn can be enhanced by combining with carbon. Atanassov et al. found that nanocrystalline Ni-Zn/Ketjenblack had twofold increase of activity in peak current density than nanocrystalline Ni-Zn [6]. Martinez et al. also found a significant enhanced electrocatalytic activity of nanocrystalline Ni-Zn/Ketjenblack with the addition of Ketjenblack [27]. Reduced graphene oxide (RGO) has large specific surface area and good electrical conductivity, which has been widely used as catalyst or carbon support for fuel cells [28]. However, up to now, few studies have focused on investigating the electrocatalytic performance of Ni-Zn/RGO for hydrazine oxidation. In contrast to the tremendous research in exploring various nanostructured catalysts, there is very limited attention on the management of the gas products on the microscopic electrode surface. This process is of great importance because the gas products accumulated on electrode surface may disturb the transport of hydrazine and result in a high pressure in the flow field, especially at high reaction rates [29,30]. It is reported that the adhesion behavior of gas bubbles can be flexibly tuned by structure and architecture of the surface. Sun et al. electrodeposited Cunanostructured film and 3D porous Ni-Cu alloy, and good catalytic hydrazine performance can be achieved due to their ‘superaerophobic’ effects [31,32]. The porous surface can afford discontinuous three phase (gas-liquid-solid) contact line, which can markedly reduce the adhesion force between the catalyst and gas products, thereby leading to the accelerated gas evolution behavior and significantly enhanced catalytic properties, especially at high reaction rates [33]. Notable developments have been made to enhance the catalytic performance of electrocatalysts towards hydrazine oxidation, however there is still substantial room for further improvement. In the present work, 3D porous superaerophobic Ni-Zn/RGO was synthesized by bubble dynamic template method during electrodeposition. The as-prepared Ni-Zn/RGO exhibits excellent activity, high stability and 100% selectivity towards the complete hydrazine oxidation. Interestingly, the mechanistic reason for the enhanced catalytic performance of Ni-Zn/RGO was discussed. 2. Experimental The graphene oxide (GO) was synthesized by a modified Hummers method [34]. Then, the RGO was electrodeposited on Ni foam by cyclic voltammogram (CV) in 0.5 g L1 GO solution using a conventional three-electrode cell (SEM image shown in Fig. S1). The Ni foam (Tianjin An Nuohe Chemical industry-Trade Co., LTD, 1.70 mm in thickness) with 1 cm2 was used as the working electrode, which has an open pore structure with an area density of about 420 g m2, and an average pore size of 0.20e0.60 mm. A platinum foil (99.99%, 1 cm2) and saturate calomel electrode (SCE) were employed as the counter electrode and reference electrode, respectively. The scan was from 1.5 V to 1.0 V at a scan rate of 50 mV s1 (40 segments). Ni-Zn/RGO was electrodeposited on RGO from the bath with 5,5-dimethylhydantoin 50 g L1, 1 1 Na4P2O7$10H2O 10 g L , ZnSO4$7H2O 2 g L , NiSO4$6H2O 10 g L1, K2CO3 20 g L1. The current density was 640 mA cm2 with the bath temperature of 323 K. The electroplating time was 2 min and the
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bath pH was 9e10. For comparison, Ni-Zn was electrodeposited on Ni foam from the same bath. Prior to electrodeposition, the Ni foam was cleaned by sonicating in ethanol to remove grease for 10 min, followed by immersing in 1.0 mol L1 HCl solution for 1 min to remove the surface oxide. The cathodic efficiency of electrodeposition (h) was calculated by Faraday's law according to equation (4), using deposit composition, charge passed and weight gained of the deposit.
h¼
ðm2 m1 ÞF X ci ni 100% I,t Mi
(4)
Where m1 (g) and m2 (g) are the weight before and after electroplating, I (A) is the total current passed in the plating time t (s), ci is the weight fraction of the element in the Ni-Zn alloy deposit, ni is the number of electrons transferred per atom of each metal, Mi is the molar mass of metal (g$mol1), and F is the Faraday's constant (96,485 C mol1). The phase structure of catalysts was identified by X-ray diffraction (XRD, D/max-gb) with Cu Ka radiation (l ¼ 0.1546 nm). Surface morphology of catalysts was observed by scanning electron microscope (SEM, Hitachi S4800) equipped with an energydispersive X-ray spectroscopy (EDX). The chemical states of the component of the catalysts were measured by X-ray photoelectron spectroscopy (XPS). The XPS spectra were taken with an ESCALAB250 system (Thermo VG, USA) at room temperature and the photoelectrons were detected with a hemispherical analyzer. The excitation source for the XPS measurements was Al Ka radiation (with photoelectron energy as 1486.6 eV). The adhesion force between the electrode interfaces and gas bubbles was determined by a high-sensitivity microelectromechanical balance system (Dataphysics DCAT11). A CHI760E electrochemical workstation was employed for the electrochemical measurements, which was carried out in a conventional three-electrode cell. The as-prepared Ni-Zn/RGO and NiZn with 1 cm2 were used as the working electrodes. A platinum foil (99.99%, 1 cm2) and Hg/HgO were employed as the counter electrode (CE) and reference electrode (RE), respectively. Linear sweep voltammetry (LSV), cyclic voltammograms (CV) and chronoamperometry (CA) were used to investigate the activity, electrochemical surface are (ESA) and stability of the catalysts. Electrochemical impedance spectroscopy (EIS) was performed in the frequency ranging from 100 KHz to 100 mHz with 5 mV amplitude. All potentials were reported with respective to the reversible hydrogen electrode (RHE) at pH 14. The volume of the anodic gas was measured by a classic water-displacement and then Faradic efficiency (FE) was calculated by comparing the experimental gas amount with theoretical value. 3. Results and discussion The bubble dynamic template method was used to produce the porous Ni-Zn and Ni-Zn/RGO on a gas-liquid-solid three phase interface during electrodeposition at large current density. Fig. 1aeb shows the surface morphologies of Ni-Zn and Ni-Zn/RGO catalysts, respectively. It is observed that uniform porous structure can be seen on Ni-Zn surface. A serious hydrogen evolution side reaction happens on the cathode owing to the large current density during electrodeposition. The insulating hydrogen bubbles will be adsorbed on the cathode surface due to the slow separation rate and will prevent the electrolyte from contacting the cathode. Therefore, the liquid-solid two-phase interface is changed to a gasliquid-solid three-phase interface. The electrodeposition of Ni-Zn can only proceed in the space between bubbles, which leads to the formation of a porous structure of Ni-Zn [35]. The porous Ni-Zn
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Fig. 1. a) SEM images of Ni-Zn catalyst, b) SEM images of Ni-Zn/RGO catalyst and c) corresponding XRD patterns of Ni-Zn and Ni-Zn/RGO catalysts.
catalyst has an average pore size of ~200 nm and a wall thickness of 20 nm (Fig. 1a). After combining with RGO, a thin layer of RGO can be observed on Ni foam surface and the porous structured Ni-Zn alloy is evenly distributed on RGO surface (Fig. 1b). This has been confirmed by the results of EDX-mapping (Fig. S2). Ni and Zn are found to be distributed uniformly on RGO surface. It is clear that RGO can significantly increase the surface active sites of Ni-Zn alloy due to its large specific surface area, which is beneficial to the full access of the electrolyte, thereby the significantly enhanced catalytic performance may be obtained. Moreover, the current efficiency of Ni-Zn and Ni-Zn/RGO is 15.4% and 18.1%, respectively. Fig. 1c displays the XRD patterns of Ni-Zn and Ni-Zn/RGO catalysts. Three diffraction peaks can be seen for both Ni-Zn and Ni-Zn/RGO, corresponding to (111), (200) and (220) planes (PDF # 70-0989), respectively. However, all peaks are shifted towards a lower angle due to the alloying with Zn. Only a-NiZn phase can be observed for both Ni-Zn and Ni-Zn/RGO catalysts. The peaks become narrow for Ni-Zn/RGO, indicating the increased particle size of Ni-Zn/RGO. TEM image of Ni-Zn/RGO (Fig. S3) shows that the interplanar spacing of the adjacent fringes was measured to be 0.178 nm and 0.207 nm, which matches well with the (200) and (111) planes of aNiZn phase, respectively. It shows an agreement with the XRD result. The electrocatalytic activities of Ni-Zn and Ni-Zn/RGO catalysts were investigated by LSV plots in 1 mol L1 NaOH solution with 0.1 mol L1 N2H4$H2O. As shown in Fig. 2a, Ni-Zn catalyst exhibits a high activity towards the hydrazine electrooxidation. The onset potential is ~ -0.098 V and the current density is 336 mA cm2 at 0.30 V vs RHE. For Ni-Zn/RGO, the onset potential negatively shifts to 0.103 V and the current density reaches to 469 mA cm2 at 0.30 V. The onset potential is reduced about 5 mV, and most importantly, the current density increases about 40% for Ni-Zn/RGO catalyst compared with Ni-Zn. The Tafel slopes (Fig. S4) are about
70 mV dec1 and 60 mV dec1 for Ni-Zn and Ni-Zn/RGO, respectively, suggesting more favorable catalytic kinetics of Ni-Zn/RGO for HzOR. It should be noted that the electrocatalytic performance of the reported Ni-Zn/RGO catalyst is at the top level among the reported electrocatalysts for the hydrazine oxidation up to date (Table 1). The RGO can accelerate the electron and mass transport, and the strongly coupled interface between RGO and Ni-Zn will lead to modified incorporation and manipulated electron structures. These effects of the RGO substrate can significantly improve the activity of the pristine active components [36e38]. N2 and H2O are the ideal products for hydrazine electrooxidation in practical DHFC development [20,39]. However, NH3 and H2 may also be generated based on Eqs. (5)e(7), leading to the decreased utilization efficiency of hydrazine. In order to investigate the possible reaction pathway of the hydrazine oxidation, and estimate the numbers of electron transfer processes during hydrazine electrooxidation, the gas generation rates of Ni-Zn and Ni-Zn/RGO at various current densities were measured. As shown in Fig. 2b, the fitting lines of generation rate vs. current density show a good consistent with the values calculated by the 4-electron-reacton for both Ni-Zn and Ni-Zn/RGO catalysts, suggesting that the Faraday efficiency of the current for hydrazine electrooxidation is almost 100% during the used range of current density for Ni-Zn and Ni-Zn/ RGO catalysts.
N2 H4 þ nOH /N2 þ nH2 O þ ð2 0:5nÞH2 þ ne
(5)
Where n is the electron number of the reaction (n ¼ 1, 2, 3 and 4).
N2 H4 /N2 þ 2H2 ðdecompositionÞ
(6)
N2 H4 /N2 þ 4NH3 ðdecompositionÞ
(7)
Fig. 2. a) LSV plots of Ni-Zn and Ni-Zn/RGO catalysts at 25 C in the electrolyte with 0.1 mol L1 N2H4$H2O and 1.0 mol L1 NaOH at a scan rate of 20 mV s1, and b) the variation of anodic gas generation rate as a function of current density for Ni-Zn and Ni-Zn/RGO catalysts.
Z. Feng et al. / Journal of Alloys and Compounds 788 (2019) 1240e1245
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Table 1 Catalytic properties of various catalysts towards hydrazine electrooxidation. Samples
NiZn/NF Ni-NSA/Ni CoNi-S/Ni Ni0.43Cu0.57/Cu Ni0.6Co0.4/Ni NiS2/TiM FeP/NF Ni-Zn/RGO
Electrolyte
Electrocatalytic performance
[N2H4] and [NaOH]/mol L1
j/mA cm2
Ea/V
O.P.b/V
1.0, 1.0, 2.0, 0.1, 3.0, 0.1, 0.5, 0.1,
320 228 118 300 292 150 125 469
0.60 0.25 1.00 0.47 0.22 0.30 0.50 0.30
0.14 0.04 0.00 0.02 0.08 0.05 0.00 0.10
1.0 3.0 0.1 3.0 0.5 1.0 1.0 1.0
Ref.
[7] [14] [25] [32] [40] [41] [42] This work
Note. a All potentials were relative to RHE at pH ¼ 14. b The onset potential is denoted as O.P..
Compared with Ni-Zn catalyst, an improved activity of hydrazine electrooxidation is observed for Ni-Zn/RGO. To gain insight onto the enhanced performance, we used the electrical doublelayer capacitance (CDL) that was measured by CV plots at various scan rates (Fig. S5) to determine the electrochemical active surface area (ESA) and measured the reaction resistance (Rct) of the catalysts by EIS plots. As shown in Fig. 3a, a markedly larger capacitance can be observed for Ni-Zn/RGO (15.49 mF cm2) than Ni-Zn catalyst (1.97 mF cm2), and the CDL of Ni-Zn/RGO is almost 8-fold higher compared with Ni-Zn catalyst. Since ESA is proportional to CDL, these results strongly confirm the above analysis that the active sites and surface area of Ni-Zn/RGO are significantly enhanced by combining with RGO. Fig. 3b shows the Nyquist plots and equivalent electrical circuit of Ni-Zn and Ni-Zn/RGO. In the equivalent electrical circuit, Rs and Rct represent the solution resistance and charge transfer resistance, respectively, CPE represents the catalyst/ solution interface. As displayed in Table S1, Ni-Zn/RGO catalyst exhibits a much lower Rct than Ni-Zn catalyst, indicating an enhanced electric conductivity. It is clear that the synergistic effects of the increased active surface area and enhanced electric conductivity are desirable for boosting the electrooxidation activity [43,44]. The catalytic stability of Ni-Zn and Ni-Zn/RGO is critical for evaluating the long-term catalytic ability of hydrazine electrooxidation. As seen in Fig. 4a, Ni-Zn/RGO can still maintain 91.4% of its initial activity after 5000 s. However, Ni-Zn only remains its initial activity of 83.6%. To gain insight into the enhanced stability, XPS analysis of the fresh and used Ni-Zn and Ni-Zn/RGO catalysts is displayed (Fig. S6). Compared with Ni-Zn/RGO, the signal of Ni (II)
for Ni-Zn is remarkably enhanced after the stability experiments, indicating that the surface oxidation of Ni-Zn/RGO can be inhibited during electrocatalysis after the combine with RGO. Thus, the superior stability of Ni-Zn/RGO catalyst may be stem from its inhibited surface oxidation during test. The CV cycles were also employed to evaluate the stability of Ni-Zn and Ni-Zn/RGO catalysts. As seen in Fig. 4b, Ni-Zn/RGO shows an excellent durability for the oxidation of hydrazine, with only 7.4% loss of its initial current density after 1000 cycles, superior to Ni-Zn (14.9%). In order to shed light on the enhanced catalytic activity of Ni-Zn/ RGO, XPS was employed to measure the chemical states of Ni-Zn and Ni-Zn/RGO. As shown in Fig. 5, two chemical states can be seen for both Ni and Zn elements in Ni-Zn and Ni-Zn/RGO. The Ni2p3/2 peak at 856.86 eV and Zn2p3/2 at 1022.26 eV can be safely assigned to Ni (II) and Zn (II) species, respectively, which is associated with the possible surface oxidation in the air. The other Ni2p3/2 peak and Zn2p3/2 peak at 852.46 eV and 1020.66 eV are assigned to Ni (0) and Zn (0), respectively. Ni-Zn/RGO catalyst shows a negative shift of 0.14 eV for the binding energy of Ni (0) and a slight positive shift of 0.28 eV for the binding energy of Zn (0) compared with Ni-Zn catalyst, resulting in easier electron transfer from Zn to Ni for Ni-Zn/RGO. This can be due to the fact that the existence of RGO is beneficial to the electron transfer from Zn to Ni. Hence, back electron donation from Ni to the antibonding orbital of N2H4 may be strengthened, leading to a more destabilization of the interaction of Ni-N2H4 and weaker bond of N-H, thereby higher electrocatalytic activity can be observed for Ni-Zn/RGO than Ni-Zn catalyst. It is noted that porous structure can reduce the contact area
Fig. 3. a) The capacitive current densities of Ni-Zn and Ni-Zn/RGO catalysts as a function of scan rate at open circuit potential, b) Nyquist curves of Ni-Zn and Ni-Zn/RGO catalysts at onset potentials.
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Fig. 4. a) CA plots of Ni-Zn and Ni-Zn/RGO catalysts at a constant-potential of 0.2 V vs RHE in the electrolyte with 3.0 mol L1 N2H4$H2O and 1.0 mol L1 NaOH, and b) cycling stability of Ni-Zn and Ni-Zn/RGO catalysts in the electrolyte with 3.0 mol L1 N2H4$H2O and 1.0 mol L1 NaOH.
Fig. 5. a) Ni 2p3/2, and b) Zn 2p3/2 XPS spectrum of Ni-Zn and Ni-Zn/RGO catalysts.
between the produced N2 and catalysts, resulting in fast removal speed of N2 gas bubbles. Therefore, the excellent electrocatalytic activity and high stability of the catalysts may be stem from the tight binding between the raw materials and substrate, and fast mass transfer for the release of the products on superaerophobic surface [45]. In order to verify the above hypothesis, the adhesionforce measurements of Ni-Zn and Ni-Zn/RGO were performed. As displayed in Fig. 6, both Ni-Zn and Ni-Zn/RGO have small adhesion force to the gas bubbles (20 mN and 10 mN, respectively). Moreover, Ni-Zn/RGO has a smaller value. The above results clearly indicate the superaerophobic nature of Ni-Zn/RGO. The lower adhesive performance of Ni-Zn/RGO can result in faster removal rate, thereby higher electrocatalytic performance can be observed for Ni-Zn/RGO compared with Ni-Zn. 4. Conclusions 3D porous superaerophobic Ni-Zn/RGO was successfully prepared on Ni foam by bubble dynamic template method during electrodeposition. The resulting porous Ni-Zn/RGO exhibits an extreme high electrocatalytic activity with excellent stability towards hydrazine electrooxidation compared to Ni-Zn catalyst. A current density of 469 mA cm2 at 0.30 V vs. RHE, a retention rate of 92.6% after 5000 s and almost 100% selectivity towards the complete hydrazine oxidation can be achieved for Ni-Zn/RGO, which is
Fig. 6. The adhesive force measurements of the gas bubbles on Ni-Zn and Ni-Zn/RGO.
at the top level among the reported electrocatalysts for hydrazine oxidation up to date. The mechanistic reason for the enhanced catalytic performance of Ni-Zn/RGO can be attributed to its active center of Ni electron richer, the large ESA, high electrical
Z. Feng et al. / Journal of Alloys and Compounds 788 (2019) 1240e1245
conductivity and most importantly, the superaerophobic surface induced by combining with RGO and its porous structure. This result may provide a good guideline for design and synthesis catalyst with high performance for hydrazine electrooxidation. Acknowledgments The authors are grateful for the supported by the Certificate of China Postdoctoral Science Foundation Grant (2018M631809), the Fundamental Research Funds for the Central Universities (N172503012), and State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2018DX03). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.03.007. References [1] L. Carrette, K.A. Friedrich, U. Stimming, Fuel cells: principles, types, fuels, and applications, ChemPhysChem 4 (2000) 162e193. [2] L. An, T.S. Zhao, Y.S. Li, Carbon-neutral sustainable energy technology: direct ethanol fuel cells, Renew. Sustain. Energy Rev. 50 (2015) 1462e1468. [3] X.L. Ji, K.T. Lee, R. Holden, L. Zhang, J.J. Zhang, G.A. Botton, M. Couillard, L.F. Nazar, Nanocrystalline intermetallics on mesoporous carbon for direct formic acid fuel cell anodes, Nat. Chem. 2 (2010) 286e293. [4] G. Rostamikia, M.J. Janik, Direct borohydride oxidation: mechanism determination and design of alloy catalysts guided by density functional theory, Energy Environ. Sci. 3 (2010) 1262e1274. [5] A. Serov, C. Kwak, Direct hydrazine fuel cells: a review, Appl. Catal. B Environ. 98 (2010) 1e9. [6] A. Serov, M. Padilla, A.J. Roy, P. Atanassov, T. Sakamoto, K. Asazawa, H. Tanaka, Anode catalysts for direct hydrazine fuel cells: from laboratory test to an electric vehicle, Angew. Chem. Int. Ed. 53 (2014) 10336e10339. [7] J.R. Varcoe, P. Atanassov, D.R. Dekel, A.M. Herring, M.A. Hickner, P.A. Kohl, A.R. Kucernak, W.E. Mustain, K. Nijmeijer, K. Scott, T. Xu, L. Zhuang, Anionexchange membranes in electrochemical energy systems, Energy Environ. Sci. 7 (2014) 3135e3191. [8] J.Y. Zhang, H. Wang, Y. Tian, Y. Yan, Q. Xue, T. He, H. Liu, C. Wang, Y. Chen, B.Y. Xia, Anodic hydrazine oxidation assists energy-efficient hydrogen evolution over a bifunctional cobalt perselenide nanosheet electrode, Angew. Chem. Int. Ed. 130 (2018) 7775e7779. [9] L. Zhang, D. Lu, Y. Chen, Y. Tang, T. Lu, Facile synthesis of Pd-Co-P ternary alloy network nanostructures and their enhanced electrocatalytic activity towards hydrazine oxidation, J. Mater. Chem. A 2 (2014) 1252e1256. [10] Y. Liang, Y. Zhou, J. Ma, J. Zhao, Y. Chen, Y. Tang, T. Lu, Preparation of highly dispersed and ultrafine Pd/C catalyst and its electrocatalytic performance for hydrazine electrooxidation, Appl. Catal. B Environ. 103 (2011) 388e396. [11] V. Rosca, M.T.M. Koper, Electrocatalytic oxidation of hydrazine on platinum electrodes in alkaline solutions, Electrochim. Acta 53 (2008) 5199e5205. [12] S.S. Narwade, B.B. Mulik, S.M. Mali, B.R. Sathe, Silver nanoparticles sensitized C60 (Ag@C60) as efficient electrocatalysts for hydrazine oxidation: implication for hydrogen generation reaction, Appl. Surf. Sci. 396 (2017) 939e944. [13] L. Zhang, W.X. Niu, W.Y. Gao, L.M. Qi, J.M. Zhao, M. Xu, G.B. Xu, Facetdependent electrocatalytic activities of Pd nanocrystals toward the electrooxidation of hydrazine, Electrochem. Commun. 37 (2013) 57e60. [14] Y. Kuang, G. Feng, P.S. Li, Y. Bi, Y.M. Li, X.M. Sun, Single-crystalline ultrathin nickel nanosheets array from in situ topotactic reduction for active and stable electrocatalysis, Angew. Chem. Int. Ed. 55 (2016) 693e697. [15] R. Liu, X. Jiang, F. Guo, N.N. Shi, J.L. Yin, G.L. Wang, D.X. Cao, Carbon fiber cloth supported micro-and nano-structured Co as the electrode for hydrazine oxidation in alkaline media, Electrochim. Acta 94 (2013) 214e218. [16] H.C. Gao, Y.X. Wang, F. Xiao, C.B. Ching, H.W. Duan, Growth of copper nanocubes on graphene paper as free-standing electrodes for direct hydrazine fuel cells, J. Phys. Chem. C 116 (2012) 7719e7725. [17] Q. Xue, Y. Ding, Y. Xue, F. Li, P. Chen, Y. Chen, 3D nitrogen-doped graphene aerogels as efficient electrocatalyst for the oxygen reduction reaction, Carbon 139 (2018) 137e144. [18] X. Xiao, T. Wang, J. Bai, F. Li, T. Ma, Y. Chen, Enhancing the selectivity of H2O2 electrogeneration by steric hindrance effect, ACS Appl. Mater. Interfaces 10 (2018) 42534e42541. [19] N.V. Rees, R.G. Compton, Carbon-free energy: a review of ammonia-and hydrazine-based electrochemical fuel cells, Energy Environ. Sci. 4 (2011) 1255e1260. [20] J. Sanabria-Chinchilla, K. Asazawa, T. Sakamoto, K. Yamada, H. Tanaka, P. Strasser, Noble metal-free hydrazine fuel cell catalysts: EPOC effect in competing chemical and electrochemical reaction pathways, J. Am. Chem. Soc.
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