Alkaline direct alcohol fuel cells (ADAFCs) have high volumetric energy density, are easy to handle, store, transport and use non-noble metals as catalysts [1]. Nickel based materials are catalyst activity for alcohol oxidation [2, 3] since they present a high surface area and improved intrinsic catalytic properties [4].

Traditionally, electrodeposition of nanoparticles is carried out under potentiostatic control [5]. In contrast, Videa et al. [6] showed that galvanostatic control leads to smaller particles and narrower size distributions. This work investigates the catalytic activity of galvansotatically deposited nickel nanoparticles towards methanol electro-oxidation in alkaline media.

A gold working electrode was cycled in 50 mM H₂SO₄. To electrodeposit nickel nanoparticles, a two second galvanostatic pulse was applied in a bath containing 50 mM NiSO₄ and 500 mM NaCl as support electrolyte. Figure1a shows the transients of the electrode potential during the application of pulses with different current density. After an initial drop, a minimum value of −1.35 V vs. Hg|Hg₂SO₄ (MMSE) was reached; it then stabilized around −1.2 V vs. MMSE. Experiments performed with current densities higher than 4.0 mAcm^-2 showed a second potential drop, reaching −1.8 V vs. MMSE. Each stage represents a different electrochemical process, complying with the current demanded by the galvanostat. Under potentiostatic conditions, electrodeposition of nickel on gold at low overpotentials occurs in a monolayer growth regime, while at higher overpotentials follows a multilayer growth [7], explaining the profile of the potential-time curves; once nickel covers the gold surface, the potential drops to allow nickel on nickel nucleation and growth.

The deposits were activated by cycling in 1 M KOH, to generate a NiOOH layer. In the typical activation voltammogram shown in Figure1b, the redox peaks I_af and I_cb correspond to the formation of NiOOH from the following redox process:

Ni(OH)₂ + OH^- ==> NiOOH + H₂O + e^-

After activation the electrode was cycled in 1 M KOH + 0.5 M methanol. The voltammogram (Figure1c) exhibited two additional anodic peaks, II_af and II_ab, corresponding to the oxidation of methanol on fresh NiOOH and NiOOH reactivated by desorption of poisonous species [8].

Following Chen et al. [9] the surface concentration of NiOOH was estimated from the charge under peak I_af. The electrocatalytic intensity was calculated adding the current of peaks II_af and II_ab. This parameter, introduced by Ding et al. [10], quantifies the electrocatalytic ability of the material. Figure1d depicts the dependence of the surface NiOOH concentration (Γ) and EI on the applied current density. The maximum value for Γ was achieved at 2.2 mAcm^-2. Taking into account that current efficiency increases at lower overpotentials [7], the deposited nickel should increase as the applied current increases. Therefore, it is reasonable to infer that the surface area obtained has is the largest. On the other hand, the deposit obtained at 8.8 mAcm^-2 reached the highest EI value, implying particles intrinsically more active than those obtained at 2.2 mAcm^-2. The material deposited in the monolayer regime has larger surface area but is less active than the nickel multilayer deposit.

All nickel nanostructured deposits obtained by galvanostatic electrodeposition showed good electrocatalytic activity towards methanol electro-oxidation, comparable to the results obtained by more complex deposition methods [3,9].

Figure 1. a) Potential transients during galvanostatic deposition of Ni (50 mM NiSO4 + 500mM NaCl) b) Cyclic Voltammogram of Ni deposit in 1 M KOH at 50 mVs^-1. c) Cyclic Voltammogram of Ni deposit in 0.5 M Methanol in 1 M KOH at 50 mVs^-1. d) Dependence of surface NiOOH concentration (Γ) and electrocatalytic intensity (EI) on the current deposition.

References

1. I. Ozoemena, RSC Adv., 6, 89523 (2016).
2. Fleischmann, K. Korinek and D. Pletcher, J.Electroanal. Chem. Interfacial Electrochem. 31, 39 (1971).
3. Wang, Q. Zhao, H. Hou, Y. Wu, W. Yu, X. Ji, L. Shao, RSC Adv., 7, 14152 (2017).
4. B. Raoof, R. Ojani, S. R. Hosseini, S. Afr. J. Chem., 66, 47 (2013).
5. Ustarroz, X. Ke, A. Hubin, S. Bals, H. Terryn J. Phys. Chem. C 116, 2322 (2012).
6. T. Martínez, G. Zavala, and M. Videa, J. Mex. Chem. Soc., 53, 7 (2009).
7. Lachenwitzer, O. M. Magnussen, J. Phys. Chem. B 104, 7424 (2000).
8. Danaee, M. Jafariana, F. Forouzandeh, F. Gobal, M.G. Mahjani, Int. J. Hydrog. Energy, 33, 4369 (2008).
9. Chen, S. D. Minteer, J. Power Sources, 284, 27 (2015).
10. Ding, Y. Zhao, L. Liu, Y. Cao, Q. Wang, H. Gu, X. Yan, Z. Guo, Int. J. Hydrog. Energy, 39, 17622 (2014).