Among the different types of fuel cells, alkaline direct alcohol fuel cells (ADAFC) have attracted significant attention. Compared to hydrogen fuel cells, ADAFC are easy to handle, store and transport and have high volumetric energy density, which makes them strong candidates to power portable devices and consumer electronics [1]. The alkaline medium of ADAFCs favors faster reaction kinetics, low over-potentials and non-noble metals can be used as catalysts [2].

Nickel based materials have shown to be good catalysts for alcohol oxidation reaction in alkaline media [3, 4]. Furthermore, studies on NiPt nanoparticles suggest that the presence of Ni enhances the catalytic properties of Pt [5,6]. The present work proposes a methodology of galvanostatic electrodeposition of bimetallic NiPt nanoparticles on gold. Traditionally, nanoparticles are electrodeposited by double pulse potential method; however, previous works in our group proved that galvanostatic control leads to the formation of smaller particles and narrower size distribution [7].

Electrodepositions were carried out applying a two second current pulse in a platting bath containing 50 mM NiSO₄ + 1.5 mM K₂PtCl₄ + 500mM NaCl at pH 2.0. Figure1a depicts the electrode potential response to pulses of different current densities. Increasing the current density pulses results in shorter times for each of the three stages transition present. During a galvanostatic experiment, the working electrode potential adjusts to a value that ensures a charge transfer rate for an electrochemical process whose kinetics is consistent with the current demanded by the experiment. Thus, it can be inferred that each potential drop is associated to a different electrochemical process.

Cycling at 1M KOH generates a layer of NiOOH, which is the active species on the electro-oxidation of methanol on Ni in alkaline media. The catalytic properties of the deposited material were tested by cyclic voltammetry in 1M KOH + 0.5 M methanol. A typical voltammogram is shown in Figure1b. The forward scan exhibits three anodic peaks: I_af, II_af and III_af. The first one corresponds to the oxidation of methanol on Pt, while the second and third peaks are caused by the formation of NiOOH and oxidation of methanol on NiOOH, respectively. During the backward scan Ni and Pt surfaces regenerate their active sites by desorption of poisonous species [8, 9] generating two anodic peaks (I_ab and III_ab). Following Fetohi et al. [6], the I_af/ I_ab ratio and the electrocatalytic intensity (EI= I_af+ I_ab) were calculated. Larger values of the I_af/ I_ab are desired since this indicates that the catalyst has a good tolerance to poisoning. As shown on Figure1c, better poisoning tolerances are achieved when the deposits are produced at higher current densities. It is noteworthy that the I_af/ I_ab obtained for the electrodeposited materials are larger than 15, while for electrodeposited Pt this ratio is 1.8. The EI parameter was introduced by Ding et al. [5] to evaluate electrocatalytic ability of the deposited material. Figure1d shows how the best EI is achieved when the deposit is obtained by applying a pulse of 4.0 mAcm^-2. Applying the same criterion to peaks III_af and III_ab suggest the highest catalytic activity of NiOOH is also achieved for the same deposit. Galvanostatic control resulted on the deposition of NiPt particles with high tolerance to CO poisoning. Electrocatalytic activity of the deposits can be tuned by adjusting the intensity of the current pulse.

Fig. 1 a) Potential transients during galvanostatic deposition of NiPt from 50 mM NiSO₄ + 1.5 mM K₂PtCl₄ +500mM NaCl with a pH of 2.0. b) Cyclic Voltammogram of NiPt deposit in 0.5 M Methanol in 1 M KOH at 50 mVs^-1. c) Dependence of the I_af/ I_ab ratio and d) Dependence of the electrocatalytic intensity on the deposition current density.

References

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