Galvanostatically Deposited Ptni Thin-Films As Electrocatalytists for the Hydrogen Evolution Reaction

Abstract

Hydrogen renders high energy density without any direct carbon emissions, which makes it an ideal energy carrier [1]. PtNi nanomaterials exhibit high catalytic activity for hydrogen production through the hydrogen evolution reaction (HER), in some cases superior to bare Pt [2] and with lower cost due to the replacement with Ni. Efforts devoted to the synthesis of PtNi nanomaterials, most of them based on solvothermal methods [3,4], involve several steps and reaction times amounting to hours. Electrochemical deposition is a fast and simple method that allows high control over the morphology of the deposits. Previously, our group used single-pulse galvanostatic electrodeposition to generate Ni thin-layers on gold electrodes; the Ni deposits exhibited high catalytic activity for methanol electro-oxidation [5]. In this work we investigate the catalytic activity of galvanostatically generated PtNi deposits on gold towards HER in alkaline media.After mechanical polishing and electrochemical cleaning of a Au electrode, the electrodeposition was carried using a 1.5 mM K2[PtCl4] + 50 mM NiSO4 + 500 mM NaCl plating solution saturated with N2. Figure 1a shows the typical E – t transient response to a two seconds current pulse. Its stair-like shape hints the presence of at least four electrochemical reactions, identified as regions I to IV. Based on the Nernst potentials (horizontal dashed lines) and the processes reported for nickel by Salinas et al. [5], regions I and II can be assigned to Pt deposition, Ni deposition and hydrogen evolution from H+ reduction to region III, and hydrogen evolution form water reduction to region IV. The effect of the applied current density on the potential transients is shown in Fig. 1b, higher values lead to shorter durations of each stage and lower potential values. The presence of Ni and Pt was confirmed by cyclic voltammetry in 1 M KOH and 50 mM H2SO4, respectively, but no particles could be detected by SEM, which suggest that thin films were formed. Liu et al. [6] and Wang et al. [7] reported the electrodeposition of Pt and Ni thin films which were self-terminated by potential pulses negative enough to favor the formation and adsorption of species blocking further growth of the deposits. Since during the galvanostatic deposition the electrode reaches potential values negative to the self-termination threshold, it its likely that a thin layered PtNi films were formed.The activity of the PtNi deposits towards HER was evaluated by linear sweep voltammetry in 1 M KOH. The overpotentials required to attain a current of 10 mA cm-2 (η 10) were 64 mV for the deposit obtained at 8 mA cm-2, while the highest overpotential, 77 mV, was obtained at 2 mA cm-2. The η 10 of the PtNi/Au deposits falls within the range reported for similar materials: 39 mV[4] to 186 mV [8]. However, the specific current at 38 mV of the most active material obtained, 6390 A gPt -1, is far larger that the one of the most active material reported in the literature of 1740 A gPt -1 [4]. This may be attributed to an optimal Pt/Ni ratio and a thin-layered deposit, achieved by the complex potential trajectories generated by galvanostatic electrodeposition. Tafel plots (Fig. 1c) exhibited two linear regions, with values independent of applied current density of deposition: 35 mV dec−1 for the first slope and 100 mV dec−1 for the second one. This suggests that HER follows the same pathway on all the deposits and the Heyrovsky reaction is the limiting step [8].Figure1 a) Typical potential transient recorded during the galvanostatic deposition of PtNi. b) Potential transients recorded during a galvanostatic deposition of PtNi on a clean Au electrode at different j dep. c) Tafel plots for the deposits produced at different current densities.