Kinetics of Atomic Hydrogen Evolution and Hydrogen Permeability of Ag–Pd Alloys in an Alkaline Medium

Abstract

Homogeneous Ag-Pd alloys are effective catalysts for the cathodic evolution of hydrogen. They are characterised by high mechanical strength and are less susceptible to hydrogen embrittlement than metallic palladium. The aim of this study was to determine the kinetics of hydrogen evolution on palladium and its homogeneous alloys with silver in an alkaline aqueous solution, and to investigate their hydrogen permeability. The behaviour of Pd and Ag-Pd alloys (XPd = 15–80 at%) in a deaerated 0.1 M KOH aqueous solution was studied using cyclic voltammetry and double step anodic-cathodic chronoamperometry. Cyclic voltammograms for Pd and Ag80Pd were similar. However, when a small amount of silver (≤ 20 at%) was introduced into palladium, the ionization rate of hydrogen decreased.
A further increase in the concentration of silver in the alloy resulted in a complete suppression of the ionisation process. For Ag–Pd alloys with palladium concentrations below 30 at%, the voltammograms did not show any hydrogen ionization peaks. The dependencies of the peak ionisation current on the potential scan rate for all the studied alloys were linear and were extrapolated to the origin of the coordinates, which indicated that the process was complicated by solid-phase diffusion. The slopes of the lines for Ag60Pd and Ag50Pd alloys were higher than the slope for the Ag80Pd alloy, which indicated the presence of silver oxides on the surface. For all the studied electrodes the dependence of the peak current potential on the
potential scan rate log linearly increased, which means that the electrochemical stage of atomic hydrogen ionization, which is complicated by solid-phase diffusion, is irreversible. The hydrogen permeability parameters of the alloys were calculated using potentiostatic cathodic and anodic current transients at different time intervals (1-10 sec.). The longer the hydrogenation time, the lower the current amplitudes on cathodic and anodic branches of the chronoamperograms. The hydrogen permeability parameters were calculated based on cathodic and anodic current transients linearised in the corresponding criteria coordinates, using the results of theoretical modelling of hydrogen injection and extraction for
semi-infi nite thickness electrodes. The phase-boundary exchange constant and the ionisation rate constant of atomic hydrogen were maximum for the alloy with the concentration of palladium of 80 at%. The hydrogen extraction rate constant changed linearly with the decrease in the concentration of palladium. The study determined that the values of hydrogen permeability for Ag-Pd alloys in alkaline solutions are lower than in acidic ones. The determining stage of the hydrogen evolution reaction on Ag–Pd alloys (XPd ≤ 40 at%) in a 0.1M KOH solution is the electrochemical stage of atomic hydrogen ionization complicated by its diffusion in the solid phase. The hydrogen permeability parameters in Ag-Pd alloys are maximum, when the concentration of palladium is ~80 at%. Therefore, such alloys can be used as materials for effi cient hydrogen purifi cation and storage

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