Kinetics of hydrogen evolution reaction on some noble metal coated platinum electrode surfaces in aqueous acetonitrile and dimethylformamide solutions by cyclic voltammetry

Abstract

Electrocatalytic activities of some noble metal [M=Pt, Pd, Rh, It, Ru and Au] coated platinum electrodes, {Pt}M, have been studied for the hydrogen evolution reaction (HER) from 0.1 m<em>M</em> aqueous perchloric acid solutions in some aqueous acetonitrile (ACN) and dimethylformamide (DMF) mixtures by determining the rate constants (<em>k</em>_s) of the reaction by cyclic voltammetry under varying sweep rates [2300 mVs^-1] in the cathodic potential range of 0-1000 mV. As on bright Pt reported earlier, the cyclic voltammograms on all the metallized surfaces, save (Pt}Rh, are characterized by single cathodic and the corresponding anodic peaks and are found to be quasi-reversible in nature. The rate constant (<em>k</em>_s) values based on apparent areas of the electrodes, have been derived by using Shain and Nicholson's formulations. As on bright Pt, the <em>k</em>_s values on all these surfaces are found to decrease with increase in the content of the non-aqueous components and that in 50 wt% DMF is less than that in 50 wt% ACN, as is expected from increased protophilicity of DMF-water solutions as compared to that of ACN-water mixtures. The relative rate constant data over different metallized surfaces show that (Pt)Pt or even bright Pt surface is more or less a better electrocatalyst as compared to others, save rhodized platinum surface (Pt)Rh. Notably, the observed sharply increased cathodic current and the absence of peak within the potential range studied is seemingly indicative of the possible capacity of (Pt)Rh surface for acting as a reversible hydrogen electrode (RHE). In fact, this contention has also been supported by the observed equilibrium emf behaviour of some galvanic cells comprising (Pt)Rh, H₂ (g, 1 atm) and Ag - AgX (X = Cl, Br and I) electrodes and [HCl], [benzoic acid (HBz), KBz and KX] and [KOH and KX] as electrolytes in 50 wt %ACN and 50 wt% DMF-water mixtures.