Hydrogen Overvoltage at Cathodes of 77 per cent Palladium and 23 per cent Rhodium

Abstract

CATHODIZING an equilibrium Pd/H/H₂ electrode (25° C.: 1 atm. hydrogen: zero volt potential with respect to a Pt/H/H₂ reference in the same solution) further increases its hydrogen content (H/Pd, atomic ratio) and its corresponding hydride vapour pressure p. These increases are reflected as a component of overvoltage (volts) given^1–3, the symbols η₂ or η_d (termed^1,3 the diffusion or hydride overvoltage) and related to p by η₂ = 0.029 log p. The large capacity of palladium for hydrogen has enabled η₂ to be obtained by extrapolation, back to zero time, of the increase of electrode potential, E, which follows interruption of cathodization, using established relationships between E, p and H/Pd (refs. 1–3). From such measurements it has been established^1,3–5 for surfaces active for rapid equilibration with hydrogen molecules dissolved in solution that at current densities i less than 0.040 amp. cm.⁻² the retarding kinetic step maintaining H/Pd in excess of its equilibrium value (1 atm. hydrogen: 25° C.) is the slow diffusion from the cathode of hydrogen molecules formed during electrolysis. Under these conditions, η₂ is related to i by: where under conditions of vigorous stirring i₀ ∼ 3 m.amp. cm.⁻². The observation^1–3 that η₂ attains an almost constant value for i greater than 0.040 amp. cm.⁻², when hydrogen is evolved as bubbles, has been associated¹ with either critical supersaturation of the diffusion layer with dissolved molecules or saturation of chemisorption sites. Experiments reported here appear to eliminate the latter possibility.