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) given1–3, the symbols η2 or ηd (termed1,3 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 established1,3–5 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 observation1–3 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.