Hydrogen Evolution on Single Crystal GaAs Electrodes

Abstract

Hydrogen evolution from at 23°C on the Ga (111) and As faces of single crystals of has been studied using current‐potential curves, differential capacity measurements and anodic charging curves. The irreproducible results frequently found are shown to be the result of surface corrosion, and it is shown that great care must be taken to preserve the crystal surface. A freshly etched and washed electrode can be taken from −1.2v to −0.3v (vs. RHE), and reproducible hydrogen evolution measurements can be obtained at both Ga and the As faces. Current‐potential curves on the Ga face show two regions: positive to ∼ −0.6v (vs. RHE) the current is almost independent of the potential; negative to ∼ −0.6v, Tafel behavior is found with a slope of 180–200 mv/decade; n‐type and p‐type electrodes show similar behavior, and there are no photoeffects.Differential capacities on the Ga face indicate substantial space charge effects at potentials positive to −0.7v, but suggest degeneracy at more negative potentials. A substantial frequency dispersion is found in the range 0.5–20 kHz and is shown to originate from surface states. The variation of the surface state contribution with potential suggests that some of them arise from adsorbed hydrogen. After allowance for space charge effects, made from the differential capacity data, the Tafel slope for the reaction in the region above −0.6v is shown to be .Anodic charging curves show a change from high to low hydrogen coverage at ∼0.7v.It is suggested that the reaction path is discharge followed by recombination where discharge is rate limiting over the range investigated, −0.4 to −1.2v. This mechanism is quantitatively consistent with all the experimental data.