  This paper introduces the combination of an advanced double-layer model with
electrochemical kinetics to explain electrolyte effects on the alkaline
hydrogen evolution reaction. It is known from experimental studies that the
alkaline hydrogen evolution current shows a strong dependence on the
concentration and identity of cations in the electrolyte, but is independent of
pH. To explain these effects, we formulate the faradaic current in terms of the
electric potential in the double layer, which is calculated using a mean-field
model that takes into account the cation and anion sizes as well as the
electric dipole moment of water molecules. We propose that the Volmer step
consists of two activated processes: a water reduction sub-step, and a sub-step
in which a hydroxide ion is transferred away from the reaction plane through
the double layer. Either of these sub-steps may limit the rate. The proposed
models for these sub-steps qualitatively explain experimental observations,
including cation effects, pH-independence, and the trend reversal between gold
and platinum electrodes. We also assess the quantitative accuracy of the
water-reduction-limited current model; we suggest that the predicted functional
relationship is valid as long as the hydrogen bonding structure of water near
the electrode is sufficiently maintained.
