Enhancement of Hydrogen Insertion into Carbon Interlayers by Surface Catalytic Poisoning

The well-known phenomenon that the adsorption of catalytic poisons can promote H electrosorption into metal electrodes is demonstrated on carbon electrodes for the first time. The use of a small amount of thiourea in the KOH electrolyte as a catalytic poison can substantially enhance the H insertion into the carbon interlayer. The mechanism of such a promotion of H insertion by the catalytic poison is proposed. The adsorbed thiourea interferes with the H + H recombination, thus the competitive hydrogen evolution step is discriminated against in favor of H insertion into the carbon interlayer. The increase of current efficiency for H insertion is the supporting evidence for such a mechanism. ac impedance and equivalent circuits are also used to investigate the surface phenomena and proton diffusion. Evidently, although the thiourea stays on the surface during reduction, the electrosorbed thiourea may become oxidized and desorbed from the surface at the oxidation overpotential. It is demonstrated that the strong surface adsorption of the poison species could significantly reduce self-discharge or leakage of hydrogen at the open-circuit voltage and favorably impact the kinetics for hydrogen oxidation.