Sub-monolayers of iridium electrodeposited on Ti2AlC substrate as catalysts for hydrogen evolution reaction in sulfuric acid solution

The hydrogen evolution reaction (HER) was investigated at sub-monolayers of iridium electrodeposited on Ti2AlC substrate. The lowest amount of electrodeposited iridium was 3 close-packed (111) monolayers (3 ML), while the highest one was 22 ML (3, 5, 10, 15 and 22 ML). The lowest and the highest amounts of iridium were electrodeposited by linear sweep voltammetry (LSV), while the other three samples were electrodeposited by controlled potential coulometry, from the solution containing 1 mM, or 3 mM K3IrCl6 + 0.5 M Na2SO4 (pH 6.2) at 70 °C. The HER was investigated by polarization and electrochemical impedance spectroscopy (EIS) measurements. Polarization curves for iridium sub-monolayers equal, or higher than 6 ML showed low Tafel slope of -14 to -16 mV dec-1 up to about -0.1 A cm-2, while at higher current densities the Tafel slopes increased, varying between -40 and -72 mV dec-1. The highest value of exchange current density (jo) was obtained for 6 ML of electrodeposited iridium, being -27.89 A g-1. The overpotential at j = -0.3 A cm-2 could be determined for samples containing 15 ML and 22 ML of iridium, being 82 mV.