Unravelling Hydrogen Adsorption Kinetics on Ir(111) Electrode in Acid Solutions by Impedance Spectroscopy

The kinetics for hydrogen (H) adsorption on Ir(111) electrode has been studied in both HClO _4 and H _2 SO _4 solutions by impedance spectroscopy. In HClO _4 , the adsorption rate for H adsorption on Ir(111) increases from 1.74 \times 10 ^-8 mol \cdot cm ^-2 \cdot s ^-1 to 3.47 \times 10 ^-7 mol \cdot cm ^-2 \cdot s ^-1 with the decrease of the applied potential from 0.2 V to 0.1 V (vs. RHE), which is ca. one to two orders of magnitude slower than that on Pt(111) under otherwise identical condition. This is explained by the stronger binding of water to Ir(111), which needs a higher barrier to reorient during the under potential deposition of H from hydronium within the hydrogen bonded water network. In H _2 SO _4 , the adsorption potential is ca. 200 mV negatively shifted, accompanied by a decrease of adsorption rate by up to one order of magnitude, which is explained by the hindrance of the strongly adsorbed sulfate/bisulfate on Ir(111). Our results demonstrate that under electrochemical environment, H adsorption is strongly affected by the accompanying displacement and reorientation of water molecules that initially stay close to the electrode surface.