Effect of Non-Specific Ion Adsorption and Parallel Faradaic Reactions on the Nucleation of H2 Nanobubbles

Abstract

We report voltammetric experiments of single H2 nanobubble nucleation at Pt nanodisk electrodes (radius < 100 nm) in the presence of highly-charged species: Ru(NH3)6 3+, La3+, Fe(CN)6 3−, and Fe(CN)6 4−. The highly charged cations are expected to be non-specifically adsorb at electrode potentials sufficiently negative for the H2 evolution reaction (HER) to proceed, thus affecting the thermodynamics and kinetics of bubble formation through alterations in interfacial solution structure. Conversely, anions are not expected to be non-specifically adsorbed during H2 evolution. We observed a more highly complex behavior than expected: (i) the presence of the two redox ions that are simultaneously reduced at potentials where HER occurs (i.e., Ru(NH3)6 3+ and Fe(CN)6 3−) results in a large overpotential for HER and bubble nucleation as well as a decrease in the H2 supersaturation required for nucleation; (ii) the presence of the electroinactive cation, La3+, results in a decrease in H2 supersaturation but no increase in HER overpotential; and (iii) Fe(CN)6 4− has only a minor effect on both HER kinetics and H2 supersaturation. We infer that the driving force of nucleation decreases in the presence of highly-charged ions. The results also suggest that the HER rate is significantly decreased when parallel redox reactions are operative.