Effect of ion implantation on the oxygen overpotential of Ni anodes

Abstract

This work investigates the use of ion implantation for decreasing the oxygen-overpotential of nickel anodes. It is part of a search for improved electrocatalysts to increase the energy efficiency of the H2O electrolysis process for producing H2 gas. A series of Ni electrodes were implanted at room temperature with various doses of 50 keV Ag+, Li+, He+, or Kr+ ions. Polarization measurements were then made in a suitable electrolysis cell over a wide range of current densities, using aqueous KOH solution (30%) at 80 °C as electrolyte. In the case of Ag+ implants, Rutherford backscattering (RBS) measurements were performed before and after electrolysis in order to monitor the amount and depth distribution of the Ag atoms. For Li+ or He+ implantation, we observe a negligible change in the measured polarization curves. For high dose Kr+ implants (10^18 ions cm^(-2)) the Ni electrode exhibits an increase in overpotential, indicating that excessive damage and/or sputtering of the surface causes some deterioration in electrode behaviour. For Ag+ implants, on the other hand, we observe a large (20–40%) reduction in the total overpotential at implant doses of 0.3–4*10^16 Ag+ cm^(-2). Furthermore, RBS measurements show that prolonged electrolysis at higher current density (24–28 h at 1A/cm^(-2)) produces only a small loss of Ag and shifts its depth profile to significantly larger depths. Supplementary nuclear microanalyses, using the 16O(d, p )17O reaction, show that the shift in Ag profile is correlated with the growth of an anodic nickel oxide (+carbon) layer during electrolysis. In one set of runs, the Ni electrodes were thermally oxidized before Ag+ implantation in order to form an ~400 Å layer of NiO at the surface. In this case, we observe a somewhat smaller reduction in overpotential following Ag+ implantation; furthermore, a large loss of Ag into the electrolyte occurs during the subsequent electrolysis.