Effect of ion implantation on the oxygen overpotential of Ni Anodes

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 (1018 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 × 1016 Ag+ cm−2. Furthermore, RBS measurements show that prolonged electrolysis at higher current density (24–28 h at 1 A/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.