Electrochemical Investigation of the Effects of Guest Metals in Nickel Hydroxide Films on Oxygen Evolution Reaction Catalysis

Abstract

The sun is the largest and cheapest green energy source worldwide with one hour of solar energy outweighing the global energy consumption per year. Thus, the generation of renewable energy using nanomaterials is a rising topic in science today especially via solar water splitting at nanostructures coupled with co-catalysts. This MS thesis focuses on investigating oxygen evolution reaction (OER) catalysts from earth abundant materials and the role of guest metal inclusion. The aim of this project is to investigate the effect of guest metals, Fe and Al, on Ni-based oxide catalysts for oxygen evolution reactions as a function of i) mode of inclusion, ii) electrolyte and iii) the presence and absence of Fe in solution. Thin Ni-oxo/hydroxo films have shown significant activity as non-noble metal OER electrocatalysts. A promoting effect has been reported with the addition of certain supplementary metals particularly Fe. Nonetheless, the role of guest metals in solution has yet to be fully and clearly understood. Earlier in this laboratory, Ni-oxo/hydroxo films in borate, KBi, were investigated in the presence and absence of Fe: as-deposited and after anodic conditioning. Differences in OER activity between co-electrodeposition and inclusion post-deposition of Fe led to the hypothesis that surface modification with Fe causes an increase in OER activity rather than its bulk inclusion. Furthermore, it was observed earlier in this laboratory that Al in solution inhibits OER. In this MS thesis, we investigate the electrochemistry and OER activity at Ni-oxohydroxo, Ni(Fe) and Ni(Al)-oxohydroxo films in KBi, KOH purified from Fe and KOH with traces of Fe. OER activity of NiOxHy and co-deposited NiAlOxHy and NiFeOxHy was studied in KOH electrolytes which were unpurified or purified from Fe impurities to eliminate any trace Fe that would modify the catalyst surface. The effect of Al and Fe added to these electrolytes was also studied. The effects of the guest metals on the redox behavior of Ni-based oxides on OER electrocatalytic activity and stability were studied using cyclic voltammetry, Tafel slope measurements, current-overpotential and turnover frequency calculations. The films were characterized using SEM coupled with EDX to examine the surface structure and morphology of the catalysts. The complete removal of Fe from the electrolyte resulted in low OER activity at both Ni and Ni(Fe) co-deposited films and thus supports our hypothesis that bulk Fe does not promote OER. The presence of Fe in 1 M KOH showed an increase in OER activity of NiOxHy, while the addition of Al to 1 M KBi inhibited OER activity. The OER activity was not sustained at Fe-activated NiOxHy in Fe-free electrolyte indicated that the presence of Fe is needed for sustainability of active sites. A deactivation mechanism consistent with the electrochemical results was proposed for OER activity. The addition of Al to 1 M KOH and 1 M Fe-free KOH did not cause an effect on OER but affected the redox behavior of the catalyst. The co-deposition of Al with Ni showed no significant effect on Tafel slopes but a varied effect on TOF at different thicknesses in the tested electrolytes.