The slow kinetics of the water oxidation half-reaction limit the efficiency of current solar water splitting technologies for hydrogen fuel generation. We have studied a variety of electrocatalysts for the oxygen evolution reaction (OER) in a thin film geometry, enabling simple and direct comparison of the activity of different catalyst materials. NiFeO was found to be the most active water oxidation catalyst in basic media, passing 10 mA cm at an overpotential of 336 mV with a Tafel slope of 30 mV dec and intrinsic OER activity roughly an order of magnitude higher than IrO control films and similar to or better than the best known OER catalysts in basic media.
The high activity is attributed to the formation of a layered NiFeOOH oxyhydroxide species with nearly every Ni atom electrochemically active. These thin-film catalysts are ideal for integration with light-absorbing photoanodes, and we have measured their optical properties under operating conditions and shown using a simple model that optical absorption by the catalyst drives optimal film thicknesses to the ultra-thin sub-nanometer range. Fe plays a critical, but yet not understood role, in enhancing the activity of the NiOOH catalyst.
We report electrochemical, electrical, and surface spectroscopic measurements of Ni and mixed Ni-Fe hydroxides to investigate the changes in electronic properties, OER activity, and structure of films as a result of Fe inclusion. Through-film conductivity measurements show little change with Fe addition, indicating that an increase in film conductivity is not the dominant mechanism for enhanced activity. The addition of Fe shifts Ni redox peaks to higher potentials and significantly increases OER activity. Measurements of activity as a function of film thickness indicate that Fe exerts an electron withdrawing effect on Ni centers, similar to that observed for noble metal electrodes. Cyclic voltammetry of Ni(OH)/NiOOH kept rigorously Fe-free shows unique Ni redox peaks and no significant OER current until >400 mV overpotential, suggesting that most previous reports of highly-active Ni(OH)-based OER catalysts are affected by Fe impurities.
We show that under rigorously Fe-free conditions, the β-NiOOH structure is less active for OER, and that the increase in activity previously reported with aging in KOH electrolyte is rather due to incorporation of Fe impurities. These results have significant implications for the design and study of Ni(OH)-based OER electrocatalysts and batteries.