Thickness-Dependent Oxygen Evolution Reaction Performance of Atomic Layer Deposition Cobalt Oxide Nickel-Oxide Heterostacks

Heterostacks of nickel oxide (NiO) and cobalt oxide (Co3O4) are promising, nonprecious oxygen evolution reaction (OER) catalysts. NiO is particularly notable for its ability to form the active (oxy)hydroxide phase and its ability to scavenge highly active electrolyte-based iron, while Co3O4 is renowned for its high activity and stability. However, the nanoscale design of the investigated structures, typically composites and nanohybrids, hampers their optimization toward efficient OER, as it is challenging to discern contributions from Co3O4, NiO, and their interface. In this respect, atomic layer deposition (ALD) offers the opportunity to systematically investigate the synergy between these materials by enabling subnanometer thickness control of thin-film heterostacks. These heterostacks demonstrate a strong interfacial coupling associated with the epitaxial growth of NiO on polycrystalline Co3O4. Electrochemical analysis in 1 M KOH of stacks with variable NiO thickness, between 0.2 and 13 nm NiO on Co3O4, reveals two distinctive OER activity regimes, with ∼2 nm NiO as the tipping point. Stacks consisting of less than 2 nm NiO completely transform into an electrolyte-permeable, flaky nickel (oxy)hydroxide film, enhancing the electrochemically active surface area (ECSA) and enabling Co3O4 to contribute to the OER activity. In contrast, stacks consisting of more than a 2 nm-thick NiO overlayer on Co3O4 exhibit incomplete conversion to the (oxy)hydroxide phase, thereby preventing exposure of Co3O4 to the electrolyte. Nevertheless, the underlying Co3O4 promotes enhanced nickel (oxy)hydroxide formation compared to a NiO thin film, resulting in all stacks outperforming NiO. These results underscore the added value of ALD model systems for the design and understanding of ternary oxides’ OER electrocatalysts.