Unraveling the Role of the Stoichiometry of Atomic Layer Deposited Nickel Cobalt Oxides on the Oxygen Evolution Reaction

Abstract Nickel cobalt oxides (NCOs) are promising, non‐precious oxygen evolution reaction (OER) electrocatalysts. However, the stoichiometry‐dependent electrochemical behavior makes it crucial to understand the structure‐OER relationship. In this work, NCO thin film model systems are prepared using atomic layer deposition. In‐depth film characterization shows the phase transition from Ni‐rich rock‐salt films to Co‐rich spinel films. Electrochemical analysis in 1 m KOH reveals a synergistic effect between Co and Ni with optimal performance for the 30 at.% Co film after 500 CV cycles. Electrochemical activation correlates with film composition, specifically increasing activation is observed for more Ni‐rich films as its bulk transitions to the active (oxy)hydroxide phase. In parallel to this transition, the electrochemical surface area (ECSA) increases up to a factor 8. Using an original approach, the changes in ECSA are decoupled from intrinsic OER activity, leading to the conclusion that 70 at.% Co spinel phase NCO films are intrinsically the most active. The studies point to a chemical composition dependent OER mechanism: Co‐rich spinel films show instantly high activities, while the more sustainable Ni‐rich rock‐salt films require extended activation to increase the ECSA and OER performance. The results highlight the added value of working with model systems to disclose structure‐performance mechanisms.