NiFe₂O₄ Spinel Thin Film Electrocatalysts: Ordered Mesoporous Networks for Driving the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is the bottleneck of electrocatalytic water splitting owing to its mechanistic complexity. Noble metal-based catalysts need to be replaced by earth-abundant elements possessing high OER activity and stability. In particular, nanostructured ferrites are inexpensive materials and thus hold promise for cost-effectively driving the OER. The general utilization of sol–gel-derived metal oxide thin film electrocatalysts on conductive substrates is beneficial over the use of powder-based catalysts, which need to be deposited postsynthetically and typically suffer under bad adhesion to the substrate. In this work, highly ordered mesoporous NiFe2O4 (NFO) thin films were prepared on conductive substrates by dip-coating and soft-templating using the diblock copolymer poly(ethylene-co-butylene)-block-poly(ethylene oxide) (KLE) without the need for citric acid as a complexing agent. The ordered mesoporous network was crack-free on the nano- and centimeter scale and provided good accessibility for the electrolyte, ensuring high mass transport at the electrode–electrolyte interface. Physicochemical characterization by various techniques confirmed a phase-pure cubic spinel structure and a nanocrystalline mesostructured NFO framework, for which especially the infiltration by the alkaline electrolyte, the availability of Ni2+ sites, and the presence of defective oxygen species are identified to be responsible for the electrocatalytic performance. In the present work, highly ordered mesoporous NFO thin films were investigated for the first time as electrocatalysts for the OER, revealing robust electrochemical stability under alkaline conditions. The synthetic approach offers a straightforward one-step strategy for the preparation of mesoporous metal oxide thin film-based electrocatalysts, avoiding the complexities associated with particle-based powder processing procedures.