Soft-templated, mesoporous Co₃O₄ thin films for electrocatalysis of the oxygen evolution reaction

In order to decrease the overpotentials required for the electrocatalytic water splitting reaction, high-performance and abundant-element based-electrocatalysts have to be developed. Especially, high-surface-area materials are considered as promising candidates to drive the oxygen evolution reaction (OER) at low overpotentials in alkaline and acidic media owing to a large amount of available active surface sites. In this context, mesoporous spinel cobalt oxide (meso-Co3O4) thin films on conductive substrates were prepared for the first time by the dip-coating and soft-templating approach using cobalte nitrate as precursor and commercially available, low-cost Pluronic (R) F-127 as structure-directing agent. By making use of the evaporation-induced self-assembly (EISA) process, citric acid was applied as structural stabilizer reported yet to be only compatible with customized polymers for formation of mesoporous thin films. The usage of commercial Pluronic (R) F-127 was reported to result in breakdown of the mesoporous framework. However, this work shows that both Pluronic (R) F-127 and citric acid are compatible and can be utilized to produce uniform mesoporous networks after annealing. The meso-Co3O4 thin film was found to be crack-free on the nano- and macroscale, thus offering a highly accessible nanoarchitecture ensuring a large interface to any electrolyte allowing for efficient mass transport. The surface and bulk morphology, crystallographic structure, and surface composition of the mesostructured Co3O4 thin film were correlated to its OER activity. The Co3O4 nanostructure showed, when applied as OER catalyst, a low overpotential of 340 mV vs. RHE at 10 mA cm(-2) in 1 M KOH. The electrochemical performance was evaluated by means of impedance spectroscopy and Tafel plot analysis confirming enhanced electron transfer at the electrode/electrolyte interface. The Co3O4 electrocatalyst exhibited a promising stability under alkaline conditions over more than 4 hours upon chronopotentiometry (OER-activity loss of only 2% at 10 mA cm(-2)). Our preparation procedure reveals the general capability of combining commercial copolymer templates with citric acid and can be transferred to other metal oxides to produce low-cost and high-surface-area materials for electrochemical applications.