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

Abstract

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® 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® F-127 was reported to result in breakdown of the mesoporous framework. However, this work shows that both Pluronic® 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.