Nanocrystalline Ordered Mesoporous Co(OH) ₂ and Co ₃ O ₄ Thin Films: Oxygen Evolution Reaction Activity from a Structural Properties Perspective

Design of nanostructured electrocatalysts is essential to improve the efficiency for driving the oxygen evolution reaction (OER) at low overpotentials. Mesoporous cobalt-based thin films are prepared by dip-coating and soft-templating using the structure-directing diblock copolymer poly(ethylene-co-butylene)-block-poly(ethylene oxide). Our temperature-dependent study reveals how the calcination temperature affects the phase formation and development of the surface and bulk morphology of the catalysts. The crystallographic structure, surface composition, and development of the mesoporous framework were correlated with the OER activities. The increase in calcination temperature significantly impacts the nanoarchitecture, changing from an amorphous and dense structure, which is composed of Co(OH)₂, to structurally intact and ordered mesoporous Co₃O₄ networks. The morphology of the mesoporous network (providing accessibility for the electrolyte), the overall surface area, and the presence of a nanocrystalline Co(OH)₂ pre-catalyst phase (allowing fast formation of electrocatalytically active species), collectively determine the OER activity. These structure-property relationships explain why Co(OH)₂ films annealed at 250 °C show the lowest overpotential of 370 mV at 10 mA cm^-2 and electrochemical stability in alkaline media. The development of the ordered mesoporous architectures in dependence on the annealing temperature demonstrates the importance of careful tailoring of the synthesis conditions to achieve optimized OER performance.