Self-limiting thin film deposition of amorphous metal oxides from aprotic solvents for oxygen evolution electrocatalysis

Abstract

In the context of global energy and environmental challenges, developing efficient and sustainable methods for energy storage and conversion is essential. Amorphous thin films of nanoporous single and mixed Ni, Co, Fe metal oxides were deposited using in situ generated superoxide from dimethyl formamide (DMF) solutions, presenting a promising advancement in oxygen evolution reaction (OER) catalysis. The OER is a critical process in technologies such as water electrolysis for hydrogen production and metal–air batteries. The oxygen reduction reaction (ORR) was investigated in DMF containing tetrabutylammonium chloride (TBAC) and transition metal chlorides (Co²⁺, Ni²⁺, Fe²⁺). The ORR exhibited pseudo-reversibility in 0.1 M TBAC/DMF due to the stable superoxide–TBA complex (TBAO₂). Diffusion coefficients for TBAO₂ and dissolved oxygen in DMF were measured with a rotating ring-disk electrode (RRDE) setup, obtaining values of 9.4 × 10⁻¹⁰ m² s⁻¹ and 4.4 × 10⁻⁹ m² s⁻¹, respectively. In the presence of metal ions, the superoxide radicals react rapidly, initiating oxidative precipitation of metal oxides. Amorphous oxide layers grow until the electrode passivation towards the ORR occurs, resulting in thin films. The as-deposited films displayed exceptional oxygen evolution reaction (OER) catalytic performance, with overpotentials and Tafel slopes as low as 330 mV (@10 mA cm⁻²) and 42 mV dec⁻¹ for NiO_x on glassy carbon (GC). The self-limiting deposition process was investigated on porous substrates, such as reticulated vitreous carbon (RVC), which demonstrated a 100 mV reduction in OER overpotential compared to GC at the same current density. This self-limiting electrodeposition method offers potential environmental benefits, including resource efficiency and reduced environmental impact. The charge transfer mechanism and long term stability of the films were explored, with results showing stability up to 160 h on GC/RVC and 600 h on Ni foam. This study reveals insights in the ORR in aprotic solvents with transition metals oxides and presents a simple, self-limiting electrodeposition method for amorphous metal oxides, paving the way for coating complex structures and advancing energy storage and conversion technologies.