Oxygen defected NiCo spinel structures for ammonia electrooxidation reaction: An experimental and theoretical approach
Sara Sumbal, Adrian Olejnik, Marek Lieder, Justyna Łuczak
Abstract
Designing highly selective, stable, and economical catalysts for the ammonia electrooxidation reaction is crucial to improve the electrochemical energy conversion and storage technologies. Hereby, we disclose oxygen-defected NiCo-based spinel structures, obtained by direct deposition of spinel on Ni foam and subsequent thermal treatment under Ar/H 2 atmosphere, as efficient electrocatalysts for AOR. The electrocatalytic surface area values (ECSA) for NiCo 2 O 4 and defective samples were higher than the sum of the ECSA values of NiO and Co 3 O 4 . This indicates more active surface area for AOR due to combining Ni and Co oxides in the spinel structure, which contributes to enhanced catalytic activity. Among the prepared electrocatalysts, NCO-350 was found to exhibit the highest AOR activity with regard to current density (125 mA⸱cm −2 ) as well as turnover frequency (1.46 s −1 ). The N 2 selectivity at 1.55 V vs RHE was more than 81 %, whereas ammonia oxidation efficiency was 93 % upon completion of the 120 h stability test. Theoretical calculations revealed that the exposure of the electrocatalysts to the Ar/H 2 atmosphere induces two types of defects: oxygen vacancies and H-atoms built into the crystal. The density of states (DOS) spectra confirmed the higher DOS for NiCo 2 O 4 at the Fermi level as compared to the constituent single-oxides. • Oxygen vacancies boost AOR performance of NiCo spinel oxides. • Oxygen vacancies in the surface layer enhanced ammonia binding energy • Surface acts as an electrophile and weakly nucleophilic NH 3 binds to the surface via N-atom. • Phase transformation of spinel NiCo 2 O 4 into hydroxides/oxyhydroxides during AOR. • Main product of AOR was N 2 with 89 % selectivity.