Water splitting electrocatalysts from manganese nodules: a structure–function analysis by operando X-ray spectroscopy

Abstract

Hydrogen production via water electrolysis using renewable energy sources has attracted significant attention as a promising pathway toward a sustainable society. The development of cost-effective and highly active oxygen evolution reaction (OER) catalysts will be an essential aspect of improving the efficiency of such systems. The present study demonstrates an OER electrocatalyst obtained from the ball milling of naturally occurring manganese nodules. This material was found to exhibit catalytic activity during the OER. X-ray diffraction data suggested that the crystal structure of the manganese nodules was similar to that of todorokite-type δ-MnO₂. Mn K-edge X-ray absorption fine structure (XAFS) analyses indicated that the Mn valence state in the nodules changed from Mn⁴⁺ to Mn³⁺ during the ball-milling process, accompanied by disordering of the fine crystal structure. Operando XAFS experiments also established that Mn³⁺ in this material was partially oxidized during the OER. On this basis, a disordered δ-MnO₂ phase containing Mn³⁺ is thought to have provided active sites for water oxidation.