Detecting structural transformation of cobalt phosphonate to active bifunctional catalysts for electrochemical water-splitting

Abstract

In recent years, several cobalt-based catalysts have been developed for water splitting because of their promising activity, stability and structural motifs. Here, we report that cobalt phosphonate represents a novel class of bifunctional single-source precursors for highly efficient alkaline electrochemical O₂ evolution (OER) and H₂ evolution reaction (HER). Inspired by its favorable catalytic OER and HER activity, an overall water-splitting device has been constructed from this precursor, showing very low cell voltage (1.62 V @ 10 mA cm⁻²) and excellent long-term stability. Depending on the applied oxidation and reduction potential on cobalt phosphonate, two distinct modified structures at the anode and cathode have been uncovered employing the quasi in situ X-ray absorption spectroscopy and ex situ methods. During OER, the phosphonate precursor reorganized itself to layered CoO_x(OH)_y structure with defects and disorders, while the contribution of the metallic Co along with Co₃O₄ spinel and Co(OH)₂ is evident to drive the HER. The presented work demonstrates the advantage of using the ‘all-in-one’ precursor approach to realize bifunctional water-splitting electrocatalysts through the evolution of different species with self-supporting interfacial structural features at the anode and cathode during electrochemical water splitting.