Dynamics and control of active sites in hierarchically nanostructured cobalt phosphide/chalcogenide-based electrocatalysts for water splitting

Abstract

The rational design of efficient electrocatalysts for industrial water splitting is essential to generate sustainable hydrogen fuel. However, a comprehensive understanding of the complex catalytic mechanisms under harsh reaction conditions remains a major challenge. We apply a self-templated strategy to introduce hierarchically nanostructured "all-surface" Fe-doped cobalt phosphide nanoboxes (Co@CoFe-P NBs) as alternative electrocatalysts for industrial-scale applications. <i>Operando</i> Raman spectroscopy and X-ray absorption spectroscopy (XAS) experiments were carried out to track the dynamics of their structural reconstruction and the real catalytically active intermediates during water splitting. Our <i>operando</i> analyses reveal that partial Fe substitution in cobalt phosphides promotes a structural reconstruction into P-Co-O-Fe-P configurations with low-valence metal centers (M⁰/M⁺) during the hydrogen evolution reaction (HER). Results from density functional theory (DFT) demonstrate that these <i>in situ</i> reconstructed configurations significantly enhance the HER performance by lowering the energy barrier for water dissociation and by facilitating the adsorption/desorption of HER intermediates (H*). The competitive activity in the oxygen evolution reaction (OER) arises from the transformation of the reconstructed P-Co-O-Fe-P configurations into oxygen-bridged, high-valence Co^IV-O-Fe^IV moieties as true active intermediates. In sharp contrast, the formation of such Co^III/IV-O-Fe^III/IV moieties in Co-FeOOH is hindered under the same conditions, which outlines the key advantages of phosphide-based electrocatalysts. <i>Ex situ</i> studies of the as-synthesized reference cobalt sulfides (Co-S), Fe doped cobalt selenides (Co@CoFe-Se), and Fe doped cobalt tellurides (Co@CoFe-Te) further corroborate the observed structural transformations. These insights are vital to systematically exploit the intrinsic catalytic mechanisms of non-oxide, low-cost, and robust overall water splitting electrocatalysts for future energy conversion and storage.