The strikingly high catalytic performance and stability of manganese substituted cobalt oxide spinel (Mn_xCo_3−xO₄) over pristine cobalt oxide spinel (Co₃O₄) for the alkaline electrochemical water oxidation is reported. The different role of cations could be uncovered along with the detection of drastic surface-structural changes during the catalysis using spectroscopic and microscopic methods.

The sustainable electrochemical production of hydrogen from water is a key element in the transition toward carbon-neutral energy systems. Application at a global scale requires the discovery of precious metal-free electrocatalysts that unify high energetic efficiency, long-term stability and economic viability. Here we report the striking properties of noble metal-free, alkali-metal cobalt borophosphates acting as robust and efficient materials for bi-functional electrocatalytic water-splitting to give hydrogen and oxygen. Alkali-metal cobalt borophosphates are porous crystalline inorganic materials with chiral DNA-like helical structures bearing two chemically distinct types of water molecules, coordinated and strands of hydrate water associated via hydrogen bonds, located in the channels, which are predestined to possess superior catalytic performance. Depending on the applied electrode potential, they can be reversibly switched between catalysis of the hydrogen and oxygen evolution reactions, both at low overpotentials. This bifunctionality provides access to technologically simple overall water-splitting systems with energetic efficiencies exceeding the 75% level (above 90% based on a higher heating value) and uncompromised long-term stability, now verified with a two-and-a-half month period.

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.