Helical cobalt borophosphates to master durable overall water-splitting†
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.