Reversible H2 oxidation and evolution by hydrogenase embedded in a redox polymer film

Efficient electrocatalytic energy conversion requires devices to function reversibly, that is, to deliver a substantial current at a minimal overpotential. Redox-active films can effectively embed and stabilize molecular electrocatalysts, but mediated electron transfer through the film typically makes the catalytic response irreversible. Here we describe a redox-active film for bidirectional (oxidation or reduction) and reversible hydrogen conversion, which consists of [FeFe] hydrogenase embedded in a low-potential, 2,2′-viologen-modified hydrogel. When this catalytic film served as the anode material in a H2/O2 biofuel cell, an open circuit voltage of 1.16 V was obtained—a benchmark value near the thermodynamic limit. The same film also acted as a highly energy efficient cathode material for H2 evolution. We explained the catalytic properties using a kinetic model, which shows that reversibility can be achieved even though intermolecular electron transfer is slower than catalysis. This understanding of reversibility simplifies the design principles of highly efficient and stable bioelectrocatalytic films, advancing their implementation in energy conversion. Integration of biocatalysts into redox films has systematically led to a loss of their intrinsic reversibility. Now, a specially designed redox hydrogel preserves the reversibility of a [FeFe] hydrogenase and the importance of this feature for energy conversion applications is demonstrated.