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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 …