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.

高效的电催化能量转换需要设备可逆地发挥作用，即以最小的过电势提供大量电流。氧化还原活性薄膜可以有效地嵌入和稳定分子电催化剂，但通过薄膜介导的电子转移通常会使催化反应不可逆。在这里，我们描述了一种用于双向（氧化或还原）和可逆氢转化的氧化还原活性薄膜，它由嵌入低电位 2,2'-紫精修饰水凝胶中的 [FeFe] 氢化酶组成。当这种催化膜在 H ₂ /O _{2中用作阳极材料时}生物燃料电池，获得了 1.16 V 的开路电压——接近热力学极限的基准值。相同的薄膜也可作为 H ₂释放的高能效阴极材料。我们使用动力学模型解释了催化特性，这表明即使分子间电子转移比催化慢，也可以实现可逆性。这种对可逆性的理解简化了高效稳定的生物电催化薄膜的设计原理，促进了它们在能量转换中的应用。