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. [Figure not available: see fulltext.]

Reversible catalysis

International audienceWe describe as "reversible" a catalyst that allows a reaction to proceed at a significant rate in response to even a small departure from equilibrium, resulting in fast and energy efficient chemical transformation. Examining the relation between rate and thermodynamic driving force is the basis of electrochemical investigations of redox reactions, which can be catalysed by metallic surfaces and synthetic or biological molecular catalysts. How rate depends on driving force has also been discussed in the context of biological energy transduction, regarding the function of biological molecular machines in which chemical reactions are used to produce mechanical work. We discuss the mean-field kinetic modeling of these three types of systems (surface catalysts, molecular catalysts of redox reactions, and molecular machines), in a step towards the integration of the concepts in these different fields. We emphasize that reversibility should be distinguished from other figures of merit, such as rate or directionality, before its design principles can be identified and used in the engineering of synthetic catalysts

The role of hydrophobicity of Os-complex-modified polymers for photosystem 1 based photocathodes

The integration of photosystem 1 in redox hydrogels based on Os-complexes modified redox polymers on electrodes yields efficient photocathodes. The generation of high photocurrent relies on high loading in PS1 and fast electron transfer rates from the electrode to PS1. The interaction between the redox polymer and PS1 influences both the loading in protein and the electron transfer rates. Since PS1 exhibits extended hydrophobic regions, polymers with similar properties may favor attractive interactions. Here we investigate three approaches to confer hydrophobicity to the redox polymer. We demonstrate that the pyridine functionality enables to switch, via basic pH values, the polymer properties from hydrophilic to hydrophobic. The transition triggers a hydrogel collapse which allows for efficient entrapment of PS1. In addition the hydrophobic-hydrophilic balance was tuned by the addition of hydrophobic group in i) the polymer backbone and ii) as substituents at the Os-complex. The increased hydrophobicity of the backbone results in higher photocurrents from PS1 integrated in the corresponding hydrogel. On the other hand, further increasing hydrophobicity of the redox relay decreases the photocurrent due to either lower mobility of the Os-complexes or poor interaction with the hydrophilic site where the redox center of PS1 is located

A gas breathing hydrogen/air biofuel cell comprising a redox polymer/hydrogenase-based bioanode

Hydrogen is an attractive alternative fuel, but many hydrogen-conversion electrocatalysts contain expensive materials. Here the authors report a dual-gas breathing hydrogen/air biofuel cell comprised of a modified polymer/hydrogenase bioanode and a bilirubin oxidase biocathode, delivering improved output