Theoretical understanding of the penetration of O2 in enzymatic redox polymer films: case of unidirectional catalysis and irreversible inactivation in a film of arbitrary thickness

Christophe Léger & Vincent Fourmond. ChemElectroChem 8, 2607-2615 (2021) doi: 10.1002/celc.202100586

Redox catalysts, including hydrogenases, can be embedded into films made of redox polymers, whose side chains mediate electrons between the catalyst and an electrode. These films can be used as bioanodes in H2-based biofuel cells, because they protect the catalyst from O2-induced inactivation: self-protection occurs because a fraction of the incoming H2 is used in the outer region of the film to catalytically produce electrons that reduce the O2 molecules that penetrate the film. Here, we focus on the case of unidirectional catalysis (e. g. H2 oxidation) by an enzyme that is irreversibly inactivated by O2, embedded in a film of arbitrary thickness. We analytically solve the reaction/diffusion system to fully describe the time evolution of the penetration of O2 and we discuss the amount of H2 consumed by the protection mechanism. We establish the relations between film thickness, electron conduction, catalyst use and life time. This provides the theoretical framework required to optimize the design of these systems.

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

Steffen Hardt, Stefanie Stapf, Dawit T. Filmon, James A. Birrell, Olaf Rüdiger, Vincent Fourmond, Christophe Léger & Nicolas Plumeré, Nature Catalysis, 2021,

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.

Nouvelle stratégie pour l’optimisation des catalyseurs électrochimiques supportés

L’amélioration des performances d’un catalyseur moléculaire peut s’envisager par la modification de sa structure, mais aussi de celle de la matrice qui le supporte, qui peut en changer totalement les propriétés. Dans le cadre d’une collaboration avec l’université de Bochum, les chercheurs montrent que dans des systèmes optimisés, ces effets peuvent prévenir complètement et durablement l’inactivation d’un catalyseur d’oxydation de l’hydrogène par l’oxygène sans compromettre l’efficacité du système. L’approche est particulièrement utile/envisageable dans le contexte des biopiles à combustibles, où le catalyseur sur une des deux électrodes oxyde le dihydrogène, mais peut être inactivé par le dioxygène qui peut traverser la membrane de la fuelcell.

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Movies in support of the calculations in J Am Chem Soc. 2019 141 16734 (2019)

This simulation shows the concentration profiles (H2 grey, oxygen red, oxidised and reduced viologen pink and blue, and inactive enzyme black) within the depth of a film of redox polymer embedding the H2-oxidation catalyst hydrogenase. xi is the normalised distance to the electrode (xi=0 is the electrode/film interface; xi=1 is the film/solution interface). The first part of the simulation shows the system reaching steady-state under anaerobic condition. When the film is exposed to O2, O2 penetrates, inactivates the enzyme, oxidises the viologen; catalytic H2 oxidation by the enzyme produces reductive equivalents that diffuse outward, and reduce the incoming O2, preventing further penetration. Note the non linear time scale (the simulation accelerates, the evolution of the system slows down). Kappa=13

Complete Protection of O2-Sensitive Catalysts in Thin Films

Huaiguang Li, Darren Buesen, Sebastien Dementin, Christophe Leger, Vincent Fourmond, Nicolas Plumere, JACS 2019 doi:10.1021/jacs.9b06790

Energy conversion schemes involving dihydrogen hold great potential for meeting sustainable energy needs, but widespread implementation cannot proceed without solutions that mitigate the cost of rare metal catalysts and the O2-instability of biological and bio-inspired replacements. Recently, thick films (>100 µm) of redox polymers were shown to prevent O2 catalyst damage, but also resulted in unnecessary catalyst load and mass transport limitations. Here, we apply novel homogeneous thin films (down to 3 µm) that provide O2-immunity while achieving highly efficient catalyst utilization. Our empirical data is explained by modeling, demonstrating that resistance to O2 inactivation can be obtained for non-limiting periods of time when the optimal thickness for catalyst utilization and current generation is achieved, even when using highly fragile catalysts such as the enzyme hydrogenase. We show that different protection mechanisms operate depending on matrix dimensions and intrinsic catalyst properties, and can be integrated together synergistically to achieve stable H2 oxidation currents in the presence of O2, potentially enabling a plethora of practical applications for bio-inspired catalysts in harsh oxidative conditions.

A novel versatile microbiosensor for local hydrogen detection by means of scanning photoelectrochemical microscopy

Fangyuan Zhao, Felipe Conzuelo, Volker Hartmann, Huaiguang Li, Stefanie Stapf, Marc M.Nowaczyk, Matthias Rögner, Nicolas Plumeré, Wolfgang Lubitz, Wolfgang Schuhmann Biosensors and Bioelectronics, Volume 94, 15 August 2017, Pages 433-437

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