Various redox proteins such as hydrogenases or photosynthetic proteins can be integrated in devices to produce electricity or chemical fuels. Redox hydrogels are among the best strategies to immobilize and electronically contact proteins to electrodes. The redox potentials and the solvation properties of viologen or Os-complex modified polymeric matrices can be tuned to enable benchmark current densities (300 μA cm^-2 for photosystem 1 [1] and up to 400 μA cm^-2 for photosystem 2 [2]) at low overpotential [3]. However, deactivation process of photosystems [4] and hydrogenases [5] limit their technological applications. We proposed the use of redox matrices for the protection of hydrogenases [6, 7] to open the possibility of utilizing these highly active but also highly sensitive enzymes in devices for energy conversion. The mechanism of protection relies on the redox buffering capacity of the matrix and the film properties can be adjusted to protect hydrogenases quasi-infinitely [8]. The main limitations are (i) the overpotential imposed by mediated electron transfer within the hydrogel, (ii) the film thickness required for protection which implies the need for a significant amount of enzyme that is not involved in the bioelectrocatalytic process and (iii) the inefficiency of the protection when the bioelectrodes are operated in a cathodic mode such as H₂ production. However, recent progresses in bioelectrode and redox hydrogel design, in particular with respect to electron transfer properties, mitigate the compromise between protection and performance in hydrogenase based energy conversion.

[1] T. Kothe, S. Pöller, F. Zhao, P. Fortgang, M. Rögner, W. Schuhmann, N. Plumeré Chem. Eur. J., 2014, 20, 11029 – 11034.

[2] K. Sokol, D. Mersch, V. Hartmann, J. Z. Zhang, M. M. Nowaczyk, M. Rögner, A. Ruff, W. Schuhmann, N. Plumeré, E. Reisner. Energy Environ. Sci., 2016, 9, 3698-3709.

[3] V. Hartmann, T. Kothe, S. Pöller, E. El-Mohsnawy, M. M. Nowaczyk, N. Plumeré, W. Schuhmann, M. Rögner Phys. Chem. Chem. Phys., 2014, 16, 11936 - 11941.

[4] M. M. Nowaczyk, N. Plumeré. Nature Chemical Biology 2016, 12, 990-991.

[5] A. Kubas, C. Orain, D. De Sancho, L. Saujet, M. Sensi, C. Gauquelin, I. Meynial-Salles, P. Soucaille, H. Bottin, C. Baffert, V. Fourmond, R. Best, J. Blumberger, C. Leger, Nature Chem. 2017, 9, 88-95.

[6] N. Plumeré, O. Rüdiger, A.A. Oughli, R. Williams, J. Vivekananthan, S. Pöller, W. Schuhmann, W. Lubitz, Nature Chem. 2014, 6, 822.

[7] A. A. Oughli, F. Conzuelo, M. Winkler, T. Happe, W. Lubitz, W. Schuhmann, O. Rüdiger, N. Plumeré, Angew. Chem. Int. Ed. 2015, 54, 12329.

[8] Fourmond, V.; Stapf, S.; Li, H.; Buesen, D.; Birrell, J.; Rüdiger, O.; Lubitz, W.; Schuhmann, W.; Plumeré, N.; Léger, C. J. Am. Chem. Soc., 2015, 137, 5494

Acknowledgement. Financial support by the Cluster of Excellence RESOLV (EXC 1069) and by the ERC Starting grant REDOX SHIELDS is gratefully acknowledged.