Hydrogen is one of the most promising alternatives for fossil fuels. However, the power output of hydrogen/oxygen fuel cells is often restricted by mass transport limitations of the substrate. Here, we present a dual-gas breathing H2/air biofuel cell that overcomes these limitations. The cell is equipped with a hydrogen-oxidizing redox polymer/hydrogenase gas-breathing bioanode and an oxygen-reducing bilirubin oxidase gas-breathing biocathode (operated in a direct electron transfer regime). The bioanode consists of a two layer system with a redox polymer-based adhesion layer and an active, redox polymer/hydrogenase top layer. The redox polymers protect the biocatalyst from high potentials and oxygen damage. The bioanodes show remarkable current densities of up to 8 mA cm-2. A maximum power density of 3.6 mW cm-2 at 0.7 V and an open circuit voltage of up to 1.13 V were achieved in biofuel cell tests, representing outstanding values for a device that is based on a redox polymer-based hydrogenase bioanode.

We report on the fabrication of bioanodes for H 2 oxidation based on [NiFeSe] hydrogenase. The enzyme was electrically wired by means of a specifically designed low-potential viologen-modified polymer, which delivers benchmark H 2 oxidizing currents even under deactivating conditions owing to efficient protection against O 2 combined with a viologen-induced reactivation of the O 2 inhibited enzyme. Moreover, the viologen-modified polymer allows for electrochemical co-deposition of polymer and biocatalyst and, by this, for control of the film thickness. Protection and reactivation of the enzyme was demonstrated in thick and thin reaction layers.

We describe the fabrication of gas diffusion electrodes modified with polymer/enzyme layers for electroenzymatic CO 2 fixation. For this, a metalfree organic low-potential viologen-modified polymer has been synthesized that reveals a redox potential of around −0.39 V vs SHE and is thus able to electrically wire W-dependent formate dehydrogenase from Desulfovibrio vulgaris Hildenborough, which reversibly catalyzes the conversion of CO 2 to formate. The use of gas diffusion electrodes eliminates limitations arising from slow mass transport when solid carbonate is used as CO 2 source. The electrodes showed satisfactory stability that allowed for their long-term electrolysis application for electroenzymatic formate production.

Hydrogenases with Ni- and/or Fe-based active sites are highly active hydrogen oxidation catalysts with activities similar to those of noble metal catalysts. However, the activity is connected to a sensitivity towards high-potential deactivation and oxygen damage. Here we report a fully protected polymer multilayer/hydrogenase-based bioanode in which the sensitive hydrogen oxidation catalyst is protected from high-potential deactivation and from oxygen damage by using a polymer multilayer architecture. The active catalyst is embedded in a low-potential polymer (protection from high-potential deactivation) and covered with a polymer-supported bienzymatic oxygen removal system. In contrast to previously reported polymer-based protection systems, the proposed strategy fully decouples the hydrogenase reaction form the protection process. Incorporation of the bioanode into a hydrogen/glucose biofuel cell provides a benchmark open circuit voltage of 1.15 V and power densities of up to 530 µW cm−2 at 0.85 V.

Well-defined assemblies of photosynthetic protein complexes are required for an optimal performance of semiartificial energy conversion devices,c apable of providing unidirectional electron flow when light-harvesting proteins are interfaced with electrode surfaces.W ep resent mixed photosystem I(PSI) monolayers constituted of native cyanobacterial PSI trimers in combination with isolated PSI monomers from the same organism. The resulting compact arrangement ensures ah igh density of photoactive protein complexes per unit area, providing the basis to effectively minimize shortcircuiting processes that typically limit the performance of PSIbased bioelectrodes.T he PSI film is further interfaced with redox polymers for optimal electron transfer,e nabling highly efficient light-induced photocurrent generation. Coupling of the photocathode with a[ NiFeSe]-hydrogenase confirms the possibility to realizel ight-induced H 2 evolution.

Variants of the highly active [NiFeSe] hydrogenase from D. vulgaris Hildenborough that exhibit enhanced O 2 tolerance were used as H 2 ‐oxidation catalysts in H 2 /O 2 biofuel cells. Two [NiFeSe] variants were electrically wired by means of low‐potential viologen‐modified redox polymers and evaluated with respect to H 2 ‐oxidation and stability against O 2 in the immobilized state. The two variants showed maximum current densities of (450±84) μA cm −2 for G491A and (476±172) μA cm −2 for variant G941S on glassy carbon electrodes and a higher O 2 tolerance than the wild type. In addition, the polymer protected the enzyme from O 2 damage and high‐potential inactivation, establishing a triple protection for the bioanode. The use of gas‐diffusion bioanodes provided current densities for H 2 ‐oxidation of up to 6.3 mA cm −2 . Combination of the gas‐diffusion bioanode with a bilirubin oxidase‐based gas‐diffusion O 2 ‐reducing biocathode in a membrane‐free biofuel cell under anode‐limiting conditions showed unprecedented benchmark power densities of 4.4 mW cm −2 at 0.7 V and an open‐circuit voltage of 1.14 V even at moderate catalyst loadings, outperforming the previously reported system obtained with the [NiFeSe] wild type and the [NiFe] hydrogenase from D. vulgaris Miyazaki F.

The incorporation of highly active but also highly sensitive catalysts (e.g. the [FeFe] hydrogenase from Desulfovibrio desulfuricans) in biofuel cells is still one of the major challenges in sustainable energy conversion. We report the fabrication of a dual‐gas diffusion electrode H2/O2 biofuel cell equipped with a [FeFe] hydrogenase/redox polymer‐based high‐current‐density H2‐oxidation bioanode. The bioanodes show benchmark current densities of around 14 mA cm−2 and the corresponding fuel cell tests exhibit a benchmark for a hydrogenase/redox polymer‐based biofuel cell with outstanding power densities of 5.4 mW cm−2 at 0.7 V cell voltage. Furthermore, the highly sensitive [FeFe] hydrogenase is protected against oxygen damage by the redox polymer and can function under 5 % O2.

The immobilization, protection, and electrical wiring of sensitive catalysts by specifically designed supporting matrixes are of particular importance for technological relevant applications. Here, we describe the protection of a DuBois-type H2 oxidation catalyst, which was covalently bound to an inert polymer matrix, against molecular O2 by forming blends together with an O2-reducing redox polymer matrix. This matrix simultaneously acts as an electron relay for shuttling electrons between the catalyst and the electrode.