The unusual ability of O 2 -tolerant hydrogenases (H 2 ase) to produce electricity from a H 2 -air mixture (when used as the anodic electrocatalyst in a simple, membrane-less fuel cell) is investigated with the aim of establishing a strategy for raising volume power density, the measure of importance for miniature devices. Compacted mesoporous carbon electrodes provide a simple and inexpensive method for obtaining a large increase in productive enzyme loading, greatly increasing current densities and stability. Operated under a 78% H 2 -22% air mixture at 25 C, typical current densities at a stationary H 2 ase anode and bilirubin oxidase cathode are 4.60 AE 0.32 mA cm À2 and 1.23 AE 0.12 mA cm À2 , respectively. The power limitation due to low O 2 concentration is addressed by re-proportioning the cathode/anode area ratio to balance the cathodic and anodic currents. At room temperature, the maximum power density of the fuel cell with an anode/cathode (A/C) ratio of 1 : 3 (1A/3C) is 1.67 AE 0.24 mW cm À2 (per anode area) or 0.42 AE 0.06 mW cm À2 (per total area). Good prospects for stability are demonstrated by the fact that 90% of the power is retained after continuously working for 24 h, and more than half of the power is retained after one week of non-stop operation. Using an even weaker O 2 mixture (89% H 2 , 11% air) the 1A/3C cell gives over 0.8 mW cm À2 (anode) or 0.2 mW cm À2 (total electrode area). The results demonstrate the feasibility of membrane-less hydrogen-air fuel cells delivering volume power densities well in excess of 1 mW cm À3 . Broader contextThe reality of a future for hydrogen in energy technologies depends upon establishing many different ways in which it can be produced and used. Fuel cells able to use hydrogen mixed with a small amount of air offer alternative ways of using hydrogen and interesting possibilities for miniaturization, but require selective electrocatalysts because they lack a membrane. Enzymes have great advantages over precious metals in terms of selectivity but their giant nature limits power density. This paper describes an important step forward in signicantly increasing both stability and power density by embedding a hydrogen-oxidizing enzyme and an oxygen-reducing enzyme into '3D' compacted mesoporous carbon electrodes and varying the relative dimensions of anode and cathode to balance electrocatalytic activities.

The performance characteristics of simple enzyme-based membrane-less hydrogen fuel cells running on non-explosive H 2 -rich air mixtures have been established using an adjustable test bed that allows multiple unit cells to operate in series or parallel. Recent advances with '3D' electrodes constructed from compacted porous carbon loaded with hydrogenase (anode) and bilirubin oxidase (cathode) have been extended in order to scale up fuel cell power to useful levels. One result is an appealing 'classroom' demonstration of a model house containing small electronic devices powered by H 2 mixed with a small amount of air. The 3D electrodes work by greatly increasing catalyst loading (at both the anode and cathode) and selectively restricting the access of O 2 (relative to H 2 ) to enzymes embedded in pores at the anode. The latter property raises the possibility of using standard hydrogenases that are not O 2 -tolerant: however, experiments with such an enzyme reveal good short-term performance due to restricted O 2 access, but low long-term stability because the root cause of O 2 sensitivity has not been addressed. Hydrogenases that are truly O 2 tolerant must therefore remain the major focus of any future enzyme-based hydrogen fuel cell technology.

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