Pushing the limits for enzyme-based membrane-less hydrogen fuel cells – achieving useful power and stability†
The performance characteristics of simple enzyme-based membrane-less hydrogen fuel cells running on non-explosive H2-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 H2 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 O2 (relative to H2) to enzymes embedded in pores at the anode. The latter property raises the possibility of using standard hydrogenases that are not O2-tolerant: however, experiments with such an enzyme reveal good short-term performance due to restricted O2 access, but low long-term stability because the root cause of O2 sensitivity has not been addressed. Hydrogenases that are truly O2 tolerant must therefore remain the major focus of any future enzyme-based hydrogen fuel cell technology.