Enzymatic fuel cells: Recent progress

There is an increasing interest in replacing non-selective metal catalysts, currently used in low temperature fuel cells, with enzymes as catalysts. Specific oxidation of fuel and oxidant by enzymes as catalysts yields enzymatic fuel cells. If the catalysts can be immobilised at otherwise inert anode and cathode materials, this specificity of catalysis obviates the requirement for fuel cell casings and membranes permitting fuel cell configurations amenable to miniaturisation to be adopted. Such configurations have been proposed for application to niche areas of power generation: powering remotely located portable electronic devices, or implanted biomedical devices, for example. We focus in this review on recent efforts to improve electron transfer between the enzymes and electrodes, in the presence or absence of mediators, with most attention on research aimed at implantable or semi-implantable enzymatic fuel cells that harvest the body's own fuel, glucose, coupled to oxygen reduction, to provide power to biomedical devices. This ambitious goal is still at an early stage, with device power output and stability representing major challenges. A comparison of performance of enzymatic fuel cell electrodes and assembled fuel cells is attempted in this review, but is hampered in general by lack of availability of, and conformity to, standardised testing and reporting protocols for electrodes and cells. We therefore highlight reports that focus on this requirement. Ultimately, insight gained from enzymatic fuel cell research will lead to improved biomimetics of enzyme catalysts for fuel cell electrodes. These biomimetics will mimic enzyme catalytic sites and the structural flexibility of the protein assembly surrounding the catalytic site.