(3c) Electrochemistry for Energy: Air-Breathing Enzymatic Electrodes for Batteries and Fuel Cells

The enzyme laccase can reduce oxygen in ambient air, making it an ideal cathode catalyst for an air-breathing battery/fuel cell hybrid in the spirit of a zinc-air battery, but on a smaller, more designable scale.  Laccase is manufactured from the common fungi Trametes versicolor.  As enzymes are obtained from cell lysis and chromatography, they may be produced on a mass scale from farmed bacteria or fungi.  Micro power sources based on catalysts with little cost limitation will allow them to be deployed ubiquitously, to power highly specialized miniature devices. 

The chief obstacle to using enzyme catalysts in man-made power sources is that their structure is suited to operation in a cellular environment.  In certain areas this is a benefit, such as functionality at physiological temperature and pH.  However, electron exchange in biological systems is handled by small diffusional redox molecules, transporting electrons via diffusion.  In contrast, batteries and fuel cells rely on electron exchange at a heterogeneous interface (an electrode).

A composite soft electrode was designed to catalyze oxygen reduction with laccase and simultaneously conduct electrons with incorporated metal coordination compounds.  Metal complex molecular structure was engineered to increase electron flow, and an optimized molecular structure was found.  Catalytic rate was further increased by improving the synthesis for complex attachment to the composite.  This proved that macro-structure was important and achieved unprecedented bioelectrocatalytic current densities of 20 mA/cm². Prototype biofuel cells from this design using both hydrogen and methanol as anode fuels were successful, with mW/cm² performance.

Going forward, enzyme catalysis must take place on gas-phase oxygen in ambient air.  The goal is continuous operation of enzyme-catalyzed air electrodes with power, stability, and lifetime appropriate for practical use.