Enzymes as working or inspirational electrocatalysts for fuel cells and electrolysis.

The concept of a fuel cell dates back to 1839, from independent studies by Grove and Schoenbein. Like a battery, a fuel cell is a device for obtaining electrical energy directly from a chemical reaction, but unlike a battery, electrical power is sustained as long as the reacting chemicals are supplied to each electrode with the cathode receiving oxidant and the anode receiving reductant or “fuel”, hence “fuel cell”. There are environmental advantages over combustion because fuel cells avoid the high temperatures that cause NOx production, and they are usually reported as operating at a higher efficiency (typically 50-60%) than internal combustion engines (20-25%). Applications of fuel cells were largely neglected until the “space age” (1960s) when there became a need for reliable electrical power in challenging, niche situations. In addition to environmental driving forces, energy demands for niche applications continue to drive fuel cell development today. Fuel cells vary greatly in their power output, ranging from large-scale (kW) building-integrated systems, known as “combined heat and power” systems, to those that provide just enough power to operate electronics in special circumstances, such as an implantable device for sensing and controlling glucose levels in the body. As we outline below, the power output of a fuel cell can be limited by the electrochemical reactions occurring at either of the two electrodes, the anode for oxidizing fuel and the cathode for reducing oxidant, and so the electrodes are usually coated with electrocatalysts. An enzyme fuel cell uses an enzyme as the electrocatalyst, either at both cathode and anode, or at just one of the electrodes. The catalytic properties of redox enzymes offer some interesting advantages in fuel cell applications, although examples of devices exploiting enzyme electrocatalysis are almost exclusively at a “proof of concept” stage. Conventional low-temperature fuel cells are generally limited to H2 or primary alcohols as fuels; however, the use of an enzyme as the anode electrocatalyst means that any substance that is oxidized by an organism can become a useful fuel because it is obvious that enzymes for carrying out that specific task must exist. Not only are enzymes capable of very high activity (on a per mole basis), but they are usually highly selective for their substrates. This simplifies the design of a fuel cell because fuel and oxidant need not be separated by an ionically conducting membrane, and they can be introduced as a mixture, that is, mixed reactant fuel cells are possible. This also makes it possible to miniaturize the fuel cell to an extremely small scale. Enzymes are also renewable, meaning that their components are fully recycled using sunlight as an energy source. The main disadvantages of enzymes as electrocatalysts are as follows. First, they are usually very large molecules, so that although the active sites may be extremely active in comparison to the catalytic site of a conventional metal electrode, the catalytic (volume) density is low, and hence multilayers of enzyme are likely to be needed to provide sufficient current. Second, the catalytically active sites are usually buried, so that fast electron transfer to or from the electrode requires either use of an intrinsic electron relay system in the protein (such as a series of FeS clusters) or an extrinsic mediator that can penetrate sufficiently close to the active site. Third, enzymes are often unstable outside ambient / To whom correspondence should be addressed. E-mail: fraser.armstrong@ chem.ox.uk. Chem. Rev. 2008, 108, 2439–2461 2439