Biocatalysts in fuel cells : why, how ?

Abstract

Fuel cells offer an alternative to fossil fuels for electricity generation. However, the need to use rare, and all too easily contaminated, metals to catalyze the reactions that convert chemical energy into electrical energy remains an obstacle to large-scale development. So why not look to nature for cheaper, more sustainable solutions? Within micro-organisms, enzymes have been identified that are excellent biocatalysts for transforming a wide range of substrates into fuels and combustibles. A fuel biopile is thus defined, where these enzymes advantageously replace the platinum-based catalyst at the anode and cathode. The first enzymatic biopiles were based on enzymes specific to the degradation and transformation of substrates present in physiological fluids, glucose and oxygen in particular. Applications were aimed at powering implanted medical devices. Today, this type of biopile is so mature that clinical trials are in lecture.

A new generation of biopile has very recently emerged, using hydrogenase - the key hydrogen-converting enzyme in many microorganisms and biotopes - as the biocatalyst at the anode. This device is a replica of low-temperature chemical fuel cells, and takes advantage of hydrogen's energetic properties compared with glucose. Although it was slow to take off due to the extreme oxygen sensitivity of most known hydrogenases until the mid-2000s, it is now undergoing rapid development thanks to the identification of highly efficient, oxygen-tolerant and, what's more, CO-resistant hydrogenases. But replacing a metal particle like platinum with an enzyme is no trivial matter. The size of the biological object, the complex but highly organized protein structure, the multiple electronic relays, are all factors that control catalysis. Knowledge of the enzyme's three-dimensional structure and the molecular basis of its connection to a current collector are necessary for efficient interfacial electron transfer, and therefore for application-compatible cell powers. Increasing current densities also requires the nanostructuring of three-dimensional conductive matrices adapted to the non-denaturing immobilization of an increased number of enzymes. Finally, the stability of the bioelectrode, and ultimately of the biopile, requires research into the biodiversity of new enzymes or active protein complexes.

Based on recent examples of biopiles, we will discuss the various avenues considered to meet the constraints of using enzymes to generate electricity. We will present a number of applications based on this type of device, focusing on the interdisciplinary research now needed to bring them to market.