Biotechnologies for energy storage: microbes and electrodes

Abstract

This lecture begins with a lengthy introduction discussing the major issues involved in energy storage. To develop renewable energies, particularly solar power, which are intermittent and diluted, and incorporate them into power grids, we need to develop energy storage technologies. The needs are enormous. Although electrochemical storage (batteries) is an important part of the overall strategy, this lecture only discusses chemical storage: decomposition of water into oxygen and hydrogen, use of biomass (biofuels),_CO2 recovery. This last point is particularly developed as it is based on an original approach to bioinspired chemistry, artificial photosynthesis. Today, synthetic chemists are tackling this issue through the construction and development of photoelectrochemical cells or electrolysis cells coupled to voltaic panels, which convert solar energy into chemical energy. The few examples of such devices, which are still insufficiently efficient, recently reported in the literature, are presented and discussed. The key chemical components of these devices are light energy collectors (photosensitizers, semiconductors) and catalysts for water oxidation (anode) and_CO2 reduction to energy-rich molecules (cathode).

The second part of this lecture attempts to show, more specifically, how these technologies can be coupled with the use of microorganisms, which are then used as "biocatalysts" or "cell factories". Indeed, the latter offer the advantage of being able to be genetically modified (metabolic engineering, synthetic biology) to divert their carbon metabolism towards molecules of interest. These modified microorganisms can either be deposited directly on electrodes (forming biofilms), from which they draw the electrons required for their metabolism (known as electrofuels, for example), or introduced into devices that produce fuels, such as hydrogen or formic acid, which they can use as an energy source. The bacteria most frequently studied are chemoautotrophs capable of_CO2 reduction (acetogens, methanogens, aerobic organisms, etc.). Some recent examples of such achievements are discussed: (i) synthesis of butanols or isopropanol by modified strains of Ralstonia fed with formic acid or electrochemically formed hydrogen(Science, 2012; PNAS, 2015); (ii) acetic acid production using biofilms of acetogens on carbon electrodes(mBio, 2010) ; first microbial photoelectrosynthesis system using a photoelectrochemical cell supplying electrons to an acetogen biofilm to produce acetic acid, used in a second step by a genetically modified strain ofE. coli to produce high-value-added molecules (fuels, polymers, drugs, etc.)(Nanoletters, 2015).