Bio-Engineering for microbial fuel cells – Bioelec

The project aims to develop engineering dedicated to MFCs in order to identify the bottlenecks that block the increase in their power and find solutions to remove them.
The low efficiency of the cathodes is a proven bottleneck. A special effort will therefore be made in the project to improve oxygen reduction cathodes, exploring both abiotic pathways and microbial catalyses.
A multi-scale approach combining theory and experimental work aims at designing an MFC prototype. The overall functioning of a MFC is first analysed in the light of the multidisciplinary competencies of the consortium. Specifications were drawn up to guide the optimization of the elements (anode, cathode and separator) and to consider different cell
architectures. The anodes and cathodes have then been adapted separately, in electroanalytical conditions to tend towards the specifications. A fundamental work has been dedicated to the understanding of the mechanisms of microbial cathodes, which may offer
promising alternatives to compensate for the low efficiency of abiotic catalysts. MFCs operate with electrolytes of low ionic conductivity, so it was anticipated that ionic transport would play a major role. The theoretical modelling of MFC is therefore based on the distribution of electrostatic potentials within the electrolyte. The comparison of the
theoretical results from the model with the experimental tests made it possible to progress in understanding the MFC behaviour. Finally, the global landscape of academic and industrial a ctors involved in the development of MFCs was analysed in order to prepare the valorisation of the results.

An original MFC architecture has produced the highest power density to date, equal to 6.4 Watt per square meter of electrode surface area. Similarly, the mastery of some fundamental mechanisms of microbial cathodes has allowed their performances to be raised beyond the
state of the art. However, rate-limiting steps have been evidenced, which condemn the MFC to deliver only small powers. In the current state of the art, it seems unrealistic to envisage massive electricity production processes based on MFC technology, unless an actual groundbreaking
concept.

The project provides solid basis for restricting MFC applications to powering small systems that require little energy. Incremental research to develop large MFCs would be futile; ground-breaking concepts are needed to remove the bottlenecks identified in the project. On the other hand, the project has highlighted unsuspected processes in the self-organization of electroactive biofilms that open innovative ways to apply MFCs to the recovery of metallic elements and the development of nanoparticles.

Practically, the project led to the installation of an automated abiotic air cathode manufacturing line at the industrial partner PaxiTech. From an academic point of view, the results led to 8 published articles, 2 submitted and 4 in preparation for peer-reviewed journals, 1 book chapter, and a total of 21 communications in congresses and seminars including 7 oral communications in international congresses..

Microbial fuel cells (MFC) can transform directly into electrical energy the chemical energy contained in various organic compounds. A MFC uses microorganisms, which adhere spontaneously on the surface of the anode and form a biofilm that oxidizes organic compounds by directly transferring the electrons to the electrode. With this new type of electrocatalysis, discovered in the early 2000s, MFCs produce electrical energy by oxidizing various organic compounds (acetate, volatile fatty acids, alcohols, (poly)saccharides ...) contained in natural environments or can be obtained from biomass.

The majority of the MFCs consists of a microbial bioanode associated with an abiotic air cathode. This configuration produces electricity by oxidizing generally acetate and using the reduction of oxygen at the cathode. The performance of these MFCs grew rapidly in the beginning, but has leveled off at a few Watt per square meter of electrode surface area from 2008. Unfortunately, no serious tracks are now available for the engineer to exceed this threshold around a few W/m2. However, some groups, including partners gathered in the previous AgriElec (ANR-08-BIOE 001) project, have developed microbial bioanodes that produced current densities beyond 50 A/m2. For comparison, the current density provided by photovoltaic panels is of the order of 100 to 200 A/m2. The partners have also produced abiotic air cathodes and microbial biocathodes at the best level of the state of the art, but they have only managed to increase the power density limit of a few W/m2.

The experience gained during the previous project brings several conclusions: bioanodes and biotic or abiotic cathodes that are designed separately in optimal conditions do not work in harmony when they met in a MFC module; engineering-oriented analyses of MFCs are rare and generally focus one or the other elements of the stack but do not embrace the overrall process; finally, cathodes represent a major bottleneck, the abiotic air cathodes have limited performance, the microbial biocathodes look promising but their potential has been little investigated.

The Bioelec project will launch an engineering approach "in the right direction." The overall MFC process will first be analyzed in terms of thermodynamic, mass transfer and electrochemical kinetics to extract the optimal operating conditions for each element. These conditions extracted from the analysis of the whole process, will then be imposed on the design of the electrodes. A first prototype will be built with an abiotic air cathode associated with a separator (or membrane) and a bioanode. This separator-electrode assembly will ensure a power of 1 Watt. A second prototype will aim to overcome the cathode bottleneck by developing an air microbial biocathode. Here, the objective is to link the concepts of gas diffusion cathode and microbial biocathode to overcome the low solubility of oxygen in solution. This is an exploratory, more ambitious goal.

The consortium is composed by five partners: an industrial company that manufactures hydrogen-air fuel cells, three laboratories that provide expertise in engineering, microbiology and physical-chemistry of surfaces and a company devoted to technology transfer.