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. 2021 Sep 20:3:100057.
doi: 10.1016/j.bioflm.2021.100057. eCollection 2021 Dec.

Microbial fuel cells and their electrified biofilms

John Greenman  1 , Iwona Gajda  1 , Jiseon You  1 , Buddhi Arjuna Mendis  1 , Oluwatosin Obata  1 , Grzegorz Pasternak  2 , Ioannis Ieropoulos  1
Affiliations

Microbial fuel cells and their electrified biofilms

John Greenman et al. Biofilm. .

Abstract

Bioelectrochemical systems (BES) represent a wide range of different biofilm-based bioreactors that includes microbial fuel cells (MFCs), microbial electrolysis cells (MECs) and microbial desalination cells (MDCs). The first described bioelectrical bioreactor is the Microbial Fuel Cell and with the exception of MDCs, it is the only type of BES that actually produces harvestable amounts of electricity, rather than requiring an electrical input to function. For these reasons, this review article, with previously unpublished supporting data, focusses primarily on MFCs. Of relevance is the architecture of these bioreactors, the type of membrane they employ (if any) for separating the chambers along with the size, as well as the geometry and material composition of the electrodes which support biofilms. Finally, the structure, properties and growth rate of the microbial biofilms colonising anodic electrodes, are of critical importance for rendering these devices, functional living 'engines' for a wide range of applications.

Keywords: Bioenergy; Electricity; Microbial fuel cell; Perfusion electrodes; Synchrony.

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Conflict of interest statement

The author(s) declare no competing financial interests.

Figures

Fig. 1
Fig. 1
Examples of membrane-based MFC designs: a) cuboid, double chamber MFC, b) cylindrical double chamber MFC, c) spherical double chamber MFC, d) cuboid MFC with open to air cathode, e) H-type, double chamber MFC. In all designs, “A” and “C” indicate anode and cathode respectively. Inputs and outputs (when these are used for continuous flow) are marked with arrows.
Fig. 2
Fig. 2
Schematic design of M-MFC: a) top view, b) cross section.
Fig. 3
Fig. 3
Non-polymeric membranes e.g. terracotta MFC with a multi-MFC modular stack.
Fig. 4
Fig. 4
Three main mechanisms for anodic electron transfer from cell reducing power (NADH/NADPH) to the anode electrode in MFC through (a) soluble mediator, (b) direct contact to an outer membrane cytochrome, (c) direct contact to a membrane cytochrome via conductive pili.
Fig. 5
Fig. 5
MFC carbon electrode structure: a) plain carbon electrodes of same geometric macro size compared and b) carbon veil and carbon felt (or mat) compared. SA:V represents the surface area to volume ratio.
Fig. 6
Fig. 6
Conventional biofilm paradigm indicating the attachment phase (1) and (2), biofilm maturation forming microcolonies (3), then matrix formation (4) and erosion (5).
Fig. 7
Fig. 7
Alternative biofilm theory for perfusion biofilms and carbon veil electrodes illustrating process of attachment, colonisation and maturity.
Fig. 8
Fig. 8
Biofilm, perfusate populations and rates of glycolysis from the Sorbarod model over time. A. Growth of S. mutans biofilm perfused with 1/5th strength TYE followed over 3 days. Closed symbols show total biofilm populations; Open symbols show perfusate numbers (cfu ml-1). B. Schematic of the Sorbarod perfusion biofilm system with electrodes. C. Shows how growth rates (h-1) vary across increasing flow rates (ml h-1); maximum specific growth rate = 0.28 h-1. D. Outputs from pH electrodes placed at the top (inflow; open symbol) and bottom (outflow; closed symbol) showing changes in H+ production with flow rate. The difference between the lines indicates that the pH gradient becomes less the faster the medium is supplied. Therefore, gradients diminish. E. Response of model to glucose pulses showing repeated responses to increasing concentration of glucose injected as 0.5 ml pulses. Note the stability of the baseline throughout the experiment. The inset shows the dose response curve.
Fig. 9
Fig. 9
Steady state power output from Geobacter sulfurreducens. The slight changes in output (<4%) are probably due to slight temperature fluctuations. Reproduced from Greenman et al. [101] with permission from the Publisher.
Fig. 10
Fig. 10
Experiment to test a carbon coated polyurethane sponge as the anode in small scale MFC. A) Temporal MFC output from the start of the experiment. Black arrows show periods when the resistor was disconnected in order to measure the open circuit voltages. Red arrows show periods of time when polarisation experiments were being conducted. B) the scheme of the MFC anode used, C) power curves performed on 12,18, 26 and 36 day of the experiment. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 11
Fig. 11
Exclusion-exfoliation theory for explaining stability. The grey coloured object represents a graphite thread or strand, one of many in carbon veil. The blue particles represent conductive electroactive species. The red particles represent heterotrophic species (typically bacteria that ferment organic substrates). Mixed culture: Inner “core” pushes out any attached heterotrophs that may themselves be dividing. Outer cores are mechanically pushed away by exfoliating daughter cells from the inner core of cells. Heterotrophs adhere to outside surface of daughter cells so wash away as an aggregate of cells and exoelectrogens. The exposed surface “becomes” the next daughter cell and also pushes away. The process repeats. Even though heterotrophs are growing, the vast majority are washed away with the rest. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 12
Fig. 12
Power output versus dilution rate. Data taken from previous work, parts of which have been published in You et al. [117].
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