Microbial Fuel Cells: Prospects as a Renewable Energy Source

Introduction

Fig. 1: Schematic of a microbial fuel cell. Image source: M. Hurley, after Schröder. [4]

A fuel cell converts chemical energy into electrical energy, producing a current. [1] This conversion process takes place in the presence of a continuous external supply of raw material (fuel), which releases electrons via a reduction-oxidation reaction. Fuel cells have long been touted as a potential alternative to fossil fuels; attention has focused primarily on versions that use hydrogen as their chemical energy source. [2] However, more recent developments include a bio-variation on this theme: the microbial fuel cell (MFC). MFCs may evade some of the challenges posed by hydrogen (such as its production, storage, and cost), and have attracted interest as a carbon-neutral source of renewable energy. [3] They do, however, offer their own set of difficulties and are at present primarily an object of research.

Fig. 1 shows a schematic of a typical MFC. [4] In solution, microbes consume organic substrate (e.g., glucose, sucrose, acetate) as fuel. As shall be discussed further below, the organisms, in the course of their metabolism, oxidize the substrate, releasing electrons which are then transported to the anode. The transfer from microbe to anode may take place through various methods, including conductive nanowires (bacterial pili), cellular-membrane-bound redox-active proteins, or secondary metabolites produced by the bacteria, all of which serve as electron shuttles. [4,5] The electrons then pass from the anode to the cathode, producing an electric current which can be coupled to an external load to act as a power source. At the cathode, reduction occurs as the electrons combine with some final oxidizing agent, most commonly oxygen. In this two-chamber configuration, the anodic and cathodic sides of the MFC are separated by a membrane which allows the permeation of certain cations (typically H⁺ ) to maintain charge balance and participate in the final reduction reaction.

Basic Physical Model

Microbes can extract energy from many organic compounds via respiration. As a concrete example commonly implemented in real MFCs, consider the breakdown of acetate; the overall reaction is [6]

The microbial metabolic pathways thus "burn" acetate to produce water, carbon dioxide, and energy, which they use to power their essential biological functions. Note that while CO₂ is generated here, the process is carbon-neutral, as it is part of a sustainable carbon lifecycle: the carbon in the CO₂ comes from organic molecules, which were created by plant life that absorbs CO₂ from the atmosphere to use as raw material.

In an MFC, the above reaction is separated into two half-reactions. At the anode, oxidation of the acetate takes place and electrons, denoted e^-, are produced:

At the cathode, on the other hand, these electrons then recombine with the hydrogen ions in the presence of oxygen to create water:

The energy released or absorbed by a reaction can be quantified in terms of its reduction potential E_r, which is related to the change in the Gibbs free energy, ΔG, as E_r = -ΔG/(nF), where n is the number of electrons transferred per reaction mole and F = 9.65 x 10⁴ C mol^-1 is Faraday's constant. [7] A reaction's "standard" reduction potential, E_r⁰, is the reduction potential as measured under a set of standard conditions (temperature of 298°K, pressure of 1 bar, concentrations of 1 M). Under a different set of reaction conditions, the reduction potential can then be calculated from the standard value via the Nernst equation:

Here R = 8.314 J K^-1 mol^-1 is the ideal gas constant, T the temperature at which the reaction occurs, and 𝛱 the reaction quotient (approximately equal to the product of the concentrations of the reaction products divided by the product of the concentrations of the reactants).

Using standard values for the Gibbs free energy of formation (-389.9 kJ mol^-1 for CH₃COOH, -237.1 kJ mol^-1 for H₂O, -386 kJ mol^-1 for CO₂, and zero for the remaining components) gives, for the (inverse) anodic reaction, a reduction potential of [8,9]

For the cathodic reaction we have similarly

To roughly calculate the reduction potentials under realistic reaction conditions, we consider concentrations [CH₃COOH] = 5 mM, [CO₂] = 5 mM, pO₂ = 0.2, and pH = 7, at a temperature of 300°K. [7] The reduction potential for the anode is then

For the cathode, we find

Ec	=	Ec0- RT 8F × ln( 1 (pO2)2 [H+]8 )
	=	0.12 V - 8.314 J °K-1 mol-1 × 300°K 8 × 9.65 × 104 C mol-1 ) &time;s ln( 1 (0.2)2 × (10-7)8 )
	=	0.8 V.

The overall voltage for the MFC is then given by the difference between the cathode and anode reduction potentials:

In the above example, we arrive at a cell voltage of E_MFC = 0.8 V - (-0.3 V) = 1.1 V.

Power Output of MFCs

Fig. 2: Representative (a) voltage vs. current and (b) power vs. current curves for an MFC. Dashed lines are schematic curves for a cell with strong Ohmic losses, showing the linear and quadratic behavior in Eext(I) and P(I), respectively. Solid curves are data from a starch-operated MFC. (Image source: M. Hurley, after Logan et al. [7])

The value E_MFC represents the largest possible voltage that can be produced by the fuel cell. This ideal value is, of course, never actually attained. First, some chunk of the released energy must be used to sustain the vital functions of the bacteria powering the cell. In principle, this number can be almost arbitrarily small: estimating 10 bacteria per μm² with a base metabolic rate of 10^-19 W per bacterium gives a power of roughly 10^-6 W/m². [10,11] This expenditure is much less than the MFC power, which is typically of order 10^-1 W/m² (see discussion below). However, in practice it is not easy to control or limit the energy intake of the microbes; this process depends on the specifics of the metabolic pathway, as well as their ability to transfer electrons to the anode. A low anode potential gives a larger external voltage that approaches the theoretical limit of E_MFC, but reduces the bacterial metabolic gain and renders the transfer of electrons from bacteria to anode more difficult: only some of the generated electrons actually enter the circuit. [7] There is thus in general a trade-off between the maximum attainable voltage and the cell's efficiency.

The power output of the MFC is calculated as

where I is the current through the circuit connecting anode and cathode and E_ext the corresponding voltage over the external load. In addition to losses associated with the bacterial metabolism, an MFC will have "Ohmic" losses caused by the internal resistance of the fuel cell's electrodes and solvent. [12] Denoting this internal resistance by R_int, by Ohm's law the corresponding internal voltage drop will be IR_int. Therefore if the overall cell voltage is some E_cell (bounded by our E_MFC above), the external emf will only be E_ext = E_cell - IR_int, which gives a power output of

The quadratic dependence of the power on the current I implies that there is some finite current (or, equivalently, some finite load) that corresponds to a maximum in the power density. Additional non-Ohmic losses will distort the shape of this power vs. current curve, but it will in general be the case that the external voltage is some decreasing function of current, placing an upper bound on the obtained power (see Fig. 2). [7] Both the maximum power and the corresponding current are typically quite small, taking values < 1 W and < 1 A, respectively.

Comparison With Other Sources of Energy

We now review MFC performance along key metrics of energy production.

• Efficiency: The energy efficiency of MFCs has reached up to 50%. [5] This performance is superior to that of most solar photovoltaic cells (10%-20%), on par with wind power (20%-50%), inferior to hydroelectric power (70%-90%), and similar to traditional fossil fuels (30%-40%). [5] Reducing Ohmic losses, improving electron transfer to the anode, and minimizing bacterial metabolic gain currently remain topics of intense research focus on this front.

• Lifetime: In contrast to batteries, which have a fixed energy supply, MFCs can in principle generate energy indefinitely so long as they have access to a source of raw material. However, in practice it is a challenge to maintain steady reaction conditions over extended periods of time; for instance, the production of H⁺ at the anode and consumption at the cathode can lead to a pH imbalance, lowering the reduction potential of the cell and possibly impacting bacterial metabolism. [12] While some MFCs have demonstrated continuous operation for multiple years without degradation, it is common to see a drop-off in the generated power over the span of hundreds of days. [13,14] This behavior far lags behind e.g., solar panels, which can last for over 20 years and degrade at a rate of <1%/year. [15]

• Power: The energy per unit time produced by the MFC is affected by many factors, including the choice of organic substrate, microbes, electrode materials, and cell design. Expressed per unit surface area of the anode, power outputs have ranged from 1 mW/m² to 3 W/m². [5,16] These values are much smaller than those of hydrogen fuel cells (thousands of W/m²), primarily due to increased internal resistance associated with electron transfer through the biofilm as well as the environmental conditions necessary for microbial growth, which can impede proton transport and oxygen reduction kinetics at the cathode. [17,18] A solar cell experiencing solar irradiance of ~1000 W/m² with an efficiency of 20% produces about 200 W/m². [19]

• Cost: Because there has so far been little to no commercialization of MFCs, data for their production cost is scarce; however, there are some reports of up to ~$600/W. [20] This cost far exceeds that of fossil fuels (~$4/W), other renewables such as solar (~$1/W), and other fuel cells (~$7/W). [21] The large cost of the MFC originates primarily from the electrode materials and cation-permeable membrane; these expenses are also common to other kinds of fuel cell, but the low power output of MFCs leads to higher cost/W.

Outlook

While attractive for many reasons, at this stage MFCs are unlikely to be a viable means of large-scale energy production. Though their efficiencies and output voltages are in line with other technologies, the maximum power exhibited to this point lags far behind competing energy sources, and they are prohibitively expensive. However, MFCs are receiving much research interest, and the advent of new electrode materials, superior cell designs, and increased control over the complex biological processes at play may enable meaningful progress. At present, they show promise for more limited applications, such as wastewater treatment: MFCs can be used to break down organic matter in water, generating electricity to reduce the energy cost relative to alternative methods. [18,22] Niche yet important use cases such as this one will serve as the initial proving ground for MFCs as practical devices.

© Matthew Hurley. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

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