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Biochemical Engineering Journal 73 (2013) 53– 64
Contents lists available at SciVerse ScienceDirect
Biochemical Engineering Journal
journa l h omepage: www.elsev ier .com/ locate /be j
eview
verview on the developments of microbial fuel cells
.B. Oliveirab,∗, M. Simõesb, L.F. Melob, A.M.F.R. Pintoa,∗
CEFT, Departamento de Eng. Química, Universidade do Porto, Faculdade de Engenharia, Rua Dr. Roberto Frias, 4200-465 Porto, PortugalLEPAE, Departamento de Eng. Química, Universidade do Porto, Faculdade de Engenharia, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
r t i c l e i n f o
rticle history:eceived 9 July 2012eceived in revised form9 December 2012ccepted 17 January 2013vailable online xxx
a b s t r a c t
Microbial fuel cells (MFCs) are a promising technology for electricity production from a variety ofmaterials, such as natural organic matter, complex organic waste or renewable biomass, and can beadvantageously combined with applications in wastewater treatment. The problem with MFCs is thatthey are technically still very far from attaining acceptable levels of power output, since the performanceof this type of fuel cells is affected by limitations based on irreversible reactions and processes occurringboth on the anode and cathode side. However, in the last years, there has been a growing amount of work
eywords:icrobial fuel cellsodellingperating conditionscale-upustainable energy
on MFCs which managed to increase power outputs by an order of magnitude.The present review article discusses a number of biological and engineering aspects related to improve-
ment of MFC performance including the effect of important parameters, such as pH, temperature, feedrate, shear stress and organic load. The recent progresses on scale-up MFC are summarized and the dif-ferent modelling approaches to describe the different biological and transport phenomena in MFCs arealso provided.
© 2013 Elsevier B.V. All rights reserved.
ontents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532. Scale-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553. Operational conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.1. Effect of pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573.2. Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.3. Organic loading rate (OLR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.4. Feed-rate and shear-stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4. MFC models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
. Introduction
There is no effective way to keep us living on fossil fuelsnd reduce greenhouse emissions through increases in energyfficiency alone. Although efficiency is an essential componentf any plan to get us back on the track of balanced growth, newnergy sources and technologies must be developed. These newolutions should produce carbon dioxide at a lower rate than thehemical cycles of our planet are able to manage in the short-term.
If we consider the simple concept that in man’s life-essentialactivities, wastes are produced, a technology that can use thesewastes and turn them into useful energy is probably the most-close-to-Nature form of energy production. Microbial fuel cells(MFCs) fulfil this requirement: they can use natural wastewaters(or other wastes) produced by the human being, which containsubstrates and microorganisms, and turn them into electricity orhydrogen via adequate metabolic pathways. Although some exter-nal resources are consumed (electrodes, electrical wires), theyrepresent only a minimal fraction in the whole environmental bal-
∗ Corresponding authors. Tel.: +351 225081675.E-mail addresses: [email protected] (V.B. Oliveira), [email protected]
A.M.F.R. Pinto).
369-703X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.bej.2013.01.012
ance equation.MFCs are similar to any other battery or fuel cell, having two
electrodes connected through an electrical wire. The difference isthat they use organic substrates as fuels, on the anode side, to
54 V.B. Oliveira et al. / Biochemical Engineering Journal 73 (2013) 53– 64
Nomenclature
aA, aC anode and cathode constantsA areaAF biofilm areabA, bC anode and cathode constantsbata biomass attachment ratebdet biomass detachment ratec voltage loss parameterCMOX oxidized mediator concentrationCO2 oxygen concentrationC0
O2inlet oxygen concentration
CS substrate concentrationC0
S inlet substrate concentrationD dilution factorDS,F diffusion coefficient of substrate in biofilmE0 thermodynamic equilibrium potentialF Faraday’s constantfX reciprocal of wash-out fractionI currentj current densityjL limiting current densitykA constant in the anode rate expressionkC constant in the cathode rate expressionKdec decay rateKdec,a decay rate of anodophilic microorganismsKdec,m decay rate of methanogenic microorganismsKMOX half velocity rate constant oxidized mediatorKO2 half velocity rate constant for oxygenKS half velocity rate constant for substrateKS,a half velocity rate constant of anodophilicsKS,m half velocity rate constant of methanogensQ substrate consumption rateqa substrate consumption rate by anodophilic
microorganismsQA anode flow rateQC cathode flow rateqm substrate consumption rate by methanogenic
microorganismsqmax maximum specific rateqmax,a maximum anodophilic reaction rateqmax,m maximum methanogenic reaction rateR gas constantrA anode reaction raterC cathode reaction rateRint internal resistanceRohmic ohmic resistancerS,E substrate rate on the electrode surfacerS,F substrate rate on the biofilmRsol anode solution resistancet timeT temperatureY bacterial yieldVA anode volumeVC cathode volumeVCell cell voltageVF biofilm volumex coordinate directionX biomass concentrationX0 inlet biomass concentrationXa concentration anodophilic microorganisms
˛A anodic transfer coefficient˛m biofilm retention constant for methanogenic
microorganisms cathodic transfer coefficient
�A anode overpotential (V)�act activation overpotential (V)
(improvement in electrode structure, new catalyst electrolytes).In the last years, advances have been made in the MFC research
and some review articles on this type of fuel cell are available,each one with a different accent or emphasis (Fig. 1). These
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Xm concentration methanogenic microorganisms˛a biofilm retention constant for anodophilic microor-
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�C cathode overpotential (V)�conc concentration overpotential (V)
produce electricity. On the anode the organic matter is oxidizedreleasing electrons and protons. The electrons flow to the cathodethrough the external electric circuit producing electrical current. Inthe cathode, oxygen (or other electron acceptor) reacts with elec-trons and protons producing water (or other reduced compound).
The ideal performance of an MFC depends on the electrochem-ical reactions that occur between the organic matter and the finalacceptor, such as oxygen. The real/actual cell potential is alwayslower than its ideal value due to three irreversible losses that occurin a MFC: the activation polarization, the Ohmic losses and the con-centration polarization. The activation polarization is dominant atlow current densities and is due to an activation energy that mustbe overcome by the reacting species. Phenomena involving adsorp-tion or desorption of reactant species, transfer of electrons and thenature of the electrode surface contribute to the activation polar-ization. In MFCs where microbes do not readily release electronsto the anode, activation polarization is enhanced, generating anenergy barrier which can be minimized by adding mediators. In themiddle of the operating range, the predominant loss is the Ohmicloss which is due to ionic and electronic conduction. This loss canbe reduced by shortening the distance between the two electrodes,by increasing the ionic conductivity of the electrolytes and usingmembranes with lower resistance. At very high current densities,the major loss is the concentration polarization which is due to theinability to maintain the initial substrate concentration in the bulkfluid and to mass transport limitations. Stirring and/or bubblingas well as the MFC design helps to reduce this loss. Polarizationcurves of an MFC indicate the various losses and the extent of eachone, pointing out to possible measures to minimize them in order toapproach the ideal potential. These measures may include selectionof new microbes, mediators, substrates and modifications in theMFC design (short spacing between electrodes) and configurations
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Fig. 1. MFC review papers based on areas of investigation.
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V.B. Oliveira et al. / Biochemica
nclude the following: microbial metabolism [1,2], biocathodessed on the cathode side [3–5], biocatalyst used in both anode andathode [6,7], microbial communities [8–13], possibilities for thisechnology [14,15], physical and biochemical properties of elec-rochemically active bacteria used in mediator less MFC systemsnd their electron transport chain [16], different MFC configura-ions and designs [2,4,6,8,10–12,17,18], different construction and
aterials, and methods of data analysis and reporting in ordero give information to the MFC developers [17], comparison ofhe conventional anaerobic digestion and the MFC technologynd the potential of the MFC systems as a complement to thenaerobic digestion technology [19], performances and optimiza-ion of operating parameters of the MFC systems, anodic electronransfer processes [20], cathodic limitations, due to irreversibleeactions and processes that occur on the cathode side that canffect the MFC performance [6,21], different substrates used inFCs [4,9,12,22], different separator materials and configurations
2,4,9,23], challenges of the MFC technology [6,10,11,18,24], differ-nt electrode materials and designs both for the anode and cathodeide [2,4,9,10,15,24,25], applications of MFCs [8,10,12,13], stackedFCs [9,18] and patents until 2008 [26].Based on the review articles already published and described
bove and due to a lack of recent work regarding developments andodelling on MFCs, the main goal of this paper is to provide a recent
eview concerning the scale-up studies, the developments in MFCsith emphasis on optimization of operational conditions (such asH, temperature, organic load, feed rate and shear stress), and MFCodelling. We sincerely hope that this review will be highly useful
or readers who are interested to initiate their work in this area, asell as, to the researches already working in this field to access therogress achieved until now.
. Scale-up
Most MFCs studies were conducted at lab scales, however, scal-ng up this type of fuel cells is inevitable to achieve levels of powereeded to power real electrical devices. In order to make MFCs suit-ble for real applications, such as wastewater treatment plants, its critical to achieve high power densities at a large scale.
The first demonstration of a MFC as an alternative and viableower supply to batteries for a low-power consuming applica-ion was reported by Tender et al. [27]. The specific applicationeported was a meteorological buoy that measures air temperature,ressure, relative humidity, and water temperature, configured foreal time line-of-sight RF telemetry of data. The specific type ofFC employed in this demonstration was the benthic microbial
uel cell, which operates on the bottom of marine environments,here it oxidizes organic matter residing in oxygen depleted sedi-ent with oxygen in the overlying water. It is maintenance free,
oes not deplete and is sufficiently powerful to operate a wideange of low-power marine-deployed scientific instruments nor-ally powered by batteries. Despite this first attempt in scale-upFCs, bringing this technology out of laboratory appears as a chal-
enge in the MFC developments, due to the high power densities andow costs materials (precious metals should be avoid) needed andince previous studies regarding this issue revealed that the powerensity decrease on the scale-up MFC systems [28,29]. Clauwaertt al. [30] presented a review paper analyzing this effect in terms of
volume-based resistivity, multiplying the internal resistance byhe reactor volume. Using the volume instead of surface area is use-ul when the exact active surface area cannot be determined and
llows a relative comparison between reactors of different volumes.hey show that increasing reactor volume leads to an increasen the volume-based resistivity. This resistivity can be analyzedn terms of a number of elements that all contribute to the totaleering Journal 73 (2013) 53– 64 55
internal resistance, like anode and cathode overpotential, con-centration overpotential, membrane resistance, and solutionresistance. Scale-up of MFC will require a better understanding ofthe effects of reactor design and operation mode on volumetricpower densities. Logan [31] also, present a review regarding theadvances and future challenges in scaling up MFCs with respectto their practical application for renewable energy production andother applications. The different types of electrodes materials andtheir surface areas, the importance of current collectors to improvepower production, and the role of separators in reactor construc-tion are reviewed. It is also, provided a review concerning the pilottests that are being undertaken to understand how to build andoperate scalable systems.
Based on the review articles mentioned above regarding thescale-up and due to a lack of a recent review work regarding theefforts made to develop low cost materials, new MFC configura-tions and to evaluate the effects of scaling up on power output thegoal of this section is to provide a recent review concerning theseissues.
To scale-up the MFC technology, single small units can be con-nected together or the volume of a single MFC unit can be increased[28,29,32–48]. However, producing larger MFCs can alter electrodespacing, and thus affect power density through changes in the spe-cific internal resistance area. Questions such as how MFCs poweroutput is related to the surface area of electrodes should, therefore,be answered. Dewan et al. [29] presented a study concerning thequantification of the relation between the surface area of the cur-rent limiting electrode of a MFC and the power generated by thiscell. The authors used a surface area on the cathode larger thanthe one of the anode to ensure that the anode was the currentlimiting electrode. Their results show a maximum power densityof 0.329 mW/cm2 for the smallest electrode used (1.92 cm2) andincreasing the electrode surface area 80 times leads to a decreaseof power density to less than half (0.141 mW/cm2). They foundthat the maximum power density generated by a MFC is notdirectly proportional to the surface area of the anode, but is insteadproportional to the logarithm of the surface area (power den-sity = − 0.0369 × ln(surface area) + 0.3371). Cheng and Logan [32],also, studied the effect of electrode surface area on the MFC perfor-mance using domestic wastewater. They found that doubling thecathode size can increase power by 62%, but doubling the anodesize only increases power in 12%. These results suggest that thecathode specific surface area is one of the most critical factors forscaling-up MFCs. Liu et al. [33] studied the effect of the electrodespacing on MFC performance. The electrode spacing and orientationwere found to be the key factor affecting the area-specific, inter-nal resistance and power density. Therefore, to maintain the powerdensity during scale-up, the larger reactor architecture must main-tain or even reduce the electrode spacing. The reduction in averageelectrode spacing (2.6 cm for a large MFC (520 mL) and 4 cm for asmall MFC (28 mL)) leads to an increase of 15% in power density(14–16 W/m3). An increase of 36% in power density (14–19 W/m3),obtained with the small MFC, was also, achieved when the electrodeorientation was changed from parallel to perpendicular, which asmainly due to a reduction of the electrode spacing (from 4 cm to2.5 cm). This work demonstrates that power output can be main-tained or even increased during reactor scale-up if the key factors,especially electrode spacing, are considered and optimized duringthis process.
To avoid the problems of electrode spacing, area and orienta-tion, when scaling up a MFC using a large volume cell, some workhas been done regarding the stack configuration and some of them
proved that this alternative may be a more effective way to achievethe power outputs needed for real applications [28,34–39].Ieropoulos et al. [28] studied different MFC sizes operating undercontinuous flow in order to propose an optimum configuration for
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6 V.B. Oliveira et al. / Biochemica
caling-up MFCs. They compared the performance of a small unitFC (6.25 mL), 5 and 10 small units connected together (31.25 mL
nd 62.50 mL, respectively), a medium unit (25 mL) and a large unit500 mL) and found that the power density per unit of anode vol-me increase 33% for 5 small units, 29% for 10 small units andecrease 35% and 72% for, respectively, medium and large units0.75 W/m3, 1.00 W/m3, 0.97 W/m3, 0.49 W/m3 and 0.21 W/m3,espectively). These results suggest that MFC scale-up may beetter achieved connecting multiple small-scale units rather than
ncreasing the size of a single unit. They, also, found that the series-arallel configuration proved to be more efficient than the seriesnd parallel configuration.
When the MFCs units are connected in stacks, voltage, currentr both can be stepped up depending on the stack size and config-ration. This is very useful since the energy requirements can beet by adjusting the size of the stack through the removal or addi-
ion of units. However, stacking multiple MFCs together in serieslso result in some problems, such as voltage reversal, contact volt-ge losses and erratic operation [35,36]. Also, the connection ofultiple MFCs may be further complicated if the units are running
nder continuous flow conditions, which involves electrically andydraulically fluid connections. Connecting multiple MFCs in con-inuous flow requires the units to be fluidically joined to an inflownd outflow stream [28]. However, Zhuang and Zhou [35] showhat there is a voltage loss around 36% when the cells are hydrauli-ally and electrically connected instead of electrically connectednd hydraulically isolated (1.1 W/m3 and 1.3 W/m3, respectively).heir work suggests that is better to separate the influent feed ofhe different fuel cells units presented in the stack. Although, thisesign for the stacked MFCs requires an individual recirculation
oop for substrate feed and discharge. Such distribution system isoo delicate and undeniably unsuitable for real scale-up MFC tech-ologies, such as wastewater treatment. To avoid these connectionroblems Liu et al. [36] described a novel configuration of stackedFCs bridged internally through an extra cation exchange mem-
rane. The performance of the stack (assembled from two singleFCs) was evaluated with focus on the electricity generation and
OD (chemical oxygen demand) removal of the feedstock. Stud-es on the external circuit connection and half cell analysis werelso presented to further characterize the stack. The MFC stackhowed a an increase on power output from 1.0 W/m3 (single MFC)o 1.4 W/m3, half optimal internal resistance and the COD removalate increased from 32% to 54%.
Zhuang et al. [37], also, developed a new design to overcomehe connection problems, a tubular air-cathode MFC stack that wasperated with real swine wastewater containing high substrateomplexity and strength. As an effort to reduce the material cost,he MFCs used MnO2 instead of Pt as cathode catalyst. Five indi-idual cells were connected in series or parallel and the maximumower density (11.2 W/m3) was achieved with a parallel connec-ion. This kind of reactor design seems to be more suitable for usingn real wastewater treatment since they are analogous to the exist-ng wastewater treatment processes, having a continuous plug flow
ith a simple water distribution system, and are highly scalable inize and flexible in shape. Based on that, Zhuang et al. [38] devel-ped a 10 l serpentine-type MFC stack using 40 tubular air-cathodeFC units in a 3D alignment pattern. The performance of the stack
n both series and series-parallel circuit is compared. A maximumower density of 4.1 W/m3 was achieved when operating the stack
n series and fed with brewery wastewater.Another problem of connecting small MFCs units was found by
eropoulos et al. [39]. In this work, they investigate the effects of dif-
erent polymer electrolyte membranes (PEM) and MFC structuralaterials on the stack performance and found that the PEM thatllowed higher energy output levels in individual MFCs (Hyflon87-03 shown to produce twice times the power output of the
eering Journal 73 (2013) 53– 64
standard PEM) was the type causing the most severe cell-reversalphenomena in stacks (better with a standard PEM). Regardingthe four polymeric structural materials (acrylo-butadiene-styrene(ABS) coated, ABS coated with methyl-ethyl-ketone (ABS-MEK) andpolycarbonate) tested in a stack of four small-scale units the resultsshow a highest power density (86 W/m3) for polycarbonate.
Another challenge for scaling up MFC technologies is developinglow-cost, simple constructions and easy to maintenance systemsthat can generate high power outputs. Many efforts have been madein order to achieve these goals [40–48].
Jiang and Li [40] developed a single-chamber MFC (SCMFC) con-figuration granular activated carbon (GAC) bed as the anode andconnecting with a single piece of carbon cathode. GAC is a com-monly used packing material in water and wastewater treatmentprocesses, is an inexpensive and durable material with a high sur-face area. They found that GAC is a good substitute for carboncloth since the GAC-SCMFC generate higher and stable power out-puts than the traditional two-chamber MFCs. Following this work,the authors presented a new design based on multi anodes andcathodes MFCs [41]. The power density of a 4-anode/cathode MFC(1.18 W/m3) was 3.2 times higher than a single-anode/cathode MFC(0.35 W/m3). This design incorporates multiple MFCs into a singleunit, which maintain high power generation at a low cost and smallspace occupation for the scale-up MFC systems.
Zuo et al. [42] presented a new approach for cathode designbased on converting tubular ultra filtration membranes into MFCcathodes. In order to reduce costs they used a nonprecious metalcatalyst (CoTMPP) on the cathode. The results show a maximumpower density of 17.7 W/m3 using graphite brush as anode elec-trode and a tubular cathode corresponding to a increase of 78% froma standard MFC (carbon paper as anode and cathode electrode andPt as cathode catalyst). This result demonstrates that an MFC designusing tubular cathodes and brush anodes is a promising design forscale-up MFC systems. Following this work, Zuo et al. [43], pre-sented a work where two different cathodes constructed from alow-cost anion (AEM) and cation (CEM) exchange membranes thatwere made electrically conductive by using graphite paint and anonprecious metal catalyst (CoTMPP), capable of oxygen reductionat the surface were studied. The authors found that when combinedwith graphite bush anodes the AEM cathodes produced compara-ble power generation (21.2 W/m3) to a carbon electrode with thesame surface area, but much higher electron recovery efficiencieswith reduced material costs. Lefebvre et al. [44] manufactured alow-cost cathode that can be used for practical applications, sincethe material selected in this work has high availability on Earthand reduced cost as compared to other noble metals such as Pt.A sputter-deposited cobalt (Co) cathode was used as catalyst anddifferent types of cathode configurations were tested. By sput-tering the catalyst on the air-side of the cathode, they increasedcontact with ambient oxygen resulting in higher electricity gener-ation (4.7 W/m3 instead of 1.8 W/m3 obtained with basic design).Zhuang et al. [45] developed a membrane-less cloth cathode assem-bly (CCA) by coating the cloth with conductive paint (nickel-basedor graphite-based) and nonprecious metal catalyst (MnO2). Theresults show a maximum power density of 9.87 W/m3 when theconductive paint used is nickel-based, since the Niquel based CCAhas lower volume resistivity. Following this work, the same authorspresent a study comparing this new membrane with an anionexchange membrane and a cation exchange membrane in order toprovide an optimum cathode configuration for MFC scaling up [46].The MFCs were compared in terms of power production, Coulombicefficiency, COD removal, internal resistance and material cost under
batch mode. The experimental results showed that the best choiceis the CCA due to its higher power density (11.0 W/m3, insteadof 7.4 W/m3 (AEM) and 6.8 W/m3 (CEM)) and lower cost (623.6USD/W, instead of 1047.7 USD/W (AEM) and 1145.2 USD/W (CEM)).
V.B. Oliveira et al. / Biochemical Engineering Journal 73 (2013) 53– 64 57
Table 1Overview of various types of MFCs: the maximum power production in relation to the reactor design, electrode and membrane types.
Reactor design Volume (L) Separator type Anode Cathode MaximumPdensity (W/m3)
Reference
Substrate Material Substrate Material
5 small MFC 0.031 PEM Acetate Carbon fibber veil Oxygen Carbon fibber veil 1.0 [28]1 large MFC 0.520 – Acetate Carbon cloth Oxygen Carbon cloth/platinum 16 [33]4 MFC 20 CEM Acetate Titanium plates Oxygen Titanium plates 144 [34]2 tubular SCMFC 1.50 MCA Acetate Graphite felt Oxygen MnO2 1.3 [35]2 MFC 0.18 CEM Glucose Graphite plates Oxygen Carbon cloth/platinum 1.4 [36]5 tubular SCMFC 0.295 CEM Swine wastewater Graphite felt Oxygen Carbon fibber cloth/MnO2 11.2 [37]40 tubular SCMFC 10 Gore-tex cloth Brewery wastewater Graphite felt Oxygen Ni-based paint/MnO2 4.1 [38]4 SCMFC 0.025 CEM Acetate Carbon fibber veil Oxygen Carbon fibber veil 86.4 [39]4-Anode/cathode 1.5 – Acetate Graphite road Oxygen Carbon cloth/platinum 1.2 [41]Tubular SCMFC 0.026 – Glucose Graphite brush Oxygen CoTMPP 17.7 [42]Tubular SCMFC 0.026 AEM Acetate Graphite brush Oxygen CoTMPP 21.2 [43]Tubular SCMFC 0.085 – Acetate Carbon cloth Oxygen Co/Nafion/Carbon black 4.7 [44]Tubular SCMFC 0.17 Canvas cloth Brewery wastewater Graphite granules Oxygen Ni-based paint/MnO2 9.87 [45]Tubular SCMFC 0.17 Gore-tex cloth Brewery wastewater Graphite granules Oxygen Ni-based paint/MnO2 11.0 [46]MFC 5 CEM Acetate Titanium plates Ferric iron Titanium plates 200 [47]
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i
CEA-MFC 0.03 Non-woven cloth Acetate Car
In order to replace the use of noble metals in MFCs, Heijne et al.47] studied an MFC where Ferric iron was used as the electroncceptor instead of oxygen. The results show a maximum powerensity of 200 W/m3 that is 38% higher than the 144 W/m3 obtainedith the same scale-up MFC but using oxygen instead of ferric iron
s electron acceptor [34]. They found that the cathode reaction wasmproved by replacing oxygen reduction with ferric iron reductionnd the MFCs cost were reduced.
Fan et al. [48] studied the performance of a 10 times larger MFCith double cloth electrode assemblies (CEAs) during 63 days in
ontinues operation. This work shows that the use of low-cost non-oven cloth separator not only reduces MFC costs but also reduces
he anode–cathode spacing and internal resistance, enhancing theower generation. The maximum power density obtained with theEA-MFC was 2080 W/m3, which, as we are aware, is the highestower density obtained in MFC scale-up studies.
The relevant issues concerning the MFC scale-up were revised inhis section and are summarized in Table 1. The studies mentionedbove focus on improvements in power generation performance,ost-effective materials and new deigns that can be used in largercale applications. Reducing costs while increasing power outputss a great challenge for scaling-up MFCs to be used in wastewa-er treatment systems. Pt or other noble metal based catalysts,hich are the common cathode catalysts for oxygen reduction reac-
ion in small MFCs, will likely not be feasible for large-scale MFCs.sing different cathode designs or other electron acceptors insteadf oxygen to increase current densities shows great promise, butore work should be done regarding these issues. Stack MFCs con-
gurations seems to be the more effective way to scale-up MFCs.owever, connecting single MFCs units also, result in some diffi-ulties, such as connection and material problems. Previous workn scale-up showed that the best materials in single cell units mayot be the best materials for stacks.
Taken together, these findings indicate that scaling-up MFCs iseasible, but additional work needs to be done on increasing powerutputs and reducing costs so that MFCs can be used in practi-al applications. Some of the newest advances in scaling up hereeviewed showed that some progress was made towards theseoals.
. Operational conditions
Performances of laboratory scale MFCs are still lower than thedeal performance. For an existing MFC it is also possible to enhance
oth Oxygen Carbon cloth/platinum 2080 [48]
MFC performance by optimization of the operating conditions, suchas organic loading, feed rate and shear stress, pH and temperature.Recent work done on the optimization of these important param-eters in order to achieve the best performance in MFCs is reviewedin this section and is summarized in Table 2.
3.1. Effect of pH
In MFCs, the anode reaction produces protons that will flow tothe cathode side where will react with oxygen (or other reducedcompound) producing water. Continuous operation of an MFCcauses acidification at the anode as a result of accumulation of pro-tons produced on the microbial oxidation of organic compounds,due to slow and incomplete proton diffusion and migration throughthe membrane. On the other hand, alkalinization is observed onthe cathode side due to the continuous consumption of protons bythe oxygen reduction reaction and due to the lack of replacementof protons from the anodic oxidation reaction. These phenomenalead to a membrane pH concentration gradient which puts anelectrochemical/thermodynamic limitation on MFC performance.The increased pH in the cathode compartment can significantlydecrease current generation since according to the Nernst equation,the potential of the oxygen reduction reaction should increase witha decrease of the pH value. Decreasing the operational pH would bebenefit for the oxygen reduction and consequently to the currentoutput from MFCs [49–55]. Also, generally, bacteria require a pHclose to neutral for their optimal growth, and bacteria respond tothe changes in internal and external pH by adjusting their activity[49,50,53,56,57]. Depending on the bacteria and growth conditions,changes in pH can cause alterations in several primary physiologi-cal parameters, such as concentration of ions, membrane potential,proton-motive force and biofilm formation [58–61]. Anodic pHmicroenvironment is, then, one of the important factors, whichcan influence substrate metabolic activity and in turn affects theelectron and proton generation mechanism. The acidified anodecan decrease bacterial activity and consequently affect the biofilmperformance and stability [53,59,60]. In this way, the pH value onthe anode side plays a crucial role for the development and cur-rent production of anodic microbial electroactive biofilms. It wasdemonstrated that only a narrow pH window ranging from 6 to
9 was suitable for growth and operation of biofilms derived frompH neutral wastewaters [58–60,62]. Although, Raghavulu et al. [51]found that MFCs operated at low anodic pH may have higher protontransfer rates increasing the availability of protons at the cathode
58 V.B. Oliveira et al. / Biochemical Engineering Journal 73 (2013) 53– 64
Table 2Studies regarding the effect of operational conditions on MFC performance.
Study area Best performance Reference
pH
Anode Ph range 6–9 [53,58–62]Cathode Low ph values [49–55]
pH control (buffers)
Most used Phosphate [66,68–70]
Low cost
Bicarbonate [67]Borax [71]Synthetic zwitterionic [72]Carbon dioxide [73,74]
Temperature Temperature range 30–45 ◦C [50,64,70,75–91]
Organic loading rate Higher OLR [50,53,64,82,89,92–101]
spo
tiam[oAarratHtawtiromho
b[lsfchatp
omapp[emttA
Feed-rate Higher feed rates
Shear-stress Higher shear rates
ide. Due to the advantages of different pH values on each com-artment of the MFC, this parameter is crucial to the MFCs powerutput.
A traditional MFC can maintain two different pH conditionso optimize the anodic and cathodic reactions. In previous stud-es on the influence of pH, dual-chambers MFCs were operatedt different anodic and cathodic pH, and the difference in perfor-ance may also stem from the difference in anodic and cathodic pH
52,54,58,59,62]. It is, however, impossible to do so in air-cathoder single-chamber MFCs, because only one electrolyte is present.ir-cathode MFCs are advantageous due to their high power outputnd simplified reactor configuration. In this type of MFC configu-ation, the pH affects both anodic microbial activities and cathodiceaction, and thus is a very important factor. The performance ofn air-cathode MFC is determined by the mixed effects of the elec-rolyte pH on both anodic and cathodic reactions [56,57,61,63,65].e et al. [56] found that the air-cathode MFC can tolerate an elec-
rolyte pH as high as 10 with optimal conditions between pH 8nd 10. The anodic bacterial activity and cathode oxygen reductionere not only affected by the pH, but also changed the elec-
rolyte pH by supplying or consuming protons. The electrochemicalmpedance spectroscopy data demonstrated that the polarizationesistance of the cathode was the dominant factor limiting powerutput. The results showed that the anodic bacterial activity is opti-al at a neutral pH, while the cathodic reaction was improved at a
igher pH. Contrarily, in a continuous flow MFC the higher powerutput was observed at acidophilic conditions (pH 6.3) [64].
Due to the poor cathodic oxygen reduction and the detrimentaluildup of a pH gradient between anode and cathode, Cheng et al.65] described and tested a new concept that can overcome theseimitations, by inverting the polarity of the MFC repeatedly in theame half cell and neutralizing, in this way, the pH effects. Theyound that a mixed culture anodophilic biofilm can also catalyze theathodic reaction of oxygen in a single bioelectrochemical system,elping in overcoming the detrimental drifting of electrolyte pHnd poor cathodic oxygen reduction. Further research is neededo explore the application of such bidirectional microbial catalyticroperties for sustainable MFC processes.
As already mentioned, the pH gradient between anode and cath-de is one of the main causes for the sink of voltage efficiency inicrobial systems. To rectify this problem, active pH control such
s acid/base dosing or addition of chemical buffer (such as phos-hate or bicarbonate buffer) is required to sustain steady currentroduction, and some work has been done regarding this issue66–74]. Due to its chemical composition and interaction withlectrodes, bacteria, and membrane the buffer affects MFC perfor-
ance. In addition, the buffer helps to reduce changes in pH inhe bulk solution and in the biofilm, and therefore it maintainshe pH in the range suitable for the growth of microorganisms.n ideal buffer should be able to maintain constant pH without
[93,95,107–109]
[103,105,106]
interfering with chemical reactions or microbial physiology, facil-itate proton transfer to the electrode for high power densities inMFC and increase the solution conductivity [66–71]. Phosphatebuffers are the most commonly used in MFC [66,68–70], and ithas been shown that increasing phosphate concentration withincertain ranges will increase power output [70]. However, additionof high concentration of phosphate buffer is expensive especiallyfor wastewater treatment and phosphates can contribute to theeutrophication conditions of water bodies if the effluents are dis-charged without the removal of these compounds. Also, the lack ofcost effective phosphate recovery techniques makes it impracticalfor wastewater treatment. Bicarbonate buffer is another useful lowcost and effective pH buffer especially for wastewater treatment,although high concentrations of carbonate can enhance the growthof methanogens [67]. A borax buffer was used by Qiang et al. [71]and they found that adding a suitable concentration of borax bufferimproves the electron recovery efficiency. Several synthetic zwit-terionic buffers, such as MES (2[N-morpholino]ethane sulfonate),HEPES (4(2-hydroxyethyl)1-piperazine ethane sulfonic acid) andPIPES (piperazine-N,N′-bis[2-ethane sulfonate]) can also be used asbuffers in MFCs. These zwitterionic buffers have advantages com-pared with conventional buffers such as phosphate and carbonatebuffers in biological studies as their pKa values are in the range ofpH 6.0 and 8.0, they are chemically stable and nontoxic, and do notinterfere with biochemical reactions. The use of a buffer with a pKa
slightly higher than the solution pH is advantageous to maintainpH control since the solution pH will increase as the substrate isconsumes [72].
Despite the different types of buffers used in MFCs and theiradvantages in MFC operation and performance, it should be men-tioned that the use of chemical buffers is not practical for realapplications since they represent a significant cost and energyinput. Also, even when pH is controlled due to the addition ofa buffer, the type of the buffer (at identical concentrations) usedhad a large impact on power production. The various buffers pro-duce different solution conductivities, resulting in different ohmicresistances and maximum power productions [72].
To overcome that and to guarantee a continuously fed of abuffer at a low cost, some work has been done in order toadd acid buffer to the cathode compartment in the form of car-bon dioxide [73,74]. Carbon dioxide is an acid that combineswith hydroxide ion in the cathode to produce bicarbonate andcarbonate, creating a carbon dioxide/carbonate or bicarbonatebuffered catholyte systems. The results showed an increase inpower density and cell voltage by feeding air with carbon diox-ide to the cathode side, with a decreased pH imbalance [73,74].
Also, the carbon dioxide is available as a waste gas in indus-trial settings making the implementation of this carbon dioxidebuffered MFC system very promising due to a significant costreduction.
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.2. Effect of temperature
Microbial fuel cells are strongly affected by temperature sincehanges in temperature may affect system kinetics and mass trans-er (activation energy, mass transfer coefficient, and conductivityf the solution), thermodynamic (free Gibbs energy and electrodeotentials) and nature and distribution of the microbial commu-ity (different species will have different optimum temperatures)50,64,70,75–91]. Although research in this field has increased dur-ng the last years, there is no systematic information regardinghe influence of temperature in microbial fuel cells. It has beenbserved that temperature is a crucial factor in the MFC perfor-ance both, for COD removal and electricity generation, showing
increase on power output and COD removal with a increase ofemperature [50,64,70,75–86]. The increase of power density with
increase of temperature may be related to the enhancement ofhe microbial metabolism and membrane permeability and to thehmic resistance reduction due to higher conductivity of the liquidolution [64,71,78,80,85,86]. Regarding the membrane permeabil-ty it was observed that a modification in the temperature barelynfluenced the permeability, so the electricity generation of the
FC seems not to be sensitive to the change in the membrane per-eability [86]. Regarding the ohmic resistance, the temperature
ffect on this parameter, normally, presents a linear trend with aecrease in the internal resistance on the MFC with an increasen temperature [64,80,81,86,87]. This decrease could be explainedy the fact that an increase of temperature leads to an increasef ionic conductivity and therefore a decrease of ohmic resistance78]. However, the change in the ohmic resistance is not sufficient toxplain the changes in MFC performance with temperature, sincehese changes are nor linear but exponential [70,76,78,80,85,86].t is known that the temperature effect on microorganism activityollows an exponential trend, so the exponential trend observedn the effect of temperature on MFC power output is due to thenhancement of the microbial activity [75,79,85,86]. This micro-ial activity can be analyzed in terms of biofilm development athe anode which has an impact on anodic biocatalytic activity. Stud-es regarding the effect of temperature on biofilm formation showhat the initial temperature of an MFC has important impact onnitial formation of biofilm and on the time needed for startup.igher temperatures lead to a stable biofilm and MFC operation
n a short period of time and consequently a higher performance79,83,88,89]. It was found that the biofilms process their maxi-
um bioelectrocatalytic activity at a temperature between 30 ◦Cnd 45 ◦C [79,83,88], being this temperature range the more advan-ageous to operate a MFC in order to achieve higher performances50,64,70,75–86]. Lower and higher operation temperatures yield
decrease on biofilm and MFC performance due, respectively, tolower metabolic rates and to irreversible denaturation processeshich may lead to full biofilm decomposition and inactivation of
he bacterial metabolic activity [79,80]. Since different electrogenicacteria had different appropriate temperature ranges and grew atifferent temperatures, the temperature during the initial growthhase of biofilm determines the abundance of the different micro-ial species as well as their distribution within the biofilm matrix.nce the biofilm formed, the microbial species were capable ofdjusting their metabolism functioning at different temperatures77,88–90]. This clearly shows the importance of the startup tem-eratures on biofilm formation and that after this period the MFCsan operate over a wide range of temperatures without a significantecrease on performance [79,88,89,91].
These findings are important for application of MFC for
astewater treatment. It is obvious that the startup proceduresan dictate system performance, so use higher temperaturesn the startup processes is better to MFC performance. Aftertar-up, the MFC can operate at lower temperatures decreasing
eering Journal 73 (2013) 53– 64 59
the associated heating costs without a significant decrease onperformance.
3.3. Organic loading rate (OLR)
The organic loading rate (OLR) and the sludge loading rate (SLR)applied during star-up are two important parameters that gov-ern the sludge characteristics. These two parameters, respectively,define the capacity of the reactor per unit volume and per unitmass of microorganisms to convert organic substrates. Since thepower density and Coulombic efficiency of the MFC depend uponthe substrate conversion rate, the SLR or the OLR applied will playan important role in controlling the performance of the MFC. TheMFCs should be operated at an optimum SLR or OLR to get betterperformance in terms of organic matter removal and power pro-duction. Some work has been done in order to explore the effectof SLR and OLR on MFC performance [50,53,64,82,86,92–101]. Itis evident from these works that the OLR and SLR have a markedinfluence on power yield and substrate degradation. Increment inpower generation and substrate degradation was noticed as theorganic load rate increase. However if the organic load rate was toohigh a decrease on power generation was observed even though theincrease on the substrate degradation occurs [50,64,82,86,92–101].The increase on the substrate removal rates observed at high load-ing rates conditions may be attributed to the direct anodic oxidation(DAO) mechanism. DAO of deprotonated substrate might activatethe formation of oxidation species on the anode surface due tomanifestation of bio-electrochemical reactions. This helps the fur-ther oxidation of substrate leading to enhanced removal rates. Theobserved higher power outputs can be attributed to the avail-ability of higher substrate concentration to sustain the metabolicactivity [94]. The enhancement on power generation and sub-strate degradation indicates that more organic matter is utilizedfor power generation at higher OLR. It was also found that theinternal resistance of the cell decreases with an increase of theOLR. This is due to an improvement of the ionic strength of theanodic liquid, resulting from higher volatile fatty acids concen-trations, and to an increased catalytic activity and density of theanodophilic microorganism [64,101]. However, the Coulombic effi-ciency increased with a decreased OLR and substrate degradation[50,64,82,86,92–101]. It is known that in MFCs, methanogens com-pete with electrochemically active microorganisms and convertorganic material to methane, thus reducing electron carbon oxi-dation. Also, the volumetric methane production rate was foundto increase with increasing OLR [64]. High OLR, corresponding tohighly saturated conditions with respect to anode surface area, maylead to competition between microorganisms involved in electric-ity production and other types of bacteria, leading to greater organicmatter removal not related to current generation, lowering theCoulombic efficiency, but increasing the substrate degradation. Atlow OLR the Coulombic efficiency is improved because less sub-strate is used for methane production, although internal resistancewas higher [64,95,101]. Thus, a better electron recovery does notnecessarily lead to a better MFC performance. The studies regardingthe effect of OLR on MFCs show that with an optimized OLR themethane production rate can be avoided and that the electricity-to-methane production rate can be successfully shifted towardselectricity production.
3.4. Feed-rate and shear-stress
Operating a MFC in continuous mode presents hydrodynamic
challenges that can affect the MFC performance [102] and thebehaviour of biofilm microbial communities [103,104]. Thus flow-rate and the resultant hydraulic retention time (HRT) as wellas the shear stress are important parameters that need to be
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ddressed before MFCs can be successfully used in wastewaterreatment plants and some work has been done regarding thesessues [93,95,102–109]. Regarding the flow rate, an increase on theow rate leads to an increase of the power output [93,95,107,108].lthough if a very high flow rate is used the power density decreases
109]. This suggests that an optimal performance is best obtainedfter microbial community has been given time to develop andhen nutrient capture and the extent of hydrolysis of substrate areore favourable. It was, also found that the increase on flow-rate
as an opposite effect on COD removal and Coulombic efficiency,ince COD removal and Coulombic efficiency decreases with anncrease of flow-rate [95,99]. As the flow rate increased the HRTecreased, so the time required by the microorganism to degradehe organic matter decrease. Therefore, the average COD removalfficiency of each culture decreased.
Hydrodynamic strength is one of the key parameters influenc-ng microbial adhesion and biofilm formation. Usually, high shearates (below the level called tensile strength) result in thicker andenser biofilms and hence can transfer electrons more efficiently
ncreasing the power output of a MFC [103,105,106]. The develop-ent of thicker biofilms may be due to increased biofilm cohesion
n response to high detachment forces and/or to an increase ofiomass production resulting from higher mass transfer. If thehear rate is very high (above the tensile strength) a decrease ofhe current and the thickness of the biofilm is observed demonstrat-ng cell detachment [103,105]. In this extreme situation, bacteriattach to the electrode transfer electrons by other mechanism, suchs via direct contacts, instead of using only redox mediators. CLSMconfocal laser scanning microscopy) and SEM (scanning electron
icroscopy) observations showed that the bacterial density on thelectrode surface for the high shear rates was significantly higherhan the one corresponding to the low shear rates, proving that theiofilm formed under these conditions is denser and thicker lead-
ng to higher electricity generation. It was also observed that higherhear rates decrease the diversity, therefore one species dominatedhe bacterial community leading to a more homogeneous biofilm104–106]. A biofilm with these characteristics not only improveshe electron via direct contact, but also enhances transfer via elec-ron shuttles as more cells might be involved in electron transfernd more shuttles can be produced. The results on this research areauggest that high shear enrichment can be used to obtain bettererforming anodic microbial consortia.
. MFC models
The MFC is a complex system involving simultaneous biologicalnd electrochemical processes, and charge, mass and energy trans-er. The power output obtained with a MFC depends of a range ofifferent parameters, such as amount of bacteria, mixing and massransfer phenomena, bacterial kinetics, cathodic reactions and thefficiency of the proton transport. Therefore, the development of
MFC involves multidisciplinary knowledge of microbiology, elec-rochemistry, material science and engineering. Investigation of theffect of each parameter independently and/or possible combina-ions of parameters using experimental work is costly and timeonsuming, so the development of mathematical models is a cru-ial tool for investigation system parameters and for the design andptimization of MFCs with reduced time and money. These mod-ls can be easily modified to simulate various configurations andperating conditions.
Modelling and simulation has been widely used to develop
arious chemical fuel cells systems and different types ofpproaches are available in literature [110,111]. Substantial infor-ation already exists on mass transfer, reactions and electricalhenomena, and this can be also adapted to MFCs. Despite the
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importance of MFCs modelling with the exception of one case overa decade ago [112], there have been, as far as we are aware, onlya few modelling studies dedicated to MFCs [113–121]. As already,referred, one of the first MFC models was proposed by Zhang andHalme [112]. This single species model considered the use of anexternal mediator to transfer electrons and established dependencebetween power output and the external mediator. The model wasdeveloped only for monitoring and control purposes and allowspredicting the dependence of the current output on the substrateand mediator concentration and other main variables. Since mod-ern MFC do not use external mediators, this line of research hasnot been pursued further. Due to the importance of the biofilm for-mation on the anode electrode, recently computational models forbiofilm based microbial fuel cells have been reported [113–116].Marcus et al. [113] developed a dynamic, one-dimensional modelwith a detailed description of bio-electrochemical reactions of amediatorless MFC based on mass balance principles, Ohm’s law anda novel Nernst–Monod expression to describe the rate of electron-donor oxidation. They explained the function of biofilm-basedMFCs considering a hypothetical biofilm electrical conductionproperty instead of diffusible mediators. The model allows studyingthe dual limitation in biofilm by the electron-donor concentra-tion and local potential. It distinguished between active and noactive anodophilic biomass but the existence of several microbialpopulations competing for the same substrate was not considered.Picioreanu et al. [114–116] developed a detailed three-dimensionalmultispecies model of anodic compartment biofilm with medi-ated electron transfer. The model [114] can describe the evolutionin time of important parameters (such as current charge, volt-age and power production, consumption of substrates, suspendedand attached biomass growth) under several operation conditions.A further extension of this model included methanogenic pro-cesses was presented by the same authors [115]. Picioreanu et al.[116], also, extended the MFC model presented in [114] in order toinclude pH calculations. The proposed model reflects the generallyobserved variations of pH, solute concentrations and current overtime in MFCs. Although, the models described by Picioreanu et al.[114–116] provided detailed descriptions of multi-species for thebiofilm anode by using partial differential equations, their practi-cal application has inherent complexity, long computational timesand may not be readily implemented by the majority of the MFCcommunity. To overcome that, simplified models consisting of ordi-nary differential equations can adequately describe MFC microbialcommunities at various operating conditions while being suitablefor process design and optimization are desirable [117–121]. Wenet al. [117] developed a basic electrochemical model of a MFC basedon polarization curve, which can be used to quantify the vari-ous voltage losses for different operating conditions. The modelpresented by Zeng et al. [118] integrates the biochemical reac-tions, Butler–Volmer expressions and mass/charge balances andsimulates both steady and dynamic behaviour of a MFC, includ-ing voltage, power density, fuel concentration, and the influence ofvarious parameters on power generation. This model is very easyto implement and can be a very useful tool to the developmentand scale-up of more efficient MFCs. Another, simple mathemati-cal model was presented by Picioreanu et al. [119], which describesthe MFC behaviour with suspended cells and electron transfer frommicrobial cells to the anode via diffusible mediator. The model for-mulation is based on mass balances for several dissolved chemicalspecies such as substrate and oxidized and reduced mediator. Themodel gives time-dependent production of current and electricalcharge, current–voltage and current–power curves, the evolution
of concentrations of chemical species and can be used to predictthe effect of different parameters (such as electrical resistance,biomass, substrate and mediator concentrations) on the MFC per-formance. A two-population model describing the competition of
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Table 3Overview of MFCs modelling: kinetic, mass transport and electrochemical expressions.
Anode Cathode Electrochemical Ref.
Kinetic Mass balance Kinetic Mass balance
Biomass Substrate
rA = kAX(
CSCS+KS
)×(
CMOXCMOX +KMOX
)dXdt
= YrAdCSdt
= rA Assumed Vcell = E0 − �a − �c − IRint [112,119]
q = qmaxX(
CSCS+KS
)×(
CMOXCMOX +KMOX
)dXdt
= D(X0 − X) + Yq + bdetAF� − bata
AFVA
dCSdt
= D(C0S
− CS) − q +1
VA
∫VF
rS,F dV +1
VA
∫AF
rS,E dA
Assumed Vcell = E0 − �a − �c − IRint [114–116]
∂CS∂t
=∂∂x
(DS,F
∂CS∂x
)+ rS,F
– – – – Vcell = E0 − (aA + bA × ln j) − (aC +bC × ln j) − (j × A × Rohmic) −[
c × ln(
jLjL−j
)][117]
rA = kA exp[− ˛AF
RT �A
]×(
CSCS+KS
)X VA
dXdt
= QA(X0−X)
fX+ AYrA − VAKdecX VA
dCSdt
=QA(C0
S− CS) − ArA
rC = −kC exp[
(ˇ − 1) FRT �C
]×(
CO2CO2
+KO2
)VC
dCO2dt
=QC (C0
O2−
CO2 ) + ArC
Vcell = E0 − �A + �C − (Rint − Rsol)I [118]
qa = qmax,aX(
CSCS+KS,a
)×(
CMOXCMOX +KMOX
)dXadt
= qaXa − Kdec,aXa − ˛aDXadCSdt
= D(C0S
− CS) −qaXa − qmXm
Assumed Vcell = E0 − �act − �conc − RintI [119,120]
qm = qmax,mX(
CSCS+KS,m
)dXm
dt= qmXm − Kdec,mXm − ˛mDXm
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nodophilic and methanogenic microbial populations for a com-on substrate in a microbial fuel cell (MFC) was presented by Pinto
t al. [120,121]. These models are easy to implement, easy to solvend can be used for process optimization and control, since theyonsist of ordinary differential equations that can describe the MFCopulations at different operating conditions.
As can be seen, different modelling approaches are availablen literature concerning the MFCs. The more complex models areransport models using differential and algebraic equations whoseerivation is based in the electrochemistry, biological and physicsoverning the phenomena taking place in the cell. These equa-ions are numerically solved by different computational methods,nvolving extensive calculations, but accurately predict all thehenomena’s occurring in the cell. The simpler ones are an ade-uate tool to understand the basic effect of different operatingonditions and design parameters on MFC performance and cane used to predict voltage losses for simple designs and can beseful tool for rapid calculations. However, they neglect some ofrucial features such as transient performance and spatial non-niformities. It should, also, be mentioned that most of the modelseveloped for MFCs and revised in this section only considered thehenomena occurring in the anode side assuming an overpotentialalue for the cathode side. Therefore, all the important phenomenahat occur on the cathode side which may affect the MFC perfor-
ance are neglected [114–116,119–121] and cannot be predictedy the model. As the authors are aware, only one work developed
coupled model for both anode and cathode chambers (similar tohe ones used for chemical fuel cells) [118]. However, the majorrawback of this work is that the biofilm formation on the anodeide is not considered. An overview of the MFC models regardinghe main kinetic, mass transport and electrochemical expressionsan be found in Table 3. Due to a lack of models describing the mainiological, mass transfer, energy and charge effects in the anode andathode compartment and taking into account the biofilm forma-ion on the anode side much effort should be directed towards theevelopment of a model considering all that.
. Conclusions
In the last years considerable developments in MFC technologyave been seen, although, significant challenges still exist before aFC can be ready to commercialization. Towards MFC commer-
ialization different scale-up studies were performed and thesetudies indicate that scaling-up MFCs is feasible, but it is impor-ant to improve power output with cost-effective materials andew designs that can be used in large scale applications. The use ofoble metal based catalysts on the cathode side will not be viableue to their high cost, so new materials and/or different electrodecceptors instead of oxygen should be used. Many, other studiesave been focused on analyzing and improving MFC performances,o the effect of different operating conditions has been tested. Thecidified anode can decrease bacterial activity affecting the biofilmerformance and stability and consequently the MFC power output.herefore, only a narrow pH window ranging from 6 to 9 was foundo be suitable for growth and operation of biofilms. In an oppositeay, on the cathode side, a decrease of the pH benefits the oxygen
eduction and consequently the current output from MFCs. Select-ng the best value of ph for each side, a pH gradient is observed
hich is pointed out as one of the main causes for the sink of voltagefficiency in microbial systems. To rectify this problem, differentypes of buffers can be used, such as acid/base dosing and chemi-
al buffers (phosphate or bicarbonate buffer). However the use ofhemical buffers is not practical for real applications since theyepresent a significant cost. Regarding the effect of temperaturen MFC performance it was concluded that higher temperatureseering Journal 73 (2013) 53– 64
are advantageous, especially, in the startup process and thereafterthe MFC can operate at lower temperatures without a considerablereduce on performance. The MFCs should operate at an optimumOLR to avoid the methane production, so the electricity-to-methaneproduction rate can be successfully shifted towards electricity pro-duction. An increase on the flow rate leads to an increase on poweroutput, a decrease of HRT, with a consequent decrease of the CODremoval efficiency. Higher shear enrichment was found to be a use-ful way to obtain a better performance of the anodic microbialconsortia. It should be mentioned that the best operating condi-tions for one type of MFC are not necessary adequate to anothertype. Therefore, when developing a MFC, the conditions need tobe carefully considered and chosen in order to achieve the bestperformance which means the higher power output.
Since the MFC is a complex system involving biological, chemicaland electrochemical processes, energy, mass and charge transfer,a better understanding of the basic transport phenomena is essen-tial to overcome these challenges and to encourage new designconcepts. Up to now, there are only a few mathematical modelsfor MFCs, with different approaches such as electrochemical mod-els, biofilm models and bulk liquid models. Most of them are basedon Nernst equation, with complex frames and model parameters.Such models need to be solved by different computational methods,involving extensive calculations, longer running times and onlydescribed the transport and biochemical phenomena of the anodeside, neglecting the cathode side. Simpler models that can provideimportant predictions and optimization about the influence of dif-ferent losses, charge and mass transport on both chambers, on theMFC performance and biofilm formation are more desirable. Suchmodels are helpful for the discovery of new cell designs and oper-ation regimes of the MFC system.
Acknowledgements
The support of “Fundac ão para a Ciência e Tecnologia (FCT) –Portugal” trough scholarship SFRH/BPD/66837/2009 is gratefullyacknowledged. POCI (FEDER) also supported this work via CEFT andLEPAE.
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