Effects of architectural changes and inoculum type on

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 9 9 e6 2 0 9

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Effects of architectural changes and inoculum type on internal resistance of a microbial fuel cell designed for the treatment of leachates from the dark hydrogenogenic fermentation of organic solid wastes Ana L. Va´zquez-Larios a, Omar Solorza-Feria b, Gerardo Va´zquez-Huerta b, Fernando Esparza-Garcıá a, Noemı´ Rinderknecht-Seijas c, Hećtor M. Poggi-Varaldo a,* a

Environmental Biotechnology and Renewable Energy R&D Group, Depto. Biotecnologıá y Bioingenierıá, Centro de Investigacioń y de Estudios Avanzados del IPN, Apdo. Postal 14-740, 07000 Me´xico D.F., Mexico b Depto. Quı´mica, Centro de Investigacioń y de Estudios Avanzados del IPN, Me´xico D.F., Mexico c ESIQIE del IPN, Divisioń de Ciencias Ba´sicas, Me´xico D.F., Mexico

article info

abstract

Article history:

A new design of a single chamber MFC-A based on extended electrode surface (larger s,

Received 26 November 2009

specific surface or surface area of electrode to cell volume) and the assemblage or ‘sand-

Received in revised form

wich’ arrangement of the anode-proton exchange membrane-cathode (AMC arrangement)

19 December 2010

and a standard single chamber MFC-B with separated electrodes were tested with several

Accepted 1 January 2011

inocula (sulphate-reducing, SR-In; methanogenic, M-In, and aerobic, Ab-In) in order to

Available online 31 March 2011

determine the effects on the internal resistance Rint and other electrical characteristics of

Keywords:

of the standard MFC-B, for all inocula used in this work. Resistances followed the order

Biohydrogen

Rint,SR-In < Rint,M-In Rint,Ab-In.

the cells. In general, the Rint of the new design cell MFC-A was consistently lower than that

Dark fermentation Electrode surface to cell volume

These results were consistent with reports on reduction of ohmic resistance of cells by decreasing inter-electrode distance. Also, the volumetric power PV output was higher for

Inoculums

the MFC-A than for MFC-B; this was congruent with doubling the s in the MFC-A compared

Internal resistance

to MFC-B. Yet, power density PAn delivered was higher for MFC-A only when operated with

Microbial fuel cell

SR-In and Ab-In, but not with M-In. The MFC-A loaded with SR-In showed a substantial

Sandwich-electrodes

improvement in PV (ca. 13-fold, probably due to the combined effects of increased s and

Separated electrodes

decreased of Rint) and a 6.4-fold jump in PAn compared to MFC-B. The improvement was higher than the expected improvement factors (or algebraic factors; 6.5 improvement expected for PV due to combined effects of increase of s and lowering the Rint; 3.25 improvement expected for PAn due to lowering the Rint). Our results point out to continuing work using the two-set, sandwich-electrode MFC and sulphate-reducing inoculum as a departing model for further studies on effects of inoculum enrichment and electrode material substitution on cell performance. Also, the MFC-A model seems to hold promise for future studies of bioelectricity generation and

* Corresponding author. Tel.: þ5255 5747 3800x4324; fax: þ5255 5747 3313. E-mail address: [email protected] (H.M. Poggi-Varaldo). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.006

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pollution abatement processing leachates produced during biohydrogen generation in dark fermentation processes of organic solid wastes. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Our modern societies mainly rely on fossil fuels in order to satisfy their present energy needs. Unfortunately, renewable energy still plays only a marginal role in the menu of commercial energy technologies, in spite of its sustainability. Fossil fuels use and abuse present several disadvantages such as high and unstable costs, increasing scarcity in the near future, and negative impacts on the environment and human health due to harmful combustion products, spills and leaks during exploration, production and transportation [1,2]. On the other hand, oil experts have predicted a plateau of oil extraction and production and the start of declining by 2020e2030 [3,4], whereas other experts in energy and hydrogen have forecasted a near future technological transition where fossil fuels importance will progressively decrease and renewable energy contribution will become more substantial [5,6]. Renewable energies, such as wind, solar, hydraulic, and biological-based energy represent an interesting alternative because of their potential lower costs and minimum environment negative impact [7,8]. Among the biologicalbased energies, biohydrogen is an attractive alternative because wastes may be used as feedstocks and the double goal of waste treatment and management and bioenergy production may be attained [6,9,10]. Sometimes this is not possible, as in the biohydrogen production from fermentation of organic wastes. In this type of process, there is just a partial biodegradation of waste to hydrogen, with the consistent production of organic metabolites remaining in the spent solids [11,12]. These metabolites, usually low molecular weight organic acids and solvents, can be used to yield additional bioenergy by a methanogenic system [13,14], by phototrophic bacteria capable of producing hydrogen as a fuel [10], or by a microbial fuel cell [9,14,15]. Microbial fuel cells (MFC) constitute a promising technology for sustainable production of alternative energy and waste treatment. A microbial fuel cell is an electro-biochemical reactor capable of directly converting organic matter into electricity. In the anodic chamber the microorganisms anoxically oxidize the organic matter and release electrons and protons. Electrons are transported to the anode that acts as an intermediate, external electron acceptor. The electrons flow through an external circuit where there is a resistor or a device to be powered, producing electricity and finally react at the cathode with the protons and oxygen producing water [16]. The corresponding protons released during the oxidation of organic compounds migrate to the cathode through the electrolyte (liquor) contained in the cell and a proton exchange membrane; in this way charge neutrality is kept [17]. The reversible thermodynamic or ideal voltage delivered by a MFC at a given temperature of operation, that is, the maximum voltage attainable, can be estimated by the Nernst equation [18]. However, the actual voltage output of an MFC

is lower than the predicted by the Nernst equation due to irreversible losses or overpotentials [17,19,20]. The most important losses associated to poor MFC performance are the activation losses, ohmic losses, and mass transport losses. These irreversibilities are usually defined as the voltage required to compensate for the current lost due to electrochemical reactions, charge transport (also known as ohmic loss), and mass transfer processes that take place in the cell; these voltages substract from the potential calculated by the Nernst equation [20,21]. So, much of the current research on MFC is devoted to overcome the limitations imposed by these irreversibilities. Ohmic potential hohmic is the ohmic loss from ionic and electronic resistances; it collectively represents the voltage lost in order to accomplish electron and proton transport in the cell. The hohmic is usually described by the Ohm’s law, that is hohmic ¼ IMFC Rohmic

(1)

where IMFC is the current intensity of the cell; Rohmic, ohmic resistance. The ohmic resistance reflects the combination of the resistances of electrodes, electrolyte(s), membrane (if any), junctions, and connections; that is, it combines the ionic and electronic resistances. In most cases Rohmic is dominated by the ionic resistance (Rion) associated to the electrolyte(s) resistance [17,21] since resistance associated to electrodes and connections is relatively low. The Rion due to electrolyte is given by the following expression [22] Rohmic z Rion ¼ r L/A ¼ (1/k) L/A

(2)

where r is the specific resistance or resistivity of the electroyte; L, distance between electrodes; A, electrode surface area; k, specfic conductance or conductivity of the electrolyte. Eq. (2) shows the key ways to lower ohmic losses, i.e., by reducing the distance that separates the electrodes (decreasing L), increasing the electrode surface area (increasing A), and increasing the conductivity of the electrolyte and materials of the proton-exchange membrane (increasing k). A plausible physical picture of the effect of inter-electrode separation would be that the protons have less distance to travel, and consequently the ohmic resistance is lowered. Thus, electrode separation has been investigated by several researchers as one way to improve MFC performance [17]. The influence of electrode spacing on performance of MFCs has been shown in several works [23e28]. For instance, Liu et al. [27] in experiments with a membrane-less MFC, observed that decreasing the distance between the electrodes from 4 to 2 cm significantly reduced the ohmic resistance and resulted in a 67% increase in the power output. Relatively high power outputs have also been achieved in MFCs with a ‘sandwich’ membrane-electrodes arrangement


(AMC, for anode-membrane-cathode setup) [28e31] that minimized the inter-electrode distance and significantly reduced the Rohmic. Liang et al. [28] compared the internal resistances of two air-cathode MFCs, one with an MEA design and the other one with a 4-cm electrode spacing. They found a significant decrease of internal resistance and a 3-fold improvement in power delivery with the MFC equipped with the AMC arrangement compared to a standard MFC where electrodes were separated 4 cm. Another variable that may lead to decreased Rohmic is the electrode area A. The latter can be expressed in terms of a variable s, the ratio of surface area of electrode to the cell volume, or specific electrode surface, as follows: s ¼ A/VMFC

(3)

where VMFC is the volume of the MFC. Since s is proportional to A (Eq. (3)) and the Rohmic is inversely proportional to A (Eq. (3)), it follows that Rohmic would be inversely proportional to s. Independently of the math, intuitively, it is plausible that a high s would be desirable, since more active electrode area is available for bioelectricity generation in a given volume of the cell, that is, the exploitation of cell volume is maximized. In this regard, flat electrodes have an inherently low s. Therefore, several works have investigated the use of electrode materials with high s such as granular and reticulated graphite and granular activated carbon [19,32]. Regarding the use of flat electrodes, the s of the cell can still be increased if more walls of the cell are fitted with electrodes. In this way, the MFC fitted with a ‘sandwich’ AMC as reported by Liang et al. [28] might have an increased performance if the two circular surfaces of the cylindrical shell of their MFC were fitted with AMC arrangements. In a research on the effects of electrode and PEM surface area on performance of a two-chamber MFC on cell performance carried out by Oh and Logan [33], it was observed ca. 10-fold increase of maximum power density with a 11-fold increase of cathode surface area (from 2 to 22.5 cm2), at fixed surface areas of anode and PEM (22.5 and 30.6 cm2, respectively.) Another factor that could influence the internal resistance of MFC is the inoculum type [15,34e36]. In particular, PoggiVaraldo et al. [15] have recently shown that performance of identical MFCs fed with a model extract similar to that of the dark, hydrogenogenic fermentation of organic solid wastes [37], depended on the inoculum type. They reported that the cell that used a sulphate-reducing inoculum outperformed the cell loaded with a methanogenic one. Therefore, the purpose of our work was to evaluate the effect of a two microbial fuel cell designs (named as MFC-A and MFC-B) and the type of three inocula (aerobic, methanogenic, and sulphate-reducing consortia) on the internal resistance of the cell.

2.

Materials and methods

2.1.

Experimental design and microbial fuel cells

The experiment was carried out as a complete randomized block (CRB) factorial design with factor ‘type of MFC’ (new

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design MFC-A and standard cell MFC-B) and factor ‘type of inoculum’ (aerobic, methanogenic, and sulphate-reducing consortia, named here-in after as Ab-In, M-In, and SR-In, respectively) with a total of six treatments and two replicates in time. The two replicates in time were treated as blocks [38]. The main variable response was the internal resistance Rint as estimated by the polarization curve method. Results were further processed by analysis of variance (ANOVA) corresponding to the CRB-two factor model, in order to assess the statistical significance of the effects of factors and interaction [38]. Both MFCs consisted of a horizontal cylinder built in Plexiglass 78 mm long and 48 mm internal diameter. In the MFC-A (new design), the two circular, opposing faces of the cylindrical shell were fitted with corresponding sets of an assemblage or circular ‘sandwich’ arrangement that consisted (from inside to outside) of an anode made of Toray carbon cloth, the proton exchange membrane (Nafion 117), the cathode made of flexible carbon-cloth containing 0.5 mg cm2 platinum catalyst (Pt 10 wt%/C-ETEK), and a perforated plate of stainless steel 1 mm thickness (Fig. 1a). For brevity, this ‘sandwich’ arrangement was coined as AMC for the Anode-proton exchange Membrane-Cathode. On the other hand, the standard cell MFC-B (Fig. 1b) was fitted with a circular anode made of stainless steel plate 1 mm thickness with a Toray flexible carbon-cloth sheet placed in one circular face and a cathode in the opposing face made of (from inside to outside): proton exchange membrane (Nafion 117), a Toray flexible carbon-cloth containing 0.5 mg cm2 platinum catalyst (Pt 10 wt%/C-ETEK), and a perforated plate of stainless steel 1 mm thickness. All the cathodes in both cells MFC-A and MFC-B were in direct contact with atmospheric air on the perforated metallic plate side. It is worth highlighting that the MFC-A had a ratio s (electrode surface area to cell volume, Eq. (3)) two-fold of that in the MFC-B. Also, the separation between electrodes in MFCA was null or minimal (‘sandwich’ arrangement) whereas the inter-electrode distance in MFC-B was 7.8 cm.

2.2.

Model extract (fuel) and biocatalysts

The cells were loaded with 7 ml from a model extract similar to that produced in the anaerobic, fermentative biological hydrogen generation from the organic fraction of the municipal solid wastes [11,39,40]. The model extract was concocted with a mixture of the following substances (in g/L): acetic, propionic and butyric acids (4 each) as well as acetone and ethanol (4 each) and mineral salts such as NaHCO3 and Na2CO3 (3 each) and K2HPO4 and NH4Cl (0.6 each). Organic matter concentration of model extract was ca. 25 g COD/L. The cells were loaded with 143 mL of mixed liquor from either a sulphate-reducing, methanogenic, or aerobic lab scale bioreactors. The latter were complete mixed, semicontinuous, suspended growh bioreactors, also known as ‘seeding’ bioreactors. The initial COD in the cell liquor were ca. 1 250 mg O2/L whereas initial biomass concentration in MFCs were 890, 940, and 1080 mg VSS/L when inoculated with the SR-In, M-In, and Ab-In, respectively.

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Fig. 1 e Schematic diagrams of single chamber cells: (a) type A (new design), and (b) type B (standard design).

2.3. Determination of internal resistance of the cells and calculations The internal resistance is one of the main characteristics of a MFC, because according with the theorem of Jacobi of maximum power delivered by an electromotive force, an MFC fitted with an external resistance equal to its internal resistance will give a maximum power output [41]. On the other hand, the Rint is crucial for a good performance of a MFC since high values tend to result in low power output. The internal resistance of cells was determined using the polarization curve method, by varying the external resistance (Rext) and monitoring both the voltage and the current intensity, according to procedures suggested by Clauwaert et al. [42] and Logan et al. [16]. In brief, each MFC was loaded with substrate and inoculum as described in Section 2.2. Each MFC was batch-operated for 7 h at 35 C. The circuit of the MFC was fitted with an external, variable resistance device. In this regard, we carried out the polarization curve of the MFC, relating mathematically the cell voltage (EMFC) and current intensity (IMFC) against the external resistance value, forwards and backwards regarding the Rext values. First, the

MFC was operated at open circuit for 1 h. Afterwards, the Rext was varied from 1000 U to 10 kU and viceversa. After this, the cell was set to open circuit conditions for 1 h in order to check the adequacy of the procedure (values of initial and final open circuit voltages should be close). The voltage was measured and recorded with a Multimeter ESCORT 3146A. The current intensity IMFC was calculated by the Ohm’s law: IMFC ¼

EMFC Rext

(4)

The delivered power was obtained as the product of the current intensity times the voltage, that is: PMFC ¼ IMFC EMFC

(5)

With the purpose to get values comparable with the works already published, the power was normalized by the anode surface area and the cell volume PAn ¼

E2MFC AAn Rext

(6)


E2MFC (7) VMFC Rext where AAn is the anode superficial area, Rext is the external resistance, and VMFC is the cell volume. It is worth noting that other researchers sometimes use the volume of the anodic chamber in Eq. (7) instead of the cell volume; there is no difference when Eq. (7) is applied to a single chamber MFC, but a significant difference may result when the MFC is a twochamber model. Analysis of variance of internal resistance results (their log-transformed values in order to minimize heteroscedasticity of the original variable [38]) was performed with the software Design-Expert v6.0 from Stat-Ease Inc., Minneapolis, MN, USA. PV ¼

2.4.

Analytical methods

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a

b

The COD, VSS, conductivity and pH of the liquors of sulphatereducing seed bioreactor and cells were determined according to the methods nos. 5220C, 2540E, 2510, and 4500-Hþ, respectively, of the Standard Methods [43]. In addition, the individual concentrations of volatile organic acids and solvents in the model extract were analyzed by gas chromatography in a chromatograph Perkin Elmer Autosystem equipped with a flame ionization detector as described elsewhere [11].

3.

Results and discussion

In general, internal resistance of the new design cell MFC-A was consistently lower than that of the standard MFC-B, for all inocula used in this work (Fig. 2, Table 1). This result was probably related to the elimination of separation between electrodes via the ‘sandwich’ AMC arrangement in our MFC-A. The significant decrease of Rint with decrease of inter-electrode distance is consistent with results from previous experiments that tested the effect of electrode spacing on internal resistance of MFCs [23e28]. The polarization curve and the plot of superficial power density with current density for both types of cells loaded with SR-In are depicted in Fig. 2a. Polarization curves were very close to straight lines; the internal resistances were estimated from the slopes of the corresponding regression lines as 1200 and 3900 U for MFC-A and MFC-B, respectively, i.e., a factor of 3.25 and a percentage decrease of 69%. In particular, the proportion of Rint decrease in our work was similar to the 68% reduction in Rint value reported by Liang et al. [28] in a comparative study of a single chamber MFC fitted with a ‘sandwich’ AMC and a second cell where the electrodes were separated 4 cm. Maximum density power generated by MFC-A and MFC-B in our work were 20.9 and 3.3 mW m2, respectively (Table 1), that is, 6.4-fold superior for MFC-A. The substantial improvement in PV (ca. 13-fold, Table 1) was probably due to the combined effects of increased s and decrease of Rint. Yet, it is interesting to note that the expected (algebraic) enhancement due to these two features would be in the order of 6.5 (6.5 ¼ (2/1) (3900 U/ 1200 U)), that is, the actual improvement factor was almost double of the mere algebraic one. It seems that there was a synergistic effect between the architecture of the cell (s) and

c

Fig. 2 e Polarization curves and anodic superficial power density delivered by cells A and B as function of current intensity: (a) with sulphate-reducing inoculum; (b) with methanogenic inoculum; (c) with aerobic inoculum. Squares correspond to new design A cell, circles correspond to standard B model.

the lower internal resistance of the ‘sandwich’ AMC arrangement on the volumetric power of the MFC, probably combined with an influence of the SR-In [15]. Polarization curve and power density plot of the cells loaded with M-In are shown in Fig. 2b; values of Rint were 5300 and 7500 U for the MFC-A and MFC-B, respectively (i.e., a reduction factor of just 1.4). Rint values were correspondingly higher than

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Table 1 e Selected electrochemical characteristics of microbial fuel cells in this work. Parameter

Rint (U) PAn-max (mW m2) PV-max (mW m3) IMFC-max (mA) EMFC-max (V) PMFC-max (mW)

Sulphate-reducing inoculum

Methanogenic inoculum

Aerobic inoculum

MFC-A

MFC-B

MFC-A

MFC-B

MFC-A

MFC-B

1200 20.90 501 0.061 0.68 0.041

3900 3.25 39 0.02 0.33 0.007

5300 5.7 137.6 0.032 0.48 0.015

7500 6.2 74.9 0.03 0.56 0.017

100,000 0.06 1.44 0.002 0.21 4.2 x104

130,000 0.048 0.59 0.002 0.18 4.2 x104

MFC-A, new design of single chamber cell; MFC-B, standard single chamber cell with separated electrodes; Rint, internal resistance; PAn-max, maximum power density; PV-max, maximum volumetric power; EMFC-max, maximum voltage output; PMFC-max, maximum power otuput.

those obtained for the cells loaded with SR-In. This finding was consistent with trends observed by Poggi-Varaldo et al. [15] in tests of characterization and batch operation of a standard MFC (single chamber, separated electrodes) with a model extract mimicking a leachate from the hydrogenogenic, dark fermentation of the organic fraction of municipal solid waste. They compared the effect of the inoculum, i.e., M-In and SR-In, and observed that the MFC seeded with SR-In outperformed the MFC loaded with the M-In. They reported that Rint of the SRIn-loaded MFC was substantially lower than that of the M-Inloaded MFC, by a factor of 3.3. The maximum PAn of cells loaded with M-In in this work did not show an advantage for the MFC-A. Indeed, PAn values were 5.73 and 6.24 mW m2 for MFC-A and MFC-B, respectively (Table 1). In contrast, the maximum volumetric power of the MFC-A was superior to that of MFC-B by a factor of 1.8; the latter was somewhat lower than the expected algebraic factor of 2.8 (2.8 ¼ (7500 U/5300 U) (2/1)). Power outputs of cells loaded with SR-In were consistently higher than those of cells loaded with M-In (Table 1) pointing out to a strong influence of inoculum type on the electrochemical characteristics of cells. Internal resistances of cells loaded with Ab-In were atypically high; their values were 100,000 and 130,000 U for the MFC-A and MFC-B, respectively (Fig. 2c and Table 1). This feature was accompanied by negligible power outputs for both types of cells. So, the Ab-In was the less adequate for the cells in our work. Yet, the MFC-A still showed better characteristics than the MFC-B (Table 1). Regarding the best power outputs obtained in this work, that is, the MFC-A loaded with SR-In, the maximum PV was in the middle to high side of the range of PV reported in the literature (Table 2). Yet, the PAn of the MFC-A still was in the low range of published results of Logan et al. [16,26,28]. It can be demonstrated that for the cell models used in this work as well as their performances with sulfate-reducing inoculum, the cost per delivered watt of the new model can be as low as 15% of the cost per watt of the standard model (proof available upon request). The log-transformed values of Rint were subjected to an ANOVA according to the model of completely randomized block two factors, in order to assess statistical significance. Both the factors ‘type of cell’ (combined influence of s and ‘sandwich’ electrode arrangement AMC) and ‘type of inoculum’ were significant (Table 3, Fig. 3) with probability values of ca. 0.038 and 0.000037, respectively. Interaction of factors

did not result significant. The effect of ‘type of cell’ (new type versus standard) was significant and statistically confirmed that the new design was associated to lower and more convenient internal resistance values Particularly for the factor type of inoculum, a comparison of means by the Duncan’s method (Montgomery) showed that the three means were significantly different when compared by pairs (procedure not shown). That is, it can be stated that the inequality relationship Rint,SR-In < Rint,M-In Rint,Ab-In

(8)

is real, not due to chance. M-In seemed to be less adequate than SR-In. There are some antecedents in the literature that reported a relatively low performance of methanogenic inocula in MFC. He et al. [53] used crushed granular methanogenic sludge for inoculating a lab scale upflow microbial fuel cell. After a long time of operation where the inoculum was acclimated to a feed with sucrose as main electron donor, it was found a power density of 170 mW m2 and high removal efficiency of organic matter. Yet, a low coulombic efficiency in the range 0.7e8% was observed. The authors suggested that electron-transfer microorganisms in their methanogenic inoculum, if present, were not able to convert all of the available carbon source into electrical energy. In another work with methanogenic inoculum [54] several approaches were studied in order to enrich electrochemically active bacteria on an electrode using anaerobic sewage sludge in a two-chambered MFC. As part of their experiments, when a porous carbon paper anode electrode was used, a power density up to 8 mW m2 was obtained. Spiking with a specific methanogen inhibitor 2-bromoethanesulfonate increased both the power density and coulombic efficiency. This result strongly suggested that methanogenic activity of the inoculum may not be desirable and is linked to lower performance of the MFC. In our experiments, the methanogenic inoculum found a cell liquor rich in acetate and bicarbonate and carbonate (CO2 or bicarbonate are recognized electron acceptors of methanogenesis whereas the acetate is the substrate of acetoclastic methanogens [55]). Such conditions could favour the deviation of the electron flow from electricity to generation of methane, thus lowering both the power density output of our MFC. On the other hand, the sulfate-reducing inoculum in a cell whose liquor lacks sulfate such as in our work, will have less chance to deviate electrons from organic matter to

Table 2 e Results from published works on microbial fuel cells. Cell Two chambers (O2) Two chambers (O2) Sediment cell (O2)

Electrodes

Substrate

Inoculum

Ref.

PV ¼ 130 mW/m PV ¼ 110 mW/m3 PV ¼ 2.8 mW/m3

Ultrex Ultrex CM 17000 Membrane-less

[44] [45] [46]

PV ¼ 700 mW/m3; PAn ¼ 28 mW/m2 PAn ¼ 720 mW/m2

Nafion 117

[47]

Membrane-less

[25]

Nafion 117 Membrane-less

[28] [48]

Membrane-less

[49]

Nafion 117

[50]

Polytetrafluoroethylene

[23]

Acetate Acetate Acetate

One chamber (O2)

Separated

Wastewater

One chamber (O2)

Separated (Toral cabon paper)

Acetate

Sandwich (Plain toray carbon paper) Separated (Carbon cloth)

Acetate Brewery wastewater

One chamber (Air cathode)

Separated (Carbon cloth)

Wastewater

Domestic wastewater

PAn ¼ 1180 mW/m2 PV ¼ 5100 mW/m3; PAn ¼ 205 mW/m2 PV ¼ 12,000 mW/m3; PAn ¼ 483 mW/m2

Two chambers biocathode (NO3) One chamber (O2)

Separated (Carbon paper)

Acetate


Acetate

Autohetero-trofic denitrifying Domestic wastewater

PV ¼ 190 mW/m3; PAn ¼ 9.4 mW/m2 PV ¼ 627,000 mW/m3; PAn ¼ 1120 mW/m2

Two chamber (O2)

Separated (Graphite felt)

Activated sludge

PAn ¼ 1.3 mW/m2

Membrane-less

[24]

Two chamber (O2)

Separated (Plain toray carbon paper)

Modified artificial wastewater (glucose and glutamate) Acetate

PAn ¼ 720 mW/m2

Membrane-less

[27]

Two chamber (O2)

Separated (Plain porous carbon paper)

Acetate

Domestic wastewater Anaerobic sludge

AMI-7001

[25]

One chamber (O2)


[15]


Methanogenic consortium Sulphate-reducing consortium Anaerobic sludge

Nafion 117

One chamber (O2)

Mixture of organic acids acids and solvents Mixture of organic acids acids and solvents Fermented acidogenic vegetable waste Acetate

PV ¼ 15,000 mW/m3; PAn ¼ 610 mW/m2 PV ¼ 13.4 W/m3; PAn ¼ 1.4 mW/m2 PV ¼ 158 mW/m3; PAn ¼ 12.3 mW/m2 PV ¼ 1560 mW/m3; PAn ¼ 57 mW/m2 PV ¼ 2.4 mW/m3

Nafion 117

[15]

Nafion 117

[51]

Nafion 117

[52]

Mixture of organic acids acids and solvents Mixture of organic acids acids and solvents Mixture of organic acids acids and solvents

Sulphate-reducing consortium Methanogenic consortium Aerobic consortium

Nafion 117

This work This work This work

One chamber (non-catalyzed graphite plates) Two chamber biocathode (NO3) One chamber (O2)

Sandwich Separated (Anode graphite plates and cathode granular graphite) Sandwich (Carbon cloth)

One chamber (O2)

Sandwich (Carbon cloth)

One chamber (O2)

Sandwich (Carbon cloth)

Domestic wastewater Sludge Wastewater

NR

PV ¼ 501 mW/m3; PAn ¼ 20.9 mW/m2 PV ¼ 137 mW/m3; PAn ¼ 5.7 mW/m2 PV ¼ 1.44 mW/m3; PAn ¼ 0.06 mW/m2

Nafion 117 Nafion 117


Membrane 3

Separated (Plain granular graphite) Separated (Graphite roads) Separated (Carbon felt)

One chamber One chamber

Sludge Sludge Efluent from a biocathode Wastewater

Performance

PAn, Power density; PV, volumetric power; NR, not reported.

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Table 3 e Analysis of variance of log-transformed values of internal resistance: effects of cell design and inoculum type. Source

Model or total Blocks A-Inoculum B-MFC design AB Pure error

Sum of squares

df

6.492 0.035 6.087 0.164 0.102 0.104

11 1 2 1 2 5

Mean of square sum value

F

p-Value

3.0435 0.1638 0.0508 0.0208

146.426 7.881 2.446

0.0000365 0.0377 0.182

df, Degrees of freedom; F, value of Fisher statistics.

anaerobic respiration. This is consistent with higher power density found for the SR-In compared with the M-In in our work and in a previous research of our group [15]. Another possibility is that the SR-In might be rich in electrochemicallyactive bacteria (EAB, also known as anodophilic or exoelectrogens) that were reported to be associated to good MFC

Fig. 3 e Main effects of factors (a) type of microbial fuel cell and (b) inoculum type on the internal resistance of cells (log of internal resistance). The error bars correspond to the standard error of the experimental design (EED), calculated as EED [ (MSE/r)1/2 where MSE stands for mean of the sum of squares of the error, and r is the number of replicates (r [ 2). Keys: SR-In, sulphate-reducing inoculum; M-In, methanogenic inoculum; Ab-In, aerobic inoculum; MFC-A, new design cell; MFC-B, standard cell with separated electrodes.

performance [34e36]. For instance, sulphate-reducing bacteria are reported to effect their electron transport based on cytochroms which have been implied in electron transfer to anodes of MFC [56,57]. The very poor results associated to Ab-In were unexpected. Conductivities and consequently the resistivities of inocula were different (Table 4), the latter followed the order rSR-In < rM-In rAb-In. Conductivity of the SR-In was the highest, most probably due to remnants of sulfate anion as well as the presence of sulfide and HS from the reduction of sulfate in the SR-seed bioreactor. The internal resistances of cells closely followed the same order (Table 4). Yet, according to the theory of electrolytes, for dilute electrolytes, i.e. range 0 < k < 2000 mS/cm, the resistance of the electrolyte that is one of the main components of the ohmic resistance in the cell should be proportional to the resistivity [22,58]; see also Eq. (2). However, the very high resistance of Ab-In seems to depart from this trend by far (Table 4). Instead, log Rint seems to be proportional to r, not the Rint (this can easily be seen by plotting values of Rint and ln(Rint) of inocula in Table 4 versus r of inocula). This strongly suggests that the high resistivity of Ab-In only partially explains the high internal resistance obtained and the poor power performance of cells seeded with Ab-In (Table 1). Another possible explanation or contribution to low power performance and higher Rint in the Ab-In could be related to its microbial community. Logically, selective pressure of continuous aeration and good mixing in the aerobic seed reactor would have lead to a microflora profile rich in aerobic microbes, with scarce presence of strict anaerobes and low proportion of facultative anaerobes [59]. It can be inferred that the capability of strict aerobes to anoxically oxidize the organic matter and transfer the electrons to the anode is very low or non existent. In this regard, SR-In and M-In microflora profiles could be richer in strict anaerobes and facultative anaerobes that are more proficient in the anoxic oxidation of organic substrates. This, in turn, would result in an enhancement of the electron flow to the anode and a boosting of the power delivered by the cell compared to the Ab-In [34e36]. The relatively low values of PAn obtained in this work (Table 2) could be due to the fact that our MFC architecture relied on a cell design with a relative large volume compared to other designs [33,60,61]. In our study Pt as a low density catalyst was used only at the cathode in order to facilitate the final reaction to produce water; the external circuit lacked platinum. Another possible factor contributing to low average power densities in this work could be lack of acclimation of inocula to the new substrate. In effect, anaerobic microbial consortia used in our experiments were acclimated to a feed rich in sucrose and

Table 4 e Typical pH, conductivities, and resistivities of inocula used in this work.

pH Average conductivity k (mS/cm) Average resistivity r (1/ohm cm) Average Rint (ohm) log Rint

SR-In

M-In

Ab-In

7.75 1767 0.000566 2550 3.4065

7.70 1032 0.000969 6400 3.8062

7.65 397 0.002519 115000 5.0607


acetic acid, as well as sodium sulfate as electron acceptor in the sulphate-reducing ‘seeding’ bioreactor, whereas the Ab-In came from a bioreactor receiving a feed with powder milk as carbon source. After transfer to the MFCs, the substrate fed was a model extract that did not contain sucrose, sulphate, or milk powder; it was rather concocted with acetic, propionic and butyric acids as well as acetone and ethanol and mineral salts. The absence of acclimation to the new substrate might have played a negative effect on MFC performance. Moreover, inocula in our work were not previously subjected to selective pressures that could lead to its enrichment in EAB. As it is known, most of those EAB are dissimilatory metal reducing microorganisms, and their presence and predominance in the consortia anchored in MFCs are associated to high power outputs [34e36].

4.

Conclusions

In general, internal resistance of the new design cell MFC-A was consistently lower than that of the standard MFC-B, for all inocula used in this work. Resistances followed the order Rint,SR-In < Rint,M-In Rint,Ab-In. These results were consistent with reports on decreasing ohmic resistance of cells by decreasing inter-electrode distance. Also, the volumetric power PV output was higher for the MFC-A than for MFC-B; this was consistent with doubling the s in the MFC-A compared to MFC-B. Yet, power density PAn delivered was higher for MFC-A only when operated with SRIn and Ab-In, but not with M-In. The MFC-A loaded with SR-In showed a substantial improvement in PV (ca. 13-fold) probably due to the combined effects of increased s and decreased Rint and a 6.4-fold jump in PAn compared to MFC-B. The improvement was higher than the expected improvement factors (or algebraic factors; 6.5 improvement expected for PV due to combined effects of increase of s and lowering the Rint; 3.25 improvement expected for PAn due to lowering the Rint). Our results point out to continuing work using the extended area, sandwich-electrode MFC and sulphatereducing inoculum as a departing model for further studies on effects of inoculum enrichment and electrode material substitution on cell performance. Also, the MFC-A model seems to hold promise for future studies of bioelectricity generation and pollution abatement from leachates produced during biohydrogen generation in dark fermentation processes of organic solid wastes.

Acknowledgements The authors express their recognition to the Editor and the anonymous referees, for their insightful comments that allowed to considerably improve the manuscript. The authors also wish to thank the help of Research Assistants and Technicians from the Environmental Biotechnology and Renewable Energies R&D Group and the Hydrogen and Fuel Cell Group. CONACYT granted a graduate student scholarship to one of the authors (ALV-L). HMP-V gratefully acknowledges the use of a free license of software Design-Expert v6.0 from Stat-Ease, Inc.

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Nomenclature A: surface area of electrode, m2 AAn: surface area of the anode, m2 AMC: ‘sandwich’ arrangement anode-proton exchange membrane-cathode Ab-In: aerobic inoculum ANOVA: analysis of variance COD: chemical oxygen demand, mg/L or g/L df: degrees of freedom EAB: electrochemically active bacteria EED: error of the experimental design, value in ohms when the response variable is the internal resistance EMFC-max: maximum voltage output of the cell, V F: value of Fisher statistics in the ANOVA, dimensionless MFC: microbial fuel cell MFC-A: single chamber microbial fuel cell of new type MFC-B: standard single chamber microbial fuel cell M-In: methanogenic inoculum

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MSE: mean of the sum of squares of the error in the experimental design (ohm)2 PAn-max: maximum power density, mW/m2 PEM: proton exchange membrane PMFC-max: maximum power output of the cell, mW PV-max: maximum volumetric power, mW/m3 Rext: external resistance, ohm Rint: internal resistance, ohm SR-In: sulphate-reducing inoculum V: volume of the cell, m3 VSS: volatile suspended solids, a measure of biomass in cells and bioreactors, mg/L Greek characters s: ratio surface-of-electrode to cell volume (also known as specific surface), 1/m k: specific conductance or conductivity, mS/cm or S/m r: specific resistance or resistivity, ohm m hohmic: ohmic overpotential, ohm