- Email: [email protected]
Optimization of a Pt-free cathode suitable for practical applications of microbial fuel cells
Bioresource Technology 100 (2009) 4907–4910
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Optimization of a Pt-free cathode suitable for practical applications of microbial fuel cells Olivier Lefebvre a, Wai K. Ooi a, Zhe Tang b, Md. Abdullah-Al-Mamun a, Daniel H.C. Chua b, How Y. Ng a,* a b
Division of Environmental Science and Engineering, National University of Singapore, Block EA #03-12, 9 Engineering Dr. 1, Singapore 117576, Singapore Department of Material Science and Engineering, National University of Singapore, 7 Engineering Dr. 1, Singapore 117576, Singapore
a r t i c l e
i n f o
Article history: Received 18 February 2009 Received in revised form 26 April 2009 Accepted 27 April 2009 Available online 22 May 2009 Keywords: Microbial fuel cell Cathode Sputtering Cobalt
a b s t r a c t Microbial fuel cells (MFCs) are considered as a promising way for the direct extraction of biochemical energy from biomass into electricity. However, scaling up the process for practical applications and mainly for wastewater treatment is an issue because there is a necessity to get rid of unsustainable platinum (Pt) catalyst. In this study, we developed a low-cost cathode for a MFC making use of sputter-deposited cobalt (Co) as the catalyst and different types of cathode architecture were tested in a singlechambered air-cathode MFC. By sputtering the catalyst on the air-side of the cathode, increased contact with ambient oxygen significantly resulted in higher electricity generation. This outcome was different from previous studies using conventionally-coated Pt cathodes, which was due to the different technology used. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Since the first introduction of mediator-less microbial fuel cells (MFCs) by Kim et al. (1999), MFC technology is getting nearer to practical applications such as wastewater treatment; however the cost of the cathode is an issue as it can represent more than half of the total cost of a lab-scale MFC due to the use of costly platinum (Pt) to accelerate the rate of oxygen reduction (Rozendal et al., 2008). The most conventional way of fabricating the cathode is by coating a carbon cloth with an ink comprising of (i) carbon particles (carbon black), (ii) clusters of Pt catalyst and (iii) Nafion or polytetrafluoroethylene (PTFE) particles used as polymer binders. Sputtering is an alternative method of catalyst deposition whereby atoms are ejected from a solid target material due to bombardment of the target by an ion beam, resulting in thin-film deposition on the area to be coated. It is used extensively in the semiconductor industry due to its reliable control of film thickness and distribution. Typically the catalyst load can be reduced by a factor of 5 in chemical PEM fuel cells through sputter-deposition without altering the performance (Brault et al., 2004; Hirano et al., 1997). Sputtering has not yet been tested with MFCs to the best of our knowledge. The aim of this study was to manufacture a low-cost cathode that can be viably scaled-up for practical applications. This implies that the cathode should be exempt of Pt and that the catalyst load should be minimized. Hence, a sputter-deposited cobalt (Co) cathode was fabricated and its configuration was optimized in * Corresponding author. Tel.: +65 65164777; fax: +65 67744202. E-mail address: [email protected] (H.Y. Ng). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.04.061
terms of cathode layer arrangement and spatial property. In particular, Co was sputtered at a very low load (0.1 mg cm 2) either on the internal (facing the electrolyte) or on the external (facing the air) side of the cathode. Co was selected due to its high availability on Earth and reduced cost as compared to other metals such as Pt. 2. Methods 2.1. MFC electrode design The anode and the cathode consisted basically of non-wetproofed plain carbon cloth (Ballard, USA). The anode was untreated whereas the cathode, unless stated otherwise, was wet-proofed in the range of 15–19% using a PTFE suspension (Gashub, Singapore). A series of layers were then applied onto the cathode using an air brush. A backing layer (typical loading of 3.6 mg cm 2) consisting of carbon black powder (Gashub, Singapore) mixed with either Nafion (weight ratio of 2:1) or PTFE (weight ratio of 3:2) was first applied on one side of the carbon cloth and the catalyst layer (Co) was sputtered on top of it. Unless stated otherwise, a gas diffusion layer (GDL) consisting of PTFE mixed with carbon black was then applied on the side of the cathode facing the air following the procedure recommended by Cheng et al. (2006). In some experiments, the catalyst layer was sputtered on the side of the cathode facing the air and, in this case the GDL was applied on top of the catalyst layer. Co catalyst was sputtered at a load of 0.1 mg cm 2 using a Denton Vacuum Discovery TM 18 Deposition System and proper distribution of catalyst particles was checked by scanning electron microscopy. A total of five types of cathodes were designed and
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O. Lefebvre et al. / Bioresource Technology 100 (2009) 4907–4910
electrolyte interface
air interface
A
B
C
E
D
carbon cloth wet-proofing Nafion
outlet inlet
PTFE catalyst (sputter-deposited cobalt) Fig. 1. Details of the preparation of the cathodes used in this experiment. A. Basic design consisting of a 15% wet-proofed carbon cloth on which a Nafion layer and a catalyst layer are successively deposited at the electrolyte interface and a PTFE layer is applied at the air interface. B. Same as A, where the Nafion layer is replaced by PTFE. C. Same as B, where the PTFE layer at the air interface is omitted. D. Design consisting of a 15% wet-proofed carbon cloth on which a Nafion layer, a catalyst layer and a PTFE layer are successively deposited at the air interface. E. Same as D, where the carbon cloth is not wet-proofed. The inset shows the channeled continuous-flow single-chambered MFC with the cathode exposed to the air.
labeled from A to E (Fig. 1). In some experiments, we also used commercially available cathodes, coated with Pt at a standard load of 0.5 mg cm 2 (E-Tek, A6 ELAT V2.1) as a positive control and plain carbon cloth without any catalyst as a negative control. The addition of the GDL on the air-side of the electrodes was the only modification performed on the control electrodes. 2.2. MFC design and experimental conditions A PEM-less single-chambered MFC (85 mL) was designed for this experiment (Fig. 1), incorporated with channels providing a serpentine pathway, similarly as in the study of Min and Logan (2004). All types of MFCs, varying only by the configuration of the cathode, were operated in duplicates. For each set of experiment, MFCs were seeded with bacteria naturally present in sewage. This inoculation step was performed in a batch-mode, the MFC being refilled with fresh wastewater when the voltage dropped below 50 mV. The inoculation step was considered accomplished when the profiles of voltage generation showed similar pattern for a number of consecutive batches. The MFC operation was then switched to continuous mode using a synthetic sodium acetate solution as the substrate. Synthetic medium was prepared according to Oh et al. (2004), in which acetate was added at various concentrations from 48 to 337 mg L 1. Sodium acetate was pumped continuously using a peristaltic pump at a flow rate of 0.3 mL min 1 (hydraulic retention time of 4.7 h) into the bottom of the MFC and was allowed to flow through the channels directly between the anode and the cathode (44 cm2 each).
black GDL was air-brushed on the other side of the carbon cloth at the air-interface. The inoculation period lasted for 25 days in batch mode using sewage. Initially the cell potential was very low but after 7 days it started to increase and reached a maximum value of around 0.2 V with an external resistance of 1000 X upon substrate replenishment, as a consequence of the system colonization by electrochemically active bacteria. At the end of the inoculation period, the substrate was switched to acetate and the MFC was operated in continuous flow during 270 days. Unlike other studies where use of synthetic wastewater resulted in carbonate salt deposition at the air-cathode and reduced performance (Pham et al., 2005), this was not observed in our study, probably due to wet proofing and use of a GDL. The acetate removal decreased with increasing acetate loading rate (ALR) from 88 ± 17% at an ALR of 0.3 ± 0.1 kg acetate m 3 d 1 to 48 ± 13% at an ALR of 1.3 ± 0.2 kg acetate m 3 d 1. Power curves at steady state were further drawn using a variable resistance (Fig. 2). Max power did not depend on the loading rate and averaged 0.18 ± 0.04 mW (external resistance of 150 X). The linear polarization curve along with a symmetrical semi-cycle power density curve is typical for a high internal resistance MFC limited by its ohmic resistance (Logan et al., 2006). As a consequence of the influence of ALR on the acetate removal efficiency, ?c averaged 24 ± 7% at the lowest ALR tested (0.3 ± 0.1 kg acetate m 3 d 1), but dropped at higher ALR, being as low as 10 ± 3% at an ALR of 1.3 ± 0.2 kg acetate m 3 d 1. This is in accordance with other studies conducted in MFCs lacking a PEM (Liu and Logan, 2004).
0.5 2.3. Analytical methods and calculations Acetate was analyzed using a gas chromatograph (Shimadzu, AOC-20i) equipped with a FIT detector. Cell potential (Ecell, V) was measured with a multimeter connected to a computer by a data acquisition system (M3500A, Array Electronic, Taiwan). Power (W) and Coulombic efficiency (?c) were calculated following Logan et al. (2006).
Power (mW)
0.4
A.
B.
D.
E.
C.
0.3 0.2 0.1 0
3. Results and discussion Basic MFC cathode design consisted of a Co layer sputtered over a Nafion-Carbon black backing layer (MFC Type A). A PTFE-carbon
0
0.001
0.002 0.003 Current (A)
0.004
0.005
Fig. 2. Steady-state power curves of microbial fuel cells equipped with different types of cathodes and operating on synthetic acetate wastewater.
O. Lefebvre et al. / Bioresource Technology 100 (2009) 4907–4910
Amendments were then done on MFC Type A, such as replacing Nafion by PTFE in the backing layer over which the catalyst was sputtered (MFC Type B) or omitting the GDL (MFC Type C). It is worth noting here that MFC type B was motivated by the lower cost of PTFE over Nafion and the fact that – unlike other studies – Nafion did not intend to play the role of a PEM in our experiment but simply acted as a backing layer for the catalyst to be sputtered, as recommended by Haug et al. (2002). Amended versions were operated in parallel to type A over the same period of time and following the same procedure (i.e., first inoculation with sewage in batch mode, then operation with acetate in continuous-flow mode). However, these two amendments had little impact on the power generation (Fig. 2), even though in the absence of the outer GDL layer (MFC type C), water leaked through the cathode and biofilm growth was observed on the outer side of the cathode. On the contrary, applying the catalyst on the air-side of the cathode (MFC Type D) resulted in a maximum power of 0.40 ± 0.07 mW, significantly higher (+150%) than that with the basic design (MFC Type A). This finding is in contradiction with the only study apart from ours that has tested the effect of the catalyst loading side, using conventional Pt cathodes (Yang et al., 2009). To sort out the reason of our different findings, we used a commercial Pt cathode as a positive control along with a plain cathode as a negative control and the variation of cell potential with time at a similar external resistance of 1000 X using different configurations (Pt/Co facing air/electrolyte) is shown in Fig. 3. When Pt faced the electrolyte, the voltage was around 0.6 V, about 70% higher than when it was applied on the air-side of the cathode, which is in accordance with Yang et al. (2009). In addition, when Pt was applied on the electrolyte side, this voltage was attained after only a few hours, whereas reversing the Pt loading side resulted in a slow increase of voltage over several days. This shows that in the first case, the catalyst was directly available for use, whereas in the second case, on the contrary, a form of impregnation of the carbon cloth material happened that slowly increased the contact between the electrolyte and the catalyst. This is a strong sign that with conventional Pt cathodes the transfer of protons from the electrolyte to the catalyst is the limiting factor at the cathode. However, when Co was used as the catalyst, we observed the opposite as voltage increased from 0.30 to 0.35 V with an external resistance of 1000 X as a result of reversing the catalyst loading towards the air. Co dissolution was checked by dosing Co in the treating effluent but Co was always below detection level. Consequently, Co dissolution cannot be held responsible for the difference observed between our experiment and that of Yang et al. (2009) and the difference is more likely due to the use of sputtering technique. Hence, it can be hypothesized that when a large amount
of catalyst is used, the transfer of protons from the anode is the limiting step and it is advisable to apply the catalyst on the electrolyte side, whereas when the catalyst is scarce, it is better to apply the catalyst on the air side to increase the reduction rate of oxygen, which becomes the critical issue. It can also be noted from Fig. 3 that the voltage obtained with Co was significantly lower than that obtained with Pt applied on the water side of the cathode. This could be explained by the nature of Co as a catalyst and/or the much lower load of Co (0.1 mg.cm 2) used in comparison to Pt (0.5 mg.cm 2). However, the catalytic effect of Co cannot be denied as it produced significantly higher voltage output than a cathode lacking the catalyst (negative control). From an economic point of view, Pt being nearly 500 times as expensive as Co (weight basis), use of Co remains advantageous for practical applications especially wastewater treatment. Finally, having the catalyst on the air-side of the cathode could increase MFC performance but wet-proofing may limit proton access to the catalyst layer in this case. Hence we tested the efficiency of non-wet proofed carbon cloth as the cathode material (MFC Type E). However, such configuration did not improve the power output and, even though protons may reach the catalyst layer more easily, power remained at 0.37 ± 0.16 mW, which confirms that, due to the scarcity of catalyst induced by sputter-deposition and/or the lowest catalytic activity of Co as compared to Pt, the oxygen reduction rate is the limiting factor at the cathode and not the proton access to the catalyst.
4. Conclusions In this study we developed a cathode that uses a low cost catalyst (Co) at a reduced load using the sputtering technology. The best performance was achieved when the catalyst was applied on the air-side of the cathode over a backing layer made of Nafion and further protected by a GDL made of PTFE. The main breakthrough consisted of applying the Co layer on the air-side of the cathode and other parameters had less influence in comparison, even though we would recommend replacing all Nafion by PTFE for economic reasons. The GDL on the outer side of the cathode did not improve the performance of the cathode but prevented leakage and biomass growth on the air-side of the cathode. More studies have to be performed using real wastewater rich in poisoning elements such as carbon oxides and hydrogen sulphide to further check the impact of such configuration on catalyst poisoning. Finally, it can be concluded from this study that sputtering is a promising technique for the development of low cost MFCs, achieving decent levels of electricity generation at a reduced catalyst loading (0.1 mg cm 2).
References
0.65 0.55
Cell potential (V)
4909
Pt (facing electrolyte)
0.45 Pt (facing air)
Co (facing air) 0.35 0.25
No catalyst
Co (facing electrolyte)
0.15 0
20
40
60 80 Time (h)
100
120
140
Fig. 3. Variations of voltage output with time using various types of microbial fuel cell cathodes (external resistance of 1000 X).
Brault, P., Caillard, A., Thomann, A.L., Mathias, J., Charles, C., Boswell, R.W., Escribano, S., Durand, J., Sauvage, T., 2004. Plasma sputtering deposition of platinum into porous fuel cell electrodes. J. Phys. D: Appl. Phys. 37, 3419– 3423. Cheng, S., Liu, H., Logan, B.E., 2006. Power densities using different cathode catalysts (Pt and CoTMPP) and polymer binders (Nafion and PTFE) in single chamber microbial fuel cells. Environ. Sci. Technol. 40, 364–369. Haug, A.T., White, R.E., Weidner, J.W., Huang, W., Shi, S., Stoner, T., Rana, N., 2002. Increasing proton exchange membrane fuel cell catalyst effectiveness through sputter deposition. J. Electrochem. Soc. 149, A280–A287. Hirano, S., Kim, J., Srinivasan, S., 1997. High performance proton exchange membrane fuel cells with sputter-deposited Pt layer electrodes. Electrochim. Acta 42, 1587–1593. Kim, B.H., Kim, H.J., Hyun, M.S., Park, D.H., 1999. Direct electrode reaction of Fe(III)reducing bacterium, Shewanella putrefaciens. J. Microbiol. Biotechnol. 9, 127– 131. Liu, H., Logan, B.E., 2004. Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ. Sci. Technol. 38, 4040–4046.
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Logan, B.E., Hamelers, B., Rozendal, R., Schrorder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W., Rabaey, K., 2006. Microbial fuel cells: methodology and technology. Environ. Sci. Technol. 40, 5181–5192. Min, B., Logan, B.E., 2004. Continuous electricity generation from domestic wastewater and organic substrates in a flat plate microbial fuel cell. Environ. Sci. Technol. 38, 5809–5814. Oh, S., Min, B., Logan, B.E., 2004. Cathode performance as a factor in electricity generation in microbial fuel cells. Environ. Sci. Technol. 38, 4900– 4904.
Pham, T.H., Jang, J.K., Moon, H.S., Chang, I.S., Kim, B.H., 2005. Improved performance of microbial fuel cell using membrane-electrode assembly. J. Microbiol. Biotechnol. 15, 438–441. Rozendal, R.A., Hamelers, H.V., Rabaey, K., Keller, J., Buisman, C.J., 2008. Towards practical implementation of bioelectrochemical wastewater treatment. Trends Biotechnol. 26, 450–459. Yang, S., Jia, B., Liu, H., 2009. Effects of the Pt loading side and cathode-biofilm on the performance of a membrane-less and single-chamber microbial fuel cell. Bioresour. Technol. 100, 1197–1202.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Optimization of a Pt-free cathode suitable for practical applications of microbial fuel cells Olivier Lefebvre a, Wai K. Ooi a, Zhe Tang b, Md. Abdullah-Al-Mamun a, Daniel H.C. Chua b, How Y. Ng a,* a b
Division of Environmental Science and Engineering, National University of Singapore, Block EA #03-12, 9 Engineering Dr. 1, Singapore 117576, Singapore Department of Material Science and Engineering, National University of Singapore, 7 Engineering Dr. 1, Singapore 117576, Singapore
a r t i c l e
i n f o
Article history: Received 18 February 2009 Received in revised form 26 April 2009 Accepted 27 April 2009 Available online 22 May 2009 Keywords: Microbial fuel cell Cathode Sputtering Cobalt
a b s t r a c t Microbial fuel cells (MFCs) are considered as a promising way for the direct extraction of biochemical energy from biomass into electricity. However, scaling up the process for practical applications and mainly for wastewater treatment is an issue because there is a necessity to get rid of unsustainable platinum (Pt) catalyst. In this study, we developed a low-cost cathode for a MFC making use of sputter-deposited cobalt (Co) as the catalyst and different types of cathode architecture were tested in a singlechambered air-cathode MFC. By sputtering the catalyst on the air-side of the cathode, increased contact with ambient oxygen significantly resulted in higher electricity generation. This outcome was different from previous studies using conventionally-coated Pt cathodes, which was due to the different technology used. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Since the first introduction of mediator-less microbial fuel cells (MFCs) by Kim et al. (1999), MFC technology is getting nearer to practical applications such as wastewater treatment; however the cost of the cathode is an issue as it can represent more than half of the total cost of a lab-scale MFC due to the use of costly platinum (Pt) to accelerate the rate of oxygen reduction (Rozendal et al., 2008). The most conventional way of fabricating the cathode is by coating a carbon cloth with an ink comprising of (i) carbon particles (carbon black), (ii) clusters of Pt catalyst and (iii) Nafion or polytetrafluoroethylene (PTFE) particles used as polymer binders. Sputtering is an alternative method of catalyst deposition whereby atoms are ejected from a solid target material due to bombardment of the target by an ion beam, resulting in thin-film deposition on the area to be coated. It is used extensively in the semiconductor industry due to its reliable control of film thickness and distribution. Typically the catalyst load can be reduced by a factor of 5 in chemical PEM fuel cells through sputter-deposition without altering the performance (Brault et al., 2004; Hirano et al., 1997). Sputtering has not yet been tested with MFCs to the best of our knowledge. The aim of this study was to manufacture a low-cost cathode that can be viably scaled-up for practical applications. This implies that the cathode should be exempt of Pt and that the catalyst load should be minimized. Hence, a sputter-deposited cobalt (Co) cathode was fabricated and its configuration was optimized in * Corresponding author. Tel.: +65 65164777; fax: +65 67744202. E-mail address: [email protected] (H.Y. Ng). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.04.061
terms of cathode layer arrangement and spatial property. In particular, Co was sputtered at a very low load (0.1 mg cm 2) either on the internal (facing the electrolyte) or on the external (facing the air) side of the cathode. Co was selected due to its high availability on Earth and reduced cost as compared to other metals such as Pt. 2. Methods 2.1. MFC electrode design The anode and the cathode consisted basically of non-wetproofed plain carbon cloth (Ballard, USA). The anode was untreated whereas the cathode, unless stated otherwise, was wet-proofed in the range of 15–19% using a PTFE suspension (Gashub, Singapore). A series of layers were then applied onto the cathode using an air brush. A backing layer (typical loading of 3.6 mg cm 2) consisting of carbon black powder (Gashub, Singapore) mixed with either Nafion (weight ratio of 2:1) or PTFE (weight ratio of 3:2) was first applied on one side of the carbon cloth and the catalyst layer (Co) was sputtered on top of it. Unless stated otherwise, a gas diffusion layer (GDL) consisting of PTFE mixed with carbon black was then applied on the side of the cathode facing the air following the procedure recommended by Cheng et al. (2006). In some experiments, the catalyst layer was sputtered on the side of the cathode facing the air and, in this case the GDL was applied on top of the catalyst layer. Co catalyst was sputtered at a load of 0.1 mg cm 2 using a Denton Vacuum Discovery TM 18 Deposition System and proper distribution of catalyst particles was checked by scanning electron microscopy. A total of five types of cathodes were designed and
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O. Lefebvre et al. / Bioresource Technology 100 (2009) 4907–4910
electrolyte interface
air interface
A
B
C
E
D
carbon cloth wet-proofing Nafion
outlet inlet
PTFE catalyst (sputter-deposited cobalt) Fig. 1. Details of the preparation of the cathodes used in this experiment. A. Basic design consisting of a 15% wet-proofed carbon cloth on which a Nafion layer and a catalyst layer are successively deposited at the electrolyte interface and a PTFE layer is applied at the air interface. B. Same as A, where the Nafion layer is replaced by PTFE. C. Same as B, where the PTFE layer at the air interface is omitted. D. Design consisting of a 15% wet-proofed carbon cloth on which a Nafion layer, a catalyst layer and a PTFE layer are successively deposited at the air interface. E. Same as D, where the carbon cloth is not wet-proofed. The inset shows the channeled continuous-flow single-chambered MFC with the cathode exposed to the air.
labeled from A to E (Fig. 1). In some experiments, we also used commercially available cathodes, coated with Pt at a standard load of 0.5 mg cm 2 (E-Tek, A6 ELAT V2.1) as a positive control and plain carbon cloth without any catalyst as a negative control. The addition of the GDL on the air-side of the electrodes was the only modification performed on the control electrodes. 2.2. MFC design and experimental conditions A PEM-less single-chambered MFC (85 mL) was designed for this experiment (Fig. 1), incorporated with channels providing a serpentine pathway, similarly as in the study of Min and Logan (2004). All types of MFCs, varying only by the configuration of the cathode, were operated in duplicates. For each set of experiment, MFCs were seeded with bacteria naturally present in sewage. This inoculation step was performed in a batch-mode, the MFC being refilled with fresh wastewater when the voltage dropped below 50 mV. The inoculation step was considered accomplished when the profiles of voltage generation showed similar pattern for a number of consecutive batches. The MFC operation was then switched to continuous mode using a synthetic sodium acetate solution as the substrate. Synthetic medium was prepared according to Oh et al. (2004), in which acetate was added at various concentrations from 48 to 337 mg L 1. Sodium acetate was pumped continuously using a peristaltic pump at a flow rate of 0.3 mL min 1 (hydraulic retention time of 4.7 h) into the bottom of the MFC and was allowed to flow through the channels directly between the anode and the cathode (44 cm2 each).
black GDL was air-brushed on the other side of the carbon cloth at the air-interface. The inoculation period lasted for 25 days in batch mode using sewage. Initially the cell potential was very low but after 7 days it started to increase and reached a maximum value of around 0.2 V with an external resistance of 1000 X upon substrate replenishment, as a consequence of the system colonization by electrochemically active bacteria. At the end of the inoculation period, the substrate was switched to acetate and the MFC was operated in continuous flow during 270 days. Unlike other studies where use of synthetic wastewater resulted in carbonate salt deposition at the air-cathode and reduced performance (Pham et al., 2005), this was not observed in our study, probably due to wet proofing and use of a GDL. The acetate removal decreased with increasing acetate loading rate (ALR) from 88 ± 17% at an ALR of 0.3 ± 0.1 kg acetate m 3 d 1 to 48 ± 13% at an ALR of 1.3 ± 0.2 kg acetate m 3 d 1. Power curves at steady state were further drawn using a variable resistance (Fig. 2). Max power did not depend on the loading rate and averaged 0.18 ± 0.04 mW (external resistance of 150 X). The linear polarization curve along with a symmetrical semi-cycle power density curve is typical for a high internal resistance MFC limited by its ohmic resistance (Logan et al., 2006). As a consequence of the influence of ALR on the acetate removal efficiency, ?c averaged 24 ± 7% at the lowest ALR tested (0.3 ± 0.1 kg acetate m 3 d 1), but dropped at higher ALR, being as low as 10 ± 3% at an ALR of 1.3 ± 0.2 kg acetate m 3 d 1. This is in accordance with other studies conducted in MFCs lacking a PEM (Liu and Logan, 2004).
0.5 2.3. Analytical methods and calculations Acetate was analyzed using a gas chromatograph (Shimadzu, AOC-20i) equipped with a FIT detector. Cell potential (Ecell, V) was measured with a multimeter connected to a computer by a data acquisition system (M3500A, Array Electronic, Taiwan). Power (W) and Coulombic efficiency (?c) were calculated following Logan et al. (2006).
Power (mW)
0.4
A.
B.
D.
E.
C.
0.3 0.2 0.1 0
3. Results and discussion Basic MFC cathode design consisted of a Co layer sputtered over a Nafion-Carbon black backing layer (MFC Type A). A PTFE-carbon
0
0.001
0.002 0.003 Current (A)
0.004
0.005
Fig. 2. Steady-state power curves of microbial fuel cells equipped with different types of cathodes and operating on synthetic acetate wastewater.
O. Lefebvre et al. / Bioresource Technology 100 (2009) 4907–4910
Amendments were then done on MFC Type A, such as replacing Nafion by PTFE in the backing layer over which the catalyst was sputtered (MFC Type B) or omitting the GDL (MFC Type C). It is worth noting here that MFC type B was motivated by the lower cost of PTFE over Nafion and the fact that – unlike other studies – Nafion did not intend to play the role of a PEM in our experiment but simply acted as a backing layer for the catalyst to be sputtered, as recommended by Haug et al. (2002). Amended versions were operated in parallel to type A over the same period of time and following the same procedure (i.e., first inoculation with sewage in batch mode, then operation with acetate in continuous-flow mode). However, these two amendments had little impact on the power generation (Fig. 2), even though in the absence of the outer GDL layer (MFC type C), water leaked through the cathode and biofilm growth was observed on the outer side of the cathode. On the contrary, applying the catalyst on the air-side of the cathode (MFC Type D) resulted in a maximum power of 0.40 ± 0.07 mW, significantly higher (+150%) than that with the basic design (MFC Type A). This finding is in contradiction with the only study apart from ours that has tested the effect of the catalyst loading side, using conventional Pt cathodes (Yang et al., 2009). To sort out the reason of our different findings, we used a commercial Pt cathode as a positive control along with a plain cathode as a negative control and the variation of cell potential with time at a similar external resistance of 1000 X using different configurations (Pt/Co facing air/electrolyte) is shown in Fig. 3. When Pt faced the electrolyte, the voltage was around 0.6 V, about 70% higher than when it was applied on the air-side of the cathode, which is in accordance with Yang et al. (2009). In addition, when Pt was applied on the electrolyte side, this voltage was attained after only a few hours, whereas reversing the Pt loading side resulted in a slow increase of voltage over several days. This shows that in the first case, the catalyst was directly available for use, whereas in the second case, on the contrary, a form of impregnation of the carbon cloth material happened that slowly increased the contact between the electrolyte and the catalyst. This is a strong sign that with conventional Pt cathodes the transfer of protons from the electrolyte to the catalyst is the limiting factor at the cathode. However, when Co was used as the catalyst, we observed the opposite as voltage increased from 0.30 to 0.35 V with an external resistance of 1000 X as a result of reversing the catalyst loading towards the air. Co dissolution was checked by dosing Co in the treating effluent but Co was always below detection level. Consequently, Co dissolution cannot be held responsible for the difference observed between our experiment and that of Yang et al. (2009) and the difference is more likely due to the use of sputtering technique. Hence, it can be hypothesized that when a large amount
of catalyst is used, the transfer of protons from the anode is the limiting step and it is advisable to apply the catalyst on the electrolyte side, whereas when the catalyst is scarce, it is better to apply the catalyst on the air side to increase the reduction rate of oxygen, which becomes the critical issue. It can also be noted from Fig. 3 that the voltage obtained with Co was significantly lower than that obtained with Pt applied on the water side of the cathode. This could be explained by the nature of Co as a catalyst and/or the much lower load of Co (0.1 mg.cm 2) used in comparison to Pt (0.5 mg.cm 2). However, the catalytic effect of Co cannot be denied as it produced significantly higher voltage output than a cathode lacking the catalyst (negative control). From an economic point of view, Pt being nearly 500 times as expensive as Co (weight basis), use of Co remains advantageous for practical applications especially wastewater treatment. Finally, having the catalyst on the air-side of the cathode could increase MFC performance but wet-proofing may limit proton access to the catalyst layer in this case. Hence we tested the efficiency of non-wet proofed carbon cloth as the cathode material (MFC Type E). However, such configuration did not improve the power output and, even though protons may reach the catalyst layer more easily, power remained at 0.37 ± 0.16 mW, which confirms that, due to the scarcity of catalyst induced by sputter-deposition and/or the lowest catalytic activity of Co as compared to Pt, the oxygen reduction rate is the limiting factor at the cathode and not the proton access to the catalyst.
4. Conclusions In this study we developed a cathode that uses a low cost catalyst (Co) at a reduced load using the sputtering technology. The best performance was achieved when the catalyst was applied on the air-side of the cathode over a backing layer made of Nafion and further protected by a GDL made of PTFE. The main breakthrough consisted of applying the Co layer on the air-side of the cathode and other parameters had less influence in comparison, even though we would recommend replacing all Nafion by PTFE for economic reasons. The GDL on the outer side of the cathode did not improve the performance of the cathode but prevented leakage and biomass growth on the air-side of the cathode. More studies have to be performed using real wastewater rich in poisoning elements such as carbon oxides and hydrogen sulphide to further check the impact of such configuration on catalyst poisoning. Finally, it can be concluded from this study that sputtering is a promising technique for the development of low cost MFCs, achieving decent levels of electricity generation at a reduced catalyst loading (0.1 mg cm 2).
References
0.65 0.55
Cell potential (V)
4909
Pt (facing electrolyte)
0.45 Pt (facing air)
Co (facing air) 0.35 0.25
No catalyst
Co (facing electrolyte)
0.15 0
20
40
60 80 Time (h)
100
120
140
Fig. 3. Variations of voltage output with time using various types of microbial fuel cell cathodes (external resistance of 1000 X).
Brault, P., Caillard, A., Thomann, A.L., Mathias, J., Charles, C., Boswell, R.W., Escribano, S., Durand, J., Sauvage, T., 2004. Plasma sputtering deposition of platinum into porous fuel cell electrodes. J. Phys. D: Appl. Phys. 37, 3419– 3423. Cheng, S., Liu, H., Logan, B.E., 2006. Power densities using different cathode catalysts (Pt and CoTMPP) and polymer binders (Nafion and PTFE) in single chamber microbial fuel cells. Environ. Sci. Technol. 40, 364–369. Haug, A.T., White, R.E., Weidner, J.W., Huang, W., Shi, S., Stoner, T., Rana, N., 2002. Increasing proton exchange membrane fuel cell catalyst effectiveness through sputter deposition. J. Electrochem. Soc. 149, A280–A287. Hirano, S., Kim, J., Srinivasan, S., 1997. High performance proton exchange membrane fuel cells with sputter-deposited Pt layer electrodes. Electrochim. Acta 42, 1587–1593. Kim, B.H., Kim, H.J., Hyun, M.S., Park, D.H., 1999. Direct electrode reaction of Fe(III)reducing bacterium, Shewanella putrefaciens. J. Microbiol. Biotechnol. 9, 127– 131. Liu, H., Logan, B.E., 2004. Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ. Sci. Technol. 38, 4040–4046.
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Logan, B.E., Hamelers, B., Rozendal, R., Schrorder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W., Rabaey, K., 2006. Microbial fuel cells: methodology and technology. Environ. Sci. Technol. 40, 5181–5192. Min, B., Logan, B.E., 2004. Continuous electricity generation from domestic wastewater and organic substrates in a flat plate microbial fuel cell. Environ. Sci. Technol. 38, 5809–5814. Oh, S., Min, B., Logan, B.E., 2004. Cathode performance as a factor in electricity generation in microbial fuel cells. Environ. Sci. Technol. 38, 4900– 4904.
Pham, T.H., Jang, J.K., Moon, H.S., Chang, I.S., Kim, B.H., 2005. Improved performance of microbial fuel cell using membrane-electrode assembly. J. Microbiol. Biotechnol. 15, 438–441. Rozendal, R.A., Hamelers, H.V., Rabaey, K., Keller, J., Buisman, C.J., 2008. Towards practical implementation of bioelectrochemical wastewater treatment. Trends Biotechnol. 26, 450–459. Yang, S., Jia, B., Liu, H., 2009. Effects of the Pt loading side and cathode-biofilm on the performance of a membrane-less and single-chamber microbial fuel cell. Bioresour. Technol. 100, 1197–1202.