STUDY ON MEMBRANES AND CATHODE CATALYSTS IN MICROBIAL FUEL CELLS LU MIN (BSc Nanjing University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2013 DECLARATIONS I hereby declare that this thesis is my original work and it has been written by me in its entirety, under the supervision of Prof Sam Li Fong Yau, Department of Chemistry, National University of Singapore, between Aug 2009 and July 2013 I have duly acknowledged all the sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously The content of the thesis has been partly published in: (1) Critical Reviews in Environmental Science and Technology (2) Biosensors & Bioelectronics (3) Journal of Power Sources Name Signature II Date ACKNOWLEDGEMENT First and foremost, I would like to extend my sincere gratitude to my supervisor Professor Sam Li Fong Yau for his guidance and support during my graduate study His open mindedness and friendly disposition will deeply impact on my life and future career I am thankful to my collaborators from Department of Civil & Environmental Engineering (CEE), Professor Ng How Yong and Shailesh Kharkwal To all the members in Professor Sam Li’s lab who provide such a suitable learning environment, encouragement and support, thank you I would like to specially mention some of them in our wastewater team: Dr Wu Huanan., Ms Guo Lin, Ms Lee Si Ni, Mr Lai Linke., Ms Zhang Lijuan, and also my past honors and UROPS students, Mr Leonard Bay and Mr Yap Chen Xi, who have injected much fun and enthusiasm in the research life I would like to thank a few important colleagues and friends in the NUS Environmental Research Institute (NERI): Ms Elaine Tay, Mdm Frances Lim, Ms Per Poh Geok who have provided support for my experiment in NERI from every aspect My heartfelt gratitude goes to Ms Suriawati Binte Sa'ad in Department of Chemistry, always helping me in my graduate study, from admission to graduation I would like to express my loving thanks to my husband Xie Xiaoji His love, encouragement and sometimes excellent ideas motivated me towards the accomplishment documented in this thesis Last, but not least, I wish to dedicate this thesis to my parents Without their love and understanding, I would not have completed my doctoral study The financial support of National University of Singapore is gratefully acknowledged III TABLE OF CONTENTS DECLARATIONS II ACKNOWLEDGEMENT III TABLE OF CONTENTS IV SUMMARY IX LIST OF TABLES XI LIST OF FIGURES XII CHAPTER INTRODUCTION 1.1 FUNDAMENTALS ABOUT MFCS 1.1.1 Thermodynamic fundamentals 1.1.2 Electrochemical losses of MFCs- an overview 1.2 MEMBRANES IN MFCS 1.3 CATHODE REACTIONS IN MFCS 10 1.3.1 Electron acceptors in MFCs 12 1.3.2 Oxygen reduction catalysts in MFCs 14 1.3.3 Summary 18 1.4 OBJECTIVES AND SIGNIFICANCE OF THIS THESIS 21 IV REFERENCES 23 CHAPTER NANOPOROUS HYDROPHILIC POLYMER MEMBRANES AS ALTERNATIVE SEPARATORS IN MICROBIAL FUEL CELLS 27 2.1 INTRODUCTION 27 2.2 EXPERIMENTAL 27 2.2.1 MFC configuration 27 2.2.2 Membrane selection 28 2.2.3 MFC operation 28 2.2.4 Analysis 29 2.3 RESULTS AND DISCUSSIONS 30 2.3.1 Membrane characterization 30 2.3.2 Power output of different MFCs 32 2.4 CONCLUSION 35 REFERENCES 37 CHAPTER CARBON NANOTUBE SUPPORTED MnO2 CATALYSTS FOR OXYGEN REDUCTION REACTION 38 3.1 INTRODUCTION 38 3.2 EXPERIMENTAL 41 3.2.1 Synthesis of MnO2 Nanomaterials 41 V 3.2.2 Electrode Fabrication 42 3.2.3 MFC Test System Setup 43 3.2.4 Electrochemical Measurement 45 3.3 RESULTS AND DISCUSSIONS 45 3.3.1 Characterization of Manganese Dioxide 45 3.3.2 Cyclic Voltammetry 50 3.3.3 Performance of the Cubic MFCs with Different Cathode Catalysts 56 3.4 CONCLUSION 58 REFERENCES 59 CHAPTER MANGANESE-POLYPYRROLECARBON NANOTUBE COMPOSITE AS OXYGEN REDUCTION CATALYST 61 4.1 INTRODUCTION 61 4.2 EXPERIMENTAL 62 4.2.1 Preparation of manganese-polypyrrole-carbon nanotube composite 62 4.2.2 Electrode fabrication 64 4.2.3 Electrochemical measurement 64 4.2.4 Air-cathode MFC set-up 64 4.3 RESULTS AND DISCUSSIONS 65 4.3.1 Synthesis and Characterization of the catalysts 65 VI 4.3.2 Electrochemical characterization of Mn-PPY-CNT composite 70 4.3.3 MFC performances with various catalysts 73 4.4 CONCLUSION 75 REFERENCES 76 CHAPTER POLYELECTROLYTE FUNCTIONALIZED-SINGLE WALL CARBON NANOTUBES AS OXYGEN REDUCTION CATALYST 77 5.1 INTRODUCTION 77 5.2 EXPERIMENTAL 78 5.2.1 Synthesis of polyelectrolyte-SCNT composite catalyst 78 5.2.2 Electrode fabrication 79 5.2.3 Electrochemical measurement 79 5.2.4 MFC setup and operation 80 5.3 RESULTS AND DISCUSSIONS 81 5.3.1 Characterization of polyelectrolyte-SCNT composites catalysts 81 5.3.2 Catalytic capability towards ORR with polyelectrolyte-SCNT composites 82 5.3.3 MFC Performances with different cathode catalysts 84 5.4 CONCLUSION 85 REFERENCES 87 VII CHAPTER CONCLUSION AND OUTLOOK 88 6.1 CONCLUSION 88 6.2 OUTLOOK OF MFC DEVELOPMENT 91 6.2.1 Bottlenecks of MFC scaling up 91 6.2.1.1 Design constraints as determined by wastewater application 91 6.2.1.2 Design constraints as determined by scaling up 92 6.2.2 Future trend of MFC development 93 REFERENCES 95 PUBLICATION & CONFERENCE 96 VIII SUMMARY Microbial fuel cell (MFC) is a device harnessing microorganisms to harvest electricity from wastewater It shows great promise because of its ability for simultaneous energy recovery and wastewater treatment However, it is still in its infancy with problems to be solved For the membrane, it needs to be selective for target molecules, corrosion-resistant and affordable In the cathode chamber, oxygen is often applied in the presence of expensive platinum-based catalyst to achieve good performance, which brings high cost and thus hinders further practical applications of MFCs In this thesis, these two elements are optimized in two parts In the first part (Chapter 2), nanoporous membranes are examined as separators to substitute ion exchange membrane It was found that membranes with different pore sizes and materials performed differently Polyethersulfone membrane-based MFC yielded the highest power, 92% comparing with that based on cation exchange membrane It also possessed the lowest internal resistance among the selected membranes possibly because of better proton conductivity Considering other parameters, polyethersulfone membrane showed less satisfactory results because of the bigger pore size allowing organics and electrons to cross over the membrane to cathode chamber, resulting in lower COD removal and lower columbic efficiency From a general point of view, polyethersulfone membrane could be a cheaper alternative as MFC separators As for other membranes, comparable power outputs with varied COD removal efficiencies were also achieved IX In the second part (Chapter 3-5), cathode catalysts in microbial fuel cells were studied Several noble metal-free catalysts, namely manganese dioxide, manganese-polypyrrole-carbon nanotube composite and polyelectrolytecarbon nanotube composite, have been synthesized and demonstrated as efficient and stable cathode catalysts for oxygen reduction reaction (ORR) Prepared by various methods, these catalysts were comprehensively characterized Subsequently, electro-catalytic capability of these novel catalysts in neutral electrolyte was investigated by cyclic voltammetry To further verify catalytic capability of these catalysts, they were utilized as the cathode catalysts in air-cathode MFCs It was found that these catalysts yielded efficient and stable performance with maximum power comparable to platinum/carbon black (Pt/C) catalyst Furthermore, the catalysts showed good long-term stability which is essential for MFC study Compared to Pt/C catalyst, these noble metal-free catalysts sacrificed electricity generation performance to some extent and reached a compromise between power output and capital cost, thus increasing the feasibility towards MFC practical applications In addition, the three catalysts developed in this dissertation represent three promising research directions for noble metal-free oxygen reduction catalysts, and more effort could be made for further improvement by applying different components In the future, novel application of MFCs such as bioremediation reactor or on-line sensors could be explored, and our cost-effective catalysts will facilitate this progress X Chapter Polyelectrolyte -Single Wall Carbon Nanotubes as Oxygen Reduction Catalyst composites, respectively Table Elemental contents of different composites Sample C / wt% N / wt% PDDA-SCNT 78.82 2.04 PEPU-SCNT 87.89 1.61 B-SCNT 93.24 N.A 5.3.2 Catalytic capability towards ORR with polyelectrolyte-SCNT composites Figure 33 CVs for ORR with different catalysts and conditions (a) Pt/C 0.5 mg Pt cm-2; (b) and (e) PDDA-SCNT mg cm-2; (c) and (f) PEPU-SCNT mg cm-2; (d) B-CNT mg cm-2 (a-d) in aerobic condition, (e,f) in anaerobic condition We further examined the catalytic capability of the polyelectrolyte-SCNT composites in neutral media by CV (Figure 33) The benchmark Pt/C catalyst yielded broad ORR peak from -0.2 V to -0.5 V in aerated electrolyte 82 Chapter Polyelectrolyte -Single Wall Carbon Nanotubes as Oxygen Reduction Catalyst both PDDA-SCNT and PEPU-SCNT catalysts showed peaks from -0.2 V to 0.5 V under aerobic condition, but no such peaks under anaerobic condition, which implied that this peak was attributed to ORR process Comparing with B-SCNT, the ORR peak intensities increased for both two composites with polyelectrolyte incorporation, indicating enhanced catalytic capability These results indicated the important role of polyelectrolytes during the process of ORR Table Detailed breakdown of N1s signal with peak position and relative composition of different nitrogen groups PDDA PDDA-SCNT PEPU PEPU-SCNT Sample Pk / eV Quaternary N Urea N At% Pk / eV At% Pk / eV At% Pk / eV At% 402.28 7.2 402.35 0.5 399.55 7.3 399.62 0.7 - - - - 402.44 9.5 402.35 0.8 Note: Pk represents “peak position”; At % represents “atomic percentage” To further investigate the catalytic mechanism of these polyelectrolyteSCNT composites, interaction between carbon nanotube and polyelectrolyte was investigated by XPS As shown in Figure 34 and Table 8, binding energy of N atoms in pure polyelectrolyte and in polyelectrolyte-SCNT composite did not vary, which indicates no direct electron transfer between C backbone and N atom in quaternary ammonium group Therefore, we believe that the catalytic activity is due to the increased oxygen affinity at positive quaternary ammonium sites In the meanwhile, the delocalized electrons of adjacent carbon backbone in SCNTs are responsible for the enhanced electron transfer from the catalyst to the adsorbed oxygen with weakened O-O covalent bond.17,18 The synergetic effect between quaternary ammonium and delocalized electrons on carbon backbone promotes the catalytic efficiency of the polyelectrolyte-SCNT composites Moreover, the better performance of PEPU-SCNT could be due to the longer linear chain structure that allows higher degree of freedom, leading to stronger interaction between PEPU and SCNT, while the five-member-ring structure of PDDA causes the steric 83 Chapter Polyelectrolyte -Single Wall Carbon Nanotubes as Oxygen Reduction Catalyst hindrance on contact to SCNT.19 Figure 34 XPS spectra of N1s for different composites (a) PDDA, (b)PEPU, (c) PDDA-SCNT, and (d) PEPU-SCNT N 1s in (a) and (c) is assigned to quaternary N species, while N1 and N2 in (b) (d) to urea N and quaternary N, respectively 5.3.3 MFC Performances with different cathode catalysts To further evaluate the catalytic capabilities of these polyelectrolyteSCNT composites in the air-cathode MFC systems, MFCs equipped with PDDA-SCNT, PEPU-SCNT and Pt/C individually applied as cathode catalyst were investigated and compared Performances of these MFCs were summarized in Table and the polarization and power curves were presented in Figure 35 OCVs were obtained as 0.617 V for PEPU-SCNT based MFCs This value was higher than that of PDDA-SCNT based MFCs (0.543 V) and approaching that of Pt/C-based MFCs (0.641 V) The internal resistances, calculated from the slope of polarization curves, are 73.3 and 69.7 (PDDA-SCNT) (PEPU-SCNT), respectively Importantly, in the presence of PEPU-SCNT catalyst, the MFCs generated a maximum power density of 84 Chapter Polyelectrolyte -Single Wall Carbon Nanotubes as Oxygen Reduction Cat Single Catalyst 270.1 mWm-2, reaching over 70% of Pt/C-based MFC (375.3 mW -2) based mWm Slightly lower power dens was observed for the MFC with PDDA density s PDDA-SCNT catalyst (188.9 mWm-2), which was consistent with the CV results Taken s together, the results obtained in the MFCs strongly indicate that PDDA indicated PDDASCNT and PEPU-SCNT could be effective MFC cathode catalysts SCNT Furthermore, the lower cost ma them more competitive in MFC practical more, made applications Figure 35 Polarization and power curves for air cathode MFCs air-cathode PDDA-SCNT mg cm-2; (b) PEPU-SCNT mg cm-2; (c) Pt/C 0.5 mg Pt cm-2 SCNT c) Table Performance of air-cathode MFCs based on different cathode catalysts Pmax / mW PDmax / mW·m-2 73.3 0.534 188.9 0.617 69.7 0.763 270.1 0.641 67.1 1.061 375.3 Catalysts Eemf / V OCV / V PDDA-SCNT 0.399 0.543 PEPU-SCNT 0.459 Pt/C 0.531 31 Rin / 5.4 CONCLUSION In this study, two types of polyelectrolyte-SCNT composites, PDDA SCNT PDDA85 Chapter Polyelectrolyte -Single Wall Carbon Nanotubes as Oxygen Reduction Catalyst SCNT and PEPU-SCNT, were prepared by a simple solution-based method The resulting nanocomposites were further studied by CV The results demonstrated that both of them had good catalytic capability towards ORR in neutral media When applied in MFCs, the performances of PDDA-SCNT and PEPU-SCNT were quite comparable to Pt/C Noticeably, PEPU-SCNT based MFCs produce 72% as much power as that of Pt-based MFCs With simple preparation, low cost and good catalytic capability, we believed that the polyelectrolyte-SCNT composite would be a promising category of MFC cathode catalysts that were worthy of further investigation 86 Chapter Polyelectrolyte -Single Wall Carbon Nanotubes as Oxygen Reduction Catalyst REFERENCES (1) Zhao, F.; Harnisch, F.; Schröder, U.; Scholz, F.; Bogdanoff, P.; Herrmann, I Electrochemistry Communications 2005, 7, 1405-1410 (2) Li, X.; Hu, B.; Suib, S.; Lei, Y.; Li, B Journal of Power Sources 2010, 195, 2586-2591 (3) Tepper, A W J W.; Milikisyants, S.; Sottini, S.; Vijgenboom, E.; Groenen, E J J.; Canters, G W Journal of the American Chemical Society 2009, 131, 11680-11682 (4) Kjaergaard, C H.; Rossmeisl, J.; Norskov, J K Inorganic chemistry 2010, 49, 3567-72 (5) Fujigaya, T.; Uchinoumi, T.; Kaneko, K.; Nakashima, N Chem Commun (Camb) 2011, 47, 6843-5 (6) Chen, Z.; Higgins, D.; Tao, H.; Hsu, R S.; Chen, Z The Journal of Physical Chemistry C 2009, 113, 21008-21013 (7) Liu, R.; Wu, D.; Feng, X.; Mullen, K Angew Chem Int Ed Engl 2010, 49, 2565-9 (8) Matter, P H.; Wang, E.; Arias, M.; Biddinger, E J.; Ozkan, U S The Journal of Physical Chemistry B 2006, 110, 18374-18384 (9) Wang, S.; Yu, D.; Dai, L.; Chang, D W.; Baek, J.-B ACS Nano 2011, 5, 6202-6209 (10) Luo, Z.; Lim, S.; Tian, Z.; Shang, J.; Lai, L.; MacDonald, B.; Fu, C.; Shen, Z.; Yu, T.; Lin, J Journal of Materials Chemistry 2011, 21, 8038 (11) Shanmugam, S.; Osaka, T Chem Commun (Camb) 2011, 47, 4463-5 (12) Jin, H.; Zhang, H.; Zhong, H.; Zhang, J Energy & Environmental Science 2011, 4, 3389 (13) Wang, S.; Yu, D.; Dai, L Journal of the American Chemical Society 2011, 133, 5182-5 (14) Lu, M.; Kharkwal, S.; Ng, H Y.; Li, S F Biosensors & bioelectronics 2011, 26, 4728-32 (15) Cheng, S.; Liu, H.; Logan, B E Electrochemistry Communications 2006, 8, 489-494 (16) Phillips, D R L a E J P Applied and Environmental Microbiology 1988, 54, 1472-1480 (17) Sharifi, T.; Hu, G.; Jia, X.; Wagberg, T ACS Nano 2012 (18) Li, Y.; Li, T.; Yao, M.; Liu, S Journal of Materials Chemistry 2012, 22, 10911-10917 (19) Lin, H.; Qu, F.; Wu, X.; Xue, M.; Zhu, G.; Qiu, S Journal of Solid State Chemistry 2011, 184, 1415-1420 87 Chapter Conclusion and Outlook Chapter Conclusion and Outlook 6.1 CONCLUSION In the first part (chapter 2), we examined the feasibility of nanoporous membrane as separators in MFCs, and it was demonstrated that nanoporous membranes could be used with comparable power output with comparable internal resistance, and could be chosen according to different application of MFCs In the second part, we developed three novel oxygen reduction catalysts Prepared by various methods, these noble metal-free catalysts were applied in MFCs for simultaneous wastewater treatment and electricity generation (Chapter 3-5) Firstly, we synthesized three types of MnO2 nanoparticles by hydrothermal method and apply them in MFCs as cathode catalysts in MFCs, performing ORR catalysis to different extents Based on the results obtained in this dissertation, it is proposed that the remarkable catalytic capability of MnO2 may be attributed to the Mn(IV) /Mn(III) redox couple in which MnO2 is first reduced to Mn(III) in MOOH which has good affinity to oxygen molecules, exchanges electrons with oxygen molecules and gets oxidized back to MnO2 This is a novel metal-oxide nanomaterial developed for ORR in neutral conditions; the remarkable stability allows continuous exploration of MnO2based MFCs as online biological oxygen demand (BOD) sensors in our group (Chapter 3) Secondly, we synthesized manganese-polypyrrole-carbon 88 nanotube Chapter Conclusion and Outlook composite by solution-based method Polypyrrole molecules were first deposited around outer surface of carbon nanotubes, and manganese atoms were inserted into polypyrrole environment The metal-polymer interaction was proposed to be responsible for enhanced ORR catalysis This was the first time a manganese-heterocycle-carbon support composite was applied in MFCs as cathode catalyst, and this could facilitate a new research direction for ORR catalysis (Chapter 4) Thirdly, we synthesized polyelectrolyte-carbon nanotube composite by simple solution-based method The good affinity of polyelectrolyte and the electron transfer ability of carbon nanotube contribute together towards the enhanced ORR catalytic ability Easy preparation, low cost and metal-free feature make this composite a good candidate as MFC cathode catalyst (Chapter 5) In conclusion, these noble metal-free catalysts offer the cost-effective choice for MFC cathode catalyst with efficient and stable performance Compared to platinum-based catalysts, these noble metal-free catalysts sacrifice electricity generation performance to some extent and reach a compromise between power output and capital cost, thus increasing the feasibility towards practical MFC applications In addition, the catalysts developed in this dissertation represent three promising directions for oxygen reduction catalyst, i.e metal oxides, metal-heterocycle-carbon support composite, heterocycle-carbon support composite, and more effect could be made for further improvement by applying different components Comparing these three catalysts, radar plots are presented as below with the six criteria described in section 1.3.3 89 Chapter Conclusion and Outlook Figure 36 Radar plot to summarize performances of catalyst in this dissertation by performances evaluating six elements Other than these six elements, toxicity is another feature to be noted Manganese is a heavy metal and on the other side a trace metal required by human body MnO2 has the highest content of Mn (63.2%), however the low dissolubility makes the catalyst a long lasting material with low loss MnPPYCNT has lower Mn content (1.5 ), therefore the toxicity is quite low (1.5‰), PEPU-SCNT has no heavy metal content and therefore has the lowest toxicity SCNT In future studies, more applications of microbial fuel cells could be explored with novel membranes and stable cathode catalysts 90 Chapter Conclusion and Outlook 6.2 OUTLOOK OF MFC DEVELOPMENT Since MFC has the potential to treat wastewater and meanwhile harvest energy from wastewater It is reasonable to consider further upscaling the reactors to harvest more energy However, currently there are still some bottlenecks, hindering the progress of scaling up.1 6.2.1 Bottlenecks of MFC scaling up 6.2.1.1 Design constraints as determined by wastewater application The first constraint could be summarized as footprint of MFCs and their energy efficiency It is evaluated from the parameter of organic removal efficiency, applied load and energy recovery rate Comparing MFC with the conventional methods, activated sludge and anaerobic digestion, each of them has its own advantages and constraints i.e For activated sludge, it has high organic removal efficiency, low applied load but high energy consumption, and producing too much sludge; for anaerobic digestion, it has moderate organic removal efficiency, high applied load, moderate energy recovery, and low sludge production; for microbial fuel cell, it has moderate organic removal efficiency, low applied load, high energy recovery up to 80% which is competitive, and low sludge production Each of these technologies has its own advantages In terms of wastewater treatment performance, MFC is not a perfect choice because of the low applied loaded,2 and modifications are necessary From the other aspect, the voltage and power generated by MFCs are really low (< V for a single cell), even when the reactors are scaled up with larger electrode size, the potential could not be increased for a single cell because of the standard potential limitation Apart from these, in real application domestic wastewater or municipal wastewater is used The low conductivity would highly reduce the proton/ion mobility, bringing high ohmic loss and reducing the energy recovery efficiency In addition, in an up-scaled reactor, the anodic acidification and cathodic alkalization also present another problem, increasing overpotential 91 Chapter Conclusion and Outlook and reducing the material stability Finally, the utilization of membrane or not is an issue, if present as a physical barrier, the cost would be increased; if eliminated, oxygen would be diffused to the anode, consumed by exoelectrogens at the anode, generating fewer free electrons, thus reducing energy recovery efficiency 6.2.1.2 Design constraints as determined by scaling up Different from lab-scale reactors, the problem of electron collection on both electrodes is amplified in an up-scaled reactor For example, a rectangular anode is increased by 100 in the length (l) from centimeter scale to meter scale, and the surface area for electron collection is increased by 100 accordingly Assuming the biofilm density is identical, the electron amount/current density (i) is increased by 100 According to the equation, η=iR =iρ× l/A (A is the cross-sectional area of electrode), the overpotential is increased by 10000 Normally carbon electrode is used because of its good biocompatibility and considerably low resistance, which is 1000 times that of Cu, yet could be neglected at lab-scale reactor However, when the reactor is scaled up, the resistance is highly increased, and cannot be neglected No matter how the reactor is scaled up, attention should also be placed on hydrodynamics and mechanics, i.e a well designed hydrodynamic system is necessary to obtain and maintain a good distribution of anolyte and catholyte to the cells from a shared manifold Whenever there is any clog or any other failure of one element, there could be change in pressure drop, a SCADA (supervisory control and data acquisition) system needs to be designed to integrate in the system for instant response 6.2.1.3 Cost and choice of materials The capital cost of anode, cathode, and membrane hinders the MFC development In the dissertation, the cheap alternatives of cathode catalyst are developed For the expensive anode (carbon paper/cloth), membrane, cheap alternatives are still needed In the past five years, the price of reverse osmosis membrane has been reduced by 50% It is reasonable to believe that along 92 Chapter Conclusion and Outlook with the MFC development, anode material and PEM would be much cheaper, allowing the future scaling up This requires the effort of scientists, engineers and microbiologists 6.2.2 Future trend of MFC development Because of the bottlenecks reviewed above, it is still not timely to scale up MFCs now Several directions are proposed Development of cheap alternatives for anode, separator and cathode Take the cathode side for example, based on the current progress for MFC research, in the near future oxygen will still be a target for bioelectricity generation due to its sustainability and easy availability, therefore alternative ORR catalysts shall be explored and utilized Similar to catalysts in this dissertation, they should be cost-effective, active and stable with easy preparation method, making commercialization a possibility It is suggested that carbon-based catalyst would be a good choice Novel configuration to amplify MFC potential by incorporating other energy Because of the footprint of MFC, the maximum voltage for a typical oxygen-based MFC could only reach V This is too low to be utilized in practical application Chemical energy could be incorporated into MFC system for voltage amplification.3 It is reasonable to design stacked reactors with continuous flow to make the voltage enhancement, or store the charge in super-capacitors to amplify the voltage during discharging Other than scaling up, scaling down is another choice, and miniature MFCs may be developed In these miniature MFCs, higher current densities are generated because of increased surface area and better adhesion of biofilm.4, Another advantage is that the miniature MFC has rapid response time6 and high throughput7 These advantages make miniature MFCs a good choice as screening system, e.g anode material optimization, microactivity test to select more active exoelectrogenic bacteria These sensors will be beneficial for MFC development Exploration of MFC anode or cathode for other applications In this way, the feature of spontaneous reactions could be made use of As mentioned 93 Chapter Conclusion and Outlook in chapter 1, the cathode reactions for MFCs were classified by their different functions including bioenergy generation, bioremediation and bioproduction Bioremediation function seems to be an economic and efficient technology for partial contaminant removal with simultaneous electricity generation Many other target molecules could be investigated to find the special strains for cathode bioremediation and incorporated into MFCs Bioproduction is also an interesting direction to explore substitute technology to conventional methods which were environmentally detrimental and require high capital and operational cost Each of these functions could be exploited and optimized independently or simultaneously (e.g bioenergy with bioremediation) to fully realize the advantages of the respective MFC designs Integrate MFCs in the wastewater and sludge treatment line MFCs not have the capacity to support the whole wastewater treatment system; however it could be designed as peripherals, to be incorporated into wastewater and sludge treatment line (1) Sensors and low power silicon circuitry could be powered to sense (temperature, pH, pO2, BOD, specific chemicals) and store or transmit time resolved data (2) Light emitting diodes or low power lamps to energize indicator lights or local sources of illumination (3) Small scale electric motors, including fans, pumps and refrigerators To summarize, MFCs present a bright future with these functions for energy and environmental applications, and there are tremendous scopes for further research 94 Chapter Conclusion and Outlook REFERENCES (1) Rabaey, K.; Angenent, L.; Schroder, U.; Keller, J Bioelectrochemical Systems: from extracellular electron transfer to biotechnological application IWA publishing, 2010 (2) Lefebvre, O.; Uzabiaga, A.; Chang, I S.; Kim, B H.; Ng, H Y Applied Microbiology and Biotechnology 2011, 89, 259-270 (3) Cusick, R D.; Kim, Y.; Logan, B E Science 2012, 335, 14741477 (4) Qian, F.; He, Z.; Thelen, M P.; Li, Y Bioresource Technology 2011, 102, 5836-5840 (5) Mink, J E.; Rojas, J P.; Logan, B E.; Hussain, M M Nano Letters 2012, 12, 791-795 (6) Wang, H.-Y.; Su, J.-Y Bioresource Technology 2013 (7) Hou, H.; Li, L.; Cho, Y.; de Figueiredo, P.; Han, A PLoS ONE 2009, 4, e6570 95 PUBLICATION & CONFERENCE Lu M., Guo L., Kharkwal S., Wu H., Ng H Y., Li S F Y., Manganese-PolypyrroleCarbon Nanotube, A New Oxygen Reduction Catalyst for Air-Cathode Microbial Fuel Cells, Journal of Power Sources, 2013, 221, 381-386 Lu M and Li S F Y., Cathode reactions and applications in microbial fuel cells: a review Critical Reviews in Environmental Science and Technology, 2012, 42, 1-22 Lu M., Kharkwal S., Ng H Y., Li S F Y., Carbon nanotube supported MnO₂ catalysts for oxygen reduction reaction and their applications in microbial fuel cells Biosensors & Bioelectronics 2011, 26, 4728-4732 Lu M., Guo L., Bay L G H., Wu H., Zhang Z., Li S F Y., Polyelectrolyte Functionalized-Single Wall Carbon Nanotubes as Effective Cathode Catalyst in Microbial Fuel Cells, submitted Kharkwal S., Lu M., Won S H., Li S F Y., Ng H Y., Novel MnO2 catalyst based microbial fuel cells type biosensors for online measurement of BOD in wastewater, submitted Lu M., Bay L G H., Li S F Y., Use of Nanoporous Hydrophilic Polymer Membranes for Energy recovery in Microbial Fuel Cells, the 7th Singapore International Chemistry Conference, 2012 Lu M Kharkwal S., Ng H Y., Li S F Y., Carbon Nanotube Supported MnO2 Catalysts for Oxygen Reduction Reaction and Their Applications in Microbial Fuel Cells, 3rd International Microbial Fuel Cell Conference, Leeuwarden, Netherland, 2011 96 ... dependent on the cell configurations (Figure 3) and electrolyte condition, while the selection of electron acceptor species and the catalysts for 10 Chapter Introduction reaction acceleration could... electrolyte in dual-chamber MFCs and always saturated, and it could also be used in single-chamber MFCs by incorporating membrane -cathode- assembly (MCA) Chapter Introduction configuration The main challenge... power production and total denitrification,10 and optimized the conditions soon after.11 Meanwhile simultaneous carbon removal, nitrification and denitrification 12 Chapter Introduction were also