Biofuel Cells
Description
This book covers the most recent developments and offers a detailed overview of fundamentals, principles, mechanisms, properties, optimizing parameters, analytical characterization tools, various types of biofuel cells, all-category of materials, catalysts, engineering architectures, implantable biofuel cells, applications and novel innovations and challenges in this sector. This book is a reference guide for anyone working in the areas of energy and the environment.
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Persons
Inamuddin, PhD, is an assistant professor at the Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India. He has extensive research experience in analytical chemistry, materials chemistry, electrochemistry, renewable energy, and environmental science. He has worked on different research projects funded by various government agencies and universities and is the recipient of multiple awards, including the Fast Track Young Scientist Award and the Young Researcher of the Year Award for 2020, from Aligarh Muslim University. He has published almost 200 research articles in various international scientific journals, 18 book chapters, and 120 edited books with multiple well-known publishers.
Mohd Imran Ahamed, PhD, is a research associate in the Department of Chemistry, Aligarh Muslim University, Aligarh, India. He has published several research and review articles in various international scientific journals and has co-edited multiple books. His research work includes ion-exchange chromatography, wastewater treatment, and analysis, bending actuator and electrospinning.
Rajender Boddula, PhD, is currently working for the Chinese Academy of Sciences President's International Fellowship Initiative (CAS-PIFI) at the National Center for Nanoscience and Technology (NCNST, Beijing). His academic honors include multiple fellowships and scholarships, and he has published many scientific articles in international peer-reviewed journals. He is also serving as an editorial board member and a referee for several reputed international peer-reviewed journals. He has published edited books with numerous publishers and has authored over 20 book chapters.
Mashallah Rezakazemi, PhD, received his doctorate from the University of Tehran (UT) in 2015. In his first appointment, he served as associate professor in the Faculty of Chemical and Materials Engineering at Shahrood University of Technology. He has co-authored in more than 140 highly cited journal publications, conference articles and book chapters. He has received numerous major awards and grants from various funding agencies in recognition of his research. Notable among these are Khwarizmi Youth Award from the Iranian Research Organization for Science and Technology (IROST), and the Outstanding Young Researcher Award in Chemical Engineering from the Academy of Sciences of Iran. He was named a top 1% most Highly Cited Researcher by Web of Science (ESI).
Content
Preface xvii
1 Bioelectrocatalysis for Biofuel Cells 1
Casanova-Moreno Jannu, Arjona Noé and Cercado Bibiana
1.1 Introduction: Generalities of the Bioelectrocatalysis 2
1.2 Reactions of Interest in Bioelectrocatalysis 3
1.2.1 Enzyme Catalyzed Reactions 3
1.2.2 Reactions Catalyzed by Microorganisms 8
1.3 Immobilization of Biocatalyst 9
1.3.1 Immobilization of Enzymes on Electrodes 9
1.3.2 Preparation of Microbial Bioelectrodes 15
1.4 Supports for Immobilization of Enzymes and Microorganisms for Biofuel Cells 17
1.4.1 Buckypaper Bioelectrodes for BFCs 20
1.4.2 Carbon Paper Bioelectrodes for BFCs 21
1.4.3 Nitrogen-Doped Carbonaceous Materials as Bioelectrodes for BFCs 22
1.4.4 Metal-Organic Framework (MOF)-Based Carbonaceous Materials as Bioelectrodes for BFCs 23
1.4.5 Flexible Bioelectrodes for Flexible BFCs 24
1.5 Electron Transfer Phenomena 25
1.5.1 Enzyme-Electrode Electron Transfer 25
1.5.2 Microorganism-Electrode Electron Transfer 31
1.6 Bioelectrocatalysis Control 34
1.6.1 Control of Enzymatic Bioelectrocatalysis 34
1.6.2 Microbiological Catalysis Control 35
1.7 Recent Applications of Bioelectrocatalysis 36
1.7.1 Biosensors 36
1.7.2 Microbial Catalyzed CO2 Reduction 37
References 39
2 Novel Innovations in Biofuel Cells 53
Muhammet Samet Kilic and Seyda Korkut
2.1 Introduction to Biological Fuel Cells 53
2.1.1 Implantable BFCs 55
2.1.2 Wearable BFCs 59
2.2 Conclusions and Future Perspectives 63
Acknowledgment 64
References 64
3 Implantable Biofuel Cells for Biomedical Applications 69
Arushi Chauhan and Pramod Avti
3.1 Introduction 70
3.2 Biofuel Cells 72
3.2.1 Microbial Biofuel Cells 72
3.2.1.1 Design and Configuration 73
3.3 Enzymatic Biofuel Cells 75
3.3.1 Design and Configurations 75
3.3.2 Factors Affecting 77
3.4 Mechanism of Electron Transfer 80
3.5 Energy Sources in the Human Body 81
3.6 Biomedical Applications 83
3.6.1 Glucose-Based Biofuels Cells 84
3.6.2 Pacemakers 85
3.6.3 Implanted Brain-Machine Interface 86
3.6.4 Biomarkers 87
3.7 Limitations 87
3.8 Conclusion and Future Perspectives 88
References 88
Abbreviations 95
4 Enzymatic Biofuel Cells 97
Rabisa Zia, Ayesha Taj, Sumaira Younis, Haq Nawaz Bhatti, Waheed S. Khan and Sadia Z. Bajwa
4.1 Introduction 98
4.2 Enzyme Used in EBFCs 99
4.3 Enzyme Immobilization Materials 103
4.3.1 Physical Adsorption Onto a Solid Surface 105
4.3.2 Entrapment in a Matrix 106
4.3.3 Sol-Gel Entrapment 106
4.3.4 Nanomaterials as Matrices for Enzyme Immobilization 107
4.3.5 Covalent Bonding 109
4.3.6 Cross-Linking With Bifunctional or Multifunctional Reagents 110
4.4 Applications of EBFCs 111
4.4.1 Self-Powered Biosensors 111
4.4.2 EBFCs Into Implantable Bioelectronics 111
4.4.3 EBFCs Powering Portable Devices 112
4.5 Challenges 114
4.6 Conclusion 116
References 116
5 Introduction to Microbial Fuel Cell (MFC): Waste Matter to Electricity 123
Rustiana Yuliasni, Abudukeremu Kadier, Nanik Indah Setianingsih, Junying Wang, Nani Harihastuti and Peng-Cheng Ma
5.1 Introduction 124
5.2 Operating Principles of MFC 125
5.3 Main Components and Materials of MFCs 126
5.3.1 Anode Materials 126
5.3.2 Cathode Materials 134
5.3.3 Substrates or Fed-Stocks 135
5.3.4 MFC Cell Configurations 135
5.4 Current and Prospective Applications of MFC Technology 136
5.5 Conclusion and Future Prospects 138
Acknowledgement 138
References 138
6 Flexible Biofuel Cells: An Overview 145
Gayatri Konwar and Debajyoti Mahanta
6.1 Introduction 145
6.1.1 Working Principle of Fuel Cell 146
6.1.2 Types of Fuel Cells 148
6.2 Biofuel Cells (BFCs) 149
6.2.1 Working Principle 149
6.2.1.1 Microbial Fuel Cell 150
6.2.1.2 Photomicrobial Fuel Cell 151
6.2.1.3 Enzymatic Fuel Cell 151
6.2.2 Applications of Biofuel Cells 152
6.3 Needs for Flexible Biofuel Cell 153
6.3.1 Fuel Diversity 153
6.3.2 Materials for Flexible Biofuel Cells 154
6.3.3 Fabrication of Bioelectrodes 156
6.3.4 Recent Advances and New Progress for the Development of Flexible Biofuel Cell 156
6.3.4.1 Carbon-Based Electrode Materials for Flexible Biofuel Cells 157
6.3.4.2 Textile and Polymer-Based Electrode Materials for Flexible Biofuel Cells 160
6.3.4.3 Metal-Based Electrode Materials 162
6.3.5 Challenges Faced by Flexible Biofuel Cell 162
6.4 Conclusion 164
References 164
7 Carbon Nanomaterials for Biofuel Cells 171
Udaya Bhat K. and Devadas Bhat P.
List of Abbreviations 172
7.1 Introduction 173
7.2 Types of Biofuel Cells 174
7.2.1 Enzyme-Based Biofuel Cell (EBFC) 175
7.2.2 Microbial-Based Biofuel Cells (MBFCs) 176
7.3 Carbon-Based Materials for Biofuel Cells 176
7.3.1 Cellulose-Based Biomass Fuel Cells 176
7.3.2 Starch and Glucose-Based Fuel Cells 177
7.3.3 Carbon Nanoparticles (NPs) 178
7.3.4 Graphite 179
7.3.5 Nanographene 179
7.3.5.1 N-Doped Graphene 182
7.3.6 Carbon Nanotubes 182
7.3.6.1 Buckypapers 187
7.3.6.2 Hydrogenases 188
7.3.6.3 N-Doped CNTs 189
7.3.6.4 Biphenylated CNTs 189
7.3.7 Nanohorns 189
7.3.8 Nanorods 190
7.3.9 Carbon Nanofibers 191
7.3.10 Nanoballs 191
7.3.11 Nanosheets 192
7.3.12 Reticulated Vitreous Carbon (RVC) 192
7.3.13 Porous Carbon 192
7.4 Applications of Biofuel Cells Using Carbon-Based Nanomaterials 193
7.4.1 Living Batteries/Implantable Fuel Cells 193
7.4.1.1 Animal In Vivo Implantation 194
7.4.1.2 Energy Extraction From Body Fluids 195
7.4.2 Energy Extraction From Fruits 197
7.5 Conclusion 197
References 198
8 Glucose Biofuel Cells 219
Srijita Basumallick
8.1 Introduction 219
8.2 Merits of BFC Over FC 220
8.3 Glucose Oxidize (GOs) as Enzyme Catalyst in Glucose Biofuel Cells 221
8.4 General Experimental Technique for Fabrication of Enzyme GOs Immobilized Electrodes for Glucose Oxidation 222
8.5 General Method of Characterization of Fabricated Enzyme Immobilized Working Electrode 223
8.6 Determination of Electron Transfer Rate Constant (ks) 224
8.7 Denaturation of Enzymes 225
8.8 Conclusions 225
Acknowledgments 226
References 226
9 Photochemical Biofuel Cells 229
Mohd Nur Ikhmal Salehmin, Rosmahani Mohd Shah, Mohamad Azuwa Mohamed, Ibdal Satar and Siti Mariam Daud
9.1 Introduction 230
9.1.1 Various Configuration of PBEC-FC 231
9.2 Photosynthetic Biofuel Cell (PS-BFC) 233
9.2.1 Various Configurations of PS-BFC 234
9.3 Photovoltaic-Biofuel Cell (PV-BFC) 238
9.4 Photoelectrode Integrated-Biofuel Cell (PE-BFC) 240
9.4.1 The Basic Mechanism of Photoelectrochemical (PEC) Reaction 241
9.4.2 Photoelectrode-Integrated BFC 242
9.4.3 Various Configuration of PE-BFC 243
9.4.4 Materials Used in PE-BFC 245
9.5 Potential Fuels Generation and Their Performance From PEC-BFC 247
9.5.1 Hydrogen Generation 247
9.5.2 Contaminants Removal and Waste Remediation 249
9.5.3 Sustainable Power Generation 251
9.6 Conclusion 252
References 253
10 Engineering Architectures for Biofuel Cells 261
Udaya Bhat K. and Devadas Bhat P.
Abbreviations 261
10.1 Introduction 263
10.1.1 Biofuel Cell 263
10.1.2 General Configuration of a Biofuel Cell 263
10.2 Role as Miniaturized Ones 264
10.3 Attractiveness 266
10.3.1 Biological Sensors 266
10.3.2 Implantable Medical Devices 267
10.3.2.1 Invertebrates 268
10.3.2.2 Vertebrates 269
10.3.3 Electronics 269
10.3.4 Building Materials 270
10.4 Architecture 270
10.4.1 Fabrication and Design 270
10.4.1.1 Modeling 271
10.4.1.2 Sol-Gel Encapsulation 272
10.4.1.3 3D Electrode Architecture 272
10.4.1.4 Multi-Enzyme Systems (Enzyme Cascades) 273
10.4.1.5 Linear Cascades 273
10.4.1.6 Cyclic Cascades 274
10.4.1.7 Parallel Cascades 274
10.4.1.8 Artificial Neural Networks (ANNs) 274
10.4.2 Single Compartment Layout 275
10.4.3 Two-Compartment Layout 275
10.4.4 Mechanisms 275
10.4.4.1 Direct Electron Transfer 275
10.4.4.2 Mediated Electron Transfer 276
10.4.5 Materials 277
10.4.5.1 Carbon Nanomaterials 277
10.4.5.2 H2/O2 Biofuel Cells 277
10.4.5.3 Hydrogenases 278
10.4.5.4 Fungal Cellulases 279
10.4.6 Characterization 279
10.4.6.1 Scanning Electron Microscopy (SEM) 279
10.4.6.2 Atomic Force Microscopy (AFM) 279
10.4.6.3 X-Ray Photoelectron Spectroscopy (XPS) 280
10.4.6.4 Fluorescence Microscopy 280
10.4.7 Metagenomic Techniques 280
10.4.7.1 Pre-Treatment of Environmental Samples 281
10.4.7.2 Nucleic Acid Extraction 281
10.4.8 Integrated Devices 282
10.5 Issues and Perspectives 282
10.6 Future Challenges in the Architectural Engineering 283
10.7 Conclusions 283
References 284
11 Biofuel Cells for Commercial Applications 299
Mohan Kumar Anand Raj, Rajasekar Rathanasamy, Moganapriya Chinnasamy and Sathish Kumar Palaniappan
Abbreviations 299
11.1 Introduction 300
11.1.1 History of Biofuel Cell 300
11.2 Classification of Electrochemical Devices Based on Fuel Confinement 303
11.2.1 Process of Electron Shift From Response Site to Electrode 303
11.2.2 Bioelectrochemical Cells Including an Entire Organism 303
11.2.3 Entire Organism Product Biofuel Cells Producing Hydrogen Gas 304
11.2.4 Entire Organism Non-Diffusive Biofuel Cells 305
11.3 Application of Biofuel Cells 307
11.3.1 Micro- and Nanotechnology 308
11.3.2 Self-Powered Biofuel Sensor 309
11.3.3 Switchable Biofuel Cells and Logic Gates 310
11.3.4 Microbial Energy Production 310
11.3.5 Transport and Energy Generation 311
11.3.6 Infixable Power Sources 312
11.3.7 Aqua Treatment 312
11.3.8 Robots 312
11.4 Conclusion 312
References 313
12 Development of Suitable Cathode Catalyst for Biofuel Cells 317
Mehak Munjal, Deepak Kumar Yadav, Raj Kishore Sharma and Gurmeet Singh
12.1 Introduction 317
12.2 Kinetics and Mechanism of Oxygen Reduction Reaction 321
12.3 Techniques for Evaluating ORR Catalyst 322
12.4 Cathode Catalyst in BFCs 326
12.5 Chemical Catalyst 327
12.5.1 Metals-Based Catalyst 327
12.5.1.1 Metals and Alloys 327
12.5.1.2 Metal Oxide 328
12.5.2 Carbon Materials 331
12.6 Microbial Catalyst 332
12.7 Enzymatic Catalyst for Biofuel Cell 333
12.8 Conclusion 334
Acknowledgements 335
References 335
13 Biofuel Cells for Water Desalination 345
Somakraj Banerjee, Ranjana Das and Chiranjib Bhattacharjee
13.1 Introduction 345
13.2 Biofuel Cell 347
13.2.1 Basic Mechanism 347
13.2.2 Types of Biofuel Cells 348
13.2.2.1 Enzymatic Fuel Cell 349
13.2.2.2 Microbial Fuel Cell 349
13.3 Biofuel Cells for Desalination: Microbial Desalination Cell 350
13.3.1 Working Mechanism 351
13.3.2 Microbial Desalination Cell Configurations 353
13.3.2.1 Air Cathode MDC 353
13.3.2.2 Biocathode MDC 354
13.3.2.3 Stacked MDC (sMDC) 355
13.3.2.4 Recirculation MDC (rMDC) 357
13.3.2.5 Microbial Electrolysis Desalination and Chemical Production Cell (MEDCC) 358
13.3.2.6 Capacitive MDC (cMDC) 359
13.3.2.7 Upflow MDC (UMDC) 360
13.3.2.8 Osmotic MDC (OMDC) 361
13.3.2.9 Bipolar Membrane Microbial Desalination Cell 362
13.3.2.10 Decoupled MDC 363
13.3.2.11 Separator Coupled Stacked Circulation MDC (c-SMDC-S) 364
13.3.2.12 Ion-Exchange Resin Coupled Microbial Desalination Cell 365
13.4 Factors Affecting the Performance and Efficiency of Desalination Cells 366
13.4.1 Effect of External Resistance 366
13.4.2 Effect of Internal Resistance 367
13.4.3 Effect of pH 367
13.4.4 Effect of Microorganisms 368
13.4.5 Effect of Operating Conditions 369
13.4.6 Effect of Membrane Scaling and Fouling 370
13.4.7 Effect of Desalinated Water Contamination 370
13.5 Current Challenges and Further Prospects 370
Acknowledgment 371
References 372
14 Conventional Fuel Cells vs Biofuel Cells 377
Naila Yamin, Wajeeha Khalid, Muhammad Altaf, Raja Shahid Ashraf, Munazza Shahid and Amna Zulfiqar
14.1 Bioelectrochemical Cell 378
14.2 Types 378
14.2.1 Fuel Cells 378
14.2.1.1 Conventional Fuel Cell (FC) 378
14.2.1.2 History 378
14.2.1.3 Principle of FC 380
14.2.1.4 Construction/Designs 380
14.2.1.5 Stacking of Fuel Cell 383
14.2.1.6 Importance of Conventional FC 384
14.2.2 Types of FC 384
14.2.2.1 Molten Carbonate Fuel Cell (MCFC) 385
14.2.2.2 Proton Exchange Membrane Fuel Cell (PEMFC) 386
14.2.2.3 Direct Methanol Fuel Cell (DMFC) 388
14.2.2.4 Solid Oxide Fuel Cell (SOFC) 389
14.2.2.5 Alkaline FC (AFC) 390
14.2.2.6 Phosphoric Acid Fuel Cell (PAFC) 391
14.2.3 Advantages of Fuel Cells 394
14.2.3.1 Efficiency 394
14.2.3.2 Low Emissions 394
14.2.3.3 Noiseless 394
14.2.4 Applications 394
14.3 Biofuel Cells 395
14.3.1 Introduction 395
14.3.2 Categories of Biofuel 395
14.3.2.1 First-Generation Biofuel 395
14.3.2.2 Second-Generation Biofuel 399
14.3.2.3 Third-Generation Biofuel 399
14.3.2.4 Fourth-Generation Biofuel 399
14.3.3 Advantages of Biofuels 399
14.4 Types of Biofuel Cells 399
14.4.1 Microbial Fuel Cell 399
14.4.1.1 Basic Principles of MFC 401
14.4.1.2 Types of MFCs 403
14.4.1.3 Mechanism of Electron Transfer 404
14.4.1.4 Uses of MFCs 405
14.4.1.5 Advantages of MFCs 406
14.4.1.6 Disadvantage of MFCs 407
14.4.2 Enzymatic Biofuel Cells (EBCs) 407
14.4.2.1 Principle/Mechanism 407
14.4.2.2 Working of EBCs 407
14.4.2.3 Immobilization of an Enzyme 408
14.4.3 Glucose Biofuel Cells (GBFCs) 409
14.4.4 Photochemical Biofuel Cell 411
14.4.5 Flexible or Stretchable Biofuel Cell 412
14.5 Conclusion 413
References 413
15 State-of-the-Art and Prospective in Biofuel Cells: A Roadmap Towards Sustainability 423
Biswajit Debnath, Moumita Sardar, Khushbu K. Birawat, Indrashis Saha and Ankita Das
15.1 Introduction 423
15.2 Membrane-Based and Membrane-Less Biofuel Cells 425
15.3 Enzymatic Biofuel Cells 429
15.4 Wearable Biofuel Cells 432
15.5 Fuels for Biofuel Cells 434
15.6 Roadmap to Sustainability 434
15.7 Conclusion and Future Direction 438
Acknowledgement 439
References 439
16 Anodes for Biofuel Cells 449
Naveen Patel, Dibyajyoti Mukherjee, Ishu Vansal, Rama Pati Mishra and Vinod Kumar Chaudhary
16.1 Introduction 450
16.2 Anode Material Properties 451
16.3 Anode 452
16.3.1 Non-Carbon Anode Materials 452
16.3.2 Carbon Anode Materials 453
16.4 Anode Modification 453
16.4.1 Anode Modification With Carbon Nanotube (CNT) 453
16.4.2 Graphite-Based Material for Anode Electrode Modification 454
16.4.3 Anode Modification With Nanocomposite of Metal Oxides 454
16.4.4 Anode Modification With Conducting Polymer 455
16.4.5 Chemical and Electrochemical Anode Modifications 456
16.5 Challenge and Future Perspectives 456
16.6 Conclusion 457
Acknowledgements 457
References 457
17 Applications of Biofuel Cells 465
Joel Joseph, Muthamilselvi Ponnuchamy, Ashish Kapoor and Prabhakar Sivaraman
17.1 Introduction 465
17.2 Fuel Cell 467
17.3 Biofuel Cells 468
17.3.1 Microbial Biofuel Cell 469
17.3.1.1 At Anode Chamber 470
17.3.1.2 At Cathode Chamber 471
17.3.2 Enzymatic Biofuel Cell 471
17.3.3 Mammalian Biofuel Cell 472
17.4 Implantable Devices Powered by Using Biofuel Cell 473
17.4.1 Implantable Biofuel Cell for Pacemakers or Artificial Urinary Sphincter 473
17.4.2 Implantable Medical Devices Powered by Mammalian Biofuel Cells 474
17.4.3 Medical Devices Using PEM Fuel Cell 475
17.4.4 Implantable Brain Machine Interface Using Glucose Fuel Cell 475
17.5 Single Compartment EBFCs 476
17.6 Extracting Energy from Human Perspiration Through Epidermal Biofuel Cell 476
17.7 Mammalian Body Fluid as an Energy Source 477
17.8 Implantation of Enzymatic Biofuel Cell in Living Lobsters 477
17.9 Biofuel Cell Implanted in Snail 477
17.10 Application of Biofuel Cell 478
17.11 Conclusion 479
References 479
Index 483
1
Bioelectrocatalysis for Biofuel Cells
Casanova-Moreno Jannu1, Arjona Noé2 and Cercado Bibiana2*
1CONACYT-Centro de Investigación y Desarrollo Tecnológico en Electroquímica S.C., Pedro Escobedo, Mexico
2Centro de Investigación y Desarrollo Tecnológico en Electroquímica S.C., Pedro Escobedo, Mexico
Abstract
Bioelectrocatalysis is the acceleration of reactions that occur on an electrode via a biological component, be it an enzyme, a cellular organelle or a whole cell. Enzymatic reactions on the anode are mainly the oxidation of saccharides and alcohols, while the oxidative metabolism of bacteria is exploited for removal of short-chain organic acids. In the cathode, the main enzyme-controlled reaction is the reduction of dioxygen, while microbial catalysis tends to obtain hydrogen and methane-like energy vectors. One of the challenges in bioelectrocatalysis is the preparation of electrodes. The techniques for immobilization of enzymes and organelles include the use of polymers and composites and the naturallyoccurring adhesion of bacteria to the solid material forming a biofilm on the electrode. Given the importance of the support material, numerous efforts have been directed to modifying materials that improve the adhesion of enzymes and bacteria, as well as electron transfer. The control of electron transfer is performed by the modification of the pH in the medium, the use of mediators, and the application of a potential difference in an electrolytic cell. The applications of electrochemical cells in bioelectrocatalytic operation include energy conversion, enzymatic sensors and gaseous fuel production in microbial bioelectrochemical systems.
Keywords: Bioelectrocatalysis, biofuel cell, enzymatic electrocatalysis, microbial electrocatalysis
1.1 Introduction: Generalities of the Bioelectrocatalysis
Electrochemical catalysis or electrocatalysis is used to describe charge transfer-based reactions occurring on an electrode. This term was employed for first time in 1936 by Santos and Schimickler [1]. The electrocatalysis is focused on increasing the reaction rate of an electrochemical process (oxidation/reduction), involving a dissociative chemisorption or a reaction step on an electrode surface and thus, the electrocatalysis depends on the ad/desorption of reactants and products, and on the formation of an electrochemical double layer. An electrocatalytic cycle is composed of three stages: 1) mass transport of electroactive species from bulk to the interface, 2) the electrocatalytic reaction, and 3) transport of products to bulk. Additionally, stage 2 involves the adsorption of reactants, the electron transfer, and the desorption of products. Consequently, the art of electrocatalysis consists of identifying the barriers of an electrochemical reaction to adjust the properties of the electrochemical interface (electrode and/or solution) with the aim of remove or at least, decrease the energy barriers (activation energies).
The practical role of electrocatalysis implies the science of designing the electrochemical interface properties. Hence, the morphological and electronic properties of the electrocatalyst, together with the electrolyte characteristics, become important to analyze. On the other hand, the activation energy of electrocatalytic reactions also depends on the electrode potential, thus enabling a fine control of the reactions. Consequently, electrocatalysis focuses on minimizing electrode overpotential, and increasing the reaction rate via the decrease of activation energies for a specific reaction.
The relation between electrocatalysis and microorganisms was presented in 1910 when a yeast was used as catalyst in a fuel cell. In the 60s microbial fuel cells (MFC) were used to exploit human waste from spacecraft, and only about 40 years later, the MFCs gained worldwide attention due to the use of industrial wastewater as fuel [2]. Entire microbial cells, organelles and biological molecules have been utilized as catalyst in biofuel cells. The molecules for energy conversion in living eukaryotic cells are utilized as biocatalyst and as model reactions. The reactions are complex and involve the action of nucleotides nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). The nucleotides in the cell are reduced to NADH and FADH2 by protons coming from a chain of oxidation reactions belonging to the microbial catabolic metabolism. The cyclic oxidation and reduction of nucleotides enables the transport of charge in the mitochondria and thus in the microbial cell. The energy pathways in prokaryote cells involve a chain of transmembrane enzymatic proteins. The c-type cytochromes in the outer cell membrane enable direct contact cell-electrode and research in molecular biology shows that cytochromes are responsible for extracellular electron transfer (EET).
The reactions occurring in the living cells involve different catalytic proteins or enzymes; thus charge transfer through biological molecules has required many years of investigation. Enzymes can act in the electrolyte, or be immobilized at the electrode, and electron transfer achieved via either mediated or direct form. The contact of the enzyme with the substrate is achieved via physical or covalent adsorption. The type of contact is a function of the location of the active site in the enzyme, which can be in the periphery or in the core of the catalytic protein. The electrode material for immobilization of the bioelectrocatalyst is one of the main issues. Thus, the intrinsic properties of the electrode such as porosity and conductivity must be improved via doping, template construction or addition of nanomaterials. Another concern in bioelectrocatalysis is the lifetime of the enzymatic electrodes, which are very sensitive to environmental conditions. Plenty of strategies using polymers have been proposed, including encapsulation, cross-linking, anchoring, and self-assembly with the aim of improving the electron transfer between the enzyme and the electrode. This process can be explained by different mechanisms like percolation though immobile redox centers, collision of mobile centers, and conduction through a conjugated backbone. The direct transfer occurs via electron tunneling from the active site in the enzyme and the electrode.
In the following sections, reactions of general interest in cells catalyzed by enzymes and microorganisms are described in the first instance. The next section focuses on advances in electrode material development, as well as enzyme immobilization and bacterial biofilm preparation strategies. Finally, in the last sections the phenomena that occur in the transfer of electrons at the enzymatic and bacterial level are described, and two cases of application of bioelectrocatalysis are presented.
1.2 Reactions of Interest in Bioelectrocatalysis
1.2.1 Enzyme Catalyzed Reactions
Oxidoreductases (EC group 1┼) are enzymes that are capable of catalyzing reactions in which electron transfer is involved and have been used as fuel cell components since the early 1960s. In 1962, Davis and Yarborough reported an increase in the potential of a cell in which glucose oxidase was present in one of the electrode compartments (the other being a Pt/O2 one) [3]. Two years later, Yahiro et al. reported the first polarization curves using glucose oxidase (GOx), D-amino acid oxidase and yeast alcohol dehydrogenase in bioanodes that they coupled with Pt cathodes for O2 reduction [4]. Since then, most of the enzymatic biofuel research has been centered in the oxidation of glucose and the reduction of oxygen. This has been driven by the abundance of both substances in our biosphere; while oxygen is abundant in the atmosphere, glucose is used as a source of energy by almost all living beings. Because of their predominance in the literature, this section will focus on the mechanistic description of enzymatic oxidation of glucose and reduction of oxygen by glucose oxidase and laccase, respectively. Other enzymes, like glucose dehydrogenase [5, 6] and bilirubin oxidase [5] can perform similar reactions through different mechanisms and can be useful in some situations. Furthermore, a variety of other fuels (e.g. carbohydrates [7, 8], alcohols [9, 10], lipids [11] and organic acids [12] and oxidants (mainly H2O2 [13]) have been employed in enzymatic biofuel cells. However, it is out of the scope of this work to review them all in detail. Rather, it is expected that the information presented here will allow the readers to perform similar literature search for their particular enzyme of interest.
Glucose can be oxidized by a variety of enzymes including glucose oxidase and glucose dehydrogenase. Of these, glucose oxidase has been the one massively preferred, due to its high specificity and good turnover and stability [14, 15]. Glucose oxidase (EC 1.1.3.4) is a dimeric flavoprotein that oxidizes ß-D-glucose into D-glucono-d-lactone while reducing molecular dioxygen (from here on simply referred to as oxygen) to hydrogen peroxide. In each subunit, an active site is deeply buried in a funnel-shaped pocket that contains a non-covalently bound flavin adenine dinucleotide (FAD) cofactor (Figure 1.1a). The N5 atom of this molecule is situated 13-18 Å from the surface and acts as the first electron acceptor in a so called "ping-pong" mechanism [16]. This first half reaction (enzyme reduction) takes place through simultaneous donation of a proton and a hydride from the glucose to the His516 residue and FAD, respectively. Although literature...
