۱- Introduction

۲- Electrode materials

۳- Mechanisms of electron transport

۴- Field standardisation and comparison techniques

۵- Conclusions

References

Abstract

Research into alternative renewable energy generation is a priority, due to the ever-increasing concern of climate change. Microbial fuel cells (MFCs) are one potential avenue to be explored, as a partial solution towards combating the over-reliance on fossil fuel based electricity. Limitations have slowed the advancement of MFC development, including low power generation, expensive electrode materials and the inability to scale up MFCs to industrially relevant capacities. However, utilisation of new advanced electrode-materials (i.e. 2D nanomaterials), has promise to advance the field of electromicrobiology. New electrode materials coupled with a more thorough understanding of the mechanisms in which electrogenic bacteria partake in electron transfer could dramatically increase power outputs, potentially reaching the upper extremities of theoretical limits. Continued research into both the electrochemistry and microbiology is of paramount importance in order to achieve industrial-scale development of MFCs. This review gives an overview of the current field and knowledge in regards to MFCs and discusses the known mechanisms underpinning MFC technology, which allows bacteria to facilitate in electron transfer processes. This review focusses specifically on enhancing the performance of MFCs, with the key intrinsic factor currently limiting power output from MFCs being the rate of electron transfer to/from the anode; the use of advanced carbon-based materials as electrode surfaces is discussed.

Introduction

Energy generation, storage and consumption are topics that are increasingly prevalent within modern research fields and are of global interest and importance [1,2]. Research into alternative renewable energy generation sources are increasing exponentially, with vast research showing promising results, in an abundance of areas including: solar [3], wind [4], tidal [5], geothermal [6] and biomass energy generation (Fig. 1) [7,8]. Currently no individual renewable energy source has the ability to compete with and replace the conventional fossil-fuel based energy generation approach, however, combining renewable energy sources such as, solar-wind hybrids and/or solar-hydrogen fuel cells may be alternative routes to be explored [9,10]. One potential alternative energy source is the use of microbial fuel cells (MFCs). MFCs follow a similar concept to traditional fuel cells (Fig. 2). However, MFCs utilise the bio-catalytic capabilities of viable microorganisms and are capable of using a range of organic fuel sources, by converting the energy stored in the chemical bonds, to generate an electrical current instead of relying for example, on the use of metal catalysts [1]. Microorganisms, such as bacteria, can generate electricity by utilising organic matter and biodegradable substrates such as wastewater, whilst also accomplishing biodegradation/treatment of biodegradable products, such as municipal wastewater [1,11]. Table 1 provides an overview of the current literature of MFCs. Clearly, significant attention has been given to MFCs cells due to their ambient operating conditions (e.g. utilisation at low temperatures) and a variety of biodegradable substrates as fuel. This review aims to highlight the current understanding of MFCs, whilst giving a thorough overview of the field. Particular emphasis is placed upon the fundamentals of MFC technologies, electrode materials, mechanism of electron transport and field standardisation.