6 Electricity Generation by Photosynthetic Biomass

Biofuel cells generate electricity through biological processes. Conventional microbial fuel cells (MFCs) operate by converting organic substrates, such as glucose, acetate, starch, and lactate, to electrical bioenergy through microbial oxidation processes [1-3]. A typical MFC consists of an anodic and a cathodic chamber with electrodes partitioned by a proton exchange membrane (PEM) or a cation exchange membrane. This membrane functions as an insulator for maintaining the redox potential and only allows specific ion exchange [4]. While MFCs use suspension cultivation of microorganisms in the anodic chamber [5], some MFCs attach microorganisms to the electrodes to form a biofilm [6]. As microbial oxidation consumes the supplied substrates, the anode surface generates electrons and conducts them to the cathode through an external circuit. The resulting cations pass through the membrane to the cathode in the electrolyte. However, the following three factors limit MFC performance: (1) electron activation on the anode and cathode surfaces, (2) electron transfer from microbial cells to the anode, and (3) internal resistances of the circuit and anions passing through the membrane. Researchers have developed electrode modification, mediator addition, and membrane-free designs to address these issues and improve MFC performance [5]. Different electrode materials produce different activation polarization losses; for example, the noble metal platinum (Pt) offers superior catalytic activity. But, graphite, graphite felt, Pt-coated graphite, and other metal-coated materials are employed as cost-effective electrodes [5,7,8]. Since the cell surfaces of microorganisms are not electrically conductive, the electrons inside the cells cannot directly transfer to the surrounding electrolyte [9,10]. For this reason, previous designs adopt several kinds of toxic and unstable electro-chemicals as electrochemical mediators, or electron shuttles: neutral red (NR), methylene blue (MB), thionint, and phenolic compounds [4,10]. However, these active chemicals are expensive and toxic, rendering them unsuitable for long-term operation [4]. Exchange membranes are the most expensive components of typical MFC [10]. These membranes insulate and separate different kinds and/or concentrations of electrolytes into two chambers and restrict specific ion exchange. Once the permeability of the exchange membrane becomes poor, the resulting increase in ohmic resistance decreases MFC performance [11]. A membrane-free system may address these concerns. In a membrane-free system, the diffusion gradient of the dissolved reactants between the anode and cathode maintains the electrochemical