Supercapacitors in bioelectrochemical systems

Bioelectrochemical systems (BESs) are devices composed of two electrodes, the anode and the cathode, a separator and the electrolyte [1, 2]. At the anode, the oxidation reaction occurs while at the cathode reduction reaction takes place. Different from classical abiotic electrochemical systems such as fuel cells, batteries, and supercapacitors, BESs have biotic components and features in at least one of the two electrodes [1, 2]. It may happen that both of the electrodes experience and interact with the biotic matter. The biotic components are generally microorganisms and enzymes that categorize fuel cells called microbial fuel cells (MFCs) [1, 2] and enzymatic fuel cells (EFCs) [3]. The latter are also known as a general term, that is, biofuel cells (BFCs). BESs dealing with microorganisms can also be exploited for the following: (i) generating hydrogen at the cathode under an external voltage supply in a microbial electrolysis cell (MEC); (ii) reducing carbon dioxide to methane, acetate, or other valuable compounds in a microbial electrosynthesis cell (MEC); (iii) desalinate water while producing electricity (MDC) [2]. In acidic or alkaline fuel cells, extreme pHs are required to accelerate the reactions and increase the efficiency of the overall system. Moreover, often the temperature is increased for increasing the reaction kinetics [4, 5]. H⁺ and OH⁻ are reactants within the redox process and therefore high concentration of those reactants can be achieved at low and high pHs respectively. In BESs in which a biological component is present, neutralor circumneutral conditions are required to operate in order for enzymes or bacteria to work efficiently. BESs can work in ubiquitous media containing organics that bacteria or enzymes can use as fuel for the electrochemical device to produce useful electricity [2, 3]. Because of presence of biological and enzymatic life, BESs are forced to operate in conditions closer to room temperature in order to preserve their activities [2, 3].