Microbial fuel cells are devices that employ bacterial or other microbial cells to generate electricity (Singh, Pratap, Baranwal, Kumar and Chaudhary, 2010). Cells generally carry out electron transfer and oxidation-reduction reactions during normal metabolic activities. Electrons generated during these reactions can be made to flow through an external circuit to generate current. Fig 1. Depicts a typical MFC. The anodic compartment consists of microbial culture in glucose or other medium (Singh et al, 2010). Wastewater can also be used as a substrate source for microbes in a MFC intended for effluent treatment application. The microbial cells breakdown complex substrates into simpler molecules such as CO2 through bio-metabolic pathways. The anode, which is usually carbon rod, accepts electrons generated in the reaction. Through the external circuit electrons flow from anode to cathode through the load. Protons are also generated in the biochemical reactions, and these H+ ions pass through the selective, semipermeable, cation exchange membrane (CEM) into the cathode chamber (Singh et al, 2010). O2 combines with the proton in the cathode chamber to form water.
Anodic chamber in a MFC is usually anaerobic, and air or O2 is supplied to the cathode side of the MFC. Ferricyanide is a common cathode used in MFCs (Singh et al, 2010). Power generated by a MFC is usually in very limited quantity, and there is a need to boost and store the electricity generated from MFC for practical applications. Basically, microbial cells transfer electrons from a substrate at lower potential to oxygen or nitrite electron acceptor at a higher potential, through electron transport chain reactions (Singh et al, 2010). The process helps microbial cells gain maximum energy, but to harvest energy we substitute the electron acceptor with a anode in an MFC. This anode must be such that there is only minimal potential difference between the substrate and anode, so that microbial cells get less energy, and the MFC’s output is high (Singh et al, 2010). Bacterial metabolic loss is one important factor that affects output from an MFC. Further, the surface area of cathode and anode, as well as the CEM determine power out based on the design of the reactor (Singh et al, 2010).
Super-capacitor
Super-capacitors are capacitors i.e. energy storing devices with a storage capacity (capacitance) several times higher than conventional capacitors. Batteries store energy by electrochemical reaction, whereas capacitors store static charge that develops when a differential potential is applied across two conductors or electrodes (Battery University, 2016). Unlike conventional capacitors with smooth surface electrodes and solid dielectric medium, super-capacitors have porous electrodes, immersed in a liquid dielectric (electrolyte) medium. Capacitance (C) of a capacitor depends on the surface area (A) of the two conductors, distance between the conductors (d) and dielectric constant (ε) of the insulator medium (Fuente, 2016). The porous surface of super-capacitor electrodes, increase the surface area enormously and hence capacitance is also increased. If a capacitor can store charge in the order of picofarads or microfarads, an ultra-capacitor’s capacitance is rated in farads (Battery University, 2016). Super-capacitors are also called electrical double layer capacitors (EDLC) or ultra-capacitors. The difference between a capacitor and a super-capacitor is depicted in Fig 2.
Super-capacitors have low charge densities when compared to batteries but they can charge and discharge very quickly. Also, repeated charge-discharge cycles do not affect its life (Battery University, 2016). Hence, super-capacitors can be used to regulate voltage, in power back-up generators, in regenerative braking systems, etc. (Battery University, 2016). Batteries can store large amount of energy, but the charging time will be very high (Fuente, 2016). Also, electrochemical batteries cannot be used under all temperature and weather conditions (Fuente, 2016). But, despite their lower energy storage capacity compared to batteries, super-capacitors can find applications in remote power generation and supply. So far activated carbon has been used as conductors in super-capacitors, but recently graphene conductors with higher surface areas are being experimented (Fuente, 2016). Graphene based super-capacitors could find a wider application due to eco-friendly nature of carbon, as well as their high-energy storage capacity comparable to batteries (Fuente, 2016). Soon, Super-capacitors would replace batteries, which are hazardous to the environment posing severe disposal issues.
References
Battery University. (2016, March 30). BU-209: How does a Supercapacitor Work? Retrieved
the_supercapacitor
Fuente, J. D. (2016). Graphene Supercapacitors - What Are They? Retrieved May 03, 2016,
Singh, D., Pratap, D., Baranwal, Y., Kumar, B., & Chaudhary, R. K. (2010). Microbial fuel
cells: a green technology for power generation. Annals of Biological Research, 1(3), 128-138. http://scholarsresearchlibrary.com/ABR-vol1-iss3/ABR-2010-1-3-128-138.pdf
Skeletontech. (n.d.). Supercapacitors -basics and applications. Retrieved May 3, 2016, from
http://www.tgz-bautzen.de/fileadmin/media/pdf/Elektromobilitaet/3_
Superkondensatoren.pdf
