Study of tea, onion and garlic plant extracts as green electron mediators in a twochambermicrobial fuel cell (MFC)

Today, energy plays an important role in all aspects of human interactions. Fossil fuels are graduallydecreasing and the demand for alternative energy production is increasing. Bio- renewable energy canbe a suitable alternative for existing resources. Microbial fuel cell (MFCs) is a part of this bioenergytechnology. MFC is a biological reactor that converts the energy of the chemical substrates intoelectrical energy in the presence of biological catalysts (for example, suitable bacteria and/orenzymes). In other words, MFC simultaneously performs the process of producing electricity andtreatment organic materials. One of the disadvantages of this technology is their relatively low power.Among the ways to increase the power of these cells is to use electron shuttles or mediators for bettertransfer of electrons from the cell wall of anode-attached microorganisms to the anode electrode andthereby to the external circuit. Electron mediators are usually chemicals secreted by the bacteriathemselves or compounds deliberately added to a MFC system to enhance electrical power. Theselected electron mediator must be satisfactorily reversible in electrochemistry, stable in solution andmost importantly non-toxic for microorganisms present as anodic biofilm. Recently, attention has beenpaid to use herbal extracts of some plants for this purpose. In this research, tea, onion and garlicextracts were used as electron mediators in a two- chamber mixed-culture glucose-feed MFC. Anodicand cathodic chambers with graphite plate electrodes are separated by a FlemionTM proton exchangemembrane. Air cathode was employed in which air was blown in the cathode chamber via a miniatureair pump. An external resistor as an electric power consumer completed the circuit. To study herbalextracts, about ۲۰ ml of all three extracts were added to the anode chamber in separate experiments.Polarization curves obtained in this research showed that the plant materials in tea has reduced themaximum output power of the MFC from about ۴.۰ μW.cm-۲ (normalized to the anode electrode geometric surface area) in the absence of any extract (possible source of any chemical performing aselectron mediator) to about ۱.۲ μW.cm-۲ in the presence of tea extract. Onion and garlic extractsenhanced the maximum power from ۴.۰ μW.cm-۲ in the absence of extract to about ۲۱.۵ μW.cm-۲ and۱۲.۱ μW.cm-۲, respectively.So, chemicals present in onion and garlic extracts perform as electron mediators that increase shortcircuit current, open circuit potential and thereby the output power of the MFC with onion showing abetter performance. While, chemical content in tea extract affects the biofilm in a reverse manner andmay deteriorate the biological paths of electron transport which leads to a subsequent decrease in themaximum output power of the MFC .In conclusion, the results obtained in this research show that cheapand available plant extracts can participate in the oxidation-reduction cycle and thereby can be used aselectron mediators in MFC systems to increase power level.