Development and characterization of graphene-based electrodes for microbial electrolysis cells

MARIELA LESCANO; AURELIEN GASNIER; M. LAURA PEDANO; MAURICIO SICA; DANIEL PASQUEVICH; MARÍA BELÉN PRADOS

Congreso; Advances in Lithium and Hydrogen Electrochemical Systems for Energy Conversion and Storage; 2017

International Society of Electrochemistry

Microbial electrolysis cells (MEC) are biocatalyzed electrolysis reactors employed in the productionof hydrogen. These systems are based on the ability of certain electrogenic microorganisms, such asbacteria of the genus Geobacter, that are capable of extracellular electron transfer while oxidizing organiccompounds. This mechanism can be used to transfer electrons to an electrode in an electrochemical cell,where they can be used for the production of value added products. Currently, MEC represent a promisingtechnology to turn the organic pollutants dissolved in wastewaters in a renewable source of energy.Bioelectrochemical wastewater treatment can capture the chemical energy present in various effluents andtransform it into storable energy, such as H 2 . However, the development of this technology has beenhampered mainly by the low rate of charge transfer to the electrodes. For this reason, various materialslike graphene, that could overcome this limitation, have arisen great interest in the area.The objective of this work was to develop graphene-based electrodes that promote the establishmentof biofilms of electrogenic bacteria and increment the performance of MEC. Graphene was obtained fromthe reduction of graphite oxide with ascorbic acid and further prepared as a hydrogel. The hydrogel wasprepared on different meshworks, such as carbon cloth or 304 stainless steel mesh. The resulting modifiedelectrodes were used as anodes in MECs. The MECs consisted of sterile anaerobic single-chamber cellswith three electrodes, where a platinum wire was used as counter electrode and a Ag/AgCl electrode wasused as reference electrode. The cells were filled with 200 ml of culture media, flushed continuously withN 2 :CO 2 and cultured at 30oC during operation. The working electrode was poised at a constant potential(+240 mV vs Ag/AgCl) and the current intensity was recorded. After 24 h, in order to initiate bacterialgrowth on the electrodes, the MECs were inoculated with stationary-phase cultures of Geobactersulfurreducens that had been grown with sodium fumarate as the electron acceptor (20% inoculum).Sodium acetate was provided as the electron donor, and no electron acceptors other than the electrodewere present. The current intensity was monitored during 10 days. The results were compared against apristine mesh of comparable size without graphene, and also against a solid graphite rod. Controls in theabsence of bacteria were also performed. The electrodes were characterized, before and after bacteriainoculation, by cyclic voltammetry, Raman spectroscopy and electron microscopy techniques.Cyclic voltammetry showed that graphene-based electrodes have larger surface area than the controlelectrodes. After inoculation, a significant increase in current intensity was observed when graphene-based electrodes were employed, indicating that the bacteria were able to adhere to the electrode surfaceand replicate. Furthermore, this increase was faster than for the respective pristine mesh and the graphiterod electrodes. The maximal current intensity was reached earlier with graphene-based electrodes and itwas five folds than the control electrodes. No significant changes in current intensities were observed inany of the cells operated without bacteria . Bacterial biofilms were established in the graphene-basedelectrodes and the graphite rod, as observed by SEM. Their thickness was similar for each electrode(approx. 20 μm), but the architecture was different. In the graphene-carbon cloth and the graphite rodelectrodes, the biofilms formed heterogeneous pillar structures. When the electrode consisted of grapheneover stainless steel mesh, G. sulfurreducens formed a thick homogenous layer of cells evenly distributed,with minimal pillar structures. Finally, electrochemical experiments and Raman spectra of grapheneindicated that graphene has been further reduced by the bacterial biofilm during the MEC operation.In conclusion, the graphene-based electrodes proved to be suitable to develop dense and conductivebiofilms and to produce high currents. Currently, we are improving the design of the electrode in order toallow longer operation times. Moreover, we are investigating how the different biofilm architecturesobserved affect the electrochemical response of the MEC, and whether the better performance of thegraphene-based electrodes is due to the wider surface area, an enhanced bacterial anchorage and/or abetter electron transfer on the cell-electrode interface.