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

LESCANO, MARIELA; GASNIER, AURÉLIEN; PEDANO, MARIA LAURA; SICA, MAURICIO P. ; PASQUEVICH, DANIEL; PRADOS, MARÍA BELÉN

La Plata

Congreso; 2O th Topical Meeting of the International Society of Electrochemistry, Advances in Lithium and Hydrogen Electrochemical Systems for Energy Conversion and Storage; 2017

the International Society of Electrochemistry

Microbial electrolysis cells (MEC) are biocatalyzed electrolysis reactors employed in the production of hydrogen. These systems are based on the ability of certain electrogenic microorganisms, such as bacteria of the genus Geobacter, that are capable of extracellular electron transfer while oxidizing organic compounds. 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 promising technology 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 and transform it into storable energy, such as H2. However, the development of this technology has been hampered mainly by the low rate of charge transfer to the electrodes. For this reason, various materials like 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 establishment of biofilms of electrogenic bacteria and increment the performance of MEC. Graphene was obtained from the reduction of graphite oxide with ascorbic acid and further prepared as a hydrogel. The hydrogel was prepared on different meshworks, such as carbon cloth or 304 stainless steel mesh. The resulting modified electrodes were used as anodes in MECs. The MECs consisted of sterile anaerobic single-chamber cells with three electrodes, where a platinum wire was used as counter electrode and a Ag/AgCl electrode was used as reference electrode. The cells were filled with 200 ml of culture media, flushed continuously with N2:CO2 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 bacterial growth on the electrodes, the MECs were inoculated with stationary-phase cultures of Geobacter sulfurreducens 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 electrode were present. The current intensity was monitored during 10 days. The results were compared against a pristine mesh of comparable size without graphene, and also against a solid graphite rod. Controls in the absence of bacteria were also performed. The electrodes were characterized, before and after bacteria inoculation, by cyclic voltammetry, Raman spectroscopy and electron microscopy techniques.Cyclic voltammetry showed that graphene-based electrodes have larger surface area than the control electrodes. 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 surface and replicate. Furthermore, this increase was faster than for the respective pristine mesh and the graphite rod electrodes. The maximal current intensity was reached earlier with graphene-based electrodes and it was five folds than the control electrodes. No significant changes in current intensities were observed in any of the cells operated without bacteria. Bacterial biofilms were established in the graphene-based electrodes 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 rod electrodes, the biofilms formed heterogeneous pillar structures. When the electrode consisted of graphene over 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 graphene indicated 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 conductive biofilms and to produce high currents. Currently, we are improving the design of the electrode in order to allow longer operation times. Moreover, we are investigating how the different biofilm architectures observed affect the electrochemical response of the MEC, and whether the better performance of the graphene-based electrodes is due to the wider surface area, an enhanced bacterial anchorage and/or a better electron transfer on the cell-electrode interface.