Bio-electrocatalysis Of Graphene Aerogel Based Microbial Fuel Cell Anodes

Microbial fuel cells(MFCs) is capable of converting chemical energy stored in organic fuels or organic wastes into electrical energy using biocatalysts. It has attracted significant interest as a renewable energy source and has been applied in many fields, particularly in wastewater treatment, and biosensors. However, the relatively poor energy conversion efficiency and low power density limit its practical applications. A lot of factors affect MFCs performance, such as chemical substrates, proton exchange material and electrode materials. Among these factors, anode is generally considered as the main limiting factor for the low power density, which is mainly due to the poor interfacial electron transfer efficiency between the bacterial cells and electrode. Thus, optimizing anode structure is very critical for facilitating bacterial adhesion and interfacial electron transfer, which is also a key strategy to enhance the performance of microbial fuel cells.To improve the power output of MFCs, three-dimensional porous anodes have been applied such as graphite fiber brush and polypyrrole or carbon nanotube-coated carbon foam. Compared with the conventional flat anodes, the three-dimensional anodes provide a larger surface area for bacterial adsorption and redox reaction. Among these 3D porous materials, self-assembled 3D graphene aerogels possess both the structural merits and unique electrochemical properties inherited from graphene sheets. In recent years, graphene has been adopted in MFC anodes to improve their performance owing to its extraordinary properties, including high specific surface area, outstanding electrical conductivity, chemical inertness, and outstanding biocompatibility.Hydrothermal reduction is the most frequently used method to prepare graphene aerogels from a solution. However, the obtained aerogels often possess considerably hydrophobic surfaces that are hard for the electrolyte to access. Furthermore, the pore size is not sufficiently for the bacterial cells moving in, and thus the loading of biocatalysts cannot be further improved. It requires a suitable strategy to produce the three-dimensional graphene aerogels with hydrophilic and biocompatible surface, thereby improving the catalytic properties of the material and ultimately improving the MFCs performance.In this work, graphene/polyaniline composites with three-dimensional porous structure and graphene aerogels reduced with biological molecules is synthesized. Then physical characterizations and electrochemical analyses are carried out. Finally, it is applied in S.putrefaciens CN32 microbial fuel cells to improve MFCs performance. The main contents and conclusions are as follows:(1) Firstly, graphene aerogels with three-dimensional porous structure is developed via freeze-drying assisted hydrothermal method. Secondly, the surface of graphene aerogels is modified with polyaniline nanorods by in-situ chemical oxidation polymerization. The structure and electrochemical properties of different graphene/polyaniline composites are investigated. From the results, with the optimized ratio of aniline in the precursor(the weight ratio of graphene and aniline is 1:7), the obtained GR/PANIⅡ composite maintains three-dimensional structure of graphene aerogels and a layer of polyaniline nanorods is uniformly grown on its inner and outer surface. As a result, it possesses a hydrophilic and biocompatible surface. According to the electrochemical analyses, the GR/PANIⅡ not only facilitates electron transport between the electrode and bacterial cells, but also improves the loading of the biocatalyst. The maximum power density of GR/PANIⅡ anode is around 1.4-fold higher than that of the GR anode. This study not only provides a new strategy to prepare graphene-based aerogels / conductive polymer composite but also provides some insights on how to build a high bacteria loading anode for MFCs.(2) As shown in the experimental results of the first work, graphene aerogels modified with polyaniline can improve MFCs performance. However, the pore size of graphene/polyaniline composites is not sufficiently for the bacterial cells to move in. To solve this problem, in the second work, graphene aerogels with three-dimensional porous structure are obtained from a one-pot synthesis with L-cysteine as the reductant at a lower temperature(80 ℃) and under an inert atmosphere. The differences corresponding to the structure and electrochemical properties of graphene aerogels when used as an anode are studied by analyzing the relevant physical and electrochemical characterizations. As shown in the relevant physical characterizations, the introduction of a suitable amount of L- cysteine apparently enlarges the pore size of the graphene aerogels and increases the electroactive surface area of material. With the optimized ratio of L-cysteine in the precursor(the weight ratio of GO and L-cysteine is 1:13), the obtained graphene aerogel has a three-dimensional porous structure suitable for bacteria moving in and adhesion and also possesses a hydrophilic and biocompatible surface. According to the electrochemical analyses in S.putrefaciens CN32 microbial fuel cells, the GIII anode not only provides more reactive sites for the redox reaction of the flavin, but also significantly improves the loading of the biocatalyst. What’s more, it also greatly facilitates electron transport between the electrode and bacterial cells. As a result, the optimized GIII anode delivers a maximum power density of 679.7 mWm-2, which is around 1.6-fold higher than that of the G0 anode(422.86 mWm-2). This study not only provides a new strategy to prepare graphene aerogels in a simple and environmental-friendly manner but also provides some insights on how to build a high bacteria loading anode for MFCs.