Microbial fuel cells : the effect of granular activated carbon and graphite as growth substrates on bioelectrogenesis by Geobacter sulfurreducens

Abstract

There is an increasing demand for environmentally sustainable alternative energy sources as the supply of fossil fuels dwindles and the concern about our carbon footprint continues to increase. Bioelectrogenesis is a process that directly converts biomass into electricity. Bioelectrochemical systems hold great potential to breach the gap into sustainable and green energy. Bioelectrochemical systems can be thought of as fuel cells with regenerative, living microbial catalysts.

The aim of this investigation was to study the effect of the addition of carbon particles to the anodic chamber of a microbial fuel cell (MFC) containing Geobacter sulfurreducens in order to change the conditions of the MFC. The effect of the crystallinity of the carbon on the efficiency of the cell was evaluated. A crystalline carbon in the form of graphite particles was added to the initial growth media to increase the electrical conductivity of the anolyte. An amorphous carbon, granular activated carbon (GAC) particles, was added to the growth media to increase microbial growth. It was then decided which parameter needed to be prioritised if only one carbon substrate were to be added. A mixture of the two carbon substrates was added to the growth media to investigate what, if both the microbial growth and the conductivity in the MFC were increased, the effect would be on bioelectrogenesis. The effect of various growth periods of G. sulfurreducens prior to inoculation into the MFCs was also investigated. The experiments was thus limited to screening the effects of the bacteria growth media and growth time for the different carbon substrates and mixtures thereof.

An increased growth period, 4 months, appeared to be more advantageous for bioelectrogenesis. The addition of GAC particles proved to be advantageous to the microbial growth as the growth was improved nearly 6 fold compared to the control MFC containing G. sulfurreducens only. FE-SEM imaging and BET analyses confirmed that a large and rough surface area is ideal due to the ample attachment sites available for microbes. The biofilm thickens after only 3 weeks of growth.

It was evident that the addition of GAC particles to the system was beneficial for bioelectrogenesis. The maximum power density of the MFC containing GAC particles with a 4 months growth period was increased by 6 times compared to the MFC containing pure G. sulfurreducens. The average total energy generated by the MFC containing GAC particles was also 41 % higher than that of the pure G. sulfurreducens control MFC. The overall outputs were improved by the mere addition of GAC particles to the growth media.

The addition of the graphite particles to the system had to the opposite effect. The microbial growth was inhibited, which directly caused the bioelectrogenesis to be extremely low. The average energy density of the graphite containing MFC is almost equal to the blank MFC, i.e., the MFC containing no microbial community. This suggests that growth must be prioritised over conductivity. Without microbial growth, increased conductivity does not help the system.

The energy density of the MFC comprising the 1:1 mixture of graphite and GAC particles as growth substrate increased the average energy density of the control MFC by 134 %. The addition of the mixture of the two carbon substrates showed a synergistic effect since adding only pure GAC to the MFC increased the average energy density by 41 % compared to the control MFC, and by the addition of pure graphite particles to the MFC had the opposite effect of producing lower energy density than the control MFC. Therefore, mixing the two neat carbon sources and adding the mixture to the MFC, caused the synergistic effect of increasing the energy density of the MFC by more than triple than that of the MFC containing only pure GAC. With the mixture of GAC and graphite particles, both parameters were improved, i.e., microbial growth was improved compared with the control MFC and the conductivity in the system was increased, increasing the electron transfer efficiency and consequently the bioelectrogenesis.

One of the industrial applications of MFC systems where the impact can be maximised, is in waste water treatment, since the outcomes include both power generation and the removal of organic compounds in waste streams. It is well documented that numerous microbial fuel cells generate power by oxidation of compounds in wastewater. One study predicts, assuming 100 % efficiency, that the wastewater from a town of 150 000 people could be used to generate approximately 2.3 MW of power; but realistically, a power of 0.5 MW can be expected. From this review it is mentioned that up to 80 % of the chemical oxygen demand (COD) of the wastewater can be removed by using an MFC and that the power generated could be used on site to power additional wastewater treatment.

The second most promising application is the use of MFC systems in the biomedical industry as implantable devices. Currently most implanted biomedical devices are powered by batteries, which need to be recharged or replaced, necessitating additional surgeries for the patients. A method for continual electricity generation within the body would revolutionise biomedical devices. The use of MFCs as power sources for implantable devices in humans is a promising focus point. MFCs offer advantages over existing technologies such as lithium-ion batteries in implantable devices such as the heart pacemakers. The MFC would ideally use a biological metabolite fuel source (i.e. glucose or lactate) which is available in physiological fluids such as blood. It is unlikely that MFCs can replace the enzymatic glucose sensors that are currently used, but it was found that a well-designed MFC system, operating in a continuous flow regime, is implanted into the large intestines and utilises the natural flora of microbes within the intestines, could provide adequate power for cardiac pacing. This is one of the most promising future research areas. There are several variables that impact MFC power outputs, therefore extensive future research is still required. The one major problem that needs to be addressed is the longevity of many types of MFCs, most of which would currently be capable of meeting demands for biomedical devices implanted for short-term applications only.