Addition of reduced graphene oxide to an activated-carbon cathode increases electrical power generation of a microbial fuel cell by enhancing cathodic performance
Introduction
Microbial fuel cells (MFCs) are innovative bioelectrochemical systems (BES) that are being developed for an energy positive wastewater treatment process that generate electricity from the organic waste with the concurrent wastewater treatment [[1], [2], [3]]. In MFCs, exoelectrogenic biofilm on the anode decomposes organic matter and releases electrons, which move to the cathode through an external circuit, and an oxygen reduction reaction (ORR) occurs on the cathode catalysts.
Platinum (Pt) has been used as a typical cathode catalyst for the ORR in the MFCs, but its high price is a bottleneck for commercialization of MFCs. One of the most promising alternative catalysts for platinum is activated carbon (AC) powder [4,5]. AC has lower catalytic activity for oxygen reduction reaction than Pt, but AC (∼$ 1.4 kg−1) is very cheaper than platinum (∼$ 69,000 kg−1) [6]. Therefore, by loading much higher amount of AC powder (AC: 26.45 mg/cm2) than Pt powder (Pt: 0.5 mg/cm2), AC cathodes with similar performance to those of Pt cathodes have been fabricated [7,8].
Along with increasing a current collector area [9], various AC cathodes with a stainless steel mesh current collector have been tested to improve cathode performance (Table 1). Maximum power production increased by physical and chemical treatments of AC powder in the cathodes. Alkaline or acidic treatments of AC powder enhanced cathodic performance and MFC power generation by increasing effective surface area. Strong alkaline treatment (KOH) of AC powder increased maximum power density (pmax) (957 mW/m2) by 19% compared to the plain AC cathode (804 mW/m2) [10]. Strong phosphoric acidic treatment (H3PO4) of AC powder increased pmax (1546 mW/m2) by 142% compared to the plain AC cathode (638 mW/m2) [11]. Functional groups were attached to the AC surface to improve catalytic reaction and electrical conductivity. Attaching the Fe-N-C framework to the AC surface improved pmax (2600 mW/m2) by 62% over the plain AC cathode (1600 mW/m2) [12].
Addition of conductive material to a cathode is very a simple and easy way to enhance cathodic performance. When FeEDTA was added instead of CB, pmax (1410 mW/m2) increased by 35% over the non-treated one (1040 mW/m2) [13]. When the iron-aminoantipyrine (Fe-AAPyr) catalyst was added to a cathode, pmax (2170 mW/m2) was 108% higher than that of the control cathode (1040 mW/m2) [14]. When the metal structure (iron-nicarbazin) was added, pmax (1850 mW/m2) was 330% higher than that of the plain AC cathode (430 mW/m2) [15]. When carbon black (CB) was added to the AC cathode (CB:AC = 1:10) without reducing the total AC weight, pmax (1560 mW/m2) improved by 16% over the control without carbon black (1340 mW/m2) [16]. Additions of AC and CB particles pre-treated at 800 °C showed the highest pmax of 1900 mW/m2, which is 18% higher than that of the non-treated control (1610 mW/m2) [17].
Because graphene is a thin carbonaceous material with superb conductivity and chemical stability, graphene has gained substantial interests for various applications [[18], [19], [20]]. Among the graphene-like materials, graphene oxide have been used as an ideal carbon-based electrode material [21]. During the thermal reduction process of graphene oxide (GO), GO with a sheet form expands due to the thermal reduction of its oxygen constituents. Through this process, its conductivity and surface area increase. When rGO is added to the cathode catalyst layer, conductivity of the catalyst layer can increase. In a previous study, rGO particles were added as a cathodic catalyst to the AC cathode by replacing AC in three different ratios, and the 10 mg/cm2 loading of rGO showed the highest pmax 2059 mW/m2, which is 102% higher than the AC cathode (1017 mW/m2) [22]. The rGO particles were used as an electrochemical reduction catalyst, and cathode performance was enhanced by replacing activated carbon.
In this study, it was hypothesized cathodic performance would increase if reduced graphene oxide (rGO) replaces CB in an optimum ratio. rGO was tested as an additive to the cathodic catalyst layer in the AC-based cathode. Because rGO has a high electrical conductivity and unique shape with flat and large area, it can be used for improving the electrical conductivity and facilitating electron transfer in the cathodic catalyst layer by intercalation into the catalyst layer in replace of CB [16]. The same amount of AC was used in all the cathodes, CB was replaced with rGO in the four different weight ratios, and these four types of cathodes were tested in the single chamber cubic MFC.
Section snippets
Preparation of reduced graphene oxide
Graphene oxide powder (GO-V30-100, Standard Graphene Inc., South Korea) was thermally reduced at 300 °C for 5 min in a tube furnace (HTF-Q85, HAN tech, South Korea) flowing 4% H2/Ar gas. After this reduction process, reduced graphene oxide (rGO) powder was obtained. The GO had 45–50% carbon, 45–50% hydrogen, less than 2% hydrogen and no nitrogen detected along with more than 7.0 μm of lateral size.
Characterizations of carbon materials
X-ray photoelectron spectroscopy (XPS) was performed to determine atomic compositions and
Power and current production
rGO15 produced higher power and current density in the MFC than any other cathodes (Fig. 1 and Table 3). rGO0 having no reduced graphene oxide showed the lowest performance. rGO15 produced a 35% higher maximum power density, 21% higher optimum current density and 10% higher maximum current density than those of rGO0. Maximum power density on average in the MFC was in the rGO15 (2642 mW/m2), which is 19%, 25% and 35% higher than those of rGO5 (2214 mW/m2), rGO30 (2120 mW/m2) and rGO0 (1955 mW/m2
Graphene chemistry
Graphene has 2-dimensional crystalline sheet structure. In a carbon atom, its 4 valence electrons are available for chemical bonding. However, in graphene, one carbon atom is connected to the other three carbon atoms on the two-dimensional sheet, leaving one electron available in the z-axis. These free electrons are called pi electrons (π electrons) and are located above and below the graphene sheet, giving high conductivity. Among graphene-like materials, graphene oxide (GO) is obtained by
Conclusion
In the AC-based cathode, carbon black (CB) is added as a conductive supporting material. To enhance the performance of the AC-based cathode, rGO replaced CB in the four different weight ratios of rGO to CB: rGO0 0:30; rGO5 5:25; rGO15 15:15; rGO30 30:0. rGO15 showed 35% higher maximum power density (2642 mW/m2) than that of the rGO0 (1955 mW/m2). During the optimum resistance operation, rGO5 showed the best power (1063 mW/m2) and current (3896 mA/m2) and rGO15 showed similar power (1061 mW/m2)
Disclamier
This article contains contents of the master thesis of Bonyoung Koo in 2017. This article also contains contests presented in an academic conference: 2017 ISBUT. The authors certify that there is no authorship dispute among the authors.
Acknowledgements
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1A1B07050567), a research grant from Gwangju Green Environment Center in Ministry of Environment (17-04-10-14-12), and a research grant from Korea Electric Power Corporation through Korea Electrical Engineering and Science Research Institute (R15XA03-04).
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