The performance and mechanism of modified activated carbon air cathode by non-stoichiometric nano Fe3O4 in the microbial fuel cell
Introduction
Microbial fuel cell (MFC) is the device which converts chemical energy to electrical energy by using electrochemical active microorganisms as the anode catalysts to oxidize the waste biomass, and this clean technology has a broad prospect in the field of wastewater treatment and new energy development such as wireless sensors and biosensors (Logan et al., 2006, Jeffrey J et al., 2008). However, the low performance and high cost are considered as the thresholds for feasible industrial application (Logan et al., 2007, Rozendal et al., 2008).The use of air cathode in MFC alleviated the above problems because the oxygen is freely available, and sustainable (Liu et al., 2008, Zhang et al., 2013). The output voltage of MFC with air cathodes often constrained by the oxygen reduction reaction (ORR) rate and internal resistance (Fan et al., 2008). Until now, platinum (Pt) is the most common catalyst for ORR. However, due to its high cost, it is not used for commercial MFC applications (Ghasemi et al., 2013).
Therefore, it is urgent to develop a kind of cheaper and more active electrode catalyst. In this respect, a broad range of alternative catalysts based on non-precious metals (Fe, Co, Ni, etc.) or metal oxides (Fe2O3, PbO2,Co3O4, MnO2, etc.) have been actively pursued (Choi et al., 2011, Jaouen et al., 2011, Médard et al., 2006, Morris et al., 2007, Zhang et al., 2009b, Zhang et al., 2014, Zhou et al., 2011). Among these materials, Fe based materials were widely studied due to lower cost. The previous study proved that iron-based Ethylene Diamine Tetraacetic Acid (EDTA) had a good catalytic activity in the air cathode, which was reversibly re-oxidized by oxygen (Xia et al., 2013). The previous studies reported that FePc exhibited a very similar catalytic activity to Pt (Birry et al., 2011, Zhao et al., 2005). Moreover, the enhanced catalytic performance for ORR was also observed in Fe based alloys. (Tang et al., 2010, Toda et al., 1999, Zhang et al., 2011). These studies indicated that ferrous materials had great catalytic effect in air cathode.
Recently, Fe3O4 has been drawn extensive attention since it owed unique catalytic and eco-friendly properties (Yang et al., 2014). In previous research, Lowy et al. found that the anode modified by iron (III) oxides had advantage over the unmodified anode in MFC. The enhancement of anode performance was attributed to the improved kinetic activity after the Fe3O4 modification (Lowy et al., 2006). It was also found that N doped graphene aerogel-supported Fe3O4 (N-GA/Fe3O4) exhibited higher cathodic density, electron transfer number and better durability than the commercial Pt/C (Wu et al., 2012). Vulcan XC–72 R, Black Pearl, Ketjen black are commonly used as catalyst support materials. Activated carbon (AC) was developed as an alternative catalyst to Pt in air cathode (Zhang et al., 2009a) because of low cost (Cheng and Wu, 2013). It is reported that defects and oxygen vacancies strongly influenced transition-metal oxide’s electronic properties (Greiner et al., 2012). Cheng et al. (2013) have shown that the catalytic activity of MnO2 for the ORR can be enhanced by generating oxygen vacancies, which could be introduced by simple heat treatment. TiO2 with low content of oxygen vacancies also improved the electron transport properties in the ORR (Tang et al., 2014). Non-stoichiometric compounds are chemical compounds with an elemental composition that cannot be represented by usual integral numbers. The oxygen vacancies could lead to the non-stoichiometric compounds. Until now, there is no research on the catalytic activity of non-stoichiometric nano Fe3O4 (NSFe3O4) in MFC.
Herein, AC air cathode modified by NSFe3O4 was prepared using a simple ultrasonic doping method. To our knowledge, this is the first report of the direct combination of NSFe3O4 and AC composite as a cathode catalyst for ORR in MFC. The effects of NSFe3O4 content on ORR and the performance of cathode were studied. The structure and morphology of NSFe3O4 were also examined.
Section snippets
Electrode materials and chemicals
The AC powder (2132 m2/g, Yihuan Carbon Co. Ltd., Fujian, China) was used to fabricate air cathode. The Fe3O4 particles were prepared via a solvothermal method as described previously (Yu et al., 2014). The NSFe3O4 was prepared by heating synthetic nano Fe3O4 under N2 gas from room temperature to 600 °C. The AC was modified by adding different content of Fe3O4 or NSFe3O4 with ethanol in ultrasonic device. The air cathodes were fabricated by rolling catalyst layer and gas diffusion layer on the
Performance of different air cathodes
A polarization curve provides useful information to understand the behavior of MFC and estimate maximum power density (MPD) that can be delivered and its internal resistance. It represents the cell voltage as a function of the current density. The results of polarization and power density were provided in Fig. 1A, which obviously showed that the modified air cathodes could significantly affect the cathode performance. The MFC with RAW-AC had a MPD of 780±38 mW/m2, which was similar to that
Conclusion
The modification of MFC cathode by nano NSFe3O4 with 5% content achieved the best performance, increasing the MPD by 83.3% from 780 mW/m2 to 1430 mW/m2. In addition to the onset potential and charge transfer kinetics of modified air cathodes raised, it was also found that the resistance was reduced greatly. Furthermore, the mechanism of ORR for the NSFe3O4/AC catalyst was a 4-electron pathway and there were more active sites for ORR on NSFe3O4 than Fe3O4. We attributed the excellent properties
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The two authors contributed equally to the work.