1. Introduction
Efficient development and utilization of the large amount of biomass energy contained in wastewater can turn the waste to treasure. Microbial fuel cells (MFCs) are one of the most promising clean energy sources to convert organic fuels, including organic wastes, into electricity using microorganisms [
1,
2,
3]. MFCs can produce electricity while processing wastewater pollutants; thus, they have become a research hotspot in the environment and energy fields [
2,
4]. The concept of using microorganisms as catalysts in fuel cells was explored in the 1910s [
4,
5]. The emergence and development of MFCs provided a new concept for efficiently developing the biomass energy in wastewater and implementing wastewater reclamation at the same time [
6,
7]. Scale-up of MFCs will require compact reactors that use inexpensive electrode materials [
2,
6] such as graphite fiber brushes [
6,
7] or electrically-conductive granules [
4,
8], and activated carbon [
1,
2]. High current densities are needed to maximize power production, as well as avoid voltage reversal when connecting reactors together in series [
6,
7,
8]. Searching for highly effective anode and cathode catalysts, improving reactor architectures, and optimizing operational conditions are crucial strategies for further enhancing the performance of MFCs [
8,
9,
10]. The practical MFC technology is currently in its infancy and there are many problems to be solved. How to improve the reactor power generation and reduce the cost remains a difficult objective for many researchers. The electrode material is a key factor in determining the performance and cost of MFCs. Therefore, it is particularly important to modify the properties of anode and cathode materials and further optimize the power generation of MFCs [
11].
Polyaniline is an excellent conducting polymer characterized by high conductivity, simple synthesis, and low cost. Owing to these advantages, polyaniline is considered to be the most promising conducting polymer [
12,
13,
14]. However, polyaniline can be problematic for its poor stability; it also easily becomes doped (impure) during the preparation process and it is difficult to process or functionalize [
12]. These disadvantages have limited the development of polyaniline.
Graphene is a two-dimensional carbon nanomaterial that forms the basic building block of graphite. Graphene provides a large surface area (2630 m
2/g) and excellent electrical, thermal and mechanical properties [
15,
16]. It can be easily prepared from cheap natural graphite [
15]. Graphene has been demonstrated to be a promising adsorbent to remove heavy metals such as uranium [
17,
18], chromium [
19], thorium [
20], and antimony [
16] from aqueous solutions. Additionally, graphene exhibits exciting adsorption abilities for removing hazardous cationic dyes such as methylene blue and safranine T from contaminated water [
18,
21]. Moreover, graphene hybrid materials have been shown to have potential applications in electronic/spintronic devices [
22,
23], touch panels [
24], gas/biosensors [
25,
26], and solar cells [
27].
The excellent plasticity and stability of graphene can make up for the deficiencies of polyaniline. The disadvantages of polyaniline as a conducting polymer therefore may be overcome by preparing polyaniline-graphene (PANI-G) composites [
12]. Hou et al. [
12] have modified the anode of MFC with PANI-G, which could minimize anode energy losses in the system and significantly increase the power density output of MFC. The power density of a MFC with PANI-G-modified anode is three times larger than that of a MFC with Carbon cloth (CC) anode. So far, PANI-G composites have been widely used in capacitors, but less used for electrode modification in MFCs and existing studies have mainly investigated dual-chamber MFCs. Moreover, most of the existing studies used potassium ferricyanide solution as the catholyte. Despite the relatively high potential of potassium ferricyanide as a cathodic electron acceptor, it is not conducive to the promotion and application of MFC technology [
28].
The best catalyst currently used in MFCs is platinum (Pt), but its expensive price can greatly increase the cost and the poisoning of Pt catalyst goes against practical applications. Transition metal macrocycles have received great attention from researchers due to excellent catalytic activity toward oxygen reduction [
29,
30]. Metal phthalocyanine is a typical transition metal macrocycle. Deng et al. [
31] prepared a Fe-Co-NC composite by mixing carbon nanotubes with iron phthalocyanine, which was then used as a catalyst to modify the MFC cathode; the maximum output power of the cell was 751 mW/ m
2, that is, 1.5 times higher than that obtained with the modified Pt/C cathode. In addition, Yuan et al. [
2] prepared a composite of polyaniline, carbon black, and phthalocyanine iron, which showed more positive potential for catalyzing oxygen reduction and greater reduction current compared with Pt electrode. Based on the above studies, sulfonated cobalt phthalocyanine (CoPcS) may have the potential to catalyze oxygen reduction and replace Pt catalyst. In the present study, we prepared electrodes using functionalized graphene, polyaniline, and CoPcS and then assessed the performance of single-chamber MFCs with modified anodes and cathodes. The aim of the study was to improve the efficiency of anode and cathode of single-chamber MFCs and enhance the power generation and output of the cells. This work has implications for the development and expanded application of MFC technology and provides a reference for research on wastewater treatment with MFCs.