Conducting Polymers As Electrode Modifiers for the Enhanced Microbial Fuel Cell Performance

The low power density of a microbial fuel cell (MFC) compared to other fuel cells has been a problem for this technology to be implanted in the real world despite the fact that it has many merits as an alternative energy source such as environmental sustainability and carbon neutrality. The main reason lies in the slow electron transfer (ET) kinetics at the anode and the cathode. Electrons produced from the oxidation of organic matters by the bacterial biofilm on the anode are transferred to the cathode where oxygen reduction reaction (ORR) takes place. Thus fast ET at both electrodes is very important to increase current density that leads to enhanced power density. Here we show how polyaniline (PANI) and polypyrrole (Ppy) can be utilized as electrode modifiers to greatly improve MFC performance.

First, we have used PANI and Ppy to modify anodes. When Ppy was applied to the anode in a mediator-type MFC using Proteus vulgaris as a biocatalyst, a large enhancement in power density was observed. A dramatic power enhancement was resulted from the electrodeposited Ppy onto the reticulated vitreous carbon (RVC) electrode. Our obtained maximum power density of 1.2 mW cm⁻³ is the highest value among the reported ones for the similar system. From impedance measurements, enhancement was attributed to the decrease in charge transfer resistance. More practical MFCs are single chamber and mediatorless type MFCs with a biofilm formed from activated sludge. We prepared polyaniline/carbon black (PANI/C) composite and applied it to carbon cloth anode. It shortened start-up period and significantly enhanced power density from 764 mW m^-2 to 1277 mW m^-2. Enhances performance was also attributed to fast electron transfer to the anode through PANI. The anode surface was characterized by employing various surface characterization techniques such as scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The power output depended upon the amount of PANI/C, showing a highest value at 0.2 mg cm^-2. Coulombic efficiency was more than 30% at 4.3 A m^-2.

Second, PANI was also utilized as an ORR catalyst for an MFC cathode. We used polyaniline nanofibers (PANInf) synthesized by interfacial polymerization and prepared PANInf/Carbon black composite for the cathode. Higher electrocatalytic activity for the oxygen reduction compared to pristine PANInf was resulted, thus leading to a large power density enhancement 185 mW m⁻² for the pristine PANInf to 496 mW m⁻² for the composite material. Although the power density was still lower than when conventional Pt catalyst was used (604.3 mW m⁻²), a facile bulk synthesis and cheaper price would make PANInf/C an alternative to Pt when a large scale application comes to an issue.

PANI was also used as a support to incorporate compounds that have high catalytic activity toward oxygen reduction. Iron phthalocyanine (FePc) has been known as effective oxygen reduction catalyst. FePc was incorporated into Polyaniline/carbon black (PANI/C) composite. Thus prepared (PANI/C/FePc) was used as a catalyst for the ORR in an air–cathode microbial fuel cell (MFC). The electrocatalytic activity of the PANI/C/FePc toward the ORR was compared with carbon-supported FePc. The ORR overpotential was decreased and current density was greatly increased, indicating that polyaniline plays a key role in ORR reactions. This PANI/FePc/C composite was also well adopted as a cathode material in MFCs. The maximum power density of 630.5 mW m⁻² with the PANI/C/FePc cathode was higher than that of 336.6 mW m⁻² with the C/FePc cathode because of enhanced ORR activity. We compared monetary aspect per power for these materials. It turned out that the power per cost of the PANI/C/FePc cathode is 7.5 times greater than that of the Pt cathode. Thus, the PANI/C/FePc can be a potential alternative to Pt in MFCs.

We found PANInf could be a useful material for modifying the interior of the three-dimensional anode. Reticulated vitreous carbon (RVC) is an ideal material in that it has many interconnected pores inside so that substrate can freely flow through the pores. The PANInf layer was easily formed on the interior surface of the RVC on which a stable and robust bacterial biofilm was formed, giving higher power density.

In conclusion, conducting polymers have been proven very promising materials for the MFC power enhancement. Electron transfer barrier at the anode and cathode can be lowered when conducting polymers are used. Modification of these polymers with foreign species can further increase power density.