Double Chamber MFC with Non-PGM F-C-N Cathode Catalyst

Due to the high cost of wastewater treatement, new and alternative low cost technologies need to be investigated. Bioelectrochemical systems and particularly microbial fuel cells (MFCs) seem to address positively this problem. In MFCs the organic waste is utilized as a fuel that is oxidised from microorganisms through their metabolism generating electricity. The main problem related with MFCs utilization is the small electricity production and high cost of the electrodes materials. These are the major reasons why MFCs are not commercialized in a large scale yet.  In order to overcome the high cost, that is mainly related to the high price of the noble metals used as catalysts at the cathode, inexpensive catalysts for oxygen reduction reaction (ORR) should be explored. These catalysts belong to the group of non platinum based catalyst that are proved as very active towards ORR. 

This work focused on the utilization of a low cost non-PGM (Fe-Aminoantipyrine) catalyst explored in the design of oxygen reducing cathode for MFCs application. The activity of this catalyst was characterized electrochemically in a three-electrode configuration varying the pH of the electroyte. Then the cathodes were introduced in double chamber MFC (Figure 1) with the cathode completely immersed in the solution. The two compartments (125 ml) of the MFC were separated by proton-exchange membrane (Nafion 211). The anode composed of carbon brush (6x4 cm projected surface area) pre-colonized with mixed cultured bacteria. The anode chamber was filled with 50% in volume PBS (50 mM) and 50% in volume of activated sludge (pH=7.5±0.1).  The non-PGM cathode (2.3 x 2.3 cm geometric area) was immersed in solution with different pHs (6, 7.5, 9 and 11) and purged with air for oxygen supply. The MFCs performance was investigated at different pHs in order to simulate possible industrial wastes with pHs different than neutral.

The catalyst used in this work was synthesized by modified sacrificial support method which was developed at UNM¹. In general, the metal precursor (iron nitrate) and nitrogen-containing low-molecular weight organic precursor (4-aminoantipiryne) are deposited on the surface of fumed silica (surface area ~120 m^-2 g^-1). the obtained composite material is heat treated in nitrogen atmosphere at T=950^°C. After heat treatment fumed silica was removed by excess amount of HF.

Potentiodynamic polarizations curve of the anode and the cathode separately were carried out using platinum mesh as a counter and a Ag/AgCl (3M KCl) as reference electrodes with scan rate 0.2 mV/s. MFC overall polarization curves were measured using a potentiostat with a scan rate of 1 mV/s. Power curves were determined using Ohm`s law (P= V x I).

The electrochemical measurements of the cathode as a result of differences in the electrolyte pH showed highest electrocatalytic activity of the catalyst at lower pHs. This confirms our previous observation for the dependance of the Fe-AAPyr activity from the electrolyte pH².

The polarization and power curves (Figure 2) of the whole MFCs followed the same trend as the cathode polarization curves showing the dominating role of the cathode over the MFCs performance. The MFCs with low pH of the catholyte demonstrated the highest power (200 μW) and the highest open circuit voltage (OCV = 850 mV).

This study demonstrated the applicability of non-PGM catalyst for the development of cathodes for MFC application. Further studies with improved MFCs design should be performed. Long-term operation test will be carried out investigating the influence of the wastewater pollutants on the cathode and subsequently the whole MFC operation and output.

References

1. A. Serov, U. Martinez, A. Falase, P. Atanassov, Electrochem. Comm. 22 (2012) 193-196.
2. S. Brocato, A. Serov, P. Atanassov  Electrochim. Acta,  87 (2013) 361-365