Sodium Cobalt Oxide As a Non-Platinum Cathode Catalyst for Microbial Fuel Cells

Abstract

Microbial fuel cell is a device that can achieve wastewater treatment and power generation simultaneously. For the practical application of microbial fuel cell, development of alternative cathode catalyst for platinum is necessary. In this study, a kind of sodium cobalt oxide, NaCo2O4was investigated, because estimated cobalt reserve is 100 times higher than platinum, and its price is about one thousandth. Catalyst ink was prepared by mixing NaCo2O4powder with carbon black powder and Nafion dispersion. Then cathode was fabricated by applying the ink to PTFE treated carbon paper. Cathode with Pt catalyst and that with activated carbon (NORIT SX PLUS) were also prepared for comparison.ORR activity of the cathodes was measured by linear sweep voltammetry (LSV). Oxygen reduction reaction (ORR) activity of NaCo2O4 cathode increased 15-25 times by the addition of carbon black as conductive assistant material in catalyst ink. Two kinds of carbon black were tested, and a cathode with Ketjen black showed better ORR activity than that with Vulcan XC-72. By increasing coating amount of catalyst ink from 2mg/cm2 (NaCo2O4) to 4mg/cm2, ORR activity increased 1.6 - 1.7 times, which was equivalent to 50-60% of Pt catalyst (Fig) . Gram based ORR activity of this catalyst was higher than that of activated carbon (NORIT SX PLUS) catalyst (Fig). The NaCo2O4 cathode having highest ORR activity was built into a pre-acclimated MFC, and then used for the treatment of synthetic wastewater. The current density was about 3A/m2, which was a little lower than the value during the operation equipped with a Pt cathode (about 4A/m2). However, any problems such as a decrease in electricity generation were not observed in several days of operation. Power density curve of the MFC equipped with the NaCo2O4 cathode was obtained by the LSV measurement , and the maximum current density was 0.58 W/m2, which is about a half of the density achieved by a Pt cathode. Further research on the preparation condition of the cathode, such as the amount of conductive assistant material, the amount of binder, and the coating amounts of the catalyst ink, would probably contribute to further improve the cathode ORR activity, which would improve electricity generation performance of a MFC. Figure 1