Microbial fuel cells (MFCs) are bioelectrochemical systems that utilize different species of microorganisms to break down organic substrates and generate electricity. Those microorganisms are able to transfer the electrons from the oxidation process to electron acceptors, such as carbon anode electrodes. The electrons are transferred through an electrical circuit from the anode to the cathode, where the oxygen reduction reaction (ORR) occurs. The rate of organic substrate removal and magnitude of electricity generation from MFCs depend on the operational efficiency of both electrodes - biotic anode and abiotic cathode.

Our study focuses on the development, characterization, and optimization of gas diffusion ORR cathodes for single-chamber MFC applications. Gas diffusion cathodes rely on the passive diffusion of oxygen from air to complete the ORR. The gas diffusion cathode developed for this study (GDC) had a novel PTFE and polyethylene (PE) diffusion layer. The gas diffusion layer was heat pressed onto a carbon cloth substrate. The carbon cloth substrate was used as a current collector and support for the catalytic layer. Different combinations and concentrations of catalyst and binder were evaluated in this study. Activated carbon, nickel, plain carbon and non-platinum based catalysts were tested. After the best catalyst had been selected, different binder materials were evaluated. These include PTFE, PE, and a PTFE/Polyethylene glycol mixture.

The initial electrochemical characterization of the cathodes was performed in a three-electrode setup with a titanium mesh counter electrode and Ag/AgCl reference electrode. 0.1M phosphate buffer solution was used as the electrolyte. Durability studies of the cathode were conducted in a single chamber MFC, with swine waste as the organic substrate and carbon brush as the anode.

Potentiostatic polarization curves were carried out to evaluate the influence of the catalyst and the binder on the cathode performance (Fig. 1). As can be seen, the non-platinum based catalyst demonstrated the highest performance, with a current density of 0.34 mA/cm² at -0.2 V vs. Ag/AgCl, followed by the activated carbon/carbon black catalytic layer with current output of 0.14 mA/cm² at the same potential. The cathode with PE as the binder (Fig. 1b) showed the highest performance and reproducibility, but had low durability. It has also been established that MFC operation under continuous flow results in a loss of cathode performance, attributed to the presence of pollutants in the wastewater.