Development and investigation of cathode for low temperature fuel cell using graphene-based oxygen reduction reaction catalysts

Abstract

The focus in recent years has been on developing high performance non-precious metal catalysts (NPMCs) to reduce the catalyst costs of fuel cells. However, little attention has been paid to improve the utilization of NPMCs. Thus, first half of this dissertation focuses on the optimization of electrode component, particularly the Nafion content. With the newly developed graphene-based oxygen reduction reaction catalyst, the catalyst inks were prepared at various Nafion content. And electrodes were fabricated by spraying suitable amounts of catalysts on the gas diffusion media. Twenty different single cells were assembled and measured for polarization, resistance and electrochemical impedance. Electrodes of 66.7 and 50.0 % Nafion content showed the highest performance for hydrogen/oxygen and hydrogen/air operation, respectively. These results were explained using the electrochemical impedance spectra, where the highest performance electrode resulted with the lowest charge transfer resistance. Moreover, negligible change in performance was observed during the 1,100 h of stability test. The optimization compositions of NPMC-based catalyst layer were very different compared to that of Pt-based catalyst layer, indicating the importance of optimization studies for the practical use of NPMCs. In the second half of this dissertation, direct methanol fuel cells were used for optimization and characterization. The fabricated electrode with 66.7 wt. % Nafion content and $5.2 mg cm^{-2}$ catalyst loading demonstrated the highest performance in oxygen and air operation. The influence of methanol concentration and characteristics of open circuit voltage were investigated systematically. The methanol tolerance and open circuit voltage were superior to the commercial Pt-based catalyst which indicates that the graphene-based catalyst could be applied as a potential catalyst for direct methanol fuel cells.