Insights into the role of active site density in the fuel cell performance of Co-N-C catalysts

Improvement of proton exchange membrane fuel cell (PEMFC) performance of Co-N-C electrocatalyst requires an in-depth understanding of performance enhancement mechanism. Herein, we synthesize a series of Co-N-C catalysts with different CoN4 densities. CoN4 active sites (electrochemical accessible ones) and CoN4 species are differentiated in the study. The power density shows a slow linear increase in the low concentration region and an accelerated increase in the high concentration region of CoN4 active sites, showing the crucial role of the high active site density to the high power density of Co-N-C. The optimized Co-N-C achieves a high power density of 826 mW cm(-2). Meanwhile, CoN4 ORR turnover frequency (TOF) is calculated to be 0.01 s(-1) at 0.8 V vs. RHE in acid media. The results allow us to predict that many non-precious metal catalysts with only moderate activity may have considerable PEMFC performance as long as possessing sufficiently dense active sites.