Effect of Carbon Xerogel Activation on Fe-N-C Catalyst Activity in Fuel Cells

Fe-N-C catalysts are an interesting option for polymer electrolyte fuel cells due to their low cost and high activity towards the oxygen reduction reaction (ORR). Since Fe-N-C active sites are preferentially formed in the micropores of the carbon matrix, increasing the microporosity is highly appealing. In this work, carbon xerogels (CXG) were activated by physical and chemical methods to favor the formation of micropores, used as carbon matrices for Fe-N-C catalysts, and investigated for the ORR. The catalysts were characterized by solid-state techniques to determine chemical composition and pore structure. Physical activation increased microporosity up to 2-fold leading to catalysts with a larger density of active sites (more than twice iron and nitrogen uptake, pyridinic N and Nx-Fe). This entailed a higher ORR intrinsic activity determined in a 3-electrode cell (80 mV better half-wave potential). At the cathode of a fuel cell, the catalysts based on activated carbon materials showed 26 % lower power density ascribed to a more hydrophilic surface, causing a larger extent of flooding of the electrode that counterbalances the higher intrinsic activity. Interestingly, a more stable behavior was observed for the activated catalysts, with up to 2-fold better relative power density retention after 20-hour operation. Activated carbon xerogels were studied as matrix for Fe-N-C catalysts. The increased microporosity lead to catalysts with a larger density of active sites, achieving up to more than twice iron and nitrogen uptake, and consequently, a higher ORR intrinsic activity. Whereas, fuel cell power density is negatively affected by a more hydrophilic character, but stability enhances with activation.image