(419e) Nitrogen-Coordinated-Iron-Carbon (FeNC) Catalysts for Oxygen Reduction Reaction in Phosphoric Acid Fuel Cells
Proton exchange membrane fuel cells (PEMFC) are an emerging sustainable energy technology targeted for vehicular applications. Within a PEMFC two separate electrochemical half-reactions take place; at the anode, hydrogen is oxidized to form protons, while at the cathode oxygen is reduced with protons to form water (ORR). PEMFC are typically operated below 100 °C to maintain the proton conductivity of the polymer membrane. Low operation temperatures limit the reaction kinetics of the ORR resulting in high catalyst loadings. The state-of-the-art catalyst for the ORR is highly dispersed Pt which is expensive and a scarce natural resource. There are a wide variety of non-precious metal catalysts that may serve as a low-cost alternative to Pt such as nitrogen-doped carbon nanostructures (CNX) or nitrogen coordinated iron-carbon (FeNC)[1, 2].
A method for increasing the reaction kinetics on the cathode is by modification of the polymer membrane. Phosphoric acid-doped polybenzimidazole membranes (PBI) have been shown to function well at higher temperatures (up to 200 °C) and provide additional benefits such as increased CO tolerance and improved water management. One major drawback to these high temperature membranes is the detrimental effect phosphate chemisorption has on the Pt catalysts. FeNC catalysts have demonstrated resistance to the adsorption of these anions and are a promising alternative to Pt in high temperature PEM and phosphoric acid fuel cells. Our current work focuses on evaluating the performance and stability of FeNC ORR catalysts in phosphoric acid containing electrolytes and understanding the nature of the active sites poisoning resistance.
Electrochemical ORR activity measurements with a rotating ring-disk electrode (RRDE) system show high tolerance of FeNC towards phosphate anion exposure in both ex-situ soaking and in-situ electrolyte additions experiments. Accelerated durability tests of FeNC in phosphoric acid electrolyte demonstrated minimal losses in ORR activity. Transmission IR spectroscopy and X-ray photoelectron spectroscopy were used to investigate phosphate functionalities on the catalysts surface. One possible reason for FeNCs high resistance to phosphate poisoning is that the iron-nitrogen active sites are buried within the pore structure of the high surface area carbon support and not accessible to the large phosphate anion. Synthesis and testing of low surface area FeNC will be used to investigate the role of the carbon-support on poisoning resistance.
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