Two-dimensional mxene as non-noble Fe-N-C catalyst support for oxygen reduction reaction in proton exchange membrane fuel cell

Abstract

MXenes are the largest family of two-dimensional (2D) materials with a unique layered structure, good mechanical properties, and excellent conductivity. Due to these outstanding properties, MXenes have the potential to be utilized as the catalyst support for the oxygen reduction reactions (ORR) in fuel cells. In general, the ORR that occurs at the cathode of fuel cells is sluggish even with the use of scarce and expensive Platinum (Pt) electrocatalyst compared to the hydrogen oxidation reaction (HOR) at the anode. Therefore, highly active and durable alternative electrocatalysts are needed. Among the Pt-free catalysts, Fe-N-C catalysts are the promising candidates. In this work, three different types of structures of Fe-N-C catalysts were studied in terms of their activity and durability, and subsequently Ti3C2Tx MXene was introduce as the catalyst support for Fe-N-C catalysts, for the application of low-temperature proton exchange membrane fuel cell (PEMFC). The objectives are to investigate the influence of Fe-N-C catalyst morphologies on the oxygen reduction reaction, to investigate the effect of anchoring Fe-N-C catalyst onto multilayer Ti3C2Tx MXene towards the ORR and to evaluate the performance of Fe-N-C/MXene in single cell stack PEMFC. Pyrolysis method was used to synthesize the Fe-N-C catalysts using three different precursors to produce three different Fe-N-C morphologies: hollow sphere structure, bulky structure, and sheet-like structure. ORR half-cell characterizations showed that the Fe-N-C with hollow sphere structure (Fe-N-C_H) exhibited the highest activity for oxygen reduction with Eonset of 0.95 V vs. RHE and 0.87 V vs. RHE in 0.1 M KOH and 0.1 M HClO4 electrolyte, respectively. In addition, the stability and durability of the Fe-N-C_H was the highest compared to the other Fe-N-C attributed to the presence of Fe-Nx active sites with large specific surface area and micropores. Further, multilayers MXene were added as the Fe-N-C catalyst support via different approaches, including mixing of synthesized Fe-N-C with MXene through the impregnation method, post-heat treatment of Fe-N-C/MXene, and one-pot pyrolysis of Fe-N-C/MXene at varying compositions and temperatures. Among the methods, post-heat treatment of Fe-N-C/MXene at the composition of 4:1, with pyrolysis temperature of 500 °C, demonstrated the highest ORR activity with improved stability with current retainment of 94% after 10 000 s of chronoamperometry test at 0.6 V. Through a PEMFC single cell testing, MXene-supported Fe-N-C catalyst enhanced the power density by obtaining 18.3 mW/cm2, while Fe-N-C could only achieve 8.1 mW/cm2 of power density. This work concluded that using MXene support could improve the Fe-N-C catalyst performance in single-cell PEMFC likely attributed to the strong metal-support interaction (SMSI) effect between the Fe-N-C catalyst and multilayer MXene support that facilitates electron transfer for the ORR to occcur.