When MOFs meet MXenes: superior ORR performance in both alkaline and acidic solutions

Abstract

The oxygen reduction reaction (ORR) plays a key role in many efficient and clean energy systems, specifically in fuel cells and metal–air batteries. As a widely used commercial ORR catalyst, Pt/C has the limitations of high price and scarce reserves. Metal–nitrogen–carbon (M–N–C) is considered as one of the most promising alternatives to replace noble metal catalysts. However, how to balance the high catalytic activity and stability is still a major challenge. In this study, we report a novel non-noble metal composited catalyst, Fe–N–C@Ti₃C₂T_x, synthesized by a facile separated pyrolysis strategy. The Fe-doped Zeolitic Imidazolate Framework-8 (ZIF-8) was converted to Fe–N–C by carbonization firstly, which was then mixed with few-layered Ti₃C₂T_x and pyrolyzed together to obtain Fe–N–C@Ti₃C₂T_x composites. This separated pyrolysis strategy can not only ensure the high carbonization temperature required by Fe–N–C for high catalytic activity but also avoid the damage of MXene caused by high pyrolysis temperature. Thus, Fe–N–C@Ti₃C₂T_x exhibited excellent ORR activity and stability in alkaline solution, including a half-wave potential of 0.887 V vs. RHE, limiting diffusion current density of 6.3 mA cm⁻², and even no attenuation after 10 000 cycles in 0.1 M KOH. Meanwhile, it also exhibited surprisingly good performance in acidic solution, with a half-wave potential of 0.777 V vs. RHE, limiting diffusion current density of 5.7 mA cm⁻², and only 11 mV attenuation after 10 000 cycles in 0.1 M HClO₄. The superior ORR performance of the synthesized Fe–N–C@Ti₃C₂T_x could be attributed to the strong coupling effect between Fe–N–C and MXene, the reduced intrinsic and charge transfer impedance, and the increased electrochemically active surface area. This work here provides a new strategy for the development of M–N–C based non-noble metal-based catalysts with high activity and stability, and a promising future for applications in fuel cells and metal–air batteries.