MXene: A two-dimensional nanomaterial for enhancing start-up performance of high temperature proton exchange membranes

Commercializing high temperature proton exchange membrane fuel cells (HT-PEMFCs) faces the challenge of slow start-up at low temperatures. Meanwhile, start-up under low relative humidity or anhydrous is beneficial for HT-PEMFCs, it can avoid lots of problems associated with water flooding. So, it requires membranes to have excellent water retention and fast proton transport pathways, which is the key to fast start-up. This study addresses this issue by incorporating MXene into membranes, achieving improved start-up and operating performance (144.1, 175.4, 225.6, 740.8 mW cm−2 at 40, 60, 80 and 160 °C under anhydrous, respectively). It attributes to MXene's excellent water retention and hydroxyl group (-OH). Through alkali treatment to add more –OH to MXene, improved proton conductivity proves that –OH has a positive affect on transporting proton. Further, by grafting sulfonic acid groups onto the MXene, start-up performance has enhanced at all temperatures (178.7, 200.0, and 280.8 mW cm−2 at 40, 60, and 80 °C under anhydrous, respectively), and power density of 160 °C was significantly improved compared to pristine membranes. The incorporation of sulfonic acid groups also led to better mechanical properties and oxidative stability of the composite membranes. MXene's potential for water retention, hydroxyl group, and modifiability – all desirable traits for accelerating start-up of HT-PEMs.