Enhancing Proton Conductivity at Subzero Temperature Through Constructing the Well-Ordered Structure Based on Carbon Dots

The layer-by-layer (LBL) self-assembly technique could nanoscopically construct the composite membranes with the layered structure. It has thus been frequently mentioned in the field of microelectronics, biology and clean energy, etc. In this research, low temperature proton exchange membranes (LTPEMs) were fabricated via the alternate deposition of polyurethane (PU) as polyanions, Nafion ® and carbon dots (CDs) as polycations. The successful construction of the multilayered LTPEMs has been demonstrated by the structure characterization and property analysis of the (PU/Nafion/PU/CDs) 200 membrane. The CDs and PU can combine phosphoric acid (PA) molecules owing to the hydrogen-bond interaction. The (PU/Nafion/PU/CDs) 200 /PA membrane was thus obtained with a large number of PA molecules doping. The well-ordered dispersion of the components reduced the proton conduction resistance and the target of enhancing the low temperature proton conductivity was thus achieved. The (PU/Nafion/PU/CDs) 200 /PA membrane exhibited the proton conductivities of (3.29±3.16)×10 -3 S/cm at -30 °C and (2.99±0.39)×10 -2 S/cm at 30 °C. Furthermore, the membranes exhibited the fine proton conductivity stability, such as the values of 3.69×10 -3 S/cm at -30 °C and 3.34×10 -2 S/cm at 30 °C even the membranes underwent a five-cycle heating/cooling process of -30 °C–30 °C. Even after a 315 h non-stop measurement, the values maintained 9.34×10 -3 S/cm at -30 °C and 1.58×10 -2 S/cm at 0 °C. The results can reveal that the LBL self-assembly technique was a prospective strategy to construct LTPEMs for the application of proton exchange membrane fuel cells at subzero temperature