Enhancing proton conductivity at subzero temperature through constructing the well-ordered structure based on carbon dots

Abstract

The layer-by-layer (LBL) self-assembly technique could nanoscopically construct the composite membranes with multilayered structures. It has been frequently mentioned in the field of microelectronics, biology and clean energy, etc. In this research, low temperature proton exchange membranes (LTPEMs) were fabricated through alternate deposition of polyurethane (PU) as polyanions, Nafion and carbon dots (CDs) as polycations. The successful preparation of LTPEMs with multilayered structure 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 the combination of a large number of PA molecules. The target of enhancing the proton conductivity at subzero temperature was achieved since the well-ordered dispersion of components could reduce proton conduction resistance. Specifically, 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 3.69 × 10−3 S/cm at −30 °C and 3.34 × 10−2 S/cm at 30 °C even if the membrane underwent a five-cycle heating/cooling process of −30 °C–30 °C. Furthermore, the values could reach 9.34 × 10−3 S/cm at −30 °C and 1.58 × 10−2 S/cm at 0 °C after a 315 h non-stop test. The results can reveal that the LBL self-assembly technique was a prospective strategy to construct LTPEMs for the application at subzero temperature.