As an ideal power source for new energy electric vehicles, hydrogen fuel cells have been elevated to the level of national energy strategy. The traditional hydrogen fuel cell technology route is hydrogen-hydrogen refueling station-fuel cell. However, the preparation of high-purity hydrogen, high-pressure storage, transportation, and the construction of hydrogen refueling stations restrict its wide application and promotion. Methanol reforming hydrogen fuel cell uses methanol as a liquid fuel, which is convenient for storage and transportation, and hydrogen is "ready to use", which can use the convenience of existing gas stations to complete methanol distribution and refueling, which is more safe and reliable. If the temperature match (200-300 oC) between the reforming hydrogen generator and the proton exchange membrane fuel cell can be achieved, the high-temperature hydrogen-rich reforming gas generated from methanol can be directly utilized, which greatly simplifies the system and improves the energy utilization efficiency. However, the service temperature of the phosphoric acid doped high-temperature proton exchange membrane is still limited to 120-200 oC, and the degradation of proton conduction and battery performance will be caused by the problems of phosphoric acid dehydration and polycondensation and membrane mechanical creep when the temperature is too high. Therefore, challenging this temperature limit and developing a new high-temperature proton exchange membrane that can operate efficiently and stably above 200oC has become the key to realize the on-site integration and integrated direct coupling of methanol reforming system and fuel cell.
鉴于此，浙江工业大学黄菲教授联合温州大学薛立新研究员采用多聚磷酸溶胶-凝胶技术，原位构筑了具有三维（3D）片层双交联结构的凝胶态磷酸掺杂聚苯并咪唑（DC-PBI-G）高温质子交换膜。 这类膜材料可以在高温下有效地锚定并且限域保留磷酸分子，成功抑制了膜内96%磷酸的脱水缩聚，大幅提升膜力学抗蠕变性能，并兼具优异的质子传导率（0.348 S/cm）与稳定性。 基于DC-PBI-G的燃料电池展现出了在200-240 oC下高达1.20-1.48 W/cm2的单池输出功率密度峰以及250小时长期运行中仅0.27 mV/h的电压衰减率，最终实现了高温甲醇重整气的高效稳定利用。 相关研究以“Double cross-linked 3D layered PBI proton exchange membranes for stable fuel cell performance above 200 °C”为题发表在期刊《Nature Communications》上。
[Double-crosslinked 3D sheet PBI proton exchange membrane that enables the fuel cell to be stably serviced at more than 200oC]
Figure 1. In this study, 3,3'-diaminobenzidine (TAB), 2,5-dihydroxyterephthalic acid (DHTA) and trimelliptrialic acid (TMA) were used as monomers, and HO- groups were introduced into the PBI network to form a cross-linked phosphate bridge, thereby anchoring the phosphoric acid molecule in situ to improve proton conduction performance. At the same time, the addition of TMA branched monomers constructed a rigid branching network to improve the creep resistance of the membrane. Under the synergistic effect of this double cross-linking structure and the film-forming mechanism, a 3D sheet DC-PBI-G high-temperature proton exchange membrane that can be stably serviced at high temperature was constructed. The authors systematically studied the structure, chemical and mechanical stability, phosphoric acid retention, proton conductivity, and fuel cell properties of the membrane and the control groups obtained by different preparation methods, including the phosphate-bridged single cross-linked gelatinous HO-PBI-G, the non-crosslinked gelatinous p-PBI-G, and the dense m-PBI membrane (m-PBI-D) prepared by the traditional organic solvent casting method.
Figure 2. The analysis of the comprehensive performance and enhancement mechanism of the DC-PBI-G membrane shows that the key to the smooth operation of the proton exchange membrane above 200oC mainly depends on the effective retention of phosphoric acid and the significant improvement of the creep resistance of the DC-PBI-G membrane. The authors performed 31P NMR and high-temperature acid retention tests on phosphoric acid doped membranes. The results showed that at 240°C, DC-PBI-G successfully anchored the confined domain of phosphoric phosphomolecule, thereby effectively inhibiting its dehydration and condensation, and the retention rate was as high as 96%. In addition, compared with single cross-linked membranes and non-cross-linked membranes, the double-crosslinked DC-PBI-G has additional proton transport channels and a more continuous three-dimensional hydrogen bond network, and the proton conductivity reaches 0.348 S/cm at 220°C, and the proton conductivity can still be stable at 0.241 S/cm after 100 h of testing at 240°C, with a retention rate of 72%.
Figure 3. Output performance and long-term operation stability of high-temperature proton exchange membrane fuel cellsFinally, the authors conducted a comprehensive performance test of high-temperature fuel cells based on double-crosslinked DC-PBI-G membranes. At 200°C and 240°C, the peak power density is as high as 1480 W/cm² and 1302 mW/cm² respectively (hydrogen/oxygen, no back pressure, no additional humidification). In addition, the membrane-based fuel cell also achieves excellent output performance with a peak power density of 636 mW/cm² under the conditions of methanol reforming/oxygen and 240°C reaction. In the high-temperature endurance test at 220°C, the voltage attenuation rate of the DC-PBI-G film was significantly lower than that of other films, and was only 0.27 mV/h after 250 hours of testing. In addition, the DC-PBI-G membrane also performs well in low temperature environments. The fuel cell achieves a peak power density of 443 mW/cm² at 40°C and maintains an extremely low attenuation rate (8.97 μV/h). Therefore, the DC-PBI-G membrane exhibits excellent fuel cell performance and operational flexibility under both high and low temperature conditions, bringing new breakthroughs to the further development of high-temperature fuel cell technology. SummaryThe authors successfully prepared a DC-PBI-G proton exchange membrane with three-dimensional sheet structure by combining the polyphosphorylsol-gel process combined with the construction of a double cross-linked network, which significantly improved the phosphoric acid stability, proton conductivity, creep resistance and fuel cell performance. This innovative research breaks the service limitations of conventional high-temperature proton exchange membrane fuel cells, provides new materials and new methods for the research and development of high-performance proton membranes, and is expected to solve the bottleneck of industrial development caused by the production, storage, transportation and filling of hydrogen, which has scientific significance and application value for the realization of the national "double carbon" strategic goal. Original link https://www.nature.com/articles/s41467-024-47627-4 Source: Frontiers of Polymer Science