Phosphate-based Composite Electrolytes For High Temperature Proton-exchange Membrane Fuel Cells

Recently,the high temperature proton exchange membrane fuel cells（HT-PEMFCs）have been attracted wide attention due to its working temperature rang from 100 to 250°C,which can accelerate the electrode reaction kinetics and improve the anti-CO ability and activity of catalyst,responsible for implifying the hydrothermal management system and reducing the total costs of fuel cells.The key material of this kind of fuel cells is high temperature proton-exchange membrane（HT-PEM）,which requires excellent proton conductivity,good chemical and thermal stability,and appropriate mechanical strength in the operating temperature range.These properties are not available for commercial proton exchange membranes such as Nafion membranes,etc..In contrast,some phosphates related to the inorganic-organic composite electrolytes have been considered in this hot field.Herein,a series of phosphate-based inorganic proton conductors and inorganic-organic composite electrolyte membranes are developed for HT-PEMFCs by improving the proton conductivity,chemical and thermal stability,and mechanical strength of the electrolyte membranes.The performance of the PEMFCs with the as-received composite electrolyte membranes were examined in the working temperature range from 100 to250°C.In particular,an integrated system comprising of a methanol steam reformer（MSR）and an HT-PEMFC was demonstrated to test the power degradation at the same working temperature range.The main results are presented as follows:1)A PSi-sPEEK inorganic-organic composite electrolyte membrane composited with phosphosilicate sol-gel and sPEEK polymer was prepared by sol-gel method combined with mechanical milling process.The proton conductivity of the as-received membranes were examined in the elevated temperatures ranges of 180–250°C under ambient conditions of 47%relative humidity（RH）and without humidity.A proton conductivity of1.1×10^-3 S cm^–1 is achieved from the composite membrane of PSi-sPEEK at 250°C and 47%H₂O/N₂ humid atmosphere.2)To improve the proton conductivity,the multi-composite electrolyte membranes comprising of CsH₅（PO₄）₂,sPEEK and phosphosilicate sol were prepared,which exhibit the high proton conductivity of1.46×10^-2 S cm^-1 at 250°C and 47%H₂O/N₂ humid atmosphere.Due to the incorporation of the sPEEK polymer,the composite membranes with the inorganic content of up to 80 wt.%exhibit the tensile strength of 6.2MPa,indicating the good mechanical performance.3)Regarding to the chemical stability and thermal stability,the composite electrolyte membranes comprising of the molten CsH₅（PO₄）₂,PBI polymer and SiO₂ powder have been prepared for high proton conductivity of 6.48×10^-2 S cm^-1.Moveover,an HT-PEMFC equipped with such a Cs/（SiO₂/PBI）composite electrolyte membrane operating at 200°C showed an open–circuit voltage of 1.08 V,and a peak power density of 120 m W cm^–2.The duration test showed that the output of voltage had little degradation under the constant output current density of 100 m A cm^–2 during the continuous working for 150 h.4)An intergrated system equipped with an MSR and an HT-PEMFC was assembled for hydrogen generating and fuel cells operating at the same working temperature.For the comprasion,the performances of the fuel cells working with pure H₂ and the H₂ generated from the MSR were examined respectively.The results showed that the integrated system with the MSR-generating fuel has a peak power density of 192 m W cm^–2 at220°C,which was similar to the density of 196 m W cm^–2 obtained by pure H₂.The fuel cells duration test results showed that the integrated system working with the MSR-generating fuel performed the voltage degradation rate of 140μV h^–1 for 150 h continuous operation at 220°C,while that of the commercial HT-PEMFC with pure H₂ was 497μV h^–1 under the same test environment.This results indicate that the intergrated system in our case（HT-PEMFCs equipped with MSR）has the competiable performance for commercializing applications.