Study On The Preparation And Properties Of Enhanced Composite Proton Exchange Membranes

Energy shortage and carbon emissions are important issues which countries around the world continue to pay attention to.Proton exchange membrane fuel cells（PEMFCs）are a clean and efficient energy conversion device with great potential in reducing carbon emissions and alleviating the energy crisis.PEMFCs have been widely used in fuel cell electric vehicles（FCEV）,ships,aerospace,military equipment and other fields due to its high energy density,quiet operation and quick cold-start.Proton exchange membrane（PEM）,as one of the most important core components in PEMFCs,has the dual functions of conducting protons and separating reaction gases.Therefore,various physical and chemical properties of PEM have an important impact on the electrochemical performance of fuel cells.At present,the most commonly used proton exchange membrane in commercial PEMFCs is produced by Gore and Du Pont corporation.Due to the late start of PEM technology,the preparation technology of high-performance PEM is still at a low level in China.Therefore,there is an urgent demand for advanced preparation technology of high-performance PEM based on domestic perfluorinated sulfonic acid（PFSA）.Although PEM based PFSA has reasonable proton conductivity and mechanical strength,rapid decline of proton conductivity under high temperature and low humidity,rapid decay of lifetime under dry/wet cycling and high manufacturing cost,seriously impair the competitiveness of PEMFCs in new energy field.Therefore,the performance,lifetime and cost of PEM are three key challenges that restrict large-scale adoption of PEMFCs.At the same time,although domestic and foreign researches on the traditional materials of PFSA have been very in-depth,and membrane fabrication technology is relatively mature,there is still a big gap between domestic enterprises and foreign countries in the mass production and downstream application of PEM.To solve these issues,in this dissertation,PEM based on domestic PFSA was modified by inorganic fillers to improve its proton conductivity at low humidity,organic materials to enhance its mechanical strength at low thickness,and inorganic-organic composite modifiers to promote its mechanical/chemical durability under long-term cycling,and the mechanisms of these reinforcement materials were investigated.This dissertation focuses on the mass production technology of PEM and successfully applies the PEM with different characteristics to different technical fields.The main research contents and results are as follows:（1）The dissolution properties of domestic PFSA in six kinds of solvents and the effect of annealing temperature on the properties of the membrane were investigated.It was found that the smaller the interaction radius Ra between the Hansen solubilities of the solvent and the PFSA side chain,the better solubility of PFSA in the solvent,based on which we found a high solubility of PFSA in dimethyl sulphoxide.At the same time,the PFSA prepared by n-methylpyrrolidone（NMP）solvent had high proton conductivity and mechanical properties due to the high main chain mobility of PFSA in NMP.Additional,thermal annealing is a necessary process for the preparation of high-performance PEM,because it can significantly improve the crystallinity of the PEM,enhancing the mechanical properties and stability of the membranes.Finally,PEM based on the production process was successfully applied to the downstream product fields such as hydrogen water cup and hydrogen water machine.（2）The lamellar structured Cd0.85PS3Li0.15H0.15（Cd PSLi H）that contained Cd vacancies and had high proton conductivity was synthesized by an alkaline ion intercalation exchange method,and different proportions of Cd0.85PS3Li0.15H0.15/PFSA composite membranes were prepared by simple solution casting.The physical properties of 3 wt%Cd0.85PS3Li0.15H0.15/PFSA composite membrane was significantly improved.The water uptake of and in-plane swelling ratio were up to 30%and as low as 5%,respectively.Due to the abundance of proton donor centers in Cd0.85PS3Li0.15H0.15,the composite membranes showed excellent proton conductivity at different humidities.When the composite PEMs enabled the peak power densities of PEMFCs to reach 1.51,1.23 and 0.27 W cm^-2 at 80°C@100%RH,80°C@30%RH and 110°C@10%RH,respectively.（3）Various PVDF/PFSA blends were prepared with commercial polyvinylidene fluoride（PVDF）powders with five different molecular weights.The effects of different molecular weights of PVDF on PFSA membranes were systematically studied for the first time.The results showed that PVDF with higher molecular weight improved the bonding strength between PVDF and PFSA,and significantly improved the mechanical strength of the blend membrane.The tensile strength of 25 wt%PVDF5130-PFSA(Mw=1100000g mol^-1)with low thickness of 15μm reached 33.5 MPa,200%and 60%higher than those of pure PFSA and Nafion 211（thickness=25μm）.The power density of H2/O2 fuel cells based on 25 wt%PVDF5130-PFSA membrane was 1.17 W cm^-2,comparable to that of Nafion 211 membrane(1.13 W cm^-2).When applied in direct methanol fuel cells fed with10 M methanol,the composite membrane gave a power density of 51 m W cm^-2.（4）As a non-toxic and antioxidant interfacial modifier abundant in nature,tannic acid（TA）was used to enhance the mechanical/chemical durability of PEMs.The coated e PTFE by hydrophilic TA（TA-e PTFE）improved the adhesion strength between the modified e PTFE and PFSA,which rendered the composite membrane a higher filling content and mechanical strength.Furthermore,TA coated Zr O2（TA@Zr O2）nanoparticles were synthesized based the redox properties of phenolic hydroxyl groups in TA.It was added to the PFSA matrix to form TA@Zr O2/e PTFE reinforced composite membrane（RCM）.The TA@Zr O2 enhanced the bonding strength of PFSA polymer,proton conduction and oxidation resistance.The Fenton experiment showed that TA@Zr O2/e PTFE RCM had good chemical durability,and the mass loss was only 13.5%after 72 hours of Fenton reaction,23%and 12%lower than those of unmodified e PTFE/PFSA RCM and Gore-Select,respectively.Accelerated durability tests showed that the TA@Zr O2/e PTFE RCM still outputted current density of 1.3 A cm^-2 at 0.55 V after 5500 dry/wet cycling,higher than that of Gore-Select(1.21 A cm^-2)under the same condition.Finally,the composite membrane was successfully applied to the PEMFC battery system.The output power of the battery system reached 10 k W@530 m A cm^-2,and the fluctuation of voltage of single cell is small,showing good consistency.