Preparation And Investigation Of Self-humidifying Membrane Electrode Assembly For Proton Exchange Membrane Fuel Cell

With the drying up of fossil fuels and the serious problem of environmental pollutioncaused by the combustion of fossil fuels，more and more attention has been paid to thedevelopment of new energy and new energy utilization technology. Proton exchangemembrane fuel cells （PEMFCs） have becoming attractive due to their advantages, such ashigh power density, zero or low exhaust, and quick startup at low temperature, etc. Membraneelectrode assembly （MEA） is the key component of PEMFC, it not only plays a key role forthe performance of the PEMFCs, but also for the reducing the cost of the fuel cells, as well asaccelerating the commercial applications. The commercial Nafion series membranes producedby Dupont company （USA） are commonly applied to the electrolyte because of its goodmechanical and thermal stability, as well as its high proton conductivity at100%relativehumidity （RH）. Actually, Nafion ionomer is also used in the catalyst layer as a binder andproton conductor. However, the proton conductivity of Nafion depends strongly on the watercontent; dehydration of the Nafion membrane will lead to a series of detrimental effects on thePEMFC’s performance. The proton conductivity of Nafion membrane will decrease and theohmic resistance of the cell will increase under low-humidity conditions, causing rapid decayin cell performance. Thus, in terms of the actual operating conditions of a PEMFC, anexternal humidifier has to be used to maintain the water content of the cell system. In otherwords, the reactant gas must pass through the external humidifier and absorb water vaporbefore entering the cell. Therefore, this approach makes the cell system complicated, makesits energy efficiency and volume efficiency reduced, and makes the cost of fuel cell increased,thereby hinders the commercialization of the fuel cells. Indeed, the development of aself-humidifying membrane electrode assembly （MEA） has become one of the most importantsubjects in the PEMFC field. It is a most important aim for PEMFC to realizeself-humidification or no humidification, which not only solves the problem of watermanagement in PEMFC, but also decreases the cost and self energy consumption of PEMFCsystem by reducing the external humidifier. Summarily, the development of self-humidifyingMEA is extremely significant for the development and commercialization of proton exchangemembrane fuel cell. In this thesis, the self-humidifying MEAs were prepared by several new methods, includingadding hydrophilic organic polymer in catalyst layer, adding hydrophilic organic polymer andinorganic oxide in the catalyst layer, adding hydrophilic organic polymer in double cathodecatalyst layer, as well as preparing a self-humidification composite membrane by alayer-by-layer assembly methods.Firstly, we prepared a novel self-humidifying membrane electrode assembly （MEA） withaddition of polyvinyl alcohol （PVA） as the hygroscopic agent into anode catalyst layer wasdeveloped for proton exchange membrane fuel cell （PEMFC）. The effect of the variety ofhydrophilic organic polymer, addition amount and the relative humidity of anode and cathodewere investigated. We found that the performance of self-humidifying MEA prepared byusing PVA as hygroscopic agent is better than using other organic polymers. The waterretention ability of self-humidifying MEAs was enhanced by adding PVA in anode catalystlayer. It is interesting that the performance of MEA PVA5hardly changes even if the relativehumidity （RH） of both the anode and cathode decreased from100%to34%. The MEAsshowed good self humidification performance, for the sample with PVA addition of5wt.%（MEA PVA5）, the maximum power density can reach up to623.3mW.cm^-2, with currentdensities of1000mA.cm^-2at0.6V and600mA.cm^-2at0.7V respectively, at50oC and34%of relative humidity （RH）. The MEA PVA5also showed good stability at low humidityoperating conditions: keeping the MEA discharged at constant voltage of0.6V for60hours at34%of RH, the attenuation of the current density is less than10%, whilst for the MEAwithout addition of PVA, the attenuation is high up to80%within5hours.Secondly, the activity of catalyst increased with the increasing of cell temperature, as wellas heat and water management would be simplified at an elevated temperarue. To decrease theconflict between improving activity of catalyst and maintaining Nafion with a good protonconductivity at an elevated cell temperature under low-humidity conditions. A novelself-humidifying membrane electrode assembly （MEA） has been successfully prepared bysimultaneously adding both a hydrophilic organic polymer （polyvinyl alcohol, PVA） and aninorganic oxide （silica） to the anode catalyst layer. Compared to the self-humidifying MEA byadding only a hygroscopic substance （PVA or SiO₂） in anode catalyst layer, this MEA showedexcellent self-humidification performance under low-humidity conditions. A sample （MPS3） containing3wt.%PVA and3wt.%silica in the anode catalyst layer achieves a currentdensity as high as1100mA.cm^-2at0.6V, and the highest peak power density was780mW.cm^-2, operating at60oC and15%relative humidity for both anode and cathode. Theperformance of the self-humidifying MEA was hardly changed even if the RHs of both theanode and cathode decreased from100%to15%at60oC. Appropriate to improve the celltemperature is advantageous to the promotion of cell performance. The sample also showedexcellent stability at low-humidity: after30h of continuous operation under the sameconditions, the current density decreases just slightly, from1100mA.cm^-2to ca.900mA.cm^-2,whereas with MEAs to which only PVA or silica alone had been added, the current densitiesafter30h is just700mA.cm^-2and800mA.cm^-2, respectively.Furthermore, a self-humidifying MEA was prepared by adding hydrophilic organic polymer（PVA） in the inner layer of the double cathode catalyst layer. The effect of adding mode ofPVA in the cathode, the addition amount, the cell temperature and the relative humidity ofanode and cathode were investigated. Compared to the MEA by adding PVA in the singlecathode layer （SC-3PVA） and adding PVA in the outlayer of double cathode layer（DC-3PVA-outer）, the concentration polarization was increaed by adding PVA in the innerlayer of double cathode layer （DC-3PVA-inner）, but the back diffusion of water from cathodeto anode was enhanced. At a cell temperature of60oC and30psi, the current density ofDC-3PVA-inner at0.6V can achieved900mA.cm^-2at20%RH, which is2.25times higherthan that of DC-3PVA-outer and4.5times higher than that of SC-3PVA.Finally, a novel self-humidifying composite membrane Nafion/（PANI/SiO₂）n was preparedby layer-by-layer assembly using doped PANI and SiO₂as the raw substances. We suggestedthat the doped PANI and SiO₂assemble on the Nafion membran in a form of electrostaticattraction. The composite membranes were characterized by Digital photos,Thermogravimetric analysis （TG）, Fourier transform infrared spectroscopy （FTIR）,Ultraviolet-visible absorption spectrum （UV-vis）, X-ray diffraction （XRD）. We found that thedoped PANI and SiO₂can be successful absorbed on the Nafion membrane, and theadsorption quantity of PANI is proportional to the assemble layer. The proton conductivity ofcomposite membranes gradually decreased with the increaing of （PANI/SiO₂） layers byelectrical conductivity tests. When the （PANI/SiO₂） layers is7, the cell performance of self-humidifying MEA is the best in the low humidity condition.