Operando Studies Of Water Transport And Stability Mechanism In Alkaline Polymer Electrolyte Fuel Cells

Fuel cell,as a device that can efficiently convert hydrogen energy to electricity,is an important part of achieving the"carbon peaking and carbon neutrality"goals.After years of research,performance and durability of proton exchange membrane fuel cells(PEMFCs)have met the preliminarily requirements of the electric vehicle application.However,Nafion,the key material of PEMFC,is expensive,and its strong acidic environment requires the use of noble metal catalysts.The high material cost severely restricts the large-scale application of PEMFC.Alkaline polymer electrolyte fuel cells(APEFCs)retain the merits of using solid electrolytes,and the less corrosive alkaline environment provides the possibility of reducing material costs,thus the development of APEFCs have attracted extensive attention.The development of key material alkaline polymer electrolyte(APE)has made significant progress in recent years.However,their device performance and stability are still fall far from application requirements.One main issue is that in APEFCs,water is generated in anode and consumed in cathode,resulting in an unbalanced water distribution across the electrode during cell operation.Meanwhile,due to the restriction of device structure,it is difficult to get time-resolving water distribution information in fuel cell using conventional methods.At present,no suitable operando method has been developed for APEFCs water management study.To prepare for APEFCs water management study,we established an operando water management study method utilizing high-precision chilled mirror hygrometer and electrochemical impedance spectroscopy(EIS),which is suitable for various APEFCs operating conditions.This method can provide qualitatively/quantitatively measurement of the water transportation and distribution within APEFCs by monitoring the pressure of exhaust gas water vapor.Meanwhile,mass transport resistance can be quantified using distribution of relaxation time(DRT)analysis of the EIS.Using this method,influences from the gas flow rate,cell temperature and back-pressure on water transport in APEFCs were studied,and corresponding relationship between water vapor pressure fluctuation and DRT results was verified.In this thesis,it is shown that reducing the thickness of alkaline exchange membrane(AEM)will reduce the ionic resistance and increase the water transportation according to report,thus improving the performance of APEFC.Operando water management measurement was used to distinguish the influence from ionic resistance and water transportation on APEFCs performance.Combining water transport coefficient of AEM and the oprando measurement results,it is shown that the quaternary ammonia poly(N-methyl-piperidine-co-p-terphenyl)(QAPPT)exhibits excellent water transport property.Reducing membrane thickness further improves the water transportation rate.According to EIS-DRT analysis,the higher water transportation rate of the thinner membrane is the main contributor to the elevation of cell performance elevation.Based on these understanding,we achieved a PEMFC comparable peak power density(3.43W/cm~2)using ultra-thin QAPPT.A breakthrough performance of1.75W/cm~2 is also achieved under self-humidification mode(both anode and cathode fed with dry gas).These results revealed the critical role of water transportation in APEFCs.With operando measurement,it was found that the cell voltage drop at the beginning of APEFCs stability test is caused by the mass transport resistance increasing in the anode.Extra inactive components are introduced to the electrode to increase the electrode hydrophobicity when using conventional hydrophobic materials(such as polytetrafluoroethylene PTFE).Here we synthesized quaternary ammonium poly(arylene perfluoroalkylene)(QAPAF)ionomer with both hydrophobicity and ionic conductivity to precisely control the hydrophilicity/hydrophobicity of the three-phase reaction interface.Operando measurement results verified that this hydrophobic ionomer could avoid the generation of liquid water accumulation in the electrode thus alleviate the gas mass transport resistance caused by the anode flooding at the beginning of the APEFCs stability test.This results in a large 140m V cell voltage gain.With this improvement,the APEFCs is capable to be operated at a"zero anode emission"mode without humidified gas at both sides.In-situ EIS-DRT measurement proved that the hydrophobic ionomer can still avoid anode flooding with this operating mode,thus increasing cell stability.These results provide a new quidance for anode electrode structure design and standsas a practical operating solution for APEFCs.After the optimization of APEFCs water management optimization,its long-term stability needs to be further investigated.So far,APEFC lifetime is still limited by APE stability.However,most study focuses on APEM,while the effect of ionomer stability is largely ignored.To explore the effect of ionomer on APEFCs stability,we synthesized two ionomers(QATPN-TMA and QATPN-Pip)with the same main chain structure but different cation functional groups.The stability of cell using QATPN-TMA and QATPN-Pip as ionomer was distinctively different.Cation degradation was found in both ionomers using X-ray photoelectron spectroscopy(XPS),but the degradation subttle difference on degradation is insufficient to explain the drastic difference in performance degradation.It was confirmed the existance of catalysts layer ionomer loss,according to the benzene signal peak in ultraviolet-visible spectrum(UV-vis).From there,an experiment was set up to simulate the ionomer loss in catalysts layer.Combining with the ex-situ APE stability test results,it was found that although QATPN-TMA and QATPN-Pip have similar alkaline stability,the Pip cation has stronger interaction with water.Therefore QATPN-Pip ionomer suffers more severe loss,leading to more obvious cell voltage decay.It was further found that the ionomer loss would cause catalysts layer structural damage and severe exfoliation in the catalysts/membrane interface using scanning electron microscopy(SEM).By introducing Nafion to form ionic crosslinking and reducing ionomer ion exchange capacity(IEC),the catalysts layer swell and ionomer loss was limited.It keeps the structural integrity of the catalyst layer(CL)and improves the cell stability.APEFCs was able to discharge continuously for nearly 300h at 1A/cm~2.This result reveals the importance of ionomer to the CL structure,and provides a direction to improve the stability of APEFCs.