Key Problem Study Of Alkaline Polymer Electrolyte Fuel Cells

Fuel cell,as an efficient and clean energy conversion device,is considered to play a key role in the construction of renewable energy-based society in the future.Among which the most mature one is proton exchange membrane fuel cell(PEMFC),it has met preliminary performance and stability requirement for application after decades of study.But its acid environment makes catalysts development restricted to Platinum-group metals(PGM),resulting very high cost.Alkaline polymer electrolyte fuel cell(APEFC)retains the advantages of PEMFC while making non-PGM catalysts utilizing possible.The development of key material,alkaline polymer electrolyte(APE),has made significant progress in recent years and drives further development of APEFC.Quaternary ammonia poly(N-methyl-piperidine-co-p-terphenyl)(QAPPT)is a newly developed APE with excellent ion conductivity,stability and mechanical properties.This dissertation conducted in-situ characterization methods establishment and key problem study of APEFC based on QAPPT,further promoted the development of APEFC.This dissertation firstly established in-situ electrochemical impedance spectrum(EIS)and electrochemical based measurement methodes for APEFC,laid down the foundation for further APEFC study.Key properties of QAPPT was measured in APEFC,showed that QAPPT has low gas permeation rate and its ion conduction has high water content dependence.A record-breaking peak power density(PPD)of 3.4 W/cm² was achieved with optimizations based on these findings.The sluggish hydrogen oxidation reaction(HOR)kinetics in alkaline causes extra issue for APEFC,previous research showed alloying Pt with Ru can largely improve its HOR activity and ensuing APEFC performance.With fuel cell and electrochemical study,we found Pt possesses higher HOR activation energy,resulting higher performance improvement when operation conditions elevated.Electrochemical analysis showed hydrogen binding energy(HBE)is the main HOR activity descriptor in alkaline,and HOR in alkaline processes through Tafel-Volmer route and Heyrovsky-Volmer route simultaneously.A PPD of 1.7 W/cm² was further achieved with an anode PGM-loading of 0.1 mg/cm²,indicating low anode PGM-loading is feasible in APEFC.Carbonation forms in APEFC when air is used as oxidant.This dissertation found APEFC performance halved while stability remains unchanged after carbonation.And carbonation overpotential is much larger than ionic resistance increase.Theoretical analysis shows no thermodynamic potential difference is formed between anode and cathode after carbonation.We further inferred from experimental results that HOR rate is largely reduced after carbonation and an APEFC carbonation model was established.Further H₂-pump measurements in APEFC confirmed our speculation,while HOR rate remain unchanged in carbonate solution.A key difference between poly-electrolyte and electrolyte solution is observed for the first time and deepened our understanding in electrode/poly-electrolyte interface property.Lowing PGM-loading and achieving non-PGM catalysts-based fuel cell is a main goal for APEFC development.A PPD of 2 W/cm² is achieved here with a total PGM-loading of 0.4mg/cm²(0.35 mg _Pt/cm²),reaching a highest catalyst specific power density in APEFC.Good performances were also obtained using Ru-based and Ir-based catalysts,further broadened catalyst options for APEFC.An APEFC performance of 1.2 W/cm² was achieved with the application of both transition metal oxide(TMO)and nitride(TMN)oxygen reduction reaction(ORR)catalysts.Structural transformation of these catalysts is observed in further stability study,offering guidance for further non-PGM ORR catalysts development.In the study of non-PGM HOR catalyst,the importance of antioxidation capability is addressed by comparing Ni₄Mo/C catalysts and carbon coated Ni@C catalysts.And a record-high PPD of 850 m W/cm²was achieved using the more oxidation-resistant Ni@C catalysts.This result casted new light on non-PGM HOR catalysts development.Stability is the biggest shortcomings for APEFC while detailed study still missing.Here EIS,X-ray photoelectron spectrum(XPS)and proton nuclear magnetic resonance(¹H-NMR)are combined to study QAPPT-based APEFC stability.Revealing that early stage voltage drop is caused by flooding.Analysis after stability test showed only minor degradation in membrane while catalysts layer(CL)ionomer cation degraded more than 20%.Further ¹H-NMR analysis of CL ionomer showed formation of phenol at QAPPT backbone.These degradation of ionomer results huge ionic resistance increase in catalysts layer,which is the main cause of APEFC performance degradation.This research established the methodology and gained important cognition for future APEFC stability improvement.