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Development of Composite Membranes for Polymer Electrolyte Fuel Cells Operated at Elevated Temperature and under Low Relative Humidity

Title
Development of Composite Membranes for Polymer Electrolyte Fuel Cells Operated at Elevated Temperature and under Low Relative Humidity
Authors
Kriansak Ketpang
DGIST Authors
Kriansak Ketpang; Shanmugam, Sangaraju
Advisor(s)
Shanmugam, Sangaraju
Co-Advisor(s)
Choi, Young woo
Issue Date
2015
Degree Date
2015. 8
Type
Thesis
Access Rights
The original item will not be provided upon request from the author
Keywords
Polymer electrolyte fuel cellsComposite membranePorous metal oxides nanotubesElectrospinningRelative humidity
Abstract
Polymer electrolyte fuel cells (PEFCs) which utilize hydrogen (H2) gas as a fuel have been one of the most promising power sources for electric vehicles because they have absolutely zero CO2 emissions and relatively high power generation efficiency which beneficially impacts on the mitigation of the serious global warming issue. In addition, PEFCs with the ability to operate at elevated temperature (100 - 200 oC) and under low relative humidity (RH) result in fast electrode kinetics, utilizing CO contained H2 fuel, simple thermal management and eliminating the need for an external humidification system, which can eventually reduce the PEFCs system cost. One of the major components which directly determine the PEFCs performance is the polymer electrolyte membrane (PEM). Nevertheless, a profound shortcoming of the current PEM is its low performance and poor durability when it is operated at elevated temperature and under low RH, which mainly results from the membrane dehydration and the presence of hydrogen peroxide (H2O2). Therefore, improvement of the performance and durability of the current PEM particularly the commercial perfluorosulfonic acid (Nafion®) and the sulfonated poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membranes, operated at elevated temperature and under dry conditions was the major objective of this thesis. This was achieved by incorporating mesoporous hygroscopic metal oxides (titanium dioxide, TiO2, cerium oxide, CeO2, and zirconium oxide, ZrO1.95,) nanotubes into PEM. Porous TiO2 nanotubes (TNT), CeO2 nanotubes (CeNT) and ZrO1.95 (ZrNT) were synthesized by calcining electrospun polyacrylonitrile nanofibers embedded with metal precursors under an air atmosphere. The nanofibers were prepared using a conventional single spinneret electrospinning technique. Their porous tubular morphology was observed by SEM and TEM analyses. HR-TEM results revealed a porous metal oxide wall composed of small particles joined together. The mesoporous structure of the samples was analyzed using BET. In addition, The tubular structure of the fabricated metal oxide nanotubes (MONT) was effectively controlled by adjusting the pyrolysing temperature. Furthermore, the diameter of the MONT was significantly controlled by changing the concentration of the precursor solution. The synthesized TNT, ZrNT and CeNT utilized as fillers were incorporated into Nafion ionomer and their PEFCs performances operated at elevated temperature and dry conditions were investigated. The smaller diameter size of the fillers resulted in the higher surface area which yielded the greater water absorption capability in composite membrane. The excellent water retention ability and facile water diffusion of the porous tubular fillers led to the effective enhancement of the proton conductivity under both fully humid and dry conditions. Compared to a commercial membrane (Nafion, NRE-212) operated under 100% RH at 80 oC, Nafion-TNT, Nafion-ZrNT and Nafion-CeNT composite membranes generated 1.3, 1.2 and 1.1 times, respectively, higher power density at 0.6 V. At 18% RH and 80 oC, the Nafion-TNT, Nafion-ZrNT and Nafion-CeNT composite membrane exhibited 3.4, 3.1 and 2.4 folds higher maximum power density, than the NRE-212 membrane. In addition, the power density (at 0.6 V) of Nafion-TNT, Nafion-ZrNT and Nafion-CeNT composite membrane was 1.9, 1.4 and 1.4 times higher than the NRE-212 membrane when operated at 110 oC under 65% RH. Compared Nafion-metal oxide nanoparticles composite membrane, the Nafion-MONT delivered better fuel cell performance under whole operating conditions. The remarkably high performance of the Nafion-MONT composite membrane is mainly attributed to the significant reduction of the ohmic resistance as well as the improvement of cathode catalyst utilization by incorporating MONTs, which greatly enhances the water retention and the water management capability through the membrane. Furthermore, Nafion-MONT membranes exhibit superior mechanical property. In addition, the incorporated MONT fillers were capable of effectively enhancing the Nafion membrane durability due to the excellent water management and the reversible redox reaction of MONT fillers. In addition to improving the performance and durability of Nafion membrane, the porous hygroscopic TNT was also incorporated into the aromatic SPESK membrane in order to enhance its performance and durability operated at elevated temperature and under low RH. In comparison with pristine SPESK and commercial NRE-212 membranes, the SPESK-TNT composite membrane exhibited approximately 3.8 and 2.8 times, respectively, higher power density at 0.6 V, when operated at 80 oC under 30% RH. At 110 oC, under 53% RH, the SPESK-TNT composite membrane generated 3.3 and 2.8 times higher power density at 0.6 V than that of the pristine SPESK and the commercial NRE-212 membranes, respectively. Compared to a SPESK-TiO2 nanoparticles composite membrane, the SPESK-TNT composite membrane also provided better fuel cell performance under both fully humid and dry conditions. Additionally, the SPESK-TNT composite membrane exhibited stable operating potential for 200 h under 30% RH at 80 oC. The enhanced fuel cell performance at elevated temperature and under low relative humidity is mainly the result of enhanced water management by the TNT filler in the membrane, improving cathode catalyst utilization, while significantly suppressing both ohmic and mass transport over potentials. Incorporation of the TNT filler also improved the thermal stability of the SPESK membrane. ⓒ 2015 DGIST
Table Of Contents
Chapter I Introduction 1-- 1.1 Global warming 1-- 1.2 Hydrogen as fuel 2-- 1.3 Fuel cells 3-- 1.4 Type of fuel cells· 5-- 1.5 Polymer electrolyte fuel cells (PEFCs) 7-- 1.6 Performance of PEFCs 10-- 1.7 Polymer electrolyte membrane (PEM) 12-- 1.8 Benefit of increasing operating temperature of PEFCs 14-- 1.9 Water management 17-- 1.10 Proton transport mechanism 18-- 1.11 Status of PEM materials 21-- 1.12 Porous metal oxide nanotubes 46-- 1.13 Objective of the present dissertation 49-- Chapter II Experimental methodology 52-- 2.1 Materials 52-- 2.2 Preparation of electrospun composite nonwoven web 52-- 2.3 Preparation of porous metal oxide nanotubes 53-- 2.4 Synthesis of SPESK copolymer 53-- 2.5 Preparation of Nafion-metal oxide nanotubes composite membranes 54-- 2.6 Preparation of SPESK-TiO2 nanotubes composite membranes 55-- 2.7 Metal oxide nanotubes characterizations 55-- 2.8 Membrane characterizations 56-- 2.9 Proton conductivity measurement 58-- 2.10 Fabrication of membrane electrode assemblies and fuel cell performance evaluation 59-- Chapter III Facile synthesis of porous metal oxides nanotubes 61-- 3.1 Introduction 61-- 3.2 Results and discussion 64-- 3.3 Summary 79-- Chapter IV 80-- 4.1 Porous titanium dioxide nanotubes modified Nafion composite membrane for fuel cells operated at elevated temperature and under low relative humidity 80-- 4.1.1 Introduction 80-- 4.1.2 Results and discussion 83-- 4.1.3 Summary 128-- 4.2 Porous zirconium oxide nanotubes modified Nafion composite membrane for fuel cells operated at elevated temperature and under low relative humidity 129-- 4.2.1 Introduction 129-- 4.2.2 Results and discussion 130-- 4.2.3 Summary 157-- 4.3 Durable Nafion-Cerium oxide nanotubes composite membrane operated in fuel cells at elevated temperature and under low relative humidity 158-- 4.3.1 Introduction 158-- 4.3.2 Results and discussion 161-- 4.3.3 Summary 181-- 4.4 Effect of porous metal oxides nanotubes modified Nafion composite membrane operated in fuel cells at elevated temperature and under low relative humidity 182-- 4.4.1 Introduction 182-- 4.4.2 Results and discussion 184-- 4.4.3 Summary 197-- Chapter V Sulfonated poly(arylene ether sulfone ketone) multiblock copolymer TiO2 nanotubes composite membrane for fuel cells operated at elevated temperature and under low relative humidity -- 5.1 Introduction 198-- 5.2 Results and discussion 201-- 5.3 Summary 220-- Chapter VI Conclusions 221-- Chapter VII References 224
URI
http://dgist.dcollection.net/jsp/common/DcLoOrgPer.jsp?sItemId=000002060722
http://hdl.handle.net/20.500.11750/1421
DOI
10.22677/thesis.2060722
Degree
Doctor
Department
Energy Systems Engineering
University
DGIST
Files:
There are no files associated with this item.
Collection:
Energy Science and EngineeringThesesPh.D.


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