Proton exchange membrane fuel cells (PEMFCs) are promising electrochemical energy ® conversion devices, which are based on high cost materials such as Nafion membranes. The high cost and limited availability of noble metals such as Pt hinder the commercialisation of PEMFCs. The research described in this thesis focused on the development of composite materials and functionalised polymer membranes for intermediate temperature PEMFCs that o operate in the temperature range of 120 to 200 C. A higher operating temperature would enhance the kinetics of the cell compared to a perfluorinated polymer membrane based cell and provide a greater opportunity to use non-noble metal electrocatalysts. Inorganic–organic composite electrolyte membranes were fabricated from Cs substituted heteropolyacids (CsHPAs) and polybenzimidazole (PBI) for application in intermediate temperature hydrogen fuel cells. Four caesium salts of heteropolyacid, (CsHPMoO X3-X1240 (CsPOMo), CsHPWO(CsPOW), CsHSiMoO(CsSiOMo) and CsHX3-X1240 X4-X1240 X4- SiWO(CsSiOW)) and an ionic liquid heteropolyacid were used to form composite X1240 , membranes with PBI. The membranes were characterised by using SEM, FTIR and XRD. The CsHPA powders were nano-size as shown in the XRD and SEM data. The CsHPA/PBI composite membranes, loaded with HPO had high conductivity, greater than that of a 34 phosphoric acid loaded PBI membrane. Cs substituted heteropolyacid salt showed better enhancement of conductivity than that provided from ionic liquid heteropolyacid salt. The conductivity increased with an increase in the percentage of powder in the composite. The 30% -1 CsPOMo/PBI/HPO exhibited a conductivity of 0.12 S cm under anhydrous conditions 34 although its mechanical strength was the poorest, but still promising with a value of 40 MPa. The performance of the hydrogen fuel cell with composite membranes was better than that with a phosphoric acid-doped PBI membrane under the same conditions. The CsPOMo gave -2 the best power density, of around 0.6 W cm with oxygen at atmospheric pressure. A novel method was used to prepare poly (ethylene oxide)/graphite oxide (PEO/GO) composite membrane aimed for low temperature polymer electrolyte membrane fuel cells without any chemical modification. The membrane thickness was 80 µm with the GO content was 0.5 wt. %. SEM images showed that the PEO/GO membrane was a condensed composite material without structure defects. Small angle XRD for the resultant membrane results showed that the d-spacing reflection (001) of GO in PEO matrix was shifted from2θ=11º to 4.5 º as the PEO molecules intercalated into the GO layers during the membrane -1 preparation process. FTIR tests showed that the vibration near 1700 cm was attributed to the -COOH groups. The ionic conductivity of this PEO/GO membrane increased from 0.086 S -1 -1 cm at 25 ºC to 0.134 S cm at 60 ºC and 100% relative humidity. The DC electrical resistance of this membrane was higher than 20 MΩ at room temperature and 100% relative humidity. Polarisation curves in a single cell with this membrane gave a maximum power -2 density of 53 mW cm at temperature around 60 ºC, although an optimised catalyst layer composition was not used. Polybenzimidazole/graphite oxide (GO /PBI), sulphonated graphite oxide/PBI and ionic liquid GO/PBI composite membranes were prepared for high temperature polymer electrolyte membrane fuel cells. The membranes were loaded with phosphoric acid to provide suitable proton conductivity. The PBI/GO and PBI/SGO membranes were characterised by XRD which showed that the d-spacing reflection (001) of SGO in PBI matrix was shifted from 2θ=11º, meaning that the PBI molecules were intercalated into the SGO layers during the membrane preparation. A low acid loading reduced the free acid in the membranes which avoided water loss and thus conductivity loss. The ionic conductivities of the GO /PBI and -1 -1 SGO/PBI and ILGO/PBI membranes, with low acid loading, were 0.027 S cm , 0.052 S cm -1 and 0.025 S cm at 175 ºC and 0% humidity. Fuel cell performance with SGO/PBI -2 membranes gave a maximum power density of 600 mW cm at 175 ºC. A quaternary ammonium PBI was synthesised as a membrane for applications in intermediate temperature (100-200°C) hydrogen fuel cells. The QPBI membrane was loaded with phosphoric acid (PA) to provide suitable proton conductivity and compared to that of a similar PA loading of the pristine PBI membrane. The resulting membrane material was characterised in terms of composition, structure and morphology by NMR, FTIR, SEM, and −1 EDX. The proton conductivity of the membrane was 0.051 S cm at 150 °C and a PA acid loading of 3.5 PRU (amount of HPO per repeat unit of polymer QPBI). The fuel cell 34 -2 performance with the membrane gave a peak power density of 440 mW cm and 240 mW −2 cm at 175 °C using oxygen and air, respectively. Inorganic–organic composite electrolyte membranes were fabricated from CsHPMoO X3-X1240 CsPOMo and quaternary diazabicyclo-octane polysulfone (QDPSU using a polytetrafluoroethylene (PTFE) porous polymer matrix for applications in intermediate temperature (100-200°C) hydrogen fuel cells. The CsPOMo/QDPSU/PTFE composite membrane was made proton conducting using a relatively low phosphoric acid loading to provide the membrane conductivity without compromising the mechanical strength to a great extent. A casting method was used to build a thin and robust composite membrane. The resulting membrane materials were characterised in terms of composition, structure and morphology by EDX, FTIR and SEM. The proton conductivity of the membrane was 0.04 S -1 cm with a PA loading of 1.8 PRU (amount of HPO per repeat unit of polymer QDPSU). 34 -2 The fuel cell performance with the membrane gave a peak power density of 240 mW cm , at 150 °C and atmospheric pressure. A composite material for phosphoric acid (PA) loaded membrane was prepared using a porous polytetrafluoroethylene (PTFE) thin film. N, N-Dimethylhexadecylamine partially - quaternised poly (vinyl benzyl chloride) (qPVBzCl ) was synthesised as the substrate for the - phosphoric acid loaded polymer membrane. The qPVBzCl was filled into the interconnected - pores of a PTFE thin film to prepare the PTFE/qPVBzCl membrane. A SEM data indicated - that the pores were filled with the qPVBzCl . The PA loading was calculated to be on average 4.67~5.12 per repeat unit. TGA results showed that the composite membrane’s was stable at intermediate temperatures of 100°C to 200 °C. The composite membrane’s tensile stress was 56.23 MPa, and Young’s Modulus was 0.25GPa. The fractured elongation was 23%. The - conductivity of the composite membrane after PA addition (PTFE/qPVBzCl /HPO) 34 -1 -1 increased from 0.085 S cm to 0.1 S cm from 105°C to 180 °C. The peak power density of the H/O fuel cell, at 175 °C under low humidity conditions (<1%), with the22 PTFE/qPVBzCl /HPO 34 membranes was 360 mW cm.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:594520 |
| Date | January 2013 |
| Creators | Xu, Chenxi |
| Publisher | University of Newcastle upon Tyne |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10443/2123 |
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