Metal–organic frameworks in proton-exchange membrane for intermediate-to-high-temperature fuel-cell applications: a review

A proton-exchange membrane (PEM) is a vital component in fuel cells as a solid electrolyte that conducts ions. The high cost and degradation of Nafion® membrane in low-temperature fuel cells limits the technology’s commercialization. The development of intermediate (IT-PEMFCs) to high-temperature (HT-PEMFCs) fuel cells operating within the range of 80–200 °C has made progress over the last few decades, and improvements in water management addressing the issues of low-temperature PEMFCs have been observed. However, these types of PEM fuel cells (IT-PEMFCs and HT-PEMFCs) still face considerable challenges, such as unsatisfactory performance stability at high temperatures. Particularly, in HT-PEMFC, despite the high acid doping level (ADL) in membranes as a potential means to improve proton conductivity, high ADL decreases the membrane’s mechanical stability. Recently, metal–organic frameworks (MOFs) have achieved satisfactory results in applications of PEM modification. This manuscript reviews the development in applying MOFs in improving the properties of composite membranes in IT- and HT-PEMFCs by using SPEEK and PBI, respectively. The synthesis strategies using MOFs in the PEM are discussed together with the electrochemical properties obtained. The success of incorporating of MOFs into PEMs could shed light on the synthesis of new-generation IT- and HT-PEMFCs, which could improve several properties such as mechanical and thermal stability, oxidative stability, and acid-retention capacity

Development of membranes for low and intermediate temperature polymer electrolyte membrane fuel cell

PhD ThesisProton 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 cmUK EPSRC Supergen progra

Composite proton exchange membranes for intermediate temperature fuel cells

Intermediate temperature (IT) proton exchange membrane fuel cells (PEMFCs) offer a future that does not rely on the burning of fossil fuels, but dictate durable and high performance component materials. At operating conditions of 120 °C and 50 % relative humidity (RH), composite proton exchange membranes (PEMs) offer increased performance due to enhanced water uptake and retention resulting from the hydrophilic filler material. This project aimed to relate measured data for composite PEMs with literature data on Nafion-graphene oxide (GO) PEMs. In order to achieve this, the membrane casting method was optimised and GO was synthesised in-house. A range of membranes were tested using a calibrated and optimised high temperature test stand. In-situ and ex-situ testing was carried out between 80°C and 120°C, and between 25 and 95 % RH. In contrast with some published data, this study found inconsistent trends between water uptake, ion exchange capacity, membrane resistance and single cell performance. Overall it was found that recast and composite membranes had higher in-plane resistance than Nafion 212, but that composite membranes with low filler loading had comparable in-situ performance to the commercial membrane. Further single cell optimisation is likely to result in further advances for composite PEMs

Poly(2,6-dimethyl-1,4-phenylene oxide)-based Semi-interpenetrating Polymer Network Proton Exchange Membranes for Direct Methanol Fuel Cells

Ph.DDOCTOR OF PHILOSOPH

Engineering Ionomer Materials for Addressing Ohmic Resistances in Electrochemical Desalination and Waste Heat Recovery

Water scarcity and energy availability present important challenges that need to be addressed in the coming centuries. In the front of water technologies, desalting brackish water is of extreme importance for thermal electric power plants, chemical manufacturing plants, and other industrial operations that treat and reuse their water utilities. Membrane capacitive deionization (MCDI) is an energy efficient desalination technique that has drawn attention from commercial entities. Most material research studies on MCDI focus on enhancing electrode performance while little emphasis is given to rationale design of ion-exchange membranes (IEMs). In this work, the ionic conductivity, permselectivity, and thickness for three different IEM chemistries (polyaliphatic, poly (arylene ether), and perfluorinated) were correlated to MCDI performance attributes. A 5-10-fold reduction in area specific resistance (ASR), with unconventional perfluorinated and poly (arylene ether) IEMs reduced the energy expended per ion removed in MCDI by a factor of two, compared to conventional electrodialysis IEMs. For further advancement in energy efficiency of operation, ohmic resistance of the spacer channel needs to be addressed for which, ion-exchange resins bound by a polymeric binder termed resin wafers were explored. A new class of ion-exchange resin wafers (RWs) fabricated with ion-conductive binders were developed that exhibit exceptional ionic conductivities - a 3-5-fold improvement over conventional RWs containing a non-ionic polyethylene binder. Incorporation into a resin-wafer electrodeionization stack (RW-EDI) resulted in an increased desalination rate and reduced energy expenditure. Overall, this work demonstrates that ohmic resistances can be substantially curtailed with ionomer binder RWs at dilute salt concentrations. With respect to energy, thermally regenerative ammonia flow batteries (TRBs) are an emerging platform for extracting electrical energy from low-grade waste heat (T \u3c 130 °C). Previous TRB demonstrations suffered from poor heat to electrical energy conversion efficiency when benchmarked against state-of-the-art thermoelectric generators. This work reports the highest power density to date for a TRB (280 W m-2 at 55 °C) with a 5.7× improvement in power density over conventional designs and thermal efficiency (ηth) values as high as 2.99 % and 37.9 % relative to the Carnot efficiency (ηth/C). The gains made in TRB performance was ascribed to the zero gap design and deploying a low-resistant, inexpensive anion exchange membrane (AEM) separator and implementing a copper ion selective ionomer coating on the copper mesh electrodes. The improved TRB power density and the use of a low-cost materials represent significant milestones in low-grade waste heat recovery using electrochemical platforms

Syntheses and Properties of Branched Sulfonated Poly(arylene ether ketone sulfone)s for Proton Exchange Membranes

Im Laufe von mehr als zwei Jahrzehnten haben sulfonierte aromatische Polymere, wie zum Beispiel sulfonierte Poly(arylether), ein erhebliches Potenzial für die Anwendung als Protonenaustauschmembranen (PEM) gezeigt. Die Einführung eines kleinen molaren Anteils eines trifunktionalen Monomers in den herkömmlichen Polymerisationsprozess dieser Polymere führt zu einem makroskopischen Netzwerk von Polymerverzweigungen. Im Vergleich zu ihren linearen Analoga weisen die hergestellten verzweigten sulfonierten Polymer-PEMs in der Regel eine verbesserte Protonenleitfähigkeit, Dimensionsstabilität und oxidative Stabilität auf. In diesem Beitrag wurde die Struktur-Eigenschafts-Beziehung von verzweigten sulfonierten Poly(aryletherketonsulfon)-PEMs mit dem Ziel untersucht, eine bessere Leistung als die kommerziellen Perfluorsulfonsäure (PFSA)-Polymer-PEMs zu erzielen. Die Auswirkungen von zwei grundlegenden Parametern, dem Verzweigungsgrad (DB) und der Ionenaustauschkapazität (IEC), auf die verzweigten Polymer-PEMs wurden durch die Synthese von zwei Serien von verzweigten Poly(arylenetherketonsulfonen) mit unterschiedlichem Gehalt an Verzweigungsstellen bzw. Sulfonsäuregruppen untersucht. Eine Erhöhung des DB und der IEC hatte einen ähnlichen Einfluss auf die meisten Eigenschaften der PEMs, mit Ausnahme deutlicher Unterschiede in der Dimensionsstabilität und der oxidativen Stabilität. Ein erhöhter DB führte zu isotropen Quellungsänderungen und erhöhter oxidativer Stabilität, während die IEC genau das Gegenteil bewirkte. Um die oxidative Stabilität zu maximieren und eine zufriedenstellende Protonenleitfähigkeit der PEMs zu gewährleisten, wurde eine Erhöhung des DB auf 12,5 % und eine anschließende Erhöhung der IEC als sinnvolle Option abgeleitet. Darüber hinaus wurde der Einfluss der Positionierung der Sulfonsäuregruppe innerhalb der verzweigten Polymerarchitektur auf die PEMs untersucht. Es wurde eine Serie von verzweigten Poly(arylenetherketonsulfonen) mit extrem dicht sulfonierten verzweigten Zentren synthetisiert, wobei ein maximaler DB-Wert von 12,5 % erreicht wurde. Im Vergleich zum analogen verzweigten Polymer mit relativ zufälliger Sulfonierung entlang der verzweigten Arme, führte die Konzentration von Sulfonsäuregruppen an den verzweigten Zentren zu einer verbesserten Dimensionsstabilität (in der Ebene und volumetrisch) und Protonenleitfähigkeit bei gleichem Wassergehalt. Die oxidative Stabilität war jedoch aufgrund der erhöhten Empfindlichkeit der Polymerkette deutlich geringer. Aufbauend auf den Ergebnissen der oben genannten Untersuchungen wurde eine Reihe von verzweigten Poly(arylenetherketonsulfonen) mit Sulfoalkyl-Seitenketten an den verzweigten Armen synthetisiert, wobei der DB-Wert konstant auf 12,5 % gehalten wurde. Die Flexibilität der Seitenketten verbesserte die Mobilität der Sulfonsäuregruppen in den hergestellten PEMs. Die PEM mit einem IEC-Wert von 1,73 meq g-1 zeigte eine mit Nafion 117, einer PFSA-Membran, vergleichbare Protonenleitfähigkeit und Dimensionsstabilität sowie eine gute oxidative Stabilität. Darüber hinaus übertraf die Protonenleitfähigkeit der PEM mit einem IEC-Wert von 1,96 meq g-1 deutlich die von Nafion 117, was zu einer besseren H2 /Luft-Einzelzellenleistung im Vergleich zu Nafion 117 führte. Zusammenfassend lässt sich sagen, dass die Polymerarchitektur der verzweigten sulfonierten Poly(arylenetherketonsulfone) schrittweise optimiert wurde, so dass letztlich eine bessere Leistung als bei kommerziellen PFSA-Membranen erzielt wurde. Die in dieser Arbeit vorgestellte Arbeit soll als Leitfaden für das strukturelle Design und die Anwendung verzweigter sulfonierter aromatischer Polymere als PEMs dienen