Development of hybrid polyelectrolytemembranes for fuel cell applications
- Entwicklung von Hybrid Polyelektrolytmembranen für Brennstoffzellenanwendungen
Filipoi, Carmen Ioana; Möller, Martin (Thesis advisor); Pich, Andrij (Thesis advisor)
Aachen (2019, 2020)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2019
This thesis is concerned with preparation and investigation of composite polyelectrolyte membranes based on 3M perfluorosulfonic acid ionomer (PFSA) for fuel cell (FC) applications. The state-of-the-art PFSA membranes provide excellent electrochemical properties and high chemical stability. An operation under fuel cells conditions requires operating temperatures above 100 °C in order to limit poisoning of anode catalyst by trace amounts of CO and low fuel humidification. At such temperatures, the PFSA membranes show a dramatic decrease of proton conductivity due to dehydration and loss of connectivity between ionic domains. In spite a lot of work in this field, the goal of 0.1 S/cm for the proton conductivity at 120 °C and 50 % relative humidity according to the U.S. Department of Energy was not achieved. This work aims at improving the properties of 3M PFSA membranes by incorporation of hydrophilic inorganic phases with good water retention properties. The conversion of a silica precursor polymer, hyperbranched polyethoxysiloxane (PEOS) to silica in the 800 EW PFSA dispersion and subsequent film casting were used to prepare composite membranes with ultrasmall silica nanoparticles well distributed within the ionic channels of ionomer, facilitating the proton transport and preserving the connectivity of ionic channels at low humidity. Up to 10 wt.-% SiO2 was incorporated into the composite PFSA membranes. Transmission electron microscopy and small- and wide-angle X-ray scattering were used to study the complex membrane morphology. In situ generation of 2-12 nm SiO2 particles concomitantly with the membrane formation influences the membrane morphology, affecting the ionic clusters in a smaller extent, but disturbing the crystallinity of PFSA matrix. An attempt to correlate the morphology with the proton conductivity and the fuel cell performance of the membranes is reported. The proton conductivity of composite membranes, measured by electrochemical impedance spectroscopy, reaches maximum at silica contents of 3-5 wt.-% in the whole temperature range from 80 to 120 °C and implicit, with the reduction of humidity from 100 to 23 %. The polarization curves of the membrane electrode assembly (MEA) from the PFSA/SiO2 composite membranes also reveal an improvement of the fuel cell performance in the range of high current density at 3 wt.-% SiO2 content, implying improved water management by incorporating a small amount of ultrafine silica particles. It was found by 1H NMR relaxometry that two main types of water molecules are confined within the membranes, namely “bound” and “free” water. The proton mobility is increased at low silica content, whereas at high silica contents it is probably affected by a disruption of the connectivity of conduction pathways around bigger silica particles and a more tortuous structure of the membranes. In order to understand the changes in the micrometer-scale structure of membranes, an investigation of translational motion of water molecules within membranes was performed by means of non-invasive pulsed field gradient NMR diffusometry. An anisotropic orientation of the water channels reflected in the anisotropy of water diffusivity is found, with a preponderantly in-plane alignment. As the silica content increases and the diffusion time over which the diffusion is measured also increases, the anisotropy of water diffusion tends to decrease. We propose a diffusion-exchange model in the approximation of two water pools to describe the water transport and the exchange between the “bound” and “free” water states within membranes. The results show that the exchange rate from “free” to “bound” water takes place faster than the opposite exchange rate for all PFSA membranes. The sorption behavior from water vapor and the mechanical properties of composite membranes, also important in defining their FC performance, were also studied. The composite membranes containing 3-5 wt.-% SiO2 present a higher water uptake, which is correlated with the membrane morphology and the water diffusion. Upon heating, the diffusion rates increase in a bigger extent in the low and medium RH range than at high RH. The activation energy of water diffusion in membranes attains an inflexion point around 50 % RH, which is related to the inverse in the microstructure of PFSA membranes and the exchange between “free” and “bound” water states. During the fuel cell operation, the membranes should prevent direct mixture of hydrogen and oxygen gases. The analysis of free volume of membranes using 129Xe NMR spectroscopy and relaxometry was correlated with the gas barrier properties of membranes. At low silica content the free volume of pendant chain domains reaches a maximum, facilitating the formation of an extensive hydrogen bond network, while the free volume of backbone domains shows a minimum, i.e. enhanced gas barrier function. The analysis of hydrogen and oxygen permeation indicates improved barrier properties of membranes as the silica content increases. An attempt to orient the ionic channels perpendicular to the membrane plane was realized by casting the membranes onto glass substrates of different surface energy by treatment with silane coupling agents. No strong dependence of proton conductivity on the surface tension of the casting substrate is observed. However, a small reduction of proton conductivity is found when the membranes were cast onto hydrophobic substrates. Different additives were incorporated within the PFSA/SiO2 composite membranes with the aim of further increase of proton conductivity, the most important property of proton exchange membranes. The additive (phosphotungstic acid, zirconium phosphate, N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole) amount was varied relative to silica content, and the as-prepared PFSA/SiO2/Additive membranes were compared to PFSA/SiO2 membranes. It was observed that the use of additives together with PEOS hardly influences the proton conductivity of PFSA membranes, most probably because of high ion exchange capacity (IEC) of PFSA itself. The sulfonation degree of ionomer cannot be arbitrarily increased, because it becomes water soluble when a certain value is exceeded. Therefore, adding strong acid additive can be an efficient way to increase the IEC of composite PEMs. Silica nanoparticles bearing sulfonate groups were introduced to PFSA membranes by in-situ sol-gel technology, using thiol-functionalized PEOS as a silica source and subsequent oxidation with 30 % H2O2 solution. The higher IEC values of PFSA/sulfonated silica membranes compared to that of PFSA/silica membranes are a direct indication of the presence of extra sulfonic acid groups covalently attached to silica. It is found that the addition of modified silica with acidic functional group improves the proton conductivity, the water uptake and water self-diffusion within composite membranes presenting a more random alignment of ionic channels.