Improving the water-handling of proton exchange membrane fuel cells

点击翻译按钮获取中文摘要

Hydrogen proton exchange membrane fuel cells (PEMFCs) are a key technology in enabling the transition to net-zero carbon energy. Hydrogen powered fuel cell stacks have been demonstrated to be eminently suitable for powering cars and especially trucks, because battery solutions are not viable for most larger vehicles. A recent Hydrogen Council report (link below) emphasises the synergy in using both battery electric vehicles (BEVs) and fuel cell electric vehicles (FCEVs) to have a major impact on transport derived CO2 emissions. The heavy duty truck market is expected to be the first to become fully commercialised FCEV application due to the less-demanding infrastructure network requirements.However, a key issue severely impacting the performance and efficiency of PEMFCs is water flooding. Water flooding occurs in the cathode-side catalyst layer whereby water generated in the cathode reaction condenses out in the pores of the catalyst support thereby blocking access for in-coming oxygen. Hydrophobic ionomer is added to the cathode catalyst layer to attempt to mitigate against flooding. In order to design the optimal formulation and fabrication process for the cathode catalyst layer, it is necessary to understand the relationship between the properties of the layer, especially pore size, pore connectivity and surface wettability/hydrophobicity, and the performance in the actual PEMFC. However, it has been found that current characterisation methods used are unable to distinguish sufficiently between the pore network and wettability characteristics of different layers to predict differences in their eventual PEMFC performance. Hence, new characterisation techniques are needed, and this is the aim of this project. The objectives of this project are to test three such candidate techniques for suitability. NMR spectroscopy and relaxometry of hyperpolarised (hp) krypton and xenon have been shown to be sensitive probes of pore size and the spatial distribution of hydro-phobic/-philic surfaces within a probed network. We, thus, intend to test hp Kr and hp Xe NMR techniques for determining the spatial distribution of ionomer and inception of water adsorption within cathode pore networks. The second candidate is adsorption calorimetry. Previous combined gravimetric and calorimetric studies of gas uptake in gas shales have suggested the technique can readily assess the spatial arrangement and juxtaposition of pore condensate in complex geometries, and its impact on mass transport. This technique will also be tried on catalyst layers. Finally, serial water adsorption and mercury porosimetry studies on catalyst pellets have revealed that this method can characterise the spatial distribution of the adsorbed water and the impact on percolation pathways within the pore network, and will thus be tested on cathode catalyst layers. We will also aim to develop a characterisation technique that can be used as a quality control, near-to-line measurement during manufacturing.