(Invited) Synchrotron X-Ray and Pulsed Neutron Imaging of Water Transport Distribution in Fuel Cells

Abstract

In order to realize a sustainable energy society, it is necessary to optimize energy and effectively utilize it by combining an electric grid and a hydrogen grid instead of the conventional fossil fuel-dependent type. Hydrogen has a high affinity with renewable energy and can be produced from various primary energies, so it is positioned as a promising energy in the future. We position fuel cell vehicles (FCVs) powered by hydrogen as the ultimate eco-friendly car, and are making vigorous efforts to develop high performance/reliability and low cost FCVs.Polymer electrolyte fuel cells (PEFCs) convert the chemical energy stored in hydrogen into electricity, with water as the only by-product. The inherent advantages of a PEFC include high electric energy efficiency and power density, low operating temperature and fast start-up, along with reduced environmental impact. PEFCs are regarded as a clean and efficient powertrain technology for automotive applications, serving as replacements for conventional internal combustion engines.PEFCs consist of an electrolyte membrane sandwiched between an anode and a cathode, and both the anode and cathode have laminated structures composed of a catalyst layer (CL) and gas channels in contact with a gas diffusion layer (GDL). When hydrogen and oxygen flow through the gas channels, an electrochemical reaction occurs in the CL, producing a by-product of water in addition to electrical power. The water generated by power generation then passes through the cathode GDL to the gas channel, and is finally discharged to the outside of the vehicle.The performance of fuel cells is greatly affected both by materials such as catalysts and electrolytes, and by how efficiently water generated by power generation is discharged, that is the essential role in water management function. To achieve a high-power density, it is really important to design sophisticated water management from water generation on the catalyst of FC to drainage outside the vehicle. Regarding the water management in PEFCs, it is necessary to manage it on a very wide scale from nanometer to millimeter. Based on the viewpoint that FC multiscale analysis linking materials, devices and vehicles is essential to achieve high performance by water management, we are currently constructing our original multiscale analysis combining quantum beam analysis using X-ray and neutron which enables liquid water visualization, as shown in Figure.In this presentation, I will exhibit two topics. First, I report improving water management in GDL through microporous layer (MPL) modifications [1]. Advanced 4D operando X-ray imaging (3D structure plus time) was employed to analyse the water content in GDLs, and demonstrated that the superior performance of cells with large MPL pores is due to the efficient formation of water pathways. The second is pulsed neutron imaging for differentiation of ice and liquid water distribution [2]. Energy-resolved neutron imaging is a methodology capable of distinguishing between liquid water and ice, and is effective for investigating ice formation in PEFCs operating in a subfreezing environment. We have enabled the observation of liquid water/ice distributions in a large field of view (300 mm × 300 mm) by manufacturing a sub-zero environment chamber that can be cooled down to -30 oC, as a step towards in situ visualition of full-size fuel cells.We hope that these studies will lead to the development of next generation FCVs. Figure 1