Search

To realize a more powerful lithium-air battery, it is necessary to clarify and improve the O2 transport phenomena in the cathode. In this study, the effect of dissolved O2 concentration in the electrolyte and precipitates on the cathode performance of a lithium-air battery is investigated. Using a beaker-type cell and micro-bubble generator to dissolve O2, O2 concentration in the electrolyte is varied. Discharge performance is improved by increasing the dissolved O2 concentration in the electrolyte. However, concentration overpotential dominates discharge at high current density. Additionally, precipitate and gas bubbles are observed in the porous cathode. The formation of these discharge products depends on local O2 concentration in the electrolyte. Concentration overpotential is lowered by using an electrolyte flow cell, which decreases transport resistance in the cathode.

The catalyst layer (CL) of polymer electrolyte fuel cells plays a critical role in their operation and also the production process of CL is of significant importance in terms of the cell performance as well as the system cost. Herein, we propose a novel approach to investigate the characteristics of catalyst inks by using magnetic resonance imaging (MRI) combined with nuclear magnetic resonance (NMR) spectroscopy. We developed a technique to acquire NMR spectra from a selected volume of a liquid sample and determine the local diffusion coefficients of component molecules. It was found that the diffusion coefficients of both water and n-propyl alcohol (NPA) remarkably changed depending on the mixuture composition, which indicates the change in molecular microstructure. In addition, we obtained MRI images of catalyst inks and the precipitated bed of carbon particles was visualized. We found the dependence of the local diffusion coefficient of water in the particle bed region on the diffusion time, which suggested that the molecular diffusion of solvent molecules were distracted by the closely-packed carbon particles. Furthermore, 19F-NMR spectroscopy of Nafion dispersions suggested the change in the dispersion state of Nafion molecuels. We demonstrated that our approach provided a unique capability to investigate the catalyst ink and its decal process. It will be proved that this technique is useful in investigating many samples prepared at various conditions and better understand the decal process.

Insufficient supply of oxygen (O2) to the cathode Pt catalyst layer can be caused by transport resistance, and is a major problem in proton exchange membrane fuel cells. To clarify O2 transport phenomena, a non-destructive technique for measuring the O2 concentration is required. In this study, the O2 concentration at the catalyst layer was measured under operating conditions, using a tiny optical fiber and oxygen indicator. The O2 concentration decreased continuously with decreasing cell voltage. It was considered that a decrease in size of the reaction area was caused by water accumulation under the rib. Local O2 concentration measurements suggested that power generation was concentrated at the channel region.

In order to achieve high current density in PEFCs, it is necessary to suppress the increase in concentration overvoltage caused by the liquid water in the gas diffusion layer (GDL), especially in substrate layer. This study presents a thin self-standing microporous layer (MPL) to reduce liquid water accumulation in the GDL. The self-standing MPL was made by filling carbon particles into a carbon mesh specifically prepared for this study. The Cell performance was improved in the case of the self-standing MPL at high current density. Moreover, results of X-ray visualization showed that liquid water accumulation in the self-standing MPL was reduced compared to the case of the SIGRACET®️ 25BC.

To clarify the behavior of liquid water in polymer electrolyte fuel cell (PEFC), X-ray computed tomography (XTM) is used. In the past, 3D visualization experiments of liquid water in substrate layer and microporous layer (MPL) were conducted. However, 3D visualization of liquid water in catalyst layer (CL) has not been conducted yet. It is important to clarify liquid water in CL because water is generated in CL. We improved two things. First, we used the CL with low platinum percentage because its X-ray absorption coefficient is low. Second, 20.5-hour CT scan was conducted to improve spatial resolution. Therefore, we succeeded the detection of liquid water in CL.

In proton exchange membrane fuel cells (PEMFCs), appropriate water management is critical to achieve high power density operation with increased robustness. For this purpose, soft X-ray radiography has been used in this study because it has been proved to be a powerful incitement for observing liquid water in PEMFCs. In this study, we focused our attention on micro porous layers (MPLs) with vapor growth carbon fiber (VGCF) impregnated in the MEA. In the experiment, we fabricated VGCF MPL coated on gas diffusion substrate (SIGRACET®24BA). We also used commercial gas diffusion layer (SIGRACET®24BC) for comparison. In situvisualization of liquid water distribution in both MEAs by soft X-ray radiography clearly shows their difference. The water tends to accumulate both at GDS and MPL for 24BC; the water tends to accumulate at GDS for VGCF.

In situ visualization of liquid water distribution in PEM fuel cell with and without MPL and its effect on the fuel cell performance was investigated. The liquid water accumulation at the interface of catalyst layer (CL) and gas diffusion media (GDM) was observed to have large impact on the performance of fuel cell without MPL. The MPL reduces the liquid water at the interface of CL and GDM and enhance the performance of the fuel cell with MPL. The effect of gas flow rate on the liquid water distribution and performance of the fuel cell with MPL was also studied. The higher water content at the cathode MPL/GDL boundary was observed and corresponds for the large decreasing in cell voltage when lower the gas flow rate.

In situ observation of liquid water accumulation in PEMFCs operating at 30ºC, 50ºC, and 70ºC was performed by using high-resolution soft X-ray radiography. A less amount of liquid water in the MPL and GDL was observed in the elevated operating temperature. It is suggested that water generated in the electrode is mainly transported as vapor at higher temperature conditions investigated. The liquid water accumulation was significantly smaller in the MPL than in the GDL, independently observed in all operating temperature.