# Imaging Liquid Water in a Polymer Electrolyte Fuel Cell with High-Energy X-ray Compton Scattering
## 4 Results and Discussion  
Figure 3 shows the images of Compton-scattered X-rays in the vicinity of the cathode CL at the different current densities. Since the width of the incident X-rays in the horizontal direction is 1 mm, the vertical width of the observed images in Figure 3 is 1 mm (seeFigure 2b). These images show the intensity distribution of the Compton-scattered X-rays from all compositions, including water, platinum, and carbon. The differences in the images between OC and CC are due to water generation by the chemical reaction (see Equation (1)) since the platinum and carbon contributions remain unchanged. In order to obtain the images of liquid water distributions, the Compton scattering images of OC are subtracted from those of CC. The subtracted images are shown in Figure 4, where positive intensity values are indicated in blue and negative values in white. The results show that the intensity of subtracted images increase with increasing current density up to 400 mA/cm\({}^{2}\), indicating that the amount of water is proportional to the current density. This fact demonstrates that X-ray Compton scattering is capable of imaging the liquid water distribution in a PEFC. Although the intensities are proportional to the current density up to 400 mA/cm\({}^{2}\), they are found to be nearly constant in the range of 400-900 mA/cm\({}^{2}\). This finding suggests that there is a threshold amount of liquid water that can be contained in the cathode CL. When it exceeds the threshold, liquid water flows into the GDL or channel. Figure 5a shows the Compton scattering images in the cathode GDL. The fibrous structure of carbon fibers is observed in Figure 5a. Figure 5b-g show the subtracted Compton-scattering images at the different current densities of 100-700 mA/cm\({}^{2}\). The case of 900 mA/cm\({}^{2}\) is not displayed because of unstable voltage. In 100  
Figure 4: Subtracted Compton-scattering images in the vicinity of the cathode catalyst layer (CL) at different current densities. The images display the liquid water distribution at different current densities. Figure 3: Compton scattering images in the vicinity of the cathode catalyst layer (CL) at different current densities. amount of liquid water is negligibly small in the GDL. On the other hand, liquid water is observed in a limited area at 500 and 700 mA/cm\({}^{2}\). The observed liquid water is considered to be the one which flows from the cathode CL to the channel. The water flow path is located at the void space of the GDL. Figure 6a shows the image of Compton scattered X-rays in the PEM. Since the membrane consists of bulk polymers, significant structures are not observed with the present spatial resolution. Figure 6b-h show the subtracted images at the current densities of 100-900 mA/cm\({}^{2}\). Additional liquid water generated by the chemical reaction (see (1)) is negligibly small at all current densities. An explanation for this result is that the water uptake in the membrane is already saturated by the humidified supply gases at the OC condition. Figure 7a,b show the images of Compton-scattered X-rays in the layer of the cathode channel and rib at 0 and 500 mA/cm\({}^{2}\), respectively. Figure 7c shows the subtracted images at the current density of 500 mA/cm\({}^{2}\). In the vertical center of Figure 7a,b, the structure of the gas flow channel with a width of 1 mm wide is observed, together with ribs above and below it. Accumulation of liquid water is observed on the wall surface of the ribs. This observation is consistent with the results of the combined, small-angle neutron scattering, and neutron imaging [20]. Figure 5: (**a**) Compton scattering image of the cathode GDL; (**b–g**) subtracted images at different current densities. A water flow path is observed in a limited area (see the yellow circle) at the current density of 500 and 700 mA/cm\({}^{2}\). Figure 6: (**a**) Compton scattering image of the PEM; (**b–h**) subtracted images at different current densities.