# Imaging Liquid Water in a Polymer Electrolyte Fuel Cell with High-Energy X-ray Compton Scattering
## 3 Experimental Details  
The CSI measurements were carried out on the High-energy Inelastic X-ray Scattering beamline (BL08W), SPring-8. The incident X-ray energy was 115 keV. Figure 2a shows the schematic drawing of the experimental setup. A PEFC was placed on the sample stage with the MEA in a horizontal (X-Y) orientation. The cathode was on the upper side. Cross-sectional images at the PEM layer, CL, and GDL were obtained after positioning the PEFC with a stepping motor along the Z direction. The incident X-ray beams were focused vertically to 10 \(\upmu\)m using a Ni compound refractive lens [31] and horizontally to 1 mm using an asymmetric Johan monochromator [32]. The Compton-scattered X-rays and fluorescent X-rays from the PEFC were detected by a two-dimensional CdTe X-ray detector (HEXITEC) through a tungsten pinhole with a diameter of 70 \(\upmu\)m. The scattering angle is 90 degrees. The pixel size of the detector is 25 \(\upmu\)m, and the number of the pixel is 80 \(\times\) 80 (6400 in total). The energy range of the detected X-rays is 4-200 keV, and the energy resolution is 1 keV at 60 keV. Since each pixel has this energy resolution, Compton-scattered X-rays and fluorescent X-rays can be separated, so that both CSI and XRF imaging can be performed by setting the region of interest (ROI) in the energy spectra. Here, we set the ROI with the energy range of 81.6-104.0 keV for Compton-scattered X-rays. The size of the MEA is 30 \(\times\) 30 mm. The observation area of the X-ray detector through the pinhole is 4.2 \(\times\) 4.2 mm. Since the horizontal width of the X-ray beams is 1 mm, the visible area is effectively limited (see Figure 2b). The left side of Figure 2c shows the perspective view of the effective area of all measured images from the X-ray detector, together with the channel and rib structure of the cathode-side separator. The right side of Figure 2c shows the perspective view of the model PEFC from the incident X-rays, displaying the stacked structure of GDLs, CLs, and PEM along the Z direction. The MEA was manufactured by FC Development Co, Ltd. (Tokyo, Japan) with an area of 3 \(\times\) 3 cm\({}^{2}\). The catalysts are TECRu(ONLY)E50 for the anode and TEC10E50E for the cathode (Tanaka Kikinzoku Kogyo, Tokyo, Japan) [33]. The loading weight of Ru of TECRu(ONLY)E50 is 0.61 mg/cm\({}^{2}\), and that of Pt of TEC10E50E is 0.60 mg/cm\({}^{2}\). The membrane is Nafion\({}^{\text{TM}}\) NR-212, and the GDLs are Toray Carbon Paper TGP-H-60. The thicknesses of the anode CL, cathode CL, PEM, and GDLs are 20, 20, 50, and 230 \(\upmu\)m, respectively. The PEFC was assembled by sandwiching the MEA using gas flow channels and stainless steel separators. Before the CSI measurements, the gas flow channels were purged using gas N\({}_{2}\) to replace the residual air, followed by aging for about 1.5 h. The supplied gases for the anode and cathode channels were H\({}_{2}\) and air, respectively. The flow rates of H\({}_{2}\) and air were 150 and 500 mL/min, respectively. The voltage was approximately 0.6 V during the aging with a current density of 500 mA/cm\({}^{2}\). The PEFC was heated to 80 \({}^{\circ}\)C and humidified to 85%RH. The temperature was measured using a thermocouple and the relative humidity was calculated from the temperature of the water tank for the humidification. The CSI measurements were performed with the following procedure. After aging, the PEFC was operated under open circuit (OC) with a current density of 0 mA/cm\({}^{2}\), and then operated under closed circuit (CC) with current density loadings. The loadings were increased to 100, 200, 300, 400, 500, 700, and 900 mA/cm\({}^{2}\) in this order. Here the current density is given as the geometric area. After the voltage became stable under each current density condition, the CSI measurement was carried out at the position of PEM, cathode CL, cathode GDL, and channel. During the operation, water is generated in the cathode CL and then transported to the channel through the cathode GDL. The measurement time was 1 h for each image.