Porous transport layers (PTL) in polymer electrolyte membrane electrolysis cells ensure gas/liquid mass transport at the anode, and determine the feasibility of high current density operation up to the limiting current density (LCD). This study examined the effect of anode PTL properties on LCD by varying porosities of 45 %, 56 %, and 72 %. Electrochemical characterizations suggest that the mass transport limitation leads to an abrupt increase in the electrolysis voltage and membrane dehydration, and also indicate that the PTL with the largest porosity of 72 % achieves the largest LCD compared to the other two PTLs with smaller porosities. Moreover, visualization experiments with a high-speed microscope captured the bubble detachment sites at the interface between flow channel and PTL, and reveal that a larger PTL porosity increases the density of the detachment sites. Additionally, a correlation emerges between the bubble flow regimes at the anode channel and periodic changes of the current density. A numerical analysis incorporating the visualized bubble behavior into the boundary condition successfully predicts the dependence of the porosity on the LCD, in agreement with the experimental results. The analysis also demonstrates that a larger porosity contributes to reducing the gas saturation and enhancing the LCD.

This study elucidates the effects of current density on membrane degradation under combined mechanical and chemical stress tests. Relative humidity (RH) cycling tests using hydrogen gas and air are conducted on a polymer electrolyte membrane fuel cell based membrane NRE211 at the open circuit voltage (OCV), 0.05 and 0.3 Acm(-2) conditions. The different current density conditions result in different in-plane membrane stresses and H2O2 formation rates during the test. After every 200 RH cycles, membrane integrity is assessed via the hydrogen crossover rate and OCV. Furthermore, catalytic combustion is analyzed during OCV measurement using a thermal imaging method employing high-transmittance glass at the cathode side. The membrane failed after 1600, 1800, and 2200 RH cycles under the OCV condition, 0.05 of 0.3 Acm(-2), respectively. The vigorous membrane degradation under OCV conditions can be attributed to higher mechanical stress and H2O2 formation rate. Hotspots created owing to the combustion between the crossover hydrogen and air were successfully captured, with a maximum temperature rise ranging from 15 to 16 degrees C compared with a given cell temperature of 80 degrees C. Moreover, a post-mortem analysis (SEM imaging) revealed the presence of pinholes, through-membrane cracks, and membrane thinning at the hotspot locations.

Introduction The PEM electrolysis cell (PEMEC) exhibits great potential to produce hydrogen gas. However, the high cost associated with cell components, such as the iridium oxide catalyst and titanium porous transport layer, poses a challenge to the commercialization of PEMEC. So, continued research is needed to improve the cost-effectiveness of PEMEC. Operation at high current densities can minimize the electrode area and reduce capital expenses. However, high current density operation is limited by limiting current density (LCD), which causes a sharp increase in electrolysis voltage. At high current densities, the generation of oxygen gas bubbles creates additional flow restrictions to the water supply and increase overvoltage. Although previous studies have achieved relatively high current densities [1-2], the limit of operating current density and its mechanism still remain unknown. In this study, it reveals the impact of operating temperature and pressure on LCD of a lab-scale PEMEC. The experiments are conducted over a range of temperatures from 80 to 90 ℃ and pressures from 0.1 to 0.3 MPa. In conjunction with experiments, a mathematical model is developed to analyze the mechanism of LCD. The model includes mass and momentum equations for liquid water and oxygen gas through the porous transport layer. And the model also involves electrochemical equations to study overvoltages associated with mass transport. By combining experimental and theoretical analysis, this study provides valuable insights into the factors affecting LCD of PEMECs and facilitates the optimization of high-performance operation. Experimental apparatus The specifications of crucial components such as membrane (PEM), catalyst layer (CL), porous transport layer (PTL) is listed in Table.1 . Nafion NR212 is used as PEM, and the thickness is 51 μm. The catalyst is contained on CL, and its loading amount are 1.5 mg/cm2 IrO2 on anode and 0.5 mg/cm2 Pt on cathode, respectively. The anodic PTL is a Titanium mesh plated by Pt (Nikko Techno, NKT-1803-03), and the carbon paper (SGL 38BA) is used as cathodic GDL. For separators, the anode separator is fabricated from titanium, and the cathode separator is carbon. The anodic and cathodic flow pattern is designed as single-serpentine channels, and the channel depth, channel width, and rib width are 1 mm ×1 mm ×1 mm, respectively. Results and discussions Fig.1 shows the theoretical and experimental I-V curves. In Fig.1, the electrolysis voltage in both experiment and simulation raises abruptly, which is corresponding to LCD. The experimental LCD is 11.6 A/cm2 as indicated by the black circle, and the predicted LCD is 10.1 A/cm2. Although the prediction of LCD is qualitative, simulation could follow the IV characteristics obtained by experiment before LCD. Fig.2 and Fig.3 shows the effect of operating temperature and pressure on LCD. The electrolysis voltage “E” is experimentally determined, while the gas saturation at interface of PTL-CH “Sg” is obtained through simulation. As shown in Fig.2, the lower operation temperature enlarges the LCD, the experimental LCD increases 8% from 90 ℃ to 80 ℃. Theoretically, high operating temperature increases oxygen bubbles and then rises the gas saturation S g at the anode CL. So, the water supply flowing to CL is insufficient, and the electrochemical performance also decrease. Therefore, the overvoltages caused by mass transport becomes larger in higher operating temperature. As shown in Fig.3, higher operating pressure shrinks the volume of oxygen gas and enhance the water supply from CH to CL, leading to a larger LCD. Under 80 ℃, the experimental LCD rises by 17 % from 0.1 MPa to 0.3 MPa. Although the theoretical analysis reproduced the experimental results in a qualitative manner, the theoretical analysis suggests that, when electrolysis current density is quite close to the LCD, water mass transport at anode reaches a limitation, indicating that water saturation drops to zero at anodic catalyst layer. References Lee J K, et al. Cell Reports Physical Science, 2020:100147. A Zinser, et al. International Journal of Hydrogen Energy, 2019, 44(52): 28077-28087. Figure 1

This study is focused on elucidating the catalytic combustion phenomenon in proton-exchange-membrane fuel cells. A visualization cell and an infrared (IR) camera are used to capture the thermal behavior under combined chemical and mechanical accelerated stress conditions in situ. Catalyst coated membrane (CCM) embedded in the cell is subjected to a relative humidity (RH) cycling test under open-circuit voltage (OCV) conditions at atmospheric pressure and at a cell temperature of 80 ?. The temperature distribution on the gas diffusion layer surface at the cathode is captured through a high-transmittance glass window (ZnS window). Continuous IR imaging revealed a hot spot at ca. 500 RH cycles, suggesting the existence of a pinhole in the degraded CCM and the occurrence of catalytic combustion there. The occurrence of the hot spot coincides with the time at which the electrochemical indicators detect membrane failure, i.e., hydrogen crossover rate, OCV. Furthermore, a post mortem analysis revealed a 105-mu m diameter pinhole, the position of which matched that of the hot spot. This pinhole is responsible for the rapid increase in the hydrogen crossover rate as well as the significant decrease in the OCV at 500 RH cycles until the end of the durability test.

This study is focused on elucidating the catalytic combustion phenomenon in proton-exchange membrane fuel cells. A visualization cell and an infrared (IR) camera are used to capture the thermal behavior under combined chemical and mechanical accelerated stress conditions in situ. A 3.4 cm 2 catalyst coated membrane (CCM) embedded in the cell is subjected to a relative humidity (RH) cycling test at atmospheric pressure and a cell temperature of 80 °C under open-circuit voltage (OCV) conditions. The temperature distribution on the gas diffusion layer surface at the cathode is captured using a high-transmittance glass window (ZnS window). Continuous IR imaging revealed a hot spot at ca. 500 RH cycles, suggesting the existence of a pinhole in the degraded CCM and the occurrence of catalytic combustion there. The occurrence of the hot spot coincides with the time at which the electrochemical indicators detect membrane failure, i.e., hydrogen crossover rate, OCV. Furthermore, the post mortem analysis revealed a 105-micrometer diameter pinhole, the position of which matched that of the hotspot. An accidental combustion during the durability test could cause the formation of a pinhole. This pinhole is responsible for the rapid increase in the hydrogen crossover rate as well as the significant decrease in the OCV at 500 RH cycles until the end of the durability test. Figure 1