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Operation at high current density of polymer electrolyte membrane electrolysis cell (PEMEC) can reduce its electrode area and capital expense. The high current density operation is limited by the limiting current density (LCD), where electrolysis voltage abruptly increases. Effects of operating temperature and pressure on the LCD were evaluated in a lab-scale PEMEC within a range from 70 to 90 °C and from 0.1 to 0.3 MPa. Also, a theoretical model was developed to predict the LCD, and to clarify what determines LCD and how the operating conditions influence the LCD. The theoretical analysis suggests that when current density is close to the LCD, water mass transport at anode reaches a limitation, which indicates that water saturation reaches almost zero at anodic catalyst layer. The analysis also finds that lower operating temperature and higher operating pressure can reduce oxygen gas saturation and raise the water saturation in porous transport layer at anode, and finally enlarge the LCD. • Limiting current density (LCD) is decided by mass transport limitation. • Oxygen gas saturation near catalyst layer is close to 1 at LCD. • Bubble overvoltage abruptly rises at LCD. • Lower temperature and higher pressure enlarge LCD. [ABSTRACT FROM AUTHOR]

Novel Ru-core Ir-shell catalyst-integrated porous transport electrodes (PTEs) for polymer electrolyte membrane water electrolysis (PEMWE) cells are prepared, in which Ru-core Ir-shell catalyst nanostructures are directly deposited onto a porous transport layer (PTL) via arc plasma deposition (APD). The PTL has a nanostructured TiO 2 surface prepared via NaOH etching, acting as a catalyst support. The performance and durability of these Ru-core Ir-shell catalysts depend strongly on the ratio of Ir and Ru. The current-voltage (I–V) characteristics of PEMWE cells were improved by applying these core-shell catalysts with a low Ir loading of around 0.1 mg cm−2. The core-shell catalyst-integrated PTEs can operate at current densities of up to 10 A cm−2 without exhibiting limiting current behavior. This unique combination of the core-shell catalyst and the PTE structure enables PEMWE cell operation with low iridium loading and high current density, potentially reducing the cost of green hydrogen. • Ru-core Ir-shell electrocatalysts were prepared on Ti-based porous transport layers. • The activation overvoltage was significantly lower compared to pure Ir electrocatalysts. • The durability of the electrocatalyst was improved by increasing the Ir-to-Ru ratio. • This unique electrode can operate at high current densities up to 10 A cm−2. [ABSTRACT FROM AUTHOR]

The high initial cost of polymer electrolyte membrane water electrolyzers (PEMWEs) has delayed their widespread commercialization. A possible means to reduce cost is by reducing the overvoltage and increasing the current density to reduce the electrode area. This study proposes a novel method in which boiling is superimposed on the oxygen evolution reaction (OER) to decrease electrolysis voltage. The vapor bubbles formed by boiling are expected to decrease the concentration overvoltage. The boiling effect was experimentally analyzed using a three-electrode cell. Although a general catalyst layer (CL) was formed on a working electrode (WE) bar, the structure of the working electrode (WE) bar was special, in which a 10-W heater was embedded and made boiling on the electrode under 1 bar condition. Increasing the electrode temperature under static OER current density slightly decreased the OER potential. However, an abrupt decrease in potential was observed when the temperature was scaled over the boiling temperature. Moreover, this abrupt decrease substantially intensified when, similar to a practical PEMWE, a porous transfer layer (PTL) and flow channel were assembled on the CL. These experimental results suggest that boiling can reduce the concentration overvoltage by reducing the oxygen concentration on the CL, especially when the mass transport resistance caused by the PTL is considerable. Innovatively and simply utilizing boiling, as proposed here, can enhance the oxygen transfer and contribute to reducing the initial cost of PEMWEs. • OER is coupled with boiling. • Three-electrode cell confirms boiling effect on OER. • Theoretical modeling qualitatively predicts the boiling mechanism. • Oxygen activity in catalyst layer is estimated electrochemically. • Boiling reduces concentration overvoltage of OER. [ABSTRACT FROM AUTHOR]

Water boiling in a polymer electrolyte membrane water electrolyzer (PEMWE) has been experimentally demonstrated to reduce the electrolysis voltage [Ito K et al. ECS Transactions. 2017; 80(8):1117]. The boiling phenomenon and formation mechanism are studied comprehensively by introducing boiling into a PEMWE model. The gas saturation at the channel/porous transfer layer (PTL) is first determined as the boundary condition for the gas transfer in the PTL. Then, the gas transfer equation can be used to determine the oxygen and hydrogen pressures at the catalyst layer (CL). The dissolved oxygen and hydrogen activities are considered in the activation overvoltages through a variable (activation fraction). Finally, the Nernst loss, activation, and ohmic overvoltage of the PEMWE over a wide temperature range (encompassing non-boiling, boiling, and gas-phase-only conditions) are determined from the oxygen and hydrogen pressures and gas saturation at the CL. The results show that the PEMWE under boiling conditions has the lowest electrolysis voltage among the non-boiling, boiling, and gas-phase conditions. Boiling can accelerate the detachment of gas bubbles composed of gaseous oxygen and water vapor in the channel (CH). Furthermore, vigorous boiling significantly reduces the Nernst loss by reducing the oxygen and hydrogen partial pressures at the CL. • Boiling is coupled in a PEMWE model. • Model quantitatively clarifies the boiling mechanism on the electrolysis voltage. • Boiling lowers the oxygen and hydrogen activities at the CL. • Anodic and cathodic Nernst losses decrease during boiling. • Boiling activates the anodic reaction under bubbles. [ABSTRACT FROM AUTHOR]

This study aimed to optimize the flow-field pattern and flow configuration of a polymer-electrolyte-membrane water electrolyzer, with a particular focus on high-temperature operation up to 120 °C. Three types of flow-field pattern (serpentine, parallel, and cascade) were tested in both the anode and cathode sides of a water electrolyzer cell, and the current-voltage characteristics and high-frequency resistance were measured to examine which overpotential components are impacted by the flow-field pattern. The experimental results revealed that the cathode flow-field pattern only affects the ohmic overpotential, while the anode flow-field pattern significantly affects the overpotential related to liquid water shortage at catalyst layer, and the flow configuration (counter- and co-flow) does not affect the electrolysis performance. Finally, under operating conditions of 120 °C and 0.3 MPa, we found that the optimized cell configuration consisted of cascade and serpentine flow-field patterns in the anode and cathode, respectively; this configuration produced the minimum electrolysis voltage of 1.69 V at 2 A/cm 2 . [ABSTRACT FROM AUTHOR]

Effects of operating conditions of a high-temperature polymer electrolyte water electrolyzer (HT-PEWE) on the electrolysis voltage are evaluated, and the optimal conditions for a high performance are revealed. A HT-PEWE unit cell with a 4-cm 2 electrode consisting of Nafion117-based catalyst-coated membrane with IrO 2 and Pt/C as the oxygen and hydrogen evolution catalysts is fabricated, and its electrolysis voltage and high-frequency resistance are assessed. The cell temperature and pressure are controlled at 80–130 °C and 0.1–0.5 MPa, respectively. It is observed that increasing the temperature at a constant pressure of 0.1 MPa does not increase the ohmic overvoltage of the cell; however, it does increase the concentration overvoltage. It is also found that the increase in the overvoltage resulting from the rise in the temperature can be suppressed by elevating the pressure. When operating the cell at a temperature of 100 °C, pressure greater than 0.1 MPa suppresses the overvoltage, and so does pressures greater than 0.3 MPa at 130 °C. This behavior suggests that keeping the water in a liquid water phase by increasing the pressure is critical for operating PEWEs at high temperatures. [ABSTRACT FROM AUTHOR]

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 °C. 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-μm 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. • ZnS window embedded in a cell enables an IR imaging. • RH cycling under OCV condition causes chemical and mechanical stress. • IR imaging revealed a hot spot, suggesting catalytic combustion at pinhole formed. • The occurrence of the hot spot coincides abrupt increase of hydrogen crossover rate. [ABSTRACT FROM AUTHOR]