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The internal reaction state of air-cooled proton exchange membrane fuel cell (PEMFC) is the key factor affecting the output performance and stability of the cell. By developing an in-situ testing device for the reaction state of air-cooled fuel cell, the real-time measurement of cell temperature and current density is realized, and the influence mechanism of hydrogen outlet pulse interval, hydrogen inlet pressure and cathode wind speed on the performance of the cell is revealed. The results show that the distribution of temperature and current density in air-cooled cells is uneven. The temperature difference can reach 20 °C, and the current density difference reaches 400 mA/cm2 when the average current density is 500 mA/cm2. As the interval between pulses decreases and the inlet pressure increases, the performance of the hydrogen outlet area and the uniformity of the distribution increase, which can reduce the fluctuation of current density in the cells and improve output stability. If the cathode wind speed is too low, the temperature in central areas is high, and the temperature distribution uniformity is reduced. However, excessive wind speed causes the generating water to be blown away. The water content of the proton exchange membrane thus decreases, and the uniformity of the current density distribution deteriorates.

The proton exchange membrane water electrolyzer (PEMWE) is one of the most promising electrochemical energy conversion devices for hydrogen production, while still limited by performance bottlenecks at high current densities, due to the lack of mass transfer insights. To investigate the mechanisms of oxygen transport inside the PEMWE at high current density and its relation to electrolytic performance. Operational in situ x-ray imaging is utilized to simultaneously characterize the bubble behavior and voltage response in a novel designed visual mini-cell, and it is identified that oxygen evolution and transport in the PEMWE follow the process of bubble nucleation, growth, and detachment. Based on the results of mini-cells with three porous transport layers (PTLs) up to 9 A cm -2 operation, it revealed that critical current densities exist for both carbon-based and titanium-based PTLs. Once exceeding the critical current density, the cell voltage can no longer be stabilized and the cell exhibits a significant oxygen overpotential. To illustrate this, the concept of interfacial separation zone (ISZ) is first proposed, which is an effective pathway for bubble growth and separation and the pattern of the ISZ exhibits specific regimes with the critical current density. Ultimately, a new approach for better understanding the mechanisms of oxygen transport is revealed., (© 2024 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)