Hydrogen crossover raises serious concerns on proton exchange membrane water electrolyzer
Green hydrogen plays a crucial role in the modern energy system, especially considering its production with the input of renewable electricity. Proton exchange membrane water electrolysis is favored for producing green hydrogen with renewable electricity, because of its high efficiency, rapid start-up, and quick adaptation to electricity changes. However, the hydrogen crossover issue gradually attracts significant attention under variable electricity input. The hydrogen crossover mechanism is proposed, with emphasis on its aggravation during the frequent start-stop cycles. The preliminary experimental observations and theoretical insights are reported to assess the hydrogen crossover rate under constant and dynamic operating conditions. Dynamic operation can particularly intensify the hydrogen permeation, increasing anode-side hydrogen volume fraction in oxygen and membrane degradation, thereby aggravating the safety risk and reducing efficiency. Several possible mitigation strategies, such as catalytic layer design, membrane material optimization, and operational condition control, are proposed.
Background
Green hydrogen plays crucial roles in transforming the future energy landscape, with applications spanning industry, transportation, and energy storage. By converting intermittent renewable energy sources like wind and solar into stable green gas (Figure 1A), it addresses their inherent variability, enhancing grid stability and energy security. The International Energy Agency projects global hydrogen demand to reach 500 million tons by 2050, with a market value exceeding $2.5 trillion.1 Thus, the innovation and large-scale deployment of green hydrogen technologies are critical for achieving sustainable development goals.
Proton exchange membrane water electrolysis (PEMWE) technology produces clean hydrogen from pure water via a solid polymer electrolyte (i.e., proton exchange membrane [PEM]) to transfer the proton. The low ohmic loss and excellent ion transport properties enable its operation over a wide dynamic power range (30%–120%), guaranteeing its direct coupling with renewables like fluctuating wind or solar. The flexibility makes PEMWE ideal for addressing intermittency issues while supporting grid stability (Figure 1B). It gains some global attention driven by successful implementations including Siemens’ offshore-based scheme (2021), whereby units are paired directly with turbines generating hydrogen even in challenging maritime environments. Similarly, China Huadian Corporation unveiled a massive 600 MW hybrid plant in Inner Mongolia (2024) tapping vast local potentials of both wind and solar energy.
Unlike other established technologies, safety remains a critical concern throughout hydrogen production. In PEMWE technology, the primary safety risk arises from hydrogen crossover—i.e., the diffusion of hydrogen through the PEM into the anode. Owing to the small molecular size and high diffusion capacity of hydrogen, it can permeate through the PEM’s water channels to the other side,2 which is prevalent in both PEM fuel cells and alkaline water electrolysis (AWE). It becomes especially critical when PEMWE systems are coupled with intermittent power sources. Beyond electrolyzer’s efficiency losses, hydrogen crossover poses critical safety hazards by elevating the hydrogen volume fraction in oxygen (𝛷𝐻2) beyond the 4 vol. % low explosion limit (LEL). In this perspective, we present a comprehensive analysis of deteriorative hydrogen crossover behavior under variable power conditions, aiming to highlight the critical safety implications associated with fluctuating operational regimes, while proposing actionable strategies to mitigate these emerging risks.
