Reaching high current densities is absolutely imperative for electrochemical energy conversion, from fuel cells to CO₂ reduction. Here, we identify the existence of a performance indicator for gas-evolving electrochemical energy conversion devices: the critical current density. The critical current density pinpoints a performance inflection point whereby both the gas saturations and mass transport overpotentials suddenly dominate cell performance and exacerbate failure. We elucidate the mass transport behavior of a polymer electrolyte membrane (PEM) water electrolyzer using in operando neutron imaging at operating current densities as high as 9 A ⋅ cm⁻². Product gases become heterogeneously distributed in the porous transport layer adjacent to the catalyst layer and promote disastrous local hotspots. Turbulent reactant flows are prerequisites for mitigating these undesired mass transport overpotentials. Optimizing new materials and cell architectures with this performance indicator may unlock higher than previously reported performances for electrochemical energy conversion.

从燃料电池到减少CO _2的电化学能量转换，绝对必须达到高电流密度。在这里，我们确定了用于气体演化电化学能量转换装置的性能指标：临界电流密度。临界电流密度可精确地指出性能的拐点，由此气体饱和和传质超电势突然会主导电池性能并加剧故障。我们阐明了在操作电流密度高达9 A⋅cm ^-2的操作中子成像中使用的聚合物电解质膜（PEM）水电解器的传质行为。产物气体在与催化剂层相邻的多孔传输层中变得异质分布，并导致灾难性的局部热点。湍流的反应物流是减轻这些不良的物质运输超电势的先决条件。使用该性能指标优化新材料和电池体系结构，可以实现比以前报道的电化学能量转换更高的性能。