In polymer electrolyte water electrolysis, the mass transport losses are the least understood overvoltage contribution. Underlying origin as well as correlation parameters with the structure of the porous transport layer (PTL) are not identified, because in the past the focus was set on the PTL bulk^1,2and not on the PTL surface properties. The application of X-ray tomographic analysis of the interface³ (an example shown in Figure 1), in combination with electrochemical characterization leads to the identification of governing parameters. PTL interface properties (interfacial contact area, membrane deformation) seem to be key properties, as they correlate with cell performance. Based on these findings, novel two layer PTL materials were designed and characterized. Increased performance and reduced mechanical deformation were obtained in comparison to conventional PTL materials, also enabling the employment of ultra-thin membranes.

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