Recent studies show that low loadings and therefore thin electrodes lead to poor electrical contact to the outer porous transport layer (PTL), which provides for electron, water, and oxygen transport to and from the anode. (11) The same mechanism is the cause for poor efficiencies of low iridium loaded anodes. (9,10) The uneven current distribution creates local activity hot spots, which may lead to accelerated degradation of the membrane electrode assembly (MEA). Also, iridium per se slowly dissolves during operation, leading to a lower catalytically active surface area and electrically isolated catalyst material. (8) The lack of durability can be explained by the intrinsic mechanical instability of the very thin anode layers (in the lower micrometer range), which in the current state-of-the-art consist of iridium-based nanoparticles and a binding ionomer applied onto the ionomer membrane. (5) However, when reducing the loading, two major challenges arise: low durability and low conversion efficiency. (2) For this reason, reducing the iridium loading at the anode from the current state-of-the-art (1–3 mg/cm 2) to values below 0.5 mg/cm 2 is a primary focus of current research and development. (7) Considering the current iridium production of only 5 t per year, it becomes evident that with state-of-the-art loadings the installed PEM water electrolysis capacity will not break any time soon into the required terawatt scale. (2) In state-of-the-art electrolysis around 0.5 kg of iridium is required per megawatt installed electrolyzer power. (6) In contrast, reducing the amount of iridium for the oxygen evolution reaction (OER) remains a key challenge for PEM water electrolysis. (5) The platinum required for the hydrogen evolution reaction (HER) can generally be reduced to values below 0.1 mg/cm 2 without significantly losing performance. (4,5) However, with decreasing costs for these components the noble metal based catalysts necessary for high conversion rates and efficiencies become a major cost driver. (1−3) Currently, bipolar plates and porous transport layers make up for more than half of the PEM water electrolyzer stack costs.
Polymer electrolyte membrane (PEM) water electrolysis is a key technology for a sustainable hydrogen economy, but costs still have to be reduced to be competitive with hydrogen production from fossil resources. Besides the improved performance, the hybrid layer also shows better stability in a potential cycling and a 150 h constant current test compared to an identically loaded nanoparticle reference. The improved performance is attributed to a combination of good electric contact and high porosity of the IrOx nanofibers with high surface area of the IrOx nanoparticles.
In spite of an ultralow overall catalyst loading of 0.2 mg Ir/cm 2, a cell with a hybrid layer shows similar performance compared to a state-of-the-art cell with a catalyst loading of 1.2 mg Ir/cm 2 and clearly outperforms identically loaded reference cells with pure IrOx nanoparticle and pure nanofiber anodes. With this hybrid design we can reduce the iridium loading by more than 80% while maintaining performance. In this work we combine IrOx nanofibers with a conventional nanoparticle-based IrOx anode catalyst layer. Significant reduction of the precious metal catalyst loading is one of the key challenges for the commercialization of proton-exchange membrane water electrolyzers.