Ionomer-free Iridium Anodes with Low Loading for Zero-gap Electrolyzers via Direct Atomic Layer Deposition
Proton exchange membrane water electrolysis (PEMWE) is among the most promising techniques to provide hydrogen as both energy carrier and feedstock for the chemical industry, due to its compatibility with intermittent renewable electricity sources. However, a major bottleneck in scaling up PEMWE is the limited supply of the anode catalyst Ir. This work shows a novel approach to tailoring ionomer-free catalyst layers based on custom-made microporous substrates and two different atomic layer deposition (ALD) processes, both using Ir(EtCp)(CHD) precursor. Thermal ALD using O3 as counter-reactant yields mostly metallic Ir/IrOx and an electrode Ir loading of 0.21 mg/cm2. The PEMWE cell-level performance is comparable to a commercial catalyst-coated membrane anode despite 7x lower loading, and is maintained over an accelerated durability test equivalent to 600 h of intermittent operation. Moreover, using a low-loading ALD-made cathode, the total platinum group metal loading is only 0.24 mg/cm2. Plasma-enhanced ALD reduces the Ir loading to 0.04 mg/cm2 by tailoring the catalyst distribution, however, yields lower-activity IrO2. Finally, direct ALD catalyst deposition needs no ionomer-bearing ink. The anodes are therefore compatible with various membrane materials, as demonstrated in a CO2 electrolysis experiment with an anion exchange membrane.