Quantification of Hydrogen and Transition Metals in the Cathode Catalyst Layer of Anion Exchange Membrane Electrolyzer By Ion Beam Techniques

Anion Exchange Membrane Water Electrolyzer (AEMWE) has increased interest in H2 production at low cost and high scalability. The efficiency is strongly dependent on the membrane electrode assembly (MEA) with appropriate ionomer and catalyst layers. Herein, we report AEMWE with MoNi4-MoO2 cathode catalyst layer in ionomer-free Catalyst Coated Substrate (CCS) configuration. The optimized MEA for AEMWE demonstrated the superior performance of 225 mA/cm2 at 2V with more than 48 hours of durability. The X-ray diffraction confirmed the NiMoO4 phase transformation to MoNi4-MoO2 and SEM-EDS was used to study the morphology and elemental composition of the deposited catalyst. X-ray photoelectron Spectroscopy demonstrated the dominant metallic MoO2 formed after H2/Ar annealing. Rutherford Backscattering Spectroscopy (RBS) was also employed to measure the depth composition profile of the catalyst layer exhibiting the reduction of Ni concentration in the bulk after annealing. Further, we introduce hydrogen depth profiling by Nuclear Reaction Analysis (NRA) via the resonant 1H(15N,αγ)12C reaction as a versatile method for the highly depth-resolved visualization of hydrogen(H) in the near-surface region, including transitions of hydrogen between the surface and the bulk, and between shallow interfaces of the nanostructured cathode catalyst layer. The diffusion of the H atoms was observed with a 40% atomic percent of Hydrogen after H2/Ar annealing of the catalyst for 180 minutes (about 3 hours) and decreased to 10% atomic percent of hydrogen after 24 hours of electrolyzer operation. This technique also enabled the measurement of the zero-point vibrational energy of hydrogen atoms absorbed on the surfaces of the catalyst layer. Overall, this work better explains the cathode catalyst degradation effects and hydrogen diffusion in the cathode catalyst layer responsible for voltage and mass transfer losses in AEM electrolyzers.