New Insights into the Hydrogen Evolution Mechanism near the Ni/YSZ Triple Phase Boundary during Steam Electrolysis: A Patterned Model Electrode Study

Solid oxide cell technologies play a crucial role in climate change mitigation by enabling the reversible storage of renewable energy. Understanding the electrochemical high-temperature reaction mechanisms and the catalytic role of the electrode and electrolyte materials is essential for advancing power-to-H₂ technologies. Despite its significance, limited <i>in situ</i> spectroscopic research focusing on nickel and yttria-stabilized zirconia (Ni/YSZ) is available. We employ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to investigate 2D porous Ni/YSZ model electrodes with variable YSZ domain sizes and triple phase boundary (TPB) lengths. Focusing on the hydrogen evolution reaction (HER), we provide a mechanistic explanation for why surface hydroxylation and electrochemical activity are correlated with the YSZ surface area and YSZ domain size and unravel the specific mechanistic role of the YSZ surface. A direct comparison of normalization of the measured total electrolysis current to the purely geometrical length of the TPB vs an electrified "catchment area" next to the TPB, exhibiting strong enough electric fields, is the key to a correct quantitative description of the individual elementary steps of water electrolysis on Ni/YSZ. By combining electrochemical impedance spectroscopy, NAP-XPS, and electric field modeling, the local water reduction process near the TPB can be described, indicating optimized structural parameters for improved HER performance.