New Insights\ninto the Hydrogen Evolution Mechanism\nnear the Ni/YSZ Triple Phase Boundary during Steam Electrolysis: A\nPatterned Model Electrode Study

Solid oxide cell technologies play a crucial role in\nclimate change\nmitigation by enabling the reversible storage of renewable energy.\nUnderstanding the electrochemical high-temperature reaction mechanisms\nand the catalytic role of the electrode and electrolyte materials\nis essential for advancing power-to-H<sub>2</sub> technologies. Despite\nits significance, limited <i>in situ</i> spectroscopic research\nfocusing on nickel and yttria-stabilized zirconia (Ni/YSZ) is available.\nWe employ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS)\nto investigate 2D porous Ni/YSZ model electrodes with variable YSZ\ndomain sizes and triple phase boundary (TPB) lengths. Focusing on\nthe hydrogen evolution reaction (HER), we provide a mechanistic explanation\nfor why surface hydroxylation and electrochemical activity are correlated\nwith the YSZ surface area and YSZ domain size and unravel the specific\nmechanistic role of the YSZ surface. A direct comparison of normalization\nof the measured total electrolysis current to the purely geometrical\nlength of the TPB vs an electrified “catchment area”\nnext to the TPB, exhibiting strong enough electric fields, is the\nkey to a correct quantitative description of the individual elementary\nsteps of water electrolysis on Ni/YSZ. By combining electrochemical\nimpedance spectroscopy, NAP-XPS, and electric field modeling, the\nlocal water reduction process near the TPB can be described, indicating\noptimized structural parameters for improved HER performance.