Engineering NiO Particle Size in Hydrogen Electrode Functional Layers for Enhanced Performance of Protonic Ceramic Electrochemical Cells

ABSTRACT Protonic ceramic electrochemical cells (PCECs) operating at intermediate temperatures (400–600°C) enable efficient hydrogen production, power generation, and chemical synthesis. To enhance electrochemical performance, engineering the electrode‐electrolyte interface is an effective approach. While substantial progress has been achieved at the oxygen electrode/electrolyte interface, understanding of the hydrogen electrode/electrolyte interface, particularly involving hydrogen electrode functional layers (HEFLs), remains limited. This study investigated the effects of nickel oxide (NiO) particle size in HEFLs on microstructural evolution and electrochemical behavior. HEFLs fabricated with smaller NiO particles enhanced electrolyte membrane densification and promoted more homogeneous and fine‐grained localized microstructures after reduction. These improvements in nano‐NiO samples reduced ohmic resistance from 0.23 Ω cm 2 to ≈0.15 Ω cm 2 , at 600°C, yielding a peak power density, 1.11 W cm −2 (fuel cell) and a current density, −1.50 A cm −2 at 1.3 V with 30% steam (electrolysis). Findings reveal clear relationship that initial NiO particle size influences the thin electrolyte sintering and localized microstructure evolution, consequently affecting gas diffusion and electrochemical interfaces, and ultimately electrochemical performance of PCECs. This work provides fundamental insights into interface and functional layer engineering, offering guidance for the rational design of optimized PCEC architectures with enhanced electrochemical performance.