Non-Equimolar High-Entropy Spinel Oxides as Anode Materials for a Direct Ammonia-Fed Solid Oxide Fuel Cell

High-entropy oxides (HEOs) have emerged as promising electrocatalysts owing to their unique cocktail effect, which enables synergistic multielement interactions, leading to enhanced catalytic activity. In this work, three novel non-equimolar high-entropy spinel ferrites-based materials, (M_0.3Ni_0.21Cu_0.21Co_0.14Mn_0.14)Fe₂O₄ (M = Fe, Mg, or Zn)-Gd_0.1Ce_0.9O_1.95 (GDC), have been adopted as anodes for direct-ammonia solid oxide fuel cells (DA-SOFCs). Synchrotron-based X-ray absorption spectroscopy (XAS) and micro-computed tomography (Micro-CT) were employed, alongside X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM), scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDX), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy, for comprehensive characterization. The XRD analysis showed that the HEO(Zn) and HEO(Mg) samples exhibited a single-phase high-entropy spinel ferrite, whereas the d-HEO(Fe) displayed a dual-phase structure comprising a predominant spinel phase (90.9 wt%) and a minor hematite (Fe₂O₃) phase. The HEO samples were treated in a reducing atmosphere to form a composite with a monometallic Fe phase (>82.4 wt%) and FeNiCu alloy. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis revealed that the Fe³⁺, Cu²⁺, Ni²⁺, and Co²⁺ cations are distributed between the octahedral and tetrahedral coordination environments within d-HEO(Fe) sample, while the Mn³⁺ ions are located at the octahedral sites. The lanthanum strontium gallium magnesium oxide (LSGM) electrolyte-supported cell with a d-HEO(Fe) anode achieved the highest peak power density (PPD) of nearly 382.0 mW/cm² at 850 °C, whereas the SOFCs with HEO(Zn) and HEO(Mg) anodes delivered lower PPD values of 218.3 and 195.7 mW/cm², respectively. Notably, the cell with the d-HEO(Fe) anode demonstrated excellent durability for over 75 h at 800 °C, with only 3.16% voltage degradation. Density functional theory (DFT) computations were carried out to scrutinize the NH₃ decomposition mechanism on the Fe(110) surface, which represents the reduced HEO anode, and were benchmarked against those on the Ni(111) surface. Interestingly, the formation of the N₂* species via recombination of N^* atoms is identified as the rate-controlling step for NH₃ cracking on Fe(110) and Ni(111), exhibiting the highest activation barriers of 2.43 and 1.63 eV, respectively, among all elementary steps considered.