Tsinghua University ACS Catal.: Mechanism and Selectivity of Direct Ammonia Oxidation Reaction on Nickel-Yttria-Stabilized Zirconia Anodes in Solid Oxide Fuel Cells

I. Key Innovations and Conclusions

Through thermodynamic analysis and microkinetic modeling, the dominant mechanism of the direct ammonia oxidation pathway on the Ni-YSZ anode was revealed. The core innovations and conclusions are as follows:

Reaction Pathway Dominance: Under typical operating conditions (high temperature, activation overpotential), the direct ammonia oxidation pathway is superior to the decomposition-oxidation pathway, and its selectivity increases with rising temperature. The entropy effect promotes H₂O formation rather than H₂.

Key Descriptor: The oxygen vacancy formation energy at the triple-phase boundary (TPB) is critical in determining the reaction activity and selectivity; reducing this energy facilitates electron transfer from oxygen ions to Ni, accelerates H diffusion and oxidation, and enhances direct oxidation selectivity.

Overpotential Regulation Effect: Applying an anode overpotential modulates the oxygen chemical potential in the TPB region, strengthens the coupling of interfacial electronic structures, lowers the reaction energy barrier, and significantly improves activity and selectivity (selectivity reaches 0.98 at 0.34 V).

Impact of Surface Coverage Species: H adsorption on Ni or TPB sites promotes the reaction, while H accumulation on the YSZ surface inhibits performance; N tends to accumulate on the Ni surface and migrate to the TPB, blocking active sites and causing catalyst deactivation due to nitridation, which is the main reason for stability degradation.

Optimization Strategy: The dominant role of electronic structure regulation on the reaction pathway was elucidated, and a new strategy was proposed to enhance DA-SOFC performance by adjusting the oxygen chemical potential, optimizing TPB design, and suppressing N* accumulation.

II. Core Content of Graphical Analysis

The study progressively reveals the reaction mechanism through a series of graphical analyses:

Reaction Pathway Model: Figure 1 proposes two pathways—indirect (ammonia first decomposes to N₂ and H₂, followed by oxidation) and direct (ammonia undergoes stepwise dehydrogenation to form NHx and H, with H* diffusing to the TPB to directly react and form water)—annotating the spatial distribution of atoms and establishing the foundation of the physical model.

Catalytic Performance Comparison: Figure 2 shows that Ni-YSZ has stronger NH₃ adsorption energy at 1,000 K, with the first dehydrogenation energy barrier being 0.14 eV lower than that of pure Ni; within 600–1,000 K, increasing temperature significantly enhances direct oxidation selectivity, and the entropy effect drives H* diffusion to dominate the reaction.

Overpotential Regulation: Figure 3 indicates that after applying a 0.34 V overpotential, the H* diffusion energy barrier decreases from 0.46 eV to 0.28 eV, and the oxygen vacancy formation energy decreases from 0.62 eV to 0.39 eV; direct oxidation activity increases exponentially, and selectivity improves from 0.90 to 0.98.

Effect of H Coverage Location: Figure 4 shows that H adsorption on Ni or TPB sites reduces energy barriers, enhancing activity and selectivity; when H occupies the YSZ side, the oxygen vacancy formation energy sharply increases to 0.97 eV, and selectivity drops from 0.99 to 0.59. The H location affects the interfacial oxygen chemical potential by shifting the O 2p band center.

N Accumulation Poisoning Effect: Figure 5 indicates that N tends to migrate to TPB oxygen vacancies and occupy active sites, increasing the reaction energy barrier; a high N coverage rate reduces the direct oxidation activity to 36% of the original value and the selectivity to 0.64. N* exhibits strong binding with Ni, and the high energy barrier for N₂ recombination can easily lead to deactivation.

Role of Oxygen Chemical Potential: Figure 6 identifies oxygen chemical potential as a key descriptor. An upward shift in the O 2p band center stabilizes H and promotes water desorption, enhancing direct oxidation performance. However, excessively high oxygen chemical potential may cause over-oxidation of Ni, compromising stability.

III. Summary and Outlook

The study systematically elucidates the competitive mechanisms and regulatory principles of the ammonia oxidation reaction pathway on Ni-YSZ anodes. It demonstrates that under typical operating conditions, the direct ammonia oxidation pathway holds thermodynamic and kinetic advantages, with selectivity determined by the competition between H coupling and H diffusion. The formation energy of oxygen vacancies is controlled by the interfacial electronic structure, and the applied overpotential can optimize the oxygen chemical potential by adjusting the O 2p band center.

Future anode material design should focus on "electronic band engineering," optimizing the TPB electronic structure through compositional tuning or interface modification to enhance lattice oxygen activity and improve intermediate management. This work provides a design blueprint for developing low-energy-consumption, high-stability ammonia fuel cells. Subsequent efforts may combine in situ characterization to validate the theory and explore the application potential of new-type composite anode systems.