Ammonia, a compound composed of nitrogen and hydrogen, is rapidly emerging as a novel hydrogen carrier. Its potential in fuel cell technology is increasingly recognized due to its high hydrogen density, ease of liquefaction, and established transportation infrastructure. Such advantages of ammonia have naturally led to the development of Direct Ammonia Solid Oxide Fuel Cells (DA-SOFCs). Despite these prospects, current research on DA-SOFCs faces critical limitations, particularly in catalyst selection. Ru, widely known for its high catalytic activity in ammonia decomposition, is a precious metal, thus raising economic concerns due to its high cost. This presents a significant challenge in the practical application and widespread employment of DA-SOFCs. Therefore, exploring alternative catalysts that can offer comparable activity to Ru at a lower cost is urgently needed.

Another significant challenge lies in the choice of promoters. For ammonia decomposition catalysts, their activity is widely known to be strongly enhanced by promoters. However, in the high-temperature and humid conditions of fuel cells, commonly used alkali and alkaline earth metal promoters are unstable and thus cannot effectively working as promoters. Moreover, while oxides or the oxygen vacancies within these oxides, are known to potentially act as promoters, their effectiveness in enhancing catalytic performance is limited. Therefore, there is also urgent needs to develop stable and effective promoters for DA-SOFCs.

Herein, we systematically designed a low-precious-metal, high-performance DA-SOFC system through DFT calculations. Our approach divided the DA-SOFC system into two components: the metal catalyst and the promoter. We analyzed the requisite properties for each component to achieve high activity and designed appropriate catalysts and promoters. For the metal catalyst, nitrogen adsorption energy was selected as a descriptor to design an alloy catalyst with superior performance to Ru. NiFe alloy was found to have a nitrogen adsorption energy very close to Ru at a certain composition (Fe = 50~75%). Furthermore, NiFe exhibited a lower activation energy than Ru in the N₂ associative desorption reaction, confirming its potential as an outstanding catalyst to replace Ru. For the promoter, the work function was selected as a descriptor to analyze electron donation capability. To ensure both superior electron donation capability and stability, we developed a strategy that involves intrinsically doping the oxide support with a donor element. This approach effectively integrates the support itself as a promoter, addressing the stability issues posed by conventional promoters in fuel cells. Nb-doped STO (NSTO) was chosen as a donor-doped oxide and found to have a work function below 1.5 eV, which is much lower than that of conventional promoters and any oxide with oxygen vacancies, demonstrating that doping oxides with donors not only produces stable promoters but also highly effective ones.

To evaluate the performance of the NiFe-NSTO system designed through DFT calculations, NiFe-STO and NiFe-NSTO cells were fabricated, and ammonia conversion test was conducted. Both NiFe-STO and NiFe-NSTO showed a high conversion rate confirming the superior performance of the NiFe catalyst. Additionally, NiFe-NSTO outperformed NiFe-STO across all temperature ranges from 500 to 700℃, validating that the doped donor acts effectively as a promoter in harsh fuel cell environments. Our research may guide the further development of highly stable and active fuel electrodes for high-performance SOFCs and suggest that donor doping in oxides is an effective strategy for producing and maintaining promoter effects in fuel cells.