Design Principles of Nickel-Based Alloy Catalysts for Ammonia Decomposition

Ammonia (NH₃) decomposition is extensively studied for clean hydrogen production and direct ammonia fuel cell (DAFC) due to its high hydrogen storage capacity and existing infrastructure. However, current ammonia decomposition proess poses several challenges. Primarily, Ruthenium (Ru) catalysts are highly effective for ammonia decomposition, but their limited supply and high cost hinder their commercialization. Consequently, nickel (Ni) catalysts are being considered as a cost-effective alternative for ammonia-based hydrogen production and anode catalyst for DAFC. To further improve the catalytic activity of Ni catalysts, Ni-based alloy catalysts with other metals are a promising approach. This study employs density functional theory (DFT) calculations to investigate the catalytic activity of 3d transition metals (Ni, Co, Cu, and Fe) and their potential alloys.<br/>According to DFT analysis, the key elementary reaction steps of overall ammonia decomposition are NH_x-H bond scission (NH_x* → NH_x-1* + H*) and N+N recombination (N* + N* → N₂*). We found that nitrogen binding energy (E_ad(N)) acts as a key descriptor for predicting the activation energies of these elementary steps, showing a volcano-like relationship between the experimentally observed catalytic activity and the DFT-calculated E_ad(N). This relationship indicates that E_ad(N) can be used to fine-tune the catalytic properties of the materials to achieve optimal performance.<br/>DFT results showed that Fe-Ni alloy catalyst exhibited a comparable catalytic activity to pure Ni catalyst. Therefore, we anticipated that a Ni-Fe alloying system, incorporating the relatively inexpensive Fe element into Ni, could be a potentially cost-efficient catalyst. To further optimize the composition of Ni-Fe system, we conducted DFT calculations and revealed the relationship between Fe concentration in the Ni-Fe system and E_ad(N). Based on this correlation, we identified a Ni_0.64Fe_0.36 composition predicted to have an optimal E_ad(N) value on the volcano plot. Experimental validation also confirmed that the Ni_0.64Fe_0.36 catalyst exhibited higher catalytic performance compared to that of pure Ni.<br/>To extract the fundamental descriptor for E_ad(N), we additionally performed DFT-based electronic structure analysis and found that as the d-band filling (f_d) of the metal surface increases, the M-N* bonding interaction weakens and the anti-bonding interaction strengthens, leading to a decreased E_ad(N). This understanding allows us to design catalysts with tailored electronic properties for optimizing their performance. Based on these results, we concluded that f_d can be a powerful descriptor for predicting E_ad(N), thereby acting as a useful and fundamental descriptor to predict the catalytic activity of Ni-based alloy catalysts for ammonia decomposition for hydrogen production and DAFC anode. Our findings offer valuable insights for optimizing Ni-based alloy catalysts for efficient hydrogen production and DAFC anode through ammonia decomposition, potentially leading to more cost-effective and sustainable hydrogen economy.