Ruthenium-Induced Strain- and Phase-Polarized Multifaceted Nickel for Nitrate-to-Ammonia Electrocatalysis

Abstract

Two-phase-engineered bimetallic alloys with tunable crystallographic defects offer a promising platform for proton-coupled electron-transfer in electrocatalytic nitrate reduction (NO3RR) to ammonia, providing a sustainable alternative to the Haber–Bosch process. Alloying Ru transforms metastable hexagonal close-packed (hcp) Ni into NO3RR-active face-centered cubic (fcc) Ni1–xRux (x = 0–0.3) alloys by modulating the nucleation and growth kinetics of Ni. Concurrently, the high density of twin boundaries and edge dislocations in hcp Ni approaches a defect-saturation limit, driving a structural transition that stabilizes a semicrystalline Ru-alloyed fcc phase with a phase fraction of 96%. Operando Raman and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectral analyses along with theoretical calculations show potential-dependent NH3 formation through stepwise *NO3 to *NO reduction at charge-modulated Niδ+-Ruδ- sites, and subsequent *N hydrogenation to *NH3 on Ruδ- sites, assisted by interfacial H2O-derived *H. Preadsorbed *H on fcc Ni74Ru26 surface facilitates *NO3 adsorption and lowers the rate-determining energy barrier. Ni74Ru26 delivers nearly 100% NH3 Faradaic efficiency from 0 to −0.4 VRHE, with a maximum of 99.6 ± 0.3% at −0.1 VRHE. NH3 yield reaches 30077 ± 4316 μg h–1 cm–2 (48201 μg h–1 mgRu–1) at −0.9 VRHE and energy efficiency of 37% at +0.1 VRHE. Ni74Ru26 retains ∼100% NH3 Faradaic efficiency over 10 cycles and 50 h at −0.1 VRHE, and −100 mA cm–2, respectively. When Ni74Ru26 is used as the cathode in a galvanic Zn-NO3– battery, it achieves a power density of 5.6 mW cm–2.