Enhanced Nitrogen Reduction to Ammonia by Surface- and Defect-Engineered Co-catalyst-Modified Perovskite Catalysts...

An electrocatalytic nitrogen reduction reaction is considered a potential approach for green ammonia production─a zero-carbon fertilizer, fuel, and energy storage for renewable energy. To harness the synergistic properties of perovskites, the inherent dipole moment due to their non-centrosymmetric structure (that facilitates better charge separation), oxygen vacancies, and the presence of Ni metal sites that permit activation and reduction of N₂ efficiently, the NiTiO₃-based nanoelectrocatalysts have been synthesized. Further, these catalysts have been modified with ultra-small metal nanocrystal co-catalysts to form heterointerfaces that not only aid to improve the charge separation but also activate N₂ and lower overpotential requirements. The appearance of peaks corresponding to (012), (104), (110), (11-3), (024), (11-6), (018), (027), and (300) confirms the formation of rhombohedral NiTiO_3. The shift in the XRD peak corresponding to the (104) plane to a smaller 2θ value and peak shifting and widening of Raman spectra imply the lattice distortion that signifies the formation of Pd-NiTiO₃ and Pt-NiTiO₃ heterojunction electrocatalysts with the loadings of 0.4 and 0.3 wt % of Pd and Pt, respectively, as confirmed by ICP-OES analysis. The detailed XPS analysis reveals the presence of Pd (0), Pd (II), and Pt (0), Pt (II) in respective electrocatalysts. The appearance of XPS peaks at 528.7 and 531.1 eV suggests the presence of oxidative oxygen species (O^2-/O^-) and the presence of oxygen defects due to oxygen vacancy. The detailed nitrogen reduction (NRR) investigation exhibits a 5-fold enhancement in ammonia yield rate (∼14.28 μg h^-1 mg^-1 at -0.003 V vs RHE), a faradic efficiency of 27% (at 0.097 V vs RHE) for Pd-NiTiO₃ electrocatalysts than that for bare NiTiO₃ (3.08 μg h^-1 mg^-1), and 9-folds higher than that of the activity shown by the commercial TiO₂ (P₂₅) (1.52 μg h^-1mg^-1). The formation of ammonia was further confirmed by using isotopic nitrogen as the feeding gas. Furthermore, the highest NRR is observed at lower cathodic potential (-0.003 V vs RHE) in the case of the Pd-NiTiO₃ electrocatalyst than that of the Pt-NiTiO₃ electrocatalyst (-0.203 V vs RHE), implying significantly reduced overpotential requirement. Such enhanced NRR activity with lower overpotential requirement in the case of the Pd-NiTiO₃ electrocatalyst is due to efficient charge separation as shown by the semicircle Nyquist plot, decreased photoluminescence emission intensity, shorter average lifetime (∼29 ns) of excitons, appropriate band bending, and improved activation of N₂ by the oxygen vacancies and heterointerface formed between Pd nanocrystals and NiTiO₃. Furthermore, no change is observed in the current density, after stabilization in the initial few seconds, even up to 2 h, which signifies that these electrocatalysts are stable. The structural and morphological integrity of the optimized catalyst remained even after the nitrogen reduction reactions, as revealed by no significant change observed in FESEM, elemental mapping, and EDS analysis.