fig4

Figure 4. Mechanistic investigation of nitrate electroreduction on HS-LFP. (A) ¹H NMR spectra of products after electrolysis in ¹⁴NO₃^- and ¹⁵NO₃^--containing electrolytes; (B) EPR spectra recorded in nitrate-free electrolyte and after NO₃^- addition. * indicates the characteristic peaks of the DMPO-H adduct; (C) In-situ FTIR spectra collected at different applied potentials; (D) Atomic structure models of FePO₄ and Fe-O₄/Fe-O₆/FePO₄; (E) Charge density difference diagrams for reactant adsorption on the two catalysts, with net electron transfer labeled; (F) PDOS of Fe d-orbitals in FePO₄ and Fe-O₄/Fe-O₆/FePO₄, with d-band center and Fermi level indicated; (G) Comparison of NO₃^- adsorption energies on the two catalysts; (H) Free energy diagrams for nitrate reduction to NH₃ on FePO₄ and Fe-O₄/Fe-O₆/FePO₄ representative models respectively. RHE: Reversible hydrogen electrode; PDOS: projected density of states; ¹H NMR: hydrogen-1 nuclear magnetic resonance; HS-LFP: the S-LFP-derived slag; EPR: electron paramagnetic resonance; FTIR: Fourier transform infrared.