Cation Substitution Strategy for Developing Perovskite Oxide with Rich Oxygen Vacancy-Mediated Charge Redistribution Enables Highly Efficient Nitrate Electroreduction to Ammonia

The electrocatalytic nitrate (NO3–) reduction reaction (eNITRR) is a promising method for ammonia synthesis. However, its efficacy is currently limited due to poor selectivity, largely caused by the inherent complexity of the multiple-electron processes involved. To address these issues, oxygen-vacancy-rich LaFe0.9M0.1O3−δ (M = Co, Ni, and Cu) perovskite submicrofibers have been designed from the starting material LaFeO3−δ (LF) by a B-site substitution strategy and used as the eNITRR electrocatalyst. Consequently, the LaFe0.9Cu0.1O3−δ (LF0.9Cu0.1) submicrofibers with a stronger Fe–O hybridization, more oxygen vacancies, and more positive surface potential exhibit a higher ammonia yield rate of 349 ± 15 μg h–1 mg–1cat. and a Faradaic efficiency of 48 ± 2% than LF submicrofibers. The COMSOL Multiphysics simulations demonstrate that the more positive surface of LF0.9Cu0.1 submicrofibers can induce NO3– enrichment and suppress the competing hydrogen evolution reaction. By combining a variety of in situ characterizations and density functional theory calculations, the eNITRR mechanism is revealed, where the first proton–electron coupling step (*NO3 + H+ + e– → *HNO3) is the rate-determining step with a reduced energy barrier of 1.83 eV. This work highlights the positive effect of cation substitution in promoting eNITRR properties of perovskites and provides new insights into the studies of perovskite-type electrocatalytic ammonia synthesis catalysts.


INTRODUCTION
Ammonia (NH 3 ), an essential chemical in the nitrogen cycle network, is a fundamental compound for human beings that has been widely used as a nitrogen source for the synthesis of fertilizers, explosives, plastics, and so on. 1 Currently, NH 3 is mainly produced via the energy-intensive Haber−Bosch process under harsh conditions (300−500 °C, 200−300 bar), 2 accompanied by an abundant amount of greenhouse gas emissions (the global average is 2.86 tons of carbon dioxide per ton of NH 3 ). 3−8 Recent research indicates that metals, alloys, nitrides, oxides, and carbides can function as potential eNRR catalysts. 9espite significant achievements in recent years, the eNRR still suffers from low Faradaic efficiencies (FEs, mostly <30%) and limited NH 3 yield rates (mostly <200 μg h −1 mg −1 cat.). 9,10−13 Owning to the high solubility (the solubility of NaNO 3 is 87.6 g in 100 mL of water at 293 K) and the weak N�O double bond (binding energy of 204 kJ mol −1 ) of nitrate (NO 3 − ), the electrocatalytic nitrate reduction reaction (eNITRR) has emerged as an alternative technology to the eNRR process that can improve both the NH 3 yield rate and the FE. 14,15−19 Therefore, finding ways to use it by the eNITRR can also tackle environmental issues simultaneously.However, designing advanced electrocatalysts with high selectivity for transforming NO 3 − to NH 3 via the eNITRR remains a challenge because the eNITRR is a complicated 8e − transfer reaction involving many intermediates (e.g., NO 2 , NO, N 2 O, N 2 , NH 2 OH, NH 3 , and NH 2 NH 2 ). 15Among the various eNITRR electrocatalysts, Cu-containing catalysts demonstrate superior performance compared to other catalysts. 20However, the issue of instability in copper-containing catalysts limits their applications, which has also sparked research interest in the reaction mechanism of Cu-containing catalysts and the design of highly stable Cucontaining catalysts.For instance, extensive research has been conducted on various aspects, such as regulating the d-band center of Cu-containing catalysts, 14 the electrochemical restructuring of Cu-containing catalysts, 20−22 and the synergistic effects between Cu and other metals. 23,24Nonetheless, addressing the stability concern associated with Cu still poses a significant challenge.
−27 Perovskite oxides (ABO 3 , where the A-site cations are alkaline-earth or rare-earth metals and B-site cations are transition metals) are conventionally prepared by ball milling or sol−gel methods, resulting in an uncontrollable and blocked structure with severe particle agglomeration. 28,29Generally, it has been demonstrated that exposing accessible active sites of perovskite oxides by constructing specific geometrical structures, such as SrNb 0.1 Co 0.7 Fe 0.2 O 3−δ nanorods, 30 BiFeO 3 nanosheets, 31 NaNbO 3 nanocubes, 32 and so on, can result in a highly efficient electrocatalytic process.Among them, the cross-linked network woven by one-dimensional (1D) perovskite oxide submicrofibers can not only facilitate reactant diffusion and electron transport during the electrocatalytic process but also avoid particle agglomeration to expose the maximum number of active sites for the adsorption and reduction of NO 3 − ions.In addition to the construction of the unique structure, the activity of the perovskite is also affected by the electronic environment around the active sites. 33As the perovskite oxides can accommodate ∼90% of metallic elements of the periodic table, their high compositional flexibility enables the incorporation of metal elements to form a B-site bimetallic perovskite.Based on the molecular orbital theory and band theory, the catalytic performance of perovskite oxides can be influenced by tailoring the octahedral structure of their [BO 6 ] units, regulating the hybridization of B−O bonds, and generating oxygen vacancies (OVs).For example, the covalency of transition metal−oxygen bonds, which reflects the adsorption strength of oxygen-related intermediates, was verified to be an important descriptor for electrocatalytic activity. 28,34Therefore, it is rational to optimize the adsorption strength of intermediates in the eNITRR process by substituting the B-site cation with other elements.Apart from the variation of the electronic structure, the B-site cation substitution strategy will also affect the work function of the catalyst surface, which is accompanied by a change in surface potential. 35,36The surface potential and corresponding work function play important roles in facilitating the transport of electrons from the catalyst to the reactant.
Inspired by the Fe active sites in both Haber−Bosch catalysts (Fe-based compounds) and nitrogenase enzymes (mainly containing the Fe−Mo cofactor), a series of Fe-rich perovskite oxides of LaFe 0.9 M 0.1 O 3−δ (M = Co, Ni, and Cu) submicrofibers (noted as LF 0.9 Co 0.1 , LF 0.9 Ni 0.1 , and LF 0.9 Cu 0.1 submicrofibers, respectively) were constructed by a B-site substitution strategy and acted as the eNITRR electrocatalysts.The LF 0.9 Cu 0.1 submicrofibers showed a higher NH 3 yield rate of 349 ± 15 μg h −1 mg −1 cat.and a higher FE of 48 ± 2% than the parent LF submicrofibers, which are attributed to the inhomogeneous charge redistribution and abundant OVs on the surface of LF 0.9 Cu 0.1 submicrofibers, as well as its more positive surface potential and lower work function for achieving an enhanced adsorption ability toward NO 3 − ions.COMSOL Multiphysics simulations also confirm this discovery from the perspective of a theoretical calculation.The reaction mechanism was investigated in detail by combining operando Fourier transform infrared (FT-IR) spectroscopy, online differential electrochemical mass spectrometry (DEMS), and density functional theory (DFT) calculations.In the deoxidation process of the reaction stages, the first proton− electron coupling step of *NO 3 + H + + e − → *HNO 3 is the rate-determining step (RDS).The substitution of Cu in the B site lowers the barrier energy in this step, thereby facilitating the reaction.

RESULTS AND DISCUSSION
The LaFeO 3 (LF) and LF 0.9 M 0.1 (M = Co, Ni, and Cu) submicrofibers were synthesized via an electrospinning technique followed by calcination, as illustrated in Figure 1a.In a typical procedure, the metal precursors as nitrate salts and polyvinylpyrrolidone were dissolved in N,N-dimethylformamide to form a homogeneous, viscous solution, which was used for the electrospinning process.Subsequently, the electrospun metal salt−polymer submicrofibers were calcined to generate 1D perovskite oxide submicrofibers (more experimental details can be found in the Supporting Information).As the scanning electron microscopy (SEM) images show in Figures 1b and S1, all these samples consist of 1D submicrofibers without obvious fracture or aggregation.As an example, LF 0.9 Cu 0.1 submicrofibers are stacked layer-by-layer and connected to form threedimensional networks (Figure 1b), with an average diameter of approximately 250 nm (Figure S2).Notably, the LF 0.9 Cu 0.1 submicrofiber was generated by concatenating abundant LF 0.9 Cu 0.1 nanoparticles, with diameters ranging between 50 and 70 nm, into an integrated structure.This is evident from the transmission electron microscopy (TEM) image (Figure 1c).The high-resolution TEM (HRTEM) image of LF 0.9 Cu 0.1 shows clear crystal fringes with a lattice spacing of ∼0.27 nm, which belongs to the (121) plane of the perovskite oxide (Figure 1d).Moreover, the energy-dispersive X-ray spectroscopy (EDX) mappings of LF 0.9 Cu 0.1 submicrofibers (Figure 1e) show that La, Fe, O, and Cu elements are dispersed throughout the nanostructure homogeneously.The perovskite crystal phases of LF, LF 0.9 Co 0.1 , LF 0.9 Ni 0.1 , and LF 0.9 Cu 0.  123), (242), and (204) planes, respectively, of orthorhombic perovskite structures (JCPDS 88−0641) without any impurity phase.It should be noted that the diffraction peaks would shift slightly after cationic substitution, such as the (121) peak, which may imply the lattice variation due to the cation substitution. 37The crystal structures were further confirmed by Rietveld refinement of the XRD patterns (Figure S4) in Table S1.By taking LF 0.9 Cu 0.1 as an example, the decrease in unit cell volume indicates that the substitution of Cu caused lattice shrinkage due to the reduced ionic radii of Cu 2+ ions.The atomic ratios of Fe/M in LF 0.9 Co 0.1 , LF 0.9 Ni 0.1 , and LF 0.9 Cu 0.1 submicrofibers were also checked to be approximately 9:1 by inductively coupled plasma mass spectrometry (ICP−MS) (Table S2).In short, the 1D perovskite submicrofibers formed by the directional accumulation of nanoparticles with tunable B-site bimetallic cations were successfully synthesized by the electrospinning technique and calcination process.
The eNITRR performance of various perovskite submicrofibers was evaluated in an H-type electrolytic cell under ambient conditions, where 0.  the eNITRR process.When the potential is more negative than −0.6 V vs the reversible hydrogen electrode (RHE), the current density for the LSV curve with NO 3 − is enhanced significantly (Figure 2a), suggesting the occurrence of the eNITRR process.Figure 2b shows the time-dependent current density curves of LF 0.9 Cu 0.1 submicrofibers at different working potentials.The negligible decay in current density indicates the considerable catalytic stability of LF 0.9 Cu 0.1 submicrofibers.From the summarized FE values and NH 3 yield rates for various samples (LF, LF 0.9 Co 0.1 , LF 0.9 Ni 0.1 , and LF 0.9 Cu 0.1 ) in Figures 2c,d, S8, and S9, the FEs for NH 3 of these samples follow a volcano-type trend as a function of applied potentials.This phenomenon can be attributed to the increased competition from the HER at more negative potentials, coupled with a decrease in the available charges at more positive potentials.Notably, the LF 0.9 Cu 0.1 submicrofibers exhibit the best eNITRR performance with a maximal FE value of 48 ± 2% and a high NH 3 yield rate of 349 ± 15 μg h −1 mg −1 cat. at −0.90 V vs RHE.Additionally, the 1 H nuclear magnetic resonance method (Figure S10) was also conducted as another quantitative measurement to check the NH 3 yield rates for all electrocatalysts at a potential of −0.9 V vs RHE, which are in good agreement with the results by the UV−vis spectroscopy method (Figure S11).The N-selectivity indicates the ratio of NH 4 + −N in converted NO 3 − −N, and is an important parameter that reflects the degree of conversion from NO 3 − to NH 4

+
. By testing the NO 3 − concentrations for various samples after 2 h eNITRR processes at −0.9 V vs RHE, it can be observed that all the perovskite oxides display high Nselectivities of NH 3 of over 80% (Figure 2e), which are comparable to or better than other reports for NO 3 − electroreduction (Table S3).The N-selectivities of the four samples for NO 2 − are close to 20%, indicating that their main soluble byproduct is NO 2 − (Figure S12).To evaluate the eNITRR performance of LF 0.9 Cu 0.1 comprehensively, the NH 3 yield rates and FEs at different concentrations were also measured.The results, as depicted in Figure S13, demonstrate that the catalyst maintains high catalytic activity even at higher concentrations.
To exclude the interference from the electrocatalyst itself or the external environment, a comparison test was performed in 0.5 M Na 2 SO 4 electrolyte without NO 3 − .As shown in Figure S14, no NH 3 can be detected, which indicates that the produced NH 3 during the eNITRR process originates from the

NO 3
− ions in the Na 2 SO 4 electrolyte.Furthermore, the electrocatalytic and structural stabilities of the LF 0.9 Cu 0.1 submicrofibers were evaluated by 6 consecutive electrolysis cycles at −0.9 V vs RHE.As shown in Figure 2f, the NH 3 yield rate and FE value of each cycle fluctuate slightly but remain stable, suggesting the excellent electrocatalytic stability of LF 0.9 Cu 0.1 submicrofibers for potential practical applications.The perovskite structure of LF 0.9 Cu 0.1 submicrofibers is also maintained well based on the XRD pattern of LF 0.9 Cu 0.1 / carbon paper after the eNITRR process (Figure S15), which illustrates the excellent structural stability of LF 0.9 Cu 0.1 submicrofibers.ICP−MS was employed for the evaluation of changes in the composition of LF 0.9 Cu 0.1 submicrofibers after the stability test.The results show that no significant changes have occurred in the composition of the catalyst, further highlighting its excellent stability (Table S4). Figure S16 shows the Nyquist plots of the LF and LF 0.9 Cu 0.1 submicrofibers.The charge transfer resistance of LF 0.9 Cu 0.1 (28.6 ± 3.9 Ω) is smaller than that of LF (40.0 ± 3.0 Ω), which indicates that the electron transfer rate is faster in LF 0.9 Cu 0.1 .To investigate the influence of active sites on the eNITRR, we calculated the electrochemically active surface areas (ECSAs) of LF and LF 0.9 Cu 0.1 .As shown in Figures S17 and S18, the double-layer capacitance of LF 0.9 Cu 0.1 is 0.31 mF cm −2 , which is almost equal to that of LF at 0.27 mF cm −2 .Based on this, the activity gap between LF and LF 0.9 Cu 0.1 was not strongly correlated with the ECSA.
The valence state and electronic structural information were explored by X-ray photoelectron spectroscopy (XPS) and synchrotron-based X-ray absorption spectroscopy (XAS) techniques.Figure 3a shows Fe 2p XPS spectra of LF and LF 0.9 Cu 0.1 submicrofibers, and the peaks at 710.2 eV (Fe 2+ 2p 3/2 ), 712.1 eV (Fe 3+ 2p 3/2 ), 723.7 eV (Fe 2+ 2p 1/2 ), and 725.5 eV (Fe 3+ 2p 1/2 ) indicate the coexistence of Fe 2+ and Fe 3+ . 41,42he Fe 3+ /Fe 2+ ratio increases with the substitution of lowvalent Cu atoms in LF because more Fe 3+ ions are required to balance the charges (Table S5). 43This phenomenon also illustrates the charge redistribution in LF as a result of the cation substitution strategy. 44Moreover, the valence state of Fe is influenced by the filling degree of the d orbitals, thereby determining the d-band center of the catalyst.Therefore, the value for the d-band center of the Fe element becomes more positive with the increase of its valence state, leading to an enhanced adsorption of NO 3 − ions. 45The Fe coordination environments of LF and LF 0.9 Cu 0.1 were also investigated by synchrotron-based XAS measurements.The X-ray absorption near-edge structure (XANES) spectra of LF, LF 0.9 Cu 0.1 , standard Fe 2 O 3 , standard FeO, and Fe foil are shown in Figure 3b.The Fe K-edge XANES spectra of LF and LF 0.9 Cu 0.1 are found to be between those of FeO and Fe 2 O 3 , suggesting that their average valence states of Fe are both between +2 and +3.Meanwhile, XANES spectra of Fe in LF 0.9 Cu 0.1 has a higher energy than that in LF, indicating that the valence state of Fe in LF 0.9 Cu 0.1 is slightly higher than that in LF, 46 which is consistent with the XPS results.The Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectra in R-space for LF and LF 0.9 Cu 0.1 are shown in Figure 3c.There are three prominent peaks at about 1.5, 3.0, and 3.5 Å for LF and LF 0.9 Cu 0.1 that originate from the scattering paths of the Fe−O, La−Fe, and Fe−O−Fe bonds, 47 respectively.With partial Fe atoms substituted by Cu atoms, the length of the Fe−O bond decreases and indicates an enhanced hybridization of the Fe 3d−O 2p orbitals. 48The wavelet transform (WT)-EXAFS measurements were also used to provide insights into the coordination structures of Fe atoms in LF and LF 0.9 Cu 0.1 (Figures 3d,e and S19).The maximum intensity is closely associated with the path length, which can provide pivotal clues for identifying the coordination environment.Specifically, the scattering path signals at [χ(R), χ(k)] of [1.56, 7.0] and [1.50, 6.9] are associated with Fe−O bonds in the LF and LF 0.9 Cu 0.1 submicrofibers, respectively.The second set of WT-EXAFS peaks at [3.0, 7.9] and [3.0, 7.8] can be assigned to the La−Fe contributions in the LF and LF 0.9 Cu 0.1 submicrofibers, respectively.The third set of WT-EXAFS peaks at [3.5, 9.3] and [3.5, 8.9] originate from the Fe−O−Fe bonds in the LF and LF 0.9 Cu 0.1 submicrofibers, respectively.It is further demonstrated that the bond length of Fe−O reduces after Cu substitution, which agrees with the results of the FT-EXAFS spectra in R space.The fitting curves at the R space and k space are consistent with the FT-EXAFS spectra of LF and LF 0.9 Cu 0.1 (Figures S20 and S21).Based on the above analysis, it is evidenced that the B-site substitution strategy with low-valent metal elements can lead to charge redistribution and curtate Fe−O bond distance. 49It can be further inferred that with the increase of Fe 3d−O 2p orbital hybridization, the lattice O p bands shift toward the E Fermi as the Fe d states shift closer to the lattice O p energy. 35,48The shift of the lattice O p band lowers the formation energy of OVs.Subsequently, it facilitates the generation of more OVs on the surface, promoting full contact between the reactant and active B-site transition metals.The OVs in LF and LF 0.9 Cu 0.1 were probed via the XPS spectra of O 1s species and electron paramagnetic resonance (EPR) spectra.The corresponding deconvolution results of O 1s are shown in Figure 3f.The peaks at 528.8, 530.4,531.4,and 532.7 eV are assigned to lattice O 2− , a highly oxidative oxygen species (O 2 2− /O − ), surface-adsorbed O 2 or hydroxyl groups, and surface-adsorbed H 2 O, respectively. 38,50Based on the relative areas of peaks (Table S6), the concentrations of surface OVs, which are correlated with the O 2 2− /O − species, 51 were calculated to be 13.6 and 17.6% for LF and LF 0.9 Cu 0.1 , respectively.These results indicate that the surface OVs slightly increased with Cu substitution.To further study the variation of OVs, the EPR spectra of LF and LF 0.9 Cu 0.1 submicrofibers were also recorded, as shown in Figure S22.Both samples exhibit EPR signals at g = 2.004, which are identified as the trapped electrons in the OVs. 52Furthermore, the EPR signal intensity of LF 0.9 Cu 0.1 is higher than that of LF, indicating that more OVs have been generated in LF 0.9 Cu 0.1 .The chemical state of Cu of the LF 0.9 Cu 0.1 submicrofibers was subsequently checked by XPS measurements (Figure 3g).The four peaks at 932.4 eV (Cu + 2p 3/2 ), 934.0 eV (Cu 2+ 2p 3/2 ), 952.8 (Cu + 2p 1/2 ), and 954.5 eV (Cu 2+ 2p 1/2 ) suggest that there are two chemical states for the Cu element in LF 0.9 Cu 0.1 submicrofibers (Table S7). 53Considering the potential stability issues associated with Cu during the eNITRR process, XPS measurements of Cu were also conducted after the stability test for LF 0.9 Cu 0.1 (Figure S23).The results demonstrate that except for a slight decrease in the oxidation state, Cu still maintains the same two chemical states (Table S8).Following the bespoke analysis, it can be concluded that the charge redistribution and increased surface OVs induced by the B-site substitution strategy are expected to promote the eNITRR catalytic activity (Figure 3h−i).
The surface potential will be changed by B-site substitution in the perovskite structure. 35,36Therefore, Kelvin probe force Journal of the American Chemical Society microscopy (KPFM) was conducted to investigate the surface potential of the LF and LF 0.9 Cu 0.1 submicrofibers.The potential distributions at the LF 0.9 Cu 0.1 and LF submicrofiber surfaces are shown in Figure 4a,b with their corresponding topography images.A rectangular region (along the direction of the arrow) was chosen to quantify the surface potential

Journal of the American Chemical Society
variation.As shown in Figure 4c, the surface potential of LF 0.9 Cu 0.1 is roughly 66 mV higher than that of LF.The above results show that the work function of LF 0.9 Cu 0.1 is about 0.066 eV lower than that of LF according to the conversion formula between the work function and surface potential. 36A lower work function of LF 0.9 Cu 0.1 submicrofibers implies a smaller energy barrier for electron transfer to the reactant (NO 3 − ). 54otably, the surface potential of LF 0.9 Cu 0.1 submicrofibers becomes more positive, which indicates that LF 0.9 Cu 0.1 is more favorable to adsorbing and enriching NO 3 − .Therefore, it is advantageous by constructing eNITRR catalysts with more positive surface potential in addressing the issue of low activity resulting from weak mass transfer and small concentration gradients near the electrode region in low-concentration NO 3 − electrolytes. 55Meanwhile, COMSOL Multiphysics simulations were used to simulate the ionic distribution after the variation of the potential on the nanoreactor surface.Since the competitive HER can inhibit the eNITRR process, the concentration of hydrogen ions on the catalyst surface is also an important factor affecting the eNITRR performance.The variations of ion concentrations are described through the Nernst−Planck equation (see the details in the Supporting Information).Before the simulation, it is assumed that the anions and cations are distributed randomly, without electrostatic force.After the surface potential becomes more positive, the distributions of the NO 3 − and H + ions are shown in Figures 4d,e and S24.The NO 3 − concentration increases gradually, indicating a NO 3 − enrichment.On the contrary, the H + concentration at the surface of LF 0.9 Cu 0.1 submicrofibers decreases progressively.These results demonstrate that a surface with more positive potential would be beneficial for not only promoting the enrichment of NO 3 − ions but also reducing the HER process (Figure 4f).
The mechanism for the eNITRR process was experimentally probed by two-dimensional operando FT-IR spectroscopy and online DEMS.The infrared signals are collected from 1100 to 2200 cm −1 during a negative scan from −0.8 to −1.2 V vs RHE (Figure 5a).It can be concluded that as the potentials become more negative, the intensity of the characteristic peaks increases.This demonstrates the emergence of the rocking mode of −NH 2 at 1190 cm −1 , 56 the wagging mode of −NH 2 at 1307 cm −1 , 57 and M−N−O groups in the range of 1800−2000 cm −1 . 58Moreover, the molecular intermediates and products were detected by online DEMS with an applied voltage between −0.6 and −1.2 V vs RHE (Figure 5b).The m/z signals of 46, 30, and 17 appear during seven subsequent cycles and correspond to the NO 2 , NO, and NH 3 species, respectively.
DFT calculations were also conducted to explore the relationship between the Cu substitution and improved eNITRR activity.The charge density difference of LF 0.9 Cu 0.1 after adsorbing NO 3 − is plotted in Figure 5c, which shows a charge transfer from Fe to adsorbed NO 3 − and provides clues into the adsorption and activation of NO 3 − for subsequent hydrogenation.Based on the findings from DEMS/FT-IR spectroscopy measurements, the reaction pathway of NO 3 − electroreduction could be deduced, and the free energy of every intermediate over LF and LF 0.9 Cu 0.1 was calculated.According to a previous study, 38,59 the eNITRR process can be considered as a hydrogenation process (Figure S25).Before the calculation of the total reaction steps, it is crucial to determine the mode of NO 3 − adsorption and hydrogenation on the LF 0.9 Cu 0.1 surface.The free energies of two different pathways to form *NO 3 and *HNO 3 were calculated.As shown in Figure 5d, the free energy values of the NO 3 − coupling and first proton−electron coupling steps of single oxygen (O1 mode) on the Fe site are lower than those of dioxygen (O2 mode).For example, the free energy values for the first proton−electron coupling step (*NO 3 + H + + e − → *HNO 3 ) with the O1 and O2 modes are 1.83 and 2.67 eV, respectively, which indicate that the O1 mode is a more optimal structure for the coupling step between the LF 0.9 Cu 0.1 catalyst and NO 3 − species, as well as the following hydrogenation.It is noted that the byproduct NO 2 (g) can be produced in the side reaction *HNO 3 + H + + e − → *OH + NO 2 (g).The free energy of this potential intermediate was also calculated with a larger free energy of −2.61 eV than that of the step for the generation of the *NO 2 species (−3.86 eV).Therefore, the side reaction is much less likely to occur than the second proton−electron coupling step (*HNO 3 + H + + e − → H 2 O + *NO 2 ).After other possibilities are excluded, the total reaction pathway is shown as follows.First, the reactant NO 3 − is chemically absorbed on the catalyst surface to form *NO 3 with a decrease of total energy, indicating the driving force of this reaction step.Then, the N−O bond is continuously cleaved by proton-coupled electron transfer to form *NO 2 and *NO.During the following hydrogenation processes, the *NO intermediate is converted to *HNO, *H 2 NO, and *O species gradually.In the final active site refreshing process, the *O intermediate is coupled with two other protons and converted to a H 2 O molecule.To be noted, different intermediates were adsorbed on the surface of catalysts, and the most stable adsorption models are employed to describe the eNITRR process (illustrations of Figure 5d).For both LF 0.9 Cu 0.1 and LF surfaces, nonspontaneous reaction steps can be clearly observed that will influence the reaction rate of the eNITRR process.Among these nonspontaneous steps, the RDS is the first proton−electron coupling step (*NO 3 + H + + e − → *HNO 3 ), where the energy barriers are 2.07 and 1.83 eV for LF and LF 0.9 Cu 0.1 , respectively.This indicates that the eNITRR process can be facilitated on the surface of LF 0.9 Cu 0.1 .Combining all experimental results with the theoretical calculations, it can be concluded that the eNITRR performance can be promoted by the B-site substitution strategy.

CONCLUSIONS
In summary, we have demonstrated that perovskite LF 0.9 Cu 0.1 submicrofibers can act as a selective and efficient electrocatalyst to reduce NO 3 − to valuable NH 3 .At the optimal potential of −0.9 V, the NH 3 yield rate, FE, and selectivity can reach 349 ± 15 μg h −1 mg −1 cat., 48 ± 2%, and 88 ± 4%, respectively.Synchrotron-based XAS revealed the charge redistribution on the L 0.9 Cu 0.1 submicrofiber and confirmed the increase of hybridization of the Fe 3d−O 2p orbital, which favors the generation of surface OVs.Combined with the results of KPFM and COMSOL Multiphysics simulations, it is shown that a more positive surface can be induced by the cation substitution strategy, which promotes the fixation of more NO 3 − anions on the catalyst surface.Based on a variety of in situ characterizations and DFT calculations, the reaction pathway was deduced.The results showed that the lower free energy for the RDS (*NO 3 + H + + e − → *HNO 3 ) for LF 0.9 Cu 0.1 led to its optimal NH 3 synthesis performance.This work opens new avenues for designing efficient perovskite-type electrocatalytic NH 3 synthesis catalysts via a cation substitution strategy.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c06402.Detailed experimental procedures, supported physical and electrochemical characterization of materials by SEM, TEM, XRD, 1

5 M 3 −
Na 2 SO 4 with 50 ppm NO 3 − −N (NO 3 − −N represents the concentration of NO expressed in terms of the nitrogen content) aqueous solution was used as the electrolyte.The concentrations of reactant NO 3 − , reductive product NH 3 , and byproduct nitrite (NO 2 − ) were detected using colorimetric methods (Figures S5−S7). 38− 40 The linear sweep voltammetry (LSV) curves of LF 0.9 Cu 0.1 submicrofibers in 0.5 M Na 2 SO 4 electrolytes with and without NO 3 − were recorded to investigate the potential window for

Figure 2 .
Figure 2. (a) LSV curves of LF 0.9 Cu 0.1 submicrofibers in 0.5 M Na 2 SO 4 electrolyte with and without NO 3 − .(b) Time-dependent current density curves of the LF 0.9 Cu 0.1 submicrofibers against various work potentials.(c) FE values of LF and LF 0.9 Cu 0.1 submicrofibers at each given potential.(d) NH 3 yield rates and (e) N selectivities of LF and LF 0.9 M 0.1 (M = Co, Ni, and Cu) submicrofibers at −0.9 V vs RHE.(f) Cycling tests of LF 0.9 Cu 0.1 submicrofibers at −0.9 V vs RHE.

Figure 3 .
Figure 3. (a) Fe 2p XPS spectra in LF and LF 0.9 Cu 0.1 submicrofibers.(b) Fe K-edge XANES spectra (inset: enlarged view of the absorption edge position) of LF, LF 0.9 Cu 0.1 , Fe foil, FeO, and Fe 2 O 3 .(c) Fe K-edge FT-EXAFS spectra of LF and LF 0.9 Cu 0.1 .WT-EXAFS at Fe K-edge of (d) LF and (e) LF 0.9 Cu 0.1 .(f) O 1s XPS spectra of LF and LF 0.9 Cu 0.1 submicrofibers.(g) Cu 2p XPS spectrum of LF 0.9 Cu 0.1 submicrofibers.(h) Schematic illustration of the charge redistribution and generation of OVs in LF 0.9 Cu 0.1 .(i) Illustration for the eNITRR on the surface of LF 0.9 Cu 0.1 submicrofibers.

Figure 4 .
Figure 4. Surface potential distribution of (a) LF 0.9 Cu 0.1 and (b) LF submicrofibers.(c) Surface potential values extracted in (a,b).Top view of the model of the variation of (d) NO 3 − and (e) H + concentrations on the fiber surface.(f) Schematic illustration of the ion movement on the catalyst surface with more positive potential.

Figure 5 .
Figure 5. (a) Two-dimensional operando FT-IR spectra of LF 0.9 Cu 0.1 .Here, the pink represents high values of transmittance, while the green represents low values of transmittance.(b) DEMS signals of gaseous intermediates/products during seven cycles.(c) Charge density difference of LF 0.9 Cu 0.1 coupled with NO 3 -.(d) Gibbs free energy diagrams of the eNITRR on the surface of LF and LF 0.9 Cu 0.1 .