Nb-doped NiO nanoflowers for nitrite electroreduction to ammonia

Summary Electrocatalytic reduction of nitrite to ammonia (NO2RR) is considered as an appealing route to simultaneously achieve sustainable ammonia production and abate hazardous nitrite pollution. Herein, atomically Nb-doped NiO nanoflowers are designed as a high-performance NO2RR catalyst, which exhibits the highest NH3-Faradaic efficiency of 92.4% with an NH3 yield rate of 200.5 μmol h−1 cm−2 at −0.6 V RHE. Theoretical calculations unravel that Nb dopants can act as Lewis acid sites to render effective NO2− activation, decreased protonation energy barriers, and restricted hydrogen evolution, ultimately leading to a high NO2RR selectivity and activity.


RESULTS AND DISCUSSION
The synthesis of NbÀNiO nanoflowers is conducted by the combined hydrothermal and calcination methods (Figure 1A).The X-ray diffraction (XRD) patterns of both pristine NiO and NbÀNiO (Figure 1B) show the characteristic diffraction peaks of cubic NiO (No. 78À0643). 33 detailed inspection reveals that NbÀNiO delivers a slightly lower peak intensity and wider full width at half maximum compared to pristine NiO, arising from the incorporation of Nb dopants in NbÀNiO (Figure S1).Representative scanning electron microscopy (SEM) (Figures 1C  and 1D) image of NbÀNiO shows a typical nanoflower structure comprising many vertically aligned nanosheets, similar to that of original NiO (Figure S2A).The thin nanosheet feature of NbÀNiO (Figure 1E) and NiO (Figure S2B) can be further revealed by the transmission electron microscopy (TEM) image showing clear wrinkles and corrugations.In addition, the high-resolution transmission electron microscopy (HRTEM) image exhibits a clear lattice fringe of 0.24 nm, correlating well with (200) crystallographic plane of cubic NiO (Figure 1F).Elemental mapping images (Figure 1G) unveil that Nb dopants are uniformly dispersed over the whole surface of NbÀNiO nanoflowers.
The X-ray absorption spectroscopy (XAS) characterizations are conducted to evaluate the coordination environment of NbÀNiO.The Nb KÀedge X-ray absorption near edge structure (XANES) spectra (Figure 2A) show that the absorption edge of Nb-NiO is situated between Nb foil and Nb 2 O 5 , indicating that Nb dopants are in oxidation state.Linear XANES fitting result reveals that the average Nb valence state is +3.4 (Figure S3).The Nb KÀedge extended X-ray absorption fine-structure (EXAFS) spectrum of NbÀNiO (Figure 2B) reveals a dominant peak at 1.54 A ˚, which is assigned to NbÀO first coordination shell.5][36] Besides, the 2.65 A ˚peak is assigned to NbÀNi second coordination shell.Similarly, the wavelet-transformed (WT, Figure 2D) profiles display that NbÀNiO exhibits two NbÀO and NbÀNi intensity maxima.EXAFS fitting analysis shows that the isolated Nb atom coordinates with five adjacent O atoms to form geometric Nb 1 ÀO 5 moiety (Figure 2C; Table S1).
Density functional theory (DFT) computations are performed to investigate the electronic structures of NbÀNiO.On the basis of XRD and HRTEM results, we select (200) plane of NiO slab for NbÀNiO structural modeling.As seen in Figure S4, by substituting a surface-exposed Ni atom with an Nb dopant, the resulting NbÀNiO shows a rather negative formation energy of À2.46 eV, suggesting that Nb dopant incorporated in NiO lattice is thermodynamically feasible. 37Charge density difference and electron location function (ELF, Figure 2E) maps of NbÀNiO exhibit the noticeable electron-deficient regions around Nb dopant.This can be further verified by the detailed charge analysis (Figure S5), in which Nb dopant (+1.12 |e|) is more positively charged than Ni (+0.77 |e|) and thus Nb dopants can serve as Lewis acid sites to activate and polarize NO 2 À during the NO 2 RR process. 38The projected density of states (PDOS, Figure 2F) analysis displays that NiO possesses a distinct band gap, indicating its semiconducting nature.In stark contrast, introducing Nb dopant in NiO generates significant electronic states crossing the Fermi level, suggesting the metallic character and improved conductivity of NbÀNiO. 39Meanwhile, as shown in Figure 2G (Figure S6), the calculated work function (F) value of NbÀNiO is 3.42 eV, which is lower than that of NiO (3.81 eV).Thus, the proton-coupled electron transfer process and the electrocatalytic NO 2 RR kinetics can be significantly facilitated on NbÀNiO. 40Moreover, AIMD simulations of NbÀNiO display the equilibrium states of energy and temperature (Figure S7), signifying the high thermodynamic stability of NbÀNiO. 40Electrochemical NO 2 RR performance of NbÀNiO is evaluated in 0.5 M Na 2 SO 4 + 0.1 M NaNO 2 solution using an H cell based on a standard procedure flow chart (Figure S8). 14 The produced liquid and gas products after NO 2 RR electrolysis are determined by colorimetric and gas chromatography methods (Figures S9ÀS11), [41][42][43][44] respectively.The linear sweep voltammetry (LSV) curves of NbÀNiO are measured firstly (Figure 3A), and a significant increase in current density (j) is observed for NO 2 À -containing electrolyte compared to NO 2

À
-free electrolyte, signifying the high NO 2 RR activity of NbÀNiO.Subsequently, the NO 2 RR performance of NbÀNiO is quantitatively evaluated at various potentials using the combined chronoamperometry (Figure 3B) and colorimetric tests.Figure 3C shows that NbÀNiO shows a maximum FENH 3 of 92.4% at À0.6 V, with the corresponding NH 3 yield rate reaching 200.5 mmol h À1 cm À2 .Such NO 2 RR performance is better than that of most reported NO 2 RR catalysts as depicted in Figure 3D and Table S2.][47][48] Regarding the NO 2 RR selectivity, NbÀNiO exhibits fairly low Faradaic efficiencies (FEs) for H 2 , NH 2 OH, and N 2 H 4 by-products relative to FENH 3 (Figure S14), confirming a high NO 2 RR selectivity of NbÀNiO toward the NH 3 generation.This finding can be further confirmed by the time-dependent NO 2 RR electrolysis (Figure S15), which shows a considerably decreased NO 2 À ÀN concentration coupled with a significantly increased NH 3 ÀN concentration as the electrolysis time increases.As a comparison, we evaluate the NO 2 RR performance of pristine NiO (Figure 3E), which exhibits much lower NO 2 RR activity and selectivity than NbÀNiO.Specifically, NbÀNiO outperforms pristine NiO by 2.3 and 1.3 times in NH 3 yield rate and FENH 3 at À0.6 V, respectively.Besides, NbÀNiO displays a higher electrochemical active surface area (ECSA, Figure S16) than NiO, while the catalyst performance normalized by ECSA (Figure S17) exhibits the same trend with Figure 3E, suggesting the high intrinsic activity of NbÀNiO toward the NO 2 RR.As for the electrocatalytic stability of NbÀNiO, slight changes in NH 3 yield rates and FENH 3 over six consecutive cycles can be seen, indicating an excellent cycling stability of NbÀNiO (Figure 3F).[54]  Theoretical calculations are conducted to elucidate the mechanism for the Nb-doping-induced enhanced NO 2 RR performance of NbÀNiO.Upon the NO 2 À adsorption on NbÀNiO (Figure S19), the electron-deficient Nb dopant, as previously determined in Figure 2E, can serve as Lewis acid site to favorably absorb Lewis base NO 2

À
[57][58] Charge density difference analysis (Figures 4A and 4B) reveals a remarkable NbÀ*NO 2 electronic coupling where Nb dopant donates À0.32 | e| to *NO 2 , in stark contrast to À0.12 |e| for NiÀtoÀ*NO 2 charge transfer.In addition, the free energy diagram (Figure 4C; Figure S20) presents that both Nb dopant of NbÀNiO and Ni site of NiO exhibit the same rate determining step (RDS) of *NO / *NHO. 59Nonetheless, Nb dopant exhibits a much reduced RDS energy barrier compared to Ni site (À2.26 eV).Besides, Nb dopant presents much lower free energies of all protonation intermediates than Ni site.Both findings demonstrate that Lewis acid Nb dopant serve as active site to significantly enhance the protonation energetics to boost the NO 2 À ÀtoÀNH 3 conversion process on NbÀNiO.Considering that HER is the main competing reaction of NO 2 RR, 60 the HER activity of NO 2 RR-active Nb-dopant site is further investigated.As displayed in Figure 4D, the binding free energy of *H on Nb dopant of NbÀNiO is calculated as 0.84 eV, which is much positive than that of *NO 2 (À0.51 eV), confirming an unfavorable HER performance of NbÀNiO, which is attributed to the Lewis acidity of Nb dopant capable of repelling the binding of positively charged H. Additionally, molecular dynamics (MD) simulations (Figure 4E) reveal that the snapshot after simulation (Figure S21) shows the aggregation of evident NO 2 À on NbÀNiO, and the calculated radial distribution function (RDF) curves (Fig- ure 4E) present a more intense NbÀNiO/*NO 2 À interaction compared to NbÀNiO/*H interaction, [61][62][63] further corroborating that NbÀNiO is able to selectively adsorb NO 2 À and suppress H coverage, thus facilitating the boosted NO 2 RR and inhibited HER.These theoretical results reveal that the Lewis acid Nb dopant of NbÀNiO plays a crucial role in enhancing the efficient adsorption and activation of NO 2

À
, boosting the protonation energetics and suppressing the HER, eventually leading to the high catalytic activity and selectivity of NbÀNiO for the NO 2 RR.

Conclusion
NbÀNiO has been proved to be an efficient and robust NO 2 RR catalyst.Theoretical computations suggest that the enhanced NO 2 RR performance of NbÀNiO originates from the key role of Lewis acid Nb dopant in suppressing the HER and enhancing NO 2 À activation and protonation.This work not only offers an in-depth understanding of the Lewis acid dopant-catalyzed NO 2 RR mechanism but also implies the great potential of constructing Lewis acid dopants in catalysts to achieve exceptional NO 2 À electroreduction and beyond.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following:

Lead contact
Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Dr. Ke Chu (chuk630@mail.lzjtu.cn).

Materials availability
This study did not generate new unique reagents.All chemicals were obtained from commercial resources and used as received.

Data and code availability
Data reported in this paper will be shared by the lead contact upon reasonable request.All original code is available in this paper's supplemental information.Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon reasonable request.

METHOD DETAILS
Synthesis of NbÀNiO

Electrochemical experiments
Electrochemical measurements were investigated with a CHIÀ760E electrochemical workstation using a conventional threeÀelectrode cell.NbÀNiO coated on carbon cloth (1 3 1 cm 2 , 0.5 mg cm À2 ) was used as the working electrode, Ag/AgCl (saturated KCl) electrode was used as the reference electrode, and Pt foil was used as the counter electrode.All potentials were referenced to reversible hydrogen electrode (RHE) by following equation: E (V vs. RHE) = E (V vs. Ag/AgCl) + 0.198V + 0.059 3 pH.The NO 2 RR measurements were carried out in 0.5 M Na 2 SO 4 + 0.1 M NaNO 2 electrolyte using an HÀtype twoÀcompartment electrochemical cell separated by a Nafion 211 membrane.After each chronoamperometry test for 0.5 h, the produced NH 3 and other possible by-product (N 2 H 4 ) were analyzed by various colorimetric methods using UV-vis absorbance spectrophotometer (MAPADA P5), while the gas products (H 2 , NH 2 OH) were analyzed by gas chromatography (Shimadzu GC2010).The detailed determination procedures are given in our previous publication. 46aradaic efficiency (FE) of NH 3 generation was calculated by the following equation:

Figure 2 .
Figure 2. Structural characteristics of NbÀNiO (A, B, and D) (A) Nb KÀedge XANES, (B) EXAFS spectra, and (D) WT profiles of NbÀNiO, Nb foil and Nb 2 O 5 .(C) EXAFS fitting analysis of NbÀNiO.(E) Charge density difference (top half) and electron location function (bottom half), yellow and red: charge accumulation, cyan and blue: charge depletion.(F and G) (F) PDOS profiles and (G) calculated work functions of NiO and NbÀNiO.

Figure 3 .
Figure 3. Electrochemical NO 2 RR tsts (A) LSV curves of NbÀNiO in various electrolytes.(B and C) (B) Chronoamperometry test of NbÀNiO at different potentials after 0.5 h electrolysis and (C) obtained NH 3 yield rates and FE NH3 .(D) Comparison of NH 3 yield rates and FENH3 between NbÀNiO and reported NO2RR catalysts.(E) Comparison of the NO 2 RR performance between NiO and NbÀNiO at À0.6 V. (F and G) (F) Cycling and (G) long-term stability tests of NbÀNiO at À0.6 V.

Figure 4 . 2 À
Figure 4. Theoretical analysis (A and B) Charge density difference plots of *NO 2 on (A) NiO and (B) NbÀNiO.Yellow: charge accumulation, cyan: charge depletion.(C) Free energy profiles of NO 2 RR process on NiO and NbÀNiO.(D) Free energies of absorbed H and NO 2 À on Nb-dopant site of NbÀNiO.(E) RDF curves of the interactions between NbÀdopant and NO 2 À /H + .

TABLE
d RESOURCE AVAILABILITY B Lead contact B Materials availability B Data and code availability d METHOD DETAILS B Synthesis of NbÀNiO B Electrochemical experiments B Characterizations 0.3 g Ni(NO 3 ) 2 $6H 2 O and 0.32 g C 10 H 5 NbO 20 were dispersed in 30 mL ethanol solution under stirring to form a transparent solution.Afterward, the solution was transferred into a 50 mL autoclave.After treatment at 150 C for 6 h, the light green precipitates were collected by centrifuging, washed with deionized water/ethanol and dried under vacuum overnight.The obtained precipitates were ground in an agate mortar and then transferred to a muffle furnace for calcination at 300 C for 4 h to obtain NbÀNiO.Pristine NiO was prepared by the same method as NbÀNiO by without addition of C 10 H 5 NbO 20 .