Interfacial Engineering of SeO Ligands on Tellurium Featuring Synergistic Functionalities of Bond Activation and Chemical States Buffering toward Electrocatalytic Conversion of Nitrogen to Ammonia

Abstract Ammonia (NH3) production from electrochemical nitrogen (N2) reduction reaction (NRR) under ambient conditions represents a sustainable alternative to the traditional Haber–Bosch process. However, the conventional electrocatalytic NRR process often suffers from low selectivity (competition with the hydrogen evolution reaction (HER)) and electron transfer bottleneck for efficient activation and dissociation. Herein, a strategy to simultaneously promote selectivity and activity through dual‐incorporation of Se and O elements onto the shell of HER‐inactive Te nanorods is reported. It is theoretically and experimentally verified that the exposure of lone‐pair electrons in the TeO2 shell of Se, O dual‐doped Te nanorods can maximize orbits overlap between N2 and Te for N‐N bond activation via π‐backdonation interactions. Further, the Gibbs free energy change indicates that the Lewis‐basic anchor ‐SeO ligand with strong electron‐donating characteristics serves as an electron reservoir and is capable of buffering the oxidation state variation of Te, thereby improving the thermodynamics of desorption of the intermediates in the N2‐to‐NH3 conversion process. As expected, a high faradaic efficiency of 24.56% and NH3 yield rate of ≈21.54 µg h−1 mg−1 are obtained under a low overpotential of ≈0.30 V versus reversible hydrogen electrode in an aqueous electrolyte under ambient conditions.

S3 PHI5000 Versa Probe system (Physical Electronics, MN), and binding energy was calibrated against reference of C1s peak at 284.8 eV. O and Se K-edge X-ray absorption fine structure spectroscopy (XAFS) were respectively carried out at 4B7B beamline at Beijing Synchrotron Radiation Facility (BSRF) China and Shanghai Synchrotron Radiation Facility (SSRF) China.
Nitrogen temperature programmed desorption (N 2 -TPD) measurements were performed using a chemisorption apparatus. 20 mg of catalysts was placed in the glass tube, and were pretreated at 150 o C for 1 h, and cooled down to 50 o C. Adsorption of N 2 was conducted in a 99.999% N 2 gas flow for 3 h at 50 o C. After purging with He gas for 0.5 h, the sample was heated from 50 to 300 o C. TPD signal was recorded using a thermal conductivity detector. Diffuse reflectance Fourier transform infrared (DRIFT) spectra were recorded at room temperature in a diffuse reflectance cell with CaF 2 windows on a BRUKER tensor 27 FTIR spectrometer equipped with a vacuum.

Electrochemical measurements.
Electrochemical measurements were carried out in three-electrode system at an electrochemical station (Gamry). 5 mg of catalyst and 40 μL of Nafion solution (5 wt%) were dispersed in 960 μL of water-ethanol solution with the volume ratio of 1:3 by sonication for 1 h. After that, 10 μL of dispersion was loaded onto a glassy carbon Electrode with the diameter of 5 mm (Note: N-free Nafion should not be a source of ammonia). For N 2 electrochemical reduction, the electrochemical measurements were carried out in an H-cell system which was separated by Nafion 115 membrane. The gases used in this work were primarily scrubbed (using water) to remove background NH 3 or NH 3 from exogenous sources, S4 as well as the NO x contaminants. After that, the level of such species present was determined using the gas chromatography (Agilent 490-pro). As shown in following Figure, two obvious peaks at the retention time ~1.5 min and ~1.5 min were respectively ascribed to O 2 and N 2 in channel I, while only the peak assigned to mixture of O 2 and N 2 can be observed in channel II. This indicates that contaminants of NO, NO 2 and NH 3 can be excluded before electrocatalytic reduction step.
Ag/AgCl electrode and Graphite rod were applied as reference electrode and counter electrode, respectively. All potentials in this study were measured against Ag/AgCl reference electrode and converted to RHE reference by E RHE = E Ag/AgCl + 0.21 V + 0.0591 × pH. N 2 electrochemical reduction was conducted in N 2 -saturated 0.1 M HCl solution. After N 2 was purged into HCl solution for at least 30 min to remove residual air, controlled potential electrolysis was performed at applied potentials for 2 h.

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Determination of ammonia. The concentration of ammonia was determined by the indophenol blue approach. 2 mL of 1 M NaOH solution containing salicylic acid and sodium citrate was added into 2 mL of electrolyte after NRR, followed by addition of 1 mL of 0.05 M NaClO and 0.2 mL of C 5 FeN 6 Na 2 O. The absorption spectrum was measured using an UV-vis spectro-photometer. The concentration of indophenol blue was determined using absorbance at wavelength of 655 nm.
Concentration-absorbance curve was calibrated using standard ammonia chloride solution with a serious of concentrations. Fitting curve (y=0.065 x + 0.033, R 2 =0.997) shows good linear relation of absorbance value with NH 3 concentration by three times independent calibrations.
Faradaic efficiency for NH 3 production was calculated at a given potential as follow: C NH3 : the measured NH 3 concentration; V: volume of the electrolyte; N: the number of electrons transferred for product formation, which is 3 for NH 3 ; F: Faraday constant, 96485 C mol -1 ; Q: quantity of electric charge integrated by i-t curve.
To identify the source of ammonia, the isotopic labeling experiment using 15 N 2 gas (99% 15 N≡ 15 N, Shanghai Yuanneng Biotechnology Co., Ltd.) as the feeding gas was thereafter performed. In view of limited supply and high expense of 15 N 2 , the velocity of 15 N 2 gas flow was set at 5 mL min -1 . After electrolytic reaction for 6h, + -contained electrolyte was detected by using 1 H (nuclear magnetic resonance, 600 MHz) NMR spectroscopy.

Calculation Details
The surfaces of Te (111)  According to the method presented by Nørskov, the Gibbs free energy diagrams were estimated by the following equation, where ∆E is the energy change between the reactant and product obtained from DFT calculations; ∆ZPE is the change of zero point energy; T and ∆S denote temperature and change of entropy, respectively. i represents three intermediates; U is the potential S7 measured against normal hydrogen electrode (NHE) at standard conditions; e is the transferred charge, T is the temperature with unit K). In here, T = 300K was considered. Se, O on shell had no effect to its original crystal phase. However, with increasing of the amount of Na 2 SeO 3 , the decreased peak intensities were ascribed the exess Se, O incoporation. S11 Figure S5. High-resolution XPS spectra of Te for pristine Te NWs and different

Supplementary Figures
STRs products, respectively. The intensities of peak ~575.95 eV assigned to Te 4+ were remarkably increased with the growth of Se, suggesting that amount of surface Se or O decoration increased with the growth of the addtion of the Se precursor. S12 Figure S6. Raman spectra of the STRs, Te NWs, and Se powder, respectively. In contrast to Raman spectra for pristine Se, disappeared Raman characteristic peaks at 236 and 458 cm −1 excluded the presence of chain-structured and annular Se in STRs S13 Figure S7. O K-edge XANES spectra of the Te-based materials. In general, the peaks at ~533, ~534-537, and ~545 eV were assigned to t 2g , e g , and sp molecular orbits (MOs), respectively. In contrast to TeO 2 reference, the peak at ~533 eV assigned to the t 2g obviously decreased in STRs materials, which was due to charge redistribution induced by hybridization with Se atoms. S14 Figure S8. Nitrogen adsorption-desorption isotherms of the different catalysts. In contrast to the BET specific surface area of 1.5 m 2 g -1 for the pristine Te NWs, the hierarchical-structured STRs catalyst should have a positive influence on BET surface area, which was almost ~3 times higher than Te NWs. S15 Figure S9. Illustration of free energy diagram for distal (red line) and alternavtive (red line) NRR pathways on Te-based samples. In distal pathway, the adsorbed N 2 will be hydrogenated by adsorbing a proton coupled with electron transfer, resulting in formation of N2H*. After that, (H + + e − ) consecutively reacts with distal N atom to formation of N2H*. NH 3 can be detached after interaction of prehydrogenated N site in N2H2*. After that, remaining N* species will be hydrogenated to second NH 3 via a similar path by another three protons coupled with electrons. In addition to TeO 2 , in view of the smaller overpotential, NRR occurring on Te-based catalysts preferred to proceed through alternating mechanism. S16 Figure S10. Catalytic performance of Te-contianing catalyts during electrocatalytic N 2 reduction process. Yield rate of NH 3 production and Faradaic efficiencies at each given potential for 2 h using different Te-based catalysts. Although pristine Te NWs exhibited the highest polarization cuurent, the relative low NH 3 production rate under various overpotential was mainly due to the sluggish kinetics of N 2 absorption and strong HER competion. In the presence of Se, NH 3 production rate increased to a peak value when suitiabe introduction of Se onto Te surface. However, due to chemical bonding effect, the NH 3 production catalyzed by the excessive Se-coated Te NWs catalysts were decreased. S17 Figure S11. UV-vis absorption spectra of electrolyte after electrolysis at −0.3 V for 2h under control conditons. To verify that reduction product was generated via NRR catalyzed by Te-based catalysts, comparing experiments were performed using carbon paper as the working electrode. Ar/N 2 gas flow is introduced into an electrochemical reaction cell at potential of -0.3 V for 1 h. UV-vis absorption spectra presented no NH 3 product is detected in these conditions. S18 Figure S12. Durability test for the STRs toward N 2 electrochemical reduction at different given potentials. S19 Figure 13. Cycling tests for the NRR using STRs catalyst.