Supporting Information for. Baozhan Zheng,,* and Xuping Sun,* Institute of Fundamental and Frontier Sciences, University of Electronic Science and

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1 Supporting Information for Efficient Electrochemical N 2 Reduction to NH 3 on MoN Nanosheets Array under Ambient Conditions Ling Zhang,, Xuqiang Ji, Xiang Ren, Yonglan Luo, Xifeng Shi, Abdullah M. Asiri, ʃ Baozhan Zheng,,* and Xuping Sun,* Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu , China, College of Chemistry, Sichuan University, Chengdu , China, College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan , Shandong, China, and ʃ Chemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia * xpsun@uestc.edu.cn (X.S.); zheng_baozhan@outlook.com (B.Z.) S1

2 Content Experimental Section SEM images. Figure S1 UV-Vis absorption spectra and calibration curves Figure S2 UV-Vis absorption spectrum figure S3 Chronoamperometry curve and NRR performances...figure S4 SEM images and XRD pattern......figure S5 XPS spectra figure S6 UV-Vis absorption spectra figure S7 UV-Vis absorption spectra figure S8 UV-Vis absorption spectra and FEs...Figure S9 Chronoamperometry curve and NH 3 yields...figure S10 XPS spectra figure S11 EDX spectra...figure S12 XRD pattern......figure S13 1 H NMR spectra......figure S14 Simulation box...figure S15 Comparison of NRR performance.....table S1 S2

3 Experimental Section Materials: HNO 3, HCl, H 2 O 2 (30 wt%) N 2 H 4 H 2 O and C 2 H 5 OH were purchased from Chengdu Kelong Chemical Reagent Factory. NH 4 Cl, salicylic acid (C 7 H 6 O 3 ), sodium citrate (C 6 H 5 Na 3 O 7 ), para-(dimethylamino) benzaldehyde (p-c 9 H 11 NO), sodium nitroferricyanide dihydrate (C 5 FeN 6 Na 2 O 2H 2 O), sodium hypochlorite solution (NaClO), sodium molybdate dihydrate (Na 2 MoO 4 2H 2 O) and thiourea (CH 4 N 2 S) were purchased from Beijing Chemical Corp. (China). All the reagents were used as received without further purification. CC was provided by Hongshan District, Wuhan Instrument Surgical Instruments business. And it was pretreated in HNO 3 and then cleaned by sonication in water and C 2 H 5 OH for several times to remove surface impurities. The water used throughout all experiments was purified through a Millipore system. Synthesis of MoN NA/CC: In a typical synthesis, Na 2 MoO 4 2H 2 O (0.242 g) was dissolved in deionized water (22 ml) in a 50 ml beaker, into which thiourea (0.305 g) was added. After gently stirred for 30 min, the solution was then transferred to a 40 ml Teflon-lined stainless steel autoclave with a piece of CC (2 cm 3 cm). The autoclave was heated at 220 C for 24 h in an oven and then cooled down to room temperature naturally. Finally, the precursor on CC was taken out, rinsed with deionized water several times and dried at 60 C in air. To prepare MoN NA/CC, the precursor was placed in the furnace and heated to 800 C for 3 h with a heating speed of 5 C min 1 under a flowing NH 3 atmosphere. The system was allowed to cool down to room temperature naturally still under a flowing NH 3 atmosphere. Finally, the black MoN NA/CC was collected. Characterizations: XRD patterns were recorded using a LabX XRD-6100 X-ray diffractometer, with a Cu Kα radiation (40 kv, 30 ma) of wavelength nm (SHIMADZU, Japan). SEM measurements were performed on a Hitachi S-4800 field emission scanning electron microscope at an accelerating voltage of 20 kv. The structures of the samples were determined by TEM images on a HITACHI H-8100 electron microscopy (Hitachi, Tokyo, Japan) operated at 200 kv. XPS measurements were carried out on an ESCALABMK II X-ray photoelectron spectrometer using Mg as the exciting source. 1 H NMR spectra were collected on a superconducting-magnet NMR spectrometer (Bruker AVANCE III HD 500 MHz) and S3

4 dimethyl sulphoxide was used as an internal to calibrate the chemical shifts in the spectra. Electrochemical measurements: Before NRR test, Nafion membrane was protonated by first boiling in water for 1 h, then in H 2 O 2 for 1 h, then in water for another hour, followed by 3 h in 0.5 M H 2 SO 4, and finally for 6 h in water. All steps were performed at 80 C. 1 The electrochemical experiments were carried out with a CHI 660E electrochemical analyzer (CH Instruments, Inc., Shanghai) using a three-electrode configuration with MoN NA/CC as working electrode, graphite rod as counter electrode and Ag/AgCl (saturated KCl electrolyte) as reference electrode. Potentials reported in this work were ir-compensated and converted to RHE scale via calibration with the following equation: E (vs. RHE) = E (vs. Ag/AgCl) ph. LSV test was performed with a scan rate of 5 mv s 1 at room temperature. The presented current densities were normalized to geometric surface area. For electrochemical NRR, potentiostatic tests were conducted in N 2 -saturated 0.1 M HCl (ph=1) solution, which was purged with N 2 for 30 min before the measurement. Pure N 2 was continuously fed into the cathodic compartment during the experiments. Determination of NH 3 : Concentration of produced NH 3 was spectrophotometrically determined by the indophenol blue method. 2 In detail, 2 ml aliquot of solution was removed from electrochemical reaction vessel. Then, 2 ml of 1.0 M NaOH solution containing 5 wt% salicylic acid and 5 wt% sodium citrate, followed by addition of 1 ml of 0.05 M NaClO and 0.2 ml of an aqueous solution of 1 wt% Na 2 [Fe(NO)(CN) 5 ].2H 2 O. After standing at room temperature for 2 hours, UV-Vis absorption spectra were measured using an UV-Vis spectrophotometer. The formation of indophenol blue was determined using absorbance at a wavelength of 655 nm. The concentration-absorbance curve was calibrated using standard NH 4 Cl solutions with a series of concentrations. The fitting curve (y = 0.353x , R 2 = 0.999) shows good linear relation of absorbance value with NH 4 Cl concentration by three times independent calibrations. NH 3 yield was calculated using the following equation. NH 3 yield rate = c NH4Cl V / t A 17 Where c NH4Cl is the measured NH 4 Cl concentration; V is the volume of electrolyte; t is the reaction time; A is the geometric area of the cathode. S4

5 Determination of N 2 H 4 : N 2 H 4 in the electrolyte was estimated by the method of Watt and Chrisp. 3 A mixture of p-c 9 H 11 NO (5.99 g), HCl (concentrated, 30 ml) and C 2 H 5 OH (300 ml) was used as a color reagent. In detail, 5 ml electrolyte removed from the electrochemical reaction vessel was added into 5 ml above prepared color reagent. After standing at room temperature for 10 min, UV-Vis absorption spectra were measured using an UV-Vis spectrophotometer at 455 nm. The concentration-absorbance curve was calibrated using standard N 2 H 4 solutions with a series of concentrations. The fitting curve (y = 0.545x , R 2 = 0.999) shows good linear relation of absorbance value with N 2 H 4 concentration by three times independent calibrations. Calculation of FE: Assuming three electrons were needed to produce one NH 3 molecule, the FE could be calculated as follows: FE = 3 F c NH4Cl V / 17 Q where F is the Faraday constant, Q is the quantity of applied electricity. Calculation details: DFT calculations were performed using the Vienna Ab initio simulation package (VASP) 4 with the projected augment wave (PAW) pseudo-potential. 5 The Perdew, Burke and Ernzerhof (PBE) exchange correlation functional 6 of the generalized gradient approximation (GGA) was employed to optimize the structures and obtain energetics of all the species. A Monkhorst Pack k-point net of was used to sample the Brillouin zone with energy cut-off of 500 ev. The force convergence was set as ev Å -1. A vacuum layer of at least 20 Å was used above the slab model (Figure S12) to avoid periodic interactions. The free energy of structure A (G A ) was computed from: G A = E A +ZPE A TS A The zero-point energy (ZPE A ) and the entropy (S A ) of adsorbed species were yielded from frequency calculation, whereas the thermodynamic corrections for gas molecules were from standard tables. 7 Generally, there are three catalytic mechanism of NRR process on electrocatalysts including dissociative Heyrovski, 8,9 associative Heyrovski, 10 and MvK mechanism. 11 Firstly, when the formation of NH 3 via the dissociative Heyrovsky mechanism, the N 2 molecule is dissociated S5

6 on the surface of catalyst and then subsequently protonated (an asterisk, *, denotes a surface site): N 2 (g) + 6(H + + e) + 2 * 2N * + 6 (H + + e ) NH * + N * + 5 (H + + e) NH * 2 + N * + 4 (H + + e) NH * 3 + N * + 3 (H + + e) NH * 3 + NH * + 2 (H + + e) NH * 3 + NH * 2 + (H + + e) 2 NH * 3 NH * 3 + NH 3 (g) + * 2 NH 3 (g) + 2 * Secondly, the associative Heyrovski mechanism is considered, in which N 2 adheres to the surface of the catalyst and then is protonated before the dissociation of nitrogen-nitrogen bond. N 2 (g) + 6 (H + + e ) + 2 * N ** (H + + e ) N 2 H ** + 5 (H + + e ) N 2 H ** (H + + e ) N 2 H ** (H + + e ) NH * 2 + NH * (H + + e ) NH * 3 + NH * 2 + (H + + e * ) 2 NH 3 NH * 3 + NH 3 (g) + * 2 NH 3 (g) + 2 * Finally, TMNs possess the advantage of being able to form NH 3 by way of a MvK mechanism, in which a surface N atom of TMNs is reduced to NH 3 and the catalyst regenerates later by reacting with dissolved N 2. The detailed process is shown in Figure 4a. S6

7 Figure S1. SEM images for bare CC. S7

8 Figure S2. (a) UV-Vis absorption spectra of indophenol assays with NH + 4 ions after incubated for 2 hours at room temperature. (b) Calibration curve used for calculation of NH 4 Cl concentrations. (c) UV-Vis absorption spectra of various N 2 H 4 concentration after incubated for 10 min at room temperature. (d) Calibration curve used for calculation of N 2 H 4 concentrations. S8

9 Figure S3. UV-Vis absorption spectrum of the 0.1 M HCl electrolyte (after charging at 0.3 V for 3 h) after incubated for 10 min at ambient conditions. S9

10 Figure S4. (a) Chronoamperometric curve of MoN NA/CC for NRR at 0.3 V for 27 h. (b) NH 3 yields and FEs for MoN NA/CC after charging at 0.3 V for 3h and 27 h. S10

11 Figure S5. (a) SEM images and (b) XRD pattern for MoN NA/CC after stability test. S11

12 Figure S6. (a) XPS survey spectrum for post-nrr MoN in 0.1 M HCl. XPS spectra for the post-nrr MoN in (b) Mo 3d and (c) N 1s regions. S12

13 Figure S7. UV-Vis absorption spectra of 0.1 M HCl electrolyte stained with indophenol indicator after charging at 0.3 V for 3 h. S13

14 Figure S8. UV-Vis absorption spectra of 0.1 M HCl electrolyte stained with indophenol indicator before and after 3-h electrolysis under N 2 atmosphere on MoN NA/CC at open-circuit potential under ambient conditions. S14

15 Figure S9. (a) UV-Vis absorption spectra of 0.1 M HCl electrolyte stained with indophenol indicator after charging at 0.3 V for 3 h under Ar and N 2 atmosphere. (b) FEs for NH 3 under Ar and N 2 atmosphere. S15

16 Figure S10. (a) Chronoamperometry curve for MoN NA/CC at potential of 0.3 V under Ar atmosphere and (b) corresponding NH 3 yields. S16

17 Figure S11. XPS spectra for MoN in (a) Mo 3d and (b) N 1s regions after long-term electrolysis in 0.1 M HCl under Ar atmosphere. S17

18 Figure S12. EDX spectra of MoN NA/CC after long-term electrolysis in (a) Ar-saturated and (b) N 2 -saturated 0.1 M HCl. S18

19 Figure S13. XRD pattern for MoN NA/CC after long-term electrolysis in Ar-saturated 0.1 M HCl. S19

20 Figure S N isotope labeling experiment. 1 H NMR spectra for the post-electrolysis 0.1 M HCl electrolytes with 15 N 2. The spectra for 15 NH 4 + and 14 NH 4 + standard samples are also shown. S20

21 Figure S15. (a) The top view of and (b) side view of the simulation box for MoN(200) surface. S21

22 Table S1. Comparison of the electrosynthesis of NH 3 activity for MoN NA/CC with other catalysts under ambient conditions. Catalyst Electrolyte NH 3 yield (mol s 1 cm 2 ) FE (%) MoN NA/CC 0.1 M HCl Ref. This work MoO M HCl µg h 1 mg 1 cat. 1.9 (8) NPC 0.05 M H 2 SO µg h 1 mg 1 cat (10) Ru/Ti 0.5 M H 2 SO mol s 1 cm 2 -- (12) BG 0.05 M H 2 SO mol s 1 cm (13) Mo nanofilm 0.01 M H 2 SO mol s 1 cm (14) TA-reduced Au/TiO M HCl µg h 1 mg 1 cat (15) Bi 4 V 2 O 11 /CeO M HCl µg h 1 mg 1 cat (16) α-au/ceo x -RGO 0.1 M HCl 8.30 µg h 1 mg 1 cat (17) NPC 0.1 M HCl 15.7 µg h 1 mg 1 cat (18) PCN 0.1 M HCl 8.09 µg h 1 mg 1 cat (19) PEBCD/C 0.5 M Li 2 SO mol s 1 cm (20) MoS 2 /CC 0.1 M Na 2 SO mol s 1 cm (9) AuHNCs 0.5 M LiClO mol s 1 cm (21) N-doped carbon nanospikes 0.25 M LiClO mol s 1 cm (22) Pd/C 0.1 M PBS 4.5 µg h 1 mg 1 cat. 8.2 (23) Fe 2 O 3 -CNT diluted KHCO mol s 1 cm (24) Au nanorods 0.1 M KOH mol s 1 cm (25) ZIF-derived carbon 0.1 M KOH mol s 1 cm (26) Ru/C 2.0 M KOH mol s 1 cm (27) S22

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