氨對於人類而言扮演了重要的角色，在農業及工業上都看的到它的身影，而在能源領域中，因具有高能量密度及高含氫量等性質被認為是新興能量載體，目前主要的氨合成方法為哈伯法，產氨時會耗費能量及排放溫室氣體，因此尋找其替代方法是必要的。
本研究合成氧化鉻/硫化鈷異質結構作為電催化劑，並在鹼性電解液中進行氮還原反應，從實驗結果可知，純氧化鉻在電位 -0.7 V時具有最高的產率6.85 µg h−1 mg−1以及法拉第效率4.42%；純硫化鈷在-0.3 V時有最高的產率2.44 µg h−1 mg−1，在-0.1 V時有20.64% 的法拉第效率；而氧化鉻/硫化鈷異質結構在電位-0.2 V時達到最高產率9.41 µg h−1 mg−1，法拉第效率29.41%，與純氧化鉻相比其產率提升了1.37倍，法拉第效率則提升6.65倍，與純硫化鈷相比其產率提升至3.86倍，法拉第效率提升至1.42倍，可以看到氧化鉻/硫化鈷異質結構大大提升了產率及法拉第效率，其原因為兩材料結合後其電子分布往硫化鈷移動，增強其鈷原子進行氮還原反應的能力並增加催化劑中活性位點；氧化鉻因損失電子而能抑制質子接近，因此在催化過程中扮演著抑制產氫反應的發生而提升整體法拉第效率。

Ammonia (NH3) plays a significant role for humans, with its presence observed in both agriculture and industry. In the field of energy, it is considered an emerging energy carrier due to its high energy density and hydrogen content. Currently, the primary method for NH3 synthesis is the Haber-Bosch process, which consumes energy and releases greenhouse gases during NH3 production. Therefore, finding alternative methods is necessary.
In this study, we demonstrate that chromium oxide/cobalt sulfide heterostructure (Cr2O3/CoS) serves as nitrogen reduction reaction electrocatalyst in alkaline electrolytes. Cr2O3/CoS exhibits an NH3 yield rate of 9.41 µg h−1 mg−1 and a Faraday efficiency of 29.41% at -0.2 V versus RHE, which is 3.86 and 1.37 times higher NH3 yield and 1.42 and 6.65 times higher FE for pure CoS and Cr2O3, respectively. The heterostructure significantly enhances the production rate and FE, which is attributed to that the electrons move from Cr to Co after combination, increasing the active sites in catalysts and boosting the NRR yields. Furthermore, the Cr2O3/CoS can inhibit Hydrogen evolution reaction and improve the FE due to the electron loss in Cr2O3, which can limit the contact of proton on catalyst. As a result, the combination of Cr2O3 and CoS as heterostructure achieves a boosted NRR performance compared to pure Cr2O3 and pure CoS.

1. Zhao X, Hu G, Chen GF, Zhang H, Zhang S, Wang H. Comprehensive understanding of the thriving ambient electrochemical nitrogen reduction reaction. Adv Mater 33, 2007650 (2021).

2. Chang F, Gao W, Guo J, Chen P. Emerging materials and methods toward ammonia‐based energy storage and conversion. Adv Mater 33, 2005721 (2021).

3. Smith C, Hill AK, Torrente-Murciano L. Current and future role of Haber–Bosch ammonia in a carbon-free energy landscape. Energy Environ Sci 13, 331-344 (2020).

4. Chen SY, et al. Identification of the highly active Co-N-4 coordination motif for selective oxygen reduction to hydrogen peroxide. J Am Chem Soc 144, 14505-14516 (2022).

5. Fu HQ, et al. Synergistic Cr2O3@ Ag heterostructure enhanced electrocatalytic CO2 reduction to CO. Adv Mater 34, 2202854 (2022).

6. Wang TT, et al. Nanoframes of Co3O4-Mo2N heterointerfaces enable high-performance bifunctionality toward both electrocatalytic HER and OER. Adv Funct Mater 32, 2107382 (2022).

7. Chu K, Luo YJ, Shen P, Li XCA, Li QQ, Guo YL. Unveiling the synergy of O-vacancy and heterostructure over MoO3-x/MXene for N2 electroreduction to NH3. Adv Energy Mater 12, 2103022 (2022).

8. Modak JM. Haber process for ammonia synthesis. Resonance 7, 69-77 (2002).

9. Erisman JW, Sutton MA, Galloway J, Klimont Z, Winiwarter W. How a century of ammonia synthesis changed the world. Nat Geosci 1, 636-639 (2008).

10. Medford AJ, Hatzell MC. Photon-driven nitrogen fixation: current progress, thermodynamic considerations, and future outlook. ACS Catal 7, 2624-2643 (2017).

11. Wang W, Zhang H, Zhang S, Liu Y, Wang G, Sun C, Zhao H. Potassium‐ion‐assisted regeneration of active cyano groups in carbon nitride nanoribbons: visible‐light‐driven photocatalytic nitrogen reduction. Angew Chem Int Ed 58, 16644-16650 (2019).

12. Shang S, et al. Atomically dispersed iron metal site in a porphyrin-based metal–organic framework for photocatalytic nitrogen fixation. ACS nano 15, 9670-9678 (2021).

13. Ma Y, et al. Synergizing Mo single atoms and Mo2C nanoparticles on CNTs synchronizes selectivity and activity of electrocatalytic N2 reduction to ammonia. Adv Mater 32, 2002177 (2020).

14. Chu K, Cheng Y-h, Li Q-q, Liu Y-p, Tian Y. Fe-doping induced morphological changes, oxygen vacancies and Ce3+–Ce3+ pairs in CeO2 for promoting electrocatalytic nitrogen fixation. J Mater Chem A 8, 5865-5873 (2020).

15. Chu K, Gu WC, Li QQ, Liu YP, Tian Y, Liu WM. Amorphization activated FeB2 porous nanosheets enable efficient electrocatalytic N2 fixation. J Energy Chem 53, 82-89 (2021).

16. Liu D, et al. Development of electrocatalysts for efficient nitrogen reduction reaction under ambient condition. Adv Funct Mater 31, 2008983 (2021).

17. Mills JP, Du C, Chen Z, Guo T, Wu YA. Catalyst design strategies for aqueous N2 electroreduction. Appl Mater Today 25, 101184 (2021).

18. Huang ZL, Rafiq M, Woldu AR, Tong QX, Astruc D, Hu LS. Recent progress in electrocatalytic nitrogen reduction to ammonia (NRR). Coord Chem Rev 478, 214981 (2023).

19. Kordali V, Kyriacou G, Lambrou C. Electrochemical synthesis of ammonia at atmospheric pressure and low temperature in a solid polymer electrolyte cell. Chem comm, 1673-1674 (2000).

20. Wan Y, Wang Z, Zheng M, Li J, Lv R. Heterogeneous crystalline–amorphous interface for boosted electrocatalytic nitrogen reduction to ammonia. J Mater Chem A 11, 818-827 (2023).

21. Kugler K, Luhn M, Schramm JA, Rahimi K, Wessling M. Galvanic deposition of Rh and Ru on randomly structured Ti felts for the electrochemical NH3 synthesis. Phys Chem Chem Phys 17, 3768-3782 (2015).

22. Nan HF, Liu YP, Li QQ, Shen P, Chu K. A Janus antimony sulfide catalyst for highly selective N2 electroreduction. Chem comm 56, 10345-10348 (2020).

23. Liu Y, et al. Unveiling the protonation kinetics‐dependent selectivity in nitrogen electroreduction: achieving 75.05% selectivity. Angew Chem Int Ed 61, 202209555 (2022).

24. Wang H, et al. Electrochemical fabrication of porous Au film on Ni foam for nitrogen reduction to ammonia. Small 15, 1804769 (2019).

25. Zhang Y, et al. High-performance electrohydrogenation of N2 to NH3 catalyzed by multishelled hollow Cr2O3 microspheres under ambient conditions. ACS Catal 8, 8540-8544 (2018).

26. Wu H, Singh-Morgan A, Qi K, Zeng Z, Mougel V, Voiry D. Electrocatalyst microenvironment engineering for enhanced product selectivity in carbon dioxide and nitrogen reduction reactions. ACS Catal 13, 5375-5396 (2023).

27. Zhou L, Boyd CE. Comparison of Nessler, phenate, salicylate and ion selective electrode procedures for determination of total ammonia nitrogen in aquaculture. Aquaculture 450, 187-193 (2016).

28. Thomas D, Rey M, Jackson P. Determination of inorganic cations and ammonium in environmental waters by ion chromatography with a high-capacity cation-exchange column. J Chromatogr A 956, 181-186 (2002).

29. LeDuy A, Samson R. Testing of an ammonia ion selective electrode for ammonia nitrogen measurement in the methanogenic sludge. Biotechnol Lett 4, 303-306 (1982).

30. Liu J, et al. Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia. Proc Natl Acad Sci USA 113, 5530-5535 (2016).

31. Yu H, Zhang G, Cai Y, Dong F. Altering the substituents of salicylic acid to improve Berthelot reaction for ultrasensitive colorimetric detection of ammonium and atmospheric ammonia. Anal Bioanal Chem 413, 5695-5702 (2021).

32. Feng Z, et al. FeS2/MoS2@ RGO hybrid materials derived from polyoxomolybdate-based metal–organic frameworks as high-performance electrocatalyst for ammonia synthesis under ambient conditions. Chem Eng J 445, 136797 (2022).

33. Fei H, Liu R, Wang J, Guo T, Wu Z, Wang D, Liu F. Targeted modulation of competitive active sites toward nitrogen fixation via sulfur vacancy engineering over MoS2. Adv Funct Mater, 2302501 (2023).

34. Li Y, et al. Local spin‐state tuning of iron single‐atom electrocatalyst by S‐coordinated doping for kinetics‐boosted ammonia synthesis. Adv Mater 34, 2202240 (2022).

35. Gupta D, et al. High yield selective electrochemical conversion of N2 to NH3 via morphology controlled silver phosphate under ambient conditions. J Mater Chem A 10, 20616-20625 (2022).

36. Chen H, et al. Ti2O3 nanoparticles with Ti3+ sites toward efficient NH3 electrosynthesis under ambient conditions. ACS Appl Mater Interfaces 13, 41715-41722 (2021).

37. Chu K, Liu Y-p, Li Y-b, Guo Y-l, Tian Y. Two-dimensional (2D)/2D interface engineering of a MoS2/C3N4 heterostructure for promoted electrocatalytic nitrogen fixation. ACS Appl Mater Interfaces 12, 7081-7090 (2020).

38. Chen H, et al. Modulating oxygen vacancies of TiO2 nanospheres by Mn-doping to boost electrocatalytic N2 reduction. ACS Sustain Chem Eng 9, 1512-1517 (2021).

39. Liu Y, et al. MoS2 nanodots anchored on reduced graphene oxide for efficient N2 fixation to NH3. ACS Sustain Chem Eng 8, 2320-2326 (2020).

40. Zhang H, Cui C, Luo Z. MoS2-supported Fe2 clusters catalyzing nitrogen reduction reaction to produce ammonia. J Phys Chem C 124, 6260-6266 (2020).

41. Jin HY, et al. Nitrogen vacancies on 2D layered W2N3: a stable and efficient active site for nitrogen reduction reaction. Adv Mater 31, 1902709 (2019).

42. Zhang L, et al. Efficient electrochemical N2 reduction to NH3 on MoN nanosheets array under ambient conditions. ACS Sustain Chem Eng 6, 9550-9554 (2018).

43. Jiang Y, et al. Distorted spinel ferrite heterostructure triggered by alkaline earth metal substitution facilitates nitrogen localization and electrocatalytic reduction to ammonia. Chem Eng J 450, 138226 (2022).

44. Li XC, Shen P, Luo YJ, Li YH, Guo YL, Zhang H, Chu K. PdFe single-atom alloy metallene for N2 electroreduction. Angew Chem Int Ed 61, (2022).

45. Wang M, et al. Over 56.55% Faradaic efficiency of ambient ammonia synthesis enabled by positively shifting the reaction potential. Nat Commun 10, 341 (2019).

46. Li C, et al. Sulfur vacancies in ultrathin cobalt sulfide nanoflowers enable boosted electrocatalytic activity of nitrogen reduction reaction. Chem Eng J 415, (2021).

47. Dou S, Wang X, Wang S. Rational design of transition metal‐based materials for highly efficient electrocatalysis. Small Methods 3, 1800211 (2019).

48. Tao L, Duan X, Wang C, Duan X, Wang S. Plasma-engineered MoS2 thin-film as an efficient electrocatalyst for hydrogen evolution reaction. Chem comm 51, 7470-7473 (2015).

49. Xiao D, et al. Effects of ion energy and density on the plasma etching‐induced surface area, edge electrical field, and multivacancies in MoSe2 nanosheets for enhancement of the hydrogen evolution reaction. Small 16, 2001470 (2020).

50. Xie J, et al. Defect‐rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution. Adv Mater 25, 5807-5813 (2013).

51. Chu K, Liu Y-p, Cheng Y-h, Li Q-q. Synergistic boron-dopants and boron-induced oxygen vacancies in MnO2 nanosheets to promote electrocatalytic nitrogen reduction. J Mater Chem A 8, 5200-5208 (2020).

52. Hori Y, Takahashi I, Koga O, Hoshi N. Electrochemical reduction of carbon dioxide at various series of copper single crystal electrodes. J Mol Catal 199, 39-47 (2003).

53. Zhai ZB, et al. Catalytically active sites of MOF-derived electrocatalysts: synthesis, characterization, theoretical calculations, and functional mechanisms. J Mater Chem A 9, 20320-20344 (2021).

54. Shi C-M, Wang T-G, Pei Z-L, Gong J, Sun C. Microstructure, interface, and properties of multilayered CrN/Cr2O3 coatings prepared by arc ion plating. J mater sci technol 30, 1193-1201 (2014).

55. Ma H, et al. Highly toluene sensing performance based on monodispersed Cr2O3 porous microspheres. Sens Actuators B Chem 174, 325-331 (2012).

56. Singh J, Verma V, Kumar R. Preparation and structural, optical studies of Al substituted chromium oxide (Cr2O3) nanoparticles. Vacuum 159, 282-286 (2019).

57. Sousa P, Silvestre AJ, Popovici N, Conde O. Morphological and structural characterization of CrO2/Cr2O3 films grown by Laser-CVD. Appl Surf Sci 247, 423-428 (2005).

58. Pei Z, Xu H, Zhang Y. Preparation of Cr2O3 nanoparticles via C2H5OH hydrothermal reduction. J Alloys Compd 468, L5-L8 (2009).

59. Sun H, Wang L, Chu D, Ma Z, Wang A. Synthesis of porous Cr2O3 hollow microspheres via a facile template-free approach. Mater Lett 140, 35-38 (2015).

60. Du H, Guo X, Kong R-M, Qu F. Cr2O3 nanofiber: a high-performance electrocatalyst toward artificial N2 fixation to NH3 under ambient conditions. Chem comm 54, 12848-12851 (2018).

61. Srouji F, Afzaal M, Waters J, O'Brien P. Single‐source routes to cobalt sulfide and manganese sulfide thin films. Chem Vap Depos 11, 91-94 (2005).

62. Bian H, et al. One-step synthesis of mesoporous Cobalt sulfides (CoSx) on the metal substrate as an efficient bifunctional electrode for overall water splitting. Electrochim Acta 389, 138786 (2021).

63. Bi E, Chen H, Yang X, Peng W, Grätzel M, Han L. A quasi core–shell nitrogen-doped graphene/cobalt sulfide conductive catalyst for highly efficient dye-sensitized solar cells. Energy Environ Sci 7, 2637-2641 (2014).

64. Wang X, Chen Y, Fang Y, Zhang J, Gao S, Lou XW. Synthesis of cobalt sulfide multi‐shelled nanoboxes with precisely controlled two to five shells for sodium‐ion batteries. Angew Chem Int Ed 58, 2675-2679 (2019).

65. Zhao L, et al. Laser synthesis of amorphous CoSx nanospheres for efficient hydrogen evolution and nitrogen reduction reactions. J Mater Chem A 10, 20071-20079 (2022).

66. Yao N, Wang G, Jia H, Yin J, Cong H, Chen S, Luo W. Intermolecular energy gap‐induced formation of high‐valent cobalt species in CoOOH surface layer on cobalt sulfides for efficient water oxidation. Angew Chem 134, 202117178 (2022).

67. Iqbal MW, Faisal MM, ul Hassan H, Afzal AM, Aftab S, Zahid T, Rehman AU. Facile hydrothermal synthesis of high-performance binary silver-cobalt-sulfide for supercapattery devices. J Energy Storage 52, 104847 (2022).

68. Wang CL, et al. Hierarchical CoS2/MoS2 flower-like heterostructured arrays derived from polyoxometalates for efficient electrocatalytic nitrogen reduction under ambient conditions. J Colloid Interface Sci 609, 815-824 (2022).

69. Meng QG, et al. Modulation of surface properties on cobalt phosphide for high-performance ambient ammonia electrosynthesis. Appl Catal B: Environ 303, 120874 (2022).

70. Xia L, et al. Cr2O3 nanoparticle-reduced graphene oxide hybrid: a highly active electrocatalyst for N2 reduction at ambient conditions. Inorg Chem 58, 2257-2260 (2019).

71. Hu WJ, Shi Q, Chen ZJ, Yin H, Zhong HF, Wang P. Co2N/Co2Mo3O8 heterostructure as a highly active electrocatalyst for an alkaline hydrogen evolution reaction. ACS Appl Mater Interfaces 13, 8337-8343 (2021).

72. Li N, et al. 1T/2H MoSe2-on-MXene heterostructure as bifunctional electrocatalyst for efficient overall water splitting. Electrochim Acta 326, 134976 (2019).

73. Wang XM, et al. Cooperative electrocatalytic N2 reduction based on WS2-covered Co9S8 hexagonal heterostructures derived from ZIF-67@POMs. Mater Today Energy 33, 101278 (2023).

74. Ling Y, et al. Strain induced rich planar defects in heterogeneous WS2/WO2 enable efficient nitrogen fixation at low overpotential. J Mater Chem A 8, 12996-13003 (2020).

75. Hanifpour F, Sveinbjornsson A, Canales CP, Skulason E, Flosadottir HD. Preparation of nafion membranes for reproducible ammonia quantification in nitrogen reduction reaction experiments. Angew Chem Int Ed 59, 22938-22942 (2020).

76. Swain G, Sultana S, Parida K. Constructing a novel surfactant-free MoS2 nanosheet modified MgIn2S4 marigold microflower: an efficient visible-light driven 2D-2D p-n heterojunction photocatalyst toward HER and pH regulated NRR. ACS Sustain Chem Eng 8, 4848-4862 (2020).

77. Hui XC, Li LF, Xia QN, Hong S, Hao LD, Robertson AW, Sun ZY. Interface engineered Sb2O3/W18O49 heterostructure for enhanced visible-light-driven photocatalytic N2 reduction. Chem Eng J 438, 135485 (2022).

78. Li X, et al. Bio-inspired NiCoP/CoMoP/Co(Mo3Se4)4 @C/NF multi-heterojunction nanoflowers : Effective catalytic nitrogen reduction by driving electron transfer. Appl Catal B: Environ 314, 121531 (2022).