English

• NH₃ is one of the most important chemicals in industry and the earth's ecological cycle^[1]. So far, the majority of NH₃ on earth has been manufactured by the Haber-Bosch process invented in the last century. This method not only needs harsh reaction conditions (high temperature and high pressure), but also consumes a large amount fossil energy and produces a lot of greenhouse gases^[2-3]. Due to the crisis of both energy and environment, it is very desirable to develop an alternative ammonia synthesis process that can be carried out gently, efficiently, environmentally friendly and low-energy.

  Up to now, there are three main ways to fix nitrogen under mild conditions, including biological nitrogen fixation^[4-6], photocatalytic nitrogen fixation^[7-9]and electrochemical nitrogen fixation^[10-11]. Among them, the production of biological nitrogen fixation is insufficient to meet the needs of human beings and limits its large-scale industrial development. Photocatalytic nitrogen fixation has a problem of low utilization rate of renewable energy. Therefore, electrocatalytic nitrogen reduction reaction (ENRR) has been paid more and more attention from researchers in recent decade^[12-14]. Nitrogen nitrogen triple bond (N≡N) in N₂ has a strong dipole moment and a high bond energy (940.9 kJ/mol), making it difficult to break, which is one of the biggest challenges for nitrogen fixation under mild conditions^[15-16]. Therefore, ENRR under ambient conditions requires a highly efficient electrocatalyst to reduce the activation energy, thereby increasing the reaction rate. In recent years, many ENRR catalysts have been developed, including noble metal catalysts, such as Au^[17], Pd^[18], Ru^[19-21], and Rh^[22], and non-precious metal catalysts such as Fe^[23-25], Mo^[26-28], Cr^[29], etc., in addition to some hetero-atom doped non-metallic catalysts^[30-32]. However, the application of noble metal catalysts is severely restricted due to their limited resources and high costs. And currently non-precious mental catalysts suffer from low production rate and Faradic efficiency (FE). So alternative non-precious metal electrocatalysts with high ammonia production rate and FE are particularly desirable. Transition metals are abundant on earth and their oxides can be prepared easily in a large scale^[33]. In the present, only few of transition metal oxides (such as NbO₂^[34], Fe₃O₄^[35], and MoO₃^[36]) have been studied as ENRR catalysts. Therefore, the study of more transition metal oxide nanocatalysts is of great significance for the developing of highly efficient and low cost electrocatalysts for NRR.

  Among them, Mn is an element that can promote nitrogenase activity of NRR, and its oxide may have excellent ENRR activity. Sun et al first reported Mn₃O₄ nanotubes as ENRR catalysts in 2018, with 3% FE in neutral electrolyte (-0.8 V vs RHE)^[37]. After that, the same team proved that the Mn (200) surface has excellent ENRR activity with FE up to 8% in the neutral solution^[38]. Recently, they further reported that the MnO₂ with O vacancies supported on a titanium mesh are also active for NRR but with limited activity^[39].

  Here, we report a highly efficient NRR electrocatalyst based on manganese oxide supported on activated carbon (MnO_x/C), prepared by a method of impregnation redox reaction and subsequent high-temperature calcination. Such catalyst shows high NRR activity (with FE up to 9.2% and ammonia production rate up to 7.8 μg_NH3/(h·mg_cat)) and remarkable durability/selectivity for ammonia production in acidic aqueous solution. Such work deeps our understanding to the Mn-based NRR process.

  1.   Experimental

  1.1   Reagents and apparatus

  Salicylic acid, NaOH, N₂H₄·H₂O, NH₄OH, sodium citrate, sodium hypochlorite (NaClO), p-dimethylaminobenzaldehyde (p-C₉H₁₁NO), sodium citrate dehydrate (C₆H₅Na₃O₇·2H₂O), sodium nitroferricyanide dihydrate (Na₂[Fe(CN)₅NO]·2H₂O), MnSO₄, MnO, Mn₂O₃, MnO₂ and KMnO₄ were purchased from Sigma-Aldrich Chemical Reagent Co., Ltd. Carbon black is BP2000. Ultrapure water, which was purified with a Millipore system.

  Element mapping analysis test instrument is Philips TECNAI G2 analytical high resolution transmission electron microscope (TEM), with operating voltage of 200 kV; scanning electron microscope (SEM) test instrument is XL 30 ESEM FEG field emission SEM; Using MgKα as the radiation source, the X-ray photoelectric spectrum (XPS) was measured on Kratos XSAM-800 spectrometer; Absorbance is measured by UV-visible (UV-Vis) absorbance photometry in Agilent Cary 60.

  1.2   Synthesis of MnO_x/C-Based catalysts

  We have made improvements on the preparation of carbon-supported manganese oxides (MnO_x/C) as described in literature^[40]. Quickly, 50 mg of carbon black was mixed with an aqueous solution of MnSO₄, then the KMnO₄ was added after stirring for 4 hours, the suspension was maintained at a controlled temperature (95 ℃) for 20 min. The obtained product was centrifuged with deionized water for several times and then dried at 110 ℃. Finally, the powder was placed into a quartz boat and then heat-treated under Ar/H₂ (5%H₂) gas atmosphere in a tube furnace. The pyrolysis products were washed with plenty of deionized water and dried at 60 ℃ under vacuum overnight.

  1.3   Synthesis of MnO/C, Mn₂O₃/C and MnO₂/C catalysts

  Mix 30 mg of MnO, Mn₂O₃ and MnO₂ with 70 mg of activated carbon BP-2000, respectively, then add an appropriate amount of deionized water, stir for several hours, centrifuge and dry the product overnight.

  1.4   Electrochemical N₂ reduction measurements

  Before the NRR test, Nafion membrane was pretreated with deionized water, H₂SO₄ and H₂O₂ to protonate it. The steps are as follows: First, it was boiled in ultrapure water for 1 hour, and then treated in an aqueous solution of H₂O₂ (5%) at 80 ℃ for 1 hour. Then, it was treated in 0.05 mol/L H₂SO₄ at 80 ℃ for 3 hours, and finally washed in water. Electrochemical measurements were performed using a standard three-electrode system using a CHI 750E electrochemical analyzer(CH Instruments, Inc) with MnO_x/C as the working electrode, graphite rod as the counter electrode and saturated Ag/AgCl (saturated KCl electrolyte) as the reference electrode. All potentials were corrected using the automatic IR compensation function on the potentiostat. All experiments were performed at room temperature. For the N₂ reduction experiment, 0.05 mol/L H₂SO₄ electrolyte was purged with N₂ for 30 minutes before each measurement. A chronoamperometry test was then performed in a N₂ saturated 0.05 mol/L H₂SO₄ solution in H-type cell, which was separated by a Nafion membrane.

  2.   Results and discussion

  In this case, based on the reaction between KMnO₄ and MnSO₄:KMnO₄+MnSO₄ → MnO_x, different amount of MnO_x was deposited on carbon support at room temperature. To study the ENRR activity of these catalysts, the linear sweep voltammetry (LSV) curves of these catalysts were obtained in Ar- or N₂-saturated 0.05 mol/L sulfuric acid. Fig.S1(in Supporting Information) and Fig. 1A show that the one with Mn of 30% (named as MnO_x-30/C) presents the largest response current to NRR, indicating the highest NRR activity.

  Figure 1

  

  Figure 1.  (A) LSV curves of MnO_x/C catalysts with different metal (Mn) loadings in N₂ saturated 0.05 mol/L H₂SO₄ electrolyte; (B) LSV curves of MnO_x-30/C after heat-treatment at different temperatures in N₂ saturated 0.05 mol/L H₂SO₄ electrolyte; (C) LSV curves of MnO_x-30/C-350 catalyst in N₂ and Ar saturated 0.05 mol/L H₂SO₄ electrolyte; (D) Chronoamperometry curve of MnO_x-30/C-350 in N₂ saturated 0.05 mol/L H₂SO₄ electrolyte at different potentials

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  To further optimize the performance of the catalyst MnO_x-30/C, the sample was further heat-treated at different temperatures under Ar-H₂ (5%) atmosphere. As shown in Fig.S2 (in Supporting Information) and Fig. 1B-1C, the catalyst obtained at 350 ℃ (named as MnO_x-30/C-350) shows the best NRR activity as indicated by the largest response current to NRR.

  To quantify the amounts of possible products (NH₃ or N₂H₄) of NRR on these catalysts, indophenol blue method and Watt and Chrisp method^[41]were adopted to obtain the standard curves for these two possible NRR product (NH₃ and N₂H₄) as shown in Fig.S3 and Fig.S4 (in Supporting Information), respectively. Based on such standard curves, the liquid NRR products on these catalysts could be quantified by fixing the electrode potential at different values for 2 hours (the working electrode was prepared based on carbon paper with catalyst loading of 0.5 mg/cm²). For the optimal catalyst MnO_x-30/C-350, to quantify the possible NRR products formed on it, as shown in Fig. 1D, the NRR products were accumulated for 2 hours at different potentials. Fig. 2A-2B show the absorption spectra of the solution obtained at different potentials. Based on the standard absorption spectra of NH₃ and N₂H₄ shown in Fig.S3A-S4A (in Supporting Information), Fig. 2A shows clearly that NH₃ is indeed produced potential-dependently as indicated by the absorption peak at 655 nm, while no N₂H₄ formation in the whole potential range as indicated by the no absorption at 460 nm shown in Fig. 2B, indicating a 100% selectivity of NRR for ammonia production. Based on the standard curves of NH₃ shown in Fig.S3B, we further calculated the yield and FE of ammonia from NRR at different potentials as shown in Fig. 2C. It shows that the maximum NH₃ yield of 7.8 μg_NH3/(h·mg_cat) was achieved at -0.66 V(vs.RHE), while the maximum FE of 9.2% was achieved at -0.56 V, higher than most of the recently reported NRR catalysts (see Support Information Table S1).

  Figure 2

  

  Figure 2.  ENRR product analysis on optimal catalyst MnO_x-30/C-350 in N₂-saturated 0.05 mol/L H₂SO₄ solution. (A) UV-Vis absorption spectra of NH₄⁺ ions solution at different potentials; (B) UV-Vis absorption spectra of N₂H₄·H₂O solution at different potentials; (C) NH₃ yield and FE at different potentials; (D) NH₃ yield and FE of MnO_x-30/C-350 in 8 cycles of test at potential of -0.56 V

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  In order to exclude the possible influence of external pollution sources, we conducted product analysis of Ar-saturated solution at optimal potential of -0.56 V and nitrogen saturated solution at open circuit voltage. As shown in Fig.S5 (in Supporting Information), no NH₃ could be detected in the above two electrolytes. Similarly, we also observed no NH₃ formation on either carbon paper (see Supporting Information Fig.S6) or pure carbon black supported by carbon paper (see Supporting Information Fig.S7). All these results confirm that the formation of NH₃ can only be attributed to the active components of MnO_x for NRR.

  We further evaluated the NRR durability of the optimal catalyst MnO_x-30/C-350 at -0.56 V. Fig. 2D shows that after 8 cycles of repeated testing (2 hours at -0.56 V in each cycle), the values of both FE and yield of NH₃ on MnO_x-30/C-350 catalyst keep almost constant, consistent with the observed stable current density at fixed potential as show in Fig.S8 (in Supporting Information). All these results indicate the remarkable durability of MnO_x-30/C-350 for long-term NRR process.

  To deeply understand such durability, SEM analysis was conducted. As the SEM images shown in Fig. 3A-3B, no obvious morphology variation could be observed after long-term (16 hours at -0.56 V) NRR process, indicating an excellent structure stability of the catalyst. The distribution of MnO_x on MnO_x-30/C-350 was also evaluated via high-angle annular dark-field scanning TEM (HAADF-STEM) and element mapping analysis. As shown in Fig. 3C-3F, MnO_x was dispersed uniformly on carbon support. Such homogeneous distribution could explain in part the observed remarkable durability.

  Figure 3

  

  Figure 3.  (A) SEM images of MnO_x-30/C-350 before testing; (B) SEM images of MnO_x-30/C-350 after long-term testing (16 hours at -0.56 V); (C-F) HAADF-STEM and element mapping images of MnO_x-30/C-350

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  To deeply understand the origin of the observed ENRR performance on catalyst MnO_x-30/C-350, NRR catalytic activities of univalent MnO/C, Mn₂O₃/C and MnO₂/C were also prepared with Mn content of 30% and studied at fixed potentials of -0.56 V and -0.66 V to obtain the FE and yield information, respectively (see Support Information Fig.S9).

  Fig. 4A shows clearly that polyvalent MnO_x presents better ENRR performance than other univalent manganese oxide. Based on such result, chemical composition of MnO_x/C treated at different calcination temperatures were investigated by XPS. As shown in Fig. 4B and 4C, three different oxidative states (Mn²⁺, Mn³⁺ and Mn⁴⁺) of Mn coexist in MnO_x-30/C, while after calcination at 350 ℃, the content of Mn⁴⁺ decreased and the contents of both Mn²⁺ and Mn³⁺ increased probably due to the reduction of Mn⁴⁺ by H₂ at high temperature. Such results along with observed higher NRR performance of MnO_x-30/C-350 than that of MnO_x-30/C (Fig. 1B) indicate that the Mn²⁺ and Mn³⁺ are the apparent main active components for the observed high NRR performance via a synergistic effect between them. To further confirm such point, the chemical compositions of MnO_x-30/C after heat-treated at different temperatures were further revealed via high resolution XPS. As shown in Table S2 (in Supporting Information) and Fig. 4D, with the temperature increase, the content of Mn⁴⁺ decreased step-by-step slowly, while the contents of both Mn²⁺ and Mn³⁺ increased gradually. Fig. 4D also shows clearly that the optimal NRR performance achieves only at an optimal composition (Mn²⁺:31.2%, Mn³⁺:29.5%, Mn⁴⁺:39.3%) obtained at 350 ℃, after the heat-treatment at higher temperature, the further content decrease of Mn⁴⁺ or the further content increase of Mn²⁺ and Mn³⁺ leads to the performance decrease inversely. Such fact indicates that the Mn⁴⁺ is also an important component for the observed high NRR performance. It means, the synergist effect for ENRR on such MnO_x/C occurs among the sites of Mn²⁺, Mn³⁺, and Mn⁴⁺ simultaneously rather than only between Mn²⁺ and Mn³⁺.

  Figure 4

  

  Figure 4.  (A) Performance test results of univalent MnO/C, Mn₂O₃/C, MnO₂/C and polyvalent MnO_x-30/C-350 (FE and yield were test at -0.56 V and -0.66 V, respectively); (B) XPS pattern of MnO_x-30/C; (C) XPS pattern of MnO_x-30/C-350. (D) Contents of Mn in different valence in MnO_x-30/C after calcinated at different temperatures and corresponding FE of NH₃

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  3.   Conclusions

  In summary, a series of MnO_x/C catalysts with multiple valence states of Mn was synthesized for highly efficient NRR process. In acidic solution, the optimal MnO_x-30/C-350 presents high FE (up to 9.2%) at -0.56 V with NH₃ yield up to 7.8 μg _NH3/(h·mg_cat) at -0.66 V and remarkable stability. It was further revealed that the high NRR performance could be due to the synergistic effect among different valence Mn ions of Mn²⁺, Mn³⁺, and Mn⁴⁺. This work presents a new catalytic system for the development of cost-effective highly efficient NRR catalysts.

  Supporing information [LSV curves, UV-Vis and XPS spectra of diferent Mn0_x, catalysts] is arailable free of charge ria the Internet at htp://yzhx.ciac.jl.cn.

  
  
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  Figure 1  (A) LSV curves of MnO_x/C catalysts with different metal (Mn) loadings in N₂ saturated 0.05 mol/L H₂SO₄ electrolyte; (B) LSV curves of MnO_x-30/C after heat-treatment at different temperatures in N₂ saturated 0.05 mol/L H₂SO₄ electrolyte; (C) LSV curves of MnO_x-30/C-350 catalyst in N₂ and Ar saturated 0.05 mol/L H₂SO₄ electrolyte; (D) Chronoamperometry curve of MnO_x-30/C-350 in N₂ saturated 0.05 mol/L H₂SO₄ electrolyte at different potentials

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  Figure 2  ENRR product analysis on optimal catalyst MnO_x-30/C-350 in N₂-saturated 0.05 mol/L H₂SO₄ solution. (A) UV-Vis absorption spectra of NH₄⁺ ions solution at different potentials; (B) UV-Vis absorption spectra of N₂H₄·H₂O solution at different potentials; (C) NH₃ yield and FE at different potentials; (D) NH₃ yield and FE of MnO_x-30/C-350 in 8 cycles of test at potential of -0.56 V

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  Figure 3  (A) SEM images of MnO_x-30/C-350 before testing; (B) SEM images of MnO_x-30/C-350 after long-term testing (16 hours at -0.56 V); (C-F) HAADF-STEM and element mapping images of MnO_x-30/C-350

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  Figure 4  (A) Performance test results of univalent MnO/C, Mn₂O₃/C, MnO₂/C and polyvalent MnO_x-30/C-350 (FE and yield were test at -0.56 V and -0.66 V, respectively); (B) XPS pattern of MnO_x-30/C; (C) XPS pattern of MnO_x-30/C-350. (D) Contents of Mn in different valence in MnO_x-30/C after calcinated at different temperatures and corresponding FE of NH₃

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