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Ampoule method fabricated sulfur vacancy-rich N-doped ZnS electrodes for ammonia production in alkaline media

Ampoule method fabricated sulfur vacancy-rich N-doped ZnS electrodes for ammonia production in... The electrochemical production of green and low-cost ammonia requests the development of high-performance electro- catalysts. In this work, the ampoule method was applied to modulate the surface of the zinc electrode by implanting defects and low-valent active sites. The N-doped ZnS electrocatalyst was thus generated by sulfurization with thiourea and applied for electrocatalytic nitrogen reduction reaction (ENRR). Given the rich sulfur vacancies and abundant Zn-N active sites on –10 −1 −2 the surface, excellent catalytic activity and selectivity were obtained, with an NH yield rate of 2.42 × 10 mol  s  cm and a Faradaic efficiency of 7.92% at − 0.6 V vs. RHE in 0.1 M KOH solution. Moreover, the as-synthesized zinc electrode exhibits high stability after five recycling tests and a 24 h potentiostatic test. The comparison with Zn foil, non-doping ZnS/ Zn and recent metal sulfide electrocatalysts further demonstrated advanced catalytic performance of N@ZnS/Zn for ENRR. By simple synthesis, S vacancies, and N-doping defects, this promising electrocatalyst would represent a good addition to the arena of transition-metal-based catalysts with superior performance in ENRR. Graphic abstract Keywords Ampoule method · Zinc-based electrocatalyst · Sulfur vacancy · Ammonia synthesis Extended author information available on the last page of the article Vol.:(0123456789) 1 3 8 Page 2 of 10 Materials for Renewable and Sustainable Energy (2021) 10:8 with the aid of abundance S vacancies, ZrS nanofibers Introduction were also proven as a stable and active catalyst for ambi- ent electrochemical N -to-NH conversion with excellent In decades, chemical nitrogen fixation has become a hot- 2 3 selectivity [14]. Apart from early stage transition metals, spot of chemistry research [1]. It is a sought-after pro- iron-series metals were adopted as robust alternatives in cess that converting atmospheric dinitrogen gas into the ENRR by diverse modifications. Through solvothermal nitrogen-containing compounds acquired by modern civi- method and plasma synthesis, the F e S nanosheet [15] lization [2]. Nowadays, the utilization of dinitrogen gas 3 4 and surface-sulfurized Fe foam [16] both exhibited out- is mainly focused on the synthesis of ammonia, a vital standing catalytic performance due to the low valence industrial raw material [3]. Emerged as one of the prom- metallic site and S vacancies with suitable Fe–Fe distance. ising alternatives to the traditional Haber–Bosch process, Because of the various bonding modes and enhanced con- electrocatalytic nitrogen reduction reaction (ENRR) has ductivity, cobalt-based composite materials, C oS @NC drawn increasing attention. Instead of the high consump- [17], CoS -N/S-C [18] and C@CoS@TiO [19], were tion of fossil fuels and massive amounts of greenhouse 2 2 certified as good candidates for ENRR with the gratifying gas emission, the ENRR provides a clean generation of NH yields [20, 21]. However, most of the sulfide elec- NH in aqueous electrolytes under ambient conditions [4]. trodes were obtained in aqueous conditions, which dis- However, the reaction efficiency is restricted by the kinetic played an inferior synthetic efficiency due to the insolubil- and thermodynamic features of dinitrogen gas under ambi- ity issue of metal sulfides. And the complicated synthetic ent conditions. And due to the inevitable electrocatalytic routes pose a significant barrier to the practical use of hydrogen emission reaction (HER) that took place in the metal sulfides in ENRR. aqueous electrolytes, the development of a practical ENRR On the other hand, the controllable synthesis of electro- technique is still challenging [5]. catalysts for various electrochemical reactions is a state- The past 5 years witnessed numerous metal-based mate- of-art technological challenge in both electrochemistry rials that were fabricated and adopted as electrocatalysts and material science [22]. Among all of the existing syn- for the ENRR process [6, 7]. Regarding the improvement thetic processes, the ampoule method owns superpassingly of the catalytic efficiency and selectivity toward ENRR, versatility in preparing self-standing electrode materials, defect engineering is widely used as an efficient protocol including alloys, nitrides, phosphides, chalcogenides, and to tune the superficial morphology and band structure of halides. Made from silica, quartz, and corrosion-resistant the nano-electrocatalysts, which facilitates the adsorp- niobium and tantalum, the ampoules can be simply self- tion and activation of the N molecule [8]. Therein, using designed according to the requirement of various materials sulfur-containing substances as synthetic precursors for synthetic processes. Meanwhile, the solid or liquid reac- producing metal-based electrocatalysts may originally tants can be loaded inside the ampoule under evacuated generate a nitrogenases-mimicking active site for enhanc- or inert gas conditions before sealing the open end. After ing the ENRR process [9]. that, the heat-up step is always implemented in a furnace As an element in the VI group, sulfur has a relatively with hetero- or homogeneous applying temperature over weak electronegativity, which determined its diversity of a wide range. By such manipulations, the reaction con- bonding form. For instance, the sulfur reacts with metal ditions and atmosphere are confined, sequentially elimi- elements to form ionic compounds, and shares electrons nating the environmental impact and thus realizing the to bond with itself to form a chain-shaped polysulfide ion 2− controllable synthesis. These days, the surface-modified S . Therefore, the structure of metal sulfides tends to be transition-metal foil materials generated from the ampoule diversified and complicated, which make sulfur a fascinat- method have already been applied to various electrocata- ing and promising doping element for functional metal lytic reactions [23–27]. electrocatalysts construction. In addition, it is reasonable With our ongoing interests in the ampoule methods for to suppose that sulfur defects and vacancies would exhibit synthesizing catalytic materials and exploration of electro- great potential in improving the activity of ENRR due to catalytic ENRR, [28–30] herein, we propose the N@ZnS/ the enhancement of ENRR performance presented by the Zn materials fabricated through the facile ampoule method oxygen vacancies. After theoretical study and experiments, that enables the electrochemical synthesis of ammonia in the MoS catalysts provided a distinctive electrocatalytic ambient conditions. The S vacancies and Zn-N activate effect in producing ammonia from N gas in an aqueous sites are speculated to facilitate the adsorption of the N solution [10–12]. Furthermore, the N-doped MoS nano- and reduction process. The adopted alkaline electrolytes flowers presented enhanced catalytic performance due to and potassium cations present enough HER inhibition, rich S vacancies and Mo–N active sites [13]. Besides, which allows efficient synthesis of NH . 1 3 Materials for Renewable and Sustainable Energy (2021) 10:8 Page 3 of 10 8 was protonated by first boiling in ultrapure water for 1 h Experimental section and treating in H O (5%) aqueous solution at 80 °C for 2 2 another 1 h in sequence. The membrane was then treated in Chemicals 0.5 m H SO for 3 h at 80 °C and finally stored in ultrapure 2 4 water overnight. The electrochemical measurements were Zinc foil (99.95% trace metals basis, thickness 0.1 mm) was conducted using CHI 760 E electrochemical worksta- purchased from Sigma-Aldrich. Potassium sodium tartrate, tion (CH Instruments, Inc., USA) with a three-electrode thiourea (CS(NH ) ), sulfur power (re-sublimation), hydro- 2 2 configuration. Before proceeding with electrocatalytic chloric acid (HCl, 36–38%), and hydrogen peroxide (H O , 2 2 tests, the synthesized materials were ultrasonicated Milli- 30%) were purchased from Tianjin Yongda Chemical Rea- Q water for 30 min and activated by electrolysis with the gent Co., Ltd. Potassium hydroxide (KOH) and Nessler’s setups of ENRR tests in Ar-saturated 0.1  M KOH elec- reagent were purchased from Tianjin Damao Chemical trolyte at − 0.7 V vs RHE for 48 h. The as-prepared and Reagent Co., Ltd. Nitrogen (N , 99.999%) and argon (Ar, activated zinc sulfide materials were used as the working 99.999%) were purchased from Shenyang Zhaote Special electrode, while the graphite rod electrode and Ag/AgCl Gas Co., Ltd. All the chemicals were used without further electrode (3.5  M KCl electrolyte) were adopted as the purification. counter and reference electrode, respectively. Unless oth- erwise specified, the 0.1 M KOH solution (pH = 13) was Zinc‑based electrode synthesis adopted as the electrolyte in the linear sweep voltammetry (LSV) and chronoamperometry tests. The measured poten- The zinc-based catalytic electrodes were synthesized tials vs. Ag/AgCl were converted to a reversible hydrogen through the ampoule method. In the beginning, the zinc electrode (RHE) scale according to the Nernst equation foil was polished and cleaned with anhydrate ethanol, ace- (E = E + 0.059pH + 0.205 = E + 0.972). RHE Ag/AgCl Ag/AgCl tone, 1 M HCl, and Milli-Q water for 5 min in sequence. Also, the outlet gas was introduced to an acid bottle with After cutting into pieces (180 mg, 2 cm × 0.4 cm × 0.01 cm, 5 mL of 2 M HCl for wet scrubbing to collect the possible length × width × height), the pretreated zinc foils were sealed leaking ammonia. with sulfurization reagents ampoule under vacuum and cal- cinated at 400 °C for 5 h. The heating rate was 5 ºC/min and the cooling process took place naturally. During this process, Determination of products the Zn atoms of the foil were sulfurized by sulfur atom vapor under pressure, leading to free-standing electrodes possess- The concentrations of the synthesized NH are detected by ing ZnS species on the surface. The material that sulfurized ammonia-sensitive electrode measurement and colorimetric with thiourea and sulfur power is named N@ZnS/Zn and method using Nessler reagent. ZnS/Zn, respectively. For the ammonia-sensitive electrode measurement, 5 mL of a solution of electrolyte or five-time diluted tail Instruments and characterization gas absorber was mixed with 5 mL 1 M KOH solution and 0.2 mL low-level ammonia pH-adjusting ISA solution (Ther- The X-ray diffraction (XRD) patterns were obtained on a mofisher). After stirring, the voltage was obtained using Bruker (Germany) D8 Advance diffractometer with Cu Kα the ammonia-sensitive selecting electrode (Thermofisher) radiation in the range of 20°–80° (2θ). The scanning electron when stable. The standard curve is constructed by measur- microscope (SEM) images and elemental mapping images ing a series of voltage for the reference solutions with 0.1 M −1 were performed in a Hitachi SU8000. Transmission elec- KOH and different NH Cl concentrations (0.8  μg  mL , −1 −1 −1 tron microscope (TEM) images were obtained from a JEOL 0.4 μg  mL, 0.2 μg  mL, 0.1 μg  mL ). JEM-2100 at an acceleration voltage of 200 kV. X-ray pho- For the colorimetric method, 25 mL solution of elec- toelectron spectroscopy (XPS) measurement was carried out trolyte and five-time diluted tail gas absorber was mixed on an ESCALAB Mk II (Vacuum generators) spectrometer with 0.5 mL 50% potassium sodium tartrate solution. Then, with an Al K X-ray source (240 W). 0.75 mL Nessler’s reagent and the mixture were allowed still for 10 min. The absorbance was tested by UV–Vis spectrom- eter (Shimadzu UV-2600) under light at 420 nm wavelength Electrochemical tests with deionized water as a reference. The standard curve is constructed by measuring a series of absorbances for the In general, the ENRR experiments were conducted in a two- reference solutions with 0.1 M KOH and different NH Cl compartment electrolysis cell that separated by the Nafion −1 −1 −1 concentrations (2.0 μg  mL, 1.6 μg  mL, 1.2 μg  mL , 117 membrane under ambient condition. The membrane −1 −1 −1 −1 0.8 μg  mL, 0.4 μg  mL, 0.2 μg  mL, 0.1 μg  mL , 1 3 8 Page 4 of 10 Materials for Renewable and Sustainable Energy (2021) 10:8 −1 0 μg  mL ). The background is corrected by a blank solu- ZnS/Zn was conducted to get rid of the impurity adsorbed tion with 0.1 M KOH and Nessler’s reagent. on the surface as well as the nitrogen-containing species. The concentration of hydrazine in the electrolyte was The morphology and microstructure of as-prepared N@ evaluated via Watt and Chrisp method. The chromogenic ZnS/Zn are examined by scanning electron microscopy agent was produced by mixing para-(dimethylamino) ben- (SEM). The rough surface of the materials is presented in zaldehyde (2 g), concentrated HCl (10 mL), and ethanol Fig. 1b, with particle size between 50 and 100 nm (Fig. 1d). (100  mL). After blending with the agent, the resulting To obtain more evidence for the effectiveness of the mate- electrolytes were detected by measuring the absorbance at rial, the high-resolution transmitting electron microscopy 455 nm. (HRTEM) is adopted and the picture is shown in Fig. 1c. Moreover, plenty of crystal phases and abundant surface defects can be observed at 5 nm resolution (Fig. 1e), further proving the stability and robustness of the electrode mate- Results and discussion rial. Meanwhile, the energy dispersive spectroscopy (EDS) was applied to compare the element ratio of the non-acti- Synthesis and characterization vated N@ZnS/Zn with activated ones After the calculation and approximation, the Zn: S: N ratio is altered from 6: 3: The synthesis of N@ZnS/Zn was following the typical pro- 2.7 to 6: 4: 1 (Figure S1 and S2). The remaining N atoms cess of the ampoule method. At first, the electrode shape on the surface indicate the formation of Zn-N bonds, which zinc foil and thiourea were sealed in an ampoule bottle under is one of the reasons for the electrode with extraordinary a vacuum line. Then the sulfurization took place when put- performance in ENRR (Figure S1). And the EDS-mapping ting the sealed ampoule bottle in the tube furnace under exhibited a homogeneous distribution for the Zn, S, and N 400 °C for 5 h, generating surface sulfurized N@ZnS/Zn atoms (Fig. 1d). material (Fig. 1a). Before characterizing and applying in the The X-ray diffraction (XRD) patterns to the surface of as- electrochemical tests, the activation of the as-prepared N@ prepared and activated N@ZnS/Zn are presented in Fig. 2. Fig. 1 Synthesis and characterization of catalysts. a Synthetic route Zn; inset: SEM–EDS elemental mapping images of Zn, S and N. e for zinc sulfide materials using ampoule method. b SEM image of HR-TEM image of N@ZnS/Zn with surface defects and cell spacing N@ZnS/Zn. c TEM image of N@ZnS/Zn. d SEM image of N@ZnS/ presented 1 3 Materials for Renewable and Sustainable Energy (2021) 10:8 Page 5 of 10 8 demonstrate the amorphous nature of the material surface. In addition, the XRD pattern of non-activated N@ZnS/Zn material is also demonstrated in Fig. 2. With more peaks of impurities presenting in the diagram, the activation process is therefore necessary. To obtain more details of the surface composition and structure, the ZnS/Zn with sulfur powder as sulfurizing agents by the ampoule method is also synthe- sized. The XRD pattern of ZnS/Zn shows the peaks at 28.5°, 47.5°, 56.3°, 76.6°, and 81.2° belong to the ZnS species, within which the lower peaks may ascribe to the moderated crystallinity. The full-range X-ray photoelectron spectroscopy (XPS) spectra of activated N@ZnS/Zn is shown in Fig. 3a, which demonstrated the presence of the elements Zn, S, and N. The Fig. 2 XRD patterns of N@ZnS/Zn and ZnS/Zn materials measured binding energies are in-line with those reported in the literature [31]. In particular, the binding energy of Zn 2p and Zn 2p is located at 1022.9 eV and 1045.9 eV, 3/2 1/2 According to the standard PDF # 80-0200, the diffraction respectively (Fig. 3b). The difference between the two core peaks at 28.5°, 47.5°, 56.3°, 69.4°, 76.6°, and 81.2° are level components of Zn 2p and Zn 2p is approximately 3/2 1/2 the character peaks attributing to the (111), (220), (311), 23 eV, indicating the bivalent state of Zn. Moreover, the (400), (331), (420) planes of the zinc sulfide crystal, which fitting of the XPS spectrum sulfur S 2 p peaks exhibits the implies the sphalerite ZnS species existing on the material main sulfur species on the surface of N@ZnS/Zn, which can surface. The relatively broad peaks and the rough baseline be divided into two main characteristic peaks at 161.6 eV Fig. 3 XPS spectra of the N@ZnS/Zn samples: a Full spectrum; b Zn 2p spectrum; c S 2p spectrum; d N 1 s spectrum 1 3 8 Page 6 of 10 Materials for Renewable and Sustainable Energy (2021) 10:8 and 162.9 eV, ascribed to the S 2p and S 2p respec- electrolyte. The divided cells were used to carry out the 3/2 1/2 tively, in a good agreement with the energy of the S-Zn experiments, and a proton conductive cation exchange bond (Fig. 3c) [31]. The existence of polysulfides, one of membrane (Nafion 117) was adopted to separate the cath- the primary reasons for emerged S vacancies, were specu- ode and anode chamber. Ahead of the electrochemical lated based on the 1.3 eV difference of the binding energies. tests, the 2 h electrolysis of N -saturated electrolytes and Meanwhile, the presence of the N 1 s characteristic peak Ar-saturated electrolytes with the working electrode was located at 395.38 eV and 396.43 eV reveals the existence conducted at open circuit potential and − 0.7 V vs RHE, of the Zn-N bond according to the literature (Fig. 3d) [32]. respectively. After that, the resulting solutions were exam- It is noteworthy that the residual nitrogen on the surface of ined by Nessler’s reagent with no obvious NH detection, N@ZnS/Zn after the activation would not be reduced into thus proving that there are no ammonia-like impurities in ammonia due to the binding. the feeding gas and reaction environment. The results from the above material characterizations At first, we explored the performances of the N@ZnS/ demonstrate that the surface modification of Zn foil by sul- Zn electrodes in ENRR through the comparison of the furization with thiourea through the ampoule method pro- linear sweep voltammetry (LSV) curves under Ar- and vides stable amorphous ZnS with abundant S vacancies and N -saturated electrolytes (Fig.  4a). The increasing cur- Zn-N binding active sites promising for electrocatalysis for rent density along with the increasing potentials is due to ENRR. the competing hydrogen reduction reaction (HER) that is enhanced gradually in both LSV tests. A higher current Electrocatalytic ENRR performances density in N -saturated electrolytes was then observed compare to the Ar-saturated ones when the applied poten- To evaluate the electrocatalytic ENRR efficiency of N@ tial was more negative than − 0.55 V vs RHE, which indi- ZnS/Zn under ambient conditions, several electrochemi- cates the electrosynthesis of ammonia proceeding in the cal tests were performed in N -saturated 0.1  M KOH N -saturated cell. This phenomenon further proves the 2 2 Fig. 4 Electrocatalytic performance of N@ZnS/Zn electrode for and NH yield rate values at controlled potentials; d Comparison of ENRR. a LSV in N - and Ar-saturated electrolytes; b It-curves from the ammonia-sensitive selecting electrode and Nessler reagent-based chronoamperometry tests under different potentials; c Calculated FE colorimetric method for the quantitative analysis of FE values 1 3 Materials for Renewable and Sustainable Energy (2021) 10:8 Page 7 of 10 8 effectiveness of the N@ZnS/Zn electrode in the electro- durability of the N@ZnS/Zn electrocatalyst over a long catalytic ENRR process. time. Furthermore, to estimate the catalytic effectiveness After confirming the validity of the working electrode in of the N@ZnS/Zn electrode, five times parallel tests for ENRR, several chronoamperometry tests were conducted in 2 h each were also carried out at − 0.6 V vs RHE. After N -saturated 0.1 M KOH solution for 2 h each under con- examining the ammonia concentration by the colori- trolled potentials (− 0.5 V, − 0.6 V, − 0.7 V, − 0.8 V, − 0.9 V metric method, the NH yield rates and FE values were vs RHE) to further evaluate the catalytic performance of N@ calculated and presented in Fig.  5b. The long-acting of ZnS/Zn material. The It-curves are shown in Fig. 4b. As the the N@ZnS/Zn electrocatalyst was thus exhibited with pH value of the electrolyte is higher than the pKa value of almost no decrease FE for the electrochemical generation ammonia, an acidic gas absorber was also installed on the of ammonia. electrochemical cell, in the case of ammonia gas emitting. To gain more insight into the relationship between After the chronoamperometry tests, the corresponding structure and function, the comparing of the catalytic NH concentrations of the electrolytes, as well as solutions performance in the electrocatalytic ENRR process was in the absorber, were all measured by spectroscopy using made through conducting chronoamperometry measure- Nessler’s reagent and ammonia-sensitive selecting electrode ments using N@ZnS/Zn, ZnS/Zn, and Zn foil as the work- methods (see more details in the Experimental section and ing electrode at − 0.6 V vs RHE in alkaline electrolytes. Figures S3, S4 in supporting information, which shows the The related NH yield rates and FE values were listed and calibration curves for confirmation of ammonia concentra- compared in Fig. 5c. As expected, the Zn foil exhibited no tion). Given that N H is a possible by-product during the effect for electrocatalytic ENRR, while ZnS/Zn electro- 2 4 electrolysis, the colorimetric method was also employed catalyst mediated the NH synthesis with 4.11% FE and –10 −1 −2 to examine the existence of N H for the electrolytes from 0.24 × 10 mol  s  cm yield rate, almost one-tenth of 2 4 the above-mentioned chronoamperometry tests. Since no the performance with N-doped. Considering the observation N H was detected in solutions, the good selectivity for the from XRD patterns, the attenuated electrocatalytic perfor- 2 4 ENRR was thus verified. After the calculations, the Faradic mance of ZnS/Zn material may attribute to the less forma- efficiency (FE) and the yield rate of ammonia for all the tion of S vacancies and active sites on the surface caused chronoamperometry tests were obtained and listed in Fig. 4c. by the lack of N dopant. In other words, the N doping not As shown, the highest FE, 7.92%, the highest yield rate of only plays a vital role in generating surface defects but also –10 −1 −2 NH , 2.42 × 10 mol  s  cm , were obtained with onset improve the band structure of the material, which would potential at − 0.6 V vs RHE. Consistent with the observation significantly improve the catalytic performance in the ENRR in the LSV curves, a gradual decrease of both FE and yield process. rate can be observed with the more negative onset poten- Furthermore, another comparison was also held to evalu- tials, which might ascribe to the dominant role of compet- ate the influence of the pH value of the electrolyte. As the ing for HER process at higher potentials. It is noteworthy Zn-based electrode is very reactive to an acidic solution, that the yield rate of NH was remaining in the range of a neutral solution, 0.1 M K SO electrolyte, was used in 3 2 4 −1 −2 1.3–2.3 mol  s  cm , suggesting the steady catalytic effi- potentiostatistic tests at − 0.6 V vs RHE with N@ZnS/Zn ciency of the electrode in alkaline solution under ambient as the working electrode with FE value and NH yield rate conditions. presented also in Fig. 5c. In contrast to alkaline electrolytes, As a similar amount of ammonia generating at each neutral solution displayed a moderated effect in N gas fixa- potential, the higher and higher electricity consumed reduces tion, which ascribe to less HER inhibition under the neutral the efficiency of this electrocatalytic N fixation. This also conditions. Intriguingly, the appearance of these demon- explains the increasing current density yet decreasing FE at strated that the possible vacancy filled by OH rather than high potentials observed in the It-curves. In the meantime, N from the alkaline electrolytes did not impede the elec- the validity and reliability of the NH concentrations that trosynthesis of NH by N@ZnS/Zn catalyzed. 3 3 measured with the ammonia-sensitive selecting electrode as Therefore, the formation of NH is plausibly triggered well as the colorimetric method using Nessler reagent were by the simultaneous adsorption and reduction of N and verified with close FE values (Fig.  4d). H O molecules. The N–N bond is further weakened by the With the evaluation of electrocatalytic performance for back-donation of zinc electrons to *π orbital with the aid ENRR in hand, the long-term stability and robustness of of e-supply from heteroatoms, sulfur and nitrogen. Further- the N@ZnS/Zn electrocatalyst were then examined. At more, an integrated reaction between the activated interme- first, a 24 h durability test was conducted at − 0.7 V vs diates achieves the generation of N–H bonds and the cleav- RHE in alkaline electrolyte, and the It-curve was shown age of the N–N bond. After the transmitting of electrons and in Fig. 5a. The few fluctuations along the current density coupling with hydrogens, the final NH molecule desorbs curve and a final 2.86% FE value illustrated the consistent and diffuses into the bulk electrolyte. 1 3 8 Page 8 of 10 Materials for Renewable and Sustainable Energy (2021) 10:8 Fig. 5 a Time-dependent current density curve for N@ZnS/Zn at ZnS/Zn in alkaline electrolytes, and N@ZnS/Zn in neutral electro- − 0.7 V vs RHE for 24 h. b 5 times repeating ENRR at − 0.6 V vs lytes for ENRR; d Comparison of the performance for ENRR with RHE; c Comparison FE and yield rate values of Zn foil, ZnS/Zn, N@ selected recently reported sulfide electrocatalysts Additionally, regarding onset potentials and the FE val- solemn product, the NH product was obtained at a yield –10 −1 −2 ues of electrochemical N -to-NH transformation, several rate of 2.42 × 10 mol  s  cm and an FE of 7.92% 2 3 selected recent works using metal sulfide electrocatalysts at − 0.6 V vs RHE. The N-doping, surface defects, and were compared graphically with N@ZnS/Zn in Fig.  5d the alkaline electrolyte are indispensable factors for the (more details in Table S1). In the diagram, the relatively achievement of high electrocatalytic ENRR performance. superior electrocatalytic performance of N@ZnS/Zn was In the end, the simple controllable synthesis of the electro- shown. In consideration of its low-cost and simple genera- catalysts, as well as their long-term stability and catalytic tion protocol, such a durable and robust electrocatalyst is activity, suggest a promising application in the practical very promising in the practical industrial use for green syn- industrial use of the functionalized zinc electrode for arti- thesis of NH .ficial N fixation. 3 2 Supplementary Information The online version contains supplemen- tary material available at https://doi. or g/10. 1007/ s40243- 021- 00193-x . Conclusion Acknowledgements This work was supported by National Natural In conclusion, we have developed a novel N-doped zinc Science Foundation of China (No. 52071171), Liaoning Revitaliza- sulfide material through the facile ampoule method. The tion Talents Program—Pan Deng Scholars (XLYC1802005), Liaoning BaiQianWan Talents Program (LNBQW2018B0048), Natural Science surface components and morphology of the material were Fund of Liaoning Province for Excellent Young Scholars (2019-YQ- examined through various characteristic techniques. Due 04), Key Project of Scientific Research of the Education Department to the rich S vacancies and abundant Zn-N activate sites, of Liaoning Province (LZD201902), Foundation for Young Scholars of Liaoning University (LDQN2019007), and Natural Science Foundation the as-fabricated N@ZnS/Zn enables the extraordinary of Liaoning Province of China (2020-MS-137). electrocatalytic performance for the ENRR process in alkaline electrolytes under ambient conditions. As the 1 3 Materials for Renewable and Sustainable Energy (2021) 10:8 Page 9 of 10 8 9. Wen, L., Ren, C., Zou, Y., Lin, W., Ding, K.: Why it is S-rich Declarations around Mo atom in the nitrogenase: a DFT investigation. Appl. Surf. Sci. 534, 147595 (2020). https:// doi. org/ 10. 1016/j. apsusc. Conflict of interest The authors certify that they have NO affiliations 2020. 147595 with or involvement in any organization or entity with any financial 10. Zhang, L., Ji, X., Ren, X., Ma, Y., Shi, X., Tian, Z., Asiri, A.M., interest, or non-financial interest in the subject matter or materials dis- Chen, L., Tang, B., Sun, X.: Electrochemical ammonia synthesis cussed in this manuscript. via nitrogen reduction reaction on a MoS Catalyst: theoretical and experimental studies. Adv. Mater. 30, 1800191 (2018). https:// Ethical statement The authors consciously assure that this work is the doi. org/ 10. 1002/ adma. 20180 0191 authors’ own original work, which has not been previously published 11. Li, X., Li, T., Ma, Y., Wei, Q., Qiu, W., Guo, H., Shi, X., Zhang, elsewhere, the paper is not currently being considered for publication P., Asiri, A.M., Chen, L., Tang, B., Sun, X.: Boosted electrocata- elsewhere, the paper reflects the authors’ own research and analysis in lytic N reduction to NH by defect-rich MoS nanoflower. Adv. 2 3 2 a truthful and complete manner, the paper properly credits the mean- Energy Mater. 8, 1801357 (2018). https:// doi. org/ 10. 1002/ aenm. ingful contributions of co-authors and co-researchers, the results are 20180 1357 appropriately placed in the context of prior and existing research, all 12. Li, X., Ren, X., Liu, X., Zhao, J., Sun, X., Zhang, Y., Kuang, X., sources used are properly disclosed (correct citation), and all authors Yan, T., Wei, Q., Wu, D.: A MoS nanosheet-reduced graphene have been personally and actively involved in substantial work leading oxide hybrid: an efficient electrocatalyst for electrocatalytic N to the paper, and will take public responsibility for its content. reduction to NH under ambient conditions. J. Mater. Chem. A 7, 2524–2528 (2019). https:// doi. org/ 10. 1039/ C8TA1 0433F 13. 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Feng, D., Zhang, X., Sun, Y., Ma, T.: Surface-defective F eS for electrochemical NH production under ambient conditions. Nano Authors and Affiliations 1 1 2 3 3 1 3 Da‑Ming Feng  · Ying Sun  · Zhong‑Yong Yuan  · Yang Fu  · Baohua Jia  · Hui Li  · Tianyi Ma * Hui Li Key Laboratory of Advanced Energy Materials Chemistry lihuichemical@gmail.com (Ministry of Education), Nankai University, Tianjin 300071, China * Tianyi Ma tianyima@swin.edu.au Centre for Translational Atomaterials, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, VIC 3122, Australia Institute of Clean Energy Chemistry, Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals

Ampoule method fabricated sulfur vacancy-rich N-doped ZnS electrodes for ammonia production in alkaline media

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Abstract

The electrochemical production of green and low-cost ammonia requests the development of high-performance electro- catalysts. In this work, the ampoule method was applied to modulate the surface of the zinc electrode by implanting defects and low-valent active sites. The N-doped ZnS electrocatalyst was thus generated by sulfurization with thiourea and applied for electrocatalytic nitrogen reduction reaction (ENRR). Given the rich sulfur vacancies and abundant Zn-N active sites on –10 −1 −2 the surface, excellent catalytic activity and selectivity were obtained, with an NH yield rate of 2.42 × 10 mol  s  cm and a Faradaic efficiency of 7.92% at − 0.6 V vs. RHE in 0.1 M KOH solution. Moreover, the as-synthesized zinc electrode exhibits high stability after five recycling tests and a 24 h potentiostatic test. The comparison with Zn foil, non-doping ZnS/ Zn and recent metal sulfide electrocatalysts further demonstrated advanced catalytic performance of N@ZnS/Zn for ENRR. By simple synthesis, S vacancies, and N-doping defects, this promising electrocatalyst would represent a good addition to the arena of transition-metal-based catalysts with superior performance in ENRR. Graphic abstract Keywords Ampoule method · Zinc-based electrocatalyst · Sulfur vacancy · Ammonia synthesis Extended author information available on the last page of the article Vol.:(0123456789) 1 3 8 Page 2 of 10 Materials for Renewable and Sustainable Energy (2021) 10:8 with the aid of abundance S vacancies, ZrS nanofibers Introduction were also proven as a stable and active catalyst for ambi- ent electrochemical N -to-NH conversion with excellent In decades, chemical nitrogen fixation has become a hot- 2 3 selectivity [14]. Apart from early stage transition metals, spot of chemistry research [1]. It is a sought-after pro- iron-series metals were adopted as robust alternatives in cess that converting atmospheric dinitrogen gas into the ENRR by diverse modifications. Through solvothermal nitrogen-containing compounds acquired by modern civi- method and plasma synthesis, the F e S nanosheet [15] lization [2]. Nowadays, the utilization of dinitrogen gas 3 4 and surface-sulfurized Fe foam [16] both exhibited out- is mainly focused on the synthesis of ammonia, a vital standing catalytic performance due to the low valence industrial raw material [3]. Emerged as one of the prom- metallic site and S vacancies with suitable Fe–Fe distance. ising alternatives to the traditional Haber–Bosch process, Because of the various bonding modes and enhanced con- electrocatalytic nitrogen reduction reaction (ENRR) has ductivity, cobalt-based composite materials, C oS @NC drawn increasing attention. Instead of the high consump- [17], CoS -N/S-C [18] and C@CoS@TiO [19], were tion of fossil fuels and massive amounts of greenhouse 2 2 certified as good candidates for ENRR with the gratifying gas emission, the ENRR provides a clean generation of NH yields [20, 21]. However, most of the sulfide elec- NH in aqueous electrolytes under ambient conditions [4]. trodes were obtained in aqueous conditions, which dis- However, the reaction efficiency is restricted by the kinetic played an inferior synthetic efficiency due to the insolubil- and thermodynamic features of dinitrogen gas under ambi- ity issue of metal sulfides. And the complicated synthetic ent conditions. And due to the inevitable electrocatalytic routes pose a significant barrier to the practical use of hydrogen emission reaction (HER) that took place in the metal sulfides in ENRR. aqueous electrolytes, the development of a practical ENRR On the other hand, the controllable synthesis of electro- technique is still challenging [5]. catalysts for various electrochemical reactions is a state- The past 5 years witnessed numerous metal-based mate- of-art technological challenge in both electrochemistry rials that were fabricated and adopted as electrocatalysts and material science [22]. Among all of the existing syn- for the ENRR process [6, 7]. Regarding the improvement thetic processes, the ampoule method owns superpassingly of the catalytic efficiency and selectivity toward ENRR, versatility in preparing self-standing electrode materials, defect engineering is widely used as an efficient protocol including alloys, nitrides, phosphides, chalcogenides, and to tune the superficial morphology and band structure of halides. Made from silica, quartz, and corrosion-resistant the nano-electrocatalysts, which facilitates the adsorp- niobium and tantalum, the ampoules can be simply self- tion and activation of the N molecule [8]. Therein, using designed according to the requirement of various materials sulfur-containing substances as synthetic precursors for synthetic processes. Meanwhile, the solid or liquid reac- producing metal-based electrocatalysts may originally tants can be loaded inside the ampoule under evacuated generate a nitrogenases-mimicking active site for enhanc- or inert gas conditions before sealing the open end. After ing the ENRR process [9]. that, the heat-up step is always implemented in a furnace As an element in the VI group, sulfur has a relatively with hetero- or homogeneous applying temperature over weak electronegativity, which determined its diversity of a wide range. By such manipulations, the reaction con- bonding form. For instance, the sulfur reacts with metal ditions and atmosphere are confined, sequentially elimi- elements to form ionic compounds, and shares electrons nating the environmental impact and thus realizing the to bond with itself to form a chain-shaped polysulfide ion 2− controllable synthesis. These days, the surface-modified S . Therefore, the structure of metal sulfides tends to be transition-metal foil materials generated from the ampoule diversified and complicated, which make sulfur a fascinat- method have already been applied to various electrocata- ing and promising doping element for functional metal lytic reactions [23–27]. electrocatalysts construction. In addition, it is reasonable With our ongoing interests in the ampoule methods for to suppose that sulfur defects and vacancies would exhibit synthesizing catalytic materials and exploration of electro- great potential in improving the activity of ENRR due to catalytic ENRR, [28–30] herein, we propose the N@ZnS/ the enhancement of ENRR performance presented by the Zn materials fabricated through the facile ampoule method oxygen vacancies. After theoretical study and experiments, that enables the electrochemical synthesis of ammonia in the MoS catalysts provided a distinctive electrocatalytic ambient conditions. The S vacancies and Zn-N activate effect in producing ammonia from N gas in an aqueous sites are speculated to facilitate the adsorption of the N solution [10–12]. Furthermore, the N-doped MoS nano- and reduction process. The adopted alkaline electrolytes flowers presented enhanced catalytic performance due to and potassium cations present enough HER inhibition, rich S vacancies and Mo–N active sites [13]. Besides, which allows efficient synthesis of NH . 1 3 Materials for Renewable and Sustainable Energy (2021) 10:8 Page 3 of 10 8 was protonated by first boiling in ultrapure water for 1 h Experimental section and treating in H O (5%) aqueous solution at 80 °C for 2 2 another 1 h in sequence. The membrane was then treated in Chemicals 0.5 m H SO for 3 h at 80 °C and finally stored in ultrapure 2 4 water overnight. The electrochemical measurements were Zinc foil (99.95% trace metals basis, thickness 0.1 mm) was conducted using CHI 760 E electrochemical worksta- purchased from Sigma-Aldrich. Potassium sodium tartrate, tion (CH Instruments, Inc., USA) with a three-electrode thiourea (CS(NH ) ), sulfur power (re-sublimation), hydro- 2 2 configuration. Before proceeding with electrocatalytic chloric acid (HCl, 36–38%), and hydrogen peroxide (H O , 2 2 tests, the synthesized materials were ultrasonicated Milli- 30%) were purchased from Tianjin Yongda Chemical Rea- Q water for 30 min and activated by electrolysis with the gent Co., Ltd. Potassium hydroxide (KOH) and Nessler’s setups of ENRR tests in Ar-saturated 0.1  M KOH elec- reagent were purchased from Tianjin Damao Chemical trolyte at − 0.7 V vs RHE for 48 h. The as-prepared and Reagent Co., Ltd. Nitrogen (N , 99.999%) and argon (Ar, activated zinc sulfide materials were used as the working 99.999%) were purchased from Shenyang Zhaote Special electrode, while the graphite rod electrode and Ag/AgCl Gas Co., Ltd. All the chemicals were used without further electrode (3.5  M KCl electrolyte) were adopted as the purification. counter and reference electrode, respectively. Unless oth- erwise specified, the 0.1 M KOH solution (pH = 13) was Zinc‑based electrode synthesis adopted as the electrolyte in the linear sweep voltammetry (LSV) and chronoamperometry tests. The measured poten- The zinc-based catalytic electrodes were synthesized tials vs. Ag/AgCl were converted to a reversible hydrogen through the ampoule method. In the beginning, the zinc electrode (RHE) scale according to the Nernst equation foil was polished and cleaned with anhydrate ethanol, ace- (E = E + 0.059pH + 0.205 = E + 0.972). RHE Ag/AgCl Ag/AgCl tone, 1 M HCl, and Milli-Q water for 5 min in sequence. Also, the outlet gas was introduced to an acid bottle with After cutting into pieces (180 mg, 2 cm × 0.4 cm × 0.01 cm, 5 mL of 2 M HCl for wet scrubbing to collect the possible length × width × height), the pretreated zinc foils were sealed leaking ammonia. with sulfurization reagents ampoule under vacuum and cal- cinated at 400 °C for 5 h. The heating rate was 5 ºC/min and the cooling process took place naturally. During this process, Determination of products the Zn atoms of the foil were sulfurized by sulfur atom vapor under pressure, leading to free-standing electrodes possess- The concentrations of the synthesized NH are detected by ing ZnS species on the surface. The material that sulfurized ammonia-sensitive electrode measurement and colorimetric with thiourea and sulfur power is named N@ZnS/Zn and method using Nessler reagent. ZnS/Zn, respectively. For the ammonia-sensitive electrode measurement, 5 mL of a solution of electrolyte or five-time diluted tail Instruments and characterization gas absorber was mixed with 5 mL 1 M KOH solution and 0.2 mL low-level ammonia pH-adjusting ISA solution (Ther- The X-ray diffraction (XRD) patterns were obtained on a mofisher). After stirring, the voltage was obtained using Bruker (Germany) D8 Advance diffractometer with Cu Kα the ammonia-sensitive selecting electrode (Thermofisher) radiation in the range of 20°–80° (2θ). The scanning electron when stable. The standard curve is constructed by measur- microscope (SEM) images and elemental mapping images ing a series of voltage for the reference solutions with 0.1 M −1 were performed in a Hitachi SU8000. Transmission elec- KOH and different NH Cl concentrations (0.8  μg  mL , −1 −1 −1 tron microscope (TEM) images were obtained from a JEOL 0.4 μg  mL, 0.2 μg  mL, 0.1 μg  mL ). JEM-2100 at an acceleration voltage of 200 kV. X-ray pho- For the colorimetric method, 25 mL solution of elec- toelectron spectroscopy (XPS) measurement was carried out trolyte and five-time diluted tail gas absorber was mixed on an ESCALAB Mk II (Vacuum generators) spectrometer with 0.5 mL 50% potassium sodium tartrate solution. Then, with an Al K X-ray source (240 W). 0.75 mL Nessler’s reagent and the mixture were allowed still for 10 min. The absorbance was tested by UV–Vis spectrom- eter (Shimadzu UV-2600) under light at 420 nm wavelength Electrochemical tests with deionized water as a reference. The standard curve is constructed by measuring a series of absorbances for the In general, the ENRR experiments were conducted in a two- reference solutions with 0.1 M KOH and different NH Cl compartment electrolysis cell that separated by the Nafion −1 −1 −1 concentrations (2.0 μg  mL, 1.6 μg  mL, 1.2 μg  mL , 117 membrane under ambient condition. The membrane −1 −1 −1 −1 0.8 μg  mL, 0.4 μg  mL, 0.2 μg  mL, 0.1 μg  mL , 1 3 8 Page 4 of 10 Materials for Renewable and Sustainable Energy (2021) 10:8 −1 0 μg  mL ). The background is corrected by a blank solu- ZnS/Zn was conducted to get rid of the impurity adsorbed tion with 0.1 M KOH and Nessler’s reagent. on the surface as well as the nitrogen-containing species. The concentration of hydrazine in the electrolyte was The morphology and microstructure of as-prepared N@ evaluated via Watt and Chrisp method. The chromogenic ZnS/Zn are examined by scanning electron microscopy agent was produced by mixing para-(dimethylamino) ben- (SEM). The rough surface of the materials is presented in zaldehyde (2 g), concentrated HCl (10 mL), and ethanol Fig. 1b, with particle size between 50 and 100 nm (Fig. 1d). (100  mL). After blending with the agent, the resulting To obtain more evidence for the effectiveness of the mate- electrolytes were detected by measuring the absorbance at rial, the high-resolution transmitting electron microscopy 455 nm. (HRTEM) is adopted and the picture is shown in Fig. 1c. Moreover, plenty of crystal phases and abundant surface defects can be observed at 5 nm resolution (Fig. 1e), further proving the stability and robustness of the electrode mate- Results and discussion rial. Meanwhile, the energy dispersive spectroscopy (EDS) was applied to compare the element ratio of the non-acti- Synthesis and characterization vated N@ZnS/Zn with activated ones After the calculation and approximation, the Zn: S: N ratio is altered from 6: 3: The synthesis of N@ZnS/Zn was following the typical pro- 2.7 to 6: 4: 1 (Figure S1 and S2). The remaining N atoms cess of the ampoule method. At first, the electrode shape on the surface indicate the formation of Zn-N bonds, which zinc foil and thiourea were sealed in an ampoule bottle under is one of the reasons for the electrode with extraordinary a vacuum line. Then the sulfurization took place when put- performance in ENRR (Figure S1). And the EDS-mapping ting the sealed ampoule bottle in the tube furnace under exhibited a homogeneous distribution for the Zn, S, and N 400 °C for 5 h, generating surface sulfurized N@ZnS/Zn atoms (Fig. 1d). material (Fig. 1a). Before characterizing and applying in the The X-ray diffraction (XRD) patterns to the surface of as- electrochemical tests, the activation of the as-prepared N@ prepared and activated N@ZnS/Zn are presented in Fig. 2. Fig. 1 Synthesis and characterization of catalysts. a Synthetic route Zn; inset: SEM–EDS elemental mapping images of Zn, S and N. e for zinc sulfide materials using ampoule method. b SEM image of HR-TEM image of N@ZnS/Zn with surface defects and cell spacing N@ZnS/Zn. c TEM image of N@ZnS/Zn. d SEM image of N@ZnS/ presented 1 3 Materials for Renewable and Sustainable Energy (2021) 10:8 Page 5 of 10 8 demonstrate the amorphous nature of the material surface. In addition, the XRD pattern of non-activated N@ZnS/Zn material is also demonstrated in Fig. 2. With more peaks of impurities presenting in the diagram, the activation process is therefore necessary. To obtain more details of the surface composition and structure, the ZnS/Zn with sulfur powder as sulfurizing agents by the ampoule method is also synthe- sized. The XRD pattern of ZnS/Zn shows the peaks at 28.5°, 47.5°, 56.3°, 76.6°, and 81.2° belong to the ZnS species, within which the lower peaks may ascribe to the moderated crystallinity. The full-range X-ray photoelectron spectroscopy (XPS) spectra of activated N@ZnS/Zn is shown in Fig. 3a, which demonstrated the presence of the elements Zn, S, and N. The Fig. 2 XRD patterns of N@ZnS/Zn and ZnS/Zn materials measured binding energies are in-line with those reported in the literature [31]. In particular, the binding energy of Zn 2p and Zn 2p is located at 1022.9 eV and 1045.9 eV, 3/2 1/2 According to the standard PDF # 80-0200, the diffraction respectively (Fig. 3b). The difference between the two core peaks at 28.5°, 47.5°, 56.3°, 69.4°, 76.6°, and 81.2° are level components of Zn 2p and Zn 2p is approximately 3/2 1/2 the character peaks attributing to the (111), (220), (311), 23 eV, indicating the bivalent state of Zn. Moreover, the (400), (331), (420) planes of the zinc sulfide crystal, which fitting of the XPS spectrum sulfur S 2 p peaks exhibits the implies the sphalerite ZnS species existing on the material main sulfur species on the surface of N@ZnS/Zn, which can surface. The relatively broad peaks and the rough baseline be divided into two main characteristic peaks at 161.6 eV Fig. 3 XPS spectra of the N@ZnS/Zn samples: a Full spectrum; b Zn 2p spectrum; c S 2p spectrum; d N 1 s spectrum 1 3 8 Page 6 of 10 Materials for Renewable and Sustainable Energy (2021) 10:8 and 162.9 eV, ascribed to the S 2p and S 2p respec- electrolyte. The divided cells were used to carry out the 3/2 1/2 tively, in a good agreement with the energy of the S-Zn experiments, and a proton conductive cation exchange bond (Fig. 3c) [31]. The existence of polysulfides, one of membrane (Nafion 117) was adopted to separate the cath- the primary reasons for emerged S vacancies, were specu- ode and anode chamber. Ahead of the electrochemical lated based on the 1.3 eV difference of the binding energies. tests, the 2 h electrolysis of N -saturated electrolytes and Meanwhile, the presence of the N 1 s characteristic peak Ar-saturated electrolytes with the working electrode was located at 395.38 eV and 396.43 eV reveals the existence conducted at open circuit potential and − 0.7 V vs RHE, of the Zn-N bond according to the literature (Fig. 3d) [32]. respectively. After that, the resulting solutions were exam- It is noteworthy that the residual nitrogen on the surface of ined by Nessler’s reagent with no obvious NH detection, N@ZnS/Zn after the activation would not be reduced into thus proving that there are no ammonia-like impurities in ammonia due to the binding. the feeding gas and reaction environment. The results from the above material characterizations At first, we explored the performances of the N@ZnS/ demonstrate that the surface modification of Zn foil by sul- Zn electrodes in ENRR through the comparison of the furization with thiourea through the ampoule method pro- linear sweep voltammetry (LSV) curves under Ar- and vides stable amorphous ZnS with abundant S vacancies and N -saturated electrolytes (Fig.  4a). The increasing cur- Zn-N binding active sites promising for electrocatalysis for rent density along with the increasing potentials is due to ENRR. the competing hydrogen reduction reaction (HER) that is enhanced gradually in both LSV tests. A higher current Electrocatalytic ENRR performances density in N -saturated electrolytes was then observed compare to the Ar-saturated ones when the applied poten- To evaluate the electrocatalytic ENRR efficiency of N@ tial was more negative than − 0.55 V vs RHE, which indi- ZnS/Zn under ambient conditions, several electrochemi- cates the electrosynthesis of ammonia proceeding in the cal tests were performed in N -saturated 0.1  M KOH N -saturated cell. This phenomenon further proves the 2 2 Fig. 4 Electrocatalytic performance of N@ZnS/Zn electrode for and NH yield rate values at controlled potentials; d Comparison of ENRR. a LSV in N - and Ar-saturated electrolytes; b It-curves from the ammonia-sensitive selecting electrode and Nessler reagent-based chronoamperometry tests under different potentials; c Calculated FE colorimetric method for the quantitative analysis of FE values 1 3 Materials for Renewable and Sustainable Energy (2021) 10:8 Page 7 of 10 8 effectiveness of the N@ZnS/Zn electrode in the electro- durability of the N@ZnS/Zn electrocatalyst over a long catalytic ENRR process. time. Furthermore, to estimate the catalytic effectiveness After confirming the validity of the working electrode in of the N@ZnS/Zn electrode, five times parallel tests for ENRR, several chronoamperometry tests were conducted in 2 h each were also carried out at − 0.6 V vs RHE. After N -saturated 0.1 M KOH solution for 2 h each under con- examining the ammonia concentration by the colori- trolled potentials (− 0.5 V, − 0.6 V, − 0.7 V, − 0.8 V, − 0.9 V metric method, the NH yield rates and FE values were vs RHE) to further evaluate the catalytic performance of N@ calculated and presented in Fig.  5b. The long-acting of ZnS/Zn material. The It-curves are shown in Fig. 4b. As the the N@ZnS/Zn electrocatalyst was thus exhibited with pH value of the electrolyte is higher than the pKa value of almost no decrease FE for the electrochemical generation ammonia, an acidic gas absorber was also installed on the of ammonia. electrochemical cell, in the case of ammonia gas emitting. To gain more insight into the relationship between After the chronoamperometry tests, the corresponding structure and function, the comparing of the catalytic NH concentrations of the electrolytes, as well as solutions performance in the electrocatalytic ENRR process was in the absorber, were all measured by spectroscopy using made through conducting chronoamperometry measure- Nessler’s reagent and ammonia-sensitive selecting electrode ments using N@ZnS/Zn, ZnS/Zn, and Zn foil as the work- methods (see more details in the Experimental section and ing electrode at − 0.6 V vs RHE in alkaline electrolytes. Figures S3, S4 in supporting information, which shows the The related NH yield rates and FE values were listed and calibration curves for confirmation of ammonia concentra- compared in Fig. 5c. As expected, the Zn foil exhibited no tion). Given that N H is a possible by-product during the effect for electrocatalytic ENRR, while ZnS/Zn electro- 2 4 electrolysis, the colorimetric method was also employed catalyst mediated the NH synthesis with 4.11% FE and –10 −1 −2 to examine the existence of N H for the electrolytes from 0.24 × 10 mol  s  cm yield rate, almost one-tenth of 2 4 the above-mentioned chronoamperometry tests. Since no the performance with N-doped. Considering the observation N H was detected in solutions, the good selectivity for the from XRD patterns, the attenuated electrocatalytic perfor- 2 4 ENRR was thus verified. After the calculations, the Faradic mance of ZnS/Zn material may attribute to the less forma- efficiency (FE) and the yield rate of ammonia for all the tion of S vacancies and active sites on the surface caused chronoamperometry tests were obtained and listed in Fig. 4c. by the lack of N dopant. In other words, the N doping not As shown, the highest FE, 7.92%, the highest yield rate of only plays a vital role in generating surface defects but also –10 −1 −2 NH , 2.42 × 10 mol  s  cm , were obtained with onset improve the band structure of the material, which would potential at − 0.6 V vs RHE. Consistent with the observation significantly improve the catalytic performance in the ENRR in the LSV curves, a gradual decrease of both FE and yield process. rate can be observed with the more negative onset poten- Furthermore, another comparison was also held to evalu- tials, which might ascribe to the dominant role of compet- ate the influence of the pH value of the electrolyte. As the ing for HER process at higher potentials. It is noteworthy Zn-based electrode is very reactive to an acidic solution, that the yield rate of NH was remaining in the range of a neutral solution, 0.1 M K SO electrolyte, was used in 3 2 4 −1 −2 1.3–2.3 mol  s  cm , suggesting the steady catalytic effi- potentiostatistic tests at − 0.6 V vs RHE with N@ZnS/Zn ciency of the electrode in alkaline solution under ambient as the working electrode with FE value and NH yield rate conditions. presented also in Fig. 5c. In contrast to alkaline electrolytes, As a similar amount of ammonia generating at each neutral solution displayed a moderated effect in N gas fixa- potential, the higher and higher electricity consumed reduces tion, which ascribe to less HER inhibition under the neutral the efficiency of this electrocatalytic N fixation. This also conditions. Intriguingly, the appearance of these demon- explains the increasing current density yet decreasing FE at strated that the possible vacancy filled by OH rather than high potentials observed in the It-curves. In the meantime, N from the alkaline electrolytes did not impede the elec- the validity and reliability of the NH concentrations that trosynthesis of NH by N@ZnS/Zn catalyzed. 3 3 measured with the ammonia-sensitive selecting electrode as Therefore, the formation of NH is plausibly triggered well as the colorimetric method using Nessler reagent were by the simultaneous adsorption and reduction of N and verified with close FE values (Fig.  4d). H O molecules. The N–N bond is further weakened by the With the evaluation of electrocatalytic performance for back-donation of zinc electrons to *π orbital with the aid ENRR in hand, the long-term stability and robustness of of e-supply from heteroatoms, sulfur and nitrogen. Further- the N@ZnS/Zn electrocatalyst were then examined. At more, an integrated reaction between the activated interme- first, a 24 h durability test was conducted at − 0.7 V vs diates achieves the generation of N–H bonds and the cleav- RHE in alkaline electrolyte, and the It-curve was shown age of the N–N bond. After the transmitting of electrons and in Fig. 5a. The few fluctuations along the current density coupling with hydrogens, the final NH molecule desorbs curve and a final 2.86% FE value illustrated the consistent and diffuses into the bulk electrolyte. 1 3 8 Page 8 of 10 Materials for Renewable and Sustainable Energy (2021) 10:8 Fig. 5 a Time-dependent current density curve for N@ZnS/Zn at ZnS/Zn in alkaline electrolytes, and N@ZnS/Zn in neutral electro- − 0.7 V vs RHE for 24 h. b 5 times repeating ENRR at − 0.6 V vs lytes for ENRR; d Comparison of the performance for ENRR with RHE; c Comparison FE and yield rate values of Zn foil, ZnS/Zn, N@ selected recently reported sulfide electrocatalysts Additionally, regarding onset potentials and the FE val- solemn product, the NH product was obtained at a yield –10 −1 −2 ues of electrochemical N -to-NH transformation, several rate of 2.42 × 10 mol  s  cm and an FE of 7.92% 2 3 selected recent works using metal sulfide electrocatalysts at − 0.6 V vs RHE. The N-doping, surface defects, and were compared graphically with N@ZnS/Zn in Fig.  5d the alkaline electrolyte are indispensable factors for the (more details in Table S1). In the diagram, the relatively achievement of high electrocatalytic ENRR performance. superior electrocatalytic performance of N@ZnS/Zn was In the end, the simple controllable synthesis of the electro- shown. In consideration of its low-cost and simple genera- catalysts, as well as their long-term stability and catalytic tion protocol, such a durable and robust electrocatalyst is activity, suggest a promising application in the practical very promising in the practical industrial use for green syn- industrial use of the functionalized zinc electrode for arti- thesis of NH .ficial N fixation. 3 2 Supplementary Information The online version contains supplemen- tary material available at https://doi. or g/10. 1007/ s40243- 021- 00193-x . Conclusion Acknowledgements This work was supported by National Natural In conclusion, we have developed a novel N-doped zinc Science Foundation of China (No. 52071171), Liaoning Revitaliza- sulfide material through the facile ampoule method. The tion Talents Program—Pan Deng Scholars (XLYC1802005), Liaoning BaiQianWan Talents Program (LNBQW2018B0048), Natural Science surface components and morphology of the material were Fund of Liaoning Province for Excellent Young Scholars (2019-YQ- examined through various characteristic techniques. Due 04), Key Project of Scientific Research of the Education Department to the rich S vacancies and abundant Zn-N activate sites, of Liaoning Province (LZD201902), Foundation for Young Scholars of Liaoning University (LDQN2019007), and Natural Science Foundation the as-fabricated N@ZnS/Zn enables the extraordinary of Liaoning Province of China (2020-MS-137). electrocatalytic performance for the ENRR process in alkaline electrolytes under ambient conditions. As the 1 3 Materials for Renewable and Sustainable Energy (2021) 10:8 Page 9 of 10 8 9. Wen, L., Ren, C., Zou, Y., Lin, W., Ding, K.: Why it is S-rich Declarations around Mo atom in the nitrogenase: a DFT investigation. Appl. Surf. Sci. 534, 147595 (2020). https:// doi. org/ 10. 1016/j. apsusc. Conflict of interest The authors certify that they have NO affiliations 2020. 147595 with or involvement in any organization or entity with any financial 10. Zhang, L., Ji, X., Ren, X., Ma, Y., Shi, X., Tian, Z., Asiri, A.M., interest, or non-financial interest in the subject matter or materials dis- Chen, L., Tang, B., Sun, X.: Electrochemical ammonia synthesis cussed in this manuscript. via nitrogen reduction reaction on a MoS Catalyst: theoretical and experimental studies. Adv. Mater. 30, 1800191 (2018). https:// Ethical statement The authors consciously assure that this work is the doi. org/ 10. 1002/ adma. 20180 0191 authors’ own original work, which has not been previously published 11. Li, X., Li, T., Ma, Y., Wei, Q., Qiu, W., Guo, H., Shi, X., Zhang, elsewhere, the paper is not currently being considered for publication P., Asiri, A.M., Chen, L., Tang, B., Sun, X.: Boosted electrocata- elsewhere, the paper reflects the authors’ own research and analysis in lytic N reduction to NH by defect-rich MoS nanoflower. Adv. 2 3 2 a truthful and complete manner, the paper properly credits the mean- Energy Mater. 8, 1801357 (2018). https:// doi. org/ 10. 1002/ aenm. ingful contributions of co-authors and co-researchers, the results are 20180 1357 appropriately placed in the context of prior and existing research, all 12. Li, X., Ren, X., Liu, X., Zhao, J., Sun, X., Zhang, Y., Kuang, X., sources used are properly disclosed (correct citation), and all authors Yan, T., Wei, Q., Wu, D.: A MoS nanosheet-reduced graphene have been personally and actively involved in substantial work leading oxide hybrid: an efficient electrocatalyst for electrocatalytic N to the paper, and will take public responsibility for its content. reduction to NH under ambient conditions. J. Mater. Chem. A 7, 2524–2528 (2019). https:// doi. org/ 10. 1039/ C8TA1 0433F 13. Zeng, L., Chen, S., van der Zalm, J., Li, X., Chen, A.: Sulfur Open Access This article is licensed under a Creative Commons Attri- vacancy-rich N-doped MoS nanoflowers for highly boosting elec- bution 4.0 International License, which permits use, sharing, adapta- 2 trocatalytic N fixation to NH under ambient conditions. Chem. tion, distribution and reproduction in any medium or format, as long 2 3 Commun. 55, 7386–7389 (2019). https:// doi. org/ 10. 1039/ c9cc0 as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes 14. Xu, T., Ma, D., Li, T., Yue, L., Luo, Y., Lu, S., Shi, X., Asiri, were made. The images or other third party material in this article are A.M., Yang, C., Sun, X.: Enhanced electrocatalytic N -to-NH included in the article’s Creative Commons licence, unless indicated 2 3 fixation by ZrS nanofibers with a sulfur vacancy. Chem. Com- otherwise in a credit line to the material. 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Feng, D., Zhang, X., Sun, Y., Ma, T.: Surface-defective F eS for electrochemical NH production under ambient conditions. Nano Authors and Affiliations 1 1 2 3 3 1 3 Da‑Ming Feng  · Ying Sun  · Zhong‑Yong Yuan  · Yang Fu  · Baohua Jia  · Hui Li  · Tianyi Ma * Hui Li Key Laboratory of Advanced Energy Materials Chemistry lihuichemical@gmail.com (Ministry of Education), Nankai University, Tianjin 300071, China * Tianyi Ma tianyima@swin.edu.au Centre for Translational Atomaterials, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, VIC 3122, Australia Institute of Clean Energy Chemistry, Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China 1 3

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