Facility green electrocatalyst: Sulfur-modied N-doped Durian Shell derived Graphene-like Porous Carbon for N 2 �xation

Undoubtedly, electrochemical ammonia synthesis without carbon footprint will be an anticipated alternative to the Haber-Bosch N 2 -xation process which is energy-intensive. Herein, the durian shell derived carbon was designed as the electrocatalyst precursor, and its graphene-like morphology features and unique hierarchic pore structure obtained by controlling calcination condition was used to trap the N 2 molecules �rmly and convert them. Furthermore, the NH 3 synthesis properties with N, S doping and co-doped were systematically tested. Detailed investigations shown the synergistic effect brought by N and S atom double doping strategy was e�cient promote the increase of electrochemical active sites and thus enhanced the electrocatalytic performance. The NH 3 yield of 32.05 µg NH3 mg cat . −1 h − 1 was obtained by double-doped strategy, which enriched the application of biomass derived carbon materials.


Introduction
The conversion of N 2 to NH 3 play a crucial role in social development. 15] Among them, ammonia synthesis by electrocatalytic nitrogen reduction reaction (ENRR) represents the most promising method, which was attributed to its mild production conditions, green hydrogen and no-carbon footprint. 6e lack of permanent dipole inertia for N 2 molecules makes it necessary to overcome a large energy barrier (with high bond energy up to 941 kJ•mol − 1 ). 7Therefore, an e cient catalyst is required to activate N 2 molecules.5][16][17] Conceptually, the main components of durian shell were cellulose, pectin and lignin.
Meanwhile, the surface of cellulose contains hydroxyl groups, which provides a platform to realize functional modi cations. 18However, carbon materials exhibit reaction inertness in catalytic reactions, so the research focus was to develop the activity of carbon materials and enhance the catalytic performance. 19The strength of interaction with substrate molecules and thus the catalytic performance of carbon materials can be tuned by heteroatom doping which will causes inhomogeneous charge distribution and surface polarity. 202][23] For example, Yang Yong et al. developed a N-doped hierarchical pores carbon from bamboo shoots as a electrocatalyst for convert N 2 to NH 3 under mild conditions, and achieved a high ammonia yield of 16.3 µg NH3 mg cat .−1 h −1 . 18rein, graphene-like hierarchical porous carbon derived prepared from durian shell was prepared by adjusting the temperature, carbon-alkali ratio, and activation time as a N 2 conversion electrocatalyst carrier.5][26] Compared with the performance of N or S doped synthetic NH 3 , the yield of codoped synthetic NH 3 was increased by 10%, which can be attributed to the synergistic effect brought by co-doping. 27The electrochemical catalytic reduction of N 2 to NH 3 was demonstrated in Scheme 1.The experiments indicated that the catalyst had both high selectivity and durability (72 h).The development of e cient and affordable electrocatalysts was fatal for the practical realization of NH 3 electrosynthesis.
This work provides a novel and promising approach to the development of such catalysts.

Preparation of DSPC
The durian shells were washed thoroughly, then dried at 120 ℃ for 48 h.It was heated in a tube furnace to 400 ℃ at N 2 atmosphere and held for 2 h for carbonization.Fully grinding, mixed with KOH in a mass ratio of 1:4 (1:3 and 1:5) and dried overnight.The mixture was heated to 900 ℃ under N 2 protection for 3 h (1 h, 2 h, 3 h and 4h) to obtain expected pore structures.After cooling, the unreacted KOH was neutralized with 1 M HCl, and then dried overnight in an oven at 120 ℃ to obtain black ground which was marked as DSPC 9-4-3.(The naming rules of carbon materials are DSPC temperature -carbon base ratioactivation time.)

Preparation of N-DSPC
DSPC 9-4-3 and urea were weighed with the mass ratio of 1:5, 1:10 and 1:20, respectively, and placed in a 50 mL reactor mixed with 35 mL deionized water, stirred for 30 min at room temperature.Then the mixture was transferred to an oven at 160°C for 4 h, centrifuged and washed with water.Afterward, the product was transfer to 80°C vacuum oven overnight.The obtained products were denoted as N-DSPC-5, N-DSPC-10, and N-DSPC-20, respectively, in which N-DSPC-10 with the best performance was marked as N-DSPC.

Preparation of S/N-DSPC
Weigh the powder of DSPC or N-DSPC and put it into a 2.5 M aqueous solution of Na 2 S 2 O 3 .Placed on a constant temperature magnetic stirrer, stirring at a constant speed at 60°C for 3 h, so that the Na 2 S 2 O 3 solution was fully immersed in the pores of DSPC or N-DSPC.Then seal the beaker with the sealing lm and placed it in the freezer, take it out after 30 minutes.Remove the pre-frozen HCl with a concentration of 1.0 M, and add HCl slowly and uniformly with a pipette under magnetic stirring until the solution appears milky white.The mixture was then refrigerated for 1 h to allow the HCl and S 2 O 3 2− in the DSPC or N-DSPC holes to fully react.The mixture was then centrifuged and cleaned by centrifuge.Afterward, dried overnight in an oven at 60°C, and the black-gray powder was recorded as S/DSPC or S/N-DSPC.

Results and discussion
The surface morphology evolutions of the DSPC with the increase of activation temperature were characterized by SEM observations (Figure S1).Generally, carbonized materials bear a complicated physical-chemical reaction process during the KOH activation process which can cause the different volume shrinkage of carbonized materials at different activation temperature.The volume shrinkage ultimately effective control the microscopic morphology of the DSPC.With the calcination time increased from 1 to 4 h, the number of holes on the surface of the carbonized materials gradually increase and the lamellar layer at the edge of the hole will become thinner, which was conducive to N 2 adsorption and electron transport.The SEM of the N-doped DSPC was presented in Figure S2, by contrast with DSPC was shown that the surface of the material had a slight change, the large hole increases, but still the pore structure of the interaction was present.Figure 1a displayed the SEM image of S/N-DSPC.It can be seen that the surface morphology of catalyst still retains the cross-linked pore structure of DSPC, and the existence of large pore provides a channel for the in ltration of electrolyte.The surface of the S/N-DSPC was attached with a slight granular shape, which was presumed to be the sulfur elemental generated on the surface of the material.The hydrophobicity of sulfur can be used to isolate the contact between the material and the water-based electrolyte to a certain extent, thus conducive to the suppression of the hydrolytic competitive reaction.The comparison TEM image of N-DSPC was emerged in Figure S3. Figure 1b exhibited the graphene-like two-dimensional structural properties of the catalyst, with signi cant folds and a large number of shadows covering the surface.Graphene-like layered folds and disordered graphite were evident in the HRTEM (Fig. 1c).The high temperature carbonization process generated a higher degree of graphitization of the carbon material, which is conducive to the electrical conductivity of the material.EDS analysis was carried out to obtain the distribution of elements in the catalyst.Figure 1d, 1e, 1f and 1g were the corresponding TEM images and the corresponding distribution of elements C, N and S, which fully con rmed that the preparation method made the nitrogen and sulfur elements evenly distributed in the S/N-DSPC.The speci c surface area and pore size distribution of carbon materials were one of the factors affecting the catalytic performance.Therefore, the effects of calcination temperature, carbon base ratio and calcination time on the speci c surface area of electrocatalyst were investigated in this experiment and plotted in Figure S4.Detailed N 2 adsorption and desorption gures and pore size distribution for different calcination times were demonstrated in Figure S5. 28N 2 sorption-desorption isotherms and Pore size distributions analysis results were shown in Figure S5a.The curve presented a typical I-type isotherm, which proved that the carbon material prepared under such conditions was a microporous material. 29After calcination for 4 h, a large number of large pores with pore size greater than 2 nm appeared, which was consistent with the phenomenon displayed in the SEM images.The disappearance of a large number of micropores is undesirable, so 3 h of holding time was the ideal calcination condition.
The crystal structure information and defect degree of DSPC, N-DSPC, S/DSPC and S/N-DSPC were determined by Powder X-ray diffraction (PXRD) and Raman spectra, respectively.Figure 2a summarized the PXRD patterns of the four materials, exhibited wide and bulge (002) characteristic peak of carbon materials at 22.3-24.5°,which proved that neither N-doped nor sulfur modi cation process changes the crystal structure of the carbon materials.However, after doping and modi cation, the characteristic peaks had a slight deviation to a small angle, especially after sulfur modi cation, the deviation angle was more signi cant.This was because the doping of heteroatoms caused the change of the layer spacing of carbon layer.1] The characterization results of Raman spectra demonstrated graphene-like characteristics which were displayed in Fig. 2b.The materials exhibited the characteristic peaks D band (1360 cm − 1 ) and G band (1580 cm − 1 ) of carbon materials, which represent the concentration of defects in the carbon material and carbon atoms in-plain vibration mode E2g, respectively. 32The ratio of the intensity was usually used to judge the defect degree of the carbon material structure.As shown in the Fig. 2b, doping and modi cation of heteroatoms increased the concentration of defects in the materials, especially the N-doped porous carbon of durian shell after sulfur modi cation displayed the largest ratio, proving that sulfur modi cation caused more defects in carbon materials. 33 X-ray photoelectron spectroscopy (XPS) spectrum of S/N-DSPC (Fig. 3) con rms the presence of C, N and S elements.XPS spectra of DSPC, N-DSPC and S/DSPC analyzed the successful doping of N and S atoms, which were signi ed in Figure S6, Figure S7 and Figure S8, respectively.A weak N-peak was detected in DSPC, which belong to amino acids in biomass materials (Figure S6a).The spectrum of N-DSPC showed an obvious N-peak which most of N was in the form of catalytic activity of pyrrole-N and pyridine-N, which fully proved successful N-doped (Figure S7c).In contrast, the XPS spectra of S/DSPC clearly revealed the presence of C-S bonds (Figure S8c).The electronegativity of sulfur (2.58) is similar to carbon (2.55), and the larger atomic radius helps to adjust the electronic properties of graphene-like carbon materials.The content of pyridine-N and pyrrole-N had slight variation, which was due to the introduction of S atoms, who changed the electronic state nearby the C atoms, caused the electron pair polarization, generates charge sites, and creates more abundant active sites, thus signi cantly improving the electrochemical performance of electrocatalysts. 36After the removal of elemental sulfur in catalyst by the compatibility of pristine sulfur and carbon disul de, the characteristic peak of sulfur is still visible and still exists in the form of C-S bond (Figure S9d), which fully proved the feasibility of S doping by low temperature impregnation method.XPS spectra of S/N-DSPC for C 1s regions spectrum exhibited three strong peaks at 284.7, 285.4 and 289.3 eV assigned to C-SO X , C-S/C-N and C-C/C = C, respectively, which con rmed the successful doping of N and S atoms in the electrocatalyst.N 1s spectrum can be deconvoluted into three peaks at 398.7, 400.8 and 405.7 eV corresponding to graphitic-N, pyrrolic-N and pyridinic-N, respectively.Among them, pyrrolidine and pyridine-N were identi ed as active-N that can enhance the catalytic activity of the electrocatalyst, while the presence of graphite-N was believed to promote the electrical conductivity of carbon materials, thus improving the electron transport rate of the catalyst in electrochemistry.XPS spectrum of S 2p can be divided into four peaks at 164.0, 165.The content of C, H, N and S element in catalyst was con rmed by CHN element analyzer (Table S1).The data showed that N content increased to 5.48% after N doping, which was close to the mass percentage after sulfur modi cation.Compared with almost no sulfur was detected before sulfur modi cation, the mass percentage of sulfur increased rapidly to about 36.8% after sulfur modi cation, con rming a large increase of sulfur in the catalyst.According to thermogravimetric analysis (TGA), N-DSPC (Figure S10a) doped with N has 18.2% mass loss during the programmed heating process, mainly owning to the removal of moisture and impurities contained in carbon materials.In addition to the inherent moisture and impurities in the catalyst, the mass loss of S/N-DSPC (Figure S10b) was still 30.5%,which was attributed to the evaporation of sulfur in the material, and similar to the sulfur content detected by the elemental analyzer.The presence of pristine sulfur coated the surface of the electrocatalyst as a hydrophobic layer, which isolated the contact between the electrocatalyst and the liquid in the electrolyte to avoid the occurrence of HER competition reactions. 38Thus, contributing to electron utilization in electrocatalytic reactions.The hydrophobicity of the catalysts was examined using the contact angle method.The test result of the contact angle of carbon paper (125.8°) was displayed in Figure S11, which represented that carbon paper was a hydrophobic material.As shown in Fig. 4, the contact angle of a water droplet on the catalyst surface increases in the order DSPC (140.5°),N-DSPC (140.9°),S/DSPC (145.8°) and S/N-DSPC (146.4°).In particular, S/DSPC and S/N-DSPC exhibited the superhydrophobic state (> 145°), possibly due to pristine sulfur accumulation on the electrocatalyst surface.
The electrocatalytic performance of S/N-DSPC was tested in an H-type electrolytic cell in 0.05 M H 2 SO 4 electrolyte, and the positive and negative electrodes were separated by Na on 117 membrane.Before each catalytic experiment, the test gas (high purity N 2 or Ar) was continuously blowed into the electrolytic cell for 30 min to saturate the test gas in the electrolyte, and the potential was calibrated to the RHE scale.The contents of NH 3 and by-product N 2 H 4 were detected by ion chromatography, indophenol blue and Watt-Chrisp methods, respectively. 39The corresponding standard curve and linear equation were shown in Figure S12, Figure S13 and Figure S14, respectively.By comparing the current density of the electrocatalyst in N 2 and Ar, the activity of the catalyst for N 2 conversion was determined to determine the range of conversion potential.The results showed that the current density measured in N 2 -saturation was higher than that in Ar-saturated electrolyte in the range of potential − 0.2 to -0.6 V, which con rmed the NRR activity of the catalyst.In the experiment, the electrolyte electrolyzed for 2 h under different potential conditions was tested, and combined with the total amount of charge used at the corresponding potential, the catalytic NRR activity was quantitatively analysed, and the volcanic column was drawn in Fig. 5b.It was worth mentioning that the NH 3 yield of S/N-DSPC could reach 32.05 µg NH3 mg cat .−1 h −1 at -0.4 V potential.It was better than the reported catalytic performance of biomass derived carbon-based NRR catalysts.In addition, the corresponding NH 3 yield was quantitatively measured by ion chromatography, and the results were close to those obtained by indophenol blue method (Figure S15). Figure 5c shown the constant voltage current density results tested at a series of potential.Due to HER competing response, the current density was larger and unstable at more negative potentials.In order to eliminate the possible in uence on the experimental results and prove the catalytic performance of sulfurmodi ed N-doped biomass derived carbon, a series of comparative experiments were carried out using the control variable method.The experimental variables were set as: pure carbon paper, open circuit voltage, Ar-saturation; Catalyst setup: N-DSPC, S/DSPC, S/N-DSPC.The results were shown in Fig. 5d.NH 3 yield of pure carbon paper, open-circuit voltage, Ar-saturation, N-DSPC, S/DSPC, S/N-DSPC was the highest, which proves its high catalytic performance of ENRR.As a contrast experiment, the relevant experimental results of S/DSPC were added in Figure S16 and Figure S17.1] Moreover, the doping of heteroatoms promoted the exposure of more active sites in carbon materials, created abundant ENRR active sites, and enhanced the catalytic activity of materials. 42der the optimal potential, the experiment alternated 2 h test cycle of electrocatalytic ammonia synthesis between N 2 saturated electrolyte and Ar-saturated electrolyte.Obviously, a large amount of NH 3 can be produced only in N 2 -saturated electrolyte, while NH 3 in Ar-saturated electrolyte can be almost negligible.4] In order to test the repeatability of the electrocatalytic performance of S/N-DSPC electrocatalyst, the same electrode was used in the experiment for six consecutive repeated electrolysis experiments.Figure 6b exhibited the corresponding ultraviolet absorption spectrum.Figure 6c demonstrated the NH 3 yield and Faraday e ciency of S/N-DSPC for six consecutive repeated tests.Obviously, the catalytic performance of S/N-DSPC was repeatable and stable.What's more, as shown in Fig. 6d, S/N-DSPC maintained a stable current density during the electrocatalytic reaction for 72 h continuously, proving that the graphene-like carbon base material had excellent physicochemical structure stable during the electrocatalytic reaction.Satisfactorily, after durability electrocatalytic test, the N content (Table S2) remained the same as before test, which proved that N 2 was the source of N element in the NH 3 .Moreover, the uniqueness of the product was the key to judge the performance of the catalyst.The detection result of N 2 H 4 as a by-product (Figure S18) indicated that no N 2 H 4 was produced in the reaction process, which further demonstrates the durability of the S/N-DSPC electrocatalyst and its excellent selectivity for synthetic ammonia reaction.

Conclusion
In summary, porous bio-derived carbon materials with graphene-like two-dimensional structure demonstrated excellent electrochemical application prospects.The double-doping strategy improved the catalytic performance of carbon materials, and nally S/N-DSPC exhibited a high NH 3 yield of 32.05 µg NH3 mg cat .−1 h −1 .This work not only demonstrated a competitive green catalyst source, but also provides exciting avenues for designing and creating biomass-derived carbon materials for modi cation and application.
2 and 168.8 eV, which are ascribed to S-O/S = O, C-SO x , C-S and C-S-C, respectively.Indicating the successful doping of S atoms into S/N-DSPC.The existence of N, S atoms in uenced the charge density of adjacent C atoms and provided strong adsorption and conversion capacity towards N 2 .37

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