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Ambient Electrosynthesis of Ammonia on a Biomass-Derived Nitrogen-Doped Porous Carbon Electrocatalyst: Contribution of Pyridinic Nitrogen ACS Energy Lett. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/23/19. For personal use only.

Cuijiao Zhao,†,‡ Shengbo Zhang,†,‡ Miaomiao Han,*,† Xian Zhang,† Yanyan Liu,†,‡ Wenyi Li,†,‡ Chun Chen,† Guozhong Wang,† Haimin Zhang,*,† and Huijun Zhao†,§ †

Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China ‡ University of Science and Technology of China, Hefei 230026, China § Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus, Southport, Queensland 4222, Australia S Supporting Information *

ABSTRACT: Here we report that alfalfa-derived nitrogen-doped porous carbon (NPC) fabricated by a pyrolysis method is electrochemically active for the N2 reduction reaction (NRR) at ambient conditions. The results demonstrate that the obtained NPC material with a nitrogen content of 6.35 atom % at 500 °C (NPC-500) exhibits high NRR activity with an NH3 yield rate of 1.31 mmol h−1 g−1cat. and a Faradaic efficiency of 9.98% at −0.4 V (vs RHE) in 0.005 M H2SO4 solution. The isotopic labeling experimental results using 15N2 feed gas and 15NPC-500 reveal that the yielded NH3 indeed resulted from the NPC-500 catalyzed NRR, and the doped pyridinic-N also contributes NH3 formation during NRR. The experimental and theoretical calculations results indicate that the doped pyridinic-N in the NPC catalyst during NRR can break away from its surface to form N vacancies in a carbon matrix as the catalytic active sites for N2 adsorption and activation.

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to generate H2, significantly relieving the production-cost, energy consumption, and CO2 emission.2 Such “green NH3 synthesis” technology has been attempted in intensive farming areas, exhibiting great potential scale-up to industrial levels.2 Additionally, an effective strategy to solve these serious aforementioned issues is to develop low-cost and energyefficient substitute technologies. However, related studies are still underway and far from a large-scale industrial level. In recent years, the electrocatalytic N2 reduction reaction (NRR) to NH3 has attracted great attention because of its several advantages, such as ambient operation conditions, abundant water as a hydrogen source, utilization of clean electrons to reduce N2, and renewable energy source (e.g., solar, wind, and hydro) derived electricity as a power supply.1,8,9 To realize efficient NRR to NH3, a prerequisite is

he cycle of nitrogen (N) (e.g., N2/NH3) on the earth is closely related to human life and social development.1−5 Among the varieties of artificial N2 fixation to NH3 technologies, a century-old Haber−Bosch process is still undisputedly dominant to date, operating at high temperature (400−500 °C) and pressure (100−200 atm).5 As we know, this widely applied NH3 production process is highly energyintensive (accounting for 1−3% of the world’s annual energy supply), excessively consumptive of natural gas (e.g., methane) for production of H2 (consuming around 3−5% of the world’s natural gas production), and simultaneously accompanied by massive CO2 emission (accounting for ∼1.5% of all greenhouse gas production).2,6,7 Therefore, effectively tackling the serious energy and environmental issues caused by the traditional Haber−Bosch process is extremely urgent at present. Recently, the utilization of renewable energy (e.g., solar, wind) powered electrolysis water to produce H2 without CO2 emission has been employed to integrate the Haber− Bosch process for NH3 production in some countries, a substitute for the traditional methane/H2O reforming process © XXXX American Chemical Society

Received: November 6, 2018 Accepted: January 2, 2019 Published: January 2, 2019 377

DOI: 10.1021/acsenergylett.8b02138 ACS Energy Lett. 2019, 4, 377−383

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Cite This: ACS Energy Lett. 2019, 4, 377−383

Letter

ACS Energy Letters to develop high-performance NRR electrocatalysts that can adsorb and activate the N2 molecule with large triple-bond energy (940.95 kJ mol−1), negative electron affinity (−1.8 eV), high ionization potential (15.8 eV), and scarcity of a permanent dipole.10−12 Additionally, the large energy gap (10.82 eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of N2 is also unfavorable for the electron transfer process.12 Up to now, almost all reported NRR electrocatalysts are exclusively noble/nonprecious metal-based materials, which exhibit high NRR activities with Faradaic efficiencies (FEs) of ∼30% in aqueous electrolytes and 32−60% in ionic liquids.13−15 Until recently, metal-free N-doped porous graphitic carbon (NPGC) materials as the electrocatalysts have demonstrated good NRR activities, promising for NH3 synthesis.16−21 However, two main issues using the NPGC electrocatalysts for NRR are still of concern to researchers: one is the NRR active mechanism and another is if the doped N in the NPGC catalyst will break away from the catalyst surface to contribute to NH3 formation during NRR. The experimental and theoretical calculations results obtained in Quan et al. revealed that the pyridinic-N and pyrrolic-N in the graphitic carbon electrocatalyst are the active sites for NH3 synthesis from the NRR.17 The reported work by Wu and co-workers illustrated that the moiety consisting of three pyridinic-N atoms (N3) adjacent with one carbon vacancy embedded in a graphitic carbon layer is capable of strongly adsorbing and activating N2, followed by a protonation process.19 To sum up, the NRR activity is highly dependent on the doped N content and type in NPGC electrocatalysts according to reported works.17,22,23 However, to obtain high-efficiency NPGC NRR electrocatalysts, more experimental and theoretical studies are still highly needed. For the NPGC electrocatalysts, another very important issue of concern to researchers is if the doped N (or which type of doped N) in NPGC will break away from the catalyst surface to form NH3 during NRR, which is still a controversial topic. Unfortunately, related experimental and theoretical confirmation has been lacking. Herein, we report that alfalfa-derived N-doped porous graphitic carbon electrocatalysts obtained at different temperatures in an Ar atmosphere with the help of CaCO3 and K2C2O4 exhibit electrocatalytic NRR activities for NH3 synthesis. Figure 1a shows the fabrication procedure of an alfalfa-derived N-doped porous graphitic carbon sample (see the Experimental Section for the details, Supporting Information), and the obtained products were denoted as NPC-X (NPC: N-doped porous carbon; X represents a pyrolysis temperature of 450, 500, 600, or 700 °C). The experimental results demonstrate that the NPC-500 with a nitrogen content of 6.35 atom % exhibits the best NRR activity with a rate of NH3 yield of 1.31 mmol h−1 g−1cat. (22.3 mg h−1 g−1cat.) and a Faradaic efficiency (FE) of 9.98% at −0.4 V (vs RHE) in 0.005 M H2SO4 solution under ambient conditions. Importantly, it was found that the doped pyridinic-N in NPC500 can leave from the catalyst surface to contribute to NH3 formation during NRR on the basis of the 15N isotopic labeling experiments, XPS analysis, and theoretical calculations results. As a proof of concept experiment, we further utilize the NPC500-assembled rechargeable Zn-air battery to drive the NPC500/carbon cloth constructed two-electrode NRR cell, giving an NH3 yield rate of 1.28 mmol h−1 g−1cat. with a FE of 9.89% at −1.95 V.

Figure 1. (a) Schematic illustration of the fabrication process of an alfalfa-derived NPC sample. (b) Surface SEM image of the NPC500. (c) TEM image of the NPC-500. (d) HAADF-STEM image and corresponding elemental mapping images of the NPC-500.

Figure 1b,c presents SEM and TEM images of NPC-500, exhibiting a good three-dimensional (3D) porous carbon structure. Other NPC samples such as NPC-450, NPC-600, and NPC-700 also display 3D porous carbon structures; moreover, higher pyrolysis temperature is more favorable for the formation of a good 3D porous carbon structure from the alfalfa precursor (Figure S1). Figure 1d shows the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the NPC-500 and corresponding elemental mapping images, indicating the presence of only C, O, and N. The XRD patterns (Figure S2a) of all NPC samples exhibit a strong diffraction peak at ∼23.8° and a very weak peak at ∼43.9°, corresponding to the (002) and (100) planes of graphitic carbon, respectively.17 The Raman spectra (Figure S2b) of all NPC samples display two characteristic peaks at ∼1330 cm−1 (D band) and ∼1590 cm−1 (G band) due to the defects and graphitic carbon in carbon materials, respectively.17,23 The intensity ratio of ID/IG for NPC-450, NPC-500, NPC-600, and NPC-700 is 0.97, 1.01, 1.03, and 1.04 respectively, indicating the existence of more defects in the NPC sample obtained at higher pyrolysis temperature, possibly owing to the removal of the doped N in the carbon structure at higher temperature. The defect-rich NPC as the electrocatalyst has been proven to be electrochemically active for the NRR to NH3.24 Figure S3a shows the N2 adsorption−desorption isotherms of the NPC samples. All samples exhibit a type-IV isotherm with a strip hysteresis loop at a wide relative pressure range, indicating the presence of hierarchical porous structures composed of micropores, mesopores, and macropores.22,24 The calculated BET surface area of the NPC-450, NPC-500, NPC-600, and NPC-700 is 144.9, 629.8, 753.7, and 1424.2 m2 g−1, and the corresponding total pore volume is 0.64, 0.93, 1.11, and 1.38 cm3 g−1, respectively. Compared to the NPC450 sample, the NPC-500, NPC-600, and NPC-700 exhibit larger surface areas, primarily resulting from their rich microporous and mesoporous structures (Figure S3b). The high surface area and good porous structure of the NPC samples are beneficial for enhancing electrocatalysis performance.17 378

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Figure 2. High-resolution C 1s (a), O 1s (b), and N 1s (c) XPS spectra of the NPC samples obtained at different pyrolysis temperatures in an Ar atmosphere.

Figure 3. (a) N2 and (b) CO2 TPD profiles of the NPC samples. (c) NH3 yield rate and FE of the NPC-500 at different potentials in N2saturated 0.005 M H2SO4 solution. (d) Recycling test of the NPC-500 at −0.4 V (vs RHE) in a N2-saturated 0.005 M H2SO4 solution.

increased, consistent with the reported results.19 The presence of the abundant pyridinic-N and pyrrolic-N in the NPC samples obtained at low pyrolysis temperatures may be very favorable for high-performance NRR. The previously reported studies have demonstrated that the pyridinic-N and pyrrolic-N in metal-free carbon electrocatalysts are the catalytic active sites, which can be considered as dangling bonds for the NRR.17,21 Therefore, we investigated subsequently the NRR activities of the as-fabricated NPC electrocatalysts. As we know, the chemisorption of N2 molecules on an electrocatalyst is a prerequisite for the following NRR hydrogenation process.17 We therefore performed the N2TPD measurements of the NPC catalysts, as shown in Figure 3a. Apparently, NPC-500 displays the strongest desorption peak centered at ∼114 °C among all investigated catalysts, evidencing its strong N2 chemisorption.17 Although NPC-450 also possesses high doping densities of pyridinic-N and pyrrolic-N (Table S1), its low surface area and inferior pore structure are disadvantageous for active site exposure, thus possibly resulting in decreased N2 chemisorption capability.

The surface survey XPS spectra (Figure S4) show the existence of only C, O, and N characteristic peaks in the NPC samples, further confirming that the alfalfa-derived NPC samples are metal-free. The C 1s XPS spectra (Figure 2a) of all NPC samples can be fitted into five peaks attributed to C− C (284.3 eV), CC (284.6 eV), C−N (285.1 eV), C−O (286.4 eV), and CO/CN (288.8 eV).25 Figure 2b presents the wide asymmetric O 1s XPS spectra of the NPC samples, and the three fitted peaks are attributed to CO (531.4 eV), C−O−C (532.8 eV), and C−OH (533.9 eV), verifying their surface-rich O-containing groups.25 The N 1s XPS spectra (Figure 2c) of the NPC samples can be divided into three feature peaks at 398.8, 400.2, and 401.1 eV, corresponding to the pyridinic-N, pyrrolic-N, and graphitic-N, respectively.25,26 The XPS analysis results are summarized in Table S1. Obviously, the N content is decreased from 7.83 atom % for NPC-450 to 3.34 atom % for NPC-700 with the pyrolysis temperature. Moreover, the contents of pyridinic-N and pyrrolic-N are obviously decreased with the pyrolysis temperature, while the content of graphitic-N is apparently 379

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Figure 4. (a) 1H NMR spectra of both 14NH4+ and 15NH4+ produced from the NRR using 14N2, 15N2, and Ar as the feeding gas and using the 15 NPC-500 as the electrocatalyst. (b) XANES spectra of the NPC-500 before and after NRR. (c) Voltage−time curve of the NPC-500assembled Zn-air battery.

H2SO4 solution at −0.7 and −0.4 V (vs RHE). As shown in Figure S8, NPC-500 exhibits the largest NH3 yield rate and FE at −0.7 and −0.4 V (vs RHE) among all investigated NPC electrocatalysts, indicating its high NRR activity. Subsequently, we investigated the influence of the concentration of H2SO4 solution on the NH3 yield rate and corresponding FE of NPC500 at −0.7 V (vs RHE), indicating the 0.005 M H2SO4 solution as the electrolyte that can contribute the largest rate of NH3 yield (1.53 mmol h−1 g−1cat.) with a FE of 4.93% (Figure S9). Therefore, we selected the 0.005 M H2SO4 solution as the electrolyte for the following experiments. Figure 3c presents the rate of NH3 yield and the corresponding FE of NPC-500 at different potentials in N2-saturated 0.005 M H2SO4 for a 2 h NRR (their corresponding current density−time curves are shown in Figure S10). It can be seen that the rate of NH3 yield is apparently increased with the applied potential, which can reach 1.54 mmol h−1 g−1cat. (26.2 mg h−1 g−1cat.) at −0.8 V (vs RHE) with a FE of 3.55%. Nevertheless, the largest FE of 9.98% can be obtained at −0.4 V (vs RHE), affording an NH3 yield rate of 1.31 mmol h−1 g−1cat. (22.3 mg h−1 g−1cat.). With increasing applied potential, the decreased FE can be ascribed to a competitive HER process on NPC-500 at more negative potentials.17 Collectively considering the NH3 production and FE, an applied potential of −0.4 V (vs RHE) was chosen for the following experiments. The recycling test results (Figure 3d) of NPC-500 at −0.4 V (vs RHE) with 2 h NRR for each recycling experiment show that the NH3 yield rate and FE have no obvious change under the given experimental conditions, indicating its superior stability for the NRR. The durability test result (Figure S11a) of NPC-500 at −0.4 V (vs RHE) shows only a slight decay in the current density after a 10 h NRR measurement, further indicating its high durability. Furthermore, the produced amount of NH3 was found to be increased with the reaction time (Figure S11b), demonstrating the robustness of NPC-500 for the NRR to NH3. Additionally, we also performed several control experiments to illustrate the formed NH3 from the NRR on NPC-500. The experimental

Similar results can be also observed for NPC-600 and NPC700 with high surface area and good porous structure, mainly attributed to their reduced doping densities of pyridinic-N and pyrrolic-N compared to NPC-500. The above N2-TPD results demonstrate that NPC-500 may be more electrochemically active for the NRR. The CO2-TPD profiles (Figure 3b) of the NPC samples indicate that NPC-500 exhibits higher acidic CO2 adsorption capability in comparison with NPC-450, NPC-600, and NPC-700, suggesting its surface-rich Lewis base sites created by the doped pyridinic-N, favorable for providing electrons for the NRR.27 Similarly, although NPC-450 has higher doping density of pyridinic-N compared to NPC-500, its low surface area and poor pore structure may result in decreased CO2 adsorption capability. Figure S5 shows linear sweep voltammogram (LSV) curves of the NPC electrocatalysts in an Ar- or N2-saturated 0.005 M H2SO4 solution. Compared to the results obtained in an Ar-saturated 0.005 M H2SO4 solution, all NPC catalysts exhibit electrocatalytic NRR activities in a N2-saturated 0.005 M H2SO4 solution. Furthermore, the onset potential for the NRR is −0.32, −0.25, −0.24, and −0.19 V (vs RHE) for NPC-450, NPC-500, NPC-600, and NPC-700, respectively. Obviously, NPC-700 indicates a more positive onset potential of the NRR compared to other NPC catalysts; however, its onset potential of the hydrogen evolution reaction (HER) in Ar-saturated 0.005 M H2SO4 is also more positive than those of other NPC catalysts, possibly meaning a high HER competition reaction, thus resulting in low NRR FE. Comparatively, NPC-500 can afford larger current densities in the investigated potential range for the NRR than those of other NPC catalysts, meaning its higher NRR activity. In this work, the obtained NRR products including possible NH3 or/and N2H4 were analyzed through the indophenol blue spectrophotometry method (Figure S6) and the Watt and Chrisp method (Figure S7), respectively. The initial analysis can confirm that only NH3 product without N2H4 can be detected during NRR. First, we compared the NRR performance using different NPC catalysts in a 0.005 M 380

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surface, consistent with the recently reported works.28,29 Similar results can be also obtained for NPC-500 after the NRR in an Ar-saturated 0.005 M H2SO4 solution for a 2 h NRR. The above XPS analysis together with the 15N isotopic labeling experimental results implies that the breaking of doped pyridinic-N in NPC-500 may result in the generation of N vacancies in the carbon matrix, which can act as catalytic active sites for the NRR.19 This was further verified by the X-ray absorption near-edge structure (XANES) spectra of N 1s of NPC-500 before and after the NRR. As shown in Figure 4b, a new absorption peak of −CN can be observed for NPC-500 after the NRR compared to NPC-500 before the NRR,30−33 suggesting the validity of our speculation. On the basis of the above experimental results, we therefore performed theoretical calculations to further verify the contribution of the doped pyridinic-N in graphitic carbon on the formation of NH3 and a N vacancy. The results reveal that two adjacent pyridinic-N species in the graphitic carbon structure can form strong interactions with a very stable structure, followed by a hydrogenation process according to N−N → N−NH → HN−NH → HN−NH2 → HN−NH3, followed by generation of an NH3 molecule and formation of a N vacancy in the graphitic carbon structure for further N2 adsorption and activation. In detail, the N−N formed interactions with a bond length of 1.42 Å, which is much larger than 1.10 Å in a N2 molecule and close to the N−N single bond length (1.40 Å). As shown in Figure S16, the first hydrogenation process happens with a free energy change of 0.68 eV, together with N−N bond breakage. Then, the second H attacks the other N atom with an energy release of 0.39 eV. In the third step, the free energy increases by 1.18 eV to form HN−NH2. At last, HN−NH3 prefers to form rather than H2N−NH2 with the first NH3 release and concurrent formation of a N vacancy in this step, and the free energy changes by 0.73 eV. On the basis of the obtained information mentioned above, the NRR active mechanism using the NPC catalyst was proposed as follows: the doped pyridinic-N in NPC can be reduced to NH3 by hydrogenation during NRR, thus generating N vacancies in a carbon matrix as NRR active sites for subsequent N2 adsorption, activation, and hydrogenation. In fact, the formation of N vacancies in the graphitic carbon structure by the loss of the doped pyridinic-N would result in the formation of unsaturated coordinated carbon sites around these formed vacancies in the carbon structure, which are the catalytic active sites for further N2 adsorption and activation using N-doped carbon NRR electrocatalysts.19 Even so, the vacancy generation approaches are different between our work (N vacancies) and the work (C vacancies) reported by Wu et al.19 It was believable that the NRR activity of the N-doped graphitic carbon electrocatalyst could be ascribed to a collective contribution of N/C vacancies in the carbon structure. In this work, our study is mainly focused on the contribution of the doped pyridinic-N to the formation of NH3 and a N vacancy. The catalytic active sites of N-doped graphitic carbon materials are very different from the transitional metal-based NRR catalysts, where the positively charged transitional metal with empty an d orbital is the catalytic active site for NRR.34−37 Compared to metal-based NRR catalysts, the biomass-derived N-doped carbon NRR catalysts are cheaper and more abundant; however, the intrinsically limited N doping content in biomass-derived carbon catalysts could result in their low NRR activity. Therefore, the enhancement of N content with controllably

results obtained in blank solution and open-circuit conditions confirm that no NH3 product is detectable (Figure S12a,b). However, a small amount of NH3 (0.19 mmol h−1 g−1cat.) can be verified to be produced at −0.4 V (vs RHE) in an Arsaturated 0.005 M H2SO4 solution for a 2 h NRR (Figure S13), meaning that the doped N in NPC-500 may be reduced to NH3 by a hydrogenation process during NRR. The above control experimental results demonstrate that the formed NH3 during NRR may be composed of two parts: primarily produced NH3 is from the NRR catalyzed by NPC-500, and the doped N in NPC-500 may also contribute a small amount of NH3 production during NRR. This can be further confirmed by isotopic labeling experiments using 15N2 as the feed gas (15N enrichment of 99%). The 1H NMR spectra (Figure 4a) reveal that the distinguishable chemical shift of doublet coupling can be ascribed to 15N in 15NH4+ (enrichment of 89.6%), indicating that the NH3 was primarily produced from the NPC-500-catalyzed NRR. Interestingly, it was found that the weak triplet peaks of 14N in 14NH4+ (enrichment of 10.4%) can be also detected in the 1H NMR spectra. The above 1H NMR sample was collected for a 2 h NRR measurement using 15 N2 as the feed gas. After that, we performed again isotopic labeling experiments using 15N2 as the feed gas for another 5 h NRR measurement in a fresh 0.005 M H2SO4 solution. Similar 1 H NMR peaks including 14N in 14NH4+ and 15N in 15NH4+ can be observed (Figure S14); however, the enrichment of 15N increases to 97.3%, and the enrichment of 14N decreases to 2.7% with long-time NRR. Moreover, after the NRR in an Arsaturated 0.005 M H2SO4 solution, the formed NH3 product only indicates the presence of triplet peaks of 14N in 14NH4+ in the 1H NMR spectra (Figure 4a). The above results suggest that the doped N in NPC-500 could contribute to NH3 formation during NRR. To further verify this, the NRR experiment was also conducted in a 14N2-saturated 0.005 M H2SO4 solution using 15N-labeled NPC-500 (denoted as 15 NPC-500; the total N content is 7.27 atom %) as the electrocatalyst. The 1H NMR spectrum shows that the synthesized NH3 is mainly from 14N in 14NH4+, while two weak peaks of the 15N in 15NH4+ can be also be observed, further confirming the speculation of the doped N contributing to NH3 formation. The above 15N-labeled experimental results validate that the formed NH3 is mainly produced from the NRR catalyzed by NPC-500, while the doped N in NPC is also capable of breaking away from the catalyst surface to form NH3 by a hydrogenation reduction process. It should be of interest to researchers which type of doped N among pyridinic-N, pyrrolic-N, and graphitic-N in the NPC will contribute to NH3 formation during NRR. Therefore, we performed XPS analysis of NPC-500 after the NRR in a N2-saturated 0.005 M H2SO4 solution for a 2 h reaction. The results show that after the NRR, the N doping content obviously decreases to 5.71 from 6.35 atom % for NPC-500 before the NRR (Figure S15 and Table S1). This can support the 15N labeling experimental results. Interestingly, it can be seen that the content of pyridinic-N in NPC-500 after the NRR is apparently decreased, while the pyrrolic-N content is obviously increased, compared to NPC-500 before the NRR. The decrease in the content of pyridinic-N means that they are unstable and readily break away from the NPC catalyst surface to contribute to formation of NH3 by hydrogenation reduction during NRR, whereas the increase of the pyrrolic-N content is due to the formation of NRR-derived N species (e.g., −NH2) on the NPC 381

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ACS Energy Letters doped type in biomass-derived N-doped carbon through a postdoping approach could be very feasible for improving NRR performance. Additionally, the incorporation of metalbased active components into biomass-derived N-doped carbon could provide more opportunities to obtain highly NRR active electrocatalysts with significantly enhanced FE in aqueous electrolyte.37 In previously reported works by our group and others, biomass-derived N-doped carbon materials have been used as electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), promising for rechargeable Zn-air batteries.38−40 In this work, alfalfa-derived NPC-500 possesses surface-rich pyridinic-N and O-containing functional groups, thus resulting in superior bifunctionality of the ORR and OER (Figure S17).41,42 As a bifunctional electrocatalyst, the NPC-500-assembled rechargeable Zn-air battery gives an open-circuit voltage of ∼1.35 V (Figure 4c), capable of lighting LED-assembled “NRR” patterns (inset in Figure 4c). In addition, the NPC-500-assembled rechargeable Zn-air battery exhibits good charging and discharging performance (Figure S18a) and high durability (Figure S18b). As a proof-of-concept experiment, two series of NPC-500-assembled rechargeable Zn-air batteries were used as the power supply to drive the NPC-500-constructed two-electrode NRR cell (commercial carbon cloth as the counter electrode) for NH3 production (inset in Figure 4c). The obtained LSV curves (Figure S19) also indicate that NPC-500 is highly electrocatalytically active for the NRR in N2-saturated 0.005 M H2SO4 electrolyte using this NPC-500-assembled two-electrode NRR system. As a result, two series of NPC-500-assembled Zn-air batteries powered by an NPC-500-constructed two-electrode NRR cell can deliver an NH3 yield rate of 1.28 mmol h−1 g−1cat. with a FE of 9.89% at −1.95 V. In summary, alfalfa-derived NPC has been proven to be electrocatalytically active for the NRR to NH3 at ambient conditions. Moreover, the doped pyridinic-N in NPC materials also contributes to the formation of NH3 during NRR, thus generating N vacancies in the carbon matrix as the catalytic active sites for the NRR. As a proof-of-concept experiment, our work also demonstrates the feasibility of utilizing a biomassderived carbon electrocatalyst for energy-integrated electrosynthesis of NH3 at ambient conditions.

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of NPC-500; NPC-500-assembled rechargeable Zn-air battery performance; and LSV curves of the NPC-500 in a two-electrode NRR system (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chun Chen: 0000-0003-3794-4497 Guozhong Wang: 0000-0002-2900-3577 Haimin Zhang: 0000-0003-4443-6888 Author Contributions

C.Z. and S.Z. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51672277 and 51432009), the CAS Pioneer Hundred Talents Program, and the CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Sciences, China. The authors would like to thank the National Synchrotron Radiation Laboratory of the University of Science and Technology of China for measurement of the X-ray absorption near-edge structure (XANES) spectra of the samples.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b02138. Experimental section; elemental composition information on the NPC samples; comparable results for carbon-based NRR electrocatalysts; SEM images of NPC-450, NPC-600, and NPC-700; XRD patterns and Raman spectra of the NPC samples; N2 adsorption− desorption isotherms and pore size distribution curves of the NPC samples; surface survey XPS spectra of the NPC samples; LSV curves of the NPC electrocatalysts; UV−vis absorbance spectra and calibration curves; current density profiles of NPC-500; durability measurement of NPC-500; UV−vis absorption spectra of the electrolytes colored with indophenol blue indicator under different conditions; surface survey XPS spectra of NPC-500 after the NRR; ORR and OER performance 382

DOI: 10.1021/acsenergylett.8b02138 ACS Energy Lett. 2019, 4, 377−383

Letter

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DOI: 10.1021/acsenergylett.8b02138 ACS Energy Lett. 2019, 4, 377−383