Ambient Electrosynthesis of Ammonia on a Biomass-Derived Nitrogen

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Letter

Ambient Electrosynthesis of Ammonia on Biomass-Derived NitrogenDoped Porous Carbon Electrocatalyst: Contribution of Pyridinic Nitrogen Cuijiao Zhao, Shengbo Zhang, Miaomiao Han, Xian Zhang, Yanyan Liu, Wenyi Li, Chun Chen, Guozhong Wang, Haimin Zhang, and Huijun Zhao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02138 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 2, 2019

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ACS Energy Letters

Ambient Electrosynthesis of Ammonia on Biomass-Derived Nitrogen-Doped Porous Carbon Electrocatalyst: Contribution of Pyridinic Nitrogen 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, QLD 4222,

Australia

Corresponding Author * [email protected], [email protected]

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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 at.% at 500 °C (NPC-500) exhibits high NRR activity with a 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 15N

2

feeding gas and 15NPC-500 reveal that the yielded NH3 is 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 NPC catalyst during NRR can break away from its surface to form N vacancies in carbon matrix as the catalytic active sites for N2 adsorption and activation.

TOC Figure


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ACS Energy Letters

The cycle of nitrogen (N) element (e.g., N2/NH3) on the earth is closely related to human life and social development.1-5 Among varieties of artificial N2 fixation to NH3 technologies, a century-old Haber- Bosch process is still undisputedly dominated 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 energy-intensive (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 accompanies with 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 developed to integrate the Haber-Bosch process for NH3 production in some countries, substitute for the traditional methane/H2O reforming process to generate H2, significantly relieving the production-cost, energy consumption and CO2 emission.2 Such “green NH3 synthesis” technology has been attempted to apply in intensive farming areas, exhibiting great potential scale up to industrial levels.2 Additionally, an effective strategy to solve these serious issues aforementioned is to develop low-cost and energy-efficient substitute technologies. However, the related studies are still on the way and far away from 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 hydrogen source, utilization of clean electrons to reduce N2, and renewable energy sources (e.g., solar, wind and hydro) derived electricity as power supply.1,

8-9

To realize efficient NRR to NH3, a prerequisite is to develop

high-performance NRR electrocatalysts which can adsorb and activate the molecule N2 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 unfavourable for the electron transfer process.12 Up to now, almost all reported NRR electrocatalysts are exclusively noble/non-precious metal-based materials, which exhibit high NRR activities with the Faradaic efficiencies (FE) 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 concerned by the researchers: one is the NRR active mechanism and another is if the doped N in NPGC catalyst will break away from the catalyst surface to contribute the NH3 formation during NRR. The experimental and theoretical calculations results obtained in Quan et al. work have 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


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ACS Energy Letters

work by Wu and co-workers illustrated that the moiety, consisted 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 protonation process.19 To sum up, the NRR activity is highly dependent on the doped N content and type in NPGC electrocatalysts according to the 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 concerned by the researchers is if the doped N (or which type of the 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 Ar atmosphere with the help of CaCO3 and K2C2O4 exhibit electrocatalytic NRR activities for NH3 synthesis. Figure 1a shows the fabrication procedure of alfalfa-derived N-doped porous graphitic carbon sample (see Experimental Section for the details, Supporting Information), and the obtained products were denoted as NPC-X (X represents the pyrolysis temperature of 450, 500, 600 or 700 °C, respectively). The experimental results demonstrate that the NPC-500 with a nitrogen content of 6.35 at.% 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 NPC-500



can leave from the catalyst surface to contribute 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 NPC-500 assembled rechargeable Zn-air battery to drive the NPC-500/carbon cloth constructed two-electrode NRR cell, giving a NH3 yield rate of 1.28 mmol h-1 g-1cat. with an FE of 9.89% at –1.95 V. Figure 1b, c presents the SEM and TEM images of NPC-500, exhibiting 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 favourable for the formation of 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 elements. 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 carbon


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ACS Energy Letters

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 NPC-450 sample, the NPC-500, NPC-600 and NPC-700 exhibit larger surface areas, primarily resulted 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 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 fitted three feature 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 have been summarized in Table S1. Obviously, the N content is decreased from 7.83 at.% for NPC-450 to 3.34 at.% 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 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 favourable 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 the 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 N2-TPD measurements of the NPC catalysts, as shown in Figure 3a. Apparently, the NPC-500 displays the strongest desorption peak centered at ~114 °C among all investigated catalysts, meaning its strong N2 chemisorption.17 Although the NPC-450 also possesses high doping densities of the pyridinic-N and pyrrolic-N (Table S1), its low surface area and inferior pore structure are disadvantageous for these active sites exposure, thus possibly resulting in decreased N2 chemisorption capability. Similar results can be also observed for the NPC-600 and NPC-700 with high surface area and good porous structure, mainly attributed to their reduced doping densities of


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ACS Energy Letters

pyridinic-N and pyrrolic-N compared to the NPC-500. The above N2-TPD results demonstrate that the NPC-500 may be more electrochemically active for the NRR. The CO2-TPD profiles (Figure 3b) of the NPC samples indicate that the NPC-500 exhibits higher acidic CO2 adsorption capability in comparison with the NPC-450, NPC-600 and NPC-700, suggesting its surface rich Lewis base sites created by the doped pyridinic-N, favourable for providing electrons for the NRR.27 Similarly, although the NPC-450 has higher doping density of the pyridinic-N compared to the NPC-500, its low surface area and poor pore structure may result in the decreased CO2 adsorption capability. Figure S5 shows the linear sweep voltammogram (LSV) curves of the NPC electrocatalysts in Ar- or N2-saturated 0.005 M H2SO4 solution. Compared to the obtained results in Ar-saturated 0.005 M H2SO4 solution, all NPC catalysts exhibit the electrocatalytic NRR activities in 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 the NPC-450, NPC-500, NPC-600 and NPC-700, respectively. Obviously, the NPC-700 indicates 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 high HER competition reaction, thus resulting in low NRR Faradaic efficiency. Comparatively, the 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. Firstly, we compared the NRR performance using different NPC catalysts in 0.005 M H2SO4 solution at -0.7 V and -0.4 V (vs. RHE), respectively. As shown in Figure S8, the NPC-500 exhibits the largest NH3 yield rate and Faradaic efficiency at -0.7 V and -0.4 V (vs. RHE), respectively, 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 the NPC-500 at -0.7 V (vs. RHE), indicating 0.005 M H2SO4 solution as the electrolyte 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 0.005 M H2SO4 solution as electrolyte for the following experiments. Figure 3c presents the rate of NH3 yield and corresponding FE of the NPC-500 at different potentials in N2-saturated 0.005 M H2SO4 for 2 h NRR (their corresponding current density-time curves 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 a 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 the NPC-500 at more negative potentials.17 Collectively considering the NH3 production and Faradaic efficiency, the applied potential of -0.4 V (vs. RHE) was chosen for the following experiments. The


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ACS Energy Letters

recycling test results (Figure 3d) of the 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 the NPC-500 at -0.4 V (vs. RHE) shows only a slight decay in the current density after 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 the 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 results obtained in blank solution and open-circuit condition 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 produce at -0.4 V (vs. RHE) in Ar-saturated 0.005 M H2SO4 solution for 2 h NRR (Figure S13), meaning that the doped N in NPC-500 may be reduced to NH3 by 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 the NPC-500 and the doped N in NPC-500 may also contribute small amount of NH3 production during NRR. This can be further confirmed by the isotopic labeling experiments using 15N2 as the feeding gas (the 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

15NH + 4

(the enrichment of 89.6%), indicating the NH3

primarily produced from the NPC-500 catalyzed NRR. Interestingly, it was found that



the weak triplet peaks of

14N

in

14NH + 4

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(the enrichment of 10.4%) can be also

detected in the 1H NMR spectra. The above 1H NMR sample was collected for 2 h NRR measurement using 15N2 as the feeding gas. After that, we performed again the isotopic labeling experiments using

15N

2

as the feeding gas for another 5 h NRR

measurement in fresh 0.005 M H2SO4 solution. Similar 1H 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 Ar-saturated 0.005 M H2SO4 solution, the formed NH3 product only indicates the presence of triplet peaks of

14N

in

14NH + 4

in the 1H

NMR spectra (Figure 4a). The above results suggest that the doped N in NPC-500 could contribute the NH3 formation during NRR. To further verify this, the NRR experiment was also conducted in labeled NPC-500 (denoted as

14N

2-saturated

15NPC-500,

0.005 M H2SO4 solution using

15N

the total N content is 7.27 at.%) as the

electrocatalyst. The 1H NMR spectrum shows that the synthesized NH3 is mainly from

14N

in

14NH +, 4

while two weak peaks of the

15N

in

15NH + 4

can be also be

observed, further confirming the speculation of the doped N to contribute NH3 formation. The above 15N labeled experimental results validate that the formed NH3 is mainly produced from the NRR catalyzed by the NPC-500, while the doped N in NPC is also capable of breaking away from the catalyst surface to form NH3 by hydrogenation reduction process. It should be of interest to the researchers which type of the doped N among pyridinic-N, pyrrolic-N and graphitic-N in the NPC will contribute the NH3 formation during NRR. Therefore, we performed the XPS analysis


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ACS Energy Letters

of the NPC-500 after the NRR in N2-saturated 0.005 M H2SO4 solution for 2 h reaction. The results show that after the NRR, the N doping content obviously decreases to 5.71 at.% from 6.35 at.% of the 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 the NPC-500 after the NRR is apparently decreased, while the pyrrolic-N content is obviously increased, compared to the 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 the 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 surface, consistent with the recently reported works.28-29 Similar results can be also obtained for the NPC-500 after the NRR in Ar-saturated 0.005 M H2SO4 solution for 2 h NRR. The above XPS analysis together with the 15N isotopic labeling experimental results imply that the breakage of doped pyridinic-N in the NPC-500 may result in the generation of N vacancies in carbon matrix which can act as the 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 the NPC-500 before and after the NRR. As shown in Figure 4b, a new absorption peak of -C≡N can be observed for the NPC-500 after the NRR compared to the NPC-500 before the NRR,30-33 suggesting the validity of our speculation. Based on the above experimental results, we therefore performed the theoretical calculations to further verify the contribution of the doped pyridinic-N in graphitic carbon on the formation of NH3 and



N vacancy. The results reveal that two ajacent pyridinic-N in graphitic carbon structure can form strong interactions with very stable structure, followed by hydrogenation process according to N-N→N-NH→HN-NH→HN-NH2→HN-NH3, then generation of a NH3 molecule and formation of an N vacancy in graphitic carbon structure for further N2 adsorption and activation. In detail, the formed N-N interactions with a bond length of 1.42 Å, which is much larger than 1.10 Å in 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 the N-N bond break. Then the second H attacks the other N atom with an energy release of 0.39 eV. In the third step, the free energy changes uphill 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 an 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 carbon matrix as NRR active sites for subsequent N2 adsorption, activation and hydrogenation. In fact, the formation of N vacancies in 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 carbon structure, which are the catalytic active sites for further N2 adsorption and activation using N-doped carbon NRR electrocatalysts.19 Even so, the vacancies generation approaches are different between our work (N vacancies) and the reported


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ACS Energy Letters

work (C vacancies) 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 carbon structure. In this work, our study is mainly focused on the contribution of the doped pyridinic-N on the formation of NH3 and N vacancy. The catalytic active sites of N-doped graphitic carbon materials are very different from the transitional metal based NRR catalysts, in where the positively charged transitional metal with empty d orbit is the catalytic active site for NRR.34-37 Compared to metal-based NRR catalysts, the biomass-derived N-doped carbon NRR catalysts are more cheap and 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 controllable doped type in biomass-derived N-doped carbon through a post-doping approach could be very feasible for improving NRR performance. Additionally, the incorporation of metal-based active components into biomass-derived N-doped carbon could provide more opportunities to obtain high NRR active electrocatalysts with significantly enhanced Faradaic efficiency in aqueous electrolyte.37 In previously reported works by our and other groups, biomass-derived N-doped carbon materials have been used as the electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), promising for the rechargeable Zn-air batteries.38-40 In this work, the 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 bifunctional electrocatalyst,



the NPC-500 assembled rechargeable Zn-air battery gives an open-circuit voltage of ~1.35 V (Figure 4c), capable of lightening 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 the 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 NPC-500 constructed two-electrode NRR cell can deliver a 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 N-doped porous carbon (NPC) has been proven to be electrocatalytically active for the NRR to NH3 at ambient conditions. Moreover, the doped pyridinic-N in NPC material also contributes the formation of NH3 during NRR, and thus generating N vacancies in carbon matrix as the catalytic active sites for the NRR. As a proof of concept experiment, our work also demonstrates the feasibility of utilizing biomass-derived carbon electrocatalyst for energy-integrated electrosynthesis of NH3 at ambient conditions. ASSOCIATED CONTENT Supporting Information


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ACS Energy Letters

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.xxxxxxx. Experimental section; Elemental composition information of 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 curve; current density profiles of the NPC-500; durability measurement of the NPC-500; UV-Vis absorption spectra of the electrolytes colored with indophenol blue indicator under different conditions; surface survey XPS spectra of the NPC-500 after the NRR; ORR and ORE performance of the NPC-500; NPC-500 assembled rechargeable Zn-air battery performance; LSV curves of the NPC-500 in a two-electrode NRR system. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Author Contributions C.Z. and S.Z. contributed equally to this work. 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 university of science and technology of China for measurement of the X-ray absorption near-edge structure (XANES) spectra of the samples. REFERENCES (1) Banerjee, A.; Yuhas, B. D.; Margulies, E. A.; Zhang, Y.; Shim, Y.; Wasielewski, M. R.; Kanatzidis, M. G. Photochemical Nitrogen Conversion to Ammonia in Ambient Conditions With FeMoS-Chalcogels. J Am. Chem. Soc. 2015, 137 (5), 2030-2034. (2) Deng, J.; Iñiguez, J. A.; Liu, C. Electrocatalytic Nitrogen Reduction at Low Temperature. Joule 2018, 2 (5), 846-856. (3) Chatt J.; Dilworth, J. R.; Richards, R. L. Recent Advances in the Chemistry of Nitrogen Fixation. Chem. Rev.,1978, 78, 589-625. (4) Legare, M. A.; Belanger-Chabot, G.; Dewhurst, R. D.; Welz, E.; Krummenacher, I.; Engels, B.; Braunschweig, H. Nitrogen Fixation and Reduction at Boron. Science 2018, 359 (6378), 896-900. (5) van Kessel, M. A.; Speth, D. R.; Albertsen, M.; Nielsen, P. H.; Op den Camp, H. J.; Kartal, B.; Jetten, M. S.; Lucker, S. Complete Nitrification by a Single Microorganism. Nature 2015, 528 (7583), 555-559. (6) Chen, X.; Li, N.; Kong, Z.; Ong, W. J.; Zhao, X. Photocatalytic Fixation of Nitrogen to Ammonia: State-of-The-Art Advancements and Future Prospects. Mater. Horiz. 2018, 5 (1), 9-27. (7) Van der Ham, C. J.; Koper, M. T.; Hetterscheid, D. G. Challenges in Reduction of Dinitrogen by Proton and Electron Transfer. Chem. Soc. Rev. 2014, 43 (15), 5183-5191. (8) Bao, D.; Zhang, Q.; Meng, F. L.; Zhong, H. X.; Shi, M. M.; Zhang, Y.; Yan, J. M.; Jiang, Q.; Zhang, X. B. Electrochemical Reduction of N2 under Ambient Conditions for Artificial N2 Fixation and Renewable Energy Storage Using N2 /NH3 Cycle. Adv. Mater. 2017, 29 (3), 1604799. (9) Brown, K. A.; Harris, D. F.; Wilker, M. B.; Rasmussen, A.; Khadka, N.; Hamby, H.; Keable, S.; Dukovic, G.; Peters, J. W.; Seefeldt, L. C., et al. Light-Driven Dinitrogen Reduction Catalyzed by a CdS: Nitrogenase MoFe Protein Biohybrid. Science 2016, 352, 448-450. (10)Chen, G. F.; Cao, X.; Wu, S.; Zeng, X.; Ding, L. X.; Zhu, M.; Wang, H. Ammonia Electrosynthesis with High Selectivity under Ambient Conditions via a Li(+) Incorporation Strategy. J Am. Chem. Soc. 2017, 139 (29), 9771-9774. (11)Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K. Ammonia Synthesis from First-Principles Calculations. Science 2005, 307, 555-558.


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ACS Energy Letters

(12)Zhao, J.; Chen, Z. Single Mo Atom Supported on Defective Boron Nitride Monolayer as an Efficient Electrocatalyst for Nitrogen Fixation: A Computational Study. J Am. Chem. Soc. 2017, 139 (36), 12480-12487. (13)Geng, Z.; Liu, Y.; Kong, X.; Li, P.; Li, K.; Liu, Z.; Du, J.; Shu, M.; Si, R.; Zeng, J. Achieving a Record-High Yield Rate of 120.9 ugNH3 mgcat.-1 h-1 for N2 Electrochemical Reduction over Ru Single-Atom Catalysts. Adv. Mater. 2018, 1803498. (14)Suryanto, B. H. R.; Kang, C. S. M.; Wang, D.; Xiao, C.; Zhou, F.; Azofra, L. M.; Cavallo, L.; Zhang, X.; MacFarlane, D. R. Rational Electrode-Electrolyte Design for Efficient Ammonia Electrosynthesis under Ambient Conditions. ACS Energy Lett. 2018, 3 (6), 1219-1224. (15)Zhou, F.; Azofra, L. M.; Ali, M.; Kar, M.; Simonov, A. N.; McDonnell-Worth, C.; Sun, C.; Zhang, X.; MacFarlane, D. R. Electro-Synthesis of Ammonia from Nitrogen at Ambient Temperature and Pressure in Ionic Liquids. Energ. Environ. Sci. 2017, 10 (12), 2516-2520. (16)Chen, S.; Perathoner, S.; Ampelli, C.; Mebrahtu, C.; Su, D.; Centi, G. Electrocatalytic Synthesis of Ammonia at Room Temperature and Atmospheric Pressure from Water and Nitrogen on a Carbon-Nanotube-Based Electrocatalyst. Angew. Chem. Int. Ed. 2017, 56 (10), 2699-2703. (17)Liu, Y.; Su, Y.; Quan, X.; Fan, X.; Chen, S.; Yu, H.; Zhao, H.; Zhang, Y.; Zhao, J. Facile Ammonia Synthesis from Electrocatalytic N2 Reduction under Ambient Conditions on N-Doped Porous Carbon. ACS Catal. 2018, 8 (2), 1186-1191. (18)Lv, C.; Qian, Y.; Yan, C.; Ding, Y.; Liu, Y.; Chen, G.; Yu, G. Defect Engineering Metal-Free Polymeric Carbon Nitride Electrocatalyst for Effective Nitrogen Fixation under Ambient Conditions. Angew. Chem. Int. Ed. 2018, 57 (32), 10246-10250. (19)Mukherjee, S.; Cullen, D. A.; Karakalos, S.; Liu, K.; Zhang, H.; Zhao, S.; Xu, H.; More, K. L.; Wang, G.; Wu, G. Metal-Organic Framework-Derived Nitrogen-Doped Highly Disordered Carbon for Electrochemical Ammonia Synthesis Using N2 and H2O in Alkaline Electrolytes. Nano Energy 2018, 48, 217-226. (20)Song, Y. A Physical Catalyst for the Electrolysis of Nitrogen to Ammonia. Sci. Adv. 2018, 4, 1700336. (21)Wang, H.; Wang, L.; Wang, Q.; Ye, S.; Sun, W.; Shao, Y.; Jiang, Z.; Qiao, Q.; Zhu, Y.; Song, P. et al. Ambient Electrosynthesis of Ammonia: Electrode Porosity and Composition Engineering. Angew. Chem. Int. Ed. 2018, 57 (38), 12360-12364. (22)Tang, J.; Salunkhe, R. R.; Liu, J.; Torad, N. L.; Imura, M.; Furukawa, S.; Yamauchi, Y. Thermal Conversion of Core-Shell Metal-Organic Frameworks: A New Method for Selectively Functionalized Nanoporous Hybrid Carbon. J Am. Chem. Soc. 2015, 137 (4), 1572-1580. (23)Zheng, F.; Yang, Y.; Chen, Q. High Lithium Anodic Performance of Highly Nitrogen-Doped Porous Carbon Prepared from a Metal-Organic Framework. Nat. Commun. 2014, 5, 5261.



(24)Liu, Y.; Quan, X.; Fan, X.; Wang, H.; Chen, S. High-Yield Electrosynthesis of Hydrogen Peroxide from Oxygen Reduction by Hierarchically Porous Carbon. Angew. Chem. Int. Ed. 2015, 54 (23), 6837-6841. (25)Li, Y.; Zhao, Y.; Cheng, H.; Hu, Y.; Shi, G.; Dai, L.; Qu, L. Nitrogen-Doped Graphene Quantum Dots with Oxygen-Rich Functional Groups. J Am. Chem. Soc. 2012, 134 (1), 15-18. (26)Liu, S.; Tian, J.; Wang, L.; Zhang, Y.; Qin, X.; Luo, Y.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Hydrothermal Treatment of Grass: A Low-Cost, Green Route to Nitrogen-Doped, Carbon-Rich, Photoluminescent Polymer Nanodots as An Effective Fluorescent Sensing Platform for Label-Free Detection of Cu(II) Ions. Adv. Mater. 2012, 24 (15), 2037-2041. (27)Guo, D.; Shibuya, R.; Akiba, C.; Saji S.; Kondo, T.; Nakamura, J. Active Sites of Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Clarified Using Model Catalysts. Science 2016, 351, 361-365. (28)Li, W.; Wu, T.; Zhang, S.; Liu, Y.; Zhao, C.; Liu, G.; Wang, G.; Zhang, H.; Zhao, H. Nitrogen-Free Commercial Carbon Cloth with Rich Defects for Electrocatalytic Ammonia Synthesis under Ambient Conditions. Chem. Commun. 2018, 54 (79), 11188-11191. (29)Ahmed, M. H.; Byrne, J. A.; McLaughlin, J. A. D.; Elhissi, A.; Ahmed, W. Comparison between FTIR and XPS Characterization of Amino Acid Glycine Adsorption onto Diamond-Like Carbon (DLC) and Silicon Doped DLC. Appl. Surf. Sci. 2013, 273, 507-514. (30)Jokic A.; Cutler J.; Anderson D.; Walley F. Detection of Heterocyclic N Compounds in Whole Soils Using N-XANES Spectroscopy. Can. J. Soil Sci. 2004, 84, 291-293. (31)Chuang, C. H.; Ray, S. C.; Mazumder, D.; Sharma, S.; Ganguly, A.; Papakonstantinou, P.; Chiou, J. W.; Tsai, H. M.; Shiu, H. W.; Chen, C. H. et al. Chemical Modification of Graphene Oxide by Nitrogenation: An X-ray Absorption and Emission Spectroscopy Study. Sci. Rep. 2017, 7, 42235. (32)Kiersch, K.; Kruse, J.; Regier, T. Z.; Leinweber, P. Temperature Resolved Alteration of Soil Organic Matter Composition During Laboratory Heating as Revealed by C and N XANES Spectroscopy and Py-FIMS. Thermochim. Acta. 2012, 537, 36-43. (33)Latham, K. G.; Dose, W. M.; Allen, J. A.; Donne, S. W. Nitrogen Doped Heat Treated and Activated Hydrothermal Carbon: NEXAFS Examination of the Carbon Surface at Different Temperatures. Carbon 2018, 128, 179-190. (34)Chen, G. F.; Ren S. Y.; Zhang, L. L.; Cheng, H.; Luo, Y. R.; Zhu, K. H.; Ding, L. X.; Wang, H. H. Advances in Electrocatalytic N2 Reduction-Strategies to Tackle the Selectivity Challenge. Small Methods 2018, 1800337. (35)Cui, X. Y.; Tang, C.; Zhang, Q. A Review of Electrocatalytic Reduction of Dinitrogen to Ammonia under Ambient Conditions. Adv, Energy Mater. 2018, 8, 1800369.


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ACS Energy Letters

Figures and Captions

(a)

CaCO3

K2C2O4·H2O

Fully grinded and mixed

NPC

Washing

Pyrolysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Mixture

Drying

(c)

(d)

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


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ACS Energy Letters

(a)

C=C C-O

C=O/C=N C-C C-N

C1s

Pyrrolic N

(b)

C=O

C-O-C

C-OH

O1s

(c)

Pyridinic N

Graphitic N

NPC-450

NPC-450

N1s NPC-450

NPC-500

NPC-500

NPC-500

NPC-600

NPC-600

NPC-600

NPC-700

NPC-700

NPC-700

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 Ar atmosphere.

(a)

(b)

(c)

(d)

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



(a)

15N 15N 2

for NRR 14N

for NRR using 15NPC-500

15N

(b)

Graphitic Pyrrolic

14N

14N 14N

14N 2

15N

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-C≡N

14N

14N

Pyridinic

15N

NRR in Ar-Saturated Solution 14N

14N

15N

14N

(c)

15N

1.35V ~1.35 V

(15NH4)2SO4 14N

14N

14N

(14NH4)2SO4

Figure 4. (a) 1H NMR spectra of both using

14N

2

,

15N

2

14NH + 4

and

15NH + 4

produced from the NRR

and Ar as the feeding gas and using the

15NPC-500

as the

electrocatalyst. (b) XANES spectra of the NPC-500 before and after NRR. (c) Voltage-time curve of the NPC-500 assembled Zn-air battery.


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ACS Energy Letters

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Ambient Electrosynthesis of Ammonia on a Biomass-Derived Nitrogen

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