Communication pubs.acs.org/JACS
Ammonia Electrosynthesis with High Selectivity under Ambient Conditions via a Li+ Incorporation Strategy Gao-Feng Chen,†,∥ Xinrui Cao,§,∥ Shunqing Wu,§ Xingye Zeng,† Liang-Xin Ding,*,† Min Zhu,‡ and Haihui Wang*,† †
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China § Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Department of Physics, Xiamen University, Xiamen 361005, China ‡
S Supporting Information *
use of enzymes has recently motivated an alternative electrochemical technology to reduce N2 to NH3.12 To date, a few strategies have been developed to optimize the NH3 production rate or to alleviate the thermodynamic requirements using different electrocatalytic approaches.13−16 Nevertheless, almost all the reported data were obtained from noble metal electrocatalysts,17−19 and these studies both theoretically and experimentally demonstrated that the N2 electroreduction reaction (NRR) does not easily proceed in aqueous electrolytes under ambient conditions (a Faradaic efficiency of less than 1%). Generally, four key points are considered the main scientific challenges for this reaction: (1) Most catalysts with weak-binding adsorption for N2 are limited in the first step of the reaction for the activation of N2 by ·N2H; (2) some catalysts with strong-binding adsorption for N2 are limited by either the protonation of intermediate species or the removal of NH3; (3) several moderate catalysts neither too strongly nor too weakly bind to N2 but are still subjected to the competing hydrogen evolution reaction (HER), leading to a compromising Faradaic efficiency for NRR; (4) the flat surfaces of a few metals (e.g., Re, Sc, Y, Ti, and Zr) are capable of suppressing the HER at appropriate potentials, but they lack effective mechanisms to obtain reasonable NRR performances.20,21 Accordingly, further efforts are needed to boost the NRR interface kinetics and to create suppression mechanisms for the HER to obtain reasonable performances. Recently, polyimides, known for their use as insulating materials, have been discovered as host materials for rechargeable lithium batteries, and they allow a charging− discharging process accompanied by the association and the disassociation of Li+ ions with oxygen atoms (i.e., most active sites for hydrogen atom adsorption).22−24 Predictably, polyimides with an intrinsic insulating behavior and Li+-occupied oxygen structures should lack sufficient surface engineering and have a high potential-energy barrier for the HER.25 Therefore, a Li+-incorporation poly(N-ethyl-benzene-1,2,4,5-tetracarboxylic diimide) (PEBCD) design for an NRR electrode is proposed in this work. The atomic processes for Li+ association with the oxygen sites in PEBCD should not only result in sluggish
ABSTRACT: We report the discovery of a dramatically enhanced N2 electroreduction reaction (NRR) selectivity under ambient conditions via the Li+ incorporation into poly(N-ethyl-benzene-1,2,4,5-tetracarboxylic diimide) (PEBCD) as a catalyst. The detailed electrochemical evaluation and density functional theory calculations showed that Li+ association with the O atoms in the PEBCD matrix can retard the HER process and can facilitate the adsorption of N2 to afford a high potential scope for the NRR process to proceed in the “[OLi+]· N2Hx” alternating hydrogenation mode. This atomicscale incorporation strategy provides new insight into the rational design of NRR catalysts with higher selectivity.
A
mmonia (NH3), an emerging energy carrier with 17.6 wt % hydrogen in liquid ammonia compared with 12.5 wt % in methanol, is likely to be a promising candidate in the future hydrogen economy.1,2 However, further developments will be impossible if NH3-based fertilizers are insufficient to feed the world’s rapidly increasing population.3 The ever-increasing ammonia demands have stimulated intensive research on artificial NH3 mass production technology over the last century.4,5 Among the existing options, the Haber−Bosch process is one of the most popular because of the convenient synthesis of ammonia from gaseous elements (N2 and H2) to realize industrial-scale production.6,7 Despite its appealing capacities, the Haber−Bosch process has been plagued for decades with problems such as the harsh conditions required, e.g., operating at high temperatures (350−550 °C) and pressures (150−350 atm), to achieve a considerable rate and yield of NH3 and the necessary H2 feedstock that is often produced from fossil fuels, leading to high CO2 emissions and serious safety concerns.8,9 Therefore, the development of a new, scalable ammonia synthesis technology to reduce the harsh condition requirements and to improve security is an urgent need. Fixation of N2 to NH3 is a fundamental, natural process occurring in microorganisms via nitrogenase enzymes, which synthesize NH3 from H2O, electrons, and atmospheric N2 under ambient conditions (N2 + 6H+ + nMg-ATP + 6e− (enzyme) → 2NH3 + nMg-ADP + nPi).10,11 Heuristically, the © 2017 American Chemical Society
Received: April 29, 2017 Published: July 11, 2017 9771
DOI: 10.1021/jacs.7b04393 J. Am. Chem. Soc. 2017, 139, 9771−9774
Communication
Journal of the American Chemical Society
Figure 3. NRR behavior of PEBCD/C. (a) Schematic reaction cell for NRR. (b) Faradaic efficiency and (c) NH3 yield rate at various potentials under 25 °C. (d) Cycling stability results.
Figure 1. (a) Illustration of the two-step synthetic route for PEBCD: (I) solution polymerization and (II) heat treatment. (b) SEM image. (c) Element mapping and EDS. (d) XPS survey spectrum. (e) FTIR spectrum of PEBCD.
Figure 4. Predicted relative energies for the electrochemical reduction of N2 to NH3 at the [OLi+] active site, where the chemical potential of [H+ + e−] is defined by the binding energy of a single hydrogen atom in H2.
chain. The high-resolution N 1s spectrum (Figure S4) shows the principle peak of CNC (399.6 eV), suggesting the successful synthesis of PEBCD. In addition, the C 1s and O 1s XPS spectra (Figure S5a,b) verify the CC and CO bonds in PEBCD.28 These results are also consistent with the Fourier transform infrared (FTIR) spectrum analysis (Figure 1e), which shows the characteristic peaks for the stretching vibrations of the CO, CN, and CNC bonds.29 The association of Li+ with the imide CO groups was first verified using the cyclic voltammetry (CV) curve recorded in 0.5 M Li2SO4 electrolyte. The typical redox peaks can be clearly observed in Figure 2b, even at low pH values, which corresponds to the association and the disassociation of Li+ with the imide CO groups.25 Moreover, the obvious cathodic peak at approximately −0.4 V, especially compared with that of in (NH4)2SO4 electrolyte (Figure S6a), indicates that the Li+ incorporation is accompanied by the partial reduction of the PEBCD polymer. As a result, the HER activity of PEBCD was significantly passivated. For example, after Li+ incorporation, the HER onset potential on PEBCD was extended to −0.61 V at pH = 3.0, which is significantly larger than that of the C cloth electrodes (Figure S6b,c) and the PEBCD/C electrode with H2SO4 electrolyte (−0.22 V, Figure S6d) in pH = 3.0. FTIR measurements were also used to validate the Li+ incorporation.
Figure 2. (a) CV curves for the PEBCD/C electrode recorded in 0.5 M Li2SO4 electrolytes with various pH values and the C cloth electrode in 0.5 M H2SO4 electrolyte. (b) FTIR spectra of PEBCD before and after Li+ incorporation. (c) Schematic illustration of Li+ association with O sites.
kinetics and a higher energy barrier for H2 formation but also provide appropriate NRR sites. The synthesis strategy for the amorphous PEBCD (Figure S1)26 includes a simple solution polycondensation of polyamic acid from dianhydride and diamine and a final heat treatment to achieve complete imidization (Figure 1a).27 After that, carbon cloth (C) fibers are covered with a layer of upright PEBCD nanosheets along the fiber growth direction (Figures 1b, S2). Energy dispersive X-ray spectroscopy (EDS) and element mapping (Figures 1c, S3) results indicate the uniform distribution of C and O elements on the PEBCD nanosheetconstructed fibers and a lower weight percentage of N (2.1%). According to the X-ray photoelectron spectroscopy (XPS) survey spectra (Figure 1d, Table S1), the atomic ratio of O:N is approximately 2.28:1 on the PEBCD surface, which is consistent with the theoretical value of the PEBCD molecular 9772
DOI: 10.1021/jacs.7b04393 J. Am. Chem. Soc. 2017, 139, 9771−9774
Communication
Journal of the American Chemical Society
corresponding Faradaic efficiency (Figure 3b) and NH3 yield rates (Figure 3c) were obtained using Nessler’s test (Figure S11). As expected, the electrochemical NRR can obtain higher selectivity in the reaction process within the working potential of −0.6 V due to the HER-restraining behavior, and the Faradaic efficiencies of the NH3 yields do not vary significantly for applied potentials from −0.4 to −0.6 V. When the applied potential is below −0.6 V, the competing HER reaction greatly reduces the Faradaic efficiency of NRR to 1.71% at −0.7 V and 0.42% at −0.8 V. However, the best NH3 yield rate of 2.01 μg h−1 cm−2 (derived from the slope of the curve for ammonia production vs reaction time, Figure S12) is obtained at an applied potential of −0.7 V due to the equilibrium effect between enhancing the Faradaic efficiency for the HER and increasing the current density (derived using the i−t curves in Figure S13). Nevertheless, an optimum potential was achieved at −0.5 V by considering the Faradaic efficiency (2.85%) and NH3 yield rate (1.58 μg h−1 cm−2), and the selectivity surpasses most of the reported data under ambient conditions (Table S2) and is even comparable to results obtained under high temperatures and pressures (Table S3). In a durability test (Figures 3d, S14), the PEBCD/C electrode showed stable behavior with an acceptable Faradaic efficiency change (82.2% performance retention) after 6 recycling tests. This finding was further confirmed by the well-retained, original morphology (Figure S15) and negligible variation in the elemental components and content (the atomic ratio of O:N was approximately 2.30:1, Table S1, Figure S16). The unique roles of PEBCD, Li+, and N2 in the NRR mechanism were validated via several control experiments run in a similar manner, such as replacing the N2 gas flow with an Ar atmosphere, using bare C cloth as the working electrode, and substituting 0.5 M Li2SO4 with diluted H2SO4 solution (pH = 3.0), 1 M KOH or 0.5 M K2SO4 as the electrolyte (Figures S17−21). In these experiments, NH3 could not be formed at the applied potential values. On the basis of the result that no N 2H4 was detected in the foregoing normal experiments (Figure S22), the following possible NRR mechanism was suggested: (i) At a negative potential, the Li+ ions are associated with O sites in PEBCD, which can, though incompletely, retard the HER process and provide higher selectivity for NRR. As a typical example, the chronoamperometry curve recorded at a potential of −0.5 V at 40 °C (Figure S23) shows a reduced current density within 1 h, which suggests that the O sites in PEBCD gradually associate with the Li+ at a negative potential and indicated competing reactions among Li+ incorporation, NRR, and HER. Subsequently, the NRR and HER are the main competing reactions after full occupation of the O sites in PEBCD, and the HER has the trend of aggrandizement because more protons exist in the solution than dissolved N2, which is accompanied by the increased current density in the chronoamperometry curve. (ii) The plausible low-energy electroreduction pathway for the conversion of N2 to NH3 follows as an alternating-hydrogenation mode (detailed discussions in Supporting Information, Figure S24):32 [A]·NN → [A]·NHNH → [A]· NH2NH2 → [A]·NH3···NH3 → [A]·NH3 + NH3 → [A] + 2NH3. It should be pointed out that, after the [N2H4] moiety generated, the desorption of N2H4 from the [OLi+] site is predicted to be endothermic by 1.00 eV, whereas the further hydrogenation reduction of [N2H4] to [NH3···NH3] releases an energy of 1.93 eV (Figure 4). Considering the existing large energy difference between these two processes, the hydro-
As shown in Figure 2b, after a negative CV sweep in 0.5 M Li2SO4 electrolyte, the CO stretching vibration peak of the PEBCD shifted to a higher wavenumber (from 1701.9 to 1707.5 cm−1), which should be attributed to electron injection into CO bond and thus formation of a weak bond between the Li+ and electron-rich CO (Li+ ← OC interaction). The result further confirms a successful Li+ association with CO groups. Moreover, the decreased peak intensity of the CO bond compared with that of the original PEBCD is also observed, indicating a decreasing CO groups. The result corresponds to the partial reduction of CO groups to an enol structure during the Li+ incorporation process.26 In addition, the Tafel slope was used to evaluate the HER kinetics. As shown in Figure S7, the Tafel slopes of the PEBCD/C electrode measured in 0.5 M Li2SO4 and 0.5 M H2SO4 aqueous solutions are 968.5 and 356.3 mV dec−1, respectively. These values are significantly higher than those of other reported electrode materials,30 indicating the inherently poor HER behavior of the PEBCD/C electrode and demonstrating that the incorporation of Li+ into PEBCD can effectively retard the HER kinetics. Previous theoretical calculations revealed that oxygen atoms are the best active sites for hydrogen atom adsorption (Hads), with the lowest energy barriers in the polyimide structure.25 In this case, an intrinsically sluggish response to H2 formation for the completely exposed oxygen sites in PEBCD was verified, with an activation barrier of 2.62 eV for a Tafel-reaction pathway (Figure S8). Furthermore, the Volmer step (H+ + e− → Hads) can be replaced by Li+ association with the O atoms under a negative potential in a Li+-rich system. Afterward, either the Tafel (2Hads → H2) or the Heyrovsky (Hads + H2O + e− → H2 + OH−) reaction path is obstructed (Figure 2c), resulting in a higher energy barrier and sluggish kinetics for HER. Then, a larger potential window can be used to achieve higher selectivity for the NRR. Further experiments were conducted to determine the NRR performance of the PEBCD/C electrode, which is the ultimate purpose in suppressing the HER. A schematic diagram of the reaction cell in this work is shown in Figure 3a. The reaction process model in this cell can be identified as follows:31 At the anode, the predominant oxygen evolution reaction (OER) can steadily convert H2O to O2 and H+: 3H 2O → 6H+ + 3/2O2 + 6e−
The generated H+ is transported through the proton membrane to react with N2 to obtain NH3 at the cathode: 6H+ + N2 + 6e− → 2NH3
Preliminarily, a comparison study of the linear sweep voltammetry (LSV) curves under an Ar and N2 atmosphere was conducted (Figure S9). The study showed a distinct current enhancement below −0.54 V under a N2 atmosphere, which is attributed to the reaction between PEBCD/C and N2 to produce NH3. To qualitatively confirm that ammonia originated from N2, an isotopic labeling experiment using N2 enriched to 98% with 15N14N as the feeding gas was conducted. The 1H NMR spectra identified the obtained 15 NH4+ according to a distinguishable chemical shift of triplet coupling of 14N and doublet coupling of 15N (Figure S10), which correspond to the calibration curves of authentic (14NH4)2SO4 and (15NH4)2SO4, substantially verifying ammonia production from N2. Moreover, more detailed electrosynthesis experiments at a series of potentials were performed. The 9773
DOI: 10.1021/jacs.7b04393 J. Am. Chem. Soc. 2017, 139, 9771−9774
Communication
Journal of the American Chemical Society
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genation route to NH3 is the dominant path, as observed experimentally. In summary, we have demonstrated a Li+ incorporation strategy to retard the HER process and afford a reaction site for the NRR. The active OLi+ sites in PEBCD were confirmed by electrochemical characterizations and density functional theory calculations. Per the detailed mechanism, Li+ association with the oxygen atoms in the PEBCD structure can effectively retard H2 formation, leading to a larger potential window for achieving a higher-selectivity NRR with the “[OLi+]·N2 Hx” alternating hydrogenation mode. The proposed strategy provides new opportunities for the development of synthetic ammonia from H2O and N2 under ambient conditions.
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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/jacs.7b04393. Detailed experimental procedures, supported physical and electrochemical characterization of materials by SEM, XRD, XPS, LSV, i−t curves, optical images of electrolytes with color reagent detection, and additional DFT results (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*
[email protected] *
[email protected] ORCID
Min Zhu: 0000-0001-5018-2525 Haihui Wang: 0000-0002-2917-4739 Author Contributions ∥
G.-F.C. and X.C. contributed equally.
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 (21406078, 51621001), National Key R&D Program (2016YFA0202601), and the Pearl River and S&T Nova Program of Guangzhou (201610010076).
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REFERENCES
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DOI: 10.1021/jacs.7b04393 J. Am. Chem. Soc. 2017, 139, 9771−9774
