Electrochemical Ammonia Synthesis from N2 and H2O

May 6, 2019 - undoped perovskites, such as CeFeO3, PrFeO3, SmFeO3, and. GdFeO3, and LaFeO3 doped with other metal elements showed poorer ...
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Research Note Cite This: Ind. Eng. Chem. Res. 2019, 58, 8935−8939

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Electrochemical Ammonia Synthesis from N2 and H2O Catalyzed by Doped LaFeO3 Perovskite under Mild Conditions Sheng Zhang,§,† Guoyi Duan,§,† Lingling Qiao,† Yang Tang,† Yongmei Chen,*,† Yanzhi Sun,*,† Pingyu Wan,† and Suojiang Zhang‡ †

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National Fundamental Research Laboratory of New Hazardous Chemicals Assessment and Accident Analysis, Institute of Applied Electrochemistry, Beijing University of Chemical Technology, Beijing 100029, China ‡ Institute of Processes, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: A green route of electrochemical ammonia synthesis was realized for the first time by using doped LaFeO3 perovskite as an electrocatalyst under mild conditions. The NH3 formation rate of 13.46 μgNH3 h−1 mgcat−1 (3.52 × 10−10 mol s−1 cm−2) was achieved with the cell voltage of 2.4 V, and a faradaic efficiency of 1.99% was obtained when a cell voltage of 1.8 V was applied. The electrochemical ammonia synthesis was based on the chemisorption of N2 molecules on oxygen vacancies in perovskites and the subsequent electrochemical hydrogenation.

1. INTRODUCTION Ammonia is one of the most important chemicals in modern society, and it is also predicted as one of the most ideal energy carriers in the future hydrogen economy.1,2 More than 150 million tons of ammonia were produced worldwide in 2017,3 while more than 95% of them were produced through the state-of-the-art Haber−Bosch process, despite its energyintensive (e.g., high temperature and high pressure)4 and environmentally unfriendly shortcomings (e.g., huge CO2 emissions).5−9 Recently, great effort and thus phenomenal progress have been made in reducing the energy needed for NH3 production by using alternative technologies.10−20 Among them is electrochemical ammonia synthesis from H2O and N2 by using renewable energy sources including solar energy and wind power. Finding a high-performance catalyst is the key problem to be solved for electrochemical ammonia syntheis (EAS) process. It is well-known that NH3 synthesis from N2 and H2 is a thermodynamically spontaneous reaction, even under standard conditions [ΔrGθm (298 K) = −32.90 kJ mol−1]. However, the process is kinetically controlled, because of the large activation energy barrier (∼954 kJ mol−1) of dinitrogen dissociation.21,22 Perovskite-type oxides have been widely used as catalysts in electrochemistry and photochemistry fields, because of their ordered structures, adjustable compositions, and abundant active sites. The oxygen vacancies in perovskite-type oxides are believed to play an important role in the EAS process. Typically, perovskites are always used in high-tempeature solid electrolyte systems. 23,24 For example, Tao used the La0.6Sr0.4Co0.2Fe0.8O3−δ−Ce0.8Gd0.18Ca0.02O2−δ composite as a cathode at 400 °C for NH3 synthesis.25 The oxygen vacancies © 2019 American Chemical Society

in perovskites make the dissociation adsorption of N2 under high temperature feasible, as confirmed by the computational work of the Michalsky group.26 Recently, the studies on photochemical route of ammonia synthesis have shown that the oxygen vacancies in oxides (TiO2 and BiOBr) could adsorb and activate N2 molecules under mild conditions, despite N2 nondissociation.27,28 Thus, the chemisorption of N2 molecules on oxygen vacancies in perovskites could weaken the N−N bonds in N2 molecules and then break the N−N bonds after subsequent electrochemical hydrogenation under mild conditions. This study was focused on using non-noble-metal-based perovskite (Cs- and Ni-doped LaFeO3, LCFN) nanoparticles loaded carbon paper as a working electrode for EAS in a nonmembrane cell with alkali aqueous electrolyte. The catalytic mechanism of perovskite for EAS was discussed based on its structural characteristics and the catalytic performance results.

2. EXPERIMENTAL SECTION 2.1. Preparation of LCFN-Loaded Carbon Paper Electrodes. LaFeO3 (LFO) particles were prepared through a sol−gel method similar to that described in reported literature:29 La(NO3)3·6H2O (4.1 mmol) and Fe(NO3)3· 9H2O (4.1 mmol) were dissolved in deionized (DI) water (100 mL). After citric acid (12.3 mmol) and EDTA (8.2 Received: Revised: Accepted: Published: 8935

February 12, 2019 April 26, 2019 May 6, 2019 May 6, 2019 DOI: 10.1021/acs.iecr.9b00833 Ind. Eng. Chem. Res. 2019, 58, 8935−8939

Research Note

Industrial & Engineering Chemistry Research

3. RESULTS AND DISCUSSION X-ray diffraction (XRD) patterns (Figure 2a) demonstrated the perfectness of the perovskite lattice (JCPDS File No. 37-1493),

mmol) were added, the solution was stirred at room temperature for 30 min and then the pH was adjusted to 6 by using a diluted ammonia solution to get a yellow clear mixture. The mixture was then heated at 200−250 °C and a dark colored sticky gel was formed. Heating was continued until a black-colored dry gel was obtained. The obtained material was heated at a speed of 5 °C min−1 and pyrolyzed at 700 °C for 2 h in air after it was grounded. LaFeO3 was obtained as a powder. Different amounts of Cs+ and Ni2+ were added to the mixture solution of La3+ and Fe3+ for synthesizing different LaaCsbFecNidO3−δ (LCFN) catalysts (where 0 < δ < 3). La0.8Cs0.2Fe0.8Ni0.2O3−δ and La0.6Cs0.4Fe0.6Ni0.4O3−δ (where, for each, 0 < δ < 3) were defined as LCFN82 and LCFN64, respectively. The undoped LaFeO3 was used as the control or reference catalyst. The as-prepared perovskite particles then were dispersed into the Nafion-containing solution to prepare catalyst inks, followed by depositing them on carbon papers, and the loadings of all of the catalysts were 1.6 mg cm−1 (see Section S1 in the Supporting Information). 2.2. Electrochemical Ammonia Synthesis. The continuous electrochemical ammonia synthesis (EAS) experiments, using the LCFN catalysts, were performed in a twoelectrode system (Figure 1b) in 2 M KOH solution with a N2 flow rate of 40 mL min−1 under the applied cell voltage of 1.6− 2.4 V within 40−100 °C. LFO was used as a reference catalyst.

Figure 2. (a) XRD patterns of LFO, LCFN82, and LCFN64; (b) N2 isotherm of LFO, LCFN82, and LCFN64 (inset shows the specific surface area of LFO, LCFN82, and LCFN64); (c−e) SEM images of LFO, LCFN82, and LCFN64; and (f) EDX element mapping images of LCFN82.

Figure 1. (a) Proposed pathway of the LCFN-catalyzed EAS process. (b) Schematic diagram of the setup for EAS.

2.3. Determination of Ammonia. The generated NH3 within 1 h was collected into a 25 mL of 0.001 M H2SO4 solution, and the quantity of generated NH4+ was determined with the Nessler’s reagent spectrophotometric method at 420 nm (see Figure S1 in the Supporting Information). Each sample was measured triple times and the relative standard deviation (RSD) of each reported data was within 5%. Furthermore, all the samples were verified by using the ion chromatographic method (Figure S2 in the Supporting Information), and the relative error between these two methods was 2.0 V. The highest EAS rate, 13.46 μgNH3 h−1 mgcat−1 (3.52 × 10−10 mol s−1 cm−2), was achieved at the cell voltage of 2.4 V. According to the previous studies,22,26 the cathodic potential at which all steps of N2 reduction are favorable is more negative than the onset potential of HER. When the cell voltage was 1.6 V, the cathodic potential was approximately −1.2 V (see Section S3 in the Supporting Information), and the HER just started (Figure 4a). However, for N2 reduction, the cathodic potential of −1.2 V is slightly more positive. Both EAS rate and FE increase with the cell voltage within 1.6−1.8 V, implying the trade-off relationship between EAS and HER at low voltages. The highest FE (1.99%) was obtained with a cell voltage of 1.8 V. The FE value decreased to 0.16% when the cell voltage was increased to 2.4 V, indicating that the competition of HER was a major factor that negatively affected the FE of the ESA process (see Reactions R2 and R4). As shown in Figure 4d, the EAS rate increased from 3.48 μgNH3 h−1 mgcat−1 to 12.79 μgNH3 h−1 mgcat−1 (0.908 × 10−10 mol s−1 cm−2 to 3.34 × 10−10 mol s−1 cm−2) with temperature within 40−80 °C. This results from the fact that all the steps in ESA process accelerated with temperature, including the diffusion of H+ from bulk to surface, the hydrogenation of the adsorbed N2 molecules (Reaction R3), and the desorption of formed NH3 molecules (M-NH3,ad). The bare change in FE within 40−80 °C indicated that the ratio of kN to kH remained constant during the temperature increase, which means that the activation energy (Ea) for the hydrogenation of the adsorbed N2 (Reaction R4) was lowered to almost the same order as that of HER. The EAS rate decreased to 8.81 μgNH3 h−1 mgcat−1 (2.30 × 10−10 mol s−1 cm−2) at 100 °C implied the quick HER and the formation of excessive H2 bubbles on the active sites of catalyst surface, resulting in the hindrance of the N2 chemisorption and thus EAS process. The comparison between this study and recently reported results are listed in Tables S2 and S4 in the Supporting Information. Other undoped perovskites, such as CeFeO3, PrFeO3, SmFeO3, and GdFeO3, and LaFeO3 doped with other metal elements

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Chen). *E-mail: [email protected] (Y. Sun). ORCID

Yang Tang: 0000-0003-0976-0519 Yongmei Chen: 0000-0003-1386-1574 Suojiang Zhang: 0000-0002-9397-954X Author Contributions §

These authors contributed equally and should be considered as cofirst authors. Funding

This work is supported by National Key R&D Program of China (No. 2018YFB0605802) and China Natural Science Funds (Nos. 21706004 and 71571010). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.iecr.9b00833 Ind. Eng. Chem. Res. 2019, 58, 8935−8939

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.9b00833 Ind. Eng. Chem. Res. 2019, 58, 8935−8939