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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Cr2O3 Nanoparticle-Reduced Graphene Oxide Hybrid: A Highly Active Electrocatalyst for N2 Reduction at Ambient Conditions Li Xia,†,‡,# Baihai Li,§,# Ya Zhang,‡ Rong Zhang,‡ Lei Ji,‡ Hongyu Chen,‡ Guanwei Cui,∥ Hongguo Zheng,⊥ Xuping Sun,‡ Fengyu Xie,*,† and Qian Liu*,§ †
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, Sichuan, China Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China § School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, Sichuan, China ∥ College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, Shandong, China ⊥ ThermoFisher Scientific, Chengdu 610023, Sichuan China
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S Supporting Information *
has focused on exploring NRR electrocatalysts operating efficiently under acidic conditions, including MoO 3 , 19 Mo2N,20 MoN,21 VN,22 and Nb2O5,23 it is of great interest to study the NRR performance of Cr2O3 at such pH. Cr2O3, however, has a wide band gap (∼3.0 eV),24 suffering from low electrical conductivity. Such an issue can be fixed by its hybrid with reduced graphene oxide (rGO), which enhances the electron transfer and serves as an attractive support to protect the in situ grown nanocatalysts from agglomeration.25,26 Here, we report a development of Cr2O3 nanoparticles on rGO as a high-property NRR electrcatalyst with outstanding selectivity under ambient conditions. In 0.1 M HCl, the Cr2O3rGO hybrid can achieve a large NH3 yield of 33.3 μg h−1 mg−1cat. and a high Faradaic efficiency (FE) of 7.33% at −0.7 and −0.6 V vs reversible hydrogen electrode (RHE), respectively, outperforming its Cr2O3 counterpart (NH3 yield, 13.9 μg h−1 mg−1cat.; FE, 1.38%). This nanohybrid demonstrates superior electrochemical and structural stability. Density functional theory (DFT) calculations were executed to gain insight into the the mechanisms. Cr2O3-rGO was synthesized by hydrothermal reaction of GO and Na2CrO4 followed by Ar annealing. Figure 1a displays the Xray powder diffraction (XRD) patterns of rGO and Cr2O3-rGO. As observed, the diffraction peak at 2θ = 25° is due to rGO. The diffraction peak of Cr2O3-rGO at 2θ = 24.6° is attributed to the rGO nanosheet;27 the other peak arises from Cr2O3 (JCPDS No. 38-1479). The Raman spectrum confirms that the GO is reduced (Figure 1b). Three prominent peaks appear in Cr2O3-rGO. A peak at about 550 cm−1 is assigned to Cr−O lattice vibrations.28 The D band at around 1340.2 cm−1 represents the defects in sp3 hybridized carbon. The G band at around 1595.5 cm−1 corresponds to the ordered in-plane vibration in sp2 hybridized carbon structures.29 Figure S1 shows Raman spectroscopy of GO. The disorder degree of carbon materials relates to the intensity ratio ID/IG. The ID/IG value is 0.93 of GO. Compared with GO results, the ID/IG of Cr2O3-rGO is 1.01 owing to higher degree of disorder within the rGO structure, indicating successful reduction of the GO to rGO. Thermal gravimetric
ABSTRACT: Electrochemical reduction is an ecofriendly alternative for energy-saving artificial N2 fixation. The development of this process requires efficient N2 reduction reaction (NRR) electrocatalysts to overcome the challenge with N2 activation. We show that a Cr2O3 nanoparticle-reduced graphene oxide hybrid (Cr2O3rGO) is as an outstanding catalyst for electrochemical N2-to-NH3 conversion under ambient conditions. In 0.1 M HCl, Cr2O3-rGO achieves a high NH3 yield of 33.3 μg h−1 mg−1cat. at −0.7 V vs RHE and a high Faradaic efficiency of 7.33% at −0.6 V vs RHE, with excellent selectivity for NH3 synthesis and stability. Density functional theory calculations were executed to gain further insight into the mechanisms.
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s the activated N2 building block, NH3 production is vital to increase the output of food and fertilizers to meet the nutritional requirements of terrestrial organisms and provide for the growing global population. It has thus become one of the most widely manufactured chemicals.1−3 NH3 is still regarded as a carbon-neutral energy carrier with 17.6 wt % hydrogen in the future hydrogen economy.4,5 In, addition, ever-increasing NH3 demands have stimulated intensive interest in N2-to-NH3 conversion.6,7 N2 is the most abundant molecule, accounting for 78% of air; it is electrochemically inert and is incapable of participating in many chemical reactions owing to the firm triple bond.8 The Haber−Bosch method for industrial-scale NH3 synthesis operates at high temperature and pressure, which is energy-intensive and emits a lot of CO2.9,10 Electrochemical reduction offers a promising approach toward ambient artificial NH3 synthesis.11,12 Also, this reaction can be driven via renewable energy sources. Development of this process, however, requires efficient N2 reduction reaction (NRR) electrocatalysts.13−15 Noble-metal-based catalysts display efficient NRR performance, but they have high cost and scarcity.16,17 It is therefore attractive to design and develop nonprecious-metal alternatives. Our recent work suggests that multishelled hollow Cr2O3 microspheres are active for electrocatalytic N2 reduction in 0.1 M Na2SO4.18 Given huge attention © XXXX American Chemical Society
Received: November 7, 2018
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DOI: 10.1021/acs.inorgchem.8b03143 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
N2H4 were spectrophotometrically tested after electrolysis according to a previous study.35 Both of the detecting fitting curves are displayed in Figures S6 and S7. Figure 2a displays
Figure 1. (a) XRD pattern of rGO and Cr2O3-rGO. (b) Raman spectrum of Cr2O3-rGO. TEM images of (c) rGO and (d) Cr2O3-rGO. (e) HRTEM image from one nanoparticle and (f) STEM image and corresponding EDX mapping images of Cr2O3-rGO.
Figure 2. (a) LSV curves of Cr2O3-rGO/CP in Ar- and N2-saturated 0.1 M HCl. Scan rate: 5 mV s−1. (b) Time-dependent current density curves over Cr2O3-rGO/CP at various potentials for 2 h in 0.1 M HCl. (c) UV−vis absorption spectra of the 0.1 M HCl stained with indophenol indicator at each potential after electrolysis for 2 h. (d) VNH3’s and FEs for Cr2O3-rGO/CP at corresponding potentials. (e) Cycling stability test over Cr2O3-rGO/CP at −0.7 V for five cycles. (f) Time-dependent current density curve for Cr2O3-rGO/CP.
analysis (Figure S2) indicates that Cr2O3-rGO is made up of 12.5 wt % rGO and 87.5 wt % Cr2O3. Figure 1c,d shows the transmission electron microscopy (TEM) images of the rGO and Cr2O3-rGO. A large quantity of nanoscale Cr2O3 particles was uniformly deposited RGO (Figure 1d). Figure 1e displays the high-resolution TEM (HRTEM) image. As observed, one nanoparticle displays a lattice spacing of 3.63 Å, corresponding to the (012) plane of Cr2O3. The scanning TEM (STEM) image is shown in Figure 1f. The corresponding energy-dispersive Xray (EDX) elemental mapping images of Cr2O3-rGO reveal that Cr, C, and O are uniformly distributed within Cr2O3-rGO. Note that the same reaction without using GO only leads to larger Cr2O3 aggregates (Figure S3), indicating rGO can effectively suppress Cr2O3 nanoparticle agglomeration during the hightemperature treatment. Figure S4a displays the X-ray photoelectron spectroscopy (XPS) survey spectrum of Cr2O3-rGO, indicating the existence of Cr, O, and C. As shown in Figure S4b, in the Cr 2p region, the peaks at binding energies (BEs) of 586.8 and 577 eV are attributed to Cr 2p1/2 and Cr 2p3/2, indicating the existence of Cr(III).30 The O 1s region (Figure S4c) can be distributed to two peaks at BEs of 532.6 and 530.3 eV, corresponding to the C− O and Cr−O species.31,32 Additionally, two peaks at 286.6 and 284.7 eV in the C 1s spectrum (Figure S4c) are ascribed to the C−O and C−C species.33,34 To evaluate the electrocatalytic NRR activity of Cr2O3-rGO, the catalyst was deposited onto carbon paper (Cr2O3-rGO/CP, Cr2O3-rGO loading: 0.1 mg cm−2) as the working electrode (Figure S5). In this work, all the potentials were converted and reported as values versus RHE. To identify the successful electroreduction of N2, generated NH3 and possible generated
linear sweep voltammetry (LSV) curves of Cr2O3-rGO in Arand N2-saturated electrolyte. As observed, a higher current density under a N2 atmosphere illustrates that Cr2O3-rGO is effective to catalyze NRR. Figure 2b displays time-dependent current density curves of Cr2O3-rGO/CP in 0.1 M N2-saturated HCl. Figure 2c shows the UV−vis absorption spectra of the electrolyte stained with indophenol indicator at each potential after 2 h. The NNR process tests at potential scopes from −0.5 to −1.0 V. Of note, NH3 yields (VNH3) and FEs at each potential are demonstrated in Figure 2d. As observed, VNH3 increases with the potential being more negative at −0.7 V, where the highest value of 33.3 μg h−1 mg−1cat. is obtained. Strikingly, the FE for NH3 production reaches a maximum value of 7.33% at −0.6 V. To further verify NH3 yields and FEs, produced NH3 was detected by ion chromatography. Figure S8 shows the corresponding calibration curves, the ion chromatogram for the electrolytes at each potential after electrolysis for 2 h, and the corresponding NH3 yields and FEs. Of note, Cr2O3-rGO achieves a large VNH3 of 32.6 μg h−1 mg−1cat. and a high FE of 7.42% at −7 and −0.6 V, respectively. This performance favorably compares to that of other NRR electrocatalysts (Table S1). Strikingly, VNH3’s and FEs significantly decrease from −0.7 V, primarily due to competition of the hydrogen evolution reaction. Of note, the highest FE in NRR at −0.7 V is attributed to that the FE of the B
DOI: 10.1021/acs.inorgchem.8b03143 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
(0.32 μg h−1 mg−1cat., 0.11%) possess poor electrocatalytic NRR activity. Figure 3d compares the amounts of NH3 produced on each electrode after 2 h tests at −0.7 V. CP has almost no electrocatalytic performance for NRR, and rGO/CP displays a very poor electrocatalytic NRR property. Cr2O3/CP is efficient to electrochemically catalyze NRR with a NH3 amount of 2.78 μg. In sharp contrast, a much larger amount (6.67 μg) of NH3 is generated over Cr2O3-rGO/CP, suggesting the greatly enhanced NRR property. Such an excellent NRR property for Cr2O3-rGO/ CP is ascribed to rGO being evenly dispersed with the Cr2O3 nanoparticles. Meanwhile, it can maximizes the reveal of active sites (Figure S17) and can be capable of more effective utilization of Cr2O3-rGO. As shown in Figure S18, the enhanced conductivity for Cr2O3-rGO contributes to its higher electrocatalytic property. DFT calculations suggest that adding the first hydrogen atom to form *NNH is the potential-determining step and the next hydrogenation steps energetically prefer to obey the distal mechanism (see Figure S19 and SI for details). In summary, Cr2O3-rGO has been demonstrated by theory and experiment as a superior electrocatalyst for artificial N2 fixation under ambient conditions. In 0.1 M HCl, Cr2O3-rGO achieves a high VNH3 of 33.3 μg h−1 mg−1cat. at −0.7 V and a high FE of 7.33% at −0.6 V, with outstanding selectivity for NH3 production and stability. DFT calculations were executed to gain further insight into the catalytic mechanism.
HER slow growth from −0.5 to −0.7 V but rapid growth beyond −0.7 V (Figure S9). It is worth noting that no N2H4 is tested (Figure S10), demonstrating the outstanding selectivity of Cr2O3-rGO/CP for NH3 production. Inductively coupled plasma-atom emission spectrometry detection displays the absence of Cr3+ in the 0.1 M HCl after NRR. Figure S11 shows the NRR property of Cr2O3-rGO in 0.1 M Na2SO4 electrolyte, indicating that the catalyst is also efficient for NRR at neutral pH. In a cycling stability test, NH3 yields and FEs barely change over five consecutive cycles (Figure 2e), implying high electrochemical stability of NRR. The time-dependent current density curve for Cr2O3-rGO/CP is shown in Figure 2f, suggesting that the current densities remain almost constant for 24 h at −0.7 V in 0.1 M HCl. XRD analysis (Figure S12) and XPS spectra (Figure S13) demonstrate that such an electrocatalyst is still the Cr2O3-rGO in nature after stability test. The TEM image (Figure S14) further suggests that Cr2O3-rGO still keeps its primary morphology after the test. To verify that tested NH3 originates from electrocatalyzed conversion of N2 over Cr2O3-rGO/CP, we performed several control experiments. We carried out electrolysis in 0.1 M HCl at open circuit potential. Meanwhile, Ar-saturated solution at −0.7 V for 2 h was tested. Figure 3a compares NH3 yields, indicating
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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/acs.inorgchem.8b03143.
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Experimental section; optical photo for the H-type cell, Raman spectrum, UV−vis absorption curves, ion chromatograms, XPS spectra, TGA and calibration curves; XRD patterns; SEM and TEM images; CVs; capacitive current densities vs scan rate; Nyquist plots; comparison of electrocatalytic N2 reduction performance (Table S1) (PDF)
AUTHOR INFORMATION
Corresponding Authors
Figure 3. (a) NH3 yields of before (blank) and after 2 h electrolysis under each conditions: open circuit in N2, −0.7 V in Ar, and −0.7 V in N2. (b) NH3 production rates and FEs of Cr2O3-rGO at −0.7 V vs RHE. (c) Curve of NH3 synthesis vs reaction time at −0.7 V vs RHE. (d) Amount of NH3 prepared with each electrode at −0.7 V after 2 h tests.
*E-mail:
[email protected] (F.X.). *E-mail:
[email protected] (Q.L.). ORCID
Baihai Li: 0000-0002-9266-1791 Xuping Sun: 0000-0002-5326-3838 Fengyu Xie: 0000-0001-7222-6680 Qian Liu: 0000-0002-7217-5083
nearly no NH3 was tested in either condition. Besides, O-G/CP was immersed in N2- and Ar-saturated tests with alternating 2 h cycles for a total of 12 h at −0.7 V. Figure 3b shows that NH3 was only acquired in the durations of N2-saturated electrolyte. Note that the amount of NH3 also increases linearly with increased electrolysis time (Figure 3c). Considering a small amount of NH3 in air and Ar/N2, we maintain high-purified gas (Ar/N2, purify: 99.999%) flowing through the 0.1 M HCl in an H-type cell with no potential. Corresponding ultraviolet−visible (UV− vis) absorption spectra reveal no NH3 was tested (Figure S15), indicating no distraction from NH3 of air and Ar/N2. All tests verify that tested NH3 is only generated from the NRR process by Cr2O3-rGO/CP. In addition, we detected the electrocatalytic NRR properties of Cr2O3/CP and rGO/CP (Figure S16). As observed, Cr2O3/CP (13.9 μg h−1 mg−1cat., 1.38%) and rGO/CP
Author Contributions #
L.X. and B.L. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21575137). REFERENCES
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DOI: 10.1021/acs.inorgchem.8b03143 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.8b03143 Inorg. Chem. XXXX, XXX, XXX−XXX
