Acidic Electrochemical Reduction of CO2 Using Nickel Nitride on

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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Acidic Electrochemical Reduction of CO2 Using Nickel Nitride on Multiwalled Carbon Nanotube as Selective Catalyst Zhuo Wang,† Pengfei Hou,†,§ Yulin Wang,†,§ Xu Xiang,∥ and Peng Kang*,†,‡ †

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Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Beijing 100190, China ‡ School of Chemical Engineering and Technology, Tianjin University, 135 Yaguan Road, Tianjin 300350, China § University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China ∥ State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Ni3N/MCNT (multiwalled carbon nanotube) nanocomposites fabricated by ammonolysis displayed high CO2 electrochemical reduction reactivity with CO Faradaic efficiencies of 89.0% and current density of 6.5 mA cm−2 at −0.73 V vs reversible hydrogen electrode (RHE). The Ni3N/MCNT catalyst could operate under weakly acidic conditions in the pH range of 2.5−7.2. Even at pH 2.5, the CO Faradaic efficiency remained at 50.1%, demonstrating the high selectivity for CO2 reduction. The high catalyst selectivity could be due to increased adsorption of CO2 on the Ni3N surface, which can compete with hydrogen evolution under acidic conditions. KEYWORDS: CO2 reduction, Electrocatalysis, Ni3N, CO, MCNT

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−0.9 V.24 Bao et al. reported high nickel loading materials by pyrolysis of Zn/Ni bimetallic ZIF-8 which yields high CO current density of 71.5 mA cm−2 at −1.03 V (vs RHE), and high CO FE of over 90%.23 Kamiya and co-workers prepared nickel−nitrogen-modified graphene for CO2RR with CO FEs over 90% in weakly acidic and neutral solutions.33 Strasser et al. investigated Ni−N−C catalysts with CO FEs of over 80%.34 Still, developing Ni-based catalyst is highly challenging because Ni-based nanocomposites are highly efficient for H2 evolution reaction (HER).35,36 Reducing CO2 under acidic conditions is important for gas phase CO2 reduction. However, reducing CO2 under acidic conditions is very challenging and much less reported. In gas phase CO2 reduction, catalyst is commonly pressed onto Nafion membrane which is highly acidic in nature, requiring high selectivity for the catalyst. In Newman’s study, Ag nanoparticles loaded on Nafion only generated H2, unless they were separated with Nafion using a buffer layer.37 Making the catalyst surface with more basic sites could increase CO2 adsorption on surface and buffer the local pH. Nickel nitride (Ni3N) has low electrical resistance and decent corrosion resistance and has been used as electrocatalyst for water splitting38,39 and electrode material for supercapacitors.40 In this work, we used Ni3N/MCNT (multiwalled carbon nanotube) nanocomposites of small particle sizes for efficient

INTRODUCTION Increasing CO2 emission is urging researchers to develop renewable energy alternatives. CO2 can serve as a renewable carbon source for fuels or commodity chemicals.1−5 Electrochemical reduction of carbon dioxide is a promising strategy for sustainable production of chemicals under mild conditions.2 However, because of the extreme stability of the linear CO2 molecule, electrochemical reduction of CO2 requires high activation energy to form a CO2•− intermediate, which causes large overpotential and competitive formation of H2.6,7 Lowering the barrier of CO2 activation is necessary for developing new catalysts. Numerous CO2 reduction catalysts based on metal and metal oxides,8−13 chalcogenides,14 nitrogen-doped/-functionalized carbons,15−17 and molecular complexes18−20 have been developed. Among them, earth abundant metals have been increasingly explored as CO2 electroreduction catalysts, such as Ni,21−25 Co13,26 and Fe.27,28 Nickel-based materials have been an important class of catalysts for CO2 reduction. Ni-based complexes have been reported; for example, nickel-1,4,8,11tetraazacyclotetradecane (Ni-cyclam) and its derivatives have shown considerable selectivity in electroreduction of CO2 to CO at Hg electrode.29−31 Recently, Li et al. reported Ni single atoms distributed in nitrogen-doped porous carbon (Ni SAs/ N−C) for selective reduction of CO2 to CO, and the highest selectivity for CO production was achieved at an overpotential of 0.89 V with Faradaic efficiency (FE) of 71.9%.32 Ni−N4 structure catalyst exhibited excellent activity for CO2RR with high FE over 90% for CO in the potential range from −0.5 to © XXXX American Chemical Society

Received: November 30, 2018 Revised: February 1, 2019

A

DOI: 10.1021/acssuschemeng.8b06278 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering CO2 electrochemical reduction with high CO selectivity, and the selectivity is sufficiently high which allows catalysis to occur at pH 2.5.

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RESULTS AND DISCUSSION Ni3N/MCNT nanocomposites were prepared as illustrated in Scheme 1. Ni(OH)2/MCNT composites were synthesized via Scheme 1. Preparation of Ni3N/MCNT

a self-assembly method, followed by ammonolysis at 300 °C for 2 h to yield Ni3N/MCNT nanocomposites. The amount of Ni loaded on MCNT was varied, and the samples were named Ni3N/MCNT-1 with Ni:C molar ratio of 1:5 and Ni3N/ MCNT-2 with Ni:C ratio of 2:5. From TEM studies, the Ni(OH)2 nanosheets were of wrinkled thin-layer structure and evenly combined with MCNT (Supporting Information Figure S2a). Upon ammonolysis, the nanosheet structure of Ni(OH)2 transformed to Ni3N nanoparticles anchored evenly on MCNT, with average size of ca. 5 nm for Ni3N/MCNT-1 (Figure 1 and Figure S1). The Ni3N/MCNT-1 nanocomposite demonstrated a rosarylike morphology (Figure 1a,b and Figure S1). HRTEM images of Ni3N/MCNT-1 in Figure 1c,d displayed clearly resolved lattice fringes with a lattice spacing of 0.2088 nm, consistent with (111) planes of Ni3N. The structure of Ni3N/MCNT-1 was further verified by HAADF-STEM images and the corresponding element mapping for C, N, and Ni atoms of Ni3N/MCNT nanocomposites was shown in Figure 1e. Using MCNT as substrate is very important for even distribution of Ni3N particles. Without MCNT, bare Ni3N was found to be large, aggregated particles (Figure S2b), suggesting that using MCNT as substrate can prevent aggregation during ammonolysis. Yet, at higher Ni loading, Ni3N/MCNT-2 became more aggregated on MCNT with larger sizes. In addition, using Ni(OH)2 nanosheets as precursor is critical for the formation of well-distributed Ni3N particles, as switching to Ni(NO3)2 precursor caused considerable aggregation on MCNT (Figure S3). XRD confirmed the formation of Ni3N/MCNT nanocomposites (Figure 2a). The characteristic peaks of Ni(OH)2/MCNT nanocomposites can be attributed to Ni(OH)2 and MCNT, similar to the previous report.39 After thermal ammonolysis, the formed Ni3N nanoparticles on MCNT were of low crystallinity, and thus no clear XRD pattern was observed. For comparison, Ni3N particles without MCNT were prepared with the same ammonolysis method, and the XRD pattern matched with JCPDS No.10-0280, suggesting that the ammonolysis is effective in forming Ni3N particles. The Ni3N/MCNT-1 material was further analyzed by X-ray photoelectron spectroscopy (XPS) as shown in Figure 2b,c. The high-resolution N 1s XPS spectrum of Ni3N/MCNT-1 was fit into five peaks at 397.1, 397.8, 399.2, 400.1, and 402.8 eV, and they were assigned to pyridinic-N, pyrrolic-N, Ni−N in Ni3N, graphitic-N, and N−O, respectively.40−42 The XPS

Figure 1. TEM (a, b), HR-TEM (c, d), and HADDF-STEM images of Ni3N/MCNT-1 and its corresponding element mapping for C, N, and Ni (e).

spectrum of N-doped MCNT was similar to that of Ni3N/ MCNT-1, only lacking the Ni−N and N−O peaks (Figure 2d). This confirms that nitrogen has been doped into Ni3N/MCNT after thermal ammonolysis of Ni(OH)2/MCNT nanocomposites. The high-resolution spectrum of Ni element in Ni3N/ MCNT is shown in Figure 2c, and the peaks at 853.3 and 870.6 eV can be assigned to Ni 2p3/2 and Ni 2p1/2 in Ni3N/ MCNT-1, which is similar to bare Ni3N particles in Figure S4 and reported Ni3N material.39 Meanwhile, the peaks located at 856.7, 861.9, 874.2, and 880.6 eV can be ascribed to Ni 2p3/2, 2p1/2, and satellite peaks in NiO, from slight surface oxidation in Ni3N.43 It is reasonable to conclude that the major phase is Ni3N with a small portion of NiO in the thin layers of the surface. The 1D nanostructure of Ni3N/MCNT-1 generated relatively large surface area for catalysis. Specific surface area and porosity of Ni(OH)2/MCNT-1 and Ni3N/MCNT-1 were measured by nitrogen adsorption experiments. These composites possessed a well-developed mesoporous structure, as reflected by the N2 sorption isotherms and BJH adsorption dV/d(log(D)) pore volume (Figure 3). The pore size distribution is centered at 40 nm, and the specific surface area is 159 m2·g−1 for Ni(OH)2/MCNT-1 and 180 m2·g−1 for Ni3N/MCNT-1, respectively. Overall, the 1D structure of Ni3N/MCNT allows more exposed active sites and increased electron-transfer rates required for electrocatalytic reactions. The electrocatalytic activity of Ni3N/MCNT for CO2 reduction was studied by cyclic voltammetry (CV) in CO2saturated 0.5 M NaHCO3 solution. As shown in Figure 4a, B

DOI: 10.1021/acssuschemeng.8b06278 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. (a) XRD patterns of Ni(OH)2/MCNT, Ni3N/MCNT-1, and bare Ni3N; XPS spectrum for N 1s (b), Ni 2p (c) of Ni3N/MCNT-1, and N 1s (d) for N-MCNT as control.

Figure 3. (a) Nitrogen adsorption isotherms and (b) pore size distributions of Ni(OH)2/MCNT-1 and Ni3N/MCNT-1 nanocomposites.

potential, the FE for CO by Ni3N/MCNT-1 initially increased until reaching the maximum of 89.0% at −0.73 V vs RHE and then decreased with further applied potentials. The current density of 6.5 mA cm−2 by Ni3N/MCNT-1 was almost three times that of Ni3N/MCNT-2 at −0.83 V (Figure 4e), even though the Ni loading of Ni3N/MCNT-1 was less, suggesting that smaller Ni3N particles can be more efficient in CO2 reduction. Similar trends have been found in reported Pd11 and Sn2 nanoparticles, in which particles with smaller size provided more active sites, while, for Ni(OH)2/MCNT, H2 was the predominant product and the maximum CO FE was only ca. 10% at −0.73 V. As a control experiment, N-doped MCNT without Ni3N was prepared after thermal ammonolysis at 300 °C, and it showed smaller CO FE of 60% at −0.83 V and much

Ni3N/MCNT-1 showed higher catalytic current density and relatively positive onset potential of −0.35 V vs RHE under CO2 compared to Ni3N/MCNT-2. Compared to CVs in Arsaturated 0.5 M NaHCO3 solution (Figure S5), the current density under CO2 for Ni3N/MCNT-1 was larger. By contrast, Ni(OH)2/MCNT showed lower current density under CO2 compared to Ni3N/MCNT (Figure S6). Controlled-potential electrolyses (CPEs) were conducted to probe the catalytic activity and product distribution. The dependence of product FEs on potential is plotted in Figure 4b−d. Both Ni3N/MCNT catalysts produced CO with selectivity significantly higher than that of Ni(OH)2/MCNT between −0.53 and −0.93 V vs RHE. The CO FE is strongly dependent on the applied potential. With increasingly applied C

DOI: 10.1021/acssuschemeng.8b06278 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. (a) CVs of Ni3N/MCNT-1 (black), Ni3N/MCNT-2 (red), and Ni(OH)2/MCNT (green) in CO2-saturated 0.5 M NaHCO3 solution at scan rate of 100 mV s−1; Faradaic efficiencies for Ni3N/MCNT-1 (b), Ni3N/MCNT-2 (c), and Ni(OH)2/MCNT (d); (e) partial current densities for CO production; (f) Tafel plots using CO partial current densities for Ni3N/MCNT-1 and Ni3N/MCNT-2.

smaller current density (1.8 mA cm−2) than Ni3N/MCNT-1 (Figure S7); thus the high CO FE must originate from the inclusion of Ni3N, not just N-MCNT itself. The electrolyte pH value has significant effect on the performance of CO2RR. Figure 5a shows LSVs at different pHs from 2.5 to 7.2. Although the pH values were different, which should change the reduction potential, the LSVs mostly overlapped with each other, suggesting that the ratedetermining step is less sensitive to pH changes. Figure 5b shows the maximum FEs for CO at different pHs. The applied potentials were between −0.9 and −1.0 V. Although more acidic conditions generated more H2, still at pH 3.7, the CO FE remained at 85.7%. Yet, at pH 2.5, CO FE dropped to 50.1%. Furthermore, CO FE was only 8% at pH 1, suggesting that overly acidic pH still favors hydrogen evolution. The wide

pH tolerance of Ni3N/MCNT-1 catalyst points to a high CO2 selectivity. Electrochemical surface areas (ECSAs) were determined by measuring the double-layer capacitance in CV experiments. In Figure 6a, Ni3N/MCNT-1 showed the highest ECSA of 10.87 mF cm−2, which was ca. 1.5-fold of Ni3N/MCNT-2. Increased ECSA indicates more active catalytic sites, which could contribute to increased catalytic activity for Ni3N/MCNT-1. Also, measurement of CO2 adsorption revealed that Ni3N/ MCNT-1 absorbs more CO2 (ca. 120 mg·g−1) than Ni(OH)2/ MCNT (ca. 90 mg·g−1), indicating that the nitriding strategy increased available base sites (Figure 6b). To obtain the mechanism of the reaction, Tafel analysis was performed (Figure 4f). The Tafel slopes were calculated as 182 and 186 mV dec−1 for Ni3N/MCNT-1 and Ni3N/MCNT-2 D

DOI: 10.1021/acssuschemeng.8b06278 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 5. (a) LSV scans at 50 mV s−1 for Ni3N/MCNT-1 in CO2-saturated NaCl solution at different pHs (adjusted with HCl or NaHCO3, ionic strength = 0.5 M). (b) FEs for CO (blue) and H2 (red) by Ni3N/MCNT-1 at various pHs.

Figure 6. (a) Charging current densities vs CV scan rates; (b) CO2 adsorption isotherms for Ni(OH)2/MCNT and Ni3N/MCNT-1.

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CONCLUSION The Ni3N/MCNT nanocomposites can achieve high current density and CO selectivity at moderate potentials for the electrocatalytic reduction of CO2 under acidic conditions. The enhanced performance of Ni3N/MCNT can be attributed to increased adsorption of CO2 on the surface of Ni3N/MCNT, which could concentrate CO2 to compete with hydrogen evolution. This study opens a new avenue for rational design of high-efficiency catalysts for CO2 electrochemical reduction. Further gas phase investigation is underway.

catalysts, comparably close to the theoretical value of 118 mV dec−1, implying a mechanism for a single electron-transfer ratedetermining step at the electrode.47 By contrast, the Tafel slope for Ni(OH)2/MCNT was 229 mV dec−1 (Figure S8), which indicates more chemical-step-controlled kinetics for CO2 reduction. Besides, the exchange current density j0 of 0.05 mA cm−2 for Ni3N/MCNT-1 was significantly higher than 0.0032 mA cm−2 for Ni3N/MCNT-2, suggesting reduced reaction barrier and more stabilized CO2•− intermediate on the surface. The experimental results together suggest that Ni3N/ MCNT-1 performs better in the reduction of CO2 by facilitating adsorption of CO2 and lowering the reduction barrier. It is commonly accepted that the adsorbed CO2 on metal electrodes44,45 is initially reduced by a single electron to form a CO2•− intermediate; therefore, the adsorption of CO2 plays a key role in the reduction process, and then the intermediate is reduced further according to the equations that follow:9,46 CO2 (g) + * + (H+ + e−) → *COOH

(1)

*COOH + (H+ + e−) → *CO + H 2O(1)

(2)

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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/acssuschemeng.8b06278.

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TEM images for Ni3N/MCNT-2, Ni(OH)2/MCNT-1, and bare Ni3N particles; XPS for Ni3N; CVs for Ni3N/ MCNT-1 and Ni(OH)2/MCNT-1; CPE for N-MCNT; Tafel plot for Ni(OH)2/MCNT-1 (PDF)

AUTHOR INFORMATION

Corresponding Author

*CO → CO(g) + *

*E-mail: [email protected].

(3)

ORCID

Xu Xiang: 0000-0003-1089-6210

where the asterisk denotes a catalytically active site. E

DOI: 10.1021/acssuschemeng.8b06278 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Peng Kang: 0000-0002-6639-8299 Funding

This work was financially supported by National Natural Science Foundation of China (Grant 21701180) and National Key R&D Program of China (Grant 2016YFB0600901). Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acssuschemeng.8b06278 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.8b06278 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX