Nitrogen dopant induced highly selective CO₂ reduction over lotus-leaf shaped ZnO nanorods

Abstract

Heteroatom doping offers great promise for boosting the electrocatalytic activity for the CO₂ reduction reaction (CO₂RR). Herein, nitrogen-doped ZnO nanorods with a lotus-leaf shape were constructed as the active electrocatalysts, demonstrating enhanced CO₂RR performance with a maximum CO faradaic efficiency of 76% at −0.7 V (vs. reversible hydrogen electrode, RHE) and excellent durable activity of more than 30 h. Density functional calculations reveal that doping nitrogen in ZnO induced the reduction of free-energy barrier for the key intermediate (*COOH) formation and the enhancement of electron exchange, thereby leading to a notable improvement in the CO₂RR activity over these low-cost N-doped ZnO nanorods.

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Introduction

The greenhouse effect due to CO₂ has seriously influenced the carbon cycle in nature and posed a severe threat to human life.^1,2 Electrocatalytic CO₂ reduction reaction (CO₂RR) is a promising technology that can convert CO₂ to valuable feedstocks and chemical fuels.^3–8 Thus far, great efforts have been devoted to searching for suitable materials for CO₂ electroreduction. Among these materials, transition metal electrocatalysts such as Co,⁹ Ni,¹⁰ Bi,¹¹ Cu,^12,13 Ag¹⁴ and Zn^15,16 are ideal candidates for converting CO₂ to chemical fuels because their d bands contain valence electrons close to the Fermi level,¹⁷ thus beneficial for overcoming the activation barriers of the reduction reaction.

In the past decades, earth-abundant and environmentally friendly Zn-based materials have attracted much attention as CO₂RR electrocatalysts due to their high adsorption affinity towards CO₂ molecules.^18,19 However, bulk Zn catalysts require large overpotentials with low reaction activity, due to fewer active sites.²⁰ To enhance the catalytic activity, significant efforts have focused on the structural design and morphology of Zn-based catalysts, including dendritic Zn,²¹ hexagonal Zn,²² nanoporous ZnO²³ and ZnO nanosheets.²⁴ Previous studies have confirmed heteroatom doping (such as B, N, P and S) to be an effective strategy for increasing the electrocatalytic activity. The introduction of dopants not only induces a great change in the electronic structure and enhances the conductivity, but also reduces the activation energy barrier of the electrocatalytic reaction.^25–27 Recently, we have developed a facile approach to dope nitrogen into Ti₃C₂ MXene nanosheets for electrocatalytic CO₂ reduction to produce CO.²⁸

In this study, nitrogen-doped ZnO nanorods with lotus-leaf shape were fabricated as active electrocatalysts for CO₂ conversion using a facile hydrothermal growth and subsequent plasma treatment process. Compared with undoped ones, N-doped ZnO nanorods exhibited a significant improvement in CO generation with a maximum faradaic efficiency (FE_CO) of 76% at −0.7 V (vs. reversible hydrogen electrode, RHE) and excellent durable activity of more than 30 h. Theoretical calculations confirm that the introduction of nitrogen dopants is beneficial for reducing the activation barrier for the key intermediate (*COOH) formation and enhancing the electron exchange between N dopants and coordinated Zn atoms, resulting in an outstanding CO₂RR performance.

Results and discussion

The N-doped ZnO nanorods were fabricated according to the previously reported method²⁹ and the schematic illustration is shown in Fig. 1a. ZnO nanorods were first grown on ITO conductive glass by the hydrothermal method, and the morphology of ZnO looks like lotus leaves as observed by scanning electron microscopy (SEM) (Fig. S1, ESI†). The N-doped ZnO nanorods were obtained by following the plasma treatment process, and these nanorods showed a similar morphology to that of lotus leaves in the undoped ones, represented in Fig. 1b (Fig. S2, ESI†) and transmission electron microscopy (TEM) (Fig. 1c) images. The corresponding high-resolution TEM (HRTEM) image (Fig. 1d) exhibits a lattice fringe distance of 0.26 nm, which corresponds to the (002) plane spacing of ZnO material. Additionally, N elements were distributed uniformly in the ZnO nanorods according to elemental mapping images from energy-dispersive X-ray spectroscopy (EDS) (Fig. 1e), and the content of nitrogen dopants in these N-doped ZnO nanorods was about 0.52 wt%.

Fig. 1 (a) Schematic illustration of the preparation, (b) SEM image, (c) TEM image, (d) high-magnification TEM image, and (e) EDS elemental mapping images of N-doped ZnO nanorods.

The crystallographic structure was investigated by XRD patterns, (Fig. 2a) and all diffraction peaks for undoped and N-doped ZnO nanorods were approximately identical with no peak shifts, corresponding to the JCPDS files #75-1526 of ZnO. Nevertheless, the change in the surface chemical structure of N-doped ZnO nanorods was also measured by X-ray photoelectron spectroscopy (XPS) and is shown in Fig. 2. The binding energies of Zn 2p_3/2 and Zn 2p_1/2 peaks for N-doped ZnO nanorods are 1022.1 eV and 1045.1 eV, respectively. Obviously, the binding energy for Zn²⁺ showed a right shift of ∼0.2 eV for N-doped ZnO nanorods compared to the undoped ones (Fig. 2b). This indicates that a part of electrons have been transferred away from Zn, which may be caused by the introduction of N after the plasma treatment. The O 1s spectrum shown in Fig. 2c exhibits a very broad peak and can be deconvoluted into three peaks at 530.6 eV, 531.6 eV and 532.6 eV, which were assigned to O²⁻ of lattice oxygen, deficient oxygen and chemically adsorbed oxygen, respectively.³⁰ The binding energy of N 1s peak in N-doped ZnO (Fig. 2d) also can be deconvoluted into two peaks at 397.6 eV and 399.4 eV, which were assigned to Zn–N³¹ and O–N³² species (∼400 eV), indicating that N atoms have been successfully doped into ZnO nanorods.

Fig. 2 The structural characterization of ZnO and N-doped ZnO nanorods. (a) XRD patterns, (b) XPS spectra of Zn 2p, XPS fitting spectra of (c) O 1s and (d) N 1s of N-doped ZnO.

The electrocatalytic CO₂ performance of the electrodes based on lotus leaf-shaped ZnO and N-doped ZnO nanorods were evaluated in a H-cell separated by a Nafion-115 proton exchange membrane with CO₂-saturated 0.5 M KHCO₃ (pH = 7.2), as previously reported by our group.³³ The linear sweep voltammetry (LSV) curves were obtained using 90% iR-offset to compensate the drop of ohmic potential caused by electrolyte resistance. All potentials were reported with respect to the RHE, and each current density was normalized by the geometric area of the electrode. From LSV curves (Fig. S3 and S4, ESI†), the N-doped ZnO electrode showed lower overpotential and higher current density under CO₂ atmosphere compared to N₂ atmosphere, indicating better electrocatalytic activity for CO₂RR. However, the undoped ZnO electrode showed lower current density under CO₂ atmosphere compared to N₂ (Ar) atmosphere, indicating better electrocatalytic activity for hydrogen evolution reaction. The N-doped ZnO electrode (Fig. 3a) exhibited a reduction current density of 5.6 mA cm⁻² at −0.7 V vs. RHE; this value is 2.4 times greater than that of the undoped one, indicating a higher catalytic activity over the N-doped ZnO nanorods as compared to the undoped counterpart.

Fig. 3 The CO₂RR performance of ZnO and N-doped ZnO nanorods. (a) LSV curves, (b) FEs of CO, CH₄ and H₂ at various applied potentials, (c) stability test for N-doped ZnO nanorods at −0.7 V vs. RHE and the FE variation for CO production, (d) charging current density differences j plotted against the scan rates, (e) CO₂ adsorption isotherms, and (f) Nyquist plots of EIS.

The products were quantitatively analyzed by gas chromatography (GC) and ¹H nuclear magnetic resonance (NMR) spectrometry. Consequently, only CO, H₂ and CH₄ were detected and no liquid products were observed. The NMR spectrum of the N-doped ZnO electrode for CO₂RR recorded at −0.7 V vs. RHE is shown in Fig. S5 (ESI†). The dependence of faradaic efficiency (FE) for different products on the applied potentials for both electrodes is shown in Fig. 3b. At a low applied potential of −0.3 V vs. RHE, low FE values for CO (11 ± 4%) and CH₄ (7 ± 4%) production were observed over the N-doped ZnO electrode, while H₂ (70 ± 4%) was the dominant product. With further increase in the applied potential, a maximum FE_CO of 76 ± 4% was achieved at −0.7 V vs. RHE, as well as a small amount of CH₄ (∼8%) and H₂ (∼14%) were detected. For observed products, the sum of FEs was above 90.0% for the syngas with a ratio of CO/H₂ of 5.4, and also generates controllable ratios of syngas with varying potential during the CO₂RR. For the reference ZnO electrode, the hydrogen evolution reaction (HER) dominated the entire potential in the range from −0.3 to −0.8 V vs. RHE, and it showed a maximum FE_CO of 28% at −0.7 V vs. RHE, indicating low activity for reducing CO₂ to CO.

The long-term stability of electrocatalysts is another crucial indicator in evaluating the CO₂ reduction performance. The current density of N-doped ZnO electrode was relatively stable with a value of 5.6 mA cm⁻² at −0.7 V vs. RHE even after 30 h (Fig. 3c). Notably, the corresponding FE for CO production was constantly maintained to be greater than 75% for a duration of 30 h. Meanwhile, the structure and morphology of the N-doped ZnO after CO₂RR are shown in Fig. S6 (ESI†). Also, the corresponding Nyquist plot after the long-term electrolysis of the N-doped ZnO electrode has been obtained and is shown in Fig. S7 (ESI†). After long-term electrolysis, the plot exhibited a little larger semicircle than before CO₂ reduction, which indicates a higher interfacial charge-transfer resistance and a little decrease in the conductivity. The observed results showed that the N-doped ZnO nanorods showed a good structural and electrochemical stability.

The electrochemically active surface area of the electrodes was determined by double-layer capacitance (C_dl) to investigate the underlying kinetic mechanism for the enhanced CO₂RR performance. The measured C_dl of the N-doped ZnO nanorods was 57.3 mF cm⁻², approximately double that of the undoped counterpart (28.6 mF cm⁻²), which confirms an increased number of catalytically active sites because of N doping and subsequently improved electroreduction activity. Furthermore, the volumetric CO₂ adsorption isotherms showed that N-doped ZnO nanorods exhibited much higher CO₂ adsorption capacity (10.3 cm³ g⁻¹) compared to that of the undoped counterpart, confirming that N-doped ZnO nanorods have much more abundant adsorption sites for CO₂ on the surface due to the doping of nitrogen. The charge-transfer resistance values at the interfaces for the two electrodes were obtained by electrochemical impedance spectroscopy (EIS). The Nyquist plot of N-doped ZnO nanorods exhibited a much smaller semicircle and a steeper slope, which indicated a lower interfacial charge-transfer resistance and better conductivity, suggesting a much faster faradaic process. The Tafel plots (Fig. S8, ESI†) show that in the linear Tafel region, N-doped ZnO nanorods exhibited a lower Tafel slope (169 mV dec⁻¹) than undoped ZnO nanorods (192 mV dec⁻¹), indicating that the single-electron transfer to CO₂ is the rate-determining step for CO₂ reduction to CO³⁴ on the two ZnO electrodes, and N-doping of ZnO improved the CO₂ reduction thermodynamic kinetics, which is consistent with the LSV analysis (Fig. 3a).

To further provide a theoretical insight into the underlying mechanism, we performed DFT calculations to calculate the density of states, differential charge density and the Gibbs free energy of the two nanorods. The projected density of states (PDOS) of Zn d-orbitals in the two nanorods are presented in Fig. 4a. N-Doped ZnO exhibits a significant increase in the density of states near the Fermi level compared to the undoped counterpart, which can enhance the electron exchange with the reactants and intermediates during the reaction of Zn atoms, thus improving the reactivity of CO₂ electroreduction.³⁵ The differential charge density of O–N–Zn in N-doped ZnO (Fig. 4b) revealed that the charge between N and active Zn atoms was accumulated, and the electron exchange between the N dopants and coordinated Zn atoms was much easier than that between the O and Zn atoms.

Fig. 4 DFT calculations. (a) Calculated PDOS of Zn d-orbital in N-doped and undoped ZnO nanorods, (b) differential charge diagram of ZnO with nitrogen dopant, the gray, red, and white spheres represent Zn, O, and N atoms, respectively. (c) A possible reaction path for CO₂RR to CO on N-doped ZnO nanorods, and (d) DFT free energy profiles of CO₂RR to CO.

On the basis of above results and Tafel values, a possible mechanism for CO₂ electroreduction to CO over N-doped ZnO (Fig. 4c) was proposed, where the overall path proceeds as CO₂ → *CO₂ → *COOH → *CO → CO, which is in agreement with the previous reports.^19,36 Initially, a CO₂ molecule gets adsorbed on the N site of ZnO surface and then generates *CO₂. After that, the adsorbed *CO₂ couples with a proton from the electrolyte to form *COOH intermediate, and this step is identified as the rate-limiting step (RLS). Finally, the *COOH intermediate leaves off the H⁺ + e⁻ reactants to form H₂O product and produce CO. DFT calculations were performed to gain the reaction free energy profile (Fig. 4d) and to analyze the kinetics of elementary steps in the CO₂RR catalyzed by these two samples. It can be clearly seen that the activation energy barrier of the *COOH intermediate for N-doped ZnO (0.31 eV) is much lower than that of undoped ZnO (0.49 eV). Owing to the difference of 0.18 eV in activation barrier, the *COOH intermediate formation is much easier over N-doped ZnO and further promotes CO formation at lower overpotentials. Therefore, the reduction of the free-energy barrier and the enhanced electron exchange induced by N dopants provide superior CO₂RR performance for N-doped ZnO nanorods with a lotus-leaf shape.

Conclusion

In summary, N-doped ZnO nanorods with a lotus-leaf shape were prepared by a simple hydrothermal synthesis and plasma treatment method. It exhibited an enhanced CO₂RR performance with a maximum CO faradaic efficiency of 76% at −0.7 V vs. RHE. Further, more than 30 h durability test confirmed its excellent stability for the CO₂RR. Theoretical calculations revealed that the nitrogen dopants enhance the electron exchange between the N dopants and coordinated Zn atoms, which induces the reduction of free-energy barriers and thus facilitates the formation of the intermediate (*COOH). Finally, these N-doped ZnO nanorods with a lotus-leaf shape show superb CO₂RR activity.

Author contributions

Xijun Liu, Hui Yang and Gaocan Qi designed the catalyst; Fang Lü, Haihong Bao and Fei He performed the experiments and DFT calculations; Jiaqiang Sun, Shusheng Zhang, Longchao Zhuo, and Jun Luo co-analyzed data; Gaocan Qi and Hui Yang co-wrote the manuscript; Xijun Liu revised the manuscript. All the authors discussed and commented on the data and contributed to the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (22075211, 21601136, 21501132, 51971157, 12074017 and 51621003), Tianjin Science Fund for Distinguished Young Scholars (19JCJQJC61800), the Science and Technology Development Fund of Tianjin Education Commission for Higher Education (2018KJ126) and Tianjin Natural Science Foundation (18JCQNJC77100). The authors acknowledge Beijng PARATERA Tech Co., Ltd for providing HPC resources that have contributed to the research results reported within this paper.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00344e

‡ These authors contributed equally to this work.

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