Electroreduction of CO₂ to syngas with controllable H₂/CO ratios in a wide potential range over Ni–N co-doped ultrathin carbon nanosheets

Abstract

The conversion of CO₂ to syngas (H₂ and CO) via electrochemical reduction has been considered a promising strategy to mitigate the greenhouse effect. However, it is a great challenge to control H₂/CO ratios over a wide voltage window. Herein, a new method of fabricating Ni–N co-doped carbon nanosheets by molten salt-assisted pyrolysis, impregnation and re-carbonization is proposed. Benefiting from their ultrathin structure and tunable Ni–N_x active site content, the H₂/CO ratios can be adjusted from 1/2 to 2/1 within a wide applied voltage range (−0.7 to −1.3 V vs. RHE). After electrochemical stability testing for 10 h, the current density and H₂/CO ratios remained almost constant, revealing robust long-term stability. This work may benefit the construction of efficient and low-budget electrocatalysts for the production of tunable syngas.

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Introduction

With the acceleration of industrialization, the excessive consumption of fossil fuels has caused an energy and environmental crisis.^1,2 To alleviate this problem, many emerging technologies such as capture and sequestration, chemical fixation, and electro/photochemical reduction have been proposed.^3,4 Of the above technologies, the electrochemical CO₂ reduction reaction (CO₂RR) has been regarded as a promising strategy for producing fuels and chemicals from renewable energy sources.⁵ In recent years, considerable advances have been made in the CO₂RR, in which the reduction of CO₂ to low-carbon hydrocarbons is an intermediate step in the further production of high-value-added products.^6,7 Among them, syngas (H₂ and CO) is key feedstock for many important chemicals; for example, when the ratio of H₂/CO is 2/1, it is an ideal choice for methanol production while the optimal ratio for dimethyl ether production is 1/1.^8,9 Therefore, the realization of adjustable syngas production is very promising and also urgently needed.

In recent years, many attempts have been made to produce syngas via the CO₂RR. For instance, Kim et al. used Ag/TiO₂ as catalysts to produce tunable syngas.¹⁰ These catalysts could regulate H₂/CO ratios from 0.1 to 1.5 by controlling oxygen vacancy contents in the range of applied potentials from −0.35 to −0.65 V (vs. RHE). He et al. developed Pd–SnO₂ nanosheet catalysts for syngas production with controllable H₂/CO ratios.¹¹ The large specific surface area of these catalysts exposes abundant active interfaces that can modulate the H₂/CO ratios from 0.28 to 4.2, when the potential is varied from −0.5 to −1.1 V (vs. RHE). However, precious metal catalysts have the disadvantages of high cost and scarce raw materials, which limit their large-scale application. Therefore, transition metal catalysts for electrosynthesis of syngas were also studied. Maeng et al. fabricated ZnO nanorod-based catalysts to prepare syngas.¹² By the surface modification of the employed catalysts, the H₂/CO ratios can be adjusted in the range of 0.25–4.5 within the applied potential range of −1.2 to −2.0 V (vs. Ag/AgCl). Chen et al. synthesized oxide-derived Cu nanowires with two-phase CuO heterostructures for syngas production.¹³ Misfit dislocation sites on the surface of Cu nanowires promote the selectivity of CO, so that the H₂/CO ratios can be regulated from 1 to 3 within a certain potential window (−0.28 to −0.649 V vs. RHE). To further improve the stability of catalysts and reduce the cost, some researchers have developed carbon-based catalysts. Han et al. prepared BPNC catalysts for producing syngas via the CO₂RR.¹⁴ By changing the precursor ratio of tetraphenylphosphonium tetraphenylborate (C₄₈H₄₀BP) to melamine, the H₂/CO ratios can be adjusted from 0.2 to 6.8 in a potential window from −0.5 to −1.0 V (vs. RHE). Li et al. designed a series of nitrogen-doped carbon foam tubular electrodes to prepare syngas with controllable H₂/CO ratios.¹⁵ These electrodes exhibit adjustable H₂/CO ratios (1/3 to 2/1) over a potential range from −0.5 to −1.3 V (vs. RHE). Inspired by the above research, the syngas ratio can be adjusted by regulating the number of active sites for CO production. Compared with the heteroatom-doped carbon catalysts, Ni–N-doped carbon materials have the advantages of higher atomic utilization, higher intrinsic activity and controllable active sites.^16–22 Benefiting from those merits, it is possible to control the amount of CO by regulating the number of Ni–N active sites, thereby adjusting the proportion of syngas.

Recently, Leverett et al. synthesized undercoordinated Ni–N_x catalysts with a holey graphene framework to adjust the H₂/CO ratio by altering the coordination of Ni and N.²³ He et al. prepared N-doped carbon-supported single-atom catalysts CoNi-NC and regulated the syngas composition by controlling the doping ratio of Co to Ni.²⁴ Although the above studies have realized the regulation of the syngas ratios to some extent, the process of large-scale application of such catalysts is limited by the narrow adjustment ratio range or high cost of raw materials.

Herein, we synthesized Ni–N co-doped ultrathin carbon nanosheets by a molten salt-assisted pyrolysis, impregnation and re-carbonization strategy. The ultrathin structure of Ni–N co-doped carbon nanosheets can shorten the mass transfer path and provide a sufficient three-phase reaction interface for the CO₂RR. The number of Ni–N active sites can be adjusted by varying the carbonization temperature, thus enabling the adjustment of the CO yield and the regulation of the resulting syngas ratios. Therefore, the ratio of H₂/CO can be effectively regulated from 1/2 to 2/1 within a wide potential range (−0.70 to −1.30 V vs. RHE), which shows great promise for practical electrosynthesis of tunable syngas.

Results and discussion

The synthetic procedure of Ni/N–C is illustrated in Fig. 1a. In a typical process, coal tar pitch (carbon source) was mixed with salts (NaCl and KCl) and ground uniformly, followed by pyrolysis under an Ar atmosphere. During pyrolysis, the strong sheer force of the molten salts converted the pitch precursors into ultrathin carbon nanosheets. Subsequently, the as-prepared carbon nanosheets were treated with concentrated nitric acid to introduce nitro groups to facilitate Ni²⁺ adsorption. Then, Ni²⁺ was introduced into the carbon nanosheets by impregnation. The carbon nanosheets, after adsorption of Ni²⁺, were then carbonized together with N sources under an Ar atmosphere. The product obtained after acid leaching and drying was labelled Ni/N–C. The Ni–C sample without the N source and the N–C sample without the Ni source were prepared by a similar route to demonstrate the significance of the interaction between N and Ni species. The structural characteristics of the obtained Ni/N–C-900 were examined by scanning electron microscopy (SEM) (Fig. 1b–d) and transmission electron microscopy (TEM) (Fig. 1e–g). Ni/N–C-900 clearly shows an ultrathin lamellar structure, which reduces the ion transport resistance and increases the accessible area of the active sites. Meanwhile, there was no metal aggregation in Ni/N–C-900. The N–C and Ni–C samples exhibit similar sheet-like structures to that of Ni/N–C-900 (Fig. S1†); the influence of material morphology on the catalytic performance can be excluded. Energy dispersive spectroscopy (EDS) mapping displays that N, O and Ni atoms were dispersed uniformly in the carbon nanosheets (Fig. 1h).

Fig. 1 (a) Schematic diagram of the synthetic procedure of the Ni/N–C catalysts. (b–d) SEM, (e–g) TEM and (h) EDS mapping images of Ni/N–C-900.

The microstructures of the Ni/N–C catalysts were further investigated by X-ray diffraction (XRD) (Fig. 2a). The XRD patterns of the Ni/N–C catalysts exhibit only two broad peaks located at around 26° and 44°, attributed to the (002) and (101) facets of graphite,²⁵ further suggesting the absence of metallic Ni nanoparticles in these catalysts. The crystal structure and defects of the prepared catalysts were evaluated with the Raman spectra. All the Ni/N–C catalysts show two characteristic bands at around 1340 cm⁻¹ (D band) and 1580 cm⁻¹ (G band) (Fig. 2b).^26–28

Fig. 2 (a) XRD patterns and (b) Raman spectra of various Ni/N–C catalysts. High-resolution XPS spectra of (c) N 1s and (d) Ni 2p in Ni/N–C-700, 800, 900 and 1000.

Specifically, the I_D/I_G values of Ni/N–C-700, 800, 900 and 1000 are 0.94, 0.95, 0.97 and 0.96, respectively. The close I_D/I_G values of these four samples indicate that they all have similar graphitized structures, but these values are significantly higher than those of Ni–C (0.91) and N–C (0.93) (Fig. S2†), proving that the introduction of Ni–N active sites leads to an increase of disorder in carbon and thus more defects are obtained.²⁹

To further reveal the elemental composition and coordination environment of Ni/N–C catalysts, X-ray photoelectron spectroscopy (XPS) measurements were carried out. The XPS wide-survey spectra of the Ni/N–C catalysts indicate the presence of C, N, O and Ni elements (Fig. S3†), which is in accordance with the EDS mapping. The C 1s spectra can be fitted into C–C (∼284.6 eV), C–N (∼285.6 eV) and C=O (∼287.8 eV) (Fig. S4†), demonstrating that the N species are successfully doped into the carbon nanosheets.^30–32 As shown in Fig. 2c, the N 1s spectra can be deconvoluted into four peaks centered at 398.3 eV (pyridinic-N), 399.5 eV (Ni–N), 400.5 eV (pyrrolic-N) and 401.3 eV (graphitic-N).^33,34 The contents of various N species are shown in Fig. S5;† the pyridinic-N was dominant in all samples. More importantly, pyridinic-N has strong adsorption capacity for acidic CO₂ molecules, thus enhancing the CO₂RR rate.¹⁴

The Ni contents of the various samples characterized by XPS are 0.09 at% (Ni/N–C-700), 0.10 at% (Ni/N–C-800), 0.16 at% (Ni/N–C-900), and 0.13 at% (Ni/N–C-1000) (Table S1†), respectively, indicating that 900 °C is a potentially ideal temperature for preparing the Ni/N–C catalysts with relatively high Ni content. Meanwhile, we calculated the relative contents of nitrogen species in various Ni/N–C catalysts. Fig. S5† shows that the relative contents of pyridinic-N are 35.7, 33.3, 27.6 and 25.9%, when the carbonization temperature is 700, 800, 900 and 1000 °C, respectively. Meanwhile, the corresponding relative contents of pyrrolic-N are 21.3, 20.0, 17.6 and 17.3%, respectively. Obviously, the contents of both pyridinic-N and pyrrolic-N in the samples decrease by increasing the carbonization temperature. In contrast, the relative contents of graphitic-N in Ni/N–C-700, 800, 900 and 1000 are 21.2, 22.3, 24.2 and 31.7%, respectively. The possible reason is that the graphitic-N is more stable and is lost at a lower rate with the increasing temperature than the unstable pyridinic and pyrrolic-N. It is noteworthy that the relative content of Ni–N_x reaches its maximum at 900 °C (30.6%), while higher carbonization temperature will reduce the number of Ni–N_x species (1000 °C, 25.1%). All these results show that 900 °C is an optimum temperature for the formation of Ni–N_x species (1.18 at%) and the maintenance of a high total nitrogen content (3.85 at%), which are beneficial for improving the catalytic performance. Thus, the number of Ni–N_x active sites can be modulated by changing the carbonization temperature, thus further adjusting the H₂/CO ratios.^35,36 Although the main active sites in Ni/N–C catalysts are Ni–N_x coordinated structures, graphitic-N, pyridinic-N and pyrrolic-N also play important roles in the CO₂RR, in which pyridinic-N is a dominant factor. Therefore, the Ni–N_x active sites and N species have synergistic effects on the CO₂RR activity in the Ni/N–C electrocatalysts.^37,38

The high-resolution spectra of Ni 2p are displayed in Fig. 2d, where the Ni 2p_3/2 peak at 855.5 eV is between those of Ni⁰ (852.5–853.0 eV) and Ni²⁺ (856 eV),^39–41 revealing that the Ni species are in an unsaturated chemical state and the potential presence of coordinated Ni–N sites in the catalysts. In addition, the Ni contents were also detected by inductively coupled plasma-optical emission spectroscopy (ICP-OES), where the Ni/N–C-900 catalyst (0.00936 wt%) possesses higher Ni content than those of Ni/N–C-700 (0.00230 wt%), Ni/N–C-800 (0.00283 wt%) and Ni/N–C-1000 (0.00381 wt%) (Table S2†), which agrees well with the XPS results. The high-resolution N 1s and Ni 2p spectra of N–C and Ni–C samples are shown in Fig. S6,† demonstrating the absence of Ni–N_x active sites. The trace amount of N species (0.00188 wt%) in the Ni–C sample comes from coal tar pitch, which is insufficient to form Ni–N_x active sites with Ni sources.

The electrochemical activities of various electrocatalysts were investigated in an H-type cell. As revealed by linear sweep voltammetry (LSV) curves in Fig. 3a, the current densities of various catalysts under a CO₂ atmosphere are significantly higher than those of catalysts under an Ar atmosphere, proving their preference towards the CO₂RR. Compared with N–C (1.25 mA cm⁻²) and Ni–C (1.47 mA cm⁻²), Ni/N–C-900 exhibits higher current density (8.37 mA cm⁻²) (Fig. S7a†), demonstrating that the ultrathin carbon nanosheets can promote the performance of the CO₂RR.⁴² To determine the composition of the resulting syngas, the H₂/CO ratios of various samples are calculated at different potentials (Fig. 3b). Obviously, the controllable ratios of H₂/CO vary from 1/2 to 2/1, providing an opportunity for tunable syngas production. Notably, most of the H₂/CO ratios of each sample at different potentials are distributed in the optimal region (0.5–2), and the syngas contents in this interval are the key feedstocks for preparing multiple chemical products.^24,43–47 The H₂/CO ratios of N–C and Ni–C (Fig. S7b†) show that most of the ratios deviated from the optimal syngas region, further demonstrating that the presence of Ni–N_x active sites can improve the selectivity for CO and thereby modulate the H₂/CO ratios.

Fig. 3 (a) Linear sweep voltammetric (LSV) curves of various samples in CO₂ and Ar-saturated 0.1 M KHCO₃ solution. (b) Dependence of the H₂/CO ratio on the applied potentials of Ni/N–C-700, 800, 900 and 1000. Partial current densities of CO (c) and H₂ (d).

The CO partial current densities (j_CO) of various samples are 0.51 mA cm⁻² (Ni/N–C-700), 0.68 mA cm⁻² (Ni/N–C-800), 2.36 mA cm⁻² (Ni/N–C-900), and 1.27 mA cm⁻² (Ni/N–C-1000) (Fig. 3c). The j_CO of Ni/N–C catalysts is higher than that of N–C (0.24 mA cm⁻²) and Ni–C (0.44 mA cm⁻²) (Fig. S7†), suggesting that the Ni–N_x coordination structure plays a vital role in the CO₂RR. Meanwhile, the H₂ partial current densities (j_H2) (Fig. 3d) and j_CO increase with the applied voltage; however, there are some differences in the magnitude of j_H2 and j_CO growth, indicating that the H₂/CO ratios of various Ni/N–C catalysts can also be modulated by changing the applied potential. The syngas production rate is also a key parameter for estimating the catalytic performance.

The production rates of CO and H₂ can vary with applied potentials (Fig. 4a and b). As predicted, the generation rates of CO and H₂ gradually increase in a wide potential window (−0.7 to −1.1 V vs. RHE). Ni/N–C-900 displays the maximum CO production rate (19.96 mmol g⁻¹ h⁻¹) compared with those of Ni/N–C-700 (6.68 mmol g⁻¹ h⁻¹), Ni/N–C-800 (9.83 mmol g⁻¹ h⁻¹) and Ni/N–C-1000 (12.55 mmol g⁻¹ h⁻¹), demonstrating that the production rates of CO are consistent with the number of Ni–N_x active sites. From 700 to 900 °C, the number of Ni–N_x active sites in the catalysts gradually increases, the reactions are mainly based on the CO₂RR, and the HER is inhibited, resulting in a gradual decrease of production rates for H₂. As the temperature continued to rise to 1000 °C, the decrease in the number of Ni–N_x active sites caused a gradually enhanced HER. To further reveal the reaction kinetics of the catalysts, the Tafel slopes for CO production over Ni/N–C catalysts are presented in Fig. 4c. The Tafel slope of Ni/N–C-900 is 243.08 mV dec⁻¹, which is lower than those of Ni/N–C-700 (315.23 mV dec⁻¹), Ni/N–C-800 (258.51 mV dec⁻¹) and Ni/N–C-1000 (257.05 mV dec⁻¹), implying that the increased number of Ni–N_x active sites accelerates the reaction kinetics.⁴⁸ Meanwhile, the Tafel slopes of Ni/N–C catalysts are significantly lower than those of N–C (322.85 mV dec⁻¹) and Ni–C (319.59 mV dec⁻¹) (Fig. S8†), further proving that the Ni–N_x active sites can accelerate the reaction kinetics of Ni/N–C catalysts in the CO₂RR.^49,50

Fig. 4 (a) CO and (b) H₂ production rates of Ni/N–C catalysts at different applied potentials. (c) Tafel plots based on j_CO of the Ni/N–C catalysts in CO₂-saturated 0.1 M KHCO₃ solution. (d) Long-term stability test of Ni/N–C-900 at −0.8 V vs. RHE for 10 h.

In addition, Fig. S9 and S10† show the electrochemically active surface areas (ECSAs) and the double-layer capacitances (C_dl) of all samples. The C_dl of Ni/N–C-900 (1.08 mF cm⁻²) is obviously larger than the C_dl of Ni/N–C-700 (0.613 mF cm⁻²), Ni/N–C-800 (0.842 mF cm⁻²), Ni/N–C-1000 (0.863 mF cm⁻²), N–C (0.365 mF cm⁻²) and Ni–C (0.426 mF cm⁻²). The results show that the ESCA of Ni/N–C-900 is the largest among the above samples, which is conducive to increasing the contact area between active sites and CO₂. The j_CO of different catalysts is normalized to the ECSA to fairly compare the catalytic activity of Ni/N–C-900 with other samples. As shown in Fig. S11,† Ni/N–C-900 displays a significantly larger ESCA-normalized CO current density than the N–C and Ni–C samples. Thus, the higher intrinsic activity of Ni/N–C-900 is attributed to its unique Ni–N_x sites. An isotope tracer measurement for the electroreduction of ¹³CO₂ was carried out to trace the origin of the products, where ¹²CO₂ was used as a reference. As shown in Fig. S12,† the dominant peak of ¹³CO (m/z = 29) was observed at m/z = 29, which was assigned to ¹³CO produced during the MS measurement. The results verify that CO detected upon the Ni/N–C materials comes from the electroreduction of the CO₂ source instead of any organic impurities from the catalyst. Furthermore, Ni/N–C-900 exhibits significant electrochemical stability in a long-term electrolysis test. When the reaction continues for 10 h at −0.8 V vs. RHE, the H₂/CO ratio remains basically stable and the current density decreases only slightly at the beginning (Fig. 4d). At the beginning of the reaction, sufficient CO₂ is adsorbed on the electrode and a large amount of CO₂ is consumed instantaneously after applying the potential. However, as the reaction proceeds, the insufficient supply of CO₂ leads to a decrease in the current density. To verify the reason for the decrease of FE_co after 10 h of electrolysis, we investigated the chemical states of N species. The relative contents of various N dopants show no obvious differences compared with the fresh Ni/N–C-900 sample. However, the atomic concentration of total N decreases from 3.85 to 3.02 at% (Fig. S13†), which may be the cause of deactivation.

In order to highlight the advantages of the stable and controlled syngas production within a wide potential window over the Ni–N co-doped ultrathin carbon nanosheet electrodes, a comparison between H₂/CO ratios in this work and those in recent representative literature reports is presented in Fig. 5.

Fig. 5 Recently reported H₂/CO ratios of various catalysts used for syngas production at different applied potentials. (a) Heteroatom-doped carbon catalysts (N-CNTs,⁴² BPNC,¹⁴ NCSP-70,⁵¹ UNCNs-900,⁵² NCN-6,⁵³ and NBC⁵⁴), (b) metal catalysts (OD-Cu NA,⁵⁵ ZnOCNT,⁵⁶ I-Ag,⁵⁷ AuCu₂/CNT,⁵⁸Au Θ_Cu ∼ 3/3,⁵⁹ and Co rate-aligened⁶⁰), (c) M–N–C single-atom catalysts (Ni-hG,²³ CoNi-NC,²⁴ Co–N–C–Ph,⁶¹ Fe–N–C,³³ P/Ni-4@Ni-NC,⁶² and Co@CoNC-900 ⁶³) and (d) Ni–N co-doped ultrathin carbon nanosheet electrodes. The shaded regions are the areas with the optimal syngas ratios.

The H₂/CO ratios over heteroatom-doped carbon catalysts and metal catalysts could be regulated in the range of 0–3.5, but it is far from the optimal range. Meanwhile, the weak intrinsic activity of heteroatom-doped carbon and the scarcity or cost disadvantages of metal catalysts still limit their large-scale application. Compared with the M–N–C catalysts in the literature, our Ni/N–C catalysts showed obvious advantages in the regulatory range, that is, it covered all the optimal syngas regions. Furthermore, the ratio of syngas on our catalysts can be controlled simultaneously by adjusting the active site content and applied potential. The unique and convenient regulation strategy of this work enables the H₂/CO ratio to be adjusted between 1/2 and 2/1 within a wide voltage window, highlighting excellent intrinsic activity and economy.

Conclusions

In summary, this work presented Ni–N co-doped carbon nanosheets as promising catalysts for the electrochemical conversion of CO₂ to tunable syngas. The number of Ni–N active sites in the catalysts is varied by adjusting the carbonization temperature of the synthesis process, enabling the effective regulation of H₂/CO ratios to the optimal range (1/2–2/1) over a wide potential window (from −0.7 to −1.3 V vs. RHE). The scheme of modulating the H₂/CO ratios by changing the number of Ni–N_x active sites shows great potential and may provide a valuable reference for future syngas production.

Author contributions

Kaining Gan: investigation, methodology, data curation, writing – original draft, review and editing. Hongqiang Li: supervision, conceptualization, methodology, writing – review and editing, and funding acquisition. Ran Li: investigation and validation. Jiabao Niu: investigation and methodology. Jun He: validation. Dedong Jia: review and editing, and funding acquisition. Xiaojun He: supervision and funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was partly supported by the National Natural Science Foundation of China [no. 22108003, 52072002, 51872005, 52002194], the Natural Science Foundation of Anhui Provincial Education Department [no. KJ2020A0254], the Research Fund for Young Teachers of Anhui University of Technology [no. QZ202007], and the Wanjiang Scholar Program. The authors would also like to acknowledge the financial support from the Anhui International Research Center of Energy Materials Green Manufacturing and Biotechnology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi00108c

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