Tuning the efficiency and product composition for electrocatalytic CO₂ reduction to syngas over zinc films by morphology and wettability†

Pengsong Li ^ab, Jiyuan Liu ^ab, Jiahui Bi ^ab, Qinggong Zhu *^ab, Tianbin Wu ^a, Jun Ma ^a, Farao Zhang ^c, Jinchao Jia ^c and Buxing Han *^abd
aBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. E-mail: hanbx@iccas.ac.cn; qgzhu@iccas.ac.cn
^bUniversity of Chinese Academy of Sciences, Beijing 100049, China
^cNingbo MaterChem Technology Co. Ltd., Ningbo, 315830, China
^dShanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200062, China

First published on 22nd January 2022

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Introduction

Syngas (CO and H₂ mixture) is a well-known industrial feedstock for producing value-added chemicals and fuels through the Fischer–Tropsch process.^1–3 With different CO/H₂ ratios, syngas can be used to synthesize various downstream products, such as aldehyde (CO/H₂ ratio of 1:1),⁴ methanol (CO/H₂ ratio of 2:1),⁵ and methane (CO/H₂ ratio of 3:1).⁶ However, most syngas is industrially synthesized by natural gas reforming and coal gasification, which also require harsh reaction conditions, such as high pressure and high temperature.^7,8 As an alternative, electrochemical reduction of CO₂ using water as the hydrogen source is a prospective strategy to address this issue.^9–11 In this process, a competing hydrogen evolution reaction (HER)^12,13 is combined into a competitive approach for producing syngas. The fabrication of electrodes with high catalyst selectivity, CO₂/H⁺ mass transfer, and charge transfer are most important for achieving high syngas yield with controllable CO/H₂ ratios.^14–16

Hitherto, many strategies have been invested to enhance syngas production rate in aqueous solutions, including hybrid modulation,^17,18 alloying,^19,20 metal oxidation,²¹ single atoms,²² bimetallic layered conjugated MOF²³ and defect adjusting.^24,25 Even though a majority of catalysts can tune the CO/H₂ yield via modulating the active sites for CO₂ reduction reaction (CO₂RR) and HER, the catalytic activity is still limited by the low concentration of CO₂ and slow diffusivity to the catalyst surface.^26,27 The unbalanced distribution of CO₂ and H₂O on the catalyst surface also leads to the lower stability of the catalysts. The design and preparation of efficient catalysts to achieve a desirable balance between CO₂RR and HER is very interesting.^9,28

Gradually, the engineering of electrode wettability has emerged as a new strategy to overcome the mass transfer limitation in CO₂RR by creating a gas–electrode–solution three-phase boundary.^29,30 Thus, concentrated CO₂ molecules in the gas phase can contact the catalyst surface directly and inhibit the HER due to the lowering proton contacts.^31,32 Hence, we assume that a controllable gas–solid–liquid contact interface can effectively balance CO₂ gas diffusion and proton supply on the active sites by tailoring the adsorption/affinity of reactants for a wide and tunable CO/H₂ ratio with high current density.^33,34

Herein, we developed a simple and facile method to synthesize Zn film electrodes with different morphologies and wettability. The effect of hydrophobicity of the three-dimensional (3D) Zn surface on the reaction was studied. The CO/H₂ ratios of the syngas could be controlled by varying the applied potential or wettability of the electrode. It was found that the nanoleaf Zn surface with a water contact angle of 91° was very efficient for CO₂RR to syngas in aqueous electrolytes. The current density could reach 90.4 mA cm⁻² with a CO/H₂ ratio of 1:1 at −1.27 V vs. RHE.

Results and discussion

The hydrophobic 3D Zn film electrode on the Zn foil substrate was fabricated by electrodeposition, PTFE modification and calcination. We prepared Zn films with different morphologies (Zn-1, Zn-2 and Zn-3) to study the hydrophobicity as an isolated parameter in promoting gas trapping and consequently the study of CO₂ reduction to syngas with tunable CO/H₂ ratios (Fig. 1a). Field emission scanning electron microscopy (SEM) was employed to study the morphology of the as-synthesized Zn catalysts on Zn foil (Fig. S1† and Fig. 1b–d). It shows that Zn-1 films grew perpendicularly on the substrate with a pile of porous nanoleaf-like nanostructures. In contrast, Zn-2 had a typical 3D open porous architecture with interconnected nanowires. Zn-3 showed a large amount of Zn nanosheets that were vertically grown on the substrate. The transmission electron microscope (TEM) images of the Zn film electrodes provided in Fig. S2† also certify the nanoleaf, nanowire and nanosheet morphologies. The high-resolution TEM (HR-TEM) images revealed the characteristic spacing of 0.21 nm for the Zn(101) lattice planes in the Zn-1 sample and 0.25 nm for Zn(002) in Zn-2 and Zn-3. The typical X-ray diffraction (XRD) patterns are shown in Fig. S3.† The peaks located at 36°, 39°, 43°, 54°, 70° and 71° can be indexed to the (002), (100), (101), (102), (103) and (110) crystalline planes of pure metal Zn (JCPDS: PDF #04-0831), which confirmed the highly crystalline nature of the as-prepared Zn film catalysts. The differences in the relative peak intensities between the (002) and (100) facets of Zn-1, Zn-2 and Zn-3 indicate the influence of morphology, which are consistent with the result of the HR-TEM study (Fig. S2†). Meanwhile, all the Zn film electrodes showed a superhydrophilic surface, as evidenced by the contact angle of 0° for water droplets on the surfaces (insets in Fig. 1b–d).

After 0.3 wt% PTFE modification, the Zn film electrodes showed a hydrophobic surface, which is denoted as Zn-1P, Zn-2P and Zn-3P. The SEM and TEM images of the modified Zn film electrodes demonstrated that the initial morphology could be well-maintained and there were some PTFE particles attached to the Zn film surface to regulate the wettability of the electrode (Fig. 1e, f and Fig. S4†). Over the Zn-1P electrode, the water contact angle increased to 91° after modification with 0.3 wt% PTFE (inset in Fig. 1e). There was also no noticeable change in the crystallinity and crystal face exposure of the PTFE-modified Zn, as confirmed by the XRD patterns and HR-TEM images (Fig. S5† and inset in Fig. 1f). This phenomenon also appeared over the Zn-2P and Zn-3P electrodes. Because metallic zinc is easily oxidized in air, a quasi in situ X-ray photoelectron spectroscopy (XPS) measurement was carried out using the homemade sample cell (Fig. S6†) to have further insight into the chemical states of Zn film electrodes. The results in Fig. 1g, h and S7† show that the Zn 2p photoelectron spectra of the Zn-1, Zn-2 and Zn-3 electrodes exhibited two main peaks at around 1044.6 and 1021.5 eV before and after PTFE modification, which are assigned to Zn 2p_1/2 and Zn 2p_3/2, respectively.³⁵ The F 1s photoelectron spectra before and after PTFE decoration revealed that PTFE was successfully combined with the Zn electrodes.³⁶

The catalysts were used directly as working electrodes for the production of syngas in the CO₂-saturated 0.5 M KHCO₃ aqueous electrolyte. Cyclic voltammetry (CV) measurements over Zn film electrodes before and after PTFE modification are shown in Fig. S8,† which indicate that the hydrophobic electrodes had better electrocatalytic performance. We then investigated the catalytic activity and selectivity change induced by the difference in gas–solid–liquid contact interface. The electrolysis performance of various Zn electrodes is illustrated in Fig. 2a. Under the reaction conditions, no obvious liquid products were detected by NMR spectroscopy, and the mainly yielded products were H₂ and CO, with a combined faradaic efficiency (FE) of around 100%. Clearly, the Zn foil electrode persistently suffered from high H₂ yield (86.1%) and low CO yield (14.2%) caused by the intrinsic hydrogen evolution reaction activity in aqueous electrolytes.^35,37,38 The current density and FE of CO were obviously improved for the structure-modified Zn-1 electrode (59%, CO) compared with the Zn foil. Interestingly, we found that the current density and FE of CO were significantly improved for the Zn-1P electrode. The maximum FE of CO could reach 92.6% with a current density of 35.3 mA cm⁻². Fig. 2b shows that the Tafel slope of the Zn-1P electrode was 22.3 mV dec⁻¹ in the potential range of −0.6 V to −0.7 V vs. RHE, which was lower than that of the Zn-1 (23.5 mV dec⁻¹) and Zn foil (33.6 mV dec⁻¹) electrodes, indicating a much faster increment of electrochemical reduction rate on Zn-1P electrode for producing syngas. We assume that with increasing hydrophobicity, high concentrated CO₂ could be delivered to the catalyst surface directly and the HER was obviously suppressed (Zn-1P).

Long-term electrolysis was also performed to verify the stability of the catalysts. As shown in Fig. 2c, the current density, FE_CO and water contact angle did not change markedly during the electrochemical reduction process, revealing that the Zn-1P electrode was stable under the CO₂RR conditions. After the reaction, the morphology of Zn-1P was well preserved, further demonstrating the robustness of the nanoleaf structure (Fig. S9†). Elemental mapping was utilized to confirm the elemental distribution of Zn-1P after 12 h electrocatalysis. As shown in Fig. S10,† the typical nanoleaf morphology was unchanged and the F element still existed on the electrode, indicating that PTFE could be firmly decorated on the surface of the Zn electrode during the CO₂RR process. Even though the Zn-1 electrode also showed long-term stability, the FE of CO was much lower than that of the Zn-1P electrode (Fig. 2c). All these results illustrate that the wettable interface engineering for the Zn film can effectively improve the catalytic performance of the catalyst for CO₂RR and maintain durability.

The FE and current density for the formation of CO and H₂ were also determined in the potential range of −0.67 V to −1.07 V vs. RHE on Zn film (Zn-1, Zn-2, and Zn-3) and 0.3 wt% PTFE-modified Zn film (Zn-1P, Zn-2P, and Zn-3P) electrodes, respectively (Fig. S11† and Fig. 3a–c). For Zn film electrodes, the catalytic activity, in terms of the generated current density, followed the trend Zn-3 < Zn-2 < Zn-1. For the Zn-1 electrode, the maximum FE_CO was 59.1% with a current density of 34 mA cm⁻² at the potential of −0.97 V vs. RHE (Fig. S11†). For modified Zn-film electrodes, the FEs of CO were all much higher than that of the Zn film electrodes in the potential range. The CO/H₂ ratio also depended strongly on the applied potential. For Zn-1P, the FE of CO increased with decreasing applied potential and reached a maximum of 92.6% at 0.97 V vs. RHE and the HER became significant with further decreasing potential. The CO/H₂ ratio of the syngas could be tuned from 1.2 to 11.4 in the potential range (Fig. 3a). In particular, CO/H₂ was about 1/1 at −1.27 V vs. RHE and the current density could reach 90.4 mA cm⁻² (Fig. S12†). A similar tendency was also found for the Zn-2P and Zn-3P electrodes, with CO/H₂ ratio tuning from 0.7 to 5.7 and 0.3 to 0.8, respectively (Fig. 3b and c). An interesting finding was that although a similar hydrophobic surface was established on each modified surface, the CO₂RR and HER activities were different. It indicates that the gas (CO₂)–solid (catalyst) interface greatly influenced the CO₂RR activity by tuning charge transfer and active sites.

To study the interfacial properties of the electrode, electrochemical impedance spectroscopy (EIS) analysis was performed and the results are shown in Fig. S13.† The result indicates that the charge-transfer resistance (R_ct) became larger after the PTFE modification over those three electrodes. This is reasonable because PTFE is an insulator that reduces the conductivity of the electrode after the incorporation. Noticeably, the R_ct followed the trend of Zn-1 < Zn-2 < Zn-3 for both Zn film and modified Zn film electrodes; the smaller R_ct resulted in the fast charge transfer to the active sites. We also determined the electrochemical active surface area (ECSA) by the electrochemical double layer capacitance (C_dl), which was calculated by measuring the cyclic voltammetry curves in the proper potential range without redox processes (Fig. 3d, S14 and S15†). It shows that the Zn-1P electrode had a much larger C_dl (34.5 mF cm⁻²) than the Zn-1 (27.3 mF cm⁻²), suggesting more electrochemical active site exposure. The ECAS corrected CO partial current density of Zn-1P was also significantly larger than that of Zn-1 (Fig. S16†), revealing a substantial enhancement of the catalytic activity after decorating with PTFE. The increase in ECSA could also be observed for the PTFE-modified Zn-2 and Zn-3 electrodes. Moreover, the C_dl of Zn-1 (27.3 mF cm⁻²) was considerably larger than that of Zn-2 (21.7 mF cm⁻²) and Zn-3 (17.6 mF cm⁻²). Zn-1 with the nanoleaf-like nanostructure could provide a large surface area to expose more active sites and shorten the diffusion distance of charge. The 3D architecture created more gas–solid–liquid three-phase contact interfaces as the gaseous CO₂ attached to the surface of the electrode replaced some electrolytes and facilitated CO₂ reduction to CO.

To gain insight into the effect of wettability on the catalytic properties, we then prepared Zn film electrodes with a range of wettability by using different mass concentrations of PTFE. The contact angle of water on PTFE-modified Zn film electrodes is depicted in Fig. S17.† It shows that the electrode surface could be modified from hydrophilic to aerophilic with increasing amounts of PTFE. For the Zn-1 electrode, the CO/H₂ ratio of syngas could be tuned from about 1.3 to 11.4 using PTFE-modified Zn-1 electrodes at the potential of −0.97 V vs. RHE (Fig. 4a, b and S18†). It can be found that FE_CO and current density were lower with 0.1 wt% and 0.2 wt% PTFE decoration due to the strong water adhesion (contact angle of water < 60°). Higher FE_CO and current density were achieved when the PTFE amount was 0.3 wt%. FE_CO gradually decreased when involving excess PTFE (0.4 wt% and 0.5 wt%) (Fig. 4b). In particular, the maximum FE of CO was 92.6% at −0.97 V vs. RHE and the current density was 35.3 mA cm⁻² for the Zn-1 electrode with a contact angle of 91°. A substantial increase in FE_CO was also observed for the Zn-2 and Zn-3 electrodes (Fig. S19 and S20†). Importantly, the Zn catalysts access a wide and tunable syngas ratio (CO/H₂) from nearly 0.09 to 11.4 by modulating the morphology and wettability of Zn electrodes, which is comparable to the recently reported state-of-the-art electrocatalysts in Table S1.† In brief, the electrode and electrolyte contact interface can be regulated by the PTFE amount for the adjustable FE_CO. Based on such electrode/electrolyte contact interface regulation, syngas could be produced efficiently.

The results reveal that the surface modification of the Zn-1, Zn-2 and Zn-3 electrodes can suppress the competitive side reaction (HER) and enhance the activity for the conversion of CO₂ to CO (Fig. 4c). The 3D architecture is crucial for the concentration regulation of the gas and liquid phases in the electrode/electrolyte interface for efficient electrocatalysis. When the solid–liquid interface occupies the whole region of the catalyst, the catalyst is immersed by the electrolyte, which is in favor of HER. This demonstrates the insufficient supply of CO₂ on the electrode surface. With suitable hydrophobicity, high concentration of CO₂ can be achieved at the catalyst surface and the HER can be obviously suppressed. Higher hydrophobicity indicates that when more PTFE was added, the catalyst surface was blocked by excess PTFE, thus reducing the binding of CO₂ molecules to the active sites and resulting in a smaller FE of CO. Overall, regulating the 3D architecture and wettability of the electrode/electrolyte interface can effectively adjust the reaction activity of CO₂RR and HER, leading to a tunable ratio of CO/H₂.

Conclusions

In summary, Zn film can be used as an efficient electrocatalyst for synthesizing syngas. Constructing the three-phase (gas–solid–liquid) contact interface of the Zn film can regulate the CO₂ and proton supply in the interface between the electrode and electrolyte, leading to tunable CO/H₂ ratios ranging from 0.09 to 11.4. In particular, the CO/H₂ ratio of the syngas can be controlled by varying the applied potential or wettability of the electrode. For the nanoleaf Zn film with a water contact angle of 91°, the adsorption of CO₂ is promoted with a suitable proton supply leading to the favorable CO pathway. A FE of CO as high as 92.6% and a current density of 35.3 mA cm⁻² are obtained with superior durability for 12 h electrocatalysis. When the CO/H₂ ratio was 1:1, the current density could reach 90.4 mA cm⁻² at −1.27 V vs. RHE. The 3D architecture results in abundant active sites, large electrochemical surface area, high CO₂/H⁺ mass transfer, and charge transfer rate, which are all favorable for the electrochemical reaction. We believe this three-phase electrocatalysis may provide a facile and competitive strategy to produce syngas.

Author contributions

P. Li, Q. Zhu and B. Han proposed the project, designed the experiments, and wrote the manuscript. P. Li performed the whole experiment. J. Liu, J. Bi, Q. Zhu, T. Wu, J. Ma, F. Zhang and J. Jia performed the analysis of experimental data. Q. Zhu and B. Han co-supervised the whole project. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the National Natural Science Foundation of China (22102192, 22022307, 22072156, and 21773267), the National Key Research and Development Program of China (2017YFA0403101 and 2017YFA0403102), the Ningbo S & T Innovation 2025 Major Special Program (2018B10044), the Chinese Academy of Sciences (QYZDY-SSW-SLH013) and the China Postdoctoral Science Foundation (BX20200336 and 2020M680680).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1gc04364a

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