Introduction

With the rapid development of the global economy, the consumption of diverse fossil energy sources has been increasing day by day [1,2,3]. CO2, as the final product of the burning of fossil fuels, its excessive emission has caused the global warming and other environmental issues [4, 5]. To address the issues, it is an inevitable trend of reducing the emissions of CO2. Electrochemical CO2 reduction, which could effectively convert CO2 to valuable petrochemicals, has been considered as one of the more attractive and promising strategies due to its simple process and mild reaction conditions [6,7,8]. However, it is still a challenge to achieve a high-efficiency CO2 reduction with a suitable electrocatalyst which contains high activity and good selectivity, due to the large thermodynamic energy barrier and difficulty of activating stable CO2 molecules as well as competitive hydrogen evolution reaction [9, 10].

In recent years, various catalysts, such as nanomaterials [11,12,13], metal [14, 15], transition-metal oxide [16, 17], metal-free materials carbon [18, 19], and metal–organic framework (MOFs) [20, 21], have been explored for electrocatalytic CO2 conversion [22,23,24]. Compared with other electrocatalysts, MOFs, as a highly porous material with a periodic network structure formed by the self-assembly of organic ligands and metal ions or clusters, exhibit a hopeful electrocatalyst for CO2 reduction due to their high surface areas, single active sites, and potential to capture CO2 [25,26,27]. However, they often suffer from poor chemical stability in the electrolyte solution and are sensitive to water, severely hindering their industrial use. Recently, balanced hydrogen-bonding interactions between the cationic frameworks and anions have been proposed to enhance the stability of MOFs [28]. And intramolecular hydrogen-bonding interactions (IHBIs) play fundamentally important roles in the fabrication of stable MOFs. To date, improving MOF stability for electrochemical CO2 reduction through IHBIs has rarely been considered. Because of Zn-based catalysts with high cost-effectiveness and low-activity hydrogen evolution reaction [29], Zn-MOF-based electrocatalysts of CO2 reduction have attracted great interest [30,31,32]. Presently, most reported Zn-MOFs show high selectivity and activity to CO or CH4 [31, 33, 34], only a few Zn-MOFs [35] exhibit high selectivity of formate which is economically feasible in view of the market value and the energy input. Investigating its reason, the coordination environment of the metal as an active site greatly affects the product selectivity of CO2 reduction. Therefore, it is significant to design and synthesize new Zn-MOFs with IHBIs for electrocatalytic conversion of CO2 to formate.

In this work, a new three-dimensional N, O-co-coordinated MOF-CH3 was obtained using Zn as the metal center and 1,2,4-triazole and 2-methyl terephthalic acid as organic ligands by a simple solvothermal method. The MOF-CH3 is isostructural with the reported FJU-40-NH2 (denoted by MOF-NH2 here) containing IHBIs and FJU-40-H (denoted by MOF-H here) without IHBIs [28]. MOF-CH3 and MOF-NH2 show far better chemical stability than MOF-H in an electrolyte solution. The electrochemical CO2 reduction test displays that all the N,O-co-coordinated Zn-based MOFs exhibit specific selectivity for conversion CO2 to formate with the faradaic efficiency (FE) more than 55% at the optimal potential, which is consistent with the reported N,O-coordinated Zn-based MOFs [35], but different from the reported N- or O-coordinated Zn-based MOFs catalysts with the primary product of CO or CH4 [31, 33, 34]. The above results indicate that the microenvironment of the catalyst could affect the electroreduction of CO2. The FEformate of 76.5% for MOF-CH3 is lower than that for MOF materials such as Zn-MOF [35], Bi-BTC-D-3.75 [36], and MIL-68(In)-NH2 [37], but higher than that for (Me2NH2+)[InIII-(TTFTB)]·0.7C2H5OH·DMF [38], Cu-MOF/GO [39], and MFM-300(In)-t/CP [40]. Although there are relatively poor CO2 reaction activities for MOF-CH3 in comparison with the reported advanced catalysts, this work provided a novel Zn-based MOF catalyst with IHBIs for electrochemical conversion of CO2 to formate.

Experiment

Materials

All reagents and chemicals were obtained commercially and used without further purification. 1,2,4-triazole, terephthalic acid, 2-amino-terephthalic acid, 2-methyl-terephthalic acid, zinc nitrate hexahydrate, and N, N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. Nafion 117 proton exchange membrane and NafionD-521 dispersion (5 wt% in lower aliphatic alcohols and water) were purchased from Shanghai Hesen Electric Co., Ltd.

Instrumentation

Powder X-ray diffraction (PXRD) was carried out with a PANalytical X’Pert3 diffractometer equipped with a Cu-sealed tube (λ = 1.541874 Å) at 40 kV and 40 mA over the 2θ range of 5–30°. The simulated PXRD pattern was produced by using the Mercury V1.4 program and single-crystal diffraction data. Thermal analysis was carried out on a METTLER TGA/SDTA 851 thermal analyzer from 30 to 600 °C at a heating rate of 10 °C min−1 under N2 flow. Fourier transform infrared spectra (FT-IR, KBr pellet) were measured in the range of 400–4000 cm−1 by a Nicolet 5700 FT-IR. Elemental (C, H, N) analytical data were obtained on a Perkin-Elmer model 240C elemental analyzer. Electrochemical test data were recorded with a Versa STAT 3F electrochemical workstation (Princeton Instruments, USA). All the gas products (CO and H2) were quantified with a gas chromatography (PANNA-A60), which was equipped with a thermal conductivity detector and flame ionization detector, and N2 was used as carrier gas. Liquid products were analyzed by quantitative 1H-nuclear magnetic resonance (NMR, Bruker AVNANCE-400) using deuterium oxide as an internal standard.

Synthesis of MOF-R (R = CH3, H2, H) for the electroreduction of CO2

MOF-R (R = CH3, H2, H) were prepared via solvothermal reaction. The detailed information was shown in Supplementary Information, including synthesis, SCXRD analysis, electrochemical measurements, and product analysis.

Results and discussion

Structures and characterization of MOF-R (R = CH3, NH2, H)

As illustrated in Fig. 1a, a series of Zn-based MOFs (MOF-R, R = CH3, NH2, H) were prepared by a facile one-step solvothermal procedure with zinc nitrate and organic ligands. The images of scanning electron microscopy (SEM) show that all the MOFs are relatively regular block structures (Fig. S1), wherein MOF-CH3 is a new MOF material. SCXRD patterns of the obtained MOF-CH3 could be ascribed to a structural type with a tetragonal P4/nnc space group which is consistent with MOF-NH2 (Table S1). Each ZnII atom with tetrahedron geometry (Table S2 and S3) is coordinated with three N atoms from three 1,2,4-triazole ligands molecules and two O atoms from a 2-methyl-terephthalic acid ligand (Fig. 1a). And the two ZnII atoms are joined by two 1,2,4-triazole ligands to form a [Zn2(1,2,4-triazole)2] unit (Fig. 1b), which connect to each other to extend further into a waved two-dimensional layer. These layers are further supported by 2-methyl-terephthalic acid ligands via Zn–O coordinated bonds to form a three-dimensional columnar layer framework (Fig. 1c, d). These Zn-MOFs have almost the same physical structure as MOF-NH2 and MOF-H, which are all columnar structures constructed with [Zn2(1,2,4-triazole)2]n as layers and 2-methyl-terephthalic acid as column (Fig. 1a). Nevertheless, their structures are slightly different due to the differences in the functional groups of terephthalic acid-R linker (Fig. 1a). For MOF-CH3 or MOF-NH2, the IHBIs with d[O(− COO)···H(− CH3)] = 2.3147 Å or d[O(-COO)···H(-NH2)] = 2.616 Å are formed between hydrogen atoms of − CH3 or NH2 and carboxyl oxygen atoms (Fig. 1a), which could shorten the distance of Zn1-O2. Specifically, the Zn1-O2 bond lengths in MOF-CH3, MOF-NH2, and MOF-H are 2.485 Å, 2.495 Å, and 2.604 Å, respectively (Table S2). A shorter Zn1-O2 bond length could make MOF-CH3 and MOF-NH2 more stable than MOF-H [28], which was confirmed by the subsequent stability test in KHCO3 electrolyte solutions via powder X-ray diffraction (PXRD).

Fig. 1
figure 1

a Synthesis and structure illustration of MOF-R (R = CH3, NH2, H), b [Zn2(1,2,4-triazole)2] units connect to each other to form a two-dimensional layer of MOF-CH3, and c, d three-dimensional open framework pillared by 2-methyl-terephthalic acid of MOF-CH3

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The PXRD data for the as-prepared MOF-CH3 or MOF-NH2 materials (Fig. 2a, b) has typical diffraction peaks at the 2θ of 6.387°, 9.128°, 13.120°, and 15.17°, which match well with the simulated PXRD pattern of the main crystal faces (002), (102), (200), and (211), respectively. MOF-H also shows the crystal faces (020), (011), (101), and (111) at the typical diffraction peaks of 2θ = 6.611°, 9.735°, 12.763°, and 13.117°, respectively (Fig. 2c). These results indicate the successful synthesis of MOF-R. The thermal stability of MOF-R was measured by thermogravimetric (TG) analysis. TG curve results display that MOF-R has pretty similar weightlessness tracks up before 600 °C (Fig. 2d). For MOF-CH3, the first weight loss at 123 °C is the loss of DMF molecules. Then, the plateau was maintained until the framework began to decompose at around 369 °C. The MOF-NH2 and MOF-H lose the first weightlessness before 100 °C, which could be ascribed to the loss of water molecules in the pore channel. For MOF-NH2, the second weight loss at 123 ℃ is the loss of DMF molecules. The last weight loss over 322 ℃ corresponds to the decomposition of the framework. For MOF-H, the molecules of DMF evaporate at ~ 112 ℃, and when heated above 345 ℃, the organic ligands start the decomposition, and the framework collapse. These results suggest that the MOFs possess excellent thermal stability. The characteristic temperatures and final weight losses of all samples are also stated in Table S4. The Fourier transform infrared spectroscopy (FT-IR) spectrums of MOF-R are displayed in Fig. 2e. All the MOFs show C-N peak at about 1130 cm−1 for 1,2,4-triazole ligand and the characteristic peaks of C = O/C-O, respectively, at about 1720/1100 cm−1 for terephthalic acid ligand. For MOF-CH3, the characteristic peaks of − CH3 group are at 2940 cm−1, 2860 cm−1, and 1380 cm−1. There are two peaks at 3470 cm−1 and 3340 cm−1 with moderate intensity, which correspond to N–H antisymmetric stretching vibration and symmetric stretching vibration on − NH2 functional group of MOF-NH2, respectively. In addition, we found that MOF-CH3 and MOF-NH2 have blunt peaks of O–H stretching vibration intramolecular hydrogen bonds at 3610 cm−1 and 3530 cm−1[28], while MOF-H does not, which is consistent with the crystal structure analysis described in Fig. 1a.

Fig. 2
figure 2

a–c Powder X-ray diffraction patterns, d thermogravimetric curves, and e Fourier transform infrared spectroscopy spectra of MOF-R (R = CH3, NH2, H)

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Electrochemical measurements and CO2RR performance

The stability of the three MOFs immersed in 0.5 mol mL−1 KHCO3 electrolyte solution was analyzed, as shown in Fig. 2a–c. The results show that MOF-CH3 and MOF-NH2 in the electrolyte solution could maintain good crystallinity for 24 h, while there are basically no diffraction peaks for MOF-H after soaking for 2 h. The stability differences of the MOFs in the electrolyte solutions should be closely related to the presence of IHBIs in their structures. For MOF-CH3 or MOF-NH2, the distance between Zn1-O2 is shortened due to the presence of IHBIs. A shorter Zn1-O2 distance could protect Zn atoms from water attack, resulting in better stability of MOF-CH3 or MOF-NH2 than MOF-H in electrolyte solutions.

Electrochemical CO2 reduction tests were conducted in 0.5 mol mL−1 KHCO3 electrolyte solution (pH = 7.2) via a liquid H-type electrochemical cell with successive CO2 bubbling at a flow rate of 20 mL min−1 [41]. The gaseous and liquid reduction products were respectively detected and quantified by gas chromatography and 1H-nuclear magnetic resonance (1H-NMR), and details were provided in supporting information. The cyclic voltammograms curves for MOF-CH3 and MOF-H (Fig. 3a, b) with the redox current peaks of Zn(II) reduced to Zn(I) [31, 35, 42] show that the onset potentials of MOF-R under CO2 atmosphere are more positive than that in Ar condition, which indicates their activities are originated from the CO2 conversion [43, 44]. Although MOF-NH2 has similar CV curves in Ar and CO2 condition, the current density values in the CO2 condition is higher than that in the Ar condition when the potential values surpass 1 V (Fig. 3c), indicating CO2 reduction reaction still is the superior reaction. Moreover, MOF-CH3 exhibits slightly larger current densities at high potential than MOF-NH2 or MOF-H from linear sweep voltammetry curves (Fig. S2). In order to study CO2 reduction activity and selectivity, chronoamperometry tests were conducted at different potentials (Fig. S3). Formate, CO, and H2 are the products of CO2 reduction over the MOF-R materials at each potential from − 0.97 to − 1.67 V vs. RHE (reversible hydrogen electrode, the same below) (Fig. 3d–f), and formate is the primary product at the most given potential, especially MOF-CH3 (Fig. 3d). The detailed data in Fig. 3d–f has also been shown in Table S5. In addition, the gas chromatograms of the gaseous products formed at the optimum potential were provided in Fig. S4 for explicitness, and 1H-NMR of the liquid phase products was shown in Fig. S5a. To confirm the formate from the electroreduction of CO2, isotopic labeling tests were further performed by using the mixture of 13CO2/12CO2 as the feedstock. The 1H-NMR signals of H13COOH (Fig. S5b) illustrate that the formate originated from the CO2 reduction.

Fig. 3
figure 3

a–c Cyclic voltammograms curves in Ar-( black line) or CO2-(red line) saturated 0.5 mol L−1 KHCO3 solutions with a scan rate of 50 mV s−1, d–f faradaic efficiency of different products, g faradaic efficiency of formate, h partial current densities of formate, and i faradaic efficiency of H2 for MOF-R (R = CH3, NH2, H) at given potentials

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As shown in Fig. 3g, h, the highest FEformate for MOF-CH3 and MOF-H is, respectively, 76.5% and 73.5% at − 1.37 V, with the partial current density of formate (jformate) of − 12.1 mA cm−2 and −14.0 mA cm−2, respectively. The maximum FEformate for MOF-NH2 is 55.7% at − 1.57 V with jformate of − 14.5 mA cm−2. As shown in Fig. 3g, the MOF-CH3 shows good selectivity of formate with FEformate of more than 50% at all applied potentials, while poorer formate selectivity of MOF-NH2 was obtained than that of MOF-CH3 or MOF-H. Also, the FEH2 for MOF-NH2 is up to 40–50% in the low potential range of − 0.97 to − 1.27 V, and MOF-H shows high FEH2 at − 0.97 V (Fig. 3i). The formate concentration of MOF-CH3 is higher than MOF-NH2 or MOF-H in a wide potential ranging from − 0.97 to − 1.67 V except − 1.27 V (Fig. S6). The concentration of formate for MOF-CH3 is 11.1 mmol L−1 at − 1.37 V, while that for MOF-NH2 and MOF-H is 7.0 mmol L−1 and 9.6 mmol L−1, respectively. Taken overall, MOF-CH3 is more efficient electrocatalysts for CO2 reduction than MOF-NH2 or MOF-H. A comparison of Zn-based MOFs and complex electrocatalysts for CO2 reduction was summarized in Table S6. These N,O-co-coordinated Zn-based MOFs exhibit specific selectivity for formate at the optimum potential (FE > 55%), different from the reported N- or O-coordinated Zn-based MOF catalysts with the primary product of CO or CH4 [31, 33, 34]. Although there is a relatively poor CO2 reduction selectivity for MOF-CH3 in comparison with the reported N, O-coordinated Zn-based MOFs [35], this work provided a novel example of Zn-MOF catalysts with IHBIs for electrochemical conversion of CO2 to formate.

To further probe the catalytic activity of Zn-MOFs during CO2 reduction, the Tafel plots, electrochemical impedance spectroscopy (EIS), the electrochemical active surface area (ECSA), and turnover frequency (TOF) [45] were studied. The reaction kinetics of the CO2 reduction process was explored by the Tafel analysis. The low Tafel slope means fast dynamics and better electrocatalytic performance. As illustrated in Fig. 4a, MOF-CH3 displayed a Tafel slope of 232.7 mV dec−1, slightly smaller than MOF-NH2 (346.9 mV dec−1) and MOF-H (234.7 mV dec−1). This indicates that MOF-CH3 has a relatively faster kinetics and excellent electrocatalytic performance for CO2 reduction. In addition, the electron transfer behavior of MOF-R during CO2 reduction was studied via EIS measurement, which is displayed in Fig. 4b. MOF-CH3 has a lower semicircular radius (R) of ~ 700 Ω than MOF-NH2 or MOF-H at − 0.37 V. The smaller R reflects the much favorable charge transfer kinetics, which also reveals that MOF-CH3 has better electrochemical capabilities [46, 47]. Additionally, the electrochemical double-layer capacitance (Cdl) measured by CV at different scan rates (Fig. S7) was used to evaluate the ECSA of the Zn-MOF electrocatalysts. A higher Cdl generally means a higher ECSA which illustrates more exposed active sites [48]. The result showed that Cdl value for MOF-CH3, MOF-NH2, and MOF-H is 4.0, 6.5, and 6.25 mF cm−2, respectively (Fig. 4c). Although MOF-NH2 or MOF-H has an obviously higher ECSA than MOF-CH3, the two electrocatalysts do not exhibit better conversion performance of CO2 to formate, as could be explained by more reaction active sites given to HER or the conversion of CO2 to CO at most applied potentials, based on the FE values of the products. Besides, TOF was used to evaluate the intrinsic activity of electrocatalysts. These TOF results for MOF-CH3 are better than that for MOF-NH2 or MOF-H at most given potentials (Fig. 4d), which could verify that MOF-CH3 has higher intrinsic actives.

Fig. 4
figure 4

a Tafel plots, b electrochemical impedance spectroscopy spectra at − 0.37 V, c charging current density at different scan rates, and d turnover frequency plots for the generation of formate for MOF-R (R = CH3, NH2, H)

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Although the metal center for these MOFs is the same and their structure is similar, these MOFs show different effects of CO2 reduction. Apart from the presence or absence of intramolecular hydrogen bonds which mainly affect the stability, they also possess different functional groups (− CH3, − NH2, − H) with regard to microenvironment. Thus, we further explore the effect of functional groups. And contact angle tests were performed to analyze the hydrophilicity of the Zn-MOFs. As shown in Fig. S8, the contact angle of 44.2° for MOF-CH3 is larger than that of the other two MOFs, indicating that the hydrophilicity of MOF-CH3 is poorer than the others. The introduction of hydrophobic groups (− CH3) could make MOFs show good chemical stability and avoid the attack from water to generate H2 to some extent, which is helpful for CO2 reduction [49]. FEH2 of MOF-CH3, that is lower than that of MOF-NH2 at most given different potentials (Fig. 3i), also illustrated that MOF-CH3 could effectively inhibit the hydrogen evolution reaction. To sum up, favorable charge transfer kinetics and intrinsic activity as well as hydrophobicity let MOF-CH3 improve electrocatalytic CO2 reduction in comparison with the other two structural analogues. It is clear that the performance could be improved by regulating the coordination microenvironment of MOF catalysts.

Conclusions

In summary, a series of N, O-coordinated Zn-based isostructuralism MOF catalysts were obtained by a simple solvothermal method for selective electroconversion of CO2 to formate under ambient conditions. Among them, MOF-CH3 is a new MOF with IHBI distance of 2.3147 Å from the SCXRD analyses. MOF-NH2 also exists as an intramolecular hydrogen bond with the distance of 2.616 Å, while MOF-H does not. PXRD results indicate that the presence of IHBIs in MOFs is helpful for keeping the good crystallinity in electrolyte solution. The related electrochemical tests for CO2 reduction show that the catalytic performance of MOF-CH3 with FEformate 76.5% at − 1.37 V is better than that of MOF-NH2 with FEformate of 55.7% at − 1.57 V or MOF-H with FEformate of 73.5% at − 1.37 V. According to the analyses of EIS, TOF, and contact angle, it was found that MOF-CH3 possesses the lowest R of ~ 700 Ω, high intrinsic actives, and the largest contact angle of 44.2°. The introduction of IHBIs and hydrophobic groups (− CH3) may be helpful for CO2 reduction. This work provides a novel Zn-MOF catalyst for the electroreduction of CO2 to formate.