Dalton Transactions This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online PAPER View Journal | View Issue Improvement of carbon dioxide electroreduction Cite this: Dalton Trans., 2023, 52, by crystal surface modification of ZIF-8† Open Access Article. Published on 15 March 2023. Downloaded on 11/3/2023 3:30:03 PM. 5234 Ting Zhang,‡a,b Hong Liu, ‡c,f Xu Han,a Martí Biset-Peiró, b Yunhui Yang,a Inhar Imaz,a Daniel Maspoch, a,d Bo Yang, *c Joan Ramon Morante b,e and Jordi Arbiol *a,d Metal–organic frameworks (MOFs) possess high CO2 adsorption properties and are considered to be a promising candidate for the electrochemical carbon dioxide reduction reaction (eCO2RR). However, their insufficient selectivity and current density constrain their further exploration in the eCO2RR. In this work, by introducing a very small proportion of 2,5-dihydroxyterephthalic acid (DOBDC) into ZIF-8, a surface modified ZIF-8-5% catalyst was synthesized by a post-modification method, exhibiting enhanced selecti- vity (from 56% to 79%) and current density (from −4 mA cm−2 to −10 mA m−2) compared to ZIF-8. Received 19th January 2023, Density functional theory (DFT) calculations further demonstrate that the boosted eCO2RR performance Accepted 13th March 2023 on ZIF-8-5% could be attributed to the improved formation of the *COOH intermediate stemming from DOI: 10.1039/d3dt00185g successful DOBDC surface modification. This work opens a new path for improving the catalytic pro- rsc.li/dalton perties of MOFs via their surface modification. 1. Introduction in the field of eCO2RR to produce CO, as both metal ions and organic ligands could influence the catalytic performance.10–14 The electrochemical carbon dioxide reduction reaction More importantly, the inherent porous confinement properties (eCO2RR) to obtain fuels and chemical feedstocks has been of MOFs are expected to induce a local CO2 concentration considered to be a promising option to mitigate the excessive enhancement, thus, facilitating eCO2RR catalysis.15,16 Despite emissions of CO2 and to balance the global carbon cycle.1–4 these advantages, the current density for CO, when using pure Among the various CO2 RR products, carbon monoxide (CO) is MOF catalysts in the eCO2RR, is still limited (usually lower one of the most promising and economically valuable candi- than 3.4 mA cm−2),12 hindering the wide utilization of MOFs dates because it can be directly utilized as a feedstock for the in electrocatalytic reactions.14 Therefore, there is still a grand preparation of value-added chemicals and complex multi- challenge to achieve MOF-based catalysts presenting high carbon products via the well-known Fischer–Tropsch current densities while maintaining a high faradaic efficiency synthesis.5–9 Recently, metal–organic framework (MOF) (FE), both properties being essentially required for practical materials have sparked considerable interest as novel catalysts applications. Two key aspects should be primarily considered to achieve high eCO2RR performance for a catalyst: (1) increasing the a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, number of the exposed active sites and (2) promoting the mass Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain. transport of CO2 or relevant species during the catalytic E-mail: arbiol@icrea.cat b Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, reaction.14,17 To achieve these two goals, surface regulation of Sant Adrià del Besòs, Barcelona 08930, Catalonia, Spain catalysts is reported as a promising method to simultaneously c School of Physical Science and Technology, ShanghaiTech University, 393 Middle increase the number of exposed active sites and improve mass Huaxia Road, Shanghai 201210, China. E-mail: yangbo1@shanghaitech.edu.cn transport of the reactants.18–20 For example, the number of d ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain e active sites has been increased by the surface nitrogen-decora- Department of Physics, Universitat de Barcelona, 08028 Barcelona, Catalonia, Spain f State Key Laboratory of Information Functional Materials, 2020 X-Lab, ShangHai tion strategy, leading to a high formate formation rate on Institute of Microsystem and Information Technology, Chinese Academy of Sciences, surface decorated Sn.21 Moreover, due to increased mass Shanghai 200050, China. diffusion and transport, an enhanced FE of CO can be † Electronic supplementary information (ESI) available: SEM, TEM, EELS, XRD, achieved on F-doped cage-like porous carbon through engin- Raman spectra, XPS, BET, gas calibration, I–t, LSV, and FE (H2/CO), partial eering the pore size distributions at the surface of a F-doped current densities of different samples, DFT results and 1H-NMR results. See DOI: https://doi.org/10.1039/d3dt00185g carbon shell.17 Therefore, a rational chemical modification of ‡ These authors contributed equally to this work. the catalyst’s surface could be an effective strategy to obtain a 5234 | Dalton Trans., 2023, 52, 5234–5242 This journal is © The Royal Society of Chemistry 2023 View Article Online Dalton Transactions Paper high eCO2RR performance, which would not largely change 2.2 Synthesis of ZIF-8 the crystal structure of the original bulk phase.21 However, The fabrication of ZIF-8 was similar to the method in published there are few reports on enhancing the eCO2RR through regu- reports.13 Typically, 1.115 g of Zn(NO3)2·6H2O was dissolved in lating the surface of MOF catalysts. In particular, experimental 50 mL of methanol under magnetic stirring at room temperature This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. and computational insights into such surface modification– to form a homogeneous solution. Then, 50 mL of methanolic performance relationship of MOF catalysts are still rare. solution containing 1.232 g of 2-mim were added into the above Here, we demonstrate that a surface modification strategy mixture solution under ultrasonication until a clear solution was to treat ZIF-8 crystals with a polyphenolic acid such as 2,5- formed. The obtained homogeneous solution reacted at room dihydroxyterephthalic acid (DOBDC) could bring about high- temperature for 24 h without stirring. Then, the white powder efficiency eCO2RR performance. As an efficient etching and was collected by centrifugation, washed with methanol several Open Access Article. Published on 15 March 2023. Downloaded on 11/3/2023 3:30:03 PM. doping agent for MOFs, on the one hand, these acids can etch times to remove the organic residual. The final products were the surface of ZIF-8 crystals to create large surface areas and a then dried in a vacuum at 60 °C overnight. high proportion of mesopores, potentially offering a large number of surface active sites for the eCO2RR and allowing the 2.3 Synthesis of Zn-MOF-74 facile diffusion of eCO2RR-relevant species to the active The Zn-MOF-74 sample was synthesized according to previous sites.22–24 On the other hand, DOBDC can adsorb onto the published protocols with minor modification.26,27 Firstly, ZIF-8 surface, and then progressively replace partially the orig- 60 mg of Zn(NO3)2·6H2O and 20 mg of DOBDC were dissolved inal organic linkers in ZIF-8 due to its ability to coordinate in 15 mL of mixed DMF/H2O/ethanol solution (v/v/v = 1 : 1 : 1) with metals,25 thus, influencing the local environment of under ultrasonication to form a homogeneous solution. Then, active sites to facilitate *COOH generation (the key intermedi- the obtained solution was transferred into a Teflon reactor and ate for CO production).13 Both experimental and theoretical heated at 120 °C for 24 h, then cooled to room temperature, results in this work reveal that this synergistic effect could brown crystals were obtained by centrifugation, and washed promote the eCO2RR performance on surface modified ZIF-8, with DMF several times. Finally, the final products were dried realizing a dual improvement of selectivity and activity. In par- in a vacuum at 60 °C overnight. ticular, through precisely controlling the DOBDC addition, when DOBDC is used at a concentration of 5% in weight with 2.4 Synthesis of modified ZIF-8-x with the DOBDC respect to ZIF-8 (hereafter denoted as ZIF-8-5%), ZIF-8-5% modification achieves a remarkable increase of the FE of CO up to 79% at −1.20 V vs. RHE, which is higher than that of the parent ZIF-8 In this procedure, 300 mg of as-prepared ZIF-8 powder was dis- (56%). More importantly, it also shows a 2.5 times higher CO persed in 45 mL of a mixture solution containing 15 mL of current density, from −4 mA cm−2 on pure ZIF-8 to −10 mA DMF, 15 ml of ethanol and 15 mL of water under ultrasound cm−2 on ZIF-8-5%. Moreover, the selectivity can be retained for 20 min at room temperature. After forming a homogeneous over 60% in a range of working potentials from −1.0 to −1.2 V solution, DOBDC with different quantities (5 mg, 15 mg, vs. RHE, proving that CO is still the main product on ZIF-8-5% 30 mg, 50 mg and 100 mg) was added into the above solution at high overpotentials. Theoretical analyses further demon- under ultrasound for 5 min at room temperature. The weights strate that ZIF-8-5% could reduce the reaction energy for of DOBDC added correspond to 1.7%, 5%, 10%, 17% and 33% *COOH intermediate formation during the eCO2RR process, vs. the ZIF-8 weight, respectively. Next, the mixed solution was thus enhancing the production efficiency of CO. We believe left in the oven at 60 °C for 7 days. After the reaction, the that this post-synthetic treatment could open a new way for powders were collected by centrifugation, washed with ethanol boosting the catalytic performance of MOF-based catalysts and DMF several times and dried in a vacuum at 60 °C over- with controllable surface modification. night. The samples have been labelled depending on the per- centage of the added DOBDC: ZIF-8-1.7%, ZIF-8-5%, ZIF-8- 10%, ZIF-8-17% and ZIF-8-33%, respectively. 2. Experimental section 2.5 Synthesis of a physical mixture of ZIF-8-5% (labelled as 2.1 Chemicals ZIF-8-5%-P) If not specified, all chemical reagents were purchased from For the preparation of the ZIF-8-5%-P sample, 15 mg of Sigma-Aldrich. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), DOBDC powder were directly added into 300 mg of ZIF-8 2-methylimidazole (2-mim), methanol, 2,5-dihydroxyterephtha- powder and mixed well. lic acid (DOBDC), N,N-dimethylformamide (DMF), ethanol and sodium bicarbonate (NaHCO3) were all of analytical grade and 2.6 Preparation of working electrodes used as received without further purification. Meanwhile, all 10 mg of the different synthesized samples and 50 μl of 5 wt% solutions were prepared with Milli-Q water (DI-H2O, Ricca Nafion solutions were dissolved in ethanol (1 mL) and ultraso- Chemical, ASTM Type I). Nafion (N-117 membrane, 0.18 mm nicated for 1 h to form even suspensions for further electro- thick) was purchased from Alfa Aesar and kept in 0.5 M NaOH chemical experiments. To prepare the working electrode, solution. Carbon paper was also purchased from Alfa Aesar. 500 µL of the above as-prepared inks were dropped onto the This journal is © The Royal Society of Chemistry 2023 Dalton Trans., 2023, 52, 5234–5242 | 5235 View Article Online Paper Dalton Transactions two sides of the carbon paper electrode with 1 × 1 cm2 and (FE-SEM) images were collected on a FEI Magellan 400 L scan- then dried at room temperature for a few minutes, giving a ning electron microscope. Transmission electron microscopy catalyst total loading mass of ∼2.5 mg cm−2. (TEM) and high angle annular dark field scanning TEM (HAADF-STEM) images were obtained in a Tecnai F20 field This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 2.7 Electrochemical measurements and calculations emission gun microscope with a 0.19 nm point-to-point resolu- The electrocatalytic performance of different catalysts was tion at 200 kV equipped with an embedded Quantum Gatan measured at room temperature using a gas-tight H-cell with two- image filter for EELS analyses. Images have been analyzed by compartments separated by a Nafion N-117 membrane with a means of the Gatan Digital Micrograph software. X-ray photo- continuous Ar or CO2 gas injection. Each compartment con- electron spectroscopy (XPS) was performed on a Phoibos 150 tained 70 mL of electrolyte (0.5 M NaHCO3 made from deionized analyzer (SPECS GmbH, Berlin, Germany) under ultra-high vacuum conditions (base pressure 4 × 10−10 mbar) with a Open Access Article. Published on 15 March 2023. Downloaded on 11/3/2023 3:30:03 PM. water). In a typical experiment, a standard three electrode setup in 0.5 M NaHCO3 solution was assembled: an Ag/AgCl electrode monochromatic aluminum Kα X-ray source. Binding energies was used as the reference electrode, a Pt sheet as the auxiliary (BE) were determined using the C 1s peak at 284.5 eV as a electrode and carbon paper modified with the different samples charge reference. Raman spectra were obtained using Senterra. as the working electrode (with a total surface area = 1 cm2). The Brunauer–Emmett–Teller (BET) surface areas were measured potentials were measured versus Ag/AgCl and converted to the using nitrogen adsorption at 77 K (TriStar II reversible hydrogen electrode (RHE) according to the following 3020-Micromeritics). Proton nuclear magnetic resonance equation: ERHE ¼ EAg=AgCl ° þ EAg=AgCl þ 0:059  pH, pH = 7.28,29 (1H-NMR) was conducted in a Bruker Advance III 400 MHz. All the electrochemical results are shown without iR-compen- sation and obtained using a computer-controlled BioLogic VMP3 2.9 DFT calculations electrochemical workstation. DFT calculations were performed using the Vienna ab initio Before the electrochemical CO2 reduction experiments, an simulation package (VASP) code31 with the projector augmented average rate of 20 mL min−1 Ar was injected to form an Ar-satu- wave (PAW) method.32–35 Generalized gradient approximation rated solution. During electrochemical CO2 reduction experi- (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange–corre- ments, CO2 gas was delivered at the same rate at room temp- lation function was used to set the plane wave basis.36,37 The erature and ambient pressure, measured downstream by a energy cutoff was 500 eV and all structures were allowed to relax. volumetric digital flowmeter. The gas phase composition was The force convergence criteria on each configuration were below analyzed by gas chromatography (GC) during potentiostatic 0.05 eV Å−1. In order to avoid interactions between molecules in measurements every 20 min using a 490 Micro GC (Agilent the periodic structures, we placed the model in a 20 Å × 20 Å × Technologies). The calibration of peak area vs. gas concen- 20 Å cell. 1 × 1 × 1 k-point grids with the Monkhorst–Pack tration was used for the molar quantification of each gaseous scheme were used for all systems. The DFT-D3 method with effluent. The liquid products were analyzed afterwards by Becke–Jonson damping was utilised to include van der Waals quantitative 1H-NMR using water as the deuterated solvent. interactions between molecules.38,39 Moreover, the MOF sample was digested by HF solution over- Molecular orbital (MO) analyses were performed with the PBE night to completely destroy the structure. The 1H-NMR of the function and dgdzvp2 basis sets in the Gaussian 09 program.40 sample was conducted using MeOD as the solvent. The ZIF-8 model was generated with four 2-mim ligands coordi- Details of the calculation are shown below. nating with the central Zn2+ to form a tetrahedral configuration, The partial current density for a given gas product was cal- which is according to the approach reported previously.13 The culated as given below:30 ZIF-8-5% structure was built with one DOBDC replacing two ni FP0 2-mim of ZIF-8. The free energy of the reaction at each elemen- ji ¼ xi  V   ðelectrode areaÞ1 RT tary step was estimated using the computational hydrogen elec- trode model,41 and the following elementary steps were con- where xi is the volume fraction of a certain product determined sidered for the electrochemical CO2RR to CO: by online GC referenced to calibration curves from three stan- dard gas samples, V is the flow rate, ni is the number of elec- CO2 ðgÞ þ  þ Hþ ðaqÞ þ e $ COOH trons involved, P0 = 101.3 kPa, F is the Faraday constant, T is temperature and R is the gas constant. The corresponding FE COOH  þHþ ðaqÞ þ e $ CO  þH2 OðlÞ at each potential is calculated by CO $ COðgÞ þ  ji FE ¼  100%: where *, COOH* and CO* represent the free site and adsorption j state of COOH and CO, respectively. The free energies of the reac- tion were calculated according to the following formula: 2.8 Characterization G ¼ EDFT þ EZPE  TS þ Esol X-ray diffraction patterns (XRD) were obtained using a Bruker D4 X-ray powder diffractometer using Cu Kα radiation where EDFT is the DFT calculated total energy, EZPE represents the (1.54184 Å). Field emission scanning electron microscopy zero-point energy, and S is the entropy. Esol is the solvation cor- 5236 | Dalton Trans., 2023, 52, 5234–5242 This journal is © The Royal Society of Chemistry 2023 View Article Online Dalton Transactions Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 15 March 2023. Downloaded on 11/3/2023 3:30:03 PM. Fig. 1 (a) Schematic illustration of the formation of ZIF-8-x samples. (b and c) FE-SEM images, (d) BF TEM, (e) HAADF STEM and EELS chemical composition maps of ZIF-8. (f and g) FE-SEM images, (h) BF TEM, (i) HAADF STEM and EELS chemical composition maps of ZIF-8-5%. Individual Zn L2,3-edges at 1020 eV (red), N K-edges at 401 eV (green), O K-edges at 532 eV (blue) and C K-edges at 285 eV (grey) as well as their composites. rection and the values used for CO* was −0.1 eV and −0.25 eV g and Fig. S1.† As shown in Fig. S1a† and Fig. 1f, g, the as-syn- for COOH*.42 thesized ZIF-8-1.7% and ZIF-8-5% samples still inherit the original rhombic dodecahedral morphology, which is similar to the one shown by the pure ZIF-8 structures, suggesting that 3. Results and discussion there is not an apparent morphology damage of these crystals after exposing them to a small proportion of DOBDC. With 3.1 Structural characterization increasing ratios of DOBDC, the ZIF-8-10% sample displays an A schematic illustration of the synthesis route followed to irregular spherical shape (Fig. S1b†), indicating the surface obtain the corresponding modified ZIF-8-x samples is shown corrosion caused by the large concentration of DOBDC. in Fig. 1a. ZIF-8 is initially synthesized based on previous lit- However, ZIF-8-17% and ZIF-8-33% samples are composed by erature.13 In a second step, the corresponding ZIF-8-x (where x some larger bulks and small spherical particles. The formation represents the weight percentage of DOBDC with respect to of these bulky structures could be attributed to the coordi- ZIF-8) are prepared by incubating a dispersion of the pristine nation between excess DOBDC and liberated Zn(II) ions ZIF-8 crystals and different amounts of DOBDC in a mixture of accompanied by further etching/dissolution of the sacrificial DMF, ethanol and water. Following this synthetic protocol, five ZIF-8, forming other crystalline species. The chemical compo- different samples denoted as ZIF-8-1.7%, ZIF-8-5%, ZIF-8-10%, sitions of ZIF-8 and ZIF-8-5% crystals were further investigated ZIF-8-17% and ZIF-8-33% are prepared. by high angle annular dark filed scanning transmission elec- The surface morphology and elementary composition of tron microscopy (HAADF STEM) and electron energy loss spec- the as-prepared samples are revealed by field emission scan- troscopy (EELS) elemental maps. STEM EELS compositional ning electron microscopy (FE-SEM) and transmission electron maps show the homogeneous distribution of Zn, N and C in microscopy (TEM). As revealed by FE-SEM (Fig. 1b and c), the ZIF-8 and ZIF-8-5% crystals (Fig. 1e and i). It is worth noting prepared ZIF-8 shows homogeneous crystals with the charac- that the presence of the element O in the ZIF-8-5% crystal indi- teristic rhombic dodecahedral morphology. In addition, TEM cates the successful doping DOBDC into ZIF-8. analyses show that the ZIF-8 crystals have a size in the range of These samples were further investigated by X-ray powder 80–200 nm (Fig. 1d). After DOBDC doping modification, diffraction (XRD). Indeed, the XRD patterns of ZIF-8-1.7%, various morphologies of ZIF-8-x samples are shown in Fig. 1f, ZIF-8-5% and ZIF-8-10% suggest similar crystal patterns to that This journal is © The Royal Society of Chemistry 2023 Dalton Trans., 2023, 52, 5234–5242 | 5237 View Article Online Paper Dalton Transactions of standard ZIF-8,13 indicating that the addition of a low con- two main peaks in both samples at around 1020 eV and 1044 centration (up to 10%) of DOBDC does not significantly eV (Fig. 2d), corresponding to Zn 2p3/2 and Zn 2p1/2, respect- change the crystal structure of ZIF-8 (Fig. 2a and Fig. S2†). ively, which indicates the presence of Zn2+ in both samples.47 However, the XRD patterns of ZIF-8-17% and ZIF-8-33% show However, for the Zn 2p XPS spectrum of ZIF-8, a slight shift This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. the appearance of some additional diffraction peaks, demon- toward the low binding energy for the Zn 2p3/2 and Zn 2p1/2 strating that introducing a high concentration of DOBDC peaks of ZIF-8-5% is ascribed to the introduced DOBDC ligand could change the inner bulk phase of ZIF-8, which are in with electron-donating ability.48 The N 1s spectra for ZIF-8 and accordance with the results of SEM. To determine the surface ZIF-8-5% are shown in Fig. 2e, which reveals that both samples area of different samples, Brunauer–Emmett–Teller (BET) mainly show three major peaks corresponding to –NH–, Zn–N measurements were performed. As shown in Fig. 2b, the and –Nv at 398.0 eV, 399.2 eV and 400.3 eV, respectively.13 measured surface area (1403 m2 g−1) of ZIF-8 is fully consistent Open Access Article. Published on 15 March 2023. Downloaded on 11/3/2023 3:30:03 PM. The O 1s XPS core level spectra for ZIF-8-5% can be deconvo- with the previously reported value.43 Obviously, when DOBDC luted into three peaks at around at 532.8, 531.7 and 530.1 eV, is introduced at a low concentration (up to 10%), all the modi- which would be related to C–O, CvO and Zn–O, respectively fied ZIF-8-x show an increased surface area compared to the (Fig. 2f ).49 These results support the successful introduction of parent ZIF-8. In addition, the porosity of the ZIF-8 and ZIF-8- DOBDC into the ZIF-8 crystal surface. In addition, as shown in 5% samples was investigated by N2 adsorption–desorption Fig. S4,† red-shifts at ca. 643.5, 1147.2, 1188, 1461.5, 1503 and measurement, as shown in Fig. S3.† Both ZIF-8 and ZIF-8-5% 1510.7 cm−1, are observed in the high-resolution Raman peaks show a typical type I adsorption/desorption isotherm curve in on ZIF-8 after surface modification by DOBDC doping. These Fig. S3a,† indicating that both samples have numerous shifts are attributed to the tiny torsion and stretching of the micropores.44,45 Notably, the existence of a higher proportion different bonds, revealing that the coordinated environment of of mesopores observed on ZIF-8-5% (Fig. S3b†) could facilitate ZIF-8 is influenced by the introduced DOBDC ligand.50 mass diffusion and transport of CO2(aq) or the eCO2RR-rele- Meanwhile, Fourier transform infrared spectrophotometry vant species in solution.17,24 The results proves that DOBDC (FT-IR) of ZIF-8-5%, as shown in Fig. S5,† proves the formation can be used to tailor the surface of ZIF-8 to provide a large of Zn–DOBDC coordination by the negligible peak from number of surface active sites for the eCO2RR and enhance the characteristic O–H vibration peaks in ZIF-8-5% compared to mass diffusion.46 DOBDC.25 Furthermore, successful introduction of DOBDC Furthermore, the chemical valence state and surface com- into ZIF-8 with a concentration of 5% has been directly investi- position of the ZIF-8 and ZIF-8-5% samples have been proved gated by 1H-NMR. We digested the ZIF-8-5% sample and ana- by X-ray photoelectron spectroscopy (XPS). The full survey scan lysed the resulting solutions by 1H-NMR. As shown in Fig. S6,† XPS spectrum shown in Fig. 2c indicates the presence of C, N, the ZIF-8-5% sample shows a small peak at 7.41 ppm attribu- O and Zn in both ZIF-8 and ZIF-8-5% samples. Remarkably, ted to the functional group of DOBDC, thus corroborating the the O signal in ZIF-8-5% is significantly larger than in ZIF-8. successful introduction of DOBDC into ZIF-8-5%. We then cal- The high-resolution XPS spectra obtained on the Zn 2p shows culated the DOBDC : 2-mim ratio for ZIF-8-5% by a comparison Fig. 2 (a) Simulated data from crystal structure and experimental XRD patterns of ZIF-8 and ZIF-8-5%. (b) The BET surface area of different samples. (c) XPS survey spectra, (d) high-resolution XPS spectra of Zn 2p, and (e) high resolution XPS spectra of N 1s on ZIF-8 and ZIF-8-5%. (f ) High resolution XPS spectra of O 1s of ZIF-8-5%. 5238 | Dalton Trans., 2023, 52, 5234–5242 This journal is © The Royal Society of Chemistry 2023 View Article Online Dalton Transactions Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 15 March 2023. Downloaded on 11/3/2023 3:30:03 PM. Fig. 3 (a) Total current densities, (b) corresponding FE for CO, (c) current densities of CO and (d) corresponding FE for H2 on ZIF-8 and ZIF-8-5% coated on carbon paper in CO2-saturated 0.5 M NaHCO3 solution. (e) Current density vs. time (I–t ) curve for ZIF-8-5% modified carbon paper at −1.20 V vs. RHE and (f ) XRD pattern of ZIF-8-5% coated on the carbon paper before and after the stability test. of the integration of the peak at 7.41 ppm corresponding to shown in Fig. 3b, the parent ZIF-8 exhibits lower FE(CO) values DOBDC and the peak at 7.17 ppm corresponding to 2-mim, similar to previous reports.11 The FE(CO) for the ZIF-8-5% indicating a DOBDC : 2-mim ratio of 0.02. All these results sample at each applied potential is higher than that of ZIF-8, suggest that the surface modification with DOBDC was suc- with the highest FE(CO) of 79% at −1.20 V vs. RHE. In cessful on ZIF-8-5%, which not only increases the surface area addition, according to Fig. 3c, the corresponding partial of the parent ZIF-8, but also successfully forms the Zn–DOBDC current densities of CO obtained on the ZIF-8-5% sample at all coordination. applied potentials are higher than those of ZIF-8, which reveals its higher reaction rate during the eCO2RR.7 3.2 eCO2RR Furthermore, the electrochemical active surface area (ECSA) The electrocatalytic activity of ZIF-8 and ZIF-8-5% coated on was measured to confirm the high activity of ZIF-8-5%. It is carbon paper with the same mass loading of 2.5 mg cm−2 was established that an increase of the electrochemical active studied in Ar or CO2-saturated 0.5 M NaHCO3 solution as the surface area often leads to the enhancement of catalytic electrolyte using a three-electrode H-cell. Before the eCO2RR activity.28,52 The ECSA can be calculated by the electrochemical electrochemical tests, the prepared electrodes were pretreated double-layer capacitance (Cdl ) of active materials, which is gen- at a constant potential of −0.90 V vs. RHE for 30 min until a erally proportional to their ECSA (ECSA = Cdl/Cs, Cs is specific stable current was reached (Fig. S7a and b†). To roughly evalu- capacitance). Therefore, the Cdl of ZIF-8 and ZIF-8-5% was ate the electroreduction ability of ZIF-8 and ZIF-8-5% samples, probed by cyclic voltammogram (Fig. S8†). By plotting the ΔJ = linear sweep voltammetry (LSV) curves under an Ar- and a CO2- Ja − Jc against the scan rate, the slope which is twice of Cdl can saturated atmosphere were obtained (Fig. S7c and d†). Under be obtained. As shown in Fig. S8c,† the Cdl of surface modified CO2 purging, an enhanced current density could be observed ZIF-8-5% is significantly larger than that of ZIF-8, indicating on ZIF-8 and ZIF-8-5%, which was higher than that observed that ZIF-8-5% could provide more active sites to contact with in an Ar-saturated solution, which confirmed the efficient cata- the reactant to accelerate the electrocatalytic CO2 RR.52 The lytic performance of the ZIF-8 and ZIF-8-5% samples. operating stability of a catalyst is a significant parameter for its Meanwhile, ZIF-8-5% shows a higher current density in CO2- practical applications. In this manner, as displayed in Fig. 3e, saturated solution than ZIF-8, delivering a higher activity the I–t curve for ZIF-8-5% is obtained, showing a negligible toward the eCO2RR. Fig. 3a shows the total current density decay of the current density (from −9.7 to −10 mA cm−2) and plotted against the applied potential of both samples. The the FE(CO) (from 79 to 70%) during continuous electrolysis current density of the ZIF-8-5% sample increases to −13 mA under a CO2-sturated solution at −1.20 V vs. RHE for 12 h, cm−2 as the applied potential shifted negatively; however, a indicating good stability of the prepared ZIF-8-5%, which was lower current density is observed at all applied potentials in further confirmed by XRD measurement before and after the the case of the ZIF-8 sample. The high current density I–t test (Fig. 3f ), in which no appreciable difference in the achieved by ZIF-8-5% in comparison with that of the pristine crystal structure was observed after 12 h of stability test in 0.5 ZIF-8 structure could be attributed to the increased active-site M NaHCO3, proving that the main structure of ZIF-8-5% had density caused by surface modification by DOBDC.46,51 As been retained. Meanwhile, 1H-NMR analyses are performed to This journal is © The Royal Society of Chemistry 2023 Dalton Trans., 2023, 52, 5234–5242 | 5239 View Article Online Paper Dalton Transactions detect the absence of any liquid byproducts after the stability optimal adsorption configurations of reaction intermediates are measurement, revealing that there is no liquid byproduct pro- presented in Fig. S14 and 15.† The Gibbs free energy profiles of duced during the CO2 RR process (Fig. S9†). The above results CO2 reduction to CO at 0 V and −1.0 V vs. RHE are shown in strongly support that after the rational surface modification Fig. 4e and f. As indicated in these two profiles, CO2 activation to This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. using DOBDC ZIF-8-5% could achieve both high current form COOH* is endergonic at 0 V vs. RHE on the two catalysts density and FE(CO) at high applied potentials. studied while the subsequent steps of CO formation and desorp- The FE for CO and H2 on different ZIF-8-x samples are tion are exergonic. Therefore, COOH* formation is the potential- shown in Fig. S10† for comparison. Apparently, other samples limiting reaction step in the electrocatalytic reduction of CO2. In show lower FE(CO) than that of ZIF-8-5%. Such decreased addition, the Gibbs energy of the COOH* formation reaction of selectivity is attributed to the concentration of DOBDC intro- ZIF-8 is 0.4 eV more positive than that of ZIF-8-5% at both 0 V and −1.0 V vs. RHE, strongly suggesting that ZIF-8-5% is much Open Access Article. Published on 15 March 2023. Downloaded on 11/3/2023 3:30:03 PM. duced. The FE (CO and H2) values of ZIF-8-1.7% at applied potentials are similar to that of ZIF-8 (Fig. S11†), although this more active than ZIF-8 towards electrocatalytic reduction of CO2 sample shows an increased surface area compared to that of to CO, which is in good agreement with the experimental results ZIF-8. In order to reveal this phenomenon, we also digested shown in the current work. ZIF-8-1.7% and analysed the resulting solutions by 1H-NMR, Further molecular orbital (MO) analysis is performed to eluci- as shown in Fig. S12.† There is no signal from the functional date the origin of distinct behaviors of ZIF-8 and ZIF-8-5% for the group of DOBDC observed in the ZIF-8-1.7% sample. formation of COOH*. The HOMO and LUMO energy levels of Therefore, its FE(CO) is limited by deficient DOBDC content, ZIF-8 and ZIF-8-5% are shown in Fig. 4g. Interestingly, ZIF-8 and leading to only the etching process occurring on the surface of ZIF-8-5% exhibit a similar level of HOMO energy. The difference ZIF-8-1.7%, while there is negligible Zn–DOBDC coordination lies in the LUMO energy wherein ZIF-8-5% shows a much lower formation because of deficient DOBDC content. However, for LUMO energy (−3.90 eV) than ZIF-8 (−0.50 eV), forming a nar- ratios above 17%, the significant decrease in their catalytic rower HOMO–LUMO energy gap for ZIF-8-5%. It is well estab- activities is due to the new crystalline species formed which lished that a narrow HOMO–LUMO energy gap is beneficial for tend to cover up the active sites and even further change the the charge transfer process, thus making electrocatalytic CO2 RR active sites of catalysts, as found in our SEM and XRD results. more efficient as observed in recent works.53–55 In addition, we further investigated the FE(CO) of a ZIF-8-5%-P Moreover, in order to further confirm that the increase of sample prepared by a physical mixture of ZIF-8 and DOBDC. catalytic activity comes from the in situ DOBDC doping modifi- As observed in Fig. S13a,† the ZIF-8-5%-P shows poor CO cation instead of the formation of new crystals of Zn-MOF-74 selectivity, lower than 40% FE(CO) at all applied potentials. assembled by DOBDC and liberating Zn2+ on the ZIF-8 surface, Therefore, we conclude that the enhanced eCO2RR activity on a pure Zn-MOF-74 sample was prepared using the DOBDC ZIF-8-5% is mainly influenced by the rational DOBDC surface ligand and Zn2+ salts (Fig. S16†). Zn–MOF-74 was also tested modification, which changes the coordinated environment of for the eCO2RR and exhibited poor performance for generating ZIF-8, thus, boosting the catalytic performance. CO (below 20%) at each applied potential (Fig. S17†). In DFT calculations were performed to reveal the origin of the addition, a control experiment was performed to verify that the excellent activity obtained on ZIF-8-5% upon rational DOBDC obtained CO comes from the reduction of CO2 on ZIF-8-5%, surface modification. The catalyst models of ZIF-8 and ZIF-8- instead of the DOBDC ligand decomposition, as shown in 5% used in the simulations are shown in Fig. 4a–d, and the Fig. S18.† Fig. 4 The clean surface and optimized adsorption configuration on (a and b) ZIF-8 and (c and d) ZIF-8-5% surface model. (e and f ) Free energy profiles for CO2RR over ZIF-8 and ZIF-8-5% at 0 V and −1.0 V vs. RHE; (g) the HOMO and LUMO energy level of ZIF-8 and ZIF-8-5% models (Zn, C, N, O atoms are represented in purple, grey, blue, and red, respectively). 5240 | Dalton Trans., 2023, 52, 5234–5242 This journal is © The Royal Society of Chemistry 2023 View Article Online Dalton Transactions Paper 4. Conclusions 4 T. Zhang, X. Han, H. Liu, M. Biset-Peiró, X. Zhang, P. Tan, P. Tang, B. Yang, L. Zheng, J. R. Morante and J. Arbiol, In summary, we systematically explored the surface modification Energy Environ. Sci., 2021, 14, 4847–4857. strategy of ZIF-8 with DOBDC and found that it indeed influences 5 D. M. Koshy, S. Chen, D. U. Lee, M. B. Stevens, This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. the activity and selectivity towards CO production. In the case of A. M. Abdellah, S. M. Dull, G. Chen, D. Nordlund, A. Gallo, the optimized ZIF-8-5% sample, an increase of selectivity towards C. Hahn, D. C. Higgins, Z. Bao and T. F. Jaramillo, Angew. the CO2 RR is observed. The product selectivity to CO increased Chem., Int. Ed., 2020, 59, 4043–4050. to 79%, which is higher than that of pristine ZIF-8 (56%). In 6 Q. He, D. Liu, J. H. Lee, Y. Liu, Z. Xie, S. Hwang, S. Kattel, addition, ZIF-8-5% also exhibited an enhanced CO partial L. Song and J. G. Chen, Angew. Chem., Int. Ed., 2020, 59, current density of −10 mA cm−2 at −1.20 V vs. RHE, leading to a 3033–3037. Open Access Article. Published on 15 March 2023. Downloaded on 11/3/2023 3:30:03 PM. boosted CO production rate. The DFT calculations suggest that 7 O. S. Bushuyev, P. De Luna, C. T. Dinh, L. Tao, G. Saur, DOBDC modification not only maintains the active sites of ZIF-8 J. van de Lagemaat, S. O. Kelley and E. H. Sargent, Joule, but also promotes the formation of COOH* during the eCO2RR 2018, 2, 825–832. in the newly synthesized ZIF-8-5%. The enhanced ZIF-8-5% per- 8 X. Han, T. Zhang, M. Biset-Peiró, X. Zhang, J. Li, W. Tang, formance can be attributed to a decrease of the energy gap P. Tang, J. R. Morante and J. Arbiol, ACS Appl. Mater. between the HOMO and LUMO. These results offer an efficient Interfaces, 2022, 14, 32157–32165. strategy to synthesize improved MOF-based materials towards 9 T. Zhang, X. Han, H. Liu, M. Biset-Peiró, J. Li, X. Zhang, CO2 electroreduction by surface modification. P. Tang, B. Yang, L. Zheng, J. R. Morante and J. Arbiol, Adv. Funct. Mater., 2022, 32, 2111446. 10 W. Geng, W. Chen, G. Li, X. Dong, Y. Song, W. Wei and Conflicts of interest Y. Sun, ChemSusChem, 2020, 13, 4035–4040. 11 Y. Wang, P. Hou, Z. Wang and P. Kang, ChemPhysChem, There are no conflicts to declare. 2017, 18, 3142–3147. 12 X. Jiang, H. Li, J. Xiao, D. Gao, R. Si, F. Yang, Y. Li, G. Wang and X. Bao, Nano Energy, 2018, 52, 345–350. Acknowledgements 13 S. Dou, J. Song, S. Xi, Y. Du, J. Wang, Z.-F. Huang, Z. J. Xu The authors acknowledge funding from Generalitat de and X. Wang, Angew. Chem., Int. Ed., 2019, 58, 4041–4045. Catalunya 2021 SGR 01581 and 2021 SGR 00457. T. Z., J. R. M. 14 S. Dou, X. Li and X. Wang, ACS Mater. Lett., 2020, 2, 1251– and J. A. acknowledge funding from the projects (PID2020- 1267. 116093RB-C42 and -C43) funded by MCIN/AEI/10.13039/ 15 D.-H. Nam, P. De Luna, A. Rosas-Hernández, A. Thevenon, 501100011033 and by “ERDF A way of making Europe”, by the F. Li, T. Agapie, J. C. Peters, O. Shekhah, M. Eddaoudi and “European Union”. This study was supported by MCIN with E. H. Sargent, Nat. Mater., 2020, 19, 266–276. funding from European Union NextGenerationEU (PRTR-C17. 16 C. S. Diercks, Y. Liu, K. E. Cordova and O. M. Yaghi, Nat. I1) and Generalitat de Catalunya. ICN2 is supported by the Mater., 2018, 17, 301–307. Severo Ochoa program from Spanish MCIN/AEI (grant no.: 17 W. Ni, Y. Xue, X. Zang, C. Li, H. Wang, Z. Yang and CEX2021-001214-S). ICN2 and IREC are funded by the CERCA Y.-M. Yan, ACS Nano, 2020, 14, 2014–2023. Programme/Generalitat de Catalunya. A part of the present 18 P. Chen, Y. Tong, C. Wu and Y. Xie, Acc. Chem. Res., 2018, work has been performed in the framework of Universitat 51, 2857–2866. Autònoma de Barcelona Materials Science PhD program. T. Z. 19 L. Cao and C. Wang, ACS Cent. Sci., 2020, 6, 2149–2158. has received funding from the CSC-UAB PhD scholarship 20 L. Jiao, J. Wang and H.-L. Jiang, Acc. Mater. Res., 2021, 2, program (201706180028). X. H. and Y. H. Y. thank China 327–333. Scholarship Council for scholarship support (201804910551 21 H. Cheng, S. Liu, J. Zhang, T. Zhou, N. Zhang, X.-s. Zheng, and 201806650010). Thanks to Guillaume Sauthier who W. Chu, Z. Hu, C. Wu and Y. Xie, Nano Lett., 2020, 20, carried out the XPS spectrum tests. 6097–6103. 22 X. Song, Z. Ou, X. Hu, X. Zhang, M. Lin, L. Wen and M. Li, ACS Mater. Lett., 2021, 3, 171–178. References 23 M. Hu, Y. Ju, K. Liang, T. Suma, J. 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