Applied Catalysis B: Environmental 314 (2022) 121451 Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb Molecular engineering to introduce carbonyl between nickel salophen active sites to enhance electrochemical CO2 reduction to methanol Zhifu Liang a, d, 1, Jianghao Wang b, *, 1, Pengyi Tang c, h, 1, Weiqiang Tang g, 1, Lijia Liu e, Mohsen Shakouri j, Xiang Wang d, Jordi Llorca i, Shuangliang Zhao g, k, Marc Heggen h, Rafal E. Dunin-Borkowski h, Andreu Cabot d, l, **, Hao Bin Wu f, *, Jordi Arbiol a, l, ** a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain b Institute of Zhejiang University - Quzhou, 78 Jiuhua Boulevard North, Quzhou 324000, China c State Key Laboratory of Information Functional Materials, 2020 X-Lab, Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China d Catalonia Institute for Energy Research – IREC, Sant Adrià de Besòs, Barcelona 08930, Catalonia, Spain e Department of Chemistry, Western University, 1151 Richmond Street, London, ON N6A5B7, Canada f Institute for Composites Science Innovation, (InCSI), School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China g State Key Laboratory of Chemical Engineering and School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China h Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute Forschungszentrum Jülich GmbH, 52425 Jülich, Germany i Institute of Energy Technologies, Department of Chemical Engineering and Barcelona, Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Catalonia, Spain j Canadian Light Source, Saskatoon S7N 2V3, Canada k Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China l ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain A R T I C L E I N F O A B S T R A C T Keywords: The electrochemical reduction of CO2 to methanol is a potentially cost-effective strategy to reduce the con­ Two dimensional π-d organic frameworks centration of this greenhouse gas while at the same time producing a value-added chemical. Herein, we detail a Atomically dispersed nickel highly efficient 2D nickel organic framework containing a large density of highly dispersed salophen NiN2O2 Carbonyl group active sites toward electrochemical CO2RR to methanol. By tuning the ligand environment of the salophen Electrocatalytic CO2 reduction NiN2O2, the electrocatalytic activity of the material toward CO2 reduction can be significantly improved. We Methanol prove that by introducing a carbonyl group at the ligand environment of the Ni active sites, the electrochemical CO2 reduction activity is highly promoted and its product selectivity reaches a Faradaic efficiency of 27% toward the production of methanol at − 0.9 V vs RHE. The salophen-based π-d conjugated metal-organic framework presented here thus provides the best performance toward CO2 reduction to methanol among the previously developed nickel-based electrocatalysts. 1. Introduction numerous uses and its liquid phase at ambient temperature that facili­ tates storage and transportation. Thus, numerous materials have been The electrochemical conversion of carbon dioxide (CO2) to valuable explored as CO2RR catalysts for methanol production, including metal chemicals using renewable electricity can enable the cost-effective oxides, [3] metal chalcogenides, [4–6] copper-based single atom cata­ capture and reuse of this greenhouse gas [1,2]. Hydrocarbons and al­ lysts, [7,8] small bio-inspired organic molecular catalysts with carbonyl cohols are the most desired products from the CO2 reduction reaction groups [9] two-dimensional metal organic frameworks, [10] or even (CO2RR). Among them, methanol is particularly appealing owing to its molecular-based catalysts such as cobalt phtholocyamine anchored on * Corresponding authors. ** Corresponding authors at: ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain. E-mail addresses: wjh7744@zju.edu.cn (J. Wang), acabot@irec.cat (A. Cabot), hbwu@zju.edu.cn (H.B. Wu), arbiol@icrea.cat (J. Arbiol). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apcatb.2022.121451 Received 2 January 2022; Received in revised form 18 March 2022; Accepted 25 April 2022 Available online 4 May 2022 0926-3373/© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). Z. Liang et al. Applied Catalysis B: Environmental 314 (2022) 121451 carbon nanotubes. These former catalysts have been applied for the deep 2.2. Preparation of 2,5-dihydroxyterephthalaldehyde (HBC) reduction of CO2 to methanol through domino reduction processes. Their results also exhibited that CO is an important intermediate for the 25 mL dichloromethane and 2,5-dimethoxyterephthalaldehyde (250 deep reduction of CO2 to methanol [11,12]. However, despite the mg, 1.285 mmol) were placed into 50 mL three-neck flask, then mag­ relative simplicity of the methanol molecule, the reduction of CO2 to netic stirred for 10 min. After that, 3.22 mL BBr3 (1 M in hexane) was methanol with high selectivity and stability has been demonstrated dropped to the above mixture slowly at ice bath. Keep stirring for 3 h at extremely challenging and mainly CO and formate have been reported room temperature, then5 mL MILI-Q water was dropped slowly. Then as the CO2RR products [13–16]. In order to achieve the goal of flow argon was used to remove organic solvents and the obtained yellow electro-chemical reduction of CO2 to methanol, the suitable active sites precipitate was vacuum filtrated and washed with water and acetone in the catalysts can not only reduce CO2 to CO, but also have a suitable several times to obtain a yellow solid with a yield of 75%. (1H NMR (400 binding activity of CO for deep reduction. The weak CO binding activity MHz, DMSO-d6, δ)): δ 10.33 (m, 4H), 7.25 (s, 2H) [35]. will lead to the desorption of CO, while the strong binding activity will block the active sites [11,17]. Therefore, designing advanced catalysts 2.3. Preparation of tetramino-benzenequinone (TABQ) with suitable CO binding activity is crucial and challenging for efficient electro-chemical conversion of CO2 into methanol. potassium phthalimide (15 g) and tetrachloro-p-benzoquinone (5.0 Most of the CO2RR catalysts developed in the last decades are com­ g) were put into 100 nL falsk, then acetonitrile (50 mL) were added and plex nanomaterials that present unclear structure-performance correla­ magnetically stirred for 12 h at 80 ◦ C. After cooled down to room tion and uncertain reaction mechanism, which complicates their temperature naturally, the precipitate was filtrated by vacuum, and optimization.[16,18–20]. More recently, molecular catalysts with washed with hot DMF for several times, then washed with MILIQ water fine-tuned steric, electronic, electrostatic and chelating proper­ and ethanol for several times. Then vacuum dried at 60 ◦ C for overnight. ties/abilities [21] have been engineered and optimized (e.g. through 10.0 g brown-yellow powder of tetra(phthalimido)-benzoquinone was ligand modification) for CO2RR. When supported on conductive sub­ got [36,37]. The obtained tetra(phthalimido)-benzoquinone was trans­ strates, some of these molecular catalysts, such as metalloporphyrin and ferred into a 100 mL round bottom flask, then 40.0 mL of hydrazine metallophthalocyanine, cobalt salophen complexes, [22,23] have hydrate solution was dropped slowly. After being magnetically stirring shown low overpotentials and high CO2RR conversion efficiency and and kept at 65 ◦ C for 2 h, the purple tetramino-benzoquinone (TABQ) selectivities [13,24–28]. In the last few years, two-dimensional (2D) was obtained with a yield of 26%. (IR -NH2: 3367 cm− 1, -C– – O: 1668 conductive metal-organic frameworks with tunable pore structure and cm-1, C-(C–– O)-C 1140 cm− 1) 1H NMR (400 MHz, DMSO-d6, δ): δ 4.55 functionality, and with abundant catalytic active sites, have arisen as (s, 8H) [38,39]. exceptional candidate catalysts for CO2RR [14,10,29–32]. While several reports have detailed the influence of the metal, ligand type and coor­ 2.4. Preparation of Ni-2D-O-SA and Ni-2D-SA dination number on CO2RR performance, [33,31,34] the analysis and exploitation of the strong influence that the ligand microenvironment HBC (166 mg, 1 mmol), TABQ (84 mg, 0.5 mmol), nickel acetate near the active center has on the CO2RR remain unexplored. tetrahydrate (1 mmol, 248.8 mg) and 5 mL NMP were placed into a 15 Herein, a conjugated nickel organic framework with abundant mL glass vial, The obtained mixture was sonicated for 30 min to form a carbonyl groups in the ligand near the nickel catalytic active site is homogenous dispersion. The glass vial was put into a 25 mL Teflon-lined described and explored as electrocatalyst CO2RR to methanol in aqueous stainless-steel autoclave. The autoclave was sealed and maintained at medium. The unique chemical structure and uniformly distributed cat­ 120 ◦ C for 72 h. The obtained black precipitate was filtrated and then alytic active centers in the proposed nickel organic framework, com­ washed with DMF and methanol for several times, Soxhlet extracted by bined with the high electrical conductivity of carbon nanotubes (CNT), methanol for 24 h, then dried under vacuum at 60 ◦ C for 24 h to give a results in electrocatalysts with high catalytic activities, excellent dura­ black powder with ~83% yield (C11N2O3NiH4.2 H2O, Elemental Anal­ bility and notable selectivity for CO2RR to methanol. The present work ysis, calculated: C, 43.05; H, 2.63; N, 9.13; Found: C, 42.04; H, 3.14; N, not only provides new insights into the synthesis atomically dispersed 8.36). material under mild and controllable conditions for electrochemical CO2 The preparation procedure of Ni-2D-SA is similar as Ni-2D-O-SA, the reduction, but also demonstrates a novel strategy to design effective precursor TABQ was replaced by 1,2,4,5-benzenetetraamine tetrahy­ CO2RR electrocatalysts by tuning the electronic structure through the drochloride (TAB) [40]. (C11N2O2NiH5⋅H2O elemental analysis Calcu­ modification of ligand chemical environment. lated: C, 48.24; H, 2.58; N, 10.23; Found: C, 47.34; H, 3.24; N, 9.26). 2. Experimental section 2.5. Preparation of Ni-2D-O-SA-CNT and Ni-2D-SA-CNT composites 2.1. Materials 30 mg Ni-2D-O-SA, 5 mL NMP and 70 mg pre-oxidized carbon nanotubes (CNTs) were added into a 15 mL glass vial. The obtained Dimethyl sulfoxide (DMSO), methanol (99.0%), N, N-dime­ mixture was sonicated for half an hour, and stirred at 100 ◦ C for 12 h, thylformamide (DMF) (98.0%), Dichloromethane (99.0%), salicylalde­ The obtained black composite was collected by vacuum filtration and hyde (99.0%) hydrazine hydrate (98.0%) and 1,2-diaminobenzene washed with ethanol for several times, and then vacuum dried at 60 ◦ C (98.0%) were brought from Alfa Aesar. Tetrachloro-p-benzoquinone for 24 h. (99%), N-methyl-pyrrolidone (NMP, 99.99%), potassium phthalimide Ni-2D-SA-CNT composites were synthesized by using the same pro­ (98.0%), 1,2,4,5-benzenetetraamine tetrahydrochloride (TAB), 2,5- cedure as for Ni-2D-O-SA-CNT, just the precursor of Ni-2D-SA was dimethoxyterephthalaldehyde (97.0%), Nafion (10.0%), hexane replaced by Ni-2D-O-SA. (95.0%), Potassium bicarbonate (99.7%) and boron tribromide (99.99%) were purchased from Sigma-Aldrich. Nickel acetate tetrahy­ 2.6. Preparation of model complex Ni-salophen (Ni-SA) drate (99.0%), were from Acros Organics. Acetonitrile (98.0%) and ethanol (99.5%) were purchase from Honeywell. All chemicals were First, the salophen ligand was synthesized by placing 108 mg (1 used directly. Carbon Nanotubes (CNTs) was ordered from Sailed mmol) of 1,2-diaminobenzene in 15 mL three-neck flask. Then 6 mL Technology in Shenzhen. The oxidation treatment of carbon nanotube Ethanol were added and the mixture was heated to 70 ◦ C under stirring, (CNTs) was followed in the literature [12]. after that, 2 mmol of salicylaldehyde were added to the above mixture. The mixture was refluxed for 24 h. After cooling to room temperature, 2 Z. Liang et al. Applied Catalysis B: Environmental 314 (2022) 121451 the obtained precipitates were vacuum filtrated and washed with EtOH The concentration of methanol (Cmethanol) was obtained using the for several times. Finally, the yellow powder was dried under vacuum at calibration curves shown in Fig. S13. The calibration curves were made 60 ◦ C overnight. To prepare the Ni-salophen (Ni-SA), 60 mg of Ni by measuring standard solutions of methanol. (CH3COO)2⋅4H2O, 90 mg of salophen ligand and 6 mL of MeOH were put The peaks were quantified by integrating the area. the relative peak into a 15 mL flask. The obtained mixture was heated to 60 ◦ C with area can be calculated as follows: singlet peak area at3.23ppm(methanol) Relative peak area ratio(methanol) = singlet peak area at2.6ppm(DMSO) magnetic stirring for one day under argon. The mixture was filtrated by vacuum and washed with MeOH for several times. Finally, the reference complex Ni-SA was dried under vacuum at 60 ◦ C overnight. 2.9. Density functional theory (DFT) calculations 2.7. Working electrode preparation All the data were calculated from density functional theory (DFT) by 1 mg of catalyst was dispersed in the mixture of isopropanol (475 μL) the Vienna ab initio Simulation Package (VASP) [42,43]. The general­ and Nafion (5%, 25 μL) by sonication for 1 h to obtain uniformly inks. ized gradient approximation of Pardew-Burke-Ernzerhof (PBE) method Then, the inks were dropped on the carbon paper (1 * 1 cm2) under the with van der Waals correlation was conducted to optimize the geometric infrared grill light. After dropping, the electrodes were further dried by structures [44]. The convergence criteria was 0.05 eV/Å in force and infrared grill light at least 20 min before test. 1 × 10− 5 eV in energy, while the plane wave cutoff was 500 eV. The Monkhorst-Pack mesh k-point grids was 1 × 1 × 1 for all models. All of the vacuum thicknesses were higher than 15 Å. 2.8. Electrochemical measurements The whole process of CO2 electrochemical reduction to methanol for comparison we choose consists the following steps: All electrochemical measurements were performed in a gas-tight H- type glass cell containing a Nafion 117 membrane using a Biologic VMP3 CO2 (g)→ ∗ OHCO→ ∗ HCOOH→ ∗ CHO→ ∗ CH2 O→ ∗ CH2 OH→ multichannel potentiostat workstation at room temperature. The elec­ ∗ CH3 OH→ ∗ + CH3 OH trolyte (0.1 M KHCO3) was saturated with CO2 before each experiment by bubbling CO2 for 20 min. In a typical three-electrode test system, a CO2 → ∗ COOH→ ∗ CO→ ∗ COH→ ∗ CHOH→ ∗ CH2 OH→CH3 OH graphite rod and an Ag/AgCl electrode were used as the counter elec­ trode and the reference electrode, respectively. All working electrodes Where the * and ∗OHCOrepresent free sites and the adsorption state of were dried by infrared grill light at least 20 min before test. Each fresh OHCO, respectively. The (g) represent the gas phase. sample was measured with a chronoamperometric step for 1 h at each The adsorption energy was calculated by potential. The roughness factors were determined by measuring the ΔE = Etotal − Esurface − Eadsorbents double-layer capacitance with cyclic voltammetry in the potential range of open circuit potential ± 50 mV at varying scan rates (2, 5, 10, 20, 50, where Etotal , Esurface , and Eadsorbents are the DFT-calculated energy for total 100 mV s− 1). All potentials were converted to the RHE using the rela­ system, pure surface, and pure adsorbents, respectively. tionship: E(RHE) = E(Ag/AgCl) + 0.197 + 0.059 *pH [41]. The pH of The reaction free energies of each step were calculated by the 0.1 M KHCO3 saturated with CO2 is 6.8, which is measured by a following formula: pH-meter. The potentiostat workstation was set to compensate 85% of the ohmic drop, and no further iR correction was performed. ΔG = ΔE + ΔEZPE − TΔS During electrolysis, CO2 gas (99.995%) was delivered into the cathodic compartment containing CO2-saturated electrolytes at a rate of where ΔEZPE is the zero-point energy, TΔS (T = 298.15 K) is the entropy 20 sccm and vented into online gas chromatography (GC) equipped with contribution. a combination of molecular sieve 5 A. Methane and carbon monoxide were detected by a methanizer-flame ionization detector, and the 2.10. Characterizations hydrogen was detected by a thermal conductivity detector. Every 20 min, 1 mL of gas was sampled to determine the concentration of gaseous The crystal structure of the materials was characterized by means of products. After electrolysis, the liquid products in catholyte were powder X-ray diffraction (XRD) measured in a Bruker AXS D8 Advance quantified by Nuclear Magnetic Resonance (NMR). Specifically, 500 μL X-ray diffractometer. (Cu-Kα radiation, λ = 1.5106 Å, 40 kV and 40 mA; of catholyte was taken out after 1 h of CO2 electroreduction. It was then Bruker, Germany). 13C cross-polarization with magic angle-spinning mixed with 150 μL of internal standard. The internal standard was ob­ (CP-MAS) solid-state nuclear magnetic resonance (NMR) spectra were tained by dissolving 2.5 mM Dimethyl sulfoxide (DMSO) into D2O sol­ measured on a Bruker ARX 400 MHz spectrometer. Thermogravimetric vent. After that, the mixture was then transferred into a NMR sample analysis (TGA) was performed under air and nitrogen gas at a heating tube for measurement. rate of 5 ◦ C/min using a Thermogravimetric Analyzer Q200. Scanning The FEmethanol was calculated by the following formula: electron microscopy (SEM) images were obtained in a Zeiss Auriga Field emission scanning electron microscope (FE-SEM) operating at 20 kV. Cmethanol × V × NA × 6e FEmethanol = High-resolution transmission electron microscopy (HRTEM) studies Ntotal were conducted in a FEI Tecnai F20 microscope at an operating voltage Where Cmethanol is the concentration of methanol in the electrolyte, V of 200 keV. High angle annular dark-field (HAADF)-scanning trans­ is the volume of electrolyte, NA is Avogadro’s constant, Ntotal is the total mission electron microscopy (STEM) images and elemental mapping number of electrons measured during the entire CO2RR. were measured in a spherical aberration corrected transmission electron 3 Z. Liang et al. Applied Catalysis B: Environmental 314 (2022) 121451 microscope FEI Titan G2 80–200 ChemiSTEM with four energy- hydroxyterephthalaldehyde (HBC), nickel acetate and tetramino- dispersive X-ray spectroscopy (EDX) detectors and operated at 80 and benzoquinone (TABQ) in N-methyl-2-pyrrolidone (NMP). The dark- 200 keV. X-ray photoelectron spectroscopy (XPS) data was obtained black precipitate obtained from this reaction indicated the formation with a Phoibos 150 MCD-9 detector. The Ni K-edge X-ray absorption fine of a conjugated polymer. This precipitate was vacuum filtrated, washed structure (XAFS) spectra were measured at the Canadian Light Source, with water and methanol to remove small molecular mass impurities beamline SXRMB. The samples were pressed onto a double-sided carbon and finally dried under vacuum at 60 ◦ C. A control sample, without the tape and the data was recorded in X-ray fluorescence mode. The XAFS carbonyl chemical groups (Ni-2D-SA), was prepared using the same data was processed with the Athena program [45]. Extend X-ray ab­ reaction process and conditions but replacing TABQ by 1,2,4,5-benzene­ sorption fine structure (EXAFS) was analyzed using the IFEFFIT package tetraamine tetrahydrochloride (TAB) (Fig. S1) [40]. [46] and the EXAFS fitting was performed with FEFF6L [47]. For EXAFS The Ni-2D-O-SA powder exhibited a crystalline and layered struc­ fitting, spectra were fitted in R-space, from 1.0 Å to 2.76 Å. The R-space ture, as observed by powder X-ray diffraction (XRD) analysis (Fig. S2). EXAFS signal was obtained by a variable kn-weighted Fourier transform The Ni-2D-O-SA XRD pattern was similar to that of graphite, with a (n = 1, 2, 3) of the EXAFS signal χ(k) over a k-range of 3.00 Å− 1 to diffraction peak at 26.29◦ corresponding to the (001) family planes, and 11.7 Å− 1. The coordination numbers of Ni-N and Ni-O paths were both thus indicating π-π layered stacking.[48] In contrast, the XRD pattern of fixed at 2, and the bond lengths, energy shift, and the Debye-Waller the control sample, Ni-2D-SA, displayed a much lower crystallinity. This factors were optimized to yield the best fit. Nitrogen result indicates that the introduction of the carbonyl groups enhances adsorption-desorption isotherms were recorded in a Tristar II 3020 the crystallinity of the nickel organic frameworks through π-π Micromeritics system at 77 K. The specific surface area was calculated interaction. by Brunauer–Emmett–Teller (BET) method. The Fourier transform infrared (FT-IR) spectra (Fig. S3) of the two samples displayed the footprint of C–– N at 1642 cm− 1 for Ni-2D-SA and 3. Results and discussions 1648 cm− 1 for Ni-2D-O-SA. At the same time, the characteristic peaks of the N-H stretching vibration (3367–3197 cm− 1) disappeared. These 3.1. Structure characterizations pieces of evidence demonstrated the completion of the Schiff-based re­ action and the formation of Ni-salophen structure units [40]. The vi­ As shown in Scheme 1, the targeted 2D nickel organic framework bration of the carbonyl group (C– – O) in Ni-2D-O-SA negatively shifted (Ni-2D-O-SA) with abundant carbonyl functional group was sol­ below 1600 cm− 1, where it overlapped with other vibrations. Besides, vothermally synthesized by a Schiff-based reaction between 2,5- the vibration peak for the C-(C– – O)-C bond in Ni-2D-O-SA, which is Scheme 1. Schematic representation of the synthesis of Ni-2D-O-SA (blue corresponds to nitrogen, red to oxygen, green to nickel and grey to carbon). 4 Z. Liang et al. Applied Catalysis B: Environmental 314 (2022) 121451 Fig. 1. (a)–(c) HAADF-STEM images of Ni-2D-O-SA displaying atomically dispersed nickel atoms as brighter spots in the image. (scale bar with 2 nm) (d) Low magnification HAADF-STEM image and EDS elemental maps for C (red), Ni (light blue), N (green) and O (blue) (scale bar with 500 nm). characteristic of the TABQ, shifted from 1140 cm− 1 to 1010 cm− 1 due to atomically dispersed and homogeneously distributed nickel atoms. The the coordination effect and the attraction between the layers. These energy-dispersed X-ray spectroscopy (EDS) elemental mapping shown in results demonstrated the formation of the organic framework containing Fig. 1d further demonstrated the uniform distribution of Ni, C, N, and O. the carbonyl groups [49]. The solid-state 13C CP/MAS NMR spectrum of The Brunauer-Emmett-Teller surface areas of Ni-2D-O-SA and Ni-2D- the Ni-2D-O-SA catalyst exhibits the presence of C– – O, C– – N, C– – C, with SA were calculated at 21 and 83 m2 g− 1, respectively [50]. Thus the the corresponding signals at 160 ppm, 149 ppm and 123 ppm, respec­ presence of the carbonyl functional group decreases the surface area of tively. Instead, for the Ni-2D-SA catalyst, the resonance signals of C– –N the material, which is consistent with the higher π-π driven organization and C– C appear at 150 ppm and 120 ppm, respectively (Fig. S4). – of the two dimensional Ni-2D-O-SA framework observed by XRD anal­ Ni-2D-O-SA displayed a very porous morphology, resembling that of ysis. The pore size distribution profiles (Fig. S10) show the approximate cotton when analyzed by scanning electron microscopy (SEM) porous structures of Ni-2D-SA and Ni-2D-O-SA. (Figs. S5a). High angle annular dark-field (HAADF)-aberration-cor­ rected scanning transmission electron microscopy (STEM) analysis (Fig. 1a–c and Fig. S6) demonstrated the presence of a high density of 5 Z. Liang et al. Applied Catalysis B: Environmental 314 (2022) 121451 Fig. 2. (a) Ni K-edge XANES spectrum of Ni-SA, Ni-2D-SA, Ni-2D-O-SA and NiO (commercial powder), (b) First derivative of the Ni K-edge XANES for Ni-SA, Ni-2D- SA, NiO and Ni-2D-O-SA. (c) Fourier transformed Ni K-edge EXAFS spectra of Ni-2D-O-SA and NiO plotted in R-space, Fourier transformed EXAFS spectra in R-space of Ni-2D-O-SA and fitted curve. 3.2. Fine structure of Ni-2D-O-SA and Ni-2D-SA The electrocatalytic properties of Ni-2D-O-SA-CNT toward CO2RR were tested in a two-compartment electrochemical cell containing a To further confirm the local coordination environment of Ni active CO2-saturated 0.1 M KHCO3 electrolyte. As shown in Fig. 3a, the Ni-2D- sites in Ni-2D-O-SA, XAFS analysis of the Ni K-edge was conducted O-SA-CNT catalyst displayed higher current densities in the CO2-satu­ (Fig. 2a-b). We first compared the spectral features of Ni-2D-O-SA with rated electrolyte than in Ar-saturated electrolyte at potentials more those of Ni-2D-SA, NiO, and a model complex Ni-salophen (Ni-SA) at the positive than − 0.9 V vs RHE, which indicates a clear CO2RR activity. near-edge (i.e. X-ray absorption near-edge structure, XANES). As shown When the applied potentials were more negative than − 0.9 V, the in Fig. 2a, the absorption energy of near K-edge in XANES spectrum for current density in a CO2-saturated electrolyte was lower than in the Ar- the Ni-2D-O-SA sample shifted to higher energy compared to that of Ni- saturated electrolyte due to the HER suppression through CO2 adsorp­ 2D-SA, which suggests that the introduction of carbonyl groups resulted tion. The liquid products of CO2RR was analyzed by 1H NMR (Fig. S14). in a higher oxidation state of the Ni active centers. The absorption onset Ni-2D-O-SA-CNT samples displayed much higher current densities of Ni-2D-O-SA occurs at slightly lower energy than that of NiO. Differ­ than Ni-2D-SA-CNT under the same test conditions (Fig. 3b and ences are more clearly seen when the first derivative of these spectra are Fig. S15a-b), which demonstrates that Ni-2D-O-SA-CNT has better ac­ plotted (Fig. 2b). The main absorption peak of Ni-2D-O-SA also has a tivity toward CO2RR than that of Ni-2D-SA-CNT. Given that Ni-2D-O-SA- lower intensity. These features indicate that, compared with NiO where CNT and Ni-2D-SA-CNT were characterized by similar electrochemical Ni is fully coordinated with O, Ni atoms within Ni-2D-O-SA are sur­ double-layer capacitances (Fig. S16), we can conclude that the presence rounded by less electronegative elements, i.e. N substituting O. The of the carbonyl group within Ni-2D-O-SA organic framework decisively EXAFS spectrum was analyzed to obtain more quantitative details of the increases its CO2RR activity. coordination environment. First the EXAFS spectrum of Ni-SA was fitted The CO2RR product distribution obtained from Ni-2D-O-SA-CNT and well with a NiN2O2 coordination structure (Fig. S7 and Table S1), the Ni- Ni-2D-SA-CNT electrocatalysts at different potentials was evaluated by 2D-SA was also fitted with a NiN2O2 coordination structure (Fig. S8 and gas chromatography and nuclear magnetic resonance. The Faradaic ef­ Table S2). The Fourier transformed EXAFS of Ni-2D-O-SA is shown in ficiency of all products are provided in Figs. S15c and S15d, which show Fig. 2c. The fitted curve suggests that Ni is coordinated with two N atoms that H2 is one of the CO2RR products, too. The Methanol faradaic effi­ and two O atoms, at a bond length 1.87 Å and 2.05 Å, respectively ciency was quantified by liquid NMR technology. As shown in Fig. 3c, (Fig. 2c), which matches well the salophen unit structure (NiO2N2). The Ni-2D-O-SA-CNT is characterized by a volcano-shaped Faraday effi­ contribution at longer radial distances comes from C atoms with a co­ ciency (FE) and partial current densities toward CH3OH in the range of ordination number of 6 and a bond length of 2.65 Å, which also belong applied potential from − 0.85 to − 1.05 V. The maximum CH3OH FE of to the Ni.C interaction (Table S3) and the distance of Ni.Ni interaction is 27% and jMeOH of 0.94 mA cm− 2 were achieved by Ni-2D-O-SA-CNT at of 3.168 Å which corresponds to the Ni in the layer-layer structure for − 0.9 and − 0.95 V vs. RHE, respectively. This excellent performance is this type of materials. Thus, the XAFS analysis confirmed the formation in contrast with the nickel organic framework without the carbonyl of the NiN2O2 salophen structure unit within Ni-2D-O-SA frameworks. group Ni-2D-SA-CNT (9.2% FE for CH3OH at − 0.95 V, Fig. 3d). The high To confirm the coordination structure stability of Ni-2D-O-SA, the performance of Ni-2D-O-SA-CNT is also in contrast with all previously EXAFS spectrum of Ni-2D-O-SA which was immersed in a KHCO3 developed nickel-based CO2RR catalysts, which mostly yielded CO aqueous solution for three days was fitted (Fig. S9 and Table S4). The Ni- instead of methanol (Table S5) [24,33]. To the best of our knowledge, 2D-O-SA maintained the NiN2O2 coordination structure, indicating its only one previous report demonstrated CO2RR to methanol using a structure stability in the KHCO3 aqueous solution. Ni-based electrocatalyst but with a moderate activity and selectivity (6% FE) [10]. Compared with Ni-2D-SA-CNT, in the potential range from 3.3. CO2RR activity − 0.85 to − 1 V Ni-2D-O-SA-CNT also exhibited a higher selectivity toward the production of CO, which is an intermediate in the production To promote the electrical conductivity of the nickel-based organic of methanol (Fig. 3e) [7,11,12]. frameworks toward their use as efficient electrocatalysts, they were To exclude the possibility that the formation of methanol came from supported on multi-walled CNTs by physical mixing in dimethylforma­ the preparation process or the decomposition of the catalyst, we mide. XRD patterns displayed just one broad diffraction peak at about analyzed the product distribution of the Ni-2D-O-SA-CNT sample in Ar- 26◦ (Fig. S11), which was assigned to the overlap between the CNT and saturated 0.1 M KHCO3. The obtained results showed that no methanol the nickel organic framework structures. The porous morphology and signal could be found, and that the Ni-2D-O-SA-CNT catalyst could only the homogenous loading of Ni-2D-O-SA on the CNTs within the obtained produce hydrogen in Ar-saturated 0.1 M KHCO3 (Figs. S17 and S18). composites (Ni-2D-O-SA-CNT) were confirmed by STEM images and These results demonstrate the excellent intrinsic catalytic selectivity EDS elemental mapping (Fig. S12). toward methanol of the Ni-2D-O-SA-CNT sample. 6 Z. Liang et al. Applied Catalysis B: Environmental 314 (2022) 121451 Fig. 3. Electrocatalytic CO2RR performance of Ni-2D-O-SA-CNT and Ni-2D-SA-CNT. (a) Linear sweep voltammetry (LSV) curves of Ni-2D-O-SA-CNT at a scan rate of 5 mV s− 1 in CO2- and Ar-saturated 0.1 M KHCO3 electrolytes. (b) LSV curves of Ni-2D-O-SA-CNT and Ni-2D-SA-CNT in CO2-saturated 0.1 M KHCO3 electrolyte. (c,d) FEs and current densities of Ni-2D-O-SA-CNT (c) and Ni-2D-SA-CNT (d) toward CH3OH at various applied potentials, from − 0.85 to − 1.05 V. (e) CO2 to CO FE of Ni- 2D-O-SA-CNT and Ni-2D-SA-CNT at various applied potentials from − 0.85 to − 1.05 V vs RHE. (f) Current-time (i-t) responses and corresponding FEs of Ni-2D-O-SA- CNT at − 0.9 V vs RHE for 5.5 h. The catalytic stability of Ni-2D-O-SA-CNT was evaluated at − 0.85 V maintained at 855.5 eV after the CO2RR test, indicating that the active vs RHE (Fig. 3f). We observed the current density to be maintained sites for methanol formation can still be Ni species with 2+ valence. In during an operating period of 5.5 h. Although the selectivity of meth­ addition, there is no presence of metallic nickel in the XPS spectra. The anol is decreased after a relatively long electrochemical test, the Ni-2D- FT-IR spectra showed that the carbonyl groups of Ni-2D-O-SA-CNT were O-SA-CNT catalyst could still maintain ~10% FE toward methanol. still maintained after the long-term stability test, implying that the To understand which are the real active sites for the formation of carbonyl groups were not lost (Fig. S20b). To further demonstrate the methanol and the reason of deactivation after the long-term stability effect of the Ni sites on the formation of methanol, we also evaluated the test, we analyzed the post reaction sample by XRD, FT-IR, and XPS. From CO2RR performance of the 2D-O-SA-CNT catalyst (without nickel) at the XRD patterns (Fig. S20a), the characteristic peaks of Ni-2D-O-SA-CNT same potentials. The results show that almost all products obtained by are well maintained after CO2RR test, and no presence of other peaks 2D-O-SA-CNT are hydrogen instead of CO2RR related products was observed, suggesting that no Ni-based metal or metal oxide nano­ (Fig. S22), demonstrating the importance of the Ni active sites for the particles were formed. XPS was further conducted to investigate the formation of methanol. Moreover, the selectivity of methanol decreased electronic structure of Ni after CO2RR (Fig. S21). We found that only a with the decay of NiN2O2, implying a positive correlation between the small Ni2p3/2 signal remained after the CO2RR test, indicating a partial selectivity of methanol and the amount of NiN2O2. Therefore, the re­ decomposition of the NiN2O2 sites, which would be responsible for the sidual NiN2O2 sites still found in the sample after the 5 h test were deactivation of our catalyst. This residual Ni2p3/2 signal is still responsible for the still kept methanol selectivity. The degradation of 7 Z. Liang et al. Applied Catalysis B: Environmental 314 (2022) 121451 methanol selectivity and Ni XPS signals is because the Ni-2D-O-SA that the Ni-2D-O-SA-CNT electrode resulted in a very notable enhancement are not directly loaded on carbon nanotubes (CNTs) are less stable of the FEs for CO2 reduction to both methanol and CO. Thus, the pres­ compared to those directly attached to the CNTs through π-π stacking ence of the carbonyl group clearly favors the electroreduction of CO2 to interactions. Namely, those nanolayers directly attached to the CNT CO, which can be considered as a first step in the formation of methanol. surface, highly improve their physical and chemical robustness [26]. But at the same time, this carbonyl group promotes a deepen electro­ Normally, molecular catalysts can be anchored on the surface of the chemical reduction, which we tentatively attribute to the stronger π-d carbon nanotube through the π-π stacking interaction, which facilitates conjugation that enhances the adsorption of the intermediates generated the electron transfer and prevent gathering of the active sites to improve in the electrochemical reduction process of CO2 to methanol over the the physical and chemical stability [33]. Unlike the small molecular nickel active centers [12,51]. The strong electronegativity of the catalysts, Ni-2D-O-SA is a two-dimensional organic material, which can carbonyl group could shift the electron cloud in nickel active centers also promote the π-π stacking interaction between its layers and the (NiN2O2), resulting in a more positive valence of the Ni sites in CNTs. Therefore, it is hard to guarantee that all these Ni-2D-O-SA 2D Ni-2D-O-SA, and thus promote the conversion of CO2 into CO, [52]. This nanostructures are loaded on the surface of CNTs with their single layers higher valence has been reported to promote a deeper CO2 electro­ promoting the π-π stacking interaction. In this way, those Ni-2D-O-SA reduction in different metals [53]. nanolayers that are not directly loaded on the surface of a CNT would not present any π-π stacking interaction and thus, they would not be as 3.4. Mechanic study stable during the CO2RR process, resulting in the decomposition of these Ni-2D-O-SA nanolayers and the corresponding degradation of the cata­ To further understand the effect of the carbonyl group, the differ­ lytic performance. ential charge distribution was estimated by density functional theory Both electrocatalysts, Ni-2D-O-SA-CNT and Ni-2D-SA-CNT yielded (DFT) calculations. The electron density difference between of Ni-2D-SA methanol with a significant FE, which indicates that the porous struc­ and Ni-2D-O-SA (Fig. 4a) illustrates that the charge transfer from Ni ion ture, the high densities of nickel active centers, and possibly the short to ligand appears, which is due to the inductive effect of carbonyl group distance between the catalytic active sites are highly suitable for the in Ni-2D-O-SA. This results are consistent to the XANES analysis, which electroreduction process of CO2 to methanol. Furthermore, the presence show that the Ni center in Ni-2D-O-SA has a higher oxidation state of the carbonyl group in the ligand between the nickel active centers in compared to that of Ni-2D-SA. The absolute free energy of H* on Ni-2D- Fig. 4. (a) Plots of electron density difference for selected segments of Ni-2D-O-SA and Ni-2D-SA. The yellow color corresponds to an isosurface of 0.00120794 e Bohr− 3 and blue of − 0.00120794 e Bohr− 3. (b) Free energy diagram of *CO to CH3OH on selected segments of Ni-2D-O-SA. 8 Z. Liang et al. Applied Catalysis B: Environmental 314 (2022) 121451 O-SA is larger than that of Ni-2D-SA, suggesting that the Ni-2D-SA support was provided by Canada Foundation for Innovation. Mohsen catalyst is more favorable for hydrogen evolution reaction (HER) Shakouri reports financial support was provided by National Research (Fig. S23). Further more, the free energy diagram in the conversion from Council. Mohsen Shakouri reports financial support was provided by *CO to methanol was calculated, and the reaction pathway was pro­ Government of Saskatchewan. Weiqiang Tang reports financial support posed based on the previous report [7]. According to our DFT calcula­ was provided by Chinese Postdoctoral Science Foundation. tion results (Fig. 4b), the conversion of *CO to methanol is thermodynamically more favorable on Ni-2D-O-SA than that on Acknowledgements Ni-2D-SA. The result of the adsorption energy for intermediates (Fig. S24) further confirms that the introduction of the carbonyl group in ICN2 is supported by the Severo Ochoa program from Spanish Ni-2D-O-SA will favor the adsorption of the intermediates compared MINECO (Grant No. SEV-2017-0706) and is funded by the CERCA Pro­ with Ni-2D-SA. On the other hand, a second reaction pathway of the gramme/Generalitat de Catalunya. Part of the present work has been conversion from CO2 to methanol based on another published work [8] performed in the framework of Universitat Autònoma de Barcelona was also calculated. As exhibited in Fig. S25, the free energy in the Materials Science PhD program. Z. Liang acknowledges funding from rate-determining step of the conversion of *OHCO to absorbed *HCOOH MINECO SO-FPT PhD grant (SEV-2013-0295-17-1). This project has is lower on Ni-2D-O-SA. Therefore we can suggest that Ni-2D-O-SA with received funding from the European Union’s Horizon 2020 Research carbonyl groups would tend to favor the electrocatalytic reduction of and Innovation Programme under grant agreement No 823717- CO2 to methanol with respect to Ni-2D-SA. ESTEEM3. The present work is supported by the I+D+I projects PID2019-105490RB-C32 and NANOGEN (PID2020-116093RB-C43), 4. Conclusion funded by MCIN/AEI/10.13039/501100011033/ and by “ERDF A way of making Europe”, by the “European Union”. X. Wang thanks the China We have detailed the synthesis and characteristics of an atomically Scholarship Council for the scholarship support. P. Tang acknowledges dispersed nickel catalyst consisting of NiN2O2 active sites within a 2D the Humboldt Research Fellowship. Authors acknowledge funding from organic framework under mild and controllable reaction condition. A Generalitat de Catalunya 2017SGR327 and 2017SGR1246. L. L ac­ molecular engineering strategy based on modifying the edge ligand knowledges the support from Natural Sciences and Engineering environment by introducing a carbonyl group was proposed to tune the Research Council Canada (NSERC, DG RGPIN-2020-06675). J. Llorca is electrocatalytic reduction of CO2. The obtained Ni-O-SA-CNT compos­ a Serra Húnter Fellow and is grateful to MICINN/FEDER RTI2018- ites exhibited excellent activity and selectivity (27% FE at − 0.9 V vs 093996-B-C31, GC 2017 SGR 128 and to ICREA Academia Program. RHE) toward the conversion of CO2 to methanol due to the carbonyl XAFS measurements were performed at the Canadian Light Source, a group was introduced in the ligand environment to tune the electronic national research facility at the University of Saskatchewan, which is structure of the nickel active sites. This work not only provides the best supported by the Canada Foundation for Innovation (CFI), the Natural nickel-based catalysts so far reported for the electrocatalytic conversion Sciences and Engineering Research Council (NSERC), the National of CO2 to methanol, but also demonstrates a novel strategy to design and Research Council (NRC), the Canadian Institutes of Health Research engineer efficient electrocatalysts to convert CO2 to valuable chemicals (CIHR), the Government of Saskatchewan, and the University of Sas­ through modulating the ligand structures. katchewan. W. Tang acknowledges the Chinese Postdoctoral Science Foundation (Nos. 2021M691008).This study was supported by MCIN CRediT authorship contribution statement with funding from European Union NextGenerationEU (PRTR-C17.I1) and Generalitat de Catalunya. Zhifu Liang: equally contributed to this work, synthesized the cat­ alysts and finish the basic characterizations and wrote the paper. Appendix A. Supporting information Jianghao Wang: equally contributed to this work, performed the cat­ alytic test. Pengyi Tang: equally contributed to this work, conducted Supplementary data associated with this article can be found in the the AC-STEM analysis. Weiqiang Tang: equally contributed to this online version at doi:10.1016/j.apcatb.2022.121451. work, conducted DFT calculation. Lijia Liu: conducted the XAFS anal­ ysis. Mohsen Shakouri: conducted the XAFS analysis. Xiang Wang: References performed the SEM measurements. Jordi Llorca: conducted XPS anal­ ysis. Andreu Cabot: Funding acquisition, Project administration, Re­ [1] O.S. Bushuyev, P. De Luna, C.T. Dinh, L. Tao, G. Saur, J. van de Lagemaat, S. sources, Writing – review & editing. Hao Bin Wu: Funding acquisition, O. Kelly, E.H. Sargent, What should we make with CO2 and how can we make it? Project administration, Resources, Writing – review & editing. Jordi Joule 2 (2018) 825–832. [2] L. Zhang, Z.J. Zhao, J. Gong, Nanostructured materials for heterogeneous Arbiol: Funding acquisition, Project administration, Resources, Writing electrocatalytic CO2 reduction and their related reaction mechanisms, Angew. – review & editing. All authors discussed the results and commented on Chem. Int. Ed. 56 (2017) 11326–11353. the manuscripts. [3] T.-Y. Chang, R.-M. Liang, P.-W. Wu, J.-Y. Chen, Y.-C. Hsieh, Electrochemical reduction of CO2 by Cu2O-catalyzed carbon clothes, Mater. Lett. 63 (2009) 1001–1003. Declaration of Competing Interest [4] E. Andrews, M. Ren, F. Wang, Z. Zhang, P. Sprunger, R. Kurtz, J. Flake, Electrochemical reduction of CO2 at Cu nanocluster/(100) ZnO electrodes, J. Electrochem. Soc. 160 (2013) H841–H846. The authors declare the following financial interests/personal re­ [5] X. Sun, Q. Zhu, X. Kang, H. Liu, Q. Qian, Z. Zhang, B. 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