Cadmium-based metal-organic frameworks for high-performance electrochemical CO2 reduction to CO over wide potential range

Xin Li,Song Hong,Leiduan Hao,Zhenyu Sun,*

1 State Key Laboratory of Organic-Inorganic Composites,Beijing University of Chemical Technology,Beijing 100029,China

2 College of Materials Science and Engineering,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Electrochemical carbon dioxide reduction Carbon monoxide Cadmium Metal-organic framework Electrocatalysis

ABSTRACT Electrochemical CO2 reduction (ECR) powered by renewable energy sources provides a sustainable avenue to producing carbon–neutral fuels and chemicals.The design and development of high performance,cost-effective,and stable catalysts for ECR remain a focus of intense research.Here,we report a novel electrocatalyst,two-dimensional cadmium-based 1,4-benzenedicarboxylate metal–organic frameworks(Cd-BDC MOFs) which can effectively convert CO2 to CO with a faradaic efficiency (FE) of more than 80.0% over the voltage range between -0.9 and -1.1 V (versus reversible hydrogen electrode,vs.RHE)in 0.1 mol.L-1 CO2-saturated KHCO3 solution with an H-type cell,reaching up to 88.9% at -1.0 V (vs.RHE).The performance outperforms commercial CdO and many other MOF-based materials demonstrated in prior literature.The catalytic property can be readily tuned by manipulating synthesis conditions as well as electrolyte type.Especially,high CO FEs exceeding 90.0% can be attained on the Cd-BDC electrode at potentials ranging from-0.16 to-1.06 V(vs.RHE)in 0.5 mol.L-1 KHCO3 solution by using a gas diffusion electrode cell system.The maximum CO FE approaches～97.6%at-0.26 V(vs.RHE)and the CO partial geometric current density is as high as about 108.1 mA .cm-2 at -1.1 V (vs.RHE).This work offers an efficient,low cost,and alternative electrocatalyst for CO2 transformation.

1.Introduction

The incessant consumption of fossil fuels has led to a rapid increase of carbon emissions and CO2levels in the air,giving rise to global warming.Atmospheric CO2concentrations are estimated to have risen by over 33.0% from a pre-industrial level of～280.0 ml∙m-3to～419.0 ml∙m-3in April 2021.Scientists and engineers are therefore making efforts to develop alternative and sustainable energy technologies and processes to reduce reliance on exhaustible fossil resources.Electrochemical carbon dioxide reduction (ECR) to chemical feedstocks provides a promising approach to mitigate anthropogenic CO2emissions and also enable carbon fixation and carbon neutrality [1–11].In addition,the ECR reaction can be performed under ambient conditions,circumventing the need for sophisticated or energy intensive plant designs.However,due to the high stability and inertness of CO2,large activation energies are required for its reduction reactions [12,13].Nucleophilic attack at a carbon atom needs to bend the linear O-C-O bond.Proton coupled electron transfer reactions can obviate the formation ofand facilitate CO2reduction.From these scenarios,tremendous efforts have been devoted to searching efficient,abundant,cheap,and stable electrocatalysts to facilitate the kinetically sluggish CO2reduction process [14–16].CO is a critical feedstock for the production of synthetic fuels (>C1hydrocarbons or alcohols) by the Fischer-Tropsch process and light (C2–C4) olefins or aromatics by a modified Fischer-Tropsch synthesis.Electrocatalytic CO2reduction to CO is considered to be economically feasible given the high CO market price and large market size[17].Precious metals,such as Au [18–20],Ag [21] and Pd [22]are capable of catalyzing the ECR to yield CO with good activity.Nonetheless,the high cost and scarcity of the noble metals hinder their widespread implementation in ECR.The design and development of active,inexpensive,and earth-abundant electrocatalysts for selective CO2conversion are therefore greatly demanded.To this end,architectures encompassing highly accessible active sites with large specific surface area,abundant porosity,and remarkable charge transfer capability offer benefits for electrocatalysis.

Metal–organic frameworks (MOFs) with high surface areas and tunable pore metrics provide an attractive platform for trapping CO2[23],favoring enriched CO2at the electrode surface [24–26],which can circumvent low CO2solubility limitation during aqueous ECR.The density of open metal sites is maximized in MOFs.In addition,the pores without walls can prompt mass transfer and easy access of CO2to active sites.Especially,the linker species in MOFs can be readily tailored,which allows one to modify the adsorption behaviors of proton and CO2reduction intermediates and thereby ECR selectivity.These unique features facilitate electrochemical CO2conversion with remarkable turnover frequency[27,28].Since 2012,Cu-,Zn-,Ni-,Re-,Fe-,and Co-MOFs have been investigated as electrocatalysts for ECR [28],however,the exploration of Cd-based MOFs in CO2electrocatalysis is still lacking.

Herein,we demonstrate facile synthesis of cadmium-based 1,4-benzenedicarboxylate (Cd-BDC) MOFs nanosheets via a simple solvothermal method.The resulting two-dimensional (2D) Cd-BDC MOFs were found for the first time to effectively catalyze ECR towards CO under ambient conditions in both H-type and flow-type reactors.Exceptional CO faradaic efficiencies (FEs) as well as current densities were achieved at low overpotentials using the Cd-BDC MOFs electrode,surpassing many other MOF-based electrocatalysts reported in prior literature.

2.Materials and Methods

2.1.Materials and reagents

All chemicals used in this work were of analytical grade and used without purification.CdCl2.2.5H2O (product number:C118631,99.95%),1,4-benzenedicarboxylic acid (H2BDC,product number:P108506,99.0%),N,N-dimethylformamide(DMF,product number:D112009,≥99.9%),methanol (MeOH,product number:M116118,≥99.9%),isopropanol (IPA,product number:I112011,≥99.7%),commercial CdO (product number:C100162,99.0%),KOH (product number:P822103,90.0%),KCl (product number:P301833,99.0%),KBr (product number:P116278,99.4%),KI (product number:P116283,99.0%),KF (product number:P164507,99.0%),K2SO4(product number:P112581,98.5%),K2HPO4(product number:P112214,99.0%),and KHCO3(product number:P110485,99.5%) were purchased from Aladdin.Nafion solution (product number:42118,5.0% (mass) in water and 1-propanol),Toray carbon paper (TGP-H-060,thickness of 0.19 mm,resistivity of～5.8 mΩ.cm,porosity of 78.0%),and Nafion membranes(product number:42180.VA,0.18 mm thick,≥0.9 meq.g-1exchange capacity)were bought from Alfa Aesar.Deionized water (18.2 MΩ∙cm) was obtained from a Millipore system.Carbon dioxide gas (99.999%purity)and argon gas(99.999%purity)were both provided by Beijing Haipu Gas Co.,Ltd.

2.2.Synthesis of Cd-BDC MOFs

In a typical procedure to prepare Cd-BDC MOFs,0.75 mmol of CdCl2.2.5H2O was dissolved in a mixture containing 48.0 ml of DMF,12.0 ml of ultrapure water,and 12.0 ml of MeOH.Then 0.75 mmol of H2BDC was added into the above solution,which was subjected to magnetic stirring for 30.0 min and then transferred into a Teflon-lined stainless-steel autoclave of 100.0 ml followed by heating at 120.0 °C for 12.0 h.After cooling down naturally to room temperature,the resulting product was separated by centrifugation and washed with DMF and MeOH for several times,and then vacuum dried at 60.0 °C overnight.

2.3.Catalyst characterization

X-ray powder diffraction (XRD) was performed with a D/MAXRC diffractometer operated at 30.0 kV and 100.0 mA with CuKα radiation.Fourier Transform Infrared(FTIR)spectroscopy was conducted over Nicolet 6700 FTIR instrument.The FTIR sample was prepared by using a potassium bromide pressing tablet method.X-ray photoelectron spectroscopy (XPS) experiments were carried out using Thermo Scientific ESCALAB 250Xi instrument.The instrument was equipped with an electron flood and a scanning ion gun.All spectra were calibrated according to the C 1s binding energy at 284.8 eV.CO2adsorption isotherms were measured on a NOVA 4000e Surface Area &Pore Size analyzer at 25.0 °C.The sample was degassed at 80.0 °C for 12.0 h prior to CO2adsorption tests.Transmission electron microscopy (TEM) was carried out using a field emission microscope (JEM-2100F) operated at 200.0 kV.High-angle annular dark-field scanning TEM (HAADF-STEM) was conducted using a JEM-ARM200F microscope with a 200.0 kV accelerating voltage.TEM and STEM samples were prepared by depositing a droplet of suspension onto a Cu grid coated with a Lacey carbon film.

2.4.Electrochemical CO2 reduction tests

Cyclic voltammograms and linear scan voltammograms in Ar or CO2atmosphere were carried out in a three-electrode system using Ag/AgCl as reference electrode,Pt wire as counter electrode,and glassy carbon as working electrode on a CHI 760E potentiostat.Rotating disk electrode (RDE) experiments were run on an AFMSRCE RDE control system(Pine Inc.,USA).The catalyst loading on working electrode is 0.135 mg.cm-2.The electrolyte is 0.1 mol.L-1KHCO3solution with Ar or CO2purged for at least 30.0 min.The electrochemical impedance spectroscopy (EIS)experiments were operated in Ar-saturated 0.1 mol.L-1KHCO3solution at an open circuit potential with frequency from 106Hz to 10.0 Hz with 5.0 mV amplitude.

Controlled potential electrolysis of CO2was carried out in an Hcell system separated by a Nafion 117 membrane at room temperature and atmospheric pressure.Toray carbon fiber paper (1.0 cm×1.0 cm),Pt wire,and Ag/AgCl electrodes were employed as working electrode,counter electrode,and reference electrode,respectively.The potentials were controlled by an electrochemical working station (CHI 760E,Shanghai CH Instruments Co.,China).All potentials in this work were measured against the Ag/AgCl reference electrode(in 3.0 mol.L-1KCl solution)and finally converted to the RHE reference scale using the equation

The cathodic electrolyte was a CO2-saturated 0.1 mol.L-1KHCO3aqueous solution unless stated otherwise and anodic electrolyte was a 0.1 mol.L-1H2SO4degassed under argon.CO2with a flow rate of 10.0 ml.min-1was purged into the cathodic electrolyte for at least 30.0 min to remove residual air in the reservoir.Controlled potential electrolysis was performed at each potential for 60.0 min with continuous CO2bubbling.To manufacture the cathode,1.2 mg of a catalyst was dispersed in 240.0 μl of IPA/H2O (V/V=1:1)and 1.2 μl of 5.0%Nafion solution to obtain a homogenous suspension,which were then deposited on a Toray carbon paper working electrode to form catalyst films with a catalyst loading of 1.0 mg.cm-2.The working electrode was left to dry at room temperature for 30.0 min.

For ECR experiments in a flow cell,2.0 mg of sample and 2.0 μl of Nafion solution(5.0%(mass))were dispersed in 400.0 μl of IPA/H2O by ultrasonication to form a homogeneous ink.The ink was then deposited on a hydrophobic carbon paper working electrode to form catalyst films with a catalyst loading of 1.0 mg.cm-2.Subsequent ECR tests were carried out using an electrochemical flow cell consisting of a gas chamber,a cathodic chamber,and an anodic chamber.The working electrode was fixed between the gas chamber and the cathodic chamber,with the catalyst layer side facing the cathodic chamber.The cathode and anode compartments in the flow cell were separated by a Nafion 117 membrane.Ag/AgCl electrode and Pt foil were used as reference electrode and counter electrode,respectively.0.5 mol.L-1KHCO3solution was used as an electrolyte for both compartments and the catholyte was circulated using a peristaltic pump with the flow rate of 3.0 ml.min-1.CO2gas was supplied to the chamber located at backside of cathode at a flow rate of 20.0 ml.min-1.

2.5.Product analysis and calculation of electrochemical CO2 reduction reaction

The ECR gas-phase products were analyzed using an Agilent 7890B gas chromatography (GC) with two thermal conductivity detectors(TCD)and one flame ionization detector(FID).The liquid products were examined by1H NMR(proton nuclear magnetic resonance,Bruker Avance III 400 HD spectrometer) using a solvent presaturation technique to suppress the water peak.NMR samples were prepared by mixing 0.5 ml of the product-containing electrolyte and 0.1 ml DMSO-d6 as the internal standard.FE was determined from the amount of charge passed to produce each product divided by the total amount of charge passed at a specific time or during the overall run.The FE was calculated by the Eq.(2) as below:

where Z is the number of electrons transferred (Z=2.0 for CO,HCOOH,and H2production),n is the number of moles for a given product,F is Faraday’s constant (96485.0 C.mol-1),Qtotalis all the charge passed throughout the electrolysis process.

The cathodic energy efficiency(EEca.)for the ECR towards CO is calculated using the Eq.(3):

where Eeq,cellis the thermodynamic equilibrium potential between the anode and cathode reactions.Eeq,cellcan be determined by Eeq,an-ode–Eeq,cathode,wherein Eeq,anodeis the thermodynamic equilibrium potential for the anode reaction (i.e.oxygen evolution reaction,Eeq,anode=1.23 V vs.RHE)and Eeq,cathodeis the thermodynamic equilibrium potential for the cathode reaction(i.e.Eeq,cathode,CO=-0.11 V vs.RHE).ηcathodeis the cathode overpotential,the potential difference between Eeq,cathodeand the applied cathode potential.

3.Results and Discussion

3.1.Physical characterization

As illustrated in Fig.1(a),the Cd-BDC MOFs were synthesized based on reaction and coordination of a metal precursor (CdCl2)and an organic linker(H2BDC)in a mixture of DMF,ultrapure water and MeOH through a modified solvothermal procedure similar to the prior literature [29].The as-obtained MOFs possess a formula unit of (C8H10CdO7)n·4H2O.Two molecules of para-benzene dicarboxylic acid are bridged by a Cd metal ion to form a single asymmetric unit.Cd2+is 7-coordinated,out of which three bonds are in coordination with water and the remaining four are in coordination with two para-benzene dicarboxylic acid linkers.Cd2+is coordinated by the carboxyl group in an alternate way,forming onedimensional zigzag chain in an axis.The distance between two Cd ions connected by BDC is 1.1377 nm.The carboxyl species,oxygen atoms of three water molecules and the carbons of benzene ring bound to the carboxyl group are planar.The XRD patterns depicted in Fig.1(b) indicate the formation of a phase of Cd-BDC MOFs with high crystallinity in orthorhombic system with pcca space group,in good accordance with simulated Cd-based MOFs[29,30].No reflection peaks originating from the CdCl2,H2BDC,and CdO are identified.The FT-IR spectroscopy analysis of Cd-BDC MOFs is given in Fig.1(c).The peaks at～2932.0 and～1382.0 cm-1are attributed to the vibrations of -CH3groups,while the components at～1010.0,～1100.0,and～1151.0 cm-1could be assigned to the C-N stretching vibrations of DMF molecules in the framework of Cd-BDC MOFs.The symmetric and asymmetric stretching vibrations at～1620.0 and～1404.0 cm-1likely arise from characteristic carboxyl species coordinated to the metal center in the formed MOFs.Besides,the weak stretching vibration bands appearing at～1735.0 cm-1correspond to free carboxyl moieties.Peaks at～3077.0,～3053.0,～832.0,and～749.0 cm-1were also noticed,resulting from the C-H stretching vibrations of arenes.The peak at～1504.0 cm-1could be ascribed to the aromatic ring vibration of C=C.Meanwhile,the peak centering at 1568.0 cm-1is associated with the C-C stretching vibrations of the aromatic rings of terephthalate ligands[31–33].The absorption peak at around 646.0 cm-1plausibly stems from the vibrations of Cd(II)-O [34].

The wide-scan XP spectrum showed the spectroscopic features of Cd,O,and C (Fig.1(d)).No other heteroelement including element Cl was detectable,suggesting that there are no impurities,unreacted precursors or byproducts in the sample.As displayed in Fig.1(e),a spin–orbit split doublet with binding energies of Cd 3d5/2at～405.4 eV and Cd 3d3/2at～412.1 eV,assigned to Cd2+,can be seen [35,36].The O 1s peaks at～531.5,～532.1,and～533.1 eV are attributed to Cd-O bond,C=O bond,and C-O bond of Cd-BDC MOFs,respectively (Fig.1(f)).

CO2absorption isotherms were further measured at 298.15 K to evaluate the CO2absorption capability of the catalysts.Notably,the as-made Cd-BDC MOFs possess a remarkably higher CO2adsorption uptake (～4.2 mg CO2.(g cat)-1at 0.1 MPa) than commercial CdO (～1.2 mg CO2.(g cat)-1at 0.1 MPa) (Fig.1(g)).The superior CO2uptake capability evidently promotes the adsorption of a large amount of CO2on the Cd-BDC MOFs catalyst surface,thereby potentially boosting catalytic turnover.

Further morphological and microstructural characterization by TEM and HAADF-STEM revealed that the Cd-BDC MOFs sample featured with a two-dimensional nanosheet structure (Fig.2(a)-(c)).A large number of flakes that stacked on top of each other with lateral sizes of 150.0 nm–1.5 μm were clearly observed(Fig.2(a)and(b)).Many of the observed nanosheets are characterized with abundant irregular longitudinal edges (Fig.2(d)).Energydispersive X-ray spectroscopy (EDS) elemental maps (Fig.2(g)-(h))along with EDS spectrum(Fig.2(i))show the uniform distribution of C,Cd,and O elements throughout the sheets.It should be pointed out that the crystallinity of Cd-BDC MOFs flakes was destroyed after exposure to electron beam illumination during TEM imaging.

3.2.Electrochemical measurements

ECR is closely related with operating conditions including the nature and properties of electrocatalyst,electrolyte type and concentration,and electrochemical cell configuration [37].The Htype reactor provides advantages in screening catalysts,understanding structure-performance relationships,and gaining mechanism insights.To investigate the inherent catalytic properties of the Cd-BDC MOFs,we first conducted the ECR in CO2-purged 0.1 mol.L-1KHCO3aqueous solution (pH 6.8) using an H-type cell with successive CO2bubbling at ambient conditions [38].The gas phase products were probed and quantified by GC while liquid products in the solution formed during CO2reduction were determined by1H NMR.

Fig.1.(a)Schematic illustration of the preparation of Cd-BDC MOFs.(b)XRD patterns of the as-obtained Cd-BDC MOFs and simulated Cd-BDC MOF.(c)FTIR spectrum of Cd-BDC MOFs.(d) Wide-survey,(e) Cd 3d,and (f) O 1s XP spectra of Cd-BDC MOFs.(g) CO2 adsorption isotherms of Cd-BDC MOFs and commercial CdO (CdO (coml)).

Fig.2.(a)Low-magnification STEM image of Cd-BDC MOFs.TEM images of(b)stacked flakes and(c)an individual nanosheet.(d)Transformed STEM image of Cd-BDC MOFs,showing the irregular edges.(e) STEM image of Cd-BDC MOFs and corresponding EDS elemental maps of (f) C,(g) Cd,and (h) O,along with spectrum (i).

The potential-dependent geometric current densities of Cd-BDC MOFs between -0.4 to -1.4 V were studied by linear sweep voltammetry,as plotted in Fig.3(a).Invariably higher cathodic currents were seen in a CO2environment than in an Ar environment over the potential range.In addition,Cd-BDC MOFs exhibited remarkably enhanced current densities compared to CdO (coml)in CO2-saturated electrolyte.Only H2and CO were detected in the gas phase,and trace amounts of HCOOH were found in liquid products at applied potentials ranging from -0.8 to -1.3 V in CO2-saturated 0.1 mol.L-1KHCO3solutions.In stark contrast,almost exclusively H2evolved from the competing hydrogen evolution reaction (HER) was produced over H2BDC and neat carbon paper electrodes.This implies that H2BDC does not play a significant role in the ECR activity of Cd-BDC MOFs and that Cd is essential for achieving selectivity towards CO.The ECR was found to dominate over hydrogen evolution at potentials between -0.9 and -1.3 V on Cd-BDC MOFs (Fig.3(b)).The FECOincreased with applied potential in the potential range of -0.8 to -1.0 V and reached up to 88.9% at -1.0 V.Nonetheless,further increase in overpotential led to drop of FECOdue to more severe competition with HER at more negative voltages.Note that at applied cathodic potentials ranging from-0.9 to-1.1 V,CO was generated in significant quantities with FE exceeding 80.0%.Meanwhile,the energy conversion efficiency of Cd-BDC MOFs was calculated to approach 53.3%,compared to that of 34.6% for commercial CdO (Fig.S1).Alternatively,CO partial geometric current density and CO production rate increased with the applied potential approaching～5.5 mA.cm-2at -1.2 V (Fig.3(c)).Strikingly,in terms of the FE for CO formation,the as-synthesized Cd-BDC MOFs substantially outweighed commercial CdO (Fig.3(d)),and many recently reported MOF-based electrocatalysts at similar overpotentials(Fig.4(a),Table S1).The cathodic CO EE of 53.3% at -1.0 V also surpasses most prior results demonstrated for MOF-based electrodes,as manifested in Fig.S2.The summary of the CO FE and CO EE of other recently published MOF-based catalysts was shown in Table S1.

The interfacial reaction kinetics was explored by Tafel analysis.As illustrated in Fig.4(b),Cd-BDC MOFs displayed a Tafel slope of～183.3 mV.dec-1,lower than that of～197.6 mV.dec-1observed for CdO(coml).This indicates that the Cd-BDC MOFs has a comparatively faster kinetics for CO2reduction.The yield of the*COOH intermediate on the surface of the catalysts may determine the reaction rate [39].

To further elucidate the superior activity of Cd-BDC MOFs,the electrochemical active surface area (ECSA) and EIS were studied.The double-layer capacitance as a reference of ECSA [40] (Fig.S3)indicated that Cd-BDC MOFs have a significantly higher ECSA than that of CdO (coml),benefiting ECR.The Nyquist plots presented in Fig.4(c)manifest that Cd-BDC MOFs possess lower charge transfer resistance compared with the CdO (coml),favoring charge exchange between the catalyst and reactants in the electrolyte.Alternated electrolysis cycling measurements between Ar-and CO2-purged 0.1 mol.L-1KHCO3electrolytes show that the evolved CO resulted from the feed gas CO2and was stable over 8.0 cycles(Fig.4(d)).The long-term performance of Cd-BDC MOFs was examined by chronoamperometric tests.It was found that both CO FE and overall current density remained steady even after 12.0 h of continuous electrolysis at -1.1 V (Fig.4(e)).Post characterization by XRD and FT-IR spectroscopy indicated that the structure of the Cd-BDC MOFs catalyst after electrolysis was largely maintained (Fig.S4).SEM and TEM observations showed that the flake morphology of Cd-BDC MOFs retained regardless of electrolysis despite occurrence of stacking and aggregation to some extent (Fig.S5).

Fig.3.(a)Linear sweep voltammetry(LSV)results of Cd-BDC MOFs,CdO(coml),and H2BDC in Ar-(dashed line)or CO2-(solid line)saturated 0.1 mol.L-1 KHCO3 solution with a scan rate of 5 mV.s-1.(b)CO FEs and(c)CO partial geometric current densities of Cd-BDC MOFs,CdO(coml),H2BDC,and bare carbon paper electrodes.(d)Production rates of CO at different potentials over Cd-BDC MOFs and CdO (coml).

The low CO2solubility in aqueous solutions(～33.4 mmol.L-1) results in limited substrate flux at the electrode–electrolyte interface,causing insufficient current density for ECR in H-type cells.This issue can be overcome with a flow-cell reactor using gas diffusion electrodes by drastically reducing the diffusion path length of CO2molecules.Therefore,we further carried out ECR over the Cd-BDC MOFs electrode in 0.5 mol.L-1KHCO3electrolyte using a flow cell.Only CO and H2were detected as major gas reduction products with small amounts of HCOOH identified as the liquid product.It is worth noting that the Cd-BDC MOFs exhibited excellent CO2reduction activity over a wide potential range from -0.16 to -1.06 V,delivering a CO FE from～90.3% to～97.6% (Fig.4(f)).Equally importantly,the CO partial geometric current density was remarkably boosted up to～108.1 mA∙cm-2at -1.06 V.The good electrocatalytic performance can be attributed to the unique porous structure of Cd-BDC MOFs with abundant accessible metallic moieties (one Cd atom is coordinated with seven oxygen atoms),thereby facilitating adsorption and activation of CO2molecules.In addition,the surface area per amount of material is maximized for a 2D structure,which favors surface accessibility to reactants.Among others,the in-plane electrical conductivity of 2D materials is improved relative to bulk counterparts.This benefits electrochemical reactions where the higher conductivity usually enhances utilization of the electrical energy.

The ECR properties were readily regulated by manipulating synthesis parameters such as reaction temperature and molar ratio of cadmium chloride and H2BDC.The optimal molar ratio of CdCl2and H2BDC for ECR was found to be 1:1.The catalyst synthesized at 120.0 °C displayed higher ECR activity than those attained at both 100.0 and 150.0 °C (Fig.5).Moreover,the impact of the nature of electrolyte on the ECR performance was investigated (Fig.5).0.1 mol.L-1KHCO3was found to be the best electrolyte candidate for ECR to CO.Addition of halide (F-,Cl-,Br-,and I-) andions in catholyte was observed to lead to a substantial decrease of CO FE and partial geometric current density.This may be associated with the strong adsorption of halides andon the surface of the Cd-BDC MOFs electrode,thus hampering the adsorption of CO2and K+.Besides,the potential discrepancy of the dispersed layer in the electric double layer may be enlarged owing to increased concentrations of the negative charge on the cathode surface.This accelerated the transport of polar water molecules instead of nonpolar CO2molecules to the electrode surface and promoted the HER,thereby suppressing the ECR.

Fig.4.(a)CO FEs of Cd-BDC MOFs and other reported MOF-based electrocatalysts.(b)Tafel plots for CO production and(c)EIS spectra of Cd-BDC MOFs and CdO(coml).The inset gives the equivalent circuit employed for fitting the data,where RS shows the combination of the resistance of electrodes and electrolyte,CPE and Rct denote the capacitance and charge transfer resistance of working electrode–electrolyte interface,respectively.(d)CO FE and corresponding partial geometric current density for Cd-BDC MOFs during cycles with an interval of 1 h in CO2-and Ar-saturated 0.1 mol.L-1 KHCO3 at-1.0 V.(e)Current density and CO FE of Cd-BDC MOFs during 12.0 h of continuous electrolysis at -1.0 V.(f) CO FE and partial geometric current density of Cd-BDC MOFs at varying potentials in 0.5 mol.L-1 KHCO3 solution with a flow-type cell.

4.Conclusions

In summary,we have demonstrated a Cd-BDC MOF catalyst for selective conversion of CO2to CO under ambient conditions.The catalytic property was tunable by modulating hydrothermal temperature and the amounts of metal precursor and linker as well as electrolyte composition and concentration.CO FEs exceeding 80.0% were attained over the voltage range between -0.9 and-1.1 V,approaching～88.9% at -1.0 V in 0.1 mol.L-1CO2-purged KHCO3solution with an H-type cell.Particularly,the CO FEs were further improved over 90.0% in a wide potential region from-0.16 to -1.06 V in 0.5 mol.L-1CO2-saturated KHCO3solution using a flow reactor system.The maximum CO FE achieved was as high as～97.6%at a low overpotential of 150.0 mV.The CO partial geometric current density reached about 108.1 mA cm-2at-1.1 V.This work highlights new possibilities for developing unique novel framework structures for efficient electrochemical CO2reduction.

Fig.5.CO FEs and partial geometric current densities of Cd-BDC MOFs obtained at different reaction temperatures and varying molar ratios of cadmium chloride and H2BDC at -1.0 V in 0.1 mol.L-1 KHCO3 electrolyte as well as in various electrolytes (KHCO3 solutions with concentrations of 0.1,0.5,and 1 mol.L-1;KOH,KCl,K2SO4,and K2HPO4 solutions with a concentration of 0.1 mol.L-1;a mixture of 0.1 mol.L-1 KHCO3 with 0.1 mol.L-1 KX (X=Br,I,or F) over the optimized Cd-BDC MOFs using an H-type cell.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No.21972010),Beijing Natural Science Foundation (No.2192039),Beijing University of Chemical Technology(XK180301),and the Foundation of Key Laboratory of Low-Carbon Conversion Science &Engineering,Shanghai Advanced Research Institute,Chinese Academy of Sciences (No.KLLCCSE-201901,SARI,CAS).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.10.013.