Chemicals
Tin (II) chloride dihydrate (SnCl2·2H2O), Tin (IV) chloride pentahydrate (SnCl4·5H2O), potassium ferricyanide (K3[Fe(CN)6]), sodium borohydride (NaBH4), L-cysteine (C3H7NO2S), hydrochloric acid (36 ~ 38%, HCl), potassium bicarbonate (KHCO3), potassium hydroxide (KOH), 1-methyl-2-pyrrolidinone (NMP), N,N-dimethylformamide (DMF), and ethanol were obtained from Sinopharm Chemical Reagent Co. Ltd. Dimethyl sulfoxide-D6 (99.9%, DMSO), deuterium oxide (D2O), and stannous octoate (C16H30O4Sn) was purchased from Adamas Reagent Co., Ltd. Formic acid (HCOOH) was purchased from Energy Chemical Co., Ltd. Pluronic® F-127, and Nafion solution (5 wt%) was obtained from Sigma-Aldrich. Graphene oxide (GO) dispersion solution (5.88 mg mL‒1) was purchased from Shandong Yuhuang New Energy Technology Co., Ltd. Calcium hydroxide (98%, Ca(OH)2) was acquired from Acros Organics Co., Ltd. All chemical reagents were used directly without further purification. Ultrapure water (> 18.25 MΩ cm) was used for the experiments.
Instrumentation
The powder X-ray diffraction (PXRD) patterns were recorded on a Miniflex600 X-ray diffractometer at 40 kV and 40 mA with Cu Kα radiation in 2θ ranging from 10o to 80o. Field emission scanning electron microscopy (FESEM) was performed on a JSM6700 microscope operated at 5 kV. Transmission electronic microscopy (TEM) and high-resolution TEM (HRTEM) equipped with energy dispersive spectrometer (EDX) were conducted on a Talos-F200X microscope operated at 200 kV. Atomic force microscopy (AFM) images were obtained on a Bruker Dimension ICON atomic force microscope. X-ray photoelectron spectroscopy (XPS) analyses were taken on Thermo Fisher Scientific XPS ESCALAB 250Xi instrument with an Al-Ka (1486.8 eV) X-ray source using C 1s (284.8 eV) as the reference. The Fourier transform infrared (FT-IR) spectra were tested on a Spectrum One Fourier transform infrared spectrometer (Perking-Elmer Instruments) by using the KBr as a background. Operando attenuated total reflection-infrared (ATR-IR) spectra were recorded on the NICOLET 6700 instrument. Raman spectra were conducted on LabRAM HR evolution with 532 nm wavelength laser source. The Brunauer-Emmett-Teller (BET) specific surface areas of the samples were measured with N2 adsorption/desorption isotherms at liquid nitrogen temperature using automatic volumetric adsorption equipment (BELSORP Max analyzer). UV-vis diffuse reflectance spectra were collected on a Hitachi U-3010 spectrophotometer using BaSO4 as a reference.
Preparation
Preparation of SnFe-PBA-NSs
In a typical process for preparation of monolayer SnFe-PBA-NSs, 135.38 g of SnCl2·2H2O and 65.85 g of K3[Fe(CN)6] were dissolved in 800 mL of distilled water, respectively. The two solutions were simultaneously and continuously added into 400 mL of distilled water using a fluid-flow control system under ultrasonic irradiation. After stirring and ultrasonic vibration for 60 min, the mixture was aged for 24 hours. Afterwards, the product was collected by filtration, washed several times with water, and dried by vacuum freeze-drying, and finally, 158.3 g of the white solid was obtained in one batch.
Preparation of Sn-ene QDs: Sn-ene QDs was obtained via an in situ electrochemical self-reconstruction process from SnFe-PBA-NSs in a three-electrode system. Typically, 10.0 mg of SnFe-PBA-NSs was dispersed in the mixture of water, ethanol and 5 wt% Nafion solution (8:1:1) with ultrasonic treatment for 30 min, and casted onto a 1.0×1.0 cm2 carbon paper with the mass loading of 2.0 mg cm− 2. The transformation was carried out in a H-type cell with two compartments separated by a piece of proton exchange membrane (Nafion 117). The working electrode and the reference electrode (saturated Ag/AgCl) were placed in the cathode compartment, and the counter electrode (Pt mesh) was placed in the anodic compartment. Each compartment contained 0.5 M KHCO3 electrolyte (15 mL). All potentials were converted to the reversible hydrogen electrode (RHE) reference scale (ERHE = EAg/AgCl + 0.198 V + 0.0591 V × pH). The cathodic transformation of SnFe-PBA-NSs to Sn-ene-QDs was performed via the electrolysis of SnFe-PBA-NSs at ‒1.17 V for 1 h in CO2-saturated 0.5 M KHCO3 electrolyte, and the corresponding loading amount of Sn-ene QDs was calculated to be about 0.93 mg cm− 2.
Preparation of SnFe-PBA-NCs
SnFe-PBA-NCs were synthesized according to a previously reported work with some modifications [67]. Typically, 140 mg of SnCl4·5H2O and 100 mg of F127 were dispersed in a mixed solution containing 16 mL of distilled water, 14 mL of ethanol, and 2 mL of 0.01 M HCl. After stirring for 15 min, 6 mL of 0.089 M K3Fe(CN)6 aqueous solution was added into the above solution and stirred for 15 min at 25°C. Afterward, the mixture solution was transferred into an autoclave and crystallized at 80°C for 10 h. The product was collected, washed with distilled water and ethanol, and then dried at 60°C for 8 h under vacuum.
Preparation of Sn NPs
2.0256 g of C16H30O4Sn was dispersed into 200 mL of DMF during vigorous stirring at 25°C, and then 2 g of NaBH4 was added this solution. After stirring for 4 h, the product was collected, washed with distilled water, and dried at 60°C for 8 h under vacuum.
Preparation of SnO2 NSs
2.0256 g of C16H30O4Sn was dispersed to 10 mL of DMF by ultrasound, and then the GO paper was immersed in this solution for 24 h. After adsorption, the GO paper was taken out and washed with DMF twice and dried at 150°C for 12 h in vacuum oven. Finally, the calcination of GO paper containing tin cations was performed in air sequentially at 500°C for 2 h and at 650°C for 1 h with a heating rate of 5°C min− 1.
Preparation of SnS2 NSs
SnS2 NSs was synthesized according to the previously reported procedure [68]. In a typical synthesis, 228 mg of SnCl2·2H2O and 349 mg of L-cysteine were dissolved in 60 mL of NMP under magnetic stirring for 3 h. Afterward, the precursor solution was solvothermally reacted at 180°C for 6 h. The product was centrifuged, washed with water and ethanol, and dried at 60°C for 12 h in vacuum oven.
Electrochemical measurements
For CO2RR measurements, the carbon fiber paper supported Bi-ene-NW was directly used as the working electrode in the typical H-type cell with two compartments. The electrolyte was pre-saturated with Ar (pH = 8.4) or CO2 (pH = 7.4). A constant CO2 flow of 20 sccm was continuously bubbled into the electrolyte to maintain its saturation during CO2RR measurements. Cyclic voltammetry and polarization curves were carried out at a scan rate of 10 mV s− 1.
In order to analyze the reduction products and calculate their Faradaic efficiency, the electrolysis was performed at selected potentials for 30 min. During the electrolysis, gaseous products were detected by an on-line gas chromatography (GC) (Agilent 7820A) equipped with a molecular sieve 5 Å and two porapak Q columns continuously. The concentration of H2 and CO was analyzed by a thermal conductivity detector (TCD) and a flame ionization detector (FID), respectively. The quantification of gaseous products was carried out using a conversion factor derived from the standard calibration gases. The Faradaic efficiency of gaseous products was calculated as below:
$$\varvec{F}\varvec{E}=\frac{\varvec{N}\times \varvec{C}\times \varvec{v}\times \varvec{F}}{{\varvec{V}}_{\varvec{m}}\times {\varvec{j}}_{\varvec{t}\varvec{o}\varvec{t}\varvec{a}\varvec{l}}}\times 100\varvec{\%}$$
1
where N is the number of electrons required to form a molecule of product, C is the measured concentration of product, v is the flow rate of CO2, F is the Faraday constant (96 480 C mol− 1), Vm is the molar volume of gas when temperature is 298 K, and jtotal is the recorded current.
Liquid products were collected at the end of each electrocatalysis and analyzed by 1H NMR (AVANCE III 400M). For the NMR, 0.5 ml of the catholyte was mixed with 0.1 ml of D2O containing dimethylsulfoxide (DMSO, 10 ppm) as the internal standard. The concentration of formate was quantitatively determined from its NMR peak area relative to that of the internal standard using the calibration curve from a series of standard formate solutions. The Faradaic efficiency of formate was calculated as follows:
$$\varvec{F}\varvec{E}=\frac{{\varvec{n}}_{\varvec{f}\varvec{o}\varvec{r}\varvec{m}\varvec{a}\varvec{t}\varvec{e}}\times \varvec{N}\times \varvec{F}}{{\varvec{Q}}_{\varvec{t}\varvec{o}\varvec{t}\varvec{a}\varvec{l}}}\times 100\varvec{\%}$$
2
where nformate is the measured amount of formate in the cathode compartment and Qtotal is total electric quantity.
Tafel slopes for formate production were calculated from the corresponding geometric current densities and the formate partial Faradaic efficiency.
The electrochemical impedance spectroscopy (EIS) was recorded at ‒0.73 V with the frequency ranging from 0.1 to 105 Hz at the AC amplitude of 5 mV.
The adsorption of OH‒ as a surrogate of CO2*‒ was examined through a cathodic LSV scan between ‒0.4 and 0.38 V in Ar-saturated aqueous 0.1 M KOH solution at a scan rate of 10 mV s‒1.
Electrochemically active surface area (ECSA) was obtained by measuring the electrochemical double-layer capacitance from the scan-rate dependence of CVs in a potential range of ‒0.18 to ‒0.08 V.
For ATR-IR measurements, firstly, SnFe-PBA-NSs supported on the glassy carbon electrode was electrochemically reduced into Sn-ene QDs. Then, the loaded Sn-ene QDs electrode was used as the working electrode, along with Ag/AgCl as the reference electrode, and platinum wire as the counter electrode.
Flow cell measurements
Flow cell measurements were performed in a custom-designed flow cell reactor. It consisted of a gas diffusion electrode (GDE) loaded with SnFe-PBA-NSs (2.0 mg cm− 2, 1.0×1.0 cm2) as the cathode, a piece of bipolar membrane as the separator, and a porous nickel foam as the anode. Ag/AgCl reference electrode was located inside the cathode compartment. During the measurements, CO2 gas was directly fed to the cathodic GDE at a rate of 20 sccm. The catholyte was 1 M KHCO3 (pH = 8.6), 1 M K2CO3 (pH = 12.7) or 1 M KOH (pH = 14). It was forced to continuously circulate through the cathode compartment at a rate of 40 mL min‒1.
Stability test
The long-term electrolysis experiment was carried out at the current density of 200 mA cm− 2, and the volumes of the cathodic and anodic electrolytes were fixed at 2 L. To eliminate the generation of CO32− from the reaction of CO2 with KOH and reduce energy loss for CO2RR in the flow cell, the enough Ca(OH)2 powder was wrapped by the multilayer filter paper and placed in the cathode reservoir.
Computational methods
Density functional theory (DFT) calculations were carried out through the Vienna ab initio simulation package (VASP) [69, 70]. The interactions between ions and electrons were described by projector augmented wave (PAW) and generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional [71, 72]. The kinetic cut-off energy for the plane-wave basis set was 500 eV. The Brillouin zones were sampled with 3×3×1 Monkhorst-Pack meshes. The structures were fully relaxed until the maximum force on each atom was less than − 0.02 eV Å−1 and 10‒5 eV. To investigate the differences between edge and facet sites, we constructed the (101), (220) and (200) Sn planes with the supercell of 3×2×1, 3×3×1 and 3×3×1 unit cells, respectively. Where 6 atomic layers with the bottom 2 atomic layers were fixed along the y and z directions and another 15 Å vacuum space was considered to avoid the periodic interaction along the y and z direction, respectively. When we explored the differences between Sn-ene QDs with or without adsorbed cyano group, 6 atomic layers with the bottom 2 atomic layers were fixed along the y and z directions, and the 15 Å vacuum space was included to avoid the periodic interaction along the y and z directions, respectively.