A Sn-stabilized Cuδ+ electrocatalyst toward highly selective CO2-to-CO in a wide potential range

Current techno-economic evaluation manifests that the electrochemical CO2 reduction reaction (eCO2RR) to CO is very promising considering its simple two-electron transfer process, minimum cost of electricity, and low separation cost. Herein, we report a Sn-modification strategy that can tune the local electronic structure of Cu with an appropriate valence. The as-prepared catalysts can alter the broad product distribution of Cu-based eCO2RR to predominantly generate CO. CO faradaic efficiency (FE) remained above 96% in the wide potential range of −0.5 to −0.9 V vs. the reversible hydrogen electrode (RHE) with CO partial current density up to 265 mA cm−2. The catalyst also had remarkable stability. Operando experiments and density functional theory calculations demonstrated that the surface Cuδ+ sites could be modulated and stabilized after introducing Sn. The Cuδ+ sites with low positive valence were conducive to regulating the binding energy of intermediates and resulted in high CO selectivity and maintained the stability of the catalyst. Additionally, scaling up the catalyst into a membrane electrode assemble system (MEA) could achieve a high overall current of 1.3 A with exclusive and stable CO generation.


Characterization
The morphologies of the catalysts were characterized by a HITACHI SU8020 scanning electron microscope (SEM) and a JEOL JEM-2100F high-resolution transmission electron microscopy (HR-TEM). X-Ray diffraction (XRD) analysis of the samples was performed on a Rigaku D/max-2500 X-ray diffractometer with Cu-Kα radiation and the scan speed was 5° min -1 . X-ray photoelectron spectroscopy (XPS) analysis was performed on the Thermo Scientific ESCA Lab 250Xi using 200 W monochromatic Al Kα radiation. XPS depth profile spectra were collected on a Nexsa X-Ray Photoelectron Spectrometer (XPS) System (Thermo Fisher Scientific) using an argon cluster beam for 30 min (8 keV energy, 300 atoms size) and using Ar + for different times. The 500 μm X-ray spot was used. The base pressure in the analysis chamber was about 3×10 -10 mbar. Typically, the hydrocarbon C1s line at 284.8 eV from adventitious carbon was used for energy referencing. The X-ray adsorption spectroscopy (XAS) measurements were performed using a modified flow cell at the 4B9A beamline at Beijing Synchrotron Radiation Facility (BSRF), China.
The data were collected in fluorescence excitation mode using a Lytle detector. Cu foil, Cu 2 O, and CuO were used as references. The acquired EXAFS data were processed according to the standard procedures using the Athena and Artemis implemented in the IFEFFIT software packages. The XES data was performed at beamline 4W1B of the Beijing synchrotron Radiation Facility, China. The storage ring ran 2.5 GeV electron with current of 250 mA. A polychromatic beam (pink beam) with an incident X-ray energy of 5 or 10-18 keV was used, and the photon flux was on the order of 10 13 phs/s. The beam spot-size (FWHM) was focused down to 50 μm by a polycapillary half-lens. The X-ray emission spectrum (XES) is performed at beamline 4W1B of the Beijing synchrotron Radiation Facility, China. The storage ring runs 2.5 GeV electron with current of 250 mA. A polychromatic beam (pink beam) with an incident X-ray energy of 10-18 keV is used, and the photon flux is on the order of 10 13 phs/s. The beam spot-size (FWHM) is focused down to 50 μm by a polycapillary half-lens. The Kβ XES data are collected by means of a compact von Hamos spectrometer based on a full-cylinder Si crystal. The dispersion axis was perpendicular to the axis of the incident beam. The analyzers diffract and focus the emitted signal onto a position-sensitive detector. The 2D spectra were recorded using a Pilatus 100K detector with a pixel size of 172 × 172 μm 2 . The XES raw data were processed by the standard procedures using the DAWN package.

Electrode preparation
To construct the cathode electrode, a catalyst slurry containing 5 mg of obtained catalysts, 1 mL of isopropanol and 20 μL of Nafion ionomer solution (5 wt% in H 2 O) was first prepared and sonicated for 1 h. Then, the catalyst slurry (0.2 mL) was slowly air-brushed onto the carbon paper with a hydrophobic microporous gas diffusion layer (YLS-30T GDL) under vacuum to achieve a catalyst loading of ~ 1.0 mg cm -2 . Ni foam were used as anode electrode.

Electrochemical study
Electrochemical reduction of CO 2 in a flow cell. Electrochemical studies were conducted in an electrochemical flow cell which was composed of a gas chamber, a cathodic chamber, and an anodic chamber, as reported in our previous work. 1 An anion exchange membrane (FumasepFAA-3-PK-130) was used to separate the anodic and cathodic chambers, and a Hg/HgO electrode and Ni foam were used as the reference and counter electrodes, respectively. The electrolysis was conducted using a CHI 660e electrochemical workstation equipped with a high current amplifier CHI 680c. The measured potentials after iR compensation were rescaled to the reversible hydrogen electrode (RHE) reference by E (versus RHE) = E (versus Hg/HgO) + 0.098 V+0.0591V/pH × pH. For performance studies, 1 M KOH was used as the electrolyte, and it was circulated through the cathodic and anodic chambers using peristaltic pumps at a rate of 30 mL min -1 . The flow rate of CO 2 gas through the gas chamber was controlled to be 20 sccm using a digital gas flow controller.
Electrochemical reduction of CO 2 in the membrane electrode assembly (MEA) system. The MEA cell consisted of a titanium anode (cathode) bipolar plate with serpentine flow field, associated nuts, bolts, and insulating kit. An anion-exchange membrane (AEM, Sustainion X37-50 Grade RT) was sandwiched by the two gas diffusion layer electrodes to separate the chambers. The AEM was activated for at least 24 h in 1 M KOH and rinsed with deionized water before the experiments. The as-prepared gas-diffusion electrode (2.5 cm × 2.5 cm, catalyst loading: 3 mg·cm -2 ) and an IrO 2 /Ti-mesh anode catalysts (3 cm × 3 cm, catalyst loading: 1 mg·cm -2 ) were employed as the cathode and anode, respectively. The anode was prepared by a dipcoating and thermal decomposition method. 2 During the experiment, 80 sccm humidified CO 2 was supplied to the cathode side, while the anode was circulated with 0.1 M KHCO 3 electrolyte at 10 mL min -1 flow rate. No iR compensation was applied.
The gaseous product of electrochemical experiments was collected using a gas bag and analyzed by GC.
Double-layer capacitance (C dl ) measurements. The electrochemical active surface area is proportional to C dl value. C dl was determined by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammogram (CV). The CV ranged from 0.62 V to 0.52 V vs. RHE. The C dl was estimated by plotting the Δj (j a -j c ) at 0.57 V vs. RHE against the scan rates, in which the j a and j c are the anodic and cathodic current density, respectively. The scan rates were 20, 40, 60, 80, 100 and 120 mV s -1 .
Product analysis. The gaseous product of electrochemical experiments was collected using a gas bag and analyzed by gas chromatography (GC, HP 4890D), which was equipped with TCD detectors using argon as the carrier gas. The liquid product was analyzed by 1 H NMR (Bruker Avance III 400 HD spectrometer) in deuteroxide with phenol and sodium 2, 2-dimethyl-2-silapentane-5-sulfonate (DSS) as internal standards.

Calculations of Faradaic efficiencies of gaseous and liquid products
Gaseous products: From the GC peak areas and calibration curves for the TCD detector, we can obtain the V % of gaseous products. Since the flow rate of the outlet was monitored to be constant, the moles of gaseous products can be calculated.
The Faradaic efficiency of gaseous product is: