Preparation of trimetallic electrocatalysts by one-step co-electrodeposition and efficient CO2 reduction to ethylene

Use of multi-metallic catalysts to enhance reactions is an interesting research area, which has attracted much attention. In this work, we carried out the first work to prepare trimetallic electrocatalysts by a one-step co-electrodeposition process. A series of Cu–X–Y (X and Y denote different metals) catalysts were fabricated using this method. It was found that Cu10La1Cs1 (the content ratio of Cu2+, La3+, and Cs+ in the electrolyte is 10 : 1 : 1 in the deposition process), which had an elemental composition of Cu10La0.16Cs0.14 in the catalyst, formed a composite structure on three dimensional (3D) carbon paper (CP), which showed outstanding performance for CO2 electroreduction reaction (CO2RR) to produce ethylene (C2H4). The faradaic efficiency (FE) of C2H4 could reach 56.9% with a current density of 37.4 mA cm−2 in an H-type cell, and the partial current density of C2H4 was among the highest ones up to date, including those over the catalysts consisting of Cu and noble metals. Moreover, the FE of C2+ products (C2H4, ethanol, and propanol) over the Cu10La1Cs1 catalyst in a flow cell reached 70.5% with a high current density of 486 mA cm−2. Experimental and theoretical studies suggested that the doping of La and Cs into Cu could efficiently enhance the reaction efficiency via a combination of different effects, such as defects, change of electronic structure, and enhanced charge transfer rate. This work provides a simple method to prepare multi-metallic catalysts and demonstrates a successful example for highly efficient CO2RR using non-noble metals.

solution of H 2 SO 4 (10 mM) , Copper(II) gluconate (100 mM), Lanthanum(III) acetate (10 mM), cesium acetate (10 mM), and 4-aminopyridine (10 mM). The deposition was carried out at a constant voltage of 4 V for desired time. Then the as-prepared electrode was washed with water and ethanol several times and dried at room temperature in a vacuum oven before use. Cu 10 La 1 and Cu 10 Cs 1 electrodes were prepared by this method with different combinations of metal salts mixed solution.
By using this method, Cu-X-Y with different metals (e.g. La, Cs, Zn, Co, Ag, Au), could be prepared.
Electrosynthesis of Cu 10 La 1 Cs 1 based GDEs: The preparation of Cu 10 La 1 Cs 1 based GDEs were similar to that to fabricate the Cu 10 La 1 Cs 1 electrode except that CP was replaced with a hydrophobic carbon paper containing a carbon black layer. After washing and drying, the sample was immersed into PTFE dispersion (10 wt %) for a few seconds to adsorb PTFE into the pores.

Characterization
X-ray diffraction patterns were acquired by an X-ray diffractometer (XRD; Rigaku Ultima VI X-ray) with Cu-Kα radiation (λ=1.54 Å). The morphologies of the samples were observed by fieldemission scanning electron microscopy (SEM) (Hitachi S4800) and transmission electron microscope equipped with EDS (TEM, JEM-2100F) operated at 200 kV. The valence states and composition of the samples were examined by X-ray photoelectron spectroscopy (XPS) on an AXIS Supra surface analysis instrument using a monochromatic Al Kα X-ray beam (1,486.6 eV). Before the XPS measurements, the catalysts were stored under ambient conditions. This effectively preserved the oxidation of the samples during the sample transfer. The content of metals in the catalysts was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 8300, Perkin-Elmer). Comparison to Cu, La, and Cs standards of known concentration allowed the determination of the respective Cu, La, and Cs contents. Electronic paramagnetic resonance (EPR) spectra were recorded at room temperature, in continuous-wave mode, on Bruker EMXplus-10/12 spectrometers, at mw power of ~0.63 mW and modulation amplitude 10 G; spectra reported herein were typically obtained over an average of 20 scans. The X-ray absorption spectroscopy (XAS) experiments were carried out at the 4B9A beamline at Beijing Synchrotron Radiation Facility (BSRF), China. Data analysis of Cu K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were conducted using the Athena software package. Pre-edge and post-edge backgrounds were subtracted from the XAS spectra, and the resulting spectra were normalized by edge height. 1

Electrochemical study
A gas-tight two-compartment H-cell separated by a proton exchange membrane (Nafion117) and a CHI 660E potentiostat workstation (Shanghai CH Instruments Co., China) were used, and the experiment was conducted at room temperature. The anode and cathode sides were filled with 30 mL of 0.1 M K 2 SO 4 and 0.1 M KCl, respectively. Before each set experiment, the catholyte was bubbled with CO 2 or N 2 for at least 30 min to form a CO 2 -saturated or N 2 -saturated solution. Linear sweep voltammetric (LSV) scans were conducted in an H-type cell with a three-electrode configuration, which consisted of a working electrode, a platinum gauze counter electrode, and an Ag/AgCl (saturated KCl solution) reference electrode. LSV measurements in gas-saturated electrolytes were carried out in the potential range of 0.2 V to -1.4 V vs. RHE at a scan rate of 20 mV s −1 . In all measurements, all electrochemical data were referenced to RHE using equations: Where E 0 (V of Ag/AgCl. vs. NHE) = 0.197 V.

Electrolysis in an H-type cell
The as-synthesized electrodes were used as the working electrode. In the experiments, the Nafion-117 membrane was used as a proton exchange membrane to separate the cathode and anode compartments. In the electrolysis experiment, the amount of electrolyte was 30 mL, and the catholyte were bubbled with CO 2 for at least 30 min to form CO 2 saturated solution, and the potentiostat electrochemical reduction was carried out under a steady stream of CO 2 (15 sccm). First, the cathode side was electrochemically reduced using the cyclic voltammetric (CV) method, which ranged from 0.6 to −1.4 V vs. RHE at a rate of 0.1 V s −1 for 5 cycles to completely reduce the possible oxidized species.

Electrolysis in a flow cell
The Cu 10 La 1 Cs 1 GDEs were evaluated in a flow-cell reactor, which consisted of a gas diffusion chamber, a cathodic counterpart, and an anodic counterpart. CO 2 was introduced to the gas diffusion chamber at 15 mL min -1 . 1.0 M KOH electrolyte (70 mL) was circulated (at 20 mL min -1 ) using a peristaltic pump through the cathodic counterpart, and another 70 mL 1.0 M KOH of electrolyte was circulated through the anodic counterpart.

Products analysis
After electrolysis, the gaseous products were collected and analyzed by gas chromatography (GC, Agilent-8890). From the GC peak areas and calibration curves of the TCD detector, the moles of a gaseous product can be calculated. The liquid products were quantified by a nuclear magnetic resonance (NMR) spectrometer. 1H NMR spectra of freshly acquired samples were collected on an NMR spectrometer (Bruker; Ascend 400-400 MHz) in deuterated water (D 2 O) with phenol as an internal standard.
After the quantification, the FE of each product was calculated as follows: (Q: the amount of charge passed through the working electrode; F: The Faraday constant (96485 C mol -1 ); n electrons : the number of electrons transferred for product formation).

Double-layer capacitances (C dl ) measurement
The value of C dl is proportional to the electrochemically active surface area. The value of C dl was determined by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammogram (CV) in an H-type cell. The CV was obtained from

Electrochemical impedance spectroscopy (EIS)
The EIS measurements were carried out in CO 2 saturated 0.1 M KCl solution at an equilibrium potential. The impedance spectra were recorded with an amplitude of 5 mV of 10 -2 to 10 5 Hz. The data obtained from the EIS measurements were fitted by the Zview software (Version 3.1, Scribner Associates, USA).

Computational Method
DFT calculations were conducted through the Vienna ab initio Simulation Package (VASP) with the projector augmented wave method. 2,3 Generalized gradient approximation of the Perdew-Burke-Ernzerhof (PBE) functional was used as the exchange-correlation functional. 4,5 The cutoff energy was set as 500 eV, and structure relaxation was performed until the convergence criteria of energy and force reached 1 × 10 -5 eV and 0.02 eV Å -1 , respectively. The Brillouin zone was sampled with 3 × 3 × 3 K points for bulk optimization and 2 × 2 × 1 K points for surface calculation. A vacuum layer of 15 Å was constructed to eliminate interactions between periodic structures of surface models. For simulating an actual surface condition, the bottom two layers of slabs were fixed, and the top two layers were allowed to relax. The van der Waals (vdW) interaction was amended by the DFT-D3 method of Grimme. The Gibbs free energy for intermediates of CO 2 reduction was calculated as ΔG = ΔE + ΔE ZPE − TΔS, where the ΔE, ΔE ZPE , and ΔS are electronic energy, zeropoint energy, and entropy difference between products and reactants. The zero-point energies of isolated and adsorbed molecules were calculated from the frequency analysis. The entropies of molecules in the gas phase were taken from the National Institute of Standards and Technology (NIST) database. Fig. S1. (a, b) SEM images of CP with different magnifications.                    Therefore, the ECSA is proportional to C dl value and can be compared via C dl value.  Randles' equivalent circuit used for fitting the experimental impedance data of the electrode prepared by in-situ electrodeposition method: solution resistance (R s ), double layer capacitance (C dl ), charge transfer resistance (R ct ), film capacitance (C f ), film resistance (R f ) and Warburg-type impedance (Z w ). For in-situ electrodeposition method, the EIS data can be well modeled with a ladder type circuit, which is used to model the uniform film with one or more adsorbed substances.         Tables   Table S1. Comparison