Doping palladium with tellurium for the highly selective electrocatalytic reduction of aqueous CO2 to CO

The doping of Pd with a small amount of Te can selectively convert CO2 to CO with a low overpotential.

(its concentration being 5 times higher than that of the precursor) was added drop-wise into the dispersion containing metal precursor (1 mg mL -1 PdCl 2 was dissolved in 0.1 M HCl solution) under bath sonication within 2 min, Pd or Te-doped Pd particles were deposited onto graphene. Subsequently, the obtained system in each case was ultracentrifuged and the collected precipitate was washed repeatedly with absolute ethanol and distilled water, and then dried at 45 °C for 12 h.
Characterization. XPS experiments were carried out using Thermo Scientific ESCALAB 250Xi instrument. The instrument was equipped with an electron flood and scanning ion gun. All spectra were calibrated to the C1s binding energy at 284.8 eV.
X-ray powder diffraction (XRD) was performed with a D/MAX−RC diffractometer operated at 30 kV and 100 mA with Cu Kα radiation. Scanning electron microscopy (SEM) was carried out using a field emission microscope (FEI Quanta 600 FEG) operated at 20 kV and equipped with an energy-dispersive X-ray spectrometer (EDX).
High-angle annular dark field scanning TEM (HAADF-STEM) was conducted using a JEOL ARM200 microscope with 200 kV accelerating voltage. STEM samples were prepared by depositing a droplet of suspension onto a Cu grid coated with a lacey carbon film. Infrared data was collected using a Nicolet 6700 ATR-IR spectrometer with liquid nitrogen-cooled MCT detector. The spectra were obtained by averaging 128 scans with a resolution of 4 cm -1 over wavenumbers ranging from 650 to 4000 cm -1 .

Electrochemical measurements.
For electrochemical reduction of CO 2 , the cyclic voltammetry (CV) and linear scan voltammogram (LSV) were performed using rotating disk electrode (RDE) by submersing working electrode in a three-electrode cell.
Controlled potential electrolysis of CO 2 was tested in an H-cell system, which was separated by Nafion 117 membrane. Toray carbon fiber paper with a size of 1 cm × 1 cm was used as working electrode. Pt wire and Ag/AgCl electrodes were used as counter electrode and reference electrode, respectively. The potentials were controlled by an electrochemical working station (CHI 760E, Shanghai CH Instruments Co., China as reported in previous literature (Fig. S13). 1 A potential of +1.2 V was chosen as the appropriate potential limit because this potential is the upper limit for the formation of Pd(OH) 2 in a solution of pH = 1. 1 The ECSA value can be determined by where GSA is the the geometric surface area, Q R is the charge density for reduction of surface Pd(OH) 2 on working electrode and Q F is the charge density for the formation of a fully covered Pd(OH) 2 layer. The value of Q F employed here is 430 μC cm -2 , the minimum of the reported values for typical single-crystal Pd surfaces. 2

ECSA-corrected
Tafel slopes for CO production (that is, j total × η CO /ECSA) were calculated from corresponding ECSA-corrected current densities for CO based on the linear sweep voltammetry curves and the CO Faradaic efficiency.

Number of active sites and turn over frequency (TOF) measurements.
To further characterize the catalytic activities of Pd/FLG, PdTe/FLG, and Te/FLG, we applied a roughness factor technique to determine the number of active edge sites of the catalysts.
Roughness factor (R f ) was estimated from the ratio of double-layer capacitance (C dl ) between the working electrode and its corresponding smooth Pd electrode (assuming that the average double-layer capacitance of a smooth Pd electrode is 20 μF cm −2 ), 3 Figure S10).
To comparatively investigate the catalytic activity and selectivity of CO 2 electrocatalysis for Pd and Te/Pd catalysts, we converted the calculated electronic energies into free energies by adding free energy corrections using Atomic Simulation Environment (ASE) code. 9 For adsorbates, these corrections include zero point energies, enthalpy, and entropy corrections, which are calculated from a harmonic oscillator approximation with a finite displacement of ±0.01 Å in x-, y-and z-directions.
All the values are summarized in Table S1. For molecules, correction values were taken from Ref. 10 We further added +0.45 eV for CO 2 molecule to correct the inaccuracy of RPBE with respect to the experimental reaction energies. 10 To include the effect of water, we added an approximate solvation corrections, where *COOH and *CO were stabilized by 0.25 and 0.1 eV, respectively. 10 We used the computational hydrogen electrode (CHE) model 11 to estimate the chemical potential of proton and electron pair since CO 2 reduction reaction consists of two sequential protonation steps. In the CHE method, the chemical potential of proton and electron (µ(H + + e -)) is estimated from the half of chemical potential of H 2 gas (0.5µ(H 2 )) and the relation between chemical and electrical potential (∆G = −eU), i.e., µ(H + + e -) = 0.5µ(H 2 ) -eU.