Protocol for fabrication and evaluation of oxide-modified Cu foils as heterostructured electrodes for electrochemical CO2 reduction.

Heterostructured catalysts based on Cu and oxides are promising for the efficient conversion of CO2 to multi-carbon products. In this protocol, we describe the fabrication and characterization of Cu/oxide heterostructured catalysts and the evaluation approach of electrochemical CO2 reduction reaction (CO2RR) performance in an H-type cell. We also provide the details of in situ surface-enhanced Raman measurement and theoretical calculations. The protocol can be useful for constructing self-supported electrodes and assessing the CO2RR performance of as-fabricated electrodes. For complete details on the use and execution of this protocol, please refer to Li et al. (2022).

Before you begin

Electrochemical measurement in a three-electrode H-type cell is a conventional approach to evaluate the performance and properties of electrocatalysts. Herein, the fabrication and performance evaluation of oxide-modified Cu electrodes are described in this protocol following a three-step procedure. At first, an oxide-modified Cu electrode is prepared by a drop-coating method; then the self-supported electrode is used as working electrode directly in an H-type cell to electrolysis CO2; afterward, the products of CO2RR are quantified by online gas chromatography (GC) and 1H nuclear magnetic resonance (1H NMR). Before one begins, the following preparations need to be performed.

Pretreatment of Nafion membrane (for electrochemical CO2 reduction device)

Timing: 5 h

Immerse Nafion 115 membrane in a 5 wt.% H2O2 aqueous solution at 80°C for 1 h to remove organic impurities.

Wash the membrane with deionized water (DI water) several times and further immerse in DI water at 80°C for 1 h to remove residual H2O2.

Immerse the membrane in 1 M H2SO4 aqueous solution at 80°C for 1 h to convert the membrane to H+-type.

Wash the membrane with DI water several times and immerse in DI water at 80°C for 1 h, and repeat this operation twice to remove residual H2SO4.

Key resources table

Materials and equipment

Temperature: 20°C–30°C, Storage Duration: ∼5 h.

Temperature: 20°C–30°C, Storage Duration: ∼5 h.

Temperature: 20°C–30°C, Storage Duration: ∼5 h.

Temperature: 20°C–30°C, Storage Duration: ∼5 h.

Temperature: 20°C–30°C, Storage Duration: ∼5 h.

Temperature: 25°C, Storage Duration: ∼1 week.

Temperature: 25°C, Storage Duration: ∼1 week.

Temperature: 25°C, Storage Duration: ∼1 week.

Temperature: 0°C–8°C, Storage Duration: ∼1 month.

The concentrations of nine regents are the same. Temperature: 25°C. It should be freshly prepared.

Storage Duration: ∼1 year.

Storage Duration: ∼1 year.

Step-by-step method details

Fabrication of oxide-modified Cu electrode

Timing: 2 h

ZrO2-modified Cu electrode is prepared by a drop-coating method according to the following steps.

Mechanically polish Cu foil with a geometric area of 1.44 cm2 (0.4 cm × 1.8 cm × 2 sides) by 3,000 mesh silicon carbide paper to remove pristine oxide layer and surface impurities.

Ultrasonically wash in DI water at 25°C for 6 min.

Wipe with dust-free paper to remove excess water (Figures 1A and 1B).

Figure 1

Photographs of the ZrO2-modified Cu electrode preparation

(A–H) Photo images of (A) pristine Cu foil, (B) mechanically polished Cu foil, (C) Cu foil with a defined working area by encapsulating the Cu foil with Teflon tape, (D) drop-coating and drying 16 μL of ZrO2 ink on one side of Cu on a hot plate at 110°C, (E) ZrO2-modified Cu electrode after first drying on a hot plate at 110°C (16 μL of ZrO2 ink), (F) ZrO2-modified Cu electrode after second drying on a hot plate at 110°C (32 μL in total), (G, H) the front and back sides of the Cu/ZrO2 electrode.

Define the working area of 0.64 cm2 (0.4 cm × 0.8 cm × 2 sides) by encapsulating the Cu foil with Teflon tape (Figure 1C).

Mechanically polished Cu foil should be prepared freshly for the following procedure to avoid re-oxidation of the Cu surface.

Prepare the ZrO2 ink.

Weigh 10 mg of ZrO2 on an analytical balance and carefully transfer to a 2 mL centrifuge tube.

Add 480 μL of ethanol, 480 μL of DI water, and 40 μL of Nafion solution (5% in a mixture of lower aliphatic alcohol and water) using a pipettor sequentially.

Sonicate the suspension using the ultrasonic cleaning machine at 25°C for 1–2 h to obtain a uniformly dispersed suspension (named ZrO2 ink).

ZrO2 ink solution should be sonicated for a few minutes before each use to avoid settling of ZrO2 nanoparticles.

Drop-coat 32 μL of ZrO2 ink on each side of Cu foil (64 μL in total) and dry in air on a hot plate at 110°C for 10 min to obtain the ZrO2-modified Cu electrode (Figures 1D–1H).

Cu foil is put on a hot plate before heating to 110°C, and the hot plate heats from 25°C to 110°C in 40 s. And the total 32 μL of ZrO2 ink is dropped onto the Cu foil in two portions, and the second portion is dropped onto the electrode after complete drying. Finally, the ZrO2-modified Cu (named Cu/ZrO2) electrode is dried on a hot plate for an additional 10 min.

Prepare other oxide-modified Cu electrodes according to the following step, which is the same as the preparation of ZrO2-modified Cu electrode (steps 1–3).

Mechanically polish Cu foil.

Weigh 10 mg of HfO2, Al2O3, SiO2, and Ga2O3 nanoparticles and disperse in the mixture of DI water, ethanol, and Nafion solution to prepare HfO2, Al2O3, SiO2, and Ga2O3 inks, respectively.

Drop-coat 32 μL of oxide inks on each side of mechanically polished Cu foil and dry in air on a hot plate at 110°C to prepare the corresponding oxide-modified Cu electrodes.

Prepare ZrO2-modified Cu electrodes with different ZrO2 loadings (named Cu/ZrO2-X, where X is the ZrO2 loadings (mg); default Cu/ZrO2 refers to Cu/ZrO2-1) according to the following step which is the same as the preparation of ZrO2-modified Cu electrode (steps 1–3).

Mechanically polish the Cu foil.

Weigh 0.5, 1, 2.5, 5, and 20 mg of ZrO2 nanoparticles and disperse in the mixture of DI water, ethanol, and Nafion solution to prepare ZrO2 inks with different concentrations of ZrO2, respectively.

Drop-coat 32 μL of ZrO2 inks on each side of mechanically polished Cu foil and dry in air on a hot plate at 110°C to prepare the ZrO2-modified Cu electrodes with different loadings.

Since 40 mg of ZrO2 nanoparticles are difficult to be dispersed uniformly in 1 mL Nafion-ethanol-aqueous solution, the Cu/ZrO2-4 electrode is prepared by drop-coating 64 μL of ZrO2 inks (20 mg/mL) on each side of mechanically polished Cu foil (128 μL in total). Moreover, the above-mentioned ZrO2 ink solution should be sonicated for a few minutes before each use to avoid the settling of ZrO2 nanoparticles.

Fabrication of surface-reconstructed Cu electrode (named Cu-Cu2O-X)

Timing: 2 h

Surface-reconstructed Cu electrode is prepared in two steps. The first step is the preparation of ZrO2-modified Cu electrode (Cu/ZrO2-X). The second step is to remove the ZrO2 coating on the Cu surface by sonication to expose the reconstructed Cu surface.

Prepare surface-reconstructed Cu electrode.

Prepare the Cu/ZrO2-X electrode according to step 5.

Remove the Teflon tape on the electrode.

Immerse the Cu/ZrO2-X electrode in a mixture of DI water and ethanol (1:1 by volume) and sonicate until the ZrO2 coating is completely removed (approximately 10 min).

Wipe with dust-free paper to remove excess water and ethanol.

Confine the working area of 0.64 cm2 by encapsulating the electrode with Teflon tape (Figure 2).

Figure 2

Photographs of the surface-reconstructed Cu electrode preparation (take Cu-Cu2O as an example)

(A–D) Photo images of the front and back sides of (A) Cu/ZrO2 electrode, (B) Cu/ZrO2 electrode with removed Teflon tape, (C) Cu/ZrO2 electrode after sonication, (D) Cu-Cu2O electrode with a defined working area by encapsulating the electrode with Teflon tape.

Dry in air on a hot plate at 110°C for 10 min to completely remove residual ethanol and water.

Cu-Cu2O-X was harvested from Cu/ZrO2-X, where X is the loading of ZrO2.

CRITICAL: The final drying operation at 110°C is essential to avoid introducing ethanol impurities into the CO2RR measurement.

Fabrication of ZrO2-modified reconstructed Cu electrode (named Cu/ZrO2-X-Y)

Timing: 2 h

ZrO2-modified reconstructed Cu electrode is prepared in three steps. The first step is the preparation of ZrO2-modified Cu electrode (Cu/ZrO2-X). The second step is to remove the ZrO2 coating on the Cu surface by sonication to expose the reconstructed Cu surface (Cu-Cu2O-X). The third step is to re-load Y mg cm-2 of ZrO2 nanoparticles on the reconstructed Cu electrode (Cu/ZrO2-X-Y).

Prepare Cu/ZrO2-1-1 (named CC/Z) electrode.

Prepare Cu/ZrO2 electrode according to steps 1–3.

Remove the Teflon tape on the electrode.

Immerse the Cu/ZrO2 electrode in the mixture of DI water and ethanol (the volume ratio is 1:1) and sonicate until the ZrO2 coating is completely removed (approximately 10 min).

Confine the working area of 0.64 cm2 by encapsulating the electrode with Teflon tape (Figure 3A).

Figure 3

Photographs of the ZrO2-modified reconstructed Cu electrode preparation (take Cu/ZrO2-1-1 as an example)

(A and B) Photo images of (A) Cu-Cu2O electrode, (B) re-loading and drying ZrO2 ink on one side of Cu-Cu2O electrode on a hot plate at 110°C.

(C) Cu/ZrO2-1-1 electrode.

Re-coat 32 μL of ZrO2 inks (10 mg/mL) on each side of Cu-Cu2O (64 μL in total) and dry in air on a hot plate at 110°C for 10 min (Figures 3B and 3C).

For Cu/ZrO2-2-0.05, the synthesis procedure is similar to that of Cu/ZrO2-1-1.

Prepare Cu/ZrO2-2 electrode according to step 5.

Remove the ZrO2 coating on the Cu surface to prepare Cu-Cu2O-2 according to step 6.

Re-coat 32 μL of ZrO2 inks (0.5 mg/mL) on each side of Cu-Cu2O-2 (64 μL in total) and dry in air on a hot plate at 110°C for 10 min.

For Cu/ZrO2-8-0.5, the synthesis procedure is slightly different from that of Cu/ZrO2-1-1 and Cu/ZrO2-2-0.05.

Drop-coat 64 μL of ZrO2 inks (20 mg/mL) on each side of Cu foil (128 μL in total) and dry in air on a hot plate at 110°C for 10 min.

Remove the ZrO2 coat by sonication and then re-coat 64 μL of ZrO2 inks (20 mg/mL) on each side of Cu foil (128 μL in total) and dry in air at 110°C.

Subsequently, remove the ZrO2 coating again to obtain the Cu-Cu2O-8 electrode.

Re-coat 32 μL of ZrO2 inks (5 mg/mL) on each side of Cu-Cu2O-8 (64 μL in total) and dry in air on a hot plate at 110°C for 10 min.

Synthesis of Cu@ZrO2 nanoparticles

Timing: 8 h

The Cu@ZrO2 nanoparticles are prepared by ultrasonically collecting the ZrO2 nanoparticles on Cu/ZrO2 electrode.

Firstly, prepare the Cu/ZrO2 electrode according to steps 1–3.

Then, immerse the Cu/ZrO2 electrode in the mixture of DI water and ethanol (the volume ratio is 1:1) and sonicate until the ZrO2 coating is completely removed.

Finally, collect the powder by centrifuging the solution containing the ZrO2 nanoparticles at 9,000 rpm for 3 min, wash once with DI water and ethanol, respectively, and dry in vacuum at 60°C for 6 h.

Material characterizations of oxide-modified Cu electrodes

Timing: 6 h

Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Transmission electron microscope (TEM), X-ray absorption spectroscopy (XAS), etc. are carried out to characterize the structure of the oxide-modified Cu electrodes.

For SEM, XRD, and XPS sample preparation, the flat electrode before and after CO2RR are directly cropped into small pieces to characterize.

Prepare the TEM sample according to the following procedures.

Collect powder sample by scraping from flat electrode surface with a blade at first.

Disperse the powder in ethanol aqueous solution and sonicate for 10 min to obtain a uniform suspension.

Drop the suspension onto the molybdenum mesh, dry at room temperature, and examine the sample by TEM.

Prepare the XAS sample according to the following procedures.

Collect powder sample by scraping from flat electrode surface with a blade at first.

Adhere the powder onto a 3 M tape and use for XAS characterization.

Moreover, Cu2O and CuO are used as references for XAS characterization. Cu2O is synthesized according to a reported method (Gao et al., 2020). CuO is prepared by calcining Cu(NO3)2·3H2O in a muffle furnace at 400°C for 2 h in air. And the heating rate is set to 15°C min-1.

For focused ion beam (FIB), a gallium ion source is performed to directly cut the Cu/ZrO2-2-0.05 surface with an area of 28 × 4 mm2.

To minimize the re-oxidation, the electrode after CO2RR needs to be vacuum sealed before being transferred for all characterizations.

Electrochemical CO2 reduction measurement of oxide-modified Cu electrodes

Timing: 1 week

Electrochemical CO2 reduction measurement is carried out in an H-type cell with a three-electrode system in 0.1 M KHCO3 electrolyte (Figure 4). All prepared self-supported electrodes (containing Cu/ZrO2-X, Cu/oxide, Cu-Cu2O-X, and Cu/ZrO2-X-Y electrodes) in this work are used as working electrodes directly without further treatment.

Figure 4

Photographs of the electrochemical CO2 reduction measurement

Bio-Logic electrochemical workstation is used to apply a cathodic potential, the CO2RR is performed in an H-type cell, the CO2 gas flow rate is controlled by a mass flow controller (MFC), and the gas products are detected by online gas chromatography.

Perform electrochemical CO2 reduction measurement in an H-type cell, which is composed of two 60 mL-compartments and separated by a Nafion 115 proton exchange membrane with an area of 1.77 cm2 (1.5 cm in diameter).

Fill the cathode and anode compartments with 30 mL of 0.1 M KHCO3 as electrolyte, and the headspace volume of each compartment after filling electrolyte is 30 mL.

Assemble a three-electrode system in the H-type cell, in which a platinum sheet is used as a counter electrode in the anode compartment, and an Ag/AgCl electrode and the above self-supported electrode are used as reference electrode and working electrode, respectively, in the cathode compartment.

Before electrolysis, saturate the catholyte with CO2 by continuously feeding CO2 gas at a constant rate of 20 mL min-1 to the catholyte for 30 min. Control the flow rate of CO2 by a mass flow controller. Stir the catholyte with a magnetic stirrer throughout the electrolysis of CO2 (default stirring speed was 500 rpm).

Salt bridge filled with saturated KCl aqueous solution is assembled with reference electrode to decrease the solution resistance between the reference electrode and working electrode.

After completing the preparation in step 15, the electrolysis of CO2 is ready to start.

Firstly, pre-activate the working electrode by performing a cyclic voltammetry test with the potential ranging from +0.2 to -0.6 V vs RHE at a scan rate of 50 mV s-1 for 40 cycles.

Then, perform chronoamperometry measurements at a selected cathodic potential for 1–2 h. The applied potential ranges from -0.8 to -1.1 V vs RHE.

To accumulate sufficient liquid products for quantification, the electrolysis time can be extended to 2 h at low negative potential.

To determine the activity of Cu and Cu/ZrO2 electrodes, perform linear sweep voltammetry (LSV) tests in an H-type cell with Ar- or CO2- saturated 0.1 M KHCO3 electrolyte.

Firstly, saturate the catholyte with CO2 or remove oxygen from the catholyte by continuously feeding CO2 or Ar gas at a constant rate of 20 mL min-1 to the catholyte for 30 min.

Then, pre-activate all working electrodes by performing cyclic voltammetry tests with the potential ranging from +0.2 to -0.6 V vs RHE at a scan rate of 50 mV s-1 for 40 cycles.

Finally, perform linear sweep voltammetry measurements at the potential ranging from -0.61 to -1.81 V vs Ag/AgCl at a scan rate of 5 mV s-1. The flow rates of CO2 and Ar during LSV tests are controlled at 20 mL min-1.

The solution resistances at all potentials are automatically compensated by the electrochemical workstation (85% iR-compensated) and potentials are converted to the reversible hydrogen electrode (RHE) scale according to Equation 1.

The pH of Ar-saturated 0.1 M KHCO3 was 7.38 and that of CO2-saturated 0.1 M KHCO3 was 6.8.

Products analysis for electrochemical CO2 reduction measurement

Timing: 1 week

Quantify the gas and liquid products of electrochemical CO2 reduction by online gas chromatography (GC) and 1H nuclear magnetic resonance (1H NMR), respectively.

Gas chromatography is equipped with the flame ionization detector (FID) and the thermal conductivity detector (TCD). The gas stream containing CO2 and gas products flows out of the electrolytic cell, passes through an autosampler, and enters the gas chromatography.

First, separate CO2 and gas products (H2, CO, CH4, C2H4) in the gas stream by the molecular sieve 5A capillary column and TDX-01 column.

The gas stream leaves the column and is first analyzed by the TCD to quantify H2.

Then, passes through a methanizer that converts CO2 and CO to methane.

Finally, into the FID that detects the carbon-containing gas products (Kuhl et al., 2012).

The online gas chromatography is equipped with an autosampler for automated injection. The gas stream out of the electrolytic cell passes through the GC 6-way valve, and the valve is switched to inject single-point gas every 20 min (the sample loop volume in our GC is 1 mL). Except for the sampling time, the gas stream is vented during the electrolysis.

Precise quantification of gas products generated during CO2RR requires a calibration curve for different gas products.

The calibration curve determines the linear relationship between gas concentration and peak area from GC. Thus, the concentration of gas products generated during CO2RR can be determined according to the peak area from GC.

As shown in Figures 5A–5E, the standard gas concentrations versus corresponding peak areas obtained from the GC traces are plotted to obtain the calibration curves for H2, CO, CH4, and C2H4 (Table 1).

Figure 5

GC traces from FID and TCD channels of standard gas

(A–D) GC traces from (A and B) FID channel and (C and D) TCD channel of (A and C) standard gas with low concentration and (B and D) standard gas with high concentration.

(E) Calibration curves for gas products.

Table 1

The concentration of standard gases and the corresponding peak areas obtained from the GC traces

Concentration (ppm)	H2	CO	CH4	C2H4	C2H6
Low concentration	103	104	104	104	104
High concentration	1005	1007	1008	1014	1018
Peak area	H2	CO	CH4	C2H4	C2H6
Low concentration	965.5	79753.5	82726.5	158689.5	157761
High concentration	9777	804667	825807.5	1604148	1579314

Figures 6A and 6B show the GC traces from FID and TCD channels of Cu/ZrO2 tested at -1.05 V vs RHE. The concentrations of H2, CO, CH4, and C2H4 are obtained using the calibration curves.

Figure 6

GC traces from FID and TCD channels of Cu/ZrO2 tested at -1.05 V vs RHE

(A and B) GC traces from (A) FID channel, and (B) TCD channel.

(C) The chronoamperogram curve (i-t curve) of Cu/ZrO2 tested at -1.05 V vs RHE.

Calculate the Faradic efficiencies of gas products according to the following equations.

Calculate the molar amount of gas sampling volume in each injection (n) at 26°C according to the ideal gas law.

Where V is the sample loop volume in our GC (V = 1 mL), P is the pressure (Pa), R is the molar gas constant (8.314 J∙mol-1∙K-1), and T is the temperature (K), which are constants under electrochemical CO2 reduction test conditions.

According to the calibration curves in Figure 5E, determine the concentration of gas products (c in ppm).

Calculate the charge consumed to get 1 mL of c ppm H2, CO, CH4, and C2H4 molecules according to Equation (3).

Where c is the concentration of the gas product, n is the molar amount of 1 mL gas, Z is the number of electrons required to form 1 mol of gas product, NA is the Avogadro constant (6.02 × 1023 mol-1), and e is the electric quantity for one electron (1.602 × 10−19 C).

Calculate the total charge consumed according to Equation (4).

Where I is the current recorded at the time of sampling, t is the time required to fill the sample loop. The sample loop volume (V) in our GC is 1 mL, and the flow rate of gas stream (v) in this work is 20 mL/min. Thus, the time required to fill the sample loop is

Finally, calculate the Faradaic efficiency for gas products according to Equation (6).

Take C2H4 in Figure 6A as an example, the peak area of C2H4 obtained from the GC trace is 742216, thus the concentration of C2H4 is 468.3 ppm calculated from the calibration curve. The charge consumed for producing 1 mL of 468.3 ppm C2H4 is calculated according to Equation (3).

Then, calculate the total charge consumed within 3 s of injection according to Equation (4). The recorded current is 15.99 mA at injection, as shown in Figure 6C.

Hence, the Faradic efficiency for C2H4 is determined as

1H nuclear magnetic resonance (1H NMR) can be used to directly detect liquid products in the presence of electrolyte. Since the peak areas of liquid products in 1H NMR spectra are often affected by shimming, phasing, and number of scans, the peak area does not use to quantify products. According to previous reports (Kuhl et al; Ren et al., 2015), the peak area ratio of internal standard and liquid product is often used to quantify the concentration of the liquid product. Therefore, peak area ratios of liquid products and internal standard versus the known concentrations of liquid products are plotted to obtain the calibration curves for liquid products quantification. Dimethyl sulfoxide (DMSO) is selected as the internal standard. Water suppression mode is used to decrease water peak intensity and make liquid products more visible in 1H NMR spectroscopy.

Obtaining calibration curves for liquid products.

Firstly, prepare 2 mM DMSO in D2O as an internal standard solution.

Secondly, prepare the standard solutions of liquid products containing potassium formate, methanol, potassium acetate, ethanol, ethylene glycol, n-propanol, acetaldehyde, acetone, and propionaldehyde with a given concentration (15 mM, 7.5 mM, 1.5 mM, and 0.5 mM, see materials and equipment).

Finally, mix 500 μL of standard solution with 150 μL of internal standard solution and transfer to the NMR sample tube for 1H NMR measurement.

Figures 7A–7D show the NMR spectra for four standard solutions. Figure 7E shows the calibration curves for various liquid products, which exhibit good linear relationships.

Figure 7

1H NMR spectra of standard solution containing a mixture of liquid products

(A–D) (A) 15 mM of liquid products, (B) 7.5 mM of liquid products, (C) 1.5 mM of liquid products, and (D) 0.5 mM of liquid products.

(E) Calibration curves for liquid products.

Although there are nine target products in the standard solution, only five calibration curves for formate, methanol, acetate, ethanol, and n-propanol are plotted due to the following reasons. (1) The overlapping peaks of acetone and acetaldehyde at 2.1 ppm, (2) the overlapping peaks of ethylene glycol and ethanol at 3.54 ppm, (3) the strong volatility of acetaldehyde and propionaldehyde, (4) the existence of acetaldehyde and propionaldehyde in form of diol and keto in water, (5) the negligible yield of acetone, acetaldehyde, ethylene glycol, and propionaldehyde from CO2RR in our system. The characteristic peaks of various liquid products in the 1H NMR spectrum can be found in the literature (Kuhl et al).

Single peak at 8.33 ppm for HCOO-, single peak at 3.23 ppm for CH3OH, single peak at 1.79 ppm for CH3COO-, triplet peak at 1.06 ppm for C2H5OH, and triplet peak at 3.44 ppm for C3H7OH are used to divide the single peak at 2.6 ppm for DMSO to obtain the peak area ratio for quantification.

Quantification of liquid products of CO2RR.

Firstly, syringe out 500 μL of catholyte from electrolytic cell after 1–2 h electrolysis to mix with 150 μL of internal standard solution and transfer to the NMR sample tube for 1H NMR measurement.

Secondly, determine the peak area ratio of liquid product and internal standard, and calculate the concentration of liquid product according to the calibration curve.

Then, calculate the concentration of liquid product (Cliquid) according to the calibration curve shown in Figure 7E.

Calculate the charge consumed for producing V mL of Cliquid mM liquid products according to the Equation (7).

Where Z is the number of electrons required to form 1 mol of liquid product, NA is the Avogadro constant (6.02 × 1023 mol-1), and e is the electric quantity for one electron (1.602 × 10−19 C).

Obtain the total charge consumed for the whole electrolysis by integrating the chronoamperogram curve (i-t curve). Hence, calculate the Faradic efficiency of liquid product according to the Equation (8).

Take C2H5OH in Figure 8 as an example, the peak area ratio of C2H5OH/DMSO is 0.445, thus the concentration of C2H5OH is 0.624 mM according to the calibration curve of C2H5OH. The charge consumed for producing 30 mL of 0.465 mM C2H5OH is calculated according to Equation (7).

Figure 8

1H NMR spectra of the catholyte after 90 min of CO2RR on Cu/ZrO2 at -1.05 V vs RHE

The total charge consumed by electrolysis in 90 min is 76.35 C by integrating the chronoamperogram curve in Figure 6C.

Hence, the Faradic efficiency for C2H5OH is is determined as

Avoid unit errors when calculating Faradaic efficiency.

Calculate the partial current density

Timing: 1 h

Partial current density is a key indicator to evaluate the catalytic activity of the CO2RR product. The electrochemical active surface area (ECSA)-corrected current density can be used to evaluate the intrinsic activity of the catalyst by excluding the effect of surface roughness.

Calculate the partial current densities of products according to the Equation (9).

where I is the current recorded at the time of sampling gas products, and S is the geometric area of the working electrode.

The ECSA-corrected current density is calculated to compare the intrinsic activity of catalysts. The ECSAs of all working electrodes (S = 1 cm2) are determined by electrochemical double-layer capacitance measurement.

Firstly, determine the double-layer capacitance (Cdl) via cyclic voltammetry measurements, which are recorded in a non-faradaic region with a potential ranging from -0.14 to -0.04 V versus open circuit potential at scan rates of 20, 40, 60, 80, 100, 150 and 200 mV s-1 (Figures 9A and 9B).

Figure 9

Evaluation of electrochemical active surface area (ECSA)

(A and B) Cyclic voltammetry measurements on Cu and Cu/ZrO2 electrodes.

(C and D) (C) Electrochemical double-layer capacitance (Cdl) and (D) electrochemical active surface area (SECSA) of Cu and Cu/ZrO2 electrodes.

Then, plot the current density versus scan rates and perform linear fitting. The slop is considered as the Cdl of the working electrode (Figure 9C).

Calculate the Cdl ratio between the modified Cu electrode and the Cu foil to obtain the value of relative roughness (Rf) for modified Cu electrode according to Equation 10.

Finally, calculate the electrochemical active surface area for modified Cu electrode by the Equation (11) (Figure 9D).

Where CdlCu is 38.2 μF cm-2, S is the geometric area of electrode (S = 1 cm2 in CV test).

Calculate the ECSA corrected current density according to the Equation (12).

Where SECSA is the electrochemical active surface area of the electrode.

All CV tests are carried out in an H-type electrolysis cell with the CO2-saturated 0.1 M KHCO3 electrolyte.

Evaluate the stability of Cu/ZrO2 electrode

Timing: 70 h

Stability is an important index to evaluate the electrochemical CO2 reduction performance of a catalyst. In this work, we evaluate the stability of Cu/ZrO2 electrode by a chronopotentiometry test.

Firstly, prepare the Cu/ZrO2 electrode according to steps 1–3. Then, perform a chronopotentiometry test at -17.2 mA cm-2 in an H-type cell. The catholyte and anolyte are 30 mL of 0.1 M KHCO3 aqueous solution and are refreshed every 6 h to detect the liquid products.

The current density (-17.2 mA cm-2) in response at -1.00 V vs RHE is chosen for the stability test.

In situ surface-enhanced Raman measurement for electrochemical CO2 reduction

Timing: 10 h

To probe the composition evolution of the Cu/ZrO2 electrode and the adsorption status of intermediates at the Cu-ZrO2 interface during CO2RR, conduct in situ surface-enhanced Raman spectroscopy measurements with the help of Au@SiO2 nanoparticles to enhance the Raman signal.

Prepare the electrode for Raman measurement.

Prepare Cu/ZrO2-8-0.5 electrode according to step 9.

To amplify the surface signal, Au@SiO2 nanoparticles are used in Raman measurements.

Specifically, mix 500 μL of Au@SiO2 solution (50 μg/mL) with 2 μL of Nafion solution, and then drop 15 μL of the mixed Au@SiO2 ink onto the surface of electrodes (Cu and Cu-Cu2O-8) and dry on a hot plate (110°C).

For Au@SiO2 modified Cu/ZrO2 electrode, mix 15 μL of Au@SiO2 ink with ZrO2 inks (5 mg/mL) and re-coat on the Cu-Cu2O-8 surface (denoted as Cu/ZrO2-8-0.5).

Perform in situ surface-enhanced Raman measurement using an inVia Reflex Raman microscope (Renishaw) equipped with a diode laser (633 nm) and a water immersion objective (50 ×) in a modified electrochemical cell (Figure 10).

Figure 10

In situ surface-enhanced Raman spectroscopy

(A and B) (A) Photograph of the in situ electrochemical Raman spectroscopy system and (B) schematic diagram of the Raman cell (Adapted with permission from Gaoss Union).

Firstly, calibrate the spectrometer by calibrating the Raman band of a silicon wafer.

Then, pre-activate working electrodes by performing cyclic voltammetry tests with the potential ranging from +0.2 to -0.6 V vs RHE at a scan rate of 50 mV s-1 for 10 cycles.

Afterward, perform the CO2RR test coupled with Raman measurement in a three-electrode system on the electrochemical station (CHI 760E) in CO2-saturated 0.1 M KHCO3 electrolyte at selected potential ranging from -0.2 to -1.2 V vs RHE for 10 min with a potential interval of -0.2 V.

Start recording Raman spectra after CV test or after 5 min of CO2 electrolysis. Each spectrum is recorded using 50% laser power, 10 s of exposure time, and by averaging 2 scans in extended mode.

DFT calculation

Timing: 1 month

Density function theory (DFT) calculations are performed to investigate the adsorption behavior of CO2RR intermediates and the reaction barriers of various reaction pathways for CO2RR at various Cu-oxide interfaces. The structure-property relationship of Cu/oxide catalyst can be elucidated by combining the results of DFT calculations with the CO2RR performance.

Perform DFT calculations by using the Vienna ab initio simulation package (VASP), using the plane-wave basis with an energy cutoff of 500 eV, and the projector augmented wave (PAW) potentials.

Apply the generalized gradient approximation with the function of Perdew–Burke–Ernzerhof (GGA-PBE) to describe the exchange-correlation function.

Adopt Grimme’s semiempirical DFT-D3 scheme of dispersion correction to describe the van der Waals (vdW) interactions.

Set the convergence criteria of the residual Hellmann–Feynman force and energy during structure optimization to 0.02 eV Å-1 and 10−5 eV, respectively.

At first, construct the computational models for Cu-oxide interfaces.

Use the model where an M4Ox (M is Zr, Hf, Al, Si, Ga) cluster is deposited on a three-layer Cu (111) slab to stimulate the Cu-oxide interface.

To reflect the true oxide state under the electrochemical environment, hydrogenate the unsaturated oxygen atoms of M4Ox.

Therefore, adopt the M4OxHy clusters as the model in the DFT calculation.

Model the Cu (111) surface using a three-layer 5 × 5 surface slab with a vacuum layer of 20 Å. Use the Monkhorst-Pack25 k-point sampling of 3 × 3 × 1 mesh in this calculation.

Secondly, optimize the geometry configuration.

During geometry optimization, the M4OxHy cluster and Cu atoms in the top one layer are allowed to relax while atoms in the bottom two layers are fixed.

Apply the COSMO-solvation effect during all calculations to modify the water-solid interface.

Finally, calculate the adsorption energy and Gibbs free energies.

Calculate the adsorption energy (Eads) for the species in the CO2RR process using the Equation (13).

Calculate the Gibbs free energy by the approach developed by Nørskov et al., and evaluate ΔG by Equation (14).

where ΔE, ΔZPE, and ΔS are the energy of reaction, the zero-point energy contribution, and entropy change, respectively. The ZPE of adsorbate is calculated based on the case of Cu (111). T is the temperature (set to 298.15 K). ΔGU = eU is the free energy contribution related to electrode potential U. ΔGpH is the correction of the H+ free energy, which can be calculated through ΔGpH = kBT × ln10 × pH. The kB is the Boltzmann constant and pH is set to 0 (acidic medium). ΔGfield is the free-energy correction resulting from the electrochemical double layer, which is negligible in this study. The proton-electron pair was assumed and the corresponding free energy was expressed using the reversible hydrogen electrode (RHE).

Expected outcomes

This protocol allows for the fabrication of oxide-modified Cu electrodes using a drop-coating method and their application in electrochemical CO2 reduction. The adsorption energies of CO2RR intermediates and the Gibbs free energies for various reaction pathways at Cu-oxide hetero-interfaces can also be calculated according to this protocol. By correlating the structure of Cu/oxide catalysts with CO2RR performance, the structure-property relationship of Cu/oxide catalysts can be elucidated. Among the screened Cu/oxide systems, Cu/ZrO2 catalyst exhibits the optimal catalytic activity and selectivity for C2+ products attributing to the enhanced adsorption of CO2/CO and decreased reaction barrier of the C-C coupling process.

Limitations

The protocol does have some limitations. First, although a flat Cu/oxide electrode is a simple model to study the structure-property relationship, the material characterizations, such as TEM, and XAS, are difficult to conduct for such a bulk material. Second, metallic Cu is easily re-oxidized after removing the cathodic potential, thus it is difficult to characterize the Cu valence state of the Cu electrode and Cu/ZrO2 electrode after CO2RR by ex situ characterizations. Third, the flat Cu/oxide electrode cannot be used in the flow cell and MEA, which hinders its practical application for CO2RR.

Troubleshooting

Problem 1

Oxygen in air affects the accuracy of the ex situ characterization for the electrode after CO2RR.

Potential solution

Transfer the electrode to characterize after vacuum sealing. See examples here.

Problem 2

The larger distance between the reference electrode and working electrode causes a larger solution resistance, which leads to a voltage drop compared to the applied voltage.

Install a salt bridge with a porous ceramic core on the reference electrode to shorten the distance between the reference electrode and the working electrode. Then, automatically compensate the solution resistance by an electrochemical workstation.

Problem 3

Adverse effects of metal ions (Ni4+, Fe3+, etc.), Cl- ions in the used H-type cell for CO2RR performance.

Before the electrolysis of CO2, the H-type cell can be washed in an HNO3 aqueous solution (pH ≈ 2) at 100°C for 1 h to remove impurities and washed in DI water at 100°C for 1 h to remove residual acid.

Problem 4

The total Faradaic efficiency for CO2RR and HER is less than 100%.

Check that the concentration range of calibration curves includes the product concentration. Precise calibration curves ensure accurate results for gas and liquid products.

Problem 5

The weak Raman signals for flat Cu electrode under CO2RR condition.

Modify flat electrodes with Au@SiO2 nanoparticles to enhance the Raman signals. Please refer to step 28 for the preparation of the Au@SiO2-modified electrode. See examples here and here.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Hao Bin Wu (hbwu@zju.edu.cn).

Materials availability

This study did not generate new unique materials.

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Potassium formate (HCOOK, 99%)	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)	CAS: 590-29-4
Methanol (CH3OH, 99.5%)	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)	CAS: 67-56-1
Potassium acetate (CH3COOK, 99.9% metals basis)	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)	CAS: 127-08-2
Ethanol (C2H5OH, 99.7%)	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)	CAS: 64-17-5
Acetone (CH3COCH3, 99.5%)	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)	CAS: 67-64-1
Acetaldehyde (CH3CHO, 99.5%)	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)	CAS: 75-07-0
Propionaldehyde (C2H5CHO, 98%)	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)	CAS: 123-38-6
Ethylene glycol (OHCH2CH2OH, 99.5%)	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)	CAS: 107-21-1
n-Propanol (C3H7OH, 99.5%)	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)	CAS: 71-23-8
Dimethyl sulfoxide (C2H6SO, 99.7% with molecular sieves, water ≤ 50 ppm)	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)	CAS: 67-68-5
Deuterium oxide (D2O, 99.9% D, for NMR)	Energy Chemical (Shanghai, China)	CAS: 7789-20-0
Copper foil (Cu, 99.5%, 0.1 mm thickness)	Tianjin Shentai Chemical Industry Co., Ltd. (Tianjin, China)	CAS: 7440-50-8
Zirconium dioxide (ZrO2, 99.99% metal basis, 40–60 nm)	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)	CAS: 1314-23-4
Silicon dioxide (SiO2, 99.5%, 20–30 nm)	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)	CAS: 14808-60-7
Hafnium oxide (HfO2, 99.99% metal basis, 40–60 nm)	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)	CAS: 12055-23-1
Gallium oxide (Ga2O3, 99.8% metal basis, 500 nm)	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)	CAS: 12024-21-4
Aluminum oxide (Al2O3, 99.99% metal basis, 20–30 nm)	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)	CAS: 1344-28-1
Potassium bicarbonate (KHCO3, 99.5%)	Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China)	CAS: 298-14-6
Potassium phosphate dibasic (K2HPO4, 98%)	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)	CAS: 7758-11-4
Potassium chloride (KCl, 99.5%)	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)	CAS: 7447-40-7
DuPont™ Nafion® PFSA polymer dispersions (D-520, 5% in a mixture of lower aliphatic alcohol and water)	DuPont company (America)	N/A
Proton exchange membrane (Nafion 115)	DuPont company (America), pretreatment conditions are shown in before you begin.	N/A
CO2 gas (99.995%)	N/A	N/A
Ar gas (99.99%)	N/A	N/A
Standard gas (containing H2, CO, CH4, C2H4, C2H6, N2)	N/A	N/A
3,000 mesh silicon carbide paper	KAFUWELL (Hangzhou, China)	N/A
Deionized water (18.25 MΩ cm)	Made by the pure water machine	N/A
Au@SiO2 (Au core 55 nm, SiO2 shell thickness 2 nm)	Shiyanjia Lad (Hangzhou, China)	FMA2109073
Tape for XAFS sample preparation	3M Scotch	N/A
Teflon tape	N/A	N/A
Software and algorithms
Vienna ab initio simulation package (VASP)	N/A	https://www.vasp.at/
Other
Gas chromatography (GC)	FULI	9790II
1H nuclear magnetic resonance (NMR, 600 MHz)	Agilent	DD2-600
Scanning electron microscopy equipped with Energy dispersive spectrometer (SEM-EDS)	SEM: HitachiEDS: Oxford	SU8010X-max80
Transmission electron microscope equipped with Energy dispersive spectrometer (TEM-EDS)	TEM: JEOLEDS: Oxford	JEM-2100FX-MaxN 80T IE250
Ultra-high resolution Scanning electron microscopy- Focused ion beam equipped with Energy dispersive spectrometer (SEM-FIB-EDS)	TESCAN	GAIA3
X-ray diffraction (XRD)	PANalytical	PANalytical X'Pert PRO
X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES)	Thermo Fisher Scientific	Escalab 250Xi
X-ray absorption fine structure (XAFS)	Beijing Synchrotron Radiation Facility (BSRF)	1W1B station
Raman	Renishaw	inVia Reflex Raman
Electrochemical station	BioLogic	MPG2
Electrochemical station	CH Instruments	CHI 760E
Ultrasonic cleaning machine	Prima	PM2-600TD
Pure water machine	ULUPURE	UPH-II-10T
Analytical balance	OHAUS	PX124ZH
High-speed centrifuge	Hettich	Universal 320
Gas mass flow controller (MFC)	HORIBA	STEC S500
H-type cell	Gaoss Union	N/A
In situ Raman electrolytic cell	Gaoss Union	C031-3

ZrO2 inks with different concentration

Reagent	Final concentration	Amount
ZrO2	0.5, 1, 2.5, 5, 10, 20 mg/mL	0.5, 1, 2.5, 5, 10, 20 mg
DI water	N/A	480 μL
Ethanol	N/A	480 μL
Nafion	N/A	40 μL
Total	N/A	1 mL

HfO2 ink

Reagent	Final concentration	Amount
HfO2	10 mg/mL	10 mg
DI water	N/A	480 μL
Ethanol	N/A	480 μL
Nafion	N/A	40 μL
Total	N/A	1 mL

Al2O3 ink

Reagent	Final concentration	Amount
Al2O3	10 mg/mL	10 mg
DI water	N/A	480 μL
Ethanol	N/A	480 μL
Nafion	N/A	40 μL
Total	N/A	1 mL

SiO2 ink

Reagent	Final concentration	Amount
SiO2	10 mg/mL	10 mg
DI water	N/A	480 μL
Ethanol	N/A	480 μL
Nafion	N/A	40 μL
Total	N/A	1 mL

Ga2O3 ink

Reagent	Final concentration	Amount
Ga2O3	10 mg/mL	10 mg
DI water	N/A	480 μL
Ethanol	N/A	480 μL
Nafion	N/A	40 μL
Total	N/A	1 mL

KHCO3 solution

Reagent	Final concentration	Amount
KHCO3	0.1 M	0.1 mol
DI water	N/A	1 L
Total	N/A	1 L

KCl solution

Reagent	Final concentration	Amount
KCl	0.1 M	0.1 mol
DI water	N/A	1 L
Total	N/A	1 L

K2HPO4 solution

Reagent	Final concentration	Amount
K2HPO4	0.1 M	0.1 mol
DI water	N/A	1 L
Total	N/A	1 L

DMSO dissolved in D2O

Reagent	Final concentration	Amount
DMSO	2 mM	0.2 mmol
D2O	N/A	100 mL
Total	N/A	100 mL

Four standard solutions for liquid products

Reagent	Final concentration	Amount
HCOOK	15, 7.5, 1.5, 0.5 mM	1.5, 0.75, 0.15, 0.05 mmol
CH3OH	15, 7.5, 1.5, 0.5 mM	1.5, 0.75, 0.15, 0.05 mmol
CH3COOK	15, 7.5, 1.5, 0.5 mM	1.5, 0.75, 0.15, 0.05 mmol
C2H5OH	15, 7.5, 1.5, 0.5 mM	1.5, 0.75, 0.15, 0.05 mmol
CH3CHO	15, 7.5, 1.5, 0.5 mM	1.5, 0.75, 0.15, 0.05 mmol
CH3COCH3	15, 7.5, 1.5, 0.5 mM	1.5, 0.75, 0.15, 0.05 mmol
C2H5CHO	15, 7.5, 1.5, 0.5 mM	1.5, 0.75, 0.15, 0.05 mmol
OHCH2CH2OH	15, 7.5, 1.5, 0.5 mM	1.5, 0.75, 0.15, 0.05 mmol
C3H7OH	15, 7.5, 1.5, 0.5 mM	1.5, 0.75, 0.15, 0.05 mmol
DI water	N/A	100 mL
Total	N/A	100 mL

Low concentration standard gas

Reagent	Final concentration	Amount
H2	103 ppm	N/A
CO	104 ppm	N/A
CH4	104 ppm	N/A
C2H4	104 ppm	N/A
C2H6	104 ppm	N/A
N2 as carrier gas	n/a	N/A
Total	n/a	N/A

High concentration standard gas

Reagent	Final concentration	Amount
H2	1,005 ppm	N/A
CO	1,007 ppm	N/A
CH4	1,008 ppm	N/A
C2H4	1,014 ppm	N/A
C2H6	1,018 ppm	N/A
N2 as carrier gas	N/A	N/A
Total	N/A	N/A

Stock reagent
Reagent	Storage conditions
Mechanically polished Cu foil	It should be freshly prepared.
Instruments (for material Characterization):
Technique	Condition
X-ray diffraction (XRD)	Using an X'Pert PRO, PANalytical diffractometer with a Cu Kα radiation source at a scan speed of 5° min−1.
Scanning electron microscopy (SEM)	Acceleration voltage: 30 kV
Transmission electron microscope (TEM)	Acceleration voltage: 200 kV
Focused ion beam (FIB)	Gallium ion source
Raman	Using an inVia Reflex Raman microscope (Renishaw) equipped with a diode laser (633 nm) and a water immersion objective (50 ×) in a modified cell. Each spectrum was recorded using 50% laser power, 10s of exposure time, and by averaging 2 scans in extended mode.
Instruments (for CO2RR):
Technique	Condition
Gas chromatography (GC)	Warming procedure: warm-up from 65°C (holding for 3.6 min) to 125°C with 15°C/min, then hold for 7.6 min at 125°C, and finally cool down to 65°C.Each spectrum is recorded with one warming procedure.
1H nuclear magnetic resonance (NMR)	Resonance frequency:600 MHz
Electrochemical measurement	Condition
Cyclic voltammetry test (CV)	Perform at the potential ranging from +0.2 V to -0.6 V vs RHE at a scan rate of 50 mV s-1 for forty cycles
Linear sweep voltammetry test (LSV)	Perform at the potential ranging from -0.61 V to -1.81 V vs Ag/AgCl at a scan rate of 5 mV s-1
Chronoamperometry test (CA)	Perform at a selected cathodic potential ranging from -0.8 V to -1.1 V vs RHE for 1 h
Chronopotentiometry test (CP)	Perform at -17.2 mA cm-2
iR-compensated	The solution resistances at all potential are automatically compensated by an electrochemical workstation (85% iR-compensated)