Selective CO2 Conversion with Novel Copper Catalyst

Abstract

The present disclosure provides hierarchical CuO-derived inverse opal (CuO—IO) electrocatalyst compositions, their synthesis, and their application to selectively convert CO2 into carbon monoxide (CO). The electrocatalyst compositions have a three-dimensional interconnected CuO backbone in hexagonal arrangement. In one embodiment, the compositions have an inverse structure of poly (methyl methacrylate) (PMMA) latex opal. In one embodiment, the electrocatalyst composition inverse-opal structure is comprised of copper-oxide nanoparticles having an average mean diameter ranging from about 15 to about 20 nm. In another embodiment, the compositions have an average cavity size of 175 to 185 nm.

Description

RELATION TO OTHER APPLICATIONS

This application claims priority benefit as a continuation of U.S. Non-Provisional patent application Ser. No. 16/835,381 filed Mar. 31, 2020, currently pending, which in turn claimed priority benefit as a Non-Provisional of U.S. Provisional Patent Application Ser. No. 62/829,446 filed Apr. 4, 2019, currently expired, both of which are incorporated by reference in their entirety herein.

GOVERNMENT INTEREST

The United States Government has rights in this invention pursuant to the employer-employee relationship of the Government to the inventors as U.S. Department of Energy employees and site-support contractors at the National Energy Technology Laboratory.

FIELD OF THE INVENTION

One or more embodiments consistent with the present disclosure relate to copper-oxide electrocatalyst compositions for conversion of CO₂ and water to CO and H₂ (syngas). Accordingly, the disclosure includes materials, methods of their preparation, and methods for using the compositions.

BACKGROUND OF THE INVENTION

Electrochemical CO₂ reduction (EC-CO₂RR) is a promising approach to convert CO₂ emissions into industrially-relevant and value-added chemicals and fuels. However, due to slow kinetics and multi-electron transfer pathway, EC-CO₂RR usually requires significant overpotentials and can suffer from poor product selectivity and competitive hydrogen evolution reaction (HER). The development of highly active, selective and robust CO₂ conversion catalysts is of vital interest to overcome these drawbacks. The activity and selectivity towards specific products strongly depend on electrocatalyst morphology, surface roughness, nature of electrochemically active sites, electronic configuration, transport limitations, local pH environment at the electrode surface, etc.

To date, numerous materials have been widely studied, including metals, oxides, and carbonaceous composites. Expensive metals such as gold and silver can selectively convert CO₂ into CO, a commodity chemical used in a variety of industrial processes, including methanol and Fischer-Tropsch synthesis, among others. Copper derived materials have also attracted much attention due to their low cost, high abundance, and ability to produce hydrocarbons or oxygenated hydrocarbons, and efforts have recently focused on structural control to improve their product selectivity. A number of different structures and dimensions of copper-based catalysts have been investigated, such as nanoparticles, nanofoams, nanowires, prisms, dendrites, etc. CuO-derived hierarchical nanostructures composed of nanowires exhibited selective CO and HCOOH production with a total FE of 82.5% at −0.55 V vs. RHE that was attributed to the 3D porous structure of catalysts. Mesoporous Cu₂O-derived foams were also found to selectively produce C₂H₄ and C₂H₆ with a maximum C₂ FE reaching 55% at −0.9 V vs. RHE owing to the presence of dominant (100) surface sites for C—C coupling and temporal trapping of gaseous intermediates inside the mesopores. Despite this progress, it is still challenging to fully understand the nature of electrochemically active sites because the product selectivity of copper catalysts strongly depends on their structure, morphology, and oxidation state.

Inverse opal (TO) materials have been widely studied for applications in catalysis, photonics, photovoltaic devices, energy conversion, and energy storage. The three-dimensional (3D) interconnected, highly porous structure of IOs are arranged in hexagonal close packed framework and offer large surface-to-volume ratio and better adsorbability of reactant molecules. Despite these benefits, few studies on IO catalysts for EC-CO₂RR have been reported. Porous mesostructured Au and Ag IO catalysts have shown improved CO selectivity due to the generation of pH gradients that reduced proton availability at the catalyst surface and suppressed competitive H₂ evolution. Zhang and coworkers found improved CO selectivity (˜45%) for cube-like Cu—IO, but the oxidation state and crystallographic orientation during EC-CO₂RR were not investigated. However, larger IO pore size significantly decreases CO FE while enhancing H₂ and C₂ formation.

Accordingly, it is an object of this disclosure to provide copper-oxide (Cu—O) based electrocatalysts for the reduction of CO₂ to CO. The Cu—O catalysts have a 3D inverse opal structure and demonstrate high Faradaic efficiencies and current densities, high electrostability, and do not rely on precious metals. It is also an object of this disclosure to provide methods for making the compositions, as well as for use of the compositions. These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.

SUMMARY OF THE INVENTION

The present disclosure provides hierarchical CuO-derived inverse opal (CuO—IO) electrocatalyst compositions, their synthesis, and their application to selectively convert CO₂ into carbon monoxide (CO). The electrocatalyst compositions have a three-dimensional interconnected CuO backbone in hexagonal arrangement. In one embodiment, the compositions have an inverse structure of poly (methyl methacrylate) (PMMA) latex opal. In one embodiment, the electrocatalyst composition inverse-opal structure is comprised of copper-oxide nanoparticles having an average mean diameter ranging from about 15 to about 20 nm. In another embodiment, the compositions have an average cavity size of 175 to 185 nm.

The catalyst compositions provide a 3˜10-fold enhancement in product selectivity with improved CO₂ conversion rates and reaction efficiency compared to currently commercially available materials and similar materials in the open scientific literature. The improvement in catalytic rates, efficiencies, selectivity, and overpotential biases address core technical issues that have prevented the development of effective electrocatalytic technologies for CO₂ utilization.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the multiple embodiments of the present invention will become better understood with reference to the following description, appended claims, and accompanied drawings where:

FIG. 1 depicts (A) SEM image, (B) HR-TEM micrograph, (C) SXRD pattern and (D) XANES Cu K-edge of as prepared CuO—IO, while spectra in (D) are bulk CuO, bulk Cu₂O and Cu foil standards.

FIG. 2 depicts photographs of PMMA film on (A) ITO substrate, (B) top-view and (C) cross-sectional SEM images of PMMA dry opal film.

FIG. 3 depicts (A, B) top-view and (C) cross-sectional SEM images of as-prepared CuO-catalyst layer on hydrophilized glass substrate.

FIG. 4 depicts Fourier-transformed R-space Cu K-edge EXAFS spectra (not phase corrected) of as-prepared CuO—IO, bulk CuO, bulk Cu₂O and Cu foil standards.

FIG. 5 depicts (A) XPS survey, (B) Cu 2p core-level, and (C) Auger Cu LMM, and (D) s-XAS Cu L-edge (in TEY and PFY modes) spectra of as-prepared CuO—IO.

FIG. 6 depicts (A) Potential-dependent Faradaic efficiencies for CO₂ reduction products over CuO—IO catalyst in CO₂ saturated 0.1 M KHCO₃. (B) Long-term electrocatalytic performance of CuO—IO catalyst at −0.6 V vs. RHE. Comparisons of (C) CO Faradaic efficiency (FE) and (D) CO partial current density (j_CO) at various negative potentials for CuO—IO, ˜50 nm CuO NPs, and bulk CuO powder.

FIG. 7 depicts chronoamperometric profiles showing total current density as function of applied constant potential for CuO—IO catalyst (loading of 2.8 mg cm_geo⁻²).

FIG. 8 depicts selectivity of CO₂ reduction products for CuO—IO catalyst at different applied potentials.

FIG. 9 depicts total Faradaic efficiency for C₁ products at various applied potentials of CuO—IO catalyst.

FIG. 10 depicts SEM images of CuO—IO/carbon paper working electrodes with CuO—IO loading of (A, B) 1.5 mg cm_geo⁻² and (D, E) 15 mg cm_geo⁻²; and (C, F) corresponding CO FE vs. applied potentials.

FIG. 11 depicts steady-state current density and FE for CO production at −0.6 V vs. RHE as using graphite counter electrode (in CO₂-saturated KHCO₃, CO₂ flow rate of 20 mL min⁻¹, catalyst loading of 2.8 mg cm_geo⁻²).

FIG. 12 depicts Faradaic efficiency for CO, CH₄ and H₂ at different cathodic potentials over bare carbon paper (in CO₂-saturated KHCO₃ electrolyte, CO₂ flow rate of 20 mL min⁻¹).

FIG. 13 depicts (A) XPS survey, (B) Cu 3p-Pt 4f, (C) Cu 3s-Pb 4f, (D) Zn 2p, and (E) Fe 2p core-level spectra of CuO—IO post-electrodes after long-term run at −0.6 V vs. RHE using Pt wire and graphite counter electrodes.

FIG. 14 depicts (A-D) SEM images of CuO—IO electrode at different spots after 24-hour electrolysis at −0.6 V vs. RHE, (E) EDX analysis.

FIG. 15 depicts (A, B) SEM images, (C) XRD patterns, and (D) XAS Cu L-edge of CuO NPs and bulk CuO.

FIG. 16 depicts Faradaic efficiency for EC-CO₂RR products as a function of potentials for ˜50 nm diameter CuO NPs.

FIG. 17 depicts Faradaic efficiency for EC-CO₂RR products as a function of potentials for bulk CuO powder.

FIG. 18 depicts CO selectivity at various negative potentials for CuO—IO, ˜50 nm diameter CuO NPs, and bulk CuO powder.

FIG. 19 depicts (A) Total current density vs. cathodic potential and (B) Tafel plot for CO production over CuO—IO, ˜50 nm CuO NPs, and bulk CuO powder.

FIG. 20 depicts (A) Cyclic voltammetry of fresh CuO—IO electrode in CO₂ saturated 0.1 M KHCO₃. (B) Potential-dependent k²-weighted R-space EXAFS analysis (no phase correction) from −0.2 V to −1.2 V vs. RHE (collected at 30 min at each potential). (C) In situ Raman spectra for tracking surface structure of CuO—IO during EC-CO₂RR at −0.6 V (using 785 nm laser source). (D) SXRD patterns of CuO—IO electrode under open circuit and steady state at −0.6 V vs. RHE (*indicates residual carbon paper features and Kapton window from background subtraction).

FIG. 21 depicts in situ Cu K-edge XANES of CuO—IO catalyst during chronoamperometry from −0.2 V to −1.2 V vs. RHE (collected at 30 min at each potential, with comparison to Cu foil standard).

FIG. 22 depicts in situ (A) Cu K-edge XANES and (B) EXAFS measurements for real time tracking the oxidation state changes of CuO—IO during EC-CO₂RR condition at −0.6 V. The results at steady state are very close to prominent features in Cu foil standard, indicating the reduction of CuO towards metallic copper under working conditions.

FIG. 23 depicts ex situ Raman spectra of CuO—IO/carbon paper electrodes before (top) and after 24-h durability test at −0.6 V vs. RHE (bottom) using 633 nm laser.

FIG. 24 depicts in situ Raman spectra of CuO—IO during EC-CO₂RR at various applied potentials (using 785 nm laser source).

FIG. 25 depicts Raman analysis of CuO—IO collected after CO₂ electrolysis at −0.6 V vs. RHE and returned to open circuit (785 nm laser source).

FIG. 26 depicts (A) In situ time-resolved SXRD data for tracking the crystallographic changes of CuO—IO from CuO phase under open circuit to metallic Cu during EC-CO₂RR at −0.6 V vs. RHE (* indicates residual carbon paper features and Kapton window from background subtraction). (B) Relative peak intensity ratio of Cu (111) and Cu (200) and crystallite size of Cu as a function of electrolysis time showing the consistency over five hours at −0.6 V vs. RHE.

FIG. 27 depicts SXRD patterns of CuO—IO electrode collected after −0.6 V reduction for 5 hours and returned to open circuit for 120 min indicating the gradual re-oxidation of copper to oxide phases over time (* indicates residual carbon paper features from background subtraction).

FIG. 28 depicts non-Faradic double layer charging current plotted against scan rate for double layer capacitance estimation for (A) CuO—IO, (B) ˜50 nm diameter CuO NPs, and (C) bulk CuO electrodes (in CO₂-saturated 0.1 M KHCO₃ electrolyte).

DETAILED DESCRIPTION OF THE INVENTION

The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best mode contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the principles of the present invention are defined herein specifically to provide description of hierarchical oxide-derived copper inverse opal (CuO—IO) electrocatalyst compositions, methods of their preparation, and methods for using such compositions.

The hierarchical oxide-derived copper inverse opal (CuO—IO) electrocatalysts provide selective CO production and strong suppression of H₂ evolution over a wide potential window compared with typical oxidized copper materials. The CuO—IO compositions are characterized by their structure, high Faradaic efficiencies, and high electrostability.

The compositions are in general terms synthesized through a three-part process. First, an opal template is supplied. Second, the catalyst material is supplied to the opal template such that the catalyst material infiltrates the structure of the opal template. Third, the catalyst structure is fixed, and the template removed. In a non-limiting example of the general method, first, an opal template is formed by evaporation-induced vertical deposition of PMMA latex. Second, the latex is infiltrated by Cu catalyst material. Third, the infiltrated latex is annealed in air to remove the opal template and form the Cu oxide inverse opal.

As noted above, inverse opal (TO) materials provide a three-dimensional (3D) interconnected, highly porous structure arranged in hexagonal close-packed framework. IO materials offer large surface-to-volume ratio and better adsorbability of reactant molecules. As noted, such IO materials are formed by the infiltration of the precursor IO material into a suitable template substrate, followed by formation of the IO and removal of the substrate. Suitable template materials include poly (methyl methacrylate) spheres, polystyrene spheres, carboxylic polystyrene spheres, poly(styrene-methyl methacrylate-3-sulfopropyl methacrylate, potassium salt) spheres, poly(n-butyl acrylate-acrylic acid) spheres, carbon spheres, silica spheres, or other suitable spherical templates. Accordingly, such materials will provide a template for the hexagonal framework of the inverse opal structure, as determined through electron microscopy, X-ray diffraction, UV-Vis-NIR absorption spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy.

The templates may be contacted with the Cu catalyst precursors using one or more of methods in order to facilitate the formation of the IO structure. Such methods include infiltration in ambient environment or under vacuum, chemical vapor deposition, chemical bath deposition, electrochemical deposition, atomic layer deposition, convective self-assembly, evaporative co-assembly, nanoparticle suspension, or other suitable techniques. In carrying infiltration as used in the examples below, the template is contacted with a solution including the catalyst precursor such that the catalyst precursor will infiltrate and fill the interstitial voids of the template. The solvent is then allowed to evaporate such that the catalyst precursor remains fixed within the template.

Following infiltration, the Cu catalyst precursor is structurally stabilized and the opal template may be removed. One such method to carry out the stabilization and removal is by thermal annealing. In annealing, the infiltrated template is heated to a temperature sufficient to fix the structure of the Cu catalyst precursor to that of the CuO catalyst and the opal template is removed by decomposition. The annealing operation thus results in catalyst retaining the final inverse opal structure. Other methods of fixing the catalyst precursor may include wet chemical etching, plasma treatment or Ozone oxidation.

The resulting hierarchical oxide-derived copper inverse opal (CuO—IO) electrocatalyst compositions have structure characterized by the inverse of the supporting template, that is, they have the structure of the interstitial voids of the template. The compositions thus have a hexagonal structure comprised of CuO nanoparticles. In one embodiment, the CuO nanoparticles have a mean average diameter of 15-20 nm.

The electrocatalyst compositions are of use in the conversion of CO₂ and H₂O to CO and H₂ (syngas). One exemplary method utilizing the electrocatalyst compositions includes loading the catalyst onto a suitable electrode material, placing the catalyst containing electrode into an aqueous electrolyte containing dissolved CO₂, and applying a negative potential to the electrocatalyst containing electrode. Another exemplary method includes constructing a gas diffusion-style electrode or membrane electrode assembly using the catalyst. This type of configuration would be used to assembly an electrolyzer-style electrode where negative electrode potentials are applied to convert humidified CO₂ gas streams into CO.

The electrocatalyst compositions of the present disclosure provide high selectivity in the production of CO. The CO selectivity is defined as the ratio between the production rate for CO and total production rate for all reduced products, including CO₂-derived products and H₂ evolved from water splitting. In one embodiment, the electrocatalyst compositions provide a CO selectivity greater than about 80%. In another embodiment, the electrocatalyst compositions provide a CO selectivity greater than about 90%.

$F{E}_i=\frac{z_i*F*n_i}{I*t}$

The electrocatalyst compositions of the present disclosure also provide high CO Faradaic efficiency. Faradaic efficiency (FE) is defined as the percentage of supplied electrons used to convert CO₂ into products such as CO, and is calculated by dividing the quantity of produced product molecules by the number of supplied electrons compared with the theoretical number of electrons required to form that quantity of product molecules. Specifically:

where _i is the number of electrons involved in the formation of i product (=2 for CO, H₂, and HCOOH, =8 for CH₄, =12 for C₂H₄, and z=14 for C₂H₆); F is the Faraday's constant; n_i is the number of moles of product i formed (determined by GC and IC); I is the total current; and t is electrolysis time. In one embodiment, the electrocatalyst compositions provide a Faradaic efficiency of greater than about 65%. In another embodiment, the electrocatalyst compositions provide a Faradaic efficiency of greater than about 70%.

In one embodiment, the electrocatalytic composition comprises copper-oxide having an inverse-hexagonal opal structure; where the inverse-opal structure is a negative replica of poly (methyl methacrylate) opal; where the electrocatalyst has an average cavity size ranging from about 175 to about 185 nm; where the composition has a Faradaic efficiency greater than about 70% at −0.6 V vs. RHE; and where the composition has a CO to H₂ selectivity up to about 90% at −0.7 V vs. RHE.

Examples

Synthesis

A poly (methyl methacrylate) (PMMA) latex was prepared by surfactant-free emulsion polymerization using a cationic free radical initiator. Deionized water (DIW) (875 mL) and methyl methacrylate (100 g) were mixed at room temperature under a nitrogen flow for 30 min and then maintained at 70° C. Subsequently, a solution containing 0.15 g of 2,2′-azobis (2-methylpropionamidine) dihydrochloride and 25 mL of DIW was quickly added. A milky white suspension was formed, and the suspension was maintained at 70° C. for 6 h to complete the polymerization. After cooling to room temperature for 1 h, the concentration of obtained PMMA latex (diameter of ca. 210 nm) was 10 wt %.

Bare glass substrates were cut into 1 cm×3 cm pieces and cleaned with a mixture of acetone, isopropanol and deionized water (DIW) for two hours and then immersed in aqueous sodium hydroxide solution (0.5 M) for at least six hours to hydrophilize the surface. The hydrophilized substrates were finally rinsed by DIW and dried under N₂ flow.

PMMA opal films were grown by the evaporation-induced vertical deposition technique. The stock PMMA colloidal suspension was diluted in DIW to achieve the concentration of 0.5 wt %. The hydrophilized substrate was partially immersed into 5 mL of PMMA solution with an angle of 45˜60° and left in an electric oven at 35° C. with controlled humidity of ˜80% for several days to form a self-assembled opal in a fcc crystalline lattice over an area of 1 cm×1.5 cm. The opal film was then sintered at 80° C. for 30 minutes to enhance the domain arrangement and mechanical stability.

The CuO—IO electrocatalyst compositions were then prepared by infiltration of copper precursor solution with the PMMA opal film. 20 μL of copper precursor solution including Cu(NO₃)₂·3H₂O (0.625 g), C₆H₈O₇·H₂O (0.375 g), and C₂H₅OH (10 mL, 200 proof) was penetrated slowly into 10°-tilted PMMA opal and naturally evaporated overnight. The infiltrated film was subsequently annealed in air at 400° C. with ramping rate of 1° C. min⁻¹ for 4 h to completely remove PMMA and reassemble hierarchical CuO inverse opal (namely CuO—IO) as a negative replica of bare PMMA opal.

Electrochemical CO₂ Reduction Measurement

Electrochemical CO₂ reduction experiments were carried out in a gas-tight, two-compartment H-cell separated by a Nafion 117 proton exchange membrane. Each compartment was filled with 50 mL of aqueous 0.1 M KHCO₃ electrolyte (99.99%, Sigma-Aldrich) and contained 100 mL headspace. The catholyte was continuously purged with CO₂ (99.999%, Butler gas) at a flow rate of 20 mL min⁻¹ (pH˜6.8) during the experiments and stirred at 200 rpm. The counter and reference electrodes were Pt wire and Ag/AgCl (saturated NaCl, BASK)), respectively. The catalyst ink was prepared by dispersing 4 mg of as-prepared CuO—IO (scraped down from the glass substrates) in 200 μL of methanol and 10 μL of Nafion® 117 solution binder (Sigma-Aldrich, 5%). Working electrodes were fabricated by drop-casting the prepared ink onto PTFE-coated carbon paper gas diffusion layer (Toray paper 060, Alfa Aesar). The as prepared CuO—IO loading on carbon paper was kept at 2.8±0.1 mg cm_geo⁻² (based on geometric area) unless otherwise noted.

CO₂ reduction examples were performed at ambient temperature and pressure using a SP-300 potentiostat (BioLogic Science Instrument). All potentials were referenced against the reversible hydrogen electrode (RHE) and the uncompensated resistance was automatically corrected at 85% (iR-correction) using the instrument software. Typical working electrode resistances were 30-40Ω. Short-term chronoamperometric examples were conducted for 30 min at each applied potential sequentially between −0.2 V and −1.2 V vs. RHE. Long-term chronoamperometric examples were conducted for 24 hours at −0.6 V vs. RHE. The total and partial current densities were normalized to the exposed geometric area. Each data point is an average of at least three independent experiments on different fresh electrodes. The evolved gas products were quantified by PerkinElmer Clarus 600GC equipped with both FID and TCD detectors, using ShinCarbon ST 80/100 Column and He as a carrier gas. The GC was calibrated regularly using a calibration mixture of gases with known composition. The liquid products in the catholytes were determined by Dionex ICS-5000+ ion chromatography using ED50 conductometric detector, ASRS suppressor in auto-generation mode, AS11-HC column and KOH eluent with a gradient of 0.4-30 mM in 45 min run.

Materials Characterizations

The electron microscopy images in FIG. 1A and FIGS. 2-3 show a three-dimensional interconnected CuO backbone with an average cavity size of 180±5 nm. The HR-TEM micrograph in FIG. 1B reveals the CuO—IO structure is composed of 15˜20 nm CuO nanoparticles with lattice spacings of 0.272 nm, 0.252 nm, and 0.232 nm that can be indexed to (110), (002), and (200) planes of polycrystalline CuO, respectively.

The synchrotron XRD pattern of CuO—IO in FIG. 1C displays several diffraction peaks representative of monoclinic CuO (space group C₂/c) with lattice constants a=4.7119 Å, b=3.4350 Å, c=5.1164 Å. Neither cuprite Cu₂O nor metallic copper phases were found, and the mean CuO crystallite size of ca. 15 nm closely matched particle sizes determined from HR-TEM imaging. Cu K-edge XANES spectra (FIG. 1D) reveal that the line shape, and the positions of pre-edge (1s→3d transition), shakedown feature (1s 4p transition) and white line for CuO-TO resemble the CuO standard, indicating Cu²⁺ oxidation state. Two maxima centered at 1.53 and 2.52 Å in corresponding Fourier transformed k²-weighted EXAFS spectrum (FIG. 4) are ascribed to Cu—O bond in the nearest neighbor shell and Cu—Cu in the next near neighbor coordination, respectively. Additional XPS, Auger and Cu L-edge XAS data in FIG. 5 also confirmed the presence of CuO.

Performance

The CO₂ electroreduction activity of the CuO—IO electrocatalyst composition was evaluated using chronoamperometry between −0.2V and −1.2V vs. the reversible hydrogen electrode (vs. RHE) as shown in FIG. 6A. High C₁ selectivity was demonstrated over an extremely wide potential range, with only minor C₂H₄ and H₂ between −0.9 V and −1.2 V and trace ethane −1.2 V (<0.1% FE) (FIGS. 8-9 and Table 1). Total geometric current densities of CuO—IO were comparable to other oxide-derived copper electrocalysts (FIG. 7); however, C₁ product selectivity was much higher than expected for copper-based catalysts. Methane and formic acid were the dominant products below −0.4 V, while CO production increased to a maximum Faradaic efficiency (FE) of 72.5% at −0.6 V and maximum CO selectivity up to nearly 90% was observed between −0.7 V and −0.8 V (FIG. 8). An average C₁ FE of 78±2% was observed between −0.2 to −1.1V vs. RHE (FIG. 9), which decreased to approximately 60% at −1.2V vs. RHE due to the increased HER at large overpotentials. The CO yield increased from 30 μmol at −0.2 V to 60˜70 mmol of CO per gram of catalyst per hour at potentials more negative than −1.0 V (Table 1). Control experiments with different CuO—IO catalyst loadings, varied catalyst layer thickness, and a graphite counter electrode resulted in similarly high CO selectivity (FIGS. 10-11). Finally, the bare carbon paper demonstrated almost exclusive H₂ production with only trace CO and CH₄ detected from −0.7 V to −1.2 V (FIG. 12).

Long-term CO₂ electrolysis demonstrated consistent CO selectivity for the CuO—IO electrocatalyst. As shown in FIG. 6B, an average 67±2% CO FE was found over 24 hours at −0.6 V with a stable current density of ca. 2.5 mA cm⁻² and no detectable H₂ evolution. XPS analysis of post-reaction electrodes after long-term runs at −0.6 V using Pt wire and graphite counter electrodes ruled out significant deposition of trace Pt, Zn, Pb or Fe elements onto the electrode surface (FIG. 13). Post reaction electron microscopy in FIG. 14 revealed the catalyst preserved its general inverse opal structure. SEM images of CuO—IO electrode at different spots after 24-hour electrolysis at −0.6 V vs. RHE demonstrated that CuO—IO retained its structure after durability test despite partial structural dissolution into nanoparticle agglomerates (occupying approximately 20-30% of entire post-electrode). Post-reaction EDX did not identify Pt. The sustained CO selectivity and current density over 24-hour operation indicates the CuO—IO catalyst is a robust CO₂-to-CO conversion catalyst.

TABLE 1
Potential dependent Faradaic efficiencies (FE, %) and formation rate (r, mmol
gcatalyst−1 h−1) of EC-CO2RR products for CuO-IO.
V vs.	CO	CH4	C2H4	C2H6	HCOOH	H2
RHE	FE	r	FE	r	FE	r	FE	r	FE	r	FE	r
−1.2	31.1	64.2	6.7	0.7	1.9	0.7	<0.1	6.6 × 10−3	25.6	39.7	20.4	50.1
−1.1	36.1	59.2	17.6	7.2	2.8	0.78	—	—	23.3	22.9	10.5	17.3
−1.0	54.9	68.0	16.0	5.3	3.6	0.9	—	—	11.4	11.3	4.3	6.1
−0.9	49.1	43.5	7.2	1.5	0.45	5.8 × 10−2	—	—	14.7	11.7	5.7	4.5
−0.8	63.2	28.2	2.7	0.3	—	—	—	—	7.8	3.9	—	—
−0.7	66.5	16.1	1.8	0.1	—	—	—	—	5.5	1.9	—	—
−0.6	72.5	8.6	1.0	2.9 × 10−2	—	—	—	—	9.2	2.9	—	—
−0.5	59.3	3.00	1.9	2.4 × 10−2	—	—	—	—	13.6	1.8	—	—
−0.4	48.2	0.8	7.0	2.7 × 10−2	—	—	—	—	23.2	0.5	—	—
−0.3	22.7	0.2	18.9	3.1 × 10−2	—	—	—	—	48.1	0.3	—	—
−0.2	9.7	3.1 × 10−2	40.7	3.1 × 10−2	—	—	—	—	21.8	0.1	—	—

For comparison, the EC-CO₂RR performance was tested of commercially available, ˜50 nm diameter CuO nanoparticles (NPs) and bulk CuO powder (˜1-5 μm). The morphology, crystal structure and oxidation state of these oxide materials were determined by SEM, SXRD and XAS measurements (FIG. 15). The spectra of FIG. 15 show high crystallinity of monoclinic cuprous oxide of both CuO NPs (˜50 nm diameter) and bulk CuO (˜1-5 μm). Small portion of cubic Cu₂O was detected in bulk CuO powder. The potential-dependent Faradaic efficiencies for all products in FIGS. 16-17 show that these traditional CuO catalysts produced mostly H₂ (FE ˜50-70%) with much smaller amounts of CO (FE<20%) at moderate negative potentials and ˜20% FE of ethylene at high overpotentials. The product distribution obtained over these catalysts is similar to those of many copper catalysts reported before. As shown in FIGS. 6C-6D and FIG. 18, the CuO—IO demonstrated substantially higher FEs and selectivities towards CO, and CO partial current density (j_CO) compared with the CuO NPs and bulk CuO catalysts. The 141 mV dec⁻¹ Tafel slope for CO production at CuO—IO was close to the 120 mV dec⁻¹ expected for a rate determining step involving the initial electron transfer to CO₂ (FIG. 19). Tafel slopes for the CuO NPs and bulk CuO were 178 and 184 mV dec⁻¹, respectively.

The Pourbaix diagram for the Cu—H₂O system indicates CuO should reduce to metallic Cu under EC-CO₂RR at potentials more negative than −0.5 V, which is consistent with the cyclic voltammogram (CV) of CuO—IO in CO₂ saturated 0.1 M KHCO₃ (FIG. 20A). The oxidation state of Cu-based catalysts during CO₂RR is still debated in the literature, and in situ XAS, Raman spectroscopy, and XRD experiments was conducted to monitor the oxidation state, surface structure, crystallographic orientation, and crystallite size of the CuO—IO catalyst under electrochemical potential control. Cu K-edge XANES and EXAFS were collected at various potentials in CO₂ saturated 0.1 M KHCO₃. The EXAFS spectra in FIG. 20B show the CuO—IO was in the Cu²⁺ oxidation state under open circuit. A reduction of the Cu—O and Cu—Cu scattering peaks of CuO at 1.53 Å and 2.52 Å, and the emergence of the first Cu—Cu coordination shell in metallic Cu at 2.21 Å indicate the onset of Cu-oxide reduction at an applied potential of −0.2V vs. RHE. These changes became more apparent with increasingly cathodic potentials, and comparison with the bulk Cu foil reference indicates near complete reduction beyond −0.6V vs. RHE. These potential-dependent spectroscopic changes are consistent with the Cu-oxide reduction peak centered at approximately −0.45 V vs. RHE in FIG. 20A, and they agree with the Cu—H₂O Pourbaix diagram. The associated Cu K-edge XANES spectra collected at different potentials and while being held at −0.6V are presented in FIGS. 21-22.

In situ Raman spectroscopy was employed as a more surface sensitive technique to probe the surface structure changes of CuO—IO during the application of electrochemical potentials. Ex situ Raman spectrum in FIG. 23 shows three indicative Ag, Big, and Beg modes for fresh CuO—IO electrode. FIG. 20C shows that the predominant Ag feature of CuO at 294 cm⁻¹ gradually disappeared during the application of at −0.6 V vs. RHE in CO₂-purged KHCO₃, and no other peaks associated with Cu₂O were found during the reaction in the region of 100-250 cm⁻¹ and 330-600 cm⁻¹. Similar results were obtained at −0.8V and −1.0 V vs. RHE (FIG. 24). Metallic copper is Raman-inactive, and the immediate decrease in intensity and subsequent disappearance under electrocatalytic potentials strongly indicate the reduction of CuO into Cu⁰ on the electrocatalyst surface. Raman spectrum collected once the catalyst returned to open circuit showed the presence of mixed Cu₂O and CuO species (FIG. 25), which reflects the reversibility of the Cu redox process

In situ SXRD collected under open circuit (FIG. 20D) identified CuO before reaction, which is consistent with both Cu K-edge XAS and Raman measurements. Under steady state operating conditions at −0.6 V vs. RHE, the presence of CuO (111), (200), (220), (311), and (222) peaks indicative of face-centered cubic Cu (space group Fm-3m) was found. The results identify metallic copper with lattice constant a=b=c=3.6102 Å and mean crystallite size of 10-11 nm during CO₂RR at −0.6V vs. RHE, which is consistent with pre-reaction TEM analysis. Only metallic Cu⁰ was identified, and no obvious signatures were observed associated with copper oxides under working conditions (FIG. 26A). The peak intensity ratio of Cu(111) to Cu(200) for an ideal polycrystalline Cu surface was reported to be ca. 3.03. The average relative intensity ratio of Cu(111) and Cu(200) peaks at during EC-CO₂RR at −0.6V vs. RHE is 3.58 (FIG. 26B), implying the dominance of closed-packed Cu(111) surface or preferential (111) orientation of the catalyst during the reaction. The crystallographic orientation and crystallite size of operating catalyst did not substantially change during five hours of measurement at −0.6V vs. RHE. Similar to Raman measurements, re-oxidation of the catalyst was observed once it was returned to open circuit after the application of cathodic potential (FIG. 27). Correlating these various in situ measurements indicates that Cu⁰ with a preferred Cu (111) orientation is a dominant species present in the CuO—IO catalyst during EC-CO₂RR.

The CuO—IO catalyst demonstrated some of the highest CO₂-to-CO selectivity reported for oxide-derived copper electrocatalysts to date (Table 2) at low to moderate overpotentials.

TABLE 2
Comparison of CO2 conversion to C1 products of present study
and previous oxide-derived Cu and Cu based catalyst reports.
	Major C1	FE/	Potential/V
Sample	product	%	vs. RHE	References
CuO-derived Cu inverse opal	CO	72.5	−0.6	This work
	HCOOH	48.1	−0.3
Cu2O-derived Cu	CO	45	−0.35	Li et al., J. Am. Chem. Soc. 2012,
	HCOOH	38	−0.55	134, 7231-7234
CuO-derived Cu nanowire	CO	50	−0.6	Ma et al., Phys. Chem. Chem.
	HCOOH	40	−0.7	Phys. 2015, 17, 20861-20867
Cu2O-derived Cu inverse opal	CO	45.3	−0.6	Zheng et al., Nano Energy 2018,
	HCOOH	34.5	−0.8	48,93-100
Electrochemically reduced CuO-	CO	62	−0.4	Cao et al., ACS Catal. 2017, 7,
derived Cu nanowire	HCOOH	25	−0.5	8578-8587
Electrochemically reduced CuO-	CO	61.8	−0.4	Raciti et al., Nano Lett. 2015, 15,
derived Cu nanowire	HCOOH	30.7	−0.6	6829-6835
3 D CuO-derived Cu hierarchical	CO	60	−0.55	Raciti et al., ACS Appl. Energy
nanostructures	HCOOH	30~40	−0.55~−0.7	Mater. 2018, 1, 2392-2398
Chrysanthemum-like Cu	HCOOH	50~70	−1.6	Xie et al., Electrochimica Acta
nanoflower				2014, 139 137-144
Cu nanoparticles/N-doped carbon	CO	23	−0.7	Song et al., ChemistrySelect
nanospike	CH4	35	−0.9	2016, 1, 6055-6061
Cu nanoparticles/reduced	CO	40	−0.6	Hossain et al., Sci. Rep. 2017, 7,
graphene oxide	HCOOH	46.2	−0.4	3184
Cu nanoparticles/reduced	CO	50	−0.4	Cao et al., J. CO2 Util. 2017, 22,
graphene oxide				231-237
Cu/carbon aerogels	CO	35	−0.81	Han et al., Electrochim. Acta
				2019, 297, 545-552
Cu nanoparticles/carbon aerogels	CO	75.6	−0.6	Xiao et al., J. Colloid Interface
				Sci. 2019, 545, 1-7

Selective EC-CO₂RR performance and strong HER inhibition demonstrated by the CuO—IO electrocatalyst compositions may be attributed to both its 3D morphology and crystallographic surface orientation. The CuO—IO catalyst is composed of small nanoparticles in a 3D interconnected porous structure that offers a large surface-to-volume ratio. As shown in FIG. 28 and Table 3, the ECSA (˜2.96 cm²) and RF (˜30.8) of CuO—IO were considerably larger than the CuO NPs and bulk CuO powder (0.37˜0.55 cm² and RF=5.3˜7.8). The preferential Cu(111) surface/orientation is also expected to demonstrate higher C₁ selectivity due to weaker binding of *CO and *COOH intermediates, whereas Cu(100) facets have favored C₂₊ production owing to a lower energetic barrier for intermediate hydrogenation. In the low and moderate overpotential range (−0.2 to −0.8V vs. RHE), the CuO—IO catalyst produced exclusive C₁ products and almost no H₂ evolution. In this potential range, the high surface roughness allowed rapid consumption of both CO₂ and H⁺ at the catalyst surface. The observed current density likely increased the local pH sufficiently to reduce the number of protons available for HER, while the Cu (111) orientation favored C₁ production over C₂ formation. In the high overpotential range, large current densities can increase the local pH at copper electrodes to 13 or higher. These basic conditions can activate additional C₂ forming reaction pathways and deplete the concentration of available CO₂ molecules. The effect of these two phenomena was observed beyond −0.8V vs. RHE with increased H₂ evolution and the emergence of C₂ product formation at the CuO—IO catalyst. While hydrocarbon formation was observed at large overpotentials, methane was substantially favored over ethylene. These results were largely consistent with the expected C₁ preference of Cu(111) facets.

TABLE 3
ECSA and RF of CuO—IO,
bulk CuO and CuO NPs electrodes.
	Sample	ECSA/cm2	RF
	CuO—IO	2.962	30.79
	Bulk CuO	0.372	5.27
	CuO nps	0.548	7.76

In summary, the roughened, porous hierarchical CuO-derived IO catalyst has shown impressive CO selectivity across a wide potential range with negligible H₂ evolution compared with bulk oxidized copper surfaces. The electrocatalyst compositions comprising a 3D interconnected porous structure made of small nanoparticles in Cu-promoted EC-CO₂RR by creating local pH gradients within the catalyst pores that deplete the local concentration of protons available for HER. In addition, the high roughness surface and the dominance of active Cu (111) surface site would facilitate a C₁ reaction path.

Having described the basic concept of the embodiments, it will be apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations and various improvements of the subject matter described and claimed are considered to be within the scope of the spirited embodiments as recited in the appended claims. Additionally, the recited order of the elements or sequences, or the use of numbers, letters or other designations therefor, is not intended to limit the claimed processes to any order except as may be specified. All ranges disclosed herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof. Any listed range is easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as up to, at least, greater than, less than, and the like refer to ranges which are subsequently broken down into sub-ranges as discussed above. As utilized herein, the terms “about,” “substantially,” and other similar terms are intended to have a broad meaning in conjunction with the common and accepted usage by those having ordinary skill in the art to which the subject matter of this disclosure pertains. As utilized herein, the term “approximately equal to” shall carry the meaning of being within 15, 10, 5, 4, 3, 2, or 1 percent of the subject measurement, item, unit, or concentration, with preference given to the percent variance. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the exact numerical ranges provided. Accordingly, the embodiments are limited only by the following claims and equivalents thereto. All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.

Claims

1. A method for the synthesis of an electrocatalyst composition, the method comprising:

providing a polymethylmethacrylate latex;

infiltrating the polymethylmethacrylate latex with a copper precursor; and,

annealing the infiltrated polymethylmethacrylate latex to provide an electrocatalyst having an inverse opal structure.

2. A method for electrochemical conversion of CO2 to CO comprising:

providing a working electrode comprising an electrocatalyst, wherein the electrocatalyst comprises a 3D interconnected porous copper inverse-opal structure;

applying a negative potential to the working electrode;

contacting the working electrode with CO2, wherein the CO2 is reduced to CO.

3. The method of claim 2 wherein the electrocatalyst has an average cavity size ranging from about 175 to about 185 nm.

4. The method of claim 2 wherein the inverse-opal structure has a hexagonal structure.

5. The method of claim 2 wherein the inverse-opal structure is a negative replica of poly (methyl methacrylate) opal.

6. The method of claim 2 wherein the 3D interconnected porous copper inverse-opal structure comprises nanoparticles with an average mean diameter ranging from about 15 to about 20 nm.

7. The method of claim 2 wherein the method has a Faradaic efficiency greater than about 65% at −0.7 V vs. RHE.

8. The method of claim 7 wherein the Faradaic efficiency is greater than about 70% at −0.6 V vs. RHE.

9. The method of claim 2 wherein the CO2 is converted to CO with a CO to H2 selectivity greater than about 80% at −0.8 V vs. RHE.

10. The method of claim 9 wherein the CO to H2 selectivity is greater than about 90% at −0.7 V vs. RHE.

11. The method of claim 2, wherein the inverse-opal structure has a hexagonal structure; wherein the inverse-opal structure is a negative replica of poly (methyl methacrylate) opal; wherein the electrocatalyst has an average cavity size ranging from about 175 to about 185 nm; wherein the method has a Faradaic efficiency greater than about 70% at −0.6 V vs. RHE; and wherein the method the CO2 is converted to CO with a CO to H2 selectivity up to about 90% at −0.7 V vs. RHE.

12. The method of claim 2 wherein the electrocatalyst comprises copper in a +2 oxidation state in the providing step.

13. The method of claim 2 wherein the electrocatalyst comprises copper in a 0 oxidation state when the negative potential is applied to the working electrode.