ELECTROREDUCTION OF CO2 IN ACIDIC CONDITIONS USING A CATALYST HAVING A DUAL CO GENERATION AND C-C COUPLING FUNCTION

Abstract

The present disclosure relates to a carbon dioxide reduction reaction (CO2RR) catalytic system having a dual catalysis function for the electroreduction of CO2 into multi-carbon products, the catalytic system comprising: a first catalyst layer comprising a support and a first metal-based catalyst that is configured to sustain reduction of CO2 into CO, wherein the first metal-based catalyst is atomically dispersed on the support; and a second catalyst layer comprising a second metal-based catalyst that is configured to sustain C—C coupling mechanisms to yield multi-carbon products from CO; wherein the first catalyst layer is positioned above the second catalyst layer to generate CO between the first catalyst layer and the second catalyst layer.

Description

TECHNICAL FIELD

The present invention generally relates to CO₂ electroreduction techniques, and more particularly to a catalyst allowing the decoupling of CO₂ reduction reactions in a single electrode.

BACKGROUND

In the field of electrochemical CO₂ reduction reaction (CO₂RR), alkaline and neutral electrolytes can be used to suppress the hydrogen evolution reaction (HER) and facilitate a C—C coupling step during the CO₂RR. Unfortunately, in this case over 75% of input CO₂ is lost chemically by reacting with hydroxyl ions (OH⁻) to form (bi)carbonates, which results from high local alkalinity at the cathode of the CO₂ electrolyzer system. This loss lowers a CO₂ single-pass conversion efficiency (SPCE) and imposes an energetic penalty to recover the lost CO₂ reactants.

Alternatively, using acidic electrolytes in place of alkaline or neutral electrolytes for CO₂RR can improve CO₂ utilization by reducing (bi)carbonate formation and CO₂ crossover via the ionic exchange membrane of the electrolyzer. In this approach, however, the kinetically favored HER typically outcompetes CO₂RR in acidic media, leading to poor CO₂RR selectivity.

WO2021/075638 A1 and US2023/243051 A1 disclose a catalyst structure for electrochemical CO₂ reduction. The catalyst structure includes carbon nanofibers doped with nitrogen (N), and copper (Cu) particles dispersed on the carbon nanofibers. At least portions of the carbon nanofibers at interfaces with the Cu particles may have a pyridinic-N structure.

US2021/115577 A1 discloses a tandem electrode for electrochemically reducing carbon dioxide. The electrode includes a first distinct catalyst layer and a second distinct catalyst layer. The first distinct catalyst is selected from the group consisting of Cu, Cu alloys, doped Cu, nitrogen doped carbon materials, boron doped carbon materials, nitrogen and boron co-doped carbon materials, and functionalized carbon materials. The second catalyst is selected from Au, Ag, Zn, ZnO, Fe—N—C, Ni—N—C, Co—N—C, N doped CNT, N doped graphene, and other materials that are selective for CO formation.

Kong Calton et al.: «Design principles of tandem cascade photoelectrochemical devices», Sustainable Energy & Fuels, vol. 5, no. 24, 7 Dec. 2021, pages 6361-6371 discloses Cascade photoelectrocatalysis (PEC) is a possible method to improve the selectivity of solar-driven CO2 reduction (CO2R) wherein different CO2R catalysts are coupled to different subcells in a multijunction photovoltaic (PV) stack. To perform a two-step conversion of CO2 to ethylene in aqueous electrolyte, via a CO intermediate, a three-terminal tandem (3TT) configuration was designed. The configuration is using Ill-V-semiconductor based subcells coupled to Au (produces CO intermediate) and Cu (converts CO to ethylene).

Liu Xinyan et al.: «A perspective of the electrocatalytic conversion of carbon dioxide to methanol with metallomacrocyclic catalysts», Journal of Energy Chemistry, vol. 64, 4 May 2021, pages 263-275, discloses the production of methanol from electrochemical CO₂R, using metallomacrocyclic molecules as the model catalysts. The document discuss the motivation for having methanol as the sole CO₂R product, the documented application of metallomacrocyclic catalysts for CO₂R applications, and recent advance in catalyzing CO₂ to methanol with cobalt phthalocyanine-based catalysts.

CO₂RR presents various challenges that are known in the field, such as the above-mentioned drawbacks that may result from the used of an acidic medium, that still need to be addressed.

SUMMARY

Operating electrochemical reduction reaction of CO₂ (CO₂RR) in acidic conditions improves single-pass carbon efficiency (SPCE). However, the competing hydrogen evolution reaction reduces the selectivity for CO₂RR. In CO₂RR, the formation of CO and its subsequent coupling is essential to achieving ensuing higher multicarbon (C₂₊) formation. These two reactions rely on distinct catalyst properties that are difficult to achieve in a single catalyst.

There is provided herein a method to decouple the CO₂-to-C₂₊, reaction into two reduction steps, including a CO₂-to-CO reduction step and a CO-to-C₂₊ reduction step. There is provided a modified cathode including a CO₂RR catalyst having two distinct catalyst layers, each being tailored to catalyze one of the two reduction steps to achieve the desired transformation of CO₂ into multi-carbon products.

In some implementations, the modified cathode includes a first catalyst layer that can comprise atomically dispersed cobalt phthalocyanine to favour reduction of CO₂ to CO, thereby increasing local CO availability of CO. The modified cathode further includes a second catalyst layer comprising metal nanoparticles, such as copper, and an ionomer to enhance the C—C coupling reduction step from the locally available CO. For example, operation of an electrolyzer including the modified cathode can achieve 54% C₂H₄ Faradaic efficiency (FE) and 80% C₂₊ FE at 800 mA cm⁻², as well as an SPCE of 87%.

More particularly, there is provided a carbon dioxide reduction reaction (CO₂RR) catalytic system having a dual catalysis function for the electroreduction of CO₂ into multi-carbon products. The catalytic system comprises:

• • a first catalyst layer comprising a support and a first metal-based catalyst
    • wherein the first metal-based catalyst comprises cobalt phthalocyanine (CoPc), nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), iron phthalocyanine (FePc) or any combinations thereof;
    • wherein the support is a carbon-based material derived from a metal organic framework,
    • wherein the first metal-based catalyst is atomically dispersed on the support as determined by energy-dispersive X-ray spectroscopy (EDS); and
  • a second catalyst layer comprising a second metal-based catalyst that comprises copper or copper-based alloys;
    wherein the first catalyst layer is positioned above the second catalyst layer to generate CO between the first catalyst layer and the second catalyst layer, and wherein the support of the first catalyst layer is doped with one or more N-containing compounds.

According to the invention, the first metal-based catalyst is configured to sustain reduction of CO₂ into CO and second metal-based catalyst is configured to sustain C—C coupling mechanisms to yield multi-carbon products from CO.

For example, the support of the first catalyst layer comprises between 0.5 wt. % and 10 wt. % of the N-containing compounds based on the total weight of the first catalyst layer and as determined by energy-dispersive X-ray spectroscopy (EDS) measurements, preferably between 1 wt. % and 8 wt. %, and further preferably between 2 wt. % and 6 wt. %.

In some implementations, the first metal-based catalyst is or comprises nanoparticles. The first metal-based catalyst can comprise one or more metals selected from Au, Ag, Co, Ni, Cu, Fe and any mixtures thereof, preferably the first metal-based catalyst comprises Co. The first metal-based catalyst can comprise a molecule having a metal center which is able to bond with four nitrogen atoms (Me-N₄) of the one or more N-containing compounds. The metal center (Me) can be Co, Ni, Cu, Fe or any combinations thereof. According to the disclosure, the first metal-based catalyst comprises cobalt phthalocyanine (CoPc), nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), iron phthalocyanine (FePc) or any combinations thereof. Preferably, the first metal-based catalyst can be cobalt phthalocyanine (CoPc).

In some implementations, the first metal-based catalyst can be uniformly dispersed on the support as determined by energy-dispersive X-ray spectroscopy (EDS).

In some implementations, the support can be a carbon-based material derived from a metal organic framework. For example, the metal organic framework can be a zeolitic metal organic framework. Preferably, the metal organic framework can be a zeolitic imidazolate framework comprising one or more metallic ions selected from Zn, Fe, Co, Cu or any combinations thereof, preferably zinc. Further preferably, the metal organic framework can be a zeolitic imidazolate framework (ZIFs) comprising one or more ZIF systems selected from ZIF-8, ZIF-67, ZIF-68, ZIF-69, ZIF-70, ZIF-90 and any combinations thereof, preferably ZIF-8.

In some implementations, the support has a hollow structure as determined by transmission electron microscopy. For example, the support can have a hollow polyhedral morphology as determined by transmission electron microscopy. Preferably, the support is a carbon support having a hollow polyhedral morphology.

In some implementations, the support can comprise pores having a diameter size of ranging between 0.3 nm and 5 nm, as determined by Brunauer-Emmett-Teller (BET) method, preferably between 0.5 nm and 4.5 nm, or between 1 nm and 4 nm.

Preferably, the support can be co-doped with one or more of P-containing compounds, and/or with one or more of S-containing compounds. For example, the support of the first catalyst layer can include between 0.25 wt. % and 5 wt. % of the one or more P-containing compounds based on the total weight of the first catalyst layer and as determined by energy-dispersive X-ray spectroscopy (EDS) measurements, preferably between 0.5 wt. % and 4 wt. %, and further preferably between 1 wt. % and 3 wt. %. For example, the support of the first catalyst layer can include between 0.25 wt. % and 5 wt. % of the one or more of S-containing compounds based on the total weight of the first catalyst layer and as determined by energy-dispersive X-ray spectroscopy (EDS) measurements, preferably between 0.5 wt. % and 4 wt. %, and further preferably between 1 wt. % and 3 wt. %.

In some implementations, the first catalyst layer has a catalyst content between 3 wt. % and 5 wt. % based on the total weight of the first catalyst layer as determined by inductively coupled plasma optical emission spectrometry, or between 3.5 wt. % and 4.5 wt. %.

In some implementations, the second metal-based catalyst can comprise copper or copper-based alloys. With preference, said copper-based alloys comprise CuAg, CuAu, CuPd, CuAl, CuBi, CuSn, CuIn, or any combinations thereof. The second metal-based catalyst can be provided as nanoparticles, nanowires, nanosheets, nanodendrites, or a combination thereof. Preferably, the second catalyst layer can further comprise an ionomer. For example, the ionomer can be a perfluorinated sulfonic acid ionomer. Preferably, the ionomer can be 1,1,2,2-tetrafluoroethene; 1,1,2,2-tetrafluoro-2-[1,1,1,2,3,3-hexafluoro-3-(1,2,2-trifluoroethenoxy)propan-2-yl]oxyethanesulfonic acid copolymer, tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer, or any combinations thereof.

In some implementations, the second catalyst layer can comprise an underlayer consisting of the second metal-based catalyst and a top layer comprising the ionomer and the second metal-based catalyst. For example, the top layer can be a three-dimensional catalyst:ionomer bulk heterojunction (Cu-CIBH) top layer consisting of Cu nanoparticles (CuNP) and perfluorosulfonic acid (PFSA) ionomer. Preferably, the catalyst-ionomer bulk heterojunction can have a CIBH thickness ranging between 50.0 nm to 25.0 μm as determined by scanning electron microscopy, preferably between 60.0 nm and 24.0 μm, further preferably between 70.0 nm and 23.0 μm, yet further preferably between 100.0 nm and 20.0 μm. For example, the catalyst-ionomer bulk heterojunction can have a ratio of catalyst material to ion-conducting polymer ranging from 0.1 to 10.0, preferably ranging from 0.2 to 9.0, further preferably ranging from 0.3 to 8.0, even further preferably ranging from 0.4 to 7.0, yet further preferably ranging from 0.5 to 5.0 or ranging from 0.5: to 2.0, even further preferably ranging from 0.6 to 2.0 or from 1.0 to 1.6.

In some implementations, the second catalyst layer can have a catalyst content between 50 wt. % and 100 wt. % based on the total weight of the second catalyst layer.

In another aspect, there is provided a modified electrode configured for the electroreduction of CO₂ into multi-carbon products in acidic conditions. The modified electrode comprises:

• • a gas diffusion layer being a porous support; and
  • the CO₂RR catalytic system as defined herein, wherein the second catalyst layer is deposited on the gas diffusion layer.

In some implementations, the porous support can be selected from polytetrafluoroethylene (PTFE), porous carbon paper, and any combination thereof. For example, the porous support can be PTFE. Preferably, the porous support shows pores with a diameter size ranging between 100 nm and 5000 nm, as determined by Brunauer-Emmett-Teller (BET) method.

In another aspect, there is provided a CO₂ electrolyzer system for reducing CO₂ into multi-carbon products. The CO₂ electrolyzer system comprises:

• • a cathodic compartment comprising:
    • a reactant inlet that is configured to supply a gas stream comprising CO₂ in the cathodic compartment,
    • a cathode being the modified electrode as defined herein,
    • a product outlet configured to release a gas-liquid mixture comprising CO₂ and the multi-carbon products from the cathodic compartment;
  • an anodic compartment comprising:
    • an anodic inlet that is configured to supply an anolyte in the anodic compartment,
    • an anode, and
    • an anodic outlet that is configured to release used anolyte from the anodic compartment; and
  • a cationic exchange membrane that is positioned between the cathodic compartment and the anodic compartment.

In some implementations, the cationic exchange membrane can be a proton exchange membrane. For example, the cationic exchange membrane can be a perfluorinated membrane.

In some implementations, the CO₂ electrolyzer system can be a one-gap catholyte-containing electrolyzer, wherein the cathodic compartment further comprises a catholyte inlet that is configured to be supplied with a catholyte. In other implementations, the CO₂ electrolyzer system can be a zero-gap electrolyzer, such as a membrane electrode assembly.

In yet another aspect, there is provided a process for operating CO₂ electroreduction into multi-carbon products in acidic conditions. The process comprises:

• • providing a CO₂ electrolyzer system as defined herein,
  • supplying a CO₂-containing gas stream to the reactant inlet of the cathodic compartment;
  • supplying an acidic electrolyte having a pk_a measured at 25° C. ranging between 3 and 12 to
  • the anolyte inlet of the anodic compartment of the CO₂ electrolyzer;
  • applying a current density to the CO₂ electrolyzer between 100 and 1200 mA·cm⁻²; and
  • recovering a gas-liquid mixture from a product outlet of the cathodic compartment, the gas-liquid mixture comprising the multi-carbon products.

For example, the acidic electrolyte can comprise H₃PO₄, KH₂PO₄, KCl, H₂SO₄, or any combinations thereof.

For example, the pk_a measured at 25° C. of the acidic electrolyte can be between 7 and 8.

For example, the applied current density can be between 100 mA·cm⁻² and 1200 mA·cm⁻², preferably between 400 and 800 mA·cm⁻².

For example, the supplying of the CO₂-containing gas stream can be performed with an inlet flow rate of CO₂ being between 1 sccm and 100 sccm.

In yet another aspect, there is provided a method for preparing the carbon dioxide reduction reaction catalytic system as herein. The method comprises:

• • a. providing a metal organic framework;
  • b. coating said metal organic framework with a ligand to obtain a chelated metal organic framework;
  • c. performing pyrolysis of the chelated metal organic framework obtained at step (b) to obtain a hollow carbon support;
  • d. dispersing one or more first metal-based compounds on the hollow carbon support obtained at step (c) to obtain a first metal-based catalyst dispersed on a support;
  • e. adding an ionomer to said first metal-based catalyst dispersed on a support to obtain an ink;
  • f. providing one or more second metal-based compounds and spraying the ink obtained at step (e) on to said one or more second metal-based compounds so as to obtain the carbon dioxide reduction reaction catalytic system as defined herein.

In some implementations, step (b) can comprise a first sub-step of dispersing said metal organic framework into one or more polar organic solvents to obtain a dispersion, a second sub-step of adding one or more anchoring agent into said dispersion, a third sub-step of adding a chelating agent and a fourth sub-step of adding one or more basic compounds. For example, said one or more polar organic can be one or more protic solvents, preferably selected from methanol and/or propanol. For example, the one or more anchoring agent can be bis(4-hydrophenyl)sulfone, bis(4-aminophenyl) ether, dicyandiamide, melamine, or any combinations thereof. For example, the chelating agent can be phosphonitrilic chloride trimer, pyrrole, aniline, dopamine, or any combinations thereof. For example, the one or more basic compounds can be triethylamine, ammonium persulphate, or a combination thereof.

In some implementations, step (b) is carried out under stirring for at least 12 hours and/or at a temperature range comprises between 15° C. and 30° C.

In some implementations, step (c) can be carried out at a temperature ranging between 800° C. and 1200° C., preferably between 900° C. and 1100° C., and/or during a time ranging between 1 h and 5 h, preferably between 2 h and 4 h. Further preferably, step (c) can be carried out under an inert atmosphere, preferably under Ar, N₂, He, more preferably under Ar.

In some implementations, step (d) can be performed by sonication for a time ranging between 15 minutes and 1 hour; and then by mechanical stirring for at least 12 hours.

In some implementations, spraying the ink during step (f) can be performed by airbrushing.

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to such embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the present description. The objects, advantages and other features of the present invention will become more apparent and be better understood upon reading of the following non-restrictive description of the invention, given with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the CO₂RR catalyst having a dual catalyst function, and related modified electrode, system and methods are represented in and will be further understood in connection with the following figures.

The Spatially-Decoupled Strategy for Acidic CO₂RR Via Tandem Catalysis

FIG. 1: Reaction free energies for H* intermediate formation on Cu surface under different CO coverages.

FIG. 2: Energy profiles for initial states, transition states (TSs), and final states of CO dimerization on Cu surface under different CO coverages.

FIG. 3: FEs of H₂, C₂H₄, and C₂₊ on the Cu electrode toward CO₂RR and CORR at different current densities in an acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl. Values are means, and error bars represent the standard deviation from three independent measurements.

FIG. 4: Schematic of the ion transport and reactions in acidic CO₂RR system with schematic illustration of the spatially-decoupled strategy via tandem catalysis in acidic CO₂RR system. PEM, proton exchange membrane.

Synthesis and Structural Analysis of Atomically Dispersed CoPc@HC.

FIG. 5: Schematic illustration of the synthesis process of CoPc@HC.

FIG. 6: SEM image of CoPc@HC.

FIG. 7: HAADF-STEM image and the corresponding EDS mapping images (Co; C; N; P; S) of CoPc@HC.

FIG. 8: Aberration-corrected HAADF-STEM image of CoPc@HC.

FIG. 9: Co K-edge XANES spectra of CoPc@HC and reference samples (Co foil, CoO, Co₃O₄, CoPc).

FIG. 10: Fourier-transformed magnitudes of Co K-edge EXAFS spectra of CoPc@HC and reference samples.

FIG. 11: Comparison of CO FE on CoPc@HC electrode and CoPc/C electrode toward acidic CO₂RR.

Acidic CO₂RR performance of CoPc@HC/Cu tandem electrode.

FIG. 12: Cross-sectional SEM image of CoPc@HC/Cu tandem electrode, with TEM image of upper CoPc@HC catalyst layer and with SEM image of lower Cu catalyst layer with catalyst-ionomer bulk heterojunction interface.

FIG. 13: Schematic of the spatially-decoupled strategy via tandem catalysis, showing the electron transfer and mass transport in acidic CO₂RR.

FIG. 14: FEs of CO₂RR products on CoPc@HC/Cu tandem electrode in an acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl. The flow rate of inlet CO₂ is 10 sccm. Values are means, and error bars represent the standard deviation from three independent measurements.

FIG. 15: FEs of CO₂RR products on Cu electrode in an acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl. The flow rate of inlet CO₂ is 10 sccm. Values are means, and error bars represent the standard deviation from three independent measurements.

FIG. 16: Partial current densities of C₂H₄ and H₂.

FIG. 17: FEs toward CO₂RR products and SPCE on CoPc@HC/Cu tandem electrode at 800 mA cm⁻² with different inlet flow rates of CO₂. Values are means, and error bars represent the standard deviation from three independent measurements.

FIG. 18: Current density toward CO₂RR products on CoPc@HC/Cu tandem electrode and Cu electrode.

FIG. 19: Comparison of the SPCE, FE and partial current density of C₂₊, partial current density of ethylene, and energy consumption for ethylene production of CoPc@HC/Cu tandem electrode with those of state-of-the-art electrodes.

DFT Calculations and In Situ Raman Studies.

FIG. 20: Free energy diagram of CO₂RR on CoN₄—C/Cu tandem electrode and Cu electrode.

FIG. 21: The corresponding atomic configurations from CO to the hydrogenation of *CHCO are presented.

FIG. 22: In situ Raman spectra of CO for a Cu electrode under different applied potentials.

FIG. 23: In situ Raman spectra of CO for a CoPc/Cu tandem electrode under different applied potentials.

FIG. 24: In situ Raman spectra of CoPc/Cu tandem electrode with high CoPc coverage on the Cu surface.

FIG. 25: In situ Raman spectra of CoPc/Cu tandem electrode with agglomerated CoPc on the Cu surface.

FIG. 26: The periodic model of Cu(111) covered a layer of water consisting of six water molecules.

FIG. 27: The initial, transitional, and final states of C—C coupling on Cu(111) at the CO coverage of 2/9 ML, 3/9 ML, 4/9 ML. The distances of C—C in the transition states are plotted and labelled.

FIG. 28: The initial, transitional, and final states of C—C coupling on Cu(111) at the *CO coverage of 2/9 ML, 3/9 ML, 4/9 ML.

FIG. 29: Acidic CO₂RR performance of the state-of-the-art carbon-supported cobalt phthalocyanine (CoPc/C) catalyst: CO and H₂ FE on CoPc/C catalyst at various current densities in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl.

FIG. 30: Acidic CO₂RR performance of the state-of-the-art carbon-supported cobalt phthalocyanine (CoPc/C) catalyst: The partial current densities of CO and H₂ on CoPc/C catalyst at various current densities in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl.

FIG. 31: The stability test of the CoPc/C catalyst at the current density of 300 mA cm⁻² toward acidic CO₂RR in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl.

FIG. 32: High-resolution transmission electrode microscopy (HRTEM) images of the CoPc/C catalyst after stability test (scale of 10 nm). The nanoparticles were observed in the post-reaction CoPc/C catalyst, suggesting the agglomeration of CoPc into Co clusters and small-size Co nanoparticles after stability test.

FIG. 33: High-resolution transmission electrode microscopy (HRTEM) images of the CoPc/C catalyst after stability test (scale of 2 nm). The nanoparticles were observed in the post-reaction CoPc/C catalyst, suggesting the agglomeration of CoPc into Co clusters and small-size Co nanoparticles after stability test.

FIG. 34: Structural characterization of N, P, S co-doped hollow carbon (HC) by transmission electron microscopy (TEM).

FIG. 35: Structural characterization of N, P, S co-doped hollow carbon (HC) by powder X-ray diffraction (XRD).

FIG. 36: Transmission electron microscopy (TEM) image of CoPc@HC.

FIG. 37: Powder X-ray diffraction (XRD) pattern of CoPc@HC in comparison to CoPc molecules.

FIG. 38: The first derivative curves of CoPc@HC and reference samples. The first derivative curves demonstrate that Co species in CoPc@HC exhibits higher oxidation state compared to pure CoPc, suggesting the electron-deficient nature of the Co center of CoPc@HC, ascribed to the strong electronic interaction between CoPc and the N species of HC substrate.

FIG. 39: Co K-edge wavelet transformed extended X-ray absorption fine structure (WT-EXAFS) contour plots of CoPc@HC and reference samples (CoPc, COO, Co₃O₄, and Co foil).

FIG. 40: Co K-edge EXAFS first-shell fitting analysis of CoPc@HC in k space.

FIG. 41: Co K-edge EXAFS first-shell fitting analysis of CoPc@HC in R space.

FIG. 42: Co K-edge EXAFS first-shell fitting analysis of CoPc in k space.

FIG. 43: Co K-edge EXAFS first-shell fitting analysis of CoPc in R space

FIG. 44: Co K-edge EXAFS fitting analysis of CoO in k space.

FIG. 45: Co K-edge EXAFS fitting analysis of CoO in R space.

FIG. 46: Co K-edge EXAFS fitting analysis of Co₃O₄ in k space.

FIG. 47: Co K-edge EXAFS fitting analysis of Co₃O₄ in R space.

FIG. 48: Co K-edge EXAFS fitting analysis of Co foil in k space.

FIG. 49: Co K-edge EXAFS fitting analysis of Co foil in R space.

FIG. 50: Acidic CO₂RR performance of the CoPc@HC catalyst. CO and H₂ FE on CoPc@HC toward acidic CO₂RR at various current densities in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl.

FIG. 51: Acidic CO₂RR performance of the CoPc@HC catalyst. The partial current density of CO and H₂ on CoPc@HC toward acidic CO₂RR in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl.

FIG. 52: H₂ FE on CoPc@HC electrode and CoPc/C electrode toward acidic CO₂RR at different current densities.

FIG. 53: The stability test of CoPc@HC at the current density of 300 mA cm⁻² toward acidic CO₂RR in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl.

FIG. 54: FEs of CO, C₂H₄ and C₂₊ on CoPc@HC/sCu tandem electrode in an acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl. The flow rate of inlet CO₂ is 10 sccm.

FIG. 55: H₂ FE on CoPc@HC/Cu electrode and Cu electrode toward acidic CO₂RR at different current densities. Error bars represent the standard deviation of three independent samples. Data are presented as mean values±standard deviation.

FIG. 56: The stability of CoPc@HC/Cu electrode at the current density of 800 mA cm-2, which shows CoPc@HC/Cu electrode maintains the stable potential and C₂H₄ FE for 16 h.

FIG. 57: The cross-sectional SEM image of the used CoPc@HC/Cu electrode after the stability test.

FIG. 58: FEs toward CO₂RR products and SPCE on Cu electrode at 800 mA cm-2 with different flow rates of CO₂ inlet in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl. Values are means, and error bars represent the standard deviation from three independent measurements.

FIG. 59: The acidic CO₂RR performance of CoPc@HC/Cu tandem electrode in slim flow cell.

FIG. 60: The acidic CO₂RR performance of Cu electrode in slim flow cell.

FIG. 61: Performance comparison CoPc@HC/Cu electrode and Cu electrode toward acidic CO₂RR in slim flow cell. C₂H₄ FE of CoPc@HC/Cu electrode and Cu electrode.

FIG. 62: Performance comparison CoPc@HC/Cu electrode and Cu electrode toward acidic CO₂RR in slim flow cell. C₂₊ FE of CoPc@HC/Cu tandem electrode and Cu electrode.

FIG. 63: Partial current densities of C₂₊ products on CoPc@HC/Cu tandem electrode and Cu electrode toward acidic CO₂RR in the slim flow cell.

FIG. 64: The energy efficiency of C₂H₄ production in acidic CO₂RR on CoPc@HC/Cu electrode and Cu electrode.

FIG. 65: The energy intensity for the production of C₂H₄ on CoPc@HC/Cu tandem electrode and Cu electrode as well as the benchmark of prior alkaline/neutral/acidic system based on the Tables 4 and 5.

FIG. 66: C₂H₄ FE of CoPc@HC/Cu tandem electrode and Cu electrode toward CO reduction reaction (CORR) performance in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl. Error bars represent the standard deviation of three independent samples. Data are presented as mean values±standard deviation.

FIG. 67: H₂ FE of CoPc@HC/Cu tandem electrode and Cu electrode toward CO reduction reaction (CORR) performance in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl. Error bars represent the standard deviation of three independent samples. Data are presented as mean values±standard deviation.

FIG. 68: Comparison of the ratio of C₂H₄ FE and H₂ FE on CoPc@HC/Cu tandem electrode and Cu electrode toward CO reduction reaction performance in the acidic buffer electrolyte of 0.5 M H₃PO₄/0.5 M KH₂PO₄ with 2.5 M KCl. CoPc@HC/Cu tandem electrode exhibits obviously higher C₂H₄ FE and lower H₂ FE in CORR compared to CO₂RR from 400 to 800 mA cm⁻², suggesting, besides reducing CO₂ to CO and suppressing the evolution of H₂ in acidic media, the CoPc@HC catalyst layer was able to accelerate C—C coupling to C₂H₄.

FIG. 69: The model of (6×6) Cu(111) covered a charged water-layer consisting of twenty-four molecules with/without two-dimensional CoN₄-Graphene structure.

FIG. 70: Free energy diagram of the production of C₂H₄ on Cu(111). Two possible reaction pathways on Cu(111).

FIG. 71: Free energy diagram of the production of C₂H₄ on Cu(111). The corresponding atomic configuration for each reaction intermediate.

FIG. 72: Free energy diagram of the production of C₂H₄ on CoN₄—C/Cu(111). The possible reaction pathway on the CoN₄—C/Cu(111). Under the influence of CoN₄—C structure, the pathway 1 similar as in FIG. 70 does not exist because the key intermediate *CHCHO is hard to stabilize adsorption on Cu(111).

FIG. 73: Free energy diagram of the production of C₂H₄ on CoN₄—C/Cu(111). The corresponding atomic configurations for each reaction intermediate. Under the influence of CoN₄—C structure, the atomic configurations of the pathway 1 similar as in FIG. 71 does not exist because the key intermediate *CHCHO is hard to stabilize adsorption on Cu(111).

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to these embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the appended claims.

DETAILED DESCRIPTION

There are provided herein techniques that promote selective C₂₊ formation following CO₂RR in acidic conditions by decoupling the CO₂RR in two consecutive reaction steps upon using a dual catalysis strategy. More particularly, there is provided a modified electrode, for example a cathode, having a dual catalytic function. The modified electrode comprises a first metal-based catalyst favoring the first reaction step of reduction of CO₂ into CO, and a second metal-based catalyst favoring the second reaction step of C—C coupling mechanisms to form C₂₊ products. The highly efficient and selective CO₂-to-CO conversion enabled by the first catalyst allows the formation of a high local concentration of CO hereby generated nearby the second catalyst surface to further favour the C—C coupling reactions.

The first and second catalysts are provided as distinct layers in the modified electrode. The modified electrode particularly includes two spatially-decoupled catalyst layers having each distinct catalytic properties. The first catalyst layer can be referred to as a CO-producing layer and the second catalyst layer can be referred to as a C—C coupling layer. The decoupling of the CO₂-to-C₂₊ reaction into two consecutive steps: CO₂-to-CO and CO-to-C₂₊, which are thus deployed on two separately-optimized distinct catalyst layers working in tandem to achieve the desired transformation into multicarbon products. The term “spatially-decoupled” can be understood as referring to a nanoscale gap/space that is present between the first catalyst layer and the second catalyst layer. The nanoscale space can thus function as a nanoreactor, thereby allowing generation of a high concentration of CO at a surface of the first catalyst layer and nearby the second catalyst layer, thus further promoting C—C coupling reactions at a surface of the second catalyst layer.

First Catalyst Function—CO₂ to CO Reduction

The first catalyst layer includes a supported catalyst, and more particularly a support and the first metal-based catalyst that is configured to sustain reduction of CO₂ into CO. The first catalyst layer favoring reduction of CO₂ to CO in acidic media can, for example, be a catalyst material with high performance for electrochemical CO₂ reduction to CO, and can include carbon-supported Au nanoparticles, Au film, carbon-supported Ag nanoparticles, Ag film, carbon-supported nickel phthalocyanine, carbon-supported iron phthalocyanine, carbon-supported copper phthalocyanine, carbon-supported cobalt phthalocyanine, carbon-supported metal single-atom (including iron, cobalt, nickel), or any combinations thereof.

Cobalt phtalocyanine (CoPc) molecules are known to improve CO selectivity in alkaline conditions but typically poorly perform in acidic media with a CO Faradaic Efficiency (FE) limited to 70% that rapidly decrease over time. Using high-resolution transmission electron microscopy (HRTEM) and referring to FIGS. 29-33, it has been found that a CoPc/C catalyst can have a poor performance in acidic medium (e.g., the performance declines rapidly over the course of 2 hours) due to agglomeration of the Co molecules (better seen in FIGS. 32-33).

To reduce or avoid agglomeration of the catalyst molecules, the first catalyst layer includes an atomic dispersion of molecules of the first metal-based catalyst on a strong interaction support to allow reaching a higher initial CO FE, e.g. 94%, and maintaining such value over the course of operation. The strong interaction support can be understood as a support including species that can form electronic interaction with the Co metal center of the CoPc molecules. The first metal-based catalyst is thus provided atomically dispersed on the support such that the first metal-based catalyst molecules are bonded to the support via a strong metal support interaction.

In some implementations, the support can be a carbon support that is doped with one or more atoms to enhance electronic interaction between the catalyst molecules and the support. For example, the support can be doped with N species providing electronic interactions with Co on the support, thereby contributing to the atomic dispersion and stabilization of CoPc molecules. For example, the support can be a hollow carbon support including N species serving as anchoring sites to stabilize the CoPc molecules, with some electrons from the Co metal center of the CoPc molecules transferring to the N species of the hollow carbon support, thereby leading to the electron-deficient nature of Co metal center and the formation of a Co—N coordination between Co metal center and N species of support.

For example, referring to FIG. 5, the first catalyst layer can include a hollow carbon support (HC) and atomically dispersed CoPc on the hollow carbon support (CoPc@HC), where the individual CoPc molecules are anchored (i.e., chemically bonded) to species of the hollow carbon support via chemical bonding.

Preferably, the support can have a hollow structure, for example a hollow polyhedral morphology as determined by transmission electron microscopy. It should be further noted that the hollow structure of the support can facilitate the mass transfer to achieve a higher performance (than in absence of the hollow structure).

It should further be noted that the support can be a carbon-based support as detailed herein and can include graphene, nitrogen doped graphene, heteroatom doped graphene, nitrogen doped carbon, heteroatom doped carbon, or a combination thereof.

In some implementations, the carbon-based support is a carbon-based material derived from a metal organic framework, for example pyrolysis of the metal organic framework. The metal organic framework can be a zeolitic metal organic framework comprising one or more metallic ions selected from Zn, Fe, Co, Cu or any combinations thereof. For example, the zeolitic metal organic framework can be a zeolitic imidazolate framework (ZIFs) comprising one or more ZIF systems selected from ZIF-8, ZIF-67, ZIF-68, ZIF-69, ZIF-70, ZIF-90 and any combinations thereof.

In addition, the support from the first catalyst layer can be characterized as being porous and can comprise pores having a diameter size of ranging between 0.3 nm and 5 nm, as determined by Brunauer-Emmett-Teller (BET) method, preferably between 0.5 nm and 4.5 nm, or between 1 nm and 4 nm.

Furthermore, the first catalyst layer can be characterized as having a catalyst content between 0.5 wt. % and 10 wt. %, preferentially between 1 wt. % and 8 wt. %, or between 2 wt. % and 6 wt. %. For example, when the first catalyst layer comprises a support of carbon and a metal-based catalyst being CoPc, the catalyst content is a CoPc weight content in the layer. If the catalyst consists of metal (i.e., no support), the catalyst content can be equal to the metal content of the first catalyst layer.

Referring to FIG. 4, in acidic CO₂RR, CO₂ that was lost to carbonate formation can be recovered by reacting with protons in the bulk electrolyte. In addition to this recovery mechanism, the first catalyst layer, such as exemplified with the CoPc@HC layer, offers an avenue to rapidly convert CO₂ into CO, which does not transform into carbonates in local alkaline conditions.

Structure Characterization of an Example CoPc@HC Catalyst Layer

For example, a first catalyst layer CoPc@HC comprising an N, P, and S co-doped HC support and atomically dispersed CoPc catalyst was studied based on transmission electron microscopy (TEM) images (see FIGS. 34 and 36), powder X-ray diffraction (XRD) pattern (see FIG. 35) and scanning electron microscopy (SEM) images (see FIG. 6). N species provides anchoring sites for the CoPc molecules while the P and S species can improve the conductivity of the support. Referring to FIGS. 6, 34 and 36, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images reveal a hollow morphology, and more particularly display a uniform hollow polyhedral morphology.

No evidence of CoPc catalyst agglomeration was detected as shown in FIG. 7. The high-angle annular dark-field scanning TEM (HAADF-SETM) image detects no agglomeration of CoPc, and the corresponding energy-dispersive X-ray spectroscopy (EDS) mappings reveal uniform distribution of Co species throughout the entire N, P, and S co-doped hollow carbon polyhedron architecture.

Referring to FIG. 8, aberration-corrected HAADF-STEM measurement was performed to directly observe the dispersion of CoPc at the atomic level, indicating that CoPc molecules were atomically dispersed on N, P, and S co-doped hollow carbon polyhedron. High-resolution Z-contrast images thus substantiate atomic dispersion of Co species, as indicated by the individual bright dots associated with the Co centers of CoPc molecules. Referring to FIG. 37, no obvious diffraction peaks assigned to CoPc were observed in the powder X-ray diffraction (PXRD) pattern of CoPc@HC.

The metal content, and more particularly the Co content in the CoPc@HC first catalyst layer was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) as 0.44 wt. %, corresponding to 4.3 wt. % of CoPc in CoPc@HC.

Synchrotron-radiation-based X-ray absorption fine structure (XAS) analysis was further carried out to investigate the electronic structure and coordination environment of the example CoPc@HC first catalyst layer of the modified electrode. FIGS. 9 and 38 show the X-ray absorption near-edge structure (XANES) spectra of CoPc@HC and other reference catalyst layers consisting of COPc, Co₃O₄, CoO or Co foil. Referring to FIG. 38, the XANES curve of CoPc@HC exhibits a higher absorption-edge energy compared with CoPc, suggesting the electron-deficient nature of the Co center of CoPc@HC, ascribed to the strong electronic interaction between CoPc and the N species of HC substrate. As shown in FIG. 10, a Co K-edge of the CoPc@HC first catalyst layer exhibits a similar Fourier transformed (FT) extended X-ray absorption fine structure (FT-EXAFS) spectrum to that of a CoPc catalyst layer, suggesting the main peak observed in approximately 1.3 Å is associated with the Co—N coordination. Furthermore, referring to FIG. 39, the wavelet transform (WT) contour plot of CoPc@HC displays one intensity maximum at about 4.3 Å⁻¹, associated with the contribution of Co—N scattering path by comparing with the WT analysis of CoPc, Co foil, CoO and CoO₄ catalyst layers. The intensity maximum at about 6.8 Å⁻¹ indexed to Co—Co coordination is not detected in CoPc@HC. Referring to FIGS. 42 to 48, quantitative least-squares EXAFS curve-fitting analysis was performed to extract the coordination configuration. As shown in Table 1, the first-shell EXAFS curve-fitting result of CoPc@HC reveals that the main peak at 1.3 Å is attributed to the superimposition of the Co—N₄ coordination from CoPc molecules and Co—N coordination from the interaction between CoPc molecules and HC substrate.

TABLE 1
Co K-edge EXAFS curve Fitting Parametersa
	sample	path	N	R (Å)	σ2 (Å2)
	Co foil	Co—Co	12	2.49	0.008
	CoOc	Co—O	6	2.11	0.010
		Co—Co	12	2.99	0.009
	Co3O4d	Co—O	5.3	1.92	0.005
		Co—Co	4	2.85	0.004
		Co—Co	8	3.36	0.007
	CoPce	Co—N	4.2	1.88	0.007
	CoN5f	Co—N	5.3	1.87	0.008
	aN, coordination number; R, distance between absorber and backscatter atoms; σ2, Debye-Waller factor to account for both thermal and structural disorders; Error bounds (accuracies) that characterize the structural parameters obtained by EXAFS spectroscopy were estimated as N ± 20%; R ± 1%; σ2 ± 20%. S02 was fixed to 0.93 as determined from Co foil fitting.
	bFitting range: 2 ≤ k (/Å) ≤ 10 and 1 ≤ R (Å) ≤ 3.
	cFitting range: 2.0 ≤ k (/Å) ≤ 9.7 and 1.0 ≤ R (Å) ≤ 3.
	dFitting range: 2.0 ≤ k (/Å) ≤ 12.0 and 1.0 ≤ R (Å) ≤ 3.50.
	eFitting range: 2.0 ≤ k (/Å) ≤ 8.0 and 1.0 ≤ R (Å) ≤ 1.9.
	fFitting range: 2.0 ≤ k (/Å) ≤ 8.0 and 1.0 ≤ R (Å) ≤ 2.0.

Thus, by studying an example CoPc@HC first catalyst layer, it was shown that the electronic catalyst-support interaction contributes to reducing Co agglomeration and improving performance of the modified electrode when operated in an acidic medium. Indeed, the electronic catalyst-support interaction offers beneficial chemical bonding and associated charge transfer at the interface between the catalyst molecule center and the support, e.g., the Co center of CoPc molecule and the support being hollow carbon support with N doping.

Second Catalyst Function—C—C Coupling

The second catalyst can be referred to as a tandem catalyst, that is configured to unite on a single support the CO₂-to-CO reduction and the C—C coupling steps.

The second catalyst is also a metal-based catalyst that is provided as a second catalyst layer in the modified electrode. Referring to the right portion of FIG. 4, the second CO-to-C₂₊ catalyst layer is positioned below the first CO₂-to-CO catalyst layer. “Below” should be understood herein as a position of the second catalyst layer being such that the first catalyst layer is an upper layer in contact with the acidic electrolyte and the second catalyst layer is in contact with a cathodic flow field supplying gaseous CO₂. The modified electrode as encompassed herein thus includes a bilayer structure having a dual catalysis function.

The bilayer structure of the modified electrode allows enrichment of CO coverage on Cu surface, thereby suppressing HER and promoting multi-carbon (C₂₊) product formation in acidic CO₂RR. The decoupling of the overall CO₂-to-C₂₊ reaction into two steps: CO₂-to-CO and CO-to-C₂₊ is enabled the dual catalysis function of the modified electrode integrating two functionally distinct catalysts.

Still referring to the right portion of FIG. 4, in view of the low solubility of produced CO in the aqueous electrolyte, by positioning the CO producing layer on top of the C—C coupling layer, the CO can be driven downwardly, from the CO producing layer and through the C—C coupling catalyst, to promote the desired reaction sequence and local enrichment in CO between the layers.

The second metal-based catalyst can be provided as a Cu-based material, including Cu nanomaterials with different morphologies, such as nanoparticles, nanowires, nanosheets. The Cu-based material can also include Cu-based alloy materials, such as CuAg, CuAu, CuPd, CuBi, CuAl, CuSn, CuIn or any combinations thereof. The second metal-based catalyst particles can have an average size between 10 nm and 1000 nm. For example, the second catalyst layer can have a catalyst content between 50 wt. % and 100 wt. % based on the total weight of the second catalyst layer.

It should be noted that the modified electrode further includes a porous support that is positioned in contact with the second and inner catalyst layer to ensure gas diffusion to the catalyst layers. The porous support can be selected, for example, from polytetrafluoroethylene (PTFE) and porous carbon paper. For example, the porous support can have pores with a diameter size ranging between 100 nm and 5000 nm, as determined by Brunauer-Emmett-Teller (BET) method.

The method to manufacture the modified electrode can thus include, for example, sputtering the second metal-based catalyst, such as copper nanoparticles, onto the porous support to form the second catalyst layer onto the porous support.

In some implementations, the second catalyst layer can further include an ionomer. For example, the ionomer can be 1,1,2,2-tetrafluoroethene; 1,1,2,2-tetrafluoro-2-[1,1,1,2,3,3-hexafluoro-3-(1,2,2-trifluoroethenoxy)propan-2-yl]oxyethanesulfonic acid copolymer, tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer, or any combinations thereof.

Regarding the method to manufacture the modified electrode, when including an ionomer, the second catalyst layer can be prepared by depositing an ionomer-containing ink comprising the ionomer and the second metal-based catalyst onto the porous support. The ionomer-containing ink can be airbrushed, or drop-casted. The second metal-based catalyst loading in the ionomer-containing ink can be between 0.1 wt. % and 2 wt. % based on the total weight of the ionomer-containing ink, preferably between 0.5 wt. % and 1.5 wt. %. The ionomer loading in the ionomer-containing ink can be between 0.5 wt. % and 5 wt. % based on the total weight of the ionomer-containing ink, preferably between 1 wt. % and 4 wt. %. In addition, the second catalyst layer can include an ionomer-containing top layer and a pure metal-based catalyst underlayer. The pure metal-based catalyst underlayer can be for example formed by sputtering a metal or metal alloy onto the porous support. The ionomer-containing top layer can be formed by depositing, e.g. by airbrushing, a ionomer-containing ink onto the pure metal-based catalyst underlayer. Alternatively, the ionomer-containing ink comprising the ionomer and the second metal-based catalyst can be directly deposited onto the porous support such that the modified electrode does not include any sputtered metal underlayer.

CO Interaction Characterization with Example CoPC@HC/Cu Modified Electrode

The interaction between the produced CO and the modified electrode was studied with an example CoPc@HC/Cu modified electrode (also referred to as a CoPc/Cu tandem electrode), including a CoPC@HC first and upper catalyst layer, and a Cu second and inner catalyst layer by probing CO adsorption on the Cu surface using in-situ Raman spectroscopy.

FIG. 22 shows the Raman spectra of CO for a Cu electrode under different applied potentials (in V). The peak at 361 cm⁻¹ is ascribed to the Cu—CO stretch. This peak emerged at −1.2 V vs. Ag/AgCl, indicating that CO₂ is converted to CO on the Cu surface. FIG. 23 shows the Raman spectra of CO for the CoPc/Cu tandem electrode. The peak at 365 cm⁻¹ is blue-shifted to 389 cm⁻¹, indicating a stronger binding of CO on the Cu sites as a result of CoPc in the vicinity of adsorbed CO. Additionally, a new peak at 532 cm⁻¹ is observed and associated to Co—CO bending vibration upon interaction between CO and the Co species in CoPc molecules. It is noticeable that the Cu—CO and Co—CO peaks appear at −0.8 V for CoPc/Cu, indicating a much earlier onset compared to that observed on the Cu surface. Thus, the presence of CoPc molecules facilitates CO₂ to CO conversion at lower overpotentials, which explains the lower CO₂RR voltage achieved using the tandem electrode as encompassed herein compared to that of the Cu electrode.

Referring to FIG. 24, control Raman spectra experiments indicated that the 532 cm⁻¹ peak is due to Co—CO, indicating that higher CoPc coverage on the Cu surface is correlated primarily with a Co—CO peak. Interestingly, referring to FIG. 25, when the CoPc molecules forms aggregates, the CoPc catalyst layer is not effective at catalyzing CO₂-to-CO conversion, and only the Cu—CO peak at 365 cm⁻¹ is observed. Thus, the Raman spectra results suggest that an atomic dispersion of the CoPc molecules is necessary to achieve the enhanced performance of CO₂ electroreduction with dual catalysis functions.

Operation of CO₂ Electroreduction in a System

The modified cathode can be used in a CO₂ electrolyzer system for reducing CO₂ into multi-carbon products; with preference, the multi-carbon products comprise ethylene. The system comprises a cathodic compartment having a reactant inlet configured for receiving a CO₂-containing gas stream and including the modified electrode. Upon operation of the system, electroreduction performance is enhanced by the dual catalysis function of the modified electrode.

For example, the system can be operated by applying a current density between 100 mA·cm⁻² and 1200 mA·cm⁻² for a cell surface between 1 cm² and 5 cm². The CO₂-containing gas stream includes gaseous CO₂ and can include one or more additional gas(es) such as CO and N₂. An inlet CO₂ flow rate between 50 v/v % and 100 v/v % can be supplied to the reactant inlet of the cathodic compartment of the system.

In some implementations, the system can be a flow cell, a slim flow cell, or a membrane electrode assembly.

CO₂-to-CO Performance in an Example Flow Cell

Referring to FIG. 11, it was found that the CoPc@HC catalyst can improve CO production with a CO FE of 94% and H₂ FE of less than 3% at an applied current density of 300 mA·cm⁻². This performance was substantially maintained (within 3%-relative) over 12 h of operation at an applied current density of 300 mA·cm⁻², as shown in FIGS. 50-53.

Table 2 shows that the resulting performance metrics exceed those of previously-reported CO₂-to-CO catalysts in acidic media.

TABLE 2
Comparison of CoPc@HC and previously reported catalysts
toward the conversion of CO2 to CO in acidic media.
	CO FE	Current density
Electrode	(%)	(mA cm−2)	Reference
CoPc@HC	94	300	Present results
Au/C	91	250	Nat. Catal. 5, 268-276 (2022)
Au/C	90	100	Nat. Commun. 12, 4943 (2021)
sputtered	32	400	Science 372, 1074-1078 (2021)
Ag/PTFE
Ag	60	100	ACS Energy Lett. 6, 4291-4298
			(2021)

CO-to-C₂₊ Performance in an Example Flow Cell

A first CoPc@HC/sCu tandem electrode consisting of the second C—C coupling catalyst layer followed by an upper layer of first CO₂-to-CO catalyst was tested. The first CoPc@HC catalyst was layered on the top of a sputtered Cu layer acting as the second catalyst layer to construct the CoPc@HC/sCu tandem electrode. The acidic CO₂RR performance of the CoPc@HC/sCu tandem electrode in a buffered acidic electrolyte containing 0.5 M H₃PO₄, 0.5 M KH₂PO₄, and 2.5 M KCl was evaluated when used to build a flow cell electrolyzer.

The dominant product generated by the CoPc@HC/sCu tandem electrode was CO, and the FE to C₂₊ reached a maximum value of 36% at 200 mA·cm⁻². Referring to FIG. 54, a further increase in current density led to a decrease in C₂₊ formation while the CO FE remained nearly constant. The lowering of the C₂₊ FE was assumed as a result of underutilized CO generated by the CoPC@HC catalyst layer.

A solution to maintain the C₂₊ FE upon increasing the applied current density can include increasing the density of Cu sites and enhancing mass transport of CO to these active sites.

For doing so, in some implementations, the second catalyst layer can be designed to include a three-dimensional catalyst:ionomer bulk heterojunction (Cu-CIBH) top layer consisting of Cu nanoparticles (CuNP) and perfluorosulfonic acid (PFSA) ionomer, and a sputtered Cu underlayer, thereby forming a CoPc@HC/Cu tandem electrode as better seen in FIGS. 12-13. For example, the catalyst-ionomer bulk heterojunction can have a CIBH thickness ranging between 50.0 nm to 25.0 μm as determined by scanning electron microscopy, preferably between 60.0 nm and 24.0 μm, more preferably between 70.0 nm and 23.0 μm, more preferably between 100.0 nm and 20.0 μm. For example, the catalyst-ionomer bulk heterojunction can have a ratio of catalyst material to ion-conducting polymer ranging from 0.1 to 10.0, preferentially ranging from 0.2 to 9.0, more preferentially ranging from 0.3 to 8.0, even more preferentially ranging from 0.4 to 7.0, most preferentially ranging from 0.5 to 5.0 or ranging from 0.5: to 2.0, even most preferentially ranging from 0.6 to 2.0 or from 1.0 to 1.6. Referring to FIG. 14, operation of the flow cell including the CoPc@HC/Cu tandem electrode led to having the FEs of C₂H₄ and C₂₊ increased from 30% to 54% and from 36% to 80%, respectively, and these were accompanied by a four-fold increase in the total current density to 800 mA cm⁻².

The improvement in performance suggests that a greater local CO availability enabled by the tandem catalysis design promotes the formation of C₂H₄ and C₂₊ products. By contrast, Cu electrode without tandem configuration displayed much lower FEs for C₂H₄ (27%), and C₂₊ (41%) (FIGS. 15-16). The CoPc@HC/Cu tandem electrode exhibited <10% H₂ FE compared with >35% in the case of the Cu electrode (FIG. 55). This indicates that the CO produced by the CoPc@HC layer during CO₂RR could suppress the competing HER on Cu surface in acidic media. These results are consistent with the following DFT predictions wherein higher CO concentrations on Cu surface lower the barrier to C—C coupling and suppress HER.

Using DFT, reaction energetics of CO₂RR on copper being used as catalyst were studied, noting the dependence on the surface concentration of the reaction intermediates (i.e., CO) and the adsorbed H (H*, * indicating a surface site). Referring to FIGS. 1 and 26-28, a greater CO coverage on copper catalyst was shown to reduce the Gibbs free energy of hydrogen adsorption (ΔG_H*), thus leading to a predicted increase in HER overpotential. Referring to FIG. 2, a lower activation energy for the C—C coupling reaction was found at greater CO coverage on the copper catalyst. Referring to FIG. 3, when the input gas was switched from CO₂ to CO, the C₂H₄ and C₂₊ production increased and the H₂ Faradaic Efficiency (FE) decreased. The forward-rate of C—C coupling is thus indeed promoted by a high presence of CO. It should be noted that the DFT calculations were performed on a bare copper model, and supporting experiments were performed on a tandem electrode as encompassed herein including the second catalyst layer having the Cu-CIBH top layer.

Stability of the C₂₊ FE was also assessed in the flow cell electrolyzer system. Referring to FIG. 56, the system maintained about 54% C₂H₄ FE for an operation duration of 16 h. Referring to FIG. 57, the scanning electrode microscopy (SEM) image shows that the bilayer structure of the modified electrode remained intact.

The example CoPc@HC/Cu tandem electrode was also used in a flow cell to study the single-pass carbon efficiency (SPCE). Referring to FIG. 17, by reducing the flow rate of CO₂ to 2 standard cubic centimeters per minute (sccm), a maximum SPCE of 87% was obtained for all the CO₂RR products. Comparing with FIG. 58 for a bare Cu electrode, the SPCE on the Cu electrode is 52%, when the system is supplied with the same flow rate of CO₂. The spatially decoupled strategy of dual catalysis enables a remarkable enhancement on the SPCE from 52% to 87% when the CoPc@HC/Cu tandem electrode was implemented in the flow cell. The improvement in SPCE allows reducing the energy penalty associated with the cathodic gas separation of unreacted CO₂ and product separation from dilute streams.

Referring to FIG. 14, the CoPc@HC/Cu tandem electrode demonstrated higher current densities of C₂H₄ and C₂₊ at the lower potentials compared to those of the bare Cu electrode. This translated to a better energy efficiency at the device level—achieving a full cell voltage of 3.8 V at 500 mA cm⁻², leading to a 16% C₂H₄ energy efficiency (EE) (FIGS. 59-64). This compares favourably to the 7% EE of the best prior report: Huang, J. E. et al. CO₂ electrolysis to multicarbon products in strong acid. Science 372, 1074-1078 (2021), as indicated on FIG. 65.

Referring to FIGS. 66-68, the C₂H₄ FE and C₂₊ FE for the CoPc@HC/Cu tandem electrode were noted to be higher than those for the bare Cu electrode in CO reduction reaction (CORR) where the availability of CO is not a limiting factor in C₂H₄ formation. Referring to the right-sided structure illustrated in FIG. 69, DFT was further used to study the CO₂RR pathway on a modified electrode model having a bilayer structure comprising a periodic Cu slab (second catalyst layer) covered by a CoN₄—C layer (first carbon-supported catalyst layer). Referring to FIGS. 20, 21, and 70-73, DFT calculations showed that the CoN₄—C/Cu model reduced the ΔG required for C—C coupling by about 0.11 eV and changed the favourable reaction pathway for the hydrogenation of *CCO intermediate to favour C₂H₄ formation. The combined DFT and empirical results suggest that CoPc working in conjunction with Cu influence C₂H₄ selectivity in addition to promoting C—C coupling.

When implemented in a flow cell system, the example nanoscale-engineered CoPc@HC/Cu tandem electrode achieved 54% C₂H₄ FE and 80% C₂₊ FE at 800 mA cm⁻² while maintaining <10% H₂ FE. The example modified electrode also enabled a record high SPCE of 87%. A C₂H₄ energy efficiency of 16% was also exhibited, corresponding to a two-fold improvement over the best prior acidic CO₂-to-C₂H₄ electrosynthesis.

Technoeconomic analysis showcased this tandem system delivered the lowest energy cost for C₂H₄ production compared to the state-of-the-art alkaline/neutral/acidic systems.

Energy Assessment

Based on technoeconomic analysis, the energy intensity of C₂H₄ production is 300 GJ/ton, which is 50% lower compared to the highest-efficiency prior CO₂-to-C₂H₄ in an acidic system, and also compares well with those of benchmark alkaline and neutral systems that suffer significant energy penalties associated with CO₂ separation (see FIGS. 19 and 65).

Energy analysis of the CO₂RR electrolyzer was carried out by using an energy assessment model akin to that reported. Here we provide a summary of the model and assumptions used to obtain the energy intensity of producing ethylene in an electrolyzer using acid electrolytes. To calculate the energy intensity of producing ethylene, we used the main performance metrics of the electrolyzer such as Faradaic efficiency, single-pass conversion efficiency, full-cell voltage, and current density as the input. We carried out the energy assessment under various operating conditions (e.g., at various CO₂ input flow rates) by using the readily achieved performance metrics as the input. We consider the presence of hydrogen produced via hydrogen evolution reaction (HER) at the cathodic product stream. We consider oxygen as the only product produced via oxygen evolution reaction (OER) at the anodic product stream. We implement a pressure swing adsorption (PSA) gas separation unit at the cathodic downstream to recover ethylene from the unreacted CO₂ and side product hydrogen. The recovered CO₂ from the cathode outlet is returned to the cathode inlet for utilization in the CO₂RR. Scaling up the electrolyte required for the lab-scale electrolyzers, we considered an electrolyte requirement of 100 L per m² of electrolyzer geometric area. The electrolyte is assumed to be circulated through a closed loop and used for 1 year without replacement. Further details of the energy calculations associated with the electrolyzer electricity and cathode separation are provided below.

There is provided an example calculation for the energy requirement associated with the electrolyzer electricity and cathode separation in the CO₂-to-ethylene conversion process in a slim-flow cell. The model uses the performance metrics of the slim-flow cell as the input. The metrics provided in Table 3 include a Faradaic efficiency of 54%, a full-cell voltage of 3.76 V, a single pass conversion efficiency of 17%, and a current density of 500 mA cm⁻². It should be noted that the full-cell voltage is to be understood as a total input voltage to power the CO₂ electrolysis in the acidic system.

TABLE 3
Product FEs on CoPc@HC/Cu tandem electrode and Cu electrode
toward acidic CO2RR under the applied current density of 500 mA cm−2 with
CO2 flow rate of 4 sccm in slim flow cell.
			CO	C2H4	CH4	EtOH	Acetate	Formate	Propanol
	Full cell	H2	FE	FE	FE	FE	FE	FE	FE
Electrodes	voltage (V)	FE (%)	(%)	(%)	(%)	(%)	(%)	(%)	(%
CoPc@HC/	3.76	13.2	9.6	53.6	0.01	16.2	1.9	3.1	2.5
Cu
Cu	4.70	42.1	9.1	30.2	0.1	11.8	1.7	2.9	2.1

Electrolyzer Electricity

The finding of the production rate of ethylene on a molar basis per second, with a production capacity of 100 tons is determined according to equations (1a) and (1b).

$\begin{array}{cc}\mathrm{Production}\mathrm{rate}\left[\frac{\mathrm{mol}}{s}\right]=\frac{\mathrm{Production}\left[\frac{g}{\mathrm{day}}\right]}{\mathrm{molecular}{\mathrm{weight}}_{\mathrm{ethylene}}\left[\frac{g}{\mathrm{mol}}\right]\times 86400\left[\frac{s}{\mathrm{day}}\right]}& \left(1a\right)\end{array}$ $\begin{array}{cc}\mathrm{Production}\mathrm{rate}\left[\frac{\mathrm{mol}}{s}\right]=\frac{100\times \frac{{10}^{6}g}{\mathrm{day}}}{\frac{28g}{\mathrm{mol}}\times \frac{86400s}{\mathrm{day}}}=41.335\frac{\mathrm{mol}}{s}& \left(1b\right)\end{array}$

The current required to produce ethylene at this rate is then determined for an experimentally achieved ethylene FE of 54% according to equations (2a) and (2b):

$\begin{array}{cc}\mathrm{Total}\mathrm{current}\mathrm{required}\left[A\right]=\frac{\begin{array}{c}\mathrm{Production}\mathrm{rate}\left[\frac{\mathrm{mol}}{s}\right]\times \\ \mathrm{electrons}\mathrm{transferred}\times \mathrm{Faraday}’s\mathrm{Constant}\end{array}}{{\mathrm{FE}}_{\mathrm{ethylene}}\left[\mathrm{decimal}\right]}& \left(2a\right)\end{array}$ $\begin{array}{cc}\mathrm{Total}\mathrm{current}\mathrm{required}\left[A\right]=\frac{41.335\frac{\mathrm{mol}}{s}\times 12\times 96485\frac{\mathrm{sA}}{\mathrm{mol}}}{0.54}=88626832A& \left(2b\right)\end{array}$

The full-cell voltage (3.76 V) was then multiplied by the value of the current to calculate the power consumed:

$\begin{array}{cc}\mathrm{Power}\mathrm{Consumed}\left[W\right]=\mathrm{Total}\mathrm{current}\mathrm{needed}\left[A\right]\times \mathrm{Cell}\mathrm{voltage}\left[V\right]=88626832A\times 3.76V=333237\mathrm{kW}& \left(3\right)\end{array}$

To calculate the energy needed to run the plant for one day and meet the production capacity of 100 tons, the previous result was then multiplied by 24 h and divided by the production capacity:

$\begin{array}{cc}\mathrm{Electricity}\mathrm{Energy}\mathrm{Requirement}\left[\frac{\mathrm{GJ}}{{\mathrm{ton}}^{-1}}\right]=\frac{\mathrm{Power}\mathrm{Consumed}\left[W\right]\times 24\left[h\right]}{\mathrm{daily}\mathrm{production}\left[\mathrm{ton}\right]}=\frac{333237\mathrm{kW}\times 24h\times 0.0036\frac{\mathrm{GJ}}{\mathrm{kWh}}}{100\mathrm{ton}}=288\frac{\mathrm{GJ}}{\mathrm{ton}}& \left(4\right)\end{array}$

Cathode Separation

To recover the ethylene from the cathodic downstream, it was assumed that a pressure swing adsorption (PSA) separation unit was used. The model accounts for the capital and operating costs of the PSA separation module based on a model for biogas upgrading. The cathodic gas stream is modelled to be composed of ethylene, unreacted CO₂, and hydrogen. The model considers a cost of $1 989 043 for a flow rate of 1000 m³ h⁻¹ by using a scaling factor of 0.7 and energy input of 0.25 kWh m⁻³. In light of these, the energy consumption was calculated using equation (5a) as follows:

$\begin{array}{cc}\mathrm{PSA}\mathrm{Operating}\mathrm{Energy}\left[\frac{\mathrm{kWh}}{\mathrm{ton}\mathrm{ethylene}}\right]=0.25\frac{\mathrm{kWh}}{m^2}\times \mathrm{flow}\mathrm{rate}\left[\frac{m^3}{h}\right]\times 24\frac{h}{\mathrm{day}}& \left(5a\right)\end{array}$

Before using this equation, the flow rate of the cathodic stream was calculated by determining the flow rate of ethylene under the standard conditions, as follows:

$\begin{array}{cc}\mathrm{Output}\mathrm{ethylene}\mathrm{flow}\mathrm{rate}\left[\frac{m^3}{h}\right]=\frac{100\times {10}^{6}g\times 8.314{\mathrm{Jmol}}^{-1}{K}^{-1}\times 298K}{28\frac{g}{\mathrm{mol}}\times 101300\mathrm{Pa}\times 24\frac{h}{\mathrm{day}}}=3639\frac{m^3}{h}& \left(6\right)\end{array}$

The flow rates of unreacted CO₂, product ethylene, and byproduct hydrogen was calculated at the cathode outlet. The flow rate of unreacted CO₂ at the cathode outlet is calculated by using the single pass conversion efficiency at a constant pressure. It is worth noting that this single pass conversion efficiency merely relates to the amount of CO₂ reduced to the CO₂ that passes through the cathode stream, unreacted. Considering a single-pass conversion of 17%, the output CO₂ flow rate has been determined as follows:

$\begin{array}{cc}\mathrm{Output}{\mathrm{CO}}_2\mathrm{flow}\mathrm{rate}\left[\frac{m^3}{h}\right]& \left(7a\right)\end{array}$ $\mathrm{ethylene}\mathrm{flow}\mathrm{rate}\left[\frac{m^3}{h}\right]\times \mathrm{molar}\mathrm{ratio}\left[\frac{{\mathrm{CO}}_2}{\mathrm{ethylene}}\right]\times (\frac{100-\mathrm{single}\mathrm{pass}\mathrm{conversion}\left[\%\right]}{\mathrm{single}\mathrm{pass}\mathrm{conersion}\left[\%\right]}$ $\begin{array}{cc}\mathrm{Output}{\mathrm{CO}}_2\mathrm{flow}\mathrm{rate}\left[\frac{m^3}{h}\right]=3639\frac{m^3}{h}\times 2\times \frac{100-17}{17}=35533\frac{m^3}{h}& \left(7b\right)\end{array}$

Since H₂ is the only byproduct at the cathode stream, the current toward H₂ was determined as follows:

$\begin{array}{cc}\mathrm{Current}\mathrm{toward}{H}_2\left[A\right]=\mathrm{Total}\mathrm{current}\mathrm{needed}\left[A\right]\times \frac{100-{\mathrm{FE}}_{\mathrm{ethylene}}\left[\%\right]}{100}& \left(8a\right)\end{array}$ $\begin{array}{cc}\mathrm{Current}\mathrm{toward}{H}_2\left[A\right]=88626832A\times \frac{100-54}{100}=40768342A& \left(8b\right)\end{array}$

The H₂ production rate can be defined as follows:

$\begin{array}{cc}{H}_2\mathrm{production}\left[\frac{\mathrm{mol}}{h}\right]=\frac{\mathrm{Current}\mathrm{toward}{H}_2\left[A\right]\times 3600\frac{s}{h}}{2\frac{\mathrm{electrons}}{H_2\mathrm{product}}\times \mathrm{Faraday}’s\mathrm{Constant}}& \left(9a\right)\end{array}$ $\begin{array}{cc}{H}_2\mathrm{production}\left[\frac{\mathrm{mol}}{h}\right]=\frac{40768342A\times 3600\frac{s}{\mathrm{hour}}}{2\frac{\mathrm{electrons}}{H_2\mathrm{product}}\times 96485\frac{\mathrm{sA}}{\mathrm{mol}}}=760563\frac{\mathrm{mol}}{h}& \left(9b\right)\end{array}$

Assuming an ideal gas under standard conditions, the flow rate of H₂ was calculated as follows:

$\begin{array}{cc}\mathrm{flow}{\mathrm{rate}}_{H2}\left[\frac{m^3}{h}\right]=\frac{H_2\mathrm{production}\left[\frac{\mathrm{mol}}{h}\right]\times 8.314\frac{J}{\mathrm{mol}\times K}\times 298K}{101.3\times {10}^3\mathrm{Pa}}& \left(10a\right)\end{array}$ $\begin{array}{cc}\mathrm{flow}{\mathrm{rate}}_{H2}\left[\frac{m^3}{h}\right]=\frac{760563\frac{\mathrm{mol}}{h}\times 8.314\frac{J}{\mathrm{mol}\times K}\times 298K}{101.3\times {10}^3\mathrm{Pa}}=18601\frac{m^3}{h}& \left(10b\right)\end{array}$

The total flow rate at the cathodic downstream was then calculated by summing the flow rate of ethylene, unreacted CO₂, and H₂ using supplementary Equations 6, 7b, and 10b.

$\begin{array}{cc}\mathrm{flow}\mathrm{rate}\left[\frac{m^3}{h}\right]=\left(3639+35533+18601\right)\frac{m^3}{h}=57773\frac{m^3}{h}& \left(11\right)\end{array}$

With the final output flow rate, the energy required per ton of ethylene produced was then calculated using equation 5b as follows:

$\begin{array}{cc}\mathrm{PSA}\mathrm{Energy}\left[\frac{\mathrm{GJ}}{\mathrm{ton}\mathrm{ethylene}}\right]=0.25\frac{\mathrm{kWh}}{m^3}\times 57773\frac{m^3}{h}\times 24\frac{h}{\mathrm{day}}\times \frac{0.0036\mathrm{GJ}{\mathrm{kWh}}^{-1}}{100\mathrm{ton}\mathrm{ethylene}}=12\frac{\mathrm{GJ}}{\mathrm{ton}\mathrm{ethylene}}& \left(5b\right)\end{array}$

Energy Calculations Toward C₂₊ Products

Energy cost of producing C₂₊ products is estimated by using the similar calculations for the electrolyzer electricity and cathode separation energy costs. As the ethylene is the dominant C₂ product, the electrolyzer electricity cost is made using the molecular mass and electron transfer numbers for ethylene, and this leads to a more conservative energy requirement. Referring to Tables 4 and 5, an extra energy input of ˜25 GJ ton⁻¹ associated with the liquid product distillation is estimated by using an empirical formula reported. This estimation reflects the highly diluted liquid product concentration in the acidic electrolytes, which is assumed to be 0.5% wt. %.

TABLE 4
Energy assessment of producing ethylene and C2+ in acidic media.
		Cu	CoPc@H			CoPc@
	Current	electrode	C/Cu	Current	Cu	HC/Cu
	best C2H4	C2H4	C2H4	best C2+	electrode	C2+
	(Science	(This	(This	(Science	C2+ (This	(This
Parameters	2021)1	work)	work)	2021)1	work)	work)
Cell voltage (V)	4.2	4.70	3.76	4.2	4.70	3.76
Faradaic efficiency (%)	29	30	54	44	46	74
Current density (mA cm−2)	1200	500	500	1200	500	500
Electrolyzer specific energy distribution (GJ/(tonne ethylene or C2+)−1)
Electrolyzer electricity	599	648	288	395	423	210
Cathode separation	16	26	12	9	15	7
Anode separation (Carbonate)	0.0	0.0	0.0	0.0	0.0	0.0
Carbonate regeneration	0.0	0.0	0.0	0.0	0.0	0.0
Liquid product distillation	—	—	—	25	25	25
Overall energy (GJ/ton)	615	674	300	429	463	242

TABLE 5
Energy assessment of producing ethylene of CoPc@HC/Cu tandem electrode
system in comparison to the benchmark of alkaline/neutral/acidic systems.
	Alkaline	Neutral	Acidic	Acidic
	media	media	media	media
	CO2RR2	CO2RR3	CO2RR1	CO2RR
	(Science 2018,	(Nature 2020,	(Science 2021,	CoPc@HC/Cu
Parameters	360, 783-787.)	577, 509-513.)	372, 1074-1078.)	(This work)
Cell voltage (V)	2.5	3.65	4.2	3.76
Faradaic efficiency (%)	70	64	29	54
Current density (mA cm−2)	150	120	1200	500
	Electrolyzer specific energy distribution (GJ/(tonne ethylene)−1)
Electrolyzer electricity	142	235	599	288
Cathode separation	144	133	16	12
Anode separation	0.0	68	0.0	0.0
Carbonate regeneration	278	0.0	0.0	0.0
Overall energy (GJ/ton)	564	436	615	300

Experimental Methods and Materials

Chemicals

Cobalt (II) phthalocyanine (CoPc) (CAS 3317-67-7), zinc nitrate hexahydrate (98%) (CAS 10196-18-6), 2-methylimidazole (98%) (CAS 693-98-1), cobalt (II) nitrate hexahydrate (98.5%) (CAS 10026-22-9), methanol (CAS 67-56-1), N,N-dimethylformamide (DMF) (CAS 68-12-2), triethylamine (CAS 121-44-8), Nafion perfluorinated resin solution (5 wt. % in a mixture of lower aliphatic alcohols and water; Sigma-Aldrich; CAS 31175-20-9), phosphoric acid (85%) (CAS 7664-38-2), potassium chloride (CAS 7447-70-7), potassium phosphate monobasic (CAS 778-77-0), sulfuric acid (CAS 7664-93-9), perchloric acid (CAS 7601-90-3) were purchased from Sigma Aldrich. Phosphonitrilic chloride trimer (98%) (CAS 940-71-6), and bis(4-hydroxyphenyl) sulfone (CAS 80-09-1) were purchased from Alfa Aesar. Copper nanoparticles (25 nm) were purchased from US Research Nanomaterials, Inc. Nafion 117 membrane, platinum mesh and gas diffusion layer (Freudenberg H23C3), and carbon powder (Vulcan XC-72R) were received from Fuel Cell Store. The polytetrafluoroethylene (PTFE) gas diffusion layer with 450 nm pore size was obtained from Beijing Zhongxingweiye Instrument Co., Ltd. Copper target (>99.99%) was purchased from Kurt J. Lesker. The conductive gas-diffusion layer was prepared by sputtering 150 nm Copper layer on the PTFE substrate using pure copper target with a deposition rate of 1 Å/sec in an Angstrom Nexdep sputtering system. The distilled water with a resistivity of 18.2 MΩ cm obtained from a Milli-Q reference water-purification system was used to prepare the aqueous solutions in all the experiments. All chemicals were used without any further purification.

Manufacture of the Example Modified Electrodes

The CoPc@HC catalyst layer that was used in most of the experiments described herein included a second catalyst layer made of CoPc atomically dispersed on a hollow carbon support. The hollow carbon support (HC) was first prepared by synthesizing ZIF-8 (zeolitic imidazolate framework-8, a class of metal-organic framework), then ZIF-8@PZS (poly(cyclotriphospazene-co-4,4′-sulfonyldiphenol), a class of polymer), and then HC.

For the synthesis of ZIF-8, 6 mmol of Zn(NO₃)₂·6H₂O and 24 mmol of 2-methylimidazole and 100 mL of methanol were mixed with vigorous stirring for 3 min at room temperature. Subsequently, the mixture was kept at 35° C. for 6 h. The precipitate was collected, washed, and finally dried in a vacuum at 80° C. for 12 h. For the synthesis of ZIF-8@PZS, 400 mg of as-prepared ZIF-8 powder was dispersed in 40 mL of methanol. Then, 100 mL of methanol containing 325 mg bis(4-hydroxyphenyl) sulfone and 152 mg phosphonitrilic chloride trimer was added and stirred for 15 min. Subsequently, 1 mL of N,N-diethylethanamine was slowly dripped into the above dispersion, followed by stirring for 15 h at room temperature. The resulting precipitate marked as ZIF-8@PZS was collected, washed and finally dried in a vacuum at 80° C. for 12 h. For the synthesis of HC, the as-prepared ZIF-8@PZS powder was placed in a quartz boat and maintained 950° C. for 3 h in a tube furnace with a heating rate of 5° C. min⁻¹ under a flowing Ar atmosphere to obtain HC.

For atomically dispersing CoPc onto the HC and for the CoPc@HC layer of the modified electrode, 4 mg of CoPc and 60 mg of HC were dispersed in 60 mL of DMF using sonication, respectively. Then, the CoPc dispersion solution was added to HC suspension. The mixture was sonicated for 30 min and then stirred for 24 h at room temperature. Subsequently, the CoPc@HC was obtained by washing with DMF many times until colorless, followed by drying in a vacuum at 80° C. for 12 h.

Various electrodes were prepared to perform the experiments escribed herein, including a state-of-the-art CoPc/C electrode, a Cu electrode, a CoPc@HC/Cu electrode, a CoPc@HC/sCu electrode, and a CoPc@HC electrode. The state-of-the-art CoPc/C electrode was prepared according to a typical procedure, wherein 82.9 mg of Vulcan carbon powder (XC-72R), 89.6 mg CoPc, and 374 μL Nafion perfluorinated resin solution were stirred and sonicated in a 35 mL absolute ethanol. Then, 10 mL of the ink was sprayed onto the gas diffusion layer (Freudenberg H23C3). The Cu electrode was prepared through airbrushing the catalyst ink consisting of 30 mg of Cu nanoparticles, 2 mL of methanol, and 22.5 μL Nafion perfluorinated resin solution onto a conductive gas-diffusion layer with a Cu nanoparticle loading of 1 mg cm⁻². For the preparation of CoPc@HC/Cu electrode, the catalyst ink comprising 15 mg of CoPc@HC, 2 mL of methanol and 45 μL Nafion perfluorinated resin solution was sprayed onto the Cu electrode with a CoPc@HC loading of 0.5 mg cm² using airbrushing. CoPc@HC/sCu electrode was prepared by spraying CoPc@HC catalyst ink onto the sputtered Cu layer. CoPc@HC electrode was prepared by spraying CoPc@HC catalyst ink onto the gas diffusion layer (Freudenberg H23C3).

Material Characterization

The morphology of catalyst layers including the first catalyst layer and the second catalyst layer can be characterized by scanning electrode microscopy (SEM) and transmission electron microscopy (TEM) measurements. The structure of the catalyst layers can be characterized by X-ray powder diffractometer (XRD) and X-ray absorption spectroscopy (XAS) measurements. The metal content of the catalysts, such as Co, Cu can be determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The contents of N, C, and optionally P and/or S, of the catalytic system can be determined by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) measurements.

Scanning electrode microscopy (SEM) images were obtained in a Hitachi FE-SEM SU 5000 microscope. The high-resolution transmission electron microscopy (HRTEM) images and the corresponding elemental mappings were collected on a JEOL JEM-2100F with an electron acceleration energy of 200 kV.

The high-resolution transmission electron microscopy (TEM) images and the corresponding energy-dispersive X-ray spectroscopy (EDS) were collected on a JEOL JEM-2100F with an electron acceleration energy of 200 kV.

The high-angle annular dark-field scanning TEM (HAADF-STEM) images were collected on a high-resolution transmission electron microscopy (JEM-ARM200F working at 300 kV), equipped with a probe spherical aberration corrector.

X-ray powder diffractometer (XRD) measurements were performed in a MiniFlex600 with Cu-Kα radiation. The metal content of the catalysts was measured by ICP-OES (iCAP6300).

X-ray photoelectron spectroscopy (XPS) measurements were performed in an ECSA device (PHI 5700) with Al Kα X-ray energy source (1486.6 eV) for excitation.

X-ray absorption spectroscopy (XAS) measurements were carried out at the 9BM beamline of the Advanced Photon Source (APS, Argonne National Laboratory, Lemont, Illinois). The XAS data were processed using ATHENA and ARTEMIS software incorporated into a standard IFEFFIT package.

In situ Raman measurements were conducted in a Renishaw inVia Raman Microscope with a water immersion objective (×63), 785 nm laser) in a modified flow cell.

Electrochemical Measurement

Without specification, all the CO₂RR and CORR measurements were carried out in an electrochemical flow cell setup by using an electrochemical station (Autolab PGSTAT302N) equipped with a current booster (Metrohm Autolab, 10 A). 0.5 M phosphate buffer solution (0.5 M H₃PO₄, 0.5 M KH₂PO₄) with 2.5 M KCl (pH=1.7) was used as the catholyte. The solution consisting of 0.5 M H₃PO₄ and 0.5 M KH₂PO₄ was used as the anolyte. Ag/AgCl (3 M KCl) and a Pt mesh were employed as the reference electrode and counter electrode, respectively. The cation-exchange membrane (Nafion 117) was used as the membrane to separate the cathode and anode chambers. Full-cell measurement was carried out in a slim flow cell setup, which consisted of an anolyte chamber, catholyte chamber, and gas flow chamber. All chambers were designed to ensure the proximity between cathode and anode electrodes to minimize the ohmic losses. The distances between the Nafion membrane and cathode electrode and the Nafion membrane and anode electrode were both about 5 mm.

For the full-cell voltage measurement, 0.05 M H₂SO₄ with 2.5 M KCl was used as the catholyte and 0.05 M H₂SO₄ was used as the anolyte. Titanium mesh-supported iridium oxide (IrOx/Ti mesh) was used as the anode electrode and was prepared by a previously reported dip coating and thermal decomposition method.

In all the electrochemical tests, the gas products were analyzed using a gas chromatograph (PerkinElmer Clarus 600).

In all the electrochemical tests, the liquid products were measured by ¹H NMR spectroscopy (600 MHz Agilent DD2 NMR Spectrometer) with dimethyl sulfoxide (DMSO) as the reference standard and deuterium oxide (D₂O) as the lock solvent.

The single pass carbon efficiency (SPCE) of CO₂ at the conditions of 298.15 K and 101.3 kPa was determined using the following equation:

$\mathrm{SPCE}=\left(j\times 60\sec \right)/\left(N\times F\right)÷(\mathrm{flow}\mathrm{rate}\left(\frac{L}{\min }\right)\times 1\left(\min \right)/(24.05\left(\frac{L}{\min }\right)$

• • where j is the partial current density of a specific group of products from CO₂ reduction, and N is the electron transfer for every product molecule.

The full-cell energy efficiency (EE) based on the production of ethylene was calculated as follows:

${\mathrm{EE}}_{\mathrm{full}\mathrm{cell},\mathrm{ethylene}}=\frac{\left(1.23+\left(-E_{\mathrm{ethylene}}^0\right)\right)\times {\mathrm{FE}}_{\mathrm{ethylene}}}{-E_{\mathrm{full}\mathrm{cell}}}$

• • where

$E_{\mathrm{ethylene}}^0\left(E_{\mathrm{ethylene}}^0=0.08V\mathrm{versus}\mathrm{RHE}\right)$

• •  is the thermodynamic potential of CO₂ to ethylene, FE_ethylene is the measured FE of ethylene, and E_{full cell} is the full-cell voltage without ohmic loss correction evaluated in the slim flow cell.

DFT Calculations

The VASP (Vienna Ab initio Package) software was used to perform all Density functional theory (DFT) calculations with the spin polarization setting. The core-valence interaction was calculated by the project augmented wave method (PAW), where the Cu(d¹⁰p¹), Co(d⁸s¹), O(s²p⁴), C(s²p²), N(s²p³) electrons were treated as valence states, and the remaining electrons were seen as a core state. The cut-off energy was set to 450 eV. The exchange-correlation correction effect was described by the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof functional (PBE). For the geometry optimization, the self-consistent iteration must reach to 10⁻⁵ eV for the energy convergence and 0.01 eV/A for the force convergence. The DFT-D3 method was used to account for dispersion correction for van der Waals force.

The (3×3) Cu(111) model consisted of four Cu atoms layers was built, in which the bottom two-atom layers were fixed to mimic bulk material and other atoms were relaxed. A charged water layer consisting of one protonated water molecule and five regular water molecules was considered to cover the Cu(111) surface. The vacuum space was about 15 Å along the z-axis. The k-point mesh was set to (3×3×1). The effect of CO coverage of 0, 1/9 ML, 2/9 ML, and 3/9 ML was accounted to calculate the adsorption energy of H* and the reaction energy barrier of C—C coupling. Here, the climbing-image Nudged Elastic Band (Cl-NEB) method was used to search the reaction pathway until the criteria of the force convergence and energy convergence reached 0.05 eV/Å and 10⁻⁵ eV, respectively.

The (6×6) Cu(111) model covered a charged water-layer consisting of twenty-four water molecules and a two-dimensional CoN₄-Graphen structure was built. The bottom Cu layer is treated as the bulk material, and two atomic layers at the top are modeled as surfaces. The vacuum space is about 15 Å along the z-axis. The k-point mesh was set to (2×2×1). The different possible reaction pathways from CO to ethylene with/without two-dimensional CoN₄—C structure were searched. The optimum reaction pathways were studied to unravel the effect of the two-dimensional CoN₄—C structure.

It should be noted that the same numerical references refer to similar elements. Furthermore, for the sake of simplicity and clarity, namely so as to not unduly burden the figures with several reference numbers, not all figures contain references to all the components and features, and references to some components and features may be found in only one figure, and components and features of the present disclosure which are illustrated in other figures can be easily inferred therefrom. The embodiments, geometrical configurations, materials mentioned and/or dimensions shown in the figures are optional, and are given for exemplification purposes only. Therefore, the descriptions, examples, methods and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only.

It is worth mentioning that throughout the following description when the article “a” is used to introduce an element it does not have the meaning of “only one” it rather means of “one or more”. It is to be understood that where the specification states that a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included.

Moreover, it will be appreciated that positional descriptions such as “above”, “below”, “left”, “right” and the like should, unless otherwise indicated, be taken in the context of the figures and should not be considered limiting.

In the following description, the term “about” means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. It is commonly accepted that a 10% precision measure is acceptable and encompasses the term “about”.

In the above description, an embodiment is an example or implementation of the inventions. The various appearances of “one embodiment,” “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment.

It should be understood that any one of the above-mentioned optional aspects of each catalyst layer, modified electrode, system and related method/process, may be combined with any other of the aspects thereof, unless two aspects clearly cannot be combined due to their mutual exclusivity. For example, the various structural elements of electrode described herein above, herein below and/or in the appended Figures, may be combined with any of the general operational steps of the operation process or manufacture electrode method descriptions appearing herein and/or in accordance with the appended claims.

All publications referred to herein are incorporated by reference.

• 1. Dinh, C. T. et al. CO₂ electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface. Science 360, 783-787 (2018).
• 2. Ren, S. et al. Molecular electrocatalysts can mediate fast, selective CO₂ reduction in a flow cell. Science 365, 367-369 (2019).
• 3. Ma, M. et al. Insights into the carbon balance for CO₂ electroreduction on Cu using gas diffusion electrode reactor designs. Energy Environ. Sci. 13, 977-985 (2020).
• 4. Cofell, E. R., Nwabara, U. O., Bhargava, S. S., Henckel, D. E. & Kenis, P. J. A. Investigation of electrolyte-dependent carbonate formation on gas diffusion electrodes for CO₂ electrolysis. ACS Appl. Mater. Inter. 13, 15132-15142 (2021).
• 5. Al-Mamoori, A., Krishnamurthy, A., Rownaghi, A. A. & Rezaei, F. Carbon capture and utilization update. Energy Technol. 5, 834-849 (2017).
• 6. Sisler, J. et al. Ethylene Electrosynthesis: A comparative techno-economic analysis of alkaline vs membrane electrode assembly vs CO₂—CO—C₂H₄ tandems. ACS Energy Lett. 6, 997-1002 (2021).
• 7. Huang, J. E. et al. CO₂ electrolysis to multicarbon products in strong acid. Science 372, 1074-1078 (2021).
• 8. Bondue, C. J., Graf, M., Goyal, A. & Koper, M. T. M. Suppression of hydrogen evolution in acidic electrolytes by electrochemical CO₂ reduction. J. Am. Chem. Soc. 143, 279-285 (2021).
• 9. Nitopi, S. et al. Progress and perspectives of electrochemical CO₂ reduction on copper in aqueous electrolyte. Chem. Rev. 119, 7610-7672 (2019).
• 10. Zhong, M. et al. Accelerated discovery of CO₂ electrocatalysts using active machine learning. Nature 581, 178-183 (2020).
• 11. Seh, Z. W et al. Combining theory and experiment in electrocatalysis: Insights into materials design. Science 355, eaad4998 (2017).
• 12. Li, F. et al. Molecular tuning of CO₂-to-ethylene conversion. Nature 577, 509-513 (2020).
• 13. Li, F. et al. Cooperative CO₂-to-ethanol conversion via enriched intermediates at molecule-metal catalyst interfaces. Nat. Catal. 3, 75-82 (2019).
• 14. Ozden, A., García de Arquer, F. P., Huang, J. E. et al. Carbon-efficient carbon dioxide electrolyzers. Nat. Sustain. 5, 563-573 (2022).
• 15. Russel, I. Undserstanding yeast fundamentals, The Alcohol Textbook, Chapter 9, 85-119 (2003).

Claims

1. A carbon dioxide reduction reaction (CO2RR) catalytic system having a dual catalysis function for the electroreduction of CO2 into multi-carbon products, the catalytic system comprising:

a first catalyst layer comprising a support and a first metal-based catalyst wherein the first metal-based catalyst comprises cobalt phthalocyanine (CoPc), nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), iron phthalocyanine (FePc) or any combinations thereof; wherein the support is a carbon-based material derived from a metal organic framework wherein the first metal-based catalyst is atomically dispersed on the support as determined by energy-dispersive X-ray spectroscopy; and

a second catalyst layer comprising a second metal-based catalyst that comprises copper or copper-based alloys;

wherein the first catalyst layer is positioned above the second catalyst layer to generate CO between the first catalyst layer and the second catalyst layer,

and wherein the support of the first catalyst layer is doped with one or more N-containing compounds.

2. The CO2RR catalytic system of claim 1, wherein the support of the first catalyst layer comprises between 0.5 w.t % and 10 wt. % of the N-containing compounds based on the total weight of the first catalyst layer and as determined by energy-dispersive X-ray spectroscopy (EDS) measurements, preferably between 1 wt. % and 8 wt. %, or further preferably between 2 wt. % and 6 wt. %.

3. (canceled)

4. (canceled)

5. The CO2RR catalytic system of claim 1, wherein the metal organic framework is a zeolitic metal organic framework.

6. The CO2RR catalytic system of claim 1, wherein the metal organic framework is a zeolitic imidazolate framework comprising one or more metallic ions selected from Zn, Fe, Co, Cu or any combinations thereof, preferably zinc.

7. The CO2RR catalytic system of claim 1, wherein the metal organic framework is a zeolitic imidazolate framework (ZIFs) comprising one or more ZIF systems selected from ZIF-8, ZIF-67, ZIF-68, ZIF-69, ZIF-70, ZIF-90 and any combinations thereof, preferably ZIF-8.

8. (canceled)

9. The CO2RR catalytic system of claim 1, wherein the support comprises pores having a diameter size of ranging between 0.3 nm and 5 nm, as determined by Brunauer-Emmett-Teller (BET) method, preferably between 0.5 nm and 4.5 nm, or between 1 nm and 4 nm.

10. The CO2RR catalytic system of claim 1, wherein the support is co-doped with one or more of P-containing compounds and/or one or more of S-containing compounds.

11. (canceled)

12. (canceled)

13. The CO2RR catalytic system of claim 1, wherein the support is a carbon support having a hollow polyhedral morphology.

14. The CO2RR catalytic system of claim 1, wherein the first catalyst layer has a catalyst content between 3 wt. % and 5 wt. % based on the total weight of the first catalyst layer as determined by inductively coupled plasma optical emission spectrometry.

15. The CO2RR catalytic system of claim 1, wherein said copper-based alloys comprise CuAg, CuAu, CuPd, CuBi, CuAl, CuSn, CuIn or any combinations thereof.

16. (canceled)

17. The CO2RR catalytic system of claim 1, wherein the second catalyst layer further comprises an ionomer.

18. The CO2RR catalyst of claim 17, wherein the ionomer is a perfluorinated sulfonic acid ionomer.

19. The CO2RR catalytic system of claim 17, wherein the ionomer is 1,1,2,2-tetrafluoroethene; 1,1,2,2-tetrafluoro-2-[1,1,1,2,3,3-hexafluoro-3-(1,2,2-trifluoroethenoxy)propan-2-yl]oxyethanesulfonic acid copolymer, tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer, or any combinations thereof.

20. The CO2RR catalytic system of claim 17, wherein the second catalyst layer comprises an underlayer consisting of the second-metal-based catalyst and a top layer comprising the ionomer and the second-metal-based catalyst.

21. The CO2RR catalytic system of claim 20, wherein the top layer is a three-dimensional catalyst:ionomer bulk heterojunction (Cu-CIBH) top layer consisting of Cu nanoparticles (CuNP) and perfluorosulfonic acid (PFSA) ionomer.

22. (canceled)

23. (canceled)

24. The CO2RR catalytic system of claim 1, wherein the second catalyst layer has a catalyst content between 50 wt. % and 100 wt. % based on the total weight of the second catalyst layer.

25. A modified electrode configured for the electroreduction of CO2 into multi-carbon products in acidic conditions, the modified electrode comprising:

a gas diffusion layer being a porous support; and

the CO2RR catalytic system as defined in claim 1, wherein the second catalyst layer is deposited on the gas diffusion layer.

26. (canceled)

27. (canceled)

28. A CO2 electrolyzer system for reducing CO2 into multi-carbon products, the system comprising:

a cathodic compartment comprising: a reactant inlet that is configured to supply a gas stream comprising CO2 in the cathodic compartment, a cathode being the modified electrode as defined in claim 25, a product outlet configured to release a gas-liquid mixture comprising CO2 and the multi-carbon products from the cathodic compartment;

an anodic compartment comprising: an anodic inlet that is configured to supply an anolyte in the anodic compartment, an anode, and an anodic outlet that is configured to release used anolyte from the anodic compartment; and

a cationic exchange membrane that is positioned between the cathodic compartment and the anodic compartment.

29. (canceled)

30. (canceled)

31. (canceled)

32. A method for preparing the carbon dioxide reduction reaction catalytic system as defined in accordance with claim 1, characterized in that said method comprises the following step:

a. providing a metal organic framework;

b. coating said metal organic framework with a ligand to obtain a chelated metal organic framework;

c. performing pyrolysis of the chelated metal organic framework obtained at step (b) to obtain a hollow carbon support;

d. dispersing one or more first metal-based compounds on the hollow carbon support obtained at step (c) to obtain a first metal-based catalyst dispersed on a support;

e. adding an ionomer to said first metal-based catalyst dispersed on a support to obtain an ink;

f. providing one or more second metal-based compounds and spraying the ink obtained at step (e) on to said one or more second metal-based compounds so as to obtain the carbon dioxide reduction reaction catalytic system as defined in accordance with claim 1.

33. A process for operating CO2 electroreduction into multi-carbon products in acidic conditions, the process comprising:

providing a CO2 electrolyzer system as defined in claim 28,

supplying a CO2-containing gas stream to the reactant inlet of the cathodic compartment;

supplying an acidic electrolyte having a pka measured at 25° C. ranging between 3 and 12 to the anolyte inlet of the anodic compartment of the CO2 electrolyzer;

applying a current density to the CO2 electrolyzer between 100 and 1200 mA·cm−2; and

recovering a gas-liquid mixture from a product outlet of the cathodic compartment, the gas-liquid mixture comprising the multi-carbon products; with preference, the multi-carbon products comprise ethylene.

34. (canceled)

35. (canceled)

36. (canceled)

37. (canceled)