Plasmonic gas diffusion reactor for co2 reduction to high-value chemicals

Abstract

Reactors for performing photocatalyzed electrochemical reactions are provided. In contrast with conventional work where only a single electrode reaction is considered, where it is easy to provide light to the single electrode, here we consider ways to provide light to reactors having two electrodes. In some embodiments internal light sources are used. In other embodiments, a transparent window is used to admit light from outside the reactor, and one of the electrodes is configured so that it does not block light from the window so that the other electrode can receive that light.

Description

[0001] Plasmonic gas di ffusion reactor for C02 reduction to high-value chemicals

[0002] FIELD OF THE INVENTION

[0003] This invention relates to reactors for performing photocatalyzed electrochemical reactions .

[0004] BACKGROUND

[0005] The threat of climate change has resulted in strong interest and investment in CO2 capture by industry, government , and private funds . However, without carbon utili zation, carbon capture represents a financial burden that precludes large-scale capture ef forts . For example , for CO2 sources like methane reforming and cement production, CO2 capture costs up to $ 120 per tonne , mostly due to costly gas separations , while transport and storage of captured CO2 can add another $ 80 per tonne . Electrochemical conversion of CO2 into hydrocarbons provides an opportunity to save on gas separation and storage costs while also producing valuable feedstocks for the chemical industry or zero-carbon fuels to decarboni ze aviation and shipping . At the same time , the expansion of intermittent renewable energy, especially solar, has resulted in hour-long periods with near- zero electricity prices and curtailment of solar energy . This creates an opportunity for inexpensive , small electrolyzers that operate primarily when renewable electricity capacity is high . SUMMARY

[0006] We consider a novel electrolyzer technology that is inexpensive and scalable . Unlike existing electrolyzers that are built to operate on large scales , resulting in long payback periods that typically require high capacity factors , our system can operate when renewable energy is in excess , generating net- zero emissions and converting pointsource CO2 emissions to high-value products .

[0007] In contrast with conventional work where only a single electrode reaction is considered, where it is easy to provide light to the single electrode , here we consider ways to provide light to reactors having two electrodes . In some embodiments internal light sources are used . In other embodiments , a transparent window is used to admit light from outside the reactor, and one of the electrodes is configured so that it does not block light from the window so that the other electrode can receive that light .

[0008] In one example , we consider a plasmon-enhanced gas di f fusion reactor based on 50 nm copper nanocubes that converts CO2 into ethylene using light and electricity . The cell includes the following components : a gas flow channel , a gas di f fusion electrode covered in copper nanoparticles , a KOH electrolyte solution and a glass window . The cell is illuminated through the glass window . The copper nanoparticle decorated gas di f fusion electrode serves as the photocathode , absorbing visible light ( 450- 800 nm) through the plasmon resonance of the copper nanoparticles . Electrons in the nanoparticle gain energy through this plasmon resonance , a collective oscillation of electrons . Facilitated by an external potential , photo-excited electrons in the copper nanoparticles reduce CO2 to ethylene. Nickel mesh is used as the counter electrode.

[0009] Applications include:

[0010] 1) Chemical production: Production of key industrial chemicals, especially ethylene, the most used petrochemical with a total market of 166 billion USD. Current approaches for CO2 reduction produce mainly carbon monoxide and hydrogen mixtures (syngas) , which is used mainly as a fuel. An alternative way to produce chemicals without fossil fuels is biomass, but biomass requires significant processing and refining to produce industrially useful chemicals. While there are isolated opportunities to produce biomass from waste materials, large-scale use of biomass requires dedicating land, water, and fertilizer to growing crops.

[0011] 2) Green fuel production: Hydrocarbon-based fuel production, especially (but not limited to) sustainable aviation fuel and fuel for trucks. Green fuels are produced commercially but most of this production is hydrogen, which is difficult to store and transport and would require significant supply chain development. Producing green fuels from biomass (biofuels) suffers the same resource (land, water) issues outlined above. It is estimated that growing enough crops to power aviation with biofuels would require a land mass equivalent to the country of India.

[0012] 3) Syngas production: Our electrolyzers can also produce syngas, a mixture of H2 and CO. Syngas can be used as a fuel or turned into hydrocarbon-based fuels using the Fischer-

[0013] Tropsch process. Significant advantages are provided:

[0014] 1)Cost: By focusing on making the electrolyzer inexpensive, the cap-ex will be paid back quickly.

[0015] 2) Size: Our approach is making small electrolyzer units that can be mass-produced and shipped easily. Scale-up can be accomplished by using multiple electrolyzers in parallel, keeping production and supply chain simple.

[0016] 3) Ambient temperature: Operation at ambient temperatures allows the electrolyzer to be started and shut down rapidly, allowing it to only operate when electricity costs are low.

[0017] 4) No purification of reactants: The electrolyzer is based on a gas diffusion electrode to operate with captured CO2 and gas mixtures containing CO2. No purification or gas separation is required beforehand, reducing upfront costs and energy consumption. Collection and purification of the produced ethylene will be required, which can be done via molecular sieves and is easier than the separation of CO2.

[0018] 5) Improvements relative to competing technologies:

[0019] 5A) Solid oxide fuel cells typically operate at higher efficiencies and production rates, however, they are expensive and need to operate at high temperatures. Both of these factors make them incompatible with intermittent operation: high cost means the cell needs to be operated at high capacity factors to pay back the initial capex. High temperatures also require longer startup times.

[0020] 5B) Proton exchange membrane electrolysis operates at room temperature but is more expensive. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIGs. 1A-B show two exemplary embodiments of the invention having an internal light source.

[0022] FIGs. 2A-D show an exemplary embodiment of the invention having a window to permit external illumination.

[0023] FIGs. 3A-D show nanoparticle characterization results.

[0024] FIG. 4 shows electrochemical characterization of the reactor .

[0025] FIGs. 5A-D show product analysis.

[0026] FIG. 6 shows product analysis of CO2 reduction in the dark and under illumination.

[0027] DETAILED DESCRIPTION

[0028] FIGs. 1A-B show two exemplary embodiments of the invention having an internal light source. The embodiment of FIG. 1A is a reactor including: a reaction electrode 102 configured as a planar gasdiffusion electrode; a counter-electrode 110 configured as a planar electrode (and preferably also configured as a gas diffusion electrode) ; an electrolyte 112 disposed in a space between the reaction electrode 102 and the counter-electrode 110, where the space is defined by the counter-electrode 110, the reaction electrode 102 and a side wall of the reactor; one or more light sources 106 (e.g., light emitting diodes (LEDs) ) located within the space and configured to illuminate the reaction electrode in operation. Here CO2 reduction products are denoted as P, and catalysts on the reaction electrode 102 and counterelectrode 110 are referenced as 104 and 108 , respectively . Catalyst 104 on the reaction electrode serves to promote the desired CO2 reduction reaction, and catalyst 108 on the counter-electrode serves to promote an oxygen evolution reaction ( OER) .

[0029] FIG . IB shows a version of this embodiment having two electrolytes -- catholyte 124 and anolyte 126 separated by an ion-permeable membrane 122 . Here reaction electrode 102 acts as the cathode and counter-electrode 110 acts as the anode . This approach allows OER and CO2 reduction products to be collected separately, simpli fying gas handling, improving safety (no explosive mixtures of O2 and hydrocarbons or H2 is formed) , and preventing CO2 reduction products from being oxidi zed again at the anode . Furthermore , the use of separate anolytes and catholytes enables optimi zing each hal f reaction separately .

[0030] FIGs . 2A-D show an exemplary embodiment of the invention having a window to permit external illumination . The basic idea is shown in the schematic side view of FIG . 2A. This example is a reactor 202 including : a reaction electrode 204 configured as a gas-di f fusion planar electrode ; a transparent window 206 facing the reaction electrode , where the reaction electrode can be illuminated from outside the reactor ( e . g . , by light 212 , which can be sunlight or from an arti ficial source such as LEDs or lasers ) ; an electrolyte 210 disposed in a space between the reaction electrode 204 and the transparent window 206 , where this space is defined by the transparent window 206 , the reaction electrode 204 and a side wall of the reactor ; a counter-electrode 208 disposed on the side wall of the reactor.

[0031] FIG. 2B is an enlarged view of reaction electrode 204 on FIG. 2A. Here 220 is a microporous substrate, 222 is a microporous layer, and 224 is the catalyst.

[0032] FIG. 2C is a partial front view showing counterelectrode 208 but not reaction electrode 204. Here 230 is the contact for counter electrode 208, 232 and 234 are holes for electrolyte flow, 236 is an optional reference electrode, and 238 is a member defining the side wall of the reactor .

[0033] FIG. 2D is a partial front view showing reaction electrode 204 but not counter-electrode 208. Here 242 is a support member on which reaction electrode 204 is disposed. 250 is the contact for the reaction electrode.

[0034] In any of these embodiments, practice of the invention does not depend critically on the reaction products. Suitable reaction products include, but are not limited to: ethylene, syngas, CO, formate, formic acid, methanol, methane, acetate, ethane, ethanol, n-propanol, i-propanol, propylene, and oxalic acid.

[0035] In any of these embodiments, practice of the invention does not depend critically on the catalyst (s) used. Suitable catalysts include, but are not limited to: gold, silver, copper, aluminum, nickel, palladium, ruthenium, iron, cobalt, platinum, tin, and alloys or mixtures thereof.

[0036] In any of these embodiments, practice of the invention does not depend critically on details of nanostructure (if present) of the catalyst (s) . A nanoparticle catalyst can have a particle size between 1 nm and 1000 nm. Such a catalyst can have particles of any shape, including spheres, cubes, prisms, octahedra, bipyramids, nanorods, and hedgehog particles .

[0037] In any of these embodiments, practice of the invention does not depend critically on the electrolyte ( s ) used. Suitable electrolytes include, but are not limited to: potassium hydroxide, sodium hydroxide, any hydroxide electrolyte, carbonate electrolytes, bicarbonate electrolytes, phosphoric acid, sulfuric acid, hydrochloric acid, chloride electrolytes, nitrate electrolytes, phosphates, biphosphates, and ionic liquids.

[0038] FIGs. 3A-D show nanoparticle characterization results. FIG. 3A shows transmission electron microscopy (TEM) micrograph of 4 nm silver nanoparticles. FIG. 3B is a histogram of the measured size distribution of the nanoparticles. FIG. 3C shows UV-vis extinction spectrum showing a plasmon peak at 410 nm. FIG. 3D is a scanning electron microscopy (SEM) micrograph of silver nanoparticles on microporous support of a gas diffusion electrode.

[0039] FIG. 4 shows electrochemical characterization of the reactor. Part a shows current vs time at -0.6 V vs RHE (reversible hydrogen electrode) as the light is turned on and off. Part b shows the counter electrode potential. Part c shows the reaction electrode resistance. Part d shows open circuit potential. For all measurements under illumination, 450 nm, 1.7 W (600 mW cm-2) light from a continuous wave diode laser was used. Parts a,b,c are from one experiment, and part d is from a different experiment, so the periods of light and dark for part d are different than for parts a,b,c. The inset is an exemplary zoom in on the open circuit potential (OCP) vs. time trace to show the small change in OCP. Here the reactor has a transparent window configuration with 50 nm copper catalyst on a Sigracet 39 BB gas diffusion electrode at the reaction electrode. FIGs. 5A-D show product analysis of CO2 reduction having CO and H2 as products. FIG. 5A shows Faradaic efficiencies for CO2 reduction at -0.4 V in dark and light. Faradaic efficiencies add up to 94% and 96% in the dark and under illumination, respectively. We attribute this number to formation of liquid products that remained in the electrolyte (especially formate) , and error in product quantification using GC . FIG. 5B shows Faradaic efficiencies at -0.6 V. FIGs. 5C and 5D show partial current densities for the measurements in FIGs. 5A and 5B, respectively. The error bars in all panels represent the standard error derived from two gas chromatography injections. Here the reactor has a transparent window configuration with 50 nm copper catalyst on a Sigracet 39 BB gas diffusion electrode at the reaction electrode.

[0040] FIG. 6 shows product analysis of CO2 reduction having CO and C2H4 (ethylene) as products, in the dark and under illumination. Under illumination, more products are generated. Notably, the amount of C2H4 increases more than CO and CH4. While this peak is small, this observation indicates that light directs the selectivity of the reaction towards the higher value ethylene product and substantially changes the reaction mechanism. Here the reactor has a transparent window configuration with 50 nm copper catalyst on a Sigracet 39 BB gas diffusion electrode at the reaction electrode .

Claims

CLAIMS1. A reactor for performing a plasmon assisted photocatalyzed electrochemical reaction, the reactor comprising : a reaction electrode configured as a gas-diffusion planar electrode; a transparent window facing the reaction electrode, whereby the reaction electrode can be illuminated from outside the reactor; an electrolyte disposed in a space between the reaction electrode and the transparent window, wherein the space is defined by the transparent window, the reaction electrode and a side wall of the reactor; a counter-electrode disposed on the side wall of the reactor .

2. The reactor of claim 1, wherein the reactor is configured to produce one or more reaction products selected from the group consisting of: ethylene, syngas, CO, formate, formic acid, methanol, methane, acetate, ethane, ethanol, n- propanol, i-propanol, propylene, and oxalic acid.

3. The reactor of claim 1, wherein the reaction electrode includes a nanoparticle catalyst with composition selected from the group consisting of: gold, silver, copper, aluminum, nickel, palladium, ruthenium, iron, cobalt, platinum, tin, and alloys or mixtures thereof.

4. The reactor of claim 1, wherein the reaction electrode includes a nanoparticle catalyst having a particle size between 1 nm and 1000 nm.

5. The reactor of claim 1, wherein the electrolyte is selected from the group consisting of: potassium hydroxide, sodium hydroxide, any hydroxide electrolyte, carbonate electrolytes, bicarbonate electrolytes, phosphoric acid, sulfuric acid, hydrochloric acid, chloride electrolytes, nitrate electrolytes, phosphates, biphosphates, and ionic liquids .

6. A reactor for performing a plasmon assisted photocatalyzed electrochemical reaction, the reactor comprising : a reaction electrode configured as a planar gasdiffusion electrode; a counter-electrode configured as a planar electrode; an electrolyte disposed in a space between the reaction electrode and the counter-electrode, wherein the space is defined by the counter-electrode, the reaction electrode and a side wall of the reactor; one or more light sources located within the space and configured to illuminate the reaction electrode in operation .

7. The reactor of claim 6, wherein the reactor is configured to produce one or more reaction products selected from the group consisting of: ethylene, syngas, CO, formate, formic acid, methanol, methane, acetate, ethane, ethanol, n- propanol, i-propanol, propylene, and oxalic acid.

8. The reactor of claim 6, wherein the reaction electrode includes a nanoparticle catalyst with composition selected from the group consisting of: gold, silver, copper, aluminum, nickel, palladium, ruthenium, iron, cobalt, platinum, tin, and alloys or mixtures thereof.

9. The reactor of claim 6, wherein the reaction electrode includes a nanoparticle catalyst having a particle size between 1 nm and 1000 nm.

10. The reactor of claim 6, wherein the electrolyte is selected from the group consisting of: potassium hydroxide, sodium hydroxide, any hydroxide electrolyte, carbonate electrolytes, bicarbonate electrolytes, phosphoric acid, sulfuric acid, hydrochloric acid, chloride electrolytes, nitrate electrolytes, phosphates, biphosphates, and ionic liquids .

11. A reactor for performing a plasmon-assisted photocatalyzed electrochemical reaction, the reactor comprising : an anode; a cathode; an ion-permeable membrane between the anode and the cathode ; a reactor housing configured such that the ion- permeable membrane separates the reactor into a cathode side and an anode side;wherein the anode side includes an anolyte between the anode and the ion-permeable membrane; wherein the cathode side includes a catholyte between the cathode and the ion-permeable membrane; one or more light sources internal to the reactor and configured to illuminate at least one of the anode and cathode in operation.

12. The reactor of claim 11, wherein the reactor is configured to produce one or more reaction products selected from the group consisting of: ethylene, syngas, CO, formate, formic acid, methanol, methane, acetate, ethane, ethanol, n- propanol, i-propanol, propylene, and oxalic acid.

13. The reactor of claim 11, wherein at least one of the anode and cathode includes a nanoparticle catalyst with composition selected from the group consisting of: gold, silver, copper, aluminum, nickel, palladium, ruthenium, iron, cobalt, platinum, tin, and alloys or mixtures thereof.

14. The reactor of claim 11, wherein at least one of the anode and cathode includes a nanoparticle catalyst having a particle size between 1 nm and 1000 nm.

15. The reactor of claim 11, wherein at least one of the anolyte and catholyte is selected from the group consisting of: potassium hydroxide, sodium hydroxide, any hydroxide electrolyte, carbonate electrolytes, bicarbonate electrolytes, phosphoric acid, sulfuric acid, hydrochloric acid, chloride electrolytes, nitrate electrolytes, phosphates, biphosphates, and ionic liquids.

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