MEMBRANE-LESS REACTOR FOR THE ELECTRO-REDUCTION OF CARBON

DIOXIDE

Field of the Invention

This invention concerns apparatus for the electrochemical reduction of carbon dioxide (ERC). More specifically, the invention relates to a membrane-less electrochemical reactor for use in processes for the electro-reduction of CO ₂ .

Background of the Invention

The electro-reduction of C0 ₂ to organic compounds such as formate/formic acid and methane normally requires an electrochemical reactor ("cell") in which the anode and cathode chambers are separated by an ion selective membrane (Synonyms for "membrane" are: ion selective membrane, ion exchange membrane, cation membrane, anion membrane, proton exchange membrane (PEM)), usually a cation membrane such as Nafion® (DuPont). The membrane is used to separate the anolyte from the catholyte and to prevent the transport of cathode products to the anode - where they may be consumed by electro-oxidation. This so-called "divided cell" approach is ubiquitous in the prior art aimed at commercially viable ERC processes

Although it is useful for the electro-reduction of C0 ₂ the presence of a membrane imposes capital and energy costs on the process, as well as increasing the complexity of its operation. In this respect it would be desirable to have a membrane- less electrochemical reactor for the electro-reduction of C0 ₂ (ERC). The prior art in this field has not considered the merits and/or possibility of eliminating the membrane from ERC reactors. Summary of the Invention.

The present invention aims to eliminate ion selective membranes from reactors for the electro-reduction of C0 ₂ (ERC) to products such as formate salts, formic acid, oxalic acid, carbon monoxide, methane, ethene, formaldehyde, methanol and other organic compounds of low molecular mass (i.e. less than ca. 80 kg/kmol).

In an embodiment, the invention comprises a membrane-less electrochemical reactor for the electro-reduction of carbon dioxide comprising: an anode; a 3D cathode, said cathode separated from said anode by an electronically insulating layer; and a cathode current feeder in electronic contact with said 3D cathode. In another aspect, the electronically insulating layer is a porous hydrophilic material or a porous hydrophobic material.

In another aspect, the anode has an electro-catalytic surface and suppresses the electro-oxidative loss of soluble reaction products. The electro-catalytic surface comprises either iridium, an adsorbed film or a nano-porous film. In another aspect, the anode has an electro-active surface and the surface has an area below a superficial area of the cathode. The surface is distributed substantially uniformly with respect to a face of the cathode.

In another aspect, the anode is pattern masked or impregnated with a polymer. The ratio of active anode area to cathode area is 0.05 to 0.5. In another aspect, the anode comprises a two-dimensional electrode. The electrode has an area about equal to a superficial area of the cathode.

In another aspect, the 3D cathode comprises an electronically conductive gas diffusion cathode layer, said reactor further comprising a micro-porous electronically insulating diaphragm fixed or pressed onto said cathode layer. It may also comprise an electronically conductive porous layer between the cathode layer and the cathode current feeder. An electrolyte solution is fed into the electronically insulating layer and a C02 containing gas is fed into the electronically conductive porous layer.

In another aspect, the reactor comprises a micro-porous diaphragm between the electronically insulating layer and the 3D cathode and wherein a liquid anolyte enters the electronically insulating layer and exits as a reaction product, while C02 gas is fed to the 3D cathode.

The foregoing was intended as a broad summary only and of only some of the aspects of the invention. It was not intended to define the limits or requirements of the invention. Other aspects of the invention will be appreciated by reference to the detailed description of the preferred embodiment and alternative embodiments and to the claims.

Brief Description of the Drawings

Figures 1(a)-(e) show longitudinal cross sections of five embodiments of this invention, each exemplified in a single cell parallel plate membrane-less electrochemical reactor.

Figure 1(a) shows a continuous reactor in which the electro-active surface of the anode is of a material that suppresses the electro-oxidation of ERC products. In other words, the kinetics of electro-oxidation of at least one of the ERC products is slow relative to the electro-oxidation of other species in the reactor.

Figure 1(b) shows a continuous reactor with differential surface area electrodes. That is, the anode is configured such that the ratio of its electro-active surface area to the superficial area of the corresponding cathode is below 1. Figure 1 (c) shows a continuous reactor for the reduction of C0 ₂ to gaseous products. In this case the gas products should have a low solubility in the electrolyte and preferably be nearly intrinsically inactive to electro-oxidation on the anode.

Figure 1 (d) shows a continuous reactor producing gaseous products on a gas- diffusion cathode in which the electro-catalytic gas-diffusion layer is held on a micro- porous diaphragm separator.

Figure 1(e) shows a continuous reactor with anode and cathode chambers separated by a porous diaphragm, through which for the anolyte liquid can flow into the cathode and thus counter the transport of cathode products into the anode chamber. Figure 2 shows a process for the electrochemical reduction of carbon dioxide to obtain CO ₂ reduction products by cathode reactions

Detailed Description of the Invention

A process for the electrochemical reduction of carbon dioxide to obtain C0 ₂ reduction products by cathode reactions is shown in Fig. 2. The reaction has the generic form: xCC- ₂ + (y-2(z-2x))H ⁺ + ye ^" CxHyOz + (z-2x)H ₂ 0 Reaction

1

where x, y and z may take integer values respectively of 1 to 3, 0 to 8 and 0 to 2, as exemplified in Table 1.

Table 1

The process of Figure 2 has an electrochemical reactor A where carbon dioxide (C0 ₂ ) is reduced according to reaction 1 , along with the associated reactor feed, recycle and product separation systems.

In Figure 2 the electrochemical reactor A may have single or multiple electrochemical cells of parallel plate or cylindrical shape, wherein each cell has an anode B, a cathode C and a separator D. An electric power source E supplies direct current to the reactor at a voltage about 2 to 10 Volt/cell. The process uses a feed tank F and product separator G. In the continuous process, fresh feed H, optionally mixed with recycle N, forms electrolyte liquid J which is passed to the reactor A along with a CO ₂ containing gas I, to be converted to output K, which is to be subsequently separated to products L and M and an optional recycle N.

In the reactor A, the cathode C, where the C0 ₂ is reduced, includes a porous electrode with an electro-catalytic specific surface in the range about 100 to 100,000 m ² /m ³ , which may include na no-structured surface embellishments, and may be in the form of a reticulate, foam, felt, matt, mesh, frit, fixed-bed, fluidized-bed, gas diffusion electrode (GDE), solid polymer electrode (SPE) or the like. The cathode operates with a mixture of a CO ₂ containing gas I and an electrolyte liquid solution J in a volumetric flow ratio from about 10 to 1000, measured at 1 bar(abs), 273 K. The gas I and liquid J may be introduced separately to the reactor, or mixed before entering the reactor, and pass through the cathode in two-phase co-current flow. The co-current fluid (l+J) flow path through the porous cathode may be preferably in the so-called "flow-by" mode with fluid flow orthogonal to the electric current or optionally in the so-called "flow-through" mode with fluid flow parallel to the electric current. The reactor may be oriented horizontally or sloped or preferably vertically, with the reactant fluid (l+J) flow preferably upward but optionally downward.

The separator D may be a porous layer of an electronically non-conductive material that is made ionically conductive by absorption of the electrolyte solution. Preferably the separator D is a porous hydrophilic material such as asbestos fibre matt, glass cloth, Zirfon ^R Perl (Agfa-Gevaert N.V.), Scimat (Freudenberg NonWovens), Celgard (Celgard LLC) or similar materials used as separators in water electrolysers and electric batteries. Alternatively the separator D may be of a hydrophobic material, such as polypropylene felt, and the electrolyte may include a suitable wetting agent, such as an ethoxylated alcohol (e.g. Makon NF12 from Stephan Company).

Depending on the desired products L, ,N and process conditions the electronically conductive anode material may be selected from those known to the art, including for example nickel, stainless steel, lead, conductive oxide (e.g. Pb0 ₂ , Sn0 ₂ ), diamond, platinum, iridium or iridium oxide and mixed oxides coated on a valve metal (e.g. titanium, zirconium) and the like. The anode may be a two-dimensional electrode or a three-dimensional electrode in the form of a reticulate, foam, felt, matt, mesh, frit, fixed-bed, fluidized-bed, gas-diffusion (GDE) or solid-polymer electrode (SPE). The desired products L,M,N and process conditions also determine the choice of the electronically conductive cathode electro-catalyst material(s), which may be selected from the exemplary lists in Table 3 or from organo-metal complexes of cobalt, copper, iron, nickel, palladium, rhenium and ruthenium such as those listed in Table 4, on conductive supports.

The anode reaction is complimentary to the cathode electro-reduction reaction 1 and may be chosen from a range of electro-oxidations exemplified by reactions 2 to 7.

Product

40H ^" 0 ₂ + 2H ₂ 0 + 4e ^" Reaction 2 oxygen

2CI ^" -» Cl ₂ + 2e ^" Reaction 3 chlorine

2H ₂ 0 -» 0 ₂ + 4H ⁺ + 4e Reaction 4 oxygen

H ₂ -> 2H ⁺ + 2e ^~ Reaction 5 proton

S0 ₂ + 2H ₂ 0 -> SO4 ^2" + 4H ⁺ + 2e ^~ Reaction 6 sulphuric acid

CH ₄ + H ₂ 0 -> CH ₄ 0 + 2H ⁺ + 2e Reaction 7 methanol

The primary reactants at the anode may be soluble ionic species as in reactions 2 and 3, neutral species as in reactions 4 to 7, or gases as in reactions 5 to 7.

The electrolyte J may be a non-aqueous solution of an electrolyte, but preferably an aqueous solution of an acid or base and/or salt with alkali metal or ammonium cations. Corresponding reagents may be for example: sulphuric, hydrochloric, hydrobromic, phosphoric, methanesulphonic or formic acid; sodium, potassium, rubidium, caesium or ammonium hydroxide or a sodium, potassium, rubidium, caesium, or ammonium salt of the above acids, including the bicarbonate and carbonate salts. The electrolyte may optionally include species to be engaged in reductive redox couples, such as, Cr ³⁺ / Cr ² * , Cu ²⁺ / Cu ¹⁺ , Sn ⁴⁺ / Sn ²⁺ , Ti ³⁺ / Ti ²⁺ , V ³⁺ / V ²⁺ , organic couples such as quinone/hydroquinone and the like, in bare, complexed or chelated forms, with a redox potential matched to that of the desired cathode process. In some embodiments the catholyte may contain chelating and/or surface active agents (surfactants) such as for example amino-carboxylates (e.g. EDTA, DTPA), phosphonates, quaternary ammonium salts (cationic), ethoxylated alcohols (non-ionic) and alkyl diphenyl sulphonates (anionic). The feed gas I may contain about 1 to 100 volume % C0 ₂ and the cathode reactant mixture (l+J) may enter and/or traverse the porous cathode C in a two-phase flow pattern such as described in Walas S., "Chemical Process Equipment", Butterworth, Boston, 1990. Page 114, as: "bubbly", "plug", "slug", "dispersed" or "froth" (i.e. foam).

Methods for separating the reactor products L,M,N may be for example gas/liquid disengagement, crystallization, filtration, liquid extraction and distillation.

The process of Figure 2, its main components and variants, are the basis for the present invention, which is described below.

Figure 1 shows five embodiments of the electrochemical reactor A from Figure 2. In Figure 1(a) the electro-oxidative loss of soluble ERC product is suppressed by the use of an anode 1 whose electro-catalytic surface promotes alternative reactions, such as the electro-oxidation of water to oxygen. Such an anode surface may be for example iridium, an adsorbed film (e.g. an organic species mono-layer) or a nano- porous film that blocks the transport of ERC products to the anode. The opposing 3D cathode 4 is in electronic contact with the cathode current feeder 3, electronically insulated from the anode 1 by a layer of porous material 2 and fed by a [C0 ₂ gas + liquid electrolyte] 2-phase stream 11 that supplies C0 ₂ for the cathode reactions. The C0 ₂ gas may be a gas mixture, such as (C0 ₂ + N ₂ + H ₂ 0), containing from about 1 to 100 volume % C0 ₂ . The electronically insulating layer 2 should have a porosity, pore size, thickness and surface energy selected to allow the electrolyte liquid from the 2-phase feed stream 11 to access the anode, to allow gas formed at the anode to escape, to suppress the by-pass of fluids 11 around the 3D cathode 4 and to prevent electronic shorting between the anode 1 and cathode 4.

The electronically insulating layer 2 may comprise for example screens, cloths, felts or reticulates of hydrophilic or hydrophobic materials, such as glass cloth, polypropylene mesh or felt, sintered oxide frit or the like, with porosity, pore size and thickness respectively about 30 to 95%, 0.1 to 5 mm and 0.1 to 3 mm. The 3D cathode 4 may be a electronically conductive reticulate (open-cell foam), felt, screen, wool, packed bed, fluidized bed or the like, as described in US Patent Application Publication No. 2012/090052 A1 , optionally having embellished electro-catalytic surfaces, with specific surface area in the range of about 100 to 100,000 m2/m3, and to which electric current is distributed by the electronically conductive current feeder 3. In the arrangement of Figure 1(a) any gas generated at the anode (e.g. oxygen) will join the main fluid stream 11 in the reactor, which may contain hydrogen or other combustible gases. However the high C0 ₂ flow dilutes the gas mixture and lowers the potential explosion hazard. The loss of ERC products due to spontaneous thermo-chemical reaction with the anode products (e.g. oxygen) may be acceptably low due to the relatively low rates of such reactions under the prevailing conditions.

In Figure 1(b) the electro-oxidative loss of soluble ERC products is suppressed by the use of an anode 5 having an electro-active surface area below the superficial area of the cathode 4, with the anode surface distributed substantially uniformly with respect to the face of the cathode 4. For example, the anode may be pattern masked or impregnated with a polymer 6 to give an active anode to cathode area ratio about 0.05 to 0.5, Here the anode 5 is separated from the 3D cathode 4 by a porous electronic insulating layer 2, as in Figure 1(a). The 3D cathode 4, current feeder 3 and [C0 ₂ gas + liquid] feed stream 1 also have the same functions as in Figure 1(a). The anode to cathode area ratio is chosen to provide satisfactory distribution of current density to the 3D cathode 4 while imposing mass transport and/or intrinsic kinetic constraints that can act with the insulating layer 2 to suppress the electro-oxidation of soluble ERC products. In Figure 1(c) the electro-oxidative loss of gaseous ERC products is suppressed by virtue of the fact that they have a relatively low solubility in the electrolyte liquid, so their rate of destruction at the anode 7 is strongly inhibited by mass transfer. The loss of some gaseous products, such as carbon monoxide and methane, is also limited by the slow intrinsic kinetics of their electro-oxidation. The porous electronically insulating layer 2, current feeder 3, 3D cathode 4 and [C0 ₂ gas + liquid] feed stream 11 have the same functions as in Figure 1(a). In this case the anode 7 should preferably be a relatively smooth two-dimensional (2D) electrode with an area about equal to the superficial area of the cathode. Gaseous ERC products can easily access the anode 7 but due to mass transfer and kinetic constraints the rate of their electro-oxidation is substantially below rate of their generation on the cathode 4, thus allowing the reactor to operate with a useful current efficiency. Any gas generated at the anode (e.g. oxygen) will join the gas of the main fluid stream 11 in the reactor, where the high C0 ₂ flow dilutes the gas mixture and lowers the potential explosion hazard. The loss of ERC products due to spontaneous thermo-chemical reaction with the anode products (e.g. oxygen) may be acceptably low due to the relatively low rates of such reactions under the prevailing conditions. In Figure 1(d) the anode 7 is separated by a porous electronically insulating layer 2 from a micro-porous electronically insulating diaphragm 8 which is fixed or pressed onto an electronically conductive gas diffusion cathode layer 9. The cathode layer 9 is in electronic communication with the current feeder 3 via an electronically conductive porous layer 10 which conducts the ERC gas reactants and products while supporting and distributing electric current to the cathode 9. In this so-called "divided cell" reactor the anode chamber 2 is fed with a suitable electrolyte solution 12, such as an acid and/or salt or hydroxide of an alkali metal, while the cathode chamber 10 is fed with a C0 ₂ containing gas 13, such as a mixture of C0 ₂ with N ₂ and Η ₂ 0 containing from about 1 to 100 volume % C0 ₂ .

The porous diaphragm 8 is preferably a hydrophilic material which becomes ionically conductive when wetted by the electrolyte solution 12, examples of such materials are: asbestos matt, glass cloth, Zirfon ® Perl (Agfa-Gevaert N.V.), SciMAT (Freudenberg NonWovens), Celgard (Celgard LLC) and some battery separators as described in Arora P., Zhang J., "Battery separators", Chem. Rev. 2004, 104, 4419- 4462. The gas-diffusion cathode 9 is prepared by known methods and is typically a porous mixture of carbon black, a fluoropolymer (e.g. PTFE), a solid electrolyte such as Nafion® and an electro-catalyst, with a thickness about 0.01 to 1 mm. The nature of the electro-catalyst depends on the desired ERC product, for example copper for methane and silver for carbon monoxide. The cathode 9 may be formed on the diaphragm 8 by heat and pressure or formed separately and subsequently pressed against the diaphragm 8 by the porous electronic conductor 10, which may consist for example of a metal mesh or open-cell foam from about 0.1 to 3 mm thick. In this reactor the gaseous products from the anode 7 and cathode 9 are largely prevented from mixing together by the capillary pressure in the diaphragm 8 while their usual low solubility suppresses anode/cathode crossover in the liquid phase.

In Figure 1(e) the anode 7 is separated from the porous cathode 4 and cathode feeder 3 by a micro-porous diaphragm 8. The diaphragm 8 is supported by a porous layer 2, which may be an electronic insulator or alternatively an electronically conductive material acting as an anode. Liquid anolyte 12 enters the reactor in upward or downward flow and leaves as a reaction product 15, while C0 ₂ gas 13 is fed to the cathode. Preferably the reactor is vertical and 12 and 13 are in countercurrent flow. The permeability of the diaphragm 8 is arranged to allow a controlled flux of anolyte 12-15 into the gas-contacting cathode 4 where it engages with the gas 13 in support of the electro-reduction of C0 ₂ to give a 2-phase [gas + liquid] product 14. With appropriate design the convective flux of anolyte into the cathode may counter the transport of cathode products into the anode chamber by diffusion and migration. For example, given an anoltye of potassium hydroxide and a cathode product of potassium formate this diaphragm flow-through configuration suppresses cross-over of formate anion (HCCV) to the anode and thus provides for an efficient ERC process.

Example 1.

A single-cell continuous electrochemical reactor was assembled as in Figure 1(a), with superficial area dimensions of 0.5 m long by 0.02 m wide for both the anode 1 and the cathode 4. The 3D cathode 4, contained by a 3 mm thick gasket, was a bed of pure lead wool with a fibre diameter, porosity and specific surface respectively about 0.2 mm, 80% and 3000 m ² /m ³ , contacted with a lead plate current collector 3 and separated from the anode 1 by a layer 2 of an 8 mesh per inch polypropylene screen plus a 3 mm thick polyester felt with porosity about 90%. The reactor was fed with a [C0 ₂ gas + liquid electrolyte] mixture 11 consisting of 100 vol% C0 ₂ gas at 120 Sml/min with 1 M aqueous potassium carbonate solution at 2 ml/minute and operated at 120 kPa(abs), 295 K. Two separate runs were carried out with this reactor, first with the anode 1 as a 1.5 mm thick 316 stainless steel plate and second with the anode 1 as a 1.5 mm thick titanium plate with a 3 micron thick electro-plated layer of iridium on the electro-active face. The results of these runs are given in Table 2.

Table 2. Experimental results for Example 1.

The data of Table 2 show that a higher current efficiency for formate production was obtained in the reactor with the iridium (Ti/lr) anode than with the 316 stainless steel anode. This effect may be due to the relatively slow kinetics of formate electro- oxidation on iridium, as indicated by the voltammetric study of the reaction reported in Ferrer J., Li V., "Electro-oxidation of formic acid on the iridium electrode as a function of pH", Electrochim. Acta, 1993, 38(12), 1631-1636. More generally, this result demonstrates that the electro-reduction of carbon dioxide may be carried out in a membrane-less reactor which is designed to suppress the electro-oxidative loss of the cathode products.

Example 2.

A single cell continuous electrochemical reactor was assembled as in Figure 1(d), with superficial electrode dimensions 0.02 m high by 0.02 m wide for both the anode 7 and cathode 9. The anode 9 was an expanded nickel mesh of about 20 mesh/inch and the cathode 9 was a 0.001 m thick gas-diffusion type porous electrode with a 60 wt% silver metal catalyst powder dispersed in Teflon to give an electronically conductive matrix of about 50% porosity. The cathode 9 was contacted to the cathode current feeder 3 by a 1 mm thick stainless steel mesh 10 of about 10 mesh/inch. In this case the porous diaphragm 8 was found to be redundant and removed. Thus the only material separating the anode from the cathode was the porous electronically insulating layer 2, consisting of a 0.005 m thick porous insulating polypropylene screen of about 20 mesh per inch. This configuration is similar to that of Figure 1 (c), except that the reactor was fed with two electrolyte streams 12 and 13.. The reactor was fed on the anode side 12 by a 0.5 M solution of potassium bicarbonate in water at a rate of 30 ml/minute and on the cathode side 13 by a 0.5 M solution of potassium bicarbonate in water at 3 ml/minute plus 25 Sml/minute carbon dioxide gas. All operations were at about 295 K and 120 kPa(abs). The reactor was operated for six hours with currents of 0.1 A to 0.2 A, corresponding to superficial current densities respectively of 250 to 500 A m ² . The product gas mixture containing carbon monoxide (CO) and hydrogen (H ₂ ) from the cathode, plus oxygen (0 ₂ ) from the anode was analysed for carbon monoxide, produced by the reaction:

C0 ₂ + 2H ⁺ + 2e ^" CO + H ₂ 0

The results of these runs are shown in Table 2A.

Table 2A.

Table 2A demonstrates that the electro-reduction of carbon dioxide to the gaseous product carbon monoxide can be performed in a reactor constructed as in Figure 1(d), using a porous insulating layer as in Figure 1(c), without an ion-exchange membrane (e.g. Nafion) to separate the anode from the cathode. Again this result shows that the reduction of carbon dioxide may be carried out in a membrane-less reactor designed to suppress electro-oxidative loss of cathode products at the anode.

Table 3 Cathode metal electro-catalyst materials

Gold/Tantalum Alloy Palladium/Gallium Alloy Silver/Tantalum Alloy

Gold/Zinc Alloy Palladium/Gold Alloy Silver/Zinc Alloy

HYDROCARBON PRODUCTION

Copper/Aluminum Alloy Copper/Tantalum Alloy Titanium Superalloy Titanium/Nickel Alloy

Copper/Antimony Alloy High Purity Copper Titanium/Aluminum Alloy Titanium/Tantalum Alloy

Copper/Nickel Alloy High Purity Titanium Titanium/Antimony Alloy

Titanium Metal Matrix

Copper/Nickel/Tin Alloy Titanium/Copper Alloy

Composite

Table 4. Organo-metal electrocatalysts.

Silver Pyrazole Supported Nitrogen Based Catalysts for the Electrochemical

CO

on Carbon Reduction of C02

Silver Phthalocyanine Nitrogen Based Catalysts for the Electrochemical

CO

Support on Carbon Reduction of C02

Silver tris[(2- Nitrogen Based Catalysts for the Electrochemical

CO

pyridyl)methyl]amine Reduction of C02

A Local Proton Source Enhances C02

Iron Tetraphenyl Porphyrin CO

Electroreduction to CO by a Molecular Fe Catalyst

Iron 5, 10, 15, 20-terakis(2',

A Local Proton Source Enhances C02

6'-dihydroxylphenyl)- CO

Electroreduction to CO by a Molecular Fe Catalyst

porphyrin

Iron 5, 10, 15, 20-tetrakis(2',

A Local Proton Source Enhances C02

6'-dimethoxyphenyl)- CO

Electroreduction to CO by a Molecular Fe Catalyst

porphyrin