ELECTROCHEMICAL REACTORS AND METHOD OF MAKING THE SAME

Field of the Invention

This invention concerns reactors, electrodes and methods for dealing with gaseous reactants in electrochemical processes involving liquid electrolytes. In particular, this disclosure is aimed at continuous reactors for the electro-chemical reduction of carbon dioxide.

Background of the Invention

The electro-reduction of carbon dioxide (ERC), which has been known for over 100 years, has now become a focus on efforts to convert C0 ₂ to useful materials, such as fuels and organic chemicals. xC02 + (y-2(z-2x))H ⁺ + ye ^" CxHyOz + (z-2x)H ₂ 0 Reaction 1

2H ⁺ + 2e- -> H ₂ Reaction 2

Reactions 1 and 2 represent respectively the generic primary and specific secondary cathode reactions in the ERC process, which may generate organic products such as those in Table 1.

Table 1.

1 0 1 CO carbon monoxide

1 2 2 CH ₂ o ₂ methanoic acid

1 4 1 CH ₄ 0 methanol

1 2 1 CH ₂ 0 methanal

2 6 1 C ₂ H ₆ 0 ethanol

Electrochemical processes with gaseous reactants and liquid electrolytes involve three- phase systems (gas- liquid - solid) that bring difficulties to the design of the corresponding electrodes and continuous electro-chemical reactors. These difficulties arise from three main factors: (i) gases have zero electrical conductivity (ii) gases often have low solubility in aqueous liquids and mass transfer from gas to solid is slow (iii) the volumetric flow rates demanded by reaction stoichiometry are high for gases relative to those of the corresponding liquids. In the prior art these difficulties are engaged with gas-diffusion electrodes (GDE), solid-polymer electrolyte (SPE) electrodes or in three- phase systems exemplified by trickle-bed electrodes. In GDEs the gas and liquid are separated by a thin micro-nano porous hydrophobic and electronically conductive barrier containing the dispersed electro-catalyst and usually supported on an ion exchange membrane. In SPEs a thin porous electronically conductive layer containing the particulate electrocatalyst in an ionically conductive solid polymer matrix is coated on an ion exchange membrane that separates the gas from a liquid electrolyte or the opposing anode. Both GDEs and SPEs have a thickness (dimension parallel to current) less than 0.1 mm. In trickle-bed electrodes a two-phase [gas + liquid] mixture is pumped through a porous electrode - usually in the so-called "flow-by" mode, with an electrode thickness up to about 10 mm. Electrochemical reactors with GDE, SPE and trickle-bed cathodes used in prior art for the electro-reduction of carbon dioxide have encountered various problems. For example, GDEs are plagued by liquid flooding, SPEs deteriorate due to in-situ crystallization of reaction products and trickle-beds require operation with a substantial pressure drop. In particular, GDEs and SPEs suffer from a drop of effectiveness due to the gradual loss of active surface area. In all the above cathodes it is found that obtaining partial current densities for C0 ₂ reduction above about 1000 A/m2 requires operation at super-atmospheric pressure.

Summary of the Invention.

The present invention offers novel gas-contacting electrodes for use in continuous electrochemical reactors with gaseous reactants to obtain gas or liquid products. Preferably, the reactors are employed for the electro-chemical reduction of carbon dioxide. In accordance with some aspects the present invention provides a process for making a gas contacting electrode comprising the steps of forming a porous matrix by applying a mixture comprising a catalyst, a hydrophobe and a micro-sized pore-former onto a substrate; and treating the matrix to remove the pore-former. In some embodiments, the step of treating the matrix to remove the pore-former may be by any one of the dissolution, decomposition, vaporization and reaction of pore forming particulates. In some embodiments, the mixture may be applied to the substrate by any one of rolling, calendaring, hot pressing, cold pressing, printing, spraying, brushing and electrostatic spraying. In some embodiments, the pore-former may comprise one or more of an inorganic and organic salt that is soluble in water but possess low solubility in alcohols. In some embodiments, the pore forming agents may be materials volatilised or decomposed by heat such as ammonium bicarbonate, aluminium chloride, naphthalene and urea. In some embodiments, the pore forming agents may be materials decomposed and solubilised by reaction, such as sodium carbonate (with acid), calcium carbonate (with acid) and aluminium (with acid or base). In some embodiments, the particle size of the pore-former in the mixture may be between 1 micrometer to 1 millimeter. In some embodiments, the amount of the pore-former in the mixture may be between about 10 and about 60% by weight. In some embodiments, the step of treating the matrix to remove the pore-former may be by any one of the dissolution, decomposition, vaporization and reaction of pore forming particulates. In some embodiments, the substrate may comprise a porous structure of any one ore more of a metal mesh, carbon cloth, carbon paper, porous carbon, porous metal, porous plastic and glass fiber.

In some aspects the present invention provides micro-porous gas-contacting electrodes produced in accordance with the above processes. In some embodiments, the micro- porous gas-contacting electrode may have pore diameters in the range about 1 micrometer to 1000 micrometer. In some embodiments, the catalyst in the micro-porous gas-contacting electrode may comprise a particulate in the size (equivalent diameter) range of 1 nanometer to 100 micrometer. In some embodiments, the catalyst in the micro-porous gas-contacting electrode may be supported or unsupported. Examples of catalyst supports are but not limited to carbon and titanium dioxide. In some embodiments, the catalyst content may be from 10 to 80 wt%. In some embodiments, the micro-porous gas-contacting electrode may further comprise carbon black. The carbon black may be in an amount ranging from 0 to 70% by weight. In some embodiments, the micro-porous gas-contacting electrode may have pore surfaces that are embellished by nano-structured catalyst. The embellishments may be obtained by one or more of electro-phoretic deposition, micelle templated electro deposition and micelle templated electro-less deposition. In some embodiments, the embellishments may comprise one or more of cadmium, lead, tin, indium, thallium, copper, silver, gold, palladium and platinum. In some embodiments, the porosity may range from 20 to 95 %. In some embodiments, the thickness of the micro-porous gas-contacting electrode may range from 0.1 to 5 mm. In some embodiments, the pore sizes of the micro-porous gas-contacting electrode may range from 0.01 to 1 mm.

In some aspects, the present invention provides an electrochemical reactor comprising a gas-contacting electrode as described herein and in which a two-phase flow of reactant gas and liquid is fed to a fluid distribution space adjacent and parallel to the electrode.

In some aspects, the present invention provides a use of a gas-contacting electrode as claimed as described herein as a cathode in an electrochemical process for the reduction of carbon dioxide to carbon monoxide. In some embodiments the use of the gas contacting electrode may be as a cathode in an electrochemical process for the reduction of carbon dioxide gas to carbon monoxide in which the carbon dioxide gas is fed to a fluid distribution space in the electrochemical reactor adjacent and parallel to the cathode. In some embodiments the use of the gas contacting electrode may be as a cathode in an electrochemical process for the reduction of carbon dioxide to formate salts. The carbon dioxide gas may be fed to a fluid distribution space in the electrochemical reactor adjacent and parallel to the cathode.

Brief Description of the Drawings

Fig. 1 is a flow diagram of an electrochemical reactor process according to the invention.

Fig. 2 shows an ERC reactor configuration. Fig. 3 is a graph showing the data from Example 1.

Fig. 4 shows an electrode for use in an ERC reactor which includes electrode embellishments.

Fig. 5 shows the cross section of an experimental reactor used for electro-reduction of C0 ₂ to CO. Fig. 6 a graph of data from example 2.

Fig. 7 a graph of data from example 3.

Fig. 8 a graph of data from example 4.

Fig. 9 a micro-graph of a hybrid diffusion electrode (HDE). Detailed Description of the Invention

Figure 1 shows a process for the electrochemical reduction of carbon dioxide to obtain C0 ₂ reduction products by cathode reactions with the generic form: xC0 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

In aqueous solutions reaction 1 is usually accompanied by the parasitic reaction 2, which lowers the Faradaic efficiency of the process for CO2 reduction.

2H ⁺ + 2e ^" H ₂ Reaction 2

The process of Figure 1 has an electrochemical reactor A where carbon dioxide (CO2) is reduced according to Reaction 1 , along with the associated reactor feed, recycle and product separation systems. In Figure 1 the electrochemical reactor A may have single or multiple electrochemical cells of parallel plate or cylindrical shape, wherein each cell is divided into an anode chamber with anode B and a cathode chamber with cathode C by a separator D. An electric power source E supplies direct current to the reactor at a voltage of about 2 to 10 Volts/cell. The process uses anode and cathode feed tanks F and G along with the respective product separators H and I. In the continuous process an anode fresh feed J, optionally mixed with recycle U, forms anolyte liquid K which is passed to the anode chamber B where it is converted to anode output L, to be subsequently separated to products M and N and an optional anolyte recycle U. Meanwhile a cathode fresh feed O, optionally mixed with recycle V, forms catholyte liquid Q which is mixed with C0 ₂ gas P and passed to the cathode chamber C where the mixture (P+Q) is converted to cathode output R, to be subsequently separated to products S and T and an optional catholyte recycle V. 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 nano-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 is fed by a C0 ₂ containing gas P and a catholyte liquid solution Q in a volumetric flow ratio from about 1 to 1000, measured at 1 bar(abs), 273 K. In a conventional GDE the gas and liquid would be kept separate by the hydrophobic electrode, while in other cases the gas P and liquid Q may be introduced separately or mixed before entering the cathode, then pass through the cathode in two-phase co-current flow. The co-current fluid (P+Q) 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 cathode fluid (P+Q) flow preferably upward but optionally downward. The separator D may be a layer of an electronically non-conductive material that is inherently ionically conductive, or made ionically conductive by absorption of an electrolyte solution. The preferred separator is an ion selective membrane such as those under the trade names Nafion, Fumasep, VANADion, Neosepta and Selemion and PEEK as detailed in Table 4, and is preferably a cation exchange membrane (CEM) such as Nafion N424, with a selectivity above about 90%. The separator may also comprise a layer of porous hydrophilic material such as asbestos, Zirfon ^R Perl (Agfa-Gevaert N.V.), Scimat (Freudenberg NonWovens), Celgard (Celgard LLC) and like materials used as separators in water electrolysers and electric batteries.

Depending on the desired anode products M,N,U and process conditions, the electronically conductive anode material may be selected from those known in the art, including, for example, nickel, stainless steel, lead, conductive oxide (e.g. Pb0 ₂ , Sn0 ₂ ), diamond, platinised titanium, iridium oxide and mixed oxide coated titanium (DSE), and the like. The anode may be a two-dimensional electrode or a three-dimensional (porous) 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 cathode products S,T,V and process conditions determine the choice of the electronically conductive cathode electro-catalyst material(s), which may be selected from the exemplary lists in Tables 2 and 3.

The anode reaction is complimentary to the cathode electro-reduction Reaction 1 and may be chosen from a wide range of electro-oxidations exemplified by Reactions 3 to 11 listed below.

Product

40H ^" 0 ₂ + 2H ₂ 0 + 4e ^" Reaction 3 oxygen

2CI ^" Cl ₂ + 2e ^" Reaction 4 chlorine

2S0 ^2" →· S ₂ 0 ₈ ^2" + 2e ^" Reaction 5 persulphate

2C0 ₃ ^2" →· C ₂ 0 ₆ ^2" + 2e ^" Reaction 6 percarbonate

2H ₂ 0 0 ₂ + 4H ⁺ + 4e ^" Reaction 7 oxygen

C ₆ H ₆ + 2H ₂ 0→· C ₆ H ₄ 0 ₂ + 2H ⁺ + 2e ^" Reaction 8 benzoquinone

C ₈ HioO + H ₂ 0→· C ₈ H ₈ 0 ₂ + 4H ⁺ + 4e Reaction 9 methoxybenzaldehyde

H ₂ ^ 2H ⁺ + 2e ^" Reaction 10 proton

CH ₄ + H ₂ 0→· CH ₄ 0 + 2H ⁺ + 2e ^" Reaction 1 1 methanol

The primary reactants at the anode may be soluble ionic species as in reactions 3 to 6, neutral species as in reactions 7 to 1 1 , "immiscible" organic liquids as in reactions 8 and 9 or gases as in reactions 10 and 1 1. Immiscible liquid and gas reactants, along with an aqueous liquid anolyte, may engender multi-phase flow at the anode which may include respectively a gas/liquid foam or liquid/liquid emulsion. The anolyte K 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 or methanesulphonic acid; sodium, potassium, rubidium, caesium or ammonium hydroxide or a sodium, potassium, rubidium, caesium, or ammonium salt of the above acids. The anolyte may optionally include species to be engaged in oxidative redox couples, such as Ag ²⁺ / Ag ¹⁺ ,Ce ⁺ / Ce ³⁺ , Co ³⁺ / Co ²⁺ , Fe ³⁺ / Fe ²⁺ , Mn ³⁺ / Mn ²⁺ , V ⁵⁺ / V ⁺ , 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 anode process.

The desired cathode products S,T,V and process conditions determine the choice of the electronically conductive cathode electro-catalyst material(s), which may be selected from the exemplary lists in Table 2 or from organo-metal complexes of cobalt, copper, iron, nickel, palladium and rhenium such as those in Table 3, on electronically conductive supports. The catholyte Q 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 catholyte may optionally include species to be engaged in reductive redox couples, such as, Cr ³⁺ / Cr ² *, Cu ²⁺ / Cu ¹⁺ , Sn ⁺ / 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 cases the catholyte may contain chelating and/or surface active agents (surfactants) such as for example amino-carboxylates (e.g. EDTA, DTPA), phosphonates and quaternary ammonium salts.

The feed gas P may contain about 1 to 100 volume % C0 ₂ and the cathode reactant mixture (P+Q) may enter and/or traverse the porous cathode in a two-phase flow pattern such as described in Walas S., Chemical Process Equipmnent, Butterworth, Boston 1990, page 114 as: "bubbly", "plug", "slug", "dispersed" or "froth" (i.e. a foam).

Methods for separating the anode and cathode products may be for example gas/liquid or liquid/liquid disengagement, crystallization, filtration, liquid extraction and distillation. Table 2. Cathode metal electro-catalyst materials

Cadmium/Antimony

Indium/Lead Alloy Tin/Aluminum Alloy Titanium/Tin Alloy Alloy

Cadmium/Tantalum Indium/Tantalum

Tin/Antimony Alloy

Alloy Alloy

CO PRODUCTION

Gallium/Aluminum Palladium/Silver Waspaloy Alloy High Purity Gallium Alloy Superalloy

Gallium/Antimony Palladium/Tantalum Zinc/Aluminum Alloy High Purity Gold Alloy Alloy

Gallium/Tantalum High Purity

Alloy Palladium Palladium/Zinc Alloy Zinc/Antimony Alloy

Gold/Aluminum Silver/Aluminum

Alloy High Purity Silver Alloy Zinc/Gallium Alloy

Silver/Antimony

Gold/Antimony Alloy High Purity Zinc Alloy Zinc/Nickel Alloy

Palladium/Aluminum

Gold/Gallium Alloy Alloy Silver/Gallium Alloy Zinc/Tantalum Alloy

Palladium/Antimony

Gold/Silver Alloy Alloy Silver/Nickel Alloy

Palladium/Gallium Silver/Tantalum

Gold/Tantalum Alloy Alloy Alloy

Palladium/Gold

Gold/Zinc Alloy Alloy Silver/Zinc Alloy

HYDROCARBON PRODUCTION

Copper/Aluminum Copper/Tantalum Titanium/Nickel

Titanium Superalloy

Alloy Alloy Alloy

Copper/Antimony Titanium/Aluminum Titanium/Tantalum

High Purity Copper

Alloy Alloy Alloy

Titanium/Antimony

Copper/Nickel Alloy High Purity Titanium

Alloy

Copper/Nickel/Tin Titanium Metal Titanium/Copper

Alloy Matrix Composite Alloy

Table 3. Organo-metal electro-catalysts

formation of CO and HCOO-

Electrochemical C02 reduction catalyzed

by ruthenium complexes

[Ru(bpy) ₂ (CO)CI] ⁺ [Ru(bpy)2(CO)2]2+ and CO, HCOO ^"

[Ru(bpy)2(CO)CI]+. Effect of pH on the

formation of CO and HCOO-

Involvement of a Binuclear Species with

the Re-C(0)0-Re Moiety in

C0 ₂ Reduction Catalyzed by Tricarbonyl

Rhenium(l) Complexes with Diimine

[Re(dmb)(CO) ₃ ] ₂ CO

Ligands: Strikingly Slow Formation of the

Re-Re and Re-C(0)0-Re Species from

Re(dmb)(CO) ₃ S (dmb = 4,4'-Dimethyl-2,2'- bipyridine, S = Solvent)

Electrocatalytic and Homogeneous

Iron Porphyrin Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

Re(bipy)(CO) ₃ CI Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

Ph ₃ PCo(tpfc) Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

CIFe(tpfc) Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

CIFe(tdcc) Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

[M(bpy) ₂ (CO)H] ⁺

Approaches to Conversion of C02 to liquid CO, HCOO ^" (M = Os, Ru)

Fuels

Electrocatalytic and Homogeneous

Rh(dppe) ₂ CI Approaches to Conversion of C02 to liquid HCOO ^"

Fuels

Electrocatalytic and Homogeneous

[Pd(triphos)(PR ₃ )](BF ₄ ) ₂ Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

[Νί ₃ (μ ₃ -Ι)(μ ₃ -ΟΝΜ _θ )(μ ₂ - Approaches to Conversion of C02 to liquid CO, C0 ₃ ^2" dppm) ₃ ] ⁺

Fuels

Electrocatalytic and Homogeneous

[Cu ₂ ^-PPh ₂ bipy) ₂ - Approaches to Conversion of C02 to liquid CO, C0 ₃ ^2" (MeCN) ₂ [PF ₆ ] ₂

Fuels Electrocatalytic Reduction of Carbon

[Re(CO) ₃ (K ² -N,N- Dioxide by a Polymeric Film of Rhenium CO

PPP)CI]

Tricarbonyl Dipyridylamine

Using a One-Electron Shuttle for the

Multielectron Reduction of C02 to

4-tert-butylpyridinium HCOO ^" , CH ₃ OH, CH ₂ 0

Methanol: Kinetic, Mechanistic, and

Structural Insights

Molecular Approaches to the

[Ni(cyclam)] ²⁺ Electrochemical Reduction of Carbon CO

Dioxide

Molecular Approaches to the

[Co(l)Porphyrin] ^" Electrochemical Reduction of Carbon CO

Dioxide

Silver Pyrazole Nitrogen Based Catalysts for the

CO

Supported on Carbon Electrochemical Reduction of C02

Silver Phthalocyanine Nitrogen Based Catalysts for the

CO

Support on Carbon Electrochemical Reduction of C02

Silver tris[(2- Nitrogen Based Catalysts for the

CO

pyridyl)methyl]amine Electrochemical Reduction of C02

A Local Proton Source Enhances C02

Iron Tetraphenyl

Electroreduction to CO by a Molecular Fe CO

Porphyrin

Catalyst

Iron 5, 10, 15, 20-

A Local Proton Source Enhances C02

terakis(2', 6'- Electroreduction to CO by a Molecular Fe CO dihydroxylphenyl)- Catalyst

porphyrin

Iron 5, 10, 15, 20-

A Local Proton Source Enhances C02

tetrakis(2', 6'- Electroreduction to CO by a Molecular Fe CO dimethoxyphenyl)- Catalyst

porphyrin

Table 4. Membrane materials

Sulphonated

Nafion N324 0.152 CEM Teflon Reinforced

Fluoropolymer

Sulphonated

Nafion N424 0.178 CEM Teflon Reinforced

Fluoropolymer

Sulphonated PTFE Monofilament

Nafion N438 CEM

Fluoropolymer Reinforced

VANADion:

Thickness

Name Type Base Material Note

(mm)

VANADion Fluoropolymer with

0.254 CEM

20 lonomer Coating

VANADion Fluoropolymer with Low Oil Composite

0.254 CEM

20L lonomer Coating Membrane

HYDRion:

Thickness

Name Type Base Material Note

(mm)

Fluoropolymer with

HYDRion

0.127 CEM Iridium or Platinum

N1 15

Coating

Fluoropolymer with

HYDRion

0.178 CEM Iridium or Platinum

N1 17

Coating

Fluoropolymer with

HYDRion

0.254 CEM Iridium or Platinum

N1110

Coating

Fumatech:

Thickness

Name Type Base Material Note

(mm)

Fumasep Specifically for

0.050-0.070 CEM Fluoropolymer

FKE Electrolysis

Fumasep Polyethylene

0.110-0.130 CEM

FKS Terephthalate

Fumasep

0.080-0.100 CEM Fluoropolymer PEEK Reinforced FKB

Fumasep

0.110-0.120 CEM Fluoropolymer PEEK Reinforced FKL

Very Low

Fumasep

0.100-0.130 AEM Fluoropolymer Resistance, PEEK FAB

Reinforced Fumasep High Mechanical FAA-3-PK- 0.130 AEM Fluoropolymer Strength, PK 130 Reinforced

Very High

Fumasep

0.200-0.250 BPM Effectiveness, High FBM

Mechanical Strength

NEOSEPTA:

Thickness

Name Type Base Material Note

(mm)

Neosepta

0.150 CEM

CIMS

Neosepta

0.1 1 AEM

ACM

Neosepta High Mechanical

0.170 CEM

CMX Strength

Neosepta High Mechanical

0.140 AEM

AMX Strength

Neosepta

0.180 AEM

ACS

Neosepta Very Low Resistance

0.160 AEM

AFN (0.5 ohms.cm2)

SELEMION Hydrocarbon:

Thickness

Name Type Base Material Note

(mm)

SELEMION

0.120 CEM lonomer

CMV

SELEMION

0.120 AEM lonomer

AMV

SELEMION

0.200 AEM lonomer

AMT

SELEMION Very Low Resistance

0.100 AEM lonomer

DSV (~1 ohms)

SELEMION

0.120 AEM lonomer Low Proton Leakage AAV

SELEMION Monovalent-lon-

0.120 AEM lonomer

ASV Selective

SELEMION

0.150 AEM lonomer Oxidant-Proof APS4 The process of Figure 1 , its main components and variants, are the basis for the present invention, which is described below.

In one embodiment the subject electrodes are made from a mixture an optional nano to micrometer sized and relatively non-electro-catalytic catalyst support, a wetting alcohol, a dispersion or solution of hydrophobic fluorinated polymer, an electro-catalyst powder, a pore former and an optional solution of solid electrolyte. The resulting paste is formed into a sheet under pressure and temperature and the pore former is removed to leave a solid macro-porous matrix of the support, electro-catalyst, hydrophobe and, if present, the solid electrolyte. For example the initial mixture may consist of Vulcan XC-72 carbon black as the catalyst support, isopropanol, PTFE dispersion, tin powder catalyst, potassium sulphate powder pore-former and, optionally, a Nafion-propanol solution. This mixture is then dried, molded to the desired thickness, hot pressed at the desired pressure and temperature and the potassium sulphate leached out to form the porous electrode, then an electronically conductive mesh, or the like, is embedded on one face of the electrode. Manipulating the composition of the initial mixture gives control of the catalyst type, content and particle size as well as the porosity, pore size, hydrophobicity, electrical conductivity and thickness of the electrode. Such an electrode can also be constructed with properties graded by thickness and/or length to match potential and/or concentration and/or temperature profiles in the reactor. The hydrophilic internal surfaces of the electrode may subsequently be embellished with a nano-structured electro-catalyst. Pore forming materials and process.

The subject electrode matrix comprises micro-scale pores of equivalent diameter in the range about 0.01 to 1 millimeter. The pores are formed from a sacrificial pore-forming agent that is mixed with the matrix components in the early stage of the electrode preparation process outlined below. The pore forming agent may be any compatible solid that can be removed from the matrix by dissolution, decomposition, vaporization or reaction. In some manifestations a solution of a sacrificial agent may be used in place of the solid. Examples of pore forming agents are: water soluble salts such as potassium sulphate, potassium nitrate, sodium acetate sodium citrate & ammonium chloride, materials volatilised or decomposed by heat such as ammonium bicarbonate, aluminium chloride, naphthalene and urea and materials decomposed and solubilised by reaction, such as sodium carbonate (with acid), calcium carbonate (with acid) and aluminium (with acid or base). The pore former is preferably in solid particulate form characterised by a particle size range and distribution that correspond to the pore size(s) in the finished electrode. The optimal pore size range and distribution depend on the electro-synthesis process conditions and desired reaction product. For example, a product in the liquid phase (e.g. potassium formate) may require pore sizes in the range about 0.1 to 1 millimeter, whereas gas phase product (e.g. carbon monoxide) may need a pore size about 0.01 to 0.1 millimeter. The pore-former should not be soluble in the reagents (e.g. alcohol) used to prepare the electrode paste or ink.

The pore forming agent specifications are critical to the performance of the finished electrode. In particular, the density and particle size range of the pore former affect the uniformity of distribution of pore former particulates in the pre-electrode paste or ink and hence the distribution of pores in the finished electrode. As a rule, larger particulates should have a lower density to suppress segregation by sedimentation in the electrode paste prior to compression of the paste and removal of the pore former. In some manifestations it may be desirable to manipulate the pore-former material, density and size distribution to tailor the pore size distribution through the electrode thickness and so optimize the performance of the three dimensional (3D) electrode. Further, the pore former specifications along with the proportions of electronic conductors and nonconductors in the matrix can be used to modify the electronic conductivity of the electrode, which in turn affects the performance of the electrode.

Process for porous electrode fabrication.

The subject electrode (e.g. cathode) is fabricated as follows:

1. List the finished electrode specifications (thickness, porosity, pore size range, catalyst load, hydrophobe, ionomer, carbon and other support content, etc.) and calculate the required amounts of each component.

2. Assemble and weigh the components (pore former, catalyst, hydrophobe,

ionomer, carbon black, support, etc.) to match the specifications.

3. Mix components to a homogeneous paste or ink by addition of an alcohol.

4. Sonicate the mixture at ambient temperature.

5. Spread the paste (e.g. by brushing, printing, spraying, rolling or calendering) onto a restricted clean surface (e.g. a mold) or onto an electronically conductive mesh (e.g. stainless steel, nickel, copper or graphite) with mesh size in the range about 20 to 100 mesh/inch or onto an electronically conductive porous structure (e.g. carbon paper, cloth or felt) or onto an electronically non-conductive porous structure (e.g. plastic mesh or glass fiber felt).

6. Optional. Compress the [paste+support] matrix to form a sheet at a selected pressure and ambient or super-ambient temperature (e.g. 1.5 tons/square inch at 20 °C to 250 °C).

7. Optional. Attach (e.g. by compression and/or heat) the electrode matrix onto the separator (e.g. ion exchange membrane or polypropylene diaphragm)

8. Remove the pore former by dissolution, decomposition, vaporization or reaction.

For example a potassium sulphate pore former is removed by immersing the sheet in warm water (e.g. 60 °C) for several hours. An ammonium bicarbonate pore former may be removed by holding the sheet at a temperature in the range about 70 to 200 °C for at least 1 hour, a calcium carbonate pore former may be removed by immersing the sheet in a solution of an acid such as acetic, formic or hydrochloric and an aluminium pore former may be removed by immersion in a caustic solution such as potassium hydroxide.

9. Clean the sheet by immersion in water at 20 °C to 80 °C for at least 1 hour.

As compared to a gas-diffusion electrode (GDE) that depends on nano-scale pores for contacting gaseous reactants the above procedure results in an electrode with micrometer sized pores, suitable for electrode reactions involving gas and liquid reactants. This new electrode is differentiated from the GDE by an acronym "HDE" = hybrid diffusion electrode.

Figure 9 shows the pore structure of a HDE prepared by the above method. Example 1.

A single cell parallel plate electrochemical reactor was assembled as shown in the vertical cross-section of Figure 2. In Figure 2 the reactor 1 consists of the porous gas- contacting cathode 2 in electronic contact with a tin plated copper mesh 3 and graphite plate with integral ribbed channels machined into each on one side to carry fluids [flow field + current feeder] 4. Adjacent the cathode 2 is an electronically non-conductive layer of 0.3 mm thick micro-porous hydrophilic cloth 5, against a Nafion N117 membrane 6 bonded to a platinum catalysed hydrogen electrode 7 forming the anode and supported on electronically conductive graphite ribbed channel [flow field + current collector] 8. A two-phase mixture of (C0 ₂ gas and liquid catholyte) 9 flows to the cathode chamber, which is occupied by the cathode 2 and tin plated copper mesh 3, and leaves as catholyte product 11 while a flow of hydrogen gas 10 is delivered to the anode 7 and exits the anode as anode product 12. The reactor is driven by the DC power supply 13, with current and voltage measurements monitored respectively by ammeter 14 and voltmeter 15.

Cathode 2 with dimensions 24 mm high by 24 mm wide was prepared according to the above method, including hot pressing at 150 °C, with 500 pound force per square inch for 5 minutes. The properties of the resulting cathode were as follows:

Thickness: about 1 mm

Catalyst: 10 micron tin powder

Catalyst loading: 20 mg/cm ²

PTFE content: about 28 wt%

Carbon black content: about 50 wt% Nafion content: zero

Pore size: 0.84 - 0.29 mm (20-50 mesh)

Porosity: about 55 % The two-phase mixture 9 was formed of 1.2 ml/minute 1.5 M sodium sulphate aqueous solution plus 20 SmI/minute 100 vol % C0 ₂ gas and was fed to the cathode 2 while hydrogen gas 10 was fed to the anode 7. The cathode product stream 11 was sampled at three minute intervals and analysed for formate during continuous operation at 2 V, 0.19 A under 105 kPa(abs) pressure at 293 K. Figure 3 shows the resulting current (Faradaic) efficiency for formate generation versus operating time.

Relative to Figure 3 the current density and current efficiency for C0 ₂ reduction products (e.g. formate) may be changed by tailoring several properties of the cathode 2. For example the catalyst support may constitute about 30 to 70 wt% of the electrode and be composed of various electronically conductive materials with a relatively low electro-chemical activity, such as carbon black. The catalyst may be selected from those known to the art, such as cadmium, lead, tin, indium, copper, silver, gold, platinum and their alloys, as well as organo-metal compounds such as cobalt tetra-3- aminophenylporhyrin and nickel tetramethyldinaphthotetraazo annulene, with particle size from about 1 to 100 micron. Further, the catalyst support may be activated by absorption of catalyst salts and their subsequent in-situ conversion to the active catalyst, for example by the thermal and/or chemical reductive conversion of absorbed nitrate salts to metals. The catalyst load may range from about 1 to 100 mg/cm ² of electrode superficial area, with an electrode thickness from about 0.1 to 5 mm. The pore former may be one of many water soluble salts, such as sodium chloride and potassium nitrate, reactive solids such as aluminium and sodium carbonate or thermally decomposable solids such as ammonium bicarbonate and urea, or other substances known to the art. The pore size and porosity may be respectively from about 0.01 to 1 mm and 20 to 95 %. The hydrophobic component may be one of several solid fluoropolymers such as PTFE and FEP and constitute about 5 to 50 wt%, along with a solid electrolyte (e.g. Nafion) content from zero to about 20 wt% of the electrode.

The activity of the electrode may also be enhanced by embellishing the hydrophilic internal surfaces with nano-structured electro-catalyst(s), as represented in Figure 4. All elements of the reactor shown in Figure 4 that are the same as shown in Figure 3 have been given identical reference numbers. In Figure 4, the cathode 2 also includes embellishment 16, as described below.

Electrocatalytic embellishment. The effectiveness of three-dimensional (3D) electrodes is largely dependent on the specific surface area of the electro-active component(s) (i.e. the catalyst(s)). In the present case the electro-active specific surface is initially fixed by the catalyst loading, the size and shape of the catalyst particles, the porosity and pore size distribution and the hydrophobe content. The specific surface may be enhanced in a second stage of the electrode fabrication that deposits nano-scale catalyst particles or structures on electronically conductive interior surfaces of the pores, as shown in Figure 4. Such nano-embellishments can both increase the specific surface and raise the kinetic activity of the catalyst for selected reactions. Nano-embellishment of the electrode pores may be done by several methods. For example by electro, electro-phoretic or preferably electro-less deposition. In electro- deposition the electrode is made to function as the cathode of an electrochemical cell with an electrolyte solution containing the desired catalyst and suitable additives (e.g. surfactants) to promote the formation of nano-structured catalyst deposits. In electro- phoretic deposition the electrode is made to function as an anode or cathode in a suspension of charged catalyst particles, where the charge comes from the native double-layer of the catalyst or by adsorption suitable additives, such as those used as flotation agents in the mineral industry.

The preferred method of nano-embellishment is electro-less deposition, which may be as follows:

1. The porous electrode is immersed in a pre-treatment solution of stannous

chloride, washed in water, immersed in a solution of palladium chloride and washed again in preparation for step 2.

2. The porous electrode is immersed in a plating solution containing a compound of the desired catalyst and a surfactant above its liquid crystalline or critical micelle concentration, along with the requisite complexing agents and pH buffers.

3. Optionally, in steps 1 and 2 the containment vessel is evacuated and/or heated and cooled to draw solution into the pores of the electrode.

4. A solution of a reducing agent is added stepwise to the plating solution, with mixing at a controlled temperature.

5. The resulting plated electrode is removed from the plating solution then washed successively in a water miscible solvent and water. For example to obtain silver catalyst nano-structures the plating solution may be an aqueous mixture of silver nitrate, 30 wt% ammonia solution and water in the mass ratio about 1/1/10 with the addition of about 60 wt% Triton X-100 non-ionic surfactant. The reducing agent may be an aqueous solution of hydrazine sulphate and 30 wt% ammonia solution and water in the mass ratio about 1/1/10, with the plating at about 50 °C over a period up to about 1 hour. The final wash to remove residual surfactant may be in warm water, followed successively by methanol and water.

Similar methods but with different reagents (salts, complexants, buffers, etc.) may be used for the electro-less plating of other catalytic metals, such as copper, nickel, iron, cobalt, gold, palladium, platinum, tin, lead and their alloys.

Example 2.

The subject electrodes were made from a mixture of an electro-catalyst powder, a wetting alcohol, a dispersion of hydrophobic fluorinated polymer, a pore former, and optionally a solution of solid electrolyte. The resulting paste was formed into a sheet under pressure and the pore former removed to leave a solid macro-porous matrix of the electro-catalyst, hydrophobe, and if present the solid electrolyte.

A single cell parallel plate electrochemical reactor was assembled as shown in the vertical cross-section of Figure 5. In Figure 5 the reactor 1 consists of the porous gas- contacting cathode 2 in electronic contact with a stainless steel current collector 4 and a plastic mesh distributor 3. Adjacent the cathode 2 is a Fumasep cation exchange membrane 5. The Fumasep CEM is separated by a thin mesh 6 from the stainless steel flow channel and anode current collector 7. A two-phase mixture of C0 ₂ gas and liquid catholyte 8 flows to the cathode chamber and leaves as cathode product 10 while an anolyte 9 flows through the anode chamber and leaves as the anode product 11.

The reactor is driven by a potentiostat 12, with current and voltage measurements.

A cathode 2 of dimensions 20 mm by 20 mm was prepared according to the above method, including pressing at 25 °C, 1.5 tonnes force per square inch for 6 minutes. The properties of the resulting cathode were as follows:

Thickness: about 0.3 mm

Catalyst: silver nano-powder

Catalyst loading: 31 mg/cm ²

Fluorinated polymer (PTFE) content: 40% wt

Carbon black content: zero

Solid electrolyte (Nafion) content: zero

Porosity: about 50%

A mixture of 15 ml/minute 0.5 M potassium bicarbonate plus 50 sml/minute 100 vol% C02 gas was fed to the cathode and 150 ml/minute 2.5 M potassium hydroxide was fed to the anode. Both anode and cathode contents were recirculated. The gas and liquid products were analysed respectively by a gas chromatograph and a UV-vis

spectrophotometer. The reactor operation was galvanostatic under a pressure of 105 kPa(abs) at 293 K. Samples were taking at currents of 0.1 , 0.2, 0.4, and 0.8 A, which correspond to superficial current densities of 250, 500, 1000, and 2000 A/m ² . Figure 6 shows the performance of this improved cathode structure compared to a commercial silver gas diffusion electrode (Gaskatel Biplex with about 100 mg/cm ² Ag) and to a conventional gas diffusion electrode fabricated in house (60 wt% Ag which gave a loading of 31 mg/cm ² + 40 wt% PTFE), all operating under similar conditions. These data include the Faradaic efficiency for CO, the reactor voltage and the ratio of hydrogen to carbon monoxide (H ₂ /CO) in the product gas as a function of the superficial current density. The H ₂ /CO ratio is important to applications of C0 ₂ electro-reduction., Relative to Figure 6 the current density and efficiency for C0 ₂ reduction products (e.g. CO) may be changed by tailoring several properties of the cathode. Different support material such as carbon black and can be used modify porosity while maintaining the electronic conductivity of the electrode matrix. Example 1 summarizes several other variations in the materials and method of fabrication that may be used to affect the reactivity and selectivity of the cathode.

Example 3.

The subject electrodes were made as in Example 2 with three different pore-forming agents and tested as cathodes in the reactor of Figure 5.

Particulate potassium sulphate, sodium acetate and sodium citrate in the size range 63- 125 urn were the pore-forming agents, with the properties of each cathode as follow:

Thickness: about 0.3 mm

Catalyst: silver nano-powder

Catalyst loading: 31 mg/cm ²

PTFE content: 40% wt Carbon black content: zero

Nation content: zero

Porosity: about 50%

The reactor was operated and tested as in Example 2, except with a cathode feed of 3 ml/minute 0.5 M potassium bicarbonate plus 25 sml/minute 100 vol% C0 ₂ gas, an anolyte flow of 30 ml/minute and superficial current density of 500 A/m2. Figure 7 shows the performance of this improved cathode structure using different pore forming agents, compared to the conventional gas diffusion electrode fabricated in house, all operating under similar conditions.

Example 4.

The subject electrodes were made as in Example 2 and tested as cathodes in the reactor of Figure 5. In this case a single pore-forming agent, potassium sulphate, was used with particle size ranges of 48-63, 63-125, 125-250, and 250-500 urn, each in cathodes with the following properties:

Thickness: about 0.2-0.5 mm

Catalyst: silver nano-powder

Catalyst loading: 31 mg/cm ²

PTFE content: 40% wt

Carbon black content: zero

Nafion content: zero

Porosity: about 40-60%

The reactor was operated and tested as in Example 3 to give the results of Figure 8. Figure 8 compares the performance the four cathodes to the conventional gas diffusion electrode fabricated in house, all operating under similar conditions. All examples 2-4 demonstrate the superiority of the subject hybrid diffusion electrode (HDE) over the gas diffusion electrode (GDE) for the electroreduction of carbon dioxide to carbon monoxide.

Based on this invention those skilled in the art will see that many levels and combinations of the properties of the subject electrode listed above can be employed to enhance the performance of reactors used in the electrochemical reduction of carbon dioxide.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention.