WO2015021553A1

WO2015021553A1 - Functional materials as additives in the electro-reduction of carbon dioxide

Info

Publication number: WO2015021553A1
Application number: PCT/CA2014/050772
Authority: WO
Inventors: Colin Oloman
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-08-14
Filing date: 2014-08-14
Publication date: 2015-02-19
Anticipated expiration: 2016-02-14

Abstract

Functional materials used as electrolyte additives in the electro-reduction of carbon dioxide to control selectivity, suppress the temporal loss of Faradaic efficiency and lower the specific energy consumption in producing organic products. Specifically, a process for electrochemically reducing carbon dioxide comprising the steps of providing an electrochemical cell comprising an anode chamber having an anode and an anolyte, a cathode chamber having a cathode and a catholyte, and a membrane separator between the anode chamber and the cathode chamber, providing carbon dioxide into the catholyte, providing a chelating agent to the catholyte, and applying an electrical potential between the anode and the cathode to reduce the carbon dioxide to a reduction product. The use of chelating agents in the catholyte in an electrochemically process for reducing carbon dioxide.

Description

FUNCTIONAL MATERIALS AS ADDITIVES IN THE ELECTRO-REDUCTION OF

CARBON DIOXIDE

Field of the Invention

The present invention concerns processes for the electro-chemical reduction of carbon dioxide. In particular, the present invention is aimed at suppressing the temporal deterioration of reactor performance and corresponding loss of Faradaic efficiency during their operation in electrochemical processes for the reduction of C0₂.

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 C0₂ to useful materials, such as fuels and organic chemicals. xC02 + (y-2(z-2x))H⁺ + ye- CxHyOz + (z-2x)H₂0 Reaction 1

2H⁺ + 2e- ^ H₂ 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:

X y z CxHyOz Name

1 4 0 CH₄ methane

2 4 0 C₂H₄ ethene

2 6 0 C₂H6 ethane

1 0 1 CO carbon monoxide

1 2 2 CH₂o₂ methanoic acid

1 4 1 CH₄0 methanol

1 2 1 CH₂0 methanal

2 6 1 C₂H₆0 ethanol These products have been obtained mostly in small scale batch laboratory experiments of short time duration (e.g. less than 1 hour) in which the temporal stability was not mentioned. Where the effect of time has been reported it is routinely observed, in both batch and continuous reactors, that the Faradaic efficiency for C0₂ reduction products decreases with increased operating time, usually in the span of about 24 hours. Various reasons for this loss of cathode selectivity have been proposed, as there have been attempts to prevent it, for example by polarity reversal, pulsed electrolysis or acid, alkali and water wash. Some of these methods have been partially successful but all are problematic with respect to operating a continuous electrochemical process for C0₂ reduction. Since industrial electrochemical reactors should operate for thousands of hours it is clear that the temporal stability of the cathode is a major hurdle in the path to the commercial exploitation of reaction 1.

It is well known that the selectivity of electrode reactions is sensitive to the electrode material and the nature of its surface, along with the composition of the electrolyte via its effect on conditions in the adjacent electric double-layer. However, the quantitative effect of these variables is difficult to predict - to the extent that the related electrode kinetic data are found almost exclusively by experimental observation. It is also known that, when added to electrolytes, low levels of some substances can have a large effect on the mechanism and kinetics (and hence on the selectivity) of electrode reactions, which can determine the success, or failure, of electrochemical processes. Examples of this phenomenon are: the 0.01 M sodium dichromate used in the electrosynthesis of sodium chlorate, the 0.01 M bisQuat phosphate (where bisQuat = hexamethylene bis(ethyldibutylammonium)) used in the production of adiponitrile and the pyridinium compounds proposed for the electrochemical reduction of C0₂ to methanol.

Experience in the prior art points to the need for method(s) to increase the temporal stability and reaction selectivity in cathodes used for the electro-reduction of C02. The objective of the present invention is to employ electrolyte additives to suppress the temporal loss of Faradaic efficiency for organic products in the electro-reduction of carbon dioxide.

Summary of the Invention.

This invention discloses the use of functional materials as electrolyte additives to control selectivity, suppress the temporal loss of Faradaic efficiency and lower the specific energy consumption for organic products in the electro-reduction of carbon dioxide.

In some aspects, the present invention provides a process for electrochemically reducing carbon dioxide comprising the steps of: providing an electrochemical cell comprising an anode chamber having an anode and an anolyte, a cathode chamber having a cathode and a catholyte, and a membrane separator between the anode chamber and the cathode chamber; providing carbon dioxide into the catholyte; providing a chelating agent to the catholyte; and applying an electrical potential between the anode and the cathode to reduce the carbon dioxide to a reduction product.

In some embodiments, the chelating agent may be an anionic chelating agent. The chelating agent may be an anionic organic chelating agent, capable of complexing metal ions, selected from the group consisting of amino-carboxylates, organophosphates, gluconates and nitrilotracetates.

In some embodiments, the chelating agent may be diethylene triamine pentaacetic acetate anion, and it may be in a concentration of about 1 mM.

In some embodiments, the chelating agent may be an organophosphonate chelating agent. The concentration of the organophosphonate chelating agent in the catholyte may be within the range of about 0.1 mM to about 10.0 mM, or within the range of about 0.5 mM to about 5.0 mM, or about 0.5 mM, or about 2 mM, or about 5 mM.

In some embodiments, the chelating agent may be Na5DTPA, and it may be in a concentration in the catholyte of about 2 mM.

In another aspect, the present invention provides the use of a chelating agent in a catholyte in an electrochemical process for reducing carbon dioxide. In some embodiments, the chelating agent may be an anionic chelating agent. In some embodiments, the chelating agent may be an anionic organic chelating agent, capable of complexing metal ions, selected from the group consisting of amino- carboxylates, organophosphates, gluconates and nitrilotracetates. In some embodiments, the chelating agent may be diethylene triamine pentaacetic acetate anion. In some embodiments, the chelating agent may be an organophosphonate chelating agent. In some embodiments, the chelating agent may be Na5DTPA.

Brief Description of the Drawings Figure 1 is a diagram of an electrochemical reactor process according to the invention.

Figure 2 is a graph showing the results of the two runs conducted in Example 1 described below, shown as a temporal comparison of the specific energy for formate production.

Figures 3 and 4 are graphs with the results of the five runs conducted in Example 2 below, shown as temporal comparisons of the current (i.e. Faradaic) efficiency (Fig. 3) for formate production and of the corresponding reactor voltage (Fig. 4).

Detailed Description of the Invention

Figure 1 shows a process for the electrochemical reduction of carbon dioxide to obtain CO₂ reduction products by cathode reactions with the generic form: xC0₂ + (y-2(z-2x))H⁺ + ye^" CxHyOz + (z-2x)H₂0 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

1 2 1 CH₂0 methanal

2 6 1 C₂H_eO ethanol

In aqueous solutions reaction 1 is usually accompanied by the parasitic reaction 2, which lowers the Faradaic efficiency of the process for C0₂ reduction.

2H⁺ + 2e → H₂ Reaction 2

The process of Figure 1 has an electrochemical reactor A where carbon dioxide (C0₂) 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 C0₂ 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 C0₂ is reduced, includes a porous electrode with an electro-catalytic specific surface in the range about 100 to 100,000 m²/m³, 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 mixture of a C0₂ containing gas P and a catholyte liquid solution Q in a volumetric flow ratio from about 10 to 1000, measured at 1 bar(abs), 273 K. The gas P and liquid Q may be introduced separately to the cathode, 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 marks 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 include a layer of porous hydrophilic material such as asbestos, ZirfonPerl (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. Pb0₂, Sn0₂), 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^" -» 0₂ + 2H₂0 + 4e^" Reaction 3 oxygen

2Cr Cl₂ + 2e Reaction 4 chlorine

2S0₄ ^2" -> S₂0₈ ^2" + 2e Reaction 5 persulphate

2C0₃ ^2"■» C₂0₆ ^2" + 2e Reaction 6 percarbonate

2H₂0 0₂ + 4H⁺ + e Reaction 7 oxygen

C₆H₆ + 2H₂0 C₆H₄0₂ + 2H⁺ + 2e^~ Reaction 8 benzoquinone

C₈HioO + H₂0 -> C₈H₈0₂ + 4H⁺ + 4e Reaction 9 methoxybenzaldehyde

H₂ -» 2H⁺ + 2e^" Reaction 10 proton

CH₄ + H₂0 - CH₄0 + 2H⁺ + 2e^" Reaction 11 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 11 , "immiscible" organic liquids as in reactions 8 and 9 or gases as in reactions 10 and 11. 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, lithium or ammonium hydroxide or a sodium, potassium, rubidium, caesium, lithium or ammonium salt of the above acids. The anolyte may optionally include species to be engaged in oxidative redox couples, such as Ag²⁺ / Ag¹⁺,Ce⁴7 Ce³⁺, Co³⁺ / Co²⁺, Fe³⁺ / Fe²⁺, Mn³⁺ / Mn²⁺, V⁵⁺ / V⁴⁺, 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, lithium or ammonium hydroxide or a sodium, potassium, rubidium, caesium, lithium 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, Cr³⁺ / Cr²*, Cu²⁺ / Cu¹⁺, Sn⁴⁺ / Sn²\ j|3+ j|2+^ y3+ / y2+ ^ _ol-g_anj_{c CO}uples 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 % C0₂ 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.

The process of Figure 1 , its main components and variants, are the basis for the present invention, which is described below.

Many of the substances that may affect the kinetics and selectivity of electrode reactions are in the categories of complexing agents, surfactants and so-called "functional materials", such as those compiled in McCutcheon's "Functional Materials", McCutcheon's Publishing Co., Glen Rock, 1996, which is incorporated herein by reference, including for example: anti-stats, bactericides, carriers, chelating agents, corrosion and scale inhibitors, coupling agents, dispersants and leveling agents. Thousands of substances fall into these categories and each can have different or unique effects in different electrochemical systems. The selection of such substances for a specific electrochemical application is by trial and error experiments, preferably based on concepts of the reaction mechanism(s).

For example, in the case of electro-reduction of CO2 it is known that the Faradaic efficiency for reaction 1 is lowered by the occurrence of reaction 2. The prior art (for example, US 6706432) shows that in some circumstances reaction 2 is inhibited in the presence of cationic species such as quaternary ammonium ions (QUATs). Thus one skilled in the art can speculate that additives like quaternary ammonium salts would suppress reaction 2 and increase the Faradaic efficiency for reaction 1. It is also possible that, since they may shift the cathode potential, cationic species would affect the selectivity for individual products of reaction 1. The additives of interest here are materials such as those listed under "Bactericides", "Corrosion inhibitors" and "Scale inhibitors" in McCutcheon's "Functional Materials", McCutcheon's Publishing Co., Glen Rock, 1996, and under "Cationics" in McCutcheon's "Emulsifiers and Detergents", McCutcheon's Publishing Co., Princeton, 2009, which is incorporated herein by reference.

On the other hand, the category of chelating agents includes many substances with anionic activity. Examples of such substances are the salts of citric acid, ethylendiamine tetraacetic acid (EDTA), hydroxyelthenediamine triacetic acid

(HEEDTA) and diethylenetriamine pentaacetic acid (DTPA), as well as various phosphates, organophosphonates, gluconates and nitrilotriacetates as listed in

McCutcheon's "Functional Materials" under "Chelating/Sequestering Agents", which is incorporated herein by reference. These materials are used in commerce for their ability to sequester polyvalent ions in aqueous solution. However, their effect in the electro-reduction of C0₂ is not disclosed in the prior art, except in Li H.,

"Development of a continuous reactor for the electrochemical reduction of carbon dioxide", Ph.D. Thesis, University of British Columbia, 2006, which is incorporated herein by reference, where chelating agents EDTA and DTPA were seen to have a negative effect on the ERC process. However, from recent experiments with the exemplary chelating agents pentasodium DTPA and an organophosphonate, it has now been surprisingly discovered that the addition of a chelating agent to the catholyte can substantially improve the ERC process, by increasing both the Faradaic efficiency and the temporal stability of the cathode, while decreasing the applied reactor voltage and specific energy consumption.

It is postulated that the positive effect of chelating agents in the electro-reduction of carbon dioxide may be due to their preventing or modifying the accumulation of passivating cathode corrosion products on that electrode's surface, or they may affect the action of impurities in the electric-double layer, or adsorb on the cathode to interfere in the electrode kinetics, or possibly act in the bulk catholyte to lower the rate of conversion of C0₂(aq) to unreactive bicarbonate/carbonate species.

The chelating agent may be an anionic organic species, such as an amino- carboxylate, organophosphate, gluconate or nitrilotracetate, capable of complexing metal ions. It may have the potential complimentary benefits of binding to the surface of metal cathodes to form a negative potential barrier that rejects hydroxide (OH^") ions and thus slows the interfacial conversion of reactant CC½ to non-electro-active HC0₃ ^" by the reaction: C0₂ + OH- -> HC0₃ ^". It may also be capable of adsorbing at the gas/liquid or gas/liquid/solid interface to create a pH gradient that slows the conversion of C0₂ to HC0₃ ^" and so promotes the transfer of reactive C0₂ to the cathode surface. It may also be capable of modifying the structure of electrode passivating metal oxide films - to promote access of C0₂ to the cathode by changes in the surface morphology or the pH profile in the boundary layer, and be stable to electrochemical reduction (as opposed to cationic organic species).

Example 1.

A single-cell continuous parallel plate electrochemical reactor was assembled with superficial active area dimensions of 0.1 m long by 0.01 m wide for both the anode and cathode. The 3D cathode, 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 m²/m³, contacted with a lead plate current collector and separated from the anode by a layer of Nafion™ 1110 membrane supported on two layers of 8 mesh per inch polypropylene screen in a 3 mm thick gasket. The anode was a 1.5 mm thick 316 stainless steel plate. The reactor was fed with an anolyte of

1 M potassium carbonate at 10 to 30 ml/minute and a two-phase catholyte mixture of 100 vol% C0₂ gas at 40 to 200 Sml/min with 1 aqueous potassium carbonate solution at 0.8 to 2 ml/minute and operated at 120 kPa(abs), 295 K with currents ranging from 0.5 to 1.1 A and voltages from 5.1 to 7.3 V.

Two separate runs were carried out with this reactor: Run 1(i) with no DTPA added to the catholyte feed. Run 1(ii) with 1 mM DTPA added to the catholyte feed. Figure

2 shows the results as a temporal comparison of the specific energy for formate production in Runs 1(i) and 1(ii). This result demonstrates a positive effect of the chelating agent on the performance of this process by decreasing the specific energy for formate production. Surprisingly the presence of the chelating agent here had the triple effect of increasing the Faradaic efficiency, increasing the temporal stability and decreasing the reactor voltage and corresponding specific electrochemical energy consumption for the C0₂ reduction product, formate. Example 2.

A single-cell continuous parallel plate electrochemical reactor was assembled as in Example 1 , except with a membrane separator of Fumasep™ FKB 130. The reactor was fed with an anolyte of 2.5 M potassium hydroxide at 30 ml/minute and a two- phase catholyte mixture of 100 vol% C0₂ gas at 150 Sml/min with 0.5 M aqueous potassium bicarbonate solution at 1.7 to 2.1 ml/minute and operated at 130 kPa(abs), 295 K with a current of 0.5 A.

Five separate runs were carried out with this reactor: Run 2(i) with no DTPA added to the catholyte feed. Run 2(ii) with 2 mM DTPA and Runs 2(iii),2(iv) and 2(v) with respectively 0.5, 2 and 5 ml/litre of a Briquest™ 543-45AS organophosphonate chelating agent (Sodium Diethylenetriaminepentakis (Methylenephosphonate) added to the cathoyte feed. Figures 3 and 4 show the results as a temporal comparison of the current efficiency (CE) for formate production and reactor voltage in Runs 2(i) to 2(v). Again, this result demonstrates a triple positive effect of the chelating agents, by increasing the Faradaic efficiency, increasing the temporal stability and decreasing the electrochemical energy consumption for the C0₂ reduction product, formate. It should be noted that the specific energy consumption is proportional to the ratio of the reactor voltage to the current efficiency.

Further, Examples 1 and 2 show that the beneficial effects of chelating agents in the electroreduction of C0₂ depend on their type and concentration, as well as on the conditions in the electrochemical process. On this basis one skilled in the art may proceed to select the combination of chelant and conditions to optimize this process for a desired outcome. Table 2. Cathode metal electro-catalyst materials

Copper/Antimony Titanium/Aluminum Titanium/Tantalum

High Purity Copper

Alloy Alloy Alloy

High Purity Titanium/Antimony

Copper/Nickel Alloy

Titanium Alloy

Copper/Nickel/Tin Titanium Metal Titanium/Copper

Alloy Matrix Composite Alloy

Table 3. Organo-metal electro-catalysts

Electrocatalytic and Homogeneous

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

Fuels

Electrocatalytic and Homogeneous

[Pd(triphos)(PR₃)](BF₄)₂ Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

[Νί₃(μ₃-Ι)(μ₃-ΟΝΜβ)(μ₂- Approaches to Conversion of C02 to liquid CO, C0₃ ^2" dppm)₃]⁺

Fuels

Electrocatalytic and Homogeneous

[Cu₂(p-PPh₂bipy)₂- Approaches to Conversion of C02 to liquid CO, C0₃ ^2" (MeCN)₂[PF₆]₂

Fuels

Electrocatalytic Reduction of Carbon

[Re(CO)₃(K²-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-, CH₃OH, CH₂0

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

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

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.

Claims

WHAT IS CLAIMED IS:

1. A process for electrochemically reducing carbon dioxide comprising the steps of:

(A) providing an electrochemical cell comprising an anode chamber having an anode and an anolyte, a cathode chamber having a cathode and a catholyte, and a membrane separator between the anode chamber and the cathode chamber;

(B) providing carbon dioxide into the catholyte;

(C) providing a chelating agent to the catholyte; and

(D) applying an electrical potential between the anode and the cathode to reduce the carbon dioxide to a reduction product.

2. The process as claimed in claim 1 wherein the chelating agent is an anionic chelating agent.

3. The process claimed in claim 1 wherein the chelating agent is an anionic organic chelating agent, capable of complexing metal ions, selected from the group consisting of amino-carboxylates, organophosphates, gluconates and nitrilotracetates.

4. The process as claimed in claim 1 wherein the chelating agent is diethylene triamine pentaacetic acetate anion.

5. The process as claimed in claim 4 wherein the concentration of the diethylene triamine pentaacetic acetate anion in the catholyte is about 1 mM.

6. The process as claimed in claim 1 wherein the chelating agent is an

organophosphonate chelating agent.

7. The process as claimed in claim 6 wherein the concentration of the

organophosphonate chelating agent in the catholyte is within the range of about 0.5 mM to about 10.0 mM.

8. The process as claimed in claim 6 wherein the concentration of the

organophosphonate chelating agent in the catholyte is within the range of about 0.5 mM to about 5.0 mM.

9. The process as claimed in claim 6 wherein the concentration of the

organophosphonate chelating agent in the catholyte is about 0.5 mM.

10. The process as claimed in claim 6 wherein the concentration of the

organophosphonate chelating agent in the catholyte is about 2 mM.

1. The process as claimed in claim 6 wherein the concentration of the

organophosphonate chelating agent in the catholyte is about 5 mM.

12. The process as claimed in claim 1 wherein the chelating agent is Na₅DTPA.

13. The process as claimed in claim 12 wherein the concentration of the

Na₅DTPA in the catholyte is about 2 mM.

14. Use of a chelating agent in a catholyte in an electrochemical process for reducing carbon dioxide.

15. The use as in claim 14 wherein the chelating agent is an anionic chelating agent.

16. The use as in claim 14 wherein the chelating agent is an anionic organic chelating agent, capable of complexing metal ions, selected from the group consisting of amino-carboxylates, organophosphates, gluconates and nitrilotracetates.

17. The use as in claim 14 wherein the chelating agent is diethylene triamine pentaacetic acetate anion.

18. The use as in claim 14 wherein the chelating agent is an organophosphonate chelating agent.

19. The use as in claim 14 wherein the chelating agent is Na₅DTPA.