US 20080223727 A1
In various embodiments, the invention provides electro-chemical processes for reduction of carbon dioxide, for example converting carbon dioxide to formate salts or formic acid. In selected embodiments, operation of a continuous reactor with a three dimensional cathode and a two-phase (gas/liquid) catholyte flow provides advantageous conditions for electro-reduction of carbon dioxide. In these embodiments, the continuous two-phase flow of catholyte solvent and carbon dioxide containing gas, in selected gas/liquid phase volume flow ratios, provides dynamic conditions that favour the electro-reduction of COs at relatively high effective superficial current densities and gas space velocities, with relatively low reactor (cell) voltages (<10 Volts). In some embodiments, relatively high internal gas hold-up in the cathode chamber (evident in an internal gas to liquid phase volume ratio >0.1) may provide greater than equilibrium CO2 concentrations in the liquid phase, also facilitating relatively high effective superficial current densities. In some embodiments, these characteristics may for example be achieved at catholyte pH >7 and relatively low CO2 partial pressures (<10 bar). In some embodiments, these characteristics may for example be achieved under near adiabatic conditions, with catholyte outlet temperature up to about 80° C.
1. An electrochemical process for reducing carbon dioxide comprising:
a) continuously passing a catholyte mixture through a cathode chamber of an electrochemical reactor, the catholyte mixture comprising carbon dioxide gas and a liquid catholyte solvent containing dissolved carbon dioxide;
b) maintaining a catholyte gas to liquid volumetric hold-up ratio, being the ratio of the volume of gas to the volume of the liquid catholyte solvent, in the cathode chamber, greater than about 0.1.
c) passing an electric current between a cathode in the cathode chamber and an anode, to reduce the dissolved carbon dioxide to form a desired product.
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a) a dissolved alkali metal hydroxide;
b) a dissolved alkali metal or ammonium salt;
c) a dissolved acid, being H2SO4, HCl, or H3PO4;
d) dissolved sulphuric acid and ammonium sulphate; or
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The invention is in the field of electrochemistry, encompassing processes for the electro-reduction of carbon dioxide in aqueous systems, and apparatus therefor.
The formate salts MHCO2 (where M is typically Na, K or NH4) and formic acid HCO2H are commercial chemicals that may be produced by industrial thermochemical processes (Kirk-Othmer—Encyclopedia of Chemical Technology, 1991). For example, sodium formate and subsequently formic acid may be obtained by reaction of sodium hydroxide with carbon monoxide, followed by acidolysis with sulphuric acid.
Formic acid may also be produced as a co-product in the oxidation of hydrocarbons and by the hydrolysis of methyl formate from the carbonylation of methanol. Processes for the synthesis of formate salts (e.g. KHCO2) by the electro-reduction of carbon dioxide are also known (Chaplin and Wragg, 2003; Sanchez et al., 2001; Akahori et al., 2004; Hui and Oloman, 2005).
Carbon dioxide Is considered the main anthropogenic cause of climate change. Methods to sequester CO2 and/or convert it to useful products are therefore needed.
Oloman and Watkinson in U.S. Pat. Nos. 3,969,201 and 4,118,305 (incorporated herein by reference) describe a trickle bed reactor for electroreduction of oxygen to alkaline peroxide. In various aspects, that electrochemical cell comprises a pair of spaced apart electrodes, at least one of the electrodes being in the form of a fluid permeable conductive mass separated from the counter electrode by a barrier wall. The electrode mass may be in the form of a bed of particles or a fixed porous matrix. It is composed of an electronically conducting material the surface which is a good electrocatalyst for the reaction to be carried out. Inlets are provided for feeding liquid electrolyte and gas into the electrode mass such that the electrolyte and gas move co-currently through the electrode mass, for example in a direction generally perpendicular to the direction of the current between the electrodes. An outlet is provided for removing solutions containing reaction products from the fluid permeable conductive mass.
In various embodiments, the invention provides electro-chemical processes for reduction of carbon dioxide, for example converting carbon dioxide to formate salts or formic acid. In selected embodiments, operation of a continuous reactor with a three dimensional cathode and a two-phase (gas/liquid) catholyte flow provides advantageous conditions for electro-reduction of carbon dioxide. In these embodiments, the continuous two-phase flow of catholyte solvent and carbon dioxide gas, in selected gas/liquid phase volume ratios, provides dynamic conditions that favour the electro-reduction of CO2 at relatively high effective superficial current densities. In some embodiments, relatively high internal gas hold-up in the cathode chamber (evident in a gas to liquid phase volume ratio >1 in the feed stream, or >0.1 within the porous electrode) may provide greater than equilibrium CO2 concentrations In the liquid phase, facilitating relatively high effective superficial current densities. In some embodiments, these characteristics may for example be achieved at catholyte pH >7 and relatively low CO2 partial pressures (<10 bar).
In alternative aspects, the invention involves continuously passing a catholyte mixture through a cathode chamber of an electrochemical reactor. The catholyte mixture may include carbon dioxide gas and a liquid catholyte solvent containing dissolved carbon dioxide. The catholyte solvent may for example be an aqueous solvent, it may include a dissolved alkali metal or ammonium bicarbonate, and may be maintained at a desired pH, such as in the range of from about 6 to about 9. A catholyte gas to liquid (G/L) volumetric ratio may be maintained, being the ratio of the volume of carbon dioxide gas to the volume of the liquid catholyte solvent. The G/L ratio may be maintained in the cathode chamber, for example In the feed stream or in a porous cathode within the chamber. For example, the process may be operated so that the G/L ratio is greater than a threshold value, such as greater than 1 in the feed, or greater than 0.1 within the porous (3D) cathode.
One aspect of the invention involves passing an electric current between a cathode in the cathode chamber and an anode, to reduce dissolved carbon dioxide to form a desired product. In some embodiments, the process may be operated so that the effective superficial current density at the cathode is greater than a threshold value, such as 1 kA/m2 (or 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5 kA/m2). The electric current in the system may for example be a direct current, driven by an electrochemical cell voltage, and in some embodiments the process may be capable of operating at relatively low electrochemical cell voltages, for example less than 10 Volts.
Various aspects of the invention work in concert to facilitate the adoption, in some embodiments, of process parameters that may improve the economics of processes of the invention. In some embodiments, the processes of the Invention may be used with relatively dilute input gas streams, for example the carbon dioxide gas concentration in the feed gas may be from 1 to 100%, or any numeric value within this range (in some embodiments yielding a carbon dioxide partial pressure in the cathode chamber less than a threshold value, such as 3, 5 or 10 Bar). Similarly, relatively low system pressures may be used, for example the cathode chamber may be maintained at a cathode pressure in the range of a minimum value such as 1, 2, 3, 4 or 5 Bar (1 Bar=100 kPa(abs)) up to a higher maximum value such as 6, 7, 8, 9 or 10 Bar. In some embodiments, it may be effective to run processes of the invention at elevated temperatures, which may avoid the necessity for cooling, for example at cathode temperatures above a desired threshold such as 20, 25, 30, 35, 40, 45 or 50° C. In this context, it will be understood that cathode chamber pressures and temperatures may vary along the cathode height. For example, the inlet pressure may be greater than outlet pressure (in some embodiments, the pressure drop may for example range from a minimum of about 10, 20, 30, 40, or 50 kPa, up to a maximum of about 500, 600, 700, 800 or 900 kPa). Similarly, the outlet temperature may be greater than inlet temperature, with a temperature rise from the inlet to the outlet of from about 1 to 100° C., or any numeric value within this range. It will be understood that the gas composition (particularly CO2 concentration) and the total pressure, fix the CO2 partial pressure. i.e. ppCO2=(CO2 fraction)×(Total pressure)
Cathodes for use in the invention may have an effective thickness in the dimension of current flow, such as a porous cathode. These may be referred to as three dimensional (3D) electrodes. Such electrodes may have a selected thickness, such as less than 6, 5, 4, 3, 2, 1 or 0.5mm, and they may have a selected porosity, or range of porosities, such as 5 to 95% or any numeric value within this range, such as 30, 40, 50, 60 or 70%. Cathodes of the invention may be made from a wide variety of selectively electro-active materials, such as tin, lead, pewter, mercury, indium, zinc, cadmium, or other materials such as electronically conductive or non-conductive substrates coated with selectively electro-active materials (e.g. tinned copper, mercury amalgamated copper, tinned graphite or tinned glass).
The anode may be in an anode chamber, and the anode chamber may be separated from the cathode chamber by an electrochemical cell membrane. The anolyte in the anode chamber may be an aqueous anolyte, and may for example include a dissolved alkali metal hydroxide, a salt (including an ammonium salt) or an acid, and may have a pH range of from about 0 to about 14, or any pH value or range within this range.
The electrochemical cell membrane may be a cation permeable membrane, for example a membrane that permits selected ions to cross the membrane to balance process stoichiometry.
The desired products of the process include formate salts, such as ammonium, potassium and sodium formate, or formic acid. The desired product may be separated from the catholyte solvent in a variety of ways. For example, a portion of the catholyte solvent, the recycling catholyte solvent, may be recycled from a cathode chamber outlet to a cathode chamber inlet, and the desired product may be separated from the recycling catholyte solvent. Similarly, at least a portion of the anolyte may be recycled from an anolyte chamber outlet to an anolyte chamber inlet, and an anode co-product may be separated from the recycling anolyte,
In selected embodiments, Joule heating of the anolyte may be used to provide heated anolyte, and the heated anolyte may be used to heat the recycling catholyte solvent to separate the desired product from the recycling catholyte solvent, for example by evaporation with fractional crystallization or vacuum distillation. In some embodiments, recycling catholyte, that includes formate, may be reacted with the anolyte, to obtain the desired product by an acidolysis reaction.
In various aspects, the invention provides a continuous reactor for electroreduction of CO2, which may for example be used in a process that converts a feed of carbon dioxide plus water to formate ion (Reaction 1) and consequently produces formate salts or formic acid.
In some embodiments, the invention may utilize an electrochemical reactor analogous to the trickle bed reactor described by Oloman and Watkinson in U.S. Pat. Nos. 3,969,201 and 4,118,305. In such embodiments, the Invention may utilize an apparatus for carrying out electrochemical reactions involving gaseous reactants comprising an electrochemical cell having a pair of spaced apart electrodes, at least one of the electrodes, such as the cathode, being in the form of a fluid permeable conductive mass and being separated from the counter electrode by an ionically conductive but electronically insulating layer (such as a membrane or porous diaphragm). The reactor may be operated in a “trickle-bed” mode, with co-current flow of reactant gas and catholyte liquid through a flow-by 3-D cathode. As illustrated in the Examples, the process parameters of the invention may be adjusted so that this reactor achieves advantageous reactant supply (evident for example as a high gas space velocity, a ratio of the volumetric gas feed flow rate over reactor volume) and mass transfer characteristics. The co-current fluid flow in the cathode may be at any orientation relative to gravity, such as upward or downward.
In reactors of the invention, an inlet may be provided for feeding a liquid electrolyte and a gas into the fluid permeable conductive mass, and an outlet may be provided for removing solutions containing reaction products from the conductive mass. The inlet and outlet may be arranged so that the electrolyte and gas move co-currently through the conductive mass, for example in a direction generally normal to the flow of electric current between the electrodes. The reactor may for example be provided with a cation membrane separator (as described for example in Hui and Oloman, 2005). In alternative embodiments, other types of reactor may be used.
Depending on the desired products and overall material balance, the process feed may also include: metal hydroxides and/or metal salts such as MOH, MCl, M2CO3, M2SO4 and M3PO4 where M is typically an alkali metal (Na, K, etc.) or NH4; acids such as H2SO4, H3PO4, or HCl; or ammonia (NH3).
Flow sheets in various degrees of detail are provided for alternative processes in
Process steps 1 to 5 in
Step 1. MIX: Continuously mixes the feed water (plus any make-up reagents) with the recycling catholyte, which is then delivered continuously to the reactor cathode chamber.
Step 2. REACT: [C] Cathode. Continuously drives reaction 1, along with the side-reaction, Reaction 2, that gives hydrogen by the electro-reduction of water.
Anode. Continuously drives the complimentary anode reaction(s) whose nature depends on the desired products from the process. For example, if the desired main product is a formate salt and the co-product Is oxygen then the anode reaction may be Reaction 3.
If the desired main product is formic acid and the co-product is oxygen or chlorine then the anode reaction may be respectively Reaction 4 or 5. Other anode reactions may include the generation of peroxy-salts of peroxy-acids, such as peroxy-disulphate (2SO4 −→S2O8 2−+2e).
The electrode chambers in the reactor may be separated by a membrane that selectively allows the transport of cations from anode to cathode in amounts that balance the desired process stoichiometry. If the desired main product is a formate salt then these cations may be alkali metal ions (e.g. Na+, K+ or NH4 +) fed to the anolyte as hydroxides, salts or NH3 gas, whereas if the desired main product is formic acid the transported cations may include protons (H+) generated in Reaction 4 and/or fed to the anolyte as an acid.
Step 3. SEPARATE: Continuously separates the main product (formate salt or formic acid) and byproduct (hydrogen) from the recycling catholyte.
Step 4. MIX: Continuously mixes required anode reagents and water with the recycling anolyte.
Step 5. SEPARATE: Continuously separates the anode co-product(s) from the recycling anolyte.
In various steps of the process, carbon dioxide and water may be consumed and/or generated In other reactions, such as Reactions 6, 7 and 8 that occur in the reactor or elsewhere in the process.
In some embodiments, the process may involve driving the reactor at a relatively high superficial current density (e.g. above 0.5 kA/m2) and current efficiency, for example for formate production (e.g. above 50%). Processes of the invention may also involve balancing the material and energy requirements of the various process steps to match the required process stoichiometry, while maintaining a low specific energy consumption. For example, processes of the invention have demonstrated 75% current efficiency for formate at 1.3 kA/m2 with a reactor voltage of 3 V at CO2 pressure of 200 kPa(abs) and temperature of 300 K. The management of water may be important to the material balance and require that water be fed to the cathode and/or anode circuits to match its rate of reaction, electro-osmotic transport and evaporation. The consumption of energy in electrochemical reaction, heating, cooling and pumping may be a contributor to the process cost, and may be kept relatively low by appropriate reactor design and by rationalizing the thermal loads In the process. In some embodiments, non-metallic catalysts may be used. For example, U.S. Pat. Nos. 5,284,563 and 5,382,332 disclose nickel alkyl cyclam catalysts that may be used for carbon dioxide reduction.
In some embodiments, a relatively high gas/liquid (G/L) phase feed volumetric flow ratio may be used in the electrochemical reactor (e.g.. G/L flow=1 to 1000 or 10 to 200), as well as a high gas space velocity (e.g. >100 h−1). In selected reactors of the invention, increasing G/L from about 5to 100 increases the voltage by less than 10%. The optimum G/L phase volume (denoted as the “G/L hold-up”) ratio depends, in general, on the balance between the effective catholyte conductivity (usually decreasing with increasing G/L hold-up), the CO2 mass transfer capacity (usually increasing with increasing G/L hold-up) and the intrinsic temperature and pH dependent kinetics of CO2 conversion to the un-reactive bicarbonate/carbonate species in the bulk catholyte liquid phase.
In various embodiments, there are two separate gas/liquid (G/L) ratios that may be of importance:
The above conditions may be modulated to allow (where CD=current density):
The “superficial current density” is the current passing through the cell divided by the projected surface area of the relevant element, such as the cathode. The “projected surface area” of an element, such as the cathode, is surface area of a projection of the element on a plane parallel to the element. For flat plate elements, the projected surface area is equal to the area of the side of that element facing the other conductive element, for example the projected surface area of the cathode facing the anode. For an element in the form of a planar mesh, the projected surface area is the area within the outline of the mesh as projected onto a continuous planar surface.
The “current efficiency” (CE) is the ratio, generally expressed as a percentage, of the actual reaction rate to the rate that would be achieved if all of the current passing through the cell were consumed by relevant reaction, such as the reduction of carbon dioxide.
In some embodiments, the invention may operate at or near adiabatic conditions (T out, up to about 90° C.). In some embodiments, while reducing the solubility of CO2 in the catholyte, an increasing temperature actually favours the intrinsic kinetics of the electroreduction of carbon dioxide (ERC), and good CE can be obtained at higher temperatures by manipulating the factors that promote CO2 mass transfer in a continuous reactor. In some embodiments, the ability to operate at high temperature may be important, because the effects of Joule heating at high CD under near adiabatic conditions in the continuous reactor may automatically increase the reaction temperature by up to about 80° C.
Galvanostatic electrolysis of CO2 was carried out with a DC power supply connecting across the anode and cathode. A voltmeter was also connected to the unit to measure the reactor voltage. All voltages included anode potential, cathode potential and IR drop. The individual electrode potentials were not measured.
An automatic pressure control valve was used in the anolyte product line to balance the pressure in the anode chamber against that in the cathode chamber. Such a pressure balance is required to prevent catholyte by-passing the 3-D cathode and/or the bursting of the membrane that can occur when the cathode pressure exceeds the anode pressure.
Most runs were conducted with the cathode outlet at the atmospheric pressure. For some runs in Reactor B a manual back pressure control valve and pressure gauge were installed in the catholyte product line to maintain superatmospheric pressure in the catholyte outlet.
Processes of the invention were performed first in Reactor A (small reactor) and then in a seven-fold big Reactor B (big reactor) to evidence the effects of scale up. Both reactors have the configuration shown in
In Reactor B, tin-coated copper mesh cathodes or fin granule cathodes were used.
Reactor B was assembled with a tin granule fixed-bed cathode, according to the following procedures: (1) A sanded tin plate (99.99 wt % Sn, 3mm thick) cathode feeder was put onto the neoprene gasket; (2) The pretreated tin granules were spread uniformly into a Durabla gasket (3.2 mm thick) on the tin plate, and layers of Netlon screen were inserted Into the entrance and exit regions of the catholyte flow to distribute the fluid and support the membrane; (3) The wet Nafion 117 membrane was put on top of the tin granule bed, and then, the PVC screen spacer, anode SS mesh, and anode feeder (SS plate) were placed on top of one another In that sequence; (4) Lastly, a cell body was put into place, and 24 ⅜ inch bolts were employed to compress the sandwiched cell uniformly.
A variety of cathode materials are available for use In alternative aspects of the invention. Carbon dioxide can be electrochemically reduced on almost all groups of metals in the periodic table to give a variety of products with different levels of selectivity. The following cathode materials, among others, may be adapted to particular embodiments: Nanostuctured Cu deposited on graphite felt; Cu/Sn alloy deposited on graphite felt; nano-structured Sn on Sn mesh, Sn coated plastic mash, Cu mesh; Sn deposited graphite felt; Sn coated copper mesh; Pb plate, shot, granules, grid and Pb-C reticulate; Sn shot and granules. The last five of the foregoing materials were used in alternative embodiments for the present Example. In some embodiments, a high (specific) surface area micro or nano-structured deposit on a 3D substrate is desirable. Other potential cathodes are. nano-structured Cu on Cu mesh, nano-structured Sn on Sn mesh, or Sn coated plastic mesh, alternatively with Pb, In or Hg as the electroactive surface.
Reactor A, using granulated tin cathodes (99.9 wt % Sn) and a feed gas of 100% CO2 showed slightly better performance than that of the tinned-copper mesh cathodes. The seven fold scaled-up Reactor B used a feed gas of 100% CO2 with the aqueous catholyte and anolyte respectively [0.5 M KHCO3+2 M KC1 ] and 2 M KOH, at inlet pressure from 350 to 600 kPa(abs) and outlet temperature 295 to 325 K. For a superficial current density of 0.6 to 3.1 kA m−2 Reactor B achieved corresponding formate current efficiencies of 91 to 63%, with the same range of reactor voltage as that in Reactor A (2.7 to 4.3 V). Up to 1 M formate was obtained in the catholyte product from a single pass in Reactor B.
An electrochemical reactor as described in Example 1 was constructed and operated as follows:
With a cathode of fresh tin granules the formate current efficiency (CE) dropped from about 60% at 30 minutes to 50% at 250 minutes operating time. Recovery of the current efficiency was achieved by:
(i). Chemical treatment and recycle of the cathode: The used cathode tin granules were treated in 11 wt % nitric acid at room temperature for 2 minutes, washed in deionized water and re-used in the reactor. Table 1 shows that this treatment regained the cathode activity at 30 minutes operating time.
Similar results for cathode recovery were obtained by treating the used tin granules with hydrochloric acid and/or potassium hydroxide.
(ii). Polarity reversal: Under similar conditions to those above, with fresh tin granules the formate current efficiency dropped from 65% at 30 minutes to 48% at 360 minutes operating time. Polarity reversal was applied to the reactor for 5 minutes at 1 A. The formate current efficiency subsequently increased and was back to 65% at 400 minutes operating time.
An electrochemical reactor as described in Example 1 was constructed and operated as follows:
Table 2 shows the performance of this reactor.
A reactor was constructed as in Example 1, operation was as in Example 2, except the anolyte was replaced by an acid sodium sulphate solution as follows:
The reactor was operated over a current range from 1 to 14 A (0.2 to 3.1 kA/m2) with corresponding formate CE from 80 to 30% and reactor voltage from 3.5 to8.0 V.
This result shows that the process can be operated with an acid anolyte. The various ratios of Na+/H+ in the anolyte gave different formate current efficiencies, thus indicating that the formate CE could be improved by manipulating the anolyte composition.
In some embodiments, the invention may utilize ammonium cations, to produce ammonium formate. A reactor was constructed as in Example 1, operation was as in Example 4, except the catholyte potassium cations were replaced by ammonium and the anolyte was replaced by an acid ammonium sulphate solution, as follows:
This result demonstrates that the process can use exclusively ammonium cations in the catholyte. The ability to use ammonium cations is illustrated in Process Flowsheets B and C, for the production of formic acid/or ammonium formate.
An electrochemical reactor as described in Example 1 was constructed and operated as follows:
Operation of this reactor over a period from 2 to 6 hours showed a constant formate current efficiency of 31+/−1%.
The process of this Example is illustrated in
Based on the concept of
An aqueous solution of (NH4)2SO4 and H2SO4 recycles through the anode circuit, supplying NH4 + and H+ cations for transport to the catholyte via the cation membrane. The co-product O2 gas is generated with protons (H+) at the anode by reaction 4 and recovered from a gas/liquid separator (U9). The recycling acid anolyte is then divided (U10) to supply H2SO4 for the acidolysis reaction (U6) from which the spent reactant is re-combined with the anolyte (U8).
A material and energy (M&E) balance for Flowsheet B operating at steady-state is shown In the stream table below. This M&E balance is based on the assumption of a formate current efficiency of 80% and 80% conversion of CO2 per pass through the electrochemical reactor.
The primary and secondary net reactions in Flowsheet B are respectively reactions 10 and 11.
The conditions of this process may be chosen to promote the main net reaction 10. The characteristics of the process of this example, to promote reaction 10 may be selected as follows:
Operation of the process will typically depend on interactions among the conditions i to x listed above. Modeling of this embodiment provided a steady-state material and energy balance, on the basis of 105 tonne/day CO2, giving a current efficiency of 80% and CO2 conversion/pass of 80%. The material and energy balance stream table corresponding to Process flowsheet B is set out below, with the Table continued across the three sub-tables.
The feed plus recycle CO2 is compressed and delivered to the cathode of the electrochemical reactor along with the recycling catholyte, an aqueous solution of NH4HCO2 (e.g. >1 M) with minor amounts of NH4HCO2 (ammonium bicarbonate—e.g. 0.1 M). The cathode outlet stream goes to separation system that recovers a solution of NH4HCO2 plus the byproduct hydrogen and recycles the spent catholyte.
Ammonia (NH3 gas or aqueous solution) is fed to the anolyte circuit where it combines to form (NH4)2SO4 (ammonium sulphate). An aqueous solution of (NH4)2SO4 and H2SO4 then recycles through the anode circuit, supplying NH4 + and H+ cations for transport to the catholyte via the cation membrane. The co-product O2 gas is generated with protons (H+) at the anode by reaction 4 and recovered from a gas/liquid separator. The ratio [NH4 +]/[H+] is maintained in the anolyte to supply these species to the catholyte at rates that balance the stoichiometry of reactions 1 and 2 and produce a catholyte solution of predominantly ammonium formate at pH in the range about 4 to 8.
The primary and secondary net reactions in flowsheet C are respectively reactions 12 and 13.
Variations of this scheme may include for example replacement of (NH4)2SO4 and H2SO4 in the anolyte by (NH4)3PO4 and H3PO4or by NH4Cl and HCl. In the later case the anode co-product may be Cl2 by reaction 5. Anode co-products may also include peroxy-compounds such as ammonium persulphate (NH4)2S2O8 or persulphuric acid H2S2O8, etc. by reaction 14.
Kirk-Othmer—Encyclopedia of Chemical Technology. John Wiley, New York, 1991.
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Y. Akahori et al. “New electrochemical process for CO2 reduction to formic acid from combustion flue gases”. Denki Kagaku (Electrochemistry) 72(4) 266-270 (2004).
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Although various embodiments of the invention are disclosed herein, many adaptations and modifications may be made within the scope of the Invention in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the invention in order to achieve the same result in substantially the same way. Numeric ranges are inclusive of the numbers defining the range. The word “comprising” is used herein as an open-ended term, substantially equivalent to the phrase “including, but not limited to”, and the word “comprises” has a corresponding meaning. As used herein, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a thing” includes more than one such thing. Citation of references herein Is not an admission that such references are prior art to the present invention. Any priority document(s) and all publications, including but not limited to patents and patent applications, cited in this specification are incorporated herein by reference as if each individual publication were specifically and individually indicated to be incorporated by reference herein and as though fully set forth herein. The invention includes all embodiments and variations substantially as hereinbefore described and with reference to the examples and drawings.