Electrolysis of carbon dioxide to solid carbon

Abstract

An electrolytic process and an apparatus for producing solid carbon and gaseous oxygen from CO2. In one aspect the process comprises using a cathode that includes a solid, semi-solid or liquid metal containing a catalytically active material for CO2 reduction that includes less than or equal to 10 wt.% of at least one of copper or silver or gold. In another aspect the process comprises using a "spouted" bed electrolysis apparatus comprising a downwardly moving packed bed of cathode particles that include a catalyst, such as copper, for CO2 reduction and an upward flow of electrolyte.

Description

[0001] ELECTROLYSIS OF CARBON DIOXIDE TO SOLID CARBON

[0002] TECHNICAL FIELD

[0003] The present invention relates to an electrolysis process and apparatus for producing solid carbon and gaseous oxygen from carbon dioxide using a solid, semi-solid or liquid metal cathode.

[0004] The present invention relates particularly, although by no means exclusively, to an electrolysis process and apparatus for producing solid carbon and gaseous oxygen from carbon dioxide using a solid, semi-solid or liquid metal cathode that contains at least one Group IB transition metal (copper, silver and gold) as active catalyst for CO2 reduction to solid carbon at an operating temperature that is below 120 °C.

[0005] The present invention also relates to the use of “spouted bed” electrolyser in an electrolysis process and apparatus for producing solid carbon and gaseous oxygen from carbon dioxide.

[0006] BACKGROUND

[0007] Climate change is driving a fundamental re-evaluation of future options across a broad spectrum of energy-intensive industries.

[0008] In addition to finding ways of preventing fossil-derived carbon dioxide generation and release, there is increasing pressure to find cost-effective ways to actively remove CO2from the atmosphere.

[0009] There is much that can (and is) being done using plants, trees, soil microbiology, ocean organisms etc to lock up additional atmospheric CO2in biological structures, but much more needs to be done.

[0010] Processes such as water electrolysis to produce green hydrogen using renewable energy are regarded as vital for the global energy transition away from fossil fuels. Although this process will no doubt contribute substantially, it is not without issues related to hydrogen storage and potentially high cost.

[0011] A parallel concept using electrolysis of CO2 instead of water to produce solid carbon is attractive in theory, because it makes the energy storage issue far easier (since it involves solid carbon in place of hydrogen).

[0012] The concept of an electrolysis cell for CO2, similar in principle to an electrolysis cell for making hydrogen from water, is not new.

[0013] A large body of information exists relating to the production of either formic acid or synthesis gas (CO) via electrolysis of CO2 (1-3). Whilst the production of synthetic fuels and chemicals via this route will no doubt contribute, this does not represent a scalable solution to the overall problem. In particular, the demand for synthetic chemicals is likely to dominate how much can be produced. The synthetic fuel route is more open in terms of demand-side tonnage, but CO2 will still ultimately be released back into the atmosphere. This approach, when based on fossil carbon dioxide, is one of “re-purposing” CO2 to use it a second time and delay its release, rather than actually dealing with the core problem. In processes of this type, the selectivity of carbon in CO2 converted to solid carbon is very low (if not actually zero).

[0014] There is currently no commercially-operating technology for electrolysing CO2 directly to solid carbon and gaseous oxygen that can operate with high selectivity to solid carbon as defined herein.

[0015] One potential reason for this is a long-standing problem of carbon fouling of cathodes of the electrolysis apparatus. Since the product of this electrolysis reaction is a solid, it is clear that electrode fouling and product removal need to be handled in a practical, cost-effective manner.

[0016] International application PCT / AU2021 / 051416 (W02022120411A1) in the name of the applicant describe a “Carbelec” process being developed by the applicant. The disclosure in the patent specification of the International application are incorporated herein by cross-reference.

[0017] The “Carbelec” process described in the International application is an electrolysis process and apparatus for producing solid carbon and gaseous oxygen from carbon dioxide using a liquid metal cathode.

[0018] An embodiment of the “Carbelec” process utilises a liquid metal cathode containing typically 1-3% of a suitable active catalyst such as Ce (cerium) for reduction of CO2 to solid carbon. The electrolyte may be aqueous or organic, typically (a) an aqueous alkali carbonate or (b) an aqueous amine-based system.

[0019] The “Carbelec” process includes management of solid carbon removal from the liquid metal cathode surface in a way that allows continuous operation of the process.

[0020] The term “solid carbon” in the context of the Carbelec process is understood herein to refer to carbon in a solid state that may contain some residual oxygen (generally < 20%, typically < 15%, by weight on a dry basis). This oxygen comes predominantly from the original carbon dioxide from which the solid carbon was produced.

[0021] The term “liquid metal” in the context of the Carbelec process is understood herein to refer to any metal-containing substance that is at least partially liquid at a selected operating temperature of the process, is electrically conductive, and is capable of catalysing reduction of CO2 to solid carbon under the operating conditions of the process.

[0022] The metal-containing substance may be a single metal, an alloy or a metal containing additives.

[0023] The term “liquid metal” extends to metal-containing substances that include a nonliquid component. The non-liquid component may be in any suitable form. For example, the metal-containing substance may be in particulate form. The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere .

[0024] SUMMARY

[0025] The applicant has explored multiple different combinations of electrolytes, cathode materials and catalysts for conversion of CO2 to solid carbon.

[0026] The work carried out by the applicant has been generally conducted on a laboratory scale with appropriate electrode and electrolyte combinations.

[0027] The applicant has found that, to date, Ce has been generally recognised in the technical literature as being the catalyst of choice for liquid metal cathodes. Ce is typically present at 1-3% by weight in liquid metal (typically a Galinstan alloy) forming the cathode and is known to be present as a combination of (i) Ce dissolved in metal and (ii) Ce-oxide nano-particles. Much of the early work was done with Ce (4) and its activity for CO2 reduction to solid carbon has been confirmed in current tests.

[0028] The following comments focus on two technical areas considered by the applicant.

[0029] These technical areas are cathode selection and solid carbon stickiness.

[0030] 1. Cathode selection

[0031] When moving from an organic electrolyte to an aqueous one containing alkali carbonates, it was noted by the applicant that some of the Ce in a liquid metal cathode, such as a cerium-containing liquid Galinstan alloy cathode, dissolved into the electrolyte.

[0032] The applicant considered this to be a significant problem, given potential issues associated with Ce recovery on a commercial plant.

[0033] Accordingly, the applicant investigated potential candidates other than Ce for providing catalytic activity in liquid metal cathodes. Other catalyst metal candidates tested (with a liquid metal cathode) included Fe, Zn, Ti and Cu.

[0034] The applicant also investigated solids for use as cathodes, as alternatives to liquid metal cathodes. Solid cathode metals including steel, copper and pure carbon were tested.

[0035] Cu was the only metal that showed any significant catalytic activity in test work with liquid metal cathodes and did not dissolve into or otherwise react with the electrolyte.

[0036] The performance of Cu was better than that of Ce: current densities around lOx higher were recorded, together with significantly lower initiation voltages for the CO2 reduction reaction.

[0037] Laboratory test work was then conducted to further verify that Cu was responsible for catalysing the reaction of CO2 to solid carbon.

[0038] The laboratory test work included mixing together gallium and 3 wt.% Cu and heating the mixture for approximately 1.5 hrs at approximately 40 °C and forming a liquid gallium-copper cathode. The liquid Ga-Cu cathode was immersed in a 0.1 M KHCO3 electrolyte solution that was saturated with CO2. An anode was positioned in contact with the electrolyte. A potential of 1.2 V was applied across the anode and the liquid Ga-Cu cathode for 15 hours. At the end of this time period, it was observed that a black deposit had formed on the Ga-Cu cathode. SEM and EDS analysis found that the deposit was mainly solid carbon.

[0039] This laboratory work established Cu as being a suitable catalytic alternative to Ce.

[0040] The applicant expects that “sister elements” of copper (such as silver and gold) will also be active for CO2 reduction in a similar way. This expectation is based on the knowledge of the applicant in the field of the invention.

[0041] It was also demonstrated by the applicant that Cu is capable of reducing CO2 to carbon in laboratory test work with Cu as a solid cathode, thereby avoiding the need for a liquid metal cathode.

[0042] In the laboratory test work, a Cu plate cathode was immersed in a 2M KHCO3 electrolyte that was saturated with CO2. An anode was positioned in contact with the electrolyte. An electrical potential was applied across an anode in contact with the electrolyte and the Cu cathode. After a reaction period, it was observed that a black deposit had formed on the Cu cathode. SEM and EDS analysis found that the deposit was mainly solid carbon.

[0043] The applicant has realised that “spouted” bed electrolysers that have been investigated in relation to the copper industry (5) could be used for electrolysis of solid carbon and gaseous oxygen.

[0044] The applicant has recognised that such devices potentially offer 2-3 orders of magnitude increase in active (cathode) surface area.

[0045] The applicant believes that it is possible that, even with low inherent rates of CO2 reduction, spouted beds using (for example) surface-treated copper beads as a solid cathode with appropriate carbon removal could offer an alternative commercialisation pathway.

[0046] The use of spouted beds proposed by the applicant includes in one embodiment a downwardly-moving packed bed of solid cathode particles, such as Cu particles (which can be described as “beads”), with solid cathode particles being formed from copper and touching one another, thereby conducting electricity and allowing the whole packed bed to achieve more or less the same electrical potential, and a counter-current flow of electrolyte containing CO2, with the CO2 being converted to solid carbon and oxygen gas and the solid carbon being deposited on the cathode particles.

[0047] An alternative approach includes the use of a liquid-solid homogeneous fluidized bed.

[0048] In this case, solid cathode particles, such as Cu particles, are in motion and each one is generally surrounded by a liquid electrolyte. However, they are still in relatively close proximity to one another, and the electrical potential of the average particle may not be too much different to that of particles in a spouted bed, especially if the liquid electrolyte has a high electrical conductivity.

[0049] An alternative approach includes the use of a multi-pronged cathode to minimise the spread in fluidized particle electrical potential.

[0050] A possible advantage of this fluidized bed variation is that smaller solid particles (with higher external surface area) can be used, whilst maintaining a sufficiently high supply rate of CO2 in freshly loaded electrolyte (as electrolyte fluidization medium).

[0051] 2. Solid carbon stickiness

[0052] Laboratory test work with liquid metal cathodes carried out by the applicant has also found that the solid carbon product produced via electrolysis can stick quite strongly to surfaces of liquid metal cathodes based on gallium or Galinstan alloy when used in conjunction with aqueous electrolytes.

[0053] This is an issue because a Carbelec electrolyser needs to be capable of easy and controllable solid carbon removal form liquid metal cathodes.

[0054] Although the mechanism is not entirely clear, it is known that gallium forms a thin oxide layer when in contact with air or water in much the same way that aluminium forms a protective alumina layer (6). It is thought that this type of oxide layer plays a significant role in the carbon sticking problem.

[0055] Carbon sticking behaviour is generally expected to be a function of electronegativity and surface properties of the host cathode metal.

[0056] The applicant considered cathode options that had minimal stickiness.

[0057] In further laboratory test work, a mercury cathode was used in conjunction with 3 wt.% copper catalyst.

[0058] The mercury-copper cathode was immersed in a 0.1 M KHCO3 electrolyte that was saturated with CO2. An anode was positioned in contact with the electrolyte. A potential of 1.2 V was applied across the anode and the mercury-copper cathode for 15 hours. At the end of a reaction time period, it was observed that a black deposit had formed on the mercury-copper cathode. SEM and EDS analysis found that the deposit was mainly solid carbon.

[0059] Mercury has a higher electronegativity than that of gallium and is not known to form the same type of oxide surface layer.

[0060] This test work showed that carbon sticking on the liquid mercury-copper cathode surface was dramatically different - far lower and potentially compatible with the type of electrolyte-swept electrolyser described in the above-mentioned earlier Carbelec patent application.

[0061] In terms of the oxidation state of the catalyst, such as copper, a redox cycle was expected by the applicant. This means the catalyst would typically be in a low oxidation state at the cathode potential, and then interact with CO2 to take up oxygen and release solid carbon.

[0062] The catalyst, at that point, would effectively be in a higher oxidation state. It would subsequently undergo reduction at the cathode, reverting to its original lower state of oxidation whilst releasing its oxygen into the electrolyte in a suitable ionic form.

[0063] This means that the catalyst would exist at least partly in an oxidized form (oxide) at the active interface with the electrolyte.

[0064] It may also be that in some embodiments some surface-oxidized catalyst is necessary for the process to commence. Either way, the presence of at least some oxidized catalyst at the active interface is considered normal (perhaps even preferable) in this process. work

[0065] In summary, the laboratory test work of the applicant found that:

[0066] (a) copper (silver and gold) is an alternative to cerium as a catalyst for CO2 reduction to solid carbon in a liquid metal cathode, with better technical performance and lower cost;

[0067] (b) copper is a viable option for a solid cathode, for example in a “spouted” bed; electrolyser;

[0068] (c) a cathode comprising mercury with a copper catalyst is less subject to solid carbon attachment than liquid metal cathodes based on gallium or Galinstan alloy; and

[0069] (d) a surface of the mercury cathode with copper catalyst could be semi-solid or totally solid and still perform in the same way.

[0070] The following description refers to an “active cathode surface”. The term is understood herein to mean a surface that, in use, contacts an electrolyte and that includes a catalytically active material.

[0071] The “surface” may have depth and, for example, may be in the form of a film.

[0072] Catalyst concentrations in a bulk cathode (e.g. a liquid metal) and at an active cathode surface with an electrolyte may be significantly different.

[0073] For example, there may be a relatively small bulk percentage of copper in a Galinstan alloy liquid metal cathode (e.g. 0.1 wt.%) but this copper may be concentrated in the cathode active surface in such a way as to present a much higher local percentage (e.g. > 5 wt.%) to the electrolyte.

[0074] This is especially true if the cathode is semi-solid or solid, and the catalytic material has been applied as a surface coating or very thin surface film. Surface concentration is considered to be more important of the two, so in this context the quantity of a catalyst required for the process will be defined in terms of the resulting percentage of the active cathode surface that comprises the catalyst.

[0075] With the above in mind, one aspect of the invention is a process for producing solid carbon and gaseous oxygen from CO2 via electrolysis, for example as generally as described in International application PCT / AU2021 / 051416 (W02022120411A1) in the name of the applicant.

[0076] In this aspect, the process for producing solid carbon and gaseous oxygen from CO2 via electrolysis includes using an electrolysis apparatus having a chamber with an electrolyte inlet, an electrolyte outlet, a liquid electrolyte containing CO2 in the chamber, and at least one cathode-anode pair, with the cathode including a solid, semisolid or liquid metal containing a catalytically active material for CO2 reduction that includes less than or equal to 10 wt.% of at least one of copper or silver or gold.

[0077] Typically, the amount of the copper or silver or gold is selected to be the minimum possible to provide a material impact on reduction of CO2 to solid carbon. A skilled person will be able to assess what is “material” in any given situation.

[0078] Typically, the process comprises supplying the electrolyte to the chamber via the inlet and discharging the electrolyte from the chamber via the outlet with the electrolyte flowing from the inlet to the outlet in fluid communication with the cathode- anode pair, applying a voltage between the cathode-anode pair and causing solid carbon to form on the cathode from CO2 in the electrolyte and gaseous oxygen to be evolved at the anode from CO2 in the electrolyte, and discharging solid carbon by transporting solid carbon from the cathode in the electrolyte to the electrolyte outlet and from the chamber via the outlet, and discharging gaseous oxygen from the chamber.

[0079] The copper or silver or gold may be less than or equal to 8 wt.% of the catalytically active material. The copper or silver or gold may be less than or equal to 6 wt.% of the catalytically active material.

[0080] The copper or silver or gold may be less than or equal to 5 wt.% of the catalytically active material.

[0081] The copper or silver or gold may be less than or equal to 4 wt.% of the catalytically active material.

[0082] The copper or silver or gold may be less than or equal to 3 wt.% of the catalytically active material.

[0083] The copper or silver or gold may be at least 0.5 wt.% of the catalytically active material.

[0084] The copper or silver or gold may be at least 1 wt.% of the catalytically active material.

[0085] The catalytically active material may comprise a liquid metal and one or more of (i) copper or silver or gold dissolved in the liquid metal and (ii) copper or silver or gold nano-particles or micro-granules in the liquid metal.

[0086] The liquid metal may be any suitable liquid metal.

[0087] Gallium is one example of a suitable liquid metal.

[0088] Mercury is another example of a suitable liquid metal.

[0089] The catalytically active material may comprise gallium and at least one of copper or silver or gold.

[0090] The process may include operating at a temperature below 120°C.

[0091] The process may include agitating the electrolyte via mechanical, ultrasonic or other means to promote removal of solid carbon from the cathode.

[0092] The process may also include periodic, short-term voltage reversal to promote carbon release from the cathode into the electrolyte.

[0093] The electrolyte may include at least 30% water.

[0094] The electrolyte may include at least one of amines and alkali carbonates.

[0095] The process may include operating at any suitable voltage between the cathode-anode pair.

[0096] The applied voltage between the cathode-anode pair in the electrolysis stage may be below that for splitting water into hydrogen and oxygen.

[0097] The process may include separating solid carbon from the electrolyte discharged from the electrolyte outlet and returning the electrolyte to the chamber via the electrolyte inlet.

[0098] The process may include regenerating the electrolyte by adding CO2 to the electrolyte before returning the electrolyte to the chamber via the electrolyte inlet.

[0099] The process may include agitating the liquid metal via mechanical, ultrasonic or other means to promote removal of solid carbon from the cathode.

[0100] Another aspect of the invention provides an electrolysis apparatus for producing solid carbon and gaseous oxygen from CO2 via electrolysis, for example as generally as described in International application PCT / AU2021 / 051416 (W02022120411A1).

[0101] In this aspect, the apparatus includes a chamber with an electrolyte inlet, an electrolyte outlet, a liquid electrolyte containing CO2 in the chamber, and at least one cathodeanode pair, a solid, semi-solid or liquid metal cathode containing a catalytically active material for CO2 reduction that includes less than or equal to 10 wt.% of at least one of copper or silver or gold.

[0102] The selection of the cathode and the catalytically active material of the cathode in any given situation depends on a range of factors including but not limited to the operating parameters of the process, such as pressure, voltage, electrolyte composition and temperature.

[0103] The cathode may be any metal-containing substance that is solid, partially liquid or fully liquid at a selected operating temperature of the process and, with copper or silver and / or gold catalyst added as described herein, is capable of catalysing reduction of CO2 to solid carbon, typically with high selectivity, and typically converting at least 90 wt.% of carbon in CO2 to solid carbon, under the operating conditions of the process.

[0104] The term “selectivity” is understood herein to mean the percentage of carbon present in CO2 that is supplied to the electrolysis process that ends up as solid carbon, such as a solid particulate form, as opposed to carbon that reports gaseous or as liquid products such as CO and formic acid.

[0105] The cathode may include an active cathode surface.

[0106] The active cathode surface may comprise at least one of copper or silver or gold on or in the surface.

[0107] The cathode may comprise a substrate and the active cathode surface on the substrate.

[0108] The active cathode surface may comprise a thin film of a liquid or semi-liquid metal.

[0109] The active cathode surface may comprise a solid metal with an intermetallic bonding of two or more metals in a thin surface film, where at least one of the metals is copper or silver or gold. The cathode may include mercury as a liquid, as a thin semi-solid film or as a bonded intermetallic component.

[0110] As noted above, mercury has been found to be effective in minimising attachment of solid carbon to the cathode.

[0111] The copper or silver or gold may be less than or equal to 8 wt.% of the catalytically active material.

[0112] The copper or silver or gold may be less than or equal to 6 wt.% of the catalytically active material.

[0113] The copper or silver or gold may be less than or equal to 5 wt.% of the catalytically active material.

[0114] The copper or silver or gold may be less than or equal to 4 wt.% of the catalytically active material.

[0115] The copper or silver or gold may be less than or equal to 3 wt.% of the catalytically active material.

[0116] The copper or silver or gold may be at least 0.5 wt.% of the catalytically active material.

[0117] The copper or silver or gold may be at least 1 wt.% of the catalytically active material.

[0118] The active cathode surface may comprise a liquid metal and one or more of (i) copper or silver or gold dissolved in the liquid metal and (ii) copper or silver or gold nanoparticles or micro-granules in the liquid metal.

[0119] The liquid metal may be any suitable liquid metal. Gallium is one example of a suitable liquid metal.

[0120] Mercury is another example of a suitable liquid metal.

[0121] The active cathode surface may comprise gallium and at least one of copper or silver or gold.

[0122] The anode may be in any suitable form.

[0123] Typically, the profile of the anode is complementary to that of the cathode to maintain the spacing between facing anode and cathode surfaces at least substantially constant.

[0124] The anode may be a plate, with or without apertures.

[0125] The anode may also be a mesh.

[0126] Another aspect of the invention is a process for producing solid carbon and gaseous oxygen from CO2 via electrolysis in a “spouted” bed electrolysis apparatus, the process including supplying an electrolyte containing CO2 to a moving packed bed of cathode particles (which may also be described as “beads”) that include a catalyst for CO2 reduction, such as copper, in a counter-current direction to a direction of movement of the packed bed, applying an electrical potential across an anode and the cathode particles and forming solid carbon on the cathode particles and oxygen gas from CO2, also supplying the electrolyte to a draft tube located in the packed bed so that there is an upward flow of the electrolyte and entrained cathode particles with deposited carbon, with the upward flow detaching at least a part of the carbon from the cathode particles, depositing cathode particles that are carried from an upper end of the draft tube on top of the downwardly-moving packed, discharging the electrolyte and detached carbon and oxygen gas that flows from the upper end of the draft tube, and recovering carbon from the electrolyte,

[0127] Another aspect of the invention is a “spouted” bed electrolysis apparatus that includes a chamber that contains a cathode in the form of a packed bed of cathode particles (which may also be described as “beads”) that include a catalyst for CO2 reduction, such as copper, a draft tube extending substantially the height of the chamber, an inlet for electrolyte containing CO2 in a lower region, and outlet for electrolyte and oxygen gas in an upper region .

[0128] The cathode particles may include a coating of a material, such as mercury, that minimises solid carbon attachment.

[0129] BRIEF DESCRIPTION OF THE DRAWINGS

[0130] The present invention is described further by way of two examples with reference to the accompanying drawings, of which:

[0131] Figure 1 is a schematic diagram of an embodiment of the electrolysis apparatus of the invention that includes an electrolysis cell with single plate-based electrode pair (Embodiment A); and

[0132] Figure 2 is a schematic diagram of an embodiment of the electrolysis apparatus of the invention that includes a spouted bed electrode (Embodiment B).

[0133] DESCRIPTION OF EMBODIMENTS

[0134] Figure 1 is a schematic diagram of Embodiment A of a process and an apparatus based on flat plate design for electrolysis of carbon dioxide into solid carbon and gaseous oxygen according to the invention.

[0135] With reference to Figure 1, the apparatus includes an electrolysis chamber 101 and a CO2saturator 102, both of which are maintained at an operating temperature of 70-90 °C.

[0136] Carbon dioxide 103 is dissolved in the electrolyte in saturator 102 via a venturi aspirator which forms part of saturator 102.

[0137] The electrolyte is water containing potassium carbonate salts. The electrolyte may be any suitable electrolyte. Loaded electrolyte 105 containing substantial dissolved carbon dioxide is fed into electrolysis chamber 101 via electrolyte pump 104.

[0138] A cathode 106 and an anode 107 are located in chamber 101.

[0139] There is an oxygen-rich gas space 108 above the cathode 106 and the anode 107.

[0140] Anode 107 contains perforations and / or apertures such that oxygen gas bubbles can escape upwards into gas space 108.

[0141] Cathode 106 comprises a large horizontal metallic solid plate. On an upper surface of this plate is an active cathode surface 109 comprising a thin film comprising mercury with 10 wt.% copper, with the copper being partially dissolved copper and nanoparticles or micro-granules of copper in the mercury. The active cathode surface 109 includes a catalytically active material.

[0142] The mercury film is supported on cathode plate 106 in such a way that is behaves essentially as a static layer, with a “wet” or semi-solid appearance on its upper surface.

[0143] Anode 107 comprises a parallel mesh or perforated flat plate set 30-80 mm above cathode active surface 109.

[0144] A power supply 110 is connected to the cathode-anode pair to maintain a voltage difference in the range 1 to 10 volts, typically 1-4 volts, more typically 1.4-2 volts.

[0145] In use, CO2 loaded electrolyte 105 is pumped from left to right as shown, at a superficial liquid velocity in a gap between the cathode active surface 109 and the bottom of anode 107 in a range 0.1-1 m / s.

[0146] There is electrolysis of CO2 in the electrolyte, with solid carbon being formed as carbon flakes 112 on the active surface 109 of the cathode 106 and gaseous oxygen being evolved. Carbon flakes 112 detach from cathode active surface 109 and are transported to the right by electrolyte convection.

[0147] As they leave the cathode-anode gap (at right hand extreme), residual oxygen bubbles rise into gas space 108 and from there pass through demister 113 where any residual electrolyte is removed and returned to cell 101.

[0148] Final oxygen product 114 is removed for compression and re-use or else vented. In this process carbon in reacted CO2 is converted into carbon flakes 112 (or other forms of particulate carbon) with a selectivity of >98%.

[0149] Electrolyte containing these carbon flakes 115 enters separation unit 116 where solid carbon is removed. Carbon product 117 is collected and removed for storage or re-use.

[0150] Figure 2 is a schematic diagram of Embodiment B of a process and an apparatus based on a spouted bed design for electrolysis of carbon dioxide into solid carbon and gaseous oxygen according to the invention.

[0151] With reference to Figure 2, the apparatus includes spouted bed electrolysis chamber 201 which is shown as a vertical cross-section (in the Figure on the left) and a front view (in the Figure on the right). This chamber contains draft tube 202 positioned within a moving packed bed 203 of particles (which may also be described as beads) in an outer region of the spouted bed chamber 201.

[0152] The draft tube 202 is located centrally in the chamber 201. The draft tube 202 extends form a floor of the chamber 201 and extends substantially the height of the chamber 201.

[0153] The chamber 201 includes an upper volume.

[0154] The chamber 201 includes an inlet in the floor to supply electrolyte into the chamber 201. Particles in the bed 203 are 0.5 mm diameter copper beads (but may be made from any other suitable material) which form a cathode.

[0155] The copper beads are surface-treated with mercury (or another suitable metal) to promote easy carbon detachment. Surface copper on the particle surface is typically 40 wt.%.

[0156] In use, electrolyte moves upwardly through the draft tube 202 from the inlet in the floor of the chamber.

[0157] The electrolyte flows outwardly from an upper end of the draft tube 202 and leaves the chamber 201 via an outlet.

[0158] The copper beads form a downward-moving packed bed 203 in the region outside the draft tube 202, with counter current upward movement of electrolyte in the interstitial spaces between them.

[0159] There is strong upward flow of electrolyte in the draft tube 202, sufficient to entrain particles in the vicinity of the bottom opening and convey them upwards with the electrolyte, ultimately depositing them on top of the outer downward-moving packed bed section.

[0160] This creates a circulatory motion of copper beads (fast up the draft tube 202 and slowly back down in the moving packed bed outer region) which defines the device as a spouted bed.

[0161] The electrolyte is water containing potassium carbonate salts in this embodiment.

[0162] The copper beads in the outer packed bed region are physically touching one another and can conduct electric current between them with only minor electrical resistance.

[0163] This moving packed bed of copper beads is bounded on one side by mesh 204 which allows only electrolyte to pass through its apertures, and on the other side by cathode plate 205 which establishes electrical contact with the bed.

[0164] Copper beads within moving packed bed 203 are therefore electrically connected to the cathode potential. Anode plate 206 completes the electrical circuit.

[0165] Dissolved CO2 feed in the form of loaded electrolyte 207 enters the bottom of the chamber 201 and CO2 is converted into solid carbon on the surface of copper beads in zone 203.

[0166] Electrically charged hydroxyl ions in the electrolyte migrate through mesh screen 204 to anode plate 206 and here gaseous oxygen bubbles 208 are liberated.

[0167] A key attribute of the spouted bed is that total cathode surface (which is effectively the total surface area of the particles in the moving packed bed) is around an order of magnitude higher than would be possible with any type of flat-plate design electrolysis apparatus.

[0168] Carbon product from the electrolysis reaction is deposited in particulate form on the surface of copper beads.

[0169] Selectivity of carbon in reacted CO2 to solid carbon as defined herein is >98%.

[0170] When these particles are accelerated up into the draft tube, this carbon generally detaches and is swept out in depleted electrolyte stream 209.

[0171] If this carbon detachment mechanism alone is insufficient to remove carbon product from particle surfaces, then additional measures (not shown) such as localised ultrasonic stirring, localised voltage reversal and / or addition of a specialised sidechamber with sustained high-shear fluidisation conditions may be used.

[0172] Depleted electrolyte 209 containing small carbon particles / flakes enters setting chamber 210 where liquid and gas separate, allowing oxygen to accumulate in gas space 211 prior to being cleaned in demister 212 and released as oxygen product 213.

[0173] De-gassed, depleted electrolyte containing carbon particles / flakes 214 enters carbon separation device 215 where carbon product 216 is removed. Device 215 may be a filter, a centrifuge or any other suitable solid-liquid separation device.

[0174] Clear depleted electrolyte 217 is re-charged with CO2 218 in saturator vessel 219 before being pumped back to spouted bed 201 to complete the electrolyte circuit.

[0175] Many modifications may be made to the embodiments described above without departing from the spirit and scope of the invention.

[0176] By way of example, further embodiments are possible, such as along the lines generally described Australian International application PCT / AU2021 / 051416 (W02022120411A1).

[0177] References

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Claims

CLAIMS1. A process for producing solid carbon and gaseous oxygen from CO2 via electrolysis using an electrolysis apparatus having a chamber with an electrolyte inlet, an electrolyte outlet, a liquid electrolyte containing CO2 in the chamber, and at least one cathode- anode pair, with the cathode including a solid or semisolid or liquid metal containing a catalytically active material for CO2 reduction that includes less than or equal to 10 wt.% of at least one of copper, silver or gold.

2. The process defines in claim 1 comprises supplying the electrolyte to the chamber via the inlet and discharging the electrolyte from the chamber via the outlet with the electrolyte flowing from the inlet to the outlet in fluid communication with the cathode-anode pair, applying a voltage between the cathode-anode pair and causing solid carbon to form on the cathode from CO2 in the electrolyte and gaseous oxygen to be evolved at the anode from CO2 in the electrolyte, and discharging solid carbon by transporting solid carbon from the cathode in the electrolyte to the electrolyte outlet and from the chamber via the outlet, and discharging gaseous oxygen from the chamber.

3. The process defined in claim 1 or claim 2 includes operating at a temperature below 120°C.

4. The process defined in any one of the preceding claims includes agitating the electrolyte via mechanical, ultrasonic or other means to promote removal of solid carbon from the cathode.

5. The process defined in any one of the preceding claims includes periodic, shortterm voltage reversal as a means to promote carbon release from the cathode into the electrolyte.

6. The process defined in any one of the preceding claims includes separating solid carbon from the electrolyte discharged from the electrolyte outlet and returning the electrolyte to the chamber via the electrolyte inlet.

7. The process defined in claim 6 includes regenerating the electrolyte by adding CO2 to the electrolyte before returning the electrolyte to the chamber via the electrolyte inlet.

8. An electrolysis apparatus for producing solid carbon and gaseous oxygen from CO2 via electrolysis includes a chamber with an electrolyte inlet, an electrolyte outlet, a liquid electrolyte containing CO2 in the chamber, and at least one cathode-anode pair, a solid, semi-solid or liquid metal cathode containing a catalytically active material for CO2 reduction that includes less than or equal to 10 wt.% of at least one of copper or silver or gold.

9. The apparatus defined in claim 8 wherein the cathode includes an active cathode surface.

10. The apparatus defined in claim 9 wherein the active cathode surface comprises at least one of copper, silver or gold on or in the surface.

11. The apparatus defined in claim 9 or claim 10 wherein the cathode comprises a substrate and the active cathode surface on the substrate.

12. The apparatus defined in any one of claims 9 to 11 wherein the active cathode surface comprises a thin film of a liquid or semi-liquid metal.

13. The apparatus defined in any one of claims 9 to 12 wherein the active cathode surface comprises a solid metal with an intermetallic bonding of two or more metals in a thin surface film, where at least one of the metals is copper, silver or gold.

14. The apparatus defined in any one of claims 8 to 13 wherein the cathode includes mercury as a liquid or a thin semi-solid film or a bonded intermetallic component.

15. The apparatus defined in any one of claims 8 to 14 wherein the electrolyteincludes at least 30% water and at least one of amines and alkali carbonates.

16. A process for producing solid carbon and gaseous oxygen from CO2 via electrolysis in a “spouted” bed electrolysis apparatus, the process including supplying an electrolyte containing CO2 to a moving packed bed of cathode particles (which may also be described as “beads”) that include a catalyst for CO2 reduction, such as copper, in a counter-current direction to a direction of movement of the packed bed, applying an electrical potential across an anode and the cathode particles and forming solid carbon on the cathode particles and oxygen gas from CO2, also supplying the electrolyte to a draft tube located in the packed bed so that there is an upward flow of the electrolyte and entrained cathode particles with deposited carbon, with the upward flow detaching at least a part of the carbon from the cathode particles, depositing cathode particles that are carried from an upper end of the draft tube on top of the downward-moving packed, discharging the electrolyte and detached carbon and oxygen gas that flows from the upper end of the draft tube, and recovering carbon from the electrolyte.

17. A “spouted” bed electrolysis apparatus that includes a chamber that contains a cathode in the form of a packed bed of cathode particles (which may also be described as “beads”) that include a catalyst for CO2 reduction, such as copper, a draft tube extending substantially the height of the chamber, an inlet for electrolyte containing CO2 in a lower region, and outlet for electrolyte and oxygen gas in an upper region.

18. The apparatus defined in claim 17 wherein the cathode particles include a coating of a material, such as mercury, that minimises solid carbon attachment.

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