TECHNICAL FIELD

This invention relates to electrolysis. In particular, though not exclusively, this invention relates to an electrolyser for producing hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of greater than one, and to a method of producing hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of greater than one.

BACKGROUND

Fossil fuels are the most important source of energy for transport vehicles. The most common primary source is petroleum oil, which is refined to produce fractions such as petrol, paraffin and diesel. These are being consumed at an increasing rate, with the result that it is estimated that the finite reserves will last for several decades at most.

One solution to this problem is to synthesise fuels such as methanol. This can be achieved using electrolysis to generate hydrogen gas and carbon dioxide gas, which can then be reacted together using a catalyst to provide methanol (and water as a by-product).

However, currently available electrolysers are only capable of producing hydrogen gas and carbon dioxide gas in a H2:CO2 ratio of 1 : 1. In order to synthesise methanol, hydrogen gas and carbon dioxide gas are required in a H2:CC>2 ratio of 3: 1.

Thus, at present a second type of known electrolyser is required to provide the remainder of the hydrogen required in order to synthesise these fuels. When this requirement is translated into industrial-sized plants, and bearing in mind the cost of electrolysers, the additional capital cost of the second electrolyser is very significant.

It is an object of the invention to address at least one of the above problems, or another problem associated with the prior art.

SUMMARY OF THE INVENTION

A first aspect of the invention provides an electrolyser for producing hydrogen gas and carbon dioxide gas in a H2:CO2 ratio of greater than one. The electrolyser comprises an anodic compartment comprising an aqueous solution of acid, a cathodic compartment, and a middle compartment arranged therebetween. The cathodic and middle compartments comprise an aqueous solution of first and second electrolytes. The first electrolyte comprises a metal carbonate. The anodic and cathodic compartments are separated from the middle compartment by first and second semi-permeable membranes. In this way, when a voltage is applied between the anodic and cathodic compartments, oxygen gas may be produced in the anodic compartment, carbon dioxide gas may be produced in the middle compartment, and hydrogen gas may be produced in the cathodic compartment. The presence of the second electrolyte in the middle and cathodic compartments may advantageously increase the conductivity of the aqueous solution in the middle and/or cathodic compartments. This may conveniently lower the voltage required for electrolysis to occur (i.e. the voltage needed to decompose the carbonate) and so minimise the electrical energy required by the process.

Moreover, the presence of the second electrolyte in the middle compartment may advantageously allow the carbonate to be decomposed (to form H2:CC>2 in a 1: 1 ratio) by part of the available current without having any serious effect on the conductivity that might require a large use of electricity (i.e. an unduly high voltage), whilst leaving a proportion of the available current to be used to decompose some of the water in the cathodic compartment to make more hydrogen, thereby enabling hydrogen gas and carbon dioxide gas to be produced in a H2:CC>2 ratio of greater than one.

This advantageously removes the need to have a second electrolyser to provide the remainder of the hydrogen required for the synthesis of methanol, thereby avoiding the additional capital costs associated with the running of a second electrolyser.

The electrolyser may advantageously be suitable for producing hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of at least 3: 1. By way of example, if conditions are arranged such that one third of the current is used to decompose all of the carbonate present and form hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of 1: 1 (e.g. providing one third of the amount of hydrogen required for methanol production), then two thirds of the maximum current available could be used to make two more volumes of hydrogen, to produce hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of 3: 1 for methanol production.

Moreover, the electrolyser may advantageously be suitable for producing hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of greater than 3: 1. For, example, conditions may be arranged such that less than one third of the current is used to decompose all of the carbonate present and form hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of 1: 1, such that more of the maximum current is available to make further volumes of hydrogen, to form hydrogen gas and carbon dioxide gas in H2:CC>2 ratio of greater than 3: 1.

The H2:CC>2 ratio may suitably refer to the volume ratio and/or the molar ratio. The anodic compartment may suitably comprise an electrode (i.e. an anode). For example, the electrode in the anodic compartment may be the electrode at which oxidation occurs.

The cathodic compartment may suitably comprise an electrode (i.e. a cathode). For example, the electrode in the cathodic compartment may be the electrode at which reduction occurs.

In some embodiments, the first electrolyte may comprise a metal carbonate. The term "carbonate" as defined herein refers to the COs ² ' (carbonate) ion, as well as to the conjugate acid thereof, i.e. the HCC (bicarbonate or hydrogen carbonate) ion. Suitably, the metal carbonate may comprise an alkali metal carbonate. For example, the metal carbonate may comprise one or more of lithium carbonate, lithium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate and/or potassium bicarbonate. Alternatively, or additionally, the metal carbonate may comprise one or more of magnesium carbonate and/or magnesium bicarbonate. Suitably, the first electrolyte may be present in the middle and/or cathodic compartments in a concentration of about 0.05 M, or about 0.1 M, or about 0.5 M, such as about 1 M.

In some embodiments, the second electrolyte may comprise a metal salt. Suitably, the metal salt may be inert. In the context of the present invention, the term "inert" means that the metal salt is inert to electrolysis in that neither the metal or the salt components are released from the anodic or cathodic compartments and, moreover, the metal salt does not interfere with the decomposition reaction of the carbonate or bicarbonate ions. In this way, the metal salt may suitably dissociate to a metal cation and a counter anion, thereby advantageously increasing the conductivity of the solution in the middle and/or cathodic compartments.

Suitably, the metal salt may be water soluble. For example, the metal salt may have a solubility in water of 1 g/dL or more, such as 50 g/dL or more, or even 100 g/dL or more at standard temperature and pressure. In some embodiments, the metal salt may comprise one or more of sodium chloride, sodium acetate, sodium sulphate and/or sodium nitrate.

Suitably, prior to starting electrolysis (i.e. prior to applying a voltage between the anodic and cathodic compartments), the second electrolyte may be present in the middle compartment in a concentration of at least 3 M, or at least 4 M, or at least 5 M, or at least 6 M, such as at least 7 M.

In some embodiments, the anodic compartment may comprise an aqueous solution of sulphuric acid. Additionally, or alternatively, the anodic compartment may comprise an aqueous solution of acetic acid. In some embodiments, the first and/or second semi-permeable membrane may comprise a cation exchange membrane. For example, the first and/or second semi-permeable membrane may comprise a perfluorinated membrane. In some embodiments, the first and/or second semi-permeable membrane may comprise polytetrafluoroethylene (PTFE). For example, the first and/or second semi-permeable membrane may comprise a PTFE backbone with side chains comprising ether groups and/or capped at one or both ends with a sulphonic acid unit. Such membranes are commercially available and sold under the trade name Nafion ^(RTM) .

In some embodiments, the first and/or second semi-permeable membrane may be substantially impermeable to anions. For example, the first and/or second semi-permeable membrane may be substantially impermeable to the counter anion of the dissociated metal salt. This may advantageously prevent the counter anion of the dissociated metal salt from entering the anodic compartment and being oxidised.

In some embodiments, the first and/or second semi-permeable membrane may be substantially impermeable to one or more of chloride anions, acetate anions, sulphate anions and/or nitrate anions.

Suitably, the first and/or second semi-permeable membrane may be generally permeable to hydrogen cations, metal cations (for example, such as lithium, sodium, potassium and/or magnesium ions) and/or water. In some embodiments, the first and/or second semi- permeable membrane may comprise a membrane that is both generally permeable to hydrogen cations, metal cations and/or water and substantially impermeable to anions such as chloride anions, acetate anions, sulphate anions and/or nitrate anions.

In some embodiments, one or more of the anodic, middle and cathodic compartments may be in fluid connection with at least one gas/liquid separator. For example, each of the anodic, middle and cathodic compartments may be in fluid connection with at least one gas/liquid separator.

In some embodiments, the middle compartment may be in fluid connection with a gas/liquid separator, and the gas/liquid separator may be in fluid connection with the cathodic compartment. In this way, water obtained from the gas/liquid separator (which may comprise any dissolved first and/or second electrolytes) may advantageously be fed into the cathodic compartment from the liquid/gas separation in fluid connection with the middle compartment.

In some embodiments, the electrolyser may comprise a conduit arranged to fluidly connect the middle and cathodic compartments. For example, the conduit may be arranged to fluidly connect a gas/liquid separator connected to the middle compartment to the cathodic compartment. In some embodiments, the conduit arranged to fluidly connect the middle and cathodic compartments may comprise a gas/liquid separator.

In some embodiments, the electrolyser may comprise a pump arranged to recirculate the aqueous solution of acid through the anodic compartment. For embodiments in which the anodic compartment is in fluid connection with a gas/liquid separator, the pump may suitably be arranged to recirculate the aqueous solution of acid through the anodic compartment and the gas/liquid separator. Recirculation of the aqueous solution of acid through the anodic compartment may advantageously maintain homogeneity of the aqueous solution of acid in the anodic compartment. This in turn may allow for heat dissipation and allow water top up.

In some embodiments, the electrolyser may comprise a source of first electrolyte. The source of first electrolyte may suitably be arranged to deliver the first electrolyte to the middle compartment. For example, the source of first electrolyte may be arranged to deliver an aqueous solution of the first electrolyte to the middle compartment.

In some embodiments, the electrolyser may comprise a direct air capture (DAC) device. The DAC device may advantageously extract carbon dioxide from the atmosphere (i.e. from external air). Moreover, the DAC device may use the carbon dioxide to synthesise the first electrolyte. In this way, the DAC device may suitably provide an aqueous solution of the first electrolyte to the middle compartment.

For example, the electrolyser may comprise a scrubber arranged to receive an aqueous solution of metal hydroxide from the cathodic compartment and carbon dioxide gas. Suitably, the scrubber may be arranged to convert the aqueous solution of metal hydroxide and carbon dioxide gas into an aqueous solution of metal carbonate. Moreover, the scrubber may advantageously deliver the aqueous solution of metal carbonate to the middle compartment. In this way, metal hydroxide by-product formed in the cathodic compartment may be converted back into metal carbonate for use in the electrolyser.

Suitably, the scrubber may comprise a packing material. The packing material may comprise one or more of plastic, metal and/or ceramic. In some embodiments, the packing material may comprise a structured packing material. For example, the structured packing material may comprise one or more of a honeycomb, waffle and/or herringbone structure. In some embodiments, the packing material may comprise a random packing material. For example, the random packing material may comprise one or more of Pall rings, Raschig rings and/or saddle rings. The presence of a packing material may advantageously increase the total surface area inside the scrubber, thereby increasing the absorption of carbon dioxide gas into the aqueous solution of metal hydroxide and thus increasing the reaction rate. Conveniently, the scrubber may be arranged to convert the aqueous solution of metal hydroxide and carbon dioxide gas from the external atmosphere (i.e. by direct air capture of carbon dioxide from the atmosphere) into an aqueous solution of metal carbonate. In some embodiments, the scrubber may comprise an air blower for directing a flow of carbon dioxide gas and/or external air into the scrubber. Suitably, the scrubber may comprise a carbon dioxide meter for monitoring the carbon dioxide concentration inside the scrubber.

In some embodiments, the scrubber may be generally elongate in structure. Suitably, the scrubber may be arranged such that it extends generally upwards (i.e. vertically) to define a tower. In such embodiments, the air blower may be arranged to direct carbon dioxide gas and/or air into the scrubber at a location at or towards a lower end of the scrubber. The aqueous solution of metal hydroxide may suitably be received at or towards an upper end of the scrubber. In this way, a counter current may advantageously be formed between the carbon dioxide gas blown up through the scrubber and the aqueous solution of metal hydroxide percolating down through the scrubber, thereby increasing the absorption of carbon dioxide gas into the aqueous solution of metal hydroxide and thus increasing the reaction rate.

Thus, as the aqueous solution of metal hydroxide reaches the lower end of the scrubber, most (i.e. in the range of from 90 to 99%) or substantially all (i.e. greater than 99%) of the aqueous solution of metal hydroxide may have been converted into an aqueous solution of metal carbonate. The aqueous solution of metal carbonate may conveniently be removed at a location at or towards a lower end of the scrubber. Moreover, the aqueous solution of metal carbonate may conveniently be delivered to the middle compartment at a location at or towards a lower end of the middle compartment. By making the aqueous solution of metal carbonate rise up through the middle compartment from the lower end, hydrogen ions entering the middle compartment from the anodic compartment may advantageously have an increased chance of decomposing the carbonate (as will be explained in more detail below).

A second aspect of the invention provides a method of producing hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of greater than one. The method comprises (i) providing an electrolyser comprising an anodic compartment comprising an aqueous solution of acid, a cathodic compartment, and a middle compartment arranged therebetween.

The cathodic and middle compartments comprise an aqueous solution of first and second electrolytes. The first electrolyte comprises a metal carbonate. The anodic and cathodic compartments are separated from the middle compartment by first and second semi- permeable membranes. The method also comprises (ii) applying a voltage between the anodic and cathodic compartments to generate: oxygen gas in the anodic compartment, wherein hydrogen ions from the aqueous solution of acid pass through the first semi-permeable membrane and enter the middle compartment; carbon dioxide gas and water in the middle compartment from the reaction of carbonate or bicarbonate ions with the hydrogen ions, wherein the metal ions pass through the second semi-permeable membrane and enter the cathodic compartment; and hydrogen gas and hydroxide ions in the cathodic compartment.

The method further comprises (iii) decomposing water in the cathodic compartment to generate additional hydrogen gas, once the carbonate or bicarbonate ions in the middle compartment have decomposed, by maintaining conductivity across the middle compartment via the second electrolyte.

Suitably, the method may produce hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of at least 3: 1. In some embodiments, the method may produce hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of greater than 3: 1.

In some embodiments, a current of 600 amps or more, or 900 amps or more, or 1200 amps or more, or even 1800 amps or more may be applied between the anodic and cathodic compartments.

In some embodiments, the method may comprise providing a gas/liquid separator in fluid connection with the middle compartment, separating carbon dioxide and water (which may comprise any dissolved first and/or second electrolytes) in the gas/liquid separator, and delivering the separated water to the cathodic compartment.

In some embodiments, the method may comprise recovering an aqueous solution comprising metal ions and hydroxide ions from the cathodic compartment. The aqueous solution comprising metal ions and hydroxide ions from the cathodic compartment may also comprise the second electrolyte.

In some embodiments, the method may comprise reacting the aqueous solution comprising metal ions and hydroxide ions with carbon dioxide to form an aqueous solution comprising metal carbonate. In some embodiments, the method may comprise delivering the aqueous solution comprising metal carbonate to the middle compartment.

In some embodiments, the method may comprise reacting the hydrogen gas and carbon dioxide gas in the presence of a catalyst to form methanol.

In some embodiments, the method may comprise using renewable electricity to apply the voltage between the anodic and cathodic compartments. Thus, fuels (such as methanol) produced from the hydrogen and carbon dioxide gas generated by the electrolysis process may advantageously be carbon neutral.

A third aspect of the invention provides an electrolyser unit comprising an anodic compartment, a cathodic compartment and a middle compartment arranged therebetween. The anodic and cathodic compartments are separated from the middle compartment by first and second semi-permeable membranes. The middle compartment is in fluid connection with a gas/liquid separator and the gas/liquid separator is in fluid connection with the cathodic compartment.

The electrolyser unit may be suitable for use in a method according to the second aspect of the invention.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other components, integers or steps. Moreover, the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of an electrolyser in accordance with a first embodiment of the invention; and

Figure 2 is a schematic diagram of an electrolyser in accordance with a second embodiment of the invention.

DETAILED DESCRIPTION

Figure 1 shows an electrolyser 100 in accordance with a first embodiment of the invention. The electrolyser 100 comprises an electrolyser unit 120.

The electrolyser unit 120 comprises anodic compartment 122, a middle compartment 124, and a cathodic compartment 126. The middle compartment 124 is disposed between the anodic and cathodic compartments 122, 126. The middle compartment 124 is separated from the anodic compartment 122 by a first semi-permeable membrane 127. The middle compartment is separated from the cathodic compartment 126 by a second semi-permeable membrane 128.

An upper end of the anodic compartment 122 is connected to a first gas/liq uid separator 131, which sits above the anodic comportment 122. An upper end of the first gas/liquid separator 131 has a gas outlet 131a. A pipe 136 connects a lower end of the first gas/liquid separator 131 to a lower end of the anodic compartment 122. A pump 137 is arranged part-way along the pipe 136.

An upper end of the middle compartment 124 is connected to a second gas/liquid separator

132, which sits above the middle compartment 124. An upper end of the second gas/liquid separator 132 has a gas outlet 132a. A pipe 138 connects a lower end of the second gas/liquid separator 132 to a lower end of the cathodic compartment 126. The middle compartment 124 is also connected to a supply 109 of aqueous sodium carbonate by a pipe 110, which connects the supply 109 to a lower end of the middle compartment 124. A pump 112 is arranged partway along the pipe 110.

An upper end of the cathodic compartment 126 is connected to a third gas/liquid separator

133, which sits above the cathodic compartment 126. An upper end of the third gas/liquid separator 133 has a gas outlet 133a. A lower end of end of the third gas/liquid separator 133 is connected to pipe 139. Prior to commencing electrolysis, the anodic compartment 122 is filled with a 1 M solution of aqueous sulphuric acid. The middle and cathodic compartments 124, 126 are filled with an aqueous mixture of 0.05 M sodium carbonate and 7 M sodium chloride. In order to start the electrolysis process, a voltage is applied between the anodic compartment 122 and the cathodic compartment 126, and the pumps 112, 137 are switched on.

The pump 137 arranged part-way along the pipe 136 connecting the lower end of the first gas/liquid separator 131 to a lower end of the anodic compartment 122 circulates the aqueous sulphuric acid through the anodic compartment 122 and the first gas/liquid separator 131. In the anodic compartment 122, the sulphuric acid is present in a dissociated form as follows:

In the anodic compartment 122, hydroxide anions arising from the self-ionisation of water are oxidised to generate oxygen gas as follows:

The aqueous sulphuric acid solution in the first gas/liquid separator 131 is pumped into the first gas/liquid separator 131 by the pump 137. The resulting oxygen gas is separated out from the aqueous sulphuric acid in the first gas/liquid separator 131, and exits the first gas/liquid separator 131 through the gas outlet 131a. The aqueous sulphuric acid solution in the first gas/liquid separator 131 is then pumped back to the anodic compartment 122 by the pump 137.

Some of the hydrogen ions formed in the anodic compartment 122 pass through the first semi-permeable membrane 127 and enter the middle compartment 124. In the middle compartment 124, the sodium carbonate is present in a dissociated form as follows:

The carbonate reacts with the hydrogen ions as follows to generate carbon dioxide gas and water: The resulting carbon dioxide gas is separated out from the aqueous solution in the second gas/liq uid separator 132, and exits the second gas/liq uid separator 132 through the gas outlet 132a. The aqueous solution in the second gas/liquid separator 132 then flows into the cathodic compartment 126 under gravity via the pipe 138 connecting a lower end of the second gas/liquid separator 132 to a lower end of the cathodic compartment 126. Moreover, some of the sodium ions present in the middle compartment 124 pass through the second semi- permeable membrane 128 and enter the cathodic compartment 126.

In the cathodic compartment 126, hydrogen ions arising from the self-ionisation of water are reduced to hydrogen gas, as follows:

2H ₂ O 2H ⁺ + 2OH-

The resulting hydrogen gas is separated out from the aqueous solution in the third gas/liquid separator 133, and exits the third gas/liquid separator 133 through the gas outlet 133a. The aqueous solution comprising sodium ions and hydroxide ions in the third gas/liquid separator 133 then exits the third gas/liquid separator 133 and is removed from the electrolyser 100 through pipe 139.

As mentioned above, the middle compartment 124 is also connected to a supply of aqueous sodium carbonate 109 by a pipe 110. During electrolysis, aqueous sodium carbonate is pumped from the supply 109 through the pipe 110 to a lower end of the middle compartment 124.

Further electrical current can be applied between the anodic and cathodic compartments 122, 126 to decompose water in the cathodic compartment and generate additional volumes of hydrogen gas. Crucially, the presence of sodium chloride in the middle and cathodic compartments 124, 126 allows electrical conductivity to be maintained between the anodic and cathodic compartments 122, 126 once all of the carbonate in the middle compartment 124 has decomposed.

Moreover, the presence of sodium chloride means that conditions may be arranged such that only part of the current applied between the anodic and cathodic compartments 122, 126 is required to decompose the carbonate in the middle compartment 124. The remainder of the current is therefore available to decompose water in the cathodic compartment 126, resulting in an increased amount of hydrogen being released. In this way, the electrolyser 100 is able to produce hydrogen gas and carbon dioxide gas in a H ₂ :CO ₂ ratio of greater than one. Figure 2 shows an electrolyser 200 in accordance with a second embodiment of the invention. The electrolyser 200 comprises a carbon dioxide capture column 202 coupled to an electrolyser unit 220.

The electrolyser unit 220 comprises anodic compartment 222, a middle compartment 224, and a cathodic compartment 226. The middle compartment 224 is disposed between the anodic and cathodic compartments 222, 226. The middle compartment 224 is separated from the anodic compartment 222 by a first semi-permeable membrane 227. The middle compartment is separated from the cathodic compartment 224 by a second semi-permeable membrane 228.

An upper end of the anodic compartment 222 is connected to a first gas/liq uid separator 231, which sits above the anodic comportment 222. An upper end of the first gas/liquid separator 231 has a gas outlet 231a. A pipe 236 connects a lower end of the first gas/liquid separator 231 to a lower end of the anodic compartment 222. A pump 237 is arranged part-way along the pipe 236.

An upper end of the middle compartment 224 is connected to a second gas/liquid separator

232, which sits above the middle compartment 224. An upper end of the second gas/liquid separator 232 has a gas outlet 232a. A pipe 238 connects a lower end of the second gas/liquid separator 232 to a lower end of the cathodic compartment 226.

An upper end of the cathodic compartment 226 is connected to a third gas/liquid separator

233, which sits above the cathodic compartment 226. An upper end of the third gas/liquid separator 233 has a gas outlet 233a. A pipe 239 connects a lower end of the third gas/liquid separator 233 to an upper end of the carbon dioxide capture column 202. A pump 240 is arranged part-way along the pipe 239 near to the lower end of the third gas/liquid separator 233. A flow meter 241 is also arranged part-way along the pipe 239, slightly further downstream from the lower end of the third gas/liquid separator 233.

The pipe 239 connects to the carbon dioxide capture column 202 via a first inlet 204 arranged in an upper end of the carbon dioxide capture column 202. A second inlet 206 arranged in a lower end of the carbon dioxide capture column 102 connects to an air blower 208. The lower end of the carbon dioxide capture column 202 also has an outlet 209 arranged on an opposite side of the carbon dioxide capture column 102 to the second inlet 206, and at a slightly lower position than the second inlet 206. The outlet 209 is connected to a pipe 210, which connects the carbon dioxide capture column 202 to a lower end of the middle compartment 224. A pump 212 is arranged part-way along the pipe 210 near to the lower end of the carbon dioxide capture column 202. The carbon dioxide capture column 202 is filled with a high surface area packing material 214. A carbon dioxide meter 216 is connected to an upper end of the carbon dioxide capture column 202. Additionally, the carbon dioxide capture column 202 is equipped with an anemometer 218 arranged at an upper end of the carbon dioxide capture column 202.

Prior to commencing electrolysis, the anodic compartment 222 is filled with a 1 M solution of aqueous sulphuric acid. The middle and cathodic compartments 224, 226 and carbon dioxide capture column 222 are filled with an aqueous mixture of 0.05 M sodium carbonate and 7 M sodium chloride. In order to start the electrolysis process, a voltage is applied between the anodic compartment 222 and the cathodic compartment 226, and the pumps 212, 237, 240, and air blower 208 are switched on.

The pump 237 arranged part-way along the pipe 236 connecting the lower end of the first gas/liquid separator 231 to a lower end of the anodic compartment 222 circulates the aqueous sulphuric acid through the anodic compartment 222 and the first gas/liquid separator 231. In the anodic compartment 222, the sulphuric acid is present in a dissociated form as follows:

In the anodic compartment 222, hydroxide anions arising from the self-ionisation of water are oxidised to generate oxygen gas as follows:

4H ₂ O 4H ⁺ + 4OH-

The aqueous sulphuric acid solution in the first gas/liquid separator 231 is pumped into the first gas/liquid separator 231 by the pump 237. The resulting oxygen gas is separated out from the aqueous sulphuric acid in the first gas/liquid separator 231, and exits the first gas/liquid separator 231 through the gas outlet 231a. The aqueous sulphuric acid solution in the first gas/liquid separator 231 is then pumped back to the anodic compartment 222 by the pump 237.

Some of the hydrogen ions formed in the anodic compartment 222 pass through the first semi-permeable membrane 227 and enter the middle compartment 224. In the middle compartment 224, the sodium carbonate is present in a dissociated form as follows: The carbonate reacts with the hydrogen ions as follows to generate carbon dioxide gas and water:

The resulting carbon dioxide gas is separated out from the aqueous solution in the second gas/liq uid separator 232, and exits the second gas/liq uid separator 232 through the gas outlet 232a. The aqueous solution in the second gas/liquid separator 232 then flows into the cathodic compartment 226 under gravity via the pipe 238 connecting a lower end of the second gas/liquid separator 232 to a lower end of the cathodic compartment 226. Moreover, some of the sodium ions present in the middle compartment 224 pass through the second semi- permeable membrane 228 and enter the cathodic compartment 226.

In the cathodic compartment 226, hydrogen ions arising from the self-ionisation of water are reduced to hydrogen gas, as follows:

The resulting hydrogen gas is separated out from the aqueous solution in the third gas/liquid separator 233, and exits the third gas/liquid separator 233 through the gas outlet 233a. The aqueous solution comprising sodium ions and hydroxide ions in the third gas/liquid separator 233 is then pumped along pipe 239 by pump 240 to the first inlet 204 of the carbon dioxide capture column 202. This aqueous sodium hydroxide solution then percolates down through carbon dioxide capture column 202 via the high surface area packing material 214.

The air blower 208 drives a flow of air through the second inlet 206 and up through the carbon dioxide capture column 202 to form a counter current flow of air and the aqueous solution comprising sodium hydroxide in the carbon dioxide capture column 202. Carbon dioxide in the air dissolves into the aqueous solution comprising sodium hydroxide and reacts with the sodium hydroxide to form sodium carbonate as follows:

Conditions are arranged such that as the aqueous solution comprising sodium hydroxide reaches the lower end of the carbon dioxide capture column 202, all of the sodium hydroxide is converted into sodium carbonate. The resulting aqueous solution comprising sodium carbonate is then pumped out of the carbon dioxide capture column 202 by pump 222, through outlet 209, and the pipe 210 to a lower end of the middle compartment 224.

Further electrical current can be applied between the anodic and cathodic compartments 222, 226 to decompose water in the cathodic compartment 226 and generate additional volumes of hydrogen gas. Crucially, the presence of sodium chloride in the middle and cathodic compartments 224, 226 allows electrical conductivity to be maintained between the anodic and cathodic compartments 222, 226 once all of the carbonate in the middle compartment 224 has decomposed.

Moreover, the presence of sodium chloride means that conditions may be arranged such that only part of the current applied between the anodic and cathodic compartments 222, 226 is required to decompose the carbonate in the middle compartment 224. The remainder of the current is therefore available to decompose water in the cathodic compartment 226, resulting in an increased amount of hydrogen being released. In this way, the electrolyser 200 is able to produce hydrogen gas and carbon dioxide gas in a H2:CC>2 ratio of greater than one.

EXAMPLES

Comparative Example A

An aqueous solution of 0.05 M sodium carbonate was used. The application of a current of 600 amps released about 4 L/min of CO2 gas at atmospheric pressure from a flowing sodium carbonate solution in the middle compartment, flowing at a rate of about 3.7 L/min. All the sodium carbonate passing through the middle compartment was decomposed. However, at this low concentration of sodium carbonate, the conductivity was not very large and so the voltage needed to provide 600 amps was significant, at about 8 volts. Hydrogen gas and carbon dioxide gas were produced in a H2:CC>2 ratio of 1 : 1.

Comparative Example B

The same conditions as for Comparative Example A were used, however, in this example the sodium carbonate concentration was increased to 2 M (around the maximum concentration possible at room temperature). By using this concentration, it was possible to reduce the voltage to about 5 volts. Again, the rate of CO2 production was about 4 L/min and by increasing the current beyond 600 amps, the rate of production of CO2 could be increased to take advantage of the higher sodium carbonate concentration. However hydrogen gas and carbon dioxide gas were still produced in a H2:CC>2 ratio of 1 : 1. Example 1 : Voltage reduction using sodium chloride

In this example, an aqueous mixture of 0.05 M sodium carbonate and 7 M sodium chloride were used. Due to the high concentration of sodium chloride, when a current of 600 amps was applied, CO2 was released at about 4L/min at about only 3 volts due to the higher conductivity. However, hydrogen gas and carbon dioxide gas were still produced in a H2:CC>2 ratio of 1 : 1.

Example 2: Voltage reduction using sodium acetate

In this example, an aqueous mixture of 0.05M sodium carbonate and 7 M sodium acetate were used. Due to the high concentration of sodium acetate, when a current of 600 amps was applied, CO2 was released at about 4L/min at about only 3 volts due to the higher conductivity. However, hydrogen gas and carbon dioxide gas were still produced in a H2:CC>2 ratio of 1: 1.

Example 3: Generating additional volumes of hydrogen using sodium chloride

In this example, conditions were arranged such that an aqueous mixture of 0.05 M sodium carbonate and 7 M sodium chloride was used. The current was raised to 1800 amps at a reasonable voltage of 5 volts by taking advantage of the higher conductivity due to the presence of sodium chloride. As only 600 amps is required to consume all the sodium carbonate, the additional 1200 amps electrolyse water to generate two more volumes of hydrogen. Thus, in this example, hydrogen gas and carbon dioxide gas were produced in a H2:CC>2 ratio of 3: 1.

Example 4: Generating additional volumes of hydrogen using sodium acetate

In this example, conditions were arranged such that an aqueous mixture of 0.05 M sodium carbonate and 7 M sodium acetate was used. The current was raised to 1800 amps at a reasonable voltage of 5 volts by taking advantage of the higher conductivity due to the presence of sodium acetate. As only 600 amps was required to consume all the sodium carbonate, the additional 1200 amps decomposed water to generate two more volumes of hydrogen. Thus, in this example, hydrogen gas and carbon dioxide gas were produced in a H2:CC>2 ratio of 3: 1.

Example 5: Generating additional volumes of hydrogen using sodium acetate (small scale) In this example, conditions were arranged such that an aqueous mixture of 0.05 M sodium carbonate and 3 M sodium acetate was used, flowing through the system at 0.75L/min. A current of 180 amps was applied between the anodic and cathodic compartments using a power controller. Of the 180 amps applied, 60 amps were used to decompose all the sodium carbonate and produce one volume of hydrogen, leaving 120 amps to produce two more volumes of hydrogen and generate hydrogen gas and carbon dioxide gas in a H2:CO2 ratio of 3: 1.