A CONTINUOUS PROCESS FOR SUSTAINABLE PRODUCTION OF HYDROGEN

Abstract

The disclosure provides a method of producing hydrogen. The method comprises conducting a thermochemical reaction by contacting a metal, or an alloy thereof, with steam to produce a metal oxide and/or a metal hydroxide and hydrogen. The method then comprises contacting the metal oxide and/or the metal hydroxide produced in the thermochemical reaction with water or a basic aqueous solution to produce a solution comprising a metal ion. Finally, the method comprises conducting an electrochemical reaction by applying a voltage across an anode and a cathode, whereby at least a portion of the cathode contacts the solution comprising the metal ion, to produce hydrogen, oxygen and the metal, or the alloy thereof.

Claims

1. A method of producing hydrogen, the method comprising: conducting a thermochemical reaction by contacting a metal, or an alloy thereof, with steam to produce a metal oxide and/or a metal hydroxide and hydrogen; contacting the metal oxide and/or the metal hydroxide produced in the thermochemical reaction with water or a basic aqueous solution to produce a solution comprising a metal ion; and conducting an electrochemical reaction by applying a voltage across an anode and a cathode, whereby at least a portion of the cathode contacts the solution comprising the metal ion, to produce hydrogen, oxygen and the metal, or the alloy thereof.

2. A method of producing hydrogen, the method comprising: contacting a metal oxide and/or a metal hydroxide with water or a basic aqueous solution to produce a solution comprising a metal ion; conducting an electrochemical reaction by applying a voltage across an anode and a cathode, whereby at least a portion of the cathode contacts the solution comprising the metal ion, to produce hydrogen, oxygen and a metal, or an alloy thereof; and conducting a thermochemical reaction by contacting the metal, or the alloy thereof, produced in the electrochemical reaction with steam to produce the metal oxide and/or the metal hydroxide and hydrogen.

3. The method according to claim 1, wherein the electrochemical reaction is conducted repeatedly, or continuously.

4. (canceled)

5. The method according to claim 1, wherein the thermochemical reaction is conducted continuously, or repeatedly.

6. (canceled)

7. The method according to claim 1, wherein the metal, or the alloy thereof, is a transition metal, a p-block metal, an f-block metal, or an alloy thereof, optionally wherein the metal, or the alloy thereof, is selected from the group consisting of tin, lead, thallium, selenium, bismuth, zinc, copper, iron, nickel, cobalt, manganese, titanium, molybdenum, cadmium, chromium, vanadium, silver, rhodium, platinum, palladium, iridium, osmium, rhenium, ruthenium, lanthanum, zirconium, cerium, gadolinium, yttrium, holmium, samarium, and terbium.

8. (canceled)

9. The method according to claim 7, wherein the metal, or the alloy thereof, is zinc or tin.

10. The method according to claim 1, wherein the metal, or the alloy thereof, is contacted with the steam at a temperature of between 100° C. and 700° C.

11. The method according to claim 1, wherein the method comprises agitating the metal, or the alloy thereof, while it is being contacted with the steam.

12. The method according to claim 1, wherein the method comprises condensing unreacted steam from a gaseous mixture obtained from the thermochemical reaction.

13. The method according to claim 1, wherein the method comprises contacting the metal oxide and/or the metal hydroxide with the basic aqueous solution and the basic aqueous solution comprises a base, and the base is an Arrhenius base, a Lewis base, or a Bronsted-Lowry base, optionally wherein the base is an Arrhenius base and comprises an alkali metal or alkaline earth metal hydroxide.

14. (canceled)

15. The method according to claim 1, wherein the method comprises contacting the metal oxide and/or the metal hydroxide with the basic aqueous solution, and the basic aqueous solution comprises a concentration of between 0.5 and 8.5 M of a base.

16. The method according to claim 1, wherein the method comprises contacting the metal oxide and/or the metal hydroxide with the basic aqueous solution, and the metal oxide and/or the metal hydroxide is contacted with the basic aqueous solution in a sufficient quantity to produce the solution comprising the metal ion, wherein the metal ion is present at a concentration of between 0.001 and 1 M.

17. The method according to claim 1, wherein the method comprises contacting the metal oxide and/or the metal hydroxide with water, and the metal oxide and/or metal hydroxide is contacted with water in a sufficient quantity to produce the solution comprising the metal ion, wherein the metal ion is present at a concentration of between 0.2 and 5 M.

18. The method according to claim 1, wherein the anode and the cathode are disposed in an undivided electrochemical cell.

19. The method according to claim 1, wherein the anode and the cathode are disposed in an electrochemical cell, and the electrochemical cell comprises a membrane disposed between the anode and the cathode dividing the cell into two portions.

20. The method according to claim 19, wherein the method comprises disposing the solution comprising the metal ion in a cathode portion of the cell, such that at least a portion of the cathode contacts the solution comprising the metal alloy and the method further comprises disposing a further electrolyte in an anode portion of the cell, such that at least a portion of the anode contacts the further electrolyte, optionally wherein the further electrolyte comprises a basic aqueous solution.

21. (canceled)

22. The method according to claim 1, wherein the method comprises applying a voltage of between 1 and 8 V across the anode and cathode.

23. The method according to claim 1, wherein the method comprises causing a current of between 0.5 and 10 A to flow through the anode, cathode and the solution comprising the metal ion.

24. An apparatus for producing hydrogen, the apparatus comprising: a thermochemical reactor, configured to hold a metal, or an alloy thereof, therein, the thermochemical reactor comprising a feed means configured to feed steam into the thermochemical reactor and thereby convert the metal, or the alloy thereof, into a metal oxide and/or a metal hydroxide; and an electrochemical cell comprising an anode and a cathode, and configured to receive a solution comprising a metal ion, such that at least a portion of the cathode contacts the solution comprising the metal ion.

25. The apparatus according to claim 24, wherein the apparatus is configured to prevent the metal, the alloy thereof or the metal oxide and/or the metal hydroxide from escaping from the thermochemical reactor while a thermochemical reaction is being conducted therein, optionally wherein the apparatus comprises a container configured to hold the metal, or the alloy thereof, therein and to be placed in the thermochemical reactor, wherein the feed means is configured to feed the steam directly into the container and the container comprises a mesh configured to allow gases to flow out of the container while being further configured to prevent the metal, the alloy thereof or the metal oxide and/or the metal hydroxide from being removed from the container.

26. (canceled)

Description

[0117] For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:—

[0118] FIG. 1 is schematic diagram of the apparatus used to conduct a thermochemical reaction;

[0119] FIG. 2 is a graph showing the effect of temperature on the volume of hydrogen produced in a thermochemical reaction;

[0120] FIG. 3 is a graph showing the effect of steam flow rate on the volume of hydrogen produced in a thermochemical reaction;

[0121] FIG. 4 is a graph showing the effect of the form of zinc on the volume of hydrogen produced in a thermochemical reaction;

[0122] FIG. 5 is a graph showing the effect of mixing zinc on the volume of hydrogen produced in a thermochemical reaction;

[0123] FIG. 6 is a graph showing the experiments hydrogen production in experiments which were run to completion;

[0124] FIG. 7 is a schematic diagram of the experimental set-up for an electrolysis reaction;

[0125] FIG. 8 shows the electrolysis cell using in the experimental set-up of FIG. 7;

[0126] FIG. 9 is a graph showing the maximum solubility of zinc oxide (ZnO) in aqueous solutions comprising different concentrations of sodium hydroxide (NaOH);

[0127] FIG. 10 is a graph showing how the conductivity of a sodium zincate solution varies with the concentration of sodium zincate and sodium hydroxide;

[0128] FIG. 11 is a graph showing how the conductivity of a sodium hydroxide solution varies with the concentration of sodium hydroxide;

[0129] FIG. 12 is a graph showing how the voltage applied across electrodes affects the volume of hydrogen produced;

[0130] FIG. 13 is a graph showing how the voltage applied across electrodes affects the current;

[0131] FIG. 14 is a schematic diagram showing the cyclic nature of the overall process;

[0132] FIG. 15 is a graph showing how the conductivity and resistance of a sodium stannate trihydate solution varies with the concentration of sodium stannate trihydate;

[0133] FIG. 16 is a graph showing hydrogen production against time for different concentrations of sodium stannate trihydate where the voltage applied was 2 V; and

[0134] FIG. 17 is a graph showing hydrogen production against time for the 1.2 M electrolyte where the voltage applied was varied.

EXAMPLE 1—THERMOCHEMICAL REACTION OF ZINC

[0135] The first experiment conducted involved reacting zinc with steam to form zinc oxide and hydrogen.

Methods

[0136] The thermochemical experiment was carried out using the apparatus 1 shown in FIG. 1. The apparatus 1 comprises a tube furnace 2 with a crucible 4 and a temperature sensor 5 disposed therein. The crucible 4 comprises a mesh cover 6.

[0137] A syringe pump 8 is disposed outside the furnace 2 and a conduit 10 extends between the syringe pump 8 and the crucible 4. The conduit 10 comprises a coiled section 12 which is disposed inside the furnace 2.

[0138] A condenser 16, measuring cylinder 18, water bath 20 and vacuum pump 22 are also disposed outside the furnace 2. The cylinder 18 is inverted in the water bath 20, and a conduit 24 extends between the base 26 of the cylinder 18 and the vacuum pump 22. A conduit 28 extends between the furnace 2 and the condenser 16 and a further conduit 30 extends between the condenser and the cylinder 18.

[0139] When a user wishes to run an experiment, they place a pre-weighed amount of zinc 32 in the crucible 4. In the experiments described below, 10 g of zinc was used for each experiment.

[0140] The user fills the syringe pump 8 with deionised water 34. The user also fills the water bath 20 with water 36 and activates the vacuum pump 22. The vacuum pump 22 removes gas from the cylinder 18 and causes it to be replaced by the water 36. The user then flushes the system with nitrogen and then activates the oven 2 and waits for it to reach a desired temperature.

[0141] Before running the experiment, the user will again activate the vacuum pump 22 to remove any gases which have entered the cylinder 18 due to gas expansion caused by the oven 2 heating up. The user will then activate the syringe pump 8. This will feed the deionised water 34 along the conduit 10. As the deionised water 34 passes through the coiled section 12 it will evaporate ensuring that steam is fed into the crucible 4 and contacts the zinc powder 32. The steam will react with the zinc 32 according to the following equation:

Zn.sub.(s)+H.sub.2O.sub.(l).fwdarw.ZnO.sub.(s)+H.sub.2(g)  1a

[0142] The mesh will prevent the zinc 32 and zinc oxide from escaping from the crucible, while allowing the gases to flow therefrom. The gases will then flow along the conduit 28 and through the condenser 16, which will condense the steam into water and remove it. The hydrogen gas 38 will continue to flow along the conduit 3o and into the cylinder 38 displacing the water 36. The user can thereby measure the volume of hydrogen gas 38 produced. In the experiments described below, a reading was taken every five minutes.

Results

[0143] The effects of temperature were studied by measuring the hydrogen generation at four different temperatures, namely 250° C., 300° C., 350° C. and 400° C. All of the experiments were carried out using zinc powder at a steam flow rate of 150 ml/min. The results from the temperature experiments show that an increase in reaction temperature leads to an increase in the volume of hydrogen produced in the reaction, see FIG. 2. All of the different temperatures follow a similar shape of an initial rapid increase in hydrogen, followed by a decrease in the rate of production. The rate of production follows the general pattern that the higher the temperature the greater the rate of hydrogen generation. A further observation is that the higher the temperature the less the delay in hydrogen production after the water is injected into the system.

[0144] The effects of steam volumetric flow rate were studied by measuring the hydrogen generation at three different rates of 50 ml/min, 100 ml/min and 150 ml/min. All of the experiments were carried out using zinc powder at a constant temperature of 350° C. The results show that an increase in steam volumetric flow leads to an increase in the volume of hydrogen produced, see FIG. 3. The results follow a pattern of rapid increase followed by a gradual decrease in hydrogen generation rate. The difference in rate and volume produced is less when compared to the effects of temperature. There is a consistent delay of 10 minutes before the start of the reaction after the water is injected into the system.

[0145] The effects of the form of the zinc were studied by comparing the volume of hydrogen generated when the zinc was provided in the form of pellets compared to a powder. All of the experiments were carried out using a constant temperature of 350° C. and steam volumetric flow rate of 150 ml/min. The results show that zinc in the form of a powder produces more hydrogen than zinc in the form of pellets, see FIG. 4. At the end of the experiment the zinc samples were removed from the crucible. When removed the pellets were found to have broken up slightly and there were clumps of zinc surrounded by zinc powder that had broken off.

[0146] During the above experiments, channelling was observed in the crucible whereby steam would flow out of the crucible in specific areas by-passing large amounts of the zinc. Accordingly, some areas of the zinc sample were exposed to the steam while others were not. In order to assess how this channelling was impacting on the rate of reaction an experiment was carried out which involved removing the zinc sample and mixing it halfway through the experiment. The experiments were carried out using zinc powder at a constant temperature of 400° C. and steam volumetric flow rate of 150 ml/min. As would be expected, the initial reaction is similar for both experiments, but the volume of hydrogen produced jumps at the point where the zinc is mixed, see FIG. 5.

[0147] To determine how long the reaction would take to complete, two experiments were run until hydrogen production ceased. The first experiment was carried out using zinc powder at 350° C. and a steam volumetric flow of 100 ml/min and run until the production plateaued, which occurred after approximately 8 hours, see FIG. 6. The second experiment was carried out using zinc powder at 400° C. and a steam volumetric flow of 150 ml/min. Similarly to the first experiment, it took approximately 8 hours for the hydrogen production to start levelling out, see FIG. 6.

EXAMPLE 2—ELECTROCHEMICAL REACTION OF ZINC OXIDE

Methods

[0148] The second experiment conducted was an electrochemical reaction using the apparatus 100 shown in FIG. 7. The apparatus 100 comprised an electrolysis cell 102 made by the University of Surrey Workshop. As shown in FIG. 8, the cell 102 comprised a Perspex™ housing 104 which was provided in two separate parts 106, 108. A cathode 110 was disposed in the first part 106 of the housing 104, and an anode 112 was disposed in the second part 108 of the housing 104. Both the cathode 110 and the anode 112 comprised a graphite plate.

[0149] A rubber O-ring 114 was disposed in the first part 106 of the housing 104, and surrounded the cathode. Prior to use, an anion exchange membrane 116 would be placed over the cathode 110 and O-ring 114, so that the edges of the membrane 116 extended over the O-ring 114. The second part 108 of the housing 104 would then be placed over the membrane 116 and the first and second parts 106, 108 of the housing would be fixed together by screwing bolts into the corresponding bolt holes 118 disposed in the first and second parts 106, 108 of the housing.

[0150] Using peristaltic pumps (not shown) sodium zincate 120 was circulated through the cathode 110 side of the cell 102 and sodium hydroxide 122 was circulated through the anode 112 side of the cell 102 throughout the experiment. Details describing the preparation of the sodium zincate 120 are provided below.

[0151] A conventional DC power supply 124 was used to apply power to the electrodes 110, 112.

[0152] Two measuring cylinders 126a, 126b were disposed inverted in baths 128 comprising water 130. Nylon tubing 132 was connected to the top of the cylinders 126, allowing them to be filled with the waster 130 using a vacuum pump 134 prior to the start of an experiment. Valves 136 could then be closed. Nylon tubing 138 extending between the cathode 110 side of the cell 102 and one of the measuring cylinders 126a is configured to carry hydrogen 140 produced in an experiment to the measuring cylinder 126a.

[0153] Similarly, nylon tubing 142 extending between the anode 112 side of the cell 102 and one of the measuring cylinders 126b is configured to carry oxygen 144 produced in an experiment to the measuring cylinder 126b.

Results

Solubility Experiment Results

[0154] Before any electrolysis cell experiments could be carried out, the best electrolyte solution to use had to be determined.

[0155] Zinc oxide is insoluble in water. However, it will react with sodium hydroxide according to the following formula:

ZnO.sub.(s)+2NaOH.sub.(aq).fwdarw.Zn(OH).sub.2(aq)+Na.sub.2O.sub.(aq)  1

[0156] The sodium oxide then reacts with the water like so:

Na.sub.2O.sub.(aq)+H.sub.2O.sub.(l).fwdarw.2NaOH.sub.(aq)  2

[0157] The excess NaOH that is available in the solution is used to dissolve the Zn(OH).sub.2 to form a pseudo compound which can be labelled as sodium zincate, Na.sub.2Zn(OH).sub.4, like so:

Zn(OH).sub.2(aq)+2NaOH.sub.(aq).fwdarw.Na.sub.2Zn(OH).sub.4(aq)  3

[0158] In order to find the range of molarities of sodium zincate solution which could be made, the solubility of sodium hydroxide (NaOH) in water had to first be determined. Experiments were carried out to find the maximum solubility of sodium hydroxide in water at room temperature. This was achieved by periodically adding sodium hydroxide to 100 ml of de-ionised water which was being continuously mixed by a magnetic stirrer. When the solution began going cloudy it was noted that dissolution had stopped.

[0159] As the dissolution of sodium hydroxide in water is exothermic an ice bath was used to keep the solution cool. Initially, sodium hydroxide pellets were added to the water in 10 g intervals, allowing the solution to cool between additions. Once the range of maximum solubility was found (in the range of 80 g-90 g) the solution could be re-made and smaller increments of sodium hydroxide added closer to the saturation point at room temperature.

[0160] It was found that a maximum mass of 82 g NaOH could be dissolved in 100 ml of water. The maximum found was less than in the literature (O'Neil, 2006) which states that 109 g can be dissolved in 100 ml of water at 20° C. To keep the solution contained and stop any water vapour escaping, Parafilm was used to cover the top of the beaker in-between the addition of sodium hydroxide pellets. However, some water droplets did form on underside of the film, which could have contributed to this discrepancy from the literature, as there was less water for the sodium hydroxide to dissolve in.

[0161] To find the maximum solubility of zinc oxide in a sodium hydroxide solution a 1M NaOH solution was first created using 100 ml of water and 4 g NaOH. Then 0.5 g of ZnO was added and was left to mix for approximately 5 minutes. If it dissolved, another 0.5 g of ZnO was added, if it didn't 4 more grams of NaOH was added to the solution. This method was carried out until no more ZnO dissolved, despite adding more NaOH to the solution, and the results are shown in FIG. 9. The maximum solubility of zinc oxide was found to be 14 g in a solution containing 100 ml of water and 64 g of NaOH (to create a 1.78M sodium zincate solution). The graph shows that as more NaOH is added to the solution more zinc oxide can react and the sodium zincate can dissolve in solution.

[0162] However, as shown in FIG. 9, as the solution reaches 30 wt % NaOH, the percentage of zinc oxide that reacts and dissolves increases more slowly and then levels off at just under 8 w % ZnO.

Conductivity Experiment Results

[0163] Sodium zincate solutions were prepared by adding either 12 g, 16 g, 20 g or 24 g of NaOH, and the required quantity of ZnO, to 100 ml deionised water. The solution was left to dissolve for between 5-10 minutes until it was clear, and the conductivity was then measured by using a Mettler Toledo conductivity probe. All readings were calibrated to 20° C. to keep results consistent and directly comparable. The conductivity probe was rinsed with D.I. water after the reading was complete to remove any sodium zincate solution, and left in D.I. water between readings to avoid contamination.

[0164] As shown in FIG. 10, as the molarity of the sodium zincate decreases the conductivity increases. Furthermore, increasing the concentration of NaOH increases the conductivity of the solution. Based on these results, a solution containing 100 ml water, 16 g NaOH and 1 g ZnO (0.12M) was chosen to scale up into a litre solution for use as the electrolyte in the cathode side of the cell. It may be appreciated that higher concentrations of NaOH could be used if a solution with higher conductivity was desired.

[0165] The conductivity of sodium hydroxide also needed to be measured to decide which sodium hydroxide solution had the best conductivity and therefore would provide the best performance for the cell. Sodium hydroxide solutions between 0.5M and 7M were prepared and the conductivity was measured. As shown in FIG. 11, the inventors observed an increase in conductive with increasing molarity until reaching a peak at 4.5M when the conductivity begins to deteriorate. Accordingly, the concentration of sodium hydroxide in the electrolyte for the anode side was selected to be 4.5M, as it gave the highest conductivity.

Cell Experiment Results

[0166] Sodium zincate (Na.sub.2Zn(OH).sub.4) solution is electrolytically decomposed in the cell by evolving hydrogen gas along with simultaneously electrodepositing zinc particles on the surface of cathode, and also evolving oxygen gas in the anode as described in the following anode-cathode summary reaction:

Na.sub.2Zn(OH).sub.4(aq).fwdarw.2NaOH+Zn+H.sub.2+O.sub.2  3a

[0167] The effects of voltage on hydrogen production were studied using voltages of 2.5V, 3.0V and 3.5V. As shown in FIG. 12, as the voltage is increased the amount of hydrogen produced also increases. From 2.5V to 3V only a small increase in hydrogen production occurred.

[0168] The effects of voltage on the current in the cell were studied for the three different voltages: 2.5V, 3.0V and 3.5V, and the results are shown in FIG. 13. The results show that as the voltage is increased the current increased. The pattern for the 2.5V experiments shows an initial decrease in current with time which then stabilises, giving an average current of 0.25 A. The current in the 3V experiment remained level at first but then slowly increased to almost 1.5 A, with an average current over the experiment being recorded as 1.22 A. The 3.5V experiment had an initial rapid rise, then continued to slowly rise with respect to time, giving an average current of 3.51 A.

[0169] The zinc produced in the above experiments was recovered from the electrode surface. Further experiments were run varying the various experimental parameters, and the results are shown in table 1.

TABLE-US-00001 TABLE 1 Mass of zinc recovered from various electrolysis experiments Electrolyte on cathode side Molarity Molarity Mass of of of sodium Experiment zinc Voltage/ sodium hydroxide/ duration/ recovered/ Experiment V zincate/M M mins g  1 2.5 0.12 3.0  60 0.21  2 3.0 0.12 3.0  60 1.83  3 3.5 0.12 3.0  60 3.44  4 5.0 0.06 3.0 120 4.48  5 4.1 0.06 3.0  65 2.47  6 2.4 0.06 3.0 120 0.42  7 2.5 0.06 3.0 120 0.85  8 2.8 0.06 3.0 120 1.11  9 2.8 0.06 4.5 120 1.47 10 2.5 0.06 3.0 120 0.84 11 2.8 0.09 3.0 120 1.82

[0170] As shown in table 1, the higher the voltage, and therefore the current, the greater the amount of zinc recovery. Comparing experiments 8 and 9, it is noted that increasing the concentration of the sodium zincate solution increases the mass of zinc recovered. It was noted that the current increased at a faster rate in experiment 9 and reached an overall higher current than in experiment 8. It is thought that this is due to the increased conductivity of the electrolyte, and led to the increased mass of zinc.

[0171] The cell efficiencies for experiments 1 to 3 were calculated so their performance could be compared, and the results are shown in table 2.

TABLE-US-00002 TABLE 2 Cell efficiencies for experiments 1 to 3 Hydrogen production Hydrogen Volume rate per unit Experiment produced/cm.sup.3 of cell/cm.sup.3 volume/hr.sup.−1 Voltage/V Current/A Cell Efficiency 1 2 2000 0.001 2.5 0.2504 1.60 2 59 2000 0.0295 3.0 1.225 8.03 3 590 2000 0.295 3.5 3.515 23.98

[0172] The trend shows that as the voltage increased and more hydrogen is produced, the cell efficiency increases.

EXAMPLE 3—ELECTROCHEMICAL REACTION OF SODIUM STANNATE

[0173] To show that the method was applicable for use with metals other than zinc, the inventors also investigated the electrochemical reaction of sodium stannate.

Method

Apparatus

[0174] An undivided electrolysis cell, was used to carry out the batch experiments. The cell comprised of a rectangular, open top perspex acrylic vessel with two cylindrical graphite electrodes disposed 6 cm apart and connected to the bottom of the vessel. The vessel had a height of 25 cm, a length of 20 cm, and a width of 12 cm. The part of the electrode which remained inside the vessel had a length of 3 cm and a diameter of 1 cm. Electric wires connected the electrodes to the power supply. The power supply set the voltage (and current) prior to each experiment. Each electrode had a cylinder (500 ml) with fitted valves placed over it. Tubing was used to connect one the cylinders to the vacuum pump, which was used to fill the cylinders with electrolyte solution prior to the start of the experiments. The valves were then closed prior to the start of the experiment.

Electrical Conductivity

[0175] The electrolyte was prepared by dissolving 55.4 g of sodium stannate (42-45% SnO.sub.2 basis), into 200 ml of distilled water. The solution produced had a concentration of 1.3M. This concentration was determined based on sodium stannate's solubility limit in water. The reaction which occurred was exothermic:

Na.sub.2SnO.sub.3(s)+3H.sub.2O.Math.Na.sub.2[Sn(OH).sub.6].sub.(aq)  4

[0176] The electrical conductivity of the electrolyte was measured using a conductivity measuring device. Then the solution was diluted with the addition of distilled water to the desired concentrations. These ranged from 1.3-0.1M, decreasing by 0.1M, at each interval. The conductivity was measured at every concentration. Using, the conductivity, the resistance was calculated using the following equation:

R=L/σ.Math.S  5

where R is electrical resistance (Ω), σ is electrical conductivity (mS/cm), L is the distance between the electrodes (cm) and S is the electrode surface area (cm.sup.2). In this experiment, the electrode surface area was fixed at 11 cm.sup.2.

Hydrogen Production

[0177] Once electrolyte had been produced with the desired concentration, as described above, it was mixed thoroughly to ensure a constant concentration gradient. It was then poured carefully into the reaction vessel and the power supply was turned on. The current was fixed at 1 A for all experiments.

[0178] Two different experimental procedures were carried out. The first experimental procedure involved measuring the hydrogen production rate, over time, at three different concentrations of electrolyte around the optimum. The optimum concentrations were determined from the results of the electrical conductivity measurements. The second experimental procedure involved measuring the hydrogen production rate, over time, at three different voltages: 2, 2.5, and 3V. Each experiment lasted for three hours, with the hydrogen production rates measured at five minute intervals.

[0179] The electrolysis which is desirable is that of sodium stannate trihydrate. However, it can exist in equilibrium with tin (IV) hydroxide and NaOH, as shown below:

Na.sub.2[Sn(OH).sub.6].sub.(aq).Math.2NaOH.sub.(aq)+Sn(OH).sub.4(aq)  6

[0180] The process was carried out at room temperature, which is beneficial as it tends to favour the formation of sodium stannate trihydrate at equilibrium. Even though the dissociation of water is relatively small in this process, it is still possible. It will lead to the production of hydrogen gas at the cathode. However, the more prominent reaction which occurs at the same time is the reduction of tin(IV) hydroxide to tin. The metal deposits on the surface of the cathode. These reactions can be seen respectively as:

4H.sub.2O.sub.(l)+4e.sup.−.Math.2H.sub.2(g)+4OH.sup.−.sub.(aq)  7

Sn(OH).sub.4(aq)+4e.sup.−.Math.Sn.sub.(s)+4OH.sup.−.sub.(aq)  8

[0181] The reaction at the anode involves the production of oxygen gas:

8OH.sup.−.sub.(aq).Math.2O.sub.2(g)+4H.sub.2O.sub.(l)+8e.sup.−  9

[0182] The summation of equations 6 to 9 simplifies to:

Na.sub.2[Sn(OH)6].sub.(aq).Math.2NaOH.sub.(aq)+Sn.sub.(s)+2H.sub.2(g)+2O.sub.2(g)  10

[0183] The tin deposited on the cathode can then be oxidised to produce tin (IV) hydroxide and hydrogen gas. It will be appreciated that this process could be carried out using the apparatus described in example 1. The equation for this reaction is:

Sn.sub.(s)+4H.sub.2O.sub.(l).Math.Sn(OH).sub.4(aq)+2H.sub.2(g)  11

[0184] The tin (IV) hydroxide produced can be used to replace the sodium stannate trihydrate which has reacted according to equation 6. By taking the summation of equations 10 and 11, the overall equation used to represent the process can simplify to:

4H.sub.2O.Math.4H.sub.2(g)+2O.sub.2(g)  12

Cell Efficiency

[0185] The cell efficiency was calculated based on a combination of measurements taken from the experiments and parameters used to conduct the experiments. The equation used to determine the cell efficiency was:

n=q/(U.Math.I.Math.t)  13

where n is the cell efficiency based on the hydrogen production rate [m.sup.3 m.sup.−3 h.sup.−1 (kWh).sup.−1], q is the hydrogen production rate per unit volume of electrolyte in electrolysis cell (m.sup.3 m.sup.−3 h.sup.−1), U is the cell voltage (V), I is the cell current (A) and t is time (hours).

[0186] The term (kWh).sup.−1, is a combination of the terms in the denominator of equation 10. It is a representation of the amount of power consumed by electrolysis.

[0187] During these experiments, the voltage varies (2, 2.5, and 3V) with a fixed electrolyte concentration, current and specified time.

Results and Discussion

Concentration and Conductivity

[0188] As explained above, the conductivity of the electrolyte was measured at various concentrations. As shown in FIG. 15, a decrease in the concentration of the electrolyte leads to a decrease in the conductivity. This is because the number of ions per unit volume decreases as the concentration decreases. Therefore, there are fewer ions to carry the electrical charge per unit volume, which results in a decrease in conductivity. Based on this fact, it is expected that the electrolyte concentration of 1.3M, the maximum concentration used in the experiment, will have the highest conductivity. However, this was not the case. The optimum conductivity was at a concentration of 1.2M. The unexpected results could be explained by the electrolyte being at its solubility limit.

[0189] Additionally, the relationship between resistance and conductivity is reciprocal.

Concentration and Hydrogen Production

[0190] Concentrations of 1.1 M, 1.2 M and 1.3 M sodium stannate trihydrate were chosen based on these electrolytes having the highest conductivities. The hydrogen production for these electrolytes was then measured.

[0191] The greatest yield of hydrogen was measured for the 1.2M electrolyte, see FIG. 16. Accordingly, the electrolyte with the highest conductivity produced the most hydrogen in a given time.

Voltage and Hydrogen Production

[0192] The three voltages considered in FIG. 17 were chosen based on the minimum cell voltage used in water electrolysis for hydrogen production, which was 2V. The hydrogen production was then measured.

[0193] The most amount of hydrogen accumulated was when the voltage was at its highest, i.e. 3V. This is expected since a larger voltage accelerates the rate at which electrolysis occurs, which leads to more ions flowing to the electrodes. Hence, more hydrogen accumulated over time. Comparing this to FIG. 16, it can be seen that voltage has a more significant effect on hydrogen production than concentration. The difference in the hydrogen production rate between 2V and 2.5V was approximately 150 cm.sup.3. This is a substantial change in hydrogen accumulated based on the scale of production for the relative change in voltage.

Cell Efficiency

[0194] The cell efficiency was calculated based on a 1.2M electrolyte solution, and the results are provided in table 3.

TABLE-US-00003 TABLE 3 The cell efficiency for a 1.2M electrolyte solution Voltage (V) Cell efficiency [m.sup.3 m.sup.−3 h.sup.−1 (kWh).sup.−1] 2 10.68 2.5 4.35 3 2.03

[0195] The first observation made is an increase in voltage leads to a decrease in cell efficiency. Even though it gives a more considerable amount of hydrogen produced, the cell is less efficient, which is undesirable. Based upon these results, a cell using sodium zincate (discussed in example 2) is considered to be more efficient. However, it is hard to make a direct comparison because the amount of electrical power consumed during the electrolysis experiment was not the same. Therefore, this will lead to some discrepancies in the results.

[0196] It appears that there is a significant drop-off in efficiency in the range 2-2.5V compared to 2.5-3V. For this reason, it would be more efficient to use smaller voltages, as this is when the cells are the most efficient. Nevertheless, at lower voltages, i.e. 2V it is apparent the cell is more efficient than a water electrolyser which could be as a result of the ionic activator in the electrolyte solution.

CONCLUSION

[0197] The inventors have shown that it is possible to generate hydrogen gas by converting zinc to zinc oxide in a thermochemical reaction, and to generate further hydrogen gas by converting zinc oxide to zinc in an electrochemical reaction. Accordingly, once an initial quantity of zinc/zinc oxide has been provided, it is possible to run these reactions in a cycle, as shown in FIG. 14, to generate large amounts of hydrogen gas where the only feedstock required is water.

[0198] The inventors have shown that this system can also be applied to other metals, such as tin. In particular, the inventors have produced tin from sodium stannate in an electrolysis reaction.