HYDROGEN GENERATOR

Abstract

The disclosure relates to an electrolysis cell for producing hydrogen. The cell comprises an electrolyte compartment and an electrolyte disposed therein. The electrolyte comprises an aqueous alkaline solution comprising a transition metal ion or p block metal ion. The cell further comprises first and second spaced apart electrodes at least partially disposed in the electrolyte.

Claims

1. An electrolysis cell for producing hydrogen, the cell comprising an electrolyte compartment; an electrolyte disposed in the electrolyte compartment, wherein the electrolyte comprises an aqueous alkaline solution comprising a transition metal ion or p block metal ion; and first and second spaced apart electrodes at least partially disposed in the electrolyte.

2. An electrolysis cell according to claim 1, wherein the first and second electrodes comprise graphite, chromium, nickel, zinc, cadmium, copper, tin, lead, rhodium, platinum, gold, palladium, iridium, osmium, rhenium, ruthenium, germanium, beryllium, silver, brass, and/or bronze.

3. (canceled)

4. An electrolysis cell according to claim 1, wherein at least one of the electrodes comprises at least a layer of a transition metal or p block metal which is the same metal as the transition metal ion or p block metal ion in the electrolyte.

5. An electrolysis cell according to any-preceding claim 1, wherein the aqueous alkaline solution comprises an Arrhenius base, a Lewis base, or a Bronsted-Lowry base, preferably a strong Arrhenius base or a Lewis superbase.

6. An electrolysis cell according to claim 5, wherein the aqueous alkaline solution comprises an Arrhenius base selected from the group consisting of potassium hydroxide, sodium hydroxide, barium hydroxide, caesium hydroxide, strontium hydroxide, calcium hydroxide, lithium hydroxide, and rubidium hydroxide, optionally wherein the ratio of the hydroxide concentration to the transition metal ion or p block metal ion concentration is between 2:1 and 50:1, between 3:1 and 40:1, between 4:1 and 30:1, between 5:1 and 25:1, or between 10:1 and 20:1.

7. (canceled)

8. An electrolysis cell according to either claim 6, wherein the aqueous alkaline solution comprises sodium hydroxide and the ratio of hydroxide concentration to the transition metal ion or p block metal ion concentration is between 10:1 and 15:1.

9. (canceled)

10. An electrolysis cell according to either claim 6, wherein the aqueous alkaline solution comprises potassium hydroxide and the ratio of hydroxide concentration to the transition metal ion or p block metal ion concentration is between 15:1 and 20:1.

11. (canceled)

12. An electrolysis cell according to claim 1, wherein the pH of the aqueous alkaline solution at 20 C. is at least 9, at least 10, at least 11 or at least 11.5 and/or wherein the concentration of the transition metal ion or p block metal ion in the electrolyte is between 0.01 M and 1.2 M, between 0.05 M and 1 M, between 0.1 M and 0.8 M, between 0.15 M and 0.6 M, or between 0.2 M and 0.4 M.

13. (canceled)

14. An electrolysis cell according to claim 1, wherein the transition metal ion or p block metal ion comprises a zinc ion, a copper ion, an iron ion, a nickel ion, a cobalt ion, a chromium ion, a cadmium ion, a vanadium ion, a titanium ion, a yttrium ion, a zirconium ion, a scandium ion, an aluminium ion, a tin ion, and/or a lead ion.

15. An electrolysis cell according to claim 14, wherein the electrolyte comprises sodium zincate and the concentration of the zinc ion is between 0.01 M and 0.6 M, between 0.1 M and 0.4 M, between 0.15 M and 0.3 M, or between 0.2 M and 0.25 M.

16. (canceled)

17. An electrolysis cell according to claim 14, wherein the electrolyte comprises potassium zincate and the concentration of the zinc ion is between 0.01 M and 1.2 M, between 0.1 M and 0.6 M, between 0.2 M and 0.5 M, or between 0.25 M and 0.45 M.

18. (canceled)

19. An electrolysis cell according to claim 1, wherein the cell comprises a power supply configured to apply a voltage across the first and second electrodes, optionally wherein the power supply is configured to apply a voltage of between 1 V and 6 V, between 1.5 V and 3 V, or between 2 and 2.5 V.

20. An electrolysis cell according to claim 19, wherein the power supply is configured to supply a direct current to the electrodes, and the apparatus comprises a control system configured to monitor the current flowing through the electrodes and to switch the direction of the current if the control system detects that the current has fallen below a predetermined level.

21. An electrolysis cell according to claim 19, wherein the power supply is configured to apply an alternating or oscillating current to the electrodes.

22. (canceled)

23. An apparatus for generating and storing hydrogen, the apparatus comprising the electrolysis cell of claim 1, a chamber for hydrogen gas storage, and a conduit configured to feed hydrogen gas from the cell to the chamber.

24. An apparatus according to claim 23, wherein the chamber comprises: a first portion comprising a first gas; a second portion comprising a second gas; and a liquid, wherein the chamber is configured to allow the liquid to flow between the first and second portions and to prevent the first gas in the first portion from exchanging with the second gas in the second portion.

25. An apparatus according to claim 24, wherein the first portion of the chamber comprises an outlet comprising a valve, optionally wherein the valve comprises a back-pressure regulating valve.

26. (canceled)

27. (canceled)

28. (canceled)

29. A method of producing hydrogen, the method comprising applying a voltage across electrodes, wherein the electrodes comprise a cathode and a spaced apart anode and are at least partially disposed in an electrolyte comprising an aqueous alkaline solution comprising a transition metal ion or p block metal ion and the anode comprises at least a layer of a transition metal or p block metal.

30. A method according to claim 29, wherein the transition metal or p block metal of the anode is the same metal as the transition metal ion or p block metal ion in the electrolyte, optionally wherein the method comprises a primary step of: applying a voltage across the electrodes wherein the electrodes are at least partially disposed in the electrolyte, and thereby causing a layer of the transition metal or p block metal to form on the cathode; and switching the electrodes to provide an anode comprising the layer of the transition metal or p block metal.

31. (canceled)

32. A method according to claim 29, wherein the method comprises adding water to the electrolyte to maintain the concentration of the transition metal ion or p block metal ion dissolved therein.

Description

[0063] 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 Figures, in which:

[0064] FIG. 1a is a schematic diagram of the experimental set-up for an electrolysis cell; and FIG. 1b is a photo of the experimental set-up;

[0065] FIG. 2a is a graph showing the change in hydrogen production rate and current over time; and FIG. 2b is a schematic diagram showing how the zinc layer on the anode changes with time;

[0066] FIG. 3 is a graph showing the conductivity and resistance of the electrolyte as a function of concentration of sodium zincate in an aqueous solution;

[0067] FIG. 4 is a graph showing the hydrogen production versus the concentration of sodium zincate at 1.8 V, 2.0 V and 2.2 V for an electrolysis system using graphite electrodes;

[0068] FIG. 5 is a graph showing the current density versus the concentration of sodium zincate at 1.8 V, 2.0 V and 2.2 V for an electrolysis system using graphite electrodes;

[0069] FIG. 6 is a graph showing the effect of the graphite electrode surface on hydrogen production rate;

[0070] FIG. 7 is a graph showing the hydrogen production versus the concentration of sodium zincate at 1.8 V, 2.0 V and 2.2 V for an electrolysis system using zinc electrodes;

[0071] FIG. 8 is a graph showing the current density versus the concentration of sodium zincate at 1.8 V, 2.0 V and 2.2 V for an electrolysis system using zinc electrodes;

[0072] FIG. 9 is a graph showing the hydrogen production versus the concentration of potassium zincate at 1.8 V, 2.0 V and 2.2 V for an electrolysis system using graphite electrodes;

[0073] FIG. 10 is a graph showing the hydrogen production versus the concentration of potassium zincate at 1.8 V, 2.0 V and 2.2 V for an electrolysis system using zinc electrodes; and

[0074] FIG. 11 is a schematic diagram of an embodiment of a hydrogen generator in accordance with the invention.

EXAMPLE 1

Production of Hydrogen Gas Using Graphite Electrodes and an Electrolyte Comprising Sodium Zincate

Materials and Methods

Apparatus

[0075] Batch experiments were carried out in a closed electrolysis cell 2, shown in FIG. 1. The cell 2 comprised a rectangular acrylic (Perspex) container 4 which was 25 cm high with a length and width of 20 cm12 cm. This ensured enough capacity to contain different volumes of solution for all of the experiments. Cylindrical graphite electrodes 6 were utilised which were 1 cm in diameter and 3 cm in height. Each electrode 6 was attached to the base of the container 4, and the electrodes were spaced apart by 3 cm. A 1 litre gas collecting tube 8 was placed over each electrode 6 to capture hydrogen gas 12 produced in the experiment. A scale on the side of the tubes 8 allowed the volume of hydrogen gas produced to be measured.

[0076] Nylon tubing 9 was connected to the top of the tubes 8, allowing them to be filled with an electrolyte using a vacuum pump 13 prior to the start of an experiment. Valves 11 could then be closed ensuring that any gas collected during the experiment would remain in the tubes.

[0077] A conventional DC power supply (DIGIMESS HY3010, 0-30V/0-10 A) 10 was used to apply voltage to the system.

Experimental Procedure

[0078] Sodium zincate solution (0.59 mole/litre) was prepared by first dissolving 660 g sodium hydroxide pellets (certified grade Sigma-Aldrich, 06203, 98% purity) in 1386 ml of distilled water. This would cause an exothermic reaction which would raise the temperature of the solution in which 66 g zinc oxide powder (certified grade Honeywell, 205532, 99.9% purity) was dissolved. Accordingly, the zinc oxide reacted with the sodium hydroxide solution according to the following formula:

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

[0079] The sodium oxide then reacts with the water like so:

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

[0080] 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

[0081] It has not yet been ascertained which compounds form under the influence of zinc hydroxide on strong bases. Some reports have denied the existence of sodium zincate on the grounds of conductivity measurements and have speculated that Zn(OH).sub.2 in NaOH instead forms a colloidal solution.

[0082] Analysis by Reichle et al [7] using Atomic Absorption Spectrophotometer have postulated the solubility of zinc hydroxide in NaOH can be interpreted in terms of Na.sub.2Zn(OH).sub.4 in equilibrium with saturated solution that contain species which can be represented by Zn.sup.2+.sub.(aq), Zn(OH).sup.+.sub.(aq), Zn(OH).sub.2(aq), Zn(OH).sub.3.sup..sub.(aq) and Zn(OH).sub.4.sup.2.

[0083] The solution would then be diluted with distilled water to achieve the desired zinc hydroxide concentrations.

[0084] The solution was poured into the electrolysis cell 2 and the two gas collecting tubes 8 were used to capture the hydrogen gas. A voltage was then applied which caused the following reactions to occur at the cathode:

Zn(OH).sub.2(aq)+2e.sup..fwdarw.Zn.sub.(s)+2OH.sup..sub.(aq) E=0.828 V4

and:

2H.sub.2O.sub.(l)+2e.sup..fwdarw.H.sub.2(g)+2OH.sup..sub.(aq) E=1.249 V5

[0085] Similarly, the following reaction occurred at the anode:

4OH.sup..sub.(aq).fwdarw.O.sub.2(g)+2H.sub.2O.sub.(l)+4e.sup.E=0.4 V6

[0086] Accordingly, the overall reaction can be written as:

Zn(OH).sub.2(aq).fwdarw.Zn.sub.(s)+H.sub.2(g)+O.sub.2(g)7

The reversible potential E, or equilibrium cell voltage, defined as the equilibrium potential difference between the anode and cathode is 1.677 V for this set of reactions. Due to cell resistance, the inventors found that the minimum operational was 1.9 V. To ensure the reaction proceeded at a reasonable rate, the inventors used a voltage of 2.5 V.

[0087] Deposition of a uniform layer of zinc on the cathode was observed due to the reduction of zinc hydroxide according to equation 4. After four hours the inventors found that the layer of zinc was sufficient to proceed to the next stage. At this point the electrodes were switched, by swapping the two (+/) ports on the power supply. Batch experiments could then be carried out. The experiments were run at 2.2 V, 2.0 V and 1.8 V for each concentration.

[0088] As indicated in equation 6 oxygen gas is produced at the anode. This then reacts with the zinc layer, like so:

2Zn.sub.(s)+O.sub.2(g).fwdarw.2ZnO.sub.(s)8

[0089] The zinc oxide then reacts with the sodium hydroxide in the solution according to equations 1 to 3 to give a solution containing zinc hydroxide and/or sodium zincate. This will react at the cathode according to equation 4. Accordingly, the following process is occurring:

Zn.sub.(s)+2H.sub.2(O).fwdarw.Zn(OH).sub.2(aq)+H.sub.2(g)9

[0090] All measurements were collected manually by reading of the scale on the gas collecting tubes 8 to determine the volume of hydrogen gas released and logging the electrical current. Readings were taken in 5 minute intervals to monitor the hydrogen production over time.

[0091] The solution was mixed regularly using a glass rod to ensure uniform gradient of zinc hydroxide. When suspended zinc solids were observed in the solution, creating a deviation from the intended concentration, the solids were removed and replaced with the equivalent molar amount of zinc oxide power. Alternatively, smaller quantities of suspended zinc were swept towards the anode, causing it to oxidise to form zinc hydroxide, according to equation 8.

[0092] It was important to track the amount of water consumed in the electrolysis cell. Since water is dissociated to produce hydrogen the concentration of the solution will change as the electrolysis progresses as water is dissociated to produce hydrogen and oxygen gas. The amount of water that is used is calculated based on the cumulative hydrogen produced, as that was the only gas that could be measured. To maintain the concentration of sodium zincate in the solution, 1 ml of water was added for every 1244 cm.sup.3 of hydrogen gas produced.

Electrode Switching

[0093] As indicated in equation 8, above, as the electrolysis reaction progresses the zinc 14 disposed on the anode 16 reacts with oxygen 18 to produce zinc oxide. Accordingly, the layer of zinc 14 on the anode 16 is reduced as the reaction progresses. FIG. 2b(i) shows the anode 16 at the start of a reaction with a thick layer of zinc 14 disposed on its outer surface. FIG. 2b(ii) shows the anode 16 after the electrolysis reaction has been run for some time, it will be noted that the layer of zinc 14 has been substantially decreased. Finally, FIG. 2b(iii) shows the anode 16 after the electrolysis reaction has been run for an even longer period of time, and the layer of zinc 14 has been completely depleted. Accordingly, oxygen bubbles 18 are now forming on the anode 16 in accordance with equation 6. The inventors have noted that when the layer of zinc is depleted then the observed current and the rate of hydrogen production decrease, as shown in FIG. 2a.

[0094] However, while the layer of zinc 14 is being depleted on the anode 16, a further layer of zinc 14 will be being deposited on the cathode, according to equation 4. Accordingly, once the level of zinc on the anode 16 was depleted, the inventors could switch the electrodes by swapping the two (+/) ports on the power supply. The new anode would then have a layer of zinc disposed thereon.

[0095] There were several observable indicators to suggest the time of switching the electrodes, namely the drop in the current, the decrease in the hydrogen production rate and the increase in the oxygen production rate. In theory these indicators are expected to appear at the same time. However, it was important to have one main indicator to ensure consistency. Accordingly, the level of the current was used as the main indicator to switch the electrodes. The inventors observed that once the layer of zinc 14 becomes so thin that some areas of the anode 16 are exposed to the solution then current begins to drop rapidly, and it was at this point the inventors switched the electrodes.

[0096] It will be appreciated that the switch time varied between experiments due to changes in the applied voltage and the concentration of the solution. Accordingly, the inventors predicted at what time they expected the current to drop, but also maintained live observation of the current so that as soon as any drop in current was detected and the electrodes were switched. This approach ensured that a consistent standard was maintained.

Results and Discussion

Effect of Sodium Zincate Concentration

[0097] Ionic transfer within the solution depends on the concentration of the solution and distance between the electrodes. The ionic resistance can be minimised by reducing the gap between the electrodes 6. The smallest practical distance using the equipment shown in FIG. 1 was 3 cm as this left room for the gas collecting tube 8 to be accommodated. The ionic resistance of the solution depends on concentration of the ions therein and can be estimated by measuring the conductivity of the solution. The direct relationship between conductivity and resistance is given by the equation R=l/A, where R is the electrical resistance, is the electrical conductivity, l is the length and A is the area. In the experimental set-up l was 0.6 cm and A was 1 cm.sup.2. Therefore the effect of changing the concentration of the sodium zincate solution on the resistance can be analysed. The result of this is illustrated in FIG. 3.

[0098] Ionic transfer is controlled by convective mass transfer in the solution. At lower concentrations (<0.25M), the conductivity is low due to the solution becoming heavily diluted resulting in a decrease in the number of ions. Conversely, at higher concentrations, the conductivity also decreases because of less mobility of the ions due to higher viscosity of the solution and also the formation of neutral ion-pairs not contributing to the overall cell conductivity. Accordingly, the optimum concentration for the solution is around 0.25M.

[0099] The ability to sustain a passage of electrical current by the electrolyte solution depends on the mobility of its constituent charged ions in the electric field between electrodes immersed in the electrolyte. Better ion mobility leads to higher reaction rates that in turn increases hydrogen production rate. Accordingly, the inventors also analysed the hydrogen production rates at different concentrations, and the results are shown in FIG. 4.

[0100] The highest hydrogen production rate observed was at 0.2M, and the trends for all three voltages suggests that the optimum concentration is between 0.1 M and 0.3 M. Below this concentration, the hydrogen production decreases as the solution becomes heavily diluted and the availability of zinc hydroxide ions at the electrode and electrolyte interface is restricted. Similarly, above this concentration, neutral ion-pairs form and these will not be drawn to the electrodes.

Effect of Voltage

[0101] The effect of the applied operating cell voltage on the hydrogen production was experimented by tuning the voltage for each experiment.

[0102] The hydrogen production increases at higher voltages as it increases the electrolysis process. The relationship between the cell voltage and current characterises the electrochemical behaviour of an electrolysis cell. The hydrogen produced in electrolysis is proportional to the amount of charge involved in the process. Therefore, according to Faraday's law the hydrogen production is directly proportional to the charge transfer, that is, the electric current. Hence when the voltage is tuned on the power supply, electric charge delivered to the electrolysis process by the power supply affects the current. Therefore by increasing the voltage, current density also increases resulting in higher hydrogen production rates as shown in Table 1.

TABLE-US-00001 TABLE 1 Current density at different voltages shown for all concentrations Average current Density (A/cm.sup.2) Concentration (M) 2.2 V 2.0 V 1.8 V 0.59 0.11 0.09 0.09 0.55 0.13 0.11 0.11 0.5 0.15 0.15 0.15 0.45 0.19 0.19 0.17 0.4 0.19 0.16 0.14 0.3 0.22 0.18 0.14 0.2 0.25 0.21 0.18 0.1 0.18 0.18 0.11

[0103] Using lower voltages is desirable, as it allows for greater energy efficiency. Due to the zinc layer covering the graphite electrode, the cell can operate at lower voltage than would otherwise be possible. For instance, the rate of hydrogen production was high when a voltage of 1.8 V was used. However, as explained above, when the anode did not comprise a zinc layer, a minimum voltage of 1.9 V was necessary to simply allow the reaction to proceed at all. Similarly, in industry water electrolysis needs the minimum voltage of 2.0V for hydrogen generation for current densities between 0.1-0.3 A/cm.sup.2 [8].

Effect of Current Density

[0104] It will be appreciated that hydrogen production rate is dependent on the current density in the electrolysis cell. Accordingly, the inventors investigated how the current density changed with the concentration of sodium zincate, and the results are shown in FIG. 5.

[0105] It will be noted that the trends observed for the average current density vs concentration closely reflects the trends observed for rate of hydrogen production vs concentration, confirming the relationship between hydrogen production and current density. In particular, the maximum average current density was observed at a concentration of 0.2M, the same concentration that the maximum hydrogen production rate was observed.

[0106] The current decreases over time as the zinc layer becomes thicker on the electrode. This increases the resistance which are of two components; the resistance of the electrode and the resistance of the electrolyte. The thick zinc layer on the electrode causes an increase in resistance at the electrode and electrolyte interface which means the freshly formed zinc hinders the evolution of hydrogen, or the availability of zinc hydroxide ions at the interface is restricted suspending the electrochemical reaction. The cell responds by decreasing the current resulting in lower rates of hydrogen production.

Electrode Material and Surface Condition

[0107] Selection of the correct electrode material is important for efficient operation of the electrolysis cell. The electrode material was selected to be graphite based as this was understood to provide adequate strength and stability against physical attacks such as erosion by the alkaline solution. At first, the electrodes had a smooth surface. However, when the electrodes were subject to long hours of applied voltage they corroded leaving a slightly porous surface. Repeating the initial experiments determined what effect this change had.

[0108] As shown in FIG. 6, an increase in hydrogen production rate was observed when the electrodes had a porous surface. In fact, an average increase of 96% was observed for same voltage and electrolyte concentration as when performed on a smooth surface. This is due to larger surface area of the electrodes. Accordingly, porous electrodes are advantageous.

Cell Efficiency

[0109] Direct comparison of electrolyser technologies can be made by comparing the energy efficiency of the different technologies. This considers the hydrogen production rate at unit volume electrolysis cell against the total electrical energy applied to the cell [8], as given in the following formula:

.sub.H.sub.2.sub.Production rate=V.sub.H.sub.2/(Uit)10

, where V.sub.H.sub.2 is the hydrogen production rate at unit volume electrolysis cell;
U is the cell voltage;
i is the cell current; and
t is time.

[0110] The units of .sub.H.sub.2 .sub.Production rate are m.sup.3m.sup.3h.sup.1kWh.sup.1. The initial m.sub.3 comes from the volume of the electrolysis container/electrolyte, the m.sup.3h.sup.1 comes from the rate of hydrogen production and the kWh.sup.1 comes from the amount of electrical power used.

[0111] The values obtained when the electrolyte comprised 0.2M sodium zincate are given in table 2 below.

TABLE-US-00002 TABLE 2 Hydrogen production rate at unit volume electrolysis cell against the total electrical energy applied to the cell at various voltages at 0.2M sodium zincate with graphite electrodes Hydrogen production rate at unit volume electrolysis cell against the total electrical energy Voltage/V applied to the cell/m.sup.3m.sup.3h.sup.1kWh.sup.1 1.8 19.5 2 9.3 2.2 6.3

[0112] The cell is considered inefficient if high voltage is required to produce the same hydrogen mass while keeping the current constant. As shown in the table above, the obtained value for this system at 2.2V is 6.3 m.sup.3m.sup.3h.sup.1kWh.sup.1, conversely a value of 2.3 m.sup.3m.sup.3h.sup.3Wh.sup.1 is typical for water electrolysers [9]. Accordingly, the cell developed by the inventors has a greater hydrogen production performance compared to conventional water electrolysers as less power is required to produce hydrogen.

Conclusion

[0113] A practical alkaline water electrolysis cell with graphite electrodes has been constructed for exploring hydrogen production using a zincate solution as an electrolyte. Optimum conditions for operation were investigated by analysing the influences of applied voltage, current and sodium zincate concentration. The experimental results show that the hydrogen production peaked at about 0.2M. Increasing the voltage also increased the hydrogen production rate. Furthermore, the formation of porous graphite surface was another factor which positively affected the hydrogen production rate.

[0114] The results demonstrate a significant hydrogen production at low voltages indicating greater energy efficiency and reducing energy per mass unit of hydrogen produced by a factor of 2.7 compared to conventional water electrolysis systems. Since the experiments represent the feasibility stage, there is further potential for the system to be optimised to achieve greater efficiency.

EXAMPLE 2

Production of Hydrogen Gas Using Zinc Electrodes and an Electrolyte Comprising Sodium Zincate

Materials and Methods

[0115] The apparatus described in Example 1 was modified by replacing the graphite electrodes with zinc electrodes [2.4 cm (h)0.9 cm (d)]. The electrodes were covered with 0.5 cm of a polyethylene hose at the base to protect the base from corrosion.

[0116] As described in Example 1, the electrodes were switched when a decrease in current was observed. However, unlike Example 1, a delay in the hydrogen production was observed. Without wishing to be bound by theory, the inventors hypothesize that this could be due to oversaturation of the electrode. To counter this, when the ports were switched the voltage was increased to 10-12 Amps until there was a hydrogen bubble and then the voltage was decreased to the desired voltage. This was noted in the results as in some cases, there was no or small amounts of hydrogen produced in the first 5 minutes. The current also decreased and the hydrogen production rate increased as this oversaturation was overcome.

[0117] Apart from these differences, the apparatus and experimental method were as described in Example 1.

Results and Discussion

Effect of Sodium Zincate Concentration

[0118] The hydrogen production rates at different concentrations are shown in FIG. 7. Again, the highest hydrogen production rate observed was at 0.2M, and the trends for all three voltages suggests that the optimum concentration is between 0.1 M and 0.3 M.

Effect of Voltage

[0119] Similarly, the inventors found that the cell could operate at low voltages.

TABLE-US-00003 TABLE 3 Current density at different voltages shown for all concentrations (sodium zincate/zinc electrode) Average current Density (A/cm.sup.2) Concentration (M) 2.2 V 2.0 V 1.8 V 0.59 0.13 0.12 0.11 0.55 0.15 0.13 0.12 0.5 0.16 0.15 0.12 0.45 0.18 0.16 0.13 0.4 0.19 0.18 0.15 0.3 0.20 0.19 0.16 0.2 0.23 0.20 0.18 0.1 0.18 0.16 0.13

Effect of Current Density

[0120] As with the graphite electrodes, the inventors investigated how the current density changed with the concentration of sodium zincate, and the results are shown in FIG. 8. Again, the maximum average current density was observed at a concentration of 0.2M, the same concentration that the maximum hydrogen production rate was observed.

Cell Efficiency

[0121] The cell efficiency was also calculated for this system, and the values obtained are given in Table X below.

TABLE-US-00004 TABLE 4 Hydrogen production rate at unit volume electrolysis cell against the total electrical energy applied to the cell at various voltages at 0.2M sodium zincate with zinc electrodes Hydrogen production rate at unit volume electrolysis cell against the total electrical energy Voltage/V applied to the cell/m.sup.3m.sup.3h.sup.1kWh.sup.1 1.8 13.8 2 13.5 2.2 9.31

Conclusion

[0122] The alkaline water electrolysis cell described in Example 1 may be used with electrodes comprising materials other than graphite. In particular, the inventors have shown that zinc electrodes may be used effectively, and using these electrodes achieved similar results to those obtained with graphite electrodes. However, it will be appreciated that electrodes comprising alternative materials may also be used.

EXAMPLE 3

Production of Hydrogen Gas Using Graphite Electrodes and an Electrolyte Comprising Potassium Zincate

Materials and Methods

[0123] The apparatus used was as described in Example 1.

[0124] Potassium zincate solution (1.25 mole/litre) was prepared by first dissolving 1430 g of potassium hydroxide in 980 ml of distilled water. This would cause an exothermic reaction which would raise the temperature of the solution in which 98 g zinc oxide powder (certified grade Honeywell, 205532, 99.9% purity) was dissolved.

[0125] It will be appreciated that the reactions which occurred will be as described in Example 1, except that the sodium would have been replaced by potassium.

[0126] Apart from these differences, the apparatus and method used were as described in Example 1.

Results and Discussion

Effect of Potassium Zincate Concentration

[0127] The hydrogen production rates at different concentrations are shown in FIG. 9. Unlike Examples 1 and 2, the highest hydrogen production rate observed was at 0.3M, and the trends for all three voltages suggests that the optimum concentration is between 0.1 M and 0.4 M.

TABLE-US-00005 TABLE 5 Hydrogen production rate at unit volume electrolysis cell against the total electrical energy applied to the cell at various voltages at 0.3M sodium zincate with graphite electrodes Hydrogen production rate at unit volume electrolysis cell against the total electrical energy Voltage/V applied to the cell/m.sup.3m.sup.3h.sup.1kWh.sup.1 1.8 4.07 2 3.64 2.2 3.40

Conclusion

[0128] The alkaline water electrolysis cell described in Example 1 may be used with alternative electrolytes. In particular, the inventors have shown that a potassium zincate solution can be used. The experimental results show that the hydrogen production peaked at about 0.3M. Again, increasing the voltage also increased the hydrogen production rate.

EXAMPLE 4

Production of Hydrogen Gas Using Zinc Electrodes and an Electrolyte Comprising Potassium Zincate

Materials and Methods

[0129] The apparatus used was as described in Example 2, and the electrolyte used was as described in Example 3.

[0130] Apart from these differences, the apparatus and method used were as described in Example 1.

Results and Discussion

Effect of Potassium Zincate Concentration

[0131] The hydrogen production rates at different concentrations are shown in FIG. 10. The highest hydrogen production rate observed was at 0.4M, and the trends for all three voltages suggests that the optimum concentration is between 0.3 M and 0.5 M.

TABLE-US-00006 TABLE 6 Hydrogen production rate at unit volume electrolysis cell against the total electrical energy applied to the cell at various voltages at 0.4M sodium zincate with zinc electrodes Hydrogen production rate at unit volume electrolysis cell against the total electrical energy Voltage/V applied to the cell/m.sup.3m.sup.3h.sup.1kWh.sup.1 1.8 5.42 2 4.82 2.2 2.84

Conclusion

[0132] The alkaline water electrolysis cell described in Example 3 may also be used with alternative electrolytes. In particular, the inventors have shown that a potassium zincate solution can be used. The experimental results show that the hydrogen production peaked at about 0.4M. Again, increasing the voltage also increased the hydrogen production rate.

EXAMPLE 5

A Small-Scale Hydrogen Generator

[0133] FIG. 11 shows a small-scale hydrogen generator 20. The generator comprises an electrolysis cell 2 comprising a plurality of graphite electrodes 6. The electrodes 6 are shown as anodes 16 and cathodes 22. However, it will be appreciated that the cathodes 16 and anodes 22 can be switched as explained above, alternatively an oscillating or alternating current may be used to continuously switch the electrodes.

[0134] The generator 20 also comprises an electrolyte solution 24 comprising sodium zincate, as described in example 1, and a conduit 26 allows a user to top up the solution 24 as necessary. A valve 28 disposed in the conduit 26 prevents hydrogen 12 produced in the cell 2 from escaping to the atmosphere.

[0135] The generator 20 also comprises a hydrogen store 30. The store 30 comprises a chamber 34 split into two portions 34a and 34b by a separator 36. The portions 34a and 34b are of roughly equal volume. The separator 36 is connected to the top and side walls but leaves a small hole 37 at the base, thereby allowing fluid communication between the two portions 34a and 34b. A liquid 38 is disposed in the chamber 34 fills about half of the volume. The height of the liquid 38 is greater than the height of the gap 37, and thereby prevents gas in the first portion 34a from mixing with gas in the second portion 34b. A vent 40 comprising a pressure safety valve 46 allows gas in the second portion 34b to vent into the atmosphere if the pressure within the chamber 34 exceeds a predetermined level. An operator can access the chamber 34 to replenish the liquid 38 as necessary.

[0136] A conduit 42 from the first portion 34a comprising a back-pressure regulating valve 44 allows gas to be selectively removed from the first portion 34a. Ideally, the pressure within the chamber 34 is maintained between 0.5 and 3 barg.

[0137] While not shown, it will be appreciated that each electrode 6 could be provided in a separate compartment with an associated conduit comprising valves configured to selectively to the atmosphere when the electrode 6 is producing oxygen gas, and to selectively transport gas to the hydrogen store 30 when the electrode 6 is producing hydrogen gas.

[0138] A conduit 32 comprising a solenoid valve 33 extends between the cell 2 and the first portion 34a of the chamber. Accordingly, when the cell 2 is producing hydrogen 12, the valve 33 will be open allowing the hydrogen 12 to flow from the cell 2 to the first portion 34a of the store 30. The hydrogen 12 will displace the liquid 38 in the first portion 34a, and it will flow through the hole 37 into the second portion 34b. The total pressure within the chamber 34 will increase due to the addition of the hydrogen gas.

[0139] Accordingly, the hydrogen gas 12 can be stored until it is required by a user, or could continuously supply hydrogen gas at a desired pressure.

REFERENCES

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