Method for operating of a regenerative bipolar membrane fuel cell, and regenerative bipolar membrane fuel cell there for

Abstract

The invention relates to a method for operating a regenerative bipolar membrane fuel cell and regenerative bipolar cell for storing and generating energy. The method according to the invention comprises: providing a regenerative bipolar membrane fuel cell comprising: a reactor with an anode compartment having an anode and a cathode compartment having a cathode; and a number of cell units separating the anode and cathode compartments, wherein the cell unit comprises an anion exchange membrane, a cation exchange membrane, and a bipolar, with the membranes defining compartments; providing a fluid on both sides of the bipolar membrane with ion concentrations such that water activity difference between the fluids on both sides of the bipolar membrane is minimized; storing energy in an energy storage state by providing an external current to the reactor such that a p H difference between fluids in contact with the bipolar membrane is achieved; switching between the energy storage state and an energy generation state; and regenerating energy in the energy generating state from the p H difference between fluids in contact with the bipolar membrane.

Claims

1. A method for operating a regenerative bipolar membrane fuel cell, the method comprising the steps of: providing a regenerative bipolar membrane fuel cell comprising: a reactor with an anode compartment having an anode and a cathode compartment having a cathode; and a number of cell units separating the anode and cathode compartments, wherein the cell unit comprises an anion exchange membrane, a cation exchange membrane, and a bipolar membrane, with the membranes defining compartments; providing a number of fluids to the cell, with at least a fluid on both sides of the bipolar membrane with ion concentrations such that the difference in water activity of the fluids on both sides of the bipolar membrane is minimized, wherein the difference in water activity of both fluids in contact with the bipolar membrane is maintained within the range of 0.015 to +0.015; storing energy in an energy storage state by providing an external current to the reactor such that a pH difference between fluids in contact with the bipolar membrane is achieved; switching between the energy storage state and an energy generation state; and regenerating energy in the energy generating state from the pH difference between fluids in contact with the bipolar membrane.

2. The method according to claim 1, wherein the difference in water activity of the fluids on both sides of the bipolar membrane is minimized in the energy generation state.

3. The method according to claim 1, wherein storing energy comprises water splitting.

4. The method according to claim 3, wherein the water splitting is performed in the bipolar membrane.

5. The method according to claim 1, wherein in use the reactor is provided with first, second and third fluids, the first fluid comprising a salt solution, the second fluid comprising an acid solution, and the third fluid comprising a base solution.

6. The method according to claim 5, wherein the salt solution comprises one or more of the following ions: Li.sup.+, Na.sup.+, K.sup.+, Mg.sup.2+, Ca.sup.2+, Ba.sup.2+, and Cl.sup., F.sup., Br.sup., I.sup., SO.sub.4.sup.2, NO.sub.3.sup., ClO.sub.4.sup..

7. The method according to claim 6, wherein the base solution comprises one or more of the following ions: Li.sup.+, Na.sup.+, K.sup.+, Mg.sup.2+, Ca.sup.2+, Ba.sup.2+, and OH.sup..

8. The method according to claim 7, wherein the acid solution comprises one or more of the following ions: H.sup.+, and F.sup., Cl.sup., Br.sup., I.sup., SO.sub.4.sup.2, NO.sub.3.sup., ClO.sub.4.sup..

9. The method according to claim 8, wherein the acid solution comprises HCl, the base solution comprises LiOH and/or NaOH and/or KOH, and the salt solution comprises LiCl and/or NaCl and/or KCl.

10. The method according to claim 9, wherein the acid solution comprises HNO.sub.3, the base solution comprises LiOH and/or NaOH and/or KOH, and the salt solution comprises LiNO.sub.3 and/or NaNO.sub.3 and/or KNO.sub.3.

11. The method according to claim 1, wherein the difference in water activity of both fluids in contact with the bipolar membrane is maintained within the range of 0.012 to +0.012.

12. The method according to claim 1, further comprising the step of supplying an additional electrolytic fluid and/or an organic compound, such that water activity of such fluid is maintained.

13. The method according to claim 1, further comprising the step of controlling the difference in water activity of both fluids in contact with the bipolar membrane.

14. The method according to claim 13, wherein controlling the difference in water activity of both fluids in contact with the bipolar membrane comprises controlling pump speed for one or more of the fluids with a pump controller.

15. The method according to claim 11, further comprising the step of supplying an additional electrolytic fluid and/or an organic compound, such that water activity of such fluid is maintained.

16. The method according to claim 11, further comprising the step of controlling the difference in water activity of both fluids in contact with the bipolar membrane.

17. The method according to claim 16, wherein controlling the difference in water activity of both fluids in contact with the bipolar membrane comprises controlling pump speed for one or more of the fluids with a pump controller.

18. The method according to claim 11, wherein the difference in water activity of both fluids in contact with the bipolar membrane is maintained in the range of 0.01 to +0.01.

Description

(1) Further advantages, features and details of the invention are elucidated on the basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, in which:

(2) FIG. 1 shows a configuration of a reactor capable of performing the method according to the present invention;

(3) FIG. 2 shows an alternative configuration;

(4) FIG. 3A-E show the difference in water activity vs. molality of some solutions;

(5) FIGS. 4-7 show some experimental results;

(6) FIG. 8 shows a hexagonal shape for the cell unit according to the present invention;

(7) FIG. 9 shows the stack voltage response to a discharge current density of 20 A/m.sup.2; and

(8) FIG. 10 shows the stack voltage at different conditions.

(9) Regenerative bipolar membrane fuel cell 2 (FIG. 1) comprises first electrode 4 and second electrode 6 in first electrode compartment 8 and second electrode compartment 10, respectively. It will be understood that another number and/or configurations of electrode could also be envisaged in accordance with the invention. Between electrodes 4, 6 a number of membranes are provided that separate electrode compartments 8, 10. In the illustrated embodiment, two cell units 12 are provided. Between second cell unit 12 and first electrode 4 cation exchange membrane 20 is provided.

(10) Each cell unit 12 comprises anion exchange membrane 22 and cation exchange membrane 24 that are separated by bipolar membrane 26. Cell unit 12 defines three compartments 28, 30, 32. In use, first compartment 28 is provided with a salt solution, such as 0.5 mol/l NaCl. In use, second chamber 30 is provided with an acid solution, such as 0.5 mol/l acid solution HCl. In use, third compartment 32 is provided with a base solution, such as 0.5 mol/l NaOH.

(11) Electrodes 4, 6 are connected via circuit 34. In the illustrated embodiment, system 2 is shown in the electrical energy generating state wherein a circuit 34 comprises load 36. Furthermore, circuit 34 comprises switch 38 for switching on and off the electrical energy generation of system 2. Electrodes 4, 6 are made of titanium mesh, e.g. with a mixed metal oxide coating like Pt.sub.xIr.sub.y, Ru.sub.xIr.sub.y, Ta.sub.xIr.sub.y.

(12) Experiments performed with a system according to system 2 with ten cell units 12 that are placed in series between electrodes 4, 6, having (additional) cation exchange membrane 20 and one of cation exchange membranes 16 of cell unit 12 as end membranes. The solution for the electrode compartments 8, 10 that was used in the experiments was 0.5 mol/l Na.sub.2SO.sub.4. Salt solution for the three chamber cell unit 12 was 0.5 mol/l NaCl, base solution was 0.5 mol/l NaOH, and acid solution was 0.5 mol/l HCl. Results with this configuration will be described later.

(13) In the illustrated embodiment system 2 comprises separate pumps 40, 42, 44 for the three specific fluids. It will be understood that also separate pumps can be provided for each individual compartment to enhance control possibilities. In the illustrated embodiment, fluids are stored in containers 46, 48, 50, 52, 54, 56. It will be understood that another configuration and/or number of containers can be provided in accordance with the invention. For illustrative reasons only, the containers for the fluids leaving system 2 are not shown in the figures.

(14) System 58 (FIG. 2) comprises electrodes 4, 6 and cell unit 12 to illustrate the principle of electrical energy generation. In the illustrated embodiment, electrode reactions at electrodes 4, 6 include at electrode 4 the reaction 4H.sub.2O+4e.sup..fwdarw.2H.sub.2+4OH.sup. and at second electrode 6 2H.sub.2O.fwdarw.4e.sup.+O.sub.2+4H.sup.+. Electrode 4 acts as cathode and electrode 6 acts as anode, in the illustrated energy generating state of system 58. In the illustrated electrical energy generation, the concentrated NaOH and HCl solutions are diluted while the NaCl fluid is concentrated in this state.

(15) Optionally, background electrolytic fluid from container 60 (FIG. 1) is provided by separate pumps 62, 64, 66 to the other three fluids. It will be understood that other configurations for the electrolytic fluid with one or more additional containers 60 and/or a different number of pumps 62, 64, 66 could also be envisaged in accordance with the present invention.

(16) For NaOH and HCl solutions water activity difference is illustrated in FIG. 3A vs. molality. For molalities up to two water activities for both solutions are about equal (see also FIG. 3B). For higher molalities water activity difference start to increase significantly. In case the method according to the present invention is performed at higher molalities, water activities have to be chosen carefully. For example, water activity of NaOH with a molality of 6 is about equal to the water activity of HCl with a molality of about 4.5. At these levels, the difference in water activity of both HCl solution and NaOH solution in contact with the bipolar membrane can be kept to a minimum. This illustrates a possible approach to manipulate the difference in water activity difference of both fluids in contact with bipolar membrane 26. As an alternative, or in addition thereto, background electrolyte fluids can be used to reduce the difference in water activity of fluids in contact with the bipolar membrane.

(17) Water activity (FIG. 3C) is shown vs. molality of suphuric acid (), LiOH (), NaOH () and KOH () with closest fits shown for the solutions. Depending on the molality of a fluid the corresponding water activities of other fluids can be shown depending on the molality. It will be understood that other solutions also show similar water activities in relation to molality, thereby enabling manipulation of the difference in water activity of both fluids in contact with bipolar membrane 26.

(18) FIG. 3D shows water activity difference versus molality for HCl versus LiOH (), HCl versus NaOH (), and HCl versus KOH () with their closest fits. Also shown is a range for water activity differences of 0.01. Highest possible molality for the illustrated water activity range is achieved with a system with KCl leading to HCl and KOH. FIG. 3E shows water activity difference versus molality for HNO.sub.3 versus LiOH (), NaOH () and KOH (), together with their closest fits and a water activity range of 0.01. It is shown that the system with NaNO.sub.3 leading to NaOH and HNO.sub.3 has the smallest difference in water activity of both fluids, comprised of NaOH and HNO.sub.3 solutions with equal molality, in contact with bipolar membrane 26, and is capable of operating at the highest molality thereby improving the overall efficiency of the method, and preventing ballooning and/or delaminating of (laminated) bipolar membrane 26.

(19) The results show several combinations that can be used in a method and/or system according to the present invention, such as KNO.sub.3 and preferably NaNO.sub.3. It will be understood that also other solutions could be used in accordance with the present invention.

(20) The aforementioned experiment is performed with a ten cell unit system, wherein an effective area of 100 cm.sup.2 for each membrane is used. Results are shown in FIGS. 4-7. The maximum power that is obtained was about 0.65 Watt. The effective electrode area was about 100 cm.sup.2. The total installed membrane area was about 3.000 cm.sup.2. The tests were being performed at room temperature. FIG. 4 shows the power vs. discharge current. FIG. 5 shows power density (Watt/m.sup.2) vs. discharge current density (A/m.sup.2), wherein the total installed membrane area was used to calculate the power density and the electrode area is used to calculate the current density. In the experiment, electrode reactions comprised a water oxidation reaction to oxygen gas and a water reduction reaction to hydrogen gas. This resulted in relatively high voltage losses at the electrodes in relation to the limited number of membrane units that are placed in series between the electrodes. By increasing the number of cell units 12 between electrodes 4, 6, these losses can be minimized.

(21) FIG. 6 illustrates the stack voltage vs. discharge current, showing a slope of the curve of about 12.9 Ohm. Stack voltage is lowered with the losses that occur at the electrodes where water oxidation and water reduction reactions take place. The slope of the curve can be decreased by increasing the number of membranes between the electrodes, since this reduces the influence of the relative voltage losses at the electrodes. The resistance distribution was calculated from the membrane resistances (Ralex) anion and cation exchange membranes and Fumatech bipolar membrane. FIG. 7 illustrates the resistance distribution of the ten membrane unit stack with a cation exchange membrane (CEM), bipolar membrane (BPM) and anion exchange membrane (AEM) per cell unit 12, and the solutions and electrodes 4, 6.

(22) The experiment illustrates the applicability of the method for storing and (re)generation of electrical energy according to the present invention.

(23) In an alternative embodiment, stack 68 of cell units 12 (FIG. 8) is provided with hexagonal shape 70. Stack 68 has inlet 72 and outlet 74 for the salt solution, inlet 76 and outlet 78 for the acid solution, and inlet 80 and outlet 82 for the base solution. Hexagonal shape 70 defines an active region 84 for stack 68. Hexagonal shape 70 reduces pressure losses at inlets and outlets of stack 68.

(24) Further experiments have been performed with an experimental setup according to system 2 with ten cell units 12 that are placed between electrodes 4, 6. In this experiment the electrodes have an electrode area of about 100 cm.sup.2. Total installed membrane area for 10 cells having three different membrane types, i.e. AEM, CEM and BPM, is about 0.3 m.sup.2. During the experiment that the maximum duration of the discharge are about 20 A/m.sup.2 was about 4 hours. Applied solutions involved 1 M NaOH and 1 M HCl. No ballooning of the BPM was experienced in the experiment(s) during discharge when energy was generated from system 2.

(25) The stack voltage response to a discharge current density of 20 A/m.sup.2 (FIG. 9) shows a smooth discharge curve. After a rapid decrease in voltage at the start, the voltage slowly stabilizes. In the experiment it was noticed that the pH of NaCl changed from neutral to 11.5, which could be caused higher co-ion transport of hydroxyls as compared to proton co-ion transport.

(26) A further experiment to test the performance of system 2 during six hours was performed. It was started with freshly prepared 1 M HCl, 1 M NaOH and 0.5 M NaCl solutions. The test comprised different phases, starting with a 4 hour discharge at 20 A/m.sup.2, followed by an equivalent 1 hour charge of 100 A/m.sup.2, then 10 minutes Open Circuit Voltage (OCV) conditions, and finally a 1 hour discharge at 20 A/m.sup.2.

(27) The stack voltage monitored in the experiment (FIG. 12) shows an effective discharge of the system for 4 hours. Then, the system can be charged, reaching pH=13 for the base and pH=3 for the acid. The OCV returns to the initial voltage. Next, a 1 hour final discharge is achieved in the experiment. Furthermore, in the experiment it was noticed that the pH of the NaCl solution decreased to pH=2 almost immediately and was maintained at this level throughout the rest of the experiment. This may indicate that co-ion transport of protons was larger than hydroxyl co-ion transport. In the first discharge in can be calculated that about 28800 C were harvested, which indicates a coulombic efficiency, , of about 30%, as compared to the (theoretical) maximum of 96485 C that are potentially present in 1 M HCl and 1 M NaOH. In the second charge and discharge, it can be calculated that the coulombic efficiency is about 25%, because of the 28800 C applied in the charge and 7140 C harvested during discharge.

(28) The experiments show the applicability of the invention. It will be understood that similar results could be achieved with different experimental setups. For example, the number of cell may be varied.

(29) The present invention is by no means limited to the above described preferred embodiments thereof. The rights sought are defined by the following claims, within the scope of which many modifications can be envisaged.