Patent Application
20130313126
The invention relates to an electrolysis process according to the features of the preamble of Patent claim 1 and to an electrolytic cell according to the features of the preamble of Patent claim 2.
U.S. Pat. No. 5,916,505 has disclosed a fuel cell arrangement in which water or electrolyte is pumped through the cell. U.S. Pat. No. 4,330,378 has disclosed an electrolytic cell in which water or electrolyte is pumped through the electrode.
A known device for the electrolysis of water with a fixed, alkaline electrolyte is known, for example, from US 2009/008261 A1 or from R. J. Davenport et al., Space water electrolysis: space station through advanced missions, Journal of Power Sources, 36 1991, 235-250. The key part of this device is the electrolytic cell. It comprises the following components:
For the further construction and function of an electrolytic cell, reference is made to the description of DE 195 35 212 C2.
DE 195 35 212 C2 has also disclosed an alkaline electrolysis process using an immobilized potassium hydroxide solution, in which the water is transferred into an electrode-membrane-electrode system by diffusion via a hydrophobic porous membrane adjacent to the cathode gas chamber. This process has the disadvantage that, when the electrolytic cell is switched off, the diffusion process can only be stopped by flushing out the entire water reservoir in the stack with an inert gas. Since the diffusion process is controlled by way of the temperature of the water, there is quite a time delay in the control.
The invention is based on the objective of providing an alkaline electrolysis process for hydrogen electrolysers by which the disadvantages of the prior art are overcome.
A further objective is to implement an electrolytic cell of a simple construction, with which it is possible to switch the electrolysis process on and off quickly without diffusion processes leading to a dilution of the immobile electrolyte solution.
These objectives are achieved by an electrolysis process according to the features of the current claim 1, and an electrolytic cell according to claim 2. Advantageous refinements are the subject of subclaims.
In accordance with the process according to the invention for alkaline electrolytic cells with an electrode-membrane-electrode arrangement comprising two porous electrodes with a membrane lying in between, one or several liquids is/are introduced directly into the electrode-membrane-electrode arrangement. According to the invention, the one or several liquids is/are conveyed directly in a structure of channels implemented in the membrane.
Hereinafter, an electrode-membrane-electrode arrangement or EME system is understood as meaning an arrangement of two electrodes and a membrane arranged between the electrodes, a liquid electrolyte being fixed in the membrane. The membrane, in this case, can be a porous membrane or an ion-exchange membrane.
The one or several liquids is/are introduced directly into the structure of channels of the membrane. This ensures that the one or several liquids is/are distributed uniformly in the membrane or electrode.
The electrolysis causes liquid, for example water, in the structure of channels of the membrane to be used up. The respective liquid, for example water, is then replenished accordingly in the structure of channels. It is to be mentioned that, according to the invention, the liquid is not passed through the structure of channels but is introduced or conducted into the structure of channels, where it may be used up.
The liquid is consequently an immobilized liquid, which when used up accordingly, for example when water is converted into hydrogen and oxygen in the electrolysis, is topped up by replenishing the corresponding liquid in the structure of channels.
According to the invention, the electrolytic cell comprises porous electrodes between which a porous membrane is arranged, a liquid electrolyte being fixed in the pores of the electrodes and of the membrane, a product gas chamber adjacent to the cathode, a further product gas chamber adjacent to the anode and an arrangement for feeding water to the electrodes. According to the invention, a structure of channels in which a distribution of the liquid, expediently water or electrolyte, is provided is implemented in the membrane.
The structure of channels may expediently be a structure of microchannels and/or nanochannels. The membrane is expediently a proton-conducting membrane or an ion-exchange membrane. In one particular embodiment, the membrane may also be implemented in a multi-layered form.
A concentrated, aqueous solution of high electrical conductivity is expediently used as an electrolyte. Preferred exemplary embodiments for this are: acids, bases and inorganic salt solutions of high electrical conductivity, such as for example: potassium hydroxide or other alkaline and alkaline-earth hydroxides in concentrations of approximately 5 to 12 mol/litre; sulphuric acid of approximately 2 to 5 mol/litre; phosphoric acid, etc.
One advantage of the invention is that, in the case of alkaline electrolytic cells with an immobilized electrolyte, water and/or electrolyte is fed to the EME system directly, so that no inert gas flushing is necessary when it is switched off and, in addition, the delayed response of the system to changes in load is reduced. Furthermore, in the case of polymer electrolyte cells, water is likewise fed directly to the EME system, so that the hydrogen and oxygen gas emerging leaves the cell as gas and no further gas/liquid separation is needed.
The invention and further advantages of the invention are explained in more detail by means of the drawings, in which:
If a voltage is then applied to the current lines 8 by means of a power source 9, water molecules from the aqueous electrolyte 22 are split into their components hydrogen, H2, and oxygen, O2. At the cathode 3 H2 is produced, which flows as a gas into the H2-gas chamber 2. At the anode 6 gaseous oxygen is produced, which flows into the O2-gas chamber 7. As this process continues, the electrolyte 22 becomes more and more concentrated unless water is replenished correspondingly. In the electrolytic cell 18 according to the invention, it is provided that water to be split is then fed to the immobilized electrolyte 22.
According to the invention, the feeding of the water to be split takes place by way of a structure of channels 20, which is implemented in the membrane 1 and/or in at least one electrode 3, 6. Through this structure of channels 20, the water to be split is introduced into the EME system 3, 1, 6 over a large surface area and is distributed in the EME system 3, 1, 6 over a large surface area. Numerous further possibilities for feeding water directly into the EME system 3, 1, 6 are conceivable.
The electrolytic cell, namely the structure of channels 20 of the membrane 1 of the electrolytic cell, is connected to a water reservoir 13 by way of a control valve 19 and a first pump 12. For specific applications, the capillary forces in the EME system 3, 1, 6 may also be sufficient to suck the water to be split from a water reservoir 13 without a pump 12.
In the case of prolonged operation of the alkaline electrolytic cell or of instances of improper operation, liquid electrolyte may be discharged from the EME system 3, 1, 6 by way of the product gas chambers, as a result of which the electrolyte concentration in the electrolytic cell is diluted and the performance of the cell drops. By way of the electrolyte reservoir 25 and the pump 26, electrolyte can expediently also be fed to a cell by way of the structure of channels.
By way of the control valve 19 and a second pump 26, the electrolytic cell 18, namely the structure of channels 20 of the membrane 1 of the electrolytic cell, is connected to an electrolyte reservoir 25. It is consequently possible for the electrolyte 22 to be introduced into the membrane 1 and distributed on the membrane surface directly by way of the structure of channels 20.
The structure of channels 20 is constructed in such a way that the water is thus uniformly distributed over the surface area of the membrane 1 of the EME system 3, 1, 6 and mixed with the electrolyte 22, and negligible concentration gradients form over the cross section of the surface area.
If the electrolysis process is then stopped and the power supply to the electrolytic cell is interrupted, no water is replenished either if the control valve 17 is closed or if the pump 12 is switched off. In this state, an alkaline electrolytic cell can go for any length of time without flushing of the electrolytic cell being necessary.
Direct supply of water to be split or of the electrolyte to the membrane 1 of the EME system 3, 1, 6 offers the following advantages here:
The same arrangement with a separate supply of water to be split to the membrane 1 of the EME system 3, 1, 6 can also be used for an electrolytic cell with a polymer electrolyte.
According to the generally known prior art, on the anode side 6 water is pumped through a structure of channels (not shown) of the bipolar plate B. A portion of the water is split. The oxygen will herein bubble into the water flow and be carried thereby out of the electrolytic cell 18. On the cathode side 3, the hydrogen enters the intermediate spaces of the bipolar plate B. For better removal of the hydrogen, water circulating in a cycle on the cathode side 3 is also often used to discharge the hydrogen. To obtain gaseous H2 and O2, this necessitates a phase separation, which requires additional equipment.
The water from the water reservoir 13 is pressurized centrally by a pump 12 and then fed individually, by way of control valves 19, to each electrolytic cell 18, namely to the structure of microchannels in the membrane (not shown). However, it is also possible for each electrolytic cell 18 to be connected to a respective pump, which in turn is connected to the water reservoir 13. In this implementation, the control valves 19 for each electrolytic cell 18 could be omitted.
In the electrolytic cell arrangement that is shown in
The cooling of an electrolytic cell 18 is effected by way of cooling cells 23, which are arranged between the individual electrolytic cells 18 and through which cooling water flows in parallel. The cooling water cycle 23a, which is isolated from the electrochemical process, consequently serves exclusively for cooling. In the case of alkaline electrolytic cells, the diffusion membrane, as described in DE 195 35 212 C2, is omitted. In the case of the polymer-membrane electrolytic cell, the O2-and/or H2-water cycle, and consequently the phase separation, is omitted.
Material tolerances and different thermal behaviour of individual cells may cause differences in the operating behaviour and the ageing of individual cells. In an extreme case, it was possible in the prior art for a single cell to fail and thereby render the entire stack of cells inoperative.
The following further advantages are provided by the water being supplied directly:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10015427.7 | Dec 2010 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/006089 | 12/6/2011 | WO | 00 | 8/9/2013 |
