Patent Application
20190106797
The present disclosure relates to electrolysis. Various embodiments may include cells for alkaline water electrolysis with in-situ anode activation.
In modern times, electrolysis is used for various purposes, for example in Hydrogen and/or Oxygen generation which are achieved by hydrogen evolution reaction (HER) and Oxygen evolution reaction (OER) during electrolysis of alkaline water used as an electrolyte in an electrolytic cell also referred to as an electrolyser. The electrolytic cell includes electrodes, i.e. at least an anode and at least a cathode, that conduct electrical energy to the electrolyte and thus decomposes the electrolyte in the electrolytic cell.
Various types of electrodes are used in the electrolytic cell, for example, commonly used electrodes in the electrolytic cell are a Nickel cathode and a Nickel anode. The efficiency of alkaline water electrolysis is dependent on overpotential of the electrolytic cell and is increased by reducing the overpotential of the electrolytic cell, hereinafter also referred to as cell overpotential. The cell overpotential is significantly constituted by overpotential of the electrodes i.e. the overpotential of the anode, hereinafter also referred to as the anodic overpotential, and the overpotential of the cathode, hereinafter also referred to as the cathodic overpotential.
The anodic and the cathodic overpotentials are optimized, generally reduced, by various approaches for example either by geometry changes in the electrodes or by applying an electrocatalytic coating forming surfaces of the electrodes. It is well known in the art of alkaline water electrolysis that coating with different catalysts reduce the over-potential of the electrode that is coated and hence energy consumption for the OER and the HER. For example, anodes are typically coated with various oxides of transition elements while cathodes are typically composed of an underlying substrate such as stainless steel on which Raney nickel, nickel sulfide, etc. are coated. The various approaches to reduce the anodic and/or the cathodic overpotentials by surface modification of the anode and/or the cathode are generally referred to as activation. It is customary to use an activated cathode as well as an activated anode in the electrolytic cells for alkaline water electrolysis to achieve an optimum efficiency of the electrolysis.
Activation of the anode and the cathode may be carried out separately before the electrodes are installed in the electrolyser and before the alkaline water electrolysis is performed. Moreover, if and when any of the electrodes are to be replaced then the replacement electrodes are also activated before the electrodes are installed in the electrolyser and restarting the electrolysis process in the electrolyser. Various methods are used to form an activated anode in addition to formation of an activated cathode, and this requirement of activating the anode before being placed in the electrolytic cell increases at least cost and time of production of the activated anode.
The teachings of the present disclosure may be embodied in a technique for in-situ anode activation that can be performed in the electrolytic cell along with the electrolysis of the alkaline water.
For example, some embodiments may include an electrolytic cell (1) for alkaline water electrolysis with in-situ anode activation, the electrolytic cell (1) comprising: an anode (10), a cathode (20) wherein at least a part of a surface of the cathode (20) comprises an electrically conducting stable material (22) and an anode catalytic material (21), the anode catalytic material (21) adapted to be released from the cathode (20) in alkaline water and be deposited at a surface of the anode (10) when an electric voltage is applied across the anode (10) and the cathode (20), and a diaphragm (30) disposed between the anode (10) and the cathode (20) wherein the anode (10) and the cathode (20) are separated by the diaphragm (30) and wherein the diaphragm (30) is gas tight and is permeable to the anode catalytic material (21).
In some embodiments, the electrically conducting stable material (22) comprises Nickel.
In some embodiments, the anode catalytic material (21) is one of Cobalt, Manganese, Molybdenum, Chromium, and a combination thereof.
In some embodiments, the anode catalytic material (21) in the cathode (20) is between 4 percentage by mass of the cathode (20) and 30 percentage by mass of the cathode (20), and particularly between 10 percentage by mass of the cathode (20) and 15 percentage by mass of the cathode (20).
In some embodiments, the part of the surface of the cathode (20) further comprises Sulfur.
In some embodiments, the anode (10) is made of only Nickel.
As another example, some embodiments may include a method (100) for in-situ anode activation of an anode (10) positioned in an electrolytic cell (1) for alkaline water electrolysis, the method (100) comprising: a step (110) of providing a cathode (20) wherein at least a part of a surface of the cathode (20) comprises an electrically conducting stable material (22) and an anode catalytic material (21), the anode catalytic material (21) adapted to be released from the cathode (20) in alkaline water, a step (120) of releasing the anode catalytic material (21) from the cathode (20) wherein a part of the anode catalytic material (21) from the cathode (20) is released from the cathode (20) into an electrolyte (40) in the electrolytic cell (1) by positioning the cathode (20) in the electrolytic cell (1) such the surface of the cathode (20) is contacted with the electrolyte (40), wherein the electrolyte (40) is alkaline water and wherein the cathode (20) is positioned such that in the electrolytic cell (1) the cathode (20) and the anode (10) are separated by a diaphragm (30) configured to be gas tight and permeable to the anode catalytic material (21), and a step (140) of depositing the anode catalytic material (21) so released wherein at least a part of the anode catalytic material (21) so released is deposited at a surface of the anode (10) by applying an electric voltage across the anode (10) and the cathode (20) such that an overpotential at the anode (10) is reduced by deposition of the anode catalytic material (21) at the surface of the anode (10).
In some embodiments, the step (120) of releasing the anode catalytic material (21) from the cathode (20) into the electrolyte (40) is performed at an open circuit potential of the electrolytic cell (1).
In some embodiments, there is a step (130) of sustained release of the anode catalytic material (21) from the cathode (20) subsequent to the step (120) of releasing the anode catalytic material (21) from the cathode (20), wherein in the step (130) of sustained release of the anode catalytic material (21) at least a part of the anode catalytic material (21) from the cathode (20) is released from the cathode (20) into the electrolyte (40) along with applying the external electric voltage across the anode (10) and the cathode (20).
In some embodiments, the external electric voltage applied across the across the anode (10) and the cathode (20) is corresponding to a pH of the electrolyte (40) and a molality of the anode catalytic material (21) present in the electrolyte (40).
In some embodiments, the electrically conducting stable material (22) comprises Nickel.
In some embodiments, the anode catalytic material (21) is one of Cobalt, Manganese, Molybdenum, Chromium, and a combination thereof.
In some embodiments, the anode catalytic material (21) in the cathode (20) is between 4 percentage by mass of the cathode (20) and 30 percentage by mass of the cathode (20), and particularly between 10 percentage by mass of the cathode (20) and 15 percentage by mass of the cathode (20).
In some embodiments, the part of the surface of the cathode (20) further comprises Sulfur.
In some embodiments, the anode (10) is made of only Nickel.
The teachings of the present disclosure are further described hereinafter with reference to illustrated embodiments shown in the accompanying drawing, in which:
In some embodiments, an electrolytic cell for alkaline water electrolysis, hereinafter also referred to as the cell, includes a pair of electrodes having an anode and a cathode and a diaphragm.
The electrolytic cell may be an individual cell or a part of an electrolyser stack. The diaphragm is disposed between the anode and the cathode and the anode and the cathode are separated by the diaphragm. The diaphragm is gas tight. The anode may be, but not limited to, formed of Nickel only.
At least a part of a surface of the cathode includes an electrically conducting stable material, for example Nickel, and an anode catalytic material, for example Cobalt, Molybdenum, Manganese, Chromium or a combination thereof. incorporating teachings of the present disclosure the anode catalytic material in the cathode may be between 4 percentage by mass of the cathode and 30 percentage by mass of the cathode, and particularly between 10 percentage by mass of the cathode and 15 percentage by mass of the cathode. The anode catalytic material is adapted to be released from the cathode in alkaline water, when present in the cell. The anode catalytic material may be released at open circuit potential of the cell and/or may be released when an external voltage is applied to the cell. The electrically conducting stable material is stable in the cathode and is not released in the electrolyte or is released in insignificant amount over long period of usage typical of electrolytic cells for alkaline water electrolysis.
In some embodiments, the diaphragm is permeable to anode catalytic material and allows passage of anode catalytic material, especially in ionic forms dissolved in or carried in the electrolyte into which the anode catalytic material gets released from the cathode, to pass through it, i.e. the diaphragm, from cathode side of the electrolyte to the anode side of the electrolyte. The anode catalytic material may be deposited at a surface of the anode when an electric voltage is applied across the anode and the cathode.
As a result of deposition of the anode catalytic material overpotential of the anode is decreased and the anode becomes active. Since the anode is being activated while in seat within the electrolytic cell, and also during the running of the alkaline water electrolysis by application of the external voltage, the activation of the anode is in-situ and occurs simultaneously with the electrolysis of the electrolyte. This at least partly obviates the requirement of ex-situ pre-electrolysis activation of the anode.
Furthermore, since the anode catalytic material is being released in the electrolyte from the cathode and since not the entire mass of the anode catalytic material present in the cathode at the start of the electrolysis is released from the cathode simultaneously, a sustained and continuous availability of the anode catalytic material in the electrolyte and subsequently at the anode for deposition on surface of the anode is possible, unlike an alternate scenario in which a similar material intended to be deposited on the anode is presented in the electrolytic cell in the electrolyte directly and thus is available all at once for deposition at the anode and thus the availability of the similar material is not sustained and continuous. The anode may be maintained in the activated state for a longer period of time due to replenishment of any deposited anode catalytic material that the anode might release during the electrolytic process. Moreover, in case there is a requirement to remove an existent anode and insert a new anode into the electrolytic cell, then activation of the new anode happens according to the described technique without needing to replenish the anode catalytic material in the electrolytic cell.
In some embodiments, since the anode catalytic material is leaving the surface of the cathode when it is released in the electrolyte, the cathode surface become porous or increased in porosity, and thus the overpotential at the cathode is also maintained or decreased due to the porosity increase in the cathode due to release of the anode catalytic material from the cathode.
In some embodiments, the part of the surface of the cathode further includes Sulfur, i.e. the cathode if formed of the electrically conducting stable material, the anode catalytic material and Sulfur for example the cathode is formed of Nickel, Cobalt and Sulfur. The sulfur or parts of the sulfur from the cathode during electrolysis in the electrolytic cells, also called electrolyser, is slowly and continuously removed from the cathode into the electrolyte in the electrolyser leaving thereby the electrically conducting stable material, for example nickel. The release of sulfur from the cathode further facilitates release of the anode catalytic material from the cathode.
Some embodiments include a method for activating an anode, for example a Nickel anode, positioned in an electrolytic cell for alkaline water electrolysis. In some embodiments, the method includes a step of providing a cathode. The cathode provided in the method may be understood same as the cathode presented in according to the previous aspect of the present technique. Subsequently, in the method, a step of releasing the anode catalytic material from the cathode is performed.
In the step of releasing the anode catalytic material a part of the anode catalytic material from the cathode is released from the cathode into an electrolyte, e.g. alkaline water, in the electrolytic cell by positioning the cathode in the electrolytic cell such the surface of the cathode is contacted with the electrolyte. The cathode is positioned such that in the electrolytic cell the cathode and the anode are separated by a diaphragm. The diaphragm is gas tight and permeable to the anode catalytic material. The anode catalytic material, for example Cobalt, Manganese, Molybdenum, Chromium, and/or a combination thereof, may be released at open circuit potential of the cell and/or may be released when an external voltage is applied to the cell.
In some embodiments, a step of depositing the anode catalytic material is performed in which at least a part of the anode catalytic material so released is deposited at a surface of the anode by applying an electric voltage across the anode and the cathode. As a result of deposition of the anode catalytic material at the anode overpotential at the anode is reduced thereby resulting into in-situ activation of the anode.
In some embodiments, a method further includes a step of sustained release of the anode catalytic material from the cathode. The step of sustained release is subsequent to the step of releasing the anode catalytic material from the cathode and may be simultaneously performed along with the step of depositing the anode catalytic material. In the step of sustained release of the anode catalytic material at least a part of the anode catalytic material from the cathode is released from the cathode into the electrolyte along with applying the external electric voltage across the anode and the cathode. The external electric voltage applied across the electrodes i.e. the anode and the cathode corresponds to a pH of the electrolyte and a molality of the anode catalytic material present in the electrolyte as a result of the release from the cathode. The anode and the cathode overpotentials, as well as the molality of the anode catalytic material present in the electrolyte, corresponds to pH, and in turn the external electric voltage applied is changed or adjusted depending on the pH of the electrolyte.
In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be noted that the illustrated embodiments are intended to explain, and not to limit the scope of the teachings herein. It may be evident that such embodiments may be practiced without these specific details.
The teachings herein may be employed to provide an in-situ activation of the anode and thus obviating ex-situ pre-electrolysis activation of the anode. In some embodiments, in an electrolytic cell for alkaline water electrolysis, a cathode is used that includes an anode catalytic material e.g. a material which when deposited on the anode will activate the anode. Furthermore, the anode catalytic material is such that it gets released, mostly in ionic forms, from the cathode into the electrolyte within the electrolytic cell.
A part of the ions of the anode catalytic material present in the electrolyte as a result of the release from the cathode, then migrate towards the anode within the electrolyte under the influence of an external voltage applied across electrodes, the voltage using which the alkaline water electrolysis is being carried out. The migrating ions of the anode catalytic material in the electrolyte pass through a diaphragm which is substantially impermeable to gas but permeable to ions in electrolyte. The migrating ions of the anode catalytic material in the electrolyte pass through the diaphragm from the cathode side of the electrolyte to the anode side of the electrolyte, the cathode and the anode sides of the electrolyte created by the diaphragm positioned between and separating the anode from the cathode. Some of the ions of the anode catalytic material in the electrolyte that pass into the anode side of the electrolyte then get deposited at the anode under the influence of the voltage applied across the electrodes of the electrolytic cell. The deposition of the anode catalytic material reduces the anodic overpotential and thus results into activation of the anode and increase in efficiency of the electrolytic cell.
In some embodiments, the electrolytic cell 1 may be an individual cell or a part of an electrolyser stack (not shown). The other electrode in the electrolytic cell 1 is a cathode 10. In the cathode 10, at least a part of a surface of the cathode 10 includes an electrically conducting stable material 21 and an anode catalytic material 22. The electrolytic cell 1 also has a diaphragm 30. The anode 10 and the cathode 20 may be positioned in a container 50 which receives the anode 10, the cathode 20, the diaphragm 30, and an electrolyte 40 e.g. alkaline water, provides a seat for performing the alkaline water electrolysis by applying an external voltage across the anode 10 and the cathode 20 and also provides a seat for in-situ activation of the anode 10 according to aspects of the present technique. The diaphragm 30 is disposed between the anode 10 and the cathode 20 i.e. the anode 10 and the cathode 20 are separated by the diaphragm 30. The diaphragm 30 is gas tight.
The diaphragm 30 divides the electrolyte 40 in the container 50 into an anode side 41 of the electrolyte 40 and a cathode side 42 of the electrolyte 40. The anode side 41 of the electrolyte 40 is the part of the electrolyte 40 surrounding the anode 10 whereas the cathode side 42 of the electrolyte 40 is the part of the electrolyte 40 surrounding the cathode 20. Oxygen is evolved in the electrolytic cell 1 as a result of the OER in the anode side 41 and Hydrogen is evolved in the electrolytic cell 1 as a result of the HER in the cathode side 42. Due to gas impermeability of the diaphragm 30, the oxygen evolved in the anode side 41 is restricted to the anode side 41 and does not pass through the electrolyte 40 into the cathode side 42, and similarly, the hydrogen evolved in the cathode side 42 is restricted to the cathode side 42 and does not pass through the electrolyte 40 into the anode side 41.
Referring to
The Co 21 is released from the cathode 20 in the alkaline water 40, i.e. the electrolyte 40, when the electrolyte 40 is present in the electrolytic cell 1 as depicted in
The diaphragm 30 is permeable to the Co 21 and allows passage of the Co 21, especially in ionic forms for example Cobalt ions with oxidation states of +2, +3 dissolved in or carried in the electrolyte 40 from the cathode side 42 to the anode side 41 of the electrolyte 40. Such diaphragms 30 are commonly used in alkaline water electrolysis and are also referred to as separator membranes. Since such diaphragms 30 are commonly used and well known in the art of alkaline water electrolysis the same has not been detailed here for sake of brevity. The migration of the Co 21 through the diaphragm 30 and from the cathode side 42 to the anode side 41 of the electrolyte 40 results from diffusion of the ions of the Co 21 and is facilitated by the external voltage applied to the electrodes 10, 20 of the electrolytic cell 1. The Co 21 when into electrolyte 40 at the anode side 41 gets deposited at a surface of the anode 10 under the influence of the electric voltage applied across the anode 10 and the cathode 20. The migration of the Co 21 ions from the cathode side 42 into the anode side 41 of the electrolyte 40 and the subsequent deposition of the Co 21 ions on the anode 10 is depicted schematically in
Also, since the Co 21 leaves the surface of the cathode 20 as depicted in
In some embodiments, the sulfur or parts of the sulfur from the cathode 20 during electrolysis in the electrolytic cell 1, also called electrolyser, is slowly and continuously removed from the cathode 20 into the electrolyte 40 in the electrolyser 1 leaving thereby the electrically conducting stable material 22 i.e. for example nickel. The sulfur in the cathode 20 is between 10 percentage and 30 percentage by mass of the cathode 20.
In some embodiments, the release of sulfur from the cathode 20 further facilitates release of the Co 21 from the cathode 20 by either detaching at least some of the Co 21 from the cathode 20 along with the Sulfur's release from the cathode 20 and/or modifies the surface of the cathode 20 in such a way, for example by creating micro- or nano-sized protrusions (not shown) on the surface which are highly unstable mechanically and thus are eventually broken off from the surface of the cathode 20 thereby releasing the Co 21 present in this broken off parts into the electrolyte 40.
Thus, the sulfur in the cathode 20 facilitates release of the Co from the cathode 20 into the electrolyte 40 during alkaline water electrolysis. Furthermore, release of sulfur increases the porosity of the cathode 20 in the same way as explained in
In some embodiments, for removal of Sulfur from the electrolyte 40, a complexing agent may be provided into the electrolyte 40. The complexing agent chemically reacts with the Sulfur present in the electrolyte 40, and also with the Sulfur present in the cathode 20, to form a coordination complex having a solid state. The coordination complex is then filtered out from the electrolyte 40 and rendering the electrolyte 40, and thus the electrolyser 1, at least partially free from the sulfur released from the cathode 20. The complexing agent may include, but not limited to, one or more of Barium hydroxide, Barium chloride, Barium nitrate, Strontium hydroxide, Strontium chloride, Strontium nitrate, Calcium hydroxide, Calcium chloride, and Calcium nitrate. Generally, the amount of complexing agent when provided to the electrolyte 40 in the container 50 is substantially equal to or less than 1 M (molar) concentration.
The complexing agent reacts with the released sulfur to form the coordination complex with the released sulfur for example as schematically depicted in the following equation:
SO42−+Ba(OH)2→BaSO4+2OH− (i)
Thus, as shown in equation (i) above, the released sulfur for example the sulfate ion in equation (i) chemically reacts with the complexing agent for example Barium hydroxide in equation (i) to form coordination complex for example in equation (i) Barium sulfate in solid state i.e. BaS04 (s) in the electrolyte 40 in the container 50. The coordination complex so formed may be removed from the electrolyte 40 by filtration.
Referring now to
In the step 120 a part of the Co 21 from the cathode 20 is released from the cathode 20 into the electrolyte 40, alkaline water, in the electrolytic cell 1 by positioning the cathode 20 in the electrolytic cell 1 such the surface of the cathode 20 is contacted with the electrolyte 40 as shown in
In some embodiments, the method 100 further includes a step 130 of sustained release of the Co 21 from the cathode 20. The step 130 is subsequent to the step 120, and simultaneous along with the step 140. In the step 130 at least a part of the Co 21 from the cathode 20 is released from the cathode 20 into the electrolyte 40 along with applying the external electric voltage across the anode 10 and the cathode 20. The external electric voltage applied across the electrodes corresponds to a pH of the electrolyte 40 and a molality of the Co 21 present in the electrolyte 40 as a result of the release from the cathode 20.
As a result of deposition of the Cobalt 21 at the anode 10, the overpotential at the anode 10 is decreased. The following table provides potential of the electrolytic cell 1, the anode 10 and the cathode 20 under different conditions:
The anode and cathode potentials are expressed in comparison to a Mercury/Mercury Oxide reference electrode. The values in the table are measured at 100 degree Centigrade, 50 wt % KOH as the electrolyte 40, external current at 0.2 A/cm square of each of the electrode, Zero-gap configuration, and total electrode area for each electrode as 25 cm square.
As can be seen in the table above, in first row both the anode 10 and the cathode 20 are not activated for example when sheets of Nickel are used as the cathode and the anode without any coating for activation or surface modification for activation, and the cell potential is 2101 mV (millivolts) which is contributed primarily by the anode potential of 657 mV and cathode potential of −1467 mV. Second row of the table only the anode 10 is activated externally ex-situ, contrary to our technique, and the cathode 20 is not activated and the cell potential is 1982 mV which is contributed primarily by the anode potential of 553 mV and cathode potential of −1454 mV. The fourth row presents a scenario where both the anode 10 and the cathode 20 are activated externally ex-situ, the cell potential is 1649 mV which is contributed primarily by the anode potential of 510 mV and cathode potential of −1135 mV. The third row represent the scenario arising from the present technique, i.e. the cathode 20 is activated and includes the anode catalytic material 21 which then is released into the electrolyte 40, migrated to the anode 10 and is deposited on the anode 10, and as can be seen from the third row, the cell potential is 1664 mV which is contributed primarily by the anode potential of 506 mV and cathode potential of −1154 mV—thus the cell potential for the present technique is substantially similar to the cell potential of fourth row, where both the anode and the cathode were ex-site activated separately.
While the present technique has been described in detail with reference to certain embodiments, it should be appreciated that the present technique is not limited to those precise embodiments.
Rather, in view of the present disclosure which describes exemplary modes for practicing the invention, many modifications and variations would present themselves, to those skilled in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.
This application is a U.S. National Stage Application of International Application No. PCT/EP2016/057047 filed Mar. 31, 2016, the contents of which are hereby incorporated by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/057047 | 3/31/2016 | WO | 00 |
