Improved in situ reduction of the passivating oxide layer on titanium components of the anode of a PEM electrolyzer

Abstract

A method for reducing the oxide layer of titanium components of an anode in an electrolysis cell, wherein the method is carried out in an electrolysis stack and / or an electrolyzer, comprising polarizing the anode to a negative potential and supplying the anode with a reducing medium, wherein the titanium surfaces are coated with metal layers of less than 1 micrometer thickness, wherein the reducing medium is a gas selected from the group consisting of hydrogen, carbon monoxide, ammonia and hydrocarbons and mixtures thereof.

Description

The present invention relates to a method for regenerating electrolysis cells, in particular PEM electrolysers or alkaline electrolysers. Furthermore, the present invention relates to an electrolytic cell for in particular a PEM electrolyser or alkaline electrolyser, wherein the titanium components of the anode are provided with very thin metal coatings of less than 1 micrometer thickness.Prior ArtAn electrolyser according to the present invention is understood in particular to mean a proton exchange membrane electrolyser (PEM electrolyser) in which anode and cathode are separated from one another by a proton-conducting membrane. The membrane material used is usually polysulphonic acid (PFSA) in a polytetrafluoroethylene (PTFE) matrix.At the potentials prevailing in the anode during normal operation, titanium forms a passivating oxide layer which increases over time. This substantially prevents a further reaction of the titanium surface with oxygen, but at the same time increases the contact resistance considerably. As a result, the ohmic losses in the cell increase greatly.Therefore, uncoated titanium surfaces on anodes are only used in laboratory electrolysers in the scientific sector which are to be operated for a maximum of a few 1000 hours. In commercial electrolysers, titanium surfaces coated on the anode are always used. The coating prevents the formation of the oxide layer and thus ensures long-term low contact resistances between the titanium layers. However, these coatings are typically very expensive, since noble metals such as gold are usually used. The entire flow distributor structure, including current distributor layers of anode and cathode, therefore nowadays makes up to 50% of the cost of a PEM electrolysis stack. In addition, coatings which bring about low contact resistances over the lifetime (about 60000 to 90000 hours) of a PEM electrolysis stack are usually made very thick in the prior art, typically in thicknesses of 1 to 5 micrometers. This leads to very high costs, especially in the case of noble metal coatings.From JP H11-165 174 A a method is known in which a deposit of an electrode of an electrolytic cell is removed by reversing the polarity. US 2010 / 0 187 122 A1 likewise discloses reversing the polarity in an electrolysis cell in an acidic environment, the electrodes comprising a titanium substrate and a coating such as, for example. The titanium oxide may be titanium oxide.Disclosure of the InventionThe present invention therefore provides a method for reducing the oxide layer of titanium components of the anode in an electrolytic cell comprising polarising the anode to a negative potential and supplying the anode with a reducing medium, the titanium surfaces being coated with metal layers of less than 1 micrometer thickness.The developers of the present invention found that a reduction of this layer can be achieved by drastically reducing the coating to thicknesses less than 1 micrometer in conjunction with regularly treating the passivating oxide layer by a reducing atmosphere and polarizing to an appropriate potential. This method thus enables a reduction in the amount of coatings otherwise used and thus a drastic reduction in costs.An electrolyser in the sense of the present invention is understood in particular to mean a proton exchange membrane electrolyser (PEM electrolyser), in which anode and cathode are separated from one another by a proton-conducting membrane. The membrane material used is usually polysulphonic acid (PFSA) in a polytetrafluoroethylene (PTFE) matrix.The cathode located on the hydrogen side of the electrolysis cell of the electrolyser typically experiences potentials in the vicinity of 0 V compared with the normal hydrogen electrode (NHE) known to the person skilled in the art. The cathode preferably consists of a catalyst layer applied to the proton-conducting membrane, a porous current distribution layer and a flow distribution structure. The porous current distribution layer is preferably comprised of a nonwoven or woven carbon fiber, a stack of thin stretched titanium sheets, titanium fibers which may be woven or non-woven, or a porous titanium sinter, or combinations thereof.In normal operation, the anode of the electrolysis cell of the electrolyser according to the invention experiences potentials in the range from +1.0 V to +2.5 V. The anode preferably consists of a catalyst layer applied to the proton-conducting membrane, a porous current distributor layer and a flow distributor structure. The porous current distribution layer is preferably comprised of a stack of thin stretched titanium sheets, titanium fibers, which may be woven or non-woven, or a porous titanium sinter, or combinations thereof. The flow distribution structure can consist of channels, which supply the liquid water to the electrode and transport away the gaseous oxygen formed, in a titanium plate or of a porous titanium structure. The flow distribution structure is preferably made of an electrically conductive polymer containing a channel structure. An electrical conductivity of the polymer can be achieved, for example, by filling with carbon, such as carbon black. The channel structure can be produced by injection molding, by embossing or by milling. Alternatively, the flow distributor structure of the cathode can also be formed from titanium. In this case, it is preferred that dense protective layers are formed over the titanium over the lifetime of the cell. Such protective layers are known to the person skilled in the art from the prior art, an example of which is a protective layer made of carbon. Such protective layers can be applied, for example, by a PVD method (physical vapor deposition). The titanium plate can serve simultaneously as a titanium sheet part of a bimetallic cell separator. If the flow distributor structure consists of a porous material, a thin titanium sheet is additionally required as bipolar plate.Titanium used in the context of the present invention can be either commercially available titanium sheet or else commercially available titanium alloys, for example Ti-6Al-4V (i.e. a titanium alloy having 6% aluminum and 4% vanadium). Preferred titanium alloys consist of at least 50% by weight titanium. Other suitable and customary titanium alloys are known to the skilled person.The cell separator may include a bimetallic sheet of a steel sheet and a titanium sheet. A bimetal is generally understood to mean a metal strip which consists of two layers of different metals or metal alloys. The layers are usually connected in a form-fitting and materially integral manner.It is preferred here that the steel used is a stainless steel, preferably selected from the group of stainless steels with the AISI designations 316, 316L, 410, 304, 303, 304L, 301, P2000 and 321, particularly preferably the AISI designations 316 and 316L. The AISI designations are the normalized designations of the American Iron and Steel Institute for the compositions of stainless steels and are commonly known to those skilled in the art. The stainless steels listed here are to be understood as being exemplary, by means of which the skilled person can select further stainless steels suitable for use in the present invention.The bimetallic sheet of the cell separator can be obtained by joining a steel sheet and a titanium sheet. The joining can be obtained by riveting, spot welding, bolting, bonding, and more preferably plating.Potentials of electrodes are generally given as a difference with respect to the NHE ("normal hydrogen electrode"), i.e. the normal hydrogen electrode. A normal hydrogen electrode is understood to mean the definition of a platinum / hydrogen electrode customary in the prior art, which is operated with 1 mol / L hydrochloric acid as electrolyte and under atmospheric conditions, i.e. at 1013 mbar under normal conditions. The deviations from Standardwassserstoffelektrode normalized to a proton activity of 1 mol / L and a hydrogen pressure of 1013 mbar at each temperature, are minimal and can be neglected. The potential of the standard hydrogen electrode or the normal hydrogen electrode is defined as 0 V as standard in the prior art. If potentials are indicated in the present description, then, as just described, these are indicated as being customary in the prior art as the difference with respect to the potential of the standard hydrogen electrode or normal hydrogen electrode.At the potentials of 1.0 V to 2.5 V prevailing in the anode during normal operation, the titanium of the corresponding components forms a passivating oxide layer which increases over time. Although this slows down significantly the further reaction of the oxygen present in the anode during normal operation with the titanium surface, the oxide layer nevertheless grows strongly over an operating time of several thousand hours. This oxide layer is reduced again in the method according to the invention, the titanium surface of the components is regenerated and in this way the contact resistances at the contact surfaces between the components of the anode are reduced, so that the latter can again operate in an economically economical range.The process according to the invention for reducing this oxide layer of titanium components of the anode of electrolysis cells comprises polarising the anode to a negative potential and supplying the anode with a reducing medium, the titanium surfaces being coated with metal layers of less than 1 micrometer thickness.The reducing medium of the present invention is a medium which under the selected conditions, in particular under the potentials applied to the anode, is capable of reducing the titanium oxide layer, but at least of supporting the reduction of the titanium oxide layer. According to the invention, the reducing medium is a gas.According to the invention, the reducing medium is a gas selected from the group consisting of hydrogen, carbon monoxide, ammonia and hydrocarbons and mixtures thereof. Suitable hydrocarbons are known to the skilled worker and can preferably be methane, ethane, propane, butane, ethene, propene, butene, ethyne, propyne, butyne and mixtures thereof. The gases mentioned can be humidified. It is often convenient to use hydrogen. The particularly preferred reducing medium of the process according to the invention is therefore hydrogen gas, which may optionally be humidified.It has now surprisingly been found that by conducting regeneration under a reducing medium upon application of an appropriate potential, the thickness of the metal coatings can be reduced very much, which means a considerable saving effect. Moreover, it has likewise surprisingly been found that, by using very thin metal coatings on the titanium surfaces, the reduction of the titanium oxide layers can even be accelerated or the negative potential at the anode can be reduced during the reduction. This synergy of a) metal coatings in thicknesses of less than 1 micrometer and b) the carrying out of a regeneration by means of applying a negative potential under a reducing medium allows, on the one hand, the drastic reduction of the expensive metal coatings to a fraction and, on the other hand, the increase of the service life of the electrolysis cell.In a preferred embodiment of the present invention, the thickness of the metal layers is less than 800 nanometers, preferably less than 500 nanometers, and more preferably less than 200 nanometers. Thicknesses of the metal layers of less than 100 nanometers are also preferred in the context of the present invention.Furthermore, it is preferred to carry out the very thin coatings with certain metals or alloys. The metals selected from the group consisting of lithium, sodium, potassium, cesium, calcium, aluminum, hafnium, vanadium, niobium, chromium, molybdenum, molybdenum oxide, manganese, manganese oxide, iron, ruthenium, ruthenium oxide, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold and alloys thereof are capable of accelerating the reduction and / or reducing the negative potential compared to the NHE necessary for reduction below, for example, a hydrogen atmosphere. This is probably due to the fact that these metals or alloys catalyze the reduction of the titanium oxides and / or reduce the free (negative) enthalpy of reaction of the titanium oxide formed. In a preferred embodiment of the present invention, the metal is therefore selected from the group consisting of lithium, sodium, potassium, cesium, calcium, aluminum, hafnium, vanadium, niobium, chromium, molybdenum, molybdenum oxide, manganese, manganese oxide, iron, ruthenium, ruthenium oxide, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold and alloys thereof. Preference is given here to ruthenium, rhodium, iridium, palladium, platinum, silver, gold and alloys thereof. Of these, particular preference is given to ruthenium, rhodium, iridium and alloys thereof. All conventional coating methods are suitable for coating and / or surface alloy. The methods of physical vapor deposition (PVD) and chemical vapor deposition (CVD), of which sputtering methods (also called cathode sputtering) are preferably suitable.As mentioned above, an anode in a PEM electrolyser is usually operated at potentials of +1.0 V to +2.5 V in normal electrolysis operation. In the method according to the invention, it is preferred that the cathode is brought to a potential of approximately 0 V and the anode to a negative potential. A potential of approximately 0 V means, for the purposes of the present invention, a potential in the range from -0.5 V to +0.5 V, preferably in the range from -0.25 V to +0.25 V, particularly preferably in the range from -0.1 V to +0.1 V and very particularly preferably in the range from -0.01 V to +0.01 V. A negative potential means a negative potential with respect to the NHE, i.e. potentials of less than 0 V and preferably lower, i.e. more negative, potentials than the potential of the cathode. Preferred potentials of the anode in the method according to the invention are therefore potentials<0 V, preferably <-0.5 V, further preferably <-1.0 V, particularly preferably <-1.5 V. In a preferred embodiment of the present invention, the cathode is therefore brought to a potential of approximately 0 V and the anode is brought to a negative potential.Since, in continuous operation, as is customary in commercial practice, the titanium oxide layers mentioned form with the described disadvantageous consequences at relatively equal time intervals of about a few thousand hours, it is expedient to carry out the process according to the invention for the degradation of these layers at just the same time intervals. In a preferred embodiment of the process of the present invention, therefore, the process is carried out in a regular regeneration cycle, preferably in a period of from all 1000 hours to all 10000 hours. In the context of the present invention, this time period means in particular every 1000 to 10000 operating hours of the electrolysis cell, of the electrolysis stack or of the electrolyser. In a further preferred embodiment of the present invention, therefore, the regeneration process is carried out in a regular regeneration cycle in the range from all 1000 hours to all 10000 hours.The regeneration can take place unsupervised or monitored in this case, such as by means of regular monitoring of the 1 kHz AC impedance. The measurement of an impedance of at least one kilohertz (1 kHz) is particularly suitable for determining the ohmic components of the contact resistors. In a preferred embodiment of the present invention, the regeneration takes place while monitoring the 1 kHz impedance of the electrolysis cell or of the anode. In this way, either the regeneration operation can be maintained until a predetermined limit value is undershot, or the regeneration procedure is triggered in all load-free states as long as said limit value is exceeded. In this case, a load-free state means that no power consumption by the electrolyser takes place. The predefined limit value is particularly preferably that value which is measured in the new state of the electrolyser and which therefore has no or only minimal titanium oxide layers, as are illustrated by way of example in FIG. 2. Alternatively, the regeneration method can also be carried out until the 1 kHz impedance no longer changes, i.e. no further regeneration takes place.The process according to the invention can preferably be carried out in a single electrolysis cell, preferably an electrolysis stack (i.e. a stack of interconnected cells) and / or an electrolyser.In a further preferred embodiment of the present invention, the reducing medium is hydrogen, the anode is brought to negative potentials, the thickness of the metal layers is less than 500 nanometers, preferably less than 200 nanometers, and the metal comprises ruthenium, rhodium, iridium, palladium, platinum, silver, gold and alloys thereof, preferably rhodium, iridium, palladium, and alloys thereof.The process according to the invention is thus excellently suitable for reducing ohmic contact resistances on titanium surfaces in electrolysis cells and thus for increasing the life of the uncoated titanium anode structures. At the same time, the use of the method according to the invention makes it possible to substantially dispense with the expensive coating of the titanium surfaces of the anodes. In this way, the process according to the invention additionally contributes to increasing the economics of such anodes or electrolysers.The present invention therefore further provides an electrolysis cell for an electrolyser comprising titanium components, the titanium surfaces being coated as described above with metal layers of less than 1 micrometer thickness.DESCRIPTION OF THE FIGURESFIG. 1 shows a cross section through the layers of the anode (1) of a PEM electrolysis cell. The flow distributor layer ( 3) and the current distributor layer ( 4) thereon are arranged directly on the bipolar plate / the cell separator ( 2) made of titanium sheet or a titanium / stainless steel bimetal. Flow distributor layer ( 3) and flow distributor layer ( 4) are usually also made of titanium. Between the current distributor layer (4) and the outer membrane (6) is the catalyst layer (5).FIG. 2 shows the oxide layers ( 7) in an anode ( 1) according to FIG. 1. The oxide layer (7) is depicted on the contact surfaces between the components (3) and (4) and between the components (4) and (5). Furthermore, the oxide layer (7) also occurs on the inner surface (8) and the inner contact layer (9) of the non-sintered fibre structures of the current distribution layer (4). The individual components ( 3), ( 4) and ( 5) are shown at a distance from one another in comparison to FIG. 1 for the purpose of illustration.FIG. 3 illustrates the sequences within a PEM electrolysis cell ( 10) during normal electrolysis operation. A potential of about 0 V is applied to the cathode (11), while a potential of about 1.5 to 2.5 V is applied to the anode (12). Water ( 13) reaches the anode through the flow distributor layer, is correspondingly decomposed, the protons thus produced migrate ( 14) through the membrane to the cathode, are converted there to gaseous hydrogen, which is discharged again via the flow distributor layer of the anode ( 15). Via the flow distributor layer of the anode, the gaseous oxygen generated in the anode is discharged together with water (16).FIG. 4 shows schematically, by way of example, the sequence of the process according to the invention in a PEM electrolysis cell ( 10). A potential of about 0 V is applied to the cathode (17), and a potential<0 V (18) is applied to the anode. This means in principle that the potential conditions during the regeneration process have reversed in comparison with the normal operation of the electrolysis cell. Optionally humidified gaseous hydrogen is passed through the flow distributor layers of cathode and anode ( 19). According to the potentials reversed in comparison to normal operation, protons now diffuse through the membrane from the cathode in the direction of the anode (20). Gaseous hydrogen exits the cathode flow distribution layer (21), while gaseous hydrogen and small amounts of gaseous oxygen are discharged from the anode flow distribution layer (22).

Claims

A method for reducing the oxide layer of titanium components of an anode in an electrolysis cell, the method being carried out in an electrolysis stack and / or an electrolyser comprising polarising the anode to a negative potential and supplying the anode with a reducing medium, the titanium surfaces being coated with metal layers of less than 1 micrometer thickness, the reducing medium being a gas selected from the group consisting of hydrogen, carbon monoxide, ammonia and hydrocarbons and mixtures thereof.The method of claim 1, wherein the cathode is brought to a potential of 0 V.The method according to at least one of the preceding claims, wherein the thickness of the metal layers is less than 800 nanometers, preferably less than 500 nanometers and more preferably less than 200 nanometers.The method according to any one of the preceding claims, wherein the metal is selected from the group consisting of lithium, sodium, potassium, cesium, calcium, aluminum, hafnium, vanadium, niobium, chromium, molybdenum, manganese, iron, ruthenium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, and alloys thereof.The method according to any one of the preceding claims, wherein the method is performed in a regular regeneration cycle ranging from all 1000 hours to all 10000 hours.The method according to any one of the preceding claims, wherein the reducing medium is hydrogen, the thickness of the metal layers is less than 500 nanometers, preferably less than 200 nanometers, and the metal comprises palladium, platinum, silver, gold and alloys thereof.Method according to at least one of the preceding claims, wherein the regeneration is monitored, in particular by regular measurement of the 1 kHz impedance.The method according to at least one of the preceding claims, wherein the method is carried out in a PEM electrolysis stack.

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