Method for 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 electrolytic cell, comprising supplying the anode with a reducing medium, wherein the cathode is brought to a potential of 0 V and the anode is brought to a negative potential.

Description

The present invention relates to a media distributor of electrolysis cells, in particular of PEM electrolysers or alkaline electrolysers.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. Other customary proton-conducting membranes which are used in electrolysers are, for example, polybenzimidazole (PBI) and those which are available under the trade names Flemion (Asahi Glass) or Dow Membrane (Dow Chemical).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.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.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 developers of the present invention found that by treating the passivating oxide layer regularly by a reducing atmosphere and polarizing to an appropriate potential, a reduction of this layer can be achieved. This method thus enables a reduction in the amount of coatings otherwise used and thus a reduction in costs.The present invention thus provides a method for reducing the oxide layer of titanium components of the anode in an electrolysis cell, comprising polarising the anode to a suitable potential and supplying the anode with a reducing medium.As a result, the potentials prevailing in the anode at the potentials prevailing in normal operation can be reduced again effectively at the oxide layers forming on the titanium. Although the oxide layers mentioned prevent further reactions of titanium with oxygen, at the same time the contact resistance increases over time in such a way that the operation of the cell would become uneconomical, which likewise represents a limiting factor for the service life of anodes in electrolysers. The process according to the invention can thus improve the service life and the economic efficiency of modern electrolysers.In commercial electrolysers, coated titanium surfaces have normally always been used up to now, which surfaces are intended to prevent the formation of the oxide layer in the long term and thus to ensure low contact resistances between the titanium layers in the long term. The method according to the invention offers a considerable potential for cost reduction here, since the otherwise very expensive coatings of noble metals such as gold can be dispensed with. This is to be considered in particular on the background that the entire flow distributor structure including current distributor layers of anode and cathode make up up 50% of the cost of a PEM electrolysis stack.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, but other proton-conducting membrane materials are also customaryThe 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. 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).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 titanium plate can serve simultaneously as a titanium sheet part of a bimetallic cell separator.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). 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 suitable potential and supplying the anode with a reducing medium.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. Preferably, the reducing medium is a gas.In a preferred embodiment of the present 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 optionally humidified hydrogen gas.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. According to the invention, 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 process according to the present invention, hydrogen is used as the reducing medium and the anode is brought to negative potentials.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.The regeneration can take place unsupervised or monitored in this case, such as by means of regular measurement of the impedance. Preferably, the impedance measurement takes place in the frequency range between 100 Hz and 10 kHz, particularly preferably at 1 kHz. 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.In a preferred embodiment of the present invention, the rain ration is monitored, in particular by regular measurement of the impedance of the anode and / or of the electrolysis cell and / or of the electrolysis stack, preferably in the frequency range between 100 Hz and 10 kHz, particularly preferably at 1 kHz.The process according to the invention can be carried out in a single electrolysis cell, a customary electrolysis stack (i.e. a stack of interconnected cells) and / or an electrolyser.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.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 bipolar plate / 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).In FIG. 2, the oxide layer (7) is depicted on the inner surface (8) and inner contact layer (9), for example unsintered fiber structures, of the titanium components of the anode of a PEM electrolyser.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 and another reducing gas are 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 again from the flow distributor layer of the cathode (21), while gaseous hydrogen and reaction products deriving from the reduction of the passivating titanium oxide layer, e.g. small amounts of gaseous oxygen or water or other oxidized or partially oxidized species of the reducing gas species added at (19), are discharged (22).

Claims

A method for reducing the oxide layer of titanium components of an anode in an electrolytic cell, comprising supplying the anode with a reducing medium, wherein the cathode is brought to a potential of 0 V and the anode is brought to a negative potential.The process of claim 1, wherein the reducing medium is a gas selected from the group consisting of hydrogen, carbon monoxide, ammonia, and hydrocarbons, and mixtures thereof.The method of claim 2, wherein the reducing medium is hydrogen.Method according to at least one of the preceding claims, wherein the method is carried out in a regular regeneration cycle.The method of claim 4, wherein the periodic regeneration cycle comprises a time period from all 1000 hours to all 10000 hours.Method according to at least one of the preceding claims, wherein the regeneration is monitored, in particular by regular measurement of the impedance of the anode and / or of the electrolysis cell and / or of an electrolysis stack, preferably in the frequency range between 100 Hz and 10 kHz, particularly preferably at 1 kHz.Method according to at least one of the preceding claims, wherein the method is carried out in an electrolysis stack, preferably a PEM electrolysis stack, and / or an electrolyser.

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