JP2014181408A - In situ-reduction method of passivation oxide layer in titanium component of anode of pem electrolytic bath - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an improved reduction method of an oxide layer which can avoid thick coating in the prior art.SOLUTION: A method for reducing an oxide layer of a titanium component of an anode in an electrolytic cell includes the steps of applying a proper potential to the anode, and supplying the anode with a reduction medium.

Description

  The present invention relates to medium distribution in electrolysis cells, in particular PEM or alkaline electrolysers.

The electrolytic cell according to the present invention is in particular understood as a Proton Exchange Membrane electrolytic cell (PEM electrolytic cell), in which the anode and the cathode are separated from each other by a proton conducting membrane. As membrane material, polysulfonic acid (PFSA) in a polytetrafluoroethylene (PTFE) matrix is usually used. Other common proton conducting membranes used in the electrolysis layer are sold, for example, as polybenzimidazole (PBI), trade name Flemion ^(R) (Asahi Glass) or Dow Membrane ^(R) (Dow Chemical). Are listed.

  Under the potential generated at the anode during normal operation, a passivation oxide layer is formed on titanium, and the passivation oxide layer increases with time. This certainly prevents further reaction with oxygen on the titanium surface, but at the same time the contact resistance is significantly increased. Thus, the uncoated titanium surface at the anode is only used in the scientific field in laboratory electrolytic cells that need only operate for up to several thousand hours. In commercial electrolytic cells, a titanium surface coated at the anode is always used. In that case, the coating prevents the formation of an oxide layer and provides low contact resistance between the titanium layers over time.

  The object of the present invention is to provide an improved method of reducing the oxide layer which can avoid thick coatings according to the prior art.

  The inventors of the present invention have found that reduction of the passivation oxide layer can be achieved by periodic treatment of the passivation oxide layer with a reducing atmosphere and polarization to an appropriate potential. Thus, the present method can reduce the commonly used coatings and thus reduce costs.

  Thus, in accordance with the present invention, a method is provided for reducing the oxide layer of the titanium component of the anode in an electrolysis cell, the method comprising polarizing the anode to an appropriate potential and supplying a reducing medium to the anode. And steps.

  Thus, the potential generated at the anode during normal operation can effectively reduce the oxide layer formed on titanium again. Certainly, the oxide layer described above prevents further reaction of titanium and oxygen, but at the same time, the contact resistance increases with time, which can make the operation of the cell uneconomical. It also represents the limiting factor regarding the life of the anode in the electrolytic cell. Thus, the method of the present invention can improve the life and economy of modern electrolytic layers.

  In commercial electrolysers, a coated titanium surface is generally always used today. Such a coated titanium surface prevents the formation of oxide layers and provides low contact resistance between the titanium layers over time. The method of the present invention provides a significant possibility for reducing costs, for example because a coating made of a noble metal such as gold, which is usually very expensive, can be omitted. This deserves consideration in the background that the overall flow distribution structure, including the anode and cathode current distribution layers, has reached 50% of the cost of the PEM electrolytic stack.

  The electrolytic cell in the frame of the present invention is particularly understood as a proton exchange membrane (Proton Exchange Membrane) electrolytic cell (PEM electrolytic cell), and the anode and the cathode in this electrolytic cell are separated from each other by a proton conducting membrane. Yes. As membrane material, polysulfonic acid (PFSA) in a polytetrafluoroethylene (PTFE) matrix is usually used, however other proton conducting membrane materials are also commonly used.

  In general, a potential of about 0 V with respect to a normal hydrogen electrode (NHE) known to those skilled in the art is applied to the cathode on the hydrogen side of the electrolytic cell of the electrolytic cell. The cathode is advantageously composed of a catalyst layer deposited on a proton conducting membrane, a porous current distribution layer, and a flow distribution structure. The porous current distribution layer is advantageously composed of a non-woven or woven fabric composed of carbon fibers, a laminate composed of a plurality of thinly stretched titanium sheets, or a woven or woven fabric. It is comprised from the titanium fiber which does not need to be comprised, it is comprised from the porous titanium sintered compact, or it is comprised from what combined them. The flow distribution structure is advantageously composed of a conductive polymer containing a channel structure. The conductivity of the polymer can be achieved, for example, by filling with carbon such as carbon black. The channel structure can be formed by injection molding, stamping or milling. Alternatively, the cathode current distribution structure can also be formed from titanium. In that case, a thick protective layer is advantageously formed on the titanium over the lifetime of the cell. Such protective layers are known to the person skilled in the art from the prior art, examples of which are protective layers made of carbon. Such a protective layer can be applied, for example, by PVD (physical vapor deposition).

  A potential in the range of +1.0 V to +2.5 V is applied to the anode of the electrolytic cell of the electrolytic cell according to the present invention during normal operation. The anode is advantageously composed of a catalyst layer deposited on a proton conducting membrane, a porous current distribution layer, and a flow distribution structure. The porous current distribution layer is advantageously composed of a laminate composed of a plurality of thinly elongated titanium sheets, or composed of titanium fibers which may or may not be woven, It is comprised from the porous titanium sintered compact, or it is comprised from what combined them. The flow distribution structure can consist of a channel in the titanium plate that supplies liquid water to the electrode and discharges the generated gaseous oxygen, or consists of a porous titanium structure be able to. At the same time, the titanium plate can be used as a titanium thin plate portion of a bimetal cell separator.

  As titanium in the frame of the present invention, a commercially available titanium thin plate similarly uses a commercially available titanium alloy such as Ti-6Al-4V (that is, a titanium alloy containing 6% aluminum and 4% vanadium). be able to. Other generally suitable titanium alloys are known to those skilled in the art.

  The cell separator can include a bimetallic sheet consisting of a steel sheet and a titanium sheet. Bimetal is generally understood to be a metal plate composed of two layers of different metals or different metal alloys. These layers are often connected by shape bonding (ie, formschluessig) and material bonding (ie, material binding: stoffschluessig).

  Preferably, the steel used is a special steel, preferably a special steel according to AISI standards 316, 316L, 410, 304, 303, 304L, 301, P2000 and 321, particularly preferably AISI standards 316 and 316L. Selected from the group. The AISI standard is a standardized standard by the American Iron and Steel Institute for the composition of special steels and is well known to those skilled in the art. It should be understood that the special steels listed here are exemplary, and based on them, those skilled in the art can select other special steels suitable for use in the present invention.

  A bimetallic sheet of a cell separator can be obtained by joining a steel sheet and a titanium sheet. This connection can be obtained by rivet joining, spot welding, screwing, particularly preferably by cladding.

  The potential of the electrode is generally expressed as NHE (“Normal Hydrogen Electrode”), ie, the difference with respect to the standard hydrogen electrode. The standard hydrogen electrode is a definition of a platinum / hydrogen electrode that operates under 1 mol / L hydrochloric acid as an electrolytic solution and atmospheric conditions, that is, operates under 1013 mbar under normal conditions, which is common in the prior art. It is understood that there is. Deviations from a standard hydrogen electrode standardized at 1013 mbar hydrogen pressure and 1 mol / L proton activity at any temperature are minimal and can be ignored. In the prior art, the potential of the reference hydrogen electrode or the standard hydrogen electrode is normally defined as 0V. In the present specification, when potentials are expressed, these potentials are expressed as a difference with respect to the potential of the reference hydrogen electrode or the standard hydrogen electrode as in the prior art as described above.

  Under a potential of 1.0 V to 2.5 V generated at the anode during normal operation, a passivation oxide layer is formed on the titanium of the corresponding component, and the passivation oxide layer increases with time. This does indeed significantly slow down further reaction of the titanium surface with oxygen present in the anode during normal operation, but nevertheless the oxide layer increases significantly over thousands of hours of operation. . This oxide layer is reduced again by the method of the invention, so that the titanium surface of the component is regenerated, and thus the contact resistance at the contact surface between the components of the anode is reduced, so that the anode is again economical. It is possible to operate within the effective and effective range.

  The method according to the invention for reducing such an oxide layer of the titanium component of the anode in an electrolytic cell comprises the steps of polarizing the anode to a suitable potential and supplying a reducing medium to the anode.

  The reduction medium of the present invention is capable of reducing the titanium oxide layer under selected conditions, particularly under the potential applied to the anode, but at least assists in the reduction of the titanium oxide layer. It is a possible medium. Advantageously, the reducing medium is a gas.

  In one advantageous embodiment of the invention, the reduction medium is a gas selected from the group consisting of hydrogen, carbon monoxide, ammonia and hydrocarbons and mixtures thereof. Suitable hydrocarbons are known to those skilled in the art and are advantageously methane, ethane, propane, butane, ethene, propene, butene, ethyne, propyne, butyne and mixtures thereof. The above gas may be humidified. Often it is preferred to use hydrogen. A particularly advantageous reduction medium of the process according to the invention is therefore hydrogen gas, which is optionally humidified.

  As described above, the anode in the PEM electrolytic cell normally operates at a potential of +1.0 V to +2.5 V during a normal electrolysis operation. In the method according to the invention, it is advantageous that a potential of about 0 V is applied to the cathode and a negative potential is applied to the anode. A potential of about 0 V is within the framework of the present invention a potential in the range of −0.5 V to +0.5 V, preferably a potential in the range of −0.25 V to +0.25 V, particularly preferably − It means a potential in the range of 0.1V to + 0.1V, very advantageously a potential in the range of -0.01V to + 0.01V. Negative potential means a negative potential with respect to NHE, ie a potential lower than 0V, and advantageously a potential lower than the potential of the cathode, ie a potential which is more negative than the potential of the cathode. Accordingly, the preferred potential of the anode in the process according to the invention is less than 0 V, preferably less than −0.5 V, more preferably less than −1.0 V, particularly preferably less than −1.5 V.

  In one advantageous embodiment of the method according to the invention, hydrogen is used as the reducing medium and a negative potential is applied to the anode.

  During constant operation, as is common within the commercial framework, the titanium oxide layer described above is formed at relatively equal time intervals of approximately several thousand hours, resulting in the disadvantageous effects described above. It is preferred that the method according to the invention is carried out to decompose those layers at just such time intervals. Thus, in one advantageous embodiment of the method according to the invention, the method is carried out with a regular regeneration period, preferably with a time interval between every 1,000 hours and every 10,000 hours. Within the framework of the present invention, this time interval means in particular the time interval from every 1,000 operating hours to every 10,000 operating hours of the electrolytic cell, electrolytic stack or electrolytic cell.

  This regeneration can be carried out without monitoring or can be carried out with monitoring, for example by regular measurement of impedance. The impedance measurement is preferably carried out in the frequency range between 100 Hz and 10 kHz, particularly preferably at a frequency of 1 kHz. The measurement of impedance of at least 1 kilohertz (1 kHz) is particularly advantageously suitable for detecting the ohmic component of contact resistance. In one advantageous embodiment of the invention, the regeneration is performed under the monitoring of the 1 kHz impedance of the electrolytic cell or anode. In this way, the regeneration operation can be maintained until a predetermined limit value is reached, or the regeneration process is triggered in any unloaded condition until such a limit value is exceeded. Here, the no-load state means that no electric power is consumed by the electrolytic cell. The predetermined limit value is particularly preferably a value measured when the electrolysis cell is new and does not have a titanium oxide layer or is minimal, as exemplarily shown in FIG. It is a value in a state having only a limited titanium oxide layer. Alternatively, the regeneration method can be carried out until the impedance of 1 kHz no longer changes, ie until no further regeneration takes place.

  In one advantageous embodiment of the invention, the impedance of the anode and / or electrolytic cell and / or electrolytic stack is preferably regulated, preferably in the frequency range between 100 Hz and 10 kHz, particularly preferably at a frequency of 1 kHz. By measuring automatically, regeneration is monitored.

  The method according to the invention can be carried out in individual electrolysis cells in a general electrolysis stack (ie a stack of cells connected together) and / or in an electrolyzer.

  The method according to the invention is therefore very suitable for reducing the ohmic contact resistance on the titanium surface in the electrolysis cell and thus extending the life of the uncoated titanium anode structure. At the same time, by using the method according to the invention, the expensive coating on the titanium surface of the anode can be omitted sufficiently. As such, the method according to the invention further contributes to an economic improvement of such an anode or electrolytic cell.

Figure 2 shows a cross-sectional view of the anode layer of a PEM electrolysis cell. FIG. 2 is an exploded view of the anode according to FIG. 1, in which an oxide layer is formed on each component. 2 shows the progress in the PEM electrolysis cell during normal operation of the electrolytic cell. 2 shows the progress of the method according to the invention in a PEM electrolysis cell.

  FIG. 1 shows a cross-sectional view of a plurality of layers of an anode 1 of a PEM electrolysis cell. A flow distribution layer 3 is disposed directly on the bipolar plate / cell separator 2 made of titanium thin plate or titanium / special steel bimetal, and a current distribution layer 4 is disposed on the flow distribution layer 3. . The flow distribution layer 3 and the current distribution layer 4 are mostly composed of titanium. A catalyst layer 5 is provided between the current distribution layer 4 and the outer membrane 6.

  In FIG. 2, an oxide layer 7 is formed on the inner surface 8 and the inner contact layer 9 of, for example, an unsintered fiber structure of the anode titanium component in the PEM electrolytic layer.

  In FIG. 3, the course in the PEM electrolysis cell 10 during normal operation of the electrolytic cell is visualized. A potential of about 0V is applied to the cathode (11), while the anode is at a potential of about 1.5V to 2.5V (12). Through the flow distribution layer, water (13) reaches the anode and is decomposed accordingly, and the protons produced there move to the cathode through the membrane (14) and turn into gaseous hydrogen at the cathode. The gaseous hydrogen is again discharged through the anode flow distribution layer (15). Gaseous oxygen generated at the anode is discharged with water through the anode flow distribution layer (16).

  In FIG. 4, the course of the method according to the invention in the PEM electrolysis cell 10 is schematically illustrated. A potential of about 0 V is applied to the cathode (17), and a potential of less than 0 V is applied to the anode (18). This in principle means that the potential ratio during the regeneration process has been reversed compared to during normal operation of the electrolysis cell. Passing through the cathode and anode flow distribution layers, gaseous hydrogen and / or other reducing gases, if necessary, are induced (19). Depending on the potential reversed compared to normal operation, protons now diffuse from the cathode through the membrane towards the anode (20). Again, gaseous hydrogen flows out of the cathode flow distribution layer (21), while the reaction product resulting from the reduction of gaseous hydrogen and the passivation titanium oxide layer from the anode flow distribution layer. For example, a small amount of gaseous oxygen or water, or other oxidized or partially oxidized species of the supplied reducing gas species represented by reference number (19) flows out (22).

Claims (8)

In a method for reducing an oxide layer of an anode titanium component in an electrolytic cell,
Polarizing the anode to an appropriate potential;
Supplying a reducing medium to the anode,
A method characterized by that.   The method 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 according to claim 1, wherein a potential of about 0 V is applied to the cathode and a negative potential is applied to the anode.   The method according to at least one of claims 1 to 3, wherein the method is carried out with a regular regeneration period.   The method according to claim 4, wherein the regular regeneration period is performed at a time interval from every 1,000 hours to every 10,000 hours. The reducing medium is hydrogen;
The method according to claim 1, wherein a negative potential is applied to the anode.   Monitoring the regeneration, in particular by regularly measuring the impedance of the anode and / or electrolytic cell and / or electrolytic stack, preferably in the frequency range between 100 Hz and 10 kHz, particularly preferably at a frequency of 1 kHz, 7. A method according to at least one of claims 1-6.   Method according to at least one of the preceding claims, carried out in an electrolytic stack, preferably in a PEM electrolytic stack and / or in an electrolytic layer.

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