Patent Publication Number: US-2023143743-A1

Title: Titanium substrate, method for producing titanium substrate, electrode for water electrolysis, and water electrolysis apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a titanium substrate excellent in conductivity and corrosion resistance, a method for producing a titanium substrate, an electrode for water electrolysis formed of the titanium substrate, and a water electrolysis apparatus. 
     Priority is claimed on Japanese Patent Application No. 2020-056277, filed Mar. 26, 2020, the content of which is incorporated herein by reference. 
     BACKGROUND ART 
     As shown in Patent Document 1 for example, titanium substrates formed of titanium or a titanium alloy are used in current-carrying members such as electrodes, in particular, in applications which require oxidation resistance (corrosion resistance). 
     However, for example, in a case of using a cathode electrode of a solid polymer electrolyte fuel cell (PEFC), an anode electrode of a water electrolysis apparatus, an electrode material for a lithium ion battery or a lithium ion capacitor, or the like in a harsh corrosive environment such as a high potential, oxygenated, strongly acidic atmosphere, there are problems in that the corrosion resistance is not sufficient and an insulative TiO 2  film is formed on the surface of the titanium substrate during use and the performance as a current-carrying member such as an electrode deteriorates. 
     Therefore, for example, Patent Document 2 proposes forming a noble metal film of gold, platinum, or the like on the surface of a substrate formed of aluminum, nickel, or titanium to improve corrosion resistance while ensuring conductivity. 
     In addition, Patent Document 3 proposes a titanium material in which an oxide film, in which an X-ray diffraction peak of TiO 2  is not observed, is formed on the surface of titanium or a titanium alloy. 
     Furthermore, Patent Document 4 proposes a titanium material having a titanium oxide layer having an oxygen/titanium atomic concentration ratio (O/Ti) of 0.3 or more and 1.7 or less on the surface of a titanium material formed of pure titanium or a titanium alloy, in which an alloy layer including at least one type of noble metal selected from Au, Pt, and Pd is formed on this titanium oxide layer. 
     Here, in a case where a noble metal film is formed as shown in Patent Document 2 and Patent Document 4, the cost increases greatly and wide use is not possible. 
     In addition, in the oxide film described in Patent Document 3, the conductivity and corrosion resistance are insufficient, thus, application is not possible as a member to be used in a harsh environment. 
     Therefore, Patent Document 5 proposes a titanium substrate on which a Magneli phase titanium oxide film formed of a Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10) is formed. 
     This Magneli phase titanium oxide has corrosion resistance equivalent to TiO 2  and conductivity equivalent to graphite and is able to be used even under harsh corrosive environments. 
     CITATION LIST 
     Patent Documents 
     [Patent Document 1] 
     Japanese Unexamined Patent Application, First Publication No. 2003-226992 
     [Patent Document 2] 
     Japanese Unexamined Patent Application, First Publication No. 2010-135316 
     [Patent Document 3] 
     Japanese Patent No. 5831670 
     [Patent Document 4] 
     Japanese Unexamined Patent Application, First Publication No. 2010-236083 
     [Patent Document 5] 
     Japanese Unexamined Patent Application, First Publication No. 2019-157273 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the titanium substrate described in Patent Document 5 described above, the Magneli phase titanium oxide film has a porous structure. 
     Here, depending on the use application of the titanium substrate, there may be a demand for excellent durability for the Magneli phase titanium oxide film in order to be used stably for a long period. In such use applications, there is a concern that, as described in Patent Document 5, the Magneli phase titanium oxide film with the porous structure may not be viable. 
     The present invention was made against the background of the above circumstances and has an object of providing a titanium substrate with particularly excellent conductivity and corrosion resistance and having superior durability, a method for producing a titanium substrate, an electrode for water electrolysis formed of this titanium substrate, and a water electrolysis apparatus. 
     Solution to Problem 
     In order to solve these problems and achieve the object described above, a titanium substrate of the present invention includes a substrate main body formed of titanium or a titanium alloy, in which a Magneli phase titanium oxide film is formed on a surface of the substrate main body, the Magneli phase titanium oxide film is formed of a Magneli phase titanium oxide represented by a chemical formula Ti n O 2n-1  (4≤n≤10), and a BET value of the substrate main body on which the Magneli phase titanium oxide film is formed is 0.1 m 2 /g or less. 
     According to the titanium substrate having this configuration, since the Magneli phase titanium oxide film formed of a Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10) is formed on a surface of the substrate main body formed of titanium or a titanium alloy and the BET value of the substrate main body on which the Magneli phase titanium oxide film is formed is 0.1 m 2 /g or less, the conductivity and corrosion resistance are particularly excellent and the durability is also excellent. 
     Thus, stable use is possible for a long period in a harsh corrosive environment such as a high potential, oxygenated, strongly acidic atmosphere. 
     Here, in the titanium substrate of the present invention, it is preferable that the Magneli phase titanium oxide film contains at least one of Ti 4 O 7  and Ti 5 O 9 . 
     In such a case, since the Magneli phase titanium oxide film contains at least one of Ti 4 O 7  and T 5 O 9 , which are particularly excellent in conductivity and corrosion resistance, it is particularly suitable as a current-carrying member used in a harsh corrosive environment such as a high potential, oxygenated, strongly acidic atmosphere. 
     In addition, in the titanium substrate of the present invention, it is preferable that the Magneli phase titanium oxide film has a film thickness in a range of 0.01 μm or more and 3.0 μm or less. 
     In this case, since the film thickness of the Magneli phase titanium oxide film is 0.01 μm or more, it is possible to secure a sufficient corrosion resistance. 
     On the other hand, since the film thickness of the Magneli phase titanium oxide film is 3.0 μm or less, it is possible to secure sufficient conductivity as a titanium substrate. 
     Furthermore, in the titanium substrate of the present invention, preferably, the substrate main body is a porous body having a porosity in a range of 30% or more and 97% or less. 
     In this case, since the substrate main body formed of titanium or titanium alloy is a porous body and the porosity thereof is set to 30% or more, the specific surface area is increased and it is possible to promote the reaction on the surface of the titanium substrate. In addition, it is possible to efficiently discharge the gas generated by the reaction. 
     On the other hand, since the porosity of the substrate main body is set to 97% or less, it is possible to secure the strength of the substrate main body. 
     A method for producing the titanium substrate of the present invention includes a TiO 2  film forming step of forming a TiO 2  film on a surface of a substrate main body formed of titanium or a titanium alloy; and a reduction treatment step of reducing the TiO 2  film formed on the surface of the substrate main body by using a microwave plasma reduction method to obtain a Magneli phase titanium oxide film, the Magneli phase titanium oxide film being formed of a Magneli phase titanium oxide represented by a chemical formula Ti n O 2n-1  (4≤n≤10), in which the titanium substrate described above is produced. 
     The method for producing a titanium substrate with this configuration is provided with a TiO 2  film forming step of forming a TiO 2  film on a surface of a substrate main body formed of titanium or a titanium alloy, and a reduction treatment step of reducing the TiO 2  film formed by a microwave plasma reduction method to obtain a Magneli phase titanium oxide film, thus, it is possible to produce a titanium substrate having a Magneli phase titanium oxide film formed of Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10). 
     The electrode for water electrolysis of the present invention includes the titanium substrate described above. 
     According to the electrode for water electrolysis with this configuration, since the configuration uses a titanium substrate on which the Magneli phase titanium oxide film formed of the Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10) is formed, the conductivity and corrosion resistance are particularly excellent, it is possible to suppress deterioration due to oxidation, and it is possible to significantly improve the service life. In addition, since the corrosion resistance is excellent, use is possible as a substitute for noble metal electrodes, and it is possible to configure an electrode for water electrolysis at low cost. Furthermore, since the BET value of the substrate main body on which the Magneli phase titanium oxide film is formed is 0.1 m 2 /g or less, the durability is excellent and stable use for a long period is possible. 
     Here, in the electrode for water electrolysis of the present invention, in a voltammetry test in which one cycle is denoted by a step of holding for 1 minute at 2.5 V and a step of holding for 1 minute at 0 V, an energy efficiency after 10 cycles is preferably 90% or more of an initial value. 
     In such a case, the durability is sufficiently excellent and stable use for a long period is possible. 
     A water electrolysis apparatus of the present invention includes the electrode for water electrolysis described above. 
     According to the electrode for water electrolysis having this configuration, since the electrode for water electrolysis formed of a titanium substrate on which the Magneli phase titanium oxide film formed of Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10) is formed is provided, it is possible to suppress deterioration of the electrode for water electrolysis due to oxidation during use. In addition, it is not necessary to use the noble metal electrode and it is possible to significantly reduce the production cost of the water electrolysis apparatus. Furthermore, since the BET value of the substrate main body on which the Magneli phase titanium oxide film is formed is 0.1 m 2 /g or less, the durability is excellent and stable use for a long period is possible. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a titanium substrate with particularly excellent conductivity and corrosion resistance and with excellent durability, a method for producing a titanium substrate, an electrode for water electrolysis formed of this titanium substrate, and a water electrolysis apparatus. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is an explanatory diagram showing an example of a titanium substrate which is an embodiment of the present invention. 
         FIG.  2    is an enlarged schematic diagram of a surface layer portion of the titanium substrate shown in  FIG.  1   . 
         FIG.  3    is an SEM image showing cross-sectional observation results of the Magneli phase titanium oxide film of the titanium substrate, which is an embodiment of the present invention. 
         FIG.  4    is a flow chart showing an example of a method for producing the titanium substrate shown in  FIG.  1   . 
         FIG.  5    is an explanatory diagram showing production steps for producing the titanium substrate shown in  FIG.  1   .  FIG.  5 ( a )  shows a substrate main body preparing step S 01 ,  FIG.  5 ( b )  shows a TiO2 film forming step S 02 , and  FIG.  5 ( c )  shows a reduction treatment step S 03 . 
         FIG.  6    is a schematic explanatory diagram of a water electrolysis apparatus provided with an electrode for water electrolysis which is an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A description will be given below of a titanium substrate, a method for producing a titanium substrate, an electrode for water electrolysis, and a water electrolysis apparatus, which are embodiments of the present invention, with reference to the accompanying drawings. 
     A titanium substrate  10  of the present embodiment is used as, for example, a current-carrying member such as a cathode electrode of a solid polymer electrolyte fuel cell (PEFC), an anode electrode of a water electrolysis apparatus, and an electrode material for a lithium ion battery or a lithium ion capacitor. 
     As shown in  FIG.  1    and  FIG.  2   , the titanium substrate  10  of the present embodiment is provided with a substrate main body  11  formed of titanium or a titanium alloy, and a Magneli phase titanium oxide film  16  formed on the surface of the substrate main body  11 . 
     In the present embodiment, as shown in  FIG.  1   , the substrate main body  11  is a porous body and is provided with a skeleton portion  12  having a three-dimensional network structure and a pore portion  13  surrounded by the skeleton portion  12 . 
     The substrate main body  11  has a porosity P in a range of 30% or more and 97% or less. The porosity P of the substrate main body  11  is calculated by the following formula. 
         P (%)=(1−( W /( V×DT )))×100
 
     W: Mass of substrate main body  11  (g) 
     V: Volume of substrate main body  11  (cm 3 ) 
     DT: True density (g/cm 3 ) of titanium or a titanium alloy which forms the substrate main body  11   
     In the present embodiment, the substrate main body  11  formed of this porous body is formed of, for example, a titanium sintered body obtained by sintering a titanium sintering raw material including titanium. 
     In addition, the pore portions  13  surrounded by the skeleton portion  12  are structured to communicate with each other and open toward the outside of the substrate main body  11 . 
     In the titanium substrate  10  of the present embodiment, as shown in  FIG.  2   , the Magneli phase titanium oxide film  16  is formed on the surface of the substrate main body  11 . 
     This Magneli phase titanium oxide film  16  is formed of a Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10). 
     In the present embodiment, the Magneli phase titanium oxide film  16  is densely structured as shown in  FIG.  3    and the BET value of the substrate main body  11  on which the Magneli phase titanium oxide film  16  is formed is 0.1 m 2 /g or less. 
     In the present embodiment, the BET value of the substrate main body  11  on which the Magneli phase titanium oxide film  16  is formed is preferably 0.09 m 2 /g or less and more preferably 0.08 m 2 /g or less. 
     In addition, in the present embodiment, the Magneli phase titanium oxide film  16  preferably contains at least one of Ti 4 O 7  and Ti 5 O 9 . It is possible to identify the titanium oxide structure in the Magneli phase titanium oxide film  16  by an X-ray diffraction analysis (XRD) method. 
     Here, in the Magneli phase titanium oxide film  16  of the present embodiment, the XRD peaks of Ti 4 O 7  and Ti 5 O 9  in X-ray diffraction (XRD) are included and the sum of the maximum peak intensities of both is greater than the maximum peak intensity of the other Magneli phase titanium oxide (6≤n≤10). 
     Here, when a film thickness t of the Magneli phase titanium oxide film  16  is thinned, the corrosion resistance is lowered but the conductivity is improved. On the other hand, when the film thickness t of the Magneli phase titanium oxide film  16  is increased, the corrosion resistance is improved but the conductivity is decreased. Therefore, the film thickness t of the Magneli phase titanium oxide film  16  is preferably set appropriately according to the properties required for the titanium substrate  10 . 
     In the present embodiment, the lower limit of the film thickness t of the Magneli phase titanium oxide film  16  is 0.01 μm or more in order to sufficiently improve corrosion resistance. In addition, the upper limit of the film thickness t of the Magneli phase titanium oxide film  16  is 3.0 μm or less in order to sufficiently improve the conductivity. 
     In order to further improve the corrosion resistance, the lower limit of the film thickness t of the Magneli phase titanium oxide film  16  is preferably 0.02 μm or more, and more preferably 0.03 μm or more. On the other hand, in order to further improve the conductivity, the upper limit of the film thickness t of the Magneli phase titanium oxide film  16  is preferably 2.0 μm or less and more preferably 1.0 μm or less. 
     A description will be given below of the method for producing the titanium substrate  10  of the present embodiment with reference to the flow chart of  FIG.  4   , the step chart of  FIG.  5   , and the like. 
     (Substrate Main Body Preparing Step S 01 ) 
     First, the substrate main body  11  formed of titanium and a titanium alloy shown in  FIG.  5 ( a )  is prepared. In the present embodiment, a porous titanium sintered body is prepared as the substrate main body  11 . 
     It is possible to produce the substrate main body  11  formed of this porous titanium sintered body, for example, by the following steps. A sintering raw material including titanium is mixed with an organic binder, a foaming agent, a plasticizer, water and, as necessary, a surfactant to prepare an effervescent slurry. This effervescent slurry is applied using a doctor blade (applying apparatus) to form a sheet-shaped molded body. This sheet-shaped molded body is heated to foam and obtain a foamed molded body. Then, the result is degreased and then sintered. Due to this, the substrate main body  11  formed of a porous titanium sintered body is prepared. (For example, refer to Japanese Unexamined Patent Application, First Publication No. 2006-138005 and Japanese Unexamined Patent Application, First Publication No. 2003-082405). 
     (TiO 2  Film Forming Step S 02 ) 
     Next, as shown in  FIG.  6 ( b ) , a TiO 2  film  26  is formed on the surface of the substrate main body  11 . In the present embodiment, a TiO 2  film  26  with a dense structure is formed by applying a physical vapor deposition method (for example, ion plating (IP) or a sputtering method). Specifically, the surface of the substrate main body  11  is irradiated with titanium evaporated by an electron gun onto through a plasma formed of argon and oxygen gases. During irradiation, the substrate main body  11  is rotated to uniformly form the TiO 2  film  26  up to the inside of the substrate main body  11 . 
     Here, the film thickness t 0  of the TiO 2  film  26  is preferably in a range of 0.01 μm or more and 3.0 μm or less. 
     (Reduction Treatment Step S 03 ) 
     Next, the TiO 2  film  26  is subjected to a reduction treatment using plasma generated by irradiating the gas with microwaves (microwave plasma reduction treatment), such that the TiO 2  film  26  is the Magneli phase titanium oxide film  16  formed of Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10), as shown in  FIG.  5 ( c ) . In order to suppress oxygen from diffusing to the substrate main body  11  side, this reduction treatment step S 03  is carried out under conditions of a substrate temperature of 400° C. or lower and a treatment time of 15 minutes or less. 
     It is possible to set the lower limit value of the substrate temperature in the reduction treatment step S 03  to 0° C., and the lower limit value of the treatment time to 0.01 minutes. 
     By subjecting the entire TiO 2  film  26  to a reduction treatment to form the Magneli phase titanium oxide film  16 , the film thickness t 0  of the TiO 2  film  26  becomes the film thickness t of the Magneli phase titanium oxide film  16 . Therefore, adjusting the film thickness t 0  of the TiO 2  film  26  in the TiO 2  film forming step S 02  makes it possible to control the film thickness t of the Magneli phase titanium oxide film  16 . 
     By the production method described above, the titanium substrate  10  is produced in which the Magneli phase titanium oxide film  16  formed of Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10) is formed on the surface of the substrate main body  11  formed of titanium or a titanium alloy. 
     Next,  FIG.  6    shows a schematic diagram of the electrode for water electrolysis and the water electrolysis apparatus according to the present embodiment. The water electrolysis apparatus of the present embodiment is a solid polymer-type water electrolysis apparatus having high energy efficiency and hydrogen purity at the time of production. 
     As shown in  FIG.  6   , a water electrolysis apparatus  30  of the present embodiment is provided with a water electrolysis cell  31  provided with an anode electrode  32  and a cathode electrode  33  which are arranged to face each other, and an ion permeable membrane  34  which is arranged between the anode electrode  32  and the cathode electrode  33 . Here, catalyst layers  35  and  36  are formed on both surfaces of the ion permeable membrane  34  (contact surface with the anode electrode  32  and contact surface with the cathode electrode  33 ), respectively. 
     Here, for the cathode electrode  33 , the ion permeable membrane  34 , and the catalyst layers  35  and  36 , it is possible to use examples used in general solid polymer-type water electrolysis apparatuses of the related art. 
     The anode electrode  32  described above is used as the electrode for water electrolysis according to the present embodiment. The anode electrode  32  (electrode for water electrolysis) is formed of the titanium substrate  10  according to the present embodiment described above, and is provided with the substrate main body  11  formed of titanium or a titanium alloy and the Magneli phase titanium oxide film  16  formed on the surface of the substrate main body  11 . In addition, the substrate main body  11  is a porous body, and has a structure provided with the skeleton portion  12  having a three-dimensional network structure and the pore portion  13  surrounded by the skeleton portion  12 . 
     Here, in the electrode for water electrolysis (anode electrode  32 ) according to the present embodiment, in a voltammetry test in which one cycle is denoted by a holding time of 1 minute at 2.5 V and 1 minute at 0 V, the energy efficiency after 10 cycles is preferably 90% or more with respect to the initial value. 
     In the water electrolysis apparatus  30  (water electrolysis cell  31 ) described above, as shown in  FIG.  6   , water (H 2 O) is supplied from the side of the anode electrode  32  and the anode electrode  32  and the cathode electrode  33  are energized. By doing so, oxygen (O 2 ) generated by the electrolysis of water is discharged from the anode electrode  32  and hydrogen (H 2 ) is discharged from the cathode electrode  33 . 
     Here, in the anode electrode  32 , as described above, water (liquid) and oxygen (gas) are circulated, thus, in order to stably circulate the liquid and gas, it is preferable to have a high porosity. In addition, since the anode electrode  32  is exposed to oxygen, there is a demand for excellent corrosion resistance. Therefore, the electrode for water electrolysis formed of the titanium substrate  10  of the present embodiment is particularly suitable as the anode electrode  32 . 
     According to the titanium substrate  10  of the present embodiment configured as described above, since the Magneli phase titanium oxide film  16  formed of Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10) is formed on the surface of the substrate main body  11  formed of titanium or a titanium alloy, the conductivity and corrosion resistance are particularly excellent. 
     In the titanium substrate  10  of the present embodiment, since the BET value of the substrate main body  11  on which the Magneli phase titanium oxide film  16  is formed is 0.03 m 2 /g or more and 0.1 m 2 /g or less, the durability is also excellent. 
     Thus, stable use for a long period is possible in a harsh corrosive environment such as a high potential, oxygenated, strongly acidic atmosphere. 
     In addition, in the present embodiment, since the Magneli phase titanium oxide film  16  contains, as the Magneli phase titanium oxide, at least one of Ti 4 O 7  and Ti 5 O 9 , which are particularly excellent in conductivity and corrosion resistance, it is particularly suitable as a current-carrying member used in a harsh corrosive environment such as a high potential, oxygenated, strongly acidic atmosphere. 
     Furthermore, in the present embodiment, since the film thickness t of the Magneli phase titanium oxide film  16  is in a range of 0.01 μm or more and 3.0 μm or less, it is possible to improve the corrosion resistance and the conductivity in a well-balanced manner. 
     In addition, in the present embodiment, since the substrate main body  11  formed of titanium or a titanium alloy is a porous body, and the porosity P thereof is set to 30% or more, the specific surface area becomes large and it is possible to promote a reaction on the surface of the titanium substrate  10 . In addition, it is possible to efficiently discharge the gas generated by the reaction. Thus, the result is particularly suitable as an electrode member. 
     On the other hand, since the porosity P of the substrate main body  11  formed of a porous body is 97% or less, it is possible to secure the strength of the substrate main body  11 . 
     Since the method for producing the titanium substrate  10  according to the present embodiment is provided with the substrate main body preparing step S 01  for preparing the substrate main body  11  formed of titanium or a titanium alloy, the TiO 2  film forming step S 02  of forming the TiO 2  film  26  on the surface of the substrate main body  11 , and the reduction treatment step S 03  of reducing the TiO 2  film  26  by using a microwave plasma reduction method to obtain the Magneli phase titanium oxide film  16  formed of a Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10), it is possible to produce the titanium substrate  10  having particularly excellent corrosion resistance and conductivity. 
     Then, in the TiO 2  film forming step S 02 , since the configuration is formed by forming the TiO 2  film  26  by a physical vapor deposition method (for example, ion plating or sputtering method), it is possible to form the TiO 2  film  26  with a dense structure. Thus, it is possible to make the Magneli phase titanium oxide film  16  formed after the reduction treatment step S 03  have a dense structure. 
     Furthermore, adjusting the film thickness t 0  of the TiO 2  film  26  formed in the TiO 2  film forming step S 02  makes it possible to accurately control the film thickness t of the Magneli phase titanium oxide film  16 . 
     Since the electrode for water electrolysis (anode electrode  32 ) of the present embodiment is formed of the titanium substrate  10  described above, the conductivity and corrosion resistance are particularly excellent and the durability is excellent, it is possible to suppress deterioration due to oxidation, and stable use for a long period is possible. In addition, since corrosion resistance is excellent, use is possible as a substitute for the noble metal electrode, and it is possible to form an electrode for water electrolysis (anode electrode  32 ) at low cost. 
     In the water electrolysis apparatus  30  of the present embodiment, since the electrode for water electrolysis formed of the titanium substrate  10  described above is used for the anode electrode  32  and the conductivity and corrosion resistance are particularly excellent and the durability is excellent, even in a use environment exposed to oxygen gas, it is possible to suppress deterioration of the electrode for water electrolysis (anode electrode  32 ) due to oxidation and to use the electrode stably for a long period. In addition, since the corrosion resistance is excellent, it is not necessary to use a noble metal electrode, and it is possible to significantly reduce the production cost of the water electrolysis apparatus  30 . Furthermore, since the titanium substrate  10  is formed of the porous body having the structure described above, it is possible to favorably circulate water and oxygen gas. 
     Although embodiments of the present invention were described above, the present invention is not limited thereto and appropriate modification is possible without departing from the technical idea of the invention. 
     For example, in the present embodiment, the substrate main body  11  was described as a porous body, but the present invention is not limited thereto and the substrate main body  11  may be in the shape of a plate, wire, rod, tube, or the like. In addition, the substrate main body  11  was described as being formed of a titanium sintered body, but, without being limited thereto, a mesh plate or the like may be used. 
     In addition, in the present embodiment, the Magneli phase titanium oxide film was described as containing at least one of Ti 4 O 7  and Ti 5 O 9 , but the present invention is not limited thereto, and the above may be formed of Magneli phase titanium oxide represented by the chemical formula Ti n O 2n-1  (4≤n≤10). 
     Furthermore, in the present embodiment, description was given assuming that the film thickness of the Magneli phase titanium oxide film was in a range of 0.01 μm or more and 3.0 μm or less, but the present invention is not limited thereto and the film thickness of the Magneli phase titanium oxide film is preferably set as appropriate according to the properties required for the titanium substrate. 
     Furthermore, in the present embodiment, a water electrolysis apparatus (water electrolysis cell) having the structure shown in  FIG.  6    was described as an example, but the present invention is not limited thereto and as long as an electrode for water electrolysis formed of the titanium substrate according to the present embodiment is provided, the water electrolysis apparatus (water electrolysis cell) may have another structure. 
     Examples 
     A description will be given below of the results of confirmation experiments performed to confirm the effects of the present invention. 
     First, the substrate main body shown in Table 1 is prepared. In Table 1, “titanium” was pure titanium having a purity of 99.9 mass % or more, and “titanium alloy” was a titanium alloy having Ti-0.15 mass % Pd. 
     The dimensions of each prepared substrate main body were width 50 mm×length 60 mm×thickness 0.3 mm. 
     Next, a TiO 2  film was formed on the surface of the substrate main body using the methods and conditions shown in Table 1. 
     Next, the substrate main body on which the TiO 2  film was formed was subjected to a reduction treatment using the methods and conditions shown in Table 1. 
     As described above, a titanium substrate was obtained which had a titanium oxide film (Magneli phase titanium oxide film in the Invention Examples) formed on the surface of a substrate main body formed of titanium or a titanium alloy. 
     With respect to the obtained titanium substrate, the identification of the titanium oxide film, and the film thickness, the conductivity, and the corrosion resistance of the titanium oxide film were evaluated as follows. 
     (Identification of Titanium Oxide in Titanium Oxide Film) 
     The titanium oxide of the titanium oxide film was identified by the X-ray diffraction analysis (XRD) method. The acceleration voltage was set to 30 keV and a Cu Ka line of 8 keV was used for measurement. The measurement range was 2θ=15° to 35°. The presence/absence of Ti 4 O 7  and Ti 5 O 9  was confirmed by the presence/absence of peaks near 21°, 26°, and 30° (Ti 4 O 7 ), 22°, 26°, and 29° (Ti 5 O 9 ), respectively. The evaluation results are shown in Table 2. 
     (BET Value of Substrate Main Body on which Magneli Phase Titanium Oxide Film is Formed) 
     The sample after forming was cut into square pieces of 2 mm×2 mm or less and approximately 0.3 g was filled into a sample folder. The sample folder was placed in an Autosorb-iQ2 manufactured by Quantachrome and degassed at a temperature of 200° C. for 60 minutes. Thereafter, krypton gas was introduced and the specific surface area was measured. 
     (Thickness of Titanium Oxide Film) 
     The sample after film formation is filled with resin and cut in the direction perpendicular to the thickness direction of the titanium oxide film to expose a cross-section thereof. This cross-section was observed by SEM, and five points were equally taken from one end to the other end of the titanium oxide film layer in the SEM image observed at a magnification of 20,000 times and the thicknesses were calculated for each point. Then, the thickness of the titanium oxide film was determined from the average value of the measured 5 points. 
     (Conductivity) 
     From the obtained titanium substrate, a strip-shaped test piece which was width 30 mm×length 40 mm×thickness 0.3 nm was prepared and the electrical conductivity was measured by the 4-probe method. When the measured electrical conductivity value was 1 S/cm or more, the evaluation was “O”, and when less than 1 S/cm, the evaluation was “x”. The evaluation results are shown in Table 2. 
     (Corrosion Resistance) 
     Cyclic voltammetry measurement was performed in a cell with a radius of 4 cm filled with 1 M sulfuric acid, with the created titanium substrate prepared as the working electrode and the coiled Pt wire as the counter electrode. The sweep was repeated between 0-2V for the Ag/AgCl electrode used as the reference electrode. Cyclic voltammetry was measured for 1,000 cycles, and when no change was seen in the CV waveform, the evaluation was “O”, and when a change was observed, the evaluation was “x”. The evaluation results are shown in Table 2. 
     (Stability) 
     Furthermore, the stability of Invention Example 6 and Comparative Examples 5 and 6 as a water electrolysis electrode was confirmed. 
     The titanium substrates described above were each used as anode electrodes to form a solid polymer type water electrolysis cell (area 4 cm×4 cm) with the structure shown in  FIG.  5   . 
     A voltammetry test in which one cycle is a step of holding for 1 minute at 2.5 V and a step of holding for 1 minute at 0 V, was carried out with respect to the water electrolysis cell. The current density that flowed through the cell due to water electrolysis was measured. The test temperature was 80° C. The evaluation results are shown in Table 3. 
     Here, Table 3 shows the current density at the first cycle as the initial value, this initial value as the reference value ( 1 . 0 ), and the ratio of the current density after each cycle to the initial value. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 TiO 2  film forming step 
                 Reduction treatment step 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Process 
                 Process 
                 Film 
                   
                 Substrate 
                 Process 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Substrate main body 
                   
                 temperature 
                 time 
                 thickness 
                   
                 temperature 
                 time 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Substance 
                 Form 
                 Method 
                 (° C.) 
                 (min) 
                 (μm) 
                 Method 
                 (° C.) 
                 (min) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Invention 
                 1 
                 Titanium 
                 Plate 
                 IP 
                 25 
                 180 
                 3.0 
                 Microwave 
                 80 
                 1   
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                 reduction 
               
               
                   
                 2 
                 Titanium 
                 Plate 
                 IP 
                 25 
                 3 
                  0.01 
                 Microwave 
                 30 
                 0.1 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 reduction 
               
               
                   
                 3 
                 Titanium 
                 Porous body 
                 IP 
                 25 
                 30 
                 0.5 
                 Microwave 
                 50 
                 0.5 
               
               
                   
                   
                   
                 (porosity 
                   
                   
                   
                   
                 reduction 
               
               
                   
                   
                   
                 60%) 
               
               
                   
                 4 
                 Titanium 
                 Porous body 
                 IP 
                 25 
                 30 
                 0.5 
                 Microwave 
                 50 
                 0.5 
               
               
                   
                   
                 alloy 
                 (porosity 
                   
                   
                   
                   
                 reduction 
               
               
                   
                   
                   
                 60%) 
               
               
                   
                 5 
                 Titanium 
                 Porous body 
                 IP 
                 25 
                 30 
                 0.5 
                 Microwave 
                 50 
                 0.5 
               
               
                   
                   
                   
                 (porosity 
                   
                   
                   
                   
                 reduction 
               
               
                   
                   
                   
                 30%) 
               
               
                   
                 6 
                 Titanium 
                 Porous body 
                 IP 
                 25 
                 30 
                 0.5 
                 Microwave 
                 50 
                 0.5 
               
               
                   
                   
                   
                 (porosity 
                   
                   
                   
                   
                 reduction 
               
               
                   
                   
                   
                 90%) 
               
               
                 Comparative 
                 1 
                 Titanium 
                 Plate 
                 IP 
                 25 
                 30 
                 0.5 
                 — 
                 — 
                 — 
               
               
                 Example 
                 2 
                 Titanium 
                 Porous body 
                 Atmospheric 
                 500  
                 180 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                   
                   
                 (porosity 
                 sintering 
               
               
                   
                   
                   
                 60%) 
               
               
                   
                 3 
                 Titanium 
                 Porous body 
                 IP 
                 25 
                 30 
                 0.5 
                 Thermal 
                 1000  
                 60   
               
               
                   
                   
                   
                 (porosity 
                   
                   
                   
                   
                 reduction 
               
               
                   
                   
                   
                 60%) 
               
               
                   
                 4 
                 Titanium 
                 Porous body 
                 — 
                 — 
                 — 
                 — 
                   
                 — 
                 — 
               
               
                   
                   
                   
                 (porosity 
               
               
                   
                   
                   
                 80%) 
               
               
                   
                 5 
                 Titanium 
                 Porous body 
                 Plasma 
                 25 
                 20 
                 2   
                 Microwave 
                 50 
                 0.5 
               
               
                   
                   
                   
                 (porosity 
                 electrolytic 
                   
                   
                   
                 reduction 
               
               
                   
                   
                   
                 80%) 
                 oxidation 
               
               
                   
                 6 
                 Titanium 
                 Porous body 
                 IP 
                 25 
                 30 
                 0.5 
                   
                 — 
                 — 
               
               
                   
                   
                   
                 (porosity 
               
               
                   
                   
                   
                 80%) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Magneli phase* 
                 Film thickness of 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Present or 
                 BET value 
                 titanium oxide film 
                   
                 Corrosion 
               
               
                   
                 absent 
                 (m 2 /g) 
                 (μm) 
                 Conductivity 
                 resistance 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Invention 
                 1 
                 Present 
                 0.06 
                 3.0 
                 ◯ 
                 ◯ 
               
               
                 Example 
                 2 
                 Present 
                 0.07 
                  0.05 
                 ◯ 
                 ◯ 
               
               
                   
                 3 
                 Present 
                 0.06 
                 0.5 
                 ◯ 
                 ◯ 
               
               
                   
                 4 
                 Present 
                 0.06 
                 0.5 
                 ◯ 
                 ◯ 
               
               
                   
                 5 
                 Present 
                 0.06 
                 0.5 
                 ◯ 
                 ◯ 
               
               
                   
                 6 
                 Present 
                 0.06 
                 0.5 
                 ◯ 
                 ◯ 
               
               
                 Comparative 
                 1 
                 Absent 
                 — 
                 0.5 
                 — 
                 — 
               
               
                 Example 
                 2 
                 Absent 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 3 
                 Absent 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 4 
                 Absent 
                 — 
                 — 
                 ◯ 
                 — 
               
               
                   
                 5 
                 Present 
                 0.67 
                 2.0 
                 ◯ 
                 ◯ 
               
               
                   
                 6 
                 Absent 
                 — 
                 0.5 
                 — 
                 — 
               
               
                   
               
               
                 *Magneli phase: Magneli phase titanium oxide represented by chemical formula Ti n O 2n−1  (4 ≤ n ≤ 10) 
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Voltammetry test 
               
            
           
           
               
               
               
               
               
            
               
                   
                 First time 
                 Second time 
                 Fifth time 
                 Tenth time 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 6 
                 1.00 
                 1.02 
                 1.04 
                 0.98 
               
               
                 Comparative 
                 1.00 
                 0.93 
                 0.88 
                 0.83 
               
               
                 Example 5 
               
               
                 Comparative 
                 — 
                 — 
                 — 
                 — 
               
               
                 Example 6 
               
               
                   
               
               
                 * Measurement voltage was 2.5 V 
               
               
                 * Current density ratio with respect to 1 cycle 
               
               
                 * Comparative Example 6 had a low current density (0.1 A/cm 2  in one cycle) and a performance as an electrode was not realized. 
               
            
           
         
       
     
     In Comparative Example 1, a TiO 2  film was formed by the ion plating method, but since microwave reduction was not applied, there was no Magneli phase and the conductivity was inferior. 
     In Comparative Example 2, the substrate main body was oxidized by the atmospheric sintering method, but the TiO 2  film was not formed and deterioration occurred due to oxygen diffusion into the titanium substrate. For this reason, use as an electrode was not possible. 
     In Comparative Example 3, a TiO 2  film was formed by the ion plating method and the thermal reduction method was used for subsequent reduction, but deterioration occurred due to oxygen diffusion into the titanium substrate. 
     In Comparative Example 4, since no film was formed, there was no corrosion resistance at all. 
     In Comparative Example 5, a TiO 2  film was formed by the plasma electrolytic oxidation method and reduced by using the microwave reduction method; however, the performance of the electrode was inferior to that of Invention Example 6 in the 10th cycle in the stability evaluation as an electrode according to the PV method. The BET value of the substrate main body on which the Magneli phase titanium oxide film was formed in Comparative Example 5 was a value six times or more larger than 0.1 m 2 /g. 
     In Comparative Example 6, a TiO 2  film was formed on the porous titanium body by the ion plating method and no subsequent reduction was performed, thus, there was no Magneli phase and the conductivity was poor. It was not possible to perform the measurement in the voltammetry test due to the poor electrical conductivity. 
     In contrast, in Invention Examples 1 to 6 in which the BET value of the substrate main body on which the Magneli phase titanium oxide film was formed was 0.1 m 2 /g or less, it was confirmed that the conductivity and corrosion resistance were excellent. 
     In addition, it was also confirmed that the material has sufficient durability in a case of being used as an electrode for water electrolysis. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 : Titanium substrate 
               11 : Substrate main body 
               16 : Magneli phase titanium oxide film 
               26 : TiO 2  film 
               30 : Water electrolysis apparatus 
               32 : Anode electrode (electrode for water electrolysis)