Patent Description:
In water electrolysis hydrogen production using an anion exchange membrane, a hydrogen reaction (H<NUM>O+e-→<NUM>/<NUM><NUM>+OH-) occurs at a cathode, and an oxygen generation reaction (OH-→<NUM>/4O<NUM>+<NUM>/<NUM><NUM>O+e-) occurs at an anode in the opposite position. Currently, noble metal oxides (IrOx) are used as electrode catalysts. To improve catalytic activity, an attempt has been made to replace IrOx with IrRuOx. This has reduced the amount of Ir used by <NUM>%, and its overvoltage has been reduced to about <NUM> V (Non Patent Literature <NUM>). However, there is a need to develop non-noble metal-supported catalysts that do not use Ir, which is expensive and scarce in reserves. In Patent Literature <NUM>, a catalyst in which non-noble metal Ni(OH)<NUM> nanoparticles are supported on conductive carbon has been developed, but the addition of carbon is necessary to provide conductivity and to form gas diffusion pathways. The accelerated degradation of carbon at operating potential (<NUM> to <NUM> V) will be a major issue in its practical use. The conductivity issue has been overcome by using NiFe metal nanoparticles and the like, and the overvoltage has been successfully reduced to less than that of IrOx. However, the electrode tends to become dense, and carbon must be added to provide diffusion pathways (void) for the oxygen generated (Non Patent Literature <NUM>). In the NiCoO and NiFeO systems, the overvoltage at the anode is successfully reduced compared to IrOx (Non Patent Literatures <NUM> and <NUM>). However, in the NiCoO system, the formation of gas diffusion pathways and the improvement of the specific surface area increase the resistance to <NUM>Ω at the operating potential (<NUM> V), causing conductivity issues (Non Patent Literature <NUM>). The NiFeO system has the issue of degradation due to the oxidation of Fe as the potential increases (Non Patent Literature <NUM>).

For example, Patent Literature <NUM> discloses a catalyst which can be used for the acceleration of an electrochemical reaction, and is characterized by being a connected body of alloy particles each comprising nickel (Ni) and at least one element selected from iron (Fe), cobalt (Co), chromium (Cr), tungsten (W) and copper (Cu). Patent Literature <NUM> discloses an electrode catalyst layer for a solid polymer fuel cell or a solid polymer water electrolysis cell which is formed using a paste for forming an electrode catalyst layer, the paste comprising a catalyst-carrying carbon powder on which a catalyst is carried, water, a hydrophilic organic solvent, and a solid polymer electrolyte. Patent Literature <NUM> relates to a composition useful in electrodes which provides higher power capability through the use of nanoparticle catalysts present in the composition. Nanoparticles of transition metals are preferred such as manganese, nickel, cobalt, iron, palladium, ruthenium, gold, silver, and lead, as well as alloys thereof, and respective oxides. Patent Literature <NUM> describes a gas decomposing element including a porous anode into which a hydrogen-containing gas such as ammonia is introduced; a porous cathode into which an oxidizing gas is introduced; and an ionic conductive material which has ionic conductivity and is interposed between the anode and the cathode. The cathode is a sintered compact obtained by sintering a compact of a metallic granule and ionic conductive ceramics. The metallic granule is based on Ni and/or Fe. Patent Literature <NUM> discloses a conductive material comprising a core made of a metal mixture of a base metal and at least one metal additive. The base metal and the at least one metal additive are selected from a group A consisting of platinum, rhodium, gold, palladium, silver, copper, iridium, ruthenium, osmium and rhenium. Patent Literature <NUM> relates to an electrochemical electrode having a conductive substrate and an active layer formed on the conductive substrate, wherein the active layer has a nickel-containing nanostructured material having a dendritic structure formed by agglomerating a plurality of primary particles, and each primary particle has a core and a shell surrounding the core wherein the core is formed of a nickel nanocrystal and the shell is formed of a nickel oxide film.

The present invention has been made in consideration of the aforementioned circumstances and provides an electrode catalyst that is excellent in durability, material conductivity, and electrical conductivity and can be produced at a low cost.

According to the present invention, there is provided an electrode catalyst in the form of a void-containing material, wherein: the void-containing material is a powder, which is an aggregate of fine particles, the void being a gap between the fine particles and a gap inside the fine particles; each of the fine particles comprises a core part and a skin layer) covering the core part; the core part is structured with metal; the skin layer is structured with an oxide containing Ni; and the fine particles themselves act as a catalyst, the fine particles are structured with a plurality of primary particles being fusion bonded to form a chain.

The electrode catalyst of the present invention has excellent durability because it does not require the addition of carbon and is structured with a metal in the core part and oxide in the skin layer. The electrode catalyst also has excellent material conductivity because it is structured with the void-containing body with a void ratio of at least <NUM>%. Furthermore, this electrode catalyst has excellent electrical conductivity because the core is metal. In addition, this electrode catalyst can be produced at a low cost because noble metals are not essential components.

Hereinafter, the embodiments of the present invention will be described with reference to the drawings. Various features described in the embodiments shown below can be combined with each other.

<FIG> shows the configuration of an anion exchange membrane electrochemical cell <NUM> according to an embodiment of the present invention. The electrochemical cell <NUM> comprises a cathode <NUM>, an anode <NUM>, and an anion exchange membrane <NUM> placed between them.

As shown in <FIG>, when a voltage is applied between the cathode <NUM> and the anode 30d and water is supplied to the cathode <NUM> in the electrochemical cell <NUM>, cathodic and anodic reactions described below occur, producing hydrogen from the cathode <NUM> and water and oxygen from the anode <NUM>. Electrons move from the anode <NUM> to the cathode <NUM> through a wiring, and OH- moves from the cathode <NUM> to the anode <NUM> through the anion exchange membrane <NUM>. In this case, the electrochemical cell <NUM> is an anion exchange membrane water electrolysis cell and is in water electrolysis operation.

Cathodic reaction: H<NUM>O+e-→<NUM>/<NUM><NUM>+OH-.

Anodic reaction: OH-→<NUM>/<NUM><NUM>O+<NUM>/4O<NUM>+e-.

As shown in <FIG>, when a load R is connected between the cathode <NUM> and the anode <NUM> and water and oxygen are supplied to the cathode <NUM> and hydrogen to the anode <NUM> in the electrochemical cell <NUM>, an electromotive force is generated by the cathodic and anodic reactions described below, and water is produced. The generated electromotive force causes electrons to move from the anode <NUM> to the cathode <NUM> through the load R, and OH- to move from the cathode <NUM> to the anode <NUM> through the anion exchange membrane <NUM>. In this case, the electrochemical cell <NUM> is an anion exchange membrane fuel cell and is in electricity generating operation.

Cathodic reaction: <NUM>/<NUM><NUM>O+<NUM>/4O<NUM>+e-→OH-.

Anodic reaction: <NUM>/<NUM><NUM>+OH-→H<NUM>O+e-.

The cathodic reaction in the electricity generating operation is the reverse reaction of the anodic reaction in the water electrolysis operation. The anode reaction in the electricity-generating operation is the reverse reaction of the cathodic reaction in the water electrolysis operation.

Thus, the electrochemical cell <NUM> can be operated as a water electrolysis cell or as a fuel cell. Thus, the electrochemical cell <NUM> can be efficiently operated by, for example, operating the electrochemical cell <NUM> as a water electrolysis cell using surplus electricity generated by solar photovoltaic generation to generate and store hydrogen and oxygen and by operating the electrochemical cell <NUM> as a fuel cell using the stored hydrogen and oxygen when electricity is needed and generating the electromotive force.

The cathode <NUM> comprises a diffusion layer <NUM>, a microporous layer <NUM>, and a catalyst layer <NUM>. The anode <NUM> comprises a diffusion layer <NUM>, a microporous layer <NUM>, and a catalyst layer <NUM>. The diffusion layers <NUM> and <NUM> are structured with a porous material and have a function to diffuse fluid (liquid or gas) supplied to the catalyst layers <NUM> and <NUM>. The microporous layers <NUM> and <NUM> have functions to further diffuse the fluid supplied to the catalyst layers <NUM> and <NUM> and to efficiently remove the liquid generated in the catalyst layers <NUM> and <NUM>. The catalyst layers <NUM> and <NUM> have a function to promote an electrochemical reaction (the cathodic or anodic reactions) by virtue of a catalyst.

One or both of the catalyst layers <NUM> and <NUM> are structured with an electrode catalyst <NUM> of the present invention described below. When one of the catalyst layers <NUM> and <NUM> is structured with the electrode catalyst <NUM>, the other of the catalyst layers <NUM> and <NUM> may be structured with any catalyst capable of promoting a desired electrochemical reaction, and for example, a catalyst, such as IrOx, mentioned in the prior art can be used. When both catalyst layers <NUM> and <NUM> are structured with the electrode catalyst <NUM>, the composition and structure of the electrode catalyst <NUM> may be the same or different from each other.

The electrode catalyst <NUM> is structured with a void-containing body having a void. Examples of the void-containing material include a porous material <NUM>, as shown in <FIG>, and powder <NUM>, as shown in <FIG>. In the present invention, the void-containing body is formed by the catalyst itself, so it is not necessary to support the catalyst on a support as in the prior art.

The porous material <NUM> is structured by forming many pores <NUM> on a substrate <NUM>, and the pores <NUM> are the void. The pores <NUM> may be regularly or irregularly arranged. The pores <NUM> may be regular (e.g., linear) or irregular in shape. The pores <NUM> may or may not penetrate the substrate <NUM>. According to the present invention, the powder <NUM> is an aggregate of fine particles <NUM> and the gap <NUM> between the fine particles <NUM> and the gap inside the fine particles are the void. The fine particle <NUM> may be spherical or other shapes. When the void-containing body is the powder <NUM>, the void-containing body is structured with the fine particles <NUM> themselves, which act as the catalyst.

A void ratio of the void-containing body is preferably <NUM>% or more, and preferably <NUM>% or more. The void ratio is, for example, <NUM> to <NUM>%, particularly, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>%, and may be in the range between the two values exemplified herein. The void ratio can be calculated by bulk density/true density. The void ratio of the powder can be measured in a molded state in a uniaxial pressure molding machine (Molded body size: <NUM> x <NUM> x <NUM>, molding pressure: <NUM> MPa or less).

The void-containing body comprises a core part <NUM> and a skin layer <NUM> covering the core part <NUM>. When the void-containing body is the porous material <NUM>, the porous material <NUM> comprises the core part <NUM> and the skin layer <NUM>. According to the present invention, the void-containing body is the powder <NUM> and each of the fine particles <NUM> comprises the core part <NUM> and the skin layer <NUM>.

The core part <NUM> is formed of metal, and the skin layer <NUM> is structured with an oxide containing Ni. Since the skin layer <NUM> contains NiO bonds, it generates NiOOH (active point) in an alkaline aqueous solution, which promotes the electrochemical reaction. The core part <NUM>, on the other hand, is formed of metal and is highly electrically conductive. The metal of the core part <NUM> may or may not contain Ni. When the metal of the core part <NUM> contains Ni, the skin layer <NUM> structured with the oxide containing Ni can be formed by reducing the entire void-containing body and then oxidizing only its surface, which facilitates production. The skin layer <NUM> may be formed by coating the oxide containing Ni to cover the core part <NUM>. In this case, the core part <NUM> may not contain Ni.

A thickness of the skin layer <NUM> is, for example, <NUM> to <NUM>, and preferably <NUM> to <NUM>. The thickness can be particularly <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>, and may range between the two values exemplified herein.

The electrical conductivity of the electrode catalyst <NUM> is preferably <NUM>/cm or more, more preferably <NUM>/cm or more, and even more preferably <NUM>/cm or more. The electrical conductivity is <NUM> to <NUM>/cm, and particularly, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>/cm, and may be in the range between the two values exemplified herein.

The metal of the core part <NUM> and the skin layer <NUM> preferably contain a transition metal with a smaller atomic number than Ni, as a transition metal other than Ni. Examples of such transition metal include Co, Fe, Mn, Cr, V, Ti, Sc, and the like, and Co or Fe is preferable. When such transition metal is contained, the Fermi level is lowered, and the electrochemical reaction is promoted. A ratio of the transition metal to the total of Ni and the transition metal is preferably <NUM> to <NUM> atomic%, and more preferably <NUM> to <NUM> atomic%. The ratio is, particularly, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> atomic%, and may range between the two values exemplified herein.

In an oxygen evolution reaction ("OER") in water electrolysis operation, the ratio of the transition metal to the total of Ni and the transition metal is preferably <NUM> to <NUM> atomic%, and especially preferably <NUM> to <NUM> atomic%. For a hydrogen evolution reaction ("HER") in water electrolysis operation, the ratio of the transition metal to the total of Ni and the transition metal is preferably <NUM> to <NUM> atomic%, and particularly preferably <NUM> to <NUM> atomic%. Co is especially preferable as the transition metal.

As shown in <FIG>, the fine particles <NUM> are a structure having a chain part <NUM> structured with a plurality of primary particles <NUM> being fusion bonded in a chain (hereinafter, "fused-aggregate network structure"). In this case, a region surrounded by the chain parts <NUM> is a void <NUM>. It is preferable that the fused-aggregate network structure has branch structures in which the chain part <NUM> is branched at a branching point <NUM>. In this case, the void <NUM> is easy to form. In addition, since the fused-aggregate network structure is formed by the catalyst itself, it is not necessary to support the catalyst on a support as in the prior art.

As shown in <FIG>, each of the plurality of primary particles <NUM> structuring the chain part <NUM> comprises the core part <NUM> and the skin layer <NUM>, and the core parts <NUM> of the adjacent primary particles <NUM> are preferably connected to each other. In this case, electrical conductivity is particularly improved.

An average size of the primary particles <NUM> is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, and even more preferably <NUM> to <NUM>. The average size is particularly, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>, and may range between the two values exemplified herein. The average size of the primary particles <NUM> can be determined by an arithmetic mean of the primary particle diameters of <NUM> or more of the primary particles <NUM>. The primary particle diameter can be measured using the TEM image as shown in <FIG>. In the TEM image shown in <FIG>, dark-colored areas indicate a part where multiple primary particles overlap. For the measurement of the primary particle diameter, a particle that is relatively light in color and whose outer circumference can be identified is focused on, and a value at which a distance between two points on the outer circumference of the particle is the longest (a length of the arrow in <FIG>) is determined as the primary particle diameter.

An average particle diameter of the fine particles <NUM> is <NUM> to <NUM>, preferably <NUM> to <NUM>. The average particle diameter of the fine particles <NUM> can be measured by a laser diffraction/scattering particle diameter distribution measuring device.

A specific surface area of the powder <NUM> is preferably <NUM><NUM>/g or more. The specific surface area is, for example, <NUM> to <NUM><NUM>/g, particularly, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM><NUM>/g, and may be in the range between the two values exemplified herein.

A repose angle of the powder <NUM> is preferably <NUM> degrees or less, and more preferably <NUM> degrees or less. In this case, the powder has the same degree of fluidity as flour and is easy to handle. The repose angle is, for example, <NUM> to <NUM> degrees, particularly, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> degrees, and may be in the range between the two values exemplified herein. The repose angle can be determined by a drop volume method.

When the electrode catalyst <NUM> is structured with the powder <NUM>, which is the aggregate of the fine particles <NUM> having the fused-aggregate network structure, the electrode catalyst <NUM> can be produced by a method that comprises a powder forming step and a reducing and surface oxidizing process. Each process is described in detail below.

First, <FIG> will be used to describe a producing apparatus <NUM> which can be used to produce the powder. The producing apparatus <NUM> comprises a burner <NUM>, a raw material supplying unit <NUM>, a reaction tube <NUM>, a collector <NUM>, and a gas reservoir <NUM>. The raw material supplying unit <NUM> comprises an outer tube <NUM> and a raw material distribution tube <NUM>.

The burner <NUM> is tubular in shape, and the raw material supplying unit <NUM> is arranged in the burner <NUM>. Burner gas 2a is distributed between the burner <NUM> and the outer tube <NUM>. The burner gas 2a is used to form a flame <NUM> at the tip of the burner <NUM> by ignition. The flame <NUM> creates a high temperature region of <NUM> or more. The burner gas 2a preferably contains a combustible gas such as propane, methane, acetylene, hydrogen, or nitrous oxide. In one example, a gas mixture of oxygen and propane gas can be used as the burner gas 2a. The temperature in the high temperature region is, for example, <NUM> to <NUM>, and is particularly, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>, and may be in the range between the two values exemplified herein.

A raw material solution 14a for producing the powder is distributed in the raw material distribution tube <NUM>. A solution containing a Ni compound and, if necessary, a compound of transition metals (e.g., Co, Fe) is used as the raw material solution 14a. Examples of the compounds include fatty acid metal salts (e.g., fatty acid Ni, fatty acid Co, fatty acid Fe). The carbon number of the fatty acid is, for example, <NUM> to <NUM>, preferably <NUM> to <NUM>, and even more preferably <NUM> to <NUM>. Octylic acid is preferrable as the fatty acid.

In the raw material solution 14a, the fatty acid metal salt is preferably dissolved or dispersed in a non-aqueous solvent. Examples of the non-aqueous solvent include organic solvents represented by turpentine. If moisture is contained in the raw material solution 14a, the fatty acid metal salts may undergo hydrolysis and degrade.

Mist gas 13a, which is used to mist the raw material solution 14a, is distributed between the outer tube <NUM> and the raw material distribution tube <NUM>. When the mist gas 13a and the raw material solution 14a are jetted together from the tip of the raw material supplying unit <NUM>, the raw material solution 14a is misted. The mist 14b of the raw material solution 14a is sprayed into the flame <NUM>, and the fatty acid metal salt in the raw material solution 14a undergoes a thermal decompose reaction in the flame <NUM> to produce an oxide powder, which is an aggregate of oxide particles <NUM> having the chain parts structured by fusion bonding crystallites <NUM> of an oxide into a chain. The crystallites <NUM>, the oxide particles <NUM>, and the oxide powder undergo the reducing and surface oxidizing process to yield the primary particles <NUM>, the fine particles <NUM>, and the powder <NUM>, respectively. The mist gas 13a is, in one example, oxygen.

The reaction tube <NUM> is provided between the collector <NUM> and the gas reservoir <NUM>. The flame <NUM> is formed in the reaction tube <NUM>. The collector <NUM> is provided with a filter 5a and a gas discharging portion 5b. A negative pressure is applied to the gas discharging portion 5b. This generates a flow that flows towards the gas discharging portion 5b in the collector <NUM> and the reaction tube <NUM>.

The gas reservoir <NUM> is tubular in shape and comprises a cold gas introducing portion 6a and a slit 6b. A cold gas <NUM> is introduced into the gas reservoir <NUM> from the cold gas introducing portion 6a. Since the cold gas introducing portion 6a is oriented along the tangent line of the inner peripheral wall 6c of the gas reservoir <NUM>, the cold gas <NUM> introduced into the gas reservoir <NUM> through the cold gas introducing portion 6a swirls along the inner peripheral wall 6c. A burner insertion hole 6d is provided in the center of the gas reservoir <NUM>. The burner <NUM> is inserted through the burner insertion hole 6d. The slit 6b is provided at a position adjacent to the burner insertion hole 6d so as to surround the burner insertion hole 6d. Therefore, when the burner <NUM> inserted through the burner insertion hole 6d, the slit 6b is disposed to surround the burner <NUM>. The cold gas <NUM> in the gas reservoir <NUM> is driven by the negative pressure applied to the gas discharging portion 5b and is discharged through the slit 6b toward the reaction tube <NUM>. The cold gas <NUM> can be any gas capable of cooling the generated metal oxide, preferably an inert gas, e.g., air.

After the oxide particles <NUM> exit the flame <NUM>, the oxide particles <NUM> are immediately cooled by the cold gas <NUM>, thus allowing to maintain the structure having the chain part. The cooled oxide particles <NUM> are captured and collected by the filter 5a. The captured oxide particles <NUM> may be subjected to heat treatment at <NUM> to <NUM> to adjust to the desired primary particle diameter.

The oxide particles <NUM>, which constitute the oxide powder obtained in the above step, are entirely oxide and have poor electrical conductivity. Therefore, in this step, after reducing the oxide particles <NUM> so that their entirety becomes metal, only the surface is oxidized to form the metal core part <NUM> and the oxide skin layer <NUM>. Consequently, the electrode catalyst <NUM> structured with the powder <NUM>, which is the aggregate of the fine particles <NUM> with the fused-aggregate network structure can be obtained.

The reduction of the oxide particles <NUM> can be performed by heat treatment of the oxide particles <NUM> under a hydrogen-containing atmosphere. The hydrogen-containing atmosphere is an atmosphere containing hydrogen, preferably one in which the hydrogen is diluted with an inert gas (e.g., nitrogen). A hydrogen content in the atmosphere is, for example, <NUM> to <NUM> %. A heat treatment temperature is preferably <NUM> to <NUM>. This process reduces the oxide particles <NUM> to obtain the metal fine particles. Then, after the reduction, the surface of the metal fine particles can be oxidized by holding them in nitrogen that contains trace amounts of oxygen during slow cooling to room temperature. A concentration of oxygen is preferably <NUM> to <NUM> ppm.

The electrode catalysts were produced by the following method, and various evaluations were conducted.

By using the producing apparatus <NUM> shown in <FIG>, the electrode catalyst <NUM> was produced. As the burner gas 2a, gas prepared by blending <NUM>/min of oxygen and <NUM>/min of propane gas was used. This gas was ignited to form the flame (chemical flame) <NUM> of <NUM> or more at the tip of the burner <NUM>. The raw material solution 14a was prepared by blending Ni octylate and Co octylate by a predetermined ratio, and then the blend was further combined with mineral spirit turpentine and dissolved. Ni octylate and Co octylate were blended so that the atomic ratio x of Co to the total of Ni and Co was <NUM>, <NUM>, <NUM>, or <NUM> (Example <NUM>, Example <NUM>, Example <NUM>, and Example <NUM>, respectively). Oxygen was used as the mist gas 13a. <NUM>/min of the mist gas 13a and <NUM>/min of the raw material solution 14a were blended and sprayed from the tip of the raw material supplying unit <NUM>, which is a spray nozzle (atomizer), towards the central part of the flame, thereby allowing combustion of the blend and generation of the oxide powder which is the aggregate of the particles <NUM>. During such, negative pressure was applied to the gas discharging portion 5b, the air was sucked through the slit 6b at a flow rate of <NUM>/min, and the generated powder was collected in the collector <NUM> (with the filter 5a). The raw material supplying unit <NUM> has a double-tube structure (overall length of <NUM>). Oxygen gas is supplied from the outer tube <NUM>, and the raw material solution 14a is supplied to the raw material distribution tube <NUM>. A fluid nozzle and an air nozzle were provided at the tip of the raw material distribution tube <NUM>, and the raw material solution 14a was converted into mist 14b at the nozzles.

The general formula of the oxide powder obtained is Ni<NUM>-xCoxO. The TEM image of the oxide particles <NUM> contained in the powder obtained for x = <NUM> is shown in <FIG>. It can be seen that the oxide particles <NUM> have the chain parts structured with the crystallites <NUM> being fusion bonded to a chain.

Then, the reduction and surface oxidation were performed on the oxide powder obtained in the above process to form the metal core part <NUM> and the oxide skin layer <NUM>. inconsequently, the electrode catalyst <NUM> structured with the powder <NUM>, which is the aggregate of the fine particles <NUM> with the fused-aggregate network structure was obtained.

The oxide particles <NUM> was reduced by heat treatment at <NUM> for <NUM> hours in the hydrogen-containing atmosphere ( atmosphere of the gas mixture containing hydrogen and nitrogen, the hydrogen content of <NUM>%. The surface oxidation was performed by holding them in nitrogen that contains trace amounts of oxygen during slow cooling to room temperature after reduction. The concentration of oxygen was set at <NUM> ppm.

The electrode catalysts <NUM> of Examples <NUM> to <NUM> were produced under the same condition as in Example <NUM>, except that Mn octylate (Example <NUM>) or Fe octylate (Example <NUM>) was used instead of Co octylate.

The electrical conductivity of the electrode catalysts <NUM> of Examples <NUM> to <NUM> obtained by the above method was measured by the following method. The results are shown in <FIG>. As shown in <FIG>, the electrical conductivity was found to be particularly high, reaching <NUM>/cm or more, when the Co content x is near <NUM>.

<NUM> samples of the electrode catalyst (hereinafter, "subject sample") were weighed precisely using a precision electronic balance and were each filled into <NUM> sample folders (<NUM> in diameter, <NUM> in depth) in a measurement jig. The measurement jig filled with the subject samples was set in a pressing device, and the subject samples were compressed with a force of <NUM> kN. By using an electrode set in the compressor of the pressing device, the resistance of the subject samples was measured by the DC two-terminal method during powder compression, and the length of the subject samples during powder compression was also measured at the same time. These procedures were performed with <NUM> or more kinds of subject samples with different weight, and the relationship between the length (x-axis) and resistance (y-axis) of the subject samples during powder compression was determined and extrapolated in the y-axis direction to obtain the y-intercept value. From the y-intercept value and the length and cross-sectional area of the compressed powder body, the resistivity of the subject samples was determined, and the electrical conductivity, which is the inverse of the resistivity, was calculated.

Using an electrochemical measurement apparatus <NUM> of a three-electrode system shown in <FIG>, the OER activity of the electrode catalysts <NUM> in Examples <NUM> to <NUM> was measured. The apparatus <NUM> comprises a glass cell 15a, a working electrode 15b, a counter electrode 15c, and a reference electrode 15d. The potential of the working electrode 15b relative to the reference electrode 15d can be adjusted by an unshown potentiostat. A KOH solution 15e with a concentration of <NUM> mol/L is contained in the glass cell 15a. Nitrogen or oxygen can be blown into the KOH solution 15e. The working electrode 15b is made of glassy carbon (GC) and is columnar in shape, and the electrode catalyst <NUM> is applied to its lower surface. The lower surface of the working electrode 15b and the counter electrode 15c are immersed in the KOH solution 15e. The reference electrode 15d is in a liquid junction with the KOH solution 15e through a salt bridge 15f.

The electrode catalyst <NUM> in a state of being dispersed in a mixture of <NUM> wt% water and <NUM> wt% ethanol was applied to the lower surface of the working electrode 15b and then dried. Nitrogen was blown into the KOH solution 15e before the measurement to purge the KOH solution 15e. During the measurement, oxygen was blown in at a flow rate of <NUM>/min, and the reference electrode 15d was rotated around its central axis. Under these conditions, the current values were measured while changing the potential of the working electrode 15b relative to the reference electrode 15d (Potential /V vs RHE). The results are shown in <FIG> (Examples <NUM> to <NUM>) and <FIG> (Examples <NUM>, <NUM>, <NUM>, and <NUM>). <FIG> also show the results when the catalyst is the Pt/C (manufactured by Tanaka Kikinzoku Kogyo K. , TEC10E50E), and is IrOx (manufactured by Tanaka Kikinzoku Kogyo K.

As shown in <FIG>, the electrode catalyst <NUM> was found to have a catalytic activity comparable to that of IrOx. The performance was also improved by adding Co and was particularly high when the Co content x was near <NUM>.

Using the same apparatus as for the OER activity measurement described above, the HER activity of the electrode catalyst <NUM> was measured. The potentiostat was set so that the potential of the working electrode 15b is negative. The results are shown in <FIG> (Examples <NUM> to <NUM>) and <FIG> (Examples <NUM>, <NUM>, <NUM>, and <NUM>).

As shown in <FIG>, the electrode catalyst <NUM> was found to have excellent catalytic activity at the level intermediate between IrOx and Pt/C. The addition of Fe or Co significantly improved the performance, and the addition of Fe improved the performance particularly significantly. When Co was added, the performance was particularly high when the Co content x was near <NUM>.

In the water electrolysis cell shown in <FIG>, Pt/C (manufactured by Tanaka Kikinzoku Kogyo K. , TEC10E50E) was used as the cathode catalyst, and the electrode catalyst <NUM> (Ni<NUM>Co<NUM>O) of Example <NUM> or a commercial IrOx catalyst (manufactured by Tanaka Kikinzoku Kogyo K. ) was used as the anode catalyst. The relationship between the voltage applied between the anode and the cathode and the current flowing during the water electrolysis reaction was measured at <NUM>, and the results are shown in <FIG>. As shown in <FIG>, the water electrolysis cell using the electrode catalyst <NUM> in Example <NUM> had a larger water electrolysis reaction rate and higher catalytic performance than the water electrolysis cell using the IrOx catalyst.

A long-term evaluation of the water electrolysis cell produced in "<NUM>. Voltage-current density characteristic evaluation of water electrolysis cell" was performed. The anode catalyst was the electrode catalyst <NUM> (Ni<NUM>Co<NUM>O) of Example <NUM>.

Claim 1:
An electrode catalyst (<NUM>) in the form of a void-containing material, wherein:
the void-containing material is a powder (<NUM>), which is an aggregate of fine particles (<NUM>), the void being a gap (<NUM>) between the fine particles (<NUM>) and a gap (<NUM>) inside the fine particles (<NUM>);
each of the fine particles (<NUM>) comprises a core part (<NUM>) and a skin layer (<NUM>) covering the core part (<NUM>);
the core part (<NUM>) is structured with metal;
the skin layer (<NUM>) is structured with an oxide containing Ni; and
the fine particles (<NUM>) themselves act as a catalyst,
the fine particles (<NUM>) are structured with a plurality of primary particles (<NUM>) being fusion bonded to form a chain (<NUM>).