JP2007031781A - Oxygen reduction electrode - Google Patents
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Abstract
Description
本発明は、例えば水や有機物の電気分解、燃料電池等の電極として用いるのに好適な酸素還元電極に関する。 The present invention relates to an oxygen reduction electrode suitable for use as an electrode for, for example, electrolysis of water or organic matter, fuel cells, and the like.
白金等の貴金属は、高い電位においても安定で、かつ触媒能が高いため、各種の電気化学システムの電極用触媒に用いられている。しかし、白金の価格が高いことや資源量が限られていることから、白金を代替できる高活性の触媒が要望されている。
このようなことから、遷移金属であるモリブデンの窒化物や（例えば、特許文献１参照）、遷移金属である鉄の窒化物と貴金属の混合物（例えば、特許文献２参照）が、電極用触媒として提唱されている。
Since noble metals such as platinum are stable even at high potentials and have high catalytic ability, they are used as catalysts for electrodes in various electrochemical systems. However, since the price of platinum is high and the amount of resources is limited, a highly active catalyst that can replace platinum is desired.
Therefore, molybdenum nitride as a transition metal (see, for example, Patent Document 1), and a mixture of iron nitride as a transition metal and a noble metal (see, for example, Patent Document 2) are used as electrode catalysts. Has been advocated.
しかしながら、上記した技術の場合、酸性電解質中での酸素還元能が不充分であり、又、触媒が活性溶解する場合があった。
従って、本発明の目的は、酸性電解質中での酸素還元能及び安定性に優れた酸素還元電極を提供することにある。
However, in the case of the above-described technique, the oxygen reducing ability in the acidic electrolyte is insufficient, and the catalyst may be actively dissolved.
Accordingly, an object of the present invention is to provide an oxygen reduction electrode excellent in oxygen reduction ability and stability in an acidic electrolyte.
本発明の酸素還元電極は、長周期表の４族、５族、及び１４族の元素の群から選ばれる１種以上の元素の窒化物を含むことを特徴とする。
30℃の0.1mol/L硫酸水溶液中で、走査速度5mV/sで電位走査したとき、可逆水素電極電位基準で0.4Vにおいて測定される酸素雰囲気での電流値をIO2とし、窒素雰囲気での電流値をIN2としたとき、R=(IO2−IN2)/IN2
で表される値が0.5以上となることが好ましい。
30℃の0.1mol/L硫酸水溶液中で、走査速度5mV/sで電位走査したとき、酸素還元電流が流れ始める時の電位が可逆水素電極電位基準で0.5V以上となることが好ましい。
前記酸素還元電極が前記窒化物の微粒子を担持して成ることが好ましい。
The oxygen reduction electrode of the present invention is characterized by containing a nitride of one or more elements selected from the group of elements of Group 4, Group 5 and Group 14 of the long periodic table.
When a potential scan was performed at a scanning speed of 5 mV / s in a 0.1 mol / L sulfuric acid aqueous solution at 30 ° C., the current value in an oxygen atmosphere measured at 0.4 V on the basis of the reversible hydrogen electrode potential was defined as I O2, and in a nitrogen atmosphere R = (I O2 −I N2 ) / I N2 where I N2 is the current value
Is preferably 0.5 or more.
When a potential scan is performed at a scanning speed of 5 mV / s in a 0.1 mol / L sulfuric acid aqueous solution at 30 ° C., the potential when the oxygen reduction current starts to flow is preferably 0.5 V or more on the basis of the reversible hydrogen electrode potential.
The oxygen reduction electrode preferably carries the nitride fine particles.
本発明によれば、酸性電解質中での酸素還元能及び安定性に優れた酸素還元電極を得ることができる。 According to the present invention, an oxygen reduction electrode excellent in oxygen reduction ability and stability in an acidic electrolyte can be obtained.
以下、本発明の実施形態について説明する。なお、以下の説明及び図表に用いる電位は可逆水素電極電位基準とし、これをRHEと表示する。
<電極に用いる窒化物>
本発明の酸素還元電極は、長周期表の４族、５族、及び１４族の元素の群から選ばれる１種以上の元素の窒化物を含む。上記元素としては、Ti,Zr,Hf,V,Nb,Ta,C,Si,Ge,Sn,Pbが具体的に挙げられる。好ましい上記元素としては、Ta,Zr,Ti,V,Nb,C,Si,Snの群から選ばれる１種以上である。
窒化物は、上記元素Ｍに対し、ＭＮｘで表される。ここで、Ｍは２種以上の上記元素であってもよく、１種の元素であってもよい。又、ｘとしては、その窒化物の最高窒化状態より低い値であることが好ましい。より好ましくは、４族及び１４族元素に対しては0.5≦ｘ<4/3であり、５族元素に対しては、0.5≦ｘ<5/3である。ｘが0.5未満であると窒化度が充分でなく、酸性電解質中で電極が溶解する場合があり、又、Ｘが上記上限値以上であると、窒化度が高くなり過ぎて導電性が低下し、又、上記元素の電子状態が変化することにより、触媒能も低下する可能性もある。
例えば、４族及び５族元素の窒化物の最高窒化状態のｘはそれぞれ4/3及び5/3である。
なお、本発明において、窒化物は窒素以外の他の元素を含まない。例えば、Ta系窒化物の場合、酸素を含む化合物（TaON）は本発明に含まない。
本発明においては、上記窒化物を１種以上含んで電極が構成される。例えば、TiN系窒化物のみを電極物質に用いてもよく、TiN系窒化物及びVN系窒化物を電極物質に用いてもよい。
Hereinafter, embodiments of the present invention will be described. Note that the potential used in the following description and chart is based on the reversible hydrogen electrode potential reference, and this is expressed as RHE.
<Nitride used for electrode>
The oxygen reduction electrode of the present invention contains a nitride of one or more elements selected from the group of elements of Groups 4, 5, and 14 of the long periodic table. Specific examples of the element include Ti, Zr, Hf, V, Nb, Ta, C, Si, Ge, Sn, and Pb. The preferable element is at least one selected from the group consisting of Ta, Zr, Ti, V, Nb, C, Si, and Sn.
The nitride is represented by MN x with respect to the element M. Here, M may be two or more of the above elements, or one element. Further, x is preferably a value lower than the highest nitriding state of the nitride. More preferably, 0.5 ≦ x <4/3 for Group 4 and 14 elements and 0.5 ≦ x <5/3 for Group 5 elements. If x is less than 0.5, the degree of nitridation may not be sufficient, and the electrode may be dissolved in an acidic electrolyte. If X is greater than the above upper limit, the degree of nitridation will be too high and the conductivity will be reduced. In addition, the catalytic ability may be reduced by changing the electronic state of the element.
For example, x in the highest nitriding state of nitrides of Group 4 and Group 5 elements is 4/3 and 5/3, respectively.
In the present invention, the nitride does not contain any element other than nitrogen. For example, in the case of Ta-based nitride, a compound containing oxygen (TaON) is not included in the present invention.
In the present invention, an electrode is formed containing one or more of the above nitrides. For example, only TiN-based nitride may be used as the electrode material, and TiN-based nitride and VN-based nitride may be used as the electrode material.
窒化物としては、結晶化したものが好ましい。この理由としては、結晶化することにより、導電性が上昇するとともに、元素の電子状態が変化し触媒活性が向上するためと考えられる。 The nitride is preferably crystallized. The reason for this is thought to be that crystallization increases the conductivity and changes the electronic state of the element to improve the catalytic activity.
窒化物は、例えば次のようにして製造することができる。まず、上記元素が遷移金属である場合、遷移金属酸化物の粉末をアンモニア気流で窒化することで窒化物が得られる。上記元素が１４族の場合も同様にして製造することができるが、通常、より高温での処理が必要となる。 The nitride can be produced, for example, as follows. First, when the element is a transition metal, a nitride is obtained by nitriding a powder of a transition metal oxide with an ammonia stream. When the element is a group 14, it can be produced in the same manner, but usually a treatment at a higher temperature is required.
上記した窒化物を酸素還元電極に用いると、酸性電解質中で使用しても窒化物が溶解せず安定である。又、本発明者らの研究により、酸性電解質中で安定な電極は触媒活性（酸素還元触媒能）を持つことが判明しているので、上記した窒化物を用いることの優位性が明らかである。
このことを、酸性電解質中での電極のサイクリックボルタモグラム（ＣＶ）を示す図１〜図３を用いて説明する。図１はTaNを用いた本発明に係る電極のＣＶを示し、図２はCoNを用いた電極のＣＶを示し、図３はWNを用いた電極のＣＶを示す。
When the above nitride is used for an oxygen reduction electrode, the nitride is not dissolved and is stable even when used in an acidic electrolyte. In addition, the inventors' research has revealed that a stable electrode in an acidic electrolyte has catalytic activity (oxygen reduction catalytic ability), and thus the superiority of using the above-described nitride is clear. .
This will be described with reference to FIGS. 1 to 3 showing cyclic voltammograms (CV) of electrodes in an acidic electrolyte. FIG. 1 shows the CV of the electrode according to the present invention using TaN, FIG. 2 shows the CV of the electrode using CoN, and FIG. 3 shows the CV of the electrode using WN.
図１より、本発明に係る電極の場合、電位走査を数１０回繰り返しても、ＣＶ曲線の形状がほとんど変化せず、酸性電解質中で窒化物が安定であることがわかる。
一方、図２より、WNを用いた場合、0.7V以上の電位で酸化電流（アノード電流）が増大する。これは、不安定な窒化物が酸化物になったか、又は、酸性電解質（硫酸）中でＷが陽イオンになって溶解していることを示す。窒化物が酸化物になると、時間と共に溶解していく。
又、図３より、CoNを用いた場合、電位走査を繰り返すと共にＣＶ曲線の形状が大きく変化し、電極物質が溶解していることがわかる。
From FIG. 1, it can be seen that in the case of the electrode according to the present invention, even when the potential scan is repeated several tens of times, the shape of the CV curve hardly changes and the nitride is stable in the acidic electrolyte.
On the other hand, as shown in FIG. 2, when WN is used, the oxidation current (anode current) increases at a potential of 0.7 V or more. This indicates that the unstable nitride has become an oxide or that W has become a cation and dissolved in the acidic electrolyte (sulfuric acid). When nitride becomes oxide, it dissolves with time.
FIG. 3 also shows that when CoN is used, the potential scan is repeated and the shape of the CV curve changes greatly to dissolve the electrode material.
<電極の電気化学的特性>
本発明においては、上記窒化物を用いた電極の電気化学的特性が以下のようになっていることが好ましい。この規定は、触媒単位量当りの酸素還元電流を評価するものであり、この値が所定値以上であれば、充分な酸素還元能を有することになる。つまり、上記窒化物を用い、以下の電気化学的特性を有する電極は、酸性電解質中での酸素還元能及び安定性がより一層優れている。
上記電極の電気化学的特性として、30℃の0.1mol/L硫酸水溶液中で、走査速度5mV/sで電位走査したとき、可逆水素電極電位基準で0.4Vにおいて測定される酸素雰囲気での電流値をIO2とし、窒素雰囲気での電流値をIN2としたとき、
R=(IO2−IN2)/IN2
で表される値が0.5以上となることが好ましい。
<Electrochemical properties of electrode>
In the present invention, the electrochemical characteristics of the electrode using the nitride are preferably as follows. This rule is for evaluating the oxygen reduction current per unit amount of the catalyst. If this value is equal to or greater than a predetermined value, the oxygen reduction current is sufficient. That is, an electrode using the above nitride and having the following electrochemical characteristics is further excellent in oxygen reducing ability and stability in an acidic electrolyte.
As the electrochemical characteristics of the above electrode, the current value in an oxygen atmosphere measured at 0.4 V on the basis of the potential of the reversible hydrogen electrode when scanning the potential at a scanning speed of 5 mV / s in a 0.1 mol / L sulfuric acid aqueous solution at 30 ° C Is I O2 and the current value in the nitrogen atmosphere is I N2 ,
R = (I O2 −I N2 ) / I N2
Is preferably 0.5 or more.
上記Ｒを規定した理由について説明する。まず、不活性で反応物のない状態で電位走査を行った場合でも、電極触媒と電解質の界面に電気二重層が生じるため、電位を走査すると電流が流れる。これはいわゆるコンデンサの充放電電流に相当する。従って、酸素還元電流（酸素の還元反応に伴う電流）を評価する際には、このコンデンサの充放電電流分を除く必要がある。
そこで、不活性な状態として窒素雰囲気で電解電流IN2を測定し、これを酸素雰囲気での電流IO2から差引くことで、正味の酸素還元電流(IO2−IN2)を求める。
The reason for defining R will be described. First, even when the potential scan is performed in an inert and no reactant state, an electric double layer is formed at the interface between the electrode catalyst and the electrolyte, and thus a current flows when the potential is scanned. This corresponds to a so-called charging / discharging current of the capacitor. Therefore, when evaluating the oxygen reduction current (current associated with the oxygen reduction reaction), it is necessary to exclude the charge / discharge current of this capacitor.
Therefore, the electrolysis current I N2 is measured in a nitrogen atmosphere as an inactive state, and this is subtracted from the current I O2 in the oxygen atmosphere to obtain the net oxygen reduction current (I O2 −I N2 ).
次に、得られた酸素還元電流を触媒単位量当りの値に変換する必要がある。ここで、実際に電極触媒として作用しているのは電極表面であるので、触媒の絶対量でなく、反応面積で規格化する必要がある。しかし、気相法で反応面積を求めるのは、試料が小さく表面積が小さいので、困難である。そこで、上記した電気二重層の充放電電流（IN2）が電極の実質的な反応面積に比例することを利用し、正味の酸素還元電流をIN2で割ることによって、電極の実表面積あたりの触媒能の指標とすることができる。このようにすると、触媒が粉体であっても、窒素雰囲気と酸素雰囲気で電流を測定することで、容易に触媒能を評価できる。 Next, it is necessary to convert the obtained oxygen reduction current into a value per unit amount of catalyst. Here, since it is the electrode surface that actually acts as an electrode catalyst, it is necessary to normalize the reaction area rather than the absolute amount of the catalyst. However, it is difficult to obtain the reaction area by the vapor phase method because the sample is small and the surface area is small. Therefore, by utilizing the fact that the charge / discharge current (I N2 ) of the electric double layer is proportional to the substantial reaction area of the electrode, the net oxygen reduction current is divided by I N2 to obtain It can be used as an index of catalytic ability. In this way, even if the catalyst is a powder, the catalytic ability can be easily evaluated by measuring the current in a nitrogen atmosphere and an oxygen atmosphere.
ここで、0.4Vにおける電流値を採用する理由は、実際の燃料電池では0.4Ｖより低い電位では空気極として使用しないため、0.4V以上の電位で、ある程度の酸素還元触媒能がないと、電極として実用的でないからである。
R値が大きいほど、単位電極表面積あたりの酸素還元電流が大きく、触媒能が高いことになる。R＝0.5とは、酸素還元電流が二重層の充放電電流の1/2以上であることを示す。Rが0.5未満であると触媒能がなく、酸素還元電極として実用に適さない。
Here, the reason why the current value at 0.4V is adopted is that an actual fuel cell is not used as an air electrode at a potential lower than 0.4V. Because it is not practical.
The larger the R value, the larger the oxygen reduction current per unit electrode surface area and the higher the catalytic ability. R = 0.5 indicates that the oxygen reduction current is 1/2 or more of the charge / discharge current of the double layer. When R is less than 0.5, there is no catalytic ability and it is not suitable for practical use as an oxygen reduction electrode.
ところで、触媒能を評価する方法としては、所定の電位における還元電流値を指標とする上記方法の他、酸素還元電流が流れ始める時の電位（酸素還元電流開始電位）の大きさを指標とする方法がある。酸素還元電流がより高電位から流れ始める程、反応の活性化エネルギーが小さい可能性があり、この場合は触媒の表面積を増大させるという工学的な改良によって還元電流値を増加させることができるからである。酸素還元電流開始電位は、可逆水素電極基準で0.5V以上であることが、実用上から好ましい。酸素還元開始電位は高ければ高いほどよい。なお、酸素還元電流は、上記したIO2−IN2で定義される。
上記したR値による還元電流値と、酸素還元開始電位のいずれか又は双方を触媒能を評価する指標として採用することが好ましい。
By the way, as a method for evaluating catalytic ability, in addition to the above method using the reduction current value at a predetermined potential as an index, the magnitude of the potential (oxygen reduction current start potential) when the oxygen reduction current starts flowing is used as an index. There is a way. As the oxygen reduction current starts to flow from a higher potential, the activation energy of the reaction may be smaller, and in this case, the reduction current value can be increased by engineering improvement that increases the surface area of the catalyst. is there. It is preferable from a practical point of view that the oxygen reduction current starting potential is 0.5 V or more on the basis of the reversible hydrogen electrode. The higher the oxygen reduction start potential, the better. The oxygen reduction current is defined by I O2 −IN 2 described above.
It is preferable to employ either or both of the above-described reduction current value based on the R value and the oxygen reduction starting potential as an index for evaluating the catalytic ability.
図４、図５は、TaNを用いた電極の還元電流の測定結果及び上記指標Rの計算結果を示す。図４より、窒素雰囲気でわずかに還元電流が流れ、酸素雰囲気では還元電流が大きいことがわかる。又、図５より、0.4V以下の電位で上記指標Rが0.5以上となっている。 4 and 5 show the measurement result of the reduction current of the electrode using TaN and the calculation result of the index R. FIG. FIG. 4 shows that a slight reduction current flows in the nitrogen atmosphere, and that the reduction current is large in the oxygen atmosphere. Further, from FIG. 5, the index R is 0.5 or more at a potential of 0.4 V or less.
なお、上記電解において、アノードとなる対極は何であってもよく、例えば、カーボン、Pt等を用いることができるが、Ptは微量に溶解することもあるので、カーボンが好ましい。 In the above electrolysis, any counter electrode can be used as the anode. For example, carbon, Pt, or the like can be used. However, carbon is preferable because Pt may be dissolved in a minute amount.
<電極の製造>
上記酸素還元電極は、例えば次のようにして製造することができる。まず、上記した窒化物の粉末を、例えば酸化タングステン、酸化イリジウム等、炭素等の導電性物質の粉末と混合し、公知の結着剤と混合してペーストとし、このペーストを担体表面に塗布、乾燥させて電極を製造する。
例えば、燃料電池用の電極としては、導電性粉末としてカーボンブラックを用い、上記窒化物の微粒子の粒径を２〜３ｎｍ程度とすると、触媒量が少量でも触媒能を発揮できるので好ましい。
<Manufacture of electrodes>
The oxygen reduction electrode can be manufactured, for example, as follows. First, the above-mentioned nitride powder is mixed with a powder of a conductive substance such as tungsten oxide, iridium oxide, etc., and mixed with a known binder to form a paste, and this paste is applied to the surface of the carrier. An electrode is manufactured by drying.
For example, as an electrode for a fuel cell, it is preferable to use carbon black as the conductive powder, and to have a particle size of the above-mentioned nitride fine particles of about 2 to 3 nm because the catalytic ability can be exhibited even with a small amount of catalyst.
本発明の酸素還元電極は、水、無機物質、有機物質の電気分解、燃料電池等の酸性電解質を用いる電気化学システムのカソード用電極として好適に使用できる。りん酸形燃料電池や高分子電解質形燃料電池等、酸性電解質を用いる際の酸化剤極として、本発明の酸素還元電極は適する。 The oxygen reduction electrode of the present invention can be suitably used as a cathode electrode for an electrochemical system using an acidic electrolyte such as water, an inorganic substance or an organic substance, or a fuel cell. The oxygen reduction electrode of the present invention is suitable as an oxidant electrode when using an acidic electrolyte such as a phosphoric acid fuel cell or a polymer electrolyte fuel cell.
以下に、実施例によって本発明を更に具体的に説明するが、本発明は以下の実施例に限定されるものではない。 EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.
<電極の作成>
直径５．２ｍｍの円柱状グラッシーカーボンを基材とし、その底面に電極物質としてＴａＮ窒化物の薄膜をスパッタにより形成させた。スパッタ条件は、Ａｒ分圧約１×１０−１Ｐａ、窒素分圧約４×１０−１Ｐａ、ターゲットＴａとした。なお、薄膜の結晶性を高めるため、スパッタ時に基材を５８０℃に加熱した。
<Creation of electrode>
A cylindrical glassy carbon having a diameter of 5.2 mm was used as a base material, and a TaN nitride thin film was formed as an electrode material on the bottom by sputtering. The sputtering conditions were an Ar partial pressure of about 1 × 10 −1 Pa, a nitrogen partial pressure of about 4 × 10 −1 Pa, and a target Ta. In order to increase the crystallinity of the thin film, the substrate was heated to 580 ° C. during sputtering.
得られた薄膜の厚みを水晶振動式膜厚計で測定したところ、４０ｎｍであった。又、薄膜の結晶構造をＸＲＤ（Ｘ線回折装置）で測定したところ、図６に示すように、ＴａＮの結晶構造を示すことが確認された。なお、基材を加熱せずに５０℃としてスパッタした場合、薄膜は明確な結晶構造を示さず、アモルファス構造を示した。なお、TaNを形成しないカーボンのみのXRDをバックグラウンドとした。
又、図７に示すように、基材を加熱した場合は、加熱しない場合に比べ、得られた電極のＴａＮ面が粗面となった。
It was 40 nm when the thickness of the obtained thin film was measured with the crystal vibration type film thickness meter. Further, when the crystal structure of the thin film was measured by XRD (X-ray diffractometer), it was confirmed that the crystal structure of TaN was shown as shown in FIG. When the substrate was sputtered at 50 ° C. without heating, the thin film did not show a clear crystal structure but showed an amorphous structure. Note that the XRD of carbon alone that does not form TaN was used as the background.
Further, as shown in FIG. 7, when the substrate was heated, the TaN surface of the obtained electrode became rough as compared with the case where the substrate was not heated.
<実施例２〜７>
スパッタ時のターゲットとしてそれぞれ、Zr,Ti,Nb,Si,Sn,Cを用いたこと以外は、実施例１とまったく同様にして電極を作成し、同様な測定を行った。これらの電極の電極物質は、それぞれ膜厚約４０ｎｍのZrN,TiN,NbN,SiN,SnN,CN（いずれも、元素と窒素がほぼ１：１の窒化物）であることをＸＲＤにより確認した。これらをそれぞれ実施例２〜７とする。
<Examples 2 to 7>
An electrode was prepared in the same manner as in Example 1 except that Zr, Ti, Nb, Si, Sn, and C were used as targets during sputtering, and the same measurement was performed. It was confirmed by XRD that the electrode materials of these electrodes were ZrN, TiN, NbN, SiN, SnN, and CN (all of which were nitrides in which element and nitrogen were approximately 1: 1) having a film thickness of about 40 nm, respectively. These are referred to as Examples 2 to 7, respectively.
<比較例１〜４>
スパッタ時のターゲットとしてそれぞれ、Co,Ni,Mo,Wを用いたこと以外は、実施例１とまったく同様にして電極を作成し、同様な測定を行った。これらの電極の電極物質は、それぞれ膜厚約４０ｎｍのCoN,NiN,MoN,WN（いずれも、元素と窒素がほぼ１：１の窒化物）であることをＸＲＤにより確認した。これらをそれぞれ比較例１〜４とする。
<Comparative Examples 1-4>
An electrode was prepared in the same manner as in Example 1 except that Co, Ni, Mo, and W were used as targets during sputtering, respectively, and the same measurement was performed. It was confirmed by XRD that the electrode materials of these electrodes were CoN, NiN, MoN, and WN each having a film thickness of about 40 nm (all of which were nitrides in which element and nitrogen were approximately 1: 1). These are referred to as Comparative Examples 1 to 4, respectively.
<比較例５>
以下の方法によりTa3N5電極を作成した。
（１）Ta3N5粉末の製造
酸化タンタルTa2O5粉末（高純度化学社製、純度99.9%、平均粒径0.5μm）を原料とし、アンモニアガスをよく通すようにこの原料を石英ウールで包んでパイレックス（登録商標）ガラス管内に保持した。パイレックス（登録商標）ガラス管内に、アンモニア（純度99.999％）と水蒸気の混合気体を導入し、ガスの流れがほぼ定常になるまで待機した後（60分程度）、850℃（昇温:10℃/min）で窒化を行った。窒化を完全に進行させ、大きさが数百nmのタンタルナイトライド（Ta3N5）粉末を回収した。
（２）電極の作成
Ta3N5粉末（電極触媒）をグラッシーカーボン電極(5.2mm径)に塗布、ナフィオン（登録商標）をコーティングした。等量塗布できるよう以下の方法で塗布した。まず、1g秤量したTa3N5粉末を水5ml中に混合した後、超音波で攪拌・懸濁し、粉末が均一分散した溶液30μＬ塗布した。この電極をカソードとして用い、実施例１とまったく同様にして測定を行った。
<Comparative Example 5>
A Ta 3 N 5 electrode was prepared by the following method.
(1) Manufacture of Ta 3 N 5 powder Using tantalum oxide Ta 2 O 5 powder (manufactured by High Purity Chemical Co., Ltd., purity 99.9%, average particle size 0.5 μm) as raw material, this raw material is quartz wool so that ammonia gas can pass well And held in a Pyrex (registered trademark) glass tube. After introducing a mixed gas of ammonia (purity 99.999%) and water vapor into a Pyrex (registered trademark) glass tube and waiting until the gas flow becomes almost steady (about 60 minutes), 850 ° C (temperature increase: 10 ° C) / min) for nitriding. Nitriding proceeded completely, and tantalum nitride (Ta 3 N 5 ) powder having a size of several hundred nm was recovered.
(2) Electrode creation
Ta 3 N 5 powder (electrode catalyst) was applied to a glassy carbon electrode (5.2 mm diameter), and Nafion (registered trademark) was coated. It was applied by the following method so that an equal amount could be applied. First, 1 g of Ta 3 N 5 powder weighed was mixed in 5 ml of water, and then stirred and suspended with ultrasonic waves to apply 30 μL of a solution in which the powder was uniformly dispersed. Using this electrode as a cathode, measurement was performed in the same manner as in Example 1.
<評価> <Evaluation>
<サイクリックボルタモグラムの測定（電極安定性）>
上記電極をカソードとして用い、対極に白金箔を用い、窒素雰囲気下、30℃の0.1mol/L硫酸水溶液を電解液として電解セルを用意した。参照電極としては、硫酸濃度が上記と同一の可逆水素電極を用いた。この電解セルを用い、電位走査速度50mV/sでサイクリックボルタモグラム（ＣＶ）を測定した。電位範囲を0.05〜1.0Ｖとした。
上記サイクリックボルタモグラム曲線から、以下の基準で電極安定性を評価した。
〇：ＣＶ曲線の形状がほとんど変化しない
×：ＣＶ曲線の形状が大きく変化するか、又は０．７〜０．８Ｖを超える電位で酸化電流が増加する。
実施例１について得られた結果を図１に示す。電位走査を１０回繰り返したが、ＣＶ曲線の形状は変化せず、この電極を硫酸水溶液中で電解しても安定であることが判明した。
<Measurement of cyclic voltammogram (electrode stability)>
An electrolytic cell was prepared using the above electrode as a cathode, a platinum foil as a counter electrode, and a 0.1 mol / L sulfuric acid aqueous solution at 30 ° C. as an electrolytic solution in a nitrogen atmosphere. As the reference electrode, a reversible hydrogen electrode having the same sulfuric acid concentration as described above was used. Using this electrolytic cell, a cyclic voltammogram (CV) was measured at a potential scanning speed of 50 mV / s. The potential range was 0.05 to 1.0V.
From the cyclic voltammogram curve, electrode stability was evaluated according to the following criteria.
O: The shape of the CV curve hardly changes. X: The shape of the CV curve changes greatly, or the oxidation current increases at a potential exceeding 0.7 to 0.8 V.
The results obtained for Example 1 are shown in FIG. The potential scan was repeated 10 times, but the shape of the CV curve did not change, and it was found that this electrode was stable even when electrolyzed in an aqueous sulfuric acid solution.
<還元電流の測定>
上記電解セルを用い、30℃の0.1mol/L硫酸水溶液中で、走査速度5mV/sで電位走査したとき、可逆水素電極電位基準で0.4Vにおいて測定される酸素雰囲気での電流値IO2、窒素雰囲気での電流値IN2を測定した。次に、
R=(IO2−IN2)/IN2
で表される値を求めた。還元電流が高いほど、触媒能が高い。
実施例１について得られた結果を図４、図５に示す。窒素雰囲気でわずかに還元電流が生じ、酸素雰囲気で還元電流が生じた。これよりRを求めると0.5であった。
又、比較例５について得られた結果を図８、図９に示す。0.4Vで窒素雰囲気と酸素雰囲気の還元電流がほぼ同様の値となり、これよりRを求めるとほぼ０であった。これは、触媒能がないことを示す。
<Measurement of reduction current>
Using the above electrolytic cell, in an aqueous 0.1 mol / L sulfuric acid solution at 30 ° C., when the potential scan was performed at a scanning speed of 5 mV / s, the current value I O2 in an oxygen atmosphere measured at 0.4 V on the basis of the reversible hydrogen electrode potential reference, The current value I N2 in a nitrogen atmosphere was measured. next,
R = (I O2 −I N2 ) / I N2
The value represented by The higher the reduction current, the higher the catalytic ability.
The results obtained for Example 1 are shown in FIGS. A slight reduction current was generated in a nitrogen atmosphere, and a reduction current was generated in an oxygen atmosphere. From this, R was 0.5.
The results obtained for Comparative Example 5 are shown in FIGS. At 0.4 V, the reduction currents in the nitrogen atmosphere and the oxygen atmosphere were almost the same value, and R was found to be almost 0 from this. This indicates that there is no catalytic ability.
<酸素還元開始電位の測定>
上記電解セルを用い、上記還元電流の測定時におけるIO2が流れ始めた時（IO2<０となる時）の電位を可逆水素電極電位基準で求めた。酸素還元開始電位が高いほど、触媒能が高い。
<Measurement of oxygen reduction initiation potential>
The used electrolytic cell was determined by a reversible hydrogen electrode potential relative to the potential when I O2 starts flowing during the measurement of the reduction current (when the I O2 <0). The higher the oxygen reduction starting potential, the higher the catalytic ability.
得られた結果を表１に示す。 The obtained results are shown in Table 1.
表１から明らかなように、各実施例の場合、ＣＶ曲線の形状がほとんど変化せず、硫酸中での電極安定性に優れていた。又、各実施例の場合、酸素還元能を示す指標であるRが0.5以上であり、電極活性が高いものとなった。
一方、比較例１（ＣｏＮ）、比較例２（ＮｉＮ）の場合、電位走査の回数が増えるとともにＣＶ曲線の形状が変化し、電極安定性に劣った。
比較例３（ＭｏＮ）、比較例４（ＷＮ）の場合、０．７〜０．８Ｖを超える電位で酸化電流が増加し、電極が溶解したため、電極安定性に劣った。
比較例５の場合、ＣＶ曲線の形状がほとんど変化せず、硫酸中での電極安定性に優れていたが、Rが0.5未満であり、電極活性が低いものとなった。
As is clear from Table 1, in each example, the shape of the CV curve hardly changed and the electrode stability in sulfuric acid was excellent. In each example, R, which is an index indicating the oxygen reducing ability, was 0.5 or more, and the electrode activity was high.
On the other hand, in Comparative Example 1 (CoN) and Comparative Example 2 (NiN), the number of potential scans increased and the shape of the CV curve changed, resulting in poor electrode stability.
In the case of Comparative Example 3 (MoN) and Comparative Example 4 (WN), the oxidation current increased at a potential exceeding 0.7 to 0.8 V, and the electrode was dissolved, resulting in poor electrode stability.
In Comparative Example 5, the shape of the CV curve hardly changed and the electrode stability in sulfuric acid was excellent, but R was less than 0.5 and the electrode activity was low.
Claims (4)
R=(IO2−IN2)/IN2
で表される値が0.5以上となることを特徴とする請求項１記載の酸素還元電極。 When a potential scan was performed at a scanning speed of 5 mV / s in a 0.1 mol / L sulfuric acid aqueous solution at 30 ° C., the current value in an oxygen atmosphere measured at 0.4 V on the basis of the reversible hydrogen electrode potential was defined as I O2, and in a nitrogen atmosphere When the current value is I N2 ,
R = (I O2 −I N2 ) / I N2
The oxygen reduction electrode according to claim 1, wherein a value represented by the formula is 0.5 or more.
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