JP2005135752A - Oxygen reduction catalyst for fuel cell - Google Patents
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Abstract
Description
本発明は、電気化学触媒、特に、液体燃料を直接燃料とする直接形燃料電池の空気極触媒に関する。 The present invention relates to an electrochemical catalyst, and more particularly, to an air electrode catalyst of a direct fuel cell using liquid fuel as a direct fuel.
燃料及び酸素含有ガスの供給及び集電を担うセパレータ間に、イオン伝導体である電解質の両側に多孔質の燃料酸化触媒層と酸素還元触媒層を接合した電極−電解質接合体を挟んで構成される燃料電池の中で、メタノールを初めとする炭素及び水素を含む液体燃料を直接燃料とする直接形燃料電池（例えば、特許文献１，２）は、構造が単純であることから、携帯用途、移動電源、分散電源への応用が進められている。 An electrode-electrolyte assembly is formed by sandwiching a porous fuel oxidation catalyst layer and an oxygen reduction catalyst layer on both sides of the electrolyte, which is an ionic conductor, between separators that supply and collect fuel and oxygen-containing gas. Among direct fuel cells (for example, Patent Documents 1 and 2) that directly use liquid fuel containing methanol and carbon and hydrogen, such as methanol, the structure is simple. Applications to mobile power sources and distributed power sources are being promoted.
直接形燃料電池の電解質膜にはパーフルオロエチレンスルホン酸膜に代表されるプロトン交換性の高分子膜が用いられている。この電解質膜は含水することによりプロトン伝導性を発現する。したがって、メタノールなどの水溶性燃料を用いる場合、電解質膜内に燃料が染み込んで、空気極の触媒上で直接化学的に燃焼するため、燃料利用率及び空気極の電位が低下するため、エネルギー変換効率が著しく低くなるという問題点がある。 A proton-exchangeable polymer membrane typified by a perfluoroethylene sulfonic acid membrane is used for the electrolyte membrane of the direct fuel cell. This electrolyte membrane exhibits proton conductivity when it contains water. Therefore, when using a water-soluble fuel such as methanol, the fuel soaks into the electrolyte membrane and directly chemically burns on the catalyst of the air electrode, so that the fuel utilization rate and the potential of the air electrode decrease, so energy conversion. There is a problem that the efficiency is remarkably lowered.
燃料の透過（クロスリーク）によるエネルギー変換効率の低下を抑制する手段として、燃料の透過を抑制した電解質膜の開発が行われている（例えば、特許文献２、３、４）。また、酸素極にPdまたはPd合金としてRu,Rh,Os,Ir,Pt,Au,Agなど水素より酸化されにくい遷移金属との合金を用いたもの（特許文献５）や、負極の燃料及び水との接触面と正極の酸素含有気体との接触面の間のいずれかの位置に、Pd膜またはPd合金膜を配置したもの（特許文献６）も開発されている。 As means for suppressing a decrease in energy conversion efficiency due to fuel permeation (cross leak), an electrolyte membrane that suppresses fuel permeation has been developed (for example, Patent Documents 2, 3, and 4). Also, an oxygen electrode using Pd or an alloy with a transition metal that is less oxidized than hydrogen, such as Ru, Rh, Os, Ir, Pt, Au, Ag, as a Pd alloy (Patent Document 5), fuel and water for the negative electrode A device in which a Pd film or a Pd alloy film is disposed at any position between the contact surface with the oxygen-containing gas and the contact surface with the oxygen-containing gas of the positive electrode (Patent Document 6) has also been developed.
上記のように、電解質膜の開発が行われているが、パーフルオロエチレンスルホン酸膜と同程度の高いイオン伝導度と安定性を持ちながら、なおかつ燃料の透過度のみを十分に低下させることは困難である。また、これらの新規電解質膜であってもプロトン伝導の媒体が水である以上はある程度の燃料の透過は避けられない。そこで、本発明は、メタノールなどの液体燃料がクロスリークした状態で高効率の酸素還元反応を行うための直接形燃料電池用酸素還元触媒を提供することを目的とする。 As described above, the development of electrolyte membranes has been carried out, but it is possible to sufficiently reduce only the fuel permeability while having the same high ionic conductivity and stability as perfluoroethylene sulfonic acid membranes. Have difficulty. Further, even with these new electrolyte membranes, a certain amount of fuel permeation is inevitable as long as the proton conducting medium is water. Accordingly, an object of the present invention is to provide an oxygen reduction catalyst for a direct fuel cell for performing a highly efficient oxygen reduction reaction in a state where a liquid fuel such as methanol is cross leaked.
本発明は、イオン伝導性電解質膜の両側に酸化反応触媒と酸素還元反応触媒を配置して構成され、酸化反応触媒での酸化反応用の燃料として炭素及び水素を含む液体燃料を供給する直接形燃料電池用の酸素還元反応触媒を提供する。本発明の酸素還元反応触媒を用いる場合、上記の新規電解質膜との併用を妨げるものではないが、必ずしも燃料の透過を抑制する必要は無い。図１に、その概念図を示す。横軸は電極電位であり、空気極での反応について示してある。電極Aは、例えば白金の如く、酸素還元反応、燃料の酸化反応の両
者に活性な触媒を、電極Bは酸素還元反応に活性であり、燃料の酸化反応に不活性な本発明の電極触媒を用いた電極を示す。
The present invention is configured by arranging an oxidation reaction catalyst and an oxygen reduction reaction catalyst on both sides of an ion conductive electrolyte membrane, and supplying a liquid fuel containing carbon and hydrogen as a fuel for the oxidation reaction in the oxidation reaction catalyst. An oxygen reduction reaction catalyst for a fuel cell is provided. When the oxygen reduction reaction catalyst of the present invention is used, the combined use with the above novel electrolyte membrane is not hindered, but it is not always necessary to suppress fuel permeation. FIG. 1 shows a conceptual diagram thereof. The horizontal axis represents the electrode potential, and shows the reaction at the air electrode. Electrode A is an active catalyst for both oxygen reduction reaction and fuel oxidation reaction, such as platinum. Electrode B is an active electrode catalyst for the oxygen reduction reaction and inactive for fuel oxidation reaction. The electrode used is shown.
なお、この例では電極Aの方がBよりも酸素還元は触媒活性が高いとして記述した。すなわち、触媒上に燃料が存在しない場合の電極の性能は図１中、電極A酸素(破線)、電極B酸素(実線)で示される如く、酸素の理論平衡電位に対して高い電位では酸化反応(酸素発生)、低い電位では還元反応(酸素還元)の曲線で表される。燃料電池の空気極反応では同じ還元電流を高い電位で得るほうが、エネルギー変換効率は高いため、酸素還元触媒活性は電極Aの方が高い。 In this example, the electrode A is described as having higher catalytic activity for oxygen reduction than B. That is, the performance of the electrode when no fuel is present on the catalyst is shown in FIG. 1 as indicated by electrode A oxygen (dashed line) and electrode B oxygen (solid line). (Oxygen generation), a low potential is represented by a reduction reaction (oxygen reduction) curve. In the air electrode reaction of the fuel cell, obtaining the same reduction current at a higher potential results in higher energy conversion efficiency, so that the oxygen reduction catalytic activity of electrode A is higher.
一方、触媒上に燃料が存在すると、燃料の酸化反応が同時に進行する。電極A燃料(破線)及び電極B燃料(実線)で示した曲線は、それぞれの電極上での燃料の反応を示す。透過Aで示される電流値が電解質膜を透過する燃料の量で決定される電極A上での酸化反応の電流値であり、電極Bは燃料の酸化に対して不活性であるため、電流は流れない。このとき、燃料の酸化に不活性な電極Bの性能は変化せず、実線で表されるのに対して、電極Aでは等しい電位における電極A燃料の酸化電流と電極A酸素の還元電流の和である電極A和(一転鎖線)で示した曲線の特性まで、性能が低下する。したがって、燃料が透過する条件では電極Bの方が電気化学的に活性な触媒であり、なおかつ燃料の損失も無い良好な触媒であることが分かる。 On the other hand, when fuel is present on the catalyst, the oxidation reaction of the fuel proceeds simultaneously. The curves indicated by the electrode A fuel (dashed line) and the electrode B fuel (solid line) indicate the reaction of the fuel on the respective electrodes. The current value indicated by permeation A is the current value of the oxidation reaction on electrode A determined by the amount of fuel permeating the electrolyte membrane, and since electrode B is inactive against fuel oxidation, the current is Not flowing. At this time, the performance of the electrode B that is inactive to the oxidation of the fuel does not change and is represented by a solid line, whereas in the electrode A, the sum of the oxidation current of the electrode A fuel and the reduction current of the electrode A oxygen at the same potential. The performance is lowered to the characteristic of the curve indicated by the electrode A sum (one-dot chain line). Therefore, it can be seen that the electrode B is an electrochemically active catalyst under the conditions where the fuel permeates, and is a good catalyst with no fuel loss.
酸性電解質中の酸素還元触媒としては、Kim KINOSHITA、“Electrochemical Oxygen Technology”、John Wiley & Sons、Inc. 1992年、54ページ等の各種総説に示される如く、白金及び白金に遷移金属を添加した白金系の合金触媒が一般的に使用されるが、白金のほかには、白金族であるパラジウム、ルテニウム、ロジウム、イリジウムなどが酸素還元触媒能を有することが知られており、この中でもパラジウムが白金に次いで酸素還元触媒能が高いとされている。 Oxygen reduction catalysts in acidic electrolytes include platinum and platinum with transition metals added as shown in various reviews such as Kim KINOSHITA, “Electrochemical Oxygen Technology”, John Wiley & Sons, Inc. 1992, p. 54. In general, platinum-based alloy catalysts such as palladium, ruthenium, rhodium, and iridium are known to have oxygen reduction catalytic ability. Among these, palladium is platinum. Next, it is said that the oxygen reduction catalytic ability is high.
本発明者らは、パラジウムに遷移金属、とくに標準酸化還元電位0V以下の遷移金属を添加することにより、メタノールを初めとする燃料の酸化に対して不活性でかつ純粋なパラジウムと比較して高い酸素還元触媒能を有する電極触媒を得ることができることを見出した。ここで、標準酸化還元電位とはAllen. J. Bard、Larry R. Faulkner、”Electrochemical Methods”、John Wiley & Sons、Inc. 1980年、700ページ等の電気化学に関する専門書に記載される水素の酸化還元反応を基準とした酸化還元のポテンシャルを表す指標であり、標準酸化還元電位0V以下の物質は平衡論的に水素より酸化されやすいことを意味する。 By adding a transition metal, particularly a transition metal having a standard oxidation-reduction potential of 0 V or less, to the palladium, the present inventors are inert to the oxidation of fuels including methanol and are higher than pure palladium. It has been found that an electrode catalyst having oxygen reduction catalytic ability can be obtained. Here, the standard oxidation-reduction potential is defined as the hydrogen oxidation described in specialist books on electrochemistry such as Allen. J. Bard, Larry R. Faulkner, “Electrochemical Methods”, John Wiley & Sons, Inc. 1980, page 700. It is an index representing the redox potential based on the redox reaction, and means that a substance having a standard redox potential of 0 V or less is more easily oxidized than hydrogen in terms of equilibrium.
パラジウムは水素を吸蔵する金属として知られる。一方、標準酸化還元電位0V以下の遷移金属は燃料が存在する環境においても、単独では酸化物の状態で安定な物質であることを意味し、酸素を吸着しやすく、メタノールを初めとする炭素及び水素を含む燃料を吸着し難く、燃料を酸化するための触媒とはならない。したがって、標準酸化還元電位の高い白金、ルテニウム、ロジウム、パラジウム、オスミウム、イリジウム、金などを添加した触媒では、添加元素が燃料の酸化触媒として働くのに対して、標準酸化還元電位0V以下の遷移金属は燃料の酸化能力が無いために酸素還元反応の選択率が高くなる。また、酸素を還元する反応による生成物は水であるので、パラジウムが水素を、標準酸化還元電位0V以下の遷移金属が酸素を捕捉することにより酸素還元反応の触媒能が高い。 Palladium is known as a metal that stores hydrogen. On the other hand, a transition metal having a standard oxidation-reduction potential of 0 V or less means that it is a stable substance in an oxide state alone even in an environment where fuel exists, and it is easy to adsorb oxygen, and carbon such as methanol and carbon. It is difficult to adsorb fuel containing hydrogen, and it does not become a catalyst for oxidizing fuel. Therefore, in the catalyst with platinum, ruthenium, rhodium, palladium, osmium, iridium, gold, etc. with high standard oxidation-reduction potential, the additive element works as an oxidation catalyst for fuel, whereas the transition with standard oxidation-reduction potential of 0 V or less Since metal does not have the ability to oxidize fuel, the selectivity of the oxygen reduction reaction is increased. In addition, since the product resulting from the reaction of reducing oxygen is water, palladium has hydrogen and a transition metal having a standard oxidation-reduction potential of 0 V or less has high catalytic ability for the oxygen reduction reaction.
上記の酸素還元反応触媒の組成としては30原子%以上かつ95原子%以下のパラジウムと、5原子%以上70原子%以下の遷移金属原子を含むことが好ましい。前記の遷移金属元素としては、コバルト、クロム、ニッケル、モリブデン、タンタルのうち、一つ以上の元素を含むことが好ましい。なかでも、40原子%以上かつ70原子%以下のパラジウムを含む酸素還元
反応触媒が酸素還元反応に対する触媒活性が高く、燃料の酸化に対して不活性な触媒である。
The composition of the oxygen reduction reaction catalyst preferably contains 30 atomic% or more and 95 atomic% or less of palladium and 5 atomic% or more and 70 atomic% or less of a transition metal atom. The transition metal element preferably contains one or more elements of cobalt, chromium, nickel, molybdenum, and tantalum. Among them, an oxygen reduction reaction catalyst containing 40 atomic% or more and 70 atomic% or less of palladium has a high catalytic activity for the oxygen reduction reaction and is an inactive catalyst for fuel oxidation.
これらのパラジウムを含む酸素還元反応触媒の特徴を表す別の指標として、燃料の酸化反応と電気化学的な酸素還元反応が競争反応として進行する条件において、酸素還元反応の選択率が高いことを考えることができる。図１に示した原理に基づいて表現すると、酸素還元反応の速度が支配的である限界拡散電流の5%から10%程度の任意の電流密度において、電解質に燃料が存在する場合と、しない場合の電位が変わらないこと、具体的には1リットル当たり0.1モルの硫酸水溶液中での前記任意の電流密度での電位に対し、1リットル当たり0.1モルの硫酸と0.1モルのメタノールの電解質中での前記任意の電流密度における電位の比が90％以上かつ100％以下であることと定義できる。 As another index expressing the characteristics of these oxygen-reducing catalysts containing palladium, it is considered that the selectivity of the oxygen-reducing reaction is high under the condition that the oxidation reaction of the fuel and the electrochemical oxygen-reducing reaction proceed as a competitive reaction. be able to. When expressed based on the principle shown in FIG. 1, when the fuel is present in the electrolyte and when it is not present at an arbitrary current density of 5% to 10% of the limiting diffusion current where the rate of the oxygen reduction reaction is dominant In the electrolyte of 0.1 mol sulfuric acid and 0.1 mol methanol per liter with respect to the potential at the above arbitrary current density in 0.1 mol sulfuric acid aqueous solution per liter. It can be defined that the ratio of the potential at the arbitrary current density is 90% or more and 100% or less.
以上の合金触媒を合成する方法として、パラジウムと遷移金属をターゲットとして炭素製電極基板上にスパッタして合金を得る方法、白金系の合金触媒を合成する方法として 特開平05-182672号公報、特開平06-124712号公報記載の方法に準じて、塩化パラジウム酸及び塩化コバルトの混合溶液と担体炭素の混合液に還元剤を加えて担体炭素上に触媒を析出させる方法などがあるが、本発明は合金触媒の合成方法を限定するものではない。 As a method of synthesizing the above alloy catalyst, a method of obtaining an alloy by sputtering on a carbon electrode substrate using palladium and a transition metal as a target, and a method of synthesizing a platinum-based alloy catalyst are disclosed in JP-A-05-182672. According to the method described in Kaihei 06-124712, there is a method in which a reducing agent is added to a mixed solution of chloropalladium acid and cobalt chloride and a mixed solution of carrier carbon to deposit a catalyst on the supported carbon, etc. Does not limit the synthesis method of the alloy catalyst.
以上の酸素還元電極触媒はイオン伝導性電解質膜中を燃料が透過しやすいメタノールを初めとするアルコール系の水溶性の液体燃料、具体的にはメタノール、エタノール、グリコール、アセタールなどを燃料とする直接形燃料電池、特に酸化反応の活性化エネルギーが小さいメタノールを燃料とする直接形燃料電池のエネルギー変換効率の向上に有効である。 The above-mentioned oxygen reduction electrode catalyst is an alcohol-based water-soluble liquid fuel such as methanol that can easily pass through the ion-conducting electrolyte membrane, specifically, methanol, ethanol, glycol, acetal, etc. This is effective in improving the energy conversion efficiency of a direct fuel cell using methanol as a fuel, particularly a fuel having a small activation energy for oxidation reaction.
以上の結果から明らかなように、パラジウムと遷移金属元素を含む本発明の酸素還元触媒は燃料のメタノールなど、水溶性液体燃料の酸化反応に対して不活性でありながら、酸素還元反応の触媒活性が高いため、直接形燃料電池の性能向上、すなわち、発電効率の向上に資することは明白である。よって、本発明は、クロスリークした燃料の酸化反応に対して不活性かつ高い酸素還元触媒活性を有する電極触媒を供することができる。 As is clear from the above results, the oxygen reduction catalyst of the present invention containing palladium and a transition metal element is inactive with respect to the oxidation reaction of a water-soluble liquid fuel such as methanol of fuel, but the catalytic activity of the oxygen reduction reaction. Therefore, it is obvious that it contributes to improving the performance of the direct fuel cell, that is, improving the power generation efficiency. Therefore, the present invention can provide an electrode catalyst that is inactive with respect to the oxidation reaction of the cross leaked fuel and has a high oxygen reduction catalytic activity.
以下、本発明を、その実施の形態に基づいて説明する。
スパッタ法にて直径5mmのグラッシーカーボン上に酸素還元反応触媒を製作した。ターゲットにはパラジウムを用い、添加する遷移金属元素として、コバルト、クロム、ニッケル、モリブデン及びタンタル片を適宜パラジウム上に乗せて酸素還元反応触媒の組成を変化させた。スパッタ時のヘリウム圧は1×10-5Pa以下とした。
Hereinafter, the present invention will be described based on the embodiments.
An oxygen reduction reaction catalyst was fabricated on glassy carbon with a diameter of 5 mm by sputtering. Palladium was used as a target, and as a transition metal element to be added, cobalt, chromium, nickel, molybdenum and tantalum pieces were appropriately placed on palladium to change the composition of the oxygen reduction reaction catalyst. The helium pressure during sputtering was 1 × 10 −5 Pa or less.
水晶振動式膜厚計を用いて、スパッタ量を計測し、酸素還元反応触媒の膜の厚さがおよそ1μmの電極を作製した。作製した触媒の組成はエネルギー分散蛍光X線法により定量した。以上の手順で作製した本発明の電極と、比較のための、直径1mmの白金線を熱王水及び純水で洗浄した比較電極について、電気化学的に酸素還元触媒能を評価した。 The amount of spatter was measured using a quartz vibration type film thickness meter, and an electrode having an oxygen reduction reaction catalyst film thickness of about 1 μm was produced. The composition of the prepared catalyst was quantified by energy dispersive fluorescent X-ray method. The oxygen reduction catalytic ability of the electrode of the present invention produced by the above procedure and a comparative electrode obtained by washing a platinum wire having a diameter of 1 mm with hot aqua regia and pure water for comparison were evaluated electrochemically.
電解質として純水にH2SO4とCH3OHがそれぞれ0.1mol dm-3となるように添加したH2SO4+CH3OH電解質及びH2SO4が0.1mol dm-3となるように添加したH2SO4電解質を用いた。基準電極として可逆水素電極、対極に白金黒付き白金電極を用いた。30℃、酸素雰囲気中における5mV s-1の電位走査を行い、本発明の電極と比較電極について評価した。 H 2 SO 4 + CH 3 OH electrolyte added to pure water so that H 2 SO 4 and CH 3 OH are each 0.1 mol dm -3 and H 2 SO 4 are 0.1 mol dm -3 as electrolytes. Added H 2 SO 4 electrolyte was used. A reversible hydrogen electrode was used as a reference electrode, and a platinum electrode with platinum black was used as a counter electrode. A potential scan of 5 mV s −1 in an oxygen atmosphere at 30 ° C. was performed to evaluate the electrode of the present invention and the comparative electrode.
図２に、H2SO4電解質中での酸素還元反応の評価結果を示す。ここでは、本発明の酸素還元反応触媒として、パラジウムが60原子%、遷移金属としてコバルト、クロム、ニッケ
ル、モリブデン、タンタルの何れかを40原子%含む電極触媒をスパッタした電極を用いた。比較として白金電極及びパラジウムのみをグラッシーカーボン上にスパッタした電極の評価結果も示した。
FIG. 2 shows the evaluation result of the oxygen reduction reaction in the H 2 SO 4 electrolyte. Here, an electrode obtained by sputtering an electrode catalyst containing 60 atomic% of palladium and 40 atomic% of cobalt, chromium, nickel, molybdenum, or tantalum as a transition metal was used as the oxygen reduction reaction catalyst of the present invention. For comparison, evaluation results of a platinum electrode and an electrode obtained by sputtering only palladium on glassy carbon are also shown.
この評価は、空気極側にメタノールなどの燃料が存在しない水素−酸素形燃料電池の空気極を模擬している。図の縦軸で負の電流が酸素還元反応の速度を示す。電位(横軸)が高いときに大きな酸素還元電流が得られる電極ほど活性が高い。すなわち、この図から比較電極である白金が一番酸素還元触媒能が高く、ついで本発明のパラジウムと各種遷移金属を含む電極触媒、最も酸素還元触媒能が低いのがパラジウムのみをスパッタしたものとなっている。 This evaluation simulates the air electrode of a hydrogen-oxygen fuel cell in which no fuel such as methanol exists on the air electrode side. On the vertical axis of the figure, a negative current indicates the rate of the oxygen reduction reaction. The higher the potential (horizontal axis), the higher the activity of the electrode that can obtain a large oxygen reduction current. That is, from this figure, platinum as a reference electrode has the highest oxygen reduction catalytic ability, then the electrode catalyst containing palladium and various transition metals of the present invention, and the lowest oxygen reduction catalytic ability is the one with only palladium sputtered. It has become.
図３に、同じ電極をH2SO4+CH3OH電解質で評価した結果を示す。本試験条件は直接形燃料電池の空気極の環境、すなわち、本発明の酸素還元触媒の使用環境を模擬したものである。比較の白金電極では電流値が0となる電位が0.8Vであり、0.8V以上では酸化電流が検出された。H2SO4電解質での結果と比較すると、図１の概念図に示したように、メタノールの酸化電流により酸素還元電位が低下していることを示す。 FIG. 3 shows the results of evaluating the same electrode with H 2 SO 4 + CH 3 OH electrolyte. This test condition simulates the environment of the air electrode of the direct fuel cell, that is, the use environment of the oxygen reduction catalyst of the present invention. In the comparison platinum electrode, the potential at which the current value becomes 0 was 0.8 V, and an oxidation current was detected at 0.8 V or more. Compared with the results with the H 2 SO 4 electrolyte, as shown in the conceptual diagram of FIG. 1, it is shown that the oxygen reduction potential is lowered by the oxidation current of methanol.
これに対して、本発明のパラジウムと各種遷移金属元素を含む電極触媒及び比較のためのパラジウムのみの電極触媒ではメタノールの酸化を示す酸化電流(図３での正方向の電流)は認められない。酸素還元の触媒能は高いものから、本発明のパラジウムと各種遷移金属元素を含む電極触媒、比較の白金電極、パラジウムのみの電極の順であり、本発明のパラジウムと各種遷移金属元素を含む電極触媒が空気極に燃料が透過する直接形燃料電池の環境で非常に高い酸素還元触媒能を持つことが確認できた。 In contrast, the electrocatalyst containing palladium and various transition metal elements of the present invention and the palladium-only electrocatalyst for comparison do not show an oxidation current (positive current in FIG. 3) indicating methanol oxidation. . From the high catalytic ability of oxygen reduction, the electrode catalyst containing palladium and various transition metal elements of the present invention, the comparative platinum electrode, and the electrode containing only palladium, and the electrode containing palladium and various transition metal elements of the present invention. It was confirmed that the catalyst has a very high oxygen reduction catalytic ability in the environment of a direct fuel cell in which the fuel permeates the air electrode.
図４には、図２及び図３と同様の試験を行い、触媒中のパラジウムの原子％に対して0.1mA cm-2の還元電流(図２、図３中で-0.1mA cm-2)を示す電位で定義する有効酸素還元電位をプロットした例を示す。塗りつぶしのプロットがH2SO4電解質中、白抜きのプロットがH2SO4+CH3OH電解質での測定結果である。 In FIG. 4, the same test as in FIGS. 2 and 3 was performed, and a reduction current of 0.1 mA cm −2 with respect to atomic% of palladium in the catalyst (−0.1 mA cm −2 in FIGS. 2 and 3). The example which plotted the effective oxygen reduction potential defined by the electric potential which shows is shown. The solid plot is the measurement result in the H 2 SO 4 electrolyte, and the white plot is the measurement result in the H 2 SO 4 + CH 3 OH electrolyte.
H2SO4電解質中での測定結果から、パラジウムと遷移金属元素からなる酸素還元触媒は30原子％以上かつ95原子％以下の広い範囲でパラジウム単体より高い触媒能を有すること、特に、H2SO4+CH3OH電解質に代表されるような燃料が存在する条件で高い酸素還元触媒活性を示すこと、これらの遷移金属元素としてコバルト、クロム、ニッケル、モリブデン、タンタルが良好な特性を示すこと、パラジウムが40原子%以上かつ70原子%以下の領域が特に触媒活性が高く、白金以上の活性を示すことが明らかとなった。 From the measurement results in the H 2 SO 4 electrolyte, the oxygen reduction catalyst composed of palladium and a transition metal element has higher catalytic ability than palladium alone in a wide range of 30 atomic% or more and 95 atomic% or less, particularly H 2 High oxygen reduction catalytic activity in the presence of fuel such as SO 4 + CH 3 OH electrolyte, and cobalt, chromium, nickel, molybdenum and tantalum as these transition metal elements have good characteristics In addition, it has been clarified that the region where palladium is 40 atomic% or more and 70 atomic% or less has particularly high catalytic activity and exhibits activity higher than that of platinum.
図５に、触媒中のパラジウムの原子％に対して図４にプロットしたH2SO4電解質中での有効酸素還元電位に対するH2SO4+CH3OH電解質での有効酸素還元電位の比をプロットして示す。図１に示した概念からも明らかなとおり、図５に示す比が1に近いほど燃料の酸化反応と比較して、酸素還元反応の選択性が高いことを示す。すなわち、本指標は触媒の選択性を示したものである。 FIG. 5 shows the ratio of the effective oxygen reduction potential in the H 2 SO 4 + CH 3 OH electrolyte to the effective oxygen reduction potential in the H 2 SO 4 electrolyte plotted in FIG. 4 against the atomic% of palladium in the catalyst. Plotted. As is clear from the concept shown in FIG. 1, the closer the ratio shown in FIG. 5 is to 1, the higher the selectivity of the oxygen reduction reaction compared to the fuel oxidation reaction. That is, this index indicates the selectivity of the catalyst.
ここで、図５で選択性が高い触媒は図４より見てH2SO4+CH3OH電解質中での有効酸素還元電位が高い、すなわち、優れた酸素還元触媒活性を示すことは明らかである。即ち、パラジウムを含む酸素還元触媒を燃料が共存する条件で使用する場合には選択性が高い電極触媒が優れた活性を有し、その評価基準として1リットル当たり0.1モルの硫酸水溶液中での酸素還元反応の限界拡散電流の5%以上、10%以下の任意の電流密度における電位に対し、1リットル当たり0.1モルの硫酸と0.1モルのメタノールの電解質中での前記任意の電流密度における電位の比が90％以上かつ100％以下であることがわかる。 Here, it is clear that the catalyst having high selectivity in FIG. 5 has a high effective oxygen reduction potential in the H 2 SO 4 + CH 3 OH electrolyte as shown in FIG. 4, that is, exhibits excellent oxygen reduction catalytic activity. is there. That is, when an oxygen reduction catalyst containing palladium is used under the condition where the fuel coexists, an electrode catalyst having high selectivity has an excellent activity. Ratio of the potential at any current density in an electrolyte of 0.1 mol sulfuric acid and 0.1 mol methanol per liter with respect to the potential at an arbitrary current density of 5% to 10% of the limiting diffusion current of the reduction reaction Is 90% or more and 100% or less.
本発明は、直接形燃料電池の高効率化に寄与するものであり、携帯機器、非常用、分散型システムなどの電源として幅広く利用できるものである。 The present invention contributes to increasing the efficiency of a direct fuel cell, and can be widely used as a power source for portable devices, emergency, distributed systems and the like.
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