JP2004119077A - Medium temperature fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell which can operate in a medium temperature region where conventional fuel cells can not operate. <P>SOLUTION: The fuel cell uses a mixed molten salt including at least one of potassium hydrogensulfate (KHSO<SB>4</SB>) and sodium hydrogensulfate (NaHSO<SB>4</SB>) as an electrolyte and is made to operate in temperature regions from a melting point of the electrolyte to 600°C. The gas supplied to the electrolyte comprises a fuel gas including either of a fuel gas which includes hydrogen or which is reformed to generate hydrogen and a gas which can directly supply proton to the electrolyte by an anodic reaction, and an oxidizer gas including oxygen. Preferably, both of the fuel gas and the oxidizer gas are humidified. <P>COPYRIGHT: (C)2004,JPO

Description

ここで、式中のｉは電流密度［ｍＡ／ｃｍ^２］、Ｐはガス分圧［−］、Ｔは作動温度［Ｋ］を表す。
【００１２】
また、ＰＡＦＣおよびＭＣＦＣの反応過電圧η_ａ＋ｃを導出するには数式２および数式３を用いることができる。これらは、現在公表されている代表的な電池性能から導出された式である。
【数２】

【数３】

ここで、式中のｍはモル分率［−］を表す。
【００１３】
また、各燃料電池の起電力Ｅは、各作動温度Ｔとガス分圧から数式４を用いて計算できる。
【数４】

ここで、Ｅ_０は標準起電力で、温度の関数であり、温度が高いほど低下する。Ｒはガス定数、Ｆはファラデー定数である。
【００１４】
したがって、電池性能を表すセル電圧Ｖ［Ｖ］は、数式で表すことができる。
【数５】

ここで、η_ＮＥはネルンストロスを表す。
【００１５】
ＰＡＦＣおよびＭＣＦＣの作動温度は、２００℃および６５０℃とし、中温域プロトン移動型燃料電池の作動温度は、３００℃とした。また、ＰＡＦＣおよびＭＣＦＣの内部抵抗ｉＲは、電力中央研究所において導出された結果によればそれぞれ０．２８３ｉ、０．２１５ｉである。さらに、ネルンストロスを、プロトン移動型のＰＡＦＣについては１０［ｍＶ］、オキサイドイオン移動型のＭＣＦＣについては１２０［ｍＶ］とした。そしてそれぞれの燃料電池と中温域作動型燃料電池について性能を比較した（図１参照）。図中において中温域作動型燃料電池をＭＡＦＣ（Ｍｏｌｔｅｎ　Ａｃｉｄ　ｓｕｌｆａｔｅ　Ｆｕｅｌ　Ｃｅｌｌの略）と示す。なお、中温域作動型燃料電池においては、電解質以外の部材をＰＡＦＣと同じにし、電解質の違いのみによる性能の差異を確認するようにした。ネルンストロスは電解質がプロトン移動型であることから１０［ｍＶ］とした。また中温域作動型燃料電池（ＭＡＦＣ）における内部抵抗は、作動温度によっては接触および腐食によりＰＡＦＣのそれよりも増大すると考えられたが、中温域作動型燃料電池ではＰＡＦＣよりも導電率が高い電解質（例えば０．１［Ｓ／ｃｍ］以上）を用いるようにしたため、ＰＡＦＣの２／３であると仮定した。
【００１６】
ここで、まずＰＡＦＣとＭＣＦＣとを比較すると、高温型であるＭＣＦＣの出力電圧は低温型であるＰＡＦＣのものよりも大幅に向上していることが判る。高温化に伴う開路電圧の低下があるものの、電圧が大幅に向上したのは、カソード反応抵抗の低下によるものであることが判る。ただし、高温型であるＭＣＦＣの場合、高温化に伴い開路電圧が熱力学的に低下すること、水蒸気がアノード側に生成するオキサイドイオン型であってネルンストロスが大きいことといった理由により（図４参照）、性能向上が抑制されていることが判った。
【００１７】
これに対し、中温域作動型燃料電池については、ネルンストロスが小さいプロトン移動型燃料電池とすることにより、材料に関する制約の少ない２００℃〜６００℃という中温域内において、高温型であるＭＣＦＣと同程度の性能が得られることが期待され、実際、図１に示すように、例えば２００℃で作動するＰＡＦＣと比較して３００℃で作動させることにより大幅な電圧向上が認められた。このように、２００℃〜６００℃という中温域で作動するプロトン移動型の燃料電池は、性能面についてのメリットが多い。しかも、この中温域内であれば電池の構成材料に高価な高温耐食材料を使用する必要がないので、製造コストを低減できるという効果が極めて大きい。
【００１８】
［２］中温域作動型燃料電池の電解質および使用条件の検討
燃料電池の内部抵抗の大部分は電解質部分の抵抗によって占められており、この電解質部分の抵抗を低減するには導電率の高い電解質を選ばなければならない。これまでの燃料電池用電解質は、０．０１［Ｓ／ｃｍ］以上の導電率を示していることから、実用的な見地からすると、中温域作動型燃料電池用の電解質の条件として、▲１▼導電の媒体がプロトンであること、▲２▼導電率が少なくとも０．１［Ｓ／ｃｍ］以上であること、▲３▼燃料電池の運転時の状況下で安定に存在すること、▲４▼電池構成部材と反応せず、化学的に安定であること、が必要である。本発明者は電解質について種々の検討を行い、その結果、上記各条件を満たしうる好適な材質として、硫酸水素カリウムと硫酸水素ナトリウムの混合物を知見するに至った。
【００１９】
硫酸水素カリウムと硫酸水素ナトリウムの混合物、中でもそれらの共晶塩（Ｋ／Ｎａ）ＨＳＯ_４（ＫＨＳＯ_４：ＮａＨＳＯ_４＝４６．５：５３．５［ｍｏｌ％］）は、単独塩にはない特異な安定性を示し、高い導電率を有することが期待された。ここで混合物の融点について説明しておくと、ＫＨＳＯ_４−ＮａＨＳＯ_４の状態図から、両端の組成となる単独塩ＮａＨＳＯ_４の融点Ｔｍ＝１７８℃であり、ＫＨＳＯ_４の融点Ｔｍ＝２０７℃であり、共晶塩の融点はＴｍ＝１２５℃と一般的にこれらの値より落ち込む。従って、ＫＨＳＯ_４リッチな組成での融点は１２５〜２０７℃であり、ＮａＨＳＯ_４リッチな組成での融点は１２５〜１７８℃となる。なお、溶融したばかりの状態での混合物は高粘性であり、電極との馴染みも良くないので作動温度は２００℃以上であることが好ましい。
【００２０】
まず、水素雰囲気中における（Ｋ／Ｎａ）ＨＳＯ_４の導電率について測定した（図２参照）。ここでは、水蒸気を添加した場合とそうでない場合とを比較するため、湿潤水素雰囲気内での導電率と乾燥水素雰囲気内での導電率とを調べた。なお、図２（および図３）中における横軸は１０００を絶対温度［Ｋ］で除した値を示す。
【００２１】
図２から明らかなように、湿潤水素雰囲気内での（Ｋ／Ｎａ）ＨＳＯ_４の導電率は、測定温度範囲（１００〜５００℃）内で０．０１［Ｓ／ｃｍ］以上であって、１００℃以上で増大し、５００℃に至るまで高い導電率（最大で０．５７［Ｓ／ｃｍ］）を示すというように、安定しかつ２００〜５００℃で高い値を示した。また、この測定中において、単独塩に見られるような塩の分解固化は生じなかった。このため、この混合物は燃料電池の電解質に適すると考えられた。
【００２２】
また、湿潤水素雰囲気内での導電率と乾燥水素雰囲気内での導電率とを比較すると、この混合物を電解質として用いる場合には加湿されていることが好ましいことが知見された。このような加湿条件の中でも、室温で飽和するために十分な程度、具体的には約２．７％以上の水蒸気が添加された状態がより好ましい。乾燥雰囲気中で高温となると電解質が脱水反応によって分解し易くなるおそれがあるが、加湿することによってこのような脱水反応が起こるのを抑制することができる。
【００２３】
さらに、燃料電池の雰囲気による（Ｋ／Ｎａ）ＨＳＯ_４の導電率の違いについて測定した結果を図３に示す。図３では、酸化雰囲気（空気雰囲気）の場合と還元雰囲気（水素雰囲気）の場合とにおける導電率をそれぞれ示している。この図からわかるように、導電率の値は、約３００℃以上では、酸化雰囲気、還元雰囲気のいずれの雰囲気においても同程度であった。しかし、３００℃以下では、含有Ｈ_２Ｏの量により、異なる導電率となった。
【００２４】
以上のように、共晶塩（Ｋ／Ｎａ）ＨＳＯ_４は、燃料電池の雰囲気中において塩の安定性が良く、しかも導電率が高いことが判明した。また、この電解質の導電率と各種燃料電池における電解質の導電率とを比較すると図４に示すようになった。以上の検討と実験の結果、硫酸水素カリウムと硫酸水素ナトリウムの混合物（Ｋ／Ｎａ）ＨＳＯ_４、特にこれらの共晶塩は、他の燃料電池の電解質と比較して高い導電率を示し、中温域作動型燃料電池の電解質として十分適用可能であることが知見された。
【００２５】
また、上記の検討により中温域作動型燃料電池の電解質として硫酸水素カリウムナトリウム共晶塩（Ｋ／Ｎａ）ＨＳＯ_４が最も好適であることが知見されたが、この場合、厳密な意味での共晶塩のみならず、共晶組成がずれた単なる混合物の場合にも中温域作動型燃料電池の電解質として適用可能なことが知見され、さらには、硫酸水素カリウムと硫酸水素ナトリウムのいずれか一方を含む混合溶融塩によっても中温域作動型燃料電池の電解質として適用可能なことが知見された。
【００２６】
本発明はかかる知見に基づくものであり、請求項１記載の中温域作動型燃料電池は、電解質が硫酸水素カリウム（ＫＨＳＯ_４）および硫酸水素ナトリウム（ＮａＨＳＯ_４）の少なくとも一方を含む混合溶融塩であり、電解質の融点から６００℃までの間の温度域で作動することを特徴とするものである。この場合、上述したように、電解質としては硫酸水素カリウムナトリウム共晶塩（Ｋ／Ｎａ）ＨＳＯ_４が好ましいが、硫酸水素カリウム（ＫＨＳＯ_４）あるいは硫酸水素ナトリウム（ＮａＨＳＯ_４）のいずれか一方のみを含む混合溶融塩である場合にも中温域で作動しうる。また、現段階で具体例は明らかでないが、何らかの添加物を含ませることによって導電率や電極反応が向上する可能性がある。
【００２７】
また、請求項２記載の発明は、請求項１記載の中温域作動型燃料電池において、電解質に供給される供給ガスが、水素を含むか若しくは改質されて水素を発生する燃料ガスおよびアノード反応で直接プロトンを電解質に供給できるガスのいずれかを含む燃料ガスと、酸素を含む酸化剤ガスとであり、燃料ガスおよび酸化剤ガスの双方とも加湿されているものであることを特徴とするものである。この場合、水素を含むか若しくは改質されて水素を発生する燃料ガスとしては、例えば天然ガス改質ガス、バイオマスガス、廃棄物ガス化ガス、石炭ガス化ガスなどがある。また、燃料ガスおよび酸化剤ガスの双方ともに加湿されている状態とすれば、低温から高温に至るまで安定しかつ高い値の導電率を得ることができる。
【００２８】
［３］燃料電池を構成する構成部材の選定
さらに本発明者は燃料電池を構成するための部材についても検討したのでこれについて述べておく。
【００２９】
電池を構成するためには、液体の電解質を保持し、アノードおよびカソード両電極で挟み込んで単位ユニットのセルを構築する必要がある。この場合、すべての部材が電解質と電池反応以外では反応しないことが理想的であるため、部材を選定するにあたっては各部材と電解質との反応性を確認する必要がある。本発明者は、作動温度が近いＰＡＦＣとＭＣＦＣの各部材を用いた試験を行い、その結果をふまえて材料を選定することとした。具体的には、ＳｉＣ（多孔質）、Ｃ（多孔質）、Ｐｔ、Ａｕ、Ｃｕ、真鍮、Ａｌ、Ａｌ_２Ｏ_３、Ｎｉ、ＮｉＯおよびＳＵＳ材のそれぞれについて試験片（５×５　ｍｍ）を作成し、４００℃湿潤空気雰囲気内における２５ｇの（Ｋ／Ｎａ）ＨＳＯ_４中に各試料片を１２時間浸漬した結果を観察し、各試料と電解質との化学的安定性を確認した。その結果を表１に示す。
【表１】

【００３０】
ＮｉおよびＳＵＳ材については、若干気体を発生し、表面が黒い膜で覆われるという反応があることが観察された（図５参照）。ただし、これらの部材は、このように反応があるものの、表面に腐食膜が形成されるのに伴い反応が収まってくるので中温域作動型燃料電池の構成部材として適用することは可能である。また、（Ｋ／Ｎａ）ＨＳＯ_４との反応が全く認められず安定であったＳｉＣ、Ｃ、Ｐｔ、Ａｕ、Ｃｕ、真鍮、ＡｌおよびＡｌ_２Ｏ_３のそれぞれは、燃料電池を構成するための部材として相応しい。具体例を挙げれば、電極にはＰｔ担持カーボン、集電極にはＡｌまたはＣ、電解質板にはＳｉＣ（多孔質）またはＡｌ_２Ｏ_３を用いることができる。なお、電池製造コスト低減の観点からすれば、低廉なＡｌ、Ａｌ_２Ｏ_３、真鍮が構成部材として好ましい。
【００３１】
【発明の実施の形態】
以下、本発明の構成を図面に示す実施の形態の一例に基づいて説明する。
【００３２】
図６に、本発明の一実施形態を示す。この中温域作動型燃料電池１においては、電解質として硫酸水素カリウム（ＫＨＳＯ_４）と硫酸水素ナトリウム（ＮａＨＳＯ_４）の共晶塩が用いられ、この共晶塩を保持する多孔体としてＡｌ_２Ｏ_３が用いられ、これらによって電解質板２が形成されている。また電極材には、アノード３としてＣｕまたはＣｕ系合金の多孔体が、カソード４としてＬｉＮｉＯｘ多孔体がそれぞれ用いられ、電解質板２を挟み込むように設けられている。また、電極材５には真鍮を材質とする集電波板（または集電波網）が取り付けられ、ハウジング６によってアノード室とカソード室が構成されることにより、燃料ガスおよび酸化剤ガスが個別に流通することが可能となっている。なお、多孔体としてはＡｌ_２Ｏ_３の代わりにＳｉＣを用いてもよい。
【００３３】
なお、上述の実施形態は本発明の好適な実施の一例ではあるがこれに限定されるものではなく本発明の要旨を逸脱しない範囲において種々変形実施可能である。
【００３４】
【発明の効果】
以上の説明より明らかなように、請求項１記載の発明によると、既存の燃料電池では作動しえなかった中温域で作動するプロトン移動型の燃料電池を実現したことにより、例えばＭＣＦＣと同等の出力電圧が得られるという高効率な燃料電池を提供することが可能となる。また、この中温域作動型燃料電池によれば、低温型燃料電池（ＰＥＦＣ、ＰＡＦＣ）のデメリットである高価な電極触媒を低減することが可能となり、更に、高温型燃料電池（ＭＣＦＣ、ＳＯＦＣ）のデメリットである作動温度による電池構成材料の制約を緩和することも可能となる。
【００３５】
また請求項２記載の発明によると、燃料ガスおよび酸化剤ガスの双方ともに加湿されている状態とすることにより、電解質が脱水反応によって分解するのを抑制し、低温から高温に至るまで安定しかつ高い値の導電率を得ることができる。
【図面の簡単な説明】
【図１】本発明に係る中温域作動型燃料電池の出力電圧や抵抗等についてＰＡＦＣおよびＭＣＦＣと比較した結果を示すグラフである。
【図２】水素雰囲気中における（Ｋ／Ｎａ）ＨＳＯ_４の導電率の測定結果を示すグラフである。
【図３】湿潤雰囲気中における（Ｋ／Ｎａ）ＨＳＯ_４の導電率の測定結果を示すグラフである。
【図４】（Ｋ／Ｎａ）ＨＳＯ_４と各種燃料電池用電解質の導電率の比較を示すグラフである。
【図５】ＳＵＳ材のＳＥＭ写真であり、（Ａ）は（Ｋ／Ｎａ）ＨＳＯ_４と反応する前、（Ｂ）は反応した後である。
【図６】本発明に係る中温域作動型燃料電池の構成例を示す概略図である。
【符号の説明】
１　中温域作動型燃料電池[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fuel cell operating at a middle temperature range. More specifically, the present invention relates to a fuel cell that can operate in a medium temperature range where existing fuel cells do not operate.
[0002]
[Prior art]
Fuel cells are high-efficiency power generators, depending on the type of electrolyte, polymer electrolyte (PEFC), phosphoric acid (PAFC), molten carbonate (MCFC), and solid oxide (SOFC). Types exist. The operating temperature of each fuel cell varies, for example, about 80 ° C. for PEFC and about 1000 ° C. for SOFC.
[0003]
In recent years, researches aimed at increasing the operating temperature of PEFC (about 80 ° C.) to 100 ° C. or higher at which water vapor does not condense, specifically 100 to 200 ° C., have been actively conducted (for example, Patent Document 1). reference). This can simplify the equipment for managing water vapor by raising the temperature to prevent condensation of water in the gas path, and furthermore, by activating the electrode reaction, an expensive platinum catalyst can be obtained. This is because it can reduce.
[0004]
On the other hand, researches aimed at lowering the operating temperature to about 600 to 800 ° C. in order to ease the use conditions of the SOFC have been actively conducted (for example, see Patent Document 2). The reason for this is that the lower the operating temperature, the wider the permissible range of use of the metal member, and the cheaper the metal member such as SUS material can be used.
[0005]
As exemplified above, when the fuel cell operates in the middle temperature range, that is, a temperature range higher than the operating temperature of the PEFC and lower than the operating temperature of the SOFC, there are great merits in terms of performance and materials. Therefore, various studies have been conducted for the purpose of increasing the operating temperature of the PEFC or the operating temperature of the SOFC.
[0006]
[Patent Document 1]
JP 2001-43879 A [Patent Document 2]
JP 10-228920 A
[Problems to be solved by the invention]
However, in practice, the operating temperature of existing fuel cells is up to 100 ° C. for PEFC, up to 200 ° C. for PAFC, and about 650 ° C. for MCFC. At present, there is no fuel cell operating in a temperature range of about 200 ° C. to 600 ° C.
[0008]
In addition, although there are mainly four types of fuel cells as described above, research and development is currently being conducted for each type, and there is almost no technical exchange between different types of fuel cells. It is in. For this reason, for example, an operating temperature of 100 ° C. to 200 ° C. is targeted for PEFC, and an operating temperature of 600 ° C. to 800 ° C. is targeted for SOFC. There is a gap in the definition of the word "temperature range" or "medium temperature range". Under these circumstances, the research currently being conducted is to gradually expand the operable temperature range such as lowering the temperature of the PEFC or lowering the temperature of the SOFC. There is no research and development on fuel cells that can operate in a completely new temperature range.
[0009]
Therefore, an object of the present invention is to provide a medium temperature range type fuel cell that can operate in a medium temperature range where existing fuel cells cannot operate.
[0010]
[Means for Solving the Problems]
In order to achieve this object, the present inventor has determined that a performance evaluation method for comparing the characteristics and the highest performance of these fuel cells with the same analytical evaluation method has been established at the Central Research Institute of Electric Power Industry of the applicant. We paid attention to. Based on this performance evaluation method, the temperature dependence of the output voltage of various fuel cells was generalized, and the performance of the proton transfer type (Nernstros was 10 mV) and the oxide ion transfer type (Nernstros was 120 mV) was predicted. Of a proton-transfer fuel cell with a small value, it is possible to achieve performance comparable to a high-temperature fuel cell (specifically, an oxide ion-transfer fuel cell such as SOFC and MCFC) even in a medium temperature range (for example, 200 ° C. to 600 ° C.). Came to know that there is. Such examination and performance evaluation
[1] Study on the effect of the fuel cell operating in the middle temperature range [2] Study on the electrolyte and operating conditions of the fuel cell operating in the middle temperature range [3] Selection of components constituting the fuel cell was performed for each item. The details of the examination and the performance evaluation will be described in detail below.
[0011]
[1] Examination of the effect of the fuel cell operating in the middle temperature range At the Central Research Institute of Electric Power Industry of the present applicant, attempts have already been made to grasp the performance of the fuel cell in general, and the reaction at the anode and cathode of fuel cells in general. An equation for deriving the overvoltage η _{a + c} has been obtained (Equation 1).
(Equation 1)

Here, i in the equation represents current density [mA / cm ² ], P represents gas partial pressure [−], and T represents operating temperature [K].
[0012]
Equations 2 and 3 can be used to derive the reaction overvoltage η _{a + c} of the PAFC and the MCFC. These are equations derived from the currently published representative battery performance.
(Equation 2)

[Equation 3]

Here, m in the formula represents a mole fraction [-].
[0013]
Further, the electromotive force E of each fuel cell can be calculated from each operating temperature T and gas partial pressure by using Expression 4.
(Equation 4)

Here, E ₀ is the standard electromotive force is a function of temperature, decreases as temperature increases. R is a gas constant and F is a Faraday constant.
[0014]
Therefore, the cell voltage V [V] representing the battery performance can be represented by a mathematical formula.
(Equation 5)

Here, η _NE represents Nernstros.
[0015]
The operating temperatures of PAFC and MCFC were 200 ° C. and 650 ° C., and the operating temperature of the medium-temperature proton transfer fuel cell was 300 ° C. The internal resistance iR of the PAFC and the MCFC is 0.283i and 0.215i, respectively, according to the results derived by the Central Research Institute of Electric Power Industry. Further, Nernstros was set to 10 [mV] for the proton transfer type PAFC and 120 [mV] for the oxide ion transfer type MCFC. Then, the performance of each fuel cell was compared with that of the intermediate-temperature operation type fuel cell (see FIG. 1). In the figure, the fuel cell of the middle temperature range operation type is shown as MAFC (Molten Acid sulfate Fuel Cell). In the fuel cell of the intermediate temperature range operation type, the members other than the electrolyte were made the same as the PAFC, and the difference in performance due only to the difference in the electrolyte was confirmed. Nernstros was set to 10 [mV] because the electrolyte was of a proton transfer type. It was thought that the internal resistance of the medium temperature operation type fuel cell (MAFC) was higher than that of the PAFC due to contact and corrosion depending on the operation temperature. (For example, 0.1 [S / cm] or more) was used, so it was assumed to be 2/3 of PAFC.
[0016]
Here, when the PAFC and the MCFC are compared first, it can be seen that the output voltage of the high-temperature type MCFC is much higher than that of the low-temperature type PAFC. Although there is a decrease in the open circuit voltage due to the increase in temperature, it can be seen that the voltage was greatly improved due to a decrease in the cathode reaction resistance. However, in the case of a high-temperature type MCFC, the open-circuit voltage decreases thermodynamically as the temperature rises, and the Nernstroth is large due to the oxide ion type in which water vapor is generated on the anode side (see FIG. 4). ), It was found that the performance improvement was suppressed.
[0017]
On the other hand, the medium-temperature operation type fuel cell is similar to the high-temperature type MCFC in a medium temperature range of 200 ° C. to 600 ° C. where there are few restrictions on materials by using a proton transfer fuel cell having a small Nernstroth. In fact, as shown in FIG. 1, a significant voltage improvement was observed by operating at 300 ° C. as compared with PAFC operating at 200 ° C., for example, as shown in FIG. As described above, the proton transfer fuel cell that operates in the middle temperature range of 200 ° C. to 600 ° C. has many advantages in terms of performance. In addition, in the middle temperature range, there is no need to use an expensive high-temperature corrosion-resistant material as a constituent material of the battery, so that the effect of reducing the manufacturing cost is extremely large.
[0018]
[2] Examination of the electrolyte and operating conditions of the fuel cell operated at a middle temperature range Most of the internal resistance of the fuel cell is occupied by the resistance of the electrolyte part, and in order to reduce the resistance of the electrolyte part, an electrolyte having a high conductivity is used. Must be chosen. Conventional fuel cell electrolytes have a conductivity of 0.01 [S / cm] or more. Therefore, from a practical point of view, the condition of the electrolyte for a fuel cell operating at a middle temperature range is as follows. (2) The conductive medium is proton, (2) the conductivity is at least 0.1 [S / cm] or more, (3) the fuel cell is stable under the operating conditions of the fuel cell, and (4) ▼ It must be chemically stable without reacting with battery components. The present inventors have conducted various studies on electrolytes and, as a result, have come to know a mixture of potassium hydrogen sulfate and sodium hydrogen sulfate as a suitable material that can satisfy the above conditions.
[0019]
Mixtures of potassium hydrogen sulfate and sodium hydrogen sulfate, especially their eutectic salt (K / Na) HSO ₄ (KHSO ₄ : NaHSO ₄ = 46.5: 53.5 [mol%]), are unique to single salts It was expected to exhibit excellent stability and to have high electrical conductivity. If you leave describes the melting point of the mixture _here, from the state diagram of KHSO 4 -NaHSO _4, has a melting point Tm = 178 ° C. sole salt NaHSO ₄ having a composition of both ends, be a melting point Tm = 207 ° C. of KHSO ₄ The melting point of the eutectic salt is Tm = 125 ° C., which is generally lower than these values. Therefore, the melting point of the composition rich in KHSO ₄ is 125 to 207 ° C., and the melting point of the composition rich in NaHSO ₄ is 125 to 178 ° C. It is to be noted that the operating temperature is preferably 200 ° C. or higher because the mixture in a state just melted has a high viscosity and is not well compatible with the electrodes.
[0020]
First, the conductivity of (K / Na) HSO ₄ in a hydrogen atmosphere was measured (see FIG. 2). Here, in order to compare the case where steam was added and the case where steam was not added, the conductivity in a wet hydrogen atmosphere and the conductivity in a dry hydrogen atmosphere were examined. The horizontal axis in FIG. 2 (and FIG. 3) indicates a value obtained by dividing 1000 by the absolute temperature [K].
[0021]
As is clear from FIG. 2, the conductivity of (K / Na) HSO ₄ in a wet hydrogen atmosphere is 0.01 [S / cm] or more in the measurement temperature range (100 to 500 ° C.) It was stable and showed a high value at 200 to 500 ° C. such that it increased at 100 ° C. or more and showed high conductivity (up to 0.57 [S / cm]) up to 500 ° C. Further, during this measurement, decomposition and solidification of the salt as observed in the single salt did not occur. For this reason, this mixture was considered suitable for the electrolyte of the fuel cell.
[0022]
Further, when comparing the conductivity in a wet hydrogen atmosphere with the conductivity in a dry hydrogen atmosphere, it was found that when this mixture is used as an electrolyte, it is preferable that the mixture be humidified. Among such humidification conditions, a state in which water vapor is added to a degree sufficient to saturate at room temperature, specifically, about 2.7% or more is more preferable. When the temperature becomes high in a dry atmosphere, the electrolyte may be easily decomposed by a dehydration reaction. However, such a dehydration reaction can be suppressed by humidification.
[0023]
FIG. 3 shows the results of measuring the difference in the conductivity of (K / Na) HSO ₄ depending on the atmosphere of the fuel cell. FIG. 3 shows the conductivity in an oxidizing atmosphere (air atmosphere) and in a reducing atmosphere (hydrogen atmosphere), respectively. As can be seen from the figure, the value of the electrical conductivity was approximately the same in both the oxidizing atmosphere and the reducing atmosphere at about 300 ° C. or higher. However, below 300 ° C., the conductivity became different depending on the amount of H ₂ O contained.
[0024]
As described above, it was found that eutectic salt (K / Na) HSO ₄ has good salt stability and high conductivity in the atmosphere of a fuel cell. FIG. 4 shows the comparison between the conductivity of the electrolyte and the conductivity of the electrolyte in various fuel cells. As a result of the above examinations and experiments, a mixture of potassium hydrogen sulfate and sodium hydrogen sulfate (K / Na) HSO ₄ , particularly a eutectic salt thereof, showed higher conductivity than other fuel cell electrolytes, It has been found that it is sufficiently applicable as an electrolyte for a zone-operated fuel cell.
[0025]
In addition, it has been found from the above study that potassium sodium hydrogen sulfate eutectic salt (K / Na) HSO ₄ is most suitable as an electrolyte for a medium temperature operation type fuel cell. It has been found that not only the crystal salt but also a mere mixture having a shifted eutectic composition can be applied as an electrolyte for a medium temperature operation type fuel cell, and furthermore, either one of potassium hydrogen sulfate and sodium hydrogen sulfate is used. It has been found that the mixed molten salt can also be used as an electrolyte for a fuel cell operated at a medium temperature range.
[0026]
The present invention is based on such knowledge, and the fuel cell according to claim 1 is a mixed molten salt in which an electrolyte contains at least one of potassium hydrogen sulfate (KHSO ₄ ) and sodium hydrogen sulfate (NaHSO ₄ ). It operates in a temperature range between the melting point of the electrolyte and 600 ° C. In this case, as described above, as the electrolyte, potassium sodium hydrogen sulfate eutectic salt (K / Na) HSO ₄ is preferable, but only one of potassium hydrogen sulfate (KHSO ₄ ) and sodium hydrogen sulfate (NaHSO ₄ ) is used. In the case of a mixed molten salt containing, it can operate in a medium temperature range. Although a specific example is not clear at this stage, the conductivity and the electrode reaction may be improved by adding some additives.
[0027]
According to a second aspect of the present invention, in the fuel cell of the intermediate temperature range operation type according to the first aspect, the supply gas supplied to the electrolyte contains or reforms a fuel gas containing hydrogen or an anode reaction. A fuel gas containing any of the gases capable of directly supplying protons to the electrolyte and an oxidizing gas containing oxygen, wherein both the fuel gas and the oxidizing gas are humidified. It is. In this case, examples of the fuel gas containing hydrogen or being reformed to generate hydrogen include a natural gas reformed gas, a biomass gas, a waste gasified gas, and a coal gasified gas. If both the fuel gas and the oxidizing gas are humidified, stable and high conductivity can be obtained from low to high temperatures.
[0028]
[3] Selection of components constituting fuel cell Further, the present inventor has examined members for constituting the fuel cell, and will be described here.
[0029]
In order to construct a battery, it is necessary to hold a liquid electrolyte and to sandwich a cell between the anode and the cathode to construct a unit cell. In this case, it is ideal that all members do not react with the electrolyte except for the battery reaction. Therefore, when selecting members, it is necessary to confirm the reactivity between each member and the electrolyte. The inventor conducted a test using each member of the PAFC and the MCFC whose operating temperatures are close to each other, and decided to select a material based on the results. Specifically, test specimens (5 × 5 mm) were prepared for each of SiC (porous), C (porous), Pt, Au, Cu, brass, Al, Al ₂ O ₃ , Ni, NiO and SUS materials. Each sample was immersed in 25 g of (K / Na) HSO ₄ in a 400 ° C. humid air atmosphere for 12 hours, and the result was observed to confirm the chemical stability of each sample and the electrolyte. Table 1 shows the results.
[Table 1]

[0030]
Regarding Ni and SUS materials, it was observed that there was a reaction that a slight gas was generated and the surface was covered with a black film (see FIG. 5). However, although these members have a reaction as described above, since the reaction stops as a corrosive film is formed on the surface, the members can be applied as constituent members of a fuel cell of a medium temperature range operation type. In addition, each of SiC, C, Pt, Au, Cu, brass, Al and Al ₂ O ₃ , which was stable without any reaction with (K / Na) HSO ₄ , was used to constitute a fuel cell. Suitable as a member. For example, Pt-supported carbon can be used for the electrode, Al or C can be used for the collector, and SiC (porous) or Al ₂ O ₃ can be used for the electrolyte plate. From the viewpoint of reducing the battery manufacturing cost, inexpensive Al, Al ₂ O ₃ , and brass are preferable as constituent members.
[0031]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the configuration of the present invention will be described based on an example of an embodiment shown in the drawings.
[0032]
FIG. 6 shows an embodiment of the present invention. In this intermediate temperature operation type fuel cell 1, a eutectic salt of potassium hydrogen sulfate (KHSO ₄ ) and sodium hydrogen sulfate (NaHSO ₄ ) is used as an electrolyte, and Al ₂ O ₃ is used as a porous body holding the eutectic salt. These are used to form the electrolyte plate 2. In the electrode material, a porous body of Cu or a Cu-based alloy is used as the anode 3 and a LiNiOx porous body is used as the cathode 4, and is provided so as to sandwich the electrolyte plate 2. A collecting plate (or a collecting network) made of brass is attached to the electrode member 5, and the anode chamber and the cathode chamber are constituted by the housing 6, so that the fuel gas and the oxidizing gas flow individually. It is possible to do. Note that SiC may be used instead of Al ₂ O ₃ as the porous body.
[0033]
The above embodiment is an example of a preferred embodiment of the present invention, but the present invention is not limited to this, and various modifications can be made without departing from the gist of the present invention.
[0034]
【The invention's effect】
As is apparent from the above description, according to the first aspect of the present invention, a proton transfer fuel cell that operates in a medium temperature range, which cannot operate with an existing fuel cell, is realized. It is possible to provide a highly efficient fuel cell capable of obtaining an output voltage. According to the fuel cell of the middle temperature range operation type, it is possible to reduce expensive electrode catalyst which is a disadvantage of the low temperature fuel cell (PEFC, PAFC), and further, it is possible to reduce the temperature of the high temperature fuel cell (MCFC, SOFC). It is also possible to alleviate the disadvantages of the battery constituent materials due to the operating temperature.
[0035]
According to the second aspect of the present invention, since both the fuel gas and the oxidizing gas are humidified, the decomposition of the electrolyte by the dehydration reaction is suppressed, and the electrolyte is stable from a low temperature to a high temperature, and High values of conductivity can be obtained.
[Brief description of the drawings]
FIG. 1 is a graph showing the results of comparing the output voltage, resistance, and the like of a fuel cell of the intermediate temperature range operation type according to the present invention with those of PAFC and MCFC.
FIG. 2 is a graph showing the measurement results of the conductivity of (K / Na) HSO ₄ in a hydrogen atmosphere.
FIG. 3 is a graph showing the measurement results of the conductivity of (K / Na) HSO ₄ in a humid atmosphere.
FIG. 4 is a graph showing a comparison of conductivity between (K / Na) HSO ₄ and various fuel cell electrolytes.
FIG. 5 is a SEM photograph of a SUS material, (A) before reacting with (K / Na) HSO ₄ and (B) after reacting.
FIG. 6 is a schematic diagram showing a configuration example of a fuel cell of the intermediate temperature range operation type according to the present invention.
[Explanation of symbols]
1 Medium temperature operation type fuel cell

Claims (2)

The electrolyte is a mixed molten salt containing at least one of potassium hydrogen sulfate (KHSO ₄ ) and sodium hydrogen sulfate (NaHSO ₄ ), and operates in a temperature range from the melting point of the electrolyte to 600 ° C. Range-operated fuel cell. The supply gas supplied to the electrolyte is a fuel gas containing any of hydrogen-containing or reformed hydrogen gas and a gas capable of directly supplying protons to the electrolyte by an anode reaction. 2. The fuel cell according to claim 1, wherein the fuel gas and the oxidizing gas are both humidified.

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