JP4155745B2

JP4155745B2 - Operating method of redox flow battery

Info

Publication number: JP4155745B2
Application number: JP2002052299A
Authority: JP
Inventors: 洋成出口; 誠司荻野; 信幸徳田
Original assignee: Kansai Electric Power Co Inc; Sumitomo Electric Industries Ltd
Current assignee: Kansai Electric Power Co Inc; Sumitomo Electric Industries Ltd
Priority date: 2002-02-27
Filing date: 2002-02-27
Publication date: 2008-09-24
Anticipated expiration: 2022-02-27
Also published as: JP2003257467A

Description

【０００１】
【発明の属する技術分野】
本発明は、レドックスフロー電池の運転方法に関するものである。特に、電池効率がより高く、ガス発生量が少ないレドックスフロー電池の運転方法に関するものである。
【０００２】
【従来の技術】
負荷平準化用途や瞬低・停電対策用途などにレドックスフロー電池を利用することが提案されている。
【０００３】
特に、バナジウムレドックスフロー電池は、▲１▼起電力が高く、▲２▼エネルギー密度が大きく、▲３▼電解液が単一元素系であるため正極電解液と負極電解液とが混合しても充電によって再生することができると言った多くの利点を有している。
【０００４】
このようなバナジウムレドックスフロー電池は、充放電を繰り返すと隔膜を通して電解液中の各種イオンや溶媒が移動し、正極及び負極の電解液量の増減が起こる。例えば、アニオン隔膜を用いた場合、通常、正極側から負極側へ液移りが起こり、電解液量がアンバランスになることで一方の電気容量が著しく低下することになる。
【０００５】
このような液移りに伴う問題を解消するため、従来、一定回数の充放電サイクルごとに正極電解液と負極電解液とを連通あるいは混合して液量調整を行っている。この液量調整に関する従来の技術としては、特開2001-167787号公報、実開平4-124754号公報、特開平11-204124号公報や、特開平2-195657号公報に記載のものが知られている。
【０００６】
例えば、特開2001-167787号公報に記載の技術では、液量が増加したタンクから液量が減少したタンクに配管を通して電解液を戻すことで、各タンクの液量変化を一定の範囲に保持している。
【０００７】
【発明が解決しようとする課題】
しかし、従来は、具体的にどの程度の液量変化でどの程度の液量を移動させるかについて詳しく検討されていなかった。そのため、従来の技術では、例えば、電池効率をより向上させることが困難である。
【０００８】
また、特開2001−167787号公報に記載の技術では、充放電前の設定条件として、充放電サイクルの繰り返しにより液量が減少するタンクの液量を予め他方のタンクの液量よりも多くした場合を主に評価している。しかし、液量が減少するタンクの液量を予め他方のタンクの液量よりも多くすると、充放電に利用されない電解液を余分に含むことになり、コストアップとなる。
【０００９】
更に、従来は、電池評価として必要な発生ガスに関する評価をほとんど実施していない。充放電の副反応として、ガスが発生する。しかし、上記従来の技術では、発生ガスの種類や発生量について明確な知見が得られていない。そのため、電池の評価項目を効率(電力、電圧、電流)と放電電力量(初期状態との比較)のみとしており、十分な電池評価が行われていなかった。
【００１０】
そこで、本発明の主目的は、ガス発生量が少なく電池効率により優れ、かつより低コストであるレドックスフロー電池の運転方法を提供することにある。
【００１１】
【課題を解決するための手段】
本発明は、充放電の開始時において正負両極の電解液貯蔵用タンクに貯蔵する電解液量を同量とし、かつ充放電サイクルの繰り返しに伴う電解液の液量変化の割合が規定の範囲を超える前に両タンクの液量を同等にすることで、上記の目的を達成する。
【００１２】
即ち、本発明は、隔膜で分離される正極及び負極と、前記正極に循環供給される正極電解液を貯蔵する正極電解液貯蔵用タンクと、前記負極に循環供給される負極電解液を貯蔵する負極電解液貯蔵用タンクとを具えるレドックスフロー電池の運転方法である。前記各タンクには、充放電の開始時において電解液を同量貯蔵する。そして、充放電サイクルの繰り返しに伴い隔膜を通して移動した電解液の液量変化の割合が−15％以上+17％以下の範囲内にあるときに各タンクの電解液を同量に戻す。特に、液量変化の割合が0％超+5％以下の範囲内にあるときに電解液を同量に戻すことが最適である。本発明において、電解液の液量変化の割合とは、充放電の開始時における両電解液の和を2で割った平均液量に対する正極電解液量から負極電解液量を引いた液量差の割合である。
【００１３】
上記構成を具える本発明は、充放電の開始時、即ち設定条件において各タンクに貯蔵する電解液を同量にすることで、コストの低減を図る。また、電解液の液量変化の割合を規定し、この規定の範囲を超える前に両タンクの電解液量を等しくするべく、液移りにより増加した液量分を減少した側のタンクに移動させることで、ガスの発生をより少なくし、電池効率をより向上させる。
【００１４】
本発明において電解液の液量変化の割合を規定し、この規定の範囲を超える前に両タンクの電解液量を同量にする理由を説明する。
従来は、充放電の開始時において両タンクの電解液量を同量にして充放電を行い、作業効率上、一定時間の充放電毎に液量調整を行うことが多かった。この液量調整の頻度は、せいぜい1日1回程度であり、どれだけの液量変化でどの程度の量を移動させれば電池効率をより向上させられるかについて、明確な指針がなかった。また、従来の技術として、一方の送液圧力を大きくし、他方の送液圧力を小さくすることで、両タンクの液量を常に同量に保つ技術がある。即ち、この技術は、一方のセルに流される電解液流量を他方のセルに流される電解液流量よりも小さくして流量差をつけるものである。しかし、セルの耐圧制限のため、特にセルの面積が大きい場合、流量を小さくせざるを得ないことがある。すると、流量を小さくすることでエネルギー密度が小さくなり、結果として電池性能が低下することがある。そこで、本発明者らが検討した結果、液量変化が一定の範囲を超える前に、液が増加したタンクから液が減少したタンクに、増加した液だけ戻して両タンクの液量を同量にすれば、より優れた電池効率が得られると共に、ガスの発生量が比較的少ないことを見出した。特に、この場合、流量を調整して各タンクの液量を常に同量に維持して運転する場合よりもむしろ電池効率、エネルギー密度がよいことも見出した。
【００１５】
また、充放電サイクルの繰り返しに伴う各タンクの液量の増減は、隔膜の種類及び正負極への送液圧力差で決まる。しかし、特開平2001-167787号公報のように隔膜の種類だけで液量が増減すると認識している場合、液量の増減を明確に把握できない。
【００１６】
上記事項及び知見に基づき、本発明は、各タンクの電解液量を同量とした設定条件において、電解液の液量変化の割合が規定の範囲に達してそれを超える前に各タンクの電解液量を同量に戻す。
【００１７】
次に、本発明において、ガス発生をより少なくする理由を説明する。バナジウムレドックスフロー電池は、充放電の副反応としてガスが発生する。例えば、正極活物質を充電しすぎると、副反応として水の分解反応により酸素が発生し、電極の酸化劣化を招き、ひいては電圧効率が低下する。
【００１８】
また、本発明者らは、負極から水素、正極から二酸化炭素などのガスが発生するとの知見も得た。水素であれば最悪の場合、発火、爆発の可能性もある。二酸化炭素であれば電極が分解しており、電池効率低下としてあらわれる前に劣化が進行していることになる。
【００１９】
更に、このようにガスが発生して長期的に蓄積すると、タンク耐圧に問題が生じる。そのため、ガスの発生をより少なくすることが好ましい。
【００２０】
本発明において電解液量を調整する方法は、公知の技術を適用すればよい。例えば、両タンクを連通管で連結し、連通管の両端を各タンクに貯蔵する電解液の液面より上の位置で連結したり(実開平4-124754号公報参照)、連通管の両端を電解液の液面より下の位置で連結したり(特開平11-204124号公報参照)、少なくとも一方の送液圧力を調整することにより隔膜を通して電解液を逆向きに移動させたりする(特開平2-195657号公報参照)とよい。また、本発明において電解液を同量に戻すには、例えば、両タンクを連通管で連結しておき、一方のタンクから他方のタンクに電解液が移動できるよう構成することが好ましい。
【００２１】
【発明の実施の形態】
以下、本発明の実施の形態を説明する。
充放電の開始時において、正極電解液量と負極電解液量とを同量としたレドックスフロー電池の充放電を行い、電池効率、液エネルギー密度、ガス発生量を測定した。
【００２２】
本試験に用いたレドックスフロー電池の概要を図1に示す。この電池は、イオンが通過できる隔膜4で正極セル1Aと負極セル1Bとに分離されたセル1を具える。正極セル1Aと負極セル1Bの各々には正極電極5と負極電極6とを内蔵している。正極セル1Aには、正極電解液を供給及び排出する正極電解液貯蔵用タンク2が導管7、8を介して接続されている。同様に負極セル1Bには、負極電解液を供給及び排出する負極電解液貯蔵用タンク3が導管10、11を介して接続されている。各電解液は、バナジウムイオンなどの価数が変化するイオンの水溶液を用い、ポンプ9、12で循環させ、正極電極5及び負極電極6におけるイオンの価数変化反応に伴って充放電を行う。
【００２３】
本試験において電解液を移動させて両タンクの電解液量を同量にする方法を説明する。まず、両極電解液貯蔵用タンクには、同量の電解液を貯蔵しておく。各タンクの側壁には、液位センサが設けてあり、タンク内の電解液量を把握することができる。この両タンクをポンプを介して連通管で連結する。そして、充放電に伴い、各タンクの電解液量が表1に示す量に達したら、この連通管を介して充放電に伴って増加した量だけを一方のタンクから他方のタンクにポンプを利用して移動させ、両タンクの電解液量を同量に調整する。表1に示す両極電解液の量(l)、及び電解液の液量変化の割合は、同量に調整する前に測定したものである。次に、本試験において電解液量の調整方法を説明する。本試験では、図1に示すポンプ12を調整して、電解液の送液圧力を調整することで液量を変化させた。具体的には、いずれの試料も正極電解液の流量を7ｌ/min、送液圧力を0.6×10⁵Paと等しくし、負極電解液の流量、送液圧力を異ならせた。正極側の電解液を増加させる試料No1〜4は、負極電解液の流量を14ｌ/min、送液圧力を1.4×10⁵Paとし、各極の液量が表1に示す量となるまで変化させた。負極側の電解液を増加させる試料No6〜8は、負極電解液の流量を7ｌ/min、送液圧力を0.7×10⁵Paとし、同様に各極の液量が表1に示す量となるまで変化させた。両極の電解液を等量に維持する試料5は、負極電解液の流量を10ｌ/min、送液圧力を1.0×10⁵Paとした。
【００２４】
【表１】

【００２５】
試験条件を以下に示す。
(電池仕様)
電極の反応面積：1000cm²×10セル
隔膜：陰イオン(アニオン)交換膜
正極電解液：4価のバナジウムイオン1.7mol/l 硫酸2.6mol/lの電解液25l
負極電解液：3価のバナジウムイオン1.7mol/l 硫酸2.6mol/lの電解液25l
【００２６】
(本試験における電解液量)
上記正極電解液と上記負極電解液とを混合して、価数バランス3.5価の電解液を50l用意する。この混合電解液を正極電解液貯蔵用タンクと負極電解液貯蔵用タンクとにそれぞれ同量25l入れる。
【００２７】
(充電方法)
電流密度100mA/cm²で定電流充電をはじめ、次に上限充電電圧1.60V/セルの条件で開放電圧1.55V/セルになるまで定電圧充電を行う。
【００２８】
(放電方法)
電流密度100mA/cm²で定電流放電を行う。下限放電電圧1.0V/セルに達したところで放電を終了する。
【００２９】
(評価方法)
上記仕様の電池を用いて、充放電開始後、各タンクの液量が表1に示す液量に変化した時点で両タンクの電解液量を同量に戻すという操作を繰り返す。このような充放電と液量調整とを繰り返す運転を1週間連続して行う。1週間の前後で、電池効率と液エネルギー密度を測定する。1週間連続充放電終了後、ガス(水素及び二酸化炭素)の発生量を測定する。
【００３０】
電池効率は、｛放電電圧(V)×放電電流(A)×放電時間(h)｝／｛充電電圧(V)×充電電流(A)×充電時間(h)｝で表される。
【００３１】
ガスは正極で二酸化炭素の発生を、負極で水素の発生量を測定した。ガス分析はガスクロマトグラフィー法によって行った。
【００３２】
試験結果を表2に示す。また、本試験における電池効率の低下量の評価基準を表3に、同液エネルギー密度低下量の評価基準を表4に、同ガス発生量の評価基準を表5に示す。更に、電解液の液量変化の割合を示す(正極電解液量-負極電解液量)/｛(正極電解液量+負極電解液量)/2｝と電池効率の関係、及び(正極電解液量-負極電解液量)/｛(正極電解液量+負極電解液量)/2｝と液エネルギー密度の関係を図2のグラフに示す。
【００３３】
【表２】

【００３４】
【表３】

【００３５】
【表４】

【００３６】
【表５】

【００３７】
表2及び図2のグラフから明らかなように、(正極電解液量-負極電解液量)/｛(正極電解液量+負極電解液量)/2｝が-0.15未満(-15％未満)、又は+0.17超(+17％超)となる前に両タンクの電解液の量を同量に戻す操作を行った試料No3、4及び6は、電池効率及び液エネルギー密度が高く、ガスの発生量も少ないことが分かる。特に、(正極電解液量-負極電解液量)/｛(正極電解液量+負極電解液量)/2｝が0超から+0.05以下(0％超から+5％以下)を超える前に各タンクの電解液を同量に戻した試料No4は、電池効率及び液エネルギー密度がより高い値を示し、ガスの発生量も極めて少ない。また、従来は、電解液を十分に流し、液量を同量に保つように運転する場合、即ち(正極電解液量-負極電解液量)/｛(正極電解液量+負極電解液量)/2｝が0となるように運転する場合(試料No5)、電池効率が最も優れていると考えられていた。しかし、液量変化が0(0％)よりも大きい範囲、より具体的には0(0％)超から+0.05(+5％)以下の範囲内で両タンクの液量を同量に戻す操作を行う試料No4の方が好ましい結果が得られることが分かった。また、液量を同量に戻す操作を行う試料No4は、この操作を行わない試料No5と比較してエネルギー密度も優れていた。
【００３８】
本例において、電解液を移動させて両極の電解液量を同量に調整する機構を具体的に説明する。本例では、連通管にポンプを設置して正負極各タンクのどちらからでも電解液を移動させることが可能な機構を用いた。図3は、各タンクを連通管で連結した状態を示す模式図である。正極電解液貯蔵用タンク20及び負極電解液貯蔵用タンク21とは2本の連通管22及び23とで連結され、連通管22及び23は、ポンプ24を具える接続管25で連結されている。
【００３９】
この場合、正極電解液貯蔵用タンク20から負極電解液貯蔵用タンク21に電解液を移動させるには、バルブ30及びバルブ31を開き、バルブ32及びバルブ33を閉じ、適宜ポンプ24を用いて行うとよい。一方、負極電解液貯蔵用タンク21から正極電解液貯蔵用タンク20に電解液を移動させるには、バルブ32及びバルブ33を開き、バルブ30及びバルブ31を閉じ、適宜ポンプ24を用いて行うとよい。この構成により、一方のタンクから他方のタンクへの電解液の移動をより効率よく行うことができる。
【００４０】
本例では、連通管にポンプ24を設けた例を示したが、ポンプ24を設けず連通管とバルブのみ設け、バルブを開くことで重力に従って液が移動するようにしてもよい。
【００４１】
【発明の効果】
以上説明したように本発明レドックスフロー電池によれば、充放電に伴う電解液の液量変化の割合が規定値を超える前に両極の電解液量を同量に戻すことで、従来に比べてガス発生が少なく、かつ電池効率が高いという優れた効果を奏し得る。また、各タンクにおいて充放電開始時の電解液量を同量とすることで、余分な電解液を含むことなく経済的である。
【図面の簡単な説明】
【図１】レドックスフロー電池の動作原理を示す説明図である。
【図２】電解液の液量変化の割合と電池効率の関係、及び電解液の液量変化の割合と液エネルギー密度の関係を示すグラフである。
【図３】正極電解液貯蔵用タンクと負極電解液貯蔵用タンクとを連通管で連結した状態を示す模式図である。
【符号の説明】
1 セル 1A 正極セル 1B 負極セル 2 正極電解液貯蔵用タンク
3 負極電解液貯蔵用タンク 4 隔膜 5 正極電極 6 負極電極
7、8、10、11 導管 9、12 ポンプ
20 正極電解液貯蔵用タンク 21 負極電解液貯蔵用タンク 22、23 連通管
24 ポンプ 25 接続管
30、31、32、33 バルブ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for operating a redox flow battery. In particular, the present invention relates to a method for operating a redox flow battery with higher battery efficiency and less gas generation.
[0002]
[Prior art]
It has been proposed to use a redox flow battery for load leveling applications, voltage sag and power failure countermeasures.
[0003]
In particular, the vanadium redox flow battery (1) has a high electromotive force, (2) a large energy density, and (3) the electrolyte is a single element system, so even if the positive and negative electrolytes are mixed. It has many advantages that it can be played by charging.
[0004]
In such a vanadium redox flow battery, when charging and discharging are repeated, various ions and solvents in the electrolytic solution move through the diaphragm, and the amount of electrolytic solution in the positive electrode and the negative electrode increases or decreases. For example, when an anion membrane is used, liquid transfer usually occurs from the positive electrode side to the negative electrode side, and the electric capacity of one of the electrolyte solution is significantly reduced due to the unbalanced amount of the electrolyte.
[0005]
In order to solve the problem associated with such liquid transfer, conventionally, the positive electrode electrolyte and the negative electrode electrolyte are communicated or mixed every predetermined number of charge / discharge cycles to adjust the liquid volume. Conventional techniques relating to this liquid amount adjustment include those described in JP-A-2001-167787, JP-A-4-124754, JP-A-11-204124, and JP-A-2-195657. ing.
[0006]
For example, in the technology described in Japanese Patent Laid-Open No. 2001-167787, the amount of liquid in each tank is kept within a certain range by returning the electrolyte through a pipe from the tank with the increased liquid to the tank with the decreased liquid. is doing.
[0007]
[Problems to be solved by the invention]
However, heretofore, it has not been studied in detail about how much liquid amount is moved with specific change in liquid amount. For this reason, it is difficult to improve battery efficiency, for example, with conventional techniques.
[0008]
Further, in the technique described in Japanese Patent Application Laid-Open No. 2001-167787, as a setting condition before charging / discharging, the amount of liquid in a tank in which the amount of liquid decreases due to repeated charging / discharging cycles is made larger than the amount of liquid in the other tank in advance. The case is mainly evaluated. However, if the liquid volume of the tank in which the liquid volume decreases is made larger than the liquid volume of the other tank in advance, an extra electrolyte solution that is not used for charging / discharging is included, resulting in an increase in cost.
[0009]
Furthermore, conventionally, evaluation about the generated gas required for battery evaluation is hardly performed. Gas is generated as a side reaction of charge and discharge. However, in the above conventional technique, clear knowledge about the type and amount of generated gas has not been obtained. Therefore, the evaluation items of the battery are only efficiency (power, voltage, current) and discharge energy (comparison with the initial state), and sufficient battery evaluation has not been performed.
[0010]
Therefore, a main object of the present invention is to provide a method for operating a redox flow battery that generates less gas and is superior in battery efficiency and at a lower cost.
[0011]
[Means for Solving the Problems]
In the present invention, the amount of electrolyte stored in the electrolyte storage tank of both positive and negative electrodes at the start of charging / discharging is the same amount, and the ratio of the change in the amount of the electrolyte accompanying repeated charging / discharging cycles is within a specified range. The above objective is achieved by equalizing the liquid volume in both tanks before exceeding.
[0012]
That is, the present invention stores a positive electrode and a negative electrode separated by a diaphragm, a positive electrode electrolyte storage tank for storing a positive electrode electrolyte circulated and supplied to the positive electrode, and a negative electrode electrolyte circulated and supplied to the negative electrode. An operation method of a redox flow battery including a negative electrode electrolyte storage tank. Each tank stores the same amount of electrolyte at the start of charging and discharging. Then, when the ratio of the change in the amount of the electrolytic solution moved through the diaphragm with the repetition of the charge / discharge cycle is within the range of −15% to + 17%, the electrolytic solution in each tank is returned to the same amount. In particular, it is optimal to return the electrolytic solution to the same amount when the rate of change in the liquid amount is in the range of more than 0% and not more than 5%. In the present invention, the rate of change in the amount of electrolyte is the difference between the amount of cathode electrolyte and the amount of cathode electrolyte with respect to the average amount of liquid obtained by dividing the sum of both electrolytes at the start of charge / discharge by 2. Is the ratio.
[0013]
The present invention having the above-described configuration aims to reduce the cost by using the same amount of electrolyte stored in each tank at the start of charging / discharging, that is, under the set conditions. In addition, the ratio of the change in the electrolyte volume is specified, and the liquid volume increased by the liquid transfer is moved to the reduced tank in order to equalize the electrolyte volume in both tanks before exceeding the specified range. Thus, the generation of gas is reduced, and the battery efficiency is further improved.
[0014]
In the present invention, the ratio of the change in the amount of the electrolytic solution is specified, and the reason why the amount of the electrolytic solution in both tanks is made the same before exceeding the specified range will be described.
Conventionally, charging and discharging is performed with the same amount of electrolytic solution in both tanks at the start of charging and discharging, and the amount of liquid is often adjusted for each charging and discharging for a certain time for work efficiency. The frequency of this liquid amount adjustment is at most once a day, and there is no clear guideline on how much the amount of liquid can be changed and how much can be moved to improve battery efficiency. Further, as a conventional technique, there is a technique that always maintains the same amount of liquid in both tanks by increasing one liquid supply pressure and decreasing the other liquid supply pressure. That is, this technique makes the flow rate difference by making the flow rate of the electrolyte flowing in one cell smaller than the flow rate of the electrolyte flowing in the other cell. However, the flow rate may be inevitably reduced because of the cell pressure limit, especially when the cell area is large. Then, the energy density is reduced by reducing the flow rate, and as a result, the battery performance may be lowered. Therefore, as a result of the study by the present inventors, before the change in the liquid amount exceeds a certain range, the increased liquid is returned from the tank in which the liquid has increased to the tank in which the liquid has decreased, so that the liquid volume in both tanks is the same. As a result, it has been found that better battery efficiency can be obtained and that the amount of gas generated is relatively small. In particular, in this case, it was also found that the battery efficiency and the energy density are good rather than the case where the operation is performed by always adjusting the flow rate and maintaining the liquid amount in each tank at the same amount.
[0015]
Moreover, the increase / decrease in the liquid amount of each tank accompanying the repetition of a charging / discharging cycle is determined by the kind of diaphragm and the pressure difference of liquid feeding to a positive / negative electrode. However, when it is recognized that the amount of liquid increases / decreases only by the type of diaphragm as disclosed in Japanese Patent Laid-Open No. 2001-167787, the increase / decrease in the amount of liquid cannot be clearly grasped.
[0016]
Based on the above matters and knowledge, the present invention is based on the setting conditions in which the amount of electrolytic solution in each tank is the same, and before the ratio of the change in the amount of the electrolytic solution reaches the specified range and exceeds the specified range, Return the liquid volume to the same volume.
[0017]
Next, the reason for reducing the gas generation in the present invention will be described. In a vanadium redox flow battery, gas is generated as a side reaction of charge and discharge. For example, if the positive electrode active material is charged too much, oxygen is generated as a side reaction due to the decomposition reaction of water, which causes oxidative degradation of the electrode and consequently voltage efficiency.
[0018]
In addition, the present inventors have also obtained knowledge that gas such as hydrogen is generated from the negative electrode and carbon dioxide is generated from the positive electrode. In the worst case, hydrogen may ignite or explode. If it is carbon dioxide, the electrode is decomposed, and the deterioration has progressed before it appears as a decrease in battery efficiency.
[0019]
Further, when gas is generated and accumulated for a long period of time, a problem occurs in the tank pressure resistance. Therefore, it is preferable to reduce the generation of gas.
[0020]
In the present invention, a known technique may be applied as a method for adjusting the amount of the electrolytic solution. For example, both tanks are connected by communication pipes, and both ends of the communication pipes are connected above the level of the electrolyte stored in each tank (see Japanese Utility Model Publication No. 4-124754), or both ends of the communication pipes are connected. It is connected at a position below the liquid surface of the electrolyte solution (see JP-A-11-204124), or the electrolyte solution is moved in the opposite direction through the diaphragm by adjusting at least one solution feeding pressure (JP-A-11-204124). 2-195657). In order to return the electrolytic solution to the same amount in the present invention, for example, it is preferable that both tanks are connected by a communication pipe so that the electrolytic solution can be moved from one tank to the other tank.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
At the start of charge / discharge, the redox flow battery was charged / discharged with the same amount of positive electrode electrolyte and negative electrode electrolyte, and the battery efficiency, liquid energy density, and gas generation amount were measured.
[0022]
An outline of the redox flow battery used in this test is shown in FIG. This battery includes a cell 1 separated into a positive electrode cell 1A and a negative electrode cell 1B by a diaphragm 4 through which ions can pass. Each of the positive electrode cell 1A and the negative electrode cell 1B contains a positive electrode 5 and a negative electrode 6. A positive electrode electrolyte storage tank 2 for supplying and discharging a positive electrode electrolyte is connected to the positive electrode cell 1A via conduits 7 and 8. Similarly, a negative electrode electrolyte storage tank 3 for supplying and discharging a negative electrode electrolyte is connected to the negative electrode cell 1B via

conduits

10 and 11. Each electrolytic solution uses an aqueous solution of ions such as vanadium ions whose valence changes, is circulated by pumps 9 and 12, and is charged and discharged in accordance with the valence change reaction of the positive electrode 5 and the negative electrode 6.
[0023]
In this test, a method of moving the electrolyte solution so that the amount of electrolyte solution in both tanks is the same will be described. First, the same amount of electrolyte is stored in the bipolar electrolyte storage tank. A liquid level sensor is provided on the side wall of each tank so that the amount of electrolyte in the tank can be grasped. Both tanks are connected by a communication pipe through a pump. When the amount of electrolyte in each tank reaches the amount shown in Table 1 along with charge / discharge, only the amount increased with charge / discharge is transferred from one tank to the other tank via this communication pipe. And adjust the amount of electrolyte in both tanks to the same amount. The amount (1) of the bipolar electrolyte solution and the rate of change in the electrolyte solution amount shown in Table 1 were measured before adjusting to the same amount. Next, a method for adjusting the amount of electrolytic solution in this test will be described. In this test, the liquid amount was changed by adjusting the pump 12 shown in FIG. Specifically, in all samples, the flow rate of the positive electrode electrolyte was 7 l / min, the feed pressure was made equal to 0.6 × 10 ⁵ Pa, and the flow rate of the negative electrode electrolyte and the feed pressure were varied. Samples Nos. 1 to 4, which increase the electrolyte on the positive electrode side, change the flow rate of the negative electrode electrolyte to 14 l / min, the liquid feed pressure to 1.4 x 10 ⁵ Pa, and change until the liquid volume at each electrode reaches the amount shown in Table 1. I let you. Samples Nos. 6 to 8 for increasing the electrolyte solution on the negative electrode side have a negative electrode electrolyte flow rate of 7 l / min and a liquid feed pressure of 0.7 × 10 ⁵ Pa. Similarly, the amount of each electrode is the amount shown in Table 1. Until changed. In Sample 5 for maintaining the same amount of electrolyte solution in both electrodes, the flow rate of the negative electrode electrolyte solution was 10 l / min, and the feeding pressure was 1.0 × 10 ⁵ Pa.
[0024]
[Table 1]

[0025]
Test conditions are shown below.
(Battery specifications)
Electrode reaction area: 1000 cm ² × 10 cell membrane: Anion (anion) exchange membrane Positive electrode electrolyte: Tetravalent vanadium ion 1.7 mol / l Sulfuric acid 2.6 mol / l electrolyte 25 l
Anode electrolyte: Trivalent vanadium ion 1.7mol / l Sulfuric acid 2.6mol / l electrolyte 25l
[0026]
(Amount of electrolyte in this test)
The positive electrode electrolyte and the negative electrode electrolyte are mixed to prepare 50 liters of a 3.5-valence electrolyte solution. The same amount of this mixed electrolyte is put into a positive electrode electrolyte storage tank and a negative electrode electrolyte storage tank in an amount of 25 liters.
[0027]
(Charging method)
Constant current charging is started at a current density of 100 mA / cm ² and then constant voltage charging is performed until the open circuit voltage is 1.55 V / cell under the condition of the upper limit charging voltage of 1.60 V / cell.
[0028]
(Discharge method)
Constant current discharge is performed at a current density of 100 mA / cm ² . Discharge is terminated when the lower limit discharge voltage reaches 1.0V / cell.
[0029]
(Evaluation methods)
Using the battery with the above specifications, the operation of returning the amount of electrolyte in both tanks to the same amount is repeated when the amount of liquid in each tank changes to the amount shown in Table 1 after the start of charging and discharging. The operation of repeating such charge / discharge and liquid volume adjustment is performed continuously for one week. Measure battery efficiency and liquid energy density around a week. After the completion of continuous charge and discharge for one week, the amount of gas (hydrogen and carbon dioxide) generated is measured.
[0030]
The battery efficiency is represented by {discharge voltage (V) × discharge current (A) × discharge time (h)} / {charge voltage (V) × charge current (A) × charge time (h)}.
[0031]
The gas was measured for carbon dioxide generation at the positive electrode and the hydrogen generation amount at the negative electrode. Gas analysis was performed by gas chromatography.
[0032]
The test results are shown in Table 2. Table 3 shows the evaluation criteria for the battery efficiency reduction amount in this test, Table 4 shows the evaluation criteria for the same liquid energy density reduction amount, and Table 5 shows the evaluation criteria for the gas generation amount. Further, the ratio of the change in the amount of the electrolyte solution (the amount of the positive electrode electrolyte−the amount of the negative electrode electrolyte) / {(the amount of the positive electrode electrolyte + the amount of the negative electrode electrolyte) / 2} and the battery efficiency and (the positive electrode electrolyte) FIG. 2 is a graph showing the relationship between the amount−the amount of negative electrode electrolyte) / {(the amount of positive electrode electrolyte + the amount of negative electrode electrolyte) / 2} and the liquid energy density.
[0033]
[Table 2]

[0034]
[Table 3]

[0035]
[Table 4]

[0036]
[Table 5]

[0037]
As is apparent from the graphs in Table 2 and FIG. 2, (amount of positive electrode electrolyte−amount of anode electrolyte) / {(amount of cathode electrolyte + amount of anode electrolyte) / 2} is less than −0.15 (less than −15%) Samples Nos. 3, 4 and 6 that had been operated to return the amount of electrolyte in both tanks to the same amount before exceeding +0.17 (+ 17%) had high battery efficiency and liquid energy density. It can be seen that the amount generated is small. In particular, before (positive electrode electrolyte amount−negative electrode electrolyte amount) / {(positive electrode electrolyte amount + negative electrode electrolyte amount) / 2} exceeds 0 to +0.05 or less (over 0% to + 5% or less). Sample No. 4 in which the electrolytic solution in each tank is returned to the same amount shows higher values of battery efficiency and liquid energy density, and the amount of gas generated is extremely small. Further, conventionally, when the operation is performed so that the electrolyte is sufficiently flowed and the amount is kept equal, that is, (amount of cathode electrolyte−amount of anode electrolyte) / {(amount of cathode electrolyte + amount of anode electrolyte) / 2} was considered to have the best battery efficiency when operated so as to be 0 (sample No. 5). However, the liquid volume in both tanks is returned to the same volume within the range where the liquid volume change is greater than 0 (0%), more specifically within the range of more than 0 (0%) to +0.05 (+ 5%). It was found that the sample No. 4 to be operated gave a preferable result. In addition, sample No. 4 that performed the operation of returning the liquid amount to the same amount was superior in energy density as compared with sample No. 5 that did not perform this operation.
[0038]
In this example, a mechanism for moving the electrolytic solution to adjust the amount of electrolytic solution in both electrodes to the same amount will be specifically described. In this example, a mechanism was used in which a pump was installed in the communication pipe and the electrolyte solution could be moved from either the positive or negative tank. FIG. 3 is a schematic diagram showing a state in which the tanks are connected by a communication pipe. The positive electrolyte storage tank 20 and the negative electrolyte storage tank 21 are connected by two

communication pipes

22 and 23, and the

communication pipes

22 and 23 are connected by a connection pipe 25 including a pump 24. .
[0039]
In this case, in order to move the electrolyte from the cathode electrolyte storage tank 20 to the anode electrolyte storage tank 21, the valve 30 and valve 31 are opened, the valve 32 and valve 33 are closed, and the pump 24 is used as appropriate. Good. On the other hand, in order to move the electrolyte from the negative electrode electrolyte storage tank 21 to the positive electrode electrolyte storage tank 20, the valve 32 and the valve 33 are opened, the valve 30 and the valve 31 are closed, and the pump 24 is appropriately used. Good. With this configuration, the electrolyte solution can be moved more efficiently from one tank to the other tank.
[0040]
In this example, the pump 24 is provided in the communication pipe. However, the pump 24 may not be provided, only the communication pipe and the valve may be provided, and the liquid may move according to gravity by opening the valve.
[0041]
【The invention's effect】
As described above, according to the redox flow battery of the present invention, the amount of electrolyte solution in both electrodes is returned to the same amount before the rate of change in the amount of electrolyte solution accompanying charge / discharge exceeds the specified value, compared to the conventional case. An excellent effect of generating less gas and having high battery efficiency can be obtained. Moreover, by making the amount of the electrolyte at the start of charging / discharging the same in each tank, it is economical without including an excess electrolyte.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing the operating principle of a redox flow battery.
FIG. 2 is a graph showing the relationship between the rate of change in electrolyte volume and battery efficiency, and the relationship between the rate of change in electrolyte volume and liquid energy density.
FIG. 3 is a schematic diagram showing a state in which a positive electrode electrolyte storage tank and a negative electrode electrolyte storage tank are connected by a communication pipe.
[Explanation of symbols]
1 cell 1A positive electrode cell 1B negative electrode cell 2 positive electrode electrolyte storage tank
3 Negative electrolyte storage tank 4 Diaphragm 5 Positive electrode 6 Negative electrode
7, 8, 10, 11 Conduit 9, 12 Pump
20 Positive electrolyte storage tank 21 Negative

electrolyte storage tank

22, 23 Communication pipe
24 Pump 25 Connection pipe
30, 31, 32, 33 Valve

Claims

A positive electrode and a negative electrode separated by a diaphragm, a positive electrode electrolyte storage tank for storing a positive electrode electrolyte circulated and supplied to the positive electrode, and a negative electrode electrolyte storage tank for storing a negative electrode electrolyte circulated and supplied to the negative electrode In the operation method of the vanadium redox flow battery connected to each other so that both tanks can adjust the amount of electrolyte in each tank,
In each tank, the same amount of electrolyte is stored at the start of charging and discharging,
It is characterized in that the electrolyte solution in both tanks is returned to the same amount when the rate of change in the amount of the electrolyte solution that has moved through the diaphragm with repeated charge / discharge cycles is within the range of -15% to + 17%. Operation method of vanadium redox flow battery.
However, the ratio of the change in the liquid volume is the ratio of the liquid volume difference obtained by subtracting the negative electrode electrolyte volume from the positive electrolyte volume with respect to the average liquid volume obtained by dividing the sum of the two electrolytes at the start of charge / discharge by 2.

2. The method of operating a vanadium redox flow battery according to claim 1, wherein the electrolyte solution in both tanks is returned to the same amount when the ratio of the change in the liquid amount is in the range of more than 0% and not more than 5%.