JP4244476B2

JP4244476B2 - Redox flow battery electrode material and electrolytic cell

Info

Publication number: JP4244476B2
Application number: JP34859699A
Authority: JP
Inventors: 誠井上; 真申小林
Original assignee: Toyobo Co Ltd
Current assignee: Toyobo Co Ltd
Priority date: 1999-12-08
Filing date: 1999-12-08
Publication date: 2009-03-25
Anticipated expiration: 2019-12-08
Also published as: JP2001167772A

Description

【０００１】
【発明の属する技術分野】
本発明は水溶液系電解液を用いたレドックスフロー電池の電解槽に用いられる電極材および電解槽に関する。特に本発明のレドックスフロー電池用電極材および電解槽は、特にバナジウム系レドックスフロー電池用の電極材および電解槽として有用である。
【０００２】
【従来の技術】
従来より、電極は電池の性能を左右するものとして重点的に開発されている。電極には、それ自体が活物質とならず、活物質の電気化学的反応を促進させる反応場として働くタイプのものがあり、このタイプには導電性や耐薬品性などから炭素材料がよく用いられる。特に電力貯蔵用に開発が盛んなレドックスフロー電池の電極には、耐薬品性があり、導電性を有し、かつ通液性のある炭素繊維集合体が用いられている。
【０００３】
レドックスフロー電池は、正極に鉄の塩酸水溶液、負極にクロムの塩酸水溶液を用いたタイプから、起電力の高いバナジウムの硫酸水溶液を両極に用いるタイプに替わり、高エネルギー密度化されたが、最近さらに活物質濃度を高める開発が進み、一段と高エネルギー密度化が進んでいる。
【０００４】
レドックスフロー型電池の主な構成は、図１に示すように電解液を貯える外部タンク６, ７と電解槽ＥＣから成り、ポンプ８, ９にて活物質を含む電解液を外部タンク６, ７から電解槽ＥＣに送りながら、電解槽ＥＣに組み込まれた電極上で電気化学的なエネルギー変換、すなわち充放電が行われる。
【０００５】
一般に、充放電の際には、電解液を外部タンクと電解槽との間で循環させるため、電解槽は図１に示すような液流通型構造をとる。該液流通型電解槽を単セルと称し、これを最小単位として単独もしくは多段積層して用いられる。液流通型電解槽における電気化学反応は、電極表面で起こる不均一相反応であるため、一般的には二次元的な電解反応場を伴うことになる。電解反応場が二次元的であると、電解槽の単位体積当たりの反応量が小さいという難点がある。
【０００６】
そこで、単位面積当りの反応量、すなわち電流密度を増すために電気化学反応場の三次元化が行われるようになった。図２は、三次元電極を有する液流通型電解槽の分解斜視図である。該電解槽では、相対する二枚の集電板１, １間にイオン交換膜３が配設され、イオン交換膜３の両側にスペーサー２によって集電板１, １の内面に沿った電解液の流路４ａ, ４ｂが形成されている。該流通路４ａ, ４ｂの少なくとも一方には炭素繊維集合体等の電極材５が配設されており、このようにして三次元電極が構成されている。なお、集電板１には、電解液の液流入り口１０と液流出口１１とが設けられている。
【０００７】
正極電解液にオキシ硫酸バナジウム、負極電解液に硫酸バナジウムの各々硫酸酸性水溶液を用いたレドックスフロー型電池の場合、放電時には、Ｖ²⁺を含む電解液が負極側の液流路４ａに供給され、正極側の流路４ｂにはＶ⁵⁺（実際には酸素を含むイオン）を含む電解液が供給される。負極側の流路４ａでは、三次元電極５内でＶ²⁺が電子を放出しＶ³⁺に酸化される。放出された電子は外部回路を通って正極側の三次元電極内でＶ⁵⁺をＶ⁴⁺（実際には酸素を含むイオン）に還元する。この酸化還元反応に伴って負極電解液中のＳＯ₄ ^2- が不足し、正極電解液ではＳＯ₄ ^2- が過剰になるため、イオン交換膜３を通ってＳＯ₄ ^2- が正極側から負極側に移動し電荷バランスが保たれる。あるいは、Ｈ⁺ がイオン交換膜を通って負極側から正極側へ移動することによっても電荷バランスを保つことができる。充電時には放電と逆の反応が進行する。
【０００８】
バナジウム系レドックスフロー電池用電極材の特性としては、特に以下に示す性能が要求される。
【０００９】
１）目的とする反応以外の副反応を起こさないこと（反応選択性が高いこと）、具体的には電流効率（η_I ）が高いこと。
【００１０】
２）電極反応活性が高いこと、具体的にはセル抵抗（Ｒ）が小さいこと。すなわち電圧効率（η_V ）が高いこと。
【００１１】
３）上記１）２）に関連するが電池エネルギー効率（η_E ）が高いこと。
η_E ＝η_I ×η_V
【００１２】
４）繰り返し使用に対する劣化が小さいこと（高寿命）、具体的には電池エネルギー効率（η_E ）の低下量が小さいこと。
【００１３】
例えば、特開昭６０−２３２６６９号公報には、Ｘ線広角解析より求めた＜００２＞面間隔が、平均３．７０Å以下であり、またｃ軸方向の結晶子の大きさが平均９．０Å以上の擬黒鉛微結晶を有し、かつ全酸性官能基量が少なくとも０．０１ｍｅｑ／ｇである炭素質材料を、電解槽用電極材として用いることが提案されている。
【００１４】
また、特開平５−２３４６１２号公報には、ポリアクリロニトリル系繊維を原料とする炭素質繊維で、Ｘ線広角解析より求めた＜００２＞面間隔が３．５０〜３．６０Åの擬黒鉛結晶構造を有し、炭素質材料表面の結合酸素原子数が炭素原子数の１０〜２５％となるような炭素質材を、鉄−クロム系レドックスフロー電池の電解槽用電極材として用いることが提案されている。
【００１５】
【発明が解決しようとする課題】
しかしながら、特開昭６０−２３２６６９号公報、特開平５−２３４６１２号公報では、炭素質材料表面と電解液との間に有効な濡れ性を発現させるために、全酸性官能基量が０．０１ｍｅｑ／ｇ以上か、あるいは炭素質材料表面の結合酸素原子数が炭素原子数の１０％以上必要であり、炭素電極材表面の官能基が多すぎるため、上記の如き接触抵抗が高くなり、その結果セル抵抗が高くなって、高い電池エネルギー効率が得られていなかった。こうした問題に対し、電極材の導電性を向上させるために、電極材の充填密度を高くすることが考えられるが、いたずらに充填密度を高くすると、電解液の流通性が悪化して通液圧損が高くなり、電解液を流通するポンプの動力損失が増加して結果として、電池の効率が低下してしまう。
【００１６】
そこで、本発明の目的はかかる事情に鑑み、レドックスフロー電池の電極材に用いられる炭素質繊維の基本的な特性を有し、かつ電解槽における接触抵抗を低減しうる電極材を提供すること、さらには当該電極材を用いて電池の内部抵抗を低減した電解槽を提供することにある。
【００１７】
【発明を解決するための手段】
本発明者らは、上記目的を達成するために鋭意研究したところ、電極材に用いる炭素集合体のポイント圧縮率を厚み方向に差を付けることで上記目的を達成できる事を見いだし、本発明を完成させるに至った。
【００１８】
すなわち、本発明は、水溶液系電解液を用いたレドックスフロー電池において該電池の電解槽に用いられる炭素集合体からなる電極材であって、炭素集合体が厚み方向にポイント圧縮保持率の異なる２層以上の一体化された層構造を有し、表裏面のポイント圧縮保持率の高低比率（低い値／高い値）が０．８０以上０．９８以下であることを特徴とするレドックスフロー電池用電極材、に関する。
【００１９】
前記ポイント圧縮保持率は表層の局所的な堅さを評価するため、先端が球状の測定子を用いて低荷重での厚み変化率を評価する方法である。ポイント圧縮保持率が高い値になるほど球状測定子による荷重厚みの変化率が大きいことを意味し、表面組織が柔らかいことを示唆している。ポイント圧縮保持率の高低比率が高いことは、即ち電極材組織の表裏面の堅さに大きな違いがあることを意味する。
【００２０】
前記のように電極材に用いる炭素集合体を、厚み方向にポイント圧縮保持率の異なる２層以上の一体化された層構造とすることにより、電極材の電解槽への組み込み時に幾何的な接触面積を増加し、さらにポイント圧縮保持率の高低比率を上記所定範囲に調整することによって、ポイント圧縮保持率の高い側では圧縮保持率が高いことから、電極材の電解槽への組み込み時に、優先して電極材が圧縮され高ポイント圧縮保持率を有する面により電極材の幾何表面積を増加するとともに、ポイント圧縮保持率の低い側では圧縮保持率が低いために、電極材の電解槽への組み込み時に強い圧縮応力によって電極材と集電板との接着性が向上し低ポイント圧縮保持率を有する面で必要な圧縮応力を得て接触抵抗を低減したものである。
【００２１】
さらに、本発明は、間隙を介した状態で対向して配設された一対の集電板間に隔膜が配設され、該集電板と隔膜の間との少なくとも一方に電極材が圧接挟持された電解槽において、前記電極材として前記電極材を用い、当該電極材のポイント圧縮保持率の高い側を集電板側に配設したことを特徴とするレドックスフロー電池用電解槽、に関する。
【００２２】
前記電極材の高ポイント圧縮保持率側を集電板側に向けることにより、低ポイント圧縮保持率部が高ポイント圧縮保持率部を押さえる形になり優先的に高ポイント圧縮保持率部が圧縮される。そこで集電板と付近の電極材中の単繊維幾何表面積が増長し電極材との接触性が増す。そのため接触抵抗が低くなりセル抵抗を低くすることが可能となる。また低ポイント圧縮保持率部は電解槽組立時に圧縮されにくくなるため充填密度の増加がなく通液圧損の低減につながり、電極全体の抵抗が減少する結果、電池のセル抵抗が低減する。この二つの効果により結果的に電池効率を高めることができる。
【００２３】
また、前記レドックスフロー電池用電極材および電解槽は、バナジウム系レドックスフロー電池に使用するのが好適である。
【００２４】
バナジウム系レドックスフロー電池は、従来の鉄−クロム系電解液を用いた電池に比べて活物質と電極材表面との反応速度が速く、電極材の接触抵抗は電極材との反応に伴う抵抗（反応抵抗）に比べて相対的に高くなる傾向にある。したがって、電極材を構成する炭素集合体や集電板表面に対する接触抵抗が特に問題となりやすい、バナジウム系レドックスフロー電池における電解槽において、上記作用効果を有する前記本発明の電解槽は特に有効なものとなる。
【００２５】
【発明の実施の形態】
本発明の電極材は炭素質繊維からなり、その組成、微細構造等は特に制限されないが、電極表面積を大きくできるものが好ましい。具体的には、紡績糸、フィラメント集束糸、不織布、編地、織地、特殊編織物（特開昭６３−２００４６７号公報に記載されているようなもの）があげられるが取扱いや加工性、製造性等から主として不織布が利用される。当該不織布は、焼成（炭化）の前に不融化あるいは耐炎化された短繊維を解繊し、カードにかけ、幾層かに重ねられたレイヤーからなるウェブを作成し、ニードルパンチ法、サーマルボンド法、ステッチボンド法等の公知の方法を組み合わせて好適に作成される。
【００２６】
電極材として用いられる炭素集合体はこうして得られた不織布等を公知の方法で焼成し必要であれば表面処理を行うことによって得られる。
【００２７】
炭素集合体の目付量は電解槽のスペーサーで設定される間隙によって異なるが、通常、１００〜１０００ｇ／ｍ² 程度であり、圧接時に必要な圧縮応力を得るために厚みは少なくとも間隙より大きいこと、望ましくは間隙の１．２倍から３．３倍の厚みに調整される。しかしながら電極材の圧縮応力が高いと隔膜の損傷や電解槽作成時のハンドリングの悪さが顕在化するため設定間隙までの圧縮応力が０．０９８ＭＰａ以下にすることが望ましい。
【００２８】
このような層構造を有する炭素集合体は、通常、厚さ０．５〜１５ｍｍ程度、好ましくは１〜１０ｍｍであり、嵩密度は、０．０５〜０．１５ｇ／ｃｍ³ 程度、好ましくは０．０６〜０．１４ｇ／ｃｍ³ とするのが、通液性とセル抵抗を両立する上で好ましい。
【００２９】
前記電極材として用いられる炭素集合体は、厚み方向にポイント圧縮保持率の異なる２層以上の一体化された層構造を有する。電極材の層構造は、表裏にポイント圧縮保持率の異なる層が２層以上あれば、その層構造は特に制限されない。たとえば、厚み方向に各層のポイント圧縮保持率が順次に高く（または低く）なるような層構造であってもよく、高ポイント圧縮保持率層／低ポイント圧縮保持率層／高ポイント圧縮保持率層、または低ポイント圧縮保持率層／高ポイント圧縮保持率層／低ポイント圧縮保持率層のようにポイント圧縮保持率の異なる層が繰り返されているような層構造であってもよい。ただし、炭素集合体は表面と裏面のポイント圧縮保持率には差がついていることが必要である。
【００３０】
高ポイント圧縮保持率層と低ポイント圧縮保持率層の区別は、表裏面のポイント圧縮保持率を測定し、どちらかの圧縮保持率の高い面を含む層を高ポイント圧縮保持率層とし、もう一方の面を含む層を低ポイント圧縮保持率層とする。電極材の層構造が３層以上ある場合にも表裏面のポイント圧縮保持率を測定し高低比率を決定する。
【００３１】
また、前記炭素集合体は、表裏面のポイント圧縮保持率の高低比率（低い値／高い値）が０．８０以上０．９８以下の範囲になるように調整したものを用いる。
【００３２】
ポイント圧縮保持率の高低比率の比が小さくなり、０．８０未満になると、ポイント圧縮保持率の低い部分は表層が極端に堅い組織になってしまうため、電解槽作成時に隔膜を損傷し電解液が混入し容量低下が起きるので好ましくない。こうした傾向があることから高低比率は、０．８２以上、さらには０．８５以上とするのが好ましい。一方、ポイント圧縮保持率の高低比率が０．９８を超える場合には、ポイント圧縮保持率の差がなくなるので電解槽作成時には電極材が均一充填となり、集電板と付近の電極材中の単繊維幾何表面積が増加せず電極材との接合性が悪くなる。そのため接触抵抗が増加しセル抵抗が増加し、結果的に電池効率が低下する。こうした傾向があることから高低比率の比は、０．９７以下、さらには０．９５以下とするのが好ましい。
【００３３】
高ポイント圧縮保持率層、低ポイント圧縮保持率層の厚さやポイント圧縮保持率は、前記高低比率となるように適宜に調整されるが、高ポイント圧縮保持率層のポイント圧縮保持率は、５〜５０％程度、好ましくはで１０〜４０％であり、厚さはスペーサーによって得られる空隙の３０〜９９％程度、好ましくは５０〜９０％とするのが通液性と接触性を確保する上で好ましい。低ポイント圧縮保持率層のポイント圧縮保持率は、０．１〜５０％程度、好ましくは０．２〜４０％であり、厚さはスペーサーによって得られる空隙の１〜７０％程度、好ましくは１０〜５０％とするのが、通液性と電解槽構成時に必要な圧縮応力を確保する上で好ましい。
【００３４】
炭素集合体として不織布を用いた場合に、ポイント圧縮保持率を調整する方法としては不織布化時のニードルパンチ用針の特性や針密度、針深度、押さえギャップなどのニードルパンチ法における不織布化条件を種々設定する方法があげられる。
【００３５】
また、ニードルパンチ法で実現できない様な圧縮保持率にするには熱プレスしたりバインダーの存在下で熱圧着する事によって達成される。
【００３６】
熱プレスを行う場合は各繊維の性質によって温度・圧力を調整する必要がある。
【００３７】
バインダーの種類は特に得資源されず、たとえば、アクリル系、でんぷんのり、ポリビニルアルコール系、エポキシ樹脂系、酢酸ビニル系、フェノール樹脂系等のバインダーがあげられる。なお、炭化後にも炭化して接着性を保持させるため前記バインダーとしては、フェノール系樹脂バインダーを用いることが最も好ましい。バインダーの不織布への添加方法は、特に制限されず、原綿の解繊後混綿工程で添加する方法、水や有機溶媒などに溶解または分散させ不織布に添着し乾燥する方法があげられ、各素材に適した条件で実施することが望ましい。
【００３８】
炭素集合体を２層以上の層構造とし、ポイント圧縮保持率の高低比率を前記範囲になるようにする方法としては、たとえば、前記方法により、異なったポイント圧縮保持率を持つ不織布を重ねてさらにニードルパンチをかけて接合する方法、不織布と別種の高いポイント圧縮保持率を有する織布、編地をあわせて接合する方法、ウェブ作成時に異なった目付のウェブを作成し目付順に積層して不織布化する方法、上記のバインダーを不織布の片方の面に塗布または散布し固化する方法があげられる。そのほかラッセル編地、マリフリーズといった表裏面にポイント圧縮保持率の差が形成される組織を用いても良い。
【００３９】
次に、本発明において採用される電極材のポイント圧縮保持率、ポイント圧縮保持率の高低比率、セル抵抗の測定方法について説明する。
【００４０】
１．ポイント圧縮保持率およびその高低比率
圧縮厚み試験機に測定子（尾崎製作所（株）製球状測定子Ｘ−１）を装着し、寸法２５ｍｍ×１０ｍｍ）の試験片を用意する。ゼロ荷重時でのゼロ点を調整した後、試験片の表面に測定子を合わせ、０．０３９２Ｎの荷重をかけ、そのときの厚みを読みとる（ｔ４）。その後、０．４９０Ｎまで圧縮し、この時の厚みを読みとる（ｔ５０）。これらのデータから式１によって表面の圧縮保持率を得る。測定したサンプルを裏返し、同様にして測定子を合わせ裏面の圧縮保持率を得る。
【００４１】
【数１】

次いで、数式２によって、ポイント圧縮保持率高低比率を算出する。
【数２】

【００４２】
２．接触抵抗
電極材を１ｃｍ×１０ｃｍの大きさにカットし、厚さ３．０ｍｍのテフロン製スペーサーを用いて厚み方向から２枚の導電板で電極材をスペーサーの厚みになるまで圧縮し、導電板の両端をデジタルマルチメーター（アドバンテスト製ＴＲ６８４６）を用いて測定する。
【００４３】
３．通液圧損
液流通型電解槽と同じ形状で通液方向に２０ｃｍ、幅方向（流路幅）１０ｃｍ、３．０ｍｍのスペーサーで形成された液流通型電解槽を用意し、作成された電極材を１０ｃｍ角に切って設置する。液量１０リットル／時のイオン交換水を流通させ、電解槽の出入口の通液圧力損失を測定する。ブランクとして電極材を設置しない系で同様に測定し、測定値とブランク測定値との差を電極材の通液圧力損失とする。
【００４４】
４．セル抵抗
上下方向（通液方向）に１ｃｍ、幅方向に１０ｃｍの電極面積１０ｃｍ² を有する小型のセルを作り、電極材の高ポイント圧縮保持率側をセルの集電板側に向けて装着する。定電流密度で充放電を行い、電極性能のテストを行う。正極電解液には２ｍｏｌ／ｌのオキシ硫酸バナジウムの３ｍｏｌ／ｌ硫酸水溶液を用い、負極電解液には２ｍｏｌ／ｌの硫酸バナジウムの３ｍｏｌ／ｌ硫酸水溶液を用いた。電解液量はセル、配管に対して大過剰とした。液流量は毎分６．２ｍｌとし、３０℃で測定を行った。
【００４５】
充電に始まり、放電で終わる１サイクルのテストにおいて、電流密度を電極幾何面積当たり４０ｍＡ／ｃｍ² （４００ｍＡ）として、１．７Ｖまでの充電に要した電気量をＱ₁ クーロン、１．０Ｖまでの定電流放電、およびこれに続く１．２Ｖでの定電圧放電で取りだした電気量をそれぞれＱ₂ 、Ｑ₃ クーロンとし、負極液中のＶ³⁺をＶ²⁺に完全に還元するのに必要な理論電気量Ｑ_thに対して放電により取りだした電気量の比を充電率とし、数式３で充電率を求める。
【００４６】
【数３】

充電率が５０％のときの電気量に対応する充電電圧ＶＣ５０、放電電圧ＶＤ５０を電気量−電圧曲線からそれぞれ求め、数式４より電極幾何面積に対するセル抵抗Ｒ（Ω・ｃｍ² ）を求める。
【００４７】
【数４】

ここで、I は定電流充放電における電流値０. ４Ａである。
【００４８】
本発明の電極材は、水溶液系電解液を使用するレドックスフロー電池に用いられるものである。当該レドックスフロー電池は、前述のように例えば間隙を介した状態で対向して配設された一対の集電板間に隔膜が配設され、該集電板と隔膜の間との少なくとも一方に電極材が圧接挟持された構造を有する電解槽を備えている。電解槽は、従来と同様のものを使用できる。たとえば、図１、図２に示した構造をしている。集電板、隔膜としては従来と同様のものを使用できる。電解槽には活物質を含んだ水溶液が水溶液系電解液として用いられる。
【００４９】
水溶液系電解液としては前述のバナジウム系電解液の他、鉄−クロム系、チタン−マンガン系、マンガン−クロム系、鉄−チタン系があげられるがバナジウム系が望ましい。本発明の電極材は特に粘度が２５℃にて０．０５Ｐａ・ｓ以上であるバナジウム系電解液、あるいは１．５ｍｏｌ／ｌ以上のバナジウムイオンを含むバナジウム系電解液を使用するレドックスフロー電池に用いられるのが有用である。
【００５０】
【実施例】
以下、本発明の構成および効果を具体的に示し、実施例等について説明する。
【００５１】
実施例１
平均繊維径１６μｍのポリアクリロニトリル繊維を、空気中、２００〜３００℃で耐炎化した後、長さ約８０ｍｍにカットし耐炎化繊維の短繊維を作成した。次いで、フォスター社製ＨＤＢ４０番の針、針密度４３．０９本／ｃｍ² 、押さえギャップ５．０ｍｍの条件で不織布化し、目付量６００ｇ／ｍ² 、厚み５．１ｍｍの不織布を作成した。これにフェノール系樹脂粉末（鐘紡（株）製：ベルパールＳ８９０）３ｇ／ｍ² を不織布上方より散布した後、不織布下方より静圧９８０．６Ｐａ、吸引速度２．０ｍ／秒の吸引サクションで吸引し、バインダーを不織布表面に固定化した。これを１８０℃のエアスルーオーブンで１０分間加熱してバインダーを接着させた。該布を窒素気流下で１００℃／分の昇温速度でそれぞれ１６００℃まで昇温し、この温度で１時間保持し炭化を行って冷却し、続いて酸素濃度５ｖｏｌ％の窒素気流下で７００℃にて重量収率９３％になるまで処理し炭素質繊維不織布を得た。
【００５２】
実施例２
平均繊維径１６μｍのポリアクリロニトリル繊維を、空気中、２００〜３００℃で耐炎化した後、長さ約８０ｍｍにカットし耐炎化繊維の短繊維を作成した。次いで、フォスター社製ＨＤＢ４０番の針、針密度４３．０９本／ｃｍ² 、押さえギャップ５．０ｍｍの条件で不織布化し、目付量６００ｇ／ｍ² 、厚み５．１ｍｍの不織布を作成した。これにフェノール系樹脂粉末（鐘紡（株）製：ベルパールＳ８９０）１０ｇ／ｍ² を不織布上方より散布した後、不織布下方より静圧９８０．６Ｐａ、吸引速度２．０ｍ／秒の吸引サクションで吸引しバインダーを不織布表面に固定化した。これを１８０℃のエアスルーオーブンで１０分間加熱してバインダーを接着させた。該布を窒素気流下で１００℃／分の昇温速度でそれぞれ１６００℃まで昇温し、この温度で１時間保持し炭化を行って冷却し、続いて酸素濃度５ｖｏｌ％の窒素気流下で７００℃にて重量収率９３％になるまで処理し炭素質繊維不織布を得た。
【００５３】
実施例３
平均繊維径１６μｍのポリアクリロニトリル繊維を、空気中、２００〜３００℃で耐炎化した後、長さ約８０ｍｍにカットし耐炎化繊維の短繊維を作成した。次いで、フォスター社製ＨＤＢ４０番の針、針密度４３．０９本／ｃｍ² 、押さえギャップ２．０ｍｍの条件で不織布化し目付量２００ｇ／ｍ² 、厚み３．０ｍｍの耐炎化不織布１を作成した。また同じ耐炎化繊維にフェノール系樹脂粉末（鐘紡（株）製：ベルパールＳ８９０）１０ｇ／ｍ² を混ぜフォスター社製ＨＤＢ４０番の針、針密度７２．８５本／ｃｍ² 、押さえギャップ３．０ｍｍの条件で不織布化し目付量４００ｇ／ｍ² 、厚み４．０ｍｍの耐炎化不織布２を作成した。これら２枚の不織布をあわせてニードルパンチにかけさらに１８０℃のエアスルーオーブンで１０分間加熱してバインダーを接着させ積層耐炎化不織布を得た。この積層耐炎化不織布を窒素気流下で１００℃／分の昇温速度でそれぞれ１６００℃まで昇温し、この温度で１時間保持し炭化を行って冷却し、続いて酸素濃度５ｖｏｌ％の窒素気流下で７００℃にて重量収率９３％になるまで処理し炭素質繊維不織布を得た。
【００５４】
実施例４
平均繊維径１６μｍのポリアクリロニトリル繊維を、空気中、２００〜３００℃で耐炎化した後、長さ約８０ｍｍにカットし耐炎化繊維の短繊維を作成した。次いで、フォスター社製ＨＤＢ４０番の針、針密度４３．０９本／ｃｍ² 、押さえギャップ２．０ｍｍの条件で不織布化し目付量２００ｇ／ｍ² 、厚み３．０ｍｍの耐炎化不織布１作成した。また同じ耐炎化繊維をフォスター社製ＨＤＢ４０番の針、針密度７２．８５本／ｃｍ² 、押さえギャップ３．０ｍｍの条件で不織布化し目付量４００ｇ／ｍ² 、厚み４．０ｍｍの耐炎化不織布２を作成し、液体フェノール樹脂（昭和高分子（株）製：ＢＲＥ１７４）０．１重量％を添着した。これら２枚の不織布をあわせてニードルパンチにかけさらに１８０℃のエアスルーオーブンで１０分間加熱してバインダーを接着させ積層耐炎化不織布を得た。この積層耐炎化不織布を窒素気流下で１００℃／分の昇温速度でそれぞれ１６００℃まで昇温し、この温度で１時間保持し炭化を行って冷却し、続いて酸素濃度５ｖｏｌ％の窒素気流下で７００℃にて重量収率９３％になるまで処理し炭素質繊維不織布を得た。
【００５５】
実施例５
平均繊維径１６μｍのポリアクリロニトリル繊維を、空気中、２００〜３００℃で耐炎化した後、長さ約８０ｍｍにカットし耐炎化繊維の短繊維を作成した。次いで、フォスター社製ＨＤＢ４０番の針、針密度４３．０９本／ｃｍ² 、押さえギャップ３．０ｍｍの条件で不織布化し目付量４００ｇ／ｍ² 、厚み４．３ｍｍの耐炎化不織布１作成した。また同じ耐炎化繊維をフォスター社製ＨＤＢ４０番の針、針密度７２．８５本／ｃｍ² 、押さえギャップ２．０ｍｍの条件で不織布化し目付量２００ｇ／ｍ² 、厚み２．７ｍｍの耐炎化不織布２を作成し、液体フェノール樹脂（昭和高分子（株）製：ＢＲＥ１７４）０．１重量％を添着した。これら２枚の不織布をあわせてニードルパンチにかけさらに１８０℃のエアスルーオーブンで１０分間加熱してバインダーを接着させ積層耐炎化不織布を得た。この積層耐炎化不織布を窒素気流下で１００℃／分の昇温速度でそれぞれ１６００℃まで昇温し、この温度で１時間保持し炭化を行って冷却し、続いて酸素濃度５ｖｏｌ％の窒素気流下で７００℃にて重量収率９３％になるまで処理し炭素質繊維不織布を得た。
【００５６】
比較例１
平均繊維径１６μｍのポリアクリロニトリル繊維を、空気中、２００〜３００℃で耐炎化した後、長さ約８０ｍｍにカットし耐炎化繊維の短繊維を作成した。次いで、フォスター社製ＨＤＢ４０番の針、針密度４３．０９本／ｃｍ² 、押さえギャップ５．０ｍｍの条件で不織布化し目付量６００ｇ／ｍ² 、厚み５．１ｍｍの不織布を作成した。これを１８０℃のエアスルーオーブンで１０分間加熱してバインダーを接着させた。該布を窒素気流下で１００℃／分の昇温速度でそれぞれ１６００℃まで昇温し、この温度で１時間保持し炭化を行って冷却し、続いて酸素濃度５ｖｏｌ％の窒素気流下で７００℃にて重量収率９３％になるまで処理し炭素質繊維不織布を得た。
【００５７】
比較例２
平均繊維径１６μｍのポリアクリロニトリル繊維を、空気中、２００〜３００℃で耐炎化した後、長さ約８０ｍｍにカットし耐炎化繊維の短繊維を作成した。次いで、フォスター社製ＨＤＢ４０番の針、針密度４３．０９本／ｃｍ² 、押さえギャップ５．０ｍｍの条件で不織布化し目付量６００ｇ／ｍ² 、厚み５．１ｍｍの不織布を作成した。これにフェノール系樹脂粉末（ベルパールＳ８９０）４０ｇ／ｍ² を不織布上方より散布しその後不織布下方より静圧４９０．３Ｐａ、吸引速度２．０ｍ／秒の吸引サクションで吸引しバインダーを不織布表面に固定化した。これを１８０℃のエアスルーオーブンで１０分間加熱してバインダーを接着させた。該布を窒素気流下で１００℃／分の昇温速度でそれぞれ１６００℃まで昇温し、この温度で１時間保持し炭化を行って冷却し、続いて酸素濃度５ｖｏｌ％の窒素気流下で７００℃にて重量収率９３％になるまで処理し炭素質繊維不織布を得た。
【００５８】
実施例および比較例で得られた炭素質繊維不織布の目付量、厚み、ポイント圧縮保持率の高低比率、通液圧損、接触抵抗、ポイント圧縮保持率の高い側を集電板方向に設置した電解槽によるセル抵抗値を表１に記載する。
【００５９】
【表１】

表１の結果から明らかなように、実施例１〜５の電極材は通液圧損、集電板との接触抵抗、セル抵抗値が小さい。また、該電極材の高ポイント圧縮保持率側を集電板側に向けた電極を用いて、電解槽を作成することによって高い電圧効率を示し、優れたエネルギー効率が得られる。
【００６０】
これに対し電極材の高ポイント圧縮保持率高低比率が０．９８より高い比較例１では表裏のポイント圧縮保持率の差がないので電解槽作成時には電極材は均一充填となり集電板と付近の電極材中の単繊維幾何表面積が増加せず電極材との接合性が悪くなる。そのため接触抵抗が増加しセル抵抗が増加する。さらに通液圧損が増加して、電解液の送液ポンプ効率ロスが増加し、結果的に電池効率が低下し好ましくない。またポイント圧縮保持率の高低比率が０．８０未満の場合ポイント圧縮保持率の低い部分は表層が極端に堅い組織になってしまうため電解槽作成時に隔膜を損傷し電解液が混入し容量低下が起きて運転不能状態になる可能性があり好ましくない。
【図面の簡単な説明】
【図１】バナジウム系レドックスフロー電池の概略図
【図２】三次元電極を有するバナジウム系レドックスフロー電池の電解槽の分解斜図
【符号の説明】
１集電板
２スペーサー
３イオン交換膜
４ａ，４ｂ通液路
５電極材
６外部液タンク（正極側）
７外部液タンク（負極側）
８，９ポンプ
１０液流入口
１１液流出口[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrode material and an electrolytic cell used in an electrolytic cell of a redox flow battery using an aqueous electrolyte solution. In particular, the electrode material and electrolytic cell for redox flow batteries of the present invention are particularly useful as an electrode material and electrolytic cell for vanadium redox flow batteries.
[0002]
[Prior art]
Conventionally, electrodes have been intensively developed as affecting the performance of batteries. There are electrode types that do not become active materials themselves but act as reaction fields that promote the electrochemical reaction of the active materials. Carbon materials are often used for this type because of their electrical conductivity and chemical resistance. It is done. In particular, an electrode of a redox flow battery, which has been actively developed for power storage, uses a carbon fiber aggregate having chemical resistance, conductivity, and liquid permeability.
[0003]
The redox flow battery has been changed from a type that uses an aqueous hydrochloric acid solution of iron for the positive electrode and an aqueous solution of chromium hydrochloric acid for the negative electrode to a type that uses an aqueous solution of vanadium sulfuric acid with a high electromotive force for both electrodes. Development to increase the active material concentration is progressing, and energy density is further increased.
[0004]
As shown in FIG. 1, the main structure of the redox flow type battery is composed of external tanks 6 and 7 for storing an electrolytic solution and an electrolytic cell EC. The

pumps

8 and 9 are used to supply an electrolytic solution containing an active material to the external tanks 6 and 7. While being sent to the electrolytic cell EC, electrochemical energy conversion, that is, charging / discharging is performed on the electrode incorporated in the electrolytic cell EC.
[0005]
In general, when charging / discharging, an electrolytic solution is circulated between an external tank and an electrolytic cell, so that the electrolytic cell has a liquid flow type structure as shown in FIG. The liquid flow type electrolytic cell is referred to as a single cell, and this is used as a minimum unit alone or in multiple layers. Since the electrochemical reaction in the liquid flow type electrolytic cell is a heterogeneous phase reaction that occurs on the electrode surface, it generally involves a two-dimensional electrolytic reaction field. If the electrolytic reaction field is two-dimensional, there is a drawback that the reaction amount per unit volume of the electrolytic cell is small.
[0006]
In order to increase the reaction amount per unit area, that is, the current density, the electrochemical reaction field is three-dimensionalized. FIG. 2 is an exploded perspective view of a liquid flow type electrolytic cell having a three-dimensional electrode. In the electrolytic cell, an ion exchange membrane 3 is disposed between two opposing current collector plates 1, 1, and an electrolytic solution is provided along the inner surface of the current collector plates 1, 1 by spacers 2 on both sides of the ion exchange membrane 3. Channels 4a and 4b are formed. At least one of the flow passages 4a and 4b is provided with an electrode material 5 such as a carbon fiber aggregate, and thus a three-dimensional electrode is configured. The current collector plate 1 is provided with a liquid inlet 10 and a liquid outlet 11 for the electrolytic solution.
[0007]
In the case of a redox flow battery using a sulfuric acid aqueous solution of vanadium oxysulfate for the positive electrode electrolyte and vanadium sulfate for the negative electrode electrolyte,²⁺Is supplied to the negative-side liquid channel 4a, and the positive-side channel 4b has V⁵⁺An electrolytic solution containing (actually ions containing oxygen) is supplied. In the flow path 4a on the negative electrode side, V in the three-dimensional electrode 5²⁺Emits electrons and V³⁺It is oxidized to. The emitted electrons pass through an external circuit and become V inside the three-dimensional electrode on the positive electrode side.⁵⁺V⁴⁺Reduction to (actually oxygen-containing ions). Accompanying this redox reaction, SO in the negative electrode electrolyte_Four ^2- Of the positive electrode electrolyte_Four ^2- Is excessive, so the SO through the ion exchange membrane 3_Four ^2- Moves from the positive electrode side to the negative electrode side, and the charge balance is maintained. Or H⁺ The charge balance can also be maintained by moving from the negative electrode side to the positive electrode side through the ion exchange membrane. During charging, a reaction opposite to discharging proceeds.
[0008]
As the characteristics of the electrode material for a vanadium redox flow battery, the following performance is particularly required.
[0009]
1) No side reactions other than the intended reaction (high reaction selectivity), specifically current efficiency (η_I ) Is high.
[0010]
2) The electrode reaction activity is high, specifically, the cell resistance (R) is small. That is, voltage efficiency (η_V ) Is high.
[0011]
3) Battery energy efficiency (η) related to 1) and 2) above_E ) Is high.
η_E = Η_I × η_V
[0012]
4) Small deterioration due to repeated use (long life), specifically battery energy efficiency (η_E ) The amount of decrease is small.
[0013]
For example, in Japanese Patent Laid-Open No. 60-232669, the <002> plane spacing determined by X-ray wide angle analysis is 3.70 mm or less on average, and the average crystallite size in the c-axis direction is 9.0 mm. It has been proposed to use a carbonaceous material having the above pseudographite microcrystals and a total acidic functional group amount of at least 0.01 meq / g as an electrode material for an electrolytic cell.
[0014]
JP-A-5-234612 discloses a pseudo-graphite crystal structure of carbonaceous fiber made from polyacrylonitrile fiber and having a <002> plane spacing of 3.50 to 3.60 mm determined by X-ray wide angle analysis. It is proposed to use a carbonaceous material having an oxygen content of 10 to 25% of the number of carbon atoms as an electrode material for an electrolytic cell of an iron-chromium redox flow battery. ing.
[0015]
[Problems to be solved by the invention]
However, in JP-A-60-232669 and JP-A-5-234612, in order to develop effective wettability between the surface of the carbonaceous material and the electrolytic solution, the total amount of acidic functional groups is 0.01 meq. / G or more, or the number of bonded oxygen atoms on the surface of the carbonaceous material is required to be 10% or more of the number of carbon atoms, and there are too many functional groups on the surface of the carbon electrode material, resulting in an increase in contact resistance as described above. The cell resistance was increased, and high battery energy efficiency was not obtained. In order to improve the conductivity of the electrode material, it is conceivable to increase the packing density of the electrode material. However, if the packing density is increased unnecessarily, the flowability of the electrolyte solution deteriorates and the flow pressure loss is reduced. As a result, the power loss of the pump that circulates the electrolyte increases, and as a result, the efficiency of the battery decreases.
[0016]
Therefore, in view of such circumstances, an object of the present invention is to provide an electrode material having basic characteristics of carbonaceous fibers used for an electrode material of a redox flow battery and capable of reducing contact resistance in an electrolytic cell, Furthermore, it is providing the electrolytic cell which reduced the internal resistance of the battery using the said electrode material.
[0017]
[Means for Solving the Invention]
The inventors of the present invention have made extensive studies to achieve the above object, and found that the above object can be achieved by making a difference in the point compression ratio of the carbon aggregate used for the electrode material in the thickness direction. It came to complete.
[0018]
That is, the present invention is an electrode material comprising a carbon aggregate used in an electrolytic cell of a redox flow battery using an aqueous electrolyte, and the carbon aggregate has a different point compression retention rate in the thickness direction. A redox flow battery characterized by having an integrated layer structure of at least layers and having a high / low ratio (low value / high value) of point compression retention on the front and back surfaces of 0.80 or more and 0.98 or less It relates to an electrode material.
[0019]
The point compression retention rate is a method for evaluating the thickness change rate at a low load using a probe having a spherical tip in order to evaluate the local hardness of the surface layer. The higher the point compression retention, the larger the rate of change of the load thickness by the spherical probe, suggesting that the surface texture is soft. A high point compression retention ratio means that there is a large difference in the hardness of the front and back surfaces of the electrode material structure.
[0020]
As described above, the carbon aggregate used for the electrode material has an integrated layer structure of two or more layers having different point compression retention rates in the thickness direction, so that geometric contact is made when the electrode material is incorporated into the electrolytic cell. By increasing the area and adjusting the ratio of the point compression retention rate to the specified range above, the compression retention rate is high on the side where the point compression retention rate is high. In addition to increasing the geometric surface area of the electrode material due to the surface having a high point compression retention rate due to the compression of the electrode material, the electrode material is incorporated into the electrolytic cell because the compression retention rate is low on the low point compression retention rate side. The adhesion between the electrode material and the current collector plate is sometimes improved by a strong compressive stress, and the contact resistance is reduced by obtaining the necessary compressive stress on the surface having a low point compression retention.
[0021]
Further, according to the present invention, a diaphragm is disposed between a pair of current collector plates arranged to face each other with a gap interposed therebetween, and an electrode material is pressed and sandwiched between at least one of the current collector plate and the diaphragm. The present invention relates to an electrolytic cell for a redox flow battery, wherein the electrode material is used as the electrode material, and a side having a high point compression retention rate of the electrode material is disposed on a current collecting plate side.
[0022]
By directing the high point compression retention rate side of the electrode material to the current collector plate side, the low point compression retention rate portion presses the high point compression retention rate portion and the high point compression retention rate portion is preferentially compressed. The Therefore, the single fiber geometric surface area in the current collector plate and the nearby electrode material is increased, and the contact property with the electrode material is increased. For this reason, the contact resistance is lowered, and the cell resistance can be lowered. Further, since the low point compression retention rate portion becomes difficult to be compressed at the time of assembling the electrolytic cell, there is no increase in filling density, leading to a reduction in liquid passage pressure loss, and as a result, the resistance of the entire electrode is reduced, thereby reducing the cell resistance of the battery. As a result, the battery efficiency can be improved by these two effects.
[0023]
The electrode material for an redox flow battery and the electrolytic cell are preferably used for a vanadium redox flow battery.
[0024]
Vanadium-based redox flow batteries have a faster reaction rate between the active material and the surface of the electrode material than batteries using conventional iron-chromium electrolyte, and the contact resistance of the electrode material is the resistance associated with the reaction with the electrode material ( The reaction resistance tends to be relatively high. Therefore, in the electrolytic cell in the vanadium redox flow battery in which the contact resistance with respect to the carbon aggregate constituting the electrode material and the current collector plate is particularly problematic, the electrolytic cell of the present invention having the above-mentioned effects is particularly effective. It becomes.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
The electrode material of the present invention is composed of carbonaceous fibers, and its composition, microstructure, etc. are not particularly limited, but those that can increase the electrode surface area are preferred. Specific examples include spun yarn, filament bundle yarn, non-woven fabric, knitted fabric, woven fabric, and special knitted fabric (as described in JP-A 63-200467). Nonwoven fabric is mainly used because of its properties. The non-woven fabric is made by defibrating short fibers that have been infusibilized or flame-resistant before firing (carbonization), applied to a card, creating a web consisting of several layers, needle punch method, thermal bond method It is suitably created by combining known methods such as a stitch bond method.
[0026]
The carbon aggregate used as the electrode material can be obtained by firing the nonwoven fabric obtained in this way by a known method and performing a surface treatment if necessary.
[0027]
The basis weight of the carbon aggregate varies depending on the gap set by the electrolytic cell spacer, but is usually 100 to 1000 g / m.² In order to obtain the necessary compressive stress during pressure welding, the thickness is adjusted to be at least larger than the gap, preferably 1.2 to 3.3 times the gap. However, when the compressive stress of the electrode material is high, damage to the diaphragm and poor handling at the time of producing the electrolytic cell become obvious, so it is desirable that the compressive stress up to the set gap is 0.098 MPa or less.
[0028]
The carbon aggregate having such a layer structure is usually about 0.5 to 15 mm in thickness, preferably 1 to 10 mm, and the bulk density is 0.05 to 0.15 g / cm.^Three Degree, preferably 0.06-0.14 g / cm^Three It is preferable to achieve both liquid permeability and cell resistance.
[0029]
The carbon aggregate used as the electrode material has an integrated layer structure of two or more layers having different point compression retention rates in the thickness direction. The layer structure of the electrode material is not particularly limited as long as there are two or more layers having different point compression retention ratios on the front and back sides. For example, a layer structure in which the point compression retention rate of each layer sequentially increases (or decreases) in the thickness direction may be employed, and a high point compression retention layer / low point compression retention layer / high point compression retention layer Alternatively, a layer structure in which layers having different point compression retention rates such as a low point compression retention layer / a high point compression retention layer / a low point compression retention layer are repeated may be used. However, it is necessary for the carbon aggregate to have a difference in the point compression retention rate between the front surface and the back surface.
[0030]
The distinction between the high point compression retention layer and the low point compression retention layer is that the point compression retention on the front and back surfaces is measured, and the layer that includes one of the surfaces with the higher compression retention is the high point compression retention layer. Let the layer containing one side be a low point compression retention layer. Even when the electrode material has three or more layers, the point compression retention on the front and back surfaces is measured to determine the height ratio.
[0031]
The carbon aggregate is adjusted so that the ratio of the point compression retention ratios of the front and rear surfaces (low value / high value) is in the range of 0.80 to 0.98.
[0032]
When the ratio of the height ratio of the point compression retention ratio becomes smaller and less than 0.80, the surface layer becomes extremely hard in the portion where the point compression retention ratio is low. This is not preferable because of a decrease in capacity caused by mixing. Because of this tendency, the height ratio is preferably 0.82 or more, more preferably 0.85 or more. On the other hand, when the height ratio of the point compression retention rate exceeds 0.98, the difference in point compression retention rate is eliminated, so that the electrode material is uniformly filled when the electrolytic cell is created, and the current collector plate and the nearby electrode material are simply filled. The fiber geometric surface area does not increase and the bondability with the electrode material becomes poor. As a result, the contact resistance increases and the cell resistance increases, resulting in a decrease in battery efficiency. Because of this tendency, the ratio of the height ratio is preferably 0.97 or less, more preferably 0.95 or less.
[0033]
The thickness and the point compression retention rate of the high point compression retention layer and the low point compression retention rate layer are appropriately adjusted so as to be the above-mentioned high / low ratio, but the point compression retention rate of the high point compression retention layer is 5 About 50%, preferably 10 to 40%, and the thickness is about 30 to 99%, preferably 50 to 90% of the gap obtained by the spacer in order to ensure liquid permeability and contact. Is preferable. The point compression retention of the low point compression retention layer is about 0.1 to 50%, preferably 0.2 to 40%, and the thickness is about 1 to 70% of the voids obtained by the spacer, preferably 10 It is preferable to set it to ˜50% in order to ensure the liquid permeability and the compressive stress necessary for the electrolytic cell configuration.
[0034]
When using a nonwoven fabric as the carbon aggregate, the method for adjusting the point compression retention rate is the characteristics of the needle punch needle at the time of making the nonwoven fabric, the needle density, the needle depth, the pressing gap, etc. Various methods can be set.
[0035]
Further, a compression retention rate that cannot be realized by the needle punch method can be achieved by hot pressing or thermocompression bonding in the presence of a binder.
[0036]
When performing hot pressing, it is necessary to adjust temperature and pressure depending on the properties of each fiber.
[0037]
The type of binder is not particularly obtained, and examples thereof include acrylic, starch paste, polyvinyl alcohol, epoxy resin, vinyl acetate, and phenol resin binders. In addition, since it carbonizes after carbonization and an adhesiveness is hold | maintained, it is most preferable to use a phenol-type resin binder as said binder. The method of adding the binder to the nonwoven fabric is not particularly limited, and includes a method of adding in the cotton blending step after defibration of the raw cotton, a method of dissolving or dispersing in water or an organic solvent, attaching to the nonwoven fabric, and drying. It is desirable to carry out under suitable conditions.
[0038]
As a method of making the carbon aggregate into a layer structure of two or more layers and making the ratio of the point compression retention ratio to be in the above range, for example, by the above method, the nonwoven fabrics having different point compression retention ratios are overlapped. A method of joining with a needle punch, a woven fabric having a high point compression retention rate that is different from that of a nonwoven fabric, a method of joining together a knitted fabric, creating webs with different basis weights when creating a web, and laminating in order of basis weight to make a nonwoven fabric And a method of applying or spraying the binder to one side of the nonwoven fabric and solidifying it. In addition, a structure in which a difference in the point compression retention rate is formed on the front and back surfaces, such as a raschel knitted fabric and a mari freeze, may be used.
[0039]
Next, a method for measuring the point compression retention rate, the height ratio of the point compression retention rate, and the cell resistance of the electrode material employed in the present invention will be described.
[0040]
1. Point compression retention and its ratio
A measuring piece (spherical measuring piece X-1 manufactured by Ozaki Seisakusho Co., Ltd.) is attached to a compression thickness tester, and a test piece having a size of 25 mm × 10 mm is prepared. After adjusting the zero point at the time of zero load, the measuring element is aligned with the surface of the test piece, a load of 0.0392 N is applied, and the thickness at that time is read (t4). Then, it compresses to 0.490N and reads the thickness at this time (t50). From these data, the compression retention of the surface is obtained by Equation 1. The measured sample is turned upside down, and in the same manner, a measuring element is combined to obtain the compression retention of the back surface.
[0041]
[Expression 1]

Next, the point compression retention ratio height ratio is calculated by Equation 2.
[Expression 2]

[0042]
2. Contact resistance
Cut the electrode material to a size of 1 cm x 10 cm, compress the electrode material with two conductive plates from the thickness direction using a 3.0 mm thick Teflon spacer to the thickness of the spacer, Is measured using a digital multimeter (TR6846 manufactured by Advantest).
[0043]
3. Fluid pressure loss
A liquid-flowing electrolytic cell having the same shape as the liquid-flowing electrolytic cell and a spacer of 20 cm in the liquid-flowing direction, 10 cm in the width direction (flow channel width), and 3.0 mm is prepared. Cut into two and install. Ion-exchanged water with a liquid volume of 10 liters / hour is circulated, and the flow pressure loss at the inlet / outlet of the electrolytic cell is measured. It measures similarly by the system which does not install an electrode material as a blank, and makes the difference of a measured value and a blank measured value the liquid pressure loss of an electrode material.
[0044]
4). Cell resistance
1 cm in the vertical direction (liquid flow direction), 10 cm in the width direction, electrode area 10 cm² A small cell having an electrode material is made, and the electrode material is mounted with the high point compression retention side facing the current collector side of the cell. Charge and discharge at constant current density and test electrode performance. A 3 mol / l sulfuric acid aqueous solution of 2 mol / l vanadium oxysulfate was used for the positive electrode electrolyte, and a 3 mol / l sulfuric acid aqueous solution of 2 mol / l vanadium sulfate was used for the negative electrode electrolyte. The amount of the electrolytic solution was excessively large with respect to the cell and the piping. The liquid flow rate was 6.2 ml per minute, and the measurement was performed at 30 ° C.
[0045]
In a one-cycle test starting with charging and ending with discharging, the current density is 40 mA / cm per electrode geometric area.² (400mA) Q is the amount of electricity required for charging up to 1.7V.₁ The quantity of electricity taken out by coulomb, constant current discharge to 1.0V, and subsequent constant voltage discharge at 1.2V is Q₂ , Q_Three Coulomb and V in the negative electrode solution³⁺V²⁺Theoretical quantity of electricity Q required for complete reduction_thThe charging rate is obtained by Equation 3 with the ratio of the amount of electricity taken out by discharging as the charging rate.
[0046]
[Equation 3]

A charge voltage VC50 and a discharge voltage VD50 corresponding to the amount of electricity when the charging rate is 50% are obtained from the amount-voltage curve of the electricity, respectively, and the cell resistance R (Ω · cm to the electrode geometric area is obtained from Equation 4.² )
[0047]
[Expression 4]

Here, I is a current value of 0.4 A in constant current charge / discharge.
[0048]
The electrode material of the present invention is used for a redox flow battery using an aqueous electrolyte solution. In the redox flow battery, as described above, for example, a diaphragm is disposed between a pair of current collector plates opposed to each other with a gap interposed therebetween, and at least one between the current collector plate and the diaphragm is provided. An electrolytic cell having a structure in which an electrode material is clamped is provided. An electrolytic cell similar to the conventional one can be used. For example, it has the structure shown in FIGS. The current collector plate and the diaphragm can be the same as the conventional one. An aqueous solution containing an active material is used as an aqueous electrolytic solution in the electrolytic cell.
[0049]
Examples of the aqueous electrolyte include iron-chromium, titanium-manganese, manganese-chromium, and iron-titanium in addition to the vanadium electrolyte described above, and vanadium is preferable. The electrode material of the present invention is particularly used for a redox flow battery using a vanadium-based electrolyte having a viscosity of 0.05 Pa · s or more at 25 ° C. or a vanadium-based electrolyte containing 1.5 mol / l or more of vanadium ions. It is useful to be able to
[0050]
【Example】
Hereinafter, the configuration and effects of the present invention will be specifically shown, and examples and the like will be described.
[0051]
Example 1
A polyacrylonitrile fiber having an average fiber diameter of 16 μm was flame-resistant in air at 200 to 300 ° C., and then cut to a length of about 80 mm to produce a flame-resistant short fiber. Next, Foster HDB No. 40 needle, needle density 43.09 / cm² A nonwoven fabric is formed under the condition of a holding gap of 5.0 mm, and the basis weight is 600 g / m.² A nonwoven fabric having a thickness of 5.1 mm was prepared. Phenol resin powder (manufactured by Kanebo Co., Ltd .: Bell Pearl S890) 3 g / m² Was sprayed from above the non-woven fabric, and then sucked from below the non-woven fabric by suction suction with a static pressure of 980.6 Pa and a suction speed of 2.0 m / sec to immobilize the binder on the non-woven fabric surface. This was heated for 10 minutes in an air-through oven at 180 ° C. to bond the binder. The fabric was heated to 1600 ° C. at a rate of 100 ° C./min under a nitrogen stream, kept at this temperature for 1 hour, carbonized and cooled, and then 700 ° C. under a nitrogen stream with an oxygen concentration of 5 vol%. The carbon fiber non-woven fabric was obtained by processing until a weight yield of 93% at 0 ° C.
[0052]
Example 2
A polyacrylonitrile fiber having an average fiber diameter of 16 μm was flame-resistant in air at 200 to 300 ° C., and then cut to a length of about 80 mm to produce a flame-resistant short fiber. Next, Foster HDB No. 40 needle, needle density 43.09 / cm² A nonwoven fabric is formed under the condition of a holding gap of 5.0 mm, and the basis weight is 600 g / m.² A nonwoven fabric having a thickness of 5.1 mm was prepared. Phenol resin powder (manufactured by Kanebo Co., Ltd .: Bell Pearl S890) 10 g / m² After spraying from above the nonwoven fabric, the binder was fixed to the nonwoven fabric surface by sucking from below the nonwoven fabric with a suction suction with a static pressure of 980.6 Pa and a suction speed of 2.0 m / second. This was heated for 10 minutes in an air-through oven at 180 ° C. to bond the binder. The fabric was heated to 1600 ° C. at a rate of 100 ° C./min under a nitrogen stream, kept at this temperature for 1 hour, carbonized and cooled, and then 700 ° C. under a nitrogen stream with an oxygen concentration of 5 vol%. The carbon fiber non-woven fabric was obtained by processing until a weight yield of 93% at 0 ° C.
[0053]
Example 3
A polyacrylonitrile fiber having an average fiber diameter of 16 μm was flame-resistant in air at 200 to 300 ° C., and then cut to a length of about 80 mm to produce a flame-resistant short fiber. Next, Foster HDB No. 40 needle, needle density 43.09 / cm² , Non-woven fabric with a holding gap of 2.0 mm and a basis weight of 200 g / m² A flameproof nonwoven fabric 1 having a thickness of 3.0 mm was prepared. In addition, phenolic resin powder (manufactured by Kanebo Co., Ltd .: Bell Pearl S890) 10 g / m on the same flameproof fiber² Mixed with Foster HDB No. 40 needle, needle density 72.85 / cm² , Non-woven fabric with a pressing gap of 3.0 mm, basis weight 400 g / m² A flameproof nonwoven fabric 2 having a thickness of 4.0 mm was prepared. These two nonwoven fabrics were put together and subjected to needle punching, and further heated in an air-through oven at 180 ° C. for 10 minutes to bond a binder to obtain a laminated flameproof nonwoven fabric. The laminated flameproof nonwoven fabric was heated to 1600 ° C. at a rate of 100 ° C./min under a nitrogen stream, kept at this temperature for 1 hour, carbonized and cooled, and then a nitrogen stream with an oxygen concentration of 5 vol%. Under treatment at 700 ° C. until a weight yield of 93% was obtained, a carbonaceous fiber nonwoven fabric was obtained.
[0054]
Example 4
A polyacrylonitrile fiber having an average fiber diameter of 16 μm was flame-resistant in air at 200 to 300 ° C., and then cut to a length of about 80 mm to produce a flame-resistant short fiber. Next, Foster HDB No. 40 needle, needle density 43.09 / cm² , Non-woven fabric with a holding gap of 2.0 mm and a basis weight of 200 g / m² A flameproof nonwoven fabric 1 having a thickness of 3.0 mm was prepared. The same flame-resistant fiber is made of Foster HDB No. 40 needle, needle density 72.85 / cm.² , Non-woven fabric with a pressing gap of 3.0 mm, basis weight 400 g / m² A flameproof nonwoven fabric 2 having a thickness of 4.0 mm was prepared, and 0.1 wt% of a liquid phenol resin (manufactured by Showa Polymer Co., Ltd .: BRE174) was attached thereto. These two nonwoven fabrics were put together and subjected to needle punching, and further heated in an air-through oven at 180 ° C. for 10 minutes to bond a binder to obtain a laminated flameproof nonwoven fabric. The laminated flameproof nonwoven fabric was heated to 1600 ° C. at a rate of 100 ° C./min under a nitrogen stream, kept at this temperature for 1 hour, carbonized and cooled, and then a nitrogen stream with an oxygen concentration of 5 vol%. Under treatment at 700 ° C. until a weight yield of 93% was obtained, a carbonaceous fiber nonwoven fabric was obtained.
[0055]
Example 5
A polyacrylonitrile fiber having an average fiber diameter of 16 μm was flame-resistant in air at 200 to 300 ° C., and then cut to a length of about 80 mm to produce a flame-resistant short fiber. Next, Foster HDB No. 40 needle, needle density 43.09 / cm² , Non-woven fabric with a pressing gap of 3.0 mm, basis weight 400 g / m² The flame-resistant nonwoven fabric 1 having a thickness of 4.3 mm was prepared. The same flame-resistant fiber is made of Foster HDB No. 40 needle, needle density 72.85 / cm.² , Non-woven fabric with a holding gap of 2.0 mm and a basis weight of 200 g / m² Then, a flame-resistant nonwoven fabric 2 having a thickness of 2.7 mm was prepared, and 0.1% by weight of a liquid phenolic resin (manufactured by Showa Polymer Co., Ltd .: BRE174) was attached. These two nonwoven fabrics were put together and subjected to needle punching, and further heated in an air-through oven at 180 ° C. for 10 minutes to bond a binder to obtain a laminated flameproof nonwoven fabric. The laminated flameproof nonwoven fabric was heated to 1600 ° C. at a rate of 100 ° C./min under a nitrogen stream, kept at this temperature for 1 hour, carbonized and cooled, and then a nitrogen stream with an oxygen concentration of 5 vol%. Under treatment at 700 ° C. until a weight yield of 93% was obtained, a carbonaceous fiber nonwoven fabric was obtained.
[0056]
Comparative Example 1
A polyacrylonitrile fiber having an average fiber diameter of 16 μm was flame-resistant in air at 200 to 300 ° C., and then cut to a length of about 80 mm to produce a flame-resistant short fiber. Next, Foster HDB No. 40 needle, needle density 43.09 / cm² The fabric weight is 600 g / m.² A nonwoven fabric having a thickness of 5.1 mm was prepared. This was heated for 10 minutes in an air-through oven at 180 ° C. to bond the binder. The fabric was heated to 1600 ° C. at a rate of 100 ° C./min under a nitrogen stream, kept at this temperature for 1 hour, carbonized and cooled, and then 700 ° C. under a nitrogen stream with an oxygen concentration of 5 vol%. The carbon fiber non-woven fabric was obtained by processing until a weight yield of 93% at 0 ° C.
[0057]
Comparative Example 2
A polyacrylonitrile fiber having an average fiber diameter of 16 μm was flame-resistant in air at 200 to 300 ° C., and then cut to a length of about 80 mm to produce a flame-resistant short fiber. Next, Foster HDB No. 40 needle, needle density 43.09 / cm² The fabric weight is 600 g / m.² A nonwoven fabric having a thickness of 5.1 mm was prepared. Phenol resin powder (Bellepearl S890) 40g / m² Was sprayed from above the non-woven fabric and then sucked from below the non-woven fabric by suction suction with a static pressure of 490.3 Pa and a suction speed of 2.0 m / sec to immobilize the binder on the non-woven fabric surface. This was heated for 10 minutes in an air-through oven at 180 ° C. to bond the binder. The fabric was heated to 1600 ° C. at a rate of 100 ° C./min under a nitrogen stream, kept at this temperature for 1 hour, carbonized and cooled, and then 700 ° C. under a nitrogen stream with an oxygen concentration of 5 vol%. The carbon fiber non-woven fabric was obtained by processing until a weight yield of 93% at 0 ° C.
[0058]
Electrolysis in which the basis weight, thickness, and point compression retention ratio of the carbon fiber nonwoven fabrics obtained in the examples and comparative examples were set in the direction of the current collector plate on the side with high pressure loss, contact resistance, and point compression retention ratio. The cell resistance value by the tank is shown in Table 1.
[0059]
[Table 1]

As is clear from the results in Table 1, the electrode materials of Examples 1 to 5 have low fluid pressure loss, contact resistance with the current collector, and cell resistance. Moreover, high voltage efficiency is shown by producing an electrolytic cell using an electrode having the high point compression retention rate side of the electrode material facing the current collector plate side, and excellent energy efficiency is obtained.
[0060]
In contrast, in Comparative Example 1 in which the high point compression retention ratio of the electrode material is higher than 0.98, there is no difference in the point compression retention ratio between the front and back surfaces. The single fiber geometric surface area in the electrode material does not increase, and the bondability with the electrode material becomes poor. Therefore, contact resistance increases and cell resistance increases. Further, the loss of liquid passing pressure is increased, the loss of efficiency of the electrolyte solution feeding pump is increased, and as a result, the battery efficiency is lowered, which is not preferable. In addition, when the point compression retention ratio is less than 0.80, the part where the point compression retention ratio is low becomes an extremely hard structure on the surface layer. It is not preferable because it may occur and become inoperable.
[Brief description of the drawings]
FIG. 1 is a schematic view of a vanadium redox flow battery.
FIG. 2 is an exploded perspective view of an electrolytic cell of a vanadium redox flow battery having a three-dimensional electrode.
[Explanation of symbols]
1 Current collector
2 Spacer
3 Ion exchange membrane
4a, 4b Fluid passage
5 Electrode material
6 External liquid tank (positive electrode side)
7 External liquid tank (negative electrode side)
8,9 pump
10 Liquid inlet
11 Liquid outlet

Claims

An electrode material comprising a carbon aggregate used in an electrolytic cell of a redox flow battery using an aqueous electrolyte, wherein the carbon aggregate is integrated into two or more layers having different point compression retention rates in the thickness direction. An electrode material for a redox flow battery, wherein the ratio of the point compression retention ratios of the front and back surfaces (low value / high value) is 0.80 or more and 0.98 or less.

The electrode material for a redox flow battery according to claim 1, which is used for a vanadium redox flow battery.

In an electrolytic cell in which a diaphragm is disposed between a pair of current collectors arranged to face each other with a gap interposed therebetween, and an electrode material is press-fitted between at least one of the current collector and the diaphragm, The electrode material according to claim 1 is used as the electrode material, and the side having a high point compression retention rate of the electrode material is disposed on the current collecting plate side.

The electrolytic cell for a redox flow battery according to claim 3, which is used for a vanadium redox flow battery.