盧穎莉，傅冠生，楊 斌，喬志軍

(寧波南車新能源科技有限公司，浙江寧波315112)

生物質(zhì)炭材料在雙電層電容器中的應(yīng)用

盧穎莉，傅冠生，楊 斌，喬志軍

(寧波南車新能源科技有限公司，浙江寧波315112)

雙電層電容器電極材料一般由炭材料組成，隨著化石能源的日益枯竭和環(huán)境污染的逐步惡化，傳統(tǒng)炭材料的生產(chǎn)和應(yīng)用受到了挑戰(zhàn)。采用綠色環(huán)保的生物質(zhì)作為碳源，制備雙電層電容器電極材料已成為研究熱點(diǎn)之一。介紹了近幾年來(lái)生物質(zhì)炭材料的制備及其在雙電層電容器中的應(yīng)用進(jìn)展，綜述了生物質(zhì)前驅(qū)體和制備方法對(duì)雙電層電容器電化學(xué)性能的影響。

雙電層電容器；生物質(zhì)炭材料；多孔炭

雙電層電容器 (electrochemical double-layer capacitors, EDLC)是依靠靜電作用力在電極上儲(chǔ)存電荷的一種儲(chǔ)能器件，與傳統(tǒng)依賴于電荷轉(zhuǎn)移反應(yīng)的電池不同，EDLC具有高功率密度、長(zhǎng)使用壽命、長(zhǎng)放置時(shí)間、寬使用溫度、高安全性等優(yōu)點(diǎn)。根據(jù)EDLC的儲(chǔ)能機(jī)理，其電容是由電極材料的靜電吸附提供，因此具有大比表面積的多孔炭是EDLC最常見(jiàn)的電極材料。多孔炭通常是從石油焦炭或煤衍生而來(lái)，但是隨著礦物日益減少以及這些傳統(tǒng)材料的價(jià)格不斷上升，人們?cè)絹?lái)越多的將目光投向新資源的開(kāi)發(fā)。

生物質(zhì)具有來(lái)源廣、價(jià)格低廉、可再生和綠色環(huán)保的優(yōu)點(diǎn)，它作為碳源制備EDLC電極材料已成為儲(chǔ)能領(lǐng)域研究的熱點(diǎn)。目前以生物質(zhì)為前驅(qū)體，采取不同方法制備多孔炭電極材料是主要發(fā)展方向。根據(jù)不同的制備方法和多孔炭孔道結(jié)構(gòu)，這些生物質(zhì)基多孔炭被分為四種：活性炭、介孔炭、分級(jí)多孔炭和炭氣凝膠。本文綜述了目前生物質(zhì)基多孔炭用于EDLC電極材料的研究進(jìn)展。

1 生物質(zhì)基多孔炭

1.1 活性炭

活性炭是生物質(zhì)炭材料的主要衍生產(chǎn)品，目前商業(yè)化的生物質(zhì)活性炭(biomass materials-based activated carbons，BMACs)主要是椰殼基活性炭系列，其商業(yè)化YP17(Kuraray Chemical Corporation)在6mol/L KOH溶液中的比電容僅為158 F/g[1]，因此開(kāi)發(fā)具有更高電容的生物質(zhì)活性炭迫在眉睫。近年來(lái)，玉米[2]、咖啡豆[3]、甘蔗渣[4]、稻米殼[5]、木頭[6]、竹子[7]、木薯皮[8]等生物質(zhì)作為炭材料被用于活性炭電極材料。采用這些原材料制備的活性炭比電容均高于椰殼基活性炭，例如馬鈴薯淀粉基活性炭在6mol/L KOH溶液中甚至具有335 F/g的比電容(電流密度50 mA/g)[9]；麥稈基活性炭在1.2mol/L MeEt3NBF4/ AN有機(jī)電解液中呈現(xiàn)251.1 F/g(2 mV/s掃速)的比電容[10]。雖然在低掃描速度或低電流密度下，生物質(zhì)活性炭具有高的比電容，然而生物質(zhì)活性炭的孔大多數(shù)是由微孔構(gòu)成，例如木頭和竹子的微孔率高達(dá)70%以上(孔徑小于2nm)[6-7]。研究表明在高掃描速度或電流密度下，因電解液離子尺度的問(wèn)題使其遷移內(nèi)阻變大，不能快速進(jìn)入微孔，導(dǎo)致比電容明顯下降，ESR增大，電壓下降明顯(IR drop)[11]。因此具有較多微孔結(jié)構(gòu)的生物質(zhì)活性炭在大電流充放電時(shí)，電容特性表現(xiàn)不佳，例如雞羽毛基活性炭的BET比表面積(SSA)和微孔比表面積分別為1 839和1 575 m2/g，其較高的微孔比表面積使其在1 A/g(1mol/L H2SO4電解液)時(shí)具有302 F/g的比電容且壓降為0.12 V，然而當(dāng)電流密度為10 A/g時(shí)，比電容為168 A/g且IR降更明顯，其容量保持率為55.6%(從1 A/g到10 A/g)，如圖1[12]所示。

圖1 雞羽毛基活性炭的透射電鏡圖(a)(b)和恒流充放電曲線(c)

1.2 介孔炭

生物質(zhì)活性炭的高微孔率使之大電流特性不佳，因此介孔炭(孔徑＞2nm)或高中孔率多孔炭成為生物質(zhì)炭材料的另一重要產(chǎn)品。H.L.Lu等[13]以硅為模板，采用葡萄糖和硼酸為前驅(qū)體制備得到孔徑分布在 2～20nm的有序介孔炭(ordered mesoporous carbons,OMCs)，雖然OMC-M-6的SSA僅為868.5 m2/g，但是其具有高達(dá)90%的中孔，結(jié)果表明OMC-M-6在30%(質(zhì)量分?jǐn)?shù))KOH電解液中在1 A/g電流密度下表現(xiàn)出202.8 F/g的比電容，在10 A/g下表現(xiàn)出172.5 F/g的比電容，容量保持率為85.1%(從1 A/g到10 A/g)且未出現(xiàn)明顯的壓降。高中孔率多孔炭與介孔炭具有相似的電化學(xué)性能，例如稻米殼基多孔炭[14]和花生殼基多孔炭(mesopore percentage＞82%)[15-16]。與BM-ACs相比，生物質(zhì)介孔炭或高中孔率多孔炭的優(yōu)異性能更體現(xiàn)在大電流密度下，但是在小電流密度下，其比電容或能量密度偏低。稻米殼ZnCl活化后，具有1 500 m2/g比表面積的兩種多孔炭，具有較低中孔率(85.2%)的多孔炭比較高中孔率(99.0%)的多孔炭擁有更高的比電容、更穩(wěn)定的循環(huán)性能和更高的容量保持率，表明多孔炭的比電容不僅與中孔有關(guān)，而且與微孔有關(guān)。因?yàn)槲⒖资请姾蓛?chǔ)存的主要場(chǎng)所，中孔是離子傳輸?shù)耐ǖ馈?/p>

1.3 分級(jí)多孔炭

具有高微孔的活性炭在小電流密度下?lián)碛懈叩谋入娙?，具有一定微孔結(jié)構(gòu)和較高中孔結(jié)構(gòu)的多孔炭在大電流密度下?lián)碛懈叩谋入娙莺腿萘勘３致?，因此多孔炭的孔徑分布合理性是其作為EDLCs電極材料的必備條件。由于分級(jí)多孔炭(hierarchical porous carbons，HPCs)的大孔(＞50nm)結(jié)構(gòu)提供電解液儲(chǔ)存場(chǎng)所，中孔(2～50nm)結(jié)構(gòu)提供離子傳輸通道，而微孔提供離子儲(chǔ)存場(chǎng)所，因此目前分級(jí)多孔炭是擁有最合理孔道結(jié)構(gòu)的電極材料。例如淀粉基HPCs[17]，香蕉皮基HPCs[18]，動(dòng)物骨頭基HPCs[19]和魚(yú)鱗基HPCs[20]。淀粉基HPCs的球形結(jié)構(gòu)、分級(jí)的孔道結(jié)構(gòu)(相互連通的大-中-微孔結(jié)構(gòu))、超高的SSA(3 251 mm2/g)和高中孔率(＞75%)使其在6mol/L KOH溶液中表現(xiàn)出良好的電化學(xué)性能，0.05 A/g時(shí)比電容為304 F/g，180 A/g時(shí)比電容為197 F/g，且10 000次循環(huán)后仍能保持98%，如圖2[21]所示。

1.4 炭氣凝膠

生物質(zhì)材料可以通過(guò)溶膠-凝膠、超臨界干燥、冷凍干燥、水熱等方法合成生物質(zhì)基水凝膠或炭氣凝膠 (hydrogels or aerogels)，例如 β-葡聚糖[22-23]、木質(zhì)纖維素[24-25]、西瓜[26]、纖維素[27]、甲殼素[28-29]、淀粉[30]等。這些炭氣凝膠具有豐富的大-中孔結(jié)構(gòu)和優(yōu)越的導(dǎo)電網(wǎng)絡(luò)，因此是EDLCs電極材料的重要選擇之一。西瓜基炭氣凝膠及其電化學(xué)性能如圖3[26]所示。

圖2 淀粉基HPCs的表觀形貌(a～e)、倍率性能(f)和循環(huán)性能(g)

圖3 西瓜基炭氣凝膠及其電化學(xué)性能

2 結(jié)論

生物質(zhì)炭材料作為前驅(qū)體被廣泛應(yīng)用于生產(chǎn)活性炭、介孔炭、分級(jí)的多孔炭、水凝膠或炭氣凝膠等，減少了化石燃料帶來(lái)的環(huán)境污染。除此之外，生物質(zhì)炭材料來(lái)源廣泛、價(jià)格低廉且具有可再生的特點(diǎn)，因此可作為合成電化學(xué)電容器電極材料的重要原材料。然而，就目前生物質(zhì)炭材料做前驅(qū)體而言仍然存在兩個(gè)主要問(wèn)題：(1)灰分較高，前期處理比較繁瑣，例如動(dòng)物骨頭(灰分33%～43%)[19]和魚(yú)鱗(灰分46%)[30]；(2)含炭量或有機(jī)炭含量偏低，產(chǎn)率較低，例如杏仁殼(殘?zhí)?5%，多孔炭產(chǎn)率17%)[31]和甘蔗渣(殘?zhí)?9.5%，多孔炭產(chǎn)率34.8%)[4]。因此，尋找一種具有低灰分和高含炭量的生物質(zhì)炭材料是拓展其應(yīng)用的重要課題。

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Application of biomass-based carbon materials for electrochemical double-layer capacitors

By the consuming of chemical energy and deteriorating of environment, even though the electrode of electrochemical double-layer capacitors(EDLC)always consist of carbon materials,the production and application for traditional carbon materials have been challenged.Therefore,using biomass-based materials as precursor to prepare porous carbon has become a hot topic for EDLCs.The preparation and application progress of the biomass-based carbon materials were introduced.The influence of different raw materials and preparation methods on electrochemical properties was reviewed.

electrochemical double-layer capacitors;biomass-based carbon materials;porous carbon materials

TM 53

A

1002-087 X(2016)06-1334-03

2015-12-05

寧波市重大科技專項(xiàng)(2013B6003)

盧穎莉(1986—)，女，陜西省人，碩士研究生，主要研究方向?yàn)槌?jí)電容器的工程化制備與性能研究。

喬志軍，男，E-mail：zjqiao@csrcap.com