生物质炭材料在双电层电容器中的应用

2016-04-05卢颖莉傅冠生乔志军

电源技术 2016年6期

关键词：电层微孔电流密度

卢颖莉，傅冠生，杨斌，乔志军

(宁波南车新能源科技有限公司，浙江宁波315112)

生物质炭材料在双电层电容器中的应用

卢颖莉，傅冠生，杨斌，乔志军

(宁波南车新能源科技有限公司，浙江宁波315112)

双电层电容器电极材料一般由炭材料组成，随着化石能源的日益枯竭和环境污染的逐步恶化，传统炭材料的生产和应用受到了挑战。采用绿色环保的生物质作为碳源，制备双电层电容器电极材料已成为研究热点之一。介绍了近几年来生物质炭材料的制备及其在双电层电容器中的应用进展，综述了生物质前驱体和制备方法对双电层电容器电化学性能的影响。

双电层电容器；生物质炭材料；多孔炭

双电层电容器 (electrochemical double-layer capacitors, EDLC)是依靠静电作用力在电极上储存电荷的一种储能器件，与传统依赖于电荷转移反应的电池不同，EDLC具有高功率密度、长使用寿命、长放置时间、宽使用温度、高安全性等优点。根据EDLC的储能机理，其电容是由电极材料的静电吸附提供，因此具有大比表面积的多孔炭是EDLC最常见的电极材料。多孔炭通常是从石油焦炭或煤衍生而来，但是随着矿物日益减少以及这些传统材料的价格不断上升，人们越来越多的将目光投向新资源的开发。

生物质具有来源广、价格低廉、可再生和绿色环保的优点，它作为碳源制备EDLC电极材料已成为储能领域研究的热点。目前以生物质为前驱体，采取不同方法制备多孔炭电极材料是主要发展方向。根据不同的制备方法和多孔炭孔道结构，这些生物质基多孔炭被分为四种：活性炭、介孔炭、分级多孔炭和炭气凝胶。本文综述了目前生物质基多孔炭用于EDLC电极材料的研究进展。

1 生物质基多孔炭

1.1 活性炭

活性炭是生物质炭材料的主要衍生产品，目前商业化的生物质活性炭(biomass materials-based activated carbons，BMACs)主要是椰壳基活性炭系列，其商业化YP17(Kuraray Chemical Corporation)在6mol/L KOH溶液中的比电容仅为158 F/g[1]，因此开发具有更高电容的生物质活性炭迫在眉睫。近年来，玉米[2]、咖啡豆[3]、甘蔗渣[4]、稻米壳[5]、木头[6]、竹子[7]、木薯皮[8]等生物质作为炭材料被用于活性炭电极材料。采用这些原材料制备的活性炭比电容均高于椰壳基活性炭，例如马铃薯淀粉基活性炭在6mol/L KOH溶液中甚至具有335 F/g的比电容(电流密度50 mA/g)[9]；麦秆基活性炭在1.2mol/L MeEt3NBF4/ AN有机电解液中呈现251.1 F/g(2 mV/s扫速)的比电容[10]。虽然在低扫描速度或低电流密度下，生物质活性炭具有高的比电容，然而生物质活性炭的孔大多数是由微孔构成，例如木头和竹子的微孔率高达70%以上(孔径小于2nm)[6-7]。研究表明在高扫描速度或电流密度下，因电解液离子尺度的问题使其迁移内阻变大，不能快速进入微孔，导致比电容明显下降，ESR增大，电压下降明显(IR drop)[11]。因此具有较多微孔结构的生物质活性炭在大电流充放电时，电容特性表现不佳，例如鸡羽毛基活性炭的BET比表面积(SSA)和微孔比表面积分别为1 839和1 575 m2/g，其较高的微孔比表面积使其在1 A/g(1mol/L H2SO4电解液)时具有302 F/g的比电容且压降为0.12 V，然而当电流密度为10 A/g时，比电容为168 A/g且IR降更明显，其容量保持率为55.6%(从1 A/g到10 A/g)，如图1[12]所示。

图1 鸡羽毛基活性炭的透射电镜图(a)(b)和恒流充放电曲线(c)

1.2 介孔炭

生物质活性炭的高微孔率使之大电流特性不佳，因此介孔炭(孔径＞2nm)或高中孔率多孔炭成为生物质炭材料的另一重要产品。H.L.Lu等[13]以硅为模板，采用葡萄糖和硼酸为前驱体制备得到孔径分布在 2～20nm的有序介孔炭(ordered mesoporous carbons,OMCs)，虽然OMC-M-6的SSA仅为868.5 m2/g，但是其具有高达90%的中孔，结果表明OMC-M-6在30%(质量分数)KOH电解液中在1 A/g电流密度下表现出202.8 F/g的比电容，在10 A/g下表现出172.5 F/g的比电容，容量保持率为85.1%(从1 A/g到10 A/g)且未出现明显的压降。高中孔率多孔炭与介孔炭具有相似的电化学性能，例如稻米壳基多孔炭[14]和花生壳基多孔炭(mesopore percentage＞82%)[15-16]。与BM-ACs相比，生物质介孔炭或高中孔率多孔炭的优异性能更体现在大电流密度下，但是在小电流密度下，其比电容或能量密度偏低。稻米壳ZnCl活化后，具有1 500 m2/g比表面积的两种多孔炭，具有较低中孔率(85.2%)的多孔炭比较高中孔率(99.0%)的多孔炭拥有更高的比电容、更稳定的循环性能和更高的容量保持率，表明多孔炭的比电容不仅与中孔有关，而且与微孔有关。因为微孔是电荷储存的主要场所，中孔是离子传输的通道。

1.3 分级多孔炭

具有高微孔的活性炭在小电流密度下拥有更高的比电容，具有一定微孔结构和较高中孔结构的多孔炭在大电流密度下拥有更高的比电容和容量保持率，因此多孔炭的孔径分布合理性是其作为EDLCs电极材料的必备条件。由于分级多孔炭(hierarchical porous carbons，HPCs)的大孔(＞50nm)结构提供电解液储存场所，中孔(2～50nm)结构提供离子传输通道，而微孔提供离子储存场所，因此目前分级多孔炭是拥有最合理孔道结构的电极材料。例如淀粉基HPCs[17]，香蕉皮基HPCs[18]，动物骨头基HPCs[19]和鱼鳞基HPCs[20]。淀粉基HPCs的球形结构、分级的孔道结构(相互连通的大-中-微孔结构)、超高的SSA(3 251 mm2/g)和高中孔率(＞75%)使其在6mol/L KOH溶液中表现出良好的电化学性能，0.05 A/g时比电容为304 F/g，180 A/g时比电容为197 F/g，且10 000次循环后仍能保持98%，如图2[21]所示。

1.4 炭气凝胶

生物质材料可以通过溶胶-凝胶、超临界干燥、冷冻干燥、水热等方法合成生物质基水凝胶或炭气凝胶 (hydrogels or aerogels)，例如 β-葡聚糖[22-23]、木质纤维素[24-25]、西瓜[26]、纤维素[27]、甲壳素[28-29]、淀粉[30]等。这些炭气凝胶具有丰富的大-中孔结构和优越的导电网络，因此是EDLCs电极材料的重要选择之一。西瓜基炭气凝胶及其电化学性能如图3[26]所示。

图2 淀粉基HPCs的表观形貌(a～e)、倍率性能(f)和循环性能(g)

图3 西瓜基炭气凝胶及其电化学性能

2 结论

生物质炭材料作为前驱体被广泛应用于生产活性炭、介孔炭、分级的多孔炭、水凝胶或炭气凝胶等，减少了化石燃料带来的环境污染。除此之外，生物质炭材料来源广泛、价格低廉且具有可再生的特点，因此可作为合成电化学电容器电极材料的重要原材料。然而，就目前生物质炭材料做前驱体而言仍然存在两个主要问题：(1)灰分较高，前期处理比较繁琐，例如动物骨头(灰分33%～43%)[19]和鱼鳞(灰分46%)[30]；(2)含炭量或有机炭含量偏低，产率较低，例如杏仁壳(残炭25%，多孔炭产率17%)[31]和甘蔗渣(残炭19.5%，多孔炭产率34.8%)[4]。因此，寻找一种具有低灰分和高含炭量的生物质炭材料是拓展其应用的重要课题。

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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

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