导电石墨烯孔道内双电层结构的研究.pdf

第41卷第6 期2024年12 月J.At.Mol.Phys.,2024,41:062005(12pp)原子与分子物理学报JOURNAL OF ATOMIC AND MOLECULAR PHYSICS导电石墨烯孔道内双电层结构的研究Vol.41 No.6Dec.2024董乐，蒋更平（武汉科技大学高性能钢铁材料及其应用省部共建协同创新中心国际钢铁研究院，武汉430 0 8 1）摘要：为有效开发和利用新能源，人们迫切需要高性能的超级电容器提供能量的存储和转换，在超级电容器中双电层结构扮演着关键性的角色：本文利用分子动力学方法通过建立开放的石墨烯纳米孔道（12 nm）,研究了KCl溶液在纳米孔道内的双电层结构,同时也比较了恒电量模拟(Q)和恒电势模拟法(U)下双电层结构的异同：结果表明在恒电势模拟法考虑了导电石墨烯壁的镜像作用使结果更符合实验中的材料系统，而石墨烯壁的镜像作用能额外吸附离子从而增强孔道内部的阴阳离子，这可能有助于电极电容的提升：通过对不同孔道高度的研究，本文发现水分子作为介电材料在水基超级电容器中发挥着决定性的作用，它能在很大程度上抵消不同离子和不同孔道高度下双电层的变化，从而在不同情况下获得了相似的电容。关键词：分子动力学模拟；超级电容器；恒电势模拟方法；石墨烯中图分类号：0 48 5Study onelectric double layers inside the conducting graphene nanopores文献标识码：AD0I:10.19855/j.1000-0364.2024.062005DONG Le,JIANG Geng-Ping(Collaborative Innovation Center for Advanced Steels,International Research Institute for Steel Technology,Wuhan 430081,China)Abstract:In order to effectively develop and utilize new energy,people urgently need high-performance super-capacitors to provide energy storage and conversion.Electric double layer structure plays a key role in superca-pacitors.In this work,the molecular dynamics simulation was taken to an open graphene nanochannel(1 2nm),and the double-layers structure of KCl solution inside the nanochannel was studied.Meanwhile,the sim-ilarities and diferences of the double-layer structure under fixed charge simulation(Q)and constant potentialsimulation(U)were compared.The results show that the mirror image effect of the conducting graphene wall isconsidered in the constant potential simulation method,which makes the results more consistent with the practicalmaterials.The mirror image effect can absorb extra ions.Thus it enhances the concentratiosn of anion and cationin the pore,which may contribute to the improvement of electrode capacitance.Through the study of differentchannel heights,it is found that water molecules play a decisive role as dielectric materials in water-based su-percapacitor.It can largely offset the change of electric double layer under different ions and different channelheights,resulting in a similar capacitance under dfferent conditions.Key words:Molecular dynamics simulation;Supercapacitor;Constant-potential simulation method;Graphenepacitor)作为超级电容器的一种，是一种新型储能1 引 言装置，其充放电过程完全没有涉及到化学变化，双电层电容器（Electrical Double-Layer Ca-超级电容器储能具有充放电速度快、功率密度大、收稿日期：2 0 2 3-0 3-2 9基金项目：国家自然科学基金青年科学基金项目（2 190 52 15）作者简介：董乐（1998 一），女，湖北武汉人，硕士研究生，研究方向为分子动力学模拟.E-mail：2 6 36 6 6 2 7 13 q q.c o m通讯作者：蒋更平.E-mail：G e n g p i n g j i a n g w u s t.e d u.c n062005-1第41卷循环稳定性优异、清洁环保等优势，因此得到人们的广泛关注：石墨烯作为最早被发现的二维材料，是一种单层的蜂窝状的碳晶体，拥有许多优良的物理、化学、电学、力学性质2-4。通过对石墨烯内部结构的研究，发现表面化学和溶液成分的调控能实现比传统材料高得多的比电容和更稳定的循环性能5。Huang等人 提出了一个在小微孔中形成的模型，即electrici n c y l i n d e r 模型，有别于传统的双电层模型Jiang等人7 通过分子动力学模拟研究发现较大的离子在较强的外加电场作用下会去溶剂化并穿透水层直接吸附于电极表面形成亥姆霍兹层Paek等人8 通过对碳纳米管和石墨烯表面双电层结构的研究揭示了电极外表面曲率半径对实验结果的影响Kalluri 等人9 展示了水合离子在纳米孔道约束下的脱水作用（h1.3nm时，孔道内同离子升高的幅度，与对离子降低的幅度相同这意味着在这一范围内，孔道的溶液以离子交换的形式极化2 9 即，以一个阳离子置换一个阴离子的形式来平衡表面电荷而当h1.3nm)，在h=1.8nm内孔道内，得到了反常的离子分布如U=+0.5V时，孔道内的阳离子浓062005-10-61.2-4-2h(A)1.41.6h(nm)02468101.82.0第41卷度基本不变（0.6 99M)溶液的极化作用仅由阴离子的注人完成的：如果进一步增大阳极的电压（U=+1.0 V)，则阳离子浓度降低的幅度（1.84 M)与阴离子升高的幅度（0.2 2 M)相匹配.也就是说在1.8 nm的阳极孔道中，首先以对离子注入完了溶液的极化，然后伴随有阴阳离子的交换过程类似的反常现象也可以在1.8 nm的阴极孔道中观察得到当U=0.5V或-1.0 V时，孔道内部的CI-浓度基本不变，始终维持在比较高的浓度（0.48 9)这一反常结果可能是应为石墨烯壁上双电层结构干涉相作用和镜像作用所共同造成的.通过以上的恒电势模拟，本文也计算了不同孔道内部双电层结构的电容，即C=Q/AU.Q为电极上平均的电荷数目在恒电势模拟中U为阴阳两电极间的电势差，这一计算得到的电容是阴阳两电极所组成的电容器的整体电容，表1为电压为U=0.5V与1.0 V时的电容我们发现这一结果与此前Jiang7的文章中恒电荷条件下计算得到的结果相差不大如U=0.5V或U=1.0V时，比电容大小为1.41 1.44F/g.而Jiang7的文章中当Q=0.041 e时，KCl 溶液的比电容也为1.44 F/g.表1不同孔道宽度下电容器的质量比电容Table 1 The specific capacitances of nanochannel with thedifferent channel heights孔道宽度(nm)0.5V11441.21431.51451.81412147比较不同孔道宽度时，计算得到的双电层电容却几乎保持不变尽管在不同孔道高度下内部的双电层有着截然不同的分布结构（离子峰的分布和高度）但最终的电极电容却几乎一致相较于离子，中性的水分子的数目在溶液中占主导地位（1:55.4）因此，水分子对超级电容器的储能性能的起到关键作用7 不同离子分布导致的电容变化，反过来被作为电介质物质的水分子所屏蔽这最终导致了类似的电极电容。董乐，等：导电石墨烯孔道内双电层结构的研究质量比电容(F/g)1.0V141144150143144第6 期4 结 论本章使用分子动力学模拟研究了纳米尺度下的石墨烯孔道内的双电层结构，采用恒电量模拟(FCM)与恒电势模拟(CPM)两种不同方法分别对不同孔道高度的下的石墨烯孔道进行了充电与不充电的模拟，研究了不同孔道宽度、电量与电极电压下水分子与阴阳离子的在孔道内的分布，主要得到以下结论：1.在不充电情况下，对比了恒电量（Q=0）与恒电势(U=0)模拟下孔道内部的溶液分布，两者有着类似的水分子的分布结构但恒电势模拟下，极板具有镜像作用这增强了孔道内部的离子浓度。2随着孔道高度的降低，离子进人孔道内部需要脱去周围的水合结构这导致了内部的离子浓度低于孔道外的溶液区域（1.3 1.8 M)同时随着孔道高度的降低，孔道的约束作用强烈，这导致了在入口处更高的能垒，这降低了孔道内的离子浓度，随着电极电压的上升，在大孔径内(h 1.3nm)溶液的极化以离子交换为主：而当h1.3nm，极化的主要方式是对离子的注人：3通过计算不同孔道高度下的电容，发现电极电容基本保持不变，因为水分子是影响电容的重要因素对于在不同孔道内由不同离子分布而产生的变化，会反过来被水分所屏蔽，这导致了类似的电极电容，参考文献：1Chen J,Wei H,Fu N,et al.Facile synthesis of nitro-gen-containing porous carbon as electrode materialsfor superior-performance electrical double-layer ca-pacitorsJJ.J.Mater.Sci.,2018,53:2137.2Huang G R,Chen J.Synthesis and applications of gra-phene J.Carbon Tech.,2009,28:35(in Chinese)黄桂荣，陈建石墨烯的合成与应用J炭素技术,2 0 0 9,2 8:353Huang G J,Chen Z G,Li M D,et al.Surface func-tionalization modification of graphene and graphene ox-ide J.Acta Chim.Sin.,2016,74:789（i n Ch i-nese）【黄国家，陈志刚，李茂东，等石墨烯和氧化石墨烯的表面功能化改性J化学学报，2 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