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上海世博会并没有给我们留下多么深刻的印象,但是世博会上批量亮相的超级电容公交车却让人眼前一亮。随着文献阅读量的增加,我们发现绝大多数的文章只是在做烧点多孔碳,测测储电性能之类的工作。而电极材料的微观结构对比电容影响的研究却是少之又少。根据传统的电容器理论,人们首先想到的就是通过提高多孔碳的比表面积来实现高的比电容。可是问题在这里出现了,目前的多孔碳材料的面积比电容远远低于理论计算值,为什么?2006年的一篇Science文章又把大伙都带到微孔碳制备的方向去,紧接着多级孔碳也成为超级电容器领域的热门。可是,对于一个理想的超级电容器材料,其孔结构究竟该是什么样子的,依然是众说纷纭。
利用表面自组装结合Bergman环化反应,我们可以在任何拓扑形貌的表面上“贴”上去一层碳纳米膜,将这些材料做成“碳”材料的样子(IP02-利用Bergman环化反应制备具三维形貌的碳纳米膜)。前期的研究结果也发现这些介孔硅支持的碳纳米膜(SS-CNM)具有良好的导电性(J. Mater. Chem. A, 2013, 1, 3171),为此,我们可以利用这些SS-CNM为电极材料,探索电极材料的孔结构对双电层型超级电容器(EDLC)性能的影响。介孔硅的制备方法已经是很成熟的了,我们选择了四种具有同样空间群(p6mm)的介孔硅制备得到四种SS-CNM。它们都具有长且直的六角孔结构,唯一的区别是孔径不一。由于碳纳米膜几乎全覆盖住了这些介孔硅中原有的微孔,使得这些SS-CNM成为孔径分布单一的纯粹介孔“碳”材料,非常适合研究孔径对比电容的影响。
实验结果发现,比电容和比表面积没有直接的关系。这其实很容易理解(尽管这个领域的研究者大多不认可这种理解),比表面积多是采用氮气吸附的方法测定,采用BET方法计算而得(当然更有甚者用DFT方法去计算,给出高得吓人的数值),而比电容则取决于带电粒子在电极表面上的吸附。一者,溶剂化的带电离子和中性的氮气分子相差甚远;二者,气/固界面的吸附行为和液/固表面的吸附行为相差甚远。用气体吸附的数值指导带电离子的集聚行为无异于南辕北辙。实验结果还发现一个有意思的现象,随着介孔孔径的增大,比电容呈现先上升后下降的现象。比电容的最高值出现在孔径4nm附近。
这个奇特的现象无法用之前的任何一种理论去解释。经过一个寒假苦苦思索,我们决定自己建立一套模型。这个决定最终导引了史上最为惨烈的投稿过程。在这个简化了模型里,我们引进了分数维度的概念,而将比表面积完全摈弃。模型给出的结果呈现出两个极大点,一个点对应于孔径和溶剂化离子直径相同的位置,而另外一个极大点则出现在孔径是溶剂化离子直径的2.5倍左右。考虑到溶剂化离子的直径可能大至1.5nm,与实验值很吻合。
根据这个模型,我们提出一个理想的超级电容器电极材料的结构模型,并取了一个好听的名字:honeycomb shaped honeycomb(具有蜂窝状介孔结构的石墨烯)。
文章数经周折,近20个月的torture后最终发表于RSC Adv(2014,40296-40300)。也许如同学生说的那样,如果单单是报道碳材料的制备和并表征一下EDLC的性能,至少可以混个JMC级别的文章,who knows?我知道这个模型的确是相当简化,不完美,只是希望它会给沉迷于提高电极材料比表面积的研究者们一点侧面提醒(或许吧)。
一点花絮调节一下气氛。
论文投稿中期时我们投给了EES,送审结果是一个同意,一个让改投低些影响因子的期刊,一个修改,自然没有通过。要求修改的那位除了提了几个问题外还指出我们没有引他们组的另外两篇文章。反正EES拒稿了,也没啥可说的。最终,论文投稿到RSC Adv时,编辑部说,你们前面投稿过EES,也送审过了,要不先回答一下EES评阅人的问题,让他们看看。于是,我们翻出那些comments,一一作出答复。前两个同意,第三个也同意,但是强烈要求我们引他们的那两篇文章(我们在答复时说那两篇文章是重复性工作,不需要引用)。以下就是双方对话了:
reviewer 3(具体页码略去了,为同为华人的reviewer讳)
The authors tried to address my previous comments by adding ... This of course I can understand. But the authors' refusal to cite the two papers (J. Phys. Chem. Lett., xxxx (2013); J. Phys. Chem. Lett., xxxx (2012) indicates that the authors didn't really understand what these papers were trying to address. The Nano Letter paper is about ionic liquids as an electrolyte, while the JPCL 2012 paper is about an organic electrolyte, that is, a salt in a solvent. This JPCL 2012 paper is therefore highly relevant to the present paper. I hope that the authors can appreciate this difference, since the behavior is very different between ionic liquids and organic electrolytes. The JPCL 2013 paper is a review to compare the ionic liquids and organic electrolytes. So I think the authors should at least cite the JPCL 2012 paper.
回复如下:
Reply: The original work from these authors was cited in our paper (xxx, Nano Lett., 2011, xxx). We are now even more confused why the referee urged us to cite his/her JPCL papers again. It is improper for a referee to do such a kind of thing in the peer reviewing system. We have carefully read through all these three papers and found that the concepts of these papers were indeed the same, the model in these papers were indeed similar to each other. We cited the Nano Lett. paper only because it was an original work. We did not cite the two JPCL papers only because they were simply follow-up work. As the referee also agreed that the difference of the papers were the electrolytes (one was ILs, the other was organic electrolytes). This difference however does not make the follow-up papers important enough to be comparable to the original one and other papers we cited. Should these two papers be cited, we may have to add hundreds more references of the same importance. Therefore, we are sorry to this referee and insist on only citing the original paper.
最终结果是RSC Adv编辑部没有再为此时纠缠下去,直接接受。
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