石墨烯气凝胶做的超级电容器，性能不仅仅是强悍，还有...
近期，科学家们报道了一种超级电容器电极的前所未有的性能结果。研究人员选用的是一种石墨烯气凝胶材料，采用3D打印机技术，构建了一个装有赝电容材料的多孔三维支架电极。在实验室测试中，新型电极实现了有史以来超级电容器报告中的最高面积电容（每单位电极表面积存储的电荷）。
利用3D打印技术制造石墨烯气凝胶/氧化锰超级电容器电极
加州大学圣克鲁兹分校和劳伦斯利弗莫尔国家实验室（LLNL）的科学家们近期报告了一种超级电容器电极具有的前所未有的性能结果。研究人员选用的是一种石墨烯气凝胶材料，采用3D打印机技术，构建了一个装有赝电容材料的多孔三维支架电极。
加州大学圣克鲁斯分校的化学和生物化学教授Yat Li表示，在实验室测试中，这种新型电极实现了有史以来超级电容器所报告的最高面积电容（每单位电极表面积存储的电荷）。而关于这项研究的成果以及支架电极的制备，于10月18日发表在《Joule》上。
作为一种储能设备，超级电容器，具有快速充电(几秒到几分钟)，且在通过数万次充电循环后依然保持其存储容量的优点。鉴于此，目前多被用于电动汽车和其他应用中的再生制动系统。与电池相比，它们不仅在相同的空间内储存的能量更少一些，而且它们的充电时间不长。但随着超级电容器技术的进步，其可能会有更广泛的应用领域，或许在某些领域上与电池形成竞争和互补关系。
在早期的研究中，UCSC和LLNL的研究人员展示了利用3D打印技术制备这种石墨烯气凝胶超快超级电容电极。在这项新研究中，他们主要使用了一种改进的石墨烯气凝胶材料来制造多孔支架，然后在支架上又装载了种常用的赝电容材料--氧化锰，从而构成了这种超快超级电容电极。
所谓的赝电容器，其实是一种超级电容器，其通过电极表面的反应来存储能量，使其比主要通过静电机制（称为电双层电容或EDLC）存储能量的超级电容器具有更像电池的性能。
Yat Li 还表示：“赝电容器的问题在于，当增加电极的厚度时，由于体结构中的离子扩散缓慢，电容会迅速下降。因此，我们目前面临的最大的挑战就是如何在增加赝电容器材料的质量负载的同时不牺牲其单位质量或体积的能量存储容量。”
针对以上的挑战，Yat Li的新研究成果表明，他们在平衡赝电容器中的质量负载和电容方面取得了突破。即研究人员能够在不影响性能的情况下，将质量负荷提高到每平方厘米100毫克以上的氧化锰的记录水平，而就目前的商用设备而言，质量负荷平均水平为每平方厘米10毫克左右。
Yat Li教授
最重要的是，面电容随着氧化锰的质量负载和电极厚度线性增加，而每克电容（重量电容）几乎保持不变。这表明，即使在如此高的质量负载下，电极的性能也不受离子扩散的限制。
论文的第一作者Bin Yao是加州大学圣克鲁斯分校李教授实验室的一名研究生，他解释说，在传统的超级电容器的商业制造中，会将一层薄薄的电极材料涂层应用到作为电流收集器的薄金属片上。但是随着涂层厚度的增加会导致性能下降，所以采用堆叠多个片材以构建电容的方法，但是由于金属集电体分布于在每一层，这不仅增加了整体重量，而且还增加了材料的成本。
Yao还表示：“通过我们的方法，我们不需要堆叠，因为我们可以通过使电极更厚同时，不牺牲性能来增加电容。”
具体来说，研究人员能够将电极厚度增加到4毫米，而没有任何性能损失。而且他们设计了具有周期性孔结构的电极，其不仅能够使材料均匀沉积，而且可以有效的离子扩散进行充放电。其中，3D打印的结构是由石墨烯气凝胶的圆柱棒构成的晶格。除了晶格结构中的孔之外，棒本身也是多孔的。然后将氧化锰电沉积到石墨烯气凝胶晶格上。
“这项研究的关键创新是使用3D打印来制造合理设计的结构，提供一个碳支架以支持赝电容材料，”Li说，“这些发现验证了使用3D打印制造储能设备的新方法。”
用石墨烯气凝胶/氧化锰电极制成的超级电容器装置显示出良好的循环稳定性，在20,000次充电和放电循环后依然可以保持超过90％的初始电容。3D打印的石墨烯气凝胶电极也具有极大的设计灵活性，因为3D打印机技术可以制成适合装置所需的任何形状。而且LLNL开发的可打印的石墨烯基油墨具有超高的表面积，轻质特性，弹性以及优异的导电性。
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原文内容
Scientists at UC Santa Cruz and Lawrence Livermore National Laboratory (LLNL) have reported unprecedented performance results for a supercapacitor electrode. The researchers fabricated electrodes using a printable graphene aerogel to build a porous three-dimensional scaffold loaded with pseudocapacitive material.
In laboratory tests, the novel electrodes achieved the highest areal capacitance (electric charge stored per unit of electrode surface area) ever reported for a supercapacitor, said Yat Li, professor of chemistry and biochemistry at UC Santa Cruz. Li and his collaborators reported their findings in a paper published October 18 in Joule.
As energy storage devices, supercapacitors have the advantages of charging very rapidly (in seconds to minutes) and retaining their storage capacity through tens of thousands of charge cycles. They are used for regenerative braking systems in electric vehicles and other applications. Compared to batteries, they hold less energy in the same amount of space, and they don't hold a charge for as long. But advances in supercapacitor technology could make them competitive with batteries in a much wider range of applications.
In earlier work, the UCSC and LLNL researchers demonstrated ultrafast supercapacitor electrodes fabricated using a 3D-printed graphene aerogel. In the new study, they used an improved graphene aerogel to build a porous scaffold which was then loaded with manganese oxide, a commonly used pseudocapacitive material.
A pseudocapacitor is a type of supercapacitor that stores energy through a reaction at the electrode surface, giving it more battery-like performance than supercapacitors that store energy primarily through an electrostatic mechanism (called electric double-layer capacitance, or EDLC). "The problem for pseudocapacitors is that when you increase the thickness of the electrode, the capacitance decreases rapidly because of sluggish ion diffusion in bulk structure. So the challenge is to increase the mass loading of pseudocapacitor material without sacrificing its energy storage capacity per unit mass or volume," Li explained.
The new study demonstrates a breakthrough in balancing mass loading and capacitance in a pseudocapacitor. The researchers were able to increase mass loading to record levels of more than 100 milligrams of manganese oxide per square centimeter without compromising performance, compared to typical levels of around 10 milligrams per square centimeter for commercial devices.
Most importantly, the areal capacitance increased linearly with mass loading of manganese oxide and electrode thickness, while the capacitance per gram (gravimetric capacitance) remained almost unchanged. This indicates that the electrode's performance is not limited by ion diffusion even at such a high mass loading.
First author Bin Yao, a graduate student in Li's lab at UC Santa Cruz, explained that in traditional commercial fabrication of supercapacitors, a thin coating of electrode material is applied to a thin metal sheet that serves as a current collector. Because increasing the thickness of the coating causes performance to decline, multiple sheets are stacked to build capacitance, adding weight and material cost because of the metallic current collector in each layer.
"With our approach, we don't need stacking because we can increase capacitance by making the electrode thicker without sacrificing performance," Yao said.
The researchers were able to increase the thickness of their electrodes to 4 millimeters without any loss of performance. They designed the electrodes with a periodic pore structure that enables both uniform deposition of the material and efficient ion diffusion for charging and discharging. The printed structure is a lattice composed of cylindrical rods of the graphene aerogel. The rods themselves are porous, in addition to the pores in the lattice structure. Manganese oxide is then electrodeposited onto the graphene aerogel lattice.
"The key innovation in this study is the use of 3D printing to fabricate a rationally designed structure providing a carbon scaffold to support the pseudocapacitive material," Li said. "These findings validate a new approach to fabricating energy storage devices using 3D printing."
Supercapacitor devices made with the graphene aerogel/manganese oxide electrodes showed good cycling stability, retaining more than 90 percent of initial capacitance after 20,000 cycles of charging and discharging. The 3D-printed graphene aerogel electrodes allow tremendous design flexibility because they can be made in any shape needed to fit into a device. The printable graphene-based inks developed at LLNL provide ultrahigh surface area, lightweight properties, elasticity, and superior electrical conductivity.