Kossenko Alexey, Lugovskoy Svetlana, Averbukh Moshe
Department of Chemical Engineering, Ariel University, Ariel 40700, Israel.
Department of Electrical/Electronic Engineering, Ariel University, Ariel 40700, Israel.
Materials (Basel). 2018 Apr 23;11(4):650. doi: 10.3390/ma11040650.
Electroconductive carbon felt (CF) material, having a permeable structure and significant electroconductive surface, is widely used for electrodes in numerous electrochemical applications such as redox flow batteries, fuel cells, electrochemical desalination apparatus, etc. The internal structure of CF is composed of different lengths of carbon filaments bonded together. This structure creates a large number of stochastically oriented and stochastically linked channels that have different lengths and cross sections. Therefore, the CF hydraulic permeability is similar to that of porous media and is determined by the internal empty volume and arrangement of carbon fibers. Its electroconductivity is ensured by the conductivity of the carbon filaments and by the electrical interconnections between fibers. Both of these properties (permeability and electrical conductivity) are extremely important for the efficient functioning of electrochemical devices. However, their influences counter each other during CF compressing. Increasing the stress on a felt element provides supplementary electrical contacts of carbon filaments, which lead to improved electrical conductivity. Thus, the active surface of the felt electrode is increased, which also boosts redox chemical reactions. On the other hand, compressed felt possesses reduced hydrodynamic permeability as a result of a diminished free volume of porous media and intrinsic channels. This causes increasing hydrodynamic expenditures of electrolyte pumping through electrodes and lessened cell (battery) efficiency. The designer of specific electrochemical systems has to take into account both of these properties when selecting the optimal construction for a cell. This article presents the results of measurements and novel approximating expressions of electrical and hydraulic characteristics of a CF during its compression. Since electrical conductivity plays a determining role in providing electrochemical reactions, it was measured in dry conditions and when the CF was immersed in several non-conductive liquids. The choice of such liquids prevented side effects of electrolyte ionic conductivity impact on electrical resistivity of the CF. This gave an opportunity to determine the influences of dielectric parameters of electrolytes to increase or decrease the density of interconnectivity of carbon fibers either between themselves or between them and electrodes. The experiments showed the influence of liquid permittivity on the conductivity of CF, probably by changing the density of fiber interconnections inside the felt.
具有可渗透结构和显著导电表面的导电碳毡（CF）材料，被广泛用于众多电化学应用中的电极，如氧化还原液流电池、燃料电池、电化学脱盐装置等。CF的内部结构由不同长度的碳丝粘结在一起组成。这种结构形成了大量随机取向和随机连接的通道，这些通道具有不同的长度和横截面。因此，CF的水力渗透率类似于多孔介质的水力渗透率，并且由碳纤维的内部空隙体积和排列决定。其导电性由碳丝的导电性以及纤维之间的电连接来保证。这两种特性（渗透率和导电性）对于电化学装置的高效运行都极为重要。然而，在CF压缩过程中，它们的影响相互抵消。增加对毡元件的压力会提供碳丝的补充电接触，这会导致导电性提高。因此，毡电极的活性表面增加，这也促进了氧化还原化学反应。另一方面，由于多孔介质和固有通道的自由体积减小，压缩后的毡具有降低的流体动力渗透率。这导致通过电极泵送电解质的流体动力消耗增加，以及电池（电池组）效率降低。特定电化学系统的设计者在为电池选择最佳结构时必须考虑这两种特性。本文介绍了CF在压缩过程中的电学和水力特性的测量结果以及新的近似表达式。由于导电性在提供电化学反应中起决定性作用，因此在干燥条件下以及CF浸入几种非导电液体中时对其进行了测量。选择这些液体可防止电解质离子电导率对CF电阻率的影响产生副作用。这使得有机会确定电解质介电参数对增加或降低碳纤维自身之间或它们与电极之间的互连密度的影响。实验表明，液体介电常数可能通过改变毡内部纤维互连的密度来影响CF的导电性。
