Diffusion layer thickness in a membrane system as determined from voltammetric and chronopotentiometric data
Diffusion layer thickness in a membrane system as determined from voltammetric and chronopotentiometric data
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DOI：
10.1134/s1023193510120074
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发表时间：
2010-12-01
影响因子：
1.2
通讯作者：
Pourcelly, G.
中科院分区：
文献类型：
--
作者：
Kozmai, A. E.;Nikonenko, V. V.;Pourcelly, G.
在远小于极限值的电流密度（delta（0））和极限电流密度（delta（lim））下比较离子交换膜表面附近溶液中扩散边界层（DBL）的厚度。膜的电流-电压曲线（CVC）的初始线性部分和其计时电位图的初始部分用于计算Δ（0）。δ（0）值在流通池中发现，其中活性膜面积为2 x 2 cm（2），三种膜为0.02 M NaCl溶液：AMX、Nafion-117和MK-40。结果表明，Nafion-117的delta（0）比AMX和MK-40小20%以上。后两种膜的δ（0）值非常接近，与对流扩散理论（Leveque方程）计算的值相差不大。在所有情况下，delta（0）显著大于通过相交切线的方法从极限电流密度的值得到的delta（lim），所述相交切线被绘制到CVC的初始段和倾斜平台。确定DBL厚度的依赖性，不仅对流体动力学条件的影响，但也膜表面的状态，进行了讨论。负责DBL厚度随电流增加而减小的主要现象被称为耦合对流，更可能是电对流。在表面的重要性质中，挑选出其电异质性和疏水性程度。阳离子和阴离子交换膜附近的电对流的不同速率与反离子的Stokes半径有关。后者解释了文献中众所周知的观察结果，即尽管Cl-离子的迁移率约为Na+离子的1.5倍，但在稀NaCl溶液中，阳离子和阴离子交换膜的极限电流密度大致相同。
The thickness of the diffusion boundary layer (DBL) in solution near the surface of an ion-exchange membrane is compared at current densities (delta(0)) much less than the limiting value and at the limiting current density (delta(lim)). The initial linear part of the current-voltage curve (CVC) of the membrane and the initial part of its chronopotentiogram are used to calculate delta(0). Values of delta(0) are found in a flow-through cell with an active membrane area of 2 x 2 cm(2) and 0.02 M NaCl solution for three membranes: AMX, Nafion-117, and MK-40. It is shown that delta(0) is more than 20% less for Nafion-117 than for AMX and MK-40. Values of delta(0) are close together for the latter two membranes and do not differ greatly from the value calculated from convective diffusion theory (the Leveque equation). In all cases, delta(0) is significantly greater than delta(lim) found from the value of the limiting current density by the method of intersecting tangents, which are drawn to the initial segment of the CVC and to the sloping plateau. The effects that determine the dependence of DBL thickness on not only hydrodynamic conditions, but also the state of the membrane surface, are discussed. The principal phenomenon responsible for the decrease in DBL thickness with increasing current is termed coupled convection, more likely, electroconvection. Among the significant properties of the surface are singled out its electrical heterogeneity and degree of hydrophobicity. The different rate of electroconvection near cation- and anion-exchange membranes is related to the Stokes radius of the counterions. The latter explains the well-known observation in the literature that the limiting current density in dilute NaCl solutions is approximately the same for cation- and anion-exchange membranes in spite of the fact that the mobility of Cl- ions is approximately 1.5 times higher than that of Na+ ions.