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Effect of an Iodine Film on Charge-Transfer Resistance during the Electro-Oxidation of Iodide in Redox Flow Batteries
ACS Applied Materials & Interfaces ( IF 8.3 ) Pub Date : 2021-01-27 , DOI: 10.1021/acsami.0c22895 Won Joon Jang 1, 2 , Jin Seong Cha 1, 3 , Hansung Kim 2 , Jung Hoon Yang 1
ACS Applied Materials & Interfaces ( IF 8.3 ) Pub Date : 2021-01-27 , DOI: 10.1021/acsami.0c22895 Won Joon Jang 1, 2 , Jin Seong Cha 1, 3 , Hansung Kim 2 , Jung Hoon Yang 1
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The use of iodide as the positive redox-active species in redox flow batteries has been highly anticipated owing to its attractive features of high solubility, excellent reversibility, and low cost. However, the electro-oxidation reaction of iodide (I–) is very complicated, giving various possible products such as iodine (I2), polyiodides (I2n+1–), and polyiodines (I2n+2) with n ≥ 1. In particular, the electro-oxidation of I–/I3– and I3–/I2 occurs in competition depending on the applied potential. Although the former reaction is adopted as the main reaction in most redox flow batteries because I3– is highly soluble in an aqueous electrolyte, the latter reaction inevitably occurs together and a thick I2-film forms on the electrode, impeding the electro-oxidation of I–. In this study, we investigate the variation of the interface between the electrode and the electrolyte during the development of an I2-film and the corresponding change in the charge-transfer resistance (Rct). Initially, the I2-film builds upon the electrode surface in the form of a porous layer and the aqueous I– ions can easily reach the electrode surface through pores inside the film. I– ions are electro-oxidized to I3– or I2 at the interface between the aqueous I– phase and electrode with a small Rct of less than 16.5 ohm·cm2. Over time, the I2-film is converted into a dense layer and I– ions diffuse through the film in the form of I3–, possibly by a Grotthuss-type hopping mechanism. I3– can then be electro-oxidized to I2 at the new interface between the I2-film and electrode, resulting in a dramatic 9-fold increase of Rct to 147.4 ohm·cm2. This increase of Rct by the dense I2-film is also observed in the actual flow battery. At high current densities above 400 mA·cm–2, the overpotential begins to show an abrupt increase in the amplitude of more than 300 mV after reaching a critical charging capacity at which the dense I2-film appears to have begun to form on the felt electrode. Therefore, the I2-film exerts a serious negative effect on the performance of the flow battery depending on the current density and electrolyte SoC (state-of-charge).
中文翻译：
氧化还原液流电池中碘化物的电氧化过程中，碘膜对电荷转移电阻的影响
由于碘化物具有高溶解度，优异的可逆性和低成本的吸引人的特性，因此人们高度期待将碘化物用作氧化还原液流电池中的正极性氧化还原活性物质。但是，碘化物（I –）的电氧化反应非常复杂，会产生各种可能的产物，例如碘（I 2），多碘化物（I 2 n +1 –）和多碘（I 2 n +2），其中n ≥1.特别是I – / I 3 –和I 3 – / I 2的电氧化在竞争中发生取决于所应用的潜力。尽管前者反应被采用作为在大多数氧化还原液流电池的主要反应，因为我3 -是在含水电解质中高度可溶，后者反应不可避免地出现一起和厚的余2的电极上-film形式，妨碍电氧化我- 。在这项研究中，我们研究了在I 2膜形成过程中电极与电解质之间的界面变化以及电荷转移电阻（R ct）的相应变化。最初，I 2膜以多孔层的形式沉积在电极表面，含水的I –离子很容易通过薄膜内部的孔到达电极表面。我-离子被电氧化成我3 -或I 2在水溶液我之间的界面-相和电极具有小的- [R CT小于16.5欧姆的·厘米2。随着时间的推移，该I 2 -film被转换成一个致密层和我-离子通过膜扩散以I形式3 - ，可能由格罗特胡斯型跳频机制。I 3 –然后可以在I 2之间的新界面处电氧化为I 2膜和电极，导致R ct显着增加9倍，达到147.4 ohm·cm 2。在实际的液流电池中也观察到由致密的I 2膜引起的R ct的增加。在高于400 mA·cm –2的高电流密度下，在达到临界充电容量后，过电位开始突然超过300 mV的幅度增加，在该临界充电容量下，密集的I 2膜似乎已经开始形成。毡电极。因此，取决于电流密度和电解质SoC（充电状态），I 2膜对液流电池的性能产生严重的负面影响。
更新日期：2021-02-10
中文翻译：
氧化还原液流电池中碘化物的电氧化过程中，碘膜对电荷转移电阻的影响
由于碘化物具有高溶解度，优异的可逆性和低成本的吸引人的特性，因此人们高度期待将碘化物用作氧化还原液流电池中的正极性氧化还原活性物质。但是，碘化物（I –）的电氧化反应非常复杂，会产生各种可能的产物，例如碘（I 2），多碘化物（I 2 n +1 –）和多碘（I 2 n +2），其中n ≥1.特别是I – / I 3 –和I 3 – / I 2的电氧化在竞争中发生取决于所应用的潜力。尽管前者反应被采用作为在大多数氧化还原液流电池的主要反应，因为我3 -是在含水电解质中高度可溶，后者反应不可避免地出现一起和厚的余2的电极上-film形式，妨碍电氧化我- 。在这项研究中，我们研究了在I 2膜形成过程中电极与电解质之间的界面变化以及电荷转移电阻（R ct）的相应变化。最初，I 2膜以多孔层的形式沉积在电极表面，含水的I –离子很容易通过薄膜内部的孔到达电极表面。我-离子被电氧化成我3 -或I 2在水溶液我之间的界面-相和电极具有小的- [R CT小于16.5欧姆的·厘米2。随着时间的推移，该I 2 -film被转换成一个致密层和我-离子通过膜扩散以I形式3 - ，可能由格罗特胡斯型跳频机制。I 3 –然后可以在I 2之间的新界面处电氧化为I 2膜和电极，导致R ct显着增加9倍，达到147.4 ohm·cm 2。在实际的液流电池中也观察到由致密的I 2膜引起的R ct的增加。在高于400 mA·cm –2的高电流密度下，在达到临界充电容量后，过电位开始突然超过300 mV的幅度增加，在该临界充电容量下，密集的I 2膜似乎已经开始形成。毡电极。因此，取决于电流密度和电解质SoC（充电状态），I 2膜对液流电池的性能产生严重的负面影响。
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