Department of Polymer Science and Engineering , University of Massachusetts Amherst , Amherst , Massachusetts 01003 , United States.
Department of Materials Science and Engineering , The Pennsylvania State University , University Park , Pennsylvania 16802 , United States.
J Am Chem Soc. 2018 Jun 27;140(25):7961-7969. doi: 10.1021/jacs.8b03979. Epub 2018 Jun 14.
As the field of anion exchange membranes (AEMs) employs an increasing variety of cations, a critical understanding of cation properties must be obtained, especially as they relate to membrane ion conductivity. Here, to elucidate such properties, metal cation-based AEMs, featuring bis(norbornene) nickel, ruthenium, or cobalt complexes, were synthesized and characterized. In addition, isothermal titration calorimetry (ITC) was used to probe counterion exchange thermodynamics in order to understand previously reported differences in conductivity. The ion conductivity data reported here further demonstrated that nickel-complex cations had higher conductivity as compared to their ruthenium and cobalt counterparts. Surprisingly, bulk hydration number, ion concentration, ion exchange capacity, and activation energy were not sufficient to explain differences in conductivity, so the thermodynamics of metal cation-counterion association were explored using ITC. Specifically, for the nickel cation as compared to the other two metal-based cations, a larger thermodynamic driving force for chloride counterion release was observed, shown through a smaller Δ H for counterion exchange, which indicated weaker cation-counterion association. The use of ITC to study cation-counterion association was further exemplified by characterizing more traditional AEM cations, such as quaternary ammoniums and an imidazolium cation, which demonstrated small variances in their enthalpic response, but an overall Δ H similar to that of the nickel-based cation. The cation hydration, rather than its hydration shell or the bulk hydration of the membrane, likely played the key role in determining the strength of the initial cation-counterion pair. This report identifies for the first time how ITC can be used to experimentally determine thermodynamic quantities that are key parameters for understanding and predicting conductivity in AEMs.
随着阴离子交换膜（AEMs）领域采用越来越多的阳离子，必须深入了解阳离子的特性，尤其是与膜离子电导率相关的特性。在这里，为了阐明这些特性，我们合成并表征了基于金属阳离子的 AEMs，其特征是含有双（降冰片烯）镍、钌或钴配合物。此外，还使用等温滴定微量热法（ITC）来探究抗衡离子交换热力学，以理解之前报道的电导率差异。这里报告的离子电导率数据进一步表明，镍配合物阳离子的电导率高于其钌和钴对应物。令人惊讶的是，体相水合数、离子浓度、离子交换容量和活化能不足以解释电导率的差异，因此使用 ITC 研究了金属阳离子-抗衡离子缔合的热力学。具体而言，与其他两种基于金属的阳离子相比，镍阳离子观察到释放氯离子的热力学驱动力更大，这表现为离子交换的ΔH 较小，表明阳离子-抗衡离子的缔合较弱。通过表征更传统的 AEM 阳离子，如季铵盐和咪唑鎓阳离子，进一步例证了 ITC 用于研究阳离子-抗衡离子缔合的方法，它们的焓响应差异较小，但总的ΔH 与镍基阳离子相似。阳离子水合作用，而不是其水合壳或膜的体相水合作用，可能在决定初始阳离子-抗衡离子对的强度方面起着关键作用。本报告首次确定了 ITC 如何可用于实验确定对理解和预测 AEM 中电导率至关重要的热力学量。
