Motoyama Daisuke, Yoshikawa Kai, Ozawa Hiroaki, Tadokoro Makoto, Haga Masa-Aki
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University , 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan.
Department of Chemistry, Faculty of Science, Tokyo University of Science , Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-8601, Japan.
Inorg Chem. 2017 Jun 5;56(11):6419-6428. doi: 10.1021/acs.inorgchem.7b00518. Epub 2017 May 11.
The judicious selection of pairs of benzimidazole-ligated ruthenium complexes allowed the construction of a rechargeable proton-coupled electron-transfer (PCET)-type redox battery. A series of ruthenium(II) and -(III) complexes were synthesized that contain substituted benzimidazoles that engage in PCET reactions. The formation of intramolecular Ru-C cyclometalation bonds stabilized the resulting ruthenium(III) complexes, in which pK values of the imino N-H protons on the benzimidazoles are usually lower than those for the corresponding ruthenium(II) complexes. As a proof-of-concept study for a solution redox battery based on such PCET reactions, the charging/discharging cycles of several pairs of ruthenium complexes were examined by chronopotentiometry in an H-type device with half-cells separated by a Nafion membrane in unbuffered CHCN/HO (1/1, v/v) containing 0.1 M NaCl. During the charging/discharging cycles, the pH value of the solution gradually changed accompanied by a change of the open-circuit potential (OCP). The changes for the OCP and pH value of the solution in the anodic and cathodic half-cells were in good agreement with the predicted values from the Pourbaix diagrams for the pairs of ruthenium complexes used. Accordingly, the careful selection of pairs of ruthenium complexes with a sufficient potential gradient and a suitably large pK difference is crucial: the charge generated between the two ruthenium complexes changes the OCP and the pH difference between the two cells in an unbuffered solution, given that the PCET reactions occur at both electrodes and that discharging leads to the original state. Because the electric energy is stored as a pH gradient between the half-cells, new possibilities for PCET-type rocking-chair redox batteries arise.
通过明智地选择苯并咪唑连接的钌配合物对，构建了一种可充电的质子耦合电子转移（PCET）型氧化还原电池。合成了一系列含有参与PCET反应的取代苯并咪唑的钌（II）和钌（III）配合物。分子内Ru-C环金属化键的形成使生成的钌（III）配合物得以稳定，其中苯并咪唑上亚氨基N-H质子的pK值通常低于相应钌（II）配合物的pK值。作为基于此类PCET反应的溶液氧化还原电池的概念验证研究，在一个H型装置中通过计时电位法研究了几对钌配合物的充放电循环，该装置的半电池由Nafion膜隔开，置于含有0.1 M NaCl的无缓冲CHCN/H₂O（1/1，v/v）中。在充放电循环过程中，溶液的pH值随着开路电位（OCP）的变化而逐渐改变。阳极和阴极半电池中溶液的OCP和pH值变化与所用钌配合物对的Pourbaix图预测值吻合良好。因此，仔细选择具有足够电位梯度和适当大的pK差值的钌配合物对至关重要：鉴于PCET反应在两个电极上均发生且放电会导致回到原始状态，两个钌配合物之间产生的电荷会改变无缓冲溶液中两个电池之间的OCP和pH差值。由于电能以半电池之间的pH梯度形式存储，PCET型摇椅式氧化还原电池出现了新的可能性。
