Slesinski Adam, Sroka Sylwia, Fic Krzysztof, Frackowiak Elzbieta, Menzel Jakub
Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Berdychowo 4, Poznan 60-965, Poland.
ACS Appl Mater Interfaces. 2022 Aug 24;14(33):37782-37792. doi: 10.1021/acsami.2c09920. Epub 2022 Aug 10.
The operando monitoring of pH during the charging and discharging of an electrochemical capacitor in an aqueous neutral salt solution is presented. Proper knowledge of transient and limiting pH values allows for a better understanding of the phenomena that take place during capacitor operation. It also enables the proper assignment of the reaction potentials responsible for water decomposition. It is shown that the pH inside the capacitor is strongly potential-dependent and different for individual electrodes; therefore, the values of the evolution potentials of hydrogen and oxygen cannot be precisely calculated based only on the initial pH of the electrolyte. The operando measurements indicate that the pH at the positive electrode reaches 4, while at the negative electrode, it is 8.5, which in theory could shift the theoretical operating voltage well beyond 1.23 V. On the other hand, high voltage cannot be easily maintained since the electrolyte of both electrode vicinities is subjected to mixing. Operando gas monitoring measurements show that the evolution of electrolysis byproducts occurs even below the theoretical decomposition voltage. These reactions are important in maintaining a voltage-advantaged pH difference within the cell. At the same time, the electrochemical quartz crystal microbalance (EQCM) measurements indicated that the ions governing the pH (OH) that initially accumulated in the vicinity of the positive electrode enter the carbon porosity, losing their pH-governing abilities. pH fluctuations in the cell are important and play a vital role in the description of its performance during the cyclability at a given voltage. This is especially noticeable in cell floating at 1.3 V, where the pH difference between electrodes is the highest (6 units). The increase of the electrode separation distance acts similarly to the introduction of a semipermeable membrane toward the increase of the capacitor cycle life. During floating at 1.6 V, where the pH difference is not as high anymore (4 units), the influence of separation in terms of electrode stability, although present, is less notable.
本文介绍了在中性盐水溶液中对电化学电容器充放电过程中pH值的原位监测。了解瞬态和极限pH值有助于更好地理解电容器运行过程中发生的现象。这也有助于正确确定导致水分解的反应电位。结果表明，电容器内部的pH值强烈依赖于电位，且各电极不同；因此，不能仅根据电解质的初始pH值精确计算氢和氧的析出电位。原位测量表明，正极处的pH值达到4，而负极处为8.5，理论上这可能会使理论工作电压远超过1.23V。另一方面，由于两个电极附近的电解质会混合，因此难以维持高电压。原位气体监测测量表明，即使在低于理论分解电压的情况下也会发生电解副产物的析出。这些反应对于维持电池内电压有利的pH差值很重要。同时，电化学石英晶体微天平（EQCM）测量表明，最初在正极附近积累的控制pH值的离子（OH）进入碳孔隙，失去其控制pH值的能力。电池中的pH波动很重要，并且在描述其在给定电压下的循环性能时起着至关重要的作用。这在1.3V浮动的电池中尤为明显，此时电极之间的pH差值最大（6个单位）。增加电极间距的作用类似于引入半透膜，可延长电容器的循环寿命。在1.6V浮动时，pH差值不再那么大（4个单位），电极间距对电极稳定性的影响虽然存在，但不太明显。
