Operando Monitoring of Local pH Value Changes at the Carbon Electrode Surface in Neutral Sulfate-Based Aqueous Electrochemical Capacitors

Abstract

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.

Ambiguous Role of Cations in the Long-Term Performance of Electrochemical Capacitors with Aqueous Electrolytes

“…It significantly increased the operating time of ECs and decreased electrochemical changes after reaching the end-of-life criterion ( C / C 0 = 80%). The results presented in the manuscript complement previous reports on sulfate-based electrolytes in EC applications. ,,, Moreover, we propose two novel approaches for EC lifetime extension.…”

“…Thus, a decrease in S BET is more likely caused by both some additional redox reactions ongoing at the electrode/ electrolyte interface (especially on the positive electrode) and by strong cation confinement/specific anion adsorption (or cation movement limitation in the negative electrode due to hydrogen sorption). 44 It seems that the Li + cation features do not follow the trends observed among other alkali metal cations (Na + , Rb + , Cs + ). Figure 4 shows the exceptional features of lithium sulfate-based electrolytes.…”

“…Various in situ techniques have already confirmed that the real size of OH − as well as that of solvated cations and anions is strongly affected by the electrolyte composition. 44,53,54 Therefore, it can be predicted that for aqueous solutions of Rb 2 SO 4 and Cs 2 SO 4 , OH − is surrounded by more water molecules. Interestingly, SO 4 2− ionic flux has not been observed in Li 2 SO 4 electrolyte, 32 and according to studies conducted with KI-based electrolyte, 53 it can be assumed that the charge storage mechanism is the same when considering other alkali metal sulfates.…”

“…Moreover, ions are mixed, and the perm-selectivity failure of carbon electrodes is widely discussed. , Hence, some perturbation of ion fluxes in the electrolyte bulk occurs, whereby cations are adsorbed and counterions are repulsed from the electrode/electrolyte interface. Thus, a decrease in S BET is more likely caused by both some additional redox reactions ongoing at the electrode/electrolyte interface (especially on the positive electrode) and by strong cation confinement/specific anion adsorption (or cation movement limitation in the negative electrode due to hydrogen sorption) …”

“…The results presented in the manuscript complement previous reports on sulfate-based electrolytes in EC applications. 30,32,34,44 Moreover, we propose two novel approaches for EC lifetime extension.…”