Quantification of the Charge Consuming Phenomena under High‐Voltage Hold of Carbon/Carbon Supercapacitors by Coupling Operando and Post‐Mortem Analyses

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.

Section: Resultsmentioning

confidence: 98%

Section: Resultsmentioning

confidence: 99%

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

Ślesiński

Sroka

Fic

et al. 2022

“…Different from macroscopic ageing research, microscopic ageing research focused on probing the relationship between the change of the materials of specific components and the degradation of SCs by disassembling or reassembling aged SCs. − Actually, electrode materials, the electrolyte and separator have been widely studied separately by postmortem analyses by scanning electron microscopy (SEM), the Brunauer–Emmett–Teller (BET) method, Raman spectra, X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), etc. Using microscopic characterization methods, the change in the microstructure and chemical elements in SC materials after ageing and the related ageing mechanism can be revealed.…”

Section: Introductionmentioning

confidence: 99%

Degeneration of Key Structural Components Resulting in Ageing of Supercapacitors and the Related Chemical Ageing Mechanism

Huang

Weng

Gong

et al. 2021

The research on supercapacitors (SCs) is one of the hot topics in the field of energy storage, and the intrinsic ageing mechanism of SCs is significant from both the economic and the scientific point of view. In this paper, the negative effects of decay of the key structural components on ageing of SCs were investigated by factorial design and analysis of variance (ANOVA). The ANOVA results showed that the degree of the negative influence on ageing of SCs could be ranked in descending order as anode > separator > cathode. The ageing would be accelerated due to the interaction between the electrode and separator, especially at a high charge−discharge current density. Further, the intrinsic chemical ageing mechanism of SCs was revealed by the morphology, microstructure, and chemical composition analyses of the fresh and aged key components (the electrode carbon materials, current collectors, and separators) with scanning electron microscopy (SEM), Brunauer−Emmett−Teller (BET), X-ray photoelectron spectra (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), etc. Moreover, the minimum pore width of electrode carbon materials suitable for electrolyte ion diffusion was obtained by density functional theory (DFT) calculations, which corroborated the assumption that the pore structure deterioration was one of the direct causes of capacitance loss for aged SCs. Generally, the ageing mechanism of key components of SCs could be a reference to develop advanced electrode materials and separators for SCs.

“…59 It has also been shown that after 10 h of constant polarization, electrosorbed hydrogen begins to recombine and form molecular hydrogen. 60 Thus, a 2 h pulsed voltage held at 1.6 V does not necessarily lead to hydrogen evolution, but as discussed in the literature, hydrogen sorption can effectively occur. 32 Taking into account all of this information, the CV shapes recorded after aging tests (Figure 5) are reasonable.…”

Section: Resultsmentioning

confidence: 91%

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

Płatek-Mielczarek

Piwek

Frąckowiak

et al. 2023

A comprehensive comparison of electrochemical capacitors (ECs) with various aqueous alkali metal sulfate solutions (Li 2 SO 4 , Na 2 SO 4 , Rb 2 SO 4 , and Cs 2 SO 4 ) is reported. The EC with a less conductive 1 mol L −1 Li 2 SO 4 solution demonstrates the best long-term performance (214 h floating test) compared to the EC with a highly conductive 1 mol L −1 Cs 2 SO 4 solution (200 h). Both the positive and negative EC electrodes are affected by extensive oxidation and hydrogen electrosorption, respectively, during the aging process, as proven by the S BET fade. Interestingly, carbonate formation is observed as a minor cause of aging. Two strategies for optimizing sulfate-based ECs are proposed. In the first approach, Li 2 SO 4 solutions with the pH adjusted to 3, 7, and 11 are investigated. The sulfate solution alkalization inhibits subsequent redox reactions, and as a result, EC performance is successfully enhanced. The second approach exploits so-called bication electrolytic solutions based on a mixture of Li 2 SO 4 and Na 2 SO 4 at an equal concentration. This concept allows the operational time to be significantly prolonged, up to 648 h (+200% compared to 1 mol L −1 Li 2 SO 4 ). Therefore, two successful pathways for improving sulfate-based ECs are demonstrated.