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

Abstract

Enhancing the operating voltage of supercapacitors (SCs), hence their specific energy, is important. However, long‐term hold at high voltage entails loss of capacitance, increase of resistance and internal pressure. Such detrimental effects could be reduced by obtaining quantitative information on the relative impact of the various mechanisms leading to the worsening of the SCs performance. Now, for a carbon/carbon supercapacitor in aqueous Li2SO4, a self‐consistent approach is used to assign leaking charge during high voltage hold to the charge: 1) distributed throughout the electrochemical cell (steady‐state leakage current measurements), 2) spent at each electrode for gases production (operando electrochemical mass spectrometry (EMS) analysis and pressure records), 3) utilized to oxidize the electrodes surface (from post‐mortem surface functionality determination by temperature programmed desorption (TPD)), and 4) used for other parasitic reactions.

High‐Voltage Supercapacitive Swing Adsorption of Carbon Dioxide

“…Previous studies have shown that increasing the voltage window of aqueous supercapacitors above 1.5–1.6 V results in carbon oxidation at the anode and hydrogen evolution at the cathode, irreversible charge losses, and high leakage currents that significantly degrades the supercapacitors long‐term performance. [ 25 — 27 ] Therefore, the optimal voltage window for SSA with GR‐AC electrodes in an NaHCO 3 electrolyte would be −1.4 V. The energy efficiency (η e ) stays above 60% up to −1.2 V, reduces to 50% at −1.4 V, and further reduces to 40% at −1.6 V (Figure 3d). Since additional energy is required to store additional charges in the pores at higher voltage, an increased energy loss is observed at higher voltages (Table S1, Supporting Information).…”

“…Reduced η c at −1.6 V may be attributed to enhanced irreversible, pseudocapacitive contributions from surface functionalities and hydrogen sorption. [26,27] By continuously running SSA at −1.4 V for multiple cycles, a coulombic efficiency of ≈97% was achieved without any performance loss in SSA adsorption capacity (Figure S5, Supporting Information), arguing that after a few cycles the most irreversibly redox-active functionalities of the activated carbon pore surfaces are removed, and the electrodes can be fully reversibly charged. After 130 h of continuous cycling at −1.4 V, the electrodes were again charged for additional 90 h at −1.4 V, and the voltage curves, current profiles, and CO 2 concentration changes show reproducible results (Figure S6, Supporting Information).…”

High-voltage supercapacitive swing adsorption of carbon dioxide

“…Previous studies have shown that increasing the voltage window of aqueous supercapacitors above 1.5-1.6 V results in carbon oxidation at the anode and hydrogen evolution at the cathode, irreversible charge losses, and high leakage currents that significantly degrades the supercapacitors long-term performance. [26][27][28] Therefore, the optimal voltage window for SSA with GR-AC electrodes in the NaHCO3 electrolyte used in the current study would be -1.4 V. The energy efficiency (ηe) stays above 60 % up to -1.2 V, reduces to 50 % at -1.4 V, and further reduces to 40 % at -1.6 V (Figure 3d). Since additional energy is required to store additional charges in the pores at higher voltage, an increased energy loss is observed at higher voltages (Table S1).…”

“…Reduced ηc at high voltage windows may be attributed to enhanced irreversible, pseudocapacitive contributions from surface functionalities, hydrogen sorption, and faradaic leakage current from water decomposition. 27,28 By continuously running the SSA cycles at -1.4 V for multiple cycles, a coulombic efficiency of 96 % was achieved without any performance loss in SSA adsorption capacity (Figure S4), arguing that after a few cycles the most irreversibly redox-active functionalities from the activated carbon surface are removed, and the electrodes can be fully reversibly charged. Previous studies have shown that increasing the voltage window of aqueous supercapacitors above 1.5-1.6 V results in carbon oxidation at the anode and hydrogen evolution at the cathode, irreversible charge losses, and high leakage currents that significantly degrades the supercapacitors long-term performance.…”

The Many Deaths of Supercapacitors: Degradation, Aging, and Performance Fading

“…The combination of EMS and TPD was also proposed to obtain quantitative and qualitative information on all mechanisms of supercapacitors during and after aging ( Figure 3 ). [ 41 ] While the in situ technique was used to monitor the gas evolution in the cell during its cycling, the post‐mortem analysis (TPD coupled with MS) was implemented after its death to measure oxygenated surface functionalities on the electrodes.…”