Processes of EDLC Degradation AGING

A particular feature of operation of EDLC with AC electrodes is the formation of a certain amount of CO2 in the course of operation at the potentials above 1V (NHE). Analysis of the gas collected in the case of EDLC with electrolyte of 38% H2SO4 showed that it consisted predominantly of N2 and O2 with traces of CO2 and H2O at potentials below 1V. Eor charged EDLC, the gas consists mostly of CO2 with small amounts of Nj, O2, and HjO vapors. Presence of water vapors shows that the performance of the capacitor can decrease with time because of loss of electrolyte, which results in a decrease in capacitance of the capacitor. This, however, can occur only in unsealed EDLCs. Carbon dioxide is a product of oxidation of carbon electrodes. Its amount strongly depends on the voltage applied. To prevent CO2 accumulation in the course of EDLC operation, a release valve is used in some schemes. In most schemes, however, no valves are used. 

The aging of carbon materials in acetonitrile-based electrolyte is very slow and at least several weeks are required to register the measurable changes even when conditions for accelerated aging (increased potential) are provided. 

In the performed study, samples A and B made of natural precursors (coconut shell) and carbon C of synthetic resin were used. Let us point out that accelerated chemical aging was achieved in the experiments by the immersion of the electrode into acetonitrile heated to 50°C and conditioning for 1 month and usual 

As seen from this table, the result of electrochemical aging for all the studied electrodes was a decrease in the specific surface area and the micropore volume herewith, these changes in the anode are much greater than in the cathode. Additional studies showed that a decrease in the volume of micropores and mesopores is the consequence of their degradation and also the result of their clogging up by the products of decomposition of electrolyte. AC made of a synthetic precursor (sample C) shows the high structural stability as compared to ACs made of natural precursors (samples A and B). 

According to chemical analysis carried out using a number of methods, especially X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy, the chemical composition of carbons for different electrodes changes variously. All changes depend on the electrode polarity negative electrodes (cathodes) age much less than positive electrodes (anodes). This must be taken into account in new supercapacitor schemes, for example, anodes must be made thicker than cathodes in an asymmetric scheme to compensate losses of capacitance or, in the case of presence of temperature gradients, to position an anode from the colder side. The most important measurements showed formation as a result of the process of the aging of C-H bonds on the cathode and bonds C-N and C-F on the anode and also polymerization of acetonitrile on the cathode and decomposition of tetrafluoroborate. The practically unavoidable 

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