Relationship Between Structural Properties and Electrochemical Characteristics of Monolithic Carbon Xerogel‐Based Electrochemical Double‐Layer Electrodes in Aqueous and Organic Electrolytes

Abstract

The impact of the micropore width, external surface area, and meso-/macropore size on the charging performance of electrochemical double-layer capacitor (EDLC) electrodes is systematically investigated. Nonactivated carbon xerogels are used as model electrodes in aqueous and organic electrolytes. Monolithic porous model carbons with different structural parameters are prepared using a resorcinol-formaldehyde-based sol-gel process and subsequent pyrolysis of the organic precursors. Electrochemical properties are characterized by utilizing them as EDLC half-cells operated in aqueous and organic electrolytes, respectively. Experimental data derived for organic electrolytes reveals that the respective ions cannot enter the micropores within the skeleton of the meso-and macroporous carbons. Therefore the total capacitance is limited by the external surface formed by the interface between the meso-/macropores and the microporous carbon particles forming the xerogel skeleton. In contrast, for aqueous electrolytes the total capacitance solely depends on the total surface area, including interfaces at the micropore scale. For both types of electrolytes the charging rate of the electrodes is systematically enhanced when increasing the diameter of the carbon xerogel particles from 10 to 75 nm and the meso-/macropore size from 10 to 121 nm.

The contribution of sulfate ions and protons to the specific capacitance of microporous carbon monoliths

“…The monolith density decreases with removal of oxygen groups and development of porosity [42]. The density found for the three monoliths is higher than that reported for other carbon monoliths, with densities usually below 0.6 g cm -3 [2,20,21,24,26,27,[29][30][31][32], and also higher than the density of pellets obtained from PVDC-based carbon powders [43,44].…”

“…as an entire piece of carbon [6,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Cells having carbon monoliths as electrodes have a number of advantages over those comprising compacted powder pellets made from powder carbon.…”

“…In general, the M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4 carbon monoliths are isotropic materials; however, some monoliths are anisotropic and show carbon walls and channels, both aligned along a specific direction of the monolith [21]. The main drawback of the carbon monoliths is their low density, typically lower than 0.5-0.6 g cm -3 [2,[20][21][22][23][24][25][26][27][28][29][30][31][32]. Consequently, their volumetric capacitance is usually low, below 100 F cm -3 in aqueous electrolytes and below 50 F cm -3 in organic ones.…”

“…The usual procedures to prepare porous carbon monoliths are: (i) From carbonization of gels obtained from several carbon precursors and catalysts [1,2,[11][12][13][14][15]18,24,25,29,31], (ii) From carbonization of gels having a "template" that is removed either thermally during carbonization, or chemically by reaction with specific reagents [4,8,23], (iii) From infiltration of an inorganic monolith with a carbon precursor followed by carbonization of the precursor and removal of the inorganic template [8,9,13,30], (iv)…”

A redox‐mediator‐doped gel polymer electrolyte applied in quasi‐solid‐state supercapacitors

“…The same results also can be deduced from the frequency‐dependant capacitance of supercapacitors. In the plots of capacitance vs. frequency, C ′ ( F ) represents the real part of the capacitance, C ″ ( F ) is the imaginary part of the capacitance, f k (Hz) is the knee‐frequency, τ(s) is the relaxation time when system obtained the frequency f k …”