Influence of KOH followed by oxidation pretreatment on the electrochemical performance of phenolic based activated carbon fibers
Introduction
Phenolic based activated carbon fibers (ACFs) have several merits over traditional granular activated carbons. ACFs have high specific surface area and adsorption–desorption rate [1], [2]. Therefore, the use of ACFs in polarizable electrodes has been paid great attention [3]. ACFs have been considered as the suitable material for the polarizable electrodes of the electric double layer capacitors (EDLCs) with high capacitance, working voltage, reliability and low leakage current [4], [5].
It is well known that pore structures of carbon electrodes affect the performance of the resulting capacitors [6], [7], [8], [9]. The effects of the specific surface area and the pore size distribution of the carbon electrodes have been discussed. Apart from that, geometric heterogeneity e.g. roughness, and the chemical characteristics e.g. the distribution of heteroatom on carbon surface could also influence the capacitor performance [10], [11], [12], [13]. It was reported that the presence of the carbonyl- or quinone-type functional groups may induce double-layer formation, faradic current and surface polarity of the resulting EDLCs [14], [15]. For increasing the capacitance of EDLCs, the surface functional groups were introduced on the surface of activated carbon fibers through many methods such as gaseous oxidation with oxidizing gases, chemical oxidation in HNO3 or H2SO4 solutions, electrochemical oxidation and cold plasma modification, etc. [16], [17], [18]. In the present study, KOH corrosion followed by oxygen treatment of ACFs was employed for achieving high capacitance of EDLCs. The performance of EDLCs fabricated using the resulting ACFs was examined. The surface morphology and oxygen functional groups were characterized using scanning electron microscopy (SEM) and a temperature programmed desorption (TPD) technique, in attempt to elucidate the effects of the morphology and oxygen functional groups during the course of double layer formation at the electrolyte-carbon interface in 7 M KOH/H2O electrolyte.
Section snippets
Preparation of ACFs
The phenolic resin was dissolved in methanol, and then dried by using a vacuum evaporation at 80 °C. The resulting phenolic resin was spun, stabilized in an acidic solution [19], [20], and then carbonized in nitrogen at 900 °C for 30 min. The ACF was prepared by activating the carbon fiber (CF) in a stream of CO2 at 900 °C for 4 h.
The original ACF was denoted ACF and the fibers oxidized at different temperatures for 3 h were denoted ACF-T (thermal treatment temperature). Chemical corrosion was
Structural characteristics of ACFs
The SBET and pore structural parameters of ACF, ACF-T, ACF-KOH and ACF-KOH-T are given in Table 1. From Table 1 it can be seen that for ACF-T series SBET and pore volume increase slightly with increasing oxidation temperature. The burn-off of these samples also increases with increasing oxidation temperature, indicating that carbon gasification plays a dominant role in these cases. For ACF-KOH and ACF-KOH-T series SBET and pore volume do not change obviously with increasing oxidation
Conclusions
The present work has demonstrated that the capacitance of ACFs in 7 M KOH can be enhanced with KOH corrosion followed by oxygen treatment at 250 °C. Heat treatment in air at 150–300 °C for 3 h had slight influence on the SBET and surface oxygen functional groups. KOH corrosion can increase the surface roughness of ACFs. The amount of surface oxygen functional groups increases with the extent of oxygen treatment after KOH corrosion, and reaches the maximum at 250 °C; the increase is contributed
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