Influence of Fluorine Doping of Activated Carbon Fibers on Their Water Vapor Adsorption Characteristics.

Leticia F Velasco,Kyung Hoon Kim,Peter Lodewyckx,Young-Seak Lee

doi:10.3389/fchem.2020.593756

Leticia F Velasco, Kyung Hoon Kim + Show 2 more

Open Access

https://doi.org/10.3389/fchem.2020.593756

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Abstract

The characterization of fluorinated carbon fibers by water sorption has been broadly investigated in this work. In brief, a pitch-based activated carbon fiber (ACF) was submitted to a fluorination process under different conditions of partial pressure (F2:N2 ratio) and temperature. This led to samples with varied fluorine content and C-F type bonding. The effect of the fluorination treatment on the textural properties of the ACF was studied by means of nitrogen and carbon dioxide adsorption at −196 and 0°C, respectively, while the changes induced in the surface chemistry of the materials were analyzed by XPS. Also, the affinity and stability of the materials toward water was evaluated by single and cycling isotherms. The obtained results show that a mild fluorination not only can preserve most of the textural properties of the parent ACF, but enhance the water uptake at the first stages of the water sorption process, together with a shift in the upswing of the water isotherms toward lower relative humidities. This indicates that fluorination under certain conditions can actually enhance the surface hydrophilicity of carbon materials with specific properties. On the contrary, higher partial pressures led to highly fluorinated fibers with lower porosity and more hydrophobic character. Moreover, they presented a lower chemical stability as demonstrated by a change in the shape of the water isotherms after two consecutive measurements. The kinetics of water sorption in the ACFs provided further insights into the different sorption phenomena involved. Hence, water sorption can definitely help to tailor the water affinity, stability and performance of fluorinated porous carbon materials under humid conditions.

Highlights

The mechanisms of water sorption in carbon pores have been the subject of extensive study during the last years (Liu et al, 2017)
The modification of the surface chemistry of the carbon fibers by the fluorination treatment was evaluated by X-Ray Photoelectron Spectroscopy (XPS)
The XPS spectra revealed the presence of three main peaks, which correspond to F1s, O1s, and C1s

Summary

INTRODUCTION

The mechanisms of water sorption in carbon pores have been the subject of extensive study during the last years (Liu et al, 2017). Recent simulation and experimental investigations on silicon carbide derived nanoporous carbons pointed toward a less straightforward and dual scenario (Farmahini et al, 2015; Shahtalebi et al, 2016) This two-fold behavior apparently arises from the fact that fluorination generates more hydrophilic carbon surfaces but at the same time enhances the energy barriers for water sorption. ) (Lee et al, 2007; Matei Ghimbeu et al, 2015) In this context, the present work aims at further investigating the role of fluorine doping on the experimental water sorption behavior of activated carbon fibers. The samples were degassed overnight at 150◦C prior to the sorption measurements On this subject, several outgassing temperatures were investigated in order to find the optimal one (i.e., to guarantee a proper cleaning of the surface to allow the diffusion of the gas, while minimizing the modification of the original material). This guarantees an optimal compromise between the cleaning of the surface and avoiding the modification of the surface chemistry (Velasco et al, 2019)

RESULTS AND DISCUSSION

CONCLUSIONS

DATA AVAILABILITY STATEMENT