Preparation and porous texture characteristics of fibrous ultrahigh surface area carbons

Abstract

Microporous carbon adsorbents with fibrous morphology and very high surface area and pore volume development were prepared from poly (m-phenylene isophthalamide) fibres. The precursor polymer was impregnated with aqueous solutions of phosphoric acid, carbonised and physically activated with CO₂ to different burn-off degrees. The char yield following pyrolysis increased with the impregnation ratio. Additionally, pre-impregnation with phosphoric acid strongly enhanced the reactivity of chars towards CO₂, there being a positive correlation between reactivity, impregnation ratio and oxygen content in the chars. The porous texture of the obtained materials was characterized by physical adsorption of N₂ at 77 K and CO₂ at 273 K. The pores enlarged slightly with increasing burn-off in CO₂, though remaining restricted to the micropore region. No pores larger than 3–4 nm were ever found in any of the activated materials, even at high burn-offs. Ultrahigh surface area carbons with BET surface areas over 2000 m² g⁻¹ and pore volumes in excess of 1.10 cm³ g⁻¹ were obtained at high burn-offs, displaying an extensive network of supermicropores considerably wider than those obtained under equivalent conditions in the absence of the H₃PO₄ additive. To complement these observations, scanning tunneling microscopy was performed on the ultrahigh surface area carbons. Visual evidence was provided of an extensive network of ∼2 nm pores, consistent with the physical adsorption measurements.