Fabrication of porous SiC(y) (core)/C (shell) fibres using a hybrid precursor of polycarbosilane and pitch

Abstract

Porous SiCy (core)/C (shell) composite fibres have been fabricated using a simple KOH controlled-activation of SiCx fibres, which were pyrolyzed from polycarbosilane-pitch blend fibres. Effects of activation conditions and pyrolysis temperatures were studied. There are distinctive interfaces observed on the cross-sections of the co-axial fibres, where Si content varies gradually from the core to the shell. The etching of Si follows a slow “core-reducing” process in N2, while in CO2, cracks are frequently observed on the shells due to the accelerated activations. V-shaped Si-free carbon fibres could be obtained when a lower pyrolysis temperature was used to produce the SiCx fibres.

Microporous carbon (MC)-based materials have received intensive attention in the context of physisorption for hydrogen storage in the coming era of hydrogen economy [1]. Recently, tremendous efforts have been made to prepare MCs with high specific surface areas (SSAs) and developed porosity, typically, high SSAs can be achieved by controlled-activation of carbon precursors [3], template-assembling using Zeolites [2], or metal-extracting from carbides (carbide-derived carbons, CDCs) [4]. All these candidates have shown brilliant future for the hydrogen storage. However, the amounts of hydrogen adsorbed are still quite limited under high pressure and ambient temperature conditions [5]. Fortunately, the modification of C surfaces through incorporation of functional groups or dopants facilitates increasing binding energy of hydrogen with MCs and most recently, B-substituted MC has been synthesized and exhibited increased H2 storage capacity, 3.2 wt.%, almost double that of MC with similar SSA [6]. As simulated [7], Si should be a good alternative dopant due to the much higher binding energy of hydrogen with SiC. However, SiC usually has low SSA and hard to fabricate porous structure. We here reported a simple one-step activation to produce porous SiC fibres using a hybrid precursor of polycarbosilane (PCS) and pitch. Interestingly, novel core–shell structured materials were obtained. The influences of processing parameters on the cross-section morphologies were discussed in this letter.Polycarbosilane and pitch were mixed with a weight ratio of 55%/45%, melt-spun into green fibres, cured in air and pyrolyzed in N2 giving C-rich SiCx fibres. Then the SiCx fibres were loaded with KOH at a definite impregnation mass ratio (R = wt. KOH/wt. fibres), and activated in N2 or CO2 at a defined temperature/duration (for experimental details, see Supplementary information).

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Identifiers

ORCID for Zheng Jiang: orcid.org/0000-0002-7972-6175

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