Synthesis of nanoporous carbon with pre-graphitic domains

Abstract

Porous carbons with tunable porosity and electrical conductivity have received considerable attention due to their potential application as anodes in lithium ion batteries, electrocatalysts in fuel cells, and high surface area electrodes in electrical double layer capacitors [1,2]. High porosities and surface areas are critical to the performances of these materials. Significant porosity and surface area are easily created by activation of non-graphitizing carbon, yet the material lacks in conductivity [3–5]. Whereas graphitic carbon possesses high conductivity, it strongly resists activation by traditional methods [5]. Recently graphitic carbons with controlled pore sizes have been obtained by pyrolysis of graphitizing carbon precursors deposited in mesoporous silica templates [2,6,7]. However, the preparation of carbon with nanopores, width less than 2 nm, is unattainable by this method, thus limiting its ultimate surface area, porosity, and performance. Graphitization of non-graphitizing carbon occurs in the presence of a variety metals, but the product is contaminated with the catalyst [8–11]. Stress graphitization of pure carbon has been reported at temperatures in excess of 2000 C [12,13]. Porosity is not preserved. Herein we describe the discovery of a new route to the synthesis of a pure nanoporous carbon containing pre-graphitic structures with both high surface area and high nanoand mesoporosity from the readily available polymer precursor polyfurfuryl alcohol. Nanoporous carbon (NPC) is synthesized by pyrolysis of polymers, such as polyfurfuryl alcohol (PFA). Due to its strong resistance to transformation to graphite even when annealed at temperature above 2000 C, it is classified as non-graphitizing, which was defined by Franklin [14]. This resistance is attributed to the presence of extensive cross-linking in the precursor, which creates kinetically frozen disorder through a chaotic misalignment of the graphenes in the carbon that result during pyrolysis [15– 17]. Disorder in NPC gives rise to porosity and the majority of pores are narrowly distributed in the range from 0.4 to 0.5 nm width [16,18,19]. The pore volume and surface area of NPC can be further developed by simple activation with CO2 [20,21]. We have found that the pre-graphitic ordering of PFA-derived NPC can be accomplished after CO2 treatment at 900 C, which generates activated NPC (a-NPC). The a-NPC provides an unexpected pathway to pre-graphitic carbon with nanoporosity by subsequent annealing at 2000 C. First, p-toluenesulfonic acid monohydrate (0.048 gm, Sigma–Aldrich) was dissolved in 5 ml of Triton X-100 (Sigma–Aldrich) by heating mildly. Then to this solution, 5 ml of furfuryl alcohol (99% Sigma–Aldrich) was added. The reaction mixture was stirred magnetically at 10 C. After polymerization for 48 h the product was transferred to a quartz boat and pyrolyzed under flowing argon in a quartz tube furnace. The sample was heated at a rate of 10 C min 1 to 800 C and held for 1 h. The carbon product was pure, without inorganic contaminants. It was ground and sieved to a particle size of < 38 lm. Activated NPC was prepared in a quartz tube furnace. 0.5 gm of carbon was heated to 900 C over 1 h in flowing argon. After 1 h of soak time the gas was switched to CO2 and soaked for an additional 3.5 h. The sample was cooled back to room temperature under argon. High temperature