Titanium carbide derived nanoporous carbon for energy-related applications
Introduction
Porous carbon is undoubtedly the most versatile porous material, not only because of the wide variety of structures that carbon offers but also due to the wide range of applications, ranging from gas storage to molecular sieves, catalyst supports, absorbents, electrodes in batteries and supercapacitors, water/air filters and medical devices [1]. Carbon produced by etching of metal(s) from a metal carbide is called carbide derived carbon (CDC) [2], [3]. It has been experimentally demonstrated that highly porous carbon with tunable pore size can be produced by chlorination of metal carbides, such as B4C [4], ZrC [5], Ti3SiC2[3], [6], Ti2AlC [7] and others.
Titanium carbide (TiC) is one of the most common and widely used carbides. It has a rock-salt structure (space group: Fm3m) where the distance between the nearest carbon atoms is 0.4328 nm [8]. Such a small and uniform carbon–carbon distance in the starting material may lead to a highly porous carbon (∼56% porosity [2], [3] and 0.56 cm3/g pore volume calculated assuming conformal transformation) with narrowly distributed small pores. CDC from TiC (henceforth called TiC–CDC) has been reported in the literature [9], [10]. Specific surface area (SSA) increases from 400 to 1000 °C chlorination temperature, followed by a drop at 1200 °C. On the other hand the total pore volume remains almost the same for samples chlorinated between 700 and 1100 °C but the pore size distributions are different. It was also reported that TiC–CDC has an onion-like structure [10]. Previous studies of CDC as supercapacitor electrodes showed high specific capacitance in aqueous H2SO4 electrolytes [11], [12]. Formation of CDC from ternary carbides containing Ti has also been reported [3], [6], however to the best of our knowledge no detailed analysis of the TiC–CDC pore structure and pore size distribution has been reported.
This paper describes a parametric study designed to determine the effect of synthesis temperature on porosity, structure and technologically relevant properties of TiC–CDC such as gas storage capacity (hydrogen, methane) and double-layer capacitance.
Section snippets
Material synthesis
Titanium carbide powder (formula weight 59.91 g/mol, density 4.93 g/cm3) with particle size 2 μm was obtained from Alfa Aesar (Stock # 40178). TiC–CDC was synthesized as described elsewhere [2], [3], [4]. TiC in a quartz boat was placed in a quartz tube furnace and heated to the desired temperature (200–1200 °C) under an argon (Airgas, UHP grade) purge. Once the desired temperature was reached, chlorine gas (Airgas, UHP grade) at 10–15 cm3/min was passed through the 1-in. diameter quartz tube for 3
CDC structure
Powder XRD data (Fig. 1) shows that the complete conversion of titanium carbide to carbon takes place at 400 °C and higher. The absence of sharp peaks corresponding to graphite, even at 1200 °C, indicates the disordered nature of TiC–CDC. Thus it can be called “amorphous carbon”.
The Raman spectra of perfectly ordered graphite shows only one peak in the range studied, the G band corresponding to in-plane stretching at 1582 cm−1. Disordered carbons generally exhibit a second disorder-induced (D)
Conclusion
The small and uniform C–C distance in the precursor TiC results in a nanoporous carbon with narrow PSD and amorphous structure at low synthesis temperatures (400 and 600 °C). Increasing the synthesis temperature to 800 °C and above results in more ordered carbon structure with larger pores and broader PSD. Nanoporous CDC consists mostly of sp2 bonded carbon. The TiC crystal structure and high theoretical density of the resulting CDC might be responsible for interlocking of carbon ribbons which
Acknowledgements
This work was supported in part by Arkema, France, and the US Department of Energy contract DE-FC36-04GO14282. XANES experiments were performed at the Advanced Light Source, Lawrence Berkeley National Laboratory under the auspices of the U.S. DOE by the University of California, LLNL under Contract No. W-7405-Eng-48. The ALS is supported by the Director, Office of Science, Office of BES, Materials Sciences Division, of the US DOE under Contract No. DE-AC03-76SF00098 at LBNL.
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