Surface-Structure Change and Charge/Discharge Behavior of Petroleum Cokes Surface-Modified by Thermally Activated ClF[sub 3] and NF[sub 3]

Abstract

Heat-treatment of oxygen-containing petroleum coke ͑PC͒ at a high temperature ͑ϳ2800°C͒ gave rise to closure of edge planes by carbon-carbon bond formation. To change the surface structure, surface modification of PC and those heat-treated at 1860, 2300, and 2800°C ͑PC1860, PC2300, and PC2800͒ has been performed by ClF 3 and NF 3 at 200-500°C. No surface fluorine was detected except in one sample treated by NF 3 , while small amounts of surface chlorine were found in all samples treated by ClF 3 . Small amounts of nitrogen were detected in two samples treated by NF 3 . Brunauer-Emmett-Teller surface areas and total mesopore volumes were reduced by surface modification. Transmission electron microscopic observation revealed that closededge planes of graphitized PC were destroyed and opened by surface modification with ClF 3 and NF 3 . Removal of closed-edge planes increased first-charge capacities of many PC samples heat-treated at 1860-2800°C by ϳ63 mAh/g ͑ϳ29.6%͒ at 150 mA/g. Application of fluorination techniques and fluorine compounds to lithium batteries is increasing in energy-generation devices such as lithium batteries and fuel cells. 1 Primary lithium batteries with graphite fluoride cathodes have been used as energy sources for many kinds of electronic instruments. 2,3 Of recent research interest is, however, the development of new materials for secondary ͑re-chargeable͒ lithium batteries. Secondary lithium batteries require such properties as high reversible capacity, high first-coulombic efficiency ͑low irreversible capacity͒, high cycleability, and high rate characteristics. First-coulombic efficiency is associated with the irreversible capacity at the first cycle involving electrolyte decomposition and subsequent formation of surface film ͓solid electrolyte interphase or interface ͑SEI͔͒ on carbonaceous anode. Some lithium ions are consumed for the formation of SEI and therefore are not intercalated in graphene layers. It is one of the important subjects for the development of new carbonaceous anodes to suppress the electrochemical decomposition of solvents. Because electrochemical reactions take place on the surface of carbon anodes, surface structure is an important factor in determining the electrochemical characteristics of carbonaceous anode. It has been shown by recent investigations that surface modification of carbon materials is an effective method to improve the electrochemical characteristics of carbonaceous electrodes. 4 Some different methods of surface modification have been attempted. They are surface oxidation, 5-7 surface fluorination, 8-17 metal or metal-oxide coating, 18-21 and carbon coating, [22][23][24][25][26][27] It was shown that surface fluorination increased the discharge capacities of natural graphite samples with surface areas of 4.8-2.9 m 2 /g. [8][9][10][11][12]16 Light fluorination of natural graphite powder samples by elemental fluorine ͑F 2 ͒ increased surface areas and mesopores with diameters of 2-3 nm, resulting in the increase in discharge capacities without decr...

Synthesis of Pt3Co Alloy Nanocatalyst via Reverse Micelle for Oxygen Reduction Reaction in PEMFCs

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