Hydrothermal synthesis of porous phosphorus-doped carbon nanotubes and their use in the oxygen reduction reaction and lithium-sulfur batteries

The many uses of carbon nanotubes (CNTs) depend not only on their intrinsic physical properties, but also on their tunable chemical components. Exploring a low-temperature method for the incorporation of phosphorus atoms in the carbon framework is expected to change the chemical properties of CNTs. Here, phosphorus-functionalized CNTs (PCNTs) were prepared by the direct hydrothermal treatment of a CNT-H3PO4 mixture at 170 oC. The PCNTs had a high phosphorus content of 1.66 at %, a specific surface area of 132 m2•g−1, and an improved thermal stability with a weight loss peak at 694 oC during oxidation in pure oxygen. They showed good electrocatalytic activity for the oxygen reduction reaction with an onset potential of −0.20 V vs Hg/Hg2Cl2, an electron transfer number of 2.60, and a larger current density as well as improved cyclic stability compared with pristine CNTs. PCNTs were also used as conductive scaffolds for the cathode in lithium-sulfur batteries. The cathode delivered an initial discharge capacity of 1106 mAh•g−1, a capacity retention of 80 % from 0.1 to 1.0 C, and a low decay rate of 0.25 % per cycle during 100 cycles.