The Ammonia-Driven Phase Transition in Bulk and Nanostructured Potassium Graphite KC 24
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The Ammonia-Driven Phase Transition in Bulk and Nanostructured Potassium Graphite KC24 Arthur Lovell1,2, Zeynep Kurban1,2, Stephen M. Bennington1,2, Gadipelli Srinivas2, Neal T. Skipper2, Ron I. Smith1, Derek Jenkins3, Christopher A. Howard2, Laurent Chapon1 1
ISIS facility, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, U.K. 2 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K. 3 Micro and Nano Technology Centre, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, U.K. ABSTRACT We report the synthesis of nanostructured stage-2 potassium graphite, KC24, by intercalation of turbostratic graphite nanofibers produced from an electrospun polymer, and compare its properties with exfoliated graphite-based KC24. The nanostructured KC24 sample has low crystalline order and slightly increased interlayer spacing of 8.76 Å, compared with 8.65 Å in the bulk sample, indicating minimal registration of the graphite planes. Time-resolved time-offlight neutron diffraction on both nanostructured and bulk KC24 under ammoniation is suggestive of a more homogeneous and faster pressure-modulated phase transition to the ternary ammoniated potassium-graphite in the nanostructured material. Following ammoniation, negligible hydrogen uptake is observed at 50 K. INTRODUCTION The kinetic and thermodynamic properties of phase changes in materials can be heavily influenced by nanostructure, and this is likely to be a key factor in improving the performance of many kinds of functional material. In this paper, we report the effects of nanostructure on the ammonia-driven phase transition in potassium-intercalated graphite as part of an investigation into the hydrogen storage [1] potential of doped carbons. Graphite intercalation compounds (GICs) are formed when guest species move into the spaces between graphite planes [2-4]. The intercalation process is governed by many factors, the most important of which are temperature, the charge transfer from or to the intercalant, its size, and internal diffusion. Less-commonly considered is the effect on intercalation of the morphology of the host graphite, which can vary from highly crystalline natural graphite flake to exfoliated flexible graphite materials such as Papyex [5]. Intercalation compounds of carbon and graphitic fibers have also been reported [6, 7]. The ternary potassium-ammonia GIC can be formed from KC24 at ambient temperature. On acceptance of the ammonia, the sample undergoes an irreversible restaging phase transition from stage 2 to 1[8,9], accommodating a dilute K-NH3 layer in all formerly filled and empty galleries. The saturated composition has been measured to be K(NH3)4.33C23.5 and the new interlayer spacing 6.62 Å, almost twice that of pure graphite. This interlayer spacing is close to that calculated to give good hydrogen uptake in metal-doped carbons [10] and we have previously investigated the ability of the saturated compound to de
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