Carbon materials are used in a wide range of applications from gas filtration to energy storage in batteries and supercapacitors. Optimization and control of carbon structures, e.g. level of graphitization, is vital for achieving peak performance of these materials for a specific application. Thus far, the structure of carbon materials was primarily determined by the synthesis conditions and reversible changes to the carbon structure were achieved at pressures in the GPa range. The capability to reversibly change the carbon structure at near-ambient pressures through the control of temperature and gas pressure would potentially open new interesting fields of application for carbon materials such as efficient gas separation, sensors etc.
Carbon materials were synthesized from carbides, i.e. carbide-derived carbons, CDCs, by chlorination method. The in situ elastic and quasi-elastic neutron scattering from CDCs was measured at degassed and hydrogen loading conditions in a temperature range from 20 K to 100 K. Three distinct Bragg peaks characteristic to the (002) reflection of graphite with different interlayer distances formed upon heating to 50 K if the hydrogen loading was applied. The formed graphitic domains corresponded to natural graphite, graphite with expanded interlayer distances, and graphite with compressed interlayer distances. The Bragg peaks corresponding to the graphitic domains disappeared upon heating to 60 K and the formation/disappearance of the graphitic domains was reproduced for the same sample at hydrogen pressures from 68 mbar to 10 bar, applied at 77 K. The probable cause for the formation/disappearance of the graphitic domains is the confinement of hydrogen in the curved carbon structure of the CDCs during heating, which causes a pressure gradient in the CDCs and the formation of the ordered graphitic domains. During further heating, to the temperature of 60 K, hydrogen is not effectively confined by the CDC. Thus, over a time span of at least 40 min, the escape of hydrogen from inside of the CDC causes the original CDC structure to be restored. The cause for the described process was determined based on complementary information from quasi-elastic and elastic neutron scattering data, and the in situ pressure reading from the sample holder. The work exemplifies how different aspects of one experiment, i.e. the simultaneous recording and analysis of both elastic as well as quasi-elastic neutron scattering signal, can be efficiently used to acquire a deep understanding of the investigated phenomenon.
Figure 1: A combination of elastic neutron scattering (left) and quasi-elastic neutron scattering (right) methods were used in this study. With the increase in temperature to 50 K clear Bragg peaks form in the graphitic carbon (002) reflection range (left). Concurently, with the increase in temperature the amount of mobile hydrogen (quasi-elastic widening, short dash line) increases, where hydrogen is completely confined at 40 K (only elastic signal, long dash line). This clearly exemplifies the dynamically induced structural order in this system and how neutron scattering is the perfect tool to investigate such phenomena.
To determine the exact cause and suitable preliminary carbon structures, which support the reversible formation and disappearance of graphitic domains, fine-tuned carbon materials will be synthesized and in situ neutron scattering methods will be used. A doctoral project will start at the University of Tartu, Estonia, in co-operation with KTH, Sweden, from autumn of 2021 focusing on the investigation of the described phenomenon with in situ neutron scattering methods. Based on further experiments the applicability of the investigated process for technical systems will be determined.
The work resulted from the collaboration between the workgroups of Enn Lust (University of Tartu, Estonia), Margarita Russina (Helmholtz Zentrum Berlin, Germany), and Martin Månsson (KTH, Sweden). The work was financed by the Swedish Research Council through the neutron project grant (Dnr. 2016-06955), by the Estonian Research Council Grants (PUTJD957 and PRG676), and through the European Regional Development Fund (Centre of Excellence, TK141 2014-2020.4.01.15-0011). Experiments were performed on NEAT at BER II, Germany.
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Carbon, 174, 190 (2021). https://doi.org/10.1016/j.carbon.2020.12.025