Intercalation/Deintercalation Dynamics in Ionic Liquid-Based Graphene Thermal Emissivity Modulators

The intercalation/deintercalation of ionic liquids into multilayer graphene-based devices modulates their thermal emissivity and complex dielectric function. This study presents the temperature-dependent behavior of electrochemically driven intercalation in multilayer graphene devices from -80 to 130 °C. The device consists of a chemical vapor deposition (CVD)-grown multilayer graphene (MLG) anode and a copper cathode separated by an alumina membrane impregnated with ionic liquid [DEME⁺][TFSI^-]. Applying positive voltages of 3.5-4 V drives the intercalation of [TFSI^-] anions into the interstitial spaces in the MLG, resulting in dramatic changes in the free carrier density and thermal emissivity of the material. The intercalation of [TFSI^-] anions improves with increasing temperature, as observed from Raman spectroscopy over the temperature range of 10-100 °C. The apparent temperature change due to intercalation also increases as a function of temperature, with a maximum temperature change of 25.5 °C, as measured by a thermal imaging camera sensitive over the wavelength range from 7.5 to 14 μm. This change in the thermal emissivity arises from a two-order-magnitude shift in the free carrier concentration upon intercalation, which ultimately modulates the complex dielectric function of the material (i.e., ε(ω) = ε₁ + iε₂). While our previous efforts were limited to static temperature measurements, the work reported here explores the temperature dependence of this phenomenon. Higher temperatures appear to enhance the extent of intercalation but have little to no effect on raising the cutoff frequencies. The operating temperature range is also limited by the boiling point of water and the freezing point of ice. The effective temperature range of the device, as well as the degree of intercalation, depends on the outside temperature and shows reduced effectiveness below a certain temperature due to reduced ion mobility and slower diffusion kinetics.