Capacitance behaviour of ionic liquids at the semi-metallic interface Jing Yang 1 and Athanasios Papaderakis 1 , Ji Soo Roh 1,3 , Ashok Keerthi 1,3 , Mark Bissett 1,3 , Radha Boya 2,3 and Robert A. W. Dryfe 1 1 Department of Chemistry and Henry Royce Institute, University of Manchester, UK 2 Department of Physics and Astronomy and Manchester Centre for Nonlinear Dynamics, University of Manchester, UK, 3 National Graphene Institute, University of Manchester, UK Room temperature ionic liquids (RTILs) are a class of materials composed only ions exhibiting a series of unique and useful properties such as high thermal and chemical stability, low volatility, non-flammability, wide electrochemical windows, which attracts extended attentions from energy storage area to fundamental research. Strong ion-ion interactions of RTILs lead to special ions arrangements at charged surface. An oscillatory ion arrangement behaviour shows closely near charged surface (normally below 4 nm) and a long experimental decay length exhibited at the distance beyond the oscillatory region, which much larger than the theoretical Debye length 1 . Capacitance of ionic liquids at charged surface is directly influenced by ion behaviours within electric double layer (EDL) and extensive work have been done. Capacitance-potential curves of ionic liquids at charged surface composed with varied materials exhibiting "camel-shape" or "bell-shape" in both experimental and simulations, which could be explained by different "lattice saturation" of ions near charged surface 2 . However, ionic liquid is not the only factor influence capacitance at interface. Electrode properties also play a crucial role, especially for semi-metallic electrodes. Many semi-metallic electrodes lack of density of states (DoS) so space charge capacitance is needed to be seriously considered into total capacitance 3 . The small space charge capacitance could suppress the total capacitance and smear the capacitance curve feature of ionic liquids down to a “U-shape”. Here, basal plane of highly oriented pyrolytic graphite (HOPG) were used as an ideal semi- metallic electrode to study capacitance of ionic liquids and diluted ionic liquids by organic solvents with different dielectric constants. All of the capacitance curves exhibit a “U-shape” and minimal capacitance below than 3-4 uF/cm 2 , which is attributed to the limitation of space charge capacitance. For further investigation, 1 to 4 layers chemical vapour deposition (CVD) graphene were applied as electrodes to analyse pure ionic liquid behaviours with graphene thickness changes. A “V-shape” capacitance curve with a clear minimal capacitance around 1 uF/ cm 2 shows on monolayer graphene which is attributed to small quantum capacitance of graphene and its unique linear DoS changes near Dirac point. As the thickness of graphene increases, Dos increases with layer numbers leading to the total capacitance increasing. Until reaching to 4 layers, capacitance approaches to the similar values of HOPG. Both HOPG and graphene cases reveal space charge or quantum capacitance dominating the total capacitance. References 1. Smith, A. M., Lee, A. A. & Perkin, S. The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration. J. Phys. Chem. Lett. 7, 2157–2163 (2016). 2. Fedorov, M. V. & Kornyshev, A. A. Ionic liquids at electrified interfaces. Chem. Rev.114, 2978–3036 (2014). 3. Islam, M. M. & Ohsaka, T. Model of electrical double layer structure at semi-metallic electrode/ionic liquid interface. Electrochim. Acta 368, 137555 (2021).
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