Self-Consistent Mean-Field Theory for Room-Temperature Ionic Liquids

Research in room-temperature ionic liquids (RTILs) has exploded in the last two decades, which are marked by an exponential increase in the number of theoretical and experimental publications in this field. The typically good thermal stability, ionic conductivity, and solvability of RTILs make them superior electrolytes in many electrochemical applications, such as metal electrodeposition and energy storage.(Armand et al., 2009; Endres et al., 2008; Ohno, 2005; Simon &Gogotsi, 2008) The efficiency of RTILs in such applications is determined by a number of factors, one of which is the relationship between the interfacial structure of RTILs and their electrochemical properties. The use of RTILs as solvents in supercapacitors is of particular interest since they can generate significant electrical double-layer (EDL) capacitance due to the formation of thin layer of ions on an Angstrom-scale at RTIL-electrode interfaces.(Atkin & Warr, 2007; Baldelli, 2008; Horn et al., 1988; Mezger et al., 2008; 2009; Rivera-Rubero & Baldelli, 2004; Santos et al., 2006; Yuan et al., 2010) The formation of ultra thin EDL is mainly caused by the absence of any solvation shells around RTILs, which also enhances the accessibility of ions to penetrate nanoporous electrodes, thus increasing their useability in electrochemical applications.(Chmiola et al., 2008; Largeot et al., 2008) The precise form of EDL structure in RTILs has been a point of argument recently. Experimental results from the sum frequency generation spectroscopy suggest a monolayer of ion formed at the interface.(Baldelli, 2008; Rivera-Rubero & Baldelli, 2004) However, a similar study using the surface force apparatus,(Horn et al., 1988) atomic force microscopy,(Atkin & Warr, 2007; Atkin et al., 2009) and x-ray reflectometry (Mezger et al., 2009) indicate that an alternating layer of ions is instead present. Analytical and numerical models also predict the formation of alternating layer of counterions and coions on polar surfaces due to strong steric and electrostatic correlations in RTILs.(Fedorov & Kornyshev, 2008; Feng et al., 2009; Kornyshev, 2007; Lauw et al., 2009; 2010; Oldham, 2008; Reed et al., 2008) Such an alternating layer typically occurs through the so-called charge overcompensation/overscreening mechanism when the electrostatic screening length is practically less than the size of the ion.(Outhwaite et al., 1980; Parsons, 1990; Stillinger & Kirkwood, 1960) In the highly charged system like RTILs, the charge overcompensation Self-Consistent Mean-Field Theory for Room-Temperature Ionic Liquids 14