(Invited) In-Situ Study of Electric Double Layer and Ionic Transport at the Solid/Liquid Interface Using Scanning Probe Microscopy

The structure and dynamics of the solid/liquid interface are of fundamental interest for electrochemical capacitors technology. Theories describing the electrical double layer (EDL) have been proposed; yet a full molecular level experimental description is lacking to fully understand its interfacial structures, dynamics and reactivity for both capacitive and pseudocapacitive processes. Due to the characteristic sizes of EDLs, this requires characterization techniques capable of resolving nm length scales. Atomic Force Microscopy (AFM) has recently shown great interest for characterizing the electrochemical storage processes at the nanoscale, where local phenomena at the tip-sample junction can be measured and analyzed. Here, we demonstrate the use of AFM based techniques to understand electrochemical processes at the solid/liquid interfaces. Using force spectroscopy reinforced with molecular dynamics (MD) simulations, we were able to image the EDL in a room temperature ionic liquid (RTIL) on electrified surfaces ( 1 ). In the dense layered structure of about 4 nanometers thick at graphite surfaces, single step edges were found to result only in short range changes in the ionic liquid ordering. At the same time, formations of dislocation type topological defects were observed on planar surfaces in a 3D manner. The influence of applied potential on the ions structure showed a complex cation/anion transition together with ionic reorientation, in excellent agreement with MD simulations. State-of-the-art AFM techniques in liquid environment also enabled to spatially resolve pseudocapacitive intercalation pathways. For the first time, we demonstrated how the ionic transport at the solid/liquid interface can be tracked using changes in mechanical properties when cations are inserted from an aqueous solution into 2D layered transition metal carbides (MXenes) ( 2 ). For Ti 3 C 2 , the insertion of ions results in a strong elastic coupling between MXene layers which results in a significant and measurable change of electrode stiffness. The tracking of spatially resolved elastic properties at the nanoscale allowed identifying fast ionic channels into MXene particles, strongly depending on the intercalated cation radius and charge. As the development of AFM techniques in liquid environment improves, achieving time-resolved tracking of ion storage in energy materials becomes more accessible, and can lead to tuning surface interactions for improved supercapacitor performance. This work was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences. References 1) J. M. Black, B. Okatan, G. Feng, P. T. Cummings, S. V. Kalinin, N. Balke. Nano Energy 2015 , 15, 737-745. 2) J. Come, Y. Xie, M. Naguib, S. V. Kalinin, Y. Gogotsi, P. R. C. Kent, N. Balke. Advanced Energy Materials 2016 , 1502290.