2018 In Situ Tracking of Partial Sodium Desolvation of Materials with Capacitive, Pseudocapacitive, and Battery-like Charge/Discharge Behavior in Aqueous Electrolytes

Authors: Pattarachai Srimuk,^†,‡ Juhan Lee,^†,‡ Öznil Budak,^†,‡ Jaehoon Choi,^†,§ Ming Chen,^∥,⊥ Guang Feng,^∥,⊥ Christian Prehal,^# and Volker Presser^*,†,‡

Journal: Langmuir 2018, 34, 13132−13143 doi: 10.1021/acs.langmuir.8b02485

Institut: 

^†INM - Leibniz Institute for New Materials, 66123 Saarbrücken, Germany

^‡Department of Materials Science and Engineering, Saarland University, 66123 Saarbrücken, Germany

^§School of Energy, Materials and Chemical Engineering, Korea University of Technology and Education, 1600 Chungjeol-or, Cheonan 31253, Republic of Korea

^∥State Key Laboratory of Coal Combustion, School of Energy and Power Engineering and ⊥Nano Interface Centre for Energy, School of Energy and Power Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China

^#Institute for Chemistry and Technology of Materials, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria

Abstract

Aqueous electrolytes can be used for electrical double-layer capacitors, pseudocapacitors, and intercalation-type batteries. These technologies may employ different electrode materials, most importantly high-surface-area nanoporous carbon, two-dimensional materials, and metal oxides. All of these materials also find more and more applications in electrochemical desalination devices. During the electrochemical operation of such electrode materials, charge storage and ion immobilization are accomplished by non-Faradaic ion electrosorption, Faradaic ion intercalation at specific crystallographic sites, or ion insertion between layers of two-dimensional materials. These processes may or may not be associated with a (partial) loss of the aqueous solvation shell around the ions. Our work showcases the electrochemical quartz crystal microbalance as an excellent tool for quantifying the change in effective solvation. We chose sodium as an important cation for energy storage materials (sodium-based aqueous electrolytes) and electrochemical desalination (saline media). Our data show that a major amount of water uptake occurs during ion electrosorption in nanoporous carbon, while battery-like ion insertion between layers of titanium disulfide is associated with an 80% loss of the initially present solvation molecules. Sodiation of MXene is accomplished by a loss of 90% of the number of solvent molecules, but nanoconfined water in-between the MXene layers may compensate for this large degree of desolvation. In the case of sodium manganese oxide, we were able to demonstrate the full loss of the solvation shell.