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- ItemTime‐Dependent Cation Selectivity of Titanium Carbide MXene in Aqueous Solution(Weinheim : Wiley-VCH, 2022) Wang, Lei; Torkamanzadeh, Mohammad; Majed, Ahmad; Zhang, Yuan; Wang, Qingsong; Breitung, Ben; Feng, Guang; Naguib, Michael; Presser, VolkerElectrochemical ion separation is a promising technology to recover valuable ionic species from water. Pseudocapacitive materials, especially 2D materials, are up-and-coming electrodes for electrochemical ion separation. For implementation, it is essential to understand the interplay of the intrinsic preference of a specific ion (by charge/size), kinetic ion preference (by mobility), and crystal structure changes. Ti3C2Tz MXene is chosen here to investigate its selective behavior toward alkali and alkaline earth cations. Utilizing an online inductively coupled plasma system, it is found that Ti3C2Tz shows a time-dependent selectivity feature. In the early stage of charging (up to about 50 min), K+ is preferred, while ultimately Ca2+ and Mg2+ uptake dominate; this unique phenomenon is related to dehydration energy barriers and the ion exchange effect between divalent and monovalent cations. Given the wide variety of MXenes, this work opens the door to a new avenue where selective ion-separation with MXene can be further engineered and optimized.
- ItemIonophobicity of carbon sub-nanometer pores enables efficient desalination at high salinity(Maryland Heights, MO : Cell Press, 2022) Zhang, Yuan; Prehal, Christian; Jiang, Huili; Liu, Yang; Feng, Guang; Presser, VolkerElectrochemical seawater desalination has drawn significant attention as an energy-efficient technique to address the global issue of water remediation. Microporous carbons, that is, carbons with pore sizes smaller than 2 nm, are commonly used for capacitive deionization. However, micropores are ineffective for capacitive deionization at high molar strength because of their inability to permselectively uptake ions. In our work, we combine experimental work with molecular dynamics simulation and reveal the ability of sub-nanometer pores (ultramicropores) to effectively desalinate aqueous media at seawater-like molar strength. This is done without any ion-exchange membrane. The desalination capacity in 600 mM reaches 12 mg/g, with a charge efficiency of 94% and high cycling stability over 200 cycles (97% of charge efficiency retention). Using molecular dynamic simulations and providing experimental data, our work makes it possible both to understand and to calculate desalination capacity and charge efficiency at high molar strength as a function of pore size.
- ItemGraphitization as a universal tool to tailor the potential-dependent capacitance of carbon supercapacitors(Hoboken, NJ : Wiley, 2014) Weingarth, Daniel; Zeiger, Marco; Jäckel, Nicolas; Aslan, Mesut; Feng, Guang; Presser, VolkerMost efforts to improve the energy density of supercapacitors are currently dedicated to optimized porosity or hybrid devices employing pseudocapacitive elements. Little attention has been given to the effects of the low charge carrier density of carbon on the total material capacitance. To study the effect of graphitization on the differential capacitance, carbon onion (also known as onion-like carbon) supercapacitors are chosen. The increase in density of states (DOS) related to the low density of charge carriers in carbon materials is an important effect that leads to a substantial increase in capacitance as the electrode potential is increased. Using carbon onions as a model, it is shown that this phenomenon cannot be related only to geometric aspects but must be the result of varying graphitization. This provides a new tool to significantly improve carbon supercapacitor performance, in addition to having significant consequences for the modeling community where carbons usually are approximated to be ideal metallic conductors. Data on the structure, composition, and phase content of carbon onions are presented and the correlation between electrochemical performance and electrical resistance and graphitization is shown. Highly graphitic carbons show a stronger degree of electrochemical doping, making them very attractive for enhancing the capacitance.
- ItemUnderstanding the charging of supercapacitors by electrochemical quartz crystal microbalance(Cambridge : Royal Society of Chemistry, 2022) Niu, Liang; Yang, Long; Yang, Jingjing; Chen, Ming; Zeng, Liang; Duan, Pan; Wu, Taizheng; Pameté, Emmanuel; Presser, Volker; Feng, GuangSupercapacitors are highly valued energy storage devices with high power density, fast charging ability, and exceptional cycling stability. A profound understanding of their charging mechanisms is crucial for continuous performance enhancement. Electrochemical quartz crystal microbalance (EQCM), a detection means that provides in situ mass change information during charging–discharging processes at the nanogram level, has received greatly significant attention during the past decade due to its high sensitivity, non-destructiveness and low cost. Since being used to track ionic fluxes in porous carbons in 2009, EQCM has played a pivotal role in understanding the charging mechanisms of supercapacitors. Herein, we review the critical progress of EQCM hitherto, including theory fundamentals and applications in supercapacitors. Finally, we discuss the fundamental effects of ion desolvation and transport on the performance of supercapacitors. The advantages and defects of applying EQCM in supercapacitors are thoroughly examined, and future directions are proposed.