2:45 PM - ES06.08.04
MXene Electrode Materials for Electrochemical Energy Storage—First-Principles and Grand Canonical Monte Carlo Simulations
Yasuaki Okada1,Nathan Keilbart2,James Goff2,Shin’ichi Higai1,Kosuke Shiratsuyu1,Ismaila Dabo2
Murata Manufacturing Co., Ltd.1,The Pennsylvania State University2
MXene compounds are drawing attention due to their advantageous characteristics, including the large surface area of their two-dimensional structure, which may enable high energy density for electrochemical storage. These compounds have the generic formula Mn + 1XnTx (n = 1–3), where M, X, Tx represent the transition-metal cation, the carbon or nitrogen atom, and the surface termination of MXene layers, respectively. Since the discovery of MXene in the laboratories of Barsoum and Gogotsi, a number of studies have shown applications of their catalytic, electronic, mechanical, and optical properties [1,2]. In parallel, first-principles simulations have delivered quantitative information about their electrochemical response [3,4,5]; however, there is still debate over the charge-transfer mechanisms at the origins of electrochemical storage in MXene-based pseudocapacitors. In particular, it is still not fully understood how charge transfer is affected by transition-metal chemistry, stoichiometry, and surface termination. In the present study, we performed first-principles calculations on MXene compounds with 12 different transition metals (Sc–Cr, Y–Mo, La, and Hf–W), three different composition ratios (M2XTx, M3X2Tx, and M4X3Tx), and two different terminal groups (oxygen and fluorine). It was found that the electronic structures, formation energies and the amount of charge transferred from the cations to the MXene electrode is dependent on composition and that their charge is stored in the bond between the MXene and adsorbing cations. Importantly, the Bader atomic charge analysis revealed that the changes in valence number of the M-elements and X-elements are rather small, while the terminal groups play an important role in the pseudocapacitive charge storage of MXene compounds. Additionally, we carried out grand canonical simulations using a dataset of first-principles free energies, taking into account solvation effects using implicit solvent models based on the self-consistent continuum solvation (SCCS) formalism and including configurational entropy contributions via finite-temperature Monte Carlo sampling in an effort to clarify the charge storage mechanisms underlying the operation of the pseudocapacitive electrode . We provide a detailed description of the sequence of desorption steps at the MXene surface as the applied voltage increases. Moreover, changes in the valence state and projected density of states were observed during the cation desorption reaction. These results provide design principles to improve the performance of MXene as an electrode material for electrochemical energy storage and to assess the electrochemical stability of MXene-based electrocatalyst under applied voltage and controlled pH.
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