Predicting the Pseudocapacitive Windows for MXene Electrodes with Voltage-Dependent Cluster Expansion Models

Abstract

MXene transition-metal carbides and nitrides are of growing interest for energy storage applications. These compounds are especially promising for use as pseudocapacitive electrodes due to their ability to convert energy electrochemically at fast rates. Using voltage-dependent cluster expansion models, we predict the charge storage performance of MXene pseudocapacitors for a range of electrode compositions. M3C2O2 electrodes based on group-VI transition metals have up to 80% larger areal energy densities than prototypical titanium-based (e.g., Ti3C2O2) MXene electrodes. We attribute this high pseudocapacitance to the Faradaic voltage windows of group-VI MXene electrodes, which are predicted to be 1.2 to 1.8 times larger than those of titanium-based MXenes. The size of the pseudocapacitive voltage window increases with the range of oxidation states that are accessible to the MXene transition metals. By similar mechanisms, the presence of multiple ions in the solvent (Li+ and H+) leads to sharp changes in the transition-metal oxidation states and can significantly increase the charge capacity of MXene pseudocapacitors.

Isolation of pseudocapacitive surface processes at monolayer MXene flakes reveals delocalized charging mechanism

“…These estimates of gravimetric capacitance are in excellent agreement with values predicted by DFT simulations ( ca . 230 F/g) 12 , 29 – 31 , 34 , 35 and previous experimental determinations (220–250 F/g) 27 , 36 . Normalizing the basal-plane capacitance values by the two-sided area of the entire monolayer flake, we obtain specific surface capacitance values of 40 ± 10 µF/cm 2 , which agrees with DFT prediction for Ti 3 C 2 T x of 45 µF/cm 2 27 .…”

“…These values are remarkably high, one to two orders of magnitude greater than any previous theoretical prediction or measurement (see Supplementary Table 5) [24][25][26]32,33 . The Ti 3 C 2 T x pseudocapacitive charging is estimated to provide about 0.4 e-per unit cell per volt of storage when both sides of a monolayer are protonated 12,29,34 . A gravimetric capacitance of 12,000 F/g would be equivalent to 14.8 e-per unit cell per volt, an unphysically large capacitance that suggests that the MXene monolayer area engaged in capacitive charging is much larger than the area of the submicron droplet contact (0.31 µm 2 ).…”

Advances in the Electrocatalytic Hydrogen Evolution Reaction by Metal Nanoclusters‐based Materials

“…As a new class of 2D transition metal carbides/nitrides, MXenes have a big surface area, high metallic conductivity, high hydrophilicity, and suitable defect sites, making them ideal candidates for supporting single atomic or sub-nanometer clusters. [153][154] Liu et al reported Ru clusters anchored in Ti 3 C 2 T x (Figure 14a, b), and the coordination number of Ru atoms was regulated from 2.1 to 2.8 by adjusting the pyrolysis temperature (300, 600, and 750 °C). The resulting Ru-Ti 3 C 2 T x @600 was an excellent HER catalyst comparable with Pt/C (Figure 14c).…”

Element‐Doped Mxenes: Mechanism, Synthesis, and Applications

“…[101,[243][244][245] Since the commercial application of lithiumion batteries (LIBs) in 1991, their important advantages such as high voltage, large capacity, long cycle life, and good safety performance have attracted much attention. [246,247] Compared with pristine MXenes, due to the adjustment of the interlayer distance by elemental doping, more abundant active sites are generated, which suppress the performance degradation due to the stacking of sheets and provide better electrochemical adsorption sites. [85,86,248,249] N, S codoping of V 2 CT x MXenes through rational structural design and optimization.…”