Computational Screening of 2D Ordered Double Transition-Metal Carbides (MXenes) as Electrocatalysts for Hydrogen Evolution Reaction

Abstract

Hydrogen evolution reaction (HER) is vital for sustainable energy production and plays a key role in achieving a hydrogen economy. Herein, density functional theory calculations are used to screen for suitable HER catalysts among 24 twodimensional double transition-metal (TM) carbide MXenes (chemical composition M 2 ′M″C 2 T x and M 2 ′M 2 ″C 3 T x ; M′ and M″ are two different metals, M′ = Cr, V, Ti, or Nb; M″ = Nb, Ta, Ti, or V; and T = O and/or OH) and determine their thermodynamic stability under HER-relevant conditions. The established surface Pourbaix diagrams, describing the chemistry on the basal planes of the MXenes, reveal the most stable terminations under the standard conditions (U = 0, pH = 0, p = 1 bar, and T = 298 K) for Mo 2 M x ″C y , Ti 2 M x ″C y , and Nb 2 Ta 2 C 3 to be O-termination, whereas Cr 2 M x ″C y and V 2 M 2 ″C 3 expose a mixed O-and OHtermination (M″ = Nb, Ta, Ti, or V; x = 1, y = 2, or x = 2, y = 3). Eighteen different carbides are predicted to be active HER electrocatalyst candidates, and Mo 2 NbC 2 O 2 showed the lowest overpotential. The Pourbaix diagrams and free energy diagrams reveal that the stability of the functional groups under HER-relevant conditions and the HER performance of the investigated double metal MXenes are closely related to its outermost TM. In other words, the outermost metal dominates the basal plane chemistry of the double TM carbide MXenes. Bader charge, density of states, and the crystal orbital Hamilton population analyses indicate that the hydrogen binding strength on different functionalized MXenes is related to the initial outer layer metal M′−O bond. A guiding observation is that the weaker the M′−O bond of the MXenes, the stronger the bonding between the terminated O* and the adsorbed H. Overall, this investigation demonstrates that the double TM carbides, as a subfamily of MXenes, provide a plethora of design opportunities, in particular as promising electrocatalysts for HER and other reactions.

Exfoliation Energy as a Descriptor of MXenes Synthesizability and Surface Chemical Activity

“…The utilization of energy-related descriptors contrasts to the more extended use of electronic-structure-based descriptors [ 23 ], such as the d -band center in transition metals [ 24 ], better than other possible descriptors such as the width corrected d -band center or the highest Hilbert transform peak [ 23 ]. In O-terminated MXenes, the p -band center has been already used as a descriptor of the electrocatalytic hydrogen evolution reaction [ 25 ].…”

Hetero‐MXenes: Theory, Synthesis, and Emerging Applications

“…Reproduced with permission. [ 163 ] Copyright 2020, American Chemical Society. d) Schematic illustration of the formation of MXene alloys.…”

“…performed effective screening of active catalysts for HER among M′ 2 M′′C 2 T x and M′ 2 M′′ 2 C 3 T x (M′ = Cr, V, Ti, Nb and M′′ = Nb, Ta, Ti, V) MXenes, and 18 of them were predicted to show active HER performance. [ 163 ] Owing to the near‐zero Gibbs free energy (≈0.003 eV) and the lowest theoretical overpotential, Mo 2 NbC 2 O 2 is estimated to be the best candidate for HER (Figure 6c). In addition to the stoichiometric double‐M MXenes, nonstoichiometric MXenes (also called as MXene alloys) have been theoretically investigated.…”

Theoretical exploration on the vibrational and mechanical properties of M3C2/M3C2T2 MXenes

“…[ 13–15 ] Some MXenes are also predicted to be suitable electrocatalyst materials for hydrogen evolution reaction, [ 16 ] and the performance could be improved by double transition‐metal substitution. [ 17 ] Due to composition diversity, many studies aim to reveal the relations between properties and the functional groups or components of MXenes. The functional groups can alter the adsorption energy for Li‐ion, thus significantly affecting the storage capability [ 18,19 ] ; the Li diffusion barrier can be adjusted by the modification of functional groups .…”