Our goal is to understand the effects of atomic and molecular intercalation in 2D layered materials from first principles, focusing on renewable energy applications such as batteries and catalysis.
Na-ion and multi-valent ion batteries
Batteries store excess energy when it is available and release it when required. The most popular Li-ion battery (LIB) uses lithium intercalated graphite as the anode, but it possess a relatively moderate Li capacity (372 mAhg−1) and suffers from a poor rate capability. Alternatives are required in order to meet the market demand, whether for electrical grid energy storage or for personal transport and computing devices.
The abundance, and subsequent low cost, of sodium is driving the development of Na-ion batteries. Even higher energy densities could be achieved through the use of multi-valent ions such as Al3+ and Ca2+. Unfortunately, graphite is not a suitable anode for post-LIBs so new host materials are required. Candidates must have a high reversible specific capacity, be robust over prolonged cycling and have a high ion diffusion rate. Finding such candidates is providing a major challenge to material scientists.
MXenes are the latest family of 2D materials to be discovered. They are formed through the selective chemical etching of a series of Mn+1AXn (i.e. MAX) phases, where M is an early transition metal, A is generally from group IIIA or IVA, X is either carbon or nitrogen and n ∈ [1,3].
Soon after their discovery they were explored as possible anode materials. Bare Ti3C2, the prototypical MXene material, was theoretically predicted to have a storage capacity of 320 mAhg−1, a value comparable to that of the current market dominator, graphite, but with a superior cycling rate due to a smaller diffusion barrier.
Finding suitable host materials for multivalent ions is a major issue. While MXene materials have been put forward as potential candidates, it is vital that the effect of experimentally relevant parameters, such as the nature of the terminating groups, or the presence of co-intercalated solvent, is understood.
This atomic-scale understanding will be crucial to the eventual use of MXene materials as next-generation multivalent-ion battery anodes.
Atomic and molecular intercalation in 2D materials
Solvents are an essential element in the production and processing of two-dimensional (2D) materials. For example, the liquid phase exfoliation of layered materials requires a solvent to prevent the resulting monolayers from re-aggregating, while solutions of functional atoms and molecules are routinely used to modify the properties of the layers. It is generally assumed that these solvents do not interact strongly with the layer and so their effects can be neglected.
Yet experimental evidence has suggested that explicit atomic-scale interactions between the solvent and layered material may play a crucial role in exfoliation and cause unintended electronic changes in the layer. Little is known about the precise nature of the interaction between the solvent molecules and the 2D layer.
Here, we use density functional theory calculations to determine the adsorption configuration and binding energy of a variety of common solvent molecules, both polar and non-polar, on two of the most popular 2D materials, namely graphene and MoS2.
Improving the Efficiency of 2D Catalysts
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