Single-layer materials for energy devices
The realisation of integrated devices for high-performance electronics requires high-purity wafer-scale 2D materials.
There has been substantial global investment in two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDC) and phosphorene. The majority of research in the UK is focused on semiconducting properties of 2D materials to realise the next generation of electronic devices. This is because 2D materials that are mechanically exfoliated from bulk crystals are of good structural quality and exhibit excellent electronic properties that could, in principle, be exploited for devices.
However, the realisation of integrated devices for high-performance electronics requires high-purity wafer-scale 2D materials. It is clear within the global community that atomically thin materials possess many of the fundamental attributes that are useful for electronic devices. But before they can be used in practical electronic devices, significant progress towards large area growth of high-quality materials, complementary metal oxide semiconductors compatible fabrication processes, and low-resistance contacts for holes and electrons must be realised. Implementation of 2D semiconductors into devices, therefore, should be viewed as a long-term goal that will require years of research.
In contrast, researchers in the USA, China, Singapore and Korea are focusing on metallic 2D materials for near-term applications in electrochemical devices such as batteries and electrocatalysis.
Transition metal dichalcogenides
There are more than 40 layered TMD compounds, of which many are metallic in their natural form. Typically, these are compounds with an odd number of valence electrons in the d-orbital of the transition metal. 2D nanosheets of purely metallic TMDC compounds such as niobium disulphide (NbS₂), vanadium disulphide (VS₂), tantalum disulphide (TaS₂) and rhenium disulphide (ReS₂), have been synthesised in large quantities via chemical and liquid phase exfoliation and studied for electrochemical applications.
Chemical exfoliation can be used to tailor surface terminations of 2D nanosheets. Post-synthesis chemical functionalisation – attachment of specific molecules – can be achieved on the nanosheets, which provides pathways for tuning the electrochemical interactions between the nanosheets and ions so that redox, intercalation, and adsorption can be controlled. These capabilities allow for the synthesis of customised nanosheets that can not only store large amounts of charge, but do it in a manner that does not lead to substantial deformation of the electrode.
Carbides and nitrides
2D carbides and nitrides, also referred to as MXenes, are interesting for energy storage because they may be used to build electrodes for a variety of rocking chair batteries and electrochemical capacitors – pseudocapacitors. Electrodes from these materials have performed well in lithium ion (Li-ion) batteries and capacitors with excellent rate-handling capabilities, which can be partially explained by low Li diffusion barriers in 2D materials. Unlike other anodes, for example silicon (Si), electrodes from 2D materials can easily expand along the c-axis, thus not suffering from intercalation-related fracture, even at high cationic loads. They also show promise in sodium (Na) and potassium (K) ion batteries and are predicted to have high capacity for multivalent ions such as calcium ion (Ca2+), magnesium ion (Mg2+) and aluminium cation (Al3+).
Three dimensional (3D) compounds with the generalised formula of Mn+1AXn, where M is an early transition metal, A is an A-group element, X is carbon (C) and/or nitrogen (N), and n has a value of 1–3, can be used to obtain 2D metallic carbide and nitrides. More specifically, by etching away the A element in the 3D Mn+1AXn compounds, it is possible to obtain 2D nanosheets of Mn+1XnTx, where Tx represents surface terminations such as hydroxide (OH), hydrogen (H) or fluorine (F). These surface terminations allow tuning of electronic and electrochemical properties. Dozens of carbides and nitrides can be easily exfoliated.
The method for synthesis of 2D carbides and nitrides such as titanium carbides (Ti2C and Ti3C2) and titanium carbon nitride (Ti3CN) in large quantities involves slowly adding bulk powders to a mixture of lithium fluoride and hydrochloric acid for mild etching of the A-layers at 40OC for 36 hours in a controlled environment. The process is completed by washing the exfoliated powders to achieve a pH of seven.
Energy storage electrodes
2D materials provide well-defined ion diffusion pathways for lithium and other ions, facilitating ion insertion and movement, which is difficult to achieve in conventional 3D electrode materials. As a result, high power densities can be achieved in 2D electrodes with rapid ion and electron transport.
Although heavier than Li, sodium ion (Na+) and potassium ion (K+) have key advantages in addition to their lower price. Their lower desolvation energy, compared with smaller Li+, improves kinetics of ion insertion into the electrode and may lead to higher power. Multivalent ions, such as Mg2+ and Al3+, may store two or three electrons per ion, but their movement in 3D bulk or porous solids is restricted due to the limited lattice space, leading to slow charge/discharge processes and inferior stability of charge storage devices.
Metallic 2D materials offer distinct advantages over semiconducting 2D materials for energy storage. They are highly conductive with a high density of states at the Fermi level, and metal-like carrier densities. They are chemically diverse and tailorable, allowing for systematic variation of their intrinsic composition and post-synthetically modified surface chemistry. Their rigidity is exceptional, with bending stiffness values comparable to graphene that are ideally suited for flexible energy storage devices. Finally, they are hydrophilic, allowing for co-assembly with polar species and enabling sustainable, green processability. These attributes make them especially promising for the next generation of thin and flexible rechargeable batteries with improved storage capability, faster charging and much longer lifetimes, even when combined with larger and higher charged ions.
Fast intercalation of sodium, potassium and multivalent ions into electrodes comprised of restacked 2D nanosheets may enable wireless technologies, where wearable internet could be powered by compatible portable energy storage devices. Additionally, inexpensive 2D nanosheets of different compositions that are rationally restacked could offer solutions for automotive and large-scale stationary storage of renewable energy.
Charge storage ability, electronic/ionic conductivity and mechanical stability that govern battery performance can be precisely controlled by selecting from a number of 2D materials that are already available.
Some 2D materials offer high conductivity and the high surface area needed for supercapacitors and pseudocapacitors for quick charging and harvesting energy from fast processes. Metallic 2D nanosheets offer fast transport of electrons – metallic conductivity and the 2D channel for the ion between the nanometre-thin layers, structural integrity, and redox capability – making them ideal for energy storage devices.
2D metals are interesting as electrocatalysts for
the hydrogen evolution – nitrogen (N₂) and carbon dioxide (CO2) reduction reactions, hydrogen evolution reaction (HER), nitrogen reduction reaction (N₂RR) and carbon dioxide reduction reaction (CO2RR) – because they are earth abundant and can be synthesised in large quantities. Electrocatalysis consists of three steps, the first being electron injection into the catalyst, then charge transport to the active site, and finally a reaction at the active site. The first two steps drive the reaction kinetics while the third step is driven by thermodynamics.
In the case of 3D metal catalysts, for example platinum (Pt) and iridium (Ir), the electron transfer resistance is low and electron transport to the active sites is highly efficient – providing fast reaction kinetics. Catalysis reactions with semiconducting 2D materials such as HER with molybdenum disulphide (MoS₂) suffer from sluggish reactions that limit the overall catalytic activity, but this can be mitigated by using metallic 2D nanosheets. The free energy of the reaction is an intrinsic property of the material determined by its electronic structure.
However, it has been shown that the free energy of a given reaction can be tuned by varying the strain in the material. For example, in the case of metallic 2D TMDs, the presence of about 3% strain leads to the change in free energy for the limiting reaction for hydrogen evolution, the ideal condition for HER. In addition to the fast reaction kinetics and tunable free energy, metallic 2D compounds also provide stability for the active site, which is typical of a chalcogen vacancy in TMDs. That is, a sufficient number of active sites can be exposed for efficient catalysis to take place without oxidising the transition metal, so that long lifetimes and chemical stability can be achieved with metallic 2D catalysts. For example, with metallic NbS₂, it is possible to achieve very high current densities – several Amps per cm² – in a stable manner in proof of concept electrolyser devices for electrocatalytically generating hydrogen. The extraordinary current densities are enabled by the exceptionally high conductivity of the 2D material. Such high current densities are required for practical hydrogen generators. Currently, metallic 2D materials are being investigated for N₂RR, CO₂RR, oxygen evolution and reduction and other reactions.
Chemically exfoliated metallic 2D materials for electrochemical applications are emerging as active topics of research worldwide. These materials are likely to find near-term applications in advances in batteries and catalysts. They will play a key role in the next generation of electric vehicles and mitigating carbon emissions using catalysis. The engagement of the UK academic and industrial communities through strategic investments will be essential for reaping the economic benefits from technologies based on these materials.