Changdong Gu

Deep eutectic solvents (DESs) are a large family of solvents that show many similarities with ionic liquids. They are distinguished by the presence of a large amount of molecular components (typically hydrogen bond donors). They are more industrially promising than ionic liquids due to their low cost and tolerance to humidity (hydrolysis or hygroscopicity). As an emerging research field, DESs have already received significant research attention from chemistry scientists. The exploration of DESs used for functional materials in energy and environmental applications is still in its early stage. This review briefly introduces the basics of DESs and how they could be promising as solvents for material scientists. We summarized the application of DESs for the synthesis of materials used for energy and environmental applications. In this review, DESs have been described in view of the three main roles they play in the solution process of functional materials. Besides DESs being widely known as inert media or reactive reagents for the synthesis of materials, they can also be directly adopted as functional materials such as electrolytes for energy storage devices or as CO₂ adsorbents. The present review focused on several categories of functional materials including noble metals, porous carbonaceous materials, transition metal compounds, and DESs themselves, which are synthesized or derived from DESs for potential applications in the energy and environmental fields. DESs have been demonstrated to be effective in guiding the formation of functional materials with unique structures and properties. In particular, we introduced our work on exploring a DES-thermal synthesis strategy, in which the DES is used as a solvent as well as a reagent. Recent theoretical and experimental work for understanding the structural basis of DESs has also been summarized. This review article aims to inspire scientists to use DESs as a powerful tool to push the frontiers in the field of materials, energy, and environmental science.

Electro-brush plating (EBP) procedure using deep eutectic solvent (DES) based electrolytes was adopted for the first time to prepare nanocrystalline Ni coatings on Fe substrates. The electro-brush plated Ni (EBP Ni) coating is featured as lower surface roughness, better bonding strength, and higher wear resistance in comparison with the traditional electroplated Ni (EP Ni) from the same DES electrolyte. Nanoindentation tests indicate the EBP Ni has a high ratio of hardness to Young's modulus (0.0551) and a high hardness of about 5.4 GPa, showing an optimum mechanical property. Moreover, the EBP Ni coating with a thickness of ∼3.6 μm exhibits an enhanced corrosion resistance over the EP Ni for protecting Fe substrate. The unique deposition dynamics at the electrode/DES interface during the EBP process is suggested as being responsible for the superior mechanical property and enhanced corrosion resistance of the EBP Ni coatings.

An easy ionothermal method is proposed to deoxygenate graphene oxide (GO) for preparation of graphene by using a choline chloride (ChCl)/urea mixture-based ionic liquid. In the synthesis process, the urea portion in the ionic liquid breaks up to release NH₃ at around 100 °C. This reacts with the oxygen functional groups of the GO sheets, leading to the formation of graphene. The ionothermal method is an environmentally-friendly and attractive strategy for reducing GO because both ChCl and urea are naturally biocompatible compounds. X-Ray photoelectron spectroscopy indicates that nitrogen atoms can be doped into the graphene skeleton accompanied by the reduction of GO. As an electrode for an electrochemical double layer capacitor in aqueous KOH, the ionothermally reduced GO exhibits a gravimetric capacitance of 155 F g⁻¹ at a current density of 0.2 A g⁻¹ and good cycling performance, which is comparable to the hydrazine reduced GO. We expect that the ionothermal method will have implications for the use of ionic liquids in the large-scale synthesis of graphene-based materials.

Novel non-aqueous thermochromic solutions were facilely prepared by dissolving transition metal chlorides, such as NiCl₂·6H₂O, CrCl₃·6H₂O, FeCl₃, and CoCl₂·6H₂O, in an inexpensive, widely available ionic liquid analogous deep eutectic solvent (DES). It was found that only the NiCl₂·6H₂O dissolved choline chloride-based DES exhibited a stable and significant thermochromic behavior over a wide working temperature ranging from room temperature to about 150 °C. Typically, the colour of the 0.1 M NiCl₂·6H₂O dissolved DES solution changed in this order from pale green (room temperature), spring green (∼70 °C), to blue (∼120 °C). An effective strategy was proposed to produce more thermochromic solutions by mixing Ni(II) with other metal ions in the choline chloride-based DES. The thermochromic PVDF composite film was demonstrated by incorporating DES-based Ni(II) solution into the microporous PVDF film. The preliminary results are expected to stimulate interest of the development of high performance thermochromic materials for the facile fabrications from DESs.

Facile galvanic replacement was adopted to grow cauliflower-like and dendrite coral-like silver films on commercial copper alloy substrates. Both types of silver films possess micro/nanobinary structures, the formation and evolution of which are achieved through the oriented attachment growth mechanism. After modification by a monolayer of n-dodecanethiol, the cauliflower-like silver film becomes hydrophobic with a contact angle (CA) of 115 ± 1° and a CA hysteresis of 74 ± 1° and the dendrite coral-like silver film exhibits the extreme hydrophobicity characterized by a CA of 158 ± 1° and a CA hysteresis of less than 2 ± 1°. Furthermore, the bouncing property of millimeter-sized water droplets on the modified dendrite coral-like silver surface is much better than that on the modified cauliflower-like silver surface. The different wetting property between the two silver films is attributed to the surface roughness. Larger surface roughness provided by the dendrite coral-like silver film means more air pockets, among the micro/nanobinary structures, that bounce water droplets back more strongly. The dynamic bouncing behavior of water droplets, such as restitution coefficient and contact time, on the superhydrophobic modified cauliflower-like silver surface was also investigated at different impact velocities.

Single-crystalline Ag dendrites are grown on a Ni/Cu substrate by using a simple templateless, surfactantless electrochemical technique. Controlling only the applied potential causes a change in the deposited silver morphology from polyhedrons to dendrites. Microstructure characterization suggests that preferential growth along the ⟨211⟩ directions by the oriented attachment of Ag nanocrystals leads to the formation of Ag dendrites, which are composed of trunks, branches, and leaves. Modifying as-grown Ag dendritic film with a thickness of about 10 μm with a self-assembled monolayer of n-dodecanethiol yields a superhydrophobic surface with a contact angle of 154.5 ± 1.0° and a tilt angle lower than 2°.