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.

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.