Rare earth elements (REEs) are in high demand as critical materials in modern green and other advanced technologies. This review focuses on the recovery of REEs from waste with ionic liquids (ILs) as green dilutants and extractants to tackle the REE supply challenge. Extraction mechanisms in different extraction systems, including pure ILs, non-functional ILs with extractants, and functional ILs, are discussed in depth. The IL-based extraction process is different from the traditional molecular solvent-based extraction process. In addition, stripping is crucial to recovering ILs, and supercritical CO₂ (Sc-CO₂) is introduced to recover REE complexes from the IL phase. Also, Sc-CO₂ can accelerate the whole extraction process by enhancing mass transfer. The challenge is to understand the extraction mechanism of the IL/Sc-CO₂ system to advance the green recovery of REEs from waste.

A 3D self-supported hierarchical architecture composed of Ni nanoparticle@nitrogen-doped carbon nanotube modified NiMoN micro-pillar arrays grown on Ni foam (Ni@NCNT/NiMoN/NF) was synthesized. Benefiting from the protection by N-doped carbon layers, and the well-designed hierarchical structure with a large surface area, high electrical conductivity and convenient mass/electron transfer paths, this composite shows excellent activity and durability under both acidic and alkaline conditions. It demands extremely low overpotentials of 15 mV and 156 mV in 1.0 M KOH, and 31 and 96 mV in 0.5 M H₂SO₄ to achieve current densities of 10 and 200 mA cm⁻², respectively, which are superior to those for commercial Pt/C and other control samples. DFT calculations prove that the synergetic effect between N-doped carbon, Ni, and NiMoN enhances H₂O adsorption and decreases hydrogen adsorption free energy (|ΔG_H*|) to facilitate the HER process. Furthermore, the composite possesses extraordinary antioxidation ability under the conditions of the cyclic current pulse and power-off–on state. This work suggests an effective pathway to enhance the catalytic activity and oxidation resistance by building a hierarchical structure encapsulated by thin N-doped carbon layers, showing promising potency as the HER electrode in practical energy conversion and storage systems of wind power and solar power.