Extremely high-efficiency solar steam generation, robust and scalable photothermal evaporator based on ZIF-67@MXene/rGO decorated rock wool

Abstract

Emerging solar steam generation techniques utilizing solar energy are considered as promising for seawater desalination and wastewater purification. Herein, for the first-time, recycled rock wool (RW) waste is used as a 3-dimensional (3D) evaporator, decorated with a novel hierarchical structure of zeolitic imidazolate framework (ZIF-67), MXene and reduced graphene oxide (rGO) as the photothermal evaporator, termed ZIF-67@MXene/rGO (ZMG). This evaporator demonstrates good thermostability, pH stability and is mechanically robust without any detachment of the RW fibers. Based on the strong light absorption, improved wettability and microchannels that facilitate accelerated water transport through the fibers, the RW/ZMG evaporator – with sample height of 3.5 cm – has a remarkable evaporation rate of 3.81 kg m⁻² h⁻¹, corresponding to a solar-thermal conversion efficiency of 153.7%, achieved under 1 sun illumination. The highly efficient solar evaporator was correlated to the cold evaporation from the sidewall surface gaining extra energy from the environment. It is important to note that the solar evaporation performance is almost two times higher that of a pristine RW sample and far superior to most reported in the literature that have a larger sample height (≥6 cm), so it could also reduce the loading mass of photothermal materials. Meanwhile, the evaporator was highly stable in 20 wt% NaCl under 1 sun illumination over 12 h continuous tests, able to maintain its evaporation rate of 3.55 kg m⁻² h⁻¹ with no salt accumulation observed, revealing the impressive long-term salt durability. Moreover, the large-scale evaporator was fabricated and tested under outdoor conditions for practicability evaluation, and demonstrates excellent salt ion rejection (99.8%), effective removal of azo and organic dyes, acidic/alkaline solution, and oil-in-water emulsions. These findings suggest that the cost-effective RW based evaporator provides a new concept for high performance solar steam generation, salt-resistant and multimedia purification applications.