Recyclable Fe₃O₄@CNT nanoparticles for high-efficiency solar vapor generation

Highlights: • Fe₃O₄@CNT nanofluids was utilized for recyclable solar thermal evaporation system. • Evaporation efficiency up to 60.3% was achieved by Fe₃O₄@CNT nanofluids. • Recovery efficiency could be controlled by magnetic fields and Fe₃O₄ particles. - Abstract: Solar power, as an important source of renewable energy during the recent years, holds a great potential to produce vapor. Solar-assisted evaporation systems usually rely on the concentration of nanofluids and the intensity of solar illumination. However, the nanofluids used in these systems can cause secondary pollution. These issues severely limit large-scale application of the solar evaporation technology. In this study, we developed a solar thermal evaporation system based on carbon nanotubes (CNTs) decorated with magnetic Fe₃O₄ nanoparticles (Fe₃O₄@CNT), which could be separated from water under the action of a magnetic force. The two components (with different ratios) of these Fe₃O₄@CNT differed in their magnetic and thermal properties. A high evaporation efficiency (60.3%) was obtained with a 0.5 g/L Fe₃O₄@CNT nanofluid under a solar illumination power of 10 sun (1 sun = 1 kW·m⁻²), which was achieved by heating the nanofluid locally rather than by increasing the temperature in bulk. A thermal receiver efficiency of 88.7% was obtained with a 0.5 g/L Fe₃O₄@CNT nanofluid under a solar illumination power of 1 sun achieved by a bulk temperature increase. The Fe₃O₄@CNT nanoparticles could be separated from water at a rate faster than that of the Fe₃O₄ nanoparticles. The recovery rate of the Fe₃O₄ nanoparticles and Fe₃O₄@CNT ranged from 97.9% to 98.7%. The Fe₃O₄ nanoparticles in Fe₃O₄@CNT enhance their ability to absorb the solar energy, thus making it higher than that of CNTs. The recovery rate and efficiency could be controlled by adjusting the magnetic field strength and the ratio of Fe₃O₄ and CNTs. Thus, this system study provides an approach to not only significantly reduce the material consumption in the design of solar evaporators, but also to realize broad solar energy applications such as seawater desalination and vapor generation.