A 3D Photothermal Structure toward Improved Energy Efficiency in Solar Steam Generation

The energy efficiency in solar steam generation by 2D photothermal materials has approached its limit. In this work, we fabricated 3D cylindrical cup-shaped structures of mixed metal oxide as solar evaporator, and the 3D structure led to a high energy efficiency close to 100% under one-sun illumination due to the capability of the cup wall to recover the diffuse reflectance and thermal radiation heat loss from the 2D cup bottom. Additional heat was gained from the ambient air when the 3D structure was exposed under one-sun illumination, leading to an extremely high steam generation rate of 2.04 kg m⁻² h⁻¹. The 3D structure has a high thermal stability and shows great promise in practical applications including domestic wastewater volume reduction and seawater desalination. The results of this work inspire further research efforts to use 3D photothermal structures to break through the energy efficiency limit of 2D photothermal materials. Tapping into solar energy to produce clean water as well as to treat wastewater is a viable solution to the ongoing global challenges of water scarcity and clean energy shortage. Solar steam generation assisted by photothermal materials is an integral part of solar distillation and many water removal processes. The energy efficiency of 2D planar photothermal materials for solar steam generation has been pushed to its limit, with diffuse reflectance and thermal radiation accounting for the major energy loss therein. Here, we introduce a 3D cup-shaped photothermal structure capable of recovering most of the lost energy in 2D photothermal materials, thus breaking the energy limit of 2D materials. The solar steam generation rate of the 3D photothermal material is further improved by purposefully harvesting heat from the ambient air. The 3D cup-shaped solar evaporator achieves near 100% energy efficiency in solar steam generation, because its wall can efficiently reabsorb the diffuse reflectance and thermal radiation from its 2D bottom part. It also gains excess energy from the surroundings by keeping most of its wall cooler than its surroundings, even under one-sun illumination. The 3D design inspires further research efforts to use 3D structures to break through the energy efficiency limit of 2D photothermal materials.