Constructing central hollow cylindrical reduced graphene oxide foams with vertically and radially orientated porous channels for highly efficient solar-driven water evaporation and purification

Although solar steam generation is an eco-friendly approach for desalinating seawater and purifying wastewater, there are still issues on how to improve the efficiency of solar energy utilization and accelerate the water and heat transport inside the solar-driven water evaporators. Herein, we design a central hollow cylindrical reduced graphene oxide (RGO) foam with vertically and radially orientated channels as a solar steam generation device for efficient water evaporation and purification. The vertically aligned porous channels accelerate upward transport of water to the top evaporation surface, while the radially aligned porous channels facilitate water transport and heat transfer along the radial directions for fully utilizing the heat accumulated inside the central cylindrical hole of the foam. The central hole of the foam plays a highly positive role in accumulating more heat for accelerating the water evaporation, and the newly generated inner sidewall resulted from the central hole can gain extra thermal energy from surrounding environment in the same way as the outer sidewall of the foam due to the surface cooling effect of the water evaporation. As a result, the vertically and radially aligned RGO foam evaporator with central hollow cylinder achieves a high solar steam generation rate of 2.32 kg·m⁻²·h⁻¹ with an exceptional energy conversion efficiency of 120.9% under 1-sun irradiation, superior to the vertically aligned RGO foam without the central hole (1.83 kg·m⁻²·h⁻¹, 96.9%) because of the enhanced water and heat transfer inside the porous channels, and the efficient utilization of environmental energy.