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.

Effective utilization of abundant solar energy for desalination of seawater and purification of wastewater is one of sustainable techniques for production of clean water, helping relieve global water resource shortage. Herein, we fabricate a vertically aligned reduced graphene oxide/Ti₃C₂T_x MXene (A-RGO/MX) hybrid hydrogel with aligned channels as an independent solar steam generation device for highly efficient solar steam generation. The vertically aligned channels, generated by a liquid nitrogen- assisted directional-freezing process, not only rapidly transport water upward to the evaporation surface for efficient solar steam generation, but also facilitate multiple reflections of solar light inside the channels for efficient solar light absorption. The deliberate slight reduction endows the RGO with plenty of polar groups, decreasing the water vaporization enthalpy effectively and hence accelerating water evaporation efficiently. The MXene sheets, infiltrated inside the A-RGO hydrogel on the basis of Marangoni effect, enhance light absorption capacity and photothermal conversion performance. As a result, the A-RGO/MX hybrid hydrogel achieves a water evaporation rate of 2.09 kg·m^-2·h^-1 with a high conversion efficiency of 93.5% under 1-sun irradiation. Additionally, this photothermal conversion hydrogel rapidly desalinates seawater and purifies wastewater to generate clean water with outstanding ion rejection rates of above 99% for most ions.