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Direct solar steam generation (DSSG) is currently one of the most promising desalination methods due to its high energy conversion efficiency. The key element of DSSG is solar thermal conversion materials (STCMs), which absorb incoming solar illumination owing to their excellent solar absorption and convert light to heat. Although many types of STCMs have been applied to DSSG, there are still some existing problems such as complex preparation, high cost, and low energy conversion efficiency. In this work, carbonized corncob, as an example of inexpensive agricultural waste, is prepared as a STCM, which has porous honeycomb‐like pores, and hydrophilic functional groups, ensuring sufficient water transportation. Different pyrolytic temperatures lead to different size pores, content of amorphous carbon, and amount of oxygen functional groups, which affect the water transportation efficiency and light‐to‐heat conversion efficiency. Benefiting from developed pores, 95.7% solar absorption and thick‐hole wall, the carbonized corncob pyrolyzed at 800 °C reaches a solar‐to‐steam conversion efficiency of 86%, and its water evaporation rate is 1.62 kg m–2 h–1, higher than most biomass reported so far. Using cost‐effective materials and methods to carry out efficient photothermal conversion is of great significance for the realization of practical seawater desalination and sewage treatment.

Solar-driven steam generation using photothermal materials is a green and promising approach to address clean water scarcity. In this work, the graphene oxide(GO) was synthesized by an improved Hummers method, and then GO was reduced by hydrothermal method and modified with nitrogen, called as NRGO. Then, we performed UV-VIS-NIR, XRD, Raman, FTIR and SEM characterization. In order to reduce the heat loss of bulk working liquid and improve the solar-to-heat conversion efficiency, the isolation system with 2D water path was used. The efficiency test of steam generation was performed under the solar intensity of 1000 Wm −2 for 1 h, and the photothermal conversion efficiency of reduced graphene oxide (RGO) and nitrogen-doped graphene (NRGO) was 73% and 87%, respectively. It was found that NRGO owned better solar absorption efficiency, lower thermal conductivity and porosity. NRGO was combined with insulated isolated system which can reduce the heat loss of bulk water, to improve photothermal conversion efficiency.

Solar-driven steam generation as a potential green technology has attracted extensive attention to solve the freshwater scarcity crisis. Photothermal materials as the key section of solar steam generation have been widely reported. However, there is still a challenge in developing easily prepared, environmental-friendly, and low-cost materials. Herein, the simple, scalable, and porous carbonized cotton was prepared as an evaporator to enhance solar-based evaporation, which has excellent light absorption ability in the range of the full spectrum (300–2,500 nm). Benefiting from 95% solar absorption and the pores between the cellulose tubes, the carbonized cotton heated by plate carbonization reaches a steam generation rate of 0.8 kg m−2 h−1, which is about 5 times that of untreated cotton. Compared with tube furnace carbonization, flat plate heating carbonization of cotton requires lower equipment requirements and does not need nitrogen protection and cleaning tar, and the photothermal conversion efficiencies of both are similar. In addition, carbonized cotton as an evaporator was heated up rapidly under 1 sun irradiation and reached a stable temperature in 20 s, greatly improving the photothermal conversion rate. Therefore, plate heating carbonized cotton provides a good idea for preparing solar photothermal conversion materials and a novel strategy for simplifying the production of biomass thermal evaporators.

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