Solar energy is the most abundant, clean, and renewable natural resource available. Solar photothermal energy conversion technology, used to generate clean water steam from seawater desalination or wastewater purification, represents a promising solution to the global scarcity of fresh water. Herein, for the first time, graphene acid (GA), a novel member of the graphene family with a defined number of carboxyl groups, is employed as a light-to-heat conversion material. To enhance efficiency, graphene acid is reacted with branched polyethylenimine (PEI), which has a high amine density, at room temperature. This process splits the graphene acid sheets and produces modified porous GA (GA-10/PEI/3D) materials. The porous materials are stabilized through both electrostatic interactions between the carboxyl groups in GA and the NH₃+ groups in the terminal chains of PEI, as well as the formation of amide bonds between the carboxyl groups and primary amine groups of PEI. The structure of the porous materials is characterized by N₂ adsorption–desorption isotherms, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), while their chemical and optical properties are analyzed using X-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy, Raman spectroscopy and UV–Vis spectroscopy. The porous materials are deposited onto a thermally insulating three-dimensional (3D) aerogel foam with low thermal conductivity (0.026 W m−1 K−1), a hydrophilic surface, and a microporous structure. The hydrophilicity, high porosity, and strong solar absorption of the prepared GA-10/PEI/3D porous materials, combined with the low thermal conductivity of the 3D aerogel, contribute to efficient conversion of absorbed solar energy into heat at the water–air interface. This enables a solar evaporation rate of 2.2 kg m−2 h−1, with a photothermal efficiency of 94% under one-sun illumination (1 kW m−2). For comparison, graphene oxide was prepared and treated with PEI. The resulting GO/PEI/3D aerogel exhibits a lower solar evaporation rate of 1.6 kg m−2 h−1 and a photothermal efficiency of 86% under the same conditions. Notably, the evaporation rate of the GA-10/PEI/3D aerogel photothermal converter is approximately nine times higher than that of seawater under natural conditions after 30 min. Cycling tests confirm that the prepared mesoporous materials maintain stable evaporation rates over 25 repeated cycles without any decrease in performance. Furthermore, the excellent stability, high solar thermal evaporation efficiency, low cost (due to the absence of noble metals), long-term durability, and simple preparation make the GA-10/PEI/3D aerogel a strong candidate for a wide range of applications in solar steam generation for seawater desalination and wastewater purification.