Desalination of seawater using solar energy is a promising solution to the global freshwater shortage. Ultrahigh surface area (up to 1740 m² g⁻¹) hierarchical porous carbons (HPC) have been prepared by the carbonization of precursors derived from the room temperature dehalogenation of low cost, widely available polyvinyl chloride (PVC) with simple, low cost, environmentally friendly processes. The broad hierarchical pores (from 2 nm to 20 μm) facilitate and ensure fast water and vapor transportation. Flexible photothermal steam generation devices were successfully fabricated with these hierarchical porous carbons on hydrophilic ultrathin (200 μm) paper. An evaporation rate record of 7.87 kg m⁻² h⁻¹ and high energy conversion of 95.8% have been obtained under the concentrated solar intensity of 5 kW m⁻². Our research leads to a new approach to converting halogenated plastics into environmentally friendly and useful porous carbon materials by simple, low-cost processes. It establishes and validates the concept of creating a sustainable and economic pathway to simultaneously recycle halogenated polymers, harvest solar energy and produce clean freshwater.