Harvesting environment energy from water-evaporation over free-standing graphene oxide sponges

Due to increasing global energy demands, environmental pollution, and the rapid development of self-powered devices, converting ubiquitous environmental energy to usable energy, e.g., electricity is attracting increasing interests worldwide. Herein, a type of porous graphene oxide (GO) sponges is fabricated using the freeze-drying method. Then an annealing treatment and UV + O-3 oxidation are carried out to achieve partially reduced GO (rGO) sponges. We find that these rGO sponges can convert environmental energy to electricity via the natural evaporation of water. The generated open-circuit voltages are measured to be as high as about 0.63 V over a single piece of rGO sponges. The maximum output power and output power density are calculated to be approximately 17.30 mu W and 1.74 mu W cm(-2), respectively. We suggest that streaming potentials, which arise from water molecule-graphene interactions, should be the underlying mechanism of water-evaporation-induced electricity generation. Furthermore, we demonstrate that ambient temperatures, airflow velocities, and evaporation-areas all can seriously influence the electricity generation. Moreover, the water-evaporation-induced voltage can be easily scaled up to as high as about 2.34 V by connecting multiple samples in series. Therefore, our work supplies a potential method of converting ubiquitous environmental energy to electricity. (C) 2019 Elsevier Ltd. All rights reserved.