Hydrothermal preparation of carbonized wood for photothermal water evaporation performance enhancement

Abstract: Interfacial photothermal evaporation is expected to resolve the water shortage, inconvenient water intake, and low water quality in most rural areas. However, the high material cost, complex preparation process, and low evaporation rate of the solar absorber are restricting the development of this novel technique. It has been reported that adjusting the water supply rate on the light-absorbing surface is an important approach to developing an efficient and continuous interfacial photothermal evaporator. In this work, the naturally evolved, porous, and photothermal material of pinus sylvestris was prepared with simple preparation, strong hydrophilicity, and evaporation rate close to the theoretical limit using a one-step hydrothermal method. Subsequently, the solar-driven interfacial water evaporation experiments were conducted to clarify the effects of carbonization degree and water supply height of the photothermal material on the interfacial evaporation performance. The surface temperature and evaporation reduction were considered at the different carbonization temperatures (100, 125, 150, 175, and 200℃), carbonization time (2, 4, 6, 8, 10, and 12 h), and the heights of the photothermal material (5, 10, and 15 mm). The underlying mechanism was then determined using the microstructure, element distribution, capillary water transport, and heat transfer. Importantly, the enhanced interfacial water evaporation was enabled by the carbonized wood. The results showed that the original pore structure of the log was retained with the rough surface formed by carbon microspheres after the mild carbonization process. There was a 2.3 ℃ temperature rise of the evaporation surface, particularly beneficial to the increase of light absorption. Moreover, the proportion of the C-H/C-C bond in the log (51.5%) was higher than that of the C-O-C/C-OH bond (37.7%), whereas, the proportion of the C-H/C-C bond (37.8%) in the carbonized wood was lower than those. It infers that the carbonization process greatly contributed to the log with more hydrophilic groups. This was because the parts of hydrophobic lignin and hemicellulose were removed during carbonization. Therefore, the climbing height of water in the carbonized wood increased from 4.2 to 22.3 mm during the climbing time (120 s). By contrast, the rising heights of water were 3.1 and 9.0 mm in the log and carbonized wood, respectively, indicating the weak capacity of transverse water transport. The data also agreed well with the classical Lucas-Washburn imbibition model. The steady-state evaporation rate of the carbonized wood-based evaporator increased by 130% and 28%, respectively, compared with the water- and log-configured evaporators. The maximum evaporation rate reached 1.24 kg/(m2?h) at the temperature of 200 ℃ after 8 h carbonization. It was noted that the excessive carbonization formed a bright and smooth surface, leading to the reflection loss of incident light with the reduced surface temperature of the evaporator. As a result, the evaporation rate dropped significantly. More importantly, the sidewall height of the material reduced the average temperature of the evaporation surface, resulting in the reduction of radiative and convective heat losses from the evaporator to the surroundings, thus improving the evaporation rate. Once the sidewall height was 15 mm, the evaporation rate reached 1.48 kg/(m2?h), and the corresponding solar-to-vapor conversion efficiency was 66.2%. This finding can also provide an important reference for the utilization of agricultural and forestry wastes, particularly for the heat-moisture balance during solar-driven interfacial water evaporation.