In situ generation of carbonized polyaniline nanowires on thermally-treated and electrochemically-etched carbon fiber cloth for high efficient solar seawater desalination
Graphical abstract
Electrochemically etched carbon fiber cloth with surface-coated carbonized polyaniline nanowires (ECFC/CPANW) shows three-dimensional porous structure, low thermal conductivity, high mechanical strength, broadband solar absorption and efficient water supply capacity, resulting in highly efficient solar steam generation performance.
Introduction
Currently, the shortage of freshwater resources is becoming more and more serious [1] and will definitely have a seriously negative impact on human life and world sustainable development [2]. In order to tackle this problem, some methods have been proposed, such as membrane treatment [3,4], advanced oxidation [5,6], tiny-fog collection [7,8] and solar steam generation [[9], [10], [11]], among which solar steam generation is an ideal technology for water treatment due to its low cost, sustainable stability, environmental friendliness and therefore has attracted enormous attentions [[12], [13], [14], [15], [16], [17]].
For ordinary water evaporation using solar energy as heat source, only a small part of the solar energy is used to heat bulk water and most of it is absorbed by the external environment, resulting in low efficiency of photothermal conversion, which severely affects the water evaporation rate [[18], [19], [20]]. In order to improve the efficiency of photothermal conversion, kinds of efficient photothermal conversion materials have been developed. Mu et al. prepared a three dimensional kapok fiber aerogels with polypyrrole as a solar steam generator, achieving a high steam generation efficiency of 82.4% under 1 sun illumination [21]. It has been proven by Xue et al. that natural wood treated with flame can be utilized as an ideal solar evaporator and its photothermal conversion efficiency can reach 72% under one solar illumination intensity [22]. Zhang et al. developed a double-layer graphite-modified polyurethane sponge as a solar evaporator, which has a simple manufacturing process, high recovery and low cost [23]. These studies indicate that efficient photothermal conversion materials need to be possessed with the following characteristics: high efficiency and broad-spectrum solar absorption, low thermal conductivity, hydrophilicity of efficient water supply, porous steam channel network, and high energy conversion efficiency. At present, the common photothermal conversion materials can be divided into the following categories: metal plasma nanoparticles [11,[24], [25], [26]], carbon-based materials [[27], [28], [29]], biomass-based materials [21,30,31] and porous polymers [16,32,33], etc. However, in most cases, the reported materials (such as porous polymers, carbonized woods, graphene oxide or metal plasma nanoparticles) have the disadvantages of high production costs, hard to scale up or complicated manufacturing methods, which greatly hinder their practical large-scale applications. Therefore, designing and manufacturing stable, efficient and inexpensive photothermal conversion materials is of great significance.
Pre-oxidized polyacrylonitrile fiber cloth (PPFC) is a stable, low-toxic, heat-insulating, easy-to-cut and eco-friendly material, but its hydrophilicity and conductivity are poor, and its mechanical strength is low [34,35]. This means that the spatial structure of the PPFC is easy to be changed, which affects the water transport and steam escape. In current work, we obtained an annealed carbon fiber cloth (ACFC) via annealing PPFC at 650 °C. Compared with PPFC, the mechanical strength and electrical conductivity of ACFC are significantly improved. Polyaniline has been proved to be an efficient water evaporation material owning to its low cost and simple synthesis process [32]. It is an excellent precursor for the preparation of nitrogen-containing carbon materials with high specific surface area via thermo-treatment [36,37]. In addition, we all know that by doping N, the hydrophilicity of carbon materials will be significantly improved [38,39].
In this paper, we prepared the etched carbon fiber cloth coated with carbonized polyaniline nanowires (ECFC/CPANW), which owned low density, efficient solar absorption, good thermal insulation and high efficiency of water transportation. In addition, the raw materials for preparing ECFC/CPANW are low in cost and the production process is mature and facile. Therefore, the production of the ECFC/CPANW composite can be scaled up easily, which make ECFC/CPANW a promising candidate for water evaporation. However, ECFC/CPANW cannot float on water surface for long time during the frequent water evaporation processes, resulting in compromised evaporation efficiency. In order to solve this problem, polyurethane foam was used as the bottom layer to achieve a solar evaporator with two advantages: the first one is to support the ECFC/CPANW on the water surface and the second one is to establish an insulation layer to avoid the heat loss. The water evaporation test for ECFC/CPANW shows evaporation rates of 1.4255 kg m−2 h−1 under 1 sun illumination, a high evaporation conversion efficiency of 93.7%, and good seawater desalination performance.
Section snippets
Materials
Pre-oxidized polyacrylonitrile fiber cloth (PPFC) was purchased from Shandong Aolong Garment Co., Ltd. Concentrated sulfuric acid and concentrated hydrochloric acid were purchased from Shenzhen Taidou Scientific Instrument Co., Ltd. Aniline was purchased from Sinopharm Chemical Reagent Co., Ltd. Polyurethane foam was purchased from Langfang Wenchang Insulation Materials Co., Ltd. Deionized water was produced by Fulham water purifier (Qingdao, China). Seawater was taken from the Yellow Sea
Results and discussion
The schematic illustration for the preparation of ECFC/CPANW is shown in Fig. 1a. and the detailed processes can be found in the experimental section. The reason why we chosen PPFC as the framework of the proposed photothermal materials is because it owns low price, low thermal conductivity, high porosity, acid-alkali corrosion resistance and chemical environment resistance. The morphology of PPFC is shown in Fig. 2a and d. Its thermal stability and surface functionalities have been
Conclusions
In summary, we have processed the pre-oxidized polyacrylonitrile fiber cloth by annealing, electro-etching, electropolymerization and carbonization to prepare ECFC/CPANW, which was then combined with polyurethane foam to achieve an evaporator for solar steam generation. The prepared ECFC/CPANW has low thermal conductivity, good hydrophilicity and broad light absorbance band, high mechanical strength and rich pore structure. The bottom layer of polyurethane foam has excellent thermal insulation
Author statement
I would like to declare on behalf of my co-authors that the submitted work was an original research that has not been published previously, and not under consideration for publication elsewhere.
Declaration of competing interest
We ensure that no conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.
Acknowledgements
The authors are thankful to funds from the Qingdao Innovation Leading Talent Program, National Natural Science Foundation of China (21805124), and Natural Science Foundation of Shandong Province (ZR2018BEM020).
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