Energy And Mass Transport Regulation And Seawater Desalination Application Of Polypyrrole-based Interfacial Solar Evaporator

With increasing fresh water shortage and serious water pollution,it is particularly critical to develop a novel freshwater production technology with high efficiency,low toxicity,low cost and environmental friendliness.Seawater desalination technology can significantly alleviate the water shortage.Compared with traditional solar desalination technologis,solar-driven interfacial heating water evaporation technology（abbreviated as interfacial solar evaporation technology）achieves higher evaporation efficiency under low solar flux,which is favored by many scholars.In recent years,through constructing novel thermodynamic structures and developing new photothermal absorbers,the existing interfacial solar evaporators have basically approached more than 90%.Nevertheless,there are still some problems that hinder the further application of interfacial solar evaporators in seawater desalination.For example:（1）The interfacial solar evaporator suffers from serious salt pollution during the long-term desalination process;（2）Compared with other seawater desalination technologies,the evaporation rate of existing interfacial solar evaporators in seawater is still low;（3）The synthesis cost of interfacial solar evaporators with high evaporation performance is high and the preparation process is relatively complex.To solve the above challenges,this paper focuses on the interfacial energy and mass transfer issues involved in the interface evaporation process,such as heat and vapor conversion and water and salt transportation,aiming to improve the evaporation rate of interfacial solar evaporators in seawater.Polypyrrole-based interfacial solar evaporators and related seawater desalination application is studied.The main research results obtained are as follows:An intermittent salt removal strategy is proposed to solve the salt pollution of traditional interfacial solar evaporators in the long-term desalination process.An interfacial solar evaporator using polypyrrole（PPy）-coated polyvinyl formal（PVF）sponge（PPy/PVF sponge）as the absorber is constructed to verify the intermittent salt removal strategy.Specifically,during the evaporation process,the salinity of seawater in the PPy/PVF sponge gradually increases,while at night,the salt ions in the PPy/PVF sponge gradually diffuse into the bulk solution under the concentration gradient.Owing to the intermittent salt removal strategy of"salt accumulation under solar irradiation-salt discharge in the dark",the evaporator enables a stable evaporation rate of 1.15 kg m^-2 h^-1in seawater under 1 k W m^-2 of solar irradiation（1 sun）.The salt concentration distribution calculation results based on Fick’s law and seawater desalination experiments show that most interfacial solar evaporators can stably desalinate seawater through intermittent salt removal strategies.The research results provide a novel direction for designing salt-rejecting interfacial solar evaporators.Micro-nano water film-enhanced interfacial solar evaporation structure and construction principles were proposed to improve the evaporation rate of traditional interfacial solar evaporators.This construction method is verified by using a polydimethylsiloxane（PDMS）sponge co-coated by PPy and polydopamine（PDA）（PPy sponge）as the absorber.The in situ optical characterization results show that water will attach to the skeleton of the PPy sponge in the form of micro-nano water film after wetting.Under 1 sun,the evaporation rate of the interfacial solar evaporators with a PPy sponge as the absorber can reach 1.89 kg m^-2 h^-1.Quantitative water supply-simultaneous evaporation-in-situ optical characterization experiments show that:（1）The thickness of the micro-nano water film is the critical factor affecting the evaporation rate;（2）Balancing the heat transfer rate and mass transfer rate to maintain the balance of the thermal and mass of the interfacial solar evaporators is one of the necessary conditions for efficient evaporation.Through the intermittent salt removal strategy,the daily water production of the outdoor device based on the PPy sponge can reach 1.91 to 2.33 kg k W^-1 h^-1 for 40consecutive days.The research results provide a novel direction for designing high-efficiency interfacial solar evaporators.Based on the heat and mass balance,an interfacial solar evaporation structure co-strengthened by micro-meniscus and micro-droplets is proposed.Using p-Ts OH as a soft template,a hydrophilic carbon cloth loaded with PPy arrays（CC-PPy array）is constructed by electrochemical deposition.The interfacial solar evaporator taking CC-PPy array as the absorber exhibited a high evaporation rate of 2.13～2.16 kg m^-2 h^-1under 1 sun.The in situ optical characterization results show that when the CC-PPy array is wetted,many micro-meniscus with a diameter of about 1-5μm and micro-droplets with a diameter of less than 1μm appear.Kinetic calculations analyze the interfacial evaporation mechanism of micro-meniscus and micro-droplet co-enhancement;that is,the micro-meniscus increases the effective evaporation area and strengthens the heat transfer performance of the evaporation interface,and micro-droplets have a higher saturated vapor pressure.Under the combined effect of the micro-meniscus and micro-droplet,the evaporation rate of the evaporator is greatly enhanced.Aiming at the weak salt-removal ability of the CC-PPy array and the failure of the"intermittent salt-removal strategy"in high-salinity brine,a flowing interfacial solar evaporation structure with high-efficiency salt removal performance is constructed.The results of seawater desalination experiments show that the flowing interfacial solar evaporator based on CC-PPy array can operate stably in 10.0 wt%Na Cl solution and seawater for more than 72 h,and its evaporation performance does not decrease significantly.It is worth noting that the flowing interfacial solar evaporation structure can be effectively coupled with most absorbers with high evaporation performance,enabling the evaporator to operate stably in high-salinity brine.This study provides a novel direction for designing efficient salt-rejecting interfacial solar evaporators.Aiming at the low evaporation performance of interfacial solar evaporators under weak solar irradiation or dark conditions,a flowing interfacial evaporator driven by multiple heat sources is proposed.Aiming at the complex preparation process and high preparation cost of the above absorbers（PPy sponge and CC-PPy array）,a low-cost,non-toxic,harmless,and scalable PPy-coated bacteria cellulose hydrogel（PPy-BCH）is prepared by a low-temperature fermentation-in-situ deposition method.Subsequently,a flowing interfacial solar evaporator,heat-driven flowing interfacial evaporators,and solar and heat-co-driven flowing interfacial evaporators are constructed using PPy-BCH as the absorber.Under the combined effect of the 60℃heating conditions and 0.4 sun,the evaporation rate of the solar and heat-co-driven flowing interfacial evaporators reaches 4.38 kg m^-2 h^-1.The research results laid a theoretical and experimental basis for the large-scale application of interfacial evaporators.