Design,Preparation And Performance Research Of Highly Efficient Photothermal Water Evaporation Material Based On Polymer Cryogels

The global pollution of water resources is increasing,and the water resources available for direct use by human beings are continuously decreasing.Therefore,there is an urgent need for environmentally friendly and effective technologies and materials for sewage purification and seawater desalination.Solar energy is a green and recyclable resource.With the assistance of photothermal materials,solar energy can be converted into thermal energy for photo thermal water evaporation,which provides a green and promising way for water treatment.In order to realize the practical application of efficient photothermal water evaporation,there are still many challenges,including photothermal materials with excellent photophysical properties,evaporation systems with high water transfer capacity,no negative effects to the environment,convenient transportation/usage and low cost for large-scale production.Generally,the photothermal water evaporation system consists of the photothermal material and the supporting substrate.Therefore,for photothermal materials,they are required to have excellent light absorption characteristics,low cost,and steady combination with the matrix;for substrate materials,they should have excellent physical and mechanical properties,thermal stability,thermal insulation properties,and rapid water absorption and transfer etc.Poly(acrylic acid)(PAAc)cryogel has good hydrophilicity and interpenetrating macroporous structure.It is prepared at the temperatures lower than the freezing point of polymerization mixture.It can quickly absorb and transport water,has low thermal conductivity,and exhibits the acid-doping ability for polyaniline(PANI)and polypyrrole(PPy),which prompts the steady combination between them.Therefore,PAAc cryogel is an excellent candidate as the matrix for photothermal water evaporation with the help of photothermal materials.In the first item of this thesis research,we prepared PAAc cryogel,introduced three photothermal materials through different ways to obtain four composite cryogels,and constructed a photothermal water evaporation system with one mushroom-like PAAc cryogel as the substrate.Under one-sunlight irradiation,the water evaporation rate and overall light-to-evaporation efficiency for PAAc-PPy composite cryogel are 1.819 kg·m-2·h-1 and 100%,respectively while the evaporation system has excellent long-term recyclability.In addition,for the photothermal evaporation of salty water,due to the rapid transmission of water,the precipitated salt solids can be dissolved spontaneously during the simulated sunlight shut-off period,thereby maintaining a long and high evaporation rate.As for the realistic calculation of energy conversion for thermal evaporation based on Energy Constant Principle,it is deduced that the decrease of real evaporation enthalpy is the dominative factor to enhance the photothermal evaporation rate,which can be reached by increasing the evaporation temperature and modulating the molecular interactions on water molecules.This research provides an ideal matrix material—PAAc cryogel for photothermal water evaporation system,and opens up a new way for water purification and seawater desalination.Usually,polymer cryogels are prepared through redox-initiated cryo-polymerization with much long duration more than 10 h,leading to low production effectiveness.Thus,in the second part of my thesis study,polyacrylamide(PAAm)cryogels were prepared through UV-radiated cryo-polymerization,whose duration was shortened to less than 1 h.Changing the concentrations of monomer,crosslinker and photo-initiator,the needed duration at the different subzero temperatures was determined and various PPAm cryogels were obtained in sheet shape.After the in-situ oxidative coupling polymerization in PPAm cryogels,composite cryogel(PAAm-PPy)sheets were produced.On the top of commercial melamine resin sponge cut into umbrella shape,PAAm-PPy cryogel sheet was integrated to have the photothermal evaporation system.The experimental results show that when the monomer mass fraction was 10 wt%,the cross-linker content was 1 wt%of monomer and the photoinitiator concentration was 2 wt%of monomer,the photothermal evaporation rate of the composite cryogel under one-sun light irradiation reached the maximum,being 1.900 kg·m-2·h-1.Additionally,during the unbroken 12-h photothermal evaporation and the repeated photothermal evaporation within 2 months,the evaporation rate was observed to be unchanged,indicating the wonderful durability.Therefore,the combination of cryogel synthesis through UV-radiation cryo-polymerization and the commercial macro-porous materials provides one easy-prepared,easy-used and low-cost photothermal evaporation device with excellent performance.