Integrated Design Of Porous Carbonized Rice Husk Foam Absorber For Efficient Solar Steam Generation

Solar steam generation has become a research focus in photo-thermal conversion technologies in recent years for its attractive efficiency and practicability.To obtain good light absorption and conversion efficiency,the key factor is to control the microstructure and performance of absorbers.About four characteristics are considered to be required for an ideal absorber,such as broadband solar absorption,abundant porous channels for water transfer,good wettability and low thermal conductivity.Being a representative biogenic carbon,carbonized rice husk,has exhibited great potential for solar steam generation owing to its high carbon contents,efficient light absorption,good wettability,renewability and unmatched reserves.But the powder state of carbonized rice husk has still resulted in the lack of water channels and relative low solar vapor efficiency.And further studies on affecting factors and solar-thermal conversion mechanism for carbonized rice husk based absorbers are also needed.In this work,a 3D carbonized rice husk foam absorber with continuous porous channels is first constructed with the engineering uniting foaming method and gel injection molding for the convenient water transfer.Meanwhile,some strategies like in-situ introducing plasmonic Au nanoparticles and growing graphitized carbon structure are also implemented to enhance the solar-thermal conversion efficiency.Furthermore,the pore structure,and hydrophilicity of foam absorbers are adjusted to investigate the influencing mechanism on solar-thermal conversion.Based on experiments,the propagation of rays in carbonized rice husk foam is simulated to analyze the key factors on light absorption.The effective light waves for solar-thermal conversion are also studied to illuminate the solar-thermal conversion mechanism during solar steam generation process.Specific studies and results are as follows:（1）A 3D carbonized rice husk foam（CRF）absorber was constructed by uniting foaming method and gel injection molding technology.The relationship between pore structure,light absorption performance,water wettability and photothermal conversion performance of CRF was studied.Some porous models were established and propagation of rays in carbonized rice husk foam was simulated using a commercially available simulation tool COMSOL Multiphysics 5.4 to reveal key factors on light transmittance and reflectivity.The effective light waves for solar-thermal conversion were also analyzed to illuminate the solar-thermal conversion mechanism for CRF.Results show:as-prepared CRF possesses continious micron pore channels,broadband solar absorption,good hydrophilicity and～71%solar-vapor conversion efficiency;the effective light waves for solar-thermal conversion are across visible and infrared light area and the solar-thermal conversion mechanism may be ascribed to the lattice vibration heating effect of carbonized rice husk;rays simulation revealed that increasing pore numbers can effectively reduce the transmittance of porous absobers,and reducing intrinsic reflection coefficient of modles can optimize the transmittance and reflectance simultaneously.（2）The Au-NPs composite carbonized rice husk foam（CRF/Au-NPs）was acquired by uniformly dispersing plasmonic Au-NPs on carbonized rice husk foam frameworks with liquid foams as carriers.The effects of introducing Au-NPs on pore structure,light absorption and solar-thermal conversion permance were studied.The effective light waves for solar-thermal conversion were also investigated to illuminate the solar-thermal conversion enhancement mechanism for CRF/Au-NPs compared to CRF.Results show:Au-NPs were uniformly dispersed in CRF/Au-NPs frameworks in the form of discrete nanometer points;introducing Au-NPs can simultaneously enhance light absorption and solar-vapor evaporation performance;the maximum solar-vapor conversion efficiency is～88%under a power density of 1kW m^-2;effective light wave study indicates that the solar-thermal enhancement mechanism of CRF/Au-NPs system is mainly ascribed to the characteristic plasma heating effects of Au-NPs across visible light.（3）Focusing on the biomass organic carbon of rice husk,the graphitized carbon modified carbonized rice husk foam（GC/CRHF）absorber was prepared by introducing transition metal Ni as catalyst to in-situ catalyze the pyrolysis carbon of rice husk to form graphitized carbon structure in carbonized rice husk foam substrate during the carbonization process.The influences of forming graphitized carbon on microstructure,light absorption and solar-thermal conversion performance of GC/CRHF were investigated.The solar-thermal conversion enhancement mechanism was also analyzed through the effective light wave study.Results show:large numbers of graphitized carbon structures were in-situ formed in GC/CRHF owing to the catalysis of Ni;the micro-morphology of graphitized carbon was influenced by the heat-treatment atmosphere;under N₂ atmosphere,it mainly forms core-shell nanosphere with nano Ni as core and graphitized carbon layer as shell,while in riched carbon atmosphere,it evolved into carbon nanotubes;the formation of graphitized carbon can improve the graphitization degree,light absorption and solar-thermal conversion performance of GC/CRHF;the maximum solar-thermal evaporation efficiency reaches at～67%;effective light wave study reveals that in-situ forming graphitized carbon can enhance the lattice vibration of rice husk carbon while absorbing solar.（4）The pore structure and wettability of GC/CRHF were adjusted to respectively optimize evaporation surface and manage solar and thermal in 3D absorbers with the engineering of chemical activation and surface hydrophobic treatment.The influences of the adjustment process on solar-thermal conversion performance of were studied.Results show:with KOH activated,large numbers of nano-pores were formed in GC/CRHF,revealing the successful adjustment of pores;the pore structure of activated GC/CRHF was covered across micron pores,mesopores and micropores;the formation of nano-pores can enrich the evaporation sites of vapor,but also increasing the water absorption and heat transfer burden between absorber-water interface which may limit the evaporation process;the surface treatment of fluorosilane can effectively adjust the interface wettability of GC/CRHF absorber;with increasing concentration of fluorosilane,the wetting process evolved from slow wetting to superhydrophobic state;dynamic wetting process and solar steam generation test demonstrated that appropriate wettability can effectively manage solar and thermal,and simultaneously ensure convenient water transfer,which can of course improve the solar-vapor conversion efficiency,while super or terrible hydrophilicity may lead to some defects,such as solar and heat loss or bad water transfer,to reduce evaporation efficiency.