Water Vapor Adsorption Of Modified Activated Carbon Fibers And The Characteristics Of Electrothermal Regeneration

Water harvesting from air is considered to be one of the important ways to solve the global water shortage.Adsorbents play a vitally important role in the system of adsorpption water harvesting from air.However,there are still some shortcomings in the study of hydrophilic adsorbents.（1）An efficient hydrophilic adsorbent for water harvesting from air,especially at low humidity levels,has not been developed;（2）The water vapor adsorption properties of hydrophilic adsorbents have been studied extensively,but its regeneration properties are less studied.The present work aims to prepare appropriate carbon-based adsorbent for water harvesting from air on account of above problems.Viscose-based activated carbon fiber（ACF）was modified by low temperature oxygen plasma（LTOP）and hydrophilic salt（Ca Cl₂）,respectively.Experimental investigation of water vapor adsorption and regeneration together with a series of characterization techniques were combined to explore the potential application of ACFs modified by LTOP and Ca Cl₂ in water harvesting from air.Firstly,a series of hydrophilic ACF-based adsorbents were modified by LTOP.Meanwhile,the water vapor adsorption performance was investigated under low relative humidity（RH）conditions（the environment of most inland dry regions in the world）.A variety of modern analysis and testing methods were used to reveal the changes in physical and chemical characteristics of ACFs before and after LTOP modification.The effects of adsorbent characteristics on its water adsorption performance were investigated.Measurement of water vapor adsorption was performed by a programmable constant temperature and humidity chamber and a vapor sorption mode of the automated gas sorption analyzer.Accordingly,the suitable effective output voltage and treatment time were obtained to improve the water vapor adsorption at low RH.The mechanism of LTOP modification on ACF surface and the adsorption mechanism of water vapor over the modified sample were proposed,respectively.Secondly,a series of composite adsorbents with different loading amounts of Ca Cl₂were prepared by an impregnation method and used to evaluate the water vapor adsorption performance under high RH conditions（the environment of islands and coastal areas）.The loading amounts of Ca Cl₂ and the pore structure of the samples were measured by inductively coupled plasma optical emission spectrometer（ICP-OES）and the automated gas sorption analyzer,respectively.The measurement of water vapor adsorption was also performed by the programmable constant temperature.Meanwhile,the water vapor adsorption and in-situ electrothermal regeneration device were constructed to evaluate the water vapor adsorption and electrothermal characteristics.The main research conclusions are as follows:（1）In the lower RH range（RH≤40%）,the LTOP modified ACF has better water vapor adsorption performance than the original sample.The initial adsorption sites,including oxygen-containing functional groups and carbon atoms with unpaired electrons,on the sample surface were increased by LTOP modification.Both the large number of oxygen-containing functional groups（OFGs）and these carbon atoms with unpaired electrons act as the nucleation sites for the formation,growth and coalescence of water clusters.Thereinto,the relative content of the carboxyl group and its derivatives increased from 4.49%to 5.26%-18.49%.The results show that when the treatment time is 15 min and the effective output voltage is 8～9 k V,the saturated adsorption capacity of the modified sample is about 0.15 g/g at 25°C and 40%RH,which is 2.5 times of the original sample（about 0.06 g/g）.（2）At low RH（RH=40%）,LTOP modified ACFs have a good performance of water vapor adsorption-regeneration cycle.Carbon atoms with unpaired electrons can chemisorb with water molecules and generate new hydrophilic oxygen-containing sites in the process of water vapor adsorption.After 6 times of adsorption and regeneration,the adsorption capacity of LTOP modified ACF was still 1.6～1.8 times that of the original sample.（3）At high RH,the saturated adsorption capacity of water vapor mainly depends on the pore structure characteristics（i.e.,the total pore volume）.Some negative effects on the pore structure of the samples were caused by LTOP modification.After LTOP modification,the specific surface area and total pore volume decreased from 1783 m²/g and 0.905 cm³/g to 990～1716 m²/g and 0.516～0.879 cm³/g,respectively.Therefore,LTOP modification technology is not conducive to improving the water vapor adsorption performance of ACF at higher RH.（4）The water vapor adsorption performance of the adsorbent can be greatly improved by modifying it with hydrophilic salt.First,water vapor enters the pore channel under the action of the capillary force of ACF carrier.And these moisture is chemisorbed with the hydrophilic salt inside the pore channel to form the hydrated salt.Second,hydrated salt further absorbs water to form a saturated salt solution,which still has a certain capacity of water vapor adsorption.At 25°C and 70%RH,the saturated adsorption capacity can be increased from 0.55 g/g to 1.70 g/g.（5）At low loading amounts of Ca Cl₂（≤10%）,adsorption performance of composite adsorbents to water vapor is mainly dominated by physical properties such as specific surface area and pore volume.When the loading amount of Ca Cl₂ is higher than 10%,the water vapor adsorption performance of the composite adsorbent is gradually changed to its chemical properties（i.e.,loading amount of Ca Cl₂）.（6）The study of electrothermal regeneration showed that the sample modified with hydrophilic salt had better water vapor adsorption performance.The above research results have a certain promoting effect on optimizing the modification of activated carbon fibers,deepening the understanding of the mechanism of activated carbon fibers adsorption of water vapor,establishing a complete theoretical analysis system and guiding the research and development of hydrophilic carbon materials.These studies indicated that LTOP modified ACFs and hydrophilic salt modified ACFs provide potential applications for water harvesting from air under low RH and high RH conditions,respectively.