Performance Analysis And Manipulation Of A Proton Exchange Membrane(PEM) Dehumidification System

There is a great demand for compact air dehumidification in small spaces,high precision manufacturing,instrumentation and high price goods storage,etc.However,the traditional dehumidification technologies are limited in application because of there complex equipment and large thermal inertia.Electrolyte membrane dehumidification is a promising novel dehumidification technology.It has the characteristics of compact structure,simplicity,cost-effectiveness and environment friendliness.It can achieve an accurate,portable and energy-saving dehumidification.This novel technique drives air humidity transfer by low DC electric field.It is essentially different from other dehumidification methods.There are also significant differences in system characteristics,operating conditions,application background and durability etc.from other PEM electrochemical systems such as fuel cells.However,the research on this dehumidification process is still in the stage of feasibility exploration.The dehumidification and electrochemical characteristics of the system are not clear.Research on the operating characteristics and the transport processes,especially the V-I characteristics and durability features that are significantly different from other PEM systems,are still limited and cannot guide performance manipulations.In view of this,the research work of this paper mainly has the following contents:?1?An electrolyte membrane dehumidification system with two gas flow channels is built,and the feasibility of dehumidification of air flow is verified.The experiment shows that the relative humidity of the air flow can be reduced from 90%RH to below 30%RH under an applied voltage of3 V,with a steady-state dehumidification performance of the system is 2 g/?J·m²?and 1.1 g/?s·V·m²?.The dehumidification rate of the system increases with the increase in temperature,flow rate,or relative humidity.The current density of the system increases at first and then decreases with the voltage.The performance analysis shows that the high water content in the cathode of the PEM module leads to the reverse diffusion quality accounting for 45⁵5%of the total transmission quality,which seriously limits the improvement of dehumidification performance.At the same time,only about 30%of the total power of the system is used for the effective dehumidification,and the remaining 35⁴5%is used for the reaction activation energy loss,and 5¹0%is used for the concentration difference of water molecules on both sides of the anode and cathode,resulting in loss,etc.This makes the system's COP??0.33?lower.?2?When the applied voltage is higher than 2.5 V,the current density and dehumidification efficiency of the system may appear to rise and fall with the applied voltage.When the applied voltage of the system increases from 1.5 V to 3.5 V,the EIS test shows that the high frequency resistance R_o of the dehumidification module can be increased from 1.69?to 2.69?.Based on this further analysis,it is found that the ohmic overpotential of PEM can be increased by 20 times,and the proton conductivity of PEM can be reduced by 13³8 times.The sharp decrease of PEM proton conductivity is the reason why the current density of the system decreases with the voltage.Based on the analysis of the V-I characteristic mechanism of the dehumidification process of the electrolyte membrane,the rapid prediction model established in this paper has a calculated deviation of current density,PEM proton conductivity and dehumidification rate within 15%.Based on the characteristics of the electrolyte membrane dehumidification system,the model can realize online monitoring,rapid estimation and direct regulation of system performance under various working conditions.?3?The durability test of the electrolyte membrane dehumidification system over 250 h shows that the current density of the system decreased by 67%,39.5%and 23%.The in situ and ex situ EIS test results at 4 V,3 V and 2 V are conducted respectively.The test result shows that the internal resistance of the dehumidification module can be increased by 3 times after the durability test,and the reaction polarization resistance can be increased 2 times.The combined analysis of XRD and in-situ XPS showed that the anode side catalyst IrO₂ is irreversibly dissolved during dehumidification.The dissolution of IrO₂ can lead to the increase of contact resistance and the decay of activity,which is the reason why the current does not rise,but fall,and the internal resistance and polarization resistance of the system increase with time.In the dehumidification process under high applied voltage or at high air humidity,IrO₂ is more likely to form volatile Ir?III?transition state oxides,which leads to a faster system performance degradation.?4?For the system performance manipulations,in this study,from improve the conductivity of the catalyst,increase the specific surface area and change the crystallinity of the anode catalyst,four kinds of anode catalyst materials,ATO-IrO₂,ND-MnO₂-IrO₂,anatase IrO₂?A-IrO₂?and rutile IrO₂?R-IrO₂?are developed respectively so as to improve the system dehumidification performance and durability.The operation experiments show that compared to A-IrO₂ and another commercial catalyst,ATO-IrO₂,can improve the dehumidification performance of the system by 20%and 45%,respectively.Compared to IrO₂,the specific surface area of ND-MnO₂-IrO₂ is increased by 40%,while the dehumidification performance of ND-MnO₂-IrO₂ is only increased by 2%⁵%.At the same time,its performance is still improved by 20%²5%compared to commercially catalysts.In the 50 h durability test under 4 V,the attenuation rate of the system using ATO-IrO₂ is 3%,and the attenuation rates using the ND-MnO₂-IrO₂ and A-IrO₂ systems are 8%and 10%,respectively.In addition,R-IrO₂with higher crystallinity of IrO₂ crystal has a stable performance and it does not decay in its system durability test.However its dehumidification performance is only about 35%of A-IrO₂.In general,the use of ATO-IrO₂ with high electrical conductivity can greatly improve the dehumidification performance and durability of the system.?5?Two kinds of superabsorbent proton exchange membranes?PEM?,N212/SiO₂ and Nafion/SiO₂,are also prepared in this study.The water absorption of PEM can be increased by20⁶1%and 40⁹2%,respectively.Under the working condition of 45%RH,N212/SiO₂ and Nafion/SiO₂ can increase system performance by 17%and 8%,respectively,compared to Nafion212.