Properties And Mechanism Of Hydrogen Generation From Al Composites By Low Temperature Hydrolysis

In recent years,hydrogen fuel cell(HFC)has developed rapidly.However,cold start at low temperature limits the use of HFC in extremely cold regions and climates.In addition,to solve the problem of cold start at low temperature,it is necessary to provide corresponding hydrogen supply device for HFC.As an alternative on-site hydrogen production technology,Al matrix composites can solve the problem of hydrogen storage and transportation.However,most of the current researches on hydrogen generation from hydrolysis of Al matrix composites are concentrated at room temperature or even high temperature.It usually takes a long induction time to start the reaction at low temperature,and the conversion rate of hydrogen production at low temperature is not high.As a real-time hydrogen generation method,considering the actual needs of extremely cold area and climate,the material system should have activity below273.15K,short induction time and high conversion efficiency at extremely low temperature.Therefore,two composite systems,Al alloy/NaCl/g-C₃N₄and Al alloy/NaCl/LiH/g-C₃N₄,were constructed by using g-C₃N₄as activated phase.The hydrogen generation performance,phase composition,microstructure and thermal properties of these materials were analyzed.The reaction mechanism of hydrogen production by hydrolysis at low temperature was discussed.The following work has been done:(1)A new type of Al alloy/NaCl/g-C₃N₄composite was prepared by mechanical ball milling by introducing g-C₃N₄.The feasibility of Al-H₂O reaction at 253.15K?373.15K was proved by thermodynamic calculation.The effect of g-C₃N₄on the hydrolysis of activated Al matrix composite was studied.In 23wt%NaCl aqueous solution at 253.15K,the hydrogen generation of Al alloy/NaCl/g-C₃N₄composites with1g g-C₃N₄addition reached 1006 mL·g Al^-1,the maximum hydrogen production rate(m HGR)was 80 mL·g Al^-1·s^-1and the reaction induction time was 90s.The results show that the addition of g-C₃N₄can effectively reduce the size of NaCl particles,promote their uniform distribution on the surface of Al matrix,and cause more cracks on the surface of Al.(2)In order to eliminate the induction time of hydrolysis of Al alloy/NaCl/g-C₃N₄composites at low temperature,a series of Al alloy/NaCl/LiH/g-C₃N₄composites were prepared by ball milling by introducing LiH.It was found that the hydrolysis reaction of Al alloy/NaCl/LiH/g-C₃N₄composite had no induction time.Among them,Al alloy/NaCl/1.5%LiH/g-C₃N₄composite had the best hydrolysis performance,with hydrogen generation of 1095 mL·g Al^-1and maximum hydrogen generation rate of 120mL·g Al^-1·s^-1.The results show that LiH can be used as the active site of the composite to react immediately at low temperature,and the heat generated by the reaction and alkaline conditions can promote the hydrolysis of Al.(3)The oxidation resistance of Al alloy/NaCl/LiH/g-C₃N₄composite was studied.It was found that the hydrogen generation of Al alloy/NaCl/1.5%LiH/g-C₃N₄composite decreased by 14.61%after 24h in dry air,which was much lower than that of Al alloy/NaCl/1.5%LiH(22.49%)and Al alloy/NaCl/g-C₃N₄(20.97%),indicating that g-C₃N₄and LiH can promote the oxidation resistance of the composites.At the same time,the humidity resistance test shows that the active sites on the surface of Al alloy/NaCl/1.5%LiH/g-C₃N₄sample react with water vapor to form corrosion holes in high humidity environment.