A Study On Hydrogen Generation By Hydrolysis Of Al-NaBH₄ Based Mixture

The hydrolysis kinetics of Al/Na BH4 system was studied in the paper. The effects of mixture composition, reaction temperature, catalysts and other factors on the hydrolysis of Al/Na BH4 system were clarified. Based on the microstructure analysis such as XRD, SEM, IR, particle size distribution and other characterization methods were used to analyze the phase, surface structure, particle size of the system.The hydrolysis mechanism of Al/Na BH4 system were systematically studied. The research results include the followings:1. Hydrogen generation from the hydrolysis of an Al/Na BH4/Co/Na Al O2 system was evaluated detailedly. Hydrogen generation performance can be altered by changing the Al/Na BH4 mass ratios, Co amount and Na Al O2 concentration. The optimized composite of Al/Na BH4/Co/ Na Al O2 system was achieved. 323 K, the 0.1 g Al/0.3 g Na BH4/0.06 g Co/4 m L 0.4 M Na Al O2 mixture(weight ratio of 1:3) generated hydrogen with 98% efficiency within 30 min; the hydrogen generation yield of 0.2 g Al/ 0.2 g Na BH4/0.06 g Co/4 m L 0.8 M Na Al O2 system reached 100% within 30 min. An activation energy of 53.3 k J mol-1 was obtained from 0.2 g Al/0.2 g Na BH4/0.06 g nano Co/4 m L 0.4 M Na Al O2 system, which confirms the control exerted by the chemical step. The weak acid hydrolysis byproducts Al(OH)3 improved Na BH4 hydrolysis performance with the synergistic of Co; alkaline Na BO2·2H2O promoted the dissolution of Al(OH)3 and the continuous hydrolysis of Al; the increase of Na Al O2 concentrations was conducive to Al(OH)3 dissolution, promoting Al hydrolysis; the interactions of Al/Na BH4 hydrolysis byproducts and synergies of Co/Na Al O2 improved the hydrolytic kinetic performance of Al/Na BH4/Co/Na Al O2 system.2. Hydrogen generation performance of solid-state Al-Li/Na BH4 activated by Co Cl2 solution was evaluated. Hydrogen generation performance can be regulated by Al-Li/Na BH4 weight ratio, Li content, Co Cl2 concentration, and hydrolysis temperature, among others. Based on XRD, SEM, IR and particle size distribution, etc., the relative hydrolysis mechanism was elaborated and the component design was optimized. 1 g Al-20 wt % Li/Na BH4 system(weight ratio, 1:1) yielded 1674 ml hydrogen with 89% efficiency in 2.5 wt% Co Cl2 solution at 303 K. The effect of Li content and Co Cl2 concentration were clarified: 1) Li hydrolysis itself generated more hydrogen; 2) Alloying of Al and Li during the milling process reduced the particle size and increased particle surface area, which promoted the hydrolysis of Al; 3) The hydrolysis byproduct Li OH improved the hydrolysis kinetics of Al. 4) The highly catalytic activity of Co2 B was achieved with the hydrolysis of Co Cl2 and Na BH4. Co2 B improved the hydrolytic kinetic Al/Na BH4 system. 5) The hydrolysis byproducts Li Al2(OH)7·x H2 O and Al(OH)3 from Al-Li/Na BH4 hydrolysis supported Co2 B and formed a good catalyst to improve the hydrolysis performance of Na BH4. 6) Co2 B was used as the cathode material to form a corrosion cell with the anode Al, promoted electrochemical corrosion of Al.3. Hydrogen generation from the hydrolysis of Al-Li-Co/Na BH4 system with different Co powder particle size and amount was evaluated. With the increase of Co content and the decrease of Co particle size, the hydrolysis kinetic of Al-Li-Co/Na BH4 system was improved significantly. Under the similiar conditions, the hydrolysis performance of Al-Li-Co/ Na BH4 system was listed as followed: Al-5 wt% Li-50 wt% nano Co/Na BH4 > Al-5 wt% Li-50wt% micro Co/Na BH4 > Al- 7.5 wt% Li-25 wt% nano Co/Na BH4 > Al-7.5 wt% Li-25 wt% micro Co/Na BH4, which corresponded to the hydrolysis activation energy: 41.8 k J mol-1, 54.8 k J mol-1, 62.5 k J mol-1, 68.4 k J mol-1 respectively. The increase of Co content and the decrease of Co particle size increased the formation of Al0.94Co1.05 alloy, decreased the particle size of product powder during the milling process, which was conducive to form more micro corrosion cell during hydrolysis process, speeded up the electric chemical corrosion of Al and improved the hydrolytic kinetic of Al correspondedly. The formation of Co/Al(OH)3 catalyst enhanced the catalytic activity of Co, and improved the hydrolysis kinetics of Na BH4. The hydrolysis byproducts of Al-Li-Co/Na BH4 system had good catalytic activity, but also had differences in the catalytic activity stability with different Co content and particle sizes. The catalytic stability of hydrolysis byproducts from the above system was given as below: Al-7. 5 wt% Li-25 wt% nano Co/ Na BH4 > Al-7.5 wt% Li-25 wt% micro Co/Na BH4 > Al-5 wt% Li-50 wt% nano Co/ Na BH4 > Al-5 wt% Li-50 wt% micro Co/Na BH4. The catalytic activity from hydrolysis by-products of Al-Li-Co/Na BH4 system came from the formation of Co/Al(OH)3/ Li Al2(OH)7·2H2O mixture. Compared with the good stability of Li Al2(OH)7·2H2O support, Al(OH)3 reacted with hydrolysis by-products Na BO2 of Na BH4, resulting in the catalytic activity decrease of Co.As a potential hydrogen source, Al/Na BH4/Co/Na Al O2 system, Al-Li/Na BH4/ Co Cl2 solution and Al-Li-Co/Na BH4 system present an inexpensive and highly efficient hydrogen generation for portable fuel cell with high hydrogen generation density and low cost compared with conventional Al or Na BH4 alkaline solution separately.