Investigation On Hydrogen Generation Behavior And Mechanism Of Mg-Al Alloy Waste By Green Regulation Strategy

Hydrogen(H2)generation from magnesium-based(Mg-based)materials is considered as an ideal transitional method to industrial application due to its high H2 capacity,convenient operation,clean reaction,and on-site process.Unfortunately,the inert oxide layers on the surface of Mg and forming by-product Mg(OH)2 layers through hydrolysis process result in slow hydrolysis kinetics and low H2 yield.Regulating hydrolysis medium and modifying Mg-based materials are effective way to promote the H2 production performance by increasing the surface active sites,improving the electrochemical reaction and facilitating H2O molecular mass transfer process.However,for hydrolysis medium,the regulation strategy only concentrates on cations effect but neglects anions effect.For Mgbased materials,on the one hand,the added catalysts increase the loss of H2 theoretical capacity.On the other hand,there is still lack of modification strategies on the basis of Mg(OH)2 nucleation and growth.At the same time,the current Mgbased systems and modification strategies ignore the cost and energy consumption of H2 production,and the hydrolysis process is not green enough.Therefore,the H2 production system of ’Mg-Al alloy waste-seawater’ is selected in this paper.The seawater is selected as hydrolysis medium owing to the abundant reserves and rich in various ions.The Mg-Al alloy waste(AZ91D,AZ31B)is selected from industrial processing residues,which significantly reduces the cost of H2 production.In this paper,the overall regulation strategy is adopted.Firstly,the hydrolysis solution is optimized with anions in seawater to improve the hydrolysis kinetics.Secondly,the Mg-Al alloy waste is modified by self-hydrolysable ternary(Mg10Ni)90Ce10 in the optimized simulated seawater to ensure the high theoretical hydrogen production capacity and improve the hydrogen production kinetics.Finally,(Mg10Ni)85Ce15 and light-weight hollow carbon nanotubes are adopted to synergistic activate the final H2 capacity and yield.An efficient activation strategy for Mg-Al alloy waste was proposed based on the Mg(OH)2 nucleation and growth theory.The main research results of this paper include the following aspects:(1)In this paper,the H2 production behaviors of Mg-Al alloy waste in various anions solutions were investigated.Through short-time and isothermal H,generation tests,the thermodynamics of Mg-Al alloy waste in different anion solutions were investigated.The structure-function relationship between H2 capacity and micro structure is analyzed.The hydrolysis activation energy(Ea)was calculated by thermodynamic fitting to explore the mechanism of anions.The results shows that AZ91D presents the best H2 capacity in 0.5 M NaCl solution.At 298 K,it can generate 727.0 mL·g-1 H2(0.77 H2 yield).Ea is calculated to be 27.8 kJ·mol-1.In contrast,the introduction of other anions into NaCl solution decreased the H2 capacity,which is negatively correlated with the concentration of introduced anions.Eα is increased simultaneously.The introduction of other anions have an obstructive effect on Cl-,with the degree of NO3->CO32->SO42-.The hydrolysis reaction occurs preferentially at the phase boundary between Mg and Mg17Al12,and Mg phase adsorbs anions during the hydrolysis process.Cl-can adsorb to the active sites on the surface of Mg and promote electrochemical hydrolysis.The formation of dense by-product layers is inhibited.However,when other anions added,the active sites for Cl-are occupied due to competitive adsorption,which prevents the adsorption process of Cl-and thus decreases the H2 capacity.(2)In this paper,hydrolysis behavior and modified mechanism of Mg-Al alloy waste surface activated by(Mg10Ni)90Ce10 were investigated.Selfhydrolysable ternary high-activity(Mg10Ni)90Ce10 with high activity was added on the surface of Mg-Al alloy waste through short-time ball milling process.The hydrolysis kinetics and H2 capacity of activated Mg-Al alloy waste were studied.The activation mechanism was investigated by particle size analysis,microstructure characterization and thermodynamic analysis.The results shows that AZ91D surface activated by(Mg10Ni)90Ce10 achieves excellent hydrolysis kinetics at high theoretical capacity.AZ91D-20(Mg10Ni)90Ce10 with bulk surface activated state and attle surface activated state can reach 808.06 and 790.4 mL·g-1 with H2 yield of 0.88 and 0.90,respectively.The maximum hydrogen production rates can reach 295.44 and 262.8 mL·g-1·min-1,and Ea can reach 27.4 and 23.3 kJ·mol-1.The main reason is that(Mg10Ni)90Ce10 releases additional heat by preferential hydrolysis on the surface of AZ91D,which raises the microscopic local temperature of hydrolysis solution,so that the hydrolysis reaction of AZ91D alloy can reach the Ea more easily,and therefore improves the hydrolysis kinetics.(3)In this paper,the H2 production behavior of Mg-Al alloy waste activated by CNTs and(Mg10Ni)85Ce15 are investigated,and the synergistic modification mechanism were explored.CNTs and(Mg10Ni)85Ce15 was individually or simultaneously added during the short-time ball milling process and the additive effects is analyzed by microstructure characterization.The H2 production curve was piecewise fitted by Avrami-Erofeev equation to investigate the rate control step.The synergistic modification mechanism was explained based on the nucleation and growth of Mg(OH)2.The results showed that the final H2 capacity and yield of AZ31B can be increased by synergistic modification.AZ31B-5CNTs10(Mg10Ni)85Ce15 can reach 660.5 mL·g-1 H2 capacity and 0.77 H2 yield(0.90 at 328 K)within only 10 min at room temperature,which was much higher than that of 0.42 before modification.Ea decreases to 15.6 kJ·mol-1.The rate-controlled step changes from a three-dimensional interfacial process to a one-dimensional diffusion process with continuous hydrolysis process.CNTs and(Mg10Ni)85Ce15 synergistically promote the ’nucleation-growth-contact’ of Mg(OH)2,(Mg10Ni)85Ce15 increases the nucleation rate,and CNTs provide sufficient growth space,which results in a cluster morphologies hydrolysate promoting the molecular transfer with H2O molecular and thus increasing the final H2 yield.This paper uses a green ’Mg-Al alloy waste’ hydrogen production system to achieve high kinetics,high capacity and high yield of hydrogen production through efficient and low cost modification strategies,which provides a theoretical and experimental basis for the realization of large-scale industrial hydrogen production and promotes the development and application of portable solid-state hydrogen generators.