2023 年 64 巻 7 号 p. 1376-1386
The development of hydrogen energy will help to reduce the use of nonrenewable energy sources and achieve global carbon neutrality. The aluminum-water reaction is an important method of producing hydrogen because aluminum has abundant reserves, a high yield, and no pollution. However, the dense passive oxide film on the surface of aluminum, on the other hand, often obstructs this reaction, which is the primary issue limiting the development of aluminum-based hydrolytic materials. Mechanochemical activation by processing severely plastic deformed aluminum-based materials is one effective approach and has been developed in recent years. This article reviews recent progress of hydrogen production from hydrolysis of severely plastic deformed aluminum-based materials. The kinetic model of aluminum-water reaction, aging protection of the materials, catalytic mechanism and stable rate control for the hydrolysis of aluminum-based materials are reviewed. Furthermore, some existing problems as well as some suggestions for future research on hydrogen production from aluminum-based materials are also discussed.
Producing hydrogen from hydrolysis of metal-based materials is an important direction in the field of hydrogen production. Metals that come before the element hydrogen in the metal reactivity series can create hydrogen through a substitution reaction with water or acid. The metal reacts chemically with water in a process known as metal hydrolysis, with the following fundamental reaction equation (M stands for the metal, and n for valence electron count of the metal ion):
| \begin{equation} \text{M} + \text{nH$_{2}$O} \to \text{M(OH)$_{\text{n}}$} + \frac{\text{n}}{2}\text{H$_{2}$} \end{equation} | (1) |
In order to further enhance the hydrogen generation performance, the mechanochemical activation techniques are developed by the combination of mechanical processes and the addition of chemical activators like low-melting-point alloying elements Ga, In, Bi, Sn, etc.10–13) Some typical mechanochemical activation techniques, which can introduce substantial strain into the material during processing and the materials are therefore severely plastic deformed, have been intensively investigated recently.14–20) The deformation-induced microstructures and the added chemical activators can synergistically catalyze the Al hydrolysis reaction after such mechanical activation.
Severe plastic deformation (SPD) is a series of applications of super plastic strain to a specific volume of metal to produce ultrafine-grained or even nanocrystalline materials. The study and research of SPD techniques such as high-pressure torsion (HPT), equal-channel angular pressing (ECAP), accumulative roll-bonding (ARB), twist extrusion (TE), and multi-directional forging (MDF) have received continuous attention in recent decades.21–26) In particular, the HPT method, which refers to the processing of materials wherein materials are subjected to a compressive force and concomitant torsional straining, is suitable to a wide range of materials. Materials used in this process exhibit a remarkable combination of multifunctional or super functional qualities that make them appealing for use in industrial applications.27)
The deformation-induced microstructures like ultrafine grain as well as high energy defects have more effective catalytic impact when compared to traditional mechanical activation methods, which is significant in catalytic process like hydrogen production and CO2 conversion. Such effects have been proved by previous works concerning photocatalytic hydrogen production and CO2 conversion using SPD processed high-entropy oxide TiZrHfNbTaO11 and high-entropy oxynitride TiZrHfNbTaO6N3.28–30)
Several investigations have also discovered that the SPD processing, particularly the HPT method, can be crucial for the catalytic hydrogen production. In this field, relevant studies have been carried out. Z. Horita et al. increased the cathodic electrocatalytic activity of TiO2-II for producing hydrogen from water via high-pressure torsion (HPT) straining.31) H.J. Lin showed that the HPT method helps to lower the hydrogenation temperature and enhance the hydrogenation kinetics of amorphous alloys. Nanoglasses regions and interfaces can be introduced by the HPT method, offering abundant pathways for fast hydrogenation.32) The influence of the HPT on various binary oxides, perovskites, high-entropy oxides and high-entropy oxynitrides on photocatalytic hydrogen production has been reported by the group of Z. Horita and M. Fuji.33–36)
Attempts of using severely plastic deformed Al-based materials via HPT processing have been carried out by Zhang et al. By (1) doping with low-melting-point alloying elements like Sn/Bi to reduce the corrosion potential of aluminum and increase the pitting tendency, (2) forming nano-sized Al–Sn/Bi galvanic cells using HPT processing, and (3) adding graphite to increase the active interface area, previous studies were able to produce high hydrogen production rate Al–Bi–C composites with nearly 100% hydrogen production rate in ultrapure water.37–39) In practice, however, such aluminum-based materials pose controllability issues due to their overly vigorous reactions, especially when applied to mobile power sources such as proton exchange membrane fuel cells, where hydrogen generation rates are often too fast to meet required hydrogen supply rates.40,41) Figure 1 illustrates how materials with high reaction rates are typically unable to maintain high rates throughout the reaction phase. Instead, the reaction rate tends to peak in a relatively short amount of time and then decline rapidly. The average reaction rates of some materials, however, are often lower because they can maintain a steady reaction rate throughout the whole reaction period.37–39,42) Similar results have also been reported in some other works.11,43–48) Therefore, an in-depth study of the hydrogen generation behavior from aluminum-water reaction is vital for the development of aluminum-based materials with stable and controllable hydrogen generation rates.
Hydrogen generation curves for HPT processed Al–Bi–C composites, where (a) is plotted up to 60 min and (b) covers only up to 5 min for close examination.42)
This paper will summarize the recent progress of hydrolytic hydrogen production on severely plastic deformed Al-based materials. In the second section, the kinetic studies of Al-water reaction will be discussed in consideration of both theoretical simulations and experiment data. In the third section, recent progress on the material protection after mechanochemical activation will be summarized. In the fourth section, both the catalytic mechanism of chemical activators and deformation-induced microstructures on the Al hydrolysis reaction will be discussed. In the fifth section, strategies to achieve stable hydrogen generation rate will be discussed from both the environmental factors and the interactions with activating elements. Some suggestions of future work will be given in the last section.
Kinetic studies on the reaction of aluminum-based materials with water have been conducted recently, but the precise mechanism of the reaction is still not fully clarified. According to literature,49) the Al-water reaction follows the following equation depending on the water temperature.
| \begin{equation} \text{2Al} + \text{4H$_{2}$O} \to \text{2AlOOH} + \text{3H$_{2}$} \end{equation} | (2) |
| \begin{equation} \text{2Al} + \text{6H$_{2}$O} \to \text{2Al(OH)$_{3}$} + \text{3H$_{2}$} \end{equation} | (3) |
| \begin{equation} \text{2Al} + \text{3H$_{2}$O} \to \text{Al$_{2}$O$_{3}$} + \text{3H$_{2}$} \end{equation} | (4) |
When the water temperature is above 80°C, the stable reaction product is AlOOH, while from room temperature to 80°C, it is usually a mixture of Al(OH)3 and AlOOH. During the early stages of the aluminum-water reaction, the reaction rate is mainly controlled by surface chemical mechanisms, while during the middle and late stages, it is largely controlled by diffusion of water molecules into the reaction interface. An important challenge for the research on hydrolysis of Al-based materials is dealing with the complexity of the potential energy surface during the Al-water reaction. Density Functional Theory (DFT) has been used to characterize the surface potential energy: Rivero et al.50) simplified the complexity of potential energy surfaces by dividing them into three regions (water cleavage, water dissociation, and compound structure). Zhao et al.51) analyzed five models for water adsorption on clusters, finding that water molecules form adsorption layers easily. Yang et al.52) discussed in detail the mechanisms of water molecule interactions with aluminum surfaces, including preferred adsorption sites, geometry optimization, charge transfer, dissociation processes, and the changes in the pathways of water molecules. Chen et al.53) reported that vacancy defects enhanced water-surface interactions and catalyzed water dissociation; Liu et al.54) examined the surface properties and dissolution trends of Al doped with low-melting-point metals. Such theoretical simulations are helpful to understand the reaction mechanism from various aspects.
Researchers have attempted to explain the kinetic process of the reaction between aluminum and water using the Shrinking Core Model (SCM), which suggests three specific reaction steps: 1) diffusion of water molecules across the surface oxide film; 2) Aluminum-water reaction at the interface; 3) Growth of reaction products and ion/molecule diffusion.55) Under modest pressure, nano and micro aluminum powders can react with water to generate hydrogen. The experiments show that reducing the particle size of aluminum helps to boost the reaction rate, however at higher temperatures, the agglomeration effect of aluminum particles slows down the reaction efficiency.56,57) Furthermore, when particle size decreases, the passivation impact on the surface of aluminum cannot be neglected. The hydrogen generation curve for such materials has a slowly increasing rate, as shown by the dashed line in Fig. 2. In this case, the hydrogen generation curves are frequently distinguished by three stages: the incubation period with a slow increase in rate, the acceleration period with a rapid increase in rate and rapid stabilization, and the plateau period with a gradual decrease, which correspond to three rate control steps.58,59) However, the model has certain shortcomings, such as the diffusion coefficient of product film is not constant in fact, so the fitting data is not agreeing well with the experimental one. Wang et al.60) developed a multi-step core-shell model that accounts for the impacts of particle size, formation and fracture of the initial porous Boehmite layer, and multi-step reaction kinetics on the total reaction rate. The modified model predicts the induction duration and hydrogen generation rate using time, temperature, and particle size functions, and gets reasonable results. Furthermore, SCM model was modified by considering the impact of single product AlOOH densification with time under high water temperature conditions, which correlates well with the actual data. Further research is needed to build a realistic kinetic model for hydrogen generation at low water temperatures.
Hydrogen generation rate against time for the three rate control steps.59)
The majority of current kinetic investigations of aluminum-water reactions are based on SCM and are performed on aluminum-based materials in acidic or alkaline environments.55,58–63) The hydrogen generation curves of Al–Bi/Sn system materials activated by ball milling and other mechanical processes frequently consist of only accelerated periods of accelerated rates and slow periods of flat rates, whereas the hydrogen generation curves of materials with stable rates are almost always in the linear stable phase of accelerated period.39,64) These kinetic differences imply that both the oxide film and the product film have no significant influence during the reaction for materials with a stable hydrogen generation rate, where the absence of the influence of oxide film in the initial phase is usually attributed to the destruction of the oxide film during the activation process.10) However, there is no clear conclusion on the influence of reaction products on the kinetics of hydrogen generation in such materials, therefore a more appropriate model that considers the formation and the nature of the reaction products is required.
Aluminum particle size reduction during ball milling increases the affinity of aluminum to water. The aluminum particles decrease in size during the initial stage of grinding and simultaneously form a laminar structure (as shown in Fig. 3(a) and (b)), the interlayer space of this laminate structure increases the contact area between aluminum and water, and thus the hydrogen production is significantly increased. As a result, hydrogen generation has grown dramatically.65) However, the change in particle size of ground aluminum in the subsequent stages is essentially minimal, and long-term grinding will ruin the laminate structure and reduce hydrogen generation efficiency. Furthermore, environmental parameters such as oxygen, water vapor in air, and acidity and alkalinity of aqueous solution cannot be overlooked in the kinetic modeling process since they have an effect on the induction time of the aluminum-water reaction and hence alter the hydrogen generation rate. For example, the kinetics of hydrogen generation in a hydrolysis reaction system including a non-homogeneous composition of finely scattered powdered aluminum and crystalline sodium silicate, is dependent on the presence of oxygen in the reaction medium.62) The reaction rate, oxidation rate, and activation rate on the aluminum surface have a substantial influence on the hydrogen generation kinetic in the presence of oxygen. While in the aluminum-water reaction driven by aluminum hydroxide suspension, the induction time lowers and the reaction rate increases with increasing the concentration, volume and temperature of suspension.66) Some researchers have also investigated the process of hydrogen production from the corrosion of aluminum sheet in acidic aqueous solution. By modifying the common SCM model, they analyzed the hydrogen kinetic behavior by considering the influence of hydroxide layer growth and texture changes during the reaction process.67,68) To minimize major differences between theoretical predictions and realistic findings, the future kinetic analysis of hydrogen generation should be based on the actual reaction situation and the objective study of hydrogen production kinetics in combination with environmental parameters.
SEM images of cross section of particles milled for 7 h after 2 h of hydrolysis. (a) Image of laminated structure, (b) enlarged image of (a).65)
Although the highly active aluminum can increase the rate of hydrogen generation, it is more sensitive to oxidation by oxygen in the air and may also react with water vapor, losing its activity. Nevertheless, there have been few investigations on the protection of activated aluminum-based materials in the air. Conventional preservation procedures, such as nitrogen atmosphere protection, are difficult to use with aluminum items.69) Wang et al.70) investigated how the reaction characteristics of aluminum powder with water changed after a storage of 6 months in different environments. Their results showed that oxygen in the air would gradually pass through the oxide film on the Al surface and react with the Al inside, leading an increase in the thickness of oxide film as well as an extension of the induction time of the Al hydrolysis reaction and lowering the hydrogen production performance. Furthermore, water vapor penetrates the oxide coating and react, lowering the effective aluminum concentration and the produced hydrogen content (as shown in Fig. 4). Water vapor, rather than oxygen, is the primary cause of the reduction in aluminum activity in industrial atmospheres.
Schematic representation of the surface structure of an as-received HTA particle (a) and the change after it is stored in oxygen (b) and in water vapor (c) for a time period, respectively.70)
Current protection techniques primarily aim to avoid the interaction with water vapor by separating Al from oxygen or adding some hydrophobic materials. Yang et al.71) found that after 20 days in an air environment without protective measures, the hydrogen production yield of Al–Bi–Sn composites lowers more than 50%, whereas the reactive aluminum composites stored in hexane successfully maintain the original reactivity. Furthermore, adding fluorocarbons to aluminum increases its age resistance and decreases its reactivity with water vapor, but the extra fluorocarbons do not prevent aluminum from being oxidized. Nanoporous aluminum created by dealloying can pose safety issues because of its high reactivity. Some researchers reduced its spontaneous combustibility by incorporating hygroscopic elements into composite particles.72) Additives connected to the surface of reactive Al-based materials are another technique to protect the Al-based materials. Chen et al.73) produced an Al–25%CaO–10%NaCl material which can avoid oxidation in air by covering the surface with CaO. Xiao et al.74) developed Al–OF–Bi composites with excellent anti-aging characteristics in air. Furthermore, mechanical processing and other methods of material structure modification can increase the oxidation resistance of aluminum powders to some extent. Wang et al.75) prepared Al–Bi–CNTs bulk materials from aluminum, bismuth, and carbon nanotubes using a combination of ball milling and spark plasma sintering (SPS). After SPS, the hydrogen yield of material can remain 65.8% after 20 days of exposure in air. It is worth noting that oxidation of aluminum surface can alter the microstructure, which influences the hydrogen generation rate and even shortens the entire hydrolysis reaction time. In a work of ball milled Al–Ga–In–Sn–NaCl, the hydrogen production curves of Al composites before and after 15 hours of air exposure show that the hydrogen production performance declines with air exposure. However, after air exposure, the total reaction time of the Al–Ga–Sn–10% NaCl composites is decreased (as seen in Fig. 5). Besides, the morphology of the Al–Ga–In–Sn–10%–NaCl composite shows more cracks and defects on the surface (as shown in Fig. 6), and the liquid GIS phases formed by Ga, In, and Sn can provide a channel for the Al-water reaction, resulting in the appearance of more GIS phases on the particle surface. As a result, after a time of air exposure, the activity of Al–Ga–In–Sn–10%–NaCl composites was improved. After 5 and 10 days of air exposure, the hydrogen generation rates of Al–Ga–In–Sn–5%NaCl and Al–Ga–In–Sn–10%NaCl composites were 100% and 94% of the initial rates, respectively.76)
Hydrogen generation curves and hydrogen generation rate curves of milled Al–Ga–In–Sn–5% NaCl and Al–Ga–In–Sn–10% NaCl composites with different exposure time in air.76) (A) Hydrogen generation curves of Al–Ga–In–Sn–5% NaCl, (A′) hydrogen generation rate curves of Al–Ga–In–Sn–5% NaCl, (B) hydrogen generation curves of Al–Ga–In–Sn–10% NaCl, (B′) hydrogen generation rate curves of Al–Ga–In–Sn–10% NaCl.
Surface image of Al–Ga–In–Sn–10% NaCl composite after exposure in air for 10 days.76)
Mechanical activation methods and chemical activators typically have a synergistic effect and can be used together to increase the catalytic efficiency. The initial dense aluminum oxide coating is destroyed by the SPD method, which also introduces numerous defects in the aluminum particles. Also, as the specific surface area of aluminum rises, the catalyst can cling to aluminum particles surfaces and raise the reactivity of aluminum even higher. Consequently, it is advantageous to choose an appropriate catalyst to increase the efficiency of the aluminum hydrogen production.
Currently, many catalysts accelerate the reaction by shortening the induction period.84) Low-melting-point metals,43,71,77–83) carbon materials,39,75,84–86) salt com-pounds,73,87,88) oxides89) and other additives, can help to some extent with the aluminum-water reaction. Metals with low-melting-point such as Zn, Li, Sn, Bi, Ga and In, form compounds with aluminum, which can accelerate the Al-water reaction rate. Carbon nanotubes, graphene, and graphite are now accessible as carbon material catalysts. Al–Bi doped carbon nanotubes can prevent aluminum particle aggregation and boost their activity.75) Aluminum-based composites made by graphene wrapped aluminum particles also have strong hydrogen generation characteristics.84,85) Salt compounds, like carbon materials, are primarily employed to catalyze the Al hydrolysis reaction by altering the microstructure or specific surface area of aluminum-based materials. The water-soluble inorganic salts can cause local pitting and rupture of the alumina coating on aluminum particles.87,88) Oxides are another sort of additives which can also enhance the activity of the aluminum during hydrolysis process, such as Al2O3, TiO2 and CuO. There are currently a variety of physical and chemical methods for forming various composites or alloys of the aforementioned materials with aluminum in order to change the aluminum-water reaction process.
For the severely plastic deformed Al-based materials, besides the catalytic effect of added catalysts, the deformation-induced microstructures may also play an important role in Al-water reaction. When the induced strain by the plastic deformation accumulates to a particular degree, the internal microstructure of the material changes, and the results of the various processing methods reveal the following consistent trend: (1) For ball milling, the decrease in grain size with increasing ball milling time, the increase in high-angle grain boundary content, and the evolution of textures between different component kinds.14–16) (2) For rolling, increasing the rolling ratio causes a decrease in grain size and a severe shear deformation inside the material, resulting in the production of a mixed texture.17,18) (3) The shear strain increases with the revolution number for high-pressure torsion. This finally results in grain refinement, an increase in the content of high-angle grain boundaries, and changes in type and content of the deformation textures. Moreover, higher residual stresses are frequently present inside such severely plastic deformed materials.19,20)
Previous works using mechanochemical activation have shown that only certain proper processing parameters (such as ball milling time, rolling ratio, and HPT revolution numbers) can result in significant hydrolytic hydrogen production, indicating that the material requires a certain amount of strain accumulation to achieve the activation effect. It is worth noticing that most of the previous studies have mainly focused on the accelerating effect of galvanic cells between Al and additives, especially when metals with low-melting-point were used. For example, in Al–Sn or Al–Bi systems, Bi/Sn will be incorporated in the Al matrix during mechanical processing to create micro/nano sized Al–Bi/Sn galvanic cells, hence accelerate the Al-water reaction. The activation-induced reduction in grain size increases the amount of such galvanic cells. As a result, the hydrogen generation rate can be further improved.74,90) This charge transfer-based electrochemical process can explain the increase in hydrogen generation rate after mechanochemical activation, but it cannot explain the simultaneous increase in hydrogen yield and the stable rate observed in some materials. In a previous work of Al–Sn–Zn, the unprocessed molten material was immersed in ultrapure water for 4 days with no significant hydrogen evolution, whereas the material after 1 pass of HPT processing showed a certain rate of hydrogen evolution and a significant yield, indicating that the charge transfer-based corrosion coupling acceleration mechanism is not the only factor controlling the hydrolysis of aluminum-based materials.38)
A steady rate indicates that densification of the reaction products on the aluminum surface does not occur over a long period of time. Therefore, in addition to the grain size factor, it is necessary to investigate the influence of the surface state of activated aluminum-based materials on material clusters in aqueous solution such as water molecules and hydroxide ions. Based on DFT simulation, J. Liu et al.54) and S. Cheng et al.91) proposed that the values of some surface parameters such as surface energy and work function of different crystalline surfaces of aluminum would change after doping with different elements, resulting in differences in the stability and solubility of different crystalline surfaces in corrosive environments. F. Li et al.92) demonstrated that the lower adsorption energy of the Al (111) surface for water molecules and hydroxide ions following Bi doping may be the reason why the reaction products cannot deposit on the surface of the aluminum substrate. However, the above studies do not adequately characterize the probable states of the various aluminum surfaces following elemental doping and lack direct experimental data. M. Yuan et al.93) examined the corrosion behavior of Al–Mg–Si/Al–Zn–Mg aluminum alloy composite plates with varying degrees of deformation using the cross accumulative roll bonding approach. The in situ EBSD results revealed that the corrosion resistance of different types of deformation textures in NaCl solution differed significantly, and better corrosion-resistant texture could be created by adjusting the deformation parameters (see Fig. 7). From the standpoint of corrosion and protection, this study presents some in situ evidence of the link between the evolution of the deformation texture and the corrosion resistance of the material following severe plastic deformation. However, the materials used in this case are Al–Mg–Si and Al–Zn–Mg composites, and the corrosion solution is sodium chloride solution, which is inconsistent with the common aluminum-water reaction system. Therefore, to investigate the influence of deformation texture on the hydrogen generation behavior of severely plastic deformed Al-based materials, the aluminum alloy itself and the composition of aqueous solution must be further considered.
In situ EBSD plots of AA6082/AA7204 composite plates processed by CARB rolling and soaked in 3.5 wt% NaCl solution for different times: (a), (c), (e) for 93.7% reduction; (b), (d), (f) for 99.2% reduction; (a), (b) before immersion; (c), (d) immersed for 10 min; (e), (f) immersed for 30 min.91)
Furthermore, as high-energy defects, high angle grain boundaries may participate in the Al-water reaction as favorable reaction sites in the first stage of the reaction.13) The influence of residual stresses in the severely plastic deformed Al-based materials on the hydrogen generation behavior has not been recorded in the field of Al-water reaction for hydrogen production, but there have been some reports in the corrosion field that it has a considerable influence on the hydrogen evolution of localized corrosion.94) It is therefore required to conduct a thorough investigation of the influence of deformation-induced microstructures on the hydrogen generation capabilities of aluminum-based materials in water following mechanochemical activation. Simultaneously, in situ observation of hydrogen generation processes using synchrotron X-ray imaging (see Fig. 8.) is expected to provide direct experimental evidence of the initial hydrogen generation sites and changes in the aluminum-water reaction. This approach has been used in corrosion investigations including the in-situ assessment of local corrosion rates in aluminum alloys.94–96)
(a)–(c) Evolution of corrosion of Mg2Si and increase in size of a hydrogen bubble with time. Hydrogen bubble generates as a result of cathodic reactions, and the volume of this bubble increases as dissolution of Mg2Si progresses. (a) After 45 mins, (b) after 50 mins, (c) after 56 mins.94)
Controlling the hydrogen generation rate to achieve stable and mild hydrolysis of aluminum-based materials is possible by adjusting external environmental factors such as the water addition method, where water temperature, volume, flow rate, and water type all have a significant impact on the rate of hydrogen generation.42–45) Because the aluminum-water reaction is exothermic, the heat released from the reaction will cause the system temperature to rise sharply under conditions of low water volume and slow water flow, resulting in a transient, uncontrolled high reaction rate.43,44)
Water quality has an effect on the accelerating effect of micro/nano galvanic cells in Al-based materials, which should be weakened in poorly conducting aqueous solutions due to restricted charge transfer, resulting in a decrease in hydrogen generation rate. However, there are some questionable results in the literature, such as the difficulty in distinguishing the effect of chloride ions or the effect of solution conductivity on the change of hydrogen generation rate in NaCl solution.42) Moreover, the hydrogen evolution behavior of aluminum-based materials is significantly influenced by trace ions and organic matter in tap water.97) It is therefore necessary to investigate the effect of environmental factors on the hydrogen evolution behavior of aluminum-based materials in water in a neutral solution based on ultrapure water.
5.2 Regulation by alloyingIn the cases of regulation via addition of alloying elements, the main activating elements used are expensive elements such as Ga and In. Furthermore, elements like Mg, Cu, and others can be added to form intermetallic compounds, reducing the contact areas Al-water and achieving a mild hydrogen generation rate.46–48) For example, the Al–10Bi–7Sn–3Cu (wt%) produced a lot of hydrogen in distilled water at 50°C (as shown in Fig. 9), reaching 856 mL·g−1 in 800 minutes, and this material is also resistant to oxidation and moisture.46) The addition of Mg to the Al–Ga–Sn–Mg alloy reduces the rate of hydrogen release, allowing the overall reaction rate to be controlled and stable (as shown in Fig. 10). Mg and Cu decrease the rate of hydrogen release, making the overall reaction rate controllable and stable. However, the addition of Mg and Cu reduces the hydrogen yield due to the formation of insoluble intermetallic compounds. An attempt of adding titanium into the Al alloy showed similar results (as shown in Fig. 11), in which Ti atoms occupy specific positions where Al comes into contact with water, forming an inert Ti layer on Al and slowing the reaction.98) When the added Ti amount is above a certain critical value, the aluminum particles are refined and the particle size of the GIS phase decreases, which speeds up the aluminum-water reaction rate. Moreover, the reaction generates a large number of H2 bubbles to prevent Ti from forming an inert Ti layer, allowing Al to react continuously with water and thus increase the yield.
Hydrogen generation curves of the Al–Bi–Sn–Cu composites in distilled water: (a) Al–10Bi–7Sn–0.5Cu, (b) Al–10Bi–7Sn–1.5Cu, (c) Al–10Bi–7Sn–3Cu at different water temperatures, (d) Al–Bi–Sn–Cu with different amount of Cu at 50°C.46)
(a) The hydrogen generation curves for Al–Ga–In–Sn–Mg alloys with different Mg amount at 40°C, (b) comparison of hydrogen generation rate for alloys with and without addition of Mg.47)
Hydrogen generation curves of Al alloys with different Ti amount at: (a) 50°C, (b) 60°C, respectively. Inserted image in (a) shows the initial reaction stage of the alloy at 50°C.98)
Other elements like Zn are also worth being further investigated in future work. In previous work of Al–Sn materials and a preliminary study of Al–Bi–X ternary materials, the majority of Zn enters the Al matrix in the form of solid solution, with a small portion distributed at the grain boundaries, which can further activate the Al-based materials and ensure a high yield.64) In addition, Al–Bi–X ternary alloys with certain compositions of Sn or Zn can obtain high yields and have a stable hydrogen evolution rate. Moreover, it has been proposed that decreasing stacking fault energy can promote the transformation of deformation-induced microstructures. Q. Gao et al.99) calculated that adding Si, Sc, Ga, Ge, Sr, Zr, In, Sn, La, Hf, Pb, and other alloying elements would reduce the stacking fault energy of aluminum. These findings lay the groundwork for future efforts to manipulate deformation-induced microstructure through elemental doping.
According to the above literature, the kinetic model, material protection, catalytic mechanism as well as the stable rate control of the hydrolysis of severely plastic deformed aluminum-based materials have been discussed in recent studies.
The kinetic studies of hydrolysis reaction are mainly based on SCM model while the aqueous solutions of such studies are primarily using acidic or alkaline solutions. Moreover, few studies have been done on the hydrolysis of bulk materials in neutral solution. Future work could combine the theoretical simulation and experimental data to set up a more reasonable kinetic model.
While higher activity of aluminum and aluminum-based materials can improve their hydrogen yield and hydrogen production rate, they are more likely to react with oxygen and water vapor in air, and few protective measures have been reported so far.
The catalytic mechanism for the hydrolysis of severely plastic deformed Al-based materials remains complex and needs more in-depth studies. The accelerating effect of micro/nano galvanic cells on Al hydrolysis cannot explain the high hydrogen yields and stable hydrogen generation rates observed in some materials. Future work could consider to investigate the evolution of deformation-induced microstructures and their influences on the hydrogen generation performance. The deformation-induced microstructure could be controlled by adjusting the process parameters in conjunction with heat treatment and elemental doping, and the mechanism of the influence of deformation-induced microstructures on the continuous and stable hydrolysis process could be investigated with the help of theoretical simulations like first-principles calculations and in situ observation of hydrogen evolution via synchrotron imaging.
Currently, most research are focused on finding materials with high instantaneous reaction rates via component regulation, limited studies on stable and mild rate control are primarily focused on the regulation of external factors such as water temperature and quantity, lacking more systematic studies. The limited exploration of the influence mechanism of alloying elements on the rate control is primarily focused on the elements like Mg and Cu, which readily form intermetallic compounds with Al and will obviously reduce the final hydrogen yield. Moreover, in such alloying system, the activating elements are always expensive elements like Ga and In. Future work could be carried out under neutral aqueous conditions using ultra-pure water in consideration of systematic investigations on the impact of external environmental factors as well as the addition of proper alloying elements.
The authors would like to thank Class III Peak Discipline of Shanghai—Materials Science and Engineering (High-Energy Beam Intelligent Processing and Green Manufacturing) for the financial support.
