Metal-based direct hydrogen generation as unconventional high density energy

Abstract

Metals are unconventional hydrogen production materials which are of high energy densities. This paper comprehensively reviewed and digested the latest researches of the metal-based direct hydrogen generation and the unconventional energy utilization ways thus enabled. According to the metal activities, the reaction conditions of metals were generalized into three categories. The first ones refer to those which would violently react with water at ambient temperature. The second ones start to react with water after certain pretreatments. The third ones can only react with steam under somewhat harsh conditions. To interpret the metal-water reaction mechanisms at the molecular scale, the molecule dynamics simulation and computational quantum chemistry were introduced as representative theoretical analytical tools. Besides, the state-of-the-art of the metal-water reaction was presented with several ordinary metals as illustration examples, including the material treatment technologies and the evaluations of hydrogen evolution performances. Moreover, the energy capacities of various metals were summarized, and the application potentials of the metal-based direct hydrogen production approach were explored. Furthermore, the challenges lying behind this unconventional hydrogen generation method and energy strategy were raised, which outlined promising directions worth of further endeavors. Overall, active metals like Na and K are appropriate for rapid hydrogen production occasions. Of these metals discussed, Al, Mg and their alloys offer the most promising hydrogen generation route for clean and efficient propulsion and real-time power source. In the long run, there exists plenty of space for developing future energy technology along this direction.

Key words： metal; hydrogen generation; hydrolysis; metal water reaction; clean energy

1 Introduction

Forced with the scarcity of fossil fuels and the deterioration of environment, the conventional energy industry can no longer keep pace with social development [1]. Considerable efforts need to be made to ensure access to affordable, reliable, sustainable, and modern energy [2]. Since hydrogen is deemed as a potential alternative for fossil fuels, there is a huge interest in finding a feasible approach for hydrogen production. Statistics shows that approximately 94% of merchant hydrogen is produced from the reforming of fossil fuels or hydrocarbons, like coal, gasoline, and methane [3]. However, the production technique is complicated and a great deal of greenhouse gas is released in the process [4–6]. Another favorite hydrogen generation method is electrolysis of water [7] whose production efficiency is quite low. In response to the disadvantages stated above, unremitting efforts have been made in exploration of renewable, environment-friendly and high-efficiency hydrogen production approaches [8]. It is a basic knowledge of high-school chemistry that active metals react with water, releasing hydrogen. Ever since 1940s, the USA has launched the research about metal-water reaction for fuel [9]. Sea water enters the combustor and reacts with metal fuel vigorously. Considering that the metal-water reaction is exothermic, the released reaction heat would evaporate the unreacted water, generating high-pressure steam which does work through expansion, thus producing thrust. Meanwhile, the reaction produces hydrogen which is a kind of alternative clean energy. Hydro-reactive metal fuel shows attractive characteristics of high energy capacity. Currently, the research actuality at home and abroad primarily focuses on aluminum/water and magnesium/water reaction. However, the potential application of hydro-reactive metal fuel is far more than that.

This paper is dedicated to presenting an overview of the hydrogen production from direct metal-water reaction, and systematically evaluating the opportunities and challenges lying behind, aiming to find more unconventional latent metal-based hydrogen generation routes.

2 Basic principles of metal-water reaction

2.1 Reaction activities of different metals

According to different reaction activities of metals, different metals react with water in different states. Generally speaking, the easy or difficult level of metal-H₂O reaction depends on the metallicity that represents the ability of metal atoms to lose electrons. The stronger metallicity of element indicates that the relative metal reacts with water more easily. To some extent, the metallicity is in analogy with the electronegativity. The lower the electronegativity value is, the stronger the metallicity is. The periodic table of electronegativity by Pauling scale in Fig. 1 can serve as a reference. In a broad sense, the trend of reactivity for metal elements in water rests with metal reactivity series which are linked to the electrode potentials. Generally, in the same period, the metal becomes less active as the atomic number increases. Metals in the same group comply with the opposite rule. In practice, metallic activity is affected by solvent, pH value, temperature, and precipitator. For example, although lithium seizes the top spot in the activity series, it reacts slowly with water. On the one hand, lithium owns a relatively high sublimation energy so that it is difficult to transform into gas ion with the dynamics resistance. On the other hand, the solubility of lithium hydroxide in water is low, which would attach on the metal surface and block the reaction.

Fig.1 Electronegative table of the elements [10]

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According to the metal reactivity, the metal-water reaction can be divided into three broad types. The alkali metals (lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), francium (Fr)) in group IA and calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), and europium (Eu) are active enough at ambient temperature to react with cold water, producing relative metal hydroxide and hydrogen. Owing to worse metallicity, magnesium (Mg), aluminum (Al), scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), and neodymium (Nd) can only react with water under some prerequisites or moderate triggering mechanism, and generate relevant metal hydroxide and hydrogen. The metals above react with water in accordance with (R.1).

(R.1) M+n{\mathrm{H}}_{2}O=M^{\mathit{n}+}+n{\mathrm{OH}}^{–}+n/{\mathrm{2H}}_2,

where M denotes the metallic element, and n is the valence of the metal in the reaction.

Zinc (Zn), iron (Fe), tin (Sn), titanium (Ti), manganese (Mn), cobalt (Co), chromium (Cr), zirconium (Zr), and lead (Pb) are duller than those metals discussed above. They can only react with water vapor at a high temperature, evolving relevant oxides and hydrogen. They react with water as (R.2).

(R.2) xM+y{\mathrm{H}}_{2}O=M_x{\mathrm{O}}_y+y{\mathrm{H}}_2.

2.2 Kinetics analysis

As for the exothermal metal-water reaction, researchers have kept a watchful eye on its kinetics. The relationship of reaction rate and temperature is corresponding to the Arrhenius equation:

(1) k=A\exp (–E_{\mathrm{a}}/RT),

where k is the rate constant, A is the pre-exponential factor, E_a is the activation energy of the reaction, R is the universal gas constant which is equal to 8.314 J/(mol∙K), and T is the reaction temperature. In Ref. [11], the maximum hydrogen production rates of metals per unit area were compared at the temperature ranging from 80°C to 200°C. Table 1 displays the activation energy of the reactions of several metal powders and water. According to the analysis in Ref. [11], a lower slope of the rate curve with respect to temperature implied a lower activation energy and weaker temperature dependence of the reaction rate. The activation energy of Mg and Al were approximately as large as 53 and 50 kJ/mol respectively, while the activation energy of Mn and Fe were only 22 and 8 kJ/mol respectively. It was noteworthy that the total hydrogen yield of Mn-water reaction experienced a steep increase as temperature rose from 120°C to 150°C. Meanwhile, as for Fe, there was no obvious proportional relationship between the total yield, maximum rate and temperature, but a sharp growth occurred when the temperature increased from 150°C to 200°C. As a whole, the results for Mn and Fe embodied a weak temperature dependence. The study in Ref. [11] provided primary estimates of the activation energy of the reaction between metal powder and water. More accurate studies could be employed in the future to clarify the metal-water kinetics.

Tab.1 Activation energy of several ordinary metal-water reactions [11]

Metal powder	E a/(kJ∙mol–1)
Mg	53
Al	50
Zn	35
Fe	8
Mn	22
Zr	35

Additionally, the experimental fact have suggested that the mechanisms for the hydrolysis of metal Al are not consistent in the low and high temperature region [12]. The essential process in the hydrolysis of activated Al appears as that electrons transfer from Al to the antibonding orbital of the water molecule. In the case of high temperature, this process is based on a mechanism with activation energy

(2) E_a={({\epsilon }_i{–}_{}{\epsilon }_{\mathrm{F}}+E_{\mathrm{r}})}^2/4E_{\mathrm{r}},

while in the case of low temperature, this process is completed by a tunneling mechanism with activation energy

(3) E_{\mathrm{a}}={\epsilon }_{i–}{\epsilon }_{\mathrm{F}},

where ε _i is the energy of the antibonding level of the (H₂O)^-; ε _F is the energy of the Fermi level of the metal; and E _r is the reorganization energy, which is aiming at transferring H₂O into (H₂O)^- by stretching and changing the angle of the molecule.

2.3 Micro-mechanism of metal-water reaction

2.3.1 Molecular dynamics analysis

To clarify the fundamental mechanism of metal-water reaction at the molecular level, abundant molecular dynamics simulations have been performed. As is known, the reactivity of metal clusters has a significant relevance with cluster sizes. Roach et al. [13] found that the reactivity of Al cluster showed remarkable variances with different geometry structures in the dissociative chemisorption of water occurred at specific surface sites. Although most clusters could absorb water molecules, Al clusters with certain sizes of Al₁₆^-, Al₁₇^-, and Al₁₈^- were preferential to react with water and to produce hydrogen. In Ref. [14], it was revealed via ab initio molecular dynamics simulation that the coexistence of a pair of Lewis-acid and base sites on the Al _n surface helped lower the activation-barrier, facilitating the hydrogen generation. This auxo-action was ascribed to the proton transport induced by a chain of hydrogen bond switching process which was similar to the Grotthuss mechanism [15]. Russo et al. [16] applied the ReaxFF force field to simulate the reaction of Al nanoclusters and water molecules. The research indicated that the dissociation of a single water molecule adsorbed on the Al nanocluster required a rather high activation energy. With the assistance of an adjacent non-adsorbed water molecule, the required activation energy decreased from 45 kJ/mol to 25 kJ/mol or less. The simulated process of unassisted and assisted water dissociation is presented in Figs. 2(a) and 2(b). In addition, it was found that the existence of oxide film would occupy the available active sites as exhibited in Fig. 2(c). Thus, Al became inert in water.

Fig.2 Illustrations of the water dissociation mechanism on Al(111) surface

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2.3.2 Computational quantum chemistry

The mechanism of metal-water reaction and the release rate of hydrogen are the bases of the kinetics model. Moreover, computational quantum chemistry is an effective arm to explore the microscopic mechanism of chemical reaction. Lv et al. [17–19] applied a quantum chemistry software Gaussian 03 in three cases of Fe, Cr, and Zr. Detailed researches follow these steps: ① scanning the potential energy surface of the total reaction, finding possible elementary reactions, and locating the transition state via structure optimization and frequency analysis; ② based on the transition state, calculating the route of the lowest energy via the theory of intrinsic reaction coordinate and verifying the contained elementary reactions; ③ implementing geometry optimization and frequency analysis on each stationary point of the minimum energy route and thereby calculating the energy of stationary point; ④ computing the activation energy of the total reaction according to the energy of stationary point; and ⑤ reaction rate constant k could be obtained via the classical transition state theory [18], expressed as

(4) k={\displaystyle \frac{k_{\mathrm{B}}T}{h}}{\displaystyle \frac{q}{{\mathrm{\Pi }}_j{q}_j}}\exp [-{\displaystyle \frac{E_{\mathrm{0}}}{RT}}],

where k _B is Boltzmann constant; T is temperature; h is Plant constant; q and q _j represent partition function of transient state and reactants respectively; E ₀ represents the difference between the activated complex and zero-point energy of activation energy, i.e., activation energy at absolute zero K; and R is gas constant.

It was acquired that the Fe-H₂O reaction and Cr-H₂O reaction contained two elementary reactions. The mechanism and reaction path are illustrated in Fig. 3(a). The results indicated that the activation energy of the second elementary reaction was much larger than that of the first one. Thus, the hydrogen production rate depended on the second elementary reaction H+FeOH → FeO+H₂. In practice, the control of hydrogen production could be obtained by regulating the rate-determining step. The Zr-H₂O reaction processed in four elementary steps as demonstrated in Fig. 3(b). The second elementary reaction was the rate-determining step of the total reaction, which is expressed as \mathrm{ZrOH}+H\to \hspace{0.17em}{\mathrm{ZrO+H}}_2_.

Fig.3 Mechanisms and paths of metal-water reactions

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In the aforementioned studies, the simulative objects in this software were metal atoms and water vapor. Nevertheless, metal does not exist in the form of atoms. Thus, the reaction rate constant calculated was larger than the practical value. The predicted result obtained by this means was conservative. However, for safety concern, it is applicable in most situations. On a micro level, it is suggested that the metal-water reaction rate should start from the elementary reaction with the highest activation energy.

3 State-of-the-art investigation on metal-water reaction for hydrogen generation

Metal-water reaction is a multiphase reaction, which proceeds in three steps: (1) the aqueous reactant diffuses into the solid reactant surface; (2) the chemical reaction happens on the solid/liquid interface; and (3) the product diffuses into solution.

3.1 Na/K-based hydrogen generation

When a pea-sized piece of metal is put into cold water, it floats on water, melts into a ball, and swims around driven by the hydrogen generated. If a drop of phenolphthalein is added, the solution turns rose-red immediately. This is a unique phenomenon for reaction of active metal and water which indicates that the reaction is rapid but uncontrollable. Amid them, Na and K are the most typical research objects.

The explosive reaction of NaK₂ and water releases huge energy. Experimental data manifested that 16% of the energy could be retrieved in the shock wave and the hydrogen bubbles generated, and that the rest was dispersed by heat transfer in the water. In the light of Mason et al. [20], high-speed camera imaging illustrated that the explosive reaction between alkali metal and water was a ‘coulomb explosion’. It could be observed that when the metal was put into water, numerous metal spikes (the so-called Rayleigh jets) formed and protruded from the surface of metal drop in sub-millisecond, which further increased the reactive surface [21]. According to the molecular dynamics simulation, it was the fast migration of electrons from the metal surface to water that facilitated the vigorous reaction of alkali metal and water. Under other similar premises, the number of alkali metal atoms exposed to water would decide the instant reaction rate. According to former assessment [22], Na possessed the largest number of exposed atoms among the alkali metals, reaching up to \mathrm{31.6}\times {\mathrm{10}}^{\mathrm{14}} in a 1 cm unit cube. However, it is anomalous that the other alkali metals are known to be more reactive than Na and Li with smaller numbers of exposed atoms [23]. As the metal-water reaction happened on the interface, a rate law was deduced to describe the relationship of the metal size and the reaction time at 25°C under argon as described in Eq. (5) [22].

(5) r=r_{\mathrm{0}}–kt,

where r is the radius of the metal sphere at any time t, r ₀ is the initial radius of the metal sphere, and k is the specific reaction rate constant. For Li and Na, k is equal to 0.036 mm/s and 0.054 mm/s respectively. The kinetics of the Li-water vapor reaction was studied in Ref. [24], and the activation energy were calculated to be 6.2 and 5.5 kcal/mole when the water vapor pressures were 50 and 100 mm∙Hg respectively. It can be concluded that the water vapor pressure would affect the activation energy of this reaction.

It has been a long time since Na was considered to be an excellent coolant in nuclear power plants [25]. However, its violent reaction behavior when it contacted with water was controversial. To guarantee the safe use of liquid Na, some measures were suggested, including changing the atmosphere or solution environment and adding other substances in metal.

In Ref. [26], the reaction of NaK₂ alloy with water in the presence and absence of oxygen were studied. As the oxygen content of the atmosphere changed from 21% to 5%–10%, the reaction induction period decreased from 80 ms to 37 ms. The same thing happened when pure water was replaced with the 0.036 M acetaldehyde solution. If both of the conditions are adopted, the explosion behavior vanished. Besides, the addition of slight surface-active agents could restrain the explosion behavior of alkali metal in water [21].

The result in Ref. [27] indicated that the addition of Ti NPs (nano particle) in liquid Na could alleviate the Na-H₂O reaction, and lower the reaction rate. Owing to the stronger attractive force of NPs at the interface than that in bulk liquid Na, the NPs are inclined to accumulate at the interface rather than inside the bulk. As shown in Fig. 4, there are two kinds of contact situations of Na and water, the bare Na region and the NPs-occupied region. In the initial stage, the TiNPs adhered to the surface of bare Na, which reduced the contact area of Na atoms and water molecules, accordingly slowing down the reaction.

Fig.4 Schematic diagram of two reaction regions between Na/Ti nanofluid with water (adapted with permission from Ref. [27])

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The Na/K-H₂O reaction processes autonomously and violently at room temperature, which is a suitable propulsion system for high-power marine vehicles. However, the controllability and safety are to be considered.

3.2 Mg-based hydrogen generation

As is known, the reserves of Mg and Al in Earth’s crust are both considerable. Moreover, the raw metal materials can act as indirect hydrogen storage materials with their low cost and low density, which is in favor of the miniaturization and safety of hydrogen production systems. Mg-water reaction inherits high hydrogen product 933 mL/g, and its hydrogen storage value reaches up to 8.2%, which is higher than 6%, i.e., the requirement of the US Department of Energy (DOE) for the onboard hydrogen storage medium [28]. Meanwhile, the hydrogen productivity of Al/H₂O reaction is 1245 mL/g, and its hydrogen storage value amounts to 11.1%, which outstrips the US DOE guidelines of the hydrogen storage density of 9% [29].

It is worthy of attention that the solution of the byproduct Mg(OH)₂ in Mg-H₂O reaction is 12 mg/L. After the initial stage, the hydrolysis reaction would be interrupted owing to the formation of a passive Mg(OH)₂ layer. Thus, necessary measures must be taken to keep the reaction processing. It is widely known that acid solution could react with Mg(OH)₂, generating soluble Mg²⁺ . But this might be a potential detriment to the equipment and operator. An apparent means is to break the passive layer mechanically. The Mg-H₂O reaction becomes faster and more intense when Mg is ball milled with certain metals, metal oxides, or metal salt. Nickel inherits low hydrogen over-potential and the formation of micro-galvanic cells between Mg and Ni elements could promote the Mg corrosion in conductive media. Therefore, Mg-Ni composite materials could elevate hydrogen yield and the kinetic of this hydrolysis reaction. Cho et al. [30] addressed the hydrogen generation from Ni-rich AZ91D Mg ingot in NaCl solution with catalyst. The experiments indicated that the increased NaCl concentration in the solution and ultrasonic vibration would evidently raise the hydrogen generation rate.

Recent works showed that the composition of electrolyte had a remarkable influence on the hydrolysis reaction of Mg, especially halogen ions [31]. In pure water, high-energy ball milling of pure Mg had no effect on the Mg reactivity. When 1 M KCl solution substituted pure water, the performance of the hydrolysis reaction was drastically enhanced. Different from the situation in pure water, ball milling time posed a conspicuous effect on the Mg powder reactivity in KCl solution. Mg powder milled for 30 min presented the best productivity of 89% which was prior to that milled for 15 or 45 min. The advantage of Mg conversion yield in KCl solution was associated with the instability of Mg(OH)₂ layer caused by chloride ions. The Cl^- ions replaced OH^- to form MgCl, which is more soluble than Mg(OH)₂. Meanwhile, localized destruction of the passive layer occurred as a result of the pitting corrosion process, exposing more fresh surfaces [32,33]. In another term, the presence of Cl^- ions facilitated the electrochemical conversion of Mg metal to univalent Mg⁺ ions [33]. Additionally, the low-grade Mg scrap could react spontaneously in citric acid-added seawater without other catalysts [34]. Furthermore, the increasing concentration of citric could remarkably improve the volume and rate of hydrogen generation.

To take full advantage of post-consumed Mg, Uan et al. [35] employed low-grade Mg alloy scraps and NaCl solution to produce hydrogen with a Pt-coated Ti net as catalyst. When the Ti net statically covered on the surface of Mg alloy, the average hydrogen generation rate was approximately 302.3 mL/(min∙g catalyst). When the Ti net kept grinding on the surface of Mg alloy, the mean hydrogen generation rate increased to 432.4 mL/(min∙g catalyst) because the grinding motion helped remove the produced Mg(OH)₂ passive layer on Mg surface. However, experimental results revealed that the performance of the catalyst degraded as the usage number rose due to the fact that the Pt film fell off the substrate surface [36]. Besides, it was verified that stainless steel could also trigger the low-grade Mg scraps to react with water and generate hydrogen [36].

Due to the fact that Mg and its alloys exhibit open circuit potential values lower than –1.45 V _SCE in aqueous solutions [37,38], Mg dissolution in aqueous solution is accompanied by abundant hydrogen generation. The involved anodic and cathodic reactions were respectively illustrated in (R.3) and (R.4).

(R.3) \mathrm{Mg}-{\mathrm{2e}}^{-}\to \mathrm{M}{\mathrm{g}}^{2+},

(R.4) {\mathrm{2H}}_2\mathrm{O}+{\mathrm{2e}}^{-}\to {\mathrm{H}}_2+{\mathrm{2OH}}^{-}.

An unusual feature was observed that hydrogen generation rate increased as the anodic polarization of the Mg surface strengthened [39], which is opposite to the standard electrochemical kinetics. Since then, researchers have become conscious of the negative difference (NDE) [40] and attached importance to it when studying Mg corrosion [32,41,42]. In Ref. [43], it was concluded that hydrogen evolution rate reached a minimum at the open circuit potential, i.e., the applied current was zero, and then grew as the applied anodic or cathodic current density increased. It was assumed that the increasing Mg dissolution rate enhanced the exchange current density, thus accelerating the hydrogen evolution.

The research on hydrogen production device is of great significance which should match the practical applications. An experimental setup generating hydrogen connected to hydrogen fuel cells is displayed in Fig. 5 [36]. Metal samples were placed in a sealed reactor, and NaCl solution was pumped into it. The hydrogen generated went through a low-temperature dehumidifier and then was connected to the fuel cell energy system. A water-cooled jacket encircled the reactor exterior to maintain an optimal temperature of about 25°C. The hydrogen flow rate and the temperature in the reactor were recorded via a flow meter and thermocouple respectively. In Ref. [44], an experimental setup of the MAGIC (MAGnesium Injection Cycle) engine was built, which could export thrust from Mg-H₂O reaction. Metal Mg inside the combustion chamber was rapidly heated to 923 K by the ohrmic heater. The water valve was open to supply water after Mg was ignited. Then the water vapor and hydrogen generated from the reaction formed a thrust, which was received as a momentum by a stainless steel load cell. The measurements demonstrated three stages of Mg combustion in the chamber, ignition, radial reaction, and depth direction. First, the ormic heating ignited Mg and the combustion wave raised the temperature of entire Mg fuel. As water was supplied from the top, the Mg on the top surface started to react with water. Then, the combustion wave penetrated in the depth direction. The output power of this Mg combustion engine could be controlled by the water supply, and experimental results indicated that the optimal molecular ratio of water to Mg for the maximum momentum was 5:1.

Fig.5 Experimental setup that produces hydrogen for hydrogen fuel cell energy system from Mg-H₂O reaction (adapted with permission from Ref. [36])

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The Mg-H₂O reaction releases hydrogen in an opportune velocity, whose rate could be controlled by water supply. The main problem is that the formation of a passive layer, Mg(OH)₂, on the surface prevents the reaction from further processing. Researches verified that ball-milling, changing the composition of electrolyte and adding catalyst were all effective ways to facilitate hydrogen production.

3.3 Al-based hydrogen generation

Of all metals, Al is considered to be the most attractive energy carrier with its high hydrogen storage capacity and high energy density of 29 MJ/kg [45]. The prominent barrier is the formation of Al₂O₃ on the surface of Al which is even denser than the passive film of Mg. So far, there have been several selectable methods to activate the Al-H₂O reaction which are introduced as follows.

3.3.1 Activated by adding alkaline

It is well known that the alkaline solution can react well with both Al and its oxide [46]. The reaction is conducted in two steps:

(R.5) \mathrm{2Al}+{\mathrm{6H}}_2\mathrm{O}\to \mathrm{2Al}{(\mathrm{OH})}_3+{\mathrm{3H}}_2\uparrow ,

(R.6) \mathrm{Al}{(\mathrm{OH})}_3+\mathrm{NaOH}\to {\mathrm{NaAlO}}_2+{\mathrm{2H}}_2\mathrm{O}.

Aleksandrov et al. studied the reaction of Al foil and powder with dilute aqueous NaOH in Ref. [47], and explored the effects of initial Al weight, alkali concentration, foil surface area, and temperature on the hydrogen generation performance. All above factors were found to have a positive correlation with the reaction rate, and the activation energy of the reaction between Al and NaOH solution was 51.5–53.5 kJ/mol. As for the Al powder, the maximum reaction rate was linearly relative to the initial metal weight, while the reaction rate at the steady stage was linearly dependent on the surface area of metal foil. The influence of reaction products was also revealed. Initially, aluminate ions at low concentration did not affect the hydrogen liberation. As the aluminate ions accumulated, the hydrogen generation rate would be restrained. Mechanical polishing of the metal helped remove the oxide layer on the Al surface, therefore shortening the induction time of this reaction. It was also found that stirring could accelerate the reaction by increasing the contact area of Al and water. Doping CaO into Al pellets also observably improved the hydrogen conversion [48]. According to Ref. [49], carbon dioxide in air would react with hydroxide ions, decreasing the hydrogen production efficiency. Thus, the working fluid should be isolated from air.

In Ref. [50], the used Al cans were utilized to produce hydrogen. Compared to the NaOH concentration, the pretreatment of Al scraps had a more significant effect on the hydrogen generation performance. The morphologies of Al scraps pretreated with different approaches were displayed in Figs. 6(a)–6(d). The un-pretreated Al scrap was covered with a layer of dense passive film or paint (Fig. 6(a)). The coat on the calcinated Al scrap wrinkled and localized fractures were observed so that more fresh surfaces were exposed (Fig. 6(b)). Mechanical polishing could widely breake and remove protective layer on the Al surface (Fig. 6(c)). Although concentrated sulfuric acid did extract the original coating of Al scrap, a new passive layer formed. Fresh Al matrixes were exposed only at the effervescence region and initial reaction area. The exposed areas of Al matrix increased in the following sequence: Al scrap-1>Al scrap-2>Al scrap-4>Al scrap-3, which was in accordance with the initial hydrogen generation rate, as presented in Fig. 6(e).

Fig.6 SEM images of Al scraps pretreated with different approaches

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To avoid the environmental pollution and instrument damage problems from high concentration of alkaline, waste Al cans ball milled with Ni additives were employed to react with alkaline in low concentration at elevating temperature [51]. Ball milling could decrease the Al particle sizes and increase the contact area of reactants. The formation of micro-galvanic cells between Ni and Al also accelerated the oxidation of Al. This hydrolysis system could achieve a hydrogen generation rate of 150 mL/(s∙g Al) and a hydrogen yield of 1350 mL/g Al, which was an alternative for portable power supply or hydrogen storage systems. In Ref. [52], Ni was replaced by Bi, and the Al foil was immersed in nitric acid for 5 h to remove the polythene film. Without mechanical ball-milling, the hydrogen generation rate decreased to 30 mL/(s∙g Al).

To investigate the effects of acid and alkaline media on the Al-H₂O reaction, Yang et al. regulated pH values of electrolyte via adjusting concentration of AlCl₃, CoCl₂, Al(OH)₃, Ca(OH)₂, and NaAlO₂ [53]. The results revealed that H⁺ (AlCl₃ and CoCl₃) and OH^- (Al(OH)₃, Ca(OH)₂ and NaAlO₂) both promoted the removal of oxide films, triggering the Al-H₂O reaction. In particular, replacement reaction occurred between CoCl₂ and Al, so that the synergistic effect of H⁺, Cl^- and the formed Co expedited the hydrogen liberation. The electrode potential difference between two metals could bring about electrochemical corrosion effect, thus, facilitating the Al oxidation. Shmelev et al. [54] employed three contact modes of copper and Al surface to explore the effect of contact corrosion on Al-H₂O reaction in KOH solution: initial chemical coating of Al surface; embedding copper into the melted Al matrix; and continuous feeding copper to Al surface. It was concluded that the presence of copper on Al surface raised the hydrogen generation rate up to 6 times, while the filtration corrosion of Cu in Al enhanced the hydrogen generation rate up to 200–250 times.

3.3.2 Activated by ball milling with additives

It can be known from former researches that ball-milling Al with additives, like inorganic salts, metallic oxides, and metal hydrides, can modify and activate Al powders. For one thing, the ball-milling process caused fractures, dislocations, defects, crevices, and other micro-structures in Al-based materials, which would increase their specific areas [55]. For another, ball-milling was able to transform the lattice structure of metal. Then, the formed defects could store high energy in the ball-milling process, and became highly active reaction sites [56]. Besides, soluble inorganic salts caused localized corrosive pitting and crevices on the external Al₂O₃ films, which facilitated the activation of Al.

NaCl is a preferred miller owing to its accessibility, non-toxicity, solubility, and eco-friendliness [57]. In the ball-milling process, keen-edged NaCl particles were fractured and driven into Al particles, thus providing local gates on newly formed Al surfaces. The gates became open in water, which triggered the Al-H₂O reaction. Meanwhile, as the size of Al particles reduced, the specific area of Al and the kinetics of hydrolysis reaction enhanced. The highest average hydrogen generation rate of 75 mL/(min∙g Al) was obtained when the mole ratio of salt to Al was 1.5 at 75°C [58].

Researchers [59] found that g-Al₂O₃-modified Al powders could continuously react with water and release hydrogen under ambient conditions. In the preparation stage, Al and Al(OH)₃ powders were mixed together and sintered at 600°C in vacuum to generate porous \mathrm{Al/}\gamma {\mathrm{-Al}}_2{\mathrm{O}}_3 composite. After being modified by \gamma {\mathrm{-Al}}_2{\mathrm{O}}_3, the dense passive oxide layer would be replaced by heterogeneous nucleated \gamma {\mathrm{-Al}}_2{\mathrm{O}}_3. Additionally, the hydrogen generation rate and volume increased as the content of \gamma {\mathrm{-Al}}_2{\mathrm{O}}_3 rose. After that, Deng et al. proposed a possible physicochemical mechanism to explain the activation effect of \gamma {\mathrm{-Al}}_2{\mathrm{O}}_3 in Ref. [60]. It was deduced that OH^- ions were the main mobile objects that could traverse through the hydrated oxide film. Limited by the soluble capacity of H in Al entity and the permeability through the hydrated oxide film, hydrogen gas bubble accumulated and grew at the Al/Al₂O₃ interface. As was known, there was a critical gas pressure in H₂ bubbles, which hinged on the environmental pressure and tensile strength of the generated oxide film. When the equilibrium pressure in H₂ bubble was large enough to break the hydrated oxide film, namely exceeding the critical gas pressure, the hydrogen generation reaction kept going. Therefore, the Al powders could continuously react with water at temperatures over 40°C or under low vacuum. As for Al particles (<60 mm) small enough, physical contact with loose alumina powder led to a noteworthy increase in the reactivity of Al [61]. In Ref. [61], Al powder was hand mixed with loose alumina, and then hydrolyzed in de-ionized water. It was found that the reaction accelerated with the amount of alumina increasing. Moreover, the alumina could be recycled and reused.

Al(OH)₃ is not merely a byproduct of the Al-H₂O reaction. Researchers found that the amorphous Al(OH)₃ produced by urea hydrolysis could disrupt the passive layer on Al surface, thus activating the Al-H₂O reaction [62]. The hydrogen yield reached up to 100% at 45°C with a purity of 97%. Chen et al. [63] obtained Al(OH)₃ with hexagonal plate-like crystal shape from the Al(NO₃)₃ precursor. The optimal Al:Al(OH)₃:H₂O ratio was proven to be 3:15:50 by weight, under which a hydrogen yield of 100% within 6 min and the highest hydrogen production rate of 1920 mL/(min∙3g Al) were achieved. Though the reaction was conducted at room temperature, the water temperature was once self-heated to 95°C due to the exothermic reaction.

Dupiano et al. [64] prepared five Al-metal oxide powders by mechanical milling, Al-Al₂O₃ and Al-MgO by shaker mill, Al-MoO₃, Al-Bi₂O₃, and Al-CuO by planetary mill. Of all the experiments with various compositions, the reaction rate in the case of Bi₂O₃ was the fastest. An average hydrogen generation rate of 164.2 mL/(min∙g Al) was obtained with a productivity of 100%.As was known from the analysis of reaction product, Bi₂O₃ was reduced to metallic Bi in the reaction process, which facilitated the Al-H₂O reaction. Further investigations are needed to clarify the influences of different milling compositions on the Al-H₂O reaction.

Enlightened by the excellent performance of CaH₂ in the hydrolysis of Mg-Al alloy [65], Liu et al. studied the hydrogen generation from the hydrolysis of ball-milled Al-CaH₂ composites [66]. After milling with CaH₂, the grain size of Al reduced and the passive layer was broken. Meanwhile, the hydrolysis of CaH₂ provided OH^- and hydrogen, which enhanced the corrosion of Al and the hydrogen capacity. In comparison with the addition of LiBH₄, NaBH₄, and MgH₂, CaH₂ displayed the optimal performance. A maximum hydrogen generation rate of 2074.3 mL/(min∙g) and a yield of 97.8% were obtained at 75°C with Al-10 mol% CaH₂ mixture milled for 15 h. Limited by the high cost and possible flammability of CaH₂, various chloride salts were added into the starting materials [67]. It could be tested that hydrocalumite, namely Ca₂Al(OH)₆Cl(H₂O)₂, was a by-product in the hydrolysis process, which was beneficial to the removal of dense Al(OH)₃ layer. Of those salts additives, NiCl₂ presented the best activation property because a replacement reaction occurred between NiCl₂ and Al, generating Ni that forms micro-galvanic cells with Al. The amount of CaH₂, NiCl₂, and milling time had significant influences on the hydrogen generation reaction. In this study, the sample composed of Al-10 mol% CaH₂-10 mol% NiCl₂ milled for 3 h was the optimum choice, achieving a yield of 92.1% and the maximum hydrogen generation rate of 1566.3 mL/(min∙g Al). Li-based activators also show pronounced facilitation on Al-H₂O reaction [68,69]. In Ref. [70], the Al-Li alloy was prepared by vacuum induction melting, and then crushed by ball milling. The alloy prepared could react with water even at 0°C. Moreover, both the content of Li and the initial water temperature were positively relative to the hydrogen generation performance. Reference [71] comprehensively investigated the effects of water-Al mass ratio, water temperature, water type and shape and size of Al particles on the hydrogen production of Al-H₂O reaction activated by Li-based activators, and verified the application feasibility of this reaction in fuel cells. To enhance the capacity of hydrogen production, Al-Li₃AlH₆ was prepared by ball milling. The highest hydrogen yield reached up to 1513.1 mL/g, and the maximum hydrogen yield rate was 2737.6 mL/(min∙g) [72]. Additionally, researchers synthesized materials like Al-NaMgH₃-Bi-Li₃AlH₆ [73], AlLi-NaBH₄-CoCl₂ [74,75], which contained hydrogen carriers besides Al, and produced alkaline in water solution, thus improving the hydrogen generation performance.

In the investigation conducted by Huang et al. [76], a kind of Al/graphite composite was proven to be a good hydrogen carrier. Spherical Al particles and laminate graphite were ball-milled together with 2 wt% NaCl, forming core-shell structures. Both the water temperature and milling time showed a positive correlation with the Al hydrolysis reactivity. This composite could start hydrolysis reaction below 45°C, and the maximum hydrogen generation rate reached up to 40 mL/(min∙g Al) at 75°C. The hydrogen production performance of Al-Si alloy prepared by centrifugal atomizer was also studied [77]. It turned out that the addition of Si would decrease the amount of produced hydrogen, but the induction time would reduce observably in the case of Al-12Si powder. The synthetical addition of Si and graphite should be a scheme to be considered.

In Refs. [58,64,78], the reaction scenario was concluded into three stages, as illustrated in Fig. 7, the induction period, the ‘fast’ reaction period, and the ‘slow’ reaction period. The reaction started with the induction period, which was relevant to the additive composition, water temperature, and grinding time. Once the passive layer fractured, water or OH group diffused into inner protected Al. Then it stepped into the stage of ‘fast’ reaction, during which the hydrogen generation rate soared up, and 70%–90% of theoretical hydrogen yield was attained. With the consumption of Al and the production of Al(OH)₃, the reaction mode converted into the ‘slow’ reaction period, and the transition was influenced by the morphology and grain sizes of reactant composites.

Fig.7 Illustration of three stages of Al-H₂O reaction according to different reaction rates (adapted with permission from Ref. [64])

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According to the aforementioned researches, lengthening milling time [66,79] or raising water temperature [64,69] could enhance the Al reactivity. However, for some activated materials, prolonging the milling time too much would lower the hydrogen productivity because the starting materials would be oxidized again [72]. Therefore, the materials prepared by mechanical milling are not suitable for long-time storage.

3.3.3 Activated by alloying

It was found that Al amalgam could spontaneously react with water at atmosphere temperature along with the liberation of hydrogen and the production of hydroxide [80]. Seoet al [81] reported the high reactivity of Al coated by amalgam and the effect of amalgam compositions on reaction rate. Fan et al. [82] prepared Al-Hg alloys by mechanical milling, and obtained a conversion efficiency of about 99%. Huang et al. adopted a particularly designed reactor in Ref. [83]. Hg or Zn amalgam was laid on the Al surface at 65°C. As Hg atoms entered into metallic Al, Al amalgam came into being on the surface layer of Al and caused electrochemical corrosion, thus activating Al-H₂O reaction. Experimental results showed that Zn amalgam presented a better facilitation than Hg on the hydrogen generation reaction because of its lower surface free energy. Besides, the activation energy was calculated to be 43.4 kJ/mol and 74.8 kJ/mol respectively for Al activated by Zn amalgam and Hg.

Limited by the toxicity, Hg faded away in current researches. In 1960s, Woodall et al. [84,85] accidentally discovered that the Al-Ga liquid alloy could react with water near room temperature, producing hydrogen, Al(OH)₃ and heat. Considering the high cost of rare metals like Ga, In, and Sn, researchers made a lot of efforts to increase Al content in the alloys, and found that solid alloys whose Al content exceeded 95% could also split water and produce hydrogen at room temperature [86–88]. In 1980s, Kolbenev et al. [89] conducted a series of quantitative experiments on melted Al alloys containing a small amount of rare metals (Ga 0.07 wt%, Bi 2 wt%, Pb 1 wt%, Sn 1 wt%, and the rest is Al.), obtaining a hydrogen generation rate of 4 L/(min∙g) and a conversion efficiency approximate to theoretical value.

At the very beginning, smelting and casting was the most common method to fabricate the Al-based alloys. Kravchenko et al. [90] prepared Al-based alloys doped with Ga, Zn, Sn, and InSn₄ by melting them at 450°C, obtaining a eutectic with melting point of 6°C. The XRD and SEM testing results indicated that liquid phase eutectic dispersed into the grain boundary, which activated the Al-H₂O reaction. The aging and deactivation were unavoidable for these alloys at ambient temperature unless they were stored at liquid nitrogen temperature. A quinary alloy Al-Ga-In-Sn-Bi was prepared by smelting and casting method at 800°C under nitrogen atmosphere [91]. The activation of Al was ascribed to the formation of Al-Ga solid solution and the eutectic reaction of Al and intermetallic compounds InSn₄ and InBi. Compared to the Al-Ga-In-Sn alloy, the hydrogen release from Al-Ga-InSn₄-InBi alloy was more stable and suitable in the field of fuel cells. Xie et al. [92] added trace amount of AlTi₅B in Al-Ga-In-Sn alloy. It turned out that Al grain size decreased from 120 μm to 40 μm, and the hydrogen generation rate reached 44 mL/(min·g) at 30°C and 460 mL/(min·g) at 60°C. The effect of Cu additives on the hydrogen generation performance of Al-Ga-InSn₄ alloy was also presented in Ref. [93]. Metals were heated up to 800°C, and maintained at this temperature for 1 h. With the addition of Cu, the Al(Ga) solid solution was further pulverized. As a result, the hydrogen generation rate rose and tended to be stable.

Developed by Benjamin [94], mechanical alloying is a high energy ball milling process to fabricate composite metal powder, resulting in ultrafine mixing and alloying. Lu and Zhang [95] explored the influence of processing control agent on the inter-diffusion of metals in the mechanical alloying process, verifying that mechanical milling could be used to produce metastable phase solid solution. Then Fan et al. [96] introduced the mechanical milling to synthesize activated Al alloys for hydrogen production. The results indicated that mechanical milling was more advantageous than the melting method in activity and pollution. By comparison, the hydrogen productivity of Al-Bi-based materials was higher than that of Al-Sn-based materials in that the Al-Bi alloy had a more negative anode potential than Al-Sn alloy [97]. Besides, hydrolysis of the added hydrides and salts generated abundant heat and conductive free-moved ions which also facilitated the hydrolysis of Al. A hydrogen generation rate of 160 mL/(min∙g Al) was achieved with Al-Bi-based materials in the first five minutes. A study of the hydrolysis reaction of ball-milled Al-Bi alloy was conducted in Ref. [98]. The hydrolysis mechanism was based on the micro-galvanic cells formed by Al and Bi. Therefore, the hydrolysis reactivity of the alloy strengthened with the increasing ionic conductivity of the reaction media. The maximum hydrogen conversion of Al-16 wt% Bi was 92.75% in 1 M NaCl and 89.88% in pure water. However, the hydrogen yield was a little or even null in methanol and ethanol. By the same token, the addition of NaCl in the milling process also had a positive effect on the enhancement of hydrogen generation [99]. As reported in Ref. [100], Al-Sn alloy has a potential of –1.54 V. With the addition of Zn and MgH₂, the activation energy of the hydrolysis reaction of Al-Sn-Zn-MgH₂ was as low as 17.57 kJ/mol, and the hydrogen yield was 790 mL/g. Fan et al. prepared Al-Bi-Li mixtures with NaCl as milling-assisted agent [101], and expounded that the newly formed phase, namely BiLi₃ and AlLi, contributed to the enhanced reactivity of Al. The experiment results showed that the amount of Li exerted a significant effect on the hydrogen evolution. As the content of Li rose to 4 wt%, the hydrogen yield reached up to 1380 mL/g. Besides, the Al-based sample containing 3 wt% Li presented the activation energy as low as 20.37 kJ/mol.

Wang et al. employed the Al-based alloys doped with Ga, In, and Sn to produce hydrogen, and clarified that its high activity was due to the embrittlement of Al by liquid Ga-In-Sn alloy and the active points introduced by intermetallic In₃Sn and InSn₄ [102]. In that work, a highest hydrogen generation rate of 1560 mL/(min∙g) was attained in the reaction of Al-Ga-In-Sn alloy and distilled water or deionized water. In addition, it was proven that the optimized mass ratio of In to Sn equaled 1:4, at which the highest hydrogen yield was obtained [103]. Then the synergistic effect of low melting point metals (Ga, In, Sn), CaO and NaCl was revealed in Ref. [104]. Low melting point metals brought about the embrittlement of Al, and shifted the negative potential of Al. NaCl particles invaded into Al matrix and destroyed the passive oxide layer on Al surface. CaO dissolved in water and provided hydroxyl ions. The solved salts and metal oxide increased the conductive ions in the solution, facilitating the anode corrosion of Al.

Furthermore, the effect of powder particle size was discussed via the experiments of Ga-In and Ga-In-Sn-Zn alloy in Ref. [105]. As was expected, the less the particle size was, the more intensively the reaction went. According to the SEM observation, the varied grinding manners would cause Al crystal to have different shapes. As presented in Fig. 8, the mixture of Al and Ga by weak mechanical treatment (low-energy, i.e., LE) generated needle-like grains [106], and the Al alloy activated in a planetary ball mill (high-energy, i.e., HE) displayed cubic shapes which possessed larger specific areas [107]. Working with Al-Ga-In alloy, the activation energy of LE-activated Al was 55±5 kJ/mol, which was larger than that of HE-activated Al, namely 35±5 kJ/mol. Besides, the experiment results demonstrated that the addition of Zn to gallam had no influences on the kinetics of Al-H₂O reaction, while the introduction of Sn improved the hydrogen evolution properties [107]. Moreover, no precautions were required if the mechanochemically gallams-activated Al powders were preserved at room temperature and an air humidity of below 70% for 2 months.

Fig.8 SEM graphs of Al powders activated by Ga

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To reveal the relations between microstructures and kinetic properties of Al-based alloys, the arc melting technique was applied to prepare Al-Ga-In-Sn alloys at argon atmosphere [108,109]. The analysis results of XRD, SEM, EDX, and DSC suggested that there existed Al-Ga solid solution and In-Sn second phases in activated Al alloys. The grain size of Al(Ga) solid solution reduced as the cooling rate rose, which changed the shape and size of Ga-In-Sn phases covering on the grain surfaces. It was also found that the reactivity of Ga-activated Al alloys grew with the reduction of grain size. The activation energy in reaction order of 0.7 decreased from 77 kJ/mol to 53 kJ/mol as the grain size dwindled from 258 to 23 μm [108]. As analyzed in Ref. [109], the Al-H₂O reaction originated from the eutectic reaction of Al and Ga-In-Sn. Since Al atoms diffused into Ga-In-Sn in the preparation stage, the hydrogen generation reaction could start at nearly 0°C [110]. But the reaction would suspend in a short period once the initial Al in Ga-In-Sn was consumed. To keep the durative of reaction, the temperature must be kept above 15°C, at which Al would diffuse into Ga-In-Sn continuously. In Ref. [111], it was proposed that the reaction temperatures depended on the melting point of grain boundary phases. The Al-H₂O reaction started at a lower temperature when the melting points of grain boundary phases were lower. In spite of this, the reaction rate was not affected by the melting points of grain boundaries, but by the compositions of low melting metals. In detail, In₃Sn and InSn₄ would exist in varied proportions by adjusting the In/Sn mass ratio. According to Ref. [112], the hydrogen generation rates were related to the contact areas of grain boundary phases on the Al surface and water temperature. When the water temperature was below 60°C, only In₃Sn melted, which dominated the Al-H₂O reaction. When the temperature exceeded 70°C and the Sn content ranged from 0.625 wt% to 0.75 wt%, InSn₄ began to melt. The co-existence of In₃Sn and InSn₄ led to the rapid reaction stage. If the Sn content was over 0.75 wt%, the reaction would be limited owing to the lack of the In₃Sn phase.

Apart from the aforementioned alloying techniques, more novel material synthesis methods were developed. Wang et al. [113] obtained a unique core/shell microstructure comprised of 80Al-10Bi-10Sn using the gas atomization method. By comparison, the hydrogen yield from the Al alloy fabricated by gas atomization was nearly three times that by ball milling. The hydrogen conversion yield reached up to 91.3% within 16 min at 30°C. In Ref. [114], the differences between Al-Sn and Al-Bi alloys attained by gas atomization were explored. The SEM observation demonstrated that Al-Bi powders presented core/shell structures, while Al-Sn formed eutectic structures. In this work, Al-Bi alloy had a better hydrogen generation performance than Al-Sn alloy did. In addition, Al-Bi alloy inherited a better oxidation resistance property resulting from miscibility gap and the sharp distinction of liner thermal expansion coefficient between Al and Bi. Zhang et al. adopted high-pressure torsion to synthesize Al-Sn alloys [115] and Al-Bi-C composites [116]. The addition of Bi boosted the effect of micro-galvanic cells and enhanced the pitting corrosion. Meanwhile, the existence of C in the form of graphite enhanced the fracture and enlarged the active surface area. The experiments proved that Ai-30 wt% Bi-10 wt% C was the optimized composite, which attained a hydrogen generation rate of 270 mL/(min∙g) and the hydrogen conversion reached up to the theoretical value at 333 K. In Ref. [117], the melt spinning technique was applied to prepare Al-Sn alloys at 800°C. Nevertheless, in this condition, the hydrogen generation reaction only processed in NaOH solution at a temperature over 100°C.

3.3.4 Directly triggered by corrosive metals

Since the aforementioned preparation approaches for activated Al-based alloys were energy-intensive and sophisticated, researchers resorted to the direct contact and penetration of liquid metal and Al so as to trigger Al-H₂O reaction [118,119]. Tests showed that the activation of Al was attributed to the surface, grain-boundary, and volume diffusion of liquid metal in Al. The surface diffusion disrupted the passive oxide film on Al surface, thus decreasing the surface energy and increasing effective reaction surface. The grain-boundary and volume diffusion induced composition changes of inner Al, thus causing the embrittlement and enhancing its reactivity with water. Particularly, the penetration velocity of liquid Ga into Al along its grain boundaries was addressed to be 0.55–8.7 μm/s [120,121]. Zhang et al. [122] and Yuan et al. [123] found that liquid metal alloys EGaIn (75 wt% Ga-25 wt% In) and Galinstan (68.5 wt% Ga-21.5 wt% In-10 wt% Sn) dipped in NaOH solution could autonomously swallow Al foil via corrosive process and release hydrogen generated from the Al-H₂O reaction. It was noted that hydrogen bubbles could provide motivation for the liquid metal alloy together with the imbalanced surface tension. This phenomenon made it possible to achieve self-powered soft machines [124–126]. Tan et al. [127] compared the hydrogen production behaviors activated by Ga-based liquid metal alloys in alkaline solution. The addition of In and Zn had no positive effect on the hydrogen generation behavior. GaSn₁₀ showed an excellent catalysis in this reaction while the reaction would suspend before it went thoroughly because of the fast-formed oxide film. In this study, the mass contents of Al were less than 1%. When Ga was recovered and reused for five times, the hydrogen generation rate and yield kept consistent, which manifested that the liquid metal alloy could be recycled in this scheme. In addition, Yang et al. [128] reported that conductive materials, like stainless steel and graphite, could accelerate this hydrogen generation behavior. For one thing, the conductive substrate attracted Al grains, thus expediting the Marangoni flow of the liquid metal alloys. For another, the conductive substrate facilitated the galvanic effect, thereby accelerating the anode corrosion of Al. To avoid the negative effect caused by the corrosiveness of NaOH, Lu et al. [129] studied the hydrogen generation using Al plates activated by liquid Ga-In alloy in NaCl solution. As exhibited in Fig. 9(a), the increasing NaCl concentration could enhance the hydrogen evolution at the outset. But hydrogen production behavior worsened when the NaCl concentration exceeded 20%. It can be seen from Fig. 9(b) that temperature had a vital influence on the hydrogen generation performance. The average hydrogen rate increased to three times as the temperature rose from 20°C to 80°C, and the activation energy of this reaction was decreased to 39.6 kJ/mol in this work.

Fig.9 Effects of different factors on hydrogen generation performances from Al-water reaction

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In practical applications, it is the key to having a good command of the hydrogen generation rate, which is closely associated with the Al-alloy compositions. In Ref. [130], Al-In-Zn-NaCl was selected to be a favored candidate as hydrogen supply for fuel cells. On the one hand, the addition of Zn shifted the negative potential of Al-In alloy from –1.1 V to –1.5 V. On the other hand, the formation of Al-Zn alloy created defects and cracks on the passive alumina film. The mixture could support the fuel cell operating at 0.96 W with a stable hydrogen supply. As noted, the addition of Ca in the milling process could inhibit the reactivity of Al-Hg alloy while the addition of Zn and Ga could enhance the reactivity of Al-In alloy [130]. Meanwhile, the Al-Hg-Gg alloy showed a good stability at moist atmosphere below 343 K and presented an excellent hydrogen generation performance at 343–373 K. Thus, the controllability of hydrogen production could be obtained through preparing diverse additives in the Al-based alloys [99].

In summary, the Al-H₂O reaction is highly favored by researchers because of the high energy density and controllable reactivity. However, the challenges exist in either the complicated depassivation process or the aging and degradation of the raw materials. As for recent researches of liquid metal directly triggered Al-H₂O reaction, the separation of byproduct and recovery of liquid metal is to be solved.

3.4 Zn-based hydrogen generation

Zn, Fe, Sn, and Pb are duller than those metals discussed above. They can only react with water vapor at a high temperature, evolving relevant oxides and hydrogen. A two-step hydrogen production route based on water-splitting thermochemical cycle has been proven to be eligible. The schematic diagram taking Zn as an example is displayed in Fig. 10. In the first step, the metal oxide is decomposed into metal element and oxygen with the solar thermal at a temperature over the melting point of the metallic oxide. In the second step, the metal is hydrolyzed, producing relevant metal oxide and hydrogen.

(R. 7) {\mathrm{M}}_x{\mathrm{O}}_y\to x\mathrm{M}+{\displaystyle \frac{y}{\mathrm{2}}}{\mathrm{O}}_{\mathrm{2}},

(R. 8) xM+y{\mathrm{H}}_2\mathrm{O}\to {\mathrm{M}}_x{\mathrm{O}}_y+y{\mathrm{H}}_2.

Here, M stands for a metal; Fe, Ti, Mn, Co, Pb, and Sn are alternative candidates; and M _xO _y is the corresponding metal oxide. Taking the high exergy efficiency into consideration, Zn/ZnO is one of the most promising oxide-redox pair. Weidenkaff et al. conducted thermogravimetric measurements on the ZnO/Zn water splitting cycle [131]. The dissociation of ZnO had to be operated at a temperature higher than 1823 K, and the dissociation rate showed a positive correlation with the temperature. It could be calculated that the activation energy ranged from 312 kJ/mol to 376 kJ/mol. A flow of inert gas could sweep the produced oxygen away, thus enhancing the dissociation reaction, but it would decrease the total energy conversion efficiency. The hydrolysis reaction was conducted at a temperature above the melting point of Zn with commercial Zn particles and solar Zn obtained from the dissociation of ZnO respectively. In solar Zn, there were dispersed ZnO impurities which would serve as nucleation sites for the oxidation that followed. Then solar Zn reacted with water faster and more completely compared to commercial Zn. Surface impurities appeared to favor the reaction rate in both steps, but their impacts on the cycling capability still remained unclear. To maximize the metal conversion, Ref. [132] developed a special test apparatus to investigate the hydrolysis process of Zn powders in different conditions. There was an initial slow stage at a rather lower temperature. As the temperature increased to around 400°C, the reaction stepped into the fast stage. Hydrogen yield was sensitive to the temperature, and it increased from 24% to 81% of the theoretical value as the temperature rose from 185°C to 560°C. With the depletion of reactant, the reaction rate slowed down. In this study, the size of solar Zn particles formed could be as small as 200 nm. Ernst et al. [133] found that the characteristics of Zn particles were regulated by Zn nucleation, condensation, and coagulation. Therefore, besides temperature, the length of condensation zone also affected the size of solar Zn particles, further influencing the hydrogen conversion. In Ref. [134], Wegner et al. prepared Zn nanoparticles with average crystallite sizes of 70–100 nm to produce hydrogen. On the one hand, the high specific surface area of Zn nanoparticles enhanced the reaction kinetics, heat transfer, and mass transfer. On the other hand, the Zn nanoparticles could be entrained in gas flow, which could realize the simple, continuous and controllable supply of reactants and the removal of by-products. This process was conducted in a tubular aerosol flow reactor with three zones: Zn-evaporation zone at temperature above the Zn(l) evaporation temperature, steam quenching zone and Zn/H₂O reaction zone at the temperature below the Zn(g) saturation temperature. In this study, the highest hydrogen yield of 70% was attained at 1023 K, with an average hydrogen production rate of 1 mg/min. It was also found that a higher hydrogen yield was attained at a lower Zn-evaporator furnace temperature. The low temperature brought about the low Zn evaporation rate, thus reducing the sizes of Zn particles and augmenting their specific surface areas. To perfect this technology, more efforts should be made on the simultaneous synthesis and hydrolysis of Zn nanoparticles [135]. Reference [136] emphatically analyzed the kinetics of the second step. The hydrolysis of Zn was separated into two steps. The first step was the dissociative adsorption of water molecules on ZnO, forming surface hydroxylic groups. In the second step, the hydroxylic groups reacted with Zn, producing ZnO and hydrogen. At a low partial pressure of water vapor, the oxide film was thin and defective so that the two sub-steps could influence the reaction rate. At a high water partial pressure, the concentration of hydroxylic groups was on the verge of saturation, and the oxide film was thick. Thus, the diffusion of Zn through the passive ZnO film dominated the hydrogen generation rate. Supposing the production of hydrogen is 100 nm³/h, the required quantity of Zn metal is 296 kg/h, and the amount of solar power reaches up to 542 kW. According to the investigation of Chambon et al. [137], the activation energy of the hydrolysis reaction of Zn was 87±7 kJ/mol in the reaction order of 3.5±0.5. Steinfeld [138] conducted exergy analysis and economic assessment on the solar hydrogen production via the two-step water-splitting thermochemical cycle of Zn/ZnO. Based on a 2nd-law analysis, the maximum exergy conversion efficiency reached up to 29% (ratio of the combustion heat of produced hydrogen to the solar power input) in this closed cycle. The cost of solar hydrogen from this approach was evaluated to be in the range of 0.11–0.17 $/kWh, which prevailed over the hydrogen production from electrolysis of water powered by solar, but was inferior to the hydrogen generation from steam-reforming of natural gas, whose cost approximated to 0.03–0.04 $/kWh. However, the solar thermochemical process eliminates the greenhouse gas emissions and ensures the high purity of generated hydrogen.

Fig.10 Schematic diagram of a two-step hydrogen generation route based on water splitting thermochemical cycle of Zn/ZnO (adapted with permission from Ref. [131])

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3.5 Fe-based hydrogen generation

The Fe-water vapor reaction is one of the most ancient approaches of hydrogen production. It occurs in two steps: (1) the exothermic oxidation of Fe in the steam, producing magnetite and hydrogen as expressed in (R.9); and (2) the endothermic reduction of magnetite to Fe in the presence of reducing gas, like carbon monoxide.

(R.9) \mathrm{3Fe}+{\mathrm{4H}}_{2}O\to {\mathrm{Fe}}_3{\mathrm{O}}_4+{\mathrm{4H}}_2,

(R.10) {\mathrm{Fe}}_3{\mathrm{O}}_4+\mathrm{4CO}\to \mathrm{3Fe}+{\mathrm{4CO}}_2\mathrm{.}

It was known from the phase boundary diagrams of Fe, FeO and Fe₃O₄ in Fig. 11 that (R.9) proceeded above 430 K [139]. A small scale ‘Sponge Iron Reactor’ was designed in Ref. [140], composed of the chemical reaction of hydrogen production and a solid oxide fuel cell. The operation results manifested that the electrical efficiency of this system was approximately 25%, and the overall efficiency was calculated to be 35%. The process was carried out in the temperature range of 750°C–900°C. Commercial sponge Fe pellets with high specific areas were converted into magnetite first, and then Fe₃O₄ was deoxygenated to original Fe again. Thereby, it was cost-saving with only carbon monoxide consumed. However, the Fe-steam process suffered from some disadvantages. On the one hand, the reaction must process in super high temperatures. On the other hand, the sintering of iron oxide decreased the reactivity of metallic Fe in the redox cycle. Furthermore, the reaction in the second step released large amounts of carbon dioxide, exacerbating the green-house effect. With per kg of hydrogen being produced, the amount of released CO₂ reached up to 21.83 kg [141], which was much larger than 10.66 kg of CO₂ in the technology of steam methane reforming [142].

Fig.11 Phase boundaries of Fe, FeO and Fe₃O₄ (adapted with permission from Ref. [139])

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Otsuka et al. [143] proposed an improved redox cycle of iron oxide for pseudo-storage, recovery and supply of hydrogen to polymer electrolyte fuel cell vehicles. The reaction of Fe and steam was integrated in a portable cassette. To decrease the required operation temperature and moderate the sintering problem, some additives like Al, Cr, Zr, Ga, and V were employed to improve the redox cycle of iron oxide. As a result, the additives encouraged the Fe-steam reaction to process at the temperature less than 400°C, rather than traditional 800°C. Moreover, the additives dramatically moderated the sintering of iron oxide in the redox cycles. Besides, it was demonstrated that the compound oxides formed between additives and iron oxides could activate hydrogen and water at the iron oxide surface, and increase the diffusion rate of O^2- in iron oxide.

Urasaki et al. [144] investigated the effects of a trace of palladium(Pd) and zirconia on the steam-Fe reaction at 723 K and atmospheric pressure. The study showed that Pd was conducive to both the reduction of iron oxide and the oxidation of metallic Fe. Meanwhile, zirconia enhanced only the oxidation of partial reduced iron oxide. What was cheerful was that the sintering of the samples was suppressed by accelerating the oxidation and reduction rate. Therefore, the addition of Pd and zirconia could modify iron oxide surface, resulting in a remarkable enhancement of the reaction activity during the redox cycle. In Ref. [145], the addition of 5–10 mol% Mo or Ce could achieve similar effect.

Additionally, certain metal oxides could be harnessed to improve the redox behavior of iron oxide [146]. In the control experiments without any additives or with the only addition of 5 wt% Al₂O₃, the agglomeration of the precipitate was detectable by SEM images, and the iron oxide was rapidly deactivated in 10 cycles. In comparison, samples containing CaO, together with 5 wt% Al₂O₃ and 7.5 wt% SiO₂ presented a markable improvement in the activation and stability. It was analyzed that SiO₂ and CaO played a role as the inhibitor of sintering. Namely, SiO₂ acted as a mechanical block, and CaO caused the formation of dispersed ferrite, avoiding the sintering neck growth.

To promote the Fe-based hydrogen production method, a new carbon dioxide-based hydrothermal Fe oxidation process was developed [141], which could operate at a modest temperature of 200°C–300°C. The reaction occurred in two steps as presented in (R.11) and (R.12). The whole process could be summarized as (R.13). The only byproduct is magnetite, which is pollution-free.

(R.11) \mathrm{Fe}+{\mathrm{CO}}_2+H_2\mathrm{O}\to {\mathrm{FeCO}}_3+H_2,

(R.12) {\mathrm{3FeCO}}_3+H_2\mathrm{O}\to {\mathrm{Fe}}_3{\mathrm{O}}_4+H_2+{\mathrm{3CO}}_2,

(R.13) \mathrm{3Fe}+{\mathrm{4H}}_{2}O\to {\mathrm{Fe}}_3{\mathrm{O}}_4+{\mathrm{4H}}_2.

To make better use of CO₂, hydroxide was utilized to convert CO₂ into {\mathrm{CO}}_{\mathrm{3}}^{-}, which facilitated (R.11). The experiments proved that reaction conditions like temperature, initial carbon dioxide pressure, and the grain size of Fe powder had a significant effect on the reaction. Reaction 12 agreed with the Le Chatelier’s principle [147], which meant that the lower concentration of CO₂ would urge the reaction to process more completely. Though the reaction performances were different at different temperatures, it almost had no obvious temperature dependence. The optimal result with a hydrogen production of 80% was obtained from the reaction with 5-μm Fe powders at a temperature of 160°C and an initial carbon dioxide pressure of 6 bar in 1 M KOH solution.

In Ref. [148], the authors presented a new carbon cycle where metaloxides or metal hydroxides could be reduced to zero-valent state via (R.14) or (R.15) with a suitable organic chemical such as glycerin.

(R.14) {\mathrm{MO}}_x+C_a{\mathrm{H}}_b{\mathrm{O}}_c\to \hspace{0.17em}{\mathrm{M}}^{\mathrm{o}}+C_a{\mathrm{H}}_{b–2x}{\mathrm{O}}_c+x{\mathrm{H}}_2\mathrm{O},

(R. 15) \mathrm{M}{(\mathrm{OH})}_x+C_a{\mathrm{H}}_b{\mathrm{O}}_c\to M^{\mathrm{o}}+C_a{\mathrm{H}}_{b–x}{\mathrm{O}}_c+x{\mathrm{H}}_2\mathrm{O}.

In the first step, hydrogen was produced from the reaction where metal was oxidized and CO₂ was reduced in hydrothermal conditions. For Mn, Zn, and Al, hydrogen was generated both in the presence and absence of CO₂. However, in the condition with Fe, the presence of CO₂ is a necessary requirement for the formation of hydrogen. The maximum hydrogen yield was as high as 99.4%. In the second step, the oxidized metal was reduced to the zero-valent state as (R. 14) and (R. 15) in the presence of bio-derived chemicals such as glycerin. In addition, it was suggested that HS^- acted as a catalyst in the hydrogen production reaction [149]. First, HS^- reacted with Fe to produce H₂ and FeS, which then reacted with OH^- to generate FeO and HS^-. Eventually, FeO reacted with water to form stable Fe₃O₄. An optimal hydrogen productivity of 34% was achieved at 300°C in 4 h. Two reaction cycles can be integrated in the case of Fe. Thus, an efficient and environment friendly cycle of zero-valent metal and oxidized metal oxides could be established.

In a word, the hydrogen production from Fe-based reaction demands high temperatures, and the production efficiency is disadvantageous. From the perspective of material cost, such cycle could be adopted in some special situations.

3.6 Mn-based hydrogen generation

Mn could react with water, producing hydrogen and Mn(OH)₂. In general, the generated Mn(OH)₂ forms a layer of passive film on the surface of metal, which prohibits the reaction processing. However, it was investigated that the addition of soluble ammonium salt, like CH₃COONH₄, (NH₄)₂SO₄, and NH₄Cl, facilitated the reaction to continue due to the coagulation effect of colloid caused by electrolyte. Anions in ammonium salt played a decisive role in the coagulation of Mn(OH)₂. The coagulation effect of sulfate ions was better than that of chloridions because its valence was higher than that of the latter. There was a greater Van der Waals’ force between acetate ions and colloidal ions so that it was easier for organic anion CH₃COO^- to adsorb on the colloidal ions, thus accelerating the coagulation. As the concentration of ammonium salt rose, the reaction rate ascended. With temperature increasing, the reaction with (NH₄)₂SO₄ accelerated with an activation energy of 17.46 kJ/mol [150], while the reaction rate with CH₃COONH₄ decreased in that the high temperature would lead to the decomposition of CH₃COONH₄ [151].

3.7 Zr-based hydrogen generation

Since the nuclear leakage accident in Japan in 2011, Zr-water reaction has drawn much attention. When the temperature rises over 900°C, Zr would react with the water vapor, producing zirconia and hydrogen.

In 1950s, scientists [152] studied the Zr-water reaction using Zr melted at the temperature from 1100°C to 4000°C. This reaction was demonstrated by two steps. In the former step, the reaction rate was controlled by the diffusion of water vapor toward metal particles and the detachment of generated hydrogen from the particles. In the later step, the reaction rate was dominated by the parabolic rate law, leading to the rapid cooling of particles. The parabolic rate law was deduced as

(6) v^{\mathrm{2}}=\mathrm{33.3}\times {\mathrm{10}}^{\mathrm{6}}t\cdot \exp (-{\displaystyle \frac{\mathrm{45500}}{RT}}),

where v is the consumed Zr in milligram per square centimeter of surface area, t is the time in second, R is the gas constant which is equal to 8.314 kJ/(mol∙K), and T is the temperature.

Additionally, as the particle size reduced to 1 mm in heated water or 0.5 mm in room-temperature water, the reaction accelerated sharply. For one thing, a smaller particle possessed a larger specific area so that the contact area between Zr and water increased. For another, the explosive hydrogen evolution propelled the particles at a rather high speed and helped overcome the gaseous diffusion barrier.

4 Opportunities of unconventional hydrogen generation

4.1 Energy capacities of various metals

Available energy is divided into two shares in the metal-water reaction. One part of the energy stores in the evolved hydrogen, and the other part is released as the reaction heat. The theoretical maximum hydrogen yields of several ordinary metals per unit mass and volume are plotted in Fig. 12(a). Additionally, the energy released from the metal-water reaction is calculated, including the released reaction heat of the metal-water reaction and the combustion heat of the generated hydrogen, both of which can be criterions for reference to select the potential candidates of energy carriers. As can be seen from Fig. 12(a), Li yields the largest amount of hydrogen per unit mass, followed by Al. Meanwhile, Fe produces the largest amount of hydrogen per unit volume, followed by Al. The heat densities stored in these metal-water systems are evaluated in Fig. 12(b), which includes the heat released in the metal-water reaction and the combustion heat of hydrogen generated from the metal-water reaction. As can be seen, Li occupies the first rank of gravimetric heat energy density, which excesses kerosene. Next, Al possesses a larger volumetric heat energy density than other metals. Li, Na, and K are awfully active while their storage requirements are harsh. Meanwhile, the energy storage capacities of Na and K are unfavorable. In perspective of volumetric energy density, Al and Zr are greatly superior to kerosene. What is frustrating is that Zr is expensive, and the Zr-H₂O reaction is unstable and nonuniform. In addition, beryllium (Be) inhabits a gravimetric energy density of 37.26 kJ/g and a volumetric energy density of 68.93 kJ/cm³ while it is of hypertoxicity. Taking the advantages of gravimetric and volumetric energy density, safety and cost together, Al is the most attractive metal to produce hydrogen. Besides, Mg is also a promising energy carrier with a relative high hydrogen yield and energy density.

Fig.12 Hydrogen production capacity and energy density stored in several typical metals

Full size|PPT slide

High purity hydrogen can be obtained from metal hydrolysis reaction at ambient temperature and pressure, which is a promising energy-provision approach. Judging from energy density, storage property, and safety, Al/water and Mg/water are the optimal choices.

4.2 Applications of metal-based direct hydrogen generation

The utilization of hydrogen covers the fields of military, transportation, electronics, chemistry, healthcare, food, and other domestic industries as displayed in Fig. 13. For Mg/Al-water reaction, the reaction condition is moderate and the operation is especially convenient. Therefore, it is a viable means to access in-time and on-demand hydrogen generation.

Fig.13 Application fields of hydrogen

Full size|PPT slide

Metal-water combustion is of tremendous potential in supplying clean propulsion and power for rockets, underwater vehicles, motors and other engines [153]. The system scale could be customized to adapt to user requirements in various orders as classified in Fig. 14.

Fig.14 Power supply from metal-combustion for engines in various scales (adapted with permission from Ref. [153])

Full size|PPT slide

Recently, an on-board hydrogen production approach for auxiliary power in passenger aircraft was introduced in Ref. [154]. There is no need to store hydrogen in tanks on board. Instead, Al must be carried, and then hydrogen can be timely achieved on demand. In this research, the Al powder was activated by adding 1%–2.5% Li-based activator, which could modified the Al surface. After that, Al reacted with water spontaneously at room temperature. Water in this system can be either pure water, or wasted water on the aircraft. It is pointed out in Ref. [154] that the wasted water generated in a typical flight could entirely satisfy the Al-H₂O reaction. For example, the volume of wasted water container on Boeing 787-8A is 1628 L [155]. Former investigation [156] showed that, in the reaction of Al powder and urine, the hydrogen production rates ranged from 150 to 700 mL/(min∙g Al), and hydrogen production could be up to 90%. This signifies that the airplane does not need to carry any extra water. The heat generated by the reaction can be used in the kitchen to heat water and food, deicing or heating aircraft fuel. Via a coarse estimation, the specific electric energy of the new energy system increases with flight duration, and can be as high as 850 Wh/kg for a 14-h flight [154].

The advantages of this technology when adopted on board are listed as follows:

• The combination of metal-based hydrogen generation and fuel cell builds a power supply without moving parts so that the operation process is rather quiet;

• It is environmentally friendly, regardless of the hydrogen production process, or the hydrogen using process. Carbon dioxide emissions would be significantly reduced;

• Hydrogen is produced in time and on demand. Therefore, there is no need to spend a lot on the storage of liquid hydrogen or high-pressure hydrogen. Moreover, the total mass of the new energy system is estimated to be lower than the equipment mass of liquid hydrogen or high-pressure hydrogen;

• Compared with the combustion of fossil fuel, this new energy supply could gain a higher power generation efficiency;

• The new energy system is skid-mounted, and multiple fuel cells can be located near the using location, thus reducing electric and pipelines;

• Large amounts of reaction heat would be released in the reaction course, which can be utilized in other departments;

• Using the new hydrogen production method for auxiliary power is in favor of decreasing the flammable vapor in the fuel tank.

The power system from the metal-based hydrogen production system has a great potential in transport. Metal-water combustion has been identified as a high-performance propellant for torpedoes for a long time [157]. Recently, scientists from Purdue University declared that this new technology would act as a leading role in marine power fields [158]. Al and liquid metal are stored on ship, and water is directly pumped from the seawater along the journey. The output of this power supply system is heat, electricity, and Al(OH)₃, which is eco-friendly. The byproduct is an available chemical product as fire retardant. Besides, Al can be regenerated from Al(OH)₃ via other chemical reactions. In addition, hydrogen-powered vehicles also represent a general trend for a clean future [159–161].

According to the report on PHY.ORG [162], SiGNa Chemistry Corporation proclaimed a new hydrogen production technology which supplied power for portable devices, like laptop and mobile phone. The prototype is presented in Fig. 13. Normally, Na-water reaction is violent and releases a great deal of heat, which brings about security problems. SiGNa found a way out by using the synthesis of Na and silicide, which mitigated the metal-water reaction. The mixture of Na and silicide was placed in a one-off cartridge initially, and hydrogen was produced immediately with the addition of water. The by-product of this reaction was sodium silicate, which was environment friendly, and could be recycled to fabricate cement, toothpaste and other industry products. Fast on-off of this reaction is feasible via controlling the addition of water. Thus, Na-Si fuel cell is suitable for an instant power boost. The combination of this hydrogen-producing cartridge and a pocket-sized fuel cell is a promising power supply for portable electronic devices and electrical bicycles. Besides, the ceramic-modified Al was also highlighted in power supply for portable devices [163].

The value of hydrogen is far more than providing power. It can also play a role in clinical medicine [164,165]. Thanks to the biodegradability of Mg-based metal implants [166], Mg-water reaction for hydrogen production has been applied in medical research for a long time. The corresponding products are Mg bar, peroral Mg capsule, water bottle containing hydrogen, and other hydrogen generation equipment. To maintain the hydrogen yield, the padding must be changed termly. Metal-based direct hydrogen generation can produce highly purified hydrogen rapidly, and increase the pH value of the water, which is considered to be healthful. Besides, metal hybrid can also be used to produce relevant products containing hydrogen by reacting with water, like Mg hybrid and Ca hybrid. There exist several flaws for Mg-based hydrogen generation, like poor timeliness, oxidative discoloration, and tending to clot. Therefore, a new kind of hydrogen production material was discovered, which was named as hydrogen-rich water porcelain. It was a composite of various natural mineral materials and metal alloys. In the fabrication process, nano-coating and microporous connection were implemented on the materials to avoid oxidation and precipitation. Hydrogen-rich water porcelain can realize moderate hydrogen production without any particular requirements on water. Thus, it is convenient and safe to use.

Additionally, hydrogen plays an important role in industry [167]. For example, hydrogen works as shielding gas in the chip fabrication process, as reducing gas in the petroleum refining process, and as raw material in many chemical synthesis processes. In summary, hydrogen has a giant application potential in transportation, medical and industry. The hydrolysis of metal could offer a direct and convenient hydrogen generation approach, which could exploit a new world for hydrogen propulsion.

5 Challenges

In the promotion of unconventional metal-based hydrogen generation, there are both opportunities and challenges. For better utilization of this new hydrogen production approach, a better controlling of the reactivity and more moderate reaction conditions should be realized. The main bottlenecks are summarized as follows.

5.1 The explosive reaction is too violent to control

Once alkaline metals contact with water, innumerable dendritic crystals grow on the surface and extend contact areas, causing explosive behaviors. On the one hand, the reaction rate is too rapid to be controlled. On the other hand, the reaction produces a large amount of heat and gas, causing the volume to dramatically soar up. Both aspects increase the risk of the metal-based direct hydrogen generation method. To mitigate the reaction, it is effective to lower the oxygen content of the atmosphere [26] or add particular organic solutions in water, like acetaldehyde [26] and surface-active agents [21].

5.2 The formation of passive films prevents further reaction

In the Al-H₂O reaction, a dense layer of Al₂O₃ would cover on the Al surface. As the Mg-H₂O reaction and Mn-H₂O reaction processed, the generated hydroxide precipitate would attach on the metal surface. These passive films would hinder the contact of inner metal and water, thus preventing the reaction from further process. Currently, several methods are proposed to remove the passive films:

• Dissolving the passive film. The most convenient way is to dissolve the passive film with specific solutions. For example, Al₂O₃ and Al(OH)₃ precipitate could react with strong base or acid solution; Mg(OH)₂ could be transferred into soluble Mg²⁺ with the help of acid solution; the addition of soluble ammonium salt facilitates the coagulation of Mn(OH)₂ via the effect of colloid aggregation so that the byproduct breaks away from metal Mn.

• Breaking the passive films and making some defects. Mechanical ball milling could destroy the dense passive films and cause some defects on the surface. Then, more fresh surface areas are exposed and contact with water, thus triggering the hydrogen generation reaction. Additionally, halide ions like Cl^- cause pitting corrosion on the metal surface, which enhances the destruction of passive layer.

• Minimizing the particle size. Former researches show that micro/nano-sized Al particles are of high reactivity because of their high specific surface areas. According to Ref. [108], the activity energy of hydrolysis reaction decreases obviously as the particle size dwindles.

5.3 Harsh conditions are required in the reaction

Due to the inferior metallic activity of metals like Fe and Zn, the required activity energy is rather high. A two-step route is adopted here. In the first step, metal oxide is decomposed into metal element and oxide. Then, the metal reacts with water, producing metal oxide and hydrogen, realizing the recycle of raw material. In this process, ultrahigh temperature over the melting point of metal oxide is an indispensable condition, which claims a harsh demand on the device material and power supply.

5.4 It is difficult to realize metal recycling and reusing

As for the direct hydrogen generation from active metals, raw metals are converted into metal hydroxide, which is almost irreversible. According to Ref. [168], the production of 1 kg of primary Al theoretically consumes 6.16 kWh power and releases at least 6.9 kg carbon dioxide. Therefore, the recycling and reusing of primary metal signify overwhelming energy and environment cost. To some extent, recovering waste metal could reduce the cost of this hydrogen generation approach.

6 Conclusions

Hydrogen is a kind of clean and sustainable energy. After combustion, the only products are water and heat. It is an attractive candidate for power supply in transportation, heat and electricity production. However, the storage and transport of hydrogen are the bottlenecks. Above all, metal-water reaction is a promising method to realize in situ and on-demand hydrogen production.

According to the metal activities, metals are classified into three grades. Active metals like Na and K react with water violently, which should be moderated by adjusting atmosphere or electrolyte solution composition. The hydrolysis reaction of metals like Mg and Al are inhibited by passive layers. Some effective actions to reactivate the metals were discussed: adding specific substances to dissolve the passive layer, mechanical ball milling with some additives, alloying preparation, corrosive penetration, and other activation treatments. The reactivity of metals like Zn and Fe is much poorer. A two-step route was developed to obtain hydrogen from the redox reaction of metals/metal oxides, which was dense energy-consuming.

The theoretical hydrogen storage capacities and heat densities of several ordinary metals were compared. Taking the released heat in the reaction and the combustion heat value of generated hydrogen into account, Li possesses the highest gravimetric heat density, and Al occupies the top of the volumetric heat density rank. Analyzing synthetically, the Al-based alloys are considered to be the optimum candidate to react with water, producing hydrogen, followed by Mg. Since the metal-water reaction could realize the real-time hydrogen production, it is appropriate for the power supply system of aviation, marine, and land vehicles. In view of the reducibility of hydrogen, the instant generated hydrogen means is favorable in health products, like hydrogen bar.

Besides, the challenges posed to this method are also discussed here. The key points lie in the control of the hydrogen liberation rate, the removal of the passive layer, and the recovery of metals. The metal-water combustion has displayed a splendid history in underwater propulsion, which is expected to play an important part in the power supply for transport and other domestic areas.

Acknowledgments

This work was partially supported by the Key Project of National Natural Science Foundation of China (Grant No. 91748206), Dean’s Research Funding, and the Frontier Project of the Chinese Academy of Sciences.

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