Abstract
In recent years, the utilization of aluminium (Al), Al alloys, and their composite powder and anode encourages the generation of green hydrogen through hydrolysis and water splitting electrolysis with zero emissions. As such, in this study, the development and characterization of Al, Al alloys, and Al-based composite powder and compacted Al composites for clean hydrogen production using hydrolysis and water splitting processes were reviewed. Herein, based on the available literature, it is worth mentioning that the incorporation of active additives, such as h-BN, Bi@C, g-C3N4, MoS2, Ni, In, Fe, and BiOCl@CNTs in the Al-based composites using ball milling, melting, smelting, casting, and spark plasma sintering technique remarkably improved the rate of hydrogen evolution and hydrogen gas conversion yield, particularly during hydrolysis of Al-water reaction. Again, Al-based electrodes with improved electrical conductivity notably results in better water splitting electrolysis, as well as fast chemical reaction in achieving hydrogen gas production at low energy consumption with efficiency. Though, notwithstanding the significance of Al, Al alloy, and Al-based composite hydrogen generation performances, there are still some challenges associated with the Al-based materials for hydrogen production via hydrolysis and water electrolysis. For example, the low current density and poor electrochemical properties of Al, which on the other hand results in long induction time, high overpotential and cost remains a gap to bridge. Hence, the authors concluded the review study with recommendations for future improvement of Al-based composite electrodes on hydrogen production and sustainability via hydrolysis and water electrolysis. Thus, the study will pave the way for further research on clean hydrogen energy generation.
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1 Introduction
In recent years, based on statistics, the world population is greatly increasing and is estimated to be about 9.6 billion in 2050 and 11.2 billion in 2100 as captured in [1]. However, it is worth knowing that energy consumption is expected to increase as the world population increases as the years go by. This suggests that, as consumption increases, demand will also increase. Hence, energy generation/production will also need to increase. On this note, it is anticipated that fossil fuels will be inadequate to meet the increasing demand as world population is increasing [2]. In addition, fossil fuels such as hydrocarbon-containing materials, i.e. coal, oil, and natural gas are usually not environmentally friendly while also impacting negatively on human health [2, 3]. A research study from Harvard University, in collaboration with the University of Birmingham, the University of Leicester, and University College London, established that over 8 million people died in 2018 from fossil fuel inhalation/pollution, suggesting that air pollution from burning fossil fuels, e.g. coal and diesel are responsible for about 1 in 5 deaths worldwide [4]. With this discovery/observation, environmentally friendly, sustainable, and clean energy sources remain of interest to researchers and industries due to the rise in global pollution [5]. However, establishing that among energy types that hydrogen is a renewable secondary energy source owing to its high specific energy density as it can provide three times more energy with zero emissions than gasoline combustion per unit mass [6]. Hydrogen has been accepted as a good alternative to conventional fossil fuels [7, 8], futuristic, it can be assumed that it is an optimistic candidate for global fuel. Along this line, seeing hydrogen as the energy source of the future, green hydrogen production could be economically feasible through renewable energy sources and reduce environmental pollution. Renewable energy sources are solar, biomass, wind, wave, tidal, and geothermal [1, 9]. Thus, of the mentioned renewable energy sources, solar energy remains the most abundant energy source and promising [10, 11]. Besides, study by Jain [12] revealed that non-conventional energy sources, for example solar and hydrogen energy will continue to exist for infinite period. Integration of solar energy/photovoltaic cells with water electrolysis and the hydrolysis of active metal and metal-based alloys is a low cost and practical technique to produce green hydrogen/clean energy [13]. In producing hydrogen employing electrolysers like alkaline water through hydrolysis and/or water electrolysis, electrode materials remain an important factor to consider. This is because, during water splitting, its molecules basically separate into hydrogen and oxygen gases via the application of an electric current, which flows through the electrodes [14]. To produce green hydrogen through water splitting electrolysis [15], electrode materials can influence more yield in hydrogen as there is always an interaction between the electrodes and the alkaline electrolyte solution. In recent times, numerous studies have been conducted on electrodes made of several materials for hydrogen production with different findings being presented. To understand the importance or the impact of electrodes on hydrogen generation and efficiency during water splitting electrolysis, stainless steel, carbon steel, graphite, copper, nickel, palladium, chromium, vanadium, MoNiFe, and aluminium and aluminium alloy electrodes have been utilized [16,17,18,19,20,21,22]. Among these based materials, aluminium-based electrodes have been demonstrated to be promising owing to their unique properties i.e. lightweight, corrosion resistance, conductivity, mechanical strength-to-weight ratio, and non-toxicity [23,24,25]. Aluminium is abundant in nature, and it is a highly recyclable, cost-effective, and electrochemically active material [24,25,26,27,28]. Utilizing aluminium-based electrodes initiates a few positive impacts on the system during the hydrogen production process. One such impact is the prevention of oxygen gas formation by the aluminium electrodes in the reactor when obtaining pure hydrogen gas [29]. Furthermore, it contributes to hydrogen production without any greenhouse gas emissions and hence their environmental friendliness. However, with the low dielectric constant, poor heat resistance, mechanical degradation, corrosion, and electrochemical properties of aluminium electrodes at subcritical conditions and long induction time for a chemical reaction to take place during hydrogen production through water electrolysis and hydrolysis, reportedly limit its needed potential for hydrogen energy generation [30, 31]. Although green hydrogen production through the water electrolysis and hydrolysis with the utilization of aluminium-based electrode materials still gains significant attention in hydrogen generation directly from alkaline water electrolysers with zero emission and without the need for hydrogen storage technology. In maintaining the usage and widespread use of aluminium-based electrodes for green hydrogen/clean energy production via water splitting, as well as Al-water reaction, several studies have been presented [32,33,34]. As such, the current review focuses on the recent advancements in the development and characterization of aluminium and aluminium-based alloy matrix electrode powder composites reinforced with metal, oxides, carbides, and carbon-based filler particles for hydrogen generation via electrolysis and hydrolysis. Additionally, challenges facing the Al-based composite powders and electrodes in hydrogen production were emphasized, hence suggestions for future improvement.
2 Aluminium and aluminium-based alloy composite powders and electrodes for hydrogen production through hydrolysis
Observing that aluminium (Al) is a promising light weight metal for energy conversion applications including its hydrogen production through its reaction with water (H2O). Manilevich et al. [35] examined the activating effect of Ga-In-Sn alloy at it stable 5 wt% and varying hexagonal boron nitride (h-BN) (1–3 wt%) on the Al composites for hydrogen generation via hydrolysis process. Characterizing the microstructure of the obtained pellet, results showed uniform dispersion of Ga and indium (In) in the pellets’ surface structure. Figure 1 depicts the scanning electron microscopy (SEM) micrograph of the Al powder that was mechanochemically activated by Ga–In–Sn eutectic and BN. In the Figure (Fig. 1), it could be seen that the interfaces between the grains of the activated powders were preserved after the activation and pressing. Cold welding of the grains was observed to be partial only. Hence, the interfaces between the grains offered a channel for water to ingress effectively into the pellets during Al-water reaction. Addition of h-BN in the Al-based composites remarkably improved the rate of hydrogen evolution during hydrolysis and the duration of induction time/hydrolysis was shorter than the hydrolysis of Al-based composite pellets without h-BN under room temperature. Additionally, increasing the weight fraction of the h-BN in the composites from 1 to 3 wt% resulted in a considerable acceleration of the hydrolysis of Al and hydrogen evolution. Herein, the maximum hydrogen yield of 98.3% (1125 ml H2/g Al) was recorded in 2 min for the Al-based composites filled with 5 wt% of Ga–In–Sn alloy and 3 w% h-BN. Generally, with the authors findings, it is worth noting that the manufacturing of energy-storage substances in the form of pellets aids in minimizing their susceptibility to the oxidation during storage and transportation, as well as simplifying the dosing of its loading in the hydrogen generators to achieve uniform rates of hydrogen release.
SEM image of the surface of the Al-powder activated with the 5 wt% eutectic alloy and 3 wt% h-BN particles [35].
Kwon et al. [20] observing the use of Al and its alloys for hydrogen production through hydrolysis, however conducted a study on the fabrication of nickel (Ni) particles-reinforced Al-based alloy electrode composites for hydrogen generation through the hydrolysis of alkaline water. The Ni/Al composites were prepared at different weight percentages of Ni particles (0, 0.5, 1, and 1.5 wt%) using the melting process. The microstructural results indicated precipitation of Ni/Al intermetallic compound along the composites grain boundaries (Fig. 2). In the figure (Fig. 2b), the diffraction pattern of the Ni/Al composites filled with 1.5 wt% Ni validates that the intermetallic precipitates formed is Al3Ni.
a The TEM bright field micrographs on the precipitates of 1.5 wt% Ni/Al with an in-set depicting their SEM micrograph and b Diffraction pattern of a 1.5 wt% Ni/Al precipitate in confirming that the formed precipitates at the grain boundaries is Al3Ni [20]
Examining the hydrogen generation of the composite samples on reacting with the NaOH electrolyte solution for 750 s to ensure stabilized hydrolysis reaction data, results revealed that the incorporation of the Ni particles up to 1 wt% into the Al-based alloy matrix promoted its hydrogen production kinetics remarkably from 2.58 to 16.6 ml/cm2 min. Beyond 1 wt% Ni addition, hydrogen production rate was reduced to 12.54 mL/cm2 min, though was far better than that of the neat Al. The mechanism resulting in the reduced hydrogen generation rate at 1.5 wt% Ni loading was not really explained by the investigators, though further research could provide more insight needed. Notwithstanding, such improvement in the hydrogen reaction rate of the composites compared to the neat Al ascribed to the formation of intermetallic precipitates along the resultant composites grain boundaries [36]. Knowing that precipitates can initiate intergranular and galvanic corrosion of Al due to the difference in corrosion potential of Al matrix and AlNi intermetallic compound [37, 38]. Moreover, research has it that galvanic corrosion exists when two metallic compounds with different electrochemical characteristics are electrically connected in a corrosive medium [39, 40]. Hence, the acceleration in the corrosion rate of the Ni/Al-based alloy composites, which in turn results in their hydrogen generation kinetic over the neat Al. Liao et al. [41] in their work investigated the effects of Bi@C at different concentrations (5, 7, 10, 13, and 15 wt%) on the hydrogen production of Al-based composites from Al-water reaction. The resultant composites were prepared using two fabrication methods (ball milling and spark plasma sintering). Therein, the difference between the two techniques is that the Bi@C/Al composites by ball milling is in powdered form while the opposite is the case in the sintered composites, which means the milled powders were consolidated to a cylindrical block of diameter 15 mm and thickness 1 mm with the application of sintering technology. Characterizing the composite morphology using SEM/EDS, results revealed that carbon is evenly distributed on the surface of the milled Bi@C/Al, but Bi is not captured, indicating that most of the Bi atoms are encased in the C shell after ball milling process. However, after consolidation of the ball mill powders using spark plasma sintering technique, Bi metal was detected and homogeneously distributed on the Bi@C/Al consolidated surface. Such observation of Bi in the in-situ from the C shell could be ascribed to plasma, joule heating, pulse current, and equivalent pressure in spark plasma sintering process. In determining the volume of hydrogen gas, which each of the samples can produce using water displacement as an advanced analysis technique, it was recorded that the addition of the Bi@C particles into the Al matrix improves its hydrolysis performance, and hence hydrogen yield increases with the increasing Bi@C content. This illustrates that Bi@C exhibits a good catalytic activity behaviour for hydrogen production from Al-H2O reaction. In comparison, the sintered Bi@C/Al composites display a higher hydrogen rate than the Bi@C/Al composites developed using ball milling method. Owing to the transition scanning microscopy characterization with energy-dispersive spectroscopy results, the mechanism resulting in the improved hydrogen gas production for sintered composites refers to the precipitation of AlBi compound along the grain boundaries. Such structure is effective and beneficial for forming an Al corrosion cell with the Bi that will speed up the Al-water reaction. Herein, Optimum hydrogen generation performance of about 1292 ml/g occurred in the sintered Bi@C/Al composites filled with 5 wt% Bi@C particles compared to the milled sample (Fig. 3). This is because of the improved oxidation resistance of the 5 wt% Bi@C particles reinforcement when compared to other reinforcements be it sintered, or ball billed [42]. Though, the agglomeration of reinforcing particles at high temperatures remains a challenge that still needs to be addressed.
Hydrogen production profile for milled and sintered 5 wt% Bi@C/Al composites [41]
Su et al. [31] reported on enhancing hydrogen production characteristics of Al-based composites for on-site hydrogen supply at low temperature. In the study, to achieve the instant hydrogen generation of Al-based composites at low temperature, low melting point metals like Ga, In, Sn, and additives (NaCl, g-C3N4, and LiH) were selected to fabricate Al-based composites with improved reactivity using mechanical ball milling process. To inhibit the produced composite samples from oxidation, the entire ball milling and sampling process were performed under the protection of an argon atmosphere. The hydrogen production test was conducted in a three-necked flask with 300 ml of 23 wt% NaCl aqueous solution placed inside a reaction bath. From the experimental test using a drainage technique, results showed that the NaCl/LiH/g-C3N4/Al-based alloy composites depicted better hydrogen generation performance under 253.15 K condition. The NaCl/LiH/g-C3N4/Al-based alloy composite sample filled with 1.5 wt% LiH displayed a maximum hydrogen production volume of 1095 mL. gAl−1 and optimal hydrogen production rate of 120 mL. gAl−1. This improved Al-based alloy matrix composites on 1.5 wt% LiH and g-C3N4 loadings was validated with linear polarization test, where NaCl/LiH/g-C3N4/Al-based alloy composite depicted higher current density. Another mechanism, which contributed to the better hydrogen performance of NaCl/LiH/g-C3N4/Al-based alloy composite sample remains its antioxidant behaviour [43,44,45,46]. Further, establishing that molybdenum disulfide (MoS2) depicts promising potential as a catalyst for hydrogen evolution reaction and storage because of its layered structure and electro-catalytic activity, as well as its behaviour in lowering the activation energy for hydrolysis [47, 48]. Thus, Kolupula et al. [49] in their study investigated the influence of MoS2 nanoparticles addition on the Al-based alloy (Al-10Bi) composites for hydrogen generation through hydrolysis. The composites were prepared at varying MoS2 contents (0, 5, 10, and 15 wt%) using ball milling and after the ball milling, the composite powders were air-dried and compacted into 1-gram pellets of 15 mm diameter. Characterizing the microstructure of the composites using scanning electron microscope, results depict no special morphology at high magnification. However, at 15 wt% MoS2 incorporation, it was noted that the MoS2 nanoparticles were evenly decorated on the Al-10Bi particle surfaces after the elemental evaluation, as could be seen in Fig. 4. As such the hydrogen gas conversion yield notably occurs with the increasing amount of MoS2 incorporation. 75% and 83% conversion yield were achieved at the pure Al-10Bi and Al-10Bi composites filled with 15 wt% MoS2 nanoparticles, respectively, under tap water environment. According to Yang et al. [49], this reported improved conversion yield in the nanocomposite could be attributed to the possible intercalation of MoS2 used, as well as the formation of nano-galvanic cell and disruption of the hydroxide layer owing to the introduction of the MoS2 particles in the Al-based matrix [50]. In another study, to improve the reactivity of Al with pH-neutral water, as well as avoiding the passivation formation issues associated with the rapid hydrogen generation during Al-H2O reaction systems, Xuan et al. [51] examined the effects of activated charcoal, graphite powder, and carbon black on the hydrogen generation performances of Al-based composites. The hydrogen generation test was conducted in a home-made setup, where 0.25 g of each of the carbon-based material reinforced Al-based composites mixture were loaded into a double-necked bottom flask in a glove box, followed by the addition of 25 mL of 1 M NaOH solution. The structural analysis of the samples revealed that the carbon fillers were uniformly distributed at the Al particles surface, particularly the activated charcoal particles. Hence, the better performance of the Al-based composites filled with the activated charcoal. The hydrogen generation kinetics of the composite systems was improved with the activated charcoal particles additions and as such its maximum conversion in a 1 M NaOH within 1 min and hydrogen generation rate of about 4556 mL/g min, which was recorded to be over 18-fold faster than that of the pure Al. Herein, improved oxidation resistance of the Al-based composites reinforced with the activated charcoal on the other hand resulted in its better hydrogen generation when compared to the pure Al and other reinforcements. An et al. [52] studied the unique occurrence of a morphology transformation process that developed in the grain boundary phase particles of Al-rich alloys doped with different amounts of indium (In) in addition with their hydrogen production performances. In the study, the Al-rich alloy doped with different indium contents (5.58, 5.63, 5.79, 5.85, and 7.50 wt%) was developed using conventional high-temperature melting method. Addition of the indium particles remarkably results in morphological changes of the grain boundaries phase of the produced resultant Al-based materials. Thus, In3Sn intermetallic compounds were formed according to the X-ray diffraction results (Fig. 5), which therein ascribe as morphology transformation grain boundary phase particles. With existence of the In3Sn intermetallic compounds in the Al-based composites due to indium addition, improvement in hydrogen release rate was recorded at lower wt% In dosage as better results were captured at 5.58 wt% In loading. This indicated that at high wt% loading of In, a reduction of energy conversion efficiency of Al occurred. The findings of An et al. work agrees with that of Kwon et al. work. Knowing that precipitates can initiate intergranular and galvanic corrosion of Al due to the difference in corrosion potential of Al matrix and In3Sn intermetallic compound [53].
Morphology of the pure Al-10Bi (a) and Al-10Bi-15MoS2 [48]
X-ray diffraction patterns of the Al-based alloys with different dosage of In a 5.58 wt% In, b 5.63 wt% In, c 5.79 wt%, d 5.85 wt%, and e ternary alloy without Sn element [52]
Wei et al. [54] carried out a study on the effect of Al2O3 on the microstructure and hydrogen generation performance of Al-riched bulk alloys via hydrolysis as Wang et al. [55] reported that Al2O3 could act as a catalyst in increasing the dynamics of hydrolysis reaction. The Al2O3 reinforced Al-based alloy matrix composites were prepared at varying Al2O3 contents (0–2.5 wt%) using conventional smelting and casting process. The XRD results indicated that the alloys contain two phases, which are Al phase and In3Sn intermetallic compound phase. Addition of the Al2O3 results in the appearance of In and InSn19 phase in the alloys and as the incorporation of Al2O3 increases, the intensity of the In and InSn19 peak gradually increases. This suggests that the introduction of Al2O3 particles remains beneficial for the formation of In and InSn19 in the Al-based alloy matrix. Additionally, in the microstructure examination using SEM, it was noted that the introduction of the Al2O3 in the Alloys resulted in the change of grain boundary particle morphology and a reduction in the size of grain boundary particles. From the instantaneous hydrogen generated profiles of the samples measured using a flowmeter advanced analysis technique, results indicated that the Al alloy composites filled with 1.0 wt% Al2O3 possessed the optimum instantaneous hydrogen generation rate of about 228 ml/min g. Meanwhile, beyond 1.0 wt% Al2O3 addition, the reaction rate of Al-water reportedly reduced due to the formation of indium (In) and InSn19 intermetallic compound phases in the resultant composites grain boundary, which in turn reduced the formation of low melting point component of gallium-indium-tin (Ga–In–Sn) in the grain boundary. As such, the active site of Al–H2O was reduced as there is no existence of low melting elements like indium that could contribute to activating the Al-water reaction that could improve hydrolysis rate. Herein, it could be concluded that the high density of the intermetallic compounds in the composite grain boundaries is detrimental to Al-water reaction and hydrolysis rate, though the impact of the intermetallic precipitates is not much at high temperature exposure as revealed in the DSC test analysis, which agrees with the eutectic reaction temperature of Al and In3Sn [56, 57]. Thus, the need for future study on the optimization production methods in producing Al-based (Al–Ga–In–Sn) electrodes for hydrogen generation via hydrolysis. In another study, Wang et al. [58] examined the impact of iron (Fe) on the hydrogen generation of Al–Bi–Sn composite powders. The composites were prepared at stable 10 wt% Bi, 7 wt% Sn, and varying wt% Fe particles (0, 0.5, 1.5, and 3 wt%) using the gas atomization technique, which involves inductively pre-melting of the Al, Bi, Sn, and Fe bulk powders using a high-frequency induction melting furnace. With the microstructure analysis results performed using SEM, it was discovered that the Fe particles were uniformly distributed throughout the Al alloy matrix. The uniform dispersion of the Fe in the matrix demonstrated the possibility of metal galvanic cell/Al formation that could accelerate the hydrolysis reaction. However, investigating the hydrogen production after the reaction of the composites with NaCl and CaCl2 electrolyte solutions, results indicated that the Al alloy reinforced with 1.5 wt% Fe powder particles extremely depicted fast hydrogen production rate at 50 °C that reached 1105 mL/g within 27 min in distilled water, 1086 mL/g in 15 min in 0.1 mol/L NaCl solution, and 1086 mL/g in 15 min in the 0.1 mol/L CaCl2 solution. Additionally, with the antioxidant test, it was noted that the incorporation of the Fe in the Al alloy did not only promote its hydrogen gas production rate but also improved its oxidation resistance. The formation of Al3Fe intermetallic compounds in the equilibrium solidification structure of the composites and their corrosion potential, which is reportedly higher than that of the Al alloy, contribute to the better hydrogen production recorded in the composites [59]. Knowing that galvanic influence could be induced between the intermetallic compound and the Al matrix powders, in turn accelerating the hydrolysis of Al to produce hydrogen [60]. More so, surface cracks, which could have existed in the reinforced Al-based composite powders with 1.5 wt% Fe, on one side results in unstable and high chemical reactivity of the resultant composite with water [61, 62]. As such, the improved hydrogen gas generation rate in the Al-based composites loaded with 1.5 wt% Fe [61] as revealed in Fig. 6.
Amberchan et al. [63] in their work examined the use of a gallium-rich Al composite in improving the formation of nano-Al particles, as well as facilitating their ability to split water to produce hydrogen at ambient conditions. The resultant composites were prepared by solution mixing processes. The findings illustrate that gallium (Ga) functioned as a solvent in dissolving the Al into nano-Al sites in a Ga matrix under room temperature with minimal energy input. These nano-Al particles formed within the framework of Ga were also confirmed using the SEM/EDX and XRD analysis. Additionally, Ga herein acts as a hindrance to the formation of passivating oxide layers, Al2O3 in particular and as such the existence of bimetallic composite for hydrogen production from water illustrates that significant amounts of hydrogen gas can be achieved in 15 min under ambient conditions. However, evaluating the water splitting and on-demand hydrogen production through the Grotthuss mechanism, it was noted that the H2O splitting reaction required no applied potential observing that splitting happened under room temperature conditions and neutral pH rapidly generate 130 mL of hydrogen per gram of Al alloy. Furthermore, the study established that all commercial Al and any available water source can be utilized in hydrogen generation. However, in line with Amberchan et al. [63] study on the use of any form of aluminium material and followed by the concept of waste-to-hydrogen as an attractive solution for producing a zero-carbon fuel from material [64,65,66]. Urbonavicius et al. [67] in their study explores the feasibility of employing Al scrap waste from the construction industry for hydrogen generation through hydrolysis. NaOH of 99% purity pellets were employed for the preparation of the alkaline solution used in the study, where distilled water was utilized as a base for the alkaline solution. For the hydrogen generation, a custom-made laboratory stand was used by reacting Al waste scrap powder with the alkaline solution at different molarity or molar concentrations (0.24, 0.5, and 1 M) and Fig. 7 depicts a schematic diagram of the reactor system. The elemental composition characterization revealed that the waste aluminium constitutes 94.3% of Al, 0.6% of Mg, 5.0% of C, and 0.1% of oxygen (O). Here, the origin of the surface carbon could be ascribed to the metal working process during the Al profiles production.
The hydrogen production reaction (hydrolysis) scheme [67]
The presence of Mg on the other hand referring herein, is use in improving the strength, corrosion resistance, and good wettability properties of the industrially used Al [68,69,70]. The reactions between the Al and water at varying temperatures (20, 30, 40, 50, 60, and 70 °C) were conducted with different alkaline molarity at 0.2 g Al and 50 mL alkali solution. The results indicated that as the temperature of the 0.24 M alkali solution increased, the kinetics of hydrogen generation remarkably improved while the cumulative change in hydrogen yield was negligible as 250 mL/h of hydrogen gas occurred at 20 °C temperature and 260 to 270 mL/h of hydrogen gas was recorded at 30–70 °C. The Al-water reaction was completed in about 140 min at 20 °C and in approximately 30 min at 60 °C and 70 °C temperature. Moreso, similar correlations were noted by increasing the solution to 0.5 and 1 M, where H2 yield reached about 260 mL/h. Based on the findings, it was deduced that temperature plays a significant role in the reaction kinetics and appropriate thermal insulation of the used vessel promotes hydrogen generation process owing to the heat dissipated in the exothermic reaction. Zhou et al. [71] studied the hydrogen generation performances of fabricated Al-based composites filled with BiOCl@CNTs particles using ball milling and spark plasma sintering process. In the study, ball milling of the Al powders with BiOCl@CNTs powders were performed in a laboratory planetary ball mill machine. Meanwhile, the BiOCl@CNTs/Al composite powders were sintered in a cylindrical block of 15 mm diameter and a thickness 1 mm using spark plasma sintering (SPS) furnace at different sintering temperatures (300, 400, 500, and 600 °C) and pressures (20, 30, and 50 MPa). Examining the morphology of the sintered composite using a transition electron microscope (TEM), it was observed that the introduction of the BiOCl@CNTs effectively prevented the aggregation of Al powders during ball milling. However, for comparison, the hydrogen performances of the BiOCl@CNTs/Al composite powders and sintered BiOCl@CNTs/Al composites were investigated through hydrolysis. The results showed that the hydrogen conversion yield of the ball milled BiOCl@CNTs/Al was only 68.4%, which corresponds to the release of hydrogen gas of about 864.9 mL/h per gram at 298.15 K. Meanwhile, the hydrogen yield and conversion efficiency of the sintered BiOCl@CNTs/Al composite were recorded to be approximately 1172.9 mL/g and 92.7%, respectively, at the same temperature conditions. Therein, with the obtained results, one can agree that the sintered composite realizes the Al-water reaction at 298.15 K and its response is better than that of the ball milled composite sample without sintering. The improved hydrogen generation performance of the sintered composites was attributed to its surface area properties and cracks that activated the activity of Al, and hence favourable to the Al-H2O reaction compared to the ball milled composite powders. Owing to the BET advanced analysis, the specific surface area and pore size/volume of the BiOCl@CNTs reportedly larger than that of the commercial BiOCl, hence, the better hydrogen generation of the Al-based composite loaded with 7 wt% BiOCl@CNTs powder particles. Additionally, the crystalline structure and crystallinity of Bi and Bi2O3 were increased through the SPS treatment compared to the ball milled sample without any sintering treatment based on the XRD advanced characterization results (see Fig. 8). The only observed challenge here is that the increased Bi2O3 compound might act as a passive oxide layer over a period, and such could reduce the corrosion of the Al in generating hydrogen. Thus, there is a need for future research that will result in more formation of intermetallic compounds, which can enhance the diffusion of Al to Al-H2O sites instead of oxide layer formation.
The X-ray diffraction patterns of the block sintered BiOCl@CNTs/Al and ball milled BiOCl@CNTs/Al powder [71]
According to Zhu et al. [72], the major problem with the application of Al alloys as an electrode in hydrogen production through hydrolysis is their low hydrogen production rate characteristics. With this observation, the authors studied the hydrogen generation performance of Al alloys filled with low melting point elements (Ga, In, and Sn). Herein, the active Al alloy composites were produced using a novel coupled technique-melting-mechanical crushing-mechanical ball milling process to improve the hydrogen generation rate at room temperature. After crushing the resultant composite into powders, the powders were further ball milled with nickel chloride (NiCl2) and cobalt chloride (CoCl2) salts. Characterizing the morphology and phase compositions of the Al-based alloy composite, as well as the reaction products using scanning electron microscope (SEM) coupled with energy dispersed X-ray spectroscopy (EDS) and X-ray diffractometer (XRD). Results indicated that the low point phases contain a high amount of Al that can function as a transmission channel for Al and enhance the diffusion of Al into the Al-water reaction sites. Furthermore, investigating the hydrogen generation performances of the Al alloy composites embedded with NiCl2 and CoCl2 using a gas mass flowmeter with the application of three-neck flask, the results demonstrated that the highest hydrogen gas generation and conversion efficiency could reach 5337 mL per gram per minute for hydrolysis of water with 1-gram active Al alloy composite powder. The formation of In3Sn compound according to the XRD results and the existence of Ni and Co after the Al alloy composite was ball milled with the chloride compounds could function as a cathode and speed the corrosion of Al in the composite powder, hence contributing to the 5337 mL/g min of hydrogen generation rate reported.
3 Aluminium and aluminium-based alloy composite powders and electrodes for hydrogen production through water electrolysis
Mutlu et al. [29] conducted a study on hydrogen generation through electrolysis under subcritical water conditions. Therein, the effect of neat Al, Al-based alloy (Al6013), and Al-based alloy (Al7075) anodes on the electrolysis of water were examined and compared with platinum (Pt) anode. Pt electrode was applied as cathode and the Ag/AgCl electrode was employed as reference electrode. Conducting the electrochemical impedance measurements of Pt, neat Al, Al6013, and Al7075 electrodes at frequency range of 101 to 105 Hz under room temperature conditions. Results indicated that the polarization resistance (Rp) of Pt is very low, meanwhile, the Al6013 alloy happened to display a lower polarization resistance value compared to that of the Al7075 and pure Al, hence one major characteristic that Al6013 will yield better hydrogen. From the cyclic voltammograms analysis of the Pt, neat Al, and the Al alloys in 1 M KOH electrolyte solution and the cathodic and anodic directions taken into consideration, it was observed that the reduction reaction common to the electrodes in the cathode region is the hydrogen ion reduction reaction (1) (Eq. 1). Theoretically, the hydrogen evolution potential under room temperature based on literature is − 0.84 [73], however, owing to overvoltage, there is formation of H2 at more -ve values than the theoretical potential. As such, under room conditions, hydrogen gas starts rising at − 1.29 V at Pt cathode, and − 2 V at the Al alloys and − 2.50 V at neat Al. The use of Pt as the cathode electrode herein could refer to its catalytically active characteristic [74]. Hence, in the anodic area, the oxygen gas formed at the Pt electrode could be expressed as shown in reaction (2) (Eq. 2). In the study, the formation of oxygen gas occurs at a more positive potential value of about 0.53 V as against its theoretical evolution potential, 0.4 V [75] due to overvoltage. Additionally, when Al anodes are used, Al oxidized primarily as presented in reaction (3) (Eq. 3). It was noted that Al starts to dissolve earlier than oxygen while hydrogen gas output of the Al electrode is earlier and that evidence the role of Al in hydrogen generation.
Thus, in alkaline electrolytes, theoretically, more hydrogen is usually ascertained from the Al as is expressed in reaction (4) (Eq. 4) and reaction (5) (Eq. 5). The hydrogen ion formed in the reaction (reaction 4), in addition with half moles of hydrogen ion, leads to cathodic corrosion of the Al according to the reaction presented in reaction 5. However, the detailed mechanism of hydrogen evolution reaction during the anodic polarization of Al could be found in [76]. From the current vs potential graphs of the electrodes at the speed of 5 mV/s (Fig. 9a), the Pt depicted the highest catalytic activity at the cathodic region among the electrodes. Meanwhile, neat Al begins to dissolve earlier than Al6013 and Al7075 alloy. Therein, examining the current against time curves of the neat Al, Al6013 and Al7075, and Pt anode electrodes against Pt as a cathode electrode during the generation of hydrogen at constant potential of 2 V, the lowest current occurred in Pt anode while the neat Al depicted the highest current (Fig. 9b). Although, the highest hydrogen yield was recorded in the Al6013 material considering each sample mass loss and Eq. (6) being applied [77]. Hence, the cost-effective and efficient electrode is the Al6013 even above room condition, particularly when comparing Pt anode and Al6013 anode electrode (Fig. 10a and b).
a Current vs potential curves and b Current vs time curves of the electrodes in 1.0 M KOH at 25 °C [29]
a Energy consumption and (b) efficiency of the electrodes during H2 generation under 130 °C and 20 bar pressure at 2 V constant potential [29]
Where \(\eta\) is the percentage yield, \(UIt\) is equal to the energy consumed (Q). \(U\), \(I\), and \(t\) represents the potential of the system, applied current, and time, respectively.
Positing that electrolysis requires external electrical energy via electrodes to split water into hydrogen and oxygen and that an efficient electrolysis needs suitable electrodes to minimize potential during hydrogen production. Saklin et al. [33] in their work explores the use of Al and Cu coated Al in hydrogen production. The Al and Cu coated Al were basically adopted at different combinations of anodes and cathodes in finding out more effective electrodes combinations in hydrogen generation through electrolysis. In the study, the solution of NaCl in rainwater was used as electrolyte. The different combinations of the anodes and cathodes used in the study were Al–Al, Al–Cu(Al), Cu(Al)–Al, and Cu(Al)–Cu(Al) electrodes, where the first electrode in all the combination is anode and the second is the cathode. For every electrolysis of rainwater with varying molarity of NaCl electrolytes (0.034, 0.051, 0.068, and 0.086 M), the electrodes were connected to an external direct current voltage supply. In each of the experiments, two different measuring cylinders containing water were positioned in a way over the electrodes so that the product of the electrolysis could be collected via the downward replacement of water in the cylinder as revealed in Fig. 11a where the volume of the hydrogen gas was obtained by the measuring cylinder. Meanwhile, as there are quite a number of expressions for computing electrolysis efficiency [78,79,80], the electrolysis efficiency was computed in the study employing the mass flow rate of hydrogen and enthalpy with respect to electrical energy (see Eq. 7).
Where the symbol \(\eta\) represents the electrolysis efficiency (%), \(Q\) denotes the mass flow rate of hydrogen (kg/sec), \(H\) stands for the enthalpy of combustion of hydrogen (J/kg), \(V\) is the applied voltage (Volt), and \(I\) is the current (A).
From the experimental results, Cu(Al)–Al electrodes combination depicted the optimum hydrogen generation of approximately 0.130 mol/hr followed by that of the Al–Al electrodes combination as depicted in Fig. 11b. This means that the Cu(Al)–Al functioned as the most sensitive electrode combination and such could be excellently explained by the primary or basic understanding of the electrical characteristics of Al and Cu. Knowing that the conductivity of a neat Cu at 20 °C is about 5.96 × 107 S/m [81, 82], which is higher than that of the virgin Al (3.77 × 107 S/m) [83]. This indicates that Cu coating on the Al enhances the conductivity of the Cu(Al) hybrid material, which agrees with the report presented in [84] on Cu clad Al wire experiment. As such, the intrinsic potential difference between the anode and cathode when compared to the other electrodes combination. Hence, the potential difference remains the mechanism responsible for the optimal generation rate and efficiency recorded at the Cu(Al)–Al electrode combination. From the experimental results, it is worth mentioning that the best results were obtained at the 0.086 M electrolyte solution and the outcome of the study evidenced that the higher the molarity concentration the better the efficiency. It was concluded that the produced electrode is easy to apply in an electrolytic cell, though required trivial effort and was reported to be costly, hence future research work is needed to maximize cost.
Furthermore, as the interest in hydrogen energy is increasingly expanding due to the rising greenhouse gas emissions and the depletion of fossil resources, Reda et al. [34] based their study on employing affordable aluminium alloys for hydrogen generation, as well as identifying the most effective and efficient heat-treated Al alloy with hydrogen production performances through water electrolysis. However, in the study, 1050-T0, 5052-T0, 6061-T0, 6061-T6, 7075-T0, 7075-T6, and 7075-T7 are the heat-treated Al alloys, which was examined as electrodes in a water electrolyser at a constant voltage of 9 V for hydrogen generation. On applying a constant of voltage 9 V to ascertain the Al alloy with less energy consumption and greater hydrogen flow rate at higher process efficiency, results demonstrated that the 6061-T0 Al alloy type possesses the best hydrogen production compared to the other Al alloys type. This is because applying the 6061-T0 Al alloy as the electrode material under a 60 °C temperature yielded the maximum hydrogen flow rate of 475 mL/min of efficiency (≤ 50%) with a low energy consumption of about 1182 J/mL. The surface area of the 6061-T0 electrode in addition with the formation of intermetallic compounds in structures as revealed by XRD analysis after the heat treatment on the other hand remains the mechanism, which attributes to its better hydrogen generation performance. In another study, Sunaryo [85] presented hydrogen as an alternative energy source and its production through the electrolysis process of sea water. Using sea water (3.5%) as the electrolyte solution, copper as the anode electrode, and Al as the cathode electrode. The authors conducted hydrogen production using electrolysis method (direct electric current power supply) [85] under electrolysis time of 2, 4, 6, and 8 min, and varying voltages (5, 10, 15, 20, and 25 V) at an operating temperature of 36 °C. Herein, it is worthy to note that all the experiment was performed in an electrolysis reactor and for the hydrogen production rate and the hydrogen yield, Eqs. (8) and (9) was adopted [85]. From the experimental results, it was noted that voltage significantly affects the dissolution of seawater into H2. Thus, the optimum H2 flow rate of approximately 6545.51 ml/h was recorded at 20 V within 8 min [85].
where the rated gas volume means final volume of hydrogen gas produced at the end of the electrolysis process and previous gas volume refers to the volume of hydrogen gas produced at the beginning of the electrolysis process.
Aziz et al. [86] in their study observed that a good electrode for utilization in the electrolysis of seawater process is one capable of conducting electricity with good corrosion resistant, hence they conducted a comparison study on the use of Cu and Al electrodes in the electrolysis of seawater technique to generate hydrogen gas. In the study, the electrolysis of seawater process in generating hydrogen gas using Cu and Al electrodes occurred at a constant voltage of 12 V. The results indicated that the Cu electrode generated 732 ml of H2 and a lifetime of 820 min with an average rate of 0.89 ml/min and 3.83% hydrogen yield under 400 to 440 min. Meanwhile the use of Al electrode yielded 637 ml of H2 and a lifetime of 680 min with an average rate of 1.02 ml/min and the highest hydrogen yield of 5.92% at 120 min. However, owing to the Al cost-effectiveness and availability [87, 88], there is a need for future study on improving its efficiency in the electrolysis of seawater for sustainable green hydrogen production. Ojaomo et al. [89] conducted a study on the hydrogen production via the electrolysis of NaCl for clean energy development. In the experimental test, direct current of 12 V was applied, and hydrogen gas was notably produced from the hydrolysis of H2O utilizing Al electrode and brine (NaCl) as an electrolyte solution. Results show that hydrogen produced from the hydrolysis of 250 g of brine in 360 min yielded a mass of 2.7 grams with an equivalent of 1.4 mole and volume of 30 L. Additionally, the study revealed that the higher the current the higher the volume of hydrogen generated when the concentration of conducting solution is constant. For the reaction process in generating the hydrogen gas when current was passed through the electrolysis of the brine (NaCl solution), it was noticed that the positively charged Na and H ions move to the cathode by gaining electrons thereby form Na and H atoms. Thus, the negatively charged chloride and oxygen ions on the other side move to the anode part by losing electrons to form Cl and oxygen atoms. These atoms were able to join up in pairs to form H and Cl molecules and as such hydrogen gas (H2) and chlorine (Cl) gas was formed at the cathode and anode, respectively [90]. Furthermore, authors like Buryakovskaya and Vlaskin [91], Aylikci et al. [92], Zhang et al. [93], and Xiao et al. [94] also deliberated on the production of hydrogen using aluminium and aluminium composite materials through hydrolysis and electrolysis and their findings were also presented in Table 1.
4 Challenges associated with the use of aluminium-based materials in hydrogen production via hydrolysis and electrolysis and recommendations for future research
In this study, the use of aluminium and aluminium-based materials in the generation of hydrogen through water splitting electrolysis and hydrolysis processes were reviewed. Study indicated that utilizing an Al anode promotes the generation of clean hydrogen as it hinders the formation of oxygen gas in an electrolyser [29, 95, 96]. This positioned aluminium as one of the representative materials for on-site and real-time hydrogen generation [97,98,99,100]. Water electrolysis and hydrolysis remains the most adopted method to produce green hydrogen using Al and Al-based composite powder and electrode materials, which involves the use of alkaline electrolysers and Al-water reaction. The use of Al-based materials either in composite powder or cylindrical block composites for the production of hydrogen are basically produced by mechanical ball milling, melting, casting, and spark plasma sintering processes [20, 28, 31, 35, 54, 72]. Aluminium availability and recyclability suggest its use in hydrogen generation, green manufacturing, and sustainable circular economic development [28, 101, 102]. Al and Al alloys have widely been used in the preparation of catalytic materials for green hydrogen production either by hydrolysis or water electrolysis. However, Al-water reaction long induction period and low conversion efficiency at low temperature have greatly limited its application for fast hydrogen production as regards previously reported study [30]. Aluminium on itself found it difficult to react with water for hydrogen generation and when reacted with water there exists the formation of a passivation problem during Al-H2O reaction, which hinders the rapid hydrogen generation [55, 103]. Moreover, study by Liao et al. [41] purported that a dense oxide film easily forms on the aluminium particles in air, which in turn impedes the Al-water reaction for efficient hydrogen generation. During the electrolysis of water for the production of hydrogen, low dielectric constant and electrochemical properties performance of Al electrodes, which results to long induction time for a chemical reaction to take place, overpotential, and cost reportedly limits the potential needed of Al for hydrogen gas generation [30, 33]. Notwithstanding, numerous studies have been conducted to improve the reactivity of Al with water and/or alkaline solution in producing hydrogen gas without the formation of passive oxide layers, like Al2O3. The fabrication of Al and Al-based alloy composites with the addition of catalysts using melting, ball milling, and spark plasma sintering method have been reported as a means of accelerating the activities of the Al site during hydrogen generation through hydrolysis [41, 51, 52, 104]. Hence, it is clear that the improvement of hydrogen gas rate and hydrogen gas conversion efficiency using Al and Al alloy composite electrodes through hydrolysis and water electrolysis remains a challenging task. However, to address these challenges associated with the Al-based electrode materials in producing hydrogen gas through hydrolysis and water electrolysis even with the integration of renewable energy sources like solar, the material combination for hydrogen production electrodes should depict good electrochemical properties including high current density, corrosion potential, low overpotential, and low cost. As such, the authors recommend the incorporation of superalloy powder additives, such as Inconel 718 and Haynes 282 either in micro or nano size during the fabrication of Al-based composites using spark plasma sintering or melting process. Knowing that little or no such studies have been reported in the literature in alignment with the United Nations sustainable development goal (SDG) on ensuring access to affordable, reliable, sustainable, and clean energy for all, as well as energy efficiency improvement. Haynes 282 is a newly developed strengthened nickel-cobalt material for use in energy applications [105, 106]. Haynes 282 offers both potential cost savings and improve manufacturability [107]. Haynes 282 material addition as an additive into the Al alloy matrix can result in intermetallic precipitates that could form along the resultant composite grain boundaries in promoting the acceleration of Al alloy corrosion rate on exposure to alkaline electrolyser and increases hydrogen kinetics of the alloy matrix. Formation of precipitates in a composite material basically initiate intergranular corrosion and/or galvanic corrosion especially when two electrode material with different electrochemical characteristics are electrically connected in a corrosive alkaline or acid solution [108,109,110]. This phenomenon reportedly results in a high rate of hydrogen gas noting that hydrogen production rate from hydrolysis of active metal in alkaline water is linearly proportional to its corrosion current density [20]. Inconel 718 on the other side exhibits similar properties with that of the Haynes 282, though they possess different chemical compositions [111]. Additionally, introduction of superalloy powder additives in the Al matrix composite electrodes is considered to improve the resultant composite dielectric constant, as well as its conductivity for fast chemical reaction during water electrolysis. Knowing that nickel-based superalloys as a unique class of engineering materials depicts good oxidation resistance and rise in electrical conductivity as a result of precipitate [105, 112,113,114,115]. Hence, the fabrication and characterization of Al-based composite electrodes with the addition of Inconel 718 and Haynes 282 powder additives as reinforcement remains a futuristic study that could be a technological breakthrough of obtaining optimized and effective resultant Al-based electrode material with improved antioxidant properties for green hydrogen generation and conversion efficiency via hydrolysis and water splitting electrolysis.
5 Conclusions and outlook
This review paper centred on the production of hydrogen using aluminium and aluminium-based materials through hydrolysis and water splitting electrolysis. However, recent reported study work on the utilization of Al and Al-based alloy composite powders and compacted Al-composite in hydrogen gas generation through Al-water hydrolysis and electrolysis were reviewed. The enhancement in the development of Al and Al-based alloy composites filled with active additives for improved hydrogen generation rate with reduced energy consumption, as well as promoting the process efficiency, remains state-of-the-art within the research community. Hydrogen could be generated employing different primary energy sources like solar, for the environmental/health benefits. Hence, the decarbonization of the energy, as well as transport system, which is of special interest remains green/clean hydrogen produced from renewable electricity by electrolysis. In the production of green hydrogen, water electrolysis and hydrolysis processes using Al and Al-based composites were noted to be effective. This depict that the introduction of active additives into the Al and Al-based alloy matrix could improve its corrosion current density and electrochemical properties acceptable for green hydrogen generation employing alkaline electrolysers. Furthermore, as composite materials are usually produced by different method, herein, it is clear that mechanical ball milling, melting, smelting, and powder metallurgy techniques, including spark plasma sintering are increasingly utilized in the preparation of active Al-based composite electrodes for hydrogen generation either by hydrolysis or water splitting electrolysis. As regards to the Al, Al alloy, and Al-based composites hydrogen generation performances and the data reported in Table 1 and potential properties of Al-based composite electrodes. There is still uncertainty regarding the use of Al and Al-based composites for hydrogen production via water electrolysis and hydrolysis as they are characterized with low dielectric constant and poor electrochemical properties performance, which results to long induction time and high overpotential for a chemical reaction to take place. And such high overpotential (overvoltage) leads to high energy consumption and cost. Hence, additional research studies adopting nickel-based superalloy powder additives in preparing Al-based composites that could improve hydrogen yield rate and efficiency with low energy consumption should be further explore. As such research might be a breakthrough in obtaining more optimized active additives and resultant Al-based composite with improved morphology and characteristics including electrode surface area for rapid hydrogen generation. Thus, the authors are of the opinion that future research study should be on bridging the gaps, which limits the hydrogen gas production by developing a suitable Al-based composites with improved electrochemical properties for effective green hydrogen production through hydrolysis and water splitting electrolysis.
Data availability
No datasets were generated or analysed during the current study.
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Acknowledgement
This study is based on the research supported by NRF of grant number 150574 and Centre for Energy and Electric Power, TUT.
Funding
This work is based on the research supported wholly/in part by the National Research Foundation of South Africa (Grant Numbers: 150574); and Tshwane University of Technology—Faculty of Engineering and Built Environment and Centre for Energy and Electric Power.
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V.E. wrote the main manuscript; O.M. prepared the figures; A.P.I. formatted the manuscript; O.M. and A.P.I. proof read the manuscript.
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Ogbonna, V.E., Popoola, O.M. & Popoola, A.P.I. Aluminium-based electrode materials for green hydrogen production through electrolysis and hydrolysis: a review. Discov Mater 5, 185 (2025). https://doi.org/10.1007/s43939-025-00263-1
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DOI: https://doi.org/10.1007/s43939-025-00263-1