Using liquid metals, it is possible to generate hydrogen and heat from aluminum-water reaction (AWR) through mechanochemical activation of the aluminum. 1 Researchers demonstrated accelerated hydrogen production from aluminum and seawater by adding accelerators like imidazole and caffeine to the solution.
Aim of the Study: To recover gallium-indium eutectic used as a surface coating to induce aluminum’s reactivity in water.
Accelerated Hydrogen Production from Aluminum and Seawater
In this study, researchers examine the recovery of gallium-indium eutectic (eGaIN). This material is used for the surface treatment of aluminum. Scientists aim to reuse this material for further aluminum activation. Further, the study evaluates how optimization of reaction conditions and chemical accelerators takes place to efficiently generate hydrogen while recovering eGaIn.
Highlights
- Activated aluminum reacts with water and generates heat, hydrogen gas, and aluminum oxyhydroxide (a non-toxic and valuable commodity).
- It is a cost-effective and efficient method of hydrogen production and transportation.
- Rapid reactions occur under 10 minutes after a small amount of imidazole is added to seawater.
- Sudden reactions enable the retrieval and reuse of more than 90% of gallium-indium eutectic.
- 99% of the anticipated hydrogen output was produced based on aluminum’s mass.
- A Swift and complete reaction of aluminum in saltwater was observed when the reaction was conducted at high temperatures.
Also, to reduce cost and improve the sustainability of the process, recycling of indium and gallium is important. To prevent corrosion, a protective oxide layer forms when aluminum comes in contact with oxygen. This oxide layer needs to be disrupted to generate hydrogen with high power densities.
Role of Gallium and Indium in Producing Green Hydrogen
AWR takes place when activation with liquid metal eutectic uses low-melting point metal alloys to weaken the aluminum. This allows water to penetrate the oxide layer. Here, gallium and indium have important roles. Gallium enters the oxide layer and indium allows the alloy to reach the grain boundaries.
Further, researchers reduce the ductility and hardness of the material by the Rehbinder effect. It results in disrupting surface oxide films and enables eGaIn to penetrate the aluminum. It is important to prevent any changes while recycling indium and gallium during the reaction.
As per prior studies, the recoverability of eGaIn in aluminum activation is ensured by enhancing AWRs. Another study concludes that, aluminum reacts with water and produces hydrogen, heat, and aluminum oxyhydroxide (AIOOH). It also offers a high energy density of 86 MJ/L, which is twice as much as diesel and 40 times more than Li-ion batteries.
Around half the energy involved in the AWRs releases in the form of gaseous hydrogen. The remaining half release as thermal energy ranging between 400-450 kJ mole of aluminum. Here is an equation to demonstrate the process. Q1 and Q2 represent the heat released by each reaction.
- Al + 2H2O/3 2 H2 + AlOOH + Q1 (Equation 1)
- Al + 3H2O/3 2 H2 + AlðOHÞ3 + Q2 (Equation 2)
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Observations and Results
Recovery in Ionic Solutions to Produce Hydrogen
After a 12-hour reaction of an activated aluminum pellet in 3.9 M NaCl solution, liquid eGaIn particles emerge. These formations and merging of these particles continued with progress in the reaction.
Electron microsopy and X-ray diffraction (XRD) analyses shows that liquid metal phase has pure gallium-indium alloy. High concentrations of aluminum and oxygen surrounds it.
However, the analysis to understand the differences between ionic solutions, including molecular structure and concentrations, is yet to be done. This will clear the rates of reaction and recovery rates.
The figure shows progression of hydrogen generation over the time. This includes AWR in different solutions like adding salts, or sulfates under isochoric conditions. There is an inverse exponential growth pattern reaching an asymptotic limit. The image below shows 2 different regimes.
- 1st regime – Standard AWR in DI water characterized by the reaction which begins after 30 seconds of induction time and completes within 5 minutes.
- 2nd regime – It was noticed in all substances containing chlorine. It shows slow reaction rate and completes within 250 to 1250 minutes (approx. 4 to 21 hours).
Observations
- Repeated experiments confirm that the reaction slows down with the presence of chlorine.
- Some sulfates decelerate the standard reaction 0.5 M MgSO4, 0.5 M CaSO4, 0.5 M Na2SO4, and 0.25 M K2SO4.
- Others maintain high reaction rates such as 0.5 M FeSO4 and 3 M Al2ðSO4Þ3.
Relation Between Ratios and Reaction Rates
The table below shows the outcomes of various experiments conducted in various solutions containing sulfates and salts. The aim of this was to analyze the relation between recovery ratios and reaction rates.
| Type of Solution | Recovery Ratio (±5) | Reaction rate (L/min/kgAl) |
| 3 M NaCl | 100.00 | 2.22 |
| 0.5 M Na2SO4 | 97.12 | 3.48 |
| 0.1 M Al2 (SO4) 3 | 0.00 | 212.65 |
| 0.1 M Fe SO4 | 0.00 | 1,159.42 |
| 0.1 M Ca SO4 | 0.00 | 12.71 |
| 0.25 M K2SO4 | 80.64 | 3.03 |
| 0.5 M Mg SO4 | 100.00 | 5.62 |
Observations
- Researchers find notable interdependence.
- High recovery ratios correlate with low reaction rates.
- Rapid reactions inhibit recovery in salt and sulfate solutions particularly.
- Recovery in DI water is low or non-existent due to the direct reaction of the eutectic with water.
With the progression of AWR, eutectic material in grain boundaries expelled as particles’ size range from micrometers to milimeters. - High zeta-potential values stabilize the suspensions by facilitating the effective repulsion of particles through electrostatic forces.
- eGaIn being negatively charged attracts positively charged ions which depend on its zeta-potentials and colloidal stability for recovery.
- A high absolute value of potential difference prevents the particles from approaching each other for coalescence.
According to the pattern, reaction kinetics have a direct influence on the efficiency of eGaIn recovery in such solutions. Although the AWR occurs in most aqueous media, various factors affect the activating elements’ recovery. These factors include the ionic species and the temperature of the solution.
Using in-vehicle reactors for generating hydrogen for transport applications seems challenging in achieving efficient eGaIn recovery and fast reactions. It is crucial for higher hydrogen production rates to power engines.
Chemical Accelerators for Green Hydrogen Production
A simple accelerator in household product testing, caffeine for the AWR ionic solutions. Luo et al. in a previous study already emphasized the use of caffeine complexes as catalysts in various cross-coupling reactions.
Moreover, recent biomedical studies have used caffeine as an agent for gallium and aluminum. It indicates caffeine’s potential in bonding interactions with metals involved in the AWR process.
Overall, the studies show that the characteristics of caffeine are safe and swift in absorbing molecules. They have the ability to form bonds with other substances. This further amplifies its attractiveness as a viable accelerator in this situation.
Observations
- Edible coffee has a higher reaction rate.
- Reseachers isolate caffeine, the main component, then testing it using high-quality reagent grade >99%.
- The reaction rates and hydrogen yields showed consistency across these different concentrations. The reaction time in this experiment was around 5 minutes in all cases.
The following figure demonstrates the results of testing caffeine.
Testing Imidazole in Salted Water
Imidazole, the cyclic component found in caffeine’s molecular structure, was tested too. Researchers tested the effect of various concentrations of the imidazole in salted water. It provided a better understanding of the microscopic mechanisms at play.
Observations
- Reaction rates were increased significantly when different amounts of imidazole were added, ranging from 0.02 to 1 m.
- Even with high salt concentration (0.6 to 4 m NaCl), the reactions occurred within 20 minutes.
- With increasing concentrations of imidazole or caffeine, the eGaIn recovery ratios notably decreased.
- The best recovery was achieved with 33% obtained at a concentration of 0.001 M.
- Better recovery ratios of around 90% were noticed with imidazole when its concentration was reduced to 0.02 M.
Fascinating insights into the influence of compounds like imidazole and coffee on the AWR in ionic solutions were revealed through this experiment. The reaction rates increase in all cases with the presence of free nitrogen atoms bonding to the surface of the metals. Moreover, the recovery ratios were affected by the strength of dipole moment, diverse molecular structure, varying geometry, and electronegativity.
Initial Temperature Effects
The recovery of eGaIn remained high at around 90% in 0.6 M NaCl at 80° C and around 77% at 90° C. Such results seem to encourage hydrogen production in vehicle engines. However, a decrease in recovery at 90° C indicates a limitation suggesting the presence of a temperature threshold that affects the eGaIn recovery efficiency. Overall, this insight is helpful in optimizing the AWR process for practical application, especially in scenarios like seawater conditions.
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Seawater Testing, Scaling Up, and Reuse of eGaIn
To verify the operability of the experiment, seawater was tested with and without accelerators. Researchers collected water from Revere Beach in Revere, MA, USA, which is fed by the Atlantic Ocean. Trials were conducted using the same methods.
Observations
- At room temperature, the 0.6 M NaCl solution and real seawater have similar behavior in terms of hydrogen production and reaction speed.
- The addition of chemical accelerators such as caffeine or imidazole and pre-heating the saltwater resulted in increased reaction rates.
- Consistent reaction rates and recovery ratios remained consistent throughout the experiment.
- Activated aluminum mass increased to more than 50 g from 5 L seawater solution.
- The weight of the recovered material was more than the eGaIn input mass. It further proves the presence of additional elements.
- After the reaction took place in DI water for 24 hours, the eGaIn separated from other materials.
- The separated eGaIn have recovery ratios from 90% to 100%.
Thereafter, the recovered eutectic was reused to activate more fresh aluminum. The consistency observed throughout the research demonstrated the possibility of eutectic being recycled several times. Through this, it is possible to activate more aluminum due to the ionic solutions.
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Conclusion
In conclusion, eGaIn recovery relies on EDL formation. Cost reduction is possible due to the reactivation of aluminum pellets with eGaIn for hydrogen production. Seawater with 0.6 M NaCl was used in hydrolysis with accelerators. Chemical accelerators like imidazole and caffeine showed positive impacts on reaction and recovery rates.
Moreover, energy balance is important for hydrogen storage. Around 2% of the total energy output is required for aluminum treatment for fuel production. It further highlights the high storage capacity required by it. Thus, researchers are continuously analyzing the cost and carbon footprint of the process. This will help in determining the economic viability and sustainability of the technology.
