A recyclable energy cycle using a pulsed laser and base-metal nanoparticles is proposed. In this energy cycle, iron nanoparticles reduced from iron oxides by laser ablation in liquid are used for hydrogen generation. The laser energy can be stored in the base-metal nanoparticles as the difference between the chemical energies of iron oxide and iron. According to the results of an experiment on hydrogen production using the reduced iron nanoparticles, the reaction efficiency of the hydrogen generation at a temperature of 673 K was more than 94% for the ideal amount of generated hydrogen.
It has been expected that the application of hydrogen, produced by using clean, renewable energy, such as solar power, will solve the problem of global warming and depletion of fossil fuels. Many researches on hydrogen production have been conducted around the world [
The metal oxides can be reduced to the metal and broken into nanoparticles by using pulsed-laser ablation in liquid [
As shown in Figure
Proposed energy cycle using solar energy.
Small-scale solar-pumped laser.
The required energy for reducing iron metal oxides is determined as follows. Fe2O3, Fe3O4, and FeO are generally known as “iron oxides.” According to crystal structure, there are many types. The chemical formula for the reduction of Fe2O3 is given as
The following metal-oxide powders were used to reduce and produce metal nanoparticles: Fe2O3 powder, 98% purity, mean size: 45
Experimental setups based on laser ablation in liquid to reduce metal-oxide powders are shown in Figure
Experimental setup for laser ablation in liquid. (a) Reducing Fe2O3 powder with 10-Hz Nd:YAG pulsed laser, and (b) reducing Fe3O4 powder with high-repetition-rate Nd:YAG pulsed laser.
The experimental setup based on laser ablation in liquid to reduce Fe3O4 powders by using a high-repetition Nd:YAG pulsed laser is shown in Figure
Fluence (J/cm2) and the intensity (W/cm2) of the irradiating laser pulses are important for reducing metal oxides by laser ablation in liquids. The fluence must be adequately more than 0.1 J/cm2 to heat the metal-oxide particles to a high temperature over their melting point, and the intensity must be over sub 0.1 GW/cm2 due to induction of avalanche ionization and extraction of electrons from the metal oxides.
The conditions for the irradiation intensity of the laser pulses and the high absorption coefficient are adequate for reducing Fe3O4 powders. However, for reducing Fe2O3 by using the high-repetition Nd:YAG pulsed laser, the absorption coefficient at 1064 nm wavelength is low, the irradiation intensity and fluence are low, and it has been found that the reduction of Fe2O3 using a high-repetition Nd:YAG pulsed laser is hard. Accordingly, in this study, a low-repetition Nd:YAG pulsed laser was used for reducing the Fe2O3 powder. Additionally, it has already been found in our experiments that the Fe3O4 powders can be reduced by using this low-repetition Nd:YAG pulsed laser.
Analysis of crystal structure by X-ray Diffraction (XRD) (MAXima_X XRD-7000, Shimadzu, Japan) was performed to check the quantities of iron in the Fe3O4 powders irradiated by the high-repetition Nd:YAG pulsed laser. K-
The method used for hydrogen production is described in the following. The chemical formula for reacting Fe3O4 powders with vaporized water is given as
Hydrogen production: (a) photo of instrument for hydrogen production and (b) experimental setup.
When iron nanoparticles were used for hydrogen production, it was necessary to heat them. Compared with these results using reduced Fe nanoparticles, micron-diameter iron powder with a weight of 100 mg irradiated laser pulses was used for the hydrogen production. The reduction process was repeated 10 times. The synthesized iron nanoparticles were also used for the hydrogen production.
The XRD analysis shows that the iron nanoparticles have a lot of
Results of analyzed Fe3O4 powder and reduced iron nanoparticles by XRD.
As for Fe2O3 reduction, it is easy to judge the state of iron oxides by their color when laser irradiation was conducted. The color of the Fe2O3 in the glass bottle changed from red to black. And some reduced iron nanoparticles became glossy. The color of the iron was silvery-white. As for Fe3O4 reductions, when the reduced iron was observed, the color of a few iron particles was gray, and most of the remaining powder was black. However, inside the reduced iron was pure iron, and only the outside of the reduced iron was oxidized. The mean size of the produced iron nanoparticles was measured by a particle-size analyzer to be a mean size in the order of 20 nm.
Density of generated hydrogen was measured by a hydrogen meter (Finch-Mono II, Japan). The obtained hydrogen was diluted once with 500 mL air, and the density was measured. The density of the hydrogen was compared with that of the hydrogen produced by electrolysis. It was thereby confirmed that the hydrogen concentration was near 99% and almost the same as that produced by electrolysis.
Hydrogen production was conducted using reduced iron nanoparticles from Fe2O3 powder. The experimental results are shown in Figure
Hydrogen generated by using Fe2O3 after laser-pulse irradiation.
Hydrogen production was produced by using 100 mg of pure iron powder in the reactor. The experimental results are shown in Figure
Hydrogen production using pure iron powder.
Hydrogen production was conducted using iron nanoparticles (with a weight of 100 mg) made from pure iron powder irradiated by a high-repetition-rate microchip Nd:YAG pulsed laser in water. The experimental results are shown in Figure
Hydrogen production using pure iron and Fe3O4 powder after laser-pulse irradiation.
Finally, hydrogen production was conducted using reduced iron nanoparticles (with a weight of 100 mg) from Fe3O4 powder irradiated by the high-repetition-rate microchip Nd:YAG pulsed laser in liquid. The reactor was heated at 523, 573, 623 and 673 K, and 25 mL of pure water was used. The experimental results are shown in Figure
Fe2O3, Fe3O4, and iron powders were reduced by using laser ablation in liquid. Iron oxide was chosen as a metal oxide for the energy cycle because the Clarke number was the third, and the iron amount in recoverable reserves, which could be mined economically and technically, is about 10 times that of aluminum oxide. Iron is easy to obtain and use.
We discuss the absorption of laser pulses to metal oxides. The absorption process of a laser pulse by metal oxides has two possibilities: normal linear absorption and multiphoton absorption. In this experiment, in laser ablation in liquids, the latter mainly occurs. Smaller particles make electrons in the metal oxides more active, and electron emission will thus occur actively. The absorption coefficients of Fe2O3 and Fe3O4 are markedly different. Fe3O4 powder should be used for the energy recycle because its absorption coefficient at 1064 nm wavelength is high, and after producing hydrogen, Fe3O4 powders are mainly produced.
If a CW laser is used for reduction of the metal oxides, the reduction efficiency will be low because the generated temperature of the metal oxides is low. The CW laser has a low peak power, and it cannot heat the metal oxides up to a melting temperature in a short time. Moreover, the low intensity results in no avalanche ionization and less electron ejection.
The coulomb explosion and thermal ablation had been proposed as the ablation mechanism of the metal oxide in the liquid. The coulomb explosion was mainly considered in this study. It has been suggested that metal-oxide particles are broken into pieces in the Coulomb explosion. Firstly, the metal oxide is heated to over its melting temperature, at which it changes to form a gel. Electrons in the atoms of the metal oxide are ejected by the intense electric field of the irradiated laser pulses. After the particles become positively charged, the metal oxides repel each other by the same positive charge. Finally, a plasma is produced and atomized. In the case of using water for laser ablation, a subreaction occurs. In the coulomb explosion, the ejected electrons and oxide atom react with the water, and OH− ions are generated. As a result, the water becomes alkaline. This phenomenon had already been observed in some experiments.
Two lasers, high- and low-repetition-rate Nd:YAG pulsed lasers, were used to reduce Fe2O3 and Fe3O4 by laser ablation in liquid. It was found that the high-repetition-rate laser attained higher reduction efficiency than the low-repetition-rate laser. The reduction efficiency of the metal oxides generated by using the high-repetition laser pulses was higher than that for the low-repetition pulsed laser for the same averaged laser power. The reduction efficiency will depend on the interaction time between the metal oxides and laser pulses. The emission time of electrons should be determined by a single factor, namely, multiplication of pulse duration and repetitive rate. Thus, it will be possible to reduce 100 mg of metal oxides in less than 10 minutes by using a high-repetition-pulse laser. Moreover, metal with more than 200 mg of oxides can be reduced in 10 minutes. Reduction of metal oxide will occur in single laser pulse.
We conducted hydrogen production by using small instrument for hydrogen production and the reduced iron. Hydrogen was produced by using a small instrument and the reduced iron. As a consequence, the obtained reaction efficiency of hydrogen generation using iron nanoparticles reduced by laser pulses was 94%, which is high enough in the case that the particles have oxide shells. It means that the reduction of the metal particles by laser ablation in liquids was almost complete. Moreover, power generation in the hydrogen fuel cell was confirmed.
If the particle sizes of the iron nanoparticles were less than 10 nm, it is predicted that iron nanoparticles will react with water completely by heating the temperature of the instrument for hydrogen production to be close to 373 K.
For producing hydrogen using iron nanoparticles reduced from Fe3O4, a dispersing agent for protecting the condensed nanoparticles is commonly used. An agent was not used in the present study because the condensation should not affect the reaction efficiency. In the case of using water for producing hydrogen, the surfaces of the produced iron nanoparticles are oxidized. However, the oxide surface did not affect the reaction efficiency of hydrogen production.
An energy loss occurs in the process of producing hydrogen for the produced iron nanoparticles. The energy loss is one eighth of the stored energy in the iron nanoparticles as the difference of the potential energy between iron oxides and iron in the reducing process. It is small and converts to heat. However, only 43 mg of water is needed to react with 100 mg of pure iron. In this experiment, the weight of the water used was 25 g. Thus, a lot of water was wasted without reacting with the iron nanoparticles. To eliminate this waste, first, the water should be confined in the reactor to improve the reaction efficiency with iron nanoparticles. Second, the reaction efficiency of the iron nanoparticles should be suppressed to 90% because the production rate of hydrogen degrades remarkably. It introduces to save the required water and electricity to heat and to recycle efficiently.
Solar energy or other natural energies are considered as sources of laser power for laser ablation in liquid. However, the most suitable lasers for energy cycles are considered to be solar-pumped lasers because common lasers have low electro-optical conversion efficiency.
In the future, we will establish a clean-energy hydrogen-production cycle that is simple, low-cost, no-carbon, and recyclable by combining a solar-pumped laser and laser ablation.
Fe2O3 and Fe3O4 powders were reduced with high efficiency by Nd:YAG pulsed lasers based on laser ablation in liquids. It was experimentally demonstrated that the produced iron nanoparticles can be used for hydrogen generation in an energy cycle. For hydrogen production using the reduced iron nanoparticles, the reaction efficiency of the hydrogen generation at a temperature of 673 K was more than 94% for the theoretically ideal amount of generated hydrogen. Fe3O4 powders remain after the hydrogen is produced, and they should be reduced by pulsed laser. Iron nanoparticles are reproduced and used for producing hydrogen again.
The laser pulses should be generated using a natural energy source, such as solar power or wind power. Furthermore, the laser pulses can be generated directly from solar light by using solar-pumped lasers. It has been expected that our proposed energy cycle by using pulsed laser and the Fe nanoparticles will be realized. The proposed energy cycle using a pulsed laser and iron nanoparticles is expected to provide simple, low-cost, and no-carbon production of recyclable hydrogen.