Hydrogen storage evaluation based on investigations of the catalytic properties of metal/metal oxides in electrospun carbon fibers
Introduction
Finding a new alternative energy source is becoming important, as evidenced by the increasing use of energy, its escalating price and the need for development of sustainable energy. Many types of energy have been investigated, including wind [1], biomass [2], solar [3], geothermal [4] and hydrogen energies [5]. Among these energies, hydrogen energy is attractive, due to its high efficiency, clean lifecycle and abundant quantity [6]. Hydrogen is considered in many countries as an important alternative energy vector and a bridge to a sustainable energy future [7], [8]. The promise of hydrogen as an energy carrier that can provide pollution-free, carbon-free power and fuel for buildings, industries and transport makes it a potentially critical player in our energy future [9], [10], [11].
In order to use hydrogen, one problem that must be solved is the efficient storage of hydrogen. Briefly, there are three kinds of methods for storing hydrogen [12]. First, a cryogenic system has been used, due to its highly efficient storage. Unfortunately, this method may ultimately use additional energy to maintain the hydrogen at cryogenic temperatures. As a second method, a special cylinder has been used to hold the hydrogen at high pressure. In this case, safety is an issue, due to the high pressures involved. The last method, which is the most promising method thus far due to its high safety and efficiency, uses metals and organic and carbon materials at room temperature. This hydrogen storage method allows the beneficial conditions of room temperature and high pressure during the storage period. Each material has its own advantages for high capacity hydrogen storage. Thus, carbon–metal composites have been investigated in order to use the advantages of each [13].
Carbon can be used as a hydrogen storage material because it can offer a high specific surface area and high porosity, making it light [14]. In order to reach higher capacities of hydrogen storage, metal/metal oxides have been introduced to carbon as a catalyst [15], [16], [17]. In particular, transition metals have been investigated to increase the adsorption of the hydrogen as a catalyst. Based on the increased use of transition metals as a catalyst, investigations regarding the different performance of metals and metal oxides have been carried out in order to determine which is more effective as a catalyst [18].
In this study, metals and metal oxides are compared as catalysts, in order to investigate their catalytic ability in improving hydrogen adsorption in electrospun carbon fibers. Porous carbon fibers were applied as a hydrogen carrier for improving the materials' hydrogen storage capacity.
Section snippets
Reagents
Polyacrylonitrile (PAN, d = 1.184, 181315, Aldrich) was used as a carbon source. N,N-dimethyl formamide (DMF) was used as a solvent because of its appropriate boiling point (426 K) and excellent conductivity (conductivity = 10.90 μS/cm, dipole moment = 3.82 Debye), which are important factors for electrospinning [14]. Metal oxides [iron oxide, magnesium oxide and copper oxide (m.p.: Fe3O4, 1811 K; MgO, 3073 K; CuO, 1337 K)] and metals [iron and magnesium (Fe and Mg)] were used as catalysts, and potassium
Surface morphology
Fig. 1 shows the FE-SEM images of the samples taken in order to investigate the residual metal oxide and the surface morphology. In the case of the ACF-IO sample, Fig. 1(a) and (b) shows that the iron oxide clusters have irregular sizes ranging from roughly 200 to 400 nm. The average diameter of the fibers is approximately 300 nm. Fig. 1(c) and (d) presents SEM images of ACF-MO. The fiber surface is quite rough, as compared to the surfaces of the other samples. It appears that the clusters of
Conclusions
PAN-based electrospun carbon fibers were prepared with various catalysts, such as metals and metal oxides (MgO, Fe3O4, Mg, Fe and CuO), in order to investigate the capacity of metals and metal oxides for increasing hydrogen storage. These catalyst-embedded electrospun fibers were carbonized and activated. Although they are activated via the same method, these fibers show different pore structure developments, due to the nature of the metals and metal oxides used. When comparing the catalytic
Acknowledgments
This research was performed for the Hydrogen Energy R&D Center, which is part of the 21st Century Frontier R&D Program, funded by the Ministry of Science and Technology of Korea.
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