Co-electrospun Pd-coated porous carbon nanofibers for hydrogen storage applications
Graphical abstract
Research highlights
► Fabrication of nanoporous carbon fibers by water vapor activation. ► One-step synthesis of Pd nanoparticle-coated carbon fibers. ► Hydrogen adsorption characteristics of nanoporous carbon fibers.
Introduction
Hydrogen is a promising energy alternative to fossil fuels because it is renewable and pollution-free. Among various technical issues, the identification of efficient hydrogen storage media poses a major hurdle to full-scale exploitation of hydrogen energy. Carbon materials such as carbon nanotubes [1], graphite [2], and fullerene [3] have attracted significant interest due to their overall safety, low mass density, and high reliability. Among these, activated carbon and carbon fibers [4], [5] provide cost-effective storage media with reasonable storage capacity, although more hydrogen can be absorbed on carbon nanotubes (CNT).
Hydrogen adsorption on carbon materials occurs due to Van der Waals interactions. The amount of adsorption is therefore proportional to the surface area as well as pressure and temperature. The effective surface area can be significantly extended by introducing nanoscale pores on the surfaces of carbon materials. Both single-wall CNT and mesoporous carbon have extremely high surface areas of ∼1300 and ∼2000 m2/g, respectively. These high surface area carbon materials are usually synthesized by either vapor phase deposition or templating processes. However, their synthesis methods are generally unsuitable for cost-effective mass production, which prevents them from being utilized for hydrogen storage.
Electrospinning produces sub-micron sized continuous fibers from a polymer solution or melt by electric force. Due to its versatility and cost-effectiveness, this process has been adopted to fabricate one-dimensional nanostructured materials. Recently, electrospun carbon fibers have been applied to uses in fuel cells [6], supercapacitors [7], electrochemical storage [8] and hydrogen storage media [9]. Porous carbon fibers are capable of hydrogen storage as much as 2.5 wt% H2 at 300 K and 30 MPa [10].
The introduction of water vapor while carbonizing fibers can assist with the formation of pores on the hydrophobic carbon surface by removing surface carbon atoms. The pore formation reaction occurs as follows:C(s) + 2H2O → CO2 (g) + 2H2C(s) + H2O → CO (g) + H2
Molina-Sabio et al. reported that nanoporous carbon containing pores less than 1 nm in diameter can be synthesized by steam activation at high temperature (∼800 °C), although the amount of surface oxygen increases slightly [11]. These slit pores contribute to the increase in the active surface area and, in turn, enhance the hydrogen storage. Furthermore, surface modification with a metal catalyst can also boost hydrogen adsorption, either by the spillover effect [12] or metal hydride formation [13]. The hydrogen molecules that are preferentially absorbed on the metal catalyst particles are dissociated, followed by migration of hydrogen atoms to remote surface sites on the carbon materials. In particular, Pd nanorparticles loaded on carbon materials can chemically absorb hydrogen by forming hydride (PdHx) [14]. Particle size also plays an important role in determining hydrogen solubility. In the present study, we prepare Pd nanoparticle-coated high surface area carbon nanofibers by a one-step co-electrospinning method. Pd salt containing solution was electrosprayed while polyacrylonitrile (PAN) fibers are drawn from the core. Water vapor was introduced during carbonization at 800 °C to obtain nanoporous carbon fibers and, at the same time, Pd salt attached to the PAN was reduced to Pd nanoparticles during cooling. We performed structural and chemical analyses to identify PAN-derived nanoporous carbon fibers and compare the H2 adsorption abilities of carbon fibers prepared under different synthesis conditions.
Section snippets
Experimental procedures
To prepare the precursor solution for electrospinning, polyacrylonitrile (PAN, Mw = 150,000, Aldrich) was dissolved in dimethylformamide (DMF, anhydrous, 99.8%, Aldrich) at 60 °C. The viscosity of the polymer solution was in the range of 1600–2000 mPa s as determined by a rheometer (AR-2000EX, TA Instruments). The polymer solution was electrospun through a stainless steel nozzle (inner diameter = 0.51 mm) by applying an electric field of 1 kV/cm using a high-voltage DC power supply unit
Results and discussion
Fig. 2a shows the thermal decomposition behavior of the electrospun polymer fiber. An abrupt weight loss (36 wt%) occurred accompanying an exothermic peak in the vicinity of 310–340 °C, followed by a gradual weight loss. The exothermic reaction at 310–350 °C can be ascribed to cyclization reaction of the nitrile groups [15]. Cyclization reactions cause the PAN polymer to transform into a ladder-structured polymer by polymer-analogue polymerization of the nitrile groups [16]. The stabilization
Conclusions
We demonstrated the synthesis of Pd nanoparticle-coated carbon nanoporous fibers by one-step co-electrospinning. Palladium salt solution was electrosprayed while the polyacrylonitrile (PAN) polymer fibers were electrospun. Uniform nanoporous carbon fibers with Pd nanoparticles of 10–50 nm in diameter were obtained by carbonization at 800 °C in Ar + H2O and reduction at 250 °C in H2 + Ar. Water vapor was utilized as a nanoscale pore former by injecting it during the carbonization at 800 °C. The
Acknowledgement
This work was supported by DAPA and ADD in Korea. It was also partially supported by the Second Stage of the Brain Korea 21 Project.
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