In situ high pressure XRD study on hydrogen uptake behavior of Pd-carbon systems
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1042-S07-03
In situ high pressure XRD study on hydrogen uptake behavior of Pd-carbon systems Vinay V Bhat1, Nidia C Gallego1, Cristian I Contescu1, E Andrew Payzant1, Adam J Rondinone1, Halil Tekinalp2, and Dan D Edie2 1 Oak Ridge National Laboratory, Oak Ridge, TN, 37830 2 Clemson University, Clemson, SC, 29634 ABSTRACT Efficient storage of hydrogen for use in fuel cell-powered vehicles is a challenge that is being addressed in different ways, including adsorptive, compressive, and liquid storage approaches. In this paper we report on adsorptive storage in nanoporous carbon fibers in which palladium is incorporated prior to spinning and carbonization/activation of the fibers. Nanoparticles of Pd, when dispersed in activated carbon fibers (ACF), enhance the hydrogen storage capacity of ACF. The adsorption capacity of Pd-ACF increases with increasing temperature below 0.4 bar, and the trend reverses when the pressure increases. To understand the cause for such behavior, hydrogen uptake properties of Pd with different degrees of Pd-carbon contact (Pd deposited on carbon surface and Pd embedded in carbon matrix) are compared with Pd-sponge using in situ XRD under various hydrogen partial pressures (60 oC). Figure 3a shows that both Pd-ACF and Pd-Sponge samples have similar hydrogen desorption behavior, but the pressure range corresponding to β α transition is different. In PdACF, the pressure at which the two phases coexist at equal concentrations is 5 mbar; in Pdsponge, the same level of transformation is achieved at 0.01mbar. This effect is clearly visible in Fig-3b. Pd-sponge, where Pd is not in contact with carbon, has a lower H2 pressure at 1:1 composition of β : α phases, compared to the two samples where Pd is in contact with carbon (Pd-AC and Pd-ACF). Between Pd-AC and Pd-ACF, Pd embedded in the carbon matrix (Pd-
ACF) has a higher pressure (5 mbar) for the 1:1 phase ratio. The sample where Pd is deposited on carbon surface (Pd-AC) reaches a 1:1 phase ratio at a lower pressure (0.5 mbar). 100 80 60 40
Phase concentration (%)
20 0 100 80
β Pd Sponge (325 A)
α β
60
Pd catalyst (40 A)
40 20
α
0 100 80
β
60
K230-Pd (475 A)
40 20 0
α
10000 1000 100
10
1
0.1
0.01 1E-3 1E-4 1E-5
Partial Pressure (mbar)
(a) (b) Fig-3: (a) XRD patterns of Pd-sponge and Pd-ACF under different hydrogen partial pressure at 25 oC. (b) α and β PdHx phase concentration as a function of hydrogen partial pressure. This observation apparently suggests that microporous carbon might be “pumping out” hydrogen from β-PdHx, and the “pumping power” depends on the extent of the carbon-Pd contact interface. In recent works, Yang and co-workers showed that introducing ‘carbonbridges’ at the interfaces between noble metal catalyst particles and the carbon receptor was helpful to enhance the hydrogen storage capacity [13]. They suggested that carbon-bridges help increase the contact between primary spillover sources (catalyst particles) and the secondary receptor, thereby enhancing the storage capacity. In case of Pd-ACF, a well dev
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