Evidence for large hydrogen storage capacity in single-walled carbon nanotubes encapsulated by electroplating Pd onto a Pd substrate

We report a study of hydrogen storage in an alternative material, representing single-walled carbon nanotubes (SWCNTs) encapsulated by thin Pd layers onto a Pd substrate. A synergetic effect resulting in combination of the Pd and the SWCNTs properties with regards to hydrogen has been achieved. Adding SWCNTs increases the ${\mathrm{H}}_2$ capacity of the Pd-SWCNT composite by up to 25% relative to Pd metal alone under electrochemical loading. This results in a storage capacity of $8–12\mathrm{wt}\%$. with regard to the added SWCNTs.

Figure 1

[(a) and (b)] SEM images of the sites at the Pd-SWCNT interface after $1\min$ etching in nitric acid: (a) magnification at $\times 50000$ and (b) magnification at $\times 120000$.

Figure 2

Hydrogen loading reproducibility for the ${\mathrm{Pd}}^{\prime }∕\mathrm{SWCNT}∕{\mathrm{Pd}}^{\prime }∕\mathrm{Pd}∕{\mathrm{Pd}}^{\prime }∕\mathrm{SWCNT}∕{\mathrm{Pd}}^{\prime }$ with SWCNT-(1) (curve 2), and for the reference sample of ${\mathrm{Pd}}^{\prime }∕\mathrm{Pd}∕{\mathrm{Pd}}^{\prime }$ (curve 1): five consecutive loading-deloading cycles. The loading: electrolysis at $j=5.0\mathrm{mA}∕{\mathrm{cm}}^2$ for $2.5\mathrm{h}$. The deloading: vacuum heating at $T=673\mathrm{K}$.

Figure 3

Effective hydrogen loading in the ${\mathrm{Pd}}^{\prime }∕\mathrm{SWCNT}∕{\mathrm{Pd}}^{\prime }∕\mathrm{Pd}∕{\mathrm{Pd}}^{\prime }∕\mathrm{SWCNT}∕{\mathrm{Pd}}^{\prime }$ sample with SWCNT-(1) vs time elapsed after completing the electrolysis at $T=290\mathrm{K}$ (curve 2); the same for the reference sample of ${\mathrm{Pd}}^{\prime }∕\mathrm{Pd}∕{\mathrm{Pd}}^{\prime }$ (curve 1). Inset: Kinetics of isothermic $(T=673\mathrm{K})$ degassing for the ${\mathrm{Pd}}^{\prime }∕\mathrm{Pd}∕{\mathrm{Pd}}^{\prime }$ (open circles—curve 1): the decay constant $\lambda \left(\mathrm{Pd}\right)=0.0087$, the effective duration $\tau \left(\mathrm{Pd}\right)=400\mathrm{s}$, and for the ${\mathrm{Pd}}^{\prime }∕\mathrm{SWCNT}∕{\mathrm{Pd}}^{\prime }∕\mathrm{Pd}∕{\mathrm{Pd}}^{\prime }∕\mathrm{SWCNT}∕{\mathrm{Pd}}^{\prime }$-(1) (solid squares—curve 2): $\lambda (\mathrm{Pd}+\mathrm{SWCNT})=0.004$, $\tau (\mathrm{Pd}+\mathrm{SWCNT})=1200\mathrm{s}$.

Figure 4

Effective hydrogen loading in the ${\mathrm{Pd}}^{\prime }∕\mathrm{Pd}∕{\mathrm{Pd}}^{\prime }$ (curve 1) and in the composite with the SWCNT-(1) (curve 2) samples vs the loading temperature. The inset shows the Arrhenius plot for the curve 2 data in the coordinates ($1∕T$, $\mathrm{Ln}\left[x\right]$). The linear fit is consistent with the activation energy of hydrogen absorption ${\epsilon }_{\mathrm{H}}=0.073\pm 0.006\mathrm{eV}∕\mathrm{H}$ atom.