Zeolite-templated carbon (ZTC) materials were synthesized, characterized, and evaluated as potential hydrogen storage materials between 77 and 298 K up to 30 MPa. Successful synthesis of high template fidelity ZTCs was confirmed by X-ray diffraction and nitrogen adsorption at 77 K; BET surface areas up to ~3600 mT2 g^(–1) were achieved. Equilibrium hydrogen adsorption capacity in ZTCs is higher than all other materials studied, including superactivated carbon MSC-30. The ZTCs showed a maximum in Gibbs surface excess uptake of 28.6 mmol g–1 (5.5 wt %) at 77 K, with hydrogen uptake capacity at 300 K linearly proportional to BET surface area: 2.3 mmol g^(–1) (0.46 wt %) uptake per 1000 m^2 g^(–1) at 30 MPa. This is the same trend as for other carbonaceous materials, implying that the nature of high-pressure adsorption in ZTCs is not unique despite their narrow microporosity and significantly lower skeletal densities. Isoexcess enthalpies of adsorption are calculated between 77 and 298 K and found to be 6.5–6.6 kJ mol^(–1) in the Henry’s law limit

Ahn, Channing C.

Cumberland, Robert W.

Fultz, Brent

Stadie, Nicholas P.

Vajo, John J.

Wilson, Andrew A.

English

Caltech Authors - Main

Zeolite-Templated Carbon Materials for High Pressure Hydrogen Storage 
 
Supporting Information 
 
 
 
 
Nicholas P. Stadie*
a
, John J. Vajo
b
, Robert W. Cumberland
b
, Andrew A. Wilson
c
, Channing C. Ahn
a
, and 
Brent Fultz
a
 
 
a
California Institute of Technology, 138-78, Pasadena, California 91125, USA 
b
HRL Laboratories, LLC, Malibu, California 90265, USA 
c
Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK 
 
*Corresponding author. E-mail address: nstadie@caltech.edu. 
 
 
 
 
 
 
  
2 
 
Introduction 
 
Additional materials characterization and data analysis were performed to supplement the results 
reported in “Zeolite-Templated Carbon Materials for High Pressure Hydrogen Storage” to assure 
reproducibility of the results, validate the templated structure of ZTCs, further analyze the micropore 
character of MSC-30 and ZTCs, and to probe differences in chemical bonding between MSC-30 and ZTCs 
that could clarify their significant difference in skeletal density. 
 
The correlation between BET surface area and excess hydrogen uptake across all temperatures and 
pressures in ZTCs and the other materials studied is central to the conclusions from this study. In 
addition to standard hydrogen adsorption/desorption measurements, we also include hydrogen cycling 
results to show full reversibility of hydrogen uptake and to verify that the experimental error in 
measurements is acceptable. Secondly, the BET method for characterizing the surface area of materials 
was supplemented by the Dubinin- Radushkevich method for determining microporous volume to 
determine the correlation between this value and excess hydrogen uptake at room temperature. 
 
Additional comparison of material properties of ZTC-3 and other ZTCs is important for validating the 
comparison of our high pressure results to those in the literature. Electron microscopy, both scanning 
(SEM) and transmission (TEM), was performed to show the similarity of ZTC-3 to “PFA-P7-H” synthesized 
by Ma et al.
38
, an approximately equivalent reference material to “P7(2)-H” of Nishihara et al.
16
 
 
Finally, some measurements were performed to determine if there were differences in sp
2
 or sp
3
 
chemical bonding in ZTC-3 and MSC-30, including x-ray photolelectron spectroscopy (XPS), electron 
energy-loss spectroscopy (EELS), and solid-state 
13
C nuclear magnetic resonance (NMR). The motivation 
for this additional work was elucidation of the nature of the different skeletal density between the two 
materials. However, no significant differences were detected that could account for the large difference 
in skeletal density. The results are consistent with those in the literature, for example by Yang et al.
33
 
(XPS results) and Ma et al.
32
 (NMR results). 
 
Hydrogen Cycling 
 
Hydrogen uptake isotherms measured up to 30 MPa, using our newly constructed Sieverts apparatus 
specific to high pressure experiments, were cycled multiple times to ensure repeatability of the results. 
Hydrogen cycling in all materials studied was achieved without any loss of capacity on adsorption and 
desorption after many cycles, as expected for pure physisorbent materials. For example, three 
consecutive hydrogen adsorption/desorption cycles in ZTC-3 at 298 K are shown in Figure S1. The 
sample was degassed before cycling, as detailed in the Experimental Methods, but was not degassed in 
between cycles. 
 
Dubinin- Radushkevich Micropore Volume 
 
Nitrogen adsorption isotherms at 77 K were further analyzed to determine the Dubinin- Radushkevich 
(DR) micropore volume
21
 of each sample. The DR method for treating the N2 adsorption data was found 
to be susceptible to similar pitfalls as the BET method, especially for MSC-30 which shows multiple 
stages of different slopes in the DR curve. However, with consistent treatment of the data for all 
samples, the hydrogen uptake at 77 K and 298 K is also found to be well approximated by the DR 
micropore volume in the same way as for BET surface area (shown in Figure S2). The trend is linear, and 
is ~5 mmol excess H2 uptake per mL of DR micropore volume at 298 K and 30 MPa. 
3 
 
 
Electron Microscopy 
 
SEM studies were performed on a Hitachi S-4800 instrument operated at 4.0 keV. Samples were 
prepared for SEM by dispersing in isopropanol on a holey carbon grid. Evidence can be seen of the 
superficial likeness between particles of ZTC and the zeolite template from which they were synthesized 
(shown in Figure S3), similar to that reported by Ma et al.
38
 
 
TEM studies were performed on a FEI Tecnai F20 instrument operated at 200 keV. Samples were 
prepared for TEM by dispersing a finely ground mixture of ZTC and isopropanol on a holey carbon grid. 
Low magnification TEM studies were consistent with the SEM data. A high magnification micrograph of a 
thin region of ZTC-3 is shown in Figure S4, with an inset showing the Fourier transform of the image. The 
spots in the transformed image confirm the periodicity of the porous structure. The pore to pore spacing 
of 1.0 nm is consistent with that calculated from DFT treatment of the N2 adsorption isotherms at 77 K 
(see Figure 3), and with data reported by Ma et al.
38
 
 
X-ray Photolelectron Spectroscopy 
 
X-ray photolelectron spectroscopy (XPS) was performed to compare ZTCs to MSC-30, but no appreciable 
difference was found between them (see Figure S5). XPS studies were performed on a Kratos AXIS Ultra 
DLD spectrometer with a monochromatic Al-Kα source operating at 150 W, with a 20 eV pass energy, 
and 0.1 eV step (after brief survey spectra were collected). The binding energy was corrected to the 
most intense peak, which is from sp
2
 hybridized carbon, at 285.0 eV. The intensity was not rescaled 
since identical instrumental conditions were used across all samples. For peak fitting analysis, a Shirley-
type background was subtracted and 8 component peaks were fitted, following a previously reported 
procedure.
33
  An example of peak fits is shown in Figure S6. The results are summarized in Table S1, 
indicating 18% and 19% sp
3
 hybridized carbon in ZTC-3 and MSC-30, respectively. 
 
Electron Energy-Loss Spectroscopy 
 
Electron energy-loss spectroscopy (EELS) was performed to compare ZTCs to MSC-30 and other carbon 
materials, shown in Figure S7. EELS measurements were performed on a FEI Tecnai F20 instrument 
operated at 200 keV and equipped with a Gatan Imaging Filter system. To acquire these spectra, the 
aperture size was 0.6 mm, the dispersion was 0.2 eV pixel
-1
, and the energy shift was 175 eV. Samples 
were prepared by dispersing a finely ground mixture of sample material and isopropanol on a holey 
carbon grid. The pre-edge peak was calibrated to 285.0 eV in all samples, a power-law background was 
subtracted, and the signal intensity was normalized to the same value at high loss. The ratio of the areas 
of the pre-edge peak to the main carbon 1s edge (>289 eV) was used to determine the relative content 
of sp
2
 and sp
3
 hybridized carbon (see Figure S8). This study also shows only a small difference in the 
amount of sp
2
 and sp
3
 hybridized carbon between ZTC-3 and MSC-30: approximately 18% and 16% sp
3
 
content, respectively. 
 
Solid-State 
13
C NMR 
 
Solid-state 
13
C nuclear magnetic resonance (NMR) experiments were performed on ZTC-3 and MSC-30 
using a Bruker DSX-500 spectrometer equipped with a Bruker 4 mm MAS probe (see Figure S9). The 
sample spinning rate for MAS experiments was 12 kHz, performed at room temperature under dry 
4 
 
nitrogen gas. The chemical shifts are reported in parts per million (ppm) externally referenced to 
tetramethylsilane. No significant sp
3
 hybridized carbon in either ZTC-3 or MSC-30 was detected. 
 
Acknowledgement 
 
We thank Carol Garland, Sonjong Hwang, and Joseph Beardslee for assistance with supplementary 
measurements. 
 
Additional References 
 
38. Ma, Z.; Kyotani, T.; Tomita, A.; Carbon 2002, 40, 2367-2374. 
 
 
 
  
5 
 
 
 
Figure S1. Equilibrium excess hydrogen adsorption/desorption isotherms for ZTC-3 during 3 cycles, 
showing complete reversibility of hydrogen uptake/delivery which is characteristic of physisorbent 
materials, and confirming instrument precision. 
 
 
 
 
Figure S2. Equilibrium excess adsorption uptake of hydrogen as a function of Dubinin- Radushkevic (DR) 
micropore volume at 298 K (30 MPa) for all materials studied. 
 
 
 
6 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure S3. SEM micrographs of zeolite precursor (left) and ZTC product (right), showing similar particle 
size and shape. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure S4. TEM micrograph of ZTC-3 showing pore-to-pore periodicity of 1.0 nm, and the Fourier 
transform of the image (inset). 
 
 
 
 Component Fraction (at%) 
Peak Position (eV) 285.0 285.7 286.4 287.3 288.1 289.4 290.2 291.5 
Component C-C sp
2
 C-C sp
3
 C-OR C-O-C C=O O=C-OR -- -- 
ZTC-3 53.4 18.0 8.6 6.0 1.1 4.2 1.0 7.7 
MSC-30 48.0 18.8 6.8 4.8 6.1 4.2 3.6 7.7 
 
Table S1. Summary of the XPS analysis results for ZTC-3 and MSC-30. 
(a) (b) 
7 
 
 
 
Figure S5. XPS data comparing ZTC-3, MSC-30, and an equivalent reference material, ZTC “0% Pt-Ac.”
33
 
 
 
 
 
Figure S6. XPS analyses for the carbon 1s regions in MSC-30 (top) and ZTC-3 (bottom). 
8 
 
 
 
Figure S7. EELS spectra showing the carbon 1s edge in ZTC-3 compared to MSC-30, graphite, carbon 
nanotubes, and the amorphous holey carbon grid. 
 
 
       
P1 Area = Iπ* 1.54 0 1.96 1.95 1.55 2.08 
P2 Area = Iσ* 4.41 1 7.30 6.97 7.75 6.09 
P1/P2 0.35 0 0.27 0.28 0.20 0.34 
Iπ*/( Iπ*+ Iσ*) 0.26 0 0.21 0.22 0.17 0.25 
Normalized 1 0 0.82 0.84 0.64 0.98 
sp
2
 Content                   
(%) 
100 0 81.8 84.2 64.2 98.3 
 
 
Figure S8. Summary of sp
2
 content in carbon materials studied by EELS, based on the integrated peak 
areas of the 1s→π* peak at ~285 eV to the 1s→σ* peak at ~292 eV, and fit to a calibration curve based 
on graphite and diamond. 
9 
 
 
 
Figure S9. Solid-state 
13
C NMR (MAS) spectra of ZTC-3 and MSC-30. 


Zeolite-Templated Carbon Materials for High-Pressure Hydrogen Storage

https://authors.library.caltech.edu/32796/2/la302050m_si_001.pdf

Zeolite-Templated Carbon Materials for High-Pressure Hydrogen Storage

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main