J. Phys. Chem. C 2010, 114, 4693–4703
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Are Carbon Nanotubes Viable Materials for the Electrochemical Storage of Hydrogen? Jeffrey B. Martin,† Ian A. Kinloch,‡ and Robert A. W. Dryfe*,† School of Chemistry, UniVersity of Manchester, Oxford Road, Manchester, M13 9PL, U.K., and School of Materials, UniVersity of Manchester, GrosVenor Street, Manchester, M1 7HS, U.K. ReceiVed: June 3, 2009; ReVised Manuscript ReceiVed: January 29, 2010
The electrochemical storage of hydrogen on a range of carbon nanotubes has been investigated using electrochemical techniques and in situ Raman spectroelectrochemistry. An aggregated, single-walled nanotube sample was found to have the highest apparent storage capacity of 0.22 wt % (3 M KOH). Raman spectroelectrochemistry confirmed that no irreversible structural changes occur upon charging. The storage was found to be primarily due to the sorption of H2 gas in the pores of the nanotube aggregate, combined with some chemisorption on the amorphous carbon impurities in the sample. It is, therefore, concluded that the observed storage capacity is due to the small pores and the presence of carbonaceous impurities. Introduction The recent global trend toward carbon neutral energy sources has led to renewed interest in the “hydrogen economy”, utilizing fuel cells to replace current combustion engines. Both the efficiency and the cost of fuel cells have improved greatly in recent years; however, a significant problem still exists with the need for a safe, low weight and compact hydrogen storage medium (greater than 6 wt % and 45 g liter-1 are the industrial targets).1 Carbon nanotubes have attracted considerable interest in a wide range of applications due to their intriguing combination of properties, including their high tensile strength, large aspect ratios, and range of electronic structures (semiconducting or metallic). One application is energy storage due to the high electrical conductivity and surface area of nanotubes, with specific applications, including supercapacitors2 and lithiumion storage materials in battery applications3 where the nested shell (multiwalled nanotubes) and bundle-like structures (singlewalled nanotubes) of the nanotubes are exploited. These properties also make nanotubes excellent candidates for hydrogen storage; Dillon et al. initially reported possible capacities of up to 14 wt % for single-walled carbon nanotubes.4 Following this and some promising early research on herringbone fibers,5 the interest in hydrogen storage on carbon nanostructures increased dramatically. However, subsequent refinements in both the quality of the nanotube samples and the measurement techniques showed that the early reported storage predictions were overoptimistic. Accepted values are now believed to lie below 0.5 wt % for undoped nanotubes,6,7 with improvements up to 2-3 wt % at cryogenic temperatures. Metal doping is reported to increase the hydrogen storage capacity to the region of 2.9 wt %.8,9 Electrochemical hydrogen storage is also of great interest due to the ability to generate hydrogen in situ, its easy integration into fuel cell systems, and the possibility of modulating storage energies/mechanisms and, therefore, charging characteristics with potential. Electrochemical charging of activated carbons in KOH has been investigated with hydrogen storage capacities * To whom correspondence should be addressed. E-mail: robert.dryfe@ manchester.ac.uk. Fax: +44 161 275 4734. † School of Chemistry. ‡ School of Materials.
up to 0.92 wt % reported,10 compared with 1.93 wt % for an ordered mesoporous carbon11 and 0.39 wt % for a single-walled nanotube (SWNT) mat,12 although higher values have been reported with purified SWNTs.13 Metal doping with nickel, copper, or gold reportedly gives increased storage capacities, up to 2.15 wt %, for various carbons.14-18 There is uncertainty, however, over several aspects of the storage mechanism, namely, (i) whether physisorption or chemisorption processes dominate, (ii) the desorption mechanisms that occur, and (iii) not least, the actual electrochemical activity of carbon nanotubes. Niessen et al. recently stated that SWNTs are essentially inert electrochemically and any storage capacity can be ascribed to metals present from residual catalyst or those added postgrowth.19 Raman spectroscopy is a useful analytical technique that is being used increasingly to understand the charge and doping behavior of carbon nanotubes under applied potential.20 Raman spectroscopy is a powerful tool for characterization of carbon nanostructures because well-defined, structure-specific vibrational modes (phonons) are observed, resulting in characteristic spectra. The main first-order vibrational modes are the radial breathing mode (RBM) and the tangential G mode. The RBM arises from a displacement of all carbon atoms along the radial axis and is only observed in small diameter nanotubes, such as single- and double-walled tubes. The G mode is so-called as it describes the graphitic-like character of the sample, more accurately, the tangential displacement of carbon atoms within the graphite plane (G+ along tube axis and G- in a circumferential direction); it also contains information on nanotube type, with all metallic SWNTs having a low wavenumber shoulder (Breit-Wigner-Fano, BWF line shape).21 The D mode describes defects in the crystallographic structure of the nanotube where there is a breakdown in symmetry. The G’ (or 2D) mode is an overtone of D; however, it also occurs without defects in the crystal structure.20 The G and D band positions are sensitive to changes in the local environment and doping because contraction and stiffening of the carbon-carbon bonds change the phonon frequency and, hence, the Raman peak shift.20 Raman spectroscopy is particularly useful for studying SWNTs as these nanotubes have well-defined van Hove singularities in their electronic structure, and as such, significant resonance enhancement of Raman spectra can be achieved when the energy of the incident photons matches (within 50 meV)
10.1021/jp905203c 2010 American Chemical Society Published on Web 02/19/2010
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J. Phys. Chem. C, Vol. 114, No. 10, 2010
Martin et al.
TABLE 1: Summary of the Doping-Induced Raman Peak Shifts Reported in the Literature for SWNTsa chemical mode RBM
n/e-
electrochemical p/h+
n/e-
p/h+
VK/Rb/C VLi/Rb too small to measure V v
v v v too small to measure D G
36
V V V v
v v v v v v v v
37 38 43 44 45 35 46 42 40 36 41
V
v v
47 38 36
VK/Rb/C vLi/Rb
VK/Rb
37 38 35 39 40 41 42
v
vI
G’
reference
vBr
a “v” denotes blue/upshift and “V” denotes red/downshift in peak position.
the energy of electronic transitions, Eii. This enhancement in the signal intensity from the resonant nanotubes means that the spectra from all the tubes not in resonance effectively become part of the background spectra. Therefore, by using Raman spectroscopy, the SWNTs being measured can be identified. The degree of signal enhancement is very sensitive to the energy difference between the excitation source and Eii; thus, changes in electronic structure due to doping and strain will result in a change of peak intensity in addition to position.20 Table 1 shows a summary of Raman peak shift data reported previously for SWNTs under chemical and electrochemical doping. It is apparent that across all vibrational modes a blue shift in peak positions is reported for anodic potentials, rationalized in terms of contraction and stiffening of the C-C bonds upon hole insertion.20 Cathodic doping, however, is less well understood, with varying trends reported, including variable red/blue shifts for a single SWNT sample under different K+/C ratios.20 Single-layer graphene samples have been investigated recently via Raman spectroscopy: analysis of an electrochemically “top-gated” transistor made from graphene has revealed blue shifts in the G peak position for both positive and negative doping.22 Tokura et al. have investigated the adsorption of atomic hydrogen produced in a heated tungsten filament on aligned SWNTs, observing a blue shift (increase in wavenumber) in D and G positions, along with a decrease in the G/D intensity ratio, interpreted as hydrogenation of the nanotube surface.23 By contrast, we note that some authors have reported no change for the G mode between a vacuum and 20 bar of hydrogen.24 Herein, the capacity and mechanism of electrochemical hydrogen storage on carbon nanotubes are analyzed. A range of both multiwalled and single-walled nanotubes were initially screened for storage capacity using cyclic voltammetry. On this basis, one SWNT material was identified as having some storage potential: this material was studied in more detail using both voltammetric and galvanostatic techniques to understand the
storage mechanism occurring, the roles of metals (residual catalyst and deliberate decoration with electrochemical catalysts), and pore structure. Finally, Raman spectroelectrochemistry is used to identify whether hydrogen storage occurred through chemi/physisorption or doping of the nanotubes with protons. Experimental Section Samples. Carbon nanotube samples were obtained from commercial suppliers (Thomas Swan & Co., Consett, U.K.; Sigma-Aldrich, Gillingham, U.K.; and MER Corp., Tucson, AZ) and are listed in Table 2, along with manufacturing method, specific surface area, and double-layer capacitances. Activated carbon (
