9460
J. Phys. Chem. B 2000, 104, 9460-9467
Thermogravimetric Measurement of Hydrogen Absorption in Alkali-Modified Carbon Materials F. E. Pinkerton,* B. G. Wicke, C. H. Olk, G. G. Tibbetts, G. P. Meisner, M. S. Meyer, and J. F. Herbst Materials and Processes Laboratory, General Motors Research and DeVelopment Center, 30500 Mound Road, Warren, Michigan 48090-9055 ReceiVed: March 13, 2000; In Final Form: July 11, 2000
We have prepared Li-doped multiwall carbon nanotubes and Li- and K-intercalated graphite and measured their hydrogen storage properties using a thermogravimetric analyzer (TGA). In a flowing H2 atmosphere Li-doped nanotubes and Li-intercalated graphite both exhibit a cyclable weight gain between 200 and 400 °C and weight loss between 400 and 500 °C characterized by a distinct and unusual temperature profile. We find, however, that neither H2 nor carbon is required to generate this TGA feature; we observe it even in Li-containing samples measured in flowing Ar without H2 and in LiOH samples measured in either H2 or Ar. Potassium-intercalated graphite shows mass cycling with a different thermal character between 40 and 250 °C, but as with Li, observation of a large cyclable feature does not rely on the presence of H2. In both cases we identify the cycling mass to be absorption/desorption of H2O present as an impurity in the TGA atmosphere. The temperature signatures we observe are strikingly similar to those reported in a recent study of Li- and K-doped carbon nanofibers in which mass uptakes as large as 20 wt % were attributed to hydrogen absorption. When the impurities in the TGA atmosphere are reduced as much as possible we do detect modest weight changes in K-intercalated graphite which we interpret as true hydrogen absorption at 1.3 wt %, of which 0.2 wt % is cyclable. This level of hydrogen absorption is consistent with pressure-composition isotherm measurements on the same material using a gas reaction controller (1.0 wt % total absorption with 0.3 wt % cyclable). We do not detect any evidence of hydrogen absorption by Li-containing carbon materials under our experimental conditions.
I. Introduction Recent claims of large hydrogen uptake at or near room temperature have stimulated intense interest in the hydrogen storage capacities of novel forms of carbon. Claims of roomtemperature hydrogen capacity range from 4 to 13 wt % in single-walled nanotubes (SWNTs)1,2 and carbon nanofibers (CNFs),3 with one CNF claim as high as 67 wt %.4,5 On the other hand, Ye et al. conclude6 that their observation of ∼8 wt % hydrogen absorption in high purity SWNTs at a temperature of 80 K and pressures exceeding 60 atm are inconsistent with observation1 of 5-10 wt % absorption at room temperature and less than 1 atm, and a study7 of a variety of novel carbons has indicated that their hydrogen capacities at or above room temperature do not exceed about 0.2 wt %. Verifiable large hydrogen storage near room temperature and at modest pressures would clearly interest the automotive industry, where high capacity, lightweight materials could provide on-vehicle hydrogen storage for fuel cell or hydrogen internal combustion engines. In a recent publication,8 Chen et al. reported that Li- or K-doped carbon nanofibers can reproducibly and cyclically absorb and desorb hydrogen in quantities up to 20 wt % (Lidoped) or 14 wt % (K-doped) at a pressure of 1 atm and modest temperatures. They infer these hydrogen storage characteristics from weight gains and losses as a function of time and temperature in flowing H2 measured using a thermogravimetric analyzer (TGA). Their CNFs were comprised of stacks of conical graphene sheets with a hollow core and outer diameters
between 25 and 35 nm. They also report smaller but still significant absorptions in alkali-doped graphite (14 wt % for Li and 5 wt % for K). Very recently Yang9 has attempted to verify these claims for the CNFs. Using CNFs prepared by a process similar to that employed by Chen et al., Yang found that he could obtain large weight gains in hydrogen (up to 20 wt %) only by adding moisture to the H2 gas. When the H2 gas was passed through a moisture trap to obtain dry hydrogen, his samples gained only about 2 wt %. He concluded that most of the weight gain observed by Chen et al. was H2O and that the hydrogen uptake was on the order of 2 wt % in Li- and K-doped CNFs. We have performed careful TGA measurements of the hydrogen storage potential of Li-doped multiwall nanotubes (MWNTs), Li-intercalated graphite, and K-intercalated graphite. Our MWNT morphology is different from the CNFs used by Chen et al.; our tubes consist of concentric cylindrical graphene sheets with outer diameters between 2 and 15 nm. Nevertheless, in all cases we observe weight gains and losses having temperature cycles essentially identical to those reported by Chen et al., albeit smaller in magnitude. We have also observed that these characteristic temperature signatures are present even when the samples are run in flowing Ar gas (i.e., in the complete absence of hydrogen). Furthermore, such temperature signatures are observed even in a sample of LiOH, whether run in H2 or Ar gas (i.e., in the complete absence of carbon). Our results provide compelling evidence that the large mass cycling observed in these Li- and K-doped carbon materials as
10.1021/jp000957o CCC: $19.00 © 2000 American Chemical Society Published on Web 09/16/2000
Thermogravimetric Measurement of H2 Absorption a function of temperature cannot be attributed to hydrogen absorption and desorption. These features arise from atmospheric impurities in the TGA which react with the Li or K to form LiOH or KOH. Ultimately, the cycling species responsible for the weight changes is H2O, in agreement with Yang.9 These materials do not store hydrogen in any technologically useful form. Furthermore, by careful observation we detect gravimetrically the true hydrogen absorption in K-intercalated graphite. We measure a total H2 uptake of about 1.3 wt %, of which about 0.2 wt % is cyclable with temperature between 200 and 320 °C. This result is in good agreement with independent accurate measurement of hydrogen sorption using a gas reaction controller (1.0 wt % total, 0.3 wt % cyclable) on the same material. We have not observed evidence of any hydrogen absorption in Li-doped carbons. II. Experimental Procedures The carbon multiwall nanotubes (MWNTs) used in this study were produced by MER Corporation using a cathodic arc process. They are specified by MER to consist of 10-40% tubes as determined by areal density in transmission electron microscope (TEM) images, with the remaining material composed of multilayer polygonal carbon nanoparticles and amorphous carbon; our own TEM examination finds tube outer diameters ranging from 2 to 15 nm. No attempt was made to open the ends of the tubes prior to doping. The MWNTs were doped using a molten Li method devised for forming graphite intercalation compounds of Li in highly ordered graphite crystals.10 Inside a controlled atmosphere glovebox the MWNTs were inserted into a stainless steel vessel along with a mass of Li metal in excess of that needed to form the stoichiometric compound LiC6. The sealed vessel was removed from the glovebox, placed into a high-temperature furnace, and heated to 365 ( 5 °C for ∼20 h. The Li-doped MWNTs were then separated from the excess lithium metal in the glovebox so as to avoid exposure to air. The Li-doped MWNTs contained a total of 11 wt % Li as determined by a wet chemical method. X-ray diffraction was performed on samples held under Ar gas in a sealed glass capillary tube using Cu KR radiation and a scintillation/CCD area detector. We collected data on the pristine MWNTs, Li-doped MWNTs, and Li-doped MWNTs after exposure to air for ∼30 min. Specimens of Li-intercalated graphite were prepared using the same process. Potassium-intercalated graphite was prepared at the KC8 stoichiometry. Graphitic material (BG-35) from Superior Graphite Company was mixed with shaved potassium (K:C ) 1:8) and molded in a tantalum tube. The mixture was loaded into a mechanical press inside an Ar atmosphere glovebox and heat treated at 250 °C for 12 h under a pressure of 69 MPa. The product was ground and heat treated for a second time under conditions similar to the first heat treatment. The product was cooled to ambient temperature, ground, and stored under Ar gas. Mass changes as a function of time and temperature were measured using a Perkin-Elmer System 7 thermogravimetric analyzer (TGA). Gas enters the TGA furnace via two paths: one flow purges the electrobalance head at the top of the instrument, while the other is introduced directly into the furnace just above the sample. We maintained a constant flow of approximately 20 sccm of Ar gas through the balance section, while a gas selector accessory allowed us to introduce either H2 or Ar gas at approximately 14-17 sccm through the sample port. Another gas line connected to the sample port was used
J. Phys. Chem. B, Vol. 104, No. 40, 2000 9461 to introduce additional gases when desired. We used high purity (99.999%) Ar and H2 and also passed these input gases through purifiers to remove any residual O2, H2O, and hydrocarbons. A mass spectrometer monitored the concentrations of gases in the furnace exhaust flow. The large (∼1500 cm3) balance head volume necessitates long purge times at 20 sccm gas flow in order to adequately purge the balance. We estimate, for example, that more than 10 h of purge are required to reduce the original balance head gas concentration below 100 ppm. We routinely purged overnight prior to inserting a sample in order to ensure that the TGA atmosphere was as clean as possible. This also affects gas switching; when changing from an H2-containing atmosphere to an Ar-only atmosphere we allowed a 9-15 h purge to reduce the H2 concentration to undetectable levels. The sample is held in a Pt weighing pan suspended by a Pt wire loop from a Nichrome hang-down wire. The presence of platinum in the TGA furnace will turn out to be of crucial importance in interpreting our results (see Section V). We also found that Li-containing samples tended to discolor the pan and adhere to it. We took the precaution of replacing the Pt pan and stirrup wire after every experiment involving Li. To protect the air-sensitive samples during loading into the TGA, the entire apparatus was enclosed in a large glovebag inflated with Ar gas. Samples were transferred into the glovebag in sealed vials and then loaded into the TGA under Ar as expeditiously as possible. While not perfect, this provides a considerable degree of sample protection during loading and also minimizes the amount of air that gets into the TGA furnace and balance head while the furnace is open. This is particularly important in the case of K-containing samples, which can undergo a rapid exothermic reaction upon exposure to air. We independently measured the sorption of hydrogen in K-intercalated graphite using a computer controlled commercial gas reaction controller manufactured by the Advanced Materials Corporation of Pittsburgh, PA. (Quantities of the Li-modified carbons were insufficient for performing gas sorption measurements using this technique.) This apparatus operates by allowing fixed amounts of H2 to fill the gas reservoir, which are then admitted to the sample chamber. After equilibrium is attained in the sample chamber, the number of molecules of H2 missing from the gas phase is determined; this corresponds to the number of molecules sorbed by the sample. The apparatus gradually increases the H2 pressure to the maximum allowed, about 4 MPa, while summing the sorbed hydrogen. The amount of hydrogen released from the sample is then determined by pumping out the gas reservoir and gradually bleeding H2 from the sample chamber into the gas reservoir. The stainless steel sample chamber allowed samples to be heated to 500 °C. III. Li-Modified Carbons in H2 The mass absorption/desorption characteristics of Li-doped MWNTs in H2 were studied in the TGA by cycling the temperature between 40 and 600 °C in a mixed flow of 20 sccm of Ar gas through the balance section and 17 sccm of H2 gas through the sample port. On first heating from 40 to 600 °C, the samples initially gained several wt % followed by a somewhat greater weight loss above 470 °C. Each sample was held at 600 °C until the weight stabilized at a value that was typically 2-4 wt % less than the starting weight. These initial weight changes will be addressed further in Section VI. Subsequently the weight cycled quite reproducibly on cooling from 600 to 40 °C and heating back up to 600 °C. The shape of the resulting temperature cycle signature is both unusual and quite striking, as shown by the open circles in
9462 J. Phys. Chem. B, Vol. 104, No. 40, 2000
Pinkerton et al.
Figure 1. Shape of the temperature cycling characteristics, normalized to the weight increase on cooling from 600 to 40 °C. Open circles: Li-doped multiwall nanotubes cycled at a temperature scan rate of 10 °C/min. Open triangles: Li-intercalated graphite at 5 °C/min. Filled circles: Li2O at 20 °C/min. The solid line is taken from Figure 1 of Chen et al. (ref 8).
Figure 1. For purposes of comparing the shape with that of similar materials, the weight has been normalized to the weight change over the cooling portion of the cycle. The actual magnitude of the cycle for these experimental conditions is 0.72 wt %; magnitudes as large as 4.2 wt % have been observed (cf. Table 1). During cooling the sample gains weight below 400 °C, with maximum absorption rate at about 300 °C, but the absorption essentially shuts off below about 200 °C. On heating, the temperature-activated absorption resumes between 200 and 400 °C. All of the absorbed weight is then desorbed between 400 and 500 °C. The weight cycling behavior of Li-intercalated graphite is very similar to that of Li-doped MWNTs, as shown by the open triangles in Figure 1. The magnitude of the cycle is somewhat smaller than that of the Li-doped MWNTs (Table 1). The lowtemperature absorption cutoff also occurs at a somewhat higher temperature (∼300 °C). For comparison, the cycling signature of Li-doped CNFs observed by Chen et al.8 is also shown as the solid line in Figure 1. Figure 1 leaves little doubt that the distinctive cycling
signature observed in our Li-modified carbon materials is essentially identical to that observed by Chen et al., although smaller in magnitude (0.6-4.2 wt % compared to 14.5 wt % reported in ref 8, cf. Table 1). While H2 is the dominant reactive gas present during cycling, mass spectrometer readings show that, despite our best efforts to protect the TGA atmosphere, small quantities of N2, O2, and H2O are also present in the exhaust stream (typically of the order of 0.1%). Possible sources are residual contamination of the TGA during loading (although this should be transient), small air exchange into the TGA through minor leaks, and H2O adsorbed onto the internal surfaces of the TGA. Contaminants in the input gases are in principle removed by the gas purifiers, but may also contribute if the purifiers are not efficient. Figure 1, therefore, does not unambiguously identify hydrogen as the cycling species responsible for large mass absorption in Limodified carbons. IV. Li-Modified Carbons Without H2 After each sample was cycled in the mixed H2/Ar atmosphere, we switched the sample port gas from H2 to Ar and repeated the cycling experiments in nominally pure Ar. To ensure that no H2 remained in the system, the TGA was purged for 14 h in the case of Li-intercalated graphite and over a weekend in the case of the Li-doped MWNTs. At least 2 h toward the end of the purge was spent with the samples held near 600 °C to eliminate any possibility that the sample itself could supply hydrogen. The residual H2 concentration after purging is predicted to be less than 2 ppm. Mass spectrometer measurements confirmed that the H2 concentration was below detectable levels (
