n e w s of t h e w e e k sion of the conventional theory of such surface interactions." Cole adds that he and others are currently working on that research problem. By exposing graphite nanofibers of various structures and other materials to 120 atm of hydrogen at room temperature and then monitoring pressure changes in a sample vessel, the Boston chemists determine a sample's hydrogen uptake. The group reports that one type of graphite with a "herringbone" platelet spongelike graphite material creat- ter-sized cross-sectional areas and mi- structure sorbed more than 20 L of hyed by researchers at Northeastern crometer-scale lengths that voraciously drogen per g of graphite at standard temperature and pressure. That corresponds I University, Boston, can soak up take up hydrogen. more than 10 times as much hydrogen as "It appears to be a very important de- to a weight change of more than 65%. By other hydrogen-storage materials \J. Phys. velopment, but there are many questions comparison, activated carbon picked up Chem. B., published May 12 ASAP, http:// to be answered," Williams says. "Can less than 2% hydrogen by weight. Values pubs.acs.org/journals/jpcbik/]. High-capa- these measurements be reproduced by near 5% have been reported by other recity hydrogen-storage systems are a long- others? Is the material reusable? And if searchers examining metal hydrides. sought goal of the automobile industry, so, for how many cycles? Can high-qualiAfter allowing the system pressure to which wants to use hydrogen-powered ty material be manufactured consistently equilibrate, Rodriguez and coworkers refuel cells to propel fuel-efficient and envi- and inexpensively?" cover stored gas by allowing it to diffuse ronmentallyfriendlyvehicles. Other issues also need to be investi- out of the samples at room temperature This novel form of graphite may en- gated, Williams notes, "but if these ques- and 1 atm. The researchers found that able larger quantities of hydrogen to be tions can be answered satisfactorily, then herringbone graphite releases about half stored in relatively small tanks at lower this work may lead to extremely impor- of its stored hydrogen. They confirmed the purity of the discharged gas by using pressure than is currently possible, com- tant technology." ments Robert H. Williams, a senior reMilton W. Cole, professor of physics gas chromatography-mass spectrometry. Explaining why this form of carbon is search scientist at Princeton University's at Pennsylvania State University, UniverCenter for Energy & Environmental Stud- sity Park, points out that, in addition to capable of storing so much hydrogen, ies. Other hydrogen-storage methods- automotive applications, the work has Rodriguez points out that the separation like compressing the gas in high-pressure important implications for basic sci- between graphite sheets in the herringcylinders or converting metals to metal hy- ence. The mechanism for hydrogen stor- bone structure is about 3.4 À—only drides—are being investigated, he adds, age in this material is not well under- slightly more than hydrogen's average or but currently these procedures are too stood, he explains. And the Northeastern kinetic diameter, 2.9 À. The close spacbulky or too heavy for practical transporta- group's report is stimulating experimen- ing and strong adsorption interaction betion applications. tal and theoretical research in physical tween hydrogen and graphite confines the gas and causes phase transitions and The team of Northeastern chemists- adsorption. including associate professor Nelly M. "If, as discussed in the paper, hydrogen a condensation process to occur at room Rodriguez, postdoctoral associates Alan fits compactly between graphitic sheets, it temperature. "We suspect that as more and more hyChambers and Colin Park, and professor is an energetically advantageous environR. Terry K. Baker—prepares highly or- ment," says Cole. "But numerous people drogen diffuses into the nanofibers, the dered graphite nanofibers by decom- have studied similar problems using com- layers start to separate," explains Rodriposing carbon-containing gases over cat- puter simulation and not found the high guez. "The graphite swells or expands like alyst particles at high temperature. The uptake reported here. If the [Northeastern an accordion—allowing even more hydroprocess results in stacks of neatly ar- team's] experiment proves correct, an ex- gen to work its way into the structure." ranged graphite platelets with nanome- planation must invoke an intriguing revi- Under these conditions, hydrogen's mobility is suppressed, she says, causing the gas to adopt liquidlike characteristics that account for the nanofibers' exceptionally high adsorption capacity. According to Rodriguez, the Northeastern chemists are continuing to examine uptake capacity and currently are exploring methods for retrieving an even greater fraction of the stored gas. Hydrogens (red) entering the slit-shaped pore formed by two of the graphite platelets In a nanoflber (left). Mitch Jacoby
HYDROGEN SUPERSPONGES
Unique structure of novel graphite nanofibers leads to unprecedented gas uptake
A
6 MAY 25, 1998 C&EN
