Hydrogen Desorption Kinetics in Metal Intercalated Fullerides

Abstract

For different hydrogenated metal intercalated fullerides (Na 10 C 60 -H, Li 12 C 60 -H, and Li 28 C 60 -H) the activation energies for hydrogen desorption were determined by DSC. The Vyazovkin advanced method (VA) was used for the calculation of the reaction model free activation energy as a function of the extent of conversion α. Activation energies are highest for low α and decrease for increasing α, between around 200−145 and 245−175 kJ/mol for the Na and Li compounds, respectively. The decrease of activation energy as a function of the extent of conversion can be explained by an increasing charge transfer to the C 60 H 36+y cage during desorption. Na intercalation leads to a significant thermodynamic destabilization for hydrogen desorption. Dehydrogenation enthalpies of 52 (Na 10 C 60 -H), 66 (Li 12 C 60 -H), and 69 kJ/mol H 2 (Li 28 C 60 -H) were determined. These values are lower compared to literature values for desorption of pure C 60 H 36 (74 kJ/mol H 2 ). The onsets of hydrogen desorption are 185°C (Na 10 C 60 -H), 260°C (Li 12 C 60 -H), and 250°C (Li 28 C 60 -H) compared to >400°C for pure C 60 H 36 .

Complex and liquid hydrides for energy storage

“…Reversible intercalation of Li or Na into the carbon can occur, stabilizing the metal phase and hence lowering the equilibrium decomposition temperature, for instance, that of NaH under 1 bar H 2 pressure from 425 to 225°C [154,171]. Also, lithium-and Nadoped fullerenes are capable of reversibly storing 5 wt% hydrogen [172,173].…”

Adsorption of sodium and cesium on aggregates of C60

“…Doping fullerenes with alkali or earth alkaline metals promises to elevate adsorption energies to several tenths of an eV [9,12,16]. Experiments have shown that as much as 5 wt% of H 2 could be reversibly adsorbed and desorbed in C 60 Li 6 and C 60 Li 12 ; the storage capacity of C 60 Na 6 and C 60 Na 10 was found to be slightly lower [17][18][19][20][21]. Moreover, desorption temperatures were reduced to about 260…”

On enhanced hydrogen adsorption on alkali (cesium) doped C60 and effects of the quantum nature of the H2 molecule on physisorption energies

“…Moreover, the temperature of the onset of desorption was reduced to about 250 -300 °C, well below the temperature (> 500 °C) at which hydrogen starts to desorb from pure fulleranes [33,34]. Optical spectroscopy, X-ray diffraction, neutron diffraction, NMR and Muon Spin Relaxation have been applied to probe the dynamics and structural changes that accompany ad-and desorption of hydrogen [23,[30][31][32][33][35][36][37][38][39]. Laser-desorption mass spectra of hydrogenated alkali-doped C60 solids show evidence for the presence of HnC60 (n up to 48) fulleranes [23]; other spectra show HnC60 + ions containing as many as 60 hydrogen atoms [32].…”

“…Experiments performed on bulk samples of lithium-and sodium intercalated C60 have proven enhanced values for hydrogen uptake compared to pure C60. As much as 5 wt % of H2 could be reversibly chemisorbed and desorbed in C60Li6 and C60Li12; the storage capacity of C60Na6 and C60Na10 was found to be slightly lower [22,[30][31][32][33]. Moreover, the temperature of the onset of desorption was reduced to about 250 -300 °C, well below the temperature (> 500 °C) at which hydrogen starts to desorb from pure fulleranes [33,34].…”

“…As much as 5 wt % of H2 could be reversibly chemisorbed and desorbed in C60Li6 and C60Li12; the storage capacity of C60Na6 and C60Na10 was found to be slightly lower [22,[30][31][32][33]. Moreover, the temperature of the onset of desorption was reduced to about 250 -300 °C, well below the temperature (> 500 °C) at which hydrogen starts to desorb from pure fulleranes [33,34]. Optical spectroscopy, X-ray diffraction, neutron diffraction, NMR and Muon Spin Relaxation have been applied to probe the dynamics and structural changes that accompany ad-and desorption of hydrogen [23,[30][31][32][33][35][36][37][38][39].…”