Tetsu Kiyobayashi

Hydrogen storage in carbon nanostructures

Abstract Carbon nanotubes have been known for more than 10 years. It is a challenge to fill their unique tubular structure with metals and gases. Especially, the absorption of hydrogen in single wall nanotubes has attracted many research groups worldwide. The values published for the quantity of hydrogen absorbed in nanostructured carbon materials varies between 0.4 and 67 mass%. With the assumption that the hydrogen condenses in the cavity of the nanotube or forms an adsorbed monolayer of hydrogen at the surface of the tube, the potential of nanotubes as a host material for hydrogen storage can be estimated. The hydrogen storage density due to condensed hydrogen in the cavity of the tube depends linearly on the tube diameter and starts at 1.5 mass% for a 0.671 nm single wall carbon nanotube. The surface adsorption of a monolayer of hydrogen leads to a maximum storage capacity of 3.3 mass%. We have investigated a large number of nanostructured carbon samples, i.e. high surface area graphite, single wall and multiwall nanotubes, by means of volumetric gas adsorption, galvanostatic charge/discharge experiments and temperature programmed desorption spectroscopy. The reversible hydrogen capacity of the carbon samples measured in an electrochemical half-cell at room temperature correlates with the specific surface area (BET) of the sample and is 1.5 mass% /1000 m 2 / g .

X-ray diffraction studies of titanium and zirconium doped NaAlH4: elucidation of doping induced structural changes and their relationship to enhanced hydrogen storage properties

Abstract X-ray diffraction patterns of NaAlH4 doped with up to 10 mol.% of either titanium or zirconium do not contain Bragg peaks for the bulk metals or their aluminum alloys. Instead the hydride lattice parameters a and c undergo significant contraction upon 2 mol.% doping and then expand as the doping level increases from 2 to 5 mol.%. These results are explained by a model that entails substitution of sodium cations by variable valence transition metal cations and the creation of Na+ vacancies in the bulk hydride lattice.

“Hybrid hydrogen storage vessel”, a novel high-pressure hydrogen storage vessel combined with hydrogen storage material

Abstract Potential of a novel hydrogen storage vessel, “hybrid hydrogen storage vessel”, combining an aluminum–carbon fiber reinforced plastic (Al–CFRP) composite vessel and hydrogen storage alloy, is reported through calculation of the weight and volume of the hydrogen storage system for 5 kg of hydrogen. Evaluation of this system showed that the concept of the hybrid hydrogen storage vessel allowed us to realize a hydrogen storage system advantageous in both gravimetric and volumetric hydrogen density compared with conventional hydrogen storage techniques. The hybrid vessel requires a hydrogen storage alloy with a higher volumetric hydrogen density as well as a higher gravimetric density, and with a higher equilibrium hydrogen pressure than the hydrogen storage alloys which have been used for conventional hydrogen storage vessels.

Hydrogen sorption by carbon nanotubes and other carbon nanostructures

Abstract We have analyzed the hydrogen storage capability of a set of carbon samples including a variety of carbon nanotubes, in the gas phase and in the electrolyte as well. The nanotube samples synthesized in our laboratory by pyrolysis of acetylene are of the multi-wall type. The hydrogen sorption properties of our synthesized nanotubes were compared with the properties of commercially available nanotubes and high surface area graphite as well. The nanotube samples and the high surface area graphite as well absorb hydrogen up to 5.5 mass% at cryogenic temperatures (77 K). However, at room temperatures this value drops to ≈0.6 mass%. The electrochemical experiments on the carbon samples showed a maximum discharge capacity of 2.0 mass% at room temperature (298 K). The hydrogen tends to covalently bind to carbon when the absorption takes place at elevated temperatures (>573 K). Therefore, hydrocarbons desorbed from the sample were analyzed by means of temperature programmed desorption measurements. We conclude that the adsorption of hydrogen on nanotubes is a surface phenomenon and is similar to the adsorption of hydrogen on high surface area graphite.

Indigo carmine: An organic crystal as a positive-electrode material for rechargeable sodium batteries

Using sodium, instead of lithium, in rechargeable batteries is a way to circumvent the lithiums resource problem. The challenge is to find an electrode material that can reversibly undergo redox reactions in a sodium-electrolyte at the desired electrochemical potential. We proved that indigo carmine (IC, 5,5′-indigodisulfonic acid sodium salt) can work as a positive-electrode material in not only a lithium-, but also a sodium-electrolyte. The discharge capacity of the IC-electrode was ~100 mAh g−1 with a good cycle stability in either the Na or Li electrolyte, in which the average voltage was 1.8 V vs. Na+/Na and 2.2 V vs. Li+/Li, respectively. Two Na ions per IC are stored in the electrode during the discharge, testifying to the two-electron redox reaction. An X-ray diffraction analysis revealed a layer structure for the IC powder and the DFT calculation suggested the formation of a band-like structure in the crystal.

Hydrogen adsorption in carbonaceous materials

A volumetric apparatus for gas phase hydrogen/helium adsorption and desorption measurement aimed at carbon materials was constructed. The performance of the apparatus was assessed using activated carbons and vapor grown carbon nanofibers, and was proved to be applicable for these materials of low apparent densities with sufficient accuracy. Materials used in this study did not show a significant storage capacity of hydrogen. The obtained result, however, will provide reference data for future study to develop the carbon material for hydrogen storage.

Metal hydride fuel cell with intrinsic capacity

An alkaline metal hydride (MH) fuel cell with built-in energy storage was constructed and its performance was examined. The cell employed a bifunctional air-cathode using a La0.6Ca0.4CoO3 perovskite catalyst. The anode contained the intermetallic compound MmNi3.5Co0.7Al0.7Mn0.1 as the active material. The cell voltage at was (air) and (oxygen). Using air, of continuous power was delivered from the fuel cell for almost . A maximum power output of (oxygen) was measured. The system acts as a fuel cell with built-in capacity because the hydrogen fuel in the anode is stored as MH. The H2-rechargeable MH-electrode showed an attractive recharge rate suitable for practical use: 87% of the full capacity was recharged within . By using a bifunctional air-electrode the MH-electrode can also be recharged electrochemically. The fuel cell can therefore be regarded as a battery which is rechargeable like a normal secondary cell, but which has the advantage of also being rechargeable with hydrogen gas.

Hydrogenation characteristics of Ti2Ni and Ti4Ni2X (X=O, N, C)

Ternary Ti4Ni2X (X=O, N, C) compounds and a binary Ti2Ni compound were compared based on their hydrogen pressure–composition–temperature (PCT) relations. It was demonstrated that the ternary compounds desorbed most of the absorbed hydrogen under moderate conditions such as room temperature and atmospheric pressure. The hydrogen desorption pressures of the Ti4Ni2X compounds were more than two orders of magnitude higher than the desorption pressure of the binary compound. In spite of the significant increase in the hydrogen desorption pressure, the slope of the PCT curve of each ternary compound was not large compared with that of the Ti2Ni one. These four Ti2Ni-based compounds and their corresponding hydrogen occlusion ones were discussed in terms of their relative thermal stability.

Reversible hydrogen absorption and desorption achieved by irreversible phase transition

Abstract Phase relations in the Zr7Ni10-H system were examined with a conventional Sieverts’-type apparatus. The hydrogen pressure–composition–temperature (PCT) relations obtained under the conditions such as a temperature of 410 K and a pressure range of 0.01–3 MPa demonstrated the existence of two hydride phases (β, γ) in the system and indicated the following unprecedented hydrogenation behavior of the Zr7Ni10 compound: (1) the β-hydride phase, which has lower hydrogen content than the other (γ), appears only in hydrogen desorption but not in hydrogen absorption; (2) the hydrogen capacity of the continuous solid solution phase (α) is 0.56H/M (H/M; the atomic ratio of hydrogen to metal) in hydrogen absorption and only 0.40H/M in hydrogen desorption; (3) The Zr7Ni10 compound absorbs hydrogen accompanied by the phase transition from α to γ (α→γ transition) and desorbs hydrogen accompanied by the γ→β and β→α transitions, however, the reverse of these three transitions, i.e. γ→α, β→γ and α→β transitions are not observed in the PCT relations under the conditions mentioned above. From the fact about the irreversibility of the phase transitions, the Zr7Ni10 compound can reversibly absorb and desorb hydrogen by the irreversible phase transition.

Molecular ion battery: a rechargeable system without using any elemental ions as a charge carrier

Is it possible to exceed the lithium redox potential in electrochemical systems? It seems impossible to exceed the lithium potential because the redox potential of the elemental lithium is the lowest among all the elements, which contributes to the high voltage characteristics of the widely used lithium ion battery. However, it should be possible when we use a molecule-based ion which is not reduced even at the lithium potential in principle. Here we propose a new model system using a molecular electrolyte salt with polymer-based active materials in order to verify whether a molecular ion species serves as a charge carrier. Although the potential of the negative-electrode is not yet lower than that of lithium at present, this study reveals that a molecular ion can work as a charge carrier in a battery and the system is certainly a molecular ion-based “rocking chair” type battery.