Choong-Nyeon Park

The hydrogen storage characteristics of Ti–Cr–V alloys

Abstract The crystal structures, the lattice parameters and the characteristics of hydrogen storage at 303 K have been investigated in ternary alloys of the Ti–Cr–V system. All of these alloys, in the range of this study, have shown a BCC structure. The hydrogen storage capacities and the effective hydrogen storage capacities of the alloys were strongly dependent on the composition ratio of Ti/Cr, showing their maximum values at a Ti/Cr ratio of about 0.75. It was also found that the lattice parameters of the alloys increased linearly with increasing Ti/Cr ratio. The differences in affinities to hydrogen and the atomic radius of each element could explain the Ti/Cr ratio dependence of the lattice parameter and hydrogen storage capacity of the alloys.

Effect of V and Zr on the electrochemical properties of La-based AB5-type metal hydride electrodes

In this paper, investigation has been carried out to find the effects of V- or Zr-addition and fluorination on the discharge capacity of AB5-type alloy with a multicomponent La–Ni–Al–Co–Mn–My (M=V, Zr; y=0.0, 0.02, 0.05, 0.1, 0.2, 0.3) system. Spectral studies have been made by EDS and SEM, and the crystal structure was also determined by XRD. In order to calculate the hydrogen storage capacity, the pressure–composition isotherms (PCT curves) were utilized. The metal hydride electrode was also used as a galvanostatic cycling test. As the amount of added V or Zr is increased, the portion of the second phase is also increased gradually, but the crystal structure, CaCu5-type Laves phase, is not changed. But, in the matrix, the main component is Ni, in one of the major phases the main component is La and in another minor phase the main component is V or Zr. It has been confirmed that the addition of V or Zr causes improvement in the discharge capacity (Zr, y≤0.05; V, y≤0.1) and the cyclability (Zr, y≤0.05; V, y≤0.1) up to 200 cycles. However, adding more than y=0.2 has some significant effect on these properties. Generally, V-containing alloys may have many second phases of a round shape appearing in dispersed form in the matrix, while Zr-containing alloys’ second phases appear with a plate or needle-like shape. La-rich particles precipitated along boundaries of the second phases which can act as sites of preferred hydrogen absorption, to increase the discharge capacity. In addition, many micro-cracks are formed around the La-rich particles at the surface of the La-rich particle itself. Therefore, such second phases are considered as catalytic sites for hydrogen penetration, surface reaction, initial activation and change transfer properties.

Uniform GeO2 dispersed in nitrogen-doped porous carbon core–shell architecture: an anode material for lithium ion batteries

Germanium oxide (GeO2), which possesses great potential as a high-capacity anode material for lithium ion batteries, has suffered from its poor capacity retention and rate capability due to significant volume changes during lithiation and delithiation. In this study, we introduce a simple synthetic route for producing nanosized GeO2 anchored on a nitrogen-doped carbon matrix (GeO2/N–C) via the sol–gel method followed by a calcination process in an inert argon atmosphere. The GeO2/N–C showed superior electrochemical performance over pure GeO2; almost 91.8% capacity retention of 905 mA h g−1 was shown after 200 cycles at the rate of C/2. Interestingly, even at a high rate of 20C, a specific capacity of 412 mA h g−1 was retained. This unique anode performance of GeO2/N–C is derived from the effective combination of nano-size GeO2 and its uniform distribution in a nitrogen-doped carbon matrix. Herein, the nitrogen doped-carbon matrix not only strengthens the structure but also promotes the lithium diffusion in the GeO2/C–N material. Further, the adaptability of GeO2/N–C as an anode in a full cell configuration in combination with the LiCoO2 cathode was demonstrated by exhibiting high specific capacity and good cyclability.

Effect of Ti and Zr additions on the characteristics of AB5-type hydride electrode for Ni–MH secondary battery

Abstract AB 5 -type intermetallic compounds were prepared by arc-melting in an argon atmosphere. The composition of a stoichiometric compound ( LMNi 3.6 Al 0.4 Co 0.7 Mn 0.3 ) M y ( LM = La - rich mischmetal , M=Ti, Zr; y=0.0, 0.02, 0.05, 0.1, 0.2, 0.3 ) with a hexagonal CaCu 5 structure was varied by stoichiometric and nonstoichiometric addition of Ti or Zr. The alloy surface was analyzed by energy-dispersive spectroscopy and scanning electron microscopy and the crystal structure was characterized by X-ray diffraction. To determine hydrogen storage capacity, the pressure composition isotherms (PCT curves) were utilized. Metal hydride electrode was characterized by galvanostatic cycling test. It was found that the addition of Ti ( y =0.05) or Zr ( y =0.02) improves the activation, discharge capacity and cycle life property. But, the alloy with Ti addition ( y ⩾0.2), containing a Zr addition ( y ⩾0.2), is found to have bad discharge capacity and cycle life. The kinetic properties are additionally increased by the formation of a Ti- or Zr-rich second phase. The improvement in discharge capacity caused by increasing the Ti or Zr content (0.1⩾ y ) in the alloy is attributed to active sites for the electrochemical reaction. Also, they seem to be strongly related with the shapes and amounts of the second phases.

Isotope effect on structural transitions of Ti1.0Mn0.9V1.1HX(DX) and Ti1.0Cr1.5V1.7HX(DX) with hydrogenation

Abstract We have investigated the structural transitions of Ti 1.0 Mn 0.9 V 1.1 H X (D X ) and Ti 1.0 Cr 1.5 V 1.7 H X (D X ) upon hydrogenation at 293 K and discussed the effect of hydrogen isotope on their crystal structures. The various hydride samples used for X-ray diffraction (XRD) investigation were obtained after measurement of the P–C isotherms by taking them out of the reactor. The crystal structures, phase abundance and lattice parameters of the hydrides were determined by the Rietveld method using XRD data. Because the hydrides of the alloy Ti 1.0 Mn 0.9 V 1.1 revealed complex peak profiles, we double-checked the structures through transmission electron microscope (TEM) investigations. The results of the TEM observation agreed well with those of XRD data. The crystal structures of corresponding isotope hydrides, the phase abundance and the lattice parameters do not depend on the kind of hydrogen isotope, but only on the hydrogen content. That is, if the corresponding isotope hydrides have the same hydrogen contents, they have also the same crystal structures, although they show a large difference between the equilibrium pressures in their P–C isotherms. At the experimental temperature, the Ti 1.0 Mn 0.9 V 1.1 alloy and Ti 1.0 Cr 1.5 V 1.7 alloy revealed different structural transition processes upon hydrogenation although the crystal structures of these two alloys are both body centered cubic (BCC). The structural transitions of the alloys Ti 1.0 Mn 0.9 V 1.1 and Ti 1.0 Cr 1.5 V 1.7 can be summarized by BCC ( a =3.0183(1) A)→body centered tetragonal (BCT) ( a =2.874(3) A, c =3.89(1) A)→face centered cubic (FCC) ( a =4.311(8) A) in alloy Ti 1.0 Mn 0.9 V 1.1 and BCC ( a =3.0212(9) A)→FCC ( a =4.261(4) A) in alloy Ti 1.0 Cr 1.5 V 1.7 . The Ti-rich phases with NiTi 2 structure and α-Ti with hexagonal close packed (HCP) structure absorbed hydrogen at relatively low hydrogen pressures and the phase abundance remained almost constant. From this fact, it can be deduced that it is desirable to decrease their amount as far as possible in order to increase the effective hydrogen storage capacities of the alloys.

Composite Gel Polymer Electrolyte Based on Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) with Modified Aluminum-Doped Lithium Lanthanum Titanate (A-LLTO) for High-Performance Lithium Rechargeable Batteries

A composite gel polymer electrolyte (CGPE) based on poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) polymer that includes Al-doped Li0.33La0.56TiO3 (A-LLTO) particles covered with a modified SiO2 (m-SiO2) layer was fabricated through a simple solution-casting method followed by activation in a liquid electrolyte. The obtained CGPE possessed high ionic conductivity, a large electrochemical stability window, and interfacial stability-all superior to that of the pure gel polymer electrolyte (GPE). In addition, under a highly polarized condition, the CGPE effectively suppressed the growth of Li dendrites due to the improved hardness of the GPE by the addition of inorganic A-LLTO/m-SiO2 particles. Accordingly, the Li-ion polymer and Li-O2 cells employing the CGPE exhibited remarkably improved cyclability compared to cells without CGPE. In particular, the CGPE as a protection layer for the Li metal electrode in a Li-O2 cell was effective in blocking the contamination of the Li electrode by oxygen gas or impurities diffused from the cathode side while suppressing the Li dendrites.

Nanostructured doped ceria for catalytic oxygen reduction and Li2O2 oxidation in non-aqueous electrolytes

Herein, we report the catalytic activities of porous nano-crystalline oxides with surface active sites synthesized by a wet chemical route. Zr doped ceria (ZDC) has been tested as active oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts in air electrodes for Li–O2 batteries. A ZDC-based air electrode exhibits a higher discharge capacity than that of a bare carbon-based air electrode. ZDC loaded in carbon air electrodes delivers a discharge capacity of 8435 mA h g−1. The higher discharge voltages of Li–O2 cells with ZDC, instead of bare carbon, could have originated from the higher oxygen reduction activity of ZDC. These oxides show a lower potential for Li2O2 oxidation than pure carbon. A significant increase in the kinetics for Li2O2 oxidation indicates the influence of the ZDC catalyst on the oxygen evolution reaction. The ORR and OER properties of the catalyst are explained in terms of the high fraction of active defect sites on its surface.

A simple method of electroless copper plating for the preparation of metal hydride electrodes

Abstract The simple method of electroless copper plating using an acidic bath containing only CuSO 4 and H 2 SO 4 has been applied to rare earth-nickel-based hydrogen storage alloy powders for the preparation of a hydride electrode. The coating characteristics and electrode properties were investigated. The coating was very easy and fast being performed within 5 min without any pretreatment for air-exposed samples. The results of chemical analyses showed a coating mechanism of an ion exchange between Cu 2+ and alloy constituents. It could be suggested from the electrochemical measurements that this copper plating method is very promising for electrode preparation in hydride batteries.

Effect of vanadium content on electrochemical properties of la-based AB5-type metal hydride electrodes

Abstract Commercial La–Ni–Al–Co–Mn–V hydrogen storage alloys have been investigated to examine the effect of non-stoichiometry on the microstructure and electrochemical properties. It is found that for the stoichiometric ‘B’-rich compound, single phase with CaCu 5 -type exists. However, for B-poor compounds, there is principally a CaCu 5 -type phase with a small amount of V-rich type phase and the amount of V-rich phase reduces with vanadium. With the increase of V y ⩽0.1 content, hydrogen storage capacity is enhanced, whereas when y =0.2–0.3 it is decreased. The discharge capacity and cyclability are increased considerably by addition of vanadium in the range 0.02–0.1 with a maximum value at about 0.02%. The decrease of capacity for high V content was also correlated with the amount of V-rich phase. The V-rich phase is consisted of La 0.1 Ni 2.6 Al 0.2 Co 2.0 Mn 0.6 V 1.3 . The improvement of kinetics is due to the catalytic effect, grain boundary diffusion effect or more pronounced alloy pulverization upon cycling. This can be explained because the improvement of capacity for alloys with low V content is due to better kinetics. These alloys have been subjected to analysis by EDS, SEM and XRD. In order to determine the hydrogen storage capacity, the pressure composition isotherms (PCT curves) have been used. The metal hydride electrodes were characterized by galvanostatic cycling test.

An electroless copper plating method for Ti, Zr-based hydrogen storage alloys

Abstract An acidic electroless copper plating method using HF has been developed and applied to the hydrogen storage alloy: Zr 0.5 Ti 0.45 V 0.54 Ni 0.87 Cr 0.15 Co 0.21 Mn 0.24 . It is shown that the copper plating mechanism is an ion-exchange process between alloy elements and copper ions. The resulting copper coating is metallic. The coated copper layer significantly improved activation properties of the alloy electrode for a Ni–MH secondary battery. Anodic polarization curves and SEM micrographs showed that the reason for this improvement is due to the dissolution of a surface oxide film by HF during the copper plating. This coating method was also successfully applied to eight other Zr(Ti) alloy powders with different compositions. It can be concluded that this electroless copper plating method can be widely applied to Ti(Zr)-based hydrogen storage alloys of various compositions which had been regarded to be difficult to coat by conventional electroless copper plating methods.