Kohei Miyazaki

Electrochemical characterization of single-layer MnO2 nanosheets as a high-capacitance pseudocapacitor electrode

A MnO2/carbon composite comprised of single layer MnO2 nanosheets and acetylene black (AB) was assembled by simply mixing a MnO2 nanosheet colloidal suspension and AB suspension. MnO2 nanosheets were synthesized by a one-pot procedure and adsorbed onto AB aggregates as a single layer (monosheets) through electrostatic interactions. The one-pot synthesis ensures no unexfoliated material, unlike previous preparations. Galvanostatic charge–discharge measurements and cyclic voltammetry in organic electrolytes demonstrate that the MnO2/AB composite exhibits an excellent specific capacity and capacitance of 400 mA h g−1 and 739 F g−1, respectively, at a scan rate of 1 mV s−1. The obtained specific capacity considerably exceeds the theoretical capacity; this can be explained by the extremely high fraction of single layer sheets from the one-pot procedure.

Single-step synthesis of nano-sized perovskite-type oxide/carbon nanotube composites and their electrocatalytic oxygen-reduction activities

Composites of nano-sized perovskite-type oxides of La1−xSrxMnO3 (LSMO) and carbon nanotubes (CNTs) were synthesized in a single step by the electrospray pyrolysis method, and their electrocatalytic activities for oxygen reduction were evaluated in an alkaline solution. The resulting LSMO nanoparticles with a diameter of less than 20 nm were well dispersed and deposited on the surface of CNTs. Elemental analysis showed that the metal-composition of LSMO/CNT composites was controlled by altering the concentrations of a precursor solution. Rotating-disk-electrode measurements revealed that the electrocatalytic activities of LSMO/CNT composites increased with an increase in a molar ratio of Sr element. Composites of LSMO nanoparticles and CNTs showed greater catalytic activities than conventional LSMO particles (1 µm) supported on carbon black for oxygen reduction. Moreover the LSMO/CNT catalyst showed larger oxygen-reduction currents even in the presence of ethylene glycol while a Pt disk electrode was affected by the oxidation currents of ethylene glycol. These results indicate that LSMO/CNT composites are a promising candidate as a cathode catalyst with a higher catalytic selectivity for oxygen reduction and a higher crossover-tolerance for use in anion-exchange membrane fuel cells.

Role of Edge Orientation in Kinetics of Electrochemical Intercalation of Lithium-Ion at Graphite

A relation between the amount of edge orientations and the kinetics of lithium-ion intercalation at the basal plane of highly oriented pyrolytic graphite (HOPG) was studied. The fraction of edge orientations (f(edge)) exposed on the basal plane of HOPG was evaluated from the kinetics of heterogeneous electron transfer of Ru(NH(3))(6)(2+/3+). Obtained f(edge) values ranged from 0.025 to 0.067, which were in good agreement with those previously reported. The charge-transfer resistance (R(ct)) for lithium-ion intercalation at the same HOPG samples was evaluated by ac impedance spectroscopy in an electrolyte consisting of 1 mol dm(-3) LiClO(4) dissolved in a mixture of ethylene carbonate/dimethyl carbonate (1:1 by vol). A clear correlation was observed between the f(edge) and R(ct) values at the basal plane of 15 HOPG samples, and the edge-area specific resistance was evaluated to be ca. 200 Ω cm(2) at the electrode potential of 0.2 V versus Li/Li(+). These results indicate that the amount of edge orientations is one of the determining factors in the kinetics of lithium-ion intercalation at graphite.

Electro-oxidation of methanol on gold nanoparticles supported on Pt/MoOx/C

Gold nanoparticles supported on Pt/MoO x /C were prepared using the chemical vapor deposition method, and their electrocatalytic activities for methanol oxidation in acid solution were evaluated. The gold nanoparticles on Pt/MoO x /C were about 4 nm in diameter. The catalytic properties of such catalysts containing gold nanoparticles were studied in 1 mol dm - 3 HClO 4 with 1 M CH 3 OH by the steady-state polarization method using a conventional three-electrode cell. Steady-state currents were measured after 1000 s from the beginning of electro-oxidation at potentials from 350 to 700 mV. The catalytic activities of Pt/MoO x /C and Pt/C were also studied for comparison. As a result, gold nanoparticles enhanced the electrocatalytic activities for methanol oxidation in the potential region below 450 mV (vs reversible hydrogen electrode) compared with the catalysts Pt/MoO x /C and Pt/C.

Hierarchically porous monoliths of oxygen-deficient anatase TiO2−x with electronic conductivity

We report an electrically conducting oxygen-deficient anatase TiO2−x monolith with macro/meso/micro trimodal pores, prepared by reducing insulating porous TiO2 monoliths at low temperature. Without adding any conductive materials, the TiO2−x monolith itself was electrically conducting enough for lithium insertion, potentially opening a new avenue for carbon-free electrode materials.

Aminated Perfluorosulfonic Acid Ionomers to Improve the Triple Phase Boundary Region in Anion-Exchange Membrane Fuel Cells

We propose an approach to improve the triple phase boundary (TPB) of catalyst layers in anion-exchange membrane fuel cells by using aminated Nafion ionomers with amine molecules of ethylenediamine (EDA) and diethylenetriamine (DETA) as anion conductor. Aminated Nafion ionomers were characterized and clarified by Fourier transform IR spectroscopy, Raman spectroscopy, and transference number measurements. The transference number of the aminated Nafion ionomers with DETA (DETA-modified Nafion, t_ = 0.89) was larger than that of the EDA-modified Nafion (t_ = 0.81). Pt/C catalyst layers with EDA- and DETA-modified Nafion ionomers were constructed, and their oxygen reduction currents were evaluated under the same conditions as in anion-exchange membrane fuel cells. Electrochemical measurements of oxygen reduction currents showed that the order of electrode performance was DETA-modified > EDA-modified > K-form Nafion (neutralized Nafion with KOH). We effectively improved the TPB region in catalyst layers by introducing aminated Nafion ionomers and revealed the relationship between the conductivity of OH- ion in the aminated Nafion ionomers and the number of amine functional groups in an amine molecule.

In situ Raman investigation of electrolyte solutions in the vicinity of graphite negative electrodes

The structure of electrolyte solutions plays an important role in the lithium-ion intercalation reaction at graphite negative electrodes. The solvation structure of an electrolyte solution in bulk has been investigated previously. However, the structure of an electrolyte solution at the graphite negative electrode/electrolyte solution interface, where the lithium-ion intercalation reaction occurs is more important. In this study, the structure of electrolyte solutions in the vicinity of a graphite negative electrode was investigated using in situ Raman spectroscopy during the 1st reduction process in 1 mol dm-3 LiClO4/ethylene carbonate (EC) + diethyl carbonate (DEC) (1 : 1 volume ratio), 1 mol dm-3 LiCF3SO3/propylene carbonate (PC), and 1 mol dm-3 LiCF3SO3/PC + tetraethylene glycol dimethyl ether (tetraglyme) (20 : 1 volume ratio). As a result, in the electrolyte solutions in which the lithium-ion intercalation reaction can occur (LiClO4/EC + DEC and LiCF3SO3/PC + tetraglyme), the Raman spectra of free solvent molecules (EC or PC) and anions showed a positive vibrational frequency shift during the co-intercalation reaction, and these shifts returned to their original positions during the lithium-ion intercalation reaction. On the other hand, there is no vibrational frequency shift in LiCF3SO3/PC, an electrolyte in which the lithium-ion intercalation reaction cannot occur. Based on our results, the relationship between the Raman shift and the solid electrolyte interphase (SEI) formation process was discussed.

Solid electrolyte interphase formation in propylene carbonate-based electrolyte solutions for lithium-ion batteries based on the Lewis basicity of the co-solvent and counter anion

Propylene carbonate (PC) is widely regarded as the superior organic solvent for lithium-ion batteries used in cold areas owing to its low melting point. However, an effective solid electrolyte interphase (SEI) is not formed in PC-based electrolyte solutions and reversible intercalation and de-intercalation of the lithium ions at the graphite negative electrode do not proceed. This leads to decomposition of the electrolyte solution and exfoliation of the graphite electrode. One solution to this problem is to control the structure of the solvated lithium ions. In this study, we focused on the Lewis basicity of the co-solvent and counter anion in the lithium salt to control the solvation in a PC-based electrolyte solution and form an effective SEI. Triglyme and tetraglyme were used as the co-solvents, and lithium bis(fluorosulfonyl)amide and lithium trifluoromethanesulfonate were used as the anion sources. SEI formation was investigated by charge and discharge measurements and in situ scanning probe microscopy; the obtained results indicated that SEI formation is strongly influenced by the Lewis basicity of the co-solvent and counter anion.Graphical Abstract

Structural insights into ion conduction of layered double hydroxides with various proportions of trivalent cations

We report herein the structural influence of in-plane cation arrangements on ion conductivities of layered double hydroxides (LDHs). Mg–Al–CO3 and Mg–Ga–CO3 LDHs with different proportions of trivalent cations were synthesized by the co-precipitation method, and we found that their ion conductivities presented a steep increase only at peculiar proportions of trivalent cations. Electron-beam diffraction patterns revealed that LDHs with such proportions had an ordered honeycomb arrangement of di- and trivalent cations in (0001) hydroxide layers. We propose a crucial insight that relates the order/disorder arrangement and ion conductivities for better understanding the nature of LDHs.

Lithium-ion intercalation and deintercalation behaviors of graphitized carbon nanospheres

As negative electrode materials for lithium-ion batteries, graphitized carbon nanospheres (GCNSs) exhibit excellent capacity retention and high-rate capability. GCNSs with diameters less than 1 μm possess core–shell structures with a low graphitized core and an outer graphitized shell arranged in a concentric orientation. In the present work, the structural changes in GCNSs with various sizes and heat-treatment temperatures during lithium-ion intercalation and deintercalation were examined. Charge–discharge measurements and in situ Raman spectroscopy were used to investigate the origin of the specific electrochemical properties of GCNSs. The results indicated that GCNSs with small sizes and low heat-treatment temperatures did not exhibit the formation of stages 4 and 3 during lithium-ion intercalation and deintercalation. In addition, phase transition between different stage structures was ambiguous. This behavior was related to the disordered stacking structures of the outer graphene layers. In addition to the decreased size that increased the specific surface area and reduced the lithium-ion diffusion length within a particle, the high-rate capability of GCNSs can be ascribed to the monotonous and moderate structural changes. The excellent capacity retention can also be attributed to the specific structure of GCNSs.