Shigenori Mitsushima

Methanol-Tolerant Oxygen Reduction Electrocatalysts Based on Pd-3D Transition Metal Alloys for Direct Methanol Fuel Cells

Palladium-based alloys, such as Pd-Co, Ni, and Cr, have been developed as a novel methanol-tolerant oxygen reduction electrocatalyst for direct methanol fuel cells. The Pd alloy electrocatalysts were fabricated by a rf sputtering method. Their electrochemical characteristics for the oxygen reduction reaction (ORR) were determined in sulfuric acid solution with and without methanol at 30°C. The Pd alloys showed a higher ORR electrocatalytic activity than Pd, although lower than Pt. The Pd alloys also had no electrocatalytic activity for methanol oxidation in the presence of methanol. The maximum electrocatalytic activities for ORR were observed for the alloy composition of ca. 60 atom % Pd in all the Pd alloys. Based on the X-ray photoelectron surface analysis, it was confirmed that the filling of the Pd d-band by alloying decreased the density of states (DOS) at the Fermi level. The decreased DOS inhibited the formation of Pd oxide on the surface of the electrocatalyst. This result should contribute to the improvement of the ORR activity of the Pd alloy electrocatalysts.

Consumption Rate of Pt under Potential Cycling

The consumption rate of Pt in sulfuric acid under potential cycling has been investigated in 1 mol dm -3 H 2 SO 4 at 40°C as a basic study to understand the consumption rate of the electrocatalyst of polymer electrolyte fuel cells. With a higher potential limit, E H was 1.5 V vs reversible hydrogen electrode (RHE), the consumption rate was a few ng cm -2 cycle -1 for rectangular and symmetric triangular waves, and the consumption rate increased with the E H up to 1.8 V vs RHE. Among the triangular waves between 0.5 to 1.8 V vs RHE, the consumption rate of symmetric waves and fast cathodic asymmetric triangular waves were a few ng cm -2 cycle -1 . However, the slow cathodic triangular wave, which were 20 V s -1 anodic and 0.5 V s -1 cathodic, showed 24 ng cm -2 cycle -1 consumption rate. The ratio in the difference of charge to the consumption showed that the electron transfer number was about 2 for the slow cathodic triangular wave, and that of the electron transfer number was about 4 for the symmetric waves and the fast cathodic triangular waves. Therefore, PtO 2 + 4H + + 2e - → Pt 2+ + 2H 2 O that follows the Pt oxidation, Pt + 2H 2 O → PtO 2 + 4H + + 4e - would occur under the enhanced condition, and Pt → Pt 4+ + 4e - would occur under symmetric conditions.

Brønsted acid-base ionic liquids and their use as new materials for anhydrous proton conductors.

Novel Brønsted acid-base ionic liquids, derived from a simple combination of a wide variety of organic amines with bis(trifluoromethanesulfonyl) amide are electroactive for H2 oxidation and O2 reduction at a Pt electrode under non-humidifying conditions, which shows the prospect of the use of protic ionic liquids as new materials for anhydrous proton conductors at elevated temperatures.

Solubilities of Nickel Oxide in Molten Carbonate

Cathode dissolution is a major problem for the development of the molten carbonate fuel cell. In order to evaluate the stability of the cathode, the solubility of NiO has been measured for several compositions of molten alkaline carbonates in the CO 2 pressure range from 10 -5 to 1 atm and the temperature range from 813 to 1023 K. The solubility depended on the CO 2 pressure, although the solubility of NiO was independent of either O 2 or H 2 O pressure

Zirconium-Based Compounds for Cathode of Polymer Electrolyte Fuel Cell

Zirconium-based compounds, such as ZrO x N y , have been investigated as a cathode of polymer electrolyte fuel cells. ZrC x N y was prepared using radio-frequency (rf) magnetron sputtering under an Ar + O 2 + N 2 with heating the substrate. The solubility of ZrO x N y deposited at 500°C was less than 5 X 10 -8 mol dm -3 in 0.1 mol dm -3 H 2 SO 4 at 30°C under atmospheric condition, indicating that ZrO x N y had a high chemical stability in acidic media. The effect of the heat-treatment during the film deposition on the catalytic activity for oxygen reduction reaction (ORR) and the properties of ZrO x N y has been examined. The ORR current density on ZrO x N y deposited at a substrate temperature of 800°C was ∼30 times greater than that at 50°C. The dependence of both the ORR current density at 0.4 V and the apparent activation energy of the ORR at 0.4 V on the substrate temperature changed near 500°C. The crystalline structure of ZrO x N y also changed near 500°C. In addition, the catalytic activity for the ORR increased with the increasing crystallinity and decreasing ionization potential of the specimens. Therefore, the crystalline structure, the crystallinity, and the ionization potential might affect the catalytic activity for the ORR.

Transition Metal Oxides as DMFC Cathodes Without Platinum

Transition metal oxides (ZrO 2 , TiO 2 , SnO 2 , Nb 2 O 5 , and Co 3 O 4 ) made by sputtering methods were investigated as new electrocatalysts for the cathode of a direct methanol fuel cell (DMFC) without platinum. The catalytic activity for the oxygen reduction of these transition metal oxides was evaluated in sulfuric acid with and without 0.1 M methanol. The sputtered metal oxides had stable states in acid medium at least in the potential range of the oxygen reduction. The metal oxides were not active for the methanol oxidation. The oxygen reduction activity of the metal oxides decreased in the following order: ZrO 2-x > Co 3 O 4-x > TiO 2-x ≈ SnO 2-x > Nb 2 O 5-x in the presence of methanol. Zirconium oxides showed the best activity for the oxygen reduction among these transition metal oxides. Moreover, in comparison with the sputtered Pt, the potential at i O2-N2 = -5 μA cm -2 of ZrO 2-x was 60 mV larger than that of the sputtered Pt. This is due to the high selectivity of the metal oxides for the oxygen reduction. The mixed potential as found on the Pt electrode was not observed on the metal oxide catalysts. Zirconium oxides could be a good cathode substituting the platinum cathode for DMFCs.

Partially Oxidized Tantalum Carbonitrides as a New Nonplatinum Cathode for PEFC–1–

Partially oxidized TaC 0.58 N 0.42 has been investigated as a cathode for polymer electrolyte fuel cells (PEFCs). Catalytic activity for the oxygen reduction reaction (ORR) significantly depends on the oxidation state of TaC 0.58 N 0.42· TaC 0.58 N 0.42 and Ta 2 O 2 had a poor catalytic activity for the ORR. The onset potential on the partially oxidized Tac 0.58 N 0.42 for the ORR had a maximum value of 0.83 V vs a reversible hydrogen electrode in 0.1 mol dm -3 H 2 SO 4 at 30°C. X-ray diffraction analysis showed that the specimens, which had a definite catalytic activity for the ORR, had both TaC x N y and Ta 2 O 5 peaks. This result indicated that an appropriate oxidation of the TaC 0.58 N 0.42 was essential to enhance the catalytic activity for the ORR.

Effect of Recast Temperature on Diffusion and Dissolution of Oxygen and Morphological Properties in Recast Nafion

We have investigated the effect of the recast temperature, i.e., heat-treatment of a polymer electrolyte, on the diffusion coefficient and solubility of oxygen in the electrolyte and also on the morphological properties of recast ion-exchange membranes for improving the cathode activity in polymer electrolyte fuel cells. Based on chronoamperometric measurements, the diffusion coefficient and solubility of oxygen were found to be deeply affected by the recast temperature. The diffusion coefficient increased with the decreasing recast temperature, while the solubility had the opposite tendency. The water uptakes and ionic cluster size also varied with the recast temperature. Based on the X-ray measurements, it is considered that the differences in the mass transport parameters, the cluster sizes, and water uptakes are due to the growth of the clusters and the crystallinity in the electrolyte.

Improvement of MCFC cathode stability by additives

Abstract Molten carbonate fuel cells (MCFCs) have shown the highest voltage efficiency among fuel cells. They could be operated over 40 000 h under ambient pressure. However, in order to obtain higher performance of MCFCs, especially for high-pressure operation, the stabilities of NiO cathode has been studied experimentally. To obtain lower solubility of Ni 2+ ion in the LiNa eutectic carbonate melt, the effect of additives has been studied. Metal oxides were added to the molten carbonate as well as to solid NiO. The addition of Fe oxide and Mg oxide to NiO was effective in reducing the solubility of NiO. The addition of rare earth oxide (La 2 O 3 ) to molten carbonate is much more effective in reducing the solubility of NiO. Considering the solubility, MCFCs could operate over 40 000 h without a Ni short even under a high-pressure condition of 12–15 atm.

Effect of tin oxides on oxide formation and reduction of platinum particles

A remarkable change in the shape of the cyclic voltammogram of Pt/SnO 2 in 1 M sulfuric acid was observed in the potential range from 0.6 to 1.0 V vs reversible hydrogen electrode. Although no change in the shape of the hydrogen adsorption/desorption region was observed during potential cycling, both the anodic current due to the formation of platinum oxides (>ca. 0.8 V) and the cathodic current due to the reduction of platinum oxides (>0.6 V) decreased. The current in the electric double-layer region gradually increased during potential cycling, and a new redox couple formed. The ratio of the hydrogen desorption charge to the oxide reduction charge of Pt/SnO 2 linearly decreased as the logarithm of the number of platinum particles per geometric unit area of SnO 2 decreased.