Hiroo Numata

Electrochemistry of molten salt corrosion

Abstract Thermodynamics and electrochemistry are of great importance in understanding and controlling corrosion. Molten salt corrosion has been mainly studied by the surface observation and gravimetric method. Electrochemical techniques have also been applied to this system in order to obtain information on the selection of materials and estimation of corrosion mechanisms. In this paper, the electrochemical theory and concepts of molten salt corrosion are briefly described as well as thermodynamics, and then an overview of the electrochemical evaluation methods of molten salt corrosion is given.

Neutron Evolution from a Palladium Electrode by Alternate Absorption Treatment of Deuterium and Hydrogen.

We observed neutron emissions from palladium after it absorbed deuterium from heavy water followed by hydrogen from light water. The neutron count, the duration of the release and the time of the release after electrolysis was initiated all fluctuated considerably. Neutron emissions were observed in five out of ten test cases. In all previous experiments reported, only heavy water was used, and light water was absorbed only in accidental contamination. Compared to these deuterium results, the neutron count is orders of magnitude higher, and reproducibility is much improved.

Low-temperature elastic anomalies and heat generation of deuterated palladium

Elastic parameters (the Young`s, shear, and bulk moduli; the Lame parameter; the Poisson ratio; and the Debye temperature) and shear damping anomalies accompanied by the generation of excess heat (not less than 6 W) were observed between 116 and 190 K in deuterated palladium, PdD{sub 0.719}, suggesting dynamic interactions among deuterons squeezed between tetrahedral and octahedral interstices in the palladium face-centered-cubic lattice. 42 refs., 8 figs.

Neutron Emission During a Long-Term Electrolysis of Heavy Water

Electrolysis of heavy water has been carried out for {gt} 4 months with special attention to neutron emission. The results of the measurements of the electrode potential of the palladium cathode, the temperatures of the palladium cathode and of the electrolytic solution, and the neutron count rate are described.

In Situ Potentiometric, Resistance, and Dilatometric Measurements of Palladium Electrodes during Repeated Electrochemical Hydrogen Absorption

Abstract The physicochemical properties of the Pd-H system were studied by in situ potentiometric, resistance, and dilatometric measurements in each of three applied pulse modes, A, B, and C, and repeated H absorption and desorption. Potential, resistance ratio, and an increase in dilation (Δl/l0) were measured simultaneously after H equilibrium was attained with the Pd electrode. During continuous absorption, structural phase transition (α [right arrow] β) and void formation occurred, and the values of the H/Pd ratio in the limiting α phase, in the α + β phase coexistence, and in the transition and the β+voids coexistence regions are consistent with those obtained from the Pd-H isotherm at 40°C. Hydrogen absorption caused the dilation, from whose slope the molar volume was obtained as 0.64 (α phase) and 0.40 (α + β phase) cm3/mol. The resistance increased in proportion to the H/Pd ratio and was kept constant at 1.7 to 1.8 over Rtr. For the first absorption through the β phase (>βmin), the electrode potential shifted with an increase in dilation, which suggests nonequilibrium PdH2-x precipitation followed by conversion to the β phase and void formation. Although there was a remarkable lack of any dependence on the number of repetitions of the values of the limiting resistance and potential corresponding to the α + β and β + void coexistence, the onset of the β phase, βmin, increased as the number of repetitions increased. The volumetric ratio for an increase in the H/Pd ratio corresponds to the absorption in high-density defect areas surrounding voids. During repeated absorption and desorption in the C applied pulse mode, the apparent molar volumes of the α + β phase coexistence show that absorption proceeds inhomogenously, in contrast to the first absorption in the A applied pulse mode.

Kinetics of oxygen reduction on fully and partially immersed Pt, Pd and Au electrodes in an NaCl + KCl eutectic melt

The kinetics of the O2− / 12O2 (Pt, Pd, Au) electrode in an NaCl + KCl eutectic melt at 1023 K was studied using the galvanostatic double-pulse and ac impedance methods. The cathodic reduction of oxygen proceeds via a two-step mechanism, and the rate of the rapid charge-transfer step was found from the galvanostatic double-pulse measurement to be of the order of 104 A m−2. The exchange current density obtained from the ac impedance method was one order of magnitude less than that obtained from the galvanostatic double-pulse method and probably corresponds to that of the slower reaction step. The cathodic polarization curves of the partially immersed O2− / 12O2 electrode exhibit an exponential form at a low concentration of oxide ions. The polarization behavior was well explained by a model in which the role of the reaction product (the oxide ion) on reaction kinetics was considered.

Cathodic Reduction of Oxygen on a Partially Immersed Electrode in Chloride and Carbonate Melts

In recent years, fuel cells have been developed as clean energy sources to help solve the problems of the exhaustion of fossil fuels and the progression of environmental pollution. Molten carbonate fuel cells (MCFC) are regarded as a promising energy supply because their advantages include high energy efficiency, low hazardous pollution, utilization of coal, gas, etc. Since the slow kinetics of the reduction reaction of oxygen is a major factor contributing to efficiency loss in fuel cells, a sintered NiO electrode is used to decrease the polarization. In the cells, the electrolyte creeps into pores of the porous electrode and a three-phase boundary, that is, liquid-solid-gas, is formed at the liquid meniscus. Although the reduction of oxygen on the three-phase boundary is a very important reaction in an electrochemical process, very few studies of it have been conducted.(1–3)