•Ionic and electronic effective conductivities of electrode layers were measured.•TLMs with electronic resistance were used for the analysis of EIS.•An alternative way to use TLMs, named “electron-electron connection”, is proposed.•The obtained ionic conductivities were compatible with those by traditional way.The ionic and electronic effective conductivities of an electrode mixture layers for all-solid-state lithium-ion batteries containing Li2SP2S5 as a solid electrolyte were investigated by AC impedance measurements and analysis using a transmission-line model (TLM). Samples containing graphite (graphite electrodes) or LiNi0.5Co0.2Mn0.3O2 (NCM electrodes) as the active material were measured under a “substrate | sample | bulk electrolyte | sample | substrate” configuration (ion-electron connection) and a “substrate | sample | substrate” configuration (electron-electron connection). Theoretically, if the electronic resistance is negligibly small, which is the case with our graphite electrodes, measurement with the ion-electron connection should be effective for evaluating ionic conductivity. However, if the electronic resistance is comparable to the ionic resistance, which is the case with our NCM electrodes, the results with the ion-electron connection may contain some inherent inaccuracy. In this report, we theoretically and practically demonstrate the advantage of analyzing the results with the electron-electron connection, which gives both the ionic and electronic conductivities. The similarity of the behavior of ionic conductivity with the graphite and NCM electrodes confirms the reliability of this analysis.

•We prepared cobalt octaethylporphyrin (Co-OEP)-modified perovskite/carbon catalysts.•ORR activity of perovskite/carbon was enhanced by Co-OEP-modification.•RRDE measurements suggested that the 2 + 2 electron reduction of O2 is promoted.•The porphyrin plays a role as a two-electron O2 reduction catalyst to give HO2−.•HO2− is further reduced to OH− by the perovskite-type oxide.Perovskite-type oxide-carbon (Vulcan XC72) mixture (La0.6Sr0.4Mn0.6Fe0.4O3/C) was modified by a metalloporphyrin (cobalt octaethylporphyrin: Co-OEP) having two-electron O2 reduction activity, and its electrochemical reduction activity for O2 (ORR) was investigated in an alkaline solution by rotating ring disk electrode (RRDE) voltammetry. The Co-OEP/La0.6Sr0.4Mn0.6Fe0.4O3/C catalyst showed improved ORR activity, with a positive shift of the onset potential. In addition, a decreased ring current compared to Co-OEP/C suggested that the quasi-four-electron reduction of O2 was also enhanced. Further experiments showed that ORR activity was also enhanced by Co-OEP-modification of other types of carbon (Ketjenblack EC600JD, Denka Black) or perovskite-type oxide (La0.6Ca0.4Mn0.6Fe0.4O3, La0.8Sr0.2Co0.6Fe0.4O3). In the case of the addition of other porphyrin complexes (cobalt tetraphenylporphyrin (Co-TPP), iron octaethylporphyrin (Fe-OEP)) to a La0.6Sr0.4Mn0.6Fe0.4O3/C catalyst, the onset potential did not shift to the positive side due to the lower activity compared to Co-OEP.

•A new structure for a redox flow battery to avoid crossover is proposed.•An indirect fuel cell using the newly proposed redox flow battery was also proposed.•The negative and positive halfs of an indirect fuel cell were demonstrated separately.A new design of a redox flow battery (RFB), which is composed of two subcells separated by a gas phase of hydrogen, is proposed to eliminate the crossover of ionic species between the anolyte and catholyte. This idea not only increases the possible combinations of the two electrolytes, but also opens up the prospect of a revival of the old idea of an indirect fuel cell, which is composed of an RFB and two chemical reactors to regenerate the electrolytes using a fuel and oxygen. This paper describes the operation of a subcell as a component of an indirect fuel cell system. In the cycling test, oxidation/reduction of the electroactive species in each electrolyte were repeated with a hydrogen electrode as the counter electrode. This result demonstrates the possibility of this newly proposed RFB without crossover. In the operation of the subcell with a chemical reactor, a molecular catalyst (a rhodium porphyrin) was dissolved in the anolyte, and then a fuel was bubbled in the anolyte reservoir. As the electroactive species was reduced by the fuel, a steady-state oxidation current was observed at the cell. This demonstrates the negative half of the newly proposed indirect fuel cell.

In this study, soluble redox couples were used as active materials for an electrode using a newly designed two-compartment cell. In this cell, liquid electrolyte was separated by a solid electrolyte diaphragm, which prevents dissolved active materials from reaching the counter electrode. To balance the apparent current density and the apparent energy density, a porous sheet made of carbon paper as a current collector was set on the side of the positive electrode with an active material impregnated into it, and Li foil was set on the side of the negative electrode. Some soluble benzoquinone derivatives were examined by charge/discharge cycling for use as active materials of the positive electrode in lithium secondary batteries. Some of them showed specific capacities close to the theoretical values, assuming two-electron reduction. Among them, 2,5-dipropoxy-1,4-benzoquinone (DPBQ) could be cycled regardless of whether the amount used exceeded the solubility (with precipitate in the electrolyte) or not (all is dissolved). This implies that the active material reacts at the surface of the current collector in the dissolved state, and the precipitated fraction also participates by dissolution into the electrolyte. The results also suggest that a good cycle performance using our two-compartment cell requires an active material with relatively high solubility.Highlights► We designed a two-compartment cell to improve the cycle performance of soluble active materials. ► Some soluble benzoquinone derivatives showed capacities close to the theoretical values. ► Precipitated 2,5-dipropoxy-1,4-benzoquinone cycled by dissolution into the electrolyte. ► High solubility of the active material is preferable for our cell.

A new method for the simultaneous measurement of the effective ionic conductivity and effective electronic conductivity in a porous sheet consisting of an electronic conductor and an ionic conductor (electrolyte) is proposed. In this method, two potentiostats and a voltage source are used, and these instruments are connected to the sample via two terminals made of the electronic conductor and two terminals made of the ionic conductor. The underlying principle of this technique does not include any assumption about charge-transfer resistance at the ionic conductor/electronic conductor interface, since the measurement is designed to let the potential difference at the interface be the OCP value at each point throughout the sample. The validity of this method was confirmed by measuring a pseudo-catalyst layer for a proton-exchange membrane fuel cell (PEMFC). The results revealed that the effective ionic and electronic conductivity depends on the relative humidity.

The proton conductivity of recast Nafion® thin films in the lateral direction (parallel to the interface) was measured in humidified atmospheres. The conductivity decreased with a decrease in the thickness of the film: e.g., the conductivity of a film with a thickness of about 100 nm was about an order of magnitude less than that of the bulk material. The dependence of the conductivity on temperature was also measured, and a thinner film showed a higher apparent activation energy for conduction. Since both the conductivity and the apparent activation energy for conduction were affected by the thickness, these phenomena may be due to an intrinsic change in the material. Based on the fact that the apparent activation energy for conduction in the bulk membrane under dry conditions is high, the high apparent activation energy for conduction in thin films may be due to the hindrance of water adsorption.

As a factor that may affect the accuracy of the measurement of electrode potentials in proton-exchange membrane fuel cells, the potential difference in a strip of Nafion® 117 membrane induced by a humidity or temperature gradient was examined. The sample was placed in a cell with two chambers, where the temperature and humidity could be controlled individually. Hydrogen gas was fed to both chambers, and the voltage between two electrodes set at the ends of the sample, which acts as a reversible hydrogen electrode, was measured. A higher potential of the electrode was observed when humidity or temperature was set lower than the other electrode.

To measure local phenomena in a PEMFC during a transitional state induced by changing of the feeding gas, a segmented cell was fabricated and the local current and local potential distribution were measured under open-circuit conditions. The anode or cathode was divided into 97 segments of 1.5 mm each. A change in the anode gas from nitrogen or oxygen to hydrogen induced momentary internal currents among the segments. The potential distribution in the electrolyte was observed simultaneously using three quasi-reference electrodes located locally. The results supported the reverse-current decay mechanism, which is known to be a mechanism of cathode degradation. Furthermore, internal currents were observed when the cathode gas was changed from nitrogen to oxygen. While the cathode was not subjected to a harmful potential, a large potential distribution was induced in the anode.

To investigate the stability of platinum particles, a flat carbon substrate coated with platinum nano-particles was made as a model of PEMFC cathode catalyst. Changes of the platinum particles caused by potential retention in acid solution were observed by FE-SEM. Stability of the carbon substrate without platinum was also observed by AFM. The position of the AFM and FE-SEM observation before and after each potential retention was identical so that morphological change could be traced like in situ experiments. Behaviors of each particle, i.e. disappearance, migration and aggregation, was observed after potential retentions. Statistical analysis of the images revealed that small platinum particles on the model electrode decreased in number, especially at high potentials and under an oxygen atmosphere.

In order to estimate the durability of Nafion® membrane as an electrolyte for direct methanol fuel cells (DMFCs), the degree of dissolution of Nafion® membranes in mixtures of methanol and water at various temperatures up to 80 °C was examined. At 80 °C, more than 30% of the membrane was dissolved in mixed solvents with methanol concentrations of higher than 80%. Dissolution of recast films made from Nafion® solution was also examined, because it is an important component of the catalyst layers of DMFCs. The effects of heat treatment on the durability of the recast films were also examined. Although high temperature (160 °C for 1 min) or long time (120 °C for 1 h) heat treatment improved significantly the durability at room temperature, the films were dissolved at 80 °C and the amounts of dissolution were larger than that of Nafion® 117 membranes.

The effect of the ionomer content on the mass transport ability in gas diffusion electrodes of proton exchange membrane fuel cells (PEMFCs) was investigated. The influence of the catalytic activity caused by a change in ionomer content was eliminated by adjusting the amount of the catalyst. The mass transport ability was evaluated by the current of the oxygen reduction reaction at 0.8–0.7 V versus RHE, after normalizing the catalytic activity of the electrodes using the current at 0.9 V as an index of the activity. From the results, the optimum content of the ionomer was obtained from the viewpoint of the compromise of the ionic access and the gas access. Furthermore, in order to clarify the extension of the carbon ∣ ionomer interface, the electrical double layer capacitance was measured using electrodes without catalyst. Based on the results of these two series of experiments, the microscopic configuration of the ionomer impregnated inside the electrodes was discussed.