In order to survey history and state of art of theoretical and kinetic studies of gas electrode reaction on solid oxide electrolyte, overview was made on the research works reported by the author and his co-investigators. Through reviewing their works, frontier of research is clarified. It was shown that electrode reaction is controlled by non-faradaic chemical reaction process, and the nature of the overpotential is the chemical potential deviation of the interface from equilibrium state. Mechanical stress at the electrode/solid electrolyte interface is considered varying with the overpotential. Importance is emphasized for mechano-electrochemistry, the field to study the relationship of electrochemistry and mechanical property of the electrochemical cells.

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A divalent strontium cation highly conducting solid was successfully developed by introducing Sr²⁺ cations into the NASICON-type HfNb(PO₄)₃ solid together with W⁶⁺ ions. The (Sr_0.05Hf_0.95)_4/3.9(Nb_1−yW_y)_5/(5+y)(PO₄)₃ solid exhibited Sr²⁺ conductivity that was 3300 times higher than that of previously reported NASICON-type Sr²⁺ conductors, which was caused by the effective reduction of electrostatic interaction between conducting Sr²⁺ and surrounding O²⁻ ions due to the existence of high valent Nb⁵⁺ and W⁶⁺ cations in the structure.

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Ba_1−xSr_xZrO₃, which is the mother phase of proton-conducting Ba_1−xSr_xZr_0.9Y_0.1O_3−δ, is prepared by the Pechini method at a sintering temperature of 1500°C. For specimens with x ≥ 0.4, the sintering densities of the samples prepared by the Pechini method are approximately 90%, whereas those of the samples prepared by the solid-state reaction method are approximately 78%. Although the difference in the mean grain size was small, clear facets are observed around the grains of the Ba_0.6Sr_0.4ZrO₃ ceramic prepared by the Pechini method and grain size distribution is less than that of the Ba_0.6Sr_0.4ZrO₃ ceramics prepared by the solid-state reaction method. The smaller grain size distribution is attributed to the formation of a Ba_0.6Sr_0.4ZrO₃ phase with smaller grains upon calcination at a temperature as low as 800°C by the Pechini method due to the homogeneous cation distribution in the precursor. Not only a decrease in the number of pores but also an increase in the grain size is observed with increasing Sr content in Ba_1−xSr_xZr_0.9Y_0.1O_3−δ, resulting in a sintering density as high as 92% for Ba_0.6Sr_0.4Zr_0.9Y_0.1O_3−δ.

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In this study, effects of operating conditions on the electrochemical reduction of CO₂ on solid oxide electrolysis cell (SOEC) were systematically investigated. Experimental results revealed that the reducing gas can influence the behavior of Ni–YSZ cermet cathode. Supply of hydrogen as a reducing gas was found to be the most promising approach to obtain better cell performance for CO₂ reduction. Even a small amount of H₂ can facilitate the CO₂ reduction due to the kinetically fast reverse water gas shift reaction. High temperature operation is favorable from thermodynamic viewpoint because the Gibbs free energy significantly decreases at elevated temperature. The partial pressure of oxygen at the cathode side is another critical factor that found to be beneficial. A considerable increase in cell voltage was observed when Ni–YSZ | YSZ | LSM–YSZ cell operated under constant current densities of −0.75 and −0.90 A cm⁻² at 900°C, whereas the stable cell performance could be possible at −0.15 A cm⁻² for 7 h. The increase in cell voltage was attributed to the degradation of anode.

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Sr²⁺, Al³⁺-doped LaScO₃ perovskite-type oxide (La_0.675Sr_0.325Sc_0.99Al_0.01O₃; LSSA), which exhibits high conductivity of both oxide-ion and proton, is expected to become an electrolyte material for high-temperature solid oxide fuel cells (SOFCs). Considering the potential of this material to serve as a solid electrolyte, its chemical compatibility with some of the typical fuel electrodes commonly used in SOFCs was investigated. Severe elemental diffusion was found at the sintering temperature of 1673 K between doped LaScO₃ and the anode material, mainly due to strontium migration. To control the elemental diffusion, we lowered the sintering temperature to 1523 K. In the case of Ni-SDC, we found no elemental diffusion at the electrolyte/electrode interface. Moreover, we tried to fabricate SOFC consisting of LSSA electrolyte and some electrodes, and we performed power generation tests above 1073 K. At 1273 K, the maximum power density was 158 mW cm⁻² as humidified hydrogen fuel. The electrolyte resistance was high due to the high thickness of 0.6–0.9 mm. Higher power density would seem possible if a thinner electrolyte is applied. Consequently, we found that LSSA is applicable as an electrolyte material for SOFCs.

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The promise of high safety and stability in solid-type electrolytes has prompted many battery researchers to find candidate inorganic compounds to replace existing liquid/polymer-based electrolytes. In this report, the alkali ion transport in tavorite-type ABTO₄X (A: Li, Na; B-T: Al-P, Mg-S; X: F) is investigated by ab initio calculation, bond valence path approach, and void space analysis in order to explore their viability as solid electrolytes for all-solid state Li/Na ion batteries. Results from nudged elastic band and bond valence pathway methods reveal that the Li analogues are capable of 1D Li ion diffusion (<0.60 eV) characterized by a zigzag pathway along c-axis and passing between BO₄F₂ pairs at each intersite jumps. However, the Na analogues show poor ionic conduction as evidenced by the relatively higher Na migration energy (>0.80 eV) for the same pathway due the larger size of the Na ion and the unchanged critical size along the conduction channel.

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There is currently much interest in advanced energy storage systems because of their potential relevance to power grid storage media. Both earth abundance and high energy density of divalent Mg²⁺ ion make Mg batteries attractive as alternatives to Li-ion batteries, but the number of host materials for reversible intercalation is very limited due to the strong coulombic interaction induced in solid matrix. In this work, we report the electrochemical properties of FePO₄ in aqueous Mg²⁺ electrolyte. The charge/discharge experiments showed that FePO₄ delivers the first discharge capacity of 90 mAh g⁻¹ with subsequent partial reversibility. The ex-situ Mössbauer spectroscopy confirmed reversible redox of Fe³⁺/Fe²⁺ during the discharge and charge processes, while little change was observed in the ex-situ X-ray diffraction patterns. Activation energy for Mg²⁺ diffusion in FePO₄ lattice was calculated to over three times larger than that of Li⁺. It is postulated that the electrochemical reaction of FePO₄ in aqueous Mg²⁺ electrolyte proceeds in part by non-topochemical Mg²⁺ (de)intercalation accompanied by the irreversible transformation from the crystalline state to the amorphous state in the vicinity of particle surface.

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An electrochemical analysis was conducted with respect to a hydrogen membrane fuel cell (HMFC) comprising proton-conducting, amorphous zirconium phosphate, a-ZrP_2.5O_8.9H_1.3, thin film electrolyte supported on a dense Pd anode. The HMFC gave rise to an OCV of 1.0 V, but the maximum power density was limited and was about 1 mW cm⁻² at 400°C. The impedance spectroscopy revealed that the interfacial polarizations were decreased by two orders of magnitude when the cell configuration was changed from the fuel cell setup to the hydrogen concentration cell with an anode symmetric configuration. These results indicated that the polarization losses at the solid-solid anode interface are not a main contribution to the voltage loss of the HMFC. The large cathode polarization might be attributed to the lessened conductivity of amorphous zirconium phosphate electrolyte thin film formed on a precious metal electrode.

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The organized structure and proton transport property in sulfonated polyimide (SPI) thin film have been studied, much less is known at the interfacial charge transport during confinement of such materials. The proton conductivity of SPI thin film is correlated well with the highly oriented polymer structures as determined by infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) and in situ RH dependent synchrotron grazing incidence X-ray diffraction (GI-XRD). The high proton conductivity of 2.8 × 10⁻¹ S cm⁻¹ at 80°C and 90% RH was achieved in the highly oriented SPI thin film. In contrast, the bulk SPI showed the drop of conductivity value at high relative humidity region, which is due to the significant structural disorder accompanied by the strong interaction between the sulfonic acid side chains and water molecules.

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Amorphous (a-LATP) powders with various nominal compositions of Li_1+xAl_xTi_2−x(PO₄)₃ (x = 0, 0.3, 0.35, 0.4, 0.45, 0.5, 0.6) in the Li₂O-Al₂O₃-TiO₂-P₂O₅ (LATP) system, were prepared directly from a mixture of Li₂O, γ-Al₂O₃, anatase-type TiO₂, and P₂O₅ as the starting powder, using mechanical milling (MM) at room temperature. Crystalline LATP (c-LATP) powder with a NASICON-type structure was formed by the heat treatment of mechanochemically prepared a-LATP powder. In addition, heat-treated c-LATP (x = 0.5, 0.6) powders included an impurity phase of crystalline Li₅AlO₄. The sintered c-LATP (x = 0) pellet without Al³⁺ doping exhibited a low electrical conductivity on the order of 10⁻⁷ S cm⁻¹ at room temperature. On the other hand, the sintered Al³⁺-doped c-LATP (x = 0.3, 0.4, 0.45, and 0.5) pellets showed high electrical conductivities on the order of 10⁻⁴ S cm⁻¹ at room temperature. The sintered c-LATP (x = 0.45) pellet exhibited the highest lithium ion conductivity (2.9 × 10⁻⁴ S m⁻¹) at 25°C and the lowest activation energy (30 kJ mol⁻¹).

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The relationship between the local structure and oxide ionic conduction of Nd₂NiO_4+δ possessing the K₂NiF₄ structure was investigated. Various oxygen nonstoichiometry samples of Nd₂NiO_4+δ prepared with different annealing oxygen partial pressures were examined. The local structure related to oxide ionic conduction was determined by the Nd K-edge extended X-ray absorption fine structure. The oxide ionic conductivity and surface exchange coefficient were estimated using electronic conductivity relaxation methods. The activation energy for the oxide ionic conductivity was found to have a direct correlation to the surface exchange coefficient. The bottleneck size for oxide ion conduction was strongly correlated to the oxide ionic conduction of interstitial oxygen and the oxygen surface exchange rate.

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Novel lithium niobium sulfide electrode materials, xLi₂S·NbS₂·(2−x)S (x = 1/2–2), were mechanochemically synthesized using Li₂S, NbS₂, and S₈ as starting materials, and their electrochemical performance was investigated. The electrodes with x = 1/2 were in amorphous phase, while those with x = 1 and 3/2 are assigned as cubic phase. Cells prepared with these electrodes charge and discharge reversibly, with a capacity of ca. 400 mAh g⁻¹. Within the wide operational voltage range 1.0–3.0 V, Li₃NbS₄ charges and discharges with ca. 4.5-electron processes between Li_0.4NbS₄ and Li_4.9NbS₄. Li₃NbS₄ discharges even at a high current density of 2000 mA g⁻¹ at 30°C.

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To evaluate the electrochemical and electronic properties of strained film specimen, La_1−xSr_xCoO_3−δ (x = 0.4, 0.5) films were evaluated by means of electrochemical impedance spectroscopy and electrical conductivity measurement. Dense polycrystalline La_1−xSr_xCoO_3−δ films were fabricated on a polycrystalline Ce_0.9Gd_0.1O_1.95 substrate by a pulsed laser deposition technique. Obtained film was (001) oriented in the pseudo-cubic perovskite unit cell and tensile strained along out-of-plane direction. It was confirmed that oxygen nonstoichiometric behavior of the film was different from that of bulk La_1−xSr_xCoO_3−δ and analyzed by assuming localized and itinerant electronic states. Localized electron model with Co²⁺, Co³⁺ and Co⁴⁺ shows good agreement with observed oxygen nonstoichiometry of the film. The electrical conductivity of La_1−xSr_xCoO_3−δ films show positive temperature coefficient like semiconductors although that of the bulk La_1−xSr_xCoO_3−δ shows a negative temperature coefficient like metals. This indicates localized electron for the La_1−xSr_xCoO_3−δ film and itinerant electron for the bulk La_1−xSr_xCoO_3−δ. Although detail mechanism of the strain effect is not fully understood, both oxygen nonstoichiometric and electronic conduction behaviors of the film could be explained by assuming localized electronic state. Plausible electronic structure of the film was proposed and compared with that of bulk La_1−xSr_xCoO_3−δ.

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Degradation mechanism of surface coating effects at the cathode/electrolyte interface is investigated using thin-film model electrodes combined with operando X-ray absorption spectroscopy (XAS). MgO-coated LiCoO₂ thin-film electrodes prepared via pulsed laser deposition at room temperature and high temperature are used as model systems. The MgO coating improves the durability of the cathode during high-potential cycling. Operando total reflection fluorescence XAS reveals that initial deterioration due to reduction of Co ions at the surface of the uncoated-LiCoO₂ thin film upon electrolyte immersion is inhibited by the MgO coating. Operando depth-resolved XAS reveals that the MgO coating suppresses drastic distortions of local structure at the LiCoO₂ surface as observed in the uncoated-LiCoO₂ during charging process. The electronic and local structure changes at the electrode/electrolyte interface for two types of surface coating morphologies are discussed.

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The oxygen chemical potential of dense Nd₂NiO_4+δ thin films on Zr_0.92Y_0.08O_1.96 electrolyte was investigated by operando X-ray absorption spectroscopy (XAS) measurements. Operando XAS at the Ni K-edge was measured under an applied voltage and various oxygen partial pressures at high temperature to simulate the operating conditions of solid oxide fuel cells (SOFCs). The absorption edge energy under various polarizations is similar to those measured under equivalent oxygen partial pressures under open circuit condition. Thus, the oxygen chemical potential changes drastically at the electrode/gas interface and the rate-determining step of this model system is the surface reaction. This study provides direct evidence for the rate-determining step of the SOFC cathode reaction.

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Ionic conductivity and proton transport number (t_H) of borosilicate and phosphosilicate glasses were measured at 500°C for intermediate temperature fuel cells. A borosilicate glass (Na₂O-B₂O₃-SiO₂) shows t_H = 0.04, which increases to 0.2 by doping 3 mol% of P₂O₅ component. On the other hand, a phosphosilicate glass (Na₂O-P₂O₅-SiO₂) shows t_H = 0.6 at the same condition. It was found that the Al₂O₃ doping is indispensable in order to improve both the t_H and the chemical durability of the phosphosilicate glasses, and a glass with t_H = 1.0 can be obtained for a mixed alkali glass (Na₂O-K₂O-P₂O₅-SiO₂). In the case of non-alkali phosphosilicate glasses (SnO-B₂O₃-P₂O₅-SiO₂), a SnP₂O₇ phase-dispersed dense glass-ceramic was successfully obtained with Sn/P = 0.52, and the glass ceramic also shows t_H = 1.0 at 500°C. By increasing the amount of SnO₂ and decreasing P₂O₅ components, Sn-rich Sn_2.5P₃O₁₂ phase was precipitated, and the glass-ceramics show a possibility of proton/electron mixed conductivity.

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Submicron particles of high voltage LiCoPO₄ are prepared by sucrose-aided combustion synthesise as a preliminary arrangement for the construction of all-solid-sate lithium-ion battery. The obtained LiCoPO₄ was assembled with Li₂O-Al₂O₃-TiO₂-P₂O₅ (LATP) solid electrolyte using spark plasma sintering (SPS) process in order to form enhanced electrical contacts. The reversible charge/discharge capacity of LiCoPO₄ was obtained by cyclic voltammetry with a sweep rate of 0.5 mV s⁻¹ at 150°C. It would be due to the enhanced interfacial contact between LiCoPO₄ and LATP electrolyte without any voids, which was observed by the cross-sectional SEM images.

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A sodium-containing layered cobalt fluorophosphate, Na₂CoPO₄F, is prepared by a solid-state method. Electrode performance of the Na₂CoPO₄F is first examined as a positive electrode material for non-aqueous Na batteries. The carbon-modified Na₂CoPO₄F delivers 100 mAh g⁻¹ of initial discharge capacity with a nearly flat voltage plateau at approximately 4.3 V vs. Na metal. The estimated energy density as a positive electrode material reaches 407 Wh kg⁻¹ based on metallic sodium. Na₂CoPO₄F exhibits the highest operating voltage in a series of Na₂MePO₄F (Me = Fe, Co, and Mn).

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