To improve the electrical conductivity in the Ba2In2O5 (BIO) system without a large volume change from room temperature to 1273 K, BIO materials dually doped with Zr4+ and Zn2+ samples were prepared by using a soft chemical method. Ba2(In0.7,(Zn0.5,Zr0.5)0.3)2O5 (BIZZO-0.3) consists of an orthorhombic phase from room temperature to 1273 K. While phase transformation with a large volume change was not observed for BIZZO-0.3 in the aforementioned temperature region, the electrical conductivity observed for BIZZO-0.3 was higher than the disordered state of BIO when the measurement temperature of conductivity was more than 923 K. The effect of multiple doping on the enhancement of electrical conductivity was characterized by using the transmission electron microscopy (TEM) analysis. Also, the aforementioned effect was discussed in relation to the atomistic simulation result to explain the TEM observation results. The combination of XRD phase analysis, TEM observation and atomistic simulation indicates that a Frenkel defect cluster (i.e. ) was formed in the ordered state of the BIO lattice. It is concluded that the formation of the Frenkel defect cluster in the BIO lattice contributes to the promotion of local disordering of oxygen vacancies at the microscopic scale and maximization of electrical conductivity in the BIO system.

Pt-CeOx nanowire (NW)/C electrocatalysts for the improvement of oxygen reduction reaction (ORR) activity on Pt were prepared by a combined process involving precipitation and coimpregnation. A low, 5 wt % Pt-loaded CeOx NW/C electrocatalyst, pretreated by an optimized electrochemical conditioning process, exhibited high ORR activity over a commercially available 20 wt % Pt/C electrocatalyst although the ORR activity observed for a 5 wt % Pt-loaded CeOx nanoparticle (NP)/C was similar to that of 20 wt % Pt/C. To investigate the role of a CeOx NW promotor on the enhancement of ORR activity on Pt, the Pt-CeOx NW interface was characterized by using hard X-ray photoelectron spectroscopy (HXPS), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). Microanalytical data obtained by these methods were discussed in relation to atomistic simulation performed on the interface structures. The combined techniques of HXPS, TEM-EELS, and atomistic simulation indicate that the Pt-CeOx NW interface in the electrocatalyst contains two different defect clusters: Frenkel defect clusters (i.e., 2Pti•• - 4Oi″ - 4Vo•• - VCe″″) formed in the surface around the Pt-CeOx NW interface and Schottky defect clusters (i.e., (PtCe″ - 2VO•• - 2CeCe′) and (PtCe″ - VO••)) which appear in the bulk of the Pt-CeOx NW interface similarly to Pt-CeOx NP/C. It is concluded that the formation of both Frenkel defect clusters and Schottky defect clusters at the Pt-CeOx NW heterointerface contributes to the promotion of ORR activity and permits the use of lower Pt-loadings in these electrocatalysts.Keywords: EELS analysis; Frenkel type defect cluster formation; heterointerface of Pt and CeOx nanowire; HXPS analysis; ORR; Pt-CeOx nanowire/C cathode;

Pt–CeOx/C (1.5 ≤ x ≤ 2) electro-catalyst is one of the most promising cathode materials for use in polymer membrane electrolyte fuel cells. To clarify the microstructure of Pt–CeOx heterointerface, we prepared Pt-loaded CeOx thin film on conductive SrTiO3 single crystal substrate by using a stepwise process of pulse laser deposition method for the preparation of epitaxial growth CeOx film followed by an impregnation method which loaded the Pt particles on the CeOx film. The electrochemistry observed for the Pt-loaded CeOx thin film on the conductive single crystal substrate was examined by using cyclic voltammetry in 0.5 M H2SO4 aqueous solution, and a cross-sectional image of the aforementioned Pt–CeOx thin film electrode was observed using a transmission electron microscope. The electrochemistry observed for Pt–CeOx thin film electrode clearly showed the promotion effect of CeOx. Also, the microanalysis indicated that unique, large clusters that consisted of C-type rare-earth-like structures were formed in the Pt–CeOx interface by a strong interaction between Pt and CeOx. The present combination analysis of the electrochemistry, microanalysis, and atomistic simulation indicates that the large clusters (i.e., 12 (PtCe′′–Vo••) + 2 (PtCe′′–2Vo••–2CeCe′)) that were formed into the Pt–CeOx interface promoted the charge transfer between Pt surface and CeOx, suggesting that the oxygen reduction reaction activity on Pt can be maximized by fabrication of C-type rare-earth-like structure that consists of the aforementioned large clusters in the Pt–CeOx interfaces.Keywords: atomistic simulation; fuel cell; microanalysis; promotion effect of CeOx; Pt cathode; Pt−CeOx interface

A cerium oxide (CeOx) nanowire of approximately 35 nm diameter was fabricated using an alco-thermal method. Platinum nanoparticles were formed at the interface between Ce(OH)3 and CeOx nanowire which consisted of slightly ionized Pt, Ce3+ and Ce4+. Additionally, Pt nanoparticle sizes were found to be less than 2 nm. The electrochemically active surface area of the Pt–CeOx nanowire/C electrode reaches 152 m2 gPt−1. It is quite high as compared with the same parameter observed for commercially available Pt/C electrodes of 51 m2 gPt−1. This indicates that both CeOx nanowire and Ce(OH)3 can provide useful interfacial reaction space at the nanoscale for formation of small Pt particles. The platinum content of Pt-loaded CeOx nanowire/C can be low such as 0.975 mg ml−1 using this interface reaction space, as compared to 3.90 mg ml−1 in current industrial fuel cell anode materials. While the Pt content in the present nanostructured Pt–CeOx nanowire/C anode is much lower than commercially available Pt/C anodes, the carbon monoxide (CO) tolerance of Pt in the present nanostructured Pt–CeOx nanowire/C anode is superior in the methanol electro-oxidation reaction, which is an important electrode reaction at the anodic side of fuel cells. Based on the experimental data, it is concluded that the interface between Pt and CeOx nanowire plays a key role in enhancement of the electrochemically active surface area of the Pt–CeOx nanowire/C electrode and improvement of CO tolerance of Pt in Pt–CeOx nanowire/C for fuel cell applications.

In the research field of proton exchange membrane fuel cells, the design of electro-catalytic activities on Pt-oxide promoter in the anode side has attracted attention for improvement of CO tolerance of Pt in anode side and a lowering of large over-potential loss of the oxygen reduction reaction on the cathode in the fuel cells. In the Pt-oxide promoter series, Pt–CeOx/C is one of the unique systems. It is because the unique behavior of CeOx such as electrochemical redox reaction between Ce3+ and Ce4+ in the anodic and cathodic reactions of fuel cell is observed. The present short review gives an overview of the recent works for improvement of the CO tolerance of Pt in the Pt–CeOx/C anodes and enhancement of the oxygen reduction reaction activity on Pt in the Pt–CeOx/C cathodes for fuel cell application. To show the design paradigm for fabrication of high quality Pt–CeOx/C electrodes, the authors re-introduced parts of our research results to highlight the important role of interface structure of Pt–CeOx based on the ultimate analysis results. The usefulness of the combined approach of microanalysis and the processing route design is presented.

Pt-CeOx/C electrocatalysts for the improvement of oxygen reduction reaction (ORR) activity on cathode were prepared by a combined process of precipitation and co-impregnation methods. The Pt-CeOx/C electrocatalysts pretreated by the optimized electrochemical conditioning process showed high ORR activity as compared with homemade Pt/C electrocatalyst. Also, it showed high stability in the cyclic voltammetry (CV) test up to 1000 cycles into 0.5 M H2SO4 aqueous solution. On the basis of the data of cyclic voltammogram of 30 cyclic sweeps, X-ray photoelectron spectroscopy, electron energy loss spectroscopy, high resolution transmission electron microscope image, and selected area electron diffraction analysis, it is concluded that the Pt-CeOx heterointerface involving the defect cluster formed by using optimized electrochemical pretreatment conditions on Pt in Pt-CeOx/C electro-catalyst contributes to the promotion of ORR activity and retention of its stability in long CV tests up to 1000 cycles.

Pt on ceria (CeOx) particles supported on carbon black (CB) were synthesized using the combined process of hot precipitation and impregnation methods. During 30 cycles of cyclic voltammetry pre-treatment in the potential ranging from −0.2 to 1.3 V (V vs. Ag/AgCl), it was observed that a small amount of CeOx, which consisted of the interface region between Pt and CeOx, remained on Pt particles. Other free CeOx particles were dissolved into H2SO4 aqueous solution. To develop the Pt-CeOx/CB catalyst, the surface chemical states, the net chemical composition, morphology and electrochemical behavior in H2SO4 aqueous solution were characterized. Our microanalysis and electrochemical analysis indicate that the active CeO2 with high specific surface area provides the continuous amorphous cerium oxide (Ce3+, Ce4+) layer with pores on the surface of Pt particles. It is concluded that the amorphous cerium oxide layer on Pt inhibits the oxidation of Pt surface and contributes to enhancement of the activity on Pt cathode. The single cell performance was also improved using the Pt-CeOx/CB cathode. Based on all data, it is expected that the design based on characterization of the interface between Pt and small amount of amorphous cerium oxide layer could help in preparation of more active Pt catalyst.Highlights► Pt-CeOx/CB cathode is electro catalyst for oxygen reduction reaction (ORR). ► Amorphous cerium oxide layer on Pt in Pt-CeOx/CB cathode inhibits the oxidation of Pt surface and contributes to enhancement of the ORR activity on Pt-cathode.

A considerable interest has been shown in the application of doped ceria (CeO2) compounds for “intermediate” (300–500 °C) temperature operation of solid oxide fuel cells. The microdomains with ordered structure of oxygen vacancy were observed in the microstructure of the M-doped CeO2-sintered bodies (where M: Gd, Y, and Dy). We have previously shown that the conductivity of doped CeO2-sintered bodies was lower when the sintered body contained large microdomains within grains. As a consequence of this observation, we have examined the grain size dependence and dopant content on conductivity in specimens where we adjust the microdomain size and a degree of oxygen vacancy ordering in the microdomains by controlling the microstructure. The microdomain size control in Dy-doped CeO2 specimens was obtained by combining pulsed electric current sintering and conventional sintering. Using these techniques, we were able to improve the conductivity in Dy-doped CeO2 specimens to a point where it became comparable to that of the more conventional Gd-doped CeO2 specimens. It is concluded that by combining ultimate high-resolution analysis of these nanostructures with the adjusting processing route design, it is possible to further develop these materials in CeO2-doped fuel cell application.

Doped ceria (CeO2) compounds are fluorite type oxides which show oxide ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in application of these materials for ‘low temperature operation (500–650 °C)’ of solid oxide fuel cells (SOFCs). In this study, YxCe1−xO2−δ (x=0.05,0.1,0.15,0.2 and 0.25) fine powders were prepared using a carbonate co-precipitation method. The relationship between electrolytic properties and nano-structural features in the sintered bodies was examined. The micro-structures of Y0.05Ce0.95O1.975, Y0.15Ce0.85O1.925 and Y0.25Ce0.75O1.875 as representative three specimens have been investigated in more detail with transmission electron microscopy (TEM). The big diffuse scattering was observed in the background of electron diffraction pattern recorded from Y0.15Ce0.85O1.925 and Y0.25Ce0.75O1.875 sintered bodies. This means that the coherent micro-domain with ordered structure is in the micro-structure. While Y0.25Ce0.75O1.875 sintered body with low conductivity and high activation energy has big micro-domains, the micro-domain size in Y0.15Ce0.85O1.925 with high conductivity and low activation energy was much smaller than that of Y0.25Ce0.75O1.875. TEM observation gives us message that the size of coherent micro-domain with ordered structure would closely relate to the electrolytic properties such as conductivity and activation energy in the specimens. It was concluded that a control of micro-domain size in nano-scale in Y2O3 doped CeO2 system was a key for development of high quality solid electrolyte in fuel cell application.

The concept of crystallographic index termed the effective index is suggested and applied to the design of ceria (CeO2)-based electrolytes to maximize oxide ionic conductivity. The suggested index considers the fluorite structure, and combines the expected oxygen vacancy level with the ionic radius mismatch between host and dopant cations. Using this approach, oxide ionic conductivity of Sm- or La-doped CeO2-based system has been optimized and tested under operating conditions of a solid oxide fuel cell. In the observation of microstructure in atomic scale, both Sm-doped CeO2 and La-doped CeO2 electrolytes had large micro-domains over 10 nm in the lattice. On the other hand, Sm or La and alkaline earth co-doped CeO2-based electrolytes with high effective index had small micro-domains around 1–3 nm in the microstructure. The large micro-domain would prevent oxide ion from passing through the lattice. Therefore, it is concluded that the improvement of ionic conductivity is reflected in changes of microstructure in atomic scale.

In the research field of proton exchange membrane fuel cells, the design of electro-catalytic activities on Pt-oxide promoter in the anode side has attracted attention for improvement of CO tolerance of Pt in anode side and a lowering of large over-potential loss of the oxygen reduction reaction on the cathode in the fuel cells. In the Pt-oxide promoter series, Pt–CeOx/C is one of the unique systems. It is because the unique behavior of CeOx such as electrochemical redox reaction between Ce3+ and Ce4+ in the anodic and cathodic reactions of fuel cell is observed. The present short review gives an overview of the recent works for improvement of the CO tolerance of Pt in the Pt–CeOx/C anodes and enhancement of the oxygen reduction reaction activity on Pt in the Pt–CeOx/C cathodes for fuel cell application. To show the design paradigm for fabrication of high quality Pt–CeOx/C electrodes, the authors re-introduced parts of our research results to highlight the important role of interface structure of Pt–CeOx based on the ultimate analysis results. The usefulness of the combined approach of microanalysis and the processing route design is presented.

A cerium oxide (CeOx) nanowire of approximately 35 nm diameter was fabricated using an alco-thermal method. Platinum nanoparticles were formed at the interface between Ce(OH)3 and CeOx nanowire which consisted of slightly ionized Pt, Ce3+ and Ce4+. Additionally, Pt nanoparticle sizes were found to be less than 2 nm. The electrochemically active surface area of the Pt–CeOx nanowire/C electrode reaches 152 m2 gPt−1. It is quite high as compared with the same parameter observed for commercially available Pt/C electrodes of 51 m2 gPt−1. This indicates that both CeOx nanowire and Ce(OH)3 can provide useful interfacial reaction space at the nanoscale for formation of small Pt particles. The platinum content of Pt-loaded CeOx nanowire/C can be low such as 0.975 mg ml−1 using this interface reaction space, as compared to 3.90 mg ml−1 in current industrial fuel cell anode materials. While the Pt content in the present nanostructured Pt–CeOx nanowire/C anode is much lower than commercially available Pt/C anodes, the carbon monoxide (CO) tolerance of Pt in the present nanostructured Pt–CeOx nanowire/C anode is superior in the methanol electro-oxidation reaction, which is an important electrode reaction at the anodic side of fuel cells. Based on the experimental data, it is concluded that the interface between Pt and CeOx nanowire plays a key role in enhancement of the electrochemically active surface area of the Pt–CeOx nanowire/C electrode and improvement of CO tolerance of Pt in Pt–CeOx nanowire/C for fuel cell applications.