Hall M, 1st pm, Day 3, Wednesday Abstracts

Session S8. Materials and systems engineering for electrical energy devices —fuel cell, battery, solar cell, and energy carriers—

The triple phase boundary (TPB) of metal, oxide, and gas phases in the anode of solid oxide
fuel cells plays an important role in determining their performance. In this study, we explored the TPB structures and reaction at TPB combining two approaches: atomic-resolution microscopy observation based on HAADF-STEM (High-angle annular dark field scanning transmission electron microscopy) and reaction dynamics simulation based on reactive force field. From HAADF-STEM observations, two distinct structures are found with different contact angles of metal/oxide interfaces, metal surfaces, and pore opening sizes, which have never been adopted in theoretical simulations in literature. Chemical reaction dynamics simulations for the hydrogen oxidation reaction (HOR) at the TPB are performed using realistic models reconstructed from HAADF-STEM observations. In addition, extensive development of accurate reactive force field parameters was conducted to accurately trace the reaction pathways at TPB. As a result, the activity of different structures towards HOR is clarified, and a higher activity is obtained on the TPB with smaller pore opening size. Three HOR pathways are identified: two types of hydrogen diffusion processes, and one type of oxygen migration process which is a new pathway.

After the Great East Japan Earthquake, there is demand for transformation to a bidirectional system introducing distributed power sources, and a fuel cell cogeneration system with advanced energy utilization has been attracting attention. Even among them, solid oxide fuel cell (SOFC) are high efficiency power generation systems and expected as a promising power source. It is necessary to reduce the raw material cost by improving the power density for further penetration of SOFC. Electrochemical reactions occur only on an effective triple phase boundary (TPB) where each network path of the electron, the oxide ion and the fuel gas connects uninterruptedly. The power generation performance was affected by the effectiveness of TPB. This work aims to realize a microstructure expanded effective TPB between the anode and electrolyte by using a commercial ink-jet 3D printer. Anode and electrolyte inks suitable for material jetting were prepared by changing the viscosity and the particle size. The anode ink formed a porous structure by adding acrylic particles, which ensured the path of the fuel gas. The electrolyte ink formed a dense structure to avoid cross leakage. The microstructure was formed by laminating linear structures in which the porous anode and the dense electrolyte lines were alternately arranged in parallel and orthogonally stacked. The microstructure was inserted between the anode and electrolyte as the anode functional layer of which width and thickness of linear structure were approximately 100 and 1 microns, respectively. The single cell with the microstructure was tested at 600 °C being fueled by dry methane and showed a high performance.

Fuel cells are one of the electrochemical energy conversion devices which can directly convert the chemical energy into electric energy with a high efficiency. Conventional solid oxide fuel cells (SOFC) using oxide ion conducting electrolyte have various difficulties arising from high operating temperatures (700–1000 ^oC) such as degradation of materials and slow start up and cooldown. Development of electrolytes which can operate at intermediate temperature (500–700 ^oC) has been desired to overcome these problems.
In this situation, researchers have investigated proton conducting solid oxide fuel cells (PCFC) with thin-film proton-conducting electrolyte to realize intermediate temperature operation. Rare earth-doped BaZrO₃ and BaCeO₃ have been intensively studied as thin-film electrolytes for PCFC because of their high proton conductivity at intermediate temperatures. However, current leakage through the electrolyte layer occurs as the electrolyte thickness becomes thinner owning to a high hole conductivity in oxidative atmosphere at air side of the cell. Therefore, development of an approach to reduce the current leakage of proton-conducting electrolytes at oxidative atmosphere is required to realize intermediate temperature operation with a high power density.
In the SOFC with ceria-based oxide-ion-conducting electrolyte, formation of bilayer electrolyte consisting of a rare earth-doped ceria layer and a thin Y-doped zirconate layer at fuel side was reported to be effective in suppression of current leakage arising from reduction of ceria. In this research, we extended the concept of the bilayer electrolyte to PCFC in which oxidation of electrolytes at air side is an origin of the current leakage. Anode-supported PCFCs with bilayer electrolyte consisting of Y-doped SrZrO₃ layer and Y-doped BaZrO₃ layer were prepared by a pulsed-laser deposition (PLD) method and current-voltage measurements and electrochemical impedance spectroscopy measurements were carried out. We will discuss the effect of the bilayer electrolyte on the cell performance of anode-supported PCFC.

To introduce renewable energy or electric vehicles, hydrogen power to gas to power (PtoGtoP) or lithium air battery have been developed for increasing energy density of batteries. However, there remains issues that hydrogen needs to be compressed to several tens of MPa or to be below -250 °C for increasing the energy density, and reduction of Li₂O₂ needs large overvoltage. Therefore, we made an idea to apply the redox reaction of CO₂ and carbon to secondary battery because of easy storage of CO₂ and high energy density of carbon. Here we proposed carbon-air secondary battery (CASB) system.
The CASB system can be composed of solid oxide fuel cell (SOFC), stored liquid CO₂ and solid carbon. The CASB system works as secondary battery by electrolysis of CO₂ and power generation using carbon directly. Storage of CO₂ can be easier and safer than storage of hydrogen because CO₂ liquefies under 6 MPa. In addition, the theoretical conversion efficiency of the redox reaction C+O₂⇄CO₂ equals to 1, so that the CASB system is expected to work efficiently.
In this research, we demonstrated the redox reaction and evaluated potential of the CASB system as large-capacity energy storage by comparing the theoretical volumetric power density and energy density with existing and developing secondary batteries and hydrogen PtoGtoP.
We prepared a coin type SOFC supported by the electrolyte, and repeated electrolysis of CO₂ and power generation using generated fuel at 800 °C, 100 mA/cm². During charge operation, analysis of Nernst potential revealed that carbon deposited by Boudouard reaction 2CO→C+CO₂ with increasing partial pressure of CO due to electrochemical reaction CO₂+2e^-→CO+O^2-. During discharge operation, carbon or CO was used for power generation. A discussion about potential of the CASB system showed that it can have larger gravimetric and volumetric energy density than secondary batteries and hydrogen PtoGtoP.

Sustainability has emerged as a keyword in all aspects of life whether it is resources or technologies and products or processes. Besides, nearly a billion new consumers join the society in 13-15 years; and the growing demand for higher standards of living make the worldwide materials consumption continuously growing. Strategic solutions are therefore required not only for addressing the gaps but also for eliminating the undesirable environmental effects of supply chain to ensure quality and sustainable living. Energy storage is currently a multibillion dollar industry and is expected to be continuously growing because (i) electrification of products and services has been emerged as an efficient strategy to mitigate carbon footprints from major emitting sectors such as automobiles and (ii) internet of things, advanced communication devices and other modern electrical appliances such as drones and robots demand efficient electrical energy storage devices. Source of primary materials supply for this large industrial sector is therefore crucial; extensive use of earthborn materials as energy storage medium would not only lead to disasters but also would result in expensive devices. Functionalization of renewable materials such as biomass carbon, cellulose, oils etc. as components of energy storage devices would ensure a sustainable living. This lecture will focus on the current state of renewable materials as an energy storage medium, both in the lecturer's laboratory and elsewhere, and foreseeable initiatives required to build efficient energy storage devices using renewable materials.