Solid oxide large-capacity power storage system required for mass introduction of renewable energy

The point

• Developed the world’s first large-capacity solid oxide fuel storage system that combines electricity storage using CO ₂ electrolysis and power generation by a chemical reaction using carbon and air.
• The theoretical discharge efficiency is 100% and has a theoretical volumetric energy density of 1,625 Wh / L, which is higher than the existing system using hydrogen gas.
• Expected to be used as a large-capacity power storage system required for large-scale use of renewable energy

Overview

Keisuke Kameda (3rd year doctoral student) and Manabu Ihara, a graduate student of the Department of Applied Chemistry, Faculty of Materials Science and Engineering, Tokyo Institute of Technology, and others store carbon dioxide (CO ₂ ) as carbon (C) by electrolysis, and the carbon and air We proposed a “Carbon / air secondary battery (CASB) system” that generates electricity using the carbon dioxide (O ₂ ) inside, and succeeded in demonstrating its charge and discharge. Research on the electrolysis of CO ₂ and research on power generation using carbon and oxygen have been reported so far, but this research is the first to develop a system that combines both.

The CASB system developed this time is attracting attention as a large-capacity storage technology required for mass introduction of renewable energy. Hydrogen / water-power-to-gas-to-power (H 2 / H 2 O-P2G2P) [Terms Compared with 1] , the theoretical volumetric energy density [Term 2] is 1,625 Wh / L, which is higher than compressed hydrogen (379 Wh / L, 20 MPa), and the overall reaction C + O 2 ? CO 2 standard theoretical discharge efficiency. [Term 3] is 100%. In addition, since the output of the power storage system and the storage capacity can be controlled independently, it can be expected as a next-generation large-capacity power storage system. Since the CO 2 generated during discharge is stored, the system is also characterized by not emitting CO 2 .

In charge / discharge experiments using solid oxide fuel cells / electrolytic cells (SOFC / EC) [Term 4] , Coulomb efficiency [Term 5] 84% and charge / discharge efficiency [Term 6] 38% were achieved.

This research result was published in the online version of ” Journal of Power Sources ” on November 5th.

Background

The introduction of renewable energy is being promoted, but one of the major issues is how to deal with the balance between supply and demand of electricity. In other words, since the amount of power generated by solar power generation and wind power generation, which are easily affected by the weather and conditions, is not stable, there are concerns about excessive power generation to meet demand and insufficient power generation when large amounts of power are required. “Large-capacity power storage technology” is required to meet this problem.

There are some points to consider in the technological development and equipment design of large-capacity storage. For example, it is required to secure as much electricity storage as possible with equipment that is as compact as possible. It is also important that no loss occurs during charging and discharging (charging / discharging efficiency). In addition, the development and introduction of energy storage technology is under consideration based on factors such as the short time required for charging and discharging and the large amount of energy that can be extracted.

Among the many technologies, the charge / discharge method using “hydrogen” has been attracting attention in recent years. Water can be electrolyzed into hydrogen and electricity can be stored as hydrogen gas, and electricity is taken out again by generating electricity using hydrogen gas. It is called water / hydrogen-Power to Gas to Power (H 2 / H 2 O-P2G2P) because it converts hydrogen gas (Gas) and electric power (Power) to each other . Although H 2 / H 2 O-P 2G 2P has the advantage that the storage capacity and output can be set independently, it is charged because the reaction entropy change [term 7] and the latent heat of vaporization of water [term 8] due to the oxidation of hydrogen are large. One of the problems is that the discharge efficiency becomes low. In addition, since the volume of gas is larger than that of solid and the energy density per volume is small, there is a problem that a place is required for storage.

While research is underway to improve the efficiency of H 2 / H 2 O-P2 G2P and make equipment more compact, it is also important to develop and study a charge / discharge method with even higher performance. In the laboratory, we have paid particular attention to the method using carbon (C). So far, we have developed a Rechargeable Direct Carbon Fuel Cell (RDCFC) that supplies carbon by thermal decomposition of hydrocarbons and repeatedly generates electricity using carbon as fuel [References 1 and 2] .

Newly In this study, high energy density, entropy change of less than 2 kJ / mol and less carbon and CO ₂ oxidation-reduction reaction C + O for ₂ ?CO ₂ focused on taking advantage of. Specifically, carbon is precipitated using the thermochemical equilibrium of the electrolytic reaction of CO ₂ and the Boudouard reaction, and the precipitated carbon is charged and discharged by generating power in the same reaction as RDCFC.

Research result

In this research, we proposed the CASB system as a large-capacity power storage system that can generate electricity by using SOFC / EC to store electricity as carbon by electrolysis of CO ₂ , and succeeded in demonstrating its charge and discharge.

The charge / discharge efficiency of 38% of the CASB system, which was successfully demonstrated in this study, was comparable to the estimated charge / discharge efficiency of H ₂ / H ₂ O-P2 G2P (20% to 54%). For practical use, further improvement in efficiency is desired, so we will continue to improve and develop the system in the future. In order to increase efficiency, it is necessary to develop an electrode that enables efficient use of carbon and has a low overvoltage even under carbon precipitation. In addition, for the implementation of the CASB system, it is necessary to study the charge / discharge process of the entire system that can increase the volumetric energy density and charge / discharge efficiency.

Original Article: Proposal and development of the world’s first “carbon air secondary battery system”

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