Significantly Improved Stability of Carbon/Air Secondary Battery System By Separating Carbon Deposition Area
Authors: Kameda Keisuke , Wakamiya Taishiro , Takagi Reimi , Manzhos Sergei , Ihara Manabu 
Published: December 2024

Introduction

The carbon/air secondary battery (CASB) system is a promising large-capacity energy storage technology [1]. The CASB system utilizes a redox reaction of C + O2 ⇄ CO2 by solid oxide fuel cells/electrolysis cells (SOFCs/SOECs), namely a combination of rechargeable direct carbon fuel cells [1, 2] and CO2 electrolysis cells. Carbon and liquid CO2 are stored in and outside SOFCs/SOECs respectively. The advantages of the CASB system are high theoretical efficiency of close to 100%, high volumetric energy density of 1625 Wh L-1 based on carbon and liquid CO2 (25oC, 6.4 MPa), and high safety due to unflammable CO2 storage. On the fuel electrode side, the electrochemical reaction of 2CO2 + 4e- ⇄ 2CO + 2O2- and the thermochemical (Boudouard) reaction of 2CO ⇄ C + CO2 proceed. On the air electrode side, the electrochemical reaction of 2O2- ⇄ O2 + 4e- proceeds. The previous study [3] demonstrated the reaction mechanism and the charging-discharging for 10 cycles without degradation. However, the carbon deposition on the triple phase boundary (TPB) made an overpotential increase, and there remains a risk of degradation of the fuel electrode in the long term.
To suppress overpotential or to avoid degradation, the carbon deposition area is required to separate from TPB. There are two ways considered, namely development of materials and systems for the CASB. We recently demonstrated that two kinds of fuel electrodes with different catalytic abilities, e.g. Ce0.9Gd0.1O2-δ (GDC) fuel electrode and Ni/GDC catalyst section, successfully controlled carbon deposition on the catalyst section selectively [4]. Controlling temperature is another way of separating carbon deposition area from TPB because the Boudouard equilibrium gas composition changes significantly in the range of 500oC to 900oC (Fig. 1). The lower the temperature, the lower the equilibrium partial pressure of CO. Therefore, lowering the temperature of the catalyst section enables selective carbon deposition on the catalyst.
In this study, the objective is to propose the carbon deposition area-separated type CASB system controlling temperature and to demonstrate its charging-discharging.

Experimental

The experimental setup was composed of a cell and a catalytic fixed-bed reactor. Ni/ GDC fuel electrodes were fabricated on an 8 mol% Y2O3 – stabilized ZrO2 disc electrolyte (20 mm diameter, 0.25 mm thickness). La0.8Sr0.2MnO3/10mol%Sc2O3-1mol%CeO2–ZrO2 air electrodes were fabricated on the counter sides. A catalyst for the Boudouard reaction was put in a quartz tube. The temperature of the cell and the catalytic reactor was controlled to be 900oC and 670oC. For charging, CO 40 sccm and CO2 10 sccm were supplied on the fuel electrode side, and the exhausted gas was supplied to the catalytic reactor. For discharging, CO2 10 sccm was supplied to the catalytic reactor, and the exhausted gas was supplied on the fuel electrode side. For both charging and discharging, O2 100 sccm was supplied on the air electrode side, and a current density of 100 mA cm-2 was applied for a couple of hours.

Results and discussion

Repetitive charging and discharging for a couple of hours were successfully demonstrated. The discharge capacity was above 300 mAh, which is more than 10 times larger than the previous demonstration of the carbon deposition area-integrated type CASB system [3]. Because only CO2 was supplied during the discharging, deposited carbon in the catalytic reactor was mainly used for the fuel. In addition, the partial pressure of CO in the space close to the cell during the charging was not enough for thermochemical carbon deposition. Those results show that lowering the temperature of the catalyst section successfully made selective carbon deposition on the catalyst.

Conclusion

In this study, the carbon deposition area-separated type CASB system was proposed and its repetitive charging and discharging was demonstrated. Lowering the temperature of the catalytic reactor to the SOFCs/SOECs enables selective carbon deposition in the catalytic reactor. Avoiding carbon deposition on the fuel electrodes can radically improve the stability of the CASB system in the long term.

References

[1] M. Ihara, K. Matsuda, H. Sato and C. Yokoyama, Solid state fuel storage and utilization through reversible carbon deposition on a SOFC anode, Solid State Ionics, 175(1-4), (2004), 51-54
[2] M. Ihara and S. Hasegawa, Quickly Rechargeable Direct Carbon Solid Oxide Fuel Cell with Propane for Recharging, Journal of The Electrochemical Society, 153, (2006), A1544-A1546
[3] K. Kameda, S. Manzhos, and M. Ihara, J. Power Sources, 516, (2021) 230681.
[4] K. Kameda. T. Wakamiya, C. Kexin, S. Manzhos, and M. Ihara, SCEJ 89th Annual Meeting, E316, (2024).

Figure 1