Influence of Reactions of Carbon Black and Fuel Diffusion Process on Power Generation Characteristics of Rechargeable Solid Carbon Fuel Cells(RSCFCs)

INTRODUCTION

Our research group proposed a Rechargeable Direct Carbon Fuel Cell (RDCFC) based on SOFCs, in which carbon is deposited in the pores of the anode by charging hydrocarbons ^[1]. A issue in RDCFCs is low utilization caused by low amount of deposited carbon. For higher fuel utilization, we also proposed a Rechargeable Solid Carbon Fuel Cell (RSCFC), in which solid state carbon black (CB) is directly supplied as fuel. In RSCFCs, the dominant electrochemical reaction apparently is CO oxidation reaction^{[2] [3]} because CB is difficult supplied close to TPB (Triple Phase Boundary), and CO was generated by Boudouard corrosion reaction (C+CO₂→2CO). In an earlier study, we reported that improvement in the maximum power density P_Max of RSCFCs depends on the amount of CB. In RSCFCs, high power density P with high fuel utilization U is achieved at the steady state current density i₀, which is reached when the CO supply rate r_sup (includes the CO generation reaction rate r_CO, and both CO and CO₂ diffusion rates r_D(CO), r_D(CO2) in the anode is comparable to the electrochemical reaction rate r_e (CO + O^2- → CO₂ + 2e^-). The introduction of carbon black into pores made the direct carbon oxidation (C + 2O^2- → CO₂ + 4e^-, C + O^2- → CO + 2e^-) enhanced and improved the power density.

In this study, we (a) analyzed mass transport in CB supply column using gas diffusion theory, (b) investigated the influence of CB supply densityη(=m/SL, m is amount of CB, S is cross-section area of column, and L is the distance of CB supplied) and the rate of anode porosity ε_e on the power density.

EXPERIMENTAL

The anode of the RSCFC cell was Ni/Gd_0.2Ce_0.8O₂ (Ni/GDC) and with added 1.85wt% CB (Ni/GDC_CB) to obtain a high rate of porosity in CB column ε_CB (=1-(true volume of CB particle / volume of column)= 1-ηρ, hereρis true density of CB). The cathode was porous La _0.85Sr_0.15MnO₃, and the electrolyte was a disk of 10% Sc₂O₃ -1 mol% CeO₂ – 89% ZrO₂ (ScSZ; 20-mm diameter, 0.3-mm thick; Japan Fine Ceramics Co., Ltd.). Solid state CB was used as fuel (MA7, Mitsubishi Chemical Corporation, Mitsubishi carbon). A constant amount of CB was added into a column and the ε_CB was varied from 90.2% to 88.2% by changing the length of column from 6 cm to 5 cm. For power generation, a gas mixture of Ar and 5% H₂ was introduced to the anode at 202STP ml/min. Power generation was started at 900°C with no gas to anode but pure O₂to the cathode. Terminal Voltage V was measured.

RESULTS AND DISCUSSION

Fig.1 shows the performance curves of the RSCFC for different ε_CB. With small ε_CB (Fig.1b), P_Max was higher. It was because of an increase in the r_sup caused by the increase in the r_CO due to increase in the total surface area of CB per unit volume of CB supply column.

Fig.2 shows the surface SEM photograph of the Ni/GDC electrode in Fig.3 (a) and Ni/GDC_{CB (1.85wt %)} in Fig.3 (b). We can see ε_eincreased by adding CB to Ni/GDC.

Fig.3 shows the IV characteristics in each power generation cycle of the cell using (a) Ni/GDC electrode and (b) Ni/GDC_{CB (1.85wt %)}. In the power generation using the Ni/GDC_{CB (1.85wt %)} electrode, which had larger ε_e than the Ni/GDC electrode, the maximum current density i_Max was improved by 30 mA/cm² and P_Max improved by 50% (to 34.6mW/cm²). The possible reasons for the enhancement were considered as (1) an enhancement of direct oxidation of CB due to the inner surface of the anode, and (2) a reduction of pore diffusion resistor inside the anode.

 

CONCLUSION

The P_Max of RSCFC with 2% less ε_CB of an anode was improved by 6.74 mW/cm². It was because of an increase in the r_sup caused by the increase in the r_CO due to increase in the total surface area of CB per unit volume of CB supply column. Meanwhile, by using large ε_e of an anode, the P_Max was increased (using Ni/GDC_1.85%CB anode, improved by 11.72 mW/cm²). The possible reasons for the improvement were (1) an enhancement of direct oxidation of CB due to the inner surface of anode, and (2) a reduction of pore diffusion resistor inside the anode.

REFERENCES

1. Fuhito Ohba, Yuki Tagawa, Yu Sakai, Akinori Yabuki, Hiroyuki Shimada, and Manabu Ihara. ECS Trans. 33 (40), 163 (2011)

2. Chikara Takei，Ying Chen, Manabu Ihara, ECS transactions, 33 (40), 175 (2011)

3.X.-Y.Zhao, Q.Yao, S.-Q.Li, N.-S.Cai, Journal of Power Sources, 185(2008)104-111