Carbon Deposition Mechanisms on Ni-Based Anode for SOFC: A Comparison Between Non-Discharge and Discharge Modes
Authors: Muto Seina , Uno Ryuji , Watanabe Hirotatsu 
Published: September 2023

The nickel/yttria-stabilized zirconia (Ni/YSZ) and nickel/scandia-stabilized zirconia (Ni/ScSZ) have widely used as an anode for solid fuel cells (SOFCs). When hydrocarbon gas is supplied to the Ni-based fuel electrode of SOFCs, carbon is deposited on the anode, and it causes anode destruction. Therefore, carbon deposition control is required.

The purpose of this study is to clarify carbon deposition mechanisms to understand how the carbon resistance appears on the Ni-based electrode in non-discharge and discharge modes. NiO/8XSZ powder with a binder was uniaxially pressed to form 0.7mm thick pellet (Ni : XSZ=50:50 vol%, X(dopant)=Y, Sc) which were subsequently sintered. After a reduction in H2 atmosphere, Ni/8XSZ sintered cells were heated in CH4/Ar (S/C ratio (Steam to Carbon) =0, CH4/Ar=0.25) and CH4/H2O/Ar atmosphere (S/C ratio=0.15, CH4/Ar=0.25) using a thermobalance, and the carbon deposition rates were measured from the weight change of the cell at 1173 K and 973 K. In this experiment, the S/C ratio was adjusted by bubbling CH4/Ar gas to set the saturated water vapor pressure at a given temperature. In addition, a power generation experiment of SOFC was conducted at 1073 K to investigate the effect of electrochemical reactions on the carbon deposition. The SOFC cells were prepared as follows. YSZ suspension was spin-coated on the NiO/8YSZ substrate. The electrolyte/electrode bilayer was co-sintered at 1673K. Then, a paste of LSM/YSZ was coated on electrolyte and sintered at 1473K. The cell was sealed to the alumina tube in the electric furnace. The anode side was fed with CH4/Ar/H2O (S/C ratio = 0.15) and the cathode side was exposed to O2. Current density was set to 126mA/cm2 for 30 min. After discharge, the anode surface was observed using SEM/EDS.

As a result, the carbon deposition rate of Ni/ScSZ was lower than that of Ni/YSZ by 34 % at 1173 K and S/C=0 under non-discharge mode. This result indicated that the Ni/ScSZ exhibited superior carbon deposition tolerance than the Ni/YSZ at 1173 K. Steam reforming reaction was also different between the Ni/YSZ and Ni/ScSZ. Previous studies showed that carbon deposition tended to progress easily at the metal/solid electrolyte interface. The difference in carbon deposition behavior between Ni/YSZ and Ni/ScSZ was caused by the difference in the interface structure. On the other hand, the carbon deposition rate of Ni/ScSZ was higher than that of Ni/YSZ at 973 K which was opposite to that at 1173 K. Moreover, the rate of carbon reforming reaction (C+H2O→CO+H2) of Ni/YSZ was higher than that of Ni/ScSZ. In fact, the Ni/YSZ anode showed superior carbon tolerance than the Ni/ScSZ at lower temperature. XRD (X-ray Diffraction) analysis showed that the ScSZ with cubic structure was partially changed to the rhombohedral structure after the carbon deposition at high temperature while YSZ cubic structure was unchanged. This implied that the rhombohedral structure at the interface inhibited the carbon deposition reaction, suggesting that the electrolyte structure influenced the carbon deposition and steam reforming reactions. Therefore, it was shown that the electrolyte structure can control the carbon deposition while the same Ni was used. These findings give an useful hint to design the carbon tolerance electrode.

Meanwhile, carbon deposition was not found on the Ni/YSZ surface after discharge at 1073 K. This indicated O2- ion improved the carbon tolerance during discharge. Comparison of carbon deposition between Ni/YSZ and Ni/ScSZ will be discussed using DFT (Density Functional Calculation).