Solid oxide fuel cells operated with direct internal reforming of carbon-containing fuels (DIR-SOFC) can achieve high energy conversion efficiency because heat generated with electrochemical reaction can be effectively used with endothermic reforming reaction [1]. Up to now, many researchers have studied DIR-SOFC with steam reforming of hydrocarbons, but recently it was reported that a DIR-SOFC system with dry reforming of methane (CH₄ + CO₂ → 2H₂ + 2CO) showed higher electric efficiency than conventional one with direct steam reforming of methane [2]. Moreover, DIR-SOFC with dry reforming of methane enables us to utilize biogas, mixture of methane and carbon dioxide, as fuel, contributing to reducing greenhouse gas emissions. However, when applying dry reforming reaction, carbon formation, which can cause significant deterioration of nickel in the anode, is more prone to occur than when operating on steam and methane fuel. Therefore, developing new anode materials with high coking resistance is desirable for practical use of DIR-SOFC with dry reforming of methane. Ceria-based oxides are widely used as anode components for DIR-SOFC due to their high O^2- and e^- conductivity in reducing atmosphere. This mixed ionic electronic conduction can expand the reaction sites over the surface of electrode, and it is also expected that carbon removal reaction is more likely to proceed. So in this study, Ni-Ce_0.8Sm_0.2O_2-δ (samarium doped ceria; abbreviated as SDC) was applied as anode material for DIR-SOFC with dry reforming of methane. In addition, CaO addition to Ni-SDC anode was examined, aiming at enhancing coking resistance and CO₂ reactivity by basicity of the additive.

SDC powder was prepared by co-precipitation method, and then it was mixed with NiO at a volumetric ratio of Ni-metal:SDC = 60:40, followed by calcination at 1300°C for 5 h in air. The obtained cermet was screen-printed with polyethylene glycol on YSZ disk (20 mm in diameter, 0.5 mm in thickness), and then calcined in air to form porous Ni-SDC electrode. CaO-modified Ni-SDC anode was fabricated by infiltrating Ca(NO₃)₂ solution on the Ni-SDC anode, followed by calcination at 1200°C for 2 h in air. Power generation performance of the cells was evaluated by current-voltage characteristics at 1000, 900, 800, 700°C with VersaSTAT3 (Ametek Inc.), feeding 0.3% humidified H₂ or the mixture of CH₄ and CO₂ on anode side and pure O₂ on cathode side. In addition, stability tests on direct dry reforming operation were conducted under a constant current load. Also, alternative current impedance spectroscopy in humidified H₂ flow was conducted under open circuit condition before and after the power generation tests to evaluate the durability of the cells.

Figure 1 shows I-V characteristics of the cells with Ni-SDC, CaO-modified Ni-SDC and Ni-YSZ anode, in hydrogen operation and direct dry reforming operation at 1000°C. Ni-SDC based anodes showed higher performance than conventional Ni-YSZ, with both hydrogen and dry reforming fuel. CaO-modified Ni-SDC showed slightly lower performance than pristine Ni-SDC possibly because CaO is an insulator and may have lowered the conductivity of the anode. In Figure 1(b), Ni-YSZ displayed lower open circuit voltage and steep I-V curve, which indicates catalytic activity of Ni-YSZ was insufficient to promote dry reforming and thus leading to low H₂ partial pressure and high O₂ partial pressure in the anode. On the other hand, for Ni-SDC based anodes, the open circuit voltage was higher and the slope of I-V curve was as small as that with hydrogen fuel. For Ni-SDC and CaO-modified Ni-SDC anodes, overvoltages were separated into ohmic resistance, electrode reaction overvoltage and gas diffusion overvoltage from AC impedance spectra. Although CaO-modified anode showed almost no difference before and after power generation test, pristine Ni-SDC anode, on the other hand, showed larger ohmic resistance and gas diffusion overvoltage after power generation, which probably due to carbon deposition on anode.

References

[1] T. Iida, M. Kawano, T. Matsui, R. Kikuchi, K. Eguchi, J. Electrochem. Soc., 154, B234 (2007).

[2] L. Barelli, A. Ottaviano, Energy, 71, 118 (2014).