Direct internal reforming is one of the promising methods of utilizing hydrocarbon fuels in solid oxide fuel cells (SOFCs). Among direct internal reforming technologies, dry reforming of methane (DRM) is attracting attention as an alternative to steam reforming of methane. In this study, a cermet material composed of nickel and samarium-doped ceria (Ni-SDC) was investigated as an SOFC anode for the direct internal DRM operation. Compared to a conventional nickel-yttria-stabilized zirconia (Ni-YSZ) anode, the Ni-SDC anode showed superior current-voltage characteristics. However, poor carbon balances were recorded in anode outlet gas analysis, indicating coke formation on the Ni-SDC anode. The addition of calcium to the Ni-SDC anode was also tested. Measured anode outlet gas compositions suggested that the calcium addition suppressed coke formation, while the power generation performance of the pristine Ni-SDC anode was maintained. Microscopic observations showed that a certain part of the calcium additive existed as CaO particles on the anode surface, which could be responsible for the improved coking resistance.

In this study, Ni-SDC (samarium-doped ceria) and CaO-modified Ni-SDC were examined as anode materials for SOFC with direct internal dry reforming of methane. The CaO addition is aimed to enhance the coking resistance in the methane direct internal dry reforming operation. Current-voltage characteristics were measured by feeding directly CH 4 -CO 2 mixture to the anodes, and overvoltage in the power generation was separated from alternative current impedance measurements. In addition, stability tests under a constant current loading were carried out to evaluate the durability. From these results, it is demonstrated that Ni-SDC based materials are promising due to their high performance, coking resistance and stability.

Metal oxide addition is one of the major techniques to improve the performance of SOFC cermet anodes. However, the effects of the metal oxide addition on hydrogen oxidation activity of the anodes have not been clarified so far. In this study, Ni-CaO/YSZ anodes were prepared to examine the interaction between YSZ and Ca species. The hydrogen oxidation activities of the prepared anodes were investigated by introducing electrochemical impedance spectroscopy. The anodic resistance of the electrochemical hydrogen oxidation was determined by impedance fitting, and its temperature dependency was analyzed by means of Arrhenius plots. It was found that activation energy of the reaction was decreased with the CaO addition, suggesting the possibility of the SOFC operation at lower temperatures.

Promoting effects of the additives such as MgO, CaO, BaO, and Cr2O3 to Ni-SDC (samarium-doped ceria) anodes were investigated on hydrogen oxidation and dry reforming of methane (DRM) activity. The additives were found to elevate current-voltage characteristics of H2-O2 fuel cells. The polarization resistance for H2 oxidation at the anodes was mainly decreased by the additives. The carbon balance in DRM reaction at open circuit voltage (OCV) approached 100% over the modified Ni-SDC anodes. The DRM activity was most promoted by the Cr2O3 additive, leading the highest OCV in direct DRM operation.

Direct internal reforming of hydrocarbon fuels on the anodes of solid oxide fuel cells (SOFCs) leads to an increase in power generation efficiency, and consequently numerous investigations have been done on this topic. Among hydrocarbon fuels, methane is a primary target fuel because it is a major component of natural gas. Steam and CO2 are adopted as a reforming reagent for a direct internal reforming SOFC. Direct reforming of methane with CO2 (dry reforming of methane, DRM) offers better power generation efficiency [1] and a compact system without a steam generator. In addition, a mixture of CH4 and CO2 is available as biogas, and therefore the applicability of SOFCs is expanded to biomass-derived energy resources. However, DRM will suffer from more severe coking than steam reforming of methane. Recently we reported calcium-modified Ni-SDC cermet anodes for DRM [2]. The addition of Ca to Ni-SDC anodes improved the activity for methane reforming by CO2 and suppressed carbon deposition on the anode, leading to superior durability of the anode in direct DRM operation. In this study, we further investigated the effect of additives such as MgO, BaO, and Cr2O3 on hydrogen oxidation and DRM on the anode. SDC powders were prepared by the co-precipitation method. The desired amount of Ce(NO3)3∙H2O and Sm(NO3)3∙6H2O (Wako Pure Chemical Industries) were mixed with oxalic acid to form co-precipitate, which was successively dried, and then calcined at 1000˚C for 2 h and 1500˚C for 5 h in air. The obtained SDC powder was mixed with NiO at the desired ratio, followed by calcination at 1300˚C for 5 h in air. As a cathode material, La0.6Sr0.4MnO3- d (LSM) was prepared [3]. As for the NiO-SDC anode preparation with additives, the anode was fabricated by infiltrating the pristine NiO-SDC anode with the respective nitrate solutions, followed by calcination at 1200˚C for 2 h in air. As the additives, Mg, Ca, Ba, and Cr were examined. Before power generation tests, the anode was exposed to H2 flow at 1000˚C to reduce NiO to Ni. The power generation performance of the cells was evaluated by current-voltage characteristics at 1000, 900, 800, 700˚C with SP-300 (Bio-Logic SAS, France), by feeding 0.3% humidified H2 or DRM gas on the anode side. Alternative current (AC) impedance spectroscopy in humidified H2 flow was conducted under open circuit condition before and after the power generation tests on direct DRM operation to evaluate the durability of the cells. The outlet gas composition analysis was also carried out in DRM power generation experiments by an on-line micro gas chromatograph (Varian, CP-4900). The power generation characteristics at 900˚C by feeding humidified hydrogen and a mixture of methane and CO2 are shown in Fig. 1(a) and (b), respectively. When humidified hydrogen was used, IV characteristics were improved by adding the basic and amphoteric elements to the Ni-SDC anode. The open circuit voltage (OCV) was identical to that of the pristine Ni-SDC, irrespective of the additives, whereas the current density was increased, especially by the addition of magnesium. Impedance analysis revealed that the polarization resistance of hydrogen oxidation was decreased by the additives. This result indicates that the oxidation reaction of hydrogen on the anode was promoted by the additives. Among the additives, MgO was found to be the most effective at reducing the polarization resistance of hydrogen oxidation. In the case of the power generation with DRM shown in Fig. 1(b), the MgO addition increased the current density from 0.49 A cm-2 on the pristine anode to 0.61 A cm-2 at 0.70 V, while the other additives exhibited almost no effect on IV. Nevertheless, OCV varies depending on the additives, which indicates the difference in the activity for CO2 reforming of methane. Higher conversion of methane and CO2 should lead to a decrease in oxygen partial pressure at the anode side, and consequently to the increase in OCV. The anode outlet gas analysis by GC under open circuit condition exposed that CH4 conversion was raised over the modified anodes; namely, the methane dry reforming reaction was promoted by the additives. It is also noteworthy that the carbon balance over the modified anodes was ±2% while the balance over the pristine anode was at -13%. References [1] L. Barelli, A.Ottaviano, Energy 71 (2014) 118. [2] T. Mishina, N. Fujiwara, S. Tada, A. Takagaki, R. Kikuchi, S.T. Oyama, J. Electrochem. Soc. 167 (2020) 134512. [3] N. Fujiwara, T. Minami, R. Kikuchi, A. Takagaki, T. Sugawara, S. Tada, S.T. Oyama, J. Electrochem. Soc. 166 (2019) F716. Figure 1