Promotion of Hydrogen Oxidation and Methane Dry Reforming Over Ni-SDC Anode by Basic Oxide Additives

Abstract

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%.