IntroductionThe extensive use of fossil fuel resources has a serious impact on the environment, and hence energy conservation and CO₂ emission reduction have become the focuses of current development. Solid oxide fuel cells (SOFCs) have attracted the research attention because of the outstanding energy conversion efficiency, wide range of fuel choices, no pollution, and high quality waste. H₂ is the ideal fuel for SOFCs, but its low volumetric energy density and the requirement of high pressure to be liquefied, making it difficult to store and transport. At ambient temperature and pressure, methanol is liquid and its volumetric energy density is much higher than that of hydrogen, which makes methanol easy to storage and transport as a good hydrogen carrier. Furthermore, methanol has been quite mature production process. The industrial methanol production was hydrogenation of CO and CO₂. SOFCs can generate power with direct internal methanol reforming. However, the main problem of direct methanol fed SOFCs is carbon deposition. The formation of carbon deposition not only reduces the cell performance, but affects the anode structure as well. In this study, the anode surface of flat-tube anode supported SOFCs was modified by Barium. The cell performance, durability, and methanol conversion of the unmodified and modified SOFCs were compared and evaluated under methanol atmosphere with a steam/carbon ratio of 0.75.MethodsThe anode support was composed of nickel oxide with 3%Y₂O₃-stabilized ZrO₂ (NiO–3YSZ, in mole), and the anode functional layer was made up of nickel oxide with 8%Y₂O₃-stabilized ZrO₂ (NiO–8YSZ). The electrolyte, barrier layer, and cathode layer were 8YSZ, Gd_0.1Ce_0.9O_2–δ (GDC), and La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ–GDC (LSCF–GDC), respectively. The active cathode area was 60 cm². The cell is denoted as NiO–3YSZ|NiO–8YSZ|8YSZ|GDC|LSCF–GDC. Using 15.68 g Ba(NO₃)₂ powder dissolved in 200 mL deionized water, being stirred at 70 ℃ for 2 h, to obtain 0.3 mol/L barium nitrate precursor solution for wet impregnation. After sealing one end of the Ni/YSZ cell with wax, the prepared barium nitrate precursor solution (70 ℃) was added into the fuel channels of through the sampler, and the other end of the cell was sealed with wax. The cell was kept in an oven at 50 ℃ for 24 hours to ensure that the Ba(NO₃)₂ solution was fully immersed in the anode channels. Then, the cell was dried at 90 ℃ for 1 h to remove the wax, and the cell obtained as above preparation processes was recorded as Ba–Ni /YSZ cell. After the cell was reduced at 750 ℃ by H₂, 0.60 L/min (SLM) H₂ was used as fuel for the initial cell performance test. When methanol was used as fuel for the test, the methanol solution with a steam/carbon ratio of 0.75 was added to the anode channels, and the temperature of the water steam generator was set at 130 ℃. At this time, the corresponding methanol flow rate was 0.30 g/min, and 5 L/min air was passed into the cathode. Fuel cell testing equipment (SOFC-100W, Wuxi Lead Equipment Co., LTD.) and electro-chemical workstation (VMP3B-20, France BioLogic Co., LTD.) were used to test the electro-chemical performance and durability of the cell under different operating condition. The frequencies of electrochemical impedance spectroscopy (EIS) tests were ranging from 20 mHz to 30 kHz. The AC amplitude of EIS testing was 10 mV.Results and discussionThe scanning electrical microscopy (SEM) with energy dispersive spectrometer analysis proved that Ba element was effectively loaded on the surfaces of Ni/YSZ anode fuel channels. Under H₂ fuel, the current voltage (I–V) characteristics of Ni/YSZ and Ba–Ni/YSZ cells at 750 ℃ indicate that the open-circuit voltage (OCV) of Ni/YSZ and Ba–Ni /YSZ cells were 1.12 V and 1.07 V, respectively, which revealed that the cell air tightness was good. At 750 ℃, the maximum power density (P_max) of Ni/YSZ and Ba–Ni/YSZ cells were 555.66 mW/cm² and 507.03 mW/cm², respectively, indicating that Ba element had no obvious influence on the cell output power of flat-tube SOFC fueled by H₂. The electro-chemical performance of Ni/YSZ and Ba–Ni/YSZ cells in methanol atmosphere [r(S/C) = 0.75] showed that the power density of Ni/YSZ and Ba–Ni/YSZ cells at 750 ℃ and 0.8V were 326.72mW/cm² and 452.87mW/cm², respectively. It can be seen that the power density of Ba-modified cell was significantly improved when fueled by methanol. Under OCV condition, the methanol conversion rates of Ni/YSZ and Ba–Ni /YSZ cell were 89.40% and 92.70%, respectively, because Ba on the surfaces of the fuel channels promoted methanol conversion within the cell. The hydrophilicity of the Ba-modified cell was enhanced, and the water cracking into OH^– groups that were adsorbed on the Ni surface was promoted. The durability of Ni/YSZ and Ba–Ni/YSZ cells was tested at 750 ℃ under methanol [r(S/C) = 0.75] and 200 mA/cm². The results showed that the electro-chemical performance of Ni/YSZ cell deteriorated sharply shortly after 3 h to 4 h, while the voltage of Ba–Ni/YSZ cell kept stable. After running for more than 500 h, the OCV value of the Ba–Ni/YSZ cell remained stable, indicating that the air tightness of cell was good, and the voltage was 0.89 V with a deterioration rate of 0.01 (%)/h. The gas composition analysis of the Ba–Ni/YSZ cell during the durability test showed that the conversion rate of methanol in the modified cell during the test was stable, and the conversion rate was larger than 90%. According to the SEM images of the Ba–Ni/YSZ cell before and after the durability test, the average Ni particle size of anode functional layer of the Ba-modified cell lowered from 0.97 μm to 0.92 μm. This deduced that the Ni particle lost in the anode function layer of the Ba–Ni/YSZ cell was obvious after the durability test, leading to the performance deterioration of the cell. Raman spectra showed that Ni/YSZ cell has detectable carbon deposition after the durability test, while Ba–Ni/YSZ has no carbon deposition. This showed that Ba content on the surface of the anode support of flat-tube SOFC could effectively inhibit carbon deposition.ConclusionsThe performance and durability of the flat-tube Ba–Ni/YSZ cell were investigated in this study. The Ba–Ni/YSZ cell was prepared by wet impregnation. The power density of the Ba–Ni/YSZ cell with methanol under r(S/C)=0.75 at 750℃ was 452.87 mW·cm^–2 at 0.8 V, larger than that of Ni/YSZ cell. The durability of Ba–Ni/YSZ with methanol under r(S/C)=0.75 at 750 ℃ reached 500 h, while the Ni/YSZ cell revealed a sudden performance degradation. The Ni particle agglomeration was detected for the Ba–Ni/YSZ cell after the durability test, causing the performance degradation during the test. No carbon was observed on the surface of the Ba–Ni/YSZ after the durability test.