Investigation Of Copper-Based Anode For Solid Oxide Fuel Cells

The high efficient conversion of chemical to electrical energy enables solid oxide fuel cell (SOFC) to be a promising power technology. However, a major limitation for operating current state-of-the-art Ni/YSZ fuel cell directly on hydrocarbon is that Ni catalyzes the formation of carbon which would paralyze the cell in the end. Owing to no catalytic activity to carbon deposit, Cu-based anode has been suggested as alternative material.In this study, impregnation method is applied to fabricate Cu-based SOFC. For the preparation of dense YSZ electrolyte//porous YSZ matrix bi-layer, Two way, sacrificed pore former and leaching Ni are investigated. Comparison between them reveals that the bi-layer made from the former has an inadequate porosity and suffers from a fragile, uneven configuration, and the anode matrix with 70% porosity made from the latter satisfies the subsequent impregnation process much more. EDX testing shows that Cu in the porous matrix after impregnation is distributed evenly but with little aggregation. Anode conductivity increases drastically with the content of Cu in the matrix, for instance, the conductivity is 1Scm^-1 at 5vol.% Cu and reaches to 4800Scm^-1 at 21vol.%. SEM micrographs and effective connection index c demonstrates the Cu particles in matrix are not necessarily connected. High-temperature treating decreases anode conductivity. Effect of the content of Cu on the performance is examined. Results show larger content of Cu enhances the performance by increasing conductivity, in particularly, by increasing TPB in anode. But an overload of Cu is not helpful to enhance cell performance any more. Long-term testing displays a stable 200mWcm^-2 performance at 700℃, in hydrogen atmosphere.To improve Cu/YSZ anode, CeO₂ is introduced as a catalyst. Impedance spectra points out electrode polarization resistance decreases to 30% and the power density increases by ¹50% after adding CeO2. Results of EDX and characterization of anode conductivity proves CeO₂ actually acts as a dispersant for CuO also, which increases the conductivity and enhances its tolerance to high-temperature treating. Influence of the quantity of impregnated species on performance is investigated. Experimental results indicate that the optimum amount is determined by competition between the increase of both TPB and conductivity and the decrease of porosity. In addition, effect of the ratio of Cu and CeO₂ in the impregnated mixture on anode performance is examined. Results show that relatively more CeO₂ in the mixture benefit the performance considerably. A CeO2-Cu/YSZ cell with all optimized parameters exhibits an excellent performance, for example, the maximum power density (MPD) is 336mWcm^-2 at 700℃, in hydrogen atmosphere. To tailor the microstructure of impregnated anode, urea is adopted to deal with the aggregation of impregnated species. By adding urea to the impregnation solution, Cu(OH)₂ and Ce(OH)₃ deposits instead of Cu(NO₃)₂ and Ce(NO)₃ crystal are formed during heating process. EDX analysis finds Cu and CeO₂ produced in such way is distributed more evenly. Cell performance is improved evidently, e.g., CeO₂-Cu/YSZ cell with urea exhibits 37% higher MPD than that without.Using CH4 as fuel is conducted. Cell performance is lower in CH₄ than in H₂ e.g., the MPD of CeO₂-Cu/YSZ cell in CH4 is 140,180mWcm^-2 at 700, 750℃, respectively. No carbon deposition has been found in anode. Long-term testing proves the fairly good stability of such cell operating on CH4. Effect of applying ethanol and liquid petroleum gas (LPG) flame to power cell is studied too. Results show CuO in anode can be successfully reduced to Cu on LPG flame but not on ethanol flame. MPD is as high as 200mWcm^-2 for operation on the ethanol flame. And it is at best 80mWcm^-2 on LPG flame. Anode reliability in reduction-oxidation cycle is investigated. During six cycle testing, the open circuit voltage remains and the MPD even increases slightly. Long-term testing reveals the working voltage decreases and fluctuates narrowly, and carbon deposition is found in surface layer.In conclusion, a suitable dense YSZ//porous YSZ bi-layer is fabricated successfully; the internal structure and performance of Cu/YSZ anode are investigated systematically; the influence of CeO₂ on Cu-CeO₂/YSZ anode is analyzed thoroughly, and then the performance of Cu/YSZ anode is improved significantly; the cell performance is further enhanced by introducing urea to improve the microstructure; at last, the application of cell on hydrocarbon is studied.