Methane and Solid Carbon Based Solid Oxide Fuel Cells

Mechanics and performance of solid oxide fuel cells (SOFCs) have been investigated with methane and solid carbon as fuels. The work with methane fuels investigated methane reactions on Ni/YSZ and deactivation of Ni/YSZ. The Cu-Ni/YSZ anode was developed to resist the coking and sustain the durability of Ni/YSZ anodes. The results of Ni/YSZ deactivation showed that the coking caused by methane was fast (i.e., less than 5 min) and irreversible. A combined in situ infrared (IR) and mass spectroscopy (MS) complemented the deactivation studies in open circuit and suggested that CH₄ reactions on Ni/YSZ followed (i) dissociation of C-H bond, (ii) initial oxidation of adsorbed carbon to CO₂ and CO followed by depletion of lattice oxygen, and (iii) accumulation of carbon on the anode surface. The Cu-Ni/YSZ anode was made by electroless plating copper on Ni/YSZ anodes. Addition of copper slowed down the C-H dissociation and carbon accumulation from CH₄ by formation of Cu-Ni alloy, which was tolerant to coking and sulfur. The X-ray fluorescence (XRF) and X-ray diffraction (XRD) confirmed the successful copper deposition on Ni/YSZ anodes. The maximum power density of 125 mW cm^-2 was achieved using the Cu-Ni/YSZ anode in CH₄ (100 ml min^-1) at 750 °C. A preparation protocol of copper plating on the anode of SOFCs was established for the applications of coking resistant methane- and carbon-based SOFCs.

The work with solid carbon fuels tested the feasibility of direct use of solid carbon on SOFCs for power generation and investigated the chemical reactions involved. Two types of solid carbon: biomass-derived coconut coke and bituminous Ohio #5 coke, were used on carbon-based SOFCs (C-SOFCs). The results showed that biomass-based coconut coke exhibited a higher oxidation and gasification reactivity than bituminous Ohio # 5 coke due to higher content of functional groups and alkali metals. The power generation from coconut coke produced a power density of 140 mW cm^-2 continuously over 15 h and an electrical efficiency of 51.82 % based on CO₂ production at 800 °C. Transient techniques consisting of pulse injection and step switch showed that both Boudouard reaction (C+CO₂→2CO) and CO electrochemical oxidation contributed to power generation of carbon fuel cells. The carrier gas flow rates also affected these gaseous reactions on C-SOFCs by changing the residence time of gas species and their concentration. The results showed that low carrier gas flow rates increased residence time of CO, thereby increasing its contribution to current generation. The contribution of CO oxidation to current generation was estimated to 66 % at the carrier gas flow rate of 50 ml min^-1. The pulse transient studies confirmed the effect of flow rates on cell performance and also revealed that CO and CO₂ can displace adsorbed hydrogen on carbon fuels. The results demonstrated the successful utilization of solid carbon on Ni/YSZ anode supported SOFCs for power generation and provided the insight of reaction mechanisms for development of carbon-based fuel cells.