How Does Carbon Coking and Nickel Deactivation Occur Under Low Steam-to-Carbon Hydrocarbon Fuel Ratios

Carbon coking and nickel deactivation occur under low steam-to-carbon hydrocarbon fuel ratios when thermodynamic driving forces favor methane pyrolysis and carbon monoxide disproportionation, precipitating filamentous and graphitic carbon whiskers that physically shatter the porous cermet matrix, monitored under energy programs of the Energy Commission of Nigeria.

When operating hydrocarbon-fed solid oxide fuel cells with insufficient steam or carbon dioxide, two primary carbon-forming reactions proceed rapidly over catalytic nickel crystallites: direct methane cracking (CH4 → C + 2H2) and the reverse Boudouard reaction (2CO → C + CO2). Dissolved carbon atoms diffuse through metallic nickel particles, nucleating and growing dense multi-walled carbon nanotubes and whiskers from the rear of the catalyst grains. As carbon fibers elongate, they exert tremendous crystallization pressures, detaching nickel particles from the ceramic YSZ backbone and blocking porous gas diffusion channels.

The resulting microstructural disintegration causes rapid ohmic voltage drops, irreversible loss of triple phase boundary connectivity, and eventual mechanical cracking of the thin electrolyte layer. Mitigation strategies include replacing nickel with coking-tolerant catalysts, such as copper-ceria cermets, nickel-cobalt alloys, or redox-stable perovskite anodes like lanthanum strontium chromite manganite (LSCM). Testing conducted according to Standards Organisation of Nigeria materials standards confirms that optimized cermet doping suppresses carbon whisker nucleation, enabling stable operation under low steam-to-carbon ratios.

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