(6cl) Challenges and Progress for Cogeneration of Power and Hydrogen from Nested Carbon-Air/Carbon-Steam Fuel Cells

To address growing energy demands while reducing CO₂ emissions and the environmental footprint of dirty solid fuels such as coal, it is essential to develop advanced technologies with high efficiencies that also facilitate easy carbon capture.  To this end, converting a solid fuel to clean hydrogen in a fuel cell with spontaneous and simultaneous generation of electricity while producing capture-ready CO₂ offers attractive opportunities and advantages. The nested air-carbon/steam-carbon fuel cell addresses this need through higher efficiencies, and because the solid fuel is kept separate from the steam and air in the cathode compartments by an ionically conducting electrolyte membrane, the effluent anode product gas is a concentrated CO₂stream that is capture ready.  Moreover, due to the downhill chemical potential gradient of oxygen across the electrolyte, steam electrolysis takes place spontaneously under a driving force of > 0.5 V in the steam-carbon fuel cell, which can be supplemented by simultaneous electrical power generation from the air-carbon fuel cell. 

Recently our group has published work demonstrating the feasibility of hydrogen production using activated carbon as the solid fuel in a steam-carbon fuel cell.¹ While this work has demonstrated the potential of this technology, the ultimate goal is to convert real fuels such as coal or biomass into clean fuel and electrical energy.  Biomass and coal contain a multitude of contaminants, many of which are potent poisons for the Ni-based cermet anodes typically used in these cells. In particular, Ni-cermet anodes, which are commonly employed for solid oxide fuel cells, show susceptibility to H₂S poisoning at concentrations as low as 1 ppm,²making their direct use with these fuels unfeasible. 

Herein we report our progress on advancing this technology through the use of solid sorbents dispersed throughout a fuel bed of coconut char (Norit®).  In particularly, by using sulfur sorbents that are highly dispersed on the solid fuel it is possible to dramatically reduce the H₂S and COS contents of the syngas in situ and hence, mitigate the sulfur burden on the catalytic anode material. Indeed, adding group IIA alkali earth–based sorbents to the fuel mitigates the adverse impact of sulfur on fuel cell performance and slows down the degradation process. The rate of current density loss at the constant cell voltage of 0.7 at 900 °C as a function of time for different IIA metal oxide sorbents, is correlated to a reduction in the rate of increase in the kinetic resistance in the anode electrocatalyst as determined from electrochemical impedance spectroscopy (EIS).  Elemental depth profiles determined using x-ray photoelectron spectroscopy (XPS) and x-ray diffraction of the anode electrocatalyst of the degraded fuel cells are used to understand the effect of sulfur poisoning on the Ni/YSZ electrocatalyst, and determine if nickel carbide (Ni₃C) also plays a role in increasing the anode kinetic resistance.  Finally, unique challenges associated with the use Ni/YSZ in contact with carbon fuels, particularly the formation of Ni₃C, are discussed for direct carbon fuel cells using a sulfur-free carbon source.

The work presented here serves to better understand sulfur mitigation with sulfur sensitive electrocatalyst, and provides an impetus for better understanding of sulfur poisoning mechanisms, development of sulfur tolerant anode materials, and the most effective means of utilizing sulfur sorbents. Through this technology, it will be possible to take dirty fuels containing high concentrations of sulfur and convert them into green energy in the form of hydrogen and electricity.

References:

(1)        Alexander, B. R.; Mitchell, R. E.; Gür, T. M. J. Electrochem. Soc. 2012, 159, F810.

(2)        Kurokawa, H.; Sholklapper, T. Z.; Jacobson, C. P.et al. Electrochem. Solid-State Lett. 2007, 10, B135.