Fuel Cells Explained

Fuels for Fuel Cells

Fuels for Fuel Cells  |  Fuel Types  |  Processing Required for Fuels

Fuels

The primary fuels that can be directly utilized within fuel cell stacks today are hydrogen, carbon monoxide, methanol, and dilute light hydrocarbons like methane, depending upon the fuel cell type. 

The table below presents various fuel cell types and the primary fuels that they are amenable to using.  Note that the presence of sulfur is not tolerated by fuel cells in general and that the SOFC is the most inherently fuel flexible of the fuel cell types.  MCFC units are also quite fuel flexible.

Table Effects of various gaseous reactants on various fuel cell types
(Source: Hirschenhofer et al., 1998).

Gas Species
PAFC
MCFC
SOFC
PEFC
H2
Fuel
Fuel
Fuel
Fuel
CO
Poison (>0.5%)
Fuela
Fuel
Poison (>10ppm)
CH4
Diluent
Diluentb
Fuela
Diluent
CO2 & H2O
Diluent
Diluent
Diluent
Diluent
S as (H2S & COS)
Poison (>50 ppm)
Poison(>0.5 ppm)
Poison(>1.0 ppm)
NO studies to date (11)

a - In reality, CO, with H2O, shifts to H2 and CO2, and CH4,
with H2O, reforms to H2 and CO faster than reacting as a fuel at the electrode.

b – A fuel in the internal reforming MCFC.

Hydrogen is the optimal fuel for all types of fuel cells.  Methanol can be used directly in a certain type of PEMFC called a direct methanol fuel cell. 

Carbon monoxide is a poison for lower temperature fuel cells, but is used as a fuel in the high temperature fuel cells (e.g., SOFC, MCFC).  CO may not actually react electrochemically within these cells.  It is commonly understood that CO is consumed in the gas phase through the water-gas shift reaction as follows:

CO + H2O Õ CO2 + H2

The hydrogen formed in this reaction is subsequently consumed electrochemically. 

Methane can be oxidized directly using a solid oxide fuel cell, however, high concentrations of CH4 lead to severe coking problems.  Therefore, only fuels containing dilute concentrations of CH4 can be oxidized directly in current SOFCs.  In addition, the oxidation of CH4, like that of CO, may not actually occur at active electrochemical sites within an SOFC.  Rather, CH4 is probably reformed within the cell through steam reformation chemistry as follows:

-----> CH4 + 2H2O    catalyst    CO2 + 4H2

 

 

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Fuel Cells Explained

 


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