(33d) The Rate of Carbonate Production in a Direct Carbon Fuel Cell

EXTENDED ABSTRACT:

Introduction:   

      A Direct Carbon Fuel Cell (DCFC)
produces electricity from a solid carbon (C) fuel source electrochemically,
without conventional combustion.  The fuel cell considered here employs a
molten sodium hydroxide (NaOH) electrolyte and produces electricity by the
reactions shown in Equations 1, 2, and 3.

                  Anode Reaction

                        C 
+  4OH^-  →  CO₂  +  2H₂O  +  4e^-                                                                                    (1)

                  Cathode Reaction

                        4e^- 
+  O₂  +  2H₂O  →  4OH^-                                                                           (2)

                  Overall Reaction

                        C 
+  O₂  →  CO₂                                                                                                  (3)

      There are several
technological hurdles to overcome before this technology becomes viable for
large scale energy production.  One of these issues is the formation of
carbonate, in the fuel cell electrolyte, through an undesirable side reaction. 
As the concentration of carbonate increases, the efficiency of the fuel cell
decreases and at high concentrations the fuel cell is fatally poisoned.  Carbon
dioxide (CO₂) produced at the anode reacts with hydroxide ions (OH^-) which are present in the electrolyte to produce carbonate ions (CO₃^2-). 
The mechanism for this reaction is not fully understood, but one likely
mechanism was proposed by J. Goret and B. Tremillion (Cao, et al.).  This
mechanism consists of a chemical process,
represented in Equation 4, and an electrochemical process represented in Equation
5.  The electrochemical process consists of two steps represented in Equations
6 and 7, with the reaction in Equation 7 being the rate determining step (Cao,
et al.).

CO₂  +  2OH^- 
↔  2CO₃^2-  +  H₂O                                                       
            (4)

C  +  6OH^- 
→  CO₃^2-  +  3H₂O  +  4e^-                                                            (5)

6OH^- 
↔  3O^2-  +  3H₂O                                                                                  (6)

C  +  3O^2- 
→  CO₃^2-  + 4e^-                                                                              (7)

Results:

      The production rate of
carbonate in a bench-scale DCFC utilizing a molten sodium hydroxide electrolyte
was investigated.  A wet chemistry procedure was developed to measure the mass
fraction of sodium carbonate (Na₂CO₃) in a solidified
sample of the electrolyte.  This procedure involved separating and weighing a
sodium carbonate precipitate from a high-molar sodium hydroxide solution.  The
measured mass fraction was used to calculate the concentration of the carbonate
ion in the molten electrolyte.  Multiple electrolyte samples were analyzed, and
the concentration of carbonate was plotted as a function of fuel cell
operation.

      It was found that the
production of carbonate is directly proportional to the energy produced by the
fuel cell.  As such, carbonate concentration increases linearly as energy is
produced by the cell.  Specifically, 0.67 mol of carbonate are produced for
every 100 kJ of energy produced.

            These data represent a
significant step in DCFC research, as the production of carbonate, while known
to be undesirable, has never been measured as a function of fuel cell
operation.  Attempts to reduce the rate of carbonate production can now be made
and compared against this base rate to judge the relative success or failure of
these attempts.

      Additionally, in an attempt
to reduce the formation of carbonate, carbonate production was investigated in a
DCFC doped with magnesium oxide (MgO).  It was found that there was a time lag
before carbonate was produced; however, once carbonate began forming, the rate
was unchanged compared to the un-doped electrolyte.

Cao, Sun, Wang. Direct carbon
fuel cell: Fundamentals and recent developments. Journal of Power Sources.
Volume 167. pp 250-257. 2007.