(434a) Determination of the Equilibrium Constant for the Decomposition of Potassium Bicarbonate In Water At near-Critical Conditions

The carbon fuel cell
can theoretically convert the chemical energy of carbon into electric power
with an efficiency of almost 100 %. As a result of this surprising aspect, different
attempts have been undertaken to develop it for more than a century. In our
laboratory, an aqueous-alkaline carbon fuel cell that works at moderate temperatures
(over 200 °C) and high pressures (≈48 bar) has been under development
since 2000.

When the electrolyte
used was potassium carbonate at a high concentration, unexpected crystals were
found. TGA analysis of this precipitate in the Hungarian Academy of Science
revealed that the crystals were mainly potassium bicarbonate. TGA showed the
decomposition of the potassium bicarbonate into potassium carbonate and CO₂
at temperatures above 200 °C. This caused us to wonder if the decomposition
also occurs in solution; thereby solving the precipitation problem.

The liquid phase
chemistry was studied by exposing solutions of potassium bicarbonate in a
pressure vessel to temperatures near 300 ^oC
at their saturation pressure, different time frames and different cooling
treatments.

Three different
analyses of the final solutions were utilized: (1) pH measurement, (2) proximate
analysis and (3) TGMS of the dry crystals in the Hungarian Academy of Science.

The significant pH
increase of the final solutions suggested the potassium bicarbonate decomposed
via the reaction:

2HCO₃^-↔ CO₃^2-+ CO₂+ H₂O

The determination of
the nature of the dry crystals by TGMS analysis confirmed the decomposition and
gives more accurate values of the extents of reaction.

All these findings can
finally be used to evaluate the temperature dependent equilibrium constant of
the reaction in saturated liquid water at temperatures near 300^oC.
And therefore, predict the behavior of the carbonate electrolyte in the carbon
fuel cell. In this presentation, we describe the results of this study and our
measurements of the equilibrium constant.