Ionic Liquids: A Potential Role In the Sulfur Family of Thermochemical Cycles

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The Sulfur-family of thermochemical water splitting cycles has already been identified as an ideal application of ionic liquid solvents for SO2 absorption.1 Here, ionic liquids are proposed as suitable reaction media in both the Sulfur-Sulfur cycle (a new cycle based upon the Sulfur-Iodine Cycle) and also the Hybrid Sulfur (Westinghouse) Cycle.

Using ionic liquids as a reaction medium for the Bunsen reaction of the Sulfur-Iodine Cycle, we identified conditions where a stream of H2S is produced based on a normally undesired side-reaction. This led to a new "Sulfur-Sulfur" thermochemical cycle based upon the production and steam reformation of H2S:2

Step 1A, liquid phase: 4I2+4SO2+8H2O ↔4H2SO4 + 8HI

Step 1B, liquid phase: 8HI+H2SO4↔H2S+4H2O+4I2

Step 2, gas phase: 3H2SO4↔3H2O+3SO2+1.5O2

Step 3, gas phase: H2S+2H2O↔SO2+3H2

Instead of the initial Bunsen reaction (Step 1A), the Hybrid Sulfur Cycle begins with an electrochemical reaction between H2O and SO2:

SO2(aq)+2H2O↔H2SO4(aq)+H2(g)

As product acid is formed, solubility of SO2 decreases, requiring an overpotential to drive the reaction forward.3 Possessing a wide electrochemical window, high conductivity, and high SO2 solubility, an ionic liquid electrolyte ([BMIM.BF4]) was used for the electrochemical reaction between H2O and SO2 in the Hybrid Sulfur Cycle. Cyclic voltammetry studies will be presented showing the electrochemical reaction at SO2 concentrations much higher than those achievable in aqueous systems.

1. Lee, K. Y.; Taek Gonga, G.; Songb, K. H.; Kima, H.; Junga, K.-D.; Kima, C. S., Use of ionic liquids as absorbents to separate SO2 in SO2/O2 in thermochemical processes to produce hydrogen. International Journal of Hydrogen Energy 2008.

2. Yokochi, A., AuYeung, N. In A Proposed New Sulfur-Sulfur Thermochemical Cycle, AIChE National Meeting, Nashville, Tennessee, Nashville, Tennessee, 2009.

3. O'Brien, J. A.; Hinkley, J. T.; Donne, S. W.; Lindquist, S. E., The electrochemical oxidation of aqueous sulfur dioxide: A critical review of work with respect to the hybrid sulfur cycle. Electrochimica Acta 2010, 55 (3), 573-591.

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