(66a) Ionic Liquids: A Potential Role In the Sulfur Family of Thermochemical Cycles

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:

                                       SO_2(aq)+2H₂O↔H₂SO_4(aq)+H_2(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.