Effect of operating variables on performance of membrane electrolysis cell for carrying out Bunsen reaction of I–S cycle

Membrane electrolysis is coming up as one of the alternatives to direct contact mode of carrying out Bunsen reaction of I–S cycle. It has potential to reduce the use of excess iodine and water. A two-compartment membrane electrolysis cell with graphite electrodes and Nafion 117 membrane was used for Bunsen reaction. Effect of six independent variables on cell voltage was determined for current density values of up to 80 A/dm2. The variables were anolyte pressure, catholyte pressure, temperature, sulphuric acid concentration, HI concentration, and I2/HI molar ratio in catholyte. Flow rate of anolyte and catholyte were identified where mass transfer resistance was not significant before performing experiments with different independent variables. Cell voltage was analysed by identifying three different regimes based on its variation with current density and current density ranges where electrode resistance or ohmic resistance dominated are identified. Current efficiency was measured for 1 A/dm2 and was found to be close to 100% irrespective of values of the independent variable. Minimum amount of heat equivalent of electric energy required for membrane electrolysis was calculated and increase in its value with increase in sulphuric acid concentration was compared with estimate of reduction in heat required for concentration of sulphuric acid.

► I–V curve of a membrane electrolysis cell for Bunsen reaction generated.
► Contribution of different transport resistances to cell voltage identified.
► Above 5 A/dm2, ohmic resistance of electrolyte and membrane dominates.
► Tafel region was identified in the current density range of 1.5–4 A/dm2.
► Cell voltage was strongly affected by sulphuric acid concentration.