KINETICS AND TRANSPORT PROCESSES IN AQUEOUS POLYSULFIDE ELECTRODE REACTIONS

The objectives of this research are to obtain information on the electrochemical and chemical reactions of the aqueous sulfide/polysulfide redox couple and to use that information to design a stable and efficient redox electrode.; The kinetics of the aqueous polysulfide redox couple were investigated using a transient potential step technique in a solution 1M in S{dollar}sp{lcub}0{rcub}{dollar} and 1.3M in total S{dollar}sp{lcub}2-{rcub}{dollar}, at pH 12. The low-overpotential region {dollar}(-25{dollar} to +25 mV) was investigated for cobalt and platinum electrodes from 25 to 80{dollar}spcirc{dollar}C. The results are consistent with the idea that the electrochemically active species is S{dollar}sbsp{lcub}2{rcub}{lcub}-{rcub}{dollar}. Deviations were found between experimental data for temperatures above 45{dollar}spcirc{dollar}C and the simple analytical form of the current response for an electrode reaction with one rate-determining step. The results agree with a new model proposed here that considers two rate-determining steps, one of which is the homogeneous chemical reaction: {dollar}{lcub}rm S{rcub}sbsp{lcub}4{rcub}{lcub}2{rcub}sbsp{lcub}rightarrow{rcub}{lcub}leftarrow{rcub} 2{lcub}rm S{rcub}sbsp{lcub}2{rcub}{lcub}-{rcub}{dollar} (1). The rate constants for this reaction determined using this new model agree with those reported for similar reactions. Exchange current densities (i{dollar}sb{lcub}rm o{rcub}{dollar}/{dollar}nu{dollar}) were on the order of 1 mA/cm{dollar}sp{lcub}2{rcub}{dollar} at 60{dollar}spcirc{dollar}C.; The steady-state current density-overpotential relationships for cobalt and platinum rotating disk electrodes were determined for a range of temperatures. A model was developed to take into account the rate of Reaction 1 as well as a finite electrochemical reaction rate. The addition of 13 mole-% DMF to the aqueous polysulfide solution was found to significantly increase the current density, at a given temperature and overpotential, by increasing the equilibrium concentration of S{dollar}sbsp{lcub}2{rcub}{lcub}-{rcub}{dollar}.; A flow cell for testing flow-through porous electrodes was constructed and operated. The effect of electrode material, temperature, flow rate, and the addition of DMF on electrode performance was studied. MoS{dollar}sb{lcub}2{rcub}{dollar}, Co and Ni were used as electrocatalyst materials. Raising the temperature and/or the addition of DMF had much larger effects than an order of magnitude increase in the flow rate.