Junction potentials in thermolytic reverse electrodialysis

Reverse electrodialysis (RED) can produce electric energy from waste heat using thermolytic solutions (e.g., NH4HCO3) where waste heat is used to regenerate the high concentration (HC) and low concentration (LC) solutions. The salinity difference between the two solutions in RED is converted into electric potential across an ion exchange membrane (IEM), exploiting the liquid junction potential. Theoretical calculation of the junction potential is cumbersome because the activity coefficients and equilibrium speciation of individual ions are complicated for highly concentrated NH4HCO3 solution. We used a simplification of the Planck–Henderson equation to approximate the junction potential in thermolytic RED systems based on conductivity measurements, and this approximation was consistent with experimentally measured junction potentials. The experimental results also found that NH4HCO3 created greater junction potentials across anion exchange membranes than (NH4)2CO3 solution for a given molar concentration ratio. The junction potential was hardly affected by the magnitude of HC as long as the concentration ratio between HC and LC was maintained. Based on the experimental findings, we recommend that thermolytic RED systems be operated under neutral pH and high concentration ratio conditions (above 1:100 ratio). These findings provide information essential for designing and operating thermolytic RED systems for future study and practical application.