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The performance of Reverse Electrodialysis (RED) stacks can be significantly degraded by the presence of divalent ions. To gain a deeper understanding of the influence of divalent ions, a RED model based on the Nernst-Planck framework was developed in this study. Using this model, a simulation study was performed when the feed solutions contain a mixture of NaCl and MgCl 2. The impact of the divalent ion Mg2+ on RED performance was examined from the perspective of ion transport. The results indicate that divalent counterions decrease membrane permselectivity and membrane potential, while increasing membrane electrical resistance by affecting ion concentrations within the membrane. The uphill transport of divalent ions occurs only when the molar fraction of divalent ions in the feed solutions is relatively low. Specifically, when the feed solution contains only MgCl 2 compared to only NaCl, the open-circuit voltage decreases by 36.2%, and the maximum gross power density decreases by 44.8%. [Display omitted] • A RED model was developed based on the Nernst-Planck framework. • Influence of divalent ions on the performance of RED was investigated. • Degradation of RED performance was examined in terms of ion transport. [ABSTRACT FROM AUTHOR]