In Situ Electrochemical Dilatometry of Phosphate Anion Electrosorption

Here we investigate the competitive electrosorption of mono- and divalent phosphate anions through electrochemical desalination- and dilatometry-based experiments. Through in situ dilatometry, we monitor the strain at the electrode surface as anions and cations are electrosorbed. Strain measurements show that the presence of divalent ions promotes a greater than anticipated electrode expansion during cation (Na⁺) electrosorption. The expansion observed with Na⁺ equaled the expansion observed with HPO₄ ^2-. Because the ionic radius of Na⁺ is smaller than that of HPO₄ ^2-, the symmetric expansion suggests that divalent anions do not completely desorb during electrode regeneration, causing the adverse interactions with the cation during co-ion expulsion. This results in a decrease in desalination performance, indicated by a decreased salt adsorption capacity. Conversely, an expected asymmetric expansion during anion and cation electrosorption occurs with monovalent phosphate anions (H₂PO₄ ^-), indicating that monovalent ions can be effectively replaced by the cation at the electrode surface.