Metal-Ion Depletion Impacts the Stability and Performance of Battery Electrode Deionization over Multiple Cycles

Abstract

Prussian blue hexacyanoferrate (HCF) materials, such as copper hexacyanoferrate (CuHCF) and nickel hexacyanoferrate (NiHCF), can produce higher salt removal capacities than purely capacitive materials when used as electrode materials during electrochemical water deionization due to cation intercalation into the HCF structure. One factor limiting the application of HCF materials is their decay in deionization performance over multiple cycles. By examining the performance of CuHCF and NiHCF electrodes at three different pH values (2.5, 6.3, and 10.2) in multiple-cycle deionization tests, losses in capacity (up to 73% for CuHCF and 39% for NiHCF) were shown to be tied to different redoxactive centers through analysis of dissolution of electrode metals. Both copper and iron functioned as active centers for Na + removal in CuHCF, while iron was mainly the active center in NiHCF. This interaction of Na + and active centers was demonstrated by correlating the decrease in performance to the concentration of these metal ions in the effluent solutions collected over multiple cycles at different pHs (up to 0.86 ± 0.14 mg/L for iron and 0.42 ± 0.17 mg/L for copper in CuHCF and 0.38 ± 0.05 mg/L for iron in NiHCF). Both materials were more stable (<11% decay for CuHCF and no decay for NiHCF) when the appropriate metal salt (copper or nickel) was added to the feed solutions to inhibit electrode dissolution. At a pH of 2.5, there was an increased competition between protons and Na + ions, which decreased the Na + removal amount and lowered the thermodynamic energy efficiency for deionization for both electrode materials. Therefore, while an acidic pH provided the most stable performance, a circumneutral pH would be useful to produce a better balance between performance and longevity.

Selective Capture of Cu2+ Using a Redox-Active CuS Cathode Material in Hybrid Capacitive Deionization

“…Until pH = 2.5 and 1.5, even after adsorption for 180 min, the values of C/C 0 are 47.51 and 78.40%, respectively, in the end, demonstrating a significant decrease in adsorption capacity. The reason may be that under the condition of low pH, the solution contains more H + , and due to the competitive effect, a large number of H + occupy the site where Cu 2+ binds to the CuS electrode, leading to the reduction of electric adsorption. ,, In addition, under the action of the shield effect of electric potential, the repulsion between adsorbed H + and Cu 2+ may also be the reason for this phenomenon. Generally, treatment of copper-containing wastewater via CDI works best under weak acid conditions …”

Pairing of Aqueous and Nonaqueous Electrosynthetic Reactions Enabled by a Redox Reservoir Electrode

“…It was reported that excess Ni ions in solution help stabilize the Prussian blue structure, preventing Ni dissolution that results in leaching of the redox-active Fe from NiHCF. 42 NiHCF has worse rate capabilities in methanol than aqueous solutions presumably due to the reduced hydrogen bonding network. 43 After studying different cycling currents, keeping the C rate as low as possible during cycling in methanol proved to be essential for maintaining the capacity of NiHCF.…”

“…When undergoing reduction in methanol, NiHCF had more severe dissolution. It was reported that excess Ni ions in solution help stabilize the Prussian blue structure, preventing Ni dissolution that results in leaching of the redox-active Fe from NiHCF . Therefore, we added 20 mM NiNO 3 into the methanol solution, which improved the cycling performance.…”

Thermodynamic and Kinetic Analyses of Ion Intercalation/Deintercalation Using Different Temperatures on NiHCF Electrodes for Battery Electrode Deionization

“…Electrochemical deionization technologies, such as capacitive deionization (CDI) and battery deionization (BDI), have been examined for deionization and nutrient recovery applications due to their good performance in selective removal of ions with low energy consumption and high thermodynamic energy efficiencies. − In CDI, ions are stored in the electrical double layer (EDL) on the surfaces of capacitive electrodes byan electric field . In BDI using battery-type electrodes, for example, Prussian blue analogues (PBA), cations are intercalated into the crystal structures of PBA by faradaic processes with optima at certain electrode potentials. ,− The performance of both CDI and BDI systems can be impacted by various operating conditions, such as charge–discharge modes, charge–discharge rates, feed solution flow rate, feed solution concentrations, and solution pH . However, the thermodynamic and kinetic properties of cation intercalation/deintercalation for PBA electrodes used in BDI applications are relatively unexplored.…”