Prussian blue analogs, e.g., nickel hexacyanoferrate, NiFe(CN)₆ or NiHCF, are promising candidates as low-cost and high-rate intercalation materials for secondary batteries.^1–4 Recently, this material class has been shown to possess tremendous potential for a novel energy-efficient water desalination approach.^5–8 Rising water demands are exacerbating water scarcity in many world regions. It is estimated that 60% of the global population will face severe water scarcity by 2025.⁹ The growing water demand necessitates new desalination technologies with high energy efficiency, low capital and operating cost and high freshwater output. In this work, we assess the performance and lifetime of electrochemical water desalination cells based on sodium intercalation into nickel hexacyanoferrate.^10–12

The battery desalination cells feature a symmetric design, with two NiHCF electrodes at opposite state-of-charge (SOC), capable of intercalating Na⁺-ions into their crystal structure. The electrodes are separated by an anion exchange membrane, a porous functionalized polyether ether ketone (PEEK) membrane, that only permits negatively charged ions, e.g., Cl^--ions, to pass. Two feed water streams with 20 mM NaCl enter the symmetric cell on either side (see Figure 1a). During charge of the symmetric cell, incoming Na⁺-ions are removed from one water stream and intercalated into the NiHCF electrode at low SOC. Simultaneously, Na⁺-ions are deintercalated from the opposite NiHCF electrode at high SOC. In order to maintain charge neutrality, Cl^‑-ions cross the anion exchange membrane. Thus, during every charge/discharge cycle, one water stream is desalinated forming a freshwater stream, while the other is enriched in NaCl forming a brine waste stream (see Figure 1b).

In order to quantify performance and lifetime of the novel battery-type water desalination cells, we define and measure objective metrics. We see that energy consumption (Wh/l) and productivity (l/h/m²) of NiHCF/NiHCF cells are superior to cells based on membrane capacity deionization (mCDI). Stable charge/discharge cycling of NiHCF/NiHCF cells can be achieved for over 500 cycles with NaCl feed water, but rapid aging is observed with CaCl₂ feeds. Synchrotron-based characterization of NiHCF/NiHCF cells is used to elucidate the reason for capacity fade. X-ray absorption spectroscopy and X-ray fluorescence spectroscopy reveal Fe dissolution from the NiHCF active material as a primary aging mode with CaCl₂ water feeds.

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Figure 1. (a) Battery-type water desalination approach in symmetric NiHCF/NiHCF cells with two salt water streams entering the cell and a brine stream and freshwater stream exiting the cell. (b) During galvanostatic charge/discharge cycling the salt concentrations of brine and freshwater stream can be monitored with microfluidic operando conductivity probes to determine important performance metrics.