Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt.
Environ Sci Technol. 2021 Apr 20;55(8):5412-5421. doi: 10.1021/acs.est.0c08629. Epub 2021 Mar 30.
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 redox-active 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.
普鲁士蓝六氰合铁酸盐（HCF）材料，如铜六氰合铁酸盐（CuHCF）和镍六氰合铁酸盐（NiHCF），在电化学水脱盐过程中用作电极材料时，由于阳离子嵌入 HCF 结构中，其脱盐能力比纯电容材料更高。限制 HCF 材料应用的一个因素是其在多次循环中的脱盐性能下降。通过在多次循环脱盐测试中在三个不同 pH 值（2.5、6.3 和 10.2）下检查 CuHCF 和 NiHCF 电极的性能，通过分析电极金属的溶解，发现容量损失（CuHCF 高达 73%，NiHCF 高达 39%）与不同的氧化还原活性中心有关。在 CuHCF 中，铜和铁都作为 Na 去除的活性中心，而铁主要是 NiHCF 中的活性中心。通过将性能下降与在不同 pH 值下（CuHCF 中高达 0.86±0.14 mg/L 的铁和 0.42±0.17 mg/L 的铜，NiHCF 中 0.38±0.05 mg/L 的铁）在多个循环中收集的流出溶液中这些金属离子的浓度相关联，证明了 Na 和活性中心之间的这种相互作用。在添加适当的金属盐（铜或镍）到进料溶液中以抑制电极溶解时，两种材料都更加稳定（CuHCF 衰减小于 11%，NiHCF 无衰减）。在 pH 值为 2.5 时，质子和 Na 离子之间的竞争加剧，这降低了两种电极材料的 Na 去除量，并降低了脱盐的热力学能量效率。因此，虽然酸性 pH 值提供了最稳定的性能，但中性 pH 值将有助于在性能和寿命之间取得更好的平衡。
