A High-Capacity Hybrid Desalination System Using Battery Type and Pseudocapacitive Type Electrodes
Authors: Yang Yu-Hsiang , Hu Chi-Chang , Huang Hung-Yi , Tu Yi-Heng , Chou Hsing Mei 
Published: December 2024

Developing cost-effective brackish water and seawater desalination technology is crucial. Capacitive deionization (CDI) and inverted capacitive deionization (i-CDI) have been recognized as a promising desalination technology with low energy consumption for brackish water.
Both systems use an electric field between two porous electrodes to remove and release ions in water reversibly.
Recently, CDI has started using pseudocapacitive or battery electrode materials to enhance the desalination performance.
Different from the desalination principle of electric double layers, the energy storage mechanisms of pseudocapacitive and battery materials generally involve ion intercalation/adsorption or compound formation for charge balance, leading to the faradaic desalination.
Low cost and high theoretical capacity make PBAs and conducting polymers be the potential materials for the faradaic desalination application.
In our previous studies, two dissimilar pseudocapacitive materials show a memory effect during brackish water desalination. This allows them to retain ion capturing or releasing states without an electric field, aiding water purification and resource recovery.
Based on above viewpoints, two materials with fundamentally different electrochemical properties are demonstrated to construct a high-capacity, hybrid, faradaic deionization system with CuHCF (battery type) as the positive electrode and PPy (pseudocapacitive type) as the negative electrode.
The deionization performance of this CuHCF//PPy cell can be improved by reversing the appropriate cell voltage during the discharge process.
The plot of specific SRC against time for the above CuHCF//PPy cell with variations in the charging cell voltage but a fixed discharging cell voltage of 0 V. In addition, Fig. 1(b) shows the plot of specific SRC against time for the same cell with variations in the discharging cell voltage but a fixed charging cell voltage of 1.2 V. From Fig. 1(a), at all charging voltages, the SRC generally decreases with the charging time, indicating the ion repelling process. The order of charging cell voltage with respect to increasing the SRC value is: 0.6 V < 0.8 V < 1.0 V < 1.2 V, revealing the impact of the charging cell voltage. From Fig. 1(b), the SRC obviously increase with prolonging the discharging time, suggesting the ion capturing process. However, the order of discharging cell voltage with respect to increasing the SRC value is: -0.4 V < 0 V < −0.1 V< −0.2 V < −0.3 V. Note that a little inverted cell voltage leads to a higher salt-removing capacity and rate in comparison with a discharge cell voltage of 0 V. However, when the inverted voltage is set at −0.4 V, the SRC profile exhibits the unstable performance, probably due to the presence of certain irreversible reactions at this cell voltage.
In the stability test, Fig 2 shows the CuHCF//PPy cell still maintained more than 90% of its original SRC after 50 cycles of testing.
The mean SRC values of this CuHCF//PPy system obtained from the 8, 15, and 30 mM solutions reached 35.536, 58.824, and 101.84 mg g−1, respectively.
This positive correlation between SRC and solution concentration reveals the higher utilization of the electroactive materials in more concentrated solutions and the very high SRC of the hybrid faradaic CuHCF//PPy system.
From Fig 3 shows the SRC values of a CuHCF//PPy cell with the charging/discharging times of 30/30 min at the charging/discharging cell voltages of 1.2/−0.2 V in the 8, 15, and 30 mM NaCl solutions.
The memory effect of electrochemically active materials can further extend the application of this system to concentrating valuable ions while purifying water, showing another advantage.
The results of ion-removing and salt-concentrating experiment with the discharge/charge times = 10 min/10 min for 10 cycles. In this 10-cycle test, the total amount of salts transferred is up to 152.9 mg g−1, revealing the dual function of purifying water and concentrating salts through this hybrid battery//pseudo-capacitive system.
This hybrid cell showed high salt removal capacities in the media containing various monovalent and divalent cations. In this work, the suitable working potential windows of both CuHCF and PPy were systematically evaluated by CV and GCD methods with the charge balance application. Moreover, this cell provides the ability in capturing other cations such as Mg2+ and Ca2+, further broadening its future potential applications. This methodology is a promising strategy for constructing a high-performance desalination cells consisting of various active materials.

Figure 1