Final Report from 2022 Young Investigator Grants – Dr. Jihye Park

Metal-Organic Framework-based Cl^– Storage Material for Electrochemical Desalination Battery

With a growing global population and increasing water scarcity, access to clean drinking water has become a significant concern. Traditional water desalination methods, such as reverse osmosis and thermal desalination, are energy-intensive, requiring more sustainable alternatives.¹ Electrochemical desalination, or desalination batteries, offers a groundbreaking concept where energy is stored and released during the desalination process through reversible redox reactions of saline water, involving the sequestration and release of Na⁺ and Cl^–.² Theoretically, electrochemical desalination holds the potential for a more energy-efficient solution. Still, it faces two major challenges: the instability of current electrode materials for Cl^– storage and slow charge transport due to low electrical conductivity and ion diffusivity.³

To address these challenges and unlock the full potential of electrochemical desalination, we aim to develop innovative Cl^– storage electrode materials using electrically conductive metal-organic frameworks (EC-MOFs). EC-MOFs are a relatively new subclass of MOFs, exhibiting extended d-π conjugation between metals and ligands, resulting in electrical conductivity.⁴ This unique characteristic, combined with their structural diversity, versatile functionality, and high surface area, makes EC-MOFs an exciting candidate for an electrochemical Cl^– storage material. Compared to conventional electrode materials, EC-MOFs offer the advantage of high electrical conductivity, up to 2500 S/cm,⁵ and their porous nature is well-suited to accommodate bulky Cl^– ions, ensuring fast diffusion in the open channel. In addition, the synthetic tunability of EC-MOFs enables fine control of their physical, chemical, and electrical properties, allowing for in-depth exploration of structure-property relationships. 

While MOFs have shown promise in clean water technologies, they have mainly been employed for applications such as water remediation and filtration,⁶ leaving the field of electrochemical desalination largely unexplored. Herein, we established essential design criteria for porous Cl^– storage electrodes, unraveling the intricate structure-property-performance relationships. Next, we scaled up the storage reaction to maximize the desalination efficiency and elucidated the ion storage mechanism in frameworks. 

To establish the prerequisites for effective Cl^– storage electrodes, we investigated three key factors: 1) metal identity, 2) electrical conductivity, and 3) pore size, as shown in Figure 1a. Our primary focus was on the metal identity driven by the anionic nature of Cl^– ions, indicating that metals with cationic properties would work as a Cl^– storage site. The multivalency of metals was of particular significance, as metal nodes within frameworks are inherently bonded to ligands. Therefore, adjustments in the valence states of metal ions become crucial to facilitate interaction with external anions.

We conducted a comparative analysis of Cl^– storage utilizing copper (Cu) as a multivalent metal and nickel (Ni) as a monovalent metal, with hexaaminobenzene (HAB) as a ligand. Linear sweep voltammetry (LSV) data demonstrated characteristic redox reactions for Cu, while Ni exhibited no such response. Within an aqueous solution mimicking seawater, a concentration of 0.6 M NaCl, only Cu displayed a significant change in concentration, approximately 0.08 M, of chloride ions during charge-discharge cycles (Figure 1b).

 

Next, we revealed the effect of electrical conductivity by introducing variations in the metal-ligand bonding environments within the same structures, namely Cu-HAB, Cu-triaminotrihydroxybenzene (TATHB), and Cu-hexahydroxybenzene (HHB). These variations enable the adjustment of electrical conductivity within the range of 10^-5 to 10^-8 S cm^-1.^7,8 The Cl^– storage experiment highlighted that higher electrical conductivity corresponded to enhanced adsorption efficiency (Figure 1c). Lastly, control over pore size was achieved by synthesizing MOFs based on HAB and hexaiminotriphenylene (HITP), resulting in pore size adjustments ranging from 11Å to 20 Å.⁴ This experimentation confirmed the relationship between larger pore sizes and improved Cl^– storage efficiency (Figure 1d).

Based on our experimental findings, we have concluded that Cu-HITP exhibits potential as an ideal electrode material for Cl^– storage. Consequently, a scaled-up reaction was conducted to demonstrate the efficiency of desalination. A carbon paper-based large working electrode was designed (as depicted in Figure 1e inset), and we investigated the Cl^– concentration ratio variation after charging/discharging based on the loading amount of Cu-HITP. Notably, Figure 1e indicates a significant correlation with the loading amount. An impressive 90% of Cl^– in the solution could be effectively stored in the working electrode upon loading 14 mg of MOFs. These findings hold significant promise for direct application in desalination technology.

Studying the charge storage mechanism via investigating structural transformations occurring during the redox process can propel advancements in developing enhanced and safe battery electrodes. We hypothesize that ions can be introduced into the channels or gaps between interlayers, as depicted in Figure 2a. In the former scenario, it is plausible that the crystal structure remains unchanged throughout the charge/discharge cycle. Conversely, in the latter case, intercalation would expand the interlayer spacing within the 2D EC-MOFs. We conducted an X-ray diffraction (XRD) analysis after charging and discharging to validate the precise location of Cl- ions. The results presented in Figure 2b reveal no substantial variation, supporting that ion insertion occurs within the channels. Furthermore, we observed an alteration in the oxidation states of Cu after discharging via X-ray photoelectron spectroscopy (XPS), shifting from Cu²⁺ to Cu⁺, indicating an interaction between Cl^– ions and Cu (Figure 2c and d). These findings will improve our understanding of the charge storage mechanism and enhance battery electrode design.

We introduced a novel class of electrode materials based on EC-MOFs for efficient Cl^– storage in desalination batteries. We found that multivalence of metal nodes, high electrical conductivity, and high pore volume are prerequisites for achieving efficient Cl^– storage, and the ideal EC-MOF could show an impressive 90% Cl storage efficiency. The successful execution of our work not only guides potential new materials but contribute to the fundamental understanding of EC-MOF synthesis/functionalization and the underlying Cl^– storage mechanism, paving the way for groundbreaking advancements in the water-energy nexus.

References

(1) Ghaffour, N.; Missimer, T. M.; Amy, G. L. Technical Review and Evaluation of the Economics of Water Desalination: Current and Future Challenges for Better Water Supply Sustainability. Desalination 2013, 309, 197–207.

(2) Nam, D.-H.; Lumley, M. A.; Choi, K.-S. Electrochemical Redox Cells Capable of Desalination and Energy Storage: Addressing Challenges of the Water–Energy Nexus. ACS Energy Lett. 2021, 1034–1044.

(3) Xu, D.; Wang, W.; Zhu, M.; Li, C. Recent Advances in Desalination Battery: An Initial Review. ACS Appl. Mater. Interfaces 2020, 12 (52), 57671–57685.

(4) Xie, L. S.; Skorupskii, G.; Dincă, M. Electrically Conductive Metal–Organic Frameworks. Chem. Rev. 2020, 120 (16), 8536–8580.

(5) Huang, X.; Zhang, S.; Liu, L.; Yu, L.; Chen, G.; Xu, W.; Zhu, D. Superconductivity in a Copper(II)-Based Coordination Polymer with Perfect Kagome Structure. Angew. Chem. Int. Ed. 2018, 57 (1), 146–150.

(6) Yao, Y.; Wang, C.; Na, J.; Hossain, M. S. A.; Yan, X.; Zhang, H.; Amin, M. A.; Qi, J.; Yamauchi, Y.; Li, J. Macroscopic MOF Architectures: Effective Strategies for Practical Application in Water Treatment. Small 2022, 18 (8), 2104387.

(7) Choi, J. Y.; Wang, M.; Check, B.; Stodolka, M.; Tayman, K.; Sharma, S.; Park, J. Linker-Based Bandgap Tuning in Conductive MOF Solid Solutions. Small 2023, 19 (11), 2206988.

(8) Park, J.; Hinckley, A. C.; Huang, Z.; Feng, D.; Yakovenko, A. A.; Lee, M.; Chen, S.; Zou, X.; Bao, Z. Synthetic Routes for a 2D Semiconductive Copper Hexahydroxybenzene Metal–Organic Framework. J. Am. Chem. Soc. 2018, 140 (44), 14533–14537.