Xu Bingang’s team in Adv. Mater. regulates electron delocalization in two-dimensional heteroligand-based copper-organic frameworks to enhance NH₄⁺ charge storage.

In August 2024, Professor Xu Bingang’s team from The Hong Kong Polytechnic University published a research paper titled “Regulation of Electron Delocalization Region in 2D Heteroligand-Based Copper-Organic Framework to Enhance NH4+ Charge Storage” in the journal Advanced Materials (impact factor > 27.4). Dr. Lu Hongyu is the first author of the paper. The study discovered a two-dimensional heteroligand-based copper-organic framework with multiple ammonium ion adsorption sites within its π-conjugated periodic framework, providing several available redox-active sites for high-performance ammonium storage. Thanks to the heteroligand’s effective regulation of electron delocalization and the intrinsic hydrogen-bond cage mechanism between ammonium ions, the battery achieved a high specific energy density of 211.84 Wh kg⁻¹ and could stably operate for 12,000 cycles at 5 A g⁻¹, lasting over 80 days. These findings offer new insights for the rational design of high-performance MOF-based ammonium-ion battery cathode materials to achieve efficient energy storage and conversion.

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Research Background

Aqueous rechargeable batteries have garnered widespread attention in large-scale energy storage applications due to their low cost, high safety, and environmental friendliness. In these systems, charge carriers facilitate energy transfer and charge transfer in aqueous electrolytes, which is crucial for optimizing the electrochemical performance of aqueous batteries. To date, metal cations such as lithium (Li), sodium (Na), potassium (K), zinc (Zn), magnesium (Mg), and aluminum (Al) have commonly been used as charge carriers for reversible energy storage in aqueous batteries. However, metal cations have several inherent drawbacks, including high molar mass, large hydrated volume, and relatively slow diffusion kinetics. In contrast, non-metallic charge carriers offer a viable alternative to metal charge carriers in aqueous batteries, imparting fast kinetics, longer cycle life, and lower manufacturing costs. Among non-metallic ions, the ammonium ion (NH₄⁺) has relatively sustainable and abundant resources on Earth. NH₄⁺ has a smaller hydrated ion size and lighter molar mass, which can enhance the diffusion kinetics of ions in aqueous battery systems. Moreover, the tetrahedral geometry of NH₄⁺, unlike the spherical shape of metal charge carriers, can lead to unique intercalation chemistry within host materials, subsequently reducing corrosivity and lowering the potential for the hydrogen evolution reaction (HER). Therefore, NH₄⁺ can be considered an ideal charge carrier for aqueous batteries. To date, a series of studies have promoted significant progress in “rocking-chair” aqueous ammonium ion batteries (AAIBs). Most studies on AAIBs have primarily focused on the development of corresponding cathode materials, such as manganese dioxide (MnO₂), vanadium pentoxide (V₂O₅), and Prussian blue analogs (PBAs). Although these materials have open frameworks and layered channel structures capable of accommodating large ions, their relatively low ion mobility and ion storage capacity still hinder the further application of AAIBs. Therefore, developing suitable cathode materials with fast NH₄⁺ reaction kinetics, excellent conductivity, and multiple redox centers is crucial for achieving high-performance NH₄⁺ storage. Two-dimensional conductive metal-organic frameworks (2D MOFs) have shown tremendous potential in energy storage for AAIBs due to their ordered molecular arrangement, highly tunable structure, and porous architecture. In particular, the potential value of 2D MOFs in AAIBs has become a promising research area. However, comprehensive studies on 2D MOFs in AAIBs are still lacking. Despite the fact that the persistent π-d conjugated coordination between metal nodes and organic linkers in 2D MOFs results in strong electron delocalization throughout the framework, thereby optimizing electron storage capacity, the regular arrangement of unit structures coexisting with open channels facilitates the exposure of active sites and ensures efficient ion storage. However, electrode materials based on 2D MOFs face limitations in the number of redox-active sites, with their charge storage mechanisms derived entirely from the metal centers or organic ligands. While designing 2D MOFs with multiple redox-active units can activate high specific capacity, these frameworks are prone to dissolution during long-term cycling due to the weak coordination bonds between metals and ligands. Therefore, developing 2D MOFs that simultaneously integrate multiple redox centers and robust frameworks is an ideal design strategy for achieving high-performance electrode materials.

Research Question

Figure 1.a) Schematic illustration of the synthesis of the Cu-HHTP-THBQ heteroligand MOF through ethylenediamine-assisted competitive coordination.b) Molecular structure of the designed Cu-HHTP-THBQ.c) Charge density contribution of Cu-HHTP-THBQ.d, e) Illustration of the single-coordination structure.f, g) Illustration of the double-coordination structure.h) Electrostatic potential of Cu-HHTP-THBQ.i) Schematic of the hydrogen-bond cage effect.

Figure 2.a) Preparation process of Cu-HHTP-THBQ.b) PXRD pattern and crystal structure of Cu-HHTP-THBQ.c) FT-IR spectra of Cu-HHTP-THBQ, HHTP, and THBQ.d) Raman spectra of Cu-HHTP-THBQ, HHTP, and THBQ.e-g) Adsorption energy calculations and charge density differences of Cu-HHTP-THBQ for NH₄⁺, H₂O, and NH₄⁺(H₂O).h) Diffusion energy barrier of NH₄⁺ in Cu-HHTP-THBQ.

Figure 3.a) Adsorption energy of NH₄⁺ on different water molecules.b) Solvation structures of NH₄⁺ coordinated with four and five water molecules.c) Adsorption energy of water molecules on Cu-HHTP-THBQ.d) Schematic illustration of solvation behavior in NH₄⁺ electrolyte.e) CV curves, and f) GCD curves of Cu-HHTP-THBQ and Cu-HHTP.g) Comparison of NH₄⁺ diffusion coefficients (D_NH₄⁺) between Cu-HHTP-THBQ and Cu-HHTP.h) Log(i) vs. log(v) curves at each redox peak, and.i) Energy barriers of NH₄⁺ diffusion in Cu-HHTP and Cu-HHTP-THBQ.

Figure 4.a) Illustration of charge transfer in the NH₄⁺ full-cell system.b) CV curves of the full-cell at a scan rate of 5 mV s⁻¹ (red for Cu-HHTP-THBQ cathode, purple for PTCDI anode at 5 mV s⁻¹).c) Rate performance of the full-cell at different current densities.d) Cycling stability of the full-cell at a current density of 5 A g⁻¹.e) Comparison of the electrochemical performance metrics of the full-cell with recent relevant reports.f) Schematic of the miniaturized conformal flexible NH₄⁺ full-cell, andg) Demonstration of the NH₄⁺ full-cell powering a wearable electronic device.

Figure 5.a-c) In situ Fourier transform infrared (FT-IR) spectra of the PTCDI//Cu-HHTP-THBQ full cell at different voltages.In situ high-resolution XPS spectra at different charge and discharge states:d) Cu 2p,e) O 1s, and.f) N 1s.g) Calculated band structure and partial density of states (PDOS) of Cu-HHTP-THBQ.h) Evolution of electronic states in the repeated coordination units of the 2D Cu-HHTP-THBQ MOF during the charge and discharge process.

Research Summary

In summary, the researchers developed a 2D π-d conjugated heteroligand-based MOF as a promising and sustainable electrode material for rechargeable AAIBs (aqueous ammonium-ion batteries). Through theoretical simulations and in-situ spectroscopic characterization, they uncovered a universal hydrogen bonding reaction mechanism between NH₄⁺ and the heteroligands. Additionally, the synthesized 2D heteroligand-based MOF modulated electron distribution, enabling precise control of delocalization effects, which ensured the efficiency and reversibility of the NH₄⁺ adsorption process. Ultimately, the PTCDI//Cu-HHTP-THBQ full cell delivered a high discharge capacity of 132.4 mAh g⁻¹ at 0.2 A g⁻¹, and demonstrated excellent stability with a capacity retention rate of 88.2% after 12,000 cycles at 5 A g⁻¹. This work enhances the understanding of the relationship between electron delocalization regulation and macro-scale electrochemical performance, providing new insights into achieving high energy density AAIBs.
Original text link：https://onlinelibrary.wiley.com/doi/10.1002/adma.202408396

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