# Benzotrithiophene-sulfonate covalent-organic frameworks: Supramolecular proton pumps for high-rate aqueous zinc-ion energy storage systems

Haijun Peng, Verónica Montes-garcía, Kaiyue Jiang, Dawid Pakulski, Shunqi Xu, Michal Bielejewski, Fanny Richard, Xiaodong Zhuang, Paolo Samorì, Artur Ciesielski

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Haijun Peng, Verónica Montes-garcía, Kaiyue Jiang, Dawid Pakulski, Shunqi Xu, et al. Benzotrithiophene-sulfonate covalent-organic frameworks: Supramolecular proton pumps for high-rate aqueous zinc-ion energy storage systems. SmartMat, 2024, 5 (6), 10.1002/smm2.1312. [https://doi.org/10.1002/smm2.1312](https://doi.org/10.1002/smm2.1312). [arXiv:2406.08732](https://arxiv.org/abs/2406.08732)

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# Benzotrithiophene-sulfonate covalent-organic frameworks: Supramolecular proton pumps for high-rate aqueous zinc-ion energy storage systems

Haijun Peng<fnref n="1" /> ID

Verónica Montes-García<fnref n="1" /> ID

Kaiyue Jiang<fnref n="2" />

Dawid Pakulski<fnref n="3" /> ID

Shunqi Xu<fnref n="1" />

Michał Bielejewski<fnref n="4" /> ID

Fanny Richard<fnref n="1" /> ID

Xiaodong Zhuang<fnref n="2" />

Paolo Samori<fnref n="1" /> ID

Artur Ciesielski<fnref n="1" />,<fnref n="3" /> ID

**Correspondence**

Paolo Samori and Artur Ciesielski, Université de Strasbourg & CNRS, 8, allée Gaspard Monge, Strasbourg 67083, France.

Email: samori@unistra.fr and

ciesielski@unistra.fr

**Funding information**

Interdisciplinary Thematic Institute
SysChem, Grant/Award Number: ANR-10-IDEX-0002; Foundation Jean-Marie Lehn
Institut Universitaire de France (IUF);
National Science Centre,
Grant/Award Numbers: 2019/35/B/ST5/01568, 2022/47/D/ST5/00261

**Abstract**

Proton chemistry is becoming a focal point in the development of zinc-ion energy storage devices due to its swift $H^+$ insertion/extraction kinetics. This characteristic feature confers to electrodes a remarkable power density, rate capability, and prolonged cycling durability. However, the storage mechanism of $H^+$ in electrodes based on covalent-organic frameworks (COFs) has not been thoroughly investigated. In this work, we introduce an unprecedented concept involving a supramolecular approach based on the design of a benzotrithiophene-sulfonate COF (COF-BTT-SO$_3$H) with remarkable storage capacity for simultaneous insertion and extraction of $H^+$ and $Zn^{2+}$. The ad hoc positioning of the -SO$_3$H groups within the COF-BTT-SO$_3$H structure facilitates the formation of a robust H-bonded network. Through density functional theory calculations and employing in situ and ex situ analyses, we demonstrate that this network functions as a spontaneous proton ion pump leading to enhanced ion-diffusion kinetics and exceptional rate performance in zinc-ion energy storage devices. COF-BTT-SO$_3$H reveals a high capacity of 294.7 mA h/g (0.1 A/g), a remarkable maximum energy density of 182.5 W h/kg, and power density of 14.8 kW/kg, which are superior to most of the reported COF-based electrodes or other organic and inorganic electrode materials in $Zn^{2+}$ energy storage devices.

**KEYWORDS**

charge storage mechanism, covalent-organic frameworks, functional porous materials, proton pump, zinc-ion energy storage devices

This is an open access article under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2024 The Author(s). *SmartMat* published by Tianjin University and John Wiley & Sons Australia, Ltd.

<footnote n="1">¹Université de Strasbourg & CNRS, Strasbourg, France</footnote>

<footnote n="2">²The Soft2D Lab, State Key Laboratory of Metal Matrix Composites, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, China</footnote>

<footnote n="3">³Laboratory for Hybrid Materials, Centre for Advanced Technologies, Adam Mickiewicz University, Poznań, Poland</footnote>

<footnote n="4">⁴Department of Physics of Molecular Systems and Radiospectroscopic Research, Institute of Molecular Physics, Polish Academy of Sciences, Poznań, Poland</footnote>
# 1 INTRODUCTION

Aqueous zinc-ion batteries and zinc-ion hybrid super-
capacitors (Zn-HSCs) have attracted considerable attention
as promising candidates for the next generation of energy
storage devices, primarily owing to their inherent safety
features, low cost, high specific capacity (820 mA h/g), and
relatively low redox potential (–0.76 V vs. standard hydrogen
electrode) of Zn anodes.¹⁻⁴ However, the design and devel-
opment of advanced cathode materials represent the primary
bottleneck toward the enhancement of the electrochemical
performance of zinc-ion energy storage devices to make
them ready for real-world applications.⁵⁻⁷ During the past
decade, a range of active cathode materials have been de-
veloped, including transition-metal oxides or sulfides (MnO₂,
V₂O₅, MoO₃, MoS₂), Prussian blue analogs, and organic
compounds comprising small molecules and polymers.
Among these materials, covalent-organic frameworks
(COFs) emerge as highly promising candidates due to their
distinct attributes. In contrast to small organic molecules,
which easily dissolve in common electrolytes, COFs exhibit
exceptional chemical and electrochemical stability owing to
their elongated molecular structures.⁸ Besides, COFs dem-
onstrate significant advantages for energy storage owing to
their high porosity, which provides ample space for ion
accommodation, and their numerous active groups that
facilitate metal ion coordination.⁹,¹⁰ The use of suitably
designed organic building blocks enables the development of
a wide range of functional nanomaterials combining high
porosity and high internal surface areas. Upon programming
the self-assembly of such components via noncovalent in-
teractions, especially π-π stacking, it has been possible to
develop COF topologies exhibiting abundant hosting sites.
Such characteristics facilitate the efficient permeation of
electrolyte ions and make it possible for the effective utili-
zation of redox moieties as coordination sites,¹¹⁻¹³ thereby
enabling efficient electrochemical storage through both far-
adaic and non-Faradaic processes.¹⁴

Generally, the energy storage mechanism of COFs
involves the coordination of the Zn²⁺ ions by the COF
active sites such as C=O or C=N groups. Hence, an
efficient strategy consists of the design of COFs whose
structures comprise specific moieties acting as supra-
molecular receptors for Zn-ions. For example, Banerjee
et al.¹⁵ engineered a β-ketoenamine HqTp-COF where
the primary active sites responsible for the Zn²⁺ storage
are the C=O groups, yielding a specific capacity of
276 mA h/g at 0.125 mA h/g. As a step forward, Sun
et al.¹⁶ designed a TAQ-BQ COF with C=N and C=O as
dual active sites for Zn²⁺ ions storage, which displayed a
reversible capacity of 157 mA h/g at 1 A/g. Nonetheless,
relying solely on Zn²⁺ ions as charge carriers presents
substantial challenges, not only for COF-based electrodes

but also for various energy storage systems. For instance,
the high charge of Zn²⁺ leads to strong electrostatic in-
teractions between Zn²⁺ ions and the host materials,
resulting in sluggish diffusion kinetics of the Zn²⁺ ions
and, therefore, triggering the formation of Zn dendrites.
Additionally, the relatively large size of Zn²⁺ ionic and its
hydrated form can potentially disrupt the crystal lattice
and induce substantial volume changes in the host ma-
terials during the insertion and extraction processes,
resulting in poor rate performance and limited cycle life.

Alongside Zn²⁺ ions, H⁺ are gathering a great interest as
charge carriers because of their exceptionally small ionic
radius and the lowest atomic mass.¹⁷ These characteristics
can impart electrodes with exceptional power density, rate
capability, and extended cycle life.¹⁸ Recently, Alshareef
et al.¹⁹ and Tao et al.¹²,²⁰ demonstrated that the co-storage of
Zn²⁺/H⁺ in COFs cathodes enables to achieve a significant
enhancement of the rate capability of ZIBs compared to the
systems relying solely on Zn²⁺ storage.²¹ Sun et al.¹⁶ reported
the fabrication of an anti-aromatic microporous COF cath-
ode material (TAQ-BQ) which contains dual-redox active
sites (C = O and C = N) for the co-insertion of Zn²⁺ and H⁺.
Such TAQ-BQ COF exhibited a high capacity of 208 mA h/g
at 0.1 A/g and 87% capacity retention after 1000 charge/dis-
charge cycles. Although there have been encouraging find-
ings, the utilization of COFs as cathode materials for Zn²⁺/
H⁺ storage is still in its infancy, and it faces challenges,
including the limited performance resulting from low con-
ductivity and the persistence of solubility issues. Hence, the
development of novel proton-rich COFs electrode materials,
that could enable novel redox processes, is crucial for en-
hancing the electrochemical performance of future-
generation zinc-ion energy storage systems.

Benzotrichlophenes (BTT) have emerged as compel-
ling supramolecular building blocks due to their planar
structure, extended π-conjugation, and their electron-
rich nature. These characteristics facilitate an enhanced
intermolecular π-stacking forming supramolecular poly-
mers and promote intermolecular charge transfer
through the supramolecular polymers. When BTT is as-
sembled to form COFs, similar characteristics are
observed.²² Moreover, Zn²⁺ ions can bind S atoms from
the benzothiadiazole units forming the S-Zn-S coordi-
nation bond between the successive COF layers.¹³ While
the continuous π-π stacking in the COFs backbone
provides an electron transfer pathway, hydrogen-bonded
chains in the pores could serve as proton transfer path-
ways. In this context, we introduce a new concept that
involves the structural design of COF electrodes ex-
hibiting pores that provide supramolecular environment
enabling simultaneous storage of Zn²⁺/H⁺. Inspired by
the robust hydrogen-bonded network formed by sulfonic
acid (-SO₃H) containing compounds, we anticipate that
the integration of sulfonic groups into COF structures will facilitate the swift transfer of H⁺ and OH⁻ within the network employing the Grotthuss mechanism.²³ This process mitigates the impact of H⁺ and OH⁻ attacks, ultimately enhancing the chemical stability of the devices. To the best of our knowledge, all the reported COFs for Zn²⁺/H⁺ storage are based on the competitive interaction of Zn²⁺/H⁺ with redox-active C = O and C = N groups.¹²,¹⁶,¹⁹,²⁴ Differently, electron-poor sulfonic acid groups will undergo protonation upon charging and H⁺ will be substituted by Zn²⁺ ions in the discharged state. Additionally, in the discharged state, Zn²⁺ ions will interact with the S of the benzothiadiazole units and with the N of the C = N bond, while H⁺ will potentially interact with the COF C = N groups.

Here, we have designed and synthesized a BTT sulfonate imine-linked COF (COF-BTT-SO₃H), capable of forming an extensive hydrogen-bonded network within the COF structure when exposed to an aqueous electrolyte environment. We demonstrate that the sulfonic acid groups within the COF-BTT-SO₃H framework can interact reversibly with both Zn²⁺/H⁺, acting as spontaneous proton pumps that continuously transport protons from the bulk phase to the interface region. A joint computational and experimental analyses have revealed for the first time that three chemical functionalities, that is, C = N, S, and SO₃⁻, can serve as active sites for the co-storage of Zn²⁺/H⁺. These findings underscore the importance of the thoughtful design of new electrochemically active COFs and offer an effective approach for the generation of high-performance cathodes for aqueous Zn-ion energy storage devices.

## 2 | RESULTS AND DISCUSSION

### 2.1 | Synthesis and physical characterization

The benzotrithiophene sulfonate COF-BTT-SO₃H was prepared by a Schiff-based solvothermal condensation reaction between BTT and SO₃H at 120 °C for 72 h (Figure 1A) (see the detailed experiment protocol in the Supporting Information). To unambiguously confirm the formation of the imine linkage and validate the COF-BTT-SO₃H structure, Fourier transform infrared (FT-IR) spectroscopy and solid-state (SS) ¹³C cross-polarization magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) spectroscopy analyses were performed.

The FT-IR spectrum of COF-BTT-SO₃H (see Supporting Information S1: Figure S1) revealed a distinct new peak at 1611 cm⁻¹, which can be attributed to the C = N stretching band of the formed imine linkage. Such

peak was absent in the spectrum of the initial monomers of BTT or SO₃H. The significant reduction of C = O stretching peak at 1662 cm⁻¹ in COF-BTT-SO₃H confirms the successful imine bond formation. The emergence of distinctive peaks at 1026 and 1080 cm⁻¹, associated to O = S = O stretching bands,²⁵ and the peak at 999 cm⁻¹ attributed to the S-OH stretching mode of the -SO₃H unit²⁶ indicate that the sulfonic acid groups remain stable after the imine polymerization process. The formation of the imine linkage in COF-BTT-SO₃H was further confirmed by the emergence of the typical imine (C = N) signal at 154 ppm in the SS-CP-MAS spectrum of COF-BTT-SO₃H (Figure 1B). The experimental and simulated ¹³C CP-MAS NMR spectrum can be seen in Figure 1B and Supporting Information S1: Figure S2, and it is in agreement with previously reported systems.²⁷,²⁸

The crystallinity of the bulk COF-BTT-SO₃H was characterized by powder X-ray diffraction (PXRD) analysis. As revealed from PXRD analyses, COF-BTT-SO₃H displays remarkable crystallinity, exhibiting the first intense peak at a low angle of 4.9° (2θ), which corresponds to the (110) reflection plane, along with minor peaks at 8.6°, 9.9°, 13.2°, and 26.1°, attributed to the (300), (220), (410), and (001) reflection planes, respectively (Figure 1C). To elucidate the crystal structure of COF-BTT-SO₃H and calculate the unit cell parameters, three types of possible 2D structures were built. These structures encompass eclipsed stacking (AA) as well as staggered stacking (AB and ABC) models which are subsequently optimized by the Materials Studio Forcite molecular dynamics module method (Supporting Information S1: Figures S3–S5). The analysis revealed that the total crystal stacking energy of the ABC stacking (116.38 kcal/mol) significantly exceeds that of the potential AB (70.33 kcal/mol) and AA (44.67 kcal/mol) stacking models. This suggests that the ABC stacked model exhibits a greater stability resulting from the robust π-π interactions among the BBT units and the extensive H-bonding network facilitated by the doped polar -SO₃H groups. As can be seen in Supporting Information S1: Figure S6, the experimental PXRD patterns for COF-BTT-SO₃H closely resemble the simulated patterns derived from the staggered stacking ABC model within the trigonal R-3 space group. Pawley refinements yielded optimized parameters (a = b = 36.12 Å, c = 3.45 Å), resulting in favorable agreement factors (Rₚ = 3.5% and Rwp = 4.66%). Supporting Information S1: Figure S7 reveals that there are no discernible peaks corresponding to the starting materials of BTT and SO₃H units in the COF-BTT-SO₃H PXRD spectrum, providing additional evidence for the complete reaction. A calculation based on Bragg's law revealed an average D₀₀₁ interlayer distance of 3.46 Å for the COF-BTT-SO₃H
<figure><img src="image_3.png" /><figcaption>FIGURE 1 Synthesis and characterization of COF-BTT-SO₃H. (A) Illustration of the synthetic route of COF-BTT-SO₃H chemical structure, (B) solid-state ¹³C NMR spectrum of COF-BTT-SO₃H, (C) powder X-ray diffraction pattern fitting of COF-BTT-SO₃H with Pawley refinement and simulated ABC-stacking patterns.</figcaption></figure>

material, which provides sufficient space for the transfer of Zn²⁺ ions (diameter of 1.48 Å). Importantly, the ABC stacking mode is characterized by a higher concentration of -SO₃H groups when compared to AA and AB stackings because there are 24 -SO₃H groups in each unit cell in ABC stacking model COF, and only 18 and 12 -SO₃H groups in each unit cell for AB and AA stacking models, respectively. Therefore, a higher number of active sites available for the interaction with Zn²⁺ or H⁺ can potentially result in higher specific capacity compared to COF-BTT-SO₃H with AA or AB stackings.

To gain insight into the chemical composition of COF-BTT-SO₃H, X-ray photoelectron spectroscopy (XPS) characterizations were then performed. The XPS survey spectrum of COF-BTT-SO₃H revealed four main peaks with the binding energy of 167.8, 285.1, 401.5, and 531.2 eV, which can be ascribed to S 2p, C 1s, N 1s, O 1s, respectively (Supporting Information S1: Figure S8). The

high-resolution C 1s spectrum (Figure 2A) displayed three distinctive peaks located at 284.6, 285.9, and 286.7 eV, which can be attributed to C-C/C = C bonds, C-S bond, and C-N/C = N bonds, respectively.²⁹ The high-resolution N1s spectrum (Figure 2B) can be deconvoluted into one major peak at 400 eV for N = C bonds, indicating the successful formation of the new imine linkage.³⁰

Furthermore, the high-resolution S 2p spectrum (Figure 2C) exhibited two sets of bands each one being characterized by two distinct subpeaks. The higher binding energy sulfur peaks at 168.6 and 167.4 eV correspond to the S-O environment within the sulfonic acid group, which are the most electron-deficient sulfur atoms. Conversely, the sulfur peaks at 164.8 and 163.7 eV are associated with the S-C environment of the BTTs, which represents the most electron-rich of these sulfur atoms.³¹,³²
<figure><img src="image_4.png" /><figcaption>**FIGURE 2** Characterization of COF-BTT-SO₃H. (A–C) High-resolution XPS spectra for (A) C 1 s, (B) N 1 s, (C) S 2 p, (D and E) SEM image and the corresponding elemental mapping analysis of C (red), N (green), S (blue), (F) high-resolution transmission electron microscopy image, (G) Brunauer–Emmett–Teller surface area, (H) Nyquist plots at 0% RH at different temperatures, (I) Nyquist plots at 25 °C and different RH levels.</figcaption></figure>

The morphology of COF-BTT-SO₃H, as investigated by field-emission scanning electron microscopy (SEM), showed a rod-like shape (Figure 2D). Elemental mapping images obtained through energy-dispersive X-ray spectroscopy (EDS) confirmed the homogeneous dispersion of carbon, nitrogen, oxygen, and sulfur elements throughout the COF structure (Figure 2E). Importantly, the high-resolution transmission electron microscopy image revealed a stacking distance of ~0.35 nm (Figure 2F), which is consistent with the stacking

distance calculated from PXRD analysis in the simulated ABC stacking COF-BTT-SO₃H structure.

Further insights into the chemical composition and structural integrity of the COF-BTT-SO₃H were obtained by thermogravimetric analysis and differential scanning calorimetry. As depicted in Supporting Information S1: Figure S9, the weight loss unfolds in a dual-phase manner. The initial phase, preceding 100 °C, corresponds to the liberation of guest hydrogen-bonded water molecules confined within the pores of the COF-BTT-SO₃H
framework. Subsequently, the second phase entails the decomposition of the COF-BTT-SO₃H framework itself, showcasing notable thermal resilience extending up to 350 °C. Furthermore, the porosity and the calculated Brunauer–Emmett–Teller surface area of COF-BTT-SO₃H were evaluated by N₂ adsorption-desorption isotherms at 77 K. As illustrated in Figure 2G, COF-BTT-SO₃H displayed a characteristic type-II adsorption isotherm, accompanied by a substantial surface area of 153.1 m²/g. The nonlocal DFT method was employed to assess the pore size distribution, and the calculated pore size was quantified as ca. 1.26 nm, being a value consistent with the theoretical pore size derived from the simulated crystal structure (Supporting Information S1: Figure S10).

The COF-BTT-SO₃H framework, characterized by a high-density of -SO₃H groups, serves as effective sites for facilitating proton transport, thereby imparting inherent proton conductivity properties to the material.33 The proton transport performance of COF-BTT-SO₃H was investigated under anhydrous conditions and at various relative humidity (RH) levels within a controlled temperature (see the detailed experiment protocol in the Supporting Information). As demonstrated in Figure 2H, under anhydrous conditions, COF-BTT-SO₃H exhibits a high conductivity of 1.7 × 10⁻⁴ S/cm at 80 °C. Higher temperatures provide more thermal energy to the system, leading to increased molecular motion and facilitating the movement of protons within the COF's structure, resulting in higher proton conductivity. Increased RH means more available water molecules. In proton-conducting materials like COF-BTT-SO₃H, water molecules can facilitate proton transport by acting as proton carriers or by promoting proton hopping between sulfonic functional groups within the COF material. Therefore, higher RH can enhance proton conductivity by facilitating proton transfer pathways within the material. Furthermore, the maximum conductivity amounts to 3.2 × 10⁻³ S/cm (Figure 2I) at relatively low humidity levels (40% RH). The activation energy (Eₐ), calculated based on the Arrhenius relationship between proton conductivity and temperature (Supporting Information S1: Equation S4) provides insights into the mechanisms underlying proton-to-proton transfer. In particular, there are two commonly recognized mechanisms, that is, the Grotthus mechanism (Eₐ < 0.4 eV) and the vehicular mechanism (Eₐ > 0.4 eV).34 The least-squares fit of the Arrhenius plot for COF-BTT-SO₃H yielded an Eₐ value of 0.29 eV (Supporting Information S1: Figure S12). This indicates that proton transport in COF-BTT-SO₃H occurs through the Grotthus mechanism, wherein protons move along the hydrogen-bonded network created by sulfonic acid (-SO₃H) groups,

implying that the -SO₃H groups on the COF-BTT-SO₃H act as inherent proton sources. This underscores the remarkable intrinsic proton conductivity of COF-BTT-SO₃H, which makes it a potential candidate for applications as a spontaneous proton pump.

## 2.2 | Electrochemical characterization

Motivated by the impressive proton conductivity of COF-BTT-SO₃H, its suitability as an electrode material for Zn²⁺/H⁺ energy storage was then explored. Toward this end, we investigated its electrochemical performance using a two-electrode configuration by assembling a COF-BTT-SO₃H cathode with a Zn foil anode and 2 mol/L zinc trifluoromethanesulfonate (Zn(CF₃SO₃)₂) aqueous electrolyte (Figure 3A). The cyclic voltamograms (CVs) of the first five cycles at 1 mV/s in the potential window of 0.5–1.5 V vs. Zn²⁺/Zn are displayed in Figure 3B. Three pairs of redox peaks at 0.81/0.79, 0.97/0.96, and 1.15/1.11 V indicate the reversible insertion/extraction process of Zn²⁺ and H⁺ in the COF structures (see charge storage mechanism section for details on the redox chemistry). Importantly, the peak positions exhibited minimal variation throughout successive cycles, while the peak currents displayed a gradual increase, which can be attributed to the activation of the COF-BTT-SO₃H electrode. Electrochemical impedance spectroscopy was utilized to investigate the activation process of the COF-BTT-SO₃H electrodes, revealing a reduction in charge transfer resistance with an increasing number of charge/discharge cycles (Supporting Information S1: Figure S15). The galvanostatic charge-discharge profiles of COF-BTT-SO₃H (Figure 3C) in the explored potential window revealed a high capacity of 294.7, 261.3, 232.8, 213.2, 194.6 mA h/g at a current density of 0.1, 0.5, 1, 2, 5 A/g, respectively, which are superior to most of the reported COF-based electrodes or other organic and inorganic electrode materials in Zn²⁺ energy storage devices (Supporting Information S1: Table S2).15,19,35–47

The long-term cyclability (Figure 3D) demonstrated a relatively stable chemical stability of COF-BTT-SO₃H electrodes upon cycling at a current density of 0.1 A/g, which can retain a 226.5 mA h/g specific capacity after 1000 cycles. Furthermore, the coulombic efficiency of the COF-BTT-SO₃H cell remained consistently high at 98.7% over the course of these 1000 charge-discharge cycles, indicating minimal occurrence of side reactions. To ascertain the stability of the COF-based electrodes, SEM and EDS analyses of COF-BTT-SO₃H cathode at the pristine state and after 1000 cycles were carried out (Supporting Information S1: Figures S16 and S17). Significantly, the SEM analysis of
<figure><img src="image_5.png" /><figcaption>FIGURE 3 (A) Scheme of COF-BTT-SO₃H//Zn(CF₃SO₃)₂//Zn energy storage system. (B) Cyclic voltammogram (CV) curves of COF-BTT-SO₃H at 1 mV/s during five cycles, (C) galvanostatic charge-discharge profiles for COF-BTT-SO₃H electrodes at various current densities, (D) long-term cycling performance at 0.1 A/g, (E) Ragone plots for the comparison of COF-BTT-SO₃H cathode to previously typical organic cathode materials for Zn²⁺ energy storage devices, (F) the CV curves of COF-BTT-SO₃H cathode at multiple scan rates from 0.2 to 1 mV/s, (G) the linear fit of log I vs log v plots to calculate b values according to the equation of i = avᵇ for COF-BTT-SO₃H cathode, (H) capacitive contribution and diffusion-controlled contribution fraction for the CV curve at 0.8 mV/s, (I) capacitive contribution and diffusion-controlled contribution of the COF-BTT-SO₃H cathode at different scan rates.</figcaption></figure>

COF-BTT-SO₃H cathodes following 1000 charge/discharge cycles revealed no apparent alterations in morphology, except for the surface deposition of a Zn layer. This observation suggests that the cathodes maintain notable stability throughout the charge-discharge process (Supporting Information S1: Figure S18).

Remarkably, the Ragone plot of COF-BTT-SO₃H-based electrodes (Figure 3E) showed an outstanding maximum energy density of 182.5 W/kg and power density of 14.8 kW/kg, which are superior to most of the previously reported organic cathode materials in Zn²⁺ energy storage devices.¹²,⁴⁸ Supporting Information S1: Table S4 presents the electrochemical performance of various cathode materials for Zn-HSCs, demonstrating that COF-BTT-SO₃H achieves

the highest specific capacity among reported materials, reaching 294.7 mA h/g at a current density of 0.1 A/g. Notably, even after 1000 charge/discharge cycles, the COF-BTT-SO₃H electrode retains a capacity of 226.5 mA h/g, surpassing most other cathode materials for Zn-HSCs. Therefore, COF-BTT-SO₃H's performance in terms of capacity, energy, and power density, and cyclability ranks among the top electrodes for Zn-HSCs according to current standards. The rate capability of COF-BTT-SO₃H was assessed under various current densities, as illustrated in Supporting Information S1: Figure S19. Notably, as the current density decreases from 5 to 0.1 A/g, a substantial portion of the initial specific capacity, specifically 95.8% (equivalent to 282.5 mA h/g), can be obtained,
indicating an excellent high-rate performance of COF-BTT-SO₃H.

**2.3 | Charge storage mechanism**

To elucidate the underlying factors contributing to the outstanding rate performance of COF-BTT-SO₃H cathodes, we conducted the quantitative analysis of the ion transfer kinetics using CV tests at scan rates ranging from 0.2 to 1 mV/s (Figure 3F). According to the procedure proposed by Wu et al., the relationship between the peak current (i) and scan rate (v) from CV complies with the equation: $i = av^b$, where a and b are variable parameters.⁴⁹ Generally, the coefficient b typically falls between 0.5 and 1.0. A b-value of 0.5 indicates a diffusion-controlled electrochemical process, whereas a b-value of 1.0 implies a capacitive-controlled process.⁵⁰ The b values of the three pair of redox peaks are close to 1 (Figure 3G), indicating that the charge-storage occurs by a capacitive controlled intercalation/extraction of Zn²⁺ and H⁺ in the COF-BTT-SO₃H active material and its kinetics are not limited by the diffusion of the cations.¹² The superior kinetics observed in COF-BTT-SO₃H can be ascribed to the -SO₃H proton pumps, which quickly coordinate and release protons upon charge/discharge cycling.⁵¹,⁵²

To further quantify the capacitive ($k_1v$) and diffusion-controlled contribution ($k_2v^{0.5}$) from the CV curves at different scan rates, the following equation can be used: $i = k_1v + k_2v^{0.5}$, where $k_1$ and $k_2$ are constant parameters.⁵³ As shown in Figure 3I, the highest capacitive contribution to the charge storage mechanism for the COF-BTT-SO₃H cathodes is 84.7% at the highest scan rate (1 mV/s). The capacitive contribution gradually decreases (82.5%, 80.1%, 76.3%, and 73.2%) at lower scan rates (0.8, 0.6, 0.4, and 0.2 mV/s, respectively) (Figure 3H,I and Supporting Information S1: Figure S21). Thus, the coexistence of capacitive and partially diffusion-controlled charge storage mechanisms confirms the hybrid nature of COF-BTT-SO₃H, which is highly beneficial for energy storage.²,⁵⁴

To shed more light onto the charge storage mechanism of Zn²⁺/H⁺ from the C = N, S, and SO₃⁻ active sites in the COF-BTT-SO₃H, a series of in situ and ex situ structural and compositional analyses were performed by means of ex situ XPS, in situ FT-IR, ex situ PXRD, and ex situ ¹H SS NMR characterization (the details are provided in Supporting information). Therefore, as proposed charge storage mechanism, Zn²⁺ ions interact with SO₃⁻ as well as with the S and N from the BTT and C = N groups during the discharging process, while H⁺ interacts with the C = N groups. Upon recharging, Zn²⁺ ions

are extracted from the COF-BTT-SO₃H, enabling the H⁺ to interact again with the SO₃⁻ groups.

To gain understanding regarding the origin of the three redox peaks (O1/R1, O2/R2, and O3/R3) observed in the CV curves of COF-BTT-SO₃H, different control experiments were performed. These experiments encompass recording the CV curves of COF-BTT-SO₃H cathodes in diverse electrolytes, including 2 mol/L Zn(CF₃SO₃)₂/acetonitrile (ACN) electrolyte, CF₃SO₃H/H₂O electrolyte, and 1 mol/L/3 mol/L Zn(CF₃SO₃)₂ aqueous electrolytes (Supporting Information S1: Figures S33–S35). In the 0.2 mol/L Zn(CF₃SO₃)₂/ACN electrolyte (Supporting Information S1: Figure S33), where there is no source of H⁺ in the electrolyte, we observed the presence of the redox peaks O2/R2 and O3/R3, which are associated with the intercalation/extraction of Zn²⁺ ions. Conversely, in CF₃SO₃H/H₂O electrolyte (Supporting Information S1: Figure S34), where Zn²⁺ ions are absent, we observed two redox peaks, O1/R1 and O3/R3, indicating the intercalation of H⁺. Furthermore, in Supporting Information S1: Figure S35, the intensity of all three redox peaks increased with higher electrolyte concentration, as it leads to a greater number of charge carriers, both Zn²⁺ and H⁺. Therefore, since O3/R3 are the only redox peaks involving both Zn²⁺ and H⁺, they can be attributed to the coordination with the SO₃⁻ groups. Consequently, O2/R2 represents the coordination between Zn²⁺ ions and the S atoms in the BTT units and N from the C = N groups, and O1/R1 involves the coordination of H⁺ with the redox-active C = N groups. To estimate the proportion of H⁺ and Zn²⁺ ions within the COF-BTT-SO₃H, we have calculated the CV curve area of COF-BTT-SO₃H electrode in different electrolytes (Supporting Information S1: Figure S36). First, we have employed an organic electrolyte (i.e., Zn(CF₃SO₃)₂ in ACN) where only Zn²⁺ ions can be stored in the COF-BTT-SO₃H. Then, we have employed the same electrolyte but in aqueous environment where the co-storage of both Zn²⁺ and H⁺ is possible. By subtracting the area of the CV curve performed in the organic electrolyte, we obtain the contribution from the H⁺ in the aqueous electrolyte. Therefore, the results show that Zn²⁺ ions contribute 62.4% and H⁺ contribute 37.6% to the total ion storage.

To demonstrate that the sulfonic acid groups in the COF-BTT-SO₃H framework can serve as active sites for H⁺/Zn²⁺ insertion/extraction and function as spontaneous proton pumps, effectively enhancing mass transfer, we carried out a control experiment where we synthesized the same COF without sulfonic acid moieties (i.e., the SO₃H monomer is replaced by 4,4'-diaminobiphenyl [DABP]), COF-BTT-DABP (Supporting Information S1: Figures S37–S41). As shown in Supporting Information S1: Figure S42, the primary redox peaks observed in the
CV curves of COF-BTT-DABP are O2/R2, which are associated with the coordination between Zn²⁺ ions and the S atoms in the BTT units and the N from the C = N groups. In contrast, O1/R1 and O3/R3 are significantly attenuated, demonstrating the absence of the proton pump effect due to the absence of sulfonic acid moieties.

The surface charge of COF-BTT-SO₃H and COF-BTT-DABP were investigated by zeta potential characterization. As shown in Supporting Information S1: Figures S43 and S44, COF-BTT-SO₃H exhibits a significantly more negative zeta potential of -62.68 mV compared to COF-BTT-DABP (-14.63 mV). The proton-pumping effect of COF-BTT-SO₃H relies on the interplay of the following processes: (i) during the discharged stage, the deprotonation of COF SO₃H groups facilitates the formation of Zn²⁺ complexes with SO₃⁻ moieties. This occurs simultaneously with COF thiophene-Zn²⁺-imine group complexation and the protonation of the N from the C = N groups. (ii) Upon recharging, Zn²⁺ ions are extracted from COF-BTT-SO₃⁻/Zn²⁺ complexes, and the protons once again interact with the SO₃⁻ groups. Additionally, OH⁻ ions swiftly transfer within the COF network through the Grotthuss mechanism, preventing their migration to the Zn anode and the formation of inactive compounds like Znₓ(CF₃SO₃)ₚ(OH)₂ₓ-ₚ × nH₂O, which could potentially hinder the movement of electrolyte ions and electrons, leading to adverse effects on the reversibility of zinc ions coordination and the device's long-term performance.⁵⁵,⁵⁶

## 2.4 | DFT calculations

DFT calculations were performed to study thermodynamic of the H⁺/Zn²⁺ insertion/extraction (the calculation details can be found in the Supporting Information). First, by using Gaussian distribution method, we computed the frontier orbital distributions, including the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of COF-BTT-SO₃H (Supporting Information S1: Figure S47A,B). For a simplified representation of COF-BTT-SO₃H, repetitive units (RU) of COF-BTT-SO₃H were employed (Supporting Information S1: Figure S46), yielding consistent HOMO/LUMO levels (Supporting Information S1: Figure S47C,D). The HOMO orbital in COF-BTT-SO₃H and RU predominantly resides within the benzothiadiazole moieties, while the LUMO orbital is primarily concentrated around the sulfonic acid groups. This localization of the LUMO suggests potential roles in electron acceptance or interaction with other molecules/ions.

To determine the specific locations where Zn²⁺ ions are coordinated, we employed the molecular electrostatic

potential (MESP) method (Figure 4A and Supporting Information S1: Figure S48).²,⁵⁷ This method, widely employed in organic chemistry to deduce electrophilic or nucleophilic reactivity, was exploited to identify the active regions within COF-based electrodes. In this context, areas exhibiting a more positive MESP are considered as nucleophilic centers, whereas those with a more negative MESP tend to favor electrophilic reactions. Given that the discharge process in energy storage systems entails the uptake of Zn²⁺ ions, regions characterized by a more negative MESP are particularly attractive for facilitating discharge reactions. Based on the MESP mapping results, it becomes evident that the -SO₃H groups in the RU and COF-BTT-SO₃H framework exhibit regions with notably negative MESP values (i.e., high electronegativity), (Figure 4A and Supporting Information S1: Figure S48).⁴⁰ Importantly, materials with high electronegativity may exhibit strong interactions with electrolyte ions, promoting faster ion diffusion and facilitating rapid charge transfer at the electrode-electrolyte interface.

Before delving into the three distinct active sites of the COF-BTT-SO₃H, our primary focus is directed toward understanding the storage mechanism of the sulfonic groups, which, to the best of our knowledge, has never been reported. The initial stage of the charge storage mechanism begins with the release of three H⁺ in the RU (namely [RU (-3 H⁺)]). Figure 4B illustrates that the -SO₃H groups exhibit significantly higher negative MESP values compared to the RU (one order of magnitude higher), indicating their heightened reactivity as active sites for Zn²⁺ ion uptake. The second stage involves the insertion of three Zn²⁺ ions to RU (-3 H⁺), which form a stable [RU (-3 H⁺) + 3 Zn²⁺]⁻ structure through strong electrostatic interactions. It is important to note that each Zn²⁺ ion interacts with two sulfonic groups from separate COF layers, yielding in a net zero charge. Thus, to mimic this aspect in the DFT calculations, we have incorporated a benzenesulfonate fragment (Figure 4C). Figure 4C shows that the three Zn²⁺ ions effectively interact with the SO₃H groups of both [RU (-3 H⁺)] and additional benzenesulfonates units. The subsequent phase in the DFT calculations involved analyzing the possibility of existence of additional active sites within COF-BTT-SO₃H, specifically the C = N and S groups. To elucidate the next step of the charge storage mechanism of [RU (-3 H⁺) + 3 Zn²⁺], the Gibbs free energy changes (ΔG) are calculated (Figure 4 and Supporting Information S1: Figure S49 and Supporting Information S1: Table S5). At this stage, two scenarios can theoretically take place: (i) three additional Zn²⁺ ions are inserted, yielding [RU (-3 H⁺) + 6 Zn²⁺]⁶⁺, followed by the incorporation of six electrons to achieve the system with net zero charge ([RU (-3 H⁺) + 6 Zn²⁺]); (ii) six electrons are initially added ([RU (-3 H⁺) + 3 Zn²⁺]⁶⁻) before inserting the three extra Zn²⁺ ions ([RU (-3 H⁺) + 6 Zn²⁺]). Note that in both cases, each Zn²⁺ ion interacts with
<figure><img src="image_6.png" /><figcaption>FIGURE 4 (A–D) Electrostatic potential map of RU after the step-by-step Zn²⁺ ions insertion.</figcaption></figure>

the imine nitrogen atom and the sulfur atom from the BTT unit. As depicted in Figure 4D and Supporting Information S1: Figure S50, the initial insertion of three extra Zn²⁺ ions is the only thermodynamically favorable process among the two possible scenarios (i.e., negative ΔG). Additionally, this insertion yields in a reduction in the HOMO-LUMO energy gap (E_g) compared to [RU (−3 H⁺) + 3 Zn²⁺], indicating improved electron acceptance capability and enhanced electronic conductivity (Supporting Information S1: Figure S50).

3 | CONCLUSION

In summary, this work demonstrates the potential of COF-BTT-SO₃H as an advanced cathode material for next-generation zinc-ion energy storage devices, with its supramolecular design, excellent proton conductivity, and superior electrochemical performance. While BTT creates a continuous π-π stacking pattern in the COFs' backbone, establishing an electron transfer pathway, the sulfonic acid groups (–SO₃H) play a crucial role in

building a robust hydrogen-bonding network. This network, in turn, facilitates the rapid transfer of H⁺ and OH⁻ within the COF's structure through the Grotthuss mechanism. Furthermore, the migration of OH⁻ to the Zn anode is prevented and consequently, the formation of inactive compounds like Znₓ(CF₃SO₃)ᵧ(OH)₂ₓ⁻ᵧ × H₂O is hindered. We demonstrated that COF-BTT-SO₃H acts as a spontaneous proton ion pump, resulting in accelerated ion-diffusion kinetics and exceptional rate performance. These qualities are crucial for preventing the formation of Zn dendrites. In terms of electrochemical performance, COF-BTT-SO₃H achieved a high specific capacity of 294.7 mA h/g at a current density of 0.1 A/g, excellent rate capability up to a current density of 5 A/g, and a remarkable cycling stability with 226.5 mA h/g capacity retention after 1000 cycles. Furthermore, COF-BTT-SO₃H exhibits both high energy and power densities of 182.5 W h/kg and 14.8 kW/kg, respectively, outperforming most COF-based electrodes and other organic and inorganic electrode materials in the realm of Zn²⁺ energy storage devices. Through a combination of DFT computations and comprehensive in situ and ex situ
assessments involving XPS, FTIR, PXRD, and ssNMR techniques, we unraveled the charge storage mechanism of COF-BTT-SO₃H. In this process, three active sites, specifically SO₃⁻, S, and C = N, were found to play a crucial role in the co-storage of Zn²⁺/H⁺. These findings highlight the importance of controlling COFs' pore supramolecular environment through structural design and also expand the horizons of proton chemistry and supramolecular coordination chemistry in energy storage systems.

## ACKNOWLEDGMENTS

This work was supported by the Interdisciplinary Thematic Institute SysChem via the IdEx Unistra (No. ANR-10-IDEX-0002) within the program Investissement d'Avenir program, the Foundation Jean-Marie Lehn, and the Institut Universitaire de France (IUF). A. C. and D. P. also acknowledge financial support from the National Science Centre in Poland (Nos. 2019/35/B/ST5/01568 & 2022/47/D/ST5/00261).

## CONFLICTS OF INTEREST STATEMENT

The authors declare no conflicts of interest.

## DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available in the supplementary material of this article.

## ORCID

Haijun Peng https://orcid.org/0000-0002-8623-4339

Verónica Montes-García https://orcid.org/0000-0002-3235-0545

Dawid Pakulski https://orcid.org/0000-0002-0136-6667

Michał Bielejewski https://orcid.org/0000-0002-9950-1221

Fanny Richard https://orcid.org/0000-0001-6783-5713

Paolo Samori https://orcid.org/0000-0001-6256-8281

Artur Ciesielski https://orcid.org/0000-0003-3542-4092

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**SUPPORTING INFORMATION**

Additional supporting information can be found online
in the Supporting Information section at the end of this
article.

**How to cite this article:** Peng H, Montes-García V, Jiang K, et al. Benzotrithiophene-sulfonate covalent-organic frameworks: Supramolecular proton pumps for high-rate aqueous zinc-ion energy storage systems. *SmartMat*. 2024;5:e1312.

**doi:**10.1002/smm2.1312