Minjie Shi, Nianting Chen, Yue Zhao, Cheng Yang, Chao Yan,*

1 School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China

2 China Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

Keywords:

ABSTRACT

1. Introduction

Recent studies have continuously explored and extended the organic materials in various applications,including energy storage,corrosion protection,electrochromic device,electromagnetic interference shielding,and so on[1–4].Attractively,the composition of organic materials differs from that of inorganic materials because they contain naturally rich elements(such as C,H,O,N and S)with a low environmental footprint,eco-efficient processability,as well as cost-effectiveness[5,6].In general,the powerful organic synthesis derived from abundant chemicals or biomass resources and exceptional synthetic modularity guarantees the organic materials with high diversity and controllability of molecular structures[7,8]. At the same time, the physical and chemical properties of organic materials are always determined by the functional groups of organic molecules rather than by the crystal structure of organic molecules [8,9]. Although some progress has been made in the development of diverse organic materials,the practical application of organic materials is still at the initial stage, so there is a crucial task to further explore suitable and multifunctional organic materials.

π-d conjugated coordination polymers, consisting of transition metallic ions coordinated with organic linkers, have been identified to be a novel class of organic materials [10–12]. With the hybridization of frontier π-orbits of conjugated ligands and the d-orbits of transition metals, the delocalization of electrons from π-d conjugated orbitals could optimize the chemical stability and electronic characteristics of the coordination polymers [13,14].Additionally, there is the coexistence of multivalent metallic ions and π-conjugated ligands with abundant electro-active groups,thereby guaranteeing the coordination polymers with multielectron redox capability[15–18].Benefiting from these aforementioned outstanding merits along with the structural versatility at the molecular level, the coordination polymers can be effectively created with desired properties by adjusting their π-conjugated ligands and metallic ions. From this point of view, it is therefore imperative to develop the π-d conjugated coordination polymers and explore their potential applications.

In this work, we synthesized a conjugated Ni-BTA coordination polymer by using a facile and mild wet-chemical strategy through the π-d hybridization with 1,2,4,5-benzenetetramine (BTA) as πconjugated ligands and Ni2+as the metallic centers, wherein the electrons can be significantly delocalized to form a large conjugated system in Ni-BTA polymer chains. As a result, the Ni-BTA coordination polymer exhibits a unique two-dimensional nanosheet-like architecture with high electrochemical activity and multi-electron redox capability.When served as the electrode material for aqueous battery application, the Ni-BTA coordination polymer can deliver a superior reversible capacity of 198 mA?h?g-1at 1 A?g-1, a high-rate capability of 150 mA?h?g-1at 50 A?g-1, and long-term cycle stability in alkali-ion aqueous electrolyte. Moreover, the Ni-BTA coordination polymer was applied as an anticorrosion additive in epoxy resin to make a Ni-BTA epoxy coating with enhanced anti-corrosion properties, especially a low corrosion rate(4.62×10-6mm?a-1)and a high corrosion inhibition efficiency (99.86%). Therefore, this study provides a significant reference for the research of polymer materials for the application of energy storage and corrosion protection.

2. Material and Methods

2.1. Preparation of Ni-BTA coordination polymer

Firstly,a solution of 2.5 mmol NiCl2?6H2O in 50 ml of ultrapure water was fully mixed with a solution of 2.5 mmol BTA?4HCl in 200 ml of ultrapure water.Thereafter,8.0 ml of concentrated aqueous ammonia was added to the above mixture by vigorous stirring under air bubbling.The reaction process was continued under continuous stirring for 1.5 h in the ambient condition. The resulting black powder was isolated by centrifugation and washed with water, anhydrous ethanol, and acetone several times, which was further dried at 80 °C under vacuum overnight to finally obtain black powder-like Ni-BTA coordination polymer.More information about Ni-BTA coordination polymer is provided in the Supplementary Material (Figs. S1–S3).

2.2. Energy storage measurements

The Ni-BTA electrode material was fabricated by mixing Ni-BTA powder, acetylene black, and polyvinylidene fluoride binders(70:20:10 mass ratio) together with the aid of Nmethylpyrrolidone solution, and the evenly mixed slurry was spread on graphite paper and dried under vacuum. A threeelectrode testing configuration was used to investigate the energy storage properties of Ni-BTA electrode in 5 mol?L-1NaOH aqueous electrolyte, with Pt wire and Ag/AgCl electrodes serving as the counter and reference electrodes,respectively.Cyclic voltammetry(CV) and galvanostatic charge/discharge (GCD) measurements were implemented at various sweeping rates and current densities.In-situ Raman spectra were acquired using a 50×objective within a backscattering arrangement, wherein all spectra were captured one by one via a mapping mode while a charging-discharging test was run under a low current density of 1.0 A?g-1.The spectrum collection time of 30 s was applied to gather the Raman signals with good signal-to-noise ratios, in which the beam was focused to a spot size of ～2.5 μm, the excitation laser was set to 532 nm, and the output power of the laser was set to 10 mW.

2.3. Anti-corrosion testing

Preparation of the Ni-BTA epoxy coating is as follows: an amount of Ni-BTA powder was completely dispersed in anhydrous ethanol with ultrasonic treatment for 30 min. After ultrasonically mixing the above suspension with epoxy resin, the mixture was heated to 40 °C to remove residual ethanol solvent. Afterward,the curing agent (the mass ratio of epoxy resin to curing agent was 2:1) was stirred with the above mixture for 1 h and then painted onto the Q235 carbon steel by applying the automatic coater.A three-electrode testing configuration was used to investigate all the anti-corrosion behaviors in 3.5% (mass) NaCl aqueous solution, with the coated Q235 carbon steel, Pt wire, and Ag/AgCl electrode serving as the working electrode, counter and reference electrodes, respectively. Electrochemical impedance spectroscopy(EIS) plots were performed at frequencies from 100 kHz to 0.01 Hz through a sinusoidal perturbation of 20 mV. More characterization methods of Ni-BTA coordination polymer are provided in the Supplementary Material.

3. Results and Discussion

3.1. Structural features of Ni-BTA coordination polymer

Ni-BTA coordination polymer was synthesized via a facile and mild wet chemical strategy, wherein the BTA organic precursor reacts with equimolar Ni2+salt at room temperature under an alkaline environment,as shown in Fig.1(a).Since the Ni-BTA coordination polymer is cross-linked in a planar manner,it tends to form a two-dimensional structure.The morphology of the as-obtained Ni-BTA coordination polymer was firstly illustrated by scanning electron microscope (SEM) and transmission electron microscope(TEM) analyses, which demonstrates a wrinkled and crinkled nanosheet-like morphology with a smooth and clean surface(Fig. 1(b) and (c)). The elemental mappings elucidate that the Ni,C, and N elements are homogeneously distributed throughout the entire Ni-BTA nanosheets (Fig. 1(d)). As seen from Fig. 1(e), the edge-area TEM image reveals that the Ni-BTA nanosheets are large in size and exceedingly thin with the thickness of ～10 nm. This lamellar structure can provide abundant and available active sites for greatly enhancing the utilization of Ni-BTA coordination polymer. X-ray diffraction (XRD) pattern of Ni-BTA coordination polymer displays a crystalline structure with three predominant peaks centered at 2θ=20.5°,23.7°,and 29.1°,while some insignificant peaks located at 2θ = 14.1°, 39.9° and 44.1° (Fig. 1(f)), which are attributed to the in-plane periodicity and long-range ordered stacking of the Ni-BTA coordination polymer [19,20].

Fig. 2(a) reveals the Fourier transform infrared spectroscopy(FT-IR) of Ni-BTA coordination polymer, wherein the triple vibration peaks of —NH2in BTA between 3000 and 3700 cm-1are replaced by—NH at 3303 cm-1after hybridization reaction,revealing the successful formation of coordination bonds between—NH—and Ni2+in the Ni-BTA coordination polymer.Besides,two obvious peaks located at around 1630 and 1387 cm-1are associated with the characteristic bonds of C=N and C—N,which indicates the partial double-bond character of C—N bond[21,22].An explanation for this observation is that Ni2+effectively coordinates with N atoms and the redistribution of electrons on N atoms,identifying the successful formation of Ni-BTA coordination polymer. X-ray photoelectron spectroscopy (XPS) was used to examine the molecular structure and bonding composition of Ni-BTA coordination polymer.As for the C 1s high-resolution spectra(Fig.2(b)),four obvious sub-peaks are attributable to the C=N(287.5 eV),C—N(286.2 eV),C—C (284.6 eV), and C=C (282.9 eV) bonds, respectively. The N 1s high-resolution spectra (Fig. 2(c)) displays that two sub-peaks at around 398.9 and 397.4 eV are related to the C—N and C=N bonds,which confirms the coexistence of C—N and C=N bonds in the Ni-BTA coordination polymer. Additionally, the high-resolution spectra of Ni 2p in Fig. 2(d) reveals that the Ni 2p1/2peak is located at 871.9 eV and the Ni 2p3/2peak is centered at 854.4 eV with a spinenergy separation of 17.5 eV, suggesting the divalent state of Ni atom [23,24]. The electron paramagnetic resonance (EPR) spectrum of Ni-BTA coordination polymer is displayed in Fig. 2(e),wherein a strong EPR signal with a g-factor of 2.006 is resulting from the unpaired electrons surrounding the coordination bonds[25]. The coordination bonds between Ni2+and BTA in the Ni-BTA coordination polymer are further studied through theoretical calculations based on the density functional theory (DFT). The resultant projected density of state (PDOS) is revealed in Fig. 2(f),with the considerable overlapping between d-orbits of Ni2+and p-orbits of N atoms, showing the π-d orbital hybridization of asprepared Ni-BTA coordination polymer [26].

Fig. 1. (a) Schematic illustration of the synthetic route, (b) SEM image, (c) TEM image, (d) EDS mapping images, and (e) the corresponding edge-area TEM image of Ni-BTA coordination polymer. (f) XRD pattern of Ni-BTA coordination polymer.

3.2. Energy storage performance of Ni-BTA coordination polymer

A three-electrode configuration was developed to evaluate the energy storage performance of Ni-BTA coordination polymer as electrode material in 5 mol?L-1NaOH aqueous electrolyte. As depicted from Fig. 3(a), the CV plots of Ni-BTA electrode depict obvious redox peaks under various sweeping rates, wherein the potentials of oxidation/reduction peaks move toward the positive/negative directions as the scan rates increase, indicating the high redox charge-transfer kinetics of Ni-BTA electrode. Besides,the contribution ratios of diffusion-limited and capacitivecontrolled processes in Ni-BTA electrode are generally quantified(Figs. 3(c) and S4), presenting a rising tendency as the sweeping rates increase successively. Meanwhile, a large capacitivecontrolled contribution (～84%) can be obtained at 2 mV?s-1(Fig.3(b)),identifying that the capacitive-controlled electrochemical process is dominated under high currents, which defines the remarkable rate capability of the Ni-BTA electrode [27]. The GCD curves (Fig. 3(d)) with distinct charging-discharging plateaus are in agreement with the aforementioned CV results.The Ni-BTA electrode achieves a large reversible specific capacity of 198 mA?h?g-1at 1 A?g-1.Even at an ultra-high current density of 20 A?g-1,the Ni-BTA electrode could still achieve an impressive specific capacity of 150 mA?h?g-1, revealing its superior energy storage capacity in alkali-ion aqueous electrolyte. The outstanding rate performance of the Ni-BTA electrode substantially outperforms those of electrode materials previously reported in aqueous alkali-ion electrolytes [28–36] (Fig. 3(d)).

Fig. 2. (a) FTIR spectra, (b) C 1s, (c) N 1s and (d) Ni 2p high-resolution XPS spectra of Ni-BTA coordination polymer. (e) EPR spectra and (f) PDOS pattern of Ni-BTA coordination polymer.

In-situ Raman analysis (Fig. 4(a)) was executed to reveal the redox process of Ni-BTA electrode. A schematic diagram of the in-situ Raman configuration shown in Fig. 4(b) displays a threeelectrode system with a glass window that allows the laser beam to transit. Fig. 4(c) and (d) illustrate the in-situ Raman spectra acquired at different charging-discharging states and the corresponding GCD curve.Before testing,an obvious signal peak located at 1468.8 cm-1belongs to C=N bond of the Ni-BTA electrode.During discharge to -0.1 V, the peak of C=N bond gradually weakens and even disappears. Accordingly, a new signal peak can be observed at 475.1 cm-1, which is attributed to the characteristic of C—N—Na bond, revealing that the Na+uptake transforms C=N bond of Ni-BTA electrode into C—N—Na bond through the oxidation reaction.This observation is also certificated by the XPS analysis (Fig. 4(e)). At the completely discharged condition, the intensity of characteristic peak (～397.4 eV) of C=N bond almost disappears, while the intensity of characteristic peak (～398.8 eV)of C—N bond is observable and strong. Evidently, the oxidation reaction causes the formation of C—N—Na bond in the Ni-BTA electrode upon Na+insertion. From the charging process in the in-situ Raman spectra,it can be seen that the peak intensity of C=N bond increases significantly and the peak intensity of C—N—Na bond gradually decreases, while the C=N bond would be perfectly reinstated after the complete charge. In this case, the Na+removal occurs in the presence of efficient reduction reaction from C—N—Na to C=N bonds.From the above analysis,we can therefore deduce that the Ni-BTA electrode exhibits the highly reversible oxidation and reduction reactions between C=N and C—N—Na bonds during the Na+insertion/extraction process.

Fig.3. (a)CV plots of Ni-BTA electrode at sweeping rates from 0.2 to 5 mV?s-1 tested in 5 mol?L-1 NaOH aqueous electrolyte.(b)Voltammetric response at 2 mV?s-1.(c)The corresponding contribution ratios of diffusion-limited and capacitive-controlled processes in Ni-BTA electrode under different sweeping rates. (d) GCD profiles of Ni-BTA electrode at current densities from 1 to 20 A?g-1. (e) Rate performance compared to other reported electrode materials.

Fig.4. (a)Digital photo and(b)schematic illustration of the in-situ Raman investigation on Ni-BTA electrode.(c,d)In-situ Raman spectra acquired at different states and the corresponding GCD curve. (e) The N 1s high-resolution XPS spectra of Ni-BTA electrode at complete discharge (state i).

Fig. 5. (a) Schematic diagram and (b) photograph of the soft-package aqueous battery device based on 3CN-HATN anode and Ni-BTA cathode. (c) Long-term cycle performance of the aqueous battery device.

As a practical application, a soft-package aqueous battery device(Fig.5(a))has been constructed by applying Ni-BTA coordination polymer as the positive electrode, 3CN-HATN organic compound(see detailed preparation method in Figs.S5–S7)as the negative electrode and 5 mol?L-1NaOH solution as the aqueous electrolyte, wherein the cellulose separator soaked with aqueous electrolyte was utilized to separate these two organic electrode materials, which were subsequently enclosed in the laminated Al-plastic sheet to construct the soft-package aqueous battery device. The fabricated battery can be operated safely with high energy storage capability (Fig. 5(b)) and superior long-term cycle performance (Fig. 5(c)) over repeated charging-discharging times,which shows the enormous potential for meeting various demands for safe, stable, and effective energy-storing technologies. Traditional Pb-acid and Ni-Cd batteries were once viable, but they contain heavy metals, which cause environmental pollution and difficult handling issues. Additionally, Li-ion batteries use highly toxic and volatile solvents as the electrolytes, which cause safety hazards when used improperly.In contrast,our suggested aqueous battery device utilizes organic compounds rather than any metal components as electrode materials, preventing the environmental pollution brought on by the presence of heavy metals and making it easier to handle the device after usage, in which the aqueous electrolyte demonstrates traits that are innately safe, accessible,and environmental benignity. More information about the energy storage performance of our fabricated soft-package aqueous battery based on the Ni-BTA coordination polymer electrode is provided in Figs. S8–S10.

3.3. Anti-corrosion performance of Ni-BTA coordination polymer

Fig. 6. (a) Polarization curves and (b) OCP evolutions of blank Q235, pure epoxy and Ni-BTA epoxy coatings.

Table 1 Corrosion potentials (Ecorr), corrosion current densities (Icorr) and corrosion rates (Vcorr) of blank Q235, pure epoxy and Ni-BTA epoxy coatings

By monitoring the corrosion kinetics of coatings, EIS methodologies were used to assess the barrier ability and corrosion resistance [40], where the corresponding Nyquist and Bode plots of pure epoxy and Ni-BTA epoxy coatings soaked in 3.5%(mass)NaCl aqueous solution are obtained for various immersion times. Fig. 7(a) and (c) shows the Nyquist plots of pure epoxy and Ni-BTA epoxy coatings,respectively.The general agreement is that a wider radius of the capacitive arc in the Nyquist plot suggests a more corrosion-resistant coating [41,42]. These two coatings are intact throughout the first immersion stage,and the aqueous solution has not yet reached the interface of coating substrates. The radius of the capacitive arc of the pure epoxy coating shrinks continuously with increasing immersion duration (Fig. 7(a)), while obvious diffusion tails appear on the 42nd and 56th days of immersion(inset in Fig.7(a)),indicating that the diffusion channel of corrosion ions is opened and the corrosion reaction occurs at the interface between pure epoxy coating and steel substrate [43]. It is important to note that the Ni-BTA epoxy coating displays a greater capacitive arc radius (Fig. 7(c)) without diffusion tails during the whole soaking time (inset in Fig. 7(c)), demonstrating its exceptional anti-corrosion performance. More information about the coating resistances is provided in Figs. S11 and S12.

In the Bode plots of pure epoxy and Ni-BTA epoxy coatings(Fig.7(b)and(d)),the impedance modulus at the lowest frequency(|Z|0.01Hz) is a key parameter to reveal the protective performance of coating [44]. As for the pure epoxy coating (Fig. 7(b)), the|Z|0.01Hzvalue significantly decreases from ～1.1 × 107Ω?cm2on the 1st day to ～4.7 × 105Ω?cm2on the 56th day of immersion,revealing the corrosion resistance of pure epoxy coating is greatly weakened during the immersion time. Since the rapid curing of epoxy resin could produce some defects, corrosive media (such as H2O, Cl-and O2) are easy to diffuse through the defects to the interface between coating and metal matrix [45]. By contrast, the Ni-BTA epoxy coating exhibits an extremely high value of|Z|0.01Hz(～5.2 × 1010Ω?cm2) on the 1st day of immersion (Fig. 7(d)), much superior to various coatings previously reported(Fig. 7(e)) [46–54]. More importantly, the |Z|0.01Hzvalues remain at a high level throughout the immersion period (Fig. 7(d)). On the 56th day of immersion, the |Z|0.01Hzvalue (～1.5 × 109Ω?cm2)of the Ni-BTA epoxy coating is four orders of magnitude higher than that of pure epoxy coating, showing the long-term anticorrosion ability of Ni-BTA epoxy coating.More information about the breakpoint frequencies of pure epoxy and Ni-BTA epoxy coatings are provided in Fig. S13.

For a real application, the pure epoxy and Ni-BTA epoxy coatings suffer from the salt spray testing over 30 d. As shown in Fig. 8(a), the pure epoxy coating undergoes severe corrosion,wherein the corrosion products and delamination are visible along the scratches. By contrast, the protective state of Ni-BTA epoxy coating is much better (Fig. 8(b)), accounting for its enhanced anti-corrosion performance. As a result of prolonged immersion in the 3.5% (mass) NaCl solution, the corrosion products were further characterized by the XRD measurement(Fig. 8(c)). In the case of pure epoxy coated steel, the corrosive mediums are able to permeate the coating and consequently cause the generation of corrosion products (Fe3O4, α-FeOOH and β-FeOOH)[55,56].However, there is no obvious corrosion product observed in the Ni-BTA epoxy coated steel. Consequently, the Ni-BTA coordination polymer in epoxy coating provides an effective barrier against corrosive mediums penetration, which is due to the following two primary reasons (Fig. 8(d)): on the one hand,the two-dimensional structure of Ni-BTA coordination polymer helps to isolate the coating from solution by filling the microscopic holes of epoxy (Fig. S14), while the ‘‘maze effect” created by the Ni-BTA nanosheets that delays corrosion by creating a convoluted diffusion channel for corrosive mediums; on the other hand, Ni-BTA coordination polymer with high redox-activity can accept the electrons released by metal dissolution to effectively from passive layers. The presence of this passivation film greatly reduces the penetration of corrosive mediums into coating.Accordingly, the Ni-BTA coordination polymer exhibits excellent anti-corrosion properties, showing great application potential in the field of anti-corrosion coatings.

Fig.7. Nyquist and Bode plots of(a,b)pure epoxy and(c,d)Ni-BTA epoxy coatings during 8 weeks of immersion in 3.5%(mass)NaCl solution.(e)The|Z|0.01Hz value of Ni-BTA epoxy coating compared with other reported coatings.

4. Conclusions

To summarize,a two-dimensional Ni-BTA coordination polymer was designed and synthesized via a convenient and mild chemical process,wherein the intramolecular electron delocalization resulting from the robust π-d conjugation endows the Ni-BTA coordination polymer with high electrochemical activity and multi-electron redox capability. Remarkably, the Ni-BTA coordination polymer possesses a wrinkled and crinkled nanosheet-like architecture,which not only exhibits a rapid, reversible, and efficient energy storage behavior with a considerably large reversible capacity for aqueous battery application, but also shows enhanced anticorrosion properties with low corrosion rate and long-term performance when it is used as anti-corrosion additive into the epoxy resin coating. Therefore, the bi-functional Ni-BTA coordination polymer offers potential applications in both energy storage and corrosion prevention.Our findings can inspire further research into the exploration of organic materials that are capable of meeting diverse engineering demands.

Fig.8. Photographs of(a)pure epoxy and(b)Ni-BTA epoxy coatings immersed in salt spray chamber after 30 d.(c)XRD patterns of Q235 steel coated with pure epoxy and Ni-BTA epoxy coatings after immersion in 3.5% (mass) NaCl solution for 8 weeks. (d) The schematic diagram about anti-corrosion mechanism of Ni-BTA coordination polymer nanosheets.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(52002157 and 51873083)and the Natural Science Foundation of Jiangsu Province (BK20190976).

Supplementary Material

Supplementary material to this article can be found online at https://doi.org/10.1016/j.cjche.2022.11.011.