Electrospinning Machine | Cesium Chemistry Enables Microporous Carbon Nanofibers with Biomimetic Ion Transport Channels for Zinc-ion Capacitors

The rapid development of electric vehicles and large-scale energy storage systems has further accelerated the demand for advanced electrochemical energy storage technologies. Rechargeable zinc-ion capacitors (ZICs) have garnered widespread attention due to their high safety, environmental friendliness, and cost-effectiveness. However, the practical performance of ZICs is often limited by the low ion storage capacity and slow ion transport of carbon cathodes. The low ion-accessible active sites and mismatched pore structure of traditional carbon cathodes reduce the storage efficiency of Zn²⁺, limiting the overall energy storage capability of ZICs.

Recently, Professor He Shuijian's team at Nanjing Forestry University published their latest research findings, titled "Cesium Chemistry Enables Microporous Carbon Nanofibers with Biomimetic Ion Transport Channels for Zinc-ion Capacitors," in the journal Green Chemistry. This study reports a cesium-directed carbonization strategy that bypasses the traditional deacetylation process, enabling the direct conversion of cellulose acetate (CA) into flexible carbon nanofiber (CNF) films. Cesium chemistry not only maintains the integrity of the fibers but also induces hierarchical micropores (0.78 nm and 1.1 nm) suitable for hydrated zinc ion storage. These ion transport channels mimic natural ion channels, enabling rapid ion migration and efficient storage.

Figure 1: (a) Schematic diagram of cesium acetate (CsAc)-assisted preparation of CNFx-T. (b-d) SEM images of CNF100-T, highlighting the intact fiber network. (e, f) TEM images of CNF100-750, (g) SAED pattern.

The synthesis process of CNFx-T (where x and T represent CsAc concentration and carbonization temperature, respectively) is shown in Figure 1a. Traditionally, preparing carbon materials from CA requires a 24-hour deacetylation process using 0.1 M KOH. This method not only requires prolonged chemical treatment but also relies on corrosive reagents. In this work, we propose a greener and more efficient strategy, which involves directly immersing the electrospun CA nanofiber membrane into a CsAc solution. This method bypasses the highly corrosive deacetylation step, providing an environmentally friendly pathway for carbon nanofiber fabrication. This approach utilizes unique "cesium chemistry," where Cs+ interacts with hydroxyl and acetyl groups on the CA matrix through ion exchange, while the alkaline environment of CsAc promotes the deprotonation of -OH and -COOH groups, thereby facilitating the electrostatic adsorption of Cs+. During the subsequent carbonization process, in-situ generated Cs compounds are embedded within the carbonaceous framework, catalyzing the condensation of acetyl groups and preserving the nanofiber morphology. 

To elucidate the role of CsAc and validate the elimination of traditional deacetylation, comprehensive spectroscopic analysis was conducted. Fourier Transform Infrared (FTIR) spectroscopy (Figure 2a) shows that CNF100 retains the characteristic functional groups of CA. Notably, the O-H band in CNF100 redshifts to 3396 cm−1, exhibiting significant broadening and intensity enhancement. The characteristic peaks of CA at 1743 cm⁻¹ and 1436 cm⁻¹, corresponding to the asymmetric and symmetric stretching vibrations of C=O, also shift in CNF100, becoming broader and more intense. Overall, these results indicate that the 0.1 M CsAc alcohol solution does not induce deacetylation of CA but stabilizes the fiber structure through interaction with Cs+.

Figure 2: (a) FTIR spectra of CNF100 and CNF. (b) TG-DTG curves of CNF100 under nitrogen. (c) Mass spectrometry curves of gaseous products during CNF100 pyrolysis. (d) Nitrogen adsorption-desorption isotherms, (e) Pore size distribution curves of CNF100-T. (f) XPS spectra of CNF100-T. (g) High-resolution O 1s XPS spectra of CNF100-T. (h) XRD patterns of CNF100-T, (i) Raman spectra.

Thermogravimetric (TG) analysis of CNFx under a nitrogen atmosphere (Figure 2b) provides in-depth insight into the thermal decomposition behavior of the fibers. Taking CNF100 as an example, the residual mass at 700 °C is approximately 47%. The mass loss between 160 and 310 °C corresponds to the rapid condensation of organic matter via cross-linking and polyketone cyclization, as well as the formation of carbonaceous structures through dehydrogenation and aromatization—characteristic features of the biomass carbonization process. Considering the larger ionic radius of Cs⁺ compared to K⁺ and Na⁺, this interaction is particularly advantageous, stabilizing the CA structure by limiting hydroxyl reactivity and enhancing thermal resistance. Mass spectrometry (MS) spectra (Figure 2c) show that gas release begins at approximately 300 °C. This early gas evolution indicates that the interaction between Cs⁺ and the ester groups of CA significantly accelerates the condensation reaction between CA molecular chains. This interaction promotes the rapid formation of carbonaceous structures, resulting in a more controllable and gradual pyrolysis process at high temperatures, effectively maintaining the fibrous morphology of the material throughout the thermal decomposition process.

Figure 3: (a) Schematic diagram of ZIC structure using CNF100-T as cathode. (b) CV curves of CNF100-T electrodes at a scan rate of 10 mV s⁻¹. (c) GCD curves of CNF100-T electrodes at 0.1 A g⁻¹. (d) Rate performance of CNF100-T electrodes from 0.1 to 20 A g⁻¹. (e) Voltage drop profiles of CNF100-T electrodes at different current densities. (f) Ragone plot of ZICs based on CNF100-750. (g) Cycling stability of ZIC based on CNF100-750 at 10 A g⁻¹ after 90,000 cycles.

The resulting CNF100-750 possesses a high micropore ratio (S_micro / S_BET of 93.9%) and suitable micropores (0.78 nm and 1.1 nm) that can accommodate hydrated zinc ions ([Zn(H₂O)₆]²⁺), similar to biological ion channels. These ion channels enhance the desolvation and transport of Zn²⁺, while oxygen-containing functional groups provide zincophilic sites for reversible Zn²⁺ adsorption/desorption. Consequently, the ZIC based on CNF100-750 exhibits a high specific capacity of 203 mAh g⁻¹ at 0.1 A g⁻¹, an energy density of 133.9 Wh kg⁻¹, and outstanding cycling stability (96.6% capacity retention after 90,000 cycles).

Figure 4: a) Nyquist plots of CNF100-T electrodes, showing charge transfer resistance and ion diffusion behavior. (b) Linear relationship between the real part of impedance (Z′) and the square root of angular frequency (ω ^-0.5). (c) Bode plot of frequency versus phase angle, showing the frequency response of CNF100-T electrodes. (d and e) DRT analysis derived from in-situ EIS measurements of the CNF100-750 electrode at different potentials. (f) DRT contour map of the CNF100-750 electrode, showing the evolution of relaxation processes. (g) Log-log plot of current versus scan rate.

To further address the overlapping electrochemical processes commonly observed in traditional Nyquist plots, Distribution of Relaxation Times (DRT) analysis was employed (Figure 4d, e), with the corresponding contour plot shown in Figure 4f. This method enables the deconvolution of individual electrochemical processes based on their intrinsic time constants. During the discharge process, the charge transfer resistance (Rct) in the low-frequency region decreases as the voltage decreases, reaching a minimum at 0.2 V. DRT analysis reveals four distinct peaks associated with different electrochemical processes. Analysis of the energy storage mechanism indicates that the zinc storage behavior of the carbon nanofibers is primarily dominated by surface electric double-layer capacitance.

Figure 5: (a) GCD curve of the CNF100-750 electrode at 0.1 A g⁻¹, highlighting three selected states for detailed analysis. (b) Ex-situ XRD patterns of the CNF100-750 electrode at different charge/discharge states. (c-f) Ex-situ XPS spectra of S 2p, Zn 2p, C 1s, and O 1s of the CNF100-750 cathode at different states. (g) Schematic diagram of the ion adsorption/desorption process on the CNF100-750 electrode in 2.0 M ZnSO₄ electrolyte, depicting reversible Zn²⁺ storage and surface redox reactions.

Through a series of ex-situ characterizations, the charge storage mechanism of the ZIC was systematically investigated, providing a detailed understanding of the pore structure and surface chemistry evolution of the CNF100-750 cathode at different discharge/charge states. These observations indicate that C=O groups undergo reversible redox reactions, playing a crucial role in the Faradaic process by promoting strong coupling interactions with electrolyte ions. Together, these observations suggest that efficient charge storage in CNF100-750 is governed by a combination of reversible ion adsorption/desorption and interfacial redox reactions.

Figure 6: (a) Self-discharge behavior of the quasi-solid-state ZIC after charging to 1.8 V. (b) Rate capability of the quasi-solid-state ZIC from 0.1 to 20 A g⁻¹. (c) CV curves of the quasi-solid-state ZIC at different bending angles, confirming its mechanical flexibility and structural integrity. (d, e) GCD curves of quasi-solid-state ZICs connected in series and parallel, compared with a single device. (f) Photograph of a quasi-solid-state ZIC powering an LED light, demonstrating practicality. (g) Cycling performance of the quasi-solid-state ZIC at 5 A g⁻¹.

To evaluate the practical applicability of CNF100-750 in quasi-solid-state ZICs, a device was assembled using a PVA/ZnSO4 gel electrolyte, and its electrochemical performance was systematically evaluated. The quasi-solid-state ZIC further demonstrated excellent flexibility and practicality, validating the robustness of this bio-inspired design. This study not only opens a sustainable pathway for direct CA carbonization but also provides theoretical insights into the rational design of ion transport channels in porous carbon materials, offering new perspectives for high-performance ZICs.

Paper link: Cesium Chemistry Enables Microporous Carbon Nanofibers with Biomimetic Ion Transport Channels for Zinc-ion Capacitors - Green Chemistry (RSC Publishing)