Electrospun PAN/PS Micro‑Nanofibers Loaded with Ammonium Metatungstate and Cobalt(III) Acetylacetonate: Porous Architecture and Enhanced Through‑Pore Distribution

Abstract

This work presents a one‑step electrospinning protocol for producing polyacrylonitrile/polystyrene (PAN/PS) micro‑nanofibers that incorporate ammonium metatungstate hydrate (AMT) and cobalt(III) acetylacetonate (Co(acac)₃). The fibers exhibit a highly porous surface, a broad distribution of through‑pores, and uniform loading of tungsten and cobalt species. Comprehensive characterization—including FESEM, FTIR, EDS, and through‑pore size analysis—confirms the successful integration of AMT/Co(acac)₃ and demonstrates that the resulting micro‑nanofibers possess key attributes desirable for electro‑catalytic applications: efficient electrolyte penetration and rapid gas bubble release.

Introduction

Nanofibers offer an exceptional combination of mechanical strength and surface area, making them attractive for a variety of functional materials. Electrospinning has emerged as a versatile technique for generating smooth, controllable‑morphology fibers from a wide range of polymers, including PAN and PS. Prior studies have leveraged electrospun fibers for catalysis, sensing, and energy storage; for instance, Cu‑ or Fe₂O₃ loaded fibers were fabricated via bubble electrospinning, and core‑shell PAN/PS architectures were reported for hierarchical design.

Ammonium metatungstate hydrate (AMT) and cobalt(III) acetylacetonate (Co(acac)₃) are well‑known for their catalytic properties in oxidation reactions and electro‑winning processes. Incorporating these species into polymer fibers can provide a homogeneous distribution of active sites while preserving the structural advantages of nanofibers. Herein, we report a facile one‑step electrospinning method that embeds AMT/Co(acac)₃ into PAN/PS micro‑nanofibers, yielding a highly porous scaffold with enlarged through‑pores that are expected to enhance electrolyte access and gas evolution during electrochemical processes.

Methods

Materials

Polyacrylonitrile (PAN, Mn = 150 kDa, Shanghai Macklin Biochemical) and polystyrene (PS, Mn = 192 kDa, Sigma‑Aldrich) were dissolved in N,N‑dimethylformamide (DMF, 73.09 g mol⁻¹, AR, Chinasun) to prepare 10–20 wt % solutions. AMT (MW = 2956.30 g mol⁻¹, 99.5 % purity) and Co(acac)₃ (MW = 356.26 g mol⁻¹, 99.5 % purity) were added to the polymer solutions in predefined molar ratios (W⁶⁺ : Co³⁺ = 1:0, 0:1, 1:1, 1:2, 1:3). All reagents were used as received.

Fabrication of AMT/Co(acac)₃-Loaded PAN/PS Micro‑Nanofibers

To obtain optimal fiber morphology, a 20 wt % PAN/PS (1:1 w/w) solution was prepared in DMF. AMT and Co(acac)₃ were incorporated at the following molar amounts: 1:0 (W⁶⁺ = 0.62 mmol), 0:1 (Co³⁺ = 0.62 mmol), 1:1, 1:2, and 1:3. After stirring overnight, the homogeneous solution was loaded into a 10 mL syringe. Electrospinning was carried out at 20 kV, 1 mL h⁻¹, with a grounded rotating roller collector, under ambient conditions (20 ± 3 °C, 40 ± 3 % RH). A schematic of the process is shown in Scheme 1.

Electrospun PAN/PS Micro‑Nanofibers Loaded with Ammonium Metatungstate and Cobalt(III) Acetylacetonate: Porous Architecture and Enhanced Through‑Pore Distribution

Measurements and Characterizations

Fiber Morphology

Fiber surfaces were examined by FESEM (Hitachi S‑4800). Samples were gold‑sputtered (90 s) before imaging. ImageJ was used to measure the diameters of 100 randomly selected fibers from each sample.

Fiber Chemical Structure

FTIR spectra were recorded on a Nicolet 5700 (Thermo) using KBr pellets, covering 400–4000 cm⁻¹ with 32 scans per sample.

Fiber Elemental Detection

EDS mapping was performed on a TM3030 SEM to confirm the presence and distribution of W and Co on the fiber surfaces.

Through‑Pore Properties

Through‑pore size and distribution were measured with a Porometer 3G (Quantachrome). 25 mm diameter membranes were saturated with liquid using the Liquid Accessory Kit (No. 01150‑10035).

Results and Discussion

Morphological Characterization of PAN/PS Fibers

The concentration of the PAN/PS solution strongly influences fiber morphology. FESEM images (Fig. 1) show that at 10 wt % the fibers display bead‑laden, wrinkled surfaces due to rapid DMF evaporation and phase separation. Increasing the concentration to 12–14 wt % reduces bead size and introduces spindle‑like structures, while 16–20 wt % solutions yield smooth, bead‑free fibers. This trend is consistent with the enhanced viscoelasticity that suppresses surface tension during jet whipping.

Morphological Characterization of AMT/Co(acac)₃-Loaded PAN/PS Micro‑Nanofibers

FESEM images reveal that all loaded fibers exhibit nanoporous surfaces attributable to phase separation. As the W⁶⁺ : Co³⁺ ratio increases, the fibers transition from ordered filaments to scattered, bead‑like morphologies, and the mean diameters decrease from 2765 ± 180 nm (W₁) to 1092 ± 112 nm (W₁Co₃). This trend indicates that the incorporated AMT and Co(acac)₃ influence the solution viscosity and solidification dynamics during electrospinning.

FTIR Analysis

FTIR spectra (Fig. 4) confirm the presence of characteristic peaks for PAN (C≡N at 2250 cm⁻¹), PS (phenyl vibrations at 750, 1454 cm⁻¹), and Co(acac)₃ (C=C at 1573 cm⁻¹, C=O+CH at 1513 cm⁻¹). The successful integration of AMT/Co(acac)₃ into the polymer matrix is further validated by the appearance of these distinct bands in the loaded fibers.

EDS Test

EDS analysis (Fig. 5) shows the expected elemental composition: pure PAN/PS fibers contain only C, N, and O; W₁ fibers lack Co peaks, while Co₁ fibers lack W peaks. In the loaded fibers, the atomic ratios of W and Co increase with the programmed molar ratios, confirming uniform loading. Elemental mapping (Fig. 6) further demonstrates homogeneous distribution of W and Co across the fiber surfaces.

Through‑Pore Structures

The average through‑pore sizes of the loaded fibers span 6.01–10.40 µm, markedly larger than the 6.01 µm observed for pristine PAN/PS. The broader distribution in the loaded membranes (Fig. 7) reflects the increased fiber diameter and surface roughness induced by AMT/Co(acac)₃. These enlarged pores facilitate rapid electrolyte transport and efficient gas release, critical for electrochemical reactions.

Conclusions

We have demonstrated a single‑step electrospinning route to produce PAN/PS micro‑nanofibers that incorporate AMT and Co(acac)₃. The resulting fibers display a porous surface, enlarged through‑pores, and a uniform distribution of W and Co, as confirmed by FESEM, FTIR, EDS, and pore analysis. These structural features are expected to enhance electrolyte access and gas bubble release, underscoring the potential of AMT/Co(acac)₃-loaded PAN/PS fibers as efficient electro‑catalytic platforms.