A robust polyaniline hydrogel electrode enables superior rate capability at ultrahigh mass loadings

Abstract

Simultaneously achieving high mass loading and superior rate capability in electrodes is challenging due to their often mutually constrained nature, especially for pseudocapacitors for high-power density applications. Here, we report a robust porous polyaniline hydrogel (PPH) prepared using a facile ice-templated in situ polymerization approach. Owing to the conductive, robust, and porous nanostructures suitable for ultrafast electron and ion transport, the self-supporting pure polyaniline hydrogel electrode exhibits superior areal capacitance without sacrificing rate capability and gravimetric capacitance at an ultrahigh mass loading and notable current density. It achieves a high areal capacitance (15.2 F·cm⁻² at 500 mA·cm⁻²) and excellent rate capability (~92.7% retention from 20 to 500 mA·cm⁻²) with an ultrahigh mass loading of 43.2 mg cm⁻². Our polyaniline hydrogel highlights the potential of designing porous nanostructures to boost the performance of electrode materials and inspires the development of other ultrafast pseudocapacitive electrodes with ultrahigh loadings and fast charge/discharge capabilities.

Here, the authors establish a design approach for porous materials with a high mass loading polyaniline electrode by using a radial porous nanostructure. The design approach allows for electrodes with high mass loading and rate capability.

Introduction

Energy storage devices urgently pursue simultaneous high energy and power density to meet the long-fast discharging demands^1,2. Supercapacitors, a typical energy storage device, have long been used as ultrafast charging/discharging storage due to their renowned high power capabilities, which enable efficient energy harvesting and delivery within seconds^3,4. However, they suffer from relatively low energy density, limiting their further application. Thus, researchers turn to redox-active polymers to improve energy density because of their effective pseudocapacitive capacitance⁵. Conductive polymers, a classic redox-active polymer, possess rapid and reversible doping/de-doping activity, offer an efficient capacitance at the polymer-electrolyte interfaces, and can theoretically serve as promising electrode materials for high power density⁶. Significantly, the conductive polymer-based materials have made noticeable progress regarding the capacitance and rate performance at the electrode material level^7–10. Nevertheless, an efficient power supply in practical devices hinges on attaining high energy/power density for the overall device, extending beyond exceptional specific capacitance and rate performance for electrode materials.

Mass loading, in this specific context, is particularly critical to the device, as the energy and power density are directly proportional to the mass ratio of electrode materials^11,12. High mass loadings are essential to realize high areal capacitance, which is conducive to large energy/power density¹³. Typically, commercial-level mass loading is around 10 mg cm⁻²; however, the reported literature often overlooks it¹⁴. Consequently, although many electrodes have demonstrated exceptional specific capacitances that approach their theoretical values and outstanding rate capability, this is only achievable at minimal active material loading (<10 mg cm⁻²) and low current density (<100 mA cm⁻²)^15–19. As a result, it ineluctably leads to inefficient areal capacitance (<10 F cm⁻²), which is unacceptable in practical application²⁰. For example, by incorporating only 2.2 mg cm⁻² mass loading and operating at a low current density ( ≤ 50 mA cm⁻²), polypyrrole-based electrodes can display a high rate capability of 82%. However, its areal capacitance is as low as 1.3 F cm⁻², which is disappointing¹⁸. Nevertheless, as the mass loading increases, the ion transport issue becomes particularly critical for the electrochemistry performance. High mass loading will inescapably lead to a significant deterioration in rate performance due to the considerable dead mass and clogged diffusion channels introduced^11,21–26. An example is that a PANI-based electrode with an ultrahigh mass loading of 50 mg cm⁻² displays only 31% capacitance retention at a low current density (30 mA cm⁻²), also resulting in an inferior areal capacitance of 3.1 F cm^−2 26. Therefore, despite numerous efforts in high mass loading and superior rate performance research, simultaneously achieving the two mutually exclusive features requires ensuring high electronic conductivity and fast electrolyte diffusion, which remains challenging.

Hence, we innovate the design of robust PPH as excellent rate performance electrodes with an ultrahigh mass loading. Using an ice-templated strategy to generate a radial porous structure for efficient electron transfer and electrolyte diffusion, we have bridged the two mutually exclusive characteristics, i.e., ultrahigh mass loadings and superior rate capability. The robust PPH electrode with a mass loading of 43.2 mg cm⁻² realizes an areal capacitance of 15.2 F cm⁻² and 92.7% capacitance retention at a current density of 500 mA cm⁻². Interestingly, the areal capacitance increases linearly with the mass loading of PANI. Moreover, the gravimetric capacitance only reduced slightly (~7.1%), indicating that ion diffusion does not noticeably limit the capacitive performance, even at a far more than commercial-level mass loading. Notably, our work establishes a design approach for porous materials that has the potential to unlock sustainable solutions for efficient electrodes in practical energy storage applications.

Results and discussion

Synthesis of robust PPH

To attempt to obtain a robust PPH, we illustrated the typical freeze-casting process in Fig. 1a. Initially, the aniline monomer was mixed rapidly with the oxidant, then a quick shake and ultrasonic for seconds to ensure uniform mixing, followed by immersion in liquid nitrogen rapidly so that freezing occurred from the outsides of the mold, afterward placed at −20 °C for in in situ polymerization growth of PANI; finally, obtained a robust PPH after the temperature reverse to room temperature. Due to the simplicity of the process and robustness of the PPH, we can get a wide variety of shapes in one step by simply controlling the size and shape of the mold. For example, it can be efficiently designed as stars, cat paws, and cylinder shapes without any follow-up (Fig. 1b–f). Furthermore, the PPH shows a solid mechanical property with which it can support a weight of 100 g without collapsing (Fig. 1e). Rheological tests also illustrate the rigid properties. G′ is considerably greater than G′′ (Fig. 1g), and the G′ is larger than the corresponding G″ until to 10% strain (Fig. 1h), which is superior to other pure conductive polymer hydrogels^27–29, indicating that the PPH can withstand severe deformation and display solid mechanical properties. Additionally, the deformation resistance of PPH is enhanced after treatment with ammonium hydroxide (Supplementary Fig. 1), attributed to the conversion of PANI from an aromatic ring structure (emeraldine, partially oxidized) to a rigid benzoquinone structure (leucoemeraldine, fully reduced). Notably, the robust PPH electrode presented in this study is synthesized using a single-step method consisting solely of pure PANI without additives. This approach is more accessible than other complex processes, such as template-based, interfacial, electrospinning, and hyper-crosslinking methods⁶.

Fig. 1. Preparation of robust PPH.

a Schematic illustration of the fabrication process for robust PPH. b–f Digital images of various shapes exhibit excellent mechanical strength and processability. g Storage modulus (G′) and loss modulus (G”) versus angular frequency. h Rheological strain sweeping measurements.

Radial porous nanostructure

Here, to elucidate the robustness of the PPH, we studied its morphology, porosity, chemical structure, and electrical conductivity and proposed possible mechanisms for radial porous nanostructures. As shown in Fig. 2a, the PPH displays a prominent radial cross-section after the cryogenic holding process, and this morphology is still retained to aerogel after lyophilization. From a view of the inside cylindrical aerogel, it demonstrates radial porous aligned lamella (Fig. 2b). The PANI blade is lamellar and ordered along one direction, as confirmed by different magnified SEM images in Fig. 2c–e, which exhibit channel widths varying from 5 to 30 μm. Zooming in on a single blade reveals several overlapping PANI lamellas with pores ranging from 50 to 100 nm (Fig. 2f–j and Supplementary Fig. 2). Furthermore, the thickness of these PANI blades, which have twisted and curled shape lamellas, ranges from about 1 to 10 μm (Fig. 2g, h). The space between porous PANI lamellas ranges from 0.1 to 3 μm (Fig. 2i). Accordingly, the PPH exhibits a highly porous structure, containing abundant micropores, mesopores (Supplementary Fig. 3), and plentiful macropores structures.

Fig. 2. Morphology model and characterizations.

a Schematic illustration and photograph of radial fracture cross-section before and after lyophilization. b Schematic illustration of the radial porous structure in PPH (cross-section and side section). c–e, g, and h are SEM images of radial structure in cross-section. f SEM images of porous lamella in the side section. i and j TEM images of lamella in cross-section and side section, respectively.

We suppose that the formation of radial porous PANI structure is due to the separation of the polymerization of PANI by lamellar-shaped ice crystals during the low-temperature polymerization process and propose a possible mechanism to gain a deep insight into the radial porous nanostructure. After freezing the blending solution in liquid nitrogen, the oxidants and monomers are trapped between the radially grown ice crystals, creating an aggregation zone (Supplementary Fig. 4a). The ice crystals remain in their original state at low temperature, supporting the in situ growth of PANI. As a result, PANI is grown at the aggregation zones of monomers and oxidants, and the ice crystals force PANI to form radial structures as well (Supplementary Fig. 4b). In these structures, PANI lamellae are closely connected by a particular gap, creating a uniform cell geometry to protect the porous structure from collapse under external forces (Supplementary Fig. 4c), which contributes to the enhanced bulk mechanical strength observed in Fig. 1^30,31. This robust mechanical strength is confirmed by comparing the PANI hydrogel produced from the same dispersion without liquid nitrogen freezing (PPH-wlnf), which exhibits inferior mechanical strength due to the absence of radial symmetry (Supplementary Fig. 5 and Supplementary Fig. 27a, b). Additionally, we have found that some changes in the preparation conditions impair the mechanical strength of PPH. For example, holding the liquid nitrogen-freeze bulk at 0 °C, or room temperature instead of −20 °C, or delaying the freezing process for more than 120 s (Supplementary Fig. 6) will all lead to a failure in the formation of the desired radial structure, and consequently, inferior mechanical properties. After lyophilization, the PPH forms a light aerogel with an average density of 225 ± 1 mg cm⁻³ and high porosity level (~76%), and can be supported by a flower bud (Supplementary Fig. 7). Fourier transform infrared (FTIR), Raman, and X-ray photoelectron spectroscopy verify that the PPH only comprises pure PANI regarding chemical composition and structure (Supplementary Fig. 8). Compared with the PANI powder prepared at 0 °C, The PPH (complete FTIR band assignments are shown in Supplementary Table 1) shows enhanced absorption peaks at 1384, 1236, and 930 cm⁻¹, which correspond to the large conjugate structure and phenazine-like segments stretching of PANI in PPH^32,33 (Supplementary Fig. 9), revealing the high molecular weight PANI (means high conductivity) successfully synthesized at low temperature^34–36. It is also supported by the fact that the conductivity of PPH (70 S m⁻¹) measured by the four-probe method is appreciably higher than that of PANI powder synthesized at 0 °C (0.32 S m⁻¹). These results validate the successful construction of a robust, conductive, and porous three-dimensional nanostructure in PPH, implying it is an expected candidate for energy storage due to its ideal ultrafast electron and ion transport platform.

Electrochemical properties of PPH

To achieve the optimal electrochemical performance of PPH, a series of synthesis conditions are determined by experiments, resulting in the production of robust PPH with high conductivity and large porosity (Supplementary Table 2 and Supplementary Fig. 10). A typical sandwich-like model is used for electrochemical testing (Supplementary Fig. 11). PPH-23.8 (mass loading: 23.8 mg cm⁻²) is chosen as an example for testing in a three-electrode system utilizing 1 M H₂SO₄ as the electrolyte. The cyclic voltammetry (CV) at various scan rates match with the potential to the redox peaks of PANI synthesized by chemical oxidative polymerization at 0 °C^27,37, and possess a consistent shape without noticeable variation till to 50 mV s⁻¹, denoting low internal resistance and exceptional rate performance (Fig. 3a). The galvanostatic charge/discharge (GCD) curves shown in Fig. 3b exhibit nonlinear behavior, in line with the redox peaks potential in CV curves, implying the pseudocapacitance contribution in the PPH. In addition, the Nyquist plot in Fig. 3c also shows a small equivalent series resistance (R_s, 0.74 Ω) and tiny charge transfer resistance (R_ct, 0.43 Ω), revealing the fast electron transfer and rapid electrolyte diffusion in PPH-23.8.

Fig. 3. Capacitive performance of PPH-23.8.

a CV curves at 5, 10, 25, 50, and 75 mV s⁻¹ scan rates. b GCD curves at different current densities. c Nyquist plot with inset showing enlarged plots. d Typical power-law dependence of cathodic and anodic peak currents from 5 to 75 mV s⁻¹. e Capacitive contribution at 25 mV s⁻¹ scan rate. f Contribution of capacitive and diffusion-controlled processes at different scan rates. g Specific capacitances vs. current densities.

To analyze the charge-storage kinetics of PPH-23.8, the logarithm of peak current is plotted against the logarithm of scan rates. The plot in Fig. 3d follows a power-law relationship with a slope value of 0.75, pointing out that surface capacitive behavior primarily governs the charge-storage process. At the scan rate of 25 mV s⁻¹, the capacitive behavior accounts for 42% of the total specific capacitance for the PPH-23.8 electrode (Fig. 3e). With the increase in scan rates from 5 to 75 mV s⁻¹, the capacitive contribution increased markedly from 34% to 90% (Fig. 3f), with details evidenced in Supplementary Fig. 12, definitively confirming its fast charge-storage kinetics. Such a result powerfully reveals that faradaic pseudocapacitive behavior primarily controls the charge storage in the PPH-23.8 sample, which is conducive to achieving superb rate performance, particularly at high current density³⁸. Based on the GCD curves (Fig. 3b), the specific capacitance of the PPH-23.8 electrode is calculated. Notably, the PPH-23.8 electrode exhibits a high specific capacitance of 9.2 F cm⁻² (387.9 F g⁻¹) at 20 mA cm⁻² (0.84 A g⁻¹) (Fig. 3g). Remarkably, the PPH-23.8 electrode demonstrates excellent rate capability, retaining 93% of its capacitance at a high current density of 500 mA cm⁻² (21 A g⁻¹), still with a specific capacitance of 8.6 F cm⁻² (360.6 F g⁻¹). These results highlight the promising pseudocapacitive behavior, high capacitance, and excellent rate capability of the PPH-23.8 electrode at large current densities. While the radial porous structure makes PPH-23.8 an ideal energy storage platform, further investigations are necessary to understand ion and electron transport in PPH with different mass loadings.

Electrochemical property with ultrahigh mass loading

To explore the impact of mass loading on the performance of PPH electrodes, PPH electrodes with different mass loadings of 6.0, 13.0, 23.8, and 43.2 mg cm⁻² are performed. The PPH-6.0, PPH-13.0, and PPH-43.2 show outstanding electrochemistry performance similar to PPH-23.8 (Supplementary Figs. 13–18). A detailed comparison of these PPH electrodes is displayed to gain a deeper understanding. As shown in Fig. 4a, the CV current densities at 10 mV s⁻¹ substantially increase with increasing the mass loading within a voltage range of 0~0.8 V, revealing growing capacitance with higher mass loading. It is also further supported by the longer discharge time with mass loading enhanced at the same current density (Fig. 4b). Inevitably, the oxidation peaks slightly shift to higher potential as the mass loading increases from 6.0 to 43.2 mg cm⁻² (Fig. 4a), suggesting some increase in electron transfer resistance to an ultrahigh mass loading. This observation aligns with the results obtained from the Nyquist plot (Fig. 4c) and capacitive contribution at different scan rates (Supplementary Fig. 19). The increased mass loading leads to an increase in R_s, R_ct, and a decrease in capacitive contribution, indicating that efficient electron transfer and electrolyte diffusion are affected by mass loading to some extent. It is also verified by the Bode plots, where a longer dielectric relaxation time (1/f, τ₀) in C′′ and a shift to a lower frequency of C′ are observed with increasing the mass loading (Supplementary Fig. 20a, b). Still, the values of internal resistance (Supplementary Fig. 21), R_s, and R_ct are comparable to those of conductive polymer-based materials in aqueous electrolytes^10,39,40, demonstrating that the ultrahigh loading of PANI has not seriously damaged the ability of efficient electron transfer and electrolyte diffusion. Noticeably, the tails of the Bode plots remain nearly vertical at low frequency till PPH-43.2, hinting that ion transport kinetics are still fast and fully capable of meeting the requirements of rapid charging and discharging.

Fig. 4. Capacitive performance of PPH electrodes: PPH-6.0 (black), PPH-13.0 (red), PPH-23.8 (green), and PPH-43.2 (blue).

a CV curves at 10 mV s⁻¹. b GCD curves at 200 mA cm⁻². c Nyquist plot with inset showing enlarged plots. d Areal capacitances vs. current densities. e and f Contrast of PPH-43.2 and other conductive polymer-based electrodes.

Consequently, the areal capacitance of PPH electrodes shows a linear increase with mass loading, reaching its maximum value at 43.2 mg cm⁻² (Supplementary Fig. 22). Importantly, under all kinds of mass loading, the PPH electrodes all maintain extraordinarily high rate capability, with values of 96.7%, 95.7%, 93%, and 92.7% from 20 to 500 mA cm⁻² for PPH-6.0, PPH-13.0, PPH-23.8, and PPH-43.2, respectively (Fig. 4d). Additionally, we conducted extra tests to assess the rate capability of PPH by maintaining different constant discharge current densities while increasing the charging current densities (Supplementary Fig. 23). The test results indicate that the rate performance decreases gradually as the constant discharge current densities and mass loading increase. However, even at a high constant discharge current of 200 mA cm⁻² and a mass loading of up to 40 mg cm⁻², the rate performance remains above 80%, further validating their exceptional rate performance characteristics. Furthermore, the gravimetric capacitances (Supplementary Fig. 24a), normalized to the mass of PANI, only suffer from a slight decrease (~7.1%) as the mass loading increases steeply from 6.0 to 43.2 mg cm⁻² and the obtained volumetric capacitances are in the range of 125~135 F cm⁻³ (Supplementary Fig. 24b). Moreover, the specific surface area normalized capacitances (Supplementary Fig. 24c) range from 670 to 780 μF cm⁻², which are notable values. These high capacitances are likely attributed to the pseudocapacitance contribution, which is less affected by the specific surface area. Additionally, the Coulombic efficiency of various PPH electrodes exceeds 90% at each current density, and the energy efficiencies are around 80% (Supplementary Fig. 25). The excellent performance of the PPH electrodes is perfectly demonstrated using six important parameters: capacitance, areal capacitance, volumetric capacitance, mass loading, highest current density, and rate capability (Supplementary Fig. 26). Impressively, the rate performance, volumetric capacitance, and mass-specific capacitance are almost independent of mass loading, and the area capacitance demonstrates a linear increase with mass loading. As the most electrochemically superior PPH, the areal capacitance, high current density, and rate capability of PPH-43.2 surpass those of previously reported conductive polymer electrodes (Fig. 4e, f)^{8,9,15–19,22–26,41–44}, details shown in Supplementary Table 2.

Furthermore, we conducted cycling stability tests on PPH and PANI film within the voltage ranges of −0.2~0.7 and 0~0.8 V (Supplementary Fig. 27). The cycling stability in the voltage range of −0.2~0.7 V is superior to that of 0~0.8 V, with PPH demonstrating better than the PANI film. It is expected that the test potential of PANI should be below 0.8 V due to its instability in the pernigraniline state⁴⁵, and it also reveals that the porous structure made up of high molecular weight PANI fibers, synthesized at low temperatures, effectively mitigates the volume shrinkage and expansion of PANI during the charging and discharging process. Moreover, we evaluated the performance of a symmetric PPH // PPH system with a mass loading of 25.8 mg cm⁻², which displays excellent rate performance and remarkable cycling stability (Supplementary Fig. 28). It should be noted that the theoretical capacitance of PANI is over 700 F g^−1 46, considerably higher than the actual test values obtained. One possible reason for this discrepancy is that the PANI network in PPH is relatively thick (>30 nm, Supplementary Fig. 2), and the PANI chains contain1,2,4-disubstituted rings and phenazine-like segments (Supplementary Fig. 9), which may lead to the inactivation of effective redox-active PANI. Therefore, the achievement demonstrated by these results is the substantial increase in areal capacitance, mass loading, and current density while maintaining rate capability and gravimetric capacitance in PPH.

Fast electron transfer and electrolyte diffusion in PPH

To investigate why PPH exhibits exceptional performance, we conducted two control experiments. In one experiment, a PPH-wlnf electrode is fabricated using a process similar to PPH but without liquid nitrogen freezing. The other experiment involves drop-casting PANI solution on a Pt sheet electrode to create a PANI film. We compared the nanostructures and electrochemical performance of PPH-6.0, PPH-wlnf, and PANI film. Surprisingly, the nanostructure of the PPH remains intact even after undergoing compression by 500 kPa (Supplementary Fig. 29). The compressed PPH retains its porous structure, albeit with smaller slits and gaps (0~3 μm), and the lamella becomes more curled and twisted, demonstrating that it can possess enough strength, toughness, and internal porosity to maintain its adequately porous structure when utilized in electrode pieces. In contrast, the morphology of PPH-wlnf, as shown in Supplementary Fig. 30a, b, differs from that of PPH. It lacks regularly arranged macropores and slits, which are crucial for electrolyte diffusion. The PANI film, as expected, displays a dense and non-porous structure (Supplementary Fig. 30c, d). These nanostructural differences play a critical role in determining the electrochemical properties. As a result, both PPH-wlnf (Supplementary Figs. 31 and 32) and PANI film (Supplementary Figs. 33 and 34) exhibit inferior electrochemical performance compared to PPH-6.0. Specifically, Fig. 5b shows that PPH-6.0 demonstrates clear redox peaks and the largest CV curve area, followed by PPH-wlnf and PANI film. PANI-6.0 exhibits the best redox activity and highest capacitance, while the PANI film performs worst. The comparison of CV curves at different scan rates in Supplementary Fig. 35 further supports it. Moreover, PPH-6.0 exhibits the highest surface capacitive contributions at different scan rates (Fig. 5c), indicating that the radial porous structure appreciably enhances the surface capacity. In contrast, the PPH-wlnf and PANI film presents lower surface capacity due to reduced electrolyte diffusion channels.

Fig. 5. Comparison of electrochemical performance between PPH-6.0 (red), PANI film (blue, 2.91 mg cm⁻²), and PPH without liquid nitrogen freezing electrodes (black, named as PPH-wlnf, 7.4 mg cm⁻²), demonstrating ultrafast electron and ion transport in PPH.

a Illustration of the fast electron transfer and rapid electrolyte diffusion in PPH. b CV curves at 25 mV s⁻¹. c Capacitive contribution at different scan rates. d Nyquist plots, the inset shows an enlargement of plots. e Imaginary capacitance. f Normalized real capacitance. g The relationship between Z′ and ω^−1/2 at low frequency. h Specific capacitance at various current densities.

The results from Nyquist plots shown in Fig. 5d also support these findings. The PPH-6.0 electrode displays the closest vertical line in the low-frequency range and the smallest x-axis intercept, demonstrating the lowest R_s and best ion diffusivity. Conversely, the PANI film shows the most inclined curve and the largest R_s, even at a low mass loading (2.91 mg cm⁻²). It is consistent with the results that the conductivity of PPH is over two hundred times higher than that of the PANI powder by four-probe measurements, implying that the PANI fibers in PPH with larger conjugated structures synthesized at lower temperatures favor electron transfer. Furthermore, the normalized imaginary and real capacitance vs. frequency plots in Fig. 5e, f show that PPH-6.0 has the shortest dielectric relaxation time in the C” region and the highest frequency in the C′ region, revealing quick ion response under polarization^47,48. It is further verified by the relationship between Z′ and ω at low frequency (Fig. 5g), where the slope of PPH-6.0 is the lowest among the three electrodes. As a result, PPH-6.0 demonstrates the best rate capability at high current densities. In contrast, PPH-wlnf maintains only 23% of its capacitance from 2.7 to 67.6 A g⁻¹, while the PANI film rapidly decreases to only 10% from 0.8 to 38.5 A g⁻¹. Although the three electrodes possess almost the exact value of specific capacitances at low current densities, the PANI film displays the lowest specific capacitance at high current densities and the poorest rate capability due to its inferior conductivity and non-porous nanostructure. Even though the PPH-wlnf has a porous structure, it lacks a regular porous lamellar structure, leading to poor rate performance due to hindered electrolyte diffusion at high currents.

These comparative studies highlight the vital role of high conductivity and radial porous structure in efficient electrode material. The unique features of the conductive porous structure, including high-quality PANI chains and radial porous nanostructure, enable sufficient and rapid ion diffusion and electron transfer (Fig. 5a). Firstly, the radial construction enhances the mechanical strength of the electrode, protecting the redox-active sites and pores from damage during electrode preparation. Secondly, the radial porous structure creates unimpeded channels for rapid electrolyte diffusion, reducing transport routes and enabling superior areal capacitance with excellent rate capability and large current density. Finally, the high quality of PANI and the connected porous framework enhance electrical conductivity and increase electrode stability, resulting in noticeably improved electrochemical performance. Using radial porous PPH as a scaffold effectively circumvents the trade-off between high capacitance, commercial-level mass loading, large current density, and superior rate capability. These results validate the construction of a practical electrode and open up possibilities for fabricating high areal supercapacitor electrodes.

In conclusion, our study demonstrates that the pure PANI scaffold can support an over commercial-level mass loading (40 mg cm⁻²), conducting to efficient electrodes with superb rate capability and notable current discharge capability. Due to high conductivity and radial porous nanostructure, the ice-templated porous structure enables efficient electron and ion transfer ability. It provides ultrahigh mass loading without sacrificing its gravimetric and areal capacitance at a large current density of 500 mA cm⁻². Even more remarkably, the areal capacitance increases linearly with the mass loading, highlighting their great potential for practical application. With the increasing demand for fast-charging electronic devices, PPH emerges as a promising candidate that can fulfill the need for high areal capacitance and mass loading while maintaining rate capability.

Methods

Materials

Aniline and ammonium persulfate (APS, AR) were purchased from Maclean Chemical Reagent Company and used as received. Hydrochloric acid (HCl, AR), and 1-methyl-2-pyrrolidinone, sulfuric acid (H₂SO₄, 98%) were purchased from Xilong Science Co., Ltd. and used as received.

Synthesis of PPH

Aniline (50 mmol) was typically dissolved in 50 mL 3 M HCl to prepare solution A. APS (25 mmol) was dissolved in 10.3 mL deionized water to prepare solution B. Then, the solutions A and B were cooled to 0 °C using ice water. To a 10 mL centrifuge tube, 4.4 mL of solution A and 1.1 mL of solution B were added and mixed quickly under shaking and ultrasound treatment for 10 s each. The mixture was then inserted into liquid nitrogen and formed an ice bulk. This process was repeated, and a total of 12 tubes were obtained. Once completely frozen, the tubes were transferred to a −20 °C refrigerator and kept there for 3 days to obtain cylindrical self-supporting hydrogels. The hydrogels were dialyzed in pure water for 24 h to remove impurities. Finally, the dialyzed hydrogels were lyophilized to prepare aerogels. For different shapes of PPH blocks, the mixed solution of A and B was poured into molds in varying amounts, then frozen using liquid nitrogen, and kept at −20 °C for 3 days to form PPH with mold shapes. PPH electrodes with mass loadings of 6.0, 13.0, 23.8, and 43.2 mg cm⁻² (denoted as PPH-6.0, PPH-13.0, PPH-23.8, and PPH-43.2) were prepared by cutting different amounts of PPH and directly pressing them into pieces (0.8 cm in diameter) without any conductive filler or binder. The pressing process involved applying a force of 500 kPa for 5 s, repeated twice. The thickness of each electrode is specified as 0.18 ± 0.01, 0.39 ± 0.01, 0.67 ± 0.01, and 1.21 ± 0.01 mm, respectively. The density of each electrode is calculated as 0.33, 0.33, 0.355, and 0.357 g cm⁻³, respectively.

Preparation of PPH-wlnf electrode

The preparation of PPH-wlnf is similar to the process of PPH, but without the liquid nitrogen freezing treatment, that is, after shaking and ultrasound treatment for 10 s each, the blend solution was immediately placed in a refrigerator at −20 °C for 3 days.

Preparation of PANI power

PANI powder was prepared according to the method reported in the literature⁹. Under an ice-water bath, 0.1 mol of aniline was added to 100 mL of 1 mol L⁻¹ HCl, and the pH was adjusted to 1.0 using HCl. Next, 51.5 mL of an APS solution, in which 0.125 mol was dissolved, was added dropwise to the solution. After the reaction proceeded for 3 h, vacuum filtration was performed. The filtrate cake solid was successively washed with deionized water and ethanol until the filtrate became colorless. Finally, the material was dried for 24 h at 60 °C to obtain PANI powder.

Preparation of PANI film electrode

The dried PANI powder was dissolved in 1-methyl-2-pyrrolidinone to create a homogeneous PANI solution (5 mg mL⁻¹). PANI film electrode was obtained by directly drying the PANI solution on a platinum plate electrode (1.5 × 1.5 cm) under 70 °C.

Electrochemical measurement

A 1 M H₂SO₄ electrolyte solution and a saturated calomel reference electrode (SCE) were used for all electrochemical tests at room temperature. For electrochemical testing, we assembled a typical sandwich-like model comprising the following components: two platinum foils employed as current collectors, two pieces of compressed round PPH (0.8 cm in diameter) with similar mass and thickness serving as the working and counter electrodes, a regenerated cellulose membrane acting as the diaphragm, and two platinum wires utilized as conducting wires to connect to the electrochemical workstation. The entire sandwich-like model was securely clamped and fixed using a polytetrafluoroethylene mold. In three-electrode systems, the samples were activated by cycling the voltage between −0.2 and 0.8 V for 20 cycles at a scan rate of 50 mV s⁻¹ before electrochemical testing and then tested at the voltage range of 0~0.8 V. For two-electrode system testing, the working and counter electrodes undergo separate activation using the three-electrode systems before testing. Before assembling the system, the electrodes to be tested were soaked overnight in the electrolyte to exchange internal water. After the electrochemical tests, the working electrode was subjected to dialysis and subsequently lyophilized for weighing.

Electrochemical characterization

Capacitances

In typical three-electrode testing, the gravimetric capacitance (C_m, F g⁻¹; C_S, F cm⁻²) of a single electrode was calculated using galvanostatic charge-discharge curves with the following equation.

$${\text{C}}_{\text{m}}=\frac{2\text{I}\int \text{V}\text{dt}}{\text{m}\mathrm{\Delta }{\text{V}}^2}$$ 1

where I is the discharge current (A), $\int Vdt$ is the area under the discharge curve, m is the mass of the single electrode (g), and $\mathrm{\Delta }V$ is the voltage range after an ohmic drop (V).

The geometric-areal normalized capacitance (C_GA, F cm⁻²), specific surface areal normalized capacitance (C_SA, µF cm⁻²), and volumetric capacitance (C_V, F cm⁻³) of a single electrode can be derived from C_m using the following equations:

$${\text{C}}_{\text{GA}},{\text{C}}_{\text{SA}}=\frac{{\text{C}}_{\text{m}}\times \text{m}}{\text{S}}$$ 2

$${\text{C}}_{\text{V}}=\frac{{\text{C}}_{\text{m}}\times \text{m}}{\text{S}\times \text{d}}$$ 3

where S is the geometric area (0.5 cm²) or specific surface area of a single electrode, and d is the thickness of a single electrode (cm).

In two-electrode testing, the capacitance calculation methods are the same as the above equations, but the mass (m) used in the equations refers to the total mass of the two electrodes.

Energy density and power density

In a two-electrode symmetric configuration, the gravimetric power density (P_m, W kg⁻¹) and gravimetric energy density (E_m, Wh kg⁻¹) were evaluated based on the total mass of the two electrodes. The gravimetric energy density (E_m) was calculated using the following equation:

$${\text{E}}_{\text{m}}=\frac{1}{2}{\text{C}}_{\text{m}}{\text{V}}^2$$ 4

where C_m is the gravimetric capacitance (F g⁻¹), and V is the operating voltage of the two-electrode symmetric pseudocapacitor system (V).

The gravimetric power density (P_m) can be determined using the equation:

$${\text{P}}_{\text{m}}=\frac{{\text{E}}_{\text{m}}}{\mathrm{\Delta }\text{t}}$$ 5

where P_m is the gravimetric power density (W kg⁻¹), E_m is the gravimetric energy density (Wh kg⁻¹), and $\mathrm{\Delta }$t is the discharge time (s).

The geometric-areal normalized volumetric energy density (E_V, F cm⁻³) and volumetric power density (P_V, Wh cm⁻³) of the system can be derived as follows:

$${\text{E}}_{\text{V}}=\frac{1}{2}{\text{C}}_{\text{V}}{\text{V}}^2$$ 6

$${\text{P}}_{\text{V}}=\frac{{\text{E}}_{\text{V}}}{\mathrm{\Delta }\text{t}}$$ 7

where E_V is the geometric-areal normalized volumetric energy density (Wh cm⁻³), C_V is the volumetric capacitance (F cm⁻³), V is the operating voltage of the two-electrode symmetric pseudocapacitor system (V), and $\mathrm{\Delta }$t is the discharge time (s).

In electrochemical impedance spectra (EIS), which were performed at open circuit potential with a frequency range from 0.01 Hz to 100 kHz, the real capacitance (C′) and imaginary capacitance (C”) of the electrode can be calculated using the following formulas:

$${\mathrm{C}}^{\prime }=\frac{-{\mathrm{Z}}^{″}}{2\mathrm{\pi }\mathrm{f}{\mathrm{\mid Z\mid }}^2}$$ 8

$$C^{″}=\frac{{\mathrm{Z}}^{\prime }}{2\mathrm{\pi }\mathrm{f}{\mathrm{\mid Z\mid }}^2}$$ 9

where Z′ is the real part of the impedance, Z” is the imaginary part of the impedance, f is the frequency (Hz), and |Z| is the magnitude of the impedance.

b-value analysis

We utilized Dunn’s method to quantify the capacitance contributions from fast-kinetic processes, such as electrical double-layer capacitive processes and fast redox reactions, as well as slow-kinetic processes involving diffusion-controlled redox reactions^49,50. According to CV measurement, the current densities (i) at different scan rates (v) but at a fixed potential follow a power-law relationship described by the equation:

$$\text{i}=\text{a}{\text{v}}^{\text{b}}$$ 10

In this equation, a is a pre-exponential constant, and b is a real number between 0.5 and 1.0. A b-value of 0.5 indicates sluggish charge-storage processes due to slow ion diffusion in the electrode, which is typical for battery electrodes. Conversely, a b-value of 1.0 suggests rapid charge-storage processes that are not diffusion-limited.

To quantify the capacitive and diffusion-controlled contributions, we analyzed the cyclic voltammetry data at different scan rates (v) using Eq. 11:

$$\text{i}\left(\text{V}\right)={\text{k}}_1\text{v}+{\text{k}}_2{\text{v}}^{1/2}$$ 11

Here, V represents the voltage, k₁v represents the surface capacitive effect (electrical double-layer capacity and pseudocapacitance), and k₂v^1/2 denotes the diffusion-controlled contribution.

By dividing ${\mathrm{v}}^{\frac{1}{2}}$ on both sides of the equation, we obtained:

$$\frac{\text{i}\left(\text{v}\right)}{{\text{v}}^{\frac{1}{2}}}={\text{K}}_1{\text{v}}^{\frac{1}{2}}+{\text{K}}_2$$ 12

Therefore, the relationship between between i(ν) and ${\mathrm{v}}^{\frac{1}{2}}$ is expected to be linear. The slope represents k₁, and the y-intercept represents k₂. Repeating these steps for other potentials and scan rates allows us to map out the slow-kinetic and fast-kinetic contributions to capacitance.

Energy efficiency (η_E) and Coulombic efficiency (η_C)

Energy efficiency (η_E) is calculated using the following relation:

$${\eta }_{\text{E}}=\frac{{\text{E}}_{\text{discharge}}}{{\text{E}}_{\text{charge}}}\times 100\%$$ 13

To measure energy efficiency, the energy charged (E_charge) and energy discharged (E_discharge) are calculated using the integral of the voltage (V) over the charge and discharge time (t):

$$\text{E}={\int }_0^{\text{t}}\text{VI}\mathrm{dt}$$ 14

Where I is the current.

Coulombic efficiency (η_C) can be expressed as a percentage:

$${\eta }_{\text{C}}=\frac{{\text{Q}}_{\text{discharge}}}{{\text{Q}}_{\text{charge}}}\times 100\%$$ 15

where Q_charge and Q_discharge are the charges during the charging and discharging processes, respectively.

the total charge (Q) transferred is calculated using:

$$\text{Q}={\int }_0^{\text{t}}\text{I}\mathrm{dt}$$ 16

Characterization

All the electrochemical measurements were conducted on a CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd.). The morphology of the composites was observed using a Verios G4 UC field emission scanning electron microscope (Thermo Scientific, US) and Talos F200X G2 transmission electron microscopy (Thermo Scientific, US). Fourier transform infrared spectra of the samples were taken by a JASCO spectrophotometer between 450 and 4000 cm⁻¹ from KBr pellets. Raman spectra were performed at room temperature with an inVia Reflex Confocal Raman Microscope (Renishaw, UK) and an argon ion laser operating at a wavelength of 532 nm as the excitation. The X-ray photoelectron spectroscopy was collected on the Thermo escalab 250Xi spectrometer (Thermo Scientific, US) using an Al K_α source with a takeoff angle of 45° from the surface plane. The specific surface area and pore size distributions were tested using the Brunauer–Emmett–Teller (BET) surface area analyzer (AUTOSORB-iQ, U.S.). Rheology measurements were carried out by using an MCR 302 Rheometer (Anton Paar, Austria). Electrical conductivity was measured on a compressed bulk piece by using an RST-9 double electric four-probe tester (Guangzhou four-point electronic technology, China).

Author contributions

J.F.W. conceived the concept, J.F.W., Y.A.G., and H.B. designed the experiments, supervised the research, and wrote the paper. L.L. and Z.T.A. performed the experiments, collected and analyzed the data, and wrote the paper. Z.W.L. collected rheological and conductivity data. M.Y.H. analyzed the experimental data. All authors discussed the results and commented on the manuscript.

Peer review

Peer review information

Nature Communications thanks Nilesh Chodankar, MinHo Yang, and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data that support the findings and conclusions of this study are available in the Article and Supplementary Information file. Additional information with relevance is available from the corresponding authors upon reasonable request. The source data underlying Figs. 1g, h, 3a–g, 4a–f, and 5b–h are provided as a Source Data file. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-50831-x.