MnO₂ Nanowires with Sub‐10 nm Thick Conjugated Microporous Polymers as Synergistic Triboelectric Materials

Abstract

MnO₂ nanowires coated with conjugated microporous polymers (CMP) are applied as triboelectric energy harvesting materials. The tribopositive performance of the CMP shells is enhanced with the assistance of MnO₂ nanowires (MnO₂ NW), likely due to cationic charge transfer from the tribopositive CMP layers to the surface Mn²⁺ and Mn³⁺ species of MnO₂ NW. This is supported by model studies. The MnO₂@CMP‐2 with sub‐10 nm thick CMP layers shows promising triboelectric output voltages up to 576 V and a maximum power density of 1.31 mW cm⁻². Spring‐assisted triboelectric nanogenerators fabricated with MnO₂@CMP‐2/PVP‐3 films are used as power supplies to operate electronic devices.

MnO₂@CMP nanowires show promising triboelectric performance. The possible cationic charge transfer from tribopositive CMP shells to the surface Mn²⁺/Mn³⁺ species of MnO₂ nanowires is proposed as a mechanistic principle. TENGs fabricated with a MnO₂@CMP‐2/PVP‐3 film show the output peak‐to‐peak voltage of 576 V and a maximum power density of 1.31 mW cm⁻².

1. Introduction

Over the past decade, research on small‐scale energy harvesting and utilization has gained considerable significance.^{[ 1 ]} For instance, triboelectric energy in everyday life can be harvested and utilized for various purposes.^{[ 2 ]} Since the Wang group developed triboelectric nanogenerators (TENGs),^{[ 3 ]} extensive research has focused on fabricating new devices.^{[ 4 ]} In addition, to improve the efficiency of TENGs, extensive studies have been conducted on triboelectric materials.^{[ 5 ]}

As triboelectric energy harvesting materials, various organic polymers have been studied.^{[ 6 ]} In addition, the surface areas and electronic properties of polymers have been tuned to enhance their triboelectric performance.^{[ 7} , ⁸ , ^{9 ]} For example, microporous organic materials with high surface areas have been studied for engineering TENGs.^{[ 7} , ^{8 ]} As a class of microporous organic materials, conjugated microporous polymers (CMPs) have been prepared by the coupling of organic building blocks.^{[ 10 ]} Recently, our research group reported that CMPs can be used as promising tribopositive materials for harvesting triboelectric energy.^{[ 11 ]}

On the other hand, porous organic polymer‐inorganic composites have been studied to achieve enhanced triboelectric performance.^{[ 12 ]} The triboelectrification performance of organic polymers can be enhanced with the assistance of inorganic nanomaterials. While organic polymer materials offer advantages in facile chemical engineering, inorganic nanomaterials can provide additional benefits such as facilitating redox reactions.

Recently, MnO₂ nanomaterials have been utilized as energy storage materials in pseudocapacitors.^{[ 13 ]} The morphology of MnO₂‐based nanomaterials has been engineered into nanowires. Usually, the surface of MnO₂ nanowires has an amorphous character and consists of Mn²⁺ and Mn³⁺ species,^{[ 14 ]} which can be further oxidized to Mn⁴⁺ species by donating electrons.

It can be speculated that core–shell MnO₂@CMP nanowires can be engineered to enhance the tribopositive performance of CMP materials with the assistance of surface Mn²⁺ and Mn³⁺ species in the inner MnO₂ nanowires. When preparing MnO₂@CMP nanowires, we observed the facile generation of static electricity in a Falcon plastic tube (Figure S1 and Movie S1, Supporting Information). In this work, we report the preparation of MnO₂@CMP nanowires and their enhanced triboelectric performance, compared to the corresponding MnO₂ and CMP materials.

2. Results and Discussion

Figure 1 displays the synthetic scheme of MnO₂@CMP nanowires.

Figure 1.

Synthesis of MnO₂@CMP nanowires and HCMP nanowires (HCMP NW).

First, MnO₂ nanowires (MnO₂ NW) were prepared by reacting KMnO₄ with H₂O₂ in acetic acid.^{[ 15 ]} Through the Sonogashira coupling of 1,3,5‐triethynylbenzene with 1.5 eq. 1,4‐diiodobenzene in the presence of MnO₂ NW, CMP layers were formed on the MnO₂ NW. With a fixed amount of MnO₂ NW (0.20 g), the amount of 1,3,5‐triethynylbenzene was gradually increased from 0.10 to 0.20 and 0.40 mmol to form three different MnO₂@CMP nanowires (denoted as MnO₂@CMP‐1, MnO₂@CMP‐2, and MnO₂@CMP‐3, respectively). As control materials, the inner MnO₂ of MnO₂@CMP‐2 was etched by treating with HCl to form hollow CMP nanowires (HCMP NW).

The morphologies of materials were examined using scanning (SEM) and transmission electron microscopy (TEM) (Figure 2 ). The SEM and TEM images of MnO₂ NW revealed long nanowires with a thickness of 15–25 nm and a length of 2–5 µm (Figure 2a–c). While MnO₂@CMPs retained wire‐like morphologies (Figure 2d,e,g,h,j,k), a closer inspection revealed a thin coating of CMP on the surface of MnO₂ NW (Figure 2f,i,l). The coating thicknesses of CMPs in MnO₂@CMP‐1, MnO₂@CMP‐2, and MnO₂@CMP‐3 were measured to be 3.5 ± 0.4, 6.9 ± 0.7, and 11.4 ± 0.7 nm, respectively (Figure S2, Supporting Information). Whilst the SEM images of HCMP NW showed wire‐like morphologies, the empty inner space could be confirmed by TEM analysis (Figure 2m–o).

Figure 2.

SEM and TEM images of (a,b,c) MnO₂ NW, (d,e,f) MnO₂@CMP‐1, (g,h,i) MnO₂@CMP‐2, (j,k,l) MnO₂@CMP‐3, and (m,n,o) HCMP NW.

The chemical and physical features of materials were investigated using various analytical techniques (Figure 3 ). Powder X‐ray diffraction (PXRD) studies on MnO₂ NW and MnO₂@CMPs showed diffraction peaks at 12.5°, 17.9°, 28.6°, 37.5°, 41.6°, 50.0°, 60.3°, and 69.6°, corresponding to the (110), (200), (310), (211), (301), (411), (521), and (541) crystalline planes of α‐MnO₂ (JCPDS# 44–141) (Figure 3a).^{[ 16 ]} In comparison, HCMP NW exhibited an amorphous feature, which is a conventional characteristic of CMPs in the literature.^{[ 17 ]}

Figure 3.

a) PXRD patterns, b) N₂ adsorption–desorption isotherm curves obtained at 77 K and pore size distribution diagrams based on the NL‐DFT method, c) IR spectra of MnO₂ NW, MnO₂@CMPs, and HCMP NW. d) A solid‐state ¹³C NMR spectrum of HCMP NW.

N₂ sorption studies were conducted to investigate the surface areas and porosity of materials (Figure 3b). The analysis of N₂ sorption isotherm curves based on the Brunauer–Emmett–Teller (BET) theory indicated that the MnO₂ NW has a surface area of 54 m² g⁻¹ and poor porosity. In comparison, the HCMP NW showed a high surface area of 310 m² g⁻¹ and microporosity (pore sizes < 2 nm) with a total pore volume (V_t) of 0.46 cm³ g⁻¹. As the amount of CMPs increased in the MnO₂@CMPs, the surface areas gradually increased from 85 m² g⁻¹ (MnO₂@CMP‐1) to 138 (MnO₂@CMP‐2) and 207 m² g⁻¹ (MnO₂@CMP‐3) with the increase of V_t from 0.11 cm³ g⁻¹ to 0.12 and 0.13 cm³ g⁻¹, respectively.

The infrared (IR) absorption spectra of MnO₂ NW and MnO₂@CMPs revealed broad peaks at 460–520 cm⁻¹, corresponding to Mn─O vibrations (Figure 3c).^{[ 18 ]} In comparison, HCMP NW showed aromatic C═C and C─H vibration peaks at 1581 and 837 cm⁻¹, respectively, in addition to the C─O vibration at 1010–1105 cm⁻¹, which was generated through the oxidation of CMP by MnO₂ at the interfaces.^{[ 19 ]} As expected, as the amount of CMP increased in MnO₂@CMPs, the corresponding vibration peaks of CMPs gradually increased (indicated by asterisks in Figure 3c). A solid‐state ¹³C nuclear magnetic resonance (NMR) spectrum of HCMP NW exhibited alkyne and aromatic carbon peaks at 91 and 124–138 ppm, respectively, consistent with the CMPs reported in the literature (Figure 3d).^{[ 20 ]}

The high‐resolution (HR)‐TEM analysis of MnO₂ NW and MnO₂@CMP‐2 showed a crystalline inner part showing the (200) crystal plane of α‐MnO₂ with an interlayer distance of 0.49 nm (Figure 4a,b).^{[ 21 ]} The surface (a depth of ≈1.8 nm) of MnO₂ NW displayed amorphous and defective features (indicated by dotted lines in Figure 4a,b). The MnO₂@CMP‐2 showed the additional coating of amorphous CMP materials on the MnO₂ NW (Figure 4b).

Figure 4.

HR‐TEM images of a) MnO₂ NW and b) MnO₂@CMP‐2. EDS elemental mapping images of c) MnO₂ NW and d) MnO₂@CMP‐2. e,f) XPS Mn 2p and O 1s orbital peaks of MnO₂ NW, MnO₂@CMPs, and HCMP NW with normalized intensities (Refer to Figure S3, Supporting Information for unnormalized ones).

Energy dispersive X‐ray spectroscopy (EDS)‐based elemental mapping studies of MnO₂ NW showed homogeneous distributions of Mn and O elements (Figure 4c). In comparison, MnO₂@CMP‐2 showed a homogeneous distribution of carbon elements on the surface of MnO₂ NW, indicating a successful coating of CMP (Figure 4d).

The chemical surroundings of materials were investigated by X‐ray photoelectron spectroscopy (XPS). The XPS Mn 2p_3/2 and 2p_1/2 orbital peaks of MnO₂ NW and MnO₂@CMPs were analyzed into three sets (Figure 4e; Figures S3 and S4, Supporting Information). Whilst the Mn 2p_3/2 and 2p_1/2 orbital peaks of Mn²⁺ species appeared at 641.0 and 652.7 eV, respectively, those of Mn³⁺ species appeared at 641.8 and 653.5 eV, respectively.^{[ 22 ]} In comparison, those of Mn⁴⁺ species were observed at 643.7 and 655.4 eV, respectively.^{[ 22 ]} The ratios of Mn⁴⁺/(Mn²⁺+Mn³⁺) of MnO₂ NW, MnO₂@CMP‐1, MnO₂@CMP‐2, and MnO₂@CMP‐3 were analyzed to be 0.31, 0.38, 0.42, and 0.44, respectively. These observations indicate that the surface of MnO₂ NW consists of defective Mn²⁺ and Mn³⁺ species. The O 1s orbital spectrum of MnO₂ NW showed two peaks at 529.3 and 530.3 eV, corresponding to the Mn─O─Mn and Mn─OH species, respectively (Figure 4f; Figures S3 and S5, Supporting Information).^{[ 23 ]} The O 1s orbital spectra of MnO₂@CMPs and HCMP NW showed additional peaks at 531.9 and 533.2 eV, corresponding to C─O species that were generated through oxidation of CMP by MnO₂ at the interfaces.^{[ 24 ]}

To study triboelectric performance, the films of materials were fabricated using polyvinylpyrrolidone (PVP) as a matrix (Figure 5a and refer to Experimental Section in the Supporting Information). First, five MnO₂ NW/PVP films containing 1, 3, 5, 7, and 10 wt.% MnO₂ NW were fabricated (corresponding films were denoted as MnO₂ NW/PVP‐1–5, respectively). As the amount of MnO₂ NW increased, the brown color of the films became more intense (Figure 5a). According to thermogravimetric analysis (TGA), the content of MnO₂ NW in MnO₂@CMP‐2 was analyzed to be 72 wt.% (Figure S6, Supporting Information). Considering the information, five brown MnO₂@CMP‐2/PVP films containing 1.4, 4.2, 6.9, 9.7, and 13.9 wt.% MnO₂@CMP‐2 (corresponding to 1, 3, 5, 7, and 10 wt.% MnO₂ NW and 0.4, 1.2, 1.9, 2.7, and 3.9 wt.% CMP‐2, respectively) were fabricated and denoted as MnO₂@CMP‐2/PVP‐1–5, respectively. In addition, five pale yellow HCMP NW/PVP films containing 0.4, 1.2, 1.9, 2.7, and 3.9 wt.% HCMP NW were fabricated and denoted as HCMP NW/PVP‐1–5, respectively.

Figure 5.

a) Photographs of MnO₂ NW/PVP‐1–5, HCMP NW/PVP‐1–5, and MnO₂@CMP‐2/PVP‐1–5 films (scale bars: 1 cm). Triboelectric output voltages of b) MnO₂ NW/PVP‐1–5, c) HCMP NW/PVP‐1–5, d) MnO₂@CMP‐2/PVP‐1–5 films. e) Comparative display of the output voltages of PVP, MnO₂ NW/PVP‐3, MnO₂@CMP‐2/PVP‐3, and HCMP NW/PVP‐3 films (a working area of 2 cm × 2 cm, pushing force of 2 kgf, pushing frequency of 0.73 Hz, RH 50%, PFA as a tribonegative material. Refer to Figure S11 (Supporting Information) for the output currents of corresponding films.

While the top view SEM images of MnO₂ NW/PVP‐1–2, MnO₂@CMP‐2/PVP‐1–2, and HCMP NW/PVP‐1–2 films exhibited flat surfaces, MnO₂ NW, MnO₂@CMP‐2, and HCMP materials were detected in the MnO₂ NW/PVP‐3–5, MnO₂@CMP‐2/PVP‐3–5, and HCMP NW/PVP‐3–5 films (Figure S7, Supporting Information). The thicknesses of MnO₂ NW/PVP, MnO₂@CMP‐2/PVP, and HCMP NW/PVP films were measured to be 25 µm (Figure S8, Supporting Information). The IR spectra of films showed exclusively the C─H, C═O, C─N, and C─O vibration peaks of the PVP matrix at 2891–2957, 1659, 1450, and 1283 cm⁻¹, respectively,^{[ 25 ]} due to the relatively small amount of MnO₂ NW, MnO₂@CMP‐2, and HCMP in the films (Figure S9, Supporting Information). While the PXRD patterns of MnO₂ NW/PVP‐1, MnO₂@CMP‐2/PVP‐1, and HCMP NW/PVP‐1–5 films showed broad diffraction peaks at 2θ of 10.3° and 20.7°, corresponding to the PVP matrix,^{[ 26 ]} those of MnO₂ NW/PVP‐2–5 and MnO₂@CMP‐2/PVP‐2–5 films revealed the original diffraction peaks of α‐MnO₂ (Figure S10, Supporting Information).

The triboelectric performance of MnO₂ NW/PVP, MnO₂@CMP‐2/PVP, and HCMP/PVP films with an area of 2 cm × 2 cm was studied (Figure 5b–e; Figure S11, Supporting Information). As a tribonegative material, perfluoroalkoxy alkanes (PFA) were used.^{[ 27 ]} In the case of pristine PVP and MnO₂ NW/PVP films, as the amount of MnO₂ NW increased, the output peak‐to‐peak voltages (V_p‐p) gradually increased from 234 V (PVP) to 302 (MnO₂ NW/PVP‐1), 329 (MnO₂ NW/PVP‐2), and 365 V (MnO₂ NW/PVP‐3), respectively (Figure 5b). The corresponding output currents (I_p‐p) increased from 16.8 µA (PVP) to 21.1 (MnO₂ NW/PVP‐1), 24.3 (MnO₂ NW/PVP‐2), and 26.0 µA (MnO₂ NW/PVP‐3), respectively (Figure S11, Supporting Information). Then, the V_p‐p of MnO₂ NW/PVP‐4 and MnO₂ NW/PVP‐5 films decreased to 331 and 265 V, respectively, with a decrease of I_p‐p to 24.1 and 19.6 µA. In the case of HCMP NW/PVP films, as the amount of HCMP NW increased, V_p‐p gradually increased from 339 V (HCMP NW/PVP‐1) to 385 (HCMP/PVP‐2) and 456 V (HCMP/PVP‐3) with an increase of I_p‐p from 23.8 to 25.9 and 31.7 µA (Figure 5c; Figure S11, Supporting Information). Then, the V_p‐p of HCMP NW/PVP‐4 and HCMP NW/PVP‐5 films decreased to 359 and 306 V, respectively, with a decrease of I_p‐p to 25.8 and 21.4 µA.

In comparison, as the amount of MnO₂@CMP‐2 increased, V_p‐p significantly increased from 375 V (MnO₂@CMP‐2/PVP‐1) to 507 (MnO₂@CMP‐2/PVP‐2) and 576 V (MnO₂@CMP‐2/PVP‐3) with an increase of I_p‐p from 25.9 to 36.1 and 39.6 µA (Figure 5d; Figure S11, Supporting Information). Then, the V_p‐p of MnO₂@CMP‐2/PVP‐4 and MnO₂@CMP‐2/PVP‐5 films decreased to 359 and 306 V, respectively, with a decrease of I_p‐p to 36.1 and 24.4 µA. These observations indicated that the MnO₂ NW/PVP‐3, HCMP NW/PVP‐3, and MnO₂@CMP‐2/PVP‐3 are the best films. Among these films, the triboelectric performance was gradually enhanced in the order of MnO₂ NW/PVP‐3 < HCMP NW/PVP‐3 < MnO₂@CMP‐2/PVP‐3 (Figure 5e).

According to Kelvin probe force microscopy (KPFM), the surface potentials gradually increased from 341 mV (PVP) to 387 (MnO₂ NW/PVP‐3), 446 (HCMP NW/PVP‐3), and 520 mV (MnO₂@CMP‐2/PVP‐3), matching with the observed trend of the tribopositive performance of films (Figure 6a). The mechanism of electricity generation through triboelectrification has been reported in the literature (Figure 6b).^{[ 28 ]} At the contact of the pressed two films, electrons transfer from the tribopositive MnO₂@CMP‐2/PVP‐3 film to the tribonegative PFA film. In the releasing state, electrons flow from the supporting metal electrode of a PFA film to the supporting metal electrode of an MnO₂@CMP‐2/PVP‐3 film, reaching an equilibrium state. During the re‐pushing state of two films, electrons flow back from the supporting metal electrode of an MnO₂@CMP‐2/PVP‐3 film to the supporting metal electrode of a PFA film. This process was repeated in the pushing/releasing cycles.

Figure 6.

a) KPFM images of PVP, MnO₂ NW/PVP‐3, HCMP NW/PVP‐3, and MnO₂@CMP‐2/PVP‐3 films. b) A mechanism of the triboelectric energy harvesting process by a tribopositive a MnO₂@CMP‐2/PVP film and a tribonegative PFA film. c) A suggested mechanism of the MnO₂ NW‐assisted tribopositive performance of MnO₂@CMP‐2.

We propose the following mechanistic principle for the enhanced tribopositive performance of MnO₂@CMP‐2 (Figure 6c). When the PFA film is in contact with MnO₂@CMP‐2, electrons transfer from the tribopositive CMP to the PFA. The surface Mn²⁺ (indicated as red balls in Figure 6c) and Mn³⁺ (indicated as orange balls in Figure 6c) species can be converted to Mn⁴⁺ (indicated as blue balls in Figure 6c) through electron transfer to the CMP layers. With the assistance of the surface Mn²⁺ and Mn³⁺ species in MnO₂ NW, the CMP layers can further enhance their role as tribopositive materials.

To investigate the possible redox behaviors of the surface Mn²⁺ and Mn³⁺ species of MnO₂ NW, we conducted the following model studies (Figure 7 ; Figures S12–S14, Supporting Information). Because PFA is a polymeric material and the generation of cationic charges on CMP through triboelectrification is an instantaneous event, XPS studies on the changes of the surface Mn²⁺ and Mn³⁺ species of MnO₂ NW are technically limited. Thus, we treated MnO₂ NW with tetracyanoquinone (TCNQ), as an electron‐deficient model compound,^{[ 29 ]} and trityl tetrafluoroborate (TritylBF₄),^{[ 30 ]} as a model carbocation (Figure 7a).

Figure 7.

a) Model reactions of MnO₂ NW with tetracyanoquinone (TCNQ) and trityl tetrafluoroborate (TritylBF₄). XPS b) Mn 2p_3/2 and c) O 1s orbital spectra of MnO₂ NW with normalized intensities before and after treatment of MnO₂ NW with TCNQ and TritylBF₄ (Refer to Figures S12 and S13, Supporting Information for detailed analysis).

When the MnO₂ NW was treated with TCNQ and TritylBF₄, the XPS Mn 2p orbital peaks significantly shifted to the higher energy region by 0.21 and 0.35 eV, respectively, indicating the conversion of the surface Mn²⁺ and Mn³⁺ species to Mn³⁺ and Mn⁴⁺ species. The detailed analysis indicated an increase in the Mn⁴⁺/(Mn²⁺ + Mn³⁺) values from 0.31 to 0.42 and 0.43 after the treatment of the MnO₂ NW with TCNQ and TritylBF₄, respectively (Figure 7b; Figure S12, Supporting Information). In addition, the O 1s orbital peaks of Mn─O─Mn species also shifted to the higher energy region by 0.18 and 0.32 eV, respectively, indicating the increased oxidation state of Mn species (Figure 7c; Figure S13, Supporting Information). Electron paramagnetic resonance (EPR) spectra confirmed the generation of anionic TCNQ radicals and trityl radicals (Figure S14, Supporting Information). These observations indicate that the surface Mn²⁺ and Mn³⁺ species of MnO₂ NW can be converted to Mn³⁺ and Mn⁴⁺ species through the interaction with electron‐deficient materials and carbocation species.

The thickness effect of CMP layers on the triboelectric performance of MnO₂@CMPs was investigated (Figure 8a,b; Figure S15, Supporting Information). The MnO₂@CMP‐1 with 3.5 ± 0.4 nm thick CMP layers showed slightly better triboelectric performance with V_p‐p of 590 V and I_p‐p of 40.8 µA than the MnO₂@CMP‐2 with 6.9 ± 0.7 nm thick CMP layers displaying V_p‐p of 576 V and I_p‐p of 39.6 µA. In comparison, the MnO₂@CMP‐3 with 11.4 ± 0.7 nm thick CMP layers showed significantly lower V_p‐p of 505 V and I_p‐p of 34.2 µA. These results indicate that the thickness of CMP layers should be sufficiently thin at a sub‐10 nm scale for efficient charge transfer to the inner MnO₂ NW during the triboelectrification process.

Figure 8.

a,b) CMP layers thickness‐dependent output voltages and currents of MnO₂@CMP/PVP films (Refer to Figure S15, Supporting Information for more studies). c) Pushing force and d) pushing frequency‐dependent output voltages and e) the retention of output voltages of MnO₂@CMP‐2/PVP‐3 film (standard working conditions: a working area of 2 cm × 2 cm, a pushing force of 2 kgf, a pushing frequency of 0.73 Hz, RH 50%). Refer to Figures S16 and S17, Supporting Information for the output currents of corresponding films.

The working condition‐dependent triboelectric performance of the MnO₂@CMP‐2/PVP‐3 was investigated (Figure 8c,d; Figures S16 and S17, Supporting Information). At relative humidities (RH) of 30% and 50%, the MnO₂@CMP‐2/PVP‐3 exhibited similar triboelectric performance with V_p‐p of 554 and 576 V, respectively, and I_p‐p of 39.2 and 39.6 µA, respectively (Figure S16, Supporting Information). When RH increased to 80%, the V_p‐p and I_p‐p significantly dropped to 310 V and 28.7 µA, respectively.

The triboelectric performance of MnO₂@CMP‐2/PVP‐3 was sensitive to the pushing force (Figure 8c; Figure S17, Supporting Information). As the pushing forces increased from 0.5 to 1, 1.5, 2, and 2.5 kgf, the V_p‐p increased from 322 to 363, 431, 576, and 600 V, respectively, with an increase of I_p‐p from 22.5 to 24.6, 30.3, 39.6, and 42.0 µA. In contrast, the MnO₂@CMP‐2/PVP‐3 exhibited similar triboelectric performance across a pushing frequency range of 0.23–2.57 Hz, maintaining V_p‐p of 575–576 V and I_p‐p of 39.6–39.8 µA (Figure 8d; Figure S17, Supporting Information).

The durability of MnO₂@CMP‐2/PVP‐3 was studied through cycling tests (Figure 8e). At the optimized working conditions (a pushing force of 2 kgf, a pushing frequency of 0.73 Hz, RH 50%), the MnO₂@CMP‐2/PVP‐3 maintained the original triboelectric performance in the range of 98.8–100% over 21 000 cycling tests.

Resistance‐dependent current and power densities generated by triboelectric MnO₂@CMP‐2/PVP‐3 films were measured, showing a maximum power density (P_max) of 1.31 mW cm⁻² at a resistance of 5 × 10⁷ Ω (Figure 9a). Very recently, microporous organic materials such as covalent organic frameworks (COFs) and CMPs have been applied as triboelectric materials, showing V_p‐p of 40–815 V and P_max of 0.364 µW cm⁻²–0.824 mW cm⁻² (Table S1, Supporting Information).^{[ 8} , ^{11 ]} In addition, metal–organic framework (MOF)‐based triboelectric materials showed the V_p‐p of 62–658 V and P_max of 0.968 µW cm⁻²–0.508 mW cm⁻² (Table S2, Supporting Information).^{[ 9 ]} In this regard, the triboelectric performance of the MnO₂@CMP‐2/PVP‐3 film, showing V_p‐p of 576 V and P_max up to 1.31 mW cm⁻², is quite promising.

Figure 9.

a) Resistance‐dependent current and power densities (a film area of 2 cm × 2 cm, a pushing force of 2 kgf) and b) charged voltages of electrolytic capacitors by the triboelectric performance of a MnO₂@CMP‐2/PVP‐3 film. c) An illustration and d) a photograph of an S‐TENG fabricated using MnO₂@CMP‐2/PVP‐3 and PFA films. e,f,g) Demonstration of S‐TENGs as power sources to turn on 100 green LED bulbs and to operate an electronic calculator (working conditions: a film area of 4 cm × 4 cm, a pushing force of 2 kgf, a pushing frequency of 0.73 Hz, RH 50%).

The triboelectric MnO₂@CMP‐2/PVP‐3 film could be utilized to charge electrolytic capacitors (Figure 9b). After 5 min, voltages of 50, 32, 15, and 5 V were achieved for 4.7, 10, 33, and 100 µF capacitors, respectively. Spring‐assisted triboelectric nanogenerators (S‐TENGs) were fabricated using MnO₂@CMP‐2/PVP‐3 and PFA films (Figure 9c,d). It was confirmed that the S‐TENG could work as a power supply to illuminate 100 green LED bulbs (Figure 9e; Movie S2, Supporting Information). Moreover, after a battery was removed from an electronic calculator, the S‐TENG was connected as a power supply. A capacitor charged to 6 V for 300 s using the S‐TENG could be used to operate a calculator (Figure 9f,g; Movie S3, Supporting Information).

3. Conclusion

In conclusion, this work shows that the triboelectric performance of CMP materials can be significantly enhanced with the assistance of MnO₂ NW. As a cooperative mechanistic principle for the triboelectrification of MnO₂@CMP, the possible redox role of the surface Mn²⁺ and Mn³⁺ species of MnO₂ NW was suggested. Especially, the CMP shell thickness of MnO₂@CMP was also critical and should be a sub‐10 nm scale to ensure efficient tribopositive materials. Model studies supported the possible electron transfer from the surface Mn²⁺ and Mn³⁺ species of MnO₂ NW to organic materials. Based on the observed results of this work, we believe that various redox‐active inorganic material@CMP systems can be developed as effective triboelectric materials.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Supplemental Movie 1

Supplemental Movie 2

Supplemental Movie 3

Jung H., Lee D.‐M., Park J., Kim T., Kim S.‐W., Son S. U., MnO₂ Nanowires with Sub‐10 nm Thick Conjugated Microporous Polymers as Synergistic Triboelectric Materials. Adv. Sci. 2024, 11, 2409917. 10.1002/advs.202409917

Data Availability Statement

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