A new class of flexible nanogenerators consisting of porous aerogel films driven by mechanoradicals

Author’s Accepted Manuscript A New Class of Flexible Nanogenerators Consisting of Porous Aerogel Films Driven by Mechanoradicals Yanfeng Tang, Qifeng Zheng, Bo Chen, Zhenqiang Ma, Shaoqin Gong www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(17)30376-2 http://dx.doi.org/10.1016/j.nanoen.2017.06.022 NANOEN2028

To appear in: Nano Energy Received date: 17 April 2017 Revised date: 6 June 2017 Accepted date: 9 June 2017 Cite this article as: Yanfeng Tang, Qifeng Zheng, Bo Chen, Zhenqiang Ma and Shaoqin Gong, A New Class of Flexible Nanogenerators Consisting of Porous Aerogel Films Driven by Mechanoradicals, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.06.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A New Class of Flexible Nanogenerators Consisting of Porous Aerogel Films Driven by Mechanoradicals Yanfeng Tanga,b,1, Qifeng Zhengb,c,1, Bo Chenb , Zhenqiang Mac,d , Shaoqin Gongb,c,e,*

a

School of Chemistry and Chemical Engineering, Nantong University, Nantong 226019, PR China. b

c

Wisconsin Institute for Discovery, University of Wisconsin–Madison, WI 53715, USA

Department of Material Science and Engineering, University of Wisconsin–Madison, WI 53706, USA.

d

Department of Electrical Engineering, University of Wisconsin–Madison, WI 53706, USA e

Department of Biomedical Engineering and Department of Chemistry, University of Wisconsin–Madison, WI 53706, USA

* Corresponding author. Tel: +01 6083164311. [email protected]

Abstract Flexible nanogenerators (NGs) that are capable of harvesting ubiquitous mechanical energy from ambient environments have attracted significant attention during the past decade.

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These authors contributed equally to this work.

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Herein, a simple and scalable technique has been demonstrated to fabricate a new class of high-performance flexible compact triboelectric NGs using porous aerogel films. These porous aerogel film-based NGs can exhibit significant electric outputs without the use of traditional piezoelectric materials or traditional triboelectric assembly (i.e., the need of airgap). To explain the high electric outputs generated from the porous aerogel film-based generators, a mechanoradical-based mechanism was proposed and a series of systematic studies were carried out to substantiate this new mechanism. These systematic studies have demonstrated that high-performance flexible NGs can be made from porous mechanoradicalgenerating polymer films. The outstanding electric outputs from this new family of compact triboelectric NGs can be attributed to the reversible and transient mechanoradicals resulting from bond breaking of polymer chain, thus leading to a significant amount of transient dipole moments, as well as the permanent electric dipole moments possessed by the mechanoradicalinduced polar groups. The elucidation of the potential mechanisms for this family of porous mechanoradical-generating polymers will lead to a new class of energy harvesting materials and high-performance, low-cost, and flexible energy generation devices.

Graphical Abstract

A new class of high-performance flexible compact triboelectric nanogenerators using porous aerogel films that exhibit significant electric outputs without the use of traditional piezoelectric materials or the need of airgap has been demonstrated.

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Keywords: Porous aerogel film, compact triboelectric nanogenerators, mechanoradicals, energy harvesting

1. Introduction With increasing consumption of non-renewable fossil energy, mankind is facing an unprecedented energy crisis. Thus, various devices capable of harvesting energy from sustainable resources such as light, heat, and mechanical vibrations have been extensively investigated during the last several decades [1-5]. Among them, piezoelectric and triboelectric nanogenerators (NGs) that are capable of effectively harvesting ubiquitous mechanical energy from the natural environment and convert it to electric energy have attracted a great deal of attention [6-7]. Since 2006, ZnO nanowire-based piezoelectric NGs have been widely investigated to covert mechanical energy to electrical energy. Recently, various perovskite materials such as PbZrxTi1-xO3 (PZT) [7-9], BaTiO3 [10], and Pb(Zr, Ti)O3 [11] that possess high piezoelectric constants have been proven capable of enhancing the output of the piezoelectric NG. However, these NGs normally suffer from inferior flexibility. In contrast, organic piezoelectric polymers such as PVDF and its copolymers usually have lower piezoelectric coefficients than inorganic ones, but they are more flexible and easier to process, particularly for large area devices [12-13]. These traditional piezoelectric materials, including both organic and inorganic materials, generally have a crystalline structure. Their piezoelectricity

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originates from the non-centrosymmetry of their crystalline structure [10]. The electric poling step is essential for these traditional piezoelectric materials in order to align the electric dipoles to achieve high piezoelectric outputs [4]. These piezoelectric NGs normally require sophisticated nanowires or nanotubes growth procedures, as well as electric poling step. Furthermore, the utilization of expensive row materials, the limitation of the output performance, and the lack of the flexibility have limited the wide-spread application of the piezoelectric NGs. The triboelectric NGs, first presented in 2012, have been rising as a novel and promising technology for energy harvesting due to their higher output performance and lower cost when compared to conventional piezoelectric NGs [14-15]. The output power density of the triboelectric NGs has been improved to milliwatt per cm2 level, which is sufficient to be used in a wide range of applications such as powering small electronics, charging energy storage devices, and driving chemical reactions [16]. Triboelectric NGs based on the combination effect of contact electrification and electrostatic induction have been studied and explained clearly in previous studies [14, 16-17]. To be specific, two dissimilar materials with different electron affinities will be oppositely charged upon contact; the subsequent separation of the two charged materials will induce a potential difference, driving the flow of electrons through an external load [14, 16-17]. This charge separation is the primary principle and is essential for triboelectric energy harvesting, which is usually achieved by introducing an airgap between these two dissimilar materials. Without an airgap, the resulting triboelectric NGs only exhibit very low output [15, 18]. However, the presence of the airgap not only requires a complicated fabrication and packaging process, but also leads to unfavorable durability and stability, which may limit their large-scale manufacturing for practical applications [19]. Thus, there is still an ongoing demand for novel NGs without the use of traditional expensive

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piezoelectric materials or traditional triboelectric assembly (i.e., the need of airgap) while providing high output performance and excellent flexibility. Herein, we report a new class of high-performance flexible compact triboelectric NGs using porous aerogel films that exhibit significant electric outputs without the use of traditional piezoelectric materials or the need of airgap. Carboxymethyl cellulose (CMC), a non-piezoelectric material, was used to prepare the porous aerogel films (i.e., porous CMC/poly(dimethyl siloxane) (PDMS) aerogel films) based NGs without any airgap (compact triboelectric NGs, Figure 1g). The porous CMC/PDMS aerogel film was first engineered by coating a compressed porous CMC aerogel film with a thin layer of PDMS, and then sandwiched between two thin PDMS films, followed by two aluminum foils, to form the flexible compact triboelectirc NG. For comparison purpose, traditional triboelectric NG with an airgap of 3 mm between porous CMC/PDMS aerogel film and top aluminum electrode was also fabricated (Figure 1(h)). Under a periodic stress of 0.05 MPa at a frequency of 10 Hz, the resulting flexible porous CMC/PDMS aerogel film-based compact triboelectric NGs (1 cm  1.2 cm  540 m) delivered an open-circuit voltage (Voc) of 30.0 V and a short-circuit current (Isc) of 4.5 A, corresponding to a power density of 1.1 W/m2. We hypothesized that the remarkable electric outputs of the porous CMC/PDMS aerogel film was attributed to the mechanoradicals generated by the porous PDMS coated on the surface of the CMC aerogel film, which can lead to a change in the electric dipole moments and consequently enhance the electric outputs. To verify this hypothesis, we have systematically studied the effects of porosity, the amount of free radical scavenger present in the porous PDMS coating of the CMC aerogel film, and the different types of polymer coatings (e.g., PDMS, thermoplastic polyurethane (TPU), and a blend of poly(n-butyl acrylate) (PBA) and poly(n-butyl methylacrylate) (PBMA)) with varying degrees of ability in generating 5

mechanoradicals on the electric outputs of the CMC aerogel film-based NGs. Furthermore, the effects of different types of aerogel films (e.g., CMC, chitosan (CTS), and polyvinyl alcohol (PVA)) on the electric outputs of the flexible porous polymer films have also been studied. These studies supported our hypothesis; namely, flexible porous polymer films capable of efficiently generating mechanoradicals can lead to high performance flexible NGs without the need for electric poling and without the need of airgap. This new class of flexible porous polymers can lead to many potential applications. This study also provides a simple, low-cost, universally applicable method for fabricating high-performance, flexible NGs.

2. Experimental Section 2.1. Materials Sodium carboxymethyl cellulose (CMC, average Mw ~90 kDa), chitosan (Mw: 50–190 kDa), poly(butyl methacrylate) (PBMA, average Mw ~337 kDa), and poly(butyl acrylate) (PBA, average Mw ~99 kDa, 25–30 wt% in toluene) were purchased from Sigma–Aldrich. The material 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was purchased from Oakwood Chemical. PDMS was purchased from Dow Corning. All other chemicals were purchased from Sigma–Aldrich and used without further purification. 2.2. Preparation of compressed CMC aerogel film CMC (0.200 g) was dissolved in 20 mL of water in an aluminum pan (diameter: 5.80 cm). The resulting CMC solution was frozen in a dry ice–acetone solution at –78 oC. The CMC aerogel was obtained after a freeze-drying process as previously reported[20-21]. The CMC aerogel samples were compressed into aerogel films under a pressure of 0.15 MPa. 2.3. Fabrication of porous CMC/PDMS aerogel film-based NGs 6

A PDMS solution was prepared from a PDMS prepolymer, curing agent (Sygard 184, Dow Corning), and ethyl acetate at a ratio of 10:1:20. Then the compressed CMC aerogel was dipped into the PDMS solution using a vacuum-assisted liquid filling method to enable the solution to penetrate into the inner pores. The well-coated aerogel film was left in a hood for 2 h to vaporize the solvent and then cured at 100 oC for 1 h. The CMC/PDMS aerogel film was spin-coated on both sides with a PDMS prepolymer and curing agent mixture (without any solvent) at a 10:1 ratio. The sample was cured again at 100 o

C for 1 h. The resulting PDMS/(porous CMC/PDMS aerogel film)/PDMS sandwich-like tri-

layer film was cut into pieces (1 cm  1.2 cm in area, 540 m in thickness). Thereafter, aluminum foil was affixed to both sides of the tri-layer film sample to obtain the NG shown in Figure 1 (g). For comparison, NGs made of either pure PDMS film or pure CMC aerogel film were also prepared. Detailed information on the preparation of other types of aerogel films and related NGs may be found in the supporting document. 2.4. Characterization of the materials and NGs A scanning electron microscope (SEM, LEO GEMINI 1530) was used to study the microstructures of these samples after gold sputtering. Thermal stability measurements were carried out using a thermogravimetric analyzer (TGA, Q50 TA Instruments, USA) from 30 to 800 ℃ at a 10 ℃/min heating rate under N2 protection. The dynamic impact with controlled force and frequency was generated by an oscillator (LDS V201, Brüel & Kjær, Denmark). The electrical output signals of the NGs were recorded using an oscilloscope (DS1102E, Rigol, China) and a potentiostat (versaSTAT-3, Princeton Applied Research, USA). All tests described in the manuscript were done at least in triplicate and the most representative curves and results were reported.

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3. Results and Discussion The fabrication procedures of the CMC/PDMS aerogel film-based compact NG are shown in Figure 1 and Figure S1. Detailed information is described in the Experimental Section. The NG was composed of five layers, as schematically shown in Figure 1 (g). The outermost layers were individual sheets of aluminum foil serving as the top and bottom electrodes. The secondary outer layers were two individual pure/solid spin-coated PDMS films which can not only significantly enhance the robustness of the NG, but also enable the NG to efficiently harvest mechanical energy[22-24]. The innermost layer––i.e., the core of the NG with a sandwich structure––was prepared by coating a layer of PDMS (~90 µm) on the porous surface of the compressed CMC aerogel film. The porous CMC/PDMS aerogel film is the most important active component of the NG. The microstructure of the CMC aerogel is shown in Figure 2 (a). The CMC aerogel displayed a highly porous and interconnected structure with a porosity of 99% (Table S1). The microstructure of the compressed CMC aerogel film is shown in Figures 2 (b) and (c). The compressed CMC aerogel film still exhibited a highly porous structure (90% porosity, Table S1), although the pore sizes decreased somewhat. The compressed highly porous CMC aerogel film with interconnected pores can greatly facilitate the absorption of the PDMS prepolymer/ethyl acetate solution, thereby leading to a complete PDMS coating on the porous surface of the compressed CMC aerogel, as shown in the SEM image of the CMC/PDMS film (Figure 2 (d)). The porosity of the resulting porous CMC/PDMS aerogel film was 40% (Table S1). Thereafter, two pure/solid PDMS thin layers (~90 μm) were spin-casted on both sides of the CMC/PDMS aerogel film as shown in Figure 2 (e). As shown in Figure 2 (f) and supplementary video S1, the CMC/PDMS aerogel film was very flexible and could easily be rolled up. The thermal stability of the porous CMC/PDMS film was investigated using a 8

thermogravimetric analyzer (TGA) in a N2 atmosphere from 30 to 800 °C and is shown in Figure S2. The porous CMC/PDMS aerogel film was stable up to 295 °C. Figure 3(a) shows the representative electric outputs of the porous CMC/PDMS aerogel film-based compact NG without an airgap under a periodic stress of 0.05 MPa at a frequency of 10 Hz. For a NG with a dimension of 1 cm  1.2 cm  540 m, the average values of the open-circuit voltage (Voc) and short-circuit current (Isc) were around 30.0 V and 4.5 A, respectively. The power density was calculated to be about 1.1 W/m2. To the best of our knowledge, this is the first time such high electric outputs have been achieved without the use of any piezoelectric materials and without the need of airgap. For comparison purpose, traditional triboelectric NG with an airgap of 3mm between porous CMC/PDMS aerogel film and top aluminum electrode was also fabricated (Figure 1(h)) and undergo the similar tests. As shown in Figure S3, the traditional triboelectric NG exhibited an open-circuit voltage (Voc) of 150 V and a short-circuit current (Isc) of 20 A, corresponding to a power density of 25 W/m2. Those values are much higher than those of the compact NGs (i.e., without an airgap), which have been well studied and were attributed to the triboelectric effect between PDMS and aluminum electrode [25-26]. However, the high output performance of the compact NGs without an airgap and without the use of traditional piezoelectric materials remain largely unexplored. Besides, the presence of the airgap not only requires a complicated fabrication and packaging process, but also leads the unfavorable durability and stability, which may limit their large-scale manufacturing for practical applications [19]. Thus, the following discussions were mainly focused on the investigation of this novel porous aerogel film-based compact NG that prepared without the need of airgap. In order to investigate the integratability of the as-developed porous CMC/PDMS aerogel

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film-based compact NGs, the output signals of two different NGs were measured in both series and parallel connections. As shown in Figure 3 (b), the Voc generated by integrating two NGs connected in series exceeded 58 V, which is similar to that of the sum of the Voc values for each NG separately. Moreover, as shown in Figure 3 (d), the Isc generated by two NGs connected in parallel reached 8.7 A, which is almost identical to the sum of the Isc values for each NG separately. To investigate the output ability of the porous CMC/PDMS aerogel film-based compact NG, it was connected to resistors with different resistance values (103–107). As shown in Figure 4 (a), the output voltage through the external resistor increased gradually with increasing resistance, and it is expected that the output voltage will reach approximately 30 V at an infinitely high resistance. In contrast, the output power on the external load by the NG exhibited a peak value of about 0.05 mW at a resistance of 1 M (Figure 4 (a)). To demonstrate a practical application of the porous CMC/PDMS aerogel film-based compact NG, the NG was used to power up eleven blue LEDs (with a turn-on voltage of about 2.6 V for each). As shown in Figure 4 (b), the eleven blue LEDs were directly connected in series to a NG through a commercial bridge rectifier in the form of the letter “W”. It should be noted that these LEDs were instantly turned on by the NG once it was subjected to an external stress (supplementary video S2). To further demonstrate the NG as an energy harvesting power source, it was used to charge a capacitor (22 F) through a bridge rectifier. As shown in Figure 4 (c), it charged the capacitor to 4.0 V in 500 seconds. Moreover, the reliability of the porous CMC/PDMS aerogel film-based compact NG was tested by continuously applying and releasing an external stress 1200 cycles per day. As shown in Figure 4 (d), the output voltages did not show an obvious change after three days, suggesting that the NGs possessed superior stability and reliability. 10

It is known that CMC is not a piezoelectric material, which was also confirmed by the negligible output voltage induced by the NG made of a CMC aerogel film (Figure 5 (a)). Figure 5 (b) shows the output voltage generated from a NG fabricated using a pure/solid PDMS film with a Voc of 5.0 V. Those small electrical signals may be attributed to the electrostatic charges at the interface between the CMC aerogel film or the PDMS film and the aluminum electrodes[27]. Figure 5 (c) shows the electric output voltage of the NG consisting of a porous CMC/PDMS aerogel film with a Voc of 17 V, which was significantly higher than the NG made of the CMC aerogel film or pure PDMS film alone. Moreover, the output voltage of the NG was further increased to 30.0 V by spin-coating a PDMS layer on both sides of the CMC/PDMS aerogel film (Figure 5 (d)). The spin-coated PDMS layer on both sides of the porous CMC/PDMS aerogel film made the surfaces of the porous CMC/PDMS film smoother, which could provide high potential barriers thereby preventing charge leakage and transfer the compressive stress more effectively to the porous CMC/PDMS aerogel film, thus leading to better output performance. The outstanding electric outputs exhibited by the porous CMC/PDMS aerogel film-based NGs were intriguing since neither CMC or solid PDMS are traditionally considered as piezoelectric materials as was confirmed in this study. We hypothesized that the outstanding electric performance could have originated from the mechanoradicals generated by the porous PDMS coating in the porous CMC/PDMS aerogel film. To investigate this hypothesis, a series of experiments were systematically carried out. As previously reported, a high-surface-area PDMS sponge can effectively generate mechanoradicals when subjected to an external stress [28-29]. On the contrary, solid PDMS does not generate free radicals as effectively as a PDMS sponge under a similar mechanical stress [28]. As illustrated in Scheme 1, the mechanical stress can lead to a homolytic cleavage of the Si–O–Si covalent bond in the porous PDMS coating of the CMC aerogel, thus 11

producing abundant Si–O• (I) and O–Si• (II) mechanoradicals leading to the formation of a large amount of transient dipole moments [30-31]. Such a dramatic electric dipole moment change before and after the cleavage of the Si–O–Si bonds under mechanical stress [31] will create an electric potential between the top and bottom electrode, inducing the accumulation of negative and positive charges at the top and bottom electrodes, thus driving current flow through an external circuit resulting in positive output signals (Scheme 1). The activation energies of the reaction between free radicals are near zero [32], thus most of these mechanoradicals will reversibly recombine to form the Si–O–Si bond [29, 32]. A smaller fraction of the Si–O• and Si• radicals can react with trace water in the air to yield silanols (III and IV), or they can extract H from neighboring CH3 groups to form silanols (V) or Si–H bonds (VI) while transferring the free radicals onto the carbon atoms (VII) [28-29]. These silanols not only exhibit permanent electric dipole moments by themselves, but can also form hydrogen bonding leading to an increase in the density of electric dipoles and consequently enhancing the electric outputs. Once the stress was released, the diminishment of the electric potential led to the back flow of the accumulated charges through the external circuit, leading to the negative electrical signals. However, in comparison to the relatively high compression rate, the porous aerogel film-based compact NGs took a considerably longer time to return to its undistorted state, resulting in a slow diminishment of the electric potential [33]. Thus, the positive electric signal was always higher than the negative one for the porous aerogel film-based compact NGs under forward connection. For comparison purpose, the output voltages of the compact NGs under reverse connection to the external circuit were also measured. As expected, the negative output signals under reverse connection were higher than the positive output signals (Figure S5). Furthermore, the Voc values under reverse connection were almost identical to 12

those obtained under forward connection, albeit the polarities of the signals were reversed. For this new class of compact NGs, an electric potential is generated upon a mechanical force through the creation of transient dipole moments resulting from the mechanoradicals, and permanent electric dipole moments exhibited by mechanoradical-induced polar groups. Since the porous aerogel film-based compact NGs were fabricated without the use of traditional piezoelectric materials, and the electricity generated by the mechanoradical effect upon mechanical stress has been previously defined as triboelectric effect [34-35], therefore the electricity generated by as-developed porous aerogel film-based NG could be classified as triboelectricity even though the NGs were fabricated without an airgap. In order to demonstrate the importance of the highly porous structure of the CMC/PDMS aerogel film, we tried our best to fill the pores of the CMC aerogel film with pure PDMS (8% porosity, Table S1), and the Voc of the nearly completely filled CMC/PDMS film-based NG was only 8 V (Figure 5 (e)). Furthermore, porous CMC/PDMS aerogel films with a porosity of 19% were also prepared and the resulting NGs exhibited a Voc of 15V (Figure 5 (f)). In contrast, as discussed previously, the Voc of the porous CMC/PDMS aerogel film (with a porosity of 40%, Table S1) was around 30 V. This observation is consistent with the previous report that porous PDMS can generate mechanoradicals much more efficiently than solid PDMS [28]. To further investigate this mechanoradical-based triboelectric mechanism, the effects of different amounts of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), a common free radical scavenger, mixed in the PDMS coating on the electric outputs of the porous CMC/PDMS aerogel film-based NGs were studied. As shown in Figure 6 (a) and Figure S4, the electric output voltages of the as-prepared porous CMC/PDMS (TEMPO) aerogel film-based NGs 13

decreased obviously with an increasing amount of TEMPO. In particular, when the amount of TEMPO reached 20 wt%, the output voltage decreased to only about 4 V. These data revealed that mechanoradicals generated from the porous PDMS coating could be effectively terminated by TEMPO, leading to reduced electric outputs. Furthermore, due to its sublimation characteristic, TEMPO lost its free radical scavenging capability gradually with time. Figure 6 (b) and Figure S6 show the change of the output voltage signals of the porous CMC/PDMS (TEMPO) aerogel film-based NGs stored in air for 7 days. It clearly demonstrated that the voltage output signals increased with the storage time, with an ultimate output voltage of 24 V. Therefore, these results support our hypothesis that the mechanoradicals generated by PDMS play a crucial role in the performance of the compact triboelectric NGs. To further explore this mechanoradical-based triboelectric mechanism, thermoplastic polyurethane (TPU), a typical polymer that does not efficiently generate free radicals under an external stress, was employed to replace PDMS to prepare porous CMC/TPU film-based NGs using a similar assembly. Theoretically, since CMC and TPU are not traditionally considered piezoelectric materials, and TPU does not effectively generate mechanoradicals, the porous CMC/TPU film-based NGs were not expected to produce significant electric outcomes. Indeed, as expected, Figure 7 shows only very weak electric signals collected from NGs using either a pure TPU film (Figure 7 (a)) or a porous CMC/TPU film (Figure 7 (b)), which may be attributed to the electrostatic charges at the interface under an external stress [23]. These results have helped to further establish the key role played by the mechanoradicals in this type of compact triboelectric NG. As shown in Figure S7, the porous CMC/TPU film-based traditional triboelectric NGs (i.e., with an airgap) also exhibited a higher output voltage than that of the NG without an airgap. However, the output voltage of the porous CMC/TPU film14

based traditional triboelectric NG was much lower than that of the porous CMC/PDMS filmbased traditional triboelectric NG, which was due to the small electron affinity difference between the TPU and aluminum. To further demonstrate the role of mechanoradicals in this type of compact triboelectric NG, we replaced the porous PDMS coating using yet another type of polymer coating; namely, a mixture of PBM and PBMA. Both PBM and PBMA are capable of efficiently generating mechanoradicals [29, 36]. However, the glass transition temperature of PBA is very low (–55 oC), thus pure PBA film was somewhat sticky. On the other hand, pure PBMA film was somewhat rigid. Thus, a PBA–PBMA blend at a 1:1 weight ratio was used as the porous CMC aerogel film coating to replace the PDMS coating for the electric NG. For comparison purposes, the NGs fabricated using only solid PBA–PBMA films were also prepared and characterized. As shown in Figures 8 (a) and (b), the output signals of Voc and Isc of the solid PBA–PBMA film-based NG were 12 V and 1.2 A, respectively, which supports the fact that PBA and PBMA can effectively generate mechanoradicals. The Voc and Isc outputs of the porous CMC/ PBA–PMBA aerogel film-based NG were 24 V and 4.0 A, respectively (Figures 8 (c) and 8 (d)). These findings again confirm the importance of porosity on the generation of mechanoradicals and, consequently, the electric outputs. Furthermore, although the electric outputs of the porous CMC/PBA–PBMA aerogel film-based compact triboelectric NG were slightly less than that of the porous CMC/PDMS aerogel film-based compact triboelectric NG, it is sufficient to demonstrate that high performance NGs can be prepared using porous polymer films capable of efficiently generating mechanoradicals. Similar to the porous CMC/PDMS film-based traditional triboelectric NG, the porous CMC/PBA–PBMA aerogel film-based traditional triboelectric NG (i.e., with an airgap) exhibited an open-circuit voltage (Voc) of 115 V and a short-circuit current (Isc) of 15 A 15

(Figure S8), which was much higher than that of the compact triboelectric NGs. According to the data shown in Figure 5 and our newly proposed mechanoradical-based triboelectric mechanism, the major role of the compressed CMC aerogel film was to provide a porous scaffold in order to form the porous PDMS coating. As shown in Figure 5, the output voltage of the NG made of a nearly solid CMC/PDMS film (8% porosity) was much lower than that of a NG made of a porous CMC/PDMS aerogel film (40% porosity); namely, approximately 8 V versus 30 V. Therefore, it is evident that the porosity of the PDMS coating was a dominant factor affecting the amount of mechanoradicals and, ultimately, the electric outputs. For that matter, we hypothesized that the CMC aerogel scaffold may be replaced by other types of aerogel scaffolds for the formation of the porous PDMS film. To test this hypothesis, two types of aerogels––namely, CTS and PVA––were prepared and used as the scaffolds to coat the PDMS films. The resulting porous CTS/PDMS and PVA/PDMS aerogel films (porosities of 39% and 38%, respectively; Table S1) were used to prepare compact NGs with a similar structure as the porous CMC/PDMS aerogel film-based compact NG shown in Figure 1 (g). As demonstrated in Figure 9, the porous CTS/PDMS aerogel film-based compact NG exhibited a Voc of 32 V and a Isc of 4.6 A, while the porous PVA/PDMS aerogel filmbased compact NG displayed a Voc of 26 V and a Isc of 3.6 A. These results support our hypothesis that the key role of the aerogel films used in this study was to provide a porous scaffold to form the porous mechanoradical-generating polymer coating (e.g., PDMS and PBA–PBMA blend), thereby enhancing their ability to generate mechanoradicals and consequently resulting in better electric outputs. Furthermore, these results suggest that coating a layer of mechanoradical-generating polymers onto the porous surface of the aerogel film can be a universal method for achieving high performance energy haversting materials. The output signals of these porous aerogel film-based compact NGs differed slightly, which 16

may be attributable to the different mechanical properties of the different polymers involved, as well as the different aerogel microstructures. However, these studies demonstrated that no matter what kind of polymer is used, as long as it can form an aerogel with desirable material properties (e.g., porosity and mechanical robustness), it can be utilized as a porous scaffold for fabricating this type of electric NG. To rationalize the high electric outputs generated from the porous CMC/PDMS aerogel film-based compact NGs, we proposed a mechanoradical-based triboelectric mechanism for this type of porous aerogel film-based compact NGs. Namely, previous studies have established that porous PDMS can effectively generate mechanoradicals [28]. The significant amount of transient dipole moments generated by the reversible and transient mechanoradicals, together with the permanent electric dipole moments exhibited by the mechanoradical-induced polar groups, led to substantial electric outputs. To investigate this possible electric generation mechanism, a series of studies were systemically carried out. First, we studied the effect of CMC/PDMS aerogel film porosity on the electric outputs of the compact NGs. We found that the output voltage of the compact NG made of a CMC/PDMS aerogel with 40% porosity was drastically higher than that of the compact NG made of a CMC/PDMS aerogel with 8% porosity; namely, a Voc of 30 V versus a Voc of 8 V. This finding supports our hypothesis. Second, we studied the effects of different amounts of free radical scavenger (i.e., 2,2,6,6-tetramethyl-1-piperidinyloxy) present in the porous PDMS coating on the electric outputs of the CMC/PDMS aerogel film-based compact triboelectric NGs and it was found that the electric outputs of the compact NGs decreased consistently with an increasing amount of free radical scavenger, which is in agreement with our hypothesis. Third, the effects of different porous polymer coatings on the CMC aerogel films with varying degrees of abilities in generating mechanoradicals (e.g., PDMS, thermoplastic polyurethane 17

(TPU), and a blend of poly(n-butyl acrylate) (PBA) and poly(n-butyl methylacrylate) (PBMA)) on the electric outputs of the CMC aerogel film-based compact NGs were studied and it was found that the electric outputs were drastically higher for NGs made of porous polymer coatings capable of efficiently producing mechanoradicals (e.g., PDMS and the PBA–PBMA blend), which again supports our hypothesis. Finally, the effects of different porous aerogel films (i.e., CMC, CTS, and PVA) on the electric outputs of the compact NGs were also studied and it was found that similar electric outputs could be achieved after coating a layer of PDMS on the porous surface of CMC, CTS, or PVA aerogel films, thus suggesting that the major role of the aerogel film used in this study was to provide a porous scaffold to conveniently prepare porous mechanoradical-generating polymer coatings (i.e., PDMS or the PBA–PMBA blend). From this perspective, we envision that high-performance electric NGs may also be fabricated using porous mechanoradical-generating polymer films prepared using other approaches, aside from the approach demonstrated in this study employing the aerogel film as the porous scaffold for polymer coatings. For instance, porous polymer film can be generated by making a composite film of a mechanoradical-generating polymer and nanoparticles (e.g., polystyrene or zinc oxide nanoparticles), followed by etching to remove the nanoparticles[25, 37]. Taken together, our systematic studies have demonstrated that highperformance, flexible electric NGs can be made from porous mechanoradical-generating polymer films without the use of traditional piezoelectric materials or the need of airgap, and the outstanding electric outputs from this new family of compact triboelectric NGs can be largely attributed to the reversible and transient dipole moments generated by the mechanoradicals, as well as the permanent electric dipole moments exhibited by the mechanoradical-induced polar groups.

4. Conclusion 18

A universal approach for fabricating high-performance, flexible porous aerogel film-based compact triboelectric NGs, utilizing mechanoradical-generating polymers that are not piezoelectric materials, was designed and fabricated without the need of airgap. The elucidation of the electric generation mechanism for this family of porous polymers will lead to a new class of energy haversting materials with a diverse range of material properties, thereby greatly broadening the choices of organic flexible materials for various applications. We have also demonstrated a versatile and simple fabrication process to prepare highperformance, low-cost, and flexible energy generation devices suitable for various applications such as self-powered electronic devices and active sensors. It is our hope that this research will provide new insights in the field of flexible energy harvesting materials, which has been growing in demand and importance.

Acknowledgements The authors gratefully acknowledge the financial support of the University of Wisconsin– Madison.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at:

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Professor Yanfeng Tang received his Ph.D. in State Key Lab of Fine Chemicals from Dalian University, Dalian, China. He was a visiting Professor in Prof. Shaoqin Gong’s laboratory at the University of Wisconsin–Madison. Prof. Tang currently works in the School of Chemistry and Chemical Engineering at Nantong University. Prof. Tang’s research interests include functional materials, fine chemicals, and smart polymers.

Qifeng Zheng received his BS degree in Chemistry from Xiamen University in 2012. He is currently pursuing his PhD degree in Materials Science under the supervision of Prof. Shaoqin Gong at the University of Wisconsin–Madison. His research focuses on engineering innovative multifunctional inorganic/polymer nanocomposites for various applications such as superabsorbents, flexible nanogenerators, sensors, supercapacitors and self-healing materials.

Dr. Bo Chen received his Bachelor’s degree in Chemical Engineering and Technology, and Ph.D. degree in Applied Chemistry both from Tianjin University, China. He was an exchange graduate student in Prof. Shaoqin Gong’s laboratory at the University of Wisconsin–Madison. He is currently a postdoc fellow in Beijing Institute of Nanoenergy and Nanosysterms, Chinese Academy of Sciences. His research currently focuses on developing nanogenerators for scavenging thermal and mechanical energy.

Professor Zhenqiang Ma received dual Bachelor’s degrees in Applied Physics and Electrical Engineering from Tsinghua University, Beijing, China, and a PhD degree in Electrical Engineering from the University of Michigan–Ann Arbor. He is currently a Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor in the Department of Electrical and Computer Engineering at the University of Wisconsin–Madison. Prof. Ma has a broad range of research interests including power electronics, flexible electronics, flexible optoelectronics, and nanophotonics. 22

Professor Shaoqin Gong received dual Bachelor’s degrees in Materials Science and Engineering and Economics and Management from Tsinghua University in Beijing, China. She also earned a Master’s degree from Tsinghua University in Materials Science and Engineering, and a PhD degree from the University of Michigan–Ann Arbor in Materials Science and Engineering. She is currently a Vilas Distinguished Achievement Professor in the Department of Biomedical Engineering and the Wisconsin Institute for Discovery at the University of Wisconsin–Madison. Her current research focuses on the development of multifunctional nanomaterials, nanomedicines, and flexible devices.

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Figure 1. Schematic illustration for the preparation of the porous CMC/PDMS aerogel filmbased electric NG. (a) A CMC solution, (b) a frozen CMC solution, (c) a CMC aerogel, (d) a compressed CMC aerogel film, (e) a porous CMC/PDMS aerogel film, (f) a sandwich-like PDMS/(CMC/PDMS aerogel film)/PDMS structure, (g) a porous CMC/PDMS aerogel filmbased compact triboelectric NG (i.e., without an airgap) with a five-layer structure, and (h) a porous CMC/PDMS aerogel film-based traditional triboelectric NG (i.e., with an airgap).

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Figure 2. Microstructure and flexibility of porous CMC/PDMS aerogel film. SEM images of (a) the bottom surface of the CMC aerogel, (b) the bottom surface of the compressed CMC aerogel film, (c) the cross-section of the compressed CMC aerogel film, (d) the cross-section of the PDMS-coated compressed CMC aerogel film (i.e., the porous CMC/PDMS aerogel film used to fabricate the NG), (e) the cross-section of the PDMS/(porous CMC/PDMS aerogel film)/PDMS tri-layer film, and (f) a photograph of the flexible porous CMC/PDMS aerogel film.

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Figure 3. (a) The electric output (Voc and Isc) measurement of the porous CMC/ PDMS filmbased compact NGs under a compressive stress of 0.05 MPa at a frequency of 10 Hz. (b) Voc and (c) Isc were combined by connecting two different NGs in series and parallel, respectively.

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Figure 4. The output performance and stability of a porous CMC/PDMS aerogel film-based compact NG. (a) Output voltage and instantaneous power as a function of the load resistance. The inset shows the equivalent circuit. (b) Eleven blue LEDs were directly and instantly turned on by a porous CMC/PDMS aerogel film-based NG. (c) The charging curve of a capacitor by a NG under a compressive stress of 0.05 MPa at a frequency of 30 Hz. (d) Stability test for the NG at the first, second, and third day (1200 cycles per day).

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Figure 5. Comparison of the electric output voltage generated by various compact NGs. Various compact NGs fabricated using (a) a CMC aerogel film, (b) a pure/solid PDMS film, (c) a porous CMC/PDMS aerogel film, (d) a pure PDMS/(porous CMC/PDMS aerogel film (40% porosity))/pure PDMS tri-layer film, and (e) a pure PDMS/(nearly solid CMC/PDMS aerogel film (8% porosity))/pure PDMS tri-layer film, (f) a pure PDMS/(porous CMC/PDMS aerogel film (19% porosity))/pure PDMS tri-layer film.

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Figure 6. The effects of TEMPO on the electric outputs of the porous CMC/PDMS (TEMPO) aerogel film-based compact NG. The output voltage of the CMC/PDMS (TEMPO) aerogel film-based NGs (a) containing different amounts of TEMPO, and (b) with 20 wt% TEMPO (relative to the weight of PDMS) as a function of storage time (in days) in air.

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Figure 7. The electric output voltage of the compact NGs fabricated with TPU instead of PDMS. The electric output voltage generated from (a) a solid TPU film-based compact NG, and (b) a porous CMC/TPU aerogel film-based compact NG with approximately the same thickness of 520 μm.

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Figure 8. The electric output (Voc and Isc) measurement of the compact NGs fabricated employing PBA-PBMA to replace PDMS. The output (a) voltage and (b) current of a solid PBA–PBMA film-based compact NG. The output (c) voltage and (d) current of a porous CMC/PBA–PBMA aerogel film-based compact NG. These two types of NGs had approximately the same thickness of 530 μm.

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Figure 9. The electric output (Voc and Isc) measurement of the compact NGs fabricated using CTS and PVA to replace CMC. The output (a) voltage and (b) current of a porous CTS/PDMS aerogel film-based compact NG. The output (c) voltage and (d) current of a porous PVA/PDMS aerogel film-based compact NG.

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Scheme 1. Schematic illustration of the reversible homolytic cleavage of the Si–O–Si bond under periodic mechanical stress leading to the formation and recombination of the mechanoradicals.

Highlights

33



A simple and scalable technique has been demonstrated to fabricate a new class of high-performance flexible compact triboelectric nanogenerators using porous aerogel films.



The porous aerogel film-based compact nanogenerators exhibit significant electric outputs without the use of traditional piezoelectric materials or the need of airgap.



A mechanoradical-based compact triboelectric mechanism has been proposed and substantiated to explain the high electric outputs.



The elucidation of the electric generation mechanisms for this family of porous mechanoradical-generating polymers will lead to a new class of energy harvesting materials and high-performance flexible energy generation devices.

34