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    2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1wileyonlinelibrary.com

    conversion that only relies on the ambient spontaneous moisture diffusion pro-cess greatly avoids the disturbances (e.g., thermal, mechanical variation, and so on) from the environment, which is of sig-nificance for the future high stable electric power supply. Therefore, to further satisfy the increasing demands for electric power, the effort on exploring and researching new versatile candidates (not just limited in a single less-conductive material, such as GO) for various vapor-electric energy generation is urgent, which has been absent so far yet.

    Conducting polymers have attracted numerous attention in the applications

    of actuators, sensors, batteries, supercapacitors, and electro-chromic devices due to their predominant mechanical, optical, redox, and electrical properties.[1215] In particular, polypyr-role (PPy) has been widely studied in energy conversion fields because of its significant advantages of easy synthesis, wide range of dopant species, and relatively good environmental stability.[16] For instance, PPys can act as a kind of hygroscopic material to capture the moisture from the ambient environ-ment,[15,17,18] where its dominant ionic conductivity enhances with the increase of relative humidity (RH). Furthermore, PPys could promote the conversion of electrical to mechan-ical energy in the form of actuation behaviors induced by the volume change resulting from a Faradaic doping and undoping process.[19] Meanwhile, the doped and undoped anions lead to different conducting states of PPy, in which the energy gap reduces from 4 to 2.5 eV according to the state from undoped (neutral) to doped (oxidation).[20,21] The reversible switching process between the doped (oxidized) state and undoped (neu-tral) state of PPy can be controlled by changing the electrical potential as well as the doping level.[22] These results indicate the strong interactions of PPy and doped anions to tune the functions of PPy-based devices that provide the opportunities for developing new generation energy-converting systems.

    In this work, we have developed a high-performance vapor-activated power generator (VaPG) based on a 3D PPy frame-work with a preformed anion gradient (named as gradient 3D PPy, g-3D-PPy). The g-3D-PPy was formed by electrolyte/elec-tric field coinduced by the gradient process of anions doped in the 3D PPy framework containing LiClO4 aqueous solu-tion (this strategy here is called electrolyte-electric annealing, EeA process).[10,22] Upon exposure to the water vapor, the open porous network can greatly facilitate the diffusion of water molecules, and meanwhile the anion-containing gradient pro-vides free ionic gradient to promote the spontaneous transport

    Vapor-Activated Power Generation on Conductive Polymer

    Jiangli Xue, Fei Zhao, Chuangang Hu, Yang Zhao,* Hongxia Luo, Liming Dai, and Liangti Qu*

    An efficient vapor-activated power generator based on a 3D polypyrrole (PPy) framework was demonstrated for the first time. By constructing the anions gradient in the PPy, this specially designed PPy framework provided free ionic gradient with the assistant of absorbing water vapor to promote the spontaneous transport of ionic charge carriers, thus leading to the intermit-tent electric output with the change of external water vapor. A high voltage output of 60 mV and power density output of 6.9 mW m2 were achieved under the moisture environment. More interestingly, it also exhibited power generation behaviors upon exposure to most of organic or inorganic vapors, indicating the potential new type of self-powered vapor sensors for practical applications.

    DOI: 10.1002/adfm.201604188

    Dr. J. Xue, Dr. F. Zhao, Dr. Y. Zhao, Prof. L. QuBeijing Key Laboratory of Photoelectronic/ Electrophotonic Conversion MaterialsKey Laboratory of Cluster ScienceMinistry of Education of ChinaSchool of ChemistryBeijing Institute of TechnologyBeijing 100081, P. R. ChinaE-mail: yzhao@bit.edu.cn; lqu@bit.edu.cnDr. C. Hu, Prof. L. DaiCenter of Advanced Science and Engineering for Carbon (Case 4Carbon)Department of Macromolecular Science and EngineeringCase School of EngineeringCase Western Reserve UniversityCleveland, OH 44106, USAProf. H. LuoDepartment of ChemistryRenmin University of ChinaBeijing 100872, P. R. China

    1. Introduction

    Electric power, as a typical clean and renewable energy, plays an important role in the modern society with the emerging new types of electronics, such as E-newsletter and electric vehi-cles and so on. Recently, ongoing efforts in the development of electric power generators have led to the birth of various novel devices based on harvesting and transforming energy from environment into electricity, including thermoelectric,[13] piezoelectric,[46] and triboelectric[79] generator. Remark-ably, we recently proposed a graphene-based moisture-acti-vated power generator (MaPG) which can directly convert the chemical potential energy derived from moisture diffusion to electric power by establishing the oxygen group gradient in gra-phene oxide (GO).[10,11] Such ideal and high efficiency energy

    Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201604188

    www.afm-journal.dewww.MaterialsViews.com

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    of ionic charge carriers, leading to the intermittent electric output with the change of external water vapor. The as-prepared g-3D-PPy is able to provide a voltage output of 60 mV and a power density of 6.9 mW m2, which even surpass the elec-tric outputs of MaPG based on GO film we reported recently.[10] More interestingly, it also exhibits power generation behaviors upon exposure to most of organic or inorganic vapors, indi-cating the potential new type of self-powered vapor sensors for practical applications.

    2. Results and Discussion

    Figure 1a illustrates the preparation process of 3D PPy by using a template-assisted electrochemical polymerization method. As an initial template, the vanadium pentoxide (V2O5) foam fabricated via a simple hydrothermal process[23] was directly immersed into the ethanol solution containing 10 vol% Py fol-lowed by solvothermal treatment in a sealed Teflon-lined auto-clave at 190 C for several hours to obtain the pyrrole (Py)-V2O5 foam (Figure S1, Supporting Information).[2325] For prepara-tion of PPy-V2O5 foam, the Py-V2O5 foam acted as the working

    electrode in a three-electrode cell with a Pt sheet as counter electrode and Ag/AgCl as reference electrode, respectively. The PPy-V2O5 foam was then formed through electrochemical polymerization of Py monomer adhered on V2O5 sheets in 0.2 m LiClO4 ethanol solution by applying a constant potential of 0.8 V.[24] After removing the V2O5 nanosheets by HCl, the 3D PPy was finally obtained.

    The as-prepared 3D PPy possesses a low density of 10 mg cm3, which is almost seven times lower than that of PPy sponges (70 mg cm3),[26] comparable to that of graphene aerogel (10 mg cm3)[27] and graphite foam (9.5 mg cm3).[28] As shown in Figure 1b, a 1.3 cm3 PPy foam stands stably on the top of a dandelion without any structure deformation of the dandelion. The scanning electron microscopy (SEM) image shows the PPy foam consists of a neat interconnected 3D network with randomly open pore structures (Figure 1c).[29] A closer look in Figure 1d reveals a smooth and clean surface of the PPy sheet, indicating the totally removal of the residual V2O5, which is also confirmed by X-ray energy dispersive spec-troscopy (EDS) spectrum (Figure S2, Supporting Information). Moreover, the walls possess a few PPy layers with a thickness of 10 nm (Figure 1d, the insert).[8] Raman spectroscopic

    Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201604188

    www.afm-journal.dewww.MaterialsViews.com

    Figure 1. a) Schematics of the fabrication process of the 3D PPy framework. b) Digital photo of 3D PPy foam standing on a dandelion. c) Typical SEM image of the 3D PPy framework architecture constructed from nanosheets. d) The corresponding magnified view of (c). Inset in panel (d) is the enlarged view of the edge wall of 3D PPy framework. e,f) Raman spectrum and the nitrogen adsorption isotherm of 3D PPy framework, respectively. Scale bars: b) 1 cm, c) 10 m, d) 10 m, inset in (d) 1 m.

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    measurements exhibit a series of typical peaks at 880, 930, 1050, 1238, 1370, 1410, and 1590 cm1 which are consistent with those of electropolymerized PPy, further confirming the successful formation of PPy (Figure 1e).[30,31] The BrunauerEmmettTeller (BET) nitrogen adsorption isotherm dem-onstrates a specific surface area of approximately 100 m2 g1 (Figure 1f) suggesting the predominant macroporous structure of the 3D PPy in consistence with SEM observation (Figure 1c).

    It is believed that PPy can be doped with anions (such as ClO4) through the electrochemical polymerization process in an LiClO4 aqueous solution.[32] When applying a positive poten-tial, the ClO4 tends to implant into the PPy molecule skeleton. The ClO4 releases from PPy when the potential is switched to negative potential,[33,34] which can be illustrated in the following reaction equation[34]

    PPy (ClO ) e PPy ClO40

    4+ + + (1)

    The above reaction process offers the possibility to construct a novel g-3D-PPy framework with gradient distribution of ClO4. Consequently, in order to form the uniform ClO4 gra-dient, the 3D PPy containing LiClO4 aqueous solution was sand-wiched between two pieces of gold (Au) electrodes connecting with external circuit in an enclos