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Hierarchical Nanostructured WO3 with Biomimetic Proton Channels and Mixed Ionic-Electronic Conductivity for Electrochemical Energy Storage Zheng Chen, Yiting Peng, Fang Liu, Zaiyuan Le, Jian Zhu, Gurong Shen, Zhang Dieqing, Meicheng Wen, Shuning Xiao, Chi-Ping Liu, Yunfeng Lu, and He Xing Li Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02642 • Publication Date (Web): 25 Sep 2015 Downloaded from http://pubs.acs.org on September 28, 2015
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Nano Letters
Hierarchical Nanostructured WO3 with Biomimetic Proton Channels and Mixed Ionic-Electronic Conductivity for Electrochemical Energy Storage Zheng Chen,1 Yiting Peng,1,2 Fang Liu,1 Zaiyuan Le,1 Jian Zhu,3 Gurong Shen,1 Dieqing Zhang,3 Meicheng Wen,3 Shuning Xiao,3 Chi-Ping Liu,4 Yunfeng Lu1 and Hexing Li2* 1
Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, CA 90095, USA; 2
3
Shanghai University of Electric Power, Shanghai 200090, China;
The Education Ministry Key Lab of Resource Chemistry, Shanghai Normal University, Shanghai 200234, China; 4
Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095, USA; *Corresponding to:
[email protected] Abstract: Protein channels in biologic systems can effectively transport ions such as proton (H+), sodium (Na+) and calcium (Ca+) ions. However, none of such channels is able to conduct electrons.
Inspired by the biologic proton channels, we report a novel hierarchical
nanostructured hydrous hexagonal WO3 (h-WO3) which can conduct both protons and electrons. This mixed protonic-electronic conductor (MPEC) can be synthesized by a facile single-step hydrothermal reaction at low temperature, which results in a three-dimensional nanostructure self-assembled from h-WO3 nanorods.
Such a unique h-WO3 contains biomimetic proton
channels where single-file water chains embedded within the electron-conducting matrix, which is critical for fast electro-kinetics. The mixed conductivities, high redox capacitance and structural robustness afford the h-WO3 with unprecedented electrochemical performance, including high capacitance, fast charge/discharge capability and very long cycling life (>50000 cycles without capacitance decay), thus providing a new platform for a broad range of applications. Key
words:
mixed
conductor,
proton
channel,
tungsten
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oxide,
pseudocapacitor
Nano Letters
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From photosynthesis to electrochemical energy storage, conversion and storage of energy is achieved mainly through chemical transformations with simultaneous translocation of electrons and ions (e.g., proton and lithium ion). Mixed ionic-electronic conductors, in this context, hold the utmost promise towards high-performance electrochemical devices.1-4 Inspired by the transport of protons in proton channels, where single-file water chains embedded within the protein molecules serve as highly effective proton-conducting wires,5 we reported herein a mixed protonic-electronic conductor (MPEC) synthesized by building the proton-conducting water chains within a matrix of electron-conducting hydrous hexagonal tungsten oxide (h-WO3). Forming a three-dimensional hierarchical nanostructure self-assembled from h-WO3 nanorods, such MPEC offers unique structure and properties which have not yet been fully disclosed. Mixed ionic-electronic conductors have been extensively investigated for solid-state fuel cells,4,6 electrochromic devices,7 chemical sensing8 and gas separations.9,10 Most of the current mixed conductors are based on fluorite or perovskite ceramics with high operation temperature (e.g., >800 °C). For low-temperature applications, mixed lithium-electronic conductors, such as LiCoO2 and LiMn2O4, are broadly used for lithium-ion batteries.11 Such materials generally exhibit low electronic conductivity (e.g.,~10-6 and 10-4 S/cm for LiCoO2 and delithiated LiCoO2, respectively12,13), and lithium insertion/desertion often causes transition of the crystalline phases, which results in slow electrode kinetics and short charge/discharge cycling lifetime (e.g., a few hundred cycles).
Low-temperature MPECs were also synthesized by integrating electron-
conducting moieties with proton-conducting moieties. Typical examples include the composites of Nafion® with conducting polymers14 or carbon nanotubes,15 mesoporous tungsten oxide (WO3) xerogel of which the random pores are filled with water,16 and hydrous ruthenium oxides (RuO2·nH2O).17 Proton conduction for the former two types of materials relies on the water molecules residing within Nafion’s hydrophilic domains or within the random pores of the WO3, which are generally in the meso- to micro- scale. Proton conduction in RuO2·nH2O, on the other hand, relies on the water molecules within the hydrous layers of RuO2 nano-domains. The combination of proton conductivity, electron conductivity and redox capability affords RuO2·nH2O a benchmark capacitance of ~ 750 F g-1.17,18 However, the high cost of ruthenium limits its broad use as electrode materials for energy storage applications.
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Nano Letters
To address the increasing demands for energy storage, we envision that high-performance electrochemical energy storage devices can be made using WO3-based MPECs at a significantly lower cost. Tungsten oxides exist as stoichiometric oxide (WO3) that is semiconducting and non-stoichiometric oxides (WOx, 2
