Hierarchical Nanostructured WO 3 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, † Dieqing Zhang, § Meicheng Wen, § Shuning Xiao, § Chi-Ping Liu, ∥ Yunfeng Lu, † and Hexing Li* ,‡ † Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, California 90095, United States ‡ Shanghai University of Electric Power, Shanghai 200090, China § The Education Ministry Key Lab of Resource Chemistry, Shanghai Normal University, Shanghai 200234, China ∥ Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States * S Supporting Information 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 WO 3 (h-WO 3 ) 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-WO 3 nanorods. Such a unique h-WO 3 contains biomimetic proton channels where single-file water chains embedded within the electron-conducting matrix, which is critical for fast electrokinetics. The mixed conductivities, high redox capacitance, and structural robustness afford the h-WO 3 with unprecedented electrochemical performance, including high capacitance, fast charge/discharge capability, and very long cycling life (>50 000 cycles without capacitance decay), thus providing a new platform for a broad range of applications. KEYWORDS: Mixed conductor, proton channel, tungsten oxide, pseudocapacitor F rom photosynthesis to electrochemical energy storage, conversion and storage of energy is achieved mainly through chemical transformations with simultaneous trans- location of electrons and ions (e.g., proton and lithium ion). Mixed ionic-electronic conductors, in this context, hold the utmost promise toward 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-WO 3 ). Forming a three- dimensional hierarchical nanostructure self-assembled from h- WO 3 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 sensing, 8 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 LiCoO 2 and LiMn 2 O 4 , 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 LiCoO 2 and delithiated LiCoO 2 , respectively 12,13 ), 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 polymers 14 or carbon nanotubes, 15 mesoporous tungsten oxide (WO 3 ) xerogel of which the random pores are filled with water, 16 and hydrous ruthenium oxides (RuO 2 ·nH 2 O). 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 WO 3 , which are generally in the meso- to microscale. Proton conduction in RuO 2 ·nH 2 O, on the other hand, relies on the water molecules within the hydrous layers of RuO 2 nanodomains. The combination of proton conductivity, electron conductivity, and redox capability affords RuO 2 · nH 2 O a benchmark capacitance of ∼750 F g −1 . 17,18 However, the high cost of Received: July 3, 2015 Revised: September 21, 2015 Published: September 25, 2015 Letter pubs.acs.org/NanoLett © 2015 American Chemical Society 6802 DOI: 10.1021/acs.nanolett.5b02642 Nano Lett. 2015, 15, 6802−6808