Letter pubs.acs.org/NanoLett
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,† 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 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 electrokinetics. 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 (>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
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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 electronconducting 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 microscale. Proton conduction in RuO2·nH2O, on the other hand, relies on the water molecules within the hydrous layers of RuO2 nanodomains. 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
rom 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 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 protonconducting water chains within a matrix of electron-conducting hydrous hexagonal tungsten oxide (h-WO3). Forming a threedimensional hierarchical nanostructure self-assembled from hWO3 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 lowtemperature 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 © 2015 American Chemical Society
Received: July 3, 2015 Revised: September 21, 2015 Published: September 25, 2015 6802
DOI: 10.1021/acs.nanolett.5b02642 Nano Lett. 2015, 15, 6802−6808
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Nano Letters
and ions (e.g., H2O, H+). Stacking of such layers provides onedimensional (1D) channels along the c-axis that can be filled with water molecules connecting into single-file water chains. Figure 1c schematically shows the (001) plane with hexagonalclose-packed channels (left) and the channel walls that are made from single-layer WO6 octahedrons (right). Similar to the biologic channels, the water chains in such oxide channels can serve as effective proton-conducting wires, enabling effective proton conduction throughout the crystal. In addition, the insertion and extraction of protons are confined within the channels, avoiding the crystal structure transitions that are commonly experienced in electrochemical energy-storage devices. This unique structure affords h-WO3·nH2O with high proton conductivity, high electron conductivity, and structural robustness, leading to pseudocapacitors with high power, high capacity, and long cycling life. As mentioned above, although hydrous WO3 has been explored as MEPC before, the conduction of protons relies on the water molecules within their random meso- and microporous channels around crystal domains, and their amorphous16 or monoclinic structure25 do not contain effective proton-conducting pathways. Despite amorphous26 or crystalline WO3 with various structures (e.g., monoclinic,27,28 orthorhombic,29 tetragonal,30 and hexagonal structures24,31,32) have been explored extensively for a broad range of applications (e.g., electrochromic device,26 catalysis,29 sensing,28 lithium ion battery32), such ordered proton conducting channels have not been discovered, and their critical roles in the broad applications remain to be elucidated. To be best of our knowledge, this finding represents the first example of singlefile proton channels found in metal oxides. In this work, the h-WO3·nH2O-based MPEC was synthesized by a one-step hydrothermal reaction with effective capping agent (see Method). Figure 2a shows representative scanning electron microscopic (SEM) images of the as-synthesized hWO3·nH2O. The particle size is mainly distributed from 1 to 3 μm. They show a unique hierarchical structure, which is spherical secondary particle self-assembled from closely packed nanorods (Figure 2b). Transmission electron microscopic (TEM) image (Figure 2c) clearly shows that the nanorods have diameter of ∼10 nm and length of 60−100 nm. Highresolution TEM and Fast Fourier Transform (FFT) images display the (110) and (002) lattice planes with a lattice space of 3.19 and 3.79 Å, respectively (Figure. 2d). Consistent with the TEM observation, powder X-ray diffraction (XRD) shows reflection peaks at 13.8, 22.8, 28.1, 33.8, 36.8, 49.7°, which agrees with the (100), (002), (200), (112), (202), and (220) lattice planes of the hexagonal WO3 with lattice constants of a = b = 7.428 Å, c = 7.618 Å (JCPDS# 85-2459) (Figure 2e).33 While the hexagonal phase is metastable,34 the crystal structure of the synthesized h-WO3 can be stable up to 400 °C after sintering the particles in air, indicating its good thermal stability (Figure S1). The h-WO3·nH2O particles possess a hierarchical porous structure as indicated by the nitrogen (N2) adsorption/ desorption isotherm (Figure 2f). The significant N2 uptake at the relative pressure (P/P0) below 0.01 confirms the existence of microporous channels, which are formed by the removal of water chain during degas process. The continuous N2 uptake (at P/P0 of 0.04−0.3) and hysteresis (at P/P0 of 0.65) suggests the existence of relatively regular mesopores, which are created by the self-assembly of h-WO3·nH2O nanorods into porous spherical particles during hydrothermal reaction. Such a porous
ruthenium limits its broad use as electrode materials for energy storage applications. 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 nonstoichiometric oxides (WOx, 2 < x < 3) that are electronically conductive.19 Proton insertion/extraction reactions, WO3 + xH+ + xe− ↔ HxWO3, converts WO3 into tungsten bronze (HxWO3, 0 < x < 1) that is electronically conductive,20 providing a theoretical capacity of 115 mAh g−1 (based on x = 1 according to Faraday’s law). To harvest such capability requires building effective protonconducting pathways within the electron-conducting matrix of tungsten bronze. However, the construction of effective protonconducting pathways, generally, has been challenging, despite proton channels do commonly exist in biological systems. In biological systems, proton transport plays essential roles in numerous physiological processes, such as photosynthesis,21 the synthesis of adenosine triphosphate (ATP),22 and maintaining the acidic gastric environment.23 The transport of protons generally involves proton channels, a class of protein molecules embedding single-file water chains as highly effective protonconducting wires. One of the simplest and most studied proton-conducting polypeptide is gramicidin A,5 of which dimerizes forming β-helix within the hydrophobic interior of the cellular lipid bilayers. Such β-helix embeds a single-file water chain enabling effective translocation of monovalent cations including H+ (Figure 1a).
Figure 1. Design and structure of h-WO3 nH2O-based MPEC. (a) Schematic illustration of the cross-sectional and planar views of a gramicidin A β-helix structure that embeds a single-file water chain for effective proton conduction. (b) Schematic illustration of a single hexagonal channel formed by six WO3 octahedral building blocks. (c) Schematic illustration of h-WO3 nH2O-based MPEC containing singlefile water chains as effective proton-conducting channels and tungsten bronze scaffold as the electron-conducting pathways.
Inspired by such an exquisite biological design, we synthesized a novel MPEC with biomimetic proton channels based on hierarchical nanostructured hydrous h-WO3 (h-WO3· nH2O). As illustrated in Figure 1b, six tungsten oxide (WO6) octahedral building blocks sharing their corners assemble into one six-membered ring with a diameter of ∼5.36 Å.24 The interstitial spaces are readily to accommodate guest molecules 6803
DOI: 10.1021/acs.nanolett.5b02642 Nano Lett. 2015, 15, 6802−6808
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Nano Letters
Figure 2. Characterization of the h-WO3 nH2O. (a−b) SEM image of as-synthesized h-WO3 nH2O particles. (c) TEM image of WO3 nH2O particles, showing that each particle consists of WO3 nH2O nanorods. (d) High-resolution TEM image and fast Fourier transform (FFT) images (inset) of a single h-WO3 nH2O nanorod. (e) XRD pattern showing a hexagonal crystalline structure. (f) Nitrogen sorption isotherm at 77 K. The inset shows pore size distribution.
Figure 3. Electronic and protonic conductivity of h-WO3 nH2O. (a) Temperature dependence of electronic conductivity. (b) Arrhenius plot showing the proton conductivity of WO3 nH2O at different temperatures.
low electronic conductivity (σe, ∼10−5 S cm−1, Figure S4) at stoichiometric status. Nevertheless, heating h-WO3·nH2O pellets (prepared by cold pressing with a porosity ∼47%) in reducing atmosphere (H2/N2 = 5:95 in volume) at 400 °C converts the oxide into none-stoichiometric tungsten oxides with high σe. Figure 3a shows the σe at diffident temperatures measured from electrochemical impedance spectroscopy (EIS). The σe decreases with increasing temperature at an average rate of ∼2.1 mS °C1−, indicating a metallic conducting behavior. A high σe (∼0.6 S cm−1) is observed at room temperature, which is about 2−3 orders of magnitude higher than those of lithium transition metal oxides (10−9−10−4 S cm−1)13 and close to that of hydrous RuO2 (1 S cm−1).17 Similar to chemical reduction, electrochemical reduction also leads to non-stoichiometric hWOx·nH2O with high σe. It was found that electrodes prepared from h-WO3·nH2O, binder (polyvinylidene fluoride) and carbon black (8:1:1 in mass) showed a charge transfer resistance (Rct) of ∼4 Ω at the open circuit potential (OCP, ∼0.2 V vs Ag/AgCl); Rct rapidly decreased to 1.1 Ω at 0 V and remained similar values at lower voltage (e.g., 1.1 Ω at −0.6 V)
structure results in a total Brunauer−Emmett−Teller (BET) surface area (SBET) of 70 ± 5 m2 g−1. This value is about 1.5 times of the hexagonal WO3·synthesized by a different method reported recently.35 The water content within the particles was determined by thermogravimetric analysis (TGA) (Figure S2). For a typical sample synthesized at 120 °C, the water content is around 4.8 wt %, corresponding to 0.65 H2O per WO3 (WO3· 0.65H2O), while the n value in WO3·nH2O may be varied from 0.5 to 0.8 depending on the synthesis and drying conditions. Based on the crystal parameters of h-WO3·nH2O from XRD results, it is calculated that 0.5 H2O per WO3 is required to form continuous single-file water chains inside the hexagonal channels, suggesting that our h-WO3·0.65H2O contains such desired water chains. The extra water (0.15 H2O) might be the surface-absorbed water due to the relatively large surface area. The existence of such continuous water channel is critical for charge storage reactions, as discussed below. X-ray photoelectron spectroscopy (XPS) suggests that the assynthesized h-WO3·nH2O possesses a nearly perfect 1:3 W/O stoichiometry (Figure S3), which is consistent with its relatively 6804
DOI: 10.1021/acs.nanolett.5b02642 Nano Lett. 2015, 15, 6802−6808
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Nano Letters
Figure 4. Electrochemical performance of h-WO3·nH2O electrodes. (a) CV curves of h-WO3·nH2O electrodes at voltage sweep rate of 10 mV s−1 in 0.2 M H2SO4, Li2SO4 and Na2SO4 electrolytes in three-electrode cells with Ag/AgCl and Pt foil as the reference and counter electrode, respectively. (b) Peak currents of the h-WO3·nH2O electrode at different voltage sweep rates showing a capacitor-like linear dependence. (c) Typical galvanostatic charge/discharge curves at current density of 10 A g−1. (d) Comparison of electrochemical capacitive performance of our h-WO3·nH2O with other WO3 structures (h-WO3 with CNT,35 h-WO3 bundle structure,47 hexagonal-cubic mixed phase WO3,48 monoclinic WO3 thin film49). (e) Capacitance retention of high-mass-loading h-WO3·nH2O electrodes (8 mg cm−2) during long-term cycling at a constant current density of 8 A g−1. (f) In situ XRD of h-WO3·nH2O electrodes during charge/discharge. Shift of diffraction peaks with various state-of-charge shows a decrease of (002) spacing and an increase of (200) spacing during proton insertion. No appreciable shift is observed for other diffraction peaks.
(Figure S5), suggesting that h-WO3·nH2O readily became electronically conductive upon proton insertion at potentials below the OCP. To better quantify this effect, electrodes were also prepared from the chemically reduced h-WOx·nH2O and binder (8:2 in mass without adding carbon black). They showed Rct of ∼10 Ω and ∼4 Ω at OCP (−0.25 V) and −0.36 V, respectively. The Rct slowly increased with potential increasing from the OCP to 0.1 V and then increased rapidly at higher potentials, e.g., 130 Ω at 0.08 V and 4300 Ω at 0.3 V (Figure S6). Based an electrode thickness of ∼400 μm, the σe of the h-WO3·nH2O in the bulk electrodes can be calculated to be ∼3 × 10−5 S cm−1 and ∼33 mS cm−1 at 0.3 V and −0.36 V, respectively. Note that the lower σe compared with the value measured in dry pellets can be attributed to the higher porosity and the use of nonconductive binder. Nevertheless, these studies do confirm that sufficient proton insertion can be achieved electrochemically, rapidly rendering the h-WO3·nH2O with excellent electronic conductivity. Similarly behavior was also reported for Nb2O5 thin films, which showed significantly enhanced conductivity (by a factor of about 1000) after Li+ insertion.36 Figure 3b shows proton conductivity (σH) of the h-WO3· nH2O pellets measured in air with different humidity and temperature (∼50% relative humility (RH) at 22 °C, ∼6.2% at 60 °C or ∼1.5% at 90 °C). The h-WO3·nH2O shows a high σH of ∼1.0 mS cm−1 at 22 °C, which increases to 2.7 mS cm−1 and 3.7 mS cm−1 at 60 and 90 °C, respectively. The activation energy (Ea) calculated from Arrhenius equation is 0.12 ± 0.02 eV, which is lower than hydrated oxides (0.2−0.4 eV),25 solid
acids (0.3−0.5 eV),37,38 various gramicidin A (∼0.3 eV),39 and comparable to proton in water (∼0.11 eV).40 The observation of high σH and low Ea is consistent with previous findings that protons can migrate in the bulk hydrated oxides according to Grotthuss mechanism.41 Such σH are among the highest values of inorganic proton conductors and comparable with Nafion at similar conditions. For example, σH of Nafion reduced from ∼90 mS cm−1 at RH = 100% to ∼4 mS cm−1 at RH = 30% under 60 °C and further decreased to
