Hydrogen-Treated Defect-Rich W₁₈O₄₉ Nanowire-Modified Graphite Felt as High-Performance Electrode for Vanadium Redox Flow Battery

Abstract

To improve the electrochemical performance of graphite felt (GF) electrodes in vanadium redox flow batteries (VRFBs), we synthesized simple, inexpensive, and conductive W18O49 nanowires (W18O49NWs) as electrocatalysts on the surface of GF through the one-step solvothermal process. Cyclic voltammetry and electrochemical impedance spectroscopy studies revealed that W18O49NWs exhibit electrocatalytic effects on a VO2 +/VO2+ redox couple on the positive side, which enhance the electrochemical kinetics of the redox reactions. To further improve the electrochemical performance of the W18O49NWs, we thermally annealed the sample with a controlled amount of H2/Ar atmosphere to form oxygen-vacancy-rich hydrogen-treated W18O49NWs (H-W18O49NWs). When used as an electrode in a VRFB single cell, this material demonstrated outstanding performance with 9.1 and 12.5% higher energy efficiency than cells assembled with W18O49NWs and treated GF, respectively, at a high current density of 80 mA cm–2. The superior performance of the H-W18O49NW electrocatalyst-based electrode can be attributed to the presence of numerous oxygen vacancies, which were proven to act as active sites for the VO2 +/VO2+ redox reaction. Moreover, the uniformly immobilized and 1D nature of the W18O49NWs facilitated the charge-transport process, enhanced hydrophilicity, and electrolyte accessibility, and thus remarkably reduced electrochemical polarization during the mass transfer of active species. The long-term cycling performance confirmed the outstanding durability of the as-prepared H-W18O49NW-based electrode with negligible activity decay after 100 cycles.

Modifying Metastable Sr1–xBO3−δ (B = Nb, Ta, and Mo) Perovskites for Electrode Materials

“…Previous studies have demonstrated that oxygen vacancies (V O ) can be generated in metal oxides under oxygen-reduced conditions resulting in changes to the electronic structure and powder appearance. 5 , 9 , 12 , 31 , 37 Therefore, to verify that visual changes are not due to the formation of V O , the binding environment of O in SNO NPs was probed ( Figure 3 a). The deconvoluted O 1s XPS spectra of as-synthesized NPs identifies four peaks assigned to SrNbO 3 lattice oxygen (529.2 eV), Sr 0.7 NbO 3−δ′ lattice oxygen (530.0 eV), oxygen defects (531.5 eV), and surface hydroxyls (532.6 eV).…”

“…EIS spectroscopy was performed to further characterize the electrical activity during the IMT for these NPs. This technique has been used to monitor changes in the electronic conductivity of reduced W 18 O 49 , 5 TiO 2 , 37 and SrTiO 3 . 41 The corresponding Nyquist plots for the reduced SNO NPs are shown in Figure 4 .…”

“…This enables the design of new energy-efficient schemes for electrochemical applications. 5 While there have been attempts to synthesize nanocrystals of these conductive perovskite oxides using solution-based methods, 1 , 6 the resultant insulating nanoparticles (NPs) possess B-site cations that are stabilized in highly oxidized states (i.e., Nb 5+ , Ta 5+ , Mo 6+ , etc.). 7 These multivalent B-site cations, present as surface/deep defects, 1 , 8 limit charge transport by modifying the available free carriers.…”

Metal and Metal Oxide Electrocatalysts for Redox Flow Batteries

“…Recently, Bayeh et al [ 107 ] presented a novel approach by synthesizing and evaluating a nonstoichiometric tungsten oxide W 18 O 48 (WO 2.72 ) as a catalyst for the V(IV)/V(V) redox reaction. As reported previously in the literature, [ 165 ] the reduction of WO 3 , which has a perfect crystalline structure with stacked layers of corner‐sharing WO 6 octahedra (see Figure ), to W 18 O 48 nanowires yields a crystalline structure with oxygen‐containing defects that consist of many hexagonal channels.…”

Graphite Felt Modified by Atomic Layer Deposition with TiO2 Nanocoating Exhibits Super-Hydrophilicity, Low Charge-Transform Resistance, and High Electrochemical Activity

“…Therefore, in order to enhance the electrochemical activity of GF modified with metal oxides, typically a post-annealing process in a reducing atmosphere (containing H 2 gas) is necessary. Because the concentration of oxygen vacancies in the lattice of metal oxides will be increased during the H 2 thermal treatment process to form defect-rich non-stoichiometric metal oxides, a lower resistance of metal oxides for obtaining higher electrochemical activity is found [ 46 , 47 ].…”