Defective by design: vanadium-substituted iron oxide nanoarchitectures as cation-insertion hosts for electrochemical charge storage

Abstract

Vanadium-substituted iron oxide aerogels (2 : 1 Fe : V ratio; VFe 2 O x ) are synthesized using an epoxideinitiated sol-gel method to form high surface-area, mesoporous materials in which the degree of crystallinity and concentration of defects are tuned via thermal treatments under controlled atmospheres. Thermal processing of the X-ray amorphous, as-synthesized VFe 2 O x aerogels at 300 C under O 2 -rich conditions removes residual organic byproducts while maintaining a highly defective g-Fe 2 O 3 -like local structure with minimal long-range order and vanadium in the +5 state. When assynthesized VFe 2 O x aerogels are heated under low partial pressure of O 2 (e.g., flowing argon), a fraction of vanadium sites are reduced to the +4 state, driving crystallization to a Fe 3 O 4 -like cubic phase.Subsequent thermal oxidation of this nanocrystalline VFe 2 O x aerogel re-oxidizes vanadium +4 to +5, creating additional cation vacancies and re-introducing disordered oxide domains. We correlate the electrochemical charge-storage properties of this series of VFe 2 O x aerogels with their degree of order and chemical state, as verified by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. We find that the disordered O 2 -heated VFe 2 O x aerogel yields the highest Li +and Na + -insertion capacities among this series, approaching 130 mA h g À1 and 70 mA h g À1 , respectively. Direct heat-treatment of the VFe 2 O x aerogel in flowing argon to yield the partially reduced, nanocrystalline form results in significantly lower Li + -insertion capacity (77 mA h g À1 ), which improves to 105 mA h g À1 by thermal oxidation to create additional vacancies and structural disorder.

Aerogel Architectures Boost Oxygen‐Evolution Performance of NiFe2Ox Spinels to Activity Levels Commensurate with Nickel‐Rich Oxides

“…Reagents were used as received. The NiFe 2 O x and Ni‐free FeO x sol–gel‐derived materials were synthesized and processed as aerogels or xerogels according to previously published protocols (see the Supporting Information) . The aerogels and xerogels (amorphous according to X‐ray studies) were heated and cooled at a rate of 2 °C min −1 to 300 or 400 °C with a 4 h dwell time at the peak temperature in either a tube furnace under flowing Ar (≈50 cm 3 min −1 ) or a muffle furnace under static air.…”

“…The aerogels and xerogels (amorphous according to X‐ray studies) were heated and cooled at a rate of 2 °C min −1 to 300 or 400 °C with a 4 h dwell time at the peak temperature in either a tube furnace under flowing Ar (≈50 cm 3 min −1 ) or a muffle furnace under static air. These processing temperatures and atmospheres were selected to eliminate organic byproducts, achieve the desired crystallinity, and minimize particle ripening and the concomitant loss in surface area . The ferrite nanomaterials are denoted according to their thermal treatments, for example, aerogels and xerogels heated to 300 °C in Ar are designated as Aero‐300‐Ar and Xero‐300‐Ar, respectively.…”

“…Because high-surface-area OER catalysts show enhanced performance for ag iven composition, [38][39][40] we expressed NiFe 2 Ox in three nanostructured forms:a erogel, xerogel, and precipitated nanoparticulate powder.T of orm the pore-solid structured forms, we synthesized NiFe 2 Ox gels in av ersatile epoxide-initiated sol-gel method [41][42][43][44][45][46][47] in which propylene oxide or epichlorohydrin scavenges protons from metal(aquo) complexes of Ni II and Fe III chlorides, which thus induces hydrolysis andc ondensation reactions that drivet he formation of metal-oxide networks. The means by which the wet ferrite gel is dried controls the nature of the final porous nanostructure.…”

“…As established in previous studies of NiFe 2 Ox and related VFe 2 Ox aerogels prepared by using the epoxide-initiated solgel method, the crystallinity and morphologyo ft he amorphous as-prepared ferrite gels can be tuned with mild heat treatments ( % 300 8C) under controlled atmospheres( Ar,a ir,o r O 2 ). [42][43][44][45] When thermally processed to 300 8Cu nder low p O 2 (flowing Ar), the aerogela nd xerogel forms of NiFe 2 Ox (designated as Aero-300-Ar and Xero-300-Ar,r espectively) convert to nanocrystalline pore-solid architectures with am agnetite-like spinel structure ( Figure 1);a verage crystallite sizes are 11 and 22 nm, respectively,a sc alculated from X-ray diffraction (XRD) peak broadening. Heatingt he as-dried NiFe 2 Ox aerogel to 400 8Cu nder flowing Ar (Aero-400-Ar) more than doubled the average spinel crystallite size to 26 nm and introduced aminority phase that indexes to metallicN i( Figure1).…”

Encapsulating Cytochrome <em>c</em> in Silica Aerogel Nanoarchitectures without Metal Nanoparticles while Retaining Gas-phase Bioactivity

“…All of the pores in solid aerogels result in much available surface area making aerogels extremely useful for any applications where surface reactions are important. When chemical or biochemical functionality is assembled within the aerogel nanoarchitecture, it has been shown that the physical porosity and enhanced surface area of the aerogels help to improve sensors, as well as electrodes for battery, fuel cell, and supercapacitor applications 11,[15][16][17][18][19][20][21][22][23] . In order to dry aerogels in a way that leaves the porous solid matrix unchanged, it is typical to remove the solvent that remains in the pores after sol-gel synthesis through supercritical solvent extraction.…”