Magnetic-Field-Assisted Fe Nanowire Conformable Aerogels Galvanically Displaced to Cu and Pt for Three-Dimensional Electrode Applications

Abstract

There is an increasing need for free-standing, conformal electrodes for practical energy storage devices. To address this, we demonstrate the magnetic-field-assisted synthesis of interpenetrating Fe nanowire (FeNW) gels without the use of templates or composite scaffold material over a range of magnetic fields. In either a wet gel or a supercritical dried state as an aerogel, the FeNWs may be pressed into thin or conformal films. Varying the applied magnetic field strength with a solenoid during chemical synthesis resulted in increased nanowire length and local orientation of the FeNWs with increasing magnetic field strength, with approximately 80 nm diameters across field strengths of 0-150 mT. Flowing K₂PtCl₄ or CuSO₄·5H₂O solutions through the wet iron gels to achieve the near complete galvanic displacement of iron to the more noble [PtCl₄]^2- and Cu²⁺ ions resulted in either platinum nanotubes (PtNTs) or copper nanowires (CuNWs) while maintaining a percolating network structure. Similar to the FeNW gels, the PtNT and CuNW gels were able to be supercritical dried and/or pressed into thin or conformal electrode films. CuNW and PtNT films demonstrated good potential as capacitive and oxygen reduction reaction electrodes, respectively. The magnetic-field-assisted synthesis of ferromagnetic iron nanowires offers a simple, rapid, and tunable method that, when combined with galvanic displacement with more noble metal ions, may enable a wide range of metal, alloy, and multimetallic nanowires and nanotubes for energy storage, sensing, and catalytic applications.

Keywords: 3D electrodes; aerogels; copper; fuel cells; iron; nanotubes; nanowires; platinum.

Figure 1

MFA synthesis scheme (a): FeCl₃·6H₂O and NaBH₄ are mixed inside a variable magnetic field strength solenoid (i) to form a metal gel (ii); the metal can be solvent exchanged and supercritical dried to form an Fe aerogel (iv), galvanically displaced in CuSO₄·5H₂O to form a Cu metal gel (iii), or galvanically displaced in K₂PtCl₄ to form a Pt metal gel (v). Metal gels or supercritical dried aerogels can be pressed into free-standing films (vi–viii). Aerogel conformal coatings (b) of an iron gel conformed on a Teflon stub (i–ii); a copper gel conformed around a stainless-steel cylinder (iii); gel from (iii) with the stainless-steel cylinder removed (iv). Proposed mechanism (c): initial Fe nanoparticle formation (i); nanoparticle alignment and coalescence along magnetic field lines (ii); and extension of coalesced nanoparticles into nanowires (iii).

Figure 2

Scanning electron micrographs of iron nanowires synthesized in magnetic fields with strengths of 0 mT (a), 9 mT (b), 19 mT (c), 37 mT (d), 75 mT (e), and 150 mT (f). Colorized nanowire angular orientation of FeNW synthesized in a 150 mT field (g). Orientation order parameter (OOP) versus image frame size based on colorized nanowire angular orientation (h). Average OOP versus synthesis field strength (i).

Figure 3

Iron nanowire diameters as a function of magnetic field strength during synthesis (a). Galvanic displacement scheme for iron nanowires as a sacrificial template (b). SEM images of Fe nanowires galvanically displaced to Cu nanowires (c, d). Energy dispersive X-ray spectroscopy (EDS) for copper nanotubes (e). SEM images of Fe nanowires galvanically displaced to Pt nanotubes (f, g). EDS spectrum for Pt nanotubes (h). Cu nanowire outer diameter distribution (i). Pt nanotube inner and outer diameter distributions (j). Pt nanotube shell thickness distribution (k).

Figure 4

XRD patterns for the FeNWs synthesized at various applied magnetic field strengths (a) and for the FeNWs, CuNWs, and PtNTs (b) along with Fe, Cu, Cu₂O and Pt reference patterns. Transmission electron microscopy images of the FeNWs (c, f), CuNWs (d, g), and PtNTs (e, h) collected at 100 kx (center row) and 800 kx (bottom row). The insets show electron diffraction patterns determined through Fast Fourier Transform (f, h) and SAED (g).

Figure 5

XPS data of the FeNWs produced through magnetic field-assisted synthesis in a 150 mT field and galvanic displacement. Survey scans (a) and high resolution O 1s scans (b) of the FeNWs, CuNWs and PtNTs, a high resolution Fe 2p scan of the FeNWs (c), a high resolution Cu 2p scan (d) and Cu LMM Auger scan (inset) of the CuNWs, and a Pt 4f high resolution scan of the PtNTs (e).

Figure 6

Nitrogen gas adsorption–desorption isotherms, cumulative pore volumes, and pore size distributions for FeNW (a, b), CuNW (c, d), and PtNT (e, f).

Figure 7

Photograph of an FeNW supercritical dried aerogel attached to a stir bar magnet (a). Vibration sample magnetometry (VSM) hysteresis curves for FeNW aerogels prepared in the presence of 0 and 150 mT fields (b). Representative stress–strain curve of an FeNW aerogel (c).

Figure 8

Electrochemical characterization of CuNWs was performed in 1 M KOH for electrochemical impedance spectroscopy (a–c) and cyclic voltammetry (d–f). EIS spectra (blue data points) are shown with the high frequency range in the inset; RLC fit is overlaid in red. The specific capacitance, C_sp, for each impedance data point from (a) is shown in (b) with the EIS fit model (c). Cyclic voltammetry curves for scan rates of 1, 5, 10, 25, and 50 mV/s (d). CV curve for 1 mV/s is shown separately (e). Current density versus (scan rate)^1/2 for the reduction peaks at approximately −0.5 V (f).

Figure 9

Electrochemical characterization of PtNTs was performed in 0.5 M H₂SO₄ for electrochemical impedance spectroscopy (a, b) and cyclic voltammetry (c–g) and rotating disk electrode linear sweep voltammograms (h, i). EIS spectra in blue data points is shown with the high frequency range in the inset; RLC fit is overlaid in red (a). The specific capacitance, C_sp, for each data point is shown in (b) with the RLC fit model in the inset. Cyclic voltammetry curves for scan rates of 0.5, 1, 5, 10, 25, and 50 mV/s (c). CV curve for 0.5 mV/s is shown separately (d). Current density versus (scan rate)^1/2 for the Pt reduction peaks at approximately 0.4–0.5 V (vs Ag/AgCl) (e). Specific capacity, C_sp, determined from the nonfaradaic region in (c) at approximately 0.2 V (vs Ag/AgCl) (f). The 1st and 1000th cycles of cyclic voltammetry were performed at 25 mV/s (g). Linear sweep voltammetry curves from 0.8 to 0 V (vs Ag/AgCl) conducted in oxygen saturated electrolyte (h). The reciprocal of current density from (h) versus (scan rate)^−1/2 (i).