Prussian blue analogues (PBAs) have recently been proposed as electrode materials for low-cost, long-cycle-life, and high-power batteries. However, high-capacity bimetallic examples show poor cycle stability due to surface instabilities of the reduced states. The present work demonstrates that, relative to single-component materials, higher capacity and longer cycle stability are achieved when using Prussian blue analogue core@shell particle heterostructures as the cathode material for Li-ion storage. Particle heterostructures with a size dispersion centered at 210 nm composed of a high-capacity K(0.1)Cu[Fe(CN)(6)](0.7)·3.8H(2)O (CuFe-PBA) core and lower capacity but highly stable shell of K(0.1)Ni[Fe(CN)(6)](0.7)·4.1H(2)O have been prepared and characterized. The heterostructures lead to the coexistence of both high capacity and long cycle stability because the shell protects the otherwise reactive surface of the highly reduced state of the CuFe-PBA core. Furthermore, interfacial coupling to the shell suppresses a known structural phase transition in the CuFe-PBA core, providing further evidence of synergy between the core and shell. The structure and chemical state of the heterostructure during electrochemical cycling have been monitored with ex situ X-ray diffraction and X-ray absorption experiments and compared to the behavior of the individual components.
We demonstrate that core-shell nanoparticles consisting of two different Prussian blue analogues, one high capacity and the other robust, can provide enhanced rate capability as cathode materials in sodium-ion batteries.
A series of photomagnetic coordination
heterostructures, based on the light-switchable Prussian blue analogue
·mH2O (RbCoFe-PBA) as the core and the ferromagnetic K
·nH2O (KNiCr-PBA) as the shell, was studied using powder X-ray diffraction,
down to 100 K, and magnetometry, down to 2 K, to investigate the influence
of the shell thickness on light-induced magnetization changes and
gain insight into the mechanism. The core material is known to undergo
a charge-transfer-induced spin transition (CTIST), and synchrotron
powder diffraction was used to monitor structural changes in both
the core and the shell associated with the thermally and optically
induced CTIST of the core. Significant lattice contraction in the
RbCoFe-PBA core upon cooling through the high-spin to the low-spin
state transition near ∼260 K induces strain on the KNiCr-PBA
shells. This lattice strain in the shell can be relieved either by
thermal cycling back to high temperature or by using light to access
the metastable high-spin state of the core at low temperature. The
different extents of strain in the KNiCr-PBA shell are reflected in
low-temperature, low-field magnetization versus temperature data in
the light and dark states. A broader magnetic transition at T
c ≈ 70 K in the dark state relative to
the light state reflects the greater dispersion of nearest-neighbor
contacts and exchange energies induced by the structural distortions
of the strained state. Analyses for different shell thicknesses, coupled
with high-field magnetization data, support a mechanism whereby the
light-induced magnetization changes in the KNiCr-PBA shell are due
to realignment of the local magnetic anisotropy as a result of the
structural changes in the shell associated with the optical CTIST
of the core. Through magnetization and structural analyses, the depth
to which the properties of the shell are influenced by the core–shell
architecture was estimated to be between 40 and 50 nm.
A one-step synthesis of Prussian blue nanoparticles possessing a concentration gradient of Gd3+ counterions, g-Gd-PB, has been developed, and the potential for the particles to perform as both MRI positive contrast agents and photothermal therapy agents is demonstrated. The synthesis of potassium/gadolinium ironhexacyanoferrate is performed under increasing concentration of Gd3+ ions forming particles with a higher concentration of gadolinium toward the outer layers. The proton relaxivity (r1) measured for the particles is 12.3 mM(-1) s(-1), and T1 weighted images of phantoms containing the particles show their potential as MRI contrast agents. In addition, the Prussian blue host can rapidly and efficiently convert energy from near-IR light into thermal energy, allowing g-Gd-PB to be used as a photothermal therapy agent. The photothermal properties are demonstrated by measuring temperature changes of particle suspensions under irradiation and by photothermal ablation of CCRF-CEM cancer cells.
The influence of particle size on the electrochemical properties of guest-ion storage materials has attracted much attention because of the extensive need for long cycle-life, high energy density, and high power batteries. The present work describes a systematic study of the effect of particle size on the guest-ion storage capabilities of a cyanide-bridged coordination polymer. A series of nickel hexacyanoferrate particles ranging from approximately 40 to 400 nm were synthesized by a co-precipitation method and were used as the cathode material for both Li-ion and Na-ion insertion/extraction experiments using organic electrolyte. A large polarization was observed for the largest particles during Li-ion cycling, indicating a heterogeneous ion concentration within the lattice. As a consequence, the available capacity of Li-ion intercalation at high rates is significantly improved by reducing the particle size. On the other hand, Na-ion intercalation shows excellent rate capability regardless of the particle size.Scheme 1 Schematic depiction of alkali ion insertion mechanism in the cubic Prussian blue analogue structure. The cyanometallate vacancies and coordinated and zeolitic water molecules are omitted for clarity.
X-ray absorption spectroscopy experiments of core@shell, photomagnetic, and Prussian Blue analogue heterostructures obtain the local structure around the magnetic transition-metal ions before and after illumination. Two samples, containing nickel hexacyanochromate (Ni−Cr) and cobalt hexacyanoferrate (Co−Fe) in Ni−Cr@Co−Fe and Co−Fe@ Ni−Cr geometries, were studied. Both materials display the wellknown photoinduced valence tautomerism for Co−Fe, accompanied by a large change in cobalt to nitrogen distances. Furthermore, these experimental results show a structural coupling of the photoactive Co−Fe layer to the passive Ni−Cr layer. Finally, the strain across the heterostructured interface in Co−Fe and Ni−Cr containing core@shell models is investigated with simulations that use custom potentials derived from density functional theory calculations.
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