Prussian Blue Analogue Hollow Structure: Advanced Design Strategies And Multifunctional Applications In Energy Storage And Catalysis

Molecular Composition And Structural Characteristics Of Prussian Blue Analogue Hollow Structure

Prussian blue analogues (PBAs) belong to a family of transition metal hexacyanometallates with the general formula A_xM_y[M'(CN)₆]_z·nH₂O, where A represents alkali or alkaline earth cations (Na⁺, K⁺, Ca²⁺), M and M' denote transition metals (Fe, Mn, Co, Ni, Cu, Zn), and □ signifies [M'(CN)₆] vacancies3. The crystal structure exhibits a face-centered cubic lattice wherein metal ions are interconnected through cyanide bridges (M-NC-M') in an orthogonal arrangement, forming a three-dimensional open framework analogous to the ABX₃ perovskite structure4. The large tetrahedral A-sites accommodate guest ions and water molecules, while the framework's inherent porosity (typically 0.32–0.46 nm channel diameter) facilitates rapid ionic diffusion2,4.

Hollow PBA structures introduce additional hierarchical porosity beyond the intrinsic micropores. These architectures can be classified into two primary categories: (1) closed hollow structures featuring large central cavities with thin shells (e.g., yolk-shell, multi-shell nanoboxes), and (2) open interconnected porous structures with continuous macroporous networks (e.g., nanocages with corner voids, nanoframes with face voids, three-dimensional ordered macroporous single crystals)1. The introduction of macropores (50–1000 nm) significantly reduces ion diffusion pathways—from bulk particle dimensions to shell thickness (typically 20–100 nm)—and increases tolerance to lattice expansion during ion insertion/extraction cycles1.

Key structural parameters influencing performance include:

• Vacancy concentration (y): Higher [M'(CN)₆] vacancy ratios (y = 0.3–0.5) generate coordinated water molecules (MN₅O octahedra) that enhance hydrophilicity but may impede ionic transport if interstitial water is not removed15
• Shell thickness: Optimal values of 20–50 nm balance mechanical stability with short diffusion distances1
• Pore interconnectivity: Three-dimensionally ordered macroporous (3DOM) structures with inverse opal topology provide superior mass transport compared to isolated cavities1
• Crystallinity: Single-crystalline hollow structures exhibit fewer grain boundaries and enhanced electronic conductivity relative to polycrystalline aggregates1

The hexagonal phase PBAs recently reported demonstrate unconventional crystal structures with larger channel dimensions (0.52 nm vs. 0.32 nm for cubic phases) and specific surface areas exceeding 1000 m²/g, representing a 1.5-fold enhancement in gas adsorption capacity6.

Synthesis Strategies For Prussian Blue Analogue Hollow Structure

Template-Assisted Methods For Controlled Hollow Architecture

The synthesis of hollow PBA structures requires precise control over nucleation and growth kinetics to prevent premature crystallization outside template interstices. Traditional PBA synthesis involves rapid precipitation upon mixing precursor solutions due to extremely low solubility products (K_sp ~ 10^-40), yielding irregular solid particles1. Template-mediated approaches overcome this limitation through spatial confinement and kinetic modulation.

Hard template strategies employ sacrificial scaffolds to define hollow morphologies:

• Polystyrene (PS) sphere templates: Close-packed PS nanospheres (200–500 nm diameter) arranged in cubic dense packing serve as molds for 3DOM PBA synthesis. A weak acid-assisted dissociation strategy slows PBA crystallization by protonating [Fe(CN)₆]⁴⁻ ions, enabling infiltration into template interstices before nucleation. Subsequent template removal via calcination or solvent dissolution yields ordered macroporous single crystals with tetrakaidecahedral morphology1. This method achieved PBA structures with 300–400 nm macropores and 30–50 nm wall thickness, delivering specific capacities of 150–170 mAh/g at 0.1C in sodium-ion batteries with 85% capacity retention after 1000 cycles1.

• Ionic liquid-mediated synthesis: Ionic liquids (e.g., 1-butyl-3-methylimidazolium tetrafluoroborate) provide polar/nonpolar microdomains that self-assemble into nanostructured templates. Slow dropwise addition of metal salt solutions into hexacyanoferrate-containing ionic liquid/organic solvent mixtures, followed by 48-hour aging at 25°C, produces hierarchical porous PBAs with tunable pore sizes (5–50 nm)10. The strong electrostatic and hydrogen bonding interactions in ionic liquids retard crystallization rates, enabling morphology control without external templates10.

• Layered double hydroxide (LDH) templates: Intercalation of [Fe(CN)₆]³⁻ ions into LDH interlayers followed by in-situ coordination with trivalent metal ions generates two-dimensional PBA nanosheets. Acid dissolution of LDH layers releases substrate-free 2D PBA sheets (0.6–0.8 μm lateral size, 20–30 nm thickness) with high surface area and unsaturated cyanide coordination sites13. This approach avoids organic ligands or inorganic substrates that block active sites, achieving specific surface areas >500 m²/g13.

Self-Templating And Etching Approaches

Kirkendall effect-based hollowing: Differential diffusion rates between core and shell components drive void formation. For example, pre-synthesized solid PBA nanocubes undergo selective etching in acidic solutions (pH 2–4, 60–80°C, 2–6 hours), where outer layers recrystallize while inner regions dissolve, creating yolk-shell or multi-shell structures1. Yamauchi and colleagues pioneered this method to produce PB yolk-shell, shell-in-shell, and yolk-double-shell architectures with cavity sizes of 50–200 nm and shell thicknesses of 10–30 nm1.

Epitaxial growth strategies: Low et al. reported self-templated synthesis of PBA nanocages and nanoframes via controlled epitaxial overgrowth on cubic PBA seeds. By adjusting precursor concentration ratios and reaction temperatures (25–60°C), selective deposition occurs at cube edges or corners, followed by core etching to yield open frameworks with 20–40 nm strut widths1. These structures exhibit 40–60% void fractions and maintain single-crystalline character.

Rapid Synthesis Via Micromixing For Monoclinic Phase PBAs

Continuous rapid mixing using micromixers enables synthesis of monoclinic (Prussian white) phase PBAs with controlled particle sizes (200–2000 nm) within minutes. A mixture of Na₄[Fe(CN)₆] and NaCl (solution A) is rapidly combined with MnCl₂ or FeCl₂ (solution B) in a micromixer, generating nano-precursor slurry that is subsequently aged at 80–160°C for 3 minutes to 2 hours5. This method produces Na_xM[Fe(CN)₆]_y·zH₂O (M = Mn, Fe; 1.5 < x < 2; 0.5 < y < 1) with monoclinic symmetry and theoretical capacities approaching 170 mAh/g after dehydration to rhombohedral Na₂M[Fe(CN)₆]5. The rapid mixing prevents uncontrolled nucleation, while high-temperature aging promotes crystallization and water removal.

Physicochemical Properties And Performance Metrics Of Hollow Prussian Blue Analogues

Electrochemical Characteristics For Energy Storage

Hollow PBA structures demonstrate superior electrochemical performance compared to solid counterparts due to shortened ion diffusion pathways, enhanced electrolyte penetration, and improved structural stability during cycling.

Sodium-ion battery cathodes: Three-dimensionally ordered macroporous PBA single crystals (Na_1.6Mn[Fe(CN)₆]_0.9·0.5H₂O) exhibit:

• Specific capacity: 152 mAh/g at 0.1C (vs. 120 mAh/g for solid particles)1
• Rate capability: 95 mAh/g at 10C (62% retention vs. 35% for solid analogs)1
• Cycle life: 85% capacity retention after 1000 cycles at 1C (vs. 60% for solid particles)1
• Voltage plateau: 3.4–3.5 V vs. Na/Na⁺5

The ordered macroporous architecture reduces Na⁺ diffusion distance from ~500 nm (solid particle radius) to ~30 nm (shell thickness), decreasing diffusion time by a factor of ~280 (assuming Fickian diffusion, t ∝ L²)1. Additionally, the interconnected pore network accommodates ~12% volume expansion during sodiation without particle fracture1.

Aqueous electrolyte batteries: PBA anodes with tunable redox potentials enable high-rate, long-cycle-life aqueous systems. Hexacyanometalate groups exhibit two distinct redox reactions at different potentials (e.g., −0.2 V and +0.6 V vs. Ag/AgCl), which can be tuned by substituting electrochemically inactive components18. Hollow PBA anodes paired with PBA cathodes in neutral aqueous electrolytes (1 M Na₂SO₄) achieve:

• Energy efficiency: >90% at 1C18
• Cycle stability: >5000 deep discharge cycles with <20% capacity fade18
• Rate capability: 80% capacity retention at 20C18

The hollow structure minimizes electrochemical decomposition of the aqueous electrolyte by reducing local current densities and facilitating heat dissipation18.

Catalytic Activity In Electrocatalytic Water Splitting

Phosphorized hollow PBA composites serve as efficient electrocatalysts for hydrogen evolution reaction (HER). NiCoP nanoparticles (30–50 nm) uniformly distributed on Mo-vacancy-rich MoS₂ nanosheets (NiCoP/MoS₂-V_Mo) are synthesized by immersing Ni-doped MoS₂ in K₃[Co(CN)₆] solution to form NiCo-PBA, followed by phosphorization at 300–350°C under Ar/H₂ atmosphere11. This composite exhibits:

• Overpotential: 78 mV at 10 mA/cm² (vs. 120 mV for bare MoS₂)11
• Tafel slope: 42 mV/dec (indicating Volmer-Heyrovsky mechanism)11
• Stability: <5% current density loss after 20-hour chronopotentiometry at 10 mA/cm²11
• Composition: Ni 0.5–2.0 wt%, Co 0.3–1.5 wt%, P 1.0–4.0 wt%11

The NiCoP nanoparticles modulate the electronic structure of MoS₂, enhancing OH⁻ adsorption and facilitating intermediate formation (H_ads), while Mo vacancies provide additional active sites11. The hollow PBA-derived structure ensures high dispersion of catalytic nanoparticles and prevents agglomeration during phosphorization.

Gas Adsorption And Separation Performance

Hexagonal phase Cu-Co PBAs with open structures demonstrate exceptional gas separation capabilities due to enlarged channel dimensions and high specific surface areas (>1000 m²/g)6. Performance metrics include:

• CO₂ adsorption capacity: 4.5 mmol/g at 298 K, 1 bar (vs. 3.0 mmol/g for cubic PBAs)6
• CO₂/CH₄ selectivity: 18:1 at 298 K (vs. 12:1 for cubic analogs)6
• C₃H₆/C₂H₄ selectivity: 25:1 at 298 K (vs. 15:1 for cubic PBAs)6
• Regeneration: >95% capacity retention after 10 adsorption-desorption cycles at 120°C6

The hexagonal phase is synthesized via co-precipitation without high-temperature treatment, yielding prism-shaped crystals with 0.52 nm channels (vs. 0.32 nm for cubic phases) and larger interstitial spaces that preferentially adsorb larger or more polarizable molecules6.

Applications Of Prussian Blue Analogue Hollow Structure Across Multiple Domains

Energy Storage Systems — Sodium-Ion And Aqueous Batteries

Sodium-ion battery cathodes: The abundance and low cost of sodium (Na: $150/ton vs. Li: $17,000/ton as of 2023) make sodium-ion batteries attractive for grid-scale energy storage. Hollow PBA cathodes address key challenges including limited rate capability and cycle life degradation caused by structural collapse during repeated Na⁺ insertion/extraction5.

Case Study: 3DOM PBA Single Crystals In Grid Storage — Energy Storage: A prototype 1 kWh sodium-ion battery module employing 3DOM Na_1.6Mn[Fe(CN)₆]_0.9 cathodes paired with hard carbon anodes demonstrated:

• Energy density: 120 Wh/kg (cell level)1
• Power density: 500 W/kg at 80% depth of discharge1
• Cycle life: 3000 cycles with 80% capacity retention at 1C, 25°C1
• Cost projection: <$80/kWh at scale (vs. $120/kWh for lithium iron phosphate)1

The ordered macroporous structure maintained structural integrity even after 12% volume expansion per cycle, with post-mortem TEM analysis revealing intact shell morphology and minimal particle cracking1. Recommended engineering strategies include:

• Electrode formulation: 85 wt% active material, 10 wt% conductive carbon (Super P), 5 wt% PVDF binder to maximize energy density while ensuring electronic percolation1
• Electrolyte optimization: 1 M NaPF₆ in EC:DEC (1:1 v/v) with 2 wt% fluoroethylene carbonate (FEC) additive to stabilize solid-electrolyte interphase (SEI) on hard carbon anodes1
• Voltage window: 2.0–