Synthesis of Monocrystalline Nanoframes of Prussian Blue Analogues by Controlled Preferential Etching

General synthesis of high-entropy single-atom nanocages for electrosynthesis of ammonia from nitrate

Given the growing emphasis on energy efficiency, environmental sustainability, and agricultural demand, there's a pressing need for decentralized and scalable ammonia production. Converting nitrate ions electrochemically, which are commonly found in industrial wastewater and polluted groundwater, into ammonia offers a viable approach for both wastewater treatment and ammonia production yet limited by low producibility and scalability. Here we report a versatile and scalable solution-phase synthesis of high-entropy single-atom nanocages (HESA NCs) in which Fe and other five metals-Co, Cu, Zn, Cd, and In-are isolated via cyano-bridges and coordinated with C and N, respectively. Incorporating and isolating the five metals into the matrix of Fe resulted in Fe-C5 active sites with a minimized symmetry of lattice as well as facilitated water dissociation and thus hydrogenation process. As a result, the Fe-HESA NCs exhibited a high selectivity toward NH3 from the electrocatalytic reduction of nitrate with a Faradaic efficiency of 93.4% while maintaining a high yield rate of 81.4 mg h-1 mg-1.

Hierarchical MOFs with Good Catalytic Properties and Structural Stability in Oxygen-Rich and High-Temperature Environments

Metal-organic framework materials are ideal materials characterized by open frameworks, adjustable components, and high catalytic activity. They are extensively utilized for catalysis. Due to decomposition and structural collapse under high temperatures and an oxygen-rich environment, the potential of thermal catalysis is greatly limited. In this research, Co-rich hollow spheres (Co-HSs) with a gradient composition are designed and synthesized to investigate their thermal catalytic properties in the ammonium perchlorate（AP）system. The results demonstrate that Co-HSs@AP exhibits good thermal catalytic activity and a high-temperature decomposition of 292.5 °C, which is 121.6 °C lower than pure AP. The hierarchical structure confers structural stability during the thermal decomposition process. Thermogravimetry-infrared indicates that the inclusion of Co-HSs successfully boosts the level of reactive oxygen species and achieves thorough oxidation of NH3. Based on the above phenomenon, macro dynamics calculations are carried out. The results show that Co-HSs can promote the circulation of lattice oxygen and reactive oxygen species and the multidimensional diffusion of NH3 in an oxygen-rich environment. This material has significant potential for application in the fields of thermal catalysis and ammonia oxidation.

Toward Ultrahigh Rate and Cycling Performance of Cathode Materials of Sodium Ion Battery by Introducing a Bicontinuous Porous Structure

The emerging sodium-ion batteries (SIBs) are one of the most promising candidates expected to complement lithium-ion batteries and diversify the battery market. However, the exploitation of cathode materials with high-rate performance and long-cycle stability for SIBs has remained one of the major challenges. To this end, an efficient approach to enhance rate and cycling performance by introducing an ordered bicontinuous porous structure into cathode materials of SIBs is demonstrated. Prussian blue analogues (PBAs) are selected because they are recognized as a type of most promising SIB cathode materials. Thanks to the presence of 3D continuous channels enabling fast Na+ ions diffusion as well as the intrinsic mechanical stability of bicontinuous architecture, the resultant PBAs exhibit excellent rate capability (80 mAh g-1 at 2.5 A g-1) and ultralong cycling life (>3000 circulations at 0.5 A g-1), reaching the top performance of the reported PBA-based cathode materials. This study opens a new avenue for boosting sluggish ion diffusion kinetics in electrodes of rechargeable batteries and also provides a new paradigm for solving the dilemma that electrodes' failure due to high-stress concentration upon ion storage.

Hollow Stair-Stepping Spherical High-Entropy Prussian Blue Analogue for High-Rate Sodium Ion Batteries

Prussian blue analogues (PBAs) are considered to be one of the most suitable sodium storage materials, especially with the introduction of the high-entropy (HE) concept into their structure to further improve their various abilities. However, severe agglomeration of the HEPBA particles still limits the fast charging capabilities. Here, an HEPBA (Nax(FeMnCoNiCu)[Fe(CN)6]y□1-y·nH2O) with a hollow stair-stepping spherical structure has been prepared through the chemical etching process of the traditional cubic structure of HEPBA. Electrochemical characterization (sodium ion battery), kinetic analysis, and COMSOL Multiphysics simulations reveal that the nature of the high-entropy and the hollow stair-stepping spherical structure can greatly improve the diffusion behavior of Na+ ions. Moreover, the hollow structure effectively mitigates the volume change of HEPBA during SIBs operation, ultimately extending the lifespan. Consequently, the as-prepared HEPBA cathode exhibits excellent rate performance (126.5 and 76.4 mAh g-1 at 0.1 and 4.0 A g-1, respectively) and stable long-term capability (maintaining its 75.6% capacity after 1000 cycles) due to its unique structure. Furthermore, the waste of the etching process can easily be recycled to prepare more HEPBA product. This processing method holds great promise for designing nanostructures of advanced high-entropy Prussian blue analogues for sodium ion batteries.

Modulating the pore and electronic structure for targeted recovery of platinum: Accelerated kinetic and reinforced coordination

Adsorption for recovery of low-concentration platinum (Pt) from the complex composition of acidic digestates was challenging because of slow kinetic and poor affinity. It was expected to be overcome by the improvement of pore size distribution and adsorption site activity. Herein, a series of Prussian blue etchings (PBE) with porosity-rich and activity-high cyano (CN) was synthesized to recover low-concentration Pt. The N2 isotherm results showed that the pore structure evolved from mesoporous to microporous. The Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations results revealed that the modulation of electronic structure converted FeII to FeIII in [FeII(CN)6]4-. The coexistence of micro- and meso-pore structures provided channels to accelerate adsorption and ensured PtII enrichment. The regulation of Fe valence state activated CN, which reinforced the strength of coordination interaction between Pt and Fe-CN- at N-atom. The adsorption rate and maximum capacity of PBE1 were 4.4 and 2.5 times higher than those of PB, respectively, due to the dual efficacy of accelerated kinetic and reinforced coordination. This study systematically analyzes the pivotal role of pore and electronic structure modulation in adsorption kinetic and affinity, which provides a novel strategy for PtII targeted recovery.

Interior-Confined Vacancy in Potassium Manganese Hexacyanoferrate for Ultra-Stable Potassium-Ion Batteries

Metal hexacyanoferrates (HCFs) are viewed as promising cathode materials for potassium-ion batteries (PIBs) because of their high theoretical capacities and redox potentials. However, the development of an HCF cathode with high cycling stability and voltage retention is still impeded by the unavoidable Fe(CN)6 vacancies (VFeCN) and H2O in the materials. Here, a repair method is proposed that significantly reduces the VFeCN content in potassium manganese hexacyanoferrate (KMHCF) enabled by the reducibility of sodium citrate and removal of ligand H2O at high temperature (KMHCF-H). The KMHCF-H obtained at 90 °C contains only 2% VFeCN, and the VFeCN is concentrated in the lattice interior. Such an integrated Fe-CN-Mn surface structure of the KMHCF-H cathode with repaired surface VFeCN allows preferential decomposition of potassium bis(fluorosulfonyl)imide (KFSI) in the electrolyte, which constitutes a dense anion-dominated cathode electrolyte interphase (CEI) , inhibiting effectively Mn dissolution into the electrolyte. Consequently, the KMHCF-H cathode exhibits excellent cycling performance for both half-cell (95.2 % at 0.2 Ag-1 after 2000 cycles) and full-cell (99.4 % at 0.1 Ag-1 after 200 cycles). This thermal repair method enables scalable preparation of KMHCF with a low content of vacancies, holding substantial promise for practical applications of PIBs.

Effect of particle dispersion on electrochemical performance of Prussian blue analogues electrode materials for sodium ion batteries

Prussian blue analogues (PBAs) have advantages such as high voltage and low cost, making them one kind of the promising positive electrode materials for sodium-ion batteries. Particle dispersion is a key physical parameter of electrode materials, and understanding its impact on electrochemical performance is a prerequisite for obtaining high-performance PBAs. In this article, two PBAs samples with different particle dispersion were synthesized through sodium citrate-assisted co-precipitation method by means of staying and stirring. The influence of particle dispersion on electrochemical performance was investigated through polarization curve and AC impedance tests. It was found that PBAs with well-dispersed particles exhibited excellent rate performance, with a capacity of ~120 mAh g-1 at 1 C rate and a capacity retention of 75 % after 100 cycles. The capacity retention rate could reach 63 % at 5 C rate, far higher than that of PBAs samples with poor particle dispersion. From the perspective of electrochemical kinetics analysis, it has been shown that PBAs with well-dispersed particles exhibit smaller electrochemical polarization and faster Na+ diffusion reaction kinetics, which are key factors in achieving excellent rate performance.

Chemically Etched Prussian Blue Analog-WS2 Composite as a Precatalyst for Enhanced Electrocatalytic Water Oxidation in Alkaline Media

The electrochemical water-splitting reaction is a promising source of ecofriendly hydrogen fuel. However, the oxygen evolution reaction (OER) at the anode impedes the overall process due to its four-electron oxidation steps. To address this issue, we developed a highly efficient and cost-effective electrocatalyst by transforming Co-Fe Prussian blue analog nanocubes into hollow nanocages using dimethylformamide as a mild etchant and then anchoring tungsten disulfide (WS2) nanoflowers onto the cages to boost OER efficiency. The resulting hybrid catalyst-derived oxide demonstrated a low overpotential of 290 mV at a current density of 10 mA cm-2 with a Tafel slope of 75 mV dec-1 in 1.0 M KOH and a high faradaic efficiency of 89.4%. These results were achieved through the abundant electrocatalytically active sites, enhanced surface permeability, and high electronic conductivity provided by WS2 nanoflowers and the porous three-dimensional (3D) architecture of the nanocages. Our research work uniquely combines surface etching of Co-Fe PBA with WS2 growth to create a promising OER electrocatalyst. This study provides a potential solution to the challenge of the OER in electrochemical water-splitting, contributing to UN SDG 7: Affordable and clean energy.

Structural/Compositional-Tailoring of Nickel Hexacyanoferrate Electrodes for Highly Efficient Capacitive Deionization

Prussian blue analogs (PBAs) represent a crucial class of intercalation electrode materials for electrochemical water desalination. It is shown here that structural/compositional tailoring of PBAs, the nickel hexacyanoferrate (NiHCF) electrodes in particular, can efficiently modulate their capacitive deionization (CDI) performance (e.g., desalination capacity, cyclability, selectivity, etc.). Both the desalination capacity and the cyclability of NiHCF electrodes are highly dependent on their structural/compositional features such as crystallinity, morphology, hierarchy, and coatings. It is demonstrated that the CDI cell with hierarchically structured NiHCF nanoframe (NiHCF-NF) electrode exhibits a superior desalination capacity of 121.38 mg g-1 , a high charge efficiency of up to 82%, and a large capacity retention of 88% after 40 cycles intercalation/deintercalation. In addition, it is discovered that coating of carbon (C) film over NiHCF can lower its desalination capacity owing to the partial blockage of diffusion openings by the coated C film. Moreover, the hierarchical NiHCF-NF electrode also demonstrates a superior selectivity toward monovalent sodium ions (Na+ ) over divalent calcium (Ca2+ ) and magnesim (Mg2+ ) ions, allowing it to be a promising platform for preferential capturing Na+ ions from brines. Overall, the structural/compositional tailoring strategies would offer a viable option for the rational design of other intercalation electrode materials applied in CDI techniques.

Iron-Locked Hydr(oxy)oxide Catalysts via Ion-Compensatory Reconstruction Boost Large-Current-Density Water Oxidation

Nickel-iron based hydr(oxy)oxides have been well recognized as one of the best oxygen-evolving catalysts in alkaline water electrolysis. A crucial problem, however, is that iron leakage during prolonged operation would lead to the oxygen evolution reaction (OER) deactivation over time, especially under large current densities. Here, the NiFe-based Prussian blue analogue (PBA) is designed as a structure-flexible precursor for navigating an electrochemical self-reconstruction (ECSR) with Fe cation compensation to fabricate a highly active hydr(oxy)oxide (NiFeO_x H_y ) catalyst stabilized with NiFe synergic active sites. The generated NiFeO_x H_y catalyst exhibits the low overpotentials of 302 and 313 mV required to afford large current densities of 500 and 1000 mA cm^-2 , respectively. Moreover, its robust stability over 500 h at 500 mA cm^-2 stands out among the NiFe-based OER catalysts reported previously. Various in/ex situ studies indicate that the Fe fixation by dynamic reconstruction process can reinforce the Fe-activated effect on the OER amenable to the industrial-level large current conditions against the Fe leakage. This work opens up a feasible strategy to design highly active and durable catalysts via thermodynamically self-adaptive reconstruction engineering.

Crystal Face Dominated Fabrication of Prussian Blue Analogue with Oriented Growth and Naturally Nonpreferred Unsaturated Coordination Center

Defects, such as unsaturated coordination centers and vacancies, can fundamentally change materials' inherent properties and growth habits. The development of defect engineering has promoted the application of many technologies, but it is still a great challenge to selectively manufacture defect sites in existing material systems. It is shown here that in situ site-directed tailoring of metal sites in Prussian blue analogs (PBA) can be achieved according to the reducibility differences of different metal atoms, forming naturally nonpreferred unsaturated coordination centers. Meanwhile, the in situ capture of small reducing molecule can realize site-directed tailoring of crystal facets during crystal growth and results in oriented 1D growth. As an oxygen evolution reaction catalyst, the resulted PBA with the nonpreferred unsaturated coordination centers shows a low overpotential of 239 mV at 10 mA cm^-2 in alkali, superior to the original PBAs and the previously reported defective PBA derivatives, which can be ascribed to the unsaturated coordination active center and the unique 1D structure. This work opens up opportunities for producing naturally nonpreferred unsaturated coordination center in nanomaterials for broad applications.

Divalent Oxidation State Ni as an Active Intermediate in Prussian Blue Analogues for Electrocatalytic Urea Oxidation

Urea degradation is one of the most crucial links in the natural nitrogen cycle. Exploring the real active species in the urea electro-oxidation process is of great significance for understanding the urea electro-oxidation mechanism and designing catalysts. A highly active and stable Prussian blue analogue catalyst (PBA@NiFe/NF) loaded on nickel foam was synthesized for electro-oxidation of urea. In situ Raman spectra revealed that Ni in PBA@NiFe/NF was able to maintain a stable divalent nickel (Ni(II)) state for up to 3.5 h during the initial urea oxidation process, which is rarely reported in previous research studies. In addition, with the participation of iron, the Ni-Fe bimetallic center significantly improves the electro-oxidation of urea. Our work provides a new idea for prolonging the Ni(II) activity in electrocatalytic oxidation of urea.

Incorporating Near-Pseudocapacitance Insertion Ni/Co-Based Hexacyanoferrate and Low-Cost Metallic Zn for Aqueous K-Ion Batteries

The limited availability of cathode materials with high specific capacity and significant cycling stability for aqueous K-ion batteries (AKIBs) hinder their further development owing to the large radius of K⁺ (1.38 Å). Prussian blue and its analogs with a three-dimensional frame structure possessing special energy storage mechanism are promising candidates as cathode materials for AKIBs. In this study, K_0.2 Ni_0.68 Co_0.77 Fe(CN)₆  ⋅ 1.8H₂ O (KNCHCF) was prepared as a cathode material for AKIBs. Both the electrochemical activity of Co ions and the near-pseudocapacitance intercalation of KNCHCF enhance K⁺ storage. Therefore, KNCHCF exhibits a superior capacity maintenance rate of 86 % after 1000 cycles at a high current density of 3.0 A g^-1 . The storage mechanism of K⁺ in AKIBs was revealed through ex situ X-ray diffraction, ex situ Fourier transform infrared spectroscopy, and ex situ X-ray photoelectron spectroscopy measurements. Moreover, the assembled K-Zn hybrid battery showed good cycling stability with 93.1 % capacity maintenance at 0.1 A g^-1 after 50 cycles and a high energy density of 96.81 W h kg^-1 . Hence, KNCHCF may be a potential material for the development of AKIBs.

Treatment dependent sodium-rich Prussian blue as a cathode material for sodium-ion batteries

In the preparation of Prussian blue analogs (PBAs), Na⁺ loss and Fe²⁺ oxidation take place when washing with water. Sodium-rich PBAs were prepared with sodium ascorbate aqueous solution as the washing solution, which can suppress the Na⁺ loss and Fe²⁺ oxidation. As the cathode of sodium-ion batteries, it exhibited excellent electrochemical performance.

Monoclinic Bimetallic Prussian Blue Analog Cathode with High Capacity and Long Life for Advanced Sodium Storage

Prussian blue analogs (PBAs) are regarded as promising cathode materials for sodium-ion batteries (SIBs), but most of them suffer from an incompatibility between capacity and structural stability. Herein, an innovative disodium ethylenediaminetetraacetate (Na₂EDTA)-assisted hydrothermal method is proposed to synthesize monoclinic Fe-substituted Ni-rich PBA (H-PBA) cathodes for Na-ion storage. The as-designed H-PBA cathode combines the merits of the low strain of a Ni-based PBA framework and the enhanced capacity of N-Fe³⁺/Fe²⁺ redox sites. It can achieve superior sodium-storage performance in terms of capacity, rate capability, and cycle stability. Moreover, ex situ measurements reveal that solid solution (2.0-3.0 V) and phase-transition (3.0-4.0 V) reactions occur during the charge/discharge process to allow almost 1.5 Na⁺ storage in the H-PBA lattice. Meanwhile, the H-PBA//NaTi₂(PO₄)₃@C full cell also delivers remarkable electrochemical properties. Prospectively, this work would promote the practical application of SIBs in grid-scale electric energy storage.

Recent Progress and Future Advances on Aqueous Monovalent-Ion Batteries towards Safe and High-Power Energy Storage

Aqueous monovalent-ion batteries have been rapidly developed recently as promising energy storage devices in large-scale energy storage systems owing to their fast charging capability and high power densities. In recent years, Prussian blue analogues, polyanion-type compounds, and layered oxides have been widely developed as cathodes for aqueous monovalent-ion batteries because of their low cost and high theoretical capacity. Furthermore, many design strategies have been proposed to expand their electrochemical stability window by reducing the amount of free water molecules and introducing an electrolyte addictive. This review highlights the advantages and drawbacks of cathode and anode materials, and summarizes the correlations between the various strategies and the electrochemical performance in terms of structural engineering, morphology control, elemental compositions, and interfacial design. Finally, this review can offer rational principles and potential future directions in the design of aqueous monovalent-ion batteries.

Oxygen-Plasma-Induced Hetero-Interface NiFe2O4/NiMoO4 Catalyst for Enhanced Electrochemical Oxygen Evolution

The electrolysis of water to produce hydrogen is an effective method for solving the rapid consumption of fossil fuel resources and the problem of global warming. The key to its success is to design an oxygen evolution reaction (OER) electrocatalyst with efficient conversion and reliable stability. Interface engineering is one of the most effective approaches for adjusting local electronic configurations. Adding other metal elements is also an effective way to enrich active sites and improve catalytic activity. Herein, high-valence iron in a heterogeneous interface of NiFe2O4/NiMoO4 composite was obtained through oxygen plasma to achieve excellent electrocatalytic activity and stability. In particular, 270 mV of overpotential is required to reach a current density of 50 mA cm-2, and the overpotential required to reach 500 mA cm-2 is only 309 mV. The electron transfer effect for high-valence iron was determined by X-ray photoelectron spectroscopy (XPS). The fast and irreversible reconstruction and the true active species in the catalytic process were identified by in situ Raman, ex situ XPS, and ex situ transmission electron microscopy (TEM) measurements. This work provides a feasible design guideline to modify electronic structures, promote a metal to an active oxidation state, and thus develop an electrocatalyst with enhanced OER performance.

3D Spatial Combination of CN Vacancy-Mediated NiFe-PBA with N-Doped Carbon Nanofibers Network Toward Free-Standing Bifunctional Electrode for Zn-Air Batteries

Constructing flexible free-standing electrodes with efficient bifunctional performance is significant for improving the performance of flexible Zinc-air batteries. Herein, a flexible free-standing bifunctional electrode (N2 -NiFe-PBA/NCF/CC-60) is constructed by the 3D spatial combination of CN vacancy-mediated NiFe Prussian Blue Analogue (NiFe-PBA) and N-doped carbon nanofibers (NCF) rooted on carbon cloth (CC). The in situ formed CN vacancies by N2 -plasma activation tune the local coordination environment and electronic structure of Ni-Fe active sites in NiFe-PBA, thus improving the oxygen evolution reaction (OER) catalytic intrinsic activity, and restraining the loss of Fe element during OER process. The combination of NiFe-PBA and NCF presents a 3D interworking network structure, which exhibits a large specific surface and excellent electrical conductivity, thus guaranteeing sufficient, stable, and efficient oxygen reduction reaction (ORR)/OER active sites. Therefore, the N2 -NiFe-PBA/NCF/CC-60 electrode delivers high-efficiency OER activity with a low overpotential (270 mV at 50 mA cm-2 ) and excellent ORR performance with a positive potential of 0.89 V at 5 mA cm-2 . The N2 -NiFe-PBA/NCF/CC-60 based Zn-air batteries display outstanding discharge/charge stability for 2000 cycles. Meanwhile, the corresponding flexible Zn-air batteries with satisfactory mechanical properties exhibit a low voltage gap of 0.52 V at 1.0 mA cm-2 .

Interstitial Water Improves Structural Stability of Iron Hexacyanoferrate for High-Performance Sodium-Ion Batteries

Prussian blue analogues (PBAs) are considered one of the promising cathodes for sodium-ion batteries because of their low cost and tunable structure. As an intrinsic characteristic, the influence of structured water in PBAs on the electrochemical properties is still controversial. Herein, low-vacancy iron hexacyanoferrate with different interstitial water contents is synthesized through the citric acid-assisted single iron source method. Ex situ Fourier transform infrared and X-ray diffraction characterization reveals that the interstitial water can stably exist in the Prussian blue framework during repeated cycling. The long-standing interstitial water can reduce the volume change during the Na⁺ insertion/extraction process, resulting in improved cycling stability. Thanks to the low Fe(CN)₆^4- vacancies and pillar role of interstitial water in the crystal framework, the HW-PB exhibits a high reversible capacity of 117 mAh g^-1 and excellent long cycle performance with a capacity retention of 91% after 1380 cycles. This work broadens the understanding of the relationship between the interstitial water in PBAs and Na-storage performances, providing guidance for the precise synthesis of high-quality PBAs.

Prussian-Blue Analogue-Derived Hollow Structured Co3 S4 /CuS2 /NiS2 Nanocubes as an Advanced Battery-Type Electrode Material for High-Performance Hybrid Supercapacitors

The facile and cost-effective fabrication of hybrid nanostructures comprised of hollow mixed metallic chalcogenides has attracted growing interest in the development of high-performance energy storage devices. Herein, multi-component (nickel-cobalt-copper-sulfides/selenides (NCCS/NCCSe)) hollow nanocubes (HNCs) are prepared via a single-step sulfurization/selenization process. The NCCS material shows interior HNCs, and the NCCSe material exhibits slightly formed porous cubes. Both the prepared materials demonstrate higher charge storage performance than the precursor NCC NCs owing to the improved surface morphology and addition of sulfur and selenium ions. Particularly, the NCCS HNCs electrode reveals superior specific capacity (capacitance) (70.32 mAh g^-1 (666.20 F g^-1 ) at 5 mA cm^-2 ) along with excellent cycling stability of 108.6% even after 10 000 cycles. Interestingly, the electrode delivers a good rate capability of 83.5% at a high current density of 20 mA cm^-2 . The feasibility of the battery-type NCCS HNCs as a positive electrode is explored by constructing an aqueous electrochemical hybrid capacitor (AEHC). The AEHC exhibits maximum energy and power densities of 23.15 Wh kg^-1 and 7899.08 W kg^-1 , respectively. Remarkably, it demonstrates superior long-life cycling stability even after 10 000 cycles (120.6% retention). The suitability of AEHC for practical application is also tested by driving electronic devices.

Fe-Based metal–organic frameworks as functional materials for battery applications

This review presents a comprehensive discussion on the development and application of pristine Fe-MOFs in lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, metal–air batteries and lithium–sulfur batteries.

Ni–Fe nanoframes via a unique structural formation induced by sonochemical etching

A Ni–Fe nanoframe exhibits unique structural merits including 3D open structure and high surface area, enhancing electrochemical properties for oxygen-evolution reaction (OER).

Post-synthetic modification of Prussian blue type nanoparticles: tailoring the chemical and physical properties

This review focuses on recent advances in the post-synthetic modification of nano-sized Prussian blue and its analogues and compares them with the current strategies used in metal–organic frameworks to give future outlooks in this field.

Environmentally Benign Formation of Nickel Hexacyanoferrate-Derived Mesoframes for Heterogeneous Catalysis

The tetramethylammonium hydroxide (TMAH)-controlled alkaline etching of nickel hexacyanoferrate (NiHCF) mesocrystals is explored. The alkaline etching enables the formation of hollow framework structures with an increased surface area, the exposure of active Ni and Fe sites and the retention of morphology. The ambient reaction conditions enable the establishment of a sustainable production. Our work reveals novel perspectives on the eco-friendly synthesis of hollow and colloidal superstructures for the efficient degradation of the organic contaminants rhodamine-B and bisphenol-A. In the case of peroxomonosulfate (PMS)-mediated bisphenol-A degradation, the rate constant of the etched mesoframes was 10,000 times higher indicating their significant catalytic activity.

Stepwise chemical oxidation to access ultrathin metal (oxy)-hydroxide nanosheets for the oxygen evolution reaction

Incorporation of ultrathin nanosheets with dopants/defects shows great potential to enable metal (oxy)-hydroxide electrocatalysts with enhanced oxygen evolution reaction (OER) performance via the regulation of atomic structure and bonding arrangements. However, it remains challenging in synthesis especially for such dual control and at large scale. In this study, we present a stepwise chemical oxidation route, involving phase transition and reconstruction processes, to access ultrathin CoOOH nanosheets with a thickness of ca. 4 nm and abundant oxygen vacancies. Other transition metals were also doped into CoOOH nanosheets through this strategy. Among them, the optimized FeCoOOH nanosheets demonstrated an efficient OER activity with overpotential as low as 252 mV (current density: 10 mA cm^-2) and excellent stability. A high and stable solar-to-hydrogen efficiency of 10.5% was acquired when FeCoOOH nanosheets were used as the anode in a constructed water splitting device driven by solar energy. This study offers a noble and facile strategy for potentially scalable preparation of atom-modulated ultrathin metal (oxy)-hydroxide nanosheets, and also demonstrates the OER applications.

Self-induced cobalt-derived hollow structure Prussian blue as a cathode for sodium-ion batteries

As advanced electrode materials for sodium ion batteries, Prussian blue and its derivatives have attracted considerable attention due to their low cost, structural stability and facile synthesis process. However, the application of commercially available Prussian blue is limited by its poor electronic conductivity as well as the structural defect induced by crystalline/interstitial water molecules. Herein, to address these drawbacks, an etching-agent free method is developed to synthesize Prussian blue with a hollow structure, and the synthesis mechanism is revealed. Owing to the stability of divalent iron ions, the shorter electron/ion diffusion pathway and fewer defect sites of the hollow structure, the obtained Prussian blue exhibits excellent electrochemical performance (specific capacity of 133.6 mA h g-1 at 1C, 1C = 170 mA g-1), which can put forward a new avenue to engineer advanced electrode materials for sodium ion batteries.

Continuous Conductive Networks Built by Prussian Blue Cubes and Mesoporous Carbon Lead to Enhanced Sodium-Ion Storage Performances

The challenges of improving electrical conductivities and enhanced rapid dynamics are active research areas in the modification of Prussian blue (PB) and Prussian blue analogues (PBAs), which are used as excellent cathodes of sodium-ion batteries (SIBs). Herein, the terephthalic acid etched stepwise hollow bulky PB cubes and the intimate contact mesoporous carbon (CMK-3) particles with the adhered minisize PB cubes can together build continuous conductive networks. The composite (donated as N-PB@CMK) has high electrical conductivity, low resistance, and ultrahigh specific surface, which can lead to high capacitive contribution ratios. The N-PB@CMK electrode can deliver a discharge capacity of 120 mAh g^-1 and maintain retention of 85.0% after cycling for 200 cycles at a current density of 100 mA g^-1. Even cycling at 1 A g^-1, the reversible capacity can be measured to 102 mAh g^-1 and exhibit stability over a long cycle. In situ Raman spectroscopy and X-ray diffraction (XRD) patterns were further measured to illustrate the phase transition of crystal structure along with the extraction/insertion processes of Na⁺ ions. Especially, the assembled full cell with NaTi₂(PO₄)₃@C anode can also show good stability and provide promising insights of applying the N-PB@CMK for energy storage systems in the future.

Self-Template Synthesis of Prussian Blue Analogue Hollow Polyhedrons as Superior Sodium Storage Cathodes

Prussian blue and its analogues with three-dimensional frame structures have been shown to be of great importance in the research and development of sodium-ion batteries (SIBs). Herein, we develop a simple and convenient self-template method to prepare a hollow-structured Prussian blue analogue (CoFe-PBA). This structure is conducive to buffer the volume changes during ion extraction and insertion processes and shorten the ion diffusion path. When further building a thin polydopamine (PDA) coating, the synthesized CoFe-PBA@PDA exhibits a high discharge capacity of 123.1 mAh g^-1 at 0.1 A g^-1 with a capacity retention of 71.5% after 500 cycles. Moreover, the capacity retention of CoFe-PBA@PDA after 100 cycles is 14.3% higher than that of the two comparison samples. In addition, the reversible structure of CoFe-PBA@PDA without forming a new phase was verified by in situ X-ray diffraction. This work may provide another design idea or strategy for improving the stability of the PBA cathodes used in SIBs.

Zn-Ion Batteries: Boosting the Rate Capability and Low-temperature Performance by Combining Structure and Morphology Engineering

Prussian blue analogues (PBAs) have been considered as one kind of the most promising cathode materials for Zn-ion batteries (ZIBs) due to their low cost, high performance, high safety, and high abundance. However, owing to the low conductivity and single electron reaction, it is a great challenge to obtain a PBA cathode material with high reversible capacity, high rate capability, and good temperature adaptability. Here, a cathode material, K_1.14(VO)_3.33[Fe(CN)₆]₂·6.8H₂O (KVHCF), with a multielectron reaction and double conductive carbon framework (DCCF) is designed and synthesized by combining structure and morphology engineering. With the multielectron reaction and high electronic conductivity simultaneously, the KVHCF@DCCF cathode material delivers a high specific capacity (180 mAh·g^-1 @ 400 mA·g^-1) and the best rate performance (116 mAh·g^-1 @ 8000 mA·g^-1) of the reported PBAs. Moreover, KVHCF@DCCF presents a high specific capacity of 132 mAh·g^-1 @ 400 mA·g^-1 at 0 °C. Even at -10 °C, it still delivers specific capacities of 127 mAh·g^-1 @ 40 mA·g^-1 and 80 mAh·g^-1 @ 400 mA·g^-1 with a retention of 86% after 700 cycles. In situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectroscopy (XPS) are carried out to investigate the charge-discharge electrochemical reaction mechanism.

A Low-Strain Potassium-Rich Prussian Blue Analogue Cathode for High Power Potassium-Ion Batteries

Most of the cathode materials for potassium ion batteries (PIBs) suffer from poor structural stability due to the large ionic radius of K⁺ , resulting in poor cycling stability. Here we report a low-strain potassium-rich K_1.84 Ni[Fe(CN)₆ ]_0.88 ⋅0.49 H₂ O (KNiHCF) as a cathode material for PIBs. The as-prepared KNiHCF cathode can deliver reversible discharge capacity of 62.8 mAh g^-1 at 100 mA g^-1 , with a high discharge voltage of 3.82 V. It can also achieve a superior rate performance of 45.8 mAh g^-1 at 5000 mA g^-1 , with a capacity retention of 88.6 % after 100 cycles. The superior performance of KNiHCF cathode results from low-strain de-/intercalation mechanism, intrinsic semiconductor property and low potassium diffusion energy barrier. The high power density and long-term stability of KNiHCF//graphite full cell confirmed the feasibility of K-rich KNiHCF cathode in PIBs. This work provides guidance to develop Prussian blue analogues as cathode materials for PIBs.

Bifunctional and Self-Supported NiFeP-Layer-Coated NiP Rods for Electrochemical Water Splitting in Alkaline Solution

Designing efficient and robust nonprecious metal-based electrocatalysts for overall water electrolysis, which is mainly limited by the oxygen evolution reaction (OER), for hydrogen production remains a major challenge for the hydrogen economy. In this work, a bimetallic NiFeP catalyst is coated on nickel phosphide rods grown on nickel foam (NiFeP@NiP@NF). This self-supported and interfacially connected electrode structure is favorable for mass transfer and reducing electrical resistance during electrocatalysis. The preparation of NiFeP@NiP@NF is optimized in terms of (i) the coprecipitation time of the NiFe Prussian blue analogue layer that serves as phosphides precursor and (ii) the phosphidation temperature. The optimized sample exhibits excellent OER performance delivering current densities of 10 and 100 mA cm^-2 at low overpotentials of 227 and 252 mV in 1.0 M KOH, respectively, and maintaining 10 mA cm^-2 for more than 120 h without obvious degradation. Moreover, it can also be operated as a hydrogen evolution electrocatalyst, requiring an overpotential of 105 mV at 10 mA cm^-2 in the same medium. Thus, the as-prepared material was tentatively utilized as a bifunctional electrocatalyst in a symmetric electrolyzer, requiring a voltage bias of 1.57 V to afford 10 mA cm^-2 in 1.0 M KOH, while exhibiting outstanding stability.

Modular Construction of Prussian Blue Analog and TiO2 Dual-Compartment Janus Nanoreactor for Efficient Photocatalytic Water Splitting

Janus structures that include different functional compartments have attracted significant attention due to their specific properties in a diverse range of applications. However, it remains challenge to develop an effective strategy for achieving strong interfacial interaction. Herein, a Janus nanoreactor consisting of TiO2 2D nanocrystals integrated with Prussian blue analog (PBA) single crystals is proposed and synthesized by mimicking the planting process. In situ etching of PBA particles induces nucleation and growth of TiO2 nanoflakes onto the concave surface of PBA particles, and thus enhances the interlayer interaction. The anisotropic PBA-TiO2 Janus nanoreactor demonstrates enhanced photocatalytic activities for both water reduction and oxidation reactions compared with TiO2 and PBA alone. As far as it is known, this is the first PBA-based composite that serves as a bifunctional photocatalyst for solar water splitting. The interfacial structure between two materials is vital for charge separation and transfer based on the spectroscopic studies. These results shed light on the elaborate construction of Janus nanoreactor, highlighting the important role of interfacial design at the microscale level.

Prussian Blue Analogs and Their Derived Nanomaterials for Electrochemical Energy Storage and Electrocatalysis

Prussian blue analogs (PBAs), the oldest artificial cyanide-based coordination polymers, possess open framework structures, large specific surface areas, uniform metal active sites, and tunable composition, showing significant perspective in electrochemical energy storage. These electrochemically active materials have also been converted to various functional metal containing nanomaterials, including carbon encapsulated metals/metal alloys, metal oxides, metal sulfides, metal phosphides, etc. originating from the multi-element compositions as well as elaborate structure design. In this paper, a comprehensive review will be presented on the recent progresses in the development of PBA frameworks and their derivatives based electrode materials and electrocatalysts for electrochemical energy storage and conversion. In particular, it will focus on the synthesis of representative nanostructures, the structure design, and figure out the correlation between nanomaterials structure and electrochemical performance. Lastly, critical scientific challenges in this research area are also discussed and perspective directions for the future research in this field are provided, in order to provide a brand new vision into the further development of novel active materials for the next-generation advanced electrochemical devices.

CNT-modified two-phase manganese hexacyanoferrate as a superior cathode for sodium-ion batteries

The two-phase KNa-MnFe(CN)₆@CNT material was synthesized via a facile concentration-gradient coprecipitation method. The outstanding electrochemical performance was achieved for KNa-MnFe(CN)₆@CNT material with the addition of CNT.

Hierarchically porous metal-organic frameworks: synthesis strategies, structure(s), and emerging applications in decontamination

Metal-organic frameworks (MOFs) with high porosity have received much attention as promising materials for many applications owing to their unique properties. However, to date, most of the reported MOFs have microporous structures, which slow down diffusion/mass transfer and limit the accessibility of bulky molecules to its internal surface. Thus, it is crucial to develop an efficient way to create larger pores (mesoporous and/or macroporous) into microporous MOFs to form hierarchical porous metal-organic frameworks (HP-MOFs), which facilitate the diffusion and mass transfer of guest molecules. HP-MOFs are excellent and promising candidates for environmental applications under the background of environmental contaminations. In this review paper, we are primarily focusing on the latest progress in the preparation of HP-MOFs by employing template-assisted and template-free synthetic approaches for environmental cleaning applications. Particularly, the adsorptive purification of the most common toxic substances, including gases, dyes, heavy metal ions, and antibiotics from the environment using HP-MOFs as adsorbents is briefly discussed. The overall results clearly showed that the superiority of HP-MOFs compared with conventional microporous MOFs. Finally, we summarize the remaining challenges and provide personal perspectives on possible future development of HP-MOFs.

Supramolecular Chiral Nanoarchitectonics

Exploration of molecular functions and material properties based on the control of chirality would be a scientifically elegant approach. Here, the fabrication and function of chiral-featured materials from both chiral and achiral components using a supramolecular nanoarchitectonics concept are discussed. The contents are classified in to three topics: i) chiral nanoarchitectonics of rather general molecular assemblies; ii) chiral nanoarchitectonics of metal-organic frameworks (MOFs); iii) chiral nanoarchitectonics in liquid crystals. MOF structures are based on nanoscopically well-defined coordinations, while mesoscopic orientations of liquid-crystalline phases are often flexibly altered. Discussion on the effects and features in these representative materials systems with totally different natures reveals the universal importance of supramolecular chiral nanoarchitectonics. Amplification of chiral molecular information from molecules to materials-level structures and the creation of chirality from achiral components upon temporal statistic fluctuations are universal, regardless of the nature of the assemblies. These features are thus surely advantageous characteristics for a wide range of applications.

Surfactant-Mediated Morphological Evolution of MnCo Prussian Blue Structures

In the preparation of nanomaterials, the kinetics and thermodynamics in the reaction can significantly affect the structures and phases of nanocrystals. Therefore, people are keen to adopt various synthetic strategies to accurately assemble the target nanocrystals, and reveal the underlying mechanism of the formation of specific structures. In this work, the total reaction time is adjusted to let the prepared MnCo Prussian blue analogous (MnCoPBA) crystals show four evolving morphological changes at different stages with the assistance of sodium dodecyl sulfate. Furthermore, it is clearly observed that the epitaxial growth along the (100) plane on the shell of MnCoPBA nanocrystals is favored, and the thermodynamics and kinetics in the morphology change process are analyzed in detail. Through the simple pyrolysis, MnCoPBA crystals can be successfully converted into the corresponding carbon composites, of which Mn₂ Co₂ C nanoparticles are evenly distributed in highly graphitized carbon matrix. Among them, PBA-III-700 performs good oxygen reduction reaction performance in alkaline solution with the half-wave potential of 0.801 V and diffusion-limited current density of 5.36 mA cm^-2 , and its zinc-air battery exhibits the peak power density of 103.4 mW cm^-2 competitive with commercial Pt/C.

Fabrication of CdS Frame-in-Cage Particles for Efficient Photocatalytic Hydrogen Generation under Visible-Light Irradiation

The design of advanced structures for semiconductor photocatalysts is an effective approach to enhance their performance toward solar-to-chemical energy conversion. Hollow and frame-like structures show advantageous features for photocatalytic reactions with enlarged surface area, shortened charge-transfer distance, promoted light-absorption ability, and enhanced mass-transfer capability. Here, a facile two-step sulfidation strategy is developed to fabricate unique CdS frame-in-cage particles for photocatalytic hydrogen generation. Cd-based Prussian blue analog (Cd-PBA) cubes are first converted to Cd-PBA cube-in-CdS cage particles, which are further transformed to CdS frame-in-cage particles. Benefiting from the novel frame-in-cage structure, the obtained CdS photocatalyst exhibits high activity under visible-light irradiation with the hydrogen generation rate of 13.6 mmol h^-1 g^-1 , which is much enhanced compared with those of the CdS cubes and cages.

Finely tuned Prussian blue-based nanoparticles and their application in disease treatment

The Prussian blue (PB) based nanostructure is a mixed-valence coordination network with excellent biosafety, remarkable photothermal effect and multiple enzyme-mimicking behaviours. Compared with other nanomaterials, PB-based nanoparticles (NPs) exhibit several unparalleled advantages in biomedical applications. This review begins with the chemical composition and physicochemical properties of PB-based NPs. The tuning strategies of PB-based NPs and their biomedical properties are systemically demonstrated. Afterwards, the biomedical applications of PB-based NPs are comprehensively recounted, mainly focusing on treatment of tumors, bacterial infection and inflammatory diseases. Finally, the challenges and future prospects of PB-based NPs and their application in disease treatment are discussed.