Microstructure and Oxygen Evolution Property of Prussian Blue Analogs Prepared by Mechanical Grinding

Solvent-free mechanochemical synthesis of efficient and low-cost double perovskite (DP), like a cage of Prussian blue (PB) and PB analogs (PBAs), is a promising approach for different applications such as chemical sensing, energy storage, and conversion. Although the solvent-free mechanochemical grinding approach has been extensively used to create halide-based perovskites, no such reports have been made for cyanide-based double perovskites. Herein, an innovative solvent-free mechanochemical synthetic strategy is demonstrated for synthesizing Fe4[Fe(CN)6]3, Co3[Fe(CN)6]2, and Ni2[Fe(CN)6], where defect sites such as carbon-nitrogen vacancies are inherently introduced during the synthesis. Among all the synthesized PB analogs, the Ni analog manifests a considerable electrocatalytic oxygen evolution reaction (OER) with a low overpotential of 288 mV to obtain the current benchmark density of 20 mA cm-2. We hypothesize that incorporating defects, such as carbon-nitrogen vacancies, and synergistic effects contribute to high catalytic activity. Our findings pave the way for an easy and inexpensive large-scale production of earth-abundant non-toxic electrocatalysts with vacancy-mediated defects for oxygen evolution reaction.

Co-Precipitation Synthesis of Co3[Fe(CN)6]2·10H2O@rGO Anode Electrode for Lithium-Ion Batteries

Rechargeable lithium-ion batteries (LIBs) are known to be practical and cost-effective devices for storing electric energy. LIBs have a low energy density, which calls for the development of new anode materials. The Prussian blue analog (PBA) is identified as being a candidate electrode material due to its facile synthesis, open framework structures, high specific surface areas, tunable composition, designable topologies and rich redox couples. However, its poor electrical conductivity and mechanical properties are the main factors limiting its use. The present study loaded PBA (Co₃[Fe(CN)₆]·10H₂O) on graphene oxide (Co-Fe-PBA@rGO) and then conducted calcination at 300 °C under the protection of nitrogen, which reduced the crystal water and provided more ion diffusion pathways. As a result, Co-Fe-PBA@rGO showed excellent performance when utilized as an anode in LIBs, and its specific capacities were 546.3 and 333.2 mAh g⁻¹ at 0.1 and 1.0 A g⁻¹, respectively. In addition, the electrode also showed excellent performance in the long-term cycle, and its capacity reached up to 909.7 mAh g⁻¹ at 0.1 A g⁻¹ following 100 cycles.

Highly efficient OER catalyst enabled by in situ generated manganese spinel on polyaniline with strong coordination

The oxygen evolution reaction (OER), as the rate-determining step of electrochemical water splitting, is extremely crucial, and thus it is a requisite to engineer feasible and effective electrocatalysts to shrink the reaction energy barrier and accelerate the reaction. Herein, monodisperse Mn₃O₄ nanoparticles on a PANI substrate were synthesized by polymerization and in situ oxidation. Combining Mn₃O₄ nanoparticles and PANI fibers can not only maximize the strong coupling effect and synergistic effect but also construct a well-defined three-dimensional structure with extensive exposed active sites, where the permeation and adherence of the electrolyte are made exceedingly feasible, thus displaying excellent OER activity. Benefiting from the outstanding structural stability, the resulting Mn₃O₄/PANI/NF is able to deliver a low overpotential of 262 mV at a current density of 10 mA cm^-2, which outperforms the commercial RuO₂ catalyst (275 mV) as well as presently reported representative Mn-based and PANI-based electrocatalysts and state-of-the-art OER electrocatalysts. The synthetic method for Mn₃O₄/PANI not only provides a brand-new avenue for the rational design of inorganic material/conductive polymer composites but also broadens the understanding of the mechanism of Mn-based catalysts for highly enhanced OER.

In situ polyaniline coating of Prussian blue as cathode material for sodium-ion battery

Prussian blue (PB) has great potential for use as a sodium cathode material owing to its high working potential and cube frame structure. Herein, this work reports a two-step method to synthesize PB with ascorbic acid as the ball-milling additive, which improves the electrochemical rate performance of PB during the traditional co-precipitation method. The obtained PB sample exhibited a superior specific capability (113.3 mAh g-1 even at 20 C, 1 C = 170 mA g-1) and a specific capacity retention of 84.8% after 100 cycles at 1 C rate. In order to enhance the cycling performance of the PB, an in situ polyaniline coating strategy was employed in which aniline was added into the electrolyte and polymerized under electrochemical conditions. The coated anode exhibited a high specific capacity retention of 62.7% after 500 cycles, which is significantly higher than that of the non-coated sample, which only remains 40.1% after 500 cycles. This development has shown a great potential as a low-cost, high-performance and environment-friendly technology for large-scale industrial application of PB.

Designing MOF Nanoarchitectures for Electrochemical Water Splitting

Electrochemical water splitting has attracted significant attention as a key pathway for the development of renewable energy systems. Fabricating efficient electrocatalysts for these processes is intensely desired to reduce their overpotentials and facilitate practical applications. Recently, metal-organic framework (MOF) nanoarchitectures featuring ultrahigh surface areas, tunable nanostructures, and excellent porosities have emerged as promising materials for the development of highly active catalysts for electrochemical water splitting. Herein, the most pivotal advances in recent research on engineering MOF nanoarchitectures for efficient electrochemical water splitting are presented. First, the design of catalytic centers for MOF-based/derived electrocatalysts is summarized and compared from the aspects of chemical composition optimization and structural functionalization at the atomic and molecular levels. Subsequently, the fast-growing breakthroughs in catalytic activities, identification of highly active sites, and fundamental mechanisms are thoroughly discussed. Finally, a comprehensive commentary on the current primary challenges and future perspectives in water splitting and its commercialization for hydrogen production is provided. Hereby, new insights into the synthetic principles and electrocatalysis for designing MOF nanoarchitectures for the practical utilization of water splitting are offered, thus further promoting their future prosperity for a wide range of applications.

Tuning of the electrocatalytic characteristics of PANI/Fe2O3 composite coating for alkaline hydrogen evolution reaction

The paper reports a simple and cost-effective strategy for the development of a stable and reproducible PANI/Fe2O3 composite coating as an efficient electrode for the electrocatalytic alkaline hydrogen evolution reaction (HER). The surface characteristics of the developed PANI/Fe2O3 composite coatings are tuned to achieve high hardness (510 HVN), thickness (26 μm), porosity, and surface roughness (Sa = 3.760 μm). The PANI/Fe2O3 composite coating with tuned surface characteristics (PANI/Fe2O3-2GL) facilitates the effective conduction of electrons from a highly conducting polymer to a metal. This increases the electron density on the coating surface and enhances the active surface area, which effectively enhances the hydrogen adsorption efficiency on the coating surface to improve HER activity. The composite coating exhibits enhanced HER activity with low overpotential (110 mV) and high exchange current density (95.32 mA cm-2). The mechanism of HER on the coating surface follows the Volmer-Heyrovsky reaction with the Heyrovsky step as the rate-determining step. The stability of the composite coating under aggressive reaction conditions even after long-term HER confirms its competency with commercial electrocatalysts.

Holey Cobalt-Iron Nitride Nanosheet Arrays as High-Performance Bifunctional Electrocatalysts for Overall Water Splitting

Designing efficient metal nitride electrocatalysts with advantageous nanostructures toward overall water splitting is of great significance for energy conversion. In this work, holey cobalt-iron nitride nanosheet arrays grown on Ni foam substrate (CoFeN_x HNAs/NF) are prepared via a facile hydrothermal and subsequent thermal nitridation method. This unique HNA architecture can not only expose abundant active sites but also facilitate the charge/mass transfer. Resulting from these merits, the CoFeN_x HNAs/NF exhibits high catalytic performance with overpotentials of 200 and 260 mV at 10 mA cm^-2 for the hydrogen evolution reaction (HER) and 50 mA cm^-2 for the oxygen evolution reaction (OER), respectively. Furthermore, when using CoFeN_x-500 HNAs/NF as both anode and cathode, the alkaline electrolyzer could afford a current density of 10 mA cm^-2 at 1.592 V, higher than many other metal nitride-based electrocatalysts. This work signifies a simple approach to prepare holey metal nitride nanosheet arrays, which can be applied in various fields of energy conversion and storage.

Oligoaniline-functionalized polysiloxane/Prussian blue composite towards bifunctional electrochromic supercapacitors

We report the preparation of a novel oligoaniline-functionalized polysiloxane/Prussian blue composite, which exhibits improved electrochromic properties and approving supercapacitor performance featuring charge storage indicating functionalization.

Electrochemically active sites inside crystalline porous materials for energy storage and conversion

This review provides references for the preparation of electroactive CPMs via rational design and modulation of active sites and the space around them, and their application in electrochemical energy storage and conversion systems.

Prussian Blue Analogs for Rechargeable Batteries

Non-lithium energy storage devices, especially sodium ion batteries, are drawing attention due to insufficient and uneven distribution of lithium resources. Prussian blue and its analogs (Prussian blue analogs [PBAs]), or hexacyanoferrates, are well-known since the 18th century and have been used for hydrogen storage, cancer therapy, biosensing, seawater desalination, and sewage treatment. Owing to their unique features, PBAs are receiving increasing interest in the field of energy storage, such as their high theoretical specific capacity, ease of synthesis, as well as low cost. In this review, a general summary and evaluation of the applications of PBAs for rechargeable batteries are given. After a brief review of the history of PBAs, their crystal structure, nomenclature, synthesis, and working principle in rechargeable batteries are discussed. Then, previous works classified based on the combination of insertion cations and transition metals are analyzed comprehensively. The review includes an outlook toward the further development of PBAs in electrochemical energy storage.