Facile synthesis of cobalt hexacyanoferrate/graphene nanocomposites for high-performance supercapacitor

A Dual Effect Additive Modified Electrolyte Strategy to Improve the Electrochemical Performance of Zinc-Based Prussian Blue Analogs Energy Storage Device

Prussian blue analogs (PBA) exhibit excellent potential for energy storage due to their unique three-dimensional open framework and abundant redox active sites. However, the dissolution of transition metal ions in water can compromise the structural integrity of PBAs, leading to significant issues such as low cycle life and capacity decay. To address these challenges, we proposed a dual-effect additive-modified electrolyte method to alleviate such issues, introducing sodium ferrocyanide (Na4Fe(CN)6) into aqueous alkaline electrolytes. It could not only capture Zn2+ dissolved on the surface of Na1.86Zn1.46[Fe(CN)6]0.87 (ZnHCF) electrode material during the cycling process but also conduct redox reactions on the electrode surface to provide additional capacitance. Through experiments and molecular simulation calculations, it showed that Na4Fe(CN)6 can restrict the movement of Zn dissolution into the electrolyte on the electrode surface. Based on this, an asymmetric supercapacitor based on ZnHCF//activated carbon was assembled with a modified electrolyte. The assembled supercapacitor displayed a specific capacitance of 1,329.65 mF cm-2, a power density of 2,900 mW cm-2, and an energy density of 388.28 mW h cm-2. This study provides a new idea for the design and construction of stable and efficient PBA energy storage materials by inhibiting the leaching of transition metals in PBA.

Novel mixed nickel/cobalt hexacyanoferrate microcubes with synergistic effects for aqueous hybrid supercapacitors

Combining different metals in coordination compounds is an efficient strategy to improve their various properties. Herein, mixed nickel-cobalt hexacyanoferrate (NixCoyHCF) microcubes of varying x : y molar ratios are synthesized via a co-precipitation route and comprehensively characterized to study their material and electrochemical properties. NixCoyHCF microcubes display the battery-type electrochemical energy storage mechanism in aqueous electrolytes. Among the samples, Ni1Co2HCF microcubes deliver the highest specific capacity of 134 mA h g-1 (1068 F g-1) at a specific current of 1 A g-1. This significant enhancement in the capacity indicates the synergistic and cooperative effects between Ni and Co sites in Ni1Co2HCF microcubes. The asymmetric supercapacitor device assembled with Ni1Co2HCF microcubes delivers an excellent energy density of 74.4 μW h cm-2 at a power density of 750 μW cm-2 and retains 87.7% of its initial capacity after 2000 cycles at a current density of 5 mA cm-2, indicating its robust structural integrity and electrochemical durability. This study highlights the promising potential of mixed-metal hexacyanoferrates as high-performance electrodes for aqueous supercapacitors.

Electrode Materials for Desalination of Water via Capacitive Deionization

Recent years have seen the emergence of capacitive deionization (CDI) as a promising desalination technique for converting sea and wastewater into potable water, due to its energy efficiency and eco-friendly nature. However, its low salt removal capacity and parasitic reactions have limited its effectiveness. As a result, the development of porous carbon nanomaterials as electrode materials have been explored, while taking into account of material characteristics such as morphology, wettability, high conductivity, chemical robustness, cyclic stability, specific surface area, and ease of production. To tackle the parasitic reaction issue, membrane capacitive deionization (mCDI) was proposed which utilizes ion-exchange membranes coupled to the electrode. Fabrication techniques along with the experimental parameters used to evaluate the desalination performance of different materials are discussed in this review to provide an overview of improvements made for CDI and mCDI desalination purposes.

Composite cathode with low-defect NiFe Prussian blue analogue on reduced graphene oxide for aqueous sodium-ion hybrid supercapacitors

Although sodium-ion hybrid supercapacitor (Na-ion HSC) has attracted great interest, exploitation of suitable cathode materials for reversible Na⁺ insertion reaction remains a challenge. Herein, a novel binder-free composite cathode with highly crystallized NiFe Prussian blue analogue (NiFePBA) nanocubes in-situ grown on reduced graphene oxide (rGO) was fabricated via sodium pyrophosphate (Na₄P₂O₇)-assisted co-precipitation and the subsequent ultrasonic spraying and chemical reduction. Profiting from the low-defect PBA framework and close interface contact of PBA and conductive rGO, the NiFePBA/rGO/carbon cloth composite electrode exhibits a high specific capacitance of 451F g^-1, remarkable rate performance and satisfactory cycling stability in aqueous Na₂SO₄ electrolyte. Impressively, the aqueous Na-ion HSC assembled with the composite cathode and activated carbon (AC) anode manifests a high energy density of 51.11 Wh kg^-1, superb power density of 10 kW kg^-1 and the intriguing cycling stability. This work may open a door for scalable fabrication of binder-free PBA cathode material for aqueous Na-ion storage.

Poly(azure C)-coated CoFe Prussian blue analogue nanocubes for high-energy asymmetric supercapacitors

Prussian blue analogues are considered as promising supercapacitor electrode materials due to their high theoretical capacitance and low cost. Yet, they suffer from poor electronic conductivity and cycling life. Here, a redox dye polymer, poly(azure C) (PAC), is in-situ grown uniformly on CoFe Prussian blue analogue (CoFePBA). As a polymer mediator, the PAC coating on each PBA not only enhances the electronic conductivity and surface area, but also improves the structural stability and specific capacitance of PBA. As a result, the optimized CoFePBA@PAC possesses ultrahigh specific capacitance (968.67 F g^-1 at 1 A g^-1), superior rate performance (665.78 F g^-1 at 10 A g^-1), and excellent long-cycling stability (92.45% capacity retention after 2000 cycles). As an application, a fabricated CoFePBA@PAC//AC asymmetric supercapacitor (AC = activated carbon) maintains 84.7% capacitance retention in 2000 cycles at 1 A g^-1 and displays a superior specific energy of 29.16 W h kg^-1 at the power density of 799.78 W kg^-1. These results demonstrate that redox dye polymer-coated PBAs with outstanding performance have a promising prospect in the field of energy storage.

Nickel hexacyanoferrate/graphene thin film: a candidate for the cathode in aqueous metal-ion batteries

A reduced graphene oxide/nickel nanoparticles nanocomposite was used as precursor to synthesize a novel graphene/nickel hexacyanoferrate thin film through a heterogeneous electrochemical reaction with ferricyanide ions in solution.

Core-shell shaped Ni2CoHCF@PPy microspheres from prussian blue analogues for high performance asymmetric supercapacitors

Recently, prussian blue analogues (PBAs), as the most classical class of metal-organic frameworks, have been widely studied by scientists. Nevertheless, the inferior conductivity of PBAs restricts the application in supercapacitors. In this work, nickel cobalt hexacyanoferrate (Ni₂CoHCF) had been produced via a simple co-precipitation approach and coated with polypyrrole on its surface. The conductivity of PBAs was improved by the polypyrrole coating. The Ni₂CoHCF@PPy-400 microspheres were demonstrated to the outstanding specific capacity of 82 mAh g^-1at 1 A g^-1. After 3000 cycles, the Ni₂CoHCF@PPy-400 microspheres had a long cycle life and 86% specific capacity retention rate at 5 A g^-1. Additionally, it was coupled with activated carbon to build high performance asymmetric supercapacitor (Ni₂CoHCF@PPy-400//AC), which displayed a high energy density of 21.7 Wh kg^-1at the power density of 888 W kg^-1and good cycle stability after 5000 cycles (a capacity retention rate of 85.2%). What is more, the results reveal that the Ni₂CoHCF@PPy-400 microspheresare a prospective candidate for exceptional energy storage devices.

Smart configuration of cobalt hexacyanoferrate assembled on carbon fiber cloths for fast aqueous flexible sodium ion pseudocapacitor

Aqueous rechargeable batteries (ARBs) have the advantages of low cost, high safety and sustainable environmental friendliness. However, the key challenge for ARBs is the narrow electrochemical stability window of the water, undoubtedly leading to the low output voltage, the underachieved capacity and a low energy density. Prussian blues and their analogues have attracted great research interest for energy storage due to the advantages of facile synthesis, versatile categories and tunable three dimensional frameworks. Herein a flexible integrated potassium cobalt hexacyano ferrates (Co-HCF) on carbon fiber clothes (CFCs) were designed through a feasible route combining the controllable electrochemical deposition and the efficient co-precipitation process. The Co-HCF@CFCs demonstrate an excellent sodium ion storage with a high reversible capacity of 91 mAh g^-1 at 1 A g^-1 and 55 mAh g^-1 at 10 A g^-1 in aqueous electrolytes. The long cycling stability at the high current demonstrate the excellent structure stability of the Co-HCF@CFCs. Analysis on the rate Cyclic voltammograms (CV) profiles reveal the fast electrochemical kinetics with the capacitive controlled process, while galvanostatic intermittent titration technique (GITT) tests fast diffusion coefficient related with the sodium ions intercalation/deintercalation in the Co-HCF@CFCs. In addition, the flexible Co-CHF@CFCs also demonstrate excellent performance for quasi-solid-state ARBs even at the high bending angles. The high quality Co-HCF@CFCs with advantage of high rate capability and excellent reversible capacity make them a promising candidate for high performance ARBs.

Cationic supercapacitance of carbon nanotubes covered with copper hexacyanoferrate

Carbon nanotubes (CNT) are uniformly covered with copper hexacyanoferrate (CuHCF) via coprecipitation to form a core shell structure. The CuHCF thickness can be tuned from 10 nm to 30 nm by changing the CuHCF loading in the hybrids from 25% to 58%. The capacitive behavior is affected by the hydrated cation radius. In 1 mol l^-1 KCl solution, CuHCF/CNT hybrids (46% CuHCF loading) show the largest specific capacitance of up to 989 F g^-1 at a discharge density of 1 A g^-1. The hybrids also possess superior rate capability with only 8.2% capacitance loss when increasing the discharge current from 1 to 20 A g^-1. The superior capacitive performance of the hybrids in the K⁺-ion solution can be attributed to the smaller hydrated radius of the K⁺ ion, which will favor the diffusion of the cation within the CuHCF lattice, leading to a larger faradic current. Besides, the cyclic stability of the hybrids is surprising, with 89.7% capacitance retention after 10000 discharge/charge cycles. The CuHCF/CNT hybrids are combined with the reduced graphene oxides (RGOs) to construct an asymmetrical supercapacitor, and its potential window can reach up to 2.0 V. More importantly, this supercapacitor exhibits a high energy density of 60.4 Wh kg^-1 at the power density of 0.5 kW kg^-1.

Coral-like hierarchical architecture self-assembled by cobalt hexacyanoferrate nanocrystals and N-doped carbon nanoplatelets as efficient electrocatalyst for oxygen evolution reaction

It is challenging to develop novel oxygen evolution reaction (OER) electrocatalysts with high performance and low cost to replace the noble metal-based catalysts for large-scale electrochemical water splitting. To settle such issue, herein, self-assembled porous coral-like architecture constructed by cobalt hexacyanoferrate (CoHCF) nanocrystals and nitrogen-doped carbon (NC) nanoplatelets network is fabricated for the first time by a facile electroless deposition approach. The porous coral-like CoHCF/NC hybrid exhibits an excellent OER electrocatalytic activity in alkaline medium with an ultra-low onset overpotential of 165 mV (vs. RHE) and a small Tafel slope of 73.97 mV dec^-1, which are much lower than that of bare CoHCF (onset overpotential of 296 mV and Tafel slope of 113.25 mV dec^-1); it also exhibits a lower overpotential of 357 mV (vs. RHE) at current density of 10 mA cm^-1 and superior durability even after 16 h. The excellent electrocatalytic performance of CoHCF/NC hybrid can be assigned to its unique coral-like architecture self-assembled by CoHCF nanocrystals and NC nanoplatelets network, which significantly increases the electrochemical active surface area and remarkably facilitates the electron and ion transfer. This work offers rational design and facile synthesis strategy for transition metal hexacyanoferrate-based nonprecious electrocatalysts with unique nano-architecture and excellent electrocatalytic efficiency towards OER.

Operando XAFS and XRD Study of a Prussian Blue Analogue Cathode Material: Iron Hexacyanocobaltate

The reversible electrochemical lithiation of potassium iron hexacyanocobaltate (FeCo) was studied by operando X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) assisted by chemometric techniques. In this way, it was possible to follow the system dynamics and retrieve structural and electronic transformations along cycling at both Fe and Co sites. These analyses confirmed that FeCo features iron as the main electroactive site. Even though the release of potassium ions causes a local disorder around the iron site, the material exhibits an excellent structural stability during the alkali ion deinsertion/insertion processes. An independent but interrelated analysis approach offers a good strategy for data treatment and provides a time-resolved picture of the studied system.

Self-Templating Synthesis of Cobalt Hexacyanoferrate Hollow Structures with Superior Performance for Na-Ion Hybrid Supercapacitors

Prussian blue (PB) and its analogues (PBA), especially with hollow structures, have attracted growing attention from the researchers of energy storage field. Herein, we have developed a facile self-templating method to synthesize hollow-structured cobalt hexacyanoferrate (CoHCF) with controllable morphologies by using water-soluble precursors as templates. The method is versatile and can be extended to synthesize various PB/PBA hollow structures with tunable composition and morphology. Profiting from the unique hollow structure, the CoHCF hollow prisms manifest exceptional electrochemical performance in the Na₂SO₄ aqueous electrolyte, including a high specific capacitance (284 F g^-1 at 1 A g^-1), a high rate capability, and an excellent cycling stability (92% retention after 5000 cycles). A hybrid supercapacitor device assembled with the CoHCF hollow prisms and activated carbon shows a high specific density of 47 W h kg^-1 at a specific power of 1000 W kg^-1 and stable cycling performance.

High-performance asymmetric supercapacitor based on hierarchical nanocomposites of polyaniline nanoarrays on graphene oxide and its derived N-doped carbon nanoarrays grown on graphene sheets

Activated carbon (AC), as a material for asymmetric supercapacitor (ASC), is the most widely used as negative electrode. However, AC has some electrode kinetic problems which are corresponded to inner-pore ion transport that restrict the maximum specific energy and power that can be attained in an energy storage system. Therefore, it is an important topic for researchers to extend the carbonaceous material with qualified structure for negative electrode supercapacitor. In this work, novel promoted ASC have been fabricated using nanoarrays of polyaniline grown on graphene oxide sheets (PANI-GO) as positive electrode and also, carbonized nitrogen-doped carbon nanoarrays grown on the surface of graphene (CPANI-G) as negative electrode. The porous structure of the as-synthesized CPANI-G can enlarge the specific surface area and progress ion transport into the interior of the electrode materials. From the other point of view, nitrogen doping can impressively improve the wettability of the carbon surface in the electrolyte and upgrade the specific capacitance by a pseudocapacitive effect. Because of the high specific capacitance and distinguished rate performance of PANI-GO and CPANI-G and moreover, the synergistic effects of the two electrodes with the optimum potential window, the ASC display excellent electrochemical performances. In comparison with the symmetric cell based on PANI-GO (40 Wh kg^-1), the fabricated PANI-GO//CPANI-G ASC exhibits a remarkably enhanced maximum energy density of 52 Wh kg^-1. Furthermore, ASC electrode exhibits excellent cycling durability, with 90.3% specific capacitance preserving even after 5000 cycles. These admirable results show great possibilities in developing energy storage devices with high energy and power densities for practical applications.

Facile synthesis of Mesoporouscobalt Hexacyanoferrate Nanocubes for High-Performance Supercapacitors

Mesoporous cobalt hexacyanoferrate nanocubes (meso–CoHCF) were prepared for the first time through a facile sacrificial template method. The CoHCF mesostructures possess a high specific surface area of 548.5 m²·g⁻¹ and a large amount of mesopores, which enable fast mass transport of electrolyte and abundant energy storage sites. When evaluated as supercapacitor materials, the meso–CoHCF materials exhibit a high specific capacitance of 285 F·g⁻¹, good rate capability and long cycle life with capacitance retention of 92.9% after 3000 cycles in Na₂SO₄ aqueous electrolyte. The excellent electrochemical properties demonstrate the rational preparation of mesoporous prussian blue and its analogues for energy storage applications.