Hexacyanoferrate Film Research Articles

Unidirectional Current in Layered Metal Hexacyanometallate Thin Films: Implication for Alternative Wet-Processed Electronic Materials.

Rectifying behavior of alternative electronic materials is demonstrated with layered structures of a crystalline coordination network whose mixed ionic and electronic conductivity can be manipulated by switching the redox state of coordinated transition-metal ions. The coordinated transition-metal ions can convey additional functionality such as (redox)catalysis or electrochromism. In order to obtain rectifying behavior and charge trapping, layered films of such materials are explored. Specifically, layered films of iron hexacyanoruthenate (Fe-HCR) and nickel hexacyanoferrate (Ni-HCF) were formed by the combination of different deposition procedures. They comprise electrodeposition during voltammetric cycles for Fe-HCR and Ni-HCF, layer-by-layer deposition of Ni-HCF without redox chemistry, and drop casting of presynthesized Ni-HCF nanoparticles. The obtained materials were structurally characterized by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy for nanoparticles, and scanning force microscopy (SFM). Voltammetry in 1 mol L-1 KCl and current-voltage curves (I-V curves) recorded between a conductive SFM tip and the back electrode outside of an electrolyte solution demonstrated charge trapping and rectifying behavior based on the different formal potentials of the redox centers in the films.

A potential-responsive ion-pump system based on nickel hexacyanoferrate film for selective extraction of cesium ions

A nickel hexacyanoferrate (NiHCF) film electrode was prepared with NiHCF, conductive carbon black, and polyvinylidene difluoride, which was coated on graphite plate substrate for selective extraction of Cs+ ions by using electrochemically switched ion exchange (ESIX) technology. A potential-responsive ion-pump system for efficient extraction of Cs+ ions was designed, and the effect of wet film thicknesses, charging modes, flow rates, and chamber widths on Cs+ ions extraction performance was investigated. In the system, the adsorption capacity and removal percentage of Cs+ ions on the NiHCF film electrode reached as high as 147.69 mg·g−1 and 92.47%, respectively. Furthermore, the NiHCF film electrode showed high selectivity for Cs+ ions and stability. After seven cycles of adsorption/desorption, the desorption percentage could reach about 100%. The excellent Cs+ extraction performance should be attributed to the strong driving force produced by the potential-responsive ion-pumping effect in the ESIX process, as well as the low ion transfer resistance of the film electrode which is caused by the special crystal structure of NiHCF. In addition, the NiHCF film electrode was implemented to work together with the bismuth oxybromide (BiOBr) film electrode to accomplish the simultaneous extraction of Cs+ and Br–. And the adsorption capacity and removal percentage of Br– ions on the BiOBr film electrode reached 69.53 mg·g−1 and 77.32%, correspondingly. It is expected that such a potential-responsive ion-pump system based on NiHCF and BiOBr film electrodes could be used for the selective extraction and concentration of Cs+ and Br– ions from salt lake brine..

Study of Growth and Properties of Electrodeposited Sodium Iron Hexacyanoferrate Films.

Sodium iron hexacyanoferrate (NaFeHCF) films were electrodeposited on Au/Cr/Si for the study of growth behavior and physical properties. The NaFeHCF films were studied by different analytical methods to prove the chemical composition, morphology and crystal structure. The grains of the film grow with a cubic structure with an average lattice parameter of 10.10 Å and the preferential growth direction along the [111] direction of the cubic cell. The films show a repeatable bipolar resistive switching behavior accompanied by high current changes (up to a factor of ~105). The different resistive states in the materials are dominated by ohmic conduction.

Stable films of zinc-hexacyanoferrate: electrochemistry and ion insertion capabilities

A stable film of zinc hexacyanoferrate is deposited on a GC (glassy carbon) substrate following a specific electrochemical protocol. The electrode maintains its characteristic even after dry and wet processes. SEM characterization confirms the cubic morphology of the materials and the IR suggests the presence of the FeII-C-N-ZnII structural unit. The electrochemical characterization indicates a very good stability of the film, thus opening application in ion exchange system. The focus is on monovalent, divalent, and trivalent ions. These results, the zinc low toxicity, and cost make zinc hexacyanoferrate films a promising candidate for many electrochemical applications.

A Ca‐Ion Electrochromic Battery via a Water‐in‐Salt Electrolyte

AbstractMultivalent‐ion batteries with electrochromic functionality are an emerging green technology for development of low‐carbon society. Compared to Mg2+, Zn2+ and Al3+, Ca2+ has a low polarization strength similar to that of Li+, therefore Ca2+ for electrochromism and battery can avoid kinetic issues caused by other multivalent‐ions with high polarization strength. Here, by exploiting Ca‐ion carriers for electrochromism and a water‐in‐salt (WIS) Ca(OTF)2 electrolyte for the first time, a new and safe aqueous Ca‐ion electrochromic battery (CIEB) has been demonstrated. The WIS Ca(OTF)2 electrolyte demonstrates enhanced anion‐cation interactions and decreased water activity. Vanadium oxide (VOx) and indium hexacyanoferrate (InHCF) films are respectively developed as anode and cathode because of their stable and high‐rate Ca2+ insertion/extraction, as well as matched electrochromism. The CIEB demonstrates a stable and high‐rate capability, a high energy density of 51.4 mWh m−2 at a power density of 1737.3 mW m−2, and a greenish yellow‐to‐black electrochromism. The presented results are beneficial for understanding redox kinetics in WIS electrolytes, and inspire researches on batteries and electrochromism with multivalent‐ions.

Ruthenium Oxide Hexacyanoferrate as an Effective Electrode Modifier for Amperometric Detection of Iodate and Hydrogen Peroxide

Ruthenium oxide hexacyanoferrate (RuOHCF) film was electrochemically deposited on to a glassy carbon (GC) surfaceusing consecutive cyclic voltammetry as a facile and green synthetic strategy. The electrochemical behaviour and electrocatalytic properties of the modified electrode Ru?HCF/GC were evaluated with regards to electroreduction of hydrogen peroxide and iodate in a strong acidic medium (pHs 1.0-2.0) by using different electrochemical techniques, including cyclic voltammetry and amperometry at a constant potential. Electrochemical studies indicated that Ru?HCF/GC possess a high catalytic activity in both studied reactions, fast response and good reproducibility of the current signal. The Ru?HCF/GC exhibits enhanced electrocatalytic behaviour compared with other modified electrodes reported before. The simple and reproducible procedure for electrode fabrication, the wide linear range, anti-interference performance and long-time stability of the Ru?HCF/GC make it a promising sensing material for practical quantitative determination of hydrogen peroxide and iodate. Remarkably, the reported modified.

Influence of electrosynthesis methods in the electrocatalytical and morphological properties of cobalt and nickel hexacyanoferrate films

Prussian Blue analogues (PBA) based on Cobalt and Nickel thin films were assembled by two distinct methodologies using direct electroprecipitation on the substrate and metallic nanoparticle surface modification. The modified electrodes were characterized by several techniques such as Infrared Reflection-Absorption and Electrochemical Impedance Spectroscopies, Scanning Electron and Atomic Force Microscopies, and Electrochemical methods. The electrocatalytical properties of the films were also evaluated and the rate constants for thiosulfate oxidation were obtained and discussed. The results pointed out that the metallic nanoparticle approach has presented higher electrocatalytic effect, suggesting this methodology towards the construction of high performance electrochemical based devices such as sensors, biosensors, fuel cells and others.

Morphology and Conductivity of Copper Hexacyanoferrate Films

Films of conductive coordination network compounds are interesting as functional materials for charge storage, electrocatalysis, electrochromic switching, or conversion of light. The electrical con...

Nickel hexacyanoferrate film coated pencil graphite electrode as sensor and electrode material for environment and energy applications

In this present work, a simple electroless deposition method is used for the fabrication of hexacyanoferrate(III)-nickel deposited pencil graphite (HCF-Ni-PGE) electrode and applied as an electrochemical sensor for selective and simultaneous determination of ortho- I and para- nitrophenol II isomers. The fabricated HCF-Ni-PGE was characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, and elemental mapping. The electrochemical analysis was carried out using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques in 0.1 M KCl containing K4[Fe(CN)6]. The redox (Fe2+/3+) peak currents were enormously increased due to the successful enhancement in the electroactive surface area of PGE from 0.4 to 1.57 cm2 for HCF-Ni-PGE. Further, CV and EIS support the HCF and Ni coating on the PGE surface enhanced the electronic conductivity of HCF-Ni-PGE. The electrochemical sensor application of HCF-Ni-PGE was analysed using CV and differential pulse voltammetry towards the determination of o- and p-NP in 0.1 M PBS (pH 6.0). The reduction currents of I and II at HCF-Ni-PGE was enhanced two times compared with the PGE with better resolution. This, the enhanced activity is due to the positive synergetic effect which increased an electroactive surface area, and electronic conductivity. The limit of detection (LOD) values at HCF-Ni-PGE for I and II isomers are 1.98 × 10−7 and 1.43 × 10−7 M with S/N = 3, respectively. The practical applicability of the electrochemical sensor has been investigated in the presence of common interfering agents such as Na+, SO42− and phenol. The simple electroless deposition used for the fabrication of HCF-Ni-PGE electrode possessed high surface area and high conductivity, it can also be applicable in the field of energy storage devices of supercapacitor, and battery, fuel cells and solar cells as electrode materials and photocatalysts.

(Invited) Morphology and Conductivity of Copper Hexacyanoferrate Films

Metal hexacyanometallates (MHCMs) form a large family of coordination network compounds [1]. They have variable and tunable physical and chemical properties due to the three-dimensional framework of alternating metal knots connected by a non-innocent small bridging ligand forming interconnected channels that can accommodate solvent molecules and counter ions. Therefore, these materials have been comprehensively studied in several different fields, including, functional materials for charge storage, electrocatalysis, electrochromic devices or light harvesting. For the mentioned applications, conductivity is critical, to which i) electronic conductivity by electrons or holes [2], ii) ionic conductivity by mobile alkali ions in the channels of the MHCMs [3], and iii) proton conductivity [4] may contribute. In larger scale for real application, the conductivity depends not only on intrinsic material properties but also on the morphology. Morphology (particularly grain boundaries) plays an important role in determining the pathway of charge carrier transport (electrons and holes) in the film. From first principles it is expected that continuous compact crystalline films will have better electrical conductivity.Here, we present results on copper hexacyanoferrate which demonstrate particularly well the variation of film morphology and its influence on conductivity as a key functional property. We succeeded in devising a simple method for preparation of metal hexacyanoferrates as a thin continuous film by a combination of sonication-assisted casting and vapor-assisted conversion, without the need for surface pre-treatment. This is especially helpful for coating substrates composed of different materials. Films obtained in this way show conductivities higher than that of powder pellets with equivalent crystal structure. Temperature-dependent and humidity-dependent measurements on powder pellets revealed the activation energies for charge transport as well as the role of zeolitic water in facilitating the charge transfer. 1. Paolella, A.; Faure, C.; Timoshevskii, V.; Marras, S.; Bertoni, G.; Guerfi, A.; Vijh, A.; Armand, M.; Zaghib, K. A Review on Hexacyanoferrate-based Materials for Energy Storage and Smart Windows: Challenges and Perspectives. Mater. Chem. A 2017, 5, 18919–18932.2. Behera, J. N.; D’Alessandro, D. M.; Soheilnia, N.; Long, J. R. Synthesis and Characterization of Ruthenium and Iron−Ruthenium Prussian Blue Analogues. Mater. 2009, 21, 1922–1926.3. Ishizaki, M.; Ando, H.; Yamada, N.; Tsumoto, K.; Ono, K.; Sutoh, H.; Nakamura, T.; Nakao, Y.; Kurihara, M. Redox-Coupled Alkali-Metal Ion Transport Mechanism in Binder-Free Films of Prussian Blue Nanoparticles. Mater. Chem. A 2019, 7, 4777–4787.4. Ohkoshi, S.-I.; Nakagawa, K.; Tomono, K.; Imoto, K.; Tsunobuchi, Y.; Tokoro, H. High Proton Conductivity in Prussian Blue Analogues and the Interference Effect by Magnetic Ordering. Am. Chem. Soc. 2010, 132, 6620–6621.

Dynamic Impedance Spectroscopy of Nickel Hexacyanoferrate Thin Films.

Dynamic multi‐frequency analysis (DMFA) is capable of acquiring high‐quality frequency response of electrochemical systems under non‐stationary conditions in a broad range of frequencies. In this work, we used DMFA to study the kinetics of (de‐)intercalation of univalent cations (Na+ and K+) in thin films of nickel hexacyanoferrate (NiHCF) during cyclic voltammetry. For this system, the classic stationary electrochemical impedance spectroscopy fails due to the instability of the oxidized form of NiHCF. We are showing that such spectra can be fitted with a physical model described by a simple two‐step intercalation mechanism: an adsorption step followed by an insertion step. The extracted kinetic parameters are depending on the state of charge as well on the nature of the inserted cation.

Electrochemical determination of l-vanillin using copper hexacyanoferrate film modified gold nanoparticle graphite-wax composite electrode

A simple and sensitive electrochemical determination of l-vanillin (VA), a powerful anti-microbial agent and a commonly employed food preservative was investigated using copper hexacyanoferrate (CuHCF) film modified on 2-mercaptoethylamine functionalized colloidal gold nanoparticles (colloid-Au) graphite-wax composite electrode. As achieved surface modification was characterized using confocal Raman spectroscopy studies which gave rise to peaks at 351, 493, 807 and 2048 cm−1 that corresponds to the characteristics vibrational modes of ν(Fe–C ≡N–Cu), ν(Cu–N), ν(Fe–C) and ν(–C≡N) for the CuHCF film. Further, the elemental analysis investigated using X-ray photoelectron spectroscopy confirms the presence of 2p peaks of Cu (II), Fe(II) and 1 s edge peaks of N for the CuHCF film modified electrode. The modified electrode exhibited a high sensitivity of 0.1817 μA μM−1 at a lower detection potential of 0.68 V towards the VA determination and the limit of detection was observed to be 2.53 × 10−7 M (S/N = 3). The VA content in commercial roasted coffee beans was also examined with the proposed sensor and the results were found to be satisfactory.

Redox‐Polysilsesquioxane Film as a New Chloride Storage Electrode for Desalination Batteries

Desalination battery is an emerging concept used in electrochemical technology to solve the problems in sea water desalination. However, identifying an efficient and reversible chloride (Cl−) storage electrode is a major challenge for the development of desalination batteries. A new membraneless, reversible desalination battery consisting of an efficient, reversible, high‐capacity Cl− storage electrode, ferrocyanide molecule immobilized polysilsesquioxane (redox‐PSQ), in combination with the cation storage material, a nickel hexacyanoferrate film in real sea water, is described. The redox‐PSQ polymer is superior over the other reported Cl− storage electrodes (Ag and Bi electrodes) in terms of cost, stability, reversibility, and Cl− ion removal capacity in real‐time applications. About 164 mg L−1 of Cl− ion is removed from natural sea water per 7.6 × 10−8 mol cm−2 surface concentration of electrode material with 98% coulombic efficiency. During the desalination process, alkali ions move toward nickel hexacyanoferrate and Cl− ions moves toward the redox‐PSQ film from the sea water. During the salination process, the stored ions are recovered from the electrode surface as a result of partial energy recovery. Furthermore, this electrode also removes the sulfate ion present in the sea water by the electrostatic adsorption phenomenon. In addition, a high‐voltage (2.3 V), Mg//redox‐PSQ polymer cell is constructed with a red light‐emitting diode that glows during the salination process.

Separation of cesium from wastewater with copper hexacyanoferrate film in an electrochemical system driven by microbial fuel cells

A electrochemical adsorption system driven by microbial fuel cell (MFC-adsorption) was developed based on copper(II) hexacyanoferrate(III) (CuHCF) film for cesium (Cs) removal from wastewater. Cs uptake and elution can be simply controlled by regulating the redox states of the CuHCF films. Chemical oxygen demand (COD) removal showed little difference as MFC was connected to adsorption system. Meanwhile, power density and coulombic efficiency of MFC were dramatically reduced. The efficiencies of Cs adsorption and desorption were undesirable. MFC-adsorption technology used for actual nuclear wastewater treatment still has far to go.

Thin layer films of copper hexacyanoferrate: Structure identification and analytical applications

Thin films of copper hexacyanoferrate (CuHCF) have been reproducibly electrodeposited on conductive substrates according to two different potentiostatic methods, here denoted as A and B. For both methods two consecutive steps are involved, the first being the electrodeposition of a thin Cu layer, the second its partial dissolution and formation of CuHCF in presence of hexacyanoferrate anion, giving as result a two layers film (CuHCF on Cu metal). The main difference, instead, consists in the applied potential values and their application times, featuring Method A lower potentials but longer processing times. Structural insights have been achieved by means of X-ray Diffraction (XRD) and X-ray Absorption Fine Structure (XAFS) measurements, from which we can deduce the presence of Prussian blue (PB) impurities in Method A, while Method B leads to a pure CuHCF phase. Two analytical applications have been considered, ion exchange and H2O2 sensing. Ion exchange has been first assayed and, although CuHCF-A shows a higher stability towards multivalent cations, CuHCF-B is suitable for small hydrated ions. PB impurities in CuHCF-A boost its sensing towards H2O2, making it more adapted to this employment.

Lead(II) ion detection in purified drinking water by nickel hexacyanoferrate-modified n-Si electrode in presence of dihydroxybenzene

To develop feasible probes for heavy metal ions has been considered as one of the most important research topics, due to the significance in monitoring and evaluating the toxicity of environmental water system, especially for the drinking water. This work provided a probe for Pb(II) ion detection in purified drinking water. Nickel hexacyanoferrate (NiHCF) films were prepared by vacuum evaporating nickel fine wire and transforming in a potassium ferricyanide solution on the front surface of platinum coated n-silicon. The transformed NiHCF film shows a reversible insertion electrochemistry and possesses electrocatalytic activity under illumination. The NiHCF-modified electrode was used as photoelectrode and dihydroxybenzene as photocurrent mediators to make up photoelectrochemical (PEC) sensor for Pb(II) ion detection in purified drinking water based on a two-electrode cell in absence of reference electrode and operated at zero working voltage. We have investigated the effects of Pb(II) ions on the photocurrent of hydroquinone or catechol in the purified water. The assay demonstrated good photocurrent responses by adding different concentrations of Pb(II) ion into purified drinking water with a linear range of 20–1560 nM and the limit of determination (based on S/N = 3) is 4.8 nM and 5.3 nM for the presence of hydroquinone and catechol, respectively.

Fabrication and Investigation of Crystalline Non-Porous Conductive Coordination Network Compounds

During the last years, surface-bound coordination network compounds (CNCs) have found application for sensing purposes (electrical, optical, and magnetic read-out), charge storage, electrochromic and optoelectronic layers, transparent coatings, bipolar switches, thermoelectric materials, catalytic, photocatalytic and light harvesting applications [1]. However, most of these applications require electrically conductive coordination network compounds (CCNCs). This requirement severely limits the choice of CNCs for the above-mentioned applications. Hexacyanometallates are coordinative networks with high electrical and ion conductivity that possess interesting redox properties and tuneable electrochemical redox transitions associated with clear colour changes [2]. We developed and characterized CCNCs such as copper hexacyanoferrate as thin crystalline films and characterized their optical, magnetic, electrical and dielectric properties. Thin films of these compounds were fabricated on gold and silicon oxide coated by self-assembled, cyanide-terminated monolayers using alternating immersion in their precursor solutions. Atomic force microscopy (AFM) confirms the formation of copper hexacyanoferrate with cubic structure on the gold surface (Fig. 1). Fig. 1: AFM image of copper hexacyanoferrate film on a cyanide-terminated self-assembled monolayer on gold surface via layer-by-layer method. X-ray diffraction (XRD) analysis confirmed the crystalline nature of the films. The thin films can be switched by exploiting the redox chemistry of the metal centres. The modulation of the electrical transport properties provides great potential benefits for numerous future applications in microelectronic device technologies. Structural and electronic changes of the films were probed by X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) before and after electrochemical oxidation and reduction [3]. Results demonstrate the possibility for electrochemical switching of the material properties for each compound but also indicates the complications in those transitions originating in the existence of different structures and defects in which (mobile) alkali metal cations may or may not reside in the pores and provide charge compensation. Acknowledgement(s) Financial supports from DFG priority programme (SPP1928, Wi1617/24) is gratefully acknowledged.

Infrared spectroelectrochemical analysis of potential dependent changes in cobalt hexacyanoferrate and copper hexacyanoferrate films on gold electrodes

Metal hexacyanometallates represent coordination network compounds with a large variety of structurally similar substances. It is well established that they can undergo multiple redox transitions accompanied by cation insertion/ejection. However, the exact assignment of structural changes has remained complicated partially due to the structural variability resulting from different preparation methods and the lack of in situ tools to follow the redox transition and structural changes simultaneously. In this communication we prepared cobalt hexacyanoferrate (CoHCF) and copper hexacyanoferrate (CuHCF) by electrochemical deposition and, additionally, CuHCF by layer-by-layer deposition. These hexacyanoferrate films have a suitable thickness for their spectroelectrochemical characterization by means of polarization modulation infrared reflection-absorption spectroscopy with electrochemical control. Potential-dependent structural changes were followed in both HCF films using the cyanide stretching mode ν(CN) whose detailed position reveals coordination and symmetry around the cyanide bridging ligand. CoHCF undergoes two redox reactions. The spectral analysis of the ν(CN) mode provides clear evidence that oxidation of N-coordinated Co(II) to Co(III) takes place in the first redox process, at E1°′=0.530V. The second redox process at E2°′=0.685V is assigned to a partial oxidation of the C-coordinated Fe(II)/Fe(III). In CuHCF films, one redox process involves predominantly Fe(II)/Fe(III) centers. However, in the electrochemically deposited film a small fraction of the Cu(II) centers is oxidized to Cu(III). The species containing Cu(III) ions are unstable and undergo electron transfer reactions leading to the oxidation of the Fe(II) to Fe(III). In addition, CuHCF shows ‘annealing’ in which some cyanide groups change the coordination to C-coordinated Cu ions and N-coordinated Fe ions.

Formation of nanoporous NiS films from electrochemically modified GC surface with Nickel Hexacyanoferrate film and its performance for the hydrogen evolution reaction

The production of Hydrogen (H2) gas for fuel cell application using electrochemical methods is attracting the attention of the researchers in recent years owing to its high calorific value. Current researchers are enthused to prepare cost effective catalysts from abundant elements in earth crust for the huge production of H2. Recently electrochemical studies are reported based on Metal sulfides and oxides such as MoS2, CoS, WO3 as best catalyst for H2 generation under diverse pH conditions. We focused on preparing nanoporous Nickel Sulfide (NiS) films from Nickel Hexacyanoferrate (NiHCF) nanocubes using electrochemical cycling of the precursor films in Sulfide medium and characterized those using spectroscopy and Microscopy techniques such as FT-IR, Raman spectroscopy, XPS, EDS, XRD, FESEM, AFM, and TEM. The cubic structure of NiHCF gets transformed into NiS cubic skeleton by etching in presence of sulfide ions (S2−). The nanoporous NiS give best results for Hydrogen Evolution Reaction (HER) in the alkali medium. The catalysts were electrochemically modified on glassy carbon surface for electrochemical characterization including Tafel polarization. For comparison, the same procedure was used to prepare other metal sulfides including MnS, FeS, and CoS to compare their catalytic activity towards Hydrogen evolution. Among them, the NiS was shown to be the most efficient electrocatalyst for hydrogen evolution and has shown promise as an alternative to platinum.

Anodically Grown Binder-Free Nickel Hexacyanoferrate Film: Toward Efficient Water Reduction and Hexacyanoferrate Film Based Full Device for Overall Water Splitting

One of the challenges in obtaining hydrogen economically by electrochemical water splitting is to identify and substitute cost-effective earth-abundant materials for the traditionally used precious-metal-based water-splitting electrocatalysts. Herein, we report the electrochemical formation of a thin film of nickel-based Prussian blue analogue hexacyanoferrate (Ni-HCF) through the anodization of a nickel substrate in ferricyanide electrolyte. As compared to the traditionally used Nafion-binder-based bulk film, the anodically obtained binder-free Ni-HCF film demonstrates superior performance in the electrochemical hydrogen evolution reaction (HER), which is highly competitive with that shown by a Pt-plate electrode. The HER onset and the benchmark cathodic current density of 10 mA cm-2 were achieved at small overpotentials of 15 mV and 0.2 V (not iR-corrected), respectively, in 1 M KOH electrolyte, together with the long-term electrochemical durability of the film. Further, a metal-HCF-electrode-based full water-splitting device consisting of the binder-free Ni-HCF film on a Ni plate and a one-dimensional Co-HCF film on carbon paper as the electrodes for the HER and the oxygen evolution reaction (OER), respectively, was designed and was found to demonstrate very promising performance for overall water splitting.