Transition Metal Hexacyanoferrates Research Articles

Highly efficient Cu-Fe containing prussian blue analogs (PBAs) for excellent electrocatalytic activity towards overall water splitting

Hydrogen generation by water electrolysis is considered a clean, green, and renewable approach for sustainable energy demands and environmental management. The efficient use of earth-abundant transition metal catalysts is cost-effective and limits the usage of noble metal catalysts like IrO2, RuO2, and Pt/C. The PBAs (Prussian Blue Analogs) are a class of transition metal hexacyanoferrates that can be tailored for specific electrochemical reactions. Both iron and copper are well-proven transition metals for catalytic activities owing to their aptitude to achieve much-pronounced reactant transformations under minor circumstances, as well as their environmental friendliness, cyclability, and long-term durability at 10 mA cm−2 in 1.0 M KOH, the greenly synthesized Fe8Cu2CN (PBAs) on NF (Nickel foam) displays a low overpotential of 150 mV (118 mV dec−1) for HER (Hydrogen Evolution Reaction) and 290 mV (112 mV dec−1) for OER (Oxygen Evolution Reaction). Fe8Cu2CN displays ultra-stable behavior (150 h) with a slight current loss of 4.2 % for OER and 3.4 % for HER, respectively. The interaction of Fe, Cu, and CN (cyanide) enhances the rate of reaction kinetics. The designing of a water electrolyzer that delivers 10 mA cm−2 at 1.60 V is made possible with the help of the improved bifunctional catalyst Fe8Cu2CN/NF. The Fe8Cu2CN/NF//Fe8Cu2CN/NF exhibits long-last durability (over 180 h) with a negligible current reduction of 5.4 %. The improved effectiveness of the synthesized catalyst is demonstrated by the solar water splitting at 1.66 V. These findings suggest that Fe8Cu2CN/NF//Fe8Cu2CN/NF can be used to produce large quantities of H2 at very affordable prices.

Selective removal of ammonium ions with transition metal hexacyanoferrate (MHCF) electrodes

This study investigated the potential of using transition metal hexacyanoferrates (MHCF) to selectively capture NH4+ over Na+ in brine for nutrient recovery from wastewater. The electrochemical characteristics of five different MHCF (M is Ni, Cu, Fe, Co or Zn), known as Prussian blue analogues (PBA), was investigated using a hybrid capacitive deionization device, and their desalting capacity, stability and energy consumption were evaluated. By adjusting the mass ratio of electrodes, applied voltage, and ion concentration, the relationship between NH4+ removal and selectivity was explored. Results showed that all five MHCF electrodes exhibited high selectivity (> 4) for NH4+ over Na+, but the selectivity is not stable due to comprehensive factors (such as the structural stability of electrode, solution composition, and current density, etc.). Among them, the NiHCF electrode showing the best performance in terms of specific desalination capacity (∼1.23 mmol/g), charge efficiency (>80%), and stability (82.5% capacity retention after 50 cycles). This study demonstrates that PBA has promising potential as NH4+ selective electrodes for nutrient removal and recovery in wastewater.

(Digital Presentation) Post Mortem Microscopical and Selective Analysis of Manganese Hexacyanoferrate Cathode Material By Transmission Soft X-Ray Microscopy

Prussian Blue (PB) and its analogues (PBAs) are a large family of transition metal hexacyanoferrates with general formula of AxM[Fe(CN)6]γ□1-γ∙zH2O. They have open framework structure, redox-active sites and strong structural stability, which makes them perspective electrode materials, as PBAs are able to perform the rapid insertion and extraction of the ions with the lowest possible lattice strain. In addition, they are safe, non-toxic and relatively inexpensive. Among simple PBAs, manganese hexacyanoferrate (MnHCF) displays a high specific capacity and redox plateaus at a high voltage against lithium and sodium in LIBs and SIBs [1]. The synthesis of MnHCF is possible by simple co-precipitation method. In our group we synthesized and investigated its structure and electrochemical performance as a cathode material. For the characterization of MnHCF several conventional and synchrotron-based techniques were used.Generally, after cycling inside the battery some changes might occur in the cathode material, including changing the charge states of the different elements. For the investigation of this phenomenon synchrotron techniques are particularly informative, such as energy-dependent full field transmission soft X-ray microscopy (TXM), which is able to give a full picture at the nanometre scale of the chemical state and spatial distribution of the elements [2]. By using X‐rays in the “soft” energy region (<3 KeV), it is possible to access transitions from the core levels of the light elements, such as the K‐edge of nitrogen, oxygen, fluorine, as well as L- and M- edges of heavier elements [3, 4]. TXM provides pixel-by-pixel absorption spectrum, making it possible to select groups of pixels and map regions with the similar spectral features [2]. Therefore, TXM studies were conducted on our material before and after cycling, and the special protocol of the data treatment was developed and applied to the results of the analysis in order to enhance the representation of the heterogeneities, if any, and then to extract the most relevant spectra.Indeed, the inhomogeneities within the samples were identified, especially in the aged electrodes. It was observed that the distribution of the heterogeneities were different for the electrodes extracted from the Li-ion system, compared to Na. Characteristic spectra from the entire samples as well as particular regions with different oxidation states were obtained and compared to one another, which made it evident, that the treated results from the iron L-edge and manganese L-edge expressed strong correlation, however, while in the Fe K-edge the distribution of heterogeneities looked quite random, a weak bulk-border effect was observed in case of the Mn K-edge.[1] A. Mullaliu, J. Asenbauer, G. Aquilanti, S. Passerini, M. Giorgetti, Small Methods 4, (2019) 1900529.[2] A. Sorrentino, L. Simonelli, A. Kazzazi, N. Laszczynski, A. Birrozzi, A. Mullaliu, E. Pereiro, S. Passerini, M. Giorgetti, D. Tonti, Appl. Sci., no. 11, p. 2791, 2021.[3] D. Tonti, M. Olivares-Marín, A. Sorrentino, E. Pereiro, L. P. Mehdi Khodaei, Ed., IntechOpen, 22 March, 2017.[1] A. Mullaliu, J. Asenbauer, G. Aquilanti, S. Passerini, M. Giorgetti, Small Methods 4, (2019) 1900529.[2] A. Sorrentino, L. Simonelli, A. Kazzazi, N. Laszczynski, A. Birrozzi, A. Mullaliu, E. Pereiro, S. Passerini, M. Giorgetti, D. Tonti, Appl. Sci., no. 11, p. 2791, 2021.[4] D. Attwood, Cambridge: Cambridge University Press, 1999. Figure 1

(Keynote, Digital Presentation) Prussian Blue Analogues as Electrode Material of Batteries

Prussian blue analogues (PBA), which composed of cyano-bridged jungle-gym type framework and alkali metal ions, are promising material for electrode of batteries, because its framework is robust against electrochemical intercalation/deintercalation of alkali metal ions. In addition, the material consists of ubiquitous elements and is easily synthesized from aqueous solution. Thus, PBAs are promising electrode material for sodium ion secondary battery (SIB). We will introduce the overall features of PBAs as SIB electrode. Furthermore, PBAs are promising material for energy harvesting. We proposed a new type of battery (tertiary battery) that can be charged by the environmental heat using the difference in the thermal coefficient (α= dV/dT) of the redox potential (V) between the anode (αanode) and cathode (αcathode) materials. The tertiary battery can convert the environmental heat energy into electric energy during the thermal cycle, i.e., (i) heating to TH, (ii) discharge at TH, (iii) cooling to TL, and (iv) discharge at TL. In the (i) heating process, the V values of the anode and cathode change by αanodeΔTand αcathodeΔT, respectively. We expect a thermally induced change in the cell voltage (V cell) as large as (αcathode - αanode)ΔT. The stored electric energy can be extracted by the (ii) discharge process at TH. We fabricated several tertiary batteries made of PBA electrodes with different a and evaluated the thermal voltage (Vcell) and discharge capacity (Qcell). For example, NaxCo[Fe(CN)6]0.87/NaxNi[Fe(CN)6]0.94 tertiary battery exhibits Vcell = 24 mV and Qcell = 2.4 mAh/g per unit weight of total active material contains in the cathode and anode. We will correlate the observed Qcell values with the electrode parameters of the tertiary batteries. Y. Shimaura, T. Shibata, and Y. Moritomo, "Interrelation between discharge capacity and charge coefficient of redox potential in tertiary batteries made of transition metal hexacyanoferrate", Jpn. J. Appl. Phys. 61, 044004 (2022)I. Nagai, Y. Shimaura, T. Shibata, and Y. Moritomo, "Performance of tertiary battery made of Prussian blue analogues", Appl, Phys. Express. 14, 094004 (2021).Y. Moritomo, Y. Yoshida, D. Inoue, H. Iwaizumi, S. Kobayashi, S. Kawaguchi, and T. Shibata "Origin of the material dependence of the temperature coefficient of the redox potential in coordination polymers", J. Phys. Soc. Jpn. 90, 063801 (2021).T. Shibata, H. Iwaizumi, Y. Fukuzumi, and Y. Moritomo, "Energy harvesting thermocell with use of phase transition” Sci. Res., 10 1813 (2020)I. Takahara, T. Shibata, Y. Fukuzumi, and Y. Moritomo, " Improved thermal cyclability of tertiary battery made of Prussian blue analogues ", Chem. Select 4, 8558-8563 (2019)Y. Fukuzumi, K. Amaha, W. Kobayashi, H. Niwa, and Y. Moritomo, “Prussian blue analogues as promising thermal power generation materials”, Energy Technology, 6 (2018) 1 – 7.T. Shibata, Y. Fukuzumi, W. Kobayashi, and Y. Moritomo, “Thermal efficiency of a thermocell made of Prussian blue analogues”, Sci. Reps. Appl. Phys. Express. 8 (2018) 14784.T. Shibata, Y. Fukuzumi, W. Kobayashi, and Y. Moritomo, Thermal power generation during heat cycle near room temperature, Appl. Phys. Express. 11, 018101 (2018).M. Takachi, T. Matsuda, and Y. Moritomo, “Cobalt hexacyanoferrate as cathode material of Na+ secondary battery”, Appl. Phys. Express 6, 025802 (2013).M. Takachi, T. Matsuda, and Y. Moritomo, “Redox reaction in Prussian blue analogues with fast Na+ intercalation”, Jpn. J. Appl. Phys. 52, 092202 (2013).

Transition Metal Hexacyanoferrate(II) Complexes as Catalysts in the Ring-Opening Copolymerization of CO2 and Propylene Oxide

The catalytic activity of four transition metal hexacyanoferrate(II) complexes (Ni2[Fe(CN)6], Co2[Fe(CN)6], KFe[Fe(CN)6] and Zn2[Fe(CN)6]) in the ring-opening copolymerization (ROCOP) of CO2 and propylene oxide (PO) is reported here for the first time and compared with that of other hexacyanometallate compounds. Complexes were prepared by coprecipitation employing tert-butanol as complexing agent. X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, elemental analysis, X-ray fluorescence, scanning electron microscopy, transmission electron microscopy and N2 physisorption were used to confirm the identity of the obtained materials. Except for Zn2[Fe(CN)6], which showed an amorphous nature, the complexes were constituted by aggregates of cubic nanocrystals with intra-crystalline micropores and inter-crystalline mesopores. Gas–solid phase titration with NH3 revealed the high potential of hexacyanoferrates as Lewis acid catalysts. In the case of Zn2[Fe(CN)6], the lack of structural organization led to an extremely high density of acid sites (43 μmol m−2). The resulting copolymers were analyzed via nuclear magnetic resonance spectroscopy and gel permeation chromatography. The studied transition metal hexacyanoferrate(II) catalysts showed mild activity in the target reaction, giving rise to polyethercarbonates with moderate CO2 content (9.3–18.1 wt%), random configuration (67.0–92.4% of polyethercarbonate linkages), modest molecular weights (MW, g mol−1 = 3400–20,200) and high dispersity (ĐM = 4.0–5.4). Cyclic propylene carbonate (PC) was also produced (1.4–19.8 wt%). Among all, the Co2[Fe(CN)6] complex stands as a potential catalyst for CO2/PO ROCOP due to its high CO2 uptake, selectivity and molecular weight of the obtained copolymer.

Interrelation between discharge capacity and charge coefficient of redox potential in tertiary batteries made of transition metal hexacyanoferrate

We fabricated eight tertiary batteries made of transition metal hexacyanoferrates and evaluated thermally-induced voltage (V cell) and discharge capacity (Q cell) per unit weight of the total active material contained in the cathode and anode. V cell and Q cell are reproduced by the values ( and ) calculated from temperature (α) and charge (β) coefficients of the redox potential (V) of cathode and anode materials. Reflecting small −β of Na x Zn3[Fe(CN)6]1.967.2H2O (ZnHCF), Q cell for Na x Co[Fe(CN)6]0.873.4H2O/ZnHCF cell reaches 4.0 mAh g−1 between 20 °C and 40 °C.

Thermo-rechargeable Batteries Fabricated Using Low-cost Materials

We fabricated three thermo-rechargeable batteries using low-cost transition metal hexacyanoferrate and determined the thermal voltage (Vcell) and discharge capacity (Qcell) per unit weight of the total active material in the cathode and anode. The obtained values were compared with the calculated values, which were determined by analyzing the temperature and capacity coefficients of the redox potential.

Interrelation between discharge capacity and charge coefficient of redox potential in tertiary batteries made of transition metal hexacyanoferrate

Abstract We systematically investigated the performance, i.e., thermally-induced voltage (Vcell) and discharge capacity (Qcell) per unit weight of the total active material, of the tertiary batteries made of transition metal hexacyanoferrates. Vcell and Qcell are reproduced by the values ( Vcell calc and Qcell calc) calculated from temperature (α) and charge (β) coefficients of the redox potential (V) of cathode and anode materials. Reflecting small -β of ZnHCF, Qcell for NCoF87/ZnHCF cell reaches 4.0 mAh/g between 20 ○C and 40 ○C.

Synthesis of nickel hexacyanoferrate nanostructure on carbon cloth with predeposited nickel nanoparticles as precursor for binder-free high-performance supercapacitor electrodes

Transition metal (TM) hexacyanoferrates (TM-HCF) have been proven efficient electrode materials for supercapacitors, especially in neutral electrolyte. In this work, nickel hexacyanoferrate (Ni-HCF) with ultra-high mass-loading was synthesized on carbon cloth (CC) with nickel nanoparticles (NiNPs) as the precursor, which were in-situ converted to Ni-HCF with graphene oxide (GO) as the oxidant and NiNPs as the reductant in the presence of potassium ferricyanide. First, NiNPs were potentiostatically electrodeposited on CC. Then, the electrodeposited NiNPs was oxidized to Ni2+ by GO. The produced Ni2+ reacted immediately with potassium ferricyanide to form nanostructured nickel hexacyanoferrate. By tuning the electrodeposition time of NiNPs, the mass of the finally produced Ni-HCF can be easily adjusted. In this work, the mass-loading of Ni-HCF supported on CC reaches up to 26.4 mg cm−2. As the positive electrode, Ni-HCF/CC possesses a high areal capacity of 6.9 C cm−2 at a discharge current density of 10 mA cm−2, excellent cycling stability with 81% of its initial capacitance after 2000 continuous charge/discharge cycles, ultrahigh energy density of 36 W h kg−1 at the power density of 200 W kg−1 and nearly 100% columbic efficiency. The superior energy storage capability of Ni-HCF indicates its promising prospect as the positive electrode material of high performance supercapacitors.

Citrate-mediated synthesis of highly crystalline transition metal hexacyanoferrates and their Na ion storage properties

In this work, highly crystalline transition-metal hexacyanoferrates (MHCFs) have been synthesized in the presence of citrate, which is found not only to mediate the precipitation reaction rate but also prevent the MHCF crystals from aggregation. This method is applicable to a wide range of MHCFs with different transition metals (e.g., Co, Ni, Mn and Zn). The highly crystalline MHCFs with little Fe(CN)6 vacancies and rich alkaline ions exhibited significantly improved electrochemical performance for Na ion storage in both aqueous and organic electrolytes. The effect of transition metals in MHCFs on the Na ion storage performance has been investigated in an aqueous electrolyte. Results showed that the CoHCF nanocube sample displayed the highest electrochemical performance than other MHCFs samples synthesized in this work. This CoHCF sample exhibits a specific capacity of 85 mAh g−1 at 1 A g−1. The CoHCF sample also displays a good cycling stability with capacity retentions of 92% after 12,000 cycles in aqueous electrolyte (0.5 M Na2SO4) and 78% after 10,000 cycles in organic electrolyte (NaClO4 in PC + EC with 5% FEC).

Advances of Prussian blue and its analogues in (bio)sensors

The aim of the present article is to review the current state of knowledge concerning advances of Prussian blue and its analogues in chemical and biological sensors. Despite deposition of these materials has been already discussed, an appearance of recent articles presenting poorly synthesized transition metal hexacyanoferrates, causes review of the synthetic procedures. The significance of Prussian blue for (bio)sensors is provided by its unique catalytic properties: both catalytic activity and selectivity in hydrogen peroxide reduction are >1000 times higher compared to platinum, whereas clinical analyzers and low-invasive monitors are equipped with Pt-based biosensors. Non-iron (nickel, cobalt, cupric etc.) hexacyanoferrates are catalytically inactive, however, they are highly attractive for stabilization of Prussian blue resulting in synthesis of the ‘super-stable’ Prussian blue-based electrocatalyst. The protocol for enzyme immobilization from water–organic mixtures with a high content of organic solvent is discussed. Combination of the most advantageous electrocatalyst (Prussian blue) and the improved immobilization protocol results in elaboration of the advanced biosensors. Their unique sensitivity, selectivity and operational stability (even compared with platinum-based ones) provide application for non-invasive diagnostics.

K2.25Ni0.55Co0.37Fe(CN)6 nanoparticle connected by cross-linked carbon nanotubes conductive skeletons for high-performance energy storage

Prussian Blue (PB) coordination compound is a well-known ideal candidate for energy storage due to its tunable open framework structure. Especially transition metal hexacyanoferrate displays excellent performance in supercapacitor. In this paper, a facile one-step co-precipitation synthesis method is developed to prepare hybrid K2.25Ni0.55Co0.37Fe(CN)6/CNTs composite, aiming at increasing the electric conductivity of electrode material by introducing carbon nanotube conductive skeleton to the Cobale-Nickel hexacyanoferrate hybrid nanoparticles. The TEM image shows that K2.25Ni0.55Co0.37Fe(CN)6 presents nanoparticles with a diameter about 20nm, which are connected along the surface of carbon nanotube. Such special structure could facilitate the electric transportation between each nanoscale K2.25Ni0.55Co0.37Fe(CN)6 particle, efficiently improving the utilization of active material. Considering the intrinsic abundant channels for ions insertion/extraction of nanoparticles and good conductivity of carbon nanotubes, our creative electrode is expected to display fantastic supercapacitor performance. The electrochemical data demonstrate that the K2.25Ni0.55Co0.37Fe(CN)6/CNTs electrode exhibits a high specific capacitance of 600Fg−1 at 0.2Ag−1and excellent rate performance of 90.4% when the current density ranged from 0.2Ag−1 to 5Ag−1. Furthermore, the K2.25Ni0.55Co0.37Fe(CN)6/CNTs composite achieves decent cycling stability with maintaining 94% of its initial specific capacitance after 2000discharge/charge cycles. The excellent energy storage property offers a great promise as supercapacitor materials.

Low Voltage Charge/Discharge Behavior of Manganese Hexacyanoferrate

Recently, Prussian blue analogues (PBAs) have been reported to exhibit a low voltage charge/discharge behavior with high capacity (300–545 mAh/g) in lithium-ion secondary batteries (LIBs) [...]

Estimation of continuity of electroactive inorganic films based on apparent anti-Ohmic trend in their charge transfer resistance

We report on the microscopy-free, pure electrochemical approach for estimation of the continuity of electroactive inorganic films. Impedance spectra of the transition metal hexacyanoferrate films, electrochemically deposited under the optimized conditions, have been fitted to the Randles-type equivalent circuit, complicated by diffusion impedance with reflective boundary conditions, connected in series to the charge transfer resistance. An apparent anti-Ohmic trend is revealed in the sharp decrease of the latter with an increase in amount of the deposited inorganic film commonly presuming its thickness. Assigning charge transfer resistance to the resistance of the electrode|film interface, its observed decrease with subsequent saturation is explained in terms of an increase of the interface area until the entire electrode is covered with the inorganic film completely. Scanning electron microscopy (SEM) confirms the “anti-Ohmic” and saturation regions correspond to the partially covered and fully covered electrode surfaces, respectively. The dependence of charge transfer resistance on the amount of the deposited film thus provides an estimation of the continuity of inorganic films, which is highly important in a view of their practical applications. Except for cost efficiency, microscopy-free tools are attractive because in some cases, for instance for recessed electrodes, microscopy is hardly applicable.

A Full Prussian Blue Cell Containing a Hexacyanomanganate(II/I) Anode and a Cosolvent Electrolyte

The continued deployment of variable generation assets including solar and wind has resulted in widespread concerns regarding electric grid stability.1-2 Energy storage in the form of batteries can smooth the volatile power outputs of solar and wind, alleviating the impact of that volatility on the rest of the grid.2 Unfortunately, existing battery technologies do not offer a long enough cycle life or high enough power to be economically used for this application. We previously showed that copper hexacyanoferrate, a Prussian Blue analogue (PBA) cathode material, can survive for over 40,000 deep discharge, high rate cycles.3 This cathode cycle life was an order of magnitude longer than incumbent batteries under consideration for renewables support. To achieve a significant lifetime advantage in a practical battery however, an anode having comparable cycle life was needed. In this presentation, we describe a new Na+cell containing PBA as both the cathode and the anode and a nonflammable aqueous-organic cosolvent electrolyte. By using a PBA as the active material for both electrodes, neither electrode limits the lifetime or kinetics of the cell. In this cell, a transition metal hexacyanoferrate cathode is paired with a new manganese(II) hexacyanomanganate(II/I) (MnHCMn) anode. Soft X-ray absorbance spectroscopy (sXAS) shows that the MnHCMn anode operates by a novel Mn(II/I) redox in which low-spin 3d6 MnI(CN)6 complexes are formed during charging (reduction). Pairing this anode with a hexacyanoferrate-based cathode results in a 1.6 V average cell voltage, which is nearly twice the 950 mV cell voltage of previously reported full PBA cells.4 Lab-scale full pouch cells containing have a hexacyanomanganate(II/I)-based anode and a hexacyanoferrate-based cathode have an average projected lifetime of over 30,000 cycles at 100% depth of discharge based on >2,000 cycles and six months of testing. This long cycle life is achieved because both of the electrodes cycle by near-zero strain single-phase insertion reactions. The addition of organic cosolvents to the aqueous electrolyte prevents dissolution of the PBAs while retaining the high Na+ conductivity and nonflammability of aqueous electrolytes. This results in an extension of the projected cell calendar life from just months to years. Finally, the rapid kinetics of the PBA electrodes and cosolvent electrolyte allow access to 80% of total cell energy above 1.15 V during 12C discharge. The long cycle life and high power capability of these cells make them particularly attractive for short-duration renewables support applications such as solar intermittency smoothing.5

Electrochemically formed 3D hierarchical thin films of cobalt–manganese (Co–Mn) hexacyanoferrate hybrids for electrochemical applications

Here we report the feasibility of forming 3D nanostructured hexacyanoferates of Cobalt and Manganese (Co–MnHCF) on GC surface by a facile electrochemical method. This 3D architecture on glassy carbon electrode characterised systematically by voltammetry and other physical characterisation techniques like Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Fourier transform Infrared spectroscopy (FTIR) etc,. Electrochemical Quartz crystal microbalance (EQCM) studies helped out to calculate the total mass change during Co–MnHCF formation. Electrochemical studies reveal that the formal redox potentials of both Co and MnHCF films remained close to that of newly formed Co–MnHCF hybrid films. These 3D modified films were successfully applied for two different electrochemical applications i) For pseudocapacitor studies in KNO3 medium ii) Investigated the electrocatalytic behaviour of redox film towards water oxidation reaction in alkaline medium. Electrochemical performances of newly formed Co–MnHCF are compared with their individual transition metal (Co, Mn) hexacyanoferrates. The resulting material shows a specific capacitance of 350 F g−1 through its fast reversible redox reaction of electrochemically formed Co–MnHCF modified film. Interestingly we showed the overpotential of 450 mV (from its thermodynamic voltage 1.2 V) to attain its optimum current density of 10 mA cm−2 for O2 evolution in alkaline medium.

Low-Cost Nickel Hexacyanoferrate Nanoparticle As Cathode Material of Lithium-Ion Batteries for Large-Scale Applications

1. Introduction Rechargeable lithium ion batteries become a solution to the world’s ever-increasing demand for energy storage and created excellent opportunities for communication via portable electric devices. However, LIBs are limited to the portable electronic devices and are still too expensive to be used in large-scale applications such as renewable energies. Transition metal hexacyanoferrates (MHCFs) are promising cathode materials because of easy production, low cost and promising electrochemical performance. MHCF were investigated as hosts for alkali ions in 1999 [1]. In 2012, Prof. J. Goodenough’s group showed promising Na ion battery using MHCF as a cathode [2]. In this work, NiHCF is synthesized via a simple co-precipitation method. This cathode showed very promising performance in LIBs with stable cycle life and flat charge/discharge plateaus. 2. Experiments NiHCF was synthesized via a simple co-precipitation method. Typically, 0.01 mol Ni(NO3)2 and 0.005 mol K3Fe2(CN)6were dissolved in 250 mL DI water, respectively. These two solutions were added dropwise into DI water in a round bottom flask under magnetic stirring at 70 °C [3]. The yellow precipitates were filtered, washed and dried in a vacuum oven at 35 °C for 72 h. The NiHCF was characterized by XRD (X’Pert PRO, PANalytical) and TEM (Libra 120, Carl Zeiss). NiHCF was homogeneously mixed with Acetylene Black (MTI Co.), polyvinylidene difluoride (Kynar) in 8:1:1 ratio and coated on a carbon current collector (Alfa Aesar), and dried in 35 °C for 24 h. The mass loading of NiHCF was around 4-5 mg/cm2. CR2032 coin cells were assembled to investigate NiHCF electrochemical behavior. Lithium chips was used as reference/counter electrode, Celgard 2400 was used as a separator and 1 M LiPF6 in EC:DEC:DMC (1:1:1 vol.) was used as electrolyte. The electrochemical investigations were carried out using galvanostat/potentiostat tester (VMP3, Biologic). 3. Results and discussion The pure NiHCF phase was confirmed from the XRD result, and sharp characteristic peaks indicated good crystallinity of NiHCF as presented in Fig. 1a. Transmission electron microscopy (Fig. 1b)showed that NiHCF consisted of agglomerated nanoparticles around 20–50 nm in size. Fig. 2 is the cyclic voltammetry of NiHCF at 0.1 mV s-1 between 2 - 4.2 V vs. Li/Li+. NiHCF has a well distinguished pair of symmetric peak around 3.3 V, which corresponds to the lithiation/delithiation of NiHCF during discharge and charge process. The charge/discharge profiles are shown in Fig. 3. The excellent flat plateaus around 3.3 V are well-matched with the CV results in Fig. 2. NiHCF showed a capacity of 52 mAh g-1at a current density of 0.2 C, which is about 87% of its theoretical capacity. Further information and results on this research will be presented at the Meeting. Acknowledgments This research was supported by the Institute of Batteries LLC, a research grant from the Ministry of Education and Science of Kazakhstan, and a Research Project from Nazarbayev University.

Controllable synthesis of coaxial nickel hexacyanoferrate/carbon nanotube nanocables as advanced supercapcitors materials

The transition metal hexacyanoferrate (MHCF, M represents as Fe, Co, Ni, et al), has been studied and applied intensively in biosensor, catalysts, ion exchanger, molecular magnets and gas storage, but as yet remains largely unexplored for electrochemical energy storage device. This study shows that NiHCF can be uniformly deposited into the matrix of poly(4-vinylpyridine) grafted onto multiwalled carbon nanotubes (MWCNTs), and forms coaxial NiHCF/MWCNT nanocables with MWCNTs as core and NiHCF as shell. A different NiHCF content was loaded onto MWCNTs, and their electrochemical behavior and capacitive performance were determined. The as-prepared NiHCF/P4VP-g-MWCNT hybrids can possess a huge specific capacitance up to 1035F·g−1 at a discharge density of 0.05A·g−1. These capacitance will decay by 10% at 25.6A·g−1. They remain 92.7% of its initial capacitance at 2A·g−1 after 10000 discharge/charge cycles. Their energy density is nearly 56.2Wh·kg−1 at a power density of 10W·kg−1. Their good electrochemical reversibility, high cyclic stability and excellent capacitive performance suggest a promising use as an advanced material for supercapacitors.

Electrochemical Energy Storage in Cs2Zn3[Fe(CN)6] and Their Use As Cathode at Na-Ion Batteries in Aqueous Media

The use of renewable energy resources represented by solar and wind must be the most ideal solution to reduce the serious energy and environmental problems. However, the available amounts of these resources are dependent of weather conditions, thus requiring energy storage systems (ESSs) for persistent power supply. While hydropower and pumped air systems are currently used as grid-scale ESSs, rechargeable batteries are expected to replace the existing technology utilizing their advantages related with higher efficiencies and simple installation. Among a variety of batteries, lithium-ion batteries (LIBs) are receiving the most significant attention due to their superior performance in various electrochemical aspects. In this context, batteries based on alternative carrier ion such as sodium (Na+) might be more suitable for grid-scale ESSs, linked with critical aspects such as abundance and cost of the raw materials. Indeed, raw materials for sodium-ion batteries (SIBs) are estimated to be about 30 times cheaper than those of LIBs. On the other hand, the use of radio-cesium (137Cs) in nuclear-power plants and for military activities have caused a critical environmental hazard as main fission product in radioactive wastes.From past few decades, transition metal hexacyanoferrates known as Prussian Blue Analogues (PBAs), have been widely used for Cs removal from waste waters. This open framework materials with the Prussian Blue crystal structure offer the high power capability and low cost synthesis. Herein, we report the synthesis of zinc hexacyanoferrate (ZHCF), which is endowed with enlarged ionic channels compared with typical PBAs. ZHCF was synthesized by precipitation method from a mixture of Na4Fe(CN)6 and Zn(NO3)2 solutions at 60°C. The solid formed was centrifuged and dried to room temperature. The solid material was mixed with a solution of CsNO3 during 24 hr. Then centrifuged again and dried. The obtained material was labeled as Cs-ZHCF. Using XRD, the crystal structure suggests that materials crystallize with a rhombohedral unit cell (R-3c space group). Their porous framework can be represented as the assembling of FeC6 octahedral and ZnN4 tetrahedral shapes. Therefore, the synthesis process promotes the assembling of octahedral-anionic blocks ([Fe(CN)6]4-) through Zn2+ cations, which links neighboring blocks at the N ends. In the structure of these materials the N–Zn–N angle is near to 108°. The formation of elliptical-like windows and ellipsoidal cavities is related to such bonding angle. The calculated cavity dimension was ca. 15.5 x 11.1 x 7.9 Å. Six elliptical windows of ca. 6.8 x 8 Å communicate neighbor cavities. Within a given cavity two exchangeable Cs-ions are found, located close to the major ellipsoid axis in order to minimize the repulsive electrostatic interactions between them.For the electrochemical characterization using cyclic voltammetry in NaNO3 solutions, the Cs-ZHCF was mixed with amorphous carbon, polyvinylidenedifluoride, and graphite in a ratio of 80:9:9:2 were prepared in THF. The obtained suspension was deposited on a glassy carbon electrode, and dried in Argon atmosphere. It was found that the i-E profiles show a reversible capacity of 56.4 mAhg-1. For the evaluation of the observed electrochemical data, it is important to note that in Cs-ZHCF, Zn is electrochemically inactive but rather only contributes to maintaining the framework structure, whereas, the iron in octahedral coordination can be oxidized and reduced during charge-discharge. These results indicate that only one Cs-ion was removed from the cavity of the material-structure, improving the diffusion of Na-ions.

Transition metal hexacyanoferrates in electrocatalysis of H2O2 reduction: an exclusive property of Prussian Blue.

The ability of Prussian Blue, ferric hexacyanoferrate (FeHCF), to sensitively and selectively detect hydrogen peroxide by its reduction in the presence of oxygen is of high importance for analytical chemistry. Success with Prussian Blue (PB) provided an appearance of contradictory reports concerning electrocatalysis of the other transition metal hexacyanoferrates (HCFs) in H2O2 reduction. Investigating thermodynamics of the catalyzed reactions as well as electrochemical properties of the hexacyanoferrates, we are able to conclude that the noniron hexacyanoferrates themselves are completely electrocatalytically inactive, except for a minor electrocatalysis in the opposite reaction, hydrogen peroxide oxidation, registered for NiHCF. Concerning the most important reaction, H2O2 reduction, the observed electrocatalytic activity (by the way, 100 times decreased compared to PB) is due to the presence of FeHCF (Prussian Blue) as defects in the structure of noniron hexacyanoferrates. This finding, considering other unique properties of transition metal HCFs, will provide a systematic search for sensing materials with improved analytical performance characteristics.