Preparation, characterization and application in cobalt ion adsorption using nanoparticle films of hybrid copper-nickel hexacyanoferrate

Gonçalves J, Breslin CB, Nossol E. Enhancing Oxygen Evolution Reaction Performance with rGO/CoNi-Prussian Blue-Derived Oxyhydroxide Nanocomposite Electrocatalyst: A Strategic Synthetic Approach

Electrochemical water splitting is a promising approach in the development of renewable energy technologies, providing an alternative to fossil fuels. It has attracted considerable attention in recent years. The benchmark materials used in water splitting are precious metals that are expensive and scarce. Therefore, this work proposes a strategic electrochemical synthesis of a reduced graphene oxide and cobalt-nickel hexacyanoferrate (rGO/CoNiHCF)-derived composite (rGO/CoNiPBd-OOH) to achieve optimized OER performance. The optimum rGO/CoNiHCF was fabricated with the Co:Ni precursors in a 3:1 ratio with a ferricyanide solution of pH = 1.0. Using an alkaline electrochemical treatment, the well-distributed globular particles of CoNiHCF over rGO sheets were converted into layered frameworks of metallic (oxy)hydroxide species, giving the final rGO/CoNiPBd-OOH nanocomposite. This nanocomposite presented favorable kinetic activity resulting in a Tafel slope of 33 mV dec-1, while rGO, CoNiPBd-OOH, and RuO2 exhibited slopes of 80, 47, and 51 mV dec-1, respectively. Although the benchmark RuO2 electrocatalyst showed a lower overpotential (240 mV dec-1) at a current density of 10 mA cm-2, the rGO/CoNiPBd-OOH performed well with an overpotential of 346 mV, combined with superior stability compared to CoNiPBd-OOH and RuO2, maintaining a current density of 10 mA cm-2 for 15 h with an overpotential loss of 6.92%. This work successfully presents an "all-electrochemical" synthesis of a rGO/CoNiHCF-derived material with remarkable electrocatalytic activity for OER assisted by a strategic preparation methodology, which helped to understand the influence of synthetic parameters and choose their conditions to achieve the optimum OER performance.

Rapid separation of the low concentration Pd from Pd-Pt coexisting systems: Cyano-group's monomer-specific affinity

Rapid separation of low concentration palladium (Pd) from Pd-Platinum (Pt) coexisting systems remains a formidable challenge, primarily due to the undifferentiated substitution of ligands in Pd/Pt complexes by adsorption sites. The development of an adsorbent featuring monomer-specific affinity adsorption sites for Pd/Pt could mitigate this drawback. Herein, Manganese hexacyanoferrate (MnHCF) possessing the sensitivity and specificity to Pd ions (Pd(II)) was synthesized via the facile co-precipitation method. MnHCF could rapidly and selectively capture 90.30 % of Pd(II) from a 10 ppm Pd-Pt coexisting system within just 5 min. Spectroscopic analyses and density functional theory (DFT) calculations indicated that cyano-group (CN) in MnHCF exhibited the monomer-specific affinity for targeted capturing Pd via the direct and strong coordination interaction (Fe-CN-PdCl2), which was co-determined by the electron-losing of C (0.06 e) and N (0.07 e) atom. At the same time, CN could neither react directly with the fully coordinated [PtCl6]2- species nor substitute the Cl- ligand, both of which contributed to the non-adsorption of Pt, thus triggering the Pd-Pt separation. This study provides a promising candidate adsorbent for practical applications in platinum group metals recovery by the design of adsorption sites with monomer-specific affinity.

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.

Removal of Cd(II) from Micro-Polluted Water by Magnetic Core-Shell Fe3O4@Prussian Blue

A novel core-shell magnetic Prussian blue-coated Fe3O4 composites (Fe3O4@PB) were designed and synthesized by in-situ replication and controlled etching of iron oxide (Fe3O4) to eliminate Cd (II) from micro-polluted water. The core-shell structure was confirmed by TEM, and the composites were characterized by XRD and FTIR. The pore diameter distribution from BET measurement revealed the micropore-dominated structure of Fe3O4@PB. The effects of adsorbents dosage, pH, and co-existing ions were investigated. Batch results revealed that the Cd (II) adsorption was very fast initially and reached equilibrium after 4 h. A pH of 6 was favorable for Cd (II) adsorption on Fe3O4@PB. The adsorption rate reached 98.78% at an initial Cd (II) concentration of 100 μg/L. The adsorption kinetics indicated that the pseudo-first-order and Elovich models could best describe the Cd (II) adsorption onto Fe3O4@PB, indicating that the sorption of Cd (II) ions on the binding sites of Fe3O4@PB was the main rate-limiting step of adsorption. The adsorption isotherm well fitted the Freundlich model with a maximum capacity of 9.25 mg·g-1 of Cd (II). The adsorption of Cd (II) on the Fe3O4@PB was affected by co-existing ions, including Cu (II), Ni (II), and Zn (II), due to the competitive effect of the co-adsorption of Cd (II) with other co-existing ions.

Electrochemical recovery of cobalt using nanoparticles film of copper hexacyanoferrates from aqueous solution

Nanoparticles film of copper metal hexacyanoferrates (CuHCF) was fabricated to electrochemically separate Co²⁺ in aqueous solutions under various conditions such as applied potential, solution pHs, initial concentrations, contact time and coexisting ions. Results showed that the removal efficiency conducted in reduction potential was obviously higher than that in oxidation potential. The optimal pH for Co²⁺ adsorption occurred at 8.0. Coexisting ions studies revealed that Co²⁺ could be removed from aqueous solutions containing Li⁺, Cu²⁺ and Al³⁺. Considering that cobalt and lithium are the main metallic elements in LiCoO₂, the effect of different ionic strengths (IS) of LiNO₃ (0.5, 1, 2, 5, 10) on adsorption was further investigated. Results showed that IS of LiNO₃ had little impact on the removal efficiency of Co²⁺, which indicated the potential of selective recovery of cobalt from LiCoO₂ in spent lithium-ion batteries. X-ray energy-dispersion spectroscopy (EDS) confirmed that the Co²⁺ could be adsorbed effectively onto CuHCF film. The adsorption was well described by Langmuir isotherm and the maximum sorption capacity is 218.82 mg/g. The kinetic rate of Co²⁺ adsorption was rapid initially and attained equilibrium within 60 min, and the data well fitted the Redlich-Peterson and the Elovich model, implying a chemisorption dominated process.