One-Step High-Temperature Electrodeposition of Fe-Based Films as Efficient Water Oxidation Catalysts

Electrolysis of water to produce hydrogen requires an efficient catalyst preferably made of cheap and abundant metal ions for the improved water oxidation reaction. An Fe-based film has been deposited in a single step by electrochemical deposition at temperatures higher than the room temperature. Until now, the electrodeposition of iron oxide has been carried out at 298 K or at lower temperatures under a controlled atmosphere to prohibit atmospheric oxidation of Fe²⁺ of the iron precursor. A metal inorganic complex, ferrocene, and non-aqueous electrolyte medium propylene carbonate have been used to achieve electrodeposition of iron oxide without the need of any inert or controlled atmosphere. At 298 K, the amorphous film was formed, whereas at 313 K and at higher temperatures, the hematite film was grown, as confirmed by X-ray diffraction. The transformation of iron of the ferrocene into a higher oxidation state under the experimental conditions used was further confirmed by X-ray photoelectron spectroscopy, ultraviolet-visible, and electron paramagnetic resonance spectroscopic methods. The films deposited at 313 K showed the best performance for water oxidation with remarkable long-term electrocatalytic stability and an impressive turnover frequency of 0.028 s^-1 which was 4.5 times higher than that of films deposited at 298 K (0.006 s^-1). The observed overpotential to achieve a current density of 10 mA cm^-2 was found to be 100 mV less for the film deposited at 313 K compared to room-temperature-derived films under similar experimental conditions. Furthermore, electrochemical impedance data revealed that films obtained at 313 K have the least charge transfer resistance (114 Ω) among all, supporting the most efficient electron transport in the film. To the best of our knowledge, this is the first-ever report where the crystalline iron-based film has been shown to be electrodeposited without any post-deposition additional treatment for alkaline oxygen evolution reaction application.

Theoretical Study on the Swelling Mechanism and Structural Stability of Ni3Al-LDH Based on Molecular Dynamics

layered double hydroxide (LDH) as a kind of 2D layer material has a swelling phenomenon. Because swelling significantly affects the adsorption, catalysis, energy storage, and other application properties of LDHs, it is essential to study the interlayer spacing, structural stability, and ion diffusion after swelling. In this paper, a periodic computational model of Ni₃Al-LDH is constructed, and the supramolecular structure, swelling law, stability, and anion diffusion properties of Ni₃Al-LDH are investigated by molecular dynamics theory calculations. The results show that the interlayer water molecules of Ni₃Al-LDH present a regular layered arrangement, combining with the interlayer anions by hydrogen bonds. As the number of water molecules increases, the hydrogen bond between the anion and the basal layer gradually weakens and disappears when the number of water molecules exceeds 32. The hydrogen bond between the anion and the water molecule gradually increases, reaching an extreme value when the number of water molecules is 16. The interlayer spacing of Ni₃Al-LDH is not linear with the number of water molecules. The interlayer spacing increases slowly when the number of water molecules is more than 24. The maximum layer spacing is stable at around 19 Å. The interlayer spacing, binding energy, and hydration energy show an upper limit for swelling: the number of water molecules is 32. When the number of interlayer water molecules is 16, the water molecules' layer structure and LDH interlayer spacing are suitable for anions to obtain the maximum diffusion rate, 10.97 × 10^-8 cm²·s^-1.

First-Principles Study on Interlayer Spacing and Structure Stability of NiAl-Layered Double Hydroxides

Interlayer spacing and structure stability of layered double hydroxides (LDHs) on their application performance in adsorption, ion exchange, catalysis, carrier, and energy storage is important. The effect of different interlayer anions on the interlayer spacing and structure stability of LDHs has been less studied, but it is of great significance. Therefore, based on density functional theory (DFT), the computational model with 10 kinds of anions intercalated Ni₃Al-A-LDHs (A = Cl^-, Br^-, I^-, OH^-, NO₃ ^-, CO₃ ^2-, SO₄ ^2-, HCOO^-, C₆H₅SO₃ ^-, C₁₂H₂₅SO₃ ^-) and four Ni _R Al-Cl-LDH models with different Ni²⁺/Al³⁺ ratios (R = 2, 3, 5, 8) were constructed to calculate and analyze interlayer spacing, structural stability, and their influence factors. It was found that the interlayer spacing order of Ni₃Al-A-LDHs intercalated with different anions is OH^- < CO₃ ^2- < Cl^- < Br^- < I^- < HCOO^- < SO₄ ^2- < NO₃ ^- < C₆H₅SO₃ ^- < C₁₂H₂₅SO₃ ^-. The hydrogen bond network between the base layer and the interlayer anions affects the arrangement structure of the interlayer anions, which affects the interlayer spacing. For interlayer monatomic anions Cl^-, Br^-, and I^- and the anion of comparable size in each direction SO₄ ^2-, the interlayer spacing is positively correlated with the interlayer anion diameter. The larger difference between the long-axis and short-axis dimensions of the polyatomic anions results in the long axis of the anion being perpendicular to the basal layer, increasing interlayer spacing. The long-chain anion C₁₂H₂₅SO₃ ^- intercalation system exhibits the largest layer spacing of 24.262 Å. As R value increases from 2 to 8, the interlayer spacing of Ni _R Al-Cl-LDHs gradually increases from 7.964 to 8.124 Å. The binding energy order between the interlayer anion and basal layer is CO₃ ^2- > SO₄ ^2- > OH^- > Cl^- > Br^- > I^- > HCOO^- > NO₃ ^- > C₁₂H₂₅SO₃ ^- > C₆H₅SO₃ ^-. The smaller the interlayer spacing, the higher the binding energy and the stronger the structural stability of LDHs. The factors affecting structural stability mainly include the bond length and bond angle of the hydrogen bond and the charge interaction between the basal layer and interlayer anion. In the CO₃ ^2- intercalated system, the hydrogen bond length exhibits the shortest of 1.95 Å and the largest bond angle of 163.68°. The density of states and energy band analysis show that the higher the number of charges carried by the anion, the stronger its ability to provide electrons to the basal layer.

Design of a high-performance ternary LDHs containing Ni, Co and Mn for arsenate removal

To cope with the current serious arsenate pollution problem, a new ternary layered double hydroxides (LDHs) containing Ni, Co and Mn with good performance was developed, guiding by DFT calculations. First, Ni, Co and Mn were screened as the metal sources to constitute the LDHs, due to their high ionic charge density. Then, Ni(II), Co(II) and Mn(III)-O octahedra were selected as the primary units for structuring the LDHs, because of their good chemical activity. Meanwhile, the ratio of metals in the ternary LDHs, favoring for arsenate removal, was optimized at 1:2:1. In addition, the synergistic effect among various metals in the LDHs was considered. The results suggested that in the case of single doping, all three metals can act as the center to promote chemical activity independently. On the contrary, when combined together, there is only one unilateral active center. Moreover, the existence of ligand covalent bonds between arsenate and LDHs was confirmed. Finally, a promising new NiCo₂Mn-LDHs with the maximum adsorption capacity of 407.23 mg/g for arsenate removal had been prepared.

Unveiling the Occurrence of Co(III) in NiCo Layered Electroactive Hydroxides: The Role of Distorted Environments

Co- and Ni-based layered hydroxides constitute a unique class of two-dimensional inorganic materials with exceptional chemical diversity, physicochemical properties and outstanding performance as supercapacitors and overall water splitting catalysts. Recently, the occurrence of Co(III) in these phases has been proposed as a key factor that enhance their electrochemical performance. However, the origin of this centers and control over its contents remains as an open question. We employed the Epoxide Route to synthesize a whole set of α-NiCo layered hydroxides. The PXRD and XAS characterization alert about the occurrence of Co(III) as a consequence of the increment in the Ni content. DFT+U simulation suggest that the shortening of the Co-O distance promotes a structural distortion in the Co environments, resulting in a double degeneration in the octahedral Co 3d orbitals. Hence, a strong modification of the electronic properties leaves the system prone to oxidation, by the appearance of Co localized electronic states on the Fermi level. This work combines a microscopic interpretation supported by a multiscale crystallochemical analysis, regarding the so-called synergistic redox behavior of Co and Ni, offering fundamental tools for the controllable design of highly efficient electroactive materials. To the best of our knowledge, this is the first computational-experimental investigation of the electronic and structural details of α-NiCo hydroxides, laying the foundation for the fine tuning of electronic properties in layered hydroxides.

DFT calculations of the synergistic effect of λ-MnO2/graphene composites for electrochemical adsorption of lithium ions

Recently, the composite of spinel-type manganese oxide (λ-MnO₂)/graphene has drawn wide attention because of its good electrochemical adsorption selectivity for low concentrations of Li⁺ ions from lake brine or seawater to cope with the fast-rising demand of lithium resources. In this composite, the synergistic effect between the good selectivity of λ-MnO₂ for Li⁺ ions and the excellent conductivity of graphene play an important role for the electrochemical adsorption of Li⁺ ions. In order to reveal the synergistic mechanism in the electronic conductivity, the ionic conductivity and the ion selectivity of the λ-MnO₂/graphene composite, density functional theory (DFT) calculations combined with electrochemical adsorption experiments were carried out. The calculation results show that the enhanced electronic conductivity of the composite is due to the decrease of the band gap (E_g) in the λ-MnO₂/graphene composite compared with pure λ-MnO₂. Meanwhile, the graphene composited with λ-MnO₂ decreased the diffusion energy barrier of Li⁺ ions in λ-MnO₂. In addition, the competitive adsorption of Li⁺, Na⁺ and Mg²⁺ ions were investigated by the nudged elastic band (NEB) method and charge distribution analysis. The results show that Li⁺ ions in λ-MnO₂ exist in their pure ion state and have the lowest diffusion energy barrier compared with Na⁺ and Mg²⁺. The results of the DFT calculations were validated by cyclic voltammetry, electrochemical impedance spectroscopy and electrochemical adsorption experiments.