Effective Construction of High-quality Iron Oxy-hydroxides and Co-doped Iron Oxy-hydroxides Nanostructures: Towards the Promising Oxygen Evolution Reaction Application

Design of iron oxyhydroxide nanosheets coated on Co species embedded in nanoporous carbon for oxygen evolution reaction

There is still a tremendous challenge in designing environmentally friendly oxygen evolution reaction (OER) catalysts that are inexpensive and high-performing for practical applications. Herein, the self-sacrificing template zeolitic imidazolate framework-67 (ZIF-67) was pyrolyzed under N2 atmosphere to generate Co species embedded in nanoporous carbon (Co-NC). Then, iron oxyhydroxide (FeOOH) was wrapped onto the Co-NC surface via electrodeposition to shape the Co-NC@FeOOH composites. Benefiting from the core-shell structure, high conductivity, and distributed active sites, Co-NC@FeOOH presents distinguished OER performance with a low overpotential (336 mV) at 10 mA cm-2 and small Tafel slope (49.46 mV dec-1). This work furnishes a rosy passage for receiving cost-effective electrocatalysts with high efficiency for OER.

Photovoltaic amorphous feroxyhyte nanostructures synthesized by atmospheric AC microplasma

Feroxyhite (δ-FeOOH) nanomaterials were successfully synthesized through the atmospheric AC microplasma method at room temperature from ferrous sulfate aqueous solutions. Various syntheses conditions, including electric voltage, electric field strength, ferrous concentration, hydrogen peroxide concentration, and reaction duration, were systematically investigated. The synthesized products were characterized through x-ray diffraction, UV-vis absorption spectroscopy, photoluminescence spectroscopy, infra-red spectroscopy, and electron microscopy. The bandgap of the produced materials were strongly dependent of the ferrous concentration while the product ratio was dependent on all experimental conditions. The synthesis mechanism was thoroughly discussed. The synthesized nanomaterials were amorphous nanospheres, showing superparamagnetic properties at room temperature. The synthesized oxyhydroxide is a potential photovoltaic material besides its reported applications in photocatalysts and supercapacitors. The application of this synthesis technique could be extended to synthesize other oxy-hydroxide nanomaterials for renewable energy applications facilely, scalablely, cost-effectively, and environmentally.

Dual-doping in the bulk and the surface to ameliorate the hematite anode for photoelectrochemical water oxidation

Aiming at the drawbacks of hematite like poor conductivity and tardy oxidation kinetics, herein, we utilized dual dopants in the bulk and surface to ameliorate the situation. Specifically, doping optimal amount of Zr⁴⁺ in the hematite (Zr:Fe₂O₃) enhances the conductivity of hematite due to the higher charge carrier density. Further, F:FeOOH could form p-n heterojunction in bulk where a potential barrier is built up that repels electrons but prompts holes transferring to F:FeOOH for water oxidation. What's more, the high electronegative of F^- would withdraw electron from the Fe site in FeOOH, and the enhanced positive electricity of Fe³⁺ is beneficial for adsorption of OH^- as well as enhance the conductivity of FeOOH to expedite holes transfer. As a result, the composite photoanode (F:FeOOH/Zr:Fe₂O₃) shows a 3.25-times enhanced photocurrent density comparing with α-Fe₂O₃. The special designation employs ultrathin F:FeOOH to act as both p-type semiconductor and efficient co-catalyst, avoiding redundant layer that would extend the migration distance of holes. On the top of that, the dual modification approach provides an extensive prospect for the further application of hematite.

Cobalt-Nickel Phosphate Composites for the All-Phosphate Asymmetric Supercapacitor and Oxygen Evolution Reaction

Recently, design of cost-effective multifunctional electromaterials for supercapacitors and oxygen evolution reaction (OER) and enhancing their functionalities have become an emphasis in energy storage and conversion. Herein, a series of cheap and functional phosphate composites with different ratios of cobalt and nickel are synthesized using a simple polyalcohol refluxing method, and their excellent capacity and OER properties are systematically studied. Notably, owing to the different major role of Co and Ni elements in the phosphate composites for capacity and OER, the optimal electroconductibility, structural adjustment, electrochemical active sites, and activities for capacity and OER are obtained from the composites with the different ratios of Co/Ni. In addition, using high-capacity BiPO₄ (BPO) as the negative electrodes, the new type of all-phosphate asymmetric supercapacitor (CNPO-40//BPO) shows a high energy density and reaches 36.84 W h kg^-1 at a power density of 254.52 W kg^-1. Its cyclic stability is also more excellent than that of the CNPO-40//AC device using commercial activated carbon as the negative electrodes. This study is beneficial to the more in-depth research on efficient dual-function electromaterials in capacity and OER and provides a high-efficient way to improve the practicality of asymmetric supercapacitors using the high-capacity Bi-based electromaterials as the negative electrodes.

Preparation and Characterization of Montmorillonite/PEDOT-PSS and Diatomite/PEDOT-PSS Hybrid Materials. Study of Electrochemical Properties in Acid Medium

The hybridization of clay minerals with conducting polymers receives great interest for different potential applications, including environmental remediation. This work studies and compares the electrochemical properties of two different clays, montmorillonite (Mont) and diatomite (Diat), and their respective clay/PEDOT-PSS hybrid materials in H2SO4 medium. The hybrid materials were prepared by electropolymerization of EDOT in the presence of PSS. The physico-chemical and electrochemical properties of both clays were analyzed by different techniques, and the influence of the clay properties on electropolymerization and the electroactivity of the resulting clay/PEDOT-PSS hybrids was investigated. Specifically, the Fe2+/Fe3+ redox probe and the oxidation of diclofenac, as a model pharmaceutical emerging pollutant, were used to test the electron transfer capability and oxidative response, respectively, of the clay/PEDOT-PSS hybrids. The results demonstrate that, despite its low electrical conductivity, the Mont is an electroactive material itself with good electron-transfer capability. Conversely, the Diat shows no electroactivity. The hybridization with PEDOT generally enhances the electroactivity of the clays, but the clay properties affect the electropolymerization efficiency and hybrids electroactivity, so the Mont/PEDOT displays improved electrochemical properties. It is demonstrated that clay/PEDOT-PSS hybrids exhibit diclofenac oxidation capability and diclofenac concentration sensitivity.

Functionalized biochar-supported magnetic MnFe2O4 nanocomposite for the removal of Pb(ii) and Cd(ii)

In this study, a novel magnetic biochar-MnFe2O4 nanocomposite (BC/FM) was prepared using low-cost corn straw and MnFe2O4 by sol-gel/pyrolyzing route using egg white, which has abundant functional groups (-NH2 and -COOH). Following that, its composition, morphology and structure was characterized by various techniques including SEM-EDX, BET, XRD, and VSM. Batch experiment of the adsorption for Pb(ii) and Cd(ii) including influence of pH, kinetics, isotherm and thermodynamics was also studied. The results demonstrated that biochar could effectively support MnFe2O4, which displayed high dispersion on the surface of the biochar and possessed abundant functional groups and high surface area contributing to superior performance on Pb(ii) and Cd(ii) removal. Therein, MnFe2O4 with high magnetism is convenient for separating the magnetic BC/FM from an aqueous medium. Adsorption experiment results indicate that Pb(ii) and Cd(ii) removal by BC/FM was closely related to pH with the best value of pH 5.0, and the process reached equilibrium in 2 h. The adsorption process is well-described by the pseudo-second-order kinetic model and Sips (Freundlich-Langmuir) model. Thermodynamic studies suggest that the adsorption process is spontaneous and exothermic. The maximum experimental adsorption capacity of BC/FM is 154.94 and 127.83 mg g-1 for Pb(ii) and Cd(ii), respectively, in single-solute system, which is higher than that of some of the other adsorbents of biochar or biochar-based composites. In bi-solute system, the preferential adsorption order of BC/FM for the two metals is Pb(ii) prior to Cd(ii). Finally, FTIR and XPS analysis verified that the main mechanism of Pb(ii) and Cd(ii) removal by BC/FM is by forming Pb/Cd-O or complexation of carboxyl and hydroxyl and ion exchange. Therefore, the prepared magnetic BC/FM composite, as an excellent adsorbent, exhibited potential applications for the removal of Pb(ii) and Cd(ii) from wastewater.

Remarkable Bifunctional Oxygen and Hydrogen Evolution Electrocatalytic Activities with Trace-Level Fe Doping in Ni- and Co-Layered Double Hydroxides for Overall Water-Splitting

Large-scale H₂ production from water by electrochemical water-splitting is mainly limited by the sluggish kinetics of the nonprecious-based anode catalysts for oxygen evolution reaction (OER). Here, we report layer-by-layer in situ growth of low-level Fe-doped Ni-layered double hydroxide (Ni_{1- x}Fe _x-LDH) and Co-layered double hydroxide (Co_{1- x}Fe _x-LDH), respectively, with three-dimensional microflower and one-dimensional nanopaddy-like morphologies on Ni foam, by a one-step eco-friendly hydrothermal route. In this work, an interesting finding is that both Ni_{1- x}Fe _x-LDH and Co_{1- x}Fe _x-LDH materials are very active and efficient for OER as well as hydrogen evolution reaction (HER) catalytic activities in alkaline medium. The electrochemical studies demonstrate that Co_{1- x}Fe _x-LDH material exhibits very low OER and HER overpotentials of 249 and 273 mV, respectively, at a high current density of 50 mA cm^-2, whereas Ni_{1- x}Fe _x-LDH exhibits 297 and 319 mV. To study the overall water-splitting performance using these electrocatalysts as anode and cathode, three types of alkaline electrolyzers are fabricated, namely, Co_{1- x}Fe _x-LDH(+)∥Co_{1- x}Fe _x-LDH(-), Ni_{1- x}Fe _x-LDH(+)∥Ni_{1- x}Fe _x-LDH(-), and Co_{1- x}Fe _x-LDH(+)∥Ni_{1- x}Fe _x-LDH(-). These electrolyzers require only a cell potential ( E_cell) of 1.60, 1.60, and 1.59 V, respectively, to drive the benchmark current density of 10 mA cm^-2. Another interesting finding is that their catalytic activities are enhanced after stability tests. Systematic analyses are carried out on both electrodes after all electrocatalytic activity studies. The developed three types of electrolyzers to produce H₂, are very efficient, cost-effective, and offer no complications in synthesis of materials and fabrication of electrolyzers, which can greatly enable the realization of clean renewable energy infrastructure.

Cobalt-Doped Goethite-Type Iron Oxyhydroxide (α-FeOOH) for Highly Efficient Oxygen Evolution Catalysis

It is an urgent challenge to develop low-cost and high-performance catalysts for the oxygen evolution reaction (OER). We synthesized nanoparticulate electrocatalysts consisting of cobalt-doped goethite-type iron oxyhydroxide (α-FeOOH) with controlled Co/Fe ratios [Co _x Fe_1-x OOH (x ≤ 0.20)] based on our own wet process. A Co_0.20Fe_0.80OOH-coated glassy carbon electrode generated a current density (j) of 10 mA cm^-2 at an overpotential (η) as small as 383 mV (1.61 V vs the reversible hydrogen electrode) in an alkaline electrolyte, with a small Tafel slope of 40 mV dec^-1 and excellent durability, whereas pure α-FeOOH required η = 580 mV to reach the same current density. This can be ascribed to the effect of Co doping, which resulted in an increase in electrochemically active surface area and a decrease in charge-transfer resistance. The content of cobalt, a scarce resource, in the catalyst is much smaller than those in most of the other Fe-based catalysts reported so far. Thus, this study will provide a new strategy of designing cost-effective and high-performance catalysts for the OER in alkaline solution.

Mixed NiO/NiCo2O4 Nanocrystals Grown from the Skeleton of a 3D Porous Nickel Network as Efficient Electrocatalysts for Oxygen Evolution Reactions

Mixed NiO/NiCo₂O₄ nanocrystals grown in situ from the skeleton of a 3D porous nickel network (3DPNN) were prepared with a simple hydrothermal method followed by a low temperature calcination, exhibiting outstanding electrocatalytic efficiencies toward oxygen evolution reactions (OER). The 3DPNN was prepared with a novel leaven dough method and served as both the nickel source for growth of the mixed NiO/NiCo₂O₄ nanocrystals and the charge transport highway to accelerate the sluggish kinetics of the OER. The mixed NiO/NiCo₂O₄ nanocrystals exhibited pronounced synergistic effects to achieve a high mass activity of 200 A g^-1 at the catalyst mass loading of 0.5 mg cm^-2, largely outperforming the corresponding single component nanocrystal systems, NiO (5.87) and NiCo₂O₄ (9.35). The NiO/NiCo₂O₄@3DPNN composite electrocatalyst achieved a low overpotential of 264 mV at the current density of 10 mA cm^-2 and 389 mV at the practically high current density of 250 mA cm^-2, which compete favorably among the top tier of previously reported OER electrocatalysts. Moreover, it exhibited good stability even at the high current density of 250 mA cm^-2, showing only 9.40% increase in working applied potential after a continuous 12 h operation. The present work demonstrates a new design for highly efficient OER catalysts with in situ growth of mixed oxide nanocrystals of pronounced synergistic effects.