Iridium-Doped Nanosized Zn-Al Layered Double Hydroxides as Efficient Water Oxidation Catalysts

Harnessing the electronic structure of active metals to lower the overpotential of the electrocatalytic oxygen evolution reaction

Despite substantial advancements in the field of the electrocatalytic oxygen evolution reaction (OER), the efficiency of earth-abundant electrocatalysts remains far from ideal. The difficulty stems from the complex nature of the catalytic system, which limits our fundamental understanding of the process and thus the possibility of a rational improvement of performance. Herein, we shed light on the role played by the tunable 3d configuration of the metal centers in determining the OER catalytic activity by combining electrochemical and spectroscopic measurements with an experimentally validated computational protocol. One-dimensional coordination polymers based on Fe, Co and Ni held together by an oxonato linker were selected as a case study because of their well-defined electronic and geometric structure in the active site, which can be straightforwardly correlated with their catalytic activity. Novel heterobimetallic coordination polymers were also considered, in order to shed light on the cooperativity effects of different metals. Our results demonstrate the fundamental importance of electronic structure effects such as metal spin and oxidation state evolutions along the reaction profile to modulate ligand binding energies and increase catalyst efficiency. We demonstrated that these effects could in principle be exploited to reduce the overpotential of the electrocatalytic OER below its theoretical limit, and we provide basic principles for the development of coordination polymers with a tailored electronic structure and activity.

Tailoring Pore Size and Catalytic Activity in Cobalt Iron Layered Double Hydroxides and Spinels by Microemulsion-Assisted pH-Controlled Co-Precipitation

Cobalt iron containing layered double hydroxides (LDHs) and spinels are promising catalysts for the electrochemical oxygen evolution reaction (OER). Towards development of better performing catalysts, the precise tuning of mesostructural features such as pore size is desirable, but often hard to achieve. Herein, a computer-controlled microemulsion-assisted co-precipitation (MACP) method at constant pH is established and compared to conventional co-precipitation. With MACP, the particle growth is limited and through variation of the constant pH during synthesis the pore size of the as-prepared catalysts is controlled, generating materials for the systematic investigation of confinement effects during OER. At a threshold pore size, overpotential increased significantly. Electrochemical impedance spectroscopy (EIS) indicated a change in OER mechanism, involving the oxygen release step. It is assumed that in smaller pores the critical radius for gas bubble formation is not met and therefore a smaller charge-transfer resistance is observed for medium frequencies.

Ru/Rh Cation Doping and Oxygen-Vacancy Engineering of FeOOH Nanoarrays@Ti3 C2 Tx MXene Heterojunction for Highly Efficient and Stable Electrocatalytic Oxygen Evolution

Oxyhydroxides hold promise as highly-efficient non-noble electrocatalysts for the oxygen evolution reaction (OER), but their poor conductivity and structural instability greatly impede their progress. Herein, the authors develop a cation-doping and oxygenvacancy engineering strategy to fabricate Ru/Rh-doped FeOOH nanoarrays with abundant oxygen-vacancies in situ grown on Ti₃ C₂ T_x MXene (Ru/Rh-FeOOH@Ti₃ C₂ T_x ) as highly-efficient OER electrocatalysts. Benefiting from Ru/Rh-cation regulation, oxygenvacancy engineering, and heterojunction synergy between MXene and modulated FeOOH, the optimized Rh/Ru-FeOOH@Ti₃ C₂ T_x electrocatalysts exhibit excellent OER activities and remarkable stabilities with 100 h. Particularly, 3%Rh-FeOOH@Ti₃ C₂ T_x electrocatalyst only needs a 223 mV overpotential at 10 mA cm^-2 and 306 mV to reach 100 mA cm^-2 , which is superior to commercial IrO₂ catalyst and most reported oxyhydroxide-based electrocatalysts. Further, systematically theoretical caculation, kinetics, thermodynamics, and microstructural analysis verify that the integration of Ru/Rh-cation doping and oxygen vacancy obviously enhances the intrinsic conductivity and lattice defects of FeOOH and expose more active sites, thereby decreasing the adsorption/desorption energy barrier and activation energy, and improving the specific activity and catalytic kinetics of electrocatalysts, whereas in situ hybridization with MXene strengthens the structural stability. This work clearly confirms that cationdoping and oxygen-vacancy engineering offers a joint strategy for the electronic structure modulation and design of highly-efficient inexpensive OER electrocatalysts.

Properties of Micro-Arc Oxidation Coatings on 5052 Al Alloy Sealed by SiO2 Nanoparticles

Micro-arc oxidation (MAO) treatment can effectively improve the wear resistance, corrosion resistance, and mechanical strength of aluminum alloy substrates. Improving the porous structure of MAO film and effectively sealing the pores is a significant research issue. In this study, the MAO treatment of 5052 aluminum alloy was carried out in silicate electrolytes. The MAO films were sealed with different concentrations of SiO2 nanoparticles. The effects of SiO2 nanoparticle content on the MAO films’ microstructure, mechanical properties, and corrosion performance were systematically investigated. When adding SiO2 nanoparticles to electrolytes, the particles were deposited at the micropores of the film, which could effectively seal the porous MAO film and significantly improve its corrosion and wear resistance. The corrosion resistance and wear resistance properties were optimal with 5.0 g/L SiO2 addition. Compared to the unsealed film, the corrosion current density and corrosion rate decreased from 1.24 × 10−9 A/cm2 and 1.47 × 10−5 mm/a to 7.78 × 10−10 A/cm2 and 9.15 × 10−6 mm/a, respectively. Moreover, the average friction coefficient of the sealed film was 0.606, which was ~19.3% lower than that of the substrate and 3.3% lower than for the unsealed film.

Effect of cobalt doping on photocatalytic water splitting activity of NiTi-layered double hydroxide

Cobalt-doping a NiTi layered double hydroxide affords CoNiTi-LDH with smaller plate sizes and a higher degree of order, which allows the band gap to shrink from 2.7 eV to 2.4 eV. CoNiTi-LDH is active for both water oxidation and reduction.

Recent Progress in Advanced Electrocatalyst Design for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers hold great significance for renewable energy storage and conversion. The acidic oxygen evolution reaction (OER) is one of the main roadblocks that hinder the practical application of PEM water electrolyzers. Highly active, cost-effective, and durable electrocatalysts are indispensable for lowering the high kinetic barrier of OER to achieve boosted reaction kinetics. To date, a wide spectrum of advanced electrocatalysts has been designed and synthesized for enhanced acidic OER performance, though Ir and Ru based nanostructures still represent the state-of-the-art catalysts. In this Progress Report, recent research progress in advanced electrocatalysts for improved acidic OER performance is summarized. First, fundamental understanding about acidic OER including reaction mechanisms and atomic understanding to acidic OER for rational design of efficient electrocatalysts are discussed. Thereafter, an overview of the progress in the design and synthesis of advanced acidic OER electrocatalysts is provided in terms of catalyst category, i.e., metallic nanostructures (Ir and Ru based), precious metal oxides, nonprecious metal oxides, and carbon based nanomaterials. Finally, perspectives to the future development of acidic OER are provided from the aspects of reaction mechanism investigation and more efficient electrocatalyst design.

Modulating the water oxidation catalytic activity of iridium complexes by functionalizing the Cp*-ancillary ligand: hints on the nature of the active species

A comparative study on the behavior of a series of iridium dimeric WOCs with modified Cp* ligands reveals the key role played by the variable substituent.