Boosting Oxygen Evolution Reaction on Metallocene-based Transition Metal Sulfides Integrated with N-doped Carbon Nanostructures

Bimetallic Ni-Co-MOF Nanostructures for Seawater Electrolysis: Unveiling the Mechanism of the Oxygen Evolution Reaction Using Impedance Spectroscopy

Designing anode electrodes with long-term stability and efficiency for seawater electrolysis is crucial for addressing key challenges in sustainable hydrogen production and clean energy systems. Here, we developed self-supporting bimetallic Ni-Co-MOF electrodes, demonstrating exceptional performance and durability in alkaline seawater electrolysis due to their high voltammetric charge density and increased electrochemically accessible active sites. The reaction kinetics of the water oxidation reaction in the presence of Cl- ions (at concentrations ranging from 0.5 M to 3.5 M) were investigated through electrochemical impedance spectroscopy (EIS) analysis, focusing on the kinetic parameters, suggesting that the rate-determining step (RDS) is the chemical process following the initial electron transfer. Notably, Cl- ions in the electrolyte medium do not alter the OER rate-limiting step, as indicated by negligible variations in the anodic transfer coefficient values. However, blocking active sites is evident from the decrease in interfacial chemical capacitance (Cchem) values with increasing Cl- concentration. These findings offer a deeper understanding of OER reaction kinetics in chloride-containing environments by correlating electrochemical kinetic parameters with active site availability. This work highlights critical considerations for designing efficient and durable anodes for seawater electrolysis.

The Renaissance of Ferrocene-Based Electrocatalysts: Properties, Synthesis Strategies, and Applications

The fascinating electrochemical properties of the redox-active compound ferrocene have inspired researchers across the globe to develop ferrocene-based electrocatalysts for a wide variety of applications. Advantages including excellent chemical and thermal stability, solubility in organic solvents, a pair of stable redox states, rapid electron transfer, and nontoxic nature improve its utility in various electrochemical applications. The use of ferrocene-based electrocatalysts enables control over the intrinsic properties and electroactive sites at the surface of the electrode to achieve specific electrochemical activities. Ferrocene and its derivatives can function as a potential redox medium that promotes electron transfer rates, thereby enhancing the reaction kinetics and electrochemical responses of the device. The outstanding electrocatalytic activity of ferrocene-based compounds at lower operating potentials enhances the specificity and sensitivity of reactions and also amplifies the response signals. Owing to their versatile redox chemistry and catalytic activities, ferrocene-based electrocatalysts are widely employed in various energy-related systems, molecular machines, and agricultural, biological, medicinal, and sensing applications. This review highlights the importance of ferrocene-based electrocatalysts, with emphasis on their properties, synthesis strategies for obtaining different ferrocene-based compounds, and their electrochemical applications.

Organic-Inorganic Hybrid Crystal-Assisted Etching of Nickel Foam for the Collectively Exhaustive Electrochemical Performance of Oxygen Evolution Reaction

In this work, an organic-inorganic hybrid crystal, violet-crystal (VC), was used to etch the nickel foam (NF) to fabricate a self-standing electrode for the water oxidation reaction. The efficacy of VC-assisted etching manifests the promising electrochemical performance towards the oxygen evolution reaction (OER), requiring only ~356 and ~376 mV overpotentials to reach 50 and 100 mA cm-2 , respectively. The OER activity improvement is attributed to the collectively exhaustive effects arising from the incorporation of various elements in the NF, and the enhancement of active site density. Furthermore, the self-standing electrode is robust, exhibiting a stable OER activity after 4,000 cyclic voltammetry cycles, and ~50 h. The anodic transfer coefficients (αa ) show that the first electron transfer step is the rate-determining step on the surface of NF-VCs-1.0 (NF etched by 1 g of VCs) electrode, while the chemical step involving dissociation following the first electron transfer step is identified as the rate-limiting step in other electrodes. The lowest Tafel slope value observed in the NF-VCs-1.0 electrode indicates the high surface coverage of oxygen intermediates and more favorable OER reaction kinetics, as confirmed by high interfacial chemical capacitance and low charge transport/interfacial resistance. This work demonstrates the importance of VCs-assisted etching of NF to activate the OER, and the ability to predict reaction kinetics and rate-limiting step based on αa values, which will open new avenues to identify advanced electrocatalysts for the water oxidation reaction.

Approach to Evaluation of Electrocatalytic Water Splitting Parameters, Reflecting Intrinsic Activity: Toward the Right Pathway

The development of transition metal-based non-precious-metal electrocatalysts for energy storage and conversion systems has received a lot of interest recently. To further this subject in the proper way given the development of electrocatalysts, a fair comparison of their respective performance is necessary. This Review investigates the parameters used for the comparison of electrocatalyst activity. Significant evaluation criteria employed in electrochemical water splitting studies are the overpotential at defined current density usually at 10 mA per geometric surface area, Tafel slope, exchange current density, mass activity, specific activity and turnover frequency (TOF). This Review will discuss how to identify the specific activity and TOF by electrochemical and non-electrochemical methods to represent intrinsic activity as well as the benefits and uncertainties of each technique, ensuring that each method is applied correctly when calculating intrinsic activity metrics.

NiS gradient distribution on arrayed porous carbonized grapefruit peel for water splitting

Metal-based catalysts on biomass carbon substrates can combine their respective advantages of composition and structure to improve the catalytic performance. Herein, NiS supported on grapefruit peel derived array porous carbon (APC) was obtained via a carbonization process without emission of toxic gases. The natural S source from grapefruit peel reacted with nickel salt solution. The gradient distribution of NiS and S on the APC substrates can be altered by the concentration of impregnating salt solution. Theoretical calculations showed that the S gradient distribution on APC could tune the electronic structure and optimize the adsorption energies of the intermediates. NiS was firmly anchored on the porous carbon framework, resulting in enhanced high intrinsic activity, exposure of more active sites, and accelerated mass transfer. The active mass density was proposed to build a relationship between active metal content and electrolyte diffusion capacity for the evaluation of catalytic properties.

MOF-Derived Cobalt@Mesoporous Carbon as Electrocatalysts for Oxygen Evolution Reaction: Impact of Organic Linker

Water electrolysis has attracted scientists' attention as a green route for energy generation. However, the sluggish kinetics of oxygen evolution reaction (OER) remarkably increases the reaction overpotential. In this work, we developed Co-based nanomaterials as cost-effective, highly efficient catalysts for OER. In this regard, different Co-based metal-organic frameworks (MOFs) were synthesized using different organic linkers. After annealing under inert atmosphere, the corresponding Co-embedded mesoporous carbon (Co/MC) materials were produced. Among them, Co/MC synthesized using 2-methyl imidazole (Co/NMC-2MeIM) expressed the highest surface area (412 m2/g) compared to its counterparts. Furthermore, it expressed a higher degree of defects as depicted by Raman spectra. Co/NMC-2MeIM exhibited the best catalytic performance toward OER in alkaline medium. It afforded an overpotential of 292 mV at a current density of 10 mA cm-2 and a Tafel slope of 99.2 mV dec-1. The superior electrocatalytic performance of Co/NMC-2MeIM is attributed to its high content of Co3+ on the surface, high surface area, and enhanced electrical conductivity induced by nitrogen doping. Furthermore, its high content of pyridinic-N and high degree of defects remarkably enhance the charge transfer between the adsorbed oxygen species and the active sites. These results may pave the avenue toward further investigation of metal/carbon materials in a wide range of electrocatalytic applications.

Synergistic catalysis of graphitic carbon nitride supported bimetallic sulfide nanostructures for efficient oxygen generation

Herein, a series of g-C₃N₄ supported bimetallic sulfide nanostructures (Ni₃S₂/MoS₂/ng-C₃N₄, n = 10, 20 and 30) was prepared by a hydrothermal method and subsequently a thermal annealing approach. Ni₃S₂/MoS₂/20g-C₃N₄ with controlled composition exhibits efficient OER activity with a low overpotential of 183 mV at 10 mA cm^-2, which outperforms the vast majority of sulfide OER electrocatalysts reported previously.

The Scalable Solid-State Synthesis of a Ni5P4/Ni2P–FeNi Alloy Encapsulated into a Hierarchical Porous Carbon Framework for Efficient Oxygen Evolution Reactions

The exploration of high-performance and low-cost electrocatalysts towards the oxygen evolution reaction (OER) is essential for large-scale water/seawater splitting. Herein, we develop a strategy involving the in situ generation of a template and pore-former to encapsulate a Ni5P4/Ni2P heterojunction and dispersive FeNi alloy hybrid particles into a three-dimensional hierarchical porous graphitic carbon framework (labeled as Ni5P4/Ni2P–FeNi@C) via a room-temperature solid-state grinding and sodium-carbonate-assisted pyrolysis method. The synergistic effect of the components and the architecture provides a large surface area with a sufficient number of active sites and a hierarchical porous pathway for efficient electron transfer and mass diffusion. Furthermore, a graphitic carbon coating layer restrains the corrosion of alloy particles to boost the long-term durability of the catalyst. Consequently, the Ni5P4/Ni2P–FeNi@C catalyst exhibits extraordinary OER activity with a low overpotential of 242 mV (10 mA cm−2), outperforming the commercial RuO2 catalyst in 1 M KOH. Meanwhile, a scale-up of the Ni5P4/Ni2P–FeNi@C catalyst created by a ball-milling method displays a similar level of activity to the above grinding method. In 1 M KOH + seawater electrolyte, Ni5P4/Ni2P–FeNi@C also displays excellent stability; it can continuously operate for 160 h with a negligible potential increase of 2 mV. This work may provide a new avenue for facile mass production of an efficient electrocatalyst for water/seawater splitting and diverse other applications.

Few-Layered WS2 Anchored on Co, N-Doped Carbon Hollow Polyhedron for Oxygen Evolution and Hydrogen Evolution

Tungsten disulfide (WS₂) is well known to have great potential as an electrocatalyst, but the practical application is hampered by its intrinsic inert plane and semiconductor properties. In this work, owing to a Co-based zeolite imidazole framework (ZIF-67) that effectively inhibited WS₂ growth, few-layered WS₂ was confined to the surface of Co, N-doped carbon polyhedron (WS₂@Co₉S₈), with more marginal active sites and higher conductivity, which promoted efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). For the first time, WS₂@Co₉S₈ was prepared by mixing in one pot of a liquid phase and calcination, and WS₂ realized uniform distribution on the polyhedron surface by electrostatic adsorption in the liquid phase. The obtained hybrid catalyst exhibited excellent OER and HER catalytic activity, and the OER potential was only 15 mV at 10 mA cm^-2 higher than that of noble metal oxide (RuO₂). The improvement of catalytic activity can be attributed to the enhanced exposure of sulfur edge sites by WS₂, the unique synergistic effect between WS₂ and Co₉S₈ on the metal-organic framework (MOF) surface, and the effective shortening of the diffusion path by the hollow multi-channel structure. Therefore, the robust catalyst (WS₂@Co₉S₈) prepared by a simple and efficient synthesis method in this work will serve as a highly promising bifunctional catalyst for OER and HER.