Increasing the Oxygen-Evolution Reaction Performance of Nanotubular Titanium Oxynitride-Supported Ir Nanoparticles by a Strong Metal–Support Interaction

Activity-Stability Relationships in Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a critical process in various sustainable energy technologies. Despite substantial progress in catalyst development, the practical application of OER catalysts remains hindered by the ongoing challenge of balancing high catalytic activity with long-term stability. We explore the inverse trends often observed between activity and stability, drawing on key insights from both experimental and theoretical studies. Special focus is placed on the performance of different electrodes and their interaction with acidic and alkaline media across a range of electrochemical conditions. This Perspective integrates recent advancements to present a thorough framework for understanding the mechanisms underlying the activity-stability relationship, offering strategies for the rational design of next-generation OER catalysts that successfully meet the dual demands of activity and durability.

Strong Metal-Support Interactions in Heterogeneous Oxygen Electrocatalysis

Molecular oxygen redox electrocatalysis involves oxygen reduction and evolution as core reactions in various energy conversion and environmental technology fields. Strong metal-support interactions (SMSIs) based nanomaterials are regarded as desirable and state-of-the-art heterogeneous electrocatalysts due to their exceptional physicochemical properties. Over the past decades, considerable advancements in theory and experiment have been achieved in related studies, especially in modulating the electronic structure and geometrical configuration of SMSIs to enable activity, selectivity, and stability. In this focuses on the concept of SMSI, explore their various manifestations and mechanisms of action, and summarizes recent advances in SMSIs for efficient energy conversion in oxygen redox electrocatalysis applications. Additionally, the correlation between the physicochemical properties of different metals and supports is systematically elucidated, and the potential mechanisms of the structure-activity relationships between SMSIs and catalytic performance are outlined through theoretical models. Finally, the obstacles confronting this burgeoning field are comprehensively concluded, targeted recommendations and coping strategies are proposed, and future research perspectives are outlined.

Tuning Adsorbate-Mediated Strong Metal-Support Interaction by Oxygen Vacancy: A Case Study in Ru/TiO2

The adsorbate-mediated strong metal-support interaction (A-SMSI) offers a reversible means of altering the selectivity of supported metal catalysts, thereby providing a powerful tool for facile modulation of catalytic performance. However, the fundamental understanding of A-SMSI remains inadequate and methods for tuning A-SMSI are still in their nascent stages, impeding its stabilization under reaction conditions. Here, we report that the initial concentration of oxygen vacancy in oxide supports plays a key role in tuning the A-SMSI between Ru nanoparticles and defected titania (TiO2-x). Based on this new understanding, we demonstrate the in situ formation of A-SMSI under reaction conditions, obviating the typically required CO2-rich pretreatment. The as-formed A-SMSI layer exhibits remarkable stability at various temperatures, enabling excellent activity, selectivity and long-term stability in catalyzing the reverse water gas-shift reaction. This study deepens the understanding of the A-SMSI and the ability to stabilize A-SMSI under reaction conditions represents a key step for practical catalytic applications.

Enrooted-Type Metal-Support Interaction Boosting Oxygen Evolution Reaction in Acidic Media

Supported metal catalysts with appropriate metal-support interactions (MSIs) hold a great promise for heterogeneous catalysis. However, ensuring tight immobilization of metal clusters/nanoparticles on the support while maximizing the exposure of surface active sites remains a huge challenge. Herein, we report an Ir/WO3 catalyst with a new enrooted-type MSI in which Ir clusters are, unprecedentedly, atomically enrooted into the WO3 lattice. The enrooted Ir atoms decrease the electron density of the constructed interface compared to the adhered (root-free) type, thereby achieving appropriate adsorption toward oxygen intermediates, ultimately leading to high activity and stability for oxygen evolution in acidic media. Importantly, this work provides a new enrooted-type supported metal catalyst, which endows suitable MSI and maximizes the exposure of surface active sites in contrast to the conventional adhered, embedded, and encapsulated types.

Recent progress in the development of advanced support materials for electrocatalysis

Electrocatalytic materials are pivotal for clean chemical production and energy conversion in devices like electrolyzers and fuel cells. These materials usually consist of metallic nanoparticles which serve as active reaction sites, and support materials which provide high surface area, conductivity and stability. When designing novel electrocatalytic composites, the focus is often on the metallic sites, however, the significance of the support should not be overlooked. Carbon materials, valued for their conductivity and large surface area, are commonly used as support in benchmark electrocatalysts. However, using alternative support materials instead of carbon can be beneficial in certain cases. In this minireview, we summarize recent advancements and key directions in developing novel supports for electrocatalysis, encompassing both carbon and non-carbon materials.

Efficient and sustainable water electrolysis achieved by excess electron reservoir enabling charge replenishment to catalysts

Suppressing the oxidation of active-Ir(III) in IrOx catalysts is highly desirable to realize an efficient and durable oxygen evolution reaction in water electrolysis. Although charge replenishment from supports can be effective in preventing the oxidation of IrOx catalysts, most supports have inherently limited charge transfer capability. Here, we demonstrate that an excess electron reservoir, which is a charged oxygen species, incorporated in antimony-doped tin oxide supports can effectively control the Ir oxidation states by boosting the charge donations to IrOx catalysts. Both computational and experimental analyses reveal that the promoted charge transfer driven by excess electron reservoir is the key parameter for stabilizing the active-Ir(III) in IrOx catalysts. When used in a polymer electrolyte membrane water electrolyzer, Ir catalyst on excess electron reservoir incorporated support exhibited 75 times higher mass activity than commercial nanoparticle-based catalysts and outstanding long-term stability for 250 h with a marginal degradation under a water-splitting current of 1 A cm-2. Moreover, Ir-specific power (74.8 kW g-1) indicates its remarkable potential for realizing gigawatt-scale H2 production for the first time.

"Nano Lab" Advanced Characterization Platform for Studying Electrocatalytic Iridium Nanoparticles Dispersed on TiOxNy Supports Prepared on Ti Transmission Electron Microscopy Grids

Aiming at speeding up the discovery and understanding of promising electrocatalysts, a novel experimental platform, i.e., the Nano Lab, is introduced. It is based on state-of-the-art physicochemical characterization and atomic-scale tracking of individual synthesis steps as well as subsequent electrochemical treatments targeting nanostructured composites. This is provided by having the entire experimental setup on a transmission electron microscopy (TEM) grid. Herein, the oxygen evolution reaction nanocomposite electrocatalyst, i.e., iridium nanoparticles dispersed on a high-surface-area TiO_xN_y support prepared on the Ti TEM grid, is investigated. By combining electrochemical concepts such as anodic oxidation of TEM grids, floating electrode-based electrochemical characterization, and identical location TEM analysis, relevant information from the entire composite's cycle, i.e., from the initial synthesis step to electrochemical operation, can be studied. We reveal that Ir nanoparticles as well as the TiO_xN_y support undergo dynamic changes during all steps. The most interesting findings made possible by the Nano Lab concept are the formation of Ir single atoms and only a small decrease in the N/O ratio of the TiO_xN_y-Ir catalyst during the electrochemical treatment. In this way, we show that the precise influence of the nanoscale structure, composition, morphology, and electrocatalyst's locally resolved surface sites can be deciphered on the atomic level. Furthermore, the Nano Lab's experimental setup is compatible with ex situ characterization and other analytical methods, such as Raman spectroscopy, X-ray photoelectron spectroscopy, and identical location scanning electron microscopy, hence providing a comprehensive understanding of structural changes and their effects. Overall, an experimental toolbox for the systematic development of supported electrocatalysts is now at hand.

Recent Progress in Metal Phosphorous Chalcogenides: Potential High-Performance Electrocatalysts

Since the discovery of graphene, research on the family of 2D materials has been a thriving field. Metal phosphorous chalcogenides (MPX₃ ) have attracted renewed attention due to their distinctive physical and chemical properties. The advantages of MPX₃ , such as tunable layered structures, unique electronic properties, thermodynamically appropriate band alignments and abundant catalytic active sites on the surface, make MPX₃ material great potential in electrocatalysis. In this review, the applications of MPX₃ electrocatalysts in recent years, including hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction, are summarized. Structural regulation, chemical doping and multi-material composite that are often effective and practical research methods to further optimize the catalytic properties of these materials, are introduced. Finally, the challenges and opportunities for electrocatalytic applications of MPX₃ materials are discussed. This report aims to advance future efforts to develop MPX₃ and related materials for electrocatalysis.

Activity-Stability Trends of the Sb-SnO2@RuOx Heterostructure toward Acidic Water Oxidation

Accomplishments of enhanced activity and durability are a major concern in the design of catalysts for acidic water oxidation. To date, most studied supported metal catalysts undergo fast degradation in strongly acidic and oxidative environments due to improper controlling of the interface stability caused by their lattice mismatches. Here, we evaluate the activity-stability trends of in situ crystallized antimony-doped tin oxide (Sb-SnO₂)@RuO_x (Sb-SnO₂@RuO_x) heterostructure nanosheets (NSs) for acidic water oxidation. The catalyst prepared by atomic layer deposition of a conformal Ru film on antimony-doped tin sulfide (Sb-SnS₂) NSs followed by heat treatment highlights comparable activity but longer stability than that of the ex situ catalyst (where Ru was deposited on Sb-SnO₂ followed by heating). Air calcination for in situ crystallization allows the formation of hierarchical mesoporous Sb-SnO₂ NSs from as-prepared Sb-SnS₂ NSs and parallel in situ transformation from Ru to RuO_x, resulting in a compact heterostructure. The significance of this approach significantly resists corrosive dissolution, which is justified by the enhanced oxygen evolution reaction (OER) stability of the catalyst compared to most of the state-of-the-art ruthenium-based catalysts including Carbon@RuO_x (which shows ∼10 times higher dissolution) as well as Sb-SnO₂@Com. RuO_x and Com. RuO₂. This study demonstrates the controlled interface stability of heterostructure catalysts toward enhancing OER activity and stability.

Nanotubular TiO x N y -Supported Ir Single Atoms and Clusters as Thin-Film Electrocatalysts for Oxygen Evolution in Acid Media

A versatile approach to the production of cluster- and single atom-based thin-film electrode composites is presented. The developed TiO _x N _y -Ir catalyst was prepared from sputtered Ti-Ir alloy constituted of 0.8 ± 0.2 at % Ir in α-Ti solid solution. The Ti-Ir solid solution on the Ti metal foil substrate was anodically oxidized to form amorphous TiO₂-Ir and later subjected to heat treatment in air and in ammonia to prepare the final catalyst. Detailed morphological, structural, compositional, and electrochemical characterization revealed a nanoporous film with Ir single atoms and clusters that are present throughout the entire film thickness and concentrated at the Ti/TiO _x N _y -Ir interface as a result of the anodic oxidation mechanism. The developed TiO _x N _y -Ir catalyst exhibits very high oxygen evolution reaction activity in 0.1 M HClO₄, reaching 1460 A g^-1 _Ir at 1.6 V vs reference hydrogen electrode. The new preparation concept of single atom- and cluster-based thin-film catalysts has wide potential applications in electrocatalysis and beyond. In the present paper, a detailed description of the new and unique method and a high-performance thin film catalyst are provided along with directions for the future development of high-performance cluster and single-atom catalysts prepared from solid solutions.

In Situ Electroplating of Ir@Carbon Cloth as High-Performance Selective Oxygen Evolution Reaction Catalyst for Direct Electrolytic Recovery of Lead

The hydrometallurgical technology provides an efficient and sustainable green lead recovery process from lead acid batteries. Methanesulfonic acid has been widely considered as a green solvent for lead electrolytic recovery. However, the competitive precipitation of PbO2 at anode and higher overpotential for OER limit the lead recovery efficiency. In this work, an anodic oxygen evolution reaction (OER) catalyst with a low Ir mass fraction of 7.2% is obtained by electroplating iridium on carbon cloth (CC), exhibiting a lower overpotential of 256 mV, longer lifetime of 10 h, and better stability in the 0.5 M MSA solution. When CC-Ir is used as an anodic catalyst for lead recovery in the lead methanesulfonate electrolyte, only a lesser Pb precipitation product with Pb atom mass fraction of 1.42% is found after electrolysis of 10 h, demonstrating the suppression effect of CC-Ir for a PbO2 side reaction. This work proves that the anodic catalyst plays an important role in the lead electrolytic recovery process, which can inhibit the side reaction, reduce the energy consumption, and increase recovery efficiency.

Construction of single tungsten/copper atom oxide supported on the surface of TiO2 for the higher activity of electrocatalytic water splitting and photodegradation of organic pollutant

Intense studies are being carried out on single atom catalysts that exhibit remarkable activity and selectivity. To enhance their catalytic abilities, one must have a thorough understanding of the properties of the single metal atom active site and its dynamics in the working state. Herein, we report single metal atom oxide (SMAO) (metal: W/Cu) anchored on TiO₂-rGO nanomaterials (SMAO-ED-TiO₂-rGO) by simple sonication process. It is efficient for the electrocatalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and photocatalytic degradation of pharmaceutical pollutant. The uniform dispersion of the tungsten/copper metal atom oxide over a TiO₂-rGO materials is detected by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The SMAO-ED-TiO₂-rGO nanocatalyst has shown impressive HER/OER activity with an overpotential of 121/295 mV at (-)10 mA cm^-2 current density and the low Tafel slope of 96/60.3 mV dec^-1 in 1 M KOH solution. Further, SMAO-ED-TiO₂-rGO nanocatalyst was used for the photocatalytic degradation of ciprofloxacin (CF). After 60 min of UV light irradiation, the SMAO-ED-TiO₂-rGO nanocatalyst efficiently photodegraded 98.5% of CF while retaining its activity for five cycles. Superoxide radicals (O₂^•-) are found to be the main reactive species required in the photodegradation of CF, according to scavenger study of the SMAO-ED-TiO₂-rGO nanocatalyst for CF degradation.

Iridium Stabilizes Ceramic Titanium Oxynitride Support for Oxygen Evolution Reaction

Decreasing iridium loading in the electrocatalyst presents a crucial challenge in the implementation of proton exchange membrane (PEM) electrolyzers. In this respect, fine dispersion of Ir on electrically conductive ceramic supports is a promising strategy. However, the supporting material needs to meet the demanding requirements such as structural stability and electrical conductivity under harsh oxygen evolution reaction (OER) conditions. Herein, nanotubular titanium oxynitride (TiON) is studied as a support for iridium nanoparticles. Atomically resolved structural and compositional transformations of TiON during OER were followed using a task-specific advanced characterization platform. This combined the electrochemical treatment under floating electrode configuration and identical location transmission electron microscopy (IL-TEM) analysis of an in-house-prepared Ir-TiON TEM grid. Exhaustive characterization, supported by density functional theory (DFT) calculations, demonstrates and confirms that both the Ir nanoparticles and single atoms induce a stabilizing effect on the ceramic support via marked suppression of the oxidation tendency of TiON under OER conditions.

Oxygen Evolution Electrocatalysts for the Proton Exchange Membrane Electrolyzer: Challenges on Stability

Hydrogen generated by proton exchange membrane (PEM) electrolyzer holds a promising potential to complement the traditional energy structure and achieve the global target of carbon neutrality for its efficient, clean, and sustainable nature. The acidic oxygen evolution reaction (OER), owing to its sluggish kinetic process, remains a bottleneck that dominates the efficiency of overall water splitting. Over the past few decades, tremendous efforts have been devoted to exploring OER activity, whereas most show unsatisfying stability to meet the demand for industrial application of PEM electrolyzer. In this review, systematic considerations of the origin and strategies based on OER stability challenges are focused on. Intrinsic deactivation of the material and the extrinsic balance of plant-induced destabilization are summarized. Accordingly, rational strategies for catalyst design including doping and leaching, support effect, coordination effect, strain engineering, phase and facet engineering are discussed for their contribution to the promoted OER stability. Moreover, advanced in situ/operando characterization techniques are put forward to shed light on the OER pathways as well as the structural evolution of the OER catalyst, giving insight into the deactivation mechanisms. Finally, outlooks toward future efforts on the development of long-term and practical electrocatalysts for the PEM electrolyzer are provided.

Spark Ablation for the Fabrication of PEM Water Electrolysis Catalyst-Coated Membranes

Proton-exchange-membrane (PEM) electrolyzers represent a promising technology for sustainable hydrogen production, owing to their efficiency and load flexibility. However, the acidic nature of PEM demands the use of platinum-group metal-electrocatalysts. Apart from the associated high capital costs, the scarcity of Ir hinders the large-scale implementation of the technology. Since low-cost replacements for Ir are not available at present, there is an urgent need to engineer catalyst-coated membranes (CCMs) with homogeneous catalyst layers at low Ir loadings. Efforts to realize this mainly rely on the development of advanced Ir nanostructures with maximized dispersion via wet chemistry routes. This study demonstrates the potential of an alternative vapor-based process, based on spark ablation and impaction, to fabricate efficient and durable Ir- and Pt-coated membranes. Our results indicate that spark-ablation CCMs can reduce the Ir demand by up to five times compared to commercial CCMs, without a compromise in activity. The durability of spark-ablation CCMs has been investigated by applying constant and dynamic load profiles for 150 h, indicating different degradation mechanisms for each case without major pitfalls. At constant load, an initial degradation in performance was observed during the first 30 h, but a stable degradation rate of 0.05 mV h−1 was sustained during the rest of the test. The present results, together with manufacturing aspects related to simplicity, costs and environmental footprint, suggest the high potential of spark ablation having practical applications in CCM manufacturing.

Supported Iridium-based Oxygen Evolution Reaction Electrocatalysts - Recent Developments

The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.

Improving the HER Activity and Stability of Pt Nanoparticles by Titanium Oxynitride Support

Water electrolysis powered by renewables is regarded as the feasible route for the production of hydrogen, obtained at the cathode side through electrochemical hydrogen evolution reaction (HER). Herein, we present a rational strategy to improve the overall HER catalytic performance of Pt, which is known as the best monometallic catalyst for this reaction, by supporting it on a conductive titanium oxynitride (TiON_x) dispersed over reduced graphene oxide nanoribbons. Characterization of the Pt/TiON_x composite revealed the presence of small Pt particles with diameters between 2 and 3 nm, which are well dispersed over the TiON_x support. The Pt/TiON_x nanocomposite exhibited improved HER activity and stability with respect to the Pt/C benchmark in an acid electrolyte, which was ascribed to the strong metal–support interaction (SMSI) triggered between the TiON_x support and grafted Pt nanoparticles. SMSI between TiON_x and Pt was evidenced by X-ray photoelectron spectroscopy (XPS) through a shift of the binding energies of the characteristic Pt 4f photoelectron lines with respect to Pt/C. Density functional theory (DFT) calculations confirmed the strong interaction between Pt nanoparticles and the TiON_x support. This strong interaction improves the stability of Pt nanoparticles and weakens the binding of chemisorbed H atoms thereon. Both of these effects may result in enhanced HER activity.

Microstructure and Electrical Conductivity of Electrospun Titanium Oxynitride Carbon Composite Nanofibers

Titanium oxynitride carbon composite nanofibers (TiON/C-CNFs) were synthesised with electrospinning and subsequent heat treatment in ammonia gas. In situ four-probe electrical conductivity measurements of individual TiON/C-CNFs were performed. Additionally, the TiON/C-CNFs were thoroughly analysed with various techniques, such as X-ray and electron diffractions, electron microscopies and spectroscopies, thermogravimetric analysis and chemical analysis to determine the crystal structure, morphology, chemical composition, and N/O at. ratio. It was found that nanofibers were composed of 2–5 nm sized titanium oxynitride (TiON) nanoparticles embedded in an amorphous carbon matrix with a small degree of porosity. The average electrical conductivity of a single TiON/C-CNF was 1.2 kS/m and the bulk electrical conductivity of the TiON/C-CNF fabric was 0.053 kS/m. From the available data, the mesh density of the TiON/C-CNF fabric was estimated to have a characteristic length of 1.0 µm and electrical conductivity of a single TiON/C-CNF was estimated to be from 0.45 kS/m to 19 kS/m. The electrical conductivity of the measured TiON/C-CNFs is better than that of amorphous carbon nanofibers and has ohmic behaviour, which indicates that it can effectively serve as a new type of support material for electrocatalysts, batteries, sensors or supercapacitors.

Noble Metal-Based Multimetallic Nanoparticles for Electrocatalytic Applications

Noble metal-based multimetallic nanoparticles (NMMNs) have attracted great attention for their multifunctional and synergistic effects, which offer numerous catalytic applications. Combined experimental and theoretical studies have enabled formulation of various design principles for tuning the electrocatalytic performance through controlling size, composition, morphology, and crystal structure of the nanoparticles. Despite significant advancements in the field, the chemical synthesis of NMMNs with ideal characteristics for catalysis, including high activity, stability, product-selectivity, and scalability is still challenging. This review provides an overview on structure-based classification and the general synthesis of NMMN electrocatalysts. Furthermore, postsynthetic treatments, such as the removal of surfactants to optimize the activity, and utilization of NMMNs onto suitable support for practical electrocatalytic applications are highlighted. In the end, future direction and challenges associated with the electrocatalysis of NMMNs are covered.

Structural Transformation of Heterogeneous Materials for Electrocatalytic Oxygen Evolution Reaction

Electrochemical water splitting for hydrogen generation is a promising pathway for renewable energy conversion and storage. One of the most important issues for efficient water splitting is to develop cost-effective and highly efficient electrocatalysts to drive sluggish oxygen-evolution reaction (OER) at the anode side. Notably, structural transformation such as surface oxidation of metals or metal nonoxide compounds and surface amorphization of some metal oxides during OER have attracted growing attention in recent years. The investigation of structural transformation in OER will contribute to the in-depth understanding of accurate catalytic mechanisms and will finally benefit the rational design of catalytic materials with high activity. In this Review, we provide an overview of heterogeneous materials with obvious structural transformation during OER electrocatalysis. To gain insight into the essence of structural transformation, we summarize the driving forces and critical factors that affect the transformation process. In addition, advanced techniques that are used to probe chemical states and atomic structures of transformed surfaces are also introduced. We then discuss the structure of active species and the relationship between catalytic performance and structural properties of transformed materials. Finally, the challenges and prospects of heterogeneous OER electrocatalysis are presented.

Sacrificial Cu Layer Mediated the Formation of an Active and Stable Supported Iridium Oxygen Evolution Reaction Electrocatalyst

The production of hydrogen via a proton-exchange membrane water electrolyzer (PEM-WE) is directly dependent on the rational design of electrocatalysts for the anodic oxygen evolution reaction (OER), which is the bottleneck of the process. Here, we present a smart design strategy for enhancing Ir utilization and stabilization. We showcase it on a catalyst, where Ir nanoparticles are efficiently anchored on a conductive support titanium oxynitride (TiON_x) dispersed over carbon-based Ketjen Black and covered by a thin layer of copper (Ir/CuTiON_x/C), which gets removed in the preconditioning step. Electrochemical OER activity, stability, and structural changes were compared to the Ir-based catalyst, where Ir nanoparticles without Cu are deposited on the same support (Ir/TiON_x/C). To study the effect of the sacrificial less-noble metal layer on the catalytic performance of the synthesized material, characterization methods, namely X-ray powder diffraction, X-ray photoemission spectroscopy, and identical location transmission electron microscopy were employed and complemented with scanning flow cell coupled to an inductively coupled plasma mass spectrometer, which allowed studying the online dissolution during the catalytic reaction. Utilization of these advanced methods revealed that the sacrificial Cu layer positively affects both Ir OER mass activity and its durability, which was assessed via S-number, a recently reported stability metric. Improved activity of Cu analogue was ascribed to the higher surface area of smaller Ir nanoparticles, which are better stabilized through a strong metal–support interaction (SMSI) effect.

Cobalt Phosphorous Trisulfide as a High-Performance Electrocatalyst for the Oxygen Evolution Reaction

Two-dimensional (2D) layered materials are currently one of the most explored materials for developing efficient and stable electrocatalysts in energy conversion applications. Some of the 2D metal phosphorous trichalcogenides (M₂P₂X₆ or MPX₃ in its simplified form) have been reported to be useful catalysts for water splitting, although results have been less promising for the sluggish oxygen evolution reaction (OER) due to insufficient activity or compromised stability. Herein, we report the OER catalysis of a series of M₂P₂X₆ (M²⁺ = Mn, Fe, Co, Zn, Cd; X = S, Se). From the series of MPX₃, CoPS₃ yields the best results with an overpotential within the range of values usually obtained for IrO₂ or RuO₂ catalysts. The liquid-phase exfoliation of CoPS₃ even improves the OER activity due to abundant active edges of the downsized sheets, accompanied by the presence of surface oxides. The influence of the OER medium and underlying substrate electrode is studied, with the exfoliated CoPS₃ reaching the lowest overpotential at 234 mV at a current density of 10 mA/cm², also able to sustain high current densities, with an overpotential of 388 mV at a current density of 100 mA/cm², and excellent stability after multiple cycles or long-term operation. Quantum chemical models reveal that these observations are likely tied to moieties on CoPS₃ edges, which are responsible for low overpotentials through a two-site mechanism. The OER performance of exfoliated CoPS₃ reported herein yields competitive values compared to those reported for other Co-based and MPX₃ in the literature, thus holding substantial promise for use as an efficient material for the anodic water-splitting reaction.

Low-Temperature Synthesis of Titanium Oxynitride Nanoparticles

The synthesis of transition metal oxynitrides is complicated by extreme reaction conditions such as high temperatures and/or high pressures. Here, we show an unprecedented solution-based synthesis of narrowly dispersed titanium oxynitride nanoparticles of cubic shape and average size of 65 nm. Their synthesis is performed by using titanium tetrafluoride and lithium nitride as precursors alongside trioctylphosphine oxide (TOPO) and cetrimonium bromide (CTAB) as stabilizers at temperatures as low as 250 °C. The obtained nanoparticles are characterized in terms of their shape and optical properties, as well as their crystalline rock-salt structure, as confirmed by XRD and HRTEM analysis. We also determine the composition and nitrogen content of the synthesized particles using XPS and EELS. Finally, we investigate the applicability of our titanium oxynitride nanoparticles by compounding them into carbon fiber electrodes to showcase their applicability in energy storage devices. Electrodes with titanium oxynitride nanoparticles exhibit increased capacity compared to the pure carbon material.

Effect of the Morphology of the High-Surface-Area Support on the Performance of the Oxygen-Evolution Reaction for Iridium Nanoparticles

The development of affordable, low-iridium-loading, scalable, active, and stable catalysts for the oxygen-evolution reaction (OER) is a requirement for the commercialization of proton-exchange membrane water electrolyzers (PEMWEs). However, the synthesis of high-performance OER catalysts with minimal use of the rare and expensive element Ir is very challenging and requires the identification of electrically conductive and stable high-surface-area support materials. We developed a synthesis procedure for the production of large quantities of a nanocomposite powder containing titanium oxynitride (TiON x ) and Ir. The catalysts were synthesized with an anodic oxidation process followed by detachment, milling, thermal treatment, and the deposition of Ir nanoparticles. The anodization time was varied to grow three different types of nanotubular structures exhibiting different lengths and wall thicknesses and thus a variety of properties. A comparison of milled samples with different degrees of nanotubular clustering and morphology retention, but with identical chemical compositions and Ir nanoparticle size distributions and dispersions, revealed that the nanotubular support morphology is the determining factor governing the catalyst's OER activity and stability. Our study is supported by various state-of-the-art materials' characterization techniques, like X-ray photoelectron spectroscopy, scanning and transmission electron microscopies, X-ray powder diffraction and absorption spectroscopy, and electrochemical cyclic voltammetry. Anodic oxidation proved to be a very suitable way to produce high-surface-area powder-type catalysts as the produced material greatly outperformed the IrO2 benchmarks as well as the Ir-supported samples on morphologically different TiON x from previous studies. The highest activity was achieved for the sample prepared with 3 h of anodization, which had the most appropriate morphology for the effective removal of oxygen bubbles.