Lithium-Directed Transformation of Amorphous Iridium (Oxy)hydroxides To Produce Active Water Oxidation Catalysts

Chemical Fermentation PoreCreation on Multilevel Bio-Carbon Structure with In Situ Ni-Fe Alloy Loading for Superior Oxygen Evolution Reaction Electrocatalysis

In the quest for high-efficiency and cost-effective catalysts for the oxygen evolution reaction (OER), a novel biomass-driven strategy is developed to fabricate a unique one-dimensional rod-arrays@two-dimensional interlaced-sheets (C1D@2D) network. A groundbreaking chemical fermentation (CF) pore-generation mechanism, proposed for the first time for creating nanopores within carbon structures, is based on the optimal balance between gasification and solidification. This mechanism not only results in a distinctive C1D@2D multilevel network with nanoscale, intersecting and freely flowing channels but also introduces a novel concept for in situ, extensive and hierarchical pore formation. The unique architecture, combined with the homogeneous dispersion of Ni-Fe nanoparticles, facilitates easy electrolyte penetration and provides abundant active sites for the anchoring and dispersion of reactive molecules or ions. Consequently, the Ni-Fe@C1D@2D porous network demonstrates an exceptional OER electrocatalytic performance, achieving a record-low overpotential of 165 mV at 10 mA cm-2 and maintaining long-term stability for over 90 h. Theoretical calculations reveal that the porous structure markedly strengthens the interaction between alloy nanoparticles and the carbon matrix, thereby significantly boosting their electrocatalytic activity and stability. These findings unequivocally validate the CF pore-generation mechanism as a powerful and innovative strategy for designing highly efficient functional nanostructures.

Perchlorate Fusion-Hydrothermal Synthesis of Nano-Crystalline IrO2: Leveraging Stability and Oxygen Evolution Activity

Iridium oxides are the state-of-the-art oxygen evolution reaction (OER) electrocatalysts in proton-exchange-membrane water electrolyzers (PEMWEs), but their high cost and scarcity necessitate improved utilization. Crystalline rutile-type iridium dioxide (IrO2) offers superior stability under acidic OER conditions compared to amorphous iridium oxide (IrOx). However, the higher synthesis temperatures required for crystalline phase formation result in lower OER activity due to the loss in active surface area. Herein, a novel perchlorate fusion-hydrothermal (PFHT) synthesis method to produce nano-crystalline rutile-type IrO2 with enhanced OER performance is presented. This low-temperature approach involves calcination at a mild temperature (300 °C) in the presence of a strong oxidizing agent, sodium perchlorate (NaClO4), followed by hydrothermal treatment at 180 °C, yielding small (≈2 nm) rutile-type IrO2 nanoparticles with high mass-specific OER activity, achieving 95 A gIr -1 at 1.525 VRHE in ex situ glass-cell testing. Most importantly, the catalyst displays superior stability under harsh accelerated stress test conditions compared to commercial iridium oxides. The exceptional activity of the catalyst is confirmed with in situ PEMWE single-cell evaluations. This demonstrates that the PFHT synthesis method leverages the superior intrinsic properties of nano-crystalline IrO2, effectively overcoming the typical trade-offs between OER activity and catalyst stability.

NaCl Modification: A Novel Strategy for Boosting Oxygen Evolution Activity of Ir Catalysts in Proton Exchange Membrane Water Electrolysis

The development of high-performance, low-content Ir catalysts is essential for enhancing the cost efficiency of anode catalysts and accelerating the widespread adoption of proton exchange membrane water electrolysis (PEMWE) for sustainable hydrogen production. Existing strategies, such as reducing catalyst particle size and alloying with non-precious metals, have shown limited success in surpassing the intrinsic activity of commercial IrO2 catalysts. This study presents a novel synthesis strategy for IrOx catalyst using NaCl as a structure modifier, delivering a catalyst (IrOx_NaCl) that achieves an impressive current density of 2.48 A cm-2 at 1.9 V, outperforming commercial IrO2 (2.35 A cm-2), even under low Ir catalyst loading in single-cell PEMWE test. Ex situ and in situ spectroscopic analyses suggested that NaCl incorporation effectively modulates the oxidation states and coordination structure of IrOx, leading to enhanced activity, improved stability, and greater cost efficiency. These findings offer a transformative pathway for designing advanced Ir-based catalysts for PEMWE applications.

Leaching-Induced Ti Trapping Stabilizes Amorphous IrOx for Proton Exchange Membrane Water Electrolysis

Transition metal nitrides are promising conductive support materials in oxygen evolution reaction (OER), whereas it is unclear if their electrochemical oxidation during prolonged test would affect the OER catalysts. Herein, we demonstrate that the leaching-induced Ti trapping effect from TiN supports effectively stabilizes the electrochemically oxidized amorphous IrOx catalyst. Structural characterizations reveal that the TiN support experiences severe leaching during OER test, leaving minor amounts of Ti-O species trapping on IrOx clusters, which mitigate the overoxidation of Ir(III) species and elongate the Ir-O bonds. This leads to 70% reduction of lattice oxygen oxidation and the substantially reduced Ir leaching. Additionally, self-thickening of the catalyst layers is found during proton exchange membrane water electrolysis (PEMWE), enabling an ultralow catalyst loading of 87 µgIr cm-2 to obtain a stable operation at 1.0 A cm-2 for 500 h. This work deepens the understanding of TiN-supported catalysts for long-term stable OER electrocatalysis.

Single-Atom Sb-Doped RuSbOx Bifunctional Catalysts for Ultra-Stable PEM Water Electrolyzers

Developing highly efficient and stable Pt/Ir-free bifunctional catalysts is very urgent for lowering the catalyst cost of proton exchange membrane water electrolyzer (PEMWE). Herein, a single-atom Sb-doped RuSbOx bifunctional catalyst is developed for ultra-stable PEMWE. RuSbOx exhibits excellent stability with a long-term operation of 150 h for oxygen evolution reaction (OER) and 300 h for hydrogen evolution reaction (HER) at 100 mA cm-2 in acidic media, respectively. Impressively, the PEMWE with RuSbOx as bifunctional catalysts only needs 1.72 to reach 1.0 A cm-2, and can maintain stable operation for 200 h at 200 mA cm-2. The in situ Raman and molecular probe methods reveal that the single-atom Sb doping can reconstruct the interfacial water structure on the surface of RuSbOx, resulting in an enriched supply of free water, accelerating the deprotonation process and reducing the local acidity of the catalyst surface, thereby improving the acidic OER activity and stability. Density functional theory calculations further confirm the above experimental results. In short, this work reveals that Sb is an outstanding structural stabilizer, and single-atom Sb-doping can maximize the OER stability of Ru-based catalysts in acid, which provides a useful strategy for designing ultra-stable electrocatalysts for PEMWE.

Atomically Dispersed Vanadium-Induced Ru-V Dual Active Sites Enable Exceptional Performance for Acidic Water Oxidation

Regulating the catalytic reaction pathway to essentially break the activity/stability trade-off that limits RuO2 and thus achieves exceptional stability and activity for the acidic oxygen evolution reaction (OER) is important yet challenging. Herein, we propose a novel strategy of incorporating atomically dispersed V species, including O-bridged V dimers and V single atoms, into RuO2 lattices to trigger direct O-O radical coupling to release O2 without the generation of *OOH intermediates. Vn-RuO2 showed high activity with a low overpotential of 227 mV at 10 mA cm-2 and outstanding stability during a 1050 h test in acidic electrolyte. Operando spectroscopic studies and theoretical calculations revealed that compared with the V single atom-doping case, the introduction of the V dimer into RuO2 further decreases the Ru-V atomic distance and weakens the adsorption strength of the *O intermediate to the active V site, which supports the more energetically favorable oxygen radical coupling mechanism (OCM). Furthermore, the highly asymmetric Ru-O-V local structure stabilizes the surface Ru active center by lowering the valence state and increasing the resistance against overoxidation, which result in outstanding stability. This study provides insight into ways of increasing the intrinsic catalytic activity and stability of RuO2 by atomically dispersed species modification.

Tackling activity-stability paradox of reconstructed NiIrOx electrocatalysts by bridged W-O moiety

One challenge remaining in the development of Ir-based electrocatalyst is the activity-stability paradox during acidic oxygen evolution reaction (OER), especially for the surface reconstructed IrOx catalyst with high efficiency. To address this, a phase selective Ir-based electrocatalyst is constructed by forming bridged W-O moiety in NiIrOx electrocatalyst. Through an electrochemical dealloying process, an nano-porous structure with surface-hydroxylated rutile NiWIrOx electrocatalyst is engineered via Ni as a sacrificial element. Despite low Ir content, NiWIrOx demonstrates a minimal overpotential of 180 mV for the OER at 10 mA·cm-2. It maintains a stable 300 mA·cm-2 current density during an approximately 300 h OER at 1.8 VRHE and shows a stability number of 3.9 × 105 noxygen · nIr-1. The resulting W - O-Ir bridging motif proves pivotal for enhancing the efficacy of OER catalysis by facilitating deprotonation of OER intermediates and promoting a thermodynamically favorable dual-site adsorbent evolution mechanism. Besides, the phase selective insertion of W-O in NiIrOx enabling charge balance through the W-O-Ir bridging motif, effectively counteracting lattice oxygen loss by regulating Ir-O co-valency.

Toward Realistic Models of the Electrocatalytic Oxygen Evolution Reaction

The electrocatalytic oxygen evolution reaction (OER) supplies the protons and electrons needed to transform renewable electricity into chemicals and fuels. However, the OER is kinetically sluggish; it operates at significant rates only when the applied potential far exceeds the reversible voltage. The origin of this overpotential is hidden in a complex mechanism involving multiple electron transfers and chemical bond making/breaking steps. Our desire to improve catalytic performance has then made mechanistic studies of the OER an area of major scientific inquiry, though the complexity of the reaction has made understanding difficult. While historically, mechanistic studies have relied solely on experiment and phenomenological models, over the past twenty years ab initio simulation has been playing an increasingly important role in developing our understanding of the electrocatalytic OER and its reaction mechanisms. In this Review we cover advances in our mechanistic understanding of the OER, organized by increasing complexity in the way through which the OER is modeled. We begin with phenomenological models built using experimental data before reviewing early efforts to incorporate ab initio methods into mechanistic studies. We go on to cover how the assumptions in these early ab initio simulations─no electric field, electrolyte, or explicit kinetics─have been relaxed. Through comparison with experimental literature, we explore the veracity of these different assumptions. We summarize by discussing the most critical open challenges in developing models to understand the mechanisms of the OER.

KIr4O8 Nanowires with Rich Hydroxyl Promote Oxygen Evolution Reaction in Proton Exchange Membrane Water Electrolyzer

The sluggish kinetics for anodic oxygen evolution reaction (OER) and insufficient catalytic performance over the corresponding Ir-based catalysts are still enormous challenges in proton exchange membrane water electrolyzer (PEMWE). Herein, it is reported that KIr4O8 nanowires anode catalyst with more exposed active sites and rich hydroxyl achieves a current density of 1.0 A cm-2 at 1.68 V and possesses excellent catalytic stability with 1230 h in PEMWE. Combining in situ Raman spectroscopy and differential electrochemical mass spectroscopy results, the modified adsorbate evolution mechanism is proposed, wherein the rich hydroxyl in the inherent structure of KIr4O8 nanowires directly participates in the catalytic process for favoring the OER. Density functional theory calculation results further suggest that the enhanced proximity between Ir (d) and O (p) band center in KIr4O8 can strengthen the covalence of Ir-O, facilitate the electron transfer between adsorbents and active sites, and decrease the energy barrier of rate-determining step from OH* to O* during the OER.

Intercalated and Surface-Adsorbed Phosphate Anions in NiFe Layered Double-Hydroxide Catalysts Synergistically Enhancing Oxygen Evolution Reaction Activity

The oxygen evolution reaction (OER), a crucial semireaction in water electrolysis and rechargeable metal-air batteries, is vital for carbon neutrality. Hindered by a slow proton-coupled electron transfer, an efficient catalyst activating the formation of an O-H bond is essential. Here, we proposed a straightforward one-step hydrothermal procedure for fabricating PO43--modified NiFe layered double-hydroxide (NiFe LDH) catalysts and investigated the role of PO43- anions in enhancing OER. Phosphate amounts can efficiently regulate LDH morphology, crystallinity, composition, and electronic configuration. The optimized sample showed a low overpotential of 267 mV at 10 mA cm-2. Density functional theory calculations revealed that intercalated and surface-adsorbed PO43- anions in NiFe LDH reduced the Gibbs free energy in the rate-determining step of *OOH formation, balancing oxygen-containing intermediate adsorption/dissociation and promoting the OER. Intercalated phosphate ions accelerated precatalyst dehydrogenation kinetics, leading to a rapid reconstruction into active NiFe oxyhydroxide species. Surface-adsorbed PO43- interacted favorably with adsorbed *OOH on the active Ni sites, stabilizing *OOH. Overall, the synergistic effects of intercalated and surface-adsorbed PO43- anions significantly contributed to enhanced OER activity. Achieving optimal catalytic activity requires a delicate equilibrium between thermodynamic and kinetic factors by meticulously regulating the quantity of introduced PO43- ions. This endeavor will facilitate a deeper comprehension of the influence of anions in electrocatalysis for OER.

3D Noble-Metal Nanostructures Approaching Atomic Efficiency and Atomic Density Limits

Noble metals have been widely used in catalysis, however, the scarcity and high cost of noble metal motivate researchers to balance the atomic efficiency and atomic density, which is formidably challenging. This article proposes a robust strategy for fabricating 3D amorphous noble metal-based oxides with simultaneous enhancement on atomic efficiency and density with the assistance of atomic channels, where the atomic utilization increases from 18.2% to 59.4%. The unique properties of amorphous bimetallic oxides and formation of atomic channels have been evidenced by detailed experimental characterizations and theoretical simulations. Moreover, the universality of the current strategy is validated by other binary oxides. When Cu2IrOx with atomic channels (Cu2IrOx-AE) is used as catalyst for oxygen evolution reaction (OER), the mass activity and turnover frequency value of Cu2IrOx-AE are 1-2 orders of magnitude higher than CuO/IrO2 and Cu2IrOx without atomic channels, largely outperforming the reported OER catalysts. Theoretical calculations reveal that the formation of atomic channels leads to various Ir sites, on which the proton of adsorbed *OH can transfer to adjacent O atoms of [IrO6]. This work may attract immediate interest of researchers in material science, chemistry, catalysis, and beyond.

Li+ Cations Activate NiFeOOH for Oxygen Evolution in Sodium and Potassium Hydroxide

The efficiency of electrolysis is reduced due to the sluggish oxygen evolution reaction (OER). Besides catalyst properties, electrocatalytic activity also depends on the interaction of the electrocatalyst with the electrolyte. Here, we show that the addition of small amounts of Li+ to Fe-free NaOH or KOH electrolytes activates NiFeOOH for the OER compared to single-cation electrolytes. Moreover, the activation was maintained when the solution was returned to pure NaOH. Importantly, we show that the origin of activation by Li+ cations is primarily non-kinetic in nature, as the OER onset for the mixed electrolyte does not change and the Tafel slope at low current density is ~30 mV/dec in both electrolytes. However, the increase of the apparent Tafel slope remains lower at increasing current densities in the presence of Li+. Based on electrochemical quartz crystal microbalance and in situ X-ray absorption spectroscopy measurements, we show that this reduction of non-kinetic effects is due to enhanced intercalation of sodium, water and hydroxide. This enhanced electrolyte penetration facilitates the OER, especially at higher current densities and for increased catalyst loading. Our work shows that mixed electrolytes where distinct cations can have different roles provide a simple and promising strategy towards improved OER rates.

Optimizing Edge Active Sites via Intrinsic In-Plane Iridium Deficiency in Layered Iridium Oxides for Oxygen Evolution Electrocatalysis

Improving catalytic activity of surface iridium sites without compromising catalytic stability is the core task of designing more efficient electrocatalysts for oxygen evolution reaction (OER) in acid. This work presents phase transition of a bulk layered iridate Na2IrO3 in acid solution at room temperature, and subsequent exfoliation to produce 2D iridium oxide nanosheets with around 4 nm thickness. The nanosheets consist of OH-terminated, honeycomb-type layers of edge-sharing IrO6 octahedral framework with intrinsic in-plane iridium deficiency. The nanosheet material is among the most active Ir-based catalysts reported for acidic OER and gives an iridium mass activity improvement up to a factor of 16.5 over rutile IrO2 nanoparticles. The material also exhibits good catalytic and structural stability and retains the catalytic activity for more than 1300 h. The combined experimental and theoretical results demonstrate that edge Ir sites of the layer are active centers for OER, with structural hydroxyl groups participating in the catalytic cycle of OER via a non-traditional adsorbate evolution mechanism. The existence of intrinsic in-plane iridium deficiency is the key to building a unique local environment of edge active sites that have optimal surface oxygen adsorption properties and thereby high catalytic activity.

Constructing regulable supports via non-stoichiometric engineering to stabilize ruthenium nanoparticles for enhanced pH-universal water splitting

Establishing appropriate metal-support interactions is imperative for acquiring efficient and corrosion-resistant catalysts for water splitting. Herein, the interaction mechanism between Ru nanoparticles and a series of titanium oxides, including TiO, Ti4O7 and TiO2, designed via facile non-stoichiometric engineering is systematically studied. Ti4O7, with the unique band structure, high conductivity and chemical stability, endows with ingenious metal-support interaction through interfacial Ti-O-Ru units, which stabilizes Ru species during OER and triggers hydrogen spillover to accelerate HER kinetics. As expected, Ru/Ti4O7 displays ultralow overpotentials of 8 mV and 150 mV for HER and OER with a long operation of 500 h at 10 mA cm-2 in acidic media, which is expanded in pH-universal environments. Benefitting from the excellent bifunctional performance, the proton exchange membrane and anion exchange membrane electrolyzer assembled with Ru/Ti4O7 achieves superior performance and robust operation. The work paves the way for efficient energy conversion devices.

Valence Oscillation of Ru Active Sites for Efficient and Robust Acidic Water Oxidation

The continuous oxidation and leachability of active sites in Ru-based catalysts hinder practical application in proton-exchange membrane water electrolyzers (PEMWE). Herein, robust inter-doped tungsten-ruthenium oxide heterostructures [(Ru-W)Ox ] fabricated by sequential rapid oxidation and metal thermomigration processes are proposed to enhance the activity and stability of acidic oxygen evolution reaction (OER). The introduction of high-valent W species induces the valence oscillation of the Ru sites during OER, facilitating the cyclic transition of the active metal oxidation states and maintaining the continuous operation of the active sites. The preferential oxidation of W species and electronic gain of Ru sites in the inter-doped heterostructure significantly stabilize RuOx on WOx substrates beyond the Pourbaix stability limit of bare RuO2 . Furthermore, the asymmetric Ru-O-W active units are generated around the heterostructure interface to adsorb the oxygen intermediates synergistically, enhancing the intrinsic OER activity. Consequently, the inter-doped (Ru-W)Ox heterostructures not only demonstrate an overpotential of 170 mV at 10 mA cm-2 and excellent stability of 300 h in acidic electrolytes but also exhibit the potential for practical applications, as evidenced by the stable operation at 0.5 A cm-2 for 300 h in PEMWE.

Unveiling the Oxygen Evolution Mechanism with FeNi (Hydr)oxide under Neutral Conditions

Large-scale solar-driven water splitting is a way to store energy, but it requires the development of practical and durable oxygen evolution reaction (OER) catalysts. The present paper aims to investigate the mechanism of the OER, local pH, high-valent metal ions, limitations, conversions, and details during the OER in the presence of FeNi foam using in situ surface-enhanced Raman spectroscopy. This research also explores the use of in situ surface-enhanced Raman spectroscopy for detecting species on foam surfaces during the OER. The acidic media around the electrode not only limit the process but also affect the phosphate ion protonation and overall catalysis effectiveness. The study proposes that FeNi hydroxides serve as true catalysts for OER under neutral conditions, rather than FeNi phosphates. However, phosphate species remain crucial for proton transfer and water molecule adsorption. Changes observed in pH at the open-circuit potential suggest new insights concerning the coordination of Ni(II) to phosphate ions under certain conditions. By extrapolating the Tafel plot, the overpotential for the onset of OER was determined to be 470 mV. Furthermore, the overpotentials for current densities of 1 and 5 mA/cm2 were 590 and 790 mV, respectively. These findings offer valuable insights into the advancement of the OER catalysts and our understanding of the underlying mechanism for efficient water splitting; both are crucial elements for the purpose of energy storage.