Ruthenium Single-Atom Modulated Protonated Iridium Oxide for Acidic Water Oxidation in Proton Exchange Membrane Electrolysers

Proton exchange membrane water electrolysers promise to usher in a new era of clean energy, but they remain a formidable obstacle in designing active and durable electrocatalysts for the acidic oxygen evolution reaction (OER). In this study, a protonated iridium oxide embedded with single-atom dispersed ruthenium atoms (H3.8Ir1- xRuxO4) that demonstrates exceptional activity and stability in acidic water oxidation is introduced. The single Ru dopants favorably induce localized oxygen vacancies in the Ir─O lattice, synergistically strengthening the adsorption of OOH* intermediates and enhancing the intrinsic OER activity. In addition, the preferential oxidation of Ru and the electronegativity of the oxygen vacancies significantly stabilize the Ir─O active sites, improving the OER stability. Consequently, the H3.8Ir1─ xRuxO4 catalyst shows an overpotential of 255 mV at 10 mA cm-2 and displays exceptional catalytic endurance in acidic electrolytes, surpassing 1100 h, representing a remarkable one-order-of-magnitude increase in stability compared to that of pristine H3.8IrO4. A proton exchange membrane electrolyser utilizing the H3.8Ir1- xRuxO4 catalyst as an anode exhibits stable performance for more than 1280 h under a high current density of 2 A cm-2.

Infrared Spectroscopic Observation of Oxo- and Superoxo-Intermediates in the Water Oxidation Cycle of a Molecular Ir Catalyst

Molecular Ir catalysts have emerged as an important class of model catalysts for understanding structure-activity relationships in water oxidation, a reaction that is central to renewable fuel synthesis. Prior efforts have mostly focused on controlling and elucidating the emergence of active species from prepared precursors. However, the development of efficient and stable molecular Ir catalysts also necessitates probing of reaction intermediates. To date, relatively little is known about the key intermediates in the cycles of the molecular Ir catalysts. Herein, we probed the catalytic cycle of a homogeneous Ir catalyst ("blue dimer") at a Au electrode/aqueous electrolyte interface by combining surface-enhanced infrared absorption spectroscopy (SEIRAS) with phase-sensitive detection (PSD). Cyclic voltammograms (CVs) from 1.4 to 1.7 VRHE (RHE = reversible hydrogen electrode) give rise to a band at ∼818 cm-1, whereas CVs from 1.4 to ≥1.85 VRHE generate an additional band at ∼1146 cm-1. Isotope labeling experiments indicate that the bands at ∼818 and ∼1146 cm-1 are attributable to oxo (IrV═O) and superoxo (IrIV-OO•) moieties, respectively. This study establishes PSD-SEIRAS as a sensitive tool for probing water oxidation cycles at electrode/electrolyte interfaces and demonstrates that the relative abundance of two key intermediates can be tuned by the thermodynamic driving force of the reaction.

Simultaneously Improved Activity and Stability for Acidic Water Oxidation of IrRu Oxides by a Dual Role of Tungsten Doping

Acidic oxygen evolution reaction (OER) remains a significant challenge due to the low activity and/or poor stability of the catalysts, even with state-of-the-art catalysts such as IrO2 and RuO2. Herein, we propose a strategy to enhance both the catalytic activity and stability of IrRu oxides for acidic OER by doping non-noble metal W. The W-doped IrRu3Ox (W-IrRu3Ox) undergoes a process of W leaching and reconstruction during the OER, leading to a more uniform distribution of elements, while the electronegative nature of W influences the electronic structures of Ir and Ru in W-IrRu3Ox. The dual role of W in promoting the formation of active site Ir5+ and inhibiting the concentration of soluble Ru>4+ ions results in a synergistic enhancement of both the activity and stability of acidic OER. Remarkably, W-IrRu3Ox exhibits outstanding catalytic activity for the OER in 0.5 M H2SO4, with a high stability of more than 500 h. This work presents a novel and feasible strategy for the development of efficient and stable catalysts for acid OER, shedding light on the design of advanced electrocatalysts for energy conversion and storage applications.

Development of Electrochemical Cells and Their Application for Spatially Resolved Analysis Using a Multitechnique Approach: From Conventional Experiments to X-Ray Nanoprobe Beamlines

Real (electro)catalysts are often heterogeneous, and their activity and selectivity depend on the properties of specific active sites. Therefore, unveiling the so-called structure-activity relationship is essential for a rational search for better materials and, consequently, for the development of the field of (electro-)catalysis. Thus, spatially resolved techniques are powerful tools as they allow us to characterize and/or measure the activity and selectivity of different regions of heterogeneous catalysts. To take full advantage of that, we have developed spectroelectrochemical cells to perform spatially resolved analysis using X-ray nanoprobe synchrotron beamlines and conventional pieces of equipment. Here, we describe the techniques available at the Carnaúba beamline at the Sirius-LNLS storage ring, and then we show how our cells enable obtaining X-ray (XRF, XRD, XAS, etc.) and vibrational spectroscopy (FTIR and Raman) contrast images. Through some proof-of-concept experiments, we demonstrate how using a multi-technique approach could render a complete and detailed analysis of an (electro)catalyst overall performance.

Enhanced Acidic Water Oxidation by Dynamic Migration of Oxygen Species at the Ir/Nb2 O5-x Catalyst/Support Interfaces

Catalyst/support interaction plays a vital role in catalysis towards acidic oxygen evolution (OER), and the performance reinforcement is currently interpreted by either strain or electron donation effect. We herein report that these views are insufficient, where the dynamic evolution of the interface under potential bias must be considered. Taking Nb₂ O_5-x supported iridium (Ir/Nb₂ O_5-x ) as a model catalyst, we uncovered the dynamic migration of oxygen species between IrO_x and Nb₂ O_5-x during OER. Direct spectroscopic evidence combined with theoretical computation suggests these migrations not only regulate the in situ Ir structure towards boosted activity, but also suppress its over-oxidation via spontaneously delivering excessive oxygen from IrO_x to Nb₂ O_5-x . The optimized Ir/Nb₂ O_5-x thus demonstrated exceptional performance in scalable water electrolyzers, i.e., only need 1.839 V to attain 3 A cm^-2 (surpassing the DOE 2025 target), and no activity decay during a 2000 h test at 2 A cm^-2 .

The Influence of Electrode Thickness on the Structure and Water Splitting Performance of Iridium Oxide Nanostructured Films

For a safe environment, humanity should be oriented towards renewable energy technology. Water splitting (WS), utilizing a photoelectrode with suitable thickness, morphology, and conductivity, is essential for efficient hydrogen production. In this report, iridium oxide (IrO_x) films of high conductivity were spin-cast on glass substrates. FE-SEM showed that the films are of nanorod morphology and different thicknesses. UV-Vis spectra indicated that the absorption and reflectance of the films depend on their thickness. The optical band gap (E_g) was increased from 2.925 eV to 3.07 eV by varying the spin speed (SS) of the substrates in a range of 1.5 × 10³-4.5 × 10³ rpm. It was clear from the micro-Raman spectra that the films were amorphous. The E_g vibrational mode of Ir-O stretching was red-shifted from 563 cm^-1 (for the rutile IrO₂ single crystal) to 553 cm^-1. The IrO_x films were used to develop photoelectrochemical (PEC) hydrogen production catalysts in 0.5M of sodium sulfite heptahydrate Na₂SO₃·7H₂O (2-electrode system), which exhibits higher hydrogen evaluation (HE) reaction activity, which is proportional to the thickness and absorbance of the used IrO_x photocathode, as it showed an incident photon-to-current efficiency (IPCE%) of 7.069% at 390 nm and -1 V. Photocurrent density (Jph = 2.38 mA/cm² at -1 V vs. platinum) and PEC hydrogen generation rate (83.68 mmol/ h cm² at 1 V) are the best characteristics of the best electrode (the thickest and most absorbent IrO_x photocathode). At -1 V and 500 nm, the absorbed photon-to-current conversion efficiency (APCE%) was 7.84%. Electrode stability, thermodynamic factors, solar-to-hydrogen conversion efficiency (STH), and electrochemical impedance spectroscopies (EISs) were also studied.

Time-Resolved Vibrational and Electronic Spectroscopy for Understanding How Charges Drive Metal Oxide Catalysts for Water Oxidation

Temporally resolved spectroscopy is a powerful approach for gaining detailed mechanistic understanding of water oxidation at robust Earth-abundant metal oxide catalysts for guiding efficiency improvement of solar fuel conversion systems. Beyond detecting and structurally identifying surface intermediates by vibrational and accompanying optical spectroscopy, knowledge of how charges, sequentially delivered to the metal oxide surface, drive the four-electron water oxidation cycle is critical for enhancing catalytic efficiency. Key issues addressed in this Perspective are the experimental requirements for establishing the kinetic relevancy of observed surface species and the discovery of the rate-boosting role of encounters of two or more one-electron surface hole charges, often in the form of randomly hopping metal oxo or oxyl moieties, for accessing very low-barrier O-O bond-forming pathways. Recent spectroscopic breakthroughs of metal oxide photo- and electrocatalysts inspire future research poised to take advantage of new highly sensitive spectroscopic tools and of methods for fast catalysis triggering.

Long-Life and pH-Stable SnO2-Coated Au Nanoparticles for SHINERS

Shell-isolated nanoparticles (SHINs) with a 37 nm gold core and an 11 nm tin dioxide (SnO₂) coating exhibited long-life Raman enhancement for 3 months and a wide pH stability of pH 2-13 in comparison with conventional SiO₂-coated SHINs. Herein, Au-SnO₂ is demonstrated as a more durable SHIN for use in the technique Shell-Isolated Nanoparticles for Enhanced Raman Spectroscopy (SHINERS).

Latest Advances in Nanoplasmonics and Use of New Tools for Plasmonic Characterization

Nanoplasmonics is a research topic that takes advantage of the light coupling to electrons in metals, and can break the diffraction limit for light confinement into subwavelength zones allowing strong field enhancements [...]

Shell isolated nanoparticle enhanced Raman spectroscopy for mechanistic investigation of electrochemical reactions

Electrochemical conversion of abundant resources, such as carbon dioxide, water, nitrogen, and nitrate, is a remarkable strategy for replacing fossil fuel-based processes and achieving a sustainable energy future. Designing an efficient and selective electrocatalysis system for electrochemical conversion reactions remains a challenge due to a lack of understanding of the reaction mechanism. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is a promising strategy for experimentally unraveling a reaction pathway and rate-limiting step by detecting intermediate species and catalytically active sites that occur during the reaction regardless of substrate. In this review, we introduce the SHINERS principle and its historical developments. Furthermore, we discuss recent SHINERS applications and developments for investigating intermediate species involved in a variety of electrocatalytic reactions.

Increasing Iridium Oxide Activity for the Oxygen Evolution Reaction with Hafnium Modification

Synthesis and implementation of highly active, stable, and affordable electrocatalysts for the oxygen evolution reaction (OER) is a major challenge in developing energy efficient and economically viable energy conversion devices such as electrolyzers, rechargeable metal-air batteries, and regenerative fuel cells. The current benchmark electrocatalyst for OER is based on iridium oxide (IrO_x) due to its superior performance and excellent stability. However, large scale applications using IrO_x are impractical due to its low abundance and high cost. Herein, we report a highly active hafnium-modified iridium oxide (IrHf_xO_y) electrocatalyst for OER. The IrHf_xO_y electrocatalyst demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than a IrO_x electrocatalyst. The highest intrinsic mass activity of the IrHf_xO_y catalyst in acid conditions was calculated as 6950 A g_IrOx^-1 at an overpotential (η) of 0.3 V. Combined studies utilizing operando surface enhanced Raman spectroscopy (SERS) and DFT calculations revealed that the active sites for OER are the Ir-O species for both IrO_x and IrHf_xO_y catalysts. The presence of Hf sites leads to more negative charge states on nearby O sites, shortening of the bond lengths of Ir-O, and lowers free energies for OER intermediates that accelerate the OER process.

Oxygen Vacancy Dynamics in Highly Crystalline Zinc Oxide Film Investigated by PIERS Effect

Surface-enhanced Raman spectroscopy (SERS) is commonly employed as an analysis or detection tool of biological and chemical molecules. Recently, an alternative section of the SERS field has appeared, called photo-induced enhanced Raman spectroscopy (PIERS). This PIERS effect is based on the production of the oxygen vacancies (V0) in metal-oxide semiconductor thin-film (or other structures) by irradiation with UV light, thus enabling a Raman signal enhancement of chemical molecules through charge transfer processes between this photo-irradiated semiconductor film (or other structures) and these chemical molecules via metallic nanoparticles deposited on this photo-irradiated substrate. The PIERS technique can enable studying the dynamics of the oxygen vacancies under ambient and operando conditions compared to conventional tools of analysis. In this paper, we present the results obtained on the formation and healing rates of surface oxygen vacancies (V0) in a highly crystalline ZnO film investigated by the PIERS effect, and we compare these results to the literature in order to study the effect of the crystallinity on these formation and healing rates of V0 in a ZnO film.

Plasmonic Core-Shell Nanoparticle Enhanced Spectroscopies for Surface Analysis

Probing the properties and components of reactive surfaces is crucial for illustrating reaction mechanisms. However, common surface analysis techniques are restricted to in situ acquisition of surface information at the molecular scale in the human environment and industrial catalysis processes. Plasmonic spectroscopies are promising tools to solve this problem. This Feature is intended to introduce the plasmonic core-shell nanoparticle enhanced spectroscopies for qualitatively and quantitatively analyzing surface trace species. Four different working modalities are designed for meeting varied needs, involving in situ surface species detection, catalytic process monitoring, labeled sensing, and dual mode analysis. These newly developed plasmonic spectroscopies show great potential not only in fundamental research but also in practical applications.

High-entropy alloys for water oxidation: a new class of electrocatalysts to look out for

High-entropy alloys (HEAs) with five or more elements can provide near-continuous adsorption energies and can be optimized for superior persistent catalytic activity. This report presents electrochemical water oxidation facilitated by employing graphene and FeCoNiCuCr HEA nanoparticle based composites prepared via the mechanical milling of graphite-metal powders. The composite efficiently facilitates water oxidation with a low overpotential of 330 mV at 10 mA cm-2, and high specific and mass activities (∼143 mA cm-2 and 380 mA mg-1, respectively, at 1.75 V). Importantly, the composites exhibit excellent accelerated cycling stability with ∼99% current retention (after 3250 cycles). The HEA-based composites are anticipated to replace noble/precious metal based traditional electrocatalysts in the future, the use of which is a major obstacle in the technological scalability of electrochemical energy conversion and storage devices.

Recent advances in understanding oxygen evolution reaction mechanisms over iridium oxide

Recent spectroscopic and computational studies concerning the oxygen evolution reaction over iridium oxides are reviewed to provide the state-of-the-art understanding of its reaction mechanism.

A Simple Method for Synthesizing Highly Active Amorphous Iridium Oxide for Oxygen Evolution under Acidic Conditions

Water splitting for hydrogen production has been recognized as a promising approach to store sustainable energy. The performance of this method is limited by the oxygen-evolution reaction. Herein, an approach for synthesizing a highly active oxygen-evolving catalyst by a one-step, low-cost, environmentally friendly, and easy-to-perform method is presented, which works by using iridium metal as the anode at a relatively high potential. The obtained IrO_x /Ir interface showed an overpotential of 250 mV at 10 mA cm^-2 in 0.1 m HClO₄ and remained stable under electrochemical conditions. The IrO_x that was mechanically separated from the surface of IrO_x /Ir metal after operation showed a threefold increase in activity compared to the current benchmark IrO₂ catalyst. Various characterization analyses were used to identify the structure and morphology of the catalyst, which suggested nanosized, porous, and amorphous IrO_x on the surface of metallic Ir. This synthetic approach can inspire a variety of opportunities to design and synthesize efficient metal oxide-based electrocatalysts for sustainable energy conversion and utilization.

Shell isolated nanoparticle enhanced Raman spectroscopy for renewable energy electrocatalysis

This review covers the use of shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) to investigate heterogeneous electrocatalytic processes.

An iridium-based nanocomposite prepared from an iridium complex with a hydrocarbon-based ligand

For the first time, a chlorobis(cyclooctene)iridium(i) dimer with only a simple hydrocarbon-based ligand is investigated as a heterogeneous catalyst for the oxygen-evolution reaction in the presence of cerium(iv) ammonium nitrate.