Molecular Insight in Structure and Activity of Highly Efficient, Low-Ir Ir–Ni Oxide Catalysts for Electrochemical Water Splitting (OER)

In situ observation of reactive oxygen species forming on oxygen-evolving iridium surfaces

In situ XAS measurements reveal that electron-deficient oxygen species form during OER on IrOx and correlate with catalytic activity.

Water splitting performed in acidic media relies on the exceptional performance of iridium-based materials to catalyze the oxygen evolution reaction (OER). In the present work, we use in situ X-ray photoemission and absorption spectroscopy to resolve the long-standing debate about surface species present in iridium-based catalysts during the OER. We find that the surface of an initially metallic iridium model electrode converts into a mixed-valent, conductive iridium oxide matrix during the OER, which contains O^II– and electrophilic O^I– species. We observe a positive correlation between the O^I– concentration and the evolved oxygen, suggesting that these electrophilic oxygen sites may be involved in catalyzing the OER. We can understand this observation by analogy with photosystem II; their electrophilicity renders the O^I– species active in O–O bond formation, i.e. the likely potential- and rate-determining step of the OER. The ability of amorphous iridium oxyhydroxides to easily host such reactive, electrophilic species can explain their superior performance when compared to plain iridium metal or crystalline rutile-type IrO₂.

Free Electrons to Molecular Bonds and Back: Closing the Energetic Oxygen Reduction (ORR)-Oxygen Evolution (OER) Cycle Using Core-Shell Nanoelectrocatalysts

Nanomaterial science and electrocatalytic science have entered a successful "nanoelectrochemical" symbiosis, in which novel nanomaterials offer new frontiers for studies on electrocatalytic charge transfer, while electrocatalytic processes give meaning and often practical importance to novel nanomaterial concepts. Examples of this fruitful symbiosis are dealloyed core-shell nanoparticle electrocatalysts, which often exhibit enhanced kinetic charge transfer rates at greatly improved atom-efficiency. As such, they represent ideal electrocatalyst architectures for the acidic oxygen reduction reaction to water (ORR) and the acidic oxygen evolution reaction from water (OER) that require scarce Pt- and Ir-based catalysts. Together, these two reactions constitute the "O-cycle", a key elemental process loop in the field of electrochemical energy interconversion between electricity (free electrons) and molecular bonds (H₂O/O₂), realized in the combination of water electrolyzers and hydrogen/oxygen fuel cells. In this Account, we describe our recent efforts to design, synthesize, understand, and test noble metal-poor dealloyed Pt and Ir core-shell nanoparticles for deployment in acidic polymer electrolyte membrane (PEM) electrolyzers and PEM fuel cells. Spherical dealloyed Pt core-shell particles, derived from PtNi₃ precursor alloys, showed favorable ORR activity. More detailed size-activity correlation studies further revealed that the 6-8 nm diameter range is a most desirable initial particle size range in order to maximize the particle Ni content after ORR testing and to preserve performance stability. Similarly, dealloyed and oxidized IrO_x core-shell particles derived from Ni-rich Ir-Ni precursor particles proved highly efficient oxygen evolution reaction (OER) catalysts in acidic conditions. In addition to the noble metal savings in the particle cores, the Pt core-shell particles are believed to benefit in terms of their mass-based electrochemical kinetics from surface lattice strain effects that tune the adsorption energies and barriers of elementary steps. The molecular mechanism of the kinetic benefit of the dealloyed IrO_x particle needs more attention, but there is mounting evidence for ligand hole effects in defect-rich IrO_x shells that generate preactive oxygen centers.

Uncovering the Stabilization Mechanism in Bimetallic Ruthenium-Iridium Anodes for Proton Exchange Membrane Electrolyzers

Proton exchange membrane (PEM) electrolyzers are attracting an increasing attention as a promising technology for the renewable electricity storage. In this work, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is applied for in situ monitoring of the surface state of membrane electrode assemblies with RuO2 and bimetallic Ir0.7Ru0.3O2 anodes during water splitting. We demonstrate that Ir protects Ru from the formation of an unstable hydrous Ru(IV) oxide thereby rendering bimetallic Ru-Ir oxide electrodes with higher corrosion resistance. We further show that the water splitting occurs through a surface Ru(VIII) intermediate, and, contrary to common opinion, the presence of Ir does not hinder its formation.

Reactive oxygen species in iridium-based OER catalysts

Exceptional reactivity of electrophilic oxygen species in highly OER-active Ir^III/IV oxyhydroxides is evidenced by room temperature CO oxidation.

Tremendous effort has been devoted towards elucidating the fundamental reasons for the higher activity of hydrated amorphous Ir^III/IV oxyhydroxides (IrO_x) in the oxygen evolution reaction (OER) in comparison with their crystalline counterpart, rutile-type IrO₂, by focusing on the metal oxidation state. Here we demonstrate that, through an analogy to photosystem II, the nature of this reactive species is not solely a property of the metal but is intimately tied to the electronic structure of oxygen. We use a combination of synchrotron-based X-ray photoemission and absorption spectroscopies, ab initio calculations, and microcalorimetry to show that holes in the O 2p states in amorphous IrO_x give rise to a weakly bound oxygen that is extremely susceptible to nucleophilic attack, reacting stoichiometrically with CO already at room temperature. As such, we expect this species to play the critical role of the electrophilic oxygen involved in O–O bond formation in the electrocatalytic OER on IrO_x. We propose that the dynamic nature of the Ir framework in amorphous IrO_x imparts the flexibility in Ir oxidation state required for the formation of this active electrophilic oxygen.

Ultrathin Laminar Ir Superstructure as Highly Efficient Oxygen Evolution Electrocatalyst in Broad pH Range

Shape-controlled noble metal nanocrystals (NCs), such as Au, Ag, Pt, Pd, Ru, and Rh are of great success due to their new and enhanced properties and applications in chemical conversion, fuel cells, and sensors, but the realization of shape control of Ir NCs for achieving enhanced electrocatalysis remains a significant challenge. Herein, we report an efficient solution method for a new class of three-dimensional (3D) Ir superstructure that consists of ultrathin Ir nanosheets as subunits. Electrochemical studies show that it delivers the excellent electrocatalytic activity toward oxygen evolution reaction (OER) in alkaline condition with an onset potential at 1.43 V versus reversible hydrogen electrode (RHE) and a very low Tafel slope of 32.7 mV decade(-1). In particular, it even shows superior performance for OER in acidic solutions with the low onset overpotential of 1.45 V versus RHE and small Tafel slope of 40.8 mV decade(-1), which are much better than those of small Ir nanoparticles (NPs). The 3D Ir superstructures also exhibit good stability under acidic condition with the potential shift of less than 20 mV after 8 h i-t test. The present work highlights the importance of tuning 3D structures of Ir NCs for enhancing OER performance.

Efficient and Stable Evolution of Oxygen Using Pulse-Electrodeposited Ir/Ni Oxide Catalyst in Fe-Spiked KOH Electrolyte

Metal oxides have been extensively explored as catalysts for the electrochemical oxygen evolution reaction (OER). Here, we present an excellent OER catalytic system consisting of pulse-electrodeposited Ir/Ni oxides in Fe(3+)-spiked 1 M KOH. In pure 1 M KOH electrolyte, the optimized catalyst, which had an Ir:Ni atom ratio of 1:1.49, could catalyze 10 mA/cm(2) of O2 production at a small overpotential (η) of 264 mV. Remarkably, we found that its OER performance could be significantly improved by adding 0.3 mM Fe(3+) into the electrolyte. At an η of just 343 ± 3 mV, a huge current of 500 mA/cm(2) was achieved. Furthermore, this catalytic system exhibited a small Tafel slope of 31 mV/dec and a large iridium mass-normalized current of 1260 mA/mgIr at η = 280 mV. We also discovered that the durability of the Ir/Ni oxide catalyst during OER (at 10 mA/cm(2) with η < 280 mV) could be maintained for more than 4.5 days by simply spiking Fe(3+), Ir(3+), and Ni(2+) into the KOH electrolyte. The figures-of-merit in this work, in terms of both activity and stability, compare favorably against values from several state-of-the-art catalysts. Hypotheses for the outstanding performance of the Ir/Ni catalyst are proposed and discussed.

Unique Sandwiched Carbon Sheets@Ni-Mn Nanoparticles for Enhanced Oxygen Evolution Reaction

A unique sandwich-like architecture, where Ni-Mn nanoparticles are enveloped in coupled carbon sheets (CS@Ni-Mn), has been successfully fabricated. In the synthesis process, a great quantity of uniform NiMnO3 nanosheets generated by a universal hydrothermal method acts as precursors and templates and the cheap, environmentally friendly and recyclable glucose functions as a green carbon source. Via subsequent hydrothermal reaction and thermal annealing, sandwiched nanocomposites with Ni-Mn nanoparticles embedded inside and carbon sheets encapsulating outside can be massively prepared. The novel sandwich-like CS@Ni-Mn possesses numerous advantages, such as an intrinsic porous feature, large specific surface area, and enhanced electronic conductivity. Moreover, as a promising NiMn-based oxygen evolution reaction (OER) catalyst, the special sandwiched nanostructure demonstrates improved electrochemical properties in 1 M KOH, including a low overpotential of about 250 mV, a modest Tafel slope of 40 mV dec(-1), excellent stability over 2000 cycles, and durability for 40 h.

Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes

Chlorine gas and sodium chlorate are two base chemicals produced through electrolysis of sodium chloride brine which find uses in many areas of industrial chemistry. Although the industrial production of these chemicals started over 100 years ago, there are still factors that limit the energy efficiencies of the processes. This review focuses on the unwanted production of oxygen gas, which decreases the charge yield by up to 5%. Understanding the factors that control the rate of oxygen production requires understanding of both chemical reactions occurring in the electrolyte, as well as surface reactions occurring on the anodes. The dominant anode material used in chlorate and chlor-alkali production is the dimensionally stable anode (DSA), Ti coated by a mixed oxide of RuO2 and TiO2. Although the selectivity for chlorine evolution on DSA is high, the fundamental reasons for this high selectivity are just now becoming elucidated. This review summarizes the research, since the early 1900s until today, concerning the selectivity between chlorine and oxygen evolution in chlorate and chlor-alkali production. It covers experimental as well as theoretical studies and highlights the relationships between process conditions, electrolyte composition, the material properties of the anode, and the selectivity for oxygen formation.

Shaped Ir–Ni bimetallic nanoparticles for minimizing Ir utilization in oxygen evolution reaction

Shaped Ir–Ni bimetallic nanoparticles were synthesized and used for electrocatalytic oxygen evolution reaction (OER).