Electrochemical and optical studies of thick oxide layers on iridium and their electrocatalytic activities for the oxygen evolution reaction

Support-free iridium hydroxide for high-efficiency proton-exchange membrane water electrolysis

The large-scale implementation of proton-exchange membrane water electrolyzers relies on high-performance membrane-electrode assemblies that use minimal iridium (Ir). In this study, we present a support-free Ir catalyst developed through a metal-oxide-based molecular self-assembly strategy. The unique self-assembly of densely isolated single IrO6H8 octahedra leads to the formation of μm-sized hierarchically porous Ir hydroxide particles. The support-free Ir catalyst exhibits a high turnover frequency of 5.31 s⁻¹ at 1.52 V in the membrane-electrode assembly. In the corresponding proton-exchange membrane water electrolyzer, notable performance with a cell voltage of less than 1.75 V at 4.0 A cm⁻² (Ir loading of 0.375 mg cm⁻²) is achieved. This metal-oxide-based molecular self-assembly strategy may provide a general approach for the development of advanced support-free catalysts for high-performance membrane-electrode assemblies.

Electrodeposition synthesis of cobalt-molybdenum bimetallic phosphide on nickel foam for efficient water splitting

A reasonable design of excellent bifunctional catalyst is an effective strategy for large-scale hydrogen production. In this study, a two-stage electrodeposition method was used to prepare a crystalline-amorphous structure cobalt molybdenum phosphide layered particles with different sizes on a nickel foam (NF) substrate. Electron rearrangement at the Co/CoMoP2@CoMoO4 heterogeneous interface can reduce the reaction energy barrier for HER and OER, and accelerate the catalytic reaction kinetics. The doping of Mo can promote the synergistic effect between Co and Mo, thereby optimizing the Gibbs free energy of hydrogen adsorption/desorption. This layered arrangement of different size particles greatly improves the active area of the catalyst. In alkaline solution, achieving a current density of 10 mA cm-2 only required overpotentials of 40 mV for HER and 278 mV for OER, respectively. The cell voltage required for the CoMo-P/NF||CoMo-P/NF electrolytic cell is only 1.53 V at 10 mA cm-2. This study provides a reference for the rapid, efficient, and environmentally friendly preparation of high-activity water splitting catalysts with large surface areas.

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

The oxygen evolution reaction (OER) is crucial to future energy systems based on water electrolysis. Iridium oxides are promising catalysts due to their resistance to corrosion under acidic and oxidizing conditions. Highly active iridium (oxy)hydroxides prepared using alkali metal bases transform into low activity rutile IrO₂ at elevated temperatures (>350 °C) during catalyst/electrode preparation. Depending on the residual amount of alkali metals, we now show that this transformation can result in either rutile IrO₂ or nano-crystalline Li-intercalated IrO_x. While the transition to rutile results in poor activity, the Li-intercalated IrO_x has comparative activity and improved stability when compared to the highly active amorphous material despite being treated at 500 °C. This highly active nanocrystalline form of lithium iridate could be more resistant to industrial procedures to produce PEM membranes and provide a route to stabilize the high populations of redox active sites of amorphous iridium (oxy)hydroxides.

Electrochemical evaluation of the de-/re-activation of oxygen evolving Ir oxide

Understanding the influence of dynamic and stationary polarization on the deactivation of state-of-the-art IrOx catalysts is imperative for the design and operation of robust and efficient proton exchange membrane water...

The rise of the electrochemical NAPXPS operated in the soft X-ray regime exemplified in the oxygen evolution reaction on IrOx electrocatalysts

Photoelectron spectroscopy offers detailed information about of the electronic structure and chemical composition of surfaces owing to the short distance that the photoelectrons can escape from a dense medium. Unfortunately,...

Surface Electron-Hole Rich Species Active in the Electrocatalytic Water Oxidation

Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru- and Ir-based anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nanofabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrO_x centers (i.e., oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally Ir^V species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (μ₁-O and μ₁-OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (μ₃-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a μ₁-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface-to-bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.

Ir-based bifunctional electrocatalysts for overall water splitting

The recent progress on Ir-based bifunctional electrocatalysts in enhancing the overall water splitting performance is reviewed mainly from the aspects of optimizing the composition and morphology.

Modified bluing treatment to produce nickel–cobalt–iron spinel oxide with promoted oxygen-evolving performance

A facile modified bluing treatment approach was proposed for low-cost, time-saving and batch synthesis of a ternary NiCoFe spinel oxide catalyst with high OER performance.

Tailored transition metal-doped nickel phosphide nanoparticles for the electrochemical oxygen evolution reaction (OER)

Foreign transition metals are doped into the nickel phosphide nanoparticles through a simple wet-chemical process to show a high level of doping without significant distortion of crystal structure and morphology, which endows excellent activity in the electrochemical oxygen evolution reaction.

Rechargeable Metal-Air Proton-Exchange Membrane Batteries for Renewable Energy Storage

Rechargeable proton-exchange membrane batteries that employ organic chemical hydrides as hydrogen-storage media have the potential to serve as next-generation power sources; however, significant challenges remain regarding the improvement of the reversible hydrogen-storage capacity. Here, we address this challenge through the use of metal-ion redox couples as energy carriers for battery operation. Carbon, with a suitable degree of crystallinity and surface oxygenation, was used as an effective anode material for the metal redox reactions. A Sn_0.9In_0.1P₂O₇-based electrolyte membrane allowed no crossover of vanadium ions through the membrane. The V⁴⁺/V³⁺, V³⁺/V²⁺, and Sn⁴⁺/Sn²⁺ redox reactions took place at a more positive potential than that for hydrogen reduction, so that undesired hydrogen production could be avoided. The resulting electrical capacity reached 306 and 258 mAh g^-1 for VOSO₄ and SnSO₄, respectively, and remained at 76 and 91 % of their respective initial values after 50 cycles.

A Flexible Electrode Based on Iron Phosphide Nanotubes for Overall Water Splitting

The design of cheap and efficient water splitting systems for sustainable hydrogen production has attracted increasing attention. A flexible electrode, based on carbon cloth substrate and iron phosphide nanotubes coated with an iron oxide/phosphate layer, is shown to catalyze overall water splitting. The as-prepared flexible electrode demonstrates remarkable electrocatalytic activity for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) at modest overpotentials. The surface iron oxide/phosphate, which is formed in situ, is proposed to improve the HER activity by facilitating the water-dissociation step and serves directly as the catalytically-active component for the OER process.

Porous cobalt-based thin film as a bifunctional catalyst for hydrogen generation and oxygen generation

A mixed-phased Co-based catalyst composed of Co phosphide and Co phosphate is successfully fabricated for bifunctional water electrolysis. The highly porous morphology in this anodized film enables efficient catalytic activity toward water splitting in an extremely low loading mass. The mixed phases in the porous film afford an ability to generate both H2 and O2 in a single electrolyzer.

Iridium-based complexes for water oxidation

Activation of organometallic Ir precatalysts can yield either homogeneous or heterogeneous water-oxidation catalysts with very high activity.

Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction

Activity, selectivity and stability of oxygen evolution catalysts for water electrolyzers: an interplay between composition, morphology, preparation and processing.

Effects of aqueous buffers on electrocatalytic water oxidation with an iridium oxide material electrodeposited in thin layers from an organometallic precursor

A thin layer of an amorphous, mixed-valence iridium oxide (electrodeposited from an organometallic precursor, [Cp*Ir(H(2)O)(3)](2+)) is a heterogeneous catalyst among the most active and stable currently available for electrochemical water oxidation. We show that buffers can improve the oxygen-evolution activity of such thin-layer catalysts near neutral pH, but that buffer identity and concentration, as well as the solution pH, remain key determinants of long-term electrocatalyst activity and stability; for example, phosphate buffer can reduce the overpotential by up to 173 mV.