Progress of Heterogeneous Iridium-Based Water Oxidation Catalysts

Abstract

The water oxidation reaction (or oxygen evolution reaction, OER) plays a critical role in green hydrogen production via water splitting, electrochemical CO 2 reduction, and nitrogen fixation. The four-electron and four-proton transfer OER process involves multiple reaction intermediates and elementary steps that lead to sluggish kinetics; therefore, a high overpotential is necessary to drive the reaction. Among the different water-splitting electrolyzers, the proton exchange membrane type electrolyzer has greater advantages, but its anode catalysts are limited to iridium-based materials. The iridium catalyst has been extensively studied in recent years due to its balanced activity and stability for acidic OER, and many exciting signs of progress have been made. In this review, the surface and bulk Pourbaix diagrams of iridium species in an aqueous solution are introduced. The iridium-based catalysts, including metallic or oxides, amorphous or crystalline, single crystals, atomically dispersed or nanostructured, and iridium compounds for OER, are then elaborated. The latest progress of active sites, reaction intermediates, reaction kinetics, and elementary steps is summarized. Finally, future research directions regarding iridium catalysts for acidic OER are discussed.

In Situ Activation Endows Orthorhombic Fluorite-Type Samarium Iridium Oxide with Enhanced Acidic Water Oxidation

“…For instance, as-prepared Sm 3 IrO 7 shows an overpotential of 302 mV at a current density of 10 mA cm –2 , which is 18 and 39 mV less than those of commercial IrO 2 and Ir black, respectively, even with a much lower content of Ir in Sm 3 IrO 7 . To better compare the electrocatalytic activity of as-prepared Sm 3 IrO 7 and commercial IrO 2 and Ir black, we have further analyzed their mass activity, which plays a critical role in practical applications. ,, At a 300 mV overpotential (1.53 V vs RHE), the mass activity of as-prepared Sm 3 IrO 7 is as high as 307 A g Ir –1 , which is 26 and 47 times higher than those of commercial IrO 2 and Ir black, respectively (Figure c). As shown in Figure d, the kinetic behaviors of the as-prepared Sm 3 IrO 7 , commercial IrO 2 , and Ir black are further investigated.…”

A brief introduction of electrode fabrication for proton exchange membrane water electrolyzers

“…In addition, another way to beneficially modify the architecture of the catalyst is by synthesizing core-shell catalysts with unique physical and chemical properties. Several advantages can be obtained from a core-shell structure, including introduction of interfacial strain that tunes the electronic structure, modified atomic vicinity that affects the charge transfer, increased surface area, providing protection to the unstable core by isolating it from the external environment, and facilitating the full utilization of the surface material [166,167]. On this ground, the core-shell structure is expected to effectively regulate the intrinsic catalytic activity and stability of the resultant material, simultaneously lowering the noble metal loading.…”

2D layered materials: structures, synthesis, and electrocatalytic applications

“…Traditionally, iridium oxides and ruthenium oxides do not exist in layered crystal phases. 84 Recently, researchers have turned attention to layered iridates and ruthenates for acidic OER, including Sr 2 IrO 4 , K 0.75 Na 0.25 IrO 2 , and NaRuO 2 . 85 The intercalation ions can be exchanged with acid to obtain protonated iridates/ruthenates and then layered iridium oxide and ruthenium oxide through further exfolia-tion.…”