Nanostructured Antireflective Iridium Oxide Coating for Water Oxidation

Abstract

Iridium oxide (IrO x ) is one of the best catalysts for the aqueous oxygen evolution reaction (OER), and its activity is greatly impacted by surface characteristics. By reactively sputtering in a high O 2 flow-rate environment, vertically oriented IrO x nanoplatelets grow several hundred nanometers high exhibiting large surface areas and antireflective optical properties across the visible spectrum. The nanoplatelet IrO x surface is electrochemically compared to other morphologies of IrO x surfaces for OER activity. It was found that the nanoplatelet IrO x surface outperforms all other tested morphologies and planar Ir-metal by exhibiting higher currents and lower overpotentials. Longevity testing of catalytic activity shows that the nanoplatelet surface is more stable in a wide pH range. Characterization with X-ray diffraction and X-ray photoelectron spectroscopy shows that the stoichiometry and oxidation states are similar between the different morphologies of IrO x , but the preferred crystallographic orientation of the rutile IrO x film changes at higher O 2 flow rates. The change from a (110) to a (101) growth direction corresponds with higher OER activity. Nanoplatelet IrO x films are therefore found to expose active sites preferable for the OER, and when combined with their antireflective properties these surfaces are promising for solar water-splitting applications.

Multiple Reaction Pathways for the Oxygen Evolution Reaction May Contribute to IrO2 (110)’s High Activity

“…[15][16][17] Theoretical studies of Ir activity are complicated by the well-known fact that at the sustained, high potentials of OER, Ir-based catalysts oxidize and often contain the rutile phase with various percentages of facet families. 15,[18][19][20] Pavlovic et al observed at potentials >1.2 V, Raman spectra of their hydrous iridium oxide samples begin to display peaks for the E g (550 cm −1 ) and B 2g modes (720 cm −1 ) specific to rutile iridium oxide; by 1.8 V, these peaks are strong and sharp. Since the lifetime of commercial proton exchange membrane electrolyzers is typically ∼5 years, operating at 1.8-2.0 V, some rutile iridium oxide is most likely present in the catalyst 20 so more complex views of what the active materials and facets are in the catalysts need to be considered.…”

Two Dimensional Ir‐Based Catalysts for Acidic OER

“…In 2018, David et al prepared an iridium oxide nanosheet. [60] The SEM images show the morphology of nanoplatelets with the thickness of 380 nm. The XRD pattern reveals its rutile phase structures.…”

Self-Supported Iridium Oxide Nanostructures for Electrocatalytic Water Oxidation in Acidic Media

“…47 Baker et al reported a IrO x nanoarray synthesized by reactive sputtering in an oxygen atmosphere. 48 However, both methods require specific equipment and complex procedures. The development of a more facile method is desired but is challenging.…”

“…For example, Berlinguette and co-workers prepared a self-supported amorphous IrO x film by the light-induced decomposition of iridium­(III) acetylacetonate . Baker et al reported a IrO x nanoarray synthesized by reactive sputtering in an oxygen atmosphere . However, both methods require specific equipment and complex procedures.…”