Effect of the Morphology of the High-Surface-Area Support on the Performance of the Oxygen-Evolution Reaction for Iridium Nanoparticles.

Leonard Moriau,Luka Suhadolnik,Marjan Bele,Martin Šala,Francisco Ruiz-Zepeda,Gorazd Koderman Podboršek,Nejc Hodnik,Primož Jovanovič,Iztok Arčon,Živa Marinko,Angelja Kjara Šurca,Janez Kovač

doi:10.1021/acscatal.0c04741

Abstract

The development of affordable, low-iridium-loading, scalable, active, and stable catalysts for the oxygen-evolution reaction (OER) is a requirement for the commercialization of proton-exchange membrane water electrolyzers (PEMWEs). However, the synthesis of high-performance OER catalysts with minimal use of the rare and expensive element Ir is very challenging and requires the identification of electrically conductive and stable high-surface-area support materials. We developed a synthesis procedure for the production of large quantities of a nanocomposite powder containing titanium oxynitride (TiONx) and Ir. The catalysts were synthesized with an anodic oxidation process followed by detachment, milling, thermal treatment, and the deposition of Ir nanoparticles. The anodization time was varied to grow three different types of nanotubular structures exhibiting different lengths and wall thicknesses and thus a variety of properties. A comparison of milled samples with different degrees of nanotubular clustering and morphology retention, but with identical chemical compositions and Ir nanoparticle size distributions and dispersions, revealed that the nanotubular support morphology is the determining factor governing the catalyst’s OER activity and stability. Our study is supported by various state-of-the-art materials’ characterization techniques, like X-ray photoelectron spectroscopy, scanning and transmission electron microscopies, X-ray powder diffraction and absorption spectroscopy, and electrochemical cyclic voltammetry. Anodic oxidation proved to be a very suitable way to produce high-surface-area powder-type catalysts as the produced material greatly outperformed the IrO2 benchmarks as well as the Ir-supported samples on morphologically different TiONx from previous studies. The highest activity was achieved for the sample prepared with 3 h of anodization, which had the most appropriate morphology for the effective removal of oxygen bubbles.

Highlights

The hydrogen cycle is a promising way to store energy from renewable sources, like solar and wind, through the splitting of water in an electrolyzer
Radio frequency (RF) plasma can be used for the oxynitridation of titanium;[22] the mechanical milling of titanium powder in air results in the formation of titanium oxynitride;[23] and low-pressure chemical vapor deposition (LPCVD) can be used to deposit titanium oxynitride on silicon substrates.[24]
We show that the electrochemical activity and stability can be substantially enhanced, compared to the commercial benchmark IrO2 material and to the other best oxygen-evolution reaction (OER) catalysts in the literature, exclusively by modifying the TiONx support morphology

Summary

INTRODUCTION

The hydrogen cycle is a promising way to store energy from renewable sources, like solar and wind, through the splitting of water in an electrolyzer. Radio frequency (RF) plasma can be used for the oxynitridation of titanium;[22] the mechanical milling of titanium powder in air results in the formation of titanium oxynitride;[23] and low-pressure chemical vapor deposition (LPCVD) can be used to deposit titanium oxynitride on silicon substrates.[24] Of the possible synthesis processes, one of the most direct and controllable is to synthesize TiO2 nanostructures, which are thermally treated in a reductive ammonia atmosphere (above 600 °C) to transform the TiO2 to TiONx.[25] The anodic oxidation of titanium is commonly used for the synthesis of immobilized nanotubular films.[26] It results in TiO2 nanotube arrays that are firmly adhered to the titanium substrate[27] and has been used to fabricate free-standing TiO2 membranes[28] and TiO2 nanotube powders.[29] Before the prepared TiO2 nanotube powders can be used as a support for Ir nanoparticles, their electrical conductivity has to be substantially increased This can be achieved with a thermal treatment in an ammonia atmosphere. We show that the electrochemical activity and stability can be substantially enhanced, compared to the commercial benchmark IrO2 material and to the other best OER catalysts in the literature, exclusively by modifying the TiONx support morphology

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES