Spectroscopic investigation of the electrochemical growth and stability of am-hydr-IrOx grown from IrTi alloys

Abstract

Amorphous hydrous iridium oxide thin films (am-hydr-IrOx) grown through electrochemical treatment of sputtered iridium thin films are an interesting materials class for the oxygen evolution reaction (OER). They boast high electrochemical activity and high mass utilisation of the precious metal iridium, however, the application of am-hydr-IrOx is limited by poor stability caused by film delamination and electrochemically enhanced corrosion. This study presents a comprehensive investigation of the behaviour of am-hydr-IrOx grown from films of co-sputtered iridium with titanium (IrTi). The growth behaviour of IrTi films is investigated in detail by employing X-ray photoelectron spectroscopy which enables unique insights into the structure and mechanism of the resultant am-hydr-IrOx species. Depth-dependent hard X-ray photoelectron spectroscopy (HAXPES) experiments using different excitation energies show that the films separate into an iridium rich hydrous oxide layer and supported by unreacted metallic alloy during growth. Complementary, element specific operando X-ray absorption spectroscopy studies at the Ir L3- and Ti K-edges in combination with electrochemical techniques are further used to ascertain details of the growth mechanisms and develop a hypothesis to connect the electrocatalyst structure to its electrochemical stability under hydrolysis-relevant conditions. Titanium oxide formation at the surfaces acts to impede the formation of am-hydr-IrOx, and above a critical concentration, the presence of titanium results in an impressive stability enhancement.