In proton exchange membrane water electrolyzers (PEMWE), the oxygen evolution reaction (OER) is considered the limiting process in water splitting. To date, iridium has been recognized as having the best OER performance in acidic conditions when regarding both, activity and stability. This instigated study of various Ir-based materials using different techniques, but some questions about the mechanisms are still debated [1]. Interesting insights into the redox Ir(III)/Ir(IV) reaction and the OER process can also be gained via Raman spectroscopy [2,3], and we apply this technique in our work using an ex situ [4] and in situ approach.

Since Ir is a scarce metal, its amount in electrocatalysis is reduced by the preparation of dispersed Ir NPs on different supports. When Raman spectroscopy of iridium has been reported in the literature, it has been recorded on metal foil [2], electrochemically deposited IrO_x film on GCE [3] or drop-casted samples on GCE [4], i.e. more bulk samples. The question arises whether it is possible to perform Raman measurements directly on supported Ir NPs. We made these measurements ex situ and showed that the formation of iridium oxide can be detected for degraded states. However, the relatively low loading of Ir NPs causes the low intensity of the iridium oxide bands (E_g, B_2g and A_1g vibrations). The two supports that are used in this investigation were carbon and TiO₂ (i.e. P25).

Consequently, we continued to investigate the performance of two commercial Ir-based compounds, Ir nanoparticles (Ir NPs) and rutile IrO₂, and the measurements were made on unsupported samples. Both compounds were drop-casted from suspensions on glassy carbon electrode (GCE) and activated in a 0.1 M HClO₄ electrolyte. The Raman spectrum of GCE/IrO₂ consists of three active modes at 557 cm^-1 (E_g), 728 cm^-1 (B_2g) and 746 cm^-1 (A_1g), while the fourth low-intensity B_1g mode at 145 cm^-1 [2-4] could not be identified. In the GCE/Ir NPs sample, the E_g mode appears at 552 cm^-1, while the B_2g and A_1g modes appear as an overlapping band at 724 cm^-1. Such spectrum indicates that the Ir NPs oxidize in air and the amorphous Ir-oxide forms on the surface. During the in situ Raman measurements of the GCE/Ir NPs a composed, broad band feature evolves. The previous works were carried out in different electrolytes, potential ranges and conditions [2,3] which makes it difficult to compare the results. We consequently decided to make a systematic study in three different potential ranges: 0.05 to 1.45 V_RHE , 0.05 to 1.6 V_RHE, and 1.1 to 1.6 V_RHE and in the 0.1 M HClO₄ electrolyte, which has not been used for in situ Raman spectroelectrochemical measurements before. The evolved broad bands are explored after initial, soaked and activated states. In addition, the possible perchlorate adsorption [4] at the electrode is considered.