Proton exchange membrane (PEM) electrolysis is considered one of the most promising technologies in the transition to more sustainable society. One of the major drawbacks of the technology are high overpotential losses due to sluggish kinetics of the oxygen evolution reaction (OER) on the anode side of the electrolyzer. The state-of-the-art catalysts for the reaction are iridium and its oxides, as they display the best ratio between activity and stability. The main problem, however, is that the world's iridium reserves are extremely small and must therefore be used optimally. In the process of development of new electro-catalytic materials, rotating disc electrode (RDE) method is usually utilized to measure these two parameters. In this thesis I tested the influence of three parameters on the measured activity of the commercial catalyst Ir Black: the effect of catalyst loading, effect of the backing electrode material and the effect of activation protocol. With this method I then measured the activity of two synthesised materials, Ir/TiONx/C and Ir/CuTiONx/C where iridium nanoparticles are dispersed on conductive TiONx support with a large surface area. The measured activities are three or even six times higher than the activity of Ir Black. Their stability was tested using a scanning flow cell, coupled to an inductively coupled plasma mass spectrometer (SFC-ICP-MS) with which we were able to measure the concentration of dissolved iridium in the electrolyte during the electrochemical experiment. From the amount of dissolved iridium we calculated the ratio between moles of evolved oxygen and dissolved iridium (S-numbers), which represent the metric for catalyst stability. Based on the S-numbers, the stability of the catalyst can be compared to other iridium materials. Although Ir/CuTiONx/C is more active, stability of both samples is the same.