Reducing noble metal loading and increasing specific activity of oxygen evolution catalysts are omnipresent challenges in proton exchange membrane (PEM) water electrolysis, which have recently been tackled by utilizing mixed oxides of noble and non-noble elements (e.g. perovskites, IrNiO x , etc.). However, proper verification of the stability of these materials is still pending. In this work dissolution processes of various iridium-based oxides are explored by introducing a new metric, defined as the ratio between amount of evolved oxygen and dissolved iridium. The so called Stability-number is independent of loading, surface area or involved active sites and thus, provides a reasonable comparison of diverse materials with respect to stability. Furthermore it can support the clarification of dissolution mechanisms and the estimation of a catalyst's lifetime. The case study on iridium-based perovskites shows that leaching of the non-noble elements in mixed oxides leads to formation of highly active amorphous iridium oxide, the instability of which is explained by participation of activated oxygen atoms, generating short-lived vacancies that favour dissolution. These insights are considered to guide further research which should be devoted to increasing utilization of pure crystalline iridium oxide, as it is the only known structure that guarantees a high durability in acidic conditions. In case amorphous iridium oxides are used, solutions for stabilization are needed.

The investigation of nickel phosphide (Ni5 P4 ) as a catalyst for the hydrogen (HER) and oxygen evolution reaction (OER) in strong acidic and alkaline environment is described. The catalyst can be grown in a 3D hierarchical structure directly on a nickel substrate, thus making it an ideal candidate for practical water splitting devices. The activity of the catalyst towards the HER, together with its high stability especially in acidic solution, makes it one of the best non-noble materials described to date. Furthermore, Ni5 P4 was investigated in the OER and showed activity superior to pristine nickel or platinum. The practical relevance of Ni5 P4 as a bifunctional catalyst for the overall water splitting reaction was demonstrated, with 10 mA cm(-2) achieved below 1.7 V.

The investigation of nickel phosphide (Ni 5 P 4 )a s ac atalyst for the hydrogen (HER) and oxygen evolution reaction (OER) in strong acidic and alkaline environment is described. The catalyst can be grown in a3 Dh ierarchical structure directly on anickel substrate,thus making it an ideal candidate for practical water splitting devices.T he activity of the catalyst towards the HER, together with its high stability especially in acidic solution, makes it one of the best non-noble materials described to date.F urthermore,N i 5 P 4 was investigated in the OER and showed activity superior to pristine nickel or platinum. The practical relevance of Ni 5 P 4 as ab ifunctional catalyst for the overall water splitting reaction was demonstrated, with 10 mA cm À2 achieved below1 .7 V.The availability of peak excess electricity from wind and solar energy makes temporal storage in high-energy chemicals am andatory task for chemistry.H ere,t he focus lies especially on hydrogen which can be "easily" electrolyzed from water, then effectively reconverted into electricity with an umber of available devices. [1] Although electrolysis of water into hydrogen and oxygen is considered to be one of the easiest and cleanest methods to obtain hydrogen, this reaction is far from optimized. Currently,t he reaction still requires high overpotentials for both the hydrogen (HER) and oxygen evolution reaction (OER) to obtain decent reaction rates.For example,a round 50 %( 1.8-2 Vi nstead of 1.23 V) excess potential is required in industrial electrolyzer cells,w hich account for less than 5% of the world production of hydrogen. [2] This excess potential already represents an energy penalty of at least 35 %i nt he first conversion step, which makes it less attractive for an energy storage scheme.In addition, common electrolyzers are based on rare noble metals,s uch as Pt alloys for hydrogen evolution and IrO 2 / RuO 2 for oxygen evolution, and ab roader distribution of such devices stays rather questionable.

The reduction in noble metal content for efficient oxygen evolution catalysis is a crucial aspect towards the large scale commercialisation of polymer electrolyte membrane electrolyzers. Since catalytic stability and activity are inversely related, long service lifetime still demands large amounts of low-abundant and expensive iridium. In this manuscript we elaborate on the concept of maximizing the utilisation of iridium for the oxygen evolution reaction. By combining different tin oxide based support materials with liquid atomic layer deposition of iridium oxide, new possibilities are opened up to grow thin layers of iridium oxide with tuneable noble metal amounts. In-situ, time-and potential-resolved dissolution experiments reveal how the stability of the substrate and the catalyst layer thickness directly affect the activity and stability of deposited iridium oxide. Based on our results, we elaborate on strategies how to obtain stable and active catalysts with maximized iridium utilisation for the oxygen evolution reaction and demonstrate how the activity and durability can be tailored correspondingly. Our results highlight the potential of utilizing thin noble metal films with earth abundant support materials for future catalytic applications in the energy sector.