Water electrolysis plays a crucial role in the energy transition from carbonized to green economy. Electrochemical water splitting comprises two half reactions: hydrogen evolution reaction (HER; in acid: 4H⁺ + 4e⁻ → 2H₂) and oxygen evolution reaction (OER; in acid: 2H₂O → O₂ + 4H⁺ + 4e⁻). The energy created by renewables during low energy demand periods can be used to generate H₂ by this reaction which could be used for future re-electrification in highly power demand periods. Electrolyzers are a key electrocatalytic technology for this purpose. In these devices, OER occurs in the anode while HER occurs in the cathode. Thermodynamically, 1.23V should be applied in this reaction between the anode and cathode. However, the process is under kinetic control, so potential must be driven away from the equilibrium. The value of the overpotential depends mainly on the nature of the electrocatalysts at which the reaction takes place. OER results in higher overpotentials than HER, therefore, more efforts are focused on the development of catalysts for the OER. The most active electrocatalysts for OER are based on Ir and Ru oxides, which are very expensive and scarce. In order to decrease their loading, mixed oxides have been studied as an alternative. Among others, perovskites with low content of precious metals have been reported to achieve high OER activity, but usually poor durability.

In this work we study the catalytic performance of pyrochlore oxides with general formula R₂Mn_2-xRu_xO₇ where R is a rare-earth metal. Pure R₂Mn_2-xRu_xO₇ (R = Y, Tb, Dy and Ho; x from 2 to 0) oxides were prepared by citrate method applying thermal treatments at 900ºC under pure oxygen (either at atmospheric pressure or at 200 bar, depending on R). The catalysts have thoroughly characterized by several techniques including XRD, XPS, IL-TEM and electrochemical techniques. The OER performance has been evaluated using the RRDE approach in 0.1 M HClO₄. The obtained pyrochlores display very high OER activity, especially Y₂MnRuO₇, recording a potential of 1.557 V to achieve 10 mA/cm²_geom, which is among the lowest values reported in the literature. In addition, the catalyst displays exceptional high durability, with only a minor activity loss after 2000 cycles in the OER, which is a remarkable observation in view that other Ru oxides such as SrRuO₃ only last 2 cycles and up to 50 cycles for Sr_1-xNa_xRuO₃.