Recently, IrV‐based perovskite‐like materials were proposed as oxygen evolution reaction (OER) catalysts in acidic media with promising performance. However, iridium dissolution and surface reconstruction were observed, questioning the real active sites on the surface of these catalysts. In this work, Sr2MIr(V)O6 (M=Fe, Co) and Sr2Fe0.5Ir0.5(V)O4 were explored as OER catalysts in acidic media. Their activities were observed to be roughly equal to those previously reported for La2LiIrO6 or Ba2PrIrO6. Coupling electrochemical measurements with iridium dissolution studies under chemical or electrochemical conditions, we show that the deposition of an IrOx layer on the surface of these perovskites is responsible for their OER activity. Furthermore, we experimentally reconstruct the iridium Pourbaix diagram, which will help guide future research in controlling the dissolution/precipitation equilibrium of iridium species for the design of better Ir‐based OER catalysts.

Recently,I r V -based perovskite-like materials were proposed as oxygen evolution reaction (OER) catalysts in acidic media with promising performance.H owever,i ridium dissolution and surface reconstruction were observed, questioning the real active sites on the surface of these catalysts.In this work, Sr 2 MIr (V) O 6 (M = Fe,Co) and Sr 2 Fe 0.5 Ir 0.5 (V) O 4 were explored as OER catalysts in acidic media. Their activities were observed to be roughly equal to those previously reported for La 2 LiIrO 6 or Ba 2 PrIrO 6 .C oupling electrochemical measurements with iridium dissolution studies under chemical or electrochemical conditions,w es howt hat the deposition of an IrO x layer on the surface of these perovskites is responsible for their OER activity.F urthermore,w ee xperimentally reconstruct the iridium Pourbaix diagram, whichw ill help guide future researchi nc ontrolling the dissolution/precipitation equilibrium of iridium species for the design of better Irbased OER catalysts.The electrochemical production of hydrogen fuel by water splitting has long been explored as ap otential way to store clean and renewable energy.T he key challenge for water splitting lies in improving the efficiency of the kinetically slow, rate-limiting oxygen evolution reaction (OER) (2 H 2 O! 4H + + O 2 + 4e À ). [1,2] While alarge variety of transition metal oxides have been studied as promising OER catalysts in alkaline media, [3][4][5][6] thedesign of active and stable catalysts in acidic media has proven challenging. [7][8][9][10][11] Thef ew materials established as suitable OER catalysts in acidic media are mostly Ir-based metal oxides,such as rutile IrO 2 , [12,13] 14] or more recently Ni-substituted IrO x . [15][16][17] Recently,Ir-based perovskites have been reported as promising candidates in acidic media. [18][19][20] Thehigh OER activity of these Ir-based perovskites was ascribed to the formation of electrophilic O (IÀ) surface species favoring the nucleophilic attack of water. [11,20,21] However,dissolution of iridium, alkali and/or rare earth elements in acidic electrolytes has been observed after close examinations of some Ir V -based perovskites,such as La 2 LiIr (V) O 6[20] and Ba 2 PrIr (V) O 6 , [7,19] indicating their drastic structural instabilities in harsh acidic conditions. [12] Furthermore,the presence of IrO 2 nanoparticles was revealed on the surface of La 2 LiIr (V) O 6 after cycling. [8] Given this array of observations,alegitimate question arises over the origin of the OER activity on the surface of these perovskites.Therefore,itisimportant to understand the OER mechanism of these Ir V -based perovskites,aswell as the effect of iridium dissolution on their catalytic behaviors,inorder to improve their performances and/or guide future research in designing new active and stable OER catalysts in acidic media.We herein investigate the catalytic behaviors of three Ir Vbased OER catalysts in acidic media, Sr 2 MIr (V) O 6 (M = Fe, Co) with ad ouble perovskite structure and Sr 2 Fe 0.5 Ir 0.5 (V) O 4 with a...