The origin of the high electrochemical activity of pseudo-amorphous iridium oxides

The origins of the superior catalytic activity of poorly crystallized Ir-based oxide material for the OER in acid is still under debate. Here, authors synthesize porous IrMo oxides to deconvolute the effect of Ir oxidation state from short-range ordering and show the latter to be a key factor

Nouvelles stratégies d’élaboration de matériaux poreux à base d’iridium pour la réaction de dégagement de dioxygène

Environmental concerns and increase in energy consumption require to develop alternatives to fossil fuels. In this regard, the dihydrogen is considered as the energy vector of the future. Water electrolysis is a promising technology to produce “green hydrogen”. However, decreasing the amount of noble metal needed for catalytic reactions, and particularly the iridium content in proton exchange membrane water electrolyzers, remains a critical challenge to address to allow for large scale development of this technology. Despite its unique and great stability and activity in anodic conditions, and because Ir is rare and expensive, iridium oxide strongly impacts the cost of electrolyzers. Besides decreasing the cost of the technology by lowering Ir loadings, developing catalysts with higher catalytic activity would limit the overpotential for the oxygen evolution reaction (OER) and drastically diminish H2 production cost. The objective of this work is then to decrease the iridium loading in PEM electrolyzers while maintaining their performance. To do so, iridium oxide based catalysts with a hierarchically porous structure have been synthesized by aerosol process and colloidal synthesis. In the case of iridium-molybdenum based catalysts, we have shown that small crystallized particles are responsible for the high activity toward the OER of iridium oxides calcined at low temperature. Combining in situ and ex situ studies has allowed us to propose a mechanism for the formation of iridium oxide nanocages from a copper oxide template. In general, this work provides a better understanding of the formation mechanisms and the structure-catalytic properties relationship of iridium based materials.La préservation de l’environnement et les besoins énergétiques croissants nécessitent le développement rapide d’alternatives aux énergies fossiles. Dans ce contexte, le dihydrogène est considéré comme le vecteur énergétique du futur. L’électrolyse de l’eau est une des voies les plus prometteuses pour produire de « l’hydrogène vert ». Parmi les défis à relever dans les technologies d’électrolyse, un enjeu important est la diminution de la charge en métaux nobles dans les couches catalytiques, et notamment de l’iridium dans les anodes des électrolyseurs à membrane échangeuse de proton (PEM). L’oxyde d’iridium est le seul catalyseur stable et actif dans les conditions anodiques d’un électrolyseur PEM, cependant, Ir est un élément rare et cher et son utilisation impacte le coût du système. Outre la diminution du coût d’investissement, augmenter l’activité catalytique des catalyseurs anodiques permettrait également de limiter la surtension de la réaction d’évolution de l’oxygène (OER) et de directement diminuer le coût de production de H2. L’objectif de ces travaux de thèse est de réduire la quantité d’iridium nécessaire dans les électrolyseurs tout en maintenant leurs performances. Pour cela, des oxydes à base d’iridium, à structure hiérarchique hautement poreuse, ont été synthétisés par processus aérosol et par voie colloïdale. Dans le cas des matériaux à base d’iridium-molybdène, nous avons montré que la présence de petites particules cristallisées pourrait expliquer l’activité vis-à-vis de l’OER des oxydes d’iridium calcinés à basse température. La combinaison d’études ex situ et in situ nous a permis de proposer un mécanisme pour la formation de nanocages d’oxyde d’iridium à partir de cubes d’oxyde cuivreux. Dans les deux cas, ces études ont permis une meilleure compréhension des mécanismes de formation et des relations structures-propriétés catalytiques des matériaux à base d’oxyde d’iridium

New strategies for the elaboration of porous materials based on iridium towards the oxygen evolution reaction

La préservation de l’environnement et les besoins énergétiques croissants nécessitent le développement rapide d’alternatives aux énergies fossiles. Dans ce contexte, le dihydrogène est considéré comme le vecteur énergétique du futur. L’électrolyse de l’eau est une des voies les plus prometteuses pour produire de « l’hydrogène vert ». Parmi les défis à relever dans les technologies d’électrolyse, un enjeu important est la diminution de la charge en métaux nobles dans les couches catalytiques, et notamment de l’iridium dans les anodes des électrolyseurs à membrane échangeuse de proton (PEM). L’oxyde d’iridium est le seul catalyseur stable et actif dans les conditions anodiques d’un électrolyseur PEM, cependant, Ir est un élément rare et cher et son utilisation impacte le coût du système. Outre la diminution du coût d’investissement, augmenter l’activité catalytique des catalyseurs anodiques permettrait également de limiter la surtension de la réaction d’évolution de l’oxygène (OER) et de directement diminuer le coût de production de H2. L’objectif de ces travaux de thèse est de réduire la quantité d’iridium nécessaire dans les électrolyseurs tout en maintenant leurs performances. Pour cela, des oxydes à base d’iridium, à structure hiérarchique hautement poreuse, ont été synthétisés par processus aérosol et par voie colloïdale. Dans le cas des matériaux à base d’iridium-molybdène, nous avons montré que la présence de petites particules cristallisées pourrait expliquer l’activité vis-à-vis de l’OER des oxydes d’iridium calcinés à basse température. La combinaison d’études ex situ et in situ nous a permis de proposer un mécanisme pour la formation de nanocages d’oxyde d’iridium à partir de cubes d’oxyde cuivreux. Dans les deux cas, ces études ont permis une meilleure compréhension des mécanismes de formation et des relations structures-propriétés catalytiques des matériaux à base d’oxyde d’iridium.Environmental concerns and increase in energy consumption require to develop alternatives to fossil fuels. In this regard, the dihydrogen is considered as the energy vector of the future. Water electrolysis is a promising technology to produce “green hydrogen”. However, decreasing the amount of noble metal needed for catalytic reactions, and particularly the iridium content in proton exchange membrane water electrolyzers, remains a critical challenge to address to allow for large scale development of this technology. Despite its unique and great stability and activity in anodic conditions, and because Ir is rare and expensive, iridium oxide strongly impacts the cost of electrolyzers. Besides decreasing the cost of the technology by lowering Ir loadings, developing catalysts with higher catalytic activity would limit the overpotential for the oxygen evolution reaction (OER) and drastically diminish H2 production cost. The objective of this work is then to decrease the iridium loading in PEM electrolyzers while maintaining their performance. To do so, iridium oxide based catalysts with a hierarchically porous structure have been synthesized by aerosol process and colloidal synthesis. In the case of iridium-molybdenum based catalysts, we have shown that small crystallized particles are responsible for the high activity toward the OER of iridium oxides calcined at low temperature. Combining in situ and ex situ studies has allowed us to propose a mechanism for the formation of iridium oxide nanocages from a copper oxide template. In general, this work provides a better understanding of the formation mechanisms and the structure-catalytic properties relationship of iridium based materials

Oxygen evolution reaction on iridium-molybdenum mixed oxide electrocatalysts

International audienc

Electrochemical study of iridum oxide materials for the oxygen evolution reaction in proton exchange membrane water electrolyzers (PEMWE)

International audienceProton Exchange Membrane Water Electrolyzers (PEMWE) is a promising hydrogen production technology due to its high efficiency and compact design that will make hydrogen energy more available [1]. However, the electrolyzer performance is impeded by the catalyst at the anode for the Oxygen Evolution Reaction (OER), and this problematic motivates the search for new catalytic materials. The latter should possess high catalytic activity and long-term stability as state of art materials, RuO2 and IrO2 [2]. Also, diminishing the iridium loading in PEMWE is crucial, however, for its ability to combine electrical conductivity, activity, and stability, Ir-based materials are still attractive as anode material [3]. The present work concerns the electrochemical study of iridium oxide materials prepared by Coordinating Etching Precipitating (CEP) and by spray-drying process, the latter been recently investigated for catalysts for PEMWE [3]. The mixed-metal oxide materials obtained by both methods were subjected to different thermal treatments in the range 400–800 ºC. The electrochemical characterization was determine using the three-electrode electrochemical setup. Samples heated between 400–500 ºC presented the best performance as compared to commercial IrO2, showing at 10 mA/cm2 an overpotential range of 140-160 mV. These results demonstrate the possible use of the synthesis techniques to fabricate porous noble-metal oxides or catalysts for PEMWEs

Electrochemical study of iridium oxide materials for the oxygen evolution reaction in proton exchange membrane water electrolyzers

International audienceProton Exchange Membrane Water Electrolyzers (PEMWE) is a promising hydrogen production technology due to its high efficiency and compact design that will make hydrogen energy more available [1]. However, the electrolyzer performance is impeded by the catalyst at the anode for the Oxygen Evolution Reaction (OER), and this problematic motivates the search for new catalytic materials. The latter should possess high catalytic activity and long-term stability as state of art materials, RuO2 and IrO2 [2]. Also, diminishing the iridium loading in PEMWE is crucial, however, for its ability to combine electrical conductivity, activity, and stability, Ir-based materials are still attractive as anode material [3]. The present work concerns the electrochemical study of iridium oxide materials prepared by Coordinating Etching Precipitating (CEP) and by spray-drying process, the latter been recently investigated for catalysts for PEMWE [3]. The mixed-metal oxide materials obtained by both methods were subjected to different thermal treatments in the range 400–800 ºC. The electrochemical characterization was determine using the three-electrode electrochemical setup. Samples heated between 400–500 ºC presented the best performance as compared to commercial IrO2, showing at 10 mA/cm2 an overpotential range of 140-160 mV. These results demonstrate the possible use of the synthesis techniques to fabricate porous noble-metal oxides or catalysts for PEMWEs.[1] S. Pei, W. Chao, J. San, S. Xueliang, Z. Jiu, Electrochemical Energy., Florida, CRC Press, 2016. [2] E. Antolini, ACS Catal. 4 (2014), 1426.[3] M. Faustini, M. Giraud, D. Jones, J. Rozière, M. Dupont, T. R. Porter, S. Nowak, M. Bahri, O. Ersen, C. Sanchez, C. Boissière, C. Tard, J. Peron, Adv. Energy Mater., 9 (2019), 1-11

Nanodiffusion in molecular and metal electrocatalytic films

International audienc

Electrochemical Active Surface Area Determination of Iridium‐Based Mixed Oxides by Mercury Underpotential Deposition

International audienceThe electrochemical surface area (ECSA) is a critical property to describe, analyze and compare electrocatalysts. The determination of the mass activity of a given catalyst is associated with this parameter which can thus lead to materials benchmarking. Reliable and robust methods to measure ECSA are needed, and those have to accommodate different structures, morphologies and compositions. In this study, we investigate mercury underpotential deposition (HgUPD) as a way to estimate ECSA for ultraporous electrocatalysts based on iridium and iridium-molybdenum electrocatalysts for the oxygen evolution reaction. Results reveal a clear agreement between physisorption measurements and HgUPD with excellent reproducibility. The method shows also that pre- and post-catalysis surface area measurements are not affected by the catalytic process on short timescale, opening the possibility of electrocalyst stability and degradation monitoring

Green Synthesis of Water Splitting Electrocatalysts: IrO2 Nanocages via Pearson's Chemistry

International audienceHighly porous iridium oxide structures are particularly well-suited for the preparation of porous catalyst layers needed in proton exchange membrane water electrolyzers. Herein, we report the formation of iridium oxide nanostructured cages, via a water-based process performed at room temperature, using cheap Cu2O cubes as template. In this synthetic approach, based on Pearson's hard and soft acid-base theory, the replacement of the Cu2O core by an iridium shell is permitted by the difference in hardness/softness of cations and anions of the two reactants Cu2O and IrCl3. Calcination followed by acid leaching allow the removal of residual copper oxide cores and leave IrO2 hierarchical porous structures with outstanding activity toward the oxygen evolution reaction. Fundamental understanding of the reaction steps and identification of the intermediates are permitted by coupling a set of ex situ and in situ techniques including operando time-resolved X-ray absorption spectroscopy during the synthesis