IrOx core-shell nanocatalysts for cost- and energy-efficient electrochemical water splitting

A family of dealloyed metal–oxide hybrid (M1M2@M1Ox) core@shell nanoparticle catalysts is demonstrated to provide substantial advances toward more efficient and less expensive electrolytic water splitting. IrNi@IrOx nanoparticles were synthesized from IrNix precursor alloys through selective surface Ni dealloying and controlled surface oxidation of Ir. Detailed depth-resolved insight into chemical structure, composition, morphology, and oxidation state was obtained using spectroscopic, diffraction, and scanning microscopic techniques (XANES, XRD, STEM-EDX, XPS), which confirmed our structural hypotheses at the outset. A 3-fold catalytic activity enhancement for the electrochemical oxygen evolution reaction (OER) over IrO2 and RuO2 benchmark catalysts was observed for the core-shell catalysts on a noble metal mass basis. Also, the active site-based intrinsic turnover frequency (TOF) was greatly enhanced for the most active IrNi@IrOx catalyst. This study documents the successful use of synthetic dealloying for the preparation of metal-oxide hybrid core-shell catalysts. The concept is quite general, can be applied to other noble metal nanoparticles, and points out a path forward to nanostructured proton-exchange-electrolyzer electrodes with dramatically reduced noble metal content.DFG, STR 596/3-1, Nanostructured mixed metal oxides for the electrocatalytic oxidation of waterBMBF, 03SF0433A, Verbundvorhaben MEOKATS: Effiziente edelmetallfreie Katalysatorsysteme basierend auf Mangan und Eisen für flexible Meerwasserelektrolyseur

Experimental Activity Descriptors for Iridium-based Catalysts for the Electrochemical Oxygen Evolution Reaction (OER)

Recent progress in the activity improvement of anode catalysts for acidic electrochemical water splitting was largely achieved through empirical studies of iridium-based bimetallic oxides. However, practical, experimentally accessible, yet general predictors of catalytic OER activity are lacking. This study investigates iridium and iridium-nickel thin film model electrocatalysts for the OER and identifies a set of general ex situ properties that allow the reliable prediction of their OER activity. Well defined Ir-based catalysts of various chemical nature and composition were synthesized by magnetron sput-tering. Correlation of physico-chemical and electrocatalytic properties revealed two experimental OER activity descriptors that are able to predict trends in the OER activity of unknown Ir based catalyst systems. More specifically, our study demonstrates that the IrIII+ and OH-surface concentration of the oxide catalyst constitute closely correlated, and generally applicable OER activity predictors. Based on these, an experimental volcano relationship of Ir-based OER electrocatalysts is presented and discussed

Synthese und Charakterisierung von Kern-Schale-Metalloxid-Nanopartikel für effiziente elektrochemische Wasserspaltung

Electrocatalytic water splitting is expected to emerge as a key technology for the storage of excess electricity from renewable sources in form of hydrogen or for production of sustainable hydrogen fuel as part of a solar refinery. In particular, polymer electrolyte membrane (PEM) electrolyzers, owing to their high voltage efficiency, high current density, good partial loading range, compact system design, high gas purity and rapid system response, will likely play a critically important role to store excess electricity in form of hydrogen. However, high catalyst cost and limited catalyst durability of PEM electrolyzer electrodes, especially at the anode side, where the oxygen evolution reaction (OER) takes place, have long hindered a broader dissemination of PEM electrolyzers. Typical OER electrocatalysts for PEM electrolyzers make use of RuOx and/or IrOx as catalytically active components. In order to reduce the catalyst costs, non-noble metal oxides, such as SnO2, Sb2O5, Ta2O5, Nb2O5, TiO2 and SiO2, have been mixed with IrO2, allowing the reduction of the noble metal content without a significant decrease in the catalytic activity and electronic conductivity. However, the noble metal content in these types of OER catalysts still remains very high. The overarching goal of this thesis is the rational design of oxide-supported IrNix nanoparticle catalysts with drastically reduced noble metal loadings for the electrocatalytic oxidation of water in acidic environments. In order to achieve this goal, different strategies were pursued: i) application of corrosion-resistant high surface-area supports, ii) design of IrM@IrOx metal-oxide hybrid core@shell nanoparticle architectures, where M represents a non-noble metal that helps to tune the intrinsic electrocatalytic activity and lower the noble metal content alike. In this work, the synthetic conditions were optimized in order to prepare homogenous IrNix nano alloys with controlled composition as intermediate state for the synthesis of IrNi@IrOx architectures. The IrNix and IrNi@IrOx nanoparticles were comprehensively studied using a wide range of characterization techniques including powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning TEM combined with energy dispersive X-ray spectroscopy (EDX), depth-resolved X-ray photoelectron spectroscopy (XPS), and high energy XRD coupled with atomic pair distribution functions (PDFs). The IrNi@IrOx core-shell nanoparticles showed a ~10x higher Ir-mass based activity at 250 mV overpotential compared to a rutile-type IrO2 nanoparticle benchmark catalyst. Moreover, oxide supported IrNi@IrOx nanoparticles provided an excellent stability under OER conditions demonstrated by a negligible degradation during a 20 hour stability test unlike various other Ir materials. Correlating the structural characteristics of the IrNi@IrOx core-shell nanoparticles with their OER activity it is proposed that the lattice contraction is responsible for the exceptional reactivity.Die elektrokatalytische Wasserspaltung hat ein großes Potential für die Speicherung überschüssigen Stroms aus erneuerbaren Quellen in Form von Wasserstoff und kann so für die Produktion großer Wasserstoffmengen als Teil einer nachhaltigen Versorgung mit Treibstoffen dienen. Die vielversprechendste Technologie in diesem Zusammenhang ist der Polymer Elektrolyt Membran (PEM) Elektrolyseur, da sie eine hohe Spannungseffizienz aufweisen, große Stromdichten erlauben, einen großen Teillastbereich abdecken, eine kompakte Bauform besitzen, hohe Gasreinheiten liefern und über kurze Ansprechzeiten verfügen. Allerdings schränken zurzeit noch hohe Kosten und eine beschränkte Lebensdauer im Bereich der Katalysatoren die großflächige Verbreitung dieser Technologie ein. Dies gilt insbesondere für denKatalysator an der Anode, an dem die Sauerstoffevolutionsreaktion (engl.: Oxygen Evolution Reaction-OER) abläuft. Als aktive Komponenten für den Katalysator an der Anode werden üblicherweise RuOx und IrOx eingesetzt. Zur Senkung der Kosten wird dabei IrO2 teilweise durch SnO2, Sb2O5, Ta2O5, Nb2O5, TiO2 oder SiO2 substituiert, wobei die katalytische OER Aktivität und die elektrische Leitfähigkeit des Katalysators nicht signifikant verringert werden. Nichtsdestotrotz ist der Edelmetallanteil immer noch vergleichsweise groß. Das übergeordnete Ziel dieser Arbeit ist die rationale Gestaltung und Optimierung von oxid-geträgerten nanopartikulären IrNix Katalysatoren mit drastisch reduziertem Edelmetallgehalt für die elektrokatalytische Oxidation von Wasserim sauren Milieu. Um dieses Ziel zu erreichen wurden die nachstehend genannten Strategien verfolgt: i) Einsatz eines korrosionsbeständigenoxidischen Trägermaterials mit großer Oberfläche, ii) Entwicklung von IrM@IrOx Metalloxid Hybrid Kern@Schale Nanopartikeln, wobei M ein nicht edles Metall darstellt mit dessen Hilfe die intrinsische OER Aktivität gesteigert und gleichzeitig der Edelmetallgehalt des Katalysators gesenkt werden kann. In dieser Arbeit wurden die Bedingungen der IrNixNanopartikelsynthese optimiert, um homogen legierte Nanopartikel mit kontrollierter Zusammensetzung als Zwischenstufe für die Synthese von IrNi@IrOx Nanoarchitekturen zu erhalten. Die IrNix und IrNi@IrOx Nanopartikel wurden durch eine Vielzahl von Techniken umfassend charakterisiert. In diesem Zusammenhang sind folgende Techniken besonders hervorzuheben: Röntgenbeugung, Transmissionselektronen-mikroskopie, Rastertransmissionselektronenmikroskopie zusammen mit Energiedispersiver Röntgenspektroskopie, tiefenaufgelöste Röntgenphotoelektronenspektroskopieund hochenergetische Röntgenbeugung in Kombination mit atomaren Paarverteilungsfunktionen. Die IrNi@IrOx Katalysatoren zeigten eine 10 fach höhere iridiumbasierte Massenaktivität bei einem Überpotential von 250 mV als ein reiner IrO2 Referenzkatalysator. Darüber hinaus zeigten oxidgeträgerteIrNi@IrOx Nanopartikel eine hervorragende Stabilität, sodass während eines 20 stündigen Stabilitätstests nur eine geringfügige Degradation zu beobachten war. Durch den Vergleich der strukturellen Eigenschaften der IrNi@IrOx Nanopartikel mit deren OER Aktivität konnte die Gitterkontraktion als wahrscheinlichste Ursache für die erhöhte OER Aktivität identifiziert werden.DFG, STR 596/3-1, Nanostructured mixed metal oxides for the electrocatalytic oxidation of wate

Oxide-supported Ir nanodendrites with high activity and durability for the oxygen evolution reaction in acid PEM water electrolyzers

Reducing the noble-metal catalyst content of acid Polymer Electrolyte Membrane (PEM) water electrolyzers without compromising catalytic activity and stability is a goal of fundamental scientific interest and substantial technical importance for cost-effective hydrogen-based energy storage. This study presents nanostructured iridium nanodendrites (Ir-ND) supported on antimony doped tin oxide (ATO) as efficient and stable water splitting catalysts for PEM electrolyzers. The active Ir-ND structures exhibited superior structural and morphological properties, such as particle size and surface area compared to commercial state-of-art Ir catalysts. Supported on tailored corrosion-stable conductive oxides, the Ir-ND catalysts exhibited a more than 2-fold larger kinetic water splitting activity compared with supported Ir nanoparticles, and a more than 8-fold larger catalytic activity than commercial Ir blacks. In single-cell PEM electrolyzer tests, the Ir-ND/ATO outperformed commercial Ir catalysts more than 2-fold at technological current densities of 1.5 A cm(-2) at a mere 1.80 V cell voltage, while showing excellent durability under constant current conditions. We conclude that Ir-ND/ATO catalysts have the potential to substantially reduce the required noble metal loading, while maintaining their catalytic performance, both in idealized three-electrode set ups and in the real electrolyzer device environments.DFG, SPP 1613, Regenerativ erzeugte Brennstoffe durch lichtgetriebene Wasserspaltung: Aufklärung der Elementarprozesse und Umsetzungsperspektiven auf technologische Konzept

Carbon-Supported IrCoO nanoparticles as an efficient and stable OER electrocatalyst for practicable CO2 electrolysis

The development of an efficient and stable oxygen evolution reaction (OER) electrocatalyst operating under pH-neutral conditions is vital for the realization of sustainable CO2 reduction reaction (CO2RR) systems in the future. For commercializing this system, it is also important to be able to use general-purpose water as an electrolyte. Here, we explore, characterize and validate a new IrCoOx mixed metal oxide as efficient and stable OER catalyst, before we investigate and proof its suitability as counter electrode to a CO2RR cathode operating under pH-neutral conditions. More specifically, carbon-supported IrCoOx core-shell nanoparticles exhibited a highly efficient OER catalytic activity and stability compared to state-of-art reference IrOx catalysts in CO2-saturated 0.5 M KHCO3 tap-water. IrCoOx/C also exhibited a significantly improved electrochemical oxidation and corrosion resistance than IrOx, resulting in a beneficial suppression of Ir dissolution. The application of IrCoOx/C in the CO2 electrolyzer displayed superior CO space-time yields over prolonged electrolyzer tests.11Nsciescopu

Electrochemical Catalyst Support Effects and Their Stabilizing Role for IrOx Nanoparticle Catalysts during the Oxygen Evolution Reaction

Redox-active support materials can help reduce the noble-metal loading of a solid chemical catalyst while offering electronic catalyst–support interactions beneficial for catalyst durability. This is well known in heterogeneous gas-phase catalysis but much less discussed for electrocatalysis at electrified liquid–solid interfaces. Here, we demonstrate experimental evidence for electronic catalyst–support interactions in electrochemical environments and study their role and contribution to the corrosion stability of catalyst/support couples. Electrochemically oxidized Ir oxide nanoparticles, supported on high surface area carbons and oxides, were selected as model catalyst/support systems for the electrocatalytic oxygen evolution reaction (OER). First, the electronic, chemical, and structural state of the catalyst/support couple was compared using XANES, EXAFS, TEM, and depth-resolved XPS. While carbon-supported oxidized Ir particle showed exclusively the redox state (+4), the Ir/IrO_<i>x</i>/ATO system exhibited evidence of metal/metal–oxide support interactions (MMOSI) that stabilized the metal particles on antimony-doped tin oxide (ATO) in sustained lower Ir oxidation states (Ir^3.2+). At the same time, the growth of higher valent Ir oxide layers that compromise catalyst stability was suppressed. Then the electrochemical stability and the charge-transfer kinetics of the electrocatalysts were evaluated under constant current and constant potential conditions, where the analysis of the metal dissolution confirmed that the ATO support mitigates Ir^z+ dissolution thanks to a stronger MMOSI effect. Our findings raise the possibility that MMOSI effects in electrochemistrylargely neglected in the pastmay be more important for a detailed understanding of the durability of oxide-supported nanoparticle OER catalysts than previously thought

Cation Effects on the Acidic Oxygen Reduction Reaction at Carbon Surfaces

Hydrogen peroxide (H2O2) is a widely used green oxidant. Until now, research focused on the development of efficient catalysts for the two-electron oxygen reduction reaction (2e– ORR). However, electrolyte effects on the 2e– ORR have remained little understood. We report a significant effect of alkali metal cations (AMCs) on carbons in acidic environments. The presence of AMCs at a glassy carbon electrode shifts the half wave potential from -0.48 to - 0.22 VRHE. This cationic induced enhancement effect exhibits a uniquely sensitive on/off switching behaviour depending on the voltammetric protocol. Voltammetric and in situ X-ray photoemission spectroscopic evidence is presented, supporting a controlling role of the potential of zero charge of the catalytic enhancement. Density functional theory calculations associate the enhancement with the stabilization of the *OOH key intermediate as a result of locally induced field effects from the AMCs. Finally, we developed a refined reaction mechanism for the H2O2 production in presence of AMCs