Stable and Active Oxygen Reduction Catalysts with Reduced Noble Metal Loadings through Potential Triggered Support Passivation

The development of stable, cost‐efficient and active materials is one of the main challenges in catalysis. The utilization of platinum in the electroreduction of oxygen is a salient example where the development of new material combinations has led to a drastic increase in specific activity compared to bare platinum. These material classes comprise nanostructured thin films, platinum alloys, shape‐controlled nanostructures and core–shell architectures. Excessive platinum substitution, however, leads to structural and catalytic instabilities. Herein, we introduce a catalyst concept that comprises the use of an atomically thin platinum film deposited on a potential‐triggered passivating support. The model catalyst exhibits an equal specific activity with higher atom utilization compared to bulk platinum. By using potential‐triggered passivation of titanium carbide, irregularities in the Pt film heal out via the formation of insoluble oxide species at the solid/liquid interface. The adaptation of the described catalyst design to the nanoscale and to high‐surface‐area structures highlight the potential for stable, passivating catalyst systems for various electrocatalytic reactions such as the oxygen reduction reaction

Analysing the relationship between the fields of thermo- and electrocatalysis taking hydrogen peroxide as a case study

Catalysis is inherently driven by the interaction of reactants, intermediates and formed products with the catalyst’s surface. In order to reach the desired transition state and to overcome the kinetic barrier, elevated temperatures or electrical potentials are employed to increase the rate of reaction. Despite immense efforts in the last decades, research in thermo- and electrocatalysis has often preceded in isolation, even for similar reactions. Conceptually, any heterogeneous surface process that involves changes in oxidation states, redox processes, adsorption of charged species (even as spectators) or heterolytic cleavage of small molecules should be thought of as having parallels with electrochemical processes occurring at electrified interfaces. Herein, we compare current trends in thermo- and electrocatalysis and elaborate on the commonalities and differences between both research fields, with a specific focus on the production of hydrogen peroxide as case study. We hope that interlinking both fields will be inspiring and thought-provoking, eventually creating synergies and leverage towards more efficient decentralized chemical conversion processes

Extension of the Rotating Disk Electrode Method to Thin Samples of Non-Disk Shape

A straightforward approach is presented to make thin electrode samples of non-disk shape compatible with the rotating disk electrode method, by merely attaching the sample on the rotating shroud and confining the area exposed to the electrolyte using chemically resistant, adhesive material. The performance of an as prepared rotating E-beam evaporated platinum film is compared with that of a conventional rotating platinum disk for three classical electrochemical reactions: the carbon monoxide oxidation, the hydrogen oxidation, and the oxygen reduction. Despite the unusual electrode morphology that deviates from the ideal rotating disk electrode configuration, the results and conclusions for the rotating E-beam evaporated film are equivalent to those for the classical rotating disk. Thus, this approach enables investigations under well controlled mass transport conditions using non-conventional thin electrode samples

Transition Metal—Carbon Bond Enthalpies as Descriptor for the Electrochemical Stability of Transition Metal Carbides in Electrocatalytic Applications

Transition metal carbides are used for various applications such as hard coating, heterogeneous catalysis, catalyst support material or coatings in fuel cell applications. However, little is known about the stability of their electrochemically active surface in aqueous electrolytes. Herein, the transition metal—carbon bond enthalpy is proposed as stability criterion for various transition metal carbides. The basis is an oxidation mechanism where the rate determining step is the metal—carbon bond cleavage under acidic conditions which was supported by a detailed corrosion study on hexagonal tungsten carbide. In situ flow cell measurements that were coupled to an inductively coupled plasma mass spectrometer corroborated experimentally the linear dependency of the oxidation overpotential on the transition metal—carbon bond enthalpy. The proposed model allows the estimation of the activation overpotential for electrochemical carbide oxidation resulting in a maximized stabilization for carbides in the 4th group (Ti, Zr, Hf). Together with the calculated thermodynamic oxidation potentials, TiC and VC exhibit the highest experimental oxidation potentials (0.85 VRHE). The model can be used for preselecting possible carbide materials for various electrochemical reactions

Core‐passivation: A concept for stable core‐shell nanoparticles in aqueous electrocatalysis

Abstract The stability of nanoparticles is a major challenge in thermal and electrocatalysis. This is especially true for core‐shell nanoparticles where only a few monolayers of noble metal protect the usually non‐noble core material. In this work, we utilize the practical nobility concept to engineer stable core‐shell nanoparticles with a self‐passivating core material. Specifically, tantalum carbide as core material in combination with a 1–3 monolayer thick platinum shell exhibits exceptional stability in aqueous media. The core‐shell catalyst shows no sign of structural changes after 10,000 degradation cycles up to 1.0 VRHE. Due to the efficient passivation of tantalum carbide at the solid/liquid interface, the dissolution reduces by a factor of eight compared to bare Pt. Our findings confirm that passivating core materials are highly beneficial for the stabilization of core‐shell nanomaterials in aqueous media. They open up new ways for the rational design of cost‐efficient but stable non‐noble core – platinum shell nanoparticles where harsh, oxidizing conditions are employed

Transition Metal-Carbon Bond Enthalpies as Descriptor for the Electrochemical Stability of Transition Metal Carbides in Electrocatalytic Applications

Transition metal carbides are used for various applications such as hard coating, heterogeneous catalysis, catalyst support material or coatings in fuel cell applications. However, little is known about the stability of their electrochemically active surface in aqueous electrolytes. Herein, the transition metal—carbon bond enthalpy is proposed as stability criterion for various transition metal carbides. The basis is an oxidation mechanism where the rate determining step is the metal—carbon bond cleavage under acidic conditions which was supported by a detailed corrosion study on hexagonal tungsten carbide. In situ flow cell measurements that were coupled to an inductively coupled plasma mass spectrometer corroborated experimentally the linear dependency of the oxidation overpotential on the transition metal—carbon bond enthalpy. The proposed model allows the estimation of the activation overpotential for electrochemical carbide oxidation resulting in a maximized stabilization for carbides in the 4th group (Ti, Zr, Hf). Together with the calculated thermodynamic oxidation potentials, TiC and VC exhibit the highest experimental oxidation potentials (0.85 VRHE). The model can be used for preselecting possible carbide materials for various electrochemical reactions

The Impact of Antimony on the Performance of Antimony Doped Tin Oxide Supported Platinum for the Oxygen Reduction Reaction

Daniel Jalalpoor, Daniel Göhl, Paul Paciok, Marc Heggen, Johannes Knossalla, Ivan Radev, Volker Peinecke, Claudia Weidenthaler, Karl J. J. Mayrhofer, “The Impact of Antimony on the Performance of Antimony Doped Tin Oxide Supported Platinum for the Oxygen Reduction Reaction”, J. Electrochem. Soc. 168, (2021) 024502 https://doi.org/10.1149/1945-7111/abd830 Abstract:Antimony doped tin oxide (ATO) supported platinum nanoparticles are considered a more stable replacement for conventional carbon supported platinum materials for the oxygen reduction reaction. However, the interplay of antimony, tin and platinum and its impact on the catalytic activity and durability has only received minor attention. This is partly due to difficulties in the preparation of morphology- and surface-area-controlled antimony-doped tin oxide materials. The presented study sheds light onto catalyst–support interaction on a fundamental level, specifically between platinum as a catalyst and ATO as a support material. By using a previously described hard-templating method, a series of morphology controlled ATO support materials for platinum nanoparticles with different antimony doping concentrations were prepared. Compositional and morphological changes before and during accelerated stress tests are monitored, and underlying principles of deactivation, dissolution and catalytic performance are elaborated. We demonstrate that mobilized antimony species and strong metal support interactions lead to Pt/Sb alloy formation as well as partially blocking of active sites. This has adverse consequences on the accessible platinum surface area, and affects negatively the catalytic performance of platinum. Operando time-resolved dissolution experiments uncover the potential boundary conditions at which antimony dissolution can be effectively suppressed and how platinum influences the dissolution behavior of the support

Shape-Controlled Nanoparticles in Pore-Confined Space

Increasing the catalyst’s stability and activity are one of the main quests in catalysis. Tailoring crystal surfaces to a specific reaction has demonstrated to be a very effective way in increasing the catalyst’s specific activity. Shape controlled nanoparticles with specific crystal facets are usually grown kinetically and are highly susceptible to morphological changes during the reaction due to agglomeration, metal dissolution, or Ostwald ripening. A strong interaction of the catalytic material to the support is thus crucial for successful stabilization. Taken both points into account, a general catalyst design is proposed, combining the enhanced activity of shape-controlled nanoparticles with a pore-confinement approach for high stability. Hollow graphitic spheres with narrow and uniform bimodal mesopores serve as model system and were used as support material. As catalyst, different kinds of particles, such as pure platinum (Pt), platinum/nickel (Pt3Ni) and Pt3Ni doped with molybdenum (Pt3Ni–Mo), have exemplarily been synthesized. The advantages, limits and challenges of the proposed concept are discussed and elaborated by means of time-resolved, in and ex situ measurements. It will be shown that during catalysis, the potential boundaries are crucial especially for the proposed catalyst design, resulting in either retention of the initial activity or drastic loss in shape, size and elemental composition. The synthesis and catalyst design can be adapted to a wide range of catalytic reactions where stabilization of shape-controlled particles is a focus

Stability and Activity of Non-noble-Metal-based Catalysts Toward the Hydrogen Evolution Reaction

A fundamental understanding of the behavior of non-noble based materials toward the hydrogen evolution reaction is crucial for the successful implementation into practical devices. Through the implementation of a highly sensitive inductively coupled plasma mass spectrometer coupled to a scanning flow cell, the activity and stability of non-noble electrocatalysts is presented. The studied catalysts comprise a range of compositions, including metal carbides (WC), sulfides (MoS2), phosphides (Ni5P4, Co2P), and their base metals (W, Ni, Mo, Co); their activity, stability, and degradation behavior was elaborated and compared to the state-of-the-art catalyst platinum. The non-noble materials are stable at HER potentials but dissolve substantially when no current is flowing. Through pre- and post-characterization of the catalysts, explanations of their stability (thermodynamics and kinetics) are discussed, challenges for the application in real devices are analyzed, and strategies for circumventing dissolution are suggested. The precise correlation of metal dissolution with applied potential/current density allows for narrowing down suitable material choices as replacement for precious group metals as for example, platinum and opens up new ways in finding cost-efficient, active, and stable new-generation electrocatalysts

Size‐Controlled Synthesis of IrO₂ Nanoparticles at High Temperatures for the Oxygen Evolution Reaction

Iridium oxide is the state‐of‐the‐art catalyst for electrochemical water oxidation in an acidic medium. Despite being one of the rarest elements in the Earth's crust, there is a pressing need to maximize the utilization and longevity of active iridium centers. While conventional low‐temperature synthesis can yield nanostructures with high mass‐specific activity, they are often insufficiently stable during water oxidation. Structurally ordered iridium oxide is one of the most stable electrocatalysts utilized in polymer electrolyte membrane water electrolysis that benefits from the chemically ordered structure. However, its preparation requires thermal treatment at high temperatures, which improves its durability but can also result in reduced surface area and altered particle morphology. In this study, the challenge of controlling nanoparticle size during the preparation of structurally ordered iridium oxide is successfully addressed, which typically requires high‐temperature thermal treatment. By utilizing a silica nanoreactor as a hard template, a precise control is achieved over the nanoparticle size during high‐temperature thermal treatment. This approach maintains high durability while avoiding the common problem of reduced surface area and altered particle morphology. Specifically, this study is able to synthesize iridium oxide nanoparticles at temperatures up to 800 °C, while keeping their dimensions below 10 nm