Advanced Bifunctional Oxygen Reduction and Evolution Electrocatalyst Derived from Surface-Mounted Metal-Organic Frameworks

Metal‐organic frameworks (MOFs) and their derivatives are considered as promising catalysts for the oxygen reduction (ORR) and oxygen evolution reaction (OER), which are important for many energy provision technologies, such as electrolyzers, fuel cells and some types of advanced batteries. In this work, a “strain modulation” approach has been applied through the use of surface‐mounted NiFe‐MOFs in order to design an advanced bifunctional ORR/OER electrocatalyst. The material exhibits an excellent OER activity in alkaline media, reaching an industrially relevant current density of 200 mA·cm ‐2 at an overpotential of just ~210 mV. It demonstrates operational long‐term stability even at a high current density of 500 mA·cm ‐2 and exhibits the so far narrowest “overpotential window” ΔE ORR‐OER : 0.69 V in 0.1 M KOH with a mass loading being two orders of magnitude lower than that of benchmark electrocatalysts

Combining impedance and hydrodynamic methods in electrocatalysis. Characterization of Pt(pc), Pt5Gd, and nanostructured Pd for the hydrogen evolution reaction

Electrochemical hydrodynamic techniques typically involve electrodes that move relative to the solution. Historically, approaches involving rotating disc electrode (RDE) configurations have become very popular, as one can easily control the electroactive species’ mass transport in those cases. The combination of cyclic voltammetry and RDE is nowadays one of the standard characterization protocols in electrocatalysis. On the other hand, impedance spectroscopy is one of the most informative electrochemistry techniques, enabling the acquisition of information on the processes taking place simultaneously at the electrode/electrolyte interface. In this work, we investigated the hydrogen evolution reaction (HER) catalyzed by polycrystalline Pt (Pt(pc)) and Pt _5 Gd disc electrodes and characterized them using RDE and electrochemical impedance spectroscopy techniques simultaneously. Pt _5 Gd shows higher HER activities than Pt in acidic and alkaline media due to strain and ligand effects. The mechanistic study of the reaction showed that the rotation rates in acidic media do not affect the contribution of the Volmer–Heyrovsky and Volmer–Tafel pathways. However, the Volmer–Heyrovsky pathway dominates at lower rotation rates in alkaline media. Besides, the HER in acidic solutions depends more strongly on mass diffusion than in alkaline media. In addition to simple and clearly defined systems, the combined method of both techniques is applicable for systems with greater complexity, such as Pd/C nanostructured catalysts. Applying the above-presented approach, we found that the Volmer–Tafel pathway is the dominating mechanism of the HER for this catalytic system

A trade-off between ligand and strain effects optimizes the oxygen reduction activity of Pt alloys

To optimize the performance of catalytic materials, it is paramount to elucidate the dependence of the chemical reactivity on the atomic arrangement of the catalyst surface. Therefore, identifying the nature of the active sites that provide optimal binding of reaction intermediates is the first step toward a rational catalyst design. In this work, we focus on the oxygen reduction reaction (ORR), an essential constituent of several energy provision and storage devices. Among the state-of-the-art ORR catalysts are platinum (Pt) and its alloys. The latter benefit from the so-called ligand and strain effects, which influence the electronic properties of the surface. Here, we “visualize” the active sites on Pt3Ni(111) in an acidic medium with a lateral resolution in the nanometer regime via an in-situ technique based on electrochemical scanning tunnelling microscopy. In contrast to pure Pt, where the active sites are located at concave sites close to steps, Pt3Ni(111) terraces contain the most active centres, while steps show activity to a comparable or lesser extent. We confirm the experimental findings by a model based on alloy- and strain-sensitive generalized coordination numbers. With this model, we are also able to assess both the composition and the geometric configuration of optimal catalytic active sites on various Pt alloy catalysts. In general, the interplay of ligand effects and lattice compression resulting from the alloying of Pt with 3d transition metals (Ti, Co, Ni, Cu) gradually increases the generalized coordination number of surface Pt atoms, thereby making (111) terraces highly active. This combination of theoretical and experimental tools provides clear strategies to design more efficient Pt alloy electrocatalysts for oxygen reduction.The authors cordially thank Mr Karl Eberle for his valuable assistance in the sample preparation and Mr Kun-Ting Song and Dr Batyr Garlyyev for helping with some of the electrochemical experiments. R. M. K., R. W. H., B. G. and A. S. B. acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG), in the framework of the project BA 5795/6-1. A. R. acknowledges funding by the DFG, project number 453903355. R. M. K., R. W. H., B. G., A. S. B., K. S., Y. B., J. V. B., and F. A. appreciate funding from the DFG through the Excellence Cluster “e-conversion”, EXC 2089/1-390776260. The grants RTI2018-095460-B-I00, María de Maeztu (MDM-2017-0767) and Ramón y Cajal (RYC-2015-18996) were funded by MCIN/AEI/10.13039/501100011033 and the European Union. This work was also partly funded by Generalitat de Catalunya 2017SGR13. The use of supercomputing facilities at SURFsara was sponsored by NWO Physical Sciences, with financial support from NWO. RMK, TOS and ASB acknowledge funding from the European Union's Horizon 2020 research and innovation programme under grant agreement HERMES No. 952184

Electrochemical top-down synthesis of C-supported Pt nano-particles with controllable shape and size: Mechanistic insights and application

In this work, we demonstrate the power of a simple top-down electrochemical erosion approach to obtain Pt nanoparticle with controlled shapes and sizes (in the range from ∼ 2 to ∼ 10 nm). Carbon supported nanoparticles with narrow size distributions have been synthesized by applying an alternating voltage to macroscopic bulk platinum structures, such as disks or wires. Without using any surfactants, the size and shape of the particles can be changed by adjusting simple parameters such as the applied potential, frequency and electrolyte composition. For instance, application of a sinusoidal AC voltage with lower frequencies results in cubic nanoparticles; whereas higher frequencies lead to predominantly spherical nanoparticles. On the other hand, the amplitude of the sinusoidal signal was found to affect the particle size; the lower the amplitude of the applied AC signal, the smaller the resulting particle size. Pt/C catalysts prepared by this approach showed 0.76 A/mg mass activity towards the oxygen reduction reaction which is ∼ 2 times higher than the state-of-the-art commercial Pt/C catalyst (0.42 A/mg) from Tanaka. In addition to this, we discussed the mechanistic insights about the nanoparticle formation pathways

Metamorphosis of Heterostructured Surface‐Mounted Metal–Organic Frameworks Yielding Record Oxygen Evolution Mass Activities

Materials derived from surface-mounted metal–organic frameworks (SURMOFs) are promising electrocatalysts for the oxygen evolution reaction (OER). A series of mixed-metal, heterostructured SURMOFs is fabricated by the facile layer-by-layer deposition method. The obtained materials reveal record-high electrocatalyst mass activities of ≈2.90 kA g$^{−1}$ at an overpotential of 300 mV in 0.1 m KOH, superior to the benchmarking precious and nonprecious metal electrocatalysts. This property is assigned to the particular in situ self-reconstruction and self-activation of the SURMOFs during the immersion and the electrochemical treatment in alkaline aqueous electrolytes, which allows for the generation of NiFe (oxy)hydroxide electrocatalyst materials of specific morphology and microstructure

Top-down surfactant-free electrosynthesis of magnéli phase Ti₉O₁₇ nanowires

International audienceTiO2 nanowires have proven their importance as a versatile material in numerous fields of technology due to their unique properties attributable to their high aspect ratio and large surface area. However, synthesis is an enormous challenge since state-of-the-art techniques rely on complex, multi-stage procedures with expensive, specialized equipment, employing high-temperature steps and potentially toxic precursor materials and surfactants. Hence, we elucidate a simple and facile top-down methodology for the synthesis of nanowires with non-stoichiometric Magnéli phase Ti9O17. This methodology relies on the electrochemical erosion of bulk Ti wires immersed in an aqueous electrolyte, circumventing the use of environmentally harmful precursors or surfactants, eliminating the need for high temperatures, and reducing synthesis complexity and time. Using multiple techniques, including transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction, we provide evidence of the successful synthesis of ultrathin nanowires with the crystal structure of non-stoichiometric Ti9O17 Magnéli phase. The nanowire width of ∼5 nm and the Brunauer–Emmett–Teller surface area of ∼215 m2 g−1 make the nanowires presented in this work comparable to those synthesized by state-of-the-art bottom-up techniques

Elucidation of structure–activity relations in proton electroreduction at Pd surfaces: Theoretical and experimental study

The structure–activity relationship is a cornerstone topic in catalysis, which lays the foundation for the design and functionalization of catalytic materials. Of particular interest is the catalysis of the hydrogen evolution reaction (HER) by palladium (Pd), which is envisioned to play a major role in realizing a hydrogen-based economy. Interestingly, experimentalists observed excess heat generation in such systems, which became known as the debated “cold fusion” phenomenon. Despite the considerable attention on this report, more fundamental knowledge, such as the impact of the formation of bulk Pd hydrides on the nature of active sites and the HER activity, remains largely unexplored. In this work, classical electrochemical experiments performed on model Pd(hkl) surfaces, “noise” electrochemical scanning tunneling microscopy (n-EC-STM), and density functional theory are combined to elucidate the nature of active sites for the HER. Results reveal an activity trend following Pd(111) > Pd(110) > Pd(100) and that the forma?tion of subsurface hydride layers causes morphological changes and strain, which affect the HER activity and the nature of active sites. These findings provide significant insights into the role of subsurface hydride formation on the structure–activity relations toward the design of efficient Pd-based nanocatalysts for the HER.</p