Electrocatalytic CO₂ Reduction on CuO_x Nanocubes Tracking the Evolution of Chemical State, Geometric Structure, and Catalytic Selectivity using Operando Spectroscopy

The direct electrochemical conversion of carbon dioxide (CO2) into multi-carbon (C2+) products still faces fundamental and technological challenges. While facet-controlled and oxide-derived Cu materials have been touted as promising catalysts, their stability has remained problematic and poorly understood. The present work uncovers changes in the chemical and morphological state of supported and unsupported Cu2O nanocubes during operation in low-current H-Cells and in high-current Gas Diffusion Electrodes (GDEs) using neutral pH buffer conditions. While unsupported nanocubes achieved a sustained C2+ faradaic efficiency of around 60% for 40 h, the dispersion on a carbon support sharply shifted the selectivity pattern towards C1 products. Operando XAS and time-resolved electron microscopy revealed the degradation of the cubic shape and, in the presence of a carbon support, the formation of small Cu-seeds during the surprisingly slow reduction of bulk Cu2O. Here, the initially (100)-rich facet structure has presumably no controlling role on the catalytic selectivity, whereas the oxide-derived generation of under-coordinated lattice defects, as revealed by the operando Cu-Cu coordination numbers, can support the high C2+ product yields

Elemental Anisotropic Growth and Atomic-Scale Structure of Shape-Controlled Octahedral Pt–Ni–Co Alloy Nanocatalysts

Multimetallic shape-controlled nanoparticles offer great opportunities to tune the activity, selectivity, and stability of electrocatalytic surface reactions. However, in many cases, our synthetic control over particle size, composition, and shape is limited requiring trial and error. Deeper atomic-scale insight in the particle formation process would enable more rational syntheses. Here we exemplify this using a family of trimetallic PtNiCo nanooctahedra obtained via a low-temperature, surfactant-free solvothermal synthesis. We analyze the competition between Ni and Co precursors under coreduction “one-step” conditions when the Ni reduction rates prevailed. To tune the Co reduction rate and final content, we develop a “two-step” route and track the evolution of the composition and morphology of the particles at the atomic scale. To achieve this, scanning transmission electron microscopy and energy dispersive X-ray elemental mapping techniques are used. We provide evidence of a heterogeneous element distribution caused by element-specific anisotropic growth and create octahedral nanoparticles with tailored atomic composition like Pt1.5M, PtM, and PtM1.5 (M = Ni + Co). These trimetallic electrocatalysts have been tested toward the oxygen reduction reaction (ORR), showing a greatly enhanced mass activity related to commercial Pt/C and less activity loss than binary PtNi and PtCo after 4000 potential cycles.P.S. acknowledges financial support by the German Research Foundation (DFG) through grant STR 596/5-1 (“Nanoscale Pt Alloy electrocatalysts with well-defined shapes”). Partial funding by the German Ministry of Education and Research (BMBF) grant “LOPLAKAT” is gratefully acknowledged. Also, this work was financially supported by the MICINN (Spain) (project 2013-44083-P). R.M.A.A. thanks the funding received from MICINN (EEBB-I-14-08240) to carry out a predoctoral stay in a foreign R&D center. M.H. thanks the Deutsche Forschungsgemeinschaft (DFG) for financial support within the grant HE 7192/1-1

Instrument for the Measurement of EUV Reflectance and Scattering - MERLIN

A new system is presented for measurements of angle resolved scatter and reflectance at 13.5 nm in the extreme ultraviolet spectral range. The system enables the at-wavelength characterization of EUV optical components. Examples are presented for Mo/Si multilayers deposited onto super polished substrates

pH Induced versus Oxygen Induced Surface Enrichment and Segregation Effects in Pt Ni Alloy Nanoparticle Fuel Cell Catalysts

We present a voltammetric, spectroscopic and atomic-scale microscopic study of how initial interfacial contact with high- and low-pH electrolytes affects the surface voltammetry, near-surface composition, CO binding, and electrocatalytic oxygen reduction reaction (ORR) of dealloyed Pt-Ni alloy nanoparticles deployed in fuel cells. The first contact of the catalyst with the electrolyte is critical for the evolution of the catalytically active surface structure, yet still insufficiently understood. Counter to chemical intuition, we find that voltammetric activation protocols in both pH 1 and pH 13 electrolytes result in similarly Ni-depleted surfaces with similar near-surface Ni/Pt ratios to a 2.5nm depth, yet vastly different ORR reactivities. Based on our combined voltammetric, scanning transmission electron microscopy with the spectroscopic mapping by energy dispersive X-ray (STEM-EDX) microscopic and X-ray photoelectron spectroscopy (XPS) analysis, we conclude that oxygen-saturated alkaline electrolytes causes a strong surface segregation of the more oxophilic Ni component toward the particles surface, however in distinctly different ways depending on the pretreatment pH. Data suggest a controlling role of the initial thickness of the Ni-depleted Pt shell for the catalysis-driven segregation process. We analyze and discuss how such subtle differences in initial surface composition can unfold such dramatic subsequent variations in ORR activity as function of pH. Our findings have practical bearing for the design of active Pt bimetallic ORR catalysts for Alkaline Exchange Membrane Fuel Cells. If the non-noble oxophilic Pt alloy component is insoluble in the alkaline electrolyte, our results call for an imperative acid-pretreatment to avoid surface blocking by oxygen-induced segregation. If the non-noble oxophilic Pt alloy component is soluble in alkaline electrolyte, acid or alkaline, even non-pretreated Pt alloy catalyst may be employed

Ionomer distribution control in porous carbon-supported catalyst layers for high-power and low Pt-loaded proton exchange membrane fuel cells

The reduction of Pt content in the cathode for proton exchange membrane fuel cells is highly desirable to lower their costs. However, lowering the Pt loading of the cathodic electrode leads to high voltage losses. These voltage losses are known to originate from the mass transport resistance of O-2 through the platinum-ionomer interface, the location of the Pt particle with respect to the carbon support and the supports' structures. In this study, we present a new Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance. The use of chemically modified carbon supports with tailored porosity enabled controlled deposition of Pt nanoparticles on the outer and inner surface of the support particles. This resulted in an unprecedented uniform coverage of the ionomer over the high surface-area carbon supports, especially under dry operating conditions. Consequently, the present catalyst design exhibits previously unachieved fuel cell power densities in addition to high stability under voltage cycling. Thanks to the Coulombic interaction between the ionomer and N groups on the carbon support, homogeneous ionomer distribution and reproducibility during ink manufacturing process is ensured

Molecular Analysis of the Unusual Stability of an IrNbO_x Catalyst for the Electrochemical Water Oxidation to Molecular Oxygen (OER)

Adoption of proton exchange membrane (PEM) water electrolysis technology on a global level will demand a significant reduction of today’s iridium loadings in the anode catalyst layers of PEM electrolyzers. However, new catalyst and electrode designs with reduced Ir content have been suffering from limited stability caused by (electro)chemical degradation. This has remained a serious impediment to a wider commercialization of larger-scale PEM electrolysis technology. In this combined DFT computational and experimental study, we investigate a novel family of iridium–niobium mixed metal oxide thin-film catalysts for the oxygen evolution reaction (OER), some of which exhibit greatly enhanced stability, such as minimized voltage degradation and reduced Ir dissolution with respect to the industry benchmark IrOx catalyst. More specifically, we report an unusually durable IrNbOx electrocatalyst with improved catalytic performance compared to an IrOx benchmark catalyst prepared in-house and a commercial benchmark catalyst (Umicore Elyst Ir75 0480) at significantly reduced Ir catalyst cost. Catalyst stability was assessed by conventional and newly developed accelerated degradation tests, and the mechanistic origins were analyzed and are discussed. To achieve this, the IrNbOx mixed metal oxide catalyst and its water splitting kinetics were investigated by a host of techniques such as synchrotron-based NEXAFS analysis and XPS, electrochemistry, and ab initio DFT calculations as well as STEM-EDX cross-sectional analysis. These analyses highlight a number of important structural differences to other recently reported bimetallic OER catalysts in the literature. On the methodological side, we introduce, validate, and utilize a new, nondestructive XRF-based catalyst stability monitoring technique that will benefit future catalyst development. Furthermore, the present study identifies new specific catalysts and experimental strategies for stepwise reducing the Ir demand of PEM water electrolyzers on their long way toward adoption at a larger scale

Unified structural motifs of the catalytically active state of Co oxyhydr oxides during the electrochemical oxygen evolution reaction

Efficient catalysts for the anodic oxygen evolution reaction (OER) are critical for electrochemical H2 production. Their design requires structural knowledge of their catalytically active sites and state. Here, we track the atomic-scale structural evolution of well-defined CoOx(OH)y compounds into their catalytically active state during electrocatalytic operation through operando and surface-sensitive X-ray spectroscopy and surface voltammetry, supported by theoretical calculations. We find clear voltammetric evidence that electrochemically reducible near-surface Co3+–O sites play an organizing role for high OER activity. These sites invariably emerge independent of initial metal valency and coordination under catalytic OER conditions. Combining experiments and theory reveals the unified chemical structure motif as µ2-OH-bridged Co2+/3+ ion clusters formed on all three-dimensional cross-linked and layered CoOx(OH)y precursors and present in an oxidized form during the OER, as shown by operando X-ray spectroscopy. Together, the spectroscopic and electrochemical fingerprints offer a unified picture of our molecular understanding of the structure of catalytically active metal oxide OER sites

A unique oxygen ligand environment facilitates water oxidation in hole doped IrNiOxcore shell electrocatalysts

The electro-oxidation of water to oxygen is expected to play a major role in the development of future electrochemical energy conversion and storage technologies. However, the slow rate of the oxygen evolution reaction remains a key challenge that requires fundamental understanding to facilitate the design of more active and stable electrocatalysts. Here, we probe the local geometric ligand environment and electronic metal states of oxygen-coordinated iridium centres in nickel-leached IrNi@IrOx metal oxide core–shell nanoparticles under catalytic oxygen evolution conditions using operando X-ray absorption spectroscopy, resonant high-energy X-ray diffraction and differential atomic pair correlation analysis. Nickel leaching during catalyst activation generates lattice vacancies, which in turn produce uniquely shortened Ir–O metal ligand bonds and an unusually large number of d-band holes in the iridium oxide shell. Density functional theory calculations show that this increase in the formal iridium oxidation state drives the formation of holes on the oxygen ligands in direct proximity to lattice vacancies. We argue that their electrophilic character renders these oxygen ligands susceptible to nucleophilic acid–base-type O–O bond formation at reduced kinetic barriers, resulting in strongly enhanced reactivities

Tracking Catalyst Redox States and Reaction Dynamics in Ni Fe Oxyhydroxide Oxygen Evolution Reaction Electrocatalysts the Role of Catalyst Support and Electrolyte pH

Ni–Fe oxyhydroxides are the most active known electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes and are therefore of great scientific and technological importance in the context of electrochemical energy conversion. Here we uncover, investigate, and discuss previously unaddressed effects of conductive supports and the electrolyte pH on the Ni–Fe­(OOH) catalyst redox behavior and catalytic OER activity, combining <i>in situ</i> UV–vis spectro-electrochemistry, <i>operando</i> electrochemical mass spectrometry (DEMS), and <i>in situ</i> cryo X-ray absorption spectroscopy (XAS). Supports and pH > 13 strongly enhanced the precatalytic voltammetric charge of the Ni–Fe oxyhydroxide redox peak couple, shifted them more cathodically, and caused a 2–3-fold increase in the catalytic OER activity. Analysis of DEMS-based faradaic oxygen efficiency and electrochemical UV–vis traces consistently confirmed our voltammetric observations, evidencing both a more cathodic O₂ release and a more cathodic onset of Ni oxidation at higher pH. Using UV–vis, which can monitor the amount of oxidized Ni^+3/+4 <i>in situ</i>, confirmed an earlier onset of the redox process at high electrolyte pH and further provided evidence of a smaller fraction of Ni^+3/+4 in mixed Ni–Fe centers, confirming the unresolved paradox of a reduced metal redox activity with increasing Fe content. A nonmonotonic super-Nernstian pH dependence of the redox peaks with increasing Fe contentdisplaying Pourbaix slopes as steep as −120 mV/pHsuggested a two proton–one electron transfer. We explain and discuss the experimental pH effects using refined coupled (PCET) and decoupled proton transfer–electron transfer (PT/ET) schemes involving negatively charged oxygenate ligands generated at Fe centers. Together, we offer new insight into the catalytic reaction dynamics and associated catalyst redox chemistry of the most important class of alkaline OER catalysts