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Layered double hydroxides (LDHs) are among the most active and studied catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, previous studies have generally either focused on a small number of LDHs, applied synthetic routes with limited structural control, or used non‐intrinsic activity metrics, thus hampering the construction of consistent structure–activity‐relations. Herein, by employing new individually developed synthesis strategies with atomic structural control, we obtained a broad series of crystalline α‐MA(II)MB(III) LDH and β‐MA(OH)2 electrocatalysts (MA=Ni, Co, and MB=Co, Fe, Mn). We further derived their intrinsic activity through electrochemical active surface area normalization, yielding the trend NiFe LDH > CoFe LDH > Fe‐free Co‐containing catalysts > Fe‐Co‐free Ni‐based catalysts. Our theoretical reactivity analysis revealed that these intrinsic activity trends originate from the dual‐metal‐site nature of the reaction centers, which lead to composition‐dependent synergies and diverse scaling relationships that may be used to design catalysts with improved performance., Catalytic activities for oxygen evolution on crystalline 3d transition metal layered double hydroxides are derived using electrochemical surface area based normalization. Density functional calculations reveal a dual‐metal‐site feature of the reaction centers that provides opportunities to design new catalysts with improved performance.

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.

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 O2 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. Reducing Pt content in cathodes for proton exchange membrane fuel cells is crucial to lower costs but results in high voltage losses. A Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance is reported.

A series of Pt/Ni-SiO₂/C catalysts with different mass proportions of Ni-SiO₂/C (0:100, 30:70, 50:50, 70:30 and 100:0) were prepared and studied towards ethanol electrochemical oxidation in acid medium. The support silica particles were initially synthesized via sol-gel and then modified with NiCl2. The Ni deposited on the silica surface plays a role promoting nucleation sites for the reduction of platinum. Pt was further chemically reduced onto Ni-SiO₂ using formic acid and loaded onto carbon Vulcan XC-72 R. The Pt/Ni-SiO₂/C catalysts were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, temperature-programmed reduction, X-ray photoelectron spectroscopy, transmission electron microscopy and inductively coupled plasma-optical emission spectroscopy. The physical characterizations reveal the formation of oxide-metal composite and strong interaction between Pt and the Ni-SiO₂ composite. The Pt/Ni-SiO₂/C catalyst with meso/macroporous structure exhibits higher electrocatalytic activity towards ethanol oxidation and better stability, after 48 h of electrolysis, than a commercial Pt/C catalyst. These improved features could be due to presence of Ni-SiO₂ composite that promotes corrosion resistance of the support and prevents the aggregation of Pt nanoparticles and their detachment from the support. The low poisoning of the Pt/Ni-SiO₂/C catalyst is probably due to the enhanced oxygen content on the composite surface. The high electrocatalytic activity and enhanced electrochemical stability of the Pt/Ni-SiO₂/C catalyst make it promising for further fuel cell applications.

Active and stable metal oxide nanoparticles supported on high surface area carriers (supports) play an important role in electrochemical energy conversion applications, for instance as anode electrocatalysts for the oxygen evolution reaction (OER) in water electrolyzers. While past studies most often focused on the activity and stability of the active oxide phase along with the surface area and durability of the support material of the combined catalyst/support couple, the influence of the immobilization method on its performance has been widely overlooked. Here, we emphasize the potential and limitations of the presented support methods and evaluate their applicability by means of controlling the metal loading, particle size and the accessibility of surface sites. Further, we present a technique applicable for tuning the loading of the metal oxide catalyst up to 20 wt. % avoiding agglomeration. We also establish a correlation between metal oxide loading and mass-based oxygen evolution activity.

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. Knowledge of the active sites in catalysts—including the sites that form under working conditions—is vital for future design and development. Here, the authors track the atomic-scale changes in a series of well-defined cobalt-based oxide electrocatalysts, showing that the structurally distinct catalysts develop a similar structural motif as they transform into the catalytically active state.

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 IrO

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. Herein we uncover 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. 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, can support the high C2+ product yields., Cu2O cubes of nanometer‐sized dimensions allow the chemical and structural factors that control the selectivity of the CO2 reduction reaction (CO2RR) to be traced. The Faradaic product efficiencies over time can be linked to changes in the chemical state at the surface and bulk and in the catalyst morphology for supported and unsupported nanocubes (S‐NC, U‐NC).

Fundamental understanding of anisotropic growth in oxide nanocrystals is crucial to establish new synthesis strategies and to tailor the nanoscale electronic, magnetic, optical, and electrocatalytic properties of these particles. While several growth investigations of metal alloy nanoparticles have been reported, mechanistic studies on the growth of ternary oxide materials are still missing. This work constitutes the first study on the evolution of anisotropic growth of manganese–cobalt oxide nanoparticles by monitoring the elemental distribution and morphology during the particle evolution via scanning transmission electron microscopy–X-ray spectroscopy. A new growth mechanism based on a “solution-solid-solid” pathway for mixed manganese cobalt oxides is revealed. In this mechanism, the MnO seed formation occurs in the first step, followed by the surface Co enrichment, which catalyzes the growth along the directions in all the subsequent growth stages, creating rod, cross-, and T-shaped mixed metal oxides, which preferentially expose {100} facets. It is shown that the interrelation of both Mn and Co ions initializes the anisotropic growth and presents the range of validity of the proposed mechanism as well as the shape-determining effect based on the metal-to-metal ratio.

This 2007 Chemistry Nobel prize update covers scientific advances of the past decade in our understanding of electrocatalytic processes on surfaces of nanoscale shape-controlled polyhedral solids. It is argued that the field of chemical reaction processes on solid surfaces has recently been paying increasing attention to the fundamental understanding of electrified solid–liquid interfaces and toward the operando study of the minute fraction of catalytically active, structurally dynamic non-equilibrium Taylor-type surface sites. Meanwhile, despite mounting evidence of acting as structural proxies in some cases, the concept of catalytic structure sensitivity of well-defined nanoscale solid surfaces continues to be a key organizing principle for the science of shape-controlled nanocrystals and, hence, constitutes a central recurring theme in this review. After addressing key aspects and recent progress in the wet-chemical synthesis of shaped nanocatalysts, three areas of electrocatalytic processes on solid shape-controlled nanocrystals of current scientific priority are discussed in more detail: the oxygen electroreduction on shape-controlled Pt–Ni polyhedra with its technological relevance for low temperature fuel cells, the CO2 electroreduction to hydrocarbons on Cu polyhedra and the puzzling interplay between chemical and structural effects, and the electrocatalytic oxygen evolution reaction from water on shaped transition metal oxides. The review closes with the conclusion that Surface Science and thermal catalysis, honored by Ertl's Nobel prize a decade ago, continue to show major repercussions on the emerging field of Interface Science.

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 2.5 nm depth, yet vastly different ORR reactivities. On the basis of 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 ele...

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 in situ UV–vis spectro-electrochemistry, operando electrochemical mass spectrometry (DEMS), and in situ 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...

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/IrOx/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 (Ir3.2+). 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 Irz+ 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.

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

Mixed Ni-Fe oxide electrocatalysts have shown high catalytic activity for the oxygen evolution reaction (OER) in alkaline electrolyte. Fundamental research on mixed Ni-Fe OER catalysts has largely focused on high pH, while the OER activity near neutral pH has remained poorly studied. Here we review the activity of an amorphous mixed Ni-Fe oxyhydroxide catalyst supported on carbon (NiFeOx/C) in 0.1 M KOH pH 13, in 0.1 M borate buffer (Bi) pH 9.2, and in 0.1 M phosphate buffer (Pi) pH 7.0. The OER catalytic performance was found to decrease in the order of pH 13 > pH 9.2 > pH 7. In contrast to pH 13 and 9.2, the catalyst cycled in pH 7 showed an instantaneous decrease in OER activity and a simultaneous loss of the Ni(OH)2/NiOOH redox peak. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) showed the formation of crystalline areas upon CV cycling, which appeared more Ni enriched after cycling in pH 7. Deactivated electrodes cycled in pH 13 recovered the OER activity along with a partial reappearance of the Ni redox peak when subsequently cycled in pH 13. SEM-EDX spectroscopy confirmed compositional changes in the bulk during cycling at different pH, with an extensive leaching of Ni in pH 7. Our study provides new insight into the OER activity upon exposure to different electrolyte conditions, which unveils a highly dynamic Ni-Fe oxide framework.

Mixed metal oxides in the nanoscale are of great interest for many aspects of energy related research topics as water splitting, fuel cells and battery technology. The development of scalable, cost-efficient and robust synthetic routes toward well-defined solid state structures is a major objective in this field. While monometallic oxides have been studied in much detail, reliable synthetic recipes targeting specific crystal structures of mixed metal oxide nanoparticles are largely missing. Yet, in order to meet the requirements for a broad range of technical implementation it is necessary to tailor the properties of mixed metal oxides to the particular purpose. Here, we present a study on the impact of the nature of the gas environment on the resulting crystal structure during a post-synthesis thermal heat treatment of manganese–cobalt oxide nanoparticles. We monitor the evolution of the crystal phase structure as the gas atmosphere is altered from pure nitrogen to synthetic air and pure oxygen. The particle size and homogeneity of the resulting nanoparticles increase with oxygen content, while the crystal structure gradually changes from rocksalt-like to pure spinel. We find the composition of the particles to be independent of the gas atmosphere. The manganese–cobalt oxide nanoparticles exhibited promising electrocatalytic activity regarding oxygen evolution in alkaline electrolyte. These findings offer new synthesis pathways for the direct preparation of versatile utilizable mixed metal oxides.

The present work explores the effect of autoclave-based autogenous-pressure vs. ambient pressure conditions on the synthesis and properties of carbon-supported Pt–Rh–Sn nanoparticle electrocatalysts. The Pt–Rh–Sn nanoparticles were characterized by X-ray spectroscopy, electron microscopy and mass spectroscopy and deployed as catalysts for the electrocatalytic ethanol oxidation reaction. Pt–Rh–Sn catalysts precipitated with carbon already present showed narrow particle size distribution around 7 nm, while catalysts supported on carbon after particle formation showed broader size distribution ranging from 8 to 16 nm, similar metal loadings between 40 and 48 wt.% and similar atomic ratios of Pt:Rh:Sn of 30:10:60. The highest ethanol oxidation activity at low overpotentials associated with exceptionally early ethanol oxidation onset potential was observed for ambient-pressure catalysts with the active ternary alloy phase formed in presence of the carbon supports. In contrast, catalysts prepared under ambient pressure in a two-step approach, involving alloy particle formation followed by particle separation and subsequent deposition on the carbon support, yielded the highest overall mass activities. Based on the observed synthesis–activity correlations, a comparative assessment is provided of the synthetic techniques at high vs. low pressures, and in presence and absence of carbon support. Plausible hypotheses in terms of particle dispersion and interparticle distance accounting for these observed differences are discussed.

The need of a sustainable society to advance towards a circular economy, in which there is a balance between the emission and capture of anthropogenic gases like carbon dioxide (CO2), is of utmost scientific and technological importance to ensure the increase or rather preservation of the current prosperity for future generations. The growing global population and increase in worldwide prosperity is tightly connected to a rise in power consumption. Traditionally, this energy demand is supplied by the combustion of fossil fuels, resulting in growing emission of greenhouse gases. In this, CO2 is a key issue which climatic influences and general mitigation is rigorously disused not only in scientific field, but in politics as well. Here, the electrochemical CO2 reduction reaction (CO2RR) is posing as one potential technology, to address questions of power storage for renewable energies and sustainable usage of natural resources. The CO2RR shows a diverse spectrum of products ranging from liquid fuels, as ethanol and propanol, to gaseous building blocks for the chemical industry, as ethylene and CO. The selectivity of this reaction is highly dependent on the nature of the catalyst and is always competing with the hydrogen evolution reaction (HER), due to the aqueous conditions. Recently, very selective and active catalysts have been developed for the production of CO and formic acid. Both compounds are comparably easy to produce, as the reduction only involves the transfer of 2 electrons. Unfortunately, the production of valuable hydrocarbons is much more difficult and suffers from losses in selectivity due to the highly complex reaction mechanism. So far, only copper showed acceptable production rates for hydrocarbons, owing to the favorable binding energies of intermediates. Recent studies have been focused on fundamental parameters to control the selectivity of this complex reaction on copper for C2+ species, ranging from catalyst design to electrolyte selection. On the catalyst side, many effects as the presence of (100) facets1, grain boundaries2 and oxides3 have been suggested to be beneficial, whereas influences of buffer capacity and local electric field brought additional advantages by choice of the electrolyte. While promising results were shown, many studies only focused on low-current density, which are far from an industrial application. Here, at least 200 mA cm-2 are needed for a functional electrolyzer. This poses as a discrepancy between research and application and raises the question if the results from fundamental studies can be transferred to full size electrolyzers.4 For this, we are presenting a cubic Cu2O catalyst, which we investigate in a comprehensive study, moving from initial tests in an H-Cell towards high currents on a Gas Diffusion Electrode (GDE) in a Flow-Cell setup. To deconvolute the parameters dictating CO2RR selectivity, we used X-Ray-Diffraction (XRD), quasi in-situ X-Ray Photon Spectroscopy (XPS) and operando X-ray Absorption Spectroscopy (XAS) to trace phase changes during reaction, in addition to morphological investigations by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). This complementary analysis depicts a highly dynamic system, in which the reduction of oxidized Cu progresses from the surface towards deeper layers, resulting in a purely metallic, defect-rich material. We further focus on performance tests of our Cu2O catalyst at high current density of up to 700 mA cm-2 in a flow-electrolyzer. By varying system parameters as mass loading and nafion content we observe strong changes in selectivity and activity during CO2RR, which we correlate to accessibility of the active copper sites and issues of mass transport. We further discuss the role of surface pH at high current density by varying electrolyte concentration and therefore buffer capacity. Our study suggests distinct differences between CO2RR in electrochemical cells for fundamental studies at low currents compared to tests at high currents in a flow-electrolyzer. Furthermore, we show how recent results from literature translate to performance at high current density and comment on the importance of electrode preparation. Y. Hori, I. Takahashi, O. Koga and N. Hoshi, Journal of Molecular Catalysis A: Chemical, 2003, 199, 39-47. A. Verdaguer-Casadevall, C. W. Li, T. P. Johansson, S. B. Scott, J. T. McKeown, M. Kumar, I. E. L. Stephens, M. W. Kanan and I. Chorkendorff, Journal of the American Chemical Society, 2015, 137, 9808-9811. H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser and B. R. Cuenya, Nature Communications, 2016, 7, 12123. T. Burdyny and W. A. Smith, Energy & Environmental Science, 2019, DOI: 10.1039/c8ee03134g.

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. The precise understanding of the active phase under reaction conditions at the molecular level is crucial for the design of improved catalysts. Now, Strasser, Jones and colleagues correlate the high activity of IrNi@IrOx core–shell nanoparticles with the amount of lattice vacancies produced by the nickel leaching process that takes place before and during water oxidation, and elucidate the underlying structural-electronic effects.

Direct ethanol fuel cells are attractive power sources based on a biorenewable, high energy-density fuel. Their efficiency is limited by the lack of active anode materials which catalyze the breaking of the C−C bond coupled to the 12-electron oxidation to CO2. We report shape-controlled PtNiRh octahedral ethanol oxidation electrocatalysts with excellent activity and previously unachieved low onset potentials as low as 0.1 V vs. RHE, while being highly selective to complete oxidation to CO2. Our comprehensive characterization and in situ electrochemical ATR studies suggest that the formation of a ternary surface site ensemble around the octahedral Pt3Ni1Rhx nanoparticles plays a crucial mechanistic role for this behavior.

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich., This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively., We report on a non-precious, two-phase bifunctional oxygen reduction and evolution (ORR and OER) electrocatalyst with previously unachieved combined roundtrip catalytic reactivity and stability for use in oxygen electrodes of unitized reversible fuel cell/electrolyzers or rechargeable metal-air batteries. The combined OER and ORR overpotential, total, at 10 mA cm(-2) was a record low value of 0.747 V. Rotating Ring Disk Electrode (RRDE) measurements revealed a high faradaic selectivity for the 4 electron pathways, while subsequent continuous MEA tests in reversible electrolyzer cells confirmed the excellent catalyst reactivity rivaling the state-of-the-art combination of iridium (OER) and platinum (ORR).

Seawater is an abundant water resource on our planet and its direct electrolysis has the advantage that it would not compete with activities demanding fresh water. Oxygen selectivity is challenging when performing seawater electrolysis owing to competing chloride oxidation reactions. In this work we propose a design criterion based on thermodynamic and kinetic considerations that identifies alkaline conditions as preferable to obtain high selectivity for the oxygen evolution reaction. The criterion states that catalysts sustaining the desired operating current with an overpotential < 480 mV in alkaline pH possess the best chance to achieve 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide is shown to satisfy this criterion at pH 13 in seawater-mimicking electrolyte. The catalyst was synthesized by a solvothermal method and the activity, surface redox chemistry, and stability were tested electrochemically in alkaline and near-neutral conditions (borate buffer at pH 9.2) and under both fresh seawater conditions. The Tafel slope at low current densities is not influenced by pH or presence of chloride. On the other hand, the addition of chloride ions has an influence in the temporal evolution of the nickel reduction peak and on both the activity and stability at high current densities at pH 9.2. Faradaic efficiency close to 100 % under the operating conditions predicted by our design criteria was proven using in situ electrochemical mass spectrometry.

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.

A NiFe layered double hydroxide (LDH) co-catalyst coated Ta3N5 nanorod electrode is fabricated by vapor deposition of Al on a Ta foil followed by anodizing the Al layer in oxalic acid, (40 V, 0.5 h), pore widening of the as-prepared porous Al2O3 by immersion in 5% aq.

Water splitting catalysed by earth-abundant materials is pivotal for global-scale production of non-fossil fuels, yet our understanding of the active catalyst structure and reactivity is still insufficient. Here we report on the structurally reversible evolution of crystalline Co3O4 electrocatalysts during oxygen evolution reaction identified using advanced in situ X-ray techniques. At electrode potentials facilitating oxygen evolution, a sub-nanometre shell of the Co3O4 is transformed into an X-ray amorphous CoOx(OH)y which comprises di-μ-oxo-bridged Co3+/4+ ions. Unlike irreversible amorphizations, here, the formation of the catalytically-active layer is reversed by re-crystallization upon return to non-catalytic electrode conditions. The Co3O4 material thus combines the stability advantages of a controlled, stable crystalline material with high catalytic activity, thanks to the structural flexibility of its active amorphous oxides. We propose that crystalline oxides may be tailored for generating reactive amorphous surface layers at catalytic potentials, just to return to their stable crystalline state under rest conditions., Understanding of catalyst structure and reactivity is important for the development of water splitting catalysts. Here, the authors report reversible structural transformation of the near-surface of crystalline Co3O4 electrocatalysts to an amorphous CoOx(OH)y during oxygen evolution.

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.

The increasing demand for solar fuels and hydrogen as most promising energy carrier requires new materials for electro-catalytic splitting of water or the back-transformation of hydrogen into electricity. Especially, the oxygen electrode caused by sluggish kinetics is hampering progress for cheap hydrogen made by electrolysis. Moreover the low abundance and the high costs of current highly active noble metal catalysts require new developments for novel cheap and abundant materials. Therefore we have developed a new two-component catalyst system consisting of Nickel-Iron layered double hydroxide (NiFe-LDH) and Fe-N doped carbon material (Fe-N-C) as non noble bifunctional catalyst for the oxygen electrode in alkaline unitized reversible fuel cells/electrolyzers (URFC). The combination of NiFe-LDH known as most active non-noble catalyst for the oxygen evolution reaction (OER) in alkaline media and Fe-N-C known as highly active catalyst for the oxygen reduction reaction (ORR) resulted in a combined overpotential with their respective advantages. NiFe-LDH was prepared by a simple and time-saving microwave assisted solvothermal synthesis route. The crystallinity was confirmed by X-ray diffraction (XRD) spectroscopy and Transmission electron microscopy (TEM) verified morphological characteristics. Rotating Ring disk (RRDE) measurement revealed high selectivity for the multi-component catalyst. We further investigated the behaviour in a membrane based reversible electrolyzer using an anion exchange membrane (AEM, Tokuyama A201). Compared to noble catalyst systems such as Iridium and Platinum the membrane electrode assembly (MEA) measurements proved the ability to compete and supported the initial high activities.

The storage of electric energy has become one of the main challenges in infrastructure development. The development of scalable, cost-efficient and robust synthetic routes toward well-defined solid state structures is a major objective in this field. Electrocatalytic water splitting is a promising way of chemical energy storage wherein the anodic oxygen evolution reaction (OER) is the determining factor. Manganese based oxides have attracted particular attention as non-noble metal catalysts for the OER. Due to their abundance, low price and ecological harmlessness, those materials are promising candidates for a commercial application. Therefore insights into the relations between synthesis conditions, composition, structure and activity as well as stability are of fundamental interest. 1,2 Here, we present the comparison of different synthesis routes for Mixed Oxide Nanoparticles (NPs) of Manganese and Cobalt to yield Mn3-xCoxOy-NPs in a range from X=0 to 3. The well characterized (TEM, EDX, XRD) catalysts were tested with respect to the relation of crystal structure to electrocatalytic activity and stability for the OER in alkaline media using Rotating Disk Electrode (RDE) measurements. We report a significant impact of the Cobalt fraction on the morphology, while the crystal structure remains unchanged. Further we found a non-linear relation between composition and activity. References (1) Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Araujo, J. F.; Reier, T.; Dau, H.; Strasser, P. Nat Commun 2015, 6. (2) Gliech, M.; Bergmann, A.; Spöri, C.; Strasser, P. Journal of Energy Chemistry 2016.

The storage of electric energy has become one of the main challenges in infrastructure development. The development of scalable, cost-efficient and robust synthetic routes toward well-defined solid state structures is a major objective in this field. Electrocatalytic water splitting is a promising way of chemical energy storage wherein the anodic oxygen evolution reaction (OER) is the determining factor. Manganese based oxides have attracted particular attention as non-noble metal catalysts for the OER. Due to their abundance, low price and ecological harmlessness, those materials are promising candidates for a commercial application. Therefore insights into the relations between synthesis conditions, composition, structure and activity as well as stability are of fundamental interest. 1,2 Here, we present the comparison of different synthesis routes for Mixed Oxide Nanoparticles (NPs) of Manganese with Iron and Cobalt. The well characterized (TEM, EDX, XRD) catalysts were tested with respect to the relation of crystal structure to electrocatalytic activity and stability for the OER in alkaline media using Rotating Disk Electrode (RDE) measurements. We report a non-linear relation between composition and activity as well as durability. Literature: [1]Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Araujo, J. F.; Reier, T.; Dau, H.; Strasser, P. Nat Commun 2015, 6.; [2]Gliech, M.; Bergmann, A.; Spöri, C.; Strasser, P. Journal of Energy Chemistry 2016.

Electrochemistry will play a critical role in the development of future sustainable energy storage and conversion technologies, where new functional nanocatalyst materials are the key components for application in fuel cells and electrolyzers. In this context, the generation of hydrogen and oxygen from seawater electrolysis is an attractive process for fuel production since the majority of liquid water available to mankind at the earth's surface is salty water [1]. Unfortunately, chlorine evolution reaction (CER) is a competing reaction to oxygen evolution reaction (OER) when seawater is used as electrolyte. Although OER is thermodynamically favored over CER, the poor kinetics of the OER makes CER the dominant reaction in many catalytic systems. NiFe layered double hydroxide (LDH) has been reported to catalyze the OER reaction in alkaline media at low overpotentials comparable with the performance of noble metals catalysts [2]. In our study, its activity and stability for seawater oxidation have been investigated with a rotating disk electrode in chloride containing electrolytes with basic and slightly basic pH. The electrochemical experiments are combined with products analysis (Figure 1) in order to estimate the selectivity for oxygen evolution. In particular, CER and hypochlorite formation are investigated by an in-line quadrupole mass spectrometer (QMS) and iodometric titration respectively. NiFe LDH shows a high selectivity for OER in alkaline electrolytes (pH 13) containing 0.5 M NaCl, a chloride ions concentration typical of seawater. In 0.3 M borate buffer (pH 9.2) with 0.5 M NaCl the electrochemical experiments show degradation in performance at current densities approaching 10 mA/cm2, despite the high selectivity at lower current densities. This degradation of the anodic current is not observed in pH 13 (0.1 M KOH) in the presence of the same concentration of NaCl. References [1] H.K. Abdel-Aal, K.M. Zohdy, M. Abdel Kareem, Hydrogen Production Using Sea Water Electrolysis, The Open Fuel Cells Journal, 3 (2010) 1-7. [2] M. Gong, Y.G. Li, H.L. Wang, Y.Y. Liang, J.Z. Wu, J.G. Zhou, J. Wang, T. Regier, F. Wei, H.J. Dai, An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation, J Am Chem Soc, 135 (2013) 8452-8455.