Activity-stability relationship in the surface electrochemistry of the oxygen evolution reaction.

Understanding the functional links between the stability and reactivity of oxide materials during the oxygen evolution reaction (OER) is one key to enabling a vibrant hydrogen economy capable of competing with fossil fuel-based technologies. In this work, by focusing on the surface chemistry of monometallic Ru oxide in acidic and alkaline environments, we found that the kinetics of the OER are almost entirely controlled by the stability of the Ru surface atoms. The same activity-stability relationship was found for more complex, polycrystalline and single-crystalline SrRuO(3) thin films in alkaline solutions. We propose that the electrochemical transformation of either water (acidic solutions) or hydroxyl ions (alkaline solutions) to di-oxygen molecules takes place at defect sites that are inherently present on every electrode surface. During the OER, surface defects are also created by the corrosion of the Ru ions. The dissolution is triggered by the potential-dependent change in the valence state (n) of Ru: from stable but inactive Ru(4+) to unstable but active Ru(n>4+). We conclude that if the oxide is stable then it is completely inactive for the OER. A practical consequence is that the best materials for the OER should balance stability and activity in such a way that the dissolution rate of the oxide is neither too fast nor too slow.

Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts.

Design and synthesis of materials for efficient electrochemical transformation of water to molecular hydrogen and of hydroxyl ions to oxygen in alkaline environments is of paramount importance in reducing energy losses in water-alkali electrolysers. Here, using 3d-M hydr(oxy)oxides, with distinct stoichiometries and morphologies in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) regions, we establish the overall catalytic activities for these reaction as a function of a more fundamental property, a descriptor, OH-M(2+δ) bond strength (0 ≤ δ ≤ 1.5). This relationship exhibits trends in reactivity (Mn < Fe < Co < Ni), which is governed by the strength of the OH-M(2+δ) energetic (Ni < Co < Fe < Mn). These trends are found to be independent of the source of the OH, either the supporting electrolyte (for the OER) or the water dissociation product (for the HER). The successful identification of these electrocatalytic trends provides the foundation for rational design of 'active sites' for practical alkaline HER and OER electrocatalysts.

In situ anomalous small-angle X-ray scattering studies of platinum nanoparticle fuel cell electrocatalyst degradation.

Polymer electrolyte fuel cells (PEFCs) are a promising high-efficiency energy conversion technology, but their cost-effective implementation, especially for automotive power, has been hindered by degradation of the electrochemically active surface area (ECA) of the Pt nanoparticle electrocatalysts. While numerous studies using ex situ post-mortem techniques have provided insight into the effect of operating conditions on ECA loss, the governing mechanisms and underlying processes are not fully understood. Toward the goal of elucidating the electrocatalyst degradation mechanisms, we have followed Pt nanoparticle growth during potential cycling of the electrocatalyst in an aqueous acidic environment using in situ anomalous small-angle X-ray scattering (ASAXS). ASAXS patterns were analyzed to obtain particle size distributions (PSDs) of the Pt nanoparticle electrocatalysts at periodic intervals during the potential cycling. Oxide coverages reached under the applied potential cycling protocols were both calculated and determined experimentally. Changes in the PSD, mean diameter, and geometric surface area identify the mechanism behind Pt nanoparticle coarsening in an aqueous environment. Over the first 80 potential cycles, the dominant Pt surface area loss mechanism when cycling to 1.0-1.1 V was found to be preferential dissolution or loss of the smallest particles with varying extents of reprecipitation of the dissolved species onto existing particles, resulting in particle growth, depending on potential profile. Correlation of ASAXS-determined particle growth with both calculated and voltammetrically determined oxide coverages demonstrates that the oxide coverage is playing a key role in the dissolution process and in the corresponding growth of the mean Pt nanoparticle size and loss of ECA. This understanding potentially reduces the complex changes in PSD and ECA resulting from various voltage profiles to a response dependent on oxide coverage.

Considerations on ultra-high frequency electric field effects on oxygen vacancy concentration in oxide thin films.

Atomistic simulations employing dynamic charge transfer between atoms are used to investigate ultra-thin oxide growth on Al(100) metal substrates in the presence of an ac electric field. In the range of 1-10 GHz frequencies, the enhancement in oxidation kinetics by ∼12% over natural oxidation can be explained by the Cabrera-Mott mechanism. At field frequencies approaching 0.1-1 THz, however, we observe a dramatic lowering of the kinetics of oxygen incorporation by ∼35% compared to the maximum oxidation achieved, which results in oxygen non-stoichiometry near the oxide-gas interface (O/Al ≈ 1.0). This is attributed to oxygen desorption from the oxide surface. These results suggest a general strategy to tune oxygen concentration at oxide surfaces using ac electric fields that could be of interest in diverse fields related to surface chemistry and applications such as tunnel barriers, thin dielectrics and oxide interfaces.

Enhancing the alkaline hydrogen evolution reaction activity through the bifunctionality of Ni(OH)2/metal catalysts.

Active in alkaline environment: The activity of nickel, silver, and copper catalysts for the electrochemical transformation of water to molecular hydrogen in alkaline solutions was enhanced by modification of the metal surfaces by Ni(OH)(2) (see picture; I = current density and η = overpotential). The hydrogen evolution reaction rate on a Ni electrode modified by Ni(OH)(2) nanoclusters is about four times higher than on a bare Ni surface.

Selective catalysts for the hydrogen oxidation and oxygen reduction reactions by patterning of platinum with calix[4]arene molecules.

The design of new catalysts for polymer electrolyte membrane fuel cells must be guided by two equally important fundamental principles: optimization of their catalytic behaviour as well as the long-term stability of the metal catalysts and supports in hostile electrochemical environments. The methods used to improve catalytic activity are diverse, ranging from the alloying and de-alloying of platinum to the synthesis of platinum core-shell catalysts. However, methods to improve the stability of the carbon supports and catalyst nanoparticles are limited, especially during shutdown (when hydrogen is purged from the anode by air) and startup (when air is purged from the anode by hydrogen) conditions when the cathode potential can be pushed up to 1.5 V (ref. 11). Under the latter conditions, stability of the cathode materials is strongly affected (carbon oxidation reaction) by the undesired oxygen reduction reaction (ORR) on the anode side. This emphasizes the importance of designing selective anode catalysts that can efficiently suppress the ORR while fully preserving the Pt-like activity for the hydrogen oxidation reaction. Here, we demonstrate that chemically modified platinum with a self-assembled monolayer of calix[4]arene molecules meets this challenging requirement.

Oxygen reduction reaction at three-phase interfaces.

The kinetics of the oxygen reduction reaction (ORR) is studied at metal-supporting electrolyte-Nafion three-phase interfaces. We first demonstrate that the sulfonate anions of Nafion are specifically adsorbed on a wide range of surfaces ranging from Pt(hkl) single-crystal surfaces, Pt-poly, Pt-skin [produced on a Pt(3)Ni(111) surface by annealing in ultrahigh vacuum, UHV] to high-surface-area nanostructured thin-film (NSTF) catalysts. The surface coverage by sulfonate and the strength of the Pt-sulfonate interaction are strongly dependent on the geometry and the nature of the Pt surface atoms. Also, they are found to behave analogous to (bi)sulfate anion-specific adsorption on these surfaces, where for the Pt(hkl) surfaces, the trend is Pt(111)>Pt(110)>Pt(100) and for the Pt-skin surface on Pt(3)Ni(111), the interaction strength is found to be Pt-skin

Engineering stable interfaces for three-dimensional lithium metal anodes.

Lithium metal has long been considered one of the most promising anode materials for advanced lithium batteries (for example, Li-S and Li-O2), which could offer significantly improved energy density compared to state-of-the-art lithium ion batteries. Despite decades of intense research efforts, its commercialization remains limited by poor cyclability and safety concerns of lithium metal anodes. One root cause is the parasitic reaction between metallic lithium and the organic liquid electrolyte, resulting in continuous formation of an unstable solid electrolyte interphase, which consumes both active lithium and electrolyte. Until now, it has been challenging to completely shut down the parasitic reaction. We find that a thin-layer coating applied through atomic layer deposition on a hollow carbon host guides lithium deposition inside the hollow carbon sphere and simultaneously prevents electrolyte infiltration by sealing pinholes on the shell of the hollow carbon sphere. By encapsulating lithium inside the stable host, parasitic reactions are prevented, resulting in impressive cycling behavior. We report more than 500 cycles at a high coulombic efficiency of 99% in an ether-based electrolyte at a cycling rate of 0.5 mA/cm2 and a cycling capacity of 1 mAh/cm2, which is among the most stable Li anodes reported so far.

Stitching h-BN by atomic layer deposition of LiF as a stable interface for lithium metal anode.

Defects are important features in two-dimensional (2D) materials that have a strong influence on their chemical and physical properties. Through the enhanced chemical reactivity at defect sites (point defects, line defects, etc.), one can selectively functionalize 2D materials via chemical reactions and thereby tune their physical properties. We demonstrate the selective atomic layer deposition of LiF on defect sites of h-BN prepared by chemical vapor deposition. The LiF deposits primarily on the line and point defects of h-BN, thereby creating seams that hold the h-BN crystallites together. The chemically and mechanically stable hybrid LiF/h-BN film successfully suppresses lithium dendrite formation during both the initial electrochemical deposition onto a copper foil and the subsequent cycling. The protected lithium electrodes exhibit good cycling behavior with more than 300 cycles at relatively high coulombic efficiency (>95%) in an additive-free carbonate electrolyte.

Femtosecond laser pulse driven melting in gold nanorod aqueous colloidal suspension: identification of a transition from stretched to exponential kinetics.

Many potential industrial, medical, and environmental applications of metal nanorods rely on the physics and resultant kinetics and dynamics of the interaction of these particles with light. We report a surprising kinetics transition in the global melting of femtosecond laser-driven gold nanorod aqueous colloidal suspension. At low laser intensity, the melting exhibits a stretched exponential kinetics, which abruptly transforms into a compressed exponential kinetics when the laser intensity is raised. It is found the relative formation and reduction rate of intermediate shapes play a key role in the transition. Supported by both molecular dynamics simulations and a kinetic model, the behavior is traced back to the persistent heterogeneous nature of the shape dependence of the energy uptake, dissipation and melting of individual nanoparticles. These results could have significant implications for various applications such as water purification and electrolytes for energy storage that involve heat transport between metal nanorod ensembles and surrounding solvents.

Functional links between stability and reactivity of strontium ruthenate single crystals during oxygen evolution.

In developing cost-effective complex oxide materials for the oxygen evolution reaction, it is critical to establish the missing links between structure and function at the atomic level. The fundamental and practical implications of the relationship on any oxide surface are prerequisite to the design of new stable and active materials. Here we report an intimate relationship between the stability and reactivity of oxide catalysts in exploring the reaction on strontium ruthenate single-crystal thin films in alkaline environments. We determine that for strontium ruthenate films with the same conductance, the degree of stability, decreasing in the order (001)>(110)>(111), is inversely proportional to the activity. Both stability and reactivity are governed by the potential-induced transformation of stable Ru(4+) to unstable Ru(n>4+). This ordered(Ru(4+))-to-disordered(Ru(n>4+)) transition and the development of active sites for the reaction are determined by a synergy between electronic and morphological effects.

Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption.

The development of hydrogen-based energy sources as viable alternatives to fossil-fuel technologies has revolutionized clean energy production using fuel cells. However, to date, the slow rate of the hydrogen oxidation reaction (HOR) in alkaline environments has hindered advances in alkaline fuel cell systems. Here, we address this by studying the trends in the activity of the HOR in alkaline environments. We demonstrate that it can be enhanced more than fivefold compared to state-of-the-art platinum catalysts. The maximum activity is found for materials (Ir and Pt0.1Ru0.9) with an optimal balance between the active sites that are required for the adsorption/dissociation of H2 and for the adsorption of hydroxyl species (OHad). We propose that the more oxophilic sites on Ir (defects) and PtRu material (Ru atoms) electrodes facilitate the adsorption of OHad species. Those then react with the hydrogen intermediates (Had) that are adsorbed on more noble surface sites.

Enhancing hydrogen evolution activity in water splitting by tailoring Li⁺-Ni(OH)2-Pt interfaces.

Improving the sluggish kinetics for the electrochemical reduction of water to molecular hydrogen in alkaline environments is one key to reducing the high overpotentials and associated energy losses in water-alkali and chlor-alkali electrolyzers. We found that a controlled arrangement of nanometer-scale Ni(OH)(2) clusters on platinum electrode surfaces manifests a factor of 8 activity increase in catalyzing the hydrogen evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional effect, the edges of the Ni(OH)(2) clusters promoted the dissociation of water and the production of hydrogen intermediates that then adsorbed on the nearby Pt surfaces and recombined into molecular hydrogen. The generation of these hydrogen intermediates could be further enhanced via Li(+)-induced destabilization of the HO-H bond, resulting in a factor of 10 total increase in activity.

Estimation of the hamaker coefficient for a fuel-cell-supported catalyst system.

Hamaker coefficients are estimated for various nanoparticle-support systems commonly used in polymer electrolyte membrane fuel cells. The interaction energies, cohesion between nanoparticles, and adhesion of nanoparticles on the support are also estimated from the experimental data. Comparison between the bulk properties of platinum metal, calculated from the optical spectra available in the literature, and the nanoparticles are provided. Measurements to obtain the optical properties of the systems of interest are also reported. Implications of the van der Waals forces on the supported catalyst structure properties are discussed. The algorithm used in calculation of the Hamaker coefficient using Lifshitz theory was evaluated with known materials, using the spectral parameters available in the literature, and the results are presented.