Oxygen evolution on Ru1 − x Ni x O2 − y nanocrystalline electrodes

Toward Realistic Models of the Electrocatalytic Oxygen Evolution Reaction

The electrocatalytic oxygen evolution reaction (OER) supplies the protons and electrons needed to transform renewable electricity into chemicals and fuels. However, the OER is kinetically sluggish; it operates at significant rates only when the applied potential far exceeds the reversible voltage. The origin of this overpotential is hidden in a complex mechanism involving multiple electron transfers and chemical bond making/breaking steps. Our desire to improve catalytic performance has then made mechanistic studies of the OER an area of major scientific inquiry, though the complexity of the reaction has made understanding difficult. While historically, mechanistic studies have relied solely on experiment and phenomenological models, over the past twenty years ab initio simulation has been playing an increasingly important role in developing our understanding of the electrocatalytic OER and its reaction mechanisms. In this Review we cover advances in our mechanistic understanding of the OER, organized by increasing complexity in the way through which the OER is modeled. We begin with phenomenological models built using experimental data before reviewing early efforts to incorporate ab initio methods into mechanistic studies. We go on to cover how the assumptions in these early ab initio simulations─no electric field, electrolyte, or explicit kinetics─have been relaxed. Through comparison with experimental literature, we explore the veracity of these different assumptions. We summarize by discussing the most critical open challenges in developing models to understand the mechanisms of the OER.

The Pivotal Role of s-, p-, and f-Block Metals in Water Electrolysis: Status Quo and Perspectives

Transition metals, in particular noble metals, are the most common species in metal-mediated water electrolysis because they serve as highly active catalytic sites. In many cases, the presence of nontransition metals, that is, s-, p-, and f-block metals with high natural abundance in the earth-crust in the catalytic material is indispensable to boost efficiency and durability in water electrolysis. This is why alkali metals, alkaline-earth metals, rare-earth metals, lean metals, and metalloids receive growing interest in this research area. In spite of the pivotal role of these nontransition metals in tuning efficiency of water electrolysis, there is far more room for developments toward a knowledge-based catalyst design. In this review, five classes of nontransition metals species which are successfully utilized in water electrolysis, with special emphasis on electronic structure-catalytic activity relationships and phase stability, are discussed. Moreover, specific fundamental aspects on electrocatalysts for water electrolysis as well as a perspective on this research field are also addressed in this account. It is anticipated that this review can trigger a broader interest in using s-, p-, and f-block metals species toward the discovery of advanced polymetal-containing electrocatalysts for practical water splitting.

Dynamic Surface Reconstruction Unifies the Electrocatalytic Oxygen Evolution Performance of Nonstoichiometric Mixed Metal Oxides

Compositionally versatile, nonstoichiometric, mixed ionic-electronic conducting metal oxides of the form A _n+1B _n O_3n+1 (n = 1 → ∞; A = rare-earth-/alkaline-earth-metal cation; B = transition-metal (TM) cation) remain a highly attractive class of electrocatalysts for catalyzing the energy-intensive oxygen evolution reaction (OER). The current design strategies for describing their OER activities are largely derived assuming a static, unchanged view of their surfaces, despite reports of dynamic structural changes to 3d TM-based perovskites during OER. Herein, through variations in the A- and B-site compositions of A _n+1B _n O_3n+1 oxides (n = 1 (A₂BO₄) or n = ∞ (ABO₃); A = La, Sr, Ca; B = Mn, Fe, Co, Ni), we show that, in the absence of electrolyte impurities, surface restructuring is universally the source of high OER activity in these oxides and is dependent on the initial oxide composition. Oxide surface restructuring is induced by irreversible A-site cation dissolution, resulting in in situ formation of a TM oxyhydroxide shell on top of the parent oxide core that serves as the active surface for OER. The rate of surface restructuring is found to depend on (i) composition of A-site cations, with alkaline-earth-metal cations dominating lanthanide cation dissolution, (ii) oxide crystal phase, with n = 1 A₂BO₄ oxides exhibiting higher rates of A-site dissolution in comparison to n = ∞ ABO₃ perovskites, (iii) lattice strain in the oxide induced by mixed rare-earth- and alkaline-earth-metal cations in the A-site, and (iv) oxide reducibility. Among the in situ generated 3d TM oxyhydroxide structures from A _n+1B _n O_3n+1 oxides, Co-based structures are characterized by superior OER activity and stability, even in comparison to as-synthesized Co-oxyhydroxide, pointing to the generation of high active surface area structures through oxide restructuring. These insights are critical toward the development of revised design criteria to include surface dynamics for effectively describing the OER activity of nonstoichiometric mixed-metal oxides.

Fundamental understanding of the acidic oxygen evolution reaction: mechanism study and state-of-the-art catalysts

The oxygen evolution reaction (OER), as the anodic reaction of water electrolysis (WE), suffers greatly from low reaction kinetics and thereby hampers the large-scale application of WE. Seeking active, stable, and cost-effective OER catalysts in acidic media is therefore of great significance. In this perspective, studying the reaction mechanism and exploiting advanced anode catalysts are of equal importance, where the former provides guidance for material structural engineering towards a better catalytic activity. In this review, we first summarize the currently proposed OER catalytic mechanisms, i.e., the adsorbate evolution mechanism (AEM) and lattice oxygen evolution reaction (LOER). Subsequently, we critically review several acidic OER electrocatalysts reported recently, with focus on structure-performance correlation. Finally, a few suggestions on exploring future OER catalysts are proposed.

Preparation and characterization of electrodeposited Ni-Ru alloys: morphological and catalytic study

Nickel-ruthenium alloys with various compositions have been deposited by electrodeposition for the first time. Cyclic voltammetry and linear stripping voltammetry measurements show that codeposition of nickel with ruthenium is possible below the potential value of nickel reduction. High-quality alloys containing nickel and ruthenium can be plated at cathodic potentials ranging from − 0.5 to − 1.0 V vs SCE. Deposited coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The diffractograms obtained show that an increase of nickel concentration in alloy will lead to a change in the phase composition and formation of NiRu (100) and (101) phases which is observed to be 78 mas.% Ni. SEM studies confirm the surface homogeneity and presence of small, regular grains. AFM observation allows the estimation of the real surface area of obtained alloys which increase with more negative electrodeposition potentials. Ni-Ru alloys were found to be highly electroactive in the water splitting process, which can be connected with the presence of the NiRu phase and a well-developed electroactive area.

Recommended Practices and Benchmark Activity for Hydrogen and Oxygen Electrocatalysis in Water Splitting and Fuel Cells

Electrochemical energy storage by making H₂ an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measurements, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state-of-the-art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selection, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic- and background-correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activities of some state-of-the-art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories.

Electrochemical characterization of manganese oxides as a water oxidation catalyst in proton exchange membrane electrolysers

The performance of four polymorphs of manganese (Mn) dioxides as the catalyst for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolysers was examined. The comparison of the activity between Mn oxides/carbon (Mn/C), iridium oxide/carbon (Ir/C) and platinum/carbon (Pt/C) under the same condition in PEM electrolysers showed that the γ-MnO₂/C exhibited a voltage efficiency for water electrolysis comparable to the case with Pt/C, while lower than the case with the benchmark Ir/C OER catalyst. The rapid decrease in the voltage efficiency was observed for a PEM electrolyser with the Mn/C, as indicated by the voltage shift from 1.7 to 1.9 V under the galvanostatic condition. The rapid deactivation was also observed when Pt/C was used, indicating that the instability of PEM electrolysis with Mn/C is probably due to the oxidative decomposition of carbon supports. The OER activity of the four types of Mn oxides was also evaluated at acidic pH in a three-electrode system. It was found that the OER activity trends of the Mn oxides evaluated in an acidic aqueous electrolyte were distinct from those in PEM electrolysers, demonstrating the importance of the evaluation of OER catalysts in a real device condition for future development of noble-metal-free PEM electrolysers.

Nanotubular Iridium-Cobalt Mixed Oxide Crystalline Architectures Inherited from Cobalt Oxide for Highly Efficient Oxygen Evolution Reaction Catalysis

Here, we report the unique transformation of one-dimensional tubular mixed oxide nanocomposites of iridium (Ir) and cobalt (Co) denoted as Ir_xCo_1-xO_y, where x is the relative Ir atomic content to the overall metal content. The formation of a variety of Ir_xCo_1-xO_y (0 ≤ x ≤ 1) crystalline tubular nanocomposites was readily achieved by electrospinning and subsequent calcination process. Structural characterization clearly confirmed that Ir_xCo_1-xO_y polycrystalline nanocomposites had a tubular morphology consisting of Ir/IrO₂ and Co₃O₄, where Ir, Co, and O were homogeneously distributed throughout the entire nanostructures. The facile formation of Ir_xCo_1-xO_y nanotubes was mainly ascribed to the inclination of Co₃O₄ to form the nanotubes during the calcination process, which could play a critical role in providing a template of tubular structure and facilitating the formation of IrO₂ by being incorporated with Ir precursors. Furthermore, the electroactivity of obtained Ir_xCo_1-xO_y nanotubes was characterized for oxygen evolution reaction (OER) with rotating disk electrode voltammetry in 1 M NaOH aqueous solution. Among diverse Ir_xCo_1-xO_y, Ir_0.46Co_0.54O_y nanotubes showed the best OER activity (the least-positive onset potential, greatest current density, and low Tafel slope), which was even better than that of commercial Ir/C. The Ir_0.46Co_0.54O_y nanotubes also exhibited a high stability in alkaline electrolyte. Expensive Ir mixed with cheap Co at an optimum ratio showed a greater OER catalytic activity than pure Ir oxide, one of the most efficient OER catalysts.

The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation

This Review addresses the technical challenges, scientific basis, recent progress, and outlook with respect to the stability and degradation of catalysts for the oxygen evolution reaction (OER) operating at electrolyzer anodes in acidic environments with an emphasis on ion exchange membrane applications. First, the term "catalyst stability" is clarified, as well as current performance targets, major catalyst degradation mechanisms, and their mitigation strategies. Suitable in situ experimental methods are then evaluated to give insight into catalyst degradation and possible pathways to tune OER catalyst stability. Finally, the importance of identifying universal figures of merit for stability is highlighted, leading to a comprehensive accelerated lifetime test that could yield comparable performance data across different laboratories and catalyst types. The aim of this Review is to help disseminate and stress the important relationships between structure, composition, and stability of OER catalysts under different operating conditions.

Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes

Chlorine gas and sodium chlorate are two base chemicals produced through electrolysis of sodium chloride brine which find uses in many areas of industrial chemistry. Although the industrial production of these chemicals started over 100 years ago, there are still factors that limit the energy efficiencies of the processes. This review focuses on the unwanted production of oxygen gas, which decreases the charge yield by up to 5%. Understanding the factors that control the rate of oxygen production requires understanding of both chemical reactions occurring in the electrolyte, as well as surface reactions occurring on the anodes. The dominant anode material used in chlorate and chlor-alkali production is the dimensionally stable anode (DSA), Ti coated by a mixed oxide of RuO2 and TiO2. Although the selectivity for chlorine evolution on DSA is high, the fundamental reasons for this high selectivity are just now becoming elucidated. This review summarizes the research, since the early 1900s until today, concerning the selectivity between chlorine and oxygen evolution in chlorate and chlor-alkali production. It covers experimental as well as theoretical studies and highlights the relationships between process conditions, electrolyte composition, the material properties of the anode, and the selectivity for oxygen formation.

Oxygen evolution on well-characterized mass-selected Ru and RuO2 nanoparticles

Oxygen evolution was investigated on model, mass-selected RuO2 nanoparticles in acid, prepared by magnetron sputtering. Our investigations include electrochemical measurements, electron microscopy, scanning tunneling microscopy and X-ray photoelectron spectroscopy. We show that the stability and activity of nanoparticulate RuO2 is highly sensitive to its surface pretreatment. At 0.25 V overpotential, the catalysts show a mass activity of up to 0.6 A mg-1 and a turnover frequency of 0.65 s-1, one order of magnitude higher than the current state-of-the-art.

Searching for active binary rutile oxide catalyst for water splitting from first principles

Water electrolysis is an important route to large-scale hydrogen production using renewable energy, in which the oxygen evolution reaction (OER: 2H(2)O → O(2) + 4H(+) + 4e(-)) causes the largest energy loss in traditional electrocatalysts involving Ru-Ir mixed oxides. Following our previous mechanistic studies on the OER on RuO(2)(110) (J. Am. Chem. Soc. 2010, 132, 18214), this work aims to provide further insight into the key parameters relevant to the activity of OER catalysts by investigating a group of rutile-type binary metal oxides, including RuNiO(2), RuCoO(2), RuRhO(2), RuIrO(2) and OsIrO(2). Two key aspects are focused on, namely the surface O coverage at the relevant potential conditions and the kinetics of H(2)O activation on the O-covered surfaces. The O coverage for all the oxides investigated here is found to be 1 ML at the concerned potential (1.23 V) with all the exposed metal cations being covered by terminal O atoms. The calculated free energy barrier for the H(2)O dissociation on the O covered surfaces varies significantly on different surfaces. The highest OER activity occurs at RuCoO(2) and RuNiO(2) oxides with a predicted activity about 500 times higher than pure RuO(2). On these oxides, the surface bridging O near the terminal O atom has a high activity for accepting the H during H(2)O splitting. It is concluded that while the differential adsorption energy of the terminal O atom influences the OER activity to the largest extent, the OER activity can still be tuned by modifying the electronic structure of surface bridging O.