Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

Quantum Chemical Design of Doped Ca2MnAlO(5+δ) as Oxygen Storage Media

Brownmillerite Ca2MnAlO5 has an exceptional capability to robustly adsorb half-molecules of oxygen and form Ca2MnAlO5.5. To utilize this unique property to regulate oxygen-involved reactions, it is crucial to match the oxygen release-intake equilibrium with targeted reaction conditions. Here we perform a comprehensive investigation of the strategy of tuning the oxygen storage property of Ca2MnAlO5 through chemical doping. For undoped Ca2MnAlO5+δ, our first-principles calculation predicts that the equilibrium temperature at a pressure of 1 atm of O2 is 848 K, which is in excellent agreement with experimental results. Furthermore, the doping of alkaline earth ions at the Ca site, trivalent ions at the Al site, and 3d transition metal ions at the Mn site is analyzed. By the doping of 12.5% of Ga, V, and Ti, the equilibrium temperature shifts to high values by approximately 110-270 K, while by the doping of 12.5% of Fe, Sr, and Ba, the equilibrium temperature is lowered by approximately 20-210 K. The doping of these elements is thermodynamically stable, and doping other elements including Mg, Sc, Y, Cr, Co, and Ni generates metastable compounds. The doping of a higher content of Fe, however, lowers the oxygen storage capacity. Finally, on the basis of our calculated data, we prove that the formation energetics of nondilute interacting oxygen vacancy in doped Ca2MnAlO5.5 scale linearly with a simple descriptor, the oxygen p-band position relative to the Fermi level. The higher-oxygen p-band position leads to a lower vacancy formation energy and thus a lower oxygen release temperature. Understanding such a relationship between fundamental quantum chemical properties and macroscopic properties paves the road to the design and optimization of novel functional oxides.

Ab Initio Approach for Prediction of Oxide Surface Structure, Stoichiometry, and Electrocatalytic Activity in Aqueous Solution

The design of efficient, stable, and inexpensive catalysts for oxygen evolution and reduction is crucial for the development of electrochemical energy conversion devices such as fuel cells and metal-air batteries. Currently, such design is limited by challenges in atomic-scale experimental characterization and computational modeling of solid-liquid interfaces. Here, we begin to address these issues by developing a general-, first-principles-, and electrochemical-principles-based framework for prediction of catalyst surface structure, stoichiometry, and stability as a function of pH, electrode potential, and aqueous cation concentration. We demonstrate the approach by determining the surface phase diagram of LaMnO3, which has been studied for oxygen evolution and reduction and computing the reaction overpotentials on the relevant surface phases. Our results illustrate the critical role of solvated cation species in governing the catalyst surface structure and stoichiometry, and thereby catalytic activity, in aqueous solution.

A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are traditionally carried out with noble metals (such as Pt) and metal oxides (such as RuO₂ and MnO₂) as catalysts, respectively. However, these metal-based catalysts often suffer from multiple disadvantages, including high cost, low selectivity, poor stability and detrimental environmental effects. Here, we describe a mesoporous carbon foam co-doped with nitrogen and phosphorus that has a large surface area of ∼1,663 m(2) g(-1) and good electrocatalytic properties for both ORR and OER. This material was fabricated using a scalable, one-step process involving the pyrolysis of a polyaniline aerogel synthesized in the presence of phytic acid. We then tested the suitability of this N,P-doped carbon foam as an air electrode for primary and rechargeable Zn-air batteries. Primary batteries demonstrated an open-circuit potential of 1.48 V, a specific capacity of 735 mAh gZn(-1) (corresponding to an energy density of 835 Wh kgZn(-1)), a peak power density of 55 mW cm(-2), and stable operation for 240 h after mechanical recharging. Two-electrode rechargeable batteries could be cycled stably for 180 cycles at 2 mA cm(-2). We also examine the activity of our carbon foam for both OER and ORR independently, in a three-electrode configuration, and discuss ways in which the Zn-air battery can be further improved. Finally, our density functional theory calculations reveal that the N,P co-doping and graphene edge effects are essential for the bifunctional electrocatalytic activity of our material.

Bio-Inspired Leaf-Mimicking Nanosheet/Nanotube Heterostructure as a Highly Efficient Oxygen Evolution Catalyst

Plant leaves represent a unique 2D/1D heterostructure for enhanced surface reaction and efficient mass transport. Inspired by plant leaves, a 2D/1D CoO x heterostructure is developed that is composed of ultrathin CoO x nanosheets further assembled into a nanotube structure. This bio-inspired architecture allows a highly active Co2+ electronic structure for an efficient oxygen evolution reaction (OER) at the atomic scale, ultrahigh surface area (371 m2 g-1) for interfacial electrochemical reaction at the nanoscale, and enhanced transport of charge and electrolyte over CoO x nanotube building blocks at the microscale. Consequently, this CoO x nanosheet/nanotube heterostructure demonstrates a record-high OER performance based on cobalt compounds reported so far, with an onset potential of ≈1.46 V versus reversible hydrogen electrode (RHE), a current density of 51.2 mA cm-2 at 1.65 V versus RHE, and a Tafel slope of 75 mV dec-1. Using the CoO x nanosheet/nanotube catalyst and a Pt-mesh, a full water splitting cell with a 1.5-V battery is also demonstrated.

Highly porous non-precious bimetallic electrocatalysts for efficient hydrogen evolution

A robust and efficient non-precious metal catalyst for hydrogen evolution reaction is one of the key components for carbon dioxide-free hydrogen production. Here we report that a hierarchical nanoporous copper-titanium bimetallic electrocatalyst is able to produce hydrogen from water under a mild overpotential at more than twice the rate of state-of-the-art carbon-supported platinum catalyst. Although both copper and titanium are known to be poor hydrogen evolution catalysts, the combination of these two elements creates unique copper-copper-titanium hollow sites, which have a hydrogen-binding energy very similar to that of platinum, resulting in an exceptional hydrogen evolution activity. In addition, the hierarchical porosity of the nanoporous copper-titanium catalyst also contributes to its high hydrogen evolution activity, because it provides a large-surface area for electrocatalytic hydrogen evolution, and improves the mass transport properties. Moreover, the catalyst is self-supported, eliminating the overpotential associated with the catalyst/support interface.

Investigations into non-precious metal catalysts for hydrogen evolution are ongoing. Here, the authors report a hierarchical, nanoporous copper-titanium electrocatalyst, and demonstrate that it catalyses hydrogen production at twice the over-all rate of commercial platinum-based catalysts.

Effect of B site coordination environment in the ORR activity in disordered brownmillerites Ba2In2-xCexO5+δ

Ba2In2O5 brownmillerites in which the In site is progressively doped with Ce exhibit excellent oxygen reduction activity under alkaline conditions. Ce doping leads to structural changes advantageous for the reaction. Twenty-five percent doping retains the ordered structure of brownmillerite with alternate layers of tetrahedra and octahedra, whereas further increase in Ce concentration creates disorder. Structures with disordered oxygen atoms/vacancies are found to be better oxygen reduction reaction catalysts probably aided by isotropic ionic conduction, and Ba2In0.5Ce1.5O5+δ is the most active. This enhanced activity is correlated to the more symmetric Ce site coordination environment in this compound. Stoichiometric perovskite BaCeO3 with the highest concentration of Ce shows very poor activity emphasizing the importance of oxygen vacancies, which facilitate O2 adsorption, in tandem with catalytic sites in oxygen reduction reactions.

Ab initio GGA+U study of oxygen evolution and oxygen reduction electrocatalysis on the (001) surfaces of lanthanum transition metal perovskites LaBO3(B = Cr, Mn, Fe, Co and Ni)

Theoretical ORR volcano of LaBO₃perovskite (001) surfaces at stable adsorbate coverage.

Reduction-induced surface amorphization enhances the oxygen evolution activity in Co3O4

Modifying crystalline Co₃O₄ by thermal H₂ annealing and air exposure produced an amorphous surface layer consisting of mixed hydrated cobalt hydroxide/carbonate species and remarkably enhanced the oxygen evolution activity.

Activity and stability trends of perovskite oxides for oxygen evolution catalysis at neutral pH

Perovskite oxides (ABO₃) have been studied extensively to promote the kinetics of oxygen evolution reaction (OER) at neutral pH. The same activity descriptors apply in alkaline and neutral media. Yet, stability is a greater challenge in neutral media.

Unraveling the effect of La A-site substitution on oxygen ion diffusion and oxygen catalysis in perovskite BaFeO3by data-mining molecular dynamics and density functional theory

The effects of La substitution on oxygen transport and catalysis in BaFeO₃are unraveled by data-driven molecular dynamics and density functional theory.

Nickel cobalt oxide hollow nanosponges as advanced electrocatalysts for the oxygen evolution reaction

Nickel cobalt oxide hollow nanosponges were synthesized through a kinetically controlled strategy, which exhibited excellent catalytic activity towards oxygen evolution reaction.

Efficient electrocatalytic oxygen evolution on amorphous nickel-cobalt binary oxide nanoporous layers

Nanoporous Ni-Co binary oxide layers were electrochemically fabricated by deposition followed by anodization, which produced an amorphous layered structure that could act as an efficient electrocatalyst for water oxidation. The highly porous morphologies produced higher electrochemically active surface areas, while the amorphous structure supplied abundant defect sites for oxygen evolution. These Ni-rich (10-40 atom % Co) binary oxides have an increased active surface area (roughness factor up to 17), reduced charge transfer resistance, lowered overpotential (∼325 mV) that produced a 10 mA cm(-2) current density, and a decreased Tafel slope (∼39 mV decade(-1)). The present technique has a wide range of applications for the preparation of other binary or multiple-metals or metal oxides nanoporous films. Fabrication of nanoporous materials using this method could provide products useful for renewable energy production and storage applications.

Water oxidation by amorphous cobalt-based oxides: volume activity and proton transfer to electrolyte bases

Water oxidation in the neutral pH regime catalyzed by amorphous transition-metal oxides is of high interest in energy science. Crucial determinants of electrocatalytic activity were investigated for a cobalt-based oxide film electrodeposited at various thicknesses on inert electrodes. For water oxidation at low current densities, the turnover frequency (TOF) per cobalt ion of the bulk material stayed fully constant for variation of the thickness of the oxide film by a factor of 100 (from about 15 nm to 1.5 μm). Thickness variation changed neither the nanostructure of the outer film surface nor the atomic structure of the oxide catalyst significantly. These findings imply catalytic activity of the bulk hydrated oxide material. Nonclassical dependence on pH was observed. For buffered electrolytes with pKa values of the buffer base ranging from 4.7 (acetate) to 10.3 (hydrogen carbonate), the catalytic activity reflected the protonation state of the buffer base in the electrolyte solution directly and not the intrinsic catalytic properties of the oxide itself. It is proposed that catalysis of water oxidation occurs within the bulk hydrated oxide film at the margins of cobalt oxide fragments of molecular dimensions. At high current densities, the availability of a proton-accepting base at the catalyst-electrolyte interface controls the rate of water oxidation. The reported findings may be of general relevance for water oxidation catalyzed at moderate pH by amorphous transition-metal oxides.

Anomalous Interface and Surface Strontium Segregation in (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ Heterostructured Thin Films

Heterostructured oxides have shown unusual electrochemical properties including enhanced catalytic activity, ion transport, and stability. In particular, it has been shown recently that the activity of oxygen electrocatalysis on the Ruddlesden-Popper/perovskite (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ heterostructure is remarkably enhanced relative to the Ruddlesden-Popper and perovskite constituents. Here we report the first atomic-scale structure and composition of (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ grown on SrTiO3. We observe anomalous strontium segregation from the perovskite to the interface and the Ruddlesden-Popper phase using direct X-ray methods as well as with ab initio calculations. Such Sr segregation occurred during the film growth, and no significant changes were found upon subsequent annealing in O2. Our findings provide insights into the design of highly active catalysts for oxygen electrocatalysis.

A hierarchical Ni–Co–O@Ni–Co–S nanoarray as an advanced oxygen evolution reaction electrode

Hierarchical Ni–Co–O@Ni–Co–S nanoarrays are developed by a facile approach and show excellent performance for water oxidation.

Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction

Activity, selectivity and stability of oxygen evolution catalysts for water electrolyzers: an interplay between composition, morphology, preparation and processing.

Activation of oxygen evolving perovskites for oxygen reduction by functionalization with Fe–Nx/C groups

A novel approach for integrating low-cost oxygen evolving and oxygen reducing electrocatalysts, and obtaining high-performing bifunctional composite materials for use in alkaline media.