Li–O
2
(Li–air) batteries are among the most promising energy storage technologies due to their high theoretical specific capacity and energy density. Key challenges with this technology include high ...overpotential losses associated with catalyzing the electrochemical reactions (i.e., oxygen reduction and evolution reactions) at the cathode of the battery. In this contribution, we report through the example of La
2
NiO
4+δ
that layered nickelate oxide materials with rod-shaped nanostructure exhibit promising electrochemical performance as cathode electrocatalysts for Li–O
2
batteries. We demonstrate the ability to control the nanostructure of La
2
NiO
4+δ
electrocatalyst at the nanoscale level using a reverse-microemulsion synthesis approach. We show that Li–O
2
batteries with cathodes containing rod-shaped La
2
NiO
4+δ
electrocatalyst exhibit lower charging potentials and higher reversible capacities when compared to batteries with carbon-only cathodes. Our studies indicate that the enhancement in the battery performance induced by the rod-shaped La
2
NiO
4+δ
electrocatalyst can be attributed to the fact that La
2
NiO
4+δ
nanorods (i) facilitate the formation of nanosized Li
2
O
2
particles during discharge, and (ii) promote the electrocatalytic activity toward the oxygen evolution reaction during charging. These findings open up avenues for the utilization of (i) reverse-microemulsion method for controlling the nanostructure of layered oxide materials, and (ii) nanorod-structured nickelate oxides as efficient cathode electrocatalysts for Li–O
2
batteries.
Oxidative coupling of methane (OCM) is a promising industrial process to upgrade natural gas to high value chemicals. In this study, Temporal Analysis of Products (TAP) and steady-state experiments ...were conducted to distinguish how the composition of surface and gas phase oxygen influence mechanistic details of the selective conversion of CH4 to C2H4 over the Mn-Na2WO4/SiO2 catalyst. The results from TAP studies indicate that methane activation on this catalyst proceeds predominantly via a short-lived, transient surface oxygen species and there is a competition for this species to form either CO or methyl radicals on the surface. This active species has a total lifetime of 3 s and is identified to have a dioxygen (e.g. O22- or O2-) form. We show that the concentration of the transient surface oxygen species significantly impacts the OCM performance. Oxygen attributed to the catalyst lattice (in a singular form e.g., O-), is found to activate methane to a lesser degree, but exclusively forms CO2. Evidence for these surface pathways for methyl radical, CO and CO2 formation identified by TAP are also validated through steady-state experiments. Finally, by distinguishing different catalyst oxygen species and their role in selective/nonselective pathways, important screening criteria have been identified for the advancement of superior catalyst formulations.
Oxygen electrocatalysis plays a critical role in the efficiency of important energy conversion and storage systems. While many efforts have focused on designing efficient electrocatalysts for these ...processes, optimal catalysts that are inexpensive, active, selective, and stable are still being searched. Nonstoichiometric, mixed-metal oxides present a promising group of electrocatalysts for these processes due to the versatility of the surface composition and fast oxygen conducting properties. Herein, we demonstrate, using a combination of theoretical and experimental studies, the ability to develop design principles that can be used to engineer oxygen electrocatalysis activity of layered, mixed ionic-electronic conducting Ruddlesden–Popper (R–P) oxides. We show that a density function theory (DFT) derived descriptor, O2 binding energy on a surface oxygen vacancy, can be effective in identifying efficient R–P oxide structures for oxygen reduction reaction (ORR). Using a controlled synthesis method, well-defined nanostructures of R–P oxides are obtained, which along with thermochemical and electrochemical activity studies are used to validate the design principles. This has led to the identification of a highly active ORR electrocatalyst, nanostructured Co-doped lanthanum nickelate oxide, which when incorporated in solid oxide fuel cell cathodes significantly enhances the performance at intermediate temperatures (∼550 °C), while maintaining long-term stability. The reported findings demonstrate the effectiveness of the developed design principles to engineer mixed ionic-electronic conducting oxides for efficient oxygen electrocatalysis, and the potential of nanostructured Co-doped lanthanum nickelate oxides as promising catalysts for oxygen electrocatalysis.
Inverted catalytic systems, in which metal oxide films are deposited on top of metallic substrates, present significant potential for catalysis, because of the ability to enhance the metal/metal ...oxide interface. Most of the reported methods for synthesizing these structures involve thermal treatment of the oxide film, which presents a significant challenge for reducible metal oxides, because of strong metal-oxide interactions, often leading to blocking of metal active sites. Herein, a solution-phase method is developed to synthesize Pd@TiO2 inverted catalytic structures where encapsulation is conducted at room temperature using a sol–gel reaction of Ti alkoxide precursors to circumvent thermal treatment. We show that key synthesis parameters can be used to control the desired physical properties of the inorganic encapsulating film to achieve enhanced catalytic performance. Significant effects on catalytic performance toward a probe reaction, hydroisomerization of 1-hexene, are reported as a function of the pore size of the TiO2 film.
Tuning catalysis at solid–solid interfaces is critical for the development of next-generation energy storage devices such as Li–O2 batteries, where solid lithium–oxygen species are formed and ...dissociated on a solid catalyst. Herein, atomically controlled synthesis is combined with theoretical calculations, electrochemical studies, and detailed characterization measurements to show that the interface between an oxide catalyst and the solid products is key to selectively control discharge product distribution, consequently affecting charge overpotentials. A surface structure-dependent electrochemical performance for nonprecious metal-containing, nanostructured lanthanum nickelate oxide (La2NiO4+δ, LNO) electrocatalysts is demonstrated. LNO nanostructures with (001) NiO-terminated surfaces exhibit lower charge overpotentials, as opposed to irregularly terminated polyhedral-shaped oxides of the same composition. It is found that these LNO nanostructures, with controlled surface structure, enhance the performance by providing a platform for stabilization of Li-deficient oxide species during discharge, consequently lowering overpotential losses associated with their oxidation during charge. Periodic density functional theory modeling of the solid–solid interface between the oxide catalyst and the lithium-discharge products suggests that stabilization of the Li-deficient products is due to the formation of a lithiated oxide surface, which is in turn facilitated by an electron transfer from the near surface Li to the surface lattice oxygen atoms. The potential-dependent stability of these lithium–oxygen species on LNO is predicted and confirmed experimentally. These results provide a framework for probing and engineering catalysis at solid–solid interfaces, and strategies for improving the efficiency of next-generation energy storage systems using nonprecious, nanostructured mixed metal oxide catalysts.
PDF
