Daniel Adjei Agyeman

An improved catalytic effect of nitrogen-doped TiO2 nanofibers for rechargeable Li–O2 batteries; the role of oxidation states and vacancies on the surface

This work deals with nitrogen-doped TiO2 nanofibers with increased ionic conductivity and good catalytic activity, as a potential cathode catalyst for lithium–air battery. The electrochemical enhancement with nitrogen-doped TiO2 in comparison with pristine TiO2 could be realized by the changed electronic properties correlated with the evolution of oxygen vacancies altering the surface oxidation state of TiO2. A discharge capacity greater than 11 000 mA h g−1(carbon) and a cyclic retention more than 25 cycles were achieved with the corresponding nitrogen-doped TiO2 catalyst.

In situ analyses for ion storage materials

Development of high performance electrode materials for energy storage is one of the most important issues for our future society. However, a lack of clear analytical views limits critical understanding of electrode materials. This review covers useful analytical work using X-ray diffraction, X-ray absorption spectroscopy, microscopy and neutron diffraction for ion storage systems. The in situ observation facilitates comprehending real-time ion storage behaviour while the ion storage system is operating, which help us to understand detailed physical and chemical properties. We will discuss how the tools have been used to reveal detailed reaction mechanisms and underlying properties of electrode materials.

High-Energy-Density Metal–Oxygen Batteries: Lithium–Oxygen Batteries vs Sodium–Oxygen Batteries

The development of next-generation energy-storage devices with high power, high energy density, and safety is critical for the success of large-scale energy-storage systems (ESSs), such as electric vehicles. Rechargeable sodium-oxygen (Na-O2 ) batteries offer a new and promising opportunity for low-cost, high-energy-density, and relatively efficient electrochemical systems. Although the specific energy density of the Na-O2 battery is lower than that of the lithium-oxygen (Li-O2 ) battery, the abundance and low cost of sodium resources offer major advantages for its practical application in the near future. However, little has so far been reported regarding the cell chemistry, to explain the rate-limiting parameters and the corresponding low round-trip efficiency and cycle degradation. Consequently, an elucidation of the reaction mechanism is needed for both lithium-oxygen and sodium-oxygen cells. An in-depth understanding of the differences and similarities between Li-O2 and Na-O2 battery systems, in terms of thermodynamics and a structural viewpoint, will be meaningful to promote the development of advanced metal-oxygen batteries. State-of-the-art battery design principles for high-energy-density lithium-oxygen and sodium-oxygen batteries are thus reviewed in depth here. Major drawbacks, reaction mechanisms, and recent strategies to improve performance are also summarized.

Pd-Impregnated NiCo2O4 nanosheets/porous carbon composites as a free-standing and binder-free catalyst for a high energy lithium–oxygen battery

A novel free-standing air electrode with various structural and electrochemical merits was designed for a highly reversible lithium–oxygen battery. Interconnected NiCo2O4 nanosheets were grown almost perpendicular to the surface of carbon foam acting as a gas diffusion layer via a hydrothermal method combined with low temperature calcination and then decorated with palladium (Pd). Basically, this novel class of heterostructured catalysts consists of hierarchical nanosheets that can provide enough catalytic surface and open space, which is advantageous for oxygen or lithium ion transfer. In addition, the intrinsic porous structure of carbon foam better facilitates barrier-free oxygen transport and electrolyte penetration, while the introduction of Pd can modify the electronic structure of NiCo2O4, thereby enhancing electron transport all over the electrode. Because Pd incorporation also evolves the surface oxygen vacancies, which helps the discharge product (Li2O2) grow into a flower-like form, its formation or decomposition in the free-standing Pd@NiCo2O4 electrode could be rendered extremely reversible, finally realizing low charge over-potential, high discharge capacity (the maximum capacity reaches about 4000 mA h g−1) and long cycle life (extremely stable cyclic retention almost up to 100 cycles under the capacity limitation of 1000 mA h g−1).

CNT@Ni@Ni–Co silicate core–shell nanocomposite: a synergistic triple-coaxial catalyst for enhancing catalytic activity and controlling side products for Li–O2 batteries

A great challenge in the application of carbon-based materials to Li–O2 batteries is to prevent the formation of carbonate-based side products, thereby extending the cycle life of Li–O2 batteries. Herein, for the first time, CNT@Ni@NiCo silicate core–shell nanocomposite is designed and used as a cathode catalyst in Li–O2 batteries. This nanocomposite shows a promising electrochemical performance with a discharge capacity of 10 046 mA h gcat−1 and a low overpotential of 1.44 V at a current density of 200 mA gcat−1, and it can sustain for more than 50 cycles within the voltage range of 2–4.7 V. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) characterizations prove that the formation of Li2CO3 and other side products are prevented, likely due to the encapsulation of CNTs by NiCo silicates and Ni nanoparticles, which may help decompose the side products. Finally, the synergistic effects, which are contributed by the high electrical conductivity of CNTs, high surface area, the high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of NiCo silicate, and the simple decomposition of side products by Ni nanoparticles enable outstanding performance of the CNT@Ni@NiCo silicate core–shell nanocomposite as a cathode catalyst for Li–O2 batteries.