Abstract

Electrochemical energy conversion technologies including Zn-air
batteries, water splitting and energy storage devices such as Li-ion batteries, supercapacitors are booming to meet the increasing energy demands owing to their high energy density, excellent durability, low cost and feasible potability. The performance of energy conversion are largely determined by the efficiency of oxygen reactions (oxygen reduction/evolution reaction (ORR/OER)) and hydrogen evolution reaction (HER) facilitated by the high-performance electrocatalysts. While the utilizations and performance of various energy storage devices are limited by the electrode materials properties. Nowadays, transition metal-based materials exhibit their high electrochemical activities due to their adjustable morphologies, controllable structures, and low cost. However, they still face some challenges such as low conductivity, low surface area, and structure collapsing. Hence, this research studies the rational design of three types of transition metal-based nanomaterials for both energy conversion and storage. By investigating physical properties via different characterizations and electrochemical measurements, obtaining a comprehension understanding of correlation of electrocatalytic performance with material’s structure and morphology while achieving the goal of “ designing one type of material for multiapplication”.
In energy conversion, the first is bimetallic CoNi alloy nanoparticles embedded in pomegranate-like nitrogen-doped carbon spheres (N-CoNi/PCS) for oxygen reactions. By studying the morphology configurations, we find that the porous structure possesses plentiful active sites and high surface area enables excellent electrochemical performance, which delivers a low half-wave potential of 0.80 V towards ORR and overpotential of 540 mV towards OER with excellent durability. By combining the results of physical and electrochemical properties, such porous structure play a key role to contribute to high electrochemical performance. Second, a unique nanostructure of N and S codoped porous carbon (N,S-Co/Zn-ZIF) derived from bimetallic ZIFs as an electrocatalyst for oxygen reactions. By studying the physical properties, we find that the nanostructure forms a rhombic dodecahedron morphology with rough surface, containing abundant active sites of sulfides nanocrystals. Owing to the special structure, such bifunctional electrocatalyst delivers a superior half wave potential of 0.86 V towards ORR and overpotential of 350 mV towards OER. Third, Ni9S8/MoS2 nanosheets decorated NiMoO4 nanorods heterostructure is developed by hydrothermal and sulfurization as an electrocatalyst for water splitting. Core-shell nanorods with diameter of 180-200 nm consisted of 2D Ni9S8/MoS2 nanoflakes as outer shell part and 1D nickel molybdate nanorods inner core, providing plentiful charge transportation channels. According to these features, such nanomaterials present high performance with low overpotentials of 190 and 360 mV for HER and OER in alkaline solution, respectively. Based on these, it’s concluded that rational design of electrocatalyst is correlated with electrochemical performance. Furthermore, to explore the electrochemical performance of transition metal-sulfides for energy storage, N,S-codoped carbon dodecahedron/transition metal sulfides are also studied as anode materials for Li-ion intercalation. Surprisingly, such nanocomposites with rough surface area and active sites still donates high-performance Li-ion intercalation with superior initial reversible capacity of 938.2 mA h g-1 with a high-capacity retention of 65.6% after 100 cycles at 150 mA g-1. At the same time, hetero-structural core-shell NiMoO4@Ni9S8/MoS2 nanorods are also studied as electrode materials for supercapacitor, which unveils unsurpassed specific capacity of 373.4 F g-1 at 10 A g-1. Such excellent electrochemical properties prove that such core shell structure is still favorable for energy storage.
The above results all prove the successful design of three types of transition metal-based nanomaterials and different structural features contribute to excellent electrochemical performance. Thus, there is a strong relationship about structure design and electrochemical performance. Since all of nanomaterials show excellent electrochemical performance in both energy conversion and storage, we have achieved goal of “multiapplication”. This research provides the effective design strategies of high-performance transition metal-based nanomaterials for multiapplication which paves a new way of development and understanding of these materials towards electrochemical energy conversion and storage.