The environmental pollution caused by the use of fossil fuels poses a major threat to human life. Therefore, substantial effort has been made to develop environmentally friendly renewable energy, and the design of electrochemical energy storage·conversion devices with high performance is essential for their successful establishment as the energy in everyday life. In this work, the study on the design of electrode materials for the electrochemical energy conversion·storage devices is carried out.
Oxygen reduction reaction (ORR) is an essential step for zinc-air batteries and fuel cells. Currently, Precious metal-based electrocatalysts are employed in order to increase the rate of ORR, while the development of alternative low-cost catalysts is needed to achieve the economical feasibility of such electrochemical energy conversion systems. Among the alternative catalysts examined so far, the MNC-type catalysts, composed of metal-nitrogen-carbon, are reported to have relatively high activity and durability. In this work, the MNC-type catalysts with atomically dispersed Fe species were prepared by employing a short-duration heat-treatment of catalyst precursor. Prepared catalysts were used as a cathode catalyst for Zn-air batteries. The precursor was prepared by impregnating FePc on a thermally exfoliated graphene oxide. The prepared precursor was heat-treated under ammonia atmosphere for a period of time to produce the catalyst. The catalyst, which heat treated the catalytic precursor for 2.5 minutes at 950°C, had the highest activity and durability. When the heat treatment time was too short, the resultant catalyst showed relatively low activity and durability, which resulted from an insufficient activation of catalyst precursor. On the other hand, when the heat treatment time was too long, the number of active site was reduced due to the aggregation of metal species, which leads to inferior activity and durability. The catalyst with the highest activity was 1.2 times more active in half-cell conditions compared to a commercial Pt catalyst, and the maximum power density was 1.4 times higher in Zn-air battery performance compared to platinum-used batteries.
Lithium-ion batteries are drawing attention as the power source for zero-emission vehicles. Graphite employed as the anode for lithium-ion batteries has a low Li ion capacity. Transition metal oxides have been intensively studied as anode active materials for lithium-ion batteries because of their high theoretical Li ion capacities. Among them, iron oxides have not only high theoretical capacities, but also are abundant and nontoxic, and therefore, they have been highlighted as a potential candidate of anode materials with high Li ion capacity. On the other hand, iron oxide has low conductivity and is accompanied by large volume changes during the insertion and deinsertion process of Li, which results in the pulverization of the electrode, leading to low cycling performance. To address this, in this work, iron oxide-metallic iron composites dispersed on carbon matrics are proposed. FeOOH was hydrothermally treated in glucose-dissolved water to yield precursors, which was pyrolyzed at high temperatures to produce the final electrode materials. Both the mass ratio of glucose to FeOOH and the pyrolysis temperature are the critical factor determining the crystal phase of materials prepared. When the mass ratio or the pyrolysis temperature is too high, the resultant sample contained high portion of metallic iron. The metallic iron phase was positive for the cycling stability, however, its Li capacity was inferior to that of iron oxide phase. Iron oxide-metallic iron composite compared to iron oxide showed high Li ion capacity and excellent cycling stability. This is because metallic Fe can not only increase electrical conductivity of materials but also serve as a buffer phase that control the volume change during Li insertion process.