Search

Efficient bifunctional catalysts for reversible oxygen electrodes, for electrocatalysis of both the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER) are envisioned to make a significant contribution towards the need for environmentally friendly and renewable energy systems. Despite tremendous efforts, there is still need for further development of bifunctional catalysts to achieve low-cost catalysts with stable satisfactory long-term performance, enhanced kinetics of the ORR and OER and high round-trip electrolytic-galvanic mode voltage efficiency. Herein, we demonstrate a simple one-pot procedure for synthesis of a bifunctional catalyst comprised of a composite of cobalt/cobalt oxide and N-doped graphene (denoted as Co/Co3O4-N-graphene), which exhibits very good performance in the electrocatalysis of both ORR and OER in alkaline solution. The prepared composite was characterized by TEM, SEM, Raman, XRD and XPS, to give insight of its structure-performance correlation. The results reveal that the structure of N-doped graphene is preserved and that both metallic Co and its oxide form of Co3O4 co-existed in the prepared composite, with metallic Co being predominant crystalline component. Generally, a high ratio of Co in the prepared composite led to improvement of the onset potential for the OER. [ABSTRACT FROM AUTHOR]

Coal is an earth-abundant energy resource, however, its direct combustion results in serious environmental pollution. Therefore, it becomes important to design value-added products from coal and to maximize its value chain. Herein, brown coal was used to develop non-precious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells as green energy conversion systems. The brown coal was first pretreated with different acids, followed by N-doping at 800 °C in a stream of NH 3 . A trace amount of Fe was further added to improve the electrocatalytic performance of the prepared catalyst towards ORR. The prepared coal-derived N-doped carbon further modified with 0.5% Fe exhibited onset potential of 0.92 V vs. RHE at a current density of −0.1 mA cm −2 and a predominantly 4-electron transfer pathway of oxygen to water in 0.1 M NaOH, which was evaluated by RDE and RRDE. The prepared electrocatalysts were further characterized by elemental analysis, XRD, Raman and XPS. The results suggest that the coal-derived ORR catalyst have convoluted graphitic and amorphous carbon structures. The N-content increased after acid-pretreatment and subsequent functionalization with nitrogen, while it slightly decreased after Fe incorporation apparently due to coordination of Fe with N. ORR activity enhancement after the incorporation of Fe is expected to mainly arise from a synergetic effect involving the interaction of Fe with N groups distributed in the carbon matrix. [ABSTRACT FROM AUTHOR]

Functional low-cost catalysts derived from earth abundant materials are vitally important for enabling mass implementation of renewable energy conversion systems, such as fuel cells and metal-air batteries. Herein, we report the activation of earth-abundant coal to realize very active and stable catalysts for the oxygen reduction reaction (ORR) through oxidative pretreatment followed by thermal treatment under ammonia forming self-doped heteroatoms and defects. The coal thermally pretreated in air at 350 °C for 30 min followed by treatment in ammonia for 2 h presented very good electrocatalytic ORR activity with onset potential of 0.978 V, half-wave potential of 0.871 V, Tafel slope of 57 mV dec−1, and predominant 4-electron reduction of O 2 to OH−. The influence of the preoxidation step prior to ammonia treatment on the properties of the coal-derived carbon and hence the ORR has been specifically studied. The optimized catalyst also displayed stable performance during the ORR in 0.1 M NaOH for a test period of 35 h and exhibited excellent tolerance to methanol. The preoxidation step facilitates elimination of aliphatic and amorphous carbon, condensation of aromatic structures and the formation of initial pore structures, and subsequent removal of unstable heteroatoms from aromatic carbon rings to form surface and edge defects as potential active sites for promoting the ORR. Using pristine coal as raw material to produce functional ORR catalysts will be of great importance for real applications. [ABSTRACT FROM AUTHOR]

Development of high-efficiency bifunctional oxygen electrocatalyst is evidently demanded in Zn-air batteries. Herein, we report the synthesis of cobalt compounds supported on B-, N- and S-doped graphene (Co-B,N,S-G) via simple hydrothermal method. The as-prepared Co-B,N,S-G exhibited excellent electrocatalytic activity and long-term stability for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Notably, scanning electrochemical microscopy (SECM) was specifically improved to allow the investigation of local ORR and OER reactivity of the Co-B,N,S-G sample at variable temperatures. Practically, the Co-B,N,S-G-based air electrode was used as cathode to construct a Zn-air battery, which presented an extremely small charge/discharge voltage gap of 0.64 V, and remained a high voltage efficiency of 67% after 500 cycles (ca. 84 h). The constructed Zn-air battery was capable of powering LED-figures for at least 30 h. Impressively, the reported work exhibited the feasible prospect of temperature-controlled SECM on evaluating electrocatalysts at submicrometer scale at different temperatures. [ABSTRACT FROM AUTHOR]