| Owing to increasing energy demands,renewable energy systems such as metal-air battery,especially zinc-air battery have drawn great attention because of their theoretically high energy densities,promising large-scale applications,low cost and environmental friendliness.However,stable and efficient bifunctional electrocatalysts for oxygen evolution reaction?OER?during charging,and oxygen reduction reaction?ORR?during discharging are closely related to practical application of Zn-air battery.Noble metals and their alloys are found to give excellent catalytic performance but their commercialization application is hard to be realized due to various restrictions.Thus,researchers focus on developing alternative catalysts with comparable electrocatalytic activity as substitutions for noble metal-based electrocatalysts.Several investigations illustrate that a non-noble metal-nitrogen-carbon?M-N-C,with M=Fe,Co,etc.?catalyst synthesized by high temperature carbonization of precursors including N,C and particular transition metals exhibit great potential for ORR catalysis.On the other hand,cobalt-based catalysts perform high OER activities due to the excellent conductivity and affluent electrochemically active sites.Here,through combining the merits of both mentioned above,polypyrrole-iron coordination complex?Fe/N/C?was firstly intergrated as precursors.Different efficient electrocatalysts were synthesized by some modifications of the synthetic method.Several tests were used to study the characteristics and electrochemical properties of the obtained materials.At the same time,the catalysts were also tested in zinc-air batteries for their practical application values.The main contents of this paper are as follows:1.A composite material derived from the combination of bimetallic zeoliticimidazolate frameworks?denoted as BMZIFs?and Fe/N/C framework is synthesized as an efficient bifunctional catalyst.Although BMZIF or Fe/N/C alone exhibits undesirable oxygen reaction activity,their combined materials show unprecedented ORR?half-wave potential of 0.85 V?as well as comparatively superior OER activities(potential@10 mA cm-2 of 1.64V),outperforming not only a commercial Pt/C electrocatalyst,but also most of reported bifunctional electrocatalysts.We then test its'practical application value in Zn–air batteries.Primary batteries exhibit a high peak power density of 235 mW cm-2 and the rechargeable batteries are able to be operated smoothly for 100 cycles at a curent density of 10 mA cm-2.The unprecedented catalytic activity can be attritued to chemical coupling effects between Fe/N/C and BMZIF and helps develop highly active electrocatalysts to be applied to electrochemical energy devices.2.An unprecedented activity of bifunctional electrocatalysisis produced by in-situ growth of nitrogen-enriched carbon nanotubes with transition metal composite.The resultant catalyst delivers surprisingly high OER(potential@10 mA cm-2 of 1.58 V)and ORR?onset potential of 0.97 V,half-wave potential of 0.86 V?performance.In aqueous Zn-air battery tests,primary batteries demonstrate high maximum power density and two-electrode rechargeable batteries also exhibit good cycle performance and low charging/discharging voltage gap.The unprecedented electrocatalyst opens up new avenues for developing highly active nitrogen-doped carbon nanotube-supported ORR and OER electrocatalysts and offers prospects for the next generation of fuel cells,metal-air batteries and photocatalysis applications. |
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