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Oxygen reduction reaction (ORR) is a crucial cathodic reaction for fuel cells. Due to its sluggish kinetics, various ORR electrocatalysts have been designed and extensively investigated. Traditionally, Pt-based materials serve as efficient ORR catalysts. However, Pt’s low abundance, high cost and poor durability have compelled people to explore other materials. For example, silver (Ag) is comparatively well-stocked and low-cost, which has drawn great attention. Although Ag shows exceptional anti-poisoning property, it is not good as Pt. Recently, enhanced electrochemical behavior of plasmonic nanostructures under illumination has been examined through both theoretical modeling and experimental investigations. As a green and sustainable energy source, solar energy can be efficiently introduced into fuel cells and metal-air batteries. Plasmonic Ag nanoparticles have strong spectral absorption in the ultraviolet and visible light bands due to unique localized surface plasmon resonance effects. Meanwhile, nitrogen-doped graphene (NG) incorporates nitrogen (N) atoms into graphene skeleton through a hydrothermal process, thereby providing active ORR sites. Also, the vacancies and defects generated by the N-doping allow silver nanoparticles to be better dispersed on the graphene as well as maintain silver nanoparticles during the reaction. Herein, plasmonic Ag nanoparticles decorated N-doped graphene (Ag/NG) were synthesized and utilized as ORR catalysts. The ORR performance of Ag/NG greatly exceeded that of pure Ag nanoparticles and NG. For instance, the half-wave potential was 0.86 V, exceeding that of commercial Pt/C (20 wt. %). Under the AM1.5G simulated sunlight, Ag/NG demonstrated photo-enhanced electrocatalytic performance with right shifting of onset potential and enhanced current density. Therefore, the Ag/NG composite provides a potential strategy for the development of solar-enhanced electrocatalysts for fuel cells and metal-air batteries.

A simple template method is proposed for the synthesis of Ag nanoparticles embedded in Fe-N-C (Ag@FeNC) as excellent oxygen reduction reaction (ORR) catalyst. Firstly, Ag@Fe3O4 nanoparticles are synthesized and loaded onto graphene oxide to form the Ag@Fe3O4-GO template. The surface of the template is then enveloped by polypyrrole. After pyrolysis at 850°C for 1 h, Ag@FeNC is synthesized and demonstrates excellent ORR catalytic activity in alkaline medium. Therefore, it provides a simple method for preparing metal@Fe-N-C hybrid catalysts.

A melt-salt-assisted method is utilized to construct an onion-like hybrid with Co/CoO nanoparticles embedded in graphitic Fe-N-doped carbon shells (Co/CoO@FeNC) as a bifunctional electrocatalyst. The iron-polypyrrole (Fe-PPy) is firstly prepared with a reverse emulsion. Direct pyrolysis of Fe-PPy yields turbostratic Fe-N-doped carbon (FeNC) with excellent oxygen reduction reaction (ORR) electrocatalysis, while the melt salt (CoCl2) mediated pyrolysis of Fe-PPy obtains onion-like Co/CoO@FeNC with a reversible overvoltage value of 0.695 V, largely superior to Pt/C and IrO2 (0.771 V) and other Co-based catalysts reported so far. The ORR activity is mainly due to the graphitic FeNC and further enhanced by Co Nx bonds, whereas the oxygen evolution reaction (OER) activity is principally due to the Co/CoO composite. Concurrently, Co/CoO@FeNC as cathode catalyst enables Zn-air battery with a high open circuit voltage of 1.42 V, a peak power density of 132.8 mW cm−2, a specific capacity of 813 mAh gZn−1, and long-term stability.