Search

Summary: Obstacles to the efficient operation of rechargeable aluminum‐air batteries include slow oxygen reduction reaction (ORR), slow kinetics, and large overpotential of oxygen evolution reactions (OER). An effective solution is to develop a new bifunctional electrocatalyst that is effective for both the ORR/OER processes. In this study, we prepared the α‐MnO2/Co3O4 bifunctional composite catalyst by the in situ generation of Co3O4 nanoparticles on α‐MnO2 nanorods. ORR/OER catalytic activity and structures of the prepared catalysts have been investigated systematically. Compared with single‐component α‐MnO2 nanorods and Co3O4 particles catalyst, the α‐MnO2/Co3O4 composite catalyst possesses the smallest average pore diameter (11.84 nm), and the largest specific surface area (41.34 m2 g−1), which is due to the restriction of the growth of Co3O4 particles by α‐MnO2 nanorods. Besides, during the ORR process, the α‐MnO2/Co3O4 composite catalyst shows the most positive onset potential of 0.833 V, the most positive half‐wave potential of 0.760 V, and the biggest limiting current density of 3.881 mA cm−2. Meanwhile, during the OER process, the composite catalyst exhibits the lowest onset potential of 1.630 V, the lowest overpotential of 554 mV, and the biggest limiting current density of 24.590 mA cm−2. Furthermore, the α‐MnO2/Co3O4‐assembled aluminum‐air battery (AAB) delivers the best cycling performance. Our work provides an idea for optimizing the bifunctional catalyst with high ORR/OER activity and low cost used in secondary AAB. Highlights: A novel α‐MnO2/Co3O4 bifunctional composite catalyst was prepared successfully.The α‐MnO2/Co3O4 composite catalyst was applied in a rechargeable Al‐air battery.The α‐MnO2/Co3O4 exhibited better ORR/OER catalytic activity than α‐MnO2 and Co3O4.The Al‐air battery using α‐MnO2/Co3O4 catalyst showed excellent cycling performances. Novelty Statement: This is our original work. Neither the entire paper nor any part of its content has been published or has been accepted elsewhere. It is not being submitted to any other journal. [ABSTRACT FROM AUTHOR]

Summary: Catalysts with cobalt (Co) and graphitic carbon hybrid structure are receiving significant attention in various electrocatalysis applications because of their stability and high activities. In addition, the size and the well dispersed distribution of metal active site are some of the most important factors for electrocatalyst. In this paper, we successfully synthesized ultra‐small and well dispersed Co nanoparticle on nitrogen‐doped carbon structure (Co/NC) adding and evaporating a Zinc (Zn) and found the optimized ratio of Co and Zn. We used a bottom‐up method to synthesize catalysts with Co on carbon substrate structure from zeolitic imidazolate frameworks (ZIF). Zinc (Zn) was also added and evaporated by pyrolysis. By addition and evaporation of Zn, Co nanoparticle became smaller and more evenly dispersed. As a result, the OER performance was significantly improved than the catalyst without Zn. Also, an optimized catalyst with abundant uniformly dispersed Co was successfully synthesized by appropriately controlling the ratio of Co and Zn comprising the ZIFs. As the ratio of Zn increased, the size and the amount of Co nanoparticles were decreased, and Co nanoparticles were more evenly dispersed. Furthermore, carbon substrate has more porous structure due to the evaporation of Zn atoms. When the Co:Zn ratio was 1:3, Co/NC showed the best electrocatalytic performance due to optimized active sites and large surface area. The optimized Co/NC showed an overpotential of 1.56 V (vs RHE) to acquire a current density of 10 mA cm−2, which is higher activity than commercial RuO2. We also analyzed the material characterization occurred as the ratio of Zn in ZIF increased, revealing the factors enhancing the OER activity. Optimized Co/NC (Co:Zn = 1:3) exhibited improved OER activity and long‐term and cycle stability in the OER. [ABSTRACT FROM AUTHOR]

Summary: The cost of platinum group metal (PGM) catalysts is one of the major obstacles in commercializing proton exchange membrane water electrolyzers (PEMWEs). The non‐PGM substituents are often more financially beneficial but low in activity and durability in the acidic environment. In this study, cobalt phosphide catalysts, which are promising non‐PGM alternatives for the hydrogen evolution reaction (HER) and have enhanced durability and single‐cell performance, were fabricated directly on carbon paper using the pulse electrodeposition method. As the dissolution potential (reported as ‐x vs saturated calomel electrode) of the pulse electrodeposition shifted in the positive direction, the P/Co ratio of the Co‐P‐x catalysts increased because of severe Co dissolution. Among the catalysts, Co‐P‐0.6, Co‐P‐0.5, and Co‐P‐0.4 (where the number indicates the negative dissolution potential) were rapidly degraded in acid, whereas Co‐P‐0.3, Co‐P‐0.2, and Co‐P‐0.1 showed high stability because of the relative amounts of CoP and Co2P phases. The acid‐dissolved Co‐P‐0.3 catalyst showed the best half‐cell performance (an overpotential of 143.85 mV at 10 mA/cm2) and durability, and the P‐Co and Coδ+ surface states are critical for its performance. Single‐cell tests using the Co‐P‐0.3 cathode revealed its remarkable performance of 1.89 A/cm2 at 2.0 Vcell, indicating its promise as a non‐PGM cathode material for PEMWEs. [ABSTRACT FROM AUTHOR]