Metal oxides of earth-abundant elements are promising electrocatalysts to overcome the sluggish oxygen evolution and oxygen reduction reaction (OER/ORR) in many electrochemical energy-conversion devices. However, it is difficult to control their catalytic activity precisely. Here, a general three-stage synthesis strategy is described to produce a family of hybrid materials comprising amorphous bimetallic oxide nanoparticles anchored on N-doped reduced graphene oxide with simultaneous control of nanoparticle elemental composition, size, and crystallinity. Amorphous Fe Co O is obtained from Prussian blue analog nanocrystals, showing excellent OER activity with a Tafel slope of 30.1 mV dec and an overpotential of 257 mV for 10 mA cm and superior ORR activity with a large limiting current density of -5.25 mA cm at 0.6 V. A fabricated Zn-air battery delivers a specific capacity of 756 mA h g (corresponding to an energy density of 904 W h kg ), a peak power density of 86 mW cm and can be cycled over 120 h at 10 mA cm . Other two amorphous bimetallic, Ni Fe O and Ni Co O , are also produced to demonstrate the general applicability of this method for synthesizing binary metal oxides with controllable structures as electrocatalysts for energy conversion.
Room-temperature sodium–sulfur (RT-Na/S) batteries hold great promise for sustainable and cost-effective applications. Nevertheless, it remains a great challenge to achieve high capacity and cycling stability due to the low activity of sulfur and the sluggish conversion kinetics between polysulfide intermediates and sodium sulfide. Herein, an electrocatalyzing S cathode is fabricated, which consists of porous core–shell structure and multisulfiphilic sites. The flexible carbon structure effectively buffers volume changes during cycling and provides enclosed spaces to store S8 with exceptional conductivity. Significantly, the multisulfiphilic sites (ZnS and CoS2) enhance catalysis toward multistep S conversion, which effectively suppresses long-chain polysulfides dissolution and improves the kinetics of short-chain polysulfides. Thus, the obtained S cathodes achieve an enhanced cycling performance (570 mAh g–1 at 0.2 A g–1 over 1000 cycles), decent rate capability (250 mAh g–1 at 1.0 A g–1 over 2000 cycles), and high energy density of 384 Wh kg–1 toward practical applications.
Nickel phosphide has a much higher catalytic activity for the hydrogen evolution reaction in strongly acidic and basic electrolytes.
Porous carbon has been widely used as an efficient host to encapsulate highly active molecular sulfur (S) in Li–S and Na–S batteries. However, for these sub‐nanosized pores, it is a challenge to provide fully accessible sodium ions with unobstructed channels during cycling, particularly for high sulfur content. It is well recognized that solid interphase with full coverage over the designed architectures plays critical roles in promoting rapid charge transfer and stable conversion reactions in batteries, whereas constructing a high‐ionic‐conductivity solid interphase in the pores is very difficult. Herein, unique continuous carbonaceous pores are tailored, which can serve as multifunctional channels to encapsulate highly active S and provide fully accessible pathways for sodium ions. Solid sodium sulfide interphase layers are also realized in the channels, showing high Na‐ion conductivity toward stabilizing the redox kinetics of the S cathode during charge/discharge processes. This systematically designed carbon‐hosted sulfur cathode delivers superior cycling performance (420 mAh g−1 at 2 A g−1 after 2000 cycles), high capacity retention of ≈90% over 500 cycles at current density of 0.5 A g−1, and outstanding rate capability (470 mAh g−1 at 5 A g−1) for room‐temperature sodium–sulfur batteries.
Room‐temperature sodium–sulfur (RT Na–S) batteries have attracted extensive attention because of their low cost and high specific energy. RT Na–S batteries, however, usually suffer from sluggish reaction kinetics, low reversible capacity, and short lifespans. Herein, it is shown that chain‐mail catalysts, consisting of porous nitrogen doped carbon nanofibers (PCNFs) encapsulating Co nanoparticles (Co@PCNFs), can activate sulfur via electron engineering. The chain‐mail catalysts Co@PCNFs with a micrograde hierarchical structure as a freestanding sulfur cathode (Co@PCNFs/S) can provide space for high mass loading of sulfur and polysulfides. The electrons can rapidly transfer from chain‐mail catalysts to sulfur and polysulfides during discharge–charge processes, therefore boosting its conversion kinetics. As a result, this freestanding Co@PCNFs/S cathode achieves a high sulfur loading of 2.1 ± 0.2 mg cm−2, delivering a high reversible capacity of 398 mA h g−1 at 0.5 C (1 C = 1675 mA g−1) over 600 cycles and superior rate capability of an average capacity of 240 mA h g−1 at 5 C. Experimental results, combined with density functional theory calculations, demonstrate that the Co@PCNFs/S can efficiently improve the conversion kinetics between the polysulfides and Na2S via transferring electrons from Co to them, thereby realizing efficient sulfur redox reactions.
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