Nonprecious bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalysts in wide pH electrolytes are crucial for the versatile use of electrochemical energy storage and conversion devices. Non-noble metal/nitrogendoped carbon (M−N/C) has been reported as a promising candidate for efficient and cost-effective bifunctional catalysts. Nevertheless, the stability and catalytic activity of M−N/C bifunctional catalysts in a wide pH range are still limited, especially in neutral and acidic electrolytes. Here, we synthesized directly grown N-doped bamboo-like carbon nanofiber-encapsulated cobalt nanoparticles on carbon flakes (Co−N/C fiber) via pyrolysis of 2D cobalt−zeolitic imidazolate framework and tubular g-C 3 N 4 . The in situ growth of N-doped carbon nanofiber incorporated with Co nanoparticles exposes large active sites and enhances charge transfer. Moreover, abundant mesopores with high loading of pyridinic-N, graphitic-N, Co@N/C, and Co−N x facilitate high catalytic activity in a wide pH condition. Co−N/C fiber shows superior ORR performance in alkaline, neutral, and acidic electrolytes with high onset potentials of 1.003, 0.820, and 0.800 V vs RHE, respectively. A comparable OER performance of Co−N/C fiber to the Ir/C benchmark in alkaline and neutral conditions can be obtained with 397 and 570 mV overpotentials at 10 mA cm −2 . Utilizing Co−N/ C fiber in alkaline and neutral Zn−air batteries exhibits power densities of 155 and 67 mW cm −2 , with excellent stability for over 180 h. This study offers a strategy for developing a bifunctional M−N/C catalyst that is applicable across a wide pH range via hybrid nanostructuring.
With the increasing demand for diversification of renewable energy sources, the high activity and stability of electrocatalysts in all pH ranges have been highlighted as one of the future directions in electrochemical energy generation and storage systems. Noble-metal-free nitrogen-doped carbon (M-N/ C) has been explored as a substitute for the expensive platinumbased oxygen reduction reaction (ORR) electrocatalysts. However, its ORR activity remains limited to alkaline electrolytes. In acidic medium, its low activity and stability correspond to metallic site dissolution and protonation of N-functional groups, which are even worse in neutral electrolytes because of low ionic conductivity and low H + concentration. This review summarizes strategies to improve the stability and activity of M-N/C as pH-universal ORR electrocatalysts. First, the ORR mechanism focusing on active site identification for each pH condition is discussed. Four strategies, including engineering pore structure, adding carbon shell wrapping, and introducing multiple nonmetal dopants and dual metallic active sites on the carbon substrate, are then evaluated to design pHuniversal ORR electrocatalysts with distinguished activity and stability. Lastly, future perspectives are given to show the viewpoint of further development and potential applications of M-N/C electrocatalysts.
Flexible supercapacitors are attracting interest in wearable technologies as they can withstand mechanical deformations while delivering their energy storage function. Among frequently investigated electrode materials for flexible supercapacitors, polyaniline/graphene composites are favorable due to their synergistic properties that assure excellent specific capacitance, cycling stability, and high rate capability. This review highlights recent strategies to advance structural designs and synthesis methods of polyaniline/ graphene electrodes for flexible supercapacitors. Firstly, the general mechanism and feature of the flexible supercapacitor will be discussed, followed by current challenges that focus on two key aspects, structural design and synthesis of the electrode. Next, by sorting the composites based on their morphological dimensionalities (i. e., one-, two-, and threedimensional), and focusing the discussion on the two key aspects, we evaluate recent and effective strategies to develop flexible supercapacitors with polyaniline/graphene composite electrode. Finally, future perspectives are given for broader applications of the flexible supercapacitors.
Zinc–air batteries with seawater electrolyte utilize abundant and cheap resources. However, it requires an electrocatalyst with high bifunctional activity in seawater. In this work, a carbon electrocatalyst is obtained via one-step pyrolysis of the shell waste of cranberry beans. During the oxygen reduction reaction (ORR) in seawater electrolyte, the cranberry bean shell-derived carbon catalyst exhibits an ORR onset potential of 0.69 V vs RHE and an ORR saturating current density of 2.93 mA cm–2, which are promising compared to the ORR performance of Pt/C in seawater electrolyte (0.78 V vs RHE and 3.15 mA cm–2). During OER (oxygen evolution reaction) in seawater electrolyte, the carbon catalyst shows an overpotential of 582 mV at 5 mA cm–2, 35 mV smaller than the commercial Ir/C catalyst (617 mV). Furthermore, when the catalyst is applied to the zinc–air battery with seawater electrolyte, the cell is able to exhibit discharge and charge voltages of 0.93 and 2.2 V, respectively, which are stable for more than 120 cycles of the cycling test. This work highlights the fabrication of metal–air batteries with cost-effective and sustainable resources.
Silicon anode is endowed with a high theoretical specific capacity. Unfortunately, its applicability in lithium-ion batteries is hindered by several inevitable problems, which are associated with volume changes (i.e., particle pulverization, solid electrolyte interphase layer instability, and electrode failure) and low electrical conductivity of silicon. Among the accessible strategies to enhance the anode performance, conductive polymer frameworks have been envisioned as solutions to overcome the problems. Conductive polymers can jointly bind and electronically connect silicon particles to regulate extensive volume changes while providing pathways for charge transport in a low-cost fashion. This review starts with a detailed summary of conductive polymer framework features in silicon anodes. The main section presents an in-depth discussion and current advancements of the most widely used conductive polymers in silicon anodes [i.e., polyfluorene, polyaniline, polypyrrole, polythiophene, and poly(3,4-ethylenedioxythiophene)] and several other conductive polymers. In the penultimate part, we review the performance evaluation of silicon anodes with five main conductive polymers and provide perspectives on future challenges of batteries from the standpoint of device manufacturing scale-up. In the final part, we briefly summarize the discussed advancements of conductive polymer frameworks in silicon anode for lithium-ion batteries.
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