Highly Dispersive Co@N‐C Catalyst as Freestanding Bifunctional Cathode for Flexible and Rechargeable Zinc–air Batteries

Abstract

The commercial applications of wearable electronics require the rapid growth of flexible, reliable energy-storage systems with high energy density and long shelf life. Rechargeable Zn-air batteries (ZABs) have promising applications in wearable electrochemical electronics, owing to a high theoretical energy density of 1086 Wh kg −1 , inherent safety, environmental friendliness, and low cost. [1][2][3] One pivotal task is the efficient interconversion of chemicals and electricity via oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes taken place at the air cathode of the rechargeable ZABs, [4] which requires the use of highly active, stable bifunctional cathode for both reactions. Precious noble-metal-based cathodes such as Pt, IrO 2, and RuO 2 are the state-of-the-art oxygen electrocatalysts, [5] but their scarcity, moderate durability, and unsatisfactory bifunctional performance severely undermine the economic benefits of ZABs. [6][7][8] With this in mind, significant efforts have been focused on exploring non-noble metal cathode for rechargeable ZABs, such as transition metal oxides/phosphides/sulfides/nitrides, [9][10][11] perovskites, [12] spinel oxides, [13] and carbons, [14] many of which are emerging as highly promising substitutes to benchmark noble metal cathodes. Particularly, cobalt (Co), as a representative transition metal with potential activities toward both ORR and OER, has attracted intensive interests. However, around 70% Co is found in the Democratic Republic of the Congo (DRC). The increasing demand for Co has put great pressure on local artisanal miners, where the child labor and unsafe working practices are rife. [15] To reduce such unintended consequences, it is encouraged to improve the utilization of Co, meanwhile develop other high-performance alternatives with rich abundance.Carbon-based bifunctional cathode has become a hot research focus because of their fast electron/mass transfer competence and tunable chemical/porous properties, such as metal organic framework derivatives, [16,17] biomass-derived carbon, [18] graphene or carbon nanotube based hybrids, [19,20] and so forth. However, most research witnesses a dilemma in cathode design, that is, how to achieve a balanced improvement of overall activity for ORR and OER in view of their distinctive catalytic sites. [21,22] Doping strategy and inclusion of highly dispersed

Progress on Bifunctional Carbon‐Based Electrocatalysts for Rechargeable Zinc–Air Batteries Based on Voltage Difference Performance

“…PA@Z8-Fe-N-C 0.89 1.77 0.89 Gao et al [ 78] Fe/Fe-NC-3 0.87 1.69 0.82 Deng et al [ 91] Co/N-Pg 0.82 1.63 0.81 Tian et al [ 92] Fe-ZIF/OCNT 0.865 1.672 0.807 Sheng et al [ 93] FeCo-N-C 0.81 1.61 0.80 Wu et al [ 94] Co@N-C 0.82 1.58 0.76 Ma et al [ 95] FeNi/NS-C 0.83 1.585 0.755 Li et al [ 96] Mn@Co-NS 0.89 1.61 0.72 Li et al [ 97] FeCo/N-CNT 0.87 1.58 0.71 He et al [ 98] CoNi@CoCN 0.86 1.57 0.71 Zhang et al [ 99] Co/CoFe@NC 0.84 1.54 0.70 Niu et al [ 100] interact with the electrons of sp 2 -C materials through the electron coupling effect, resulting in an activation effect. [86] In the M-N-C catalysts, the N atoms have lone electron pairs, which can form strong interactions with the metal atoms and facilitate the immobilization of metal atom dopants in the carbon skeleton.…”

“…The resulting rechargeable ZAB has a high peak power density (160 mW cm −2 ), a large specific capacity (759.7 mAh g −1 at 10 mA cm −2 after 120 h testing), and 277 h of stable performance. [95] Different from Co-N-C sites, cobalt nanoparticles (NPs) can promote the degree of graphitization of the carbon shell layer and enhance the transfer of electrons to the carbon shell layer, thus greatly increasing the electrical conductivity of nitrogendoped carbon. [117,118] Therefore, the Co nanoparticles/Co-SAC structure may possess better performance than Co-N-C. As an example, Tian et al prepared a conductive carbon matrix for hosting cobalt oxide nanoparticles and Co-N-C active sites by hydrothermal treatment and further pyrolysis.…”

“…The prepared cellulose aerogels possess hyperfine dispersion of Co, balanced distribution of N‐C species, hierarchical structure of porous structures, and increased proportion of mesopores, as well as a moderate amount of defects. [ 95 ] These structural features ensure an improved mass/electron transfer pathway and abundant rechargeable oxygen electrocatalytic sites, resulting in good catalytic activity and stability for both ORR and OER. As a result, E 1/2 of the cathode is 0.82 V and the oxygen evolution overpotential is 350 mV at 10 mA cm −2 , with Δ E (0.76 V) smaller than that of commercial Pt/C+RuO 2 (0.80 V) and most reported Co‐based electrocatalysts.…”

Interface Electronic Modulation of Monodispersed Co Metal-Co7Fe3 Alloy Heterostructures for Rechargeable Zn–Air Battery

“…In the face of global environmental and energy issues, it is imperative to develop advanced clean and renewable energy conversion/storage devices. , Rechargeable zinc-air batteries (ZAB) have received extensive attention due to their high energy density, low price, and intrinsic safety, as well as their environmental friendliness. − However, the development of ZAB is severely constrained by huge overpotential and sluggish reaction kinetics of two half-reactions, oxygen reduction and oxygen evolution reactions (ORR/OER), on the air electrode during charging/discharging . Although noble-metal-based electrocatalysts can promote these reactions (i.e., Pt/C and RuO 2 /IrO 2 for ORR and OER, respectively), their earth scarcity, high price, and lack of durability/bifunctionality are severely unsatisfying. − Therefore, it is urgently needed to design cost-effective oxygen catalysts with excellent bifunctionality and durability.…”

Synergistic Bimetallic CoCu‐Codecorated Carbon Nanosheet Arrays as Integrated Bifunctional Cathodes for High‐Performance Rechargeable/Flexible Zinc‐Air Batteries

“…[14][15][16] However, the formation of homogeneous and sufficient Co-N x active moieties is generally difficult, as the metallic loading in these Co-N-C catalysts is always restricted due to the high tendency of self-agglomeration and irreversible fusion of cobalt nanoparticles (NPs) during common pyrolysis synthesis. [17,18] By taking advantage of high specific surface area, excellent conductivity, and mechanical strength, heteroatoms-doped 2D nanocarbon could be useful to support and well distribute Co NPs for more exposed active sites, directional electron transfer and accelerated mass diffusion across the interspaces between the carbon nanosheets (CNSs), thus boosting the reversible oxygen reactions. [19,20] Besides, feasible strategies, including morphological control, heterostructure, surface reconfiguration, and interface…”