稀有金属(英文版) 2020,39(06),613-615

Surface coating of electrocatalysts boosts battery performance

Ji-Zhen Ma Jin-Tao Zhang

Key Laboratory for Colloid and Interface Chemistry(Ministry of Education),School of Chemistry and Chemical Engineering,Shandong University

作者简介：*Jin-Tao Zhang,e-mail:jtzhang@sdu.edu.cn;

Surface coating of electrocatalysts boosts battery performance

Ji-Zhen Ma Jin-Tao Zhang

Key Laboratory for Colloid and Interface Chemistry(Ministry of Education),School of Chemistry and Chemical Engineering,Shandong University

Bifunctional electrocatalyst is crucial to boost the performance of a zinc-air battery.Alternative electrocatalysts beyond noble metals are highly desirable to lower the energy barriers for oxygen reduction and evolution reactions in a zinc-air battery,while maintaining the good stability in the harsh alkaline electrolyte.Writing in Journal of American Chemical Society,Profs.Wan,Hu and their co-workers show a general strategy to prepare welldispersed transition-metal nanocrystals with graphitic carbon coating on nitrogen-doped graphene sheets for the long-lasting zinc-air battery with the record power density [ 1] .

Over the past few decades,the increasing global research efforts have been devoted to developing aqueous rechargeable zinc-air batteries as a promising technology to meet the energy requirements for electric vehicles and other energy-demanding devices [ 2] .However,the complement of rechargeable zinc-air commercially is not straightforward because of the unsatisfactory performance and poor durability.This difficulty stems from that the thermodynamic ally uphill reactions involving oxygen reduction reaction (ORR) and oxygen reduction evolution(OER) that require high overpotentials,are the bottlenecks to achieve the good performance of a zinc-air battery [ 3] .Although noble metal electrocatalysts (e.g.,Pt/C,IrO₂)exhibited good electrocatalytic activity towards ORR or OER,rechargeable Zn-air batteries have to separate ORR with OER in a tri-electrode configuration for improving the cycling stability [ 3] .It is worthwhile to replace the expensive noble metals with stable and efficient bifunctional electrocatalysts for achieving the high reversibility over long charging and discharging cycles in a Zn-air battery.Along with the development of heteroatom-doped carbon electrocatalysts with bifunctional activities [ 4] ,transition-metal (e.g.,Fe,Co,Ni) nanoparticles anchored in nitrogen-doped carbons has shown superior bifunctional activities.The high-density metal species with small size are the key to distributing the high-active sites as high as possible for boosting battery performance.To this end,metal components should be well-designed in nanoscale through strong interaction with the active substance to confine the size from nanocrystals to single-atom clusters [ 5, 6] .However,the direct pyrolysis of the carbonaceous precursors with various metal species commonly resulted in the inevitable agglomeration of metal nanoparticles.Additionally,at the large voltage over 2.0 V in the harsh alkaline electrolyte for Zn-air batteries,the oxidation of metal nanoparticles would also deteriorate the catalytic activity and stability of an electrocatalyst.

Wan et al.introduced a general strategy to prepare the well-defined transition-metal nanocrystals encapsulated in nitrogen-doped carbon shells (M@NC) that enables the record power density of a zinc-air battery and one of the longest cycling life reported (Fig.1) [ 1] .The innovative idea would stem from that the stability of metal nanostructures can be remarkably improved by the surface coating.Initially,the ultrathin Co₂Fe₁ hydroxide nanosheets on graphene matrix were coated with the polydopamine (PDA) layer.Under the thermal pyrolysis,it has been revealed that the graphene sheets would be etched in the presence of metal species due to the catalytic decomposition of carbons along with the aggregation of metal nanoparticles [ 7, 8] .In contrast,the issues have been welladdressed by the PDA coating in the present work.Especially,the in situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) results clearly revealed the temperature-dependent phase transition into the metastable rocks alt oxide (RS CoFe).The unique rocks alt oxide was crucial for the formation of well-defined metallic nanocrystals during the thermal reduction process due to its strong interaction with the graphene substrate according to the density functional theory (DFT) calculation results.Thus,the irreversible migration and aggregation of metal nanoparticles can be avoided with the gradual coating of graphitic carbon layer during the pyrolysis process.In contrast,the weak interaction of spinel oxides with the graphene substrate in the absence of PDA coating resulted in the severe aggregation of metal alloy nanoparticles with irregular morphology.The results suggest that the PDA coating not only performed as the protection layer for the gradual reduction of metal-hydroxide nanosheets into welldispersed metal nanocrystals,but also provided carbon sources to form the porous carbon network for further stabilizing Co₂Fe₁ alloy nanoparticles at the elevated temperature.The unique structure with the high-density metal nanoparticles warped by porous carbon network would stabilize sufficient active sites to proceed the electrocatalytic reactions.

Fig.1 a Schematic illustration for synthesis of Co₂Fe@NC,b,c long-term cycling performance of Zn-air battery at a current density of2 mA·cm^-2.Reproduced from Ref. [1] .Copyright (2020) American Chemical Society

Remarkably,Hu et al.found that the Zn-air battery using the as-prepared bifunctional electrocatalysts operated stably for 250 h and delivered the maximal power density of 423.7 mW·cm^-2.The enhanced catalytic activity would be ascribed to the metallic cores boosted C-N_x sites rather than M-N_x sites according to the SCN^-poisoning test.The graphitic carbon coating effectively prevents the oxidation and corrosion of the metallic cores,consequently improving the cycling stability of a Zn-air battery.Furthermore,the electrical conductivity of the electrocatalysts would be enhanced by the presence of carbon coating and network,leading to the enhanced power density [ 9] .

The comprehensive study provided an efficient strategy to boost the bifunctional electrocatalysis through anchoring the uniformly dispersed metal nanoparticles within the well-designed porous carbon networks.The approach opens up new pathways for stabilizing the active sites,thereby improving cycling stability and power density,which could be an important step towards the development of long-lasting and affordable Zn-air battery.With the combination of theoretical calculations,the in situ techniques were performed to uncover the underlying principles of interface chemistry for the precise structural design,which imparts the homogeneity and stability of active sites for enhancing electrocatalysis.The basic principles would promote the rational design of advanced nanomaterials for efficient electrocatalysis.

参考文献

[1] Tang T,Jiang WJ,Liu XZ,Deng J,Niu S,Wang B,Jin SF,Zhang Q,Gu L,Hu JS,Wan LJ.Metastable rocksalt oxidemediated synthesis of high-density dual-protected M@NC for long-life rechargeable zinc-air batteries with record power density.J Am Chem Soc.2020.https://doi.org/10.1021/jacs.0c01349.

[2] Cheng F,Chen J.Metal-air batteries:from oxygen reduction electrochemistry to cathode catalysts.Chem Soc Rev.2012;41(2):2172.

[3] Li Y,Gong M,Liang Y,Feng J,Kim JE,Wang H,Hong G,Zhang B,Dai H.Advanced zinc-air batteries based on high-performance hybrid electrocatalysts.Nat Commun.1805;2013:4.

[4] Zhang J,Zhao Z,Xia Z,Dai L.A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions.Nat Nanotechnol.2015;10:444.

[5] Chen Y,Ji S,Zhao S,Chen W,Dong J,Cheong WC,Shen R,Wen X,Zheng L,Rykov AI,Cai S,Tang H,Zhuang Z,Chen C,Peng Q,Wang D,Li Y.Enhanced oxygen reduction withsingle-atomic-site iron catalysts for a zinc-air battery and hydrogen-air fuel cell.Nat Commun.2018;9:5422.

[6] Jiang WJ,Hu WL,Zhang QH,Zhao TT,Luo H,Zhang X,Gu L,Hu JS,Wan LJ.From biological enzyme to single atomic Fe-N-C electrocatalyst for efficient oxygen reduction.Chem Commun.2018;54(11):1307.

[7] Shu X,Chen S,Chen S,Pan W,Zhang J.Cobalt nitride embedded holey N-doped graphene as advanced bifunctional electrocatalysts for Zn-air batteries and overall water splitting.Carbon.2020;157:234.

[8] Zhang B,Wang H,Zuo Z,Wang H,Zhang J.Tunable CoFe-based active sites on 3D heteroatom doped graphene aerogel electrocatalysts via annealing gas regulation for efficient water splitting.J Mater Chem A.2018;6(32):15728.

[9] Deng D,Yu L,Chen X,Wang G,Jin L,Pan X,Deng J,Sun G,Bao X.Iron encapsulated within pod-like carbon nanotubes for oxygen reduction reaction.Angew.Chem.Int.Ed.2013;52:371.