Giant Strain Of Two-Dimensional Pd Nanomesh Towards Boosted Oxygen Reduction Reaction

Sustainable energy conversion and storage technologies have been acknowledged as the alternatives to the traditional fossil fuel-based energy to adapt the global climate and environmental change challenges.The sluggish reaction kinetics in the electrodes of these sustainable energy devices.Particularly,the cathode oxygen reduction reaction（ORR）occurred in fuel cells and metal-air batteries has become a bottleneck that severely restricts the overall efficiency for their real-world applications.Thus,efficient catalysts have to be employed for a low energy barrier operation of sustainable energy devices.While the platinum group metals（PGM）have been deemed to be the most active catalysts in various catalytic reactions,the proper design of this family of catalysts with higher activity and less dosage is yet the current research target.In recent years,palladium（Pd）has been considered the most ideal replacement metal for platinum（Pt）in the field of electrocatalytic oxygen reduction reactions（ORR）due to its abundant reserves and flexible electronic structure.Although the activity of Pd-based catalysts is far less than that of Pt-based catalysts,it was found that changing the surface electronic structure of Pd-based catalysts could improve their electrocatalytic activity to be comparable to that of Pt-based catalysts.Since the electrocatalytic ORR occurs on the catalyst surface,the surface state of the catalyst is crucial to determine its electrocatalytic performance.Therefore,through morphology engineering,defect engineering,strain engineering and other strategies to regulate the microscopic morphology of Pd-based nanomaterials,improve the local structure and electronic structure of catalyst active sites,and design catalysts with high specific surface activity for improving electrocatalytic performance.In response to these strategies,this research took Pd-based nanomaterials as the research object,and adopts the regulation strategies of morphology engineering and strain engineering to improve the electrochemical activity specific surface area（ECSA）,specific activity（SA）,and mass activity（MA）of the material.Starting from these problems,the microscopic morphology,local structure,and electronic structure of palladium-based nanomaterials were regulated,so as to achieve the improvement of catalyst performance.Specifically,the controllable preparation of Pd-based nanomaterials with different dimensions and two-dimensional（2D）porous Pd nanomesh were prepared in this paper.The ORR performance in alkaline electrolytes and zinc-air battery（ZAB）performance of these catalysts were focused on,and the structure-performance relationships were thoroughly explored by means of transmission electron microscopy and first-principles calculations（DFT）,as follows:（1）The controllable preparation of Pd-based nanomaterials in different dimensions was achieved based on the morphological engineering regulation strategy.Different morphologies of Pd nanomaterials,including zero-dimension（0D）Pd nanoparticle（Pd NP）,one-dimension（1D）Pd nanodendrite（Pd ND）and 2D Pd nanosheet（Pd NS）,were synthesized through the modulation of different ligands and different synthesis environments.It was demonstrated that the unique morphologies of Pd NS and Pd ND provided great ECSA,exposed more electrochemical active sites,and significantly enhanced their electrical conductivity,thus exhibiting higher MA than commercial Pt/C and Pd/C catalysts.in addition,due to the exposure of more catalytically active crystalline facets,the Pd ND catalysts exhibited greater SA that was2.9 times higher at 0.9 V_RHE than that of the Pd NS catalysts.In addition,DFT calculations confirmed that the large number of exposed crystalline facets at the edges of Pd ND significantly improved its surface electronic structure,alleviated the excessive binding of oxygen intermediates to the surface,and fully exposed a large number of unsaturated catalytic active sites,thus significantly boosting its ORR performance.（2）The 2D porous Pd nanomesh（Pd NM）were prepared controllably based on the strain engineering strategy.Based on the synthesis of Pd NS and Pd ND,Pd NM with a 2D porous morphology was synthesized by modulating the precursors and the synthesis environment,and the analysis of the transmission electron microscopy（TEM）results clearly showed that the edges of the nanopores had multiple lattice steps,and a large amount of literature showed that the high-index crystalline facets constituted by these steps provided more active sites for the catalytic reaction and significantly enhanced the catalytic performance.In addition,giant tensile strains were found in the outermost crystalline facets of the nanopore edges,which were counted to be up to～27.23%.Pd nanomaterials with～5%tensile strain were reported to favor the ORR performance.It was demonstrated that the unique 2D morphology of Pd NM provided great ECSA,exposed more electrochemical active sites,and exhibited great MA,which was84.5 and 73.6 times higher than that of commercial Pt/C catalysts and Pd/C catalysts at 0.9 V_RHE,respectively.In addition,the large number of lattice steps and tensile strains at its pore edges significantly improve its surface electronic structure,alleviate the over-binding of oxygen intermediates to the surface,and fully expose a large number of unsaturated catalytic active sites,thus significantly enhancing its ORR performance,as confirmed by DFT calculations.（3）2D Pd NM catalysts were applied as cathodes in alkaline ZAB.The 2D Pd NM synthesized by the guidance of strain engineering exhibited extremely superior ORR catalytic activity.To investigate its practical application prospects,a series of ZAB performance tests were conducted using Pd NM as the air cathode catalyst for alkaline ZABs.The battery exhibited a high open-circuit voltage of 1.440 V over a period of up to 10 hours.In addition,the battery exhibited a high specific capacity of 797.43 m Ah g^-1 and a peak power density of220.23 m W cm^-2.We further investigated its long-lasting durability at a current density of 5m A cm^-2,and the results showed little performance loss even after 150 hours of charge/discharge cycles（450 cycles）.To summarize,the growth mechanism and control factors for Pd-based nanomaterials with highly promising practical applications were explored by morphological engineering and strain engineering in this research and the controlled construction of Pd-based nanocatalysts with different morphologies were realized;the relationship between microstructure and ORR catalytic performance of Pd-based nanomaterials was investigated;the structural sources of the excellent ORR catalytic performance of Pd ND and Pd NM were elucidated by electron microscopy combined combining with DFT.The study will provide scientific basis and experimental data for the design and construction of high performance noble metal-based catalysts at the atomic scale and promote the rapid development of energy conversion technology,which has broad application prospects and great social and economic benefits.This research provided scientific basis and experimental data for the design and construction of high-performance PGM at the atomic scale,and promotes the rapid development of energy conversion technology with broad application prospects and great social and economic benefits.