Shape-Engineered CO2 electroreduction over Silver Nanostructures
Authors: Liu Subiao , Luo Jing-Li 
Published: February 2020

CO2 makes up the largest portion of greenhouse gases (GHG) which are primarily responsible for climate change and global warming. The concept of utilizing CO2 as a feedstock fuel has drawn attention in research communities worldwide [1]. Thus, an innovative energy-efficient CO2 conversion system has therefore become a critical step towards the ultimate goal of CO2 capture and utilization. Electrochemical CO2 reduction reaction (CO2RR) is regarded as one of the most promising methods for CO2 conversion due to the increased availability of low-cost renewable energy. Currently, the major issues associated with electrochemical CO2RR include a broad distribution of the products, low catalytic activity, large required overpotentials, collectively leading to the low conversion efficiencies, high energy consumption and insufficient catalyst durability. Therefore, it is desirable to develop new highly active and selective catalysts capable of efficiently converting CO2 into high-value fuels at ambient temperature.

For crystalline catalysts, the particle size [2-3] and surface crystallography (grain boundary [4] and vacancies [5]) are important factors in determining the catalytic activity, reaction products, reaction kinetics and selectivity [6-7]. However, there have been limited studies on CO2RR over metal NPs regarding the influence of particle shape, this warrants further exploration since the presence of edge and corner sites varies as the shape changes. Shape control has received extensive attentions for Ag with particular emphasis on triangular Ag nanoplate (Tri-Ag-NP) because of their unique structure-related optical properties and potential applications.

This study demonstrates a predominant shape-dependent electrocatalytic reduction of CO2 to CO on triangular silver nanoplates (Tri-Ag-NPs) in 0.1 M KHCO3. Compared with similarly sized Ag nanoparticles (SS-Ag-NPs) and bulk Ag, Tri-Ag-NPs exhibited an enhanced current density and significantly improved Faradaic efficiency (96.8%) and energy efficiency (61.7%). Additionally, CO starts to be observed at an ultralow overpotential of 96 mV, further confirming the superiority of Tri-Ag-NPs as a catalyst for CO2RR towards CO formation. Density functional theory (DFT) calculations reveal that the significantly enhanced electrocatalytic activity and selectivity at lowered overpotential originate from the shape-controlled structure. This not only provides the optimum edge-to-corner ratio, but also dominates at the facet of Ag (100) where it requires lower energy to initiate the rate-determining step. This study demonstrates a promising approach to tune electrocatalytic activity and selectivity of metal catalysts for CO2RR by creating optimal facet and edge site through shape-controlled synthesis.

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