Mass Transport And Catalysis Of Porous Gold For CO₂ Electrochemical Reduction

Nanoporous metal materials prepared by dealloying have broad application prospects in driving,sensing and catalysis due to their excellent electrical conductivity and multi-scale interconnected channel structure.In order to cope with the shortage of renewable energy and the increasing demand for energy,it is necessary to develop a catalytic material that can efficiently catalyze the reduction of carbon dioxide（CO₂）.Nanoporous metal shows high catalytic selectivity and current density in CO₂ electrochemical reduction（CO₂ER）,which is an ideal catalyst.At the same time,studying the mass transport in porous channels can also further understand the kinetic process of catalytic reactions.On one hand,the catalytic performance and product selectivity of nanoporous metals can be regulated by the surface structure,such as surface ligament size,crystal facets,functional group modification and surface self-reconstruction.On the other hand,the hierarchically porous metals with specific surface area are conducive to electron and ion transport,mass diffusion and exchange,which can greatly improve the catalytic current density.However,the electrochemical behavior of nanoporous metals varies with different pore parameters.Therefore,understanding the relationship between pore size and surface structure is important for the accurate design of hierarchically porous metals.For finding the solution to the above problems,four kinds of porous gold with different nanostructures were designed in this thesis.The catalytic performance and mass transport process were systematically studied and discussed.（1）Mesoporous gold was prepared by nanoporous gold with annealing at different temperatures,and pore sizes were 186～1020 nm.The selectivity of CO₂ER decreased with the increase of pore size,but the current density of carbon monoxide（CO）increased first and then decreased.The size effect of current density was caused by the mass transport process in the mesochannel and the change of the electrode surface structure.The change of pore size led to the change of active sites and mass transport.On one hand,the increase of pore size improved the efficiency of mass transport.On the other hand,it increased the radius of curvature of ligament and decreased the number of active sites.Therefore,when the pore size was 300 nm,the partial current density of CO is the largest,which was 1.5times that of 186 and 727 nm mesoporous porous gold,and 3.3 times that of 1020 nm.（2）Nanoporous gold films with pore size of 22～56 nm and thickness of 100 nm were prepared by controlling the dealloying time.Due to the limitation of nanochannels on the mass transport,the reaction kinetics of nanostructure was limited.The nanoporous gold film with ultra-thin thickness could effectively avoid the influence of thickness on catalytic kinetics.Therefore,a comprehensive parameter was defined to replace the original electrochemically active surface area（ECSA）,eliminating the influence of different crystal planes on the catalytic activity,thus determining the intrinsic contribution of the mass transport in the nanoscale channel to the catalytic activity.The partial current density of CO increased with the increasing of pore size and reached saturation at 46 nm.（3）In order to explore the effect of thickness on the mass transport in the CO₂ER process,nanoporous gold electrode with a thickness range of 100～600 nm was prepared by stacking a nanoporous gold film with a thickness of 100 nm.The inhibition of hydrogen evolution（HER）process was significantly greater than that of CO₂ER process.Because the mass transport of protons was significantly different from that of CO₂,the thickness had different effects on the HER and CO₂ER processes.When the thickness was 400 nm,the catalytic current density and selectivity showed a saturation trend,and the current density dropped sharply at a thickness of 600 nm.Due to the great difference between the measurement method of ECSA and the mass transport of catalytic reaction,ECSA would be larger than the actual area involved in the reaction,resulting in a sharp drop in current density.（4）By applying a high-frequency pulse voltage to the mesoporous gold skeleton in an acidic solution,it formed a layer of nanoporous structure with a pore size of 35 nm by redox.Hierarchically porous gold with high CO selectivity and a Faraday efficiency of 90%at a low overpotential of 0.294 V was obtained.The results showed that the mesochannels of hierarchically porous gold can provide effective mass transport,and only 22%of the catalytic current decayed during the long-term reaction of 22 hours.Through crystal facets analysis,its excellent stability was due to the conversion of some low-activity crystal facets to high-activity（110）crystal facets during CO₂ER.