Theoretical Study And Applications Of Statistical Kinetic Effects In Nano-Electrocatalytic System

Electrocatalytic reactions have promising applications in solving the global energy crisis and mitigating climate change.Nanocatalysis has become a hot topic because of its much higher performance than conventional electrocatalysis.For decades,researchers have focused on catalyst design strategies that enhance electrocatalytic performance by modulating energetic factors such as reaction energy barriers.However,plenty of works have recently found that the performance of nanoelectrocatalysts is not simply energy dependent.For nanoelectrocatalytic systems,the size of catalyst is comparable to the characteristic scales of diffusion layer and electro-double-layer.Thus,electron transfer,diffusion and migration are coupled with each other,making the catalytic performance also subject to complex reaction mechanisms due to kinetic effects.Revealing the kinetic effects in nanoelectrocatalysis helps us to understand and regulate the kinetic behaviour of the system,and is of great significance for the rational design of nanocatalysts with high performance.In this paper,we employ the principles and methods of statistical mechanics to develop a full kinetic model covering the important electrocatalytic processes such as adsorption,desorption,surface electron transfer and mass transfer,systematically investigate the effects of catalyst surface microstructure and macro-configuration on the activity and selectivity of some important electrocatalytic systems,including carbon monoxide electroreduction reaction(CORR),hydrogen evolution reaction(HER),methanol oxidation reaction(MOR)and carbon dioxide electroreduction reaction(C02RR).We identify new phenomena and mechanisms by which local field effect and confinement effect regulate the mass transfer and thus influence the activity and selectivity of these important electrocatalytic systems,and propose new ideas for the design of high-performance catalysts with ordered configurations,three-dimensional nanochannel configurations and nanocavity structures.A brief summary is as follows,·The effect of catalyst surface roughness on electrocatalytic selectivitySurface structure modulation of nanocatalysts is one of the most important methods to enhance the activity and selectivity of electrocatalytic reactions.Some recent experimental work has shown that enhancing the "surface roughness" of catalysts can enhance the selectivity of carbon products in the CORR system.However,the underlying mechanism remains unclear.In this paper,we develop a reaction diffusion equation describing the kinetic process of the CORR system on the catalyst surface and systematically investigate the effect of roughness on CORR selectivity by simulating the surface roughness with high curvature active sites.It is shown that the roughness-regulated selectivity results from a ’local field effect’:the presence of a local electric field on the surface of the nanocatalyst leads to a directional transfer of OH-generated by the reaction away from the sites and redistributes them on the catalyst surface;the low intensity of the directional transfer due to the local field on the surface of a high roughness catalyst leads to an accumulation of OH-around the sites,which significantly inhibits intrinsic activity of the side reaction,HER,and thus effectively enhances the selectivity of CORR.Through further kinetic simulations,we found that CORR selectivity is also influenced by factors such as the characteristic size of the local electric field.Our work reveals the regulatory mechanism of local field-induced material transport on the kinetics of the system,and proposes a new design idea to optimize the reaction performance of nanoelectrocatalytic systems using local field effects.·Effect of two-dimensional ordered configuration of catalysts on electrocatalytic activityRecent years,many experimental works have found that the assembly of catalysts such as nanowires or nanotubes can effectively enhance the electrocatalytic reaction activity.The overall activity of the assembled system is not always a simple linear sum of individual nanoparticle activities,suggesting that the overall configuration of the catalyst at the assembly level has an important influence on the catalytic reaction performance,but there is still a lack of understanding of the underlying mechanism.In this paper,we take the nanowire-catalyzed MOR system as an example and establish a reaction diffusion kinetic equation including the spatial distribution of nanowires to systematically investigate the effect of the configuration of the catalyst on the electrode surface on the overall activity.The theoretical study shows that when the nanowires exhibit a two-dimensional ordered spatial arrangement,an ordered periodically distributed local electric field can be formed around them,leading to the directional transfer of reactants and their aggregation on the catalyst surface,thus significantly enhance the reaction activity.Our theoretical predictions were experimentally verified by our collaborators.This work reveals the mechanism by which the macroscopic configuration of the catalyst regulates the overall kinetics of the system,and provides a new strategy for the subsequent combination of theory and experiment for nanocatalyst design.·Effect of nanochannel configuration of catalysts on electrocatalytic activityThe above work has shown that the overall catalytic performance can be effectively optimised when nanowires are assembled into a 2D ordered configuration.What are the new implications of the catalyst configuration on the catalytic performance of the system when the 2D ordered nanowires are further assembled into a channel configuration to maximize the exposure of the active surface?To answer this question,we extended the two-dimensional reaction diffusion kinetic equations developed in the previous paper to a channel configuration catalyst system and systematically investigated the regulation of the channel configuration on the performance of the system by considering the effect of the pore structure on the electron transfer rate.Interestingly,we found a non-linear dependence between reactivity and pore size similar to that of a volcano curve,suggesting that there is an optimum channel size for the highest catalytic activity.It is further shown that this optimum size arises from the competition between the mass transfer and electron transfer processes of the system:on the one hand,an increase in pore size enhances the directional transport of reactants induced by the local electric field;on the other hand,an increase in pore size also leads to an increase in catalyst impedance and thus weakens the electron transfer rate.Our theoretical predictions were verified experimentally by our collaborators.The present work further reveals the regulation of catalyst assembly configuration on the kinetic behaviour and catalytic performance of the system,which provides a new approach for the design of electrocatalysts.·The effect of nanocavity structure on electrocatalytic selectivityRecent work has shown that the confinement effect of nanocavities and nanopores can break the thermodynamic scaling relationships in electrocatalytic systems,thus effectively optimising the performance of electrocatalytic reactions.In this paper,the kinetic equations for the reaction diffusion of the CO2RR system were established using the nano-cavity structure as an example,and the influence of the domain-limiting effect on the reaction selectivity was investigated in depth.It is found that the confinement effect of the nano-cavity can change the diffusion of CO and thus increase its local concentration,leading to an increase in the selectivity of the C2 product.It was further shown that the confinement effect is also influenced by the size of the cavity opening angle and the size of the cavity.Based on the above mechanism,we have designed a nanocavity modified Cu foil catalyst with high C2 product selectivity by combining the kinetic factors of the confinement effect and the energetic factors of the energy barrier reduction,which was experimentally verified by the collaborators.This work provides insight into the connection between the complex structure of catalysts and the kinetic behaviour of the system,and provides new ideas for the rational design of high-performance catalysts.