Controllable Synthesis Of Single-atom Catalysts And Their Applications In Oxygen Evolution Reaction

Nowadays,the increasingly severe shortage of energy and environmental issues have stimulated the search for sustainable and green energy storage and conversion systems.The key to elevate the overall energy-conversion efficiency is to develop highly-efficient catalysts.In recent years,single-atom catalysts(SACs)have drawn extensive attentions due to their unique electronic structures and extraordinary performance in catalysis.This thesis aims at developing facile and controllable methods to synthesize SACs.In the meanwhile,we precisely manipulate the valence state and coordination environment of SACs for applications in oxygen evolution reaction.Besides,with the aid of advanced techniques and theoretical calculations,we interpret the structure-performance relationship between the electronic structures and catalytic performance of SACs at the atomic level.The in-depth understanding into the key factors in tuning the activities of SACs provides us pathways to design novel SACs with desired performance.This thesis will discuss the following issues in detail:1.A universal method to fabricate SACs was developed using electrochemical deposition.Through this method,fabricating a variety of transitional metal SACs is possible on different substrates.Besides,the valence state and coordination environment of SACs can be tuned by controlling the conditions in electrochemical deposition.In addition,the SACs from cathodic electrochemical deposition exhibited excellent performance in hydrogen evolution,while SACs from anodic electrochemical deposition was highly productive in oxygen evolution.Furthermore,when the cathodically and anodically deposited Ir1/Co0.2Fe0.8Se2 SACs on Ni foam was integrated into a two-electrode system,a record-low potential was achieved in overall water splitting.2.An iron-based SACs with high valence state was fabricated,which exhibited high activities in oxygen evolution.The iron-based SACs(Fe1(OH)x/P-C)was obtained by electrochemical deposition of iron atoms on porous carbon.The oxygen-evolving activity of Fe1(OH)x/P-C was two orders of magnitude,compared with that of iron-based Fe2O3 and FeOOH.Electrochemical mechanism studies and theoretical calculation:discovered the transformation from Fe3+to.e4+under a very low voltage due to the existence of Fe-O-C bonding.Fe4+with a near-unity e8 filling shoWS a more appropriate adlorpti11 strength to reaction intermediates than Fe3+,accelerating the reaction kinetics in oxygen evolution.3.Symmetry breaking activates the Co sites in LiCoO2 in electrochemical oxygen evolution.Lathanum(La)with larger ion radius was introduced into the lattice of LiCoO2 via high-temperature pyrolysis.The difference between La and Co ions in ion radius induces the distortion of CoO6 octahedrons in adjacent to La ions,breaking the symmetry of CoO6 octahedrons.Resultantly,the comparatively inactive Co sites are activated,improving its activity towards oxygen evolution.Further studies showed the distorted CoO6 octahedrons strengthened the conductivity of LiCoO2 and the hybridization between Co-O orbitals,which are beneficial for promoting oxygen-evolving activities.