Research On Optical Imaging Methods Of Surface Refractive Index For Electrochemical Reaction Visualization

Surface refractive index optical sensing based on evanescent wave is a high-sensitivity,high-resolution,high-throughput and real-time detection technology,which has been widely applied in biomolecule detection and other fields.In the field of new energy,research on high-performance flow batteries and high-efficiency water-splitting technology are effective methods to tackle the energy crisis.Among them,electrode is an electrochemical reaction site,and its reaction activity distribution affects the power density and energy conversion efficiency of batteries.The distribution of water-splitting catalysts makes an impact on the onset potential of the hydrogen evolution reaction and oxygen evolution reaction.Therefore,this paper studies the surface refractive index optical sensing method and its application into in situ detection of electrode activity distribution,hydrogen evolution reaction and oxygen evolution reaction kinetics distribution.Compared with the existing techniques,the advantage of this method is that it can simultaneously realize in situ detection of electrochemical reaction distribution imaging.Based on the theory of surface refractive index optical sensing,instrumented total internal reflection imaging sensors and surface plasmon resonance sensors are built.We propose the concept of angle-pixel resolution to improve the refractive index resolution of surface plasmon resonance sensors based on angle modulation.Among them,the long range surface plasmon resonance sensor can achieve the best refractive index resolution of 4.37×10^-7RIU.For the in situ detection of electrode activity and electrochemical reaction kinetics distribution of a vanadium redox flow battery,we utilize a total internal reflection imaging sensor to map local current distribution of the electrode in the vanadium redox flow battery during cyclic voltammetry,enabling the activity and reversibility distribution imaging with the spatial resolution of a single fiber.The feasibility of the sensor is verified under different concentrations of electrolyte,different potential scan rates,different types of active electrodes and microscopic imaging of a single graphite fiber.In addition,the in-situ detections of the reaction kinetics of the electrode on its surface and side dimensions respectively during the operation of the vanadium redox flow battery are conducted and the electrode current density distributions are obtained.It is helpful for further studying the self-discharge,side reactions,electrolyte diffusion and transport process.For the in situ detection of the oxygen evolution reaction kinetics distribution,the experimental results show that the number of bubbles from oxygen evolution reaction oscillates periodically.The onset oxygen generation/consumption potential is consistent with the onset oxidation/reduction potential obtained from the cyclic voltammetry curve,which indicates that the total internal reflection imaging sensor provides a tool for measuring the onset potential of the oxygen evolution reaction.Besides,it shows that during long term cyclic voltammetry,the introduction of oxygen-containing functional groups in the oxygen evolution reaction can improve the electrode’s activity and uniformity distribution.In addition,the higher activity and irreversibility of the electrode facilitate the oxygen evolution reaction.For the hydrogen evolution reaction process,the onset potential distribution can also be obtained.This optical imaging method of surface refractive index is expected to realize in situ detection of electrode activity distribution and electrochemical reaction kinetics distribution in the fields of flow batteries,water-splitting for hydrogen production,electrocatalysis,and electrochemical corrosion,which will be helpful for the researches on electrode modification,catalysts refresh and so on.