| Hydrogen is a new energy with great potential with the advantages of wide range of source,high heat value and environmentally friendly combustion products.However,as combustible gas,it has a wide explosion limit(4-75 vol.%)and low ignition energy(0.02mJ).Once leakage is happened and not be detected in time,it is certain to explode and brought damage.Mixed-potential hydrogen sensor has attracted extensive attention of researchers because of its simple structure,stable properties and low cost.At present,the main means to optimize the performance of sensors include the development of high-performance sensitive materials,the design of sensor electrode microstructure,and the pretreatment of electrode solid electrolyte interface.However,the current understanding of the response mechanism,especially the dynamic response process,is too little to provide a reasonable explanation for the causes of some response behaviors,and it is hard to design the sensor according to the properties of electrode materials and specific electrode microstructure.The main points of this paper are as follows:(1)Based on the complete form of Bulter volmer(BV)equation,the charging and discharging process of interface capacitance is simulated,the finite response model of mixed potential hydrogen sensor is established,and the method of numerical simulation of the model for specific parameters is designed and implemented.Several parameters directly related to the interface reaction dynamics in the model and the effects of interface capacitance and temperature on the dynamic response process are analyzed by using the control variable method.The simulation results show that the increase of standard reaction rate constant,transfer coefficient,standard equilibrium potential and gas concentration will accelerate the electrochemical reaction,so as to shorten the electrochemical response time and electrochemical recovery time,while the larger differential capacitance will lead to the opposite change.Under Tafel dynamics,the power-law concentration dependence of electrochemical response/recovery time is predicted.These findings reveal the key factors affecting the sensing dynamics of mixed potential gas sensor.(2)Based on Fick’s second law,the gas diffusion model in porous electrode is established,the steady-state equation and dynamic diffusion equation of hydrogen concentration distribution in electrode are solved,and the effects of several key parameters of electrode morphology on hydrogen concentration at three-phase interface(TPB)are analyzed.The interfacial hydrogen concentration decreases with the increase of interfacial reaction rate constant,electrode thickness and heterogeneous reaction rate constant,increases with the increase of Knudsen diffusion coefficient,and the diffusion time decreases with the increase of interfacial reaction rate constant,heterogeneous reaction rate constant and Knudsen diffusion coefficient,and increases with the increase of electrode thickness.Coupling the intrinsic response model and electrode diffusion reaction model,the diffusion reaction response model is obtained,and the interaction between diffusion process and interface reaction is quantitatively analyzed.According to the different relationship between diffusion rate and intrinsic response rate,the dynamic response process is divided into two cases:interface reaction control and diffusion control.Mathematical modeling and simulation analysis are conducted for these two situations respectively.Under interface reaction control conditions,the influence of diffusion process on dynamic response mainly comes from the concentration changes caused by diffusion,and the behavior is similar to the intrinsic response model.Under diffusion-control conditions,the reaction current is mainly limited by the diffusion process,and changes in interfacial reaction activity will not have a significant impact on the dynamic response process.The dynamic response recovery curves of five sensors were simulated,and the dynamic curves of the sensors were in good agreement with the theoretical model.The logarithmic linear relationship between the response time measured in the experiment and the hydrogen concentration confirmed the conclusion of the theoretical model. |
