Experimental Research And Mechanism Analysis Of Ferri/Ferrocyanide Aqueous Thermocells

Thermocell is a kind of low-grade waste heat utilization technology with high Seebeck coefficient and flexible structure.In recent years,researches have focused on the optimization of thermoelectric performance of thermocell,and important breakthroughs have been made in Seebeck coefficient,power density,thermoelectric efficiency and other aspects.However,current studies mainly focus on the influence of single factor in electrode materials,electrolyte formulas or cell structures on thermoelectric performances,ignoring the interaction between different factors,such as the interaction among the concentrations of different components in electrolyte solutions and the cell structures.At the same time,it is difficult to analyze the electrochemistry and mass transfer mechanism inside thermoelectric chemical cells through black box experiment,so the mechanism behind experimental phenomena,such as thermodiffusion,which is unique in thermoelectric chemical cells,cannot be deeply analyzed.In this paper,a classical convection-free and square-cavity thermocell,which consists of platinum electrodes and potassium ferri/ferrocyanide aqueous electrolyte,is studied.Firstly,the experimental platform of thermocell is built.And then the influences of key factors such as the concentrations of two redox ions and the structure of experimental device on the thermocell are explored through experiments.At last,a non-isothermal mechanism model of thermocell is established based on experimental data,and the mechanism analysis of experimental results is carried out.The main research contents and conclusions of this paper are as follows:1.Concentration experimental study is carried out in U-type and straight-type thermocell experimental devices.The experimental devices with two kinds of structures are established.The solubility of the electrolyte is studied and the influences of ion concentrations on the open-circuit voltage,internal resistance,power density and electrode potential are revealed.The experimental results under two kinds of structures are compared.Results show that 1)In multi-ions and concentrated electrolyte solution,equilibrium electrode potential cannot be calculated using the Nernst equation,due to the importance but immeasurability of ion activity coefficients.In order to obtain equilibrium electrode potential values under different conditions of temperature and concentration,direct measurement of equilibrium electrode potential and its temperature coefficient and then establishing the regression equations is an effective solution;2)Taking the power density as the index,the electrolyte solution concentration should reach saturation as far as possible.In the U-shaped structure,the redox ions concentration ratio can be adjusted to achieve a better thermoelectric performance,and in the straight structure,the 1:1 concentration ratio is the optimal state.3)Time-varying exists in both open circuit voltage and output current in the straight device,of which the former confirms the existence of thermodiffusion phenomenon;4)The Utype device is a special case of the straight device under the condition of large spacing,which can ignore the thermodiffusion and the time-varying of voltage and current,and the Seebeck coefficient is equal to the temperature coefficient of the equilibrium electrode potential.2.Based on non-isothermal mechanism model,the time-varying mechanisms of voltage and current are analyzed.A non-isothermal mechanism model is established,and the transport properties in the mass transfer equation are identified.The results show that: 1)Non-isothermal model can describe thermodiffusion and reproduce open-circuit voltage changes caused by thermodiffusion;2)The identification of thermodiffusion coefficients is multimodal,and there are optimal solutions with very close fitness functions in three parameter spaces;3)Thermodiffusion is the cause of time-varying of open circuit voltage,which changes the equilibrium electrode potential and liquid phase potential simultaneously.Electrode reaction is the cause of time-varying of current,and the influence of thermodiffusion near the maximum power density condition can be ignored.