Study On Electrochemical And Thermal Transport Coupling Characteristics Of Proton Exchange Membrane Fuel Cell Based On Modified Graphene Electrode

The renewable energy systems such as Proton Exchange Membrane Fuel Cell（PEMFC）to replace conventional fossil energy generation systems is an effective way to alleviate energy shortage,which is also the major challenge for countries around the world.However,the most widely used electrode catalyst material in commercial is platinum carbon（Pt/C）,but it suffers from high cost and severe non-uniformity of electrode surface temperature after long operation.Therefore,how to design and prepare an electrode with both efficient catalytic and high thermal conductivity properties of non-precious metals is a major bottleneck in the current research that has not yet been solved and has become a major constraint to the popularization and application of this technology.Among the many physicochemical properties of graphene,the efficient electrical and thermal conductivity properties provide a theoretically feasible basis for solving the above-mentioned challenges.In this study,the electrochemical and thermophysical coupling properties of modified graphene interface materials are studied,and the changes of electrochemical and heat transfer properties of modified graphene electrode materials prepared by non-equivalent doping of non-metallic atoms and carbon nanoparticle interlayer spacers are investigated.The specific contents include:（1）The structural stability,electronic properties and adsorption-dissociation behavior of binary co-doped graphene are investigated based on the first nature principle.It is found that the doped phosphorus（P）and boron（B）atomic positions are less electronegative than carbon（C）atoms thus easy to forming catalytic sites and can be used as active centers for the adsorption of oxygen molecules.The positive charge is significantly higher than that of the other doping conditions,and there is a large increase in the positive charge at the position of the active center of the P atom after oxygen molecule adsorption in almost all the conditions of N/P co-doped graphene.N/P co-doped graphene was found to be more favorable for oxygen molecule adsorption,and the dissociated O atoms tend to move in favor of P atoms and away from N atoms in a trend similar to the adsorption process of free oxygen molecules.However,the overly strong adsorption effect can also lead to the direct dissociation of oxygen molecules and the formation of strong bonds,hindering the migration reaction process in one step.（2）The mechanism of adsorption activity of non-equivalent P,N co-doped graphene and the thermodynamic process of Oxidation Reduction Reaction（ORR）were calculated by Density Functional Theory（DFT）.It can be seen that sp³hybridized doped graphene is more favorable than sp³d hybridized formation of doped graphene in terms of thermodynamic spontaneity.The calculations further show that the oxygen molecules can dissociate directly on G-PN₂C₁ and G-PN₃ due to the strong hybridization of the p orbitals of P and O atoms.This adsorption conformation can omit the OOH*formation process present on general adsorption surfaces such as G-PN₃C₁ and G-PN₄.The analysis of the results also reveals that for the first H₂O formation pathway,the ER mechanism pathway for the H⁺reaction on the surface of the G-PN₂C₁ and G-PN₃ structures is superior to the LH mechanism pathway due to the presence of a lower energy barrier.After comparing the Gibbs free energy of different intermediate steps,it can be found that the adsorption reaction step of OH*formation is considered as the limiting step of ORR reaction for the G-PN₃C₁ and G-PN₄ conformations because the the minimum thermodynamic energy change reaches,while the reaction limiting steps of G-PN₂C₁ and G-PN₃ can exist in two reaction processes.（3）In this study,the physicochemical properties of N,P-G as interlayer spacers with different contents of Activated Carbon（AC）were synthesized by one-step thermal reduction method.It was found that the surface interlayer free dispersion of N,P-G with the addition of 10%AC was better than that without AC.With the increase of annealing temperature,the proportion of P-N bond in P element gradually decreases,while the proportion of P-N bond in N element gradually increases,and its non-equivalent P,N doping ratio corresponds to the proportional structure formulae of G-P₁N_1.5,G-P₁N_2.1 and G-P₁N_2.9 at 700 ^oC,800 ^oC and 900 ^oC,respectively.From the analysis of electrochemical experimental results,it can be found that the material with the addition of 10%AC has good ORR performance.In addition,it can be developed that the performance of the 10%AC@N,P-G modified electrode is maintained above80%after 10,000s of ORR process,indicating that the catalytic stability of the N/P non-equivalent doped graphene material with 10%AC addition is better than that of the doped graphene and conventional Pt/C at some condition.（4）This study also investigates the thermal homogeneity of composite gas diffusion electrode（GDE）and the coupling effect mechanism on the output power based on AC@N,P-G.The experimental results show that the 10%AC@N,P-G possesses a high thermal conductivity,which is significantly increased by 57.14%compared with the conventional Pt/C electrode（Pt/C）.The thermal response time of the 10%AC@N,P-G based GDE surface is much shorter than that of the carbon black based GDE surface,and the Heat Transfer Factor（HTF）value of the 10%AC@N,P-G base at the stable moment is 1.688,which is also larger than that of the carbon black base of 1.165.The Non-equilibrium Heat Transfer Rate（NTR）of the electrode surfaces of the carbon black base and 10%AC@N,P-G base are stable at 17.62%and12.78%,respectively.Therefore,the thermal transport performance of the electrode surface under 10%AC@N,P-G substrate is better than that of the carbon black substrate.It was found that the 10%AC@N,P-G substrate GDE can fully demonstrate the advantages of graphene as a substrate catalyst in the ORR process only at the optimal mass ratio.Finally,the stability test shows that the maximum fluctuation rate of 10%AC@N,P-G substrate for PEMFC single-cell system is less than 0.005,which has high stability.（5）The coupling characteristics of electrochemistry and heat and mass transfer of PEMFC at different ambient temperatures are systematically investigated for the anisotropy of the electrode material thermal conductivity.For the anisotropic electrode thermal conductivity condition,decreasing the thermal conductivity in the Z-direction is the most obvious change to the overall output performance of the system compared to the other directions.In addition,the current density increases first along the velocity inlet direction at low operating voltage and then smoothly or decreases,while it keeps decreasing along the velocity inlet direction at high operating voltage.Combined with the analysis of the molar concentration distribution of liquid water,it can be found that the molar concentration of liquid water corresponding to the anisotropic thermal conductivity operating condition with higher average temperature is slightly lower than that at isotropic case,and a suitable anisotropic thermal conductivity along the electrode Z-direction at lower ambient temperature can effectively enhance the output performance of PEMFC,but too high ambient temperature tend to limit the enhancement of electrochemical reaction performance.In summary,this work explores the mechanism of electrode interface materials with efficient catalytic and thermal stabilization synergistic properties to improve the safety and reliability of PEMFC systems with modified non-equivalent P,N-doped graphene substrate materials,and the research results have important theoretical reference value for the comprehensive promotion with commercial applications of new clean energy systems such as fuel cells.