Parametric optimization of a coupled system integrating solid oxide fuel cell and graphene thermionic energy converter

In this work, a high-efficiency coupled system by integrating solid oxide fuel cell with graphene thermionic energy converter is proposed and evaluated. Based on theories of the electrochemistry, thermionic emission, heat transfer, and first law of thermodynamics, the formulas for the overall power density and energy conversion efficiency of the proposed system are derived, and the thermal and electrical characteristics of the coupled system are studied. The three special states such as ideal heat transfer, opened graphene thermionic energy converter, and shorted solid oxide fuel cell are discussed. The maximum power density and efficiency and the corresponding optimal conditions are determined. As the subsystems work independently, the dependences of the electrical parameters on the temperature of the solid oxide fuel cell, the current density of the graphene thermionic energy converter, and the coupled system's power density and efficiency are given, and the parametric optimal designs are presented. The effects of work function on the optimal performances are revealed. The results obtained in this work are of great significance to design and optimize the coupled energy cascade utilization system.