Abstract
A two-dimensional, single-phase, isothermal, multicomponent, transient model is built to investigate the transport phenomena in unitized regenerative fuel cells (URFCs) under the condition of switching from the fuel cell (FC) mode to the water electrolysis (WE) mode. The model is coupled with an electrochemical reaction. The proton exchange membrane (PEM) is selected as the solid electrolyte of the URFC. The work is motivated by the need to elucidate the complex mass transfer and electrochemical process under operation mode switching in order to improve the performance of PEM URFC. A set of governing equations, including conservation of mass, momentum, species, and charge, are considered. These equations are solved by the finite element method. The simulation results indicate the distributions of hydrogen, oxygen, water mass fraction, and electrolyte potential response to the transient phenomena via saltation under operation mode switching. The hydrogen mass fraction gradients are smaller than the oxygen mass fraction gradients. The average mass fractions of the reactants (oxygen and hydrogen) and product (water) exhibit evident differences between each layer in the steady state of the FC mode. By contrast, the average mass fractions of the reactant (water) and products (oxygen and hydrogen) exhibit only slight differences between each layer in the steady state of the WE mode. Under either the FC mode or the WE mode, the duration of the transient state is only approximately 0.2 s.
Highlights
unitized regenerative fuel cells (URFCs) based on Proton exchange membrane (PEM) electrolyte are reversible electrochemical devices capable of operating in water electrolysis (WE) mode and H2 /O2 FC mode
This study investigates the transient behavior of URFCs under operation mode switching with a numerical simulation technique
(2D) model is selected because the study aims to observe the transport phenomena along the gas flow channel and the vertical location of the PEM; (2) water is maintained in the gaseous state to simplify the model into single-phase [28,29]; (3) the change in the internal temperature is not considered because the main objective of this study is to investigate the mass and electric transport phenomena; (4) the model involves multicomponent species; (5) operation mode switching is a transient process
Summary
URFCs based on PEM electrolyte are reversible electrochemical devices capable of operating in water electrolysis (WE) mode and H2 /O2 FC mode. In the WE mode, water molecules are split into hydrogen and oxygen with the assistance of external voltage. In the FC mode, hydrogen and oxygen molecules are combined to form water and generate electricity [1,2,3,4]. URFCs are used extensively, for space or military applications, and for solar-powered aircraft, satellites, and micro-spacecraft applications [5,6,7]. Proton exchange membrane (PEM) is a convenient electrolyte technology for both WEs and FCs, as it allows for low operational temperature, quick start, fast response, high power and energy densities [8]. Hu et al [13] built a comprehensive
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