Abstract
Knowledge concerning the complicated changes of mass and heat transfer is desired to improve the performance and durability of unitized regenerative fuel cells (URFCs). In this study, a transient, non-isothermal, single-phase, and multi-physics mathematical model for a URFC based on the proton exchange membrane is generated to investigate transient responses in the process of operation mode switching from fuel cell (FC) to electrolysis cell (EC). Various heat generation mechanisms, including Joule heat, reaction heat, and the heat attributed to activation polarizations, have been considered in the transient model coupled with electrochemical reaction and mass transfer in porous electrodes. The polarization curves of the steady-state models are validated by experimental data in the literatures. Numerical results reveal that current density, gas mass fractions, and temperature suddenly change with the sudden change of operating voltage in the mode switching process. The response time of temperature is longer than that of current density and gas mass fractions. In both FC and EC modes, the cell temperature and gradient of gas mass fraction in the oxygen side are larger than that in the hydrogen side. The temperature difference of the entire cell is less than 1.5 K. The highest temperature appears at oxygen-side catalyst layer under the FC mode and at membrane under a more stable EC mode. The cell is exothermic all the time. These dynamic responses and phenomena have important implications for heat analysis and provide proven guidelines for the improvement of URFCs mode switching.
Highlights
Science and technology are developing by leaps and bounds, leading to higher energy storage demands
The results indicated that the highest temperature exists in the catalyst of the cathode, the temperature difference among membrane electrode assembly (MEA) is lower than 1 K
The unitized regenerative fuel cells (URFCs) is switched from the fuel cell (FC) mode to the electrolysis cell (EC) mode at 2 s
Summary
Science and technology are developing by leaps and bounds, leading to higher energy storage demands. The unitized regenerative fuel cell (URFC) is perceived as one of the cleanest and most effective energy storage and conversion device, whose special energy is several times higher than that of the lightest secondary battery [1,2]. URFCs have the advantages of no self-discharge or cell capacity limitation. The high special energy of URFCs indicates limitless applications for some weight-critical and time-consuming portable applications such as space energy systems (>40 Wh·kg−1 ) [3]. URFCs can be developed for and applied in high-altitude long-endurance solar aircraft, hybrid energy storage spacecraft propulsion systems, remote area energy storage systems without relying on the grid, power systems for power grid peak adjustment, and portable power source systems [4,5,6].
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