Abstract:
Miniaturized redox flow batteries have seen interest in electronic applications
because of their potential to simultaneously deliver electric power and remove
heat. For these applications, the flow battery has to be constructed on a side of a
computer chip, with components such as flow channels, manifolds, supply tubes,
electrodes, membranes and current collectors. There are efforts underway to
optimize the design of these components for different objectives such as
maximum power density or maximum efficiency. Due to space constraints, design
rules employed for classical redox flow batteries are only of limited use in
microfluidic systems. Since experimentation with micro-scale components is
especially expensive and time-consuming, there is a need to develop
computational tools to understand trade-offs in the design and operation of these
flow batteries. Computational fluid dynamics study of redox flow batteries using
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COMSOL Multiphysics software has been done. PEM fuel cells are very similar to
flow batteries. The high temperature PEM fuel cell was studied first to understand
the electrochemistry involved in such systems.
The anolyte and catholyte fluid flows are modeled using Navier-Stokes equations
of mass and momentum conservation. Species diffusion through porous
electrodes is modeled using species-dependent diffusion coefficients, which are
calculated based on the fluid state and the electrode geometry. The
electrochemical reactions are modeled using the Butler-Volmer equation with a
pre-defined exchange current density. The species concentrations, velocities and
pressures are solved in the simulation. We found that the flow channel geometry
and manifold design has an important effect on the flow distribution. It also affects
the pressure drop, thereby affecting the pumping power required to keep the
battery operational. The effect of different operating conditions such as
electrolyte flow rate and state of charge has been simulated and it has been found
that these parameters has significant effect on flow battery performance. Based
on these findings, we make observations regarding cell design and operation. We
expect that these simulations, once validated with experiments, lead to a better
understanding of the trade-offs involved in the design and operation of miniature
redox flow batteries for electronic applications.