Fuel Cell & Electrolyzer Module

Built-In User Interfaces

COMSOL Multiphysics^® provides users with prebuilt user interface combinations that define a set of equations together with settings for mesh generation, solvers, and results. For the Fuel Cell & Electrolyzer Module, these combinations include hydrogen fuel cells and water electrolyzers.

By selecting any of the Hydrogen Fuel Cell and Water Electrolyzers interfaces, the transport and reaction properties for the oxygen and hydrogen gas diffusion electrodes are defined automatically. Only the domains for the electrodes, electrolyte, separator, and gas channels need to be selected. Chemical species and additional reactions can also be added to the oxygen and hydrogen gas diffusion electrodes, such as steam or carbon dioxide. The model equations solve for the electrode (electronic conduction) and electrolyte phase (ionic conduction) potentials as well as for the mole fractions of the gas mixtures in the system.

Concentrated Electrolyte Transport

The Concentrated Electrolyte Transport interface can be used for modeling transport in electrolyte solutions containing an arbitrary number of charged and uncharged species. This electrochemistry interface is based on concentrated solution theory, where the transport equations are defined using binary Maxwell–Stefan diffusion coefficients under the assumption of local electroneutrality. In contrast to the Nernst–Planck equations, concentrated solution theory does not assume that electrolyte species are diluted in a neutral solvent of constant concentration. Typical applications include ionic liquids, molten salts, and highly concentrated solutions with significant concentration gradients of the charge-carrying species. This interface is well suited for analyzing multicomponent transport behavior in advanced electrochemical systems.

Multiphase and Single-Phase Flow in Free and Porous Media

One of the specific phenomena in low-temperature fuel cells and water electrolyzers is the concurrent transport of liquid and gaseous water (steam). In fuel cells, the flow needs to drive water out of the cell as well in order to avoid flooding of the electrodes. Similarly, in water electrolysis, inadequate transport of the produced gas may render parts of the cell inactive. In both cases, it is important to model two-phase flow in the porous electrodes and in the open channels.

The Fuel Cell & Electrolyzer Module features the mixture, bubbly flow, and Euler–Euler models for dispersed multiphase flows, as well as phase transport in porous media. These features enable the modeling of multiphase flow in porous media (electrodes) as well as in open free media (channels). For more information on these multiphase flow models, view the CFD Module.

Primary, Secondary, and Tertiary Current Distribution

The space-dependent simulations (1D, 2D, and 3D) can account for ohmic losses (primary), ohmic and activation losses (secondary), as well as ohmic, activation, and mass transport losses (tertiary). For tertiary current distributions, it is possible to define systems with supporting electrolytes, dilute electrolytes, and concentrated electrolytes. The transport equations — the Nernst–Planck equations — can be combined with the electroneutrality condition or Poisson's equation.

The electrode kinetics can be defined using the Tafel equation, the Butler–Volmer equation, or arbitrary functions of the overpotential and the concentration of chemical species. Multiple reactions may be defined on an electrode surface (an arbitrary number).

The current distribution interfaces can be used in combination with porous electrodes, gas diffusion electrodes, and planar electrodes.

Gas Diffusion Electrodes

Modeling gas diffusion electrodes (GDEs) in the Fuel Cell & Electrolyzer Module is very straightforward. The transport equations in the gas phase and in the pore electrolyte are automatically defined in the user interface based on the boundary conditions added. The software contains separate domain features for defining the hydrogen and oxygen electrodes. The main electrode reactions are predefined, with the option to change the kinetics and add parasitic reactions or other reactions.

The transport of species in the gas phase is automatically coupled with the transport in the gas channels. The fluid flow is defined for the gas channel and for the porous structure using the Brinkman equations to model fully coupled free and porous media flow.

The charge balance in the electrolyte (separator) and the pore electrolyte (the electrolyte in the active layer or in the GDE) are also defined. They are automatically coupled to the transport equations in the gas phase through the electrochemical reactions and Faraday's law.

Built-In Thermodynamics

The content of the gas mixtures in the hydrogen and oxygen electrodes may vary for different processes and different operating conditions. The Fuel Cell & Electrolyzer Module contains a built-in thermodynamic properties database for hydrogen mixtures and oxygen mixtures. For instance, nitrogen, water, carbon dioxide, or carbon monoxide can be implemented as additional species for a hydrogen mixture in order to model byproducts from reforming reactions. The same additional species are available for an oxygen mixture. When the composition is selected and the reference partial pressures are defined, the software can compute the equilibrium electrode potential for the hydrogen and oxygen electrode reactions and thereby the equilibrium potential for the cell.