One of the most promising technologies for successfully integrating renewable energies into the grid is the production of hydrogen through electrolysis to store surplus electric energy. Low-temperature alkaline water electrolysis (AWE) technology, with greater maturity and the larger commercial outreach, has been in use for several years for hydrogen production. Mostly the preceding research primarily focus on the conventional AWE which is based on two non-porous electrodes separated by a liquid electrolyte in which gas bubbles moves freely. However, this design has a drawback since it has a high ionic (ohmic) resistance due to the relatively wide bubble containing electrolyte gaps between the electrodes and the separator membrane. In recent trends, modern AWE electrolyzers are based on zero-gap configuration in which position of two porous electrodes are directly adjacent to a hydroxide ion conducting membrane or separator. This configuration forces gas bubbles to be escaped from the backside of the porous electrodes, thereby, considerably lowering the ohmic resistance contribution from the electrolyte between the two electrodes. Unfortunately, there is a scarcity of research on two phase modeling of zero-gap AWE combined with electrochemical reactions, and thus, accurate hydrodynamic modelling of the gas-liquid flows is necessary since the local distribution of gas affects the amount of electrical energy needed to produce hydrogen.
A two-dimensional (2D) two-phase model of zero-gap AWE cell containing 30% potassium hydroxide (KOH) solution has been developed using COMSOL® software. The model embodies electrochemical kinetic for both cathode and anode electrode, the two-phase flow model for gas and electrolyte, and the transport model for species including diffusion/convection/migration. A numerical two-phase model based on the extended Darcy approach was used to simulate two-phase flow in the porous media during the cell’s operation. This multiphysics approach allows the model to simultaneously evaluate the electrochemical, fluid flow, and species transport phenomena occurring in an electrolysis cell.
A comparative study was conducted to investigate the effect of two-phase flow on zero-gap AWE performance while considering various cell parameters. In particular, the electrical response was evaluated in terms of polarization curve (voltage vs. current density) at different catalyst layer thickness and porosity. Furthermore, the gas crossover across a separator which is considered as a critical aspect related to AWE performance was studied. Simulation results showed that the cell with a large catalyst layer thickness and high porosity, and small catalyst layer thickness and low porosity exhibited better electrical response. Besides that, the gas crossover across a separator lied well below the critical limit for the typical AWE’s current density 0.3 A/cm2. The current full 2D AWE model provides a comprehensive information of the electrochemical and transport phenomena, making it a viable tool for large-scale AWE cell and multi-megawatt stack design.