Johan Ko

A three dimensional, transient, non-isothermal model of all-vanadium redox flow batteries

A three-dimensional (3-D) transient non-isothermal model of VRFBs is developed by rigorously accounting for the electrochemical reactions of four kinds of vanadium ions (V2+, V3+ VO2+ and VO2+) and resultant mass and heat transport processes. Particular emphasis is placed on analyzing various heat generation mechanisms, including irreversible and reversible heat generations due to vanadium redox reactions and joule heating arising from the solid electronic and electrolyte ionic resistances. The 3-D model is first validated against voltage evolution curves measured under charging and discharging processes. The model predictions compare well with the experimental data over a wide range of SOCs, and further reveal key electrochemical and transport phenomena inside VRFBs through multi-dimensional contours of solid /electrolyte potentials, species concentration, and temperature. This full 3-D comprehensive VRFB model can be applied to multi-cell stacks in order to find optimal design and operating conditions.

Effects of porous properties on cold-start behavior of polymer electrolyte fuel cells from sub-zero to normal operating temperatures

In this investigation, a parametric study was performed using the transient cold-start model presented in our previous paper, in which the ice melting process and additional constitutive relations were newly included for transient cold-start simulations of polymer electrolyte fuel cells (PEFCs) from a sub-zero temperature (−20°C) to a normal operating temperature (80°C). The focus is placed on exploring the transient cold-start behavior of a PEFC for different porous properties of the catalyst layer (CL) and gas diffusion layer (GDL). This work elucidates the detailed effects of these properties on key cold-start phenomena such as ice freezing/melting and membrane hydration/dehydration processes. In particular, the simulation results highlight that designing a cathode CL with a high ionomer fraction helps to retard the rate of ice growth whereas a high ionomer fraction in the anode CL is not effective to mitigate the anode dry-out and membrane dehydration issues during PEFC cold-start.

Large-scale cold start simulations for automotive fuel cells

In this study, the three-dimensional (3-D) transient cold start model is applied to the real-scale polymer electrolyte fuel cell (PEFC) geometry and transient cold-start simulations are carried out from subzero to normal temperatures. In order to reduce the computational turnaround time involving a large numerical mesh with millions of grid points, the cold start code is parallelized for parallel computing. The simulation results clearly show the evolution of ice, water content, temperature, and current density contours at different cold start stages characterizing freezing, melting, hydration, and dehydration processes. In addition, the model predictions emphasize beneficial influence of vapor phase diffusion from the cathode catalyst layer (CL) to gas diffusion layer (GDL) during cold start, which can contribute to reducing the amount of ice accumulation in the cathode CL. As the effect of vapor phase diffusion is substantial, more ice is accumulated in the cathode GDL rather than in the cathode CL. Therefore, the total amount of ice accumulated inside a cell is not always proportional to the amount of ice in the cathode CL, depending on the strength of vapor phase diffusion.

A Numerical Investigation of Effects of Methanol Concentration Fluctuation in Active-type Direct Methanol Fuel Cell (DMFC) Systems

In this study, we develop a one-dimensional (1-D), two-phase, transient-thermal DMFC　model to investigate the effect of methanol concentration fluctuation that usually occurs in active-type direct methanol fuel cell (DMFC) systems. 1-D transient simulations are conducted and time-dependent behaviors of DMFCs are analyzed under various DMFC operating conditions such as anode/cathode stoichiometry, cell temperature, and cathode inlet humidification. The simulation results indicate that the effect of methanol concentration fluctuation on DMFC performance can be mitigated by proper control of anode/cathode stoichiometry, providing a guideline to optimize operating conditions of active DMFC systems.