Modeling and simulation of electrolyte transport in alkaline fuel cells

A mathematical model accounting for liquid electrolyte transport has been developed and is applied for the simulation of alkaline fuel cell (AFC). Numerical results were obtained by using commercial CFD software, in conjunction with the user defined functions that calculate the source terms of the transport equations. Calculations show that this model possesses a good capability to capture the electrolyte transport phenomena inside AFC. An order of magnitude analysis is conducted for the energy transport in the separator and electrode regions, suggesting that cross-stream conduction and stream-wise convection are important for thermal management of AFC.; The mass-transfer limiting step is dominated by how fast oxygen can dissolve, which is a function of the gas solubility and the gas-liquid specific area. As the current density approaches its limit and the electrolyte concentration becomes relative high on the cathode side, a considerable overpotential loss occurs at the cathode due to a lack of dissolved oxygen resulted from the fact that the gas solubility decreases exponentially with an increase in the electrolyte concentration. The non-uniformity of the local primary current as well as the variation in concentration of dissolved gases is caused mainly by the species transport resistance between the electrodes. In addition, the validity of ideal solution assumption was evaluated with comparison to non-ideal solution.; The numerical calculations also captured the presence of a shunt current at the separator entrance and exit. The presence of the local shunt current is to satisfy the potential equation in a region where the primary current is lacking and where a large electrical potential gradient is present. Provisions to reduce shunt currents are suggested.; The influences of operating conditions and the sensitivity of design parameters on the performance of the AFC were presented.; Because of no physical properties of mixed electrolytes available for numerical simulations as well as no experimental study to investigate mixed electrolytes effect on the AFC performance, an experimental test for a 30W AFC stack was implemented to examine three different tasks: the effect of different electrolyte, the effect of different KOH concentration, and the effect of mixed electrolyte KOH and NaOH.