PEMFC Subzero Cold Start in a Quasi-Adiabatic Single Cell Fixture
Common single cell testing methodologies of testing PEMFCs’ materials for subzero cold start applications include freeze/thaw cycling for durability and isothermal water fill tests for cold start approximation. These testing protocols are limited because of the massive thermal sink of the fixtures endplates. In a stack of 350 or more repeat cells there is exothermic heating from adjacent cells. An adiabatic single cell fuel cell fixture was developed in 2004 at United Technologies Research Center. The goal of the proposed work presented herein is to further refine the design and testing methodologies.
A quasi-adiabatic single-cell fuel cell fixture was designed, built, and validated to emulate cold starts at the stack level, see Figure 1. This cell meets or exceeds internal requirements for pressure distribution at the MEA/GDL interface based on both modeling and mechanical testing. Balsa wood (vs synthetic alternatives) was used as a rigid insulation material. With the current fixture geometry (16cm2 active area), stresses do not exceed the ultimate stress of the weakest piece of balsa. To examine thermal distribution, the cell was frozen to a uniform temperature and heated using embedded heating pads as guard-heaters with a constant heat flux. This temperature data was then input into a finite element model in COMSOL. It was found that a large degree of insulative and guard-heat ability was obtained noted by the zero heat flux condition of the model results within the graphite flow field. Galvanic and potentiostatic cold starts were performed with the fuel cell fixture and show limitations in the MEA and the fuel cell test station. The galvanic tests set to 600 mA cm-2 were dominated by sufficiently high membrane resistances such that the fuel cell test station could not apply a constant current condition forcing the cell to be ramped up from a voltage minimum of less than<0.1V. Potentiostatic cold starts were set to 0.1V till the current attained a value of 600 mA cm-2.