Leaching of Ions from Fuel Cell Vehicle Cooling System and Their Removal to Maintain Low Conductivity 2003-01-0802
The deionized water/ethylene glycol coolant used in the Ford Focus Fuel Cell Vehicle (FCV) requires very low conductivity (< 5 μS/cm) to avoid current leakage and short circuiting, presenting a unique water chemistry issue. The coolant's initially low conductivity increases as: 1) ions are released from system materials through leaching, degradation and/or corrosion, and 2) organic acids are produced by ethylene glycol degradation. Estimating the leaching potential of these ions is necessary for design and operation of fuel cell vehicles. An on-board mixed-bed, ion exchange resin filter is used to maintain low conductivity by removing leached or produced ions.
Various candidate materials were evaluated for leaching potential by exposing them to coolant at the design operating temperature for several months and periodically analyzing the coolant for ions. There was a wide range of leaching potential among the materials evaluated, which led to careful selection of materials for the cooling system. Ethylene glycol was also found to slowly degrade to organic acids, which are dissociated to organic ions, and significantly affect the conductivity of the coolant.
The ion exchange capacity of several anion and cation exchange resins were also assessed under typical system operating conditions to determine their ion exchange capacity.
Recommendations were made for cooling system manufacture, including a system wash prior to installation of the ion exchange resin. In addition, the size and frequency of replacement of the on-board ion exchange resin filter was determined. A conservative estimate of the expected useful life of the on-board ion exchange resin filter is 60 days. The expected life can be extended to as much as 180 days by an initial rinse of the system.