Modeling and Validation of Assisted Cold Start for a PEMFC Coupled to a Thermochemical Preheater

Abstract

Cold start from sub-zero temperatures poses a challenge for automotive application of polymer electrolyte fuel cells (PEMFC), as product water freezes inside the cells hereby endangering operation safety. Therefore, ice formation in the cathode pores has to be avoided by preheating of the fuel cell above sub-zero temperatures. The concepts for such assisted cold start can be divided into internal and external heating strategies [1]. Amongst the latter, especially thermochemical preheating by metal hydrides presents a compelling yet efficient technology, as the heat is provided by exothermal hydrogen absorption without consumption of additional external energy.In this work, assisted PEMFC cold start is simulated with the transient 2D numerical model for a single fuel cell thermally coupled to a thermochemical preheater via a coolant fluid developed by Gießgen and Jahnke [2]. Here, the numerical model is validated against cold start experiments of the coupled system for both cell behavior and thermal balance of the heat transfer fluid. Starting from an initial temperature of -5°C, fuel cell behavior is then studied for 300 s of open circuit voltage (OCV) followed by a current density ramp up to 0.33 A/cm² in 150 s and a subsequent constant current period up to 500 s. Model validation is done with respect to temperature and voltage evolution as well as the energy balance of the coupled system. When the cold start is unassisted, a significant voltage drop down to 0.6 V is observed to occur already at 0.22 A/cm² and thus before reaching the maximum current density by the end of the ramp. Whereas difficult to deduce from experiments, this intermediate voltage drop can be linked to massive ice formation blocking up to 87% of the cathode catalyst pores, eventually causing a significant decrease of the electrochemically active surface (ECSA). For this ECSA reduction, an exponential dependence on the ice saturation is identified, suggesting that initial ice nucleation primarily occurs on the macroporous catalyst surface, thereby blocking the nanoporous surfaces of the primary particles within. When the cold start is assisted, a preheating phase of 41 s is performed prior to the current ramp. As the metal hydride reactor effectively heats up the heat transfer fluid, the fuel cell temperature in turn raises rapidly above the freezing point in less than 20 s. Consequently, ice formation is observed to be effectively reduced to saturations of less than 1%. As a result of this assisted cold start, the voltage drop is significantly attenuated and a minimum voltage of 0.65 V is now reached for a maximum of 0.33 A/cm² only at the end of the current ramp.