Investigation of Current Density Distributions of PEM Fuel Cells during Cold Start-up

Abstract

The possibility to start-up polymer fuel cells from freezing temperatures is an essential requirement of the automotive application for fuel cell power trains. Even though car manufacturers have found a practical solution to the cold start issue by purging and thereby drying the cells during shut down - thereby avoiding strong degradation by ice formation in gas diffusion layers and flow fields - not much is known about the electrochemical processes during cold start-up. This is however required to avoid gradual degradation and life-time limitations. In this study we investigate the current density distribution of a stack during the start-up from -20 °C. A short stack is exposed at freezing temperatures and investigated in during start-up in a fuel cell test station with segmented sensor plates. Before freezing the cells are dryed (see Figure 1). The found heterogeneities are correlated with integral polarization and cell resistance tests at different flow conditions. Furthermore, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) provide some additional information about the changes during the start-up after freezing conditions. In addition we investigate the microscopic conductivity distribution of Nafion® membranes by conductive atomic force microscopy at freezing temperatures below -15 °C. The conductivity structure is correlated with extensive investigations available at elevated temperatures. The implications for start-up procedures will be shortly discussed.