Increased Performance of PEFCs with Engineered Mass-Transport Pathways

Autor: Michael P. Manahan, Matthew M. Mench
Rok vydání: 2011
Předmět:
Zdroj: ECS Transactions. 41:569-581
ISSN: 1938-6737
1938-5862
DOI: 10.1149/1.3635590
Popis: A long-standing technical challenge in polymer electrolyte fuel cells (PEFCs) is proper water management. Excessive drying leads to accelerated degradation and poor ionic conductivity of the polymer membrane; excessive liquid water prevents reactant gases from accessing catalyst reaction sites. The diffusion media (DM) has been identified as a key component to proper water management. Under low humidity conditions, the DM must deliver inlet water vapor to the catalyst layer (CL) and membrane to ensure proper hydration of the membrane. Under high humidity conditions, the DM must remove excessive water vapor and condensate to ensure an adequate supply of reactant gas to the CL. Several studies hypothesize an ideal DM to consist of both larger pores for liquid water transport/storage and smaller pores for gas transport (1,2). The present study experimentally investigates the introduction of laser-cut perforations in the DM in attempt to create engineered pathways for improved gas and liquid flow. Conceptually, the perforations allow for increased gas and vapor access to the CL at low current, and at high current they act as water conduits for removing excess liquid water. This effect has been studied in this work using electrochemical impedance spectroscopy (EIS), neutron radiography (NR), and steady state polarization testing. In this study, the cathode-side DM was modified by introducing laser perforations with diameters ranging from 40 μm to 300 μm. Figure 1a shows the Nyquist plot for perforated DM (100 and 300 μm) and unaltered (virgin) DM under 100% inlet relative humidity (RH) at 0.2 A cm. Even at such a low current density, the 300μm DM shows a low-frequency arc (0.3 to 15 Hz) that is approximately 7 times larger diameter than the arc of the virgin DM and 100μm DM. The low frequency arc is traditionally attributed to the mass-transport related processes involved in the fuel cell due to their relatively long timescale compared to faster processes (e.g., charge transfer) in the fuel cell (3). Accompanying polarization curves (not shown) confirm this large arc is indeed attributed to excessive flooding in the 300-μm diameter perforations at the high-humidity conditions. The 100-μm and virgin DM arcs in Figure 1a are nearly identical, indicating that only minor differences in transport characteristics exist at all frequencies ranges. Figure 1b shows EIS spectra for the same cells, except with the inlet RH of the anode and cathode held at 50%. This condition yields minimal liquid water due to the sub-saturated conditions throughout the cell. The plot shows a drastic decrease in arc diameter of the 300-μm DM in the low frequency range, indicating the absence of the mass-transport limitations observed at 100% RH. Furthermore, both the 100-μm and 300-μm perforations have arc diameters smaller than the virgin DM. This suggests that the perforations enhance the mass-transport properties at low humidity conditions by increasing the gas access to the CL and shows promise for the implementation of advanced DM structure modification to improve PEFC performance. Corresponding polarization curves at 50% inlet RH (not shown) indicate that the 100-μm perforations increase the limiting current by 7%, as well as increase the cell voltage by 7% at lower currents compared to the virgin DM. While the 300-μm perforations show an increased voltage at lower currents, mass-transport losses are evident at higher currents. Optimization of perforation diameter for enhanced mass-transport properties under all humidity and current conditions is sought, and further experiments on 40-μm perforations are currently underway. In summary, the data show that well-engineered, tailored DM structural modifications yield desirable improvements in water management for PEFCs.
Databáze: OpenAIRE
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