Effect of Water Conductivity on Proton Exchange Membrane (PEM) Catalyst Durability Using Thermal Degradation Resistant Polymer Membranes in Combat Applications

Abstract

The U.S. Army is taking an increasing interest in fuel cell technology as vehicles require more electrical power to support additional capabilities for different combat roles and adversarial threats. These additional power requirements include silent watch (long term mounted surveillance), advanced radios/jamming devices and exportable power supporting stationary applications and vehicle-to-grid connectivity. Fuel cells are attractive in these roles as they generate a lower thermal and acoustic signature while having the capability of being more efficient than current internal combustion engines. The goal is to have increased energy savings and decreased fuel transport logistic burdens. Despite these advantages, significant work needs to be completed before fuel cell technology can be integrated into combat vehicles. In addition to the thermal-cycling and stack sealing issues leading to stack power degradation, the U.S. Army has unique challenges engineering fuel cells into vehicles. One challenge is that air flow for prime mover heat rejection for combat vehicles is significantly restricted (due to many components competing for the same space and the use of ballistic grills) compared to civilian automobiles. This can increase the internal temperature inside the stack to 140°C when operating in hot environments. One commercial market approach the U.S. Army is analyzing to address increased stack internal temperature is the incorporation of silicon (as quartz or silica) into the polymer membrane, which literature studies have shown can reduce polymer thermal degradation. Even though polymer thermal degradation has been shown to be reduced when silicon is incorporated, it is likely not completely eliminated, and the elevated temperatures the U.S. Army expects to experience could easily degrade membranes, even with the added silicon, and produce side products which are rejected into the exhaust water generated by fuel cells. This is a concern since many polymer materials, such as Perfluorosulfonic Acid and other novel proprietary membranes, incorporate acid side chains into the polymer chemistry to enhance their material properties for increased fuel cell power and durability. These acid side chains, or smaller compounds which contain these side chains, have the potential to be removed under thermal degradation. The removed compounds would contaminate the water produced by the stack, making it weakly acidic and electrically conductive. Studies have shown that strong acids in contact with PEM catalysts, with current cycling remove platinum electrocatalyst materials through dissolution and lower the overall stack power. Since there is the potential for PEM stack catalyst degradation and overall loss of vehicle capabilities due to elevated operating temperatures, this effect of weakly acidic and electrically conductive water was investigated in this study. Quartz containing Polybenzimidazole polymer membranes were sputter-coated with a platinum layer 6nm thick, on a single side of each sample, to simulate the boundary layer between the nano-particle electrocatalysts coated on carbon supports and polymer membrane in fuel cells. These 6nm thick sputter-coatings were applied to polymer membrane samples as thin strips (0.635cm in length and 0.238cm in width) to direct the applied electrical current, which simulated fuel cell operating current passing through the electrocatalysts generated by electrochemical reactions inside the fuel cell. Water, combined with various amounts of acetic acid, was placed in contact with the platinum coated membrane samples while electrical current cycles, between 50 and 400, were applied through sample coatings. Each cycle used a square wave profile which consisted of current being applied for 5s and then removed for 5s. Figure 1 shows Energy Dispersive Spectroscopy (EDS) scans (1a,b) and optical microscopy images (1c,d) of platinum coating levels before and after electrical currents (1a) 0.7mA and (1b,c,d) 30mA were applied through sample coatings. EDS results show that both 0.7mA and 30mA currents were sufficient to remove platinum when in contact with water containing 5 vol% acetic acid. Results showed a 38% loss of platinum when 400 cycles of 0.7mA electrical current was applied and platinum degradation was even more sever, only requiring optical microscopy images to observe the platinum loss, when using a 30mA current and a minimum of 50 cycles. Removal of platinum from membranes was also a fast process that was completed in ~8 minutes for 50 cycles and ~66 minutes for 400 cycles. Overall sample parameter testing indicated water electrical conductivity was the primary driving force for catalyst degradation. Figure 1