Nathan Ashcraft Chemical Engineering Grad Student B.S. in Chemical Engineering at University of Missouri (2004) Hometown: Springfield, MO CV

Layer-by-layer (LBL) Thin Films for Fuel Cell Applications

The increasing global focus on alternative energy sources has led to a renewed interest in fuel cells, especially hydrogen and methanol powered fuel cells. At the core of these devices is a proton exchange membrane (PEM) that must allow ion conduction through the polymer matrix while preventing fuel crossover. A promising, versatile method for fabricating thin polymer films with tunable properties is layer-by-layer (LbL) assembly. This technique consists of building a polymer film by sequential dipping into polymer solutions with complementary interactions, for example electrostatic interaction or hydrogen bonding. The properties of the film can be manipulated by adjusting the pH and/or ionic strength of the solutions used, allowing the film to be tuned to possess the desired properties. In my thesis work, the transport processes in multilayer films is explored – namely, ion conduction, water transport, and fuel crossover.

To demonstrate the use of high ionic conductivity and methanol resistance of the LbL films we have developed, we applied them as coatings for commercial PEMs in direct methanol fuel cells (DMFCs). The DMFC performance of a single cell fuel cell with unmodified PEMs and devices where the PEMs are coated with two different LBL films are shown in Figure 1. All three devices were tested at the same conditions with 10% methanol/water (v/v) fed to the anode and air to the cathode at 25°C. We found that for each LBL system used, there are an optimal number of bilayers that result in the best DMFC performance for coating the commercial PEM, Nafion®. For LPEI/sPPO, the optimal coating was ten bilayers corresponding to a 0.15 μm thick film on both sides of Nafion®, while three bilayers of PDAC/sPPO, which is 0.13 μm of film, is the optimum for this LBL system. The best performing system, PDAC/sPPO coated Nafion®, had a peak power of 11.3 mW cm-2, which is 31.4% higher than plain Nafion®. The LPEI/sPPO coating had a more modest improvement in peak power of 22.1% above plain Nafion®. At a typical operating voltage of 0.3 V, the PDAC/sPPO coated Nafion® DMFC produced 53.2% more power than unmodified Nafion®, while the LPEI/sPPO coated device improved 36.2% in power output. Also, the open circuit voltage (OCV) for both Nafion® coated devices improved to 493 mV, which is almost 40 mV higher than plain Nafion®. We attribute the higher OCV of the coated membranes to the fact that the LBL systems have lower methanol permeability values than Nafion®.

Figure 1.  Power curves of single MEA DMFCs comparing unmodified Nafion® devices with Nafion® membranes coated with LBL films of LPEI/sPPO and PDAC/sPPO.  For each LBL pair, there are an optimum number of bilayers to maximize the power output.  At a typical operating voltage of 0.3 V, the PDAC/sPPO coated Nafion® DMFC produced over 50% more power than unmodified Nafion®, while the LPEI/sPPO coated device improved 36.2% in power output.  Also, the OCV of both Nafion® coated devices improved by 40mV as a result of lower methanol permeability.

References
(1) Decher, G., Fuzzy nanoassemblies: Toward layered polymeric multicomposites. Science 1997, 277, (5330), 1232-1237.
(2) Lutkenhaus, J. L.; Hammond, P. T., Electrochemically enabled polyelectrolyte multilayer devices: from fuel cells to sensors. Soft Matter 2007, 3, (7), 804-816.
(3) Argun, A. A.; Ashcraft, J. N.; Hammond, P. T., Highly Conductive, Methanol Resistant Polyelectrolyte Multilayers. Advanced Materials 2008, 20, (8), 1539-1543.