Engineering the Ionic Polymer Phase Surface Properties of a PEM Fuel Cell Catalyst Layer

Abstract

During high power density operations, the performance of polymer electrolyte membrane fuel cells (PEMFCs) may be limited by high water saturation levels in the cathode catalyst layer due to high wettability of the ionic polymer phase. A new heat-treatment method was used to create and lock-in the surface structure of Nafion 212. Several surface characterization techniques were used to verify the membrane's surface after heat-treatment, including contact angle, atomic force microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. We found that specific heat-treatment conditions led to the formation of either a hydrophobic or hydrophilic surface. The modified membrane's surface remained intact even after the membranes were boiled in water for 1 h. Next, a 4-point conductivity technique was used to verify that the heat-treatment conditions which led to a hydrophobic surface did not negatively impact the membrane's internal conductivity. Finally, this novel heat-treatment method was applied to the cathode catalyst layer of a H 2 -Air PEMFC to create a hydrophobic polymer-gas interface inside the gas pores of the cathode catalyst layer. Preliminary results showed 33% increase in peak power. The results of this research will guide the design of a new class of PEMFC catalyst layers. PEMFCs offer the advantages of high power density and energy conversion efficiency, simplicity in design and operation, the added environmental benefits such as zero carbon emissions, and the production of benign by-products such as water when using the H 2 -O 2 /(Air) fuel cell.1,2 Additionally, reversible fuel cells (flow batteries) offer a viable solution to the highly desired need for economical grid energy storage in order to take full advantage of load leveling. [3][4][5] As the use of intermittent energy sources such as wind and solar power continue to rise throughout the world, the need for reliable, efficient and economical energy storage solutions will grow. Excessive liquid water buildup in the fuel cell catalyst layer (CL) at high current densities can lead to electrode flooding, thus restricting transport of gaseous reactants to the catalyst reaction sites. In order to realize the economic viability of fuel cells, the fuel cell CL needs to be redesigned to overcome the negative hydration effects common with PEMFCs.Water management in a PEMFC is important for peak performance and long lifetime.6 A large amount of research has been generated to minimize the impact of water production in the PEMFC catalyst layer. One of the first areas of exploration to improve water management pertains to membranes. Critical membrane parameters include ionic conductivity, water and gas permeability, and mechanical strength. Di Vona and Iwai et al. demonstrated how cross-linking polymers greatly stabilizes the polymer in terms of thermal, mechanical, and hydrolytic degradation. 7,8 Pintauro et al. demonstrated reduced swelling and increased strength by electrospinning nanofiber composite membranes together.9 By melting an inert polymer ...

Layer-by-Layer Polydimethylsiloxane Modification Using a Two-Nozzle Spray Process for High Durability of the Cathode Catalyst in Proton-Exchange Membrane Fuel Cells

“…Furthermore, it showed the tendency of decreasing contact angle value as time goes by because the Nafion ionomer comprises a hydrophobic poly­(tetrafluoroethylene) backbone and hydrophilic sulfonic acid head groups (−SO 3 H), which absorb water. This result is similar to that in a previously reported study that measures the contact angle of Nafion . The Pt/C electrode with PDMS 0.2 mg cm –2 shows the highest superhydrophobicity with a contact angle of 162° (Figure S3), which exceeds that of the Pt/C electrode with Nafion ionomer and PDMS 0.2 mg cm –2 (152°) (Figure f).…”

“…This result is similar to that in a previously reported study that measures the contact angle of Nafion. 22 The Pt/C electrode with PDMS 0.2 mg cm −2 shows the highest superhydrophobicity with a contact angle of 162°(Figure S3), which exceeds that of the Pt/C electrode with Nafion ionomer and PDMS 0.2 mg cm −2 (152°) (Figure 3f). This can be due to the fact that the Pt/C electrode with PDMS 0.2 mg cm −2 has more remarkable porosity and roughness for air-pocket 23 and low surface energy due to the absence of Nafion ionomer.…”

Water Distribution in Nafion Thin Films on Hydrophilic and Hydrophobic Carbon Substrates

“…This is the first report on the depth profile of the water distribution in Nafion thin films on hydrophobic and hydrophilic carbon in water vapor and bulk water. This depth profile information is crucial for the development of properties in PEMFCs such as proton conductivity, flooding, and discharge of water …”

Performance Evaluation of a Hydrogen-Vanadium Reversible Fuel Cell

“…Solution-cast membranes (NR211 & NR212) are known to have a more hydrophilic surface (due to a higher concentration of sulfonate ion groups at the membrane's surface) than extruded membranes (N115 & N117). 20 The increased hydrophilicity of solution-cast membranes leads to increased ionic conduction near the membranes surface, therefore improving ionic and mass transport. Figure 6 graphs the polarization and power density curves for the reversible hydrogen-vanadium fuel cell with a CNT vanadium electrode and NR212 membrane.…”