(Invited) Build Effective Cathode Nanocatalysts to Improve Performance of Proton Exchange Membrane Fuel Cells

Abstract

Fuel cells (FCs) are green energy devices capable of conversion of chemical to electrical energy, releasing heat and water. One design of FCs are proton exchange membrane (PEM), which consist of a solid nafion membrane as the electrolyte, utilizing H2or bio-fuel as a fuel supply; and noble metal as the catalysts. PEMFCs provide direct electricity for stationary and portable applications with high energy conversion efficiency, low pollutant emissions, simple design and operation, and flexible fuel supply. However, current PEMFC designs encounter problems in materials compatibility, manufacturing cost, and performance degradation. Numerous attempts have been made to improve the performance and durability of PEMFCs. Commonly-used catalysts are carbon supported Pt and its alloy, but are plagued by high expense and low resistance to carbonaceous species or use of Pt alloy (which has low resistance to corrosion). Thus, use of non-Pt metal catalysts is desired; however, their durability and carbonaceous species tolerance is unknown requiring further investigation. The novelty of this approach is application of ACNTs which allows improved performance due to rapid gas diffusion and chemisorption of the gas reactants. ACNT hydrophobicity also provides a new tool to prevent cathode flooding, resulting in long-term device stability. The aligned carbon nanotubes are synthesized using chemical vapor deposition (CVD) with a xylene-ferrocene solution as the precursor. Xylene is the carbon source, while ferrocene provides the iron metal nanoparticles, which function as the seeds for the nanotubes growth. Three 5 cm2quartz substrates are placed inside an 1 inch diameter quartz reaction tube. The tube is placed in a two stage furnace and tightly sealed to air. The first stage of the furnace is at a temperature of 225 ºC, which is enough to vaporize the solution. The second stage is held at 725 ºC and is used to carbonize the vaporized solution, depositing the iron nanoparticles on the quartz substrates, and allowing the carbon nanotubes to grow around the iron seeds. The solution with the chemicals is injected into the reaction tube, on the low temperature stage, using argon and hydrogen as the carrier gases, at flow rates of 100 and 50 mL/min respectively. The chemicals injection rate is 0.225 and 0.250 mL/min. The anodic and cathodic materials were then place on two sides of the Na-form Nafion membrane. Hot pressing technique was then used to fabricate the membrane electrode assembly (MEA) under temperature of 210 ºC and pressure of 600 pound-forces per square inch gauge (pisg) for 5 to 10 mins. The surface morphology, cross-sectional images and the thickness of the aligned carbon nanotubes were determined using a field emission scanning electron microscope (JSM7600) equipped with X-ray energy dispersive spectroscope (Chemistry Department, Texas A&M University-Kingsville). Raman/Fourier transform infrared spectroscopy (FTIR, Hariba Jobin-Yvon LabRam IR system) confocal microscope (Materials Characterization Facility, Texas A&M University) was employed to obtain highly specific fingerprints for precise chemical and molecular characterization and identification. The single cell is attached to an Electrochem Inc. test stand to record the I-V polarization curves, which gives a measurement of the MEA’s performance. The PEMFCs device is initially conditioned for about 2 hours until it reaches the desired temperature and humidity conditions, using argon gas in the anode and oxygen gas in the cathode, at a constant voltage of 0.4 V, according to the US Fuel Cell Council test protocol. After a constant current is reached, the polarization curves are measured by potentiostatically cycling the voltage between 0.2 and 1 V. The Pt-functionalized aligned carbon nanotubes cathodic catalyst was obtained by the chemical vapor deposition, wet chemistry impregnation, and followed by heat-treatment at 300 °C. The catalyst was characterized through scanning electron microscopy and Raman spectroscopy to determine the unique structure and molecular interaction between C and Pt atoms. The results show that the dense nanotubes layers have been directly grown on cathode materials with highly aligned structure, which favors the gas diffusion. The diameter of the tubes is observed approximately 20-50 nm and its length varies from 10-30 µm according to the different growth time period. The Pt nanoparticles are uniformly distributed on the surface of tubes, whose size is ranged from 1 nm to 8 nm. The single PEMFC displayed the maximum power density >850 mV/cm2 and >320 mV/cm2 with the O2 and air oxidant introduced into cathode, respectively. Figure 1