Michael Gasda

Sputter-Deposited Pt/CrN Nanoparticle PEM Fuel Cell Cathodes: Limited Proton Conductivity Through Electrode Dewetting

. Polarization curves exhibit dE/dlog i slopes b �100 and 150 mV/dec for high E 0.75 V and low E 0.5 V potentials, respectively, but show an anomalous drop with b �420 mV/dec in the intermediate voltage range. This is attributed to poor proton conduction associated with a reversible dewetting of electrode pores during low current operation. Quantitative analyses of rate-dependent polarization curves and electrochemical impedance spectra show that the time scale for pore filling by process water is 10 3 s, and that the ionic resistance RC within the electrode increases by a factor of 4, from RC 0.2 to 0.8 cm 2 ,a sE increases from 0.5 to 0.8 V. The increasing electrode resistance is attributed to a low water production rate at low current, which allows the relatively hydrophobic CrN to expel water from the electrode pores, resulting in a higher resistance for ionic transport. These results show that even ultrathin sputtered catalyst layers can exhibit incomplete flooding.

Nanorod PEM Fuel Cell Cathodes with Controlled Porosity

Arrays of 1 μm long C nanorods were grown by glancing angle deposition on flat and patterned Si wafers, coated with 0.1 mg/cm 2 Pt catalyst by magnetron sputtering, removed from the substrates, and tested as cathode electrodes in proton exchange membrane (PEM) fuel cells. Deposition on flat substrates yields a nearly fully dense nucleation layer with 0.55 V. However, the cathodes deposited on the patterned substrates yield considerably higher currents at low potential, with a 2 times higher limiting current density i L = 0.73 A/cm 2 than those grown on flat substrates. The higher current in the mass-transport-limited regime is attributed to the 10 times wider engineered pores that facilitate O 2 transport to the active catalyst sites, resulting in a 5 times lower mass transport resistance R MT = 1.5 Ω cm 2 at E = 0.50 V, as quantified by electrochemical impedance spectroscopy.

Pore formation by in situ etching of nanorod pem fuel cell electrodes

Layers of 150 nm wide and 0.5–1.5 m long carbon nanorods were grown by glancing angle deposition on Si substrates, sputter-coated with 0.10 mg/cm2 Pt, and transferred to polymer electrolyte membranes for testing as cathode electrodes in fuel cells. The rods were etched within fully assembled cells by applying a potential above the reversible H2/O2 voltage, which leads to polarization curves that show a 4–7 times higher current at 0.40 V. The current increase is attributed to the opening of pores within the electrode, which facilitates easy oxygen transport and leads to a reduction in mass transport resistance by a factor of 360, as determined by electrochemical impedance spectroscopy. Etching sequences with increasing voltage VE indicate that VE 1.6 V yields water electrolysis and Pt oxidation that facilitates Pt agglomeration and migration of Pt ions into the electrolyte, while VE = 1.7 V results in removal of C and the formation of pores within rods that facilitate oxygen transport to reaction sites, yielding a 400–700% increase in fuel cell output current at low potential. These results suggest that the controlled etching of temporary scaffolds to create pores in an operating fuel cell may be an effective approach to reduce mass transport limitations.