Combinatorial Exploration Of Non-Platinum Catalysts For Oxygen Reduction At Low Temperatures
Fuel cells present an intriguing method of transforming stored chemical energy into electricity. In PEM fuel cells that operate near room temperature, kinetic limitations in the oxygen reduction reaction (ORR) limit efficiencies. This study attempted to increase the ORR rate by replacing the platinum traditionally used to catalyze it at the cathode. Despite the high cost, platinum has had the lowest kinetic overpotential of any metal for the ORR, in addition to good conductivity and superior chemical/mechanical resistance. This study used a high-throughput method to investigate possible non-platinum ORR catalysts. Multiple elemental targets were simultaneously sputter deposited to produce thin film combinatorial spreads covering a range of elemental compositions. By including a fluorescent indicator dye into the testing solution, an optical sweep technique could determine local regions of high ORR activity. Such regions corresponded to unique compositions that were examined in further detail using cyclic voltammetry and a battery of other characterization steps. The ORR kinetics of palladium was increased through ligand and strain effects on its electronic structure. It was hot sputtered with tungsten to generate Pd90W10, which dealloyed at the surface to form a skeletal/skin layer of 4-5% W. Its activity was increased even further by adding a compressive strain of less than 0.1% through room temperature sputtering. A 3rd element was added into the Pd90W10 bulk to replace the sputtered compressive strain. In aerated 0.05 M H2SO4, Pd87W6V6 had a current density at 850 vs. RHE mV at least 30 times greater than that of platinum, along with much better resistance to methanol crossover. Conductive Pb-Ru pyrochlores were studied using both composition and temperature gradients. An optimal concentration of Pb2(Ru 1.5Pb.5)O6.5 had a binary Pb:Ru ratio of 50:50 on the surface. The high activity likely came from a weakened Pb-Ob bond favoring the displacement reaction of ObH in Pb-ObH-Ru by HO2. With a ternary Pb:Ru:Pd ratio of 25:50:25 on the surface, the activity was increased above platinum at low overpotentials in aerated 0.05 M H2SO4. In all cases, continuous gradients were leveraged with a general understanding of ORR catalysis to more easily and quickly focus on promising materials. .
dissertation or thesis