Nanostructured catalyst design to improve ultrafine Pt nanoparticle utilization and stability for PEMFCs

Polymer electrolyte membrane fuel cells (PEMFCs) is an electrochemical device which efficiently converts chemical energy stored in hydrogen and oxygen into electricity and water. Widely regarded as promising alternative clean and sustainable energy source due zero emission and fast refueling time. Nevertheless, large scale commercialization of PEMFCs is plagued due insufficient long-term stability of precious metal catalyzing oxygen reduction reaction (ORR). The increasing demand of high performance and operationally stable electrocatalyst materials for PEMFCs requires significant research and development in designing and studying new electrocatalysts. Hitherto, Pt is best available material to catalyze ORR, and state-of-art commercial Pt/C cannot meet the crucial targets set by the U.S. Department of Energy (DOE) in terms of activity and stability, and moreover, the excessive use of precious metal platinum is not economically feasible for the widespread application of PEMFCs. Pt based catalyst suffers dramatic loss in terms of activity and stability due particle agglomeration, Ostwald ripening and dissociations from support material in addition to the loss of precious metal. The technical challenges associated with Pt electrocatalysts need to be addressed by studying and designing new electrocatalysts with advanced technologies. Therefore, development of commercial Pt/C or replacement with novel and more effective electrocatalysts is desirable. In this thesis two approaches have been investigated for the development of new electrocatalyst material to catalyze ORR in PEMFCs. First study consists of designing sulfur doped graphene (SG) as a unique support material with goal to investigate replacement of carbon support to reduce carbon corrosion and then deposit Platinum-palladium core-shell structure to improve activity and stability towards ORR. Second study involved the designing and developing new type of electrocatalyst where catalytically active nanoparticle was deposited inside pore of model carbon support material and this study was further extended for the doping of sulfur and Pt deposition inside pore of carbon support. First study involves synthesis of SG material by Flash heat treatment followed with quenching. Platinum-palladium core-shell structure was deposited on SG support using solvothermal methods. Pt-Pd core-shell structure was interacted with sulfur incorporated in graphene as -C-S-C strengthen the metal support interaction. The core-shell structure exhibits improvement in ECSA and long-term stability of the catalyst due to lower oxidation potential of Pd (0.92 V vs RHE) than Pt (1.19 V vs RHE), therefore prevents active material Pt oxidation. Pt0.9Pd0.1/SG exhibits an improved ECSA of 78 m2 g-1 compared to 65 m2 g-1 of commercial Pt/C, while mass activity of 0.371 A mg-1 (@0.9V vs RHE) compared to 119 A mg-1 for Pt/C catalyst. Moreover, after 10K cycles, Pt0.9Pd0.1/SG exhibits mass activity of 0.271 A mg-1 which is 80 percent of its initial activity, while Pt/C retains only 19 percent of its initial activity. Due to large size of core-shell structure around 15-20 nm, where Pt as a shell was around 5-10 nm in size, lack of technology to synthesize graphene at large scale, to decrease particle size as to increase active site density and to reduce Pt content, we focused on development of smaller Pt nanoparticle (< 2nm) on commercially available carbon support to improve activity and durability. In second study, we have developed a facile catalyst design to embed ultrafine Pt nanoparticles inside the nanopores of the carbon support towards increased Pt utilization and suppress Pt agglomeration/Ostwald ripening through pore-confinement effect. The novel strategy endows the resultant Pt nanoparticle with optimized electronic structure, which further accelerates the ORR kinetics and results in excellent activity. Due to these attributes, the as-prepared Pt nanoparticle inside pore (Ptinside/KJ600) catalyst have shown larger ECSA (113.4 m2 g-1) and initial mass activity of 0.558 A mg-1Pt (@0.9V vs RHE), which is 3.30 times higher than commercial Pt/C and outperforms most of the reported solo Pt catalysts. Beyond that, the catalyst also exhibits significantly improved durability with 9 mV negative shift (0.373 A mg-1) in half-wave potential after 20K cycles, while commercial Pt/C benchmark displays a 52 mV negative shift (0.0619 A mg-1). The structure characterization shows negligible increase in particle size distribution when conducted on the tested catalyst after 20K cycle confirmed that pore-confinement design can effectively inhibit the particle agglomeration. Higher activity before and after durability test confirmed there was no mass transfer or reaction kinetics limitations due to Pt nanoparticle embedded inside the carbon nanopore, in fact improved stability. In order to further improve activity and stability of the catalyst we further investigated sulfur doping inside pore of KJ600 (Sin/KJ600), followed with Pt deposition inside nanopore (Pt@Sin/KJ600). First study has proved that sulfur doping improve activity by altering d-band center and improve stability by strengthening metal support interaction. We have first time synthesized and reported synthesis of Pt@Sin/KJ600 catalyst where both Sulfur doping and Pt embedded inside pore of carbon support. Pt@Sin/KJ600 demonstrated an excellent mass and specific activity of 0.654 A mg-1 and 5.26 A m-2 compared to Pt/C 0.169 A mg-1 and 2.45 A m-2 respectively. Moreover, catalyst lost merely 1 mV of half-wave potential compared to 52 mV loss in Pt/C after 20K cycles. Mass activity of Pt@Sin/KJ600 catalyst after 20K cycles was more than 10 times higher than Pt/C catalyst. The structural characterization after durability test also confirmed insignificant changes in catalyst morphology. Spatial pore-confinement and metal-support interaction kept the Pt nanoparticles intact and optimized electronic structure to imparts excellent ORR activity and stability. We believe this catalyst design is not limited to Pt based catalyst but can easily applicable to the synthesis of Pt based alloy (with transition metal) to further improve activity and different catalysis applications