Nanostructured oxygen reduction catalyst designs to reduce the platinum dependency of polymer electrolyte fuel cells

Polymer electrolyte fuel cells (PEFCs) are electrochemical devices that efficiently convert hydrogen and oxygen into electricity and water. Their clean point of operation emissions and fast refueling times have resulted in PEFCs being highly touted as integral components of sustainable energy infrastructures, most notably in the transportation sector. The issues associated with hydrogen production and distribution aside, the commercial viability of PEFCs is still hindered by the high cost and inadequate long term operational stability. A main contributor towards both of these issues is the platinum-based electrocatalysts used at the cathode to facilitate the inherently sluggish oxygen reduction reaction (ORR). These expensive precious metal catalysts comprise almost half of the overall PEFC stack cost, and undergo degradation under the cathode environment that is very corrosive due to the acidic and potentiodynamic conditions. There is therefore ample room for cost reduction if reduced platinum ORR catalysts can be developed with sufficient activity and durability to meet the technical targets set for the use of PEFCs in automobiles. \n\tIn this work, two classes of nanostructured catalysts are investigated. The first is high activity platinum or platinum alloy materials with the objective of surpassing the activity of conventional catalysts on a precious metal basis to achieve cost reductions. The second is non-platinum group metal (non-PGM) catalysts, that while intrinsically less active than platinum, can still provide high power output at moderate operating voltages, such as those encountered during automobile operation. These two catalyst technologies are developed and delivered with the ultimate objective of integrating them together into platinum/non-PGM hybrid electrodes to provide excellent PEFC performance with a reduced platinum dependency. \n\tIn Chapter 4, titanium nitride – carbon nanotube (TiN-CNT) core-shell nanocomposites developed by a simplistic two step fabrication procedure are reported. These materials are physicochemically characterized by a variety of microscopy and spectroscopy techniques and used as platinum nanoparticle elelectrocatalyst supports (Pt/TiN-CNT) for the ORR. Through half-cell electrochemical testing in acidic electrolyte, improved ORR activity was demonstrated for Pt/TiN-CNTs compared with state of the art commercial Pt/C. The one-dimensional morphology of the TiN-CNT supports is also conducive for integration into highly porous electrode structures with excellent interconnectivity to ensure reactant access and electronic conductivity throughout the catalyst layer, respectively. The long term stability of this catalyst however remains questionable, likely due to oxidation of the titanium nitride surface that results in a thin passivating layer. \n\tIt is becoming increasingly evident that corrosion of platinum nanoparticle supports is inevitable during fuel cell operation. To overcome this, a focus was then placed on the development of supportless nanostructured platinum catalyst designs. Platinum cobalt nanowires (Pt-Co-NWs) were prepared by simplistic, template free microwave-irradiation process as discussed in Chapter 5. Using cobalt as an alloying element was undertaken owing to the documented ability of this transition metal to modulate the adsorptive properties of platinum and induce increased ORR activity. The one-dimensional anisotropic nanostructure can also provide increased platinum stability owing to the reduced surface energies in comparison to zero dimensional nanoparticles. The Pt-Co-NWs displayed promising ORR activity a through half-cell testing in 0.1 M HClO4. Most notably, using harsh accelerated durability testing (ADT) that consisted of 1,000 electrochemical potential cycles from 0 to 1.5 V vs. RHE at 50 °C, the Pt-Co-NWs maintained the majority of their ORR activity, highlighting exemplary stability. While simple, the drawback of this synthesis approach is that it did not allow for nanowire diameters that were below 40 nm. This resulted in inaccessible platinum atoms within the nanowire cores, highlighting the fact that further improved ORR activity on a platinum mass basis could be achieved with reduced diameters. \n\tTo accomplish this, the electrospinning approach was used to prepare PtCoNWs (please note the nomenclature distinction). Through investigations in which synthesis parameters were systematically investigated, electrospinning was found to provide a versatile platform for the synthesis of nanowires with tunable diameters and atomic compositions. PtCoNWs with a near unity stoichiometric ratio, excellent atomic distribution and an average diameter of 28 nm were evaluated for ORR activity. Over a four-fold enhancement in Pt mass-based activity at an electrode potential of 0.9 V vs RHE is obtained in comparison to pure platinum nanowires, highlighting the beneficial impact of the alloying structure. A near 7-fold specific activity increase is also observed in comparison to commercial Pt/C catalyst, along with improved electrochemically active surface area retention through repetitive (1,000) potential cycles. Electrospinning is thereby an attractive approach to prepare morphology and composition controlled PtCoNWs that could potentially one day replace conventional nanoparticle catalysts. \n\tWith the development of PtCoNWs established, developing non-PGM catalysts that can be hybridized with the high activity platinum-based catalysts was required. In Chapter 7, single crystal cobalt disulfide (CoS2) octahedral nanoparticles supported on graphene/carbon nanotube composites were prepared as ORR catalysts. During the simplistic, one-pot solvothermal synthesis, the nanostructured carbon supports were also simultaneously doped with nitrogen and sulfur. Time dependent studies elucidated the growth process of the {111} facet encased octahedra that could only be prepared when carbon support materials were incorporated into the reaction mixture. The impact of carbon support on ORR activity was clear, with the graphene/carbon nanotube composite supported CoS2 octahedra (CoS2-CG) outperforming CoS2 supported on just graphene or carbon nanotubes. Additionally, CoS2-CG provided an on-set potential (0.78 V vs. RHE) and half-wave potential (0.66 V vs. RHE) that was 60 mV and 150 mV higher than the CoS2 particle agglomerates formed when no carbon support was included during catalyst preparation. By combining the synergistic properties of the graphene/carbon nanotube composite and unique shape controlled single crystal CoS2 nanoparticles, CoS2-CG comprises the highest activity non-precious metal transition metal chalcogenide reported to date, and is presented as an emerging catalyst for the ORR in fuel cells. \n\tChapter 8 provides a summary of the conclusions of this body of work, along with strategies that can be employed to capitalize on the scientific advancements made through this thesis. The delivery of PtCoNWs and CoS2-CG that can be reliably prepared by simple techniques provides the crucial first step towards the development of platinum/non-PGM hybrid electrodes. Future projects should focus on the integration of these two catalysts into new electrode arrangements in an attempt to exploit their individual properties. Through this approach, it is hypothesized that synergistic coupling of these two catalysts can lead to PEFC systems with reduced activation losses from the PtCoNWs, along with CoS2-CG providing increased maximum power densities at lower cell voltages, all at reduced platinum contents in comparison to state of the art PEFC cathodes.