Performance and Durability of Electrodes with Platinum Catalysts in Polymer Electrolyte Cells Prepared by Ultrasonic Spray Deposition

Catalysts in the electrodes of polymer electrolyte cells serve a critical function in reactions which can be used to either generate electrical energy from chemical fuels or convert electrical energy into chemicals. For low temperature electrochemical fuel cells, platinum is often utilized for its exceptional catalytic activities towards hydrogen oxidation and oxygen reduction reactions of the anode and cathode, respectively. However, the limited supply of platinum and high demand result in prohibitive costs plaguing commercialization of this technology. Therefore, minimal amounts of the catalyst should be used to achieve the maximum output to reduce expenses. Electrochemical behavior is governed by the available reactive surfaces of the catalyst. A conductive material with high surface areas can be used in a composite approach to maximize nanostructured electrocatalyst sites. The state-of-the-art makes use of carbonaceous materials as the support. However, parasitic corrosion reactions of these materials in the electrode cause irreversible loss of activity that limits the useful lifetime of the cell, ultimately leading to its failure. Unfortunately, the supported platinum also can promote the catalytic oxidation of the carbon support where it is connected. Design of more resilient platinum catalysts could provide significant cost savings. An engineering challenge arises from their design and integration into the electrode. Electron conductive paths in the support, proton channels in the electrolyte, and porous space are required in joint formation of active catalyst sites in the electrode. Phase boundaries and interfaces with the catalyst are critical to the design of this composite structure. Acidic ionomers, known as Nafion®, were chosen in this study for their facile proton conductivity, thermo-mechanical properties, and commercial availability. An automated process for depositing thin uniform electrodes directly on the polymer electrolyte membrane by ultrasonic spray deposition was developed. The material processing and deposition methods were refined for a variation of electrodes built with both supported and unsupported platinum catalysts. Characterizations of electrochemical performance were conducted to evaluate catalyst behaviors in working membrane electrode assemblies. The catalyst should lower the activation energy of the reaction without being consumed. Preservation of electrocatalyst activity is critical to the durability of the electrochemical cell. Better platinum supports are needed for more reliable long-term performance. In proton exchange membranes, the electrocatalyst can experience high potential and low pH, limiting the selection of stable catalyst support materials. Platinum is a noble metal which has good intrinsic stability, but carbon is not in a thermodynamic equilibrium under these conditions. It is particularly problematic, as are many platinum alloys with less-noble metals which tend to be sacrificed to protect platinum during passivation. Ideally, the performance of catalysts should come without sacrifices to its stability. The nature and bonding of carbon atoms used in the support framework are an important determination factor of its corrosion resistance. Surfaces of carbon supports can also be functionalized to enhance their interactions with the catalyst. When stablemetal oxide phases are combined with carbon, useful junctions within electrocatalyst composites can be formed. An alternative catalyst support construction to the conventional carbon black with high surface area and conductivity is viewed as an important goal in the development of performance and durability of electrodes. Carbon nanotubes offer some advantages in their material structure and properties. Multi-walled carbon nanotubes were selected for exceptional mechanical and transport behavior in the electrode, and relatively low production cost. A long range graphitic order can reduce the carbon corrosion kinetics. However, even graphitized carbon is still susceptible to corrosion and it bonds relatively weakly with platinum catalysts. This can lead to loss of active surface area through diffusion and detachment of the catalysts. To prevent this, a second phase was included into composite supports. Titania, a common name for titanium oxides, was first chemically bonded to the surfaces of carbon nanotubes to help anchor the catalysts through strong metal-support interactions. Advantages from the carbon nanotube and titania supports toward performance and durability were contrasted against a set of control samples and demonstrated in the cathode and anode. A thorough literature review on the role of titania interfaces with the catalyst suggested application for the anode could provide further insight to its role in stability and activity. In this electrode, platinum catalyzed hydrogen oxidation suffers from contamination by trace amount of impurities that adsorb strongly on its surfaces. Carbon monoxide (CO) is one of the most persistent contaminants from reformation reactions and CO is also an intermediate in oxidation of other hydrocarbons, but despite its source, its adsorption onto reactive surfaces causes severe catalyst poisoning, limiting reactive sites. In order to restore activity, potential, temperature, and/or oxygen pressure are applied to remove adsorbed contaminants. Electrochemical stripping requires oxidizing conditions that can also corrode electrocatalysts. Strong interactions between platinum and titania can potentially limit this oxidative process while also opening active sites near their interface through a bifunctional mechanism. These interfaces can be characterized as Schottky junctions that result in a synergistic relationship between activity and stability. In order to enhance charge separation across the Schottky barrier formed with n-type semiconductors, a doped form of titania was synthesized from niobium substitution in the transition metal oxide phase. Niobium was selected for its coordination, ionic radius, passivation behavior, and ability to form shallow donors. Advanced diagnostics were used to study titania supports in an electrochemical hydrogen pump to evaluate advantages for CO tolerance. Material characterizations of electrocatalysts were used to correlate the effects of support construction on resilient performance. Enhancements to the bifunctional reaction for CO oxidation as well as stability are proposed from the metal-metal oxide junction formed between catalyst and support. Performance and durability of electrochemical cells is improved by applying the science of materials and interfaces to the construction of catalyst supports for platinum in working electrodes, serving as an example for further progress and optimization

Application of Electrospinning Technique in the Fabrication of a Composite Electrode for PEMFC

The Pt/C catalysts were mixed with carbon nanotube (CNT), Nafion™ dispersion and a fiber former, polymer poly-acrylic acid (PAA), to form an ink. The ink was deposited onto aluminum foil attached to a rotating collector via an electrospinning process. The ink composition, mixing procedure, and the E-spin parameters were studied for producing a uniform nanofiber mat on the aluminum substrate. The fiber-mat containing active catalyst ingredients was heat treated and decal transferred onto a Nafion membrane to form a membrane electrode assembly (MEA)

Investigation of Carbon Corrosion Resistance of CNT Containing Electrode

Carbon support corrosion is one of the major degradation mechanisms of polymer electrolyte membrane (PEM) fuel cell. Carbon oxidation occurs in PEM electrode and is accelerated at high potential created by adverse operating conditions and improper distribution of reactants and products [1, 2, 3]. Carbon corrosion can lead to the thinning of the electrode layer and severe performance degradation. The detailed mechanisms of carbon support corrosion induced performance loss are still not fully understood; it is believed that the following events contribute to the decay: (1) structural collapse of the porous electrode due to the loss of carbon; (2) carbon surface modification due to the formation of hydrophilic surface groups which can induce water accumulation and flooding of the electrode; (3) detachment and dissolution of platinum, which results in the reduction of platinum surface area. Together, these processes contribute to the loss of electrode performance

Biphilic Nanoporous Surfaces Enabled Exceptional Drag Reduction and Capillary Evaporation Enhancement

Simultaneously achieving drag reduction and capillary evaporation enhancement is highly desired but challenging because of the trade-off between two distinct hydrophobic and hydrophilic wettabilities. Here, we report a strategy to synthesize nanoscale biphilic surfaces to endow exceptional drag reduction through creating a unique slip boundary condition and fast capillary wetting by inducing nanoscopic hydrophilic areas. The biphilic nanoporous surfaces are synthesized by decorating hydrophilic functional groups on hydrophobic pristine multiwalled carbon nanotubes. We demonstrate that the carbon nanotube-enabled biphilic nanoporous surfaces lead to a 63.1% reduction of the friction coefficient, a 61.7% wetting speed improvement, and up to 158.6% enhancement of capillary evaporation heat transfer coefficient. A peak evaporation heat transfer coefficient of 21.2W/(cm2 K) is achieved on the biphilic surfaces in a vertical direction

Maternal and Breastmilk Viral Load: Impacts of Adherence on Peripartum HIV Infections Averted—The Breastfeeding, Antiretrovirals, and Nutrition Study

Antiretroviral interventions are used to reduce HIV viral replication and prevent mother-to-child transmission. Viral suppression relies on adherence to antiretrovirals

Carbon Monoxide Tolerant Platinum Electrocatalysts on Niobium Doped Titania and Carbon Nanotube Composite Supports

In the anode of electrochemical cells operating at low temperature, the hydrogen oxidation reaction is susceptible to poisoning from carbon monoxide (CO) which strongly adsorbs on platinum (Pt) catalysts and increases activation overpotential. Adsorbed CO is removed by oxidative processes such as electrochemical stripping, though cleaning can also cause corrosion. One approach to improve the tolerance of Pt is through alloying with less-noble metals, but the durability of alloyed electrocatalysts is a critical concern. Without sacrificing stability, tolerance can be improved by careful design of the support composition using metal oxides. The bifunctional mechanism is promoted at junctions of the catalyst and metal oxides used in the support. Stable metal oxides can also form strong interactions with catalysts, as is the case for platinum on titania (TiOx). In this study, niobium (Nb) serves as an electron donor dopant in titania. The transition metal oxides are joined to functionalized multi-wall carbon nanotube (CNT) supports in order to synthesize composite supports. Pt is then deposited to form electrocatalysts which are characterized before fabrication into anodes for tests as an electrochemical hydrogen pump. Comparisons are made between the control from Pt-CNT to Pt-TiOx-CNT and Pt-Ti0.9Nb0.1Ox-CNT in order to demonstrate advantages

Composite carbon nanotube and titania catalyst supports for enhanced activity and durability

In polymer electrolyte cells, an approach is shown for construction of resilient electrocatalysts. In the anode where the hydrogen oxidation reaction is subject to poisoning from fuel impurities like carbon monoxide (CO), increased tolerance and stability for the catalyst is revealed by modification of the support structure and properties. A carbon nanotube framework serves as the foundation for metal oxide addition, namely titania and niobium doped form. The corrosion resistant transition metal oxides form a strong bond with platinum catalysts through unique electronic interactions, measured by XPS. Several other material characterizations are also included to make comparisons between composites. Composite supports contribute improved reactivity towards oxidation of CO, especially in reduced titania. Carbon corrosion resistance is also measured and shown to be the greatest for this support. Synergistic combination of effects is observed directly by preparation of electrocatalysts into working membrane electrode assemblies measured for their performance & durability

Enhancing Graphene Reinforcing Potential in Composites by Hydrogen Passivation Induced Dispersion

To take full advantages of the structural uniqueness and exceptional properties of graphene as reinforcement in composites, harvesting well-dispersed graphene is essential. On the other hand, it is challenging to achieve simultaneously high stiffness, strength and toughness in engineered materials because of the trade-off relations between these properties. Here we demonstrate that the graphene reinforcing potential can be significantly enhanced through the excellent dispersion of graphene sheets in the matrix material and the strong graphene-matrix bonding by the coupled hydrogen passivation and ultrasonication technique. The fabricated graphene/epoxy composites exhibit simultaneously remarkable increase in elastic modulus, fracture strength, and fracture energy. We found that the inlet hydrogen atoms in the hydrogen passivation serve as a source of the second atoms to terminate the C dangling bonds and form more stable C-H bonds, separating graphene flakes and promoting the binding with the matrix material

Application of Electrospinning Technique in the Fabrication of Catalyst Layer of Membrane Electrode Assemblies

Inspired by the original work published by Pintauro\u27s group, the authors investigated the application of an electrospinning (E-spin) method for the fabrication of the catalyst layer of proton exchange membrane fuel cell electrode. Electrochemical performance of the membrane electrode assemblies (MEA) produced by E-spin was characterized can compared with MEA produced with an ultrasonic spraying technique. A simple polarization curve analysis was conducted to help resolve the sources of over-potential. The platinum loading, electrode morphology were characterized by X-ray fluorescence and scanning electron microscopy. The electrospinning technique was found to yield high performance electrode with low platinum loading. The E-spin method resulted in an electrode with high electrochemical area and platinum utilization