Bimetallic porous porphyrin polymer-derived non-precious metal electrocatalysts for oxygen reduction reactions

The development of efficient and stable electrocatalysts on the basis of non-precious metals (Co, Fe) is considered as one of the most promising routes to replace expensive and susceptible platinum as the oxygen reduction reaction (ORR) catalyst. Here we report a synthetic strategy for the precursor controlled, template-free preparation of novel mono- (Fe; Co) and bimetallic (Fe/Co) nitrogen-doped porous carbons and their electrocatalytic performance towards the ORR. The precursors are composed of metal–porphyrin based conjugated microporous polymers (M-CMPs with M = Fe; Co; Fe/Co) derived from polymerization of metalloporphyrins by the Suzuki polycondensation reaction, which enables the synthesis of bimetallic polymers with alternating metal–porphyrin units for the preparation of carbon-based catalysts with homogenously distributed CoN4 and FeN4 centres. Subsequent pyrolysis of the networks reveals the key role of pre-morphology and network composition on the active sites. 57Fe-Mössbauer spectroscopy was conducted on iron catalysts (Fe; Fe/Co) to determine the coordination of Fe within the N-doped carbon matrix and the catalytic activity-enhancing shift in electron density. In acidic media the bimetallic catalyst demonstrates a synergetic effect for cobalt and iron active sites, mainly through a 4-electron transfer process, achieving an onset potential of 0.88 V (versus a reversible hydrogen electrode) and a half-wave potential of 0.78 V, which is only 0.06 V less than that of the state-of-the-art Pt/C catalyst

The superhydrophobicity of polymer surfaces: Recent developments

Superhydrophobicity is the extreme water repellence of highly textured surfaces. The field of superhydrophobicity research has reached a stage where huge numbers of candidate treatments have been proposed and jumps have been made in theoretically describing them. There now seems to be a move to more practical concerns and to considering the demands of individual applications instead of more general cases. With these developments, polymeric surfaces with their huge variety of properties have come to the fore and are fast becoming the material of choice for designing, developing, and producing superhydrophobic surfaces. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1203–1217, 201

Self-similarity of contact line depinning from textured surfaces

The mobility of drops on surfaces is important in many biological and industrial processes, but the phenomena governing their adhesion, which is dictated by the morphology of the three-phase contact line, remain unclear. Here we describe a technique for measuring the dynamic behaviour of the three-phase contact line at micron length scales using environmental scanning electron microscopy. We examine a superhydrophobic surface on which a drop’s adhesion is governed by capillary bridges at the receding contact line. We measure the microscale receding contact angle of each bridge and show that the Gibbs criterion is satisfied at the microscale. We reveal a hitherto unknown self-similar depinning mechanism that shows how some hierarchical textures such as lotus leaves lead to reduced pinning, and counter-intuitively, how some lead to increased pinning. We develop a model to predict adhesion force and experimentally verify the model’s broad applicability on both synthetic and natural textured surfaces.National Science Foundation (U.S.) (CAREER Award 0952564)DuPont MIT AllianceNational Science Foundation (U.S.). Graduate Research Fellowship ProgramNational Science Foundation (U.S.) (Award ECS-0335765

Chemistry at silicone-inorganic oxide interfaces

This dissertation describes research performed using siloxane polymers. This includes the reactions of siloxane polymers with inorganic oxide surfaces to form covalently attached monolayers, and the electrical properties of crosslinked silicone composite films fabricated by compounding with nickel particles. In addition to these topics, the use of contact line pinning as a practical and controllable method for the deposition of materials on superhydrophobic and chemically patterned surfaces is also described. The first chapter provides a general review of siloxane polymer chemistry, focusing in particular on the relationship between molecular structure and physical properties. The use and fabrication of silicone composite materials is also discussed, including typical methods for crosslinking siloxane polymers and the effects of filler materials. Finally, contact angle hysteresis and contact line pinning phenomena are presented. Following this introduction, four separate but interrelated projects are presented. First, the surface modification of titania via hydridomethylsiloxanes is discussed. This work represents an extension of the reaction of hydridosilanes and provides an environmentally clean method for the hydrophobization of titania. Linear and cyclic hydridomethylsiloxanes, as well as hydridomethylsiloxane-co-dimethylsiloxane polymers, are used as reagents and the resulting surfaces are discussed. Unpredicted results from this method lead to the consideration of poly(dimethylsiloxane) as a previously unconsidered reagent presented in the next project. The second project discusses the covalent attachment of siloxane polymers, particularly poly(dimethylsiloxane), to a range of inorganic oxide surfaces, including titania, nickel oxide, alumina, and silica. This reaction is presented as a thermally activated equilibrium process, and offers insight into certain aging processes found in silicone materials. Particular focus is made on the development of a highly reproducible method for the fabrication of low contact angle hysteresis surfaces. Furthermore, this reaction is shown to be general for the siloxane bond through the reaction of functional and cyclic siloxanes. The third project describes the preparation of electrically conductive silicone coatings, containing nickel and titania particles. The effect of nickel concentration and geometry on the electrical properties of these coatings is examined and the effects on the percolation threshold are presented. In addition to this, the addition of titania nanoparticles to diminish electrical conductance is also investigated. The fourth project discusses the contact line pinning of liquids on hydrophobic surfaces. In this chapter, the use of ionic liquids exhibiting no vapor pressure is used to experimentally determine the de-wetting process of liquids from pillared, superhydrophobic surfaces through micro-capillary bridge rupture. Furthermore, this technique is used as a preparative technique for the fabrication of individual salt crystals supported on pillared surfaces

Contact angle hysteresis: a different view and a trivial recipe for low hysteresis hydrophobic surfaces

Contact angle hysteresis is addressed from two perspectives. The first is an analysis of the events that occur during motion of droplets on superhydrophobic surfaces. Hysteresis is discussed in terms of receding contact line pinning and the tensile failure of capillary bridges. The sign of the curvature of the solid surface is implicated as playing a key role. The second is the report of a new method to prepare smooth low hysteresis surfaces. The thermal treatment of oxygen plasma-cleaned silicon wafers with trimethylsilyl-terminated linear poly(dimethylsiloxane) (PDMS - commercial silicone oils) in disposable glass vessels is described. This treatment renders silicon/silica surfaces that contain covalently attached PDMS chains. The grafted layers of nanometre scale thickness are liquid-like (rotationally dynamic at room temperature), decrease activation barriers for contact line motion and minimize water contact angle hysteresis. This simple method requires neither sophisticated techniques nor substantial laboratory skills to perform