Co-reporter:Ellen E. Benn, Bernard Gaskey, and Jonah D. Erlebacher
Journal of the American Chemical Society March 15, 2017 Volume 139(Issue 10) pp:3663-3663
Publication Date(Web):February 17, 2017
DOI:10.1021/jacs.6b10855
Electroreduction of small molecules in aqueous solution often competes with the hydrogen evolution reaction (HER), especially if the reaction is driven even moderately hard using a large overpotential. Here, the oxygen reduction reaction (ORR) was studied under proton diffusion-limited conditions in slightly acidic electrolytes: a model system to study the relative transport kinetics of protons and reactants to an electrocatalyst and the relationship between transport and catalytic performance. Using dealloyed nanoporous nickel–platinum (np-NiPt) electrodes, we find the hydrogen evolution reaction can be completely suppressed even at high overpotentials (−400 mV vs RHE). In addition, the mechanism of oxygen reduction can be changed by using buffered versus unbuffered solutions, suggesting the reaction selectivity is associated with a transient rise (or lack thereof) in the interface pH at the np-NiPt surface. Independently controlling reactant transport to electrocatalyst surfaces at high overpotentials exhibited a surprisingly rich phenomenology that may offer a generalizable strategy to increase activity and selectivity during electroreduction reactions.
Co-reporter:Ian McCue, Bernard Gaskey, Pierre-Antoine Geslin, Alain Karma, Jonah Erlebacher
Acta Materialia 2016 Volume 115() pp:10-23
Publication Date(Web):15 August 2016
DOI:10.1016/j.actamat.2016.05.032
Abstract
Liquid metal dealloying (LMD) has recently emerged as a novel technique to fabricate bulk nanostructures using a bottom-up self-organization method, but the literature lacks fundamental studies of this kinetic process. In this work, we conduct an in-depth study of the kinetics and fundamental microstructure evolution mechanisms during LMD using TiTa alloys immersed in molten Cu as a model system. We develop a model of LMD kinetics based on a quantitative characterization of the effects of key parameters in our system including alloy composition, dealloying duration, and dealloying temperature. This work demonstrates that the dealloying interface is at or near equilibrium during LMD, and that the rate-limiting step is the liquid-state diffusion of dissolving atoms away from the dealloying interface (diffusion-limited kinetics). The quantitative comparison between theoretically predicted and measured dealloying rates further reveals that convective transport and rejection of the dissolving element during coarsening of the structure also influence the dealloying kinetics.
Recent advances in oxygen reduction reaction catalysis for proton exchange membrane fuel cells (PEMFCs) include i) the use of electrochemical dealloying to produce high surface area and sometimes nanoporous catalysts with a Pt-enriched outer surface, and ii) the observation that oxygen reduction in nanoporous materials can be potentially enhanced by confinement effects, particularly if the chemical environment within the pores can bias the reaction toward completion. Here, these advances are combined by incorporating a hydrophobic, protic ionic liquid, [MTBD][beti], into the pores of high surface-area NiPt alloy nanoporous nanoparticles (np-NiPt/C + [MTBD][beti]). The high O2 solubility of the [MTBD][beti], in conjunction with the confined environment within the pores, biases reactant O2 toward the catalytic surface, consistent with an increased residence time and enhanced attempt frequencies, resulting in improved reaction kinetics. Half-cell measurements show the np-NiPt/C+[MTBD][beti] encapsulated catalyst to be nearly an order of magnitude more active than commercial Pt/C, a result that is directly translated into operational PEMFCs.
Journal of the American Chemical Society 2012 Volume 134(Issue 20) pp:8633-8645
Publication Date(Web):April 25, 2012
DOI:10.1021/ja3019498
We present a comprehensive experimental study of the formation and activity of dealloyed nanoporous Ni/Pt alloy nanoparticles for the cathodic oxygen reduction reaction. By addressing the kinetics of nucleation during solvothermal synthesis we developed a method to control the size and composition of Ni/Pt alloy nanoparticles over a broad range while maintaining an adequate size distribution. Electrochemical dealloying of these size-controlled nanoparticles was used to explore conditions in which hierarchical nanoporosity within nanoparticles can evolve. Our results show that in order to evolve fully formed porosity, particles must have a minimum diameter of ∼15 nm, a result consistent with the surface kinetic processes occurring during dealloying. Nanoporous nanoparticles possess ligaments and voids with diameters of approximately 2 nm, high surface area/mass ratios usually associated with much smaller particles, and a composition consistent with a Pt-skeleton covering a Ni/Pt alloy core. Electrochemical measurements show that the mass activity for the oxygen reduction reaction using carbon-supported nanoporous Ni/Pt nanoparticles is nearly four times that of commercial Pt/C catalyst and even exceeds that of comparable nonporous Pt-skeleton Ni/Pt alloy nanoparticles.
Co-reporter:F. Kertis, S. Khurshid, O. Okman, J. W. Kysar, L. Govada, N. Chayen and J. Erlebacher
Journal of Materials Chemistry A 2012 vol. 22(Issue 41) pp:21928-21934
Publication Date(Web):31 Aug 2012
DOI:10.1039/C2JM34527G
We present a theory and experiments that help clarify the origin of the effectiveness of nanoporous substrates in the heterogeneous nucleation of protein crystals. The central idea tested here is that when a substrate (or “nucleant”) possesses pores of the order of the hydrodynamical radius of a protein, then the entropic penalty associated with nucleating a protein crystal on that surface may be alleviated. Model experiments using lysozyme and nanoporous gold (NPG) substrates suggest that there is indeed a reduction in the entropy associated with creating critical nuclei, but the magnitude of the reduction is small. Taken together with further examination of protein crystallization with NPG nucleants using four other proteins, our aggregate results suggest that surface chemistry and surface area effects play the dominant role in nucleation when using these nanoporous nucleants.
Nanoporous metals made by dealloying possess significant geometric complexity—they are random, bicontinuous structures that also possess bubbles within ligaments, regions of very high negative, positive, and saddlepoint curvature, and significant polyfaceting. Here we introduce methods to geometrically quantify the structure of nanoporous metals employing simulated model nanoporous metals generated via large-scale kinetic Monte Carlo simulations as the basis of discussion. A method is introduced to transform these simulated structures into smooth triangulated meshes using new mesh-smoothing algorithms that hybridize mean curvature flow and signal processing approaches to mesh fairing. The technique is assessed by comparing the exact genus of high-genus meshes with the genus calculated via the Gauss–Bonnet formula, and works well to find the local curvature at all points of simulated surfaces of high topological genus. Specific geometric quantification of nanoporous metals is discussed for two quantities: (i) the relative surface area fraction of different crystal facets, which is important for catalysis; and (ii) the curvature distribution on the surface of porous metals, important for applications in which high curvature features are active (e.g. optical sensing).
Co-reporter:Joshua D. Snyder and Jonah D. Erlebacher
Langmuir 2009 Volume 25(Issue 16) pp:9596-9604
Publication Date(Web):July 22, 2009
DOI:10.1021/la9007729
The cyclic voltammetry characterizing underpotential deposition (UPD) of Ag onto Au(111) varies in the literature with respect to the characteristic UPD peaks in both position and number. Rooryck et al.(1) confirmed that the discrepancy in terms of peak position, specifically the initial UPD to which a third of a monolayer of deposition is attributed, is due to a variation in the quality of the surface. Clean, smooth Au(111) surfaces yield a peak position of 0.53 V vs Ag0/Ag+, while rough disordered surfaces yield a peak position of 0.61 V vs Ag0/Ag+. Repetitive potential cycling in the UPD region resulted in a gradual shift in peak position, with time as the deposited Ag alloyed with, and was stripped from the surface leaving vacancies. We provide a methodology for tracking the rate at which UPD Ag alloys with the Au(111) surface without the use of continuous potential cycling. A simple kinetic model is developed for the surface alloying of Ag on Au(111), from which we extract an activation barrier and attempt frequency for this process. Notably, we introduce a novel technique for the inexpensive parallel fabrication of Au(111) single crystals that allowed us to build statistics and ensured reproducibility of our data.
The growth of thin (1–10 nm) films of Pt on Au(1 1 1) was studied in order to understand and clarify differences in growth mode observed in ultra-high vacuum (UHV) studies and in electrochemical deposition studies. It was found that on flat Au(1 1 1), Pt grows in a layer-by-layer growth mode, but if the gold substrate is exposed to an acidic environment prior to Pt deposition, then the substrate becomes nanoscopically rough (islanded) and Pt growth follows a pseudo-Stranski–Krastanov (SK) growth mode in which an initially thin wetting layer becomes rougher with increasing film thickness. An analysis of curvature effects on epitaxial growth mode shows that thermodynamic curvature effects involving surface stress are negligible for the Pt/Au(1 1 1) system. Rather, the apparent SK growth is linked to kinetic effects associated with inhomogeneous in-plane elastic relaxation of Pt films on rough surfaces that drive Pt atoms from pits to the tops of islands in the early stages of growth. Implications for the control of epitaxial film roughness are discussed.
Co-reporter:Roswitha Zeis, Anant Mathur, Greg Fritz, Joe Lee, Jonah Erlebacher
Journal of Power Sources 2007 Volume 165(Issue 1) pp:65-72
Publication Date(Web):25 February 2007
DOI:10.1016/j.jpowsour.2006.12.007
Platinum-plated nanoporous gold leaf (Pt-NPGL) is made by coating a conformal, atomically thin skin of platinum over the high surface area pores of a thin membrane of nanoporous gold. Because Pt loading in Pt-NPGL can be controlled down to 0.01 mg cm−2 using only simple benchtop chemistry, the material holds promise as a low Pt loading, carbon-free electrocatalyst. Here, we report successful use of Pt-NPGL as a catalyst in proton exchange membrane (PEM) fuel cells. Stable and high performance Pt-NPGL/Nafion membrane electrode assemblies (MEAs) were made using a stamping technique. The performance of Pt-NPGL MEAs is comparable to conventional carbon-supported nanoparticles-based MEAs with much higher loading, generating an output power density of up to 4.5 kW g−1 Pt in our non-optimized test configuration. Correlations between the performance of Pt-NPGL MEAs, the electrochemically accessible surface area, and material microstructure are discussed. Our success in using Pt-NPGL as a fuel cell catalyst suggests that creating precious metals skins over nanoporous metal supports is a viable strategy for designing new catalysts for PEM fuel cells. This promising approach allows tailoring catalytic activity by engineering precious metal/substrate interactions, employs materials with dual functionality acting both as current collector and catalyst, and may avoid the sintering problems plaguing conventional nanoparticle-based catalysts.
Co-reporter:Yi Ding;Anant Mathur;Mingwei Chen Dr. Dr.
Angewandte Chemie 2005 Volume 117(Issue 26) pp:
Publication Date(Web):18 MAY 2005
DOI:10.1002/ange.200463106
Ein Netz aus Platinnanoröhren mit Durchmessern von etwa 15 nm und Wanddicken von 1 nm, die sich zu einer offenen, doppelt bikontinuierlichen Struktur verbinden (siehe hochaufgelösten Elektronenmikrograph), entsteht, wenn Platin auf ein nanoporöses Goldtemplat aufgebracht und dieses anschließend aufgelöst wird. Die Struktur ist bei 125 °C mindestens 24 Stunden stabil; bei 150 °C setzt Deformation ein und schließlich werden Nanopartikel gebildet.
Co-reporter:Yi Ding;Anant Mathur;Mingwei Chen Dr. Dr.
Angewandte Chemie International Edition 2005 Volume 44(Issue 26) pp:
Publication Date(Web):18 MAY 2005
DOI:10.1002/anie.200463106
A network of platinum nanotubes with diameters of about 15 nm and walls 1 nm thick that interconnect to form an open, doubly bicontinuous structure (see high-resolution electron micrograph) is formed by coating platinum on a nanoporous gold template then dissolving away the mold. The structure is stable at 125 °C for at least 24 hours, but starts to deform at 150 °C and eventually forms nanoparticles.
A free-standing nanoporous gold (NPG) membrane is made by dealloying commercially available white-gold leaf in nitric acid (see Figure). This porous material has an unusual combination of characteristics in that it is metallic with a continuous crystal lattice throughout the porous network, and has a pore size that is adjustable via simple room-temperature post-processing. This ultra-high-surface-area material is potentially very useful for applications such as electrocatalysis and sensing.
Co-reporter:Jonah Erlebacher,
Michael J. Aziz,
Alain Karma,
Nikolay Dimitrov
and
Karl Sieradzki
Nature 2001 410(6827) pp:450
Publication Date(Web):
DOI:10.1038/35068529
Dealloying is a common corrosion process during which an alloy is 'parted' by the selective dissolution of the most electrochemically active of its elements. This process results in the formation of a nanoporous sponge composed almost entirely of the more noble alloy constituents1. Although considerable attention has been devoted to the morphological aspects of the dealloying process, its underlying physical mechanism has remained unclear2. Here we propose a continuum model that is fully consistent with experiments and theoretical simulations of alloy dissolution, and demonstrate that nanoporosity in metals is due to an intrinsic dynamical pattern formation process. That is, pores form because the more noble atoms are chemically driven to aggregate into two-dimensional clusters by a phase separation process (spinodal decomposition) at the solid–electrolyte interface, and the surface area continuously increases owing to etching. Together, these processes evolve porosity with a characteristic length scale predicted by our continuum model. We expect that chemically tailored nanoporous gold made by dealloying Ag-Au should be suitable for sensor applications, particularly in a biomaterials context.
Co-reporter:Roswitha Zeis, Tang Lei, Karl Sieradzki, Joshua Snyder, Jonah Erlebacher
Journal of Catalysis (1 January 2008) Volume 253(Issue 1) pp:132-138
Publication Date(Web):1 January 2008
DOI:10.1016/j.jcat.2007.10.017
Nanoporous gold (NPG) made by dealloying silver/gold alloys is a mesoporous metal combining high surface area and high conductivity. Recently, NPG has been shown to exhibit some of the high catalytic activity previously associated only with supported gold nanoparticles. Here we describe how NPG acts as a catalyst for the oxygen reduction reaction in both gas phase (in fuel cells) and aqueous environments (using rotating disk electrochemistry). NPG was found to reduce oxygen via an effectively 4-four electron route comprised of a first reduction of oxygen to hydrogen peroxide, and then an unusually active further catalytic reduction of hydrogen peroxide to water.
Co-reporter:F. Kertis, S. Khurshid, O. Okman, J. W. Kysar, L. Govada, N. Chayen and J. Erlebacher
Journal of Materials Chemistry A 2012 - vol. 22(Issue 41) pp:NaN21934-21934
Publication Date(Web):2012/08/31
DOI:10.1039/C2JM34527G
We present a theory and experiments that help clarify the origin of the effectiveness of nanoporous substrates in the heterogeneous nucleation of protein crystals. The central idea tested here is that when a substrate (or “nucleant”) possesses pores of the order of the hydrodynamical radius of a protein, then the entropic penalty associated with nucleating a protein crystal on that surface may be alleviated. Model experiments using lysozyme and nanoporous gold (NPG) substrates suggest that there is indeed a reduction in the entropy associated with creating critical nuclei, but the magnitude of the reduction is small. Taken together with further examination of protein crystallization with NPG nucleants using four other proteins, our aggregate results suggest that surface chemistry and surface area effects play the dominant role in nucleation when using these nanoporous nucleants.
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