Electrochemical Potential Cycling Research Articles

Syntheses of Ni-Based Layered-Double Hydroxide Materials Via Liquid Phase Deposition Method for Alkaline Water Electrolysis

For the sustainable society, hydrogen production has received much attention during decades. The alkaline water electrolysis process has been recognized as one of the important processes for hydrogen products. For the achievements of an effective water electrolysis system, the decrease of the overpotential on the oxygen evolution reaction (OER), which is the four-electron and proton transfer process. These complex 4-electron sluggish processes lead to the large energy losses in the whole water electrolysis. Although the noble catalysts such as IrO2 or RuO2 exhibit high OER activity, their large-scale applications are limited due to cost problem. From this point of view, the low-cost transition metal materials, especially Ni-based catalysts, have been extensively studied. And also, it has been reported that the well-defined layered double hydroxide (LDH) structures can promote water oxidation through the interlayer anion effects. Despite this fact, the specific procedures for the preparation of Ni-based LDH structures have not been well established. Therefore, in this study, we have investigated the preparation of Ni-based LDH structures in the aqueous solution at room temperature, the so-called liquid phased deposition (LPD) method.Through the LPD method, we have synthesized various types of Ni-based LDH structures. As an example, the Ni-Al LDH structures were synthesized on the conductive substrate. It was found that our synthesized Ni-Al LDH exhibited relatively high OER activity and durability against electrochemical potential cycling [1]. In addition, the dependencies of the OER activity on the interlayer anion species were clearly demonstrated. In addition, the other types of Ni alloy LDHs were also prepared and their OER activity was investigated. Through the electrochemical measurements, we have confirmed the advantages of our materials prepared by LPD methods as an anode for alkaline water electrolysis.

Employing Electrochemical Potential Cycling to Controlling Surface to Volume Ratio of Nanoporous Gold and Photoelectron Injection into Electrolyte

Nanoporous metals made by alloy corrosion are bicountinuous porous networks that possess nanoscale ligaments and pores and thus a large surface-to-volume ratio. Their ligament and pore size can be tuned between a few nanometers and several microns via thermal annealing [1], which makes them attractive material systems for a variety of applications, including catalysis, sensing, and energy storage. In particular, dealloying-derived nanoporous metals, present novel opportunities for exploring the impact of surfaces on electro-photocatalytic activity of metallic nanostructures [2]. Yet, achieving the ligament and pore sizes in the dealloyed metallic networks below 10 nm, which is especially crucial for understanding an efficient electrochemical charge transfer, is challenging so far.In this study, we demonstrate a successful fabrication of nanoporous gold (npAu) thin films via electrochemical dealloying with the ligament size down below 10 nm on quartz substrate. We study the material by means of in situ photoelectrochemical setup while simultaneously varying the electrode potential in 0.5 M H2SO4 electrolyte. To probe the size effect on the photocurrent response, we exploit cyclic voltammetry (CV) in order to promote coarsening of the ligaments of npAu networks. By varying a number of CV cycles, we were able to induce subtle changes in the ligament size, therefore modifying the surface-to-volume ratio of the nanoporous metals at the lower limit of the nanoscale.Our results show that this approach provides an excellent foundation for future optical investigations of surface effects on the internal quantum efficiency of high-surface area metallic photoelectrodes.Our study demonstrates that modifying the surface-to-volume ratio of the sample by electrochemistry allows us to control the hot electron generation and injection processes in nanoscale plasmonic systems. Moreover, the internal quantum efficiency of the npAu network photoelectrode is significantly influenced by the size of its ligaments. In contrast to prior findings that suggested efficiency would level off with decreasing ligament size [3], we have observed a continuous nonlinear increase in quantum efficiency as the ligament size decreased from 16 to 8 nm. This result is attributed to the increase in absorption by the electrons colliding with the surface (Landau damping) and increase of emission of this electrons into the electrolyte due to possibility of multiple collisions with the surface.

Electrochemical cycling durability of platinum overlayers formed by self-terminating electrodeposition

Platinum thin films comprising nominally 1 to 20 atomic overlayers were formed on a gold-coated substrate using the self-terminating electrodeposition (STED) method. The platinum overlayer loading and morphology were characterized using X-ray Photoelectron Spectroscopy (XPS), Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES), Field Emission Scanning Electron Microscopy (FESEM), Cyclic Voltammetry (CV) and Chronoamperometry (CA). The durability of these platinum overlayer samples was estimated by subjecting the samples to ex-situ electrochemical potential cycling. The extent of platinum overlayer growth is lower than expected till four STED cycles and then increases non-linearly, leading to increased roughness. The shifts of platinum redox peaks in CVs and Pt4f signals in XPS spectra indicate surface-alloying of gold and platinum. After the durability test, the morphological evolution of platinum overlayers conforms with the extent of platinum dissolution estimated from CVs. Our results reveal that more than four STED cycles are needed to form platinum overlayers that meet the US Department of Energy (DOE) criteria for durability.

Use of Boron Doped Diamond Corrosion Free Supports to Evaluate Fuel Cell Electrocatalyst Stability Under Accelerated Stress Testing

Proton exchange membrane fuel cell (PEMFC) electrocatalysts are typically Pt or Pt alloy nanoparticles (NPs) supported on high surface area carbon black powders, the latter which contain a significant sp2 carbon component. One of the barriers to PEMFC commercialisation is the lifetime of the catalyst-support system, which should be at least 5,000 hours for automotive applications.[1] During the start-up and shut-down of the PEMFC, the cathode is subjected to significant oxidative potentials (greater than 1 V vs RHE) which can lead to corrosion of the predominantly sp2 bonded carbon support, a key factor which can limit the lifetime of PEMFCs. [2] High quality boron doped diamond (BDD) is an sp3 bonded network with above 1:1000 B:C ratio, which is required for metal-like electronic conductivity. BDD has many favourable properties for electrochemical experiments, most notably: higher electrochemical corrosion resistance with respect to sp2 carbons, low background currents, and a wide aqueous solvent window.[3] In this work we first explore the electrochemical corrosion stability of BDD when undergoing electrochemical cycling in acid solutions. To achieve this BDD substrates (BDD TEM grid) suitable for both electrochemical experiments and high magnification (single atom resolution) transmission electron microscopy (TEM) experiments were produced using precision ion polishing (Gatan PIPS-II). The surface was characterised using X-ray photoelectron spectroscopy to ensure the polishing process did not significantly alter the diamond substrate. Under aggressive electrochemical potential cycling in perchloric acid and sulfuric acid solutions, both TEM morphological analysis and Electron Energy Loss Spectroscopy (EELS) thickness measurements showed no evidence of BDD corrosion. [4] The BDD-TEM substrate was then used as a platform for investigating the degradation of a PEMFC electrocatalyst (Pt NPs) under accelerated stress testing (AST) [1] conditions on an atomic level, using the corrosion free carbon support. This enables the influence of other degradation pathways [2] (such as aggregation, Ostwald Ripening & direct dissolution) to be explored in more detail, free from issues associated with corrosion of the support. Pt NPs, in the size range 1 – 4 nm, were sputter coated onto the BDD TEM grid. Identical location ex-situ TEM (IL-TEM)[5] in combination with image analysis was used to probe NP changes e.g. size, shape, position, on an individual basis before, and after, AST (Fig. 1, which contains c.a. 200 NPs for analysis). Such measurements were complemented by Inductively Coupled Plasma – Optical Emission Spectroscopy analysis for any dissolved Pt, and electrochemical cyclic voltammetry measurements of the Pt to highlight changes in electrocatalytic behaviour (hydrogen evolution reaction and oxygen reduction reaction) due to AST cycling.Figure 1 – High magnification annular dark field (ADF) identical location images of Pt NPs, a) Pt/BDD before AST, b) Pt NPs after AST, c) Cyclic voltammogram of the 1000 cycle AST from 0.8 to 1.4 V vs Ag|AgCl (1.6 to 1.6 V vs RHE).

Understanding the Degradation Mechanisms of Pt Electrocatalysts in PEMFCs by Combining 2D and 3D Identical Location TEM.

The evolution of Pt nanoparticles in proton-exchanged membrane fuel cells is monitored before and after electrochemical potential cycling, using 2D and 3D identical location aberration-corrected transmission electron microscopy. This work demonstrates that 2D images might be a challenge to interpret due to the 3D nature of the carbon support. Thus, it is critical to combine both 2D and 3D observations to be able to fully understand the mechanisms associated with the durability of Pt catalyst nanoparticles. In particular, this investigation reveals that the mechanism of particle migration followed by coalescence is operative mainly across short distances (<0.5 nm). This work also shows that new Pt particles appear on the carbon support, as the result of Pt dissolution, followed by the formation of clusters, which grow by Ostwald ripening. This mechanism of Ostwald ripening is also responsible for changes in shape and particle growth, which later may result in coalescence.

An ultra-dispersive, nonprecious metal MOF\u2013FeZn catalyst with good oxygen reduction activity and favorable stability in acid

Development of the more stable nonprecious oxygen reduction reaction (ORR) catalyst is of great significance nowadays. Herein, a high-performance iron-doped integral uniform macrocyclic organic framework (MOF–FeZn) catalyst is synthesized through a combined hydrothermal and pyrolysis process, showing favorable ORR activity and stability in acid. This as-synthesized MOF–FeZn catalyst displays high porous and graphitic structures with sufficient catalytic active dopants of pyridinic N, Fe–N, pyrrolic N, graphitic N, making it a promising ORR candidate catalyst with high electrochemical stability. The onset potential, half-wave potential and limited diffusion current density of MOF–FeZn are 0.93 V @ 0.1 mA cm−2, 0.768 V@ 2.757 mA cm−2 and 5.5 mA cm−2, respectively, which are comparable to the state-of-the-art nonprecious catalyst and commercial Pt/C. ORR catalysis on MOF–FeZn follows the nearly four-electron path. What is more, MOF–FeZn can sustain the 10,000 cycles electrochemical potential cycling process in acid with the half-wave potential changed only 21 mV, superior to the reduction of 149 mV for Pt/C. The well-developed integral uniform structures, homogeneously dispersed carbides and nitrides protected by the highly graphitic carbon layers and the better agglomeration suppression of nanoparticles by the confined graphitic carbon layers on catalyst can significantly enhance the catalytic activity and stability of MOF–FeZn.

Editors’ Choice—Diffusion Media for Cation Contaminant Transport Suppression into Fuel Cell Electrodes

Polymer electrolyte membrane fuel cells provide an alternative option to fossil fuel-based energy conversion devices. However, the corrosion of fuel cell components, specifically the bipolar plates, introduces contaminants (e.g., Fe, Ni) into the membrane electrode assembly (MEA). These contaminants accelerate the ionomer degradation by acting as a Fenton’s reagent, decreasing the fuel cell’s durability. This study presents the mechanism and the diffusion media properties affecting the transport of cation contaminants into the MEA. Cation contaminant transport was studied after altering the gas diffusion layers (GDLs) wettability, emulating the GDL properties after prolonged operation, by ex situ hydrogen peroxide treatment or in situ electrochemical potential cycling. A GDL with crack-free microporous layer (MPL) showed a lower cation transport rate to the catalyst layer than MPL with cracks after both ex situ and in situ treatment. A novel GDL was developed from modification of the conventional GDL via the addition of a hydrophobic layer to the GDL substrate, which suppressed the contaminant cation transport significantly. This novel GDL also showed improved fuel cell performance.

Electrochemical Response of Clay/Polyelectrolyte Composite Barrier Coatings

Composite materials made of polymer and clay are effective at blocking mass transport. In this study, the blocking efficacy of layer-by-layer (LbL) coatings of exfoliated montmorillonite (MMT) and polyethylenimine (PEI) was studied using cyclic voltammetry and a redox couple, indigo carmine (IC). The pH of the MMT solution was varied from 4 to 10 to prepare LbL coatings of different surface roughness on metal substrates. It was found that the coated electrode had a lower redox peak current value than without the coating, demonstrating the reduction of the mass transport of IC to the metal surface. The peak values decreased with decreasing the coating’s roughness and increasing the number of layers, indicating that the blocking capability can be controlled by changing the deposition conditions. Smooth LbL coatings deposited with MMT at pH 4 showed the highest blocking efficacy up to 97.5%. The IC adsorbed at the interface between the coating and the metal substrate was found to cause the peak current measured for the coated electrode. It was also confirmed that the same coating on the copper substrate reduced the corrosion of the copper during the electrochemical potential cycling.

Electrochemical restructuring of Gold electrodes with redox active species to create electrocatalytically active nanostructured surfaces

Electrochemical potential cycling of gold electrodes in commonly used redox active species to produce a nanostructured surface is introduced. A commercially available gold electrode when cycled in three redox active species including hexaammine ruthenium chloride, ferrocene methanol and ferricyanide shows rapid conversion of the surface into a nanostructured form within 50 cycles which usually requires hundreds of cycles in their absence. The active gold nanostructured electrode was characterized by cyclic voltammetry in acidic and alkaline electrolyte indicating the presence of different basal sites depending on the redox species employed. In addition, the active gold nanostructured surface shows higher electrocatalytic activity than the pristine polycrystalline gold electrode for glucose and ascorbic acid oxidation in alkaline media.

Structurally Modulated Graphitic Carbon Nanofiber and Heteroatom (N,F) Engineering toward Metal-Free ORR Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells

The present study, designates the heteroatoms (N,F) doped various graphitic carbon nanofibers (GNF) viz. GNF-linear segmented platelets, antlers, herringbone type and their structural deformations from pristine fiber with many open-edge active centers as metal-free, cost-effective electrocatalysts for ORR in polymer electrolyte membrane fuel cells. Heteroatoms introduction to GNF frameworks enlarge the lattice spacing between graphene platelets and brings the structural modulation. The developed GNF/N-F catalysts show excellent ORR activity with insensitive to CH3OH and demonstrated outstanding electrochemical potential cycling stability of 10,000 cycles with well-retained ORR kinetics without much loss in the activity. X-ray photoelectron spectroscopy investigation of GNF/N-F catalysts explicit the highly active forms of N (pyridinic, pyrrolic, graphitic-N) and semi-ionic, ionic C-F of F in the catalysts. The deep-rooted synergistic effect among N and F atoms induces to create more active-centres entrenched with extensive C-C bond polarization, larger charge delocalisation with larger spin density differences accomplished in GNF/N-F catalysts. Wide open-edge cavities, opened-tips and many extensively accessible facets collectively enhance the ORR activity of GNF-H/N-F catalyst. The present study unlocks a deep insight towards understanding of advanced metal-free electrocatalysts for efficient ORR in PEMFCs and metal-air batteries.

Poly (3, 4-ethylene dioxythiophene) Supported Palladium Catalyst prepared by Galvanic Replacement Reaction for Methanol Tolerant Oxygen Reduction

Herein, we propose a facile electrochemical approach for the synthesis of Pd loaded poly 3, 4-ethylenedioxythiophene (PEDOT) electrodeposited on glassy carbon electrode (GCE) resulting in high surface area. The catalyst preparation is initiated with EDOT polymerization on GCE surface by electrochemical potential cycling method, followed by the electrodeposition of Cu from a 2 mM solution of CuSO4 in 0.1 M NaClO4 at a constant potential of +0.34 V vs. SHE in the form of Cu nanocubes on the PEDOT surface. Pd-PEDOT catalyst was then prepared by the partial substitution of copper by galvanic displacement with various concentrations of PdCl2. The prepared Pd/PEDOT electrocatalyst is found to be methanol resistant indicating its usefulness as fuel cell cathode. The prepared catalyst supports two electron transfer of oxygen reduction reaction in 0.5 M H2SO4. The effects of Pd and Cu contents and the quantity of PEDOT, mass and specific activities were studied. At a relatively low Pd loading of 0.57 ng/cm2, the Pd/PEDOT should be a cost-effective alternative cathode catalyst for direct methanol fuel cells, DMFCs. This work explains the usefulness of PEDOT as good catalyst supporting material which is prepared by an eco-friendly electrochemical route.

Oxygen Reduction on Methanol-Tolerant Carbon-Supported PtxSy Nanoparticles Prepared by a Single-Step Low-Temperature Method

In direct methanol fuel cells (DMFCs), the methanol crossover from the anode to the cathode is a major cause of power density loss because of the overpotential arising due to the parasitic reaction of methanol oxidation at the cathode. Catalysts modified with S have shown better methanol tolerance; however, the preparation routes often require high temperatures or pressures and very long times, making these expensive and unlikely to be used in large scale. Here, we report on a single-step low-temperature method used to prepare a carbon-supported PtxSy catalyst. Moreover, we show that the catalyst shows lower depolarization in the presence of methanol and study the effect of reductive thermal treatment and electrochemical potential cycling.

Evolution of the degradation mechanisms with the number of stress cycles during an accelerated stress test of carbon supported platinum nanoparticles

Development of high performance electrodes for polymer electrolyte membrane fuel cells (PEMFCs) requires fast screening of the catalysts for their catalytic activity and durability by subjecting them to a suitable accelerated stress test (AST). Electrochemical potential cycling in acidic media is one of such frequently used ASTs of platinum nanoparticle-based catalysts. The activity degradation of such catalysts during AST takes place through different mechanisms including particle size growth, dissolution and detachment of catalyst particles, electronic connectivity loss, etc. Here, we present a quantification of these degradation mechanisms with the number of potential cycles (N) during the AST of carbon supported platinum catalyst. Among various mechanisms, the activity degradation due to particle size growth attributed to the Ostwald ripening is found to be the most prominent during the initial part of the AST (N<500). On the other hand, the loss due to dissolution/detachment varies linearly during the AST cycling. Such understanding of the evolution of degradation mechanisms with N may be utilized effectively towards development of high performance PEMFC electrocatalysts.

Electrochemical Degradation of Pt–Ni Nanocatalysts: An Identical Location Aberration-Corrected Scanning Transmission Electron Microscopy Study

The evolution of Pt-Ni nanoparticles supported on amorphous carbon is investigated before and after electrochemical potential cycling (0.6-1.1 V), using aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS). During voltage cycling and due to the dissolution of nanoparticles, single ions/atoms and ionic/atomic clusters emerge and diffuse across the carbon support toward larger nanoparticles, where they redeposit. We observe that the preferred locations for the dissolution are the steps and corners of the nanoparticles. On the other hand, the redeposition process happens often on {111} type planes. In addition, contrary to the conventional view, where larger particles grow isotropically from smaller ones, this research work shows that anisotropic growth of smaller particles occurs during potential cycling. The reason for this behavior seems to be related to the fact that smaller particles with thicker Pt-rich shells trigger the nucleation and deposition of Pt.

Surface alloying of Pt monolayer on nanoporous gold for enhanced oxygen reduction

The introduction of stabilizing elements, such as Au, has been recognized as an effective approach to increasing the durability of Pt electrocatalysts toward oxygen reduction reaction (ORR). However, significant tensile strain involved in this system leads to an undesirable decrease of its intrinsic ORR activity as compared to the native Pt, due to a ∼3.8% tensile strain of Pt layer on Au core. In this work, we demonstrate that for a core-shell structured Pt monolayer on nanoporous gold (NPG), electrochemical potential-cycling can induce PtAu surface alloy formation, which results in unexpected improvement of its ORR activity. Theoretical calculations suggest that PtAu surface alloys possess more suitable configurations that give relatively low changes of free energy for the corresponding overpotential-determining step during ORR relative to Pt monolayer on Au. The improved ORR performance and high utilization of Pt make the nanoporous metal-based electrodes promising for green energy technologies.

Tuning Surface Structure of 3D Nanoporous Gold by Surfactant-Free Electrochemical Potential Cycling.

3D dealloyed nanoporous metals have emerged as a new class of catalysts for various chemical and electrochemical reactions. Similar to other heterogeneous catalysts, the surface atomic structure of the nanoporous metal catalysts plays a crucial role in catalytic activity and selectivity. Through surfactant-assisted bottom-up synthesis, the surface-structure modification has been successfully realized in low-dimensional particulate catalysts. However, the surface modification by top-down dealloying has not been well explored for nanoporous metal catalysts. Here, a surfactant-free approach to tailor the surface structure of nanoporous gold by surface relaxation via electrochemical redox cycling is reported. By controlling the scan rates, nanoporous gold with abundant {111} facets or {100} facets can be designed and fabricated with dramatically improved electrocatalysis toward the ethanol oxidation reaction.

Electrocatalytic oxidation and flow injection analysis of isoniazid drug using a gold nanoparticles decorated carbon nanofibers-chitosan modified carbon screen printed electrode in neutral pH

Isoniazid (INZ) is an effective anti-tuberculosis drug that has been widely used for chemotherapy of tuberculosis. Development of new and simple electroanalytical method for routine analysis of INZ is a continued research interest in the field of analytical and pharmaceutical chemistry. Although there have been several electroanalytical reports, flow injection analysis coupled electrochemical detection of INZ, which is an advanced electrochemical technique useful for practical applications, is rarely reported in the literature. Instability problem associated with the underlying electrode by the amino-functional group of the INZ is the prime reason for the limitation. Herein, we report a gold nanoparticles decorated carbon nanofibers-chitosan modified carbon screen printed electrode, designated as Aunano@CNF-CHIT/SPE, prepared by drop-casting of Au3+ ion directly on CNF-CHIT/SPE underlying electrode followed by electrochemical potential cycling, as an elegant electrochemical detector system for high stable FIA of INZ in pH7 phosphate buffer solution. The modified electrode showed about fifty times higher electrocatalytic current and 700mV reduction in the over-potential values than that of a polycrystalline gold modified electrode towards INZ. Under an optimal hydrodynamic FIA condition, the present ECD showed a wide linear range from 1μM to 1mM with regression coefficient and sensitivity values of 0.9958 and 16.1nAμM−1 respectively. The calculated limit of detection and limit of quantification values are 172nM (i.e., 472pg/20μL) and 570nM (i.e., 1.57μg/20μL), respectively. The present FIA-ECD was successfully applied for the real sample analysis by detecting INZ in two pharmaceutical formulations with satisfactory good recovery values.

Unraveling Fundamental Descriptors for the Structural and Morphological Degradation of Pt Nanoparticles on Indium Tin Oxide Using in Situ Techniques

Enhancing stability of cathode catalysts for PEMFC applications under operating conditions is of great importance because state-of-art supports for nanoparticular Pt electrocatalysts suffer from carbon corrosion especially during start-stop procedure. Metal oxides have been proven to be viable alternatives because of their corrosion resistance and observed strong metal support interaction which is thought to enhance activity as well as stability of the catalyst [1-3]. Tin-based oxides excel by their thermal stability as well as high conductivity and are therefore promising candidates as alternative fuel cell support materials [4]. But the structural evolution of Pt-based nanoparticles under electrochemical conditions on oxides is still unknown. On high surface area carbon it has been shown by using in-situ SAXS studies that Pt nanoparticles grow due to Ostwald ripening and a dissolution/redeposition mechanism decreasing the electrochemical active surface area (ECSA) [5]. Here, we report on metal oxide supported Pt nanoparticles focussing on the structural evolution under electrochemical potential cycling applied to mimic fuel cell operating conditions. The Pt nanoparticles were synthesized using a solvothermal route and deposited on Indium Tin Oxide (ITO) resulting in homogenously distributed particles with well defined size. The electrocatalysts were extensively tested with respect to long-term electrochemical stability in order to investigate its potential as alternative carbon-free support material. We determined the evolution of crystal structure, composition, crystallite size and particle size distribution of the oxide support and the Pt nanoparticles by using in situ XRD and ASAXS. Additionally, we were able to follow the dissolution of the catalyst components during the electrochemical protocol using SFC ICP-MS setup. We can show that in case of ITO the size of Pt nanoparticles increases only slightly, whereas the mass-based catalytic activity decreases primarily due to changes of the support. Furthermore, our results show that we are able to unravel fundamental aspects of morphological degradation of Pt nanoparticles on non-carbon supports under electrochemical conditions. These results will help to understand stability determining properties of oxide supports leading to enhanced cathode electrocatalysts for a PEMFC. [1] A. Kumar, V. Ramani, J. Electrochem. Soc. 2013, 160, F1207-F15. [2] A. Lewera, L. Timperman, A. Roguska, N. Alonso-Vante, J. Phys. Chem. C 2011, 115, 20153-59. [3] T.T.H. Van, C.J. Pan, J. Rick, W.N. Su, B.J. Hwang, J. Am. Chem. Soc. 2011, 133, 11716-24. [4] H.-S. Oh, H.N. Nong, P. Strasser, Adv. Funct. Mater. 2015, 25, 1074-81. [5] X. Tuaev, S. Rudi, V. Petkov, A. Hoell, P. Strasser, Acs Nano 2013, 7, 5666-74.

Electrochemically shape-controlled transformation of magnetron sputtered platinum films into platinum nanostructures enclosed by high-index facets

A new method based on transformation of magnetron sputtered platinum thin films into platinum nanostructures enclosed by high-index facets, using electrochemical potential cycling in a twin working electrode system is reported. The controllable formation of various Pt nanostructures, described in this paper, indicates that this method can be used to control a selective growth of high purity Pt nanostructures with specific shapes (facets or edges). The method opens up new possibilities for electrochemical preparation of nanostructured Pt catalysts at high yield.

Performance and Durability of Pt-Ti Catalyst for Oxygen Electroreduction

Fuel cells are expected to be a key next-generation energy source used for vehicles and homes, offering high energy conversion efficiency and minimal pollutant emissions. The high efficiency arises from the fact that fuel cells convert chemical energy directly into electrical energy without the Carnot limitation of thermal engines. However, due to the extreme operating environment, Pt was almost exclusively the only practical catalyst for molecular oxygen electroreduction in PEM fuel cells. Recently, considerable efforts have been made to investigate synergy effects of platinum alloyed with base metals to improve the sluggish kinetics of oxygen reduction reaction of Pt catalyst. The more active Pt-alloy catalysts, in return, may arise the durability issue under the extreme environment of PEM fuel cell operation. In this presentation, results of a more active yet durable PtTi alloy catalyst will be discussed for oxygen reduction reaction in acidic media, including (i) combinatorial high-throughput discovery of PtTi alloy composition; (ii) synthesis and characterization of nano-scale PtTi particles with controllable size, phase, and individual particle composition; and (iii) electrochemical STM and ICP characterization of coarsening and corrosion of PtTi alloy catalyst in comparison with Pt and PtCo catalysts. A novel combinatorial high-throughput electrocatalyst discovery workflow and associated tools were developed [1-4], where thin films of alloys can be made through a Multi-sources Physical Vapor Deposition system [5], and the resulted materials can be electrochemically screened by a Multi-channel Rotating Disk Electrode system [6]. PtTi binary system has been screened using this methodology for activity-stability-composition relationship and the best alloy composition was identified. In addition to the discovery of alloy compositions, engineering the PtTi alloy particles in nanoscale has been a challenge. Several synthesis technologies were studied and developed to achieve capabilities of controlling particle size and particle microcomposition [7]. The results show that by careful engineering the particle size and microcomposition in nanoscale, it is able to achieve the superior electrocatalytic activity predicted by the combinatorial high-throughput discovery. Electrochemical STM technique, in combination with electrochemical potentiostating, potential cycling, and ICP analysis, was used to monitor the coarsening and base metal corrosion under dynamic fuel cell operation conditions and compared with phenomena observed from Pt and PtCo catalysts [8-10]. The results show that PtTi not only is more active than Pt and much stable than PtCo, but also can mitigate the catalyst coarsening. Reference [1] T. He and E. Kreidler, Phys. Chem. Chem. Phys. 10, 3731 (2008). “Combinatorial screening of PtTiMe ternary alloys for oxygen electroreduction”. [2] E. Kreidler and T. He, in Catalysts for Oxygen Electroreduction – Recent Developments and New Directions (Ed. T. He, Transworld Research Network) Chapter 4, p.63 (2009). “Oxygen reduction on Pt alloys: high throughput combinatorial screening of Pt binary alloys”. [3] T. He, E. Kreidler, L. Xiong and E. Ding, J. Power Sources 165, 87 (2007). “Combinatorial screening and nano-synthesis of platinum binary alloys for oxygen electroreduction”. [4] T. He, E. Kreidler, L. Xiong, J. Luo and C.J. Zhong, J. Electrochem. Soc. 153, A1637 (2006). “Alloy electrocatalysts: combinatorial discovery and nano-synthesis”. [5] T. He, E.R. Kreidler and T. Nomura, US 8,944,002 (2015). “High throughput physical vapor deposition system for material combinatorial studies”. [6] T. He, US 7,077,946 (2006). “High throughput multi-channel rotating disk or ring-disk electrode assembly and method”. [7] E. Ding, K.L. More and T. He, J. Power Sources 175, 794 (2008). "Preparation and characterization of carbon-supported PtTi alloy electrocatalysts". [8] L. Tang, B. Han, K. Persson, C. Friesen, T. He, K. Sieradzki and G. Ceder, J. Am. Chem. Soc. 132, 596 (2010). “Electrochemical stability of nanometer-scale Pt particles in acidic environments”. [9] Q. Xu, E. Kreidler, D.O. Wipf and T. He, J. Electrochem. Soc. 155, B228 (2008). "An in-situ electrochemical STM study of potential-induced coarsening and corrosion of platinum nano crystals". [10] Q. Xu, T. He and D. Wipf, Langmuir 32, 9098 (2007). "An in-situ electrochemical STM study of the coarsening of platinum islands at double-layer potentials".