Electrochemical Behavior and Shape Evolution of Structured Pd Nanoparticles in Alkaline Media─Influence of Electrochemically Absorbed Hydrogen.

We report on the electrochemical behavior and shape evolution of Pd nanocubes (Pd NCs) and Pd nanooctahedrons (Pd NOs) with an average size of 9.8 and 6.9 nm, respectively, in aqueous alkaline medium in the potential range of the underpotential deposition of H (UPD H) and H absorption. While the Pd NCs and Pd NOs remain stable in the potential region of the UPD H, H absorption and desorption of absorbed H (Habs) induce structural changes to the Pd NPs, as indicated by the results of electrochemical measurements and identical location-transmission electron microscopy (IL-TEM) analyses. Because both Pd NCs and Pd NOs are known to be stable in the potential region of H absorption and Habs desorption in acidic medium and maintain their structure, the irreversible structural changes are attributed to their interfacial interaction with the aqueous alkaline medium. In the alkaline medium, the nanoparticle surface/electrolyte interfacial structure plays an essential role in the mechanism of Habs desorption that is observed at higher potentials than that in the acidic medium. Hydrogen desorption is substantially hindered due to the structure of the water network adjacent to the Pd nanoparticles or the interaction between hydrated cations and adsorbed OH on the nanoparticle surface, resulting in the trapping of a small amount of H (incomplete Habs desorption). It is proposed that H trapping and associated structural strain lead to the deformation of the Pd nanoparticles and the loss of their initial structure.

Tunable Method for the Preparation of Layered Double Hydroxide Nanoparticles and Mesoporous Mixed Metal Oxide Electrocatalysts for the Oxygen Evolution Reaction.

Preparation of high-performance and durable electrocatalysts for anion exchange membrane water electrolysis is a crucial step toward the broad implementation of this technology. Here, we present an easily tunable, one-step hydrothermal method for the preparation of Ni-based (NiX, X = Co, Fe) layered double hydroxide nanoparticles (LDHNPs) for the oxygen evolution reaction (OER), using tris(hydroxymethyl)aminomethane (Tris-NH2) for particle growth control. The LDHNPs are used as building blocks of mesoporous mixed metal oxides (MMOs) with a block copolymer template (Pluronic F127), followed by thermal treatment at 250 °C. NiX MMOs have a significantly larger surface area compared to the analogous LDHNPs. NiX LDHNPs and MMOs exhibit excellent performance and long-term cycling stability, making them promising OER catalysts. Moreover, this versatile method can be easily tailored and scaled up for the preparation of platinum group metal-free electrocatalysts for other reactions of interest, which highlights the relevance of this work to the field of electrocatalysis.

Controlled-Atmosphere Flame Fusion Single-Crystal Growth of Non-Noble fcc, hcp, and bcc Metals Using Copper, Cobalt, and Iron.

The growth of noble-metal single crystals via the flame fusion method was developed in the 1980s. Since then, there have been no major advancements to the technique until the recent development of the controlled-atmosphere flame fusion (CAFF) method to grow non-noble Ni single crystals. Herein, we demonstrate the generality of this method with the first preparation of fcc Cu as well as the first hcp and bcc single crystals of Co and Fe, respectively. The high quality of the single crystals was verified using scanning electron microscopy and Laue X-ray backscattering. Based on Wulff constructions, the equilibrium shapes of the single-crystal particles were studied, confirming the symmetry of the fcc, hcp, and bcc single-crystal lattices. The low cost of the CAFF method makes all kinds of high-quality non-noble single crystals independent of their lattice accessible for use in electrocatalysis, electrochemistry, surface science, and materials science.

Enhancement of the Capabilities of Inductively Coupled Plasma Mass Spectrometry Using Monosegmented Flow Analysis.

Flow injection (FI) in combination with inductively coupled plasma mass spectrometry (ICPMS) is advantageous for the analysis of volume-limited samples and is invaluable for the analysis of corrosive samples that would prematurely degrade ICPMS components. However, the dispersion process with 50-µL injections in FI degrades ICPMS sensitivity. Monosegmented flow analysis (MSFA), where the sample plug is in the middle of 1 mL of air, eliminates dispersion while preserving the rinsing effect of the carrier. More reproducible as well as sharper, narrower, and more symmetrical peaks result with MSFA than FI, leading to a 2-fold improvement in detection limit and a 5-fold increase in sample throughput versus FI. Furthermore, by facilitating the formation of small droplets during nebulization, the air surrounding the sample even enhances sensitivity by 20-40%, depending on the element, compared to that obtained with direct sample aspiration. Coupling MSFA to ICPMS, which does not degrade analytical performance, is advantageous for the determination of Pt in 0.50 M H2SO4 electrolyte from a simulated fuel cell. It also enables the multielement analysis of a 150-µL buffer sample containing as little as 60 µg of plant protein, thus further extending the range of applications of ICPMS.

Influence of the Surface Roughness of Platinum Electrodes on the Calibration of the Electrochemical Quartz-Crystal Nanobalance.

The electrochemical quartz-crystal nanobalance (EQCN) is an in situ technique that measures mass changes (Δm) associated with interfacial phenomena. Analysis of Δm sheds light on the mass balance (in addition to the charge and energy balances) and provides new insight into the nature of electrochemical processes. The EQCN measures changes in frequency (Δf) of a quartz-crystal resonator, which are converted into Δm using the Sauerbrey equation containing the characteristic constant (Cf). The value of Cf is determined by physical parameters of the crystal and refers to an atomically smooth surface. However, real resonators are not smooth and electrodes have their intrinsic roughness. Thus, the conversion of Δf to Δm should be done using an experimentally determined characteristic constant (Cf,exp) for a given value of the surface roughness factor (R). Here, we calibrate the system using Ag electrodeposition on Pt electrodes of gradually increasing R; the latter is adjusted through Pt electrodeposition. The surface morphology of the Pt substrates prior to and after Ag electrodeposition is examined using atomic force microscopy. The values of Cf,exp are determined by analyzing the slopes of charge density versus Δf plots for the Ag electrodeposition. They are different than Cf and increase logarithmically with R. The Cf and Cf,exp values are used in a comparative analysis of the mass changes (δΔm) for complete cyclic voltammetry profiles covering the 0.05-1.40 V range. This reveals that the employment of Cf instead of Cf,exp provides inaccurate values of δΔm, and the magnitude of the discrepancy increases with R.

Interfacial structure of atomically flat polycrystalline Pt electrodes and modified Sauerbrey equation.

The electrochemical quartz-crystal nanobalance (EQCN) measures in situ mass changes associated with interfacial electrode processes. Real electrodes are not atomically flat, thus their surface roughness affects the conversion of frequency variations (Δf) to mass changes (Δm) associated with electrochemical processes. Here, we analyze Δm associated with the electrochemical H adsorption/desorption and surface oxide formation/reduction on Pt electrodes of gradually increasing surface roughness using the EQCN and cyclic-voltammetry in an aqueous H2SO4 solution. These two interfacial processes are ideal to probe changes in the electrochemically active surface area. The surface roughness of Pt-coated resonators is fine-tuned through Pt electrodeposition and examined using atomic force microscopy. The results acquired using Pt electrodes of increasing roughness factor (1.61 ≤ R ≤ 13.0) reveal a linear relationship between Δm and R. Extrapolation of this relationship to R = 1.00 leads to the determination of Δm associated with H adsorption/desorption and oxide formation/reduction on an atomically flat polycrystalline Pt electrode. The values of Δm associated with these processes are analyzed in terms of the number of H, O, water, and ionic species interacting with each Pt atom of the electrode surface. We find that the charge densities associated with these electrochemical processes and mass variations do not scale up by the same factor. This leads to a modified version of the Sauerbrey equation for Pt electrodes, which takes into account the intrinsic surface roughness.

Modelling oxide formation and growth on platinum.

We present a mathematical model of oxide formation and growth on platinum. The motivation stems from the necessity to understand platinum dissolution in the cathode catalyst layer of polymer electrolyte fuel cells. As is known, platinum oxide formation and reduction are strongly linked to platinum dissolution processes. However, a consistent model of the oxidation processes on platinum does not exist. Our oxide growth model links interfacial exchange processes between platinum and oxygen ions with the transport of oxygen ion vacancies via diffusion and migration. A parametric analysis is performed to rationalize vital trends in oxide growth kinetics. The rate determining step of oxide formation and growth is identified as the extraction of platinum atoms at the metal-oxide interface. A kinetic effect is observed while adjusting the potential when growing the oxide layer, and the solution indicates that a structural change occurs at high potentials, around 1.5 VRHE. The model compares well to experimental data for various materials from various sources.

Identification and Analysis of Electrochemical Instrumentation Limitations through the Study of Platinum Surface Oxide Formation and Reduction.

Anodic polarization of Pt electrodes in aqueous H2SO4 leads to the formation of a surface oxide (PtO). Herein, the surface oxide growth is accomplished using three different approaches: (i) chronoamperometry (CA); (ii) chronocoulometry (CC); and (iii) a combination of cyclic voltammetry (CV) and CA. The PtO reduction is accomplished potentiodynamically using voltammetry. The oxide growth takes place at defined polarization potentials (E(p)), polarization times (t(p)), and temperatures (T). The oxide charge density (q(ox)) is determined for both the formation (q(ox,form)) and reduction (q(ox,red)) processes. The oxide reduction CV profiles are integrated to determine the charge density values for oxide reduction (q(ox,red,CV)) which are compared with the q(ox,form,CA) and q(ox,form,CC) values. The values of q(ox,form,CC) are greater than those of q(ox,form,CA), but both potentiotatic methods (CA and CC) produce q(ox,form) values that are consistently lower than those of q(ox,red,CV). In the case of oxide formation with combined CV and CA, the values of q(ox,form,CV+CA) are found to be lower than the values of q(ox,red,CV), although the difference is small. Electrochemical quartz crystal nanobalance (EQCN) is used to monitor the mass variation at the electrode surface during the oxide formation and reduction process at E(p) = 1.20 V with various t(p) values. Equal mass changes during oxide formation and reduction are detected by the EQCN. The nature of the differences in q(ox,form) and q(ox,red) encountered with the different experimental methods are discussed in terms of instrumental limitations.

Flow Injection Single Particle Inductively Coupled Plasma Mass Spectrometry: An Original Simple Approach for the Characterization of Metal-Based Nanoparticles.

In recent years, single-particle inductively coupled plasma mass spectrometry (spICPMS) has emerged as a reliable tool that can both count metal-containing nanoparticles and measure their mass, thereby allowing sizing if their shape, density, and composition are known. However, the methodology associated with the current spICPMS approach for mass determination requires determination of both the sample uptake rate and the sample introduction efficiency of the nebulization system. In this paper, the proof of concept of a novel approach based on flow injection (FI) analysis coupled to ICPMS, i.e., FI-spICPMS, is presented. Unlike the established technique, this method does not require a determination of the transport efficiency and of the sample uptake rate for the accurate measurement of particle mass. It also only requires a measurement of the transport efficiency for determination of the particle number. Unlike the traditional spICPMS approach, the measurement of transport efficiency by FI-spICPMS is not affected by changes in sample uptake rate. The efficiency of FI-spICPMS is demonstrated through accurate determination of the particle number and size of 60 nm citrate-coated gold nanoparticles suspended in high-purity water. Despite being simpler, the method provides similar results to those obtained by the established spICPMS method. With a 5 ms dwell time and 200 µs settling time, the size detection limit is 20 nm, i.e., the same as with spICPMS.

Limits of Detection and Quantification of Electrochemical Quartz-Crystal Nanobalance in Platinum Electrochemistry and Electrocatalysis Research.

The electrochemical quartz-crystal nanobalance has been used in electrochemistry research for over three decades. It provides an atomic/molecular level insight into the nature of interfacial electrochemical phenomena by measuring in situ mass changes on the nanogram scale. The sensitivity of this technique remains unknown because there have been no attempts to determine its limits of detection (LOD) or quantification (LOQ). We propose an experimental approach for determining the values of LOD and LOQ for Pt electrodes in aqueous H2SO4 solutions that employs cyclic voltammetry and frequency variation measurements. However, this methodology is also appropriate to other electrode materials and electrolytes. The LOD and LOQ values depend on the electrolyte concentration and decrease (i.e., the sensitivity increases) as the concentration decreases. Knowledge of the LOD and LOQ values determines the applicability of this technique in research on the oxidation and degradation of Pt catalysts employed in fuel cells.

Electrochemical behavior of unsupported shaped palladium nanoparticles.

The potential range in which hydrogen electro-adsorption, electro-absorption, and evolution reaction occur is examined in an acidic medium using cyclic-voltammetry (CV) and Pd nanoparticles with controlled size and shape distributions. The three processes give rise to unique features in CV profiles and are observed in distinct potential ranges. This behavior is not observed for bulk Pd materials and arises due to the nanoscopic nature of the Pd materials.

Surface oxide growth on platinum electrode in aqueous trifluoromethanesulfonic acid.

Platinum in the form of nanoparticles is the key and most expensive component of polymer electrolyte membrane fuel cells, while trifluoromethanesulfonic acid (CF3SO3H) is the smallest fluorinated sulfonic acid. Nafion, which acts as both electrolyte and separator in fuel cells, contains -CF2SO3H groups. Consequently, research on the electrochemical behaviour of Pt in aqueous CF3SO3H solutions creates important background knowledge that can benefit fuel cell development. In this contribution, Pt electro-oxidation is studied in 0.1 M aqueous CF3SO3H as a function of the polarization potential (E(p), 1.10 ≤ E(p) ≤ 1.50 V), polarization time (t(p), 10(0) ≤ t(p) ≤ 10(4) s), and temperature (T, 278 ≤ T ≤ 333 K). The critical thicknesses (X1), which determines the applicability of oxide growth theories, is determined and related to the oxide thickness (d(ox)). Because X1 > d(ox) for the entire range of E(p), t(p), and T values, the formation of Pt surface oxide follows the interfacial place-exchange or the metal cation escape mechanism. The mechanism of Pt electro-oxidation is revised and expanded by taking into account possible interactions of cations, anions, and water molecules with Pt. A modified kinetic equation for the interfacial place exchange is proposed. The application of the interfacial place-exchange and metal cation escape mechanisms leads to an estimation of the Pt(δ+)-O(δ-) surface dipole (µ(PtO)), and the potential drop (V(ox)) and electric field (E(ox)) within the oxide. The Pt-anion interactions affect the oxidation kinetics by indirectly influencing the electric field within the double layer and the surface oxide.

Roughening and long-range nanopatterning of Au(111) through potential cycling in aqueous acidic media.

Electrochemical treatment of Au(111) in aqueous H2SO4 solution by repetitive application of oxide formation-reduction cycles (OFRC) generates nanopatterned surfaces with long-range order. The pattern development depends on the lower and upper potential limits (EL, EU), the number (n) of OFRCs, and the potential scan rate (s). Surface patterning of Au(111) initially (n = 1-2) generates small islands and holes that are one atomic step in height. As n increases to 5, the number of islands decreases and the holes become larger; after n = 10 OFRCs, the islands become inexistent and large, randomly distributed holes are observed. Increase of OFRCs to n = 20 generates surface structures that reside within three atomic layers and resemble phase separation through a spinodal decomposition mechanism. As the number of OFRCs rises to n = 50, a network of interconnected islands and holes emerges; the islands and holes are two-three atomic steps in height, and are located within topmost five monolayers. Further increase of the number of OFRCs to n = 100 creates a network of interconnected trigonal pyramids that are pointed in the same direction. The size of the pyramids depends on the electrolyte composition and the number of OFRCs. In the case of n = 100, the pyramids are 12-25 nm in base length and 0.4-1.6 nm in height in 0.1 M aqueous H2SO4, and 20-50 nm in base length and 0.8-1.6 nm in height in 0.1 M aqueous HNO3. The number of OFRCs and scan rate play an important role in patterning of Au(111), and complete nanopattern development requires a large number of OFRCs and low scan rates.

In Situ Sonoactivation of Polycrystalline Ni for the Hydrogen Evolution Reaction in Alkaline Media.

In this investigation, we report on the development of a method for activating polycrystalline metallic nickel (Ni(poly)) surfaces toward the hydrogen evolution reaction (HER) in N2-saturated 1.0 M KOH aqueous electrolyte through continuous and pulsed ultrasonication (24 kHz, 44 ± 1.40 W, 60% acoustic amplitude, ultrasonic horn). It is found that ultrasonically activated Ni shows an improved HER activity with a much lower overpotential of -275 mV vs RHE at -10.0 mA cm-2 when compared to nonultrasonically activated Ni. It was observed that the ultrasonic pretreatment is a time-dependent process that gradually changes the oxidation state of Ni and longer ultrasonication times result in higher HER activity as compared to untreated Ni. This study highlights a straightforward strategy for activating nickel-based materials by ultrasonic treatment for the electrochemical water splitting reaction.

Evidence of an Eley-Rideal mechanism in the stripping of a saturation layer of chemisorbed CO on platinum nanoparticles.

The oxidative stripping of a saturation layer of CO(chem) was studied on platinum nanoparticles of high shape selectivity and narrow size distribution. Nanospheres, nanocubes, and nano-octahedrons were synthesized using the water-in-oil microemulsion or polyacrylate methods. The three shapes allowed examination of the CO(chem) stripping in relation to the geometry of the nanoparticles and presence of specific nanoscopic surface domains. Electrochemical quartz crystal nanobalance (EQCN) measurements provided evidence for the existence of more than one mechanism in the CO(chem) stripping. This was corroborated by chronoamperometry transient for a CO(chem) saturation layer at stripping potentials of E(strip) = 0.40, 0.50, 0.60, and 0.70 V. The first mechanism is operational in the case of CO(chem) stripping at lower E(strip) values; it proceeds without adsorption of anions or H(2)O molecules and corresponds to desorption of a fraction of CO(chem) in the form of a prepeak in voltammograms or in the form of an exponential decay in chrono-amperometry (CA) transients. The second mechanism is operational in the desorption of the remaining CO(chem) at higher E(strip) values and gives rise to at least two voltammetric peaks or two CA peaks. Analysis of the experimental data and modeling of the CA transients lead to the conclusion that the stripping of a saturation layer of CO(chem) first follows an Eley-Rideal mechanism in the early stage of the process and then a Langmuir-Hinshelwood mechanism.

Electro-oxidation of CO(chem) on Pt nanosurfaces: solution of the peak multiplicity puzzle.

An understanding of the oxidation of chemisorbed CO (CO(chem)) on Pt nanoparticle surfaces is of major importance to fuel cell technology. Here, we report on the relation between Pt nanoparticle surface structure and CO(chem) oxidative stripping behavior. Oxidative stripping voltammograms are obtained for CO(chem) preadsorbed on cubic, octahedral, and cuboctahedral Pt nanoparticles that possess preferentially oriented and atomically flat domains. They are compared to those obtained for etched and thermally treated Pt(poly) electrodes that possess atomically flat, ordered surface domains separated by grain boundaries as well as those obtained for spherical Pt nanoparticles. A detailed analysis of the results reveals for the first time the presence of up to four voltammetric features in CO(chem) oxidative stripping transients, a prepeak and three peaks, that are assigned to the presence of surface domains that are either preferentially oriented or disordered. The interpretation reported in this article allows one to explain all features within the voltammograms for CO(chem) oxidative stripping unambiguously.

Discovery of the potential of minimum mass for platinum electrodes.

Electrochemical quartz-crystal nanobalance (EQCN) analysis of the behavior of Pt in aqueous H(2)SO(4) reveals that the interfacial mass reaches a minimum, the potential of minimum mass (E(pmm)), at 0.045 V. A similar behavior is observed for Pt in aqueous HClO(4) and NaOH. E(pmm) is a new parameter describing the electrochemical interface. The value of E(pmm) coincides with the completion of the saturation layer of electroadsorbed H (H(UPD)) and the commencement of H(2)(g) generation or H(2)(g) electro-oxidation. The value of E(pmm) and the structure of the Pt/electrolyte interface are discussed in terms of the interactions of the anions H(3)O(+), H(UPD), H(OPD), and H(2)O with Pt. The layer of H(UPD) embedded in the Pt surface lattice minimizes the surface dipole-water dipole and surface charge-water dipole interactions, thus reduces the wetting ability of Pt. Consequently, the discharge of H(3)O(+) in the electrolytic formation of H(2)(g) or the dissociative adsorption of H(2)(g) that precedes its electro-oxidation to H(3)O(+) proceed easily on Pt, because the species do not have to displace H(2)O molecules. Effective and inexpensive non-platinum electrocatalysts for the electrolytic H(2)(g) generation in water electrolyzers or H(2)(g) electro-oxidation in polymer electrolyte membrane fuel cells should mimic the interfacial behavior of Pt by minimizing the interaction of H(2)O molecules with the electrode.

Theoretical investigations of the oxygen reduction reaction on Pt(111).

Computational modeling can provide important insights into chemical reactions in both applied and fundamental fields of research. One of the most critical processes needed in practical renewable energy sources is the oxygen reduction reaction (ORR). Besides being the key process in combustion and corrosion, the ORR has an elusive mechanism that may proceed in a number of complicated reaction steps in electrochemical fuel cells. Indeed, the mechanism of the ORR on highly studied Pt(111) electrodes has been the subject of interest and debate for decades. Herein, we first outline the theory behind these types of simulations and then show how to use these quantum mechanical approaches and approximations to create a realistic model. After reviewing the performance of these methods in studying the binding of molecular oxygen to Pt(111), we then outline our own results in elucidating the ORR and its dependence on environmental parameters, such as solvent, thermodynamic energies, and the presence of an external electrode potential. This approach can, in principle, be applied to other equally complicated investigations of other surfaces or electrochemical reactions.

Characterization of platinum nanoparticles for fuel cell applications by single particle inductively coupled plasma mass spectrometry.

The most effective utilization of platinum (Pt) in fuel cells is achieved through the use of nanoparticles (NPs) that offer a large electrochemically active surface area. Because the stability of NPs decreases as they become smaller, their size and size distribution must be known in order to optimize the catalysts' durability, while offering high catalytic activity. Single particle inductively coupled plasma mass spectrometry (spICPMS) can quantify the mass of metallic NPs suspended in aqueous medium, which can then be converted into a size if the NPs' shape, density and composition are known. In this study, for the first time, spICPMS was compared to transmission electron microscopy (TEM) for the characterization of 10 nm Pt NPs. After verifying the accurate sizing of commercial Pt NPs with diameters of 30, 50 and 70 nm, spICPMS with solution calibration was applied to laboratory-synthesized 10 nm Pt NPs possessing a near spherical shape and 10 ± 2 nm diameter according to TEM. The same NPs were also analyzed by spICPMS with Pt size calibration using Pt NPs standards. Irrespectively of the calibration strategy, spICPMS measured the entire population of 659 Pt NPs (6-65 nm), while TEM analyzed the 500 Pt NPs that appeared isolated in the field of view (6-18 nm). Analysis of the size distribution histograms revealed that the modal and mean diameters were respectively 10 and 11 ± 2 nm using solution calibration, and 12 and 13 ± 2 nm using particle size calibration. Both of these mean diameters are in agreement with the TEM measurements according to a Student's t-test at the 95% confidence level, demonstrating that spICPMS, with a size detection limit of 6 nm, can accurately quantify 10-nm Pt NPs while at the same time analyzing the entire sample.

Single particle inductively coupled plasma mass spectrometry with and without flow injection for the characterization of nickel nanoparticles.

This work compares the performance of transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), single particle inductively coupled plasma mass spectrometry (spICPMS) and flow injection (FI) coupled to spICPMS for the characterization of synthetic ferromagnetic Ni nanoparticles (NPs) prepared with and without polyvinylpyrrolidone (PVP) stabilizer. Whereas single NPs measurement by XRD yielded nominal diameters of 13.7 and 16.6 nm with and without PVP respectively, a diameter of 100-130 nm was obtained by TEM and SEM with or without PVP, indicating extensive agglomeration during preparation for microscopy. In contrast, without PVP stabilization, mean and mode sizes of respectively 35 ± 18 and 21 nm by spICPMS and 33 ± 15 and 20 nm by FI-spICPMS were obtained for suspensions of Ni NPs using external calibration with Ni standard solutions. With PVP stabilization, the mean and mode sizes respectively decreased to 27 ± 12 and 18 nm by spICPMS and 25 ± 10 and 16 nm by FI-spICPMS. Mass balance taking into account the amount of dissolved Ni was verified in all cases. No degradation in performance resulted from using FI-spICPMS instead of spICPMS, even though measurement of NPs mass by FI-spICPMS is done without knowledge of the transport efficiency and the sample uptake rate. This is the first time that spICPMS and FI-spICPMS are demonstrated to be suitable for the characterization of ferromagnetic NPs.