Radoslav R. Adzic

Hydrogen‐Evolution Catalysts Based on Non‐Noble Metal Nickel–Molybdenum Nitride Nanosheets

Hydrogen production through splitting of water has attracted great scientific interest because of its relevance to renewable energy storage and its potential for providing energy without the emission of carbon dioxide. Electrocatalytic systems for H2 generation typically incorporate noble metals such as Pt in the catalysts because of their low overpotential and fast kinetics for driving the hydrogen evolution reaction (HER). However, the high costs and limited world-wide supply of these noble metals make their application in viable commercial processes unattractive. Several non-noble metal materials, such as transition-metal chalcogenides, carbides, and complexes as well as metal alloys have been widely investigated recently, and characterized as catalysts and supports for application in the evolution of hydrogen. Nitrides of early transition-metals have been shown to have excellent catalytic activities in a variety of reactions. One of the primary interests in the applications of nitrides in these reactions was to use them in conjunction with low-cost alternative metals to replace group VIII noble metals. For example, the function of molybdenum nitride as a catalyst for hydrocarbon hydrogenolysis resembles that of platinum. The catalytic and electronic properties of transition-metal nitrides are governed by their bulk and surface structure and stoichiometry. While there is some information concerning the effect of the bulk composition on the catalytic properties of this material, there is currently little known about the effects of the surface nanostructure. Nickel and nickel–molybdenum are known electrocatalysts for hydrogen production in alkaline electrolytes, and in the bulk form they exhibited exchange current densities between 10 6 and 10 4 Acm , compared to 10 3 Acm 2 for Pt. Jaksic et al. postulated a hypo-hyper-d-electronic interactive effect between Ni and Mo that yields the synergism for the HER. Owing to their poor corrosion stability, few studies in acidic media have been reported.With the objective of exploiting the decrease in the overpotential by carrying out the HER in acidic media, we have developed a low-cost, stable, and active molybdenum-nitride-based electrocatalyst for the HER. Guided by the “volcano plot” in which the activity for the evolution of hydrogen as a function of the M H bond strength exhibits an ascending branch followed by a descending branch, peaking at Pt, we designed a material on the molecular scale combining nickel, which binds H weakly, with molybdenum, which binds H strongly. Here we report the first synthesis of NiMo nitride nanosheets on a carbon support (NiMoNx/C), and demonstrate the high HER electrocatalytic activity of the resulting NiMoNx/C catalyst with low overpotential and small Tafel slope. The NiMoNx/C catalyst was synthesized by reduction of a carbon-supported ammonium molybdate [(NH4)6Mo7O24·4H2O] and nickel nitrate (Ni(NO3)2·4H2O) mixture in a tubular oven in H2 at 400 8C, and subsequent reaction with NH3 at 700 8C. During this process, the (NH4)6Mo7O24 and Ni(NO3)2 precursors were reduced to NiMo metal particles by H2, and then they were mildly transformed to NiMoNx nanosheets by reaction with ammonia. The atomic ratio of Ni/Mo was 1/4.7 determined by energy dispersive X-ray spectroscopy (EDX) on the NiMoNx/ C sample. The transmission electron microscopy (TEM) images, as shown in Figure 1a, display NiMo particles that are mainly spherical. The high-resolution TEM image, as shown in the inset of Figure 1a, corroborated the presence of an amorphous 3 to 5 nm Ni/Mo oxide layer (see Figure S4 in the Supporting Information for resolved image), whereas NiMoNx is characterized by thin, flat, and flaky stacks composed of nanosheets with high radial-axial ratios (Figure 1b and Figure S5 in the Supporting Information for a magnified image). Figure 1c shows that some of the nanosheets lay flat on the graphite carbon (as indicated by the black arrows), and some have folded edges that show different layers of NiMoNx sheets (white arrows). The thickness of the sheets ranged from 4 to 15 nm. The average stacking number of sheets measured from Figure 1b is about [*] Dr. W.-F. Chen, Dr. K. Sasaki, Dr. J. T. Muckerman, Dr. R. R. Adzic Chemistry Department, Brookhaven National Laboratory Upton, NY 11973 (USA) E-mail: ksasaki@bnl.gov

Core-Protected Platinum Monolayer Shell High-Stability Electrocatalysts for Fuel-Cell Cathodes

More than skin deep: Platinum monolayers can act as shells for palladium nanoparticles to lead to electrocatalysts with high activities and an ultralow platinum content, but high platinum utilization. The stability derives from the core protecting the shell from dissolution. In fuel-cell tests, no loss of platinum was observed in 200?000 potential cycles, whereas loss of palladium was significant.

Highly active and durable nanostructured molybdenum carbide electrocatalysts for hydrogen production

In an attempt to tailor low-cost, precious-metal-free electrocatalysts for water electrolysis in acid, molybdenum carbide (β-Mo2C) nanoparticles are prepared by in situ carburization of ammonium molybdate on carbon nanotubes and XC-72R carbon black without using any gaseous carbon source. The formation of Mo2C is investigated by thermogravimetry and in situ X-ray diffraction. X-ray absorption analysis reveals that Mo2C nanoparticles are inlaid or anchored into the carbon supports, and the electronic modification makes the surface exhibit a relatively moderate Mo–H bond strength. It is found that carbon nanotube-supported Mo2C showed superior electrocatalytic activity and stability in the hydrogen evolution reaction (HER) compared to the bulk Mo2C. An overpotential of 63 mV for driving 1 mA cm−2 of current density was measured for the nanotube-supported Mo2C catalysts; this exceeds the activity of analogous Mo2C catalysts. The enhanced electrochemical activity is facilitated by unique effects of the anchored structure coupled with the electronic modification.

Enhanced Electrocatalytic Performance of Processed, Ultrathin, Supported Pd–Pt Core–Shell Nanowire Catalysts for the Oxygen Reduction Reaction

We report on the synthesis, characterization, and electrochemical performance of novel, ultrathin Pt monolayer shell-Pd nanowire core catalysts. Initially, ultrathin Pd nanowires with diameters of 2.0 ± 0.5 nm were generated, and a method has been developed to achieve highly uniform distributions of these catalysts onto the Vulcan XC-72 carbon support. As-prepared wires are activated by the use of two distinctive treatment protocols followed by selective CO adsorption in order to selectively remove undesirable organic residues. Subsequently, the desired nanowire core-Pt monolayer shell motif was reliably achieved by Cu underpotential deposition followed by galvanic displacement of the Cu adatoms. The surface area and mass activity of the acid and ozone-treated nanowires were assessed, and the ozone-treated nanowires were found to maintain outstanding area and mass specific activities of 0.77 mA/cm(2) and 1.83 A/mg(Pt), respectively, which were significantly enhanced as compared with conventional commercial Pt nanoparticles, core-shell nanoparticles, and acid-treated nanowires. The ozone-treated nanowires also maintained excellent electrochemical durability under accelerated half-cell testing, and it was found that the area-specific activity increased by ~1.5 fold after a simulated catalyst lifetime.

Highly stable Pt monolayer on PdAu nanoparticle electrocatalysts for the oxygen reduction reaction

Stability is one of the main requirements for commercializing fuel cell electrocatalysts for automotive applications. Platinum is the best-known catalyst for oxygen reduction in cathodes, but it undergoes dissolution during potential changes while driving electric vehicles, thus hampering commercial adoption. Here we report a new class of highly stable, active electrocatalysts comprising platinum monolayers on palladium-gold alloy nanoparticles. In fuel-cell tests, this electrocatalyst with its ultra-low platinum content showed minimal degradation in activity over 100,000 cycles between potentials 0.6 and 1.0 V. Under more severe conditions with a potential range of 0.6-1.4 V, again we registered no marked losses in platinum and gold despite the dissolution of palladium. These data coupled with theoretical analyses demonstrated that adding a small amount of gold to palladium and forming highly uniform nanoparticle cores make the platinum monolayer electrocatalyst significantly tolerant and very promising for the automotive application of fuel cells.

Ordered mesoporous porphyrinic carbons with very high electrocatalytic activity for the oxygen reduction reaction

The high cost of the platinum-based cathode catalysts for the oxygen reduction reaction (ORR) has impeded the widespread application of polymer electrolyte fuel cells. We report on a new family of non-precious metal catalysts based on ordered mesoporous porphyrinic carbons (M-OMPC; M = Fe, Co, or FeCo) with high surface areas and tunable pore structures, which were prepared by nanocasting mesoporous silica templates with metalloporphyrin precursors. The FeCo-OMPC catalyst exhibited an excellent ORR activity in an acidic medium, higher than other non-precious metal catalysts. It showed higher kinetic current at 0.9 V than Pt/C catalysts, as well as superior long-term durability and MeOH-tolerance. Density functional theory calculations in combination with extended X-ray absorption fine structure analysis revealed a weakening of the interaction between oxygen atom and FeCo-OMPC compared to Pt/C. This effect and high surface area of FeCo-OMPC appear responsible for its significantly high ORR activity.

Size-Dependent Enhancement of Electrocatalytic Performance in Relatively Defect-Free, Processed Ultrathin Platinum Nanowires

We report on the synthesis, characterization, and electrocatalytic performance of ultrathin Pt nanowires with a diameter of less than 2 nm. An acid-wash protocol was employed in order to yield highly exfoliated, crystalline nanowires with a diameter of 1.3 +/- 0.4 nm. The electrocatalytic activity of these nanowires toward the oxygen reduction reaction was studied in relation to the activity of both supported and unsupported Pt nanoparticles as well as with previously synthesized Pt nanotubes. Our ultrathin, acid-treated, unsupported nanowires displayed an electrochemical surface area activity of 1.45 mA/cm(2), which was nearly 4 times greater than that of analogous, unsupported platinum nanotubes and 7 times greater than that of commercial supported platinum nanoparticles.

Improving Electrocatalysts for O2 Reduction by Fine-Tuning the Pt−Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) Single-Crystal Alloy

We improved the effectiveness of Pt monolayer electrocatalysts for the oxygen-reduction reaction (ORR) using a novel approach to fine-tuning the Pt monolayer interaction with its support, exemplified by an annealed Pd(3)Fe(111) single-crystal alloy support having a segregated Pd layer. Low-energy ion scattering and low-energy electron diffraction studies revealed that a segregated Pd layer, with the same structure as Pd (111), is formed on the surface of high-temperature-annealed Pd(3)Fe(111). This Pd layer is considerably more active than Pd(111); its ORR kinetics is comparable to that of a Pt(111) surface. The enhanced catalytic activity of the segregated Pd layer compared to that of bulk Pd apparently reflects the modification of Pd surfaces electronic properties by underlying Fe. The Pd(3)Fe(111) suffers a large loss in ORR activity when the subsurface Fe is depleted by potential cycling (i.e., repeated excursions to high potentials in acid solutions). The Pd(3)Fe(111) surface is an excellent substrate for a Pt monolayer ORR catalyst, as verified by its enhanced ORR kinetics on PT(ML)/Pd/Pd(3)Fe(111). Our density functional theory studies suggest that the observed enhancement of ORR activity originates mainly from the destabilization of OH binding and the decreased Pt-OH coverage on the Pt/Pd/Pd(3)Fe(111) surface. The activity of Pt(ML)/Pd(111) and Pt(111) is limited by OH removal, whereas the activity of Pt(ML)/Pd/Pd(3)Fe(111) is limited by the O-O bond scission, which places these two surfaces on the two sides of the volcano plot.

Platinum-Monolayer Shell on AuNi0.5Fe Nanoparticle Core Electrocatalyst with High Activity and Stability for the Oxygen Reduction Reaction

We describe a simple method for preparing multimetallic nanoparticles by in situ decomposition of the corresponding Prussian blue analogue, which is adsorbed on carbon black. The example involves the AuNi(0.5)Fe core of the Pt(ML)/Au(1)Ni(0.5)Fe core-shell electrocatalyst for the oxygen reduction reaction. The core contains 3-5 surface atomic layers of Au, which play an essential role in determining the activity and stability of the catalyst. The Pt(ML)/AuNi(0.5)Fe electrocatalyst exhibited Pt mass and specific activities of 1.38 A/mg(Pt) and 1.12 × 10(-3) A/cm(2)(Pt), respectively, both of which are several times higher than those of commercial Pt/C catalysts. Its all-noble-metal mass activity (0.18 A/mg(Pt,Au)) is higher than or comparable to those of commercial samples. Stability tests showed an insignificant loss in activity after 15,000 triangular-potential cycles. We ascribe the high activity and stability of the Pt(ML)/AuNi(0.5)Fe electrocatalyst to its hierarchical structural properties, the Pt-core interaction, and the high electrochemical stability of the gold shell that precludes exposure to the electrolyte of the relatively active inner-core materials.

Optical and Electrochemical Studies of Underpotential Deposition of Lead on Gold Evaporated and Single‐Crystal Electrodes

Abstract : Linear sweep voltammetry and reflectance spectroscopy have been used to examine the under potential deposition of lead on gold in Pb(2+) containing HC104 solutions. The voltammetry curves and reflectance change data provide evidence that the lead is first deposited as ions although their effective ionic charge is probably reduced substantially from +2 through their strong interaction with the band structure of the gold substrate. At more cathodic potentials, still well below the reversible potential of bulk lead, a sharp transition is observed over a 5 to 10 mV range. This transition appears to involve a two dimensional phase transition leading to a metallic-like lead layer. Adsorption isotherms have been evaluated from the reflectance changes. A.c. electromodulation techniques and complex plane analysis have been used to examine the kinetics of the lead adsorption-desorption and to evaluate the apparent exchange current density for the process. (Modified author abstract)