Nancy N. Kariuki

Aligned carbon nanotubes with built-in FeN4 active sites for electrocatalytic reduction of oxygen

The electrocatalytic site FeN4, which is active towards the oxygen reduction reaction, is incorporated into the graphene layer of aligned carbon nanotubes prepared through a chemical vapour deposition process, as is confirmed by X-ray absorption spectroscopy and other characterization techniques.

Bimetallic Pd-Cu oxygen reduction electrocatalysts

A series of Vulcan carbon-supported Pd-Cu catalysts with various molar ratios of Pd to Cu was prepared by co-impregnation followed by a reduction in a hydrogen atmosphere at three different temperatures. The degree of alloying between the two metals, alloy composition, and particle size and size distribution were characterized by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The electrocatalytic activity for the oxygen reduction reaction (ORR) for these various compositions was determined using the thin-film rotating disk electrode technique. Our study reveals that the Pd-Cu bimetallic electrocatalysts, with a suitable degree of alloying, offer a greatly enhanced ORR activity compared to the Pd monometallic electrocatalyst. The best electrocatalytic activities were observed for the bimetallic catalysts that showed alloy nanoparticles with a Pd-Cu molar ratio of approximately 1:1.

Ternary alloy nanoparticles with controllable sizes and composition and electrocatalytic activity

The ability to tailor the size and composition of metal particles at the nanoscale could lead to improved or new catalytic properties. This paper reports the results of an investigation of the preparation and the characterization of platinum–nickel–iron (PtNiFe) ternary alloy nanoparticles. The synthesis of monolayer-capped ternary PtNiFe nanoparticles involved controlling the ratios of metal precursors, capping agents, and reducing agents in a single organic phase. The average diameters of the resulting nanocrystalline cores were well controlled between 1.4 and 1.8 nm with high monodispersity (±0.2–0.4 nm). The catalysts were prepared by loading the as-synthesized PtNiFe nanoparticles onto a high surface area carbon support and subsequent thermal treatment optimized for achieving effective shell removal and alloy formation, as well as controllable size, composition and metal loading. The catalysts were characterized using TEM, DCP-AES, FTIR, TGA, and XRD techniques. The catalysts were also examined for their intrinsic kinetic activities for the electrochemical oxygen reduction reaction. It is shown that the ternary catalysts are highly active towards molecular oxygen electrocatalytic reduction. Implications of our findings for the exploration of the new nanostructured catalyst materials for applications in fuel cell catalysis are also discussed.

Electrocatalytic reduction of oxygen: Gold and gold-platinum nanoparticle catalysts prepared by two-phase protocol

This paper describes recent results of an investigation of gold (Au) and gold-platinum (AuPt) nanoparticle electrocatalysts for fuel cell reaction at the cathode, i.e., oxygen reduction reaction (ORR). The Au nanoparticles and AuPt nanoparticles with different bimetallic ratios were prepared by a two-phase protocol and supported on carbon black materials. The catalysts were thermally activated under controlled calcination temperatures. The electrocatalytic ORR activities were characterized using voltammetric and rotating disk electrode techniques. We have also attempted an initial comparison of the electrocatalytic activities of our Au/C and AuPt/C catalysts with commercially-available Pt/C and PtRu/C catalysts (E-tek) under the same voltammetric measurement conditions. The results revealed important insights into the electrocatalytic activity of our catalysts, and have important implications to the design of highly active fuel cell catalysts.

Synthesis, processing, assembly and activation of core-shell structured gold nanoparticle catalysts

This paper describes recent progress of an investigation of the synthesis, processing, assembly and activation of gold nanoparticles that are of potential interest to fuel cell catalysis. Core-shell type gold nanoparticles of a few nanometer core size with organic monolayer encapsulation are highlighted. The activation of the core-shell nanoparticle assemblies towards nanostructured catalyst for potential fuel cell catalytic reactions is discussed. The understanding of the control factors in terms of nanocrystal size and interparticle spatial properties has important implications to the design of highly active nanogold catalysts for practical applications.

Oxygen Reduction Reaction Electrocatalytic Activity of Glancing Angle Deposited Platinum Nanorod Arrays

The electrocatalytic oxygen reduction reaction (ORR) activity of vertically-aligned Pt nanorods has been evaluated utilizing cyclic voltammetry (CV) and rotating-disk electrode (RDE) techniques in a 0.1 M HClO4 solution at temperatures ranging from 20 to 60 � C. A glancing angle deposition (GLAD) technique was used to fabricate Pt nanorod arrays on glassy carbon (GC) electrodes. GLAD catalyst nanorods, without any carbon support, have been produced at different lengths varying between 50 and 400 nm, corresponding to 0.04–0.32 mg/cm 2 Pt loadings, with diameter and spacing values ranging from about 5 up to 100 nm. The scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) results reveal that Pt nanorods are well-isolated, vertically aligned, and single-crystal. Crystal orientation analysis demonstrates that large surface area Pt nanorod sidewalls are mainly dominated by Pt(110) planes, which is known to be the most active crystal plane of Pt for the ORR. Compared to a commercial high-surface-area-supported Pt (Pt/C) catalyst, the CV results show that the Pt-nanorod electrocatalyst exhibits a more positive oxide reduction peak potential, indicating that GLAD Pt nanorods are less oxophilic. Moreover, the nanorods exhibit enhanced stability against loss of electrochemically-active surface area as a result of potential cycling in acidic electrolyte as compared to the Pt/C catalyst. Specific ORR activities determined by the RDE technique for GLAD Pt nanorods of different lengths are analyzed and compared to literature values for polycrystalline Pt, nano-structured thin film Pt (3M NSTF Pt), and to those measured for Pt/C. RDE results reveal that Pt-nanorod electrocatalysts exhibit higher area-specific activity, higher electron-transfer rate constant, and comparable activation energy for ORR than those of Pt/C due to their larger crystallite size, single-crystal property, and dominance of the preferred crystal orientations for ORR. However, Pt nanorods show lower mass specific activity than that of Pt/C electrocatalyst due to the large diameter of nanorods. V C 2011 The Electrochemical Society. [DOI: 10.1149/1.3599901] All rights reserved.

In Situ Anomalous Small-Angle X-ray Scattering Studies of Platinum Nanoparticle Fuel Cell Electrocatalyst Degradation

Polymer electrolyte fuel cells (PEFCs) are a promising high-efficiency energy conversion technology, but their cost-effective implementation, especially for automotive power, has been hindered by degradation of the electrochemically active surface area (ECA) of the Pt nanoparticle electrocatalysts. While numerous studies using ex situ post-mortem techniques have provided insight into the effect of operating conditions on ECA loss, the governing mechanisms and underlying processes are not fully understood. Toward the goal of elucidating the electrocatalyst degradation mechanisms, we have followed Pt nanoparticle growth during potential cycling of the electrocatalyst in an aqueous acidic environment using in situ anomalous small-angle X-ray scattering (ASAXS). ASAXS patterns were analyzed to obtain particle size distributions (PSDs) of the Pt nanoparticle electrocatalysts at periodic intervals during the potential cycling. Oxide coverages reached under the applied potential cycling protocols were both calculated and determined experimentally. Changes in the PSD, mean diameter, and geometric surface area identify the mechanism behind Pt nanoparticle coarsening in an aqueous environment. Over the first 80 potential cycles, the dominant Pt surface area loss mechanism when cycling to 1.0-1.1 V was found to be preferential dissolution or loss of the smallest particles with varying extents of reprecipitation of the dissolved species onto existing particles, resulting in particle growth, depending on potential profile. Correlation of ASAXS-determined particle growth with both calculated and voltammetrically determined oxide coverages demonstrates that the oxide coverage is playing a key role in the dissolution process and in the corresponding growth of the mean Pt nanoparticle size and loss of ECA. This understanding potentially reduces the complex changes in PSD and ECA resulting from various voltage profiles to a response dependent on oxide coverage.

Electroactivity of Cu2+ at a thin film assembly of gold nanoparticles linked by 11-mercaptoundecanoic acid

Abstract Core-shell nanoparticles are emerging advanced materials for developing novel electroanalytical platforms. This paper describes the results of an investigation of the electroactivity of Cu 2+ ions on electrodes coated with thin films assembled from thiolate-encapsulated gold nanoparticles of 2 nm core size (Au 2-nm ) and a carboxylic functionalized alkyl thiol linker, i.e. 11-mercaptoundecanoic acid (MUA). The high surface-to-volume ratio and the 3-D ligand network properties are potentially useful as sensitive and selective nanomaterials for the monitoring and removal of environmental heavy metals. The nanostructured MUA–Au 2-nm film is sensitive to Cu 2+ below 1 ppm. The selectivity of the electroactivity is also probed using mixed-metal systems such as Cu 2+ and Fe 3+ and Cu 2+ and Zn 2+ . Issues related to the electrochemical activity of these metal ions are also discussed.

Best Practices and Testing Protocols for Benchmarking ORR Activities of Fuel Cell Electrocatalysts Using Rotating Disk Electrode

AbstractThin-film-rotating disk electrodes (TF-RDEs) are the half-cell electrochemical system of choice for rapid screening of oxygen reduction reaction (ORR) activity of novel Pt supported on carbon black supports (Pt/C) electrocatalysts. It has been shown that the magnitude of the measured ORR activity and reproducibility are highly dependent on the system cleanliness, evaluation protocols, and operating conditions as well as ink formulation, composition, film drying, and the resultant film thickness and uniformity. Accurate benchmarks of baseline Pt/C catalysts evaluated using standardized protocols and best practices are necessary to expedite ultra-low-platinum group metal (PGM) catalyst development that is crucial for the imminent commercialization of fuel cell vehicles. We report results of evaluation in three independent laboratories of Pt/C electrocatalysts provided by commercial fuel cell catalyst manufacturers (Johnson Matthey, Umicore, Tanaka Kikinzoku Kogyo—TKK). The studies were conducted using identical evaluation protocols/ink formulation/film fabrication albeit employing unique electrochemical cell designs specific to each laboratory. The ORR activities reported in this work provide a baseline and criteria for selection and scale-up of novel high activity ORR electrocatalysts for implementation in proton exchange membrane fuel cells (PEMFCs). Reproducibility of ORR mass activity for three Pt/C catalysts between three laboratories using best practices and standardized measurement protocols.Graphical Abstract

Performance Improvement in PEMFC using Aligned Carbon Nanotubes as Electrode Catalyst Support

A novel membrane electrode assembly (MEA) using aligned carbon nanotubes (ACNT) as the electrocatalyst support was developed for proton exchange membrane fuel cell (PEMFC) application. A multiple-step process of preparing ACNT-PEMFC including ACNT layer growth and catalyzing, MEA fabrication, and single cell packaging is reported. Single cell polarization studies demonstrated improved fuel utilization and higher power density in comparison with the conventional, ink based MEA.