Kevin C. Leonard

Electrocatalytic Activity of Individual Pt Nanoparticles Studied by Nanoscale Scanning Electrochemical Microscopy

Understanding the relationship between the structure and the reactivity of catalytic metal nanoparticles (NPs) is important to achieve higher efficiencies in electrocatalytic devices. A big challenge remains, however, in studying these relations at the individual NP level. To address this challenge, we developed an approach using nanometer-scale scanning electrochemical microscopy (SECM) for the study of the geometric property and catalytic activity of individual Pt NPs in the hydrogen oxidation reaction (HOR). Herein, Pt NPs with a few tens to a hundred nm radius were directly electrodeposited on a highly oriented pyrolitic graphite (HOPG) surface via nucleation and growth without the necessity of capping agents or anchoring molecules. A well-defined nanometer-sized tip comparable to the dimensions of the NPs and a stable nanogap between the tip and NPs enabled us to achieve lateral and vertical spatial resolutions at a nanometer-scale and study fast electron-transfer kinetics. Specifically, the use of t...

Unbiased Photoelectrochemical Water Splitting in Z-Scheme Device Using W/Mo-Doped BiVO4 and ZnxCd1−xSe

Photoelectrochemical water splitting to generate H2 and O2 using only photon energy (with no added electrical energy) has been demonstrated with dual n-type-semiconductor (or Z-scheme) systems. Here we investigated two different Z-scheme systems; one is comprised of two cells with the same metal-oxide semiconductor (W- and Mo-doped bismuth vanadate), that is, Pt-W/Mo-BiVO4, and the other is comprised of the metal oxide and a chalcogenide semiconductor, that is, Pt-W/Mo-BiVO4 and Zn(0.2)Cd(0.8)Se. The redox couples utilized in these Z-scheme configurations were I(-)/IO3(-) or S(2-)/S(n)(2-), respectively. An electrochemical analysis of the system in terms of cell components is shown to illustrate the behavior of the complete photoelectrochemical Z-scheme water-splitting system. H2 gas from the unbiased photolysis of water was detected using gas chromatography-mass spectroscopy and using a membrane-electrode assembly. The electrode configuration to achieve the maximum conversion efficiency from solar energy to chemical energy with the given materials and the Z-scheme is discussed. Here, the possibilities and challenges of Z-scheme unbiased photoelectrochemical water-splitting devices and the materials to achieve practical solar-fuel generation are discussed.

Evaluating the Electrochemical Capacitance of Surface-Charged Nanoparticle Oxide Coatings

While transition metal oxides have been thoroughly investigated as coatings for electrochemical capacitors due to their pseudocapacitance, little work has been done investigating other oxide coatings. There exists a whole class of nanoporous oxides typically synthesized by sol-gel chemistry techniques that have very high differential capacitance. This high differential capacitance has been attributed to the surface potential of these materials and the close approach of counterions near the surface of these oxides. This study focuses on investigating the electrochemical capacitance of non-transition metal oxide nanoparticle coatings when deposited on supporting electrodes. Here, we show that, by adding coatings of SiO(2), AlOOH, TiO(2), and ZrO(2) nanoparticles to graphite support electrodes, we can increase the electrochemical capacitance. We also show that the measured electrochemical capacitance of these oxide-coated electrodes directly relates to the electrophoretic mobility of these materials with the lowest values in capacitance occurring at or near the respective isoelectric pH (pH(IEP)) of each oxide.

Nano-size layered manganese–calcium oxide as an efficient and biomimetic catalyst for water oxidation under acidic conditions: comparable to platinum

Inspired by Natures catalyst, a nano-size layered manganese-calcium oxide showed a low overvoltage for water oxidation in acidic solutions, which is comparable to platinum.

The study of multireactional electrochemical interfaces via a tip generation/substrate collection mode of scanning electrochemical microscopy: the hydrogen evolution reaction for Mn in acidic solution.

We report a new method of scanning electrochemical microscopy (SECM) that can be used to separate multireactional electrochemical interfaces, i.e., electrodes at which two or more reactions occur (and hence two partial currents flow) at the same time. This was done with a modified tip generation/substrate collection mode where the two reactions occur on the tip electrode, and the substrate electrode is held at a potential to collect only one of the products, allowing the determination of the individual partial currents. Thus, by using the substrate electrode current and the difference between the tip and substrate electrode currents, the two reactions occurring on the tip electrode can be separated. As a test case for this new method, we investigated proton reduction on Mn, a reaction that, because of the highly corrosive nature of Mn, to our knowledge has never before been directly measured. This test was carried out using a Mn tip electrode and a Pt substrate electrode. Using a three-dimensional COMSOL Multiphysics simulation, we were able to accurately determine the tip/substrate distance with this electrode, and by fitting simulations to experimental data, we were able to determine an exchange current density, log(j(0)) = -4.7 ± 0.7 A cm(-2), for proton reduction on Mn in strong acid. This result corrects a literature value and was used in a pattern recognition algorithm reported in a companion manuscript.

Pattern Recognition Correlating Materials Properties of the Elements to Their Kinetics for the Hydrogen Evolution Reaction

Here we demonstrate the use of a previously reported pattern recognition algorithm to evaluate correlations between 50 different materials properties of the elements and their kinetics for the hydrogen evolution reaction in acid. We determined that the melting point and bulk modulus of the elements quantitatively gave the highest correlations of all materials properties investigated. We also showed that the melting point and bulk modulus correlations held true for a popular hydrogen evolution catalysts alloy, NiMo, and a previously untested material, MoSi2. In addition, we quantified the previously known relationship between the d-band center of an element and its kinetics for hydrogen evolution, and found that the melting point and bulk modulus correlations have correlations that are similar to but slightly stronger than those of the d-band center.

Examining ultramicroelectrodes for scanning electrochemical microscopy by white light vertical scanning interferometry and filling recessed tips by electrodeposition of gold.

In this paper, we present a technique to rapidly and directly examine ultramicroelectrodes (UMEs) by white light vertical scanning interferometry (VSI). This technique is especially useful in obtaining topographic information with nanometer resolution without destruction or modification of the UME and in recognizing tips where the metal is recessed below the insulating sheath. Two gold UMEs, one with a metal radius a = 25 μm and relative insulating sheath radius RG = 2 and the other with a = 5 μm and RG = ∼1.5, were examined, and the average depth of the gold recessions was determined to be 1.15 μm and 910 nm, respectively. Electrodeposition of gold was performed to fill the recessed hole, and the depth was reduced to ∼200 nm. With the electrodeposited gold electrode and a conventional microelectrode (a = 25 μm) as a tip and substrate, respectively, a tip/substrate distance, d, of 600 nm was achieved allowing scanning electrochemical microscopy (SECM) in positive feedback mode at a close distance, which is useful for measuring fast kinetics.

Nanometer Scale Scanning Electrochemical Microscopy Instrumentation

We report the crucial components required to perform scanning electrochemical microscopy (SECM) with nanometer-scale resolution. The construction and modification of the software and hardware instrumentation for nanoscale SECM are explicitly explained including (1) the LabVIEW code that synchronizes the SECM tip movement with the electrochemical response, (2) the construction of an isothermal chamber to stabilize the nanometer scale gap between the tip and substrate, (3) the modification of a commercial bipotentiostat to avoid electrochemical tip damage during SECM experiments, and (4) the construction of an SECM stage to avoid artifacts in SECM images. These findings enabled us to successfully build a nanoscale SECM, which can be utilized to map the electrocatalytic activity of individual nanoparticles in a typical ensemble sample and study the structure/reactivity relationship of single nanostructures.

Microwave-assisted synthesis of a nanoamorphous (Ni0.8,Fe0.2) oxide oxygen-evolving electrocatalyst containing only “fast” sites

Nickel–iron oxyhydroxides (Ni1−xFexOOH) are non-precious metal electrocatalysts for the oxygen evolution reaction (OER) that have high efficiency in alkaline media. It has been suggested that the layered-double hydroxide (LDH) crystal structure of Ni0.8Fe0.2OOH contains two types of catalytic sites, “fast” Fe sites and “slow” Ni sites, which may limit the overall activity because only 20% of the catalytic surface is highly active. Herein, we report a facile microwave-assisted synthesis route of creating a nanoamorphous nickel–iron oxide electrocatalyst that contains only “fast” catalytic sites. Benchmarking experiments on flat electrodes (roughness factors <1.4) showed that the microwave-assisted, nanoamorphous (Ni0.8,Fe0.2) oxide had a low OER overpotential of 286 mV at a current density of 10 mA cm−2. We measured the kinetic rate constant of the active sites directly with the surface interrogation mode of scanning electrochemical microscopy (SI-SECM). We show that the microwave-assisted, nanoamorphous (Ni0.8,Fe0.2) oxide has only one type of catalytic site with an OER kinetic rate constant of 1.9 s−1 per site. We compared this to a crystalline Ni0.8Fe0.2OOH that was synthesized via electrochemical conditioning of crystalline Ni0.8:Fe0.2 oxide, and verified that the Ni0.8Fe0.2OOH contained two types of catalytic sites – “fast” sites with an OER rate constant of 1.3 s−1 per site and “slow” sites with an OER rate constant of 0.05 s−1 per site. The percentage of “fast” sites in the crystalline Ni0.8Fe0.2OOH was well matched to the total iron atom content, while 100% of the sites were “fast” in the microwave-assisted, nanoamorphous (Ni0.8,Fe0.2) oxide.

CHAPTER 6:Rapid Screening Methods in the Discovery and Investigation of New Photocatalyst Compositions

Discovery of an efficient, stable, and inexpensive photocatalyst is a key issue in the design of a practical photoelectrochemical (PEC) system for converting solar energy into chemical fuels, e.g. hydrogen production from water splitting. Despite over 40 years of enormous efforts in the area, no photocatalyst has yet been found as an optimized material for water photolysis. In this chapter, combinatorial rapid synthesis and screening of semiconducting materials to discover and improve photocatalysts for water photolysis are discussed. While several different techniques are briefly touched upon, the focus is on using modified scanning electrochemical microscopy (SECM) for combinatorial rapid screening. In this application of SECM, a piezo-dispensing tip and a fiber optic are placed in the SECM for rapid synthesis and fast scanning of the semiconductor spot arrays. The rapid synthesis and screening of electrocatalysts decorating the semiconducting photocatalysts, which is required to further enhance the water splitting reactions, i.e. hydrogen/oxygen evolution reactions (HER/OER), is also explained. In addition, several modes of SECM used to study material properties and chemical reactions on photocatalysts/electrocatalysts are generally introduced. Finally, factors affecting the activity of photocatalysts are briefly discussed to guide the rapid screening of materials in PEC systems.