Shyam S. Kocha

Electrochemical Investigation of the Gallium Nitride‐Aqueous Electrolyte Interface

GaN (E{sub g} = {approximately}3.4 eV) was photoelectrochemically characterized and the energetic position of its bandedges determined with respect to SHE. Electrochemical impedance spectroscopy was employed to analyze the interface, determine the space charge layer capacitance, and, subsequently obtain the flatband potential of GaN in different aqueous electrolytes. The flatband potential of GaN varied at an approximately Nernstian rate in aqueous buffer electrolytes of different pHs indicating acid-base equilibria at the interface.

Displacement of the Bandedges of GaInP2 in Aqueous Electrolytes Induced by Surface Modification

Photoelectrolysis of water at ambient temperature (25 C) into hydrogen and oxygen thermodynamically requires a free energy of 1.23 eV. GaInP{sub 2} was identified as a promising material since its bandgap (1.8 to 1.9 eV) is ideal for this reaction. However, previous work determining the flatband potential of p-GaInP{sub 2} has revealed that the position of the bandedges are from 100 to 400 meV too negative for water splitting. The surface of epi layer p-GaInP{sub 2} electrodes was treated using 8-quinolinol, cupferron, and ferrocyanide, producing a modified surface directed at varying the Helmholtz layer charge. Mott-Schottky and photocurrent-voltage measurements were carried out to determine if there was any shift in the flatband potential or change in the onset of photocurrent due to the altered surface charge. Treatments with 8-quinolinol and cupferron were found to shift the flatband to more positive potentials; treatments with ferrocyanide produced a negative shift. The quinolate-modified interface had flatband potentials that were pH independent in the range 5 through 8. Photoluminescence studies on electrodes that were etched, treated with 8-quinolinol, and exposed to air for long periods showed no degradation of the luminescence intensity or photoluminescence decay time, in contrast to untreated electrodes.

Investigations of the Fe1.99Ti0.01O3-electrolyte interface

The Fe1.99Ti0.01O3–electrolyte interface is investigated using electrochemical impedance spectroscopy. The analysis of the frequency dispersion of the real and imaginary parts of the complex impedance at various electrode potentials allows us to define the equivalent circuit for the electrochemical cell and calculate its associated parameters. The capacitance of the space-charge layer in the semiconductor electrode is isolated, and the limiting step of the electrode process is determined.

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

Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum–Nickel Nanowires through Synthesis and Post-Treatment Optimization

For the first time, extended nanostructured catalysts are demonstrated with both high specific activity (>6000 μA cmPt–2 at 0.9 V) and high surface areas (>90 m2 gPt–1). Platinum–nickel (Pt—Ni) nanowires, synthesized by galvanic displacement, have previously produced surface areas in excess of 90 m2 gPt–1, a significant breakthrough in and of itself for extended surface catalysts. Unfortunately, these materials were limited in terms of their specific activity and durability upon exposure to relevant electrochemical test conditions. Through a series of optimized postsynthesis steps, significant improvements were made to the activity (3-fold increase in specific activity), durability (21% mass activity loss reduced to 3%), and Ni leaching (reduced from 7 to 0.3%) of the Pt—Ni nanowires. These materials show more than a 10-fold improvement in mass activity compared to that of traditional carbon-supported Pt nanoparticle catalysts and offer significant promise as a new class of electrocatalysts in fuel cell applications.

Impedance analysis of surface modified Ga0.5In0.5P—aqueous electrolyte interface

Abstract In this work electrochemical impedance spectroscopy was carried out on pre- and post-etched Ga 0.5 In 0.5 P (1.8–1.9 eV) as well as quinolinol-modified surfaces, over seven decades of frequencies. The spectra were examined in the Bode as well as complex impedance planes and deconvoluted using nonlinear-least-squares fitting to obtain equivalent circuits that isolated the space charge from the surface states. Chemical wet-etching of Ga 0.5 In 0.5 P was found to remove surface states as evidenced in the removal of the low frequency time constant from the spectra. Treatment with 8-quinolinol was found to shift the flat-band potential in the positive direction vs. she by about 300mV in near neutral electrolytes.

Polymer Electrolyte Membrane (PEM) Fuel Cells, Automotive Applications

Since the discovery of fuel cells in the nineteenth century, they have been designed for operation with liquid alkaline, acid, and solid oxide ion conducting electrolytes in different temperature ranges to produce electrical power for stationary, portable, and automotive applications. The liquid acid that provides ionic conduction has been replaced by fairly thin proton conducting membranes such as polystyrenes and perfluorosulfonic acids (PFSAs) like Nafion and more recently with hydrocarbon-based polymers. These fuel cells incorporating a proton-conducting membrane rather than liquid electrolyte to separate the anode and cathode (forming a 3-layer sandwich or catalyst coated membrane) are referred to as PEMFCs. PEMFCs are preferred for use in automotives for a multitude of reasons including their high volumetric and gravimetric power density.

The role of nitrogen doping on durability in the Pt-Ru/HOPG system.

Improving catalytic activity and durability are two major issues that must be addressed for fuel cells to become commercially viable. Surface modifications and doping of the catalyst support has been shown to effectively address both of these issues through significant improvements in the catalyst-support interactions [1, 2]. In this work we discuss the role of nitrogen doping via ion-implantation on the stability of a Pt-Ru nanoparticle catalyst phase supported on model highly-oriented pyrolytic graphite (HOPG) substrates.

Photoelectrochemical decomposition of water using modified monolithic tandem cellsfn2fn2Presented at the International Symposium on Hydrogen-Fuel Cell-Photovoltaic Systems i, 31 August–4 September 1997, Cancun, Mexico.

Abstract Photovoltaic tandem cells, consisting of a gallium indium phosphide (GaInP 2 ) homojunction grown epitaxially on a gallium arsenide (GaAs) homojunction with a Ga Astunnel diode interconnect, were modified with an additional top p -layer of GaInP 2 . These cells were used as electrodes to photoelectrochemically decompose water into hydrogen and oxygen in 1M, 5M and 11M KOH electrolyte solutions. The hydrogen reaction was catalyzed at the semiconductor surface with a photoelectrochemically deposited thin layer of platinum and ruthenium. Gas chromatography and electrochemical experiments demonstrate that the modified tandem cells produce hydrogen and oxygen with a light-to-hydrogen conversion efficiency of up to 6%. Both the efficiency and the stability of these cells are discussed.