GovindaSamy TamizhMani

Physical, Chemical and Electrochemical Characterization of Heat-treated Tetracarboxylic Cobalt Phthalocyanine Adsorbed On Carbon-black As Electrocatalyst for Oxygen Reduction in Polymer Electrolyte Fuel-cells

Tetracarboxylic cobalt phthalocyanine (CoPcTc) has been adsorbed on carbon black (C). The resulting CoPcTc/C has been heat-treated in Ar at various temperatures ranging from 100 to 1100 degrees C in order to produce catalysts for the electroreduction of oxygen in polymer electrolyte fuel cells. Heat-treated CoPcTc/C materials have been characterized by TGA, DSC, bulk elemental analyses, XRD, XPS and ToF SIMS. Their electrocatalytic properties have been evaluated by rde and gde measurements. The highest activity is found for CoPcTc/C heat-treated between 500 and 700 degrees C. In this temperature range, the catalytic site can be traced back either to the intact polymer (< 600 degrees C) or to phthalocyanine fragments still containing Co, even as CoN4 chelates. However, short term life tests on the initially most active catalysts indicate that these catalysts are not stable compared to those obtained after pyrolysis of CoPcTc/C at 900 degrees C. The active site of the latter catalysts is related to inorganic cobalt present as metal and oxides. TEM reveals that inorganic cobalt is surrounded by a protecting graphite shell rendering it chemically stable in acidic media.

Improved Electrocatalytic Oxygen Reduction Performance of Platinum Ternary Alloy‐Oxide in Solid‐Polymer‐Electrolyte Fuel Cells

The effect of base metal oxide (nonnoble metal oxide) in a Pt-Cr-Cu alloy catalyst is investigated for the oxygen reduction reaction in a solid-polymer-electrolyte fuel cell. The cathode mass activities at 0.9 V for Pt, Pt-Cr alloy, Pt-Cr-Cu alloy, and a mixture of Pt-Cr-Cu alloy with base metal oxide are compared. The enhancement factor is largest (about 6 times) for the mixture of Pt-Cr-Cu alloy with base metal oxide, compared to Pt-Cr and Pt-Cr-Cu alloys (about 2 times). The higher electrocatalytic activity of this material may be due to the combined effects of the Pt-Cr-Cu alloy and the base metal oxide. The physical and electrochemical characterizations are carried out using various techniques like X-ray diffraction, transmission electron microscopy, cyclic voltammetry, polarization, and ac impedance.

Influence of Loading on the Activity and Stability of Heat‐Treated Carbon‐Supported Cobalt Phthalocyanine Electrocatalysts in Solid Polymer Electrolyte Fuel Cells

The influence of loading on the activity and stability of heat-treated carbon-supported cobalt phthalocyanine electrocatalysts for oxygen reduction was studied. The Co loading on the carbon support was varied from 0 to 8 weight percent (w/o), while heat-treatment temperatures ranging from 500 to 1100 C were studied. There is an optimum value of the Co loading on the carbon support (about 3.5 w/o) over which the catalytic activity of the Co on carbon catalyst decreases. This trend is observed in both half-cell (rotating disk electrode measurements) and H{sub 2}/O{sub 2} fuel cell measurements. The optimum value of the Co loading on the carbon support is independent of the heat-treatment temperature. An explanation is proposed for this observation. The authors have also performed short-term (1.5 to 18 h) life tests with the 3.5 w/o Co on carbon-support materials heat-treated at various temperatures. It is shown that the current density lost after 18 h of operation is a function of the heat-treatment temperature being lower the higher the temperature. The observed increase in the stability of the material with the heat-treatment temperature is best explained by considering the protective role played by the graphitic-like layer surrounding the Co metal particles.

Electrocatalytic Activity of Nafion‐Impregnated Pyrolyzed Cobalt Phthalocyanine A Correlative Study Between Rotating Disk and Solid Polymer Electrolyte Fuel‐Cell Electrodes

Carbon supported electrocatalysts for the reduction of oxygen can be screened quickly by using a rotating disk electrode (RDE) when an appropriate material is used as a binder to hold the catalyst on the disk electrode. The results obtained with the RDE in an oxygen-saturated H{sub 2}SO{sub 4} solution mimic those measured for a full-cell H{sub 2}/O{sub 2} solid polymer electrolyte setup. This result is demonstrated with a cobalt phthalocyanine-on-carbon black catalyst pyrolyzed at various temperatures ranging from 400 to 1,100 C. When the catalyst is held on the disk electrode with electropolymerized pyrrole, a broad maximum of activity for the reduction of oxygen is observed for pyrolysis temperatures ranging between 600 and 900 C. When the catalyst is dispersed in a Nafion film on the disk electrode, a sharp maximum in its activity is observed at the pyrolysis temperature of 600 C, in agreement with its behavior in a full-cell setup.

Performance test of amorphous silicon modules in different climates - year four: Progress in understanding exposure history stabilization effects

In a round robin outdoor exposure experiment carried out in three different climates, we have previously demonstrated that amorphous silicon (a-Si) PV modules reach higher stabilized performance levels in warmer climates. The four-year experiment involved three identical sets of thin-film a-Si modules from various manufacturers deployed outdoors simultaneously in three sites with distinct climates. Each PV module set spent a one-year period at each site before a final period at the original site where it was first deployed. The experiment aimed to determine the light-induced degradation and stabilization characteristics of a-Si regarding specific history of exposure, and to compare degradation rates in different climates. We propose that after the initial sharp degradation associated with the Stabler-Wronski effect (SWE) has passed, the subsequent stabilized performance levels attained will depend largely on light exposure and a characteristic temperature associated within a coherent time-scale. PV modules which were first deployed at the lowest-temperature site for one year, reaching a stabilized state, and were then further deployed at higher temperature sites for two more years, experienced considerable recovery in output parameters (Pmax and FF). However, when further deployed back at the original, lowest-temperature site, performance degraded back to the first year, original level.

Photoelectrochemical properties of semiconducting cadmium mercury telluride thin films with bandgaps between 1.47 and 1.08 eV

This paper reports on films of Cd-rich cadmium mercury telluride (Cd{sub {ital x}}HG{sub 1{minus}{ital x}}Te, CMT) 0.1 to 2 {mu}m thick, formed on conducting glass (SnO{sub 2}) and cadmium substrates by electrochemical (potentiostatic) deposition from an aqueous bath. 1 {minus} {ital x} ranged from 0 to 0.25. The photoresponse of CMT electrodes of n- and p-type was investigated in electrolytes containing different redox couples. In 1{ital M} polysulfide solution, the onset of photocurrents was found at {minus}1.1 V vs. SCE, independent of the optical bandgap. In the plateau range of the photocurrent potential curves, quantum yields of charge collection up to 0.45 were measured for Cd{sub 0.9}Hg{sub 0.1}Te. The long-wavelength onset of photoresponse varied with the bandgap (1.47--1.08 eV) for 1 {minus} {ital x} between 0.01 and 0.2. The drawback of the small minority-carrier diffusion length of CMT grown by electrocyrstallization can be circumvented with stacked, semitransparent thin-film electrodes. Experiments show that with such a configuration of thin CMT films grown on transparent substrates, the total photocurrent output can be increased considerably.

Transition metal phosphide semiconductors for their possible use in photoelectrochemical cells and solar chargeable battery (Saur Viddyut Kosh V)

The principle of a solar chargeable battery and the necessity for development of new materials for such a battery are discussed in this paper. A survey of the literature is made for the phosphides of those metals which are cheap and chemically stable in acid and/or alkali. For the first time the phosphides of cobalt, iron, moybdenum, nickel, vanadium and tungsten are considered as suitable materials for use as photoelectrodes in such cells. The preparation and characterization for the band gap, empirical formula, band diagram and photoresponse of these semiconducting materials are discussed in this paper.

Physical properties of electrochemically deposited cadmium mercury telluride films

Abstract Films of cadmium-rich cadmium mercury telluride (CdxHg1−xTe, CMT) 0.1–2 μm thick were formed on conducting glass (SnO2) by electrochemical (potentiostatic) deposition from an aqueous bath. 1−x ranged from 0 to 0.25. Dependingon the mercury content, a preferential orientation of the films was detected by X-ray diffraction studies. The homogeneous composition of the layers was confirmed by electron microprobe and secondary ion mass spectrometry measurements. The optical properties are in accordance with a semiconductor of variable band gap, 1 1−x > 0. CMT layers of n- and p-type were obtained by appropriate choices of deposition potential and annealing conditions, as shown by the photovoltaic effect of CMT/Au junctions.

Potential-Induced Degradation (PID): Incomplete Recovery of Shunt Resistance and Quantum Efficiency Losses

Potential-induced degradation (PID), specifically PID leading to shunts (PID-s), has recently been identified as one of the major field durability issues of photovoltaic (PV) modules. The industry is attempting to address this issue at the module/cell production level by modifying the cell, glass, and/or encapsulant properties, as well as at the system level through the application of reverse potential at night. However, there is a lingering question on the full recovery of the cells through the reverse potential application technique. The results obtained in this study indicate that the near-full recovery of efficiency at high irradiance levels can be achieved, but the full recovery of efficiency at low irradiance levels, shunt resistance, and quantum efficiency (QE) at low wavelengths could not be achieved. The wavelength-dependent QE response after PID and recovery has been modeled based on experimental data. We address the challenge in measuring accurate QE of shunted cells and the input impedance of traditional QE test equipment. A new very low impedance method minimizes, but does not totally eliminate, the scaling error in the QE system data for solar cells that have very low shunt resistances. We also evaluate previously proposed models on the effects of sodium experimentally and through simulation.

Investigation of Dominant Failure Mode(s) for Field-Aged Crystalline Silicon PV Modules Under Desert Climatic Conditions

The first step in developing a life prediction model for photovoltaic (PV) modules is the identification of dominant failure modes/mechanisms for given environmental and operating conditions. Although important, the literature is very scarce. The Jet Propulsion Laboratory (JPL) approach consists of identifying the weakest link in module construction and the failure modes or mechanisms susceptible to the link are considered dominant. The failure mode and effects analysis/failure mode and effects criticality analysis approach, proposed and tried by a few authors, provides a more analytical alternative. It uses the risk priority number (RPN) as a ranking metric for failure modes prioritization. The RPN is a product of three parameters: severity of a failure (S), occurrence of the failure (O), and detection of the failure (D). Typically, the values for S, O, and D are assigned based on qualitative analyses. As such, the values assigned for the failure modes to those factors are highly subjective, leading to considerable variations from one analyst or design team to another. The main objective of this study is to move as far away as possible from this traditionally subjective approach to a formal, objective, and data-driven determination of RPN. The approach described in this paper relies on quantitative measures and sizable datasets. For the hot and dry climatic conditions of Phoenix, Arizona USA, solder bond failures and encapsulant discoloration are found to be the dominant failure modes.