Au Anode Research Articles

Understanding Direct Borohydride – Hydrogen Peroxide Fuel Cell Performance with a Calibrated Numerical Model

Hydrogen peroxide – direct borohydride fuel cells (H2O2-DBFCs) have high theoretical energy density and cell voltage [1-3], yet these desirable characteristics have not been demonstrated in a practical system. Parasitic chemical reactions competing with the desired electrochemical reactions are partly to blame [4]. Efforts to improve H2O2-DBFC performance have been hindered by its complexity; the reactions have many intermediate steps and pathways that are difficult to resolve experimentally [5, 6]. Furthermore, the relative rates of electrode reactions are influenced by transport in the reactant flow channels, where advection, diffusion, and migration occur [7]. A numerical model can make the complexity manageable and offer new insights by revealing relationships that are difficult to measure. To this end, a H2O2-DBFC model has been developed and calibrated to experiments with a single-cell H2O2-DBFC. The calibrated model was used to investigate the relationships between transport in the flow channels, parasitic side reactions, and DBFC performance.In the H2O2-DBFC shown in Fig. 1, a Nafion 117 membrane separates parallel flow channels. The channel walls are graphite electrodes coated with electro-deposited catalysts. Fuel (1-50 mM NaBH4 / 2 M NaOH) is oxidized at the Au anode and oxidizer (10-40 mM H2O2 / 1 M H2SO4) is reduced at the Pd:Ir cathode, while Na+ and H2O cross the membrane.The steady-state finite-difference model includes diffusion, migration, and advection in the flow channels, transport across the membrane, multiple charge transfer reactions at each electrode to yield mixed potentials, and chemical parasitic reactions at each electrode. It is formulated in MATLAB and solved by a modified Newton solver in SUNDIALS.Model features were chosen to reflect experimental observations. For example, experiments showed that low cathode potential caused by oxidizer-lean stoichiometry can lead to H+ reduction at the cathode, therefore this reaction was included in the model. The model was calibrated to experiments by fitting predicted polarization curves to measured curves. Electrode reaction rates were modeled with global expressions that summarized many intermediate steps, and the fitted parameters were the rate constants in those global expressions. Predicted vs. measured polarization curve comparisons showed good agreement (R2 ≥ 0.920) over the fuel cell operating space. The predicted and measured limiting current densities agreed, indicating transport was modeled accurately. Results from the calibrated model show compact concentration boundary layers in both channels, which limit reactant utilization and current density. Transport limits are most pronounced at the cathode because BH4 - has higher diffusivity than H2O2, BH4 - transport is aided by migration, and BH4 - can provide up to 8e- while H2O2 only consumes up to 2e-. Decreasing inlet concentrations, flow rates, and cell voltage result in lower losses to parasitic side reactions as these changes decrease reactant concentrations near the electrodes, which favors charge transfer reactions. Peak conversion efficiency coincides with peak power density because thermodynamic efficiency and parasitic reaction rates both decrease (relative to charge transfer reaction rates) with increasing current density. This work shows that advection, migration and diffusion must be included in H2O2-DBFC models, and that global reactions are sufficient to capture the essential features of complex H2O2-DBFC reactions.

A calibrated hydrogen-peroxide direct-borohydride fuel cell model

A numerical model with global reaction rates is calibrated to measurements from a simple hydrogen-peroxide direct-borohydride fuel cell (H2O2-DBFC), and then used to unravel complex electrochemical and competing parasitic reactions. In this H2O2-DBFC, fuel (1–50 mM NaBH4/2 M NaOH) is oxidized at a Au anode and oxidizer (10–40 mM H2O2/1 M H2SO4) is reduced at a Pd:Ir cathode. Polarization curves and electrode potentials, as functions of fuel and oxidizer feeds support global reaction rate parameter fitting. The measurements and calibrated model showed H2O2 decomposition at the cathode depresses open circuit voltage from 3.01 V theoretical to 1.65 V, and when H2O2 supply is limited, cathode potentials are sufficiently negative to make H+ reduction to H2 thermodynamically favorable. Calibrated model results show that thin concentration boundary layers limit reactant utilization and current density. Decreasing the inlet concentrations, flow rates, and cell voltage slow parasitic reactions and favor desirable charge transfer reactions. Peak conversion efficiency and peak power density coincide because thermodynamic efficiency and parasitic reaction rates decrease (relative to charge transfer reaction rates) with increasing current density. We conclude that the performance of a fuel cell with parasitic side reactions can be predicted through numerical modeling.

Visualization of Electrode–Electrolyte Interfaces in LiPF6/EC/DEC Electrolyte for Lithium Ion Batteries via in Situ TEM

We report direct visualization of electrochemical lithiation and delithiation of Au anodes in a commercial LiPF6/EC/DEC electrolyte for lithium ion batteries using transmission electron microscopy (TEM). The inhomogeneous lithiation, lithium metal dendritic growth, electrolyte decomposition, and solid-electrolyte interface (SEI) formation are observed in situ. These results shed lights on strategies of improving electrode design for reducing short-circuit failure and improving the performance of lithium ion batteries.

Effect of Mn and Cu Addition on Lithiation and SEI Formation on Model Anode Electrodes

The effect of addition of Mn2+ to the electrolyte on the lithiation of a model battery anode was studied using voltammetry, electrochemical quartz crystal microbalance (EQCM), scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Cyclic voltammetry of a Au anode showed that the presence of one equivalent monolayer of Mn2+ in the electrolyte reduces the battery capacity by as much as 48%, a result which is recapitulated in SEM images of the anode surface after five cycles which show that the presence of Mn2+ blocks lithiation of the anode. AES demonstrates the presence or lack of Mn on the surface. EQCM analysis demonstrates greater initial mass gain on the anode with increasing concentration of Mn2+ in the electrolyte while MALDI-TOF MS shows no observable differences in the solid electrolyte interface (SEI) between the electrolyte with or without Mn2+. The addition of Cu2+ to the electrolyte exhibits an effect similar to Mn2+ addition.

Effect of contact preparation on the profile of the electric field in CdZnTe detectors

We used the transient-current technique to delineate the effects of different contact metals on the formation of space charge and on the profiles of the electric field in CdZnTe radiation detectors. In contrast to existing results on the polarization of semiconductor radiation detectors, we find that detectors with ohmic (Au/Au) contacts may experience a larger distortion of the internal electric field than those having Schottky (Au/In) contacts. We explain this difference by postulating the presence of a deep hole trap, ET, below the Fermi energy EF, which captures holes generated by the weakly injecting Au anode. The observed behaviour was described successfully by a numerical model relating the energy difference between ET and EF and the band bending at the contacts. We also present results on aging effects that limit the detector's performance and stability over long times.

Self-Poisoning during BH4– Oxidation at Pt and Au, and in Situ Poison Removal Procedures for BH4– Fuel Cells

Borohydride (BH4–) is rapidly attracting attention as an alternative fuel molecule for fuel cells due to its high gravimetric and volumetric energy densities, high power density, and avoidance of the carbon monoxide (CO) poisoning seen in hydrogen (H2) and methanol (MeOH) fuel cells. Here we describe an as-yet unreported poisoning process that occurs during BH4– oxidation at Pt and Au anodes at low potentials (high fuel cell voltages). Though such poisoning can compromise the long-term performance of BH4– fuel cells, we also demonstrate an in situ cleaning procedure for a BH4– fuel cell using a Pt anode.

Flexible Top Emission Organic Light Emitting Diodes with Ni and Au Anodes Deposited on a Cellulose Paper Substrate

Cellulose paper was prepared and utilized as a substrate for flexible top emission organic light-emitting diodes (TE-OLEDs). We successfully fabricated flexible TE-OLEDs with Ni and Au anodes on SiO2/cellulose paper substrate. The SiO2 barrier layer coated on the cellulose paper substrate enhanced the surface roughness of the cellulose paper substrate. The surfaces of 100 nm thick Ni and Au films on SiO2/cellulose paper substrate become continuous on the SiO2/cellulose paper substrate, resulting in low sheet resistance of the Ni and Au films. While the maximum luminance of the flexible TE-OLED with Au anode was about 3516 cd/m2 at a driving voltage of 12 V, that of the flexible TE-OLEDs with Ni anode was 1280 cd/m2 at a driving voltage of 10 V.

Improved Performance of ITO-Free Organic Solar Cells Using a Low-Workfunction and Periodically Corrugated Metallic Cathode

The indium–tin-oxide-free organic solar cells (OSCs) have been fabricated using a semitransparent Au film as anode. A comparative study between the OSCs with different metal cathodes shows that formation of an ideal ohmic contact at low-workfunction Ca and the organic interface reduces series resistance and increases the carrier collection. Moreover, a periodic corrugation was introduced into the device structure, which leads to an enhanced light absorption in the OSCs by elongating the optical path of the incident light inside the absorber material. The highest power conversion efficiency of an optimal OSC with a Au anode and a corrugated Ca/Ag cathode is 1.4%, which is significantly improved in contrast to 0.61% for the OSC with a planar Al cathode.

Facilitated Electro-Oxidation of Formic Acid at Nickel Oxide Nanoparticles Modified Electrodes

This study addresses the electrocatalytic oxidation of formic acid (FA) at nickel oxide nanoparticles (nano-NiOx) modified Pt, Au, GC anodes. FA oxidation proceeds at the unmodified Pt electrode with the appearance of two oxidation peaks at 0.25 and 0.65 V corresponding to the direct oxidation of FA to CO2 and the oxidation of the poisoning intermediate, CO, to CO2 with a current ratio of the two peaks less than 0.2. Interestingly, this ratio jumps up to more than 50, upon modifying Pt with nano-NiOx. This highlights the essential role of NiOx in enhancing the direct oxidation of FA (at 0.25 V) at Pt substrate. On the other hand, unmodified GC and Au anodes as well as those modified with nano-NiOx exhibit no catalytic response toward FA oxidation. This highlights the essential role of the underlying substrate and depicts also that nano-NiOx behaves as a catalytic mediator which facilitates the charge transfer during the oxidation of FA at Pt anode. Optimization of the surface coverage of nano-NiOx at Pt is achieved, aiming at maximizing the rate of the direct oxidation pathway of FA while suppressing the indirect oxidation route producing the poisoning CO. Moreover, nano-NiOx/Pt anode maintains its high catalytic activity for a prolonged time of continuous oxidation of FA.

Enhanced performance and stability of inverted organic solar cells by using novel zinc-benzothiazole complexes as anode buffer layers

Performance of inverted bulk-heterojunction solar cells with widely used tris(8-quinolinolate)aluminium(III) (Alq3) is compared with a series of novel zinc-benzothiazole (Znb2) derivatives as buffer layer. The devices including a Znb2 thin layer between the poly(3-hexylthiophene) (P3HT):C61-butyric acid methyl ester (PCBM) blend and the Au anode show a significant improvement in the power conversion efficiency (η), which is enhanced by 30% compared to Alq3cells. Moreover, by combining both Alq3 and Znb2 in the device as a double buffer layer prior to the metal electrode deposition, the efficiency improves by 40%. The reasons for the enhanced performance of Znb2 cells are attributed to the efficient charge transport and electron/exciton blocking properties. Furthermore it is expected that the deposition of Znb2 modifies the Au work function to facilitate the hole transport and collection at the anode, and encapsulate the P3HT/PCBM blend during the electrode deposition. The presented photovoltaic cells also show high stability in ambient air conditions over a period of 245 days, which evidences the need of Znb2 buffer layers for long-term device durability.

Energy level alignments and photocurrents in crystalline Si/organic semiconductor heterojunction diodes

We investigate electronic energy level alignment and photocurrent in crystalline silicon/organic/semitransparent metal heterojunction diodes. Optically thin films of poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), poly(9,9-dioctylfluorene) [PFO], pentacene (Pc), and C60 were deposited on n and p type Si wafers and diode structures were formed by depositing either a Au anode or Al cathode onto the organic film. The energy level alignment was assessed using built-in potential and capacitance-voltage measurements. In all cases, the results are consistent with near ideal vacuum energy level alignment between the organic and inorganic semiconductor. The diode current-voltage (I-V) characteristics are consistent with the electronic structure of the heterojunction interface. For n-Si/MEH-PPV/Au, the I-V curves are quantitatively described by an organic device model. For photocurrent measurements the diodes were illuminated through the semitransparent metal contact with optical wavelengths from 350–1100 nm. The photocurrent in the diode structure can be due to absorption either in the organic layer or Si substrate. For n-Si diodes, the 0 bias photocurrent is small with external quantum efficiencies (EQEs) less than 5×10−3 in all cases. The photocurrent is dominated by absorption in the organic layer for MEH-PPV, PFO, and C60 and by absorption in Si for Pc. For p-Si diodes, the 0 bias photocurrent is large with EQEs of ∼0.2 and is dominated by absorption in silicon for all organic layers. Both MEH-PPV and PFO form type I heterostructures with Si and photocurrent due to organic exciton dissociation is less efficient than in commonly used type II organic/organic heterostructures. Silicon/Pc and C60 heterojunctions are most likely type II with small valence (Pc) or conduction (C60) energy level differences. Surprisingly, no photocurrent was observed due to optical absorption in Pc most likely due to a chemical reaction between Pc and Si that prevented exciton dissociation at the heterojunction interface. In n-type/C60 structures photoconductive gain with EQE >3 was obtained and in p-type structures, the EQE was >0.15 and the spectral response of the photocurrent could be changed significantly from broad band absorption in Si to relatively narrow band absorption in C60.

Hyperthermal atomic hydrogen and oxygen etching of vertically oriented graphene sheets

Carbon nanosheets have previously been shown to be promising high current field emission cathodes for a variety of potential applications. The vertically oriented planar sp2 carbon nanosheets grown by rf plasma-enhanced chemical vapor deposition terminate with one to seven graphene sheets and grow to ∼1 μm in height. High current field emission, Je∼0.15 mA mm−2 (8 V μm−1), conducted within an ultrahigh vacuum system in a diode configuration in line-of-sight to a mass spectrometer, shows that CH4, CO2, and CO are generated as a result of cathode bombardment by hyperthermal oxygen and hydrogen neutrals and ions generated by electron stimulated desorption at the Cu anode. Confirmation of the mechanism was achieved by repeating the experiments using a Au anode. Simultaneous acquisition of I-V data and the partial pressures of reaction products in the mass spectrometer have shown repeatable, sustained CH4, CO2, and CO production. As these hyperthermal atomic hydrogen and oxygen species impinge on the sidewalls and edges of the carbon nanosheets, they bond to various sites throughout the sp2 carbon array. Progressively, as further hydrogen and oxygen arrive, CH4, CO2, and CO are formed and desorbed, thereby etching the film. Raman spectroscopy has confirmed a corresponding increase in defect sites (ID/IG increased from 0.57 to 0.81) over the test interval. Scanning electron microscopy cross sections of carbon nanosheet cathodes before and after high current lifetime testing (>200 h) show the average etching rate to be ∼1.7×10−3 nm/s.

Top-emission Si-based phosphor organic light emitting diode with Au doped ultrathin n-Si film anode and bottom Al mirror

We report a highly efficient top-emission Si-based phosphor organic light emitting diode (PhOLED) with an ultrathin polycrystalline n-Si:Au film anode and a bottom Al mirror. This anode is formed by magnetron sputtering followed by Ni induced crystallization and then Au diffusion. By optimizing the thickness of the n-Si:Au film anode, the Au diffusion temperature, and the other parameters of the PhOLED, the highest current and power efficiencies of the n-Si:Au film anode PhOLED reached 85±9 cd/A and 80±8 lm/W, respectively, corresponding to an external quantum efficiency of 21±2% and a power conversion efficiency of 15±2%, respectively, which are about 60% and 110% higher than those of the indium tin oxide anode counterpart and 70% and 50% higher than those of the bulk n+-Si:Au anode counterpart, respectively.

Development of inverted OLED with top ITO anode by plasma damage-free sputtering

A newly-developed damage-free sputtering system, Hyper-thermal Neutral Beam (HNB) sputtering, was evaluated for the production of organic light emitting diode (OLED) devices. An indium tin oxide (ITO) pixel layer as an anode electrode was deposited directly on the Hole Transport Layer (HIL) of an Inverted Bottom emission OLED (IBOLED) structure. Two types of IBOLED devices were fabricated to assess the level of process damage by the HNB sputtering deposition process. The characteristics of the leakage current at reverse bias and the UV exposure test confirmed that HNB ITO sputtering does not damage the underlying organic layers in the IBOLED device with the top ITO anode deposited by HNB sputtering, which is in comparison with a normal IBOLED with a top Au anode deposited by conventional thermal evaporation. However, the IBOLED device using HNB sputtering process showed some degradation of the turn-on characteristics and current efficiency. This degradation was not induced by damage from the HNB sputtering process but was generated by the permeation of Fe impurities from the stainless steel chamber and the low conductivity of HNB sputtered ITO thin film.

Organic photodetector arrays with indium tin oxide electrodes patterned using directly transferred metal masks

We demonstrate the patterning of indium tin oxide (ITO) electrodes with submicron resolution onto both planar and hemispherical plastic surfaces using a process based on the direct transfer of a metal film etch mask via cold welding. Using this process, we fabricate an organic (copper phthalocyanine/C60) photodetector focal plane array on a hemispherical surface in an architecture that mimics the size and form of the human eye. The low absorption of the ITO anodes results in an increase in external quantum efficiency by up to 65% in the visible as compared with that of a previously demonstrated photodetector array with semitransparent Au anodes. The maximum photodetector detectivity is 1.25×1011 cm Hz1/2 W−1.

Hole Injection in a Model Fluorene–Triarylamine Copolymer

AbstractRecent developments in synthesis and purification have yielded conjugated polymers with hole mobilities exceeding 0.01 cm2 V−1 s−1. Essential to harvesting the potential of these materials in organic light emitting diodes (OLEDs) is the identification of suitable ohmic contacts. Using a model fluorene copolymer that shows high‐mobility, non‐dispersive hole transport, it is demonstrated that electrodes commonly used as anodes in OLEDs are very poor hole injectors. Injection from Au and indium tin oxide anodes is limited by energy barriers of 0.75 and 0.65 eV, respectively, and the injected current is found to be temperature independent—a prediction that was not reproduced by the leading injection model for disordered organic semiconductors. Injection from a poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) anode, on the other hand, is found to become less efficient with electric field, a behavior which is currently not understood. In thinner poly[(9,9′‐dioctylfluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐sec‐butyl))diphenylamine)] films, which are of relevance to OLEDs, ohmic losses on the PEDOT:PSS layer are found to limit the flow of current. These results illustrate the opportunity to further improve the performance of OLEDs as well as the challenge posed by high mobility conjugated polymers for the design of hole injection layers.

A First Principles Study of Direct Oxidation of Aqueous Borohydride on Au and Pt Surfaces

Direct Borohydride Fuel Cells (DBFCs) have the potential to generate high power densities for use in portable power applications. Despite the potential of DBFC's, current applications are limited in part, by the lack of an effective anode electrocatalyst. Though a number of pure metals and alloys have been tested as anodes, gold and silver electrodes are uniquely capable of completing direct oxidation at near 100% coulombic efficiency. Overpotentials on Au anodes, however, limit the overall cell efficiency and little is known of the elementary electrocatalytic mechanisms, reaction intermediates, or limiting elementary steps . In this study, we apply density functional theory (DFT) calculations to examine borohydride electro-oxidation over the Au(111), and compare with Pt(111) surfaces for which non-selective hydrolysis paths compete with direct oxidation. Possible stable surface intermediates and limiting elementary steps are identified and results are discussed in comparison with experimental data from the literature.

Indium Tin Oxide Modified by Au and Vanadium Pentoxide as an Efficient Anode for Organic Light-Emitting Devices

Au/Vanadium pentoxide (V2O5) films on indium tin oxide (ITO) as composite anodes for hole injection in organic light-emitting devices (OLEDs) have been investigated. The device with ITO/Au(6 nm)/V2O5 (6 nm) anode shows improved current density-voltage characteristics as compared with the device with ITO/Au as anode. Hole injection is significantly reduced when Au was added on ITO as an anode. However, while a thin V2O5 film is deposited on a Au anode, the barrier height is substantially reduced. The hole injection is facilitated, and the driving voltage of the device decreased by 10 V. In addition, the maximum current efficiency for the ITO/Au/V2O5 OLED is ~3.5 cd/A, which is higher than that of the ITO/Au anode device (~1.1 cd/A) and ITO/V2O5 OLED of ~2.8 cd/A.

A direct borohydride fuel cell based on poly(vinyl alcohol)/hydroxyapatite composite polymer electrolyte membrane

A new poly(vinyl alcohol)/hydroxyapatite (PVA/HAP) composite polymer membrane was synthesized using a solution casting method. Alkaline direct borohydride fuel cells (DBFCs), consisting of an air cathode based on MnO 2/C inks on Ni-foam, anodes based on PtRu black and Au catalysts on Ni-foam, and the PVA/HAP composite polymer membrane, were assembled and investigated for the first time. It was demonstrated that the alkaline direct borohydride fuel cell comprised of this low-cost PVA/HAP composite polymer membrane showed good electrochemical performance. As a result, the maximum power density of the alkaline DBFC based on the PtRu anode (45 mW cm −2) proved higher than that of the DBFC based on the Au anode (33 mW cm −2) in a 4 M KOH + 1 M KBH 4 solution at ambient conditions. This novel PVA/HAP composite polymer electrolyte membrane with high ionic conductivity at the order of 10 −2 S cm −1 has great potential for alkaline DBFC applications.

The Effects of H2 and Isotopically Labeled HD and D2 Fuels with Combinations of H2O and D2O on SOFC Au Anode Performance

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