Slow Electron Transfer Research Articles

Solvothermal Synthesis of a Hollow Micro-Sphere LiFePO₄/C Composite with a Porous Interior Structure as a Cathode Material for Lithium Ion Batteries.

To overcome the low lithium ion diffusion and slow electron transfer, a hollow micro sphere LiFePO4/C cathode material with a porous interior structure was synthesized via a solvothermal method by using ethylene glycol (EG) as the solvent medium and cetyltrimethylammonium bromide (CTAB) as the surfactant. In this strategy, the EG solvent inhibits the growth of the crystals and the CTAB surfactant boots the self-assembly of the primary nanoparticles to form hollow spheres. The resultant carbon-coat LiFePO4/C hollow micro-spheres have a ~300 nm thick shell/wall consisting of aggregated nanoparticles and a porous interior. When used as materials for lithium-ion batteries, the hollow micro spherical LiFePO4/C composite exhibits superior discharge capacity (163 mAh g−1 at 0.1 C), good high-rate discharge capacity (118 mAh g−1 at 10 C), and fine cycling stability (99.2% after 200 cycles at 0.1 C). The good electrochemical performances are attributed to a high rate of ionic/electronic conduction and the high structural stability arising from the nanosized primary particles and the micro-sized hollow spherical structure.

(Invited) Carbon Nanotube-Based Microelectrodes for Enhanced Neurochemical Detection

Carbon-fiber microelectrodes (CFMEs) have been traditionally used with fast scan cyclic voltammetry (FSCV) to detect neurochemicals in vivo. However, there are several challenges in detecting neurochemical changes using bare CFMEs. First and foremost, fouling has been shown to significantly decrease sensitivity through the formation of biopolymers from serotonin and other analytes that block sites for analyte absorption. The loose sheets of graphene that make up the carbon fiber also have relatively low conductivities, which result in slow electron transfer for electrode materials and low temporal resolution. Carbon Nanotubes (CNTs) have shown to be promising for use as electrode materials. Carbon nanotubes (CNTs) are highly sp2 hybridized, which is very advantageous for measuring fast neurochemical changes in vivo. The CNTs also contain a high percentage of edge-plane carbon that is the catalytic site for neurotransmitter oxidation, which is fouling resistant. Furthermore, the cylindrical structure of CNTs provide a high aspect ratio, which allows for more sites for analyte adsorption and high sensitivity to detect low basal levels of neurochemicals. Several methods have been developed to incorporate CNTs onto CFMEs such as dipcoating; however, the method is highly variable and produces noise. Wetspinning CNTs with polymers or acids to form CNT fiber has been shown to be highly efficacious in developing novel electrode materials. For example, poly(ethyleneimine) CNT fiber microelectrodes were fouling resistant to both serotonin and 5-hydroxyindoleacetic acid (5-HIAA). They were also able to detect exogenously applied serotonin onto brain slices. Acid spun CNT fiber microelectrodes were formed with either sulfuric acid or chlorosulfonic acid that preclude the use of sonication, polymers, or surfactants, which act as impurities that reduce sensitivity.

New Fluorinated Carbon Materials for Binder-Free Ultra-High Capacity Cathodic Materials

Low capacity of current cathodic materials is a bottle-neck for the next generation electrochemical energy storage devices. LiMO2, LiMPO4, and mixed metal oxide lithium salts are commonly used in current lithium batteries. The heavy nature of these transition metal elements limits their capacity below 300 mAh/g. Fluorinated carbon materials are mainly used in primary lithium batteries since the 1970s. It is a high capacity cathodic material that surfers low conductivity and slow electron transfer kinetics. The preparation method, high temperature reaction between graphite and fluorine gas, also possesses significant safety threat for the production of current carbon fluoride materials. To solve the low conductivity and low discharge rate problem for carbon fluoride based cathodic materials, we designed a new type of fluorinated carbon materials that integrate conducting polymers, electron-transfer catalysts, and energy storage units, yet still have the similar composition of carbon fluoride to maintain the high specific capacity. Thus, this materials does not require additional mechanical binder and conducting materials for the cathode. The materials were characterized by electrochemical, spectroscopic, and scanning electron microscopic (SEM) methods. Electrochemical experiments were done in an Argon filled glovebox with residual O2 level less than 1 ppm. Three electrode systems were used for the characterization of this carbon fluoride cathodic materials on glassy carbon (GC) and aluminium electrodes in both 1,2-difluorobenzene (DFB)/TBAPF6 and propylene carbonate (PC)/LiPF6solution. Figure 1 shows galvanostatic discharge curve of the new carbon fluoride materials on GC WE in PC/1.0 M LIPF6 with two pieces of Li foil as CE and RE respectively at C/8 rate. The discharge result shows that this new cathodic material possesses specific capacity of 1.03 Ah/g with average voltage of 2.1 V (vs. Li+/Li) at 0.125 C rate, giving specific energy of 2.16 Wh/g. We also observed that the galvanostatic discharge of this material does not involve an initial voltage dipping as observed with current carbon fluoride materials. Searching and optimizing the electrolyte solution for rechargeable application is currently underway in this laboratory. We hope that this presentation will shed new light to the discovery of new cathodic materials. Figure 1

Characterization of OmcA Mutants from Shewanella oneidensis MR‐1 to Investigate the Molecular Mechanisms Underpinning Electron Transfer Across the Microbe‐Electrode Interface

AbstractElectricity production in microbial fuel cells (MFCs) is an emerging green alternative to the use of fossil fuels. Shewanella oneidensis MR‐1 (SOMR‐1) is a Gram‐negative bacterium, adapted to MFCs due to its ability to link its bioenergetic metabolism through the periplasm to reduce extracellular electron acceptors. OmcA is a highly abundant outer‐membrane cytochrome of SOMR‐1 cells and is involved in the extracellular electron transfer to solid acceptors and electron shuttles. To investigate electron transfer performed by OmcA towards final acceptors, site directed mutagenesis was used to disturb the axial coordination of hemes. Interactions between OmcA and redox partners such as iron and graphene oxides, and electron shuttles were characterized using nuclear magnetic resonance and stopped‐flow experiments. Results showed that solid electron acceptors do not come into close proximity to the hemes, in agreement with experimentally observed slow electron transfer. In contrast, mutation of the distal axial ligand of heme VII changes the driving force of OmcA towards electron shuttles and reduces the affinity of the FMN:OmcA complex. Overall, these results reveal a functional specificity of particular hemes of OmcA and provide guidance for the rational design of mutated SOMR‐1 strains optimized for operating in different microbial electrochemical devices.

Simple Graphite and Thermoplastic Mixtures for Complicated Electrochemical Systems

A new carbon composite electrode will be presented. The advantages of composite electrodes include low cost, ease of catalyst incorporation, and facile fabrication. Persistent common problems with composite electrodes include low conductivity, sluggish electron transfer, difficulty in patterning, and complications with incorporation into advanced sensors such as microfluidics. To overcome these issues a solvent assisted process was developed to fabricate thermoplastic electrodes (TPEs) using polymethylmethacrylate (PMMA) combined with graphite. After fabrication, we tested the ability to pattern these electrodes using common manufacturing processes. It was discovered that these materials maintained the properties of the thermoplastic binder, enabling the electrodes to be patterned with channels and pillars of micron dimensions using hot embossing. The “chewing gum” like consistency of the semi-dried electrode material lends itself to simple templating techniques, were the shape and arrangement of the electrode is fully customizable. In this work, a CO2 laser used for templating electrode dimensions of ~150 µm, the resolution limit of the laser. The CO2 laser allowed for easy fabrication of wall-jet, band, and individually addressable microelectrode arrays. The entire process is easily scaled up and can create tens to hundreds of working electrodes in a single batch, or very large centimeter to meter sized electrodes. Electrode characterization was performed with electrochemistry, conductivity measurements, and various forms of spectroscopy (XPS, SEM/EDS, Raman, XRD). Emphasis was placed on understanding the relationship between particle loading and particle size. The particle loadings ranged from 20 to 85 mass % graphite, using graphite particle sizes of 0.5 to 100 µm. Interestingly, adjusting these simple parameters had an enormous impact on both electrochemical performance and conductivity. High conductivities of ~800 S/m were achieved with 11µm particles, as well as stable and enhanced electrode kinetics, while 0.5 and 100 mm particles generated lower conductivity and somewhat slower electron transfer. Initial experiments with the Fe(CN)6 3-/4- redox couple using impedance spectroscopy and cyclic voltammetry show that the TPE electrode out performs a DropSens commercial screen printed electrode as well as glassy carbon. The low charge transfer resistance of the TPE (<5 Ωcm2) more resembles the kinetics of a platinum electrode. The capacitance and solvent window were also measured, as they are critical parameters of the performance of electrochemical sensors. Both properties were highly dependent on the particle size, graphite purity, and graphite mass loading. An in-depth electrochemical investigation of the electrode surface following the redox mediator flow chart of McCreery was performed to give insight into the chemical composition of the electrode surface. Evidence for surface oxides (quinones), metallic impurities, and both basal and edge character from the graphite lattice were observed. The enhanced electrochemical reactivity of TPEs towards common analytes of dopamine, ascorbic acid, urate, NADH, thiols, and hydrazine is similar to that of carbon nanotubes and graphene. A major finding of this work is that electrochemical activity of the TPE can be tuned when the surface is activated through sanding, polishing or plasma cleaning. Finally, as a proof of concept, electrochemical cells were fabricated to analyze the oxidative load of aerosol particles through the use of the dithiothreitol(DTT) assay. The TPE devices enable ultralow volume sampling (~300 µL) by utilizing a microelectrode array. The high activity of the electrodes realized electrochemical sensing without the need for the commonly used cobalt phthalocyanine catalyst. This unique TPE sensing system achieved unprecedented resolution of the DTT Assay, allowing for measurements to be taken at ~10 second intervals. The rugged, reusable, complex sensor required less than one man hour to fabricate, with a materials cost of <$1. Figure 1

Oxygen Electrocatalyst Based on Non-Precious Bifunctional Hybrid Material

Synthesis of new functional materials for the development of energy conversion and storage devices like fuel cells, metal-air batteries received significant interest in the last decade. The limited availability of traditional fossil fuel demands renewable energy resources. The electrochemical oxygen reduction and oxygen evolution reaction are the key processes in energy conversion and storage devices like metal-air batteries. The development of rechargeable metal-air batteries requires efficient bifunctional electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Traditionally, Pt-, Ru- and Ir-based materials have been used for ORR and OER.1 The large overpotential due to the slow electron transfer kinetics, high cost and the electrochemical instability of these traditional precious metal-based electrocatalysts demands for active non-precious bifunctional catalysts. Moreover, the use of these traditional catalysts as an oxygen electrode is limited as they are not able to catalyse both the ORR and OER simultaneously. In the recent past, the oxides of transition metals such as Co, Mn, etc. including mixed valence spinel and perovskites have been proposed as alternate catalyst for these reactions.2 However, the poor electronic conductivity and the recyclability of these oxides limits their potential application.3 Recently, the hybrid electrocatalyst based on carbon and transition metal catalyst is emergingfor ORR and OER reactions.4 Our group is interested in the development of new functional materials for fuel cell and supercapacitor applications.2a,5 In continuation of our earlier efforts, herein we demonstrate the synthesis of two types of low-cost transition metal (Co, Fe) based hybrid bifunctional electrocatalyst. The two catalysts cobalt and nitrogen dual-doped carbon (Co-N-C) and reduced graphene oxide-CoFe (rGO-CoFe) were synthesized by chemical method followed by thermal annealing at an optimized temperature. The Co-N-C catalyst was obtained from cobalt tetra-aza macrocycle (Co-TET-A) whereas the rGO-CoFe catalyst was synthesized from multimetal complex, cobalt hexacyanoferrate. Both the catalysts were characterized by analytical techniques including XPS, TEM and XRD. The electrocatalytic response towards ORR and OER was evaluated by hydrodynamic voltammetry. The Co-N-C and rGO-CoFe catalysts show the limiting current density of -4.37 mA/cm2 and -5.58 mA/cm2, respectively and both the catalysts promote the 4-electron pathway for the reduction of oxygen. The OER activity was evaluated by measuring the overpotential at which 10 mA/cm2 current density was observed. Interestingly both Co-N-C and rGO-CoFe catalysts show the benchmark current density of 10 mA/cm2 at a low overpotential of 0.28 V and 0.22 V, respectively. The overall oxygen electrode activities were evaluated by calculating the difference in potential (ΔE) between the OER current density at 10 mA/cm2 and the ORR current density at −3 mA/cm2. We could achieve a ΔE of 0.92 V and 0.82 V on Co-N-C and rGO-CoFe catalyst respectively, confirming the efficient bifunctional activity of these catalysts. It is demonstrated that the synergistic effect between the metal and carbon and the porous nature of the catalysts plays an important role in the bifunctional activity. The detailed synthetic and bifunctional activity will be discussed.

Effective Area and Charge Density of Iridium Oxide Neural Electrodes

The effective electrode area and charge density of iridium metal and anodically activated iridium has been measured by optical and electrochemical techniques. The degree of electrode activation could be assessed by changes in electrode colour. The reduction charge, activation charge, number of activation pulses and charge density were all strongly correlated. Activated iridium showed slow electron transfer kinetics for reduction of a dissolved redox species. At fast voltammetric scan rates the linear diffusion electroactive area was unaffected by iridium activation. At slow voltammetric scan rates, the steady state diffusion electroactive area was reduced by iridium activation. The steady state current was consistent with a ring electrode geometry, with lateral resistance reducing the electrode area. Slow electron transfer on activated iridium would require a larger overpotential to reduce or oxidise dissolved species in tissue, limiting the electrodes charge capacity but also reducing the likelihood of generating toxic species in vivo.

Electrochemical modification of a pyrolytic graphite sheet for improved negative electrode performance in the vanadium redox flow battery

Abstract The vanadium redox flow battery is a promising technology for buffering renewable energies. It is recognized that negative electrode is the limitation in this device where there are problems of slow heterogeneous electron transfer (HET) of V3+/2+ and parasitic H2 evolution. Any methods aimed at addressing one of these barriers must assess the effects on the other. We examine electrochemical enhancement of a common commercially available material. Treatment of Panasonic pyrolytic graphite sheets is through oxidation at 2.1 V vs. Ag/AgCl for 1 min in 1 M H2SO4. This increases the standard HET rate for V3+/2+ from 3.2 × 10−7 to 1 × 10−3 cm/s, one of the highest in literature and shifts voltammetric reductive peak potential from −1.0 V to −0.65 V in 50 mM V3+ in 1 M H2SO4. Infrared analysis of the surfaces indicates formation of C OH, C O, and C O functionalities. These groups catalyze HET with V3+/2+ as hypothesized by Skyllas-Kasacos. Also of significance is that electrode modification decreases the fraction of the current directed towards H2 evolution. This proportion decreases by two orders of a magnitude from 12% to 0.1% as measured at the respective voltammetric peak potentials of −1.0 V (pristine) and −0.65 V (modified).

Cytochrome P450-3A4/Copper-Poly(Propylene Imine)-Polypyrrole Star Co-Polymer Nanobiosensor System for Delavirdine - A Non-Nucleoside Reverse Transcriptase Inhibitor HIV Drug

HIV and AIDS are among the world’s pandemics that pose serious concern to almost every individual in the world. With the current level of availability of anti-retroviral (ARV) drugs and the ease of accessibility of treatment in many countries such as South Africa, the disease can be controlled by suppressing the viral load of an infected individual. These anti HIV drugs such as delavirdine are metabolised by enzymes which are found in the liver microsomes, particularly those of the cytochrome P450 family. Due to the fact that the metabolic rate of a patient determines the effect of the drug, the drug could either have a beneficial or an adverse effect once it is administered. It is therefore imperative that the metabolic profile of a patient is determined to ensure proper dosing of the ARV drugs. In this study a nanobiosensor system was devised and used for the determination of the metabolism of delavirdine (DLV), a non-nucleoside reverse transcriptase inhibitor (NNRTI) ARV drug. The nanobiosensor was prepared by the entrapment of the isoenzyme CYP3A4 into a pre-formed electro active carrier matrice consisting of a dendrimeric copper generation-2 poly(propylene imine)-co-polypyrrole star copolymer (Cu(G2PPI)-co-PPy). The metallo-dendrimer was used as a host for the enzyme and provided the necessary bio-compatible environment that allowed the direct transfer of electrons between the enzyme’s active centres and platinum electrode surface. (Cu(G2PPI)-co-PPy) was prepared by the incorporation of the copper metal into the G2PPI and the electropolymerization of pyrrole onto the Cu(G2PPI). The incorporation of Cu into G2PPI was determined by Fourier transform infrared (FTIR) spectroscopy which did not show the presence of the Cu but showed an increase in the intensities of the peaks after the incorporation. The surface morphology of Cu(G2PPI-2Py) was confirmed by the use of high resolution scanning electron microscopy (HRSEM) which showed a difference in the surface morphology of the dendrimer moiety with the addition of the copper metal. The HRSEM images after Cu incorporation resulted in the change from rough surface to smooth surface with open cavities which were essential for the entrapment of the biological systems (CYP3A4). The energy band gap of (Cu(G2PPI)-co-PPy) were determined to be 3.85 eV, signifying that the copolymer is characteristic of a biocompatible semiconductive platform for applications in biosensors. The star copolymer (Cu(G2PPI)-co-PPy) was characterized using cyclic voltammetry where it was confirmed that the material was electroactive and conducting due to electron movement along the polymer chain. A diffusion co-efficient (Do) value of 8.64 x 10-5 cm2/s was determined for the material indicating a slow electron transfer kinetics within the diffusion layer. The resultant nanobiosensor parameters include a dynamic linear range (DLR) of 0.01-0.06 nM, a limit of detection (LOD) of 0.025 nM and a sensitivity value of 0.379 μA/nM.

Enzyme Immobilization and Mediation with Osmium Redox Polymers.

Enzymatic electrodes are becoming increasingly common for energy production and sensing applications. Research over the past several decades has addressed a major issue that can occur when using these biocatalysts, i.e., slow heterogeneous electron transfer, by incorporation of a redox active species to act as an electron shuttle. There are several advantages to immobilizing both the enzyme and mediator at the enzyme surface, including increased electron transfer rates, decreased enzyme leaching, and minimized diffusion limitations. Redox polymers consisting of a redox active center attached to a polymer backbone are a particularly attractive option because they have high self-exchange rates for electron transfer and tunable redox potential. Osmium (Os) polymers are the most well studied of this type of polymer for bioelectrocatalysis. Here, we describe the methods to synthesize one of the most common Os redox polymers and how it can be used to fabricate glucose oxidase electrodes. Procedures are also outlined for evaluating the enzymatic electrodes.

Electron Transport Behavior Analysis of Bis(terpyridine) Metal Complex Wires on Electrodes

Molecular wires with rapid and smooth charge transport ability are desired to achieve high performance and low energy consumption of molecular electronic devices. Various kinds of molecular wires with superior electron transport abilities have been reported. They exhibit the tiny dependencies of electron transfer rate constants on the wire lengths, in other words the small attenuation factors, β. We have reported that bis(terpyridine)metal complex [M(tpy)2] (M=Fe, Co, tpy=2,2’:6’,2’’-terpyridine) oligomer wires show the small β values because M[(tpy)2] unit works as an electron hopping site and promotes intrawire electron transport. The quantitative evaluation of charge transport is important to develop superior electron transport system. In this research, we prepared two types of gold electrode-[Fe(tpy)2]-ferrocene molecular arrays as the simplest electron transport model of [M(tpy)2] wire system. The prepared arrays were characterized using cyclic voltammetry and scanning tunneling microscope. The electron transfer between the gold electrode and the terminal ferrocene moiety via [Fe(tpy)2] was evaluated by the potential step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS). The analysis of Tafel plots derived from PSCA based on Marcus theory revealed the greater electron transfer rate constant and smaller reorganization energy than a ferrocene-terminated alkanethiol self-assembled monolayer system. The parameters extracted by the fitting of the observed impedance spectra using an electric circuit model suggested the additional factor for the rapid electron transport of [Fe(tpy)2] system. On the other hand, in the present electronics field, semiconductors are used as major materials for electronic devices. Therefore, the investigation of the electron transport behaviors of Fe(tpy)2 wires on a semiconductor electrode will contribute to develop molecular devices based on the present electronics. Ferrocene- and triarylamine-terminated [Fe(tpy)2] wires were prepared on hydrogen-terminated silicon electrodes using the stepwise coordination method. Their electron transport behaviors between the terminal redox species and silicon electrodes via [Fe(tpy2)] wires were analyzed based on the cyclic voltammograms recorded at various scan rates. The ferrocene-terminated wire gave the redox wave derived from Fc+/Fc with largely-separated oxidation and reduction peaks indicating slow electron transfer while the triarylamine-terminated wire exhibited the reversible redox couple of triarylamine cation radical/triarylamine derived from the rapid electron transport. In addition, the separation of oxidation and reduction peak potentials of Fc+/Fc redox wave showed the linear dependency on the natural logarithm of scan rates at the high scan rate region whereas the that of triarylamine redox couple did not. These different electron transport phenomena were discussed based on the band structure of silicon electrode and the energy levels of wire components. At the redox potential of ferrocene moiety, the band structure of silicon is largely bended, and the electron injection is prevented. In the case of triarylamine moiety, the band deformation is insignificant. In addition, the applied potential is close to the HOMO level of Fe(tpy)2, and might allow the smooth electron injection from the wire to the electrode.

Specific Response of Quinone Redox at 2-Aminoethanethiol Modified Electrode

Self-assembled monolayers of alkanethiols on electrode surface have been the subjects of intensive studies in both fundamental interests of surface chemistry and their technological significance. The self-assembly is one of the most widely used techniques to construct ordered molecular structures and to control surface characteristics, due to its facile procedure. 2-aminoethanethiol has been used as an anchor to introduce functional molecules to electrode surfaces as it has a reactive amino group and its carbon chain is short. We have found that a 2-aminotehanethiol modified Au electrode shows a specific electrochemical response to p-hydroquinone in aqueous solution. That is, a peak separation (ΔE p) in the CV appeared to be very large at the first potential cycle, but it drastically decreased upon successive potential cycling. We have been studying condition where the specific response takes place and its mechanism. Figure 1A shows typical CVs of an aminoethanethiol modified Au disk electrode in a 0.1 M sodium perchlorate solution containing 5 mM hydroquinone at 100 mV s-1. The ΔE p for quinone redox was found to be 530 mV for the first cycle, indicating a slow electron transfer (Fig. 1A curve a). After application of +0.9 V vs. Ag/AgCl for 30 min at which hydroquinone is oxidized, the ΔE p largely decreased to 120 mV (Fig. 1A curve b) and the CV overlapped with that of a bare electrode. On the other hand, application if –0.5 V was applied, the CV remained virtually constant. Interestingly, when the electrode was left in the solution at open circuit or –0.5 V for 3 h, the ΔE p increased to the original value (530 mV). These results indicate that the changes in the ΔE p is reversible and not originating from peeling of the film, and the increase can be attributed to an interaction between aminoethanethiol and hydroquinone, whereas the decrease is due to oxidation of hydroquinone. A Au electrode modified with 2-dimethylanimoethanethiol showed similar behavior (Fig. 1B), indicating that Schiff base formation from amino group of 2-aminoethanethiol and p-quinone is not the dominant cause of the changes in the electrochemical response. At the current stage, we speculate two possible mechanisms for the changes in the electrochemical response. The first one is changes in structure of aminoethanethiol due to changes in an interaction with quinone, although it is known that short alkyl chains on the electrode surface do not block electron transfer. The second one is a chemical reaction between hydroquinone and aminoethanethiol, which produces derivatives with slow electrode reaction rate constant. We currently investigate those two possibilities with STM measurements for an aminoethanethiol modified electrode before and after the electrolysis and with NMR measurements for the reaction. Figure 1

Rate and Stability of Photocatalytic Water Oxidation using [Ru(bpy)3]2+ as Photosensitizer

The kinetics of homogeneous photocatalytic water oxidation is reported using [Ru(bpy)3]Cl2 as photosensitizer, Na2S2O8 as sacrificial electron acceptor, and three different water-oxidation catalysts: the ruthenium catalyst [Ru(bda)(isoq)2] ([1], H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid, isoq = isoquinoline), Co(NO3)2 ([2]), and [Ir(Cp*)(dmiz)(OH)2] ([3], Cp* = pentamethylcyclopentadienyl, dmiz =1,3-dimethylimidazol-2-ylidene). At pH 7.0, in a phosphate buffer and under blue light irradiation, the production of O2 at the catalyst is rate determining when [2] or [3] is used as a water-oxidation catalyst. However, when [1] is used as the catalyst under identical conditions the turnover at the water-oxidation catalyst is not the rate-limiting step of the photocatalysis. Instead, the step limiting dioxygen production is the transfer of electrons from the catalyst to the photooxidized photosensitizer [Ru(bpy)3]3+. Due to the instability of [Ru(bpy)3]3+ in neutral aqueous solutions, slow electron transfer ...

Incorporation of graphene into SnO2 photoanodes for dye-sensitized solar cells

In dye-sensitized solar cell (DSSC) photoanodes, tin dioxide (SnO2) structures present a promising alternative semiconducting oxide to the conventional titania (TiO2), but they suffer from poor photovoltaic (PV) efficiency caused by insufficient dye adsorption and low energy value of the conduction band. A hybrid structure consisting of SnO2 and reduced graphene oxide (SnO2-RGO) was synthesized via a microwave-assisted method and has been employed as a photoanode in DSSCs. Incorporation of RGO into the SnO2 photoanode enhanced the power conversion efficiency of DSSC device by 91.5%, as compared to the device assembled without RGO. This efficiency improvement can be attributed to increased dye loading, enhanced electron transfer and addition of suitable energy levels in the photoanode. Finally, the use of RGO addresses the major shortcoming of SnO2 when employed as a DSSC photoanode, namely poor dye adsorption and slow electron transfer rate.

Enhanced dopamine detection sensitivity by PEDOT/graphene oxide coating on in vivo carbon fiber electrodes

Dopamine (DA) is a monoamine neurotransmitter responsible for regulating a variety of vital life functions. In vivo detection of DA poses a challenge due to the low concentration and high speed of physiological signaling. Fast scan cyclic voltammetry at carbon fiber microelectrodes (CFEs) is an effective method to monitor real-time in vivo DA signaling, however the sensitivity is somewhat limited. Electrodeposition of poly(3,4-ethylene dioxythiophene) (PEDOT)/graphene oxide (GO) onto the CFE surface is shown to increase the sensitivity and lower the limit of detection for DA compared to bare CFEs. Thicker PEDOT/GO coatings demonstrate higher sensitivities for DA, but display the negative drawback of slow adsorption and electron transfer kinetics. The moderate thickness resulting from 25 s electrodeposition of PEDOT/GO produces the optimal electrode, exhibiting an 880% increase in sensitivity, a 50% decrease in limit of detection and minimally altered electrode kinetics. PEDOT/GO coated electrodes rapidly and robustly detect DA, both in solution and in the rat dorsal striatum. This increase in DA sensitivity is likely due to increasing the electrode surface area with a PEDOT/GO coating and improved adsorption of DA's oxidation product (DA-o-quinone). Increasing DA sensitivity without compromising electrode kinetics is expected to significantly improve our understanding of the DA function in vivo.

Photoelectrochemical Screening of Solar Cell Absorber Layers: Electron Transfer Kinetics and Surface Stabilization

Redox electrolyte contacts offer a simple way of testing the photocurrent generation/collection efficiency in partially completed thin-film solar cells without the need to complete the entire fabrication process. However, the development of a reliable quantitative method can be complicated by the instability of the semiconductor/electrolyte interface. In the case of Cu(In,Ga)Se2 (CIGSe) solar cells, these problems can be overcome by using samples that have undergone the next processing step in solar cell fabrication, which involves chemical bath deposition of a thin (ca. 50 nm) CdS buffer layer. The choice of redox system is also critical. The frequently used Eu3+/2+ redox couple is not suitable for reliable performance predictions since it suffers from very slow electron transfer kinetics. This leads to the buildup of photogenerated electrons near the interface, resulting in electron–hole recombination. This effect, which can be seen in the transient photocurrent response, has been quantified using inten...

Application of GUITAR on the Negative Electrode of the Vanadium Redox Flow Battery: Improved V3+/2+ Heterogeneous Electron Transfer with Reduced Hydrogen Gassing

GUITAR (Graphene from the University of Idaho Thermolyzed Asphalt Reaction) has the classical basal and edge plane morphology of graphites and thin layer graphenes with similar X-ray photoelectron spectroscopy (XPS), Raman and IR characteristics. However previous investigations indicated GUITAR is different electrochemically from graphenes and classical graphites. GUITAR has faster heterogeneous electron transfer across its basal plane and an electrochemical window that exceeds graphitic materials by 1 V. These beneficial properties are examined for application in the negative electrode of the vanadium redox flow battery (VRFB). Graphitic materials in this application suffer from hydrogen gassing and slow electron transfer kinetics for the V2+/3+ redox couple. Cyclic voltammetry of the V2+/3+ redox couple (0.05 M V3+ in 1 M H2SO4) on bare KFD graphite felt gives an estimated standard rate constant (k0) of 8.2 × 10−7 cm/s. The GUITAR-coated KFD graphite felt improves that quantity to 8.6 × 10−6 cm/s. The total contribution of the cyclic voltammetric currents at −1.0 V vs. Ag/AgCl to hydrogen evolution is 3% on GUITAR-coated KFD graphite felt. On bare KFD graphite felt, this is 22%. These results establish GUITAR as an excellent alternative material for the negative electrode in the vanadium redox flow battery.

New Hypothesis for SOFC Ceramic Oxygen Electrode Mechanisms

There is a lot of discussion and little agreement about the mechanisms for the oxygen reduction/oxide ion oxidation reaction ceramic electrodes in solid oxide cells, such as perovskite electrodes like the lanthanum strontium manganate – yttria stabilized zirconia composite electrode or mixed ion and electron conducting electrodes like (La1-xSrx)sCoO3- d (LSC). Many mechanisms with similarities such as electron transfer being rate determining and involving surface oxide vacancies have been proposed. Often the proposals were based on studies of oxygen isotope (16O – 18O) exchange at perovskite and fluorite structured metal oxide surfaces, which actually is not an electrically driven process. Even though oxygen exchange - on a non-polarized surface of a ceramic that is not electrically connected to an electrolyte - has electrochemical character, it does not necessarily behave the same way as an electrochemically polarized electrode. Furthermore, most proposed mechanisms assumed that the oxygen electrode reaction took place on a real perovskite surface, but during recent years, it has become generally accepted that perovskites and many other Ruddlesden-Popper phases have a thin (few nanometer) and continuous surface layer of A-site oxides, e.g. SrO and La2O3in the case of the popular mixed conducting LSC. This layer is observed to increasingly impede the kinetics of the reversible oxygen electrode as the AO layer grows thicker. As the electrode reaction does not stop, but just decreases with increasing SrO thickness, we take these literature reports as an indication that the electrode reaction takes place through an AO layer even though the AO bulk phase is both ion and electron insulating. Our hypothesis is that a thin AO layer, like a 1 nm thick SrO layer, is electron and ion conducting due to space charge layers both on the oxygen gas side and on the perovskite side of it. Thus, the SrO layer will probably contain O- and O2 -ions, which act as electron holes, and request oxygen vacancies, which enables the oxide ion conductivity. This otherwise insulating layer then becomes a mixed conductor. We further hypothesize that the actual specific conductivity by oxygen ions as well as by electrons will be very sensitively dependent on the oxygen vacancy concentration in, and the electron Fermi level of, the perovskite layer. Apart from the change in defect chemistry with polarization of the perovskite electrode, the SrO layer will also change stress state because of stoichiometric expansion/contraction of the perovskite due to the change in the perovskite stoichiometry with polarization. Straining of the SrO lattice may also change its conductivity. On this background, another rate limiting process is suggested: O2(gas) + 2 e-(surface) → 2 O-(ads,SrO) fast (1) O-(ads, SrO) + VO(surface) → O-(SrO,subsurface) slow (2) O-(SrO) + e- → O2-(LSC) fast (3) This mechanism is sketched in the Figure. The slow rate determining process of eq. 2 is not limited by slow electron transfer and it is not a simple ion transfer, but rather a kind of diffusion or conduction process through the thin segregated SrO surface layer. This hypothesis seems to be able to qualitatively explain a number of otherwise puzzling observations reported in the literature. This will be explained in the paper together with more details. The Figure shows a much simplified sketch of the proposed mechanism for the (La0.6Sr0.4)0.98CoO3- d oxygen electrode. The electrons may meet the O- - ions inside the SrO layer. VO means oxide vacancies and is not attempting to account for the charge imbalance. Figure 1

Structuring Au nanoparticles on two-dimensional MoS2 nanosheets for electrochemical glucose biosensors

Two-dimensional (2D) bioelectronics is an emerging field of research which fuses the advantages of 2D nanomaterials with those of nanobiotechnology. Due to the various physical and chemical properties present in layered counterparts of 2D materials, including high charge density, large surface area, remarkable electron mobility, ready electron transport, sizeable band gaps and ease of hybridisation, they are set to become a versatile tool to fabricate sensitive and selective novel biodevices, which might offer an unique advantages to tackle key energy, medical and environmental issues. Current 2D bioelectronics research is focused on the design of simple-to-use and cheaper biodevices, while improving their selectivity, sensitivity and stability. However, current designs generally suffer from a lack of efficiency, relatively low sensitivity, slow electron transfer kinetics, high background charging current and low current density arising from poor mass transport. Here, we report a nanoparticle-structured MoS2 nanosheet as an ideal semiconductor interface, which is able to form a homogenous layer on the electrode surface for the assembly of gold nanoparticles. This not only enhances electrocatalytic reactions, but also provides excellent electrochemical properties such as high faradic-to-capacitive current ratios, high current density and electron mobility, and faster mass transport, due to the dominance of radial diffusion. The MoS2/Au NPs/GOx bioelectrode exhibits a linear response to glucose from 0.25 to 13.2mM, with a detection limit of 0.042µM (S/N=3) and sensitivity of 13.80µA/µM/cm2.

Electrochemistry of Fe3+/2+ at highly oriented pyrolytic graphite (HOPG) electrodes: kinetics, identification of major electroactive sites and time effects on the response.

The electrochemistry of the Fe3+/2+ redox couple has been studied on highly oriented pyrolytic graphite (HOPG) samples that differ in step edge density by 2 orders of magnitude, to elucidate the effect of surface structure on the electron transfer (ET) kinetics. Macroscopic cyclic voltammetry measurements in a droplet-cell arrangement, highlight that the Fe3+/2+ process is characterised by slow ET kinetics on HOPG and that step edge coverage has little effect on the electrochemistry of Fe3+/2+. A standard heterogeneous ET rate constant of ∼5 × 10-5 cm s-1 for freshly cleaved HOPG was derived from simulation of the experimental results, which fell into the range of the values reported for metal electrodes, e.g. platinum and gold, despite the remarkable difference in density of electronic states (DOS) between HOPG and metal electrodes. This provides further evidence that outer-sphere redox processes on metal and sp2 carbon electrodes appear to be adiabatic. Complementary surface electroactivity mapping of HOPG, using scanning electrochemical cell microscopy, reveal the basal plane to be the predominant site for the Fe3+/2+ redox process. It is found that time after cleavage of the HOPG surface has an impact on the surface wettability (and surface contamination), as determined by contact angle measurements, and that this leads to a slow deterioration of the kinetics. These studies further confirm the importance of understanding and evaluating surface structure and history effects in HOPG electrochemistry, and how high resolution measurements, coupled with macroscopic studies provide a holistic view of electrochemical processes.