Professor Scott Donne

Green hydrogen is a clean fuel aiming to revolutionize the transportation industry in the next decades. It can be produced by several processes with solar electrochemical (EC) and solar thermochemical hydrogen (SCTH) as the most attractive ones. High-temperature solid oxide electrolysis (SOE) is a relatively mature technology; however, these systems suffer low durability due to delamination, build-up of nonconductive phases and separation of metallic electrode contacts. Alternatively, two-step solar thermochemical hydrogen (STCH) exhibits ample durability and benefit from cheaper thermal energy, although solar-to-hydrogen efficiencies remain low. Herein, a novel electrochemically assisted solar thermochemical hydrogen production process (EC-STCH) is proposed, presenting a fundamental thermodynamic analysis of this novel route and a comparison with conventional SCTH and SOE, assuming a fixed H2O-to-H2 conversion of 10% and 50%. The thermodynamic model is based on fundamental thermodynamic principles and demonstrates that for materials which require a ¿T > 500 °C to conduct both reactions in STCH, could operate at lower temperatures and ¿T = 0 °C. Reaction conditions were evaluated showing that it may also be possible to increase the oxygen partial pressure while still achieving high fuel conversion, which would not be possible operating under the conventional SOE and STCH conditions.

Biochars have been highlighted as a means of carbon sequestration, which is significant for achieving carbon neutrality. Mixtures of wood chips and either bentonite or kaolin were co-pyrolysed at temperatures of 350 °C and 550 °C, and the microstructural characteristics and the carbon sequestration potential of the resultant biochar were explored in the study. The addition of minerals promoted the formation of a stable carbon structure in biochar, especially the proportion of Si[sbnd]C bonds in the high-temperature mineral-composited biochar increased by 3.56¿3.82 times compared with the original biochar. After bentonite or kaolin was added to wood chips pyrolysed at 550 °C, the carbon loss after H2O2 oxidation was reduced to no more than 19.2%, and the Recalcitrance Index (R50) of biochar increased to no less than 0.89. The combined action of high temperature and minerals promoted the formation of highly aromatic structures of biochar (H:C < 0.4) and reduced the amount of dissolved organic carbon to 4.89 mg g-1. Furthermore, minerals directly covered the surface of biochar, and the content of Si[sbnd]C bond increased, thus strengthening the chemical and thermal stability of biochar. However, the addition of minerals had no significant effect on the biological stability of biochar. The study indicates that the pre-pyrolysis mineral addition is an effective way to increase the carbon sequestration potential of biochar.

Herein we report on the charge storage behaviour of a non-porous planar glassy carbon electrode (GCE) in an aqueous electrolyte of 0.5 M Na2SO4 at temperatures in the range 25 °C to 50 °C. Preliminary cyclic voltammetry (CV) data indicate a decrease in electrode performance at increasing temperatures, and this was supported by detailed analysis of step potential electrochemical spectroscopy (SPECS) data. Kinetic analysis of the deconvoluted SPECS data using the Arrhenius equation has indicated that diffusional charge storage is not thermally activated, instead being possibly influenced by thermal scattering of electrolyte ions. Entropimetric analysis of the SPECS data has generated entropy and enthalpy data for charge storage in this system. These results are discussed in terms of charge storage at this interface.

This paper reports on the charging mechanism of an activated carbon electrode in a symmetric electrochemical capacitor operating in an ionic liquid electrolyte (EMIm+TFSI-). With the application of step potential electrochemical spectroscopy (SPECS) and electrochemical dilatometry, it is determined that the electrical double-layer formation mechanism strongly depends on the porous structure of the electrode material. Furthermore, SPECS calculations allow us to separate and quantify the charge component responsible for the ionic movement.

This paper investigates the impact of kaolin, a dominant coal mineral, on the thermal and electrochemical behaviour of the molten ternary carbonate eutectic ((Li,Na,K)2CO3) in the direct carbon fuel cell (DCFC) as a means to simulate long-term operation with a continuous coal feed. Thermogravimetric and differential thermal analysis, coupled with kinetic modelling using the Friedman method, shows a substantial decrease in activation energy for eutectic melting with the addition of kaolin. Electro-oxidation of carbon (graphite) is also enhanced with kaolin in the electrolyte, increasing from 17.68 mA/cm2 at 0 wt% kaolin to the highest value of 162 mA/cm2 with 15 wt% kaolin added. It is shown that the improvement is due to increasing oxide concentration resulting from kaolin dissolution in the electrolyte.

Excessive amounts of animal manures and production of a large volume of biosolids pose serious environmental issues in terms of their safe disposal and management. Thermochemical treatment of bio-waste materials via pyrolysis can convert them into value-added products such as biochar-based fertilizers. In this study, fourteen biochars were produced from one biosolid and thirteen animal manures by slow pyrolysis at 300 °C. All feedstock and biochar samples were characterized by determining the yield, and physicochemical and surface properties, including the C-containing functional groups. Principal component and cluster analyses were used to classify the feedstock/biochar materials based on their mineral constituents. The biochar yield of various feedstocks ranged from 39 to 81%, with the highest yield for grain-fed cow manure. The highest N and K content was found in chicken manure biochar (57.8 and 29.2 g kg¿1, respectively), while the highest P was found in biosolid biochar (40.5 g kg¿1). The specific surface area of biochars ranged from 96.06¿110.83 m2 g-1. Hierarchical analyses of the chemical compositions of feedstocks and biochars enabled grouping of the materials respectively into four and five distinguished clusters. Three principal components (PC) explained 86.8% and 83.3% of the variances in the feedstocks and biochars, respectively. The PC1 represented the content of the major nutrients (N, P and K), whereas PC2 and PC3 represented other nutrients (secondary and micronutrients) contents and physicochemical properties (pH and EC). The results of this study suggested that biochars produced from different manures and biosolids may potentially be a source of soil nutrients and trace elements. In addition, different biochars may be applied to different nutrient-deficient soils to avoid plausible nutrient and potentially toxic element contamination.

The mechanisms of charge storage in four typical electrochemical capacitor systems are compared and contrasted. These systems are based on activated carbon, ruthenium dioxide, manganese dioxide and nickel hydroxide. Charge storage is discussed in terms of charge delocalization either on the surface or throughout the electrode material. Electrical double layer formation, such as on activated carbon, is considered an example of charge delocalization, with charge distributed over the electrolyte accessible surface irrespective of the applied potential. Ruthenium dioxide also stores delocalized charge, in this case through the reversible Ru(IV)/ Ru(III) redox couple. Manganese dioxide is unique in that in alkaline (battery) electrolytes charge is localized in specific structural domains, while in neutral (capacitor) electrolytes charge is delocalized over the material structure. Nickel hydroxide in an alkaline electrolyte is an example of charge localization when redox cycling due to its two-phase redox mechanism. The impact of these differing charge storage mechanisms on electrochemical performance is discussed.

Herein we report on the use of step potential electrochemical spectroscopy (SPECS) to characterize the dynamics of charge storage on a planar non-porous glassy carbon electrode in an aqueous electrolyte. Comparison between the spatial requirements of solvated electrolyte ions and the amount of charge added to the electrode surface implies that electrolyte charge storage is diffuse in nature, extending a substantial distance into the electrolyte, even for a concentrated electrolyte. This is contrary to the expected parallel planes of charge expected for double layer formation at the interface. The implications of this on the use of porous materials is also discussed.

Herein the semiconducting nature of electrodeposited manganese dioxide (¿-MnO2) is examined for its effects on pseudo-capacitive electrode behaviour in neutral electrolytes. Electrochemical analysis of the manganese dioxide electrode is achieved using a combination of cyclic voltammetry (CV), step potential electrochemical spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS), in particular the combination of SPECS and EIS in a single experiment, enabling the determination of electronic properties across the full potential window. After establishing stable cycling with CV, electrode performance is examined using SPECS. EIS data, recorded under quasi-equilibrium conditions at the end of each potential step rest period, is then analyzed using the Mott-Schottky equation, leading to the identification of both n-type and p-type behaviour for the manganese dioxide electrode. Flat band potentials are determined (Vfb,n = -0.34 V vs SCE and Vfb,p = 0.98 V vs SCE), and conclusions about the electrochemically active surface area being more closed related to the geometric surface area were reached.

Porous carbons are widely used as electrode materials for electrical double layer (EDL)-type supercapacitors. Differentiation of capacitance contributions from micropores and mesopores is important to understand porous carbon capacitive behavior. However, this remains challenging to accomplish. Here, we present an improved step potential electrochemical spectroscopy method to investigate the contributions to capacitance from hierarchical pores and redox processes.

Herein the charge storage capabilities of a planar non-porous glassy carbon electrode (GCE) in electrolytes of aqueous 0.5 M Na2SO4 and 1 M tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile are compared using a combination of cyclic voltammetry (CV), step potential electrochemical spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS). In all techniques the resultant capacitance was substantially higher for the GCE in 1 M TEABF4 compared to 0.5 M Na2SO4, with the differences due to solvation of the adsorbing electrolyte ions. Adsorbing Na+ cations in the aqueous system have a substantial solvation sheath that increases the resistance associated with capacitive charge storage, decreases the capacitance, as well as inhibits the packing of similar electrolyte ions in the vicinity of the GCE. Conversely, the adsorbing BF4- ions in the acetonitrile-based electrolyte exhibit a low resistance to charge storage, a greatly enhanced capacitance, as well as exhibit an improved ability to pack at the electrode-electrolyte interface. The impact of this behaviour on electrode performance has also been discussed.

Halloysite nanotubes (HNT) and ball-milled biochar (BC) incorporated biocompatible mesoporous adsorbents (HNT-BC@Alg) were synthesized for adsorption of aqueous heavy-metal ions. HNT-BC@Alg outperformed the BC, HNT, and BC@Alg in removing cadmium (Cd), copper (Cu), nickel (Ni), and lead (Pb). Mesoporous structure (?7.19 to 7.56 nm) of HNT-BC@Alg was developed containing an abundance of functional groups induced from encapsulated BC and tubular HNT, which allowed heavy metals to infiltrate and interact with the adsorbents. Siloxane groups from HNT, oxygen-containing functional groups from BC, and hydroxyl and carboxyl groups from alginate polymer play a significant role in the adsorption of heavy-metal ions. The removal percentage of heavy metals was recorded as Pb (?99.97 to 99.05%) > Cu (?95.01 to 90.53%) > Cd (?92.5 to 55.25%) > Ni (?80.85 to 50.6%), even in the presence of 0.01/0.001 M of CaCl2 and Na2SO4 as background electrolytes and charged organic molecule under an environmentally relevant concentration (200 µg/L). The maximum adsorption capacities of Ni, Cd, Cu, and Pb were calculated as 2.85 ± 0.08, 6.96 ± 0.31, 16.87 ± 1.50, and 26.49 ± 2.04 mg/g, respectively. HNT-BC@Alg has fast sorption kinetics and maximum adsorption capacity within a short contact time (?2 h). Energy-dispersive X-ray spectroscopy (EDS) elemental mapping exhibited that adsorbed heavy metals co-distributed with Ca, Si, and Al. The reduction of surface area, pore volume, and pore area of HNT-BC@Alg (after sorption of heavy metals) confirms that mesoporous surface (2-18 nm) supports diffusion, infiltration, and interaction. However, a lower range of mesoporous diameter of the adsorbent is more suitable for the adsorption of heavy-metal ions. The adsorption isotherm and kinetics fitted well with the Langmuir isotherm and the pseudo-second-order kinetic models, demonstrating the monolayer formation of heavy-metal ions through both the physical sorption and chemical sorption, including pore filling, ion exchange, and electrostatic interaction.

The nature of the electrified interface between a planar non-porous glassy carbon electrode and an aqueous electrolyte of 0.5 M Na2SO4 is examined using the combination of step potential electrochemical spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS) across the full potential window of stability. This combination of techniques provides insight into the dynamics of interface formation (SPECS), as well as interfacial structure (EIS), highlighting the variation in capacitance across the potential window. EIS frequency also plays a significant role in determining capacitance via the role that it plays in determining ionic and dipole movement. High frequencies stimulate electron cloud movement, followed by dipole rotation and ionic movement as the frequency progressively decreases. Outcomes further highlight the role that mass transport has on the amount of charge storage at the interface.

The incorporation of nitrogen into hydrothermal carbon with (NH4)2SO4 is shown to have a significant influence on its chemical composition and surface characteristics. This in turn boosts the pseudo-capacitive behavior of hydrothermal carbons and their overall electrochemical stability. A combination of X-ray photoelectron spectroscopy, Fourier transform infra-red spectroscopy (FTIR) and scanning electron microscopy (SEM), yielded insights on the influence of nitrogen doping on surface functionalities. 1- and 2-D solid state NMR established the molecular-level structure of both doped and non-doped hydrothermal carbon. Cyclic voltammetry and electrochemical impedance spectroscopy has established the electrochemical behaviour of these hydrothermal carbons, indicating that nitrogen doping enhances not only the capacitance but also the stability of the hydrothermal carbons.

Step potential electrochemical spectroscopy (SPECS) has been applied to a range of different electrolytic manganese dioxide (EMD) samples to examine the effects that material composition, structure and morphology have on the performance of these materials as electrochemical capacitor electrodes. The applications of SPECS enabled deconvolution of the electrode behaviour into double layer and diffusional or pseudo-capacitive effects. The technique was even sensitive enough to resolve double layer charge storage associated with either the geometric or porous surface area. The relationships between each of the contributions to total capacitance and the EMD material properties have been described and discussed.

The electrochemical conversion of CO<inf>2</inf> to carbon through the reductive electrolysis of molten carbonate-containing salts has been studied by a range of groups. These groups have examined the yields, mechanisms of deposition and physical characteristics of carbon synthesized through electrolysis using a variety of electrolytes, substrates, temperatures, current densities and deposition potentials. The findings of these research groups have been compiled and compared, with particular significance being placed on findings relating to the influence of variables on the physical properties of carbon obtained in this manner. Research on potential applications of carbon derived from the electrolysis of molten carbonate-containing salts has been presented and the energetics of these carbons have been discussed. The possibility of using this form of carbon synthesis as a manner of permanent waste CO<inf>2</inf> sequestration has been considered.

Six different phases of manganese dioxide have been examined to determine the different contributions made by double layer capacitance and pseudo-capacitive redox reactions (or diffusion limited processes) to the total charge storage. These processes were separated using a novel combination of step potential electrochemical spectroscopy (SPECS), in which the electrode is charged and discharged in a series of small (±25 mV) potential steps, combined with modelling of the current response to determine the different charge storage processes occurring at the electrode. The manganese dioxide phases were all cycled in 0.5 M Na2SO4 between 0.0-0.8 V (vs SCE). The results indicate that the different properties of the samples, such as crystal structure, surface area and porosity affect the relative contributions from the each charge storage process.

Development of an electrochemical method for separating the contributions made by faradaic and non-faradaic processes to the total specific capacitance of an electrochemical capacitor electrode is reported. The method is based on step potential electrochemical spectroscopy (SPECS), and its application is demonstrated on activated carbon electrodes in aqueous media. The method involves applying a sequence of small potential steps (±25 mV) across the electrochemical window of the electrode material, with sufficient rest time between steps to allow for current equilibration. Modelling of the resultant i-t data using combined resistive, capacitive, diffusional and residual current terms allows for separation of the faradaic and non-faradaic terms. The behavior of these contributions across the electrode potential windowis reported and discussed in comparisonwith conventionalmethods of electrochemical analysis, such as cyclic voltammetry.

Multilayered films of hybrid manganese dioxide have been synthesised in an anodic pulse deposition study with the addition of phosphotungstic acid, H<inf>3</inf>PW<inf>12</inf>O<inf>40</inf> (PW) to an acidified MnSO<inf>4</inf> solution. These hybrid materials exhibit tuneable layered morphologies, which is not present in material synthesised in the absence of PW. Cyclic voltammetry analyses have been performed in situ on single pulse thin film depositions synthesised in the presence of varying concentrations of PW, where minor amounts of PW improves the cycle stability of the material, however larger concentrations lead to a degradation in material performance. In situ electrochemical analysis shows that the incorporated PW clusters do not affect material activity, however they do play a role in defining both meso- and micro-structure.

A systematic assessment of the electrochemical activity of two different parent coal types, pyrolysed at temperatures between 500 and 900 °C higher heating temperature (HHT), is presented in this work. Analysis shows that certain coal chars are catalytically activated in molten carbonate media at 600 °C, however activity does not appear to follow trends established for ashless carbon sources. It is seen here that it is not possible to predict activity based solely on electrical resistance, surface functionalization, or the BET surface area of pyrolysed coals. Instead, it is suggested that coal ash type, abundance and distribution plays a pivotal role in activating the coal char to allow fast electrochemical oxidation through a catalytically enhanced pathway. Activation from ash influence is discussed to result from wetting of the molten carbonate media with the carbon surface (change in polarity of electrode surface), through ash mediated oxide adsorption and transfer to carbon particles, or possibly through another catalytic pathway not yet able to be predicted from current results.

The electrodeposition mechanism of manganese dioxide has been investigated in-situ using transmission mode small angle X-ray scattering (SAXS). Manganese dioxide was deposited anodically from an acidic MnSO<inf>4</inf> solution onto a platinum substrate using an electrochemical cell assembled with a short X-ray beam path length over the substrate. The evolving morphology of the film was characterized by taking short time base SAXS measurements over the duration of the electrodeposition process. Modelling of the SAXS data was discussed, with results indicating that the film morphology was found to change during the electrodeposition, with the average smallest pore size increasing from 4.3 to 5.3 Å, a size range that is consistent with previous literature. This suggests that with ongoing electrodeposition the material becomes less porous, has a lower surface area, and more crystalline.

The carbonate eutectic mixture of Li<inf>2</inf>CO<inf>3</inf>, K<inf>2</inf>CO<inf>3</inf>, and Na<inf>2</inf>CO<inf>3</inf> is commonly used as an electrolyte within the direct carbon fuel cell. Here, seven different minerals common to the ash content of Australian bituminous coals (anatase TiO<inf>2</inf>, SiO<inf>2</inf>, CaCO<inf>3</inf>, CaSO<inf>4</inf>, Fe<inf>2</inf>O<inf>3</inf>, FeS, and kaolin) were used to modify the ternary carbonate eutectic to explore the thermodynamics of the carbonate melting process. Thermal effects were examined using differential thermal analysis, where it has been shown that dissolution of the contaminant leads to liquid-phase disruption, the extent of which varies with dopant type. Furthermore, modeling of the melting process carried out using different heating rates allowed determination of the activation energy for melting in the presence of the various contaminants, where it was shown that the contaminants can dramatically affect the activation energy and, subsequently, the kinetics of the melting process.

Biochars are complex heterogeneous materials that consist of mineral phases, amorphous C, graphitic C, and labile organic molecules, many of which can be either electron donors or acceptors when placed in soil. Biochar is a reductant, but its electricaland electrochemical properties are a function of both the temperature of production and the concentration and composition of the various redox active mineral and organic phases present. When biochars are added to soils, they interact with plant roots and root hairs, micro-organisms, soil organic matter, proteins and the nutrient-rich water to form complex organo-mineral-biochar complexes Redox reactions can play an important role in the development of these complexes, and can also result in significant changes in the original C matrix. This paper reviews the redox processes that take place in soil and how they may be affected by the addition of biochar. It reviews the available literature on the redox properties of different biochars. It also reviews how biochar redox properties have been measured and presents new methods and data for determining redox properties of fresh biochars and for biochar/soil systems.

In this work the anodic electrodeposition of manganese dioxide from a solution of 0.01 M MnSO4 in 0.1 M H2SO4 onto a platinum substrate has been examined in order to determine the mechanism of deposition. Electrochemical data indicates that the dilute acid solution favours the formation of MnO2 via a MnOOH intermediate (hydrolysis). The electrodeposited material was examined further using AFM and TEM which showed that the oxidation of Mn2+ occurs in a number of steps. When a potential is applied, Mn2+ is electrochemically oxidised to soluble Mn3+. When the concentration of Mn3+ at the electrode reaches saturation, it chemically precipitates onto the electrode as MnOOH, causing an increase in the electroactive surface area. This stage of the deposition is characterised by the nucleation of MnOOH crystallites on the platinum surface. As the deposition proceeds, MnOOH undergoes solid state oxidation to MnO2. This later stage of the deposition favours the growth of existing MnO2 crystallites, leading to a decrease in the surface area of the electrodeposited material. © 2014 Published by Elsevier Ltd.

The oxidation mechanism for carbon in a carbonate melt was modelled using an electrochemical kinetic approach. Through the Butler-Volmer equation for electrode kinetics, a series of expressions was derived assuming each step of the proposed carbon oxidation mechanism is in turn the rate determining step (RDS). Through the derived expressions the transfer coefficient and Tafel slope were calculated for each possible RDS of the proposed mechanism and these were compared with real data collected on carbon based electrodes including graphite and coal. It was established that the RDS of the electrochemical oxidation process is dependent on both the carbon type and the potential region of oxidation. The simplified kinetic analysis suggested that the RDS in the main oxidation region is likely to be the first or second electron transfer on a graphite electrode surface, which occurs following initial adsorption of an oxygen anion to an active carbon site. This is contrary to previous suggestions that adsorption of the second anion to the carbon surface will be rate determining. It was further shown that use of a coal based carbon introduces a change in mechanism with an additional reaction region where a different mechanism is proposed to be operating. ©2014 Published by Elsevier Ltd.

The Hybrid Sulfur Cycle is a thermo-electrochemical process designed for the large scale production of hydrogen. The two-step process is essentially based on water splitting using various sulfur species as intermediates. The limiting step in the overall process is the electrochemical oxidation of sulfur dioxide to form sulphuric acid, which suffers from a substantial (~0.4V) anodic overpotential. Here we report on various aspects of sulfur dioxide oxidation in an acidic media including the effects of electrode preconditioning, the electrode substrate and electrolyte effects, the combination of which has allowed development of a sulfur dioxide oxidation mechanism which is described and discussed. Additionally, the electrochemical oxidation of sulfur dioxide has been shown to be an oscillating reaction, which is also a novel finding. © 2014 Hydrogen Energy Publications, LLC.

Thin film Mn-modified polypyrrole (PPy) composite electrodes have been prepared by chronoamperometric electrodeposition and characterized in terms of their physico-chemical and electrochemical properties and performance. Analysis of the chronoamperometric data shows that electrodeposition of the thin film results in a relative increase in electrochemically active surface area of up to 30 times. This finding was supported by transmission electron microscopy (TEM), atomic force microscopy (AFM) and profilometry analysis of the films. Electrochemical quartz crystal microbalance (EQCM) studies have allowed for the direct determination of electrode mass, both during deposition and electrochemical performance evaluation, which has enabled analysis of electrode properties, including film growth (up to 26 µg/cm2), density (~2 g/cm3), and the charge storage during electrochemical cycling, including the rates of mass uptake/removal with charge. The characteristics of the composite electrodes were compared with PPy-only electrodes throughout. © 2014 Elsevier B.V.

© 2014 American Chemical Society. This work presents a temperature-dependent and time-resolved Xray and neutron diffraction study of the thermally induced structural rearrangement of ¿-MnO2. Here, we study electrochemically prepared ¿-MnO2, the manganese dioxide phase used in the majority of battery applications, which we find to be ~64% ramsdellite [a = 4.4351(6) Å, b = 9.486(2) Å, c = 2.8128(7) Å, and V = 118.33(3) Å3] and ~36% pyrolusite [a = 4.718(3) Å, c = 2.795(2) Å, and V = 62.22(8) Å3]. Taking a deeper look at the kinetics of the structural rearrangement, we find two steps: A fast transition occurring within 4-8 min with a temperature-dependent ramsdellite to pyrolusite transformation (rate constant 0.11-0.74 min-1) and a slow transition over 4 h that densifies (with changes in unit cell and volume) the ramsdellite and pyrolusite phases to give structures that appear to be temperature-independent. This effectively shows that ¿/ß-MnO2 prepared in the range of 200-400°C consists of temperature-independent structures of ramsdellite, unit cell a = 4.391(1) Å, b = 9.16(5) Å, c = 2.847(1) Å, and V = 114.5(6) Å3, and pyrolusite, unit cell a = 4.410(2) Å, c = 2.869(2) Å, and V = 55.79(4) Å3, with a temperature-dependent pyrolusite fraction between 0.45 and 0.77 and increasing with temperature. Therefore, we have linked the temperature and time of heat treatment to the structural evolution of ¿-MnO2, which will aid the optimization of ¿/ß-MnO2 as used in Li-primary batteries.

The roles of alkylation and redox cycling in quinone toxicity were investigated. In general the more cytotoxic quinones produced the highest responses in an assay monitoring redox activity. No evidence of alkylation of high molecular weight protein thiols was detected. We conclude quinone toxicity is dominated by redox cycling.