Aaron T. Marshall

Accumulation of Gold Nanoparticles in Brassic Juncea

Enzymatic digestion is proposed as a method for concentrating gold nanoparticles produced in plants. The mild conditions of digestion are used in order to avoid an increase in the gold particle size, which would occur with a high-temperature process, so that material suitable for catalysis may be produced. Gold nanoparticles of a 5–50-nm diameter, as revealed by transmission electron microscopy (TEM), at concentrations 760 and 1120 ppm Au, were produced within Brassica juncea grown on soil with 22–48 mg Au kg−1. X-ray absorption near edge spectroscopy (XANES) reveals that the plant contained approximately equal quantities of Au in the metallic (Au0) and oxidized (Au+1) states. Enzymatic digestion dissolved 55–60 wt% of the plant matter. Due to the loss of the soluble gold fraction, no significant increase in the total concentration of gold in the samples was observed. However, it is likely that the concentration of the gold nanoparticles increased by a factor of two. To obtain a gold concentration suitable for catalytic reactions, around 95 wt% of the starting dry biomass would need to be solubilized or removed, which has not yet been achieved.

Iridium oxide-based nanocrystalline particles as oxygen evolution electrocatalysts

Iridium-based oxides are highly active as oxygen evolving electrocatalysts in PEM water electrolyzers. In this work XRD reveals that Ir-Sn oxides contain a single rutile phase with lattice parameters between those of pure IrO2 and SnO2. Addition of Ru leads to the synthesis of a core-shell type material due to the strong agglomeration of Ru colloids during the preparation procedure. The shell of this material consists of an Ir-Sn-Ru oxide deficient in Ru relative to the bulk. This leads to a decrease in the surface noble metal concentration (as found by XPS), which in turn results in a significant reduction in electrochemically active surface area. Polarization analysis indicates that the addition of Ru can influence the rate-determining step or mechanism by which oxygen is evolved. In a PEM water electrolysis cell, small additions of Sn do not significantly reduce the operating performance, however larger additions cause a performance loss due to a reduction in active surface area and increased ohmic resistance. When a pure IrO2 anode is used, a cell voltage is 1.61 V at 1 A cm−2 and 90°C.

Materials for Electrocatalysis of Oxygen Evolution Process in PEM Water Electrolysis Cells

Proton exchange membrane (PEM) water electrolysis offers several advantages compared to the traditional alkaline technologies including higher energy efficiencies, considerably higher specific production rates leading to more compact design, and avoiding a liquid and corrosive electrolyte. The oxygen electrode is the critical part in the energy consumption of such cells. To obtain high performance, electrocatalytically very active anode materials have to be developed for oxygen evolution. The most promising electrocatalytic materials are based on IrO2 and RuO2, preferably in mixtures with other transition metal oxides with electronic conductivity. Excellent performance has been obtained by using nanocrystalline electrocatalysts based on iridium oxide with additions of ruthenium oxide and/or tin oxide, forming rutile structures, or mixed with tantalum pentoxide.This concept has been applied extensively in our work and has been successful in understanding oxygen evolution performance variations in IrO2-RuO2, IrO2-SnO2, IrO2-RuO2-SnO2 and IrO2-RuO2-Ta2O5 systems.

Gaseous pollutant treatment and electricity generation in microbial fuel cells (MFCs) utilising redox mediators

Microbial fuel cell (MFC) is an emerging technology for sustainable energy generation and waste treatment. This paper reviews the potential of a gaseous substrate when it is combined with a mediator in an MFC to generate electricity and to treat toxic gaseous pollutants. Most MFCs for waste water treatment often cannot use mediator to enhance the electron transfer from the microbe to the anode because of the difficulty in recovering the expensive and potentially toxic compound. Combining gas feeds with mediators is possible since the soluble mediator would remain in the anode chamber as the gas passes through the reactor. In addition, this type of MFC is possible to be integrated into an anaerobic biofiltration system (BF-MFC), where the biofilter removes the gaseous contaminant and produces the reduced mediator and the MFC produces the electricity and recycles the reoxidised mediator. This paper also talks about the past research on gaseous feed MFCs, and reviews the mechanism and strategies of electron transfer in MFC using redox mediator. The advantages, process parameters and challenges of BF-MFC are discussed. This knowledge is very much required in the design and scale up of BF-MFC. This paper will be useful for those who work in the area of gaseous pollutant treatment and electricity generation.

Electrocatalytic Oxygen Evolution on Electrochemically Deposited Cobalt Oxide Films: Comparison with Thermally Deposited Films and Effect of Thermal Treatment

Electrocatalytic cobalt oxide layers have been prepared on nickel substrates using thermal decomposition and electrochemical deposition methods. Importantly, it was confirmed that the electrochemical deposition method could be applied to nickel foam substrates for use in zero-gap alkaline water electrolysis cells. The oxide layers produced were then investigated for their activity towards the oxygen evolution reaction in 30 wt % KOH solution and found to be superior compared with the uncoated nickel substrate. Layers produced by both methods had similar electrochemical behaviour, provided that the layers were annealed at temperatures ≥350 ∘C. This thermal treatment was required to mechanically stabilise the electrochemically deposited cobalt oxide layer. Due to this finding, the effect of annealing temperature was investigated for the electrochemically deposited layer, and it was found that the overpotential for oxygen evolution increased with increasing annealing temperature. Using cyclic voltammetry and impedance spectroscopy, it is concluded that the decrease in performance with increasing annealing temperature is largely caused by the corresponding decrease in active surface area. However, for annealing temperatures ≥400 ∘C, additional resistances are introduced that cause lower performance. The impedance data suggest that these additional resistances are caused by either a decrease in the conductivity of the cobalt oxide layer itself, or the formation of a passivating-like nickel oxide layer between the active cobalt oxide and the nickel substrate, or both. The resistances’ dependence on potential suggests that they originate from a semi-conducting material and these additional resistances ultimately give rise to non-linear Tafel behaviour.

Through-Mask Electrochemical Micromachining of Aluminum in Phosphoric Acid

Aluminum micro-channels have been machined in phosphoric acid via mass-transfer limited electrochemical dissolution through photoresist masks. The results of shape evolution experiments using a rotating disk electrode are presented in terms of the dimensions, shape profile and uniformity of the machined micro-channels. The influences of applied potential, cumulative passed charge and hydrodynamic conditions on the shape evolution process are discussed. Experimental results are compared with a shape evolution model assuming the rate of aluminum removal is solely controlled by diffusive mass transfer. The degree of agreement between experimental and simulated results depends mainly on the hydrodynamic conditions in the electrochemical cell and indicates a shift from purely diffusive to mixed convective-diffusive mass transfer. The feasibility of electrochemical aluminum micromachining is demonstrated by fabricating microfluidic test structures with well-defined geometries and smooth surfaces.

Spontaneous Deposition of Iridium onto Nickel Substrates for the Oxygen Evolution Reaction

Spontaneous deposition of Ir onto Ni substrates was investigated as a method to produce electrocatalytic layers for the oxygen evolution reaction in 30 % KOH solution. UV/Vis spectroscopy, cyclic voltammetry and other electrochemical methods are used to investigate the deposition process and the activity of the electrocatalytic coating towards the oxygen evolution reaction. From three solutions (IrCl3+HCl, H2IrCl6+HCl and H2IrCl6), H2IrCl6 is shown to give the most active and stable coating, with deposition times of 45 min at 60∘C enough to increase the activity of the Ni substrate for the oxygen evolution reaction. It is proposed that Ir deposition can occur via the reduction of the Ir precursor coupled to Ni oxidation, as well as the hydrolysis and localised precipitation of the Ir precursor due to the increase in surface pH during Ni dissolution.

Investigating the Kinetics and Mechanism of Organic Oxidation in Parallel with the Oxygen Evolution Reaction

AbstractIn this paper, the mechanism of organic oxidation in parallel with the oxygen evolution reaction at an electrode following the “active” anode mechanism is investigated. The active anode (IrO2-Sb2O5-SnO2/Ti) was prepared via standard thermal decomposition method and 4-nitrophenol (4-NP) chosen as the model organic compound. It is firstly confirmed that this anode does follow the “active” anode mechanism, with the rate of 4-NP oxidation being dependent on the coverage adsorbed oxygen on the surface of the anode. This surface coverage can be estimated by fitting steady-state polarisation curves with a micro-kinetic model describing the oxygen evolution behaviour of the anode. This surface coverage dependent oxidation rate can only be observed at relatively low overpotentials where mass transport limitations are avoided. At high overpotentials, the rate of oxidation is completely controlled by mass transfer of 4-NP to the anode surface, with the measured and calculated rate constants agreeing closely. It is also shown that the instantaneous current efficiency can be directly calculated from the measured pseudo first-order rate constant in both the kinetic and mass transport limited regimes. Using this analysis method, it was found that the instantaneous current efficiency for 4-NP oxidation is less than 100% in both regimes and only approached 100% at very low overpotentials. This finding is important as in prior literature, it is often believed that the instantaneous current efficiency of electrochemical wastewater oxidation will be 100% provided that mass transfer does not limit the process, due to an underlying assumption that the rate of organic oxidation is much larger than the OER. Graphical AbstractThe surface coverage of intermediates of the oxygen evolution reaction control the oxidation rate of 4-nitrophenol