Geoff Kelsall

LABORATORY STUDY OF ELECTRO-COAGULATION–FLOTATION FOR WATER TREATMENT

An electro-coagulation-flotation process has been developed for water treatment. This involved an electrolytic reactor with aluminium electrodes and a separation/flotation tank. The water to be treated passed through the reactor and was subjected to coagulation/flotation, by Al(III) ions dissolved from the electrodes, the resulting flocs floating after being captured by hydrogen gas bubbles generated at cathode surfaces. Apparent current efficiencies for Al dissolution as aqueous Al(III) species at pH 6.5 and 7.8 were greater than unity. This was due to additional reactions occurring in parallel with Al dissolution: oxygen reduction at anodes and cathodes, and hydrogen evolution at cathodes, resulting in net (i.e. oxidation + reduction) currents at both anodes and cathodes. The specific electrical energy consumption of the reactor for drinking water treatment was as low as 20 kWh (kg Al)(-1) for current densities of 10-20A m(-2). The water treatment performance of the electrocoagulation process was found to be superior to that of conventional coagulation with aluminium sulphate for treating a model-coloured water, with 20% more dissolved organic carbon (DOC) being removed for the same Al(III) dose. However, for a lowland surface water sample, the two processes achieved a similar performance for DOC and UV-absorbance removal. In addition, an up-flow electrocoagulator configuration performed better than a horizontal flow configuration, with both bipolar and monopolar electrodes.

High‐Performance, Anode‐Supported, Microtubular SOFC Prepared from Single‐Step‐Fabricated, Dual‐Layer Hollow Fibers

Microtubular solid oxide fuel cells (SOFCs) have been developed in recent years mainly due to their high specifi c surface area and fast thermal cycling. Previously, the fabrication of microtubular SOFCs was achieved through multiple-step processes. [ 1–3 ] A support layer, for example an anode support, is fi rst prepared and presintered to provide mechanical strength to the fuel cell. The electrolyte layer is then deposited and sintered prior to the fi nal coating of the cathode layer. Each step involves at least one high-temperature heat treatment, making the cell fabrication time-consuming and costly, with unstable control over cell quality. For a more economical fabrication of microtubular SOFCs with reliability and fl exibility in quality control, an advanced dry-jet wet-extrusion technique, i.e., a phase inversion-based coextrusion process, was developed. Using this technique, an electrolyte/electrode (either anode or cathode) dual-layer hollow fi ber (HF) can be formed in a single step. Generally, the electrolyte and electrode materials are separately mixed with solvent, polymer binder, and additives to form the outer and inner layer spinning suspensions, respectively, before being simultaneously coextruded through a triple-orifi ce spinneret, passing through an air gap and fi nally into a non-solvent external coagulation bath. In the mean time, a stream of nonsolvent internal coagulant is supplied through the central bore of the spinneret. The thickness of the two layers is largely determined by the design of the spinneret and can be adjusted by the corresponding extrusion rate, while the macrostructure or morphology of the prepared HF precursor can be controlled by adjusting coextrusion parameters such as suspension viscosity, air gap, and fl ow rate of internal coagulant. The dual-layer HF precursor obtained is then co-sintered once at high temperature to remove the polymer binder and form a bounding between the ceramic materials. In previous work, [ 4–6 ] a dual-layer HF support for microtubular SOFCs, which consisted of an electrolyte outer layer of approximately 80 μ m supported by an asymmetric anode inner layer with 35% fi ngerlike voids length, was successfully fabricated using the coextrusion and cosintering process. A single cell that was obtained after deposition

Scanning electrochemical microscopy of a fuel-cell electrocatalyst deposited onto highly oriented pyrolytic graphite

The hydrogen evolution reaction (HER) has been examined on a platinum electrocatalysts (Johnson Matthey HSA platinum black) dispersed onto a flat highly oriented pyrolytic graphite (HOPG) electrode using an atomic force microscope (AFM) modified to perform scanning tunneling microscopy (STM) and scanning electrochemical microscopy (SECM). For both STM and SECM experiments the same Pt/Ir tips produced by electrochemical etching of Pt/Ir wire followed by coating with varnish have been used. The coating process leaves only the very end of the tip exposed. Positioning the SECM tip 42 nm from one of the particles allows monitoring of hydrogen evolution from that particle as a function of substrate potential. In a separate experiment the substrate has been polarized at a potential at which hydrogen evolution occurs and the SECM tip rastered over the surface to obtain images of the local concentration of hydrogen. This map indicates the activity of hydrogen production as a function of position.

Carbon nitride nanosheet/metal–organic framework nanocomposites with synergistic photocatalytic activities

Heterogeneous photocatalysis plays a key role in the implementation of novel sustainable technologies, e.g. CO2 conversion into fuel, H2 production from water or organics degradation. The progress of photocatalysis relies on the development of tuneable photocatalysts and particularly the ability to build nanocomposites exhibiting synergistic properties with reduced electron–hole recombination rates. We report for the first time the in situ synthesis of nanocomposites of carbon nitride nanosheets (CNNSs) and metal–organic frameworks (MOFs) for application as photocatalysts. This approach leads to the ‘nano-scale mixing’ of the components, thereby enabling a greater performance compared to other types of 2D materials/MOF composites typically obtained via physical mixing. The objective is to take advantage of the complementary features of the materials while forming a heterojunction. The structural, chemical, photophysical and electrochemical properties of the nanocomposites are characterized and compared to those of the parent materials and their physical mixture. The nanocomposites retain the high specific surface area and strong visible light absorbance of MIL-100(Fe). The intimate contact between the CNNSs and the MOF particles is found to promote the electron–hole separation significantly due to the formation of a heterojunction. Hence, more efficient photocatalytic dye degradation is achieved over the composites than the physical mixture.

Reduction of tiIV species in aqueous sulfuric and hydrochloric acids i. Titanium speciation

The uv-visible absorption of aqueous TiIV and TiIII was measured in the aqueous H2SO4 and HCl supporting electrolytes used in subsequent batch recycle electrolyses for the electrosynthesis of TiIII. Thermodynamic predictions of the speciation were calculated from published stability constants for the weak sulfate and chloride complexes. However, when TiIV and TiIII sulfate solutions were mixed, the resulting species absorbed more strongly at visible wavelengths than the TiIII species alone, possibly indicating the formation of a TiIV-TiIII-sulfate charge transfer complex; no such behaviour was evident in chloride media. The visible absorption behaviour was modelled as a function of TiIII, TiIV, proton, and sulfate concentrations to obtain an estimate of the equilibrium constant for the formation of TiIV-TiIII-sulfate complex and its molar absorptivity. As TiIII species are oxidised by oxygen/air, resulting in possible losses in their generation and utilisation efficiencies, the reaction kinetics were determined spectrophotometrically; again the formation of a TiIV-TiIII-sulfate complex was implied. The voltammetric behaviour of TiIV and TiIII species at a hanging mercury drop electrode in sulfate and chloride media was highly irreversible and indicated the formation of an adsorbed intermediate during TiIV reduction, which precluded relating the voltammetric behaviour to bulk solution speciation.

From millimetres to metres: the critical role of current density distributions in photo-electrochemical reactor design

0.1 × 0.1 m2 tin-doped hematite photo-anodes were fabricated on titanium substrates by spray pyrolysis and deployed in a photo-electrochemical reactor for photo-assisted splitting of water into hydrogen and oxygen. Hitherto, photo-electrochemical research focussed largely on the fabrication, properties and behaviour of photo-electrodes, whereas both experimental and modelling results reported here address reactor scale-up issues of minimising inhomogeneities in spatial distributions of potentials, current densities and the resultant hydrogen evolution rates. Such information is essential for optimising the design and photon energy-to-hydrogen conversion efficiencies of photo-electrochemical reactors to progress their industrial deployment. The 2D and 3D reactor models presented here are coupled with a modified micro-kinetic model of oxygen evolution on hematite thin films both in the dark and when illuminated. For the first time, such a model is applied to a scaled-up photo-electrochemical reactor and validated against experimental data.

Model of Multiple Metal Electrodeposition in Porous Electrodes

An electrochemical process is being developed for recovering metals from shredded waste electrical and electronic equipment by leaching and electrowinning. In a membrane-divided electrochemical reactor, chlorine is generated at the anode and used as oxidant in an external leach reactor, in which the metals are dissolved in an acidic chloride solution. As the resulting metal ion concentrations are relatively low, a porous (e.g., graphite felt) cathode with a large specific surface area and high mass-transport rates is required to achieve acceptable rates and efficiencies of electrodeposition, the counter reaction to the anodic evolution of chlorine. Hence, as a design tool, a mathematical model was developed to predict potential, concentration, current density, and current efficiency distributions for individual metals within the (flow-through) porous cathode, as well as cell voltages and specific electrical energy consumptions of the electrochemical reactor as functions of cathode feeder potential, cathode thickness, porosity, concentrations, and flow rate and direction. To maximize current efficiencies and productivities of the predominant metal, copper, simulations suggest using an initial cathodic feeder electrode potential of -0.5 V (standard hydrogen electrode) to metallize the by electrodeposition of the bulk of the metal at -0.3 V (standard hydrogen electrode), optimal felt thicknesses depending on reactant concentrations.

Evaluation and Modeling of a Photo-Electrochemical Reactor for Hydrogen Production Operating under High Photon Flux

Photo-electrochemical hydrogen production has been a challenging process since it was initially discovered by Fujishima and Honda in the 1970s. Due to materials limitations, the solar to hydrogen conversion efficiency is low and hence research over the past 30 years has focused on the development and improvement of photoanodes and photocathodes. Whilst the development of improved materials will continue, there appears to be a lack of concurrent efforts being made in the design and development of associated reactor systems. In order to achieve the highest conversion efficiencies, both materials and reactor design need to be considered together. In particular we will show how different operating/construction parameters, such as photon flux, applied bias and film thickness can influence the operation efficiency of a photo-electrochemical reactor. To this effect a 1-D model has been devised to take into account the above system parameters and to return various kinetic constants associated with the intrinsic photo-electrochemical (PEC) process.

Co-Extrusion / Phase Inversion / Co-Sintering for Fabrication of Hollow Fiber Solid Oxide Fuel Cells

We have used a co-extrusion / phase inversion process, followed by co-sintering and then NiO reduction with hydrogen, to fabricate anode (Ni-CGO) / cerium-gadolinium oxide electrolyte (CGO) dual-layer hollow fibers (HFs) with inner diameters < 1 mm. This is the first time such dual-layer fibers have been produced as components of SOFCs, thereby decreasing the number of fabrication steps compared with single layer extrusion, while ensuring a gas-tight electrolyte layer of controlled thickness (ca. 60-80 μm). A La0.6Sr0.4Fe0.8Co0.2O3 (LSCF)-CGO cathode ca. 100 μm thick was deposited using slurry coating. Preliminary measurements of the performance of the dual-layer microtubular HF-SOFCs were made with hydrogen flowing at 5 cm min and air at 40 cm min, achieving maximum power densities of 420 W m, 800 W m and 1000 W m at 450C, 550C and 580C, respectively. A proposed stack design shows how these microtubular SOFCs could be connected in parallel to increase currents and in series for scaling up voltages, aiming at the design of practical stacks.

Electrochemical recovery of nickel from nickel sulfamate plating effluents

Nickel sulfamate solutions are widely used for industrial nickel plating, when electrodeposits with low stress are required. Partial decomposition of sulfamate with decreasing pH below ca. 2.5 degrades the properties of nickel electrodeposits, decreases the charge yield and results in spent solutions, from which nickel must be recovered before they could be discharged to sewers. Results are reported of charge yields for nickel recovery from an industrial sulfamate effluent, using an electrochemical reactor operated at constant current in batch-recycle mode and incorporating a nickel mesh cathode, a Ti/Ta2O5–IrO2 mesh anode and a cation-permeable membrane to prevent anodic oxidation of sulfamate. A micro-kinetic model was developed, treating the processes of nickel(II) and proton reduction in sulfamate solutions as two multi-step reactions involving adsorbed intermediates, NiadsI and Hads, respectively. The unknown kinetic parameters were obtained using gPROMS software by iterative fitting of the model to experimental data obtained over a range of nickel(II) concentrations and bulk solution pH, enabling evaluation of nickel(II) reduction charge yields as a function of nickel(II) concentration, bulk pH and electrode potential. A model combining the micro-kinetic equations with mass and charge balances on the reactor was used to determine the control parameters for electrochemical recovery of elemental metal from nickel(II) in batch-recycle mode. It was determined experimentally that a decrease in catholyte pH to values below ca. 2.5 resulted in a decrease in nickel(II) reduction charge yields to values below 0.9. The decrease in catholyte pH, caused by the flux of protons from the anolyte where they were generated via anodic oxygen evolution, was obviated by continuous addition of NaOH at a rate determined by the model, permitting nickel(II) recovery with an average charge yield of 0.94.