Nigel P. Brandon

Electrochemical Characterization of La0.6Sr0.4Co0.2Fe0.8 O 3 Cathodes for Intermediate-Temperature SOFCs

The electrochemical properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) have been assessed for its application as a cathode in intermediate-temperature solid oxide fuel cells. van der Pauw dc conductivity, two-electrode impedance, and three-electrode measurements were carried out to investigate the kinetics of the oxygen reduction reaction at various temperatures, oxygen partial pressures, and polarization values. A change in cathode behavior at temperatures around 600°C was observed. This is interpreted in terms of LSCF behaving as a mixed ionic electronic conductor at temperatures above around 600°C, oxygen reduction being stimulated by the formation of oxygen vacancies with increasing cathode overpotential. However, at temperatures below 600°C the contribution of mixed conductivity is low, and cathode behavior can then be interpreted in terms of the classical triple-phase-boundary model.

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.

Measurement of the current distribution along a single flow channel of a solid polymer fuel cell

We present a method of performing high spatial and time-resolution, non-intrusive and dynamic current measurements along the length of a single flow channel in a solid polymer fuel cell. Current profiles at different cell polarisations and reactant flow rates are examined along with the dynamic response of the fuel cell upon introduction of reactant gases.

Fuel cells for micro-combined heat and power generation

Micro-combined heat and power (CHP) holds great potential for lowering energy cost and CO2 emissions in the residential housing sector. Of the various micro-CHP technologies, fuel cells, and in particular solid oxide fuel cells, show great promise due to their high electrical efficiency and resulting low heat-to-power ratio that is better suited to residential applications. However, fuel cells are still under development and the capital cost of units available today remains high. This paper looks at the technological aspects and operating modes of fuel cells relevant to micro-CHP as well as examining the state of commercial development, life cycle issues and the techno-economics of fuel cells for micro-CHP at the residential scale.

Engineering porous materials for fuel cell applications

Porous materials play an important role in fuel cell engineering. For example, they are used to support delicate electrolyte membranes, where mechanical integrity and effective diffusivity to fuel gases is critical; they are used as gas diffusion layers, where electronic conductivity and permeability to both gas and water is critical; and they are used to construct fuel cell electrodes, where an optimum combination of ionic conductivity, electronic conductivity, porosity and catalyst distribution is critical. The paper will discuss these characteristics, and introduce the materials and processing methods used to engineer porous materials within two of the leading fuel cell variants, the solid oxide fuel cell and the polymer electrolyte membrane fuel cell.

Growth kinetics of bubbles electrogenerated at microelectrodes

The growth kinetics of electrogenerated hydrogen, oxygen and chlorine gas bubbles formed at microelectrodes, were determined photographically and fitted by regression analysis to the equation;r(t)=βtx, wherer(t) is the bubble radius at timet after nucleation,β the ‘growth coefficient”, andx the ‘time coefficient’. The coefficientx was found to decrease from a short time (< 10 ms) value near unity, typical of inertia controlled growth, through 0.5, characteristic of diffusional control, to 0.3, expected for Faradaic growth, at long times (\s> 100 ms). The current efficiency for bubble growth increased with bubble lifetime, reflecting the decrease in local dissolved gas supersaturation. The pH dependency of the bubble departure diameter indicated that, in surfactant-free electrolytes, double layer interaction forces between the negatively charged hydrogen evolving cathode or positively charged oxygen/chlorine evolving anode and positively (pH \s< 2) or negatively (pH \s> 3) charged bubbles, were the determining factor. The effect of addition of an increasing concentration of cationic (DoTAB) or anionic (SDoS) surfactant was to progressively reduce the pH effect on departure diameter, due to surfactant adsorption on the bubble and, to a lesser extent, on the electrode.

Localized impedance measurements along a single channel of a solid polymer fuel cell

A method is presented, for the first time, for measuring the localized electrochemical impedance spectroscopy response over a frequency range of 0.1 Hz to 10 kHz as a function of position in a solid polymer fuel cell. The highly idealized fuel cell on which the measurements were performed is composed of a single linear flow channel. Measurements have been made at both 0.8 and 0.6 V. A distribution of impedance characteristics is seen along the channel with evidence of mass transport effects that are not evident from localized dc measurements. The membrane conductivity does not vary with position at both potentials, as is expected from the fact that reactant gases are fully humidified. A time constant characteristic of convective transport within the flow channel dominates the low frequency response at low potentials. This response is caused by consumption of reactant upstream of the point at which the measurement is made.

Towards intelligent engineering of SOFC electrodes: a review of advanced microstructural characterisation techniques

Abstract Solid oxide fuel cells (SOFCs) are high temperature electrochemical devices with the potential for clean and efficient power generation. The electrodes, which support the electrochemical reactions, play a vital role in determining the performance and durability of these devices. Effective electrode materials must balance a spectrum of criteria, including cost, thermal and chemical stability, electronic conductivity and catalytic activity. A number of successful electrode materials have been identified; the most widely adopted materials are composite structures providing electronic, ionic and gas phase percolation, which promotes electrochemical activity throughout the bulk of the electrode. The contiguous contact of electronic, ionic and gas phases at so called triple phase boundaries provides a direct indication of the electrochemical activity of the electrode. Improvements in tomography techniques have allowed SOFC electrode microstructures to be characterised in three dimensions, giving unprecedented access to a wealth of microstructural information on the nature of triple phase contact and percolation. With improved availability of advanced tomographic techniques, fuel cell developers are increasingly equipped to link processing routes to electrode microstructure and in turn electrochemical performance, such that the intelligent engineering of SOFC electrodes is becoming a reality. Here we review the development and application of these advanced microstructural characterisation techniques.

A review of domestic heat pumps

Heat pumps are a promising technology for heating (and cooling) domestic buildings that provide exceptionally high efficiencies compared with fossil fuel combustion. There are in the region of a billion heat pumps in use world-wide, but despite their maturity they are a relatively new technology to many regions. This article gives an overview of the state-of-the-art technologies and the practical issues faced when installing and operating them. It focuses on the performance obtained in real-world operation, surveying the published efficiency figures for hundreds of air source and ground source heat pumps (ASHP and GSHP), and presenting a method to relate these to results from recent UK and German field trials. It also covers commercial aspects of the technologies, the typical savings in primary energy usage, carbon dioxide emissions abatement that can be realised, and wider implications of their uptake.

Functionally graded cathodes for solid oxide fuel cells

Functionally graded Solid Oxide Fuel Cell cathodes have been prepared from mixtures of strontium doped lanthanum manganite (LSM) and yttria stabilised zirconia (YSZ) using screen printing techniques. Samples were characterised using scanning electron microscopy, elemental dot mapping, and electrochemical impedance spectroscopy. Characterisation using AC impedance techniques showed that each cathode could be analysed in terms of a low frequency, mid frequency and high frequency response. Results showed that as the level of YSZ-LSM grading within the cathode increased, the polarisation resistance decreased. No region of the graded cathode should contain less than 20 wt% LSM to prevent an accompanying increase in series resistance.