Daniel J.L. Brett

Fe–N-Doped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions

High surface area N-doped mesoporous carbon capsules with iron traces exhibit outstanding electrocatalytic activity for the oxygen reduction reaction in both alkaline and acidic media. In alkaline conditions, they exhibit more positive onset (0.94 V vs RHE) and half-wave potentials (0.83 V vs RHE) than commercial Pt/C, while in acidic media the onset potential is comparable to that of commercial Pt/C with a peroxide yield lower than 10%. The Fe-N-doped carbon catalyst combines high catalytic activity with remarkable performance stability (3500 cycles between 0.6 and 1.0 V vs RHE), which stems from the fact that iron is coordinated to nitrogen. Additionally, the newly developed electrocatalyst is unaffected by the methanol crossover effect in both acid and basic media, contrary to commercial Pt/C. The excellent catalytic behavior of the Fe-N-doped carbon, even in the more relevant acid medium, is attributable to the combination of chemical functions (N-pyridinic, N-quaternary, and Fe-N coordination sites) and structural properties (large surface area, open mesoporous structure, and short diffusion paths), which guarantees a large number of highly active and fully accessible catalytic sites and rapid mass-transfer kinetics. Thus, this catalyst represents an important step forward toward replacing Pt catalysts with cheaper alternatives. In this regard, an alkaline anion exchange membrane fuel cell was assembled with Fe-N-doped mesoporous carbon capsules as the cathode catalyst to provide current and power densities matching those of a commercial Pt/C, which indicates the practical applicability of the Fe-N-carbon catalyst.

In-operando high-speed tomography of lithium-ion batteries during thermal runaway

Prevention and mitigation of thermal runaway presents one of the greatest challenges for the safe operation of lithium-ion batteries. Here, we demonstrate for the first time the application of high-speed synchrotron X-ray computed tomography and radiography, in conjunction with thermal imaging, to track the evolution of internal structural damage and thermal behaviour during initiation and propagation of thermal runaway in lithium-ion batteries. This diagnostic approach is applied to commercial lithium-ion batteries (LG 18650 NMC cells), yielding insights into key degradation modes including gas-induced delamination, electrode layer collapse and propagation of structural degradation. It is envisaged that the use of these techniques will lead to major improvements in the design of Li-ion batteries and their safety features.

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.

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.

Raman Spectroscopy as a Probe of Temperature and Oxidation State for Gadolinium-Doped Ceria Used in Solid Oxide Fuel Cells

Raman spectroscopy is a noninvasive and highly sensitive analytical technique capable of identifying chemical compounds in environments that can mimic SOFC operating conditions. Here we demonstrate the use of Raman spectroscopy to perform local thermal and temporal measurements, both of which are essential if phase formation diagrams are to be mapped out and compared to thermodynamic phase stability predictions. We find that the time resolution of the Raman technique is more than sufficient to capture essential dynamic effects associated with a change of chemical composition.

Review of Materials and Characterization Methods for Polymer Electrolyte Fuel Cell Flow-Field Plates

The role of the flow-field plate is of major importance in determining the performance of a polymer electrolyte fuel cell. The flow-field plate constitutes the largest volumetric and gravimetric proportion of the fuel cell stack and has a strong bearing on the cost and efficiency of the system. This review considers the materials being used to make flow-field plates and the methods used to characterize materials properties and performance.