Keith Scott

Anion-exchange membranes in electrochemical energy systems†

This article provides an up-to-date perspective on the use of anion-exchange membranes in fuel cells, electrolysers, redox flow batteries, reverse electrodialysis cells, and bioelectrochemical systems (e.g. microbial fuel cells). The aim is to highlight key concepts, misconceptions, the current state-of-the-art, technological and scientific limitations, and the future challenges (research priorities) related to the use of anion-exchange membranes in these energy technologies. All the references that the authors deemed relevant, and were available on the web by the manuscript submission date (30th April 2014), are included.

The impact of mass transport and methanol crossover on the direct methanol fuel cell

The performance of a liquid feed direct methanol fuel cell based on a Nafion® solid polymer electrolyte membrane is reported. The cell utilises a porous Pt–Ru-carbon supported catalyst anode. The effect of cell temperature, air cathode pressure, methanol fuel flow rate and methanol concentration on the power performance of a small-scale (9 cm2 area) cell is described. Data reported is analysed in terms of semi-empirical models for the effect of methanol crossover by diffusion on cathode potential and thus cell voltage. Mass transfer characteristics of the anode reaction are interpreted in terms of the influence of carbon dioxide gas evolution and methanol diffusion in the carbon cloth diffusion layer. Preliminary evaluation of reaction orders and anode polarisation agree with a previous suggested mechanism for methanol oxidation involving a rate limiting step of surface reaction between adsorbed CO and OH species.

The degree and effect of methanol crossover in the direct methanol fuel cell

A simple model is presented to describe the permeation of methanol from the anode to the cathode in direct methanol fuel cell (DMFC). Measured permeation rates of water and methanol through Nafion® 117 under varied pressure differentials across the membrane are used to determine key parameters in the model. This model is able to explain the effect of oxygen pressure at the cathode and methanol concentration at the anode on the measured cell voltage-current response of the DMFC.

Performance and modelling of a direct methanol solid polymer electrolyte fuel cell

Abstract The performance and modelling of a direct methanol fuel cell based on a solid polymer electrolyte membrane (SPE) is reported. Two sizes of cell are used: a small cell with an area of 9 cm 2 and a large single cell with an area of 250 cm 2 . The fuel cell utilises a vapourised methanol fuel at a porous carbon/Pt-Ru catalyst electrode. The performance of the fuel cell is affected by the cross-over of methanol from the anode to the cathode through the polymer membrane and this behaviour is modelled. To evaluate cell performance, mathematical models are constructed which describe mass transport in the porous electrode structures and the potential and concentration distributions in the electrode regions. These models are used to predict the cell voltage and current density response of the fuel cell.

Engineering aspects of the direct methanol fuel cell system

The direct methanol fuel cell presents several interesting scientific and engineering problems. There are many engineering issues regarding eventual application concerning cell materials, feed and oxidant requirements, fuel utilisation and recovery, scale up, etc. This paper looks at several of these issues starting from the point of current, typical, cell performance. A small-scale flow cell and a large scale cell, both with a parallel channel flow bed design, are used. The structure of the direct methanol fuel cell (DMFC) is a composite of two porous electrocatalytic electrodes; Pt–Ru–C catalyst anode and Pt–C catalyst cathode, on either side of a solid polymer electrolyte (SPE) membrane. Flow visualisation on small scale and intermediate scale (100 cm2) cells has been used in the design of a new large-scale cell of 225 cm2 active area. We discuss several important engineering factors in the successful design of large scale DMFCs including the use of vapour and liquid feeds, thermal management, gas management, methanol fuel management, hydrodynamics and mass transport.

A study of the anodic oxidation of methanol on Pt in alkaline solutions

Abstract In this study, the electro-oxidation of methanol was carried out on a platinised electrode in various alkaline media to examine the role of OH ads species and the influence of anions in the electrolyte on the methanol oxidation reaction. Cyclic voltammetry, steady state and quasi-steady state polarisation, and electrochemical impedance spectroscopy methods were used to investigate the reaction kinetics and mechanism. The activity of the methanol oxidation reaction in aqueous alkaline systems was observed to vary with the pH or OH species coverage on the electrode surface. The activity decreased in the order of NaOH>Na 2 CO 3 >NaHCO 3 .

A single-chamber microbial fuel cell as a biosensor for wastewaters.

The traditional 5-day test of the biochemical oxygen demand (BOD(5) test) has many disadvantages, and principally it is unsuitable for process control and real-time monitoring. As an alternative, a single-chamber microbial fuel cell (SCMFC) with an air cathode was tested as a biosensor and the performance analysed in terms of its measurement range, its response time, its reproducibility and its operational stability. When artificial wastewater was used as fuel, the biosensor output had a linear relationship with the BOD concentration up to 350 mg BOD cm(-3); very high reproducibility; and stability over 7 months of operation. The system was further improved by reducing by 75% the total anolyte volume. In this way a response time close to the hydraulic retention time (HRT) of the biosensor (i.e. 40 min) was reached. When the small volume SCMFC biosensor was fed with real wastewater a good correlation between COD concentration and current output was obtained, demonstrating the applicability of this system to real effluents. The measurements obtained with the biosensor were also in accordance with values obtained with standard measurement methods.

Dynamics of the direct methanol fuel cell (DMFC) : experiments and model-based analysis

Abstract A laboratory-scale liquid-feed direct methanol fuel cell (DMFC) was operated with different methanol feeding strategies. A proton exchange membrane (PEM) was used as the elecytrolyte. The cell voltage response to dynamic feeding of methanol revealed that a significant voltage increase can be obtained from dynamic changes in methanol feed concentration. The observed fuel cell behaviour was analysed with a mathematical model which consists of anode mass balances, charge balances of both electrodes and electrode kinetic expressions. Anode kinetics were derived from a four-step reaction mechanism with several intermediates bound to the catalyst surface. The model also accounts for the undesired cross-over of methanol, through the PEM, towards the cathode catalyst layer. First, the model is applied to predict steady-state current–voltage characteristics. Then, the cell voltage response to dynamic changes of methanol feed concentration is simulated. The simulated results are in full agreement to experimental observations. It turns out that methanol cross-over can be reduced by periodically pulsed methanol feeding.

Performance of the direct methanol fuel cell with radiation-grafted polymer membranes

Abstract This paper reports performance data for the direct methanol fuel cell (DMFC) using membrane electrode assemblies using radiation-grafted proton exchange membranes based on polyethylene and ETFE. These membranes exhibited low methanol diffusion coefficients and were thus felt to be potentially useful in reducing possible methanol cross-over from anode to cathode. The membrane electrode assemblies were based on Nafion ® -bonded carbon-supported catalyst; platinum/ruthenium for the anode and platinum for the cathode. The cell voltage performance of the DMFC, for short duration ( ® under identical operating conditions. However, the stability of contact between the membrane and the catalyst layer requires improvement before these membranes become real alternatives to materials such as Nafion ® , for the DMFC.

Carbon dioxide evolution patterns in direct methanol fuel cells

Carbon dioxide gas management is an important issue in the development of the liquid feed direct methanol fuel cell. Data from a flow visualisation study, designed to study carbon dioxide gas evolution and flow behaviour are reported. Two different cell designs were used, one based on a simple parallel flow channel concept and the second based on a heat exchanger design concept. With the aid of a high-speed video camera, appropriate computer software and transparent acrylic cells, gas evolution was recorded in a fuel cell working environment. The influence of current density and liquid flow rate are considered. Gas evolution mechanisms and gas management techniques are discussed. The effect of scale-up on the power performance of the parallel flow channel cell is reported.