Dario R. Dekel

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.

A Pd/C-CeO2 Anode Catalyst for High-Performance Platinum-Free Anion Exchange Membrane Fuel Cells

One of the biggest obstacles to the dissemination of fuel cells is their cost, a large part of which is due to platinum (Pt) electrocatalysts. Complete removal of Pt is a difficult if not impossible task for proton exchange membrane fuel cells (PEM-FCs). The anion exchange membrane fuel cell (AEM-FC) has long been proposed as a solution as non-Pt metals may be employed. Despite this, few examples of Pt-free AEM-FCs have been demonstrated with modest power output. The main obstacle preventing the realization of a high power density Pt-free AEM-FC is sluggish hydrogen oxidation (HOR) kinetics of the anode catalyst. Here we describe a Pt-free AEM-FC that employs a mixed carbon-CeO2 supported palladium (Pd) anode catalyst that exhibits enhanced kinetics for the HOR. AEM-FC tests run on dry H2 and pure air show peak power densities of more than 500 mW cm(-2) .

The Effect of Ambient Carbon Dioxide on Anion-Exchange Membrane Fuel Cells

Over the past 10 years, there has been a surge of interest in anion-exchange membrane fuel cells (AEMFCs) as a potentially lower cost alternative to proton-exchange membrane fuel cells (PEMFCs). Recent work has shown that AEMFCs achieve nearly identical performance to that of state-of-the-art PEMFCs; however, much of that data has been collected while feeding CO2 -free air or pure oxygen to the cathode. Usually, removing CO2 from the oxidant is done to avoid the detrimental effect of CO2 on AEMFC performance, through carbonation, whereby CO2 reacts with the OH- anions to form HCO3- and CO32- . In spite of the crucial importance of this topic for the future development and commercialization of AEMFCs, unfortunately there have been very few investigations devoted to this phenomenon and its effects. Much of the data available is widely spread out and there currently does not exist a resource that researchers in the field, or those looking to enter the field, can use as a reference text that explains the complex influence of CO2 and HCO3- /CO32- on all aspects of AEMFC performance. The purpose of this Review is to summarize the experimental and theoretical work reported to date on the effect of ambient CO2 on AEMFCs. This systematic Review aims to create a single comprehensive account of what is known regarding how CO2 behaves in AEMFCs, to date, as well as identify the most important areas for future work in this field.

A Simulator for System-Level Analysis of Heat Transfer and Phase Change in Thermal Batteries I. Computational Approach and Single-Cell Calculations

We present a complete and detailed thermal simulator designed for the computational analysis of thermal batteries from the level of a single cell up to that of the entire system. Our simulator is based on a comprehensive transient and two-dimensional (axisymmetric) mathematical heat-transfer model, with significant flexibility in the geometrical modeling and the materials used. The model accounts for different aspects of heat transfer, including conduction, joule heating, heat of reactions, and latent heat of fusion associated with electrolyte phase change (salt solidification). It is supported by a simplified mass balance involving the current drawn from the battery and accounting for the mass-transfer resistance of each of the cells components. Results presented include model verification-and-validation calculations as well as single-cell thermal battery simulations performed under realistic ? operating conditions. The latter reveal the significance of the phase-change process to heat transfer and thus to the prediction of its operation time. Solidification dynamics are found to be different in each of the cells components, emphasizing the necessity of accounting for details at the subcell level. Additional results uncover the effect of heat of reactions as well as joule heating on single-cell battery thermal behavior.

Chemical stability of poly(phenylene oxide)-based ionomers in an anion exchange-membrane fuel cell environment

In recent years, intense research interest has been focused towards the development of anion exchange membrane fuel cells (AEMFCs) due to their potential to circumvent the need for expensive platinum catalysts, tackling the high cost that impedes mass commercialization of fuel cells. However, AEMFCs are not yet practical due to the low chemical stability of the quaternary ammonium (QA) cationic groups during cell operation. Several functionalized polymers for anion exchange membranes (AEMs), including substituted poly(phenylene oxide) (PPO), have been proposed as suitable ionomeric materials, as they present good stability in strong alkaline solutions. However, while they perform well in ex situ stability tests in aqueous solutions, they still present limited performance during AEMFC operation. As the current density in the fuel cell increases, more water is consumed at the cathode side, reducing the hydration level and, in turn, increasing the nucleophilicity of OH− and its capability to attack the QA groups. Here, using our recently reported ex situ stability protocol that simulates the low-hydration environment of an AEMFC during operation, the alkaline stability of PPO-based anion exchange ionomers is measured and compared. Good agreement with previously studied QA molecules tested using the same protocol was found. Yet, the degradation processes in these ionomers are further accelerated compared to the small QA molecules as a consequence of the lower polarity of the polymer environment, which further increases the hydroxide reactivity. This study demonstrates the competence of this new ex situ stability protocol to test not only QA molecules, but also ionomers and membranes, showing alkaline stability results that are comparable to those obtained in real AEMFC tests.