A transient PEMFC model with CO poisoning and mitigation by O2 bleeding and Ru-containing catalyst

Al-Fetlawi

2010

Electrochimica Acta

Self Cite

261

183

a b s t r a c tA model for hydrogen evolution in an all-vanadium redox flow battery is developed, coupling the dynamic conservation equations for charge, mass and momentum with a detailed description of the electrochemical reactions. Bubble formation at the negative electrode is included in the model, taking into account the attendant reduction in the liquid volume and the transfer of momentum between the gas and liquid phases, using a modified multiphase-mixture approach. Numerical simulations are compared to experimental data for different vanadium concentrations and mean linear electrolyte flow rates, demonstrating good agreement. Comparisons to simulations with negligible hydrogen evolution demonstrate the effect of gas evolution on the efficiency of the battery. The effects of reactant concentration, flow rate, applied current density and gas bubble diameter on hydrogen evolution are investigated. Significant variations in the gas volume fraction and the bubble velocity are predicted, depending on the operating conditions.

Section: Equation In the Porous Carbon Electrodementioning

confidence: 99%

Dynamic modelling of hydrogen evolution effects in the all-vanadium redox flow battery

Al-Fetlawi

2010

Electrochimica Acta

Self Cite

261

183

“…The reader is referred to earlier work [74][75][76][77] for further details of the standard conservation principles.…”

Section: Model Equationsmentioning

confidence: 99%

Modeling and Simulation of the Degradation of Perfluorinated Ion-Exchange Membranes in PEM Fuel Cells

Ralph

2009

J. Electrochem. Soc.

119

107

A polymer electrolyte membrane ͑PEM͒ fuel cell model that incorporates chemical degradation in perfluorinated sulfonic acid membranes is developed. The model is based on conservation principles and includes a detailed description of the transport phenomena. A degradation submodel describes the formation of hydrogen peroxide via distinct mechanisms in the cathode and anode, together with the subsequent formation of radicals via Fenton reactions involving metal-ion impurities. The radicals participate in the decomposition of reactive end groups to form carboxylic acid, hydrogen fluoride, and CO 2 . Degradation proceeds through unzipping of the polymer backbone and cleavage of the side chains. Simulations are presented, and the numerical code is shown to be extremely time-efficient. Known trends with respect to operating conditions are qualitatively captured, and the exhibited behavior is shown to be robust to changes in the rate constants. The feasibility of a chemical degradation mechanism based on peroxide and radical formation is discussed. The proton exchange membrane ͑PEM͒ fuel cell has long been recognized as an important component of the future hydrogen economy. Although significant progress has been made on issues related to performance, the commercial viability of the PEM fuel cell is largely dependent upon overcoming a host of durability and degradation issues; 1 these include poisoning of the anode by carbon monoxide and hydrogen sulfide, 2-6 platinum sintering and dissolution, 7,8 degradation of carbon supports, 9,10 and membrane failure.11,12 Indeed, the lifetime of the PEM fuel cell is highly dependent on the lifetime of the ion-exchange membrane. The majority of membranes used in PEM fuel cells have a perfluorinated backbone and are modified to include sulfonic groups that facilitate the transport of protons. Typical examples of such materials are Aciplex, Flemion, Dow, and DuPont's Nafion, shown in Fig. 1.There are several known failure modes for these perfluorosulfonic acid ͑PFSA͒ membranes during fuel cell operation, involving mechanical, thermal, and electrochemical processes. Mechanical failure can occur in many forms, including tears, cracks, punctures, pinholes, and inadequate sealing, which can lead to reactant crossover and unwanted reactions. Among the causes of mechanical failure are manufacturing imperfections, 13 The most cited cause of membrane failure is chemical degradation, although in reality failure is likely to be a combination of both mechanical and chemical phenomena, the interplay between which is not fully understood. The two paramount steps in the sequence that leads to chemical degradation are ͑i͒ formation of the attacking species and ͑ii͒ attack by the species on the membrane structure. Both remain the subject of much debate. There is, however, a consensus that the chemical degradation of PFSA membranes is initiated by free radical attack on reactive end groups.11,12 Carboxylic acid ͑or other H-containing͒ end groups can form during polymerization or as a result of chemi...

“…Suitably validated tools can be used to inform cell testing programmes, aid optimisation and cell design, and, moreover, provide insight into the fundamental processes inside a working cell. In contrast to conventional batteries such as the lead-acid and lithium-ion [7][8][9][10][11][12] and fuel cells [13][14][15][16], there are few examples RFB modelling. A transient, two-dimensional model based on conservation principles, incorporating the fundamental modes of transport and a kinetic model for reactions involving vanadium species, was developed in [17] for the all-vanadium RFB.…”

Section: Introductionmentioning

confidence: 99%

Non-isothermal modelling of the all-vanadium redox flow battery

Al-Fetlawi

2009

Electrochimica Acta

Self Cite

222

133

Redox flow battery Vanadium Modelling and simulation Energy balance Temperature a b s t r a c tAn non-isothermal model for the all-vanadium redox flow battery (RFB) is presented. The twodimensional model is based on a comprehensive description of mass, charge, energy and momentum transport and conservation, and is combined with a global kinetic model for reactions involving vanadium species. Heat is generated as a result of activation losses, electrochemical reaction and ohmic resistance. Numerical simulations demonstrate the effects of changes in the operating temperature on performance. It is shown that variations in the electrolyte flow rate and the magnitude of the applied current substantially alter the charge/discharge characteristics, the temperature rise and the distribution of temperature. The influence of heat losses on the charge/discharge behaviour and temperature distribution is investigated. Conditions for localised heating and membrane degradation are discussed.