Polymer electrolyte fuel cells electron transport

During the operation of a polymer-electrolyte fuel cell, many interrelated and complex phenomena occur. These processes include mass and heat transfer, electrochemical reactions, and ionic and electronic transport. Only through fimdamental modeling, based on physical models developed from experimental observations, can the processes and operation of a fuel cell be truly understood. This review examines and discusses the various regions in a fuel cell and how they have been modeled. 

Polymer-electrolyte fuel cells (PEFC and DMFC) possess a exceptionally diverse range of applications, since they exhibit high thermodynamic efficiency, low emission levels, relative ease of implementation into existing infrastructures and variability in system size and layout. Their key components are a proton-conducting polymer-electrolyte membrane (PEM) and two composite electrodes backed up by electronically conducting porous transport layers and flow fields, as shown schematically in Fig. 1(a). 

The membrane is the heart of the fuel-cell sandwich and hence the entire fuel cell. It is this electrolyte that makes polymer-electrolyte fuel cells (PEFCs) unique and, correspondingly, the electrolyte must have very specific properties. Thus, it needs to conduct protons but not electrons as well as inhibit gas transport in the separator but allow it in the catalyst layers. Furthermore, the membrane is one of the most important items in dealing with water management. It is for these reasons as well as for others that modeling and experiments of the membrane have been pursued more than any other layer [1],... 

In the polymer electrolyte fuel cell (PEFC), the electrolyte membrane, allowing transport of protons from anode to cathode, serves at the same time as a separator for electrons and reactant gases. Proton transport is largely determined by the ion-exchange... 

Fig. 4.26 The elements of the hydrogen fuel cell. Note-, (i) The student is reminded that a chemical species which loses electrons is oxidized one that gains electrons, reduced, (ii) The proton H is transported through the polymer electrolyte attached to water molecules, (H20)nH which causes a water management problem. Current research is aimed at developing proton conductors able to operate in the region of 200 °C when the proton migrates unattached to water molecules.

Micro fuel cells intended for use, e.g., with portable electronics, will be mentioned below in section 3.6, as they are often based on direct methanol fuel. Direct methanol fuel cells are also PEM fuel cells, as they are based on the transport of hydrogen ions through a solid polymer electrolyte. 

The best catalysts for the electrochemical oxidation and production of hydrogen are platinum metal and the hydrogenase enzymes. Both catalyze the reaction of two protons with two electrons to form H2, as shown in Equation 7.1. Because of its superior catalytic rates and overpotentials compared to other metals and because of its high stabihty compared to hydrogenase enzymes, platinum is currently used as the catalyst for both half reactions (the oxidation of H2 and the reduction of O2) in polymer electrolyte membrane (PEM) fuel cells, which have been proposed for automotive transportation [1]. However, the high cost of platinum provides a strong impetus for developing less expensive alternatives. 

The research into proton conducting polymer electrolytes has consistently increased in recent years due to the transport characteristics which make them promising for various electrochemical applications of interest for the electronics market, including sensors and, particularly, fuel cells [113]. Nevertheless, the proton conductivity of the known polymer systems still remains below the upper limit of proton conductivity in liquids. The major problems arise from the numerous additional requirements, other than proton conductivity, which must be met for any specific application. 

Of importance to fuel cell applications is the electron conductivity of carbons, when used as electron pathway in the porous electrode. Moreover, their functional groups with respect to hydrophobicity and water transport phenomena are also important. Also their defect density plays a role, when these defects function as nucleation sites during the synthesis of nanoparticulate catalysts and also when anchoring the particles to hinder Ostwald ripening during degradation. This chapter is further subdivided into three parts corresponding to the carbon s three main functions in polymer electrolyte membrane fuel cell (PEMFC) ... 

PEMFGs use a proton-conducting polymer membrane as electrolyte. The membrane is squeezed between two porous electrodes [catalyst layers (CLs)]. The electrodes consist of a network of carbon-supported catalyst for the electron transport (soHd matrix), partly filled with ionomer for the proton transport. This network, together with the reactants, forms a three-phase boundary where the reaction takes place. The unit of anode catalyst layer (ACL), membrane, and cathode catalyst layer (CCL) is called the membrane-electrode assembly (MEA). The MEA is sandwiched between porous, electrically conductive GDLs, typically made of carbon doth or carbon paper. The GDL provides a good lateral delivery of the reactants to the CL and removal of products towards the channel of the flow plates, which form the outer layers of a single cell. Single cells are connected in series to form a fuel-cell stack. The anode flow plate with structured channels is on one side and the cathode flow plate with structured channels is on the other side. This so-called bipolar plate... 

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