Outline of this Thesis

a PEFC, the power density and fuel utilization have to be increased. Therefore, a solution has to be found for one of the main performance losses of DMFCs, namely the fuel crossover through the membrane. The permeated methanol oxidizes on the cathode electrode leading to a lowered fuel efficiency and cell voltage. This problem could be avoided by identifying methanol-tolerant cathode catalysts that only catalyze the desired oxygen reduction reaction (ORR) but not the parasitic methanol oxidation reaction on the cathode side. The investigation of a promising ORR-selective catalyst based on ruthenium modified with selenium (RuSex) is one topic of this thesis. Since this catalyst is free of platinum, which is a very expensive noble metal, a cost reduc-tion could be achieved by using RuSex.

Water management in a PEFC: The faster kinetics of the hydrogen oxidation com-pared to the methanol oxidation in conjunction with insignificant hydrogen crossover results in a much higher power density of the PEFC. One challenge of a PEFC sys-tem is to avoid the dehydration of the polymer membrane to ensure a proper protonic conductivity which forces the operating temperature below 100^◦C. This temperature limit dictates the operation of the fuel cell in the range where water also exists in liq-uid phase. Liqliq-uid water in the gas diffusion media and electrode can cause mass transport limitations of the reactants, especially at high current densities. Thus, wa-ter accumulation has to be avoided. The impact and transport of liquid wawa-ter in the porous media, its coupling with the protonic conductivity of the membrane and kinet-ics are discussed in the second part of this work. The aim is to provide a better water management strategy and improved component design for an enhanced fuel cell per-formance and reliability by obtaining a better understanding of the complex interaction of water with other processes occurring within the fuel cell.

1.3 Outline of this Thesis

Chapter 2 provides a theoretical background to the fundamentals of a PEM fuel cell.

All relevant fuel cell components, their properties and functions are discussed. A brief description of the basic principles of the electrochemistry in a PEM fuel cell is given.

In Chapter 3 the applied characterization techniques used in this thesis are illustrated in a compact form. The development of a novel fuel cell with embedded reference electrode is presented in Chapter 4. A proof of concept of the laser ablation tech-nique for isolating reference electrodes from a catalyst coated membrane is given.

By means of this new developed test fuel cell the complex kinetics of a DMFC anode can get separated from the cathode processes, presented in Chapter 5. The mea-sured dynamic behavior of the anode polarization is analyzed by a newly developed mathematical model that accounts for mixed potential formation and catalyst poison-ing. Chapter 6 highlights the problem of water management in a PEFC. By means of dynamic measurements and imaging techniques the two phase flow in porous media

1 Introduction

is investigated, supported by a sophisticated multi-phase model. The improved un-derstanding of the liquid water transport leads to a new GDL design, tested in single cell and stack experiments. Chapter 7 concludes this work.

2

Fundamentals of a Polymer Electrolyte Membrane Fuel Cell

PEM fuel cells use a proton conducting polymer membrane as electrolyte. The mem-brane is squeezed between two porous electrodes (catalyst layers). The electrodes consist of a network of carbon supported catalyst for the electron transport (solid ma-trix), partly filled with ionomer for the proton transport. This network together with the reactants form a three-phase boundary where the reaction takes place. The unit of anode catalyst layer (ACL), membrane and cathode catalyst layer (CCL) is called membrane-electrode assembly (MEA)¹. The MEA is sandwiched between porous, electrically conductive gas diffusion layers (GDLs), typically made out of carbon cloth 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 a fuel cell stack, that makes the anode flow plate with structured channels on one side to the cathode flow plate with structured channels on the other side. From the electrical point of view, this plate is on the one hand the “positive” cathode and on the other hand the “negative”

anode and therefore called bipolar plate (BP).

A schematic diagram of the fuel cell configuration and basic operating principles of a hydrogen PEM fuel cell (PEFC) and a direct methanol fuel cell (DMFC) are shown in Fig. 2.1.

A short overview about the fundamentals of a PEM fuel cell is given in the following.

1Since the catalyst layers are often brought onto the GDL and then hot-pressed with the membrane (5-layer MEA), the 3-layer structure of directly prepared CLs on the membrane is often called catalyst coated membrane (CCM).

2 Fundamentals of a Polymer Electrolyte Membrane Fuel Cell

Figure 2.1: Schematic of a PEM fuel cell. The cathode and anode reactions are given for a hydrogen PEFC and a DMFC. At high current density or high liquid water saturation of the GDL the shading of the catalyst layer by the rib area causes mass transport limitation, here illustrated for the cathode side.

2.1 PEM Fuel Cell Components, their Properties and Functions

2.1.1 Bipolar Plate (BL)

The outer layer of a fuel cell is the current collector, or bipolar plate, for the case of a cell in a stack. As the name implies, the bipolar plate carries the electrons from the anode of a cell to the cathode of the adjacent cell. Additionally, a flow channel structure is embedded in the bipolar plate to deliver oxygen and fuel, respectively, over the cell area. Also, produced liquid water gets removed by the gas stream in the channels. By means of an elaborated flow field design, the appearance of inho-mogeneities in reactant concentrations, humidity and temperature within the cell area can be reduced to a certain degree. Gas velocity as well as pressure loss in the flow channel strongly impact liquid water removal in the channel itself and in the subjacent GDL by gas shortening. The bipolar plates give the fuel cell mechanical stability and therefore assure for homogeneous contact pressure within the cell area. The bipolar plates are made out of metal or graphite compound. The material has to be gas tight, so that in a stack no reactants can permeate from the anode to the adjacent cathode or vice versa.

2.1 PEM Fuel Cell Components, their Properties and Functions

2.1.2 Gas Diffusion Layer (GDL)

The gas diffusion layer acts as gas diffuser and as electron carrier. In Fig. 2.1 it can be seen that large areas of the catalyst layer would be covered by the ribs of the flow field if no GDL is attached. This would lead to blocking of reactants to the catalyst layer under the ribs and therefore to huge dissipation of active area. With a GDL of sufficient thickness, reactants can flow by diffusion or convection, depending on the flow field design, to the shadowed area under the ribs. A second functionality of the GDL is to provide the electron pathway from the catalyst layer to the bipolar plates or vice versa. The GDL is typically made out of carbon cloth or carbon paper, impregnated by polytetrafluoroethylene (PTFE) to increase the hydrophobicity. A non-wetting surface of the GDL fibers is essential for a proper liquid water transport from the catalyst layer to the gas channel. The GDL also provides mechanical support for the flexible MEA, especially under the channel region.

2.1.3 Catalyst Layer (CL)

The heart of a fuel cell is the catalyst layer where the chemical energy of the reactants is converted into electrical energy. The catalyst layer forms a very complex structure and provides several functions: (1) the CL should provide a large active area (three-phase boundary) to minimize the activation overpotential for a given current density;

(2) a catalyst is needed to improve the kinetics; (3) a sufficient reactant and educt pathway should be available. All three requirements should be met, provided that low costs are achieved. Therefore, a technical gas diffusion electrode for fuel cell appli-cations is a highly porous structure of dispersed carbon supported catalyst, generally platinum-based. The carbon network is partially filled with ionomer, acting as binder in the layered structure and providing proton pathway. The porous CL with a thickness of generally10−20µmincreases the active area for several orders of magnitude with regard to the geometrical area of length×width, which results in high current density values. A SEM image of a typical catalyst layer is shown in Fig. 2.2.

The CL shows mixed wettabilities, because the carbon support has a non-wetting surface, whereas the ionomer is strongly hydrophilic. For insufficient liquid water re-moval, the liquid water can cover the active sites, clog the void space and therefore hinders gas diffusion, which lowers the electrode performance.

Depending on the fuel, the CL can suffer from catalyst poisoning. For example, a small amount of carbon monoxide (CO) in the hydrogen stream, which is the case for reformat gas, or as intermediate species in the methanol oxidation reaction in DMFC application would lead to strong bonded CO on the catalyst sites, where the latter becomes inactive. This phenomenon is called catalyst poisoning.

2 Fundamentals of a Polymer Electrolyte Membrane Fuel Cell

Figure 2.2: SEM image of a catalyst layer shows the complex structure of pore space and solid matrix.

2.1.4 Polymer Membrane

The membrane of a PEM fuel cell acts as separator of the anode and cathode com-partment and as proton conducting electrolyte. The most well known polymer mem-brane is Nafion^R (DuPont^TM), against which others are judged. The molecular struc-ture based on polytetrafluoroethylene (PTFE), sulphonated by adding a side chain of sulphonic acidHSO₃ (see Fig. 2.3(a)). TheHSO₃ group is ionically bonded (SO₃⁻) on the PTFE backbone, and therefore called an ionomer. The presence ofSO₃⁻andH⁺ ions leads to highly hydrophilic clusters within a generally hydrophobic structure. The hydrophilic regions can adsorb a large quantity of water due to interaction of water, having a dipole character, with theSO⁻₃ group. Within these hydrated regions theH⁺ ions are relatively weakly bonded on the side chains, which leads Nafion^R to a proton conductive material. The proton conductivity is a strong function of its hydration level [1], shown in Fig. 2.3(b) where the conductivity of Nafion^R at different temperatures is plotted against the relative humidity of the environment in which the membrane is exposed. Nafion^R exhibits high chemical and thermal stability, and is stable against chemical attack in strong bases, strong oxidizing and reducing environments at tem-peratures up to 125^◦C.

The separation of the reactants by the polymer membrane is sufficient in case of a hydrogen PEM fuel cell (PEFC), where the gas crossover reduces the open circuit potential at most about 200mV. Problematical is the use of liquid methanol as fuel.

Due to the similar physical/chemical properties of methanol and water, the uptake and permeation of methanol through the membrane is as high as for water, which results in strong performance losses due to mixed-potential formation. To prevent this prob-lem, new membrane materials have to be developed which do not need the uptake of water for a good ionic conductivity and therefore can be of hydrophobic nature. This would inhibit the methanol diffusion through the membrane. A second alternative to prevent mixed potential on the electrode is the development of selective catalyst for