Membrane-electrode assemblies current density

The theoretical power density of a DMFC at 0.5 V is about 1600 Wh per kg of methanol fuel, but in practice, small DMFCs for portable applications have achieved much less. If small DMFCs are designed like conventional PEM cells, including a membrane-electrode assembly (MEA), two gas diffusion layers, fuel and air channels with forced flows and current collectors, they may achieve power densities of about 0.015-0.050 W cm at temperatures in the range of 23-60°C (Lu et al., 2004), consistent with the value found at 85°C in Fig. 3.53. 

The fuel cell is basically a two-scale system. The small and large scales are determined, respectively, by membrane-electrode assembly (MEA) thickness and by the length of the feed channel. The Q3D model is designed to investigate the interplay of small- and large-scale processes in PEFC/DMFC, so that the fully 3D model of the cell is split into a model of a cell cross section (internal model) and a model of the flow in the channel (channel model). The two models are coupled via the local current density along the channel and the overall Q3D solution is obtained by iterations. 

Fig. 16 Single fuel cell characterisation at 90-150°C of membrane electrode assemblies prepared using sPEEK-ZrP (20wt. % ZrP prepared in situ in sPEEK solution). Noncommercial electrodes, 1.2 mgcm PtRu anode, 1.2 mgcm Pt at cathode. 1 M methanol feed, 2.5 bar oxygen, 3 bar. Measurements at 90 °C, A lOO C, 110°C, 120 °C, 130 °C, A 140 °C, 150 °C. Unbroken line is current density vs. cell voltage dashed line is power density vs. cell voltage...

For a GDM made of carbon fiber (e.g., carbon paper), the in-plane conductivity is normally much higher than the through-plane conductivity because most of the carbon fibers stay in the planar direction. Fortunately, since the thickness of the GDM is around 0.2 mm, the voltage loss caused by the through-plane resistance is not significant. For example, if the through-plane conductivity is 20 S cm then the voltage drop caused by one membrane electrode assembly (MEA) (with two pieces of GDM) will be 2 mV at a current density of 1 A cm. ... 

Usually, the starting point of model derivation is either a physical description along the channel or across the membrane electrode assembly (MEA). For HT-PEFCs, the interaction of product water and electrolyte deserves special attention. Water is produced on the cathode side of the fuel cell and will either be released to the gas phase or become adsorbed in the electrolyte. As can be derived from electrochemical impedance spectroscopy (EIS) measurements [14], water production and removal are not equally fast Water uptake of the membrane is very fast because the water production takes place inside the electrolyte, whereas the transport of water vapor to the gas channels is difiusion limited. It takes several minutes before a stationary state is reached for a single cell. The electrolyte, which consists of phosphoric add, water, and the membrane polymer, changes composition as a function of temperature and water content [15-18]. As a consequence, the proton conductivity changes as a function of current density [14, 19, 20). 

In general, PFSA membranes are characterized by excellent performance, electrochemical stability, suitable mechanical properties, and allow rapid startup. However, it appears necessary to ameliorate the PFSA membranes and ionomers to improve the operating efficiency of membrane-electrode assemblies of PEM electrolysers at practical current densities useful to reduce capital costs. PESA membranes used in electrochemical devices are essentially based on Nafion however, several alternative PSFA membranes with shorter pendant side chain have been developed by Dow, 3 M, Gore, Asahi Glass, Solvay Specialty Polymers, etc.. 

Aquivion E87-12S short-side chain perfluorosulfonic acid (SSC-PFSA) membrane with equivalent weight (EW) of 870 g eq and 120 pm thickness produced by Solvay Specialty Polymers was tested in a polymer electrolyte membrane water electrolyser (PEMWE) and compared to a benchmark Nation N115 membrane (EW 1100 g eq ) of similar thickness [27]. Both membranes were tested in conjunction with in-house prepared unsupported Ir02 anode and carbon-supported Pt cathode electrocatalyst. The electrocatalysts consisted of nanosized Ir02 and Pt particles (particle size 2-4 nm). The electrochemical tests showed better water splitting performance for the Aquivion membrane and ionomer-based membrane-electrode assembly (MEA) as compared to Nafion (Fig. 2.21). Lower ohmic drop constraints and smaller polarization resistance were observed for the electrocatalyst-Aquivion ionomer interface indicating a better catalyst-electrolyte interface. A current density of 3.2 A cm for water... 

As shown in Figure 8.7, all three of the sulfur impurities, SOj, HjS, and COS caused the same loss of current density versus time within experimental errors. Nagahara et al. (2008) also reported similar voltage decay rates for FCs exposed to SOj and HjS when measured at constant current density. All three membrane electrode assemblies (MEAs) underwent a rapid loss of activity within the first 3 h of... 

FIGURE 5.24 Membrane conductivity as a function of current density at 120 °C and 1.0 atm backpressure, with different RHs. Nafion -112-based membrane electrode assembly with an active area of 4.4 cm. The thickness of the Nafion - 12 membrane was taken to be 50 pm [45]. (For color version of this figure, the reader is referred to the online version of this book.)... 

FIGURE 9.1 Oxygen partial pressure as a function of current density at 70 °C and 100% RH with different backpressures. Gore -membrane-based membrane electrode assembly (MEA) area 46 cm. Gas diffusion layer (GDL) 25-DC. Stoichiometries of H2 and air 1.2 and 2.5, respectively. Single serpentine flow channel with both a width and depth of 1.0 mm [1]. 

Membrane-electrode assembly (MEA) is an anode-membrane-cathode composite structure in which each layer is made as thin as possible without losing the mechanical integrity of the composite structure or the electrochemical activity of the two electrodes. The functional advantages of such a structure include short diffusion paths of air and fuel gas to the electrochemical reaction sites, intimate contact of electrodes with the polymer electrolyte and low ionic resistance of the membrane. All these properties are needed to sustain high current and power densities [7]. 

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