Fuel cell operating conditions

The choice of immobilization strategy obviously depends on the enzyme, electrode surface, and fuel properties, and on whether a mediator is required, and a wide range of strategies have been employed. Some general examples are represented in Fig. 17.4. Key goals are to stabilize the enzyme under fuel cell operating conditions and to optimize both electron transfer and the efficiency of fuel/oxidant mass transport. Here, we highlight a few approaches that have been particularly useful in electrocatalysis directed towards fuel cell applications. 

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

Since the conductivity of Ni is more than 5 orders of magnitude greater than that of YSZ under the fuel cell operating conditions, the electrical conductivity of a porous Ni-YSZ cermet anode changes several orders of magnitude, usually from -0.1 S/cm... 

Still, the CO tolerance is too low for practical purposes. Ideally, 1000 ppm CO or more should be tolerated without a voltage loss exceeding 20 mV. Moreover, the stability of binary and ternary catalysts under fuel cell operating conditions is an issue. 

For both reactions to occur, a three-phase boundary is required where the reactant gas, protons, and electrons react at the catalyst surface. The CLs should be able to facilitate transport of protons, electrons, and gases to the catalytic sites. Under normal PEM fuel cell operating conditions (<80°C), the reactants are gaseous phase H2 and O2 (from air), and the product is water, primarily in the liquid phase. Water removal is a key factor affecting catalyst... 

Apart from mechanistic aspects, we have also summarized the macroscopic transport behavior of some well-studied materials in a way that may contribute to a clearer view on the relevant transport coefficients and driving forces that govern the behavior of such electrolytes under fuel cell operating conditions (Section 4). This also comprises precise definitions of the different transport coefficients and the experimental techniques implemented in their determination providing a physicochemical rational behind vague terms such as cross over , which are frequently used by engineers in the fuel cell community. Again, most of the data presented in this section is for the prototypical materials however, trends for other types of materials are also presented. 

Gemmen R.S. (2003) Analysis for the effect of inverter ripple current on fuel cell operating condition. Journal of Fluids Engineering 125, 576-585. 

EIS has played an important role in fuel cell technology development, as one of the most important research tools for fuel cell diagnosis. EIS can help to identify the contributions from different components or processes to the total impedance of a PEM fuel cell. Such information is very helpful for understanding the fundamental processes within the fuel cell, the performance-structure relationships, and the contributions of various components to performance loss, as well as the associated failure modes and mechanisms EIS thus assists with fuel cell design optimization and selection of the most appropriate fuel cell operating conditions. In this chapter, we will present some typical examples of the applications of EIS in PEM fuel cell research, and an overview of EIS spectra analysis. 

Under concentration control, the reversible hydrogen electrode exhibits Nemstian reversibility. This provides for a potential shift of 29.75 mV at room temperature, which translates to a shift of 46.8 mV at 200 °C for each decade of change in hydrogen concentration. Under fuel-cell operating conditions with highly dispersed electrocatalysts, it is possible to approach the kinetic rate determined by the dual-site dissociation of the hydrogen molecule, viz. ... 

When connected through an external circuit, the net result of these two half-cell reactions is the production of H2O and electricity from H2 and O2. Heat is also generated in the process. In the absence of a proper catalyst, however, neither of these two half reactions takes place at meaningful rates under PEM fuel cell operating conditions (50 to 80°C, 1 to 5 atm). Despite decades of effort in search of cheaper alternatives, platinum is still the catalyst of choice for both the HOR and ORR. 

Polyarylenes, in particular different types of poly(arylene ether ketone)s, have been the focus of much hydrocarbon membrane research in recent years. - - With good chemical and mechanical stability under PEM fuel cell operating conditions, the wholly aromatic polymers are considered to be the most promising candidates for high-performance PEM fuel cell applications. Many different types of these polymers are readily available and with good process capability. Some of these membranes are commercially available, such as poly(arylene sulfone)s and poly(arylene... 

We have demonstrated the importance of system component compatibility and manufacturing variables using examples from our product development experience (figure 12.12)." 3 Other groups have also shown the effects of non-MEA components on stack life. Stack components must be chemically and mechanically stable under fuel cell operating conditions so that they will not leak or leach out species that poison the electrode catalysts, be harmful to membrane stability and its proton conductivity, or have adverse effects on the electrode/GDL properties, such as hydrophilic/hydrophobic character. Stanic and Hoberecht linked membrane edge... 

Little can be gleaned about the nature of the alloy interface from only the cyclic current-potential curves. An important question that needs to be addressed is whether or not the cychc vol-tammograms are accompanied by changes in the surface composition of the alloy while a qualitative solution to this problem can easily be obtained from multiple voltammetric scans, a quantitative answer is fundamentally necessary. In fact, a more critical matter involves the stability the PtsCo alloy under fuel-cell operating conditions that is, after prolonged use at the OCP in an 02-saturated solution. All of these issues can be simultaneously tackled if the surface composition of the PtsCo alloy is monitored as a function of time at a given applied potential. For such measurements, the alloy electrode is withdrawn from the 02-saturated electrolyte at the test potential and, prior to transfer into the surface analysis chamber, rinsed in deaerated ultrapure (Millipore) water to remove emersed sulfuric acid. The results are shown in Fig. 11. 

Following the idea of Yu. Volfkovich, a model of stationary water flows in the membrane with account of porous structure-related aspects and inhomogeneous water distribution was developed [16,83]. This model will be presented in some detail below. Its implications on water-content profiles and current-voltage performance under fuel cell operation conditions will be compared to the effective diffusion models. 

Initially, poly(styrenesulfonic acid) (PSSA) and sulfonated phenol-formaldehyde membranes were used for PEFCs, but the useful life of these materials was limited because of significant degradation under fuel-cell operating conditions. A critical breakthrough was achieved with the introduction of Nafion , a perfluorinated polymer with side chains... 

Mukherjee, J., Linic, S. First-principles investigations of electrochemical oxidation of hydrogen at solid oxide fuel cell operating conditions. J. Electrochem. Soc. 2007,154, B919-24. 

Pure nickel tends to sinter at the fuel cell operating conditions, resulting in loss of surface area and pore growth. Additives have been used to control the sintering. For example, the addition of Cr203 to nickel has been shown to effectively prevent anode sintering, by the formation of submicrometer LiCr02 on the nickel surface. Additions of Co, Cr, and Cu metals to nickel powder have also been tried and a Ni-10% Cr alloy has become the standard anode. ... 

Several properties of cellulose phosphate have been evaluated. The material has good thermal stability and low hydrogen crossover, two requirements that are important to meet DOE fuel cell program targets. Further characterization of the material is required, especially the determination of proton conductivity. In addition, testing of the material in an MEA will allow the effect of acid stability and swelling properties of cellulose phosphate be evaluated under typical PEM fuel cell operating conditions. 

It is well known that nncoated stainless steel bipolar plates exhibit high transition impedances dne to formation of an oxide layer by corrosion under fuel cell operation conditions. 

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