Fuel cell oxidants

In a hydrogen fuel cell, oxidation of H2 at the anode releases electrons into the circuit and produces aqueous H3 O ". Reduction of O2 at the cathode consumes electrons and generates OH , which combines with H3 O " to produce H2 O. The schematic diagram shows these processes. 

FC ventilation fuel cell system area (leaks into ventilation air) 5A— FC oxidant outlet fuel cell oxidant outlet (emissions)... 

The combusted air and fuel stream (stream 12) from the high-pressure fuel cell are expanded (stream 13) in a turbine expander. The work of this turbine is used to drive the low- and high-pressure air compressors. The reduced pressure exhaust stream (stream 13) is utilized as the low-pressure fuel cell oxidant stream. Although vitiated, it still has 15% oxygen. The low-pressure TSOFC operates at 0.62 volts per cell, and fuel and air utilizations of 78 and 21.9%, respectively. The spent air and fuel effluents are combusted and sent (stream 14) to the low-pressure power turbine. The turbine generator produces approximately 1.4 MW AC. The low-pressure exhaust (stream 15) still has a temperature of 649°C (1200°F) and is utilized to... 

Parker, W.G., Bevc, F.P., SureCELLTM integrated Solid Oxide Fuel Cell / Oxidation Turbine power plants for distributed power applications, in Proceedings 2nd International Fuel Cell Conference, Kobe, Japan, February 5-8, 1996, pp. 275-278. 

It is well known that catalyst support plays an important role in the performance of the catalyst and the catalyst layer. The use of high surface area carbon materials, such as activated carbon, carbon nanofibres, and carbon nanotubes, as new electrode materials has received significant attention from fuel cell researchers. In particular, single-walled carbon nanotubes (SWCNTs) have unique electrical and electronic properties, wide electrochemical stability windows, and high surface areas. Using SWCNTs as support materials is expected to improve catalyst layer conductivity and charge transfer at the electrode surface for fuel cell oxidation and reduction reactions. Furthermore, these carbon nanotubes (CNTs) could also enhance electrocatalytic properties and reduce the necessary amount of precious metal catalysts, such as platinum. 

The electrolyte membrane presents critical materials issues such as high protonic conductivity over a wide relative humidity (RH) range, low electrical conductivity, low gas permeability, particularly for H2 and O2, and good mechanical properties under wet-dry and temperature cycles has stable chemical properties under fuel cell oxidation conditions and quick start-up capability even at subfreezing temperatures and is low cost. Polyperfluorosulfonic acid (PFSA) and derivatives are the current first-choice materials. A key challenge is to produce this material in very thin form to reduce ohmic losses and material cost. PFSA ionomer has low dimensional stability and swells in the presence of water. These properties lead to poor mechanical properties and crack growth. 

Steam reforming of small organic molecules, to facilitate indirect electrochemical oxidation via H2, involves some thermodynamic inefficiency as well as formation, usually, of some CO in the H2 produced. Special catalysts for the fuel-cell oxidation of the H2 thus formed are then necessary, namely, catalysts that can effect dissociative adsorption of H from H2 in the presence of small but significant concentrations of CO in the H2. In recent years, such catalysts have been engineered (95) that allow oxidation of H2 at rates of several amperes per square centimeter in the presence of traces of CO. Similarly, a variety of modified noble metal catalysts have been developed that allow CH3OH oxidation to proceed with improved performance with respect to avoidance of self-deactivation behavior. Doping of Pt by Sn02 or Ru has been effective in this direction (96. 97). 

The relatively slow rate of hydrocarbon fuel cell oxidations prompted an intensive examination of the adsorption characteristics of organic reactants in the 1960s. Because of the low potential for the development of hydrocarbon fuel cells, such studies have largely subsided today and no modern surface analysis techniques have been applied to characterize intermediates. Conventional adsorption studies of carbonaceous species have been reviewed repeatedly (7, 9-12, 100 -, therefore, we summarize here only some essential adsorption features for fuel cell and selective electrocatalytic oxidations. 

FJ. Perez, M.P. Hierro, D. Duday, C. Gomez, M. Romero, L. Daza, Hot-corrosion studies of separator plates of AISI-310 stainless steels in molten carbonate fuel cells, Oxid. Met. 53 (2000) 375-398. 

It is not possible to simply replace Pt with a non-noble metal as these metals (in their metallic state) will quickly corrode at the cathode of a PEM fuel cell. Oxides also have a strong tendency to dissolve in such an acidic environment. Some oxides, however, are acid resistant, but despite important advances recently reported in performing ORR in acid medium on these catalysts, they are still insufficiently active to replace Pt in fuel cells [7]. A detailed review on this topic can be found in Chap. 13. Promising contenders for the substitution of Pt-based catalysts for the cathode are Fe (or Co)-based electrocatalysts since they were recently reported to exhibit relatively high activity, stability, and high power in the potential range useful for transport and portable applications of PEM fuel cells [8-10]. 

A semantic point should be mentioned the term electrocatalysis cannot strictly be applied to electron transfer steps or simple electron transfer reactions since there cannot be any noncatalyzed equivalent pathway in the absence of an electrode surface. However, adsorption effects specific to the electrode surface can influence the kinetics of an electron transfer step involving adsorbed products and/or reactants and in this limited sense electrocatalysis could arise in an electron transfer reaction. The term electrocatalysis, however, applies more correctly to the influence of electrode material and the state of electrode surfaces on the behavior of those types of chemical-catalyzed steps, e.g., dissociative chemisorption in some fuel cell oxidations, that are coupled with electron transfer processes. 

A recent approach to the study of fuel cell oxidations has used mass spectroscopy directly coupled to the cell [29], since the mass spectrum allows a clear definition of whether an observed current is due to the formation of CO2 or merely the oxidation of hydrogen. In situ IR spectroscopy is another promising method these experiments clearly show that adsorbed carbon monoxide is formed during the pulsed oxidation of all small molecules containing carbon. Unfortunately, the time taken to record a spectrum, typically 10 minutes, and the pulse modulation essential to obtain the sensitivity, make uncertain the interpretation of these observations in the context of data obtained on clean surfaces after only a few milliseconds. 

Anion-Exchange Membrane Fuel Cells, Oxide-Based Catalysts... 

Shimizu Y (2012) Anion-exchange membrane fuel cells oxide-based catalysts. In Savinell R, Ota K, Kreysa G (ed) Encyclopedia of applied electrochemistry. Springer, Berlin/Heidelberg. www. springerreference.com, doi 10.1007/Springer-Reference 303656. Accessed 17 Jan 2012... 

Anion-Exchange Membrane Fuel Cells, Oxide-Based Catalysts, Fig. 1 Oxygen reduction mechanism in alkaline system... 

Anion-Exchange Membrane Fuel Cells, Oxide-Based Catalysts, Fig. 3 RRDE curves for (a, a ), carbon-only electrode b, b ), 50 wt% LaMnOs-carbon electrode (c, c ), 80 wt% LaMnOs-carbon electrode... 

Hydrocarbon Membranes for Polymer Electrolyte Fuel Cells Platinum-Based Cathode Catalysts for Polymer Electrolyte Fuel Cells Polymer Electrolyte Fuel Cells, Oxide-Based Cathode Catalysts... 

Neutron Scattering Studies of Fuel Cell Oxides... 

---