Reactants, fuel cells

Priestnall, M., Kotzeva, V., Fish, D., and Nilsson, E. Compact Mixed Reactant Fuel Cells, Journal of Power Sources, 106,21 (2002). 

All the other gaseous reactant fuel cell systems are incomplete in that they do not have circulators, and move reactants and products by irreversible diffusion. Systems such as the proton exchange fuel cell and its companion the direct methanol fuel cell are doubly incomplete, since they lack a hydrogen mine which can produce cheap hydrogen and cheap methanol from natural gas (see Section A.1.4 and Eigure A.4, and Barclay, 2002). 

Priestnall MA, Kotzeva VP, Fish DJ, Nilsson EM (2002) Compact mixed-reactant fuel cells. J Power Sources 106 21-30... 

Papageorgopoulos DC, Liu F, Conrad O (2007) Reprint of A study ofRhxSy/C andRuSex/C as methanol-tolerant oxygen reduction catalysts for mixed-reactant fuel cell applications . Electrochim Acta 53 1037-1041... 

Advances in mixed-reactant fuel cells. Fuel Cells, 4, 436-447. 

Aziznia A, Bonakdarpour A, Gyenge EL, Oloman CW (2011) Electroreduction of nitrous oxide on platinum and palladium towards selective catalysts for methanol-nitrous oxide mixed-reactant fuel cells. Electrochim Acta 56 5238-5244... 

SOFCs have largely converged on standard configurations, such as tubular or planar, with the structural support provided by the electrolyte, the anode, the metallic intercormector, or an inert porous support material. Each of these concepts has its own combination of advantages and disadvantages. In this section, some unconventional SOFC configurations and devices are discussed, and their performance and potential applications are considered in comparison with the more conventional approaches. This will include microtubular fuel cells, mixed reactant fuel cells, micro-planar fuel cells, and dual proton-oxygen ion fuel cells. 

THE PRINCIPLE OF MIXED-REACTANT SUPPLY MIXED-REACTANT FUEL CELLS... 

Fuel cells involve use of gaseous reactants to produce electricity - most often H2-O2 within a porous electrode. Secondary cells are rechargeable. The most important systems are... 

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

When the fuel gas is not pure hydrogen and air is used instead of pure oxygen, additional adjustment to the calciJated cell potential becomes necessary. Since the reactants in the two gas streams practically become depleted between the inlet and exit of the fuel cell, the cell potential is decreased by a term representing the log mean fugac-ities, and the operating cell efficiency becomes ... 

Further, as the current density of the fuel cell increases, a point is inevitably reached where the transport of reactants to or products from the surface of the electrode becomes limited by diffusion. A concentration polarization is estabhshed at the elec trode, which diminishes the cell operating potential. The magnitude of this effect depends on many design and operating variables, and its value must be obtained empirically. 

In recent years there has been a continued interest in the use of alkali metals, notably sodium and lithium, as heat exchange media in nuclear reactors and fusion systems respectively and as chemical reactants in fuel cells. This interest is reflected in the proceedings of several major conferences which are referenced in the bibliography (see p. 2.109). 

Fuel cells [2], in contrast with the previous cells described above, operate in a continuous process. The reactants — nowadays often hydrogen and oxygen — must be fed continuously into the cell from outside. 

The problem was solved by Francis Bacon, a British scientist and engineer, who developed an idea proposed by Sir William Grove in 18.39. A fuel cell generates electricity directly from a chemical reaction, as in a battery, but uses reactants that are supplied continuously, as in an engine. A fuel cell that runs on hydrogen and oxygen is currently installed on the space shuttle (see Fig. L.l). An advantage of this fuel cell is that the only product of the cell reaction, water, can be used for life support. 

Practical galvanic cells can be classified as primary cells (reactants are sealed inside in a charged state), secondary cells (can be recharged), and fuel cells. 

Porous electrodes are commonly used in fuel cells to achieve hi surface area which significantly increases the number of reaction sites. A critical part of most fuel cells is often referred to as the triple phase boundary (TPB). Thrae mostly microscopic regions, in which the actual electrochemical reactions take place, are found where reactant gas, electrolyte and electrode meet each other. For a site or area to be active, it must be exposed to the rractant, be in electrical contact with the electrode, be in ionic contact with the electrolyte, and contain sufficient electro-catalyst for the reaction to proceed at a desired rate. The density of these regions and the microstmcture of these interfaces play a critical role in the electrochemical performance of the fuel cells [1]. 

In solid electrolyte fuel cells, the challenge is to engineer a large number of catalyst sites into the interface that are electrically and ionically connected to the electrode and the electrolyte, respectively, and that is efficiently exposed to the reactant gases. In most successful solid electrolyte fuel cells, a high-performance interface requires the use of an electrode which, in the zone near the catalyst, has mixed conductivity (i.e. it conducts both electrons and ions). Otherwise, some part of the electrolyte has to be contained in the pores of electrode [1]. 

Accordingly, serious commercially oriented attempts are currently being made to develop special gas-phase micro and mini reactors for reformer technology [91, 247-259], This is a complex task since the reaction step itself, hydrogen formation, covers several individual processes. Additionally, heat exchangers are required to optimize the energy balance and the use of liquid reactants demands micro evaporators [254, 260, 261], Moreover, further systems are required to reduce the CO content to a level that is no longer poisonous for a fuel cell. Overall, three to six micro-reactor components are typically needed to construct a complete, ready-to-use micro-reformer system. 

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