Solid oxide fuel cell electrolytes conventional

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

The principles behind this membrane technology originate from solid-state electrochemistry. Conventional electrochemical halfceU reactions can be written for chemical processes occurring on each respective membrane surface. Since the general chemistry under discussion here is thermodynamically downhill, one might view these devices as short-circuited solid oxide fuel cells (SOFCs), although the ceramics used for oxygen transport are often quite different. SOFCs most frequently use fluorite-based solid electrolytes - often yttria stabUized zirco-nia (YSZ) and sometimes ceria. In comparison, dense ceramics for membrane applications most often possess a perovskite-related lattice. The key fundamental... 

The conductivity of the prototype zirconia-type electrolytes becomes acceptable (with values of about 0.15 S/cm), only at temperatures above 900°C. For this reason the working temperature of fuel cells having such an electrolyte is between 900 and 1000°C. Such fuel cells are called conventional solid oxide fuel cells in the following chapters. 

The anodes consisting of a nickel catalyst and of cermet mixed with yttria-doped zirconia electrolyte that are used in conventional solid oxide fuel cells also lose their ability to work at lower temperatures because of a loss of conductivity by the ceramic. This suggests that, for the ceramic in the anode, a material having a higher conductivity at intermediate temperatures should be used. It was in fact shown that an anode made with a nickel/samaria-doped ceria cermet has a much lower polarization than the conventional variant. 

Lan and Tao [22] successfully applied a novel fuel cell type with an alkaline membrane to oxidize ammonia at room temperature. Compared to solid oxide fuel cells, the alkaline membrane fuel cell is less brittle and can be operated at low temperatures. As an advantage of alkaline membrane fuel cells over conventional alkaline fuel cells, no KOH-based electrolyte is needed. The researchers used two types of anodes first platinum and ruthenium deposited on carbon and sec-raid chromium-decorated nickel. The ammraiia sources were either ammraiia gas or a 35 wt% aqueous ammonia solution. 

Ion exchange membranes work in the temperature range of conventional fluid electrolytes, e.g., in electric cars from 0 °C to -1-80 °C and perhaps in the future up to -1-130 °C. This must not be confused with solid electrolytes, which are used in solid oxide fuel cells (SOFC) as oxygen ion conductors at up to 1,000 °C. Lithium ionconducting polymers are important components of high-power lithium ion secondary batteries, but that is not object of this entry. 

Solid Oxide Fuel Cells, Direct Hydrocarbon Type, Fig. 3 F iedicted gas constitution versus position in the SOFC anode support, for a current density of 1 A/cm. The top shows a cell with a conventional NiYSZ support and the bottom a Sro.8Lao.2Ti03 support Both cell types had a NiYSZ anode functional layer, YSZ electrolyte, and LSMYSZ (LSM = Lao.8Sro.2Mn03) cathode (From Ref [34])... 

As constructive alternatives, SOFCs can be used as planar or tubular SOFCs, respectively. In the past period, the planar SOFCs become more attractive for the commercialization because of their high power density and low production costs [2]. The planar SOFCs may also be divided into (i) electrolyte-supported and (ii) anode-supported solid oxide fuel cells. Also, more and more attention was focused on solid oxide fuel cells operating at low and/or intermediate temperatures. The decrease of temperature demands an electrolyte with higher ionic conductivity than, for example, the conventional YSZ. 

On the other hand, there is a great demand for alternative fuel cells operating at moderate temperatures. In this context, intermediate temperature (400-800 °C) fuel cells are very attractive since they combine the advantages of both high- and low-temperature fuel cells such as fast electrode kinetics, fuel flexibility, and fewer degradation problems [7]. Furthermore, the tendency of lower temperatures makes conventional ceramic fuel cells (mainly solid oxide fuel cells SOFCs) a leading candidate for applications such as stationary power plants but also the possibility to replace internal combustion engines in vehicles [8]. Ceramic fuel cells based on ceria-carbonate salt composite electrolytes have been intensively studied for the past decade... 

Solid Electrolytes Versus Fuel Cells Ceramic solid electrolytes are used in the production of electricity in high-temperature solid oxide fuel cells (SOFC), in which electrochemical combustion reactions occur. With around 60 % efficiency of generation (compared to 35 % in conventional methods), the SOFC have already become a useful secondary source of electric energy. Single cells are, as a mle, miniature devices (Fig. 1.11). In local power plants with power outputs of 1-10 MW... 

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