Fuel cells high-pressure solid oxide

In high-temperature and larger-size (250 kW-3 mW) fuel cells, carbonate or solid oxide materials are used. Their operating temperatures range from 650 to 980°C (1,200-1,800°F), and their byproduct is high-pressure steam. 

Satisfactory conductivity is maintained up to 1800 °C in air but falls off at low oxygen pressures so that the upper temperature limit is reduced to 1400 °C when the pressure is reduced to 0.1 Pa. A further limitation arises from the volatility of Cr2C>3 which may contaminate the furnace charge. The combination of high melting point, high electronic conductivity and resistance to corrosion has led to the adoption of lanthanum chromite for the interconnect in high temperature solid oxide fuel cells (see Section 4.5.3). 

The stability of ceramic materials at high temperatures has made them useful as furnace liners and has led to interest in ceramic automobile engines, which could endure overheating. Currently, a typical automobile contains about 35 kg of ceramic materials such as spark plugs, pressure and vibration sensors, brake linings, catalytic converters, and thermal and electrical insulation. Some fuel cells make use of a porous solid electrolyte such as zirconia, Zr02, that contains a small amount of calcium oxide. It is an electronic insulator, and so electrons do not flow through it, but oxide ions do. 

Commercial alkaline electrolysis occurs at temperatures up to 150 °C and pressures to 30 bar,96 and super critical electrolysis to 350 °C and 250 bar.102 Although less developed than their fuel cell counterparts which have 100 kW systems in operation and developed from the same oxides,103 zirconia and related solid oxide based electrolytes for high temperature steam electrolysis can operate efficiently at 1000 °C,104,105 and approach the operational parameters necessary for efficient solar... 

The solid oxide fue( cell (SOFC) have been under development during several decades since it was discovered by Baur and Preis in 1937, In order to commercialise this high temperature (600 - 1000°C) fuel cell it is necessary to reduce the costs of fabrication and operation. Here ceria-based materials are of potential interest because doped ceria may help to decrease the internal electrical resistance of the SOFC by reducing the polarisation resistance in both the fuel and the air electrode. Further, the possibility of using less pre-treatment and lower water (steam) partial pressure in the natural gas feed due to lower susceptibility to coke formation on ceria containing fuel electrodes (anodes) may simplify the balance of plant of the fuel cell system, and fmally it is anticipated that ceria based anodes will be less sensitive to poising from fuel impurities such as sulphur. 

The use of gas diffusion electrodes is another way to achieve high current densities. Such electrodes are used in the fuel-cell field and are typically made with porous materials. The electrocatalyst particles are highly dispersed inside the porous carbon electrode, and the reaction takes place at the gas/liquid/solid three-phase boundary. COj reduction proceeds on the catalyst particles and the gas produced returns to the gas compartment. We have used activated carbon fibers (ACF) as supports for metal catalysts, as they possess high porosity and additionally provide extremely narrow (several nm) slit-shaped pores, in which nano-space" effects can occur. In the present work, encouraging results have been obtained with these types of electrodes. Based on the nanospace effects, electroreduction under high pressure-like conditions is expected. In the present work, we have used two types of gas diffusion electrodes. In one case, we have used metal oxide-supported Cu electrocatalysts, while in the other case, we have used activated carbon (ACF)-supported Fe and Ni electrocatalysts. In both cases, high current densities were obtained. 

The low ionic resistivities of these materials (reported to be under 10 Q cm at 1000°C in some compositions) make them very attractive candidates for use in electrochemical devices such as the solid oxide fuel cell. Their proton conductivity is highly dependent on the partial pressure of water in the atmosphere. Whether these materials exhibit longterm stability in highly oxidizing and/or highly reducing atmospheres remains to be seen. Many of the preparation techniques discussed for the oxygen ion conductors should be applicable to this relatively new class of ionic conductors. 

The production of synthesis gas from methane oxidation was also studied overFe catalyst in fuel cell using solid electrolyte (YSZ) at 850-950°C at atmospheric pressure [8]. The anodic electrode was Fe and the cathode that was exposed to air was Pt. Reduced iron was more active than oxidized iron for synthesis gas formation. The maximum CO selectivity and yield were nearly 100% and 73%, respectively. Carbon deposition was reported at high methane to oxygen ration. 

Since these first reports, Iwahara and other investigators have studied the conductivities (both ionic and electronic), conduction mechanism, deuterium isotope effect, and thermodynamic stability of these materials. The motivation for most of this work derives from the desire to utilize these materials for high temperature, hydrogen-fiieled solid oxide fuel cells. In a reverse operation mode, if metal or metal oxide electrodes are deposited onto a dense pellet of this material and heated to temperature T, the application of an electric potential to the electrodes will cause a hydrogen partial pressure difference across the pellet according to the Nemst equation ... 

After the analysis of PCFB-1.0 plant design documentation, the circuit of new hybrid co-generation power plant with use of PCFB gasifier, solid oxide fuel cells, and gas turbine power plant with built-in air recuperator was proposed (see Fig. 7). Thermal capacity of power plant will be 1.14 MW at gasifier operation under pressure of 0.35 MPa and Ukrainian bituminous coal consumption of 222.6 kg/h. Electric capacity of solid oxide fuel cell module will be 375 kW and of electric capacity of high-speed gas turbine plant will be 125 kW. 

The endothermic reaction is favored by high temperature and low pressure and is accelerated by the presence of nickel or iron catalysts. NH3 can be burned directly in combustion engines or used in solid oxide fuel cells without preprocessing [238]. In alkaline and PEM fuel cells, the ammonia has first to be decomposed according to the above reaction. For the PEM cell, even trace amounts of ammonia left in the gas after decomposition must be removed [239]. 

In the case of a solid oxide fuel cell, the anode or cathode support can be relatively thin because the component does not need to bear a very high load. However, in the case of oxygen transport membranes, the porous support needs to withstand a differential pressure of 20 atm or greater. Therefore, porous supports which are several millimeters thick are often considered. Alternatively, other concepts for strengthening the support structure are considered. These can include internal structures such as multichannel tubes, distinct solid porous inserts in tubes [21] and support braces in planar geometries. Examples of such structures are shown in Fig. 6.5 and 6.6. 

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