Fuel Cell Mode

The SEMR combines the membrane reactor concept with the working principle of a 

with oxygen being reduced to anions at the cathode side and transported 

In this process, air and fuel are fed to the reactor at opposite sides of a dense oxygen ion-conducting solid electrolyte membrane. The theoretical electromotive force (EMF), Eth or f ocv is calculated from the Nernst equation (1.01 V at 800 °C with pure hydrogen at the anode and air at the cathode). The voltage output (U) under load conditions obeys the following equation  

Because ofthe high temperature of operation, one major advantage of the SOFC, in comparison with other fuel cell types, is its flexibility in the choice of fuel natural gas and other hydrocarbons can be reformed within the cell stack [13], Internal reforming involves the conversion of hydrocarbons to hydrogen and carbon monoxide  

Generally, a pre-reforming ofthe fuel is carried out, whereby part of hydrocarbon is reformed in an external reactor and the remainder is internally reformed. Methane and other hydrocarbons can also be converted to hydrogen by partial oxidation (POX) this is an exothermic process which is often combined with endothermic steam reforming, and leads to an autothermic conversion of methane. 

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Fuel cells. In the fuel-cell mode of operation, reactants are fed into the cell (or battery) continuously while reaction products are removed continuously. Hence, fuel cells (the more appropriate term, e/ battery, is not commonly used) can deliver current continuously for a considerable length of time that depends primarily on external reactant storage. 

The ultimate catalysis challenge is provided by the reversible SOFC, which can be switched between electrolyzer and fuel cell mode. In this case, to achieve high efficiency in both modes, the electrodes should combine catalytic activity... 

The oxygen anode (in the regenerative mode) as a metal oxide is also able to work in the oxidic form as hydrogen anode (in the fuel cell mode). 

FUEL CELL MODE H2 + O - H20 + Electricity Produced ELECTROLYZER MODE H20 + Free Solar Energy- H2 + O... 

On the other hand, when solar electricity is insufficient and needs to be supplemented, the RFC is switched into the fuel cell mode of operation where the oxidation of one mol of hydrogen will generate 237.1 kj/mol of electrical energy plus 48.7 kj/mol of thermal energy. This waste heat can also be used for heating buildings or for preheating boiler feedwater. 

The one area where oxyhydrogen combustion is desirable is where high flame temperatures are required, such as in welding. In all other applications the most efficient system is to use fuel cells and the least expensive configuration is to use my new reversible fuel cell design (Section 1.3.5.4), which can operate both in the electrolyzer and the fuel cell modes and uses free solar energy to drive the electrolyzer. 

On the H2 side of the plant, the main inventions involve the fuel cells. They include the idea of using the "dual-function" electrolyzer and fuel cell combination units, which I call reversible fuel cells (RFCs) (Figure 4.2). This way, in the electrolyzer mode, the RFCs convert electricity into H2/ and in the fuel cell mode, they generate electricity from the H2 in storage. 

Proton exchange membranes, whether operating in electrolysis mode or fuel cell mode, have the property of higher efficiency at lower current density. There is a 1 1 relationship in electrolysis between the rate of hydrogen production and current applied to the system. 

Hydrogen Generation Mode Fuel Cell Mode ... 

Methane can be catalytically oxidized in the fuel cell mode to simultaneously generate electricity and C2 hydrocarbons by dimerization of methane using a yttria-stabilized zirconia membrane. A catalyst, used as the anode, is deposited on the side of the membrane that is exposed to methane and the cathode is coated on the other side of the membrane. When the catalyst Ag>Bi2C>3 is used as the anode for the reaction at 750> 900X and atmospheric total pressure, the selectivity to ethane and ethylene exceeds 90%. But this high selectivity is at the expense of low power output and low overall methane conversion (less than about 2%). 

An extensive review has been made on catalytic and elccu ocatalytic methane oxidation with solid oxide membranes including the fuel cell mode [Eng and Stoukidcs, 1991]. 

Catalytic reactors, where the incorporation of an appropriate catalyst can increase the conversion and lead to an increase in selectivity. These reactors can operate according to either a pumping mode or a "fuel-cell mode . 

Figure 1 shows data from cells operated with a 65% hydrogen/35% steam mixture at two different temperatures. Operation was primarily in electrolysis mode with daily intervals in fuel cell mode. The mixture choice of fuel composition corresponds to the approximate midpoint in oxygen potential in a complete system as shown in the second part (b) of the figure. Characterization at extreme conditions was not considered during this phase. 

Principles of PCMR for methane coupling (a) pumping mode (b) fuel cell mode (c) permeation mode. 

The actual operating domain in the fuel cell mode is a cell voltage Vhetween zero and the thermodynamic cell voltage Vq, i.e., F > 0, or and the current density i... 

In the absence of current (internal as well as external), V = Vq, the thermodynamic equilibrium cell potential, while for an external current 1=0, but for a finite internal (crossover) current Ix, V = OCV (Vilekar Datta, 2010). As the external current I is drawn in the fuel cell mode, the drop in potential registered V is equal to Vq minus the sum of the potential drops across all the branches in series. Thus, from V = >c — and using the definition of overpotentials along with V = 4>c,o — A,o ... 

The electrocatalytic membrane reactor in the fuel cell mode polymer-electrolyte membrane (PEM) fuel cell... 

Figure 15.23 Computed (Eqn (15.20)) performance and losses in a H2—O2 and a H2—Br2 URFC assuming same parameters for electrolysis and fuel cell modes. ...

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