Water electrolysis reverse fuel cell operation

3 Water electrolysis reverse fuel cell operation 

Conventional electrolysis uses an aqueous alkaUne electrolyte, e.g. KOH or NaOH at a weight percentage of around 30, with the positive and negative electrode areas separated by a microporous diaphragm (replacing ear- 

Efforts are being made to increase the efficiency to 80% or higher. One method is to increase the operating temperature, typically to above 1500°C, and to optimise electrode design and catalyst choice. 

The alkaline electrolysers described here are one t5q)e of fuel cells (described as such in section 3.4). Although a detailed discussion of the quantum chemical processes involved in such devices is the subject of Chapter 3, some explanation will be given here of the mechanisms responsible for gas dissociation and the electrode losses appearing in (2.20). How do the water molecules and ions of the electrolyte behave near the electrode surfaces, or 

Consider a metal surface (electrode or catalyst) with a regular lattice atomic structure. As an electrode in an electrolyser, the external voltage applied to the system causes a build-up of charges on the metal surface. In the classical description of an electrochemical device, charges of opposite sign. 

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Fuel cells operate in a manner reverse to that of electrolysis, discussed in Chapter 2, combining fuel to make electricity. The basic design consists of two electrodes separated by an electrolyte. The oldest type of fuel cell is the alkaline fuel cell where an alkaline electrolyte like potassium hydroxide is used. The hydrogen enters through the anode compartment and oxygen through the cathode compartment. The hydrogen is ionized by the catalytic activity of the anode material and electrons are released into the external circuit. The protons react with the hydroxyl ions in the electrolyte to form water. The reaction can be written as ... 

Electrolysis of water, mentioned above, had been described by the British chemists WilUam Nicholson (1753-1815) and Sir Anthony Carlisle (1768-1842) in 1800. But Grove s experiment seemed to go in the opposite direction. This reverse eleoctrolysis is the basic operation of the fuel cell—the combination of hydrogen gas (H ) and oxygen gas (O ) to produce water and energy, as described in the following chemical equation ... 

High-temperature steam electrolysis is based on reversed high-temperature fuel cells (Herring et al., 2007). The electrical energy needed to split water at 1000°C is considerably lower than electrolysis at 100°C. This means that a high-temperature electrolyser can operate at significantly higher overall process efficiencies (in... 

Equation (3) and (4) mean that the supply of the energetic e is needed to split water. This is the basic principle of water-electrolysis. The PEMFC is just the reverse operation of the SPE. Hydrogen fuel is decomposed into 2e and 2H+ by the catalytic cathode. The protons pass through the solid polymer (electrolyte) and arrive at the anode (A) to react with the electrons and the supplied oxygen. Then, water is produced. The electrons come to A via the external resistance. This fuel cell generates, ideally, about 1 V-direct current power. A stack of the cells is constructed to give the output power with, for example, 25 kW, which is set together to drive the vehicles. 

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