Alkaline electrolysis cell

Alkaline electrolysers use an aqueous KOH solution as an electrolyte (Floch et al., 2007). Alkaline electrolysis is best suited for stationary applications that are operating at pressures up to 25 bar. Alkaline electrolysers have been commercially for a long time. The following electrochemical reactions take place inside the alkaline electrolysis cell ... 

Alkaline electrolysis cell the electrolyte is a conductive hydroxide salt (mainly KOH) and the process works around 100°C. 

TVvo main processes are considered to produce hydrogen from water electrolysis at quite low temperature (about 100°C). These processes are the most advanced ones and, up to now, one is already commerdafised the alkaline electrolysis cell. The second one, PEMEC, seems to be very promising, especially for coupling with renewable energy. Both systems presented below are based on the fuel cell technology assodated and described in Section 15.3. 

Electrolysis cell. This is shown in Fig. VI, 31, 1 and is almost self-explanatory. The cylindrical cell of Pyrex glass (6" long by 2 " diameter) is cooled by immersion in a cooling bath. The electrodes consist of two platinum plates (4 cm. X 2-5 cm. X 0-3 mm.), which are placed about 2 mm. apart. The temperature of the electrolyte is maintained at 30-35° by means of the internal cooling coil and also by immersion of the cell in ice-water. A current of 1 5-2 0 amperes is passed until the electrolyte becomes slightly alkaline, which normally takes about 20-50 per cent, longer than the calculated time on the basis of the current and the amounts of acid employed. It is advantageous to reverse the direction of the current occasionally. 

Figure 7.2 Cell designs, (a) Conventional alkaline electrolysis (b) advanced alkaline electrolysis (zero gap) (c) SPE configuration (acid electrolysis).

Water electrolysis is usually carried out at constant current so that the performance of electrocatalysts should be constantly stable. Nevertheless, it has been shown that shutdown of cells in alkaline electrolysis results in the temporary dissolution of Ni cathodes. This implies that any variation of the electrolysis regime results in weakening of the resistance of materials to aggressive conditions. In other words, perturbations in the conditions of electrolysis are responsible for the appearance and the growth of the instability [cf AVt in Equation (7.16)]. 

The average conditions extracted from Figure 7.17 are summarized in Table 7.2. Overall, actual alkaline electrolysis requires an energy consumption of 4.0-4.9 kW hm of H2. Consumption somewhat lower than 4.0kWhm has recently been claimed for SPE cells [73]. The current yield and H2 purity are seen to be close to 100% in alkaline electrolysis. 

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 ... 

One of the technically and commercially most important cation-exchange membranes developed in recent years is based on perfluorocarbon polymers. Membranes of this type have extreme chemical and thermal stability and they are the key component in the chlorine-alkaline electrolysis as well as in most of today s fuel cells. They are prepared by copolymerization of tetrafluoroethylene with perfluorovinylether having a carboxylic or sulfonic acid group at the end of a side chain. There are several variations of a general basic structure commercially available today [11]. The various preparation techniques are described in detail in the patent literature. 

Alkaline electrolysis is a mature technology. It features a good efficiency (-66% LHV), an excellent lifetime of cell (above 20 years currently), and a production of 99.8% pure hydrogen at 30 bars. This leads to a global efficiency of -24% (based on a heat/electricity conversion efficiency of -35%). The main issue is the large fraction of the production cost (-80%) tied to the consumption of electricity (typically 2.6 out of EUR 3.2/kg H2 at EUR 54/MWh) [3.4 out of USD 4.2/kg H2 at USD 70/MWh], Besides, progress is sought to reduce the investment cost. 

The thermochemical cycles (S-I > 850°C) or hybrid cycles (S-electrolysis > 850°C) still feature many uncertainties in terms of feasibility and performances. Uncertainties still exist in parts of the flow sheet and technologies needed to provide high temperature heat whether from solar or nuclear nature. Potential assets of thermochemical cycles lie in a theoretical potential for a global efficiency above 35% and a scaling law of the hydrogen plant after the volume of reactants instead of the total surface of electrolytic cells. In return, their practical feasibility and economic viability have to be entirely demonstrated. Especially, a global efficiency above 30% is to be demonstrated to compete with alkaline electrolysis. Moreover, the safety of co-located nuclear and chemical plants has to be demonstrated. 

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... 

Key to meeting the market requirements is reducing the cost of electrolysis. Stuart s patented alkaline water electrolysis cell technology is designed to achieve the cost targets demanded by transportation fuels. The Double Electrode Plate... 

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