Conventional water electrolysis

In chloralkali electrolysis and conventional water electrolysis, the catholyte is strongly alkaline (cNaon, Ckoh = 30 wt%), whereas in orga-noelectrosynthesis it is neutral or acidic depending on whether a divided or undivided cell is used. 

In addition the investment cost for conventional water electrolysis is rather high, about 1 000 DM/kWe] in 1989. This would give a total investment for a world-size plant capacity 1 200 t/d of ammonia, which is about 2.5 times the value of a natural-gas-based steam reforming ammonia plant [590],... 

To evaluate the advantage of the chemical H2 carrier system depicted in Figure 4, the system was compared with conventional H2 production process using water electrolysis. Enthalpy balances of those systems per 1 mole of H. production were shown in Figure 5. Conventional water electrolysis in Figure 5(a) consumes electricity for water electrolysis process of 282 kJ-electric/H.-mol, and H2 compression of 29 kJ-e/H2-mol. The compression is assumed isentropic and 5 stages compression up to 700 bar. Total enthalpy of 311 kJ/H2-mol is required. The H2 carrier system in Figure 4(b) needs... 

Proposed H2 carrier system Conventional water electrolysis system... 

Fig. 34.5b shows the relation between the electrolytic current and the applied voltage at 800-900 °C. A rather high voltage was necessary to electrolyse the water vapour due to insufficient conductivity of the electrolyte. However, the polarization, except ohmic loss, is rather low, as indicated with dotted lines. This suggests that, if the thickness of the electrolyte is reduced, the electric power required for electrolysis can be reduced to less than that of conventional water electrolysis. 

Thus, Giddey et al. (2010) performed experiments without and with airflow to the cathode. When there was no air flowing to the cathode, H2 was produced there via conventional water electrolysis (Figure 15.24). However, with the introduction of air in the cathode, there was no H2 produced, and the cell voltage dropped in accordance with Eqn (15.36) as aresult of ORR occurring there, as described in Eqn (15.35). These... 

Eig. 6. Comparison of current density and cell voltage characteristics of the electrolysis systems where lines A and B represent steam electrolysis and the use of SPE, respectively, the conventional KOH water electrolysis, and, 2ero-gap cell geometry employing 40% KOH, at 120—140°C. 

Srinivasan S., Salzano F.J., Prospects for hydrogen production by water electrolysis to be competitive with conventional methods, Int. J. Hydrogen Energ., 2,53-59,1977. 

European industry is well placed with regards to conventional generation of hydrogen from fossil fuels and via water electrolysis. 

The term water electrolysis implicitly means that the electrochemical reactor does not contain pure water only. Conventional electrolysis requires that the solution should be electrically conducting for the process to proceed. This implies that an electrolyte should be dissolved in water. Whereas in other cases, for example electrochemical organic or inorganic processes, the presence of an inert electrolyte may constitute a problem for the separation of products, this is not the case for water electrolysis since gaseous products are obtained. Nevertheless, the electrolyte can give other kinds of problems, such as corrosion phenomena, poisoning of electrodes and so on. 

The conventional technology of water electrolysis makes use of alkaline solutions [7]. In particular, about 30% KOH is used at about 80 °C. The use of KOH, although more expensive than NaOH, is dictated by two reasons (1) KOH is more conductive (about 1.3 times) than NaOH and (2) KOH is chemically less aggressive than NaOH. A 30% concentration is used because the conductivity exhibits a maximum there. 

Figure 7.17 shows a summary of the available conditions of water electrolysis [72]. For each configuration there exists a range of performance. Conventional electrolyzers, which nevertheless are still the most common in the current production of H 2 on the intermediate and small scale, show high overpotential and a relatively small production rate. Membrane (SPE) and advanced alkaline electrolyzers show very similar performance, with somewhat lower overpotential but a much higher production rate. Definite improvements in energy consumption would come from high temperature (steam) electrolysis, which is, however, still far from optimization because of a low production rate and problems of material stability. 

The standard formation enthalpy for water is equal to 286 kj/mole H2 relative to the formation of liquid water and corresponding to (HHV) of H2. The theoretical voltage for pure water decomposition is 1.23 V. However, the majority of conventional electrolysis devices need at least 2.0 V when economically reasonable current densities are maintained. This value translates into a water electrolysis Faraday s efficiency of about 74%. If a thermal-to-electric conversion efficiency of 45% is assumed, the total equivalent heat requirement corresponds to a heat input of 859 kj/mole H2. 

Hydrogen production via conventional electrolysis largely depends upon the availability of cheap electricity (e.g., from hydroelectric generators). Consequently, only about 5% of the world hydrogen production is via electrolysis. The only complete hydrogen production process that is free of CO2 emissions is water electrolysis (if the electricity is derived from nuclear or renewable fuels). However, 97% of the hydrogen currently produced is ultimately derived from fossil energy. Currently, the... 

Figure 5. Enthalpy balance of H2 systems, (a) conventional IE system using water electrolysis, (b) the zero CO2 emission H2 carrier system, (c) conventional methane steam reforming.

Conventional electrolytic cells use aqueous solutions of KOH or NaOH or NaCl or use solid polymer matrices as the electrolyte. In industrial plants, the alkaline medium is preferred, because corrosion is more easily controlled and cheaper materials can be used than in acidic electrolysis technology. The alkaline water electrolysis using a 25 % potash lye as the electrolyte consumes about 4 kWh/Nm including energy losses and related energy demands for ancillary equipment. It is a mature technology since decades. 

---