Water electrolysis reactors

With solid (and particularly polymeric) electrolytes which at the same time function as separators, one can appreciably reduce the distance between the electrodes and hence increase the electrode area per unit of reactor volume. Very compact equipment for water electrolysis which has no liquid electrolyte has been designed. 

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. 

Limited pH changes may occur if water electrolysis reactions (Equations 3 and 4) occur at the same rate and efficiency. In a completely mixed reactor, the proton produced at the anode should neutralize the hydroxyl ion produced at the cathode. However, the results indicated that the pH decreased to less than 5.5 even under completely mixed conditions in fed-batch reactors. The pH drop indicate less hydroxyl production at the cathode, either because different electrolysis reactions occurred (other than Equation 4) or because of biochemical reactions in the reactor. The type and concentrations of ions in the solution will impact the pH changes and require further investigation. Sodium bicarbonate was used and was effective in buffering the system for the range of electric field strengths studied. 

Helium temperature at the reactor core inlet in case of steam methane reforming/ high-temperature water electrolysis, °C 950... 

High-pressure water electrolysis (HPE) Advanced light water reactor (ALWR) Yes... 

Kodym, R., Bergmann, H. and Bouzek, K. (2006) Results of modelling electrodes and reactors for the direct electrochemical drinking water electrolysis. Proceedings 57th Annual Meeting of the International Society of Electrochemistry, 27 Aug. to 1 Sept., Edinburgh/UK, p. S5-P16. 

The separation of Dg from ordinary water is carried out in stages, involving a successive reduction of the original volume to about one seventh. As electrolysis proceeds the proportion of Dg in the evolved gas rises. When it reaches 0.02%, the gas is burnt in oxygen and the HgO/DgO mixture added to the electrolyte of an earlier stage. Such an electrolytic separation has produced most of the considerable quantities of DgO already in use for moderating fast neutrons in heavy water atomic reactors. 

The main advantage of a heavy water reactor is that it eliminates the need for building expensive uranium enrichment facilities. However, D2O must be prepared by either fractional distillation or electrolysis of ordinary water, which can be very expensive considering the amount of water used in a nuclear reactor. In countries where hydroelectric power is abundant, the cost of producing D2O by electrolysis can be reasonably low. At present, Canada is the only nation successfully using heavy water nuclear reactors. The fact that no enriched uranium is required in a heavy water reactor allows a country to enjoy the benefits of nuclear power without undertaking work that is closely associated with weapons technology. 

Hydrogen can be obtained firom different sources as fossil fuels (natural gas reforming, and coal gasification), renewable fuels (biomass), algae, and vegetables or water (electrolysis and thermo-chemical cycles). Many different energy sources can be used in most of these processes heat from fossil fuel or nuclear reactors, electricity from several sources as solar energy. 

Establishing an interesting reference case for multijunction solar water splitting, multijunction PV cells with adequate useable photopotential to directly drive electrolysis have been coupled with electrolyzer systems, which can be separate or fully integrated. This approach is compatible with the PV-electrolysis reactor types described in Sect. 7.2.4. However, since the PV output of the multijunction cell is designed to have direct compatibility with electrolysis, no power conditioning units are required. A device level schematic is shown in Fig. 7.26 for the case of a triple-junction PV cell driving the electrolyzer reactions. The useable potential... 

D Eha Camacho et al. (2011) proposed a novel concept using an assisted electrochemical reaction to produce atomic hydrogen from water electrolysis for different heterorganic compounds conversion. The electrochemical reactor is divided into two compartments by a palladium membrane in which atomic hydrogen is absorbed and permeated. Organic sulfur in the oil can be desulfurized and transformed to H2S in the electrochemical compartment. In addition, Lam et al. (2012) recently presented a review of electrochemical desulfurization technologies for fossil fuels. Various electrodes and electrolytes that have been used for desulfurization accomphshed by oxidation, reduction, or both were summarized by Lam et al. in their paper. Some electrochemical desulfurization processes for transportation fuels were chosen for listing in Table 14.2. 

Figure 15.20 Electrocatalytic membrane reactor (eCMR) for electrochemical hydrogenation (eHyd) of furfural with protons produced from water electrolysis Reprinted with permission from Green et al. (2013).

The electrochemical process offers the possibilities to produce ammonia with milder working conditions than Haber-Bosch process. Ammonia can be electrochemically synthesised under atmospheric pressure. There is no thermodynamic limitation in the electrochemical process. As mentioned before, the ammonia industry depends very much on natural gas, and consequentiy releases a huge amount of CO2. With the demand for environmentally friendly industry and the depletion of fossil fuels, the use of renewable feedstock and electricity is encouraged. Recently, renewable feedstock such as H2O or H2 from water electrolysis was found to be usable in an electrocatalytic membrane reactor (Lan, Irvine, Tao, 2013). 

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