Solid polymer electrolyte electrolysis cell

D.J. Eames and J. Newman, Electrochemical conversion of anhydrous HC1 to Cl2 using a solid-polymer-electrolyte electrolysis cell, J. Electrochem. Soc., 1995, 142, 3619-3625 I. Uehara, Y. Kawami, N. Wakabayashi, M. Motone and H. Takenaka, Hydrochloric acid electrolysis using solid polymer electrolysis. I. Cell voltage characteristics, Denki Kagaku (J Electrochem. Soc. Jpn.), 1990, 58, 360-366 II. Gas purity and current efficiency, ibid., 1990, 58, 459-467. 

Sedlak J, Austin J, LaConti A (1981) Hydrogen recovery and purification using the solid polymer electrolyte electrolysis cell. Int J Hydrog Energy 6 45-51... 

Another design that is used in chlo-ralkali electrolysis, water electrolysis, and electro-organic synthesis [95-97] is the solid polymer electrolyte (SPE) cell, where an ion exchanger membrane, for example, Nafion , serves as the electrolyte, Fig. 9. The microporous catalytic reaction layers are pressed directly onto the membrane with porous current collectors allowing transport of dissolved reactants and gaseous products into and out of the reaction layer. 

Fuel cells (hydrogen-oxygen, hydrogen-air, methanol-air) and industrial electrolysis (water, chlor-alkali) using ion-exchange membranes are the most demanding applications for the membranes. In these apphcations, the membranes have often been designated as SPE, which can be read as solid polymer electrolyte or solid... 

The solid polymer electrolyte is a solid plastic material which has ion exchange characteristics that make it highly conductive to hydrogen ions. The particular material that is used for the current electrolysis cells is an analogue of TFE teflon to which sulfonic acid groups have been linked. This plastic sheet is the only electrolyte required, there are no free acidic or caustic liquids, and the only liquid used in the system is distilled water. 

In polyelectrolyte gels the variation of pH or salt concentration (cs) causes a swelling or shrinkage. Therefore, in this case chemical energy is transformed to mechanical work (artificial muscles). An increase of cs (or a decrease of temperature) makes the gel shrink. Usually, the shrinking process occurs smoothly, but under certain conditions a tiny addition of salt leads to the collapse of the gel [iii, iv]. Hydration of macroions also plays an important role, e.g., in the case of proton-conductive polymers, such as -> Nafion, which are applied in -rfuel cells, -> chlor-alkali electrolysis, effluent treatment, etc. [v]. Polyelectrolytes have to be distinguished from the solid polymer electrolytes [vi] (- polymer electrolytes) inasmuch as the latter usually contain an undissociable polymer and dissolved small electrolytes. 

When cells with solid polymer electrolytes (SPE) are used, no electrolytes need to be dissolved in the medium, and solvents, which usually cannot be employed in electrolysis, may be used [74—79]. 

Industrial cells are mainly bipolar consisting of a large number of individual plate cells connected back to back and coupled in blocks according to the filter press principle. If the electrolysis is carried out under pressure, the energy consumption can be reduced by 20%. Further recent developments are the use of porous electrodes, high temperature steam electrolysis and the SPE-process (solid polymer electrolyte). Heavy water, D2O, can be produced as a byproduct in water electrolysis through enrichment in the electrolyte. 

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. 

Ion exchange membranes have been used in various industrial fields, and have great potential for use in new fields due to their adaptable polymer membrane. As mentioned in the Introduction, membranes are characterized mainly by ion conductivity, hydrophilicity and the existence of carriers, which originate from the ion exchange groups of the membrane. Table 6.1 shows reported examples of applications of ion exchange membranes and the membrane species used in various fields. Various driving forces are usable for separation electrochemical potential, chemical potential, hydraulic pressure such as piezodialysis and pervaporation, temperature difference (thermo-osmosis), etc. Of these, the main applications of the membrane are to electrodialysis, diffusion dialysis, as a separator for electrolysis and a solid polymer electrolyte such as in fuel cells. 

Fig. 5-10 Schematic of a solid polymer electrolyte water electrolysis module consisting of 5 cells with a total electrode area of 500 cm, production capacity is 2 Nm /h, from [59]...

Perfluorosulphonic Nation membrane separators are used in direct contact with electrodes as solid polymer electrolytes (SPE) in fuel cells . In this case, the membrane is both the electrolyte and the separator. The use of perfluorosulphonic membranes as SPE started 30 years ago with the US space program Gemini and the realization of low temperature H2/O2 SPE fuel cells. Since then, the feasibility of operating the SPE fiiel cells on hydrogen/halogen couples has been demonstrated. In addition, the introduction of perfluorinated membranes for use in water and brine electrolysis and more recently in organic synthesis has taken place . 

Ahn, J. and Holze, R. (1992) Bifunctional electrodes for an integrated water-electrolysis and hydrogen-oxygen fuel cell with a solid polymer electrolyte. J. Appl. Electrochem., 22, 1167-1174. 

Fig. 5 7 Solid-polymer electrolyte cells for water electrolysis, (a) Reactions, (b) The cell arrangement (c) A demonstration electrolyser module which incorporates 34 cells and will generate up to 14 m h of hydrogen. (Courtesy CJB Developments Ltd.)...

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. 

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