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Cell Components (Electrolytes and Separators), Design and Performance

Cell Components

As shown in Figure 2.17, the typical cell components for overall water electrolysis in alkaline liquid electrolytes consist of an external power source, an anode, a cathode, a conductor (current collector), a separator, and an electrolyte. The external power source provides the energy required for water electrolysis. The two half reactions of water splitting occur on the anode (OER) and cathode (HER) as described in Section 2.2. The conductor, also called current collector, is used to support the catalyst and to ensure electrical contact of the supported catalysts to the electrodes and to

Schematic of a water electrolysis system with alkaline electrolyte

FIGURE 2.17 Schematic of a water electrolysis system with alkaline electrolyte.

compensate their weak in-plane conductivity. Various porous and electron conducting materials, such as carbon or metal foams, fleeces, or sintered bodies can be used as current collectors. The prime requirement for a current collector is conductivity. Typically, the electric conductivities of metals are above 104 S cm-1, which are more than four orders of magnitude higher than the ionic conductivities of electrolytes. In other words, the Ohmic drops in the current collectors have minor contributions to the overall Ohmic drop in electrolysis cells. The electrolyte provides the ionic conductivity between the electrodes as well as within the porous structures of the electrodes. The main function of the separator is to separate the generated oxygen and hydrogen evolved in the anodic and cathodic compartments.

Electrolytes and Separators

Besides the anode/cathode catalysts, electrolytes and separators are also critical components for water splitting. Ionic conductivity between the electrodes and within the porous structures of catalysts is provided by the electrolytes. Hence, the criteria of a suitable electrolyte are high ionic conductivity and non-corrosive to electrodes. In this regard, alkali (potassium or sodium hydroxide) solutions are the common commercial electrolytes for alkaline water electrolysis systems because they are more conductive than the other alkali metal bases (with respect to equal molarities) and unreactive to electrodes, hence, avoiding large corrosion loss.114115

During the process of water electrolysis, hydrogen ions move toward the cathode and are reduced to H2, while hydroxide ions move toward the anode and are oxidized to 02. In order to collect the generated hydrogen and oxygen and avoid the formation of a H2/02 mixture, a porous separator (diaphragm) is typically employed. The adopted porous separator should be filled with the liquid electrolyte in order to enable ionic conduction between the electrodes, simultaneously providing ionic conductivity and ensuring the separation of the gases.116 Generally, separators used for alkaline water electrolysis should satisfy the following properties:

  • 1. Rich porosity and high wettability, which favors surface affinity and filling of the electrolyte into the porous structure.117
  • 2. Small pore diameters to avoid the penetration of bubbles into the diaphragm and reduce electrolyte permeability driven by the differential pressure.
  • 3. High electrical resistance to avoid parasitic currents caused by electron conduction between the electrodes."8 "9
  • 4. Ultra-thin in dimension to ensure small Ohmic drop.120
  • 5. High pore volume to enable high ionic conductivity.
  • 6. High flexibility to make sure that the diaphragm does not break during operation or cell assembly.
  • 7. Good mechanical stability.
  • 8. Excellent chemical stability for long-life application.
  • 9. Excellent corrosion resistance in concentrated alkali solution.

To date, Zirfon is the most prominent material for separators. It is a composite material of zirconia and polysulfone.121 Endowed with the flexibility of the polymer together with the stiffness and wettability of ceramic zirconia, Zirfon is high performing and stable for alkaline water electrolysis. Hence, the separator made of Zirfon is highly stable in concentrated KOH solutions even at elevated temperatures. It is possible to produce extremely flexible separators with attractive mechanical properties by using a high loading of Zr02. Commercially, this type of separator is available from AGFA Corp (produced using a film-casting technique). The product “Zirfon PERL UTP 500” is with a thickness of approximately 500 ± 50 pm, a porosity of 50% ± 10%, and pores of approximately 0.15 ± 0.05 pm in diameter.

 
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