DSA - Cathode Electrodes Used in Chlor-Alkali Electrolyzers
Cathode electrodes used in chlor-alkali electrolyzers are usually made of carbon steel in diaphragm cells and nickel in membrane cells. A detailed examination of the cell voltage contributions shows a considerable impact of cathode overvoltage, influenced mainly by the electrode material and the operating current density. Commercialization of DSA anodes worldwide and increasing energy prices, provided impetus in “hydrogen evolution” applications to develop materials such as mixed oxides and high surface – high performing nickel based electrodes as cathodes to reduce energy consumption.
Current membrane cells cathodes are characterized by the following technical solutions:
- Improved electrode geometry regarding electric performances, reducing gas stagnation and improving protection of membrane separator;
- Reduction of the average interelectrodic distance – zero gap configuration;
- Implementation of an elastic cathode structure to achieve zero gap in any operating condition;
- Increase of the contact points between the electrode and the current collector, decreasing the IR drop through a more homogeneous distribution of the electric current;
- Application of catalytic cathodic coating, capable of reducing the overvoltage of hydrogen evolution reaction.
Presently, catalytic cathode coating plays a key role in CA operations in:
- Lowering power consumption;
- Ensuring operational robustness capable of withstanding plant upsets;
De Nora has recently developed and introduced to the market a new family of cathode coatings manufactured and sold under the trade-mark NRG with the following peculiarities: use of innovative chloride-free precursors, more ordered crystal lattice, enhanced surface coverage, improved activity, stability and robustness to plant upset (including “CURRENT REVERSAL” phenomena sometimes occurring during plant shutdown in case of insufficient polarization).
The new catalyst family (including NRG-R, NRG-V, NRG-S products) is capable of ensuring an HOV (Hydrogen Over Voltage) value of 60 mV (@ 3kA/m², NaOH 32%, 90°C CISEP protocol) with a considerable energy saving if referred to electrodes using Ni-Raney based coatings (HOV of 100 ÷ 120 mV) or simply sandblasted nickel (280 ÷ 350 mV).