Capacitor Plague - Development of Electrolytic Capacitors With Water-based Electrolytes

Development of Electrolytic Capacitors With Water-based Electrolytes

The first electrolytic capacitor produced was an aluminum electrolytic capacitor with a liquid electrolyte, invented by Charles Pollak in January 1896. Modern electrolytic capacitors are based on the same fundamental design. On an aluminum anode, a dielectric of a very thin aluminum oxide layer is deposited chemically. A liquid electrolyte in intimate contact with the dielectric layer forms the liquid cathode of the capacitor. A spacer made of paper prevents direct contact of the oxide layer with a second aluminum foil (cathode foil), which provides the electrical connection to the liquid cathode. Hermetically sealed and provided with terminal connections, this design is used in billions of inexpensive and reliable (within their specified life span) capacitors used for electronic devices.

The electrolyte as an ionic conductor causes most of the ohmic (resistive) losses in the capacitor. Great efforts have been made over the years to reduce these losses thus increasing the ripple current handling capability, because without such improvements the most important target of development — size reductions (volumetric efficiency) — cannot be realized.

With this goal in mind, some Japanese manufacturers have developed a new, low-ohmic water-based class of electrolytes. The conductivity of water-based electrolytes compared to electrolytes with organic solvents like GBL was significantly improved. Water, with its relatively high permittivity of ε = 81, is a powerful solvent for electrolytes. As such, it dissolves salts in high concentration. The high concentration of dissolved salt ions in the electrolyte increases the conductivity. But water will react quite aggressively and even violently with unprotected aluminum. It converts metallic aluminum (Al) via a highly exothermic reaction into aluminum hydroxide (Al(OH)₃). This is accompanied by strong heat and gas development in the capacitor, and may even lead to the explosion of the capacitor. Therefore, the main problem in the development of new water-containing electrolytes is to hinder the aggressiveness of the water against aluminum, to get capacitors having a sufficiently good long-term stability.

The Japanese manufacturer Rubycon was a leader in the development of new water-based electrolyte systems with enhanced conductivity in the late 1990s. In 1998, Rubycon announced two series, ZL and ZA, of the first capacitors on the market using an electrolyte with a water content of about 40%, which were suitable for a wide temperature range from -40 to +105°C.

The improvement achieved in the conductivity of the new electrolyte can be seen by a comparison of two capacitors, both of which have a nominal capacitance of 1000 µF at 16 V nominal voltage in a package with a diameter of 10 mm and a height of 20 mm. The capacitors of the Rubycon YXG series are provided with an electrolyte based on an organic solvent, and can attain an impedance of 46 milliohms when loaded with a ripple current of 1400 mA. ZL series capacitors with the new water-based electrolyte can attain an impedance of 23 milliohms with a ripple current of 1820 mA, an overall improvement of 30%.

Other manufacturers, such as NCC, Nichicon, and Elna followed with their own new products a short time later. The new type of capacitor was called "Low-ESR" or "Low-Impedance", "Ultra-Low-Impedance" or "High-Ripple Current" series in the data sheets. The highly competitive market in digital data technology and high-efficiency power supplies adopted these new components rapidly, because of their improved performance. Even better, by improving the conductivity of the electrolyte, capacitors not only can withstand a higher ripple current rating, they are even cheaper to produce, since water is very low cost compared to other solvents. Better performance and low cost drove widespread adoption of the new capacitors for high volume products like PCs, LCD screens and power supplies.