A stable and highly conducting electrolyte for electrochromic devices such as 'smart' windows has been made by scientists in Portugal.

Electrochromic materials can respond to electricity by changing colour. This is the principle that allows so-called smart windows to change between transparent and opaque states at the simple flick of a switch. 

Electrochromic devices contain electrodes separated by an ion-conducting electrolyte. A burst of electricity triggers a rush of cations through the electrolyte and into the electrochromic material (usually tungsten trioxide) where they find a home within its structure. This electrochemical reaction changes the transparency of the electrochromic material and another burst of electricity can change it back. 

An important part of the smart window set-up is the electrolyte, which is the focus of research being carried out at the University of Trás-os-Montes and Alto Douro. Verónica de Zea Bermudez, who lead the research, explained, 'the electrolyte must display, not just high ionic conductivity, but also high electrochemical stability.' And this is exactly what has been achieved by their use of the sol-gel technique to make new electrolytes.

The sol-gel technique is a versatile method of making materials and de Zea Bermudez's team used it to produce an organically modified silicate polymer known as a di-ureasil ormolyte. The researchers doped this silica-based hybrid material with potassium triflate (KCF₃SO₃) and investigated its effect on potassium ion conductivity.

On optimising the level of doping, the researchers said they were surprised by just how high their ion conductivity measurements were. After further promising results showing good electrochemical stability, the ormolyte was tested within a prototype smart window device, fully demonstrating its potential. 

Michel Armand, an expert in ion conducting materials at the University of Picardy Jules Verne, France, explained that the difference between this and conventional electrolytes is the use of potassium cations instead of lithium or hydrogen. He added that the higher conductivity and the fact that residual water has little or no effect on it, were the major advantages of the material.

Gavin Armstrong