Multivalent-Ion versus Proton Insertion into Nanostructured Electrochromic WO₃ from Mild Aqueous Electrolytes

Area of Science:

• Electrochemistry
• Materials Science
• Sustainable Energy

Background:

• Mild aqueous electrolytes with multivalent metal salts are explored for eco-sustainable energy devices.
• The role of multivalent metal ions in the electrochemical reactivity of transition metal oxides in protic electrolytes remains debated.

Purpose of the Study:

• To investigate the charge storage mechanisms in nanostructured gamma-WO3 thin films.
• To compare the electrochemical reactivity in mild aqueous electrolytes of varying composition and pH.
• To clarify the role of multivalent metal ions versus protons in electrochromic reduction.

Main Methods:

• Spectroelectrochemistry was employed to study transparent nanostructured gamma-WO3 thin films.
• Electrochemical performance was evaluated in mild aqueous electrolytes with different compositions and pH levels.
• Proton and multivalent metal ion insertion mechanisms were analyzed.

Main Results:

• Reversible proton insertion was identified as the sole charge storage mechanism across a wide pH range.
• This proton insertion is effective in electrolytes using organic or inorganic Bronsted acids.
• Mechanisms involving multivalent metal ion insertion, particularly Al3+ and Zn2+, were refuted.

Conclusions:

• Proton insertion is the dominant charge storage mechanism in gamma-WO3 electrochromic reduction in mild aqueous electrolytes.
• The findings challenge existing theories on multivalent metal ion involvement.
• This research provides fundamental insights for designing advanced aqueous-based energy devices, including smart windows.