Is the imidazolium cation a unique promoter for electrocatalytic reduction of carbon dioxide?

Shufeng Zhao, Mike Horne, Alan Bond, Jie Zhang

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There has been considerable recent interest in the use of the imidazolium cation as a promoter in the heterogeneous and homogeneous electrocatalysis of CO2 reduction. However, despite its widespread use for this purpose, the mechanism by which imidazolium operates is not yet fully established. The present work reveals that enhanced catalytic activity is achieved by addition of many cations other than imidazolium. Under cyclic voltammetric conditions at a Ag electrode in acetonitrile solutions (0.1 M n-Bu4NPF6), 2.0 mM concentrations of imidazolium, pyrrolidium, ammonium, phosphonium, and (trimethylamine)-(dimethylethylamine)-dihydroborate cations can all enhance the kinetics of catalytic CO2 reduction with imidazolium and pyrrolidium being the most active. Analysis of the voltammetric data suggests that imidazolium cations achieve their impact by directly acting as cocatalysts with Ag whereas the other cations affect the reaction rate by modifying the electrochemical double layer. The results also confirm that the active form of the cocatalyst is the reduced imidazolium radical which forms a complex with CO2 before being further reduced to CO or other products at Ag and not an imidazolium carboxylate formed between an imidazolium carbene and CO2. In fact, imidazolium is deactivated during CO2 reduction by the latter reaction. Addition of water inhibits this deactivation pathway allowing the imidazolium cation to remain active in a long-term for CO2 reduction. In contrast, the pyrrolidium cation, where enhanced catalysis is attributed to an electrochemical double layer effect, retains its catalytic activity for very long periods of time regardless of the presence or absence of water.

Original languageEnglish
Pages (from-to)23989-24001
Number of pages13
JournalJournal of Physical Chemistry C
Issue number42
Publication statusPublished - 27 Oct 2016

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