The Selective Electrochemical Conversion of Preactivated CO2 to Methane

Oana R Luca,Nathan F Dalleska,Carl A Koval,Charles C L Mccrory

doi:10.1149/2.0371507jes

Abstract

This work reports the selective electrochemical conversion of CO2 to methane, the reverse reaction of fossil fuel combustion. This reaction is facilitated by preactivation of the CO2 molecule with an N-heterocyclic carbene (NHC) to form a zwitterionic species in the first step. In the presence of Ni(cyclam)2+ and CF3CH2OH, this species is shown to undergo further electrochemical reduction of the bound-CO2 fragment at glassy carbon cathodes in dichloromethane electrolyte solution. Labeling studies confirm the origin of the carbon and protons in the methane product are the preactivated CO2 and trifluoroethanol respectively.

Highlights

N-heterocyclic carbenes (NHCs) are strong Brønsted bases with conjugate acids with pKa values that vary between 17 and 25, comparable to alkoxides.[24]. They are capable of reacting with ketenes forming [1,2]-dipolar species that can undergo subsequent reactions with electrophiles and nucleophiles (Figure 1a).[24]
In a directly analogous reaction, N-heterocyclic carbenes can reversibly react with CO2 (Figure 1b), their subsequent reactions with electrophiles and nucleophiles have not been explicitly explored
In order to overcome the kinetic limitations and the consequent high overpotentials typically observed for electrochemical reduction of CO2, we explored the possibility of converting CO2 into a different species, potentially circumventing the challenges generally encountered in the direct reduction of CO2

Summary

Introduction

Intermittent power sources such as solar and wind become an increasingly important part of the global energy portfolio, there is a concurrent need for efficient energy storage.[1,2,3] many storage strategies exist, one of the most promising is the electrochemical production of carbon-containing chemical fuels from carbon dioxide (CO2) using earth-abundant catalysts.[4,5,6,7,8,9] selective electrochemical conversion of CO2 by greater than 2 e− to products such as methanol (CH3OH, 6 e−)[10,11] and methane (CH4, 8 e−) has not yet been achieved.[12,13,14,15] We report the first selective conversion of CO2 to an 8 e− reduced product by preactivating CO2 with organic compounds to form CO2-adducts with lower activation barriers to subsequent reduction steps.[11,16,17,18,19,20,21,22] We used N-heterocyclic carbenes (NHCs), 1,3-bis (2,6-diisopropylphenyl)imidazole, that are known to readily react with CO2 to form neutral zwitterionic NHC-CO2 adducts, reported as intermediates in organocatalytic reduction chemistry.[16,17,18,19,20,21,22]. We report the direct electrochemical conversion of CO2 to CH4 via the reduction of 1,3-bis (2,6-diisopropylphenyl)imidazolium carboxylate, NHC-CO2, at a carbon electrode in the presence of a [Ni (cyclam)]2+ mediator[23] with CF3CH2OH as a proton source. The sole observed gas product of this electrochemical conversion is CH4 with >93% faradaic efficiency.

Results

Conclusion