Selective conversion of CO2 to fuel precursors using an inexpensive bismuth-based electrocatalyst

Rosenthal, JoelHeterogeneous electrochemical reduction of CO2 to CO, which can be coupled to liquid fuel production, provides a pathway to address current issues in solar energy storage. Precious metals such as silver and gold have been found to be active cathodes for this reaction with high current densities and energy efficiencies. However, the price of these materials prohibits their use on the scale required for widespread solar fuel production. By comparison, bismuth represents an inexpensive and environmentally benign metal, whose CO2 chemistry has been largely unexplored. To further characterize the electrocatalytic ability of bismuth materials, we have prepared bismuth modified electrodes from either an ex situ or in situ deposition procedure. These bismuth-based carbon monoxide evolving catalyst (Bi-CMEC) platforms can be easily prepared and display high energy efficiency for the conversion of CO2 to CO in the presence of an imidazolium ([IM]+) based ionic liquid (IL). The kinetics and efficiency with which Bi-CMEC drives electroreduction of CO2 is comparable to that observed using expensive precious metals, with Faradaic efficiencies and current densities for CO production of FECO ~ 80% and jCO ~ 5–30 mA/cm2, respectively at applied overpotentials (η) of ~ 300 mV. By adopting the Bi-CMEC deposition technique, we have developed a general method to prepare Sn, Bi, Pb, and Sb films from their respective triflate salts in MeCN solutions. The ability of the four materials to reduce CO 2 in the presence of IL was probed. Displaying similar electrocatalytic features as Bi, the Sn deposited material is identified as an efficient platform for CO evolution with jCO ~ 5 mA/cm2 at η ~ 300 mV. Neither the Sb nor Pb modified materials were found to be robust platforms for CO evolution. These results highlight that critical interactions occur between the electrode surface, the [IM]+ IL, and CO2. Using rotating disk electrode voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy, we have worked to reveal the primary factors that lead to CO generation by the Bi–[IM] + system. These experiments suggest that pre-organization of [IM] + ions at the electrode/electrolyte interface facilitates rate-limiting CO2 reduction to CO2•–. Further, impedance measurements predict that the formation of Bi–CO2---[IM] + adsorbates is key for minimizing the resistances associated with CO2 reduction. The [IM]+ cations are also shown to provide the protons necessary for the proton-coupled electron transfer (PCET) reduction of CO2. While high proton availability is necessary for PCET, non-[IM]+ proton donors of similar acidity such as 2,2,2-trifluroeothanol (TFE) are unable to promote the efficient evolution of CO at Bi surfaces. Impedance and surface analysis suggests that unlike [IM]+, TFE does not form the necessary stabilizing adsorbates. Instead, TFE can work cooperatively with dilute IL solutions to promote the in situ regeneration of [IM]+, permitting CO evolution at IL concentrations that have previously been prohibitively low. As part of our ongoing efforts to understand the nature of the Bi-IL system, we have isolated a series of cyanoalkyl and fluoroalkyl [IM] + ILs. Incorporation of these electron withdrawing groups increases the ILs proton donating ability and allows for the PCET reduction of CO 2 to occur at potentials ~300 mV more positive. While providing a more acidic environment, functionalization with cyanoalkyl groups inadvertently destabilized the [IM]+ to reductive decomposition, limiting the usefulness of [IM]+ functionalization. As an alternative to [IM]+, we have identified the conjugate acid of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), [DBU–H]+, as a promising IL for the reduction of CO2. Electrolysis of [DBU-H]+ using a bismuth electrode promotes the reduction of CO2 to yield predominantly formate (HCOO –, FA) with FEFA ~ 70% at j tot ~ 20–50 mA/cm2 while suppressing CO production (FECO ~ 20%) in either MeCN or MeCN/H2O (95:5) solutions. These results highlight the versatility of bismuth–IL systems for non-aqueous electrochemical CO2 reduction.University of Delaware, Department of Chemistry and BiochemistryPh.D