Energetically efficient electrocatalysts with high product selectivity are desirable targets for sustainable chemical fuel generation using renewable electricity. Recycling CO 2 by reduction to more energy dense products would support a carbon-neutral cycle that mitigates the intermittency of renewable energy sources. Conversion of CO 2 to more saturated products typically requires proton equivalents. Complications with product selectivity stem from competitive reactions between H + or CO 2 at shared intermediates. We describe generalized catalytic cycles for H 2 , CO, and HCO 2 – formation that are commonly proposed in inorganic molecular catalysts. Thermodynamic considerations and trends for the reactions of H + or CO 2 at key intermediates are outlined. A quantitative understanding of intermediate catalytic steps is key to designing systems that display high selectivity while promoting energetically efficient catalysis by minimizing the overall energy landscape. For CO 2 reduction to CO, we describe how an enzymatic active site motif facilitates efficient and selective catalysis and highlight relevant examples from synthetic systems.

Current methods for CO2 capture and concentration (CCC) are energy intensive due to their reliance on thermal cycles, which are intrinsically Carnot limited in efficiency. In contrast, electrochemically driven CCC (eCCC) can operate with much higher theoretical efficiencies. However, most reported systems are sensitive to O2, precluding their practical use. In order to achieve O2-stable eCCC, we pursued the development of molecular redox carriers with reduction potentials positive of the O2/O2 – redox couple. Prior efforts to chemically modify redox carriers to operate at milder potentials resulted in diminished CO2 binding. To overcome these limitations, we used common alcohol additives to anodically shift the reduction potential of a quinone redox carrier, 2,3,5,6-tetrachloro-p-benzoquinone (TCQ), by up to 350 mV, conferring O2 stability. Intermolecular hydrogen-bonding interactions with the dianion and CO2-bound forms of TCQ were correlated to alcohol pK a to identify ethanol as the optimal additive, as it imparts beneficial changes to both the reduction potential and CO2-binding constant, the two key properties of eCCC redox carriers. We demonstrated a full cycle of eCCC in aerobic simulated flue gas using TCQ and ethanol, two commercially available compounds. Based on the system properties, an estimated minimum of 21 kJ/mol is required to concentrate CO2 from 10 to 100% or twice as efficient as state-of-the-art thermal amine capture systems and other reported redox carrier-based systems. Furthermore, this approach of using hydrogen-bond donor additives is general and can be used to tailor the redox properties of other quinone/alcohol combinations for specific CO2-capture applications.

Electrochemical measurements on tris(cyclopentadienyl) thorium and uranium compounds in the +2, +3, and +4 oxidation states are reported with C5H3(SiMe3)2, C5H4SiMe3, and C5Me4H ligands. The reduction potentials for both U and...