Across the Board: Rui Cao on Electrocatalytic CO2 Reduction

Integrated Capture and Electrocatalytic Conversion of CO2: A Molecular Electrocatalysts Perspective

The ever-increasing concentration of atmospheric CO2, primarily driven by anthropogenic activities, has raised urgent environmental concerns, spurring the development of carbon capture and utilization (CCU) technologies. This review focuses on the integrated capture and electrochemical conversion of CO2 (ICECC), a promising approach that combines carbon capture with its direct electroreduction into value-added products. By eliminating energy-intensive steps such as CO2 release, compression, and transportation, ICECC offers a more energy-efficient and cost-effective alternative to conventional CCU methods. In this review, particular attention is given to molecular electrocatalysts, which offer high tunability and selectivity in electrochemical CO2 reduction reaction (eCO2RR). The role of capturing agents, including both external and dual-functional molecular systems, is critically examined to understand their influence on CO2 binding and catalytic efficiency. Whereas ICECC has significant potential, research in this area remains underexplored compared to conventional CO2 reduction methods. The review discusses the mechanistic insights into ICECC processes, highlighting key challenges and potential future research directions for improving catalyst design, enhancing capture efficiency, and scaling up ICECC technologies. These developments can make ICECC a critical component in achieving carbon neutrality and addressing climate change.

Molecular-scale CO spillover on a dual-site electrocatalyst enhances methanol production from CO2 reduction

Cobalt phthalocyanine (CoPc) is recognized for catalysing electrochemical CO2 reduction into methanol at high Faradaic efficiency but is subject to deactivation. Cobalt tetraaminophthalocyanine (CoPc-NH2) shows improved stability, but its methanol Faradaic efficiency is below 30%. This study addresses these limitations in selectivity, reactivity and stability by rationally designing a dual-site cascade catalyst. Here we quantify the local concentration of CO, a key intermediate of the reaction, near a working CoPc-NH2 catalyst and show that co-loading nickel tetramethoxyphthalocyanine (NiPc-OCH3) with CoPc-NH2 on multiwalled carbon nanotubes increases the generation and local concentration of CO. This dual-site cascade catalyst exhibits substantially higher performance than the original single-site CoPc-NH2/carbon nanotube catalyst, reaching a partial current density of 150 mA cm-2 and a Faradaic efficiency of 50% for methanol production. Kinetic analysis and in situ sum-frequency generation vibrational spectroscopy attribute this notable performance improvement to molecular-scale CO spillover from NiPc-OCH3 sites to methanol-active CoPc-NH2 sites.

Improving Electrocatalytic CO2 Reduction over Iron Tetraphenylporphyrin with Triethanolamine as a CO2 Shuttle

Delivering CO2 molecules to catalyst sites is a vital step in the CO2 reduction reaction (CO2RR). Achievements have been made to develop efficient catalysts, but few efforts have been dedicated to improving CO2 delivering in solutions. Herein, we report on electrocatalytic CO2-to-CO conversion using Fe tetraphenylporphyrin (FeTPP) as a catalyst and triethanolamine as a CO2 shuttle. Compared to ethanol, the electrocatalytic CO2RR current with triethanolamine increases by more than three times. We show that triethanolamine can effectively capture a CO2 molecule to form a zwitterionic alkylcarbonate through the collaboration between its tripodal alcohol and amine units. This alkylcarbonate can release the bound CO2 molecule for activation at the Fe site upon its interaction with FeTPP. In addition to shuttling CO2, alkylcarbonates can also provide protons to assist the C-O bond cleavage. Therefore, this work is significant to demonstrate a new strategy to improve electrocatalytic CO2RR by shuttling CO2.

Hinged Carboxylate in the Artificial Distal Pocket of an Iron Porphyrin Enhances CO2 Electroreduction at Low Overpotential

To efficiently capture, activate, and transform small molecules, metalloenzymes have evolved to integrate a well-organized pocket around the active metal center. Within this cavity, second coordination sphere functionalities are precisely positioned to optimize the rate, selectivity, and energy cost of catalytic reactions. Inspired by this strategy, an artificial distal pocket defined by a preorganized 3D strap is introduced on an iron-porphyrin catalyst (sc-Fe) for the CO2-to-CO electrocatalytic reduction. Combined electrochemical, kinetic, and computational studies demonstrate that the adequate positioning of a carboxylate/carboxylic group acting in synergy with a trapped water molecule within this distal pocket remarkably enhances the reaction turnover frequency (TOF) by four orders of magnitude compared to the perfluorinated iron-tetraphenylporphyrin catalyst (F20Fe) operating at a similar low overpotential. A proton-coupled electron transfer (PCET) is found to be the key process responsible for the unexpected protonation of the coordinating carboxylate, which, upon CO2 insertion, shifts from the first to the second coordination sphere to play a possible secondary role as a proton relay.

Strong effect-correlated electrochemical CO2 reduction

Electrochemical CO2 reduction (ECR) holds great potential to alleviate the greenhouse effect and our dependence on fossil fuels by integrating renewable energy for the electrosynthesis of high-value fuels from CO2. However, the high thermodynamic energy barrier, sluggish reaction kinetics, inadequate CO2 conversion rate, poor selectivity for the target product, and rapid electrocatalyst degradation severely limit its further industrial-scale application. Although numerous strategies have been proposed to enhance ECR performances from various perspectives, scattered studies fail to comprehensively elucidate the underlying effect-performance relationships toward ECR. Thus, this review presents a comparative summary and a deep discussion with respect to the effects strongly-correlated with ECR, including intrinsic effects of materials caused by various sizes, shapes, compositions, defects, interfaces, and ligands; structure-induced effects derived from diverse confinements, strains, and fields; electrolyte effects introduced by different solutes, solvents, cations, and anions; and environment effects induced by distinct ionomers, pressures, temperatures, gas impurities, and flow rates, with an emphasis on elaborating how these effects shape ECR electrocatalytic activities and selectivity and the underlying mechanisms. In addition, the challenges and prospects behind different effects resulting from various factors are suggested to inspire more attention towards high-throughput theoretical calculations and in situ/operando techniques to unlock the essence of enhanced ECR performance and realize its ultimate application.

The meso-substituent electronic effect of Fe porphyrins on the electrocatalytic CO2 reduction reaction

We report Fe porphyrins bearing different meso-substituents for the electrocatalytic CO2 reduction reaction (CO2RR). By replacing two and four meso-phenyl groups of Fe tetraphenylporphyrin (FeTPP) with strong electron-withdrawing pentafluorophenyl groups, we synthesized FeF10TPP and FeF20TPP, respectively. We showed that FeTPP and FeF10TPP are active and selective for CO2-to-CO conversion in dimethylformamide with the former being more active, but FeF20TPP catalyzes hydrogen evolution rather than the CO2RR under the same conditions. Experimental and theoretical studies revealed that with more electron-withdrawing meso-substituents, the Fe center becomes electron-deficient and it becomes difficult for it to bind a CO2 molecule in its formal Fe0 state. This work is significant to illustrate the electronic effects of catalysts on binding and activating CO2 molecules and provide fundamental knowledge for the design of new CO2RR catalysts.

Kinetic isotope effect offers selectivity in CO2 reduction

A binuclear Ni complex with N,O donors catalyzes CO2 reduction via its Ni(I) state. The product distribution when H2O is used as a proton source shows similar yields for CO, HCOOH and H2. However, when D2O is used, the product distribution shows a ∼65% selectivity for HCOOH. In situ FTIR indicates that the reaction involves a Ni-COO* and a Ni-CO intermediate. Differences in H/D KIEs on different protonation pathways determine the selectivity of CO2 reduction.

Catalytic CO2 Reduction with Heptacoordinated Polypyridine Complexes: Switching the Selectivity via Metal Replacement

The discovery of molecular catalysts for the CO2 reduction reaction (CO2 RR) in the presence of water, which are both effective and selective towards the generation of carbon-based products, is a critical task. Herein we report the catalytic activity towards the CO2 RR in acetonitrile/water mixtures by a cobalt complex and its iron analog both featuring the same redox-active ligand and an unusual seven-coordination environment. Bulk electrolysis experiments show that the cobalt complex mainly yields formate (52 % selectivity at an applied potential of -2.0 V vs Fc+ /Fc and 1 % H2 O) or H2 (up to 86 % selectivity at higher applied bias and water content), while the iron complex always delivers CO as the major product (selectivity >74 %). The different catalytic behavior is further confirmed under photochemical conditions with the [Ru(bpy)3 ]2+ sensitizer (bpy=2,2'-bipyridine) and N,N-diisopropylethylamine as electron donor, where the cobalt complex leads to preferential H2 formation (up to 89 % selectivity), while the iron analog quantitatively generates CO (up to 88 % selectivity). This is ascribed to a preference towards a metal-hydride vs. a metal-carboxyl pathway for the cobalt and the iron complex, respectively, and highlights how metal replacement may effectively impact on the reactivity of transition metal complexes towards solar fuel formation.

Redox-Active Ligand Assisted Multielectron Catalysis: A Case of Electrocatalyzed CO2-to-CO Conversion

The selective reduction of carbon dioxide remains a significant challenge due to the complex multielectron/proton transfer process, which results in a high kinetic barrier and the production of diverse products. Inspired by the electrostatic and H-bonding interactions observed in the second sphere of the [NiFe]-CODH enzyme, researchers have extensively explored these interactions to regulate proton transfer, stabilize intermediates, and ultimately improve the performance of catalytic CO2 reduction. In this work, a series of cobalt(II) tetraphenylporphyrins with varying numbers of redox-active nitro groups were synthesized and evaluated as CO2 reduction electrocatalysts. Analyses of the redox properties of these complexes revealed a consistent relationship between the number of nitro groups and the corresponding accepted electron number of the ligand at -1.59 V vs. Fc+/0. Among the catalysts tested, TNPPCo with four nitro groups exhibited the most efficient catalytic activity with a turnover frequency of 4.9 × 104 s-1 and a catalytic onset potential 820 mV more positive than that of the parent TPPCo. Furthermore, the turnover frequencies of the catalysts increased with a higher number of nitro groups. These results demonstrate the promising design strategy of incorporating multielectron redox-active ligands into CO2 reduction catalysts to enhance catalytic performance.