Aqueous CO2 Reduction with High Efficiency Using α‐Co(OH)2‐Supported Atomic Ir Electrocatalysts

Electrochemical reduction of CO₂ into energy‐dense chemical feedstock and fuels provides an attractive pathway to sustainable energy storage and artificial carbon cycle. Herein, we report the first work to use atomic Ir electrocatalyst for CO₂ reduction. By using α‐Co(OH)₂ as the support, the faradaic efficiency of CO could reach 97.6 % with a turnover frequency (TOF) of 38290 h^?1 in aqueous electrolyte, which is the highest TOF up to date. The electrochemical active area is 23.4‐times higher than Ir nanoparticles (2 nm), which is highly conductive and favors electron transfer from CO₂ to its radical anion (CO₂^.?). Moreover, the more efficient stabilization of CO₂^.? intermediate and easy charge transfer makes the atomic Ir electrocatalyst facilitate CO production. Hence, α‐Co(OH)₂‐supported atomic Ir electrocatalysts show enhanced CO₂ activity and stability.     

Electrons Mediate the Gas‐Phase Oxidation of Formic Acid with Ozone

Gas‐phase reactions of CO₃^.? with formic acid are studied using Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry. Signal loss indicates the release of a free electron, with the formation of neutral reaction products. This is corroborated by adding traces of SF₆ to the reaction gas, which scavenges 38 % of the electrons. Quantum chemical calculations of the reaction potential energy surface provide a reaction path for the formation of neutral carbon dioxide and water as the thermochemically favored products. From the literature, it is known that free electrons in the troposphere attach to O₂, which in turn transfer the electron to O₃. O₃^.? reacts with CO₂ to form CO₃^.?. The reaction reported here formally closes the catalytic cycle for the oxidation of formic acid with ozone, catalyzed by free electrons.     

Electrocatalytic CO2 Reduction with a Half‐Sandwich Cobalt Catalyst: Selectivity towards CO

We present herein a Cp*Co(III)‐half‐sandwich catalyst system for electrocatalytic CO₂ reduction in aqueous acetonitrile solution. In addition to an electron‐donating Cp* ligand (Cp*=pentamethylcyclopentadienyl), the catalyst featured a proton‐responsive pyridyl‐benzimidazole‐based N,N‐bidentate ligand. Owing to the presence of a relatively electron‐rich Co center, the reduced Co(I)‐state was made prone to activate the electrophilic carbon center of CO₂. At the same time, the proton‐responsive benzimidazole scaffold was susceptible to facilitate proton‐transfer during the subsequent reduction of CO₂. The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H⁺ reduced product HCOOH, in contrast to the only known similar half‐sandwich CpCo(III)‐based CO₂‐reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70% while the overpotential for CO production was found to be 0.78 V, as determined by controlled‐potential electrolysis.     

Regulation of Coordination Number over Single Co Sites: Triggering the Efficient Electroreduction of CO2

The design of active, selective, and stable CO₂ reduction electrocatalysts is still challenging. A series of atomically dispersed Co catalysts with different nitrogen coordination numbers were prepared and their CO₂ electroreduction catalytic performance was explored. The best catalyst, atomically dispersed Co with two‐coordinate nitrogen atoms, achieves both high selectivity and superior activity with 94 % CO formation Faradaic efficiency and a current density of 18.1 mA cm^?2 at an overpotential of 520 mV. The CO formation turnover frequency reaches a record value of 18 200 h^?1, surpassing most reported metal‐based catalysts under comparable conditions. Our experimental and theoretical results demonstrate that lower a coordination number facilitates activation of CO₂ to the CO₂^.? intermediate and hence enhances CO₂ electroreduction activity.     

Theoretical Study on the Spectroscopic Properties of CO3.−.nH2O Clusters: Extrapolation to Bulk

Vertical detachment energies (VDE) and UV/Vis absorption spectra of hydrated carbonate radical anion clusters, CO₃^.?.n H₂O (n=1–8), are determined by means of ab initio electronic structure theory. The VDE values of the hydrated clusters are calculated with second‐order Moller–Plesset perturbation (MP2) and coupled cluster theory using the 6‐311++G(d,p) set of basis functions. The bulk VDE value of an aqueous carbonate radical anion solution is predicted to be 10.6 eV from the calculated weighted average VDE values of the CO₃^.?.n H₂O clusters. UV/Vis absorption spectra of the hydrated clusters are calculated by means of time‐dependent density functional theory using the Becke three‐parameter nonlocal exchange and the Lee–Yang–Parr nonlocal correlation functional (B3LYP). The simulated UV/Vis spectrum of the CO₃^.?.8 H₂O cluster is in excellent agreement with the reported experimental spectrum for CO₃^.? (aq), obtained based on pulse radiolysis experiments.     

Voltammetric Behavior of Nitrofurazone at Highly Boron Doped Diamond Electrode

The electrochemical behavior of nitrofurazone (NFZ) at a highly boron doped diamond (BDD) electrode was studied in Britton‐Robinson (BR) buffer using cyclic voltammetry. NFZ was directly reduced to the amine derivative (RNH₂) in the pH range of 2.0 to 4.0 in a process involving six (6.0±0.4) electrons and six protons. In the range of pH 7.0 to 12 and, predominantly aqueous medium, the reduction step split into its two components: the reduction of NFZ to the radical anion (RNO₂^.?) and reduction of RNO₂^.? to hydroxylamine derivative (RNHOH) in processes involving one and three (3.1±0.1) electrons, respectively. On the anodic scan of the voltammograms and at pH 8.0, the oxidation of the hydroxylamine to the nitroso derivative (RNO), was observed in a process involving 2 (1.7±0.2) electrons and 2 protons. In addition and unreported in the literature on any electrode material, was the detection of a new oxidation peak at pH>8.0, which was observed regardless of whether NFZ had been previously reduced or not. The calculation of n, number of electrons, involved in each electrochemical step was satisfactorily accomplished using the Randles‐?evcik equation.     

Isolated Square‐Planar Copper Center in Boron Imidazolate Nanocages for Photocatalytic Reduction of CO2 to CO

Photocatalytic reduction of CO₂ to value‐added fuel has been considered to be a promising strategy to reduce global warming and shortage of energy. Rational design and synthesis of catalysts to maximumly expose the active sites is the key to activate CO₂ molecules and determine the reaction selectivity. Herein, we synthesize a well‐defined copper‐based boron imidazolate cage (BIF‐29) with six exposed mononuclear copper centers for the photocatalytic reduction of CO₂. Theoretical calculations show a single Cu site including weak coordinated water delivers a new state in the conduction band near the Fermi level and stabilizes the *COOH intermediate. Steady‐state and time‐resolved fluorescence spectra show these Cu sites promote the separation of electron–hole pairs and electron transfer. As a result, the cage achieves solar‐driven reduction of CO₂ to CO with an evolution rate of 3334 μmol g^?1 h^?1 and a high selectivity of 82.6 %.     

NHC‐Containing Manganese(I) Electrocatalysts for the Two‐Electron Reduction of CO2

The synthesis and characterization of the first catalytic manganese N‐heterocyclic carbene complexes are reported: MnBr(N‐methyl‐N′‐2‐pyridylbenzimidazol‐2‐ylidine)(CO)₃ and MnBr(N‐methyl‐N′‐2‐pyridylimidazol‐2‐ylidine)(CO)₃. Both new species mediate the reduction of CO₂ to CO following two‐electron reduction of the Mn^I center, as observed with preparative scale electrolysis and verified with ¹³CO₂. The two‐electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2′‐bipyridine)(CO)₃. Catalytic current enhancement is observed at voltages similar to MnBr(2,2′‐bipyridine)(CO)₃.     

Electrocatalytic Reduction of CO2 to Acetic Acid by a Molecular Manganese Corrole Complex

The controlled electrochemical reduction of carbon dioxide to value added chemicals is an important strategy in terms of renewable energy technologies. Therefore, the development of efficient and stable catalysts in an aqueous environment is of great importance. In this context, we focused on synthesizing and studying a molecular Mn^III‐corrole complex, which is modified on the three meso‐positions with polyethylene glycol moieties for direct and selective production of acetic acid from CO₂. Electrochemical reduction of Mn^III leads to an electroactive Mn^II species, which binds CO₂ and stabilizes the reduced intermediates. This catalyst allows to electrochemically reduce CO₂ to acetic acid in a moderate acidic aqueous medium (pH 6) with a selectivity of 63 % and a turn over frequency (TOF) of 8.25 h^?1, when immobilized on a carbon paper (CP) electrode. In terms of high selectivity towards acetate, we propose the formation and reduction of an oxalate type intermediate, stabilized at the Mn^III‐corrole center.     

Aqueous Photoelectrochemical CO2 Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst

We report a precious-metal-free molecular catalyst-based photocathode that is active for aqueous CO₂ reduction to CO and methanol. The photoelectrode is composed of cobalt phthalocyanine molecules anchored on graphene oxide which is integrated via a (3-aminopropyl)triethoxysilane linker to p-type silicon protected by a thin film of titanium dioxide. The photocathode reduces CO₂ to CO with high selectivity at potentials as mild as 0 V versus the reversible hydrogen electrode (vs RHE). Methanol production is observed at an onset potential of −0.36 V vs RHE, and reaches a peak turnover frequency of 0.18 s⁻¹. To date, this is the only molecular catalyst-based photoelectrode that is active for the six-electron reduction of CO₂ to methanol. This work puts forth a strategy for interfacing molecular catalysts to p-type semiconductors and demonstrates state-of-the-art performance for photoelectrochemical CO₂ reduction to CO and methanol.     

Boosting Electrochemical CO2 Reduction on Metal–Organic Frameworks via Ligand Doping

Electrochemical CO₂ reduction relies on the availability of highly efficient and selective catalysts. Herein, we report a general strategy to boost the activity of metal–organic frameworks (MOFs) towards CO₂ reduction via ligand doping. A strong electron‐donating molecule of 1,10‐phenanthroline was doped into Zn‐based MOFs of zeolitic imidazolate framework‐8 (ZIF‐8) as CO₂ reduction electrocatalyst. Experimental and theoretical evidences reveal that the electron‐donating nature of phenanthroline enables a charge transfer, which induces adjacent active sites at the sp² C atoms in the imidazole ligand possessing more electrons, and facilitates the generation of *COOH, hence leading to improved activity and Faradaic efficiency towards CO production.     

Ag@Au Core‐Shell Nanowires for Nearly 100 % CO2‐to‐CO Electroreduction

Electrochemical reduction of carbon dioxide (CO₂) to CO is regarded as an efficient method to utilize the greenhouse gas CO₂, because the CO product can be further converted into high value‐added chemicals via the Fisher–Tropsch process. Among all electrocatalysts used for CO₂‐to‐CO reduction, Au‐based catalysts have been demonstrated to possess high selectivity, but their precious price limits their future large‐scale applications. Thus, simultaneously achieving high selectivity and reasonable price is of great importance for the development of Au‐based catalysts. Here, we report Ag@Au core–shell nanowires as electrocatalyst for CO₂ reduction, in which a nanometer‐thick Au film is uniformly deposited on the core Ag nanowire. Importantly, the Ag@Au catalyst with a relative low Au content can drive CO generation with nearly 100 % Faraday efficiency in 0.1 m KCl electrolyte at an overpotential of ca. ?1.0 V. This high selectivity of CO₂ reduction could be attributed to a suitable adsorption strength for the key intermediate on Au film together with the synergistic effects between the Au shell and Ag core and the strong interaction between CO₂ and Cl^? ions in the electrolyte, which may further pave the way for the development of high‐efficiency electrocatalysts for CO₂ reduction.     

Unearthing the Mechanism of Prebiotic Nitrile Bond Reduction in Hydrogen Cyanide through a Curious Association of Two Molecular Radical Anions

HCN is clearly associated with the prebiotic chemical evolution of life. It has been known for decades that the radiolysis of HCN solutions produces sugars, amino acids and nucleobases. Remarkably, recent experimental studies have shown that the photolytic reduction of aqueous HCN by a photoredox reagent [Cu(CN)₃]^2? specifically yields sugars, which are the essential building blocks of RNA. Although a mechanistic understanding of such reductions with solvated electrons is poor, the general consensus is that they involve neutral free radicals. We show herein through the use of electronic structure studies and molecular simulations that the reduction of the nitrile bond of HCN is initiated through the formation of a molecular dipole‐bound anion from the photoredox reagent. Our theoretical studies show how HCN binds to the photoexcited reagent and then extracts an electron from the reagent and is ultimately detached as a dipole‐bound anion. The dipole‐bound anionic form of [HCN]^? can easily convert into a solvated valence‐bound form of [HCN]^?. After the formation of solvated [HCN]^?, an extraordinary chemical event ensues through a counter‐intuitive coupling of two valence‐bound anions to form a solvated molecular dianionic intermediate, [HCN]₂^2?. Finally, a proton‐coupled electron transfer occurs within the dianionic entity to complete the reduction. This mechanistic scenario is applicable to the reduction of other prebiotic nitrile species and avoids neutral radical‐based pathways, thereby preventing the proliferation of reactive species and preserving chemical selectivity. Furthermore, we show how such similar nitrile reduction pathways operate to yield the sugar precursors.     

SDS‐modified Nanoporous Silver as an Efficient Electrocatalyst for Selectively Converting CO2 to CO in Aqueous Solution

Selectively electrochemical conversion of CO₂ into organic fuel using renewable electricity is one of the most sought‐after processes. In this paper, we report the electrochemical reduction of CO₂ (CO₂RR) on the nanoporous Ag electrodes made of compacted Ag nanoparticles (AgNPs), which were prepared by one‐step reduction in the water phase with or without the surfactant sodium dodecyl sulfate (SDS). The scanning electron microscope (SEM) characterizations show that the compacted Ag electrodes have the nanoporous morphology formed by stacking AgNPs. Compared with the nanoporous Ag electrode without SDS modification (C‐AgNPs), the SDS‐modified AgNPs electrode (C‐AgNPs‐SDS) is highly effective in improving selective CO production in a wide range of potentials (–0.69 V — –1.19 V, vs. RHE), with a Faradaic efficiency of 92.2% and a current density of –8.23 mA·cm^–2 for CO production at –0.79 V (vs. RHE). C‐AgNPs‐SDS is also catalytically stable with only less than 7% deactivation after 8 h of continuous electrolysis.     

Organic Polymer Dots Photocatalyze CO2 Reduction in Aqueous Solution

Developing low-cost and efficient photocatalysts to convert CO₂ into valuable fuels is desirable to realize a carbon-neutral society. In this work, we report that polymer dots (Pdots) of poly[(9,9′-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-thiadiazole)] (PFBT), without adding any extra co-catalyst, can photocatalyze reduction of CO₂ into CO in aqueous solution, rendering a CO production rate of 57 μmol g⁻¹ h⁻¹ with a detectable selectivity of up to 100 %. After 5 cycles of CO₂ re-purging experiments, no distinct decline in CO amount and reaction rate was observed, indicating the promising photocatalytic stability of PFBT Pdots in the photocatalytic CO₂ reduction reaction. A mechanistic study reveals that photoexcited PFBT Pdots are reduced by sacrificial donor first, then the reduced PFBT Pdots can bind CO₂ and reduce it into CO via their intrinsic active sites. This work highlights the application of organic Pdots for CO₂ reduction in aqueous solution, which therefore provides a strategy to develop highly efficient and environmentally friendly nanoparticulate photocatalysts for CO₂ reduction.     

G‐quadruplex Nanowires To Direct the Efficiency and Selectivity of Electrocatalytic CO2 Reduction

DNA as a medium for electron transfer has been widely used in photolytic processes but is seldom applied to dark reaction of CO₂ reduction. A G‐quadruplex nanowire (tsGQwire) assembled by guanine tetranucleotides was used to host several metal complexes and further to mediate electron transfer processes in the electrochemical reduction of CO₂ catalyzed by these complexes. The tsGQwire modified electrode increased the Faradaic efficiency of cobalt(II) phthalocyanine (Co^IIPc) 2.5‐folds for CO production than bare Co^IIPc electrode, with a total current density of 11.5 mA cm^?2. Comparable Faradaic efficiency of HCOOH production was achieved on tsGQwire electrode when the catalytic center was switched to a GQ targeting Ru complex. The high efficiency and selectivity of electrocatalytic CO₂ reduction was attributed to the unique binding of metal complexes on G‐quadruplex and electron transfer mediated by GQ nanowire to achieve efficient redox cycling of catalytic centers on the electrode.     

Direct Evidence of Local pH Change and the Role of Alkali Cation during CO2 Electroreduction in Aqueous Media

We report, for the first time, utilizing a rotating ring‐disc electrode (RRDE) assembly for detecting changes in the local pH during aqueous CO₂ reduction reaction (CO₂RR). Using Au as a model catalyst where CO is the only product, we show that the CO oxidation peak shifts by ?86±2 mV/pH during CO₂RR, which can be used to directly quantify the change in the local pH near the catalyst surface during electrolysis. We then applied this methodology to investigate the role of cations in affecting the local pH during CO₂RR and find that during CO₂RR to CO on Au in an MHCO₃ buffer (where M is an alkali metal), the experimentally measured local basicity decreased in the order Li⁺ > Na⁺ > K⁺ > Cs⁺, which agreed with an earlier theoretical prediction by Singh et al. Our results also reveal that the formation of CO is independent of the cation. In summary, RRDE is a versatile tool for detecting local pH change over a diverse range of CO₂RR catalysts. Additionally, using the product itself (i.e. CO) as the local pH probe allows us to investigate CO₂RR without the interference of additional probe molecules introduced to the system. Most importantly, considering that most CO₂RR products have pH‐dependent oxidation, RRDE can be a powerful tool for determining the local pH and correlating the local pH to reaction selectivity.     

Stable Aqueous Photoelectrochemical CO2 Reduction by a Cu2O Dark Cathode with Improved Selectivity for Carbonaceous Products

Photocatalytic reduction of CO₂ to produce fuels is a promising way to reduce CO₂ emission and address the energy crisis. However, the H₂ evolution reaction competes with CO₂ photoreduction, which would lower the overall selectivity for carbonaceous products. Cu₂O has emerged as a promising material for suppressing the H₂ evolution. However, it suffers from poor stability, which is commonly regarded as the result of the electron‐induced reduction of Cu₂O. This paper describes a simple strategy using Cu₂O as a dark cathode and TiO₂ as a photoanode to achieve stable aqueous CO₂ reduction with a high Faradaic efficiency of 87.4 % and a selectivity of 92.6 % for carbonaceous products. We have shown that the photogenerated holes, instead of the electrons, primarily account for the instability of Cu₂O. Therefore, Cu₂O was used as a dark cathode to minimize the adverse effects of holes, by which an improved stability was achieved compared to the Cu₂O photocathode under illumination. Additionally, direct exposure of the Cu₂O surface to the electrolyte was identified as a critical factor for the high selectivity for carbonaceous products.     

Controllable Synthesis of Few‐Layer Bismuth Subcarbonate by Electrochemical Exfoliation for Enhanced CO2 Reduction Performance

Two‐dimensional (2D) engineering of materials has been recently explored to enhance the performance of electrocatalysts by reducing their dimensionality and introducing more catalytically active ones. In this work, controllable synthesis of few‐layer bismuth subcarbonate nanosheets has been achieved via an electrochemical exfoliation method. These nanosheets catalyse CO₂ reduction to formate with high faradaic efficiency and high current density at a low overpotential owing to the 2D structure and co‐existence of bismuth subcarbonate and bismuth metal under catalytic turnover conditions. Two underlying fast electron transfer processes revealed by Fourier‐transformed alternating current voltammetry (FTacV) are attributed to CO₂ reduction at bismuth subcarbonate and bismuth metal. FTacV results also suggest that protonation of CO₂^.? is the rate determining step for bismuth catalysed CO₂ reduction.     

High‐Yield Electrochemical Production of Formaldehyde from CO2 and Seawater

The catalytic, electrocatalytic, or photocatalytic conversion of CO₂ into useful chemicals in high yield for industrial applications has so far proven difficult. Herein, we present our work on the electrochemical reduction of CO₂ in seawater using a boron‐doped diamond (BDD) electrode under ambient conditions to produce formaldehyde. This method overcomes the usual limitation of the low yield of higher‐order products, and also reduces the generation of H₂. In comparison with other electrode materials, BDD electrodes have a wide potential window and high electrochemical stability, and, moreover, exhibit very high Faradaic efficiency (74 %) for the production of formaldehyde, using either methanol, aqueous NaCl, or seawater as the electrolyte. The high Faradaic efficiency is attributed to the sp³‐bonded carbon of the BDD. Our results have wide ranging implications for the efficient and cost‐effective conversion of CO₂.