A Graphene-Supported Single-Atom FeN5 Catalytic Site for Efficient Electrochemical CO2 Reduction.

Electrochemical conversion of CO2 into valued products is one of the most important issues but remains a great challenge in chemistry. Herein, we report a novel synthetic approach involving prolonged thermal pyrolysis of hemin and melamine molecules on graphene for the fabrication of a robust and efficient single‐iron‐atom electrocatalyst for electrochemical CO2 reduction. The single‐atom catalyst exhibits high Faradaic efficiency (ca. 97.0 %) for CO production at a low overpotential of 0.35 V, outperforming all Fe‐N‐C‐based catalysts. The remarkable performance for CO2‐to‐CO conversion can be attributed to the presence of highly efficient singly dispersed FeN5 active sites supported on N‐doped graphene with an additional axial ligand coordinated to FeN4. DFT calculations revealed that the axial pyrrolic nitrogen ligand of the FeN5 site further depletes the electron density of Fe 3d orbitals and thus reduces the Fe–CO π back‐donation, thus enabling the rapid desorption of CO and high selectivity for CO production.

The electrochemical catalytic reduction of CO2 into CO could be achieved with excellent selectivity and rate in acetonitrile in the presence of phenol with cobalt 2,2′:6′,2″:6″,2‴-quaterpyridine complex [CoII(qpy)(H2O)2]2+ (Co) acting as a molecular catalyst. Upon using cyclic voltammetry at low and high scan rate (up to 500 V/s) two catalytic pathways have been identified. At a low concentration of phenol (<1 M), catalysis mainly occurs after the reduction of Co with three electrons. In that case, the selectivity for CO production is ca. 80% with 20% of H2 as by product, along with a turnover frequency of 1.2 × 104 s–1 for CO production at an overpotential η of ca. 0.6 V. The triply reduced active species binds to CO2 and the C–O bond is cleaved thanks to the acid. At very large concentration of phenol (3 M), another pathway becomes predominant: the doubly reduced species binds to CO2, while its reductive protonation leads to CO formation. As already shown, this later process is endowed with fast rate at low overpotential (turnover frequency of 3 × 104 s–1 at η = 0.3 V) and 95% selectivity for CO production. By varying the phenol concentration and the scan rate in voltammetry experiments, it was thus possible to identify, activate, and characterize several pathways for the CO2-to-CO conversion and to decipher Co electrochemical reactivity toward CO2.

• Ni-based SACs exhibit excellent FE in CO production, but their high overpotential requires high energy input. • By breaking the electronic symmetry of Ni active sites, we enhance the adsorption and desorption of intermediates at them. • A novel electron tug effect around Ni active centers is obtained by Oxygen coordination and loading Au nanoparticles. • The induced distortion in the electronic structure facilitates the desorption of intermediates and reduces overpotential. • This innovation broadens the applied potentials of CO 2 -to-CO conversion. Modulating the coordination of atomically dispersed MN 4 moieties to enhance electron asymmetry presents a promising strategy for improving catalytic performance in the electrochemical reduction of carbon dioxide (eCO 2 RR). By combining small amounts of Au nanoclusters with lateral oxygen coordination in the first coordination shell, the enhanced electron delocalization on Ni centers improves both activity and selectivity. Experimentally, the optimized catalyst demonstrates exceptional catalytic performance, achieving over 95% Faradaic efficiency (FE) for CO across a broad potential range from − 0.50 to − 0.85 V vs. RHE. It also achieves over 90% FE for CO at an overpotential of 340 mV, outperforming state-of-the-art Ni-based single-atom catalysts (SACs). Moreover, the catalyst shows promising potential at a higher current density (∼150 mA cm −2 ) in a flow cell, maintaining high CO selectivity (over 90%). Structural characterizations and theoretical calculations indicate that this structure enhances electron redistribution around Ni sites through a unique electron tug effect. This effectively stabilizes *COOH intermediates, favoring CO production during eCO 2 RR at low applied potentials. This work offers a valuable method that extends beyond the first coordination shell, augmenting the complexity of electronic distribution on metal centers, which could be adapted for further fine-tuning the catalytic behavior of SACs in various reactions.

Electrochemical CO2 reduction can produce value-added chemical or fuels in moderate reaction environment. For efficient CO2 reduction to a target product with high selectivity, vast research works have been carried out over a few decades. Particularly, CO2 conversion performance using Au and Ag based catalysts for CO production has reached to economically viable level according to recent techno-economic analysis [1]. In various routes to improve catalytic property, metal alloys are shown effective by tailoring intermediate binding energy of CO2 reduction reaction (CO2RR). For example, bimetallic AuCu alloys, such as AuCu, AuCu3 and Au3Cu, have shown synergetic effects which lead to higher catalytic activities (i.e. overpotential, stability) although CO selectivity is still lower than state-of-art records (over 95%) [2-4]. Herein, we demonstrate the dilute Cu alloying in Au nanostructure for highly selective CO production with long-term stability. To synthesize nanostructured Au with controlled Cu alloying, we firstly formed Cu/Au bilayers by employing the electrochemical deposition (ED) and underpotential deposition (UPD) of Cu on 200-nm-thick Au thin films on Si substrates. ED and UPD processes were conducted at applied potentials of –0.1 and 0.3 V (vs. Ag/AgCl), respectively, in 50 mM H2SO4 electrolyte with 50 mM CuSO4. We carried out electrodepositon for 10s and 60s while underpotentially deposited Cu sample were only prepared for 60s. The Cu-covered Au surfaces were then electrochemically treated for nanostructuring with the same method previously reported by our research group [5]. Specifically, we electrochemically oxidized the Cu/Au bilayers at 2.5 V (vs. RHE) for 40 minutes in 0.2 M KHCO3 solution (pH = 8.5). Then electro-reduction was sequentially followed under constant current density of –0.5 mA cm–2 for about 10 minutes in the same solution. We denote the prepared samples by Nano-ED-CuAu-10s, -60s and Nano-UPD-CuAu in accordance with the type of Cu deposition processes. Additionally, nanostructured Au without Cu was prepared for the comparison. As a result of the electrochemical treatment, nanoporous structures were formed for all samples. The heights of the nanostructures were 200, 215, 225 and 240 nm for Nano-Au, Nano-UPD-CuAu, Nano-ED-CuAu-10s and 60s. XPS investigations clearly show the appearance of Cu 2p peak and slight shift of Au 4f peak position to the higher binding energy (~0.1 eV) when Cu was incorporated in Au nanostructures. In addition, the surface concentration of Cu were found to be 2, 11 and 13% for Nano-UPD-CuAu, Nano-ED-CuAu-10s and 60s, respectively. Our nanostructured CuAu and Au exhibit low CO2RR overpotential and high selectivity for CO in CO2 saturated 0.2 M KHCO3. For instance, the selectivity of CO2RR for all electrodes is around 50 – 70% (Nano-Au : 47%, Nano-UPD-CuAu : 65% Nano-ED-CuAu-10s : 69%, Nano-ED-CuAu-60s : 57%) at low overpotential (–0.34 V (vs. RHE)). It is noted that bare Au thin film shows almost no CO production. At higher applied potential (–0.59 V), the CO selectivity increases over 90% and notably, Nano-UPD-CuAu shows almost unity CO production (~99.5%) with nearly completely suppressing H2 evolution. In addition, Cu-Au nanostructures show the remarkably improved durability as compared to nanostructured Au catalysts. Whereas the CO selectivity at –0.49 V for Nano-Au drops from 92% at 30 minutes to below 80% in about 4.5 hours, Nano-UPD-CuAu retains the same performance for about 7 hours. Surprisingly, Nano-ED-CuAu samples are stable for 12 hours showing the selective CO production over 80%. These results indicate that dilute Cu-Au alloy nanostructures can be effective to improve stability with highly selective CO production. Additionally, in the presentation, we will introduce an in-situ tailoring technique to re-active the degraded Au catalysts by utilizing Cu impurities in electrolyte during CO2 electroreduction process for robust and efficient CO2 reduction. [1] Jouny et al., Ind. Eng. Chem. Res. 57, 2165 (2018) [2] Kim et al., Nat. Commun. 5, 4948 (2014) [3] Kim et al., Appl. Catal. B, 213, 211 (2017) [4] Kim et al., ACS Energy Lett., 3, 2144 (2018) [5] Kim et al., J. Mater. Chem. A., 6, 5119 (2018)

Electrochemical conversion of CO2 into valuable product is regarded as an attractive approach to fix and utilize atmospheric CO2, but it has been hampered due to small current density, poor selectivity, and poor durability of catalyst. Herein, a 3D nanoarrays, cobalt phthalocyanine anchored by a N-doped porous carbon nanorod (N–C–CoPc NR), is designed as an excellent electrocatalyst for efficient electrochemical reduction of CO2 into CO. The prepared N–C–CoPc NR structure not only strengthens the electron transfer rate but also exposes more active sites, which could be greatly improve the stability and activity for electrochemical CO2 reduction. The N–C–CoPc NR exhibits an excellent overall current density of 30 mA/cm2 and a lower overpotential of 180 mV for CO2 reduction to CO in 0.1 M KHCO3 electrolyte, and the maximal faradaic efficiency for CO at −0.7 V vs RHE is 85.3% with an excellent stability. The theoretical calculations confirm that cobalt phthalocyanine is the dominating active center for intermediate *COOH formation as well as the CO desorption.

Metal-organic frameworks (MOFs) as a promising platform for electrocatalytic CO2 conversion are still restricted by the low efficiency or unsatisfied selectivity for desired products. Herein, zirconium-based porphyrinic MOF hollow nanotubes with Cd sites (Cd-PCN-222HTs) are reported for electrocatalytic CO2 -to-CO conversion. The dispersed Cd species are anchored in PCN-222HTs and coordinated by N atoms of porphyrin structures. It is discovered that Cd-PCN-222HTs have glorious electrocatalytic activity for selective CO production in ionic liquid-water (H2 O)-acetonitrile (MeCN) electrolyte. The CO Faradaic efficiency (FECO ) of >80% could be maintained in a wide potential range from -2.0 to -2.4V versus Ag/Ag+ , and the maximum current density could reach 68.0mA cm-2 at -2.4V versus Ag/Ag+ with a satisfied turnover frequency of 26220 h-1 . The enhanced efficiency of electrocatalytic CO2 conversion of Cd-PCN-222HTs is closely related to its hollow structure, anchored Cd species, and good synergistic effect with electrolyte. The density functional theory calculations indicate that the dispersed Cd sites anchored in PCN-222HTs not only favor the formation of *COOH intermediate but also hinder the hydrogen evolution reaction, resulting in high activity of electrocatalytic CO2 -to-CO conversion.

Designing earth-abundant electrocatalysts toward highly efficient CO2 reduction has significant importance to decrease the global emission of greenhouse gas. Herein, we propose an efficient strategy to anchor non-noble metal single atoms on Zr6-cluster-porphyrin framework hollow nanocapsules with well-defined and abundant metal-N4 porphyrin sites for efficient electrochemical CO2 reduction. Among different transition metal single atoms (Mn, Fe, Co, Ni, and Cu), Co single-atom anchored Zr6-cluster-porphyrin framework hollow nanocapsules demonstrated the highest activity and selectivity for CO production. The rich Co-N4 active centers and hierarchical porous structure contribute to enhanced CO2 adsorption capability and moderate binding strength of reaction intermediates, thus facilitating *CO desorption and CO2-to-CO conversion. The Co-anchored nanocapsules maintain high efficiency and well-preserved stability during long-term electrocatalysis tests. Moreover, the Co-anchored nanocapsules exhibit a remarkable solar-to-CO energy conversion efficiency of 12.5% in an integrated solar-driven CO2 reduction/O2 evolution electrolysis system when powered by a custom large-area [Cs0.05(FA0.85MA0.15)0.95]Pb0.9(I0.85Br0.15)3-based perovskite solar cell.

A visible-light-driven CO2 reduction optical fiber is fabricated using graphene-like nitrogen-doped composites and hollow quartz optical fibers to achieve enhanced activity, selectivity, and light utilization for CO2 photoreduction. The composites are synthesized from a lead-based metal-organic framework (TMOF-10-NH2) and g-C3N4 nanosheet (CNNS) via electrostatic self-assembly. The TMOF-10-NH2/g-C3N4 (TMOF/CNNS) photocatalyst with an S-type heterojunction is coated on optical fiber. The TMOF/CNNS coating, which has a bandgap energy of 2.15eV, has good photoinduced capability at the coating interfaces, high photogenerated electron-hole pair yield, and high charge transfer rate. The conduction band potential of the TMOF/CNNS coating is more negative than that for CO2 reduction. Moreover, TMOF facilitates the CO desorption on its surface, thereby improving the selectivity for CO production. High CO2 photoreduction and selectivity for CO production is demonstrated by the TMOF/CNNS-coated optical fiber with the cladding/core diameter of 2000/1000µm, 10 wt% TMOF in CNNS, coating thickness of 25µm, initial CO2 concentration of 90 vol%, and relative humidity of 88% RH under the excitation wavelength of 380-780nm. Overall, the photocatalytic hollow optical fiber developed herein provides an effective and efficient approach for the enhancement of light utilization efficiency of photocatalysts and selective CO2 reduction.

Highly selective palladium-copper bimetallic electrocatalysts for the electrochemical reduction of CO2 to CO

Ag/C composite catalysts derived from spray pyrolysis for efficient electrochemical CO2 reduction

For the electrochemical CO2 reduction reaction (CO2 RR), the single-metal atom catalysts (SACs) on N-doped carbon are considered promising alternatives to conventional catalysts owing to their unique electrocatalytic properties. However, environmentally friendly methods to prepare SACs are still required. Herein, Ni SAC was synthesized using lignin derived from biomass whose structural and chemical properties render it suitable as both a base carbon matrix and a metal chelating agent. The coordination environment of active Ni-Nx sites was readily manipulated by controlling thermal activation. The Ni SAC on lignin-derived N-doped carbon achieved an outstanding CO Faradaic efficiency of 98.2 % at -0.9 V vs. RHE, which is comparable to those of conventional SACs. Experimental results combined with DFT calculations demonstrate the optimal conditions for manufacturing Ni SAC which is highly selective for CO2 -to-CO conversion and the effect of the electronic structure of Ni atom on CO2 RR kinetics.

The chemical and fuel industry today relies on fossil fuels as its major feedstock and applies a variety of energy-intense thermal / catalytic processes to convert this feed into different intermediates or bulk chemicals and fuels. Many of these processes are responsible for a sizeable fraction of the anthropogenic CO2 emissions that are contributing to global warming and associated issues such as climate change, rising sea levels, and more erratic weather patterns. In contrast, the use of CO2 as the feedstock for the production of bulk chemicals such as CO, ethylene, and ethanol via electrochemical CO2 reduction not only utilizes some of the CO2 that otherwise would be emitted in the atmosphere, it also avoids the sizeable CO2 emissions associated with many of the aforementioned energy-intense processes that use fossil fuels as the feed [1].Over the past decade, a range of active electrocatalysts for the selective reduction of CO2 to different product have been identified. For CO production selectivity easily exceeds 95%, and current densities exceeding 500 mA/cm2 can be achieved at overall energy efficiencies of 45-60% [2,3]. Also, ever more active and selective catalysts for ethylene / ethanol production are being developed. Recently we reported an electrodeposited copper-silver alloy catalyst able to produce ethylene and ethanol at a combined selectivity exceeding 80% (3:1 ethylene to ethanol) at a rate of 170 mA/cm2 [4]. Undoubtedly, research will continue to yield ever more active and selective catalysts for different products. A number of techno-economic analyses have indicated that stability / durability of catalysts and electrodes over thousands of hours will be crucial to achieve economic feasibility (see for example [5]).This presentation will summarize state-of-the-art electrocatalysts for the reduction of CO2 to CO, to ethylene / ethanol, and other products, and how a number of factors, such as electrolyte composition, pH, and electrolysis cell design help optimize electrocatalytic performance. Subsequently, it will review some of the challenges related to enhancing the stability/durability of catalysts and especially of gas diffusion electrodes typically used in electrolysis cells. The presentation will also further explore the techno-economic and life-cycle prospects of CO2 electroreduction technology: which product can be produced in an economically viable and close to carbon neutral fashion using energy from the grid, which only in part originates from renewable sources [6]. A key finding is that reducing the energy requirement of the anode side (by replacing the energy-intense oxygen evolution reaction with a different electrochemical conversion) can be hugely beneficial to achieving economic feasibility and carbon neutrality. For example, co-electrolysis that involves reduction of CO2 on the cathode paired with oxidation of, for example, glycerol (a waste product of biofuel production) at the anode, reduces overall the overall energy requirement of the process by 40-50%. Indeed, techno-economic and life-cycle analyses indicate that co-electrolysis approaches that involve oxidation of organic substrates (biomass, waste streams from industry) on the anode drastically enhance the prospects of CO2 electroreduction technology to be key to a future carbon neutral chemical industry.(1) P.J.A. Kenis, A. Dibenedetto, T.R. Zhang, ChemSusChem, 2017, 18(22), 3091-3093.(2) S. Verma, X. Lu, S. Ma, R.I. Masel, P.J.A. Kenis, PhysChemChemPhys, 2016, 18 (10), 7075-7084.(3) S. Verma, Y. Hamasaki, C. Kim, W. Huang, S. Lu, H.R.M. Jhong, A.A. Gewirth, T. Fujigaya, N. Nakashima, P.J.A. Kenis, ACS Energy Lett., 2018, 3, 193-198.(4) T.T.H. Hoang, S. Verma, S. Ma, T.T. Fister, J. Timoshenko, A.I. Frenkel, P.J.A. Kenis, A.A. Gewirth, J. Am. Chem. Soc., 2018, 140, 5791-5797.(5) S. Verma, B. Kim, H.R. Jhong, S. Ma, P.J.A. Kenis, ChemSusChem, 2016, 9 (15), 1972-1979.(6) S. Verma, S. Lu, P.J.A. Kenis, Nature Energy, 2019, 4, 466–474.

Carbon-based metal-free catalysts exhibit great applications in electrochemical CO2 reduction (ECR), while most studies merely focus on large-sized carbons with limited ratio of surface atoms for engineering to create surface active centers. Here, we report a joint treatment of C60 by heating and plasma to induce a dramatic performance promotion during ECR. The electrochemical measurements indicate the Faraday efficiency for ECR toward CO production kept at high level over 80% in a wide potential region from − 0.4 to − 0.7 V versus RHE, with the highest value reaching 96.8%. The physical characterization reveals that fragmentation of C60 occurs together with N/O doping, both of which could induce change in electron structure, causing the formation of *COOH intermediate and ultimately leading to the optimized activity and selectivity for CO production. This treatment route was further revealed to be applicative for other carbon materials, e.g., single-walled carbon nanotube, to promote its ECR activity and selectivity.

Integration of carbon capture with utilization technologies can lead the way to a net-zero carbon economy. Nevertheless, direct chemical conversion of chemically captured CO2 remains challenging due to its thermodynamic stability. Here, we demonstrate CO2 capture from flue gas/air and its direct conversion into syngas under solar irradiation without any externally applied voltage. The system captures CO2 with an amine/hydroxide solution and photoelectrochemically converts it into syngas (CO:H2 1:2 (concentrated CO2), 1:4 (simulated flue gas), and 1:30 (air)) using a perovskite-based photocathode with an immobilized molecular Co-phthalocyanine catalyst. At the anode, plastic-derived ethylene glycol is oxidized into glycolic acid over a Cu26Pd74 alloy catalyst. The overall process uses flue gas/air as carbon source and discarded plastic waste as electron donor, opening avenues for integrated carbon-neutral/negative solar fuel and waste upcycling technologies.

A local acidic environment at the copper/molten salt interface enabling the efficient CO2 to CO conversion