Liquid-Liquid Interface -Driven Reconstruction of CuAg Nanocomposites for Selective CO2 to C2H4 Electroreduction.

Quantifying Electrocatalytic Reduction of CO2 on Twin Boundaries

Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

CO2 electroreduction to multicarbon products from carbonate capture liquid

Carbon dioxide emission from fossil fuel combustion poses a major threat to global environment and ecological systems.Carbon capture, sequestration and conversion technologies are widely pursued as possible solutions to mitigate negative impact by CO2. The electrochemical CO2 reduction reaction (CO2RR) to fuels and chemicals using renewable electricity offers attractive “carbon-neutral” and “carbon-negative” mitigation strategies. Various catalysts have been investigated as the electrocatalysts for CO2RR. Key challenges facing the current catalyst and electrolyzer designs include insufficient energy efficiency and low single product selectivity.CO2RR to C2+ chemicals represent a highly important area for CO2 reduction and utilization. For example, ethanol, ethylene, propanol, etc. are among the most produced chemicals by the industry and are widely used for various applications. Using CO2 as raw material for chemical production through electrocatalysis not only improves the carbon cycling, but also reduces the overall emissions from chemical production. While CO2RR via two proton-electron pairs (PEPs), such as the conversion of CO2 to CO or formate, have been proven high selective with fast kinetics, conversions to C2+ chemicals are significantly more difficult due to the rapid escalation of required PEPs to much higher numbers (for example, 12 for ethanol and 18 for propanol), in addition to C-C bond coupling. The increased PEPs substantially complicate the conversion by required multiple steps along the electrochemical coordinate, leading to a high probability of branching reactions and low single product Faradaic efficiency (FE).To address these challenges, the design criteria for CO2RR to C2+ chemicals should be based on high uniformity of active center for directing identical catalytic path and suppressing competing reactions, strong catalyst-reactant binding in capturing the transient species through multiple PEP transfers, and microenvironment with nanoconfinement for retaining reaction intermediates during extended catalytic processes.Recently, we developed a new amalgamated lithium metal (ALM) synthesis method of preparing highly selective and active CO2RR catalyst and achieved > 90% FE for conversion of CO2 to ethanol [1]. Our have since expanded the approach to other C2 + chemicals including acetate, acetone, glycerol, isopropanol, etc., all with FE at or higher than 80%. To better formulate our catalyst design strategy, we not only measured the CO2RR performance over a variety of electrocatalysts, but also investigated their activity-structure relationship through advanced material characterizations, combined with the first-principle computation. We found many interesting properties uniquely associated to CO2RR mechanism and kinetics, such as catalyst-size and electro-potential modulated single selectivity. In this presentation, we will share our recent discoveries and future perspective on the development of highly selective CO2RR catalysts for C2+ chemical conversions. Acknowledgement: This work is supported by U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy - Industrial Efficiency & Decarbonization Office and by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357.[1] “Highly selective electrocatalytic CO2 reduction to ethanol by metallic clusters dynamically formed from atomically dispersed copper” Haiping Xu, Dominic Rebollar, Haiying He, Lina Chong, Yuzi Liu, Cong Liu, Cheng-Jun Sun, Tao Li, John V. Muntean, Randall E. Winans, Di-Jia Liu and Tao Xu, (2020) Nature Energy, 5, 623–632

Electrochemical CO2 reduction reaction (CO2RR) has gained significant attention as a possible carbon-neutral way of producing chemical feedstock in conjunction with renewable energy. The enormous efforts have been mainly focused on catalyst research, resulting in a spectrum of efficient catalysts for CO2RR into CO, an attractive feedstock due to the relatively preferable reaction conditions with two-electron and proton transfers. However, most catalysts were shown to promote CO2RR only in a CO2-saturated less-acidic aqueous solution or with an anion exchange membrane to suppress hydrogen evolution reaction, which impedes the implementation of a scalable and cost-effective nafion-based membrane electrode assembly (MEA). Another underlying challenge is that reported catalysts usually require multi-step and often complex synthetic conditions, which push the production costs. Additionally, the subsequent performance evaluation and cell optimization are often laborious because of a vast optimization space, which may delay the feasibility study of catalysts. Such constraints impose practical difficulty and raise the overall cost of CO2RR deployment.Here, we present scalable catalyst design and machine-learning-assisted cell optimization to address the challenges mentioned above. First, we report a straightforward one-pot synthesis of cobalt and organic [poly-4-vinylpyridine (P4VP)] precursors with carbon supports as a catalyst compatible with nafion-based MEA [1]. Electrochemical studies indicated that this catalyst performs CO2RR predominantly over HER across a wide range of bulk pH from 2 to 7 due to lower pH dependence of CO2RR reaction rate. The structural investigation revealed that CoO@Co nanoparticles and pyridine moieties co-exist, forming a local environment that is preferable for CO2RR. Pourbaix-diagram analysis indicates that concomitant reduction of Co2+ and pyridine moieties are likely involved in a reaction step, including proton-decoupled electron transfer. The initial optimization of the Co-P4VP-derived catalyst and MEA reached the remarkable performance of Faradaic efficiency (FE) of 92% at 85 mA/cm2, and a preliminary durability test showed stable FE for 20 h operation. We postulate that these outstanding performances reflect the synergistic effect of CoO and pyridine moieties that were achieved by coordinating P4VP to Co during the appropriate thermal process.To further optimize the MEA and accelerate the feasibility study cycle of the Co-P4VP-derived catalyst, we introduced a machine-learning-assisted, goal-oriented optimization procedure with energy efficiency (EE) and partial current density for CO2RR into CO (jCO2RR) as target features. This method includes data annotation, feature selection, model selection, data sorting, statistical stability test, and experimental validation. We selected the feature based on multiple non-linear correlation analyses, which highlighted the importance of both component and process parameters. While a conventional cross-validation technique did not provide a significant difference in model accuracy, the distribution of predicted data in the reverse engineering process heavily depended on the investigated models. Accordingly, we chose the model that best reflected experimental results and our understanding of the physical limitations of the MEA cell. The predicted data points were sorted with target-oriented criteria, including the production cost of CO, and several were selected based on a statistical stability test. The ensuing experiments successfully validated the effectiveness of this process as we achieved the promising performance values of 57% for EE and 115 mA/cm2 for jCO2RR with nafion-based MEA. Results from data feedback in the algorithm predicts those values are close to optimization within the given parameter space which was reached much more quickly than by conventional unguided use of machine learning.We expect this comprehensive study to serve as an effective example of how to incorporate exploratory catalyst design, analysis-based knowledge extraction, and ML-assisted cell optimization to accelerate material-based electrochemical advancement in developing areas such as CO2RR.[1] N. Fujinuma, A. Ikoma, S.E. Lofland, Adv. Energy Mater. 10 (2020) 2001645.

Various electrocatalysts are extensively examined for their ability to selectively produce desired products by electrochemical CO2 reduction reaction (CO2RR). However, an efficient CO2RR electrocatalyst doesn't ensure an effective co-catalyst on the semiconductor surface for photoelectrochemical CO2RR. Herein, Bi2S3 nanorods are synthesized and electrochemically reduced to Bi nanoplates that adhere to the substrates for application in the electrochemical and photoelectrochemical CO2RR. Compared with commercial-Bi, the Bi2S3-derived Bi (S-Bi) nanoplates on carbon paper exhibit superior electrocatalytic activity and selectivity for formate (HCOO-) in the electrochemical CO2RR, achieving a Faradaic efficiency exceeding 93%, with minimal H2 production over a wide potential range. This highly selective S-Bi catalyst is being employed on the Si photocathode to investigate the behavior of electrocatalysts during photoelectrochemical CO2RR. The strong adhesion of the S-Bi nanoplates to the Si nanowire substrate and their unique catalytic properties afford exceptional activity and selectivity for HCOO- under simulated solar irradiation. The selectivity observed in electrochemical CO2RR using the S-Bi catalyst correlates with that seen in the photoelectrochemical CO2RR system. Combined pulsed potential methods and theoretical analyses reveal stabilization of the OCHO* intermediate on the S-Bi catalyst under specific conditions, which is critical for developing efficient catalysts for CO2-to-HCOO- conversion.

Various electrocatalysts have undergone extensive examination regarding their capacity to selectively produce desired products via the electrochemical CO2 reduction reaction (CO2RR). Nonetheless, the efficiency of a CO2RR electrocatalyst does not guarantee the effectiveness of a co-catalyst on the semiconductor surface for photoelectrochemical CO2RR. Herein, we synthesize Bi2S3 nanorods and electrochemically reduce them to Bi nanoplates that adhere to substrates for use in electrochemical and photoelectrochemical CO2RR applications. Compared with bare Bi, the Bi2S3-derived Bi (S-Bi) nanoplates on carbon paper exhibit superior electrocatalytic activity and selectivity for formate (HCOO-) in the electrochemical CO2RR, achieving a Faradaic efficiency exceeding 93%, with minimal H2 production over a wide potential range. This highly selective S-Bi catalyst is being employed on the Si photocathode to investigate the behavior of electrocatalysts during photoelectrochemical CO2RR. The strong adhesion of the S-Bi nanoplates to the Si nanowire substrate and their unique catalytic properties afford exceptional activity and selectivity for HCOO- under simulated solar irradiation. The selectivity observed in electrochemical CO2RR using the S-Bi catalyst correlates with that seen in the photoelectrochemical CO2RR system. Combined pulsed potential methods and theoretical analyses reveal stabilization of the OCHO* intermediate on the S-Bi catalyst under specific conditions, which is critical for developing efficient catalysts for CO2-to-HCOO- conversion. These results emphasize the potential of S-Bi as an effective catalyst for CO2 reduction reactions and provide valuable insights into optimizing the reaction mechanisms and enhancing the product selectivity, especially in conjunction with co-catalysts for Si photocathodes.

Cu-based nanocatalysts for electrochemical reduction of CO2

Metallic carbonyls: L. Mond, H. Hirtz, and M. D. Cowap. Trans. Chem. Socy., Tom, xcvii, 198

BiO _2-x Nanosheets with Surface Electron Localizations for Efficient Electrocatalytic CO ₂ Reduction to Formate

Harnessing in-situ oxidation of nanostructured mxene to modulate the electronic structure for improved selectivity for electrochemical carbon dioxide reduction

The need of a sustainable society to advance towards a circular economy, in which there is a balance between the emission and capture of anthropogenic gases like carbon dioxide (CO2), is of utmost scientific and technological importance to ensure the increase or rather preservation of the current prosperity for future generations. The growing global population and increase in worldwide prosperity is tightly connected to a rise in power consumption. Traditionally, this energy demand is supplied by the combustion of fossil fuels, resulting in growing emission of greenhouse gases. In this, CO2 is a key issue which climatic influences and general mitigation is rigorously disused not only in scientific field, but in politics as well. Here, the electrochemical CO2 reduction reaction (CO2RR) is posing as one potential technology, to address questions of power storage for renewable energies and sustainable usage of natural resources. The CO2RR shows a diverse spectrum of products ranging from liquid fuels, as ethanol and propanol, to gaseous building blocks for the chemical industry, as ethylene and CO. The selectivity of this reaction is highly dependent on the nature of the catalyst and is always competing with the hydrogen evolution reaction (HER), due to the aqueous conditions. Recently, very selective and active catalysts have been developed for the production of CO and formic acid. Both compounds are comparably easy to produce, as the reduction only involves the transfer of 2 electrons. Unfortunately, the production of valuable hydrocarbons is much more difficult and suffers from losses in selectivity due to the highly complex reaction mechanism. So far, only copper showed acceptable production rates for hydrocarbons, owing to the favorable binding energies of intermediates. Recent studies have been focused on fundamental parameters to control the selectivity of this complex reaction on copper for C2+ species, ranging from catalyst design to electrolyte selection. On the catalyst side, many effects as the presence of (100) facets1, grain boundaries2 and oxides3 have been suggested to be beneficial, whereas influences of buffer capacity and local electric field brought additional advantages by choice of the electrolyte. While promising results were shown, many studies only focused on low-current density, which are far from an industrial application. Here, at least 200 mA cm-2 are needed for a functional electrolyzer. This poses as a discrepancy between research and application and raises the question if the results from fundamental studies can be transferred to full size electrolyzers.4 For this, we are presenting a cubic Cu2O catalyst, which we investigate in a comprehensive study, moving from initial tests in an H-Cell towards high currents on a Gas Diffusion Electrode (GDE) in a Flow-Cell setup. To deconvolute the parameters dictating CO2RR selectivity, we used X-Ray-Diffraction (XRD), quasi in-situ X-Ray Photon Spectroscopy (XPS) and operando X-ray Absorption Spectroscopy (XAS) to trace phase changes during reaction, in addition to morphological investigations by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). This complementary analysis depicts a highly dynamic system, in which the reduction of oxidized Cu progresses from the surface towards deeper layers, resulting in a purely metallic, defect-rich material. We further focus on performance tests of our Cu2O catalyst at high current density of up to 700 mA cm-2 in a flow-electrolyzer. By varying system parameters as mass loading and nafion content we observe strong changes in selectivity and activity during CO2RR, which we correlate to accessibility of the active copper sites and issues of mass transport. We further discuss the role of surface pH at high current density by varying electrolyte concentration and therefore buffer capacity. Our study suggests distinct differences between CO2RR in electrochemical cells for fundamental studies at low currents compared to tests at high currents in a flow-electrolyzer. Furthermore, we show how recent results from literature translate to performance at high current density and comment on the importance of electrode preparation. Y. Hori, I. Takahashi, O. Koga and N. Hoshi, Journal of Molecular Catalysis A: Chemical, 2003, 199, 39-47.A. Verdaguer-Casadevall, C. W. Li, T. P. Johansson, S. B. Scott, J. T. McKeown, M. Kumar, I. E. L. Stephens, M. W. Kanan and I. Chorkendorff, Journal of the American Chemical Society, 2015, 137, 9808-9811.H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser and B. R. Cuenya, Nature Communications, 2016, 7, 12123.T. Burdyny and W. A. Smith, Energy & Environmental Science, 2019, DOI: 10.1039/c8ee03134g.

Local Weak Hydrogen Bonds Significantly Enhance CO ₂ Electroreduction Performances of a Metal–Organic Framework

The electrochemical CO2 reduction reaction (CO2RR) occurs at the nanoscale interface of the electrode-electrolyte. Therefore, tailoring the interfacial properties in the interface microenvironment provides a powerful strategy to optimise the activity and selectivity of electrocatalysts towards the desired products. Here, the microenvironment at the electrode-electrolyte interface of the flow-through Ag-based hollow fibre gas diffusion electrode (Ag HFGDE) is modulated by introducing surfactant cetyltrimethylammonium bromide (CTAB) as the electrolyte additive. The porous hollow fibre configuration and gas penetration mode facilitate the CO2 mass transfer and the formation of the triple-phase interface. Through the ordered arrangement of hydrophobic long-alkyl chains, CTAB molecules at the electrode/electrolyte interface promoted CO2 penetration to active sites and repelled water to reduce the activity of competitive hydrogen evolution reaction (HER). By applying CTAB-containing catholyte, Ag HFGDE achieved a high CO Faradaic efficiency (FE) of over 95 % in a wide potential range and double the partial current density of CO. The enhancement of CO selectivity and suppression of hydrogen was attributed to the improvement of charge transfer and the CO2/H2O ratio enhancement. These findings highlight the importance of adjusting the local microenvironment to enhance the reaction kinetics and product selectivity in the electrochemical CO2 reduction reaction CO2RR.

The electrochemical reduction of CO2 is a technology of great environmental and economic interest as it provides a promising solution for reducing the concentration of CO2 with producing valuable chemical compounds. Cu and its alloys have attracted much attention because of their moderate CO binding energy for reducing CO2 to multi-carbon (C2+) products[1]. The reactivity and product selectivity of the CO2 reduction reaction (CO2RR) depend on the interfacial structure and the nano-scale morphology of the catalyst as well as on the catalyst material composition. Especially for the gas diffusion electrodes (GDEs), the catalyst morphology has a significant influence on the formation of triple-phase boundary that increases CO2 reactivity and product selectivity[2,3]. However, the influence of catalyst morphology including catalyst thickness, porosity, particle size on CO2RR has not been fully understood yet because of the complex CO2RR mechanism and catalyst structure in GDEs. In this study, we investigated the relationship between product selectivity and the thickness of the Cu controlled by sputtering techniques and characterized nanostructure of the Cu-GDEs.The Cu catalyst layer (CL) was deposited on a commercial carbon-based gas diffusion layer (GDL) with a micro porous layer (MPL) by magnetron sputtering from a pure Cu target at room temperature. The geometric area of the deposited Cu was 0.5 cm2. The nanostructures of GDEs were evaluated using scanning transmission electron microscopy equipped with energy dispersive X-ray analyzer (STEM-EDX). Electrochemical experiments were carried out in a three-electrode setup using a Biologic-VSP instrument. A home-made three-compartment flow cell was used with the Cu-GDE as the working electrode between the gas and cathode compartments. An Ag/AgCl electrode was placed in the cathode compartment as the reference electrode and a Pt mesh as the counter electrode in the anode compartment. 1 M KCl was used as the catholyte, saturated KHCO3 was used as the anolyte and a proton exchange membrane (Nafion 117) was used to separate the anode and cathode compartments. The CO2 gas flow to a gas compartment was kept at 30 sccm. Electrochemical CO2RR was performed by chronopotentiometry. The gas products were quantified using a gas chromatography with thermal conductivity detector. As for the liquid products, alcohols were evaluated using a gas chromatography with flame ionization detector and organic acids were detected by high performance liquid chromatography with conductivity detector, respectively.The faradaic efficiencies (FEs) of CO2RR products using GDEs with various CL thickness of 70, 300, 1000, 2000 nm were measured under current density of 400 mA cm-2. The GDE with CL thickness of x nm is written as CLxGDE. The major gas and liquid products for all the GDEs were ethylene and ethanol with the FE of around 37-42% and 26-31%, respectively. As decreasing CL thickness from 2000 nm, FEs for C2+ products (FEC2+), such as ethylene, ethanol, n-propanol etc., were increased from that of CL2000GDE of 69%. The maximum FEC2+ reaching 81% was achieved for CL300GDE. The FEC2+ for CL70GDE was slightly decreased down to 76%. In comparison with the FEs for CL2000GDE, the FEs of ethylene, ethanol, and n-propanol were each increased by 2-5%, while FE of H2 was decreased for CL300GDE.To clarify the details of CL morphology, the surface structure of CL300GDEs was observed before and after the CO2RR measurement using STEM-EDX. Figures (a) and (b) show cross-sectional images of nano-scale annular dark field (ADF)-STEM and EDX mapping (purple color show Cu and cyan blue color show C) before the CO2RR measurement. The Cu layer was uniformly stacked on MPL with the designed thickness of 300 nm. Figures (c) and (d) show cross-sectional images of ADF-STEM and EDX mapping after the CO2RR measurement. Cu nanoparticles less than 50 nm were distributed on the surface and inside of the MPL. The stacked Cu layer as observed before the CO2RR measurement was not observed after the CO2RR. These results indicate that Cu atoms migrate during the CO2RR and distributed Cu nanoparticles exhibit the high FEC2+. We will discuss the influence of Cu migration during the CO2RR on product selectivity from the results of synchrotron-based analysis and nanostructure observation in the presentation.[1] A. Bagger et al., ChemPhysChem. 2017, 18, 3266-3273.[2] N. T. Nesbitt et al., ACS Catal. 2020, 10, 14093-14106.[3] A. Inoue et al., EES Catal. 2023, 1, 9–16.Fig. Cross-sectional images of (a), (c) ADF-STEM and (b), (d) EDX mapping before and after CO2RR for CL300GDEs, respectively. Purple color show Cu and cyan blue color show C. Figure 1