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Spatiotemporal active phase evolution for CO2 electrocatalysis

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Nowadays, high‐valent Cu species (i.e., Cuδ⁺) are clarified to enhance multi‐carbon production in electrochemical CO2 reduction reaction (CO2RR). Nonetheless, the inconsistent average Cu valence states are reported to significantly govern the product profile of CO2RR, which may lead to misunderstanding of the enhanced mechanism for multi‐carbon production and results in ambiguous roles of high‐valent Cu species. Dynamic Cuδ⁺ during CO2RR leads to erratic valence states and challenges of high‐valent species determination. Herein, an alternative descriptor of (sub)surface oxygen, the (sub)surface‐oxygenated degree (κ), is proposed to quantify the active high‐valent Cu species on the (sub)surface, which regulates the multi‐carbon production of CO2RR. The κ validates a strong correlation to the carbonyl (*CO) coupling efficiency and is the critical factor for the multi‐carbon enhancement, in which an optimized Cu2O@Pd2.31 achieves the multi‐carbon partial current density of ≈330 mA cm⁻² with a faradaic efficiency of 83.5%. This work shows a promising way to unveil the role of high‐valent species and further achieve carbon neutralization.
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The promoted activity and enhanced selectivity of electrocatalysts is commonly ascribed to specific structural features such as surface facets, morphology, and atomic defects. However, unraveling the factors that really govern the direct electrochemical reduction of CO2 (CO2RR) is still very challenging since the surface state of electrocatalysts is dynamic and difficult to predict under working conditions. Moreover, theoretical predictions from the viewpoint of thermodynamics alone often fail to specify the actual configuration of a catalyst for the dynamic CO2RR process. Herein, we re‐survey recent studies with the emphasis on revealing the dynamic chemical state of Cu sites under CO2RR conditions extracted by in situ/operando characterizations, and further validate a critical link between the chemical state of Cu and the product profile of CO2RR. This point of view provides a generalizable concept of dynamic chemical‐state‐driven CO2RR selectivity that offers an inspiration in both fundamental understanding and efficient electrocatalysts design.
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With promising activity and stability for the oxygen reduction reaction (ORR), transition metal nitrides are an interesting class of non-platinum group catalysts for polymer electrolyte membrane fuel cells. Here, we report an active thin-film nickel nitride catalyst synthesized through a reactive sputtering method. In rotating disk electrode testing in a 0.1 M HClO4 electrolyte, the crystalline nickel nitride film achieved high activity and selectivity to four-electron ORR. It also exhibited good stability during 10 and 40 h chronoamperometry measurements in acid and alkaline electrolyte, respectively. A combined experiment-theory approach, with detailed ex situ materials characterization and density functional theory calculations, provides insight into the structure of the catalyst and its surface during catalysis. Design strategies for activity and stability improvement through alloying and nanostructuring are discussed.
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To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO2 reduction (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO2R.
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In this study, we demonstrate the initial morphology of nanoparticles can be transformed into small fragmented na-noparticles, which were densely contacted to each other, during electrochemical CO2 reduction reaction (CO2RR). Cu-based nanoparticles were directly grown on a carbon support by using cysteamine immobilization agent, and the syn-thesized nanoparticle catalyst showed increasing activity during initial CO2RR, doubling Faradaic efficiency of C2H4 production from 27 % to 57.3 %. The increased C2H4 production activity was related with the morphological transfor-mation over reaction time. 20 nm cubic Cu2O crystalline particles gradually experienced in-situ electrochemical frag-mentation into 2~4 nm small particles under the negative potential, and the fragmentation was found to be initiated from the surface of the nanocrystal. Compared to Cu@CuO nanoparticle/C or bulk Cu foil, the fragmented Cu-based NP/C catalyst achieved enhanced C2+ production selectivity, accounting 87 % of the total CO2RR products, and sup-pressed H2 production. In-situ X-ray absorption near edge structure studies showed metallic Cu0 state was observed un-der CO2RR, but the fragmented nanoparticles were more readily re-oxidized at open circuit potential inside of the elec-trolyte, allowing labile Cu states. The unique morphology, small nanoparticles stacked each other, is proposed to pro-mote C-C coupling reaction selectivity from CO2RR by suppressing HER.
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The electrochemical reduction of CO2 to higher hydrocarbons is a very challenging process that has high potential for the storage of large amounts of renewable energy with a high gravimetric and volumetric energy density. The distribution of hydrocarbons from the electrocatalytic reduction of CO2 is primarily determined by the interaction of the cathode material with the CO2 in the electrolyte. While the research on the electrochemical CO2 reduction focuses on the cathode metal and surface structure of the metals, recently evidence was found that the metal itself may not be the active species but rather the product formed from the metal and CO2. In this paper, we report about the synthesis, catalytic activity and selectivity of nanostructured metal carbonate, i.e. malachite, as a highly active catalyst for the electrochemical synthesis of C2 hydrocarbons. These first results obtained on Cu2(OH)2CO3 nanorod-structured “trees” show that carbonate, not the pure metal, is the active catalytic species. This new catalyst favors the production of ethylene (C2H4) and ethane (C2H6) with significantly higher Faradaic efficiency than that of the pure Cu surface.
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Oxygen-Cu (O-Cu) combination catalysts have recently achieved highly improved selectivity for ethylene production from the electrochemical CO2 reduction reaction (CO2RR). In this study, we developed anodized copper (AN-Cu) Cu(OH)2 catalysts by a simple electrochemical synthesis method and achieved ~40% Faradaic efficiency for ethylene production, and high stability over 40 hours. Notably, the initial reduction conditions applied to AN-Cu were critical to achieving selective and stable ethylene production activity from the CO2RR, as the initial reduction condition af-fects the structures and chemical states, crucial for highly selective and stable ethylene production over methane. A highly negative reduction potential produced a catalyst maintaining long term stability for the selective production of ethylene over methane, and a small amount of Cu(OH)2 was still observed on the catalyst surface. Meanwhile, when a mild reduction condition was applied to the AN-Cu, the Cu(OH)2 crystal structure and mixed states disappeared on the catalyst, becoming more favorable to methane production after few hours. These results show the selectivity of eth-ylene to methane in O-Cu combination catalysts is influenced by the electrochemical reduction environment related to the mixed valences. This will provide new strategies to improve durability of O-Cu combination catalysts for C-C coupling products from electrochemical CO2 conversion.
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Oxide-derived (OD) Cu catalysts have high selectivity towards the formation of multi-carbon products (C2/C3) for aqueous electrochemical CO2 reduction (CO2R). It has been proposed that a large fraction of the initial oxide can be surprisingly resistant to reduction and that these residual oxides play a crucial role in promoting the formation of C2/C3 products. We investigate the stability of residual oxides by synthesizing 18O enriched OD Cu catalysts and testing them for CO2R. These catalysts maintain a high selectivity towards C2/C3 products (~60%) for up to 5h in 0.1 M KHCO3 at -1.0 V vs RHE. However, secondary ion mass spectrometry measurements show that only a small fraction (< 1%) of the original 18O content remains, showing that residual oxides are not present in significant amounts during CO2R. Furthermore, we show that OD Cu reoxidizes rapidly in the absence of a reducing potential, which could compromise the accuracy of ex-situ methods for determining the true oxygen content.
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Atmospheric pressure x-ray spectroscopy techniques based on soft x-ray excitation can provide interface-sensitive chemical information about a solid surface immersed in a gas or liquid environment. However, x-ray illumination of such dense phases can lead to the generation of considerable quantities of radical species by radiolysis. Soft x-ray absorption measurements of Cu films in both air and aqueous alkali halide solutions reveal that this can cause significant evolution of the Cu oxidation state. In air and NaOH (0.1M) solutions, the Cu is oxidized towards CuO, whilst the addition of small amounts of CH3OH to the solution leads to reduction towards Cu2O. For Ni films in NaHCO3 solutions, the oxidation state of the surface is found to remain stable under x-ray illumination, and can be electrochemically cycled between a reduced and oxidized state. We provide a consistent explanation for this behavior based on the products of x-ray induced radiolysis in these different environments, and highlight a number of general approaches that can mitigate radiolysis effects when performing operando x-ray measurements.
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Significance A most promising approach to boosting both efficiency and selectivity for electrochemical reduction of CO 2 (CO 2 RR) is using Cu 2 O-based electrodes, and the surface Cu ⁺ is believed to play an essential role that is totally unclear from both experiment and theory. We find that the surface Cu ⁺ by itself actually deteriorates the performance of CO 2 RR. Instead we propose a Cu metal embedded in oxidized matrix (MEOM) model and show that it is synergy between surface Cu ⁺ and surface Cu ⁰ present in the MEOM model that improves significantly the kinetics and thermodynamics of both CO 2 activation and CO dimerization, thereby boosting the efficiency and selectivity of CO 2 RR. The MEOM model serves as a unique platform for design of better electrocatalysts for CO 2 RR.
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Watching batteries fail Rechargeable batteries lose capacity in part because of physical changes in the electrodes caused by electrochemical cycling. Lim et al. track the reaction dynamics of an electrode material, LiFePO 4 , by measuring the relative concentrations of Fe(II) and Fe(III) in it by means of high-resolution x-ray absorption spectrometry (see the Perspective by Schougaard). The exchange current density is then mapped for Li ⁺ insertion and removal. At fast cycling rates, solid solutions form as Li ⁺ is removed and inserted. However, at slow cycling rates, nanoscale phase separation occurs within battery particles, which eventually shortens battery life. Science , this issue p. 566 ; see also p. 543
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Nanostructured copper cathodes are among the most efficient and selective catalysts to date for making multi-carbon products from the electrochemical carbon dioxide reduction reaction (CO2RR). We report an in situ X-ray Absorption Spectroscopy (XAS) investigation of the formation of a copper nanocube CO2RR catalyst with high activity that highly favors ethylene over methane production. The results show that the precursor for the copper nanocube formation is copper(I)-oxide, not copper(I)-chloride as previously assumed. A second route to an electrochemically similar material via a copper(II)-carbonate/hydroxide is also reported. This study highlights the importance of using oxidized copper precursors for constructing selective CO2 reduction catalysts, and shows the precursor oxidation state does not affect the electrocatalyst selectivity towards ethylene formation.
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A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
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In this work, we present DFT simulations that demonstrate the ability of Cu to catalyze CO dimerization in CO2 and CO electroreduction. We describe a previously unreported CO dimer configuration that is uniquely stabilized by a charged water layer on both Cu(111) and Cu(100). Without this charged water layer at the metal surface, the formation of the CO dimer is prohibitively endergonic. Our calculations also demonstrate that dimerization should have a lower activation barrier on Cu(100) than Cu(111), which, along with a more exergonic adsorption energy and a corresponding higher coverage of *CO, is consistent with experimental observations from Schouten et al. that Cu(100) has a high activity for C-C coupling at low overpotentials. We also demonstrate that this effect is present with cations other than H+, a finding that is consistent with the experimentally observed pH independence of C2 formation on Cu.
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CO2 conversion is an essential technology to develop a sustainable carbon economy for the present and the future. Many studies have focused extensively on the electrochemical conversion of CO2 into various useful chemicals. However, there is not yet a solution of sufficiently high enough efficiency and stability to demonstrate practical applicability. In this work, we use first-principles-based high-throughput screening to propose silver-based catalysts for efficient electrochemical reduction of CO2 to CO while decreasing the overpotential by 0.4-0.5 V. We discovered the covalency-aided electrochemical reaction (CAER) mechanism in which p-block dopants have a major effect on the modulating reaction energetics by imposing partial covalency into the metal catalysts, thereby enhancing their catalytic activity well beyond modulations arising from d-block dopants. In particular, sulfur or arsenic doping can effectively minimize the overpotential with good structural and electrochemical stability. We expect this work to provide useful insights to guide the development of a feasible strategy to overcome the limitations of current technology for electrochemical CO2 conversion.
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An algorithm is presented for carrying out decomposition of electronic charge density into atomic contributions. As suggested by Bader [R. Bader, Atoms in Molecules: A Quantum Theory, Oxford University Press, New York, 1990], space is divided up into atomic regions where the dividing surfaces are at a minimum in the charge density, i.e. the gradient of the charge density is zero along the surface normal. Instead of explicitly finding and representing the dividing surfaces, which is a challenging task, our algorithm assigns each point on a regular (x,y,z) grid to one of the regions by following a steepest ascent path on the grid. The computational work required to analyze a given charge density grid is approximately 50 arithmetic operations per grid point. The work scales linearly with the number of grid points and is essentially independent of the number of atoms in the system. The algorithm is robust and insensitive to the topology of molecular bonding. In addition to two test problems involving a water molecule and NaCl crystal, the algorithm has been used to estimate the electrical activity of a cluster of boron atoms in a silicon crystal. The highly stable three-atom boron cluster, B3I is found to have a charge of −1.5e, which suggests approximately 50% reduction in electrical activity as compared with three substitutional boron atoms.
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The electronic structure of Cu(2)O and CuO thin films grown on Cu(110) was characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The various oxidation states, Cu(0), Cu(+), and Cu(2+), were unambiguously identified and characterized from their XPS and XAS spectra. We show that a clean and stoichiometric surface of CuO requires special environmental conditions to prevent loss of oxygen and contamination by background water. First-principles density functional theory XAS simulations of the oxygen K edge provide understanding of the core to valence transitions in Cu(+) and Cu(2+). A novel method to reference x-ray absorption energies based on the energies of isolated atoms is presented.
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In this paper, we reported an anodization method for the fabrication of novel uniform Cu(OH)2 nanowires, CuO nanoparticles, and CuO shuttle-like nanoparticles with advanced structures. The possible formation mechanism of Cu(OH)2 nanowires, CuO nanoparticles, and CuO shuttle-like nanoparticles was proposed. The good catalytic properties of CuO nanoparticles converted from Cu(OH)2 nanowires and the CuO shuttle-like nanoparticles were confirmed by evaluating their catalytic ability on the C–N cross coupling of amines with iodobenzene.Highlights► Electrochemical fabrication of uniform Cu(OH)2 and CuO nanostructures. ► The formation mechanism of Cu(OH)2 nanowires and CuO nanostructures was proposed. ► CuO nanostructures exhibited good catalytic ability on the C–N cross coupling.
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The electrochemical reduction of CO2 into hydrocarbons and alcohols would allow renewable energy sources to be converted into fuels and chemicals. However, no electrode catalysts have been developed that can perform this transformation with a low overpotential at reasonable current densities. In this work, we compare trends in binding energies for the intermediates in CO2 electrochemical reduction and present an activity “volcano” based on this analysis. This analysis describes the experimentally observed variations in transition-metal catalysts, including why copper is the best-known metal electrocatalyst. The protonation of adsorbed CO is singled out as the most important step dictating the overpotential. New strategies are presented for the discovery of catalysts that can operate with a reduced overpotential.