Cu Based Dilute Alloys for Tuning the C2+ Selectivity of Electrochemical CO2 Reduction.

Electrochemical CO2 reduction is a promising technology for replacing fossil fuel feedstocks in the chemical industry but further improvements in catalyst selectivity need to be made. So far, only copper-based catalysts have shown efficient conversion of CO2 into the desired multi-carbon (C2+) products. This work explores Cu-based dilute alloys to systematically tune the energy landscape of CO2 electrolysis toward C2+ products. Selection of the dilute alloy components is guided by grand canonical density functional theory simulations using the calculated binding energies of the reaction intermediates CO*, CHO*, and OCCO* dimer as descriptors for the selectivity toward C2+ products. A physical vapor deposition catalyst testing platform is employed to isolate the effect of alloy composition on the C2+/C1 product branching ratio without interference from catalyst morphology or catalyst integration. Six dilute alloy catalysts are prepared and tested with respect to their C2+/C1 product ratio using different electrolyzer environments including selected tests in a 100-cm2 electrolyzer. Consistent with theory, CuAl, CuB, CuGa and especially CuSc show increased selectivity toward C2+ products by making CO dimerization energetically more favorable on the dominant Cu facets, demonstrating the power of using the dilute alloy approach to tune the selectivity of CO2 electrolysis.

Evaluating the stability and activity of dilute Cu-based alloys for electrochemical CO2 reduction.

Cu-based catalysts currently offer the most promising route to actively and selectively produce value-added chemicals via electrochemical reduction of CO2 (eCO2R); yet further improvements are required for their wide-scale deployment in carbon mitigation efforts. Here, we systematically investigate a family of dilute Cu-based alloys to explore their viability as active and selective catalysts for eCO2R through a combined theoretical-experimental approach. Using a quantum-classical modeling approach that accounts for dynamic solvation effects, we assess the stability and activity of model single-atom catalysts under eCO2R conditions. Our calculations identify that the presence of eCO2R intermediates, such as CO*, H*, and OH*, may dynamically influence the local catalyst surface composition. Additionally, we identify through binding energy descriptors of the CO*, CHO*, and OCCO* dimer intermediates that certain elements, such as group 13 elements (B, Al, and Ga), enhance the selectivity of C2+ species relative to pure Cu by facilitating CO dimerization. The theoretical work is corroborated by preliminary testing of eCO2R activity and selectivity of candidate dilute Cu-based alloy catalyst films prepared by electron beam evaporation in a zero-gap gas diffusion electrode-based reactor. Of all studied alloys, dilute CuAl was found to be the most active and selective toward C2+ products like ethylene, consistent with the theoretical predictions. We attribute the improved performance of dilute CuAl alloys to more favorable dimerization reaction energetics of bound CO species relative to that on pure Cu. In a broader context, the results presented here demonstrate the power of our simulation framework in terms of rational catalyst design.

CO2 reduction on pure Cu produces only H2 after subsurface O is depleted: Theory and experiment.

We elucidate the role of subsurface oxygen on the production of C2 products from CO2 reduction over Cu electrocatalysts using the newly developed grand canonical potential kinetics density functional theory method, which predicts that the rate of C2 production on pure Cu with no O is ∼500 times slower than H2 evolution. In contrast, starting with Cu2O, the rate of C2 production is >5,000 times faster than pure Cu(111) and comparable to H2 production. To validate these predictions experimentally, we combined time-dependent product detection with multiple characterization techniques to show that ethylene production decreases substantially with time and that a sufficiently prolonged reaction time (up to 20 h) leads only to H2 evolution with ethylene production ∼1,000 times slower, in agreement with theory. This result shows that maintaining substantial subsurface oxygen is essential for long-term C2 production with Cu catalysts.

Electrocatalysis at Organic-Metal Interfaces: Identification of Structure-Reactivity Relationships for CO2 Reduction at Modified Cu Surfaces.

The limited selectivity of existing CO2 reduction catalysts and rising levels of CO2 in the atmosphere necessitate the identification of specific structure-reactivity relationships to inform catalyst development. Herein, we develop a predictive framework to tune the selectivity of CO2 reduction on Cu by examining a series of polymeric and molecular modifiers. We find that protic species enhance selectivity for H2, hydrophilic species enhance formic acid formation, and cationic hydrophobic species enhance CO selectivity. ReaxFF reactive molecular dynamics simulations indicate that the hydrophilic/hydrophobic modifiers influence the formation of surface hydrides, which yield formic acid or H2. These observations offer insights into how these modifiers influence catalytic behavior at the non-precious Cu surface and may aid in the future implementation of organic structures in CO2 reduction devices.

Stereoelectronic Effects on the Binding of Neutral Lewis Bases to CdSe Nanocrystals.

Using 31P nuclear magnetic resonance (NMR) spectroscopy, we monitor the competition between tri- n-butylphosphine (Bu3P) and various amine and phosphine ligands for the surface of chloride terminated CdSe nanocrystals. Distinct 31P NMR signals for free and bound phosphine ligands allow the surface ligand coverage to be measured in phosphine solution. Ligands with a small steric profile achieve higher surface coverages (Bu3P = 0.5 nm-2, Me2P- n-octyl = 2.0 nm-2, NH2Bu = >3 nm-2) and have greater relative binding affinity for the nanocrystal (binding affinity: Me3P > Me2P- n-octyl ∼ Me2P- n-octadecyl > Et3P > Bu3P). Among phosphines, only Bu3P and Me2P- n-octyl support a colloidal dispersion, allowing a relative surface binding affinity ( Krel) to be estimated in that case ( Krel = 3.1). The affinity of the amine ligands is measured by the extent to which they displace Bu3P from the nanocrystals ( Krel: H2NBu ∼ N- n-butylimidazole > 4-ethylpyridine > Bu3P ∼ HNBu2 > Me2NBu > Bu3N). The affinity for the CdSe surface is greatest among soft, basic donors and depends on the number of each ligand that bind. Sterically unencumbered ligands such as imidazole, pyridine, and n-alkylamines can therefore outcompete stronger donors such as alkylphosphines. The influence of repulsive interactions between ligands on the binding affinity is a consequence of the high atom density of binary semiconductor surfaces. The observed behavior is distinct from the self-assembly of straight-chain surfactants on gold and silver where the ligands are commensurate with the underlying lattice and attractive interactions between aliphatic chains strengthen the binding.

Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films.

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, we present a protocol for the fabrication of this material via the low-pressure vapor assisted solution process (LP-VASP) method, which yields ~19% power conversion efficiency in planar heterojunction perovskite solar cells. First, we report the synthesis of methylammonium iodide (CH3NH3I) and methylammonium bromide (CH3NH3Br) from methylamine and the corresponding halide acid (HI or HBr). Then, we describe the fabrication of pinhole-free, continuous methylammonium-lead halide perovskite (CH3NH3PbX3 with X = I, Br, Cl and their mixture) films with the LP-VASP. This process is based on two steps: i) spin-coating of a homogenous layer of lead halide precursor onto a substrate, and ii) conversion of this layer to CH3NH3PbI3-xBrx by exposing the substrate to vapors of a mixture of CH3NH3I and CH3NH3Br at reduced pressure and 120 °C. Through slow diffusion of the methylammonium halide vapor into the lead halide precursor, we achieve slow and controlled growth of a continuous, pinhole-free perovskite film. The LP-VASP allows synthetic access to the full halide composition space in CH3NH3PbI3-xBrx with 0 ≤ x ≤ 3. Depending on the composition of the vapor phase, the bandgap can be tuned between 1.6 eV ≤ Eg ≤ 2.3 eV. In addition, by varying the composition of the halide precursor and of the vapor phase, we can also obtain CH3NH3PbI3-xClx. Films obtained from the LP-VASP are reproducible, phase pure as confirmed by X-ray diffraction measurements, and show high photoluminescence quantum yield. The process does not require the use of a glovebox.

Bimetallic coordination insertion polymerization of unprotected polar monomers: copolymerization of amino olefins and ethylene by dinickel bisphenoxyiminato catalysts.

Dinickel bisphenoxyiminato complexes based on highly substituted p- and m-terphenyl backbones were synthesized, and the corresponding atropisomers were isolated. In the presence of a phosphine scavenger, Ni(COD)2, the phosphine-ligated syn-dinickel complexes copolymerized α-olefins and ethylene in the presence of amines to afford 0.2-1.3% α-olefin incorporation and copolymerized amino olefins and ethylene with a similar range of incorporation (0.1-0.8%). The present rigid catalysts provide a bimetallic strategy for insertion polymerization of polar monomers without masking of the heteroatom group. The effects of the catalyst structure on the reactivity were studied by comparisons of the syn and anti atropisomers and the p- and m-terphenyl systems.