Correlating the Experimentally Determined CO Adsorption Enthalpy with the Electrochemical CO Reduction Performance on Cu Surfaces

Steering the Selectivity of CORR from Acetate to Ethanol via Tailoring the Thermodynamic Activity of Water

The electrochemical conversion of carbon monoxide (CO) into oxygenated C2+ products at high rates and selectivity offers a promising approach for the two-step conversion of carbon dioxide (CO2). However, a major drawback of the CO electrochemical reduction in alkaline electrolyte is the preference for the acetate pathway over the more valuable ethanol pathway. Recent research has shed light on the significant impact of thermodynamic water activity on the electrochemical CO2 reduction reaction pathways, but less is understood for the electrochemical reduction of CO. In this study, we investigated how the water activity at the electrified interface can be enhanced to adjust the selectivity between acetate and ethanol. We employed an ionomer modifier to lower the local concentration of alkali ions (via Donnan exclusion), successfully enhancing ethanol production while suppressing acetate formation. We observed a remarkable improvement in the Faradaic efficiency of ethanol and alcohol (i. e. ethanol, propanol etc), which reached 42.5 % and 55.1 %, respectively, at a current density of 700 mA cm-2. The partial current densities of ethanol and alcohol reached 698 and 942 mA cm-2 at 2000 mA cm-2. Furthermore, we achieved a 3.7-fold increase in the ethanol/acetate ratio, providing clear evidence of our successful modulation of product selectivity.

Repairing Noise-Contaminated Low-Frequency Vibrational Spectra with an Attention U-Net

Low-frequency vibrational modes in infrared (IR) and Raman spectra, often termed molecular fingerprints, are sensitive probes of subtle structural changes and chemical interactions. However, their inherent weakness and susceptibility to environmental interference make them challenging to detect and analyze. To tackle this issue, we developed a deep learning denoising protocol based on an attention-enhanced U-net architecture. This model leverages the inherent correlations between high- and low-frequency vibrational modes within a molecule, effectively reconstructing low-frequency spectral features from their high-frequency counterparts. We demonstrate the effectiveness of this method by recovering low-frequency signals of trans-1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on an Ag surface, a representative system for surface enhancement Raman spectroscopy (SERS). Notably, the trained model exhibits promising transferability to SERS spectra acquired under different surface and external field conditions. Furthermore, we applied this method to experimental IR and Raman spectra of BPE, achieving high-quality, low-frequency spectral recovery.

Enhanced Carbon-Carbon Coupling at Interfaces with Abrupt Coordination Number Changes

Cu-catalyzed electrochemical CO2 reduction reaction (CO2RR) produces multi-carbon (C2+) chemicals with considerable selectivities and activities, yet required high overpotentials impede its practical application. Here, we design interfaces with abrupt coordination number (CN) changes that greatly reduce the applied potential for achieving high C2+ Faradaic efficiency (FE). Encouraged by the mechanistic finding that the coupling between *CO and *CO(H) is the most probable C-C bond formation path, we use Cu2O- and Cu-phthalocyanine-derived Cu (OD-Cu and PD-Cu) to build the interface. Using operando X-ray absorption spectroscopy (XAS), we find that the Cu CN of OD-Cu is ~11, favoring CO* adsorption, while the PD-Cu has a COH*-favorable CN of ~4. Operando Raman spectroscopy revealed that the interfaces with abrupt CN changes promote *OCCOH formation. As a result, the designed catalyst achieves a C2+ FE of 85±2 % at 220 mA cm-2 in a zero-gap CO2 electrolyzer. An improvement of C2+ FE by 3 times is confirmed at the low potential regime where the current density is 60-140 mA cm-2, compared to bare OD-Cu. We report a 45-h stable CO2RR operation at 220 mA cm-2, producing a C2+ product FE of ~80 %.

Engineering NH2-Cu-NH2 Triple-atom Sites in Defective MOFs for Selective Overall Photoreduction of CO2 into CH3COCH3

Selective photoreduction of CO2 to multicarbon products, is an important but challenging task, due to high CO2 activation barriers and insufficient catalytic sites for C-C coupling. Herein, a defect engineering strategy for incorporating copper sites into the connected nodes of defective metal-organic framework UiO-66-NH2 for selective overall photo-reduction of CO2 into acetone. The Cu2+ site in well-modified CuN2O2 units served as a trapping site to capture electrons via efficient electron-hole separation, forming the active Cu+ site for CO2 reduction. Two NH2 groups in CuN2O2 unit adsorb CO2 and cooperated with copper ion to functionalize as a triple atom catalytic site, each interacting with one CO2 molecule to strengthen the binding of *CO intermediate to the catalytic site. The deoxygenated *CO attached to the Cu site interacted with *CH3 fixed at one amino group to form the key intermediate CO*-CH3, which interacted with the third reduction intermediate on another amino group to produce acetone. Our photocatalyst realizes efficient overall CO2 reduction to C3 product acetone CH3COCH3 with an evolution rate of 70.9 μmol gcat -1 h-1 and a selectivity up to 97 % without any adducts, offering a promising avenue for designing triple-atomic sites to producing C3 product from photosynthesis with water.

Influence of electric double layer rigidity on CO adsorption and electroreduction rate

Understanding the structure of the electric double layer (EDL) is critical for designing efficient electrocatalytic processes. However, the interplay between reactant adsorbates and the concentrated ionic species within the EDL remains an aspect that has yet to be fully explored. In the present study, we employ electrochemical CO reduction on Cu as a model reaction to reveal the significant impact of EDL structure on CO adsorption. By altering the sequence of applying negative potential and elevating CO pressure, we discern two distinct EDL structures with varying cation density and CO coverage. Our findings demonstrate that the EDL comprising densely packed cations substantially hinders CO adsorption on the Cu as opposed to the EDL containing less compact cations. These two different EDL structures remained stable over the course of our experiments, despite their identical initial and final conditions, suggesting an insurmountable kinetic barrier present in between. Moreover, we show that the size and identity of cations play decisive roles in determining the properties of the EDL in CO electroreduction on Cu. This study presents a refined adaptation of the classical Gouy-Chapman-Stern model and highlights its catalytic importance, which bridges the mechanistic gap between the EDL structure and cathodic reactions.

Importance of Site Diversity and Connectivity in Electrochemical CO Reduction on Cu

Electrochemical CO2 reduction on Cu is a promising approach to produce value-added chemicals using renewable feedstocks, yet various Cu preparations have led to differences in activity and selectivity toward single and multicarbon products. Here, we find, surprisingly, that the effective catalytic activity toward ethylene improves when there is a larger fraction of less active sites acting as reservoirs of *CO on the surface of Cu nanoparticle electrocatalysts. In an adaptation of chemical transient kinetics to electrocatalysis, we measure the dynamic response of a gas diffusion electrode (GDE) cell when the feed gas is abruptly switched between Ar (inert) and CO. When switching from Ar to CO, CO reduction (COR) begins promptly, but when switching from CO to Ar, COR can be maintained for several seconds (delay time) despite the absence of the CO reactant in the gas phase. A three-site microkinetic model captures the observed dynamic behavior and shows that Cu catalysts exhibiting delay times have a less active *CO reservoir that exhibits fast diffusion to active sites. The observed delay times and the estimated *CO reservoir sizes are affected by catalyst preparation, applied potential, and microenvironment (electrolyte cation identity, electrolyte pH, and CO partial pressure). Notably, we estimate that the *CO reservoir surface coverage can be as high as 88 ± 7% on oxide-derived Cu (OD-Cu) at high overpotentials (-1.52 V vs SHE) and this increases in reservoir coverage coincide with increased turnover frequencies to ethylene. We also estimate that *CO can travel substantial distances (up to 10s of nm) prior to desorption or reaction. It appears that active C-C coupling sites by themselves do not control selectivity to C2+ products in electrochemical COR; the supply of CO to those sites is also a crucial factor. More generally, the overall activity of Cu electrocatalysts cannot be approximated from linear combinations of individual site activities. Future designs must consider the diversity of the catalyst network and account for intersite transportation pathways.

The plasmonic effect of Cu on tuning CO2 reduction activity and selectivity

Copper (Cu) has been widely used for catalyzing the CO2 reduction reaction (CO2RR), but the plasmonic effect of Cu has rarely been explored for tuning the activity and selectivity of the CO2RR. Herein, we conducted a quantitative analysis on the plasmon-generated photopotential (Ehv) of a Cu nanowire array (NA) photocathode and found that Ehv exclusively reduced the apparent activation energy (Ea) of reducing CO2 to CO without affecting the competitive hydrogen evolution reaction (HER). As a result, the CO production rate was enhanced by 52.6% under plasmon excitation when compared with that under dark conditions. On further incorporation with a polycrystalline Si photovoltaic device, the Cu NA photocathode exhibits good stability in terms of photocurrent and syngas production (CO : H2 = 2 : 1) within 10 h. This work validates the crucial role of the plasmonic effect of Cu on modulating the activity and selectivity of the CO2RR.

Mechanistic Implications of Low CO Coverage on Cu in the Electrochemical CO and CO2 Reduction Reactions

Electrochemical CO or CO2 reduction reactions (CO(2)RR), powered by renewable energy, represent one of the promising strategies for upgrading CO2 to valuable products. To design efficient and selective catalysts for the CO(2)RR, a comprehensive mechanistic understanding is necessary, including a comprehensive understanding of the reaction network and the identity of kinetically relevant steps. Surface-adsorbed CO (COad) is the most commonly reported reaction intermediate in the CO(2)RR, and its surface coverage (θCO) and binding energy are proposed to be key to the catalytic performance. Recent experimental evidence sugguests that θCO on Cu electrode at electrochemical conditions is quite low (∼0.05 monolayer), while relatively high θCO is often assumed in literature mechanistic discussion. This Perspective briefly summarizes existing efforts in determining θCO on Cu surfaces, analyzes mechanistic impacts of low θCO on the reaction pathway and catalytic performance, and discusses potential fruitful future directions in advancing our understanding of the Cu-catalyzed CO(2)RR.

CO Binding Energy is an Incomplete Descriptor of Cu-Based Catalysts for the Electrochemical CO2 Reduction Reaction

CO binding energy has been employed as a descriptor in the catalyst design for the electrochemical CO2 reduction reactions (CO2 RR). The reliability of the descriptor has yet been experimentally verified due to the lack of suitable methods to determine CO binding energies. In this work, we determined the standard CO adsorption enthalpies (Δ H C O ∘ ${\Delta {H}_{CO}^{^\circ{}}}$ ) of undoped and doped oxide-derived Cu (OD-Cu) samples, and for the first time established the correlation ofΔ H C O ∘ ${\Delta {H}_{CO}^{^\circ{}}}$ with the Faradaic efficiencies (FE) for C2+ products. A clear volcano shaped dependence of the FE for C2+ products onΔ H C O ∘ ${\Delta {H}_{CO}^{^\circ{}}}$ is observed on OD-Cu catalysts prepared with the same hydrothermal durations, however, the trend becomes less clear when all catalysts investigated are taken into account. The relative abundance of Cu sites active for the CO2 -to-CO conversion and the further reduction of CO is identified as another key descriptor.

Efficient electroreduction of CO to acetate using a metal-azolate framework with dicopper active sites

Electrochemical reduction of CO to value-added products, especially C2 products, provides a potential approach to achieve carbon neutrality and overcome the energy crisis. Herein, we report a metal-azolate framework (CuBpz) with dicopper active sites as an electrocatalyst for the electrochemical CO reduction reaction (eCORR). As a result, CuBpz achieved an impressive faradaic efficiency (FE) of 47.8% for yielding acetate with a current density of -200 mA cm-2, while no obvious degradation was observed over 60 hours of continuous operation at a current density of -200 mA cm-2. Mechanism studies revealed that the dicopper site can promote C-C coupling between two C1 intermediates, thereby being conducive to the generation of the key *CH2COOH intermediate.