The presence and role of the intermediary CO reservoir in heterogeneous electroreduction of CO2

Exploring the influence of cell configurations on Cu catalyst reconstruction during CO2 electroreduction

Membrane electrode assembly (MEA) cells incorporating Cu catalysts are effective for generating C2+ chemicals via the CO2 reduction reaction (CO2RR). However, the impact of MEA configuration on the inevitable reconstruction of Cu catalysts during CO2RR remains underexplored, despite its considerable potential to affect CO2RR efficacy. Herein, we demonstrate that MEA cells prompt a unique reconstruction of Cu, in contrast to H-type cells, which subsequently influences CO2RR outcomes. Utilizing three Cu-based catalysts, specifically engineered with different nanostructures, we identify contrasting selectivity trends in the production of C2+ chemicals between H-type and MEA cells. Operando X-ray absorption spectroscopy, alongside ex-situ analyses in both cell types, indicates that MEA cells facilitate the reduction of Cu2O, resulting in altered Cu surfaces compared to those in H-type cells. Time-resolved CO2RR studies, supported by Operando analysis, further highlight that significant Cu reconstruction within MEA cells is a primary factor leading to the deactivation of CO2RR into C2+ chemicals.

Visualization of CO2 electrolysis using optical coherence tomography

Electrolysers offer an appealing technology for conversion of CO2 into high-value chemicals. However, there are few tools available to track the reactions that occur within electrolysers. Here we report an electrolysis optical coherence tomography platform to visualize the chemical reactions occurring in a CO2 electrolyser. This platform was designed to capture three-dimensional images and videos at high spatial and temporal resolutions. We recorded 12 h of footage of an electrolyser containing a porous electrode separated by a membrane, converting a continuous feed of liquid KHCO3 to reduce CO2 into CO at applied current densities of 50-800 mA cm-2. This platform visualized reactants, intermediates and products, and captured the strikingly dynamic movement of the cathode and membrane components during electrolysis. It also linked CO production to regions of the electrolyser in which CO2 was in direct contact with both membrane and catalyst layers. These results highlight how this platform can be used to track reactions in continuous flow electrochemical reactors.

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.

Multiscale effects in tandem CO2 electrolysis to C2+ products

CO2 electrolysis is a sustainable technology capable of accelerating global decarbonisation through the production of high-value alternatives to fossil-derived products. CO2 conversion can generate critical multicarbon (C2+) products such as drop-in chemicals ethylene and ethanol, however achieving high selectivity from single-component catalysts is often limited by the competitive formation of C1 products. Tandem catalysis can overcome C2+ selectivity limitations through the incorporation of a component that generates a high concentration of CO, the primary reactant involved in the C-C coupling step to form C2+ products. A wide range of approaches to promote tandem CO2 electrolysis have been presented in recent literature that span atomic-scale manipulation to device-scale engineering. Therefore, an understanding of multiscale effects that contribute to selectivity alterations are required to develop effective tandem systems. In this review, we use relevant examples to highlight the complex and interlinked contributions to selectivity and provide an outlook for future development of tandem CO2 electrolysis systems.

Facet-Defined Dilute Metal Alloy Nanorods for Efficient Electroreduction of CO2 to n-Propanol

Electroreduction of CO2 into liquid fuels is a compelling strategy for storing intermittent renewable energy. Here, we introduce a family of facet-defined dilute copper alloy nanocrystals as catalysts to improve the electrosynthesis of n-propanol from CO2 and H2O. We show that substituting a dilute amount of weak-CO-binding metals into the Cu(100) surface improves CO2-to-n-propanol activity and selectivity by modifying the electronic structure of catalysts to facilitate C1-C2 coupling while preserving the (100)-like 4-fold Cu ensembles which favor C1-C1 coupling. With the Au0.02Cu0.98 champion catalyst, we achieve an n-propanol Faradaic efficiency of 18.2 ± 0.3% at a low potential of -0.41 V versus the reversible hydrogen electrode and a peak production rate of 16.6 mA·cm-2. This study demonstrates that shape-controlled dilute-metal-alloy nanocrystals represent a new frontier in electrocatalyst design, and precise control of the host and minority metal distributions is crucial for elucidating structure-composition-property relationships and attaining superior catalytic performance.

Electrochemical C-N Bond Formation within Boron Imidazolate Cages Featuring Single Copper Sites

Electrocatalysis expands the ability to generate industrially relevant chemicals locally and on-demand with intermittent renewable energy, thereby improving grid resiliency and reducing supply logistics. Herein, we report the feasibility of using molecular copper boron-imidazolate cages, BIF-29(Cu), to enable coupling between the electroreduction reaction of CO2 (CO2RR) with NO3- reduction (NO3RR) to produce urea with high selectivity of 68.5% and activity of 424 μA cm-2. Remarkably, BIF-29(Cu) is among the most selective systems for this multistep C-N coupling to-date, despite possessing isolated single-metal sites. The mechanism for C-N bond formation was probed with a combination of electrochemical analysis, in situ spectroscopy, and atomic-scale simulations. We found that NO3RR and CO2RR occur in tandem at separate copper sites with the most favorable C-N coupling pathway following the condensation between *CO and NH2OH to produce urea. This work highlights the utility of supramolecular metal-organic cages with atomically discrete active sites to enable highly efficient coupling reactions.

Operando High-Energy-Resolution X-ray Spectroscopy of Evolving Cu Nanoparticle Electrocatalysts for CO2 Reduction

Advances in electrocatalysis research rely heavily on building a thorough mechanistic understanding of catalyst active sites under realistic operating conditions. Only recently have techniques emerged that enable sensitive spectroscopic data collection to distinguish catalytically relevant surface sites from the underlying bulk material under applied potential in the presence of an electrolyte layer. Here, we demonstrate that operando high-energy-resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) is a powerful spectroscopic method which offers critical surface chemistry insights in CO2 electroreduction with sub-electronvolt energy resolution using hard X-rays. Combined with the high surface area-to-volume ratio of 5 nm copper nanoparticles, operando HERFD-XAS allows us to observe with clear evidence the breaking of chemical bonds between the ligands and the Cu surface as part of the ligand desorption process occurring under electrochemical potentials relevant for the CO2 reduction reaction (CO2RR). In addition, the dynamic evolution of oxidation state and coordination number throughout the operation of the nanocatalyst was continuously tracked. With these results in hand, undercoordinated metallic copper nanograins are proposed to be the real active sites in the CO2RR. This work emphasizes the importance of HERFD-XAS compared to routine XAS in catalyst characterization and mechanism exploration, especially in the complicated electrochemical CO2RR.

How Temperature Affects the Selectivity of the Electrochemical CO2 Reduction on Copper

Copper is a unique catalyst for the electrochemical CO₂ reduction reaction (CO2RR) as it can produce multi-carbon products, such as ethylene and propanol. As practical electrolyzers will likely operate at elevated temperatures, the effect of reaction temperature on the product distribution and activity of CO2RR on copper is important to elucidate. In this study, we have performed electrolysis experiments at different reaction temperatures and potentials. We show that there are two distinct temperature regimes. From 18 up to ∼48 °C, C2+ products are produced with higher Faradaic efficiency, while methane and formic acid selectivity decreases and hydrogen selectivity stays approximately constant. From 48 to 70 °C, it was found that HER dominates and the activity of CO2RR decreases. Moreover, the CO2RR products produced in this higher temperature range are mainly the C1 products, namely, CO and HCOOH. We argue that CO surface coverage, local pH, and kinetics play an important role in the lower-temperature regime, while the second regime appears most likely to be related to structural changes in the copper surface.

Potential-Dependent Temporal Dynamics of CO Surface Concentration in Electrocatalytic CO2 Reduction

Beyond the identity and structure of an intermediate, changes in its concentration on and near the electrode surface with time are a critical component to understand and improve selectivity and reactivity in electrochemical transformations. We apply pulsed-potential electrochemical Raman scattering microscopy to measure the potential-dependent temporal evolution of CO formed during electrocatalytic CO₂ reduction in acetonitrile on Ag electrodes. At driving potentials positive of the onset potential as determined by cyclic voltammetry, CO accumulates on the electrode surface at time scales longer than 1 s. Near the ensemble onset potential, CO resides on the electrode surface for approximately 100 ms. At potentials known to evolve CO from the electrode surface, CO remains adsorbed on the electrode for less than 10 ms. The time scales accessible in our strategy are nearly 3 orders of magnitude faster than transient Raman or infrared measurements, allowing direct measurement of the temporal evolution of intermediates.

The spatial distribution of cobalt phthalocyanine and copper nanocubes controls the selectivity towards C2 products in tandem electrocatalytic CO2 reduction

The coupling of CO-generating molecular catalysts with copper electrodes in tandem schemes is a promising strategy to boost the formation of multi-carbon products in the electrocatalytic reduction of CO₂. While the spatial distribution of the two components is important, this aspect remains underexplored for molecular-based tandem systems. Herein, we address this knowledge gap by studying tandem catalysts comprising Co-phthalocyanine (CoPc) and Cu nanocubes (Cu_cub). In particular, we identify the importance of the relative spatial distribution of the two components on the performance of the tandem catalyst by preparing CoPc-Cu_cub/C, wherein the CoPc and Cu_cub share an interface, and CoPc-C/Cu_cub, wherein the CoPc is loaded first on carbon black (C) before mixing with the Cu_cub. The electrocatalytic measurements of these two catalysts show that the faradaic efficiency towards C₂ products almost doubles for the CoPc-Cu_cub/C, whereas it decreases by half for the CoPc-C/Cu_cub, compared to the Cu_cub/C. Our results highlight the importance of a direct contact between the CO-generating molecular catalyst and the Cu to promote C-C coupling, which hints at a surface transport mechanism of the CO intermediate between the two components of the tandem catalyst instead of a transfer via CO diffusion in the electrolyte followed by re-adsorption.

Multiple C-C bond formation upon electrocatalytic reduction of CO2 by an iron-based molecular macrocycle

Molecular macrocycles are very promising electrocatalysts for the reduction of carbon dioxide into value-added chemicals. Up to now, most of these catalysts produced only C₁ products. We report here that iron phthalocyanine, a commercially available molecule based on earth-abundant elements, can produce light hydrocarbons upon electrocatalytic reduction of CO₂ in aqueous conditions and neutral pH. Under applied electrochemical potential, C₁ to C₄ saturated and unsaturated products are evolved. Isotopic labelling experiments unambiguously show that these products stem from CO₂. Control experiments and in situ X-ray spectroscopic analysis show that the molecular catalyst remains intact during catalysis and is responsible for the reaction. On the basis of experiments with alternate substrates, a mechanism is proposed for the C-C bond formation step.

Correlating CO Coverage and CO Electroreduction on Cu via High-Pressure in Situ Spectroscopic and Reactivity Investigations

The absolute coverage of CO has been a missing piece in the mechanistic puzzle of the CO reduction reaction (CORR) on Cu. For the first time, we revealed the upper bound of the CO coverage under electrocatalytic conditions to be 0.05 monolayer at atmospheric pressure and the saturation CO coverage to be ∼0.25 monolayer by conducting surface enhanced infrared spectroscopy at CO pressures up to 60 barg in a custom-designed spectroelectrochemical cell. CORR activities on Cu were also determined in the same pressure range. Calculated reaction orders of C₂₊ products with respect to adsorbed CO are substantially less than unity, clearly indicating that the coupling of adsorbed CO is not the rate-determining step leading to multicarbon products. The increase in CO coverage can reduce the C affinity on the Cu surface and favor the selectivity towards oxygenates, especially acetate, over ethylene. Uncommon products, including ethane, glycolaldehyde, and ethylene glycol, were detected in appreciable amounts, likely due to a new C-C coupling mechanism taking place at elevated CO pressures.

Probing the Dynamics of Low-Overpotential CO2-to-CO Activation on Copper Electrodes with Time-Resolved Raman Spectroscopy

Oxide-derived copper electrodes have displayed a boost in activity and selectivity toward valuable base chemicals in the electrochemical carbon dioxide reduction reaction (CO2RR), but the exact interplay between the dynamic restructuring of copper oxide electrodes and their activity and selectivity is not fully understood. In this work, we have utilized time-resolved surface-enhanced Raman spectroscopy (TR-SERS) to study the dynamic restructuring of the copper (oxide) electrode surface and the adsorption of reaction intermediates during cyclic voltammetry (CV) and pulsed electrolysis (PE). By coupling the electrochemical data to the spectral features in TR-SERS, we study the dynamic activation of and reactions on the electrode surface and find that CO₂ is already activated to carbon monoxide (CO) during PE (10% Faradaic efficiency, 1% under static applied potential) at low overpotentials (−0.35 V_RHE). PE at varying cathodic bias on different timescales revealed that stochastic CO is dominant directly after the cathodic bias onset, whereas no CO intermediates were observed after prolonged application of low overpotentials. An increase in cathodic bias (−0.55 V_RHE) resulted in the formation of static adsorbed CO intermediates, while the overall contribution of stochastic CO decreased. We attribute the low-overpotential CO₂-to-CO activation to a combination of selective Cu(111) facet exposure, partially oxidized surfaces during PE, and the formation of copper-carbonate-hydroxide complex intermediates during the anodic pulses. This work sheds light on the restructuring of oxide-derived copper electrodes and low-overpotential CO formation and highlights the power of the combination of electrochemistry and time-resolved vibrational spectroscopy to elucidate CO2RR mechanisms.

Photoelectrochemical CO2 Reduction toward Multicarbon Products with Silicon Nanowire Photocathodes Interfaced with Copper Nanoparticles

The development of photoelectrochemical systems for converting CO₂ into chemical feedstocks offers an attractive strategy for clean energy storage by directly utilizing solar energy, but selectivity and stability for these systems have thus been limited. Here, we interface silicon nanowire (SiNW) photocathodes with a copper nanoparticle (CuNP) ensemble to drive efficient photoelectrochemical CO₂ conversion to multicarbon products. This integrated system enables CO₂-to-C₂H₄ conversion with faradaic efficiency approaching 25% and partial current densities above 2.5 mA/cm² at -0.50 V vs RHE, while the nanowire photocathodes deliver 350 mV of photovoltage under 1 sun illumination. Under 50 h of continual bias and illumination, CuNP/SiNW can sustain stable photoelectrochemical CO₂ reduction. These results demonstrate the nanowire/catalyst system as a powerful modular platform to achieve stable photoelectrochemical CO₂ reduction and the feasibility to facilitate complex reactions toward multicarbons using generated photocarriers.