CO2 from carbonate-based capture solutions requires a substantial energy input. Replacing this step with (bi)carbonate electrolysis has been commonly proposed as an efficient alternative that coproduces CO/syngas. Here, we assess the feasibility of directly integrating air contactors with (bi)carbonate electrolyzers by leveraging process, multiphysics, microkinetic, and technoeconomic models. We show that the copresence of CO32- with HCO3- in the contactor effluent greatly diminishes the electrolyzer performance and eventually results in a reduced CO2 capture fraction to <= 1%. Additionally, we estimate suitable effluents for (bi)carbonate electrolysis to require 5-14 times larger contactors than conventionally needed contactors, leading to unfavorable process economics. Notably, we show that the regeneration of the capture solvent inside (bi)carbonate electrolyzers is insufficient for CO2 recapture. Thus, we suggest process modifications that would allow this route to be operationally feasible. Overall, this work sheds light on the practical operation of integrated direct air capture with (bi)carbonate electrolysis.

The electrocatalytic carbon dioxide reduction reaction (CO2RR) has the potential to convert carbon dioxide to higher value fuels and chemicals, offering a pathway for carbon neutral (or negative) synthesis of carbon monoxide, ethylene, and several other compounds. A common challenge for CO2RR research is driving the selective CO2RR over the competing hydrogen evolution reaction (HER). This key performance metric is quantified by the catalyst’s faradaic efficiency (FE): the number of electrons that go toward producing a specific product divided by the total number of electrons passed during electrolysis. FE is often determined by gas and liquid chromatography. However, these methods have long response times (minutes to hours) and therefore prevent selectivity analysis in real-time. Instead, rotating ring disk electrodes (RRDE) have emerged as a rapid and sensitive method for quantitative CO2RR selectivity measurements with negligible time delays. To date, work has focused on a polycrystalline Au RRDE, especially for bench scale mechanistic studies. The Au disk electrode catalyzes both CO2RR and HER, while the concentric Au ring electrode senses the CO produced at the disk and oxidizes it back to CO2. By comparing the ring and disk currents, FECO and FEHER can be determined using the subtraction method. However, comparison across several works have reported a wide range for FECO values, 60-83%, despite identical conditions: CO2 saturated 0.1 M NaHCO3 (pH = 6.8), Edisk = -0.6 V vs RHE, and rotation at 2500 rpm [A. Goyal, et al., J. Am. Chem. Soc., 142, 4154–4161 (2020); R. E. Vos and M. T. M. Koper, ChemElectroChem, 9, e202200239 (2022)]. This work investigates the possible causes of this poor reproducibility in RRDE selectivity measurements by comparing protocols with different electrochemical methods, reagent purities, and glassware cleaning procedures. Most notably, we observed rapid loss of selectivity toward CO production during chronoamperometry measurements. Lastly, we propose operational bounds for RRDE selectivity measurements on Au under CO2RR conditions and provide our perspective on next steps for developing this into a robust analytical technique.

Continued advancements in the electrochemical reduction of CO2 (CO2RR) have emphasized that reactivity, selectivity, and stability are not explicit material properties but combined effects of the catalyst, double-layer, reaction environment, and system configuration. These realizations have steadily built upon the foundational work performed for a broad array of transition metals performed at 5 mA cm(-2), which historically guided the research field. To encompass the changing advancements and mindset within the research field, an updated baseline at elevated current densities could then be of value. Here we seek to re-characterize the activity, selectivity, and stability of the five most utilized transition metal catalysts for CO2RR (Ag, Au, Pd, Sn, and Cu) at elevated reaction rates through electrochemical operation, physical characterization, and varied operating parameters to provide a renewed resource and point of comparison. As a basis, we have employed a common cell architecture, highly controlled catalyst layer morphologies and thicknesses, and fixed current densities. Through a dataset of 88 separate experiments, we provide comparisons between CO-producing catalysts (Ag, Au, and Pd), highlighting CO-limiting current densities on Au and Pd at 72 and SO mA cm(-2), respectively. We further show the instability of Sn in highly alkaline environments, and the convergence of product selectivity at elevated current densities for a Cu catalyst in neutral and alkaline media. Lastly, we reflect upon the use and limits of reaction rates as a baseline metric by comparing catalytic selectivity at 10 versus 200 mA cm(-2). We hope the collective work provides a resource for researchers setting up CO2RR experiments for the first time.

The electrochemical reduction of bicarbonate to renewable chemicals without external gaseous CO2 supply has been motivated as a means of integrating conversion with upstream CO2 capture. The way that CO2 is formed and transported during CO2-mediated bicarbonate reduction in flow cells is profoundly different from conventional CO2 saturated and gas-fed systems and a thorough understanding of the process would allow further advancements. Here, we report a comprehensive two-phase mass transport model to estimate the local concentration of species in the porous electrode resultant from homogeneous and electrochemical reactions of (bi)carbonate and CO2. The model indicates that significant CO2 is generated in the porous electrode during electrochemical reduction, even though the starting bicarbonate solution contains negligible CO2. However, the in situ formation of CO2 and subsequent reduction to CO exhibits a plateau at high potentials due to neutralization of the protons by the alkaline reaction products, acting as the limiting step toward higher CO current densities. Nevertheless, the pH in the catalyst layer exhibits a relatively smaller rise, compared to conventional electrochemical CO2 reduction cells, because of the reaction between protons and CO32- and OH- that is confined to a relatively small volume. A large fraction of the CL exhibits a mildly alkaline environment at high current densities, while an appreciable amount of carbonic acid (0.1-1 mM) and a lower pH exist adjacent to the membrane, which locally favor hydrogen evolution, especially at low electrolyte concentrations. The results presented here provide insights into local cathodic conditions for both bicarbonate cells and direct-CO2 reduction membrane electrode assembly cells utilizing cation exchange membranes facing the cathode.

Over the past decade, the accessible current densities of electrochemical CO2 reduction (CO2R) and CO reduction (COR) systems have increased by orders of magnitude, following the adaptation of the Gas Diffusion Electrode (GDE) architecture. As GDEs increase in size from cm2 to m2, cell stability and performance can change due to physical and chemical spatial variations within the cell, which can be exacerbated by the chemical complexity of carbonaceous electrochemical systems. Here, we present two approaches to measuring spatial activity: testing iteratively larger electrodes and harmonizing those results with a multi-port sampling reactor capable of measuring product concentration along the length of the reactor. This dual-pronged approach allows for benchmarking of electrochemical performance and selectivity down the channel as reaction conditions vary. We examine selectivity changes through the lens of fundamental electrochemical properties and provide subsequent recommendations for device operation as devices approach > kW scale. Experimental results are aligned with a 2-D (path length and electrode thickness) transport model to compare physical simulations with measured electrochemical signatures. Feedback loops between experiment and simulation were developed to incorporate cell inhomogeneities into the transport model. We identify potential causes for poor catalyst utilization in large electrodes for both COR and CO2R and provide recommendations for translating bench-top scale results to pilot scale results.

The deployment of gas diffusion electrodes (GDEs) for the electrochemical CO2 reduction reaction (CO2RR) has enabled current densities an order of magnitude greater than those of aqueous H cells. The gains in production, however, have come with stability challenges due to rapid flooding of GDEs, which frustrate both laboratory experiments and scale-up prospects. Here, we investigate the role of carbon gas diffusion layers (GDLs) in the advent of flooding during CO2RR, finding that applied potential plays a central role in the observed instabilities. Electrochemical characterization of carbon GDLs with and without catalysts suggests that the high overpotential required during electrochemical CO2RR initiates hydrogen evolution on the carbon GDL support. These potentials impact the wetting characteristics of the hydrophobic GDL, resulting in flooding that is independent of CO2RR. Findings from this work can be extended to any electrochemical reduction reaction using carbon-based GDEs (CORR or N2RR) with cathodic overpotentials of less than -0.65 V versus a reversible hydrogen electrode.

Detailed knowledge about the semiconductor/electrolyte interface in photoelectrochemical (PEC) systems has been lacking because of the inherent difficulty of studying such interfaces, especially during operation. Current understandings of these interfaces are mostly from the extrapolation of ex situ data or from modeling approaches. Hence, there is a need for operando techniques to study such interfaces to develop a better understanding of PEC systems. Here, we use operando photoelectrochemical attenuated total reflection Fourier transform infrared (PEC-ATR-FTIR) spectroscopy to study the metal oxide/electrolyte interface, choosing BiVO4 as a model photoanode. We demonstrate that preferential dissolution of vanadium occurs from the BiVO4/water interface, upon illumination in open-circuit conditions, while both bismuth and vanadium dissolution occurs when an anodic potential is applied under illumination. This dynamic dissolution alters the surface Bi:V ratio over time, which subsequently alters the band bending in the space charge region. This further impacts the overall PEC performance of the photoelectrode, at a time scale very relevant for most lab-scale studies, and therefore has serious implications on the performance analysis and fundamental studies performed on this and other similar photoelectrodes.

Copper hollow fibers were prepared via dry-wet spinning of a polymer solution of N-methylpyrrolidone, Polyetherimide, Polyvinyl Pyrolidone, and copper particles of sizes in the range of 1–2 µm. To remove template molecules and to sinter the copper particles, the time of calcination was varied in a range of 1–4 h at 600 °C. This calcination temperature was determined based on Thermal Gravimetric Analysis (TGA), showing completion of hydrocarbon removal at this temperature. Furthermore, the temperature of the subsequent treatment of the fibers in a flow of 4% H2 (in Ar) was varied in the range of 200 °C to 400 °C, at a fixed time of 1 h. Temperature programmed reduction experiments (TPR) were used to analyze the hydrogen treatment. The Faradaic Efficiency (FE) towards CO in electrochemical reduction of CO2 was determined at −0.45 V vs. RHE (Reversible Hydrogen Electrode), using a 0.3 M KHCO3 electrolyte. A calcination time of 3 h at 600 °C and a hydrogen treatment temperature of 280 °C were found to induce the highest FE to CO of 73% at these constant electrochemical conditions. Optimizing oxidation properties is discussed to likely affect porosity, favoring the CO2 gas distribution over the length of the fiber, and hence the CO2 reduction efficiency. Treatment in H2 in the range of 250 to 300 °C is proposed to affect the content of residual (subsurface) oxygen in Cu, which leads to favorable properties on the nanoscale.

Results of a 2-D transport model for a gas diffusion electrode performing CO2 reduction to CO with a flowing catholyte are presented, including the concentration gradients along the flow cell, spatial distribution of the current density and local pH in the catalyst layer. The model predicts that both the concentration of CO2 and the buffer electrolyte gradually diminish along the channels for a parallel flow of gas and electrolyte as a result of electrochemical conversion and nonelectrochemical consumption. At high single-pass conversions, significant concentration gradients exist along the flow channels leading to large local variations in the current density (>150 mA/cm(2)), which becomes prominent when compared to ohmic losses. In addition, concentration overpotentials change dramatically with CO2 flow rate, which results in significant differences in outlet concentrations at high conversions. The outlet concentration of CO attains a maximum of 80% along with 5% CO2 and 15% H-2, although the maximum single-pass conversion is limited to below 60% due to homogeneous consumption by the electrolyte. Fundamental and practical implications of our findings on electrochemical CO2 reduction are discussed with a focus on the trade-off between high current density operation and high single-pass conversion efficiency.

Electrochemical CO2 electrolysis to produce hydrocarbon fuels or material feedstocks offers a renewable alternative to fossilized carbon sources. Gas-diffusion electrodes (GDEs), composed of solid electrocatalysts on porous supports positioned near the interface of a conducting electrolyte and CO2 gas, have been able to demonstrate the substantial current densities needed for future commercialization. These higher reaction rates have often been ascribed to the presence of a three-phase interface, where solid, liquid, and gas provide electrons, water, and CO2, respectively. Conversely, mechanistic work on electrochemical reactions implicates a fully two-phase reaction interface, where gas molecules reach the electrocatalyst's surface by dissolution and diffusion through the electrolyte. Because the discrepancy between an atomistic three-phase versus two-phase reaction has substantial implications for the design of catalysts, gas-diffusion layers, and cell architectures, the nuances of nomenclatures and governing phenomena surrounding the three-phase-region require clarification. Here we outline the macro, micro, and atomistic phenomena occurring within a gas-diffusion electrode to provide a focused discussion on the architecture of the often-discussed three-phase region for CO2 electrolysis. From this information, we comment on the outlook for the broader CO2 electroreduction GDE cell architecture.

Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm-2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is -0.25 VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until -0.5 VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3- wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3- bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light.

Electrochemical CO2reduction has received an increased amount of interest in the last decade as a promisingavenue for storing renewable electricity in chemical bonds. Despiteconsiderable progress on catalyst performance using nanostructured electrodes,the sensitivity of the reaction to process conditions has led to debate on the originof the activity and high selectivity. Additionally, this raises questions onthe transferability of the performance and knowledge to other electrochemicalsystems. At its core, the discrepancy is primarily a result of the highlyporous nature of nanostructured electrodes, which are vulnerable to both masstransport effects and structural changes during the electrolysis. Both effectsare not straightforward to identify and difficult to decouple. Despite thesusceptibility of nanostructured electrodes to mass transfer limitations, wehighlight that nanostructured silver electrodes exhibit considerably higheractivity when normalized to the electrochemically active surface in contrast togold and copper electrodes. Alongside, we provide a discussion on howactive surface area and thickness of the catalytic layer itself can influencethe selectivity, stability, activity and mass transfer inside and outside ofthe three dimensional catalyst layer. Key parameters and potential solutionsare highlighted to decouple mass transfer effects from the measured activity inelectrochemical cells utilizing CO2 saturated aqueous solutions.

This study evaluated the performance of the commonly used strong buffer electrolytes, i.e. phosphate buffers, during CO2 electroreduction in neutral pH conditions by using in-situ surface enhanced infrared absorption spectroscopy (SEIRAS). Unfortunately, the buffers break down a lot faster than anticipated which has serious implications on many studies in the literature such as selectivity and kinetic analysis of the electrocatalysts. Increasing electrolyte concentration, surprisingly, did not extend the potential window of the phosphate buffers due to dramatic increase in hydrogen evolution reaction. Even high concentration phosphate buffers (1 M) break down within the potentials (-1 V vs RHE) where hydrocarbons are formed on copper electrodes. We have extended the discussion to high surface area electrodes by evaluating electrodes composed of copper nanowires. We would like highlight that it is not possible to cope with high local current densities on these high surface area electrodes by using high buffer capacity solutions and the CO2 electrocatalysts are needed to be evaluated by casting thin nanoparticle films onto inert substrates as commonly employed in fuel cell reactions and up to now scarcely employed in CO2 electroreduction. In addition, we underscore that normalization of the electrocatalytic activity to the electrochemical active surface area is not the ultimate solution due to concentration gradient along the catalyst layer.This will “underestimate” the activity of high surface electrocatalyst and the degree of underestimation will depend on the thickness, porosity and morphology of the catalyst layer.

The field of electrochemical CO2 conversion is undergoing significant growth in terms of the number of publications and worldwide research groups involved. Despite improvements of the catalytic performance, the complex reaction mechanisms and solution chemistry of CO2 have resulted in a considerable amount of discrepancies between theoretical and experimental studies. A clear identification of the reaction mechanism and the catalytic sites are of key importance in order to allow for a qualitative breakthrough and, from an experimental perspective, calls for the use of in-situ or operando spectroscopic techniques. In-situ infrared spectroscopy can provide information on the nature of intermediate species and products in real time and, in some cases, with relatively high time resolution. In this contribution, we review key theoretical aspects of infrared reflection spectroscopy, followed by considerations of practical implementation. Finally, recent applications to the electrocatalytic reduction of CO2 are reviewed, including challenges associated with the detection of reaction intermediates.

CO2 electroreduction is a promising technology to produce chemicals and fuels from renewable resources. Polycrystalline and nanostructured metals have been tested extensively while less effort has been spent on understanding the performance of bimetallic alloys. In this work, we study compositionally variant, smooth Au–Pd thin film alloys to discard any morphological or mesoscopic effect on the electrocatalytic performance. We find that the onset potential of CO formation exhibits a strong dependence on the Pd content of the alloys. Strikingly, palladium, a hydrogen evolution catalyst with reasonable exchange current density, suppresses hydrogen evolution when alloyed with gold in the presence of CO2. Cyclic voltammetry, in situ surface enhanced infrared absorption spectroscopy, and potential-dependent online product analysis strongly suggest that by alloying Au with Pd a significant increase in the surface coverage of adsorbed CO occurs with increasing Pd content at low overpotentials (e.g., approximatel...

Over the past decade, electrochemical carbon dioxide reduction has become a thriving area of research with the aim of converting electricity to renewable chemicals and fuels. Recent advances through catalyst development have significantly improved selectivity and activity. However, drawing potential dependent structure–activity relationships has been complicated, not only due to the ill-defined and intricate morphological and mesoscopic structure of electrocatalysts, but also by immense concentration gradients existing between the electrode surface and bulk solution. In this work, by using in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and computational modeling, we explicitly show that commonly used strong phosphate buffers cannot sustain the interfacial pH during CO2 electroreduction on copper electrodes at relatively low current densities, <10 mA/cm2. The pH near the electrode surface was observed to be as much as 5 pH units higher compared to bulk solution in 0.2 M phosphate buffer at potentials relevant to the formation of hydrocarbons (−1 V vs RHE), even on smooth polycrystalline copper electrodes. Drastically increasing the buffer capacity did not stand out as a viable solution for the problem as the concurrent production of hydrogen increased dramatically, which resulted in a breakdown of the buffer in a narrow potential range. These unforeseen results imply that most of the studies, if not all, on electrochemical CO2 reduction to hydrocarbons in CO2 saturated aqueous solutions were evaluated under mass transport limitations on copper electrodes. We underscore that the large concentration gradients on electrodes with high local current density (e.g., nanostructured) have important implications on the selectivity, activity, and kinetic analysis, and any attempt to draw structure–activity relationships must rule out mass transport effects.

In this work, the highly selective and stable electrocatalytic reduction of CO2 to CO on nanostructured Ag electrocatalysts is presented. The Ag electrocatalysts are synthesized by the electroreduction of Ag2CO3 formed by in situ anodic-etching of Ag foil in a KHCO3 electrolyte. After 3 min of this etching treatment, the Ag2CO3-derived nanostructured Ag electrocatalysts are capable of producing CO with up to 92% Faradaic efficiency at an overpotential as low as 290 mV, which surpasses all of the reported Ag catalysts at identical conditions to date. In addition, the anodic-etched Ag retained ∼90% catalytic selectivity in the electroreduction of CO2 to CO for more than 100 h. The Ag2CO3-derived Ag is able to facilitate the activation of CO2 via reduction of the activation energy barrier of the initial electron transfer and provide an increased number of active sites, resulting in the dramatically improved catalytic activity for the reduction of CO2 to CO.

Aqueous-phase electrochemical reduction of carbon dioxide requires an active, earth-abundant electrocatalyst, as well as highly efficient mass transport. Here we report the design of a porous hollow fibre copper electrode with a compact three-dimensional geometry, which provides a large area, three-phase boundary for gas-liquid reactions. The performance of the copper electrode is significantly enhanced; at overpotentials between 200 and 400 mV, faradaic efficiencies for carbon dioxide reduction up to 85% are obtained. Moreover, the carbon monoxide formation rate is at least one order of magnitude larger when compared with state-of-the-art nanocrystalline copper electrodes. Copper hollow fibre electrodes can be prepared via a facile method that is compatible with existing large-scale production processes. The results of this study may inspire the development of new types of microtubular electrodes for electrochemical processes in which at least one gas-phase reactant is involved, such as in fuel cell technology.