Abstract: Due to the ability to produce sustainably carbon-based chemicals and fuels, CO₂ electrolysis and the closely related CO electrolysis are advancing rapidly from fundamental studies toward industrial applications. Many near-room temperature CO₂ and CO electrolysis (CO₍₂₎E) technologies adopt features from proton exchange membrane fuel cells and H₂ electrolyzers. However, CO₍₂₎E's selectivity and overall performance are highly sensitive to a multitude of parameters, adding an extra degree of complexity. One often-overlooked parameter in optimizing these devices is the ion exchange membranes (IEM). Here we critically review the IEM performance variables of most relevance to CO₍₂₎E, which leads to identifying several parameters in need of substantial more scientific understanding. We begin with a summary of the working principles of the three main IEM types for CO₍₂₎E, then focus on anion exchange membranes (AEM) since AEMs provide the most favorable local alkaline environment for CO₍₂₎E at the cathode. Critical issues for AEMs in CO₂E include (i) ion and water transport in the membrane, (ii) ionic conductivity, and (iii) chemical stability. We conclude with an overview of the state-of-the-art IEM reported in high current density (j ≥ 100 mA cm⁻²) CO₂ and CO electrolysis devices.

Abstract: Linear scaling relations have led to an understanding of trends in catalytic activity and selectivity of many reactions in heterogeneous and electro-catalysis. However, linear scaling between the chemisorption energies of any two small molecule adsorbates is not guaranteed. A prominent example is the lack of scaling between the chemisorption energies of carbon and oxygen on transition metal surfaces. In this work, we show that this lack of scaling originates from different re-normalized adsorbate valence energies of lower-lying oxygen vs higher-lying carbon. We develop a model for chemisorption of small molecule adsorbates within the d-band model by combining a modified form of the Newns–Anderson hybridization energy with an effective orthogonalization term. We develop a general descriptor to a priori determine if two adsorbates are likely to scale with each other.

Abstract: Determining ab initio potential-dependent energetics is critical to the investigation of mechanisms for electrochemical reactions. While methodology for evaluating reaction thermodynamics is established, simulation techniques for the corresponding kinetics is still a major challenge owing to a lack of potential control, finite cell size effects, or computational expense. In this work, we develop a model that allows for computing electrochemical activation energies from just a handful of density functional theory (DFT) calculations. The sole input into the model are the atom-centered forces obtained from DFT calculations performed on a homogeneous grid composed of varying field strengths. We show that the activation energies as a function of the potential obtained from our model are consistent for different supercell sizes and proton concentrations for a range of electrochemical reactions.

Abstract: sustainable production of valuable chemicals and fuels. In this roadmap, we review recent progress in fundamental understanding, catalyst development, and in engineering and scale-up. We discuss the outstanding challenges towards commercialization of electrochemical CO₂ reduction technology: energy efficiencies, selectivities, low current densities, and stability. We highlight the opportunities in establishing rigorous standards for benchmarking performance, advances in in operando characterization, the discovery of new materials towards high value products, the investigation of phenomena across multiple-length scales and the application of data science towards doing so. We hope that this collective perspective sparks new research activities that ultimately bring us a step closer towards establishing a low- or zero-emission carbon cycle.

Abstract: Electrochemical CO₂ reduction is a potential approach to convert CO₂ into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel-nitrogen-doped carbon (Ni-N-C) is an efficient catalyst for CO₂ electro-reduction to CO, and the single-site Ni-N x motif is believed as the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)-derived Ni-N-C catalysts, for which the Ni-N x content could be adjusted by the pyrolysis temperature. The combination of high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure evidenced the presence of Ni single-sites, and quantitative X-ray photoemission addressed the relation between active site density and turnover frequency.

Abstract: CO is the simplest product from CO₂ electroreduction (CO₂R), but the identity and nature of its rate-limiting step remain controversial. Here we investigate the activity of transition metals (TMs), metal–nitrogen-doped carbon catalysts (MNCs) and a supported phthalocyanine, and present a unified mechanistic picture of the CO₂R to CO for these catalysts. Applying the Newns–Andersen model, we find that on MNCs, like TMs, electron transfer to CO₂ is facile. We find CO₂* adsorption to generally be limiting on TMs, whereas MNCs can be limited by either CO₂* adsorption or by the proton–electron transfer reaction to form COOH*. We evaluate these computed mechanisms against pH-dependent experimental activity measurements on the CO₂R to CO activity. We present a unified activity volcano that includes the decisive CO_2* and COOH* binding strengths. We show that the increased activity of MNC catalysts is due to the stabilization of larger adsorbate dipoles, which results from their discrete and narrow d states.

Abstract: Advancing reaction rates for electrochemical CO₂ reduction in membrane electrode assemblies (MEAs) have boosted the promise of the technology while exposing new shortcomings. Among these is the maximum utilization of CO₂, which is capped at 50% (CO as targeted product) due to unwanted homogeneous reactions. Using bipolar membranes in an MEA (BPMEA) has the capability of preventing parasitic CO₂ losses, but their promise is dampened by poor CO₂ activity and selectivity. In this work, we enable a 3-fold increase in the CO₂ reduction selectivity of a BPMEA system by promoting alkali cation (K⁺) concentrations on the catalyst’s surface, achieving a CO Faradaic efficiency of 68%. When compared to an anion exchange membrane, the cation-infused bipolar membrane (BPM) system shows a 5-fold reduction in CO₂ loss at similar current densities, while breaking the 50% CO₂ utilization mark. The work provides a combined cation and BPM strategy for overcoming CO₂ utilization issues in CO₂ electrolyzers.

Abstract: We present a scheme to extract the adsorption energy, adsorbate interaction parameter and the saturation coverage from temperature programmed desorption (TPD) experiments. We propose that the coverage dependent adsorption energy can be fit using a functional form including the configurational entropy and linear adsorbate–adsorbate interaction terms. As one example of this scheme, we analyze TPD of CO desorption on Au(211) and Au(310) surfaces. We determine that under atmospheric CO pressure, the steps of both facets adsorb between 0.4–0.9 ML coverage of CO*. We compare this result against energies obtained from five density functionals, RPBE, PBE, PBE-D3, RPBE-D3 and BEEF-vdW. We find that the energies and equilibrium coverages from RPBE-D3 and PBE are closest to the values determined from the TPD.
 

Abstract: We present a joint theoretical–experimental study of CO coverage and facet selectivity on Au under electrochemical conditions. With in situ attenuated total reflection surface-enhanced IR spectroscopy, we investigate the CO binding in an electrochemical environment. At 0.2 V versus SHE, we detect a CO band that disappears upon facet-selective partial Pb underpotential deposition (UPD), suggesting that Pb blocks certain CO adsorption sites. With Pb UPD on single crystals and theoretical surface Pourbaix analysis, we eliminate (111) terraces as a possible adsorption site of CO. Ab initio molecular dynamics simulations of explicit water on the Au surface show the adsorption of CO on (211) steps to be significantly weakened relative to the (100) terrace due to competitive water adsorption. This result suggests that CO is more likely to bind to the (100) terrace than (211) steps in an electrochemical environment, even though Au steps under gas-phase conditions bind CO* more strongly. The competition between water and CO adsorption can result in different binding sites for CO* on Au in the gas phase and electrochemical environments.

Abstract: As electrochemical CO₂ reduction studies progress from beaker or H-cell devices operating at low current densities to gas diffusion electrode (GDE)-based devices that sustain high reaction rates and provide an avenue toward commercialization, the overall system becomes significantly more complex. While the current densities may vary for the different approaches, it is essential to maintain the same scientific rigor when analyzing these systems. The mass transfer optimizations used in GDE based approaches necessarily add complexity and provide new challenges that need to be analyzed and overcome in terms of both engineering as well as analysis techniques. This Account puts into perspective our recent works analyzing high current density CO₂ electrolysis performance via a comprehensive investigation of the entire system.
In particular, we show the importance of monitoring (i) the gas flow rates at the outlet of the cathodic compartment, (ii) the anodic gas composition for CO₂/O₂ ratio, and (iii) pH variations in the electrolyte. A rigorous analysis of these parameters allows us to achieve a complete carbon balance, in addition to accounting for a total of 100% Faradaic efficiency. By analyzing both the cathode outlet and anodic CO₂:O₂ ratio, we demonstrate that these methods can be used to self-validate results providing robustness. We show that this analysis approach holds for both a zero-gap membrane electrode assembly device and a flowing-catholyte device. In addition, a comprehensive monitoring approach reveals that having an alkaline environment in the vicinity of the cathode can absorb substantial amounts of CO₂, which may greatly distort Faradaic efficiencies if not accounted for. While monitoring the outlet flow rate of a reactor appears a simple task, the mixed gases and small flow rates in lab-scale reactors can add challenges and we discuss various methods to measure these flow rates.
While pH is well-known to play a role in the activity and selectivity of CO₂ reduction, we demonstrate that (i) the operational pH is not necessarily the pH of the initial electrolyte, (ii) there are long transients in pH before steady state is reached (on the order of hours), and (iii) the pH of the anolyte and catholyte can be significantly different over the duration of the electrolysis.
By varying the membrane type in a flowing-catholyte reactor (anion exchange, cation exchange, or bipolar membrane), we can use this monitoring approach to quantitatively identify the major differences in CO₂ reduction performance related to these distinct membrane types. The overall conclusion is that complex engineering processes entail that a thorough monitoring of parameters is necessary to accurately analyze the performance of high current density electrochemical CO₂ reduction devices.

Abstract: In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO₂ reduction in GDE-type flow electrolyzers. By using different ion-selective membranes, we show nearly identical catalytic selectivity for CO₂ reduction, which is primarily due to a similar local reaction environment created at the cathode/electrolyte interface via the introduction of a catholyte layer. In addition, based on a systematic exploration of gases released from electrolytes and the dynamic change of electrolyte speciation, we demonstrate the explicit discrepancy in carbon balance paths for the captured CO₂ at the cathode/catholyte interface via reaction with OH
when using different ion-selective membranes: (i) the captured CO₂ could be transported through an anion exchange membrane in the form of CO₃^2-
, subsequently releasing CO₂ along with O₂ in the anolyte, and (ii) with a cation exchange membrane, the captured CO₂ would be accumulated in the catholyte in the form of CO₃^2
-, while (iii) with the use of a bipolar membrane, the captured CO₂ could be released at the catholyte/membrane interface in the form of gaseous CO₂. The unique carbon balance path for each type of membrane is linked to ion species transported through the membranes.

Abstract: The direct electroreduction of CO₂ to pure CO streams has attracted much attention for both academic research and industrial polymer synthesis development. Here, we explore catalytically very active, coral-structured Ag catalyst for the generation of pure CO from CO₂-feeds in lab-bench scale zero-gap CO₂ electrolyzer. Coral-shaped Ag electrodes achieved CO partial current densities of up to 312 mA cm⁻², EE_CO of 38%, and FE_CO clearly above 90%. In-situ/operando X-ray Absorption Spectroscopy revealed the sustained presence of Ag⁺ subsurface species, whose local electronic field effects constitute likely molecular origins of the favorable experimental kinetics and selectivity. In addition, we show how electrode flooding in zero-gap CO₂ electrolyzer compromises efficient CO₂ mass transfer. Our studies highlight the need for a concomitant consideration of factors related to intrinsic catalytic activity of the active phase, its porous structure and its hydrophilicity/phobicity to achieve a sustained high product yield in AEM zero-gap electrolyzer.