Anion dependent CO/H 2 production ratio from CO 2 reduction on Au electro-catalyst

Recent advances in the role of interfacial liquids in electrochemical reactions

The interfacial liquid, situated in proximity to an electrode or catalyst, plays a vital role in determining the activity and selectivity of crucial electrochemical reactions, including hydrogen evolution, oxygen evolution/reduction, and carbon dioxide reduction. Thus, there has been a growing interest in better understanding the behavior and the catalytic effect of its constituents. This minireview examines the impact of interfacial liquids on electrocatalysis, specifically the effects of water molecules and ionic species present at the interface. How the structure of interfacial water, distinct from the bulk, can affect charge transfer kinetics and transport of species is presented. Furthermore, how cations and anions (de)stabilize intermediates and transition states, compete for adsorption with reaction species, and act as local environment modifiers including pH and the surrounding solvent structure are described in detail. These effects can promote or inhibit reactions in various ways. This comprehensive exploration provides valuable insights for tailoring interfacial liquids to optimize electrochemical reactions.

Local reaction environment in electrocatalysis

Beyond conventional electrocatalyst engineering, recent studies have unveiled the effectiveness of manipulating the local reaction environment in enhancing the performance of electrocatalytic reactions. The general principles and strategies of local environmental engineering for different electrocatalytic processes have been extensively investigated. This review provides a critical appraisal of the recent advancements in local reaction environment engineering, aiming to comprehensively assess this emerging field. It presents the interactions among surface structure, ions distribution and local electric field in relation to the local reaction environment. Useful protocols such as the interfacial reactant concentration, mass transport rate, adsorption/desorption behaviors, and binding energy are in-depth discussed toward modifying the local reaction environment. Meanwhile, electrode physical structures and reaction cell configurations are viable optimization methods in engineering local reaction environments. In combination with operando investigation techniques, we conclude that rational modifications of the local reaction environment can significantly enhance various electrocatalytic processes by optimizing the thermodynamic and kinetic properties of the reaction interface. We also outline future research directions to attain a comprehensive understanding and effective modulation of the local reaction environment.

Theoretical understanding of the effect of specifically adsorbed halide anions on Cu-catalyzed CO2 electroreduction activity and product selectivity

Initial CO2 electroreduction into CO and its subsequent electroreduction pathways were selected to study the effect of specifically adsorbed halide anions X- (X = F, Cl, Br, I) on CO2 electroreduction activity and product selectivity at Cu(111)/H2O interfaces via DFT calculations. The calculated results show that the presence of halide anions can exert a notable effect on the CO2 adsorption characteristics and that chemically adsorbed CO2 molecules can be formed. Furthermore, the halide-anion-modified Cu(111)/H2O interfaces could significantly enhance the initial CO2 electroreduction into CO activity, which is regarded as the rate-determining step during CO2 electroreduction at clean Cu(111)/H2O interfaces. Analysis of the initial CO2 electroreduction and Volmer reaction pathways showed that the halide-anion-modified Cu(111)/H2O interfaces could suppress the HER and thus improve the CO2 electroreduction activity and product selectivity. It is speculated that the enhanced initial CO2 electroreduction activity at the F--, Cl--, Br--, and I--modified Cu(111)/H2O interfaces may originate from the decreased work functions and anion radical ·CO2- formations. Simultaneously, we concluded that dimer OCCO formations in the presence of halide anions were more favorable than CHO during CO electroreduction according to the order of I- > Br- > Cl- > F- and could result in the production of C2 product, suggesting an improved CO2 electroreduction product selectivity. The present analyses of electronic structure may explain the more favorable OCCO formations in the order of I- > Br- > Cl- > F-. The present understanding of this effect will provide an improved scientific guideline for the control of CO2 electroreduction pathways and design of more efficient electrocatalysts.

Boosting Electroreduction of CO2 over Cationic Covalent Organic Frameworks: Hydrogen Bonding Effects of Halogen Ions

We present the first example of charged imidazolium functionalized porphyrin-based covalent organic framework (Co-iBFBim-COF-X) for electrocatalytic CO₂ reduction reaction, where the free anions (e.g., F^- , Cl^- , Br^- , and I^- ) of imidazolium ions nearby the active Co sites can stabilize the key intermediate *COOH and inhibit hydrogen evolution reaction. Thus, Co-iBFBim-COF-X exhibits higher activity than the neutral Co-BFBim-COF, following the trend of F^- <Cl^- <Br^- <I^- . Particularly, the Co-iBFBim-COF-I^- showed nearly 100 % CO₂ selectivity at a low full-cell voltage of 2.3 V, and achieved a high CO₂ partial current density of 52 mA cm^-2 with a turnover frequency of 3018 h^-1 at 2.4 V in the anion membrane electrode assembly, which is 3.57 times larger than that of neutral Co-BFBim-COF. This work provides new insight into the importance of free anions in the stabilization of intermediates and decreasing the local binding energy of H₂ O with active moiety to enhance CO₂ reduction reaction.

Room Temperature Nanographene Production via CO2 Electrochemical Reduction on the Electrodeposited Bi on Sn Substrate

Electrochemical reduction of carbon dioxide (CO₂RR) to crystalline solid carbon at room temperature is challenging, but it is a providential CO₂ utilization route due to its indefinite storage and potential applications of its products in many advanced technologies. Here, room-temperature synthesis of polycrystalline nanographene was achieved by CO₂RR over the electrodeposited Bi on Sn substrate prepared with various bismuth concentrations (0.01 M, 0.05 M, and 0.1 M). The solid carbon products were solely produced on all the prepared electrodes at the applied potential -1.1 V vs. Ag/AgCl and were characterized as polycrystalline nanographene with an average domain size of ca. 3-4 nm. The morphology of the electrodeposited Bi/Sn electrocatalysts did not have much effect on the final structure of the solid carbon products formed but rather affected the CO₂ electroreduction activity. The optimized negative potential for the formation of nanographene products on the 0.05Bi/Sn was ca. -1.5 V vs. Ag/AgCl. Increasing the negative value of the applied potential accelerated the agglomeration of the highly reactive nascent Bi clusters in situ formed under the reaction conditions, which, as a consequence, resulted in a slight deviation of the product selectivity toward gaseous CO and H₂ evolution reaction. The Bi-graphene composites produced by this method show high potential as an additive for working electrode modification in electrochemical sensor-related applications.

Identification of electrochemically adsorbed species via electrochemical microcalorimetry: sulfate adsorption on Au(111)

We investigate compositional changes of an electrochemical interface upon polarization with electrochemical microcalorimetry. From the heat exchanged at a Au(111) electrode upon sulfate adsorption, we determine the reaction entropy of the adsorption process for both neutral and acidic solutions, where the dominant species in solution changes from SO₄²⁻ to HSO₄⁻. In neutral solution, the reaction entropy is about 40 J mol⁻¹ K⁻¹ more positive than that in acidic solution over the complete sulfate adsorption region. This entropy offset is explicable by a deprotonation step of HSO₄⁻ preceding sulfate adsorption in acidic solution, which shows that the adsorbing species is SO₄* in both solutions. The observed overall variation of the reaction entropy in the sulfate adsorption region of ca. 80 J mol⁻¹ K⁻¹ indicates significant sulfate‐coverage dependent entropic contributions to the Free Enthalpy of the surface system.

Understanding the Role of Copper Vacancies in Photoelectrochemical CO2 Reduction on Cuprous Oxide

Controlling the electronic and photoexcited properties of cuprous oxide (Cu₂O) through slight modifications of the synthesis method can impact a wide range of emerging technologies. Herein, we consider copper vacancies in Cu₂O as a prototype of a p-type oxide semiconductor for studying the impact of crystal and electronic structure on carbon dioxide photoreduction. Oriented films of copper vacancy modulated Cu₂O consisting of nano twin structures are electrodeposited by changing the potential in an aqueous alkaline copper(II)-lactate solution. The copper vacancies introduce tail states inside the band gap, improving the hole concentration and facilitating the charge separation and transfer in the Cu₂O photocathode. This study gives an in-depth view of how a cation-deficient structure regulates and promotes photoelectrochemical activity toward CO₂ reduction.

Electrochemical Reduction of CO2 to HCOOH over Copper Catalysts

Although great progress has been made in the field of electrochemical CO₂ reduction reaction (eCO₂RR), inducing product selectivity is still difficult. We herein report that a thiocyanate ion (SCN^-) switched the product selectivity of copper catalysts for eCO₂RR in an H-cell. A cuprous thiocyanate-derived Cu catalyst was found to exhibit excellent HCOOH selectivity (faradaic efficiency = 70-88%) over a wide potential range (-0.66 to -0.95 V vs RHE). Furthermore, it was revealed that the formation of CO and C₂H₄ over commercial copper electrodes could be dramatically suppressed with the presence of SCN^-, switching to HCOOH. Density functional theory calculations disclosed that SCN^- made the formation of HCOO* easier than COOH* on Cu (211), facilitating the HCOOH generation. Our results provide a new insight into eCO₂RR and will be helpful in the development of cheap electrocatalysts for specific utilization.

Time-Resolved Local pH Measurements during CO2 Reduction Using Scanning Electrochemical Microscopy: Buffering and Tip Effects

The electrochemical reduction of CO₂ is widely studied as a sustainable alternative for the production of fuels and chemicals. The electrolyte's bulk pH and composition play an important role in the reaction activity and selectivity and can affect the extent of the buildup of pH gradients between the electrode surface and the bulk of the electrolyte. Quantifying the local pH and how it is affected by the solution species is desirable to gain a better understanding of the CO₂ reduction reaction. Local pH measurements can be realized using Scanning Electrochemical Microscopy (SECM); however, finding a pH probe that is stable and selective under CO₂ reduction reaction conditions is challenging. Here, we have used our recently developed voltammetric pH sensor to perform pH measurements in the diffusion layer during CO₂ reduction using SECM, with high time resolution. Using a 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) functionalized gold ultramicroelectrode, we compare the local pH developed above a gold substrate in an argon atmosphere, when only hydrogen evolution is taking place, to the pH developed in a CO₂ atmosphere. The pH is monitored at a fixed distance from the surface, and the sample potential is varied in time. In argon, we observe a gradual increase of pH, while a plateau region is present in CO₂ atmosphere due to the formation of HCO₃ ^- buffering the reaction interface. By analyzing the diffusion layer dynamics once the sample reaction is turned "off", we gain insightful information on the time scale of the homogeneous reactions happening in solution and on the time required for the diffusion layer to fully recover to the initial bulk concentration of species. In order to account for the effect of the presence of the SECM tip on the measured pH, we performed finite element method simulations of the fluid and reaction dynamics. The results show the significant localized diffusion hindrance caused by the tip, so that in its absence, the pH values are more acidic than when the tip is present. Nonetheless, through the simulation, we can account for this effect and estimate the real local pH values across the diffusion layer.

An industrial perspective on catalysts for low-temperature CO2 electrolysis

Electrochemical conversion of CO₂ to useful products at temperatures below 100 °C is nearing the commercial scale. Pilot units for CO₂ conversion to CO are already being tested. Units to convert CO₂ to formic acid are projected to reach pilot scale in the next year. Further, several investigators are starting to observe industrially relevant rates of the electrochemical conversion of CO₂ to ethanol and ethylene, with the hydrogen needed coming from water. In each case, Faradaic efficiencies of 80% or more and current densities above 200 mA cm^-2 can be reproducibly achieved. Here we describe the key advances in nanocatalysts that lead to the impressive performance, indicate where additional work is needed and provide benchmarks that others can use to compare their results.

Looking Back and Looking Ahead in Electrochemical Reduction of CO2

Electrochemical reduction of carbon dioxide (CO₂ ) to valuable organic compounds is promising as to recycling of carbon source of CO₂ and technical compatibility with systems using renewable energy resources. In recent years, considerable efforts have been devoted to the research field of CO₂ conversion using electrocatalysis. This personal account particularly focuses on the recent progress that has been achieved by the Ertl Center and a number of groups in South Korea that becomes one of the larger CO₂ emitters. The research trends of catalyst development divided into different categories according to the primary products are presented first. Afterwards, several studies on theoretical calculations and electrolytic reactors are reviewed taking into account the fundamental understanding and feasibility of the CO₂ electroreduction. Finally, a perspective on the challenges and needs in achieving the advanced level of research and development is presented.

Electrochemical CO2 Reduction into Chemical Feedstocks: From Mechanistic Electrocatalysis Models to System Design

The electrochemical reduction of CO₂ is a promising route to convert intermittent renewable energy to storable fuels and valuable chemical feedstocks. To scale this technology for industrial implementation, a deepened understanding of how the CO₂ reduction reaction (CO₂ RR) proceeds will help converge on optimal operating parameters. Here, a techno-economic analysis is presented with the goal of identifying maximally profitable products and the performance targets that must be met to ensure economic viability-metrics that include current density, Faradaic efficiency, energy efficiency, and stability. The latest computational understanding of the CO₂ RR is discussed along with how this can contribute to the rational design of efficient, selective, and stable electrocatalysts. Catalyst materials are classified according to their selectivity for products of interest and their potential to achieve performance targets is assessed. The recent progress and opportunities in system design for CO₂ electroreduction are described. To conclude, the remaining technological challenges are highlighted, suggesting full-cell energy efficiency as a guiding performance metric for industrial impact.

Solvents and Supporting Electrolytes in the Electrocatalytic Reduction of CO2

Different electrolytes applied in the aqueous electrocatalytic CO₂ reduction reaction (CO₂RR) considerably influence the catalyst performance. Their concentration, species, buffer capacity, and pH value influence the local reaction conditions and impact the product distribution of the electrocatalyst. Relevant properties of prospective solvents include their basicity, CO₂ solubility, conductivity, and toxicity, which affect the CO₂RR and the applicability of the solvents. The complexity of an electrochemical system impedes the direct correlation between a single parameter and cell performance indicators such as the Faradaic efficiency; thus the effects of different electrolytes are often not fully comprehended. For an industrial application, a deeper understanding of the effects described in this review can help with the prediction of performance, as well as the development of scalable electrolyzers. In this review, the application of supporting electrolytes and different solvents in the CO₂RR reported in the literature are summarized and discussed.