The rates of many electrocatalytic reactions can be strongly affected by the structure and dynamics of the electrochemical double layer, which in turn can be tuned by the concentration and identity of the supporting electrolyte’s cation. The effect of cations on an electrocatalytic process depends on a complex interplay between electrolyte components, electrode material and surface structure, applied electrode potential, and reaction intermediates. Although cation effects remain insufficiently understood, the principal mechanisms underlying cation-dependent reactivity and selectivity are beginning to emerge. In this Perspective, we summarize and critically examine recent advances in this area in the context of the hydrogen evolution reaction (HER) and CO2-to-CO conversion, which are among the most intensively studied and promising electrocatalytic reactions for the sustainable production of commodity chemicals and fuels. Improving the kinetics of the HER in base and enabling energetically efficient and selective CO2 reduction at low pH are key challenges in electrocatalysis. The physical insights from the recent literature illustrate how cation effects can be utilized to help achieve these goals and to steer other electrocatalytic processes of technological relevance. 
                        more » 
                        « less   
                    
                            
                            Concept of Utilizing Ionic Liquids for the Co‐electroreduction of Carbon Dioxide and Nitrogen‐containing Compounds
                        
                    
    
            Formation of C‐N bonds through the electrochemical utilization of CO2 and nitrogen containing compounds (N‐compounds) is appealing for the purpose of converting waste and readily available sources or pollutants into value added chemicals at ambient conditions. Existing research predominantly explores these electrochemical reactions independently, often in aqueous electrolytes, leading to challenges associated with competitive hydrogen evolution reaction (HER), low product selectivity, and yield. Functional electrolytes such as those containing ionic liquids (ILs) present selective solubility to the solute reactants and present unique interactions with the electrode surface that can suppress the undesired side reaction HER while simultaneously co‐catalyzing the conversion of CO2 and N‐compounds such as N2, NO, NO2, andNO3. In this concept paper, we discuss how the microenvironment enabled by ILs can be  leveraged to stabilize reaction intermediates at the electrode‐electrolyte interface, thereby promoting C‐N bond formation on an active electrode surface at reduced overpotential, with the case study of CO2 and N‐compounds co‐catalysis to generate urea. 
        more » 
        « less   
        
    
                            - Award ID(s):
- 2045111
- PAR ID:
- 10532523
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- ChemCatChem
- ISSN:
- 1867-3880
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
- 
            
- 
            Interfacial microenvironments critically define reaction pathways for electrocatalytic processes through a combination of electric field gradients and proton activity. Non-aqueous ionic liquid electrolytes have been shown to sustain enhanced interfacial electric field gradients at intermediate ion concentration regimes of around 1 M, creating local environments that promote CO2 electroreduction. Notably, water at low concentrations absorbed by non-aqueous electrolytes is usually assumed to be the proton donor for CO2 reduction. Consumption of protons causes proton donors to become more negative by one unit charge, which significantly modifies the local concentration of charged species and hence should strongly impact local electric fields. Yet, how the coupling between proton donation and changing interfacial electric fields influences electrocatalytic processes in non-aqueous electrolytes remains largely unexplored. In this work, we show that the high activity of 1,3-dialkylimidazolium ionic liquids for CO2 reduction in acetonitrilebased electrolytes stems from the ability to act as cationic proton donors that release neutral conjugate bases. Using in situ electrochemical surface-enhanced Raman spectroscopy, we find that the formation of neutral conjugate bases from imidazolium cations preserves local electric field strengths at electrode-electrolyte interfaces, providing a powerful strategy to maintain an active local microenvironment for CO2 reduction. In contrast, conditions where water behaves as the primary proton donor generates [OH]- anions as negative “co-ions” in the electric double layer, which weakens the interfacial electric field and significantly compromises the steady-state CO2 reduction activity. Our study highlights that electrochemical driving forces are highly sensitive to the charge state of both reactant and product species and highlights the fact that the generation of interfacial co-ions plays a key role in determining electrochemical driving forces.more » « less
- 
            Abstract This study examines the activity of chemisorbed CO2 species in the microenvironment formed by bifunctional ionic liquids (ILs) in the reactive capture and conversion (RCC) of CO2 to CO on silver. Comparative electroanalytical measurements with imidazolium based ILs were performed to probe the impact of electrostatic interactions, anion and cation basicity, and hydrogen bonding on RCC. Particularly, ILs with 1-ethyl,3-methylimidazolium ([EMIM]+) and 1-ethyl, 2,3-methylimidazolium ([EMMIM]+) cations and aprotic heterocyclic anions of 2-cyanopyrrolide ([2-CNpyr]) and 1,2,4-triazolide ([1,2,4-Triz]) were examined for RCC. It was found that anion–CO2 carbamate complexes facilitate RCC at significantly lower overpotentials compared to cation–CO2 carboxylate complexes. Additionally, [EMIM]+ was found to better stabilize anion–CO2 complexes than [EMMIM]+. Furthermore, it was found that 2-CNpyrH that naturally forms in CO2 absorption competes for electrode surface adsorption with the anion–CO2 carbamate complex, thereby reducing the electrochemical activity of the anion–CO2 complex. These results highlight the importance of IL structure in tuning the interfacial interactions and suggest that ILs with anion-dominated CO2 chemisorption enhances CO2 utilization in RCC applications.more » « less
- 
            CO2 electroreduction (CO2ER) by using renewable energy resources is a promising method to mitigate the CO2 level in the atmosphere as well as producing valuable chemicals. Local environment at the electrode-electrolyte interface plays a key role in CO2ER activity and selectivity along with its competing hydrogen evolution reaction (HER). In addition to the catalyst and reactor design, electrolyte has also a significant impact on the interface. Herein, electrolyte additives were used to modify the local environment around the Cu catalyst during CO2ER. To this purpose, 10mM of ionic additives with bis(trifluoromethylsulfonyl)imide ([NTF2]-) and dicyanamide ([DCA]-) as anions and 1-butyl-3-methylimidazolium ([BMIM]+), potassium (K+), or sodium (Na+) as cations have been added to an aqueous potassium bicarbonate solution (0.1 M KHCO3). COMSOL Multiphysics was also used to calculate the local pH and CO2 concentration at electrode-electrolyte interface in different electrolytes. Results showed that the local environment modifications by the electrolyte additives altered the activity and selectivity of Cu in CO2ER. It was found that the CO2ER activity at -0.92 V was enhanced when using anion with high CO2 affinity and high hydrophobicity such as [NTF2]–. Among [NTF2]–-based additives, [BMIM][NTF2] had a higher faradaic efficiency (FE) for formate (38.7%) compared to K[NTF2] (23.2%) and Na[NTF2] (18.5%) at -0.92 V likely due to the presence of imidazolium cation which can further stabilize the intermediates on the surface and enhance CO2ER. Electrolytes containing [DCA]–-based additives with high hydrophilicity and low CO2 affinity had a very high HER selectivity (>90% FEH2) and low CO2ER selectivity regardless of the cation nature. This observation is attributed to the presence of hydrophilic [BMIM][DCA] in the vicinity of the catalyst which impacts the microenvironment around the catalyst. We observed that [DCA]– anions have a high affinity to adsorb on Cu catalysts as soon as the catalyst is submerged in the electrolyte. Although FTIR showed that [DCA]– anions desorb from the surface at negative potentials, it is likely that [DCA]– anions still remain in the proximity of the electrode, next to the adsorbed cations, impacting the transport of H2O and CO2, and altering the product selectivity. COMSOL calculations showed that the local pH is directly proportional to the H2 evolution activity. Also, hydrophilic salts such as those with the [DCA]– anion had a more alkaline local pH which leads to a lower CO2 concentration in the vicinity of the catalyst.more » « less
- 
            Abstract Rigorous electrokinetic results are key to understanding the reaction mechanisms in the electrochemical CO reduction reaction (CORR), however, most reported results are compromised by the CO mass transport limitation. In this work, we determined mass transport-free CORR kinetics by employing a gas-diffusion type electrode and identified dependence of catalyst surface speciation on the electrolyte pH using in-situ surface enhanced vibrational spectroscopies. Based on the measured Tafel slopes and reaction orders, we demonstrate that the formation rates of C 2+ products are most likely limited by the dimerization of CO adsorbate. CH 4 production is limited by the CO hydrogenation step via a proton coupled electron transfer and a chemical hydrogenation step of CO by adsorbed hydrogen atom in weakly (7 < pH < 11) and strongly (pH > 11) alkaline electrolytes, respectively. Further, CH 4 and C 2+ products are likely formed on distinct types of active sites.more » « less
 An official website of the United States government
An official website of the United States government 
				
			 
					 
					
 
                                    