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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.
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Effect of Electrolyte Cations on Organic Electrosynthesis: The Case of Adiponitrile Electrochemical Production
Electrolyte ions have a profound impact on the reaction environment of electrochemical systems and can be key drivers in determining the reaction rate and selectivity of electro-organic reactions. We combine experimental and computational approaches to understand the individual effect of the size and concentration of supporting alkali cations, as well as their synergies with other electrolyte ions on the electrosynthesis of adiponitrile (ADN). The size of supporting alkali cations influences the surface charge density, availability of water molecules, and stability of reaction intermediates. Larger alkali cations can help limit hydrogen evolution and the early protonation of intermediates by lowering the availability of water molecules in the near electrode region. A selectivity of 93% towards ADN was achieved at −20 mA cm−2in electrolytes containing cesium phosphate salts, ethylenediaminetetraacetic acid, and tetraalkylammonium ions (TAA ions). Electrolytes containing only supporting phosphate salts promote the early hydrogenation of intermediate species leading to low ADN selectivities (i.e., <10%). However, the combined effect of alkali cations and selectivity-directing ions (i.e., TAA ions) is essential in the enhancement of ADN synthesis. The insights gained in this study provide guidelines for the design of aqueous electrolytes that improve selectivity and limit hydrogen evolution in organic electrosynthesis.
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- Award ID(s):
- 1943972
- PAR ID:
- 10370655
- Publisher / Repository:
- The Electrochemical Society
- Date Published:
- Journal Name:
- Journal of The Electrochemical Society
- Volume:
- 167
- Issue:
- 15
- ISSN:
- 0013-4651
- Page Range / eLocation ID:
- Article No. 155526
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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