Quantifying local pH changes in carbonate electrolyte during copper-catalysed $$\hbox {CO}_2$$ electroreduction using in operando $$^{13}\hbox {C}$$ NMR

Abstract

The electrochemical carbon dioxide reduction on copper attracted considerable attention within the last decade, since Cu is the only elemental transition metal that catalyses the formation of short-chain hydrocarbons and alcohols. Research in this field is mainly focused on understanding the reaction mechanism in terms of adsorbates and intermediates. Furthermore, dynamic changes in the micro-environment of the catalyst, i.e. local pH and $$\hbox {CO}_2$$ CO 2 concentration values, play an equivalently important role in the selectivity of product formation. In this study, we present an in operando$$^{13}\hbox {C}$$ 13 C nuclear magnetic resonance technique that enables the simultaneous measurement of pH and $$\hbox {CO}_2$$ CO 2 concentration in electrode vicinity during electroreduction. The influence of applied potential and buffer capacity of the electrolyte on the formation of formate is demonstrated. Theoretical considerations are confirmed experimentally and the importance of the interplay between catalyst and electrolyte is emphasised.

Interplay of Local pH and Cation Hydrolysis during Electrochemical CO2 Reduction Visualized by In Operando Chemical Shift-Resolved Magnetic Resonance Imaging

“…The product analysis was performed using 1 H NMR with water suppression. 27,38 ■ RESULTS AND DISCUSSION Correction of Chemical Shift Profiles. Figure 3a represents an exemplary frequency-encoded 1 H profile of the electrochemical cell.…”

“…After the in operando measurements were conducted, electrodes were removed and the deuterated NMR reference trimethylsilylpropanoic acid (TSP) was added to the NMR tube to result in a concentration of 1 mM. The product analysis was performed using 1 H NMR with water suppression. , …”

“…For the calculations of the FE of the formed liquid products, residuals of impurities in the pristine solutions were subtracted from the product concentrations. 27 In general, formate, acetate, dissolved methane, and acetaldehyde were detected in all samples. The most pronounced difference between product distributions with NaHCO 3 and KHCO 3 electrolytes was that alcohols, i.e., ethanol and, to a lower extent, methanol, were found only in KHCO 3 electrolytes, while the FE for acetate was larger for NaHCO 3 electrolytes.…”

“…Hereby, the 13 C and 23 Na resonances of the bicarbonate electrolyte are assessed in terms of the local chemical environment at a Cu WE. Our previously described pH determination method 27 is extended by the application of MRI pulse sequences while maintaining spectroscopic information. The 13 C measurements are thereby spatially resolved, enabling the observation of the evolution of the concentration profiles of the HCO 3 − /CO 3 2− equilibrium as well as of dissolved CO 2 (aq), which are then utilized to determine the local pH profiles.…”

Direct Imaging of Local pH Reveals Bubble-Induced Mixing in a CO2 Electrolyzer

“…For example, absorption spectroscopy can determine the pH at plate cathode surfaces , or inside a bipolar membrane (BPM) , Operando Raman microscopy has allowed us to measure the local (bi)­carbonate concentrations and pH values depending on the distance from the cathode in a liquid-fed or gas-fed CO 2 electrolysis flow cell. This technique is limited by the relatively low intensity of the Raman effect, which restricts the imaging speed (typical acquisition time: ≥10 min). , Fluorescence microscopy, in contrast, can use the strong fluorescence signal of suitable probe molecules to measure spatially resolved intensity more rapidly, allowing much shorter acquisition times (e.g., 5 s) .…”