An electrochemical cell for in operando <sup>13</sup>C nuclear magnetic resonance investigations of carbon dioxide/carbonate processes in aqueous solution

Abstract

Abstract. In operando nuclear magnetic resonance (NMR) spectroscopy is one method for the online investigation of electrochemical systems and reactions. It allows for real-time observations of the formation of products and intermediates, and it grants insights into the interactions of substrates and catalysts. An in operando NMR setup for the investigation of the electrolytic reduction of CO2 at silver electrodes has been developed. The electrolysis cell consists of a three-electrode setup using a working electrode of pristine silver, a chlorinated silver wire as the reference electrode, and a graphite counter electrode. The setup can be adjusted for the use of different electrode materials and fits inside a 5 mm NMR tube. Additionally, a shielding setup was employed to minimize noise caused by interference of external radio frequency (RF) waves with the conductive components of the setup. The electrochemical performance of the in operando electrolysis setup is compared with a standard CO2 electrolysis cell. The small cell geometry impedes the release of gaseous products, and thus it is primarily suited for current densities below 1 mA cm−2. The effect of conductive components on 13C NMR experiments was studied using a CO2-saturated solution of aqueous bicarbonate electrolyte. Despite the B0 field distortions caused by the electrodes, a proper shimming could be attained, and line widths of ca. 1 Hz were achieved. This enables investigations in the sub-Hertz range by NMR spectroscopy. High-resolution 13C NMR and relaxation time measurements proved to be sensitive to changes in the sample. It was found that the dynamics of the bicarbonate electrolyte varies not only due to interactions with the silver electrode, which leads to the formation of an electrical double layer and catalyzes the exchange reaction between CO2 and HCO3-, but also due to interactions with the electrochemical setup. This highlights the necessity of a step-by-step experiment design for a mechanistic understanding of processes occurring during electrochemical CO2 reduction.

Quantifying Local pH Changes in Carbonate Electrolyte during Copper-Catalysed CO2 Electroreduction Using In Operando 13C NMR

“…This constitutes a shift of the HCO -3 /CO 2 equilibrium towards HCO -3 , caused by an increasing local pH. The FWHM decreases due to faster exchange between carbonate species, which has been observed before by in operando NMR studies with silver working electrodes 25 . The fitting parameters exhibit fluctuations that can be attributed to gas bubble formation on the electrode during the electrochemical experiments.…”

“…The cell holder presented in our previous publication 25 was adjusted such that the cell can be inserted from the top of the magnet with a narrow-bore sample lift installed. During CA, 13 C spectra were continuously recorded using 30°-pulses with 1 H decoupling (WALTZ-16 sequence with 128 repetitions every 2 s), resulting in acquisition of one spectrum every 6 min.…”

“…In this study, the measurement of pH is demonstrated in an electrochemical cell and during CO 2 electrolysis by in operando 13 C Nuclear Magnetic Resonance (NMR), which has proven capable of investigating the CO 2 (aq)/HCO -3 /CO 2 -3 equilibrium in such an environment 25 . In contrast to methods that only consider the CO 2 (aq)/HCO -3 22 or the HCO -3 /CO 2 -3 equilibirum 24 , the presented method provides data for a wide pH range by considering both equilibria, including an overlap region inbetween.…”

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

“…This constitutes a shift of the / equilibrium towards , caused by an increasing local pH. The FWHM decreases due to faster exchange between carbonate species, which has been observed before by in operando NMR studies with silver working electrodes 25 . The fitting parameters exhibit fluctuations that can be attributed to gas bubble formation on the electrode during the electrochemical experiments.…”

“…In this study, the measurement of pH is demonstrated in an electrochemical cell and during electrolysis by in operando nuclear magnetic resonance (NMR), which has proven capable of investigating the (aq)/ / equilibrium in such an environment 25 . The method takes advantage of the high spectral resolution and sensitivity afforded by this cell design even in the presence of electrically conductive material 26 .…”