An Electrochemical cell for in operando 13C NMR investigations of carbon dioxide/carbonate processes in aqueous solution

In operando NMR spectroscopy is a method for the online investigation of electrochemical systems and reactions. It allows a real-time observation of the formation of products and intermediates, and it grants insight into the interactions of substrate and catalyst. An in operando NMR setup for the investigation of the electrolytic reduction of CO2 on 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 reference electrode, and a graphite counter electrode. The setup can be adjusted for the use of different 5 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 was investigated in comparison to 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. The effect of conductive components on 13C NMR experiments was studied using a CO2 saturated solution of aqueous bicarbonate 10 electrolyte. Despite theB0 field distortions caused by the electrodes, line widths of ca. 1 Hz could be achieved. This enables the investigation of interactions in the sub-Hertz range by NMR spectroscopy. It was found that the dynamics of the bicarbonate electrolyte change due to interaction with the electrochemical setup, by catalyzing the exchange reaction between CO2 and HCO3 and affecting the formation of an electrical double layer.


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The anthropologically driven atmospheric CO 2 increase is considered one of the major contributions to global warming (Rockstrom et al., 2009;Hansen et al., 2008). A decline in anthropological CO 2 emissions is seen as improbable due to socioeconomic factors (Grundmann, 2016). Thus, the recycling of CO 2 by electrochemical conversion to energy-rich materials is of particular interest (Jhong et al., 2013;Olah et al., 2009;Whipple and Kenis, 2010;Zhu et al., 2016;Higgins et al., 2018).
A promising candidate in terms of cost and variability is the electrolytic reduction of CO 2 , which is usually performed in an Jhong et al., 2013;Kortlever et al., 2015). There are two main issues. First, there is a complex equilibrium of CO 2 and carbonate species in aqueous systems, which depend on pH, temperature and partial pressure. These parameters vary not only with time during the electrolysis but also spatially as a function of distance from the electrode surface (Hori, 2008;Wang et al., 2010;Xiang et al., 2012;Gibbons and Edsall, 1963;Holmes et al., 1998;Jiao and Rempe, 2011;Lucile et al., 2012;Mani et al., 2006;Toan et al., 2019). Second, the electrolytic CO 2 reduction suffers further from a high overpotential, which can be mitigated by 30 a few selected metal catalysts. There is evidence that the cause of the overpotential is the formation of a CO .− 2 radical as an intermediate. (Hori, 2008;Janik and Tripathi, 2016;Baruch et al., 2015).
To gain inside into the dynamic processes of an electrochemical system it is imperative to monitor the system as a whole during operation, i.e. using in operando spectroscopic techniques (Bañares, 2005;Britton, 2014). NMR spectroscopy is a flexible and powerful method for chemical and reaction analysis (Williamson et al., 2019;Stanisavljev et al., 1998). The NMR 35 study of batteries are often associated with broad line widths inherent to solid state materials. For in operando investigations of liquid state electrolysis systems, high spectral resolution is a limiting factor. The determination of structural information for small molecules relies on the visibility of minor changes in chemical shift as well as J-couplings in the range of a few Hertz, which are not visible in case of broad signals. Thus, signal line width was of concern since the earliest publication in this field.
Several experiment setups for the electrochemical reduction and/or oxidation of organic molecules are published (Bussy and 40 Boujtita, 2015; Falck and Niessen, 2015). The first in operando flow cell for the investigation of electrochemical processes was described by Richards et al. in 1975 and consisted of a two-electrode setup inside a 5 mm NMR tube (Richards and Evans, 1975). A Pt/Hg wire working electrode outside the sensitive volume was contained in a 3 mm tube concentric to the NMR tube. At the bottom of the 3 mm tube, a capillary releases the reaction products into the sensitive volume of the 5 mm tube. The setup allowed sample spinning, which was required due to the low spectral resolution of the spectrometer at that time. This 45 approach was adapted by Mairanovsky et al. in 1983 for the investigation of anion radical decays and improved by using three electrodes (Mairanovsky et al., 1983).
A new approach for an in operando setup was published in 1990 by Mincey et al. (Mincey et al., 1990). Thin film electrodes were employed to minimize distortions of the B 0 and B 1 field by the conductive parts of the electrolysis cell, and thus 1 H line widths of 0.9 Hz could be achieved (Prenzler et al., 2000;Webster, 2004;Zhang and Zwanziger, 2011). As a further evolution 50 of the setup, Webster et al. introduced radio frequency (RF) chokes to minimize interactions between NMR and the potentiostat (Webster, 2004). However, due to its prerequisites manufacturing of thin film electrodes is not easily adaptable. An alternative setup was constructed by Klod et al., which aimed for accessibility for non-specialized NMR users (Klod et al., 2009). The electrolysis cell employed carbon fiber electrodes with high surface area, and could be set up without the need for special 2 https://doi. org/10.5194/mr-2021-18 Discussions Open Access Preprint. Discussion started: 15 February 2021 c Author(s) 2021. CC BY 4.0 License. equipment (Bussy et al., 2013). However, the use of carbon fiber electrodes limits the variety in electrocatalysts that can be 55 investigated.
A different technique for the coupling of electrochemistry and magnetic resonance is hyphenated electrochemical NMR, first presented by Albert et al. in 1987 and later continued by other groups (Albert et al., 1987;Falck et al., 2013;Simon et al., 2012), where the electrochemical cell is physically separated from the NMR spectrometer by passing the electrochemically generated species to an NMR probe by flow. This technique does not suffer from B 0 and B 1 distortions, but there is a time 60 delay between generation and detection of the electrochemical species due to the physical separation.
Most research in this field has been done using 1 H NMR due to its high sensitivity compared to other nuclei, with only few attempts made to investigate 13 C systems (Albert et al., 1987;Nunes et al., 2014). 13 C NMR offers a high spectral width and thus increased separation between signals, but suffers from a low natural abundance of the nucleus. In a recent study, Nunes et al. developed a setup based on Klod et al. to investigate the electrochemical reduction of 9-chlroantracene by 13 C 65 NMR (Nunes et al., 2014). They employed steady-state free precession (SSFP) to achieve high signal-to-noise ratio during short measurement times despite using non-enriched samples. Instead of NMR, electron paramagnetic resonance (EPR) spectroscopy was employed in a recent study by Neukermans et al. as a screening tool for electrocatalysts (Neukermans et al., 2020).
Despite the appearance in 1975, in operando NMR investigations of liquid state systems are still a niche application with focus on method development and thus tested on well-studied, simple redox systems (Richards and Evans, 1975;Mairanovsky 70 et al., 1983;Albert et al., 1987;Mincey et al., 1990;Prenzler et al., 2000;Webster, 2004;Klod et al., 2009;Zhang and Zwanziger, 2011). Only in recent years in operando NMR has been used to study biological systems (Zhang and Zwanziger, 2011;Bussy et al., 2013;Falck et al., 2013). However, this method has not yet been utilized for the investigation of industrial and energy applications, e.g. the electrolytic reduction of CO 2 .
In operando 13 C NMR spectroscopy is ideally suited to study the electrolytic reduction of CO 2 to CO, which requires 75 high resolution to monitor changes in educt structure and the ability to use high sensitivity NMR equipment. To directly measure processes of interest, the working electrode needs to be placed in the sensitive volume of the NMR coil. On the other hand, conductive components in the sample lead to distortions of B 0 and B 1 . These effects can be minimized by choosing a proper placement and orientation of the electrode and by pulse sequences that are robust against B 0 and B 1 field distortions (Romanenko et al., 2014;Hargreaves et al., 2011;Jungmann et al., 2017;Britton et al., 2013). For a versatile cell setup, ease of 80 construction, adaptability to other metal electrodes and the applicability in unmodified NMR liquid state probes is desirable.
This work aims to reduce the effort required for the construction of an in operando NMR setup and apply it to investigate the CO 2 electrolysis on a molecular level. Thus, an electrolysis cell for the in operando NMR investigation of electrolytic CO 2 reduction is presented. The cell is constructed inside a 5 mm NMR tube and consists of a three-electrode setup, which can be adapted without the need of special tools. The setup was evaluated for electrochemical performance by characterizing the 85 chemical system of CO 2 in 1M KHCO 3 electrolyte with and without electrochemical equipment connected. T 1 and T 2 as well as the exchange time between CO 2 and HCO − 3 were determined to investigate the mobility and interactions of the reactant and electrolyte molecules.

Electrolysis cell 90
A three-electrode electrolysis cell has been build that fits a standard 5 mm NMR tube. It consists of a 2.5 x 4 x 0.05 mm silver foil (GoodFellow, Hamburg, Germany) with an area of 10 mm 2 as working electrode, a graphite rod of 1 mm in diameter and 50 mm in length (GoodFellow, Hamburg, Germany) as counter electrode. A chlorinated silver wire tip with a diameter of 0.25 mm (GoodFellow, Hamburg, Germany) was employed as micro Ag/AgCl reference electrode. All electrodes were connected to a silver wire with a diameter 0.25 mm insulated using polytetrafluoroethylene (PTFE) (GoodFellow,Hamburg,95 Germany) with an insulation thickness of 0.024 mm. The graphite counter electrode was chosen because metals can dissolve in small quantities during electrolysis and deposit at the working electrode resulting in a change of catalytic properties (Benke and Gnot, 2002). This process is more pronounced in small setups with half cell reactions that are not separated by a membrane, as it is the case for the in operando electrolysis cell, since species from the counter electrode diffuse sufficiently fast towards the working electrode.

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To join the silver lead wire and the silver foil used as working electrode, first the wire insulation was stripped off over a length of about 1-2 mm. Afterwards, the uninsulated wire tip was pressed on the silver foil while heating them up to 450°C for a few seconds using a soldering iron. The counter electrode was connected by soldering where ca. 2 cm of the silver wire insulation was removed and wrapped around one end of the graphite rod.
The reference electrode was prepared by cleaning the stripped tip (ca. 2 mm) of a silver wire in concentrated nitric acid 105 for 30 seconds and thereafter subsequently placed in an 1M aqueous solution of potassium chloride (≥ 99.5 purity; Sigma Aldrich, Munich, Germany) for 30 minutes. During this process, a thin layer of silver chloride (AgCl) is formed, creating a micro Ag/AgCl reference electrode (Inzelt, 2013). The averaged potential of the micro Ag/AgCl reference electrode was determined to be 0.132 ± 0.004 V vs. a Ag/AgCl (3M KCl) reference electrode in 1M KHCO 3 (aq). Subsequent potentials presented in the results and discussion section are provided vs. the micro Ag/AgCl electrode. The commercial electrode was 110 specified with a potential of 0.210 V vs. normal hydrogen electrodes (NHE), resulting in a potential of 0.342 ± 0.004 V vs.
NHE for the micro Ag/AgCl reference electrode.
The electrodes were arranged in a geometry as shown in Figure 1a and fixed using PTFE tape and shrinking tube. The distance between the center of the working electrode and the reference electrode was adapted to the height of the sensitive volume and the position of the working electrode inside the 5 mm tube was adjusted to match the center of the NMR coil. This 115 minimizes the content of conductive material inside the NMR coil, thus reducing distortions of B 0 and interactions with B 1 .
Additionally, a short distance between the reference and the working electrode ensures a small uncompensated resistance of 5 ± 2 Ω and correspondingly a small iR drop for all electrochemical measurements.
The lead wires of the electrodes were passed through a drilled opening of an NMR tube cap. Cellulosenitrate glue (UHU HART, UHU, Bühl, Germany) was applied to the top of the tube cap and the protruding connection wires in order to fix the 120 position of the electrodes inside the 5 mm tube and seal the drilled opening in the cap. Additionally, ethyl-cyanacrylate glue (Loctite 406, Henkel, Düsseldorf, Germany) was applied on top after the cellulosenitrate glue hardened.

Cell holder
The holder for the electrolysis cell is shown in Figure 2. The setup enables an easy and stable connection between the thin silver 125 wires of the cell and the shielded coaxial cables of the potentiostat. Furthermore, it increases the structural stability of the cell by reducing the weight and strain as well as vibrations of the coaxial cables. The frame of the cell holder was 3D-printed using Acrylnitrile-Butadiene-Styrene (ABS) copolymer (Filamentworld, Neu-Ulm, Germany). For each electrode a non-magnetic SMA coaxial connector (model 23_SMA-50-0-13/111_NE, Huber+Suhner, Herisau, Switzerland) was fixed to the frame using non-magnetic screws. For connection of the electrolysis cell, the silver wires were soldered to the connector pins. The bottom 130 hole of the cell holder was adjusted to the diameter of the NMR tube and the tube cap. The 5 mm tube containing the electrolysis cell is mounted into the cell holder from the top opening and the cell is then fixed by tightly clamping the tube cap at the top end of the NMR tube into the bottom hole of the holder.
To stabilize the sample inside the magnet and to achive a mechanical separation of probe and cell, a dismounted turbine of a magnet lift was fixed on top of the probe. A spinner was attached to the in operando cell, placed inside the turbine and inserted 135 into the magnet.

Noise reduction assembly
The in operando cell was connected to the potentiostat using shielded coaxial cables with SMA connectors. The top opening of the magnet was closed with a copper plate containing RF feed-throughs for the cables to the potentiostat (NMR Service, Erfurt, 140 Germany). Additionally, three low pass radio frequency filters (models SLP-5+, SLP-15+, SLP-30+, Mini Circuits, New York, USA) were connected to each cable in order to reduce RF noise from the potentiostat and environmental sources. A total of three low pass filters were connected to each cable. One low pass filter (SLP-5+, <5 MHz) was connected to the copper plate connections at the top of the magnet and two filters each (SLP-15+, <15 MHz & SLP-30+, <30 MHz) were placed directly at the potentiostat connections. As the connection cables and corresponding banana plugs attached to the potentiostat are un-145 shielded, a silver cloth was wrapped around the unshielded cables. In addition, the body of the probe and the NMR magnet as well as the potentiostat were connected to a common ground. The shielding setup is shown in Figure 3.

B 1 field simulation and nutation behavior
The distortion of the B 1 field in the proximity of the metal electrode was numerically simulated using EMpro (Version 2020, 150 Keysight Technologies). A square Helmholtz coil was designed to mimic a homogeneous RF field in the vicinity of the electrode consisting of two parallel square shaped wires with a distance of 0.5445 times the length for each side of the square.
An ideal conductor served as coil material, and both squares of the coil were driven synchronously by a current source. The silver electrode was placed in the center of the coil as shown in Figure 4d. The simulation was performed for three different angles (0°, 45°, 90°) between the B 1 field and the electrode plane. Data points were acquired with a resolution of 0.4 mm.

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The complex magnetic field vectors of the simulated volume were exported by means of a Python script (Python 3.7, Python Software Foundation) for data processing.
Eddy currents are formed on the metal surface ( Figure 4) caused by the oscillating B 1 field. In turn, the eddy currents induce a magnetic field that distorts the amplitude and phase of the excitation pulse. The distortion of the B 1 field strongly depends on the angle between the electrode and the RF field (Romanenko et al., 2014;Britton, 2014). For a parallel configuration, i.e. Concluding from the results of the simulation, an electrode orientation planar to B 1 can be considered optimal. For different angles, amplitude and phase of the B 1 field distortions depend on the spatial orientation. However, the distortions are mainly significant in the vast proximity of the electrode (0.6-0.8 mm from the surface). Therefore, the majority of the volume inside a 5 mm NMR tube is considered distortion free for the chosen electrode geometry and thus can be probed by NMR spectroscopy 175 without additional measures. Thus, for the current setup a minute adjustment of the orientation is not necessary. It should be noted that the distortions of the B 1 field do not depend only on orientation but also on electrode size. Adjustments may be required for larger in operando electrolysis cells.
A nutation experiment of a CO 2 saturated 1M KHCO 3 solution in the in operando cell was performed using a Bruker 180 Avance III HD spectrometer with a 14.1 T wide bore magnet (150.9 MHz RF frequency for 13 C) and a broadband gradient probe (Bruker DiffBB). Data points were acquired in pulse length steps of 1 µs at a constant pulse power of 59 W. The nutation curve shows the integral of the 13 C HCO − 3 due to its higher signal-to-noise ratio compared to the CO 2 signal (Figure 5a). The

Methods and materials
A 1M aqueous solution of 98% 13 C enriched KHCO 3 (Sigma Aldrich, Munich, Germany) was used as electrolyte. The electrolyte was pre-chilled inside a polyethylene vial in a 10°C water bath. Ca. 1 mL of chilled electrolyte was filled into a 5 mm 190 NMR tube and saturated with 99% 13 C enriched CO 2 (Cambridge Isotope Laboratories, Tewksbury, USA) by bubbling for 20 minutes at a temperature of 10°C if not stated otherwise. The CO 2 was bubbled into the electrolyte using a 1/16 inch PEEK tube, and the flow rate was adjusted to ca. 0.3 mL/s. The three electrode setup was placed inside the 5 mm tube filled with CO 2 saturated electrolyte, ensuring that the contact between counter electrode and silver wire is not immersed in liquid. Prior to sealing, the gas phase inside the tube was aerated with 13 C labeled CO 2 gas. All preparation steps were performed under 195 ambient condition.
The electrochemical experiments were performed using a BioLogic SP-200 potentiostat (BioLogic Science Instruments, Seyssinet-Pariset, France) at a temperature of 10°C, controlled by a surrounding water bath. The electrochemical perfor- wards. In between the electrochemical experiments the system was allowed to relax for 5 minutes. For reference, an equivalent chronopotentiometry experiment was performed using a 1 cm 2 silver electrode and identical reference and counter electrodes.
The reference chronopotentiometry experiment was performed in a cleaned glass beaker filled with 60 mL of aqueous CO 2 saturated 1M KHCO 3 electrolyte, which is denoted as bulk cell in later sections. In the bulk cell, working and counter electrode were arranged in a parallel geometry with a distance between working and reference electrode identical to the in operando  and the reference electrode potential was averaged over 10 minutes. Both reference electrodes were equilibrated for 10 minutes prior to the experiment.
All 13 C NMR measurements were performed using a Bruker Avance III HD spectrometer with a 14.1 T widebore magnet 210 (150.9 MHz RF frequency for 13 C) and a broadband gradient probe (Bruker DiffBB). Spectra were post processed applying a 1 Hz line broadening. NMR experiments were performed at a temperature of 10°C if not stated otherwise. Concentrations of the carbon species in the CO 2 saturated electrolyte with CO 2 atmosphere were determined in a sealed NMR tube using sodium trimethylsilylpropanesulfonate (DSS) (Sigma Aldrich, Munich, Germany) as reference (c(DSS) = 61.62 mmol/L) and a 1 H WALTZ-16 sequence for decoupling (Shaka et al., 1983;Tenailleau and Akoka, 2007). The chemical shift scale of all 215 future 13 C spectra was referenced using the frequency offset of DSS in this experiment. DSS was not included for in operando experiments, because the organic salt may change electrochemical behavior. The CO 2 saturated electrolyte was examined using T 1 and T 2 relaxation and exchange time measurements. Relaxation time constants were determined using a saturation recovery pulse sequence for the determination of T 1 and a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence for T 2 measurements (Carr and Purcell, 1954;Meiboom and Gill, 1958). The exchange time between HCO − 3 and solvated CO 2 was assessed by 220 a 1D EXSY sequence, which uses a shaped Gauss pulse for the selective inversion (100 Hz bandwidth) of the bicarbonate resonance at 160.7 ppm. The center frequency of the selective inversion pulse was adjusted in case of a HCO − 3 frequency shift. The exchange time constant T exc was determined by fitting the evolution of the CO 2 signal integral I(CO 2 ) as a function of the mixing time τ m to

Electrochemical performance of the in operando electrolysis cell
The time dependent potential curves for the chronopotentiometry measurements are shown in Figure 6a. The potentials ob- The non-parallel geometry of the electrodes in the in operando cell may be causing the first two deviations. In a parallel geometry as employed in the bulk cell, the distance of the working electrode to the counter electrode is approximately constant across the whole WE surface. Thus, the iR drop is constant across the electrode and the surface potential is uniform. However, in a non-parallel geometry, there is a spatially dependent iR drop between working and counter electrode, which leads to non-240 uniform potential distribution across the electrode surface. As a result, electrolysis preferably takes place at the closest region between working and counter electrode, where the iR drop is minimal. The CO 2 concentration in that region decreases during electrolysis, and thus the concentration overpotential increases. At a point in time, the concentration overpotential exceeds the increase in iR drop for a more distant region, which results in the electrolysis taking place at that location.
For the electrode setup used in the in operando cell, the region of the working electrode closest to the counter electrode is the 245 edge of the silver sheet. For a current density of -0.25 mA/cm 2 , the electrolytic CO 2 reduction at the silver foil edge takes place at the same overpotential compared to the bulk cell . However, the electrolysis at the electrode edge is not sustainable because of the small area and therefore the diffusive CO 2 transport becomes limited. As a result, the CO 2 concentration is depleted after 2 minutes of electrolysis. At this point, CO 2 reduction takes place at the next-nearest region to the counter electrode, which is a portion of the silver foil plane, where CO 2 can be more readily regenerated by diffusion. However, the iR drop at 250 the silver foil plane are increased compared to the edge, and thus the potential decreases.
With increasing current density, CO 2 is consumed at an increasing rate. Therefore growing portions of the silver electrode surface with an increasing distance to the counter electrode participate in the electrolytic reduction reaction. This results in a rising iR drop, and the overpotential increases further compared to the bulk cell. This effect may also lead to the unstable potential of the in operando cell at a current density of -4 mA/cm 2 . It is important to separate this effect from the expected 255 increase in concentration overpotential with increasing current density, which was also observed for the bulk cell.
The oscillations and increased noise observed for the potential curve of the in operando cell at higher current densities are caused by the formation of gaseous products, i.e. CO and H 2 , in the confined cell geometry. The gas bubbles tend to stick to the glass walls, electrodes or the connection wires, until they grow to a sufficient size to rise to the top. Diameters of up to 1/3 of the size of the electrode surface were observed for the gas bubbles. As a result, the bubbles blocked significant fractions of the 260 electrodes from participating in the electrolysis reaction, thus affecting the electrochemical measurements. For the bulk cell the larger cell geometry didn't promote the gas bubbles to stick, and the larger electrode size was not affected by the comparatively smaller gas bubbles.
For the in operando cell, the Tafel slope was determined as 148 mV per decade from current-voltage (IV) curves of the LSV experiments in the low current density region as shown in Figure 6b. In this region no mass transport limitations for the 265 electrolytic reduction of CO 2 were found. The slope of the Tafel-plot translates to a charge transfer coefficient of α = 0.38. of 0.41 -0.45 for comparable systems under room temperature. (Hori, 2008;Hori et al., 1987;Endrődi et al., 2017;Lu et al., 2014;Hatsukade et al., 2014;Hsieh et al., 2015). The small discrepancies between measured and literature values may originate from the lower temperature at which the shown experiments were performed, resulting in lower thermal energy for 270 the activation of processes and lower diffusion rates.
Overall, the in operando cell shows a comparable performance to a bulk electrolysis cell in the low current density range, i.e.
below -1 mA/cm 2 . Due to the non-parallel cell geometry there is a spatially dependent iR drop distribution across the working electrode surface, which is unfavorable for precise electrolysis experiments. To minimize this effect, an electrode with a small geometry was chosen. Nonetheless, the potential is unstable at higher current densities, and electrochemical measurements 275 may be distorted by gas bubbles stuck in the confined glass tube.

NMR Evaluation of the in operando electrolysis setup
The 13 C spectrum of CO 2 saturated electrolyte is shown in Figure 7a. The two signals in the spectra are assigned to HCO − 3 at 160.7 ppm and solvated CO 2 at 124.7 ppm (Holmes et al., 1998;Mani et al., 2006;Seravalli and Ragsdale, 2008;Abbott et al., 1982;Liger-Belair et al., 2003). The low solubility of carbon dioxide in water causes a weaker signal compared to bicarbonate.

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No signal of gaseous carbon dioxide could be observed, as the gas phase inside the NMR tube is outside of the sensitive volume of the RF coil. For a measured pH of 8.2 ± 0.15 after CO 2 aeration of the electrolyte, about one percent of soluted carbonate is expected. However, due to the fast exchange of HCO − 3 and CO 2− 3 signals for both species coalesced into one peak. The concentrations of the carbon species in the CO 2 saturated electrolyte are shown in Table 1 (Sander, 2015;Dodds et al., 1956). This uncertainty is caused by a combination of a weak signal-to-noise ratio of the DSS methyl group reference signal and imperfections of the temperature control (≤ 1°C). Total carbon 1.87 ± 0.14 As the solubility of CO 2 in water is low, the 13 C signal of CO 2 is weak even when using 13 C labeled and fully saturated samples. It is therefore essential to optimize the signal-to-noise ratio before performing further experiments. To investigate 290 the effect of the in operando setup on 13 C spectra, the signal-to-noise ratio of the HCO − 3 signal was determined for different conditions (Table 2). Decreasing the temperature from 22°C to 10°C significantly improved the signal-to-noise ratio. As Table 2. signal-to-noise ratio of the 13 C HCO − 3 signal under different conditions.

Temperature [°C]
Conditions signal-to-noise ratio [-] 22 No conductive material 1247 In operando cell 397 In operando cell and connection cables 290 10 No conductive material 1994 In operando cell and connection cables 399 Full in operando electrolysis setup with shielding 2510 CO 2 possess a higher solubility at lower temperatures (cf. c 1013 hPa, 22°C = 38.0 mmol/L) (Sander, 2015;Dodds et al., 1956), the signal-to-noise ratio of the CO 2 signal increased by a factor of ca. 1.4. The equilibrium constant for the CO 2 /HCO − 3 equilibrium changed by only 1% due to the decrease in temperature. Therefore an increase in CO 2 concentration causes 295 similar increases of HCO − 3 in solution. The decrease of 12°C also increases the equilibrium magnetization and thus reduces thermal noise what led to an increase in the signal-to-noise ratio by about 6%.
After introducing the electrodes into the magnet the signal positions shifted downfield by 1.1 ppm and line widths become significantly broader but could be reduced down to about 1 Hz after shimming except for a downfield shoulder. The signal-tonoise ratio of the spectrum was reduced significantly by 68%. As the concentration of the carbon species remained unchanged, 300 the decrease in the signal-to-noise is a combined effect of increased noise levels and a reduced quality factor, Q, of the NMR circuit caused by the conductive components. As shown by Figure 7b, the main contribution is the introduction of external RF noise due to the metallic components and cables acting as radio antenna. Coherent external RF noise in the frequency range of 13 C NMR at 14.1 T (150.9 MHz) is caused by mobile radio communication (Bundesnetzagentur, 2019). Introducing additional connections to the setup as well as connecting the cell directly to a powered potentiostat further decreased the signal-305 to-noise ratio despite using shielded coaxial cables. A highly shielded setup as described in in Figure 3 is therefore necessary to decrease RF noise originating from external sources in order to obtain signal-to-noise ratios comparable to experiments without conductive materials. Using just single elements of the shielding setup, i.e. only the copper plate for the top opening of the magnet, the silver cloth, the common ground or the filters, does not restore the signal-to-noise ratio to original values.
First, as a reference for the CO 2 saturated electrolyte, longitudinal relaxation times and exchange rates were determined 310 using a standard NMR tube without the electrolysis setup. In a second step the electrodes and leads were present but not connected. In the final step dataa was collected with the full electrolysis setup shown in Figure 3. The results of all experiments are summarized in Table 3. The errors of the CO 2 rates are caused by a low signal-to-noise ratio of the carbon dioxide signal.
Both species in the CO 2 saturated electrolyte show similar values of longitudinal relaxation rates. The longitidunal relaxation rates for HCO − 3 and CO 2 are unchanged within error boundaries after insertion of the electrodes. On the other hand, the 315 exchange time constant of the chemical equilibrium between CO 2 and HCO − 3 decreased from 4.55 s to 3.10 s. The decreased exchange time affects the transverse relaxation time constant and as a result decreases T 2 for both HCO − 3 and CO 2 . The change in exchange time is assumed to be caused by an interaction of HCO − 3 with the polarizable silver metal electrode surface. The interaction of ions with metal surfaces by induction of dipoles is reported in literature to extend up to 1 nm from the metal (Bonzel, 1988;Mendonca et al., 2012;Seitz-Beywl et al., 1992). To effect the bulk solution, the exchange between free ions 320 and HCO − 3 -metal complexes has to be sufficiently fast in addition to a high catalytic activity of the HCO − 3 /metal surface. On the one hand, the positively charged metal surface acts as a catalytic center for the CO 2 /HCO and increases the reaction rate by six to seven orders of magnitude (Lindskog, 1997;Grisham and Garrett, 2010). On the other hand, the fast exchange between surface and bulk species may be caused the RF pulses which induce eddy current in the silver metal. These eddy currents increase the temperature of the electrolyte in the electrode vicinity what in turn creates a convection flow and causing a mixing of the solution.
During the measurements employing the full in operando electrolysis setup, the cell was connected to a potentiostat. The A more probable source for the altered relaxation behavior is the changed capacity of the electrode assembly. Cables and filters can contain or act as capacitors, which changes the capability of the setup to dissipate or provide electrons at the 345 electrodes. As OCV is an electrostatic mode of operation, the assembly may act as a additional power supply and thus affect double layer formation. This in turn may affect the whole electrolyte, e.g. via changing the equilibrium between the ionic species, which may lead to an altered pH of the system. This is known to sensitively affect relaxation properties for aqueous carbonate solutions (Moret et al., 2013). While a detailed analysis of these processes is outside the scope of the current study, it highlights the sensitivity of 13 C NMR to investigate fundamental processes occurring during CO 2 electrolysis, thereby 350 justifying the efforts necessary to achieve sufficient sensitivity and resolution for in operando experiments. At the same time, this also demonstrates the importance for a proper designed electrolysis setup to avoid unwanted side effects.
The results also show that any sort of measurement setup may affect an electrochemical system. While the in operando electrolysis setup does not disturb the NMR measurements, it can affect the state of the electrodes and thus their interaction with the CO 2 saturated electrolyte. However, the NMR measurements of the unconnected and connected electrolysis setup 355 show that even the equipment which is imperative for electrochemical testing can affect the equilibrium state of the electrolysis. This is particularly pronounced at low current densities as OCV.

In operando NMR of the OCV evolution
The 13 C NMR spectra of the aqueous HCO − 3 /CO 2 sample during OCV and the potential between working and reference electrode are shown as a function of time in Figure 8. The current density between working electrode and counter electrodes Figure 8. Time evolution of the 13 C signals for HCO − 3 (a) and CO2 (b) during the OCV stage versus electrochemical potential and current density. In each sub panel the time dependent potential and current density is shown on the left, and the corresponding spectra are shown on the right. After a relaxation period, the potential remains at a stable potential of 47 mV.

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remains fixed at 0 mA/cm 2 during measurements. During the first 5 hours of the experiment the potential drops from -31 mV to -42 mV vs. Ag/AgCl. After 12 hours the potential plateaued at -45 mV vs. Ag/AgCl and approaches equilibrium of -47 mV vs. Ag/AgCl after 17 hours.
The 13 C NMR peaks of HCO − 3 and the CO 2 are at the same initial position compared to the reference measurements. A narrow main peak with broader shoulder persisted throughout the OCV stage. Fitting both signals with a Lorantzian line shape, 365 a peak separation of 0.04 ppm (6.1 Hz) is obtained at 14.1 T. The shoulder peak is assumed to be caused by B 0 field distortions in the proximity of the working electrode, which cannot be easily corrected by shimming.
The HCO − 3 signal drifted downfield about 7 ppb during the first 5 hours (Figure 8a), whereas the CO 2 signal shifted by 2 ppb (Figure 8b). Therefore, the evolution of the HCO − 3 signal position is not caused by a magnet drift. The integral of theHCO − 3 signal ( Figure 9a) drops by 1%. The HCO − 3 signal integral increases ca. 1% compared to the initial intensity. The evolution of 370 the HCO − 3 signal position and the intensity imply an evolution of the CO 2 /HCO − 3 /CO 2− 3 equilibrium since a higher chemical shift is associated with an increase of the CO 2− 3 concentration (Abbott et al., 1982). The intensity of the CO 2 peak continuously decreases during the OCV stage. After 12 hours the CO 2 signal integral decreased to 78% of the initial value (Figure 9b). Using 55.0 mmol/L as the initial concentration, as was determined in the reference experiment, this equals a concentration of 42.9 mmol/L. After 25.6 hours of OCV, the CO 2 signal intensity de-375 creased to 62% (34.1 mmol/L). This behavior is unexpected as no CO 2 has been converted by electrolytic processes during the OCV stage.
Leakage of such a large amount of CO 2 gas during the NMR experiment is unlikely. Permeation of carbon dioxide through the polypropylene tube cap or the glue used for sealing can be excluded, as the CO 2 gas permeability for these materials is low (Hasbullah et al., 2000). Furthermore, any CO 2 loss should be compensated by the CO 2 /HCO − 3 equilibrium reaction, thus 380 decreasing the HCO − 3 concentration. However, no sustained decrease in HCO − 3 concentration is observed. Furthermore, the total amount of all carbon species is unchanged after 12 hours of OCV, and therefore no CO 2 was lost to the environment.
It can be therefore concluded that initially, the CO 2 saturated electrolyte is not at equilibrium, which can be confirmed by the following considerations. Directly after preparation, the pH value of the electrolyte was 8.2 ± 0.15. Given a total concentration of 1.87 mol/L for all carbon species, the equilibrium concentration of solvated CO 2 at that pH value is 33.7 mmol/L. Thus the 385 initial CO 2 concentration of 55.0 mmol/L is above the equilibrium value. The CO 2 /HCO − 3 system reaches equilibrium at the end of the OCV experiment, when the CO 2 concentration reaches 34.1 mmol/L All changes in the HCO − 3 and CO 2 signal integrals and the HCO − 3 signal position occur in accordance to the variations of the potential during OCV. Changes in HCO − 3 and CO 2 signal are associated with a shift in equilibrium, what indicates an shift in the electrochemical potential during the OCV stage caused by an evolution of the CO 2 /HCO − 3 /CO 2− 3 equilibrium.

390
All relaxation and exchange times of CO 2 and HCO − 3 during the OCV stage are given in Table 4. Compared to the test of the in operando cell, the exchange time slightly decreased after the system approached electrochemical equilibrium, which corresponds to the increased HCO − 3 concentration. The decomposition rate of bicarbonate, i.e. the inverse of the exchange time, is linearly proportional to the HCO − 3 concentration. The decreased exchange time between CO 2 and HCO − 3 affects the transverse relaxation process and decreases T 2 time constants. Longitudinal relaxation time constants decreased only slightly 395 as result of the change in equilibrium and are overall comparable to the test of the in operando setup.

Conclusions
In this study, a setup for the in in operando NMR study of the electrochemical CO 2 reduction was developed, which was specifically designed to observe molecular dynamics changes in proximity to the working electrode. A key feature of the in operando setup is the suppression of noise and external radio frequency signals that were introduced by conductive materials, 400 enabling the observation of low concentration species. Relaxation and exchange experiments provide a sensitive probe for the interaction of ionic species and metal electrodes under different electrochemical conditions. The results of those experiments indicated that the electrochemical measurement equipment itself may affect a reaction and molecular dynamics. Finally, in operando NMR was used to monitor an aqueous CO 2 /HCO − 3 system for electrolytic CO 2 reduction at open circuit voltage. It was revealed, that (electro-)chemical equilibrium in solution evolves for considerable time after sample preparation.