In operando ¹³C NMR investigations of carbon dioxide/carbonate processes in aqueous solution

In operando measurements are becoming increasingly important for evaluating electrochemical device performance under realistic operating conditions.^1,2 However, there are a number of challenges for many analytical techniques for such measurements, including sample positioning, compatibility with the overall cell and device structure, and the suitability of the analytical method for dealing with the often complex information.

To this end, a novel in operando Nuclear Magnetic Resonance (NMR) spectroscopy approach has emerged that enables the investigation of CO₂ electrolytic reduction on silver electrodes.¹ The new electrochemical cell was designed to observe products and intermediates in the catalytic cycle, addressing interference issues from external radio fields that are common in standard electrochemical cells.

Carbon dioxide (CO₂) production is considered to be one of the major anthropogenic factors contributing to global climate change.³ While numerous governments, societies, and individuals are making efforts to decrease CO₂ emissions, there are still concerns that, due to socioeconomic factors, achieving a real global reduction in CO₂ emissions may range from highly unlikely to impossible.⁴

The implication that CO₂ emissions are unlikely to decrease significantly enough to impact climate change emphasizes the urgent need for CO₂ recycling or reduction technologies. Some possibilities include the transformation of CO₂ into useful chemical products, which can be achieved with the use of a bicarbonate electrode to make hydrocarbons and alcohols.⁵ Other possibilities include the use of silver or gold electrodes to make CO, which is another valuable chemical feedstock.

However, due to its innate complexity, electrochemical CO₂ reduction is still not fully understood. Its complexity arises from the sensitivity of the process to environmental factors such as species concentration, pH, and temperature. Furthermore, these parameters are not only time-dependent but also spatially dependent on the distance from the electrode. Addressing these challenges is crucial to advancing our understanding of electrochemical CO₂  reduction and maximizing its potential impact on mitigating climate change.¹

NMR is a crucial analytical technique for studying CO₂ reduction, offering opportunities for precise chemical identification and quantification. However, conducting in operando measurements with NMR presents challenges due to the subtle chemical shifts between products and reacting species. Identifying these shifts is particularly tricky without exceptional spectral resolution, a difficult achievement within electrochemical cells.

Fortunately, newer thin film cells have overcome these hurdles, enabling higher resolution in operando measurements. Among these techniques, ¹³C NMR stands out for its effectiveness in analyzing the electrochemical reduction of CO₂ to CO due to the sensitivity of both the starting material and the product.

Methodology

The novel approach utilizes a new type of electrochemical cell that fits into the NMR coil on the instrument. The cell was designed to fit into a 5 mm NMR tube and consisted of three electrodes. The performance of the cell was then evaluated both with and without electrochemical equipment for CO₂ in 1 M KHCO₃ solution. Silver and silver chloride electrodes were used with extra shielding to prevent the disruption of the NMR fields by the cell.
The NMR measurements were conducted using a Bruker Avance III HD Spectrometer equipped with a 14.1 T widebore magnet and a Bruker DiffBB broadband gradient probe. All NMR measurements were ¹³C measurements, employing a constant pulse power of 59 W.

Results

A series of chronopotentiometry measurements were taken to determine the voltage conditions marking the onset of CO₂ reduction. This allowed for a comparison between results obtained from both bulk cells and in operando conditions. In the in operando measurements, higher overpotentials were recorded. Additionally, longer equilibration times were observed when the current was either introduced or removed from the cell, leading to greater oscillations of potential and increased noise due to bubble formation.

The subsequent NMR results showed the formation of bicarbonate species (HCO₃-), which could then undergo exchange with CO₂ in the electrolyte solution. Notably, no gaseous CO₂ was detected in the measurements. The rate of exchange was found to be distance-dependent from the electrode, and the positively charged metal was identified as a catalytic center for CO₂/HCO⁻ exchange.

Discussion

The differences in behavior between the bulk and operando cells were attributed to specific inhomogeneities in potentials between the electrodes, particularly between the working and counter electrodes, within the operando cells. This potential drop creates a spatially favorable region for the electrolysis, leading to a decrease in the local CO₂ concentration. Consequently, the reduced CO₂ concentration raises the concentration overpotential, further influencing the location of electrolysis.

Conclusions

Shielding and improvements in the experiment’s signal-to-noise were crucial to using the ¹³C NMR on the electrolysis cell, particularly as many of the deviations in the concentrations of the various CO₂ species were on the order of a few percent. Overall, the new in operando cells offered similar performance to the bulk cells, particularly in the low current density range.

The high level of performance offered by the Bruker BioSpin instrumentation was key to facilitating these measurements. With the most advanced NMR instruments and software on the market, Bruker Biospin has a number of solutions available for even the most complex of reaction-tracking applications.

To find out how Bruker BioSpin’s probe and spectrometer range could enhance your measurement capabilities to incorporate realistic reaction conditions, contact Bruker BioSpin today.

1. Jovanovic, S., Schleker, P. P. M., Streun, M., Merz, S., Jakes, P., Schatz, M., Eichel, R.-A., and Granwehr, J.: An electrochemical cell for in operando 13C nuclear magnetic resonance investigations of carbon dioxide/carbonate processes in aqueous solution, Magn. Reson., 2, 265–280, https://doi.org/10.5194/mr-2-265-2021
2. Liu, D., Shadike, Z., Lin, R., Qian, K., Li, H., Li, K., ... & Li, B. (2019). Review of recent development of in situ/operando characterization techniques for lithium battery research. Advanced Materials, 31(28), 1806620. https://doi.org/10.1002/adma.201806620
3. Rockstrom, J., Steffen, W., Noone, K., Persson, A., III, F. S. C., Lambin, E. F., Lenton, T. M., Scheffer, M., Folk, C., Schnellnhuber, H. J., Nykvist, B., Wit, C. A. de, Hughers, T., Leeuw, S. van der, Rodhe, H., Sorlin, S., Snyder, P. k., Constanza, R., Svedin, U., … Foley, J. A. (2009). A safe operating space for humanity. Nature, 461, 472–475. https://doi.org/10.1038/461472a
4. Grundmann, R. (2016). Climate change as a wicked social problem. Nature Geoscience, 9, 562-563. https://doi.org/10.1038/ngeo2780
5. Hori, Y. I. (2008). Electrochemical CO2 reduction on metal electrodes. Modern aspects of electrochemistry, 89-189. https://doi.org/10.1007/978-0-387-49489-0_3