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| Home > Publications database > Measuring local pH gradients using $^{13}C$ magnetic resonance imaging |
| Conference Presentation (After Call) | FZJ-2022-04706 |
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2022
Abstract: In the light of the ever-increasing number of new catalyst materials for the CO2 reduction reaction (CO2RR), determination of local conditions in electrode proximity is crucial to understand and improve the performance of electrolysis. Especially the widespread use of KHCO3 in low concentrations clearly demonstrates the importance of the choice of electrolyte for the catalysis of this reaction, since its low buffer capacity leads to increased pH in proximity to the electrode. This promotes C2+ product reaction pathways, while simultaneously suppressing unfavourable CH4 and H2 formation. Measuring local pH values on CO2RR-catalyst surfaces has been attempted by optical methods and by scanning probe microscopy. In our recent work, we presented a NMR method for determining local pH in KHCO3 electrolyte at a Cu electrode using the 13C resonances of the CO2/HCO3-/CO32- equilibrium. The present study adds a spatial dimension to this technique in order to investigate evolution of local pH and concentration gradients over time in the electrochemical cell illustrated in Fig. 1a.Spatially resolved 13C spectra of the averaged carbonate (HCO3-/CO32-) resonance are presented in Fig. 1b. The electrode was placed at z = 0 mm. Before electrolysis, the carbonate peak had a constant chemical shift along the z-direction. As a constant potential was applied, the peak locally shifted downfield, which corresponds to a local pH increase. These pH gradients are quantified by fitting Lorentzian functions to the peaks. In Fig. 1c, resulting z-profiles of the chemical shift of the carbonate peak and their development over time are depicted. A sudden increase of near-electrode pH at the beginning of the electrolysis was observed, followed by an assimilation of local and bulk values. In this study, it will be shown that chemical shift imaging is successfully applied in operando to resolve the spatial distribution of pH value and electrolyte concentrations in the vicinity of a Cu electrode during CO2RR. The evolution of these values as a function of time are in accordance with theory.
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