In operando NMR investigations of the aqueous electrolyte chemistry during electrolytic CO2 reduction

Abstract

The electrolytic reduction of CO₂ in aqueous media promises a pathway for the utilization of the green house gas by converting it to base chemicals or building blocks thereof. However, the technology is currently not economically feasible, where one reason lies in insufficient reaction rates and selectivities. Current research of CO₂ electrolysis is becoming aware of the importance of the local environment and reactions at the electrodes and their proximity, which can be only assessed under true catalytic conditions, i.e. by in operando techniques. In this work, multinuclear in operando NMR techniques were applied in order to investigate the evolution of the electrolyte chemistry during CO₂ electrolysis. The CO₂ electroreduction was performed in aqueous NaHCO₃ or KHCO₃ electrolytes at silver electrodes. Based on ¹³C and ²³Na NMR studies at different magnetic fields, it was found that the dynamic equilibrium of the electrolyte salt in solution, existing as ion pairs and free ions, decelerates with increasingly negative potential. In turn, this equilibrium affects the resupply rate of CO₂ to the electrolysis reaction from the electrolyte. Substantiated by relaxation measurements, a mechanism was proposed where stable ion pairs in solution catalyze the bicarbonate dehydration reaction, which may provide a new pathway for improving educt resupply during CO₂ electrolysis.

Fig. 1. ¹³C spectrum of a CO₂ saturated 1 mol L⁻¹ KHCO₃ electrolyte.

The spectrum consists of the solvated CO₂ signal at 125.9 ppm and the coalesced ${{\mathrm{HCO}}_3}^{-}/{{\mathrm{CO}}_3}^{2-}$ signal at 161.8 ppm.

Fig. 2. Overview of ¹³C signal evolution during the chronoamperometry (CA) stage.

Time evolution of voltage and current density as well as the corresponding ¹³C NMR signals at B₀ = 14.1 T of ${{\mathrm{HCO}}_3}^{-}$ and CO₂ during CA with a constant voltage of -1.1 V. A new peak emerges for the ${{\mathrm{HCO}}_3}^{-}$ signal.

Fig. 3. Overview of ¹³C signal evolution during the chronopotentiometry (CP) stage.

Time evolution of voltage and current density as well as the corresponding ¹³C NMR signals at B₀ = 14.1 T of ${{\mathrm{HCO}}_3}^{-}$ and CO₂ during CP with a constant current density of -10 μA cm⁻². The separation between the ${{\mathrm{HCO}}_3}^{-}$ signal components increases during this stage.

Fig. 4. Signal deconvolution of the ¹³C HCO3− NMR signal at 14.1 T (150.9 MHz resonance frequency).

The ${{\mathrm{HCO}}_3}^{-}$ evolves from the open circuit voltage (OCV) stage a over the chronoamperometry (CA) stage b to the chronopotentiometry (CP) stage c. Two signal components are assigned to ${{\mathrm{HCO}}_3}^{-}$ existing as free ions (blue) and in an ion pair (red). For high exchange rate between these two states, i.e., during OCV, the two signal components coalesce into a single peak (violet). An additional signal component (green) represents the B₀ distortion in proximity to the working electrode.

Fig. 5. Signal deconvolution of the ¹³C HCO3− NMR signal at 9.4 T (100.6 MHz resonance frequency).

The ${{\mathrm{HCO}}_3}^{-}$ evolves from the open circuit voltage (OCV) stage a over the chronoamperometry (CA) stage b to the chronopotentiometry (CP) stage c in a similar fashion to Fig. 4, where the free ion are depicted in blue, ion pairs in red, and coalesced signal in purple. An additional signal component (green) represents the B₀ distortion in proximity to the working electrode. Due to the weaker B₀ field strength, the separation of the free ion and ion pair signal is not as pronounced.

Fig. 6. Deconvolution of the ²³Na NMR resonance caused by Na⁺ at 14.1 T (158.7 MHz resonance frequency).

The Na⁺ cation signal evolves from a coalesced state (purple) in the open circuit voltage (OCV) stage a over the chronoampoerometry (CA) stage b to the chronopotentiometry (CP) stage c by splitting into the two components of free ions (blue) and ion pairs (red), similar to the ${{\mathrm{HCO}}_3}^{-}$ signal in Fig. 4. However, due to the inherently broader line widths of ²³Na, the peak splitting is not as pronounced and the B₀ distortion artifact cannot be observed separately.

Fig. 7. Evolution of the ¹³C T₁ relaxation time constants for CO₂ (green circles) and HCO3− (purple, red, and blue circles), starting at the open circuit voltage (OCV) stage.

At the chronoamperometry (CA) stage, the ${{\mathrm{HCO}}_3}^{-}$ signal splits into two components for which different relaxation time constants for free ions and xSIPs can be distinguished. With increasingly negative potential at the CA and chronopotentiometry (CP) stage, CO₂ approaches the ${{\mathrm{HCO}}_3}^{-}$ xSIPs in terms of T₁. The error bars represent the fitting error received for T₁ determination. The term xSIP represents ion pairs with one (x = 1) or two (x = 2) solvent layers between anion and cation.

Fig. 8. Proposed reaction scheme of the bicarbonate dehydration.

The reaction is catalyzed by the cation of a xSIP, exemplary shown for Na⁺. A similar catalytic reaction is known from the enzyme carbonic anhydrase. The term xSIP represents ion pairs with one (x = 1) or two (x = 2) solvent layers between anion and cation.

Fig. 9. Correlation of equilibrium reaction rates during electrolysis.

Evolution and correlation of the CO₂/${{\mathrm{HCO}}_3}^{-}$ equilibrium reaction rate ($k_{\mathrm{exc}}^{{\mathrm{CO}}_2/{{\mathrm{HCO}}_3}^{-}}$) and the exchange rate between free electrolyte ions and xSIPs ($k_{\mathrm{exc}}^{\mathrm{FI}/\mathrm{IP}}$) as a function of potential. With increasingly negative potential, it was found that $k_{\mathrm{exc}}^{\mathrm{FI}/\mathrm{IP}}$ decreases, while $k_{\mathrm{exc}}^{{\mathrm{CO}}_2/{{\mathrm{HCO}}_3}^{-}}$ increases simultaneously. Using T₁ relaxation time experiments, it was shown that CO₂ preferably is formed from ${{\mathrm{HCO}}_3}^{-}$ that is composing a xSIP, which suggests a catalytic activity of the electrolyte cation, e.g. Na⁺. The term xSIP represents ion pairs with one (x = 1) or two (x = 2) solvent layers between anion and cation.