Examination of Near-Electrode Concentration Gradients and Kinetic Impacts on the Electrochemical Reduction of CO₂ using Surface-Enhanced Infrared Spectroscopy

Abstract

Localized concentration gradients within the electrochemical double layer during various electrochemical processes can have wide-ranging impacts; however, experimental investigation to quantitatively correlate the rate of surface-mediated electrochemical reaction with the interfacial species concentrations has historically been lacking. In this work, we demonstrate a spectroscopic method for the in situ determination of the surface pH using the CO2 reduction reaction as a model system. Attenuated total reflectance surface-enhanced infrared absorption spectroscopy is employed to monitor the ratio of vibrational bands of carbonate and bicarbonate as a function of electrode potential. Integrated areas of vibrational bands are then compared with those obtained from calibration spectra collected in electrolytes with known pH values to determine near-electrode proton concentrations. Experimentally determined interfacial proton concentrations are then related to the resultant concentration overpotentials to examine their impact on electrokinetics. We show that, in CO2-saturated sodium bicarbonate solutions, a concentration overpotential of over 150 mV can be induced during electrolysis at −1.0 V vs RHE, leading to substantial losses in energy efficiency. We also show that increases in both convection and buffering capacity of the electrolyte can mitigate interfacial concentration gradients. On the basis of these results, we further discuss how increases in concentration overpotential affect the mechanistic interpretations of the CO2 reduction electrocatalysis, particularly in terms of Tafel slopes and reaction orders.

In‐Situ Infrared Spectroscopy Applied to the Study of the Electrocatalytic Reduction of CO2: Theory, Practice and Challenges

“…In addition, if there is not enough supply of CO 2 via convection, the concentration of CO 2 near the electrode surface decreases considerably at high current densities, which leads to an additional concentration overpotential. Figure , shows the concentration overpotentials due to H + ( ) and CO 2 ( ), as well as the total overpotential, due to the consumption of both H + and CO 2 at the electrode surface, as a function of both the electrode potential (Figure a) and the current density (Figure b) . In an unstirred electrolyte, the sum of the concentration overpotentials is around 0.17 V at a potential of −0.8 V vs RHE.…”

“…This makes the ATR configuration particularly well suited for the in-situ detection of adsorbates at electrode surfaces with minimum interference from the bulk electrolyte [40] However, any species present within the probed distance in sufficiently high concentration and/or with a large enough absorption coefficient can be detected. For example, formic acid in the electrolyte above the electrode can be detected using ATR-SEIRAS at concentrations well below 0.1 M, [49][50][51] and ATR-SEIRAS has been used to determine the local pH during the electroreduction of CO 2 at the electrode-electrolyte interface by probing the ratio between the concentrations of either CO 2 and HCO 3 À [52] or HCO 3 À and CO 3 2À [53] in the electrolyte just above the electrode.…”

“…Concentration overpotentials caused by the production of hydroxide and depletion of CO 2 and protons near the electrode surface in CO 2 -saturated 0.5 M NaHCO 3 as a function of the electrode potential (a) and the current density (b) without stirring. Reproduced with permission from Ref [53]…”

Electrochemical CO2 Reduction on Nanostructured Metal Electrodes: Fact or Defect?

“…Dunwell et al they measured the local pH near a chemically-deposited thin Au film during CO 2 electroreduction by using in-situ surface enhanced infrared spectroscopy (SEIRAS). 28 The chemically deposited thin Au film (<100 nm)…”

“…1-D and 3-D modelling results for nanostructured electrodes are schematically depicted in Figure 3a and 3b, in the light of other experimental and theoretical studies. 13,14,27,28,35,45 It is important to note that this is an idealization of the real system; in reality, the electrocatalysts most likely have a much more complex and heterogeneous concentration, activity and selectivity gradient along the catalyst layer. Figure 4a illustrates a pH and CO 2 concentration gradient along a porous catalyst layer, where the former is usually more dramatic due to the smaller diffusion coefficients of buffer anions.…”