Potential Dependence of the Local pH in a CO₂ Reduction Electrolyzer

Abstract

Quantifying the local pH of a gas diffusion electrode undergoing CO2 reduction is a complicated problem owing to a multitude of competing processes, both electrochemical- and transport-related, possibly affecting the pH at the surface. Here, we present surface-enhanced Raman spectroscopy (SERS) and electrochemical data evaluating the local pH of Cu in an alkaline flow electrolyzer for CO2 reduction. The local pH is evaluated by using the ratio of the SERS signals for HCO3 – and CO3 2–. We find that the local pH is both substantially lower than expected from the bulk electrolyte pH and exhibits dependence on applied potential. Analysis of SERS data reveals that the decrease in pH is associated with the formation of malachite [Cu2(OH)2CO3, malachite] due to the presence of soluble Cu­(II) species from the initially oxidized electrode surface. After this initial layer of malachite is depleted, the local pH maintains a value >11 even at currents exceeding −20 mA/cm2.

Absence of CO2 electroreduction on copper, gold and silver electrodes without metal cations in solution

“…At more cathodic potentials, a small amount of metal oxide dissolution was observed and the traces of Ag + and Cu 2+ ions released into the electrolyte led to the production of CO in 1 mM H 2 SO 4 (Supplementary Discussion and Supplementary Fig. 7) 36 . This happens as during the SECM approach, the electrode surface is exposed to air, which can lead to the formation of (a very small amount) of oxide on the surface.…”

Electrochemical CO2 reduction toward multicarbon alcohols - The microscopic world of catalysts & process conditions

“…Figure 9 shows the FEs of EtOH and H 2 as a function of the selected electrolyte, with an increase observed for ethanol in the order K 2 HPO 4 < KHCO 3 < KClO 4 < KCl. Studies on an electrode with electrodeposited copper showed that the local pH at the oxidized copper electrode decreases from 10.4 to 9.3 with increasing negative applied potential ranging between −0.4 and −1.2 V and using 1 M KOH ( Henckel et al., 2021 ). The decrease in pH is due to the formation of HCO 3 − , while at the same time, malachite is formed at the electrode at the beginning of the reduction of the copper oxide.…”

Quantifying Local pH Changes in Carbonate Electrolyte during Copper-Catalysed CO2 Electroreduction Using In Operando 13C NMR

“…Measuring local concentrations of protons and molecules that take part in electrochemical reactions has been attempted, for example, using scanning probe techniques and various optical methods. 20 Recent studies investigating CO 2 RR apply Scanning Electrochemical Microscopy (SECM) 21 , Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS) 22,23 and Surface-Enhanced Raman Spectroscopy (SERS) 24 . SECM offers direct measurement of the proton concentration by using the pH-sensitive electrochemical response of an inert metallic material.…”

“…In this study, the measurement of pH is demonstrated in an electrochemical cell and during CO 2 electrolysis by in operando 13 C Nuclear Magnetic Resonance (NMR), which has proven capable of investigating the CO 2 (aq)/HCO -3 /CO 2 -3 equilibrium in such an environment 25 . In contrast to methods that only consider the CO 2 (aq)/HCO -3 22 or the HCO -3 /CO 2 -3 equilibirum 24 , the presented method provides data for a wide pH range by considering both equilibria, including an overlap region inbetween. The sensitive volume of in operando NMR measurements is not limited to a few nanometres from the electrode like surface-enhanced optical methods and can provide spatial resolution when magnetic resonance imaging techniques are applied.…”