pH Matters When Reducing CO₂ in an Electrochemical Flow Cell

Abstract

The pH at the electrocatalyst surface plays a key role in defining the activity and selectivity of the CO 2 reduction reaction (CO 2 RR). We report here operando Raman measurements of the catalyst surface in a customized CO 2 RR flow cell that enable the measure of pH. Using this flow cell, we were able to measure surface pH as a function of time, current density, and proximity to the catalyst surface during the electrolysis of bicarbonate solutions. We observed that increasing the current density from 0 to 200 mA cm −2 increased the surface pH from 8.5 to 10.3. We also show here that operation at elevated temperatures (70 °C) results in an increased surface pH and serves to suppress the competing and undesirable hydrogen evolution reaction.

Promoting CO2 methanation via ligand-stabilized metal oxide clusters as hydrogen-donating motifs

“…We noted that stronger-binding organic ligands on Co(II) did not prevent Co metal formation under negative cathodic potentials in prior literature reports 42 ; however, in these prior studies, the electrolyte was acidic phosphate buffer (pH 2.2), and the cathodic reaction was hydrogen evolution, compared to the 1 M KHCO 3 solution (pH 8.9) studied in CO 2 reduction herein. The local pH is higher during electrochemical CO 2 reduction, particularly so at high current density (such as 225 mA cm −2 when we obtained the best CH 4 FE) 43 . The stability of Co δ+ based catalysts, including Co δ+ complexes 44 and ultra-small/thin cobalt oxides 45 , 46 , under negative potentials for electrocatalytic CO 2 reduction, has previously been reported.…”

Electrolytic Conversion of Bicarbonate Solutions to CO at >500 mA cm-2 and 2.2 V

“…We sought to bypass these challenges by developing a CO2RR electrolyzer that uses a liquid bicarbonate feed instead of gaseous CO 2 [23][24][25][26] . The efficient electrolysis of bicarbonate solutions (e.g., KHCO 3(aq) ), a common eluant of carbon capture units, can form the same CO2RR products as an electrolyzer fed with gaseous CO 2 , but without the need for CO 2 regeneration and pressurization (Fig.…”

Boosting proton reduction via isolated Cu atoms in Bi lattice for efficient electroreduction of CO2 to formate

“…Notice that the pH near the catalyst surface would increase more signi cantly in the electrolyte with weaker buffer capacity. The FE formate increases as the buffer capacity decreases for both BiCu-0.5 and Bi, evidencing that a higher pH environment favours the formation of formate, which can be attributed to the lower sensitivity of proton concentration for CO 2 RR owing to its sluggish and complex kinetics 30 . Besides, the BiCu-0.5 delivers a substantially higher FE formate than Bi in the same electrolyte, and a more pronounced enhancement was achieved by BiCu-0.5 when NaHCO 3 was replaced by Na 2 SO 4 .…”

“…During the electrolysis, the pH near the cathode would arise owing to the rapid consumption of proton. To monitor the pH change during electrolysis, we conducted in situ Raman spectroscopy measurements, where the Raman signals of HCO 3 -(the peak at 1015 cm -1 ) and CO 3 2-(the peak at 1068 cm -1 ) can act as indicators to re ect the change in pH 29,30 . The spectra were recorded after 600 s electrolysis at each potential in CO 2 -saturated 0.5 M NaHCO 3 solution (Fig.…”