Tuning the Interfacial Reaction Environment for CO2 Electroreduction to CO in Mildly Acidic Media

A considerable carbon loss of CO2 electroreduction in neutral and alkaline media severely limits its industrial viability as a result of the homogeneous reaction of CO2 and OH– under interfacial alkalinity. Here, to mitigate homogeneous reactions, we conducted CO2 electroreduction in mildly acidic media. By modulating the interfacial reaction environment via multiple electrolyte effects, the parasitic hydrogen evolution reaction is suppressed, leading to a faradaic efficiency of over 80% for CO on the planar Au electrode. Using the rotating ring-disk electrode technique, the Au ring constitutes an in situ CO collector and pH sensor, enabling the recording of the Faradaic efficiency and monitoring of interfacial reaction environment while CO2 reduction takes place on the Au disk. The dominant branch of hydrogen evolution reaction switches from the proton reduction to the water reduction as the interfacial environment changes from acidic to alkaline. By comparison, CO2 reduction starts within the proton reduction region as the interfacial environment approaches near-neutral conditions. Thereafter, proton reduction decays, while CO2 reduction takes place, as the protons are increasingly consumed by the OH– electrogenerated from CO2 reduction. CO2 reduction reaches its maximum Faradaic efficiency just before water reduction initiates. Slowing the mass transport lowers the proton reduction current, while CO2 reduction is hardly influenced. In contrast, appropriate protic anion, e.g., HSO4– in our case, and weakly hydrated cations, e.g., K+, accelerate CO2 reduction, with the former providing extra proton flux but higher local pH, and the latter stabilizing the *CO2– intermediate.


Characterization of the Au disk and ring electrode
The Au disk and ring electrode were characterized by cyclic voltammetry in 0.1 M H 2 SO 4 respectively, to inspect possible variation of the electrode surface due to the interfacial pH measurements and the dopamine coating.As shown in Figure S1a-b, the CVs before and after dopamine coating on Au ring and disk electrode agree well with each other, signifying that dopamine residue has been totally removed and this coating process hardly influences the surface of Au ring and disk electrode.Figure s1c demonstrates that no discernable changes was traced on the surface of the Au disk during interfacial pH and FE measurements.As the peaks observed in the CVs signify the interplay between Au surface atoms, OH ads and O ads as well as the formation of Au oxide, they are surface sensitive.Each facet exhibits unique peak signatures.Since there is no observable deviation in the CVs, we conclude that the surface structure remains unchanged.

Calculation of the interfacial pH on the disk
Peak potentials of the 4-NSTP/4-HATP redox couple during CO2RR on the Au disk was recorded.
According to the calibration curve of the redox couple (Figure S2a), the interfacial pH on the ring electrode can be obtained.To calculate the interfacial pH on the disk electrode, the relationship between pH on the ring and pH on the disk needs to be figured out.According to the convective-diffusion equation of RRDE, the ratio of the average concentration of products on the ring and the disk is defined to be the detection efficiency N D as displayed in Eq. s2, 1 where μ is the normalized concentration of products (Eq.s1), is the bulk concentration (The subscript "d" and "r" symbolize the ring and disk electrode, C ∞ respectively).N D is determined by the geometry of the electrode (Eqs.s3-4), in which r 1 , r 2 and r 3 are the radii of the disk, the inner ring and the outer ring, respectively. 2In this work, the N D of the RRDE tip employed here (r 1 = 5.0 mm, r 2 = 6.5 mm, r 3 = 7.5 mm) is 0.23.
Besides, the homogenous reactions involving protons or hydroxyl ions also impact the pH gradient between ring and disk electrode.In CO 2 -saturated perchlorate, when the interfacial environment turns alkaline, OH -generated on the disk is partially consumed by either CO 2 or HCO 3 -on its way to the ring.Also, in CO 2 -saturated electrolyte with protic anions, such as HSO 4 2− H 2 PO 4 − in this work, the OH - generated by CO2R is consumed collectively by the protic anions and CO 2 and HCO 3 -.To avoid an underestimation of the interfacial pH on the disk, influences from the homogeneous buffering reactions (Eqs 2 and 4) need to be compensated. 3For CO 2 -saturated perchlorate, the normalized concentration μ here was redefined as Eq.s5 and the equation for N D was also modified (Eq.s6).As CO 2 is continuously purged into the electrolyte during the measurements, the total carbon concentration (TC, see Eq. s7), which is the sum of the concentration of the intrinsic bicarbonate electrolyte and the saturated CO 2 concentration from extrinsic bubbling, is supposed to be constant throughout the measurements.Hence the TC for 0.1 M perchlorate under continuous CO 2 bubbling is 0.135.Concentrations of the different carbonaceous species were estimated as a function of pH (Eqs.s8-s10) by combining dissociation equilibrium constants of CO 2 (pKc 1 = 6.35, pKc 2 = 10.33). 4 As a result, the interfacial pH of the disk electrode was derived from Eqs. S6-s10.The theoretical correlation between the pH on the ring and disk in 0.1 M Sodium perchlorate is plotted in Figure S2b.
For CO 2 -saturated sulfate, buffering from sulfuric species and carbonate species is compensated, the normalized concentration μ here was redefined as Eq.s11 and the equation for N D was also modified (Eq.s12).TS is defined as the total concentration of sulfuric species (Eq.s13), and the concentrations of each sulfuric species were estimated as a function of pH (Eqs.s14-s15) by combining dissociation equilibrium constants of HSO 4 − (pK S1 = 1.99).As a result, the interfacial pH of the disk electrode was derived from Eqs. s12-s15.The theoretical correlation between the pH on the ring and disk in sodium sulfate with different SO 4 − concentration is plotted in Figure S2c.[HSO Similarly, for CO 2 -saturated phosphate, buffering from phosphate species, carbonate species is compensated, the normalized concentration μ here was redefined as Eq.s16 and the equation for N D was also modified (Eq.s17).TP is defined as the total concentration of sulfuric species (Eq.s18), and the concentrations of each sulfuric species were estimated as a function of pH (Eqs.s19-s21) by combining dissociation equilibrium constants of H 2 PO 4 − (pK P1 = 2.14 pK P2 = 7.20, pK P3 = 12.37).Specially, As the measurements in phosphate is performed at bulk pH 4, H 3 PO 4 is ignored because of its extremely small concentration to simplify the calculation.The interfacial pH of the disk electrode was derived from Eqs. S18-s21.The theoretical correlation between the pH on the ring and disk in 0.1 M phosphate is plotted in Figure S2d.

Ring pH data
Here we show the ring pH data corresponding to Figures 1a, 2a, 3c, 4a, 5b and 6b.The ring pH is markedly dependent on the electrolyte conditions, in agreement with the disk pH data.Noteworthy, as suggested in Figure S3a-b, the ring pH in the mass transport limited region in Na 2 SO 4 is higher than that in NaClO 4 (and as a result, the corresponding disk pH is also higher in Na 2 SO 4 ), illustrating the great influence of the current density on corresponding reaction environment during CO2RR.The limiting current density in Na 2 SO 4 is higher due to the extra proton flux donated by HSO 4 − , leading to a less acidic reaction environment.Once the ring pH reaches around 6, as shown by the calibration curves in Figure S2, the disk pH rises quickly up to 13, due to the lack of a proper buffer anion and the weak buffer strength of the carbonaceous buffer in the electrolytes.

Calculation of Faradaic Efficiency of CO
As a CO-producing metal, Au is also a good catalyst for CO oxidation. 5Hence the Au ring electrode can easily deconvolute CO from other products on the disk electrode, since H 2 and CO are the only two products generated on the Au planar electrode.The partial current density for CO2RR on the Au disk can be calculated from the experimental ring current (i ring ) and the detect efficiency N of the Au ring (Eq.s22).The partial current density and Faradaic efficiency of HER can be determined from Eq. s23-24 (s24) As mentioned in the experimental section, after finishing FE and interfacial pH measurements, the collection efficiency N was measured in 5 mM K 3 Fe(CN) 6 dissolved in 0.1 M NaHCO 3 , during which the disk was cycled from 0.27 to 1.27 V vs RHE, while the ring potential was set to 0.96 V vs RHE.The collection efficiency was determined for each rotation rate and was calculated according to Eq. s26.

Figure S1 .
Figure S1.Cyclic voltammograms of a) the Au ring and b) the Au disk recorded in Ar-saturated 0.1 M H 2 SO 4 at 100 mV s -1 before and after the dopamine coating.c) Cyclic voltammograms the Au ring and the Au disk recorded in Ar-saturated 0.1 M H 2 SO 4 at 100 mV s -1 before the interfacial pH measurement and the FE measurement.

Figure S3 .
Figure S3.The ring pH data recorded during measurements in a) 0.1 M CO 2 -saturated NaClO 4 with a bulk pH of 3, corresponding to the disk pH data in Figure1a, b) 0.1 M CO 2 -saturated Na 2 SO 4 with a bulk pH of 3, corresponding to the disk pH data in Figure2a, c) electrolytes with 200 mM Na + and different SO 4 2− concentrations with a bulk pH of 3, corresponding to the disk pH data in Figure3c, d) 0.1 M CO 2 -saturated NaH 2 PO 4 with a bulk pH of 4, corresponding to the disk pH data in Figure4a, e) 0.1 M CO 2 -saturated sulfate with different cation identity with a bulk pH of 3 corresponding to the disk pH data in Figure5b, f) 0.1 M CO 2 -saturated Na 2 SO 4 with a bulk pH of 3 with different rotation rates, corresponding to the disk pH data in Figure6b.The scan rate is 2 mV s -1 , and the rotation rate is 2500 rpm unless specially mentioned.
The Faradaic efficiency can be calculated by Eq. s23, where the i disk is the current obtained on the disk during experiments.(s23)   =    * |  | * 100% To confirm the stability of the Au ring CO sensor, FE measurements during chronoamperometry were conducted in 0.1 M CO 2 -saturated Na 2 SO 4 , where CO2RR on the disk electrode is turned ''on'' and ''off'' at different potentials from − 0.3 V to − 1.4 V vs RHE every 200s.The rotation rate of RRDE is 2500 rpm.As depicted in FigureS4, the results agree well with the ones during cyclic voltammetry in Figure2.FE of CO2RR increases up to ~60% along the mass transport-limited region (from − 0.6V ~ −1.2 V), and starts to decay once the water reduction sets in.During steady-state electrolysis, the ring current stays stable.

Figure S4. a )
Figure S4.a) Current density on the disk (black curve) and ring (black curve) and b) FE of CO during chronoamperometry in 0.1 M CO 2 -saturated Na 2 SO 4 (pH=3).