CO2 Electrolysis via Surface-Engineering Electrografted Pyridines on Silver Catalysts

The electrochemical reduction of carbon dioxide (CO2) to value-added materials has received considerable attention. Both bulk transition-metal catalysts and molecular catalysts affixed to conductive noncatalytic solid supports represent a promising approach toward the electroreduction of CO2. Here, we report a combined silver (Ag) and pyridine catalyst through a one-pot and irreversible electrografting process, which demonstrates the enhanced CO2 conversion versus individual counterparts. We find that by tailoring the pyridine carbon chain length, a 200 mV shift in the onset potential is obtainable compared to the bare silver electrode. A 10-fold activity enhancement at −0.7 V vs reversible hydrogen electrode (RHE) is then observed with demonstratable higher partial current densities for CO, indicating that a cocatalytic effect is attainable through the integration of the two different catalytic structures. We extended the performance to a flow cell operating at 150 mA/cm2, demonstrating the approach’s potential for substantial adaptation with various transition metals as supports and electrografted molecular cocatalysts.

. a) In situ synthesis of silver nanoparticles onto glassy carbon surface using AgNO3 (1 mM) and NaHCO3

Ag-Ar
Ag-CO     The homogeneous electrocatalytic activity of compounds Py-x (5 mM) was evaluated at both GCE and Ag working electrodes in 0.1 M KHCO3 after saturation first with Ar, then with CO2 ( Figure   S10 and S11). Under CO2-saturation, a distinct one-electron reduction wave was observed, which is attributed to the catalytic current of CO2 electroreduction and/or HER.   Comparison of the CV of Py-x ( Figure S12a) and Ag-Py-x ( Figure S12b) shows the catalytic activity of Py-3 is slightly greater than that of Py-1 and Py-2. The same trend was observed in the case of using silver electrode (Ag-Py-x) confirming the integral role of extra carbon chain in improving the catalytic efficiency of the pyridine species. Exposure of the catalysts to CO2 resulted a dramatic increase to the current density beginning at ~-0.8 V vs RHE with glassy carbon electrode. Replacing the glassy carbon working electrode with silver, a higher current density experienced with a noticeable negative shift at lower overpotential energy (~-0.7 V vs RHE), which could be due to the activity of Ag towards CO2RR.

Ag-Ar
Ag-CO 2 1-CO 2 To determine the optimal potential for electrochemical CO2 reduction, chronoamperometry studies of Py-x ( Figure S14) and Ag-Py-x ( Figure S15) were performed at -0.5, -0.6, -0.7, -0.8, -0.9 and -1.0 V vs RHE. Using the GCE working electrode, after purging the electrolyte with CO2, H2 was  Using the Ag working electrode, H2 and CO were achieved as the primary products and no liquid products were observed within our detection limits. Among the catalysts performed in Figure S16 and S17, Ag-Py-3 demonstrated the highest catalytic selectivity with FECO: 41%; and j: 1.7 mA/cm 2 at -0.8 V vs RHE, which is slightly higher than bare Ag (FECO: 36%, j: 1.1 mA/cm 2 ) at the same potential. There were no noticeable differences between catalyst 1 and 2 (FECO: ~38%, j: 1.7 mA/cm 2 ) at the same overpotential. The higher selectivity achieved by catalyst Py-3 could be due to having longer chain. This observation of an earlier onset potential for Ag compared to glassy carbon electrode is also in agreement with the better performance of the sliver electrode catalyst.  X 10 -6 X 10 -6 X 10 -5 X 10 -6 The electrochemically active surface area (ECSA) of compound 2 compared to 1 and 3 was calculated before and after electrografting with the pyridine complexes through Eq. 1: 4,5 Eq. 1: A = slope/(268600 × n 3/2 × D 1/2 × c) Where n is the number of electrons transferred in the redox reaction (n = 1), D is the diffusion coefficient of ferrocene probe (7 × 10 -6 cm 2 /s) and c is the concentration of ferrocene (2.5×10 -3 mol/cm 3 ).     DFT energies and calculated reaction energies are tabulated in Table S1 to S3. As depicted in

Qualitative enhancement in charge distribution of the surface
To investigate the effect of adding adsorbents to the surface, charge distributions of isolated pyridine molecules and Ag surface have been subtracted from that of the electrografted Ag-EPyx species. The results have been depicted in Figure 4b, and Figure S25 Figure 4b and Figure S25-S26.    Figure S34, Table S7). CO and formate products were observed at all applied potentials. Using bare Ag electrode, the highest FECO was observed initially at 10 and 25 mA/cm 2 ; however, as the current density increased, CO production decreased substantially from 75% at 10 mA/cm 2 to 30% at 150 mA/cm 2 (Figure 5b). The reverse trend was observed using the electrografted Ag-EPy-2. With the influence of the pyridine layer, H2 production was successfully suppressed from 55% in Ag to 33% in Ag-EPy-2 at 150 mA/cm 2 . Overall, the amount of CO produced was improved from 30% in Ag to 58% in Ag-EPy-2 at the same current density.
Considering the trade-off of increased current densities with the drawbacks of lower selectivity and electrode stability, 150 mA/cm 2 was determined to be the best for CO2RR in the case of Ag-EPy-2.        Figure S43. High Performance Liquid Chromatography (HPLC) chromatogram example of formic acid obtained during CO2 electroreduction.