Organic Thin Films Enable Retaining the Oxidation State of Copper Catalysts during CO2 Electroreduction

A key challenge in electrocatalysis remains controlling a catalyst’s structural, chemical, and electrical properties under reaction conditions. While organic coatings showed promise for enhancing the selectivity and stability of catalysts for CO2 electroreduction (CO2RR), their impact on the chemical state of underlying metal electrodes has remained unclear. In this study, we show that organic thin films on polycrystalline copper (Cu) enable retaining Cu+ species at reducing potentials down to −1.0 V vs RHE, as evidenced by operando Raman and quasi in situ X-ray photoelectron spectroscopy. In situ electrochemical atomic force microscopy revealed the integrity of the porous organic film and nearly unaltered Cu electrode morphology. While the pristine thin film enhances the CO2-to-ethylene conversion, the addition of organic modifiers into electrolytes gives rise to improved CO2RR performance stability. Our findings showcase hybrid metal–organic systems as a versatile approach to control, beyond morphology and local environment, the oxidation states of catalysts and energy conversion materials.


6)
Quasi in situ XPS spectra of pristine Cu before and after CO2RR at -1.0 VRHE in 0.1 M KHCO3 7) Quasi in situ XPS spectra of layer-Cu before and after CO2RR at -1.0 VRHE in 0.1 M KHCO3 8) Operando Raman spectra of pristine oxygen plasma treated copper foil 9) Disentangling the effects of bromide anions and phenanthrolinium cations from 1-Br2 electrolyte modifier on CO2RR performance 10) Determination of the carbon source for CO2RR 11) Electrochemical surface areas of pristine Cu and layer-Cu 12) CO2RR performance of examined catalytic systems in a wide potential range 13) Equivalent circuits for fitting potentiostatic electrochemical impedance spectra 14) CO2RR onset shift in the presence of the organic modifier in the electrolyte 15) Quasi in situ XPS spectra of layer-Cu after CO2RR at -1.0 VRHE in 1-Br2-modified 0.1 M KHCO3 16) Enhanced CO2RR performance stability of layer-Cu in 1-Br2-modified 0.1 M KHCO3 17) Nyquist plots of all examined catalytic systems at various potentials 18) Quasi in situ XPS spectra of layer-Cu after 3h of CO2RR at -1.0 VRHE in 0.1 M KHCO3

1) Synthesis of the 1-Br2 organic modifier
The synthetic protocol of N, N'-ethylene-phenanthrolinium dibromide (1-Br2) was adapted from a prior work. 1 Specifically, 1.5 g of 1,10-phenanthronline (Merck, ≥99%) and 15 mL of dibromoethane (Sigma Aldrich, ≥98%) were added in a round flask, then stirred for 18 h at 110 °C.After washing with hexane and acetone each for 3 times and centrifuging at 9000 rpm for 10 min, a yellow product was obtained and kept in oven at 50 °C to dry out the solvent.The high purity of as-sythesized 1-Br2 modifier was confirmed by nuclear magnetic resonance spectroscopy (NMR), see Figure S1 below.

2) Electrodeposition of the organic thin film on Cu electrodes
The polycrystalline Cu electrode was functionalized by performing linear sweep voltammetry in 10 mM 1-Br2 containing bicarbonate solution.The solid red curve presents the first LSV acquired in the modifiercontaining electrolyte, the broad peaks are ascribed to the layer formation, which is absent in the modifierfree electrolyte (black curve).However, the second sweep in the presence of modifiers appears to be smooth, indicating that the deposited layer is non-conductive and its growing process is self-limiting. 2 In the previous work by Peter and Agapie 1 , PEIS cycles were actively-performed at OCP, inducing restructuring of the underlying Cu surface and cube-formation, prior to performing CO2RR during which the thin film was formed in situ.In our work, we directly performed LSV to both accelerate the formation of the organic thin film and prevent Cu electrode reconstruction.Upon thin film preparation, the sample was rinsed with DI water and dried, prior to performing CO2RR experiments in separate electrolyte environment.Scheme S1.The electrochemical reduction of 1-Br2 to its dimer and oligomer.

3) Ex situ AFM characterization: thickness of the as-prepared layer
Figure S3a is the AFM images of a pristine Cu, featuring a lower surface roughness (RMS roughness: 0.23) than that of layer-Cu (Figure 1a, RMS roughness: 0.76).To measure the thickness of the layer, contact mode is applied for scratching the organic film in an area of 500×500 nm, then tapping mode is applied for imaging the scratched area in a larger scale of 2.0×2.0 μm, as shown in Figure S3b. Figure S3c shows the height profile of the horizontal line in Figure S3b, the flat bottom of the valley indicates the Cu substrate exposed after the scratching procedure, surrounded by piled layer residues.The as-deposited layer thickness of ca.
12 nm is determined by the height difference between the scratched and unscratched area.

17) Nyquist plots of all examined catalytic systems at various potentials
For each catalytic systems, PEIS was measured at varied potentials.Generally, the semicircles attenuate as the potential steps cathodically, which indicates decreased resistance.It is noted that the overall resistance decreases sharply from -0.9 V to -0.1V, consistent with the cathodically shifted onset potential in the ligandcontaining electrolyte (Figure S11), suggesting additional energy input is required for the reaction in the presence of the ligand in the electrolyte.

Figure S2 .
Figure S2.Electrodeposition of the organic film on the polycrystalline Cu electrode.(a) Linear sweep voltammetry recorded on a Cu foil in 0.1 M KHCO3 without and with 10 mM 1-Br2.

Figure S3 .
Figure S3.Morphology of pristine Cu and thickness of the as-deposited layer on Cu.Ex situ AFM images of (a) pristine Cu; (b) layer-Cu with the organic layer being stratched using contact mode with deflection setpoint of 2.0 V; (c) height profile of the horizontal line in (b).All the AFM images are processed with WSxM software.3

Figure S5 :
Figure S5: Contact angle measurements with a droplet of 0.1 M KHCO3 aqueous electrolyte on the asprepared (a) pristine Cu and (b) layer-Cu.

Table S2 .
Extrapolated parameters from the impedance spectra recorded on pristine Cu in 0.1 M KHCO3 by fitting with equivalent circuit I in FigureS10.

Table S3 .
Extrapolated parameters from the impedance spectra recorded on layer-functionalized Cu in 0.1 M KHCO3 by fitting with equivalent circuit II in FigureS10.

Table S4 .
Extrapolated parameters from the impedance spectra recorded on layer-functionalized Cu in ligand-containing 0.1 M KHCO3 by fitting with equivalent circuit III in FigureS10.