In Situ Probing of CO2 Reduction on Cu‐Phthalocyanine‐Derived CuxO Complex

Abstract An in situ measurement of a CO2 reduction reaction (CO2RR) in Cu‐phthalocyanine (CuPC) molecules adsorbed on an Au(111) surface is performed using electrochemical scanning tunneling microscopy. One intriguing phenomenon monitored in situ during CO2RR is that a well‐ordered CuPC adlayer is formed into an unsuspected nanocluster via molecular restructuring. At an electrode potential of −0.7 V versus Ag/AgCl, the Au surface is covered mainly with the clusters, showing restructuring‐induced CO2RR catalytic activity. Using a measurement of X‐ray photoelectron spectroscopy, it is revealed that the nanocluster represents a Cu complex with its formation mechanism. This work provides an in situ observation of the restructuring of the electrocatalyst to understand the surface‐reactive correlations and suggests the CO2RR catalyst works at a relatively low potential using the CuPC‐derived Cu nanoclusters as active species.

Electrochemical scanning tunneling microscopy (EC-STM) measurements: All EC-STM experiments were performed in CO2-saturated 0.1 M KHCO3 electrolyte using RHK-STM (ATM300) and Autolab bipotentiostat (PGSTAT302N).We used the insulation material (Apiezon wax)-coated Pt-Ir tip in EC-STM measurement to minimize the influence of residual faradaic current.In the EC-STM cell, a working electrode was prepared by immersing the flame-annealed Au(111) electrode into the immersion solution.The prepared electrode obtained a highly-ordered CuPC adlayer on Au(111) surface.Pt wire was set up as reference and counter electrodes in the EC-STM cell.

Cyclic voltammetry (CV) measurements:
We carried out CV measurements in a three-compartment electrochemical cell with N2 atmosphere using Autolab potentiostat (PGSTAT302N).We prepared working electrodes in the same way as the case of EC-STM measurements.Ag/AgCl (3 M KCl) and Pt mesh were utilized as reference and counter electrodes, respectively.
X-ray photoelectron spectroscopy (XPS) measurements: Binding energies and elements were analyzed using XPS (Thermo VG Scientific, K-alpha) with Al K-alpha X-ray source (1486.7 eV).We prepared the Cu-modified Au(111) electrodes before and after the application of potential in CO2saturated 0.1 M KHCO3 electrolyte.The prepared electrodes were loaded into XPS chamber while minimizing the contamination by air.The spectra presented in this work were calibrated against versus the Au 4f7/2 core level peak to 84.0 eV.

Figure S2 .
Figure S2.Coverage and number of cluster as a function of applied electrode potential.At E = -0.7 V vs Ag/AgCl, the coverage and number of clusters greatly increased.

Figure S3 .
Figure S3.EC-STM images of NiPC/Au(111) system in CO2-saturated 0.1 M KHCO3 electrolyte at OCV and E = -0.52V vs Ag/AgCl.Regardless of the application of electrode potential, the NiPC adlayer exhibited a well-ordered configuration.

Figure S5 .
Figure S5.STM images of CuPC adlayer on Au(111) (a) before and (b, c) after electrochemical reaction.The STM images in Figure S5b and S5c were acquired by scanning the sample after the application of a potential of -0.7 V vs Ag/AgCl in CO2-and Ar-saturated 0.1 M KHCO3 electrolyte, respectively.

Figure S6 .
Figure S6.CV curves of H2PC-modified Au(111) electrode in CO2-saturated 0.1 M KHCO3 electrolyte and CuPC-modified Au(111) electrode in Ar-saturated 0.1 M KHCO3 electrolyte.The dotted arrow indicates scan direction, and the scan rate is 0.03 V/s.In the measured CV results, we did not find a reduction peak.

Figure S7 .
Figure S7.EC-STM images of CuPC/Au(111) system in CO2-saturated 0.1 M KHCO3 electrolyte at different electrode potentials.The EC-STM image at OCV was obtained after the application of an electrode potential of -0.8 V vs Ag/AgCl.

Figure S8 .
Figure S8.XPS spectrum of N 1s core level on H2PC-modified Au(111) electrode.The Nim atom indicate the iminic N atom of pyrrolic ring in an isoindole group of H2PC molecule.