A Polymer-Derived Co(Fe)Ox Oxygen Evolution Catalyst Benefiting from the Oxidative Dehydrogenative Coupling of Cobalt Porphyrins

Thin films of cobalt porphyrin conjugated polymers bearing different substituents are prepared by oxidative chemical vapor deposition (oCVD) and investigated as heterogeneous electrocatalysts for the oxygen evolution reaction (OER). Interestingly, the electrocatalytic activity originates from polymer-derived, highly transparent Co(Fe)Ox species formed under operational alkaline conditions. Structural, compositional, electrical, and electrochemical characterizations reveal that the newly formed active catalyst greatly benefited from both the polymeric conformation of the porphyrin-based thin film and the inclusion of the iron-based species originating from the oCVD reaction. High-resolution mass spectrometry analyses combined with density functional theory (DFT) calculations showed that a close relationship exists between the porphyrin substituent, the extension of the π-conjugated system cobalt porphyrin conjugated polymer, and the dynamics of the polymer conversion leading to catalytically active Co(Fe)Ox species. This work evidences the precatalytic role of cobalt porphyrin conjugated polymers and uncovers the benefit of extended π-conjugation of the molecular matrix and iron inclusion on the formation and performance of the true active catalyst.


Oxidative Chemical Vapor Deposition (oCVD)
The oCVD reaction was performed in a custom-built oCVD reactor, represented in Scheme S1, and described elsewhere [1][2][3][4] .5,15 di-substituted cobalt(II) porphyrins with different substituents (Scheme 1 in the main manuscript), were obtained from PorphyChem (98%) and used without further purification.Based on previous reports, 1,3 iron(III) chloride (97%, Sigma-Aldrich) was chosen as the oxidant.Table S1 summarizes the deposition conditions used for each porphyrin investigated.The temperature used to sublime the oxidant was 170 °C in all cases.Alternatively, copper(II) chloride (anhydrous, powder, ≥99.995% trace metals basis, Sigma-Aldrich) was used as oxidant for control experiments, with a sublimation temperature of 300 °C.
Glass microscope slides (Menzel-Gläser Superfrost®), interdigitated chips (OFET Gen4, Fraunhofer) and fluoride-doped tin oxide coated glass (FTO TEC 15, Ossila) were used as substrates.Prior deposition, all the substrates were cleaned with absolute ethanol (99.98%,VWR chemicals®) and dried with nitrogen gas.The substrate holder was kept at 150°C for all the depositions.The pressure inside the oCVD reactor was kept at 10 -3 mbar, under Argon (99.999 %, Air Liquide) atmosphere.The deposition time was set to 30 minutes for all experiments.Additionally, reference sublimed porphyrin monomers thin films were obtained under the same conditions, without supplying any oxidant.All the oCVD samples (asprepared and after electrochemical characterizations) were stored under vacuum conditions until their subsequent analysis.

Thin films characterization
The ultraviolet-visible-near infrared (UV-Vis-NIR) spectra of the sublimed and oCVD films deposited on glass slides were recorded with in a Perkin-Elmer Lambda 1050 spectrometer, in the transmission (T) mode, in the 300-2500 nm wavelength interval.The absorbance (A) was calculated as: A= -log(T).The as-deposited thin films were further rinsed with dichloromethane (anhydrous, ≥99.8%,Sigma-Aldrich) and the solvent-soluble fraction was measured in a transparent quartz cuvettes of 3.5 mL capacity and 1 cm of light path.The rinsed glass slides were also analyzed for comparison with the as-deposited films.
Electron Microscopy and SIMS imaging were performed with Helium-Ion Microscopy (Nanofab, Zeiss, Peabody) coupled to Secondary Ion Mass Spectrometer developed at LIST (HIM-SIMS). 5Measurements were performed using an 25 keV He + beam (0.5 pA) and 25 keV Ne + beam (8 pA) for secondary electron (SE) and SIMS analysis respectively.The secondary ions were detected, simultaneous for each polarity, in the multicollection system.On the same region of interest, the positive ions were 39 K, 56 Fe, 59 Co and 120 Sn.In negative mode the ions recorded were 16 O, 12 C, 35 Cl and the cluster 12 C 14 N. EM images were acquired in a matrix of 1024x1024 pixels and a field of view of 10x10µm², with a counting time of 10µs/pixel repeated 4 time per line (line average).SIMS images were acquired at a size of 10 x 10 μm² in a matrix of 512x512 pixels (20 nm/pixel) and a counting time of 2 ms/ pixel (~8 min of acquisition per polarity).
X-ray photoelectron spectroscopy (XPS) measurements were performed a Nexsa -G2 instrument (ThermoFisher Scientific, UK) using a monochromatic Al Kα X-ray source (E = 1486.6eV) and a 400 μm spot size.The binding energy of the spectra was referenced by fixing the carbon (C 1s) to 285.0 eV.
Raman spectra was recorded at room temperature with an inVia Raman Microscope (RENISHAW), using a 633 nm laser excitation.
Laser desorption/ionization high-resolution mass spectrometry (LDI-HRMS) measurements were performed using an AP-MALDI UHR ion source (MassTech, Inc.) coupled to an LTQ/Orbitrap Elite (ThermoScientific).In-source fragmentation (E = 70 V) was used to prevent the formation of clusters.The measurements were performed on Si wafer substrates coated either with the sublimed porphyrin monomer and oCVD film, which were directly placed on the sample holder.][4] The lateral conductivity of the thin films was assessed by 2-point current-voltage scans, using a microprobe station (Cascade Microtech, PM8) and a Keithley (2401) source-meter.The data were recorded by sweeping the voltage from -4 V to 4 V and back (hysteresis scan) at a scan rate of 500 mV s -1 , and the geometry of the channel was 2.5 μm (length, ), 10 mm (width, ) and 40 nm (height, ℎ).The lateral conductivity was evaluated from a linear fit and the channel geometry as: [  −1

Electrochemical characterizations
All the electrochemical measurements were performed using an AUTOLAB potentiostat/galvanostat, in a three-electrode cell consisting in an Ag/AgCl (3 M KCl) reference electrode, a Pt wire as a counter electrode and the fused-metalloporphyrin coating on FTO substrate as working electrode.A 1 M potassium hydroxide (ACS reagent, ≥85%, pellets, Sigma-Aldrich) solution of pH 13.6 was used as electrolyte.Cyclic voltammetry (CV) measurements were recorded at a scan rate of 50 mV s -1 with 5 mV step, and linear sweep voltammetry (LSV) measurements were performed at 10 mV s -1 scan rate.All the potentials were referred to the Reversible Hydrogen Electrode (RHE) through the Nernst equation:   =  / +  / 0 + 0.059 × .
Gas evolution measurements were performed in a sealed cell coupled to an Agilent microgas chromatograph (GC).Prior to GC measurements, the electrolyte (1M KOH) was purged with argon, and a constant inlet argon flow of 12 mL min -1 was kept in the sealed cell during the experiments.The oxygen evolution was monitored every 5 min during a chronoamperometric measurement performed at 1.6 V vs RHE applied potential.The faradaic efficiency (FE) for gas evolution at the electrode surface was estimated from the ratio between the experimental and the theoretical evolved gas amount calculated with the Faraday´s Law: , where  is the recorded current by chronoamperometry,  the time,  is the number of transferred electrons, and  is the Faraday constant (96 485 C mol -1 ).

Density functional theory calculations.
7][8] The DFT calculations were performed using the BP86 9, 10 functional with Karlsruhe valence triple-zeta basis set "def2-TZVP" [11][12][13] and Weigend's auxiliary basis set 14 .Dispersion effects were considered by Grimme approximation 'D3'. 15,16 o simply speed up the iteration, RIJCOSX approximation is included. 17,18 he optimized geometries were confirmed attaining local minima by confirming absence of negative frequencies after numerical frequency analysis.Docking experiments were performed using Hex 8.0 docking software.Scheme S1.Schematic of the oCVD reactor used for the preparation of the porphyrin-based thin films from the gas-phase.

Figure S2 .
Figure S2.CV of the reference sCoDPP electrode before (black) and after EC-aging (red).

Figure S6 .
Figure S6.(a) Chronoamperometry recorded during the gas evolving measurements on the EC-aged pCoDPP electrode, performed at 1.6 V vs RHE applied potential in 1M KOH.(b) Calculated oxygen evolution rate and faradaic efficiency.

Figure S7 .
Figure S7.XPS spectra on the Fe 2p core region of the as-prepared pCoDPP (using FeCl3 as the oxidant), after EC-ageing and rinsed with acetone.

Figure S8 .
Figure S8.XPS spectra on the Cu 2p core region of the as-prepared pCoDPP (using CuCl2 as the oxidant) and after EC-ageing.

Figure S9 .
Figure S9.XPS spectra on the a) Co 2p, b) Fe 2p and c) N 1s regions of the pCo-4-BrDPP and pCoDPFPP thin films as prepared and after EC-aging.

Figure S10 .
Figure S10.(a), (c) and (e) Digital pictures of the sublimed reference (left side) and oCVD (right side) coatings as deposited, and after rinsing a section of the films with dichloromethane (DCM).The DCM-rinsed area is marked inside a dash-lined box.(b), (d) and (f) Absorbance spectra of the sublimed porphyrin monomers (s), and the oCVD polymer films (p) as deposited and after rinsing with DCM.

Figure S12 .
Figure S12.XPS spectra on the valence band region of the reference (sublimed) (black dots) and oCVD (red dots) coatings.

Figure S13 .Figure S14 .
Figure S13.Lateral conductivity of the oCVD thin films coated on OFET substrates, calculated from the 2-points probe method (length of the chip= 2.5 µm)

Figure S15 .
Figure S15.Water docking distance calculations showing the interaction of water molecule with the optimized a-c) monomers and d-f) triply fused dimers, considering intramolecular cyclization.

Figure S16 .
Figure S16.Water docking distance calculations showing the interaction of water molecule with the optimized a-b) monomers, c-d) doubly and e-f) triply fused dimers, without considering intramolecular cyclization.

Table S1 .
Deposition conditions for the chemical vapor deposition of the porphyrin thin films.

Table S2 .
Elemental composition from XPS analysis of fresh and EC-aged coatings.