Covalent Organic Framework (COF) Derived Ni‐N‐C Catalysts for Electrochemical CO2 Reduction: Unraveling Fundamental Kinetic and Structural Parameters of the Active Sites

Abstract Electrochemical CO2 reduction is a potential approach to convert CO2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel‐nitrogen‐doped carbon (Ni‐N‐C) is an efficient catalyst for CO2 reduction to CO, and the single‐site Ni−N x motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)‐derived Ni‐N‐C catalysts, for which the Ni−N x content could be adjusted by the pyrolysis temperature. The combination of high‐angle annular dark‐field scanning transmission electron microscopy and extended X‐ray absorption fine structure evidenced the presence of Ni single‐sites, and quantitative X‐ray photoemission addressed the relation between active site density and turnover frequency.


Synthesis of Tp
Tp was synthesized following our previous literature approach. [1] 15.1 g hexamethylenetetramine, 6.0 g phloroglucinol, and 90 mL trifluoroacetic acid were refluxed under N2 at 100 °C for 2.5 h. 150 mL of 3M HCl was added slowly and the solution was heated at 100 °C for another 1 h. After cooling down, the solution was filtered through Celite and extracted with 350 mL dichloromethane. Then, the solution was evaporated under reduced pressure to afford 2.4 g of an off-white powder. Purification was carried out by sublimation.

Synthesis of TpDt-COF
1,3,5-triformylphloroglucinol (90 mg, 0.42 mmol), 3,5-diamino-1,2,4-triazole (45 mg, 0.45 mmol) and 1.5 mL dioxane were put into a Pyrex tube and the mixture was sonicated for 5 min. 0.5 mL DMA and 1.5 mL mesitylene were added to the above solution and sonicated for 5 min. Following this, 0.5 mL of 6 M aqueous acetic acid was added. This mixture was sonicated for another 10 minutes to yield a homogenous dispersion. The tube was then flash frozen in a liquid nitrogen bath and degassed by three freeze-pumpthaw cycles. The tube was sealed and then placed in an oven at 120 °C for 3 days. After the mixture was cooled to room temperature, the reddish brown precipitate was collected and washed with hot DMF, methanol and acetone. The powder collected was then dried at 80 °C under vacuum for 12 hours to obtain a deep red colored powder (81%, isolated yield).

Synthesis of TpDt-Ni
100 mg TpDt-COF was dispersed in 20 mL of Ni(NO3)2 6H2O (2M) aqueous solution and sonicated for 30 min and stirred for 10 h. The impregnated TpDt-COF was then separated by vacuum filtration and washed with distilled water four times to remove weakly adsorbed ions. After drying at 80 o C, the TpDt-Ni was obtained.

Synthesis of pyrolyzed C-TpDt-Ni
The TpDt-Ni was placed in a tube furnace and annealed at 900 °C (800 and 1000 °C) for 2 h under N2 atmosphere (80 sccm) at a heating rate of 5.0 °C min -1 . The carbonized COF was dispersed in aqueous HCl (ca. 1 M) and stirred for 1 day. The leached sample was collected and washed with DI water until the pH value was close to neutral. The obtained materials were denoted as C-TpDt-Ni-T (where T represents the temperature).

Synthesis of C-TpPa-COF-900
Synthesis of TpPa-COF 1,3,5-triformylphloroglucinol (63 mg, 0.3 mmol), paraphenylenediamine (48 mg, 0.45 mmol), 1.5 mL of mesitylene, 1.5 mL of 1,4-dioxane, 0.5 mL of 3 M aqueous acetic acid were placed into a Pyrex tube and the mixture was sonicated for 20 min to obtain a homogenous dispersion. The tube was then flash frozen in a liquid nitrogen bath and degassed by three freeze-pump-thaw cycles. The tube was sealed off and then placed in an oven at 120 °C for 3 days. After the mixture was cooled to room temperature, the red precipitate was collected and washed with THF and acetone. The powder collected was then dried at 80 °C under vacuum for 12 hours.

Synthesis of C-TpPa-Ni-900
100 mg TpPa-COF was dispersed in 20 mL of Ni(NO3)2 6H2O (2M) aqueous solution and sonicated for 30 min and stirred for 10 h. The impregnated TpPa-COF was then separated by vacuum filtration and washed with distilled water four times to remove weakly adsorbed ions. After drying at 80 o C, the TpPa-Ni was obtained. The TpPa-Ni was then placed in tube furnace and annealed at 900 °C for 2 h under N2 atmosphere (80 sccm) at a heating rate of 5.0 °C min -1 . The carbonized COF was dispersed in aqueous HCl (ca. 1 M) and stirred for 1 day. The leached sample was collected and washed with DI water until a pH value close to neutral.

Synthesis of NiPc/CNT
NiPc/CNT was synthesized following the analogous protocol reported in the literature. [2] 1 mg NiPc was mixed with 30 mg MWCNTs (multi wall carbon nanotubes) in 30 mL DMF solution and kept stirred for 24 hours. The suspension color turned from the initial violet to transparent. The final suspension was dried to obtain the NiPc/CNT catalyst.

Synthesis of PANI-Ni-900
PANI-Ni-900 was synthesized following our previous literature approach. [3] 3 ml of aniline, 5 g NiCl2ꞏ6H2O and 5 g ammonium persulfate (APS, (NH4)2S2O8) was added to 0.5 L of 1 M HCl and stirred for 1 hour. Then, the suspension was stirred for 48 hours along with 0.4 g of dispersed activated Ketjen 600 carbon black support (washed in HCl for purification and HNO3 for oxygen doping). Afterwards, the suspension was dried at 95 °C for 24 hours. After drying, the solid mixture was ball-milled with ZrO2 balls for 20 min. The pyrolysis is carried in a furnace with a ramp of 30°C min -1 to 900 °C and kept at this temperature for 1 hour, in N2 condition, and followed by acid washing steps (2M H2SO4 at 90 °C for overnight) to remove the excess Ni particles. In our synthesis, 4 times heat treatment (HT) and 3 times acid washing was performed by turn, and the catalyst is obtained after the 4 th pyrolysis. Aberration-corrected STEM images were recorded by using a high-angle annular dark-field (HAADF) detector equipped with a 54-200 mrad collection semi-angle at Oak Ridge National Laboratory.

X-ray Photoelectron Spectra
XPS was performed on a K-Alpha X-ray photoelectron spectrometer system (Thermo Scientific) with Hemispheric 180° dual-focus analyzer with 128-channel detector. X-ray monochromator was microfocused Al Kα radiation. The samples were pasted and pressed onto the sample holder using carbon tapes for measurement.

X-ray absorption Spectra
XAS measurements at Ni K-edge (8333 eV) were performed at P64 beamline at PETRA-III synchrotron radiation facility (Hamburg). Measurements were performed in transmission mode. Intensities of incident radiation and transmitted radiation were measured with ionization chamber detector I0 and I1 filled with pure N2. For data alignment, Ni foil's XAS spectrum was acquired in transmission mode simultaneously with the spectra for Ni samples. I2 ionization chamber used for such reference measurements was also filled with pure N2. Si (111) monochromator was used for energy selection. All measurements were performed in air at room temperature. ATHENA software was used for data alignment, normalization, and XAS spectra extraction.

Electrode preparation
Carbon paper (1 cm × 2.5 cm, Freudenberg C2H23) was sonicated in ethanol and deionized water for 15 min and dried as the electrode substrate. The catalyst ink is prepared using 4.0 mg catalyst mixed with 60 μL Nafion solution (5% in ethanol, Sigma-Aldrich), 200 μL isopropanol, and 200 μL DI water. After 15 min sonification, the ink was deposited on the micro-porous-layer side of carbon paper to achieve an area of 1 cm 2 with catalysts loading of 1 mg cm -2 .

Electrochemical measurement
The CO2RR performance screening was carried out in a regular 3-electrode H-cell, divided by a Nafion N117 membrane. The working electrode was the catalysts-coated carbon paper mentioned above, and a Pt mesh was deployed as the counter electrodes. A leak-free Ag/AgCl electrode was used as the reference.

Cathode potential
The working potential is controlled by the Biologic SP-300 potentiostat against the Ag/AgCl reference electrode. Before the bulk electrolysis, the ohmic resistance between cathode and reference electrode was measured using PEIS (potentiostatic electrochemical impedance spectroscopy) module at -1.0 V vs.
Ag/AgCl Ref, and the frequency was set from 100 k Hz to 1 Hz. Subseqently, 50% of the ohmic drop was automatically corrected, and the other half was corrected manually. All potentials were rescaled to reversible hydrogen electrode (RHE) by Eq. S1.

Computational methods
Density functional theory calculations were performed using Vienna Ab-initio Software Package (VASP). [4] Core electrons were described using Projector Augmented Waves (PAW) potentials. [5] Valence electrons were described using plane-waves with kinetic energy up to 500eV. Gaussian smearing with a width of 0.1eV was used. The RPBE [6] functional was used for all calculations. All calculations were run with spinpolarization.
Structures were prepared using the Atomic Simulation Environment (ASE). [7] The lattice constant of graphene was optimized using a 12x12x12 Monkhorst-Pack k-point mesh grid. [8] A 3×3 single-layer graphene structure was made with the obtained lattice parameter. All structures were then treated with The computational hydrogen electrode (CHE) [9] was used to determine reaction energies as a function of potential for reactions with an electron in the reactant or product. The chemical potential of the proton and electron is related to that of H2 at 0 V vs RHE using: Eq. S6 Microkinetic modelling was performed using CatMAP. [10] The rate of a given elementary step was: where + indicates the forward reaction andthe reverse reaction. The rate constants can be given as + = exp (− ,+ ) and − = exp (− ,− ). In the absence of electrochemical barriers, the free energy is used, which is given as = + + Δ field , where is the free energy for the reaction at the potential of zero charge (pzc), is the number of proton-electron pairs transferred and Δ field is the dipole-field contribution.
A multi-precision Newton root finding algorithm was used to determine the steady-staterates and coverages. A decimal precision of 100 along with a convergence tolerance value of 10 −25 was used.
* Resistance is too high, out of measurement range. and NiPc/CNT. The profile of PANI-Ni-900 is identical with our earlier work. [3] Group VI: > 404 eV.
Aromatic ring formation was revealed by the peak shift from 284.4 eV to 284.8 eV. [11] The fitted high-resolution N 1s profiles of the pyrolyzed catalyst precursors are shown in Figure S12b (detailed fitted parameters are presented in Table S3 pyridinic, pyrrolic, graphitic, and quaternary N. Species in group VI (404 eV) should be assigned to oxide N moieties. [12] Clearly, the pyrolysis temperature controls the resulting N species in all C-TpDt-Ni samples.
In the unpyrolyzed TpDt-Ni sample, the group I and III signals can be assigned to imine and amine groups.
The pyrolysis treatment significantly transformed those into the in-plane sp2 hybridized N moieties.    For quantitative EXAFS analysis we perform non-linear least square fitting to theoretical standards, as implemented in FEFFIT code, see Figure S14. [15] Theoretical phases and amplitudes were obtained in selfconsistent ab-initio calculations with FEFF8.5 code [16] for reference materials. The complex exchangecorrelation Hedin-Lundqvist potential and default values of muffin-tin radii as provided within the FEFF8.5 code were employed.
Fitting of EXAFS spectra χ(k)k 2 was carried out in R-space in the range from Rmin = 1.0 Å up to Rmax, where Rmax was set to 3.0 Å for NiO and NiPc and to 2.5 Å for metallic Ni, unpyrolyzed Ni COF and Ni COF pyrolyzed at 900 0 C. For Ni COF pyrolyzed at 800 0 C we set Rmax to 1.8 Å to exclude from fitting the contributions beyond the first coordination shells which are too weak in this sample to be reliably identified. In all cases Fourier transform was carried out in the k range from 3.0 Å -1 up to 12 Å -1 .
Guided by the insight from WT-EXAFS analysis in Figure 2c, following paths were included in the fitting: Ni-Ni path for metallic Ni, Ni-O and Ni-Ni paths for NiO, Ni-N, Ni-C and longer Ni-N path for NiPc, Ni-O path for unpyrolyzed sample, Ni-N path for sample pyrolyzed at 800 0 C, and Ni-N and Ni-Ni paths for sample pyrolyzed at 900 0 C. For each path the refined parameter were coordination number N, bond-length R and disorder factor σ 2 . In addition, correction to photoelectron reference energy ΔE0 was also fitted.
Amplitude reduction factors due to many-electronic processes (S0 2 factors) were estimated based on the fitting of EXAFS samples for reference materials with known coordination numbers.
The results of such fitting are summarized in Figure S15 and Table S6. Good agreement between experimental and modeled data ( Figure S15), and low values of fit R-factors (Table S6) give us confidence in the chosen fitting models.
It is challenging to distinguish between EXAFS contributions from elements that are neighbors in Periodic In addition to these structural motifs, the existence of metallic Ni clusters in pyrolyzed samples is confirmed by EXAFS data fitting. In particular, for sample pyrolyzed at 900 0 C the inclusion of Ni-Ni scattering path was found to be necessary to obtain a good fit. The obtained value of Ni-Ni interatomic distance (2.458 ±0.006 Å) is close to Ni-Ni distance in fcc metallic nickel (2.481 ±0.001 Å). The fact that Ni-Ni distance is slightly shorter in pyrolyzed Ni COF sample in comparison to that in bulk Ni metal, may imply the small sizes of formed Ni clusters and/or their strongly disordered nature.
Due to the low contribution of Ni-Ni scattering path, the obtained Ni-Ni coordination number has large uncertainty and cannot be used for a reliable estimation of the concentration of metallic Ni clusters.
However, the fraction of metallic Ni can be estimated indirectly from Ni-N coordination number. Since the measured EXAFS signal is averaged over all Ni species in the sample, in the case when Ni-Nx motifs coexist with metallic Ni, the Ni-N coordination number obtained in the EXAFS fitting − differs from the true number of N neighbors ̃− , and is related to the concentration of metallic Ni w as − = (1 − )̃− . Assuming that in Ni-Nx motifs Ni is coordinated with 4 N atoms (i.e., ̃− = 4), the fraction of metallic Ni can be estimated as 1 − − /4. As a result, we can estimate that in the sample pyrolyzed at 800 0 C, concentration of metallic Ni is ca. 22%, while in the sample pyrolyzed at 900 0 C concentration of metallic Ni increased to ca. 33%. These estimates are in a good agreement with the aforementioned estimates from XANES analysis. Good agreement between EXAFS and XANES results indicates the validity of our assumption that Ni-N4 motifs are the main N-Nx species in the pyrolyzed catalysts.  (3) NiPc (3 rd shell, Ni-N) 6 3.32(4) 0.007 (7) TpDt-Ni 6.9(7) 2.04 (  Schematic illustration of CO2 mass transfer limitation due to in-pores OHformation (by electrochemical H2 and CO evolution) and retention.