Hydrophobicity Tailoring of Ferric Covalent Organic Framework/MXene Nanosheets for High‐Efficiency Nitrogen Electroreduction to Ammonia

Abstract Electrocatalytic nitrogen reduction reaction (NRR) represents a promising sustainable approach for NH3 synthesis. However, the poor NRR performance of electrocatalysts is a great challenge at this stage, mainly owing to their low activity and the competitive hydrogen evolution reaction (HER). Herein, 2D ferric covalent organic framework/MXene (COF‐Fe/MXene) nanosheets with controllable hydrophobic behaviors are successfully prepared via a multiple‐in‐one synthetic strategy. The boosting hydrophobicity of COF‐Fe/MXene can effectively repel water molecules to inhibit the HER for enhanced NRR performances. By virtue of the ultrathin nanostructure, well‐defined single Fe sites, nitrogen enrichment effect, and high hydrophobicity, the 1H,1H,2H,2H‐perfluorodecanethiol modified COF‐Fe/MXene hybrid shows a NH3 yield of 41.8 µg h−1 mgcat. −1 and a Faradaic efficiency of 43.1% at −0.5 V versus RHE in a 0.1 m Na2SO4 water solution, which are vastly superior to the known Fe‐based catalysts and even to the noble metal catalysts. This work provides a universal strategy to design and synthesis of non‐precious metal electrocatalysts for high‐efficiency N2 reduction to NH3.

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were 2 taken on Tecnai G 2 F20 and Nova Nano SEM 230 electron microscope. Atomic force microscopy (AFM) was performed on Bruker Multimode 8. microscopy. X-ray photoelectron spectroscopy (XPS) was taken on Thermo Fisher ESCALAB Xi+ spectrometer. Contact angles were measured on JY-82C apparatus. Ultraviolet-visible (UV-vis) spectra were taken on Mapada V-1200 spectrometer. Electrochemical tests were performed on CHI660D electrochemical workstation.

Synthesis
COF nanosheet. Tp (15.7 mg) was dissolved in CH 2 Cl 2 (100 mL) in a 500 mL beaker, to which water (60 mL) was slowly added as the spacer-layer. Bpy (20.8 mg) was dissolved in a mixture of MeCN (30 mL) and water (70 mL), which was then added to the Tp solution and maintained at room temperature for 3 days. The COF nanosheets were afforded and collected after washing with water, DMF, acetone, and THF, respectively. [1] COF-Fe nanosheet. The as-synthesized COF nanosheet (100 mg) was ultrasonically dispersed in MeOH (50 mL) for 30 min. Different amounts of FeCl 3 (20, 30, and 40 mg) were dissolved in MeOH (20 mL), which were then added to the methanol suspension of COF nanosheet. The reaction systems were stirred at room temperature for 1 day. The product was washed with fresh MeOH and HCl water solution (0.1 M). The Fe contents of different COF-Fe nanosheets were determined to be ~4.6, ~7.0, and ~7.1 wt% by ICP-OES.

Detection of ammonia
The synthesized ammonia was quantitatively determined by various approaches. Generally, the linear relationship of analytic signal and ammonia concentration was established accurately via a series of standard ammonia solutions to calculate the ammonia concentration in electrolyte.
Nessler's reagent approach. [2] 3 mL of post-electrolysis electrolyte was mixed with 0.3 mL of potassium sodium tartrate solution (0.2 M), which was then added with the Nessler's 3 reagent (0.3 mL). After the obtained solution was kept in the dark for 20 min, UV-vis absorption spectra were measured to analyze the intensity at 425 nm.
Indophenol blue method. [3] Solution A is a mixture of 1 M NaOH solution containing 5 wt% salicylic acid and 5 wt% sodium citrate. Solution B is a NaClO (0.2 M) water solution.
Solution C is a 1 wt% sodium nitroferricyanide aqueous solution. The post-tested electrolyte solution (2 mL) was sequentially added with solution A (2 mL), solution B (1 mL) and solution C (0.2 mL). After keeping for 2 h at room temperature, the UV-vis absorption spectra of this mixed solution were recorded. The intensity of maximum absorbance at 655 nm was applied to determinate the ammonia yield.
1 H NMR spectra. [4] The ammonia solution (1 mL) was mixed with a H 2 SO 4 water solution (1 mL, 0.1 M). Then, 0.5 mL of the mixed solution was added with DMSO-d 6 (0.05 mL), which was further analyzed by 1 H NMR spectra. The NRR test was measured under N 2 atmosphere at -0.5 V versus RHE for 4 h. The electrolyte was then concentrated to 3 mL for 1 H NMR test.

Detection of hydrazine
The Watt and Chrisp method was used for quantitative analysis of N 2 H 4 . [5] A mixed solution of para-(dimethylamino) benzaldehyde (4 g), concentrated HCl (20 mL), and EtOH (200 mL) was prepared as the color reagent. The hydrazine solution (5 mL) and the color reagent (5 mL) were mixed together for 15 min at room temperature and further measured the corresponding UV-vis absorption spectrum. The absorbance intensity at λ = 458 nm was employed to establish a linear equation with the concentration of hydrazine by diverse standard hydrazine concentrations. The UV-vis spectra of the electrolyte were collected to determine the hydrazine concentration.

Calculation of ammonia production rate and Faradaic efficiency
The Faradaic efficiency was calculated by the number of electric charges in ammonia synthesis divided by the total electric charges passing through the electrodes during 4 electrolysis: The ammonia production rate ( NH 3 ) is defined as the following equation: in which F is the Faraday constant (96485 C mol -1 ), NH 3 is the ammonia concentration, V is the volume of electrolyte, Q is the total transferred charge during NRR, t is the electrochemical NRR time, and m cat. is the mass of catalyst.  Figure S1. FT-IR spectrum of the alkylamino group modified MXene.                                        Table   Table S1.