Iron-doping on Cu–N–C composite with enhanced CO faraday efficiency for the electrochemical reduction of CO2

https://doi.org/10.1016/j.jcou.2020.101418Get rights and content

Highlights

  • Iron-doped catalyst FexCu–N–C were prepared from an N-rich Fe-Cu-BTT precursor.

  • The introduction of Fe significantly increases the BET surface and the total pore volume.

  • FexCu–N–C shows abundant Fe–Nx active sites.

  • FexCu–N–C increases the selectivity of CO2-to−CO.

  • Fe0.07Cu–N–C800 exhibits the highest CO faradaic efficiency.

Abstract

Fe–N–macrocycles have been viewing as the most promising catalyst for CO2ER. It is of great importance to explore the performance of composite CuFe–N–C in CO2ER. Fe-Cu–BTT precursor was prepared by introducing the ferrous ion to a microporous N-rich MOF. It exhibits a lower plateau temperature of 800 ℃ than the prototype. The pyrolysis product of FexCu–N–C increases the selectivity of CO2-to−CO due to the increase of the BET surface area, the total pore volume, and the Fe–Nx sites, as well as a lower density of Cu NPs in the carbon matrix. The Fe0.07Cu–N–C800 exhibits the highest FECO of 48.5 %.

Graphical abstract

Iron-doped catalyst FexCu–N–C were prepared from an N-rich Fe-Cu-BTT precursor. The introduction of Fe significantly increases the selectivity of CO2-to−CO due to the increase of the BET surface, the total pore volume, the Fe–Nx sites, as well as the decrease of Cu NPs density in the carbon matrix.

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Introduction

In the past few years, the electrochemical reduction of CO2 (CO2ER) into high-energy liquid or gas fuels as a way to store intermittent energy has gained intense attention. Due to the inertia of CO2 and the fact that the process requires multiple-electron and proton transfers to be converted, its practical application remains a significant challenge [1,2]. The high over-potential of the reaction and the low activity of existing catalysts still prevent the process from approaching comercialization.

At present, it is urgent to develop high-efficient catalysts and find appropriate reaction conditions (such as solvent, temperature, pressure, etc.) to effectively promote the transfer of electrons/protons, inhibit the hydrogen evolution reaction (HER) and ensure the high selectivity of catalysts [[3], [4], [5]]. Cu is the only known catalyst capable of conversion of CO2 into hydrocarbons with a reasonably high Faraday efficiency (FE). Both experimental and theoretical studies (i.e., focus on the key intermediates, different reaction pathways, etc.) are devoted to improving the over-potential and selectivity of CO2ER over copper catalysts [[6], [7], [8]]. Bimetallic catalysts have shown the capability to turn the binding strength of key intermediates due to the electronic and geometric effects produced by alloying, therefore tune its CO2ER catalytic activity and selectivity [9]. In this context, Cu-based alloys such as Cu–Co, Cu–Ni, Cu–Ag, Cu–Ru, Cu–Pd, Cu–Au, have been investigated [[10], [11], [12], [13], [14]]. Since Fe is one of the most earth-abundant metals, the alloying element of Fe in Cu-based alloys has remained undeveloped.

Many researchers found that the N-doped C-supported transition metals, i.e., M–N–C, have unusual catalytic activity for the CO2ER, due to that it has a similar active site to M–N–macrocycles and their advantages of low cost, adjustable specific surface [[15], [16], [17], [18]]. In consideration that Fe–N–macrocycles have been viewing as the most promising catalyst for CO2ER, it is of great importance to explore the performance of composite CuFe–N–C in CO2ER [19].

Metal-organic frameworks (MOFs) are a kind of porous inorganic materials with a high specific surface area. When using it as the precursor, it could achieve a good dispersion of metal ions due to its uniform arrangement of the crystal lattice [20]. Pyrolyzing MOFs is a relatively simple and straightforward method to prepare functional porous hybrid materials of metal/metal oxide–C [[21], [22], [23], [24], [25], [26], [27]]. HKUST-1 and ZIF-8 are two types of models in this subject [17,22,26,[22], [23], [24], [25], [26], [27]]. A nitrogen-rich (37.4 %) precursor of 3D rigid MOF Cu–BTT ([Cu(DMF)6][(Cu4Cl)3(BTT)8(H2O)12], H3BTT = 1,3,5-benzenetristetrazolate; DMF= N, N-dimethylformamide) has been employed in our group, which guarantee the one-step preparation of Cu–N–C material [28]. Well-dispersed Cu nanoparticles (20–40 nm) in the N-rich carbon matrix were obtained under different pyrolysis temperatures, i.e., Cu–N–CT (T = 600–1100 °C), among which the Cu–N–C1100 exhibits the highest FE for CO2ER product (FECO =40.8 % and FEHCOOH = 38.1 %).

In this work, the ferrous ion with different contents was introduced to the precursor Cu–BTT for obtaining Fe-Cu–BTT and pyrolysis products of FexCu–N–C. The presence of ferrous ions in the precursor lowers the pyrolysis temperature to 800 °C and increases the selectivity of CO2-to−CO. The Fe0.07Cu–N–C800 exhibits the highest efficiency of 48.5 % for transferring CO2 to CO.

Section snippets

Synthesis of FexCu–BTT

H3BTT ligand was synthesized followed a similar process as reported [29]. A certain mass of CuCl2·2H2O, FeCl2·4H2O, and 142.0 mg H3BTT was dissolved in 32.0 mL DMF to obtain the yellow-green suspension. The mole amount of FeCl2·4H2O was kept at 0.3, 0.5, and 0.8 mmol, respectively, and the total amount of CuCl2·2H2O and FeCl2·4H2O was maintained at 1.6 mmol. The suspension was stirred at room temperature for 30 min to disperse it evenly. The solution was then placed in a 100 mL Teflon reactor

Characterizations of FexCu–BTT and FexCu–N–C800

X-ray powder diffraction patterns (PXRD) of FexCu–BTT exhibit a similar diffraction peak with that of Cu–BTT and the simulated one, indicating the structure keeps intact after the introduction of Fe2+ (Fig. 1a). The TG-DSC curves for FexCu–BTT reaches the plateau at a lower temperature of 800 ℃, substantially lower than 1000 ℃, which is required for Cu–BTT (Fig. S1). Herein, we choose 800 ℃ as the pyrolysis temperature for obtaining the derived material. After the carbonization, the framework

Conclusions

In summary, we have synthesized series of Fe-Cu–BTT precursors by introducing the ferrous ion to microporous nitrogen-rich MOFs of Cu–BTT. Followed by a pyrolysis route, FexCu–N–C products with different content of Fe–Nx sites were obtained. Detailed characterization analyses indicate that Fe-doping induced the change of the structure, increase the BET surface area and the total pore volume, leading to a lower density of Cu NPs and higher content of Fe–Nx sites in the carbon matrix. As a

CRediT authorship contribution statement

Si-Min Cao: Data curation, Validation. Hua-Bo Chen: Data curation, Writing - original draft. Meng-Jie Liu: Software, Validation. Bao-Qi Feng: Software. Bao-Xia Dong: Writing - review & editing, Supervision. Qiu-Hui Zheng: Software. Wen-Long Liu: Supervision. Yun-Lei Teng: Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work is financially supported by the National Natural Science Foundation of China (21671169), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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