Enhanced photoelectrochemical hydrogenation of green-house gas CO2 to high-order solar fuel on coordinatively unsaturated metal-N sites containing carbonized Zn/Co ZIFs

https://doi.org/10.1016/j.ijhydene.2019.06.102Get rights and content

Highlights

  • Coordinatively unsaturated sites containing C–Zn/Co ZIF is used in a CO2 PEC.

  • The 1h-C-Zn/Co ZIF has the highest selectivity (84%) to high-order solar fuel.

  • CoN3V site is proved to be the most selective site to high-order solar fuel.

Abstract

Porous carbon-based catalysts facilitate CO2 photoelectrochemical reduction reaction (CO2PRR) through the high charge transfer ability and CO2 adsorption ability. However, the design of catalysts with high selectivity towards high-order solar fuels (such as ethanol and propanol) remains challenging. Herein, catalysts of carbonized Zn/Co Zeolitic Imidazolat Frameworks with various pyrolysis hours (xh-C-Zn/Co ZIFs) were synthesized and accessed for high selective CO2PRR for the first time. XRD and TEM analyses show that the C containing functional groups in pristine Zn/Co ZIF are carbonized and turned into amorphous porous carbon, while many Co and ZnO nanoparticles are formed during the pyrolysis process. Electrochemical characterizations of the catalysts prove that increasing pyrolysis time of Zn/Co ZIF from 1 h to 3 h, resulting in a decreased light current density from 3.3 mA/cm2 to 2.3 mA/cm2, which is much higher than that of Zn/Co ZIF (1.9 mA/cm2). Meanwhile, increasing pyrolysis time of Zn/Co ZIF from 1 h to 3 h results in decreased BET surface area and CO2 adsorption ability. XPS spectra suggest a decreased content of metal-nitrogen bonds in C–Zn/Co ZIF, which leads to the formation of coordinatively unsaturated metal-nitrogen sites. Density functional theory (DFT) calculations reveal that the coordinatively unsaturated CoN3V sites in C–Zn/Co ZIF had the highest catalytic activity towards high-order organics. The total carbon atom conversion rate reaches 5459 nmol/h·cm2 when 1h-C-Zn/Co ZIF is employed as catalyst in the CO2PRR system and the selectivity towards high-order solar fuel reaches 84%.

Introduction

Fossil fuel shortage and global warming are the two major concerns in modern society. The fixation of CO2 via the photoelectrochemical reduction in a photoelectrochemical cell (PEC) [1], which is also known as artificial photosynthesis, has received a considerable attention for its application in the CO2 reduction recently [2], [3]. Among various options for CO2 conversion and utilization, CO2 hydrogenation to alcohol fuels [4], [5] (methanol, ethanol and propanol) has been considered as the favorite since these alcohol fuels can be easily transported and used as fuel or as an intermediate to produce valuable chemicals. Intense research effort has been made towards CO2 hydrogenation to alcohol fuels. However, achieving a high product selectivity towards the production of high-order alcohol fuels (ethanol and propanol) is still challenging.

Nowadays, various cathode catalysts, including Cu, Zn, graphene, ZIF, and MOF [6], [7], [8], [9], [10] have been investigated to be used in the CO2 photoelectrochemical reduction reaction (CO2PRR) to generate valuable products. However, the development of efficient and high-selective CO2PRR catalysts, which are able to activate the inert CO2 molecule into high-order liquid fuels at low overpotential or even spontaneously, remains a challenge.

Among the potential candidates, transition metal complexes based on nitrogen-donor ligand catalysts have attracted a large attention [11]. N-doping can effectively alter the catalyst electronic and geometrical properties and thus improve its catalytic activity [11], [12]. The change of these properties of the catalysts will lead to the stronger bonding of intermediates on the surface, resulting in the much easier generation of high-order products [13]. N-doping can be effectively achieved either by etching the carbon material in a nitrogen-containing gas or via the direct pyrolysis of N-containing carbon sources, such as ZIFs and MOFs [14], [15]. In both methods, the pyrolysis conditions are known to have a significant effect on the composition and on the structure of the resultant catalysts. The structure changes can have large impact on the activity of the catalysts in CO2PRR.

Herein, a strategy to facilitate the CO2PRR via the construction of coordinatively unsaturated transition metal-nitrogen active sites [11], [16], [17], [18] within porous carbon, derived from the pyrolysis of the Zn/Co bimetallic Zeolitic Imidazolate Framework (Zn/Co ZIF) is proposed. Moreover, this technique is used to reduce CO2 in a photoeletrochemical cell for the first time. The effects of the applied biases and of the pyrolysis duration were studied by placing the carbonized Zn/Co ZIF (C–Zn/Co ZIF) samples in the PEC system. DFT calculations were also performed to understand the high catalytic activity and the selectivity of the catalyst for the CO2PRR.

Section snippets

Materials

The analytical grade chemicals were used without any further purification. Zn(NO3)2·6H2O, Co(NO3)2·6H2O, NH4F, NaHCO3, NaCl, absolute ethanol, ethylene glycol, and isopropanol were purchased from Sinopharm Chemical Reagent Co. Ltd (China). 2-methylimidazole was purchased from Sigma Aldarich Co. Ltd. The Nafion 117 membrane and the Nafion membrane solution were purchased from Shanghai Hesen Co. Ltd. The copper foam (CF, 100 ppi) was purchased from Taili Suzhou Co. Ltd., and the 99.89% Ti foil

Catalyst structure and morphology

The XRD patterns of the Zn/Co ZIF and of the three C–Zn/Co ZIF catalysts were obtained. According to Fig. 1, the XRD pattern of the Zn/Co ZIF (Fig. 1(a)) shows similar characteristic peaks as ZIF-8, indicating that the Zn/Co ZIF and the ZIF-8 have a similar crystal structure [9], [23]. The XRD patterns of the C–Zn/Co ZIFs (Fig. 1(b)) show diffraction peaks located at 2θ = 31.8, 34.5, 36.3, 47.6, 56.7, 62.8, and 68°. These peaks have been attributed to ZnO. While the XRD patterns of the

Conclusions

In summary, carbon-based Zn/Co ZIF (C–Zn/Co) catalysts with various pyrolysis durations were successfully synthesized by pyrolyzing the pristine Zn/Co ZIF in an N2 atmosphere. Moreover, the CO2 photoelectrochemical reduction reaction at ambient temperature for these compounds was achieved for the first time. The XPS measurement shows that several coordinatively unsaturated Co–N sites can be formed upon the increase of the pyrolysis duration, and this greatly affectes the catalyst activity. The

Acknowledgements

This study was supported by the National Natural Science Foundation of China (51676171), National Key Technologies Research and Development Program-China (2016YFE0117900).

References (32)

  • F. Bai et al.

    Preparation and carbon dioxide uptake capacity of N-doped porous carbon materials derived from direct carbonization of zeolitic imidazolate framework

    Carbon

    (2014)
  • K. Kaneko et al.

    Superhigh surface area determination of microporous solids

    Colloids Surface

    (1992)
  • B. Liu et al.

    Metal–organic framework (MOF) as a template for syntheses of nanoporous carbons as electrode materials for supercapacitor

    Carbon

    (2010)
  • J. Cheng et al.

    Photoelectrocatalytic reduction of CO2 into chemicals using Pt-modified reduced graphene oxide combined with Pt-modified TiO2 nanotubes

    Environ Sci Technol

    (2014)
  • T. Yamamoto et al.

    Photoelectrochemical reduction of CO2 in methanol with TiO2 photoanode and metal cathode

    ECS Transactions

    (2017)
  • Y. Song et al.

    High-selectivity electrochemical conversion of CO2 to ethanol using a copper nanoparticle/N-doped graphene electrode

    ChemistrySelect

    (2016)
  • Cited by (11)

    • Nanoarchitectonics of low-dimensional metal-organic frameworks toward photo/electrochemical CO<inf>2</inf> reduction reactions

      2022, Journal of CO2 Utilization
      Citation Excerpt :

      Due to the adjustable chemical composition [12,13], pore structure [14], and large specific surface area [15,16], MOFs have been widely used in various applications, such as catalysis, gas adsorption, drug delivery, energy storage, etc [15,17–21]. Furthermore, these attractive characteristics can increase the adsorption ability for CO2 and provide more catalytic active sites [22–26]. Nowadays, most of the studies on the application of MOFs for CO2 reduction were conducted on three-dimensional (3D) MOFs, but their catalytic activities are limited by the large thickness and small number of exposed active sites on the 3D MOF surface.

    • Bimetallic metal–organic frameworks and MOF-derived composites: Recent progress on electro- and photoelectrocatalytic applications

      2022, Coordination Chemistry Reviews
      Citation Excerpt :

      These include: (i) an improved light harvesting efficiency under visible light, (ii) an augmented photogenerated efficiency of charge separation by selecting proper organic ligands and metal ions for MOFs, and (iii) an optimized structure of photoelectrodes due to high tunability, synthetic adjustability, porosity, and CO2 adsorption ability of MOFs. A Zn/Co bimetallic MOF from a Zeolitic Imidazolate Framework family (Zn/Co ZIF) was used to synthesize a porous carbon-based composite (C-Zn/Co ZIFs) through a high-temperature pyrolysis process [243]. This composite was studied in a photoelectrochemical system (Fig. 27a), exhibiting an efficient reduction of CO2 at ambient temperature into various chemical products including CO, CH3OH, HCOOH, C2H5OH, CH3COOH, C3H7OH, and H2 (Fig. 27b).

    • Metal-organic frameworks and their derivatives-modified photoelectrodes for photoelectrochemical applications

      2021, Coordination Chemistry Reviews
      Citation Excerpt :

      Similarly, due to the 3D structure and high adsorption, copper foam is often used as a collector electrode [93]. In related research, Cheng and his colleagues [94] mixed C-Zn/Co-ZIF catalyst, Nafion and Deionized water (DI) together to coat on the foam copper. Copper foam was used as a 3D conductive support, which could uniformly disperse the C-Zn/Co-ZIF catalyst and effectively prevent the catalyst from agglomeration.

    View all citing articles on Scopus
    View full text