Co3(hexaiminotriphenylene)2: A conductive two-dimensional π–d conjugated metal–organic framework for highly efficient oxygen evolution reaction

https://doi.org/10.1016/j.apcatb.2020.119295Get rights and content

Highlights

  • A highly conductive two-dimensional MOF Co3(HITP)2 is fabricated.

  • Co3(HITP)2 exhibits highly electrical conductivity (1150 S m−1), exceeding holey graphene (∼1000 S m−1).

  • Co3(HITP)2 affords prominent OER activity, superior to commercial RuO2 and IrO2.

  • Co3(HITP)2 displays ultrahigh coverage of Co-N4 active sites (the Co loading is 23.44 wt. %).

Abstract

The application of metal–organic frameworks (MOFs) in electrocatalysis is mainly limited by their poor electrical conductivity. Herein, we report an intrinsically conductive π–d conjugated two-dimensional (2D) MOF Co3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), which possesses well-defined porous networks and much larger number of active sites for oxygen evolution, i.e. Co-N4 sites (23.44 wt. % of Co element). Co3(HITP)2 exhibits high electrical conducting behavior (1150 S m−1), outperforming holey graphene (∼1000 S m−1). DFT theoretical calculation verifies the metallic behavior of Co3(HITP)2 and demonstrates that the π–d conjugation contributes to the excellent conductivity. Additionally, Co3(HITP)2 displays prominent oxygen evolution reaction (OER) activity (an overpotential of 254 mV vs. RHE at a current density of 10 mA cm−2 and a Tafel slope of 86.5 mV dec-1) in alkaline electrolyte, which is comparable or even superior to most cobalt-based materials reported thus far as well as commercial RuO2 and IrO2. These promising results suggest the great potential of pristine π–d conjugated 2D MOFs as electrocatalysts.

Graphical abstract

A highly conductive π–d conjugated two-dimensional MOF Co3(HITP)2 (HITP means 2,3,6,7,10,11-hexaaminotriphenylene) is successfully fabricated, which displays well-defined porous networks and ultrahigh loading of Co-N4 sites (23.44 wt. % of Co element). Co3(HITP)2 exhibits highly electrical conductivity (1150 S m−1) and prominent oxygen evolution reaction activity, which is superior to commercial RuO2, IrO2 electrocatalysts and most cobalt-based materials reported thus far.

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Introduction

Metal–organic frameworks (MOFs) materials, a kind of organic coordination polymers formed with metal ions and organic ligands, have received great research interests due to their unique properties, such as high surface area, tunable chemical structure and functionality, etc. [[1], [2], [3], [4], [5], [6], [7], [8]] Despite their promising features, it is still a huge challenge for directly applying MOFs materials in electrocatalysis with excellent catalytic performance. Their low electrical conductivity, poor mechanical stability under the electrocatalytic conditions, blockage of active metal centers by organic ligands, low mass permeability etc. seriously limit their utilization in electrocataltysis [9]. Among the factors, poor conductivity is a major factor influencing the development of MOFs as effective electrocatalysts. Therefore, great efforts have been devoted to improve the conductivity of MOFs and expand the application of MOFs in electrocatalysis, such as combining MOFs with electrical conductive compounds, using MOFs as precursors to obtain graphited carbon supported metal oxide or nitride, etc. [10,11] However, these methods need further treatment of MOFs, which either destroy their original chemical structures or require complicated processes, and are not cost effective, either [[12], [13], [14]]. Up to now, very limited MOFs with high intrinsic conductivity have been developed and utilized directly in electrocatalysis. From this perspective, pristine MOFs with high electrical conductivity hold great potential promise in electrocatalysis.

Recently, two-dimensional (2D) MOFs have attracted intensive research attention due to their superior conductivity [[15], [16], [17]], such as Ni3(HITP)2 [18], Cu3HITP2 [19,20], NiPc-M [21], Fe3(THT)2(NH4)3 [22], Ni3HHTP2 [23], Cu-HAB [24], {[Cu2(6-Hmna)(6-mn)]·NH4}n [25], Cu-HHTP [26] and so on. The high conductivity is ascribed to the in-plane delocalized π-d conjugation, which is achieved by the hybrid of frontier orbits of conjugated ligands and d-orbits of the transition metals [19,27]. These conductive 2D MOFs are mainly applied in chemiresistive sensing and supercapacitors. In terms of 2D MOFs related electrocatalysts, only CoBHT [28], Cu-BHT [29], THTNi 2DSP [30], NiAT [31], Co3(THT)2 [32] etc. are experimentally reported as efficient hydrogen evolution reaction (HER) electrocatalysts. The active centers for these materials focus on metal-sulfur sites (M-Sx). Besides, Ni3(HITP)2 [[33], [34], [35]] is reported to exhibit excellent ORR (oxygen reduction reaction) performance thanks to the H atoms directly bonded to the N atoms. The ORR performance of Ni3(HITP)2 is further increased by part replacement of Ni with Co [36]. Very recently, Co3(HITP)2 is also theoretically predicted as an efficient electrocatalyst towards ORR [[37], [38], [39], [40]]. According to the summary of related references, the application of conductive 2D MOFs in electrocatalysis is very limited and there is still great room for further exploration of 2D conductive MOFs in the field of electrocatalysis.

Oxygen evolution reaction (OER) is usually regarded as the bottleneck of water splitting due to the kinetically sluggish and intrinsically limited efficiency. The Co-N4 sites have been identified as effective OER active sites because of its high capacity to facilitate the intermediate transition and charge transfer during the oxygen evolution process [[41], [42], [43], [44]]. Therefore, many groups devote to the construction of Co-N4 sites, which is usually seen in single-atom-based catalysts [12,[45], [46], [47], [48]]. The amount of Co-N4 active sites for single-atom-based catalysts is small (less than 5 wt. %), which is still a challenge for further improving the efficiency of single-atom-based electrocatalysts.

Bearing above considerations in mind, we rationally select the conductive 2D MOF Co3(HITP)2 (HITP is the short name of 2,3,6,7,10,11-hexaaminotriphenylene) as the research target, which displays ultrahigh loading of Co-N4 sites (23.44 wt. % of Co element). The as-prepared Co3(HITP)2 is highly conductive with the conductivity up to 1150 S m−1, which is even larger than that of holey graphene (∼1000 S m−1) and activated carbon under room temperature [18,49]. DFT theoretical calculation verifies the metallic behavior of Co3(HITP)2 and demonstrates that the π–d conjugation is responsible for the excellent electrical conductivity. Furthermore, Co3(HITP)2 affords prominent OER activity (an overpotential of 254 mV vs. RHE at a current density of 10 mA cm−2 and a Tafel slope of 86.5 mV dec-1) in alkaline electrolyte, which is comparable or even superior to most cobalt-based materials reported thus far as well as commercial RuO2 and IrO2. The high OER performance of Co3(HITP)2 originates from high conductivity, porous structure and the high coverage of electrochemically active Co-N4 sites.

Section snippets

Chemicals

All the chemicals including 2,3,6,7,10,11-hexaaminotriphenylene hexahydrochloride (HITP · 6HCl), CoCl2, KOH and aqueous ammonia are of analytical grade and were used as obtained.

Pre-treatment of carbon cloth

A piece of carbon cloth (CC) (1 cm × 2.5 cm) was used as the substrate, and was freshly prepared as the following steps before each experiment: firstly dipped in the mixed solution of HNO3 and H2SO4 with a volume ratio of 1:3 for several hours, then rinsed with distilled water and ethanol in an ultrasonic bath for

Results and discussion

The synthesis of Co3(HITP)2 is accomplished by a simple method at room temperature (Scheme 1). A solution of CoCl2 in distilled water is mixed with HITP·6HCl at room temperature, followed by the addition of concentrated aqueous ammonia under constant stirring (the detailed description can be found in the experimental section). The crystal phase of Co3(HITP)2 is characterized by X-Ray powder diffraction (XRD), no diffraction peaks can be distinguished, indicating the poor crystallinity of Co3

Conclusion

In conclusion, a π–d conjugated 2D MOF material (Co3(HITP)2) is characterized and studied in terms of OER. DFT calculations suggest that Co3(HITP) displays high electronic conductivity, which is consistent with the result of four probe measurement (1150 S m−1). In addition, as a pristine porous MOF material, Co3(HITP)2 presents much more active Co-N4 sites (23.44 wt. % of Co element) than do the reported single-atom-based catalysts. Therefore, Co3(HITP)2 exhibits excellent and stable OER

CRediT authorship contribution statement

Danning Xing: Investigation, Methodology, Data curation, Formal analysis, Project administration, Validation, Writing - original draft, Writing - review & editing, Visualization. Yuanyuan Wang: Software. Peng Zhou: Methodology, Validation. Yuanyuan Liu: Conceptualization, Supervision, Writing - review & editing, Project administration, Funding acquisition. Zeyan Wang: Methodology. Peng Wang: Validation. Zhaoke Zheng: Validation. Hefeng Cheng: Investigation. Ying Dai: Software. Baibiao Huang:

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.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. U1832145, 51972195, 21832005, 21972078, 11374190, and 51321091), Young Scholars Program of Shandong University (2016WLJH16, 2020QNQT012) and Taishan Scholar Foundation of Shandong Province, China.

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