N-functionalized hierarchical carbon composite derived from ZIF-67 and carbon foam for efficient overall water splitting

https://doi.org/10.1016/j.jiec.2021.09.024Get rights and content

Highlights

  • An inexpensive and facile approach to synthesize a bimetallic electrocatalyst.

  • Carbon foam physically mixed with ZIF-67 generates Co-Zn-CNTs after pyrolysis.

  • The Co-Zn-CNTs demonstrate remarkable catalytic activity towards OER and HER.

  • Co3ZnC species contribute to the excellent performance of the electrocatalyst.

  • Co3ZnC species are associated with enhanced durability.

Abstract

An efficient and facile approach to synthesize a bi-functional electrocatalyst via combining carbon foam with cobalt-centered zeolitic imidazolate framework (ZIF-67) is reported. The carbon foam was synthesized via dehydration of sugar utilizing zinc nitrate, forming Co3ZnC in the carbon matrix. To obtain Co hybridized and Co particles covered with carbon nanotubes embedded in a carbon matrix (Co-Zn-CNTs), physical mixing of both defines the critical point after pyrolysis. Higher content of N-related species and transition metal species in polyvalent states and well-grown multi-wall carbon nanotubes for charge transfer are achieved after the pyrolysis process. The obtained Co-Zn-CNTs catalyst was employed as cathode and anode for overall water splitting (HER and OER) and showed excellent performances. This development offers a low-cost and straightforward strategy to synthesize catalyst material for large-scale fuel cells and water splitting technologies. This development affords a precise method for effectively improving the electrocatalyst performance derived from the ZIF-67 precursor.

Introduction

Hydrogen (H2) is a potential alternative clean energy fuel, which combats the increasingly severe environmental problems caused by fossil fuel combustion [1], [2]. Producing hydrogen and oxygen via overall water splitting involves two important half-reactions, namely hydrogen evolution reaction (HER) [3], [4] and oxygen evolution reaction (OER) [5], [6], offering an efficient, reliable energy technology. However, relatively high cathode overpotential and the sluggish kinetics at the anode necessitates the development of competent electrocatalysts. The noble metals such as platinum [7], iridium [8], ruthenium [9], etc., are well-known active metals for HER and OER; however, their limited reserves and high costs hinder their large-scale utilization. Developing low-cost, stable, and efficient bifunctional or multifunctional electrocatalysts to eliminate the dominance of noble metal catalysts is highly challenging and desirable.

Zeolitic imidazolate frameworks (ZIFs), a tetrahedral framework formed by crosslinking transition metals with organic imidazoles, are promising and widely applied as precursors to synthesize electrocatalysts due to the commercial availability with inexpensiveness of the linker and their high metal content [10], [11]. Among numerous ZIFs, ZIF-8 and ZIF-67 containing zinc and cobalt, respectively, are typically and predominantly used as precursors to synthesize catalysts [12], [13], [14], [15]. During the last few years, homogeneously doped transition metal–carbon materials prepared by high temperature graphitized ZIFs, have attracted considerable research efforts due to the high crystallinity and conductivity of the carbon matrix and, thus, resulting in pronounced catalyst performance [16], [17], [18], [19]. Additionally, the structure and morphology of the modified carbon electrocatalyst have a significant influence on its properties [20]. Further development included the preparation of composite materials or self-composites based on ZIFs (composited@ZIFs, ZIFs@ZIFs, etc.). These materials were synthesized, aiming to enhance the complementarity of raw materials and improve material properties with well-defined deficiencies. For instance, Chen et al. synthesized an effective HER electrocatalyst by pyrolyzing graphene oxide (GO) wrapped core–shell Co/Zn-ZIF precursor (ZIF-67@ZIF-8@GO, ZIF-8 as the core and ZIF-67 as the shell) [21]. After carbonization at 900 °C, core–shell Co/Zn-ZIF precursor was evolved entirely into a unique structure of 0-D (cobalt nanoparticles), 1-D (N-doped carbon nanotubes) composite, and protruding growth on both sides of reduced graphene oxide (rGO). Finally, 3D architecture materials containing cobalt nanoparticles encapsulated by nitrogen-doped carbon nanotubes were generated and distributed evenly on the graphene substrate. The resultant material exhibits very close properties to commercial noble metal catalysts either in 0.5 M H2SO4 or 1 M KOH, with almost no performance degradation or morphology destruction after 100 h of cycling. Similarly, Pan et al. utilized the same core–shell structure ZIF-67@ZIF-8 precursor after combining a three-step heat treatment process and finally obtained the hollow polyhedron structure decorated with abundant N-doped carbon nanotubes [22]. The as-synthesized electrocatalyst CoP/NCNHP showed excellent HER performances with overpotentials of 140 mv in 0.5 M H2SO4 and 115 mV in 1 M KOH (at the current density of 10 mA·cm−2), and considerable OER properties (310 mv overpotential, under the current density of 10 mA·cm−2). After 36 hours of cycling test, the electrocatalytic performance of the material almost did not decline. Although a new type of bifunctional catalysts had been obtained by graphene doping and complex heat treatment, it seemed not quite in line with the original intention of simple, efficient, and low-cost material synthesis. On the other hand, the element zinc present in ZIF composites holds two main functions according to the recent literature: i) Zinc liberated during the thermal treatment at high-temperature results in a nitrogen-rich micro/mesoporous carbon, and ii) Zinc accelerates the growth of carbon nanotubes on the particles during the evaporation process [23], [24]. In summary, no matter the kind of function, zinc plays a role during the carbonization process and also on the properties of carbonized materials. However, in recent reports, it was suggested that zinc presence might produce some unexpected results and even formed bimetallic heterogeneous catalysts, which have good electrocatalytic properties [25], [26]. Except for metallic elements, some particular types of carbon substrate such as graphene [27], carbon nanotubes [28], and porous carbons [29] are also helpful to enhance the performance of HER or OER catalysts. The improved electrochemical performance is mainly due to their excellent conductivity, corrosion resistance, and fast electron transfer property [30]. Nevertheless, only a few works have focused on the catalyst performance aspect via improving the synergy of Zn and carbon materials of the catalyst systems. Therefore, the synthesis and combination of carbon materials to form hybrid materials with active electrocatalyst merits remain a tremendous challenge. Likewise, it is to be noted that Co containing electrocatalyst has been developed in the context of CO2 reduction to C-2 products coupled with water oxidation. Thus, choice of Co as another metal in the synthesis of a nano-bimetallic phase with Zn in the N-doped C-matrix in the context of HER/OER was a logical development.

Herein, we provided a facile and novel one-pot route to prepare the low-cost nano-(bi)metallic particle catalysts dispersed in an N-doped carbon matrix (Co-Zn-CNTs) derived from a pyrolysis combination of carbon foam and ZIF-67. This hybridized material has never been reported to the best of our knowledge and, consequently, has not been utilized as an electrocatalyst for OER and HER. The synthetic carbon foam, directly generated from a fast-pyrolysis mixture of sucrose and zinc nitrate source, was first prepared as the additional carbon source with metal nanoparticles (ZnO) and served as a protectant of the ZIF-67 morphology. ZIF-67 was selected as the most suitable precursor for its abundance of Co-N species and ideal over other ZIFs precursors [31]. On this basis, we developed a novel N-doped Co/Zn -embedded porous carbon nanotube material (Co-Zn-CNTs) as an excellent electrocatalyst. On the one hand, carbon foam as an additional carbon source for the production of CNTs avoids the situation where the ZIF-67 was a single carbon resource, thus effectively partially preventing the morphological collapse of the ZIF-67 during the pyrolysis process. On the other hand, the uniformly distributed zinc oxide was successfully introduced into the material precursor, which was the basis for the subsequent synthesis of hybrid catalysts with excellent properties. Thus, benefitting material from the diverse, active species; the higher contents of Co–Nx and Co0 ratios, Co-Zn-C active components, and production of CNTs, the catalyst exhibit superior OER and HER performance. Moreover, this novel method may also provide new thoughts for improving the performance of other MOF derived catalysts.

Section snippets

Synthesis materials

ZIF-67: The synthesis process is similar as previously reported by our group [32]. Typically, 0.66 g 2-methyl imidazole (8 mmol) dissolved in 15 mL methanol to form a transparent homogeneous solution. In the meantime, 0.291 g Co(NO3)2·6H2O (1 mmol) is dissolved in another 15 mL methanol. The two solutions are directly mixed and continuously stirred at room temperature for 24 h. The purple solution is separated via centrifugal (8000 rpm, 3 min.) and washed with MeOH several times till the

Material synthesis

The overall synthetic procedure for Co-Zn-CNTs-N is illustrated in Scheme 1. The dodecahedral ZIF-67 with nanometer sizes (200–500 nm based on SEM images) was firstly synthesized, as shown in Fig. S1. The crystal structure of the synthesized material was analyzed via powder X-ray diffraction, revealing a high crystallinity. The crystals are isostructural with the simulation and previously reported ZIF-67 (Fig. S2). The synthesized ZIF-67 demonstrated high thermal stability up to 550 °C (under N2

Conclusion

In summary, we successfully fabricated a hybrid with Co and Co3ZnC nanoparticles embedded in an N-doped carbon matrix, which is wrapped with CNTs. This straightforward procedure, via the addition of low-cost carbon foam obtained from pyrolysis of sugar, followed by physical mixing with well-known zeolitic imidazolate frameworks (ZIF-67) and pyrolysis, generates hybrid materials suitable as advanced electrocatalysts for HER and OER after direct pyrolysis of the physical mixture. The hybrid

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

We are grateful for the financial support of the State Key Lab of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology) and the National Natural Science Foundation of China (Grant number 21950410754).

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