Elsevier

Electrochimica Acta

Volume 296, 10 February 2019, Pages 418-426
Electrochimica Acta

Homogeneous cobalt and iron oxide hollow nanocages derived from ZIF-67 etched by Fe species for enhanced water oxidation

https://doi.org/10.1016/j.electacta.2018.11.024Get rights and content

Highlights

  • Hollow CoFe oxide thin-shell nanocages were firstly prepared via Fe etching ZIF-67.

  • Fe is homogeneously incorporated into CoO lattice.

  • CoFe-OHNCs exhibit excellent OER performance and stability.

  • The enhancement is attributed to hollow morphology and strong electronic interaction.

Abstract

To develop highly efficient non-noble-metal electrocatalysts for oxygen evolution reaction is still a great challenge. Herein, cobalt and iron oxide with hollow structure has been fabricated using ZIF-67 as template, via reflux process followed by calcination. The simultaneous structural and electronic modulation are achieved due to the homogenous Fe incorporation. The Co sites in higher oxidation states and Fe sites with higher electron density can enhance the adsorption of H2O molecules and OOH species, and thus promote the water oxidation. The unique hollow structure also facilitates the mass transfer at the interface of electrolyte and active atoms. The optimal cobalt and iron oxide hollow nanocages exhibit superior OER performance, delivering an ultralow overpotential of 274 mV at 10 mA cm−2 and a very small Tafel slope of 31 mV dec−1 in alkaline media, which outperform the state-of-the-art RuO2. This work provides an efficient approach to design and explore non-precious electrocatalysts for electrochemical energy conversion and storage.

Introduction

The oxygen evolution reaction (OER) has stimulated enormous attention in recent years due to its promising performance in various applications, such as water splitting to produce hydrogen fuels, fuel cells and metal-air batteries [[1], [2], [3], [4], [5]]. However, the OER suffers from kinetically sluggish and significant overpotential loss due to the four electrons transfer process. IrO2 and RuO2 are state-of-the-art catalysts for OER so far, but the high cost and unsatisfactory durability hinder their large scale applications [6,7].

In this regard, a myriad of catalysts based on first-row transition metals, like Co-based oxides, have been investigated as the promising candidates [[8], [9], [10], [11]]. Some recent studies demonstrated that incorporating a second metallic element into the metal oxides could effectively modulate the local coordination environment to further boost the catalytic performance [[12], [13], [14], [15]]. Specifically, binary metal materials with homogenous composition far from undesired phase separation are much more favorable to tune the electronic structure and the consequent energetics of the OER [[16], [17], [18]]. Although some bimetallic oxides with good OER activity have been reported, a throughout understanding about the roles of homogenously doping a foreign metal atom on modulating the structural and electronic properties is still highly desired.

Furthermore, more active sites, which could be created via surface geometric and structural construction, are also highly desired during the OER process [[19], [20], [21], [22]]. Hollow nanostructures with thin shells, large inner cavities and tailored compositions are attractive candidates [[23], [24], [25], [26]], such as CoS-NS hollow nanoboxes [23], ZnS−Sb2S3@C core-double shell hollow structure [25] and Co3O4-x-Carbon hollow polyhedrons [26]. Because these hollow architectures can not only improve the utilization efficiency of interior atoms, but also reduce the diffusion distance of electrolyte towards electroactive centers. The metal-organic frameworks (MOFs), especially ZIF-67 combined with cobalt center and methyl imidazole, have drawn much attention as sacrificial templates for constructing hollow architecture of cobalt-based binary nanomaterials. However, the second component for hollow polyhedral is confined among the metals of Ni, Zn, Cu, Mg and Mn [23,27,28].

Herein, Fe was firstly selected to configure homogeneous cobalt and iron oxide nanocages with ZIF-67 as template. The binary oxide exhibits a hollow nanostructure with thin shells, in favor of mass transfer from electrolyte to active sites. The Fe incorporation was systematically investigated combining experiments and theory calculations. Fe doping can simultaneously tune the crystallinity of Co oxide to expose more accessible active sites and the electronic structure of metal centers for boosting the intrinsic activity, thus facilitating the electrocatalytic process. As expected, this Fe-incorporated cobalt oxide shows remarkable improvements on the catalytic performance for OER in basic electrolyte.

Section snippets

Chemicals

Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 2-methylimidazole (C4H6N2), iron acetate tetrahydrate (Fe(OAc)2·4H2O) and hexamethylenetetramine (HMTA) (C6H12N4) were purchased from Alfa Aesar Chemicals Co. Ltd. Ethanol (C2H5OH) was purchased from Tianjin Kemiou Chemical Reagent Co. Ltd. Potassium hydroxide (KOH, ≥99.999% metal basis) was purchased from Aladdin Chemical Reagent Co. Ltd. Nafion solution (5%) was purchased from Dupont Corporation. All chemicals were used as received without any

Results and discussion

The fabrication of CoFe(x)-OHNCs is schematically illustrated in Fig. 1. First, Well-defined ZIF-67 was synthesized via a modified coprecipitation method by mixing Co(NO3)2 and 2-methylimidazoleare in methanol under ambient conditions. The initial ZIF-67 particles show a uniform rhombic dodecahedral shape with a smooth surface and an average size of ca. 300 nm (Fig. S1). The solid nature of ZIF-67 particles is revealed by the uniform contrast in Fig. S2. The XRD pattern of the as-prepared

Conclusions

In summary, we developed cobalt and iron oxide hollow nanocages (CoFe(x)-OHNCs) with homogeneous Fe incorporation as high-performance catalysts for water oxidation. The homogeneous Fe introduction can modulate the electronic structures and redox behavior of Co oxide. The optimal CoFe(2.5)-OHNCs deliver an excellent OER activity with a low overpotential of 274 mV at 10 mA cm−2, superior reaction kinetics with an ultrasmall Tafel slope of 31 mV dec−1 and high stability in alkaline electrolyte,

Notes

The authors declare no competing financial interest.

Acknowledgment

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21203137).

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