Elsevier

Journal of Catalysis

Volume 380, December 2019, Pages 307-317
Journal of Catalysis

Pt (1 1 1) quantum dot engineered Fe-MOF nanosheet arrays with porous core-shell as an electrocatalyst for efficient overall water splitting

https://doi.org/10.1016/j.jcat.2019.09.038Get rights and content

Highlights

  • Self-assembled porous cuboids of Fe-MOF nanosheet arrays with Pt QDs as a core-shell structure were in situ grown on NF.

  • Pt QDs are evenly dispersed in Fe-MOF nanosheet arrays to reduce aggregation of Pt QDs for improve stability and activity.

  • Fe-O bond between the carboxylic group of terephthalic acid and Fe3+ ions is integrated during the preparation of the catalyst.

  • Ordered structure of Fe-MOF as an electron conductor leaded to high activity even with an ultralow Pt content (1.84 μg cm−2).

Abstract

The preparation of Pt quantum dots (Pt QDs) is an effective way that can solve the high cost problem by reducing the high dosage requirement of noble metal Pt. On the other hand, organic metal framework (MOF) can be used for the stabilization of metal nanoparticles with small size and uniform dispersion. Herein, we successfully synthesized a bifunctional electrocatalyst via a facile one-step hydrothermal treatment. Pt QDs cores are uniformly coated with Fe-MOF nanosheet arrays shell (Pt QDs @Fe-MOF) with a porous cuboids structure on Ni foam (NF). With the quantum size and core-shell structure, the self-assembled porous cuboids of Pt QDs @Fe-MOF exhibited perfect electrochemical performance for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and overall water splitting. Particularly, the electrocatalyst with ultralow content of Pt QDs (1.84 μg cm−2) only needed the overpotential of 33 mV and 191 mV to achieve 10 and 100 mA cm−2 in 1 M KOH, respectively. In addition, the Pt QDs @Fe-MOF/NF electrodes possessed remarkable activity and stability to deliver a current density of 10 mA cm−2 at 1.47 V for overall water splitting of at least 100 h.

Introduction

With the increasing of environmental pollution and the rapid consumption of fossil fuels, it is of most significant for our humanity to develop sustainable and eco-friendly clean energy [1], [2], [3]. Hydrogen is considered to be an ideal clean energy to replace traditional fossil fuels [4], [5], [6]. Among the techniques of producing hydrogen, electrochemical water splitting is one of the relatively efficient and clean methods for large-scale hydrogen production [7], [8]. It is required to develop highly active bifunctional electrocatalysts to reduce the influence of slow reaction kinetics for high overpotentials [9], [10]. Currently, Pt is regarded as the state-of-the-art catalytic material for HER, because water molecules could effectively be adsorbed by Pt to form Pt-Had in active sites [11], [12]. Moreover, the (1 1 1) plane of Pt was more active than other plane of Pt, particularly when it is combined with other metals as Pt-based bimetallic or polymetal metals electrocatalyst [13], [14]. Pt nanoparticle has high surface energy, however, it is easy aggregated to cause performance degradation [15]. To a certain extent, the stability of Pt is also affected by its high surface energy when it is used for a long time under strong alkali conditions [16]. In addition, the widespread application of Pt has been limited by the high cost [17], [18], [19]. One of the effective ways to solve these problems is constructing a core-shell structure with Pt nanoparticles as the core and the other material as the shell [20], [21].

Metal-organic framework (MOF) could form coordination functional groups and unsaturated metal ion centers in the form of self-assembly by connecting organic ligands with metal ions. As a function material, MOF is considered to have an immeasurable application prospect for electrocatalysis, due to its high porosity, large surface area and good stability [22], [23], [24], [25]. Moreover, the porous MOF provides an opportunity to fabricate core-shell structures. He et al. [26] reported a core-shell Ag @ MOF-5 nanoparticles for highly selective sensing property, and Ag nanoparticles were dispersed in excellent form in MOF-5. Other researchers had also explored coat nanoparticles on MOF materials to achieve uniform dispersion and excellent properties of small nanoparticles [27], [28], [29], [30]. Therefore, when the core-shell structure is formed by coating Pt with MOF, the quantum dot Pt could be evenly dispersed in MOF. Thus, the content of Pt usage can be drastically reduced. In additional, MOF in shell as electron conductor will shorten diffusion distance between charges and electrolyte ions to fast charge transport [31]. Meanwhile, the Pt QDs in core structure with different feature can change the interface contact of core-shell structure differently [32]. However, MOF particles are easily aggregated during the preparation process at high temperature, which makes the formed MOF aggregates unfavorable to the electronic conductivity in electrocatalytic reaction [33], [34], [35]. To resolve the problem, in situ growth of MOF materials on Ni foam (NF) at low temperature could prevent particle aggregation and maintain the orientation of MOF [36], [37], [38], [39]. Cai et al. [40] reported one-dimensional MOF nanorod arrays which were grown on NF, showing excellent electrical conductivity. Zhang et al. [37] adopted two-dimensional MOF nanosheets which were grown on NF, and it had been proved improving the adsorption of water molecules onto the catalyst and promoting gas diffusion to enhance the electrocatalytic performance. In addition, metal Ni in the NF could bond with Fe species to increase the active sites for OER [34], [41]. So, it will be an interesting idea if the electrocatalyst could be prepared by introducing Fe into Pt QDs @MOF on NF electrode. Thus, the Fe-MOF as an electron conductor in shell would improve the electron transfer rate and play a major role for OER in the MOF system on NF. What’s more, Fe-MOF would provide the opportunity for the formation excellent dispersion of ultralow Pt QDs in core to increase the stability and active sites of Pt for HER under alkaline conditions. To the best of our knowledge, core-shell Pt QDs @Fe-MOF nanosheet arrays in situ growth on NF for overall water splitting has rarely been reported.

In this work, we successfully synthesized Pt QDs @Fe-MOF with a porous cuboids structure on NF by a facile one-step hydrothermal treatment. The Pt QDs cores uniformly were coated with Fe-MOF nanosheet arrays shell. The Pt DQs @ Fe-MOF/NF only needed the 1.84 μg cm−2 of content of Pt to achieve a current density of 100 mA cm−2 at 191 mV for HER, which is the lowest noble metal content electrocatalyst reported so far. Moreover, the Pt DQs @ Fe-MOF/NF with the porous cuboids core-shell structure exhibited remarkable activity and stability for overall water splitting. Therefore, it has a promising future in electrocatalysis for water splitting in industrial practice.

Section snippets

Reagents and materials

NF was provided by Shanghai Zhonghui Foam Aluminum Products Co. Ltd (Shanghai, China). Chloroplatinic acid hexahydrate (H2PtCl6) was obtained from Aladdin Reagent (Shanghai) Co. Ltd. FeCl3·6H2O, terephthalic acid, N, N-Dimethylformamide (DMF) hydrochloric acid (HCl), potassium hydroxide (KOH), and ethanol were obtained from Nanning Blue Sky Experimental Equipment Co. Ltd (Nanning, China). All chemical regents were in the analytical grade and used without further purification. Deionized water

Material characterizations

The self-assembled porous cuboids of core-shell Pt DQs@Fe-MOF nanosheets arrays on NF were synthesized via a facile one-step hydrothermal treatment (Scheme 1). XRD patterns of NF, Fe-MOF/NF and Pt DQs @Fe-MOF/NF are shown in Fig. 1. The characteristic diffraction peaks of NF were located at 44.369°, 51.594° and 76.082°, corresponding to the (1 1 1), (2 0 0) and (2 2 0) planes of Ni, respectively (PDF No. 01-1258). Moreover, the three characteristic diffraction peaks shown in Fe-MOF/NF and Pt

Conclusion

In this work, self-assembly porous cuboids of core-shell Pt DQs @Fe-MOF nanosheets arrays supported on NF were synthesized by a facile one-step hydrothermal treatment with in situ grown. The Pt QDs cores uniformly are coated with Fe-MOF nanosheet arrays shell for efficient overall water splitting. The Pt DQs @Fe-MOF/NF had efficient catalytic performance and excellent stability, due to the special porous core-shell MOF structure and the (1 1 1) plane of Pt QDs, which effectively promoted the

Acknowledgements

This work was financially supported by the National Science Foundation of China (Nos. 21367002, 51707021), Guangxi Natural Science Foundation (Nos. 2016GXNSFAA380212, 2017GXNSFBA198186, 2018GXNSFAA294062 and 2018GXNSFAA281290), China Postdoctoral Science Foundation Grant (No. 2018M633295) , and Young Teachers Innovation Cultivation Program (BRP180261) from Guangxi Bossco Environmental Protection Technology Co., Ltd., and Open Fund of Guangxi Key Laboratory of Clean Pulp & Papermaking and

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