A one-pot carbon-coating-ex-solution route to efficient Ru-MnO@C nanowire electrocatalysts with enhanced interfacial interactions

https://doi.org/10.1016/j.cej.2022.136816Get rights and content

Highlights

  • Heat-treatment in C2H2 flow causes simultaneous Ru ex-solution and carbon coating.

  • Ex-solution of Ru nanoclusters remarkably improves the electrocatalytic stability.

  • Ru embedding enhances the interfacial interaction and the graphitization of carbon.

  • The increased HER activity originates from promoted interfacial charge transfer.

Abstract

The immobilization of metal nanoclusters in an inorganic matrix has attracted intense research interest because of its usefulness in exploring highly efficient electrocatalyst materials. In this study, a novel one-pot carbon-coating-ex-solution route to robust high-performance electrocatalysts of metal-nanocluster-embedded carbon-coated inorganic nanostructures was developed by the C2H2 treatment of metal-substituted MnO2 nanowires at elevated temperatures. The calcination of Ru-substituted Mn1-xRuxO2 nanowires under a flow of C2H2/Ar led not only to the simultaneous carbon coating and reductive formation of MnO, but also to the ex-solution embedding of Ru nanoclusters between the MnO substrate and the coated carbon layers. The resulting Ru-embedded Ru-MnO@C nanowires showed a promising electrocatalytic performance with an improved stability for the hydrogen evolution reaction (HER), and this performance was superior to those of Ru-deposited Ru-MnO@C nanowires, Ru-free MnO@C nanowires, and Mn1-xRuxO2 nanowires. The Ru-MnO@C nanowires possessed an excellent electrochemical stability due to the formation of strong interfacial Ru4+-O-Mn2+ bonding and the promoted crystallization of the tight graphitic carbon layer by Ru anchoring. Furthermore, in situ surface-enhanced Raman scattering and electron paramagnetic resonance analyses provided strong evidence for the improved interfacial charge transfer kinetics and enhanced electron injection from MnO to the Ru nanoclusters. This is responsible for the beneficial effect of the simultaneous carbon-coating-ex-solution process on the HER electrocatalyst functionality.

Introduction

Metal nanoclusters have attracted a great deal of research activity because of their excellent catalyst functionalities for diverse electrocatalytic reactions such as the hydrogen evolution reaction (HER), the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), and the CO2 reduction reaction (CRR) [1], [2], [3]. Since outstanding catalyst performances are frequently obtained using noble metal elements, many research efforts have focused on improving the catalytic activities of noble metal catalysts [4], [5], [6]. Although decreasing the particle size is effective in enhancing the electrocatalyst performance of a noble metal catalyst via an increase in the surface-to-volume ratio [7], [8], small metal nanoparticles suffer from severe agglomeration during the electrocatalytic reaction owing to their large surface energy [9]. One of the most promising methods to circumvent the drawbacks of metal nanoclusters is the immobilization of metal nanoclusters in an inorganic matrix [10], [11]. Thus, diverse deposition methods have been developed for the anchoring of noble metal nanoclusters on the inorganic matrix [12], [13], [14], [15]. Since the adhesion of deposited metal nanoclusters to inorganic substrate is not strong enough, the deposition methods cannot prevent completely from the agglomeration of noble metal nanoclusters during the repeated use of electrocatalysts [16], [17]. Instead, the adhesion strength of metal nanoparticles to inorganic substrates can be greatly enhanced by employing an ex-solution synthetic strategy, in which the noble metal components diffuse out into the surface of an inorganic substrate, and some part of metal nanoparticles remain embedded inside the inorganic substrate [18], [19], [20]. In addition to the ex-solution method, the carbon coating of metal nanoclusters can provide an alternative way to improve the electrocatalytic activity and electrochemical stability by enhancing the charge transfer kinetics and protecting the metal surface, respectively [21], [22]. Taking into account the reductive power of the C2H2 molecule, the acetylene treatment of noble metal-containing inorganic nanostructures at elevated temperatures is expected to induce the simultaneous ex-solution of noble metal components and carbon coating on the resulting metal nanocluster-embedded nanostructure, thereby providing a novel single-step carbon-coating-ex-solution route. In this context, one-dimensional (1D) MnO2 nanowires (NWs) are promising substrates for this synthetic strategy [23] because of their facile scalable synthesis and their ability to accommodate diverse noble metal substituents. The reductive C2H2 treatment of noble metal substituted MnO2 NWs could therefore lead to simultaneous carbon coating and the reductive formation of MnO, in addition to promoting the ex-solution embedding of Ru metal nanoclusters between the MnO NWs and the carbon layers [24]. The resulting formation of carbon-coated noble metal-embedded MnO NWs is expected to be effective in synthesizing novel efficient electrocatalyst materials. Despite the many advantages of noble metal–metal oxide@carbon nanocomposites, at the time of this submission, we are not aware of any other report on the development of a one-pot carbon-coating-ex-solution route to novel metal-inorganic nanocomposites that exhibit efficient electrocatalytic performances.

Thus, we herein report the development of a novel synthetic methodology for a single-step carbon-coating-ex-solution process by the C2H2 treatment of 1D Ru-substituted Mn1-xRuxO2 NWs at elevated temperature. The obtained Ru-embedded Ru-MnO@C NWs are then applied as HER electrocatalysts to verify the usefulness of the present synthetic method for exploring efficient energy-functional materials. Systematic spectroscopic investigations, including in situ surface-enhanced Raman scattering (SERS) and in situ electron paramagnetic resonance (EPR) spectroscopic analyses are also carried out for the obtained Ru-MnO@C NWs to characterize the interfacial chemical interactions between the Ru metal nanoclusters and the MnO NWs, and to elucidate the mechanism of operation during the HER process.

Section snippets

Synthesis

The precursor materials of the 1D Ru-substituted Mn1-xRuxO2 NWs were synthesized by hydrothermal reaction as follows: KMnO4 and RuCl3·H2O (KMnO4/RuCl3·H2O: 0.58 g/0 g, 0.51 g/0.074 g, 0.44 g/0.14 g, 0.37 g/0.21 g) were dissolved in 50 mL distilled water and then obtained mixture was transferred to hydrothermal bomb for reaction at 140 °C for 12 h [24]. After hydrothermal reaction, the obtained black powders were collected by centrifugation, washed thoroughly with distilled water, and then

Effects of Ru substitution and C2H2 treatment on the crystal structure and morphology of the Mn1xRuxO2 NWs

The structural evolutions of the 1D Mn1-xRuxO2 NWs upon Ru substitution and C2H2 treatment were probed by powder XRD analysis. As presented in Fig. S1, all RM0, RM10, and RM20 materials with a Ru range of 0 ≤ x ≤ 0.2 displayed the typical Bragg reflections of the pure α-MnO2 phase, whereas an increase in the Ru (x) content to 0.3 resulted in the appearance of intense XRD peaks corresponding to the β-MnO2 phase, as well as weak reflections arising from the α-MnO2 phase. These results confirm the

Conclusions

In this work, efficient hybrid electrocatalysts of one-dimensional Ru-MnO@C nanowires (NWs) were synthesized via a novel one-pot carbon-coating-ex-solution methodology. Heat treatment of the Ru-substituted Mn1-xRuxO2 NWs under a flow of C2H2/Ar induced not only the ex-solution of Ru4+ ions into Ru metal nanoclusters embedded between the carbon coating layer and the MnO NW matrix, but also the simultaneous carbon coating on the surfaces of the Ru-MnO NWs. Combined X-ray absorption near edge

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 supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2020R1A2C3008671, No. NRF-2017R1A5A1015365). This work was also supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (No. 2021M3H4A1A03049662). The research was supported by the Yonsei Signature Research Cluster Program of 2021 (2021-22-0002). The experiments at PAL were supported in part by

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