Rational design of Ru aerogel and RuCo aerogels with abundant oxygen vacancies for hydrogen evolution reaction, oxygen evolution reaction, and overall water splitting

doi:10.1016/j.jpowsour.2021.230600

Journal of Power Sources

Volume 514, 1 December 2021, 230600

https://doi.org/10.1016/j.jpowsour.2021.230600 Get rights and content

Highlights

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A facile method for synthesizing Ru and RuCo aerogel is developed.
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Ru aerogel exhibits comparable HER performance with Pt/C.
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Ru_0.7Co_0.3 aerogel has the superior OER performance to RuO₂.
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Oxygen vacancies play a critical role for enhancing OER performance.
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Ru/Ru_0.7Co_0.3 aerogels outperform Pt/C + RuO₂ couple in water splitting.

Abstract

Developing efficient, robust, and cost-effective catalysts to boost the electrocatalytic properties of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for overall water splitting is fundamentally important yet very challenging. Herein, we report a facile method to prepare Ru-based aerogels for HER, OER, and water electrolysis. Specifically, Ru aerogel exhibits comparable HER performance with Pt/C, evidenced by a close overpotential at 10 mA cm⁻², a smaller Tafel slope, and an outperformed long-term stability. Among a series of RuCo aerogels, the Ru_0.7Co_0.3 aerogel has the best OER performance superior to the RuO₂ benchmark catalyst, with a very small overpotential of 272 mV at 10 mA cm⁻², a low Tafel slope value of 41.6 mV dec⁻¹, and the improved long-term durability. The excellent OER performance of the Ru_0.7Co_0.3 aerogel is mainly attributed to the RuCo synergistic catalytic effect, the abundant oxygen vacancies, and the structural merits of the sample. Notably, in the practical overall water splitting test, the combined Ru and Ru_0.7Co_0.3 aerogel catalyst outperforms the Pt/C + RuO₂ couple. This study can shed light on preparation of metal aerogel-based bifunctional electrocatalyst for overall water splitting and beyond.

Graphical abstract

Introduction

The heavy reliance and massive usage of fossil fuels have caused serious energy problems and global environmental issues such as climate change, which have stimulated worldwide interest to develop green and sustainable energy sources and technologies [1]. Hydrogen (H₂) is considered as a very promising renewable, zero-carbon emission and efficient energy source to resolve the energy and environmental problems [2]. Electrochemical water splitting is an attractive means for producing hydrogen, thanks to its advantages such as industrial maturity, low pollution, high purity of the generated hydrogen, and can couple with other renewable energy sources such as solar, wind, hydroenergy, tide, and so on [3]. However, the efficiency and industrial applications of water splitting are greatly restricted by the sluggish kinetics of two half reactions, namely, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [[4], [5], [6]]. Note that, both reactions require highly active electrocatalysts to reduce the large overpotential.

Currently, Pt-based materials have been regarded as the most preferred catalysts for water splitting due to its quite low HER overpotential and robust stability [7]. For instance, Pt/C is the-state-of-art HER catalyst because of its incomparable activity and low Tafel slope. Unfortunately, the limited abundance in Earth and associated skyrocketing price of Pt significantly limited the large-scale application [8]. Recently, extensive research efforts have been devoted to developing transition-metal based materials as alternatives, but the activity especially the long-term stability of them is still inferior to the platinum group metal (PGM) based materials [9]. As a PGM metal, ruthenium (Ru) based catalysts have been garnerring more and more research attentions for replacing Pt-based materials, mainly thanks its quite low price (about 1/30 of Pt), Pt-like HER activity, and outstanding stability [10,11]. Significant research endeavours have been made to modulate the size, morphology, electronic structure, and subtle local chemical environment of Ru-based catalysts to achieve the optimal HER performance. More importantly, Ru-based materials have also demonstrated excellent OER performance. For instance, Zhu et al. prepared Ru/RuO_X nanoclusters by a one-pot pyrolysis reaction, which exhibited an overpotential of 266 mV at 10 mA cm⁻² and Tafel slope of 73.45 mV dec⁻¹ for OER in 1.0 M KOH [12]. Peng et al. reported the single-atomic Ru sites anchored onto Ti₃C₂T_x MXene nanosheets for OER, exhibiting a small overpotential of 290 mV at 10 mA cm⁻² and Tafel slope of 37.9 mV dec⁻¹ in acidic media [13]. Hu et al. found that nitrogen atoms can act as stabilizer and promoter for Ru clusters to enhance the OER performance [14]. To further reduce the amount of Ru and improve the OER performance, it has been proved that alloying Ru with transition metal can be effective due to the modulated electronic properties and optimized structure. For example, Sarkar et al. reported a RuCo@NC catalyst with low overpotential of 280 mV at 10 mA cm⁻² under alkaline media by the CoRu alloys encapsulated within nitrogen-doped carbon polyhedra [15]. Feng et al. prepared a RuCo@CD via rational modification of the bulk and surface electronic structures of Ru, achieving a very low overpotential of 257 mV for OER at 10 mA cm⁻² in 1.0 M KOH [10].

Note that, to further enhance the catalytic efficiency of RuM (M is transition metal) material for OER, several applicable principles must be taken into account: Increasing the intrinsic activity, improving the exposure of active site, and accelerate the electron transfer and mass transport during the catalytic process. Aerogel, with the unique merits including large surface area, well-defined porosity, and self-supporting properties, offers an ideal platform for designing electrocatalyst meeting the above three principles [16]. In the past decades, metallic aerogels have demonstrated great potential for electrochemical energy storage and conversion as mono-functional or bi-functional electrocatalysts. For instance, Du et al. reported a core-shell structured Au-Ir aerogel synergizing of the highly conductive gold core-network and the exposed active Ir shell, and such catalyst manifested a considerably small overpotential and Tafel slope for OER in 1.0 M KOH (245 mV and 36.9 mV dec⁻¹) [17]. In another study, Du et al. prepared a Au–Rh aerogel with low overpotential of 22 mV and small Tafel slope of 47 mV dec⁻¹ for HER in 1.0 M KOH, owing to the synergy of porous structure and high catalytic activity of the noble metal [18]. Shi et al. synthesized nanovoid incorporated Ir_xCu bimetallic aerogels, achieving a 298 mV overpotential with Tafel slope of 47.4 mV dec⁻¹ for OER in 0.1 M HClO₄ [19].

Inspired by the above findings, herein, we report the facile preparation of Ru aerogel for HER, RuCo aerogel for OER, and the combined catalyst toward overall water splitting. The structure and morphology of the Ru aerogel and the RuCo aerogel series were characterized by SEM, TEM, XRD, XPS and other techniques. Ru aerogel exhibited comparable activity but superior stability to the Pt/C catalyst, while among the RuCo aerogel series with different Ru-to-Co ratios, the Ru_0.7Co_0.3 aerogel had the best OER performance, markedly outperforming the RuO₂ benchmark catalyst. Multiple characterizations revealed that abundant oxygen vacancies are present in the Ru_0.7Co_0.3 aerogel, which plays a critical role in boosting the OER performance. Moreover, in the water splitting test, the Ru aerogel and Ru_0.7Co_0.3 aerogel combination outperformed the Pt/C + RuO₂ couple.

Section snippets

Materials

Ruthenium (III) chloride (RuCl₃) and sodium borohydride (NaBH₄) were obtained from Energy Chemicals (Shanghai, China). Cobalt (II) chloride hexahydrate (CoCl₂·6H₂O) and the Nafion solution (5 wt%) were purchased from Damao Chemical Reagents (Tianjin, China). Tricobalt tetraoxide (Co₃O₄) and Ru/C (5 wt %) were acquired from Macklin Biochemicals (Shanghai, China). Commercial ruthenium (IV) oxide (RuO₂) was bought from Aladdin (Shanghai, China) and commercial Pt/C (20 wt %) was obtained from Alfa

Results and discussions

Ru aerogel and RuCo aerogels were prepared by an excessive amount of reducing agent strategy. Note that, the much more extra amount of NaBH₄ not only provide sufficient capability for reduction, but also has the salting-out capability to initiate the aggregation and fusion of nanoparticles [17]. The synthetic route for preparing the Ru and RuCo aerogels is shown in Fig. 1, and a series of RuCo aerogel samples with various Ru-to-Co ratios were prepared (See details in the Experimental Section).

Conclusions

In summary, we report a facile approach to prepare Ru-based aerogels for HER, OER, and water splitting. Ru aerogel exhibits comparable HER activity but superior stability with Pt/C, and the active center is the metallic Ru species. Among a series of RuCo aerogels, the Ru_0.7Co_0.3 aerogel has the best OER performance superior to the RuO₂ benchmark catalyst, and such excellent OER performance is mainly attributed to the RuCo synergistic catalytic effect, the abundant oxygen vacancies, and the

CRediT authorship contribution statement

Zongshan Lin: Conceptualization, Characterization and Performance test, Writing – original draft. Shilong Liu: Characterzation, Methodology. Yonggang Liu: Characterzation, Methodology. Zhe Liu: Characterzation, Methodology. Shuidong Zhang: Resources. Xiaofeng Zhang: Resources. Yong Tian: Funding acquisition, Resources. Zhenghua Tang: Conceptualization, Funding acquisition, Writing – original draft.

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 study is supported by the Opening Project of Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, South China University of Technology, China. Y. T thanks the grant from Natural Science Foundation of Guangdong Province (No. 2020A1515010546). Z. T. acknowledges the financial support from Guangzhou Science and Technology Plan Projects (No. 201804010323).

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Rational design of Ru aerogel and RuCo aerogels with abundant oxygen vacancies for hydrogen evolution reaction, oxygen evolution reaction, and overall water splitting

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Results and discussions

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgements

Renew. Sustain. Energy Rev.

Mater. Today

Mater. Today Phys.

Electrochim. Acta

Electrochim. Acta

Chin. J. Catal.

Electrochim. Acta

Electrochim. Acta

Adv. Colloid Interface Sci.

Microporous Mesoporous Mater.

Nano Energy

Nature

Chem. Eng. J.

ACS Sustain. Chem. Eng.

Nano-Micro Lett.

Chem. Lett.

Chem. Rev.

Nano-Micro Lett.

J. Mater. Chem. A

e2002888

Small

ChemCatChem

Green Chem.

e1804881

Adv. Mater.

Nat. Commun.

Adv. Energy Mater.

ACS Energy Lett.

Appl. Catal., B

ACS Nano

J. Mater. Chem. A

Angew. Chem. Int. Ed. Engl.

Angew. Chem. Int. Ed. Engl.

Angew. Chem. Int. Ed. Engl.

ACS Appl. Mater. Interfaces

ACS Appl. Mater. Interfaces

Chem. Commun.