Ni-B coupled with borate-intercalated Ni(OH)2 for efficient and stable electrocatalytic and photocatalytic hydrogen evolution under low alkalinity

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Highlights

  • Ni-B coupled with borate-intercalated Ni(OH)2 as a novel HER catalyst in alkaline system.

  • Intercalated borate enhances electro- and photo-catalytic HER activity and stability of the composite.

  • Interlayer borate improves proton transport from Ni(OH)2 to Ni-B and stabilizes Ni(OH)2.

  • This work provides new insights for design new composite HER catalysts at alkalinity.

Abstract

To convert solar energy into storable chemical fuel hydrogen via water splitting, it is highly required to develop efficient, low-cost and stable HER (hydrogen evolution reaction) catalysts and systems. For practical application, the HER catalysts can work in low alkaline or neutral reaction systems. However, in these reaction systems, water dissociation into proton is a rate-determining step, which can be overcome by loading metal oxide or hydroxides onto the HER catalysts. Ni(OH)2 is a well-reported cocatalyst to assist water dissociation. Herein, Ni-B@Ni(OH)2 [Ni-B with loaded Ni(OH)2] as a HER catalyst at weak alkalinity has been investigated. Very interestingly, we find that when borate is added into the above reaction systems, the resultant catalyst Ni-B@Ni(OH)2-BI shows much better HER activity and stability than Ni-B@Ni(OH)2. The reason is that borate ions can intercalate into Ni(OH)2 interlayers, which increases proton transport ability and stability of Ni(OH)2 loaded on Ni-B. These findings provide new insights for design new composite catalysts of HER in low alkaline reaction systems.

Introduction

Sunlight is the most abundant and sustainable energy source on the earth. Conversion of solar energy into storable chemical fuel hydrogen via water splitting is considered as the most promising approach to solve the energy crisis of fossil fuels and environmental pollution [1], [2], [3], [4], [5], [6]. Solar-driven water splitting can be achieved through thermolysis, photolysis (photocatalysis and photoelectrocatalysis), and electrolysis (photovoltaic-driven electrolysis) [7]. For the photolysis and electrolysis of water, efficient, stable and low-cost HER catalysts and reaction systems are required. Pt is a well-established HER catalyst. However, it is rare and expensive, limiting its large-scale application. Thus, it is highly desirable to develop low-cost transition-metal-based HER catalysts to substitute Pt.

At present, water electrolysis in industry for hydrogen production is carried out in high alkaline electrolytes which are highly corrosive to reaction equipment. Reducing the alkalinity of the electrolytes is important to practical application. On the other hand, photocatalytic hydrogen production via dye-sensitization is a facile method for visible-driven hydrogen evolution, which is in nature similar to the photosynthesis of natural plants. In general, the regeneration of the photoexcited dyes usually can be effectively achieved by weakly alkaline organic sacrificial agents such as trimethylamine and triethanolamine [8], [9], [4], [10], [11], [12], namely the HER catalysts should work in the low alkaline reaction system. Therefore, the development of weakly alkaline HER catalysts is of great significance.

Ni and B are abundant and low-cost elements on earth. Ni-B alloys are good HER and OER (oxygen evolution reaction) catalyst [13], [14], [15]. However, under alkaline conditions, its HER performance becomes poor. This is because water dissociation into proton often becomes a rate-determining step with the highest energy barrier [16], [17], [18], [19]. Therefore, the development of Ni-B based HER catalysts capable of activating H2O at low alkaline media is most desired. Although nickel oxide and hydroxide are not good HER catalysts, they can effectively promote the water dissociation under alkaline condition. When they were loaded on the surface of Pt, the composites show excellent HER activity under alkaline condition, near to the activity under acidic condition [13], [16], [17], [18]. These works inspires us to load Ni(OH)2 on the surface of Ni-B ([Ni-B@Ni(OH)2]) to prepare efficient Ni-B based HER catalysts under low alkaline condition. However, we found that nickel hydroxide can be reduced to nickel metal at high reduction potentials (for high H2 output in industry) in water electrolysis system and reduced by the reduced dye species EY· (EY: Eosin Y) in our dye-sensitization system. As a result, the water dissociation function of nickel hydroxide is gradually lost, which leads to lowing the HER activity of the composite catalyst. Thus, enhancing the activity and stability of the Ni(OH)2 loaded on Ni-B is an urgent issue.

Herein, we find that when borate is added into the reaction systems, not only the stability but also HER activity of N-B@Ni(OH)2 can be greatly improved. The reason is that borate ions can intercalate into Ni(OH)2 interlayers, which increases the proton transport ability and stability of Ni(OH)2 loaded on Ni-B. These findings provide new insights for design new composite HER catalysts at high reduction potentials.

Section snippets

Preparation of Ni-B@oxide and Ni-B

Surface-oxidized Ni-B powder was prepared by chemical reduction of NiCl2 with NaBH4 in air atmosphere. In a typical procedure, 2.30 g of NiCl2·6H2O and 1.02 g of NaBH4 were dissolved in 100 mL of deionized water, respectively. Next, the resultant NaBH4 solution was dropped into the NiCl2·6H2O solution under stirring within 10 min. After the addition, the mixture was stirred for 20 min. Then, the black precipitate was collected on a filter and then washed with deionized water, ethanol and

Composition and structure of Ni-B@oxide

Fig. 1a shows the XRD pattern of Ni-B@oxide. The broad diffraction peak at around 45° demonstrates that the as-prepared Ni–B alloy is amorphous [20]. No peaks for crystalline species can be observed. Fig. 1b-c represents XPS spectra of Ni 2p and B 1 s of Ni-B@oxide, respectively. For the Ni 2p, the peak at 855.9 eV assigned to Ni2+ is strong, whereas the peak at 852.1 eV indexed to Ni of Ni-B is very weak [21], [22]. For the B 1 s, only a strong peak at 192.0 eV (B of B-O bonding) occurs, while

Conclusions

In air atmosphere, the obtained Ni-B can be partly oxidized into the surface coating of amorphous Ni(OH)2 and Ni3(BO3)2 (Ni-B@oxide). The coating can be converted into Ni-B@Ni(OH)2 with crystalline α-Ni(OH)2 in alkaline solutions and into borate-intercalated Ni(OH)2 in alkaline borate solutions. The composite Ni-B@Ni(OH)2 shows better electro- and photo-catalytic HER activity than pure Ni-B, but its stability is very poor. When borate is added into the reaction systems, both the electro- and

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2018YFB1502004), and the National Natural Science Foundation of China (21962010, 21563019).

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    These authors contributed equally to this work.

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