In-situ synthesis of porous Ni2P nanosheets for efficient and stable hydrogen evolution reaction

https://doi.org/10.1016/j.ijhydene.2019.01.042Get rights and content

Highlights

  • Porous Ni2P nanosheets have been in-situ grown on Ni foam.

  • Ni2P nanosheets show strong mechanical adherence with Ni foam substrate.

  • The porous structure of Ni2P nanosheets exhibit high activity towards HER.

  • Ni2P nanosheets also show superior HER stability at the high current density.

Abstract

The design and development of highly efficient and stable non-noble metal electrocatalysts for hydrogen evolution reaction (HER) have attracted increasing attention. However, some key issues related to large overpotential, high cost and poor stability at high current density still remains challenging. In this work, we report a facile in-situ integration strategy of porous Ni2P nanosheet catalysts on 3D Ni foam framework (Psingle bondNi2P/NF) for efficient and stable HER in alkaline medium. The two-step method can creates high density of ultra-thin porous Ni2P nanosheets firmly rooted into Ni foam substrate which can guarantee excellent electrical contacts, strong substrate adherence and large amount of active sites. Such a binder-free flexible HER cathode exhibits superior electrocatalytic performance with an overpotential of 134 mV at current density of 10 mA cm−2. It also shows superior stability at higher current densities of 100 and 500 mA cm−2 for at least 48 h and negligible performance degradation is observed.

Graphical abstract

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Porous Ni2P nanosheets in-situ grown on Ni foam have been obtained to act as a promising HER electrode with excellent electrocatalytic activity and robust stability at high current density in alkaline medium.

Introduction

The global energy crisis and its associated environmental problems have aroused an urgent demand for clean, economic and sustainable energy sources [1], [2], [3], [4], [5]. Hydrogen is known as the clean energy in 21st century and regarded as an ideal energy alternative to replace fossil fuels owing to its high energy density and environmentally friendly [6], [7], [8], [9], [10]. The electrocatalytic water splitting to realize large-scale hydrogen production from abundant water sources has been regarded as a facile pathway to achieve this object. Currently, Pt-group novel metals have been demonstrated to be the most effective electrocatalysts for the hydrogen evolution reaction (HER). However, their insufficiency and high cost greatly limit their wide spread applications [6], [11]. Therefore, developing alternative alkaline catalysts with low-cost, analogous efficiency and good stability for HER is extremely desirable.

Up to now, transition metal phosphides (TMP), including Ni2P [12], Ni12P5 [13], CoP [14], Co2P [15], NiCoP [16], FeP [17], Cu3P [18] and MoP [19], etc. are widely used as an alternative to Pt-based catalysts owing to their metalloid characteristics, highly electrocatalytic performance and significant earth-abundance [19], [20], [21]. However, the monotonous morphology, small specific surface area, as well as the low electrical conductivity and inferior stability drastically restrict their practical applications [22], [23]. In order to solve the aforementioned problems and further enhance the electrochemical activity, several strategies focusing on above key issues have been proposed and some progress has been obtained till now. For example, the increase of specific surface area of TMP nanostructures via surface roughening or porous structure can not only create more exposed active sites, but also enhance the diffusion of water molecules and the rapid release of gaseous products [22], [24], [25]. Hao et al. reported that porous Fe-doped CoP nanosheet array on carbon cloth exhibited excellent electrolyzed water performance with an overpotential of 0.359 V at the current density of 100 mA cm−2 [26]. Li et al. synthesized a three-dimensional Ni2P polyhedron with a hierarchically porous structure, which was obtained from a nickel centered metal−organic framework (MOF-74-Ni) precursor via low-temperature phosphorization. This special structure exhibits excellent electrochemical performance with a low overpotential of 158 mV to produce the cathodic current density of 10 mA cm−2 [27]. Additionally, the increase of interfacial adhesion and conductivity between the catalyst and the supporter can significantly enhance the stability and durability of electrodes, especially at a high current density for practical application [28]. Following this path, Ma et al. synthesized NiCoP@Cu3P nanowires on copper foam through chemical oxidation, hydrothermal reaction and phosphorization procedure to obtain the good electrical contacts and mechanical stability between catalyst and supporter [29]. In the same way, Yu et al. directly prepared NiFesingle bondLDH on Cu nanowires rooted into Cu foam, which effectively increases the stability and durability of electrode [28]. However, the relatively complex synthesis route and uncontrollable morphology greatly limit the practical efficiency and large-scale preparation [30]. Therefore, extensive efforts are still required to develop low cost, well-defined and self-supported TMP electrodes with outstanding catalytic activity and robust stability for HER [31], [32].

In this work, we report the design and integration of 3D porous Ni2P nanosheets arrays on NF with excellent HER performance in alkaline medium. Nickel hydroxide nanosheets rooted into NF with strong interfacial adhesion were first prepared by a low-cost hydrothermal process, and then porous Ni2P nanosheets with large active areas were in-situ obtained through a feasible low-temperature phosphorization method. Benefitting from the strong interfacial adhesion and porous nanostructures, Psingle bondNi2P/NF exhibits outstanding alkaline HER activity with the demand of an overpotential of only 134 mV to a drive current density of 10 mA cm−2 in 1.0 M KOH. Most importantly, the designed Psingle bondNi2P/NF electrode also shows superior stability with negligible performance degradation at high current densities of 100 and 500 mA cm−2 after 48 h. The excellent electrocatalytic activity makes the Psingle bondNi2P/NF as a promising cathode towards large scale and efficient hydrogen evolution via electrochemical water splitting.

Section snippets

Preparation of Ni(OH)2/NF precursor and Psingle bondNi2P/NF electrode

NFs (10 mm × 30 mm) were first washed with 2 M HCl, acetone, DI water and ethanol by ultrasonication for about 30 min, respectively. CO(NH2)2 (0.1 g) was then dissolved in 40 ml DI water in a beaker by continuously stirring for 30 min. The prepared aqueous solution and NF were transferred into a 50 ml Teflon-lined stainless steel autoclave and maintained at 120 °C for 6 h in drying oven for Ni(OH)2/NF. To obtain Psingle bondNi2P/NF, NaH2PO2 and Ni(OH)2/NF precursors were put into two alumina crucibles,

Results and discussion

To improve the efficiency and stability of Ni2P catalysts towards HER, the in-situ synthesis and surface area increasing strategy are proposed. Fig. 1 illustrates the schematic diagram that describes each key step involved in the in-situ growth of Psingle bondNi2P/NF electrode. At first, ultrathin nickel hydroxide nanosheets were prepared on Ni foam using hydrothermal method as the precursors of Psingle bondNi2P. In this process, the Ni foam itself directly provides the Ni source for the formation of Ni(OH)2

Conclusions

In summary, we demonstrated a novel synthetic strategy for the in-situ growth of ultrathin porous Ni2P electrochemical catalyst on conductive Ni foam substrate towards enhanced HER based on facile hydrothermal reaction and low-temperature phosphorization process. The template synthesis of Psingle bondNi2P/NF electrode maintains well the nanosheet morphology and 3D structure of Ni(OH)2/NF precursor. The strong interfacial adhesion and porous feature of Ni2P nanosheet electrode significantly enhance the

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (No.51702326, 51872296), the Open Fund of the State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-Sen University) with grant No. OEMT-2017-KF-02, the Basic Science Innovation Program of Shenyang National Laboratory for Materials Science (Grant No. 2017RP25), and the Program for Leading Talents in Science and Technology Innovation of Chongqing City (No. cstc2014kjcxljrc0023).

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