Elsevier

Journal of Catalysis

Volume 377, September 2019, Pages 600-608
Journal of Catalysis

Coupling NiSe2-Ni2P heterostructure nanowrinkles for highly efficient overall water splitting

https://doi.org/10.1016/j.jcat.2019.08.005Get rights and content

Highlights

  • The coupled NiSe2-Ni2P heterogeneous nanowrinkle is constructed on Ni foam.

  • NiSe2-Ni2P/NF exhibits excellent catalytic performances toward both OER and HER.

  • NiSe2-Ni2P/NF also shows outstanding overall water splitting efficiency.

  • DFT calculations confirm that coupling promotes hydrogen/water adsorption energies.

Abstract

Developing catalysts with high efficiency and low-cost toward overall water splitting is critical for the sustainable energy conversion. Herein, a facile wet-chemical route followed by successive selenization and partial phosphorization treatments is presented to synthesize the heterostructure combining NiSe2 and Ni2P coupled nanowrinkles on Ni foam (NiSe2-Ni2P/NF). DFT calculation results confirm that the NiSe2 coupled with Ni2P in NiSe2-Ni2P/NF can promote the water adsorption process, speeding up the whole catalytic kinetics for overall water splitting. Benefitting from the synergistic effects between NiSe2 and Ni2P, the optimal NiSe2-Ni2P/NF electrode exhibits excellent catalytic performances toward both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media. Impressively, for OER such heterogeneous nanowrinkles merely require an ultralow overpotential of 220 mV to reach the current density of 50 mA cm−2. Moreover, when utilized as a bifunctional catalyst for overall water splitting, the current density of 10 mA cm−2 can be delivered at an ultralow cell voltage of 1.50 V, which is among the lowest values of current non-precious metal catalysts. This work provides a unique strategy for exploring highly efficient heterostructure catalysts toward water splitting and beyond.

Graphical abstract

Both experimental and theoretical results indicate that constructing NiSe2-Ni2P heterostructure nanowrinkles not only achieves the coupling effects between two counterparts, but also optimizes the hydrogen adsorption and water adsorption energies, ultimately enhancing the catalytic activities of host catalysts.

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Introduction

Electrocatalytic water splitting consisting of the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER) has been extensively regarded as a feasible approach for generating and conversing of sustainable energy [1], [2], [3], [4]. Unfortunately, the efficiency of water splitting is seriously restricted by the sluggish kinetics of these two half reactions, especially by OER [5], [6], [7], [8]. Till now, the state-of-the-art catalysts are noble-metal based materials (Pt-based candidates for HER and Ir/Ru oxides for OER), which are still greatly impeded in wide applications owing to the prohibitive cost and scarcity [9], [10], [11], [12], [13]. To address this issue, it is extremely desirable to exploit alternatives with low-cost and earth-abundance. Although extensive efforts have been dedicated to developing catalysts in acidic or alkaline solutions, only a few of the instances are capable of simultaneously propelling both the HER and OER in the same medium, resulting in inefficiency of the electrolyzer because of incompatibility [5], [14], [15], [16], [17], [18]. Therefore, the development of cost-effective yet highly active bifunctional catalysts for water splitting in the same electrolyte is essential and worthwhile.

Transition-metal dichalcogenides (TMDs), especially the paramagnetic NiSe2 material, are emerging as one of the most promising bifunctional catalysts due to the suitable d-electron configuration, low cost and the abundant reserves [18], [19], [20]. However, the catalytic performance of NiSe2 toward overall water splitting is still unsatisfactory because of the poor HER activity [20], [21]. In this regard, numerous studies disclose that integrating TMDs with HER active materials to form heterostructure represents an efficient pathway to enhance OER, HER and the consequent water splitting activities of host catalysts, especially integrating with the typical Ni2P [14], [15], [22], [23]. For instance, Liu et al. demonstrated that integration with Ni2P could optimize the hydrogen adsorption and water adsorption kinetics and dramatically enhance the water splitting activity in comparison to pristine Ni3S2 [14]. In addition, constructing nanostructures on highly conductive Ni foam (NF) has been widely evidenced to dramatically promote the electrochemical performances, due to the porous structure and the consequent multidimensional electron transport pathway of NF [24], [25]. As demonstrated in recent reports, fabricating nanostructures on three-dimensional NF can fulfill the synergistic effect in heterostructures, because it not only provides large surface area and abundant active sites, but also effectively enhances the charge transfer, electrolyte penetration and gas diffusion [14], [24], [26]. Thus, it is reasonable to believe that constructing NiSe2-Ni2P coupled heterostructure on NF is capable of significantly enhancing the water splitting activity of host NiSe2. Unfortunately, there have been few reports focusing on coupling NiSe2 with Ni2P on NF for overall water splitting.

In this work, we present a synthetic strategy of NiSe2-Ni2P heterogeneous nanowrinkle structures on Ni foam (denoted NiSe2-Ni2P/NF). The NiSe2-Ni2P is directly grown on NF through hydrothermal processes and then successive selenization and moderate phosphorization treatments. Benefitting from the promoted water adsorption process, optimized hydrogen adsorption energy, and the synergistic effects between NiSe2 and Ni2P, the resulting NiSe2-Ni2P/NF electrode presents extremely high electrochemical activities toward both OER and HER in alkaline electrolytes, especially for OER. Most crucially, the electrolyzer assembled by the same material exhibits excellent performance toward overall water splitting accompanying with the remarkable durability, superior to that of the most current non-precious metal based electrocatalysts.

Section snippets

Synthesis and structural characterization of NiSe2-Ni2P/NF

NiSe2-Ni2P/NF heterostructure catalysts were synthesized through the partial phosphorization of NiSe2, which is schematically illustrated in Fig. 1. NiSe2 as the precursor was prepared by selenization treatments of Ni(OH)2/NF obtained by a hydrothermal process. Then, the synthesized NiSe2 was partially phosphatized to NiSe2-Ni2P/NF in the tube furnace with NaH2PO2·H2O as the phosphorus source. The as-prepared products were further analyzed by powder X-ray diffraction (XRD) patterns. As depicted

Conclusion

In summary, we successfully synthesized the NiSe2-Ni2P heterostructure catalysts supported on Ni foam. Both experimental and theoretical results indicate that constructing heterostructure not only achieves the coupling effects between two counterparts, but also optimizes the hydrogen adsorption and water adsorption energies, ultimately enhancing the catalytic activities of host catalysts. Impressively, the optimal NiSe2-Ni2P/NF catalyst shows not only excellent OER and HER catalytic

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 51672204), and the Fundamental Research Funds for the Central Universities (No. 2019-YB-012).

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