Extraction of nickel from NiFe-LDH into Ni2P@NiFe hydroxide as a bifunctional electrocatalyst for efficient overall water splitting.

Rui-Rui Liu,Fang-Shuai Zhang,Jia-Wei Wang,Chun-Ting He,Tong-Bu Lu,Jun Luo,Zhi-Ming Zhang

doi:10.1039/c7sc04569g

Abstract

The development of highly efficient, low-cost and stable electrocatalysts for overall water splitting is highly desirable for the storage of intermittent solar energy and wind energy sources. Herein, we show for the first time that nickel can be extracted from NiFe-layered double hydroxide (NiFe-LDH) to generate an Ni2P@FePO x heterostructure. The Ni2P@FePO x heterostructure was converted to an Ni2P@NiFe hydroxide heterostructure (P-NiFe) during water splitting, which displays high electrocatalytic performance for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1.0 M KOH solution, with an overpotential of 75 mV at 10 mA cm-2 for HER, and overpotentials of 205, 230 and 430 mV at 10, 100 and 1000 mA cm-2 for OER, respectively. Moreover, it could afford a stable current density of 10 mA cm-2 for overall water splitting at 1.51 V in 1.0 M KOH with long-term durability (100 h). This cell voltage is among the best reported values for bifunctional electrocatalysts. The results of theoretical calculations demonstrate that P-NiFe displays optimized adsorption energies for both HER and OER intermediates at the nickel active sites, thus dramatically enhancing its electrocatalytic activity.

Highlights

To address energy and environmental problems, hydrogen fuel has been regarded as a promising alternative to fossil fuels for its high energy density and cleanness.[1]
At rst, the vertically aligned NiFe-LDH nanosheets were uniformly grown on the NF substrate by a hydrothermal method, as observed by Scanning electron microscopy (SEM) (Fig. S1†), and its phase purity was con rmed by the X-ray diffraction (XRD) measurement (Fig. S2†)
The surface oxidation of Ni2P into Ni(OH)[2] and the transformation of amorphous iron phosphate into Fe(OH)[3] in Ni2P@FePOx a er controlled-current electrolysis (CCE) were further con rmed by the results of Raman spectra measurements (Fig. S15†), which show very weak characteristic peaks of NiFe hydroxide, indicating the presence of trace amounts of NiFe hydroxide on the surface of the Ni2P nanoparticles.13c,23 The results of electrochemical impedance spectroscopy (EIS) measurements indicate that the conductivity of P-NiFe a er hydrogen evolution reaction (HER) is dramatically increased compared to that of NiFe-LDH a er HER (Fig. S16a†), which can be attributed to the existence of highly conductive metallic Ni2P nanoparticles in PNiFe

Summary

Introduction

X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition on the surface of Ni2P@FePOx. the XPS of as-prepared Ni2P@FePOx material exhibits a binding energy peak at 853.5 eV, and this peak is absent in the XPS pattern of NiFe-LDH, which can be attributed to metallic nickel in the nickel phosphide (Ni2P).15b The presence of Ni2P in the Ni2P@FePOx sample can be further con rmed by the X-ray diffraction (XRD) measurements, which show the characteristic diffraction peaks of Ni2P (Fig. S5†).

Results

Conclusion