A nickel iron diselenide-derived efficient oxygen-evolution catalyst

Introduction

Sunlight-driven water splitting or carbon dioxide (CO2) reduction to make solar fuels is a promising solution to solar energy storage1. Essential to the water splitting and CO2 reduction reactions is the oxygen-evolution reaction (OER). This reaction is kinetically sluggish and demands an efficient electrocatalyst2. Although noble metal-based catalysts such as IrO2 and RuO2 exhibit good OER activity, their scarcity and high cost pose great constrains for large-scale applications. Tremendous efforts have been made in recent years to develop non-precious OER catalysts3,4. Despite the progress, a significant overpotential is still required for state-of-the-art catalysts. To reach 10 mA cm−2, a widely used figure of merit equivalent to ∼12% solar to hydrogen efficiency, nearly all non-precious catalysts need an overpotential of more than 250 mV.

The majority of non-precious OER catalysts are metal oxides and (oxy)hydroxides3,4. Recently, a few non-oxide-based OER catalysts including metal phosphides, sulfides and selenides are reported5,6,7,8,9,10,11. Given the limited stability of these compounds under highly oxidative potentials in alkaline solutions, questions have arisen on the nature of the true active species. Indeed, we and others showed that the surfaces of Ni2P and CoP were transformed into metal oxides during catalysis, which were responsible for the catalytic activity5,6,7. For Ni, Co and Fe sulfides, Chen et al. showed that they were entirely transformed into the corresponding metal oxides during OER8. Since metal selenides have similar chemical reactivity to metal sulfides, we were surprised by previous reports which suggested stability of bulk NiSe, Ni3Se2 and CoSe2 materials under OER9,10,11. Using a post-catalytic analysis, we show here that NiSe is completely converted into nickel hydroxides during OER, indicating that metal oxides or hydroxides are the active and final forms of metal selenides pre-catalysts in OER. This knowledge promoted us to purposely use metal selenides as templating precursors to highly active metal oxide OER catalysts, because methods to produce ultrasmall nanostructured metal selenides are readily available12,13,14,15. Following this strategy, we synthesize a hitherto unknown selenide, nickel iron diselenide (NixFe1−xSe2), which upon in situ transformation into oxides, catalyses OER with an overpotential of only 195 mV for a current density of 10 mA cm−2. This is until now the most active single-phase OER catalyst in alkaline solutions. The high activity of this NixFe1−xSe2-derived catalyst is largely due to its desirable nanostructure, inherited from its selenide precursor.

Results

Active form of NiSe in OER

NiSe was synthesized via a hydrothermal approach using Ni foam as the precursor9. The electrocatalytic activity of NiSe towards OER was investigated in 1 M KOH using a three-electrode electrochemical system. Galvanostatic scan at the current density of 10 mA cm−2 was used to activate the catalyst. The activity was then measured by linear sweep voltammetry (LSV) at a scan rate of 1 mV s−1. The overpotentials to reach 10 mA cm−2 was ∼253 mV (Supplementary Fig. 1), in agreement with previous reports. The morphology and composition of the catalyst after catalysis for 12 h were then examined. Transmission electron microscopy (TEM) image showed that single-crystal nanowires of NiSe were converted to polycrystalline particles made of ultrathin nanosheets (Fig. 1a,b). Selected area electron diffraction (SAED) pattern of the sample after OER can be indexed to (111), (103) and (301) planes of α-Ni(OH)2 (Fig. 1b) (space group: P-31m, JCPDS No. 22-0444). The different lattice fringes in high-resolution TEM (HRTEM) images of samples before and after OER confirmed the total conversion of NiSe into Ni(OH)2. Similarly, the elemental mapping of the sample after OER showed that the Se content decreased from 50.2% to 4.0%, but the oxygen content increased from 2.4 to 52.8% during the transformation (Fig. 1c,d). Thus, Se was nearly completely removed while oxygen was incorporated during OER. The above data indicate that NiSe is entirely converted into Ni(OH)2 under OER conditions, which is the active form of the catalyst. Interestingly, the activity of the NiSe-derived Ni(OH)2 is higher than the most active Ni(OH)2 nanoparticles prepared by direct synthesis, which requires 300 mV to reach 10 mA cm−2 (refs 16, 17). This result suggests that metal selenides may serve as templating precursors to metal oxides or hydroxides with superior OER activity than those prepared by other methods. With this in mind, we turn our attention to nickel iron selenides, as NiFeOx is one of the most active OER catalysts18,19,20,21.

Figure 1: Structural and compositional characterization of NiSe.

(a,b) TEM and HRTEM images and SAED patterns before (a) and after (b) OER. (c,d) Elemental mapping images and corresponding EDS before and after OER. Colours in elemental mapping images: green for Ni; and blue for Se. Cu and C signals in the EDS are from the copper grid and carbon film that are used to support the sample for TEM measurements. K is due to the residual electrolyte (1 M KOH). The inset in the spectra shows the elemental atomic percentages. Scale bar: (a) 50 nm; and (b) 50 nm. Insets in a,b, 2 nm; and c, 20 nm. EDS, energy-dispersive spectroscopy.