Nano Research DOI 10.1007/s12274-016-1112-z

Ternary NiCoP nanosheet arrays: An excellent bifunctional catalyst for alkaline overall water splitting Yingjie Li1, Haichuan Zhang1, Ming Jiang1, Yun Kuang1 (), Xiaoming Sun1,2 (), and Xue Duan1 1 2

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Institute for New Energy Materials & Low-Carbon Technologies, Tianjin University of Technology, Tianjin 300384, China

Received: 23 March 2016

ABSTRACT

Revised: 13 April 2016

Exploring bifunctional catalysts for the hydrogen and oxygen evolution reactions (HER and OER) with high efficiency, low cost, and easy integration is extremely crucial for future renewable energy systems. Herein, ternary NiCoP nanosheet arrays (NSAs) were fabricated on 3D Ni foam by a facile hydrothermal method followed by phosphorization. These arrays serve as bifunctional alkaline catalysts, exhibiting excellent electrocatalytic performance and good working stability for both the HER and OER. The overpotentials of the NiCoP NSA electrode required to drive a current density of 50 mA/cm2 for the HER and OER are as low as 133 and 308 mV, respectively, which is ascribed to excellent intrinsic electrocatalytic activity, fast electron transport, and a unique superaerophobic structure. When NiCoP was integrated as both anodic and cathodic material, the electrolyzer required a potential as low as ~1.77 V to drive a current density of 50 mA/cm2 for overall water splitting, which is much smaller than a reported electrolyzer using the same kind of phosphide-based material and is even better than the combination of Pt/C and Ir/C, the best known noble metal-based electrodes. Combining satisfactory working stability and high activity, this NiCoP electrode paves the way for exploring overall water splitting catalysts.

Accepted: 15 April 2016 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2016

KEYWORDS bifunctional catalysts, water splitting, NiCoP nanosheets, superaerophobic electrode

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Introduction

Splitting water into hydrogen and oxygen is a critical reaction for the conversion and storage of various types of energy, including solar and electrical energy [1–5]. The water splitting reaction consists of two half-reactions: the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), both of which

require catalysts to improve their efficiency and lower the overpotential [6, 7]. Noble metal-based catalysts with ultrahigh electrocatalytic activity for the HER and OER, like Pt and Ir, are not extensively employed in practical applications due to their high cost and scarcity [8–10]. In addition, Pt-group catalysts usually exhibit remarkable HER activities but poor OER performance, which is similar to Ir catalysts that only

Address correspondence to Yun Kuang, [email protected]; Xiaoming Sun, [email protected]

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exhibit good OER activities. Moreover, integrating different catalytic materials into a single water-splitting device requires a complicated preparation process, in which electrode mismatch always happens, resulting in poor electrolytic efficiency [11–13]. Therefore, it is imperative to exploit highly efficient and low-cost bifunctional electrocatalysts for overall water splitting, particularly those based on abundant elements, which could not only simplify the preparation process but also decrease the cost. Metal phosphide-based composites (MPCs) are known as excellent HER catalysts [8, 14–20] due to their moderate affinity to hydrogen, with appropriate adsorption of the H atoms and desorption of H2 [19, 21]. Recently, MPCs have been reported to show good catalytic OER performance, attributable to the formation of metal oxo/hydroxo species on the catalyst surface, which create a heterostructure with high OER activity [12, 22]. While significant progress has been made in the study of binary transition metal phosphides as bifunctional catalysts [12, 22, 23], the application of ternary metal phosphides in hydrogen or oxygen evolution reactions has been scarcely reported, to the best of our knowledge. In addition, previous spectroscopic and theoretical studies demonstrated that the incorporation of additional metal atoms into HER and OER catalysts is crucial for enhancing their electrocatalytic activity by tuning of the electronic structure [24–27]. Considering the excellent catalytic potential of phosphide-based materials with electronic structure tunable by atom doping, ternary metal phosphides should be superior bifunctional catalysts for overall water splitting, compared with their binary counterparts. Herein, we prepared ternary NiCoP nanosheet arrays (NSAs) via a facile urea precipitation method followed by phosphorization. Due to their excellent intrinsic electrocatalytic activity, fast electron transport, and unique superaerophobic structure, NiCoP NSA bifunctional catalysts exhibit ultrahigh water splitting performance in alkaline media. For cathodic HER performance, the NiCoP NSA electrode shows a low overpotential of ~133 mV to reach a current density of 50 mA/cm2. When used as an OER catalyst, this material provides a current density of 50 mA/cm2 at an overpotential of 308 mV, being one of the most

active non noble metal catalysts. As expected, an alkaline electrolyzer requiring a low potential of ~1.77 V to achieve a current density of 50 mA/cm2 can be successfully fabricated by using NiCoP NSAs as both OER and HER catalysts. In view of the satisfactory stability arising from its unique superaerophobic array structure, this NiCoP electrode paves the way for high-performance overall water splitting devices.

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Results and discussion

A typical fabrication procedure of NiCoP NSAs is schematically shown in Fig. 1(a), along with the scanning electron microscopy (SEM) characterization of the nanosheet structure. Briefly, the NiCoP nanosheets were fabricated on Ni foam via phosphorization of the hydrothermally prepared Ni-Co precursor using NaH2PO2 as a source of P. As seen in Figs. 1(b) and 1(c), the nanosheet morphology of the Ni-Co precursor can be well preserved after phosphorization, while its surface becomes rougher compared to that of the Ni-Co

Figure 1 (a) Schematic illustration of the NiCoP NSA fabrication process. High-resolution SEM images of (b) the Ni-Co precursor and (c) NiCoP NSAs. The large-scale SEM images of the Ni-Co precursor and NiCoP NSAs are insets in (b) and (c), respectively.

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precursor, which is further confirmed by transmission electron microscopy (TEM) imaging (Fig. S1 in the Electronic Supplementary Material (ESM)). The lowmagnification SEM image in the inset of Fig. 1(c) shows vertically aligned ~20 nm thick NiCoP nanosheets that have been uniformly grown on Ni foam. For comparison, Ni2P and Co2P nanosheet arrays were also prepared by the same method, with the corresponding characterization of these compounds provided in the supplementary information (Fig. S2 in the ESM). Figure 2(a) shows the X-ray diffraction (XRD) pattern of NiCoP NSAs. The characteristic peaks at 41.0°, 47.6°, and 54.5° can be indexed to (111), (210), and (300) planes of hexagonal NiCoP (JCPDS No. 71-2336) [28], respectively, and the two peaks at 44.5° and 51.8° arise from Ni foam (JCPDS No. 65-2865). The 1.37:1:1.07 atomic ratio of Ni, Co, and P was confirmed by the energy-dispersive X-ray (EDX) spectrum of NiCoP NSAs scraped off from the substrate after phosphorization (Fig. S3 in the ESM). A slight excess of elemental Ni could be attributed to the substrate Ni foam, revealing the formation of a 3D NiCoP NSAs/Ni foam electrode. The energy dispersive X-ray

spectroscopy (EDS) elemental mapping images for NiCoP NSAs in Fig. 2(b) further reflect the homogeneous distribution of the Ni, Co, and P elements, indicating a uniform transformation of the Ni-Co precursor into NiCoP nanosheets. The high-resolution TEM (HRTEM) and the corresponding selected area electron diffraction (SAED) images recorded for NiCoP NSAs show a well-resolved lattice fringe spacing of 0.220 nm, assigned to the (111) plane of NiCoP and further demonstrating the successful conversion of the Ni-Co precursor into NiCoP NSAs. X-ray photoelectron spectroscopy (XPS) analysis was then utilized to probe the surface chemistry of NiCoP NSAs. The high-resolution P 2p spectrum shows two main peaks at 134.1 and 129.3 eV (Fig. 2(d)), with the lower binding energy (BE) peak corresponding to metal phosphides, and the other reflecting the oxidized metal phosphate species on the surface, arising from exposure of superficial NiCoP to air [23, 29]. After peak deconvolution of the Co 2p3/2 core level region, two main peaks appear at 778.8 and 781.7 eV, with the peak located at 794.2 eV assigned to oxidized metal phosphate (Fig. S4(a) in the ESM) [28]. In the high-

Figure 2 (a) Wide-angle XRD pattern, (b) EDS elemental mapping images, and (c) high-resolution TEM and (inset) selected area electron diffraction images of NiCoP NSAs. (d) High-resolution XPS spectra of the P 2p region of NiCoP NSAs. www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano

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resolution Ni 2p region, two peaks appear at 853.1 and 856.5 eV, with another contribution at 852.1 eV assignable to the exposed Ni foam (Fig. S4(b) in the ESM) [30]. The peaks at 778.8 eV for Co 2p3/2, 853.1 eV for Ni 2p3/2, and 129.3 eV for P 2p3/2 are associated with the typical binding energies for Co 2p, Ni 2p, and P 2p contributions in NiCoP, respectively [29]. All of the above characterizations and analyses confirm the homogeneous conversion of the Ni-Co precursor to NiCoP NSAs on Ni foam. To gain insight into the water electrolysis activity of NiCoP NSAs, electrocatalytic hydrogen evolution was first evaluated using a typical three-electrode system in 1 M KOH. The NiCoP NSAs grown on Ni foam can be directly used as a working electrode, without employing extra substrates (such as the glassy carbon electrode) or binders (like Nafion). A saturated calomel electrode and Pt foil were used as reference and counter electrodes, respectively. Figure 3(a) shows the polarization curve of NiCoP NSAs obtained from

linear sweep voltammograms (LSV) at a scan rate of 5 mV/s. As a control, the electrocatalytic activities of Ni2P and Co2P NSAs, commercial Pt, and Ni substrates were also measured. To eliminate the effect of ohmic resistance, an iR compensation was applied to all original HER data to reflect the intrinsic catalytic behavior. The polarization curve recorded for NiCoP NSAs shows an overpotential onset as small as 80.9 mV. Despite the slightly lower onset potential of commercial Pt (with an overpotential close to zero), the NiCoP NSA electrode shows a fast increase of current density, largely outperforming commercial Pt at applied potentials negative than –0.12 V versus the reversible hydrogen electrode (RHE), featuring a much better catalytic performance. In sharp contrast, both of the metal phosphides, Ni2P and Co2P, exhibit relatively weak catalytic activities with an overpotential of 150 mV, although the electrochemical double-layer capacitances (EDLC) for Ni2P (~4.75 mF/cm2, Fig. S5(d) in the ESM) and Co2P (~4.32 mF/cm2, Fig. S5(f) in the

Figure 3 (a) Polarization curves of NiCoP NSAs, Ni2P NSAs, Co2P NSAs, Pt, and Ni foam electrodes for HER (iR-corrected), scan rate of 5 mV/s, inset: corresponding EIS. (b) Tafel pots of NiCoP NSAs, Ni2P NSAs, and Co2P NSAs. (c) Polarization curves of NiCoP NSAs, Ni2P NSAs, Co2P NSAs, Ir/C, and Ni foam electrodes for OER (iR-corrected). (d) Chronoamperometric response curves of NiCoP at a constant overpotential of ~160 mV for HER and ~350 mV for OER. | www.editorialmanager.com/nare/default.asp

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ESM) are larger than that of ~4.02 mF/cm2 for NiCoP nanosheets (Fig. S5(b) in the ESM) [31–33]. The NiCo-based oxide (Ni-Co-O), obtained by annealing the Ni-Co precursor in absence of a P source, displays low HER activity, with a large onset overpotential of ~130 mV vs. RHE (Fig. S6 in the ESM), indicating that successful transformation of the Ni-Co precursor into the ternary NiCoP can significantly improve HER activity. The catalysis kinetics for HER can be further examined by Tafel plots. The linear portions of the Tafel plots were fitted to the Tafel equation (η = blogj + a, where j is the current density and b is the Tafel slope), yielding Tafel slopes of ~68.6, ~84.9, and ~114.5 mV/dec for NiCoP, Co2P, and Ni2P NSAs, respectively, which suggests that electrochemical recombination with an additional proton is limiting the reaction rate [10, 11]. Noticeably, the Tafel slope of NiCoP NSAs is smaller than that of Co2P and Ni2P, indicating its high inherent electrocatalytic activity for driving a large catalytic current density at low overpotential [11]. In addition, the charge transfer resistance (Rct) in the electrochemical impedance spectrum (EIS) of NiCoP is much smaller than that of Ni2P and Co2P NSAs (inset of Fig. 3(a) and Fig. S7 in the ESM), further indicating a faster electron transfer [34–36]. The outstanding HER activity of the ternary NiCoP NSA electrode can be attributed to the incorporation of metal atoms into binary metal phosphides, which may modify their electronic structure as well as tune the hydrogen adsorption energy [24], leading to the improvement of intrinsic electrocatalytic performance and electron transport. The electrocatalytic activity of NiCoP NSAs in OER was also evaluated in 1 M KOH by applying a representative iR-corrected polarization curve with a sweep rate of 5 mV/s. As shown in Fig. 3(c), the NiCoP NSA electrode exhibits excellent OER performance, not only outperforming commercial Ir/C, but also being superior to Ni2P and Co2P nanosheets, despite their similar electrochemical surface area (ECSA) values (Fig. S5 in the ESM). Moreover, an overpotential of only ~308 mV is required for NiCoP NSAs to generate a current density of 50 mA/cm2. For comparison, the overpotentials to reach 50 mA/cm2 for Ni2P, Co2P, and

Ir/C are ~376, ~330, and ~349 mV, respectively, indicating superior OER performance of the NiCoP catalyst. The excellent OER electrocatalytic activity of NiCoP NSAs is attributable to the Ni-Co oxo/hydroxo species, which are key OER intermediates during oxygen evolution and are partially derived from the oxidization of Ni and Co atoms on the catalyst surface. This assumption is verified by the corresponding XRD pattern (Fig. S8 in the ESM), which shows the presence of NiOOH, Ni(OH)2·0.75H2O, and Co3O4 composites in the NiCoP electrode after a long-term OER test [37]. The Raman spectrum in Fig. S9 (in the ESM) shows that the 478 and 559 cm–1 peaks result from the presence of NiOOH on the surface of the NiCoP catalyst, and the characteristic peak at 524 cm–1 arises from Co(OH)2, further confirming the emergence of Ni-Co oxo/ hydroxo species [38, 39]. The oxidation of surface Ni and Co atoms could also be evidenced by the Ni(II) →  Ni(III) or/and Co(II) → Co(III)/Co(IV)-oxo oxidation peaks between 1.10 and 1.40 V in the LSV curves of all samples (Fig. 3(c)) and the disappearance of phosphide peaks in the P 2p region of the XPS spectrum (Fig. S10 in the ESM), which is consistent with previous studies [12, 22, 40–42]. The OER behavior of NiCoP NSAs is comparable and even superior to that of the previously reported Janus Ni2P, porous CoP/phosphate, and CoP nanorods [12, 22, 23]. A detailed comparison of different OER catalysts with various configurations is listed in Table S1 (in the ESM), further verifying the outstanding catalytic behavior of NiCoP NSAs. Long-term stability is a crucial criterion to evaluate the performance of catalysts [43–45]. At first, continuous HER at a static overpotential of ~160 mV was conducted. As shown in Fig. 3(d) (black line), the chronoamperometric response reveals high HER stability of NiCoP NSAs, with current attenuation of less than 10% even after a long time period (~10 h). Subsequently, the working stability of OER was also tested under similar conditions, except for the applied overpotential of ~350 mV. As expected, no obvious loss of current density happens concomitantly with the increase of working time (red line in Fig. 3(d)). The excellent HER and OER stabilities observed demonstrate that NiCoP NSAs directly grown on the conductive substrates are strongly bonded to them

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[46]. The ordered array morphology can also provide negligible bubble adhesion (Fig. S11 in the ESM) and enough bubble channels to promote the release of gas bubbles away from the electrode surface without accumulation, thus preventing bubbles from peeling off catalysts from substrates [47, 48], as shown in Fig. S12 (in the ESM). In contrast, commercial Pt/C and Ir/C electrodes, fabricated by drop casting on Ni foam, show a large bubble adhesive force (Fig. S13 in the ESM). Strong adhesion to gas bubbles prevents them from leaving the catalyst surface, and the accumulated bubbles peel off catalysts as they escape from the surface, leading to a large decrease of activity. Based on the above results, we could reasonably deduce that NiCoP NSA electrodes can serve as bifunctional catalysts for water splitting. Accordingly, an electrolyzer containing 1 M KOH was examined using NiCoP NSAs as both anode and cathode. From the current-potential response of this electrolyzer (Fig. 4(a)), water electrolysis for NiCoP NSAs with a current density of 50 mA/cm2 takes place at a cell voltage of ~1.77 V (without iR-correction), comparable to previously reported work (Table S2 in the ESM) [6, 12, 40]. For comparison, bifunctional Ni2P and Co2P catalysts, used as both anodic and cathodic materials, as well as the integration of commercial Pt/C as cathode and Ir/C as anode, were also examined (Fig. 4(a)). It is worth noting that our NiCoP||NiCoP electrodes exhibit superior activity compared with Ni2P||Ni2P, Co2P||Co2P, and state-of-the-art Pt/C||Ir/C at all applied electrolysis potentials, which can be attributed to

the modified electronic structure of multicomponent phosphide-based materials and the unique superaerophobic structure of the three-dimensional nanosheet array electrode. The electrolyzer stability was evaluated at 2.0 V in 1 M KOH solution. The chronoamperometric response demonstrates the advantages of NiCoP NSAs over the post-coated noble metal-based catalysts, which are prone to a large loss of current even at the originally small current density of ~47 mA/cm2 (Fig. 4(b)).

3

Summary

In summary, ternary NiCoP NSAs were successfully prepared by an easily scalable and cost-effective hydrothermal reaction and phosphorization process, and the novel catalyst exhibits significant bifunctional catalytic efficiency for both HER and OER. The excellent HER catalytic performance of NiCoP NSAs can be attributed to its high intrinsic activity exemplified by a low Tafel slope of ~68 mV/dec and favorable kinetics with a small charge transfer impedance. The observed OER activity is superior to some state-ofthe-art catalysts, generating a 50 mA/cm2 current density at an overpotential of only ~300 mV, due to the NiCo oxo/hydroxyl species generated in situ during surface oxidation under catalytic conditions. When used as water electrolysis catalysts, NiCoP NSAs offer a current density of 50 mA/cm2 at ~1.77 V, which provides an alternative pathway for future photo- and electrochemical water splitting devices using noble metal free catalysts as anode and cathode.

Figure 4 (a) Polarization curves of an alkaline electrolyzer using NiCoP||NiCoP, Ni2P||Ni2P, Co2P||Co2P, and Pt/C||Ir/C as two electrodes in 1 M KOH, with a scan rate of 5 mV/s. (b) Chronoamperometric electrolysis in 1 M KOH at a constant potential of ~2.0 V for NiCoP||NiCoP and commercial Pt/C||Ir/C over 10 h. | www.editorialmanager.com/nare/default.asp

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Acknowledgements This work was support by the National Natural Science Foundation of China (Nos. 21125101 and 21520102002), the Program for Changjiang Scholars and Innovative Research Team in the University, and the Fundamental Research Funds for the Central Universities, and the long-term subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC. Electronic Supplementary Material: Supplementary material (experimental details and supplementary figures) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-016-1112-z.

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