Hierarchically Porous Urchin-Like Ni2P Superstructures Supported on

Dec 21, 2015 - The development of high-performance nonprecious electrocatalysts with both H2 and O2 evolution reaction (HER and OER) activities for ov...
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Hierarchically Porous Urchin-like Ni2P Superstructures Supported on Nickel Foam as Efficient Bifunctional Electrocatalysts for Overall Water Splitting Bo You, Nan Jiang, Meili Sheng, Winona Bhushan, and Yujie Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02193 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 23, 2015

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Hierarchically Porous Urchin-Like Ni2P Superstructures Supported on Nickel Foam as Efficient Bifunctional Electrocatalysts for Overall Water Splitting Bo You, Nan Jiang, Meili Sheng, Winona Bhushan, and Yujie Sun* Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States E-mail: [email protected]; Fax: +1-435-797-3390; Tel: +1-435-797-7608

ABSTRACT: The development of high performance nonprecious electrocatalysts with both H2 and O2 evolution reaction (HER and OER) activities for overall water splitting is highly desirable but remains a grand challenge. Herein, we report a facile two-step method to synthesize three dimensional hierarchically porous urchin-like Ni2P microsphere superstructures anchored on nickel foam (Ni2P/Ni/NF) as bifunctional electrocatalysts for overall water splitting. The Ni2P/Ni/NF catalysts were prepared by template-free electrodeposition of porous nickel microspheres on nickel foam followed by phosphidation. The hierarchically macroporous superstructures with 3D configuration can reduce ion transport resistance and facilitate the diffusion of gaseous products (H2 and O2). The optimal Ni2P/Ni/NF exhibited remarkable catalytic performance and outstanding stability for both HER and OER in alkaline electrolyte (1.0 M KOH). For HER, the Ni2P/Ni/NF afforded a current density of 10 mA cm−2 at a low overpotential of only −98 mV. When served as an OER electrocatalyst, Ni2P/Ni/NF was partially oxidized to nickel oxides/hydroxides/oxyhydroxides (mainly NiO) on the catalyst surface and exhibited excellent OER activity with small overpotentials of 200 and 268 mV to reach 10 and 100 mA cm−2, respectively. Furthermore, when Ni2P/Ni/NF was employed as the electrocatalysts for both cathode and anode, a water splitting electrolyzer was able to reach 10 and 100 mA cm−2 in 1.0 M KOH at cell voltages of 1.49 and 1.68 V, respectively, together with robust durability. Various characterization techniques and controlled experiments indicated that the superior activity and strong stability of Ni2P/Ni/NF for overall water splitting originated from its electrochemically active constituents, 3D interconnected porosity, and high conductivity.

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KEYWORDS: superstructure, hydrogen evolution, oxygen evolution, water splitting, nickel phosphide, electrocatalysis

INTRODUCTION The increasing global energy demands and climate change resulting from fossil fuel utilization prompt intense research efforts in the exploration of various catalytic systems for the conversion and storage

of

renewable

and

carbon-neutral

energies.1-3

In

particular,

electrochemical

or

photoelectrochemical water splitting with renewable energy input, such as solar and wind, to produce dihydrogen (H2) offers a promising strategy for this purpose.4-6 Efficient water splitting requires high performance electrocatalysts to facilitate its two half reactions: hydrogen evolution reaction (HER) at cathode and oxygen evolution reaction (OER) at anode.7 Currently, the state-of-the-art catalysts to split water are iridium and ruthenium oxides for OER and platinum for HER, requiring an overall potential of ~1.50 V to reach a current density of 10 mA cm-2.8 However, the price and scarcity of these noble metalbased catalysts significantly prohibit their large-scale commercialization.8-10 It is therefore highly desirable to develop efficient and low-cost electrocatalysts for both HER and OER. Most recent efforts have mainly focused on transition metal-based electrocatalysts owing to their abundance, diversity, potential stability, and theoretically high catalytic activity.11 For example, transition metal sulfides, carbides, phosphides, and selenides including MoS2,12-14 WC,15 CoS,16,17 Co-FeS2,18 NiPx and NiSx,19 MoCx,20 MoP,21 CoPx,7,22,23 and CoSe224 have shown promising HER catalytic performance in acidic electrolytes. On the other hand, transition metal oxides/hydroxides/perovskites such as NiFeOx,8 Ba0.5Sr0.5Co0.8Fe0.2O3-σ,25 Co3O4/C,26 Mn3O4,27 and NiFe layered double hydroxide (LDH)28 were reported to be active for OER in alkaline electrolytes. However, the currently prevalent strategies often result in incompatible integration of the HER and OER catalysts in the same electrolyte and hence mediocre overall performance.7,8 Using bifunctional OER and HER catalysts under the same condition to achieve overall water splitting has advantages of simplifying the electrolyzer system and lowering the overall cost. As the overpotential of OER tends to be several-fold higher than that of HER, most nonprecious OER 2 ACS Paragon Plus Environment

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catalysts are vulnerable in acidic solution, and electrolyte conductivity is much higher at extreme pH values than that of neutral electrolytes, we reason it is more economically viable to conduct overall water splitting in strongly alkaline electrolyte than in acidic or neutral media.7,29 In this regard, our group and others were motivated to develop overall water splitting catalysts functioning under basic conditions (1.0 M KOH). For example, Grätzel and co-workers have recently reported that NiFe layered double hydroxide grown on nickel foam (NiFe LDH/NF) acted as a bifunctional catalyst in 1.0 M KOH to split water at a voltage of 1.7 V to deliver a current density of 10 mA cm−2.29 Shalom and co-workers showed that Ni5P4-based alkaline electrolyzer (1.0 M KOH) approached 10 mA cm−2 at ~1.67 V.30 Our group recently reported that the voltage of overall water splitting required to achieve 10 mA cm−2 was only 1.64 V for an electrodeposited cobalt-phosphorous-derived film.7 Despite these exciting progresses, most reported bifunctional electrocatalysts still need a relatively higher overpotential to maintain a specific current density than that of the Pt/IrO2 couple.8 To further promote the catalytic reaction kinetics and improve the energy efficiency of overall water splitting, an optimal electrocatalyst structure with rationally designed reaction interface is essential for both triphase HER and OER processes (solid, liquid, and gas).26 Toward these goals, herein we report 3D hierarchically porous urchin-like Ni2P superstructures anchored on nickel foam (Ni2P/Ni/NF) as bifunctional electrocatalysts for overall water splitting, which were readily prepared by template-free electrodeposition of porous Ni microspheres on nickel foam followed by low-temperature phosphidation. The hierarchically macroporous structure with 3D configuration buffers electrolyte to reduce ion transport resistance and facilitates gas (H2 and O2) diffusion to boost the utilization efficiency of active species.28,31,32 With the tailored architecture, when directly used as bifunctional electrocatalysts in 1.0 M KOH our Ni2P/Ni/NF electrocatalysts can achieve a current density of 10 mA cm−2 at overpotentials of −98 mV for HER and 200 mV for OER, respectively. Moreover, a water electrolyzer utilizing Ni2P/Ni/NF as electrocatalysts for both cathode and anode approached 10 and 100 mA cm−2 at cell voltages of 1.49 and 1.68 V, respectively, along with excellent stability, superior to most reported systems employing nonprecious bifunctional electrocatalysts. 3 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION For the fabrication of Ni2P/Ni/NF (see the Experimental Section for details), the porous Ni microsphere superstructures with open space were firstly grown on commercial 3D porous nickel foam by a facile template-free cathodic electrodeposition at a constant current density of −1.0 A cm−2 for 500 s. the concomitant hydrogen bubble during electrodeposition served as a template accounting for the formation of porous Ni microsphere superstructures. Subsequently, the resulting porous Ni/Ni foam (Ni/NF) was subjected to low-temperature phosphidation to obtain the 3D hierarchically porous Ni2P/Ni/NF frameworks. X-ray diffraction (XRD) pattern of Ni/NF confirmed the formation of metallic Ni (Figure S1 in the Supporting Information). As revealed by the scanning electron microscopy (SEM) images shown in Figures 1a-d, Ni/NF exhibited 3D interconnected open macropores with a continuous size distribution ranging from 100 to 350 µm, which is similar to that of the pristine nickel foam. In addition, there are abundant smaller macropores with diameters of appropriately 10 µm on the interconnected opened macropore walls of Ni/NF (Figure 1b), in sharp contrast to the smooth surface of the pristine nickel foam (Figure 1a inset). A closer inspection of the smaller macropore in a high-magnification SEM image (Figures 1c-d) revealed an open porous structure composed of stacked Ni microspheres with smooth surface. These key differences in morphology clearly demonstrated the successful electrodeposition of porous Ni microsphere superstructures on nickel foam. Upon low-temperature phosphidation, the metallic Ni microspheres were partially transformed to Ni2P (denote as Ni2P/Ni/NF), as revealed by the XRD patterns (Figure 1e). The SEM images (Figure 1f) indicated that the resulting Ni2P/Ni/NF inherited the overall 3D hierarchically porous morphology of the original Ni/NF. High-magnification SEM images of Ni2P/Ni/NF (Figures 1g-h) indicated that most of the stacked microparticle superstructures with smooth surface became rougher (similar to urchin) after phosphidation and consisted of numerous Ni2P nanosheets, drastically different from the featureless morphology of a control sample prepared by direct phosphidation of a nickel foam (Figure S2), named as Ni2P/Ni. The high-resolution SEM image of a Ni2P nanosheets in Ni2P/Ni/NF revealed the presence of plentiful holes (Figure 1h inset). Figure 1i showed the corresponding elemental mapping images of Ni and P of Ni2P/Ni/NF demonstrated that both Ni and P 4 ACS Paragon Plus Environment

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were uniformly distributed throughout the whole sample, strongly verifying the successful chemical conversion of Ni/NF to Ni2P/Ni/NF via the low-temperature phosphidation. ICP analysis indicated the incorporation of P with an amount of 0.523 mmol cm-2. X-ray photoelectron spectroscopic (XPS) analysis revealed the presence of Ni and P in Ni2P/Ni/NF (Figure S3), consistent with the elemental mapping results (Figure 1i). The high-resolution Ni 2p3/2 spectrum was deconvoluted into three sub-peaks at binding energies of 852.8, 856.8, and 861.9 eV (Figure S3a), assignable to Niδ+ in Ni2P, oxidized Ni species, and Ni 2p3/2 satellite peak of Ni2P, respectively.19,33 Similarly, the high-resolution P 2p spectrum (Figure S3b) could be fitted by three sub-peaks at 129.1, 129.9, and 134.1 eV, corresponding to P 2p3/2, P 2p1/2, and oxidized phosphorus species (arising from superficial oxidation due to air contact), respectively.19 The Ni 2p peak at 852.8 eV is positively shifted from that of metallic Ni, and the P 2p peak at 129.2 eV is negatively shifted relative to that of elemental P (130.2 eV). It suggests Ni in Ni2P/Ni/NF has a partial positive charge (δ+) while P has a partial negative charge (δ-), implying the transfer of electron density from Ni to P, which is consistent with previous reports.19,33 It is anticipated that the unique superstructure character of our Ni2P/Ni/NF is beneficial for electrocatalysis. First, the 3D interconnected configuration can facilitate substrate transport/diffusion to the interior surfaces of the urchin-like Ni2P microparticle superstructures.32 Second, the large primary macropores (100 to 350 µm) and complementary macropores (~10 µm) will benefit the diffusion of gaseous products (H2 and O2) during water splitting. Third, the open urchin-like Ni2P in the porous nickel foam is able to effectively interact with substrates because of the high accessibility of active sites due to the interconnected macropores.26 Fourth, direct anchor of Ni2P/Ni composites on conductive nickel foam eliminates the use of an extra polymer binder and thus minimizes the overpotential loss resulting from the electrocatalyst/electrode interface.

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Figure 1. (a and inset) SEM images of nickel foam (NF). (b-d) SEM images of Ni/NF at different magnification. (e) XRD pattern of Ni2P/Ni/NF with the corresponding standard patterns of Ni2P and Ni. (f-h) SEM images of Ni2P/Ni/NF at different magnification. (i) SEM and the corresponding elemental mapping of Ni2P/Ni/NF.

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Figure 2. (a) HER polarization curves of Ni/NF, Ni2P/NF, Ni2P/Ni/NF, and Pt/C catalysts at a scan rate of 2 mV s−1. The inset shows the corresponding Tafel plots of Ni2P/NF, Ni2P/Ni/NF, and Pt/C. (b) The Nyquist plots of Ni2P/NF and Ni2P/Ni/NF measured at the overpotential of −120 mV. Cyclic voltammograms of (c) Ni2P/NF and (d) Ni2P/Ni/NF at scan rates from 5 to 100 mV s−1. (e) Scan rate dependence of the current densities of Ni2P/NF and Ni2P/Ni/NF at 0.314 V vs RHE. (f) Chronopotentiometric curve of Ni2P/Ni/NF at J = −10 mA cm−2. All experiments were carried out in 1.0 M KOH.

The electrocatalytic HER performance of our Ni2P/Ni/NF was first studied by steady-state linear sweep voltammetry (LSV) in H2-saturated 1.0 M KOH. We varied the electrodeposition duration and phosphidation temperature to optimize the electrocatalytic performance of Ni2P/Ni/NF for water splitting. As shown in Figure S4, the best HER and OER activity was achieved with an electrodeposition time of 500 s and phosphidation temperature of 400 oC (denoted as Ni2P/Ni/NF for brevity). Hereafter, all the electrochemical studies were conducted using Ni2P/Ni/NF prepared under the optimal conditions unless 7 ACS Paragon Plus Environment

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noted otherwise. In Figure 2a, the polarization curve of the Ni2P/Ni/NF demonstrated a much smaller onset potential of ∼ −40 mV vs the reversible hydrogen electrode (RHE) and greater catalytic current than those of Ni/NF and Ni2P/NF samples, highlighting the important role of the tailored nanostructure. Noticeably, the HER current density of Ni2P/Ni/NF beyond −112 mV also largely exceeded that of a commercial 20 wt% Pt/C catalyst, although Pt/C exhibited a smaller onset potential (~0 mV). More importantly, our Ni2P/Ni/NF afforded a current density of 10 mA cm−2 at an overpotential (η) of only −98 mV, which is much lower than those of Ni/NF, Ni2P/NF, and most reported nonprecious HER catalysts in 1.0 M KOH, such as H2-CoCat (> −385 mV),5 MoCx (−151 mV),20 CoP/CC (−209 mV),23 NiFe LDH (> −200 mV),29 Ni5P4 (−150 mV),30 Ni2P nanoparticles (−221 mV)33 and CoOx@CN (−232 mV).34 A more detailed comparison on HER activity of Ni2P/Ni/NF and other reported nonprecious catalysts is included in Table S1. Additionally, the smaller Tafel slope (72 mV dec−1) and larger exchange current density (0.845 mA cm-2) compared to those of Ni2P/NF (93 mV dec−1 and 0.607 mA cm-2) and Pt/C (109 mV dec−1 and 0.824 mA cm-2) suggested the more favorable HER kinetics on Ni2P/Ni/NF (Figure 2a). The much higher HER activity of our Ni2P/Ni/NF compared to that of Ni2P/NF was also supported by its smaller semicircular diameter in the electric impedance spectrum (Figure 2b), implying smaller contact and charge transfer impedance of the former. To further understand the improved HER performance of Ni2P/Ni/NF compared to that of Ni2P/NF, their electrochemically active surface areas (ECSA) were estimated from the electrochemical double-layer capacitance (Cdl) by collecting cyclic voltammograms in a non-Faradaic region of 0.264 to 0.364 V vs RHE (Figures 2c-d).7,8,17,28 It is widely accepted that the ECSA of a material with similar composition is proportional to its Cdl, which can be derived from the linear slope of its current density versus scan rate.35 Figures 2c-d clearly demonstrated that Ni2P/Ni/NF exhibited much higher capacitance current at the same scan rate, implying a larger Cdl. The calculated Cdl of Ni2P/Ni/NF was ~ 27 mF cm−2, significantly higher than that of Ni2P/NF (12 mF cm−2, Figure 2e). These results unambiguously demonstrated that Ni2P/Ni/NF possessed higher ECSA which allowed the more effective accessibility of its active sites. Furthermore, the 3D hierarchically macroporous configuration of Ni2P/Ni/NF could facilitate gas release as well as mass transport, collectively resulting in 8 ACS Paragon Plus Environment

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much higher HER activity. To elucidate the important role of this 3D hierarchically macroporous superstructures, a commercial Ni foil without 3D features was employed to fabricate a control sample (named as Ni2P/Ni/Ni foil) by using the same preparation procedure of Ni2P/Ni/NF. Electrochemical measurements of the Ni2P/Ni/Ni foil control sample exhibited much inferior catalytic HER and OER activities compared to those of Ni2P/Ni/NF (Figure S5). These results unambiguously demonstrated the beneficial effect of the unique architecture of Ni2P/Ni/NF for electrocatalysis. Besides excellent HER efficiency, Ni2P/Ni/NF also showed robust long-term electrochemical stability in 1.0 M KOH. As plotted in Figure 2f, a chronopotentiometry experiment of Ni2P/Ni/NF was conducted at a current density of −10 mA/cm2 and a stable overpotential of ~ −114 mV was maintained during the 20 h electrolysis.

Figure 3. (a-c) SEM images of Ni2P/Ni/NF after 20 h HER stability testing at different magnifications (post-HER). (d) SEM and the corresponding elemental mapping of post-HER Ni2P/Ni/NF. 9 ACS Paragon Plus Environment

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SEM, XRD, and XPS were employed to probe the structure and composition details of the Ni2P/Ni/NF electrocatalyst after the 20 h HER stability test (denoted as post-HER Ni2P/Ni/NF). The lowmagnification SEM image in Figure 3a revealed that the post-HER Ni2P/Ni/NF still maintained the overall 3D hierarchically porous structure. High magnification SEM images (Figures 3b-c) suggested their urchin-like morphology were similar to that of the fresh Ni2P/Ni/NF. The corresponding elemental mapping results indicated the retained uniform spatial distribution of Ni and P in the post-HER Ni2P/Ni/NF (Figure 3d). XRD patterns demonstrated the presence of Ni2P composition in the post-HER Ni2P/Ni/NF, nearly identical to that of the as-prepared counterpart (Figure 4a). In addition, the similarity of high-resolution Ni and P XPS spectra (Figures 4b-c) of the fresh and post-HER Ni2P/Ni/NF samples also confirmed the retention of the electrocatalysts in terms of morphology and composition, corroborating its superior robustness for HER electrocatalysis (Figure 2f).

Figure 4. (a) XRD pattern and High-resolution (b) Ni 2p3/2 and (c) P 2p XPS spectra for fresh, post-HER and post-OER Ni2P/Ni/NF samples.

Different from the aforementioned HER investigation, when Ni2P/Ni/NF was used as an OER electrocatalyst in the same electrolyte (O2-saturated 1.0 M KOH), an initial activation phenomenon was observed (see below, Figure 5f), which is similar to our previously reported Co-P films.7 Therefore, all the following polarizations were collected after the cessation of the catalyst activation. As shown in 10 ACS Paragon Plus Environment

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Figure 5a, all the three samples (Ni/NF, Ni2P/NF, and Ni2P/Ni/NF) showed quasi reversible oxidation and reduction peaks prior to OER catalytic current beyond 1.5 V vs RHE. In order to avoid the interference of the catalyst oxidation feature, the cathodic sweep curves (reverse scans) in the corresponding cyclic voltammograms of the three catalysts were employed for quantitative comparison n Figure 5a inset. Apparently, Ni2P/Ni/NF exhibited a much smaller onset potential at ~1.42 V vs RHE and greater catalytic current than those of Ni/NF and Ni2P/NF, manifesting the positive synergistic effects of electrodeposited porous Ni microsphere superstructures and subsequent phosphidation. Noticeably, our Ni2P/Ni/NF afforded a current density of 10 mA cm−2 at a very small overpotential of 200 mV, substantially lower than those of Ni/NF, Ni2P/NF, and most reported nonprecious catalysts including NiMoFe-(b) (320 mV),6 Co-P film (345 mV),7 NiFeOx (>220 mV),8 Co3O4/C (220 mV),26 NiFe LDH/NF (210 mV),29 CoOx@CN (260 mV),34 NiFe NS (302 mV),35 NiCo-r NS (320 mV),36 CoMn LDH (324 mV),37 Co3O4/N-rmGO (310 mV),38 [Ni-Fe] LDH (260 mV)39, and ECT-Co0.37Ni0.26Fe0.37O (232 mV)40 (see Figure 5b and Table S2 for more details). To compare the intrinsic activity, turnover frequencies (TOFs) were also estimated. By integrating the (nominally) Ni3+/2+ anodic redox peak in the CV curves (Figure 5a), the calculated TOF of Ni2P/Ni/NF at η = 350 mV is 0.015 s-1, higher than that of Ni2P/NF (0.013 s-1).41 Remarkably, Ni2P/Ni/NF was able to reach catalytic current densities of 100 and 1000 mA cm−2 at overpotentials of 268 and 375 mV, respectively. To the best of our knowledge, such an outstanding OER activity of a nonprecious catalyst in 1.0 M KOH has rarely been reported.

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Figure 5. (a) CV curves of Ni/NF, Ni2P/NF, and Ni2P/Ni/NF catalysts at a scan rate of 2 mV s−1. The inset shows the corresponding cathodic sweep curves which were used to calculate the overpotential and also avoided the interference of catalyst oxidation features. (b) Comparison of the overpotential requirement to achieve 10 mA cm−2 between our Ni2P/Ni/NF and other reported nonprecious OER catalysts. Cyclic voltammograms of (c) Ni2P/NF and (d) Ni2P/Ni/NF at scan rates from 5 to 100 mV s−1. (e) Scan rate dependence of the current densities of Ni2P/NF and Ni2P/Ni/NF at 1.014 V vs RHE. (f) Chronopotentiometric curve of Ni2P/Ni/NF at 10 mA cm−2. The inset shows the catalyst oxidation activation and the expanded zig-zag chronopotentiometric curve due to the growth and release of O2 bubbles on the catalyst surface. All experiments were carried out in 1.0 M KOH.

The electrochemically active surface area (ECSA) of a post-OER Ni2P/Ni/NF was then evaluated from the electrochemical double-layer capacitance (Cdl) by using cyclic voltammetry in a non-Faradaic region from 1.014 to 1.116 V vs RHE to get insight of its excellent OER performance. As exemplified in Figures 5c-d, the post-OER Ni2P/Ni/NF exhibited much higher current density compared with a post-OER Ni2P/NF at the same scan rate, implying a larger Cdl of the former. The calculated Cdl of the post-OER 12 ACS Paragon Plus Environment

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Ni2P/Ni/NF was ~ 22 mF cm−2, significantly higher than that of the post-OER Ni2P/NF (6 mF cm−2, Figure 5e). The higher ECSA of the post-OER Ni2P/Ni/NF rendered a large functioning surface area of its catalytic active sites, excellent gas bubble dissipation ability, and thus superior catalytic performance. The much inferior OER activity of Ni2P/Ni/Ni foil compared to that of Ni2P/Ni/NF also corroborated this speculation (Figure S5). Next, the long-term stability of Ni2P/Ni/NF for OER was evaluated by chronopotentiometry at a current density of 10 mA cm−2 (Figure 5f). During the first 3 h, the required potential gradually increased without evolution of observable O2 bubbles on the electrode surface, as revealed by the smoothness of the expanded chronopotentiometric curve (left inset in Figure 5f). These initial passed charges were ascribed to the oxidation of Ni2P/Ni/NF to transform it to the OER active state, most likely involving the formation of nickel oxides/hydroxides/oxyhydroxides on the catalyst surface (see below). Further prolonging the testing time, the chronopotentiometric curve became stable, accompanied with the rigorous evolution of O2 bubbles on the surface of Ni2P/Ni/NF. The squiggle of an expanded chronopotentiometric curve also implied the formation and release of O2 bubbles on the catalyst surface (right inset in Figure 5f). The low-magnification SEM image showed that the post-OER Ni2P/Ni/NF inherited the overall 3D hierarchically porous configuration (Figure 6a). While a close inspection of its high-magnification SEM images (Figures 6b-d) revealed the presence of featureless monoliths besides urchin-like microparticles, different from the fresh and post-HER samples. Elemental mapping results (Figure 6e) demonstrated that the post-OER Ni2P/Ni/NF mainly consisted of Ni and P, plus a large concentration of O over those featureless monoliths. Its XRD pattern exhibited additional peaks mainly assignable to NiO (Figure 4a). Furthermore, the high-resolution Ni 2p XPS spectrum of the post-OER Ni2P/Ni/NF showed an intensity decrease at 852.8 eV (assignable to Niδ+ in Ni2P) and an increase at 856.8 eV (corresponding oxidized Ni species),19 confirming the partial oxidation of Ni2P (Figure 4b).33 This oxidation phenomenon was also revealed by the intensity increase ascribed to oxidized phosphorous species in its high-resolution P 2p XPS spectrum (Figure 4c).7,33 The collective results indicated that the original Ni2P in the fresh Ni2P/Ni/NF was partially oxidized to nickel oxides/hydroxides/oxyhydroxides (on the catalyst surface) 13 ACS Paragon Plus Environment

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during OER. It should be noted that such an oxidative transformation has been reported for a number of other OER electrocatalysts.7,40,42 Similar to the CoP-transformed OER catalyst, we believe that this in situ formed oxidized nickel species (mainly NiO) in our Ni2P/Ni/NF function as the primary OER active sites. The presence of adjacent phosphate species might shuffle the proton/hydroxide transfer and stabilize the local structure of the catalyst during repeating catalytic OER turnovers, contributing to the effective OER kinetics and robust durability of Ni2P/Ni/NF.42 Indeed, the synergetic effect between oxided nickel species and nickel phosphate has been proposed to be beneficial for OER electrocatalysis.42

Figure 6. (a-d) SEM images of Ni2P/Ni/NF after a 20 h OER stability testing at different magnifications (post-OER). (e) SEM and the corresponding elemental mapping of post-OER Ni2P/Ni/NF. With the aforementioned excellent HER and OER catalytic performance of Ni2P/Ni/NF in hand, we were confident that it could act as a bifunctional electrocatalyst for overall water splitting. To test this hypothesis, a water splitting electrolyzer employing Ni2P/Ni/NF as both anode and cathode catalysts was constructed using 1.0 M KOH as the electrolyte. As shown in Figure 7a, the Ni2P/Ni/NF catalyst couple exhibited high performance for overall water splitting with cell voltages of only 1.49, 1.54, 1.63, and 1.68 V to afford 10, 20, 50, and 100 mA cm−2, respectively, dramatically lower than those of recently reported nonprecious bifunctional catalysts including Co-P films (1.64 V for 10 mA cm−2),7 Ni5P4 films (1.70 V for 10 mA cm−2),30 Ni2P nanoparticles (1.63 V for 10 mA cm−2),33 CoOx@CN (1.90 V for 50 mA cm−2),34 NiSe/NF (1.63 V for 10 mA cm−2)43, Co-P/NC (1.85 V for 50 mA cm−2),44 and comparable to NiFeOx 14 ACS Paragon Plus Environment

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(1.70 V for 100 mA cm−2).8 Moreover, the Ni2P/Ni/NF couple also maintained excellent stability as manifested by the steady chronopotentiometric curve at 20 mA cm−2 for 20 h and subsequently at 10 mA cm−2 for another 20 h (Figure 7c). The nearly overlap of the polarization curves collected at the beginning, after 20 h testing at 20 mA cm−2, and after another 20 h testing at 10 mA cm−2 confirmed the robust stability of our Ni2P/Ni/NF for overall water splitting in 1.0 M KOH (Figure 7b).

Figure 7. (a) Polarization of Ni2P/Ni/NF served as both cathode and anode electrocatalysts (Ni2P/Ni/NF catalyst couple) in a two-electrode configuration at a scan rate of 2 mV/s in 1.0 M KOH (inset shows the expanded region around the catalytic onset). (b) Polarization curves of the Ni2P/Ni/NF catalyst couple before and after long-term overall water splitting electrolysis. (c) The corresponding chronopoteniometric curve of the Ni2P/Ni/NF catalyst couple at a current density of 20 mA/cm2 for the first 20 h and 10 mA/cm2 for the following 20 h.

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CONCLUSION In summary, we have successfully synthesized new 3D hierarchically porous Ni2P/Ni/NF frameworks as bifunctional electrocatalysts for overall water splitting by template-free electrodeposition of porous Ni microparticles on nickel foam and subsequent low-temperature phosphidation. The hierarchically macroporous structure with 3D configuration can reduce ion transport resistance and facilitate gas (H2 and O2) diffusion to boost the utilization efficiency of active species. The open urchin-like Ni2P in the porous NF with high accessibility due to the interconnected macropores can effectively interact with substrates to improve reactivity. With the tailored architecture, the optimal Ni2P/Ni/NF exhibited remarkable catalytic performance for both HER and OER in an alkaline electrolyte (1.0 M KOH) as well as robust stability. For HER, our Ni2P/Ni/NF required an overpotential of only −98 mV to reach the current density of 10 mA cm−2. When served as an OER catalyst, Ni2P/Ni/NF was first partially oxidized, and then exhibited excellent OER activity with small overpotentials of 200 and 268 mV to reach 10 and 100 mA cm−2, respectively. Furthermore, a Ni2P/Ni/NF catalyst couple-based alkaline water electrolyzer approached 10, 20, and 100 mA cm−2 at cell voltages of 1.49, 1.54, and 1.68 V, respectively, along with strong robustness. Various characterization techniques and control experiments revealed that the superior activity and strong stability of Ni2P/Ni/NF originated from its electrochemically active constituents, 3D interconnected porosity, high electrochemical surface area, and intimate interaction between the catalyst and the conductive nickel foam.

EXPERIMENTAL SECTION Materials Synthesis. All chemicals were used as received without further purification. Deionized water (18 MΩ cm-1) was used in all experiments. The Ni2P/Ni/NF catalysts were prepared by a facile template-free cathodic electrodeposition of porous Ni microsphere arrays on nickel foam followed by phosphidation. Typically, the electrodeposition of 3D porous Ni microspheres on nickel form (Ni/NF) was performed in a standard two-electrode glass cell at room temperature with an electrolyte consisting of 16 ACS Paragon Plus Environment

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2.0 M NH4Cl and 0.1 M NiCl2, a piece of commercial nickel foam (MTI Corporation, purity > 99.99%) with a size of 0.5 cm × 0.5 cm was used as the working electrode and a Pt wire as the counter electrode. The electrodeposition was carried out at a constant current of -1.0 A cm−2 for 500 s to obtain Ni/NF samples. Subsequently, the resulting Ni/NF was placed at the center of a tube furnace, and 1.0 g NaH2PO2.H2O was placed at the upstream side and near to Ni/NF. After flushed with Ar gas, the center of the furnace was quickly elevated to the reaction temperature of 400 °C with a ramping rate of 10 oC min-1 and kept at 400 oC for 2 h to convert the metallic nickel to nickel phosphides. After cooling down to room temperature, the final product named as “Ni2P/Ni/NF-500-400” was obtained. A series of “Ni2P/Ni/NF-xy” samples, wherein the “x” and “y” represent the electrodeposition time and phosphidation temperature, respectively, were prepared to optimize the electrocatalytic performance for overall water splitting. Then the HER and OER activities of Ni2P/Ni/NF-100-400, Ni2P/Ni/NF-300-400, Ni2P/Ni/NF-500-400 (denote as Ni2P/Ni/NF for brevity in main text), Ni2P/Ni/NF-500-400, Ni2P/Ni/NF-500-300, and Ni2P/Ni/NF-500500 are compared in Figure S4. The commercial nickel foam was directly subjected to phosphidation with the similar condition and the obtained sample was named as “Ni2P/NF”. Characterization. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) measurements were collected on a FEI QUANTA FEG 650 (FEI, USA). X-ray diffraction (XRD) patterns were obtained on a Rigaku MinifexII Desktop X-ray diffractometer. Phosphorous analysis was conducted on a Thermo Electron iCAP inductively coupled plasma

spectrophotometer at the Analytical Laboratory of USU. The X-ray photoelectron spectroscopy analyses were performed using a Kratos Axis Ultra instrument (Chestnut Ridge, NY) at the Surface Analysis Laboratory, University of Utah Nanofab. The samples were affixed on a stainless steel Kratos sample bar, loaded into the instrument's load lock chamber, and evacuated to 5 × 10−8 torr before it was transferred into the sample analysis chamber under ultrahigh vacuum conditions (~10−10 torr). X-ray photoelectron spectra were collected using the monochromatic Al Kα source (1486.7 eV) at a 300 × 700 µm spot size. Low resolution survey and high resolution region scans at the binding energy of interest were collected for each sample. To minimize charging, all samples were flooded with low-energy 17 ACS Paragon Plus Environment

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electrons and ions from the instrument's built-in charge neutralizer. The samples were also sputter cleaned inside the analysis chamber with 1 keV Ar+ ions for 30 seconds to remove adventitious contaminants and surface oxides. XPS data were analyzed using CASA XPS software, and energy corrections on high resolution scans were calibrated by referencing the C 1s peak of adventitious carbon to 284.5 eV. Electrochemical measurement. Electrochemical measurements in 1.0 M KOH were performed by a computer-controlled Gamry Interface 1000 electrochemical workstation with a three-electrode cell system and a scan rate of 2 mV s-1. The resulting Ni/NF, Ni2P/NF, or Ni2P/Ni/NF was used as the working electrode, a Ag/AgCl (sat. KCl) electrode as the reference electrode, and a Pt wire as the counter electrode. The electrolyte (1.0 M KOH) was saturated with H2 and O2 for HER and OER evaluation, respectively. All potentials reported herein were quoted with respect to reversible hydrogen electrode (RHE) through RHE calibration. For overall water splitting (both HER and OER) tests, Ni2P/Ni/NF was used as both anode and cathode electrodes and the potential scan range was from 1.0 to 1.85 V. iR (current times internal resistance) compensation was applied in all the electrochemical experiments to account for the voltage drop between the reference and working electrodes using Gamary Framework™ Data Acquisition Software 6.11. The electrochemical double-layer capacitance (Cdl) of the resulting electrocatalysts was evaluated by using cyclic voltammetry in a non-Faradaic region (0.923 ~ 1.023 V vs RHE for HER and 1.014 ~ 1.116 V vs RHE for OER) at different scan rates ranging from 5, 10, 20, 50 to 100 mV s-1. By plotting the difference between the anodic and cathodic current density at the middle potential (0.973 vs RHE for HER and 1.065 V vs RHE for OER) versus scan rate, the resulting linear slope is twice of the Cdl.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Supplemental figures and tables

AUTHOR INFORMATION 18 ACS Paragon Plus Environment

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Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Utah State University and the Ralph E. Powe Junior Faculty Enhancement Award (ORAU). N.J. acknowledges the Governor’s Energy Leadership Scholars Grant of the Utah Energy Research Triangle. Y.S. thanks the support from the Microscopy Core Facility at Utah State University. REFERENCES 1. Chu, S.; Majumdar, A. Nature 2012, 484, 294-303. 2. Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath. Y.; Teets, T. S.; Nocera, D. G. Chem. Rev. 2010, 110, 6474-6502. 3. Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2001, 103, 15729-15735. 4. Wang, X.; Kolen’ko, Y. V.; Bao, X. Q.; Kovnir, K.; Liu, L. Angew. Chem. Int. Ed. 2015, 54, 8188-8192. 5. Cobo, S.; Heidkamp, J.; Jacques, P. A.; Fize, J.; Fourmond, V.; Guetaz, L.; Jousselme, B.; Ivanova, V.; Dau, H.; Palacin, S.; Fontecave, M.; Artero, V. Nat. Mater. 2012, 11, 802-807. 6. McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2015, 137, 4347-4357. 7. Jiang, N.; You, B.; Sheng, M.; Sun, Y. Angew. Chem. Int. Ed. 2015, 54, 6251-6254. 8. Wang, H.; Lee, H. W.; Deng, Y.; Lu, Z.; Hsu, P. C.; Liu, Y.; Lin, D.; Cui, Y. Nat. Commun. 2015, 6, 7261. 9. Zheng, Y.; Jiao, Y.; Li, L. H.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Z. ACS Nano 2014, 8, 5290-5296. 10. Ma, W.; Ma, R.; Wang, C.; Liang, J.; Liu, X.; Zhou, K.; Sasaki, T. ACS Nano 2015, 9, 19771984. 11. Jiao, Y.; Li, Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Chem. Soc. Rev. 2015, 44, 2060-2086. 12. Kibsgaard, J.; Chen, Z. B.; Reinecke, B. N.; Jaramillo, T. F. Nat. Mater. 2012, 11, 963-969. 13. Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. I. Science 2007, 317, 100-102. 14. Lassalle-Kaiser, B.; Merki, D.; Vrubel, H.; Gul, S.; Yachandra, V. K.; Hu, X.; Yano, J. J. Am. 19 ACS Paragon Plus Environment

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