Controlled Electrodeposition Synthesis of CoâNiâP ... - ACS Publications

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Controlled Electrodeposition Synthesis of Co−Ni−P Film as a Flexible and Inexpensive Electrode for Efficient Overall Water Splitting Yu Pei,† Yang Yang,† Fangfang Zhang,† Pei Dong,‡ Robert Baines,‡ Yuancai Ge,† Hang Chu,† Pulickel M. Ajayan,‡ Jianfeng Shen,*,† and Mingxin Ye*,† †

Institute of Special Materials and Technology, Fudan University, Shanghai 200433, P. R. China Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States

‡

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S Supporting Information *

ABSTRACT: Synthesis of highly efficient and robust catalysts with earth-abundant resources for overall water splitting is essential for large-scale energy conversion processes. Herein, a series of highly active and inexpensive Co−Ni−P films were fabricated by a one-step constant current density electrodeposition method. These films were demonstrated to be efficient bifunctional catalysts for both H2 and O2 evolution reactions (HER and OER), while deposition time was deemed to be the crucial factor governing electrochemical performance. At the optimal deposition time, the obtained Co−Ni−P-2 catalyst performed remarkably for both HER and OER in alkaline media. In particular, it requires −103 mV overpotential for HER and 340 mV for OER to achieve the current density of 10 mA cm−2, with corresponding Tafel slopes of 33 and 67 mV dec−1. Moreover, it outperforms the Pt/C//RuO2 catalyst and only needs −160 mV (430 mV) overpotential for HER (OER) to achieve 200 mA cm−2 current density. Co− Ni−P electrodes were also conducted for the proof-of-concept exercise, which were proved to be flexible, stable, and efficient, further opening a new avenue for rapid synthesis of efficient, flexible catalysts for renewable energy resources. KEYWORDS: Co−Ni−P film, electrodeposition, HER, OER, flexible selenides,16 phosphides,17−19 nitrides,20−22 carbides,23 and oxides/hydroxide.24 Recently, cobalt and nickel based catalysts have attracted more and more attention thanks to their superior catalytic efficiency for HER and OER. For instance, CoO,25 Co3O4,26−29 NiCo-LDH,30 Co(OH)2,31 Co1−xFex(OH)2,32 SrNb0.1Co0.7Fe0.2O3−δ,33 and NiFe layered double hydroxides34,35 have been demonstrated to be quite efficient for OER, while CoP8,36,37 and CoS238 have proven to be outstanding for HER. On the other hand, compared to common oxides, sulfides, and hydroxides, phosphide-based materials have faster kinetics and have recently spurred a fair amount of studies. Among phosphide materials, more attention has been centered on the synthesis and electrocatalytic applications of compounds like Ni2P,39 FeP,40 MoP,41 WP,42 MnP,43,44 CoMnP,45 Co−Fe−P,46 and Ni2−xCoxP.47 A few studies have demonstrated that transition metal phosphides

1. INTRODUCTION Studying alternative energy sources to address present energy shortages and environmental problems is critical for ensuring social health in the near future. It is widely acknowledged that hydrogen provides a potential solution since it has a higher energy density compared to other energy sources and yields only water as a result of its combustion.1−5 Electrocatalytic water splitting is considered as a clean and effective energy conversion method, primarily consisting of H 2 and O2 evolution reactions (HER and OER).6,7 Generally, Pt is considered to be the most efficient catalyst for HER under acidic conditions, while IrO2 or RuO2 is considered to be the most efficient catalyst for OER under basic conditions.8,9 However, due to the high cost and limited availability, they are not suitable to be used in large-scale water splitting devices. Thus, the development of economical, earth-abundant electrocatalysts that retain excellent catalytic properties is indispensable.10−13 Currently, electrocatalysts for HER and OER are mainly focused on transition metal compounds, including sulfides,14,15 © 2017 American Chemical Society

Received: June 27, 2017 Accepted: August 29, 2017 Published: August 29, 2017 31887

DOI: 10.1021/acsami.7b09282 ACS Appl. Mater. Interfaces 2017, 9, 31887−31896

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic representation of the synthesis process of Co−Ni−P film by electrodeposition method (a, b, c, d, e). SEM and AFM images of Ti sheet and Co−Ni−P films with different deposition times: pure Ti sheet (f, k), deposition time of 10 min (g, l), 20 min (h, m), 30 min (i, n), and 40 min (j, o). SEM and BSE images of Co−Ni−P film sections with different deposition time (p, q, r, s).

theless, the performance of the films was still modest, and their preparation processes were complex. In this paper, we pored through the fabrication of a series of cobalt−nickel−phosphorus (Co−Ni−P) films by the electrodeposition method. The films were deposited by a constant current method that greatly reduced the formation time necessary for uniform coating. Conducting electrocatalytic performance tests on the films testified that the as-prepared Co−Ni−P films could be directly used as electrocatalysts for both HER and OER in alkaline solution. Moreover, compared to Co−P and Ni−P films prepared in the same deposition conditions, the Co−Ni−P catalyst exhibited better overall water splitting properties. Even more, with proper deposition time, the Co−Ni−P film outperforms those of the Pt/C HER catalyst and RuO2 OER catalyst. Deposition time was demonstrated to be the main factor that dominates the morphology and catalytic performance of the Co−Ni−P films.

could be suitable for flexible electrode materials. For instance, Ye et al. reported a chemical vapor deposition method to prepare a nanohybrid catalyst of carbon framework-wrapped cobalt phosphide on carbon cloth (Co2P@C/CC) as a flexible electrocatalyst for the HER.48 Similarly, Pu et al. synthesized nickel diphosphate nanosheet arrays on carbon cloth (NiP2/ CC) as flexible electrodes for water splitting.49 Unfortunately, most of the previously enumerated phosphide catalysts have only one catalytic application: either OER or HER. In addition, there are also obvious drawbacks regarding the synthesis process of phosphorus-based materials, including the need for expensive systems, complex synthesis process, low efficiency, long reaction time, and required toxic chemical reagents. These factors greatly restricted large-scale application of phosphidebased electrocatalysts. To simplify electrocatalytic water splitting systems and simultaneously reduce operating expenses, multifunctional catalysts for both HER and OER are highly desired. Electrodeposition offers many advantages over conventional synthesis techniques. Straightforwardness, low power consumption, fast reaction time, simple experimental process, and almost no additional chemical reagents are a few of the benefits of electrodeposition. In spite of its frequent use for preparing transition-metal-based compounds, studies rarely concern electrodeposition as it relates to water splitting of phosphorus-based transition metal materials. Recently, a few reports detailed the electrodeposition synthesis of transition metal phosphate-based materials as an electrocatalytic water splitting catalyst.50−55 These reports indicate that Ni−P film (low onset overpotential and long-term stability in both neutral and alkaline aqueous solutions), NiFe film (exhibiting high OER activity), Co−Se film (an inexpensive bifunctional electrocatalyst for the overall water splitting reaction), and Ni−Fe−P film deposited by cyclic voltammetry (CV) are promising candidates for overall electrocatalytic water splitting. Never-

2. EXPERIMENTAL SECTION Materials. Cobalt chloride (CoCl2·6H2O), nickel chloride (NiCl2· 6H2O), sodium hypophosphite (NaH2PO2·H2O), ammonium chloride (NH4Cl), and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC). All reagents were of analytical grade and used without further purification. N2 with a purity of 99.9% was purchased from Shanghai Jifu Gas Co. Ltd. Titanium (Ti) sheets were obtained from Qingyuan Metal Materials Co., Ltd. Electrodeposition of Co−Ni−P, Co−P, and Ni−P Films. Co− Ni−P films were fabricated by a constant current density deposition process. First, the substrate (Ti sheets) surface was cleaned ultrasonically in 20% HCl solution to remove surface oxides. Then, the sheets were ultrasonically cleaned with ethanol and DI water to remove trace surface pollutants. The electrodeposition solution consisted of CoCl2·6H2O (0.2 M), NiCl2·6H2O (0.2 M), NaH2PO2· H2O (0.2 M), and NH4Cl (0.25 M) with DI water. For the threeelectrode electrodeposition system, Ti sheets served as the working electrode, platinum wire as the counter electrode, and calomel electrode as the reference electrode. Electrodeposition of Co−Ni−P 31888


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Figure 2. XPS spectra of the as-prepared Co−Ni−P-2 catalyst: (a) Co 2p region, (b) Ni 2p region, and (c) P 2p region. films was attained via constant current with 10 mA cm−2 current density. Different Co−Ni−P samples were fabricated by controlling the electrodeposition times (10, 20, 30, and 40 min), which are herein labeled as Co−Ni−P-1, Co−Ni−P-2, Co−Ni−P-3, and Co−Ni-P-4, respectively. After electrodeposition, these samples were rinsed with deionized water and dried in a vacuum oven. The Co−P and Ni−P counterpart films were prepared through the same procedure (current density 10 mA cm−2 for 20 min). Preparation of Pt/C and RuO2. Pt/C and RuO2 catalysts were fabricated by methods elaborated elsewhere.55 In brief, 10 mg of 20% Pt/C or RuO2 was dissolved in a 2 mL solution of 0.8 mL of DI water, 1.08 mL of anhydrous ethanol, and 0.12 mL of 5% Nafion solution. That mixture was then ultrasonicated for 30 min to form a homogeneous catalyst ink. Subsequently, 0.05 mL of the ink was loaded onto the Ti substrate sheet 20 times using rotating deposition. Lastly, the sample was placed in an oven and dried for further testing. Characterization. X-ray diffraction (XRD) patterns were obtained by a Bruker D8 Advance X-ray diffractometer, with Cu Kα radiation and a scan rate of 8°/min. Surface morphology analysis was investigated by field-emission scanning electron microscopy (FESEM, Tescan, Maia3 XMH) and atomic force microscopy (AFM, Bruker). We conducted characteristic element image mapping with an energy-dispersive spectrometer (EDS, Bruker, XFlash660). Xray photoelectron spectroscopy (XPS) was accomplished using an IncaX-max50, Oxford. Selected-area electron diffraction (SAED) pattern and transmission electron microscopy (TEM) images were obtained on a JEOL 2010. Electrochemical Measurements. Electrochemical experiments were performed on an electrochemical workstation (Autolab PG 302N) in a standard three-electrode system. The as-prepared samples were used as the working electrode, while saturated calomel electrode and platinum wire purchased from CH Instruments were applied as the reference electrode and counter electrode, respectively. All potentials appearing in this paper were converted to vs reversible hydrogen electrode (RHE). The values were calibrated by adding a value of 0.242 + 0.059 × pH to vs RHE. Polarization curves were corrected regarding the iR compensation within the electrolyte. HER and OER measurements were carried out in 1 M KOH by linear sweep voltammetry (LSV) with a 10 mV s−1 scanning rate.

Figure 1a−e denoted the growth process of Co−Ni−P film. AFM and SEM were used to investigate surface morphology changes of the Co−Ni−P films as a function of increasing deposition time. Figure 1f and k features the surface of a pure Ti sheet. Obviously, its surface is craggy and undulating. The uneven surface is favorable for the adhesion of metal ions on the surface and the growth of thin film. After 10 min of electrodeposition (Figure 1g and l), the Ti sheet’s morphology changed dramatically since Co−Ni−P spheres proliferate across its surface. Yet these spheres are small and isolated, thus the surface still contains distinct low-lying patches. When the deposition time increases to 20 min (Figure 1h and m), the surface of Co−Ni−P film further transforms. As the Co−Ni−P spheres’ vertical growth distends, the extent of surface depression is reduced. After 30 min of electrodeposition, the Co−Ni−P spheres extend horizontally (Figure 1i and n), and the degree of surficial unevenness is normalized. Finally, at 40 min deposition time (Figure 1j and o), Co−Ni−P spheres stack with each other, fill in low-lying areas, and render the surface smoother. AFM characterization and schematic diagrams at different electrodeposition times allow us to delineate the growth process of Co−Ni−P film. Namely, Co2+, Ni2+, and H2PO2− in the cathode obtain electrons and grow into Co−Ni−P pellets on the Ti sheet. Moreover, our analysis shows that as electrodeposition time increases the Co−Ni−P pellets grow vertically. Simultaneously, the particles expand to uniformly cover the Ti sheet. We also studied the change of film thickness at different deposition times by SEM and back-scattered electron imaging (BSE). As shown in Figure 1p,q,r,s, the thickness of the Co−Ni−P film increases as a function of deposition time. Average thicknesses of 900 nm, 1.63 μm, 2.3 μm, and 2.7 μm correspond to the 10, 20, 30, and 40 min deposition times, respectively. We conducted XRD to index the crystal phases of the Ti sheet and the four Co−Ni−P films, as illustrated in Figure S2. XRD patterns of the Ti sheet feature characteristic peaks at 34.93°, 40.56°, 58.7°, and 70.16°, which may be attributed to (111), (200), (220), and (311). It indicates that the main crystalline structure present on the surface of Ti was titanium hydride phase (according to the standard card JCPDS 650708). All the films demonstrated a distinct characteristic peak at 2θ = 44.35°. Thus, the XRD results are consistent with previous literature, suggesting that our films were in fact Co− Ni−P alloy films. According to the relevant literature, the peak at 2θ = 44.35° is consistent with the (111) spacing in facecentered Ni and the (002) spacing in the hexagonal closepacked structured Co lattice.58 Meanwhile, the characteristic

3. RESULTS AND DISCUSSION A constant current density deposition methodology facilitated simple fabrication of Co−Ni−P films from a metal salt solution, as shown in Figure 1. The Ti sheet is chosen as the substrate because of its good mechanical properties and chemical stability. It is often used as a substrate to prepare catalyst. For instance, Sun et al. prepared a CoO nanowire array on Ti mesh (CoO/Ti) as a 3D bifunctional catalyst electrode;56 Moshfegh et al. once prepared cobalt oxide nanoflakes on titanium sheets.57 Additional details on the synthesis procedure can be found in the Experimental Section. 31889


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Figure 3. SEM and elemental mapping images of the Ti sheet and as-prepared Co−Ni−P film: (a, b) pure Ti sheet. (c, d) Co−Ni−P-2 film. (e) Elemental mapping of Co−Ni−P film. (f) Cross-section SEM image and elemental mapping of an as-prepared Co−Ni−P-4 film. (g) HR-TEM image and (h) elemental mapping of Co−Ni−P-2 film.

Figure 3c and d illustrates how the surface of the sheet flattens out, and potholes become covered. EDS surface element analysis pinpoints the presence of three main elements (Figure 3e). It can be seen from Figure S4 that Co, Ni, and P elements are uniformly distributed on the surface. Exploring the cross-section of the prepared Co−Ni−P4 in Figure 3f draws attention to a surficial division on the Ti sheet, and two distinct parts clearly exist. Figure S5 features the standout layer, which is about 2−3 μm thick and has a section morphology similar to corn grain accumulation. Such a disparity in elemental distribution is again verified by the EDS mappings in Figure 3f and Figures S5 and S6. Obviously, Co, Ni, and P elements are concentrated primarily on the surface film, which are separated from the distribution of the Ti element on the substrate, validating the successful formation of Co−Ni−P film on the Ti sheet. High-resolution TEM (HRTEM) characterization of the Co−Ni−P film in Figure 3g affirms the crystalline structure of the sample by showing different orientations of lattice planes. The lattice spacings between the two fringes highlighted in yellow in Figure 3g are 0.207 and 0.203 nm. We attribute these particular spacings to diffraction of the (111) and (002) lattice planes, respectively. Figure 3h indicates uniform spatial distribution of Co, Ni, and P atoms on the surface of the Co−Ni−P film through both TEM and EDX elemental mapping, indicating the successful synthesis of Co−Ni−P film with a crystalline structure. Moreover, XRD, XPS, SEM,

peaks of the Ti sheet gradually attenuate as deposition time increases, which could be attributed to a thickened electrodeposited layer. The elemental components of the Co−Ni−P film were further characterized by XPS analysis. Figure S3 displays the typical XPS profile of Co−Ni−P-2 film. It can be found that the film is mainly composed of three elements: Co, Ni, and P. The Co 2p XPS spectrum (Figure 2a) displayed two peaks: one at 778.3 and the other at 793.4 eV, corresponding to Co 2p3/2 and Co 2p1/2 states, respectively.59 The peaks at 852.3 and 869.5 eV arise from metallic Ni (Figure 2b), while Ni 2p binding energies at 855.7 and 873.5 eV correspond to NiII.51 As shown in Figure 2c, the P 2p spectrum exhibits its primary peak at 130.0 eV, indicating the phosphide signal.59,60 The broad feature at approximately 133.6 eV is a consequence of phosphate airoxidation when the sample was removed from the deposition solution, rinsed with water, and transferred to the XPS instrument.61,62 To further verify that the Co−Ni−P films were successfully prepared on Ti sheet and explore their surface morphologies, we performed FESEM and EDS on both the Ti sheet and the series of Co−Ni−P films with varying deposition times. Figure 3a and b displays the surface morphology of the Ti sheet at different scales. Clearly, the surface of the Ti sheet is rough, and many potholes pepper its surface. After electrodeposition for 20 min, significant changes occurred in the surface of the Ti sheet. 31890


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Figure 4. Electrocatalytic activities of various catalysts: (a) LSV curves of the Pt/C and Co−Ni−P catalysts for HER in 1 M KOH. (b) The corresponding Tafel plots of HER polarization curves for Pt/C and Co−Ni−P catalysts. (c) The OER performance of the RuO2 and Co−Ni−P catalysts in alkaline media (1.0 M KOH). (d) The corresponding Tafel plots of OER for the RuO2 and Co−Ni−P catalysts. (e) Nyquist plots for the Co−Ni−P catalysts with different electrodeposition times. (f) The chronopotentiometric curves of Co−Ni−P-2 catalyst at −10 mA cm−2 in 1 M KOH. Inset: the morphology and composition of the Co−Ni−P film after 10 h HER electrolysis.

to achieve a larger 200 mA cm−2 current density, the Co−Ni− P-2 film only required 160 mV, while the Pt/C catalyst needed 292 mV. In fact, each of our synthesized films demonstrated small potential at 200 mA cm−2 current density: 211 mV for Co−Ni−P-1, 234 mV for Co−Ni−P-3, and 254 mV for Co− Ni−P-4, indicating the universality of this deposition method. Figure 4b demonstrates the corresponding Tafel plots for the HER. Notably, Co−Ni−P-2 has a very small Tafel slope of only 33 mV dec−1, which is among the smallest reported Tafel slopes for outstanding HER catalysts in alkaline media (Table S1). Furthermore, Tafel slopes of Co−Ni−P-1, Co−Ni−P-3, and Co−Ni−P-4 are only 49, 64, and 81 mV dec−1, respectively. As a standard of comparison, the Tafel slope of Pt/C is 106 mV dec−1. OER plays another central role during overall water splitting. We characterized the OER of the different catalysts via line sweeping, with a scan range of 0 to 1 V, across analogous solution conditions (1 M KOH). Figure 4c profiles the OER

AFM, and TEM characterizations ensure the successful preparation of a series of Co−Ni−P films in which deposition time dominates film thickness and surface morphology. With the successful electrodeposition of Co−Ni−P film on the Ti sheet, its surprising performance for water splitting was studied extensively. First, we analyzed HER catalytic activities of the Co−Ni−P films under different deposition time prepared in alkaline solution (1 M KOH). The HER catalytic activity of the pure Ti sheet and Pt/C electrodes served as a control case. Figure 4a underscores the poor catalytic activity with the pure Ti sheet before −0.4 V versus reversible hydrogen electrode (RHE). On the other hand, the Pt/C catalyst exhibited excellent catalytic performance with a small overpotential (η). By contrast, when our synthesized Co−Ni−P film served as the electrode material, the cathode current rose rapidly beyond 75 mV. Specifically, Co−Ni−P-2 required −103 mV overpotential to reach the current density of −10 mA cm−2, which is slightly higher than that of the Pt/C catalyst. However, 31891


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Figure 5. Electrocatalytic activities of different electrodeposition catalysts: (a) polarization curves of HER for Co−Ni−P-2, Co−P, and Ni−P catalysts obtained in 1 M KOH. (b) The corresponding Tafel plots of HER polarization curves. (c) LSV curves for OER catalytic properties of different catalysts in 1 M KOH. (d) The corresponding Tafel plots of OER polarization curves. (e) Nyquist plots for the Co−Ni−P-2, Co−P and Ni−P catalysts. (f) The electrochemically active surface area values of Co−Ni−P films with different deposition times, as well as those for Co−P and Ni−P films.

(Figure 4e) to pinpoint the resistances during experimental HER and OER. A Randles equivalent circuit was leveraged to obtain the values of the ionic and charge transfer resistances of the HER and OER catalyst in 1 M KOH electrolyte (insets). Obviously, in the high frequency region, the low value of Rs (Ω) represents uncompensated solution resistances, which were similar for all the catalysts at −1.2 V. The charge-transfer resistance (Rct) in the lower frequency region was determined from the fitted values for the equivalent circuit. Through these data, we interpreted the reaction kinetics of the catalysts. As anticipated, the Co−Ni−P-2 catalyst displayed the minimum charge transfer resistance of 12.6 Ω, while the values for Co− Ni−P-1, Co−Ni−P-3, and Co−Ni−P-4 were slightly higher (55, 30.7, and 28.5 Ω). To this end, resistances acquired from the EIS spectra of the various catalysts revealed that the Co− Ni−P-2 catalyst had significantly lower impedance and, thus, markedly enhanced HER kinetics.

performance of the Co−Ni−P catalysts at different electrodeposition times, as well as a conventional oxygen evolution catalyst, RuO2. Notably, RuO2 needed a minimum overpotential of 230 mV to achieve the 10 mA cm−2 current density. It is followed by Co−Ni−P-2 catalyst at 340 mV. More interestingly, Co−Ni−P-2 catalyst reached larger current densities of 100, 200, and 500 mA cm−2 at lower overpotentials of 410, 430, and 490 mV, respectively. In the Tafel plots describing OER behavior (Figure 4d), Co−Ni−P-2 has a very small Tafel slope of 67 mV dec−1 between 1.5 and 1.6 V, while Co−Ni−P-1, Co−Ni−P-3, and Co−Ni−P-4 have 88, 91, and 98 mV dec−1 Tafel slopes, respectively, outperforming RuO2 (145 mV dec−1 between 1.45 and 1.55 V). It is widely accepted that HER and OER performance in an electrolyte are governed by ionic and transport resistances. To further probe the reason for the extinguished performance of Co−Ni−P catalysts, we recorded Nyquist plots at −1.2 V 31892


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emphasizes the degree to which Co−Ni−P film is the superior OER catalyst. Furthermore, the ionic and transport resistances of Co−Ni− P, Co−P, and Ni−P electrodes were studied. Nyquist plots at −1.2 V were recorded to characterize the effects caused by each catalyst’s resistances during HER and OER. As we reported previously, Rundles equivalent circuit indicates the values of the ionic and charge transfer resistances of the HER and OER catalyst in 1 M KOH electrolyte (insets). Figure 5e and Figure S13 track the reaction kinetics of the catalysts. Clearly, Co− Ni−P-2 displayed a minimum charge transfer resistance of 12.6 Ω, while those for the Co−P and Ni−P were higher (55 Ω and 4.68 kΩ). Accordingly, resistances acquired from the EIS spectra of the various catalysts reveal that the Co−Ni−P-2 catalyst had appreciably lower impedance, which explains the increased HER kinetics compared with Co−P and Ni−P catalysts. Electrochemical active surface area (EASA) is a primary determinant for electrochemical reactivity. Measuring electrochemical double layer capacitance is an easy and inexpensive technique to estimate the ASA. We calculated the electrochemical active surface areas of the as-prepared catalysts by measuring their electrochemical double layer capacitances (Cdl) using a simple CV method according to previous reports.63,64 Perspective was thus gained on the excellent electrochemical properties of Co−Ni−P-2 film relative to other Co−Ni−P, Co−P, and Ni−P films. The capacitance measurements were performed in a −0.2 ∼ −0.3 V potential range vs RHE since no standout electrochemical features corresponding to Faradaic current were observed in this region for any of the catalysts. For Cdl measurements, CV was recorded in the same potential range at 10, 25, 50, 75, 100, and 125 mV s−1 scan rates. The CV curves of Co−Ni−P, Co−P, and Ni−P films and the differences in current density versus scan rate are displayed in Figures S14, S15, and S16. EASA values are summarized in Figure 5f. Notably, Co−Ni−P-2 possesses the largest active area, which is conducive to excellent charge transfer efficiency. Taking all these results into account, we attribute the remarkable electrocatalytic activity and stability of the Co− Ni−P film to the following factors: (i) Compared with bare Co−P, the addition of nickel element speeds electron transport, rendering enhanced ionic conductivity and reducing internal resistance (Figure 5e). This is conducive to the rapid reaction and reduces the potential required for the reaction. (ii) As the nickel element has a certain catalytic performance, addition of nickel (Figure 5f) increases the availability of electrochemical sites and improves its catalytic performance. (iii) The bimetallic phosphate compositions in the compound further contribute to HER activity and ameliorate OER activity. Meanwhile, we can find that the electrodeposition time will affect the surface morphology and thickness of the films, thus affecting their impedance and electrochemical active area. Similar experimental results have also been reported. For example, Jin et al. synthesized amorphous cobalt−iron hydroxide (CoFe−H) nanosheets on graphite substrates by facile electrodeposition as an efficient catalyst for electrochemical water oxidation. They studied the effect of different deposition times (5, 10, 20, 40, and 60 min) on their electrochemical performance. The results indicated that the high electrochemical performance was obtained by a low but effective loading.65 The practical performance of bifunctional Co−Ni−P electrodes for overall water splitting is presented in Figure 6. As the Ti sheet is a flexible material, we anticipate that Co−Ni−P film

To determine the electrochemical HER stability of Co−Ni− P-2 catalyst, we conducted a long-term hydrogen evolution reaction at −10 mA cm−2 in 1 M KOH media. Figure 4f replicates Co−Ni−P-2 catalyst’s behavior as it underwent steady HER activity. No noticeable potential change is observed for more than 10 h of HER process. It indicates that this electrode can well retain its catalytic activity in electrolysis of water for long-term running. To further substantiate the superiority of the electrodeposition method, we investigated the morphology and composition of the Co−Ni−P film after HER via FESEM and EDS. As shown in Figure 4f (inset), the Co− Ni−P film remained uniformly dispersed across the Ti sheet. However, after the HER process, a layered structure appeared on the surface of the Co−Ni−P film (Figure S7a,b,c). Through elemental analysis of damaged spots on the surface (Figure S7d), we observed that Co, Ni, and P elements were concentrated in the film-covered areas, while Ti resided on defects. EDS (Figure S8) also confirmed the distribution of Co, Ni, and P in the surface after HER. We also compared the catalytic activities of Co−P, Ni−P, and Co−Ni−P-2 films under similar conditions. Taking Co−P film as an example, it can be seen that uniform film spans the surface of the Ti sheet. The surface morphology of the film is similar to the accumulation of spherical modules present in the analysis of Co−Ni−P-2 film. A sheet-like structure on the film surface was observed at different scales (Figure S9a, b, and c). We then assessed the surface elements of the films with EDX (Figure S9d and Figure S10) and found that evenly distributed Co and P elements in the area covered the film. The remaining elemental composition was identified as titanium. Inasmuch, we demonstrated the successful electrodeposition of Co−P film. Ni−P film was similarly characterized (Figure S11). Interestingly, the Ni−P surface appears smoother than that of Co−P. Meanwhile, EDS (Figure S11d and Figure S12) highlights how Ni and P elements are evenly distributed in the surface of the Ti sheet in this film as well. After confirming the successful preparation of the Co−P and Ni−P counterparts, we further investigated their catalytic activities. Co−P and Ni−P catalysts demand −121 mV and −202 mV to reach a 10 mA cm−2 current density, which are much higher than that of Co−Ni−P2 catalyst (Figure 5a). As to a much higher current density of 200 mA cm−2, the overpotential required for Co−Ni−P-2 is −160 mV, which is much lower than the −205 mV and −257 mV associated with Co−P and Ni−P. Studying the Tafel plots allows us to deeply understand the HER activities of these catalysts. As shown in Figure 5b, the Co−Ni−P-2 catalyst presents a Tafel slope of 33 mV dec−1, which is much lower than those of Co−P and Ni−P (46 and 62 mV dec−1). The lower Tafel slope of Co−Ni−P reinforces that it is a more effective HER catalyst than either Co−P or Ni−P. On the other hand, the OER activities of Co−P, Ni−P, and Co−Ni−P catalysts were also investigated in 1 M KOH solution via LSV with a 10 mV s−1 potential scanning rate. Figure 5c displays the linear sweep curves of OER for each catalyst. Co−Ni−P requires overpotential of 340 mV to attain a current density of 10 mA cm−2. For Co−P and Ni−P films, that value of overpotential is 380 mV and 480 mV, respectively. To achieve an even higher current density of 200 mA cm−2, Co−Ni−P requires only 430 mV overpotential, which is much lower than the 520 mV for Co−P and 650 mV for Ni−P. Figure 5d juxtaposes the Co−Ni−P 33 mV dec−1 Tafel slope with the Co−P 80 mV dec−1 and the Ni−P 130 mV dec−1 slopes. It 31893


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successfully and could further serve as a flexible electrode material.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09282. More Characterizations of Co−Ni−P, CoP, and NiP films (PDF) Movies of water splitting process with the bent Co−Ni− P electrodes (ZIP)

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Figure 6. (a) LSV curves of overall water electrolysis of various catalysts in 1.0 M KOH, Co−Ni−P electrodes with different bending angels, and Pt/C//RuO2 electrodes. (b, c, d) Photographs of Co−Ni− P with flat state, bent 90°, and bent 180°.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mingxin Ye: 0000-0002-4532-2594

could act as a flexible bifunctional electrocatalyst for overall water splitting. Figure 6a gathered and demonstrated the water splitting performances of Co−Ni−P//Co−Ni−P and Pt/C// RuO2 under different bending circumstances, and a water splitting process with the bent Co−Ni−P electrodes was recorded (Movies S1−S3). Under the flat state (Figure 6b), the Co−Ni−P electrode achieved a water-splitting current density of 10 mA cm−2 at a small cell voltage of 1.65 V in 1.0 M KOH electrolyte, which was larger than that of Pt/C//RuO2 electrodes (1.58 V). However, to deliver 100 mA cm−2, the cell voltage of the Co−Ni−P was 1.79 V. At the same voltage, the current density of Pt/C//RuO2 electrodes was only 40 mA cm−2. The water splitting properties of Co−Ni−P electrodes at different bending angles to 90° (Figure 6c) and 180° (Figure 6d) were further characterized. Interestingly, the polarization curves of the bent Co−Ni−P electrodes (90° and 180°) were almost the same as the curve elicited before bending. These results provide the new recognition that under reasonable design the Co−Ni−P electrodes can indeed serve as efficient and stable flexible electrode materials.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS National Natural Science Foundation of China (51202034) financially supported this work. REFERENCES

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4. CONCLUSIONS In summary, we successfully fabricated a series of Co−Ni−P films on the Ti sheet via a simple electrodeposition method by varying each film’s deposition time. The films were determined to be highly efficient water splitting catalysts. Particularly, with optimized deposition time, Co−Ni−P-2 film stood out amidst the batch of synthesized catalysts due to its uniform growth and enhanced charge transport. Namely, Co−Ni−P-2 presented admirable HER activity in strongly alkaline solution, only requiring η = −103 mV to reach 10 mA cm−2 current density. The corresponding Tafel slope for HER was 33 mV dec−1. In addition, the Co−Ni−P-2 film also exhibited high OER activity, necessitating η = 340 mV to reach 10 mA cm−2 in 1 M KOH. The corresponding Tafel slope for OER was 67 mV dec−1. Alongside admirable performance, Co−Ni−P film exhibited stability at low overpotentials over long periods. The comprehensive performance of Co−Ni−P-2 film outperforms the Pt/C and RuO2 counterparts. Moreover, their electrochemical behaviors were deeply verified by calculating and comparing their active areas. The bifunctionality, superb stability, inexpensiveness, earth abundance, and ease of fabrication of the electrodeposited electrocatalyst suggest it could be applied to large-scale water splitting devices 31894


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