Interweaved Nickel Phosphide Sponge as an Electrode for Flexible

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Interweaved Nickel Phosphide Sponge as an Electrode for Flexible Supercapattery and Water Splitting Applications Subramani Surendran,†,‡ Sathyanarayanan Shanmugapriya,† Sangaraju Shanmugam,*,‡ Leonid Vasylechko,§ and Ramakrishnan Kalai Selvan*,† †

Energy Storage and Conversion Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore 641 046, Tamil Nadu, India ‡ Department of Energy Science Engineering, Daegu Gyeongbuk Institute of Science and Technology, Daegu 711-873, South Korea § Semiconductor Electronics Department, Lviv Polytechnic National University, 12 Bandera Street, Lviv 79013, Ukraine S Supporting Information *

ABSTRACT: A unique multifunctional electrode made of welldefined highly crystalline Ni−P nanoparticles interweaved apiece to form sponge-like structure prepared by the single-step hydrothermal method. The distinct phases of interlinked nanospherical Ni−P compounds (Ni2P, Ni2P/Ni12P5, and Ni12P5) were obtained at 140 °C with different reaction time periods. The Ni2P exhibits supreme specific capacity of 206 mA h g−1 (1354 F g−1) at 5 mA cm−2, which seems to be the highest among the Ni2P reported so far. Biomass-derived activated carbon was prepared as a negative electrode (135 F g−1 at 1 mA cm−2) to fabricate a flexible supercapattery gadget, which delivered remarkable energy density and power density of 42 W h kg−1 and 2856 W kg−1 respectively, at 3 mA cm−2 even after 10000 cycles. The Ni−P coated carbon cloth has shown immense mechanical strength and durability, which was tempting to use for electrocatalytic application. Both oxygen evolution reaction and hydrogen evolution reaction trials resulted in the evolution of an enormous amount of gas bubbles with low overpotentials of 278 and 234 mV to achieve a current density of 10 mA cm−2, respectively. Hence, the dominant Ni−P electrodes were made multifunctional by demonstrating its potential application for efficient appliances. KEYWORDS: multifunctional, nickel phosphide, spongy material, flexible electrode, alkaline electrolyte, supercapattery, bifunctional electrocatalyst, water splitting

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reaction.2−6 Though nickel-based compounds give the impression of enjoying concrete benefits, they have more or less critical traits such as the sluggish kinetic rate of oxides that do not encourage fast electron transfer, which is an essential factor to achieve high power density.7 This crucial criterion also affects the kinetic rates of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalytic activity. Apart from the limitations in oxides, nickel-based sulfides tend to decompose in strong alkaline electrolytes and also exhibit an effective loss in retaining their initial capacitance. For avoiding this deficit of the oxide- and sulfide-based compounds, nickel phosphide is explicitly recognized as an ntype semiconductor, which gives the impression of being kinetically favorable. It also has lured interests as an electrochemically active material in the wider arena of supercapacitors, lithium-ion batteries, and electrocatalyst due

INTRODUCTION “Energy” is considered to be one of the biggest assets in this contemporary world due to the depletion of nonrenewable resources and widespread environmental pollution.1 This has enforced a look out for an alternative source to conserve the socalled asset of the word “Energy” for future use in an ecofriendly manner. To shield the depletion of nonrenewable resources, it is essential to make use of the naturally abundant materials as an ingenious electrode material for various energy storage and conversion systems. Among the transition metals, nickel seems to lure more interest due to its abundance in nature, low price tag, and preference with rich valence states (Ni0/Ni2+/Ni3+) holding a tremendous electroactive decorum.2,3 Moreover, Ni metal by means of its ions having an arresting divalent state can effortlessly outburst into dominant compounds with hydroxides, oxides, sulfides, and phosphides. These nickel-based compounds have been decoyed in diverse electrochemical applications especially supercapacitors, lithiumion batteries, nickel−metal hydride batteries, and water electrolyzers due to its tempting highly reversible redox © XXXX American Chemical Society

Received: October 8, 2017 Accepted: December 11, 2017

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DOI: 10.1021/acsaem.7b00006 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Apart from the progress of energy storage systems, energy conversion is also an essential tool to pave the way for producing renewable energy resources. Among the energy conversion systems, water splitting is considered as an essential route owing to its approaching application in the production of ultrahigh pure oxygen and hydrogen gas.27,28 At present, noble metals such as ruthenium/iridium and platinum are employed as an efficient electrocatalyst for the OER and HER, respectively.29−31 The huge asking price and limited resources of these noble metals restrict the existence of this application in the commercial market. Hence, replacing these noble metals by non-noble or metal-free electrocatalysts and achieving unfaltering performance are the dynamic tasks of the present scenario.32,33 In concern with electrocatalysis, You et al. have prepared hierarchical porous urchin-like Ni2P superstructures by electrodeposition followed by high-temperature pyrolysis for overall water splitting.18 Menezes et al. have demonstrated nickel phosphide electrodes for a highly stable water splitting system by two-step hydrothermal method.34 Read et al. have also studied the OER and HER performance of the Ni2P prepared by electrodeposition followed by pyrolysis of phosphide under bright conditions.35 To further exploit the catalytic behavior, we have also demonstrated the improved catalytic activity, superior kinetic rate, and resilient durability of the prepared Ni−P electrodes into a laboratory scale water electrolyzer. Herein, a single-step hydrothermal route is adopted to prepare thermodynamically stable three different crystal structures (Ni2P, Ni2P/Ni12P5, and Ni12P5) of nickel phosphide nanoparticles by varying reaction times without using any surfactants and reducing reagents. Among the Ni−P structures, Ni2P has ensured a maximum specific capacity of 206 mA h g−1 at 5 mA cm−2 in an aqueous 1 M KOH electrolyte with an excellent stability and specific capacity retention (at a high current density ranging from 5 to 15 mA cm−2). The prepared biomass-derived activated carbon rendered a maximum specific capacitance of 135 F g−1 in an aqueous 1 M KOH electrolyte over the negative region with an exceptional stability and specific capacity retention (at a current density ranging from 1 to 8 mA cm−2). Then both electrodes were combined to design a laboratory scale flexible aqueous supercapattery gadget resembling the commercial device, which delivered a maximum energy density of 42 W h kg−1 at 2856 W kg−1 power density with the high cyclic rate of 10000 cycles. After the supercapattery investigation, the Ni−P electrodes were laid open to the electrocatalytic scrutiny. The Ni−P compound also performed well as an electrocatalyst showing excellent strength and stability during both OER and HER reactions with improved catalytic activity, superior kinetic rate, and resilient durability.

to its physicochemical properties, special metalloid characteristics, superior electrical conductivity, and high theoretical capacity of 1886 F g−1.3,6−10 Also, nickel phosphide can embark on numerous thermodynamically resilient crystal structures and stoichiometric forms including Ni3P, Ni5P2, Ni2P, Ni12P5, NiP2, Ni5P4, NiP, and NiP3.11 In recent years, Ni−P has been prepared via numerous ways and means through reduction of phosphates and phosphinates, solid-state metathesis at high temperatures, low-temperature solvothermal methods, plasma methods, electrodeposition, electrochemical- phosphorization, chemical conversion, hydrothermal, and solvothermal methods.10,12−22 As of the works mentioned above, mostly nickel phosphides were prepared via more than one step followed by high-temperature treatment, which increases the production cost that ruins the commercialization of the nickel phosphide nanomaterials. Therefore, considering the importance of Ni−P and to overcome the drawbacks mentioned above, here we focused on preparing Ni−P by the custom of a single-step hydrothermal method and were enticed to go through the supercapacitor and water electrolyzer applications. The electrochemical capacitors, also known as supercapacitors or ultracapacitors, are considered as the most promising contenders among the energy storage systems, luring curiosity due to their superior power density (10 kW kg−1), express charging and discharging mechanism, excellent cycling stability, and an ecofriendly spirit.23−25 Even though they enjoy high power density, it is regretable that they provide low energy density in comparison with batteries.25 Retaining electrochemical capacitors’ natural property of high power density and expanding the energy density of these devices are among the major tasks for budding researchers. Nowadays, flexible hybrid capacitors (FHCs) or flexible supercapatteries are introduced to trounce the deficient energy density phenomenon. The supercapatteries are nothing but the combination of electric double layer capacitor (EDLC) based materials and the battery-type materials used as an anode and cathode, respectively. The anode embraces high power density, while the cathode is responsible for improved energy density. The implementation of flexible hybrid capacitors not only resolves the energy density criteria but also entices interests in applications from a number of open choices due to its highly flexible traits. These flexible hybrid supercapatteries can provide both high power and energy density in any form such as bending, stretching, twisting, and folding. So far, only limited studies have been reported on Ni−P compounds for the supercapacitor applications. Notably, Zhang et al. have reported the successful electrodeposition of nickel nanoparticles on carbon fiber (CF) followed by low-temperature phospidation that resulted in a maximum specific volumetric capacitance of 817 F cm−3 at 2 mA cm−2.7 Wang et al. have prepared Ni2P by ball milling process followed by annealing method that was ensued with a maximum specific capacitance of 843.25 F g−1 at 1 A g−1.11 Also Du et al. have testified a solvothermal route to prepare Ni2P nanoparticles that was ensued with a maximum specific capacitance of 668.7 F g−1 at 1 A g−1.26 As we mentioned above, these results are obtained by the NiP produced by augmented steps and overpriced reagents, which remains as a barrier to commercialization. Therefore, the aim of the present work is to prepare Ni−P by simple synthesis procedure using economically viable precursors that can encourage commercialization, delivering improved energy density with respect to power density.

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EXPERIMENTAL SECTION

Preparation of Ni−P Structures. Stoichiometric amounts of nickel nitrate and red phosphorus were dispersed in 40 mL of distilled water under stirring using a magnetic stirrer to blend the composition thoroughly. The resultant mixture was transferred into the Teflonlined autoclave, sealed tightly, and maintained at 140 °C for 12 h. After the completion of reaction, the product was centrifuged, and the obtained black colored product was dried at 80 °C under vacuum. The same procedure was repeated at different reaction periods of 24 and 48 h to obtain diverse structures of Ni−P nanoparticles. Preparation of Biomass-Derived Activated Carbon. A 2 g amount of powdered Prosopis julif lora pods was taken with 40 mL of 1 M conc. H2SO4 (98%) in a Teflon-lined autoclave and maintained at B


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ACS Applied Energy Materials 180 °C for 24 h. The obtained hydrochar was thoroughly washed with water and ethanol and dried under vacuum at 80 °C for 12 h. For the KOH activation, the weight ratio of the sample and KOH was taken as 1:1 (wt %), where the KOH was dissolved in 10 mL of DI water. The resultant mixture was kept under stirring for 24 h and finally dried in vacuum at 80 °C overnight. Subsequently, the KOH impregnated sample was carbonized at 700 °C for 1 h at a rate of 10 °C min−1 in a tubular furnace under a constant flow of Argon (100 mL min−1 approximately). The obtained activated carbon was washed with 2 M HCl before being washed thoroughly with water and ethanol to obtain neutral pH. The pH-adjusted sample was dried at 80 °C under vacuum for overnight to get the final product, named as biomass-derived activated carbon (BDAC). Characterization Techniques. The phase formation was identified using powder XRD (Rigaku, MiniFlex 600) with Cu Kα radiation (λ = 1.5418 Å). The morphologies of the catalysts were observed using FESEM (Hitachi-S4800) at an accelerating voltage of 3 kV and HRTEM (Hitachi HF-3300, 300 kV). The XPS measurements were performed to determine the oxidation states and chemical bonding between the elements, using a Thermo-Scientific ESCALAB 250Xi apparatus, a vacuum of 10−9 mbar, and Mg as the exciting source. The Bio-Logic VMP3 multichannel electrochemical workstation was used for analyzing the electrochemical properties of the samples. Electrode Preparation. The positive and negative flexible electrodes were prepared by the following methods. The active material (Ni−P or AC), carbon black, and polyvinylidene difluoride (PVDF) were mixed in the ratio of 80:10:10 using 0.4 mL of Nmethyl-2-pyrrolidine (NMP). Then the mixture was ground well for an hour to make a homogeneous slurry. Then, the obtained slurry was coated on the surface of the carbon cloth using a paintbrush and dried at 100 °C for overnight in a vacuum oven. All the electrochemical performance of this work was done in 1 M KOH electrolytes. Here, Pt and Hg/HgO were used as counter and reference electrodes, respectively. All potentials reported in the electrocatalyst analysis were against the RHE, which was converted from the Hg/HgO scale using a calibration.

temperature was increased from 120 to 140 °C for 12 h, to obtain the highly crystalline Ni2P particles. Further holding the same temperature with different reaction periods of 24 h (Figure 1b) and 48 h (Figure 1c), the transitional Ni−P phase of Ni2P/Ni12P5 and single-phase Ni12P5, respectively, were obtained. The crystallite size of the prepared Ni−P materials was calculated from the XRD data using Scherrer’s formula, D = 0.9λ/(β cos θ). Here, λ is the wavelength of the X-ray (1.54060 Å), β is the full width half-maximum of the high-intensity peak, and θ is the Bragg angle. The calculated crystallite size of Ni2P and Ni2P/Ni12P5 phases resembles almost a similar crystallite size of ∼10 nm, whereas Ni12P5 possesses the comparably increased size of ∼16 nm. The obtained Ni−P materials possess better structural properties than the other reported Ni−P nanostructures prepared by the same hydrothermal method in terms of small crystallite size and high crystalline nature.10,36,37 The Ni−P samples prepared at 140 °C for three different reaction hours (12, 24, and 48 h) are subjected to Rietveld analysis. XRD phase analysis of the 12 h sample revealed hexagonal Ni2P as the main phase. Besides, the detectable diffraction peak at 2θ ∼ 49° indicates a possible presence of the minority tetragonal Ni12P5 phase. Significant broadening of the diffraction maxima of the Ni2P phase suggests the nanocrystalline character of the powder. Full profile Rietveld refinement confirms the phase composition of the sample. For a simultaneous two-phase Rietveld refinement procedure, the atomic positions in the Ni2P (space group, P6̅2m) and Ni12P5 (space group, I4/m) structures reported in refs 38 and 39 were used as starting models. During the refinement procedure, the lattice parameters, coordinates, and displacement parameters of atoms in the Ni2P structure were refined, whereas for the minority Ni12P5-phase structural parameters were fixed according to ref 39. Simultaneous two-phase Rietveld refinement procedure led to the good agreement between experimental and calculated XRD patterns, as evidenced by Figure 2a. The amounts of Ni2P and Ni12P5 phases as derived from this quantitative phase analysis are 97.8(2) and 2.2(2) wt %, respectively. Refined structural parameters of the main Ni2P phase as well as corresponding interatomic distances are given in Tables S1 and S2 (Supporting Information), respectively. There are two types of Ni and P sites in the hexagonal Ni2P structure as charted in Table S1 (Supporting Information) and shown in Figure 2a (inset). The Ni1 atoms are located inside of the distorted NiP4 tetrahedra formed by four nearest phosphorus species located at the distances 2.212−2.268 Å, whereas the Ni2 atoms are surrounded by five P neighbors 1P1 and 4P2 species at 2.368 and 2.461 Å, respectively forming a pyramidal environment around the central atom (Figure 2a (left inset)). Second coordination spheres of Ni1 and Ni2 species include correspondingly eight and six Ni atoms located at the distances 2.612−2.679 Å in Table S2 (Supporting Information). Both P1 and P2 atoms are surrounded by nine Ni neighbors located at the distances 2.268−2.368 and 2.212− 2.461 Å, respectively in Table S2 (Supporting Information); corresponding polyhedra are shown on the right inset of Figure 2a. According to X-ray powder diffraction examination, the 24 h sample consists of two nickel phosphideshexagonal Ni2P and tetragonal Ni12P5 phases in the amounts of 77.7(5) and 22.3(3) wt %, respectively. Corresponding percentages were derived by the quantitative full profile Rietveld refinement (Figure 2b). During the refinement procedure, the lattice parameters, positional and displacement parameters of atoms in both

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RESULTS AND DISCUSSION The obtained Ni−P samples are examined via XRD analysis. Comparing the XRD pattern in Figure 1, the Ni−P prepared at

Figure 1. XRD patterns of Ni−P samples prepared at 140 °C for (a) 12, (b) 24, and (c) 48 h.

140 °C for 12 h (Figure 1a) seems to be highly pure and crystalline with strong peaks forming the hexagonal structure of Ni2P and it is well-matched with the standard data (JCPDS No. 01-089-4864). Initially, at a low temperature of 120 °C, the predicted single-phase Ni2P nanoparticles resulted in minor peak shifts and impurities. Thus, to get rid of the defects, the C


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Besides, the sample contains a detectable amount of the Ni2P phase. Because it was derived by simultaneous two-phase Rietveld refinement (Figure 2c), the amount of tetragonal Ni2P phase in the 48 h sample was 25.0(2) wt %. In the refinement procedure, the lattice parameters, coordinates and displacement parameters of atoms in the Ni12P5 and Ni2P structures, were refined. Refined structural parameters of the primary Ni12P5 phase of the 48 h sample, well-corresponding interatomic distances, are given in Tables S3 and S4 (Supporting Information), respectively. There are two types of Ni and P sites in the tetragonal Ni12P5 structure (Table S3 (Supporting Information) and Figure 2c (inset)). The Ni1 atoms, in general 16i, are surrounded by the four nearest phosphorus atoms at distances 2.194−2.467 Å and eight Ni atoms located at 2.526−2.725 Å (Table S4 (Supporting Information) and Figure 2c (inset)). There are 11 atoms forming coordination polyhedra around the Ni2 sitetwo nearest P atoms at 2.260−2.283 Å, five Ni atoms at 2.517−2.575 Å, two P atoms at 2.619 Å, and two outstanding Ni atoms at 2.725 Å. The P1 atoms in the Ni12P5 structure have 10 Ni atoms in the nearest environment located at the distances 2.194−2.619 Å, whereas P2 is located in the centers of almost regular cubes [PNi8] (Table S4 (Supporting Information) and Figure 2c (inset)). The refined lattice parameters of Ni2P and Ni12P5 phases in the 12, 24, and 48 h samples investigated are collected in Table S5 (Supporting Information). For comparison, literature data for the corresponding structures are given as well. One can observe that the obtained lattice parameters of the Ni2P phase in all three samples are detectable higher than those reported for a single-phase specimen in ref 38 and agree very well with the values reported for two-phase Ni12P5 + Ni2P alloys. According to refs 38 and 40, Ni2P has a narrow homogeneity range and the increasing phosphorus content led to a contraction of the unit cell dimensions. X-ray photoelectron spectroscopy investigated the chemical states of Ni and P in the prepared Ni−P samples (Ni2P, Ni2P/ Ni12P5 and Ni12P5), and the obtained results were displayed in Figure S1 (Supporting Information). All of the Ni 2p spectra were deconvoluted taking into account the spin−orbital splitting of Ni 2p3/2 and Ni 2p1/2 lines along with the presence of corresponding satellite peaks. The observed two significant peaks at 856.78 and 875.18 eV for Ni2P, 856.83 and 875.23 eV for Ni2P/Ni12P5, and at 856.63 and 875.03 eV for Ni12P5 correspond to the spin−orbital splitting of Ni 2p3/2 and Ni 2p1/2, respectively. The respective binding energies substantiate the existence of surface-oxidized nickel species in the Ni2+ oxidation state.11,41−44 Also, it is observed that the peaks obtained at 852.21, 852.90, and 852.35 eV in Ni 2p spectra of Ni2P, Ni2P/Ni12P5, and Ni12P5, respectively, are quite close to the elemental-zero valence state of nickel, which conveys the small positive charge (Niδ+) of Ni species in the prepared samples.45−47 The P 2p spectrum of Ni2P shows a prominent peak around 133.62 eV and a marginal hump at 129.20 eV, which can be attributed to the oxidized P species and metallic P, respectively, whereas the P 2p spectra show two prominent peaks around 129.77, 134.34 eV on behalf of Ni2P/Ni12P5 and 129.82, 134.32 eV intended for Ni12P5, which could be assigned to Pδ− in the form of metal phosphide and oxidized P species observed at the surface due to the partial passivation of phosphide particles.7,48−50 The P 2p binding energy at ∼129.20 eV is less than that of elemental P (130.20 eV), which indicates that the P species in Ni2P has a minimal negative charge (Pδ−, 0 < δ < 1).48,50 These results suggest that there is an electron

Figure 2. (a) Graphical results of two-phase Rietveld refinement showing coexistence of Ni2P (97.8 wt %) and Ni12P5 (2.2 wt %) in the 12 h sample. (Insets, panel a) Crystal structure of Ni2P: (left) coordination polyhedra of Ni atoms and Ni−P bonds; (right) coordination polyhedra of P atoms. (b) Fragment of diffraction pattern showing presence of 77.7(5) wt % Ni2P (blue) and 22.3(3) wt % Ni12P5 (red) in the 24 h sample. (c) Fragment of diffraction pattern showing presence of 75.0(7) wt % Ni12P5 (red) and 25.0(2) wt % Ni2P (blue) in the 48 h sample. (Inset, panel c) Crystal structure of Ni12P5: coordination polyhedra of Ni and P atoms are shown. For panels a−c the experimental XRD pattern is shown in comparison with the calculated patterns. The difference between measured and calculated profiles is shown as a curve below the diagrams. Short vertical bars indicate the positions of diffraction maxima of Ni12P5 and Ni2P and phases (upper and lower rows, respectively).

Ni2P and Ni12P5 structures, were refined. The diffraction maxima of the Ni2P phase are considerably broader than those of Ni12P5 (Figure 2b). This observation points to nanoscale grain size of the Ni2P phase, whereas the Ni12P5 phase is evidently microcrystalline. XRD phase analysis of the 48 h sample revealed tetragonal Ni12P5 as the primary phase. D


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Figure 3. FESEM images of (a) Ni2P, (b) Ni2P/N12P5, (c) N12P5, (d, e) low- and high-magnification HRTEM images of Ni2P, (f) TEM images showing interplanar spacing, and (g−i) EDX mapping images of Ni2P.

transfer from Ni to P in all three nickel phosphide phases. Because of the XPS studies taken for the aged Ni−P samples after a year, the conspicuous presence of oxidized Ni and P peaks have been attributed to the surface oxidation of the sample. Xiao et al.51 have reported a similar kind of issue with Mo−P in XPS. The peak corresponding to Pδ− in the form of metal phosphide, which was present in the P 2p spectrum of asprepared Mo−P was absent for the aged Mo−P sample, and they could find only the existence of oxidized P species at ∼134.20 eV.51 Therefore, they further characterized the phase formation of the aged Mo−P sample with XRD. No discernible peaks of the oxides were observed in the results, which confirmed the surface oxidation of the prepared sample. Taking this into contemplation, the XRD analysis is carried out for all of our three aged Ni−P samples (Figure S2 (Supporting Information)), which exactly matches with the XRD pattern of the freshly prepared samples (Figure 1), since it does not indicate any traces of the oxide phase of nickel or phosphorus. Hence, the existence of oxidized Ni and P species in all of our three samples is evidently due to the surface oxidation. The FESEM images (Figure 3a,b) reveal the highly absorptive sponge-like morphology for the Ni2P and Ni2P/ Ni12P5 samples. On the other hand, Ni12P5 in Figure 3c shows a lack of sponge formation. As Scheme 1 illustrates, when the reaction procedure is through 12 h at a constant temperature of 140 °C, a highly spongy Ni2P is formed. The spongy nature of the material was attained by the integration of individual particles one over the other, leaving room to craft the obtained structure. Thus, Figure 3a reveals that sponge-like morphology, which is composed of interconnected spherical particles with

Scheme 1. Represents the Structural and Morphological Phase Transformation of the Ni−P Compound with Respect to the Reaction Time (12, 24, and 48 h) at a Constant Temperature of 140 °C

some elongations creating the tendency to grow further. These interconnected Ni2P nanoparticles interweaved together forming the more significant bulk of an open spongy structure.52 By the side of 24 h, Scheme 1 clarifies the phenomena of integration of individual particles getting merged making hefty particles leading to reduced porosity with the support of sufficient reaction time. This results in the emergence of Ni12P5 phase in the Ni2P crystal structure, picked up earlier. Hence, Figure 3b shows more fusion of particles with each other concerning reaction time, which limits the size of the E


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Figure 4. (a) CV curves of all Ni−P structures and bare carbon cloth at a scan rate of 1 mV s−1 (b) CV curves of Ni2P with different scan rates. (c) GCD curves of all Ni−P structures. (d) GCD curves of Ni2P with different current densities, (e) Current density vs Specific capacity plot of Ni−P electrodes, (f) Nyquist plot of Ni2P electrode with Z-fit (inset(a) shows the resultant circuit and inset(b) shows Nyquist plot of Ni2P, Ni2P/Ni12P5 and Ni12P5 electrodes) (g) CV curves of BDAC with different scan rate, (h) GCD curves of BDAC with different current densities and (i) Current density vs Specific capacitance plot of biomass-derived activated carbon.

relatively meager BET surface area of 10.85 m2 g−1 and specific capacity of 588 C g−1 at 1 A g−1. Also, a novel hollow nanocapsule morphology was attained with Ni12P5 nanoparticles by a one-pot hydrothermal technique.8 However, the specific capacitance of the electrode provides 949 F g−1 at 1 A g−1. Figure 3d shows the TEM image of the Ni2P nanoparticles. The spongy nature of the Ni2P nanoparticles were clearly visualized from Figure 3e, and in addition Figure 3f elucidates that the obtained Ni2P nanoparticles are highly crystalline showing a well-defined diverse crystal lattice with prominent interplanar spacings of 0.50, 0.33, 0.28, 0.20, and 0.16 nm, corresponding to the d-spacings of (100), (001), (101), (201), and (300) crystal plane of hexagonal Ni2P, respectively. The HRTEM image also comprises with the EDS mapping of the Ni2P nanoparticles. The mapped images in Figure 3g−i show the uniformly dispersed Ni and P signals endorsing the formation of Ni2P. Figure 3g displays the standard TEM image that is focused on mapping the composition of Ni and P elements with red and green signals respectively, existing in the formed Ni2P compound. The widespread red signal in Figure 3h corresponds to the uniform distribution of the Ni element all over the compound with 68 at. % as recorded from the EDAX spectrum in Figure S3 (Supporting Information). The green signal representing the P elements in Figure 3i also shares

pores formed. This may be due to the increased reaction time and inequitable presence of the Ni2P structure. On further increasing the reaction time to 48 h, the particles are entirely amalgamated to form a bulk particle wrapping all the pores created at the initial stages as demonstrated in Scheme 1. Here the initial Ni2P structure is completely changed to the Ni12P5 structure. Therefore, increasing the reaction time guides one to completely fused Ni12P5 particles without any sponge-like structure as shown in Figure 3c. Overall, the Ni2P nanoparticle possesses a sponge-like morphology with increased pore size.53,54 As the reaction time increases, the fusion of particles increases which suppresses the pore size. A further increase in the reaction time leads to the formation of nonspongy Ni12P5 particles. Therefore, compared to all three compounds, Ni2P shows improved morphology with respect to the FESEM characterization. Earlier, the Ni2P nanospheres and nanorods were prepared by a surfactant-assisted hydrothermal method followed by the phoshodization procedure.10 Even though the nanostructures were obtained with a well-defined morphology, the electrode properties such as specific capacitance and capacitance retention were not comparably exceptional. Similarly, Hu et al. have fabricated the Ni2P particles by a hydrothermal method with uniform and smaller particle size along with a mass of tunnels between the particles.37 Despite the noble morphology, the Ni2P nanoparticles exhibit a F


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Ni(III) state, and further reversing the potential, it recoups its original Ni(II) state by the transfer of electrons.18,60 The Faradaic electrochemical reaction involved is as follows.25

equal space with respect to the Ni element holding 31 at. % in the composition. The texture data of the prepared Ni−P samples were investigated with Brunauer−Emmett−Teller (BET) analysis to obtain the specific surface area and pore size distribution. Panels a and b of Figure S4 (Supporting Information) show the BET isotherm and pore size distribution of all the prepared Ni−P compounds. Corroborating the degrading porous morphology with increasing reaction temperature observed in FESEM micrographs, the surface area and pore volume of the Ni−P samples take the order of Ni2P (13.68 m2 g−1; 0.59 cm3 g−1) > Ni2P/Ni12P5 (10.29 m2 g−1; 0.46 cm3 g−1) > Ni12P5 (3.44 m2 g−1; 0.24 cm3 g−1). Also, the existence of a hysteresis loop with an unsaturated tail portion at a relative pressure of 0.5−1.0 demonstrates the presence of both meso- and macropores in the samples. The obtained average pore sizes of Ni2P (15.62 nm), Ni2P/Ni12P5 (13.56 nm), and Ni12P5 (29.16 nm) are further validating the presence of mesopores, which could facilitate the charge storage capability of the material. Since the electrode materials with a large specific surface area are found to deliver high specific capacitance, the Ni2P is believed to perform better than the other two phases of Ni−P material. Moreover, the obtained BET surface area of the Ni2P is comparably higher than the reported values using the same method with different conditions.37,55 Du et al. have synthesized the Ni2P nanoparticles through a solvothermal method making use of three solvents such as oleylamine, oleic acid, and anisole. Further, the reaction was also carried out at a relatively high temperature of 200 °C and a period of 24 h, and the prepared Ni2P nanoparticles provided a low BET surface area of 5.05 m2 g−1.55 Similarly, Hu et al. have synthesized the Ni2P particles by a single-pot hydrothermal method at a different reaction condition (10 h at 200 °C). Though the particles possess a better surface area of 10.85 m2 g−1, a meager pore volume of 0.02 cm3 g−1 was acquired.37 The FESEM images of the prepared BDAC before and after activation with KOH are shown in Figure S5 and Figure S6 (Supporting Information), respectively. The BDAC before activation shows solid particles with irregular shapes. After activation of BDAC with KOH, it seems to appear as a porous structure with uniform pore distribution, which facilitates the BDAC to function in the supercapacitor application. The EDAX spectrum of BDAC represented in Figure S7 (Supporting Information) shows 89.09 at. % of carbon composition in the prepared sample. Figure S8 (Supporting Information) reveals the Raman spectrum of the biomassderived activated carbon. The two pronounced peaks obtained at ∼1350 and ∼1600 cm−1 of the Raman spectrum are assigned to D and G bands of carbon. The calculated intensity ratio between the D and G bands (ID/IG) was ∼1.18, which implies the highly disordered nature of the carbon. To investigate the electrochemical performance, a sequence of electrochemical trials was carried out by the common threeelectrode system.56 Figure 4a represents the cyclic voltammetry (CV) curves of the substrate (CC), Ni2P, Ni2P/Ni12P5, and Ni12P5 electrodes at a scan rate of 1 mV s−1 with a potential frame stretching from 0 to 0.6 V (vs Hg/HgO). The CV curves show a pair of prominent redox peaks confirming the batterylike nature of the Ni−P electrodes.57−59 The reversibility of the system is elucidated by the same behavior of the anodic and cathodic peaks. These peaks emerge due to the oxidation of Ni atom present on the surface of the electrode when a specific potential is achieved. It leads to the formation of Ni(II) state to

Ni 2P + 2OH− ↔ Ni 2P(OH−)2 + 2e−

(1)

Comparing the behavior of all three electrodes, it was visibly apparent that the Ni2P electrode sounds invincible by delivering a high specific capacitance of 1312 F g−1 at 1 mV s−1, while Ni2P/Ni12P5 and Ni12P5 stand behind with 631 and 850 F g−1 at 1 mV s−1, respectively. Figure 4b shows the CV curves of the Ni2P electrode with a different scan rate (2−10 mV s−1). The intensity of the anodic and cathodic peak current increases linearly concerning the square root of the scan rate, which follows the Randles−Sevick equation yielding an adjacent R2 value of 0.998 (Figure S9a (Supporting Information)). This linear increase in the peak current with respect to the square root of the scan rate substantiates the diffusion-controlled process of the system ensuing a potential disparity (ΔEa,c) observed between the oxidation and reduction peaks. This results in a noticeable polarization of the electrode material, which is a quite familiar occurrence in most of the battery-type electrode materials that are obviously due to the charge diffusion polarization within the electrode material.61,62 This polarization tends to increase larger depending on the factors, such as crystal structures, the surface morphology, the contact resistance between the electrode and substrate, and increasing loading mass of active materials.63 Thus in Figure 4a, the polarization of Ni2P is larger owing to its unique crystal structure and enriched surface morphology over the other two materials. Figure 4c signifies the galvanostatic charge−discharge (GCD) curves of the Ni−P electrodes with a static current density of 10 mA cm−2 at a potential of 0.55 V (vs Hg/HgO). The formation of the plateau region in the GCD curves further confirms the battery behavior of the Ni−P electrodes, supporting the CV curves. Even though they all perform in the same trait, regarding the storage concern, the specific capacity of Ni2P surpasses that of Ni2P/Ni12P5 (77 mA h g−1) and Ni12P5 (107 mA h g−1) electrodes with an extraordinary margin of 206 mA h g−1 at 5 mA cm−2. Hence, the Ni2P electrode is exposed to a variable current density of 5−15 mA cm−2 with a potential limit of 0.55 V vs Hg/HgO as shown in Figure 4d. The high specific capacity of the Ni2P electrode may be attributed due to the spongy nature of the material itself.25,64 The apertures in the spongy Ni2P act as a reservoir, which can accumulate a greater number of ions on the surface of the electrode that result in a high capacity of the electrode.64 Hence, the pores present in the compound naturally are more than a benefit, which is liable to the high specific capacity of the Ni2P electrode. Additionally, the Ni2P electrode acts by thriving in response to the altered high current densities without moving away from its nature. Figure 4e displays the specific capacity retention of the prepared electrodes. It was found that even at a high current density of 15 mA cm−2, the Ni2P electrode delivered a high specific capacity of 165 mA h g−1, by upholding 80% of its initial specific capacity, whereas its companions Ni2P/Ni12P5 and Ni12P5 have endured with 79% and 74% of its initial specific capacity. Further, to investigate the succession rate of the Ni−P electrodes, the cyclic stability tests were carried out for 1000 cycles at 15 mA cm−2 (Figure S9b (Supporting Information)). Herein, as we expected the Ni2P electrode ran riot throughout the cycle of just losing 20% of its initial specific capacity, which appears to be much better G


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ACS Applied Energy Materials Scheme 2. Schematic Representation of Charging and Discharging Modes of the Supercapattery Device

Figure 5. (a) CV curves of both positive and negative electrodes at 5 mV s−1, (b) CV curves of the fabricated supercapattery with different scan rate, (c) GCD curves of the device with different load current, (d) stability curve up to 10000 cycles, (e) Nyquist impedance plot of BDAC//Ni2P device before and after 10000 cycles [(inset) GCD curves of first and 10000th cycles], and (f) Ragone plot comprising the Ni−P compounds reported in recent times with the current work.

for the prepared BDAC, which executes as a negative electrode. Figure 4g represents the CV curves obtained at different scan rate with a potential ranging from 0 to −1 V (vs Hg/HgO). The formation of the rectangular region symbolizes the EDLC behavior of the electrode.57,58 Hence, the charge storage mechanism is by means of nonfaradaic reaction.58 The nonfaradaic electrochemical reaction is involved as follows.25

than the other reported Ni2P electrodes. The Ni2P/Ni12P5 and Ni12P5 electrodes also resulted in a massive loss percentage compared to the Ni2P electrode as shown in Figure S9b (Supporting Information). The poor performance of these electrodes compared to Ni2P can be enlightened by the impedance spectra of all three electrodes. Figure 4f shows the EIS spectra of Ni2P fitted with an equivalent circuit to obtain the Rct value. The values obtained by equivalent circuit are tabulated in Table S6 (Supporting Information). The inset of Figure 4f elucidates that the semicircle of Ni2P is very much smaller with its diameter stretching around 10 Ω compared to the Ni2P/Ni12P5 (∼80 Ω) and Ni12P5 (∼70 Ω) electrodes. The smaller the semicircle, the larger the 1/Rct value and hence the higher the conductivity that fastens reaction kinetics.65 Therefore, with support from the impedance spectra, high conductivity, faster kinetics, and the surplus spongy nature, Ni2P electrode has exhibited a high specific capacity along with remarkable retention and cyclic stability compared to other Ni−P electrodes. As reported for the above positive electrodes, the same tests were carried out

BDAC + x K+ + x e− ↔ BDAC(x e−) x K+

(2)

The prepared negative electrode provides a specific capacitance of 148 F g−1 at 1 mV s−1. The GCD curves of increasing current density at a potential of −1 V (vs Hg/HgO) has been shown in Figure 4h, which also exposes the EDLC nature of the electrode material. The specific capacitance of 135 F g−1 at 1 mA cm−2 was obtained from the GCD curves. The prepared BDAC provide a high specific capacitance retention, retaining 92% of its initial specific capacitance after a high current density of 8 mA cm−2, as shown in Figure 4i. It also visibly displays the high cyclic stability by losing just 4% of its primary specific capacitance after 1000 cycles shown in Figure H


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Table 1. Detailed Disparity among the Ni−P Reported and Current Work with Respect to the Substrate Used, Electrolyte Concentration, Working Potential, and Device Performance electrolyte

working potential (V)

energy density (initial) (W h kg−1)

power density (initial) (W kg−1)

energy density (final) (W h kg−1)

power density (final) (W kg−1)

cyclic performance (retention)

Fe2O3//Ni2P (Ni foam)

2 M KOH

1.6

17.3

8000

35.5

400

Fe2O3//Ni5P4 (Ni foam)

2 M KOH

1.6

13.9

8000

29.8

400

AC//amorphous Ni−P (Ni foam) AC//NiP@CoAl-LDH NTAs (Ni foam) AC//NiCoP (Ni foam)

2 M KOH

1.6

11.8

8000

29.2

400

2 M KOH

1.6

23.27

4680

37.18

450

6 M KOH

1.4

18

5586

32

351

BDAC//Ni2P carbon cloth

1 M KOH

1.6

25.34

3266

42.36

2856

96% after 1000 cycles 86% after 1000 cycles 84.5% after 1000 cycles 95.50% after 4000 cycles 91.8% after 3000 cycles 100% after 10000 cycles

electrode material (substrate)

ref 11 11 36 70 37 this work

The Ni 2p spectrum of the Ni2P electrode after the cyclic stability indicates two significant peaks at 875.56 and 856.84 eV corresponding to the spin−orbital splitting of Ni 2p1/2 and Ni 2p3/2, respectively. The recognized binding energies authenticate the presence of surface Ni at the +2 oxidation state as it is observed in the electrochemical studies. Hence, the valence state of the nickel in Ni2P remains similar even after 10000 cycles, validating the stable nature of the prepared Ni2P electrode. Figure 5e shows the impedance spectra of the device, before and after the sequential cyclic stability. From the spectra it is clear that the diameter of the semicircle is reduced after 10000 cycles, resulting in faster kinetics comprised of high conductivity to achieve high specific capacity. The inset of Figure 5e displays the GCD curves of the first and 10000th cycles, which validates the considerable difference in the discharge time. Because of this increase in initial specific capacity, the energy density of the device was also found to increase while the power density gets altered to some extent. This phenomenon symbolizes the activation effect of the Ni2P electrode, which is responsible for the energy density contribution of the device. After 10000 cycles, the obtained energy density was 42 W h kg−1 at a power density of 2856 W kg−1. Scheme 2 displays the schematic illustration of the mechanism involved in the charging and discharging modes of the device. The charging mode implicates migration of negative (OH−) and positive (K+) ions of the KOH electrolyte in the direction of oppositely charged Ni2P and BDAC electrodes. This results in the occurrence of oxidation reaction (Ni(II) to Ni(III)) at the cathode (Ni2P) and charges separation (+|−) at the anode (BDAC). Due to the highly spongy nature of the Ni 2 P and BDAC electrodes, a number of ions get accommodated on the electrode surface and get oxidized or separated, enlighting the charge stored by the device, whereas, during the discharging mode, the oxidized and separated ions at the cathode and anode get reduced (Ni(III) to Ni(II)) and detached back into the electrolyte solution, respectively. This results in the flow of electrons from the anode to cathode triggering the LED to glow. The Ragone plot in Figure 5f comprises a comparison of the prepared hybrid capacitor with reported hybrid capacitors. The reported hybrid capacitors show a sufficient loss of initial power density with respect to an increase in energy density. However, our device has grabbed an immense control over it, with a negligible loss of initial power density with respect to its increase in energy density. Table 1 replicates the results in the Ragone plot by a statistical

S10 (Supporting Information). Hence, the prepared BDAC material is recognized to contribute as a good negative electrode for the construction of a full cell. Successful investigation of both positive and negative electrodes has led to a curious paradigm in which an aqueous supercapattery gadget by separating both the electrodes by a separator bounded by an aqueous electrolyte medium and sealed as illustrated in Scheme 2.25,66 The fabricated gadget was examined by the CV and GCD analysis with a wide range of operating voltage stretching from 0 to 1.6 V. Based on the specific electric quantity values, the calculated mass ratio of negative to positive electrode is ∼3:1 mg (Figure 5a).25 Therefore, the mass loading of the negative electrode material is adjusted with respect to the positive electrode, correspondingly. The specific capacities of the gadget obtained from the CV and GCD curves are 121 F g−1 at 10 mV s−1 and 37 mA h g−1 at 0.5 mA, correspondingly (Figure 5b,c). From this analysis, an equivalent energy density of 25 W h kg−1 with respect to the power density of 3266 W kg−1 thrived by the designed gadget at 3 mA. The cyclic stability of the fabricated gadget was analyzed by performing charging and discharging trials for 10000 cycles as shown in Figure 5d. The initial specific capacity of 26 mA h g−1 obtained at 3 mA was established to increase for around 7000 cycles to reach a high specific capacity of 72 mA h g−1 which later get condensed to 57 mA h g-1 on reaching 10000 cycles. The increasing specific capacity with augmenting cycle numbers is attributed to the activation of the prepared Ni2P electrode. The activation phenomenon is related to the particle cracking due to the alloy lattice expansion upon its hydrogenation. The particle pulverization activates the electrode material by breaking the native oxide layer present at the surface of the particles and by increasing the effective surface area and the available active sites of the electrode contributing to the increased specific capacitance of the material on cycling.67−69 The surface morphology of the Ni2P electrode after cycling was analyzed to substantiate the stable morphology and structure of the proposed electrodes, and the corresponding FESEM images are given in Figure S11 (Supporting Information). Even though the particles congregated to form an agglomerated appearance on cycling, the morphology and structure of the electrode did not appear to be modified considerably even after 10000 cycles, which depicts the promising morphological stability of the Ni2P electrode. Further, the valence state of the Ni element was witnessed after the cycling test (Figure S12 (Supporting Information)). I


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Figure 6. (a) LSV curves of Ni−P and bare carbon cloth for OER, (b) necessary overpotential to achieve a current density of 10 mA cm−2 for different electrocatalysts, (c) corresponding OER Tafel plots, and (d) chronoamperometry of NiP.

and 1.45 V (vs RHE) is attributed to the oxidation reaction related to Ni2+/Ni3+ redox couples.75 The Ni2 P/Ni 12P 5 electrode has shown excellent catalytic activity compared to that of the carbon cloth with an onset potential at 1.45 V (vs RHE) from where the oxygen gas bubbles start to blow out from the surface of the electrode, and it requires an overpotential of 278 mV to attain a current density of 10 mA cm−2 at a scan rate of 5 mV s−1, which is relatively low compared to other reported Ni-based electrodes listed in Table 2.8 A tremendous amount of gas bubbles are seen to flow as the

approach. It also conveys the disparity in the reported and the present work with respect to the substrate used, electrolyte concentration, energy and power density and cyclic performance. From the above results, it is clear that the fabricated BDAC∥Ni2P on carbon cloth substrates has delivered an improved performance compared to other reports. The fabricated BDAC∥Ni2P has outperformed all the other reported Ni−P devices with exceptional values of capacitance, energy density, and power density because of the synergistic effect of both positive and negative electrode materials. The phase pure Ni2P with a small crystallite size of 10 nm was prepared with a spongy interweaved spherical morphology possessing a good BET specific surface area of ∼13 m2 g−1 as a positive electrode material. The Ni2P being an efficient faradaic electrode material is endowed with high specific capacitance of 1312 F g−1 at 1 mV s−1 (206 mA h g−1 at 5 mA cm−2), proficient cyclic stability of 1000 cycles with 80% capacity retention even at a high current density of 15 mA cm−2 and a good electrical conductivity with a very small resistance of ∼10 Ω. On the other hand, the activated carbon nonfaradaic electrode was prepared with a remarkable specific surface area of about 900 m2 g−1 and hierarchical porous structure to facilitate the storage capability and easy ion accessibility of the negative electrode material. The prepared BDAC material being a virtuous EDLC electrode also shows a good specific capacitance value of 148 F g−1 at 1 mV s−1 (135 F g−1 at 1 mA cm−2) and high specific capacity retention of 92% after 1000 cycles. Thus, the optimal choice of this novel combination of electrode materials has resulted an outclassing fabricated BDAC∥Ni2P supercapattery device. Further, the examination of the electrocatalytic activity of the Ni−P electrodes was executed. The oxygen evolution reaction was carried out with the common three-electrode system.71−74 Figure 6a shows the LSV curves of the carbon cloth and Ni−P coated carbon cloth electrodes. A peak observed between 1.35

Table 2. Comparison of OER Performance in 1.0 M KOH with Other Non-noble-metal Electrocatalysts catalyst

current (mA)

η (mV)

ref

Ni2P/Ni12P5 Ni12P5 Ni2P NiCoP Ni5P4 Ni2P NiP0.62S0.38 NiCoP Ni3Se2

10 10 10 50 10 10 10 10 10

278 311 336 308 330 400 240 280 310

this work this work this work 85 86 87 88 89 90

potential increases further. Figure 6b illustrates the required OER overpotential to achieve a current density of 10 mA cm−2 for the prepared electrocatalysts. Tafel slope is an important parameter to determine the ratedetermining step, which is obtained by fitting the plot to a Tafel equation (η = b log j + a), where j represents the current density and b is the Tafel slope. Figure 6c shows the Tafel plot for OER of Ni−P electrodes. A minimum Tafel slope of 112 mV dec−1 is obtained for Ni2P/Ni12P5, which implies that the rate-determining step in the electrochemical system is a singleelectron transfer step. In general, the Ni-based electrocatalysts J


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Figure 7. (a) LSV curves of Ni−P and bare carbon cloth for HER, (b) necessary overpotential to achieve a current density of 10 mA cm−2 for different electrocatalysts, (c) corresponding HER Tafel plots, and (d) chronoamperometry of NiP.

are anticipated to follow the most approving OER mechanism accompanied by Ni(II) → Ni(III) → Ni(IV) transformation that takes place through the formation of oxy-hydroxide intermediate β-NiOOH as follows,76 MIIIOOH + OH− → MIVO(OH)2 + e−

(3)

MIVO(OH)2 + OH− → MIVO(OH)2 ]+ + e−

(4)

Table 3. Comparison of HER Performance in 1.0 M KOH with Other Non-noble-metal Electrocatalysts

[MIVO(OH)2 ]+ + 2OH− → [MIIIO]+ + O2 + 2H 2O + 2e−

[MIIIO]+ +OH− → MIIIOOH

(5)

catalyst

current (mA)

η (mV)

ref

Ni2P/Ni12P5 Ni12P5 Ni2P MoS2+x/FTO NiSe Ni3S2 nanorods/NF Ni3S2 nanorods/AT-NF

10 10 10 10 10 10 10

234 248 258 310 270 300 200

this work this work this work 91 92 93 93

discharge reaction (b ≈ 120 mV/dec):

(6)

M + H 2O + e− → MHads + OH−(Volmer)

The chronoamperometry (CA) in Figure 6d was run at a constant potential of 1.56 V (vs RHE) for 25 h. The continuous evolution of gas bubbles from the electrode surface stood up to the completion of CA scrutiny, as only negligible changes were noted throughout the cycle. The negligible changes in the behavior ensure the highly stable nature of the Ni2P/Ni12P5 electrode for OER activity. To evaluate the HER catalytic activity of the Ni−P electrodes, the LSV curves on the negative potential ranging from −0.1 to −0.45 V (vs RHE) were obtained as shown in Figure 7a.77 The onset potential for HER of Ni2P/Ni12P5 was seen emerging at −0.17 V (vs RHE) at a scan rate of 5 mV s−1 from where the gas bubbles start evolving and an overpotential of 234 mV was required to drive 10 mA cm−2, which is relatively low compared to other reported Ni-based HER electrodes listed in Table 3. Figure 7b illustrates the required HER overpotential to achieve 10 mA cm−2 for the prepared electrocatalysts. The Tafel plot for hydrogen evolution reaction of Ni−P electrodes is presented in Figure 7c. Generally, for the HER process, the possible reaction steps under alkaline conditions follow the Volmer−Heyrosky−Tafel mechanism.78,79

(7)

electrochemical desorption reaction (b ≈ 40 mV/dec): MHads + H 2O + e− → H 2 + M + OH−

(Heyrovsky) (8)

and, moreover, recombination reaction (b ≈ 30 mV/dec): 2MHads → H 2 + 2M

(Tafel)

(9) −1

In the case of Ni2P/Ni12P5, a Tafel slope of 98 mV dec is obtained, which validates the Volmer−Heyrosky mechanism. The CA analysis shown in Figure 7d was studied for a constant potential of −0.28 V (vs RHE) for 25 h, which illustrated a slight rise in the current density rather than a drop elucidating the extraordinary strength of the Ni2P/Ni12P5 electrode. The evolution of gas bubbles was seen from the surface of the electrode throughout the CA analysis, which conveys the excellent electrocatalytic property of Ni2P/Ni12P5 electrode. The valence state of the Ni element after electrocatalysis was revealed in the Ni 2p spectrum of Ni element in Ni2P/Ni12P5 (Figure S13 (Supporting Information)). The observed two significant peaks at 875.26 and 857.12 eV correspond to the K


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h kg−1 at an improved power density of 2856 W kg−1 even after 10000 cycles, which stands higher compared to formerly reported results. Given the water splitting approach, the Ni−P electrodes with improved catalytic activity and resilient durability requested low overpotentials of 278 and 234 mV to accomplish a current density of 10 mA cm−2 for OER and HER activities, respectively. We firmly believe that ample upgraded outcomes based on these Ni−P electrodes could be thinkable by engineering the morphological fine-tuning and other essential structures to enterprise many flexible electrode materials for a diversity of practical applications.

spin−orbital splitting of Ni 2p1/2 and Ni 2p3/2, respectively. The perceived binding energies substantiate the presence of surface Ni at the +2 oxidation state as it is observed before the electrocatalytic studies. Hence, the valence state of the nickel in Ni2P/Ni12P5 remains the same, substantiating the stable nature of the prepared Ni2P/Ni12P5 electrode. Similarly, the FESEM image of Ni2P/Ni12P5 electrode after CA specifies that the particles appear to get flocked with one another, which is quite natural on subjection to a durability test for a long period of time (Figure S14 (Supporting Information)). Other than that the electrode has retained its morphology without any major changes even after the continuous evolution reaction for 25 h. Overall, it is evident that Ni−P crystal structures have an efficient electrochemical behavior. The most fundamental prerequisite of an electrode material in the case of the supercapacitor is the phase stability since the crystalline phase of a material has a dominant influence over the electrochemical property. Here, the Ni2P phase has better phase stability over the other phases of the Ni−P material,2,80,81 which enables it to retain its initial capacity even after 10000 cycles. Also, the more enhanced catalytic behavior of Ni2P/Ni12P5 than those of the other phases can be attributed to the existence of the (i) ligand and (ii) ensemble effects that would often occur together and contribute to the changes in the nature and composition of the surface adsorbed intermediate species. The former is the effect caused by the electronic interaction between the active metal and the partner element, which would influence the nature of the adsorption bond.82 From XPS analysis, it is observed that the peaks obtained at 852.21, 852.90, and 852.35 eV in the Ni 2p spectrum of Ni2P, Ni2P/Ni12P5, and Ni12P5, respectively, are quite close to the elemental zero valence state of nickel, which conveys the small positive charge (Niδ+) of Ni species in the prepared samples.45 The δ value of the obtained three crystal phases takes the order of δ(Ni2P/Ni12P5) > δ(Ni12P5) > δ(Ni2P). This small positive charge of Ni species in Ni2P/ Ni12P5 compared to the other two phases favors the weak ligand effect that allows a high activity for the dissociation of molecular oxygen and hydrogen from the electrocatalytic active sites during the evolution.83 The rich ensemble effect of sufficient P species on the surface of the prepared Ni2P/Ni12P5 safeguards the number of active Ni sites on the surface by defending it from surface poisoning, i.e., the deactivation induced by the coverage of strongly bound adsorbed intermediates.83,84 Thus, it is observed that the Ni2P phase performs well for supercapattery application, whereas Ni2P/ Ni12P5 was good for water splitting applications. For supercapattery, the electrode should inhibit the hydrogen/oxygen evolution reactions (HER/OER) as much as possible, aiming to achieve a wider cell voltage of the device, which is significantly achieved by the Ni2P phase. In contrast, the evolution reactions are highly desirable to minimize the HER/OER onset overpotentials to reduce energy input, which is satisfied by the Ni2P/Ni12P5 phase among the prepared Ni−P phases.

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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/acsaem.7b00006. Additional crystallographic data, XRD patterns, SEM images, EDAX, XPS and Raman spectra, and additional electrochemical profiles and formulas (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.K.S.). *E-mail: [email protected] (S. Shanmugam). ORCID

Sangaraju Shanmugam: 0000-0001-6295-2718 Ramakrishnan Kalai Selvan: 0000-0002-0989-2805 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S. Surendran thanks DGIST for providing the “Summer Internship” program to carry out part of the research work at the Advanced Energy Materials Laboratory, Department of Energy Science Engineering, DGIST, South Korea.

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CONCLUSIONS In conclusion, a simple low-temperature one-pot synthesis route was developed to prepare three well-defined highly crystalline Ni−P structures. The spongy nature of the prepared Ni−P has been credited as a multifunctional electrode comprising grander specific capacity and salient electrocatalytic activity. The laboratory scale flexible supercapacitor gadget was fabricated, resembling the commercial device, with Ni2P and BDAC electrodes that delivered a high energy density of 42 W L


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