Research Article pubs.acs.org/acscatalysis
Uncovering the Nature of Active Species of Nickel Phosphide Catalysts in High-Performance Electrochemical Overall Water Splitting Prashanth W. Menezes,† Arindam Indra,† Chittaranjan Das,‡ Carsten Walter,† Caren Göbel,† Vitaly Gutkin,§ Dieter Schmeiβer,‡ and Matthias Driess*,† †
Department of Chemistry: Metalorganics and Inorganic Materials, Technische Universität Berlin, Straße des 17 Juni 135, Sekr. C2, 10623 Berlin, Germany ‡ Applied physics and sensors, Brandenburg University of Technology Cottbus, Konrad Wachsmann Allee 17, 03046 Cottbus, Germany § The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel S Supporting Information *
ABSTRACT: A systematic structural elucidation of the near-surface active species of the two remarkably active nickel phosphides Ni12P5 and Ni2P on the basis of extensive analytical, microscopic, and spectroscopic investigations is reported. The latter can serve as complementary efficient electrocatalysts in the hydrogen (HER) versus oxygen evolution reaction (OER) in alkaline media. In the OER Ni12P5 shows enhanced performance over Ni2P due to the higher concentration of nickel in this phase, which enables the formation of an amorphous NiOOH/Ni(OH)2 shell on a modified multiphase with a disordered phosphide/phosphite core. The situation is completely reversed in the HER, where Ni2P displayed a significant improvement in electrocatalytic activity over Ni12P5 owing to a larger concentration of phosphide/phosphate species in the shell. Moreover, the efficiently combined use of the two nickel phosphide phases deposited on nickel foam in overall electrocatalytic water splitting is demonstrated by a strikingly low cell voltage and high stability with pronounced current density, and these catalysts could be an apt choice for applications in commercial alkaline water electrolysis. KEYWORDS: overall water splitting, electrocatalysis, nickel phosphide, structural rearrangement, surface structure
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INTRODUCTION
Oxide-based materials of manganese, cobalt, and nickel have been extensively studied for bioinspired water oxidation while TM sulfides, nitrides, and phosphides represent major classes of compounds in efficient electrocatalytic proton reduction (HER).14−19 Continuous interest in improving the catalytic activity has led to the exploration of new materials for the OER, HER, and eventually overall water splitting. 20 As the functionality of an electrocatalyst largely depends on the surface properties, the evolution of the active surface species from the precatalyst plays a key role in water-splitting reactions. Structural studies of the surface species of cobalt and nickel oxides revealed that structural rearrangements on the surface of the electrocatalysts occur to form hydroxide/oxyhydroxide species which are the real active catalysts in electrochemical water oxidation.21−23 As the surfaces of sulfides, nitrides, and phosphides undergo similar transformations in alkaline media under electrochemical conditions, they could serve as efficient electrocatalysts for the OER as well. In fact, recent studies on
Design and application of transition-metal (TM)-based electrocatalysts for water splitting has become a center of attraction in the search for alternative, clean, and sustainable energy sources.1,2 In recent years, growing interest in the oxygen evolution reaction (OER) with TM catalysts has been discerned.3 However, the OER is kinetically difficult to control, involving higher energy intermediates and multiple protoncoupled electron transfer steps.4−6 In addition, TM-based precatalysts have also become an attractive choice for efficient, environmentally friendly, and low-cost catalysis of the hydrogen evolution reaction (HER). 7−9 So far, the benchmark precatalysts for the OER as well as HER are represented by precious-metal (Rh, Ir, Ru, and Pt)-based materials, but the high cost and low abundance greatly limits their large-scale applications.10−12 A dual-function precatalytic material which can be employed both as an efficient anode and as a cathode for the OER and HER and, moreover, is suitable for overall electrocatalytic water splitting is scarce and thus challenging.13 Therefore, presently, vast efforts have been devoted to the advancement of overall water-splitting systems, especially to replace precious metals by TM-based catalysts. © XXXX American Chemical Society
Received: September 17, 2016 Revised: November 12, 2016 Published: November 17, 2016 103
DOI: 10.1021/acscatal.6b02666 ACS Catal. 2017, 7, 103−109
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ACS Catalysis such Mn, Co, and Ni materials indicate that they can be highly efficient OER electrocatalysts.14,24 Recent studies on metal phosphides prompted us to explore the two remarkably active nickel phosphides Ni12P5 and Ni2P as both anode and cathode materials and bifunctional catalysts for the OER and HER as well as for overall electrocatalytic water splitting and, furthermore, to comprehend the influence of the structural modification and to uncover the real catalytically active species at the near-surface region. Lately, various morphologies of Ni2P have been studied for either the HER or OER; however, suitable overall water-splitting catalysts with them are uncommon.25−27 Hu et al. demonstrated the importance of nanoparticles and nanowires of Ni2P, whereas Liu et al. described the Ni2P skeletons covered with vertically aligned Ni5P4-NiP2 nanosheets.28,29 Sun et al. presented a porous Ulrich-like Ni2P superstructure supported on a nickel foam, and Shalom et al. reported the synthesis and application of Ni5P4 films.30,31 In addition, Qu et al. indicated that the NixPy catalysts with controllable phases could be of high significance and, further, an electrodeposited Ni−P film prepared by Sun et al. has also proven to be efficient for overall water splitting.32,33 Although the catalytic activity of nickel phosphides for the OER, HER, and overall water splitting has been rarely reported, diminutive attention has been given to the structure−activity relationships and the modification of the near-surface structure. Recently, we have dedicated ourselves to investigating the active structure of the catalysts during photochemical and electrochemical water splitting.21−23 We were particularly interested in the structural rearrangements on the surface (shell) as well as in the chemical nature of the core of the particles during both the OER and HER to deduce a structure− activity relationship toward a better understanding of the overall electrochemical water-splitting phenomena.
Figure 1. (a) SEM image of Ni12P5 showing the flakelike morphology, (b) TEM image of Ni12P5 with netlike structure. (c) HRTEM image of Ni12P5, corresponding FFTs, and the filtered images of FFTs.
environment around the Ni centers (Figure S6 in the Supporting Information). The L3-edge peak (∼853 eV) could be ascribed to the regions of nickel in the zero oxidation state, and the broad peak at ∼858 eV is also typical for nickel phosphide with a δ+ oxidation state.35 A splitting of the L3-edge peak is more prominent for Ni12P5, indicating different types of electronic environments around the nickel centers. Similarly, the broad L2-edge peak (∼871 eV) corresponds to the phosphide structures (δ−) and has been well documented in the literature.36 The surface structure of the precatalysts was further analyzed by X-ray photoelectron spectroscopy (XPS) (Figure S7 in the Supporting Information). Deconvolution of the Ni 2p3/2 edge confirmed the presence of Niδ+ at 852.8 eV for Ni12P5 and 853 eV for Ni2P. Correspondingly, the P 2p edge also validated the presence of Pδ− at ∼130 eV.34 In addition to this, the peaks at ∼856.6 and ∼133.7 eV in the Ni 2p3/2 and P 2p regions, respectively, could be ascribed to Ni2+ ions interacting probably with phosphate ions resulting from surface passivation in air.37 The Ni:P ratio determined from XPS is 2.41:1 for Ni12P5 and 1.98:1 for Ni2P. Therewith, the amount of nickel centers present on the surface was significantly higher for Ni12P5 than for Ni2P; for both precatalysts, the BET surface areas are similar (∼8 m2/g). The electrochemical OERs were carried out with electrophoretically deposited catalyst films on fluorinated tin oxide (FTO) glass anodes in 1 M KOH solution. Oxygen evolution was initiated at an overpotential of ∼200 mV for both precatalysts (Figure S8a in the Supporting Information), and a current density of 10 mA cm−2 was reached with an overpotential of 295 mV for Ni12P5 and 330 mV for Ni2P (Figure S8b). The superior performance of Ni12P5 could be explained by the presence of higher amounts of nickel centers on the surface of the particles of Ni12P5, as revealed by XPS analysis (Figure S7 in the Supporting Information). The prepeak at ∼1.4 V vs RHE defines the oxidation of Ni2+ to Ni3+ sites (Figure S8c). Interestingly, both catalysts produced higher catalytic activities for OER in comparison to the commercially available RuO2, IrO2, and Pt (Figure 2 top). In fact, the
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RESULTS AND DISCUSSION Ni12P5 and Ni2P were synthesized using a facile hydrothermal approach (Experimental Section). The phase purity of these materials was confirmed using powder X-ray diffraction (PXRD), and the obtained diffraction patterns are in good agreement with those reported for Ni12P5 (JCPDS 22-1190) and Ni2P (JCPDS 3-953) (Figure S1 in the Supporting Information). Ni12P5 crystallizes in the tetragonal system (I4/ m, No. 87) with terminated layers of Ni4P5, whereas Ni2P crystallizes in the hexagonal system (P321, No. 150) with Ni3Pterminated layers (Figure S2 in the Supporting Information). Scanning electron microscopic studies displayed a flakelike structure (Figure 1a and Figure S3 in the Supporting Information), while transmission electron microscopy (TEM) revealed a more netlike structure combining particles of different sizes for both catalysts (Figure 1b and Figure S4 in the Supporting Information). The compositions of nickel and phosphorus were additionally confirmed by energy dispersive X-ray (EDX) analysis (Figure S5 and Table S1 in the Supporting Information). A very thin layer of thickness 1−2 nm could be detected on the surface of the particles due to surface passivation, in air but the majority was purely crystalline (Figure 1c).34 Fast Fourier transform (FFT) patterns indicated three different faces ((400), (310), (101)) for Ni12P5 and the (111) face for Ni2P. The filtered images of FFT (Figure 1c) also assured the existence of the ordered structure. L2,3-edge X-ray absorption spectroscopic (XAS) analyses of Ni12P5 and Ni2P were performed to understand the electronic 104
DOI: 10.1021/acscatal.6b02666 ACS Catal. 2017, 7, 103−109
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ACS Catalysis
current density occurred over 16 h of reaction time (Figure S9 in the Supporting Information). As mentioned in the Introduction, our goal was not only to synthesize remarkably active catalysts (Table S4 in the Supporting Information) but also to understand the nearsurface phenomena, gathering insights into the nature of the active species formed during the electrochemical OER and HER and to establish a structure−activity relationship. Therefore, both catalysts were postcharacterized extensively after the electrochemical CA experiments to uncover the structural transformations. In the case of OER CA, the EDX studies (Figure S11 in the Supporting Information) exhibited the presence of both nickel and phosphorus, whereas TEM and HRTEM studies of the catalysts showed an increased thickness of an amorphous layer on the surface of the particles. For Ni12P5, the thickness of the amorphous layer was much more pronounced, ranging from ∼10 to 15 nm (Figure S12 in the Supporting Information), whereas a ∼8−10 nm amorphous shell was detected for Ni2P (Figure S13 in the Supporting Information). The cores of the particles of both Ni12P5 and Ni2P were crystalline with significant structural modifications, although no change in particle morphology could be observed. The FFT of the crystalline core suggested that a partial oxidation of Ni12P5 and Ni2P occurred which probably transforms some of the cores of the phosphide particles to phosphite, Ni(HPO3)(H2O), under electrochemical conditions. In addition, the filtered FFT images revealed the presence of a multiphase weakly disordered structure (Figures S12 and S13). TEM studies were further supported by Ni L2,3-edge XAS analysis, where the peak at ∼858 eV, corresponding to Ni phosphide, started to disappear after OER CA (Figure 3). A
Figure 2. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) of Ni12P5 and Ni2P for the OER (top) and HER (bottom) in 1 M KOH solution with a scan rate of 20 mV/s on FTO substrates (loading ∼1 mg).
overpotential obtained for Ni12P5 is significantly lower than that recently reported for nickel phosphide and other non-noblemetal-based materials (see Table S2 in the Supporting Information).12,13,25,26,28−33 In additoin, Tafel slopes for both Ni12P5 (∼106 mV/dec) and Ni2P (∼112 mV/dec) are similar in the lower current density region, whereas the linearity in slopes is also maintained in the higher current density (>50 mA/cm2) region, indicating fast electron and mass transport between the catalysts and electrolyte solution during water oxidation. Apart from the higher catalytic activity, exceptional stability of the catalyst films was also established, as shown by chronoamperometric (CA) studies at a constant potential of 1.52 V for Ni12P5 and 1.56 V for Ni2P (maintained at 10 mA cm−2). For Ni2P ∼99% of the initial current density was retained even after 16 h of CA experiments, whereas a continuous increase in current density was observed for Ni12P5 (Figure S9 in the Supporting Information). Under similar alkaline conditions, HER was also performed on Ni12P5 and Ni2P cathodes (Figure 2, bottom). The situation is completely reversed for the HER in comparison to the OER, where Ni2P displayed significant improvement in catalytic activity with an overpotential of merely −176 mV in comparison to Ni12P5 (−270 mV) at −10 mA cm−2 (Figure S10 in the Supporting Information). The overpotential achieved for Ni2P was also significantly lower than those for recently reported nickel phosphides and non-noble-metal-based catalysts (see Table S3 in the Supporting Information).2,9,12,16,27,33,37−41 The excellent stabilities of the catalysts were also demonstrated by CA, where only a ∼ 0.5% drop in
Figure 3. Ni L2,3-edge X-ray absorption spectra of Ni12P5 and Ni2P as prepared and after chronoamperometric electrochemical OER measurements, displaying structural rearrangement.
new peak at ∼855 eV grew in, indicating the formation of Ni(OH)2.42 The splitting of the Ni L2-edge peak was perceived in both catalysts. Two well-resolved peaks at 870.3 and 871.6 eV could be attributed to Ni2+.43,44 A complete understanding of the near-surface structure and the nature of active species was deduced from the XPS analyses (Figure S14 in the Supporting Information). Deconvolution of Ni 2p3/2 peaks showed a substantial decrease in the amount of Niδ+, whereas NiOOH and Ni(OH)2 were the major contributors at the nearsurface along with a slight amount of NiO (Table S5 in the Supporting Information).45 In addition, the results also showed 105
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ACS Catalysis that the surface Ni/P ratios of the films were also decreased from 2.41 to 2.2 for Ni12P5 and from 1.98 to 1.8 for Ni2P. In the P 2p XPS, the Pδ− of phosphides (Figure S15 in the Supporting Information) disappeared by forming a broad peak at ∼134.3 eV that could directly be attributed to the formation of phosphite (PO33−) on the surface (also confirmed by HRTEM).46 The O 1s spectra were deconvoluted only to hydroxide and oxohydroxide species (Figure S15), confirming that only NiOOH/Ni(OH)2 species are present on the surface of the electrodes.45 In the case of HER CA, different structural rearrangements could be observed. TEM and HRTEM analysis revealed that there was nearly no amorphous shell formation on the surface of the precatalysts, apart from surface passivation (Figures S16−S18 in the Supporting Information). The FFT of the crystalline phosphides showed a modified feature, as seen by the used catalysts after OER. Both Ni12P5 and Ni2P were partially oxidized to nickel phosphates and the hydroxide hydrate phase (Figures S16−S18). This observation suggests that the oxidation of the phosphides occurs mainly by placing the electrodes in highly alkaline electrolyte for a prolonged period rather than just from the electrochemical process. XAS analyses also produced spectra comparable to those of the OER catalysts (Figure S19 in the Supporting Information). The peak at ∼858 eV of phosphides started disappearing after HER CA, suggesting a partial phase transformation. Although the first peak of the Ni L3-edge corresponds to the as-prepared materials, a new peak could be observed at ∼855.4 eV, proving partial oxidation. The Ni L2-edges at 870.2 and 871.4 eV indicate the oxidation and formation of phosphate species.43,44 The surface of the electrodes was further analyzed by XPS (Figures S20 and S21 in the Supporting Information), where the Ni 2p3/2 spectra of Ni12P5 and Ni2P exhibited two major peaks. The peak at ∼855.5 eV could be assigned to Ni2+ (of nickel(II) phosphate), whereas the peak at ∼856.7 eV clearly belongs to Ni(OH)2.42 This type of phenomenon has been already observed for Ni2P/K-USY (K-promoted Y-type zeolite), where Ni species are present as Ni(OH)2/Ni2+ in the passivated samples.47 The P 2p peak at ∼133 eV confirms the formation of phosphate (PO43−) on the surface, which is in good agreement with the phosphate materials reported in the literature.48,49 The O 1s spectra were deconvoluted into two broad peaks with largely dominating hydroxide and surface water.50 From HRTEM, XAS, and XPS, it was clear that, after OER CA, a thick amorphous shell is formed depending on the amount of nickel available on the surface. Ni12P5 contains higher concentrations of Ni which can be oxidized to give catalytically more active species, i.e. NiIIIOOH and NiII(OH)2, providing acceleration in activity for the OER. On the other hand, Ni2P shows a slight decrease in activity in comparison to Ni12P5 mainly because of the lower amount of Ni species at the surface for the OER. However, the cores of phosphide particles are also prone to slight oxidation, forming a multiphase disordered phosphide/phosphite. Nonetheless, the situation was completely reversed in HER CA. Ni2P displayed a significant improvement in catalytic activity over Ni12P5 due to the increasing atomic percentage of phosphorus on the surface which, in turn, creates a negative charge to trap protons and influences the desorption of H2. In addition to this, the phosphide materials were slightly oxidized and transform themselves to nickel phosphates in highly alkaline media. The formation of hydroxide at the surface was also unavoidable.
Therefore, apart from the structural rearrangements, the principal difference of the active catalysts after OER and HER is determined by the concentrations of Ni and P on the surface. With a higher concentration of Ni species on the surface, thicker and larger amorphous shell formation of NiOOH/Ni(OH)2 was observed, leading to a better activity of Ni12P5 in comparison to Ni2P in the OER. In contrast, higher amounts of P at the surface are beneficial in influencing the catalytic activity by directly participating in the HER and decreasing the number of exposed active sites of nickel.27 In addition, recently Li et al. have shown that an element with relatively high electronegativity favors the formation of H2, provided their crystalline metallic properties are retained.51 Therefore, the superior activity of HER is attributed to a smaller positive charge of Ni (
