Synthesis of Nickel Phosphide Electrocatalysts from Hybrid Metal

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Synthesis of Nickel Phosphide Electrocatalysts from Hybrid Metal Phosphonates Rui Zhang, Patricia Alexandra Russo, Michael Feist, Patrick Amsalem, Norbert Koch, and Nicola Pinna ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01178 • Publication Date (Web): 30 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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ACS Applied Materials & Interfaces

Synthesis of Nickel Phosphide Electrocatalysts from Hybrid Metal Phosphonates Rui Zhang,1 Patrícia A. Russo,*1 Michael Feist,1 Patrick Amsalem,2 Norbert Koch,2 and Nicola Pinna*1 1

Institut für Chemie and IRIS Adlershof, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany 2

Institut für Physik and IRIS Adlershof, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 6, 12489 Berlin, Germany

KEYWORDS: hydrogen evolution reaction, oxygen evolution reaction, transition-metal phosphide, nickel phosphide, cobalt phosphide, transition-metal phosphonate ABSTRACT: Transition-metal phosphides (TMPs) have recently emerged as efficient and inexpensive electrocatalysts for electrochemical water splitting. The synthesis of nanostructured phosphides often involves highly reactive and hazardous phosphorous-containing compounds. Herein, we report the synthesis of nickel phosphides through thermal treatment under H2(5%)/Ar of layered nickel phenyl- (NiPh) or methylphosphonates (NiMe) that act as single-source precursors. Ni12P5, Ni12P5-Ni2P and Ni2P nanoparticles with sizes of ca. 15-45 nm coated with a thin shell of carbonaceous material were produced. Thermogravimetric analysis coupled with mass spectrometry (TG-MS) showed that H2, H2O, P2 and –C6H5 are the main compounds formed during the transformation of the precursor under argon, while no hazard phosphorous-containing compounds are created, making this a simple and relatively safe route for fabricating nanostructured TMPs. The H2 most likely reacts with the –PO3 groups of the precursor to form H2O and P2, and the latter subsequently reacts with the metal to produce the phosphide. Ni12P5-Ni2P and Ni2P NPs efficiently catalyze the hydrogen evolution reaction (HER), with Ni2P showing the best performance and generating a current density of 10 mA cm-2 at an overpotential of 87 mV and exhibiting long-term stability. Co2P and CoP NPs were also synthesized following this method. This approach may be utilized to explore the rich metal-phosphonate chemistry for fabricating phosphide-based materials for electrochemical energy conversion and storage applications.

INTRODUCTION Water electrolysis is a suitable method for large scale production of H2, a chemical considered a promising environmentally-friendly fuel for future energy supply.1 The widespread application of electrocatalytic water splitting to produce H2 is however conditioned by the development of active, stable and low-cost catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), able to compete with the highly active but expensive noble-metal based catalysts (e.g. Pt, Ir).2 In 2013, Popczun et al. 3 showed that Ni2P is a very active HER catalyst in acidic medium and, more recently, the ability of this material to efficiently catalyze the OER in alkaline medium was demonstrated by Stern et al..4 These promising results have prompted intensive research on the application of transition-metal (Ni, Co, Fe, Mo, Cu) phosphides (TMP) as HER and OER catalysts and on other electrochemical applications, such as Li-ion batteries. 5-19 Ni phosphides, which can have a broad range of compositions (ranging from Ni3P to NiP3), are among the most active TMP electrocatalysts and have thus attracted much interest.5-7, 14, 19-23

Several approaches have been described in the literature for the synthesis of phosphides, including the direct reaction of the elements, solid-state metathesis or chemical vapor deposition. 15,24-26 A simple and therefore attractive synthetic procedure is the reduction of metal phosphates to the corresponding phosphides by H2 gas at elevated temperatures.27-30 Bulk phosphides are often produced through this and similar methods involving high reaction temperatures (>350 °C) due to sintering. TMP nanoparticles and nanorods may be prepared via solution-phase routes involving the decomposition of metal and phosphorous sources (e.g. organic phosphines) in organic solvents and in the presence of surfactants, at temperatures above 220 °C.3,8,9,31-34 Single-source molecular precursors have been employed in combination with this method. 35-38 This approach enables the synthesis of monodispersed nanostructures with well-controlled morphology, although the high reactivity and toxicity of the phosphorous sources and/or of their decomposition products, and consequent experimental requirements, constitutes a drawback that makes the scale up of the process difficult.2b,11 Well-defined phosphide nanostructures can also be produced by phosphorization of pre-formed metal oxides with PH3, which may

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be generated in situ from the decomposition of hypophosphite.10,12,15,39-41 This route is very versatile, as the morphology of the metal oxides are retained on the final phosphides. Its disadvantage is that it employs extremely toxic PH3, and consequently post-synthesis treatments of the tail gas are necessary.41

with a heating ramp of 1 °C/min and maintained at the final temperature for 3 h under Ar/H2 flow. After cooling to ambient temperature the black products were collected and ground. The phosphonate precursors were also treated under pure Ar (99.999%, AirLiquide) flow and under static air (with the tube closed) for 3 h.

Herein, we report the synthesis of nickel phosphides by thermal treatment of lamellar nickel-organophosphonate hybrid materials that act as single-source precursors. Treatment of Ni phenylphosphonate or methylphosphonate between 450 °C and 700 °C under H2(5%)/Ar leads to the formation of Ni12P5-Ni2P and Ni2P nanoparticles with size ca. 15-45 nm that efficiently catalyze the HER in acidic medium. The transformation proceeds without formation of significant amounts of hazardous phosphorous compounds. Furthermore, the primary phosphorous sources used in this approach, i.e. phosphonic acids, are relatively safe phosphorous compounds, commercially available or readily synthetized and inexpensive, making the overall synthetic process fairly benign. The approach was extended to fabricate cobalt phosphides, and likely can be extended to other metal phosphides, as phosphonic acids react with a wide range of mono to hexavalent metals to form metal-organophosphonate materials.42,43

Characterization. X-ray powder diffraction patterns were recorded with a STOE MP diffractometer in transmission configuration using Mo Kα radiation (λ=0.07093 nm). TEM images were acquired on a Philips CM 200 microscope at 200 kV. Fourier transformed infrared spectra of the samples were measured on a Thermo Scientific Nicolet iS50 spectrometer in the range of 4000- 400 cm-1 (4 cm-1 resolution), using pellets of the solid diluted in KBr. Carbon elemental analyses were performed on a HEKAtech Euro EA CHNSO Elemental analyzer. The X-ray photoelectron spectroscopy (XPS) measurements were performed in an analysis chamber (base pressure: 3.10-10 mbar) with an overall energy resolution of ca. 1.2 eV, using a SPECS Phoibos 100 hemispherical energy analyzer and the Mg Kα radiation (1253.6 eV) generated from a twin anode X-ray source. The thermal behaviour was studied by simultaneously coupled TA-MS measurements. A NETZSCH thermoanalyzer STA 409 C Skimmer system, equipped with a BALZERS QMG 421, was used to record the thermoanalytical curves (T, DTA, TG, DTG) together with the ionic current (IC) curves in the multiple ion detection (MID) mode.44 A DTA-TG sample carrier system with platinum crucibles (beaker, 0.8 ml) and Pt/PtRh10 thermocouples was used. A sample of 20.17 mg was measured versus empty reference crucible. A constant purge gas flow of 70 ml/min Argon 5.0 (AirLiquide) and a constant heating rate of 10 K/min were applied. The raw data have been evaluated utilizing the manufacturer’s software PROTEUS (v. 4.3) and QUADSTAR 422 (v. 6.02) without further data treatment, e.g. such as smoothing. The base line shift of the IC signals at higher temperatures (e.g. m/z 18 in Fig. 3) is characteristic of the Skimmer system where the gas sampling system and the sample holder have the same temperatures, thus avoiding condensation phenomena. Consequently, the MS signal becomes temperature-dependent, and increasing baseline IC intensities at higher temperatures do not indicate generation of gas from the sample, except if intensity maxima sitting on the increasing base line are clearly expressed.44c

EXPERIMENTAL SECTION Materials. All reagents were used as received. Nickel (II) acetylacetonate (95 %), cobalt (II) acetylacetonate (99 %), phenylphosphonic acid (98 %), methylphosphonic acid (98 %) benzyl alcohol (99 %), sulfuric acid (95-98 %) potassium hydroxide (>85 %), Nafion perfluorinated resin solution (5 wt.%) were purchased from Sigma Aldrich. Purified water with resistivity 18.2 MΩ cm-1 was used for electrochemical measurements. Synthesis of the transition-metal phosphonates. The metal phosphonates were prepared via a microwave-assistant method using benzyl alcohol as solvent. In a typical synthesis of nickel(II) phenylphosphonate: 0.5 mmol of nickel acetylacetonate, 0.5 mmol of phenylphosphonic acid and 15 mL of benzyl alcohol were mixed in a 30 mL borosilicate glass vial and sealed with a silicon cap. The mixture was stirred for 30 minutes, and then sonicated until a homogenous solution was obtained. The solution was heated in a microwave reactor (Anton Paar Monowave 300) at 180 °C for 30 min with a 5 min heating ramp and subsequently quenched with compressed air. The product was collected by centrifugation, washed with ethanol, and dried at 65 °C overnight. Synthesis of the transition-metal phosphides. The transition-metal (Ni, Co) phosphides were prepared via a simple heating process using the corresponding metal phosphonate as precursor. In a typical synthesis, 60 mg of metal phosphonate was placed in a combustion boat in a tubular oven (Nabertherm) and purged with 5% H2 in Ar (99.999%, AirLiquide,) for 30 min. Subsequently the sample was heated to a temperature between 350 and 700 °C

Electrochemical measurements. All electrochemical measurements were performed with a Bio-Logic VMP3 Potentiostat-Galvanostat having a built-in electrochemical impedance spectroscopy analyzer. The electrochemical activity and stability of catalysts were measured in a threeelectrode electrochemical cell using a 3 mm diameter glassy carbon rotating disc electrode (RDE). A platinum wire was used as the counter electrode and an Ag/AgCl (3 M KCl) was used as reference electrode. The catalysts inks were prepared as follow, 5 mg of the sample catalyst was added to 300 μL of H2O, 290 μL of ethanol and 10 μL of 5 wt.% Nafion solution, and then the mixture was dispersed by sonication for 30 min to obtain a homogeneous ink. The

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electrodes were prepared by depositing 3 µL of catalyst ink onto the GC disc and drying at room temperature to achieve a catalyst loading of 0.42 mg cm2. Prior to catalyst deposition, the GC electrode was polished with 1 µm alumina slurry followed by 0.05 µm alumina slurry, and then rinsed with deionized water, sonicated for 3 min and dried in air. For comparison, a commercial 20 wt. % Pt/C (Johnson Matthey) was deposited on the glassy carbon with the same loading. The hydrogen evolution reaction measurements were conducted in 0.5 M H2SO4 at 25 °C with the RDE rotated at 2000 rpm. Cyclic voltammetry (CV) experiments were performed in the potential window -0.6 V to 0 V vs. Ag/AgCl with a scan rate of 20 mV s-1. Linear sweep voltammetry (LSV) curves were measured in the same potential window with a scan rate of 10 mV s-1. The stability of the catalysts was evaluated by performing 1000 CV sweeps between -0.4 and 0 V vs. Ag/AgCl with a scan rate of 100 mV s-1. The long term stability was evaluated through chronopotentiometry experiments in which the potential was recorded during 11 h with the current density maintained at 10 mA cm-2. The oxygen evolution reaction measurements were conducted in 1 M KOH electrolyte at 25 °C, with the RDE rotated at 1600 rpm. CVs were measured in the potential window 0 to 0.7 V vs. Ag/AgCl at a scan rate of 20 mV s-1. LSV was measured in the same potential window after CV measurements at a scan rate of 10 mV s-1. All data was corrected for the ohmic drop. The uncompensated resistance was determined by electrochemical impedance spectroscopy (EIS). The measured potentials of both HER and OER were converted with respect to reversible hydrogen electrode (RHE). RESULTS AND DISCUSSION Synthesis and Characterization of the Phosphide Materials. Nickel phenylphosphonate (NiPh) and nickel methylphosphonate (NiMe) were prepared by reacting Ni(II) acetylacetonate with phenylphosphonic acid or methylphosphonic acid in benzyl alcohol at 180 °C under microwave irradiation. The phosphonate hybrid was subsequently submitted to thermal treatment at temperatures ranging from 350 to 700 °C under H2(5%)/Ar. Metal phosphonate materials are extended structures formed by metal cations coordinated to the oxygens of bridging or cross-linking organophosphonate ligands.42,43 The powder XRD patterns of the NiPh (Figure 1a) and

NiMe phosphonates (Figure S1) shows an intense reflec

Figure 1. Powder X-ray diffraction patterns of (a) nickel phenylphosphonate, and (b) Ni12P5-Ph, (c) Ni12P5-Ni2P-Ph and (d) Ni2P-Ph synthesized by thermal treatment of nickel phenylphosphonate under H2(5%)/Ar at 450 °C, 500 °C and 550 °C, respectively, measured using Mo radiation (λ=0.7093 Å). Vertical bars represent reference patterns: green-Ni12P5 (JCPDS card no. 022-1190); red-Ni2P (JCPDS card no. 074-1385). tion at low angles, followed by lower intensity reflections corresponding to interplanar distances (d) that are 1/2, 1/3, and 1/4 of that of the most intense reflection. These diffraction patterns are typical of lamellar structures. In the case of nickel phenylphosphonate the reflections at 2.80, 5.62, 8.48 and 11.19 ° 2θ correspond to diffraction by the (010), (020), (030) and (040) planes, respectively. For nickel methylphosphonate the same reflections occur at 3.97. 7.90, 11.5 and 17.02 ° 2θ, respectively. The metal is coordinated by five oxygens from the phosphonate ligands and one oxygen from a water molecule, with the inorganic layers composed by CPO3 tetrahedra corner- and edge-shared with four NiO6 octahedra and the organic groups located in between the inorganic layers, interacting through van der Waals and dispersion forces.45,46 The interlayer distance, related to the size of the organic and given by the d spacing of the most intense reflection (010), is 1.45 nm and 1.0 nm for NiPh and NiMe, respectively. For NiMe the interlayer distance is slightly larger than previously reported

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(0.87 nm).46 The carbon contents of NiPh and NiMe are 35.8 and 9.4 wt.%, respective-

Figure 2. TEM images of (a) nickel phenylphosphonate, and (b) Ni12P5-Ph, (c,d) Ni12P5-Ni2P-Ph and (e,f) Ni2P-Ph synthesized by thermal treatment of nickel phenylphosphonate under H2(5%)/Ar at 450 °C, 500 °C and 550 °C, respectively. Insets in b, c, and e show the SAED patterns (white-Ni2P; yellow-Ni12P5). Arrows indicate the carbonaceous shell around the nanoparticles. ly, which are also higher than the values expected from the formulas NiO3PC6H5.H2O (30.9 wt. %) and NiO3PCH3.H2O (7.0 wt. %), proposed for these materials.45,46 These differences suggest that some benzyl alcohol molecules are trapped in the interlamellar space, which has a higher impact on the interlayer distance of NiMe. The presence of a small amount of benzyl alcohol molecules on the phosphonates was confirmed by TG-MS analysis. NiPh is made of rectangular particles (Figure 2a) and NiMe consists of smaller irregular sized particles (Figure S2). The layered structure of NiPh is seen on the TEM images (Figures 2a and S3), with the darker regions corresponding to the inorganic layer and the lighter to the organic layer. The interlayer distance measured from TEM matches well the one calculated from the XRD data. It should be noted that owing to the instability of the materials under the electron beam it is difficult to obtain images at high magnifications. The XRD patterns and TEM images of the products obtained by thermal treatment of NiPh under H2(5%)/Ar gas at 450 °, 500 °C and 550 °C are shown in Figures 1 and 2,

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respectively. A crystalline Ni phosphide phase was not produced at temperatures below or equal to 400 °C. Ni12P5 is formed at 450 °C and a mixture of Ni12P5 and Ni2P phases is formed at 500 °C (Figure 1b,c). TEM reveals that in both cases the products consist of nanoparticles together with what seems to be the remains of the parental phosphonate that was not transformed, or carbonized organic (Figure 2b,c,d). The TEM image of Ni12P5-Ni2P at higher magnification shows that the nanoparticles are surrounded and coated by the carbonaceous material (Figure 2d). Pure phase Ni2P nanoparticles are produced at 550 °C and higher temperatures, with sizes ranging from ca. 15 to 45 nm and fairly uniform morphology, considering the high synthesis temperature (Figure 1d and Figure 2e,f). Further increase of the synthesis temperature to 700 °C did not cause significant changes on the final size of the Ni2P NPs (Figure S4). Selected area electron diffraction (SAED) measurements were performed to confirm the structure of Ni12P5 and Ni2P (insets in Figure 2 b, c, and e). The diffraction patterns show the (200) and (112) planes of the tetragonal Ni12P5 in Figure 2b, the (100), (111), (201), (210) and (300) of hexagonal Ni2P in Figure 2e, and a mixture of Ni12P5 and Ni2P in Figure 2c. A thin layer of carbon material covers the surface of the Ni2P nanoparticles (Figure 2f). The organic ligand likely plays a role on the size and morphology control of the phosphide particles during the synthesis. The small size of the NPs, i.e. the low sintering degree even at temperatures as high as 700 °C, may be partially attributed to the organic/carbon surrounding the particles during their formation and growth that hinder their coalescence. The nickel phosphide phase formed depends on the temperature. If a sufficient amount of phosphorous is present in the system, as in our system, Ni12P5 forms at lower temperatures and Ni2P at higher temperatures.34 An amorphous Ni-P intermediate material was produced at low temperatures, prior to the formation of

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Ni12P5. When nickel methylphosphonate is used as precursor, the results, including the phosphide phase evolution with increasing temperature, are identical to those described for NiPh (Figures S1 and S2).

Figure 3. FT-IR spectra of nickel phenylphosphonate (NiPh), and Ni12P5-Ph, Ni12P5-Ni2P-Ph and Ni2P-Ph synthesized by thermal treatment of NiPh under H2(5%)/Ar at 450 °C, 500 °C and 550 °C, respectively.

The products synthetized at 450 °C and 5oo °C from NiPh and NiMe have carbon contents of ca. 4-5 wt. %, and the materials (Ni2P) synthesized at 550 °C or temperatures above contain 1-2 wt. %. Therefore, most of the organic component of the hybrid precursor decomposes at temperatures below 450 °C. Fourier-Transformed Infrared (FT-IR) spectroscopy analysis of the samples was performed in order to determine the nature of the carbon-containing material that remains on the products after thermal treatment. Figure 3 shows the FT-IR spectra of the NiPh precursor and nickel phosphides synthesized at 450 °C, 500 °C and 550 °C. Similar results were obtained with NiMe. The spectrum of NiPh shows bands characteristic of the aromatic organic compound, P-O and P-C bonds: the bands between ca. 1300 cm-1 and 980 cm-1 are ascribed to the stretching vibration of P-O bonds; the band at 1438 cm-1 is attributed to P-C bonds; the bands in the region between 400 and 800 cm-1 are attributed to the aromatic ring, including out-of-plane C-H bending vibrations (750, 719 cm1) and out-of-plane ring C=C bending (696 cm-1). Bands associated with the C=C skeletal vibrations are found at ca. 1600 cm-1 and 1488 cm-1. Bands arising from the vibration of O-H groups from adsorbed water are observed at 30003600 cm-1.46 Ni-O vibrations occur in the region of the spectra below 400 cm-1. Bands typical of the organic phenyl group, P-C and P-O bonds are still visible on the spectrum of the phosphide synthetized at 450 °C, indicating that the product at this temperature still contains a small amount of unreacted phosphonate. The intensity of these bands significantly decreases or even completely disappears from the spectra of the phosphides synthetized at 500 and 550 °C. Thus, these samples possess pyrolyzed carbon, likely still containing O and P, that forms a thin shell around the nanoparticles.

The surface composition and valence oxidation state of the Ni phosphonate and Ni phosphide nanoparticles were characterized by X-ray photoelectron spectroscopy (XPS). Figure 4 displays the Ni 2p and P 2p XPS spectra of NiPh and Ni2P-Ph NPs. The XPS data confirms the presence of carbon on the surface of the Ni2P-Ph particles and the reduction of the Ni-P species upon thermal treatment of the phosphonate (Figures 4 and S5). In Figure S5 a, the C 1s in NiPh is observed at 288.2 eV binding energy (BE). We note that this core level, as well as the Ni and P core levels were observed to shift to higher BE when increasing film thickness. Therefore, the presently high binding energy measured for all the elements results from differential charging upon photoionization. To correct for this effect, all the core level spectra were then shifted rigidly by 4.2 eV so that the corrected C 1s level is aligned at 284 eV BE, as expected for C within phenyl ring. 47 In Figure 4, the corresponding energetically-corrected Ni 2p and P 2p spectra are presented. Changes in the Ni 2p and P 2p regions of the spectra of the phosphonate and phosphide are observed. Contributions at lower binding energies in the Ni 2p and P 2p regions of the Ni2P spectrum compared to the Ni phosphonate reflect the reduction of Ni-P species. Three contributions are found on the Ni 2p region of the Ni2P spectrum. The contribution at 853.2 eV BE results from Ni in Ni phosphide, which is very close to the binding energy of zero valence state Ni. The contribution at 855.9 eV BE indicates the existence of surface oxidized Ni species, while the contribution at 861.1 eV BE is a satellite peak. The P 2p region shows two doublets. The doublet at 129.8 eV BE, which is close to binding energy of elemental P, is attributed to phosphorous in the phosphide material. The doublet at 133.6 eV BE indicates the presence of oxidized P species on the surface of the Ni2P-Ph nanoparticles, likely phosphates that are formed at the surface by exposure to air.5,6

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Figure 4. Ni 2p and P 2p X-ray photoelectron spectra of (a,b) nickel phenylphosphonate and (c,d) Ni2P nanoparticles. The approach used for the synthesis of the Ni phosphides was extended to fabricate cobalt phosphides. Cobalt phosphides, Co2P and CoP, were prepared from the corresponding metal phenylphosphonate (Figures S6 and S7). Similarly to what was observed for the nickel phosphonates, the properties of the final product, such as the phosphide phase, are influenced by the thermal treatment conditions, especially the temperature. In addition to the treatment under H2(5%)/Ar flow, the nickel phenylphosphonate was heated at different temperatures under pure argon or under air, for 3 h. The exact composition of the product obtained by heating under Ar depends on the temperature but a nickel phosphide phase is always formed above 500 °C. At 550 °C and 600 °C Ni3P is synthesized (Figure S8) while at lower and higher temperatures mixtures of phosphides and other compounds such as phosphates or metallic Ni are produced. Mixtures of nickel oxides and phosphides were fabricated at temperatures between 400 and 500 °C under air. Figure S9 shows the XRD pattern of the material synthesized by heating NiPh at 500 °C for 3 h under static air, consisting of NiO, Ni2P and NiP2. Although at sufficiently high temperatures and reaction times only oxides and phosphates are formed, it is somewhat surprising that oxides and phosphates are not always the exclusive products of the reactions performed under oxidizing conditions. The phosphonate precursor is therefore reduced, at least partially, even in the absence of H2/Ar gas flow. Reaction Mechanism. Thermogravimetric analysis coupled with mass spectrometry (TG-MS) of the phosphonates was performed in order to determine the species generated during their conversion into phosphides and to get mechanistic insights. The TG curve of nickel phenylphosphonate shown in Figure 5 was measured under Ar flow. XRD characterization of the sample after the TG-MS analysis confirmed that nickel phosphide was produced during the measurement. The TG curve shows two pronounced mass losses occurring at temperatures near DTG /(%/min) Ion Current *10-10 /A [5] 1.00 1.4 0

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the IC curves are shifted and the DTA trace is omitted (two weakly expressed endothermal effects). 300 °C and 600 °C. MS data reveals that the main species generated during heating have m/z of 2, 18, 62 and 77, which are ascribed to the formation of H2, H2O, P2, and C6H5, respectively. Additional carbon-containing compounds were detected, e.g. methane, which were produced from further decomposition of the organic ligand. Benzyl alcohol was also released during heating. The formation of several phosphorous-containing species including PH3, organic phosphines and phosphine oxides was monitored, but only P2 was detected. Furthermore, the amount of P2 detected is small compared to that of H2, H2O and C6H5, suggesting that most of the sample phosphorous remains in the solid state during heating. The water released at ca. 250 °C corresponds to structural water, since none of the other species are observed simultaneously at this temperature. The formation of hydrogen likely results from the thermal decomposition of the organic component through a radical mechanism, since the pyrolysis of organic compounds frequently proceed through radical mechanisms,48 or from the transformation of organic species catalyzed by the metal centers.15 The production of hydrogen is not exclusive of the phenylphosphonate ligand, as it was also detected during TGMS analysis of nickel methylphosphonate, which exhibits a thermal behavior similar to that of nickel phenylphosphonate. The H2 generated likely reacts with the – PO3 groups to form P2 and H2O,29 which accounts for the water and P2 detected simultaneously with H2 formation. P2 subsequently reacts with the metal to produce the phosphide. The production of H2 in situ during the thermal decomposition of the phosphonate explains why phosphide phases can be produced under Ar or even under air: the phosphonate produces its own reduction agent. However, pure metal phosphide compounds cannot be synthesized under air owing to the presence of O2 in the system, which reacts with the precursor to produce oxides and phosphates, and because the H2 generated is only sufficient to partially reduce the precursor. The fact that no particularly toxic phosphorous-containing compounds are produced during the transformation of the inorganic-organic hybrid into the phosphide makes this a non-hazardous procedure. Electrocatalytic Performance. The electrocatalytic activity of the Ni phosphide materials synthesized from the

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nickel

ACS Applied Materials & Interfaces phenylphosphonate

precursor

under

Figure 6. (a) Polarization curves and (b)Tafel plots of Ni12P5-Ni2P-Ph, Ni2P-Ph, Ni2P-Me and Pt/C in 0.5 M H2SO4 (scan rate: 10 mV s-1; 2000 rpm); (c) polarization curves of the Ni2P-Me NPs measured before and after 1000 CV cycles (between -0.2 and 0.2 V vs. RHE; scan rate: 100 mV s-1) in 0.5 M H2SO4; (d) variation of the overpotential as a function of time for Ni2P-Me, measured at a constant cathodic current density of 10 mA cm-2. H2(5%)/Ar at 450 °C (Ni12P5-Ph), 500 °C (Ni12P5-Ni2P-Ph) and 550 °C (Ni2P-Ph) was evaluated on the HER in acidic electrolyte. For comparison the catalytic activity of the Ni2P nanoparticles synthesized using Ni methylphosphonate as precursor (Ni2P-Me) was also investigated. Figure 6a shows the HER polarization curves, measured in 0.5 M H2SO4 using a three-electrode set-up and a glassy carbon rotating disc electrode as working electrode, rotated at 2000 rpm. The corresponding Tafel plots, η vs. logj, are displayed in Figure 6b. The performance of a commercial Pt/C (20 wt.% Pt) was examined as a reference. Pt/C shows the best performance; its polarization curve and Tafel slope of 31 mV dec-1 agree with the data previously reported.2b,13 The nickel phosphonates are not active HER catalysts (Figure S10) and the performance of Ni12P5-Ph is poor (data not shown), although Ni12P5 NPs have been found before to be active HER electrocatalysts.5 On the contrary, Ni12P5Ni2P-Ph, Ni2P-Ph and Ni2P-Me are active HER catalysts in acidic medium. The Ni12P5-Ni2P-Ph, Ni2P-Ph and Ni2P-Me catalysts generate a current density of 10 mA cm-2 at overpotentials of 132 mV, 92 mV and 87 mV, respectively; overpotentials of 151, 110 and 111 are required to attain a current density of 20 mA cm-2. Ni2P is the most active catalyst and the Ni2P NPs synthesized from both NiPh and NiMe have identical activity, consistent with the similar characterization results of these samples. The different catalytic activ-

ity of the Ni-based catalysts can be explained in the following way. On the one hand, in Ni phosphides the metal has a small partial positive charge, while the phosphorous has a small partial negative charge.49 Therefore, the surface of the catalysts contains metal centers and basic phosphorous centers in their proximity. It has been considered that the metal centers function as hydride-acceptors and the phosphorous as proton-acceptor centers that promote HER.12 Different nickel phosphide phases exhibit different catalytic activity. Specifically, Ni12P5 was found to be less active than Ni2P.49 This has been attributed to the partial positive charge of Ni and atomic percentage of P in Ni2P being higher than on Ni12P5 that results in a lower intrinsic activity of the latter for the HER. Thus, the presence of the Ni12P5 nanoparticles on the N12P5-Ph and Ni12P5-Ni2P-Ph catalysts contributes to their lower activity compared to the Ni2P catalysts. On the other hand, the samples Ni12P5-Ph and Ni12P5-Ni2P-Ph contain precursor or partially carbonized precursor that was not converted into phosphide (Figure 2). This is due to the lower synthesis temperatures of these samples, which affects the amount of carbonaceous material as well as its degree of carbonization. Ni12P5-Ph still contains completely non carbonized phosphonate according to the FT-IR data (Figure 3). Since the metal phosphonate precursors do not catalyze the hydrogen evolution reaction (Figure S10) they have a detrimental effect on the activity of the phosphide catalysts. To demonstrate this detrimental effect, the activity of a physical mixture of Ni2P NPs and 15 wt. % Ni phosphonate was evaluated. The presence of the phosphonate led to a significant decrease of the HER catalytic performance of the Ni2P-Me catalyst (Figure S10). Thus, the presence of unreacted or partially carbonized phosphonate on the Ni12P5-Ph and Ni12P5-Ni2P-Ph samples also contributes to their poorer performance compared to the Ni2P catalysts. It is generally accepted that the HER is a two-electron process proceeding in two steps. The first step is an electrochemical hydrogen adsorption or discharge step (Volmer reaction) which is followed by an electrochemical desorption step (Heyrovský reaction) or by a recombination step (Tafel reaction). Tafel slopes of 116, 38 or 29 mV per decade respectively identify the Volmer, Heyrovský or Tafel reaction as the rate-determining step of the HER.2b Ni12P5-Ni2P-Ph, Ni2P-Ph and Ni2P-Me have Tafel slopes of 85, 59 and 64 mV per decade, respectively, which has been considered indicative that the HER proceeds via a VolmerHeyrovský mechanism. The smaller Tafel slopes of the Ni2P electrocatalysts confirm their higher HER activity and suggest faster proton discharge kinetics compared to that of Ni12P5-Ni2P-Ph.50 A broad range of Tafel slopes has been reported for Ni phosphides HER electrocatalysts (Table S1). The Tafel slopes of our phosphide catalysts are similar or smaller than those previously reported for Ni phosphide electrodes (Table S1), and consistent with active electrocatalyts. The performance of the Ni2P electrocatalysts synthesized in this work is superior or comparable to that of most

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nickel phosphide-based catalysts recently reported (Table S1). Owing to the synthetic method used here to fabricate the metal phosphide nanoparticles, the surface of the Ni2P NPs is in close contact with a thin layer of carbon. Several works have demonstrated that the combination of Ni2P NPs with carbon materials enhances the HER catalytic activity of the metal phosphide due in part to the increase of the conductivity of the catalyst that facilitates charge transfer.6,22,23 Pan et al.51 showed that the HER activity of Ni2P NPs supported on carbon spheres gradually improved with increasing carbon content until an optimum carbon content was reached. The performance improvement was attributed to the increase of the surface area and electrical conductivity of the catalyst. The beneficial effect of carbon is enhanced by a more intimate and extensive contact between phosphide and carbon material. For example, carbon-encapsulated Ni2P NPs were found to be considerable more active (η10=87 mV) than uncoated nanoparticles (η10 ~220 mV),22 and the catalytic activity improvement is larger than that observed for the dispersion of nanoparticles on a carbon support.6,22,51 Therefore, the intimate contact of the carbon layer surrounding the phosphide nanoparticles in the Ni2P-Ph and Ni2P-Me catalysts contributes to the high HER activity of these catalysts. These results demonstrate that the simple and non-hazardous synthesis approach reported here allows the fabrication of highly active nanostructured Ni phosphide HER catalysts. Stability is an important requirement for an electrocatalyst. Therefore, the stability of the Ni2P NPs (Ni2P-Me) was evaluated by potential cycling and chronoamperometry. The potential was swept between -0.20 and 0.20 V vs. RHE at a scan rate of 100 mV s-1. After 1000 continuous cycles the overpotential required to achieve a current density of 10 mA cm-2 increased to 97 mV, corresponding to an increase of only 10 mV (Figure 6c). The stability of the Ni2P electrode was further investigated by performing continuous electrolysis at a fixed current density of 10 mA cm-2 (Figure 6d). The electrode overpotential increases by less than 30 mV after 11 h of continuous operation. Some degradation of Ni phosphide electrodes activity with potential cycling or continuous electrolysis has been reported. 3,21 The loss of activity may be attributed to some dissolution of the Ni phosphide catalyst into the acidic electrolyte. For instance, Wang et al.52 reported that Ni2P nanorods became blunt and shorter after stability tests probably as a result of partial dissolution and/or aggregation. In addition, the authors reported that most probably structural changes occurred during the stability tests as a consequence of phosphorous loss, leading to the some conversion of Ni2P into less active Ni12P5. For evaluating possible structural and morphological changes after the stability tests, we performed TEM and electron diffraction analysis of the Ni2P catalyst after continuous electrolysis (Figure S11). No significant morphological changes are observed from the TEM images of the Ni2P-Me catalyst after the stability test. Moreover, the nanoparticles remain coated by the carbon film after electrocatalysis. The diffraction rings in the

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SAED pattern are indexed to Ni2P and no other Ni phosphide phase was detected. Therefore, although some loss of activity occurs during electrolysis, likely due to dissolution at the surface of the catalyst, the degradation is less extensive compared to some other similar catalysts. This is most probably due to the presence of the carbon layer surrounding the nanoparticles that partially hinders the transfer of Ni and P into the electrolyte. The HER catalytic activity in acidic electrolyte of the catalysts derived from cobalt phenylphosphonate was also evaluated; the polarization curves and Tafel plots are shown in Figures S12 and S13, respectively. The activity of the Co-based catalysts as a function of the synthesis temperature follows a trend similar to that of the Ni phosphide catalysts. Thus, the amorphous material produced at the lowest temperature acts as a very poor HER catalyst, as it consists of partially carbonized Co phosphonate without any metal phosphide phase. A mixture of Co2P and CoP NPs and pure phase Co2P NPs were synthesized at 550 °C and 600 °C, respectively. Both these materials are active HER catalysts. Similarly to what was observed for the Nibased electrocatalysts, the single phase Co2P prepared at higher temperature exhibits the best performance, with an overpotential of 144 mV at j=10 mA cm-2 and a tafel slope slightly lower than that of Ni2P (56 mV dec-1), indicative of fast electrode kinetics. The OER activity of the Ni phosphide catalysts in alkaline medium was also investigated. The LSV curves measured in 1 M KOH are shown in Figure S14. For comparison, the LSV of the Ni phenylphosphonate precursor is shown in Figure S15. All the curves show an oxidation peak prior to the OER onset that is generally observed for Ni-based electrodes in alkaline electrolyte and attributed to the oxidation of nickel.53 The electrochemical behavior of the Ni phosphide catalysts reported here is identical to that reported for other Ni-based electrodes.54,55 It has been shown that the active state of Ni –based electrodes consists of Ni oxyhydroxides formed in situ under OER conditions in basic medium.56,57 In the case of Ni phosphide nanoparticles, the surface of the phosphide particles is converted into a metal oxyhydroxide at the high anodic potentials.4,19 The actual catalysts are therefore core-shell nanoparticles comprising a Ni phosphide core and a hydroxide shell. All the materials tested exhibit good OER activity, including the hybrid Ni phenylphosphonate. Ni2P-Ph and Ni2P-Me have identical catalytic activity, which is consistent with the similar physical and chemical properties of these materials. Although the performances of all the different catalysts are close, the mixture Ni12P5-Ni2P5-Ph and the Ni12P5Ph material perform slightly better than the pure phase Ni2P catalysts. Ni12P5 has been found to be more active than Ni2P for the OER owing to the higher Ni atomic percentage of the former,58 and thus the presence of Ni12P5 NPs enhances the OER activity in comparison with that of the pure phase Ni2P catalysts. Additionally, the Ni phosphonate is also active, which means that the presence of unreacted or partially carbonized phosphonate on the Ni12P5-

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Ni2P5-Ph and Ni12P5-Ph catalysts has no detrimental effect on their OER activity, contrary to what happens for the HER. The LSV curve of the Ni2P electrode was additionally measured after 1000 potential cycles between 0.1 and 0.7 V vs Ag/AgCl (Figure S16). An anodic shift of the Ni oxidation peak and a slight improvement of the catalytic activity occurs after 1000 cycles. It was recently demonstrated that this common electrochemical behavior for Ni electrodes is caused by the incorporation of Fe impurities from the KOH electrolyte.59 The potential cycling data shows that the catalyst exhibits good cycling stability during the oxygen evolution reaction. CONCLUSIONS A facile, non-hazardous and scalable approach for fabricating transition-metal phosphides by using metal-organophosphonates as single-source precursors was presented. Ni12P5-Ni2P and Ni2P NPs were synthesized from Ni phenyland methylphosphonate at temperatures  500 °C under H2(5%)/Ar. The phosphides were very efficient HER catalysts in acidic medium, with Ni2P achieving a cathodic current density of 10 mA cm-2 at an overpotential of 87 mV. Co2P and CoP were synthetized from the corresponding Co phenylphosphonate following the same approach. TG-MS data revealed that H2, H2O and P2 were generated during the thermal transformation of the precursor, suggesting that the H2 released is able to reduce (at least partially) the phosphonate. No additional phosphorous-containing species were detected. The metal-phosphonate chemistry is very rich, owing to i) the strong affinity of the phosphonate group for many metal ions, ii) to the large number of organophosphonic acids available, including bi- and triphosphonic acids and those containing additional heteroelements (e.g. S, N), and iii) to the facile synthesis of multimetal phosphonate materials. This approach can be utilized to explore the rich metal phosphonate chemistry for fabricating phosphide-based materials for electrochemical energy conversion and storage applications.

ASSOCIATED CONTENT Supporting Information. XRD patterns and TEM images of Ni methyl- and phenylphosphonate and Ni phosphides; XPS data of Ni phenylphosphonate and Ni2P NPs; XRD, TEM and HER activity of Co phenylphosphonate and Co phosphides; XRD patterns of the products synthesized under argon and air from Ni phenylphosphonate; OER activity of the Ni phosphide catalysts; HER activity of TMP electrocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

ACKNOWLEDGMENT

R.Z. acknowledges the fellowship from the China Scholarship Council (CSC).

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Table of Contents


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Synthesis of Nickel Phosphide Electrocatalysts from Hybrid Metal

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