One-Step Facile Synthesis of Cobalt Phosphides ... - ACS Publications

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One-Step Facile Synthesis of Cobalt Phosphides for Hydrogen Evolution Reaction Catalyst in Acidic and Alkaline Medium Afriyanti Sumboja, Tao An, Hai Yang Goh, Mechthild Lübke, Dougal Peter Howard, Yijie Xu, Albertus Denny Handoko, Yun Zong, and Zhaolin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01491 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

One-Step Facile Synthesis of Cobalt Phosphides for Hydrogen Evolution Reaction Catalyst in Acidic and Alkaline Medium Afriyanti Sumboja,a Tao An,a Hai Yang Goh,b Mechthild Lübke,a,c Dougal Peter Howard,c Yijie Xu,a,c Albertus Denny Handoko,a Yun Zonga* and Zhaolin Liua* a

Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634

b

c

School of Applied Science, Temasek Polytechnic, 21 Tampines Avenue 1, Singapore 529757

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK

KEYWORDS cobalt phosphide; hydrogen evolution reaction; electrocatalysis; phosphidation; water splitting

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ABSTRACT Catalysts for hydrogen evolution reaction (HER) are in demand to realize the efficient conversion of hydrogen via water electrolysis. In this work, cobalt phosphides were prepared using a one-step, scalable and direct gas-solid phosphidation of commercially available cobalt salts. It was found that the effectiveness of the phosphidation reaction was closely related to the state of cobalt precursors at the reaction temperature. For instance, high yield of cobalt phosphides obtained from the phosphidation of cobalt (II) acetate was related to the good stability of cobalt salt at the phosphidation temperature. On the other hand, easily oxidisable salts (e.g. cobalt (II) acetylacetonate) tended to produce a low amount of cobalt phosphides and large content of metallic cobalt. The as-synthesized cobalt phosphides were in nanostructures with large catalytic surface areas. The catalyst prepared from phosphidation of cobalt (II) acetate exhibited an improved catalytic activity as compared to its counterpart derived from phosphidation of cobalt (II) acetylacetonate, showing overpotential of 160 and 175 mV in acidic and alkaline electrolyte, respectively. Both catalysts also displayed an enhanced long-term stability, especially in alkaline electrolyte. This study illustrates the direct phosphidation behaviour of cobalt salts, which serve as a good vantage point in realizing large-scale synthesis of transition metal phosphides for high performance electrocatalysts.


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INTRODUCTION Hydrogen (H2), a versatile, clean and renewable fuel, has the highest gravimetric energy density (~120 MJ kg-1). Apart from its prominent use in proton-exchange membrane fuel cells, hydrogen also has a wide range of other industrial applications, such as hydrodesulfurization and hydrocracking in petroleum-refining operations and ammonia production. Currently, the bulk of commercial hydrogen is generated via steam reforming of natural gas, releasing CO2 greenhouse gas as a by-product.1-2 A promising alternative and arguably cleaner method for hydrogen production is water electrolysis, in which hydrogen is harvested from water in either acidic (2H+ + 2e- H2) or alkaline/neutral (2H2O + 2e- H2 + 2OH-) media. The electrolysis of water, however, is sluggish due to the high overpotential of the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Hence, catalysts to promote both reactions are essential to realize a high hydrogen conversion efficiency. For commercial viability, the catalysts also need to be cost-effective, abundant, easy to synthesize and durable over a long-period of electrolysis. Additionally, nanoscopic and porous catalysts are preferred to increase the catalytic active sites per unit area, affording a more compact electrolyser design.2-4

While it is beneficial to run HER in acidic media, in which the activity of the catalyst is boosted by faster kinetics of H+ ions reduction, the reported OER catalysts mostly exhibit high activity in alkaline media.5-7 Thus, it is beneficial to develop efficient HER catalysts in alkaline media to realize industrial scale of hydrogen production through water electrolysis.3, 8-10 To date, the benchmark HER catalysts are still the Pt-based materials that exhibit high hydrogen adsorption/desorption rates at all pH values.8,

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However, their feasibility for large-scale

industrial application is questionable due to their high cost and scarcity. Hence, a number of non-


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precious catalysts, e.g. transition metal phosphides (TMPs), have been investigated recently and showed promising HER catalytic activities.12-19 It is proposed that the phosphorus (P) anions of the TMPs possess negative charges which trap protons and increase dissociation of H2 upon application of negative bias at the catalytic active sites.20 In addition to activity enhancement, the P anions may have protected the TMPs from dissolving and hence enabled the long-term stability of the HER catalyst.21 Hybridization of TMPs with carbon-based materials have shown to improve the catalytic activity during HER due to the enhanced conductivity and improved dispersion of the active phase.22-26

Among TMPs, cobalt phosphides are particularly promising for their good catalytic activity and long term stability in HER, especially in acidic media.14, 27-29 Current synthesis methods of cobalt phosphides typically require two or more steps, i.e. the synthesis of their own initial precursors to obtain certain intricate morphologies (e.g. nanostructures of Co3O4), which are then followed by a phosphidation process using sodium hypophosphite or trioctylphosphine.14, 27-39 A direct consequence is the low yield from complicated and laborious steps, compromising their utility in real industrial applications. Herein, we report a facile, scalable, and safe synthesis of cobalt phosphides via a direct gas-solid phosphidation of the commercial cobalt salts. The composition and the yield of cobalt phosphides were found to be related to the state of cobalt salt precursors at the phosphidation temperature. High content of nanostructured cobalt phosphides (up to 92.5 wt %) with large catalytic surface area were obtained. They show good HER catalytic activity and stability in both acid and alkaline electrolytes.


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RESULTS AND DISCUSSION In the proposed one-step phosphidation method, small amounts of PH3 gas liberated from the in-situ decomposition of sodium hypophosphite react directly with the cobalt salts of acetate or acetylacetonate during heat treatment. Thermogravimetric analysis (TGA) data reveal that in-situ degradation of the cobalt salts occurs concurrently with the phosphidation reaction (Figure S1S2), allowing for rapid formation of cobalt phosphides at a modest temperature of around 300 °C.

Figure 1. XRD data (Mo Kα source) and scanning electron micrographs of (a-b) Co-P I (cobalt acetate precursor) and (c, d) Co-P II (cobalt acetylacetonate precursor). The nanostructured catalysts of cobalt phosphides which are synthesized using acetate and acetylacetonate cobalt precursors are labelled as Co-P I and Co-P II, respectively. X-ray diffraction pattern confirmed successful synthesis of cobalt phosphides from the phosphidation of cobalt (II) acetate tetrahydrate (Figure 1a). Two phases of cobalt phosphides, Co2P and CoP were identified along with trace amounts of CoO and metallic Co. In contrast, the XRD pattern


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of Co-P II which was prepared from phosphidation of cobalt (II) acetylacetonate shows significantly larger amount of metallic Co co-existing with Co2P and CoP (Figure 1c). The formation of different cobalt phosphide phases could be due to the coarse and non-uniform cobalt salt precursors (Figure S3) which may had different accessibility for PH3 during the phosphidation process. The use of non-agglomerated and uniform size of cobalt salt precursors may improve the particle size distribution of the cobalt phosphides.

The metallic Co found in Co-P I and Co-P II likely came from a concomitant reduction of cobalt precursors in the presence of PH3, a well-known reducing agent for inorganic metal ions, halogens and aromatic nitro compounds.40 This is evidenced by the absence of metallic cobalt in the control experiments in which no sodium hypophosphite was used (Figure S4). The use of different cobalt salts as the precursor, despite the same cobalt to hypophosphite molar ratio, leads to distinct composition variation in Co-P I and Co-P II. The Quantitative Rietveld analysis of the XRD data shows that Co-P I consists primarily of cobalt phosphide variants (CoP: 48.4%; Co2P: 44.1 wt%, Co: 3.0 wt%; CoO: 4.5 wt%), while the content of metallic cobalt is fairly significant in Co-P II (CoP: 52.9 wt%; Co2P: 10.6 wt %, Co: 36.5 wt%).

Scanning electron microscopy (SEM) images reveal a porous feature of both samples. In contrast to the alveolar sac like morphology of Co-P I (Figure 1b), Co-P II consists of randomly shaped nanoparticles (Figure 1d). The nanostructures enlarge the catalytically active surface area, which facilitates the transportation of electrolyte ions during HER. The BET measurement gives the specific surface area of Co-P I and Co-P II as 23.4 and 11.1 m2 g-1, respectively. These are lower than CoP nanowires (27.5 m2 g-1)28 and WP2 microparticles (37.4 m2 g-1),41 but higher


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than CoP nanosheets (8.0 m2 g-1),28 CoP nanoparticles (6.0 m2 g-1),28 Ni2P nanoparticles (6.3 m2 g-1),13 and MoP nanoparticles (5.4 and 8.4 m2 g-1).13, 42

Figure 2. (a) XPS survey scan of Co-P I. (b) Co 2p and (c) P 2p spectra of Co-P I. (d) XPS survey scan of Co-P II. (e) Co 2p and (f) P 2p spectra of Co-P II. Si 2s signal is from the substrate used during XPS.

The core level XPS spectra of Co 2p and P 2p regions are consistent with the previous reports on cobalt phosphides and zero-valent Co (Figure 2).43-45 Exposure of the samples to the air resulted in the superficial oxidation of cobalt phosphides,30,

46

which is confirmed by the

presence of O in the XPS survey scan. XPS survey scan of both samples detected the presence of C, which is likely to originate from the carbonaceous residue of the cobalt salts during the heat treatment (Figure 2a and 2d). Quantitative analysis on the Co/P ratio suggests higher amount of cobalt phosphides in Co-P I as compared to Co-P II, agreeable with the XRD refinement results.


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The HER catalytic activity of TMPs can be due their similar structure to hydrogenases which use pendant bases proximate to the metal centers as the active sites.20-21, 47 In the case of cobalt phosphides, they may feature pendant base P (δ-) moieties within the proximity of metal center Co (δ+).10, 28, 33 Further XPS analysis is performed to investigate the electronic property of Co and P in the samples. Both Co-P I and Co-P II have similar set of peak positions. In addition to the oxidized P, the P 2p region (Figure 2c and 2f) displays the peaks of P 2p1/2 (129.7 eV) and P 2p3/2 (130.7 eV). The P 2p1/2 binding energies are negatively shifted from that of elemental P (130.2 eV), indicating the P in both samples carry negative charge.10, 28, 33, 48 The Co 2p3/2 and Co 2p1/2 peaks in Co 2p region can be deconvoluted into 4 peaks (Figure 2b and 2e). Peak of metallic and oxidized Co in the Co 2p3/2 peak can be observed at 778.7, 782.3 and 785.1 eV, respectively. The positively shifted Co 2p3/2 peaks of both samples from that of metallic Co (778.1-778.2 eV) suggest that the former carry partial positive charge.10,

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Thus, the electron

transfer from Co to P occurs in Co-P I and Co-P II, suggesting their similar structure and electronic analogy to the active site of hydrogenases.28

Variation in the composition of Co-P I and Co-P II can be attributed to the different state of their cobalt precursors during the phosphidation reaction. When PH3 was generated in-situ from disproportionation of sodium hypophosphite at 260-270 °C,49 cobalt (II) acetate for Co-P I was mostly in the form of basic cobalt acetate Co(OH)x(CH3COO)2-x (Figure S1 and S4a) which can be readily phopshidised at the temperature. In contrast, cobalt (II) acetylacetonate converted rapidly to CoO above 200 °C (Figure S2 and S4b), which is prone to reduction to metallic Co upon contact with reducing agent at about 250 °C.50 Hence, at 260-270 °C when PH3 is in-situ


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produced, the formation of metallic cobalt competes with the phosphidation reaction and reduces the total content of cobalt phosphides in the final product. We note that the phosphidation of metallic Co requires a temperature of >450 °C.51

Direct phosphidation of transition metal salts has not been commonly reported. A notable example on cobalt salt was reported by Liu et al.46, where cobalt chloride was phosphidised at 600 °C to form CoP2. A Similar method was carried out at a lower temperature (250 °C) and used a higher amount of phosphorus precursor produced Co2P.45 The gas-solid phosphidations of pre-synthesized Co3O4 precursors (i.e. a two-step synthesis) were mostly reported to form CoP.2830, 32-33, 52

Interestingly, the gas-solid phosphidation of cobalt acetylacetonate or cobalt acetate

tetrahydrate at 330 °C in this work yielded a mixture of Co2P, CoP, CoO and Co, with each of cobaltous species is predicted to bring benefit to the overall HER activity.


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Figure 3. (a, b) TEM and HRTEM image of Co-P I. (c) TEM image of Co-P I showing the atomic ratios of Co to P at two different locations and the corresponding EDS mapping at the marked regions.

The transmission electron microscopy image of Co-P I shows rounded edges for the alveolar sac morphology (Figure 3a). Lattice spacing of 2.53 Å and 1.81 Å in HRTEM micrograph of CoP I correspond to d(200) of CoP and d(301) of Co2P, respectively (Figure 3b). Homogenous distribution of Co and P is evidenced by EDS elemental mapping (Figure 3c). The EDS quantitative analysis gives the Co/P ratios as 68:32 and 51:49 for the two mapped regions, indicating the presence of mixed CoxP which is agreeable with the XRD results. Nanostructures of various shapes and sizes were observed in TEM image of Co-P II (Figure 4a and 4c). HRTEM image of Co-P II reveals a lattice spacing of 1.75 Å which corresponds to the (103) plane of CoP (Figure 4b). EDS elemental mapping suggests the larger spherical particles (darker contrast) as


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metallic Co with the rest being CoxP particles. The mapping on Co and P in the area of metallic cobalt only shows a very small amount of P (Figure 4c), which could be just tethered to the surface of the large metallic Co particles. These materials characterization data have unambiguously confirmed the successful phosphidation of the two commercially available cobalt salt precursors.

Figure 4. (a, b) TEM and HRTEM image of Co-P II. (c) TEM image of Co-P II showing the atomic ratios of Co to P at two different locations and the corresponding EDS mapping at the area (i).

Co-P I and Co-P II were further assessed with respect to their catalytic activity towards HER both in acidic and alkaline electrolytes using rotating disc electrode in a three electrode configuration. iR corrected linear scan voltammetry (LSV) scans were conducted to determine


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the overpotential required to achieve geometric current density of -10 mA cm-2. The potentials were calibrated and expressed with respect to the reversible hydrogen electrode (RHE). LSV scans were also performed on Pt/C (20 wt% of Pt on C) as the benchmark catalyst. The LSV data indicates a clear HER activity for the samples in both acidic and alkaline medium, with Co-P I and Co-P II had overpotential of 160 and 169 mV in acidic electrolyte (i.e. 0.5 M H2SO4), respectively (Figure 5a).

Figure 5. LSV curves of the cobalt phosphides and Pt/C in (a) 0.5 M H2SO4, (b) 1.0 M KOH and their corresponding Tafel slopes (c and d). The measurements were carried out at a scan rate of 5 mV s-1 and rotating speed of 1600 rpm.

The enhanced HER performance for Co-P I in acid is ascribed to its higher CoxP content (92.5 wt%) compared to Co-P II (63.5 wt%), as the HER activity of cobalt phosphides is the best


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among the other cobaltous phases.27, 53 The overpotential of Co-P I and Co-P II are comparable to other forms of cobalt phosphides measured in acidic electrolytes, such as urchin-like CoP (105 mV),31 CoP nanowires (110 mV),28 CoP nanosheets (164 mV),28 CoP microparticles (202 mV),32 CoP nanoparticles (221 mV),28 CoP microsphere (226 mV),33 and previously reported TMPs (Table S1).12, 45-46, 52

Reports on cobalt phosphides as HER catalyst in alkaline and neutral media are rather limited as compared to those in acidic media.30, 34, 45-46 Active HER catalyst in alkaline electrolyte is of interest especially for alkaline water electrolyser.54 Both of our cobalt phosphide catalysts are also able to perform HER favourably in neutral media (Figure S5) and alkaline conditions. Co-P I and Co-P II showed a favourable overpotential of 175 and 188 mV respectively in 1.0 M KOH (Figure 5b), which are comparable to the reported TMPs measured in alkaline electrolyte, such as FeP2 nanowires (189 mV),15 FeP nanowires (194 mV),15 and MoP nanoparticles (276 mV) (Table S2).42

In general, the overpotentials required for Co-P I and Co-P II catalysts to reach -10 mA cm-2 are lower in acidic than in alkaline media. This can be attributed to the more facile catalytic reaction via the hydronium reduction process (2H3O+ + 2e- H2 + 2H2O) in acidic condition. On the other hand, HER proceeds through H2O reduction in alkaline media (2H2O + 2e- H2 + 2OH-). In this case, H2O has to undergo a bond breaking process to form H+ and OH- ions, which is a more energy demanding process with higher overpotential.55 The presence of cobalt oxides in our Co-P samples is proposed to be beneficial for HER activity in alkaline condition, as CoOx has been shown to assist the dissociation of H2O due to the stronger electrostatic affinity of OH-


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to Co2+ and Co3+.56 Additionally, small amounts of metallic cobalt may be able to facilitate the H2 desorption process due to their low binding energy to H.8, 56

Broadly, the HER process mechanism can be distinguished as either a Volmer-Tafel or Volmer-Heyrovsky mechanism, depending on the rate determining step. In the former, the initial M-H bond formation (Volmer step) is followed by the non-electrochemical dimerization of 2 adsorbed H (Tafel step), while in the latter, the adsorbed H reacts with proton source in the electrolyte (Heyrovsky step).57 The rate determining step of the HER mechanism can be inferred from the Tafel slope of the catalysts (Table 1).20, 57 We note that the Tafel slope values of the measured Pt/C benchmark (30 and 67 mV dec-1 in 0.5 H2SO4 and 1.0 M KOH respectively) are consistent with the literature values.32-33, 46 On the other hand, the large Tafel slopes of Co-P I and Co-P II (>40 mV dec-1) in both acidic and alkaline medium indicate that their HER is likely to proceed via Volmer-Heyrovsky mechanism (Figure 5c and 5d).33, 57

Table 1. Performance of the cobalt phosphides as HER catalyst in acid and alkaline medium

ECSA (cm2)

BET surface area (m2 g-1)

Tafel slope (mV dec-1)

Degradation after 13h stability test (%)

Electrolyte

Catalyst

Overpotential @ -10 mA cm-2 (mV)

0.5 M H2SO4

Co-P I

160

15

23.4

56

13.1

Co-P II

169

12.5

11.1

65

15.5

Co-P I

175

10

23.4

84

3.9

Co-P II

188

7.5

11.1

97

4.4

1.0 M KOH


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The enhanced HER performance of Co-P I as compared to Co-P II may also be partly attributed to the large surface area of the former, leading to the exposure of more active sites during HER. In addition to BET, the electrochemical active surface area (ECSA) as one of the various methods to estimate the number of active sites present in the electrocatalysts was also measured. ESCA is calculated using electrochemical double layer capacitance extracted from the CV scans of various scan rates at a potential region where no faradic process are observed (Figures S6-S7).5, 58 The ECSA of the samples are presented in Table 1, in which Co-P I shows a larger ESCA as compared to Co-P II in both acidic and alkaline electrolyte. Although BET surface area of Co-P I is more than twice of the surface area of Co-P II, ECSA of Co-P I is only slightly larger than ECSA of Co-P II (Table 1), suggesting the presence of electrochemically inactive area in Co-P I which results in the limited improvement of HER activity of Co-P I. The ECSA of both samples are found to be comparable with previously reported CoP films (11.2 cm2ECSA)58 and CoP nanoparticles (8.95 cm2ECSA),29 but lower than ultrathin porous CoP nanosheets (196.75 cm2ECSA).29

Figure 6. Stability of the cobalt phosphides and Pt/C as HER catalyst in different electrolyte media under a constant current of 10 mA cm-2 for over 13 hours: (a) acidic and (b) alkaline.


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The long term durability of the HER catalysts is important for their practical application. The stability of Co-P I, Co-P II and Pt/C were tested using chronoamperometry method at constant current density of -10 mA cm-2. The time-dependant potentials of the samples in acidic and alkaline electrolyte during 13 h of stability tests are given in Figure 6. In 0.5 M H2SO4, both CoP I and Co-P II showed potential degradation of 13.1 and 15.5 % respectively, much less severe than the degradation of Pt/C catalyst. The early degradation of Pt/C catalyst can be associated to the dissolution and physical loss of the loosely attached catalysts on the glassy carbon. Both catalysts also showed good durability in alkaline electrolyte. Our Co-P catalyst show only 4 % degradation after 13 h of testing, about five times less than the Pt/C catalyst (19 % degradation). Furthermore, both catalysts maintain similar potentials after 13 h of stability test, showing only 2 to 6 % degradation after an extended 44 h of stability test in alkaline electrolyte (Figure S8). The slight increase of Co/P ratio extracted from XPS analysis indicates the loss of cobalt phosphides during the prolonged stability test (Figure S9). SEM and TEM imaging also suggest the morphology changes in both samples (e.g. agglomeration and erosion). However, the changes are more severe on the catalysts that were tested in acidic electrolyte (Figure S10 and S11), leading to the inferior stability of the catalysts in acidic electrolyte as compared to those in alkaline electrolyte.

CONCLUSIONS In summary, a simple, safe and scalable, one-step phosphidation method for the synthesis of cobalt phosphides from commercial cobalt salts is reported in this work. We found that the effectiveness of the phosphidation reaction is closely related to the state of cobalt precursor at the


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reaction temperature. Higher CoxP yield obtained from cobalt (II) acetate is linked to its better stability at the temperature of in-situ generation of PH3. Easily oxidisable salts like cobalt (II) acetylacetonate tend to give lower phosphidation yield but larger metallic cobalt content. The results can be extended to one-step phosphidation of other transition metal salts and to optimize the phosphidation parameters.

The cobalt phosphides obtained from phosphidation of cobalt (II) acetate tetrahydrate (Co-P I) showed an improved performance as compared to its counterpart derived from phosphidation of cobalt (II) acetylacetonate (Co-P II). The enhanced HER activity of Co-P I is attributed to its high content of cobalt phosphides (92.5 wt%) and large surface area that lead to the high density of the electrochemically accessible sites. Furthermore, both Co-P catalysts displayed superior long term stability, especially in alkaline electrolyte. We believe the important insight provided by this study on the direct phosphidation behaviour of cobalt salts establish a good vantage point in realizing large-scale synthesis of transition metal phosphides for high performance electrocatalysts.

EXPERIMENTAL SECTION Synthesis of Cobalt Phosphides: Cobalt acetate tetrahydrate (Co(C2H3O2)2)•4H2O, Alfa Aesar) and cobalt acetylacetonate (Co(C5H7O2)2, Sigma Aldrich 97 %) were used as starting precursors for the phosphidation reaction with sodium hypophosphite (NaH2PO2•H2O, Alfa Aesar). A molar ratio of 1:10 (cobalt salt:NaH2PO2•H2O) was used for the phosphidation reaction. The precursors were first prepared by hand milling to obtain a fine powder. The precursors and sodium hypophosphite were placed in separate position in a porcelain crucible,


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one facing the other. The crucible was then closed with a lid and placed in a furnace (CWF13/13 with a Eurotherm 3216P1 controller, Carbolite GERO). The temperature was first set to 100 °C for one hour (ramp 5 °C min-1) to remove residual water and oxygen from the system and followed with an increase of temperature towards 330 °C for 2 hours (ramp 5 °C min-1). The reaction was conducted in an inert nitrogen atmosphere and copper (II) sulfate (Sigma Aldrich) solution was used as the post-treatment at the end of the gas circle to scrub the excess of PH3 gas. Samples were washed five times with DI water to remove any soluble impurities present in the samples. Freeze-drying was used to obtain the powdered samples. Materials Characterization: The freeze-dried powders were used without any further treatments. X-ray diffraction (XRD) patterns of all samples were obtained on a STOE diffractometer using Mo-Kα radiation (λ = 0.70926 Å) over the 2θ range 2 to 40° with a step size of 0.5° and step time of 10 s. The size and morphology of the samples were determined by scanning electron microscopy (FESEM JEOL 7600F) and transmission electron microscopy (TEM JEOL JEM 2100 with LaB6 filament). Thermogravimetric analysis (TGA) was performed using a TGA Q500 instrument (TA instruments) under nitrogen with a flow rate of 40 mL min-1. The test started from room temperature to 400 °C with a heating rate of 20 °C min-1. BrunauerEmmett-Teller (BET) surface area analysis (N2 adsorption) of the powders was obtained using a TriStar II PLUS system (Micromeritics) and processed using MicroActive™ software. Samples were degassed overnight at 150 °C under flowing N2. XPS analysis was conducted on a Thermo Scientific VG ESCALAB200i-XL spectrometer with monochromatized Al Kα (hν = 1484.6 eV). Electrochemical Measurements: Rotating disc electrode (RDE, diameter: 5 mm) in a three electrode set-up was employed to quantify the catalytic activity of the samples during hydrogen evolution reaction (HER). The catalyst ink was prepared by adding 3 mg of catalyst, 1.33 mL of


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H2O:IPA:Nafion solution of 2.5:1:0.094 ratio, and 1 mg of carbon black into a small glass vial. Carbon black was added into the solution in order to improve the conductivity of the electrocatalyst and aid in the dispersion of its catalytic active sites. Catalyst ink solution was then sonicated till it became a homogenous solution. The ink was then drop-casted onto the glassy carbon RDE and air-dried at room temperature to attain a mass loading of 0.2 mg cm-2. The same procedure was employed to prepare 20 wt% Pt/C (Alfa Aesar) sample which was used as the benchmark catalyst for this study. RDE, carbon rod counter electrode and Ag/AgCl (3 M KCl) reference electrode were immersed in the electrolytes (i.e. 0.5 M H2SO4 or 1.0 M KOH) that were continuously purged with N2. The HER activity of the samples was investigated via linear sweep voltammetry (LSV) and chronopotentiometry using a galvanostat/potentiostat (PGSTAT302, AUTOLAB, Metrohm, Utrecht, Netherlands). Prior to the LSV scans, the working electrodes were first subjected cyclic voltammetry scans (about 5 scans) until stable CV curves were obtained. LSV and CV scans were performed at a rotating speed of 1600 rpm and scan rates of 5 mV s-1. Electrochemical impedance spectroscopy measurements (200 kHz – 0.05 Hz) were used to correct the iR drop of the LSV curves before the measurements. The stability of the catalyst was investigated using chronopotentiometry tests at constant current density of 10 mA cm-2and rotating speed of 1600 rpm. All the measured potentials from the electrochemical experiments were calibrated to the reversible hydrogen electrode (RHE). The experimental data are converted to RHE by experimentally measuring the potential of Ag/AgCl against hydrogen reference electrode (Gaskatel GmbH, Germany) in 0.5 M H2SO4 and 1.0 M KOH solutions.

ASSOCIATED CONTENT


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Supporting Information. The following files are available free of charge. TGA and SEM of cobalt salts (PDF) XRD of heat-treated cobalt salts in N2 (PDF) HER performance of the samples in neutral electrolyte (PDF) ECSA of the samples (PDF) Stability test of the samples (PDF) XPS analysis of the samples after stability test (PDF) SEM and TEM images of the samples after stability test (PDF) Performance of transition metal phosphides in alkaline and acidic electrolytes (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT This research was supported by Advanced Energy Storage Research Programme (IMRE/122P0503 and IMRE/12-2P0504), Institute of Materials Research and Engineering of A*STAR, Singapore. H. Y. Goh acknowledges the scholarship awarded by A*STAR and internship under A*STAR science award scheme. Luke Goh Xu Jie is thanked for graphical support. Dr. Ding Ning (IMRE) is thanked for his assistance in HRTEM imaging. Debbie Seng Hwee Leng (IMRE) is thanked for her support in XPS analysis.

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XRD data (Mo Kα source) and scanning electron micrographs of (a-b) Co-P I (cobalt acetate precursor) and (c, d) Co-P II (cobalt acetylacetonate precursor). 230x165mm (150 x 150 DPI)


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(a) XPS survey scan of Co-P I. (b) Co 2p and (c) P 2p spectra of Co-P I. (d) XPS survey scan of Co-P II. (e) Co 2p and (f) P 2p spectra of Co-P II. Si 2s signal is from the substrate used during XPS. 306x192mm (150 x 150 DPI)



(a, b) TEM and HRTEM image of Co-P I. (c) TEM image of Co-P I showing the atomic ratios of Co to P at two different locations and the corresponding EDS mapping at the marked regions. 184x194mm (150 x 150 DPI)


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(a, b) TEM and HRTEM image of Co-P II. (c) TEM image of Co-P II showing the atomic ratios of Co to P at two different locations and the corresponding EDS mapping at the area (i). 174x194mm (150 x 150 DPI)



LSV curves of the cobalt phosphides and Pt/C in (a) 0.5 M H2SO4, (b) 1.0 M KOH and their corresponding Tafel slopes (c and d). The measurements were carried out at a scan rate of 5 mV s-1 and rotating speed of 1600 rpm. 166x141mm (150 x 150 DPI)


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Stability of the cobalt phosphides and Pt/C as HER catalyst in different electrolyte media under a constant current of 10 mA cm-2 for over 13 hours: (a) acidic and (b) alkaline. 161x68mm (150 x 150 DPI)



Table of Content 178x75mm (150 x 150 DPI)


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One-Step Facile Synthesis of Cobalt Phosphides ... - ACS Publications

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