Surface Oxidized Cobalt-Phosphide Nanorods As an Advanced

Oct 16, 2015 - †State Key Laboratory of Electroanalytical Chemistry, ‡Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemist...
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Surface oxidized cobalt-phosphide nanorods as an advanced oxygen evolution catalyst in alkaline solution Jinfa Chang, Yao Xiao, Meiling Xiao, Junjie Ge, Changpeng Liu, and Wei Xing ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02076 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 19, 2015

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ACS Catalysis

Surface oxidized cobalt-phosphide nanorods as an advanced oxygen evolution catalyst in alkaline solution Jinfa Changƚ, Yao Xiaoƚ, Meiling Xiaoƚ, Junjie Geǂ,*, Changpeng Liuǂ, and Wei Xingƚ,* ƚ

ǂ

State Key Laboratory of Electroanalytical Chemistry, Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 (P.R. China) ABSTRACT: Electrochemical water splitting in alkaline solution plays a growing role in alternative energy devices due to the need for clean and sustainable energy. However, catalysts that are active for both hydrogen evolution and oxygen evolution reactions are rare. Herein, we demonstrate that Cobalt-Phosphide (CoP), which was synthesized via the hydrothermal route and has been shown to have hydrogen evolution activity, is highly active for oxygen evolution. A current density of 10 mA cm-2 was generated at an overpotential of only 320 mV in 1 M KOH for a CoP nanorod-based electrode (CoP NR/C), which was competitive with commercial IrO2. The Tafel slope for CoP NR/C was only 71 mV dec-1, and the catalyst maintained high stability during a 12 h test. This high activity was attributed to the formation of a thin layer of ultrafine crystalline cobalt oxide on the CoP surface. KEYWORDS: cobalt phosphide, nanorods, electrocatalysts, oxygen evolution reaction (OER), water splitting The oxygen evolution reaction (OER) has attracted significant research attention in recent years because of its vital role in various energy conversion and storage technologies, such as water splitting for hydrogen production, regenerative fuel cells and metal-air batteries.1-5 However, the OER has intrinsically very sluggish reaction kinetics due to multi proton-couple electron transfer steps and requires the use of an electrocatalyst to promote the reaction rate.6-8 Precious-metal electrocatalysts, such as IrO2 and RuO2, are good OER catalysts, but their low abundance and high cost prohibit them from large-scale applications. The hydrogen evolution reaction (HER) also requires the presence of proper electrocatalysts to increase the reaction speed to a practical rate.9,10 The development of new and efficient water splitting catalysts both for OER and HER, particularly catalysts based on earth-abundant elements, has been an active area of energy research.11-13 Recently, Nickel-based compounds have been discovered as good OER catalyst candidates. Great efforts have been devoted to study the OER property of Ni-based structures, including nickel oxides, nickel-hydroxide, nickel-oxyhydroxide, Nibased layer double hydroxide (LDH) and nickel sulfides. Although significant progress has been made in this area, until now there have been very few catalysts that are capable of catalyzing both HER and OER in the same media.14-17 The Janus Co-based electrocatalyst material15 has recently been shown to be a bifunctional catalyst for water splitting in pH 7 phosphate buffer. Transformation of the morphology and composition of the catalyst surface due to a potential change was found to be responsible for the switching of the catalyst activity. Co oxo-hydroxyl phosphate species at reductive potentials and Co-Pi at oxidative potentials have been reported to be the active sites for HER and OER, respectively.18 An analogous behavior for amorphous Ni oxohydroxyl film in pH 9.2 borate buffer has been reported14. NiFe LDH has also recently been shown to be a bifunctional catalyst 16 for water splitting in alkaline conditions. However, to our best knowledge, transition metal phosphides (TMPs) have been rarely reported as OER catalysts19,20, although TMPs have been widely used as catalysts for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) as well as anode materials for Li-ion batteries (LIBs) and HER catalysts. Cobalt Phosphide, as a TMP, has been widely used as an efficient HER catalyst both in acidic, neutral and alkaline solutions.2124 However, the characteristics of TMPs acting as OER catalysts are still largely unknown. Recently, Hu’s group found that Ni2P

nanoparticles are Janus catalysts for water splitting in 1 M KOH, and this high activity was attributed to the core–shell (Ni2P/NiOx) structure.19 Du’s group20 found that Ni2P is an effective precursor for water oxidation; also, Cobalt Phosphides with controllable morphology are usually synthesized through tedious steps in the presence of different organic solvents and stabilizers, which are difficult to remove and thus block the active site.25 Herein, we show that CoP, which has recently emerged as a new and highly efficient HER catalyst, can catalyze OER in alkaline medium with remarkable activity and stability. Both CoP Nanoparticles (NPs) and CoP Nanorods (NRs) mixed with carbon exhibited comparable OER performance with the IrO2 catalyst in 1 M KOH. Specifically, the CoP NP/C catalyst yielded a current density of 10 mA cm-2 at an overpotential of 340 mV, and the CoP NR/C catalyst yielded a current density of 10 mA cm-2 at an even lower overpotential of 320 mV, ranking among the most active non-precious OER catalysts. The Tafel slope for CoP NR/C was only 71 mV dec-1, and the catalyst exhibited a highly active and stable performance during a 12 h duration test. The activity of CoP in OER provides promise for applications to water oxidation. CoP NP and CoP NR were synthesized by a solid phase reaction/low-temperature phosphorization of hydrothermally obtained Co3O4 precursor methods26. It is important to note that both CoP materials were passivated in a 1.0 mol % O2/ N2 mixture at 20 mL min-1 for 3 h [See Supporting Information (SI) for details]. Figure S1 shows the X-ray diffraction (XRD) patterns for CoP NP and CoP NR, in which both samples show diffraction peaks corresponding to orthorhombic CoP (JCPDS No. 29-0497) and cubic CoO (JCPDS No. 43-1004). Figure 1a and Figure 1b show Co 2p and P 2p X-ray photoelectron spectroscopy (XPS) data for CoP NP and NR. No obvious change in binding energy was noticed between CoP NP and CoP NR samples. The peaks at 778.9 (2p3/2) and 793.9 (2p1/2) eV were assigned to the binding energies (BEs) of Co 2p in CoP27. The Co2+ peaks at 782.1 and 798.1 eV as well as the two satellite peaks (at ca. 786.5 and 803.8 eV) correspond to CoO in the sample and are consistent with previous reports28. It should be noted that the percentage of CoO in CoP NR was 48.2%, which was much higher than CoP NP (44.1%). The highresolution P(2p) region showed two peaks that were assigned to phosphide signal at 129.7 eV 27 and phosphate (POx or P-O species) at 133.8 eV.29 The high-resolution O (1s) regions for both CoP NP and CoP NR were clearly observed and can be deconvoluted into two components: the lattice oxygen and adsorbed O

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species. (Figure 1c). The elemental contents (at. %) derived from XPS are shown in Table S1, where the atomic ratio for Co and P shifted from 1:1 due to the presence of O. We also measured the elemental contents of both CoP materials using ICP-OES (Table S1), and the results showed that the atomic ratio between Co and P was approximately 1:1. The seemingly controversial results from the two measurements indicated a high possibility of forming a CoP core and a CoOx shell structure because XPS mainly probes the surface composition, while ICP-OES provides bulk content information. Figure 1d shows the nitrogen adsorption/desorption isotherms of CoP NP and CoP NR. The BrunauerEmmett-Teller (BET) specific surface areas (SSAs) of CoP NP and CoP NR were determined to be 58.5 and 87.2 m2 g-1, respectively. The Barrett-Joyner-Halenda (BJH) pore size distribution curves (Figure S2) of CoP NP and CoP NR confirmed the nanoporous nature of the CoP materials.

Figure 1. High-resolution XPS patterns for (a) Co 2p, (b) P 2p and (c) O 1s of CoP NP and NR. (d) Nitrogen adsorption/desorption isotherm plots of CoP NP and CoP NR.

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is FFT corresponding to the characteristic (011) facet of CoP, and the inset in the red framed area of (f) corresponds to the characteristic (111) facet of CoOx. The spots observed on the FFT are indicative of the registry order and the crystallinity. The lattice fringe spacings of the materials were determined using FFT. The CoP NP had an average particle size of approximately 25 nm and was coated by a thin layer (Figure 2a). Figure 2b shows a high-resolution TEM (HR-TEM) image of CoP NP, revealing clear lattice fringes with an interplane distance of 0.283 nm, corresponding to the (011) plane of CoP. The lattice fringes with an interplane distance of 0.246 nm correspond to the (111) plane of CoOx. From Figure 2b, CoP NP was found to have a CoP core with a diameter of approximately 23 nm and a CoOx shell with a thickness of approximately 1.7 nm, which was consistent with the XPS and ICP results. The corresponding selected area electron diffraction (SAED) pattern (Figure 2c) exhibited disordered rings due to the co-existence of CoP and CoOx. The SEM and elemental mappings of CoP NP are shown in Figure S3, in which Co, P and O elements are uniformly distributed throughout CoP NP. Figure 2d and Figure 2e show typical SEM and TEM images of CoP NR with a porous surface, which could be due to gas release and dehydration of the precursor during annealing. The result is consistent with the observation from Figure 1d, in which CoP NR has a much higher BET value than CoP NP. The HR-TEM image (Figure 2f) showed that CoP NR also had a lattice fringe with an interplane spacing of 0.283 nm, corresponding to the (011) plane of CoP; also, there was a characteristic interplane distance of 0.246 nm, corresponding to the (111) plane of CoOx. The inset in the framed area of Figure 2f shows two characteristic FFT; the yellow framed area corresponds to the (011) facet of CoP and the red framed area to the (111) facet of CoOx. The spots observed on the FFT were indicative of the registry order and crystallinity, which further indicated that CoP NR possessed a CoP core and a CoOx shell. The diffraction rings in the SAED pattern (Figure 2g) recorded for the CoP NR were identified as the (011), (111), (211) and (013) planes of an orthorhombic CoP structure. The corresponding energy-dispersive X-ray (EDX, Figure S4) results verified that the atomic ratio (%) of Co, P and O was 44.65:41.83:13.50, which was consistent with the XPS and ICP results. Figure 2h shows the scanning TEM (STEM) image and Figure 2i shows the elemental mapping images of Co, P and O for CoP NR; both the P and Co elements were uniformly distributed throughout the NRs, and it is worth noting that the O atoms were mainly located on the surface of CoP NR. The STEM further confirmed the presence of a surface oxide layer (CoOx) around the metallic cobalt phosphide core due to the passivation process; such a structure was confirmed to be important and beneficial for OERs, as discussed in the following section.

Figure 2. TEM (a), HR-TEM (b) and SAED (c) images of CoP NP; the inset in the framed area of (b) is FFT (fast fourier transform) obtained from the yellow framed area of Figure 2b. SEM (d), TEM (e), HR-TEM (f), SAED (g), STEM (h), and elemental mapping (i) of CoP NR; the inset in the yellow framed area of (f)

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ACS Catalysis

Figure 3. Polarization curves (a) of pure CoP NP, pure CoP NR, the physical mixture of CoP NP and carbon (CoP NP/C), the physical mixture of CoP NR and carbon (CoP NR/C), commercial IrO2, carbon loaded onto a GC electrode and bare GC in an oxygen-saturated 1 M KOH solution with a scanning rate of 5 mV s-1. Potentials were corrected with iR drop. Tafel slopes (b) for CoP NP, CoP NR, CoP NP/C, CoP NR/C and commercial IrO2. Timedependent potential curves for CoP NP/C (c) and CoP NR/C (d) under a static current density of 10 mA cm-2 for 12 h. The inset shows polarization curves of initial CV scans and scans after 3000 CV scans between 1.4 and 1.8 V vs RHE with a scanning rate of 100 mV s-1; the iR drop was not corrected. Calculated versus actual oxygen production catalyzed by CoP NP/C (e) and CoP NR/C (f) at a constant oxidative current of 10 mA cm-2 in 1 M KOH. Figure 3a shows the polarization curves of CoP NP, CoP NR, bare glass carbon (Bare GC), Vulcan XC-72 carbon (C) and commercial IrO2 (loading, 0.71 mg cm−2) in 1 M KOH at a scan rate of 5 mV s-1. The initial polarization curves and curves after iR correction are shown in Figure S5. Bare GC clearly did not have catalytic activity, and pure carbon had negligible activity for OER. For CoP NP and CoP NR, the onset potential was very positive, which may be ascribed to the poor electronic conductivity of CoP materials. Hence, we physically mixed these two CoP materials with carbon (mass ratio, 1:1) to enhance electronic conductivity. As expected, both of these two CoP/C catalysts showed excellent onset potential and overpotential for OER. Specifically, the CoP NP/C catalyst yielded a current density of 10 mA cm-2 at an overpotential of 340 mV, and the CoP NR/C catalyst yielded a current density of 10 mA cm-2 at an even lower overpotential of 320 mV, which was comparable to IrO2. This overpotential was compared with some state-of-the-art catalysts and was shown to be more efficient than the best nanostructured cobalt oxides 30 and NiCo2O4 spinel catalysts (See Table S2). At 1.65 V (vs RHE), the OER specific activities for CoP NP, CoP NR, CoP NP/C and CoP NR/C were 1.1, 1.8, 32.8 and 46.0 mA cm-2, respectively. The CoP NR/C catalyst outperformed CoP NR by more than 25 times and was much higher than the reduced Co3O4 NWs 28 (13.1 mA cm-2 at 1.65 V) and IrOx nanoparticles31 (11.5 mA cm-2 at 1.65 V). The Tafel plots are shown in Figure 3b. The measured value for IrO2 was approximately 76 mV dec-1, which was consistent with

reported values 32. The Tafel slopes for CoP NP and NR were 96 and 84 mV dec-1, respectively. The CoP NP/C exhibited a Tafel slope of 99 mV dec-1, and the Tafel slope for CoP NR/C was only 71 mV dec-1, which was slightly better than commercial IrO2. Although CoP has been reported to be a good electronic conductor, it may not be sufficient for a real OER in basic solution. Figure S6 compares the Nyquist plots for CoP NP and CoP NR with (or without) carbon at an overpotential of 350 mV. Among these catalysts, CoP NR/C had the smallest diameter, implying the smallest charge transfer resistance. Furthermore, a typical fourprobe method was used to probe and compare the electrical resistance of IrO2 and CoP; the conductivity of IrO2 was 32 S/cm, while a much lower value of 2.2 S/cm was observed for CoP. The lower conductivity of CoP can be compensated by mixing with carbon (as shown above) or other conductive materials (see subsequent discussion), through which the triple phase boundary length can be increased for the reaction and transportation of the reactant (water), product (O2), and electron. We then investigated the durability of CoP NP/C and CoP NR/C through chronopotentiometry; the time-dependent voltages under a static current density of 10 mA cm-2 are shown in Figure 3c and Figure 3d, which suggest that CoP NP/C and CoP NR/C maintained their catalytic activity for at least 12 h. The insets of Figure 3c and Figure 3d depict the continuous cyclic voltammetry (CV) results, and after 3000 CV sweeps, the polarization curve showed a negligible difference compared with the initial curve, suggesting superior stability of CoP NP/C and CoP NR/C in the long-term electrochemical process. To calculate the Faradaic efficiency (FE) of the electrocatalytic oxygen evolution process (Figure S7 shows an optical photograph detailing the generation of oxygen bubbles during the CoP NR/C FE test), we compared the amount of experimentally quantified oxygen with theoretically calculated oxygen (assuming 100% FE). As shown in Figure 3e and Figure 3f, the agreement between both of the values suggests FEs of close to 100% for both CoP NP/C and CoP NR/C. In order to evaluate potential applications, a two electrode electrolytic cell with an alkaline electrolyzer was constructed using CoP NR as the catalyst for both HER and OER. CoP NR was sprayed onto a titanium felt sheet (1 *2 cm) at a loading of 5 mg cm-2; Figure 4a shows the current-potential response of this electrolyzer. As seen, a current density of 10 mA cm-2 was obtained at ca. 1.587 V, which indicated that a combined overpotential of only 0.357 V was needed for CoP NR. The small combined overpotential, which was much better than other bifunctional catalysts, was ascribed to the high catalytic activity of CoP NR and the good electronic conductivity of the Ti substrate.15 Figure 4b indicates that the potential was stable at ca. 1.63 V during the 24 hour galvanostatic electrolysis experiment. Hence, the as-prepared CoP NR as a bifunctional catalyst possessed both excellent activity and ultrastrong stability in water splitting.

Figure 4. Current-potential (a) response of the electrolyzer using CoP NR as the catalyst for both OER and HER in 1 M KOH solutions. Two 1*2 cm titanium felt sheets were used as supports and the catalyst loading was 5 mg cm-2 for both. Galvanostatic electrolysis (b) in 1 M KOH at a constant current density of 10 mA cm-2 for 24 hours. The excellent catalytic activities and stabilities of CoP NP/C and CoP NR/C can be rationalized as follows: (1) the active form

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of the catalyst for OER was a core-shell CoP/CoOx assembly, and the CoOx that formed on the outer layer during the passivation process was responsible for electrocatalysis. Transition of a metal cation to a higher valence state/higher coordination number is known to catalyze water electrolysis at the same time. As for Co, the CoIII/II and CoIV/III pairs are both possible candidates for the OER, with reaction rates of 0.19 and >2 s−1 toward water oxidation, respectively.18 In the CoP NP/NR catalyst, the surface passivation process led to a layer of oxidation, as confirmed by the bulk-surface composition deviation observed above. Further, the formation of Co oxide in the plane was stunted by the presence of phosphate anions during the passivation process. As a result, more edge and defect sites of Co were acquired, where their coordination numbers were more easily tuned by the electrode potential. (2) The porosity and rough surface of CoP NR not only led to the exposure of more active sites but also facilitated sufficient transport of reactants and products. In addition, the surface content of CoO in CoP NR was higher than that of CoP NP, thus leading to better OER activity and stability on CoP NR. (3) Utilizing materials with high electron conductivity, including carbon and Ti toward the CoP catalyst significantly increased the conductivity of the CoP materials, thus favoring fast electron transport along CoP NP and NR. In summary, we showed that CoP was capable of catalyzing OERs in alkaline conditions with high activity and durability. CoOx at the outer layer of CoP NP/NR was responsible for the OER. Adding carbon into the CoP catalyst significantly increased the conductivity and decreased the RCT, thus favoring fast electron transport during the OER. The OER activity for CoP NR/C reached 10 mA cm-2 with an overpotential of only 320 mV. These findings highlight the potential of CoP as cathode and anode catalysts for future (photo) electrochemical water splitting devices.

ASSOCIATED CONTENT Supporting Information. Experimental details, Tables S1-S2 and Figures S1-S7 can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

* [email protected] (W. Xing); [email protected]

(J. Ge)

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS The work was supported by the National High Technology Research and Development Program of China (863 Program, 2012AA053401), the National Natural Science Foundation of China (21373199, 21433003), the Strategic priority research program of CAS (XDA0903104) and the Recruitment Program of Foreign Experts (WQ20122200077).

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