Electropolymerization Fabrication of Co Phosphate Nanoparticles

Research Article pubs.acs.org/journal/ascecg

Electropolymerization Fabrication of Co Phosphate Nanoparticles Encapsulated in N,P-Codoped Mesoporous Carbon Networks as a 3D Integrated Electrode for Full Water Splitting Zhixiong Cai,† Wei Xu,† Feiming Li,† Qiuhong Yao,§ and Xi Chen*,†,‡ †

MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, College of Chemistry and Chemical Engineering, and ‡State Key Laboratory of Marine Environmental Science, Xiamen University, No. 422, Siming South Road, Xiamen, Fujian, China, 361005 § Xiamen Huaxia University, 288 Tianma Road, Jimei Cultural and Educational District, Xiamen, Fujian, China, 361024 S Supporting Information *

ABSTRACT: The development of high-performance nonprecious electrocatalysts toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of great significance for overall water splitting but remains a grand challenge. In this study, Co phosphate (CP) nanoparticles encapsulated in three-dimensional porous N,P-codoped carbon networks (NPC) were fabricated by direct growth on carbon cloth (CC), which were further used as an integrated threedimensional (3D) electrode for cost-effective and energy efficient HER and OER both in acid and alkaline medium. Impressively, the as-obtained flexible integrated electrode exhibited excellent activities and robust stability, due to the unique 3D architecture with improved electron transport capability, high number of active sites, and channels for reactant/product transfer. Our experimental results show significantly enhanced performance for such engineered nanostructures due to the synergistic effect from nanoparticles encapsulation and nitrogen and phosphorus doping on carbon structures. Such an versatile electrode can serve as a bifunctional catalyst for overall water splitting with excellent catalytic performance and durability in a more direct and simple way, which reduces the production cost of practical technological devices. The new design demonstrated here opened avenues for simple, low-cost, and scalable manufacture of high performance bifunctional catalysts for renewable energy technologies. KEYWORDS: N,P-codoped carbon, Carbon encapsulated, Co phosphate, Electrochemical water splitting, 3D electrode

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INTRODUCTION In order to meet the challenges of the increasing global demand for energy and diminishing sources of fossil fuel conversion, electrocatalytic splitting of water into hydrogen and oxygen represents a promising and appealing solution.1−3 Electrochemical water splitting, including the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), is traditionally carried out with Pt-based and Ru/Ir-based compounds, respectively. However, the high cost and scarcity of noble metal catalysts restrict the industrial applications.4 In order to replace noble metal-based catalysts, it is highly desirable to explore nonprecious metal catalysts and metal-free heteroatom-doped carbon catalysts for HER and OER with low overpotential and long-term stability.5−7 Moreover, since most HER catalysts work under strongly acidic conditions and OER catalysts under strongly basic conditions, producing a simple and efficient bifunctional electrocatalyst working at all pH values with high activity toward both OER and HER is still a challenging issue. In the past several years, although a great deal of effort and progress have been made to realize the above goals using earth-abundant materials,8−10 until now there have been © XXXX American Chemical Society

very few catalysts capable of catalyzing both HER and OER in the same medium to accomplish overall water splitting.11−13 Recently, carbon nanomaterials doped with various heteroatoms, as metal-free catalysts, have been recognized as a new class of electrocatalysts with unique merits, such as tailorable molecular structures, abundance, being environmentally benign and having strong tolerance to acid/alkaline environments.14−16 In particular, it is revealed that N,P-codoped carbon possesses the electrocatalytic activity for water splitting.17,18 In addition, the gap of catalytic ability between carbon materials and metalbased catalysts should not be ignored. In another impressive case, the compounds coupled by the elements cobalt and phosphorus exhibit unprecedented capacity for electrocatalyzing both HER and OER. Typically, Co phosphide complexes are reported to work as HER and OER catalysts with activities rivalling Pt-based catalysts.19−21 Co phosphate (CP) has been investigated as an efficient OER catalyst owing to its ability to Received: August 15, 2016 Revised: November 8, 2016 Published: December 5, 2016 A

DOI: 10.1021/acssuschemeng.6b01952 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the synthetic process of the CC/NPC/CP, including (i) electrodeposition of the PANI/PA on carbon cloth via potentiostatic deposition, (ii) ion adsorption of Co2+ by negative PA, and (iii) formation of the CC/NPC/CP after carbonizing at 1000 °C.

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facilitate rapid proton transfer.22−25 In addition, Co-phosphorus containing Co phosphide and Co phosphate also have beed developed to be bifuctional for both OER and HER.10,26,27 Despite their outstanding performance, transition metal based electrocatalysts are still plagued by the complex preparation, low electrical conductivity, and poor mechanical or chemical stability in harsh conditions. Encapsulating metal-based electrocatalysts inside various carbons to construct highly integrated hybrids could principally address the stability issue.28,29 The excellent chemical and thermal stability of carbon materials outside the metal-based catalysts could prevent them from possible corrosion in a harsh environment.30 Furthermore, the carbon frameworks could facilitate electron transfer at the interface with catalytic activity further elevated. In the present work, we developed a facile method for the fabrication of 3D self-supported N,P-codoped mesoporous carbon foams (NPC is the abbreviation of N,P-codoped carbon), with CP nanoparticles (NPs) embedded in the carbon framework (NPC/CP). By an electrostatic effect, aniline monomers form electrostatic interactions with phytic acid (PA) molecules and anchor onto carbon cloth (CC) by in situ potentiostatic electropolymerization to form polyaniline− phytic acid (PANI-PA). The grafted phytic acid enables highly efficient cobalt ion adsorption and, more importantly, allows the incorporation of P atoms in the graphene lattice. The final calcination step renders the formation of N,P-doped carbon which wraps around the CP nanoparticles. The integrated binder-free electrode contains N,P-codoped porous carbon and CP NPs, either of which are expected to combine both efficient HER and OER activities in the same materials. We demonstrated a full water splitting activity of CC/NPC/CP in all pH value electrolytes. The new design demonstrated here opened avenues for simple, low-cost, and scalable manufacture of high performance bifunctional catalysts.

EXPERIMENTAL SECTION

Preparation of the CC/NPC/CP. CC/PANI aerogel was prepared via electropolymerization using a three-electrode configuration with concentrated acid treated CC (1 × 3 cm2), Pt wire, and SCE as a working electrode, a counter electrode, and a reference electrode. Typically, the electrolyte was prepared by dissolving 2 mL phytic acid in 17 mL H2O and then adding 1 mL aniline to form a uniform solution after sonication for 10 min. The constant potential of 0.9 V vs SCE was applied to the CC electrode for 30 min. Subsequently, the whole working electrode was washed using water and freeze-dried for 24 h, followed by immersing in 0.1 M cobalt nitrate solution for 6 h. After a full adsorption, excess solution was removed and the PANI/ PA/Co2+ hybrid was dried. Next, the as-prepared PANI-PA-Co hybrid was carbonized at different temperatures (700, 800, 900, and 1000 °C) for 2 h under an Ar atmosphere. Finally, the carbonized products were soaked in 3 M HCl for 12 h to remove unstable cobalt species. Characterization. SEM (HITACHI S-4800) and TEM (FEI Tecnai-F30 FEG) were used for the morphological observation. The chemical compositions of the hybrids were checked using XPS analysis with a PHI Quantum 2000 Scanning ESCA Microprobe and a monochromatised microfocused Al X-ray source. All the binding energies were calibrated using C 1s as the reference energy (C 1s = 284.6 eV). Electrochemical measurements were performed with a CHI 660B Electrochemical Analyzer, a conventional three electrode system including a GCE coated with catalyst film, a Pt auxiliary electrode, and a saturated calomel reference electrode (SCE). All potentials reported in this paper refer to the SCE scale. Electrochemical Measurement. The test solutions were performed with a CHI 660E electrochemistry workstation (Shanghai CHI Instruments Company, China) in a standard three-electrode system. The as-prepared CC/NPC/CP worked as the working electrode directly, and a Pt wire and a SCE electrode were used as the counter and reference electrode. To prepare a Pt/C loaded electrode, Pt/C catalysts were dispersed in a mixture containing water and Nafion (5%) (v:v 4:0.1) to form a 0.5 mg/mL catalyst ink. Two μL catalyst ink was deposited on a glassy carbon electrode (0.071 cm2) that was polished prior to catalyst deposition by 0.3 and 0.05 μm alumina powder and rinsed by sonication in ethanol and in deionized B


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Figure 2. SEM images with different magnifications of (a−c) the CC/PANI/PA after electrodeposition and (d−f) the CC/NPC/CP after carbonization.

Figure 3. Morphology and chemical composition of the CC/NPC/CP. (a, b) TEM images of the individual arm of the CC/NPC/CP network. (c) HRTEM image of the CC/NPC/CP, showing the carbon coating and the d spacing for CP. (d) Energy dispersive X-ray spectroscopy analysis of the CC/NPC/CP. (e) TEM elemental mapping of C, N, O, P, and Co. water. All potential measurements were calibrated to RHE using the following equation:

E(RHE) = E(SCE) + 0.24V + 0.059pH

and enhanced its interaction with aqueous substances (Figure S1 and S2). The solubilization of aniline was then carried out in an aqueous solution containing phytic acid, as a result of the formation of soluble anilinium salt (aniline−phytic acid) via a reaction between the protonated nitrogen groups and the deprotonated phytic acid. Subsequently, polymerization of aniline was triggered by potentiostatic deposition with the oxidized CC as a working electrode in the aniline−phytic acid electrolyte (Figure 2a−c). Because each phytic acid molecule interacted with more than one PANI chain, this cross-linking effect resulted in the formation of a mesoporous hydrogel network and the uniform dispersion of phytic acid in the PANI network. After Co ions were adsorbed by an excellent metalchelator, phytic acid, the subsequent pyrolysis of the prepared

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Polarization curves were obtained using LSV with a scan rate of 5 mV s−1 and no activation was used before the polarization curve records. Current density was calculated based on the geometric area of CC.

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RESULTS AND DISCUSSION A typical procedure for making a CC/NPC/CP is schematically shown in Figure 1 with three simple steps: electrodeposition, ion adsorption, and carbonization. Commercial CC was first mildly oxidized (Figure S1 and S2), which created abundant functional groups on the surface to improve its hydrophilicity C


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Figure 4. XPS survey spectrum (a). High-resolution scans of (b) C 1s; (c) N 1s; (d) O 1s; (e) P 2p; and (f) Co 2p electrons of CC/NPC/CP.

nm.24,27,31 The HRTEM image also indicates that CP NPs were coated with carbon matrix or carbon shells. Furthermore, elemental mapping based on energy dispersive X-ray spectroscopy analysis results, as shown in Figure 3d, revealed that the element contents of C, N, O, P, and Co were atomic fractions of 79.11, 6.13, 12.56, 1.13, and 1.07%. These results clearly showed that the NPC/CP hybrids consisted of C, N, O, P, and Co elements (Figure 3e). C, N, O, and P were evenly distributed with the contours of a high-angle annular dark-field image, and the N,P element was indeed doped into the porous carbon framework. In addition, as shown in Figure 3a, the TEM micrograph revealed the location of Co discretely, which was consistent with the deep color sites in the P, O mapping image. These results further confirmed that the wrapped particles were CP NPs. An X-ray photoelectron spectroscopy (XPS) survey of the asprepared CC/NPC/CP showed all the anticipated elements (Figure 4a), indicating the formation of the N,P-codoped carbon. Deconvolution of the high-resolution scan of the C 1s (Figure 4b) revealed a main graphitic sp2 carbon peak at around 284.8 eV and the C−O peak at around 285.9 eV. This result suggested the presence of hydrophilic oxygen-containing groups (such as hydroxy/epoxy) on the carbon framework but not the oxygenated groups (typically O−CO). The high resolution N 1s spectrum (Figure 4c) could be well-fitted into three peaks at ∼398, 400.4, and 401.3 eV, corresponding to the pyridinic-N, pyrrolic-N, and graphitic-N.32 These results indicated that N was successfully incorporated into the porous carbon matrix. The high-resolution O 1s XPS spectrum (Figure 4d) of the CC/NPC/CP showed the formation of three types of O-containing functional groups: CO, CO, PO. The XPS of the P 2p spectrum (Figure 4e) confirmed the existence of PC and PO bonds.33 The PO bond seen from the P 2p and O 1s XPS spectrum was assigned to phosphate, and the PC bond further suggested the successful doping of P heteroatoms into the carbon network. The Co 2p XPS spectrum (Figure 4f) could be identified at 781.2 and

precursor in Ar at the elevated temperature afforded one-step formation of CC/NPC/CP with different properties for direct electrochemical usage (Figure 2d−f). Figure 2 shows a different magnification scanning electron microscopy (SEM) image of the PANI-derived material before (Figure 2a−c) and after (Figure 2d−f) calcination. The results indicated that the PANIderived carbon dendrites with an average diameter of ∼400 nm were interconnected into a porous network foam. Figure S1 also presents digital images of the CC/NPC/CP before and after the calcination process. As can be seen in Figure 2, pyrolysis caused a slight shrinkage and smoother surface of the branches. No significant change in porous morphology could be found. To determine the optimal conditions for the 3D CC/ NPC/CP network formation, different ratios of aniline and phytic acid were also examined. Based on the SEM images as shown in Figure S5, the cancellate structure of the final aerogels depended on the mole ratio of aniline monomer to phytic acid. The ligament of the porous structure gradually changed from coralliform to spherical micelles on fibers. Phytic acid could serve both surfactant and doping functions. The surfactant function seemed to play an important role in the formation of the PANI hydrogel. In the absence of phytic acid, the final electrodeposited PANI network seemed more stacked (Figure S3), while with only phytic acid involved, the final production showed no porous structure (Figure S4). During the pyrolysis process, the N- and P-containing aniline and phytic acid precursors, respectively, led to simultaneous codoping of N and P into the final porous graphitic carbons. Meanwhile, thermal crystallization of the phytic acid-chelated Co ions occurred to form CP NPs during annealing. Figure 3a−c shows three representative transmission electron microscopy (TEM) images of the resulting NPC/CP hybrids. It is clear that some dark-contrast objects, that were most likely CP NPs, were well embedded in the porous carbon framework (gray matrix; Figure 3a and b). From the high-resolution TEM (HRTEM, Figure 3c) observation, CP (102) crystal planes could be clearly observed with a lattice spacing of ≈0.29 D


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Figure 5. (a) LSV curves for bare CC, CC/NC, CC/NPC, CC/NPC/CP, and Pt/C with a scan rate of 5 mV s−1 for HER in 0.5 M H2SO4 aqueous solution. (b) Corresponding Tafel slopes. (c) 90 000 s durability test with constant current density of 20 mA cm−2 in 0.5 M H2SO4. (d) LSV curves for CC/NPC/CP before and after 90 000 s durability test.

the Tafel plots in Figure 5b. The corresponding Tafel slope of the CC/NPC/CP with 60 mV dec−1 was significantly reduced as compared to those of the CC/NC (139 mV dec−1) and the CC/NPC (115 mV dec−1), implying a faster HER rate for the CC/NPC/CP. These results indicated that both N, P-doped carbon coating and CP NPs played an important role in the enhancement of the HER activity. Figure 6b further shows a temperature-dependent HER activity for the CC/NPC/CP samples. The catalytic activity increased with the increase of the pyrolysis temperature (700−1000 °C). At 1000 °C, the CC/ NPC/CP1000 exhibited the best HER performance. The lower pyrolysis temperature (for example at 700 °C) may have caused the relatively poor activity of the CC/NPC/CP700 since it failed to implant heteroatoms of N,P into the carbon matrix at this temperature. Remarkably, the CC/NPC/CP also exhibited good HER activity in a neutral solution (0.2 M pH 7.0 PBS) or in an alkaline solution (0.1 M KOH) (Figure 6a), although their HER activities were slightly lower than those in acid solution. Additionally, long-term durability testing was then performed using the CC/NPC/CP galvanostatically at 10 mA cm−2 for 90 000 s (Figure 5c), and no obvious decrease in the HER activity could be found. The HER catalytic activity for the CC/NPC/CP was stable over long-term testing. The polarization curve for the CC/NPC/CP remained almost the same after long-term durability testing (Figure 5d). The 3D hierarchical porous structure was also well-preserved (Figure S6). A high roughness factor resulted in electrocatalytic performance enhancement. The measured capacitive current densities were plotted as a function of scan rates and are shown

796.3 eV, corresponding to the Co 2p3/2 and Co 2p1/2 binding energies. The CC/NPC/CP could be directly used as a working electrode for both HER and OER without extra substrates or binders. For comparison, similar measurements with control samples, such as bare CC, CC/NC (calcined CC/PANI), CC/ NPC (calcined CC/PANI/PA) and 20 wt % Pt/C, were also performed. The water electrolysis activity was first confirmed using HER activity in acid (0.5 M H2SO4) electrolyte as shown in Figure 5. As expected, the commercial Pt/C displayed the highest electrocatalytic activity, while both the bare CC and the CC/NC showed very limited HER activity (Figure 5a). In sharp comparison, the N,P-codoped CC/NPC and the CC/ NPC/CP exhibited remarkably superior current densities and earlier onset overpotentials, which suggested that the P element substantially influenced HER performance. The linear sweep voltammetric (LSV, Figure 5a) results indicated that the HER onset overpotential (ηonset) of the CC/NPC/CP could be estimated as about 50 mV, and the maximum current density (jgeo) normalized to the geometric area could reach 20 mA cm−2 at an overpotential of 100 mV. This η was found to be 180 mV superior to that of the CC/NPC and only 50 mV inferior to Pt/C. This result indicated that the wrapped CP NPs could boost the HER rate on the surface. As shown in the photograph (inset of Figure 5a), many small bubbles appeared near the electrode at the current density of 20 mA cm−2. The small bubbles of H2 were easily released from the catalyst surfaces owing to their porous structure. Further insight into the catalytic activity of the CC/NPC/CP was obtained from E


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revealed by a durability test performance galvanostatically at 10 mA cm−2 for 90 000 s (Figure 7d). After a long-term durability test, the CC/NPC/CP retained a polarization curve similar to the initial one with negligible loss of cathodic current density (Figure 7e). Remarkably, the CC/NPC/CP also exhibited an OER activity under acidic conditions, which had almost the same performance as that under alkaline conditions, whereas it showed less activity in neutral solution (Figure 7c). Further investigation of the morphologies and composition of the CC/NPC/CP after chronoamperometry tests showed the robust and stable characteristics of the porous films for catalytic reactions. As shown in Figure S6, the porous structure was still maintained and no apparent collapse or aggregates were observed. After the HER durability test, its Co 2p XPS spectrum was more similar to the Co 2p peaks (Figure S8) compared to those of the as-prepared one (Figure 4f), implying the major composition of CP was preserved as metallic cobalt during HER. Furthermore, the phosphate peak at 133.8 eV originally observed for the as-prepared CP (Figure 4e) became debilitated. Its debilitation was likely caused by the partial dissolution of CP under cathodic conditions, which suggested the embedment of the CP involved in the reaction. In contrast, after the OER durability test, the Co 2p spectrum revealed two peaks at 780.9 and 796.5 eV (Figure S9), which could be assigned to oxidized cobalt, Co3O4, plus its satellite peaks at 786.5 and 803.1 eV. The P 2p spectrum showed a similar phosphate peak at 133.8 eV. The results indicated that the original cobalt in the CP was partially oxidized to Co3O4 during OER. Based on the above results, we anticipated that the CC/ NPC/CP could act as a bifunctional electrocatalyst for overall water splitting at all pH values, and could deliver both efficient HER and OER activity in a single electrolyzer. Therefore, we constructed a two-electrode electrolytic system using the CC/ NPC/CP as both the anode and cathode (CC/NPC/CP// CC/NPC/CP) to go one step closer to a real overall watersplitting system in 0.1 M KOH (Figure 8). Figure 8 displays the OER polarization (LSV) curve, which needed a cell voltage of 1.66 V to afford a water splitting current density of 10 mA cm−2, with vigorous gas evolution on both electrodes (Figure 8 inset). The excellent electrocatalysis performance in the water splitting for the as-prepared 3D hierarchical porous CC/NPC/ CP could be attributed mainly to their improved electronic/ chemical properties and the exposure of abundant active sites, which are related to their unique, highly porous morphology. First, the hierarchical porous structure not only endowed a large electrode−electrolyte contact area but also facilitated sufficient transport of reaction-relevant species. Second, a high electrical conductivity of the carbon substrate and the intimate contacted carbon hybrid networks enabled a good mechanical adhesion and fast electron transport, which granted high stability and electroconductivity during the electrolysis reaction. Third, previously reported work supports the benefits of N,Pcodoped graphitic carbon for HER and OER.35−37 The presence of N and P in graphitized nanocarbon can certainly provide superior electrocatalytic properties for both HER and OER, presumably because of a synergistic effect between the heteroatoms. Finally, the in situ formed CP NPs were crucial to enhance the OER and HER activity, similar to the recently reported electrodeposited cobalt−phosphorus-derived films.10 Moreover, the wrapped CP NPs might change the electronic density of states around the carbon, resulting in the generation of additional catalytically active sites from the carbon atoms.30

Figure 6. (a) Comparison of LSV curves for CC/NPC/CP in 0.5 M H2SO4, 0.1 M KOH, and 0.2 M PBS (pH 7) aqueous solution. (b) Polarization curves of CC/NPC/CP calcined at 700, 800, 900, and 1000 °C in 0.5 M H2SO4.

in Figure S7. The linear slope was equivalent to twice the electrochemical double-layer capacitance (Cdl), which was typically used to represent the electrochemically active surface area.34 The calculated capacitance of CC/NPC/CP was found to be 88 mF cm−2. Thus, the large current density value of CC/ NPC/CP benefitted from both its large surface area and its large Cdl. The OER performance of the CC/NPC/CP was further investigated in an alkaline solution (0.1 M KOH) using LSV, and the corresponding Tafel plots were listed. The CC/NPC/ CP afforded a sharp onset potential at 1.52 V for OER and required an overpotential of 360 mV to reach a current density of 20 mA cm−2. Although it was slightly worse than that of the CC/NPC/CP, the CC/NPC exhibited a superior current density and earlier onset of catalytic current with respect to the bare CC, Pt/C, and the CC/NC, indicating the critical role of P doping for OER catalytic activity (Figure 7a). Further insights into the OER kinetics were accessible through the analysis of the corresponding Tafel plots for such electrodes (Figure 7b). The Tafel slope for the CC/NPC/CP was 80 mV dec−1, which was significantly less than that of the CC/NC (247 mV dec−1) or the CC/NPC (101 mV dec−1), implying a faster OER rate for the CC/NPC/CP as a result of the participation of CP. In addition, the CC/NPC/CP featured excellent stability, as F


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Figure 7. (a) LSV curves for bare CC, CC/NC, CC/NPC, CC/NPC/CP, and Pt/C with a scan rate of 5 mV s−1 for OER in 0.1 M KOH aqueous solution. (b) Corresponding Tafel slopes. (c) Comparison of LSV curves for CC/NPC/CP in 0.1 M KOH, 0.5 M H2SO4, and 0.2 M PBS (pH 7) aqueous solution. (d) 90 000 s durability test with constant current density of 10 mA cm−2 in 0.1 M KOH. (e) LSV curves for CC/NPC/CP before and after the 90 000 s durability test.

catalyst exhibited outstanding water splitting performance with high activity toward HER and OER at all pH values. This work provided a novel effective paradigm for the design and optimization of full water splitting electrocatalysts based on metal elements and carbonaceous materials promoted by heteroatoms. Such a facile method to fabricate heterogeneous catalysts with implanted transition metal phosphate -based composites reported here provides a new vision for the design of earth-abundant catalysts applying for HER and OER.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01952. Comparison of SEM images obtained from different reaction conditions; XPS spectra of catalyst after HER and OER durability (PDF)

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Figure 8. Two electrode OER polarization curves of CC/NPC/CP// CC/NPC/CP with a scan rate 5 mV s−1 and an electrode image with gas evolution (inset).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

The CP NPs were not in contact with the harsh electrolyte, which ensured that the metals could not be dissolved in an acidic electrolyte or oxidized in an alkaline electrolyte.

ORCID

Xi Chen: 0000-0002-3013-4558

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Notes

The authors declare no competing financial interest.

CONCLUSIONS In summary, a low-cost and scalable approach was developed to prepare flexible CC/NPC/CP electrodes, via pyrolysis of an electrostatic aggregate of electropolymerized PANI, phytic acid, and cobalt ions. Owing to the large specific surface area, superior mass, electron transportation rate, and a synergistic effect of catalytic active sites, the obtained CC/NPC/CP

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ACKNOWLEDGMENTS This research was financially supported by the National Nature Scientific Foundation of China (No. 21375112), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13036), the Foundation for Innovative G


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(17) Zhang, Y.; Mori, T.; Ye, J.; Antonietti, M. Phosphorus-Doped Carbon Nitride Solid: Enhanced Electrical Conductivity and Photocurrent Generation. J. Am. Chem. Soc. 2010, 132, 6294−6295. (18) Zhang, J.; Qu, L.; Shi, G.; Liu, J.; Chen, J.; Dai, L. N,P-Codoped Carbon Networks as Efficient Metal-free Bifunctional Catalysts for Oxygen Reduction and Hydrogen Evolution Reactions. Angew. Chem. 2016, 128, 2270−2274. (19) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode over the Wide Range of pH 0−14. J. Am. Chem. Soc. 2014, 136, 7587−7590. (20) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles. Angew. Chem. 2014, 126, 5531−5534. (21) Chang, J.; Xiao, Y.; Xiao, M.; Ge, J.; Liu, C.; Xing, W. Surface Oxidized Cobalt-Phosphide Nanorods As an Advanced Oxygen Evolution Catalyst in Alkaline Solution. ACS Catal. 2015, 5, 6874− 6878. (22) Surendranath, Y.; Kanan, M. W.; Nocera, D. G. Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. J. Am. Chem. Soc. 2010, 132, 16501−16509. (23) Cobo, S.; Heidkamp, J.; Jacques, P. A.; Fize, J.; Fourmond, V.; Guetaz, L.; Jousselme, B.; Ivanova, V.; Dau, H.; Palacin, S.; Fontecave, M.; Artero, V. A Janus cobalt-based catalytic material for electrosplitting of water. Nat. Mater. 2012, 11, 802−807. (24) Chang, Y.; Shi, N.; Zhao, S.; Xu, D.; Liu, C.; Tang, Y.; Dai, Z.; Lan, Y.; Han, M.; Bao, J. Coralloid Co 2 P2 O7 Nanocrystals Encapsulated by Thin Carbon Shells for Enhanced Electrochemical Water Oxidation. ACS Appl. Mater. Interfaces 2016, 8, 22534−22544. (25) Kanan, M. W.; Nocera, D. G. In situ formation of an oxygenevolving catalyst in neutral water containing phosphate and Co2+. Science 2008, 321, 1072−1075. (26) Han, G.; Li, X.; Liu, Y.; Dong, B.; Hu, W.; Shang, X.; Zhao, X.; Chai, Y.; Liu, Y.; Liu, C. Controllable synthesis of three dimensional electrodeposited Co-P nanosphere arrays as efficient electrocatalysts for overall water splitting. RSC Adv. 2016, 6, 52761−52771. (27) Yang, Y.; Fei, H.; Ruan, G.; Tour, J. M. Porous Cobalt-Based Thin Film as a Bifunctional Catalyst for Hydrogen Generation and Oxygen Generation. Adv. Mater. 2015, 27, 3175−3180. (28) Tavakkoli, M.; Kallio, T.; Reynaud, O.; Nasibulin, A. G.; Johans, C.; Sainio, J.; Jiang, H.; Kauppinen, E. I.; Laasonen, K. Single shell carbon-encapsulated iron nanoparticles: synthesis and high electrocatalytic activity for hydrogen evolution reaction. Angew. Chem., Int. Ed. 2015, 54, 4535−4538. (29) Yang, W.; Liu, X.; Yue, X.; Jia, J.; Guo, S. Bamboo-like carbon nanotube/Fe3C nanoparticle hybrids and their highly efficient catalysis for oxygen reduction. J. Am. Chem. Soc. 2015, 137, 1436−1439. (30) Zhou, W.; Zhou, J.; Zhou, Y.; Lu, J.; Zhou, K.; Yang, L.; Tang, Z.; Li, L.; Chen, S. N-Doped Carbon-Wrapped Cobalt Nanoparticles on N-Doped Graphene Nanosheets for High-Efficiency Hydrogen Production. Chem. Mater. 2015, 27, 2026−2032. (31) Forsyth, J. B.; Wilkinson, C.; Paster, S.; Wanklyn, B. M. The magnetic structure of cobalt diphosphate Co2P2O7. J. Phys.: Condens. Matter 1989, 1, 169. (32) Qu, L.; Liu, Y.; Baek, J. B.; Dai, L. Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321−1326. (33) Wu, J.; Yang, Z.; Li, X.; Sun, Q.; Jin, C.; Strasser, P.; Yang, R. Phosphorus-doped porous carbons as efficient electrocatalysts for oxygen reduction. J. Mater. Chem. A 2013, 1, 9889−9896. (34) Chen, R.; Wang, H. Y.; Miao, J.; Yang, H.; Liu, B. A flexible high-performance oxygen evolution electrode with three-dimensional NiCo2O4 core-shell nanowires. Nano Energy 2015, 11, 333−340. (35) Ma, T. Y.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. Z. PhosphorusDoped Graphitic Carbon Nitrides Grown In Situ on Carbon-Fiber Paper: Flexible and Reversible Oxygen Electrodes. Angew. Chem., Int. Ed. 2015, 54, 4646−4650.

Research Groups of the National Natural Science Foundation of China (Grant No. 21521004), the Marine high-tech industry development projects of Fujian Province (No. 2015-19), and the Major Projects Science and Technology of Fujian Province (No. 2011YZ0001-1). We would also like to extend our thanks to Professor Bin Ren for his valuable suggestions and Professor John Hodgkiss of The University of Hong Kong for his assistance with English.

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REFERENCES

(1) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of electrocatalysts for oxygen-and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060−2086. (2) Bloor, L. G.; Molina, P. I.; Symes, M. D.; Cronin, L. Low pH Electrolytic Water Splitting Using Earth-Abundant Metastable Catalysts That Self-Assemble in Situ. J. Am. Chem. Soc. 2014, 136, 3304−3311. (3) Chen, D.; Chen, C.; Baiyee, Z. M.; Shao, Z.; Ciucci, F. Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices. Chem. Rev. 2015, 115, 9869−9921. (4) Zhang, J.; Xia, Z.; Dai, L. Carbon-based electrocatalysts for advanced energy conversion and storage. Sci. Adv. 2015, 1, 1500564. (5) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Hydrogen evolution by a metal-free electrocatalyst. Nat. Commun. 2014, 5, 3783. (6) Wang, T.; Liu, L.; Zhu, Z.; Papakonstantinou, P.; Hu, J.; Liu, H.; Li, M. Enhanced electrocatalytic activity for hydrogen evolution reaction from self-assembled monodispersed molybdenum sulfide nanoparticles on an Au electrode. Energy Environ. Sci. 2013, 6, 625− 633. (7) Long, X.; Li, J.; Xiao, S.; Yan, K.; Wang, Z.; Chen, H.; Yang, S. A Strongly Coupled Graphene and FeNi Double Hydroxide Hybrid as an Excellent Electrocatalyst for the Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53, 7584−7588. (8) Li, J.; Wang, Y.; Zhou, T.; Zhang, H.; Sun, X.; Tang, J.; Zhang, L.; Al-Enizi, A. M.; Yang, Z.; Zheng, G. Nanoparticle Superlattices as Efficient Bifunctional Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14305−14312. (9) Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G. Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science 2011, 334, 645−648. (10) Jiang, N.; You, B.; Sheng, M.; Sun, Y. Electrodeposited CobaltPhosphorous-Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 6251−6254. (11) Wang, H.; Lee, H. W.; Deng, Y.; Lu, Z.; Hsu, P. C.; Liu, Y.; Lin, D.; Cui, Y. Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting. Nat. Commun. 2015, 6, 7261. (12) Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. In situ Cobalt-Cobalt Oxide/N-Doped Carbon Hybrids As Superior Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 2688−2694. (13) Yang, Y.; Fei, H.; Ruan, G.; Tour, J. M. Porous Cobalt-Based Thin Film as a Bifunctional Catalyst for Hydrogen Generation and Oxygen Generation. Adv. Mater. 2015, 27, 3175−3180. (14) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-doped carbon nanomaterials as non-metal electrocatalysts for water oxidation. Nat. Commun. 2013, 4, 2390. (15) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Porous C3N4 Nanolayers@N-Graphene Films as Catalyst Electrodes for Highly Efficient Hydrogen Evolution. ACS Nano 2015, 9, 931−940. (16) Ge, J. M.; Zhang, B.; Lv, L. B.; Wang, H. H.; Ye, T. N.; Wei, X.; Su, J.; Wang, K. X.; Li, X. H.; Chen, J. S. Constructing holey graphene monoliths via supramolecular assembly: Enriching nitrogen heteroatoms up to the theoretical limit for hydrogen evolution reaction. Nano Energy 2015, 15, 567−575. H


Research Article

ACS Sustainable Chemistry & Engineering (36) Zheng, Y.; Jiao, Y.; Li, L. H.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Z. Toward Design of Synergistically Active Carbon-Based Catalysts for Electrocatalytic Hydrogen Evolution. ACS Nano 2014, 8, 5290−5296. (37) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat. Nanotechnol. 2015, 10, 444−452.

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