Nanoporous CoP3 Nanowire Array: Acid Etching

Jul 30, 2018 - Moreover, it also shows a good durability for at least. 60 h. KEYWORDS: Nanoporous, Nanowire array, Acid etching preparation, Hydrogen ...
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Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Nanoporous CoP3 Nanowire Array: Acid Etching Preparation and Application as a Highly Active Electrocatalyst for the Hydrogen Evolution Reaction in Alkaline Solution Yuyao Ji,† Li Yang,‡ Xiang Ren,‡ Guanwei Cui,§ Xiaoli Xiong,*,† and Xuping Sun*,‡ †

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, 610068 Sichuan, China Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China § College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan, 250014 Shandong, China

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‡

S Supporting Information *

ABSTRACT: Transition-metal phosphides have been intensively and extensively studied as earth-abundant catalysts for effective hydrogen evolution electrocatalysis, but it is highly desired to explore a new strategy to improve the catalytic activity. In this work, a nanoporous CoP3 nanowire array on Ti mesh (np-CoP3/TM) was derived from MnO2− CoP3/TM by acid etching of MnO2 that acts as a poreforming agent. As a non-noble-metal catalyst for the hydrogen evolution reaction, the resulting np-CoP3/TM demonstrates enhanced performance with the need of an overpotential of 76 mV (j = 10 mV cm−2), 45 mV less than that needed by MnO2−CoP3/TM. Moreover, it also shows a good durability for at least 60 h. KEYWORDS: Nanoporous, Nanowire array, Acid etching preparation, Hydrogen evolution reaction, Alkaline solution

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INTRODUCTION

Herein, we show the nanoporous CoP3 nanowire array on Ti mesh (np-CoP3/TM) derived from MnO2−CoP3/TM via an acid etching strategy. The central ideal lies in the fact that MnO2 and CoP3 are different in stability against oxalic acid, and the selective etching of MnO2 as the pore-forming agent produces the nanoporous CoP3 nanowire. The resulting npCoP3/TM shows superior HER activity over MnO2−CoP3/ TM, and it needs an overpotential of 76 mV (j = 10 mV cm−2). Moreover, it also exhibits strong stability to maintain for 60 h for the HER.

Recently, much attention has been focused on finding green and sustainable ways to deal with the environmental problems caused by fossil fuels.1,2 Hydrogen is considered as such an abundant candidate.3,4 Water electrolysis is an attractive carbon-neutral technique for hydrogen production but requires efficient electrocatalyst to achieve high current density at low overpotential for the hydrogen evolution reaction (HER).5−7 The most active Pt catalyst, however, suffers from scarcity, stimulating researchers to search for earth-abundant alternatives.8−10 As a vital class of compounds with metalloid properties, transition-metal phosphides (TMPs) have been widely used for the hydro-desulfurization reaction.11,12 In recent years, considerable research attention has also focused on exploring TMPs as low-cost catalysts for efficient hydrogen evolution electrocatalysis, and among such catalysts, Co phosphides attract a lot of attention because of high activity.7,8,13−18 Both element doping19−21 and surface/interface engineering22 are proven effectively to improve the HER activity of such catalysts. Porous nanostructures have obvious advantages of high surface area,23,24 providing good benefit to improve the electrocatalytic HER performance. It is thus believed that creating nanopores would be a good way to boost the HER activity of Co phosphide catalysts, which, however, has not been explored before. © XXXX American Chemical Society

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RESULTS AND DISCUSSION X-ray diffraction (XRD) patterns for MnO2−CoP3 and npCoP3 scratched down from TM are shown in Figure 1a. The CoP3 presents six peaks at 27.4°, 32.5°, 40.3°, 44.6°, 70.2°, and 76.4° indexed to the (210), (240), (245), (420), (540), and (630) facets of CoP3 (JCPDS 30-0443). The diffraction peaks at 44.7° and 65.7° are indexed to the (360) and (400) lattice plane, respectively. In contrast, np-CoP3 only shows diffraction peaks characteristic of CoP3 (JCPDS 42-1169), suggesting the successful etching of MnO2 by oxalic acid.25 The SEM image shows that the MnO2−CoP3 nanowire arrays are anchored on TM (Figure 1b). Note that the resulting np-CoP3/TM still Received: April 16, 2018 Revised: June 11, 2018 Published: July 30, 2018 A

DOI: 10.1021/acssuschemeng.8b01714 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

of P 2p1/2 and P 2p3/2 appear at 133.7 and 129.3 eV, respectively.28 This shows that the Co has partial positive charge but P partial negative charge, so this implies that the electron density is transferred from the Co to P.29,30 The POx or P−O species at 134.5 eV may be due to air exposure resulting in surface oxidation of CoP3.31 We also examined the electrochemical HER activity for npCoP3/TM (CoP3 loading quality: 1.8 mg cm−2). Bare TM, MnO2−CoP3/TM, and Pt/C on TM were also tested for comparison. As observed, Pt/C on TM shows excellent HER activity, but bare TM shows a negligible property. MnO2− CoP3/TM is also efficient to electrochemically catalyze the HER and needs overpotential of 121 mV (j = 10 mV cm−2). The np-CoP3/TM has superior catalytic activity to MnO2− CoP3/TM and requires overpotential of only 76 mV (j = 10 mV cm−2). Our np-CoP3/TM outperforms reported Co-based HER catalysts like CoP nanowire/CC,8 Co2P nanorods,15 FeCoP/Ti,18 CoP3 NAs/CFP,31 CoOx@CN,32 Co-PP/Au,33 and CoP2/RGO,34 etc. A more detailed comparison is listed in Table S1. As shown in Figure S3, CoP3/TM with a lower CoP3 loading (1.2 mg cm−2) needs a much larger overpotential of 142 mV to afford 10 mA cm−2, suggesting that catalyst loading has a heavy influence on the catalytic performance. Figure 3b

Figure 1. (a) XRD patterns for MnO2−CoP3 and np-CoP3. SEM images of (b) MnO2−CoP3/TM and (c) np-CoP3/TM. TEM images taken from one single nanowire of (d) MnO2−CoP3 and (e) npCoP3. (f) HRTEM image taken from np-CoP3. (g) EDX elemental mapping images of np-CoP3/TM. (h) Barrett−Joyner−Halenda (BJH) pore-size distribution curves and (i) nitrogen adsorption/ desorption isotherm plots of MnO2−CoP3 and np-CoP3.

preserves its nanowire array feature (Figure 1c). A crosssection analysis for np-CoP3/TM shows that the nanoarray is about 1.7 μm in height (Figure S1). Further transmission electron microscopy (TEM) analysis reveals the nanoporous CoP3 nanowire after oxalic acid etching. Figure 1d,e shows that the high-resolution TEM (HRTEM) provides the interplanar distance of 0.404 nm matching with the (220) plane of CoP3 (Figure 1f). The energy-dispersive X-ray (EDX) images (Figure 1g) further show the presence of Co and P elements. As shown from the Brunauer−Emmett−Teller (BET) poresize distribution curves of MnO2−CoP3 and np-CoP3 (Figure 1h), np-CoP3 shows a broad peak centering at 9.6 nm, correlating quite well with TEM observations. From the corresponding nitrogen adsorption/desorption isotherm plots (Figure 1i), the BET surface areas for MnO2−CoP3 and npCoP3 were determined as 67.7 and 206.6 m2 g−1, respectively. As shown in Figure S2, the Fourier transform infrared spectroscopy (FTIR) spectrum for the product only shows peaks characteristic of CoP3,26 suggesting the complete removal of oxalic acid after acid etching. All of these data strongly support the successful formation of MnO2−CoP3derived nanoporous CoP3 nanowires with high surface area after acid etching. Figure 2a shows the two peaks of the Co 2p region for npCoP3.27 Two peaks at 796.7 and 795.2 eV can be indexed to the binding energies (BEs) of Co 2p1/2 and Co 2p3/2. The BEs

Figure 3. (a) Linear sweep voltammetry (LSV) curves of MnO2− CoP3/TM, np-CoP3/TM, Pt/C on TM, and bare TM. (b) Tafel plots of MnO2−CoP3/TM, np-CoP3/TM, and Pt/C on TM. (c) LSV curves of np-CoP3/TM before and after 500 cyclic voltammetry cycles. (d) Chronopotentiometry curve for np-CoP3/TM at overpotential of 86 mV.

shows the Tafel plots of Pt/C on TM (32 mV dec−1), npCoP3/TM (45 mV dec−1), and MnO2−CoP3/TM (50 mV dec−1), respectively. We also probed the oxygen-evolving activity of np-CoP3/TM in 1.0 M KOH. This suggests that the np-CoP3/TM is also active for water oxidation electrocatalysis with the need of overpotential of 336 mV to achieve 10 mA cm−2 (Figure S4a). Using this bifunctional catalyst as both cathode and anode, we constructed a two-electrode water electrolyzer. In 1.0 M KOH, this system requires a cell voltage of 1.68 V (j = 10 mV cm−2) water-splitting current (Figure S4b). We investigated the HER activity of np-CoP3/TM in 1.0 M PBS and 0.1 M KOH, and this catalyst needs overpotentials of 121 and 134 mV (j = 10 mV cm−2), respectively (Figures S5 and S6). After 500 cyclic voltammetry cycles, there are almost no loss changes (Figure 3c), confirming its superior stability. Figure 3d

Figure 2. XPS spectra for np-CoP3 in the (a) Co 2p and (b) P 2p regions. B

DOI: 10.1021/acssuschemeng.8b01714 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering shows that np-CoP3/TM exhibits excellent long-term stability for maintaining 60 h for the HER. SEM (Figure S7), XRD (Figure S8), and XPS (Figure S9) analyses conclude that the catalyst can maintain its nanoarray feature and also is still npCoP3 in nature after HER electrolysis. For an evaluation of the effective surface area of MnO2− CoP3/TM and np-CoP3/TM, the bilayer capacitances of two electrodes at the solid/liquid interface were measured.35 The capacitance for MnO2−CoP3/TM and np-CoP3/TM was estimated as 2.3 and 9.8 mF cm−2, respectively. The electrochemically active surface areas (ECSAs) for MnO2− CoP3/TM and np-CoP3/TM were calculated to be 0.09 and 0.17 m2, respectively (Figure 4c). Compared with MnO2−

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Additional data and figures including SEM image, FTIR spectra, LSV curves, XRD pattern, and XPS spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaoli Xiong: 0000-0002-5407-4350 Xuping Sun: 0000-0002-5326-3838 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21575137).

Figure 4. Cyclic voltammograms (CVs) of (a) MnO2−CoP3/TM and (b) np-CoP3/TM at the different scan rates (from inner to outer 10, 50, 90, 130, 170). (c) Corresponding capacitive currents at 0.09 V vs Ag/AgCl as a function of scan rates for MnO2−CoP3/TM and npCoP3/TM. (d) Nyquist plots.

CoP3/TM, np-CoP3/TM has much larger surface area.35 Nyquist plots (Figure 4d) show a smaller charge-transfer resistance (Rct) of this MnO2−CoP3/TM than that of npCoP3/TM, indicating a better electron-transporting capability.36,37 For an estimation of the intrinsic activity of np-CoP3/ TM, turnover frequency (TOF) normalized by per surface site is evaluated.38 As shown in Figure S10, np-CoP3/TM shows a higher value of 0.13 s−1 than MnO2−CoP3/TM (0.06 s−1) at overpotential of 100 mV, revealing its superior intrinsic activity.

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CONCLUSIONS In summary, selective acid etching is proposed as an effective strategy to fabricate a nanoporous CoP3 nanowire array with superior catalytic activity for the HER. Such a np-CoP3 nanoarray drives 10 mV cm−2 with the need of overpotential of 76 mV. This study is of significance because it not only gives us an attractive catalyst for full water splitting, but also opens up an exciting new opportunity for the rational design of porous TMPs nanostructures for applications.

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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.8b01714. C

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DOI: 10.1021/acssuschemeng.8b01714 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX