Phosphorus-Doped Co3O4 Nanowire Array: A Highly Efficient

Feb 6, 2018 - It is vitally essential to design highly efficient and cost-effective bifunctional electrocatalysts toward water splitting. Herein, we r...
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Phosphorus-Doped Co3O4 Nanowire Array: A Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting Zhichao Wang, Hongli Liu, Ruixiang Ge, Xiang Ren, Jun Ren, Dongjiang Yang, Lixue Zhang, and Xuping Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03594 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Catalysis

Phosphorus-Doped Co3O4 Nanowire Array: A Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting Zhichao Wang,†,§ Hongli Liu,ʃ Ruixiang Ge,§ Xiang Ren,§ Jun Ren,║ Dongjiang Yang,ʃ Lixue Zhang,†,* and Xuping Sun§,£,* †

College of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, Shandong, China, §College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China, ʃCollaborative Innovation Center for Marine Biomass Fibers Materials and Textiles of Shandong Province, Institute of Marine Biobased Materials, School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, Shandong, China, ║School of Chemical and Environmental Engineering, North University of China, Taiyuan 030051, Shanxi, China, and £Institute of Fundamental and Frontier Science, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China. KEYWORDS: phosphorus-doping, Co3O4 nanoarray, DFT calculation, electrolysis, overall water splitting

ABSTRACT: It is vitally essential to design highly efficient and cost-effective bifunctional electrocatalysts towards water splitting. Herein, we report the development of P-doped Co3O4 nanowire array on nickel foam (P-Co3O4/NF) from Co3O4 nanowire array through low-temperature annealing using NaH2PO2 as the P source. As a 3D catalyst, such P-Co3O4/NF demonstrates superior performance for oxygen evolution reaction with a low overpotential (260 mV at 20 mA cm–2), a small Tafel slope (60 mV dec–1) and a satisfying durability in 1.0 M KOH. Density functional theory calculations indicate that P-Co3O4 has a much smaller reaction free energy value than that of pristine Co3O4 for potential determining step of oxygen evolution reaction. Such P-Co3O4/NF also performs efficiently for hydrogen evolution reaction, and a two-electrode alkaline electrolyzer assembled by P8.6-Co3O4/NF as both anode and cathode needs only 1.63 V to reach 10 mA cm–2 water-splitting current.

Hydrogen is identified as a one of the most important fuels in a new energy economy.1–4 For the clean and large-scale production of hydrogen in future, electrochemical water splitting approach has attracted numerous attention, but it is a thermodynamic uphill reaction5 and highly active catalysts are of prime importance for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) to lower the energy consumption.6–8 Currently, Pt is the most active HER catalyst9 while RuO2 and IrO2 are deemed as the state-of-the-art OER catalysts,10 but the large-scale application of such catalysts is obstructed by the high cost and scarcity. To this end, it is of significant importance to develop low-cost and earth-abundant alternatives.11,12 Compared with HER process at cathode, OER process at anode suffers from a more sluggish electron transfer kinetics because it is a four step electron transfer process, resulting in a high overpotential requirment.13,14 Co3O4 has attracted tremendous interest as an OER catalyst for its attractive catalytic activity and electrochemical durability in alkaline electrolyte.15–17 However, its performance is restricted by its poor electrical conductivity and low active sites exposured.15 The electrocatalytic activity of Co3O4 depends mainly on its electronic states and surface area.17 The electronic states of Co3O4 can be tuned by heteroatom-doping,18,19 facet control,20 and oxygen vacancies.17,21 Among them, non-metal doping is regarded as an effective approach to optimize the electrochemical performance of Co3O4. Xu et al reported N-doped Co3O4 nanosheets with improved electronic conductivity and OER performance.22 Compared with N, however, the larger atomic size and higher electron-donating ability of P endow it an in-

triguing dopant candidate to enhance the electrochemical performance by modulating the electronic structure of the catalysts.23,24 Moreover, nanoarray structures can facilitate the diffusion of electrolyte and expose more active sites.25,26 However, no attention has been paid to develop P-doped Co3O4 nanoarray for overall water splitting, until very recently Xiao et al. reported that filling the oxygen vacancies in Co3O4 with P by treatment with Ar plasma in the presence of a P precursor leads to enhanced water-splitting activities.27 In this Letter, we demonstrate that P-doped Co3O4 nanowire array on nickel foam (P-Co3O4/NF) can be facilely derived from Co3O4/NF via low-temperature annealing using NaH2PO2 as the P source. Such P-Co3O4/NF shows more remarkable OER activity than pure Co3O4/NF in 1.0 M KOH. Density functional theory (DFT) calculations show that P-Co3O4 has much lower free energy change than that of pure Co3O4 for the potential determining step (PDS) of OER. Besides, the high HER activity for P-Co3O4/NF enables it as a high-efficiency bifunctional electrode for overall water splitting, and for the alkaline electrolyzer using P8.6-Co3O4/NF as both anode and cathode, a stable 20 mA cm–2 water-splitting current can be achieved at around 1.7 V. P-Co3O4/NF was derived from Co3O4/NF by a thermal process under Ar with the presence of NaH2PO2 (see Experimental Section in Supporting Information). By varying the amount of NaH2PO2, the P contents in Px-Co3O4/NF can be tuned, where x is the atomic percentage of P contents measured by inductively coupled plasma mass spectrometry (Table S1). In the following, P8.6-Co3O4/NF was selected as a typical sample for demonstration use. As observed from the X-ray

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diffraction (XRD) patterns (Figure 1a), P8.6-Co3O4/NF presents almost the same diffraction peaks with Co3O4/NF at 31.1°, 36.7°, 59.1°, and 65.0°, which can be assigned to the (220), (311), (511), and (440) lattice planes of Co3O4 (JCPDS no. 421467), respectively. The additional peaks should be attributed to nickel foam,28 and no impurity diffraction peaks are observed. Interestingly, with increasing the doping amounts of P, the diffraction peaks of Px-Co3O4 present slightly negative shifts in a gradual manner compared with Co3O4 (Figure S1), implying the doping of more P into the lattice of Co3O4. Scanning electron microscopy (SEM) images show that NF is completely covered with Co(OH)F (Figure S2) and Co3O4 (Figure 1b) nanowire array. Note that the nanoarray feature is well preserved after P doping (Figure 1c, S3 and S4). The cross-sectional SEM image (Figure S3b) shows that the length of P8.6-Co3O4 nanowire is approximately 5-7 µm. Transmission electron microscopy (TEM) image confirms the nanowire structure of P8.6-Co3O4 (Figure 1d) and shows distinct lattice fringes, and the interplanar distance is measured to be 0.248 nm (Figure 1e), which should be assigned to the (311) plane of Co3O4 phase. The corresponding selective area electron diffraction pattern (Figure S5) results further confirm that P8.6Co3O4 maintains the pristine Co3O4 crystalline after P doping. The homogenous distribution of Co, P, and O elements in the catalyst can be validated by the energy-dispersive X-ray spectroscopy (EDX) analysis (Figure 1f).

and S8) due to inevitable air contact,31 the peaks of P 2p1/2 (130.0 eV) and P 2p3/2 (129.1 eV), corresponding to the combination of P and Co in Px-Co3O4 samples with various Pdoping amount, could be clearly observed (Figure S6d and S8).32,33 These results demonstrate that P is successfully doped into Co3O4, leading to the decrease of electron density around Co as well as the increase of Co3+ numbers, which can generate enriched empty d orbitals to promote the adsorption of OH group and H atom.34 Such strengthened OH adsorption and H binding are conducive to improve the kinetics of OH adsorption and Volmer step in HER process, and thus decrease the Tafel slopes of OER and HER processes on P-doped Co3O4, respectively.35

Figure 2. (a) LSV polarization curves of Co3O4/NF, P8.6Co3O4/NF, RuO2/NF, and bare NF. (b) Tafel plots for Co3O4/NF, P8.6-Co3O4/NF, and RuO2/NF. (c) The chronopotentiometric responses of P8.6-Co3O4/NF at different current densities (50 to 500 mA cm–2 with an increment of 50 mA cm–2). (d) The chronoamperometric curve of P8.6-Co3O4/NF at 1.53 V vs. RHE.

Figure 1. (a) XRD patterns of Co3O4/NF and P8.6-Co3O4/NF. SEM images of (b) Co3O4/NF and (c) P8.6-Co3O4/NF. (d) and (e) TEM images of P8.6-Co3O4/NF nanowire and (f) EDX mapping images of P8.6-Co3O4/NF.

X-ray photoelectron spectroscopy (XPS) survey pattern of P8.6-Co3O4 shows an obvious P 2p signal after the doping of P element (Figure S6a). In the Co 2p region of P8.6-Co3O4 (Figure S6b), the peaks located at 796.8 and 781.0 eV are assigned to Co 2p1/2 and Co 2p3/2, respectively. The peaks located at 780.9 and 796.7 eV should be ascribed to the Co3+, and the peaks at 783.2 and 798.7 eV with satellites (abbreviated as “Sat.”) should be assigned to the Co2+, which indicates the coexistence of the Co2+ and Co3+.29 Note that the Co 2p peaks for all Px-Co3O4 samples exhibit higher binding energies compared with pristine Co3O4 and the increased Co3+/Co2+ ratios (Figure S7), implying the decrease of electron density of Co species.30 In the O 1s region of XPS spectra for Co3O4 and P8.6-Co3O4 (Figure S6c), the fitted peaks at 530.1, 531.4, and 532.6 eV are attributed to lattice O from metal-oxygen bonds, adsorbed O from surface hydroxy and/or adsorbed oxygen species and O from the surface of absorbed H2O for P8.6Co3O4, respectively.17 The lattice O in P8.6-Co3O4 shows much lower intensity than that of Co3O4, indicating the partial replacement of lattice O by P atom in Co-O.19 Despite the partial oxidation evidenced by the peak of P-O (133.8 eV, Figure 6d

Linear sweep voltammetry (LSV) was firstly adopted to investigate the OER activities of Px-Co3O4/NF and other catalysts at a scan rate of 2 mV s–1 in a conventional threeelectrode setup, and the initial data are calibrated to eliminate the ohm effect on the intrinsic behavior of the catalyst.36 Although the OER activity of P8.6-Co3O4/NF is not comparable with that of RuO2 on NF (RuO2/NF), it only requires an overpotential of 260 mV to reach 20 mA cm–2, meanwhile the overpotential of Co3O4/NF is 315 mV, clearly indicating the enhancement of OER activity of Co3O4 by P doping (Figure 2a). The observed OER activity of P8.6-Co3O4/NF outperforms many other Co-based OER electrocatalysts, such as Ar-plasma engraved Co3O4 (η = 300 mV at 10 mA cm–2),17 CoOx@CN (η = 260 mV at 10 mA cm–2),37 CoP NR/C (η = 320 mV at 10 mA cm–2),38 and CoN (η = 290 mV at 10 mA cm–2).39 The Tafel slope of P8.6-Co3O4/NF is 60 mV dec–1, clearly lower than those for RuO2/NF and pure Co3O4/NF, suggesting the favorable reaction kinetics for OER on P8.6-Co3O4/NF electrode (Figure 2b).28 It is clear that all P-doped Co3O4/NF electrodes present enhanced OER activities compared with Co3O4/NF and P8.6-Co3O4/NF shows the best activity among all PxCo3O4/NF samples (Figure S9). To elucidate the intrinsic enhancement of OER performance for P-doped Co3O4/NF, the recorded LSV curves were normalized by the BrunauerEmmett-Teller (BET) surface areas of the electrocatalysts. As observed, P8.6-Co3O4/NF shows lower onset potential and higher OER activity than Co3O4/NF, suggesting the intrinsic

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ACS Catalysis enhancement of catalytic activity caused by the P doping into Co3O4 rather than the increased surface areas (Figure S10 and S11).40 Figure 2c presents the multi-current steps chronopotentiometric curve of P8.6-Co3O4/NF. The potential rapidly stabilizes at 0.62 V vs. Hg/HgO, and remains steady for the following 500 s. The consequent chronopotentiometric processes also present analogous response, reflecting the excellent mass transportation property of P8.6-Co3O4/NF electrode.41,42 The durability of the P8.6-Co3O4/NF electrode was examined after 1000 cycles of accelerated cyclic voltammetry scanning. As seen, the polarization curves almost keep constant before and after the cyclic voltammetry scanning (Figure S12), and corresponding SEM images demonstrate that P8.6-Co3O4 still retains its nanowire morphology after the measurement (Figure S13). Besides, the chronoamperometry curve of P8.6-Co3O4/NF at 1.53 V vs. RHE suggests that the developed electrode could keep its electrocatalytic activity in a long-term test with no obvious phase change (Figure 2d and S14). Whereas, inductively coupled plasma mass spectrometry analysis shows the atomic percentage of P species in the tested catalyst is about 2.21%, implying the loss of P after long-term anodic electrolysis (Table S2). XPS analysis further demonstrates the increased peak intensities of Co3+ and P-Ox in Co 2p and P 2p regions (Figure S15), implying partial oxidation/hydroxylation of the sample after long-term OER test. The turnover frequency (TOF) at a fixed overpotential was then calculated to further evaluate the intrinsic OER catalytic activities of P8.6-Co3O4/NF and Co3O4/NF.43 As to the electrochemical cyclic voltammetry scanning of Co3O4/NF (Figure S16a) and P8.6-Co3O4/NF (Figure S16b), a linear relationship is observed between the oxidation peak currents and the scan rates (Figure S16c), and the corresponding slope can be used to derive the number of active sites.44 The TOF values normalized by the BET surface area of P8.6-Co3O4/NF at the overpotential of 350 mV is 2.03×10–4 s–1, which is almost 3-fold larger than that of Co3O4/NF (0.70×10–4 s–1) (Figure S16d). To better elucidate the enhancement of OER performance for PCo3O4/NF, the electrochemical active surface areas (ECSAs) are compared via measuring the electrochemical double-layer capacitances (Cdl) of Co3O4/NF and Px-Co3O4/NF (Figure S17).45 The Cdl of P8.6-Co3O4/NF was calculated to be 60.54 mF cm–2, which is higher than that of Co3O4/NF (3.15 mF cm– 2 ), P2.1-Co3O4/NF (4.64 mF cm–2), P6.0-Co3O4/NF (17.25 mF cm–2), and P11.5-Co3O4/NF (26.64 mF cm–2) (Figure S17f), indicating that Px-Co3O4/NF have larger active surface areas than Co3O4/NF.25 We also performed the electrochemical impedance measurements for Co3O4/NF and P8.6-Co3O4/NF to further investigate the reason for the enhancement by P doping. As shown in Figure S18, the smaller semi-circular of P8.6Co3O4/NF than Co3O4/NF indicates a lower charge transfer resistance and faster catalytic kinetics on P8.6-Co3O4/NF.46 Hence, P doping can efficiently reduce the charge transfer resistance and improve the conductivity of P-Co3O4/NF, and thus enhance the OER activity.

Figure 3. Structural diagrams of (a) Co3O4 and (b) P-Co3O4. Blue, red, white, and pink spheres represent Co, O, H, and P atoms, respectively. Free energy diagrams for oxygen evolution reaction on (c) Co3O4 and (d) P-Co3O4.

To verify the effects of P doping theoretically, DFT calculations were conducted to investigate the characteristic structural features and the adsorption Gibbs free energy (∆G) profiles of pure Co3O4 and P-Co3O4 by constructing the correlative theoretical models (Figure 3a, 3b and S19). In alkaline environment, it is accepted that the overall OER process at anode could be described by the following four-step associative mechanism:47 OH– + * → OH* + e– (1) OH* + OH– → O* + H2O + e– (2) O* + OH– → OOH* + e– (3) OOH* + OH– → O2 + H2O + e– (4) where * and M* represent the active site and the adsorbed intermediate on the surface, respectively, and the theoretical structures with intermediates adsorbed during OER process were shown in Figure S20 for comparison. In this scenario, the ∆G values of different steps for OH*, O*, and OOH* were calculated to illustrate the origin of catalytic activities (Table S3). The theoretical onset overpotentials of OER on both Co3O4 (110) and P-Co3O4 (110) are determined by the equation (ηOER = max[∆G1, ∆G2, ∆G3, ∆G4]/e – 1.23 [V]), where ∆Gn (n = 14) is the free energy change along the OER processes. For the overall OER reaction, one step displays the largest ∆Gn is referred as the PDS.47 The free energy profiles of OER on Co3O4 show that the formation of O* should be regarded as the PDS (Figure 3c), and its free energy change is 1.87 eV, corresponding to a theoretical overpotential of 0.64 V. However, at PCo3O4 (110), the PDS for OER should be the formation of O2 molecule from OOH* (Figure 3d), and the free energy change is 1.63 eV, corresponding to an overpotential of 0.40 V. Therefore, DFT calculations agree well with the experimental observations and validate the positive effect of P doping into Co3O4 for catalyzing OER.

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Figure 4. (a) LSV curves of NF, Co3O4/NF, P8.6-Co3O4/NF, and Pt/C on NF towards HER at 2 mV s–1 in 1.0 M KOH. (b) LSV curves of P8.6-Co3O4/NF before and after 1000 cyclic voltammetry scanning, and the chronoamperometric curve at overpotential of 140 mV. (c) Polarization curves of Co3O4/NF||Co3O4/NF, P8.6Co3O4/NF||P8.6-Co3O4/NF, and RuO2/NF||Pt/C for water splitting in 1.0 M KOH electrolyte at 2 mV s–1 (without iR correction). (d) The chronopotentiometric curve of water electrolysis for P8.6Co3O4/NF||P8.6-Co3O4/NF-based electrolyzer at 20 mA cm–2.

Attractively, the developed Px-Co3O4/NF also exhibit high activity towards HER. Figure 4a shows the LSV curves of NF, Co3O4/NF, P8.6-Co3O4/NF, and Pt/C deposited on NF toward HER. Pt/C shows the best activity toward HER among them. P8.6-Co3O4/NF exhibits much superior HER performance over bare NF and Co3O4/NF, and it only demands an overpotential as low as 97 mV to reach 10 mA cm–2, meanwhile the overpotentials are 231 and 165 mV for NF and Co3O4/NF, respectively. Figure S21 shows that the Tafel slope of P8.6-Co3O4/NF is 86 mV dec–1, which is much smaller than that of Co3O4/NF (129 mV dec–1), implying a faster HER kinetics on P8.6Co3O4/NF. It is found that all P-doped Co3O4/NF electrodes present enhanced HER activities compared with Co3O4/NF electrode, and the HER activities increased with increasing the P contents (Figure S22). In addition, the superior HER catalytic activity of P8.6-Co3O4/NF normalized by the BET surface areas further confirms the intrinsic enhancement of HER activity of by P doping (Figure S23). As shown in Figure 4b, the LSV curve shows no obvious fading in the HER activity of P8.6-Co3O4/NF after 1000 continuous cyclic voltammetry scanning and the catalytic activity can be maintained with negligible decrease over 20 h. Although the P content of P8.6Co3O4 decreased slightly after a long-term cathodic electrolysis (Table S2), there are no obvious changes in crystalline phase (Figure S24) and chemical states (Figure S25), indicating the remarkable stability of P8.6-Co3O4/NF during HER. However, the HER catalytic performance of P8.6-Co3O4/NF declined after 25 h anodic electrolysis (Figure S26), implying an inactive HER layer possibly formed at the surface of P8.6Co3O4/NF as previously reported.48 Inspired by the superior activity and durability of P8.6Co3O4/NF towards both OER and HER, we assembled a twoelectrode electrolyzer using P8.6-Co3O4/NF as both electrodes for overall water splitting. This P8.6-Co3O4/NF||P8.6-Co3O4/NF setup delivers 10 mA cm–2 at 1.63 V during water splitting (Figure 4c). Although this value is bigger than that of RuO2/NF||Pt/C-based electrolyzer (1.54 V), it is smaller than the electrotrolyzers based on Co3O4 (1.66 V), CoOx@CN (1.70 V),37 a-CoSe/Ti (1.65 V),49 and NiCo2S4 NA (1.68 V).50 Fur-

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thermore, the long-term performance of this P8.6-Co3O4/NFbased system was probed at 20 mA cm–2, which shows no obvious potential change for a period of 25 h (Figure 4d). In summary, a facile approach to significantly enhance the OER activity of Co3O4 nanoarray by straightforward P-doping has been demonstrated, leading to the modifications of its electronic structure and consequently catalytic performance. The resulting P8.6-Co3O4/NF shows outstanding OER activity in 1.0 M KOH, e.g., an overpotential of 260 mV can bring a current of 20 mA cm–2. DFT calculations show that P-Co3O4 processes much smaller free energy change than that of pure Co3O4 for PDS in the four step reactions of OER, confirming the positive effect of P doping. Notably, in addition to its high HER performance, P8.6-Co3O4/NF is deemed as a bifunctional electrocatalyst for overall water splitting, and the assembled P8.6-Co3O4/NF-based overall water splitting device can reach 10 mA cm–2 at only 1.63 V. This study opens up an exciting new avenue to explore P as an effective dopant to enhance the electrocatalytic performances of metal oxides for future applications.

ASSOCIATED CONTENT Supporting Information Experimental section; XRD patterns; SEM images; selective area electron diffraction pattern; XPS spectra; LSV curves; Tafel plots; N2 sorption isotherms; cyclic voltammograms; plots of capacitive current vs. scan rate; TOF data; plots of oxidation peak current vs. scan rate; Nyquist plots; structural diagrams; Tables S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (L.Z.); [email protected] (X.S.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21775078 and 21575137), Shandong Provincial Natural Science Foundation of China (Grant No. ZR2016JL007 and JQ201713), and Taishan Scholars Program for D. Yang, and Qingdao Municipal Science and Technology Bureau (Grant No. 16-5-1-44-jch)

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Table of Contents

P-doped Co3O4 nanowire array on nickel foam (P-Co3O4/NF) behaves as a robust 3D bifunctional catalyst for oxygen evolution reaction and hydrogen evolution reaction with high activity. Its twoelectrode electrolyzer needs a cell voltage of only 1.63 V for 10 mA cm–2 water-splitting current at room temperature in 1.0 M KOH.

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