Mn Doping of CoP Nanosheets Array: An Efficient Electrocatalyst for

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Mn-Co-P Nanosheets Array: An Efficient Electrocatalyst for Hydrogen Evolution Reaction with Enhanced Activity at All pH Values Tingting Liu, Xiao Ma, Danni Liu, Shuai Hao, Gu Du, Yongjun Ma, Abdullah M. Asiri, Xuping Sun, and Liang Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02849 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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Mn Doping of CoP Nanosheets Array: An Efficient Electrocatalyst for Hydrogen Evolution Reaction with Enhanced Activity at All pH Values Tingting Liu,† Xiao Ma,† Danni Liu,† Shuai Hao,† Gu Du,‡ Yongjun Ma,§ Abdullah M. Asiri,ǁ Xuping Sun,†,* and Liang Chenʃ,* †

College of Chemistry, Sichuan University, Chengdu 610064, China, ‡Chengdu Institute of Geology and Mineral Resources, Chengdu 610064, China, §Analytical and Test Center, Southwest University of Science and Technology, Mianyang 621010, China, ǁChemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia, and ʃNingbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

ABSTRACT: Heteratom doping is a possible way to tune the hydrogen evolution reaction (HER) catalytic capability of electrocatalysts. In this work, we report the first development of Mn-doped CoP (Mn-Co-P) nanosheets array on Ti mesh (Mn-Co-P/Ti) as an efficient 3D HER electrocatalyst with good stability at all pH values. Electrochemical tests demonstrate that Mn doping leads to enhanced catalytic activity of CoP. In 0.5 M H2SO4, this Mn-Co-P/Ti catalyst drives 10 mA cm-2 at overpotential of 49 mV, which is 32 mV less than that for CoP/Ti. To achieve the same current density, it demands overpotentials of 76 and 86 mV in 1.0 M KOH and phosphate buffer saline, respectively. The enhanced HER activity for Mn-Co-P can be attributed to its more thermo-neutral hydrogen adsorption free energy than CoP, which is supported by density functional theory calculations. Keywords: Mn doping; CoP; hydrogen evolution reaction; electrocatalysts; all pH values

Increased concern over environmental pollution has triggered an urgent need for searching clean and sustainable energy carriers alternative to fossil fuels. Hydrogen is considered as an ideal such candidate featuring high energy density and zero emission of greenhouse gas.1,2 Hydrogen can be produced on a large scale by water electrolysis, but an efficient electrocatalyst for the hydrogen evolution reaction (HER) is the prerequisite for affording high cathodic current at low overpotential.3,4 Pt as the most active catalyst in acid suffers from scarcity and high cost, which limits its wide use in proton exchange membrane-based water electrolysis units operating under strongly acidic conditions.5 Microbial and alkaline electrolysis cells however require HER catalysts working in neutral6 and alkaline7 media, respectively. Therefore, it is of high importance to develop earth-abundant HER catalysts operating efficiently over the whole pH ranges. The electrochemical performance of a catalyst is strongly related to its electrical conductivity. Transition metal phosphides (TMPs), formed by the alloying of metal and phosphorus, have metalloid characteristics with good electrical conductivity,8 which is a good point for electrochemical applications. Although TMPs have been intensively utilized in hydrodesulfurization and hydrodenitrogenation catalysis as well as Li-ion batteries,9,10 their wide applications for hydrogen evolution electrocatalysis have come into attention only recently.11-13 Among such TMPs, Co phosphides have received more research interest for their good catalytic activity and durability.14-21 Recent work demonstrated enhanced HER activity for CoP after the introduction of electrochemically

active Fe.22-24 While CoMnP appears active for oxygen evolution, the electrochemical performances of such material toward the HER remains unclear.25 In this study, we present our recent finding that Mn-doped CoP nanosheets array on Ti mesh (Mn-Co-P/Ti) behaves as an efficient and stable 3D HER electrocatalyst over the wide range of pH 0-14, with superior activity to CoP/Ti. Overpotentials of 49, 76, and 86 mV mV are demanded for Mn-Co-P/Ti to approach 10 mA cm-2 in 0.5 M H2SO4, 1.0 M KOH, and 1.0 M phosphate buffer saline (PBS), respectively. Density functional theory (DFT) calculations reveal that Mn doping weakens the interactions between Co and H atoms, leading to more thermo-neutral hydrogen adsorption free energy (∆GH*). Mn-Co-P/Ti was converted from its hydroxide precursor electrodeposited on Ti mesh (Scheme S1). After electrodeposition, the color of Ti mesh turned from gray to bluish-green and further changed to black after phosphidation (Figure S1). The X-ray powder diffraction (XRD) pattern for the precursor (Figure S2) shows diffraction peaks at 19.05, 32.47, 37.91, 38.66, 51.35, and 57.91° indexed to the (001), (100), (101), (002), (102), and (110) planes of brucite-like βCo(OH)2 (JCPDS No. 30-0443, space group: P-3m1, ao = 3.183 Å, co = 4.652 Å), respectively.26-30 After phosphidation, the product shows diffraction peaks characteristic of orthorhombic CoP phase (JCPDS No. 29-0497, space group: Pnma, ao = 5.077 Å, co = 5.587 Å) (Figure 1a).31-33 Inductively coupled plasma atom emission spectrometry (ICP-AES) analysis suggests that the atomic ratio of Mn:Co:P is

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0.08:0.92:1. Thus, Mn replacing Co does not alter the crystal structure of orthorhombic CoP due to their very similar atomic sizes. Figure 1b shows the scanning electron microscopy (SEM) images of bare macroporous Ti mesh with rough surface. SEM images for Ti mesh after electrodeposition (Figure 1c) demonstrate that it is fully covered with hydroxide nanosheets array. Note that the phosphided product still retains the original morphology, as shown in Figure 1d. The scanning transmission electron microscopy (STEM) image and the corresponding energy-dispersive X-ray (EDX) elemental mapping images further illustrate that Mn, Co, and P elements are uniformly distributed throughout the product (Figure 1e). The TEM image of the resulting Mn-Co-P confirms its nanosheet nature (Figure 1f). Figure 1g displays the highresolution TEM (HRTEM) image taken from such nanosheet and the well-resolved lattice fringes with interplanar distances of 2.79 and 2.44 Å can be indexed to the (002) and (102) planes of CoP, respectively. The selective area electron diffraction (SAED) image (Figure 1h) shows well-defined rings indexed to the (112), (200), (011), and (101) planes of orthorhombic CoP. All these results suggest the formation of Mn-doped CoP nanosheets array (8% Mn doping ratio) on Ti mesh. CoP/Ti was also made but without using Mn salt for precursor preparation (Figure S3).

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respectively.36 Figure 2c shows the Co 2p spectrum. The Co 2p spectrum ranging from 792.9 to 809.3 eV and from 776.1 to 790.4 eV are associated with Co 2p1/2 and Co 2p3/2 regions, respectively.37,38 The intense peaks for Co 2p3/2 are situated at 778.2 and 781.3 eV, suggesting the co-existence of Co2+ and Co3+.39,40 The Co 2p1/2 region shows one peak at 798.9 eV accompanied with satellite peak at 802.6 eV.41 The P 2p region (Figure 2d) shows two peaks at 129.4 and 131.1 eV corresponding to the binding energies (BEs) of P 2p3/2 and P 2p1/2, respectively.42,43 The peak at 129.4 eV can be assigned to cobalt phosphide while the peak at 132.1 eV arises from Mn doping.35,44 And the relatively broader peak at 134.5 eV could be assigned to POx or P-O species42-44 arising from surface oxidation.45

Figure 2. (a) XPS survey spectrum for Mn-Co-P. XPS spectra in the (b) Mn 2p, (c) Co 2p, and (d) P 2p regions for Mn-Co-P.

Figure 1. (a) XRD pattern for Mn-Co-P. SEM images for (b) bare Ti mesh, (c) electrodeposited hydroxide precursor/Ti and (d) MnCo-P/Ti. (e) STEM image and EDX elemental mapping of Mn, Co, and P for Mn-Co-P. (f) TEM image of Mn-Co-P film. (g) HRTEM image and (h) SAED pattern taken from the Mn-Co-P nanosheet.

As shown in Figure 2a, the X-ray photoelectron spectroscopy (XPS) spectrum of Mn-Co-P suggests the existence of Mn, Co, and P elements and signals of C and O elements may arise from contamination/surface oxidation of the product.34 The Mn 2p3/2 spectrum (Figure 2b) exhibits a peak at 641.3 eV, which can be assigned to oxidized Mn species such as MnO or Mn2O3 arising from surface oxide.35 The broadening at 644.0 eV is assigned to Mn 2p3/2 energy losses. The Mn 2p1/2 core level region shows two main peaks at 654.8 and 655.9 eV corresponding to Mn4+ and Mn3+,

Electrocatalytic activity of the Mn-Co-P/Ti (Mn-Co-P loading: 5.61 mg cm-2) electrode toward the HER was evaluated by linear scan voltammetry (LSV) using a threeelectrode system in 0.5 M H2SO4 solution with a scan rate of 5 mV s-1. For comparison, commercial Pt/C (10% Pt) on Ti mesh, CoP/Ti, and bare Ti mesh were also tested. Note that all as-measured reaction currents presented were corrected against the effect of ohmic resistance.46 Figure 3a shows the polarization curves on the reversible hydrogen electrode (RHE) scale. The Pt/C is highly active for HER with onset potential of almost 0 mV while bare Ti mesh has little HER activity. Note that Mn-Co-P/Ti exhibits high catalytic activity toward the HER with the need of overpotential of only 49 mV to attain 10 mA cm-2, which is 32 mV less than that for CoP/Ti. This overpotential compares favorably to the behaviors of CoP nanowire/CC (67 mV),14 CoP/Ti (90 mV),17 CoP CPHs (133 mV),31 CoP2/RGO (70 mV),34 np-CoP NWs/Ti (78 mV),47 Co2P NWs (~ 100 mV),48 Co2P nanorods (134 mV),49 CoP NTs (129 mV),50 WO2-carbon nanowires (58 mV),51 MoS2(1-x)Se2x/NiSe2 (69 mV),52 Ni2P-NRs/Ni (131 mV),53 Au@NC (130 mV),54 MoP@NC (~130 mV),55 and Mo2C QDs/NGCLs (136 mV).56 And it also surpasses other Co-based TMPs catalysts (Table S1). Figure 3b shows the Tafel plots for Mn-Co-P/Ti, CoP/Ti and Pt/C on Ti mesh. The Tafel slope of 55 mV dec-1 for Mn-Co-P/Ti is lower than that

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for CoP/Ti (60 mV dec-1), suggesting that the HER occurs on both electrodes through a Volmer-Heyrovsky mechanism57,58 with a faster kinetics on Mn-Co-P/Ti.

Figure 3. (a) Polarization curves for Mn-Co-P/Ti, bare Ti mesh, CoP/Ti, and Pt/C on Ti mesh in 0.5 M H2SO4 with a scan rate of 5 mV s-1. (b) Tafel plots for Mn-Co-P/Ti, CoP/Ti, and Pt/C on Ti mesh. (c) The capacitive current densities at +0.20 V as a function of scan rate for Mn-Co-P/Ti and CoP/Ti (△j = ja – jc). (d) Nyquist plots of Mn-Co-P/Ti and CoP/Ti. (e) Polarization curves for MnCo-P/Ti before and after 1000 CV cycles. (f) Time-dependent current density curve of Mn-Co-P/Ti under static overpotential of − 72 mV for 10 h (without iR correction).

The increased activity could be attributed to the larger electrochemical active surface area (ECSA). One approach to estimate ECSA is to measure the double layer capacitance (Cdl).59,60 Based on the cyclic voltammograms (CVs) (Figure S4) in the region of +0.16 and +0.24 V (vs. Ag/AgCl), where the current response are only due to the charging of the double layer, the capacitances of Mn-Co-P/Ti and CoP/Ti are calculated to be 50.32 and 12.30 mF cm-2, respectively, revealing a higher surface area and hence more catalytic active sites for Mn-Co-P/Ti (Figure 3c). Electrochemical impedance spectroscopy (EIS) measurements (Figure 3d) show that the Mn-Co-P/Ti has a smaller polarization resistance than that of CoP/Ti, suggesting enhanced electron transfer rate and faster catalytic kinetics of Mn-Co-P/Ti.61 Figure S5 shows a multistep potential polarization curve for Mn-Co-P/Ti. The current density immediately levels off at 5.6 mA cm-2 at the start current value and remains unchanged for the rest 1000 s and the other steps also show similar results, implying the good mechanical robustness, conductivity and mass transportation of Mn-Co-P/Ti electrode. The strong durability is another key factor to evaluate the performance of electrocatalysts. We thus probed durability of the Mn-Co-P/Ti electrode by conducting continuous cyclic voltammetry (CV) scanning between +0.16 V and -0.22 V vs. RHE with a scan rate of 100 mV s-1 in 0.5 M H2SO4 at 298 K. After 1000 cycles, the Mn-Co-P/Ti still

performs efficiently with small loss of cathodic current density (Figure 3e). Also, the chronoamperometric test of this electrode by electrolysis at a fixed overpotential of 72 mV also suggests its good durability (Figure 3f). Further SEM, XPS and XRD analyses for Mn-Co-P/Ti after HER electrolysis demonstrate that this catalyst is still Mn-Co-P in nature with the preservation of its nanoarray feature, as shown in Figure S6-S8.

Figure 4. (a) Top and (b) side view Mn-Co-P(101) with two Mn atoms replacing the sub-surface Co atoms. Purple, blue and light gray balls represent Mn, Co and P atoms, respectively. (c) Free energy diagram for HER on pristine CoP(101), Pt, and Mn-CoP(101) with 8.3% Mn doping.

To further understand the influence of the doped Mn atoms on the catalytic HER activity, we applied DFT to calculate the ∆GH*, which is a key descriptor for the HER performance. It is well accepted that an optimal HER catalyst should have a ∆GH* close to zero.62 Pt as the most active HER catalyst has a ∆GH* of approximately -0.09 eV. In our calculations, we first evaluated the possibility of doping Mn into the CoP lattice by calculating the formation energy defined as: Ef (Ef = ECoP - µCo + µMn - EMnCoP). It is found that the replacement of 8.3% Co by Mn is energetically favorable as the calculated formation energy is 1.12 eV. Note that the doped Mn atoms prefer the replacement of sub-surface Co atoms (Figure 4a and 4b). The ∆GH* of pristine CoP(101) surface is found to be -0.14 eV, which is in good agreement with a recent study.22 As shown in the free energy diagram in Figure 4c, ∆GH* was slightly changed to -0.11 eV when Mn atoms are doped into the favorable sub-surface. The H atoms prefer to be adsorbed on the Co-Co bridge sites. Clearly, the catalytic activity of HER is enhanced since the ∆GH* moves closer to thermoneutral. The further analysis of electronic structures indicates that the doped Mn atoms can donate electrons to nearby Co and P atoms. According to the Bader charge analysis, the adjacent Co atoms obtain 0.38 electrons from Mn atoms.63 Correspondingly, the interaction between Co and H atoms is weakened64,65 and the value of ∆GH* increases to be much closer to zero. Note that our calculations are based on the

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replacement of one out of 12 Co atoms by Mn atom, which is very close to the 8% Mn doping ratio in the experiments.

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) than CoP/Ti (89 mV dec-1), as shown in Figure 5d. Its stability of operating under neutral solution was also tested. As observed, little loss in current density occurs after 1000 continuous CV cycles and its activity can be maintained for at least 10 h (Figure 5f).

In conclusion, Mn doping of CoP has been proven to be an effective way to improve the HER activity over the whole pH range of 0-14. To drive 10 mA cm-2, Mn-Co-P nanoarray integrated Ti mesh requires overpotentials of 49, 76, and 86 mV in 0.5 M H2SO4, 1.0 M KOH, and 1.0 M PBS, respectively, with good durability. DFT calculations reveal that the enhanced HER activity of CoP is associated with the weakened interactions between Co and H atoms and a more thermo-neutral ∆GH* after Mn doping. This work points out new directions in the design and development of Mn-doped TMPs nanoarrays as attractive catalyst materials for applications.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (X.S.); [email protected] (L.C.)

Notes The authors declare no competing financial interest. Figure 5. Polarization curves of Mn-Co-P/Ti (I), Pt/C on Ti mesh (II), CoP/Ti (III), hydroxide precursor/Ti (IV), and bare Ti mesh (V) in 1.0 M (a) KOH and (b) PBS with a scan rate of 5 mV s-1. Tafel plots of Mn-Co-P/Ti, Pt/C on Ti mesh, CoP/Ti, and hydroxide precursor/Ti in 1.0 M (c) KOH and (d) PBS. Polarization curves of Mn-Co-P/Ti before and after 1000 CV cycles and chronoamperometric curves of Mn-Co-P/Ti under static overpotential in 1.0 M (e) KOH and (f) PBS (without iR correction).

The HER performances for Mn-Co-P/Ti, Pt/C, CoP/Ti, hydroxide precursor/Ti, and bare Ti mesh were also tested in 1.0 M KOH. Figure 5a shows the polarization curves. It is obvious that bare Ti mesh has poor catalytic activity toward the HER and hydroxide precursor/Ti also shows pretty low HER activity. CoP/Ti is highly active for the HER with the need of overpotential of 108 mV to drive 10 mA cm-2. Mn-CoP/Ti demonstrates superior activity to CoP/Ti and it only requires overpotential of only 76 mV for the same current density. It compares favorably to the behaviors of CoP nanowire/CC (209 mV),14 CoP2/RGO (88 mV),34 np-CoP NWs/Ti (100 mV),46 Co2P NWs (~140 mV),47 Co2P nanorods (~150 mV),48 and some other Co-based phosphides catalysts in 1.0 M KOH (Table S2). Figure 5c shows the Tafel plots of Mn-Co-P/Ti, Pt/C, CoP/Ti, and hydroxide precursor/Ti. Note that the Mn-Co-P/Ti electrode maintains long-term durability for the HER in 1.0 M KOH (Figure 5e). It is of importance to mention that the Mn-Co-P/Ti electrode also performs well in 1.0 M PBS for the HER. It requires overpotential of only 86 mV to drive 10 mA cm-2 (Figure 5b), which much exceeds that of CoP/Ti (152 mV) and those of reported catalysts, including CoP/CC (106 mV),14 Ni3S2/Ni foam (170 mV),66 Co9S8/CC (175 mV),35 WP/CC (200 mV),67 and some other non-noble-metal catalysts in neutral condition (Table S3). Mn-Co-P/Ti shows lower Tafel slope (82 mV dec-

ASSOCIATED CONTENT Supporting Information Experimental section; scheme; optical photograph; XRD patterns; SEM images; CVs; Tables S1−S3; multi-potential curve; XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21575137).

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