Nitrogen-doped cobalt phosphide for enhanced hydrogen evolution

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Nitrogen-doped cobalt phosphide for enhanced hydrogen evolution activity Ling Wang, Haijun Wu, Shibo Xi, Sing Teng Chua, Fenghe Wang, Stephen J. Pennycook, Zhi Gen Yu, Yonghua Du, and Jun Min Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01235 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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ACS Applied Materials & Interfaces

Nitrogen-Doped Cobalt Phosphide for Enhanced Hydrogen Evolution Activity Ling Wang,† Haijun Wu,† Shibo Xi,‡ Sing Teng Chua,† Fenghe Wang,† Stephen J. Pennycook,† Zhi Gen Yu,*& Yonghua Du,*‡ and Junmin Xue*†

†

Department of Materials Science and Engineering, Faculty of Engineering, National

University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore. ‡

Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong

Island, Singapore 627833, Singapore. &

Institute of High Performance Computing, A*STAR, Singapore 138632

Corresponding Authors *Email: [email protected] (Dr. J. Xue) *Email: [email protected] (Dr. Y. Du) *Email: [email protected] (Dr. Z. G. Yu)

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ABSTRACT Development of highly efficient and durable hydrogen evolution reaction (HER) electrocatalysts has a direct impact on water splitting efficiency and cost-effectiveness. In this work, N-doped CoP2 is successfully synthesized for efficient HER in alkaline electrolyte, which needs an overpotential of only 64 mV to drive a current density of 10 mA cm-2, with a small Tafel slope of 47.4 mV dec-1, and excellent stability for 15 h without any performance loss in 1 M KOH. This represents one of the best HER catalysts in the alkaline electrolyte so far. The successful doping of N into CoP2 is confirmed by XPS (X-ray photoelectron spectroscopy), XANES (X-ray absorption near-edge structure) and STEM (scanning transmission electron microscopy) characterizations. It is revealed by first-principle calculation that the partial replacement of P with N not only facilitates electron transfer, but also optimizes the Gibbs free energies of H*, H2O and OH* adsorption on the P active sites, and thus facilitating the HER process. This work highlights that anion modification of transition metal phosphides (TMPs) would be an effective and feasible method to enhance their HER activities and provide new insights for the design of novel HER electrocatalysts. KEYWORDS: N doping, CoP2, porous carbon cloth, Gibbs free energy, enhanced HER activity

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INTRODUCTION Electrolysis of water has been regarded as an effective and clean method to produce H2 due to its merits of high hydrogen production efficiency, high purity product and no emission of polluted gases.1, 2 The hydrogen evolution reaction (HER) could be effectively promoted by noble metal catalysts such as Pt, with low overpotentials, small Tafel slope, and good stability.3 However, the wide use of noble metals for water splitting is primarily limited owing to their scarcity and high cost. As such, development of earth-abundant and low-priced HER electrocatalysts which possess comparable or better performance than noble metals becomes necessary and has attracted intensive interest recently. In the past decades, tremendous efforts have been spent on studying the feasibility of using transition metal compounds as HER catalyst. Through these efforts, a large number of efficient HER catalysts have been reported, including various transition metal sulphides,4-6 phosphides,7-10 borides,11-13 nitrides,14-16 and selenides,17-19 etc. However, it is noted that the majority of the research works are focused on HER performance in the acidic electrolyte, whereas the development of workable HER catalysts in the alkaline electrolyte is far behind. Among these reported HER catalysts, transition metal phosphides (TMPs), including CoP,8, 2024

Ni2P,25, 26 FeP,27, 28 Cu3P,29, 30 and MoP31 have been reported as promising candidates for

HER in alkaline electrolytes. While, most of these reported TMPs presented overpotentials around 100 mV to reach the current density of 10 mA cm-2 in the alkaline electrolyte, leaving substantial room for improvement as compared to those in acidic electrolytes. To enhance the HER activity of TMPs in alkaline electrolyte, intensive efforts have been invested to introduce a second transition metal element to form bimetal phosphides, with an attempt to weaken the H bonds on P active sites with more thermoneutral Gibbs free energy of hydrogen adsorption (ΔGH*), and thus promoting HER activity.32, 33 For example, Mn-doped CoP nanosheet arrays reported by Chen’s group34 exhibited a much lower overpotential of 86 mV in 1 M KOH than

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that of CoP (152 mV). In another reported work of Zn-doped CoP, it was observed that ΔGH* on (101) surface of CoP could be effectively improved from -0.36 to -0.08 eV with embedded Zn, leading to a significant decrease in overpotential from 99 to 67 mV to reach a current density of 10 mA cm-2.35 It has been well recognized that the active sites of TMPs for HER is P,36-39 thus it could be hypothesized that the modification on anion sites would have a more direct effect on HER activity than metal sites. This leads to a new approach of introducing a second anion into TMPs to form dianionic compounds for improved HER activity. For instance, the sulphur doped nickel phosphide reported by Huang’s group40 presented a much lower overpotential of 73 mV to obtain the current density of 10 mA cm-2 than that of the pristine nickel phosphides (220 mV). Other dianionic transition metal compounds like N-doped CoP, palladium phosphorous sulphide and iron phosphorous sulphide also exhibited enhanced HER activities.41-43 Even though intensive research works have adopted this new approach and much improved HER performance has been achieved recently, the role of the second anion in HER performance of TMPs is unclear. Therefore, it is essential to explore the mechanism of the effect of anion modification on HER activities of TMPs, and as such, a comprehensive approach integrating detailed structure characterizations, simulation calculation, and proper property analysis is highly demanded. In this work, by taking CoP2 as the model example, the effect of N doping at P site on HER activity of CoP2 is studied. The N-doped CoP2 was supported at porous carbon cloth (PCC) substrate through facile hydrothermal process and pyrolysis process. The obtained N-doped CoP2@PCC electrocatalyst showed superior HER activity, which needed an overpotential of only 64 mV to reach the current density of 10 mA cm-2 in 1 M KOH, with a small Tafel slope of 47.4 mV dec-1, and excellent stability for 15 h at a fixed current density of 10 mA cm-2. The above-mentioned results represent one of the top-performing non-precious HER 4


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electrocatalysts in alkaline electrolyte. The successful doping of N into the anion site of CoP2 was confirmed using XPS and XANES spectrum analysis. A structure model of N doped CoP2 was then established and first-principle calculation was carried out to explain the effect of N doping of CoP2 on HER activity. It is revealed that the N doping in anion site could not only facilitate electron transfer but also lower the Gibbs free energy of hydrogen adsorption (ΔGH*) on both Co and P sites, thus resulting in the improved HER activity. As with the deep understanding of the effect of N doping on HER activity of CoP2, this work also has a reference value for other transition metal based electrocatalysts with anion site doping.

EXPERIMENTAL SECTION Materials. Carbon cloth (CC) without microporous layer was purchased from CeTech Company (Taichung, Taiwan). Other chemicals, such as iron chloride hexahydrate (FeCl3·6H2O), sodium nitrate (NaNO3), hydrochloric acid (HCl, 37 wt %), sodium dihydrogen phosphate

dihydrate

(NaH2PO4·2H2O),

sodium

phosphate

dibasic

heptahydrate

(Na2HPO4·7H2O), cobalt chloride hexahydrate (CoCl2·6H2O), ammonium fluoride (NH4F), urea and sodium hypophosphite monohydrate (NaH2PO2·H2O) were bought from SigmaAldrich, and no additional treatment was applied. Fabrication of Porous carbon cloth (PCC). Firstly, Fe2O3 nanoparticles were grown onto CC (denoted as Fe2O3@CC) via hydrothermal method by using the mixture of 0.15 M FeCl3 and 1 M NaNO3 solution at 200 °C for 6 h. The Fe2O3@CC was then calcined at 900 °C for 2 h under N2 atmosphere, after cooling down to room temperature, the Fe3O4@CC was achieved. Lastly, using 2 M HCl to dissolve the iron oxide on CC and porous carbon cloth (PCC) was obtained.

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Preparation of N-doped CoP2 on PCC. Briefly, N-doped CoP2@PCC was prepared through the hydrothermal method and followed with calcination process. In a typical procedure, CoCl2·6H2O (1.189 g), NH4F (0.370 g) and urea (1.5 g) were added into 50 mL deionized water and stirred 30 min to generate a homogeneous solution, then the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave. The obtained PCC (3 x 3 cm2, 0.1092 g) was placed vertically inside a sealed autoclave which was kept at 120 °C for 4 hours. Then the purple-colour Co(OH)2@PCC precursor was obtained after rinsed by deionized water and oven-dried. The mass weight of Co(OH)2@PCC was 0.1582 g. The Co(OH)2@PCC was further subjected to a calcination process and then converted to N-doped CoP2@PCC. Specially, 0.03 g of urea and 0.37 g of NaH2PO2·2H2O were put into a ceramic boat and evenly distributed at the bottom, and a piece of hydrothermally prepared Co(OH)2@PCC was placed into the boat which was separated by a plain carbon cloth. Afterward, the ceramic boat was loaded into a multi-functional tube furnace. The furnace was then purged with N2 and heated to 400 °C with an increasing rate of 5 °C min-1 and kept at 400 °C for 30 min. After that the resultant sample was rinsed with deionized water and ethanol and subsequently dried at 80 °C for 12 hours. In this way, the N-doped CoP2@PCC was obtained with a mass weight of 0.1638g, and the loading amount of N-doped CoP2 on PCC was calculated to be ~6 mg cm-2. Four samples with different mass ratios of urea to NaH2PO2·2H2O during the calcination of Co(OH)2@PCC were also prepared for comparison (sample A(0.1/0.3) with 0.10 g of urea and 0.30 g of NaH2PO2·2H2O, sample B(0.2/0.2) with 0.20 g of urea and 0.20 g of NaH2PO2·2H2O, sample C(0.3/0.1) with 0.30 g of urea and 0.10 g of NaH2PO2·2H2O, and sample D(0.37/0.03) with 0.37 g urea and 0.03 g NaH2PO2·2H2O). Pure CoP2@PCC and CoN@PCC were prepared via a similar calcination process from Co(OH)2@PCC using 0.4 g NaH2PO2·2H2O and 0.4 g urea as precursor, respectively.

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Material characterizations. Scanning electron microscopy (SEM) images were gained through a Zeiss Supra 40 field-emission scanning electron microanalyzer. Transmission electron microscopy (TEM) images were obtained using a JEOL 100CX instrument (200 kV). Scanning transmission electron microscopy (STEM) with energy-dispersed X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) were conducted using JEOL-ARM200F equipped with a cold field-emission gun, a new ASCOR 5th order aberration corrector, Oxford X-max 100TLE SDD EDS and Gatan Quantum ER spectrometer. X-ray diffraction (XRD) tests for structure identification were carried out using Cu K radiation (40 kV, 40 mA). The surface elements states were performed by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD XPS equipped with an Al K X-ray source). All XPS data were calibrated based on the C 1s data at 284.6 eV and fitted by the software package Avantage. Co K-edge X-ray absorption near edge structure (XANES) was conducted at the X-ray Absorption Fine Structure for catalysis (XAFCA) beamline of Singapore Synchrotron Light Source (SSLS) using transition mode. Each sample was scanned for three times, and energy calibration was performed with Co foil. Electrochemical measurements. Hydrogen evolution reaction (HER) activities of the catalysts in 1 M KOH solution were conducted in a standard three-electrode electrochemical cell via a VMP3 electrochemical workstation (Bio logic Inc) at room temperature (25 ± 1 °C). The obtained samples (1 × 1 cm2) were applied directly as the working electrodes, graphite plate and Hg/HgO (1 M KOH) were used as the counter and reference electrode, respectively. For comparison, commercial Pt/C (20 wt%) on graphite paper with the same loading amount of N-doped CoP2@PCC as a benchmark HER electrocatalyst is prepared. 25 mg of Pt/C powder and 20 L of Nafion were dissolved in 1 mL of deionized water/isopropanol alcohol mixture with a molar ratio of 5:1 and sonicated for 1 h. 245 L of this solution was drop-cast 7



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onto graphite paper (1 × 1 cm2) and the solvent was evaporated under 80 °C, yielding a loading amount of 6 mg cm-2. Linear sweep voltammetry (LSV) measurement was performed with a scan rate of 5 mV s-1. All the potentials vs. Hg/HgO were calibrated with a reversible hydrogen electrode (RHE) based on the Nernst equation ERHE = EHg/HgO + 0.059 × pH + E°Hg/HgO, where E°Hg/HgO = 0.098 V at 25 °C, EHg/HgO is the experimental potential that tested over Hg/HgO reference electrode, and ERHE is the calibrated potential. The current densities were normalized over the geometrical area of the catalysts. The polarization curves were corrected by 85% iRcompensation. The overpotential (η) was calculated as η = ERHE - 0. Tafel slope was obtained based on the Tafel equation  a + b𝑙𝑜𝑔 𝑗, where a is Tafel constant, b is the Tafel slope and j is the current density. The long-term durability test was conducted at a fixed current density of 10 mA cm-2. Faradaic efficiency (FE) test. The measurement of H2 generated during the reaction process was performed through a drainage method. A stable reduction current of 20 mA was applied for 2 h. Faradaic efficiency was calculated through the equation FE = 2 × 𝑁𝐻2(𝑚𝑜𝑙) × 𝐹(𝐶 𝑚𝑜𝑙 ―1)/𝑄(𝐶). Where 𝑁𝐻2 was the total amount of hydrogen produced, F is Faradaic constant, and Q (C) was the total amount of charge that passed through the cell. Calculation of active sites. The number of actives sites was obtained through cyclic voltammetry (CV) method which was proposed by Hu and his co-workers.44,

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Cyclic

voltammetry (CV) curves of the samples were firstly measured in phosphate buffer solution (pH = 7) in the potential range of -0.2 V to 0.6 V vs. RHE with a scan rate of 50 mV s-1 to obtain the voltammetric charge (Q). The mole of active sites (n) was then calculated through n = Q/2F, where F is Faradaic constant (96485.3 C mol-1). Turnover frequency (TOF, s-1) was 𝐽𝐴

calculated with the equation TOF = 2𝑛𝐹, where J is the current density, and A is the geometrical

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electrode surface area (1 cm2).44, 45 The mass activity (A g-1) of N-doped CoP2@PCC was calculated by J/m, where m is the loading amount of active materials. Theoretical computation. In this section, first-principle calculations were conducted using the Vienna ab initio simulation package (VASP) according to density functional theory (DFT).46 Projector augmented wave pseudopotentials (PAWs) were applied to model electronion interactions.47 The generalized gradient approximation (GGA) functional of Perdew, Burke and Ernzerhof (PBE) were used to approximate exchange and correlation effects for structural relaxation.48,

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The CoP2 unitcell geometry optimization was calculated using a 8×8×6

Monkhorst-Pack (MP) k-point grid. A plane-wave basis set with a 500 eV cutoff energy. With these setting parameters, a satisfied convergence was achieved. The total energy was converged to 1.0×10-5 eV/atom. The stress on the cell was lower than 0.1 kbar, and the forces on every atom were smaller than 0.01 eV/Å. We cleaved a (110) slab to build the surface models for studying the hydrogen adsorption on the pristine and N-doped CoP2 surfaces. The nearest distance between adatom (H) and its neighboring images in adjacent supercell was ~ 15 Å, and a vacuum separation of 18 Å was used to ensure a negligible interaction between H intermediate and its images in the adjacent supercell along the vacuum direction. The Bader method50 was used to analyze the electronic structure as well as the charge transfer between N and CoP2. To evaluate HER activity of the electrocatalysts, free energy graph of hydrogen evolution at equilibrium (U = 0) was plotted by calculating the hydrogen adsorption free energy (ΔGH*) on various N- doped CoP2 surface sites, which could be used as a critical descriptor to assess the efficiency of the hydrogen evolution process.35 The free energy for hydrogen adsorption (ΔGH*) is calculated based on the equation: ΔGH* = ΔEH + ΔEZPE – TΔSH. Hydrogen chemisorption energy is computed using ΔEH = E(surface + H) – E(surface ) – ½ 𝐸(𝐻2), where E(surface + H)

and E(surface) are the total energies of the surface with one hydrogen atom that adsorbed on 9



the pristine surface, and 𝐸(𝐻2) is the energy of hydrogen gas phase. ΔSH and ΔEZPE are the differences in entropy and zero-point energy between the adsorbed H* and gas phase H2.48 The calculated correction value of ΔEZPE – TΔSH at the temperature (T) of 300 K is 0.19 eV for pristine CoP2 (110) surface and 0.18 eV for N-doped CoP2 (110) surface, respectively. The adsorption free energy of water molecule can be calculated by using ∆𝐺H2O = 𝐸H2O ∗ ―

(𝐸 ∗

+ 𝐸H2O) + ∆𝑍𝑃𝐸 ―𝑇∆𝑆, here 𝐸H2O ∗ , 𝐸 ∗ , and 𝐸H2O are the total energies of CoP2 surface

with one water molecule, clean surface and an isolated water molecule, respectively. Based on the chemical reaction of H2O + 𝑒 ― →H ∗ + OH ― , the adsorption energy of OH* can be

(

1

)

calculated through ∆𝐺OH ∗ = 𝐸OH ∗ ― 𝐸 ∗ + 𝐸H2O ― 2𝐸H2 + ∆𝑍𝑃𝐸 ―𝑇∆𝑆, here 𝐸OH ∗ and 𝐸H2 are the total energies of CoP2 surface with one hydroxyl group adsorption and one hydrogen molecule adsorption, respectively.

RESULTS AND DISCUSSION

Scheme 1. The illustration of the synthesis process of porous carbon cloth and cobalt-based nanoneedles@PCC.

In this work, the preparation procedure of N-doped CoP2@PCC (porous carbon cloth) could be divided into two main steps: (1) fabrication of PCC (porous carbon cloth) and (2) growth of cobalt-based active species at PCC, as illustrated in Scheme 1. The pores on carbon cloth were created by thermal reduction between Fe2O3 nanoparticles and carbon cloth (Fe2O3 + C →

10


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Fe3O4 + CO), followed by acid treatment. Through a hydrothermal process, the surface of carbon cloth was uniformly decorated with Fe2O3 nanoparticles (Figure S1). Figure S2 shows the XRD results of Fe2O3@CC. In the thermal reduction process, carbon cloth reacted with Fe2O3 nanoparticles and numerous crevice-pores were formed on CC surface, as shown in Figure S3. After removal of Fe3O4 nanoparticles by acid treatment, the evenly distributed pores were created, as shown in Figure 1a. In the second step, flower-like Co(OH)2 consisting of numerous nanoneedles were grown at PCC through another hydrothermal process. The morphology and XRD result of Co(OH)2@PCC are shown in Figure S4 and Figure S5, respectively. The N-doped CoP2@PCC was obtained through calcination of Co(OH)2@PCC with NaH2PO2·2H2O and urea precursors. Two control samples of CoP2 and CoN were also synthesized for comparison. The details are available in the experimental section. After calcination, the flower-like morphology was maintained for N-doped CoP2@PCC (Figure 1b), and the corresponding XRD result is shown in Figure S6. The obtained diffraction peaks can match well with CoP2 (JCPDS No. 26-0481). The XRD results of the pure CoN@PCC and CoP2@PCC are presented in Figure S7a (CoN JCPDS No. 16-0116) and 7b (CoP2 JCPDS No. 26-0481), respectively, confirming the successful formation of these two control samples. Figure 1c is the TEM image of the N-doped CoP2, showing the typical nanoneedle shape with a length of 2 m. The inset low-magnification scanning transmission electron microscopy (STEM) annular bright field (ABF) image reveals that the nanoneedles are formed with nanoscale polycrystals. Well-resolved lattice fringe spaces of 0.242, 0.275, 0.252, and 0.248 nm are observed, as shown in Figure 1d and its enlarged images in Figure 1e1-e3, which could be indexed as (-121), (020), (002) and (200) planes of CoP2 (JCPDS No. 26-0481), respectively. Based on the XRD and STEM results, it can be seen that there is no obvious change in the crystal structure of CoP2 after N doping. The local chemical features of the N-doped CoP2 were further studied by aberration-corrected STEM with energy dispersed X-ray spectroscopy (EDS)

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and electron energy-loss spectroscopy (EELS). The STEM-EELS/EDS elemental mappings of N-doped CoP2 in Figure 1f1-f6 exhibit a uniform distribution of Co, N, and P in the whole structure.

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Figure 1. (a) SEM images of PCC. (b) SEM images of N-doped CoP2@PCC. (c) TEM images of N-doped CoP2, with a low-magnification STEM-ABF image inset. (d) High-magnification STEM-ABF image of N-doped CoP2. (e1-e3) Enlarged images from the marked region of (d) showing the individual grains. (f) HAADF-STEM EELS/EDS mapping of N-doped CoP2: (f1f6) STEM high angle annular dark field (HAADF) and its respective EELS/EDS elemental mappings of N-doped CoP2.

The surface chemical states of N-doped CoP2@PCC were studied by XPS, as shown in Figure 2. The wide-scan XPS spectra (Figure 2a) reveal the existence of Co, P, N, C, and O elements. For Co 2p spectra (Figure 2b), the peaks positioned at 778.3 and 780.8 eV belong to Co 2p3/2 species, and the other two peaks at 793.3 and 797.6 eV corresponded to Co 2p1/2 species, while the peaks at 784.2, 787.2 and 802.0 eV were due to satellite shake-up peaks of oxidized Co, respectively.51 The high-resolution image of P 2p region (Figure 2c) displays two peaks at 129.0 and 129.8 eV, belonging to the P 2p3/2 and P 2p1/2 states in CoP2, respectively, with a peak intensity ratio of 2:1. At the binding energy of 130.8 eV, there was a minor peak which was due to the P 2p1/2 from the unreacted phosphorous.52 Moreover, the two peaks at 780.8 and 797.6 eV in Co 2p region, along with one peak at 133.2 eV in P 2p region, were attributed to oxidized Co and phosphorous species from the inescapable superficial oxidation of CoP2 upon exposure to air.53-55 The N 1s peaks for N-doped CoP2 are depicted in Figure 2d, in which, there are three peaks located at 398.4, 400.0 and 401.7 eV, respectively. The main peak at 398.4 eV belong to Co-N bond,56 the peaks appeared at 400.0 and 401.7 eV, corresponding to pyrrolic N bonded with two carbon atoms and graphitic N bonded with three carbon atoms, respectively.57, 58

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Figure 2. XPS results of N-doped CoP2@PCC: (a) full survey scan, (b) Co 2p, (c) P 2p, and (d) N 1s.

To gain deeper insight into the atomic structure of CoP2 with N doping, the sample was further characterized using XANES and extended X-ray absorption fine structure (EXAFS) spectroscopies, by benchmarking against the pristine CoP2 and CoN. As shown in Figure 3a, the Co K-edge XANES spectrum of N-doped CoP2 is almost identical to that of pristine CoP2, with a similar intensity of pre-peaks (between 7706-7715 eV), edge peaks, as well as the peaks in the high incident energy range, demonstrating the similar coordination environment of cobalt species. The coordination environment of Co atoms in cobalt nitride, cobalt phosphide, and Ndoped CoP2 were further confirmed using Fourier transform (FT) of the k3-weighted EXAFS spectra. The spectra show a radial distribution function (RDF) of the R3c phase. As shown in

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Figure 3b, CoP2 showed a peak at the radial distance of 1.81 Å contributed by Co-P bond. While, the Co-P peak for N-doped CoP2 showed an apparent decrease at 1.76 Å compared to undoped CoP2, which could be ascribed to the weaker photoelectron scattering amplitude of the N atom and the shorter Co-N bond length.59 The existence of Co-N bond in the sample was proved based on the above-mentioned XPS results. Moreover, the Co K-edge XANES spectrum of N-doped CoP2 was compared against the linear combination of CoP2 and CoN spectra (Figure S8). As shown in Figure S8, the two spectra could not be matched, suggesting that there is no CoN compound formed in the sample, which is in accordance with the XRD results. The result also supports that the Co-N bonds in the sample are not from CoN, instead, they originate from the partial replacement of P with N in the CoP2 crystal structure.

Figure 3. (a) Normalized Co K-edge XANES spectra of pristine CoP2, N-doped CoP2, and CoN. (b) FT of EXAFS spectra of pristine CoP2 (blue), CoN (grey) and N-doped CoP2 (red).

The electrocatalytic HER activity of N-doped CoP2@PCC (loading amount, ~6 mg cm-2) was assessed in alkaline media (1 M KOH) using a three-electrode setup, where the sample was directly applied as working electrode. Pt/C (20 wt%), CoP2 and CoN were also evaluated

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for comparison. All the potentials were calibrated on a reversible hydrogen electrode (RHE) scale based on the Nernst equation with 85% iR-compensation. The polarization curves were collected from LSV with a scan rate of 5 mV s-1 (Figure 4a). As shown in Figure 4a, N-doped CoP2@PCC electrocatalyst shows superior HER activity with an overpotential () of 64 mV to drive the current density of 10 mA cm-2, which is lower than those required by CoP2@PCC (87 mV) and CoN@PCC (215 mV) at the same current density. Pt/C (20 wt%) presented the lowest overpotential of 40 mV at 10 mA cm-2. Besides, the HER activities of four control samples with different N doping amount were available in Figure S9, as shown in the figure, the activity increased with the decrease of N doping amount, and the N-doped CoP2@PCC with the smallest amount of N doping presented the best activity. This N-doped CoP2@PCC electrocatalyst represents one of the best HER catalysts in the alkaline electrolyte. More detailed comparisons are presented in Table S1 and S2. Moreover, the HER activities of pure CoP2@PCC and N-doped CoP2@PCC were further investigated in 0.5 M H2SO4 and 1 M phosphate buffer solution (PBS), as shown in Figure S10, the N-doped CoP2@PCC presented better HER activity than that of pure CoP2@PCC in both acidic and neutral electrolyte. The HER kinetics of the electrocatalysts were investigated according to Tafel plots (Figure 4b, log j ~ ). A Tafel slope of ~ 47.4 mV dec-1 was measured for N-doped CoP2@PCC, which was lower than that of CoN@PCC (~ 180.5 mV dec-1) and CoP2@PCC (~ 60.0 mV dec-1). For Pt/C (20 wt%), the Tafel slope was ~ 55.9 mV dec-1. The Tafel slope of N-doped CoP2@PCC implies that HER occurs via a Volmer-Heyrovsky mechanism.60 Apart from the low overpotential and Tafel slope, the N-doped CoP2@PCC electrocatalyst also shows excellent durability in 1 KOH electrolyte, with almost no potential fluctuation under the fixed current density of 10 mA cm-2 within 24 h (Figure 4c). The durability test of the electrocatalyst was further confirmed at 20 mA cm-2 for 24 h (Figure S11-S13). In addition, the Faradaic Efficiency (FE) for HER catalyzed by N-doped CoP2@PCC was evaluated by comparing the theoretically

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calculated H2 (assuming 100% FE) with experimentally measured H2 by chronopotential cathodic electrolysis at 20 mA. The excellent agreement of experimental and theoretical data indicates that FE was ~100%,61-63 as shown in Figure 4d.

Figure 4. (a) LSV curves and (b) Tafel plots of Pt/C (20 wt%), CoN@PCC, CoP2@PCC, and N-doped CoP2@PCC for HER in 1 M KOH with a scan rate of 5 mV s-1. The inset of (a) represents the LSV curves in the current density range of 0 to -50 mA cm-2. (c) Chronopotential curve of N-doped CoP2@PCC at the fixed current density of 10 mA cm-2 for 24 h. (d) The molar amount of hydrogen experimentally measured versus theoretically calculated for Ndoped CoP2@PCC at a fixed current of 20 mA.

To explore the intrinsic catalytic activity of N-doped CoP2, the turnover frequency (TOF) for active sites was estimated by using the methods reported previously.46, 47 The details are available in the experimental section. The CV curves of all the samples in a phosphate buffer

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solution (pH = 7) with a scan rate of 50 mV s-1 are shown in Figure 5a. The integrated charge obtained from CV curves is proportional to the total amount of active sites. The calculated TOFs are shown in Figure 5b. As the most effective HER electrocatalyst, Pt/C shows a TOF of 0.8 s-1 at   0 mV.64 To reach a TOF of 1.5 s-1, N-doped CoP2@PCC required an overpotential of ~67 mV, which is much smaller than that of the pure CoP2@PCC (84 mV). Besides, to achieve a TOF of 4 s-1, the obtained N-doped CoP2 required an overpotential of 88 mV, which is also smaller than that of the reported cobalt phosphide nanowire arrays (240 mV).65 Additional, the electrochemical impedance spectroscopy (EIS) of N-doped CoP2@PCC, CoP2, CoN, and Pt/C (20 wt%) were studied within a frequency range of 0.1-105 Hz and an amplitude of 10 mV. As shown in Figure 5c and 5d, Pt/C (20 wt%) presented the smallest charge transfer resistance (RCT) ~ 0.87 Ω, indicating the fastest electron transfer which is of critical importance for HER. N-doped CoP2@PCC, CoP2@PCC, and CoN showed the impedance value of 3.1, 4.7 and 280 Ω, respectively.

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Figure 5. (a) CV curves of N-doped CoP2@PCC, CoP2@PCC, and CoN@PCC in pH = 7 phosphate buffer solution (PBS) between -0.2 and 0.6 V vs. RHE at a scan rate of 50 mV s-1. (b) Turn over frequency (TOF) of N-doped CoP2@PCC, CoP2@PCC, and CoN@PCC. (c) Electrochemical impedance spectroscopy (EIS) of N-doped CoP2@PCC, CoP2@PCC, CoN@PCC, and Pt/C (20 wt%) with a frequency range of 0.1-105 Hz and an amplitude of 10 mV. (d) Fitted EIS plots of N-doped CoP2@PCC, CoP2@PCC, and Pt/C (20 wt%). (e) Equivalent electric circuits of the electrode surface.

To gain a better understanding of the mechanism of N doping effects on HER activity of CoP2, we performed first-principles calculations to study the hydrogen adsorption on pristine and N-doped CoP2. The fully optimized CoP2 unitcell (a=b=5.535 Å, and c=5.609 Å; =114.45)

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is expanded to 2×2×2 supercell with 32 Co and 64 P atoms for N doping, and the unitcell is also used to build (110) surface for hydrogen adsorption study. To find the possible doping site for N, we performed DFT calculations to compute the formation energy of N doping at the possible embedding sites, and the corresponding atomic model is shown in Figure S14 and the calculated defect formation energies are shown in Figure S15. The chemical bonding states between N and Co, and the stability of N doping CoP2 were evaluated by calculating the charge difference between N and host matrix. More details are described in supporting information. The Gibbs free energy of hydrogen adsorption (ΔGH*) on the active sites of an electrocatalyst could be taken as an indicator of HER activity.66 The optimal HER activity is supposed to be obtained at ΔGH* ≈ 0. The higher ΔGH* suggests the weak bonds between the protons and active sites, while lower value indicates large surface coverage of hydrogen atoms, both of which will lead to slow HER kinetics.32, 67, 68 As described in Figure 6a, the Gibbs free energy ΔGH* in pristine CoP2 structure is far beyond zero level, and the absolute ΔGH* value of Co site (0.503 eV) is much higher than that of P site (-0.192 eV), suggesting that the latter is much more catalytically active for HER. After doping N at CoP2 on (110) surface, both the Gibbs free energies of hydrogen adsorption at Co and P sites are shifted closer to zero levels, as shown in Figure 6b, suggesting the significant improvement in HER activity of N-doped CoP2. A Bader charge analysis is also conducted over the adsorption models to understand the formation bonds through charge transfer in N-doped CoP2, and the calculated charge difference is plotted in Figure 6c. The purple region represents charge accumulation and the light yellow region denotes the charge depletion. The computed results reveal that N gains ~1.452 electrons from adjacent Co atoms. The strong charge transfer between N atom and its bonded Co atoms indicate that the chemical bonds are formed between N and neighboring Co atoms. This leads to the increase in bond strength of the adjacent Co-P bond and thus weaken the bond between

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protons and P actives sites, which is reflected in the decrease of the absolute value of Gibbs free energy of hydrogen adsorption (ΔGH*). ΔGH* constitutes a critical parameter in determining HER performance. While in alkaline media, the adsorptions of the water molecule and hydroxyl group are also critical in deciding the efficiency and durability of a catalyst. A moderate adsorption energy of water molecule and a low attractive force for hydroxyl group on active surface are desirable for an optimized catalyst. To fully consider the HER mechanism under alkaline media, we further calculated the adsorption of H2O and OH* on P sites of both pristine CoP2 and N-doped CoP2. According to DFT calculations, the Gibbs free energies of H2O adsorption at P sites are 0.08 and 0.05 eV in pristine CoP2 and N-doped CoP2, respectively. The lower Gibbs free energy of H2O adsorption shows the less energy required to dissociate water molecule and the lower activation barrier. Besides, the calculated Gibbs free energies of OH* adsorption at P sites are -0.12 eV and -0.08 eV in pristine CoP2 and N-doped CoP2, respectively. The less negative Gibbs free energy of OH* adsorption at P site of N-doped CoP2 reveals that OH* bonds weakly with the active site after N doping, and more active sites will be exposed to facilitate the Volmer reaction and the subsequent Heyrovsky or Tafel step.69 Our DFT calculations reveal that the introducing of N into CoP2 structure can optimize the H, H2O and OH adsorption energies on active sites, thus resulting in the improved HER activity, which consistently supports the experimental results.

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Figure 6. (a) Free energy diagram for hydrogen evolution of (a) pristine CoP2 and (b) N-doped CoP2 at equilibrium potential (U = 0). The calculated Gibbs free energies of intermediate H are based on Co, P, and N atoms that served as active sites for HER. (c) The calculated charge difference between N and host elements. Here, the electron accumulation and depletion in Ndoped CoP2 are indicated in purple and light yellow, respectively. The dark blue, light blue and golden balls denote to Co, P, and N atoms, respectively.

CONCLUSIONS In summary, N was successfully doped into CoP2 as an efficient HER electrocatalyst in alkaline electrolyte. The first-principle calculations suggest that the partial replacement of P with N not only facilitate electron transfer but also optimize the Gibbs free energies of H*, H2O and OH* adsorption on N-doped CoP2 active site, thus facilitating HER process. The obtained N-doped

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CoP2@PCC electrocatalyst exhibited enhanced HER performance than pristine CoP2 in 1 M KOH, it showed an overpotential of only 64 mV to reach the current density of 10 mA cm-2 compared to the value of 87 mV for CoP2@PCC, which are in good agreement with our simulation results. This catalyst represents one of the best HER electrocatalysts in the alkaline electrolyte so far. This work provides an effective and stable electrocatalyst for HER through anion modification of transition metal phosphides (TMPs). The strategy demonstrated in this study has great potential to accelerate the finding and the design of low-priced and earthabundant HER electrocatalysts.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website. SEM images of bare carbon cloth, Fe2O3@CC; XRD results of Fe2O3@CC; SEM images of Fe3O4@CC and Co(OH)2@PCC; XRD results of Co(OH)2@PCC, N-doped CoP2@PCC, pure CoN@PCC, and CoP2@PCC; Linear fitted result of Co K-edge spectrum of N-doped CoP2 with CoP2 and CoN; First-principle calculations explanation and atomic structure of N-doped CoP2 supercell (Co32P63N); The calculated defect formation energy for N at the three possible embedding sites; HER activity comparison tables. Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT We greatly acknowledge the financial support provided by Singapore MOE Tier 1 Funding R284-000-162-114. The computing facilities were provided by the A*STAR Computational Resource Centre (A*CRC) and the National Supercomputing Centre (NSCC) Singapore.

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Nitrogen-doped cobalt phosphide for enhanced hydrogen evolution

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