Electrocatalyst of two-dimensional CoP nanosheets embedded by carbon nanoparticles for hydrogen generation and urea oxidation in alkaline solution

Journal Pre-proofs Full Length Article Electrocatalyst of two-dimensional CoP nanosheets embedded by carbon nanoparticles for hydrogen generation and urea oxidation in alkaline solution Jinlong Zheng, Kaili Wu, Chaojie Lyu, Xin Pan, Xiaofang Zhang, Yuchen Zhu, Andi Wang, Woon-Ming Lau, Ning Wang PII: DOI: Reference:

S0169-4332(19)33794-8 https://doi.org/10.1016/j.apsusc.2019.144977 APSUSC 144977

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

17 October 2019 20 November 2019 5 December 2019

Please cite this article as: J. Zheng, K. Wu, C. Lyu, X. Pan, X. Zhang, Y. Zhu, A. Wang, W-M. Lau, N. Wang, Electrocatalyst of two-dimensional CoP nanosheets embedded by carbon nanoparticles for hydrogen generation and urea oxidation in alkaline solution, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144977

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Electrocatalyst of two-dimensional CoP nanosheets embedded by carbon nanoparticles for hydrogen generation and urea oxidation in alkaline solution Jinlong Zheng, Kaili Wu, Chaojie Lyu, Xin Pan, Xiaofang Zhang, Yuchen Zhu, Andi Wang, Woon-Ming Lau, Ning Wang* Center for Green Innovation, Beijing Advanced Innovation Center for Materials Genome Engineering, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China ABSTRACT: Hydrogen (H2) producing from the electrolysis of water at cathode is limited by the sluggish kinetics of oxygen evolution reaction (OER) at anode. It is essential to search a facile anodic reaction to replace OER in the hydrogen generation process. Urea oxidation reaction (UOR) is an alternative strategy to instead of OER in the water splitting reaction. Based on this, we successfully prepare two-dimensional (2D) CoP nanosheets embedded by carbon nanoparticles (CoP/C) composites as bifunctional catalysts to achieve hydrogen evolution reaction (HER) at cathode and UOR at anode simultaneously. Benefiting from the presence of carbon particles and 2D sheet structure of CoP in the composites, the contact area of electrolyte/electrode is enlarged, the exposed active sites are increased, and charge transfer rate is improved. As the anode and cathode catalysts in a two-electrode system with continuous electrolyte supplementation by a flow reactor, the CoP/C-3 composite only need 1.40 V to reach the current density of 10 mA cm-2 in the 1 M KOH electrolyte with 0.1 M urea. This study on hydrogen generation and urea oxidation at a low potential provides a promising method to prepare bifunctional catalysts for practical applications. Keywords: hydrogen evolution reaction, oxygen evolution reaction, urea oxidation reaction, transition-metal phosphides, two-dimensional nanosheet

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1. Introduction The increasingly serious energy crisis and environmental pollution are closely related to the excessive use of carbon-based fossil energy, which arose widespread attention and motivated the advance of new energy technologies[1-2]. As we all know, hydrogen (H2) fuel exhibits advantages of zero-emission and sustainable in the course of usage, which has been considered as an alternate energy server[3]. Electrolytic water splitting, which involved two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), has been used as a promising effective strategy to produce high-purity H2. Much efforts have been dedicated to enhancing the efficiency of electrolytic water splitting, mainly displayed in fabricating highperformance electrocatalysts. Compared to non-noble-metal electrocatalysts, noble metalbased electrocatalysts, such as Pt, Ir, and their alloys, explicitly reveal a higher performance towards water splitting, which can be attributed to their lower overpotential, smaller Tafel slope, higher activity, and better stability[4]. However, some drawbacks of noble metal electrocatalysts, including high cost, low reverse, and scarcity, have hampered their large-scale use[5-7]. Considering these factors, it is urgent to develop non-noble-metal electrocatalysts with properties of high efficiency and natural abundance, realizing high current density at a low overpotential for the sluggish kinetics of two-electron transfer in HER, as well as the high energy barrier for breaking the O-H and forming the O-O bond in OER. Up to now, tremendous researches have been conducted to develop efficient earth-abundant electrocatalysts, such as transition-metal chalcogenides[8, 9], carbides[10], borides[10], selenides[11], and transitionmetal phosphide[12, 13]. Transition-metal phosphides have relatively high similarity with hydrogenase that could be used as electrocatalysts into overall water splitting[14, 15]. For example, Cobalt phosphides (CoP) exhibit an intriguing electrocatalytic performance for HER and OER. The low cost, high electrical conductivity, and metalloid characteristics properties of CoP increase the possibility of practical application[16-18]. As we all known, urea has been widely used in agricultural 2

production and did harm to water environment, it dramatically enhanced the electrochemical performance and gradually degraded in overall water splitting. The urea-based overall water splitting, which consisted of the anodic urea oxidation reaction (UOR, Co(NH2)2 + 6OH- → N2 + 5H2O + CO2 + 6e-) and cathodic hydrogen evolution reaction (2H+ + 2e- → H2), have been adopted to not only degrade the contaminant urea in water, but also to enhance the efficiency of hydrogen evolution reaction[19]. What’s more, transition-metal phosphides with the same element compositions exhibit a different electrocatalytic performance based on different morphologies. 0-dimensional (e.g. nanoparticles) and 1-dimensional (e.g. nanowires and nanorods) transition-metal phosphides electrocatalysts are easily aggregated and reveal a poor stability in the electrochemical process[20-22]. Traditionally, 2D (e.g. nanosheets) electrocatalysts possessed advantages of larger specific surface area, more reaction sites and faster electron transport compared to three-dimensional electrocatalysts[23-26]. Additionally, some carbon-based materials were often used as carbon support to not only prevent electrocatalysts from aggregation and help preserve its morphologies[27, 28], but also accelerate electron transfer during HER or OER process, the formation of nanocomposites based on carbon-based support and transition-metal phosphides nanoparticles exhibited a synergistic effect based on their respective advantages[29, 30]. Inspired by the above discoveries, we fabricated 2D CoP sheets that derived from the phosphorization of Co(OH)2 sheets. In order to improve the dispersibility and electrochemical performance of sheets, carbon particles (Vulcan XC-72R) was adopted as the support to enhance the electron conductivity in the electrocatalytic process. The electrochemical performance of the composites with different mass ratios of CoP sheets to carbon particles were investigated. Compared to pure CoP and other composites, the as-prepared CoP/C-3 composite exhibited a superior electrocatalytic performance. Additionally, in order to achieve the ambition of high H2 production at low voltage, urea was used as the additive of electrolyte in our work. The optimized CoP/C-3 catalyst was demonstrated to possess superior electrochemical performance in the urea-containing electrolyte and had a promising future in the field of overall water splitting. 2. Experimental Section 3

Fabrication of Co(OH)2 nanosheets and pretreatment of carbon particles The Co(OH)2 nanosheets were synthesized by a simple wet-chemical method and the detailed procedures were as followed: 0.5 mmol of CoCl2·6H2O and 0.666 g of polyvinyl pyrrolidone (PVP, K29-32, Acros) were dissolved in 100 mL of deionized water at room temperature followed by ultrasonic treatment for 10 min. Then, 3 mL of hydrazine monohydrate liquid (N2H4·H2O, 80%) and 10 mL of sodium hydroxide (NaOH) aqueous solution (0.12 mol·L-1) were added into the above mixture solution in sequence. After reaction under 75 oC for 2 h in a round-bottom flask, Co(OH)2 nanosheets were finally obtained by centrifugation and dried at 50 oC overnight. In order to enhance the dispersibility in H2O, the carbon particles were dispersed into the nbutylamine with continuous stirring for 24 h, and the finally hydrophilic particles were obtained after centrifugation and freeze drying. The pretreated carbon particles were used in the following experiments. Fabrication of CoP/C nanocomposites with different proportions The as-obtained Co(OH)2 nanosheets were mixed with carbon particles with the mass ratios of 1:9, 2:8, 3:7, 4:6, and 5:5 for the preparation of Co(OH)2/C-1, Co(OH)2/C-2, Co(OH)2/C-3, Co(OH)2/C-4, and Co(OH)2/C-5, respectively. These mixtures were dispersed in deionized water and sonication for 1 h to form a homogeneous suspension. After freeze-drying, a series of Co(OH)2/C powders were obtained. The final CoP/C composites (CoP/C-1, CoP/C-2, CoP/C-3, CoP/C-4, and CoP/C-5) were transformed from the Co(OH)2/C composites with different mass ratios of Co(OH)2 to carbon particles after the phosphorization process: a porcelain boat with the precursor of Co(OH)2/C was placed in the downwind direction of the furnace, and another porcelain boat with sodium hypophosphite (NaH2PO2·H2O) was placed in the upwind direction of furnace. The mass ratio of Co(OH)2/C to NaH2PO2·H2O was set as 1:20, and the reaction was proceeded under N2 atmosphere with a heating rate of 2 oC/min, and maintained at 300 oC for 2 h. The CoP/C composites were obtained after natural cooling. Characterization

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The crystal structure and composition of all as-prepared samples were performed on a Rigaku D/max 2200 X-ray diffractometer (Cu Kα radiation, λ = 0.15418 nm) to conduct the typical X-ray diffraction (XRD) measurements. The microstructures and morphologies of all samples were observed on a scanning electron microscope (SEM, FEI Quanta 250) and a transmission electron microscopy (TEM, JEOL JEM-2100F). The X-ray photoelectron spectroscopy (XPS) technique was adopted to analyze the chemical states of the synthesized samples by a PHI 5000 Versaprobe III. The Raman scattering spectra were recorded on a LABRAM-HR Raman spectrometer. Electrochemical measurements Electrochemical measurements were carried on a CHI 760E electrochemical workstation (Shanghai Chenhua Apparatus Co. Ltd). A typical three-electrode cell with an electrolyte solution of 1.0 KOH was used to perform the OER and HER performance, while a carbon rod electrode and a saturated Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively. Different CoP/C composites (2 mg) were coated on a Ni foam (1 cm × 1 cm) as the working electrode. In this work, the potentials for OER were calculated versus the RHE by the following equation: ERHE = EAg/AgCl + 0.197 + 0.0591 × pH - 1.23 V the potentials for HER were calculated versus the RHE by the following equation: ERHE = EAg/AgCl + 0.197 + 0.0591 × pH where EAg/AgCl is the measured potential referring to the Ag/AgCl reference electrode. The linear sweep voltammetry (LSV) tests of OER and HER were carried out in 1.0 M KOH aqueous solution with and without 0.1 M urea at a scan rate of 1 mV·s-1. The overall water splitting was also proceeded in 1 M KOH aqueous solution with and without 0.1 M urea. The electrochemical impedance spectroscopy (EIS) of the electrodes was obtained by AC impedance spectroscopy with the amplitude of 5 mV and the frequency range from 0.1 Hz to 100 kHz. For the hydrogen production measurement, drainage method was used to quantitatively collected the H2 gases produced at 10 mA·cm-2. Faradic efficiency is calculated by the following equation: FE = mnF / It ×100% 5

where m, n, F, I, and t are the number of moles of produced hydrogen, the number of electrons transfer, the Faraday constant, the current, and the time, respectively. The electrochemically active surface areas (ECSAs) were estimated by measuring the capacitive current associated with the double layer capacitance (Cdl) from the scan-rate dependence of CVs. The CVs measurements of working electrodes were carried out in non‐ Faradic potential range of 1.118 and 1.218 V vs. RHE with the scan rates of 10-60 mV s-1 in 1.0 M KOH[12]. 3. Results and discussion

Scheme 1. Illustration of the preparation process of CoP/C composite.

Co(OH)2 nanosheets were synthesized by a wet-chemical method. Fig. S1 shows the scanning electron microscopy (SEM) image of Co(OH)2 sheets, which reveals the hexagonal shape with the size of about 150 nm. The X-ray diffraction (XRD) patterns shown in Fig. S2 confirm that the prepared Co(OH)2 sheets were hexagonal phase structure (JCPDS:45-0031). The CoP/C composites were obtained via the process displayed in scheme 1. The as-prepared Co(OH)2 sheets and hydrophilic carbon particles were dispersed and mixed in deionized water to form the homogeneous suspension. Then, the Co(OH)2/C composites were achieved after freezedrying. The mass ratios of Co(OH)2 to carbon were set as 1:9, 2:8, 3:7, 4:6, and 5:5, subsequently, the corresponding composites were named Co(OH)2/C-1, Co(OH)2/C-2, Co(OH)2/C-3, Co(OH)2/C-4, and Co(OH)2/C-5, respectively. The SEM images in Fig. S3 show the morphology features of the composites and pure carbon particles. Compared to pure carbon particles (Fig. S3f), the Co(OH)2 nanosheets can be observed obviously form the five composites (Fig. S3a-3e). The amount of nanosheets increase from Co(OH)2/C-1 to Co(OH)2/C-5 composites, and the Co(OH)2 nanosheets are few in Co(OH)2/C-1 and Co(OH)2/C-2 composites while the Co(OH)2 nanosheets are aggregate in Co(OH)2/C-4 and 6

Co(OH)2/C-5 composites. The distribution of Co(OH)2 nanosheets and carbon particles in Co(OH)2/C-3 composite is more homogeneous than the other four composites. The Co(OH)2/C composites are transformed into CoP/C composites via the phosphorization process in a tube furnace by using NaH2PO2 as the phosphorus source. CoP/C-1, CoP/C-2, CoP/C-3, CoP/C-4, and CoP/C-5 form from the precursors of Co(OH)2/C-1, Co(OH)2/C-2, Co(OH)2/C-3, Co(OH)2/C-4, and Co(OH)2/C-5, respectively. The SEM images of CoP/C composites are shown in Fig. S4. It is seen clearly that CoP can maintain the sheet structure without obvious aggregation after the phosphating reaction. The amount and the distribution of nanosheets in various CoP/C composites are similar to Co(OH)2/C composites. The characterizations for CoP/C-3 composite were carried out in detail, owing to the CoP/C-3 composite exhibited better electrochemical performance than other composites.

Fig. 1 (a) XRD patterns of CoP/C-3 composite, and the peak marked by asterisk came from carbon. (b) SEM image, (c) TEM image, and (d) Selected area electron diffraction (SAED) patterns of CoP/C-3 composite. (e) High-resolution TEM (HRTEM) image deriving from the marked region in Fig. 1c. (f) STEM image and EDX elemental mapping of Co, P, and C for CoP/C-3 composite.

Fig. 1a shows the XRD patterns of CoP/C-3 composite and CoP in the composite was orthorhombic structure. No any other peak can be found from the patterns, indicating the complete conversion from Co(OH)2 to CoP. The SEM image (Fig. 1b) of CoP/C-3 composite 7

shows that the CoP particles are sheet structure and evenly distributed by carbon particles. Fig. 1c shows the TEM image of the CoP/C-3 composite and the CoP hexagonal nanosheets can be found from the TEM image. Some pores appear on the surface of CoP sheets, which may arise from the gas release and the dehydration of the Co(OH)2 precursor during the phosphorization process. Furthermore, these CoP sheets are separated by the carbon particles, contributing to expose more surface sites of the sheets. The carbon particles act as the charge transfer mediums between two CoP sheets and improve the charge transfer rate in the electrochemical measurement. The obvious diffraction rings in the SAED patterns (Fig. 1d) represents the crystalline phase of CoP sheets, according with the XRD results. The HRTEM image (Fig. 1e) shows the interplane spacing of the lattice fringes was ~0.283 nm, corresponding to the (011) plane of CoP. The EDX elemental mapping for CoP/C composite reveals that the Co and P elements are uniform distributed around the CoP sheet (Fig. 1f).

Fig. 2 (a) XPS spectrum of wide scan of CoP/C-3. (b) High-resolution XPS spectrum of Co 2p of CoP/C-3. (c) High-resolution XPS spectrum of P 2p of CoP/C-3. (d) Raman spectra of C, Co(OH)2/C-3 and CoP/C-3.

X-ray photoelectron spectroscopy (XPS) technique was adopted to analyze the change of chemical states from Co(OH)2/C to CoP/C. Fig. 2a and Fig. S5a exhibit the survey spectra of 8

the synthesized Co(OH)2/C-3 and CoP/C-3, proving the existence of Co, O, H, C elements in Co(OH)2/C and Co, P, C elements in CoP/C, respectively. The binding energies of Co 2p (Fig. S5b) in Co(OH)2 at 780.3 and 796.4 eV eV can be assigned to the Co 2p3/2, and Co 2p1/2, and the binding energies of 780.4 and 801.8 are the satellite peaks, respectively[31]. The binding energies of O 1s (Fig. S5c) located at 530.2 and 531.0 correspond to the Co-O bond and the OH bond in Co(OH)2, and the 533.1 eV binding energy belongs to the absorbed oxygen species due to contact with air[32]. The binding energies of C 1s (Fig. S5d) located at 284.8, 285.0, and 285.7 eV correspond to the bonds of C-C, C-N, and C-H (C-N and C-H bonds came from the n-butylamine on the surface of carbon particles). As shown in Fig. 2b, the peaks at 778.0 and 794.2 eV are attributable to Co 2p3/2 and Co 2p1/2 of CoP, while the peaks at 779.7 and 795.3 eV are attributable to Co 2p3/2 and Co 2p1/2 of Co species from oxidation[33]. Fig. 2c shows that P 2p region is decomposed into four peaks, the left broad band is fitted by two peaks at 134.0 and 134.9 eV, which are attributed to P-C and P-O, respectively. The P-C comes from the combination of phosphorus and carbon particles, while the oxidized phosphorus species (PO) may be from the superficial oxidation process of CoP due to the exposure to the air[34, 35]. The peaks at 129.5 and 130.5 eV of P 2p signal can be indexed to P 2p3/2 and P 2p1/2 of CoP[36]. Co(OH)2/C-3 and CoP/C-3 composites were also investigated by the Raman spectroscopy. As shown in Fig. 2d, the position and intensity of D band and G band for Co(OH)2/C-3 are not obviously different from those for carbon particles, because there are no chemical reaction other than physical mixing in the preparation of Co(OH)2/C-3 composite. However, the position of G band for CoP/C-3 composite shifts from 1598 cm-1 to 1581 cm-1, suggesting the formation of Co-P after phosphorization and the charge transfer from carbon particles to Co-P sheets[36]. The peaks shift in XPS spectrums also prove the charge transfer phenomenon (Fig S6). The Co 2p3/2 (778.0 eV, Fig. S6a) of CoP/C-3 shifts from Co 2p3/2 ( 781.3 eV, Fig. S6c) of pure CoP, while the P 2p3/2 (129.5 eV, Fig. S6b) shifts from that P 2p3/2 (130.1 eV, Fig. S6d) of pure CoP, implying a transfer of electron density from Co to P[37]. What’s more, we also compared the C 1s of pure carbon particles with C 1s of CoP/C-3. The peak of C 1s of pure carbon particles were calibrated by C-C bond at 284.8 eV and the C 1s of CoP/C-3 were calibrated by the same corrected value of pure carbon. The C-C bond (285.6 eV) of CoP/C-3 9

shifts from that of 284.8 eV of carbon particle (Fig S6e and f). The shift peak reveals the charge transfer from carbon particles to Co-P sheets and this result is in good agreement with the Raman analysis. The OER activities of the different CoP/C composites were tested in 1 M KOH solution using a typical three-electrode system with a scan rate of 1 mV s−1. The samples of 2 mg were coated on cleaned Ni foams (1 cm × 1 cm) and used as work electrodes, while a graphite electrode and a saturated Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively. The OER activities for different CoP/C composites are shown in Fig. 3. From the linear sweep voltammetry (LSV) curves (Fig. 3a), the CoP/C-3 only requires an overpotential of 280 mV to reach 10 mA cm-2, which is better than those of CoP/C-1 (337 mV), CoP/C-2 (311 mV), CoP/C-4 (285 mV), CoP/C-5 (306 mV), and bare Ni foam (396 mV). As can be seen from Fig. 3a, the redox peaks at ~0.13 V correspond to the Ni2+/Ni3+ transformation. In order to exclude the influence of the Ni foam electrode on the experimental results, OER performance tests were also carried out on a glassy carbon electrode (Fig. S7). The results show that the data measured on the glassy carbon electrode has no oxidation peak and is consistent with the performance sequence measured on the Ni foam, which means that the overpotential of CoP/C-3 (276 mV) is less than CoP/C-1 (327 mV), CoP/C-2 (318 mV), CoP/C-4 (284 mV) and CoP/C-5 (305 mV). Despite the better performance of the glassy carbon electrode, the Ni foam electrode has more potential for practical application. The reasons why CoP/C-3 has the best performance may be that the active materials of CoP in CoP/C-1 and CoP/C-2 are fewer than CoP/C-3 and the aggregations of CoP sheets in CoP/C-4 and CoP/C-5 are more serious than CoP/C-3. To better elucidate the enhancement of OER performance for CoP/C-3, the ECSAs are compared by measuring the electrochemical Cdl of different composites (Fig. S8a-e). The Cdl of CoP/C-3 is calculated to be 14.1 mF cm-2, which is higher than those of CoP/C-1 (10.8 mF cm-2), CoP/C-2 (9.5 mF cm-2), CoP/C-4 (5.5 mF cm-2), and CoP/C-5 (3.5 mF cm-2) (Fig. S8e), indicating that CoP/C-3 has larger active surface areas than other composites[12]. The overpotential value of 280 mV for CoP/C-3 is also much lower than those of most reported Co-based catalysts in basic solution, such as rGO/CB/Co-Bi (350 mV)[38], CDs@Co3O4 NPs (353 mV)[39], Co3O4-C/rGO-W (382 mV)[40], EG/Co(OH)2/ZIF10

67 (280 mV)[41], and Co9S8/NSG-7 (360 mV)[42]. More detailed comparisons are shown in Table S1.

Fig. 3 (a) LSV curves of different CoP/C composites for OER with a scan rate of 1 mV s -1 in 1 M KOH solution. (b) Corresponding Tafel plots of different CoP/C composites. (c) Nyquist plots of different CoP/C composites at an overpotential of 0.4 V vs. RHE. (d) Chronoamperometric curve of CoP/C-3 composite recorded at the overpotential of 282 mV vs. RHE for 12 h. The inset was the LSV curves of the CoP/C-3 composite before and after 1000 CV cycles.

Besides, the OER kinetics of the CoP/C were examined by the Tafel plots (η vs. log |j|) derived from the LSV curves. As shown in Fig. 3b, the Tafel slopes of CoP/C-1, CoP/C-2, CoP/C-3, CoP/C-4, and CoP/C-5 composites are 124.8, 122.8, 92.9, 117.8, and 120.7 mV dec1

, respectively, indicating the fastest reaction kinetics of CoP/C-3 among the five composites.

The detailed performance comparisons of CoP/C-3 composite with other reported Co-based catalysts are illustrated in Table S1. To further study the reaction kinetics, the electrochemical impedance spectroscopy (EIS) analysis was presented as the Nyquist plots (Fig. 3c). As disclosed, the CoP/C-3 composite shows the smallest value of charge-transfer resistances (Rct), which suggests a better charge transfer ability compared with other composites. In addition, stability and durability are two important parameters of electrocatalysts for practical 11

application. We carried out the chronoamperometry tests at the overpotential of 282 mV vs. RHE to evaluate the catalytic stability of CoP/C-3 composite (Fig. 3d). There are no obvious decay after continuous test for 12 h, confirming the good electrochemical stability of CoP/C-3 composite. The LSV curves of the CoP/C-3 composite before and after 1000 CV cycles are shown in the inset. The two curves are almost overlapped, meaning the outstanding long-term durability of CoP/C-3 composite. Above results show that the CoP/C-3 composite is a valuable OER catalyst with high activity and stability for practical application. We also tested the HER performance of different CoP/C composites in 1 M KOH solution. As shown in Fig. S9a, the CoP/C-3 composite possess the best performance with the overpotential of 109 mV at 10 mA cm-2, while the CoP/C-1, CoP/C-2, CoP/C-4, and CoP/C-5 composites are 249, 210, 137, and 159 mV, respectively. The value of 109 mV is much better than those of most reported phosphorus-based catalysts, such as porous CoPO nanosheets (158 mV)[43], hollow porous NiCoFeP nanocubes (131 mV)[44], Co4NiP nanotubes (129 mV)[45] and 3D porous Ni/Ni8P3 (130 mV)[46]. The Tafel plots (Fig. S9b) show the CoP/C-3 composite exhibits a much faster reaction process than other composites. These results suggest the CoP/C3 composite is not only an excellent OER material, but also a satisfying HER catalyst.

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Fig. 4 (a) LSV curves and (b) Corresponding Tafel plots of CoP/C-3 composite for OER and UOR in 1 M KOH solution with and without 0.1 M urea. (c) LSV curves and (d) Corresponding Tafel plots of CoP/C-3 composite for H2 evolution in 1.0 M KOH with and without 0.1 M urea.

OER process is a four-electron transfer reaction in alkaline media, and the energy consumption in any step is higher than the electrocatalytic oxidation of some organic chemicals, such as formate and urea. Thus, we carried out the UOR and OER tests for CoP/C3 composite under a similar three-electrode system. It can be seen from Fig. 4a, the overpotential is only 124 mV to obtain 10 mA cm-2 in 1 M KOH solution with 0.1 M urea for CoP/C-3 composite, which is much smaller than the value in 1 M KOH solution without urea (280 mV, 10 mA cm-2). The Tafel slope is another evidence that the UOR (49.3 mv dec-1) is much easier than OER (92.9 mv dec-1) process in alkaline condition. The result was consistent with the relevant results recently reported[47, 48]. To further determine the effect of additional urea in 1 M KOH solution for HER performance of the CoP/C-3 composite, HER measurements in the electrolyte with and without urea were performed under a three-electrode system. As demonstrated in Fig. 4c-d, almost overlapped LSV curves and nearly equal Tafel slopes obtained in the electrolyte with and without urea suggest that the added urea bring rarely impact on HER for the CoP/C-3 composite cathode.

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Fig. 5 (a) Schematic of a continuous flow reactor using CoP/C-3 composite as cathode and anode catalysts for the HER and UOR. (b) Polarization curves of the two-electrode system in 1.0 M KOH with and without 0.1 M urea. (c) Chronoamperometry test of CoP/C-3||CoP/C-3 system at the voltage of 1.40 V for 12 h in 1.0 M KOH with 0.1 M urea (inset: Measured H2 quantity compared with theoretically calculated quantity vs. time for CoP/C-3 cathode at 10 mA cm-2 under the two-electrode system).

To better practical application on a large scale, a continuous flow reactor for H2 production coupled with UOR was constructed using CoP/C-3 composite as the bifunctional catalyst (Fig. 5a). Continuous electrolyte supplementation could keep the concentration of urea at a constant level. As shown in Fig. 5b, the voltage of 1.62 V is needed to attain 10 mA cm-2 in 1 M KOH solution. After adding 0.1 M urea in the electrolyte, the cell voltage decreases to 1.40 V to reach 10 mA cm-2. This implies that the energy conversion efficiency of UOR is much higher than that of OER, and the energy consumption for large-scale hydrogen production is lower. The long-term stability of the reactor was measured by a chronoamperometry technique (Fig. 5c). Negligible difference is observed for the current density at a voltage of 1.40 V after 12 h in this continuous flow system with the electrolyte flow rate of 1.0 mL min-1 through a peristaltic pump, indicating the excellent catalytic durability of CoP/C-3 composite. We collected the H2 produced from the electrode and compared with the calculated values (inset of Fig. 5c), and the Faradaic efficiency was nearly 100 %. Above results showed that the CoP/C3 composite is a valuable bifunctional catalyst for HER and UOR to realize the H2 generation and saving energy. Conclusions In conclusion, CoP/C composites were synthesized by phosphorization of the Co(OH)2/C precursors with different ratios of Co(OH)2 and carbon particles. 2D CoP nanosheets with porous rough surface interspersed by the added carbon particles was beneficial to increase the catalytic activities by exposing more active sites, was favorable to improve the contact area between the electrolyte and the catalytic surface, and was advantageous to accelerate the electron transfer rate in reaction process. All these remarkable features promised the outstanding performance of CoP/C composite for HER and UOR. CoP/C-3 composite exhibited better performance than other composites because of the more suitable ratio of CoP 14

sheets and carbon particles than those of other composites. In a two-electrode system, it only needed 1.40 V to reach the current density of 10 mA cm-2 for hydrogen evolution and urea oxidation. The results highlight that UOR have great potential application as a substitute for OER in water splitting to realize high efficiency and energy saving of hydrogen production. Acknowledgements This work was financially supported by the Fundamental Research Funds for the Central Universities (FRF-TP-18-079A1), and the National Natural Science Foundation of China (NSFC No. 51873020, 21575009). References [1] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater., 10 (2011) 780786. [2] J. Wang, F. Xu, H. Jin, Y. Chen, Y. Wang, Non-noble metal-based carbon composites in hydrogen evolution reaction: fundamentals to applications, Adv. Mater., 29 (2017) 1605838. [3] L.Z Ouyang, W. Chen, J.W. Liu, M. Felderhoff, H. Wang, M. Zhu, Enhancing the regeneration process of consumed NaBH4 for hydrogen storage, Adv. Energy Mater., 7 (2017) 1700299. [4] J. Mo, Z. Kang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green, Jr., M.M. Mench, F.-Y. Zhang, Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting, Sci. Adv., 2 (2016) e1600690. [5] C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H.L. Xin, J.D. Snyder, D. Li, J.A. Herron, M. Mavrikakis, M. Chi, K.L. More, Y. Li, N.M. Markovic, G.A. Somorjai, P. Yang, V.R. Stamenkovic,

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Author contributions: Jinlong Zheng: Conceptualization, Data curation, Writing- Original draft preparation. Kaili Wu, Chaojie Lyu, Xin Pan, and Andi Wang: Validation, Formal analysis. Xiaofang Zhang and Yuchen Zhu: Investigation. Woon-Ming Lau and Ning Wang: Writing- Reviewing and Editing.

Highlights 

Composites of 2D CoP nanosheets embedded by carbon particles were prepared.

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Composites could be used as bifunctional catalysts for HER and UOR.

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Owing to composite structure, active sites and charge transfer rate were improved.

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A flow reactor with continuous electrolyte supply was used for H2 generation.

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CoP/C-3 needed 1.40 V to reach 10 mA cm-2 in 1 M KOH solution with 0.1 M urea.

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Graphical abstract

There are no declaration of interest.

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