NC hybrids as highly efficient electrocatalysts for water oxidation

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Amorphous CoFeP/NC hybrids as highly efficient electrocatalysts for water oxidation Rui Yao a, Yun Wu a, Muheng Wang a, Na Li a, Fei Zhao a, Qiang Zhao a, Jinping Li a, Guang Liu a,b,* a

Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, Shanxi, 030024, PR China b Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin, 300071, China

highlights

graphical abstract


CoFeP/NC catalysts were prepared by phosphorized CoFe-based coordination polymers.
The synergistic effects between CoFeP and N-doped carbon led to high OER activity.
Optimal Fe contents and N-doped carbon resulted in optimal electronic structure.
Surface structural evolution during OER process attributed to the enlarged ECSA.

article info

abstract

Article history:

The rational design and regulate structure and composition are pivotal for the develop-

Received 5 July 2019

ment of highly efficient oxygen evolution reaction (OER) catalysts for water splitting. In this

Received in revised form

study, amorphous CoFeP/NC hybrid electrocatalyst has been synthesized by a simple and

30 August 2019

effective phosphorization of a CoFe-based coordination polymer under N2 atmosphere. The

Accepted 27 September 2019

synergistic effects between the CoFeP and N-doped carbon has led to high electronic

Available online xxx

conductivity attributed to the optimal Fe contents with N-doped carbon and enlarged electrocatalytic active surface area aroused by the nanostructure of CoFeP/NC, as well as

Keywords:

the surface structural evolution of oxyhydroxide/phosphate during OER process. The

N-doped carbon

resulting Co0.35Fe0.17P0.48/NC electrocatalyst can attain a current density of 10 mA/cm2 at an

Phosphate

overpotential of 275 mV with a Tafel slope of 31 mV/dec on glassy carbon electrode and

Hybrids

228 mV on Ni foam electrode in 1 M KOH solution, long-term OER stability of this

Electrocatalyst

Co0.35Fe0.17P0.48/NC under the applied potential of 1.53 V vs. RHE demonstrates no obvious

Oxygen evolution reaction

decline in current densities of 110 mA/cm2 within 17 h, which outperforms those of the contrast electrocatalysts in this work and also comparable to that of many of the reported

* Corresponding author. Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, Shanxi, 030024, PR China. E-mail address: [email protected] (G. Liu). https://doi.org/10.1016/j.ijhydene.2019.09.214 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Yao R et al., Amorphous CoFeP/NC hybrids as highly efficient electrocatalysts for water oxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.214

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electrocatalysts in the literatures. This Co0.35Fe0.17P0.48/NC electrocatalyst highlights the rational modulation of optimal composition and electronic structure with homogeneous incorporation of the foreign metal-doped and N-doped carbon for the synthesis of highly efficient electrocatalysts toward to the water oxidation reactions. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen, which applied as an abundant and renewable fuel, is becoming an ideal candidate for replacing the increasingly exhausted fossil fuel [1e3]. Hydrogen produced from water electrolysis has received widespread attentions due to its simple process and low cost [4]. Nevertheless, the slow kinetics caused by multiple proton-couple electron transfer process of oxygen evolution reaction (OER) at the anode results in high energy consumption and overpotential [5], which becomes a bottleneck of water splitting driven by electricity. It is imperative for us to seek a suitable electrocatalyst with highly efficient OER performances to solve such problem. Up to now, noble metal ruthenium (Ru)/iridium (Ir)-based materials such as IrO2 and RuO2 are still identified as the stateof-the-art OER catalysts for water splitting [6,7]. However, the high price and scarcity of such noble metal based OER catalysts limit their application on a massive scale. Therefore, great efforts should be paid to develop non-precious metal based OER materials (Co, Ni, Fe, and etc.) [3,8e17]. To this end, transition metal phosphides (TMP), with chemical bonds ranging from ionic for the alkali and alkaline earth metals to metallic or covalent for the transition elements [18], thereby having emerged as a promising HER&OER catalysts in the field of electro-water splitting [12,19e27]. For example, Du et al. used nickel phosphide (Ni2P) as an high efficient water oxidation electrocatalyst [28], which can attain a current density of 10 mA/cm2 under an overpotential of 400 mV with a Tafel slope of 60 mV/dec. Liu and co-workers fabricated FeP nanorods via a convenient phosphorization process with iron oxyhydroxide precursors and they found that the FeP catalyst exhibited excellent water oxidation catalytic activity by using carbon fiber paper as the current collector [29], which delivered current densities of 10 mA/cm2 under the overpotential of 380 mV with a Tafel slope of 64 mV/dec. Although the OER activities of mono-metal phosphides (e.g. Ni2P [23], FeP [29], CoP [30e32]) have been achieved to realize efficient water splitting, these electrocatalysts still need high overpotential (often >300 mV at 10 mA/cm2) to perform highly efficient water oxidation reaction. Recently, some researches indicated that the incorporation of hetero atoms (e.g. transition metals [33e42], non-metal such as C [43e46], N [47e49], O [50] and S [51,52], etc.) with transition-metal-based materials can dramatically promote the water oxidation activities of the resulted bimetallic phosphides (such as FeCoP, FeNiP, CoNiP) [25,53] or hetero-atom-doped metal phosphides [11,31,54e56]. It is well accepted that the doping of foreign atoms could tune the electronic structure and coordination environment, which

thereby modify the OER performances of these electrocatalysts towards water splitting. Despite these strategies have been demonstrated to be effective to enhance the electrocatalytic activities of transition-metal phosphides, there are only very limited reports on controlled synthesis of multi-component hybrids based on nanostructured bimetallic phosphides coupled with non-metal elements (e.g. N, C), like FeeNi2P/graphene composites and NiFeP/NC nanoflakes, which were fabricated by reacting pre-obtained carbon supported metal oxide composites with inorganic phosphine or at high temperature or pyrolyzing highly poisonous metal organophosphonate (MOP) precursor at high temperature [57,58]. To date, rational design and synthesis of binary-transition-metal phosphides with homogeneous doping of nonmetal (especially N doped carbon) is still very challenging. In this contribution, we expect that, through rational design and synthesis of TMP with novel nanostructures as well as optimal composition and electronic modification with homogeneous incorporation of the foreign metal and N-doped carbon, the water oxidation activities of TMP electrocatalyst could be further dramatically improved. Herein, we report a facile and controllable method to synthesis of amorphous CoFeP/NC hybrid electrocatalysts via low temperature phosphorization of CoFe-based coordination polymer precursors, which was derived from solvothermal treatment of CoFe ions with nitrilotriacetic acid (NTA). Taking advantage of the synergistic effects between CoFeP and Ndoped carbon, that is the nanostructure of CoFeP/NC with proper Fe-doping contents, favored electronic conductivity originated from the in-situ formed N-doped carbon as well as enlarged electrochemical surface active area, the obtained Co0.35Fe0.17P0.48/NC deposited on glassy carbon (GC) electrode can attain a current density of 10 mA/cm2 at an overpotential of 275 mV with a Tafel slope of 31 mV/dec in 1 M KOH solution, long-term OER stability of this Co0.35Fe0.17P0.48/NC under the applied potential of 1.53 V vs. RHE demonstrates no obvious decline in current densities of 110 mA/cm2 within 17 h.

Experimental methods Materials synthesis All the chemicals were from Sinopharm Chemical Reagent Company and used without further purification. The CoFenitrilotriacetic acid (CoFe-NTA) coordination polymer was synthesized using a previously established strategy [59]. Initially, stoichiometric amounts of CoCl2$6H2O, FeSO4$7H2O and nitrilotriacetic acid (NTA) were dispersed into a mixed

Please cite this article as: Yao R et al., Amorphous CoFeP/NC hybrids as highly efficient electrocatalysts for water oxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.214

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solvent of isopropyl alcohol (IPA, 45 mL) and deionized water (15 mL). The mixture was sonicated for 10 min and stirred for 20 min. Then the suspension was sealed into a Teflon-lined autoclave and heated at 180  C for 12 h. The product was obtained by filtered and washed several times with water and ethanol, dried under vacuum. In contrast, CoFe-NTA coordination polymers with different Co/Fe ratios were also synthesized by the same method. The CoFeP/NC hybrid electrocatalysts were fabricated by a method of gas-solid phase reaction. Particularly, CoFe-NTA powder (50 mg) and NaH2PO2$H2O (2.0 g) were placed separately on both sides of a crucible boat, with the NaH2PO2$H2O at the upstream side of the furnace. These two samples were annealed at the optimum temperature of 400  C (the compared calcination temperatures were 350  C, 450  C) for 2 h with a heating speed of 5  C/min under a flowing N2 atmosphere. Subsequently, the product was naturally cooled to room temperature and washed with water and ethanol for several times to obtain Co0.26P0.74/NC, Co0.43Fe0.10P0.47/NC, Co0.19Fe0.04P0.77/NC, Co0.35Fe0.17P0.48/NC, Co0.16Fe0.15P0.69/NC, Fe0.34P0.66/NC, with the different Co/Fe atomic ratios, respectively.

Materials characterization Scanning electron microscopy (SEM, 3 kV, Hitachi-SU8010), transmission electron microscopy (TEM, 200 kV, JEOL2010FEF) and energy dispersive X-ray spectrometry (EDS, IXRF SDD 2610 system) were employed for characterizing and observing the morphology and chemical composition of the as-prepared electrocatalysts. X-ray powder diffraction (XRD) (Bruker D8 ADVANCE) and Raman spectroscopy (514.5 nm excitation, argon laser, Renishaw inVia) were also carried out to analyze the structure of the samples. X-ray photoelectron spectroscopy (XPS, Thermo VG ESCALAB250) was used to characterize the surface binding energy of the catalysts. The wettability of the electrocatalysts was performed on a contact angle analyser with 1 M KOH solution (Kru¨ss, DSA 100).

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electrocatalysts (working electrode) formed a standard three electrode system at 25  C, and the 1 M KOH solution was used as electrolyte for the electrochemical testing at pH ¼ 13.6. All the potentials measured were standardized to the values versus the reversible hydrogen electrode (ERHE). The equation is ERHE ¼ EAg/AgCl þ 0.059*pH þ 0.197. For Cyclic voltammograms (CV) measurements, the scan rate was set as 20 mV s1 from 0 to 0.8 V versus the Ag/AgCl and the circulation was carried out for about 30 cycles to be stabilized. Linear scan voltammograms (LSV) curves were also obtained at a scan rate of 1 mV s1 from 1.0 to 1.8 V vs. RHE. Overpotential (h) was calculated as h ¼ ERHE -1.23 V. Electrochemical impedance spectra (EIS) was analyzed at 1.58 V vs. RHE with the frequency range of 1.0e105 Hz. Chronoamperometric (h ¼ 300 mV) testing was employed to investigate the long-term stability of the electrocatalysts on NF electrode. Unless otherwise specified, 95%-iR compensation was applied to all the potentials in this work. The Tafel analysis was carried out with the equation, h ¼ a þ b * log(j), where h is the overpotential, a is the overpotential at a current density of 10 mA/cm2, b is the Tafel slope, and j is the current density to calculate the Tafel slope. To calculate the electrochemical active surface area, cyclic voltammograms at different scan rates v (10, 20, 40, 60, 80, 100 mV s1) were obtained with the potential ranged from 0.85 to 0.95 V vs. RHE. Then the equations are as follows [59]: ic ¼ vCdl ECSA ¼

Cdl Cs m

(1)

(2)

where ic is the double-layer charging current obtained from the CVs, Cdl represents the double layer capacitance, Cs is a specific capacitance value of 0.040 mF/cm2 in 1 M KOH solution based on a typical reported value [60] and m is the mass loading of 0.2 mg/cm2 on GC electrode in this work.

Results and discussion Electrochemical measurements Materials characterizations To prepare the electrocatalyst loaded on glassy carbon (GC) electrode, a mixed solvent of 200 mL ultrapure water, 50 mL ethanol and 11 mL Nafion solution (5 wt%, Sigma-Aldrich) containing 2 mg of the electrocatalyst was ultrasonicated for 50 min to form a homogeneous ink. Then, 5.1 mL of the catalyst ink was drop-casted onto the cleaned GC electrode (0.196 cm2) and dried overnight under air (mass loading of ~0.2 mg/cm2). To prepare electrocatalyst loaded on Ni foam (NF) electrode, the fresh NF was pretreated at first. Primitively, pieces of NF were ultrasonically washed with acetone, ethanol, and ultrapure water for several times to remove organics and oxides from the surface. Then the catalyst was deposited on the NF (1 cm *1 cm) by referring to the above method of the catalyst loaded on GC electrode, resulting in a mass loading of 1.0 mg/cm2. All electrochemical measurements were carried on a Princeton VersaSTAT 3 electrochemical workstation, where a platinum foil (counter electrode), saturated Ag/AgCl electrode (reference electrode) and a GC or NF electrode loaded with

As depicted in Scheme 1, the CoFeP/NC hybrid electrocatalysts were prepared through the following facile two-step reaction in Scheme 1. In order to understand the influences of temperature during the phosphorization process, XRD patterns of CoFe-NTA (Co/Fe atomic ratio is 6/4) and Co0.35Fe0.17P0.48/NC were obtained at different annealing temperature in Fig. 1a. It can be observed that the original peak of CoFe-NTA becomes wider as the temperature increases, which almost remains only one broaden diffraction peak between 15 and 25 at 400  C, inferring that the structure of the CoFe-NTA transformed into amorphous in the calcination-phosphorization process, which may has a favorable effect on OER because the long-range disordered and short-range ordered internal structural features in the amorphous phase can effectively increase surface defects and coordination unsaturated sites [61]. Further, similarly XRD results for CoFeP/NC electrocatalysts with different atomic ratio of Co/Fe were also shown in Fig. S1a, it is found that a broaden diffraction peak around

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Scheme 1 e Schematic illustrations of the fabrication process for CoFeP/NC hybrid electrocatalysts.

Fig. 1 e (a) XRD patterns of Co0.35Fe0.17P0.48/NC obtained at different calcination temperatures and (b) Raman spectra of CoFeP/NC hybrid electrocatalysts obtained at 400  C.

15~25 can be also observed. It is evidenced that D peak of Raman spectra is correlated with defect sites (sp3-hybridized carbon) and G peak intensity is linked with the sp2-hybridized carbon in carbon materials [62]. Two broad and distinct peaks at 1350 cm1 and 1590 cm1 representing the D-band and Gband were observed for Co0.26P0.74/NC, Co0.43Fe0.10P0.47/NC, Co0.19Fe0.04P0.77/NC, Co0.35Fe0.17P0.48/NC, and Fe0.34P0.66/NC nanorods in Fig.1b and Fig. S1b, which confirmed the successful synthesis of carbon hybrid materials. The SEM image of CoFe-NTA (Co/Fe atomic ratio is 6/4) was shown in Fig. 2a and b, the well-ordered nanorods with a diameter of ~150 nm can be clear observed, indicating the CoFe-NTA precursor has a great coordination structure, which was uniform and smooth nanorod without rough nanoparticles on its surface. Similar morphology with different nanorod size because of the different atomic ratios of Co/Fe can be further observed in Figs. S2aee. The Co0.35Fe0.17P0.48/NC nanorod was further observed by SEM, as shown in Fig. 2c and d. The surface of the nanorod was rough and the edges were blurred without obviously cross-linking and polymerization phenomenon, indicating that the phosphorization process preserves the nanorod morphologies of the original precursor and completely changes the roughness of the catalyst at the same time, which may be conducive to the escape of gas bubbles and benefited to the OER kinetics [63]. It can be further confirmed that the significant advances of the two-step synthetic strategy for morphology control and surface modification by the observed SEM images of CoFeP/NC nanorods in Figs. S2fej. Meanwhile, the uniform distribution of C, N, O, Co, Fe, and P elements on the surface of Co0.35Fe0.17P0.48/NC nanorods can

be well observed from the corresponding SEM-EDS mapping in Fig. S3, indicating the successful introducing of the P and the formation of CoFeP/NC hybrid materials after phosphorization process. Furthermore, the weight ratio of Co/Fe in Co0.35Fe0.17P0.48 nanorods was determined to be 23.9/11.2 according to the quantitative analysis of EDS (Fig. S3), that are about 1:2 in atomic ratio and consistent with the original atomic ratio of the Co/Fe in CoFe-NTA. Moreover, for the purpose of further observing the exact structure of the nanorod, the microstructure Co0.35Fe0.17P0.48/NC nanorods was further investigated by TEM, as shown in Fig. 3. It is revealed that the obtained Co0.35Fe0.17P0.48/NC nanorods are formed by stacking nanoparticles (Fig. 3a). In addition, no significant diffraction fringes for Co0.35Fe0.17P0.48/NC electrocatalyst were observed by the HRTEM image (Fig. 3b), and the corresponding selected area electron diffraction (SAED, inset in Fig. 3a) and fast-Fourier transformation (FFT, inset in Fig. 3b) patterns also showed a diffusion ring instead of the crystal diffraction spot, both of which are well consistent with the XRD results and further indicate the amorphous phase of the Co0.35Fe0.17P0.48/NC sample. X-ray photoelectron spectroscopy (XPS) was then employed to inspect the chemical composition and surface information of Co0.35Fe0.17P0.48/NC. As shown in the Co 2p XPS spectra in Fig. 4a, the peak at 778.3 eV is assigned to CoeP [64], indicating the successfully synthesis of CoP during phosphorization process. The peak at 782.6 eV and 787.2 eV are the binding energies of Co2þ and its satellite peaks [64], indicating the formation of oxides on the catalyst surface. The Fe 2p spectra in Fig. 4b shows the binding energy of Fe3þ and its satellite peaks at 712.8 eV and 716.3 eV, demonstrating the

Please cite this article as: Yao R et al., Amorphous CoFeP/NC hybrids as highly efficient electrocatalysts for water oxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.214

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Fig. 2 e SEM images of (aeb) Co/Fe-NTA (6/4) and (ced) the corresponding Co0.35Fe0.17P0.48/NC phosphorized at 400  C.

Fig. 3 e (a) TEM (inset is the SAED pattern) and (b) HRTEM (inset is the corresponding FFT pattern) images of Co0.35Fe0.17P0.48/ NC nanorods obtained at 400  C.

presence of iron (III) oxides on the catalyst surface [65]. The P 2p spectrum (Fig. 4c) shows the binding energy of PeO exclusively at 134.0 eV and 134.9 eV [65], while the binding energy of M  P did not occur, probably because of the oxidation of phosphide to phosphate in the air. From the spectrum of O 1s (Fig. 4d), the binding energies of 531.2 eV, 531.9 eV and 533.1 eV can be corresponded to the M  O

binding energy, hydroxyl oxygen and surface physical & chemical adsorption of oxygen in Co0.35Fe0.17P0.48/NC [66]. The peaks of N 1s (Fig. 4e) relate to the binding energies of 398.5 eV, 400.5 eV, and 402.7 eV can be assigned to pyridinic N, pyrrolic N, and graphitic nitrogen [59], respectively. It is evidenced that both pyridinic N and pyrrolic N could coordinate with metallic atoms to form coordination structures, thereby inhibiting the

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Fig. 4 e High-resolution XPS spectrum of (a) Co 2p; (b) Fe 2p; (c) P 2p; (d) O 1s; (e) N 1s; (f) C 1s for Co0.35Fe0.17P0.48/NC nanorods.

degradation of electrocatalysts during OER [67,68]. The successful doping of N atoms into carbon species could be further confirmed by the C 1s XPS spectrum (Fig. 4f), which is confirmed the presence of CeC, C]N binding energy at 284.9 eV and 286.2 eV [69]. Combined with the XRD, SEM, Raman and TEM characterization, it is concluded the successful synthesis of CoFeP/NC hybrid structures during the phosphorization process.

Electrochemical performances Electrochemical performances of CoFeP/NC electrocatalysts coated on glass carbon electrode were evaluated in 1 M KOH solution using a three-electrode system. The LSV curves of the series of CoFeP/NC materials are shown in Fig. 5a. It is

suggested that the different Co/Fe ratios have distinct effects on their corresponding OER performance and the optimal Fe doping content could demonstrate the most excellent water oxidation activities. Expectedly, as the Fe content increases, the current density increases early and then decreases. The current densities reach the maximum value when the Co/Fe feed ratio is 6/4, that is Co0.35Fe0.17P0.48/NC demonstrates the best OER performance, which has the lowest overpotential of 275 mV at 10 mA/cm2 (Table 1), even superior to that of precious metal oxide catalyst RuO2 (317 mV@10 mA/cm2). In addition, Co0.35Fe0.17P0.48/NC has the lowest Tafel slope (31mV/dec) compared with those of other CoFeP/NC electrocatalysts (Fig. 5b and Table 1), indicating the electrocatalytic OER process of Co0.35Fe0.17P0.48/NC electrode requires the lowest overpotential and has the best water oxidation

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Fig. 5 e Electrochemical characterizations of the (a) polarization curves, (b) Tafel plots, (c) EIS curves and (d) Charging current density plots versus different scan rates for CoFeP/NC (variations on Fe amounts) electrocatalysts in 1 M KOH.

performance under the same kinetic current density. Furthermore, the water oxidation activities of Co0.35Fe0.17P0.48/ NC in different electrolyte concentrations (0.1 M and 30 wt% KOH) were also studied. As shown in Fig. S4, it is found that Co0.35Fe0.17P0.48/NC demonstrates pretty well OER performances under 0.1 M and 30 wt% KOH solutions. Especially in 30 wt% KOH solution, it could attain a current density of 10 mA/cm2 at the overpotential of 222 mV. In addition, the relationship of phosphorization degree and the OER performances for CoFeP/NC electrocatalysts by using 1.5 g, 2.0 g and 2.5 g NaH2PO2$H2O as the phosphorization reagent to obtain CoFeP/NC materials with different phosphorization degree and the corresponding OER performances were shown in Fig. S5. It is found that the optimal phosphorization degree has distinct influence on the OER performance. Moreover, the polarization curves and Tafel plots of CoFe-NTA (6/4) coordination polymer and Co0.35Fe0.17P0.48/NC nanorods obtained at different calcination temperatures were also investigated, as shown in Fig. S6 and Fig. S7, it is demonstrated that the Co0.35Fe0.17P0.48/NC electrocatalyst obtained at 400  C exhibits the better water oxidation activity than those of other

electrocatalysts. It is concluded that the water oxidation performances of the Co0.35Fe0.17P0.48/NC not only outperform the state-of-the-art electrocatalyst of RuO2, but also show comparable OER performances to the values of many other reported OER catalysts in recent literatures (Table S2). Furthermore, EIS analyses of these electrocatalysts were performed under overpotential of 350 mV in a 1 M KOH solution to further investigate the electrocatalytic kinetics. As shown in Fig. 5c, all EIS plots of these electrocatalysts include the solution resistance (Rs) and the charge transfer resistance (Rct). With the Fe content increases, the semi-circular which representing the internal resistance of charge transfer becomes smaller, which is consistent with the change trends of these polarization curves. Interestingly, the charge transfer internal resistances of CoFeP/NC electrocatalysts consist of two semicircular arcs, and the Co0.35Fe0.17P0.48/NC has the smallest radius of semicircular arc among these electrocatalysts in Table S1. Such phenomenon could be caused by the internal structure of the electrocatalysts: one part is the internal resistance generated by the surface formed oxyhydroxide/phosphate during electrochemical testing (Rct, out),

Table 1 e Comparison of OER properties of different catalysts on a GC electrode. Catalysts Co0.26P0.74/NC Co0.43Fe0.10P0.47/NC Co0.19Fe0.04P0.77/NC Co0.35Fe0.17P0.48/NC Co0.16Fe0.15P0.69/NC Fe0.34P0.66/NC

h@10 mA/cm2 (mV)

Tafel slope (mV/dec)

ECSA (m2/g)

RF

441 347 338 275 308 440

48.8 49 56 31 55 48.7

1.4 1.6 1.8 2.4 2.1 1.3

7.14 8.16 9.18 12.7 10.71 6.6

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and the other part is the internal resistance of the inner layer of amorphous CoFeP/NC (Rct, in), as shown in Fig. S8. As expected, the Rct of Co0.35Fe0.17P0.48/NC reaches the minimum of 3.204 U and 2.968 U (Table S1), indicating such electrocatalyst demonstrates the fastest charge transfer ability and thus behaving the most excellent water oxidation activities. It has been evidenced that the performance of OER catalysts is closely related to its surface properties, such as electrochemically active area (ECSA), roughness (RF), and surface wettability [70e72]. Based on this perspective, we determined the double-layer capacitance (Cdl) and ECSA of different CoFeP/NC catalysts at different sweep rates (10~100 mV s1) in the same non-Faradaic potential region (Fig. S9). By comparing these CoFeP/NC catalysts with different iron content, it is obviously shown in Cdl plots (Fig. 5d) that Co0.35Fe0.17P0.48/NC possesses the highest capacitance value, and thus demonstrates the largest surface of 2.4 m2/g and the greatest roughness among these electrocatalysts in this work (Fig. S10 and Table 1). It is accepted that the larger ECSA could provide more active surface OER sites for water splitting and the greater roughness could contribute to the diffusion of bubbles during the gas evolution reaction such as OER [72,73], all of which thereby promoting the water oxidation performances of Co0.35Fe0.17P0.48/NC. In addition, the contact angle analysis was also carried out to explore the effect of surface wettability of the electrocatalysts on the water oxidation performance before and after phosphorization process, as shown in Fig. 6a and b. As expected, the Co0.35Fe0.17P0.48/NC holds a smaller contact angle

(48.6 ) than that of CoFe-NTA catalyst (69.7 ), indicating the surface wettability of the electrocatalyst was enhanced during the phosphorization process, which could be attributed to phosphate groups on the surface of the Co0.35Fe0.17P0.48/NC nanorods. Though the exact mechanism for enhancing the surface wettability of the catalyst is unclear, previous study has certificated that the introducing of phosphate groups can distort geometry of nickel or iron with open coordinate sites, thus favoring the water adsorption ability and enhance water oxidation activity ultimately [74,75]. Considering that the powder catalyst on GC electrode possibly peeled off by the bubbles generated during the water oxidation, it is agreed that nickel foam (NF) has a special three-dimensional structure and the adhesion of powder catalyst on such three-dimensional nickel foam could provide a higher active surface area within a compact interspace, thus attain a larger current density, which is favored by practical application [76]. Therefore, Co0.35Fe0.17P0.48/NC electrocatalyst was coated on nickel foam to form a three-dimensional electrode. The larger surface area contributes to interface contact and charge transport, thereby improving the OER performance of the catalyst, which can further expect the efficient OER performance and stability of the catalyst. Fig. 6c shows the comparison of LSV curves for Co0.35Fe0.17P0.48/NC electrocatalyst loaded on NF and GC electrodes as well as the commercial RuO2 catalyst loaded on NF electrode. As expected, the overpotential to attain a current density of 10 mA/cm2 for Co0.35Fe0.17P0.48/NC on NF electrode is determined to be only 228 mV, which is superior to that of the catalyst on GC

Fig. 6 e Contact angle measurements for (a) CoFe-NTA (6/4) and (b) Co0.35Fe0.17P0.48/NC catalysts; (c) Polarization curves of Co0.35Fe0.17P0.48/NC loading on Ni foam and GC electrode; (d) Chronoamperometric response under the applied potential of 1.53 V vs. RHE of Co0.35Fe0.17P0.48/NC nanorods on NF and on GC and commercial RuO2 on NF. Please cite this article as: Yao R et al., Amorphous CoFeP/NC hybrids as highly efficient electrocatalysts for water oxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.214

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Fig. 7 e (a) HR-TEM image, (b) XRD pattern of the post-OER Co0.35Fe0.17P0.48/NC electrocatalyst. electrode (275 mV) and also better than that of the commercial RuO2 electrocatalyst (270 mV). In addition, the chronoamperometric curves (Fig. 6d) of Co0.35Fe0.17P0.48 nanorod catalyst and RuO2 on NF electrodes were tested at overpotential of 300 mV to investigate the stability of the catalyst. It is found that the current density of Co0.35Fe0.17P0.48/NC electrode under the applied potential of 1.53 V vs. RHE remains stable around 110 mA/cm2 for 17 h, which is far superior to that of RuO2 catalyst (~5 mA/cm2) under the same potential condition, demonstrating the obtained Co0.35Fe0.17P0.48/NC catalyst has excellent stability in the process of water oxidation. Moreover, a series of characterizations of Co0.35Fe0.17P0.48/ NC catalyst after the OER test was carried out to further explore the OER process of the electrocatalyst under alkaline condition. As shown in Fig. S11 and Fig. S12, the SEM and TEM images of the post-OER Co0.35Fe0.17P0.48/NC catalyst show a hierarchical structure of pony-like nanosheets on the surface of granular nanoparticles, suggesting redox corrosion etching occurred for Co0.35Fe0.17P0.48/NC during the oxygen evolution reaction, which could benefit to the larger electrochemical active surface area and thereby promoting the OER activities of Co0.35Fe0.17P0.48/NC. Additionally, obvious amorphous carbon layer can be seen from the HRTEM (Fig. 7a) of the post-OER A derived Co0.35Fe0.17P0.48/NC, and the lattice fringe of 2.43
from HRTEM image for post-OER Co0.35Fe0.17P0.48/NC is in good agreement with the Bragg diffraction (021) of the corresponding CoOOH. The surface formed CoOOH after long-term OER stability is also evidenced by the XRD pattern of Co0.35Fe0.17P0.48/NC (Fig. 7b), further indicates some of the CoFeP was oxidized into its corresponding oxyhydroxide. Besides, the formation of Co3þ and Co2þ confirmed by the Co 2p peak of the post-OER Co0.35Fe0.17P0.48/NC (Fig. S13a) further proves the generation of CoOOH during the OER process, while the coexistence of Fe3þ and Fe2þof Fe 2p orbit (Fig. S13b) might be due to the formation of FeOOH or other surface oxides, which is consistent with the previous study that the metal oxide/oxyhydroxide is the active species of the OER process under alkaline conditions [77]. It should be pointed out that the P 2p spectrum (Fig. S13c) demonstrates a degree of surface phosphate corrosion after the long-term OER testing, which matches the SEM and TEM characterization results and is also favored to enlarge the surface electrochemical active sites.

Furthermore, the presence of N-doped carbon is further supported by the N 1s (Fig. S13e) and C 1s (Fig. S13f) XPS spectrum.

Conclusions A high-efficiency, low cost gas-solid phase method was used to synthesize novel CoFeP/NC hybrid electrocatalysts as efficient oxygen evolution catalysts. In brief, we attribute the excellent OER activities of Co0.35Fe0.17P0.48/NC to several factors: i) the synergistic effects between CoFeP and N-doped carbon that is the nanostructure of CoFeP/NC with proper Fedoping contents, has led to enhanced electrocatalytic performance toward the OER kinetics for water splitting. ii) The favored electronic conductivity originated from the in-situ formed N-doped carbon as well as enlarged electrochemical surface active area, as supported by the decreased charge transfer resistance in the EIS analysis and Cdl performance. iii) The structural evolution of the formation of CoFe-oxyhydroxides and surface phosphorus/phosphate corrosion during OER process could also generate more electrochemical active OER sites for water splitting, thus promoting the water oxidation performance of Co0.35Fe0.17P0.48/NC electrocatalyst. The gas-solid phase reaction method plays an important guiding role on the surface and structural modification of the OER electrocatalyst, fully demonstrating that the great potential of surface modification-wettability control by the phosphorization process, which makes it a simple and effective strategy for modification of metal oxide or hydroxide catalysts with enhanced electrocatalytic performance.

Acknowledgements We appreciate the financial funding supported by the National Natural Science Foundation of China (No.21878204).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.214.

Please cite this article as: Yao R et al., Amorphous CoFeP/NC hybrids as highly efficient electrocatalysts for water oxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.214

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