Controllable synthesis of three dimensional electrodeposited Co–P nanosphere arrays as efficient electrocatalysts for overall water splitting

Abstract

Novel three dimensional (3D) electrodeposited Co–P nanosphere arrays on FTO (Co–P/FTO) have been successfully prepared as efficient bifunctional electrocatalysts for overall water splitting in alkaline media. The morphologies and properties of the 3D Co–P nanosphere arrays can be controlled by the electrolyte concentration. At the middle concentration, Co–P nanospheres have a more homogeneous size and array distribution and a rough surface, implying a larger surface area and an increased number of active sites for water splitting. The electrochemical measurements confirm the best electrocatalytic performances of Co–P/FTO at the middle concentration. They show excellent activity, with an overpotential of 125 mV for HER, 420 mV for OER and Tafel slopes of 54 mV dec⁻¹ and 83 mV dec⁻¹, respectively. The fabricated bifunctional systems of Co–P/Co–P can efficiently catalyse HER and OER at the same time, solving the incompatible problem of different media between HER and OER. Therefore, controlling the synthesis of 3D Co–P/FTO nanosphere arrays through electrodeposition can provide a facile way for the bifunctional electrocatalysis of both HER and OER.

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Introduction

Hydrogen, as a green and sustainable energy carrier with zero carbon emissions and a high combustion value, can be a clean chemical form to store energy for further use.^1,2 So far, hydrogen can be produced through water electrolysis (H₂O → H₂ + O₂),^3,4 which is composed of two half reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).⁵ In order to generate hydrogen efficiently, both HER and OER are needed for the reaction to work perfectly.⁶ However, a high energy barrier must be overcome for both of the two half reactions,⁷ especially for OER, which is a complex reaction involving the transfer of four electrons.^8–10 Thus, it is necessary to find useful electrocatalysts for HER and OER to lower their overpotentials and accelerate their reaction rates.^11–13 Traditionally, the most efficient electrocatalysts for HER and OER are precious metal-based materials, such as platinum for HER^14,15 and ruthenium oxide for OER.^16,17 But their wide applications are hindered by their scarcity and high price.¹⁸ Therefore, the most fundamental but essential aspect of water electrolysis is to find suitable and cheap electrocatalysts with abundant reserves.

Up to now, much effort has been made to develop efficient electrocatalysts for HER in acidic media and electrocatalysts for OER in basic media. It has been demonstrated that transition metal chalcogenides (TMCs),^19–22 transition metal phosphides (TMPs)^23–25 and transition metal nitrides (TMNs)^26,27 can be efficient electrocatalysts for HER, for example, Wang’s group have synthesized MoS₂ nanosheets with high efficiency for HER in acidic media.²⁰ Schaak’s group has prepared multifaceted CoP nanoparticles as particularly active electrocatalysts for HER under strongly acidic conditions.²⁸ Lee’s group has reported that Mo₂N-based materials can be highly active and stable electrocatalysts for HER in 0.5 M H₂SO₄.²⁹ However, OER catalysts are mostly based on metal oxides/hydroxides,^30–33 for example, Suib’s group has investigated MnO₂ as an efficient OER electrocatalyst in alkaline media.³⁰ Jin’s group has reported an active OER electrocatalysts-Ni_xCo_1−x(OH)₂ in alkaline media.³²

Therefore, there remains an incompatibility problem between HER and OER because an active HER electrocatalyst in acidic media may be poor in alkaline media and an efficient OER electrocatalyst in basic solution may be unstable in a low pH.³⁴ Thus, it is desirable to find electrocatalysts for HER and OER that work in the same media. Also, using different electrocatalysts as cathode and anode materials may lead to a low efficiency. Thus, it is necessary to find a bifunctional electrocatalyst for both HER and OER in the same media. It has been proven that TMPs, such as Ni–P and Co–P can be bifunctional electrocatalysts for overall water splitting in alkaline media, for example, Hu’s group has synthesized and proven that the Ni₂P, an excellent hydrogen evolving catalyst, is also a highly active electrocatalyst for OER.³⁵ Sun’s group has electrodeposited a Co–P film on Cu foil as a highly efficient bifunctional electrocatalyst for both HER and OER.³⁶ Owing to the increased nucleation energy of TMPs,³⁷ the traditional synthesis methods, such as wet route,^38,39 hydrothermal process,⁴⁰ solvothermal method⁴¹ or a series of complicated steps involving multiple procedures,^23,42 often involve harsh conditions such as high temperature, high pressure, toxic phosphorus precursors or some dangerous organic solvents. Thus, it is necessary to develop a mild and green route to prepare TMPs.

Electrodeposition has roused much attention because it is facile, mild, easy to control and simple to operate.^33,43 Also, during the electrocatalytic measurements, TMPs obtained through the electrodeposition process are more advantageous than the powder counterpart obtained from other methods because they do not require a binder, which may affect the conductivity.⁴⁴ However, it is difficult to control the morphologies of TMPs through the electrodeposition process and the obtained nanostructures are usually two dimensional (2D) structures which aggregate easily. Compared with these 2D structures, the 3D nanostructured TMPs can provide more active sites for water splitting. But very few people have obtained the uniform 3D nanostructured TMPs through electrodeposition until now.

Thus, we have synthesized uniform 3D Co–P nanosphere arrays on FTO using a facile electrodeposition method. The distribution and diameter of the 3D Co–P nanospheres can be largely affected by the variation of electrolyte concentration. The obtained Co–P/FTO can catalyze the overall water splitting as a bifunctional electrocatalyst with excellent activity and stability in the same alkaline media and, the fabricated Co–P/Co–P shows high electrocatalytic activity for both HER and OER at the same time.

Experimental

Materials and characterization

Cobalt(II) chloride hexahydrate (CoCl₂·6H₂O), sodium hypophosphite monohydrate (NaH₂PO₄·H₂O), sodium acetate anhydrous (CH₃COONa) and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were used as received without further purification. The water used throughout all experiments was deionized water purified through a Millipore system.

X-ray diffraction (XRD) was performed on a panalytical X’pert PROX-ray diffractometer with Cu Kα monochromatized radiation (λ = 1.54 Å) and operated at 45 kV and 40 mA. The scan rate was 8 min⁻¹ and the 2θ scan range was from 5° to 76°. The morphology of the samples was examined with field-emission scanning electron microscopy (SEM, Hitachi, S-4800). X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALABMK II spectrometer using an Al Kα (1486.6 eV) photon source.

Sample preparation

Co–P nanospheres arrays were electrodeposited by an electrochemical workstation (Gamry Reference 600 Instruments, USA), using a traditional three-electrode system, with Pt as the counter electrode, Ag/AgCl (3 M KCl) as reference electrode and FTO as the working electrode, as shown in Scheme 1. Prior to electrodeposition, FTO was ultrasonicated in acetone, ethanol and deionized water (DI) for 20 min, respectively. Then the FTO electrodes were stored in vacuum under 25 °C. The electrodeposition media consisted of 50 mM CoCl₂·6H₂O, 0.5 M NaH₂PO₄·H₂O and 0.1 M NaOAc. The cyclic voltammograms (CVs) technique was used as the electrodeposition method, with a potential range from −0.3 V to −1.0 V vs. Ag/AgCl. The electrodeposition curve is shown in Fig. S1.† Then the obtained Co–P/FTO is rinsed gently with DI and dried naturally at room temperature. The sample was marked as Co–P/FTO (2). According to the previous literature,^45,46 the mechanism of the electrodeposited Co–P can be described as follows:

H₂PO₂⁻ + Co²⁺ + 3e⁻ = Co–P + 2OH⁻

Scheme 1 Illustration of the mechanism of the electrodeposited Co–P nanosphere arrays at different concentrations.

Next, we investigated the effect of concentration on the morphologies and their electrocatalytic activity for water splitting. A lower concentration (25 mM CoCl₂·6H₂O, 0.5 M NaH₂PO₄·H₂O and 0.1 M NaOAc) and a higher concentration (100 mM CoCl₂·6H₂O, 1.0 M NaH₂PO₄·H₂O and 0.1 M NaOAc) were tested. The obtained samples were marked as Co–P/FTO (1) and Co–P/FTO (3).

Electrochemical measurements

Electrochemical measurements were performed in a three-electrode system, using electrodeposited Co–P/FTO as the working electrode, a Pt plate as the counter electrode, and an SCE (saturated KCl) as the reference electrode, at an electrochemical station (Gamry Reference 600 Instruments, USA). All measurements were performed in 40 mL of 1.0 M KOH (aq.) electrolyte prepared using 18 MΩ deionized water.

For the HER measurement, the solution of KOH was purged with N₂ before the measurement for half an hour and maintained during the entire measurements. All potentials reported in this paper were converted from vs. SCE to vs. RHE according to the equation of E(RHE) = E(SCE) + 0.245 V + 0.059pH. iR (current time internal resistance) compensation was applied in polarization and controlled potential electrolysis experiments to account for the voltage drop between the reference and working electrodes using Gamry Framework Data Acquisition Software 6.11. Linear sweep voltammetry (LSV) polarization curves were conducted from 0.05 V to −0.25 V vs. RHE with a scan rate of 10 mV s⁻¹. Electric impedance spectroscopy (EIS) measurements were carried out at −0.08 V vs. RHE from 10⁵ to 10⁻¹ Hz with an AC potential amplitude of 5 mV. Cyclic voltammograms (CVs) taken with various scan rates (40, 80, 120, 160 and 200 mV s⁻¹) were collected in the 0–0.1 V vs. RHE region and were used to estimate the double-layer capacitance. Stability was conducted at the overpotential of −140 mV for 10 h.

For OER measurement, the solution of KOH was purged with O₂ before the measurement for half an hour and maintained during the whole measurements. Linear sweep voltammetry (LSV) polarization curves were conducted from 1.3 V to 1.9 V vs. RHE with a scan rate of 10 mV s⁻¹. Electric impedance spectroscopy (EIS) measurements were carried out at 1.50 V vs. RHE from 10⁵ to 10⁻¹ Hz with AC potential amplitude of 5 mV. Cyclic voltammograms (CVs) taken with various scan rates (40, 80, 120, 160 and 200 mV s⁻¹) were collected in the 1.13–1.23 V vs. RHE region and were used to estimate the double-layer capacitance. Stability was conducted at the overpotential of 440 mV for 10 h.

For comparison, the Pt–C electrode as working electrode was prepared as follows: 5 mg of the 20% Pt–C was dispersed in 1 mL mixed solution composed of deionized water and ethanol (1 : 1 in volume ratio) containing 20 μL 5 wt% Nafion solution. The Pt–C inks were obtained after sonication for 30 min. Glassy carbon electrodes (GCE) were ultrasonicated in deionized water and absolute ethanol for 10 min several times after being successively polished with 1.0, 0.3 and 0.05 μm alumina powder. Then, 10 μL of catalyst slurry was pipetted and spread on the top of GCE. The catalyst-coated GCE was dried in air for further use.

Also, the Co–P/Co–P system was fabricated as is shown in Scheme 2, using another Co–P/FTO to alternate the Pt plate as the counter electrode. The corresponding measurement conditions for HER and OER are the same as mentioned above.

Scheme 2 Mechanisms of Co–P/Co–P couple as a bifunctional electrocatalyst at the same time in 1 M KOH.

Results and discussion

Fig. 1 shows the XRD patterns of the obtained three different Co–P/FTO samples and the blank FTO. Compared with the blank FTO, new peaks are presented in the electrodeposited Co–P/FTO samples. As is shown in Fig. 1, the new peaks at 41.6°, 44.5° and 47.5° corresponded to the (100), (002) and (110) of the hexagonal close-packed (hcp) Co structure (JCPDS no. 00-05-0727). Also, as the concentration increases, so does the intensities of these three peaks, indicating that a higher concentration can lead to better crystallinity of Co.

Fig. 1 XRD patterns of blank FTO and the three different Co–P/FTO samples.

Black coverage of Co–P on FTO can be seen from the photograph of Co–P/FTO and blank FTO (Fig. S2†). Concentrations have a great effect on the morphologies of the prepared Co–P, as shown in Fig. 2. At the three different concentrations, the obtained Co–P arrays are all composed of uniform 3D Co–P nanospheres. It can be clearly seen that the size of the 3D Co–P spheres has changed from nanospheres to microspheres with the increase in concentration. In the lowest concentration, the obtained Co–P nanospheres have the smallest diameters, ranging from 300 nm to 500 nm (Fig. 2a). In the middle concentration, the diameter of the prepared Co–P is about 800 nm (Fig. 2c). And in the highest concentration, Co–P microspheres have been obtained with the largest diameters ranging from 800 nm to 1200 nm (Fig. 2e). Also, the distribution of these Co–P spheres on the surface of FTO becomes denser and denser with the increase in concentrations. At the highest electrolyte concentration, these microspheres have aggregated together to form a whole film (Fig. 2e). From the higher magnified SEM images shown in Fig. 2b, d and f, the surface morphology of a single sphere can be clearly seen and the surfaces are also different with the increase in concentration. In the lower concentrations, the surface of Co–P nanospheres is rough, with many edges and grooves on the surface of the globular structure (Fig. 2b and d), these may enhance the surface area and provide more active sites for water splitting. But at the highest concentration, the obtained Co–P surface is smooth (Fig. 2f), which may be disadvantageous for the electrocatalytic activity. Thus, in the middle concentration, the electrodeposited 3D Co–P arrays may be the most efficient electrocatalyst, owing to the more homogeneous nanospheres with rough surface and the array distribution.

Fig. 2 SEM images of the three different Co–P/FTO samples: (a, b) Co–P/FTO (1); (c, d) Co–P/FTO (2); (e, f) Co–P/FTO (3).

The surface composition of the electrodeposited Co–P/FTO (2) at middle concentration is analyzed through X-ray photoelectron spectroscopy (XPS) (Fig. 3). The XPS survey (Fig. 3a) shows the presence of Co and P. From the magnified Co 2p region spectrum (Fig. 3b), two peaks at 778.2 eV and 793.2 eV can be assigned to metallic Co 2p_3/2 and Co 2p_1/2.⁴⁷ The other peaks are associated with the oxidant cobalt (Co₃O₄) as the previous literature reported. Fig. 3c shows the P 2p region. Two peaks centered at 129.5 eV and 130.4 eV correspond to P 2p_3/2 and P 2p_1/2, which can be attributed to the phosphide.⁴⁸ The peak at 133.6 eV is due to the presence of orthophosphate, probably forming the cobalt salt Co₃ (PO₄)₂.³⁶

Fig. 3 XPS of the Co–P/FTO (2): (a) survey; (b) Co 2p; (c) P 2p of the Co–P.

The electrochemical properties of different Co–P/FTO samples have been examined as electrocatalysts for HER, as shown in Fig. 4. From LSV curves (Fig. 4a), it can be seen that the overpotentials (j = 10 mA cm⁻²) of Co–P/FTO (1), Co–P/FTO (2) and Co–P/FTO (3) are 174 mV, 125 mV and 148 mV, following the trend of Co–P/FTO (2) > Co–P/FTO (3) > Co–P/FTO (1). The overpotential of Co–P/FTO (2) is only 40 mV, slightly higher than that of Pt–C catalyst. Tafel slopes of the three samples are extracted from Fig. 4a. As shown in Fig. 4b and Table 1, Pt–C has the lowest Tafel slope of 41 mV dec⁻¹. The Tafel slope of Co–P/FTO (2) is 54 mV dec⁻¹, indicating that the HER reaction is via a fast Volmer step followed by a rate-determining Heyrovsky step.^49,50 Compared with Co–P/FTO (1) (65 mV dec⁻¹) and Co–P/FTO (3) (61 mV dec⁻¹), the lower Tafel slope of Co–P/FTO (2) indicates a better application. As conductivity is another important issue, EIS measurements were conducted as presented in Fig. 4c. The diameter of the semi-circle of Co–P/FTO (2) is 40 Ω, which is lowest among the three Co–P/FTO samples. Thus, the activity of the electrodeposited Co–P/FTO follows the order of Co–P/FTO (2) > Co–P/FTO (3) > Co–P/FTO (1).

Fig. 4 HER activity of the three Co–P/FTO samples and Pt/C for comparison measured in 1 M KOH: (a) polarization curves; (b) Tafel slopes derived from polarization curves; (c) Nyquist plots at 1.50 V vs. RHE.

Table 1 The corresponding HER and OER activity of the three different Co–P/FTO samples in 1 M KOH

HER activity	ηj=10 mA cm^−2/mV	b/mV dec^−1	Rct/Ω	Cdl/μF cm^−2
Co–P/FTO (1)	174	65	70	11 000
Co–P/FTO (2)	125	54	40	13 000
Co–P/FTO (3)	148	61	53	12 500
Pt/C	40	41

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OER activity	ηj=10 mA cm^−2/mV	b/mV dec^−1	Rct/Ω	Cdl/μF cm^−2
Co–P/FTO (1)	440	109	22	7250
Co–P/FTO (2)	420	83	10	9820
Co–P/FTO (3)	450	115	13	4810
Pt/C	480	180

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According to the previous literature,⁵¹ cyclic voltammetry at the non-faradaic region is able to probe the electrochemical double layer as a means of estimating the electrochemical active surface area (ECSA). Thus, the electrochemical surface areas are compared through the double layer capacitances (C_dl) as they are proposed to be proportional. C_dl are extracted by plotting the Δj = j_a − j_c at a given potential (0.05 V vs. RHE) against the CV scan rates (40–200 mV s⁻¹) as shown in Fig. 5a–c. The obtained slopes are extracted in order to compare the C_dl of these samples. From Fig. 5d, the calculated C_dl are 11 000 μF cm⁻², 13 000 μF cm⁻² and 12 500 μF cm⁻², corresponding to Co–P/FTO (1), Co–P/FTO (2) and Co–P/FTO (3), respectively. Thus, the ECSA follows the trend of Co–P/FTO (2) > Co–P/FTO (3) > Co–P/FTO (1), which is consistent with the trend of LSV (Fig. 4a). Thus, the enhanced HER property of Co–P/FTO (2) is not only based on the higher LSV activity, but also on the higher ECSA. This may be owing to the special morphology obtained in the middle concentration (Fig. 2c and d), with its more homogeneous size, more grooves on the surface and a more uniform array distribution.

Fig. 5 Cyclic voltammograms (CVs) for HER of the three different Co–P/FTO samples obtained in different mixed solvent in the non-faradaic region: (a) Co–P/FTO (1); (b) Co–P/FTO (2); (c) Co–P/FTO (3); (d) scan rate dependence of the current densities (at 0.05 V vs. RHE).

The OER electrocatalytic activities of the three Co–P/FTO samples have also been investigated, as shown in Fig. 6. The best OER activity is exhibited in Co–P/FTO (2), with a lowest overpotential of 420 mV at j = 10 mA cm⁻² (Fig. 6a), a reference value in evaluating OER catalysts as it represents 10% efficiency of a solar water splitting device.⁵² The overpotentials of Co–P/FTO (1), Co–P/FTO (3) and Pt–C needed to reach 10 mA cm⁻² are 440 mV, 450 mV and 490 mV (Table 1). The Tafel slope (Fig. 6b) of Co–P/FTO (2) is only 83 mV dec⁻¹, which is much lower than Pt–C (165 mV dec⁻¹) and other Co–P/FTO samples (109 mV dec⁻¹ and 115 mV dec⁻¹). From the EIS measurements (Fig. 6c), it can be seen that the Co–P/FTO (2) has the smallest semi-circle diameter, indicating the best conductivity is presented in Co–P/FTO (2). Also, the calculated C_dl of the three Co–P/FTO samples are 7250 μF cm⁻², 9820 μF cm⁻² and 4810 μF cm⁻² (Fig. 7). Thus, the 3D Co–P/FTO (2) arrays electrodeposited at the middle concentration have best OER activity with the lowest overpotential, lowest Tafel slope, best conductivity and a highest ECSA, which may be attributed to the uniform and unique arrays distribution.

Fig. 6 OER activities of the three different Co–P/FTO samples and Pt/C for comparison measured in 1 M KOH: (a) polarization curves; (b) Tafel slopes derived from polarization curves; (c) Nyquist plots at 1.50 V vs. RHE.

Fig. 7 Cyclic voltammograms (CVs) for OER of the three different Co–P/FTO samples obtained in different mixed solvent in the non-faradaic region: (a) Co–P/FTO (1); (b) Co–P/FTO (2); (c) Co–P/FTO (3); (d) scan rate dependence of the current densities (at 1.18 V vs. RHE).

Apart from the activity, durability is also an important issue to be considered. The durability of Co–P/FTO (2) for HER and OER is shown in Fig. 8. It can be seen that after long time I–t measurement of 4 h, Co–P/FTO (2) shows nearly no current loss for both HER (Fig. 8a) and OER (Fig. 8b). After 10 h, the current loss of Co–P/FTO (2) is only 5% for HER and 6% for OER. Thus, the as-prepared Co–P/FTO (2) can be efficient electrocatalyst for water splitting, with not only good activity, but also excellent stability.

Fig. 8 Stability of the Co–P/FTO (2): (a) for HER at η = −140 mV; (b) for OER at η = 440 mV.

As the prepared Co–P nanospheres arrays can be electrocatalysts for both HER and OER in the same alkaline media, it is assumed that it could act as a bifunctional electrocatalyst for overall water splitting at the same time. Thus, these prepared Co–P nanospheres arrays were used as both the working electrode and counter electrode, as fabricated in Scheme 2. The HER and OER catalytic activity of the fabricated Co–P/Co–P couple were examined as shown in Fig. 9. The HER activity of the samples is compared, as shown in Fig. 9a and b. The overpotentials needed to reach j = 10 mA cm⁻² of the three Co–P/Co–P samples and Pt–C/Pt–C are 187 mV, 82 mV, 123 mV and 23 mV, respectively. Although the overpotential of Pt–C/Pt–C to reach 10 mA cm⁻² is the lowest, its increase in current density is the slowest. Thus, at a higher current density, the fabricated Co–P/Co–P electrodes show more efficient HER activity. The Tafel slopes of the three Co–P/Co–P samples and Pt–C/Pt–C are 201 mV dec⁻¹, 91 mV dec⁻¹, 100 mV dec⁻¹ and 34 mV dec⁻¹ (Fig. 9b). The conductivities in HER conditions of these samples are exhibited in Fig. 9c, following the trend of Co–P/Co–P (2) > Co–P/Co–P (3) > Co–P/Co–P (1). Thus, the Co–P/Co–P (2) has the best charge transfer ability. The OER electrocatalytic properties are also compared in Fig. 9d–f. As is shown in Fig. 9d, the OER activities of three Co–P/Co–P electrodes are higher than Pt–C/Pt–C. At j = 10 mA cm⁻², for the sample of the Co–P/Co–P (2), the overpotential needed is only 400 mV, which is even smaller than the Co–P (2)/Pt. The overpotential needed to reach j = 10 mA cm⁻² is 500 mV for Pt–C/Pt–C. The other Co–P/Co–P samples have middle values of 480 mV and 450 mV (Table 2). The Tafel slope of Co–P/Co–P (2) is 83 mV dec⁻¹, which is much lower than Pt–C/Pt–C (183 mV dec⁻¹), Co–P/Co–P (1) (123 mV dec⁻¹) and Co–P/Co–P (3) (99 mV dec⁻¹) (Fig. 9e). Also, the diameters of the semi-circles show the sequence of Co–P/Co–P (2) > Co–P/Co–P (3) > Co–P/Co–P (1) (Fig. 9f). Thus, the prepared Co–P/FTO can be an efficient bifunctional electrocatalyst for overall water splitting and Co–P/FTO (2) has the best HER and OER electrocatalytic activity.

Fig. 9 HER and OER activity of the three different Co–P/Co–P two electrodes and the Pt–C/Pt–C two electrodes for comparison: (a) polarization curves of HER; (b) Tafel slope derived from polarization curves of HER; (c) Nyquist plots of HER at −0.08 V vs. RHE; (d) polarization curves of OER; (e) Tafel slope derived from polarization curves of OER; (f) Nyquist plots of OER at 1.50 V vs. RHE.

Table 2 The corresponding HER and OER activity of the two electrode fabrications of the three different Co–P/Co–P

HER activity	ηj=10 mA cm^−2/mV	b/mV dec^−1	Rct/Ω
Co–P/Co–P (1)	187	201	43
Co–P/Co–P (2)	82	91	20
Co–P/Co–P (3)	123	100	29
Pt/C	23	34

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OER activity	ηj=10 mA cm^−2/mV	b/mV dec^−1	Rct/Ω
Co–P/Co–P (1)	450	123	22
Co–P/Co–P (2)	400	83	10
Co–P/Co–P (3)	480	99	13
Pt/C	500	183

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Conclusion

In summary, the Co–P nanosphere arrays as bifunctional electrocatalysts for water splitting can be obtained through a facile electrodeposition method. The electrolyte concentration has been proven to greatly influence the morphology and properties of electrodeposited Co–P. The prepared 3D Co–P/FTO nanospheres at middle concentration have the best electrocatalytic properties for both HER and OER owing to the more homogeneous size, more grooves on the surface and a more uniform array distribution.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (U1162203 and 21106185) and the Fundamental Research Funds for the Central Universities (15CX05031A).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04478f

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