An Efficient Ternary CoP2xSe2(1-x) Nanowire Array for overall Water Splitting

Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the ethical guidelines, outlined in our author and reviewer resource centre, still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. Accepted Manuscript


Introduction
Compared with fossil fuels, hydrogen energy is one of the promising alternatives for renewable fuel due to its sustainable and environmentally friendly characteristics. [1][2][3][4][5][6][7] Electrocatalytic water splitting is an efficient route to produce pure hydrogen and oxygen gas. Typically, it is composed of hydrogen evolution reaction (HER, 2H + + 2e -→H 2 ) in cathode electrode and oxygen evolution reaction (OER, 2H 2 O → 4e − + 4H + + O 2 ) in anode electrode. 8- 10 To date, Pt-based and Ir/Rubased materials are regarded as the benchmarks for HER and OER, respectively. Unfortunately, their high cost hinders the practical application in large-scale. It is, thus, extremely urgent to develop inexpensive and earth-abundant alternative electrocatalysts. In the last few years, great efforts have been made in developing non-noble metal catalysts, including transition metal chalcogenides, 11-19 phosphides, 20-24 and carbides 25,26 for HER, as well as transition metal phosphate, 27 oxides 28-31 and hydroxides [32][33][34][35] for OER. Notably, overall water splitting must be operated in the same electrolyte in the practical process. 36 What's more, water splitting conducted in alkaline solution has emerged as a much more effective method for commercial hydrogen production due to the scare electrocatalyst and much more energy-intensive of OER in acidic media. 37,38 Therefore, it's attractive to develop novel non-noble bifunctional electrocatalysts with high activity and stability for overall water splitting in alkaline solution. Nowadays, transition metal dichalcogenides (TMDs) such as NiSe 2 39 and CoS 2 40, 41 and transition metal phosphides (TMPs) like CoP, 21,42,43 FeP, 44 and Ni 12 P 5 45 have been extensively developed for HER with high performance and stability in both acidic and basic media due to their excellent catalytic performance and high stability. Unlike the binary materials, component controllably synthesized ternary materials, such as WS 2(1−x) Se 2x , 46 MoS 2(1−x) Se 2x , 47 CoPS, 48,49 NiP x Se 2−x 50 and CoS 2x Se 2(1−x) 51 are found to require a much lower overpotentials. Encouraged by this and considering the fact that substitution of the P atom in CoSe 2 would tangibly modify the electronic structure of the resulting ternary material, we designed a rational synthesis of CoP 2x Se 2(1−x) . Benefitting from the advantage of the controllability of our method, the optimum P to Se ratio can improve the activity and stability via tailoring the electronic structure and allowing the exposure of more active sites. It is therefore reasonable to predict that CoP 2x Se 2(1−x) could be a promising candidate for HER.
Herein, we successfully synthesize the ternary necklace-like CoP 2x Se 2(1−x) nanowire arrays, which provide much more electrochemical active area, on carbon fiber via phosphorization and selenization reaction. Being an electrocatalyst for HER in acidic media, the CoP 2x Se 2(1−x) electrode achieves current density of 10 mA cm −2 at overpotential of 70 mV in acidic solution and shows high stability. Significantly, the CoP 2x Se 2(1−x) electrode also exhibits excellent electrocatalytic activity and durability in alkaline condition. It needs only 98 mV to reach current density of 10 mA cm −2 which is very close to the HER performance in acidic solution. Furthermore, to realize a practical utilization, we designed overall water splitting setup in such a way that the CoP 2x Se 2(1−x) NWs and Co(OH) 2 NWs served as cathode and anode, respectively. Accordingly, this configuration requires a cell voltage of 1.65 V to reach a current density of 10 mA cm −2 , suggestive of its promising feature for practical realization of water splitting.

Synthesis of Co(OH) 2 NWs
The Co(OH) 2 nanowires on a CF was synthesized by a method that we have reported in our previous work. 52

Synthesis of CoP 2x Se 2(1−x) NWs
A horizontal quartz tube furnace is utilized to convert the Co(OH) 2 NWs into CoP 2x Se 2(1−x) NWs. The Co(OH) 2 NWs on CF was placed at the downstream side of the furnace and a mixture of phosphorus and selenium powder was placed at the upstream side of the furnace. At the beginning, the tube furnace was flushed under a 100 sccm Ar flow for three times to create an oxygen-free environment. Subsequently, the zones of the substrate and powder were quickly heated to 450 °C and 270 °C in 20 min, respectively. The reaction temperature maintained for 60 min to fully convert the Co(OH) 2 NWs into CoP 2x Se 2(1−x) NWs, followed by natural cooling down. During the synthesis process, the flow of Ar is kept at a rate of 100 sccm.

Characterizations
The morphologies of Co(OH) 2 NWs and CoP 2x Se 2(1−x) NWs were characterized by Hitach S-4800 scanning electron microscopy (SEM) under 20 kV and TecnaiF20 transmission electron microscopy (TEM) at 200 kV. STEM-EDX elemental mapping was characterized by TecnaiF20 and the energy dispersive X-ray spectroscopy (EDX) was performed on Hitach S-4800. X-ray diffraction (XRD) patterns (Philips X'Pert Pro Super) were obtained using Cu Kα radiation (λ = 1.5418 Å) and X-ray photoelectron spectroscopy (XPS) was tested on ESCALAB250Xi. The pH of the electrolyte was performed by the METTLER TOLEDO pH meter (FE20).

Electrochemical measurements
The test for hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) were performed in a typical threeelectrode configuration at an electrochemical station (CHI 660D The ternary CoP 2x Se 2(1−x) NWs were prepared through selenizing and phosphorizing Co(OH) 2 NWs, simultaneously. (see the Experimental Section). 51,52 In general, we first grew Co(OH) 2 NWs ( Fig. S1a-b), which is utilized as the precursor for achieving CoP 2x Se 2(1−x) NWs ( Fig. S2a and b), on carbon fiber (CF) through hydrothermal method. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) were used to observe the morphologies of Co(OH) 2 NWs (Fig. S1a-b) and CoP 2x Se 2(1−x) NWs. Low magnification TEM (Fig. 1a) reveals that the diameter of the CoP 2x Se 2(1−x) NW is around 200 nm with the length of several micrometers. Different with the smooth surface of Co(OH) 2 NWs (Fig. S1b), the resulting CoP 2x Se 2(1−x) NWs have a rough surface ( Fig.  1a) with the necklace-like morphology. Moreover, the highresolution TEM (HRTEM) image in Fig. 1b demonstrates the lattice fringes with the interplanar spacing of 0.244 nm, which belongs to crystal plane of (012). The scanning TEM (STEM) image and the corresponding energy-dispersive X-ray (EDX) elemental mapping images ( Fig. 1c)  Remarkably, we observed that the XRD pattern of CoSe 2 is consistent with orthorhombic marcasitestructural phase (PDF# 10-0408) (Fig. 1f left) and cubic pyritestructural phase (PDF# 88-1712) (Fig. 1f right), indicating that the CoP 2x Se 2(1−x) NWs consist of those two phases. To further confirm the chemical composition of CoP 2x Se 2(1−x) , we also conducted X-ray photoelectron spectroscopy (XPS) analysis. Fig. 2a shows that the two peaks for Co 2p 3/2 and 2p 1/2 are located at binding energies of 779.05 eV and 794.01 eV. And the peaks of Se 3d 5/2 and Se 3d 3/2 are located at 54.55 eV and 55.36 eV, respectively (Fig. 2b). Compared with the reported data of Co and Se in binary CoSe 2 , 53 this results show a very slight shift due to the incorporation of P element. The deconvulated P 2p spectrum presents two peak regions (Fig. 2c).
One of the regions has two peaks with one located at the binding energy of 129.60 eV and the other at 130.1 eV (P 2p 3/2 and 2p 1/2 ), which corresponds to phosphorus anions. The second region is centred at 133.6 and 135.5 eV (unresolved doublet) and these two peaks can be assigned to the phosphate-like P. This is agree with the reported results. 49 Meanwhile, the quantitative analysis also shows that the ratio of P and Se is near to 2 : 1 and this result is consistent with the EDX data. The HER activity of CoP 2x Se 2(1−x) NWs was evaluated in 0.5 M H 2 SO 4 electrolyte using a typical three-electrode setup at room temperature. The CoP 2x Se 2(1−x) NWs on CF was directly employed as the working electrodes, a Pt wire and a saturated calomel electrode (SCE) used as counter electrode (CE) and reference electrode (RE), respectively. As comparison, bare CF and Pt electrodes were also measured under the same condition. In order to reflect the intrinsic activity of the catalysts, all initial data are presented after iR correction. 54 Fig. 3a shows the polarization curves of CoP  (Table S1). 51 We also compared the HER activity of CoP 2x  (Fig. S3 and Fig. S4a). The overpotential of CoP 1.37 Se 0.63 NWs grown on CFs at a catalytic current density of 10 mA cm −2 is lowest among the samples with various atomic ratios (Fig. 3b), implying the best HER performance of CoP 1.37 Se 0.63 NWs. In order to assess the kinetics of the electrodes, Tafel plots of CoP 1.37 Se 0.63 NWs, Pt (Fig. 3c) S4b) were extracted from their corresponding polarization curves. Accordingly, the Tafel plots (Fig. 3c)  to 200 mV s −1 ) (Fig. S5a-b). As shown in Fig. S5c Fig. 3d shows that the decrease of catalytic current density is negligible for up to 12 h of electrolysis, indicating the good stability of the CoP 1.37 Se 0.63 NWs for HER in acid solution. The SEM image and XRD pattern ( Fig. S6a and Fig. S7) of the CoP 1.37 Se 0.63 NWs after the stability measurement also verify the robustness of CoP 1.37 Se 0.63 NWs. . The HER performance of the CoP 1.37 Se 0.63 NWs is also better than some reported binary Co-based materials. 40,43,60 In comparison with CoSe 2 (Fig. S3) (Fig. S4c), CoP 1.37 Se 0.63 NWs exhibits the best HER activity in alkaline solution. As shown in Fig. 3f, the overpotential required to achieve a current density of 10 mA cm −2 for CoP 1.37 Se 0.63 NWs is much lower than CoSe 2 (362mV), CoP 0. 45   We then assessed the OER activity for Co(OH) 2 NWs in alkaline solution. As an OER electrode, the Co(OH) 2 NWs requires an overpotential of 290 mV to achieve a catalytic current density of 10 mA cm −2 and its OER performance is even better than Pt (Fig. 4a). The corresponding Tafel slope of Co(OH) 2 NWs is 71 mV dec −1 (Fig.  4b), which is smaller than most of the reported metal hydroxide, Please do not adjust margins verifying a favorable OER kinetics for Co(OH) 2 NWs. Fig. 4c shows a multi-step chronopotentiometric curve for Co(OH) 2 NWs in 1.0 M KOH. The current is increased from 50 to 450 mA cm -2 and remains 500s for each increment of 50 mA cm −2 . The potential immediately levels off at 1.62 V at the start current of 50 mA cm −2 and remains steady for the rest 500 s and the other steps also show similar results, indicating the excellent mass transportation and mechanical robustness of the Co(OH) 2 NWs. The time-dependent current density curve (Fig. 4d) was also studied to probe the long-term stability of Co(OH) 2 NWs. According to Fig. 4d, there is no obvious current density change for up to 12 hours OER test, implying the excellent stability of Co(OH) 2 NWs for OER in strong alkaline solution.   (Fig. 5b). Accordingly, there is no obvious change in the current density with in 12 hours test for overall water splitting in 1.0 M KOH, suggesting the excellent stability of CoP 1.37 Se 0.63 /Co(OH) 2 . Fig. 5c presents a photograph to illustrate that an voltage of 1.6V can drive overall water splitting with a large amount of H 2 bubbles on the cathode and O 2 bubbles on the anode, confirming the high performance of the CoP 1.37 Se 0.63 /Co(OH) 2 .

Conclusions
In summary, ternary CoP 2x Se 2(1−x) NW arrays on carbon fiber was successfully fabricated via simultaneous phosphorization and selenization of Co(OH) 2 NWs. The CoP 2x Se 2(1−x) NWs can be directly used as an excellent and durable electrode for HER under both acidic and alkaline conditions. After optimizing the composition, CoP 1.37 Se 0.63 NWs can afford a current density of 10 mA cm −2 for HER at 70 mV and 98 mV in acidic and alkaline media, respectively. Furthermore, the Co(OH) 2 NWs also shows markedly high and stable catalytic activity toward OER in alkaline solution and requires only 290 mV to reach a current density of 10 mA cm −2 . The overall electrochemical water splitting in a strong alkaline solution (1.0 M KOH) is further conducted with CoP 1.37 Se 0.63 NWs as cathode and Co(OH) 2 NWs as anode. This setup requires a voltage of 1.65 V to generate 10 mA cm −2 water-splitting current density. Our work offers an earth-abundant, binding-free material and efficient catalysts for practical overall water splitting in alkaline media.

Table of contents:
Varying compositions of ternary CoP 2x Se 2(1-x) nanowires (NW) are synthesized via hydrothermal method followed by CVD method. Among them, CoP 1.37 Se 0.63 NW needs overpotentials of only 70 mV and 98mV to achieve a current density of 10 mA/cm 2 in acidic and alkaline solution, respectively. This evidences the fact that this ternary material shows high catalytic activity in both acidic and alkaline condition.