Bifunctional Electrocatalysts Based on Mo-Doped NiCoP Nanosheet Arrays for Overall Water Splitting
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Freestanding Mo-doped NiCoP nanosheets are designed as bifunctional electrocatalysts for overall water splitting.
Remarkable electrocatalytic performances are achieved by Mo doping, where a low-water-splitting voltage of 1.61 V at 10 mA cm−2 is obtained.
KeywordsWater splitting Bifunctional electrocatalyst Electronic structure Freestanding Metal phosphides
Electrochemical water splitting, consisting of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), is a promising approach to generate sustainable H2 from water [1, 2, 3, 4]. Even though Ir-/Ru-based oxides and Pt catalysts show excellent electrocatalytic performances for OER and HER, the high cost and the scarcity of these noble catalysts greatly hinder their practical applications [5, 6, 7, 8]. When different electrodes are used for a single water-splitting device, the incompatibility and different reaction kinetics in OER and HER may lead to inferior efficiency [9, 10, 11]. Thus, it is highly imperative to design low-cost and high-efficiency electrocatalysts for HER and OER simultaneously.
Among various alternative materials, transition metal phosphides (TMPs) have gained considerable attention, owing to the suitable d-electron configuration and rich chemical states [12, 13, 14]. In particular, NiCoP with the enhanced electrical conductivity and the synergistic effect from Ni and Co shows good catalytic performances for overall water splitting [15, 16]. To further improve the catalytic activity, considerable efforts have been devoted to increasing the number of active sites and improving the intrinsic activity of each site . On the one hand, nano-/microstructure design has been conducted to increase the number of active sites [18, 19, 20]. However, most of current works focus on preparing powder-formed electrocatalysts, and the possible aggregation and involved polymeric binders may block the active sites and hinder the electron transfer [19, 20]. To solve this concern, a promising strategy is to develop freestanding arrays directly on conductive substrates, thereby assembling a binder-free electrode, which favors gas evolution and boosts electron transfer [21, 22]. However, simply increasing surface areas could not primarily change the intrinsic activity of each site. On the other hand, the element doping is a promising strategy to essentially improve the intrinsic activity of active sites, which could modify the electronic structure and accelerate the reaction kinetics [23, 24]. For example, O, Fe, and carbon dot-doped NiCoP have been fabricated, which showed enhanced electrocatalytic performances [25, 26, 27]. According to the reported research , Mo-based phosphides show the best HER performances among various binary phosphides. In this case, it is expected that the doping Mo on freestanding NiCoP nanosheets could achieve the improvement in electrocatalytic performances.
2 Experimental Section
2.1 Synthesis of Mo-Doped NiCoP Nanosheets on Carbon Cloth
Prior to synthesis, carbon cloth (2 × 5 cm2) was treated by concentrated nitric acid at 100 °C for 3 h, then washed with deionized water and ethanol and dried in the air . The whole synthesis process is shown in Scheme 1. At first, Mo-doped NiCo precursors were synthesized by the hydrothermal process. Typically, NiCl2·6H2O (4 mmol), CoCl2·6H2O (4 mmol), and Na2MoO4·6H2O were dissolved in 60 mL deionized water to form a pink solution. To optimize the Mo doping, different contents of Na2MoO4·6H2O (0, 0.1, 0.2, 0.3, and 0.4 mmol) were conducted to synthesize various Mo–NiCo precursors (shortly named as Mo–NiCo-precursor-1–4). The pink solution with treated carbon cloth was transferred to 100-mL Teflon-lined stainless-steel autoclave, heated at 100 °C for 6 h. After that, the precursors were collected, washed with deionized water and ethanol for several times, and dried in vacuum overnight.
Secondly, the Mo–NiCo precursors were converted to corresponding Mo–NiCoP by the phosphation process (shortly named as Mo–NiCoP-1–4). The as-prepared precursors and 0.5 g NaH2PO2·H2O were placed at both ends of a tubular furnace, and NaH2PO2·H2O was at the upstream side. The phosphation process was conducted at 300 °C for 1 h with heating rate of 5 °C min−1 under continuous Ar flow.
Finally, the core-branched Mo–(Ni,Co)OOH arrays were in situ transformed from Mo–NiCoP by the electrochemical activation in the three-electrode cell (shortly named as E-Mo–NiCoP-1–4). The electrochemical activation was conducted by cyclic voltammetry (CV) under a potential window of 0–1 V versus Hg/HgO in 6 M KOH at the scan rate of 10 mV s−1. Different CV cycles (0, 12, 25, 50, and 75) were applied to optimize the OER performances of obtained samples.
2.2 Material Characterizations
The structure and morphology of as-obtained samples were analyzed by scanning electron microscopy (SEM, Helios NanoLab 600i) and transmission electron microscopy (TEM, Tecnai G2 F30). The phases were characterized by X-ray diffraction (XRD, D8 Advance). The chemical states were verified by X-ray photoelectron spectroscopy (XPS, Thermo Fisher) measurements.
2.3 Electrochemical Measurements
All tests were performed on the CHI760E electrochemical station. Both HER and OER were measured in a three-electrode configuration using 1 M KOH as electrolyte. The carbon cloth with active materials was directly employed as the working electrode in electrochemical measurement, while the graphite bar and Hg/HgO electrode were used as the counter and reference electrodes, respectively. The polarization curves were recorded at 2 mV s−1 and were compensated with iR correction. All potentials were converted into the reversible hydrogen electrode (RHE). Electrochemical impedance spectroscopy (EIS) tests were conducted in the frequency range from 0.1 Hz to 100 kHz. The mass loading of NiCoP or Mo-doped NiCoP on carbon cloth was about 2 mg cm−2. The reference electrodes of Pt/C and RuO2 with 2 mg cm−2 mass loading were also prepared on carbon cloth, and the prepared method was according to previously reported researches [32, 33].
3 Results and Discussion
3.1 Structural and Morphological Characterizations
To investigate the chemical states and electronic structures, XPS measurements are taken to comparably analyze the obtained samples. As for all Mo–NiCoP samples in Fig. S5, the binding energies of Ni 2p have been positive-shifted with gradually increasing Mo doping. As for Ni 2p in Mo–NiCoP-3 (Fig. 1h), the peaks at 853.7 and 870.9 eV are from the Niδ+ in NiCoP, while the peaks at 857.0 and 874.9 eV are attributed to oxidized Ni species, as well as two peaks from the satellites [34, 35]. In Co 2p spectra (Fig. 1i), the Co 2p1/2 region shows the two peaks at 793.8 and 798.4 eV as well as a satellite peaks. For Co 2p3/2, the peak at 779.0 eV is attributed to Co–P bond, while the peak at 781.8 is assigned to Co-oxidized state [35, 36]. As for P 2p in Fig. 1j, the peaks at 130.5, 129.4, and 134.1 eV are from P 2p1/2, P 2p3/2, and surface-oxidized P–O species . In addition, the Mo 3d3/2 at 235.5 eV and Mo 3d5/2 at 232.2 eV in Mo 3d spectrum (Fig. 1k) demonstrate the main presence of Mo6+ . Thus, Mo element exists in the form of atomic substitution by Ni or Co. To investigate the Mo mass loading in each samples, we also conducted the inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. The Mo mass loadings for Mo–NiCoP-1–4 are 0.078, 0.155, 0.199, and 0.257 mg cm−2 by ICP analysis. According to previous researches [25, 38, 39], the XPS peak shift shows the changes of electronic configuration and electron transfer between different cations, further implying the electronic interactions after Mo doping. By comparing Ni 2p and Co 2p spectra in NiCoP and Mo–NiCoP-3, it can be found that the binding energies have been positive-shifted by about 0.6 eV. It demonstrates that Mo doping could effectively modify the electronic structure of Ni and Co centers in obtained samples [24, 39]. Such positive shift in binding energies could be attributed to electron transfer owing to the presence of Mo6+, suggesting the electron transfer from Mo atoms to nearby atoms [40, 41]. The fast electron transfer brought by Mo doping could be beneficial for OER and HER process [40, 42].
3.2 The Effect of Mo Doping on HER Performances
To bridge the structural properties and intrinsic activity, the electrochemical surface area (ECSA) was estimated by the electrochemical double-layer capacitance (Cdl). Based on the cyclic voltammograms (CVs) versus the scan rate in Fig. S9, the Cdl values are calculated in Fig. 2c. Obviously, Mo–NiCoP-3 electrode shows the high Cdl value up to 76.0 mF cm−2, which is much higher than that of pure NiCoP (21.3 mF cm−2). This comparison suggests that Mo doping in NiCoP nanosheet arrays could greatly enhance the active sites, leading to performances in HER. Figure 2d shows the consecutive multi-step chronoamperometric tests for Mo–NiCoP-3, where the potential remains steady in each step. It suggests the excellent mass transport and mechanical properties of Mo–NiCoP-3 electrode in HER tests. The durability of Mo–NiCoP-3 electrode in HER was tested in 10, 20, and 50 mA cm−2 for each 15 h, as shown in Fig. 2e. Obviously, in each current density, the potentials of Mo–NiCoP-3 electrode remain stable with small degradation. In addition, SEM, XRD, and TEM results in Fig. S10 show that the morphology, structure, and phases of Mo–NiCoP-3 are well retained after longtime HER tests, again demonstrating the good durability. To investigate the charge transfer kinetics during HER, the EIS tests were conducted at − 0.27 V versus RHE. The Nyquist plots in Fig. 2f unravel that Mo–NiCoP-3 electrode shows the lowest charge-transfer resistance (Rct), suggesting the fastest electron transport for HER [39, 43]. It may be contributed to the modified electronic structure of NiCoP by Mo doping. And, the lowest Rct further confirms the fastest evolution kinetics and smallest Tafel slop in HER.
3.3 The Effect of Electrochemical Activation on OER Performances
As for all E-Mo–NiCoP samples (Fig. S14a), it can be found that the binding energies for Ni 2p have been positive-shifted with gradually increasing Mo doping. It demonstrates that Mo doping could effectively modify the electronic structure of Ni centers in obtained samples. And no obvious P 2p signal could be found in all E-Mo–NiCoP samples (Fig. S14b), which suggests that P element has been leaked during electrochemical activation. Detailed XPS analysis on E-NiCoP-3 is shown in Fig. S15b-f. The energy separation up to about 17.6 eV in Ni 2p spectrum demonstrates the formation of NiOOH (Fig. S15b) . As for Co 2p in Fig. S15c, there are obvious satellite signals, suggesting the presence of low-spin Co3+ state . As shown in Fig. S15d, no obvious P 2p signal could be found, even increasing the etching time. In addition, the Mo 3d3/2 and Mo 3d5/2 in Fig. S16e demonstrate the main presence of Mo6+. Further, three peaks (Fig. S15f) at 529.7, 531.5, and 532.6 eV are attributed to the lattice oxygen, hydroxyls, and oxygen defects [45, 46]. Thus, based on the above analysis, core-branched Mo-doped (Ni,Co)OOH arrays are in situ formed after electrochemical activation.
In summary, we have demonstrated cation doping and in situ electrochemical activation to boost the HER and OER performances of transition metal phosphides. Exemplified by freestanding NiCoP nanosheets on carbon cloth, Mo incorporation could effectively modulate the electronic configuration and increase the electroactive sites. Further, by electrochemical activation, the core-branched Mo-doped (Ni,Co)OOH arrays are formed, which could fully boost the OER activities. Consequently, the optimal Mo-doped NiCoP nanosheet arrays show the enhanced electrocatalytic performances as (pre-) electrocatalyst for efficient water splitting. Our work provides new insights for designing nonprecious bifunctional electrocatalysts by cation doping and in situ electrochemical activation.
The support from the National Natural Science Foundation of China (Nos. 51575135, 51622503, U1537206, and 51621091) is highly appreciated.
- 11.Q. Hu, X. Liu, B. Zhu, L. Fan, X. Chai et al., Crafting MoC2-doped bimetallic alloy nanoparticles encapsulated within N-doped graphene as roust bifunctional electrocatalysts for overall water splitting. Nano Energy 50, 212–219 (2018). https://doi.org/10.1016/j.nanoen.2018.05.033 CrossRefGoogle Scholar
- 13.J. Li, W. Xu, J. Luo, D. Zhou, D. Zhang, L. Wei, P. Xu, D. Yuan, Synthesis of 3D hexagram-like cobalt–manganese sulfides nanosheets grown on nickel foam: a bifunctional electrocatalyst for overall water splitting. Nano-Micro Lett. 10, 6 (2018). https://doi.org/10.1007/s40820-017-0160-6 CrossRefGoogle Scholar
- 14.Y. Huang, X. Song, J. Deng, C. Zha, W. Huang, Y. Wu, Y. Li, Ultra-dispersed molybdenum phosphide and phosphosulfide nanoparticles on hierarchical carbonaceous scaffolds for hydrogen evolution electrocatalysis. Appl. Catal. B-Environ. 245, 656–661 (2019). https://doi.org/10.1016/j.apcatb.2019.01.034 CrossRefGoogle Scholar
- 30.T. Wang, G. Nam, Y. Jin, X. Wang, P. Ren et al., NiFe (Oxy) hydroxides derived from NiFe disulfides as an efficient oxygen evolution catalyst for rechargeable Zn–air batteries: the effect of surface S residues. Adv. Mater. 30, 1800757 (2018). https://doi.org/10.1002/adma.201800757 CrossRefGoogle Scholar
- 45.L. Hou, Y. Shi, C. Wu, Y. Zhang, Y. Ma et al., Monodisperse metallic NiCoSe2 hollow sub-microspheres: formation process, intrinsic charge-storage mechanism, and appealing pseudocapacitance as highly conductive electrode for electrochemical supercapacitors. Adv. Funct. Mater. 28, 1705921 (2018). https://doi.org/10.1002/adfm.201705921 CrossRefGoogle Scholar
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