Direct electrodeposition of phosphorus-doped nickel superstructures

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Direct electrodeposition of phosphorus-doped nickel superstructures from choline chloride-ethylene glycol deep eutectic solvent for enhanced hydrogen evolution catalysis Changbin Sun, Junrong Zeng, Hao Lei, Wenqiang Yang, and Qibo Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05302 • Publication Date (Web): 20 Nov 2018 Downloaded from http://pubs.acs.org on November 20, 2018

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ACS Sustainable Chemistry & Engineering

Direct electrodeposition of phosphorus-doped nickel superstructures from choline chloride-ethylene glycol deep eutectic solvent for enhanced hydrogen evolution catalysis Changbin Sun∇a, Junrong Zeng∇a, Hao Leia, Wenqiang Yanga, Qibo Zhanga,b* a

Key Laboratory of Ionic Liquids Metallurgy, Faculty of Metallurgical and Energy Engineering, Kunming University of Science

and Technology, Kunming, 650093, P.R. China b State

Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming, 650093, P.R.

China

*Corresponding Author: Key Laboratory of Ionic Liquids Metallurgy, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, No.68 WenChang Road, 121 Street, Kunming, 650093, P.R. China. Email address: [email protected] (Q. B. Zhang), Tel: +86-871-65162008; Fax: +86-871-65161278

Abstract Hydrogen produced by electrochemical water splitting offers a hopeful and renewable solution to address the global energy crisis; however, development of highly efficient hydrogen generation electrocatalysts remains a big challenge. Herein, self-supported P-doped nickel superstructure films (NiPx) developed on Cu foil were prepared via a facile one-step electrodeposition route from the choline chloride-ethylene glycol (Ethaline)-based deep eutectic solvent (DES). Two depositional patterns including potentiostatic deposition and a consecutive potential cycling approach were compared, and the latter model with a potentiodynamic control was found to be a valid electrochemical protocol to create crack-free NiPx films which were highly active for catalyzing hydrogen evolution reaction (HER) under an alkaline condition. The optimal deposited sample with a Ni : P ratio of 1 : 0.056 achieved a low overpotential of 105 mV to deliver a current density of 10 mA cm-2 with a small Tafel slope of 44.7 mV dec-1, and excellent catalytic stability for at least 60 hours. Detailed experimental investigations coupled with theoretical analyses revealed that the high-performance catalytic activity of the NiPx films was originated from the enriched active sites and enhanced electronic conductivity induced by P-doping, which also altered the surface electronic structure of the material, and resulted in a lower energy barrier for water dissociation and favorable H adsorption free energy. This study provides a new electrochemical potentiodynamic strategy performed in DES for the 1

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fabrication of transition-metal-phosphide-based catalysts for enhancing HER catalysis.

Keywords: Nickel-phosphide films; Hydrogen evolution reaction; Deep eutectic solvent; Electrodeposition; DFT simulations

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Introduction Electro-splitting of water driven by electrical energy is believed to be an efficient and practical approach for largescale hydrogen production that is recognized as the cleanest energy resource to replace current fossil fuels.1-3 However, practical hydrogen generation by the water-splitting technology is faced with sluggish hydrogen evolution reaction (HER) kinetics. To smooth the process, highly efficient and active electrocatalysts are vitally required to lower the overpotential.4-6 The state-of-the-art HER electrocatalysts are Pt-based materials due to their ideal hydrogen adsorption properties with a near-zero HER overpotential. Unfortunately, their limited reserves and high cost make them unsuitable for large-scale applications.7-9 Thus, the development of earth abundant and lowcost catalysts with high electrocatalytic performance toward HER is an imperative but challenging issue.10-11 Nickel has emerged as an interesting Pt alternative metal for its abundant reserves and potential catalytic activity toward HER, particularly in alkaline solutions;12 however, the hydrogen adsorption on metallic Ni is too strong, causing desorption of hydrogen product gas to be kinetically sluggish.13 Therefore, its activity is still insufficient for the large-scale hydrogen production and needs to be further improved. To accomplish this, intensive efforts have been made to enhance the HER catalysis of these nickel based materials.14-18 Engineering the electronic structure of a catalyst to optimize hydrogen adsorption free energy has been demonstrated to be an efficient approach in improving the intrinsic activity, which can be achieved through non-metal-anion (S, P, Se) doping/alloying.19-26 For instance, P doping/substitution into the Ni structure was found to tune the electronic structure of the catalyst and reduce the energy barrier for hydrogen adsorption, thus enhancing its HER activity.27 It was proposed that the enhancement was related to an ensemble effect within it, where the P and Ni acted as the proton-acceptor and hydride-acceptor centers, respectively, that worked together to facilitate the HER.28 Another efficient strategy is to build nanostructures with three-dimensional interconnected architectures.29-30 Such nanostructured catalysts combine the futures of (1) a high active surface area and plenty of catalytic sites, (2) handy ion- and electrontransport pathways, and (3) high electrical conductivity with favorable reaction kinetics. In this regard, a variety of nickel-based nanomaterials, such as oxides,31 hydroxides,32 sulfides,33-34 phosphides,35-36 nitrides,37 and selenides,38 etc., for enhancing HER catalysis have been reported in the past. However, the majority of those catalysts are synthesized by the solvothermal/hydrothermal method, which generally involves time-consuming and tedious synthesis process.20 Alternatively, electrodeposition offers a facile and scalable appoarch to controllably construct diverse nanostructures.39-40 More importantly, the electrochemical deposition approach is more suitable for the preparation of self-surpported monolithic catalysts, which are of excellent electrical conductivity and structural 3



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stability and are, therefore, more efficient than their powder counterparts and appreciated for their practical applicability.17-21, 41-43 In the present work, we report the development of self-supported P-doped Ni superstructures on Cu foil (NiPx/Cu) by the electrodeposition process performed in Ethaline-based DES, as highly efficient catalysts for HER catalysis in alkaline electrolyte. Our results demonstrate that potentiodynamic deposition control is an effective strategy for fabricating crack-free NiPx films via artful P doping. Benefiting from the remarkable synergistic effects between the P and Ni, the catalyst exhibits excellent HER catalytic performance, requiring a low overpotential of 105 mV to reach a current density of 10 mA cm-2 with a small Tafel slope of 44.7 mV dec-1 (without IR-correction). Moreover, it also displays superior long-term stability, maintaining the activity for at least 60 h with negligible degradation.

Results and discussion The electrochemistry of Ni(II) ions in Ethaline/100 mM NiCl2∙6H2O without and with various concentrations of NaH2PO2 (from 2 to 50 mM) was firstly studied by CV measurement. Fig. 1a shows the typical CVs recorded at a Pt disk working electrode. In all cases, a well-defined redox couple assigned to the NiII/Ni0 redox reaction is obtained.44-45 It is interesting to find that the electrochemical reduction process is promoted with introduction of NaH2PO2, leading to a positive shift in the initial reduction potential for Ni(II) species. A more pronounced effect is observed at a higher NaH2PO2 concentration. The additional anodic peak obtained at 0.54 V vs. Ag/Ag+ could be ascribed to the stripping of the deposited Ni that has been alloyed with P (NiPx). Further investigations of the CVs obtained at different scan rates (Fig. S1) show that the jp and square root of the scan rate meet a linear correlation with the line passing through the origin for all cases (Fig. 1b), indicating that the electrochemical reduction of Ni(II) species in Ethaline is controlled by diffusion. The corresponding diffusion coefficient (D) of Ni(II) species and charge transfer coefficient (α) of the associated reduction process are determined by Randles-Sevcik and its related equations,46 respectively (see more detail in supporting information, Tables S1 and S2). According to the results obtained at various addition levels of NaH2PO2, both the diffusivity of active species and charge transfer process are accelerated upon the introduction of P until it reached 30 mM, as shown in Fig. 1c. The enhancement in the diffusivity of Ni(II) species should be due to the changes on the conductivity and viscosity with the addition of NaH2PO2, which favors fast mass transport. In addition, since the α is known as the fraction of the interface potential at an electrode/ electrolyte interface that functions in lowering the free energy barrier for the electrochemical reaction.47 A larger α value with the incorporation of P represents more efficiency on the potential utility, thereby facilitating rapid reaction kinetics. Moreover, the transformation in the nucleation and growth model for the 4


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deposition process from heterogeneous surfaces to homogenous surfaces (more energetically favorable) is also observed in the course of time.48 From the potential cycling dependence on the CVs (Fig. S2), it is clear that the onset of the reduction peak shifts towards more positive potentials, which becomes stable after 3 rounds of continuous scanning. For further deposition study, two depositional patterns including potentiostatic deposition (PSD) and potentiodynamic deposition (PDD) were performed. Fig. 1d shows current-time curves for the PSD of Ni and NiPx from Ethaline/100 mM NiCl2∙6H2O in the absence and presence of 10 mM NaH2PO2 at -0.80 V vs. Ag/Ag+ and 333 K. The introduction of P is found to smooth the deposition process by shortening the deposition time at a control charge density of 4.25 C cm-2 (inset of Fig. 1d), which is consistent with our CV findings. A similar promotion trend for the deposition process is also observed for the PDD accomplished by a consecutive potential cycling approach, shown in Fig. 1e. The potential scan region (between -0.3 and -1.0 V vs. Ag/Ag+) was selected to allow the formation of homogeneous deposits on the electrode surface and simultaneously avoid their mass dissolution. Upon cycling, behavior similar to that seen in Fig. 1a and Fig. S2 is obtained whereby the current density of the reduction peak increases in succession and the onset of the reduction peak steps to more positive potentials. The most probable explanation is the proliferation of active surface area upon the deposition progresses, and lower reaction barrier required for homogenous nucleation at the later stage. The films deposited from each solution using both deposition models produce gray or silver grey layers with fairly good adhesion on the Cu substrate (Fig. S3). The structure and morphology characterizations of the deposits were accessed by XRD, SEM and TEM. Fig. 2a shows the XRD pattern of the as-obtained samples. Except for the signal from the Cu substrate, the diffraction peaks at 43.5° and 51.8° for the films deposited from the NaH2PO2-free solvent are well-indexed to the (111) and (200) planes of the fcc-structured Ni (JCPDS 04-0850), respectively. In contrast, with the addition of NaH2PO2, the diffraction peaks located at 43.5° for Ni (111) slightly displace to a higher angle of 44.7° (Fig. 2b). Such a right shift of diffraction peak suggests the atom incorporation of P into the Ni lattice via partial substitution,49-50 resulting in the constriction of unit cells, as P atom (1.10 Å) holds a smaller ionic radius in comparison with Ni (1.15 Å). This result reveals the successful formation of NiPx layers via electrodeposition. The broadening and low intensity of the diffraction peaks are due to the nanometer size of the deposits and their thin thickness. For both cases, the deposit thickness under a control charge density of 4.25 C cm-2 is measured to be ca. 1 μm. The PSD_Ni-10P sample obtained by PSD shows a nanoparticle-assembled surface filled with crevices (Fig. 2c), which is mainly because of the large internal stress generated during the deposition process under a constant potential.45 As the percentage of P atoms increases, more cracks with a widened gap are obtained (Fig. S4), accompanied by a trend of crystal transition from 5



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the crystalline state to an amorphous form (Fig. S5). Impressively, PDD is found to be a valid route to relieve the effects of internal stress, leading to the creation of crack-free NiPx films, as evidenced by the PDD_Ni-10P sample fabricated via a consecutive potential cycling approach (Fig. 2d and Fig. S6). The corresponding EDX mapping (Fig. 2e) indicates its uniform elemental distribution of Ni, P, and O in bulk. Further TEM characterization taken from the peel-off layers suggests that the NiPx films are essentially composed of nanoparticles with an average size of 5.71 nm (Fig. 2f). Fig. 2g presents the HRTEM image of PDD_Ni-10P, where two sets of well-resolved lattice fringes with interplanar distances of 0.197 and 0.170 nm are noticed, corresponding to the (111) and (200) planes of Ni, respectively (slight contraction in the lattice spacings was observed, as these values were reported to be 0.203 nm and 0.176).21,51 The associated HAADF-STEM image (Fig. 2h) reveals the nanoparticle-packed architecture of the NiPx films featuring as a nanoporous structure, and the EDS line-scan profile (inset of Fig. 2h) also proves the homogeneous distribution of Ni, P and O elements in the layers. The surface elemental states of the as-synthesized samples were ascertained by XPS. Fig.3 plots the highresolution XPS spectra of Ni 2p, P 2p, and O 1s, respectively. From Fig. 3a, the XPS spectrum of Ni 2p for PDD_Ni10P can be divided into five fitted peaks, the weak one at 853.0 eV assigns to elemental Ni.52 The two main characteristic peaks at 856.3 and 874.1 eV correspond to Ni 3p3/2 and Ni 2p1/2, respectively,53-54 together with two satellite peaks at 861.5 and 880.5 eV, indicating the dominance of Ni-O species on the surface arising from superficial oxidation. This profile is similar to those for PDD_Ni and PSD_Ni-10P. Interestingly, the relatively larger peak at 853.0 eV for PSD_Ni-10P indicates that the films produced with this method are more resistant to oxidation. For the P 2p spectra (Fig. 3b), the broad peak at 133.6 eV reveals the presence of P-O bonding,55 while the peak at 130.4 eV can be assigned to the P bonded to Ni with a small negative charge of Pδ- (0 < δ < 1).56 Accordingly, the Ni 2p binding energies of 856.3 and 874.1 eV for the P-doped sample (PDD_Ni-10P) are up-shifted from that of pure deposited Ni (856.0 and 873.7 eV), suggesting a partial positive charge of Niδ+ (0 < δ < 2).57 This implies a transfer of electron density from Ni to P within the NiPx films. Such an electronic structure is similar to those of metal complex HER catalysts with intrinsic high HER catalytic capability.58 In all cases, the O 1s spectrum (Fig. 3c) exhibits one peak at 531.8 eV,59 attributed to the lattice oxygen in NiO species, also confirming the superficial oxidation of the samples. Bulk elemental analysis performed with ICP-OES reveals that the amounts of Ni and P in PSD_Ni-10P sample are 0.607 and 0.023 mg cm-2, and the case for PDD_Ni-10P are 0.536 and 0.016 mg cm-2, resulting in a Ni/P atomic ratio of 1 to 0.072 and 1 to 0.056, respectively. Notably, the amount of NaH2PO2 used for deposition is observed to play an important role in the formation of high-performance NiPx films with the optimal addition amount of 10 mM. A higher addition amount of NaH2PO2 is in favor of the P doping level (Fig. S7), however leads to 6


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obtain either non-uniform coarse or cracking and peeling NiPx films (Fig. S4 and Fig. S6), which both deteriorate the catalytic performances (Fig. S8). To get access into the catalytic performance of the deposited NiPx films, the electrocatalytic activity of PDD_Ni10P (mass loading: 0.55 mg cm-2) and PSD_Ni-10P (mass loading: 0.63 mg cm-2) were evaluated for HER in 1.0 M KOH with a typical three electrodes system. For comparison, the activities of commercial Pt/C catalyst, Cu foil, PDD_Ni and PSD_Ni were also included, as shown in Fig. 4a. As expected, Pt/C catalyst demonstrates excellent HER activity, while the Cu foil shows little catalytic activity for the HER. In both cases, the deposited pure Ni samples, PDD_Ni and PSD_Ni exhibit inferior catalytic activity in comparison with their P-doped counterparts, revealing a synergistic promoting effect of P and Ni within the NiPx films. The incorporation of P into the Ni skeletons results in much lower overpotentials (107 and 105 mV for PSD_Ni-10P and PDD_Ni-10P, respectively) at 10 mA cm-2 in comparison to those of PDD_Ni (246 mV) and PSD_Ni (242 mV). In contrast to the PSD_Ni-10P electrode, PDD_Ni10P shows a more excellent catalytic performance, as evidenced by a faster increment of the HER catalytic current with increasing overpotential. With aspect to the HER kinetics, the Tafel slopes of the above samples were measured based on the corresponding LSV polarization curves. From Fig. 4b, the measured value for PDD_Ni-10P is 44.7 mV dec-1, which is close to the Pt/C catalyst (36.6 mV dec-1), and smaller than the other deposited samples (PSD_Ni-10P, 56.6 mV dec-1; PDD_Ni, 90 mV dec-1; PSD_Ni, 107.3 mV dec-1) as well as most Ni-P-based catalysts previously reported (Table S3), revealing its favorable catalytic kinetics. Such a small Tafel slope indicates that the HER occurs on the electrode surface principally via a Volmer-Heyrovsky mechanism.60 The charge transfer properties of the asprepared samples were further studied by electrochemical impedance spectroscopy (EIS). Fig. 4c shows the representative Nyquist plots at an overpotential of 150 mV. EIS data, including charge transfer resistance (Rct) and solution resistance (Rs), were calculated by fitting the Nyquist plots with an equivalent circuit model (indicated in Fig. 4c). The PDD_Ni-10P exhibits reduced Rct and Rs values in comparison with PSD_Ni-10P, PSD_Ni, and PDD_Ni, suggesting the favorable charge transfer kinetics on the P-doped Ni surface and its better electrical transport properties, which is in accordance with the results of LSV measurements and Tafel plots. To understand the doping effect of P on the enhancement of HER activity, the electrochemically active surface area (ECSA) of the prepared samples was also evaluated. The ECSA of each material was calculated by measuring the double-layer capacitance (Cdl), which is proportional to the ECSA,61 via a CV method in a non-Faradaic potential region (see more detail in Fig. S9). From Fig. 4d, the PDD_Ni-10P shows a much larger Cdl value of 55.5 mF cm-2 than that of PSD_Ni-10P (42.6 mF cm-2), PSD_Ni (11.0 mF cm-2), and PDD_Ni (12.9 mF cm-2), indicating that a significant increase of ECSA is achieved by P doping. The larger ECSA of the PDD_Ni-10P with respect to PSD_Ni-10P could be 7



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attributed to the different structural features induced by two kinds of electrochemical deposition models. This corroborates that the potentiodynamic deposition model performed in DES is a more effective way to create Pdoped Ni catalyst with more active sites for catalytic reactions. Moreover, the turnover frequency (TOF) for each active site was further evaluated to show the intrinsic activity of the prepared materials.62 The calculated TOF versus potential plots (Fig. S10) were obtained by a previous method (for more details in Supporting Information). The TOF values at η = 300 mV of PDD_Ni-10P (25.0 s-1) and PSD_Ni-10P (16.3 s-1) are greatly improved after P doping than that of those counterparts (PDD_Ni, 2.9 s-1; PSD_Ni, 2.0 s-1), which also surpasses some recently reported P-based catalysts, including CoFePO (16.87 s-1),49 CoP/CC (5.0 s-1 at η = 270 mV),7 and Ni2P/Ti (1.2 s-1 at η = 230 mV).63 The much higher TOF value obtained for PDD_Ni-10P further conﬁrms the role of P doping in enhancing the intrinsic HER catalytic activity. In addition to the catalytic activity, the HER durability of the P-doped materials was examined by multistep chronoamperometric curves with the current density step increasing from 10 to 120 mA cm-2 (10 mA cm-2 increment per stage for 0.5 h) (Fig. 4f). The responsive potential remains stable over a wide current density range for both PDD_Ni-10P and PSD_Ni-10P with relative to the cases without P doping, implying their excellent mass transportation and mechanical robustness features. Intuitively, PDD_Ni-10P exhibits much lower responsive potential at a given current density in comparison with that for PSD_Ni-10P, which also demonstrates the superiority of the PDD protocol. The accelerated degradation test based on multiple CV scanning was measured at a scan rate of 100 mV s-1 between -0.20 V and 0.20 V vs. RHE. As shown in Fig. 4g, the polarization curve shows negligible differences after 200, 600 and 1000 CV sweeps, revealing the high stability of PDD_Ni-10P. Moreover, chronopotentiometric curves of PDD_Ni-10P recorded at both low (10 mA cm-2 for 60 h) and high (50 mA cm-2 for 60 h) current densities are given in Fig. 4h. After long-term electrolysis, the overpotentials show only slight degradations (6 and 27 mV for 10 and 50 mA cm-2, respectively), which is likely related to the residual H2 bubbles trapped on the catalyst surface, hindering the electrochemical reaction. The generated H2 gas is in good accordance with the theoretically expected amount (Fig. S11), indicating a nearly 100% Faradaic efficiency, which suggests that the observed current is indeed from the catalytic water splitting. Interestingly, the post-HER PDD_Ni-10P is observed to undergo a structure rearrangement, where the nanoparticle-packed surface of the original films is developed into a rough surface consisting of nanosheet arrays (Fig. S12). However, the compositions of the PDD_Ni-10P before and after HER test remains essentially unchanged as manifested by the EDX (Fig. S12), XPS (Fig. S13), and ICP-OES (a slight change on the Ni/P atomic ratio from 1: 0.056 to 1: 0.054) analyses, revealing that no obvious dissolution happened. 8


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To further elucidate the essential role of P doping on the excellent catalytic HER performance of the deposited NiPx under alkaline conditions, the DFT calculations for the P-doped Ni sample with P- and Ni-sites were performed (Fig. S14) to investigate the prior Volmer process (water dissociation to form H* intermediates) and follow-up Tafel process (hydrogen generation), as shown in Fig. 5a. For comparison, calculations on the as-built model for the (111) facet of Ni metal (pure Ni deposited sample) was also included. The pure Ni metal exhibits a large water dissociation energy barrier (ΔGH2O) up to 0.98 eV, leading to sluggish kinetics for the dissociation of water molecule into H* and OH-. Notably, the ΔGH2O on the P-doped Ni sample on the Ni-site is significantly reduced to 0.34 eV. This favorable energetics signifies that the sluggish Volmer process on the pure Ni metal can be dramatically accelerated after the incorporation of P. According to our findings from XPS (Fig. 3a), the doping of P can induce a higher-positive-chargestate Ni (δ+) that is beneficial for water adsorption and ease the O-H bond in adsorbed H2O, thus promoting the water dissociation (step 1 in Fig. 5b). Moreover, the introduction of P dopants also results in a more optimal hydrogen adsorption free energy (ΔGH*) of -0.23 eV on the P-site, which makes it more favorable for H2 generation and release. Compared with the value (ΔGH* = -0.44 eV) obtained on the Ni-site, the P-site is performing as the active site (proton-accepter center) for enhancing the HER activity. This means the H atom, which is formed after water dissociation with the aid of a free electron, is preferred to transfer onto a nearby P site (δ-) and becomes an adsorbed H (Hads), while the released OH- is adsorbed on the Ni site (δ+), step 2. As a result, the whole reaction progress is mainly kinetic-limited by a balance between the Hads recombination on the P site, followed with H2 generation and release, and the OH- desorption from the Ni site, step 3. The above calculation results reveal that the synergistic effect induced by P doping can efficiently lower the energy barrier for the water dissociation and optimize the H adsorption free energy, and thus lead to favorable catalytic kinetics for alkaline HER. Based on the above investigations and DFT calculations, the high HER catalytic performance of the NiPx could be rationalized to the synergistic effects between the P and Ni within it as follows. (1) The implantation of P into the Ni skeletons effectively increases the ECSA and active sites with high TOFs, favoring fast charge-transfer process. (2) The enhanced electronic conductivity induced by P-doping facilitates electron transport from the substrate to the active NiPx films. (3) The interfacial electronic structure engineered by P incorporation allows to optimize both the water dissociation free energy and hydrogen adsorption free energy, which leads to favorable catalytic kinetics. (4) The self-supported feature enables a strong interaction between active phases and the substrate, guaranteeing a good structural stability and robust durability during continuous catalytic reactions.

Conclusions 9



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In summary, potentiodynamically fabricated NiPx films directly grown on Cu foil with a consecutive potential cycling approach performed in Ethaline-based DES has been demonstrated to be high-performance electrocatalysts towards HER in alkaline media. By technical incorporation of P into Ni superstructures, the NiPx hybrid films possess much more active sites and enhanced electronic conductivity to promote the electron-transfer kinetics for HER due to their synergistic effects. The catalytic activity of the NiPx is highly correlated with the P-doping amount with an optimum Ni : P ratio of 1 : 0.056. Such a catalyst exhibits superior HER catalytic activity in 1.0 M KOH with low potentials (ηonset = 55 mV and η10 = 105 mV), a small Tafel slope (44.7 mV dec–1), and high catalytic efﬁciency (TOF of 25 s−1 at η = 300 mV vs RHE). Moreover, excellent electrochemical stability (continuous operation at 10 mA cm-2 for 60 hours with ignorable activity decay) is also achieved. Further DFT calculations reveal that the enhanced catalytic activity is also associated with the doping effect of P which helps to accelerate water dissociation and optimize the adsorption free energy of H. The facile potentiodynamical-deposition strategy performed in DES is expected to provide a general way to the rational fabrication of other high-efficiency transition-metal-phosphidebased materials for boosting HER catalysis.

Conflicts of interest There are no conflicts to declare.

Author Contributions ∇These

authors contributed equally.

Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (51464028), Candidate Talents Training Fund of Yunnan Province (2017PY269SQ), and the Independent Research Funds for the State Key Laboratory (CNMRCUTS1601).

Supporting Information The Supporting Information includes experimental sections, CV and LSV curves, optical photographs, systematic experimental explorations, SEM images, XRD patterns, XPS spectra, DFT calculations details, and Tables S1-S3.

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Fig. 1 (a) Comparison of CVs recorded at Pt disk electrode for the reduction and oxidation of Ni and NiPx from Ethaline/100 mM NiCl2∙6H2O without and with various concentrations of NaH2PO2 at a scan rate of 20 mV s-1. (b) Linear correlations of the cathodic peak current density against the square root of sweep rate at various concentrations of NaH2PO2. (c) The influence of NaH2PO2 on the charge transfer and diffusion coefficient of Ni(II) species at various concentrations. (d) Current-time curves of PSD_Ni and PSD_Ni-10P obtained by potentiostatic electrolysis. The inset shows the deposition charge as a function of time. (e) Potentiodynamic deposition for the preparation of PDD_Ni and PDD_Ni-10P at a scan rate of 15 mV s-1.

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Fig. 2 (a) XRD patterns of PDD_Ni, PSD_Ni, PDD_Ni-10P, and PSD_Ni-10P. (b) The typical peak shift after P doping. SEM images of (c) PSD_Ni-10P, and (d) PDD_Ni-10P at different magnifications. (e) Elemental mapping of Ni, P, and O in PDD_Ni-10P. (f) Low-magnification TEM and (g) HRTEM image of PDD_Ni-10P. The inset indicates the associated particle size histogram. (h) HAADF-STEM image and compositional line profiles across the surface along the line marked by an arrow.

Fig.3 High-resolution XPS spectra of (a) Ni 2p, (b)P 2p, and (c) O 1s of the PDD_Ni, PDD_Ni-10P, and PSD_Ni-10P.

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Fig. 4 (a) Polarization curves of commercial Pt/C catalyst, Cu foil, PDD_Ni, PSD_Ni, PDD_Ni-10P, and PSD_Ni-10P at a scan rate of 5 mV s-1 in 1 M KOH without IR correction. (b) Corresponding Tafel slope with linear fitting. (c) Nyquist plots of PDD_Ni, PSD_Ni, PDD_Ni-10P, and PSD_Ni-10P obtained at an overpotential of 150 mV. (d) The difference of the current density (Δj = ja - jc) between the anodic and cathodic sweeps of PDD_Ni, PSD_Ni, PDD_Ni-10P, and PSD_Ni-10P at 0.18 V vs. RHE depend on the scan rate. (e) The calculated TOF at an overpotential of 300 mV for PDD_Ni, PSD_Ni, PDD_Ni-10P, and PSD_Ni-10P. (f) Multi-step chronoamperometric curves for PDD_Ni, PSD_Ni, PDD_Ni-10P, and PSD_Ni-10P from 10 mA cm-2 to 120 mA cm-2 with an increment of 10 mA cm-2 every 0.5 h. (g) Polarization curves of PDD_Ni-10P before and after 200, 600, and 1000 CV cycles at a scan rate of 100 mV s-1 between -0.20 V and 0.20 V vs. RHE. (h) Time dependent potential curves of PDD_Ni-10P at 10 and 50 mA cm-2, respectively.

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Fig. 5 (a) Calculated adsorption free energy diagram of the alkaline HER (Volmer and Tafel processes) on the pure Ni metal and P-incorporated Ni sample with P- and Ni-sites, (b) Schematic illustration for the reaction paths.

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For Table of Contents Use Only

Potentiodynamic deposition carried out in Ethaline-based DES creates crack-free NiPx films with highperformance electrocatalysts towards HER in alkaline media. The doping of P can enrich active sites, enhance the electronic conductivity, and alter the surface electronic structure of the nickel superstructures, leading to boost the intrinsic HER activity.

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Direct electrodeposition of phosphorus-doped nickel superstructures

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