Sulfur-Doped Nickel Phosphide Nanoplates Arrays: A Monolithic

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Sulfur Doped Nickel Phosphide Nanoplates Arrays: a Monolithic Electrocatalyst for Efficient Hydrogen Evolution Reaction Jinfa Chang, Kai Li, Zhijian Wu, Junjie Ge, Changpeng Liu, and Wei Xing ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08068 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Applied Materials & Interfaces

Sulfur Doped Nickel Phosphide Nanoplates Arrays: a Monolithic Electrocatalyst for Efficient Hydrogen Evolution Reaction Jinfa Chang†, §, Kai Li‡, Zhijian Wu‡, Junjie Ge†, Changpeng Liu† and Wei Xing†,* †

State Key Laboratory of Electroanalytical Chemistry, &Laboratory of Advanced Power

Sources, Jilin Province Key Laboratory of Low Carbon Chemical Power Sources, Changchun Institute of Applied Chemistry, 5625 Renmin Street, Changchun 130022, PR China. ‡

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China.

ACS Paragon Plus Environment

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ABSTRACT: Searching for cost-efficient electrocatalysts with high catalytic activity and stability for hydrogen generation by means of water electrolysis would make a great improvement on energy technologies field. Herein, we report a high performance hydrogen evolution reaction (HER) electrocatalysts based on sulfur doped Ni5P4 nanoplate arrays grown on carbon paper (S-Ni5P4 NPA/CP). This ternary, robust, monolithic S-Ni5P4 NPA/CP exhibits remarkable performance for HER compared to nickel phosphide and nickel sulfide catalysts. The S-Ni5P4 NPA/CP with ~6% S presents the most promising behavior for water electrolysis applications. Specifically, it shows an onset potential of 6 mV, needing overpotentials (η) of 56 and 104 mV to attain current densities of 10 and 100 mA cm-2 with a Tafel slope of 43.6 mV dec1

. The turnover frequency of 6% S-Ni5P4 NPA/CP is about 0.11 s-1 at overpotential of 100 mV,

which is ca. 10 and 40 times that of Ni5P4 NPA/CP and NiS2 NPA/CP, respectively. It also shows remarkable stability and durability in 0.5 M H2SO4 solution. Results indicate that S and P tune the electronic properties mutually and produce an active catalyst phase for hydrogen evolution reaction. Furthermore, the density functional theory (DFT) calculations show S-Ni5P4 NPA/CP exhibits only 0.04 eV of hydrogen adsorption free energy(∆GH*), which is more suitable than Pt (~ -0.09 eV). We propose that the S-doping not only restrain the surface oxidation and dissolution of S-Ni5P4 NPA/CP in acid solution, but also reduce the ∆GH*. We believe that our work will provide a new strategy to design transition metal phosphides (TMPs) composite materials for practical applications in catalysis and energy fields. KEYWORDS:

hydrogen

evolution

reaction,

electrocatalysis,

sulphur-doping,

nickle

phosphosulfides, density functional theory


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1. INTRODUCTION Water electrolysis (WEs) provides an efficient chemical route for renewable energy storage.1-3 As the cathode half-reaction of WEs, the hydrogen evolution reaction (HER) requires effective electrocatalysts to achieve a high current density at relative lower over-potential. Precious-metals such as Pt materials are good HER catalysts, but the low abundance and high cost limit their widespread application.4-6 Therefore, the exploration of cheap and easily prepared catalysts based on cheap elements with high activity and long-term stability as well as a low overpotential for hydrogen production is of paramount importance and highly challenging.7-12 Many WEs devices and electrolyzers based on the proton exchange membrane technology are designed to operate under strong acidic conditions.13 Great attentions have been paid on developing Ni-based materials as HER catalysts in acid conditions, such as nitrides,14-15 sulfide,16-17 selenides18-20 and phosphides.21-26 Particularly, transition metal phosphides (TMPs) with metalloid characteristics are important compounds and have been widely used to promote the HER due to their good electrical conductivity.27-35 A series of polymorphic nickel phosphide with amorphous, monocrystalline and polycrystalline structures have been widely studied for HER due to the flexible valence state of P.36-39 Recent study indicated that in a metal phosphide structure with a P-terminated surface, the polarization-induced partial negative charges localized on P centers attracts protons as a base and make their discharge easier, and thereby promoting the HER.40-41 However, the surface of transition-metal phosphides is readily oxidized,23-24 which result in unsatisfactory the (electro)chemical stability during long time and continuous operation. Sulfur (S) and phosphorus (P) have a lot of similar physicochemical properties.36 Transition metal phosphosulfides have recently been demonstrated as outstanding electrocatalytic HER catalysts through crossing substitutions between phosphorus and sulfur.42 The modified


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electronic structure and tuned hydrogen adsorption free energy are the main reason for enhanced HER activity.43 For example, in 2014, Jaramillo’s group reported that the annexer of sulfur on the surface of molybdenum phosphide (MoP) will mitigate surface oxidation of the MoP.44 Soon afterwards, Jin’s group synthesized a ternary pyrite-type structure (CoPS) possessed distinct (PS)3- anions which produce the optimal binding energy for hydrogen atoms on the Co sites.45 Furthermore, Jin’s group investigated the HER performance of pyrite-phase NiPxSey materials,46 they found that NiP1.93Se0.07 has the best HER activity due to the substitution of P element by Se element, which modifies the electronic structure of NiP2. Wang’s group designed pyritestructured CoPS nanoparticles grown on carbon nanotubes, experiments and theoretical studies showed that phosphorus substitution is a fateful step to the CoS|P/CNT hybrid material due to the stronger metal-ligand bonding.47 Besides, Wang’s group investigated the HER performance of iron phosphosulfide nanoparticles with crystal structure of either the FeS or the FeP. Different from Jaramillo’s result, they found that a higher P/S ratios on the surface of the iron phosphosulfide than in the bulk, the oxidation of Fe and S atoms on the surface is suppressed due to the presence of P atoms.42 Moreover, Wang’s group developed NiS-Ni2P2S6 nanosheets as OER and HER catalysts in basic media.48 Luo et al found that NiP0.62S0.38 is an efficient HER and OER catalyst in basic media, benefiting from the electronegative of phosphosulfide species which shift ∆GH* to the thermo-neutral position and give a balance of hydrogen adsorption and desorption on nickel sites.49 Zhang’s group developed a chemical vapor deposition reaction method to prepare nickel phosphosulphide materials (Ni5P4|S), which shows excellent HER performance in wide pH ranges.50 Li’s group developed a facile method to prepare surface sulfur-containing Ni2P, the activity and the selective for hydrodesulfurization and hydrogenation


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performance of Ni2P-S was much better than the catalyst prepared by the temperatureprogrammed reduction method.51 However, because the surface sulfidation of Ni5P4 is more difficult than that for MoP,50 up to now, the synthesis of surface sulfuretted Ni5P4 is still rarely reported. The dependence on the S-doped ratio in Ni5P4 for electrocatalytic activity is rarely reported and understood. In addition, the binding force of the S-H bond (363 kJ mol-1) is stronger than P-H (322 kJ mol-1), making the H desorption on metal sulfide harder than on metal phosphide during hydrogen evolution reaction. Thus, the development of catalyst based on Ni, P, S with appropriate atomic ratio using a simple and easy method is promising but still challenging. Herein, we report S-doped Ni5P4 nanoplate arrays growing on conductive carbon paper (SNi5P4 NPA/CP) as an efficient HER catalyst in 0.5 M H2SO4, via a one-step calcination of the pre-prepared nickel hydroxide at low temperature using sodium hypophosphite and sublimed sulphur as the P and S sources, respectively. We found that the morphology of S-Ni5P4 NPA/CP and the correspondence HER performance is strongly related to the sulfur doping ratio. Specially, the S-Ni5P4 NPA/CP with ~6% S shows outstanding electro-catalytic performance for HER in acid solutions. A Pt-like HER activity which needs onset potential of only 6 mV and a small Tafel slope of 43.6 mV dec-1 is achieved, and overpotentials (η) of only 56 and 104 mV are needed to attain 10 and 100 mA cm-2, respectively, keeping its catalytic activity for at least 100h or 5000 cycles accelerated stability testing (AST) in acidic media. Theoretical calculations indicated that ∆GH* of S-Ni5P4 NPA/CP electrocatalyst is only 0.04 eV, more suitable than Pt. Our result indicates that the S-doping not only increases surface area of electrode, but also restrains surface oxidation of the Ni5P4. Thus, the synthesized S-Ni5P4 NPA/CP catalysts are ideal HER electrocatalysts for practical applications. We believe that this work provides a


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feasible method for the synthesis of cheap and valid HER ternary electrocatalysts. 2. EXPERIMENTAL SECTION 2.1 Synthesis of nickel precursor nanoplates on carbon paper (Ni(OH)2·0.75H2O). Carbon paper (CP) was processed as our recently work.52 2.5 mmol of Ni(ac)2·4H2O, 1.0 mmol NH4F, and 1.0 mmol (NH2)2CO were added in 35 ml of ultrapure water. Afterwards, the above solution was transfered to a 50 mL PTFE-lined autoclave containing the CP (1*1 cm2), and then the autoclave was sealed and heated at 120 ◦C for 5 h. After cooling down to room temperature, the obtained green products were washed by ethanol and water thoroughly and dried under vacuum oven. 2.2 Synthesis of S-Ni5P4 NPA/CP. An alumina boat containing 2.0 mmol NaH2PO2·H2O and 0.415 mmol sulphur powder was placed in the center of a fused silica tube reactor. The Ni(OH)2·0.75H2O was placed at the downstream end of the tube. The furnace was then heated to 500 ◦C with 2 ◦C min-1 for 1 h to convert these precursors to S-Ni5P4 NPA/CP under Ar atmosphere with a flow rate of 80 cc min-1. After cooling, the products were washed with 0.5 M HCl, ethanol and water respectively several times. The loading of S-Ni5P4 NPA on CP was ~0.2 mg cm-2. Besides, the Ni5P4 NPA/CP hybrid materials with different S-doped level (from 2% to 10%) with of ~0.2 mg cm-2 catalyst loading on carbon paper were also prepared through adjust the content of sulphur powder (0.135 mmol, 0.270 mmol, 0.565 mmol and 0.725 mmol sulphur powder for 2%, 4%, 8% and 10% S-doped Ni5P4 NPA/CP). 2.3 Synthesis of Ni5P4 NPA/CP and NiS2 NPA/CP. The Ni5P4 NPA/CP and NiS2 NPA/CP was synthesized through the same method to synthesized the S-Ni5P4 NPA/CP, unless the sulphur powders or NaH2PO2·H2O is absent.


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The details about the physical characterization, electrochemical measurements and DFT calculation are shown in Supporting Information.

Scheme 1. Schematic illustration of the synthesis of S-Ni5P4 NPA/CP. 3. RESULTS AND DISCUSSION 3.1 Catalyst synthesis and microstructure. A series of S-Ni5P4 NPA/CP with different S-doped level were prepared via conversion reaction from their nickel hydroxide precursors. The fabrication procedure is illustrated schematically in


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Scheme 1, and the experimental details are described in the Experimental Section. Figure S1 presents the Ni(OH)2·0.75H2O nanoplate arrays on carbon paper. The nanoplates have uniform morphology with a thickness of about 100 nm, and they are vertically aligned and interconnected with each other on the skeletons of CP. XRD patterns (Figure S2) further indicated the successful synthesis of Ni(OH)2·0.75H2O in rhombohedral phase. The XRD patterns (Figure 1a) of S-Ni5P4 NPA/CP synthesized with 6% S feeding ratios is consistent with a Ni5P4 standard pattern (JCPDS No. 65-2075) with hexagonal structure but slight shift to low 2 theta value due to Sdoping. Figure S3 and Figure S4 shows the XRD patterns and enlarged XRD patterns of Ni5P4 NPA/CP and S-Ni5P4 NPA/CP with different S content. The diffraction peaks of S-Ni5P4 NPA/CP are slightly shift towards the lower 2 theta value with the increase of S doped level, implying successful S doping of Ni5P4 NPA/CP. No other diffraction peaks can be found, indicating a single phase nature of as-prepared products. The energy-dispersive X-ray (EDX) spectrum shows the existence of Ni, P and S (Figure 1b) and inductively coupled plasma mass spectrometry (ICP-MS) results indicate nearly a 1:0.8:0.111 atomic ratio for Ni:P:S (Table S1). Figure 1c shows a scanning electron microscopy (SEM) image of 6% S-Ni5P4 NPA/CP, indicating uniform coverage of the entire CP with nanoplate arrays. The high-magnification SEM image (Figure 1d) further indicates that such nanoplates typically exhibit a thickness of about 50 nm. Transmission electron microscopy (TEM) image demonstrates that the nanoplate has rough surface (Figure 1e). High resolution TEM (HR-TEM) image taken from the nanoplate (Figure 1f) displays lattice fringe with an interplanar distance of 1.73 Å indexed to the (303) plane of Ni5P4. The existence of S in S-Ni5P4 can be further proved by the EDX spectrum (Figure 1b), elemental mapping (Figure 1h), XPS (Figure S8a and Figure 2c) and ICP tests. The selected area electron diffraction (SAED, Figure 1g) demonstrated the monocrystalline structure of S-


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Ni5P4 NPA/CP nanoplate where ordered dots were observed, which indexed to the (210) and (303) planes of Ni5P4 phase, suggesting a highly ordered single crystalline nature of the nanoplate. Scanning TEM (STEM) image and EDX elemental mapping images (Figure 1h) for S-Ni5P4 NPA/CP confirm the well distribution of Ni, P, and S within the whole nanoplate, and Figure S5 further confirmed the good amount of oxygen exist in materials.

Figure 1. Structural characterizations of the 6% S-Ni5P4 NPA/CP. XRD pattern, ‘♦’ represents the graphite and carbon paper (a), EDX spectrum (b), SEM image (c, d), TEM image (e), HR-TEM image (f), SAED pattern (g), STEM image and EDX elemental mapping (h) and Nitrogen adsorption-desorption isotherms (i). Inset in (i) is pore distribution curve obtained based on the Barrett-Joyner-Halenda (BJH) model.


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We also made NiS2 NPA/CP, Ni5P4 NPA/CP and S-Ni5P4 NPA/CP with other S-doped level ratios for comparison study. It is clear that all nanoplates have uniform morphology and thickness (Figure S6-Figure S8). However, as shown in Figure S8, with the increasing of S feeding ratios in S-Ni5P4 NPA/CP (8% and 10%), peapod-like nanoparticles grown on nanoplate was gradually formed, which would result in decreased BET and catalytic active site (Figure S9). Figure 1i shows the nitrogen adsorption/desorption isotherms of 6% S-Ni5P4 NPA/CP, a type IV isotherm was observed, which verified the mesoporous structure of the S-Ni5P4 NPA/CP. The Brunauer-Emmett-Teller (BET) specific surface areas (SSAs) were determined to be 25.63 m2 g1

, much larger than Ni5P4 NPA/CP(14.56 m2 g-1) and NiS2 NPA/CP (14.30 m2 g-1) (Figure S10).

The Barrett-Joyner-Halenda (BJH) pore size distribution curve (inset in Figure 1i) of S-Ni5P4/CP nanosheet indicates the pore size ranged from 2 to 30 nm with a mean size at 4.4 nm. The higher BET of S-Ni5P4 NPA/CP than Ni5P4 NPA/CP and NiS2 NPA/CP is believed to be associated with the preferential dissolution of residual Ni(OH)2·0.75H2O ingredient in acid aqueous solution, which further formed mesoporous structure. Thus, the S-doping will increase surface area of as prepared materials and facilitate mass diffusion/transport and contributing to a highly active surface area for electrocatalytic applications. Raman spectra (Figure S11) show no feature peaks between 200~600 cm-1 for S-Ni5P4 NPA/CP and Ni5P4 NPA/CP are observed. For NiS2 NPA/CP, on the contrary, typical nickel pyrite NiS2 peaks (272 cm-1, 277 cm-1 and 491 cm-1)53 are noticed, further certified the formation of S-Ni5P4 NPA/CP with atomic scale S-doping, and excluded the possibility of forming the core-shell structure or a mixture of Ni5P4 and NiS2. 3.2 XPS analysis Figure S12a shows the XPS survey spectrum of the 6% S-Ni5P4 NPA/CP, which confirms the exist of Ni, P, and S elements with an elemental composition of Ni : P : S close to 5:4:1,


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indicating that S mainly exist at the surface. Figure S12b and Figure S12c confirm the successful synthesis of Ni5P4 NPA/CP and NiS2 NPA/CP. Figure 2 shows the high resolution XPS spectra of Ni, P and S of the three studied samples. For NiS2 NPA/CP, the signal at 856.7 eV with a satellite peak at 861.9 eV in the Ni 2p3/2 spectra region is assigned to oxidized Ni or Ni bounded to O (Ni-O, Figure 2a);37 similar binding energy at 856.1 eV and 861.1 eV are observed in Ni5P4 NPA/CP. However, for S-Ni5P4 NPA/CP, a new signal emerged at 852.3 eV which can be ascribed to Ni in phosphide or metallic Ni, except for the signal of Ni-O at 855.5 eV and 860.8 eV in Ni 2p3/2 area. It is known that transition metal phosphide is readily oxidized due to the exposure of the sample to air.29, 37, 54-56 Here, the XRD and ICP-MS demonstrate the successful synthesis of the target products; however, surface oxidation obviously occurred in all samples, as XPS mainly probes the surface composition. From above results, the S-doping will suppress the surface passivation of Ni5P4 through electronic effect between S, P and Ni. In addition, compare with NiS2 NPA/CP and Ni5P4 NPA/CP, the binding energy of Ni in Ni 2p3/2 areas for S-Ni5P4 NPA/CP is obviously negatively shifted due to the synergistic electronic effect between P and S. This is in consistent with previous work44 that P and S tune the electronic properties mutually, thus resulting in an active catalyst phase and make S-Ni5P4 NPA/CP a highly stable HER catalyst. The P 2p XPS spectrum of the Ni5P4 NPA/CP shows one peak region (Figure 2b), at the binding energy of 133.1 eV and 134.2 eV, assignable to the characteristic of P-O species. The existence of the high oxidation state P could be ascribed to surface oxidation in the air as stated above. In contrast, the P 2p core level spectrum of the S-Ni5P4 NPA/CP displays two peak regions, with one located at the binding energy of 134.1 eV assigned to P-O species, and the other newly emerged at 129.3 eV attributed to P in phosphide.


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Figure 2. XPS analysis for the different materials. XPS Ni 2p (a), P 2p (b) and S 2p (c) spectra for S-Ni5P4 NPA/CP, Ni5P4 NPA/CP and NiS2 NPA/CP. The S 2p core level spectrum of the NiS2 NPA/CP shows binding energy components at 161.2 eV and 162.6 eV (S 2p3/2 and S 2p1/2) attributable to sulfide species. As contrasted with NiS2 NPA/CP, the S 2p core level XPS spectrum of the S-Ni5P4 NPA/CP shows lower binding energy components at 163.8 eV attributed to sulfide species and higher binding energy components at 169.2 eV attributed to S-O species. It is obvious that sulfide is more easily oxidized than phosphide, and the presence of S in S-Ni5P4 NPA/CP suppress the degradation of Ni5P4. In addition, compare with P 2p and S 2p core level of Ni5P4 NPA/CP and NiS2 NPA/CP, the binding energy is positively shifted for S-Ni5P4 NPA/CP, which indicates strong electronic


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effect between Ni, P and S. This strong electronic effect results in negative shift of Ni in S-Ni5P4 NPA/CP.

Figure 3. Electrocatalytic hydrogen evolution over the different catalyst samples. (a) Linear sweep voltammetric curves of S-Ni5P4 NPA/CP with different S-doping level in Ar-saturated 0.5 H2SO4; scan direction from higher to lower potentials with 90 % iR correction. (b) Overpotential required for J = 10 mA cm-2 (η@J=10 mA cm-2) and J = 100 mA cm-2 (η@J=100 mA cm-2). I, II, III, IV and V represent the S-doping level is 2%, 4%, 6%, 8% and 10%, respectively. (c) Linear sweep voltammetric curves of 6% S-Ni5P4 NPA/CP, Ni5P4 NPA/CP, NiS2 NPA/CP, Pt/C and bare CP


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in Ar-saturated 0.5 H2SO4 with scan rate of 5 mV s-1 and with 90 % iR correction. (d) The corresponding Tafel plots derive from (c). (e) Chronopotentiometric curve at a constant current density of 10 mA cm-2 for 100 h. (f) The theoretically calculated (black line) and experimentally measured (gray line) amount of evolved hydrogen versus time for the 6% S-Ni5P4 NPA/CP at current density of 10 mA cm-2. All catalysts loading are ~ 0.2 mg cm-2 and current was normalized to geometry area of the electrode. 3.3 Electrocatalytic hydrogen evolution reaction. Firstly, we investigated the conductivity of these different S-Ni5P4 NPA/CP materials, as shown in Figure S13. The resistance is decreased with the increasing of S-doped level, further indicated that S-doping will increase the conductivity of Ni5P4 materials. By contrast, the resistances for all S-Ni5P4 NPA/CP are ~2 Ω. Then, HER electrocatalytic activity of the Ni5P4 NPA/CP hybrid material with different S-doped level (from 2% to 10%) in Ar-saturated 0.5 M H2SO4 aqueous solution is performed. Figure 3a shows the polarization curve of all S-Ni5P4 NPA/CP hybrid materials at a scan rate of 5 mV s-1 with a mass loading of ~0.2 mg cm-2. The HER electrocatalytic activity increases with the content of S increased from 2% to 6%, and subsequently decays gradually when the S content is increased to 8% and 10% (Table S2), due to the decreased ESA, smaller CDL and reduced active sites. The 6% S-Ni5P4 NPA/CP shows the best performance (Figure 3b), the onset potential (Figure 3b) is only 6 mV and the Tafel slope (Figure S14 and Table S2) is as low as 43.6 mV dec-1. To reach current density of 10 and 100 mA cm-2, overpotentials of just 56 and 105 mV are needed, which is much better than other SNi5P4 NPA/CP. To compare the Ni5P4 NPA/CP hybrid material with different S-doped level, cyclic voltammetry (CV) scans were used to determine the double-layer capacitance (CDL), as shown in


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Figure S15 - Figure S18. The Pt/C was also investigated as reference baseline (Figure S19). Both the CDL and electrochemical surface area (ESA) shows a volcano-shaped dependence on Sdoping in the Ni5P4 NPA/CP, as shown in Figure S17. The CDL of Ni5P4 NPA/CP with 2%, 4%, 6%, 8% and 10% S-doping are calculated to be 22.6, 29.8, 45.0, 33.1 and 20.4 mF cm-2, respectively. And the value of ESA is 565, 745, 1125, 827 and 510 cm2 for S-Ni5P4 NPA/CP with 2%, 4%, 6%, 8% and 10% S-doping, respectively. Both the CDL and ESA reached the maximum when the S doping level is 6% in Ni5P4 NPA/CP. We further tested the Nyquist plots of Ni5P4 NPA/CP with different S-doping level at overpotential of 100 mV, as shown in Figure S20a, the results indicated that the 6% S-Ni5P4 NPA/CP have the smallest charge-transfer resistance (RCT) among all S-doped Ni5P4 NPA/CP samples, indicating the fastest Faradaic process in HER kinetics. Based on these observations, we focused our study on the S-Ni5P4 NPA/CP with 6% S doping thereafter. To highlight the superiority of 6% S-Ni5P4 NPA/CP, the catalytic performance of bare carbon paper (CP), Ni5P4 NPA/CP, NiS2 NPA/CP, and state-of-art Pt/C (20 wt% Pt on XC-72) were also tested for comparison. Figure 3c presents the LSV curves of all these samples, it is clear that bare CP shows no catalytic activity for HER before -0.4 V; meanwhile, Pt/C undisputed shows the most outstanding performance with 0 V onset potential, and the Ƞ@10 mA cm-2 and Ƞ@100 mA cm-2 are 18 and 77 mV (Table S2), respectively. The Ƞ@10 mA cm-2 of 95 and 204 mV are revealed for Ni5P4 NPA/CP and NiS2 NPA/CP, much bigger than 6% SNi5P4 NPA/CP (56 mV). Moreover, at a given overpotential in the entire test range, the 6% SNi5P4 NPA/CP delivered the highest current density (normalized to electrode geometry area) among all the samples (except Pt/C). As summarized in Table S3, S-Ni5P4 NPA/CP almost has the lowest Ƞ@10 mA cm-2 and Ƞ@100 mA cm-2 in all nickel phosphide HER electrocatalysts in acid media reported thus far.


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In addition, the polarization curves were further normalized to ESA, as can be seen in Figure S21, it is obvious that the trend of activity (current is normalized to ESA) is same to Figure 3a and Figure 3c where the current is normalized to electrode geometry area, and the 6% S-Ni5P4 NPA/CP still shows the best HER performance (Table S4). Moreover, we focused on the turnover frequency (TOF) of all samples, as shown in Figure S22 and Table S5, the TOF of 6% S-Ni5P4 NPA/CP is calculated to be 0.11 s-1 at η = 100 mV, which is ca. 2.8 ~ 17 times that of other S-Ni5P4 NPA/CP samples, and it is even 13 and 40 times that of NiS2 NPA/CP and Ni5P4 NPA/CP, respectively, which further confirms the superiority of 6% S-Ni5P4 NPA/CP. The results of TOF indicate that 6% S-Ni5P4 NPA/CP can facilitate effective reactions at a given time in acid conditions. Figure 3d presents the Tafel slopes of the samples derived from the polarization curves at a scan rate of 5 mV s-1. 6% S-Ni5P4 NPA/CP shows a Tafel slope of 43.6 mV dec-1, indicating the HER route follows the Volmer-Heyrovsky pathway.9 It should also be pointed out that the Tafel slope of the 6% S-Ni5P4 NPA/CP is slightly higher than that of Pt/C (30 mV dec-1) but distinctly smaller than Ni5P4 NPA/CP (97.4 mV dec-1) and NiS2 NPA/CP (90.7 mV dec-1). The lower Tafel slope of 6% S-Ni5P4 NPA/CP indicates a higher catalytic activity and much faster reaction kinetics for HER compared with other counterparts. This result is in accordance with the observations of the largest CDL, ESA, roughness factor and TOF (Table S6) and the smallest charge-transfer resistance (Figure S20) of the 6% S-Ni5P4 NPA/CP. The stability is studied firstly by chronopotentiometric curve at constant current density of 10 mA cm-2 for 100 h. As shown in Figure 3e, the NiS2 NPA/CP material is unstable during HER test in 0.5 M H2SO4. Under constant current density operation at 10 mA cm-2, the overpotential increased drastically from 0.15 V to 0.35 V accompanied substantial dissolution of the NiS2 into


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the electrolyte. A Ni concentration of 7.2 ppm in the electrolyte was measured by ICP-MS after 10 h of HER tests (Figure S23a), corresponding to the dissolution of about 47 wt.% of the NiS2. The concentration of dissolved Ni gradually increased to 7.8 ppm (~51.1 wt.%) over 100 h of HER operation. This is consistent with previous reports that CoS2/CNT is also unstable during HER test in acid solution.47 In contrast, both Ni5P4 NPA/CP and S-Ni5P4 NPA/CP were able to keep a current density of 10 mA cm-2 during 100 h of continuous HER operation, which maintained their catalytic activity at relatively stable overpotential. It should be pointed out that the ultimate overpotential for 6% S-Ni5P4 NPA/CP is as low as 0.07 V in comparison with the initial 0.09 V, which may be ascribed to the gradual self-optimization process which produce much more accessible active site and minor surface rearrangement of the S-Ni5P4.57 Regarding the fact that the Ni dissolution of S-Ni5P4 sample is only ~2.6 wt.% even after 100 h of HER catalysis (see below), a possible reason is the gradual slight dissolution of S from the surface of 6% S-Ni5P4 which produced more vacancy defects and more accessible catalytic active sites. Another possible reason is that the 6% doping in S-Ni5P4 is still not the best doping ration, while a smaller that 6% but higher 4% S doping content is the optimal one. In all, the increased HER during the testing should be attributed to a balanced interaction between the S and Ni5P4.58-61 Thus, the S-doping will restrain the dissolution of sample and the 6% S-Ni5P4 NPA/CP is more stable than the Ni5P4 NPA/CP catalyst. As shown in Figure S23b, ICP-MS results indicate that after 100 h of HER catalysis, the amount of Ni leaching into the electrolyte for S-Ni5P4 NPA/CP (~2.6 wt.%) was much lower than that of the NiS2 NPA/CP (51.1 wt.%) and Ni5P4 NPA/CP (10.8 wt.%) samples. The high durability is further certified by continuously cycling for 5000 times. Figure S24 shows the cyclic voltammery curves of the complete 5000 cycles of the stability test at a scan rate of 100 mV s-1 for S-Ni5P4 NPA/CP, Ni5P4 NPA/CP and NiS2 NPA/CP.


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Compare with Ni5P4 NPA/CP and NiS2 NPA/CP, no obvious decay of the activity was observed on 6% S-Ni5P4 NPA/CP, further suggesting the high catalytic durability of this novel 6% S-Ni5P4 NPA/CP electrocatalyst in acidic electrolyte. It should be noted that the multi-CV curves (Figure S24a) indicates the performance of 6% S-Ni5P4 NPA/CP has decreased slightly, which is seemingly contradicts with the CP test result shown in Figure 3e. However, we should notice that multi-CV curves test is operated at high current density (~250 mA cm-2) and high overpotential (-0.2 V), which is much larger/higher than that of the CP test with a relative smaller/lower current density of 10 mA cm-2 and -0.09 V overpotential. In our previous study,62 we had demonstrated that the much higher over potential/current density will results in faster water splitting performance degradation. Hence, it is reasonable that the multi-CV test results show a slight decreases performance during a 5000 cycles test. The catalysts after 5000 cycles accelerated stability test were examined by XRD, SEM, TEM and XPS characterization. As shown in Figure S25 and Figure S26, the crystal, morphology, microstructure and electron binding energy of S-Ni5P4 NPA/CP are well preserved after operation at severe conditions, further indicating the super durability of this novel 6% S-Ni5P4 NPA/CP as efficient HER catalyst.


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Figure 4. DFT-calculated HER activities of NiS2, Ni5P4 and S-Ni5P4. The top view of the structure of NiS2 (a), Ni5P4 (b and c), S-Ni5P4 (d-f) with the possible hydrogen (H*) adsorption sites on the surface. (g) The calculated free-energy diagram of HER at the equilibrium potential for three catalysts and Pt reference. 3.4 Density functional theory calculations Last, we performed density functional theory (DFT) calculations to get the insight mechanism. The HER pathway can be presented by a three-state step, including an primal state H+ + e-, an intermediate adsorbed H*, and a final product ½H2.63 The Gibbs free energy changes of the intermediates, described as ∆GH*, has been considered as an important descriptor for assessing the HER activity.64 The ideal value of ∆GH* should be zero, which well benefits a fast


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formation of adsorbed hydrogen and a rapid concomitant hydrogen release. Till now, the most efficient material for HER is Pt, with its value of ∆GH* of ca. -0.09 eV.7, 64-65 The possible freeenergy changes of H atom on different catalyst surfaces are shown in the Figure 4. For the (220) surface of NiS2, the S will be out of surface after geometry optimization (Figure S27). Therefore, the H will be likely adsorbed at the S site rather than Ni. In Figure 4a the ∆GH* on NiS2 is -1.34 eV, which is far from the ideal value. Similar results are also obtained on the Ni5P4 (303), the ∆GH* are -0.59 V and 0.39 eV for Ni-bridge (Figure 4b) and Ni-top sites (Figure 4c), respectively. For the S-Ni5P4, there are three possible configurations for the S doped in Ni5P4, i.e. Figure 4d (S with 7 coordination), Figure 4e (S with 5 coordination) and Figure 4f (S with 4 coordination), meanwhile the adsorption of H on the nearest Ni site are also investigated. The ∆GH* in Figure 4f is closer to the 0 eV (~0.04 eV) than the other configurations of -0.77 eV (Figure 4d) and 0.23 eV (Figure 4e), even than that of Pt (-0.09 eV). This indicates the ideal activity of S-Ni5P4 is comparable or even better than Pt. It is seen that the activity is largely depended on the structures of S-Ni5P4 and the components of S-Ni5P4 in experiment are not single, however, the activity of S-Ni5S4 for HER can be further improved. 4. CONCLUSION In conclusion, we have uncovered that sulfur doped nickel phosphide nanoplates arrays, as a non-precious catalyst, exhibits exceptionally high activity and stability for HER in acidic solution. The extremely high hydrogen evolution performance of S-Ni5P4 NPA/CP catalyst is attributed to the existence of the electronegative sulfur atoms within the Ni5P4. The S-doping not only increase surface area of electrode thus to large extent of active sites, but also restrain the surface oxidation and dissolution of S-Ni5P4 in acid solution; further, the S-doping also reduce ∆GH* of this novel S-Ni5P4 NPA/CP. We propose that non-precious S-Ni5P4 NPA/CP may


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successfully replace the expensive noble metal materials with the promise of higher performance in HER. This study also opens up a new avenue for design and enhanced stability of TMPs hybrid materials for HER. ASSOCIATED CONTENT Supporting Information. Experimental details, SEM images; XRD patterns; EDX spectrum; Nitrogen adsorptiondesorption isotherms; Raman and XPS spectra; Nyquist plots; Tafel slope; calculation of capacitance, ECSA, exchange current density and TOF; polarization curve; and Tables S1-S6. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author *E-mail: [email protected] ORCID Jinfa Chang: 0000-0002-5066-3625 Junjie Ge: 0000-0002-1561-4852 Changpeng Liu: 0000-0002-5335-0157 Wei Xing: 0000-0003-2841-7206 Present Addresses §

Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science

and Technology (AIST), Ikeda, Osaka 563-8577, Japan. AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), Yoshida, Sakyo-ku, Kyoto 6068501, Japan. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The work was supported by the National Natural Science Foundation of China (21633008, 21733004), the “Strategic Priority Research Program” of CAS (XDA09030104), Jilin Province Science

and

Technology

Development

Program

(20140203012SF,

20160622037JC,

20170520150JH), the “Recruitment Program of Foreign Experts” (WQ20122200077) and Hundred Talents Program of Chinese Academy of Sciences. REFERENCES 1.

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Table of Contents Graphic and Synopsis


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Sulfur-Doped Nickel Phosphide Nanoplates Arrays: A Monolithic

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