3D Nanoporous Co9S4P4 Pentlandite as a Bifunctional

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Energy, Environmental, and Catalysis Applications

3D Nanoporous Co9S4P4 Pentlandite as a Bifunctional Electrocatalyst for Overall Neutral Water Splitting Yongwen Tan, Min Luo, Pan Liu, Chun Cheng, Jiuhui Han, Kentaro Watanabe, and Mingwei Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17961 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Applied Materials & Interfaces

3D Nanoporous Co9S4P4 Pentlandite as a Bifunctional Electrocatalyst for Overall Neutral Water Splitting Yongwen Tan, †‡ Min Luo, § Pan Liu, £ Chun Cheng, ‡ Jiuhui Han, ‡ Kentaro Watanabe, ‡ and Mingwei Chen*‡¶⊥ †School

of Materials Science and Engineering, Hunan University, Changsha 410082, China

‡ Advanced

§ Department

£ State

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan of Physics, Shanghai Second Polytechnic University, Shanghai 201209, China

Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China ¶ CREST,

⊥Department

JST, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan

of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21214, USA

KEYWORDS: nanoporous materials, cobalt pentlandite, overall water splitting, neutral water, hydrogen evolution reaction

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ABSTRACT: Significant progress has recently been achieved in developing noble-metal-free catalysts for electrochemical water splitting in acidic and alkaline electrolytes. However, high performance bifunctional catalysts towards both hydrogen evolution and oxygen oxidation reactions of neutral water have not been realized in spite of the technical importance for electrochemical hydrogen production in natural environments powered by renewable energy sources of wind, solar, and so on. Here we report a nanoporous Co9S4P4 pentlandite with three-dimensional bicontinuous nanoporosity for electrochemical water splitting in neutral solutions. The three dimensional binder-free catalyst shows a negligible onset overpotential, low Tafel slope and excellent poisoning tolerance for hydrogen evolution reaction, comparable to or even better than commercial Pt catalysts. Remarkably, the new catalyst also has excellent catalytic activities towards oxygen evolution and, hence, can be used as both anode and cathode for overall neutral water splitting. These extraordinary catalytic activities toward neutral water splitting have never been obtained from non-noble-metal catalysts before. The bifunctional and low-cost catalyst holds great promise for practical applications in electrochemical water splitting in natural environments.

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INTRODUCTION Electrochemical water splitting via hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) provides a feasible approach to produce high-purity hydrogen gas as sustainable fuels.1-4 The key challenge of this technique is to develop highly efficient electrocatalysts that can overcome sluggish reaction kinetics and accelerate the evolution rates of H2 and O2 at low overpotentials. It has been long known that noble metals (e.g., Pt, Rh, Ir) and noble metal oxides (e.g., IrO2, RuO2) have desirable HER and OER activities.5 However, the scarcity and high costs of these noble metals hinder their large-scale applications. Accordingly, it is highly desirable to develop low-cost electrocatalysts for water splitting with high efficiency.6-21 Additionally, for commercial water-splitting devices developed these days, they are operated at either in an acidic solution with pH ≈ 0 for proton exchange membrane electrolyzers, or in an alkaline environment with pH > 14 for alkaline electrolyzers. Electrodes in these cells usually suffer from serious corrosion problems.22Alternatively, microbial electrolysis cells are emerging as an attractive hydrogen-producing technology through organic waste degradation of wastewater by microbes. However, the microbial electrolysis cells work


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in neutral environments and require highly active electrocatalysts for HER and OER in neutral electrolytes.23 Different from hydrogen evolution in acidic solutions, the high kinetic energy barrier for initial water dissociation (H2O+e- → Had+OH-), Volmer step at pH=7) and strong surface adsorption of resultant OH- result in sluggish HER kinetics in neutral

electrolytes.24

Therefore,

it

is

highly

desirable

to

develop

low-cost

electrocatalysts with highly catalytic performance for overall neutral water splitting. Amongst many Co-based bifunctional catalysts,25,

26

the pentlandite-type cobalt

sulfide (Co9S8) is a well-known mineral and has shown a promising performance as a water-splitting electrocatalyst. However, the further improvement of its electrocatalysis is limited by a low surface active exposure and less competitive stability.27-29 A number of strategies, such as regulating specific hybrid materials27 and encapsulating carbon protect-layer,28,

29

have been developed to address these challenges in acidic and

alkaline environments. It is known that developing multiple component transition metal compounds is one of the most efficient ways to enhance the catalytic activity and stability of catalysts. In addition to changing their chemical compositions, the deliberate introduction of foreign atoms in the transition metal compounds provides the possibility


4

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of boosting electrical conductivity and increasing the number and intrinsic activity of active sites by modulating the electronic and surface structures of the host materials. It has been found that pyrite-type cobalt phosphosulphides (CoPS and CoS2|P) have high HER activities in the presence of harsh acidic electrolytes.30-32 However, these pyritetype cobalt phosphosulphides have not been approved to be catalytically active for electrochemical water splitting in neutral solutions. Herein, we report a new pentlanditetype cobalt phosphorsulphide (Co9S4P4) with a bicontinuous nanoporous structure for HER and OER in neutral solutions. The nanoporous Co9S4P4 (np-Co9S4P4) shows a negligible onset overpotential and a low Tafel slope of 51 mV per decade (mV dec-1) with an exchange current density of 0.25 mA cm-2. In addition, the catalyst only requires the overpotentials of 87 mV and 174 mV for stable current densities of 10 mA cm-2 and 100 mA cm-2, respectively. More importantly, the bifunctional np-Co9S4P4 electrocatalyst enables long-term lifetime without obvious degradation in the overall water splitting activity. These extraordinary catalytic activities toward neutral water splitting have never been obtained from non-noble-metal catalysts before. RESULTS AND DISCUSSION


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The np-Co9S4P4 catalyst is synthesized by phosphatizing nanoporous cobalt pentlandite (Co9S8) which is prepared by an electrochemically selective phase dissolution method (Fig.1a

Figure 1. (a) Schematic illustration of the fabrication process of np-Co9S8-xPx for HER catalysis. (b) High-angle annular dark-field scanning TEM image of the rapidly solidified ribbons. (c) SEM image of np-Co9S8. (d) SEM image of np-Co9S4P4. Scale bars: (b), (c), and (d) 200 nm.

and Fig. S1-S3).11, 33 The bicontinusous open porous structure with a large surface area promotes the fast diffusion of P into nano-sized Co9S8 ligaments for the formation of chemically uniform np-Co9S8-xPx (x=0 - 4.7) (Fig.1b-d). The atomic ratio of P/S in the ternary compound is tuned by controlling the loading amount of NaH2PO2 precursor


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during phosphatization (Fig. S4) and is determined by careful chemical analysis using energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). In comparison with a number of intermediate np-Co9S8-xPx (x=0 - 4.7) phases, the Co9S4P4 compound is a stable phase and also shows the highest HER and OER activities as detailed below. Scanning electron microscope (SEM) and transmission electron microscope (TEM) characterizations confirm that np-Co9S4P4 remains the bicontinuous nanoporous morphology of the np-Co9S8 precursor after phosphatization (Fig. 2a). X-ray diffraction (XRD) shows the characteristic peaks of a pentlandite structure with the lattice parameters nearly identical to those of Co9S8 (Fig. S5), indicating that the substitution of S by P does not alter the crystal structure and lattice parameters due to the very similar atomic sizes and chemical characteristics of S and P. The lattice structure of np-Co9S4P4 is imaged by high-resolution TEM (HRTEM) (Fig. 2b). The crystal structure derived from the HRTEM image further demonstrates that the phosphorus treated Co9S8 keeps the pentlandite structure. The chemical homogeneity of the ternary pentlandite is verified by scanning TEM-EDS (Fig. 2c). The well matched distributions of Co, S and P in the elemental mappings suggest the uniform chemistry of


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the heterogeneous nanoporous structure. The structure of the np-Co9S8-xPx is also investigated by Raman spectroscopy. The characteristic Raman bands at 464, 507, and 668 cm−1 in the np-Co9S4P4 spectrum could be from Co9S8 (Fig. 2d).29 In comparison with np-Co9S8, both Eg and Ag Raman bands of np-Co9S8-xPx slightly shift to higher wavenumber with increase of phosphorus substitution (Fig. 2d and Fig. S6) as a result of the formation of P–S dumbbells. X-ray photoelectron spectroscopy (XPS) is employed to investigate the chemical composition and binding status of np-Co9S8-xPx. The Co 2p core level spectra of npCo9S8 and np-Co9S4P4 are shown in Figure 2e. The Co 2p3/2 and 2p1/2 core level peaks of the np-Co9S8 are observed at binding energies of 778.3 eV and 793.8 eV, respectively, together with satellite features, consisting with the two spin–orbit doublets characteristics of Co2+ and Co3+.34 In contrast, a positive shift of the fitted Co2+ and Co3+ peaks takes place in the np-Co9S4P4, compared to those of bare np-Co9S8, and results in slightly higher binding energies of Co 2p3/2 and 2p1/2 peaks at 778.7 eV and 793.5 eV, respectively (Fig. 2e). The minor changes in the Co 2p spectra verify that the oxidation state of Co is not affected obviously by the P substitution. In the S 2p core


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level spectra (Fig. 2f), both lower binding energy components at 161.3 eV and 162.4 eV (S 2p3/2 and 2p1/2) from sulfide species and higher binding energy components at 168.6 eV and 169.8 eV from sulfate species are observed in the np-Co9S8.35 The sulfate components are ascribed to the oxidization of the np-Co9S8 catalyst. However, the obvious decrease of oxidation state features of the S species of np-Co9S4P4 indicates that the incorporation of phosphorus could effectively prevent surface oxidation of npCo9S4P4.32 In comparison, the P 2p core level spectrum of the np-Co9S4P4 shows two peak regions (Fig. 2g). One centers at the binding energy of 128.3 and 129.6 eV (P 2p3/2 and 2p1/2), which can be assigned to the P bounded with Co, and the other locates at 135.9 eV (unresolved doublet) from phosphate-like P. The existence of the high oxidation state P could be ascribed to surface oxidation under ambient conditions as often observed from metal phosphides.9 The average P/S atomic ratios of np-Co9S8xPx

are also estimated by the XPS measurements, together with SEM-EDS (Fig. S7).


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Figure 2. Structural characterization of np-Co9S4P4. (a) Dark-field STEM image of np-Co9S4P4. (b) HAADF-STEM image of np-Co9S4P4. The inserted fast Fourier transform (FFT) pattern can be indexed by a cubic crystal structure of Co9S8. (c) STEM-EDS chemical mappings of npCo9S4P4. (d) Raman spectra of np-Co9S8 and np-Co9S4P4. (e-g) XPS spectra of np-Co9S8 and npCo9S4P4. Scale bars: (a) 200 nm, (b) 2 nm, and (c) 50 nm.


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The HER electrocatalysis of np-Co9S8-xPx was characterized in 1.0 M phosphate buffer solution (PBS, pH=7.0). Figure 3a shows the polarization curves with iRcorrection of np-Co9S8 and np-Co9S8-xPx (Fig. S8), together with a commercial Pt/C catalyst as the benchmark. The onset and operation overpotentials (η) of np-Co9S8-xPx gradually decrease and approach to those of the Pt catalyst when the P/S ratio increases from 0.0 up to 1.0. Further increasing the P content leads to the decline of HER activities probably because of the formation of a Co4S3 phase (Fig. S5). The npCo9S4P4 shows the best HER performance in terms of the extremely low overpotentials of 87 mV and 174 mV versus the reversible hydrogen electrode (RHE) at the electrode current densities (j) of -10 mA cm-2 and -100 mA cm-2, respectively, which are better than all known noble-metal-free catalysts in the neutral solution (Fig. S9).9, 11, 22, 23, 29, 3646

In comparison, the np-Co9S8 requests the overpotential of -264 mV for j = -10 mA cm-

2

electrode current. Significantly, the np-Co9S4P4 electrode shows a negligible onset

overpotential versus RHE in the neutral solution (Fig. S10), which can only be observed from Pt-based catalysts before. The extrapolation from the linear region of the overpotential versus log j plot (Fig. 3b) gives a small Tafel slope of 51 mV per decade


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for np-Co9S4P4, close to Pt/C (35 mV per decade) and much lower than np-Co9S8 (118 mV per decade). From the intercept of the linear portion of the Tafel plots, the exchange current density (j0, geometrical) of np-Co9S4P4 is determined to be 0.25 mA cm-2 (Fig. S11), which is over eight times larger than that of np-Co9S8 (0.031 mA cm-2). Such unbeatably higher HER activities place np-Co9S4P4 at the top of all reported noblemetal-free HER catalysts for neutral media (Fig. 3c and Table S1).8,

22, 23, 29, 36-46

It is

worth noting that the P substitution of np-Co9S8 not only offers superior HER catalytic activities, but also improves the catalytic durability and poisoning tolerance of cobalt pentlandite. After the catalysts work at a constant current density of 10 mA cm-2 for 20 hours, the increment of applied voltage is only ~40 mV for np-Co9S4P4 but as high as ~145 mV and 150 mV for np-Co9S8 and commercial Pt/C catalysts, respectively (Fig. 3d). The obvious deactivation of Pt in the neutral buffer electrolyte is mainly due to the poisoning of phosphoric acid anions.43 As Figure S12 shown, the np-Co9S4P4 electrode can retain the intrinsic feature after long-term HER testing. Quantitative measurements suggest that the ratio of P/S peak intensity slightly increases after the long-term testing due to the loss of S by the formation of surface sulfate. However, for comparison, the


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np-Co9S8 shows the significant formation of sulfate with intense sulfate peaks after the long-term testing. The XPS results clearly verified that the P modification obviously prevents the formation of sulfate and thus renders the excellent catalytic durability (Fig. S12). The HER stability of the np-Co9S4P4 electrode is also confirmed by the fact that detectable structure changes cannot be seen after tested at a constant current density of 10 mA cm-2 for 100 h in the neutral electrolyte (Fig. S13 and S14). In addition to the neutral solution, np-Co9S4P4 is also highly active toward HER in acidic and basic media (Fig. S15).

At the current density of 10 mA cm-2, the

overpotentials are as low as 58 and 96 mV, together with small Tafel slopes of 45 and 54 mV dec-1, in the acidic and basic solutions, respectively. It is worth noting that such high catalytic activities in acidic and basic media are comparable and even superior to best noble-metal-free HER catalysts reported in the literature (Table S2 and Table S3).616, 30, 47-50Additionally,

np-Co9S4P4 shows excellent stability in both media (Fig. S16).

Overall, these results unambiguously demonstrate that np-Co9S4P4 is a highly versatile and efficient HER electrocatalyst in a wide pH range (pH 0–14), which is very similar to


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the commercial Pt catalysts but has better poisoning tolerance in the neutral and basic solutions.

Figure 3. HER performances of the np-Co9S8-xPx catalysts with different P/S ratios in a neutral solution, along with a commercial Pt/C catalyst for comparison. (a) Polarization curves; and (b) corresponding Tafel plots. (c) The comparison with the state-of-the-art electrocatalysts, reported in the literature, for HER in the neutral solution. (d) Long-term stability testing of the np-Co9S8, np-Co9S8-xPx and Pt/C electrocatalysts at a constant current density of 10 mA cm-2. References cited in (c): Co-S (Ref.23), CoP nanowire (Ref.40), Ni-Mo-S (Ref.42), Ni-S (Ref.43), FeP nanorod (Ref.44), and MoS2@N-GR (Ref.45).


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Efficient OER catalysts in neutral solutions, such as cobalt sulfide, have been well investigated,51, 52 but the OER activities of P modified cobalt sulfide have not been well explored before. The OER activity of np-Co9S4P4 was tested in 1.0 M PBS and shows an obviously lower overpotential than that of np-Co9S8, together with a decreased Tafel slope of 106 mV per decade from 137 mV per decade of np-Co9S8 (Fig. 4a and c). In particularly, the OER performance of the np-Co9S4P4/ np-Co9S4P4 pair is much better than the Pt/ IrO2, the benchmark catalyst pair of commercial electrolyzer, in terms of over potentials and Tafel slopes (Fig. 4c), and outperforms all the reported OER electrocatalysts in the neutral electrolyte (Table S4) at the current density of 25.9 mA cm−2 at 570 mV (1.8 V vs. RHE). Together with the other half reaction of HER, the bifunctional np-Co9S4P4/np-Co9S4P4 couple only requires a low cell voltage of only ~1.67 V to deliver a current density of 10 mA cm−2, which is much lower than that for the noble metal-based Pt/C/IrO2 couple (1.72 V). Over 24 h of galvanostatic electrolysis at 10 mA cm−2, the applied voltage of the np-Co9S4P4/ np-Co9S4P4 couple only has an augmentation of ~0.02 V (Fig. 4d). To understand the origins of the outstanding catalytic activities of np-Co9S4P4, we measured the geometric specific surface areas by nitrogen adsorption/desorption isotherm and electrochemically active surface area (ECSA) by CV measurements. As show in Figure S17, the phosphorus modification leads to more than two times increase of ECSA of np-Co9S4P4 in comparison with the precursor np-Co9S8. In contrast, the


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changes in the specific surface areas are not significantly (Fig. S18). Therefore, the P modification does not change the geometric surfaces of the nanoporous catalysts too much but significantly enhances the electrochemical activity of the catalyst surfaces. The underlying mechanisms could be associated with changes in the electron structures of S and Co atoms by P doping as well as the enhanced proton dynamics as suggested by the following DFT calculations (Fig. 4). Electrochemical impedance spectroscopy (EIS) of the np-Co9S8-xPx electrodes with different P concentrations further confirm that the introduction of P gives rise to low internal resistance and fast charge transfer for a low onset potential and high HER kinetics (Fig. S19).


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Figure 4. (a) and (b) OER performance of np-Co9S8 and np-Co9S4P4 electrode in 1.0 M PBS. (a) polarization curves, (b) corresponding to Tafel plot. (c) Overall water splitting characteristics of np-Co9S4P4

and

Pt/C(-)/IrO2(+)

electrodes

in

a

two-electrode

configuration.

(d)

Chronopotentiometry curves of the np-Co9S4P4/ np-Co9S4P4 electrode under a current density of 10 mA cm-2. All experiments were carried out in 1.0 M PBS. Density functional theory (DFT) calculations were carried out to gain insights into the fundamental mechanisms of the outstanding HER catalysis of Co9S4P4. On the basis of the Co9S8-xPx atomic models shown in Figure S20 and Table S5, we calculated the kinetic energy barriers of the prior Volmer step ( △ G(H2O)) and concomitant


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combination of adsorbed H* into molecular hydrogen ( △ G(H)) , which is often considered as a descriptor for HER.53 As shown in Figure 5 and Table S6, Co9S8 has a large energy barrier for the Volmer step ( △G(H2O) = 1.121 eV) and a strong hydrogen adsorption free energy (| △G(H)| = 0.180 eV), which result in the poor HER activities in the neutral solution. Substituting S atoms with P atoms dramatically decreases the △

G(H2O) and | △G(H)| values. The △G(H2O) and | △G(H)| barrier energies of Co9S4P4 are as low as 0.746 and 0.042 eV, suggesting that the kinetics of the initial water dissociation and the hydrogen adsorption can be effectively promoted by Co9S4P4. Moreover, the calculated density of states suggests that the P substitution gives rise to enhanced excitation of charge carriers to the conduction band (Fig.5c, d and Fig. S21), benefiting for electronic conductivity. The higher electrical conductivity will be favour of the improvement of the electron transfer capacity, bring the enhanced electrochemical performances.54 Besides the intrinsically improved catalysis of the P modified cobalt pentlandite, the nanoporous structure fabricated by the selective phase dissolution plays important roles in both the synthesis and electrocatalysis of Co9S8-xPx compounds. The large surface


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area and open porosity, together with nano-size Co9S8 ligaments and thus reduced diffusion length, promote fast diffusion of P for the formation of chemically uniform Co9S8-xPx compounds. The bicontinuous and free-standing feature of the nanoporous catalysts also provides facile paths for both electrons and reactants, which benefits the HER kinetics (Fig. S22).55 Moreover, the np- Co9S4P4 is fabricated in the form of freestanding ribbons and can be directly used as working electrodes without any binders and additives, which inherently has better mechanical and electrochemical stability than conventional discrete particulate HER catalysts.10


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Figure 5. (a) DFT calculations of the adsorption energy diagram for HER on Co9S8-xPx catalysts. The calculated diagram shows the reactant initial state, intermediate state, final state, and an additional transition state for water dissociation under the neutral condition. (b) Calculated freeenergy diagrams (△G(H)). (c) Spin resolved projected density of states (PDOS) for np-Co9S8-xPx obtained from DFT calculations. (d) Fermi energy of np-Co9S8-xPx with P concentrations. CONCLUSIONS In summary, we have successfully developed a new np-Co9S4P4 catalyst towards high-efficiency HER and OER in neutral solutions by phosphatizing nanoporous cobalt pentlandite. The partial replacement of S by P in the multicomponent catalysts leads to significant improvement in the electrocatalysis of cobalt pentlandite by increasing electrochemically active sites and modifying the electronic structure. Moreover, the bicontinuous nanoporosity, produced by selective phase dissolution, offers a large specific surface area, high electric conductivity and electrochemical/mechanical stability, leading to excellent HER and OER performance in neutral solutions and long-term operation. The low-cost catalyst with outstanding HER and OER activities and excellent stability holds a great promise for electrochemical hydrogen production in neutral environments for renewable energy applications.


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METHODS

Synthesis and characterization of nanoporous Co9S8 and np-Co9S4P4. The Co90S10 alloy ingot was prepared by arc melting pure Co and CoS under an argon atmosphere. The designed Co to S atomic ratio is 90:10. A melt spinning technique was used to rapidly quench the re-melt alloy ingot by the cold surface of a spinning copper roller at a rotation speed of 5000 revolution per minute. Ribbons with the dimensions of ~1 mm wide, 10 cm long and 20 m thick were achieved. Scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD) characterization reveal that the ribbons contain a Co phase and a cubic cobalt sulfur (Co9S8) phase with bicontinous net structure. The ribbons were cut into thin plates with dimensions of 1 mm wide and 10 mm long and electrochemically etched in 0.5 M HCl solution in a standard threeelectrode configuration with an Ag/AgCl electrode as the reference electrode and a graphite sheet as the counter electrode by using an electrochemical workstation (lvium Technology). The selective phase dissolution was controlled by the applied potential


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that was determined according to the critical oxidation potentials of the FCC Co phase and the Co9S8 compound. The np-Co9S8 were obtained at a dealloying voltage of 0 V vs. Ag/AgCl for 3000 s after the full dissolution of the Co phase. The dealloyed samples were rinsed by deionized water for more than three times to remove the residual chemical substances within nanopore channels.

Nanoporous Co9S8 ribbons were placed in a ceramic crucible in which NaH2PO2 as the P source was positioned at the upstream side. The samples and NaH2PO2 were heated at 693 K for 3 hours with Ar gas flowing at 150 s.c.c.m and cooled down naturally with the furnace. The concentration of P in resulting Co9S8-xPx is controlled by the loading amount of NaH2PO2. Microstructure and chemistry of the np-Co9S8-xPx were inspected with a JEOL JIB-4600F SEM equipped with an Oxford energy dispersive Xray spectroscopy. The STEM images were taken by a 200 kV JEOL JEM-2001F with double spherical aberration (Cs) correctors for both the probe-forming and imageforming objective lenses. XRD analyses were conducted on a Rigaku Ultima X-ray diffractometer with Cu Ka radiation. The chemical state and composition of the samples


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were characterized by using X-ray photoelectron spectroscopy (XPS, AxlS-ULTRADLD) with Al Ka (mono) anode at energy of 150 W. Surface areas and pore sizes of the samples were measured by the Brunauer–Emmett–Teller (BET) method and BarrettJoyner- Hallender (BJH) method at 77.0 K using a BELSORP-mini II (BEL. JAPAN. INC). The horizontal axis was normalized with the vapor pressure of nitrogen (P0) at 77.0 K (= 0.101 MPa). The samples were heated at 80°C under vacuum for 8 hours before the measurements.

Electrochemical Characterization. All electrochemical measurements were performed by using an electrochemical workstation (lvium Technology) in a typical three-electrode cell equipped with an Ag/AgCl reference electrode, a graphite sheet as counter electrode. The acidic, neutral and basic solutions used in this study were 0.5 M H2SO4, 1.0 M potassium phosphate buffer solution (PBS) and 1.0 M KOH, respectively, which were prepared with deionized water and continuously purged with high-purity Ar for 4 hours before electrochemical measurements. The reference electrode was calibrated to reversible hydrogen potential (RHE) using a standard platinum electrode for both


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working and counter electrodes in the same electrolytes, and converted to RHE according to the Nernst equation (ERHE=EAg/AgCl +0.0591pH+ 0.198). The overpotential (η) of OER in neutral electrolyte was calculated according to the following formula: η (V) =ERHE -1.23 V. The sweep rate used in the cyclic voltammetry (CV) studies was 2 mV/s. All CV curves presented here were corrected for iR losses unless noted. The current densities

were

calculated

based

on

geometric

areas

of

tested

electrodes.

Electrochemical impedance spectroscopy (EIS) was performed in a potentiostatic mode at 100 mV versus RHE, applying a sinusoidal voltage with an amplitude of 10 mV and scanning frequency from 100 kHz to 0.01 Hz. For comparison with the commercial Pt/C catalyst, the np- Co9S4P4 was first grinded to form the Co9S4P4 nanoparticles and then the 1 mg Co9S4P4 nanoparticles (or 2 mg 1:1 wt% Carbon and Co9S4P4 nanoparticles mixed powders) of catalysts are dispersed on a solution containing water (400 μL), CH3CH2OH (100 μL) and 5 wt% Nafion solution (30 μL) to generate a homogeneous ink in a ultrasonication bath and then loaded onto a glassy carbon (GC) electrode. The loading amount is about ~1.0 mg cm-2. The GC electrode is assembled on a rotation dish electrode (RDE), while the working electrode was rotated at 1,600 r.p.m. To


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prepare the Pt/C and IrO2 electrode, 4 mg of commercial Pt/C or IrO2 was mixed with Nafion and ethanol to form a homogeneous ink, which was then casted onto the GC electrode. The typical catalyst loading amount was ~1.0 mg cm-2.

DFT calculations. Spin-polarized density functional theory calculations were performed using a slab structural model by the Vienna ab initio simulation package (VASP) with the generalized gradient approximation (GGA). The plane wave pseudopotential with a cutoff energy at least 500.0 eV and a 8×8×1 Monkhorst-Pack k grid were adopted in the self-consistent convergence. By the structure relaxation the atomic geometries were fully optimized until the Hellmann-Feynman forces were less than 0.01 eV/Å. The hydrogen adsorption free energy ∆GH was calculated as below：

1 ∆EH = E𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + H ― E𝑠𝑢𝑟𝑓𝑎𝑐𝑒 ― EH2 2

∆GH = ∆EH + ∆EZPE ― T∆SH

where EH2 is the energy of a gas phase hydrogen molecule, ∆EZPE is the zero-point energy difference between the adsorbed state of the system and the gas state, and ∆SH


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is the entropy difference between the adsorbed state of the system and the gas phase standard state (300 K, 1 bar). As the contribution from the vibrational entropy of H in the adsorbed state is negligibly small, the entropy of hydrogen adsorption is defined as ∆SH ≈1/2SH, where SH is the entropy of H2 in the gas phase at the standard conditions. The Gibbs free energy with the overall corrections was taken as: ∆GH = ∆EH +0.29eV. The adsorption energy (Eads) was calculated by subtracting the energies of gas phase species and the clean surface from the total energy of the adsorbed system; Eads = E(adsorbate/slab) – [E(adsorbate) + E(slab)], and a more negative Eads indicates a more stable adsorption. To evaluate the energy barrier, the transitional state (TS) was located using the nudged elastic band (NEB) method. The TS configurations were verified by vibration analysis and, in all cases, only one imaginary frequency was found. The barrier (Ea) and reaction energy (△Er) were calculated according to Ea = ETS–EIS and △Er = EFS–EIS, where EIS, EFS, and ETS were the energies of the corresponding initial state (IS), final state (FS), and transition state (TS), respectively. All the calculated adsorption Gibbs free energies (G) of H2O, H, and OH as well as energy barriers (△G) of H2O dissociation on different catalysts are listed in Table S5.


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Electrocatalytic HER mechanisms in acidic, neutral and basic solutions:

HER in acidic solution:

H++e-+cat

H*- cat

2H*-cat

Volmer step

H2

H*-cat+H++e-

Tafel step

cat + H2

Heyrovsky step

HER in neutral and basic solutions:

H2O +e-+cat

H*-cat

H*-cat + -OH

Volmer step

H2

H*-cat+H2O+e-

Tafel step

cat+-OH+H2

Heyrovsky step

ASSOCIATED CONTENT

Supporting Information.

This material is available free of charge via the Internet at http://pubs.acs.org.


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Additional information includes the supplementary figures and tables

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Funding Sources The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was sponsored by JST-CREST “Phase Interface Science for Highly Efficient Energy Utilization”, Japan Science and Technology Agency; and World Premier International (WPI) Research Center Initiative for Atoms, Molecules and Materials, MEXT, Japan. M.C. is supported by Whiting School of Engineering, Johns Hopkins University.

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TOC A nanoporous Co9S4P4 pentlandite with a bicontinuous nanoporous structure is exploited for hydrogen evolution reaction in neutral solutions. The three-dimensional binder-free catalyst shows a negligible onset overpotential, low Tafel slop and excellent poisoning tolerance, comparable to or even better than commercial Pt catalysts. These extraordinary catalytic activities toward neutral water splitting have never been obtained from non-noble-metal catalysts before.


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3D Nanoporous Co9S4P4 Pentlandite as a Bifunctional

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