Influence of the Fe: Ni Ratio and Reaction Temperature on the

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Influence of the Fe : Ni Ratio and Reaction Temperature on the Efficiency of (FexNi1-x)9S8 Electrocatalysts Applied in the Hydrogen Evolution Reaction Stefan Piontek, Corina Andronescu, Aleksandr Zaichenko, Bharathi Konkena, Kai junge Puring, Bernd Marler, Hendrik Antoni, Ilya Sinev, Martin Muhler, Doreen Mollenhauer, Beatriz Roldan Cuenya, Wolfgang Schuhmann, and Ulf-Peter Apfel ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02617 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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ACS Catalysis

Influence of the Fe : Ni Ratio and Reaction Temperature on the Efficiency of (FexNi1-x)9S8 Electrocatalysts Applied in the Hydrogen Evolution Reaction Stefan Piontek a,, Corina Andronescu b,c,, Aleksandr Zaichenko d, Bharathi Konkena b, Kai junge Puring a,e, Bernd Marler f, Hendrik Antoni g, Ilya Sinev h, Martin Muhler g, Doreen Mollenhauer d, Beatriz Roldan Cuenya h,i, Wolfgang Schuhmann b, Ulf-Peter Apfel a* a

Ruhr-Universität Bochum, Inorganic Chemistry I, Universitätsstrasse 150, D-44780 Bochum (Germany)

b

Ruhr-Universität Bochum, Analytical Chemistry - Center for Electrochemical Sciences (CES), Universitätsstrasse 150, D-44780 Bochum (Germany)

c

University Politehnica of Bucharest, Department of Bioresources and Polymer Science, 1-7 Gh. Polizu Street, 011061, Bucharest (Romania) d

Justus-Liebig-Universität Gießen, Institute of Physical Chemistry, Heinrich-Buff-Ring 17, D35392 Gießen (Germany) e

Fraunhofer UMSICHT, Osterfelder Strasse 3, D-46047 Oberhausen

ACS Paragon Plus Environment

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f

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Ruhr-Universität Bochum, Department of Geology, Mineralogy and Geophysics, Universitätsstrasse 150, D-44780 Bochum (Germany)

g

Ruhr-Universität Bochum, Laboratory of Industrial Chemistry, Universitätsstrasse 150, D44780 Bochum (Germany)

h

Experimental Physics IV, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44780 Bochum (Germany)

i

Department of Interface Science, Fritz-Haber Institute of the Max Planck Society, 14195 Berlin (Germany)

these authors contributed equally to this work

Abstract Inspired by our recent finding that Fe4.5Ni4.5S8 rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe : Ni ratio on the performance of the catalyst. We herein describe the synthesis of (FexNi1-x)9S8 (x = 0 - 1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry, we show that the increase in the iron or nickel content, respectively, lowers the activity of the electrocatalyst towards HER. Electrochemical surface area analysis (ECSA) clearly indicates the highest amount of active sites for a Fe : Ni ratio of 1 : 1 on the electrode surface pointing at an altered surface composition of iron and nickel for the other materials. Specific metal-metal interactions seem to be of key importance for the high electrocatalytic HER activity, which is supported by DFT calculations of several surface structures using the surface energy as a descriptor of catalytic activity. In addition, we show that a temperature increase leads to a significant decrease of the overpotential and gain


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in HER activity. Thus, we showcase the necessity to investigate the material structure, composition and reaction conditions when evaluating electrocatalysts.

Keywords Iron nickel sulfide, pentlandite, surface composition, hydrogen evolution, electrocatalysis, electrochemistry, HER

Introduction A future efficient energy economy is one of the major goals for a modern society.1 Especially, hydrogen, which possesses the highest energy density of all common fuels, is envisioned as a superior energy storage and transport molecule.2,3 Thus, hydrogen generation with minimum energy effort offers a low-cost, clean and environmental friendly alternative to fossil fuels.4 Furthermore, the efficient production of hydrogen makes it suitable to store energy and to provide power on demand. Along this line, electrocatalytic hydrogen generation can compensate for electricity over-production by buffering surplus energy from alternative energy sources like windmill-powered plants or solar collectors and serves as an efficient storage. Low-cost and robust electrocatalysts that minimize the overpotential for the hydrogen evolution reaction (HER) are therefore a necessity.5 In contrast to platinum and its alloys as benchmark-electrocatalysts for the HER in acidic media, metal chalcogenides,6–10 carbides,11–13 nitrides,14,15 borides,16 phosphides17,18 and metal-free19–21 electrocatalysts were successfully implemented but partly suffer from their low conductivity, low durability and the extensive nano-structuring that is required. Recently, we showed that synthetic pentlandite (Fe4.5Ni4.5)S8 is an efficient and robust electrocatalyst, possessing low overpotential (190 mV at -10 mA cm-2), high current density (up to -650 mA cm-2 at 600 mV overpotential) and high long-time stability (> 200 h).22 Notably, density functional theory (DFT) calculations suggested a significant influence of specific metal-metal interactions on the binding of protons to a model surface. These assumptions are supported by operando phonon studies revealing a sulfur replacement by protons and subsequent hydride formation.23 Similar effects are


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well-known from the natural enzymes and can be rationalized by their metal composition.24 While [FeFe]-hydrogenases reveal high turnover frequencies (TOF) for HER,25–27 [FeNi]-hydrogenases show distinctively lower H2 production rates.28,29 Likewise, altered metal-metal interactions, sulfur binding stability and HER activities can be expected in (FexNi1-x)9S8 (x = 0 - 1) when changing the Ni : Fe ratio. This hypothesis is in line with a recent report of Waterson et al. modeling the pentlandite (111) surface by DFT.30 According to this study, the ligand binding of ethyl xanthate and water as well as the overall surface composition severely changes upon alteration of the Fe : Ni ratio. Notably, cooperative metal-metal interactions on an electrode surface were suggested to be a key factor for the enhanced catalytic performance in metal/C catalysts.31 We assumed that similar cooperative metal-metal interactions will affect the electrochemical performance of the (FexNi1-x)9S8 catalysts. Herein, we report on the alteration of the electrocatalytic HER performance by variation of the Fe : Ni content in pentlandite-like (FexNi1-x)9S8 composites. Furthermore, we showcase the influence of the reaction temperature as an important tool to modulate the catalytic HER performance of (FexNi1-x)9S8 composites.

Experimental Section Materials. All chemicals were obtained from commercial vendors and used without further purification. Iron, nickel and sulfur powders were supplied by Sigma Aldrich with purities of at least 99.98 %. Concentrated sulfuric acid was diluted with deionized water generated by the water purification system Direct-Q from Millipore. Preparation of (FexNi1-x)9S8. The different iron-nickel sulfides (FexNi1-x)9S8 (x = 0 - 1) were synthesized from high purity elemental powders. The appropriate mixture was ground until a homogeneous mixture was obtained, sealed in a 10 mm quartz tube and kept at 1.4 x 10-2 mbar for one day. Subsequently, the mixture was heated up to 700 °C and the temperature was held for 3 h. Then, the temperature was raised to 1100 °C (5 K min-1) and maintained for additional 10 h. The mixture was then allowed to cool to room temperature and used as is. Electrode assembly.32 The pentlandite composites were ground to a fine powder and pressed into cylindrical pellets (size of 2 x 3 x 4 mm) with a pressure of 800 kg cm-². The obtained pellets were placed in a Teflon case (82 x 5 mm) containing a brass contact (Fig. S1). Electrical contact between the brass contact and the pentlandite pellet was established using a conducting two-component


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glue (Polytec EC 151 L). Subsequently, the electrode was dried for 1 day at 60 °C and polished with a 0.3 µm lapping film from 3M to obtain a flat and defined surface of 0.071 cm². Electrochemical measurements. Electrochemical experiments were performed with a GAMRY Reference 600 potentiostat or an Autolab potentiostat/galvanostat (PGSTAT12) using a threeelectrode setup. As prepared iron-nickel sulfide electrodes were used as working electrode, Ag/AgCl (3 M KCl) or Ag/AgCl (sat. KCl) as reference and a Pt grid as counter electrode. All experiments were performed in 0.5 M H2SO4 as electrolyte. Working potentials were converted from Ag/AgCl (3 M KCl or sat. KCl) to the reversible hydrogen electrode (RHE) according to ERHE = X + 0.059 x pH (X = 0.197 V, sat. KCl or X = 0.210 V, 3 M KCl). In case of temperature dependent measurements, a Ag/AgCl (3 M KCl or sat. KCl) was recalibrated to include temperature shifts of the reference electrode according to ERHE = X + 0.059 x pH (X = E(Ag/AgCl25 °C) ± E(Ag/AgCl0-90 °C) (Figure S2). In all experiments iR compensation was applied to account for the iR drop between the reference and working electrode. Before performing linear sweep voltammetry at 5 mV s-1, the electrode was conditioned by performing cyclic voltammetry (20 cycles, at 100 mV s-1) in the potential range -0.8 V - 0.0V vs. Ag/AgCl (3 M KCl or sat. KCl). All long-term experiments were performed applying controlled potential coulometry for 20 h at a potential of -0.5 V vs RHE. The electrochemical surface area was derived from the cyclic voltammograms collected in the non-faradaic region ( -0.18 to + 0.22 V vs RHE). Electrochemical impedance measurements were recorded at the corresponding open circuit potential, at constant current or potential in a frequency range from 100 kHz to 0.1 Hz. Temperature dependent measurements were performed by placing the electrochemical cell in an oil bath and controlling the temperature within the cell. After each heating step, the solution was cooled to its previous temperature and an additional linear sweep voltammogram (LSV) was recorded to check the consistency of the obtained data. Electrochemical impedance spectra (EIS) at different temperatures were recorded after a preconditioning of the electrode by applying a current of -0.2 mA for 5 min in order to reach steady state. After this step, galvanostatic EIS was performed at -0.2 mA in the 100 kHz – 0.1 Hz frequency range. Thermal analysis. The prepared iron-nickel sulfides and pure element mixtures were investigated by differential scanning calorimetry (DSC) using a NETZSCH STA 449 F3 Jupiter.


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Approximately 50 mg of a sample were placed in a closed corundum crucible and processed from room temperature to 1000 °C and vice versa at 10 K min-1 in a continuous N2 gas flow. Physical methods. The scanning electron microscope (SEM) LEO (Zeiss) 1530 Gemini FESEM scanning electron microscope was operated at a voltage of 20 kV (SEM) and 4.4 kV (electron dispersive X-ray spectroscopy, EDX). Powder X-ray diffraction (PXRD) was performed using a diffractometer from HUBER with Mo Kα radiation (0.709 Å) and the Bruker Advance D8 with Cu-Kα radiation (0.154 Å) scanning in an angle range of 3 - 50 ° with a step size of 0.03 ° s-1. All reflex positions were converted from Mo to Cu radiation via Bragg’s law. Mössbauer spectra were recorded at 298.15 K using a 57Co radiation source in a Rh matrix in a SeeCo constant acceleration spectrometer. Isomer shifts are referred to α-Fe metal at room temperature. Data were fitted to a single line superposed on a symmetric quadrupole doublet using a least-square routine with the WMOSS program. X-ray photoelectron spectroscopy (XPS) measurements were carried out in two ultra-high vacuum (UHV) setups equipped with monochromatic Al Kα X-ray sources (hν = 1486.6 eV), operated at 14.5 kV and 300 W, using either high resolution Gammadata-Scienta SES 2002 or Specs Phoibos 150 analyzers. The base pressure during the measurements was maintained at about 5 ×10-10 mbar. A flood gun was used to compensate the charging effects. High-resolution XPS spectra from the C 1s, O 1s, S 2p, Fe 2p, and Ni 2p regions were recorded. The C 1s signal of the adventitious carbon was used an external standard attributed to 285 eV binding energy. The Casa XPS software with a pseudo-Voigt Gaussian-Lorentzian product (oxide species) or asymmetric LA (metal sulfide species) functions and Shirley background subtraction was used for peak deconvolution. Atomic ratios were calculated from XPS intensities corrected by the corresponding Scofield photoemission cross-sections. The Fe and Ni composition for all different samples were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Thermo Scientific iCAP 6500 Duo equipped with CETAC ASX 520 autosampler. Data acquisition was carried out on iTEVA and analyzed on Origin 9.0G. Standards for Fe (10 ppm in 10% v/v nitric acid, Multi-element standard) and Ni (10 ppm in 10% v/v nitric acid, Multi-element standard) were purchased from Sigma Aldrich. Calibration curves were prepared in double deionised water (ddw) with 3% nitric acid, range between 5000 and 100 ppb (6 points). Standards and samples were freshly prepared in ddw with 3% metal free nitric acid. Readings were made in no-gas mode. Samples (1 mg) were diluted in 3 mL metal-free aqua regia, placed in 55 ml TFM


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vessels and digested in a CEM Mars Xpress microwave (160°C, 15 min ramp, 15 minutes hold). Digested mixtures were then diluted to 10 mL by addition of double distilled water. Computational details. All density functional theory (DFT) calculations were performed with periodic boundary conditions using the Vienna ab initio simulation package (VASP) version 5.4.1.33–36 The exchange-correlation functional Perdew-Burke-Ernzerhof (PBE) within the framework of generalized gradient approximation (GGA) was applied.36,37 Projector-augmented wave (PAW) potentials and a plain wave energy cutoff of 325 eV were employed for the electron structure calculations.38,39 Bulk calculations were performed with a 4×4×4 Monkhorst-Pack kpoint mesh, surface calculations with a 4×4×1 Monkhorst-Pack k -point mesh. The conjugategradient algorithm was used for structure calculations, the convergence criteria for the total energy was 10-5 eV. The bulk structure of the pentlandite Fe4.5Ni4.5S8 (𝐹𝑚3𝑚) with a Ni : Fe ratio of 1 : 1 was taken from reference [30] in which the energetically most stable bulk structure was determined with a homogenous distribution of Ni and Fe atoms. The lattice parameters of the bulk structure were calculated to be a = 14.18 Å, b = 14.18 Å, c = 17.37 Å and agree well with experimental data (a = 14.20 Å b = 14.20 Å c = 17.38 Å).40 Powder X-ray diffraction (Figure 1) and previous theoretical studies of the pentlandite Fe4.5Ni4.5S8 showed that the (111) facets are dominant, thus, in this work we concentrated on the Fe4.5Ni4.5S8 (111) surface.30 We have modeled the (111) surface by a slab of six layers (204 atoms). The slab was separated by a vacuum region of 12.6 Å. Very small total energy differences of less than 10 meV were obtained by increasing the vacuum region. For the structure optimizations either three-fourth of the bottom layers or three-fourth of the top layers were fixed, all other atomic positions were freely relaxed. The surface energies were calculated with the formula: 𝛾=

+,-,/0()1

&'()*

*,++,3,/0()1

2&'()*

56107

4&'()* 48&*9(:

;
?@A is the bulk energy pro atom, 𝐸B@C> KLIHM

with relaxed top or bottom layers, 𝐸B@C>

>EDDEJ,GH@CI and 𝐸B@C> are the energies of the slab

is the energy of the unrelaxed slab with all atomic

positions fixed. Furthermore, N is the number of atoms and A represents the surface area. Equation (1) was chosen because the number and positions of the Fe and Ni atoms vary for the top and bottom layers. With equation (1) a mixed surface energy containing bottom and top surface contributions was obtained.


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The same parameters and procedures were taken to calculate the bulk and surface structures of Fe6Ni3S8 and Fe3Ni6S8. The bulk structures were prepared by replacing randomly Fe or Ni atoms of the pentlandite Fe4.5Ni4.5S8.

Results and Discussion Characterization of (FexNi1-x)9S8. A series of (FexNi1-x)9S8 (x = 0 - 1) mixed metal sulfides was prepared via high temperature synthesis following the synthetic route we reported previously for Fe4.5Ni4.5S8.22 Figure 1a displays the powder X-ray patterns of the (FexNi1-x)9S8 samples in comparison to the well-known pentlandite Fe4.5Ni4.5S8 (Fig. 1b).22 The (FexNi1-x)9S8 (x = 0.22 – 0.77) samples reveal equivalent reflexes of Fe4.5Ni4.5S8 indicating an identical overall structure. We were unable to isolate pure pentlandite-like structures either for higher nickel (Fe1Ni7S8 and Fe2Ni6S8) or higher iron (≥ Fe7Ni2S8) contents. Hence in case of Fe8Ni1S8, Fe7Ni2S8, Fe1Ni8S8 and Fe2Ni6S8, mixtures of different iron and nickel sulfides are favored (Fig. 1a). Attempts to obtain pure pentlandite composites with (FexNi1-x)9S8 (0.22 > x < 0.66) utilizing altered temperature conditions failed as well. In lieu, formation of iron sulfides or nickel sulfides containing either unreacted nickel or iron, respectively, was observed. For example, in case of the desired Fe8Ni1S8, the formation of pure non-magnetic troilite (FeS) with traces of nickel (Fig. 1a), was achieved. This result is in line with the theoretical observation by Waterson et al. suggesting that a high iron or nickel content leads to a destabilization of the pentlandite surface.30 Thus, thermodynamic more stable mineral phases will be formed.


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Figure 1. (a) PXRD pattern (Cu Kα) comparison of (FexNi1-x)9S8 samples with pentlandite sharing equal reflexes. (b) Crystal structure of Ni4.5Fe4.5S8 in which iron and nickel share the same positions (Ni, Fe – brown, S – yellow).

Scanning electron microscopy (SEM) was used for all powder samples and electrode surfaces showing no periodical shape and surface structure (Figs. S3 and S4). Energy dispersive X-ray spectroscopy (EDX) reveals the expected Fe : Ni ratio at the mineral surface as well as a very homogeneous distribution of the elements (Fig. S5 and S6). Additionally, inductively coupled plasma optical emission spectrometry (ICP-OES) confirmed the ratio of Fe and Ni in the different materials compositions (Table S1). Furthermore, we performed Mössbauer spectroscopy to evaluate the iron environment in our materials. In the (FexNi1-x)9S8 series (Fig. S7) the isomer shifts of the octahedral Fe ions vary only slightly and fall between those values previously reported for octahedrally coordinated high-spin Fe2+ centers in FeS and low-spin Fe2+ in pyrite.41 In addition, tetrahedral Fe centers show isomer shifts in between those of tetrahedrally coordinated Fe3+ and Fe2+ in NaFeS2 and FeCr2S4, respectively.41 While we cannot differentiate between the various oxidation states based on such data, it is obvious that with increasing Ni content, Fe mainly occupies the tetrahedral sites within the pentlandite lattice. Such changes will eventually lead to a significant alteration of the metal distributions on the bulk surface. Differential scanning calorimetry (DSC) was performed to investigate the phase purity as well as thermal stability of our materials. DSC-thermograms were acquired during heating and cooling cycles in the range of 25 ˚C to 1000 ˚C of the different (FexNi1-x)9S8 samples (Fig. S8) in argon. All (FexNi1-x)9S8 (x = 0.22 – 0.77) materials showed two main phase transitions at ~610 ˚C and ~860 ˚C upon heating. The data is comparable to that of pure Fe4.5Ni4.5S8 and suggest overall similar phase compositions. Notably, samples with lower or higher iron content do not reveal the expected pentlandite phase and clearly indicate phase impurities, which is also in accordance to the obtained PXRD data obtained. Thus, further in the manuscript, solely pentlandite-phase containing samples will be discussed ((FexNi1) S ; 3 ≤ x ≤ 6).

x 9 8

Additionally, we have acquired X-ray photoelectron spectroscopy (XPS) data of the Fe-Ni samples prior to reaction. These data were used to determine the initial oxidation state of the samples, to corroborate the presence of Fe and Ni in sulfide and to estimate the Fe : Ni and Fe : S and Ni : S ratios on the surface. All samples showed the presence of significant amounts of both carbon and oxygen along with iron, nickel and sulfur from bimetallic sulfide structure (see Figure S9, S10 and Table S2). The amount of oxygen in the different samples measured was found to vary non-


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monotonically with the Fe : Ni ratio indicating that its origin is not related to sample preparation issues and varies between 37-48%. Fe 2p XPS spectra (Figure 2a) of all samples show doublets at 707.1 and 720.4 and at 711.4 and 724.6 eV, representing iron sulfide and oxidic iron species, respectively. While the latter is persistent in all the samples, the FeS doublets change their intensity non-monotonically, with the largest contribution being found for Fe4.5Ni4.5S8 sample measured after shorter air exposure. Likewise, the Ni 2p region presented in Figure 2b shows the presence of NiS, which is represented by a doublet at 853.2 and 870.5 eV, and oxidic Ni2+ species at 856.0 and 873.5 eV. A detailed analysis of the high-resolution Fe 2p and Ni 2p XPS spectra (Figures 2c and 2d, correspondingly) indicates severe surface oxidation of both metals. Thus, the Fe 2p3/2 spectrum consists of an intense and broad peak at ca. 711.6 eV, and two minor shoulders at 707.2 and 704.6 eV. The former intense peak can be ascribed to Fe3+ oxidic species, since there are no hints of the presence of a shake-up satellite structure, characteristic of the Fe2+ oxides state around 716 eV.42 The shoulder at 704.6 eV, or so-called ‘pre-peak’, was also reported to appear in iron oxide spectra.42 Finally, the peak at 707.2 eV fits well to the value reported for iron disulfide species.43 Similarly, Ni 2p3/2 appears to be severely oxidized. Peaks at 856.0 and 861.8 eV correspond to the main peak and shake-up satellite of oxidic Ni2+ species likely in Ni(OH)2 according to the peak shape.44 The binding energy of the peak at 853.3 eV is well in line with the value reported for nickel disulfide.45


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Figure 2. (a) Fe 2p and (b) Ni 2p XPS spectra of bulk (FexNi1-x)9S8 materials. Examples of the spectral deconvolution fits for Fe3Ni6S8 are shown in (c) for the Fe-2p core level and (d) for Ni-2p.

The S 2p spectra (Figure 3a) are similar to those recently reported for synthetic pentlandite rocks,22 showing an intense peak between 160.0 and 165.0 eV assigned to sulfide S species. More detailed analysis of the S 2p spectra, reveals three 2p doublets at 161.5 and 162.7 eV, 162.4 and 163.5 eV and at 164.0 and 165.2 eV (Figure 3b), typical for oxysulfide films46 and a mildly oxidized iron pyrite surface.47 The structure at higher binding energy is deconvoluted in one doublet, at 168.4 and 169.6 eV, corresponding to sulfate49 species (Figure 3b). As it was mentioned above, the oxygen content in this sample series changes non-monotonically with the Fe : Ni ratio. Similarly, the content of Fe- and Ni-oxides/hydroxides does not follow any specific trend. The overall Fe : Ni ratio detected by XPS (Table S2), as well as the ratio of metal sulfide species obtained by deconvolution of the corresponding 2p3/2 lines from the XPS data (Table 1), differ only slightly from the composition expected and observed in EDX measurements for all the samples. Notably, the observed oxidation states of the iron centers on the materials surface are in accordance with those observed in the bulk material by Mössbauer spectroscopy.


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Figure 3. S 2p XPS spectra of FexNi9-xS8 samples. (a) Comparison of different sample compositions; (b) Spectral deconvolution exemplified for Fe3Ni6S8.


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Table 1. Distribution of disulfide species detected by XPS. FeS and NiS fractions of the overall observed Fe and Ni species are reported along with their ratio.

Sample

%FeS of total Fe

%NiS of total Ni

Fe : Ni

Fe : Ni expected

Fe3Ni6S8

16

14

0.6

0.5

Fe4Ni5S8

12

12

0.9

0.8

Fe5Ni4S8

14

14

1.4

1.3

Fe6Ni3S8

11

13

2.4

2.0

Fe4.5Ni4.5S8

17

19

1.2

1.0

Based on our experimental data in combination with theoretical suggestions of heterogeneously dispersed Fe and Ni ions, it seems plausible that less Fe···Ni neighboring metal pairs are present at the surface when deviating from the evenly distributed Fe : Ni ratio in the bulk of the material. HER performance. Due to the altered surface composition, a different HER performance can be anticipated. Hence, we conducted electrochemical experiments of all (FexNi1-x)9S8 compositions using a three-electrode setup in 0.5 M H2SO4.22 Improving our previous electrode design, we ground the raw material into fine powder. The powder was then pressed into cylindrical pellets (size of 2 x 4 mm), attached into a Teflon frame and polished to afford a plain surface (Fig. S1 and S3).32 Thus, we obtained a well-defined surface area with a size of 0.071 cm², which was additionally confirmed by electrochemical measurements. In the as-synthesized bulk electrodes the recorded HER overpotentials of all materials are virtually equivalent (Fig. 4a and 4b). For all materials, current densities of -10 mA cm-² are achieved at ~ 390 mV overpotential. Notably, after maintaining the materials for 20 h at a constant potential of -500 mV vs RHE, a significant change in the HER overpotential was observed (Fig. 4a) with a significant shift to more positive values for all materials (Fig. 4b). Although performed under the same experimental conditions, the observed potential shifts are not equivalent for all composites leading to distinguishable overpotentials and altered HER activity. The highest activity associated with the lowest overpotential of 146 mV required to reach -10 mA cm-2 was found for Fe4.5Ni4.5S8. Similar initial HER activities and overpotentials were observed for Fe4Ni5S8 (154 mV, -10 mA cm-2) and Fe5Ni4S8 (160 mV, -10 mA cm-2). However, at higher current densities, the Fe4.5Ni4.5S8 sample shows the highest HER catalytic performance and reveals faster kinetics compared with the other samples.


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All other iron-nickel sulfides revealed significantly higher overpotentials at current densities of 10 mA cm-2 (> 200 mV). Notably, electrochemical impedance spectroscopy measurements indicate smaller charge transfer resistance as well as smaller faradaic resistances of the electrodesorption or recombination reactions for Ni4.5Fe4.5S8 compared with all others pentlandite-containing compounds, which indicates that HER is enhanced on this surface (Fig. S11, Table S3). Moreover, it can be assumed that also the surface topology has an influence on the catalytic performance of the materials. We hence derived the electrochemical surface area (ECSA) of the different iron-nickel sulfides from the electrochemical double-layer capacitance (Cdl) of the electrodes using cyclic voltammetry (Fig. 4c), which is proportional to the slope of the capacitive current as a function of the scan rate. For conductive compounds, ECSA was found to be a reliable method to determine the active surface area of different conductive catalysts.50 As expected from HER activity measurements, the slope is highest for Fe4.5Ni4.5S8 (0.03249), followed by Fe5Ni4S8 (0.02253) and Fe4Ni5S8 (0.02398), respectively (Figure S12, S13). This means that the ECSA and thus the number of exposed active sites on the surface are highest on these three materials. Contrary, the other materials show significantly diminished ECSA, suggesting a smaller number of exposed Ni-Fe surface sites, which is in agreement with the measured XPS data. One might expect that due to the suggested lower surface stability with higher Ni or Fe contents the long-term stability of the electrodes during HER will be altered. While (FexNi1-x)9S8 (x = 0.22 – 0.77) composites were stable for at least 20 h at a potential of -0.5 V vs RHE with current densities reaching from -200 to -600 mA cm-2 (Figure 4d), samples with either low or high Fe content showed severe loss in activity after a short time. Notably, the surface of such electrodes showed significant and visible corrosion accompanied by loss of structural integrity of the electrode pellets. This observation is again in line with the theoretical observations by Waterson et al. and reveals a destabilization of the material with high Ni and/or Fe content.30


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ACS Catalysis

Figure 4. (a) LSV of FeS2 and NiS2 and of as synthesized and activated pentlandite containing samples (FexNi1-x)9S8 recorded at sweep rates of 5 mVs-1. (b) Catalytic activity plot expressed as η (V) required to achieve a current density of -10 mA cm-2 as a function of the Fe : Ni ratio for as synthesized (error bars obtained from triplicate measurements are labeled in red) and activated (FexNi1-x)9S8. (c) ECSA slopes dependent of scan rates from 10-60 mVs-1. (d) Activation test for 17 h at a constant potential of -0.5 V vs RHE. The scan quality here is influenced by the formation of hydrogen bubbles on the electrode surface under HER conditions.

Based on the analytic data as well as electrochemical properties, a reasonable explanation for the observed phenomenon is that the heterogeneous Fe-Ni assembly on the catalyst surface is of utmost importance. The cooperative metal-metal effects thus established seem to be responsible for the high HER activity of Fe4.5Ni4.5S8. The more the Ni and Fe content approaches the 1 : 1 ratio, the more Fe-Ni sites can be statistically found on the electrode surface (Figure 5). Control experiments were performed to gain insight into a possible Pt contamination of the catalyst surface. Recorded currents obtained during the activation of Fe4.5Ni4.5S8 using a Au or glassy carbon counter electrode (Figure S14) show similar values as the ones registered during the activation using the


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Page 16 of 32

Pt CE indicating that Pt is not responsible for the increased activity measured. XPS and EDX were used to investigate the possible Pt contamination of the pentlandite. Both techniques have detection limits of ~ 1000 ppm. The XPS survey spectrum registered on activated Fe4.5Ni4.5S8 catalyst at 0.6 V overpotential shows that Pt is not found on the catalysts surface (Figure S15). Likewise, SEMEDX spectra also support that no Pt is deposited on the catalyst surface after applying 0.5 V overpotential for 20 h (Figure S16) and PXRD shows that the catalysts shows similar diffraction pattern before and after electrochemistry (Figure S17).

Figure 5. Schematic surface area of (FexNi1-x)9S8 with enriched nickel surface (left) and enriched iron content (right). The pentlandite (middle) serves as idealized system with a recurring pattern sequence and active centers.

Temperature dependence of HER. While alteration of the Fe : Ni ratio did not allow for an improved HER performance, we subsequently investigated the temperature influence on the HER activity. Notably, while commonly HER tests are performed at ambient conditions, industrial electrolyzers are usually operated at temperatures of ~ 70 – 90 ˚C. Such elevated temperatures offer several opportunities, like a decrease in charge-transfer resistance as well as a facilitated H2 bubble removal and lead to decreased overpotentials.51,52 In addition, increase of the double-layer capacity is usually observed.53,54 Thus we wondered if instead of altering the Ni : Fe ratio, increasing the operating temperature is a more suitable strategy to improve the performance of pentlandites towards HER catalysis. A significant improvement of the HER activity of Fe4.5Ni4.5S8 is observed with increasing temperatures, which is displayed by significantly reduced overpotentials. While at 0˚C Fe4.5Ni4.5S8 showed an overpotential of 377 mV at -10 mA cm-2 for HER, the overpotential gradually decreased with increasing temperature reaching a value of solely 138 mV at 90˚C under otherwise identical experimental conditions (Fig. 6a). Additionally, it is worth to mention that the change of the catalytic activity is induced just by the temperature and not by material activation, since same catalytic activity is observed when the temperature is increased and subsequently decreased to its initial value. As expected, along with the temperature increase, the double-layer capacity / electrochemical surface area increases, which is visible by a steeper slope of the f(j) ~ sweep rate (Fig. 6b).


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Galvanostatic EI spectra for the HER were registered at -0.2 mA for all studied temperatures. At this current, HER just occurs and the amount of generated H2 bubbles still allows acquisition of less noisy spectra. Still, with the increase in temperature, this becomes more difficult as can be seen from the Bode plots obtained at lower frequency (Figure 6c). From the Nyquist plots, in the high-frequency range, one can see a small shift of the plots towards lower values with increasing temperature, which is associated with a slow decrease of the solution resistance. While at lower temperature the overall depressed semicircle tends to close, at higher temperatures a linear trend can be observed in the low-frequency range, suggesting the presence of diffusion-controlled processes. While one may assume that at higher temperatures different hydrogen formation mechanisms can be accounted as responsible for the higher activity, Tafel analysis (η = ρ log j + log j0, overpotential η, current density j, Tafel slope ρ and exchange current density j0) that allow for an initial mechanistic hypothesis clearly indicates that the overall mechanism remains unaltered (Fig. 6d). Thus independent of the applied temperature, a fast Volmer-type discharge reaction (H3O+ + e- à Hads + H2O) followed by a rate-limiting recombination step (Hads + Hads à H2) has to be assumed and is in line with previous reports.22


17

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a

b

2.0

0

1.6

j (mA cm-2)

-15 -20 -25 -30 -0.3

-0.2

c

-0.1

0.0

0.02311 0.01997 0.01699 0.01390 0.01065 0.00932 0.00761 0.00703

0.2

0.1

10

0°C 10°C 20°C 30°C 40°C 50°C 60°C 70°C 80°C

45

160

)

0.8

40

-Phase (°)

35 30 25 20 15 10 5 0 10-1

60

100

101

102

103

104

20

105

Frequency (Hz)

30

-0.20

82 mV dec-1 -0.25

81 mV dec-1 76 mV dec-1

80 mV dec-1

40

60

80

100

120

140

160

180

70

83 mV dec-1


-0.35

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60

-0.15

20

20

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40

Scan rate (mV s-1)

d

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80

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0.0272

0.4

ERHE (V)

100

1.2

0.03094

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1.4

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ERHE (V)

j (mA cm-2)

0°C, 377 mV 10°C, 365 mV 25°C, 343 mV 30°C, 335 mV 40°C, 307 mV 50°C, 273 mV 60°C, 241 mV 70°C, 214 mV 80°C, 179 mV 90°C, 138 mV

-10

140

0°C 10°C 25°C 30°C 40°C 50°C 60°C 70°C 80°C 90°C

1.8

-5

-Z'' (

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

Z' ( )

82 mV dec-1 10

-j (mA cm-2)

10°C 20°C 30°C 40°C 50°C 60°C 70°C 80°C 100

Figure 6. Temperature-dependent electrochemical characterization (a) Linear sweep voltammograms of Fe4.5Ni4.5S8 at sweep rate of 5 mV s-1 in 0.5 M H2SO4 at different temperatures. The overpotentials at -10 mA cm-2 are provided along with the temperature. The potential was referenced according to temperature dependend RHE measurements. The current density was nomalized to the geometrical surface area of the electrodes (0.071 cm²). (b) Charging current density differences (∆J = ja-jc) plotted against scan rates. The linear slope, equivalent to twice of the double-layer capacitance Cdl, was used to represent the ECSA. (c) Temperaturedependend Nyquist plots of EIS for Fe4.5Ni4.5S8 and corresponding Bode plots (inset) (d). Tafel plot derived from linear sweep experiments at sweep rates of 1 mV s-1.

DFT calculations of surfaces. In order to understand the catalytic activity of the (FexNi1-x)9S8 surfaces we calculated the (111) surface energies of Fe4.5Ni4.5S8, Fe6Ni3S8 and Fe3Ni6S8 surfaces as a descriptor. The surface energy is influenced by the degree of coordinative unsaturation of the surface atoms. As such a higher surface energy of a potential catalyst can be assigned to a more reactive surfaces.55 All calculated surface structures are shown from the top- and bottom-view in Figure 7. Application of the surface energy to describe the catalytic activity was recently benchmarked as suitable method for the hydrogen evolution reaction (HER) of Au, Pd, Pt, WC, W2C, and Mo2C comparable to “volcano plot” calculations following Sabatier’s principles.55 The calculated (111) surface energies of Fe4.5Ni4.5S8, Fe6Ni3S8 and Fe3Ni6S8 surfaces are summarized in Table 2. It can be clearly seen that the surface energy of the Fe4.5Ni4.5S8 is the highest among all


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ACS Catalysis

surface structures in line with the highest HER catalytic activity for the Ni : Fe ratio of 1 : 1. The Fe6Ni3S8 (111) surface was modeled with a randomly slightly higher number of Fe-Ni sites than the Fe3Ni6S8 (111) surface and revealed a higher surface energy that indicates a higher HER activity. While both surface energies are smaller than that observed for Fe4.5Ni4.5S8, the different surface energies found in Fe3Ni6S8 and Fe6Ni3S8 also reveals the importance of the elemental composition on the surface. The more neighboring Fe-Ni sites on the surface, the higher the HER activity was foreseen.

Figure 7. (Left) Optimized bulk structure of pentlantide Fe4.5Ni4.5S8. (Right) Slab structures (ab plane) of top-view (T) and bottomview (B) (111) surfaces of Fe4.5Ni4.5S8 (A), Fe6Ni3S8 (B) and Fe3Ni6S8 (C). The Fe and Ni positions of B and C were randomly generated based on the Fe4.5Ni4.5S8 bulk phase. All bulk and surface structures were optimized at PBE/pw(PAW P) level of theory. Color code of the surface structures: Fe (brown), Ni (grey), S (yellow). Table 2. (111) Surface energies of Fe4.5Ni4.5S8, Fe6Ni3S8 and Fe3Ni6S8 surfaces calculated at PBE/PAW-P (pw) level of theory. The average number (top and bottom of slab) of Fe-Ni sites is given.

Fe/Ni ratio

Number of Fe-Ni sites

γ/eV/Å2

4.5/4.5

4

0.054

6/3

3

0.048

3/6

2.5

0.042

Thus, the catalytic activity interpreted from the surface energy correlates very well with the experimental outcome and shows that the more Fe-Ni sites are present at the surface the higher the catalytic activity is.


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§ Conclusions We herein report a series of (FexNi1-x)9S8 (x = 0 – 1) mixed metal sulfides. These materials vary in their Fe : Ni content and commonly reveal pentlandite-like structures. All samples show high efficiency for the hydrogen evolution reaction (HER), low intrinsic charge transfer resistance and were stable for at least 20 h at -0.5 V vs RHE. The electrochemical surface area clearly depended on the Fe : Ni ratio. The highest amount of active sites was found with a 1 : 1 ratio. The higher the Ni or Fe content was, the lower was the amount of active sites on the electrode surface and the lower was the HER activity of the electrocatalyst at a given potential. The experimental data presented herein suggest that neither the surface topology nor the conductivity are responsible for the high HER activity of Ni4.5Fe4.5S8. Instead, specific metal-metal interactions seem to be of high importance for a superior electrocatalytic HER activity. This is supported by the DFT calculations of (111) surface structures with different compositions using the surface energy as a descriptor for the catalytic activity. While we were not able to improve the performance of Fe4.5Ni4.5S8 (x = 0 – 1) composites by altering the Fe : Ni ratio, the overpotential was drastically reduced to 138 mV at -10 mA cm-2 when working at elevated temperatures. The results show that both the surface composition of the electrode and to the same extent the operating conditions of the electrolyzer play a pivotal role when elucidating noble-metal free HER catalysts. Thus, both criteria should be considered equally when non-noble metal catalysts are evaluated.

§ Author Information Corresponding Author Ulf-Peter Apfel *Phone: -49(0)234 32-24187 Mail: [email protected]

§ Acknowledgments


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The authors thank the financial support of the Fonds of the Chemical Industry (Liebig grant to U.-P.A.) and the Deutsche Forschungsgemeinschaft (Emmy Noether grant to U.-P.A., AP242/2-1). W. S., I. S., H. A., M. M. and B. R. C. acknowledge the Cluster of Excellence RESOLV at RUB (EXC 1069) funded by the Deutsche Forschungsgemeinschaft. A. Z. and D. M. thank the high performance center and the Laboratory of Material Research (LaMa) of the Justus-Liebig University Giessen (Germany). We thank the central facility for scanning electron microscopy at the RUB for performing SEM and EDX measurements. Additionally, we thank Dr. J. J. Soldevialla Barreda for measuring the ICP-OES of all composites.

Supporting information Electrode preparation (Figure S1); Temperature dependence of Ag/AgCl electrodes (Figure S2); SEM images of polished electrodes and bulk compounds (Figure S3, S4); SEM-EDS and EDX spectra (Figure S5, S6), ICP-OES measurements of the composite materials (Table S1), Mössbauer spectra (Figure S7) and DSC (Figure S8) of (FexNi1-x)9S8 compounds; survey XPS spectrum of Fe3Ni6S8 (Figure S9); surface composition in % observed by XPS (Table S2); survey XPS spectrum of Fe4.5Ni4.5S8 (Figure S10); Nyquist plots (Figure S11); parameters for fitting EIS data (Table S3); ECSA plots of FeNiS composites (Figure S12); estimation of ECSA for Fe4.5Ni4.5S8 (Figure S13); activation of Ni4.5Fe4.5S8 using a Pt-grid or Au plate as CE (Figure S14); survey XPS after activation of Fe4.5Ni4.5S8 (Figure S15); EDX/EDS analysis of the electrode surface after activation (Figure S16); powder XRD after activation for 20 hours (Figure S17).

§ References (1) (2) (3) (4) (5) (6)

Bockris, J. Int. J. Hydrog. Energy 2002, 27, 731–740. Momirlan, M.; Veziroglu, T. Int. J. Hydrog. Energy 2005, 30, 795–802. Edwards, P. P.; Kuznetsov, V. L.; David, W. I. F.; Brandon, N. P. Energy Policy 2008, 36 , 4356–4362. Yin, S. F.; Xu, B. Q.; Zhou, X. P.; Au, C. T. Appl. Catal. Gen. 2004, 277, 1–9. Li, X.; Hao, X.; Abudula, A.; Guan, G. J. Mater. Chem. A 2016, 4, 11973–12000. Voiry, D.; Yamaguchi, H.; Li, J.; Silva, R.; Alves, D. C. B.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Nat. Mater. 2013, 12, 850–855.


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(7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30)

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Li, J.; Zhou, X.; Xia, Z.; Zhang, Z.; Li, J.; Ma, Y.; Qu, Y. J. Mater. Chem. A 2015, 3, 13066– 13071. Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. J. Am. Chem. Soc. 2011, 133, 7296– 7299. Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Nano Lett. 2013, 13, 1341–1347. Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Nat. Mater. 2012, 11, 963–969. Garcia-Esparza, A. T.; Cha, D.; Ou, Y.; Kubota, J.; Domen, K.; Takanabe, K. ChemSusChem 2013, 6, 168–181. Chen, W.-F.; Wang, C.-H.; Sasaki, K.; Marinkovic, N.; Xu, W.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Energy Environ. Sci. 2013, 6, 943–951. Tang, C.; Wang, W.; Sun, A.; Qi, C.; Zhang, D.; Wu, Z.; Wang, D. ACS Catal. 2015, 5, 6956–6963. Xie, J.; Li, S.; Zhang, X.; Zhang, J.; Wang, R.; Zhang, H.; Pan, B.; Xie, Y. Chem. Sci. 2014, 5, 4615–4620. Chen, W.-F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Angew. Chem. Int. Ed. 2012, 51, 6131–6135. Vrubel, H.; Hu, X. Angew. Chem. Int. Ed. 2012, 51, 12703–12706. McEnaney, J. M.; Chance Crompton, J.; Callejas, J. F.; Popczun, E. J.; Read, C. G.; Lewis, N. S.; Schaak, R. E. Chem. Commun. 2014, 50, 11026–11028. Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. J. Am. Chem. Soc. 2013, 135 , 9267–9270. Zheng, Y.; Jiao, Y.; Li, L. H.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Z. ACS Nano 2014, 8 , 5290–5296. Cui, W.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X. Chem. Commun. 2014, 50, 9340–9342. Shalom, M.; Gimenez, S.; Schipper, F.; Herraiz-Cardona, I.; Bisquert, J.; Antonietti, M. Angew. Chem. Int. Ed. 2014, 53, 3654–3658. Konkena, B.; junge Puring, K.; Sinev, I.; Piontek, S.; Khavryuchenko, O.; Dürholt, J. P.; Schmid, R.; Tüysüz, H.; Muhler, M.; Schuhmann, W.; Apfel, U.-P. Nat. Commun. 2016, 7:12269. Zegkinoglou, I.; Zendegani, A.; Sinev, I.; Kunze, S.; Mistry, H.; Jeon, H. S.; Zhao, J.; Hu, M. Y.; Alp, E. E.; Piontek, S.; Smialkowski, M.; Apfel, U.-P.; Körmann, F.; Neugebauer, J.; Hickel, T.; Roldan Cuenya, B. J. Am. Chem. Soc. 2017, 139, 14360–14363. Ogata, H.; Nishikawa, K.; Lubitz, W. Nature 2015, 52 , 571–574. Madden, C.; Vaughn, M. D.; Díez-Pérez, I.; Brown, K. A.; King, P. W.; Gust, D.; Moore, A. L.; Moore, T. A. J. Am. Chem. Soc. 2012, 134 , 1577–1582. Reifschneider-Wegner, K.; Kanygin, A.; Redding, K. E. Int. J. Hydrog. Energy 2014, 39, 3657–3665. Adam, D.; Bösche, L.; Castaneda-Losada, L.; Winkler, M.; Apfel, U.-P.; Happe, T. ChemSusChem 2017, 10, 894-902. Vincent, K. A.; Parkin, A.; Armstrong, F. A. Chem. Rev. 2007, 107, 4366–4413. Shafaat, H. S.; Rüdiger, O.; Ogata, H.; Lubitz, W. Biochim. Biophys. Acta BBA - Bioenerg. 2013, 1827, 986–1002. Waterson, C. N.; Sindt, J. O.; Cheng, J.; Tasker, P. A.; Morrison, C. A. J. Phys. Chem. C 2015, 119 , 25457–25468.


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(31) Posada-Pérez, S.; Ramírez, P. J.; Evans, J.; Viñes, F.; Liu, P.; Illas, F.; Rodriguez, J. A. J. Am. Chem. Soc. 2016, 138, 8269–8278. (32) junge Puring, K.; Piontek, S.; Smialkowski, M.; Burfeind, J.; Kaluza, S.; Doetsch, C.; Apfel, U.-P. J. Vis. Exp. 2017, e56087, doi:10.3791/56087. (33) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558–561. (34) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251–14269. (35) Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15–50. (36) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169–11186. (37) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396–1396. (38) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953–17979. (39) Kresse, G.; Joubert, D. Phys Rev B 1999, 59, 1758–1775. (40) Evans, H. T.; Clark, J. R. Am. Mineral. 1981, 66, 376–384. (41) Knop, O.; Huang, C.-H.; Reid, K. I. G.; Carlow, J. S.; Woodhams, F. W. D. J. Solid State Chem. 1976, 16 , 97–116. (42) Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S. Surf. Interface Anal. 2004, 36, 1564–1574. (43) Brion, D. Appl. Surf. Sci. 1980, 5, 133–152. (44) Biesinger, M. C.; Payne, B. P.; Lau, L. W. M.; Gerson, A.; Smart, R. S. C. Surf. Interface Anal. 2009, 41, 324–332. (45) Heide, H. van der; Hemmel, R.; Bruggen, C. F. van; Haas, C. J. Solid State Chem. 1980, 33, 17–25. (46) Benoist, L.; Gonbeau, D.; Pfister-Guillouzo, G.; Schmidt, E.; Meunier, G.; Levasseur, A. Thin Solid Films 1995, 258, 110–114. (47) Wittstock, G.; Kartio, I.; Hirsch, D.; Kunze, S.; Szargan, R. Langmuir 1996, 12, 5709–5721. (48) Wagner, C. D.; Taylor, J. A. J. Electron Spectrosc. Relat. Phenom. 1982, 28, 211–217. (49) Siriwardane, R. V.; Cook, J. M. J. Colloid Interface Sci. 1985, 104, 250–257. (50) Jung, S.; McCrory, C. C. L.; Ferrer, I. M.; Peters, J. C.; Jaramillo, T. F. J. Mater. Chem. A 2016, 4, 3068–3076. (51) Heins, T. P.; Harms, N.; Schramm, L.-S.; Schröder, U. Energy Technol. 2016, 4, 1509– 1513. (52) Pierozynski, B.; Mikolajczyk, T. Electrocatalysis 2015, 6, 51–59. (53) Li, A.; Chuan, X. Acta Geol. Sin. - Engl. Ed. 2017, 91, 161–162. (54) Fletcher, S. I.; Sillars, F. B.; Carter, R. C.; Cruden, A. J.; Mirzaeian, M.; Hudson, N. E.; Parkinson, J. A.; Hall, P. J. J. Power Sources 2010, 195 , 7484–7488. (55) Zhuang, H.; Tkalych, A. J.; Carter, E. A. J. Phys. Chem. C 2016, 120 , 23698–23706.


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a

b (111)

(311)

(222) (331)

* Fe4.5Ni4.5S8

(200)

(220)

*

*

*

(400)

*

*

(420)

**

(511) (422)

*

*

FeS (Fe8Ni1S8) Fe7Ni2S8 Fe6Ni3S8 Fe5Ni4S8 Fe4Ni5S8 Fe3Ni6S8 Fe2Ni7S8 Fe1Ni8S8 10

15

20

25

30

35

2(degrees)

40

45

50


ACS Catalysis

a

b

Fe 2p

Ni 2p

NiS2

FeS2

Ni(OH)2

Intensity (a.u.)

Intensity (a.u.)

Fe2O3

Fe4.5Ni4.5S8

Fe3Ni6S8 Fe4Ni5S8

Fe4.5Ni4.5S8

Fe3Ni6S8 Fe4Ni5S8

Fe5Ni4S8

Fe5Ni4S8

Fe6Ni3S8

745

Fe6Ni3S8

740

735

730

725

720

715

710

705

895

890

885

880

Binding energy (eV)

c

Fe 2p

875

870

865

860

855

850

845

Binding energy (eV)

d

Fe2O3

Ni 2p

Ni(OH)2

Intensity (a.u.)

NiS

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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FeS

shake-up satellite

"pre-peak"

736

732

728

724

720

716

712

708

704

700

885

Binding energy (eV)

880

875

870

865

Binding energy (eV)


860

855

850

Page 27 of 32

a

b

oxisulfide species

Fe4.5Ni4.5S8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Fe3Ni6S8 Fe4Ni5S8 Fe5Ni4S8

SO42-

Fe6Ni3S8

175

170

165

160

Binding energy (eV)

170

165

Binding energy (eV)


160

ACS Catalysis

a

b 0 -0.15

-150 -200 -250

Activated Fe3Ni6S8 Fe4Ni5S8 Fe4.5Ni4.5S8 Fe5Ni4S8 Fe6Ni3S8

-300 -350 -400

Activated (FexNi1-x)9S8 -0.25

Activation at -0.5 V vs RHE -0.30

As synthezised (FexNi1-x)9S8

-0.35

-0.40

-0.45

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

ERHE (V)

c 2.2

-0.20

S2

Activated

1.8 1.6 1.4 1.2

S8 S8 S8 i 4S 8 i 3S 8 5 Ni 6 Ni 5 Ni 4. e 5N e 6N 5 Fe 3 Fe 4 F F . Fe 4

-600

Fe3Ni6S8 Fe4Ni5S8 Fe4.5Ni4.5S8 Fe5Ni4S8 Fe6Ni3S8

2.0

Fe

d 49 32 0 . 0 8 239 0.0 253 0.02 82 65 0.01 0.014

1.0 0.8

NiS

2

Fe5Ni4S8

Fe4.5Ni4.5S8

-500

j (mA cm-2)

j (mA cm-2)

-100

 at -10 mA cm-2 (V)

As synthesized Fe3Ni6S8 Fe4Ni5S8 Fe4.5Ni4.5S8 Fe5Ni4S8 Fe6Ni3S8 FeS2 NiS2

-50

j (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 28 of 32

Fe4Ni5S8

-400 -300

Fe6Ni3S8

-200

0.6

Fe3Ni6S8

0.4

NiS2

-100

0.2

FeS2

0.0 0

10

20

30

40

50

60

0

2

Scan rate (mV s-1) ACS Paragon Plus Environment

4

6

8

10

Time (h)

12

14

16

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

ACS Catalysis

High Iron Content

High Nickel Content


ACS Catalysis

a

b 2.0

0

0°C 10°C 25°C 30°C 40°C 50°C 60°C 70°C 80°C 90°C

1.8 1.6

j (mA cm-2)

j (mA cm-2)

-5

0°C, 377 mV 10°C, 365 mV 25°C, 343 mV 30°C, 335 mV 40°C, 307 mV 50°C, 273 mV 60°C, 241 mV 70°C, 214 mV 80°C, 179 mV 90°C, 138 mV

-10 -15 -20 -25 -30 -0.3

-0.2

1.0 0.8

0.02311 0.01997 0.01699 0.01390 0.01065 0.00932 0.00761 0.00703

0.4 0.2

-0.1

0.0

10

0.1

-Phase (°)

35 30 25 20 15 10 5

80

40

50

0 10-1

100

101

60

102

103

104

105

Frequency (Hz)

70

Scan rate (mV s )

83 mV dec-1

90 mV dec-1 82 mV dec-1 82 mV dec-1

-0.25


-0.30 40

80 mV dec-1

20

-0.35

0 0

60

-1

-0.20

ERHE (V)

0°C 10°C 20°C 30°C 40°C 50°C 60°C 70°C 80°C

40

100

30

-0.15

45

160

20

d

180

120

1.2

0.0272

0.6

ERHE (V)

c

140

1.4

0.03094

0.0 -0.4

-Z'' ()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 30 of 32

20

40

60

80

100

Z' ()

120

140

160

180


1

82 mV dec-1

10

-j (mA cm-2)

10°C 20°C 30°C 40°C 50°C 60°C 70°C 80°C 100

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11

ACS Catalysis


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13


Page 32 of 32

Influence of the Fe: Ni Ratio and Reaction Temperature on the

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