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Scalable Two-Step Synthesis of Nickel-Iron Phosphide Electrodes for Stable and Efficient Electrocatalytic Hydrogen Evolution Wai Ling Kwong, Cheng Choo Lee, and Johannes Messinger J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09050 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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The Journal of Physical Chemistry

Scalable Two-Step Synthesis of Nickel-Iron Phosphide Electrodes for Stable and Efficient Electrocatalytic Hydrogen Evolution Wai Ling Kwonga, Cheng Choo Leeb, and Johannes Messingera,c* a

Department of Chemistry, Kemiskt Biologiskt Centrum (KBC), Umeå University, S-90187 Umeå, Sweden b

Umeå Core Facility for Electron Microscopy, Umeå University, S-90187 Umeå, Sweden

c

Department of Chemistry-Ångström Laboratory, Uppsala University, S-75120 Uppsala, Sweden

*Corresponding author; Tel.: +46-90-786-5933; Fax: +46-90-786-7655; Email: [email protected]

1 Environment ACS Paragon Plus


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Abstract Development of efficient, durable, and inexpensive hydrogen evolution electrodes remains a key challenge for realizing a sustainable H2 fuel production via electrocatalytic water splitting.

Herein, nickel-iron phosphide porous films with

precisely controlled metal content were synthesized on Ti foil using a simple and scalable two-step strategy of spray-pyrolysis deposition followed by low-temperature phosphidation.

The nickel-iron phosphide of an optimized Ni:Fe ratio of 1:4

demonstrated excellent overall catalytic activity for hydrogen evolution reaction (HER) in 0.5 M H2SO4, achieving current densities of -10 mA cm-2 and -30 mA cm-2 at overpotentials of 101 mV and 123 mV, respectively, with a Tafel slope of 43 mV dec-1.

Detailed analysis obtained by X-ray diffraction, electron microscopy,

electrochemistry, and X-ray photoelectron spectroscopy revealed that the superior overall HER activity of nickel-iron phosphide compared to nickel phosphide and iron phosphide was a combined effect of differences in the morphology (real surface area) and the intrinsic catalytic properties (electronic structure). Together with a long-term stability and a near-100% Faradaic efficiency, the nickel-iron phosphide electrodes produced in this study provide blueprints for large-scale H2 production.


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Introduction Molecular hydrogen (H2) is widely regarded as the fuel of future.1 The oxidation of H2 in a fuel cell generates about three times as much energy per weight (~140 kJ g-1) as compared to that generated by the oxidation of fossil fuel (~43 kJ g-1),2,3 with zero carbon emission.1

H2 production via photovoltaic-powered electrocatalytic water

splitting is a promising method for realizing a sustainable energy production system.4 In this regard, the catalysts responsible for performing each half-reaction of water splitting play key roles in determining the efficiency of the reaction.

For the

hydrogen evolution reaction (HER), platinum (Pt) is the most efficient catalyst, but its rarity and thus high costs prevent large-scale implementation.5 Intensive research is currently ongoing to develop efficient and stable catalysts from earth-abundant materials using low-cost synthesis methods.

Metal phosphides have recently emerged as promising substitutes for Pt for electrocatalytic HER.

Examples of this expanding family of catalysts include

phosphides of nickel,6-16 iron,17-24 cobalt,25-28 tungsten,29 and molybdenum.30 These phosphides are not only active for HER, but also highly stable during operation. Nonetheless, further improvement of their activity is needed in order to outperform Pt. The efficiency of HER is dependent on the hydrogen adsorption energy and the kinetic energy barrier of H2 desorption, which are influenced by the electronic properties of the surface-exposed catalyst.31-33 This shows that the activity of a catalyst may be improved by modifying its electronic properties. In fact, this has been shown viable by incorporating a third element to form bimetallic phosphides,3338

which have shown superior activity compared to their parent monometallic

phosphides. While tremendous attention is placed on the monometallic phosphides,



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the studies on the HER activity of bimetallic phosphide are limited. The progress is possibly hindered by the search for uncomplicated, economical, and high-throughput synthesis methods that offer effective control over the elemental composition. Metal phosphides in the form of powders can be synthesized via wet-chemical and solidstate reactions.9,13,16,22,25,27,29,30,36 However, practical application of these phosphide powders requires the use of binders (e.g., Nafion, acetoneacetyl) in the preparation of electrodes, which not only adds cost to the synthesis process, but is also likely to increase electrical resistance due to a poor connection between the catalyst and the current collector.24 This problem can be minimized by growing the metal phosphide directly on the current collector, which is used as electrode.

Herein, we synthesized nickel-iron phosphide porous films directly on Ti foil (current collector)

using

spray-pyrolysis

deposition

followed

by

low-temperature

phosphidation. This method offers scalability and easy control of the metal content in the bimetallic phosphide. We showed that the HER activity of nickel-iron phosphide in acidic electrolyte was improved compared to nickel phosphide and iron phosphide and was strongly dependent on the Ni:Fe ratio. In addition to the high catalytic activity, the nickel-iron phosphide also exhibited a near-100% Faradaic efficiency and an excellent stability. The origin of improvement of the HER performance of nickeliron phosphide was investigated.


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Experimental Section Synthesis of metal phosphide films supported on Ti foil

A two-step synthesis was performed, in which NiCl2·6H2O (98%; Alfa Aesar), FeCl3·6H2O (≥99%; Sigma-Aldrich), NaH2PO2·H2O (≥99%; Sigma-Aldrich), and Ti foil (99%; 0.127 mm thick; Alfa Aesar) were used as received.

The first step involved the spray-pyrolysis deposition of metal oxide films on Ti foils. Metal chloride aqueous solution was used as the spray solution, which was carried by compressed air of 0.5 bar at a flow rate of 11.5 mL min-1. The Ni:Fe molar ratio of the solution was adjusted to 5:0, 4:1, 3:2, 2:3, 1:4, or 0:5 (total metal concentration was kept at 0.05 M). The nozzle of the sprayer was placed at a distance of 45 cm and tilted at 45º from Ti foil, which was maintained at 350ºC on a hotplate during the deposition. Unless stated otherwise, four spray cycles were performed, where each cycle consisted of a continuous spray for 4 min followed by 1-min break to restore the Ti foil temperature. In the second step, the as-sprayed sample (geometric area 1×2 cm2) was placed together with 0.027 g NaH2PO2·H2O powder in a sealed flask to be heated for 30 min at 450ºC in a static Ar environment. Heating NaH2PO2·H2O produced PH3 gas [Caution: PH3 self-ignites in air and therefore should be handled only in air-free condition], which transformed the metal oxide to metal phosphide. Excessive NaH2PO2·H2O (0.081-0.162 g) also was used, as specified accordingly, in a study of its effect on the phosphidation process. Finally, the samples were cooled to room temperature in Ar environment, rinsed with distilled water, and dried in air.



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Characterizations

The morphology of the samples was characterized using field-emission scanning electron microscopy (SEM; Carl Zeiss Merlin) and the elemental composition was quantified using energy-dispersive X-ray spectroscopy (EDS; X-Max Oxford Instruments). X-ray photoelectron spectra (XPS) were collected using Kratos Axis Ultra DLD spectrometer with a monochromatic Al Kα source. X-ray diffraction (XRD) was performed using a PANalytical X’Pert PRO MPD diffractometer with Cu Kα radiation (45 kV; 40 mA) and Bruker D8 Advance with Cu Kα radiation (40 kV; 40 mA). Transmission electron microscopy (TEM) images and energy-filtered TEM (EFTEM) elemental maps were collected using a Schottky field-emission electron microscope (JEOL JEM-2100F) operated at 200 kV equipped with a Gatan Ultrascan 1000 CCD camera and a post-column imaging filter (Gatan Tridiem 863).

The

position of Fe L2,3, P L2,3 , Ni L2,3 and O K edges was determined by electron energy loss spectroscopy (EELS) in advance. Three-window method was used to extract the core-loss information to obtain the elemental maps. For TEM analysis, a portion of the film was scratched from the Ti substrate, dispersed in water by ultrasonication, and subsequently drop-casted onto a TEM Cu grid with carbon supporting film and dried at room temperature.

Electrochemical measurements

The electrochemical measurements were performed using a computer-controlled potentiostat (Autolab PGSTAT302N) in combination with a single-compartment, three-electrode electrochemical cell.

The working electrode was prepared by


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connecting a tinned Cu wire to the back of the sample (uncoated side of Ti foil) using an aqueous-based graphite conductive adhesive (Alfa Aesar). A Pt wire and an Ag/AgCl/1 M KCl were used as the counter and the reference electrodes, respectively. Linear sweep voltammograms (LSVs) were recorded in the cathodic direction at 2 mV s-1 with correction for the ohmic drop due to uncompensated resistance (iRu). The value of Ru was determined using electrochemical impedance spectroscopy (EIS).

Unless specified otherwise, the current densities were

normalized by the geometric surface area (~0.2 cm2) of the sample that was exposed to the electrolyte. All potentials reported were in reference to reversible hydrogen electrode (RHE) via a conversion of VRHE = VAg/AgCl + 0.222 V + 0.059 × (electrolyte pH). The electrolyte used was 0.5 M H2SO4 (pH 0.3). EIS data were obtained at an AC potential with amplitude of 20 mV and frequencies ranging between 100 kHz and 0.1 Hz.

Quantitative measurements of H2 gas

Quantification of the H2 gas evolved during the electrocatalytic water splitting was performed using a membrane-inlet mass spectrometer (MIMS; ThermoFinnigan Delta plus XP).

A gas-tight, two-compartment electrochemical cell (Pine Research

Instrumentation) was used to house the working and reference electrodes in the main compartment and the counter electrode in the other compartment.

The two

compartments, which were separated by a glass frit, contained identical electrolyte that was purged with N2 prior to use. Assembly of the electrochemical cell was done in a N2-filled glove-box at 21ºC. A constant current density of -10 mA cm-2 was applied for 2 h during which gas-phase aliquots were extracted from the headspace of



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the main compartment at 20-min intervals using a gas-tight syringe and were analyzed using MIMS.

The sensitivity of MIMS towards H2 was calibrated with known

amounts of H2 gas mixture (0.5% H2 in Ar).

The Faradaic efficiency for H2 evolution was obtained as follows:

Faradaic efficiency =

measured amount of ‫ܪ‬ଶ calculated amount of ‫ܪ‬ଶ

The measured amount of H2 was the sum of the H2 in the headspace of the main compartment and the H2 dissolved in the electrolyte calculated using Henry’s law (7.8 µmol m-3 Pa-1 for H2).39 The calculated amount of H2 was determined from the total charge passed by following a two-electron reduction at an assumed 100% Faradaic efficiency.

Results and Discussion We synthesized nickel-iron phosphide films of different Ni:Fe ratios by varying the Ni:Fe molar ratio of the spray solution using our two-step synthesis method (see Experimental Section). These samples were labeled as 5Ni-P (Ni:Fe 5:0; 0% Fe), 4Ni-1Fe-P (Ni:Fe 4:1; 20% Fe), 3Ni-2Fe-P (Ni:Fe 3:2; 40% Fe), 2Ni-3Fe-P (Ni:Fe 2:3; 60% Fe), 1Ni-4Fe-P (Ni:Fe 1:4; 80% Fe), and 5Fe-P (Ni:Fe 0:5; 100% Fe). Figure 1a-f shows the scanning electron microscopy (SEM) images of the freshly prepared samples. All samples consisted of irregular-shaped agglomerates, which interconnected to form porous films of thicknesses that decreased from 20-8 µm with increasing Fe content. For samples that contain Ni, the average agglomerate size decreased with increasing Fe content (Table 1). This is possibly due to the inhibited


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grain growth caused by the presence of Fe.40 The porous structure of the films is beneficial for facilitating a large contact area with the electrolyte for HER.

Figure 1. SEM images of freshly prepared (a) 5Ni-P, (b) 4Ni-1Fe-P, (c) 3Ni-2Fe-P, (d) 2Ni-3Fe-P, (e, g) 1Ni-4Fe-P, and (f, h) 5Fe-P. Scale bar: 400 nm (a-f), 200 nm (gh).

Table 1. Summary of the average agglomerate size, mass loadings (metal oxide; before phosphidation), and EDS results of the samples. Agglomerate size

Mass loading /

Fe/(Ni+Fe) /

P/(Ni+Fe+P) /

/ nm

mg cm-2

at. %

at. %

5Ni-P

1200±400

2.6

0.0

38.8

4Ni-1Fe-P

250±90

2.6

10.7

51.0

3Ni-2Fe-P

50±10

2.6

36.8

52.3

2Ni-3Fe-P

39±7

1.8

63.2

53.6

1Ni-4Fe-P

29±6

1.1

82.9

54.3

5Fe-P

25±7

1.0

100.0

49.5

Sample

The elemental composition of all samples was examined using energy-dispersive Xray spectroscopy (EDS). As shown in Table 1, the concentrations of Fe in the total



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metal content are consistent with those of the spray solutions, except for 4Ni-1Fe-P, where the measured Fe content (~11%) was much lower than the expected value of 20%.

This could be due to inhomogeneity of metal distribution caused by the

formation of clusters with a high Ni content. As the agglomerate size decreased, the effect of inhomogeneity diminished. As seen in Figure S1, the elemental mapping of 1Ni-4Fe-P shows that Ni, Fe, and P were homogeneously distributed over the scanned area of 12.5 × 8.75 µm2. The overall good agreement of the Fe content in the samples with that of the spray solution demonstrates the effectiveness of spray pyrolysis deposition in control of the metal ratio, which is shown subsequently in the X-ray diffraction (XRD) measurement to affect the mineralogical composition of the bimetallic phosphide.

Figure 2 shows the XRD patterns of the freshly prepared samples. The diffraction peaks of 5Ni-P indicated the formation of mixed hexagonal Ni2P and hexagonal Ni5P4.41,42 Increasing the Fe content led to broadening and decrease of the intensities of Ni2P and Ni5P4 peaks, which is consistent with the decreasing agglomerate size (Figures 1a-e) and the decreasing Ni content. For samples with Fe content between 40-60%, in addition to Ni2P and Ni5P4, broad peaks assigned to orthorhombic FeP,43 are spotted at ~46.2º, ~48.0º, and ~56.1º 2θ. No shift in the Ni2P and Ni5P4 peaks was observed for samples with 40-100% Ni content, which suggests that the formation of alloy phosphide was unlikely to occur. The diffraction pattern of 1Ni-4Fe-P (80% Fe) showed a major FeP phase and no crystalline peaks of Ni2P and Ni5P4 while 5Fe-P exhibited FeP and Fe2O3 phases. The presence of metal phosphates as by-products were detected in all samples. However, they were found to partly dissolve into the


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acidic electrolyte (see explanation below) during the electrochemical measurements and shown to be less active for HER than their phosphide counterparts (Figure 4).



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Figure 2. XRD patterns of freshly prepared (a) 5Ni-P, (b) 4Ni-1Fe-P, (c) 3Ni-2Fe-P, (d) 2Ni-3Fe-P, (e) 1Ni-4Fe-P, and (f) 5Fe-P. Reference patterns of (g) Ni2P,44 (h) Ni5P4,42 (i) FeP,43 (j) γ-Fe2O3,45 (k) α-Fe2O3,46 (l) Ni(PO3)2,47 and (m) Fe3(PO4)24H2O also are shown and the corresponding peaks are labeled as Na, Nb, F, ^, “, #, and + in (a-f).48 Asterisk (*) indicates the peak attributed to Ti foil.


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Since Ni-containing phases were not observed for 1Ni-4Fe-P, Ni could be incorporated into the crystal structure of FeP.

However, this did not result in

noticeable peak shift with respect to 5Fe-P, which could be due to the small difference (~6%) between the ionic radius of Fe3+ (0.0785 nm) and Ni2+ (0.083 nm).49 Further investigation by EFTEM elemental mapping (Figure 3a) reveals a homogeneous distribution of Ni, Fe, and P in the agglomerates of 1Ni-4Fe-P. The high-resolution TEM (HRTEM) image of the agglomerate (Figure 3b) reveals lattice spacing of 0.258 nm and 0.269 nm, which correspond respectively to the (200) and (011) planes of FeP.43

The EFTEM and HRTEM measurements further show that Ni has

incorporated into the FeP structure in 1Ni-4Fe-P.

Figure 3. Characterization of 1Ni-4Fe-P (after activation). (a) TEM image and the corresponding EFTEM elemental maps (Ni, Fe, and P). (b) HRTEM image taken in the area marked by the blue rectangle in (a).



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We observed that the samples required activation to catalyze HER efficiently in our work. Figure 4a shows a series of linear sweep voltammograms (LSVs) recorded in 0.5 M H2SO4 using freshly-prepared 1Ni-4Fe-P as representative sample. After 6 cathodic LSV scans (less than 15 min), a successive reduction of the overpotential (η) was observed: η required to generate a current density (j) of -10 mA cm-2 decreased by 13% to a stable value of 101 mV after the activation. This activation procedure, done by running the LSVs scans, was performed to activate all samples except for 5Fe-P (this is explained below).


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Figure 4. (a) LSVs (iRu-corrected) of 1Ni-4Fe-P measured in 0.5 M H2SO4 during the activation for HER. High-resolution XPS spectra of 1Ni-4Fe-P before (freshly prepared) and after the activation obtained in (b) Ni 2p, (c) Fe 2p, and (d) P 2p energy regions. Assignment of peaks: (b) Ni 2p3/2 (853.8 eV) and Ni 2p1/2 (871.0 eV) of phosphide,9,15,50 Ni 2p3/2 (856.3 eV) and Ni 2p1/2 (874.1 eV) of phosphate;16,51 (c) Fe 2p3/2 (707.2 eV) and Fe 2p1/2 (720.1 eV) of phosphide, Fe 2p3/2 (711.8 eV) and Fe 2p1/2 (725.5 eV) of phosphate;19,21,50,52 (d) P 2p3/2 (129.5 eV) and P 2p1/2 (130.3 eV) of phosphide, P 2p of phosphate (133.6 eV).15,19,50,53,54

The surface analysis performed using XPS (Figures 4b-d) on 1Ni-4Fe-P as representative sample shows that, after the activation, the intensities of the peaks assigned to the phosphide phase in the Ni 2p, Fe 2p, and P 2p regions increased in parallel to the decrease of intensities of the peaks assigned to phosphate. Since EDS



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analysis of the electrolyte after the activation process shows the presence of Ni, Fe, P, and O (Figure S2), the activation process is most likely due to the dissolution of metal phosphate and the resultant exposure of electrocatalytically active metal phosphide on the surface of the sample. Examinations of the activated 1Ni-4Fe-P by XRD (Figure S3) and HRTEM (Figure 3) show that the bulk of the sample retained the FeP phase. Thus, the increased phosphide-to-phosphate proportion at the sample surface and the associated improvement in the catalytic activity (Figure 4a) after the activation clearly demonstrates that metal phosphide is the active catalyst for HER. Interestingly, an activated 1Ni-4Fe-P that was stored for 13 days in ambient atmosphere produced a similar LSV to that obtained immediately after its activation (Figure S4).

This

indicates that the sample is stable against oxidation via air contact at ambient temperature.

The HER activity of all samples was evaluated by measuring the LSVs shown in Figure 5a. The LSVs of Pt foil and Ti foil also are presented as reference. A summary of the HER performance is shown in Table 2. As expected, Pt foil showed the highest activity with η = 72 mV and 92 mV to achieve j = -10 mA cm-2 and -30 mA cm-2, respectively, while Ti foil produced negligible current density in the measured potential range. The samples exhibited metal ratio-dependent HER activity, where 1Ni-4Fe-P appeared as the best-performing sample, achieving j = -10 mA cm-2 and -30 mA cm-2 at η = 101 mV and 123 mV, respectively.

Comparative

measurements also were carried out for samples with ≥70% Fe (Figure S5) and the results show that Ni:Fe 1:4 is the optimal metal ratio for achieving a maximal HER activity.


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Figure 5. (a) LSVs (iRu-corrected) of the samples measured in 0.5 M H2SO4. (b) Tafel plots derived from the LSVs in Figure 5a. (c) LSVs (iRu-corrected) of the samples with current density normalized by the real surface area (jrsa). (d) Tafel plots derived from the LSVs in Figure 5c.

Table 2.

Summary of HER performance normalized by geometric surface area

(Figures 5a, b; marked by ^) and normalized by real surface area (Figures 5c, d; marked by *). η@-10 mA cm-2/

η@-30 mA cm-2/

Tafel slope/

η@-2 µA cm-2/

Tafel slope/

Jexchange/

Sample -1

mV^

mV^

mV dec ^

mV*

mV dec *

µA cm-2*

5Ni-P

161

195

53

121

51

0.0085

4Ni-1Fe-P

148

181

52

129

51

0.0055

3Ni-2Fe-P

133

162

48

158

47

0.0011

2Ni-3Fe-P

116

140

47

125

47

0.0045

1Ni-4Fe-P

101

123

43

113

44

0.0051

5Fe-P

144

205

61

153

59

0.0086

Pt foil

72

92

31

-

-

-

η: overpotential; Jexchange: normalized exchange current density 17 Environment ACS Paragon Plus

-1


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A comparison of the HER activity of 1Ni-4Fe-P with previously reported state-of-theart metal phosphide and chalcogenide catalysts is presented in Table S1. It shows that among all nanoparticulate catalysts, our 1Ni-4Fe-P requires the lowest η to deliver an operationally relevant j = -10 mA cm-2, with the next best having a value of η = 107 mV.6,9,11,13-16,20,22,29,30,55 This overpotential of 1Ni-4Fe-P is also comparable to a number of high-surface-area catalysts prepared in the form of nanostructured (nanosheet, nanorod, nanotube, nanowire) arrays supported on flat or 3-D (i.e., nickel foam, carbon fiber paper) substrates, which reach values of 75-96 mV at j = -10 mA cm2 10,17,19,21

.

Thus, the performance of our 1Ni-4Fe-P is notable, considering the

simplicity of our electrode preparation procedure that does not involve any morphological (1- or 2-D nanostructuring) and geometrical (3-D substrates) optimizations during the fabrication process.

The 5Fe-P electrode was found to be unstable, where the film started to crack and detach from the Ti foil upon H2 evolution, thereby causing the LSV curve to shift cathodically in the successive scans (only the first scan is shown in Figure 5). The detachment is possibly due to the corrosion of Fe2O3 that would occur under acidic condition (pH 0.3) according to the Pourbaix diagram of iron.56 The process would weaken the attachment of the film to the Ti foil, thereby leading to poor electrical transport within the electrode, as illustrated by the results of electrochemical impedance spectroscopy (EIS) in Figure S6, where a two-time-constant behavior was observed. Excessive NaH2PO2·H2O has been used in the phosphidation process to minimize the presence of Fe2O3 in 5Fe-P. Despite being unstable, the HER activities of 5Fe-P with decreased presence of Fe2O3 (prepared with 0.081-0.162 g


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NaH2PO2·H2O; see Figure S7 for XRD data) were fairly similar to that of 5FeP prepared with 0.027 g NaH2PO2·H2O (Figure S8). This suggests that Fe2O3 had a minimal role in the HER performance of the electrodes.

It is known that increasing the catalyst loading of an electrode would improve its HER performance.37,57

However, this effect is considered to be minimal in the

present work because the result in Figure 5 shows that 1Ni-4Fe-P, despite having a lower mass loading (see Table 1), exhibited a higher HER activity compared to all other samples with higher or similar mass loadings. By varying the number of spray cycles, samples of comparable mass loadings (approximately 1.0 mg cm-2) also were prepared. The HER activities of these samples (Figure S9) followed the same trend as that seen in Figure 5, demonstrating that 1Ni-4Fe-P is the best sample of this series. Further increasing the mass loading of 1Ni-4Fe-P has resulted in minor improvement in the HER activity (Figure S10) and a poor adhesion of the film to the Ti substrate.

The different overall HER activity, normalized by geometric surface area, of our samples in Figure 5a may be attributed to differences in the morphologies (Figure 1) and the intrinsic catalytic properties, i.e., the numbers and efficiencies of their catalytic sites. To examine the possibility of the latter, Tafel plots were derived from the LSVs in Figure 5a and are depicted in Figure 5b. Linearity was observed in the 40 mV ≤ η ≤ 200 mV range. The Tafel slope of Pt foil was 31 mV dec-1, which agrees with the values reported in the literature.11,19,21,24 The Tafel slopes for all samples (43-61 mV dec-1; Table 2) indicate that the catalysis for HER occurred via a Volmer-Heyrovsky mechanism where a fast proton discharge was followed by a ratelimiting reaction with an additional proton and electron to form H2.58 For bimetallic



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phosphide samples, the Tafel slopes were lower than those of monometallic phosphide samples and were found to decrease with increasing Fe content. This is attributed to the synergy between Ni and Fe in the bimetallic phosphide samples that led to improvement in the charge transfer kinetics.

To further examine the intrinsic catalytic properties of the samples, we normalized the LSVs by their real surface areas to decouple the contribution from the differences in morphologies. The real surface areas of the samples, which were determined using EIS measurements (Figure S6 and Table S2), generally increased with increasing Fe content. This is consistent with the morphological evolution seen in Figure 1 that a sample consisting of smaller agglomerates is generally associated with larger real surface area,59 which is expected to provide larger number of active sites per geometric area. 3Ni-2Fe-P showed an extraordinarily large real surface area likely due to its high surface roughness.

The LSVs normalized by the real surface areas of the samples are shown in Figure 5c and are summarized in Table 2. 1Ni-4Fe-P exhibited the highest intrinsic catalytic activity, followed by 5Ni-P, 2Ni-3Fe-P, 4Ni-1Fe-P, 5Fe-P, and 3Ni-2Fe-P. The Tafel slopes obtained by normalization by real surface area (Figure 5d; Table 2) are consistent with those obtained by normalization by geometric surface area (Figure 5b; Table 2). It is noted that the normalized exchange current densities (Table 2) of bimetallic phosphide samples were lower than those of monometallic phosphide samples, suggesting an increase in the intrinsic charge transfer resistance for HER. The highest intrinsic catalytic activity of 1Ni-4Fe-P is a result of optimized intrinsic charge transfer resistance and kinetics. This observation indicates that the metal ratio


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and the consequent mineralogical composition play an important role in the intrinsic catalytic properties and therefore needed to be optimized in order to achieve maximal intrinsic catalytic performance.

The high intrinsic catalytic activity of 1Ni-4Fe-P is expected to be related to the differences in electronic structures compared to 5Ni-P and 5Fe-P. To examine this, the chemical states of Ni, Fe, and P in 5Ni-P, 1Ni-4Fe-P, and 5Fe-P were examined using XPS (Figure 6). In the case of 5Ni-P and 1Ni-4Fe-P, the activated samples were examined, while for 5Fe-P, the freshly prepared sample was used because of the aforementioned stability issue in acidic condition. However, since the surface of freshly prepared 5Fe-P comprises already mostly phosphide phase (Figures 6b-c), the activation procedure is expected to have only a minimal effect on the comparison of XPS data. The 2p3/2 phosphide peak positions of Ni, Fe, and P for all 3 samples are shifted to higher, higher, and lower binding energies compared respectively to those of elemental Ni (2p3/2 at 852.8 eV),60 Fe (2p3/2 at 706.8 eV),50,61 and P (2p3/2 at 130.0 eV), as marked by the vertical dotted lines in Figure 6 and tabulated in Table 3.61 This shows that both Ni and Fe carried a partial positive charge (Niδ+, 0 < δ < 2; Feδ+, 0 < δ < 3),16,19 while P carried a partial negative charge (Pδ–, 0 < δ < 1).16,18



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Figure 6. High-resolution XPS spectra of activated 5Ni-P and 1Ni-4Fe-P, and freshly prepared 5Fe-P in (a) Ni 2p, (b) Fe 2p, and (c) P 2p energy regions. The vertical dashed lines mark the phosphide peak positions of 1Ni-4Fe-P, while the vertical dotted lines mark the peak positions of elemental (a) Ni 2p3/2, (b) Fe 2p3/2, and (c) P 2p3/2.

Table 3. Summary of the binding energy of Ni 2p3/2, Fe 2p3/2, and P 2p3/2 phosphide peaks in Figure 6. Values in parentheses represent the peak shift relative to the respective element.

60

Elemental Ni Elemental Fe50,61 Elemental P61 5Ni-P 1Ni-4Fe-P 5Fe-P

Ni 2p3/2 852.8 853.5 (+0.7) 853.8 (+1.0) -

Binding energy / eV Fe 2p3/2 706.8 707.2 (+0.4) 707.6 (+0.8)

P 2p3/2 130 129.6 (-0.4) 129.5 (-0.5) 129.8 (-0.2)

Compared to 5Fe-P and 5Ni-P, the positions of Ni 2p, Fe 2p, and P 2p phosphide peaks of 1Ni-4Fe-P (marked by vertical dashed lines in Figure 6 and tabulated in Table 3) were at a higher, a lower, and a lower binding energy, respectively. This shows that 1Ni-4Fe-P had an increased δ+ at the Ni sites, a decreased δ+ at the Fe sites, and an increased δ– at the P sites. Using density functional theory (DFT) calculations, nickel phosphide has been reported to catalyze HER in an analogous


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manner to [NiFe] hydrogenase, where the nickel sites (hydride-acceptor) and the phosphorus sites (proton-acceptor) work cooperatively to result in an efficient catalytic activity.62 In our study, it is expected that the decreased δ+ at the Fe sites of 1Ni-4Fe-P promoted the proton adsorption by reducing the electric repulsion while the increased δ– at the P sites facilitated proton trapping on the surface of 1Ni-4Fe-P. Although the Ni sites of 1Ni-4Fe-P had an increased δ+ compared to that of 5Ni-P, it constituted only 20% of the metal content and therefore may have a less significant role as hydride-acceptor than Fe sites.

The interpretation above is further supported by comparison to the XPS spectra of 3Ni-2Fe-P (Figure S11), which serves as an example that consisted of non-optimized Ni:Fe ratio.

For this composition, we observed an increased δ+ at the Ni sites

(compared to 5Ni-P), an unchanged δ+ at the Fe sites (compared to 5Fe-P), and an unchanged (compared to 5Fe-P) or decreased (compared to 5Ni-P) δ– at the P sites. Consistent with the arguments made above, these properties would result in a poor intrinsic catalytic performance compared to those of monometallic phosphide samples, which is in full agreement with our experimental observations shown in Figures 5c,d.

It is important to achieve reproducible quality of 1Ni-4Fe-P for HER activity and this is confirmed by the similar LSVs (η for j = -10 mA cm-2 is within 1.5% relative standard deviation) obtained using five individual 1Ni-4Fe-P samples that were synthesized in different batches (Figure S12). For practical application, it is also equally important for 1Ni-4Fe-P to work stably for HER without any undesirable side-reactions. To verify this, the LSVs of 1Ni-4Fe-P were taken before and after



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subjecting the sample to a 24-h chronopotentiometry (CP) test. As seen in Figure 7a, the η required to achieve j = -10 mA cm-2 increased by only 4% after the CP test, therefore demonstrated excellent stability. Comparison of the surface morphology before (Figure 7b) and after the CP tests (Figures 7c) showed negligible changes. The H2 evolved by 1Ni-4Fe-P during operation at j = -10 mA cm-2 was quantified and compared to the quantity of H2 calculated by assuming a 100% Faradaic efficiency (Figure S13). The Faradaic efficiency attained was 94±6%, which means that nearly all the charges passed were consumed for HER.

Figure 7. (a) Comparison of the LSVs (iRu-corrected) of 1Ni-4Fe-P before and after a 24-h CP test in 0.5 M H2SO4. Inset shows the CP test (iRu-corrected) measured at j = -10 mA cm-2. SEM images of 1Ni-4Fe-P (b) before and (c) after a 24-h CP test. Scale bar: 200 nm.

Conclusions A series of nickel-iron phosphide porous films of varied metal contents have been successfully prepared on Ti foil using a simple and scalable approach of spraypyrolysis deposition followed by low-temperature phosphidation. The nickel-iron phosphide with an optimized Ni:Fe ratio (1:4) exhibited remarkable overall catalytic


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activity for HER in 0.5 M H2SO4 (j = -10 mA cm-2 and -30 mA cm-2 at overpotentials of 101 mV and 123 mV, respectively, and Tafel slope of 43 mV dec-1) as compared to nickel phosphide and iron phosphide. It also showed a near-100% Faradaic efficiency and an excellent catalytic stability. The superior overall HER activity of nickel-iron phosphide relative to nickel phosphide and iron phosphide is a combined contribution from the larger real surface area (thus a higher number of reaction sites for HER) and the higher intrinsic catalytic activity for HER. The latter was revealed by XPS analysis to be a result of a decreased partial positive charge at Fe sites and an increased partial negative charge at P sites, which make the Fe and P sites better hydride- and proton-acceptors, respectively. The two-step synthesis method reported herein offers a convenient approach for exploring multi-transition metal phosphides for expanding the library of low-cost and efficient catalysts.

Supporting Information. Additional EDS, XRD, XPS, electrochemical and gas measurement data, and the comparison of HER activity of various catalysts.

Acknowledgement This work was supported by the Strong Research Environment Solar Fuels (Umeå University), the Artificial Leaf Project Umeå (Knut & Alice Wallenberg foundation), and Energimyndigheten. The authors acknowledge the support from the Umeå Core Facility for Electron Microscopy (UCEM), the X-ray Photoelectron Spectroscopy laboratory at the Chemical Biological Centre (KBC), and the X-ray laboratory at the Department of Applied Physics and Electronics at Umeå University. The Knut & Alice Wallenberg foundation is acknowledged for providing the electron microscopy facilities at Stockholm University.

We thank Andrey Shchukarev for the XPS



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measurement, Cheuk-Wai Tai (Arrhenius Lab, Stockholm University) for the HRTEM measurement, and Jekabs Grins (Arrhenius Lab, Stockholm University) for the XRD measurement.


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