The in situ preparation of iron phosphide using ionic liquids as iron and phosphorus sources for efficient hydrogen evolution reactions

Abstract

Ionic liquids (ILs) were utilized as iron and phosphorus sources for the preparation of iron phosphide for the first time. The IL trihexyl(tetradecyl)phosphonium tetrachloroferrate ([P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄]) and carbon nanotubes (CNTs) were applied as precursors for the in situ preparation of Fe₂P_(IL6)/CNTs. This material has good catalytic activity and stability for the hydrogen evolution reaction, including a low onset overpotential (75 mV) and Tafel slope (68 mV dec⁻¹). Moreover, this catalyst exhibits current densities of 10 and 20 mA cm⁻² at overpotentials of 115 and 150 mV, respectively. The phosphidation process using [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄] was also investigated. All experimental results indicate that Fe₂P can be formed in situ on the CNTs using this IL, and that the CNTs help the formation of the Fe₂P nanoparticles and improve the electrical conductivity. This IL-based in situ preparation strategy is facile and environmentally friendly and does not require the addition of other reagents. This method holds great promise for application in other electrochemical studies.

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Introduction

Due to the environmental impact of fossil fuels and the energy crisis, the development of some clean, low-carbon emission, renewable and efficient energy sources to replace fossil fuels is required. Hydrogen is a sustainable and green energy source and a carrier with a high energy density, which could enable the world to meet future energy demand. Water electrolysis is a simple and reliable method to generate H₂ without carbon emissions and normally needs a high overpotential.¹ In order to decrease the required overpotential and to increase the current density, electrocatalysts for the hydrogen evolution reaction (HER) are highly desired. Noble metal-based catalysts have excellent HER activities but their high price and low abundance have restricted their practical application.² Also, the proton exchange membrane used for water splitting demands that the HER electrocatalyst is acid-stable.³ Thus, it is necessary to develop some inexpensive, stable and highly efficient catalysts for the HER.⁴ Recently, transition metal phosphides (TMPs) have been introduced as a promising catalyst material due to their low-cost, good catalytic efficiency and stability.⁵ Various kinds of TMPs have been prepared as electrocatalysts for the HER, such as MoP, WP, NiP, FeP and CoP.^6–11 Amongst the transition metals, iron is the cheapest and most abundant, so iron phosphide-based catalysts, including FeP, Fe₂P, Fe₃P and FeMnP, have been introduced and widely applied in the HER.^12–14 To prepare iron phosphide, different iron salts (e.g. FeCl₃ and FeSO₄) and phosphorus compounds (e.g. trioctylphosphine and NaH₂PO₂) have frequently been used as the Fe and P sources, respectively.^12–17 However, some of these preparation methods have adopted complicated procedures to obtain a good HER performance.^6,12–17 Hence, it is vital to develop a simple approach to the synthesis of iron phosphide with novel Fe and P sources.

Ionic liquids (ILs) are organic molten salts that possess low melting points (<100 °C). Various ILs have been applied in the HER due to their high conductivities and stabilities.^18–20 In our previous study, an IL with a tetrachloroferrate anion was synthesized and applied as the iron source to prepare FeP for use in the HER. The resulting FeP showed good catalytic activity.²¹ However, this synthetic process is tedious and time consuming, and an additional P source is needed for the preparation of FeP. Thus, it is essential to use some novel metal and phosphorus sources to simplify the preparation route. In this study, an in situ iron phosphide preparation method was developed, where ILs containing a phosphonium cation and a tetrachloroferrate anion were utilized as a dual iron and phosphorus source for the first time. Generally, trihexyl(tetradecyl)phosphonium tetrachloroferrate ([P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄]) and tetrabutylphosphonium tetrachloroferrate ([P(C₄H₉)₄][FeCl₄]) were quickly synthesized, and then these two ILs were mixed with carbon nanotubes (CNTs) to prepare the Fe₂P_(IL6)/CNTs and Fe₂P_(IL4)/CNTs materials via low temperature pyrolysis (Scheme 1). These as-prepared iron phosphides exhibited good HER activity and stability. During the preparation, the CNTs facilitated the formation of the Fe₂P nanoparticles as well as improving the conductivity of these catalysts. This IL-based in situ preparation strategy is uncomplicated, quick, and environmentally friendly without needing the addition of other organic solvents and harmful reagents. It holds great promise for applications in other electrochemical studies.

Scheme 1 The preparation of Fe₂P_(IL4)/CNTs and Fe₂P_(IL6)/CNTs.

Experimental

Chemicals

Trihexyl(tetradecyl)phosphonium chloride ([P(C₆H₁₃)₃C₁₄H₂₉][Cl]), tetrabutylphosphonium chloride ([P(C₄H₉)₄][Cl]), iron(III) chloride hexahydrate (FeCl₃·6H₂O) and other reagents were purchased from Titansci Co. China. Multi-walled carbon nanotubes (CNTs) were supplied by Shenzhen Nanotech Port Ltd, Co. China.

Synthesis of ionic liquids and the preparation of Fe₂P

Two ionic liquids (ILs), namely trihexyl(tetradecyl)phosphonium tetrachloroferrate ([P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄]) and tetrabutylphosphonium tetrachloroferrate ([P(C₄H₉)₄][FeCl₄]), were synthesized via a previously reported procedure.²² Generally, [P(C₆H₁₃)₃C₁₄H₂₉][Cl] was mixed with FeCl₃·6H₂O in methanol for 24 h at room temperature and the dark red product was automatically separated from the solvent. After removing the solvent, the homogeneous product was dried under vacuum to afford [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄]. Then, this IL was mixed with CNTs in a mortar to obtain a IL6/CNTs mixture, which was subsequently pyrolyzed in a tube furnace for in situ preparation of Fe₂P_(IL6)/CNTs (400 °C in an Ar atmosphere for 2 h) (Scheme 1). Similarly, [P(C₄H₉)₄][Cl] was reacted with FeCl₃·6H₂O to synthesize [P(C₄H₉)₄][FeCl₄], which was further mixed with CNTs and pyrolyzed to obtain Fe₂P_(IL4)/CNTs. For comparison, [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄] was directly pyrolyzed without CNTs to prepare Fe₂P_(IL6) under the same conditions. FeCl₃·6H₂O was mixed with CNTs and sodium hypophosphite (NaH₂PO₂) and was then pyrolyzed to prepare FeP/CNTs under the same conditions.

Electrocatalytic performance

All electrochemical measurements were conducted on an electrochemical workstation (CHI-760E) with a three-electrode configuration at ambient temperature, where a glassy carbon electrode (GCE, 0.126 cm²), a carbon rod and a saturated calomel electrode (SCE) were employed as the working electrode, the counter electrode and the reference electrode in 0.5 M H₂SO₄, respectively. 10 μL of a catalyst ethanol solution (5 mg ml⁻¹) with Nafion (5 wt%) was dropped on the clean GCE and dried to prepare the working electrode. For each analysis, the measurement was repeated at least three times. The linear sweep voltammograms (LSV) were performed at a scan rate of 2 mV s⁻¹ (from 0.2 to −1 V). The electrochemical impedance spectroscopy (EIS) measurements of the catalysts were recorded at −400 mV over a frequency range of 0.1–1000 kHz with a 5 mA amplitude of current perturbation.

Characterization

The morphology and structure of the material were studied using field emission scanning electron microscopy (FE-SEM, JEOL-7800F), transmission electron microscopy (TEM, JEM-2100) and energy dispersive X-ray spectroscopy (EDX, INCA X-Max 250). The measurement of the X-ray diffraction (XRD) patterns was conducted using a RigakuD/Max 2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) characterization was performed using a ESCALAB 250Xi spectrometer with Al Kα (150 W, 1486.6 eV) radiation. The specific surface area was analyzed using a nitrogen adsorption method (BET method) through a TriStar II 3020 gas sorption analyzer.

Results and discussion

The morphology of the Fe₂P_(IL6)/CNTs was examined using FE-SEM and TEM. As shown in Fig. 1 and S1 in the ESI,† Fe₂P was uniformly generated in situ on the CNTs. These Fe₂P nanoparticles are sphere-shaped with a diameter of approximately 40–70 nm. These images clearly illustrate that the Fe₂P nanospheres are wrapped in the CNTs. Moreover, a size distribution histogram based on the TEM images was obtained (Fig. S2†), showing that the diameters of the particles almost ranged from 40–60 nm. The energy dispersive spectroscopy (EDS) analysis reveals that C, O, P and Fe are the main elements of Fe₂P_(IL6)/CNTs. The EDS mapping results indicate that the Fe and P are evenly spread on the CNTs (Fig. 2 and S3†). It also suggests that during the preparation, the CNTs uniformly disperse the [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄] for in situ formation of the Fe₂P nanospheres. Furthermore, the EDS pattern (Fig. 2a) shows that the atomic percentage of Fe and P in Fe₂P_(IL6)/CNTs was 36.6 and 18.35%, respectively. These results indicate that the Fe and P molar ratio in the catalyst is 2 : 1, yet this ratio is 1 : 1 in the [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄] IL. A possible reason for this change in the Fe : P ratio could be that during the preparation process (Scheme 1) some of the phosphorus formed may react with other atoms (such as Cl and H) to form volatiles that are transferred out of the tube furnace by the carrier gas (Ar). Moreover, this change in stoichiometric ratio preliminarily demonstrates the formation of Fe₂P. For comparison, SEM, TEM and EDS analyses were also performed for Fe₂P_(IL4)/CNTs, FeP/CNTs and Fe₂P_(IL6). As shown in Fig. 1, 2 and S4–S6,† Fe₂P_(IL6)/CNTs has a smaller particle size and a higher Fe₂P content than Fe₂P_(IL4)/CNTs and FeP/CNTs. The FeP/CNTs nanoparticles were aggregated and the Fe₂P_(IL6) sample has a large sheet morphology.

Fig. 1 (a) An SEM image of Fe₂P_(IL6)/CNTs; and (b) a TEM image of Fe₂P_(IL6)/CNTs (scale bar: 50 nm).

Fig. 2 (a) The EDS pattern of Fe₂P_(IL6)/CNTs; and (b): EDS mapping of Fe and P in Fe₂P_(IL6)/CNTs.

In addition, XRD and XPS characterizations of Fe₂P_(IL6)/CNTs were carried out to further confirm the formation of Fe₂P. The broad peak in the XRD pattern from 20 to 28° can be assigned to the presence of CNTs in this material. The XRD pattern of Fe₂P_(IL6)/CNTs displayed characteristic diffraction peaks at 40.4° (111), 44.2° (201), 47.3° (210), 52.9° (002), 54.1° (300), 54.6° (211), 73.6° (212) and 79.1° (302), which is in agreement with the Fe₂P reference pattern (PDF#51-0943) (Fig. 3).²³ The small positive shift of the Fe₂P_(IL6)/CNTs peaks may result from the oxygen dopant observed in the EDS data.¹⁰ Some of the iron could be oxidized during the phosphidation and so the unidentified peak around 43° may be attributed to the iron oxidized species. A Scherrer analysis of the XRD data was conducted, and the results indicated that the average grain size is 52.7 nm (Fig. S2†), which is in accordance with the TEM data. Bonding information relating to Fe₂P_(IL6)/CNTs was investigated through XPS analysis. In the Fe 2p spectrum (Fig. 4a) two broad peaks were observed at binding energies (BEs) of 712.6 eV and 726.7 eV. These are most likely related to the amorphous iron oxide species (Fe–O) in the material, indicating that the Fe³⁺ was not totally reduced to a zero valence state.²⁴ Two sharp peaks were found at 707.5 eV (Fe 2p_3/2) and 720.3 eV (Fe 2p_1/2). Also, the P 2p spectrum reveals two peaks located at 129.4 eV (P 2p_3/2) and 130.1 eV (P 2p_1/2). The Fe 2p_3/2 peak (707.5 eV) is a little higher than that of elemental Fe (707.0 eV) while the P 2p_3/2 peak (129.4 eV) is a little lower than that of elemental P (130.2 eV), which is consistent with the structure of Fe₂P.^25–27 Also, it suggests that electrons were transferred from iron to phosphorus, where Fe and P have a partial positive charge (δ+) and a partial negative charge (δ−), respectively.^28,29 Another peak located at 133.9 eV may arise from the oxidized P species, suggesting the formation of phosphorous based oxides (Fig. 4b).^28,29 The existence of oxidized Fe and P can be assumed to arise from the surface oxidation of Fe₂P under ambient conditions.²⁸ In addition, the relative area ratio of the P 2p peak area at 133.9 eV is higher than that of the other P 2p peaks. Its higher proportion could be the possible reason for these broad peaks and the peak shift observed in the XRD patterns.

Fig. 3 The XRD pattern of Fe₂P_(IL6)/CNTs.

Fig. 4 (a) The Fe 2p XPS spectrum of Fe₂P_(IL6)/CNTs; and (b) the P 2p XPS spectrum of Fe₂P_(IL6)/CNTs.

In this study, [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄] was utilized as a dual P and Fe source for in situ preparation of Fe₂P. Previous investigations have demonstrated that the quaternary phosphonium cation of an IL can decompose to a trialkylphosphine (TAP) and that the metal-based anion can be reduced to give metal nanoparticles (M–NPs).^30,31 This TAP, utilized as an organophosphine to prepare transition metal phosphides (TMPs), can exhibit a strong coordination interaction with the M–NPs to form a complex, such as Ni–TAP or Co–TAP.^32,33 In this case, Fe–TAP was generated during the pyrolysis step and the P–C bond could be broken to form phosphorus. Finally, the metal and phosphorus react to obtain the TMP.⁵ Accordingly, the possible synthetic route for Fe₂P using [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄] is described in Scheme 1. During the preparation, the IL can also be used as a solvent and a stabilizer to prevent the aggregation of the nanoparticles.^19,34 Furthermore, according to the TEM and SEM images of all four materials, Fe₂P_(IL6) is a large bulky material on the micrometer scale while the other three catalysts with CNTs are on the nanometer scale. Thus, the CNTs are favorable for the formation of iron phosphide nanoparticles by dispersing the IL and avoiding aggregation.

The HER activities of Fe₂P_(IL6)/CNTs and Fe₂P_(IL4)/CNTs were studied. Fe₂P_(IL6) and FeP/CNTs, pyrolyzed from [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄] and FeCl₃·6H₂O/CNTs/NaH₂PO₂ respectively, were introduced as control groups. As shown in Fig. 5a, Fe₂P_(IL6)/CNTs possesses the best HER activity, holding the lowest onset overpotential of 75 mV. It can also achieve current densities of 10 and 20 mA cm⁻² at overpotentials of 115 and 150 mV, respectively. On the contrary, Fe₂P_(IL6) displays a much lower activity than Fe₂P_(IL6)/CNTs and Fe₂P_(IL4)/CNTs. Thus, the CNTs played a pivotal role in improving the HER performance. It is highly possible that the CNTs can disperse the IL to small droplets, which is beneficial for forming the Fe₂P nanoparticles.

Fig. 5 (a) Linear sweep voltammograms at the working electrode using the four materials; (b) Tafel plots for the HER using the four materials; and (c) electrochemical impedance spectroscopy data for the four materials. Data were collected for the electrode at a HER overpotential of 370 mV; Fe₂P_(IL6)/CNTs (black), Fe₂P_(IL4)/CNTs (red), FeP/CNTs (blue), and Fe₂P (green). (d) Linear sweep voltammograms at the working electrode using Fe₂P_(IL6)/CNTs; initial trial: black, trial after 700 scanning cycles: red. Catalyst loading: 50 μg. Electrolyte: 0.5 M H₂SO₄. IL to CNTs ratio: 5 : 1.

Tafel plots for these materials were investigated, and the corresponding Tafel slopes were calculated (Fig. 5b). The Tafel slope for Fe₂P_(IL6) is higher than 120 mV dec⁻¹, suggesting that a Volmer step (proton adsorption) is the rate-limiting step.³⁵ On the contrary, the Tafel slope of Fe₂P_(IL6)/CNTs is the lowest (68 mV dec⁻¹), revealing that this process is a Volmer–Heyrovsky reaction and that the rate-limiting step is a Heyrovsky step (charge transfer step). Therefore, enhancing the conductivity and electron transfer ability of the material could significantly improve the catalytic activity of Fe₂P_(IL6)/CNTs. In addition, the low Tafel slope of Fe₂P_(IL6)/CNTs demonstrates good hydrogen evolution kinetics and a low energy barrier for H₂ electrochemical desorption.

Electrochemical Impedance Spectroscopy (EIS) analysis was conducted to further evaluate catalytic performance. As shown in Fig. 5c, Fe₂P_(IL6)/CNTs possesses the lowest faradaic impedance (∼30 Ω) while Fe₂P_(IL6) has a much higher impedance. This demonstrates that Fe₂P_(IL6)/CNTs possesses a high electrical conductivity and good electron transfer ability which, given that charge transfer is the rate-limiting step for this catalyst, can dramatically improve the catalytic activity. Thus, it is worth noting that the CNTs can not only facilitate the fabrication of the Fe₂P nanoparticles, but also improve the HER performance.

Additionally, both Fe₂P_(IL6)/CNTs and Fe₂P_(IL4)/CNTs have a better HER performance than FeP/CNTs, showing that the IL can improve HER activity (Table 1). The possible reason could be the carbonization of the IL cation during pyrolysis, which increases the electrical conductivity of the materials.¹⁶ Also, the EDS results indicate that Fe₂P_(IL6)/CNTs and Fe₂P_(IL4)/CNTs have a higher iron phosphide content than that of FeP/CNTs, improving the HER performance.

Table 1 A comparison of the HER performance of Fe₂P_(IL6)/CNTs with other HER electrocatalysts

Catalyst	Onset overpotential (mV)	Tafel slope (mV dec^−1)	j (mA cm^−2)	η at the corresponding j (mV)	Ref.
Fe2P(IL6)/CNTs	75	68	10	115	This work
			20	150
Fe2P(IL4)CNTs	120	130	10	290
			20	340
FeP/CNTs	350	143	10	520
			20	600
Fe2P	350	184	10	570
FeP/C	63	57	10	110	8
			20	147
FeP NPs	85	64	10	124	16
FeP NPs/graphene	65	67	10	123	37
FeP NP-candle soot	38	58	10	112	38
FeP hollow microsphere	—	58	10	114	39
FeP and Fe2P nanowires	50	39	10	96	25
FeP nanorods	150	55	10	120	40
3D-graphene aerogel CoP	—	50	10	121	41
MoP2 nanosheets	90	81.5	10	150	9
Ni2P nanoparticles	75	46	10	102	19

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When compared with [P(C₄H₉)₄][FeCl₄] (colloidal solid), P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄] (liquid) is more convenient to mix with the CNTs, forming smaller IL droplets that subsequently generate smaller Fe₂P particles. Brunauer–Emmett–Teller (BET) measurements were carried out. Fe₂P_(IL6)/CNTs has a higher surface area (163 m² g⁻¹) than that of Fe₂P_(IL4)/CNTs (145 m² g⁻¹), which can expose more active sites to enhance the HER activity.³⁶ These BET results also suggest that Fe₂P_(IL6)/CNTs has a smaller particle size than Fe₂P_(IL4)/CNTs, resulting in better HER performance. Moreover, the EDS analysis indicates that Fe₂P_(IL6)/CNTs has a higher Fe₂P content than Fe₂P_(IL4)/CNTs (Fig. 2b and S4†), which could be another reason for the better HER performance.

The stability of Fe₂P_(IL6)/CNTs in the HER was evaluated, and cyclic voltammetry (CV) over 700 scanning cycles was performed at the scan rate of 100 mV s⁻¹. After scanning, no significant change in the LSV was observed, indicating that this catalyst possesses a good cycle life during the HER (Fig. 4d). Therefore, Fe₂P_(IL6)/CNTs can serve as an effective and stable HER electrocatalyst, and its catalytic activity is comparable to other TMP catalysts used for the HER (Table 1). Moreover, Fe₂P_(IL6) was prepared in situ on CNTs using the IL as a dual source of Fe and P, which is simple, fast and green and does not require the addition of other reagents.

The IL/CNTs ratio and pyrolysis temperature for Fe₂P_(IL6)/CNTs preparation were optimized. The IL to CNTs ratio was varied from 1 : 1 to 10 : 1, and the best ratio was 5 : 1 (Fig. 6a). It is possible that a high ratio of IL to CNTs could produce more Fe₂P_(IL6) particles on the CNTs, while excess IL would form extra Fe₂P_(IL6) that deteriorates the HER performance. Next, the pyrolysis temperature was increased from 300 to 500 °C (Fig. 6b), which is, depending on the iron and phosphorus precursors, the most suitable temperature range for preparing iron phosphide.^12–17 In this study, pyrolysis at too low a temperature will not obtain the final product, while too high a temperature will waste energy and increase the annealing time. Thus, 400 °C was used for phosphidation.

Fig. 6 (a) Linear sweep voltammograms at the working electrode using Fe₂P_(IL6)/CNTs where different IL/CNTs ratios were used to prepare the catalyst; IL : CNTs = 1 : 1 (red), IL : CNTs = 2.5 : 1 (blue), IL : CNTs = 5 : 1 (black), and IL : CNTs = 10 : 1 (green). (b) Linear sweep voltammograms at the working electrode using Fe₂P_(IL6)/CNTs where different pyrolysis temperatures were used to prepare the catalyst: 300 °C (red), 350 °C (blue), 400 °C (black), and 450 °C (green). Catalyst loading: 100 μg. Electrolyte: 0.5 M H₂SO₄.

Conclusions

Fe₂P_(IL6)/CNTs was successfully prepared from [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄]/CNTs, where [P(C₆H₁₃)₃C₁₄H₂₉][FeCl₄] was used as the dual source of Fe and P to fabricate Fe₂P for the first time. Fe₂P_(IL6)/CNTs was then utilized as an electrocatalyst for the HER where it displayed good HER activity and stability. CNTs not only improve the catalytic activity but are also favorable for the preparation of the Fe₂P nanoparticles. Moreover, the IL-based in situ preparation is facile and eco-friendly and does not require the addition of any other reagent. This method possesses great promise for application in other electrochemical studies. Future studies will focus on the trapping of volatiles and using gas chromatography-mass spectrometry (GC-MS) or other characterization methods to analyze the off-gases during the preparation process. This will be applied to further investigate the phosphidation mechanism.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 51909012) and “Fundamental Research Funds of the Central University” (XDJK2019C004).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra05666a

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