Synergistic effect of surface reconstruction and rGO for FeS₂/rGO electrocatalysis with efficient oxygen evolution reaction for water splitting

2.1 Materials preparation

2.1.2

2.1.2 Preparation of FeS₂-450/rGO-x% (x = 5, 10 and 15) materials

The synthesis procedure of FeS₂-450/rGO-x% (x = 5, 10 and 15) materials is similar to preparing the FeS₂-450 sample, excepting a mount of GO was added into the initial FeCl₃ solution. The synthesis process of the FeS₂-450/rGO-x% catalysts has been displayed in Scheme 1 and can be explained according to Eq. (1) to (3). Under the alkaline environment, Fe³⁺ adsorbed on GO converted into Fe₂O₃ during the hydrothermal stage, while the GO was reduced to rGO, forming Fe₂O₃/rGO (Eqs. (1) and (2)), as shown in Fig. S1. Under the inert atmosphere of 450 °C, the mixed Fe₂O₃/rGO and S powder transformed into FeS₂-450/rGO and H₂O (Eq. (3)). The pieces were marked with FeS₂-450/rGO-5%, FeS₂-450/rGO-10% and FeS₂-450/rGO-15%, with the GO are 0.005, 0.01 and 0.014 g, respectively.

(1)

$${\text{NH}}_3·{\text{H}}_2\text{O}\iff {\text{NH}}_4^{+}+{\text{OH}}^{-}$$

(2)

$${\text{Fe}}^{3+}+3{\text{OH}}^{-}+\text{GO}\stackrel{\text{Hydrothermal}}{\to }{\text{Fe}}_2{\text{O}}_3/\text{rGO}$$

(3)

$${\text{Fe}}_2{\text{O}}_3/\text{rGO}+\text{S}\stackrel{\text{inert}\text{atmosphere,}\text{450}°\text{C}}{\to }{\text{FeS}}_2-450/\text{rGO}+{\text{H}}_2\text{O}$$

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Scheme 1

The synthesis procedure of the FeS₂-450/rGO nanocomposites.

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2.2 Characterizations

The X-ray diffraction patterns(XRD) ranging from 2θ = 20° to 70° were obtained by a powder diffractometer (D8 Advanced, Bruker Co, Germany) with Cu Kα radiation. The surface structure and morphology were characterized by the field emission scanning electron microscope (FESEM, Zeiss Sigma, Japan). The interior structure was obtained by the high-resolution transmission electron microscope (HRTEM, JEOL F200, Japan). The chemical status was acquired by X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi).

2.3 Electrochemical measurements

All the electrochemical measurements were performed using an electrochemical workstation of Gamry with a standard three-electrode system at room temperature. In the three-electrode cell, the Hg/HgO was used as the reference electrode; the graphite rod served as the counter electrode, while the pretreated foam Ni covered with samples was the working electrode. The working electrode was prepared as follows: First, the 10 mg prepared sample was dispersed into a mixture of 1 mL ethanol and 50 μL of Nafion solution (5%, DuPont) with ultrasound for 20 min to get a homogeneous catalyst ink. Second, 100 µL ink was extracted with a micropipette and uniformly dropped onto the pretreated foam Ni (1 × 1 cm²). Linear sweep voltammetry (LSV) was conducted at a sweep rate of 5 mV s⁻¹ by working the voltage range of 0–1 V (vs Hg/HgO). All LSV curves were 95% iR-corrected in this paper. The electrochemical double-layer capacitance (C_dl) of the samples was calculated by performing the cyclic voltammetry (CV) analysis at the scan rates of 20, 40, 60, 80, and 100 mV s⁻¹ in 1.0 mol/L (M) KOH. Electrochemical impedance spectroscopy (EIS) was performed in a 0.61 V potentiostatic state with a frequency range of 0.01–100 kHz. The OER stability was carried out using the Chronopotentiometry tests for 18000 s at a potential of 1.527 V vs RHE in 1.0 mol/L (M) KOH solution.

All the presented potential in this work was converted to the reversible hydrogen electrode (RHE) potential according to the following two-equation:

Overall water splitting was performed in a two-electrode system, in which FeS₂-450/rGO-10% was the anode electrode while the Pt/C served as the cathode electrode.

3.1 Fabrication and characterization before OER

The phase of FeS₂ and FeS is clearly identified from XRD patterns for the synthesized samples, Fig. 1a. The FeS₂ phase can be observed at 400 °C, 450 °C and 550 °C, where the diffraction lines at 28.52°, 33.05°, 37.08°, 40.77°, 47.43°, 56.28°, 59.01°, 61.69° and 64.29° index to the plans of (1 1 1), (2 0 0), (2 1 0), (2 1 1), (2 2 0), (3 1 1), (2 2 2), (0 2 3) and (3 2 1) of FeS₂ (JCPDS#71–1680), respectively. In contrast, the phases of FeS (JCPDS#89–6926) is found in samples at 650 °C and 750 °C, Fig. 1a.

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Fig. 1

XRD patterns of prepared samples.

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As reveled by SEM, the FeS₂-400, FeS₂-450 and FeS₂-550 samples are nanospheres with the diameter ranging from 50 to 100 nm, Fig. 2a-d. Among them, the FeS₂-450 exhibits more uniform dispersion and smooth surface, which can facilitate exposure of more active sites to obtain excellent OER performance, Fig. 2b-c. As the temperature increased further, the FeS-650 and FeS-750 samples present irregular micro-scale blocks, which may be caused by aggregation of catalysts, Fig. 2e-f.

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Fig. 2

The SEM images of prepared (a) FeS₂-400 (b,c) FeS₂-450 (d) FeS₂-550 (e) FeS-650 (f) FeS-750.

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3.2 Electrocatalytic performance for OER

The electrocatalytic OER activities of Ni foam, RuO₂, FeS₂-400, FeS₂-450, FeS₂-550, FeS-650 and FeS-750 catalysts were carried out using 1 M KOH as electrolyte (pH = 11.83) at room temperature. The LSV curves show that FeS₂-450 exhibits superior OER performance with smaller potentials and larger current density, indicating the excellent conductivity, as displayed in Fig. 3a. The overpotentials of the Ni foam, RuO₂, FeS₂-400, FeS₂-450, FeS₂-550, FeS-650 and FeS-750 catalysts at 10 mA/cm² are 365 mV, 340 mV, 291 mV, 253 mV, 298 mV, 305 mV and 307 mV, respectively, Fig. 3b. Remarkably, the overpotential of FeS₂-450 largely reduced to 253 mV. Further, the overpotentials of the FeS₂-450 at 50 mA/cm² and 100 mA/cm² are 315 mV and 369 mV, respectively, even superior to commercial precious metal electrocatalyst RuO₂, suggesting the outstanding OER performance. To get insight into understand the intrinsic activity, we calculated the turnover frequency (TOF) of the FeS₂-450, FeS-750 and RuO₂ catalysts at the overpotential of 369 mV and relevant calculation process is provided in the supporting information (Rebekah et al., 2020; Suen et al., 2017). The TOF of the FeS₂-450 reaches 0.311 O₂ per s per site, greater than FeS-750 (0.0114) and RuO₂ (0.0071), demonstrating the highest intrinsic activity in FeS₂-450. All the above results suggest that FeS₂-450 exhibits the best electrocatalytic OER activity in 1 M KOH among these catalysts.

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Fig. 3

OER measurement. (a) LSV (b) the overpotentials to achieve current densities of 10, 50, and 100 mA cm⁻² (c) Tafel plots, (d) Nyquist plots (e) electrochemical double-layer capacitance (C_dl) of the prepared Ni foam, FeS₂-400/NF, FeS₂-450/NF, FeS₂-550/NF, FeS-650/NF and FeS-750/NF and (f) i-t cures of FeS₂-450 sample under a constant potential of 1.527 V vs. RHE.

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Tafel plots, as a common descriptors to reflect the reaction kinetic of OER, are calculated by the linear formula of η = b log|j| + a, Fig. 3c. The Tafel plot of the FeS₂-450 is 72 mV/dec, which is lower than those of Ni foam (243 mV/dec), FeS₂-400 (89.6 mV/dec), FeS₂-550 (74.3 mV/dec), FeS-650 (74.2 mV/dec) and FeS-750 (79.8 mV/dec) catalysts, proving the remarkable OER catalytic kinetics over FeS₂-450. The charge-transfer resistance (R_ct) at the catalyst/electrolyte interface are further conducted by the Nyquist plots analysis, Fig. 3d. The decreased order of R_ct is FeS₂-450 < FeS₂-400 < FeS₂-550 < FeS-650 < FeS-750, demonstrating accelerated charge-transfer on highly conductive FeS₂-450 (Liu et al., 2019; Li et al., 2021).

The electrochemically active areas (ECSA) are positively correlated to the electric double-layer capacitance (C_dl), which can be calculated by the CV curves at different scan rates. The CV curves of Ni-foam, FeS₂-400, FeS₂-450, FeS₂-550, FeS-650 and FeS-750 catalysts were measured in the non-Faraday region at scan rates of 20 mVs⁻¹, 40 mVs⁻¹, 60 mVs⁻¹, 80 mVs⁻¹ and 100 mVs⁻¹, Fig. S2. Apparently, the C_dl value for FeS₂-450 is 0.45 mFcm⁻², significantly larger than that of control catalysts, suggesting uniform nanoparticles dispersion of FeS₂-450 offer much more accessible active sites on the surface, Fig. 3e (Shi et al., 2020). The electrochemical OER stability of FeS₂-450 was tested at 1.527 V vs RHE in 1 M KOH by chronoamperometry, Fig. 3f. Apparently, FeS₂-450 kept high OER stability within 5 h and without an appreciable current attenuation.

To further enhance the electrocatalytic performance of OER, rGO with high conductivity was introduced to prepared novel FeS₂-450/rGO-x% electrocatalysts. The XRD patterns of FeS₂-450/rGO-x% (x = 5, 10 and 15) samples are displayed in Fig. 1b and corresponding diffraction peaks index to FeS₂, consistent with fabricated FeS₂ at the same calcination temperature. The morphology and microstructure of the FeS₂-450/rGO-10% catalyst was characterized by the SEM, TEM and HRTEM. The SEM indicates that the FeS₂-450/rGO-10% is composed of FeS₂-450 nanoparticles and rGO nanosheets and FeS₂-450 nanoparticles are relatively uniformly dispersed on the surface of rGO nanosheets by zoom-in SEM image, Fig. 4a-c. The elemental mapping images reveals Fe and S elements are homogeneously distributed, while the elemental mapping image of exposed trace C further confirms that rGO has been surrounded by the FeS₂, which is conducive to accelerate electron transfer and improve catalytic activity, Fig. 4d. TEM images of FeS₂-450/rGO-10% verified that the catalyst is composed of nanoparticle and nanosheets, Fig. 4e-f. Moreover, the FeS₂-450 in FeS₂-450/rGO-10% has a clear surface and the lattice fringes with plane distances of 0.280 nm are assigned to (2 0 0) interplanar spacing of FeS₂, Fig. 4g-h.

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Fig. 4

(a-c) SEM images, (d) EDX surface mapping scans, (e, f) TEM and (g, h) HRTEM images of the prepared FeS₂-450/rGO-10% sample.

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The OER activity of the FeS₂-450/rGO-5%, FeS₂-450/rGO-10% and FeS₂-450/rGO-15% catalysts were also evaluated by LSV using 1.0 M KOH electrolyte and the corresponding overpotentials were displayed in Fig. 5a, b, with Ni foam, RuO₂ and FeS₂-450 as the contrast. Remarkably, the overpotentials of FeS₂-450/rGO-10% at current density of 10 mA/cm², 50 mA/cm², and 100 mA/cm² are 140 mV, 310 mV and 370 mV, respectively, significantly lower than the commercial RuO₂, FeS₂-450 and previous Fe/S-based catalysts toward OER in Table 1, indicating the outstanding OER performance after introducing highly conductive rGO. Compared with FeS₂-450 (73 mV/dec), FeS₂-450/rGO-10% showed a lower Tafel slope (71 mV/dec), demonstrating that the rGO accelerate the reaction kinetics, Fig. 5c. The The order of R_ct values is FeS₂-450/rGO-10% < FeS₂-450/rGO-15% < FeS₂-450 < FeS₂-450/rGO-5% < RuO₂ < Ni foam, indicating that FeS₂-450/rGO-10% has the minimum charge transfer resistance for OER, Fig. 5d. Moreover, the CV curves of Ni-foam, FeS₂-450, FeS₂-450/rGO-5%, FeS₂-450/rGO-10% and FeS₂-450/rGO-15% catalysts in the non-Faraday region at different scan rates (20 mVs⁻¹, 40 mVs⁻¹, 60 mVs⁻¹, 80 mVs⁻¹ and 100 mVs⁻¹) were presented in Fig. S3. FeS₂-450/rGO-10% achieved the maximum C_dl value of 1.71 mF cm⁻², which could be assigned to enhanced electron transport between the active sites and the electrolyte owing to the importing of rGO, Fig. 5e. The i-t curve in Fig. 5f shows that FeS₂-450/rGO-10% catalyst kept OER stability within 5 h without an appreciable current attenuation and the current density is higher than that of FeS₂-450 in Fig. 3f.

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Fig. 5

OER measurement. (a) LSV, (b) the overpotentials to achieve current densities of 10, 50, and 100 mA cm⁻², (c) Tafel plots, (d) Nyquist plots (e) electrochemical double-layer capacitance (C_dl) of the prepared Ni foam, FeS₂-450/NF, FeS₂-450/rGO-5%/NF, FeS₂-450/rGO-10%/NF and FeS₂-450/rGO-15%/NF and (f) I-t curve of FeS₂-450/rGO-10% under a constant potential of 1.527 V vs. RHE.

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As discussed above, the FeS₂-450/rGO-10% presents superior electrocatalytic performance toward OER. Meanwhile, an electrolyzer in a two-electrode FeS₂-450/rGO-10%/NF//Pt/C/NF was assembled in 1 M KOH for the overall water splitting, while the RuO₂/NF//Pt/C/NF and Ni foam//Ni foam system as the comparison. As expected in Fig. 6a and b, the voltage at 10 mA cm⁻² for the FeS₂-450/rGO-10%/NF//Pt/C/NF system is 1.52 V, significantly better than those of RuO₂/NF//Pt/C/NF (1.55 V) and Ni foam//Ni foam (1.86 V) systems, indicating that the FeS₂-450/rGO-10%/NF//Pt/C/NF system possess higher electrocatalytic activity toward overall water splitting.

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Fig. 6

(a) LSV curves of FeS₂-450/rGO-10%/NF//Pt/C/NF, RuO₂/NF//Pt/C/NF and Ni foam//Ni foam and (b) the corresponding potential at current densities of 10 mA cm⁻² under a constant potential of 1.6 V.

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3.3 Characterization after OER

To get insight to understand the OER catalytic activity and stability, a systematic investigation of the catalyst after OER was conducted. The FeS₂-450/rGO-10% catalyst after stability test maintain nanoparticles and nanosheets morphology, Fig. 7a-e, demonstrating its high stability. Moreover, the elemental mapping of catalysts after OER in Fig. 7c indicates the uniform distribution of Fe, S, C, and O elements. It is worth noting that the clear surface of catalyst disappeared considerably, although lattice fringes with plane distances of 0.280 nm is also assigned to (2 0 0) interplanar spacing of FeS₂. At the same time, an amorphous layer with some defects appeared, which may be associated with Fe oxo/hydroxide species or S defects, implying the surface reconstruction of the catalyst.

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Fig. 7

(a, b) SEM images, (c) EDX surface mapping scans, (d, e) TEM and (f-h) HRTEM images of the prepared FeS₂-450/rGO-10% after OER.

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To figure out the structure and electronic structure of the catalyst before and after stability test, we have performed a series of studies, Fig. 8. First, the chemical state of the catalyst before and after the reaction was investigated through XPS. The full spectrum in Fig. 8a suggests the coexistence of Fe 2p, S 2p, O 1s and C 1s elements. The binding energies at 707.5 eV and 720.3 eV assigned to the Fe(_II)-S 2p3/2 and Fe(_II)-S 2p1/2 while the other two peaks at 712.3 eV and 725.9 eV belong to the Fe³⁺(Chen et al., 2022), and the latter could be assigned to Fe-OOH bonds, Fig. 8b. The peaks intensity and area of Fe³⁺ after OER is dramatically stronger and larger compared to catalyst before the reaction, indicating the formation of Fe-oxo/hydroxide species during the catalytic process (Yu et al., 2017), Fig. 8b. The binding energies at 163.0 eV and 164. eV can ascribe to S 2p_3/2 and S 2p_1/2 in metal sulfide, while the two peaks at 168.4 eV and 169.7 eV belong to sulfate (Wang et al., 2017), resulting from particle oxidation of FeS₂, Fig. 8c. The peak at 529.8 eV belonging to O^2– (lattice O) after OER became more vigorous, which is consistent with the increased peak of Fe-OOH bonds, further confirming the existence of Fe-oxo/hydroxide species and surface reconstruction of the catalyst, Fig. 8d.

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Fig. 8

XPS spectra of (a) survey (b) Fe 2p (c) S 2p (d) O 2p (e) the percentages of Fe, S, C and O with the prepared FeS₂-450/rGO-10% before and after OER and (f) XRD with the prepared FeS₂-450/rGO-10% before and after OER.

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On the other hand, the comparison of EDX elemental analysis before and after the reaction clearly shows that the S relative content sharply reduced, in Fig. 8e. In contrast, the O content increased after OER, further verifying the surface reconstruction of catalysts, which is consistent with the above discussion in XPS. In addition, XRD was used to characterize the phase composites of the catalysts before and after OER. No significant difference was found in FeS₂-450/rGO-10% after OER compared with that of before OER, suggesting that the FeS₂ in FeS₂-450/rGO-10% catalyst kept its phase and the reconstruction of the catalyst only occurred on the surface, Fig. 8f.