Nanostructured Iron Sulfide/N, S Dual-Doped Carbon Nanotube-Graphene Composites as Efficient Electrocatalysts for Oxygen Reduction Reaction

Nanostructured FeS dispersed onto N, S dual-doped carbon nanotube–graphene composite support (FeS/N,S:CNT–GR) was prepared by a simple synthetic method. Annealing an ethanol slurry of Fe precursor, thiourea, carbon nanotube, and graphene oxide at 973 K under N2 atmosphere and subsequent acid treatment produced FeS nanoparticles distributed onto the N, S-doped carbon nanotube–graphene support. The synthesized FeS/N,S:CNT–GR catalyst exhibited significantly enhanced electrochemical performance in the oxygen reduction reaction (ORR) compared with bare FeS, FeS/N,S:GR, and FeS/N,S:CNT with a small half-wave potential (0.827 V) in an alkaline electrolyte. The improved ORR performance, comparable to that of commercial Pt/C, could be attributed to synergy between the small FeS nanoparticles with a high activity and the N, S-doped carbon nanotube–graphene composite support providing high electrical conductivity, large surface area, and additional active sites.


Introduction
The oxygen reduction reaction (ORR) is crucial for electrochemical energy conversion and storage devices including fuel cells and lithium-air batteries [1,2]. The fuel cell is a promising energy conversion device due to its high energy density, rapid start-up, and zero emissions [3][4][5]. ORR typically occurs at the cathode of fuel cells heavily loaded with platinum [6,7], but its high cost, low abundance, and instability have made the fuel cell a highly expensive device [8]. Thus, it is essential to develop non-precious metal-based ORR catalysts offering high activity and stability for more rapid dissemination of fuel cells [9].
A variety of materials have been investigated as alternative non-Pt catalysts for ORR, including oxides [10], nitrides [11][12][13], sulfides [14,15], carbides [13,16] of transition metals, metal-nitrogen-carbon catalysts (MNC) [17][18][19], and metal-free catalysts [20,21]. Various transition metal sulfides (TMS) of Mo, Fe, Co, Ni, and V have been explored as ORR catalysts due to their earth-abundance, low cost, and considerable activity [14]. Nanostructured TMS have also been considered to further improve the ORR activity, due to increased number of active sites compared to their bulk counterparts. Another way to improve ORR activity is to combine TMS with carbon supports including carbon nanotube (CNT), graphene (GR), and amorphous carbon [15]. Carbon supports can provide high electrical conductivity and large surface area to disperse TMS, enhancing ORR activity [15,22,23]. They can also act as a growth mediator, reducing TMS particle aggregation. Heteroatom (N and S) doping into the carbon supports further increases active sites and hence ORR activity Graphene oxide (GO) was synthesized by Hummer's method [27] and commercial CNT (CMP-301F, Hanwha Nanotech, Incheon, Korea) was acid treated with 90% nitric acid and 99% sulfuric acid solution, 1:3 v/v ratio, at 393 K for 3 h to eliminate residual metal species before use [28]. Typically, GO (116 mg) and CNT (116 mg) were dispersed in 5 mL ethanol. FeCl 2 ·4H 2 O (1 g) was dissolved in 5 mL ethanol and added to the CNT-GO solution. Thiourea (1.5 g) was added to the solution and the solution was stirred for 1 h. The resulting solution was dried in an oven at 373 K to evaporate ethanol and annealed at 973 K (at a heating rate of 10 • C min −1 ) for 3 h under N 2 atmosphere. The sample was stirred in 0.5 M sulfuric acid solution for 30 min to obtain pure, crystalline phase FeS loaded onto the N,S:CNT-GR support. FeS/CNT and FeS/GR were prepared following an identical method except only CNT or GO was included during the synthesis. Bare FeS was synthesized identically without any carbon support. Nominal weight content of FeS in the supported FeS catalysts was fixed to 50%, and measured weight contents (by inductively coupled plasma optical emission spectroscopy, ICP) were 52, 50, and 54 wt% for FeS/N,S:CNT-GR, FeS/N,S:CNT, and FeS/N,S:GR, respectively.

Electrochemical Characterization
Electrochemical characteristics were measured in a conventional three electrode cell with N 2 or O 2 saturated 0.1 M KOH solution, using a potentiostat (Ivium technologies, EIN, The Netherlands) equipped with a rotating disk electrode (Pine research, Durham, NC, USA). Ag/AgCl (3 M NaCl) electrodes and Pt wire were used as reference and counter electrodes, respectively. All potentials were referred to the reversible hydrogen electrode (RHE) without specification. Working electrodes were prepared by dispersing 20 mg catalyst in 2 mL water/ethanol solvent (1:1 v/v) and 40 µL 5% Nafion solution, and then 20 µL catalyst slurry was pipetted onto a glassy carbon electrode (0.19635 cm 2 ). Linear sweep voltammetry (LSV) measurements were performed at 5 mV/s scan rate of 1600 rpm, measured after 20 cyclic voltammetry tests from 0 to 1.2 V to stabilize the current. Durability was investigated by subjecting the samples to 6000 cycles of repeated potential ramp from 1.2 to 0 V.

Preparation and Physical Chracterizaton of FeS Catalysts
Scheme 1 shows the schematic fabrication procedure for bare and carbon-supported FeS catalysts. For FeS/N,S:CNT-GR, iron precursor reacts with ethanol to form Fe-ethoxide and was then mixed with CNT-GO in ethanol solution. Fe-thiourea complex was generated on the CNT-GO support by adding thiourea, and a powder product was obtained by annealing the Fe-thiourea complex/CNT-GO at 973 K for 3 h under flowing N 2 . The annealing produced mixed FeS, Fe x C, and Fe x N crystalline phases ( Figure S1 of Supporting Information), but Fe x C and Fe x N phases disappeared after the acid treatment, leaving pure FeS phase due to better chemical stability of FeS under acid solution than Fe x C and Fe x N. Crystallization of FeS and reduction of GO to GR proceeded simultaneously during annealing. The N and S-doping into CNT-GR supports was also achieved using thiourea as a source of N and S. This synthetic procedure produced FeS nanoparticles with an average size of 24 nm dispersed on CNT-GR supports. Other carbon-supported FeS catalysts, FeS/N,S:CNT, and FeS/N,S:GR, were prepared following the same synthetic method employed with either CNT or GO, exclusively. Bare FeS was also similarly prepared without carbon support. The CNT-GR hybrid support can provide a large surface area for enhanced contact between FeS nanoparticles and electrolyte [12,29].

Electrochemical Characterization
Electrochemical characteristics were measured in a conventional three electrode cell with N2 or O2 saturated 0.1 M KOH solution, using a potentiostat (Ivium technologies, EIN, Netherlands) equipped with a rotating disk electrode (Pine research, Durham, NC, USA). Ag/AgCl (3 M NaCl) electrodes and Pt wire were used as reference and counter electrodes, respectively. All potentials were referred to the reversible hydrogen electrode (RHE) without specification. Working electrodes were prepared by dispersing 20 mg catalyst in 2 mL water/ethanol solvent (1:1 v/v) and 40 µL 5% Nafion solution, and then 20 µL catalyst slurry was pipetted onto a glassy carbon electrode (0.19635 cm 2 ). Linear sweep voltammetry (LSV) measurements were performed at 5 mV/s scan rate of 1600 rpm, measured after 20 cyclic voltammetry tests from 0 to 1.2 V to stabilize the current. Durability was investigated by subjecting the samples to 6000 cycles of repeated potential ramp from 1.2 to 0 V.

Preparation and Physical Chracterizaton of FeS Catalysts
Scheme 1 shows the schematic fabrication procedure for bare and carbon-supported FeS catalysts. For FeS/N,S:CNT-GR, iron precursor reacts with ethanol to form Fe-ethoxide and was then mixed with CNT-GO in ethanol solution. Fe-thiourea complex was generated on the CNT-GO support by adding thiourea, and a powder product was obtained by annealing the Fe-thiourea complex/CNT-GO at 973 K for 3 h under flowing N2. The annealing produced mixed FeS, FexC, and FexN crystalline phases ( Figure S1 of Supporting Information), but FexC and FexN phases disappeared after the acid treatment, leaving pure FeS phase due to better chemical stability of FeS under acid solution than FexC and FexN. Crystallization of FeS and reduction of GO to GR proceeded simultaneously during annealing. The N and S-doping into CNT-GR supports was also achieved using thiourea as a source of N and S. This synthetic procedure produced FeS nanoparticles with an average size of 24 nm dispersed on CNT-GR supports. Other carbon-supported FeS catalysts, FeS/N,S:CNT, and FeS/N,S:GR, were prepared following the same synthetic method employed with either CNT or GO, exclusively. Bare FeS was also similarly prepared without carbon support. The CNT-GR hybrid support can provide a large surface area for enhanced contact between FeS nanoparticles and electrolyte [12,29]. Single carbon support CNT or GO tends to bundle or stack together by itself, significantly limiting the carbon surface to form active sites and thus lowering electrocatalytic activity [30][31][32]. In contrast, the CNT-GR composite support created a three-dimensional open structure, avoiding bundling and stacking [29]. The CNT-GR composite also provides a good electron conducting pathway for the FeS nanoparticles. N and S-doping to carbon supports can enhance the ORR performance by redistributing spin and charge  [25,26]. Thus, N,C:CNT-GR could be an effective catalyst support to enhance ORR activity, combining high conductivity and large surface area. Figure 1 shows typical TEM images for prepared catalysts. The TEM image of bare FeS in Figure 1a shows a lattice spacing of 0.299 nm corresponding to the FeS (100) plane. Bare FeS particles were aggregated forming large clusters with approximately 700 nm diameter. In contrast, substantially reduced particle aggregation occurs for carbon-supported FeS catalysts, as shown in Figure 1b-d, with much smaller FeS nanoparticles (20-30 nm) distributed on each carbon support. Metal precursors are attracted by oxygen-containing functional groups within the carbon supports (CNT and GO). Hence FeS particles grow selectively on carbon supports (CNT, GR, and CNT-GR). Strong coupling between FeS particles and carbon supports mitigates FeS nanoparticle aggregation, e.g., Figure 1b shows FeS nanoparticles (28 nm) anchored on CNT without severe aggregation. No free-standing particles occurred, indicating that the CNT support mediated FeS growth and suppressed particle aggregation. Figure 1c shows GR layers with a wrinkled paper-like morphology and FeS nanoparticles (36 nm) distributed on the GR layers. Figure 1d  Single carbon support CNT or GO tends to bundle or stack together by itself, significantly limiting the carbon surface to form active sites and thus lowering electrocatalytic activity [30][31][32]. In contrast, the CNT-GR composite support created a three-dimensional open structure, avoiding bundling and stacking [29]. The CNT-GR composite also provides a good electron conducting pathway for the FeS nanoparticles. N and S-doping to carbon supports can enhance the ORR performance by redistributing spin and charge densities [25,26]. Thus, N,C:CNT-GR could be an effective catalyst support to enhance ORR activity, combining high conductivity and large surface area. Figure 1 shows typical TEM images for prepared catalysts. The TEM image of bare FeS in Figure 1a shows a lattice spacing of 0.299 nm corresponding to the FeS (100) plane. Bare FeS particles were aggregated forming large clusters with approximately 700 nm diameter. In contrast, substantially reduced particle aggregation occurs for carbon-supported FeS catalysts, as shown in Figure 1b No free-standing particles occurred, indicating that the CNT support mediated FeS growth and suppressed particle aggregation. Figure 1c shows GR layers with a wrinkled paper-like morphology and FeS nanoparticles (36 nm) distributed on the GR layers. Figure  1d shows a mixed CNT and GR morphology with FeS nanoparticles (24 nm      peaks at 30°, 34 and 52° can be indexed to hexagonal FeS (JCPDS 03-065-3408). Carbon-supporte samples show broad peaks at 26°, originating from CNT or GR. As mentioned, imp peaks such as FexC or FexN were not present after acid treatment.  Figure 2b shows Raman spectra from the prepared catalysts. Intense peaks oc 1350 (D) and 1580 cm −1 (G) and the numbers indicate their intensity ratios, i.e., ID/IG [33]. The D peak is related to sp 2 ring disorder or defects, and the G peak to first scattering from sp 2 domains E2g mode. ID/IG measures the degree of disorder, whe creased ID/IG implies sp 2 carbon restoration and smaller sp 2 domains due to GO redu [34]. Thus, the increased ID/IG ratio in the FeS/GR (1.191) and the FeS/CNT-GR ( compared with of GO (0.937) verifies thermal reduction of GO to GR during the synt Chemical states of FeS/CNT-GR were investigated by X-ray photoelectron spe copy (XPS). Figure 2c shows high resolution Fe 2p XPS spectra for FeS/CNT-GR peaks centered around 710.1 and 723.5 eV are due to Fe 2+ 2p3/2 and Fe 2+ 2p1/2 of FeS, re tively. The peaks at 713.3 eV (Fe 3+ 2p3/2) and 727.3 eV (Fe 3+ 2p1/2) suggest partial oxid of the catalyst surface. [35,36]. Figure 2d shows N 1s spectra, with peaks at 398.5, 401.1, and 403.8 eV corresponding to pyridinic, pyrrolic, graphitic, and oxidized N cies, respectively [37]. Surface nitrogen content due to N-doping was 6.0 at% from th survey scan. N-doping to carbon improves ORR performance by enhancing electrica ductivity of carbon or increasing defect sites of ORR activity [38,39]. Figure S3a s high resolution S 2p XPS spectra for FeS/N,S:CNT-GR, which could be deconvolute five peaks. Peaks at 161.2 and 162.8eV are related to S 2− 2p3/2 and S 2− 2p1/2 in FeS, re tively; Peaks at 164.5 and 165.5 eV originate from polysulfide S in the carbon plan peak at 168.2 eV is attributed to sulphate (SO4 2− ) species due to acid treatment or p oxidation of sulfide upon air exposure [40,41]. In S 2p spectra of N, S-doped CN  Figure 2b shows Raman spectra from the prepared catalysts. Intense peaks occur at 1350 (D) and 1580 cm −1 (G) and the numbers indicate their intensity ratios, i.e., I D /I G ratios [33]. The D peak is related to sp 2 ring disorder or defects, and the G peak to first order scattering from sp 2 domains E 2g mode. I D /I G measures the degree of disorder, where increased I D /I G implies sp 2 carbon restoration and smaller sp 2 domains due to GO reduction [34]. Thus, the increased I D /I G ratio in the FeS/GR (1.191) and the FeS/CNT-GR (1.190) compared with of GO (0.937) verifies thermal reduction of GO to GR during the synthesis.
Chemical states of FeS/CNT-GR were investigated by X-ray photoelectron spectroscopy (XPS). Figure 2c shows high resolution Fe 2p XPS spectra for FeS/CNT-GR. The peaks centered around 710.1 and 723.5 eV are due to Fe 2+ 2p 3/2 and Fe 2+ 2p 1/2 of FeS, respectively. The peaks at 713.3 eV (Fe 3+ 2p 3/2 ) and 727.3 eV (Fe 3+ 2p 1/2 ) suggest partial oxidation of the catalyst surface. [35,36]. Figure 2d shows N 1s spectra, with peaks at 398.5, 399.6, 401.1, and 403.8 eV corresponding to pyridinic, pyrrolic, graphitic, and oxidized N species, respectively [37]. Surface nitrogen content due to N-doping was 6.0 at% from the XPS survey scan. N-doping to carbon improves ORR performance by enhancing electrical conductivity of carbon or increasing defect sites of ORR activity [38,39]. Figure S3a shows high resolution S 2p XPS spectra for FeS/N,S:CNT-GR, which could be deconvoluted into five peaks. Peaks at 161.2 and 162.8eV are related to S 2− 2p 3/2 and S 2− 2p 1/2 in FeS, respectively; Peaks at 164.5 and 165.5 eV originate from polysulfide S in the carbon plane; the peak at 168.2 eV is attributed to sulphate (SO 4 2− ) species due to acid treatment or partial oxidation of sulfide upon air exposure [40,41]. In S 2p spectra of N, S-doped CNT-GR made without Fe precursor ( Figure S3b), FeS-related peaks disappeared while the peaks attributed to polysulfide S in the carbon plane and sulphate species were maintained. The results indicate that the thiourea was the sulfur source for FeS crystallization and S-doping to the carbon supports. Figure S4 shows XPS spectra for FeS/N,S:CNT and FeS/N,S:GR.
They exhibit similar Fe 2p, N 1s, and S 2p peaks to FeS/N,S:CNT-GR, suggesting similar chemical states of FeS for all the carbon-supported catalysts.
Textural properties for the prepared catalysts were investigated by N 2 adsorptiondesorption isotherms, and compared with other TMS/carbon catalysts in Table S1 of Supporting Information. Figure 3a shows that carbon-supported FeS samples exhibited the type IV isotherm with a typical hysteresis of the isotherms, indicating the presence of mesopores, whereas bare FeS does not show clear hysteresis. Brunauer-Emmett-Teller (BET) surface area for bare FeS was 14 m 2 g −1 , whereas FeS/N,S:CNT-GR achieved significantly improved BET surface area of 191 m 2 g −1 after introducing the N,S:CNT-GR support. FeS/N,S:CNT and FeS/N,S:GR also showed increased BET surface areas of 174 and 137 m 2 g −1 , respectively. The larger surface area of FeS/N,S:CNT-GR compared with FeS/N,S:CNT and FeS/N,S:GR was due to a synergy between CNT and GR acting as spacers for each other, alleviating CNT's bundling and GR layers' stacking [12]. Pore size distribution was determined using the desorption isotherm following the Barrett-Joyner-Halenda method. Average pore size for all carbon-supported FeS catalysts was~4 nm (Figure 3b), and pore volume varied as FeS/N,S:CNT-GR (0.5568 cm 3 /g) > FeS/N,S:CNT (0.5067 cm 3 /g) > FeS/N,S:GR (0.4077 cm 3 /g). The large surface area with abundant mesopores will stabilize smaller FeS nanoparticles, leading to improved catalytic activity for ORR [42].
Materials 2021, 14, x FOR PEER REVIEW 6 of 10 made without Fe precursor ( Figure S3b), FeS-related peaks disappeared while the peaks attributed to polysulfide S in the carbon plane and sulphate species were maintained. The results indicate that the thiourea was the sulfur source for FeS crystallization and S-doping to the carbon supports. Figure S4 shows XPS spectra for FeS/N,S:CNT and FeS/N,S:GR. They exhibit similar Fe 2p, N 1s, and S 2p peaks to FeS/N,S:CNT-GR, suggesting similar chemical states of FeS for all the carbon-supported catalysts. Textural properties for the prepared catalysts were investigated by N2 adsorptiondesorption isotherms, and compared with other TMS/carbon catalysts in Table S1 of Supporting Information. Figure 3a shows that carbon-supported FeS samples exhibited the type IV isotherm with a typical hysteresis of the isotherms, indicating the presence of mesopores, whereas bare FeS does not show clear hysteresis. Brunauer-Emmett-Teller (BET) surface area for bare FeS was 14 m 2 g −1 , whereas FeS/N,S:CNT-GR achieved significantly improved BET surface area of 191 m 2 g −1 after introducing the N,S:CNT-GR support. FeS/N,S:CNT and FeS/N,S:GR also showed increased BET surface areas of 174 and 137 m 2 g −1 , respectively. The larger surface area of FeS/N,S:CNT-GR compared with FeS/N,S:CNT and FeS/N,S:GR was due to a synergy between CNT and GR acting as spacers for each other, alleviating CNT's bundling and GR layers' stacking [12]. Pore size distribution was determined using the desorption isotherm following the Barrett-Joyner-Halenda method. Average pore size for all carbon-supported FeS catalysts was ~4 nm (Figure 3b), and pore volume varied as FeS/N,S:CNT-GR (0.5568 cm 3 /g) > FeS/N,S:CNT (0.5067 cm 3 /g) > FeS/N,S:GR (0.4077 cm 3 /g). The large surface area with abundant mesopores will stabilize smaller FeS nanoparticles, leading to improved catalytic activity for ORR [42].    . Bare FeS showed poor ORR activity with an Eonset of 0.8 V and E 1/2 of 0.58 V. Thus, carbon supports enhanced ORR activity substantially, demonstrating their critical importance, providing high conductivity and large surface area for the loaded FeS nanoparticles. Besides, dual N, S-doping to the carbon supports further enhanced the activity due to changing the charge distribution in the carbon framework and improving electrical conductivity compared with undoped carbon supports [25,26,43,44]. These effects were maximized in FeS/N,S:CNT-GR, achieving a top performance compared to previously reported iron or other TMS-based catalysts (Table S2). electrical conductivity compared with undoped carbon supports [25,26,43,44]. These effects were maximized in FeS/N,S:CNT-GR, achieving a top performance compared to previously reported iron or other TMS-based catalysts (Table S2).  Figure 4b shows that the current density of FeS/N,S:CNT-GR increases with increasing rotation speed at the same potential due to enhanced oxygen diffusion on the electrode. Figure S5a shows Koutecky-Levich (K-L) plots for FeS/N,S:CNT-GR described as

Electrochmical Chracterizaton of FeS Catalysts
where J is measured current density; JD is diffusion-limited current density; JK is kinetic current density; ω is electrode rotation speed;    Figure 4b shows that the current density of FeS/N,S:CNT-GR increases with increasing rotation speed at the same potential due to enhanced oxygen diffusion on the electrode. Figure S5a shows Koutecky-Levich (K-L) plots for FeS/N,S:CNT-GR described as where J is measured current density; J D is diffusion-limited current density; J K is kinetic current density; ω is electrode rotation speed; is the K-L plot slope, where n is electron transfer number, F is Faraday's constant (96,486 C mol −1 ), C 0 is bulk concentration of oxygen in 0.1M KOH solution (1.2 × 10 −6 mol cm −3 ), D is diffusion coefficient of oxygen in 0.1M KOH (1.9 × 10 −5 cm 2 ·s −1 ), and υ is kinetic viscosity of oxygen in 0.1M KOH (1.0 × 10 −2 cm 2 s −1 ) [45]. The obtained electron transfer numbers are close to 4.0 ( Figure S5b), indicating the dominant four electron ORR catalytic pathway proceeds for the FeS/N,S:CNT-GR catalyst. Figure 4c shows LSV curves for FeS/N,S:CNT-GR after 6000 potential cycles between 1.2 and 0 V. Activity loss for FeS/N,S:CNT-GR was marginal with slightly decreased E 1/2 = 0.821 V from 0.827 V, whereas commercial Pt/C recorded a 30 mV decrease in E 1/2 . Similarly, the current loss for the FeS/N,S:CNT-GR was~6% after continuous operation for 10,000 s at 0.7 V (Figure 4d), whereas commercial Pt/C showed much faster current decay of 27%, indicating that our FeS/N,S:CNT has a high ORR activity comparable to Pt/C, and much better ORR stability than Pt/C. This excellent ORR activity and stability of the FeS/N,S:CNT-GR catalyst could arise from synergy between small FeS nanoparticles and the N, S dual-doped CNT-GR support. The N,S:CNT-GR support provides a high electrical conductivity and a large surface area, improving FeS activity and electrolyte contact to active sites (FeS). Indeed, in Figure S6, FeS/N,S:CNT-GR achieved considerably higher double layer capacitance C dl of 36.6 mF/cm 2 compared with FeS/N,S:GR, FeS/N,S:CNT, and bare FeS (C dl values of 10.5, 24.6, and 1.16 mF/cm 2 , respectively) [46]. The C dl value is proportional to contact area between electrode and electrolyte (or electrochemical surface area, ECSA), hence the N,S:CNT-GR support efficiently increases the contact area, alleviating CNT bundling and GR layers stacking [12,39]. Furthermore, simultaneous N and S-doping to CNT-GR was easily achieved by employing thiourea, improving the ORR performance by redistributing spin and charge densities [21,24]. The CNT-GR support also mediates FeS growth, reducing particle aggregation and further increasing FeS reaction sites. Figure S7 shows that the N, S dual-doped CNT-GR prepared by identical synthetic methods without Fe precursor has its own ORR activity, achieving E 1/2 of 0.762 V. Hence, combining it with FeS considerably improved ORR activity of the FeS/N,S:CNT-GR catalyst, indicating a synergy between FeS and N,S:CNT-GR. Thus, considering the facile synthetic method and high ORR performance, FeS/N,S:CNT-GR catalysts constitute a potential candidate to substitute commercial Pt/C catalyst.

Conclusions
This work successfully prepared a non-precious metal catalyst for ORR comprising FeS nanoparticles dispersed onto N, S dual-doped CNT-GR composite supports through a simple annealing and acid treatment, achieving simultaneous FeS formation and N, S dual-doping into CNT-GR. The synthesized FeS/N,S:CNT-GR catalyst exhibited the highest ORR performance among prepared FeS-based catalysts with a small E 1/2 value of 0.827 V, comparable to commercial Pt/C. Improved ORR performance was attributed to a synergy between the small FeS nanoparticles with high activity and N, S dual-doped CNT-GR support providing improved high electrical conductivity, large surface area, and its own ORR performance caused by modified electronic structure by the dual doping. Thus, N,S:CNT-GR composite support mediates FeS growth, reducing particle aggregation, and further increasing reaction sites. This FeS/N,S:CNT-GR catalyst offers a potential non-precious metal ORR catalyst of a high activity with a good stability.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/ma14092146/s1, Figure S1. XRD patterns of FeS/N,S:CNT-GR catalyst before and after acid treatment., Figure S2. EDS elemental mapping images of FeS/N,S:CNT-GR for C, Fe, S, and N., Figure S3. XPS S 2p spectra of (a) FeS/N,S:CNT-GR and (b) N,S:CNT-GR., Figure S4.  Table S1. Comparison of BET surface area, pore volume, and average pores size of the prepared catalysts with TMS-based electrocatalysts., Table S2