Synthesis of Fe2O3 Nanorod and NiFe2O4 Nanoparticle Composites on Expired Cotton Fiber Cloth for Enhanced Hydrogen Evolution Reaction

The design of cheap, noble-metal-free, and efficient electrocatalysts for an enhanced hydrogen evolution reaction (HER) to produce hydrogen gas as an energy source from water splitting is an ideal approach. Herein, we report the synthesis of Fe2O3 nanorods–NiFe2O4 nanoparticles on cotton fiber cloth (Fe2O3-NiFe2O4/CF) at a low temperature as an efficient electrocatalyst for HERs. Among the as-prepared samples, the optimal Fe2O3-NiFe2O4/CF-3 electrocatalyst exhibits good HER performance with an overpotential of 127 mV at a current density of 10 mA cm−2, small Tafel slope of 44.9 mV dec−1, and good stability in 1 M KOH alkaline solution. The synergistic effect between Fe2O3 nanorods and NiFe2O4 nanoparticles of the heterojunction composite at the heterointerface is mainly responsible for improved HER performance. The CF is an effective substrate for the growth of the Fe2O3-NiFe2O4 nanocomposite and provides conductive channels for the active materials’ HER process.


Introduction
The use of fossil fuels for energy requirements not only declines its reserves but also causes environmental problems [1,2].The developments of green and renewable energy sources are alternative strategies to deal with these issues [3,4].Hydrogen is regarded as an efficient and promising energy source due to its high mass-specific energy density, environmental friendliness, and zero-carbon emission on combustion [5].Hydrogen produced from water splitting is one of the most suitable and sustainable processes [6,7].However, the electrochemical water splitting hydrogen evolution reaction (HER) proceeds at the cathode and requires some overpotential.A highly active and stable catalyst is required to reduce the overpotential for the HER [8][9][10].Pt-based materials are highly active electrocatalysts and only require small overpotentials for HERs [11,12].However, the high cost of precious metals and the scarcity of reserves are obstacles to large-scale applications.It is essential to develop cheaper, readily available, and efficient electrocatalyst materials for HERs [13][14][15].
Over the past few decades, many efficient precious-metal-free electrocatalytic materials have been designed and investigated for HERs with good performance and stability [16][17][18].The transition 3d metals, especially Fe, Co, and Ni-based materials, have attracted the attention of researchers because of their low price and excellent catalytic performance [19][20][21].For example, Mondal et al. reported a NiO electrocatalyst for HER performance and good stability in alkaline solution.However, single 3d metal-based materials require large overpotentials as compared to bimetallic-based composites.The 3d transition metal nanocomposites were reported as having enhanced electrocatalytic performance due to the synergetic effect at the interface of heterogeneous composites [22,23].For example, Shi et al. reported the NiFe oxides-based materials as bifunctional electrocatalysts for both HERs and OERs in an alkaline medium.However, most FeNi-based materials possess low electroconductivity and still do not reach the benchmark.
To further enhance the HER performance, the heterojunction catalytic components can be grown on the conductive substrate.Nickel foam or carbon materials such as carbon cloth, carbon fiber paper, carbon nanotubes, or graphene are used as substrates for the growth of NiFe catalyst.For example, Yan et al. [24].synthesized a layered porous Ni x Fe-S/NiFe 2 O 4 heterogeneous electrocatalyst on a three-dimensional carbon cloth by electrodeposition.The optimal sample of Ni 1/5 Fe-S/NiFe 2 O 4 showed the best HER performance due to the synergistic effect of the bimetallic heterostructure.Our group reported Ni 3 S 2 /Fe 2 O 3 /NC on nickel foam through a facile one-step thermal process as bifunctional catalysts for enhanced HERs and OERs.Therefore, heterojunction catalysts grown on substrates become outstanding electrodes for electrocatalytic HER performance [25][26][27].However, these substrates are relatively expensive.It is highly desirable to use cost-effective substrates for the growth of catalytic nanomaterials.The NiFe-based composites grown on cotton fiber (CF) cloth are rarely reported in electrolysis systems, especially in HERs.
Here, we report the synthesis of a Fe  CF catalyst shows excellent HER performance with an overpotential of only 127 mV at a current density of 10 mA cm −2 , a small Tafel slope of only 44.9 mV dec −1 , and good stability in 1M KOH solution.Its excellent performance is attributed to the synergistic effect between Fe 2 O 3 nanorods and NiFe 2 O 4 nanoparticles and the electron transfer between Ni and Fe species at the interface in composites, resulting in high active sites produced for HER performance.

Synthesis and Characterization
The Fe 2 O 3 -NiFe 2 O 4 /CF composite was prepared by a simple one-step process.During synthesis, the (NiCl   S2. The crystal structure of the prepared samples was characterized and analyzed using the XRD technique (Figure 1a).The XRD patterns of samples showed peaks concentrated at 22.65 • , 14.9 • , 16.6 • , and 34.4 • indexed to (200), (1-10), (110), and (004) of crystalline cellulose, respectively.Due to the high diffraction peaks of the CF substrate, the diffraction peaks of the other materials cannot be observed.The nanomaterial of the Fe 2 O 3 -NiFe 2 O 4 /CF-3 sample was collected from the surface of the substrate by sonication, and the XRD pattern was recorded (Figure 1b).The peaks around 24.1  , and 63.0 • are consistent with the (220), (311), (400), (422), (511), and (440) crystal planes of NiFe 2 O 4 (JCPDS NO.74-2081).These diffraction peaks showed that the material is a composite of Fe 2 O 3 and NiFe 2 O 4 .However, when Fe salts are used alone in the reaction mixture, the XRD patterns of the collected powder are matched with JCPDS NO.02-1035 of cubic Fe 3 O 4 , while when using Ni precursors in the reaction mixture, the XRD patterns are consistent with JCPDS NO.44-1159 of hexagonal NiO.All the samples showed broad peaks at 2θ around 24 • .This may be due to amorphous carbon materials formed at 150 • C on CF and peeling from the CF surface during sonication.matched with JCPDS NO.02-1035 of cubic Fe3O4, while when using Ni precursors in the reaction mixture, the XRD patterns are consistent with JCPDS NO.44-1159 of hexagonal NiO.All the samples showed broad peaks at 2θ around 24°.This may be due to amorphous carbon materials formed at 150 °C on CF and peeling from the CF surface during sonication.The morphology and microstructure of the samples were observed by SEM.The pure CF surface was cleaned, and no nanomaterial was observed on it (Figure 2a,b).Dipping the CF in FeCl2 solution, its surface was modified, and nanorods could be observed on the surface (Figure 2c,d).On the other hand, only nanoparticles could be seen on the CF (Figure 2e,f) when it was immersed in the NiCl2 solution.When aqueous solutions of both FeCl2 and NiCl2 were used for the synthesis, the nanoparticles were observed on the nanorods' surface (Figure 2g,h).Some nanoparticles were also observed in the surface on the CF.The morphology of the Fe2O3-NiFe2O4/CF-1 (Figure S1a), Fe2O3-NiFe2O4/CF-2 (Figure S1b), Fe2O3-NiFe2O4/CF-4 (Figure S1c), and Fe2O3-NiFe2O4/CF-5 (Figure S1d) samples was very similar when using different concentrations of FeCl2.However, it is obvious that the amount of nanorods gradually increased on the CF with the increasing concentration of FeCl2 in the reaction mixture.From the SEM observation of catalysts, it is suggested that the nanorods and nanoparticles are Fe2O3 and NiFe2O4, respectively.The atomic contents of Ni and Fe in the composite samples were confirmed by ICP-OES.The atomic ratio of Ni and Fe is 1:0.17,1:0.48,1:0.83,1:1.13, and 1:1.47 in Fe2O3-NiFe2O4/CF-1, Fe2O3-NiFe2O4/CF-2, Fe2O3-NiFe2O4/CF-3, Fe2O3-NiFe2O4/CF-4, and Fe2O3-NiFe2O4/CF-5, respectively.The slightly high atomic contents of Ni compared to the final composition of the Fe2O3-NiFe2O4 catalyst may be due the presence of Ni material contamination.
The Fe2O3-NiFe2O4 sample removed from the surface of the CF was characterized by TEM to further confirm morphology.As shown in Figure 3a, the nanorods are distributed in a light gray material-like structure.The diameters of the nanorods are tens of nanometers and the length is a few micrometers.The light-gray-like structure may be amorphous carbon materials and peeled from the CF during sonication.The nanoparticles loaded on the nanorods can be seen in Figure 3b.The HRTEM images showed that the lattice fringes of nanoparticles are 0.209 nm, corresponding to the (400) lattice plane of NiFe2O4 (Figure 3c).The lattice fringes with a spacing of 0.148 nm are consistent with the (214) lattice plane of Fe2O3 nanorods.The HRTEM results further indicate that the composites consisted of Fe2O3 nanorods and NiFe2O4 nanoparticles. Figure 3d displays the dark-field TEM images and corresponding elemental mapping images of Fe (Figure 3e), Ni (Figure 3f), and O (Figure 3g).It can be seen that the Fe and O signals are uniformly observed in the nanorods, and the Ni signal is uniformly observed on the nanorods due to the very small size The morphology and microstructure of the samples were observed by SEM.The pure CF surface was cleaned, and no nanomaterial was observed on it (Figure 2a,b).Dipping the CF in FeCl 2 solution, its surface was modified, and nanorods could be observed on the surface (Figure 2c,d).On the other hand, only nanoparticles could be seen on the CF (Figure 2e,f) when it was immersed in the NiCl 2 solution.When aqueous solutions of both FeCl 2 and NiCl 2 were used for the synthesis, the nanoparticles were observed on the nanorods' surface (Figure 2g,h).Some nanoparticles were also observed in the surface on the CF.The morphology of the Fe 2 O 3 -NiFe 2 O 4 /CF-1 (Figure S1a), Fe 2 O 3 -NiFe 2 O 4 /CF-2 (Figure S1b), Fe 2 O 3 -NiFe 2 O 4 /CF-4 (Figure S1c), and Fe 2 O 3 -NiFe 2 O 4 /CF-5 (Figure S1d) samples was very similar when using different concentrations of FeCl 2 .However, it is obvious that the amount of nanorods gradually increased on the CF with the increasing concentration of FeCl 2 in the reaction mixture.From the SEM observation of catalysts, it is suggested that the nanorods and nanoparticles are Fe   The Fe 2 O 3 -NiFe 2 O 4 sample removed from the surface of the CF was characterized by TEM to further confirm morphology.As shown in Figure 3a, the nanorods are distributed in a light gray material-like structure.The diameters of the nanorods are tens of nanometers and the length is a few micrometers.The light-gray-like structure may be amorphous carbon materials and peeled from the CF during sonication.The nanoparticles loaded on the nanorods can be seen in Figure 3b.The HRTEM images showed that the lattice fringes of nanoparticles are 0.209 nm, corresponding to the (400) lattice plane of NiFe 2 O 4 (Figure 3c).The lattice fringes with a spacing of 0.148 nm are consistent with the (214) lattice plane of Fe 2 O 3 nanorods.The HRTEM results further indicate that the composites consisted of Fe 2 O 3 nanorods and NiFe 2 O 4 nanoparticles.Figure 3d displays the dark-field TEM images and corresponding elemental mapping images of Fe (Figure 3e), Ni (Figure 3f), and O (Figure 3g).It can be seen that the Fe and O signals are uniformly observed in the nanorods, and the Ni signal is uniformly observed on the nanorods due to the very small size of the nanoparticles, which further confirms that the composite material is composed of Fe 2 O 3 nanorods and NiFe 2 O 4 nanoparticles.
of the nanoparticles, which further confirms that the composite material is composed of Fe2O3 nanorods and NiFe2O4 nanoparticles.4b).The binding energy peaks at 710.9 and 714.2 eV of the Fe 2p 3/2 band suggested the existence of Fe 2+ and Fe 3+ oxidation states, respectively [28,29].The binding energy peaks 723.9 and 727.1 eV were also associated with oxidation states of Fe 2+ and Fe 3+ of the Fe 2p 1/2 band, respectively [30].The peaks at 718.8 and 727.1 eV can be assigned to satellites of Fe 2p [31].The high-resolution XPS spectrum in the Fe 2p region of Fe 3 O 4 /CF showed binding energy peaks at 714.1 and 710.8 eV for the Fe 2p 3/2 and 726.9 and 723.showed peaks of the Fe, Ni, C, and O elements, compared with the survey XPS spectrum of Fe3O4/CF and NiO/CF (Figure 4a).The high-resolution XPS spectrum in the Fe 2p region of the Fe2O3-NiFe2O4/CF-3 samples can be deconvoluted into different peaks (Figure 4b).The binding energy peaks at 710.9 and 714.2 eV of the Fe 2p3/2 band suggested the existence of Fe 2+ and Fe 3+ oxidation states, respectively [28,29].The binding energy peaks 723.9 and 727.1 eV were also associated with oxidation states of Fe 2+ and Fe 3+ of the Fe 2p1/2 band, respectively [30].The peaks at 718.8 and 727.1 eV can be assigned to satellites of Fe 2p [31].The high-resolution XPS spectrum in the Fe 2p region of Fe3O4/CF showed binding energy peaks at 714.1 and 710.8 eV for the Fe 2p3/2 and 726.9 and 723.7 eV for the Fe 2p1/2 band.The Fe 2p3/2 and Fe 2p1/2 bands of Fe3O4/CF also existed in the Fe 2+ and Fe 3+ oxidation states.The satellites peaks of Fe 2p show peaks at binding energies of 718.7 and 727 eV.The binding energy of the Fe 2p3/2 and Fe 2p1/2 bands slightly shifted towards the lower energy of Fe3O4/CF compared to Fe2O3-NiFe2O4/CF-3.This indicates that there is interaction and electron transfer between Ni and Fe species in composites.The XPS spectra of the Ni 2p region of Fe2O3-NiFe2O4/CF-3 and NiO is shown in Figure 4c.The peak centered on 854.7 eV is related to Ni 2p3/2 of the Ni 2+ valence states [32].The peaks at a binding energy of 860.2 eV were the satellite of Ni 2p.There was no obvious change in the Ni 2p spectra of the Fe2O3-NiFe2O4/CF-3 and NiO samples.As shown in Figure 4d, the O 1s spectrum can be attributed to O2 2-at 529.4 eV, representing the M-O bond.The peak of 531.9 eV may be due to the absorption of oxygen or water molecules, and 534.9 eV indicates the O-O bond [33][34][35].

Electrocatalytic HER Performance
The HER performance of as-prepared samples and Pt/C catalyst was studied in 1.0 M KOH solution using a three-electrode setup.The LSV curves of the catalysts are shown

Electrocatalytic HER Performance
The HER performance of as-prepared samples and Pt/C catalyst was studied in 1.0 M KOH solution using a three-electrode setup.The LSV curves of the catalysts are shown in Figure 5a.The commercial Pt/C catalyst has the best HER performance of all the tested samples, with an overpotential of 38 mV at a current density of 10 mA cm −2 .Among the as-prepared samples, Fe 2 O 3 -NiFe 2 O 4 /CF-3 displayed good HER performance and needed an overpotential of 127 mV to reach a current density of 10 mA cm −2 (Figure 5b).The Fe 2 O 3 -NiFe 2 O 4 /CF-3 catalyst exhibited comparable or better performance in alkaline solutions than the previously reported NiFe 2 O 4 catalysts (Table S3).The Furthermore, the ECAS of these samples was assessed by electrochemical doublelayer capacitors (Cdl).The Cdl values are positively correlated with ECAS and are therefore commonly used to describe ECAS. Figure S2a-i showed CV curves of different samples with different scan rates and Cdl values were calculated according to these CV curves.The Cdl value (Figure 6b) decreased in the order of Fe2O3-NiFe2O4/CF-3 (1.281 mF cm −2 ) > Fe2O3-NiFe2O4/CF-2 (0.818 mF cm −2 ) > Fe2O3-NiFe2O4/CF-4 (0.775 mF cm −2 ) > Fe2O3-NiFe2O4/CF-1 (0.773 mF cm −2 ) > Fe2O3-NiFe2O4/CF-5 (0.751 mF cm −2 ).(Figure 6a) > Fe3O4/CF (0.623 mF cm −2 ), NiO/CF (0.582mF cm −2 ) > CF (0.464 mF cm −2 ) > CF (150 °C) (0.442 mF cm −2 ).The Fe2O3-NiFe2O4/CF-3 sample has the highest Cdl value and this indicates a high number of active sites for enhanced HER performance.
Electrochemical impedance spectroscopy (EIS) of the prepared sample was measured in order to determine electron transfer at the electrode/electrolyte interface, as shown in Figure 6b.The Nyquist plots were fitted (illustrated inset Figure 6b) in the equivalent circuit.The charge transfer resistance (Rct) of the prepared samples was revealed.A small Tafel slope is conducive to practical application since it will lead to a faster increment of the HER rate with low overpotential.According to classical theory and recent reports, in alkaline/neutral conditions, the Volmer and Heyrovsky (Equations ( 1) and ( 2)) reactions show Tafel slope values of 120 and 40 mV dec −1 , respectively.In comparison, the Tafel slope value of the Tafel reaction is about 30 mV dec −1 and remains the same for all pH values [36,37].
H 2 O + e − + M → M-H ads + OH − (Volmer) H 2 O + e − + M-H ads → H 2 + OH − + M (Heyrovsky) M-H ads + M-H ads →H 2 + M (Tafel) where M is a catalytic active material and H ads represents an adsorbed hydrogen on the surface of the electrocatalyst.The molecular hydrogen produces either combinations of Volmer-Heyrovsky reactions or Volmer-Tafel reactions in the HER process.According to the Tafel slope values, the Tafel slope value of our Fe 2 O 3 -NiFe 2 O 4 /CF-3 composite was 44.9 mV dec −1 and possibly follows the Volmer-Heyrovsky reaction pathway for the HER process and rate determination step [38,39].The other Fe 2 O 3 -NiFe 2 O 4 /CF composites' values were from 55.8.2 to 83.7 mV dec −1 and indicated that the composites follow the same reaction pathway during the HER process.Furthermore, the ECAS of these samples was assessed by electrochemical double-layer capacitors (C dl ).The C dl values are positively correlated with ECAS and are therefore commonly used to describe ECAS. Figure S2a-i 20.69, 17.8, 13.91, 19.11, 25.73, 25.98, 24.41, 29.18, and 37.43 Ω, respectively.Among all samples, the Fe 2 O 3 -NiFe 2 O 4 /CF-3 sample exhibited lower R ct values than other materials and this indicates more efficient charge transfer at the electrode/electrolyte interface for improved catalytic performance.
The stability of the electrocatalyst is an important criterion for the HER.The stability of the Fe 2 O 3 -NiFe 2 O 4 /CF-3 composite catalyst was confirmed by CV cycle and amperometry (i-t) tests.The LSV curves initially and after 1000-8000 CV cycles were measured, as shown in Figure S3.There is a slight reduction in overpotential at a current density of 10 mV cm −2 as the number of cycles increased (Figure 6c).This suggests that Fe 2 O 3 -NiFe 2 O 4 /CF-3 exhibited good stability under alkaline conditions.The nanorods can be observed on surface of the CF in the SEM images after 8000 CV cycles (Figure S4a).This indicates that the morphology of the nanorods is not obviously changed.The TEM image confirmed that nanoparticles were on the surface of nanorods (Figure S4b).The i-t test was measured with 24 h of continuous operation (Figure 6d).The i-t test showed that 96.1% of the current density was maintained after 24 h.This further confirmed the good stability of the sample.
observed on surface of the CF in the SEM images after 8000 CV cycles (Figure S4a).This indicates that the morphology of the nanorods is not obviously changed.The TEM image confirmed that nanoparticles were on the surface of nanorods (Figure S4b).The i-t test was measured with 24 h of continuous operation (Figure 6d).The i-t test showed that 96.1% of the current density was maintained after 24 h.This further confirmed the good stability of the sample.

Materials and Chemical Reagents
Nickel chloride hexahydrate (NiCl2•6H2O) and potassium hydroxide (KOH) were purchased from McLean Biotechnology Limited, Shanghai, China.Ferrous chloride tetrahydrate (FeCl2•4H2O) and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.Deionized water was used in all experimental work.An expired cotton shirt of Uniqlo brand was used.

Preparation of Fe2O3-NiFe2O4/CF Composites
CF was cut into pieces of 1 × 3 cm 2 from a 100% cotton expired commercial T-shirt.The CF pieces were cleaned by washing with a deionized water/ethanol mixture under sonication for 20 min and then dried at 150 °C for 12 h in a nitrogen atmosphere in order to enhance its conductivity.An amount of 0.5 g of FeCl2•4H2O and 0.6 g of NiCl2•6H2O were dissolved in 150 mL deionized water and dried, and the CF was suspended in it.The    S1).

Figure 1 .
Figure 1.(a) XRD pattern of as-prepared samples and CF.(b) XRD pattern of Fe2O3-NiFe2O4/CF-3, Fe3O4/CF, and NiO/CF samples' powder removed from the surface of the CF.

Figure 1 .
Figure 1.(a) XRD pattern of as-prepared samples and CF.(b) XRD pattern of Fe 2 O 3 -NiFe 2 O 4 /CF-3, Fe 3 O 4 /CF, and NiO/CF samples' powder removed from the surface of the CF.

Figure 3 .
Figure 3. (a,b) TEM and (c) HRTEM images of Fe2O3-NiFe2O4/CF-3 sample.(d) Dark-field TEM image and element mapping.(e) Fe, (f) Ni, and (g) O images of Fe2O3-NiFe2O4/CF-3 sample.XPS analysis of the sample was performed to determine its valence state and elemental composition.The survey XPS spectrum of the Fe2O3-NiFe2O4/CF-3 samples

Figure 3 .
Figure 3. (a,b) TEM and (c) HRTEM images of Fe 2 O 3 -NiFe 2 O 4 /CF-3 sample.(d) Dark-field TEM image and element mapping.(e) Fe, (f) Ni, and (g) O images of Fe 2 O 3 -NiFe 2 O 4 /CF-3 sample.XPS analysis of the sample was performed to determine its valence state and elemental composition.The survey XPS spectrum of the Fe 2 O 3 -NiFe 2 O 4 /CF-3 samples showed peaks of the Fe, Ni, C, and O elements, compared with the survey XPS spectrum of Fe 3 O 4 /CF and NiO/CF (Figure 4a).The high-resolution XPS spectrum in the Fe 2p region of the Fe 2 O 3 -NiFe 2 O 4 /CF-3 samples can be deconvoluted into different peaks (Figure4b).The binding energy peaks at 710.9 and 714.2 eV of the Fe 2p 3/2 band suggested the existence of Fe 2+ and Fe 3+ oxidation states, respectively[28,29].The binding energy peaks 723.9 and 727.1 eV were also associated with oxidation states of Fe 2+ and Fe 3+ of the Fe 2p 1/2 band, 7 eV for the Fe 2p 1/2 band.The Fe 2p 3/2 and Fe 2p 1/2 bands of Fe 3 O 4 /CF also existed in the Fe 2+ and Fe 3+ oxidation states.The satellites peaks of Fe 2p show peaks at binding energies of 718.7 and 727 eV.The binding energy of the Fe 2p 3/2 and Fe 2p 1/2 bands slightly shifted towards the lower energy of Fe 3 O 4 /CF compared to Fe 2 O 3 -NiFe 2 O 4 /CF-3.This indicates that there is interaction and electron transfer between Ni and Fe species in composites.The XPS spectra of the Ni 2p region of Fe 2 O 3 -NiFe 2 O 4 /CF-3 and NiO is shown in Figure 4c.The peak centered on 854.7 eV is related to Ni 2p 3/2 of the Ni 2+ valence states [32].The peaks at a binding energy of 860.2 eV were the satellite of Ni 2p.There was no obvious change in the Ni 2p spectra of the Fe 2 O 3 -NiFe 2 O 4 /CF-3 and NiO samples.As shown in Figure 4d, the O 1s spectrum can be attributed to O 2 2− at 529.4 eV, representing the M-O bond.The peak of 531.9 eV may be due to the absorption of oxygen or water molecules, and 534.9 eV indicates the O-O bond [33-35].

12 Figure 5 .
Figure 5. (a) LSV curves of as-prepared samples, Pt/C, CF, and CF heated at 150 °C (CF 150 °C).(b) Comparison of overpotentials at a current density of 10 mA cm 2 of different samples.(c) The Tafel slopes and (d) comparison of Tafel slopes of different samples.

Figure 6 .
Figure 6.(a) Cdl values of the different samples.(b) Nyquist plot of samples and inset equivalent circuit fitting experimental data.(c) Overpotential (V vs. RHE) at constant current density of 10 mA cm -2 of Fe2O3-NiFe2O4/CF-3 sample before and after 1000-8000 CV cycles and (d) i-t test of Fe2O3-NiFe2O4/CF-3 composite.

Figure 6 .
Figure 6.(a) Cdl values of the different samples.(b) Nyquist plot of samples and inset equivalent circuit fitting experimental data.(c) Overpotential (V vs. RHE) at constant current density of 10 mA cm −2 of Fe 2 O 3 -NiFe 2 O 4 /CF-3 sample before and after 1000-8000 CV cycles and (d) i-t test of Fe 2 O 3 -NiFe 2 O 4 /CF-3 composite.

3. Experimental Section 3 . 1 .
Materials and Chemical Reagents Nickel chloride hexahydrate (NiCl 2 •6H 2 O) and potassium hydroxide (KOH) were purchased from McLean Biotechnology Limited, Shanghai, China.Ferrous chloride tetrahydrate (FeCl 2 •4H 2 O) and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.Deionized water was used in all experimental work.An expired cotton shirt of Uniqlo brand was used.

3. 2 .
Preparation of Fe 2 O 3 -NiFe 2 O 4 /CF Composites CF was cut into pieces of 1 × 3 cm 2 from a 100% cotton expired commercial T-shirt.The CF pieces were cleaned by washing with a deionized water/ethanol mixture under sonication for 20 min and then dried at 150 • C for 12 h in a nitrogen atmosphere in order to enhance its conductivity.An amount of 0.5 g of FeCl 2 •4H 2 O and 0.6 g of NiCl 2 •6H 2 O were dissolved in 150 mL deionized water and dried, and the CF was suspended in it.The solution was heated at 40 • C under magnetic stirring in an oil bath.After 2 h, the CF was removed, cleaned several times with deionized water and ethanol, and then placed in a vacuum drying oven at 60 • C for 12 h, to obtain the Fe 2 O 3 -NiFe 2 O 4 /CF catalyst.By changing the addition amounts of FeCl 2 •4H 2 O salt, the influence on the catalytic performance was explored.The catalyst was named Fe 2 O 3 -NiFe 2 O 4 /CF-X (where X indicates the weight of FeCl 2 •4H 2 O in grams, Table 2 O 3 nanorod/NiFe 2 O 4 nanoparticle composite on the cotton fiber cloth of expired shirts (Fe 2 O 3 -NiFe 2 O 4 /CF) for enhanced HER performance.The composite consists of NiFe 2 O 4 nanoparticles loaded on Fe 2 O 3 nanorods to form a heterointerface catalyst.The optimized Fe 2 O 3 -NiFe 2 O 4 2 •6H 2 O) and (FeCl 2 •4H 2 O) precursors were dissolved into Fe 2+ and Ni 2+ ion aqueous mediums.Fe 2+ reacted with oxygen dissolved in the water to grow Fe 2 O 3 nanorods on the CF.Simultaneously, Ni 2+ and some Fe 2+ also reacted with oxygen and formed NiFe 2 O 4 nanoparticles.Most of these NiFe 2 O 4 nanoparticles were loaded on Fe 2 O 3 nanorods.Finally, the Fe 2 O 3 -NiFe 2 O 4 /CF heterointerface composite material was formed on the CF.The amounts of different catalysts deposited on the CF during synthesis are listed in Table