CuCo2S4@B,N-Doped Reduced Graphene Oxide Hybrid as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions.

In this report, a facile synthetic route is adopted for typically designing a hybrid electrocatalyst containing boron, nitrogen dual-doped reduced graphene oxide (B,N-rGO) and thiospinel CuCo2S4 (CuCo2S4@B,N-rGO). The electrocatalytic activity of the hybrid catalyst is tested with respect to oxygen evolution (OER) and oxygen reduction (ORR) reactions in alkali. Physicochemical characterizations confirm the unique formation of a reduced graphene oxide-non-noble-metal sulfide hybrid. Electrochemical evaluation by cyclic voltammetry (CV) and linear-sweep voltammetry (LSV) reveals that the CuCo2S4@B,N-rGO hybrid possesses enhanced ORR and OER activity compared to the B,N-rGO-free CuCo2S4 catalyst. The synthesized CuCo2S4@B,N-rGO hybrid demonstrates remarkable enhancement in catalytic performance with an improved onset potential (1.50 and 0.88 V) and low Tafel slope (112 and 73 mV dec-1) for both OER and ORR processes, respectively. In addition, the catalyst exhibits a diminutive potential difference (0.81 V) between the potential corresponding to the 10 mA cm-2 current density for OER and the half-wave potential for ORR. The superior catalytic activity and high durability of the hybrid material may be attributed to the synergistic effect arising from the metal sulfide and dual-doped reduced graphene oxide. The present study illuminates the possibility of using the dual-doped graphene oxide and metal sulfide hybrid as a competent bifunctional cathode catalyst for renewable energy application.

Introduction

The increasing energy demands, depleted energy resources, and environmental hazards that arose due to the combustion of fossil fuels compelled us to develop renewable and sustainable energy technology. Regenerative fuel cells,[1] water electrolyzers,[2,3] and metal–air batteries[4−6] are the most promising renewable and environmentally benign alternative energy technologies. However, oxygen evolution reaction (OER) on the anode and oxygen reduction reaction (ORR) on the cathode are crucial and challenging issues for water electrolyzers and fuel cells. Moreover, the efficiency of oxygen electrodes (cathode) plays a vital role in renewable energy technology. Nevertheless, the process typically suffers from the intrinsic sluggish kinetics of ORR and OER due to multiple electron-transfer processes.[6,7] To overcome the high activation energy of the ORR/OER processes, noble-metal-based catalysts remain the ideal candidate. However, the main hindrance for the large-scale usage of this renewable energy technology is the presently marketed precious noble-metal-based catalysts Pt for ORR and oxides of Ir and Ru for OER, and their lower stability in operating conditions.[8,9] Additionally, designing efficient bifunctional ORR/OER electrocatalysts essential to the regenerative operation is a challenging task because optimum ORR and OER performance is often shown by different metals and in different pH mediums.[10] Hence, sustainable development of nonprecious bifunctional oxygen catalysts having improved catalytic ability and long-term stability is an absolute necessity.

The literature review reveals that a nonprecious material possesses brilliant catalytic activity for either ORR or OER, but the bifunctional catalyst that exhibits both ORR and OER activity is still scarce. Transition metal oxides,[11,12] sulfides,[13,14] and selenides[15] are exploited as oxygen reduction catalysts, but only transition metal oxides are extensively studied as bifunctional catalysts.[16,17] On the other hand, transition metal sulfides typically emerge as an excellent candidate for electrochemical energy conversion and storage and are widely used in supercapacitors,[18,19] lithium-ion batteries,[20] water electrolyzers,[21,22] etc. Compared to single-component sulfides, ternary sulfides show enhanced activity due to the presence of more active redox centers with a more rapid electron-transfer process offering better kinetics.[23] Previous reports have typically found that octahedral sites of spinels are predominantly electrocatalytically active in nature, whereas a tetrahedral site does not take part in the electrocatalytic reaction.[24] CuCo2S4 is a normal thiospinel possessing the general formula AB2S4, where octahedral “B” sites are typically occupied by trivalent Co3+ ions whereas divalent Cu2+ ions occupy the tetrahedral “A” site. Since the high-spin states of Co3+ occupy the octahedral sites, which are the main active centers for electrochemical reaction, CuCo2S4 becomes an ideal choice for energy conversion and as a storage catalyst.[25] CuCo2S4 shows significant performance for oxygen evolution reaction, supercapacitor application, etc. For example, Meenakshi Chauhan et al. convincingly demonstrate that mixed-metal thiospinel CuCo2S4 exhibits excellent performance for the oxygen evolution reaction in a high-pH medium, exhibiting a lower overpotential and higher durability.[26] The excellent catalytic activity arises due to the incorporation of Cu in the Co3S4 lattice, which can significantly enhance the OER process. Cu2+ acts as an active site for −OH and −OOH species adsorption and increases the active high-spin state of Co3+ necessary for OER.[26] However, the oxygen reduction ability of CuCo2S4 is still not satisfactory due to its poor electron conductivity.

On the other hand, carbon materials possessing high surface areas and a lower cost are arising as a new category of metal-free catalysts, and as a possible replacement of Pt-based catalysts, particularly for the oxygen reduction reaction.[27] The heteroatom (N, P, B) doping in the carbon structure further improves the potentiality of the catalyst. Several published reports show that N-doping can notably boost electrocatalytic activity.[28,29] Dual doping further enhances the activity.[30] For example, Ma et al. compare the oxygen reduction capabilities of different dual-doped graphene materials.[31] Sun et al. developed B,N-codoped nanocarbon as an excellent metal-free bifunctional catalyst for both OER and ORR. The porous structure and defect-rich carbon skeleton are the possible reasons behind the profound performance, which eventually surpasses the performance of Pt/C.[32] Though heteroatom-doped carbon material shows commendable ORR performance due to its excellent mechanical and chemical stability, it typically exhibits less efficient OER activity due to carbon corrosion at a high positive potential.[33] The unique combination of carbon and a metal oxide or metal sulfide has emerged as a new class of potential catalysts that can effectively improve the bifunctional performance.[34,35] Han et al. successfully synthesized NiCo2S4@g-C3N4-CNT hybrid electrocatalysts, which show outstanding performance for ORR and OER.[36] Nevertheless, further investigation and fabrication of different hybrid materials are still needed.

From this viewpoint, in the present study, hybrid catalysts composed of a transition metal sulfide and dual heteroatom-doped reduced graphene oxide (GO) are fabricated via a simple hydrothermal technique. The synthesized hybrid catalyst shows significant ORR/OER performance in alkaline media.

Experimental Section

Chemicals

Copper(II) nitrate trihydrate (Cu(NO3)2.3H2O, 99.0%), graphite powder, potassium hydroxide (99.99%), sulfuric acid (95–97%), boric acid (≥99.5%), cobalt(II) nitrate hexahydrate (Co(NO3)2.6H2O, 99.5%), thiourea (≥99.0%), isopropanol (≥99.5%) and ethanol (≥99.5%) were bought from Merck, India. Ethylenediamine (99%) was bought from SRL, India. Nafion (5 wt %) and IrO2/C (99.9%) were bought from Sigma Aldrich. All solutions were prepared with deionized water from Millipore.

Synthesis of N-Doped Reduced Graphene Oxide (N-rGO)

N-doped reduced graphene oxide (N-rGO) was synthesized using exfoliated graphene oxide (GO) (prepared by the improved Hummer’s method[37] from graphite powder) and urea as a precursor. For this typical synthesis, homemade GO powder was taken in 70 mL of water. After 30 min of sonication, a certain amount of urea was added to the exfoliated GO suspension and stirred for 45 min. Then the whole solution was carefully transferred to the 100 mL autoclave and heated at 180 °C for 12 h. The product was collected by centrifugation at 5000 rpm after washing with water and ethanol solvent thrice each. The collected product was dried at 60 °C for 12 h. The detailed synthesis process was described in the previous report.[38]

Synthesis of B,N Dual-Doped Reduced Graphene Oxide (B,N-rGO)

For co-doping with boron, synthesized N-rGO was annealed with the heteroatom-containing precursor boric acid (the mass ratio of N-rGO to boric acid is 1:20) at 900 °C in Ar atmosphere for 5 h. Since it is reported that the possible formation of a B–N bond often reduces the activity of the catalyst,[39,40] sequential doping processes are typically adopted to avoid B–N bond formation.

Synthesis of CuCo2S4 and CuCo2S4@B,N-rGO Catalyst

In this typical synthesis procedure, presynthesized boron, nitrogen dual-doped reduced graphene oxide (B,N-rGO) (20 wt %) was dissolved in 30 mL of deionized water (0.4 mg mL–1) by ultrasonication for 30 min. Next, cobalt nitrate and copper nitrate in (2:1) mole ratio were taken in 30 mL of water and stirred for 30 min. Then, 4 mol thiourea was added and stirred gently. Subsequently, 4 mL of ethylenediamine was carefully added to the solution. Next, the brown-colored solution was transferred to the 100 mL autoclave and heated at 200 °C for 12 h. After that, the reaction mixture was allowed to cool down up to room temperature. Finally, the material was carefully collected by centrifugation (at 5000 rpm for 10 min), washed with distilled water and ethanol, and dried for 8 h in a vacuum oven. A similar synthetic protocol was followed with graphene oxide (GO) and nitrogen-doped graphene oxide (N-rGO) to synthesize CuCo2S4@rGO and CuCo2S4@N-rGO catalysts. The CuCo2S4 catalyst is also prepared without the addition of any carbon material for comparison. The synthesis procedure is summerized in Scheme .

Scheme 1

Synthesis Procedure for the CuCo2S4@B,N-rGO Hybrid Catalyst

Physicochemical Characterization

The crystal structure of the catalysts was determined by X-ray diffraction (XRD) study using a PANALYTICAL diffractometer. A Cu Kα (1.54108 Ǻ) radiation source operated at 40 kV and 40 mA was used, and the range of 2θ was 10–80°. Raman spectroscopy was performed using 532 nm excitation lasers from the instrument Lab-RAM Horiba, France. The XPS spectra of CuCo2S4@B,N-rGO catalyst were recorded with a photoelectron spectrometer of Physical Electronics. Al Kα radiation (1486.6 eV) was implemented as an X-ray source operated at 150 W (12 kV, 12.5 mA). Field emission scanning electron microscopy (FESEM) (FEI, Quanta 200) equipped with an energy-dispersive X-ray unit (EDS unit) was employed to examine the morphology of the catalyst. A high-resolution transmission electron microscope (HR-TEM) (JEOL 2010 and operating at 200 kV) was used to examine the shape and size of the catalyst.

Electrochemical Characterization

All electrochemical measurements (ORR and OER) of the catalysts were determined using a computer-controlled multichannel potentiostat system of Biologic (VSP: 300) equipped with a rotator from Pine Instrument, at room temperature. A three-electrode cell assembly having Hg/HgO/OH– (E0 = 0.1 V vs RHE) as the reference electrode, graphite rod as the counter electrode, and a catalyst-coated rotating disk (RDE, with a geometric surface area of 0.196 cm2) and a rotating ring disk (RRDE, having a geometric surface area of 0.196 and 0.125 cm–2 for the disk and the surrounded Pt ring, respectively) as the working electrode was used. Catalyst ink was typically prepared by dispersing 4 mg of synthesized catalysts in 1 mL of solvents containing water (790 μL), isopropanol (200 μL), and 5 wt % of Nafion binder (10 μL) by ultrasonication for 15 min to obtain a homogeneous suspension. Then, 10 μL inks were drop cast on the previously mirror-polished RDE/RRDE electrode and dried at room temperature. The uniform mass loading of 0.2 ± 0.05 mg cm–2 was maintained adequately for all potential catalysts. Commercial Pt/C (20 wt % Pt, Johnson Matthey) and IrO2/C (20 wt %) catalysts were also tested for comparison.

Cyclic voltammetry (CV), linear-sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) study were properly executed in a 0.1 M KOH electrolyte. Furthermore, to properly evaluate the OER performance, an LSV study was carried out with N2-saturated 0.1 M KOH electrolyte in the potential window −0.1 to +1 V vs Hg/HgO/OH– with 1600 rpm rotation speed and a scan rate of 10 mV s–1.

For ORR analysis, O2 gas with high purity was purged for 45 min before the measurement to obtain an O2-saturated electrolyte, and the continuous flow of O2 was maintained over the solution during the test; before that, the ORR data was recorded in N2-saturated electrolytes and used for background correction.

The Koutecky–Levich eq is typically used to evaluate the number of electrons transferred during ORR. For this, J–1 vs ω–1/2 values were carefully plotted at various electrode potentials and the number of electrons transferred (n) was precisely calculated from the slopes of the best linear fit curves.where JK = kinetic current density, JL = diffusion-limiting current density, ω = angular velocity of the rotating disk (ω = 2πN, N represents the linear rotating speed in rpm), n = electron transferred number, F = Faraday constant (96,485 °C mol–1), C0 = bulk concentration of O2 (1.2 × 10–6 mol cm–3), D0 = diffusion coefficient of O2 (1.9 × 10–5 cm2 s–1), ν = kinematic viscosity of the electrolyte (0.01 cm2 s–1), and k = electron-transfer rate constant.

The rotating ring-disk electrode (RRDE) experiment was conducted using an RRDE electrode, where the disk electrode was scanned cathodically at a rate of 10 mV s–1 and the ring was polarized at +1.55 V constant potential vs RHE for oxidizing any HO2– intermediate. The percentage of production of the HO2–intermediate and the number of electrons transferred (n) were calculated according to eqs and 3where Id = disk current, Ir = ring current, and N = current collection efficiency of the Pt ring (0.37).

The long-term stability of the catalysts for OER and ORR was evaluated by the chronoamperometric (CA) method in an N2 and O2 saturated 0.1 M KOH electrolyte at a potential corresponds to 10 mA cm−2 current density for OER, and half wave potential for ORR, respectively, with 1600 rpm rotating speed. The EIS analysis was performed in the frequency range of 10–5 to 0.01 Hz with an amplitude voltage of 10 mV.

The electrochemically active surface areas (ECSAs) were calculated by measuring the double-layer capacitance from the CV curves measured at different scan rates (2, 5, 10, 25, 50, 75, 100, and 125 mV s–1) in the non-Faradic regions. All potentials reported in this work were converted from Hg/HgO to the RHE scale usingThe OER potentials were iR corrected by following the equationwhere i = current and R = uncompensated electrolyte Ohomic resistance, which was accurately measured via high-frequency AC impedance.

The overpotential of OER at 10 mA cm–2 was determined according to eq

Results and Discussion

Structural and Morphological Characterization

X-ray powder diffraction (XRD) study is properly executed to reveal the crystallographic pattern of CuCo2S4 and heteroatom-doped reduced graphene oxide-mixed-metal thiospinel hybrid catalysts as depicted in Figure a. The prominent peaks recognized at 16.2, 26.6, 31.3, 38.0, 47.0, 50.0, and 54.8° for all of the synthesized catalysts typically correspond to the highly crystalline (111), (022), (113), (004), (224), (115), and (044) planes of the cubic phase of CuCo2S4 (according to JCPDS card No. 42-1450), which are consistent with previously reported literature values.[2,21,41,42] In addition, the two notable peaks (* marked) appearing at around 29.9 and 52° may be due to the formation of the (311) and (511) planes of cobalt sulfide.[43] The absence of the signature peak of reduced graphene oxide around 2θ = 25° in the hybrid catalysts (CuCo2S4@rGO, CuCo2S4@N-rGO, CuCo2S4@B,N-rGO) may be due to the presence of CuCo2S4 on the graphene sheets, which impedes the rGO stack and weakens the diffraction feature. This also confirms that the loading of rGO is lower and undoubtedly has substantial interference with the (022) peak of CuCo2S4, exerting a negligible effect on the crystal phase of CuCo2S4.[44]

Figure 1

(a) XRD pattern for (i) CuCo2S4, (ii) CuCo2S4@rGO, (iii) CuCo2S4@N-rGO, and (iv) CuCo2S4@B,N-rGO; dotted lines represents the standard 2θ value for CuCo2S4 (b) Raman profile for (i) CuCo2S4, (ii) CuCo2S4@rGO, (iii) CuCo2S4@N-rGO, and (iv) CuCo2S4@B,N-rGO.

The existence of rGO in the catalyst matrix is carefully verified by the Raman spectroscopy study shown in Figure b. All of the potential catalysts containing reduced graphene oxide show typical D (corresponding to the defect mode) and G (associated with the E2g mode) bands at 1349 and 1586 cm–1, respectively.[44,45] The corresponding ID/IG ratio varies appreciably in the order CuCo2S4@rGO (0.97) < CuCo2S4@N-rGO (0.99) < CuCo2S4@B,N-rGO (1.14). The increased ID/IG ratio typically indicates the possible formation of more defective domains performing as active centers for the synthesized catalyst. Additionally, all synthesized catalysts exhibit three allowed Raman transitions around 453 and 511 cm–1 corresponding to the vibrational modes of the Co-S bond, and the one at 670.8 cm–1 is due to the S–S stretching mode.[46,47] The notable absence of peaks corresponding to the Cu–S species either at 422 and 467 cm–1 or at >680 cm–1 further confirms the formation of a pure phase of CuCo2S4 and the hybrid catalyst.

XPS characterization is efficiently performed to adequately evaluate the surface’s oxidation state and elemental composition of the best-performing catalyst, CuCo2S4@B,N-rGO. The XPS survey spectrum for CuCo2S4@B,N-rGO (Figure a) shows prominent peaks revealing precisely the presence of Cu, Co, and S along with C, B, N, and O. The deconvoluted C 1s spectra comprise four characteristic peaks located at around 284.6, 285.5, 286.3, and 288.7 eV, which may be attributed to the C–C, C–N, C=O, and O–C=O bonds, respectively,[38,45] as depicted in Figure b. The deconvoluted peak of the O 1s spectra typically reveals the presence of O–C, O=C, and O–C=O at binding energies of 531.8, 532.2 and 533.8 eV, respectively, as displayed in Figure c. Figure d shows the N 1s spectra comprising four prominent peaks at 398.9, 399.8, 400.6, and 404.1 eV, which can be ascribed to pyridine-type N, pyrrole-type N, graphitic N, and oxidized N, respectively.[38,45] No signal at the lower binding energy around 398.2 eV for B–N bonding is found, eliminating the possibility of B–N bond formation. In the B 1s spectra, five B-containing species are clearly detected, which correspond to the B–C (185.3 eV), B4C (186.7 eV), BC3 (188.6 eV), BC2O (190.5 eV), and BCO2 (192.9 eV) groups.[48,49] The calculated overall atomic concentrations of N and B elements are around 5.73 and 3%, as obtained from Table S1 of the Supporting information. These observations signify the successful dual doping of B, N in the reduced graphene oxide matrix.

Figure 2

XPS analysis data for the representative CuCo2S4@B,N-rGO catalyst. (a) Survey spectra and deconvoluted spectra for (b) C 1s, (c) O 1s, (d) N 1s, (e) B 1s, (f) Cu 2p, (g) Co 2p, and (h) S 2p.

Furthermore, two sets of spin–orbit peaks typically correspond to Cu 2p3/2 (932.2 and 934.6 eV), and Cu 2p1/2 (951.7 and 953.3 eV) of Cu+ and Cu2+ respectively is observed along with the satellite peak at around 955.7 eV. On the other hand, Co3+ and Co2+ oxidation states of cobalt are confirmed from the two sets of signature peaks at binding energies of (778.6 and 793.7 eV) and (780.9 and 797.5 eV), corresponding, respectively, to the Co 2p3/2 and Co 2p1/2 species, with two shakeup satellites at 785.2 and 802.4 eV in the core-level Co 2p spectra (Figure f).

The S 2p core spectrum shows two prominent peaks at around 161.3 and 162.4 eV corresponding to S2– 2p3/2 and S2– 2p1/2, respectively, confirming the spin–orbit coupling of metal sulfides as presented in Figure h. The notable peak at 164.1 eV typically represents the Cu–S or Co–S metal–sulfur bonds having a low coordination of S2– at the surface. On the other hand, the peak at higher binding energy, 169.1 eV, corresponds to the oxidized-S (SO–) species. Previous reports reveal that the M–S bonds positively influence the oxygen reduction process, but the oxidized-S species are generally inactive.[2,20,50,51] Additionally, from XPS analysis, we further verify the surface composition of the prepared CuCo2S4@B,N-rGO catalyst, as presented in Table S1 of “Supporting information.”

The FESEM image of CuCo2S4 displayed in Figure a exhibits a unique structure. For confirming the composition of the synthesized sample, EDS analysis is carried out. The observed EDS spectrum confirms Cu, Co, and S with an atomic ratio of around 1:2:4 along with C, O, N, and B for the CuCo2S4@B,N-rGO hybrid catalyst (Figure b). Furthermore, the elemental mapping analysis indicates precisely the uniform allocation of Cu, Co, and S elements over the whole region, corroborating a homogeneous phase of CuCo2S4.

Figure 3

(a) FESEM images of CuCo2S4. (b) EDS profile of the CuCo2S4@B,N-rGO catalyst; elemental analysis profiles for the CuCo2S4@B,N-rGO catalyst: (c) Cu mapping, (d) Co mapping, (e) S mapping, (f) C mapping, (g) B mapping, and (h) N mapping.

Figure a represents the TEM images of CuCo2S4@B,N-rGO, which typically indicate the presence of CuCo2S4 along with dual-doped reduced graphene oxide, confirming the hybrid nature of the catalyst. The absence of an rGO sheet is clearly visible for the CuCo2S4 catalyst (Figure b). The wrinkled surface of the B,N-rGO catalyst is amply demonstrated in Figure S1 of the Supporting information. The HR-TEM analysis reveals distinct lattice fringes for dual-doped reduced graphene oxide and CuCo2S4. The (113) plane of CuCo2S4 is identified as having a d spacing of 0.290 nm (Figure c). The selected area electron diffraction (SAED) pattern shows discrete spots of the (113), (004), and (115) planes of cubic CuCo2S4 (Figure d).

Figure 4

(a) TEM images of the CuCo2S4@B,N-rGO catalyst; (b) TEM image of CuCo2S4; (c) HR-TEM images of CuCo2S4@B,N-rGO; and (d) SAED pattern of CuCo2S4@B,N-rGO.

ORR Analysis

A typical CV profile is observed in the O2- and N2-saturated 0.1 M aqueous KOH electrolyte using RDE at a scan rate of 50 mV s–1, as depicted in Figure S2 of “Supporting information.” A noticeable cathodic peak at around 0.5V is observed in voltammograms taken in the O2-saturated electrolyte compared to voltammograms obtained for the N2-saturated electrolyte.

To further evaluate the ORR activity, linear-sweep voltammetry experiments are performed using the RDE electrode rotated at 1600 rpm speed in 0.1 M KOH at a sweep rate of 10 mV s–1, as depicted in Figure a. The LSV polarization profile shows a more positive shift in onset potential (0.88 V), half-wave potential (0.73 V), and higher diffusion-limited current density (5.44 mA cm–2), indicating that the CuCo2S4@B,N-rGO hybrid possesses enhanced electrocatalytic activity towards ORR than other similarly synthesized catalysts, as is evident from Table S2 of the Supporting information. Furthermore, only a 0.10 V negative shift in the half-wave potential compared to the commercial Pt/C catalyst further indicates the remarkable capability of ORR of the CuCo2S4@B,N-rGO catalyst. In addition, the hybrid catalyst showed a superior performance than the recently reported metallic atomic thickness of the CuCo2S4 nanosheets.[52]

Figure 5

(a) Linear-sweep voltammograms of Pt/C, CuCo2S4@rGO, CuCo2S4@N-rGO, and CuCo2S4@B,N-rGO catalysts on RDE rotating at 1600 rpm in a 0.1 M KOH solution at 10 mV s–1 scan rate. (b) Tafel plots drawn from the LSV curve. (c) LSV curves for CuCo2S4@B,N-rGO at various rotating speeds. K–L plots obtained from LSVs at different potentials are shown in the inset. (d) Comparison of the average electron-transfer number of various synthesized catalysts. (e) RRDE voltammograms for Pt/C (20%), CuCo2S4, CuCo2S4@rGO, CuCo2S4@N-rGO, and CuCo2S4@B,N-rGO at 1600 rpm; the ring electrode was polarized at 1.5 V, and 10 mV s–1 scan rate was applied. (f) Electron transfer number (n) as a function of the electrode potential. (g) Percentage of peroxide yield (%H2O–) of the synthesized catalysts. (h) Current–time chronoamperometric response of CuCo2S4@B,N-rGOand commercial Pt/C in an O2-saturated 0.1 M KOH solution at a potential corresponds to half-wave potential with a rotating speed of 1600 rpm.

To investigate the ORR kinetics of the resulting electrocatalysts, Tafel plots are drawn from the LSV curves of Figure a, which exhibit a variation in the slopes with an increase in potential, as presented in Figure b. A considerably lower slope (73 mV dec–1) for CuCo2S4@B,N-rGO is observed in the lower overpotential region than for the other synthesized catalysts, again establishing the catalyst’s superiority. Furthermore, the CuCo2S4@B,N-rGO catalyst exhibits a lower slope than previously reported Co-based mixed-sulfide catalysts,[53] which indicates better kinetics for the oxygen reduction reaction.

The number of electrons transferred (n) is accurately calculated using least-squares fitted slopes obtained from the Koutecky–Levich equation (eq ).[54] For this, LSV curves are carefully recorded at different rotating speeds typically ranging from 200 to 2400 rpm at a sweep rate of 10 mV s–1, as presented in Figures c and S3a–d of the Supporting information. With an increase in the rotation speed, the current density increases. Koutecky–Levich (K–L) plots (J–1 vs ω–1/2) from LSVs given in the insets of Figures c and S3a–d of the Supporting information demonstrate good linearity and parallelism. The calculated average electron-transfer number (n) during ORR is around 4 (Figure d), suggesting that the CuCo2S4@B,N-rGO catalyst favors a desirable four-electron oxygen reduction pathway, similar to that of Pt/C, while the other catalyst follows a complex combination of two- and four-electron paths to reduce the number of oxygen molecules in the water.

Rotating ring-disk (RRDE) experiments are carefully performed to evaluate the amount of peroxide yield of the corresponding peroxide species (HOO–) during the ORR process, as well as the number of electrons transferred per oxygen molecule (n). Figure e shows the recorded ring currents at 1600 rpm in 0.1 M KOH for the synthesized catalysts along with the commercial (20%) Pt/C catalyst, which demonstrates a smaller ring current (peroxide oxidation) for the CuCo2S4@B,N-rGO catalyst, indicating the lesser formation of intermediate byproducts during ORR. The number of electrons transferred (n) calculated from Figure f is about 3.8 for the CuCo2S4@B,N-rGO catalyst, which substantially agrees with the results determined by the K–L plots. On the other hand, as observed from Figure g, a much lower percentage yield of H2O2 is generated (<10%) for the CuCo2S4@B,N-rGO catalyst, indicating that the four-electron pathway predominantly generates OH– ions as the end product on the catalyst surface. Furthermore, Figure h displays only 76% retention of current density for the CuCo2S4@B,N-rGO catalyst after a 14 h chronoamperometric test. On the other hand, commercial Pt/C shows a rapid decrease initially at 75% retention of current density after 14 h of operation.

The excellent ORR activity of CuCo2S4@B,N-rGO may be originating from the synergism between the dual-doped reduced graphene oxide and ternary sulfide. A previous study reveals that nitrogen doping facilitates superoxide ion formation, which weakens the O–O bond facilitating the dissociation, thereby promoting the ORR activity.[55,56] In another theoretical study, DFT calculation revealed that the B–C–N bond formation is crucial, where C atoms are first polarized by the effect of N. Further, charge transfer occurs from C to B atoms, activating the B atoms for ORR. On the other hand, the configuration where B is directly bonded to N to give a B–N configuration has a poor ORR activity due to the absence of bridging of C atoms.[40,57,58] The formation of the B–N bond in the present case is ruled out from the XPS analysis, which implies the significance of the sequential doping process. Moreover, the introduction of dual-doped graphene into the catalyst matrix enhances the electrical conductivity, generates more catalytically active sites, and provides higher surface areas, which are the key parameters for the successful enhancement of ORR for a hybrid catalyst.

OER Analysis

The oxygen evolution ability of the synthesized CuCo2S4@B,N-rGO catalyst has been assessed by linear-sweep voltammetry measurement performed in N2-saturated 0.1 M KOH electrolyte by applying a 10 mV s–1 sweep rate at 1600 rpm rotational speed. For comparison, other fabricated catalysts such as CuCo2S4, CuCo2S4@rGO, CuCo2S4@N-rGO, and commercial IrO2/C also have been tested in similar conditions as presented in Figure a. As illustrated in Figure a, among the synthesized catalysts, CuCo2S4@B,N-rGO demonstrates the lowest onset potential and overpotential of 1.50 and 1.54 V, respectively, which is close to those of the benchmark IrO2/C catalysts (1.47 and 1.51 V). The crucial index overpotential (potential at 10 mA cm–2 current density) for assessment of the OER catalyst, the value for the fabricated CuCo2S4@B,N-rGO hybrid (0.310 V), is lower than that for the previously reported cobalt-nickel sulfide spinel nanocatalysts[59] and N-doped graphene-supported Co9S8 (N-Co9S8/G) catalyst (0.409 V).[60] However, at a high overpotential (above 0.450 V), the CuCo2S4@B,N-rGO hybrid surpasses the current density of the benchmark IrO2/C catalyst. The superior OER activity indicates that the doping of the heteroatom in the carbon matrix definitely provides some additional active sites that enhance the hybrid catalyst’s OER activity. In addition, the synergy between metal sulfides and carbon material facilitates electron conduction, hence efficiently improving the OER activity.[61]

Figure 6

(a) iR-corrected LSV curve of IrO2/C, and CuCo2S4, CuCo2S4@rGO, CuCo2S4@N-rGO, and CuCo2S4@B,N-rGO in N2 saturated 0.1 M KOH solution at a scan rate of 10 mV s–1 on an RDE electrode rotating at 1600 rpm. (b) Tafel plots drawn from the LSV curve measured at 1 mV s–1 scan rate in an N2-saturated 0.1 M KOH solution. (c) Nyquist plot is drawn at 1.50 V for the synthesized catalysts. (d) Current–time chronoamperometric response of CuCo2S4@B,N-rGO and IrO2/C catalyst in an O2-saturated 0.1 M KOH solution at a potential corresponds to 10 mA cm−2 current density with a rotating speed of 1600 rpm.

Tafel slope is a vital parameter to interpret the kinetics of a catalytic reaction. Figure b represents the corresponding Tafel slopes drawn from the LSV curve of Figure a, revealing that CuCo2S4@B,N-rGO possesses the smallest Tafel slope (112 mV dec–1) among the synthesized hybrid catalysts and the rGO-free CuCo2S4 (159 mV dec–1) commercial IrO2/C (128 mV dec–1) catalyst. The catalysts exhibit a lower Tafel slope value than the previously published MnCo2S4 nanowire array[62] and cobalt-iron sufides covalently entrapped in N-doped mesoporous carbon.[63] The results further imply a better kinetic process.

The electrochemically active surface area (ECSA) is calculated by measuring the electrochemical double-layer capacitance (Cdl) at the solid/liquid interface for the fabricated catalyst. The Cdl value is determined by measuring the cyclic voltammograms in the non-Faradic region and is presented in Figure S4a–e of the Supporting information. According to the formula jc = v CDL (a plot of jc as a function of v yields a straight line with a slope equal to Cdl), the Cdl value for the synthesized catalyst increases in the following order CuCo2S4 < CuCo2S4@rGO < CuCo2S4@N-rGO < CuCo2S4@B,N-rGO. Among the synthesized catalysts, CuCo2S4@B,N-rGO exhibits a much larger surface area.

The electrochemical impedance spectroscopy (EIS) analysis reveals that the charge transfer resistance decreases in the following order: CuCo2S4 > CuCo2S4@rGO > CuCo2S4@N-rGO > CuCo2S4@B,N-rGO. As expected, among the synthesized catalysts, the CuCo2S4@B,N-rGO hybrid exhibits the lowest charge-transfer resistance, implying a faster electron-transfer kinetics. The OER stability of the best-performing CuCo2S4@B,N-rGO hybrid catalyst is tested by amperometric (i–t) analysis and is shown in Figure d. As shown in Figure d, CuCo2S4@B,N-rGO exhibits impressive stability after 14 h of operation with 75% retention of current density. At the same time, IrO2/C retains only 40% of the current density. After 6 h of operation, the activity of IrO2/C drastically reduced, which may be due to the catalyst detachment from the RDE.

Therefore, it is reasonable to conclude that both CuCo2S4 and B,N dual-doped reduced graphene oxide are active species and promote the activity. Specifically, carbon atoms bonding with N will be positively charged and adsorb the OH– and accelerate the electron transfer between the catalyst surfaces and the reaction intermediates, increasing the OER activity.[64] Furthermore, CuCo2S4 with a normal spinel crystal structure is suitable for electron transportation between Co3+ and Cu2+ ions and can directly provide lattice oxygen as one of the sources of the generated O2 during OER, thus promoting OER activity.[65] Notably, the XPS study confirms the existence of the M-S bond (peak at 164.1 eV), which favorably boosts the OER performances.

Bifunctional Performance

The essential condition to properly judge an excellent bifunctional catalyst is to carry out both ORR and OER reactions with the smallest difference in the potential. Hence, the bifunctional performance for the CuCo2S4@B,N-rGO catalyst is assessed by the difference of potential (ΔE) between the potential at 10 mA cm–2Ej=10 for OER and half-wave potential E1/2 for ORR. As a result, the CuCo2S4@B,N-rGO catalyst exhibits a small ΔE value of 0.81 V, surpassing the other reported bifunctional electrocatalysts listed in Table . These results indicate that CuCo2S4@B,N-rGO is a competent cost-effective, bifunctional catalyst offering good potential for practical applications.

Table 1

Comparison of Bifunctional Oxygen Activities of the Synthesized CuCo2S4@B,N-rGO Hybrid Catalyst with Various Previously Reported Hybrid Catalysts

SL no	electroatalyst	ORR E1/2 (V)	OER E (V) at I = 10 mA cm–2	oxygen electrode (OER–ORR) E (V)	reference
1	NiCo2S4@N/S-rGO	0.76	1.70	0.94	(32)
2	CuCo2S4 NSs	0.74	1.58	0.84	(51)
3	CuCo2S4 NSs	0.70	1.52	0.82	(52)
4	CoS2(400)/N,S-GO	0.79	1.61	0.82	(66)
5	CuCo2S4@B,N-rGO	0.73	1.54	0.81	this work

Conclusions

In summary, dual heteroatoms-doped carbon and mixed-metal sulfide hybrid catalysts (CuCo2S4@B,N-rGO) are successfully synthesized by a facile hydrothermal synthesis and established as a competent bifunctional oxygen electrocatalyst. Physicochemical analysis by using several spectroscopic and microscopic studies revealed the formation of a hybrid material typically containing dual-doped reduced graphene oxide and ternary metal sulfides. The hybrid catalyst displayed an improved onset and half-wave potential for ORR as well as a lower overpotential for OER. The catalyst shows good durability in an alkaline medium. The enhanced performance may be attributed to the synergistic effect of the dual-doped graphene and metal sulfide. Moreover, dual-doped graphene provides a large surface area and increased number of active sites, enhancing the electron conductivity and boosting the catalyst’s performance. The importance of the present work lies in the simple and effective synthesis of hybrid catalysts, which can simultaneously boost the oxygen evolution and oxygen reduction reaction in alkali.