Bifunctional nanoporous ruthenium-nickel alloy nanowire electrocatalysts towards oxygen/hydrogen evolution reaction

A class of ruthenium-nickel alloy catalysts featured with nanoporous nanowires (NPNWs) were synthesized by a strategy combining rapid solidification with two-step dealloying. RuNi NPNWs exhibit excellent electrocatalytic activity and stability for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in which the RuNi-2500 NPNWs catalyst shows an OER overpotential of 327 mV to deliver a current density of 10 mA cm^?2 and the RuNi-0 NPNWs catalyst requires the overpotential of 69 mV at 10 mA cm^?2 showing the best HER activity in alkaline media. Moreover, the RuNi-1500 NPNWs catalyst was used as the bifunctional electrocatalyst in a two-electrode alkaline electrolyzer for water splitting, which exhibits a low cell voltage of 1.553 V and a long-term stability of 24 h at 10 mA cm^?2, demonstrating that the RuNi NPNWs catalysts can be considered as promising bifunctional alkaline electrocatalysts.     

PBA derived FeCoP nanoparticles decorated on NCNFs as efficient electrocatalyst for water splitting

Transition metal phosphides have been known as promising electrocatalysts for hydrogen evolution and oxygen evolution reactions (HER and OER) due to their high catalytic activity. In this work, the FeCoP nanoparticles decorated on N-doped electrospun carbon nanofibers (FeCoP@NCNFs) was successfully synthesized through depositing Fe, Co-based Prussian blue analogue Co₃[Fe(CN)₆]₂·10H₂O (FeCo-PBA) onto the electrospun PVP/PAN nanofibers via layer-by-layer approach, followed by carbonization and phosphorization treatments. Benefiting from the high electrical conductivity, abundant catalytic active sites and the synergistic effect between FeCoP nanoparticles and N-doped carbon nanofibers network, the obtained FeCoP@NCNFs displays good bifunctional electrocatalytic activity. In 1 M KOH, the FeCoP@NCNFs achieves 10 mA cm^?2 at an overpotential of 290, 226 mV for OER and HER, respectively. Moreover, it demands overpotential of 196 mV to achieve 10 mA cm^?2 for HER in 0.5 M H₂SO₄. The FeCoP@NCNFs is used as both anode and cathode for overall water splitting, it requires a low voltage of 1.65 V to achieve a current density of 10 mA cm^?2 and maintains outstanding stability over 10 h. Herein, a strategy for preparing bifunctional electrocatalysts of compositing transition metal phosphides with carbon nanofibers is proposed, and the application of metal-organic framework in electrocatalytic field is further extended.     

Bifunctional electrocatalysts for water splitting from a bimetallic (V doped-NixFey) Metal–Organic framework MOF@Graphene oxide composite

An ongoing challenge still lies in the exploration of proficient electrocatalysts from earth-abundant non-precious metals instead of noble metal-based catalysts for clean hydrogen energy through large-Scale electrochemical water splitting. However, developing a non-precious transition metals based, stable electrocatalyst for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) is important challenge for modern energy conversion technology. In this report Vanadium doped bimetallic nickel-iron nanoarray, fabricated by carbon supported architecture through carbonization process for electrochemical water splitting. Three types of catalysts were prepared in different molar ratio of Ni/Fe. The electrocatalytic performance demonstrated that the catalyst with equal mole ratio (0.06:0.06) of Ni/Fe possess high catalytic activity for both OER and HER in alkaline and acidic medium. Besides, our findings revealed that the doping of vanadium could play a strong synergetic effect with Ni/Fe, which provide a small overpotential of 90 mV and 210 mV at 10 mA cm^?2 for HER and OER respectively compared to the other two catalyst counterparts. Also, the catalyst with 1:1 (Ni/Fe) molar ratio showed a high current density of 208 mA cm^?2 for HER at 0.5 M H₂SO₄ and 579 mA cm^?2 for OER at 1 M KOH solution, the both current densities are much higher than the other two catalysts (different Ni/Fe ratio). In addition, the presented catalysts showed extremely good durability, reflecting in more than 20 h of consistent Chronoamprometry study at fixed overpotential η = 250 mV without any visible voltage elevation. Similarly, the (Ni/Fe) equal ratio catalyst showed better corrosion potential 0.209 V vs Ag/AgCl and lower current density 0.594 × 10^?12 A cm^?2 in high alkaline medium. The V-doping, MOF/GO surface defects are significantly increased the corrosion potential of the V-Ni_xFe_y-MOF/GO electrocatalyst. Besides, the water electrolyzed products were analysed by gas chromatography to get clear insights on the formed H₂ and O₂ products.     

Bimetallic metal-organic framework derived electrocatalyst for efficient overall water splitting

The development of bifunctional catalysts that can be applied to both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is widely regarded as a key factor in the production of sustainable hydrogen fuel by electrochemical water splitting. In this work, we present a high-performance electrocatalyst based on nickel-cobalt metal-organic frameworks for overall water splitting. The as-obtained catalyst shows low overpotential to reaches the current density of 10 mA cm⁻² with 249 mV for OER and 143 mV for HER in alkaline media, respectively. More importantly, when the electrolyzer was assembled with the as-prepared catalyst as anode and cathode simultaneously, it demonstrates excellent activity just applies a potential of 1.68 V to achieve 10 mA cm⁻² current density for overall water splitting.     

NiCoP–CoP heterostructural nanowires grown on hierarchical Ni foam as a novel electrocatalyst for efficient hydrogen evolution reaction

The design and manufacture of effective non-noble metal catalysts for the H₂ evolution reaction (HER) are urgent for realizing a cost-effective hydrogen production. We report herein on flower-like structures consisting of NiCoP–CoP heterostructural nanowires grown directly on the hierarchically porous nickel framework (NiCoP–CoP/Ni/NF) to achieve a highly efficient HER in alkaline solution (1.0 M KOH). The NiCoP–CoP/Ni/NF is synthesized by electrodeposition of porous Ni layers on Ni foam, followed by simple hydrothermal reaction and phosphorization. For HER, the binder-free NiCoP–CoP/Ni/NF electrode can reach 10 mA cm^?2 current density at a quite low overpotential of 49 mV, because of the combination of porous Ni layers and highly active NiCoP–CoP nanowires. In addition, the NiCoP/CoP heterostructures exhibited remarkable stability under the long-term durability test. This work provides a new strategy that combines electrodeposition and hydrothermal reaction to synthesize effective HER catalysts.     

Tailoring cation vacancies in Co,Ni phosphides for efficient overall water splitting

High-efficiency water splitting catalysts are competitive in energy conversion and clean energy production. Herein, a bifunctional water splitting catalyst CoNiP with cation vacancy defects (CoNiP–V) is constructed through defect engineering. The results show that abundant cation vacancy defects in CoNiP–V are bifunctional active centers in the process of water electrolysis, which enhance the activity of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In 1.0 mol L^?1 potassium hydroxide, CoNiP–V requires a pretty low overpotential of 58 mV to reach a geometrical current density of 10 mA cm^?2 for HER. To deliver a current density of 100 mA cm^?2, only 137 mV and 340 mV of overpotential are needed for HER and OER, respectively. Moreover, the cell with CoNiP–V as both cathode and anode exhibits good stability, which only needs 1.61 V to achieve a current density of 100 mA cm^?2, and the cell voltage barely rises 10% after 100 h’ test under 100 mA cm^?2. Therefore, CoNiP–V is promising for the development of efficient water splitting catalysts.     

Ni3S2-MoSx nanorods grown on Ni foam as high-efficient electrocatalysts for overall water splitting

In order to solve the problem of large overpotential in water electrolysis for hydrogen production, transition metal sulfides are promising bifunctional electrocatalysts for hydrogen evolution reaction/oxygen evolution reaction that can significantly reduce overpotential. In this work, Ni₃S₂ and amorphous MoS_x nanorods directly grown on Ni foam (Ni₃S₂-MoS_x/NF) were prepared via one-step solvothermal process, which were used as a high-efficient electrocatalyst for overall water splitting. The Ni₃S₂-MoS_x/NF composite exhibits very low overpotentials of 65 and 312 mV to reach 10 mA cm⁻² and 50 mA cm⁻² in 1.0 M KOH for HER and OER, respectively. Besides, it exhibits a low Tafel slope (81 mV dec⁻¹ for HER, 103 mV dec⁻¹ for OER), high exchange current density (1.51 mA cm⁻² for HER, 0.26 mA cm⁻² for OER), and remarkable long-term cycle stability. This work provides new perspective for further the development of highly effective non-noble-metal materials in the energy field.     

MoS2/NiFeS2 heterostructure as a highly efficient electrocatalyst for overall water splitting at high current densities

Searching for efficient, stable and low-cost nonprecious catalysts for oxygen and hydrogen evolution reactions (OER and HER) is highly desired in overall water splitting (OWS). Herein, presented is a nickel foam (NF)-supported MoS₂/NiFeS₂ heterostructure, as an efficient electrocatalyst for OER, HER and OWS. The MoS₂/NiFeS₂/NF catalyst achieves a 500 mA cm⁻² current density at a small overpotential of 303 mV for OER, and 228 mV for HER. Assembled as an electrolyzer for OWS, such a MoS₂/NiFeS₂/NF heterostructure catalyst shows a quite low cell voltage (≈1.79 V) at 500 mA cm⁻², which is among the best values of current non-noble metal electrocatalysts. Even at the extremely large current density of 1000 mA cm⁻², the MoS₂/NiFeS₂/NF catalyst presents low overpotentials of 314 and 253 mV for OER and HER, respectively. Furthermore, MoS₂/NiFeS₂/NF shows a ceaseless durability over 25 h with almost no change in the cell voltage. The superior catalytic activity and stability at large current densities (>500 mA cm⁻²) far exceed the benchmark RuO₂ and Pt/C catalysts. This work sheds a new light on the development of highly active and stable nonprecious electrocatalysts for industrial water electrolysis.     

Interface regulation of Pt quantum dots doped nickel phosphide and cobalt hydroxide to promote electrocatalytic overall water splitting

The transition metal phosphates are earth-abundant minerals that have been shown to perform well in electrocatalytic water splitting, whereas these catalysts still tend to have excessively high overpotentials and slow kinetics in HER and OER processes. In the present work, hybrid catalysts consisting of Pt quantum dots doped NiP (NiP-Pt) nano-embroidery spheres and Co(OH)₂ nanosheets were successfully prepared by two-step electrodeposition method. The excellent catalytic performance of the catalyst relies principally on the synergistic interaction between NiP and Pt quantum dots. Additionally, the NiP-Pt exhibits strong electronic interactions at the interface with Co(OH)₂. Consequently, the catalyst has a strong catalytic performance in terms of HER and OER catalytic performance. In terms of HER, an overpotential of only 40 mV is required when the current density reaches 10 mA cm^?2, corresponding to a Tafel slope of 49.85 mV·dec^?1. At the same time, the catalyst also performs well at OER, with a current density of 10 mA cm^?2 at an overpotential of 186 mV and a Tafel slope of 53.049 mV·dec^?1 much less than most electrocatalysts. This study involving electrodeposition and doping of quantum dots provides a new idea for the efficient synthesis of fundamental HER and OER bifunctional catalysts.     

High throughput preparation of Ni–Mo alloy thin films as efficient bifunctional electrocatalysts for water splitting

The synthesis of cost-effective and high-performance electrocatalysts for water splitting is the main challenge in electrochemical hydrogen production. In this study, we adopted a high throughput method to prepare bi-metallic catalysts for oxygen/hydrogen evolution reactions (OER/HER). A series of Ni–Mo alloy electrocatalysts with tunable compositions were prepared by a simple co-sputtering method. Due to the synergistic effect between Ni and Mo, the intrinsic electrocatalytic activity of the Ni–Mo alloy electrocatalysts is improved, resulting in excellent HER and OER performances. The Ni₉₀Mo₁₀ electrocatalyst shows the best HER performance, with an extremely low overpotential of 58 mV at 10 mA cm^?2, while the Ni₄₀Mo₆₀ electrocatalyst shows an overpotential of 258 mV at 10 mA cm^?2 in OER. More significantly, the assembled Ni₄₀Mo₆₀//Ni₉₀Mo₁₀ electrolyzer only needs a cell voltage of 1.57 V to reach 10 mA cm^?2 for overall water splitting.     

Spatially confined growth of ultrathin NiFe layered double hydroxide nanosheets within carbon nanofibers network for highly efficient water oxidation

The rational design of catalysts with low cost, high efficient and robust stability toward oxygen evolution reaction (OER) is greatly desired but remains a formidable challenge. In this work, a one-pot, spatially confined strategy was reported to fabricate ultrathin NiFe layered double hydroxide (NiFe-LDH) nanosheets interconnected by ultrafine, strong carbon nanofibers (CNFs) network. The as-fabricated NiFe-LDH/CNFs catalyst exhibits enhanced OER catalytic activity in terms of low overpotential of 230 mV to obtain an OER current density of 10 mA cm^?2 and very small Tafel slope of 34 mV dec^?1, outperforming pure NiFe-LDH nanosheets assembly, commercial RuO₂, and most non-noble metal catalysts ever reported. It also delivers an excellent structural and electrocatalytic stability upon the long-term OER operation at a large current of 30 mA cm^?2 for 40 h. Furthermore, the cell assembled by using NiFe-LDH/CNFs and commercial Pt/C as anode (+) and cathode (?) ((+)NiFe-LDH/CNFs||Pt/C(?)) only requires a potential of 1.50 V to deliver the water splitting current of 10 mA cm^?2, 130 mV lower than that of (+)RuO₂||Pt/C(?) couple, demonstrating great potential for applications in cost-efficient water splitting devices.     

Mo-incorporated three-dimensional hierarchical ternary nickel-cobalt-molybdenum layer double hydroxide for high-efficiency water splitting

Flowers-like 3D hierarchical ternary NiCoMo-layered double hydroxide (NiCoMo-LDH) spheres have been fabricated in substrate-free route via a one-pot hydrothermal method and utilized as efficient electrocatalysts for the OER and HER. The well-structured 3D hierarchical flowers were composed of numerous two-dimensional nanosheets, which inherently possess considerable electrochemical active sites, thereby enhancing catalytic activity. NiMo and CoMo binary LDHs, with similar morphology, were also prepared to illustrate the efficiency of the ternary LDH. The results indicate higher electrocatalytic activity for the ternary LDH as compared to binary LDHs under alkaline conditions. The NiCoMo-LDH required an overpotential as low as 202 and 93 mV to deliver a constant anodic and cathodic current density of 10 mA cm^?2 for the OER and HER, respectively. Furthermore, the NiCoMo-LDH exhibited remarkable HER activity, affording a low overpotential of 198 mV at a current density of ?100 mA cm^?2. Moreover, it could offer a stable current density of 10 mA cm^?2 for overall water splitting at 1.62 V in 1 M KOH with long-term stability for 20 h. The double-layer capacitance (C_dl) value indicated that the NiCoMo-LDH significantly influenced interface conductivity and the electrochemical active surface area. The ternary NiCoMo-LDH electrode yielded low Tafel slope values of 54 and 51 mVdec^?1 for the OER and HER. Owing to the efficient incorporation of Ni, Co, and Mo in a layered structure, synergetic effect, and high electrochemical surface area, the NiCoMo-LDH exhibited remarkable electrocatalytic activity. Such eco-friendly ternary LDHs can be used in rechargeable metal–air batteries for industrial applications.     

Binder-free bifunctional SnFe sulfide/oxyhydroxide heterostructure electrocatalysts for overall water splitting

Developing robust non-noble catalysts towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is vital for large-scale hydrogen production from electrochemical water splitting. Here, we synthesize Sn- and Fe-containing sulfides and oxyhydroxides anchored on nickel foam (SnFeS_xO_y/NF) using a solvothermal method, in which a heterostructure is generated between the sulfides and oxyhydroxides. The SnFeS_xO_y/NF exhibits low overpotentials of 85, 167, 249, and 324 mV at 10, 100, 500 and 1000 mA cm^?2 for the HER, respectively, and a low overpotential of only 281 mV at 100 mA cm^?2 for the OER. When it serves as both anode and cathode to assemble an electrolyzer, the cell voltage is only 1.69 V at 50 mA cm^?2. The sulfides should be the efficient active species for the HER, while the oxyhydroxides are highly active for the OER. The unique sulfide/oxyhydroxide heterostructure facilitates charge transfer and lowers reaction barrier, thus promoting electrocatalytic processes.     

Autogenous growth of highly active bifunctional Ni–Fe2B nanosheet arrays toward efficient overall water splitting

Developing an effective and low-cost bifunctional electrocatalyst for both OER and HER to achieve overall water splitting is remaining a challenge to meet the needs of sustainable development. Herein, an electroless plating method was employed to autogenous growth of ultrathin Ni–Fe₂B nanosheet arrays on nickel foam (NF), in which the whole liquid phase reduction reaction took no more than 20 min and did not require any other treatments such as calcination. In 1.0 M KOH electrolyte, the resulted Ni–Fe₂B ultrathin nanosheet displayed a low overpotential of 250 mV for OER and 115 mV for HER to deliver a current density of 10 mA cm^?2, and both OER and HER activities remained stable after 26 h stability testing. Further, the couple electrodes composed of Ni–Fe₂B could afford a current density of 10 mA cm^?2 towards overall water splitting at a cell voltage of 1.64 V in 1.0 M KOH and along with excellent stability for 26 h. The outstanding electrocatalytic activities can be attributed to the synergistic effect of electron-coupling across Ni and Fe atoms and active sites exposed by large surface area. The effective combination of low cost and high electrocatalytic activity brings about a promising prospect for Ni–Fe₂B nanosheet arrays in the field of overall water splitting.     

Preparation of NiCo-LDH@NiCoV-LDH interconnected nanosheets as high-performance electrocatalysts for overall water splitting

Alkaline water electrolysis is a promising strategy for the production of hydrogen and oxygen. However, developing high-efficiency non-precious electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is still a big challenge. Here, we report a nickel foam-based electrode coated with NiCoV-LDH and NiCo-LDH nanosheets (denoted as NiCo-LDH@NiCoV-LDH/NF) by a two-step method for efficient water splitting performance. The NiCo-LDH@NiCoV-LDH/NF with unique nanosheet-on-nanosheet construction can enlarge the electrochemical active specific surface area greatly, and thus accelerate the charge transfer of electrocatalytic reactions. Besides, the doping of vanadium could also improve the OER performance. The electrode only requires a low overpotential for OER (260 mV at 100 mA cm^?2), and HER (80 mV at 10 mA cm^?2) reactions in 1.0 mol/L KOH solution at room temperature. Furthermore, in the two-electrode water splitting test, a current density of 10 mA cm^?2 was achieved at 1.55 V using 1.0 mol/L KOH solution, with excellent durability of 40 h. This work provided a facile method for developing new bifunctional catalysts.     

Multi-active site CoNi-MOFs as non-noble bifunctional electrocatalysts for highly efficient overall water splitting

The high-efficiency non-precious metal catalysts for oxygen evolution (OER) and hydrogen evolution (HER) are of great significance to the development of renewable energy technologies. Herein, a multiple active sites CoNi-MOFs-DBD electrocatalyst modified by low temperature plasma (DBD) was successfully synthesized by converting metal hydroxyfluoride on nickel foam into a well-arranged MOFs array using vapor deposition. The as-prepared CoNi-MOFs-DBD electrode showed better HER and OER catalytic activity, super hydrophilicity, and excellent stability. In an alkaline medium, the overpotential of HER is 203 mV at 10 mA cm^?2 and that of OER is 168 mV at 40 mA cm^?2. When CoNi-MOFs-DBD was used as a bifunctional electrocatalyst for overall water splitting in a two-electrode system, a current density of 10 mA cm^?2 can be achieved at a low voltage of 1.42 V, which shows great potential in electrocatalytic water splitting.     

Porous NiFe alloys synthesized via freeze casting as bifunctional electrocatalysts for oxygen and hydrogen evolution reaction

Binder-free NiFe-based electrocatalyst with aligned pore channels has been prepared by freeze casting and served as a bifunctional catalytic electrode for oxygen and hydrogen evolution reaction (OER and HER). The synergistic effects between Ni and Fe result in the high electrocatalytic performance of porous NiFe electrodes. In 1.0 M KOH, porous Ni₇Fe₃ attains 100 mA cm⁻² at an overpotential of 388 mV with a Tafel slope of 35.8 mV dec⁻¹ for OER, and porous Ni₉Fe₁ exhibits a low overpotential of 347 mV at 100 mA cm⁻² with a Tafel slope of 121.0 mV dec⁻¹ for HER. The Ni₉Fe₁//Ni₉Fe₁ requires a low cell voltage of 1.69 V to deliver 10 mA cm⁻² current density for overall water splitting. The excellent durability at a high current density of porous NiFe electrodes has been confirmed during OER, HER and overall water splitting. The fine electrocatalytic performances of the porous NiFe-based electrodes owing to the three-dimensionally well-connected scaffolds, aligned pore channels, and bimetallic synergy, offering excellent charge/ion transfer efficiency and sizeable active surface area. Freeze casting can be applied to design and synthesize various three-dimensionally porous non-precious metal-based electrocatalysts with controllable multiphase for energy conversion and storage.     

Cadmium sulfide embedded Prussian Blue as highly active bifunctional electrocatalyst for water-splitting process

Hydrogen production by water-splitting has limited commercial application as substantial amount of energy is required for the favorable kinetics of the process. We present an interface engineering strategy for constructing a bifunctional electrode material for an efficient water splitting process. Designed cadmium sulphide and Prussian blue nanorods (CdS-NRs@PBNPs) heterostructures acts as bifunctional electrocatalyst improved water splitting performance, for both HER and OER. For HER, the optimized hybrid CdS-NRs@PBNPs (1:1) showed significantly a low overpotentials of 126 mV and 181 mV at current densities of 10 mA cm^?2 and 20 mA cm^?2 respectively. For OER it displays an overpotential of 250 mV and 316 mV at current densities of 10 mA cm^?2 and 20 mA cm^?2. Additionally, the CdS-NRs@PBNPs (1:1) has demonstrated long-term stability. The hybrid's enhanced OER and HER activity is attributable to a synergetic impact between CdS-NRs and PBNPs, as well as the active site modification due to the presence of Cadmium and iron in the hybrid.     

Transition metal carbides coupled with nitrogen-doped carbon as efficient and stable Bi-functional catalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing non-precious metal catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for proton exchange membrane fuel cell (PEMFC), metal-air batteries and water splitting. Here, we report a in-situ simple approach to synthesize ultra-small sized transition metal carbides (TMCs) nanoparticles coupled with nitrogen-doped carbon hybrids (TMCs/NC, including WC/NC, V₈C₇/NC and Mo₂C/NC). The TMCs/NC exhibit excellent ORR and HER performances in acidic electrolyte as bi-functional catalysts. The potential of WC/NC at the current density of 3.0 mA cm^?2 for ORR is 0.814 V (vs. reversible hydrogen electrode (RHE)), which is very close to Pt/C (0.827 V), making it one of the best TMCs based ORR catalysts in acidic electrolyte. Besides, the TMCs/NC exhibit excellent performances toward HER, the Mo₂C/NC only need an overpotential of 80 mV to drive the current density of 10 mA cm^?2, which is very close to Pt/C (37 mV), making it the competitive alternative candidate among the reported non-precious metal HER catalysts.     

Preparation of ultrathin molybdenum disulfide dispersed on graphene via cobalt doping: A bifunctional catalyst for hydrogen and oxygen evolution reaction

The development of highly active and low-cost catalysts for hydrogen evolution reaction (HER) is significant for the development of clean and renewable energy research. Owing to the low H adsorption free energy, molybdenum disulfide (MoS₂) is regarded as a promising candidate for HER, but it shows low activity for oxygen evolution reaction (OER). Herein, graphene-supported cobalt-doped ultrathin molybdenum disulfide (Co–MoS₂/rGO) was synthesized via a one-pot hydrothermal method. The obtained hybrids modified electrode exhibits a high HER catalytic activity with a low overpotential of 147 mV at the current density of 10 mA cm⁻², a small Tafel slope of 49.5 mV dec⁻¹, as well as good electrochemical stability in acidic electrolyte. Meanwhile, the catalyst shows remarkable OER activity with a low overpotential of 347 mV at 10 mA cm⁻². The superior activity is ascribed not only to the high conductivity originated from the reduced graphene, but also to the synergistic effect between MoS₂ and cobalt.