Bimetallic Nickel Cobalt Sulfide as Efficient Electrocatalyst for Zn–Air Battery and Water Splitting
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Bimetallic nickel cobalt sulfide (Ni,Co)S2 nanosheet arrays were demonstrated as a multifunctional catalyst for OER, HER, and ORR.
First principle calculations were performed to probe the rate-limiting step, which involves the formation of *OOH from HO− on the (Ni,Co)S2 surface.
A water-splitting system was designed with the (Ni,Co)S2 serving as both cathode and anode, and a Zn–air battery cathode electrocatalyst.
Keywords(Ni,Co)S2 nanosheet arrays DFT calculations Zn–air batteries Water splitting
The ever-worsening environmental issues and non-renewability of fossil fuels have stimulated extensive investigations for the development of sustainable energy in future energy conversion and storage technology [1, 2, 3]. The high-rate oxygen reduction or evolution reaction (ORR or OER) and hydrogen evolution reaction (HER) at lower overpotentials are of great importance to the enhancement of energy utilization rate and output power in these green energy systems. At present, the bottleneck of both water-splitting technologies and rechargeable metal–air batteries is the availability of highly efficient and durable electrocatalysts. Zn–air batteries have the merits of high theoretical energy density, environmental friendliness, and high safety for the next-generation energy storage systems [4, 5], where its development is still hampered by a low working voltage owing to the sluggish rate of ORR/OER [6, 7]. Here, HER, which is a crucial electrochemical reaction in water splitting and requires highly efficient electrocatalysts, is equally important [8, 9]. Pt-based materials exhibit excellent catalytic efficiency for HER and ORR, while Ru- and Ir-based materials are the best electrocatalysts for OER reactions [10, 11, 12]. However, their high scarcity, high cost, and insufficient long-term stability are limiting the large-scale commercial applications [13, 14]. Therefore, earth-abundant, durable, and highly efficient trifunctional (ORR, OER, and HER) electrocatalysts are urgently required [15, 16].
As a class of low-cost alternatives, transition metal-based materials, such as transition metal phosphides [17, 18], oxides [19, 20, 21], sulfides [22, 23], selenides [24, 25], nitrides [26, 27], borides [28, 29], hydroxides [30, 31], and others [32, 33, 34], have attracted overwhelming research interests recently. In particular, transition metal sulfides, such as CoS2 and NiS2, are considered a group of low-cost and eco-friendly electrocatalysts for ORR, OER, and HER owing to their high electrocatalytic activity, high stability, and cost-effectiveness [35, 36, 37]. Substitution of the transition metals with other dopants (such as V, Mn, and Cu) has been proved to enhance their electrocatalytic performance because of the synergistic effects among the metallic atoms [38, 39, 40]. Caban-Acevedo et al.  recently demonstrated that the replacement of S atom by P atom in CoS2, forming CoPS, could alter the electronic structure and dramatically enhance the HER performance. Liang et al.  also revealed that their bimetallic NiCoP nanostructures show superior catalytic activity toward both HER and OER in alkaline media compared to monometallic Ni2P. Although similar efforts are expected to be made for the bimetallic NiCoS, compared with the monometallic counterparts, challenges exist in the design of multifunctional catalysts.
As is known, both CoS2 and NiS2 have the same crystal structure, and the chemical nature and atomic radius of Ni and Co atoms are very similar, which would enable the formation of bimetallic NiCoS. In this work, we present a detailed study on the synthesis of single-phase bimetallic nickel cobalt sulfide (denoted as (Ni,Co)S2) nanosheets by the hydrothermal process and subsequent post-sulfuration. The resulting (Ni,Co)S2 shows the desired trifunctional electrocatalytic activities in OER, ORR, and HER as an electrocatalyst, and therefore has promising potential as a cathode in Zn–air batteries and water-splitting catalysis. In addition, it demonstrates excellent OER activity with an overpotential of 270 mV at 10 mA cm−2 and a notable outstanding potential difference (ΔE = Ej=10–E1/2) between E1/2 for ORR and Ej=10 for OER of only 0.79 V, thus outperforming many of the bifunctional electrocatalysts. The air electrode made of (Ni,Co)S2 nanosheets exhibited superior performance in both primary and rechargeable Zn–air batteries, showing a specific capacity of 842 mAh g Zn −1 at 5 mA cm−2, a high and stable open circuit potential of 1.48 V, a large peak power density of 152.70 mW cm−2, and excellent cycling stability without any decrease in polarization even after 480 h. The rechargeable Zn–air batteries using (Ni,Co)S2 as the cathode could efficiently power an electrochemical water-splitting unit catalyzed by the (Ni,Co)S2 nanosheets grown on a carbon cloth for both OER and HER, thus demonstrating its potential as an integrated green energy system.
2.1 Synthesis of NiS2, CoS2, and (Ni,Co)S2
2.1.1 Preparation of Precursors
Precursors for (Ni,Co)S2 were synthesized on a carbon cloth by modifying a reported procedure . First, 1.5 mmol NiCl2·6H2O, 3.0 mmol NH4F, 7.5 mmol (NH2)2CO, and 1.5 mmol Co(NO3)2·6H2O were dissolved in 50 mL de-ionized water. Then, 16 mL of the solution was transferred to a 23 mL PTFE-lined stainless steel autoclave containing the substrate leaning against the autoclave wall. The sealed autoclave was heated at 110 °C for 5 h. After cooling, the substrate was taken out, washed with water and ethanol, and dried in an oven at 60 °C for 30 min. The precursor of NiS2 or CoS2 was prepared by the same above-mentioned process, except without the addition of Co(NO3)2·6H2O or NiCl2·6H2O, respectively.
2.1.2 Thermal Conversion
A carbon cloth covered with the as-grown precursor was placed in the center of a fused silica tube in a tube furnace equipped with gas flow controllers. An alumina boat containing 10 mmol of sulfur powder was placed at the furthest upstream position within the reactor tube. The tube was then purged three times with argon gas and maintained at 101.3 kPa under a steady flow of Ar carrier gas (99.999%) at 25 sccm (standard cubic centimeter per minute). The temperature of the furnace was ramped to 500 °C and held for 60 min. After cooling under Ar flow, the sample was removed and rinsed with CS2 (99.9%) for 10 min, then washed with ethanol, and dried in an oven at 60 °C for 1 h.
2.2 Preparation of Electrocatalyst Ink
The catalyst ink was typically made by dispersing 10 mg of the catalyst and 10 mg of carbon black (Vulcan XC72) in 50 mL petroleum ether, and then dropped them on a carbon cloth. After drying, 18 mg of catalyst, 90 μL Nafion-117 solution, and 4410 μL N, N-dimethylformamide (DMF) were added into a 10 mL container and ultrasonicated for 30 min.
2.3 Calculation Details
The DFT calculations were performed by Vienna ab initio simulation package (VASP). The standard generalized-gradient approximation (GGA) in the form of the Perdew–Burke–Ernzerhof (PBE) exchange model was used. The energy cutoff for the plane-wave basis set and the convergence threshold to obtain the wave functions were 400 and 10−5 eV, respectively. Further, 3d electrons of Ni were treated using the GGA + U method with a Ueff (U–J) of 5.76 eV. Ionic relaxations were conducted until all the force components became < 0.02 eV Å−1. For the density of states (DOS), the Brillouin zone is represented by the set of 5 × 5×5 k points for geometry optimizations. A rectangular supercell of size 11.00 × 11.00 Å2 was used to calculate the OER activity with active sites on the (100) surface.
3 Results and Discussion
The CV scan results are shown in Fig. 4b. The curve measured in a N2-saturated electrolyte solution is smooth, indicating no oxygen reduction reaction. However, in the O2-saturated electrolyte solution, a sharp cathodic peak appeared at 0.75 V, revealing the occurrence of an ORR. Under the same test conditions, the oxygen reduction peaks of NiS2 and CoS2 are 0.74 and 0.67 V, respectively (Fig. S17). To explore the reaction mechanism of oxygen reduction, LSV curves with various speeds (from 400 to 2400 rpm) were measured, and the results shown in Fig. 4c indicate that the current density increases with increasing O2 diffusion rate. According to the K–L equation , the calculated electron transfer number (n) is 3.8, which indicates that a four-electron process dominates the oxygen reduction for (Ni,Co)S2. Table S3 lists the ORR parameters of the three catalysts. The ORR path was further verified with a rotating ring-disk electrode (RRDE) at 1.3 V at a rate of 2 mV s−1. As shown in Fig. 4d, the n value (3.9) thus estimated is consistent with the result obtained from the K–L equation. It clearly indicates that the oxygen reduction proceeds via an efficient four-electron pathway. A comparison of the ORR performance of (Ni,Co)S2 with the performances of some of the reported catalysts is shown in Table S4. As a bifunctional electrocatalyst, the overall oxygen activity of (Ni,Co)S2 is evaluated by the potential difference (ΔE = Ej=10–E1/2) between E1/2 for ORR and Ej=10 for OER. In general, an efficient reversible oxidation reaction requires a small ΔE, with the ΔE of commercial state-of-the-art electrocatalysts reported as 0.94 V for Pt/C and 0.92 V for Ir/C and Ru/C . Figure 4e shows that the ΔE of (Ni,Co)S2 is 0.79 V, which is much lower than those of the reported precious electrocatalysts (Table S5) as well as the ΔE of NiS2 (0.95 V) and CoS2 (0.94 V). This further indicates the excellent electrocatalytic characteristics of (Ni,Co)S2 as a multifunctional electrocatalyst.
In summary, single-phase bimetallic (Ni,Co)S2 nanosheets were successfully synthesized by a hydrothermal route followed by thermal conversion to sulfide. With the purposely tuned nanosheet morphology, electronic structure, enhanced electrical conductivity, and active sites in the bimetallic sulfides, the (Ni,Co)S2 nanosheets demonstrated a superior electrocatalytic performance for oxygen evolution, oxygen reduction, and hydrogen evolution in an alkaline electrolyte. First principle calculation results indicate that the adsorption of HO− to form *OOH on the (Ni,Co)S2 surface is the potential limiting step in the OER. When used as an electrode in a Zn–air battery, it demonstrated a small charge/discharge voltage gap of 0.45 V at 2 mA cm−2, a high peak power density of 153.5 mW cm−2, a specific capacity of 842 mAh g Zn −1 at 5 mA cm−2, and excellent cycling stability even after 480 h. The high efficiency demonstrates the application potential of the rechargeable Zn–air battery in powering an electrochemical water-splitting unit made of the same (Ni,Co)S2 nanosheets as both the electrodes, which exhibited a low cell voltage of 1.71 V at 10 mA cm−2. This work is helpful for improving the Zn–air battery performance and the utilization of new energy in the future.
This work is supported by the National Natural Science Foundation of China (Grant Nos. 11474137 and 11674143), Program for Changjiang Scholars and Innovative Research Team in University (IRT 16R35), the Fundamental Research Funds for the Central Universities (Grant Nos. LZUMMM2018017, lzujbky-2018-121). John Wang acknowledges the support of Ministry of Education (MOE2016-T2-2-138, Singapore), for research conducted at the National University of Singapore.
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