Yusong Choi*ab,
Tae-Young Ahna,
Ji-Youn Kimac,
Eun Hye Leea and
Hye-Ryeon Yuad
aDefense Materials and Energy Development Center, Agency for Defense Development, Yuseong P.O. Box 35, Daejeon, 34060, Korea. E-mail: richpine87@gmail.com
bDepartment of Defense System Engineering, University of Science and Technology, Daejeon 34113, Korea
cDepartment of Chemical and Biomolecular Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
dDepartment of Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon 34134, Korea
First published on 15th June 2023
In this study, a nickel (Ni)-doped 1T-MoS2 catalyst, an efficient tri-functional hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) catalyst, was massively synthesized at high pressure (over 15 bar). The morphology, crystal structure, and chemical and optical properties of the Ni-doped 1T-MoS2 nanosheet catalyst were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ring rotating disk electrodes (RRDE), and the OER/ORR properties were characterized using lithium-air cells. Our results confirmed that highly pure, uniform, monolayer Ni-doped 1T-MoS2 can be successfully prepared. The as-prepared catalysts exhibited excellent electrocatalytic activity for OER, HER, and ORR owing to the enhanced basal plane activity of Ni doping and formidable active edge sites resulting from the phase transition to a highly crystalline 1T structure from 2H and amorphous MoS2. Therefore, our study provides a massive and straightforward strategy to produce tri-functional catalysts.
Moreover, MoS2 has been studied as a promising hydrogen evolution reaction (HER) catalyst meeting both technical and economic requirements11–13. As the world is moving towards using carbon-free energy sources, called the ‘hydrogen economy’, the demand for efficient and non-precious HER catalysts has increased.14,15 Many attempts have been made to clarify the catalytic mechanism with various crystallographic structures and morphologies of MoS2, including chemically exfoliated MoS2,16,17 nanostructured particles,18–21 heterostructures, and amorphous and doped MoS2. To date, experimental and computational studies have reported that the edge sites of crystalline MoS2 are catalytically active, whereas their basal planes are inert. Therefore, the best approaches for improving the catalytic functionality of MoS2 are to generate an active edge site, incorporate doping elements, or convert crystal structures.
For instance, the metallic 1T-MoS2 octahedral symmetry improves the HER performance of MoS2, as it has great potential for activating basal planes in contrast to the traditional trigonal prismatic semi-conducting 2H-phase MoS2 (see the crystal structure of 2H and 1T shown in Fig. S1†). Many reports have indicated the high performance of metallic 1T-MoS2, especially when the basal planes are tethered with a single-atom catalyst. Single-atom tethering or doping of the MoS2 basal plane can effectively enhance the catalytic activity of 1T-MoS2.
Recently, NiMoS (Ni–MoS2) heterostructure electrodes have been studied as water-splitting electrodes due to their excellent HER22 and oxygen evolution reaction (OER) activities.10–12 In addition, Ni-doped MoS2 nanorods23 and nanoflakes synthesized in Ni-doped forms have been reported. However, there are no reports on the use of massively synthesizable tri-functional Ni-doped 1T-MoS2 as electrocatalysts. Therefore, in this study, to utilize the excellent electrocatalytic properties of Ni-doped MoS2 for next-generation energy synthesis, Ni-doped MoS2 was fabricated using Ni particles, and their electrocatalytic properties were investigated.
The HRTEM image of Ni-doped MoS2 after heat treatment is shown in Fig. 2(d). The edge of the Ni-doped MoS2 nanoparticle shows a 2H phase, while the centre shows a 1T phase, as shown in Fig. 2(e) and (f).
The crystal structure of the Ni-doped 1T-MoS2 was investigated (Fig. S3†), and the peaks at 21.8°, 31.4°, 38.3°, 49.7°, and 50.1° are assigned to the (101), (101), (003), (113), and (211) planes of MoS2 (JCPDS 44-1418).25,26 Conversely, the peaks at 44.5°, 51.8°, and 76.4° were consistent with those of Ni (JCPDS 11-0099). In the X-ray diffraction (XRD) peaks of Ni-doped 1T-MoS2, in contrast to pure MoS2, the diffraction peaks of Ni-doped MoS2 shifted slightly above 44.63°. These shifts indicate that the lattice parameters of doped MoS2 increase because of the larger ion size of Mo than that of Ni. This 1T Ni-doped MoS2 structure changes after heat-treatment of Ni-doped MoS2, as shown in Fig. S4.†
An XPS analysis was performed to investigate the chemical valence states of these elements (the full XPS spectrum has been provided in Fig. S4†). The XPS spectrum of the MoS2 series in the Mo 3d region can be divided into four peaks (Fig. 3(a)). The peak at 225 eV corresponds to S 2s of the MoS2 series.27 The minor peak (236.0 eV) is attributed to Mo6+, which corresponds to the slight oxidation of the Mo edges of MoS2 upon exfoliation, as reported by Gopalakrishnan et al.28 The Mo 3d peaks of the received MoS2 and Ni-doped MoS2 as a function of heat treatment, were observed as shown in Fig. 3(a) and (b), respectively. The two intense peaks at approximately 231 and 228 eV being ascribed to Mo 3d3/2 and Mo 3d5/2, respectively, should be attributed to the Mo4+ in the MoS2 series.29–31 As observed in Fig. 3(a), the MoS2 showed a 2H-phase based on the Mo 3d5/2 and Mo 3d3/2 orbitals with peaks at 228.6–228.7 eV and 231.8–231.9 eV, respectively.
Fig. 3 The XPS spectra of Mo 3d for (a)MoS2 and (b) Ni-doped MoS2 series as-synthesized after heat treatment and annealing. |
However, a noticeable red shift of Mo 3d5/2 and Mo 3d3/2 orbitals was observed for the Ni-doped MoS2, according to the heat-treatment condition (in Fig. 3(b)), which is believed to represent the 2H to 1T phase shift of MoS2.32,33 It has been reported that, when the phase transition from 2H to 1T occurs, the 3d5/2 and 3d2/3 components of Mo–S bonding in XPS is measured to be about 1 eV lower.34,35 Therefore, this result is consistent with the decrease in binding energy (∼1.1 eV) of Mo 3d3/2 upon heat-treatment. The Raman analysis results demonstrate the 1T in the 2H MoS2 structure occurrence after heat-treatment (Fig. S5†).
Based on XPS and XRD results, it can be concluded that the crystal structure transformation of Ni-doped MoS2 from the 2H to 1T phase occurred during heat treatment. EDS element mapping analysis showed a uniform distribution of Ni, Mo, and S throughout the entire grid of Ni-doped 1T-MoS2 specimens (see Fig. S6†).
Fig. 4(a) shows HER Tafel plots of Ni-doped MoS2. Pt/C was also studied for comparison. Fig. S7† shows the RHE calibration. The potential was swept at 1 mV s−1 in H2-saturated 1 M KOH. Pt wires were used as working and counter electrodes, while Ag/AgCl was used as a reference electrode. For detailed electrochemical analysis, RHE calibration was conducted with Pt wires and Ag/AgCl (saturated KCl) reference electrodeFig. S7.† The RHE calibration was calculated using the following eqn (1),
ERHE = Emeasured by Ag/AgCl + 1.029 V | (1) |
Among all the catalysts tested, both as-synthesized and heat-treated Ni-doped MoS2 exhibited inferior catalytic activities for the HER with overpotentials. However, the stability of the heat-treated Ni-doped MoS2 was much higher than that of Pt/C over 2500 s. The semicircle of the heat-treated Ni-doped MoS2 in the Nyquist plot shown in Fig. 4(c) decreased significantly following heat treatment. The decrease in resistance is ascribed to the increase in crystallinity and the phase transition from 2H to the metallic 1T structure of MoS2.
Pattenggale et al.36 and Huang et al.37 reported Ni@1T-MoS2 for HER. Pattengale reported remarkable HER performance compared to Pt/C in both acidic and alkaline solutions. Pattengale et al. reported that the remarkable HER performance is attributed to the Ni sites in the basal plane of Ni@1T-MoS2, which are active towards HER. Pattengale et al. and Huang et al. synthesized Ni@1T-MoS2 by hydrothermal synthesis; however, they reported two steps. First, the NiMo6 precursor was synthesized, and then sulfuration to NiMo6 was carried out.
OER properties were also studied, as shown in Fig. 4(d)–(f). For Ni-doped MoS 2 after heat-treatment, a lower Tafel slope of 87 mV dec−1 is observed. In contrast, higher Tafel slopes of 114 and 127 mV dec−1 were obtained for IrO2 and as-synthesized Ni-doped MoS2, respectively. Ni-doped MoS2 exhibited ORR activity with a low overpotential of 68 mV. It is worth noting that Ni-doped MoS2 series show superior activity to those of the reported MoS2, Ni-doped MoS2 nanorods, nanosheets, and most of the reported non-noble metal-based ORR catalysts. The corresponding Tafel plots were used to estimate the ORR kinetics of these electrodes (Fig. 4(g)).
To evaluate the OER/ORR stability of Ni-doped MoS2 in 1 M KOH solution, we measured the time-dependent potential curve at 10 mA cm−2. As shown in Fig. 4(e) and (h), even after the 2700 s continuous release of oxygen, the potential of network Ni-doped MoS2 after heat-treatment is still well maintained, implying excellent long-term durability for the OER and ORR. The semicircles of heat-treated Ni-doped MoS2 in the Nyquist plot shown in Fig. 4(f) and (i) decreased significantly following the heat treatment. The decrease in resistance accounts for the increase in crystallinity and the phase transition from 2H to the metallic 1T structure of MoS2. The results suggest that the prepared Ni-doped MoS2 series catalyst exhibited strong catalytic activity and stability during the OER and ORR processes. The specific surface area (SSA) plays a vital role in electrochemical activities. To better understand the intrinsic properties of each catalyst, their SSA was estimated using the Brunauer–Emmett–Teller (BET) technique, as shown in Fig. S8.† The Ni-doped MoS2 after the H/T catalyst shows much higher specific activity than the Ni-doped MoS2 no H/T catalyst for HER, ORR, and OER. Interestingly, the specific current density of the Ni-doped MoS2 after the H/T catalyst is much higher than the state-of-art Pt/C and IrO2 for the ORR and OER process. Overall, the Ni-doped MoS2 after the H/T catalyst is an excellent electrocatalyst with great promise for practical application.
Furthermore, it affords only a 0.835 V polarization gap between OER and ORR for Ni-doped MoS2 after heat treatment, making it among the most active reported electrocatalysts. For comparison, Pt/C and IrO2 were also measured, and the total polarization of Pt/C IrO2 was 0.837 V (see Fig. S9†).
The OER and ORR performance of Ni-doped MoS2 according to the heat-treatment effect was studied using full cells. For comparison, a Pt/C/IrO2 cell was also fabricated, as shown in Fig. 5. After the filtration and drying, the cathode was examined using TEM. Fig. 5(a)–(d) shows the TEM analysis results. Fig. 5(a) and (b) show the CNT-only sample of dark field TEM image and TEM EDS mapping image, respectively. As shown in Fig. 5(a) and (b), only CNT and carbon from CNT exist. However, in Fig. 5(c) and (d), the Ni-doped MoS2 catalysts adhere well to the CNT fibres. These samples were fabricated to the full cell (CR 2032) to investigate the real OER and ORR performance. For comparison, IrO2 and Pt/C were also fabricated with the filtration method.
All cells were cycled at a 0.05 mA h cm−1 cut-off voltage of 2 V, and the results are shown in Fig. 5(e). The Ni-doped MoS2 cells exhibited better cycle performance than Pt/C/IrO2 cells. The Pt/C/IrO2 cell cycled for 160 h, and synthesized Ni-doped MoS2 cycled for 247 h, which is a 55% increase. However, above 500 and 600 °C, the heat-treatment times to reach the cut-off (2 V) were 400 and 560 h, respectively. The cycle performance improved significantly with increased heat-treatment temperature and time.
After the cycle test, the cathodes were disassembled, and XRD analysis was conducted, as shown in Fig. S10.† There is no carbon electrode decomposition evidence of Li2CO3 by the reaction with oxygen as well as carbon has not been found. The drastically enhanced OER and ORR performance of Ni-doped MoS2 over Pt/C/IrO2 can be ascribed to the increase in crystallinity and the phase shift from 2H to 1T-MoS2 following heat treatment. The results indicate that the proposed massively synthesizable Ni-doped 1T-MoS2 tri-functional catalyst has robustness, good cyclability, and applicability as a next-generation lithium–air battery electrode.
Based on the structural and catalytic analysis results for the hydrothermally synthesized Ni-doped MoS2, we investigated the proposed structure tailoring path, as shown in Fig. 6. Ni-doped MoS2 microsized particles in 2D amorphous MoS2 were synthesized through conventional hydrothermal synthesis (no stirring and low pressure at 4–5 bar). However, Ni-doped MoS2 nanoparticles in 2D amorphous MoS2 synthesized at high pressure (over 15 bar) with stirring were applied during hydrothermal synthesis. Heat treatment at 500 °C was conducted on the Ni-doped MoS2 nanoparticles in 2D amorphous MoS2, synthesized at high pressure and converted to a highly crystalline 2D nanosheet with a 1T structure, which showed stable and efficient tri-functional catalytic performance for HER, OER, and ORR.
Fig. 6 The schematic of the proposed Ni-doped MoS2 morphology and crystal structure varies according to the hydrothermal synthesis conditions (applying the stirring and pressure control). |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra03016d |
This journal is © The Royal Society of Chemistry 2023 |