Efficient Hydrogen Evolution on Cu Nanodots-Decorated Ni3S2 Nanotubes by Optimizing Atomic Hydrogen Adsorption and Desorption

Low-cost transition-metal dichalcogenides (MS2) have attracted great interest as alternative catalysts for hydrogen evolution. However, a significant challenge is the formation of sulfur–hydrogen bonds on MS2 (S–Hads), which will severely suppress hydrogen evolution reaction (HER). Here we report Cu nanodots (NDs)-decorated Ni3S2 nanotubes (NTs) supported on carbon fibers (CFs) (Cu NDs/Ni3S2 NTs-CFs) as efficient electrocatalysts for HER in alkaline media. The electronic interactions between Cu and Ni3S2 result in Cu NDs that are positively charged and can promote water adsorption and activation. Meanwhile, Ni3S2 NTs are negatively charged and can weaken S–Hads bonds formed on catalyst surfaces. Therefore, the Cu/Ni3S2 hybrids can optimize H adsorption and desorption on electrocatalysts and can promote both Volmer and Heyrovsky steps of HER. The strong interactions between Cu and Ni3S2 cause the Cu NDs/Ni3S2 NTs-CFs electrocatalysts to exhibit the outstanding HER catalytic performance with low onset potential, high catalytic activity, and excellent stability.

1. Weekly Group Discussion. Paper Presentation… Date: 30 th Oct ’20

2. What they have planned !! Typical process of HER in alkaline media on the electrocatalysts can be summarized as two radical steps via Volmer−Heyrovsky pathway. The Volmer step, namely the cleavage of O−H bonds of H 2 O to form H atoms adsorbed on the surface of catalysts (H ads ), is crucial. In addition, as the H 2 is the target product, the Heyrovsky step, namely the H ads combining H from H 2 O into H 2 gas, is also important for HER. *** Ohh I see they have targeted those two equations…*** Continued…

3. An ideal catalyst for HER in alkaline media should satisfy the following two requirements The catalyst should own the strong functions to activate and cleave O−H bonds of water molecules to form H atoms, and the H atoms should be easily adsorbed on the surface of catalysts (H ads ). The H ads cannot be securely immobilized on the surfaces of catalysts since H 2 gas should be readily released for efficient HER. Therefore, optimizing atomic hydrogen adsorption and desorption on the electrocatalysts is very important for efficient hydrogen evolution. What impression we have from those equations… So they hoped to do the development by combining Low-cost transition-metal dichalcogenides (MS 2 ) with tiny Copper heads to make the radar more effective in catching H 2 O and metal chalcogens taking care of the desorption.

4. How it looks like ? The CFs were used as a working electrode (0.5 cm × 2 cm). ZnO nanorods (NRs) were fabricated on CFs by cathodic electrodeposition at a constant current of 1.0 mA cm −2 in the solution of 0.01 M Zn(NO 3 ) 2 + 0.05 M NH 4 NO 3 (10 mL) at 70 °C for 90 min. Ni 3 S 2 layers were then coated on the surfaces of ZnO NRs to form ZnO@ Ni 3 S 2 NRs-CFs by cathodic electrodeposition in the solution of 0.01 M NiCl 2 + 0.1 M Na 2 S 2 O 3 (10 mL) at 1.0 mA cm -2 for 8 min at 70 °C. Ni 3 S 2 nanotubes (NTs)-CFs were fabricated by etching ZnO from ZnO@Ni 3 S 2 NRs CFs in 2.5 M NaOH solution (15 mL) for 2 h. Finally, Cu NDs were decorated on the surfaces of Ni 3 S 2 NTs to fabricate Cu NDs/Ni 3 S 2 NTs-CFs by using chemical reduction method in the solution of 5 mM CuAc 2 + 1 wt % polyvinylpyrrolidone (PVP) + 0.1 g NaBH 4 (10 mL) for 10 min at 30 °C. The samples were dried in nitrogen flow at 120 °C to remove excess moisture.

5. What we are getting? The diameters of Cu NDs/Ni 3 S 2 nanotubes are ∼450 nm. The wall thicknesses of Cu NDs/Ni 3 S 2 NTs are ∼30 nm The sizes of Cu NDs are ∼10 nm. The atomic ratio of Ni and S in the nanotube area is ∼1.57:1, indicating Ni and S serve as Ni 3 S 2 . The atomic ratio of Cu and O in the ND area is ∼27:1, indicating the existence of high pure metal Cu.

6. (g) TEM image of Cu NDs/Ni 3 S 2 NTs. (h) TEM image with higher magnification of the wall of Cu NDs/Ni3S2 NTs. ( i −k) Elemental mappings of Ni, Cu and S in Cu NDs/Ni3S2 NTs, demonstrates that the Cu NDs are decorated on the surfaces of Ni3S2 NTs. (l) SAED pattern of Cu NDs/Ni3S2 NTs. (m) HRTEM image of Cu/Ni3S2 border.

7. The deconvoluted Ni 2p 3/2 profiles clearly show the peaks at 856.71 and 859.09 eV correspond to Ni and Ni 2+ in Ni 3 S 2 [(Ni 2+ ) 2 (Ni )(S 2− ) 2 ], respectively. Ni 2p 1/2 profiles show the peaks at 875.63 and 876.96 eV correspond to Ni0 and Ni 2+ in Ni 3 S 2 , respectively. XPS spectrum of S 2p of Cu NDs/Ni 3 S 2 NTs- CFs shows the peak at 162.5 eV corresponds to S 2p 3/2 of S 2− and the peak at 163.9 eV corresponds to S 2p 1/2 of S 2− . XPS spectrum of Cu 2p of Cu NDs/Ni 3 S 2 NTs-CFs is shown in Figure S4c. The peaks at 932.64 and 952.54 eV can be attributed to Cu 2p 3/2 and 2p 1/2 of metal Cu, respectively

8. For Cu NDs/Ni 3 S 2 NTs- CFs, the onset overpotential is only ∼60 mV, and the overpotential at the current density of 10 mA cm −2 is only 128 mV, which is much smaller than those of Ni 3 S 2 NTs-CFs (189 mV), Cu NDs-CFs (>300 mV), and CFs (>300 mV). Cu NDs/Ni 3 S 2 NTs-CFs also give much higher current densities than Ni 3 S 2 NTs-CFs, Cu NDs, and CFs at the same overpotentials (e.g., at −0.15, −0.20, and −0.25 V) as shown in Figure 2a, indicating Cu NDs/Ni 3 S 2 NTs-CFs own much higher catalytic activity than Ni 3 S 2 NTs-CFs, Cu NDs-CFs, and CFs. The Tafel slope of Cu NDs/Ni 3 S 2 NTs-CFs at the overpotential interval between 0.01 and 0.20 V is only ∼76.2 mV/ dec , which is much smaller than those of Ni 3 S 2 NTs-CFs (125.7 mV/ dec ), Cu NDs-CFs (166.7 mV/ dec ), and CFs (168.4 mV/ dec ) as shown in Figure 2b.

9. To investigate the catalytic kinetics under the HER process, the EIS measurements were carried out from 100 MHz to 0.01 Hz at an overpotential of 200 mV. Nyquist plots of Cu NDs/ Ni 3 S 2 NTs-CFs, Ni 3 S 2 NTs-CFs, and Cu NDs-CFs are shown in Figure 2c, which shows that the charge-transfer resistance of Cu NDs/Ni 3 S 2 NTs-CFs is smaller than those of Ni 3 S 2 NTs-CFs and Cu NDs-CFs, indicating more favorable catalytic kinetics for Cu NDs/Ni 3 S 2 NTs-CFs. The turnover frequency (TOF) values of Cu NDs/Ni 3 S 2 NTs-CFs at different overpotentials were also calculated as shown in Figure S9, which shows that Cu NDs/Ni 3 S 2 NTs-CFs own much larger TOF values than those of Ni 3 S 2 NTs-CFs, Cu NDs-CFs, and CFs at different overpotentials .

10. The electrochemically active surface areas (ECSAs) of Cu NDs/Ni 3 S 2 NTs-CFs, Ni 3 S 2 NTs- CFs, and Cu NDs-CFs were determined by measuring the double-layer capacitance and Figure S10 shows that the ECSA of Cu NDs/Ni 3 S 2 NTs-CFs is much larger than those of Ni 3 S 2 NTs-CFs and Cu NDs-CFs.

11. For Cu NDs/Ni 3 S 2 NTs-CFs, the characteristic Raman bands at 196.7, 218.4, 298.1, 319.8, and 345.8 cm −1 are relative to Ni−S bonds of Ni 3 S 2 . Compared with those of Ni 3 S 2 NTs-CFs, the peaks of Cu NDs/Ni 3 S 2 NTs-CFs all show red-shifts as shown in Figure 3a. The peaks of Ni 2p 3/2 and Ni 2p 1/2 of Cu NDs/Ni 3 S 2 NTs-CFs show negative shifts of ∼0.62 and 0.98 eV, respectively, compared with those of Ni 3 S 2 NTs-CFs as shown in Figure 3b. The peaks of S 2p 3/2 and S 2p 1/2 of Cu NDs/Ni 3 S 2 NTs-CFs show negative shifts of 0.21 and 0.24 eV, respectively, compared with those of Ni 3 S 2 NTs-CFs as shown in Figure 3c. The peaks of Cu 2p 3/2 and Cu 2p 1/2 of Cu NDs/ Ni 3 S 2 NTs-CFs show positive shifts of 0.47 and 0.50 eV, respectively, compared with those of Cu NDs-CFs as shown in Figure 3d.

12. To further evaluate the long-term activity retention of electrocatalysts, the chronoamperometry experiments of Cu NDs/Ni 3 S 2 NTs-CFs were further carried out at the different overpotentials of 100, 150, 200, and 250 mV for 30 h as shown in Figure 2d, which shows the current densities almost remain unchangeable at the different overpotentials , further indicating high stability of catalytic activity of Cu NDs/Ni 3 S 2 NTs-CFs. The polarization curves of Cu NDs/Ni 3 S 2 NTs-CFs before and after HER reaction of 30 h are shown in Figure S11, which shows almost no change for these polarization curves, indicating that Cu NDs/Ni 3 S 2 NTs-CFs own excellent longterm stability of catalytic activity. Stability and Performance.

13. Other messurements showing nearly same morphology and other electronic features.

14. The Cu/Ni 3 S 2 hybrid, the positive charge on Cu increases, the positive charge on Ni of Ni 3 S 2 decreases, and the negative charge on S of Ni 3 S 2 increases compared with the individual Cu, Ni, and S, respectively, indicating the electron density on Cu decreases and the electron density on Ni 3 S 2 increases. The Cu/Ni 3 S 2 hybird owns the lowest water adsorption energy, suggesting that the water adsorption on Cu NDs/Ni 3 S 2 NTs-CFs is easier than those on Cu NDs-CFs and Cu NDs/Ni 3 S 2 NTs-CFs.

15. Before HER, the weak peaks of CuO and Cu 2 O at 523 and 606 cm −1 are seen. for Cu NDs/Ni 3 S 2 NTs-CFs, no peak is seen, indicating the electronic interactions between Ni 3 S 2 and Cu can efficiently prevent from the oxidation of Cu. After HER 30 h, Cu NDs/Ni 3 S 2 NTs-CFs shows a peak at 468 cm −1 , suggesting the existence of Cu−OH. However, for Cu NDs-CFs and Ni 3 S 2 NTs-CFs, no peak is observed at400−650 cm −1 after HER (the disappearence of CuO and Cu 2 O peaks for Cu NDs-CFs can be attributed to the reduction of H 2 ). The peak located at ∼1640 cm −1 can be assigned to the bending vibrations of O−H bonds of adsorbed water on the surfaces of catalysts. The peak of the adsorbed water on Cu NDs/Ni 3 S 2 NTs-CFs at 1636 cm −1 shows a red-shift of ∼3 cm −1 and ∼7 cm −1 compared with those of Ni 3 S 2 NTs-CFs and Cu NDs- CFs, respectively.

16. In DFT calculations, the adsorption of H on Ni 3 S 2 was found to be too strong, whereas it was too weak on Cu, resulting in low HER electrocatalytic activities on Ni 3 S 2 and Cu, respectively. For Cu/Ni 3 S 2 hybrid, it possesses appropriate adsorption of H ads shown in Figure 5a. It is worth noting that the Raman shift of S− H ads for Cu NDs/Ni 3 S 2 NTs-CFs (2542 cm −1 ) is ∼21 cm −1 red shifted compared with that of Ni 3 S 2 NTs-CFs (2563 cm −1 ). Noting that the Raman shift of S− H ads for Cu NDs/Ni 3 S 2 NTs-CFs (2542 cm −1 ) is ∼21 cm −1 red shifted compared with that of Ni 3 S 2 NTs-CFs (2563 cm −1 ), indicating that the S− H ads bonds on Cu NDs/Ni 3 S 2 NTs-CFs are weaker than those on Ni 3 S 2 NTs-CFs.

17. Figure 5c,d illustrates the promotion role of Cu/Ni 3 S 2 hybird on the improvments of H 2 O adsorption and activation and the optimization of H adsorption and desorption for efficient HER compared with the individual Ni 3 S 2 . the electronic interactions between Cu and Ni 3 S 2 will make Cu positively charged and Ni 3 S 2 negatively charged. The positively charged Cu can effectively adsorb and activate water molecules and will benefit H−O cleavage, and the negatively charged Ni 3 S 2 can weaken S− H ads bonds and thus will optimize H adsorption and desorption.

18. CONCLUSIONS: E ffi cient HER electrocatalysis in alkaline media by using Cu NDs/Ni 3 S 2 NTs-CFs as catalysts. The electronic interactions between Cu and Ni 3 S 2 make Cu positively charged and will bene fi t water adsorption and activation, while Ni 3 S 2 is negatively charged and will weaken S − H ads bonds formed on the surfaces of catalysts. The above electronic interactions can well optimize H adsorption and desorption on the surfaces of metal sul fi de-based electrocatalysts and can e ffi ciently promote Volmer and Heyrovsky steps of HER and realize e ffi cient HER in alkaline media. Cu NDs/Ni 3 S 2 NTs-CFs catalysts exhibit signi fi cantly improved electrocatalytic activity and durability for HER, such as a low onset overpotential of ∼ 60 mV, a low overpotential of 128 mV at 10 mA cm -2 , and excellent durability with current density increase of only ∼ 3% for HER 30 h at the overpotential of 250 mV. the enhanced electrocatalytic activity of Cu NDs/Ni3S2 NTs-CFs compared with those of Ni 3 S 2 NTs-CFs, Cu NDs-CFs, and CFs can be attributed to a smaller charge-transfer resistance, larger TOF value, and larger ECSAs. The electronic interactions between Ni 3 S 2 and Cu will first promote water adsorption that not only will improve the Volmer step to generate H ads ( Volmer Step) but also will improve the Heyrovsky step to transfer S− H ads into H 2 (Heyrovsky step).