**3. Hydrogen evolution reaction (HER) based on TMC electrodes**

In order to improve HER performance, three main factors including the number of active sites, intrinsic catalytic activity, and the conductivity of TMCs play crucial roles. In 2005, Hinnemann et al. suggested the active sites for HER only exist in the edges of TMCs by density functional theory (DFT) calculation [30]. Thus, many research works are dedicated to enrich the defect sites and/or active sites by nanostructural engineering [31–33]. Zhang et al. synthesized the edge-rich 3D MoS2 coupling with conductive polymer polyaniline (PANI) as catalyst (**Figure 6**) [34].

The MoS2 grown on 3D PANI substrate tends to grow vertically and expose abundant edge sites for HER. Consequently, excellent HER performance can be achieved with a low onset potential of 100 mV and a small Tafel slop of 45 mV dec<sup>1</sup> .

Additionally, MoS2/PANI achieved superior stability for HER electrocatalysis. Although the promising catalytic activity of MoS2 for HER was achieved by creating abundant edge sites and/or active sites, the performance was still limited by its intrinsic properties such as poor electrical transport and inefficient electrical contact to the catalyst [35]. Lukowski et al. reported that the metallic nanosheets of 1T-MoS2, which were chemically exfoliated by lithium intercalation of semiconducting 2H-MoS2 nanostructures, dramatically enhanced HER performance (**Figure 7**) [36]. The current density of 10 mA cm<sup>1</sup> can be reached at a low overpotential of

attributed to the favorable kinetics, metallic conductivity, and increasing number of active sites in the metallic 1T-MoS2 nanosheets, which was proven by the dramatically decrease of charge-transfer resistance from 232 Ω of 2H-MoS2 nanostructures to 4 Ω of metallic 1T-MoS2 nanosheets. This finding proves that the metallic 1T polymorph of TMCs is competitive to earth-abundant catalysts in heterogeneous

*(a) The schematic illustration for the synthesis of beaded stream-like CoSe2 nanoneedles, (b) the SEM images of beaded stream-like (CoSe2) nanoneedle array, and (c) the electrochemical measurements of Co-BSND, CoSe2-*

. The excellent performance can be

187 mV with a Tafel slope of 43 mV dec<sup>1</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.92045*

*Transition Metal Chalcogenides for the Electrocatalysis of Water*

catalysis.

**Figure 8.**

**83**

*PA, and CoSe2-BSND electrodes [37].*

#### **Figure 6.**

*The defect-rich 3D MoS2/PANI catalyst for HER (a) the morphologies of synthesized 3D MoS2/PANI and (b) the electrochemical performance of 3D MoS2/PANI catalyst [34].*

#### **Figure 7.**

*(a) Electron microscopy characterization of as-grown 2H-MoS2 nanostructures. (b) Comparison of as-grown and exfoliated MoS2 nanosheets. (c) Electrocatalytic performance of chemically exfoliated and as-grown MoS2 nanosheets [36].*

### *Transition Metal Chalcogenides for the Electrocatalysis of Water DOI: http://dx.doi.org/10.5772/intechopen.92045*

The MoS2 grown on 3D PANI substrate tends to grow vertically and expose abundant edge sites for HER. Consequently, excellent HER performance can be achieved with a low onset potential of 100 mV and a small Tafel slop of 45 mV dec<sup>1</sup>

*The defect-rich 3D MoS2/PANI catalyst for HER (a) the morphologies of synthesized 3D MoS2/PANI and*

*(a) Electron microscopy characterization of as-grown 2H-MoS2 nanostructures. (b) Comparison of as-grown and exfoliated MoS2 nanosheets. (c) Electrocatalytic performance of chemically exfoliated and as-grown MoS2*

*(b) the electrochemical performance of 3D MoS2/PANI catalyst [34].*

**Figure 6.**

*Advanced Functional Materials*

**Figure 7.**

**82**

*nanosheets [36].*

.

Additionally, MoS2/PANI achieved superior stability for HER electrocatalysis. Although the promising catalytic activity of MoS2 for HER was achieved by creating abundant edge sites and/or active sites, the performance was still limited by its intrinsic properties such as poor electrical transport and inefficient electrical contact to the catalyst [35]. Lukowski et al. reported that the metallic nanosheets of 1T-MoS2, which were chemically exfoliated by lithium intercalation of semiconducting 2H-MoS2 nanostructures, dramatically enhanced HER performance (**Figure 7**) [36]. The current density of 10 mA cm<sup>1</sup> can be reached at a low overpotential of 187 mV with a Tafel slope of 43 mV dec<sup>1</sup> . The excellent performance can be attributed to the favorable kinetics, metallic conductivity, and increasing number of active sites in the metallic 1T-MoS2 nanosheets, which was proven by the dramatically decrease of charge-transfer resistance from 232 Ω of 2H-MoS2 nanostructures to 4 Ω of metallic 1T-MoS2 nanosheets. This finding proves that the metallic 1T polymorph of TMCs is competitive to earth-abundant catalysts in heterogeneous catalysis.

#### **Figure 8.**

*(a) The schematic illustration for the synthesis of beaded stream-like CoSe2 nanoneedles, (b) the SEM images of beaded stream-like (CoSe2) nanoneedle array, and (c) the electrochemical measurements of Co-BSND, CoSe2- PA, and CoSe2-BSND electrodes [37].*

Lee et al. developed earth-abundant nanostructuring beaded stream-like cobalt diselenide (CoSe2) nanoneedles (CoSe2-BSND) as electrocatalyst for HER [37]. The CoSe2 nanoneedles derived from the cobalt oxide (Co3O4) nanoneedle array directly formed on flexible titanium foils after selenization treatment (**Figure 8**). The CoSe2-BSND can drive the HER at a current density of 20 mA cm<sup>2</sup> with a small overpotential of 125 mV. Also, it possesses a small Tafel slope of 48.5 mV dec<sup>1</sup> suggesting that the HER follows the Volmer-Heyrovsky mechanism where a fast discharge of protons is followed by rate-determining electrochemical desorption. Moreover, the CoSe2-BSND electrode achieved great stability in an acidic electrolyte for 3000 cycles. The enhanced electrochemical activity is attributed to the highly accessible surface active sites, the improved charge transfer kinetics, and the super hydrophilic surface of CoSe2-BSND electrode.

required to reach 10 mA cm<sup>2</sup> was 290 mV, suggesting that this catalyst exhibits its competitivity among the oxide-based electrocatalysts. The catalytic ability of Ni3Se2 can be further improved through the modification of Se-deficient phase in Ni3Se2. Moreover, electrodeposited Ni3Se2 catalysts exhibited exceptional stability under OER for 42 hours. The effect of the underlying substrates such as glassy carbon, ITO-coated glass, and Ni foam on OER was also investigated. The results revealed that the glassy carbon substrate exhibited the lowest onset potential and the highest

*Transition Metal Chalcogenides for the Electrocatalysis of Water*

*DOI: http://dx.doi.org/10.5772/intechopen.92045*

*(a) The morphological characterization of Ni3Se2 grown by electrochemical deposition and (b) the crystal*

*(a) The structural and morphological characterizations of different P-doped CoSe2, (b) electrochemical OER activities of the different P-doped CoSe2 and standard RuO2 electrodes, (c)* in situ *STEM images of the P-doped CoSe2 catalyst taken at different times after immersing in the KOH solution, and (d)* in situ *Co K-edge XANES*

*spectra of different P-doped CoSe2 electrodes for HER and OER processes [3].*

*structure identification and OER performance of Ni3Se2 electrocatalyst [43].*

**Figure 10.**

**Figure 11.**

**85**
