**4. Oxygen evolution reaction (OER) based on TMC electrodes**

The TMC materials have shown promising performance toward HER as we mentioned above. Researchers are searching for the possibility of bifunctional electrocatalysts for both HER and OER to perform the overall water-splitting reaction. Moreover, TMC electrocatalysts have attracted tremendous attentions since Alonso-vante and coworker discovered that Mo4Ru2Se8 had a ORR activity comparable to platinum [38]. As compared to ruthenium (Ru) and Rhenium (Rh), the low cost and earth abundant transition metals such as iron-, nickel- and cobalt-based TMCs have much attention for OER [39–41]. Liu and coworkers discovered that electrodeposited CoS nanosheet films on Ti mesh show high activity toward OER (**Figure 9**) [42]. The CoS nanosheets tend to drive a current density of 10 mA cm<sup>2</sup> with an overpotential of 361 mV. In addition, this electrode maintains highly catalytic activity for at least 20 hours. The superior catalytic activity along with excellent stability of CoS nanosheets offers the good opportunity to become a costeffective and industry-feasible electrode toward OER. On the other hand, Swesi et al. first reported that the catalytic activity of OER was observed by the nickel selenide (Ni3Se2) in alkaline condition (**Figure 10**) [43]. The low overpotential

#### **Figure 9.**

*(a) The surface morphology and crystal structure of electrodeposited CoS nanosheets and (b) the electrochemical performance and stability toward OER on CoS nanosheets [42].*

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

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

#### **Figure 10.**

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.

*Advanced Functional Materials*

**Figure 9.**

**84**

**4. Oxygen evolution reaction (OER) based on TMC electrodes**

The TMC materials have shown promising performance toward HER as we mentioned above. Researchers are searching for the possibility of bifunctional electrocatalysts for both HER and OER to perform the overall water-splitting reaction. Moreover, TMC electrocatalysts have attracted tremendous attentions since Alonso-vante and coworker discovered that Mo4Ru2Se8 had a ORR activity comparable to platinum [38]. As compared to ruthenium (Ru) and Rhenium (Rh), the low cost and earth abundant transition metals such as iron-, nickel- and cobalt-based TMCs have much attention for OER [39–41]. Liu and coworkers discovered that electrodeposited CoS nanosheet films on Ti mesh show high activity toward OER (**Figure 9**) [42]. The CoS nanosheets tend to drive a current density of 10 mA cm<sup>2</sup> with an overpotential of 361 mV. In addition, this electrode maintains highly catalytic activity for at least 20 hours. The superior catalytic activity along with excellent stability of CoS nanosheets offers the good opportunity to become a costeffective and industry-feasible electrode toward OER. On the other hand, Swesi et al. first reported that the catalytic activity of OER was observed by the nickel selenide (Ni3Se2) in alkaline condition (**Figure 10**) [43]. The low overpotential

*(a) The surface morphology and crystal structure of electrodeposited CoS nanosheets and (b) the electrochemical*

*performance and stability toward OER on CoS nanosheets [42].*

*(a) The morphological characterization of Ni3Se2 grown by electrochemical deposition and (b) the crystal structure identification and OER performance of Ni3Se2 electrocatalyst [43].*

#### **Figure 11.**

*(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].*

current density, suggesting that the interaction between the underlying substrate and the Ni3Se2 may play a role in O2 evolution reaction.

Although the electrocatalytic performance of TMCs toward OER was significantly enhanced, the reaction mechanism and actual active sites responsible for the reaction were in dispute. Recently, operando or *in situ* experiments like *in situ* Fourier transform infrared spectroscopy, *in situ* Raman spectroscopy, and *in situ* X-ray absorption/diffraction are commonly carried out to provide atomic-level information [3, 44, 45]. Zhu and coworkers [3] conducted *in situ* X-ray absorption spectroscopy, *in situ* liquid-phase TEM, and *in situ* Raman spectroscopy, revealing that P-doped CoSe2 in an alkaline solution was acting as the "pre-catalyst" rather than the real reactive species, which has been debated for a while (**Figure 11**). They found that the introduction of phosphorus would generate more vacancies, which facilitated the structural transformation into the real active electrocatalyst, such as metallic cobalt for HER and cobalt oxyhydroxide (CoOOH) for OER. CoSe1.26P1.42 shows the best OER performance among all catalysts, which requires an overpotential of 255 mV to reach the current density of 10 mA cm<sup>2</sup> . Furthermore, the CoSe1.26P1.42 catalyst with a Tafel slop of 87 mV dec<sup>1</sup> exhibits a slightly lower than those of the other P-doped CoSe2 catalysts. Such performance is comparable to many leading earth-abundant HER and OER catalysts in alkaline electrolyte.
