**5. Conclusion**

Electrocatalysts for HER and OER play an important role for sustainable energy, which require converting renewable energy to storable chemical fuels or employing clean energy. In this article, we have partially reviewed the promising candidates, transition metal (di)chalcogenides (TMCs), from preparation methods to electrochemical measurements toward HER and OER. It can be concluded that a good electrocatalyst should possess good conductivity and moderate adsorption energy to reactive species and/or intermediates. In addition, the large number of active/defect sites would be favorable for catalytic ability. The future challenges for TMCs toward HER and OER are summarized as follows. The realization of the mechanisms toward the HER and OER plays a key role to design a perfect electrocatalyst. The development of *in situ* or operando techniques shines the light on this difficulty. Most importantly, the understanding of HER and OER mechanisms offers the significant information to many other fields due to the similar operating concepts in electrocatalytic and photocatalytic applications.

### **Acknowledgements**

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under grant numbers 107-2113-M-845-001-MY3.

**Author details**

**87**

Chi-Ang Tseng<sup>1</sup> and Chuan-Pei Lee<sup>2</sup>

provided the original work is properly cited.

\*

2 Department of Applied Physics and Chemistry, University of Taipei, Taiwan

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

1 Department of Chemistry, National Taiwan University, Taiwan

\*Address all correspondence to: cplee@utaipei.edu.tw

*Transition Metal Chalcogenides for the Electrocatalysis of Water*

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

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

current density, suggesting that the interaction between the underlying substrate

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>

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.

Electrocatalysts for HER and OER play an important role for sustainable energy, which require converting renewable energy to storable chemical fuels or employing clean energy. In this article, we have partially reviewed the promising candidates, transition metal (di)chalcogenides (TMCs), from preparation methods to electrochemical measurements toward HER and OER. It can be concluded that a good electrocatalyst should possess good conductivity and moderate adsorption energy to reactive species and/or intermediates. In addition, the large number of active/defect sites would be favorable for catalytic ability. The future challenges for TMCs toward HER and OER are summarized as follows. The realization of the mechanisms toward the HER and OER plays a key role to design a perfect electrocatalyst. The development of *in situ* or operando techniques shines the light on this difficulty. Most importantly, the understanding of HER and OER mechanisms offers the significant information to many other fields due to the similar operating concepts in

This work was supported by the Ministry of Science and Technology (MOST) of

. Furthermore,

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

and the Ni3Se2 may play a role in O2 evolution reaction.

*Advanced Functional Materials*

electrocatalytic and photocatalytic applications.

Taiwan, under grant numbers 107-2113-M-845-001-MY3.

**5. Conclusion**

**Acknowledgements**

**86**
