Ternary Sulfides as Electrocatalysts for Water Splitting

The intermittency and variability in the supply of renewable energy^1,2 necessitate the use of energy storage, and a clean energy carrier such as hydrogen (H₂), which can be stored, transported, and utilized as a continuous energy source without generating harmful emissions. Electrochemical water splitting for hydrogen production is an emission-free, efficient energy conversion technology^2,3. However, a big issue with this process is that the best catalysts are relatively scarce, noble metals and related compounds, which hinders the productivity and cost-effectiveness of the technology at a large scale^4,5. Thus, there is a clear need to explore the earth-abundant, transition metal-based compounds for their catalytic activity towards water splitting.<br/>A large number of oxide perovskite and related phases with the general formula ABO₃, where A is either an alkaline earth metal or a rare earth metal and B is a transition metal, have been studied and reported to exhibit interesting electrical, magnetic, and electrochemical properties^5–7. However, the physical and chemical properties of analogous chalcogenide counterparts have been rarely reported. Recently, several articles have reported the physical properties of perovskite chalcogenides with early transition metals to show attractive optoelectronic properties^8–10, which raises the interest in ternary chalcogenides for electrocatalysis.<br/>We have synthesized a series of ternary sulfides (La-M-S) with first-row late transition metals (M = Mn, Fe, Co, Ni). These materials have been synthesized by first preparing the corresponding ternary oxides and then carrying out carbon disulfide (CS₂) sulfurization at elevated temperatures. While similar compositions have been reported in some early articles^11,12, a thorough study elucidating the structure details has not been reported. This presentation will report detailed diffraction studies on these compounds using high-quality synchrotron X-ray diffraction (XRD) data. We will also report chemical analyses on these compounds based on X-ray absorption near edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) measurements. These iso-structural series of hexagonal compounds have been tested as catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and exhibit encouraging overpotentials of 330-360 mV for HER and around 370mV for OER in the early studies. The ability to catalyze both half-reactions of water electrolysis makes these materials promising candidates for bifunctional catalysts and calls for further work on these compounds to improve their performance.<br/><u>References:</u><br/>¹ H. Yuan, Electrocatalytic Water Splitting to Produce Fuel Hydrogen, Michigan State University (2017).<br/>² P. Colbertaldo, S.B. Agustin, S. Campanari, and J. Brouwer, Int. J. Hydrogen Energy 44, 9558 (2019).<br/>³ F. Dawood, M. Anda, and G.M. Shafiullah, Int. J. Hydrogen Energy 45, 3847 (2020).<br/>⁴ X.P. Li, C. Huang, W.K. Han, T. Ouyang, and Z.Q. Liu, Chinese Chem. Lett. 32, 2597 (2021).<br/>⁵ H. shan Nan, X. ying Hu, and H. wei Tian, Mater. Sci. Semicond. Process. 94, 35 (2019).<br/>⁶ J. Xu, C. Chen, Z. Han, Y. Yang, J. Li, and Q. Deng, Nanomater. 9, 1161 (2019).<br/>⁷ J. Wang, Y. Liu, X. Chen, C. Chen, P. Chen, Z. Wang, and Y. Duan, ChemPhysChem 20, 2580 (2019).<br/>⁸ D. Tiwari, O.S. Hutter, and G. Longo, J. Phys. Energy 3, 034010 (2021).<br/>⁹ Q. Xu, D. Yang, J. Lv, Y.-Y. Sun, and L. Zhang, Small Methods 2, 1700316 (2018).<br/>¹⁰ A. Swarnkar, W.J. Mir, R. Chakraborty, M. Jagadeeswararao, T. Sheikh, and A. Nag, Chem. Mater. 31, 565 (2019).<br/>¹¹ T. Takahashi, T. Oka, O. Yamada, and K. Ametani, Mater. Res. Bull. 6, 173 (1971).<br/>¹² T. Murugesan, S. Ramesh, J. Gopalakrishnan, and C.N.R. Rao, J. Solid State Chem. 38, 165 (1981).