Lanthanum Transition-Metal Sulfide Electrocatalysts for Overall Water Splitting

Hydrogen has been identified as a sustainable energy source for the future with the potential of replacing fossil fuels due to its high gravimetric energy density and zero carbon emissions. Water electrolysis is a promising approach to the clean production of pure hydrogen, however, the benchmark catalysts of the half reactions of water electrolysis are noble-metal based, which limit the application of this technique. 1 As well, the high energy barrier and sluggish kinetics of the oxygen evolution reaction limit the efficiency of the system. Therefore, it is crucial to develop robust, stable, and cost-effective materials for water electrolysis to be scalable. Lanthanum transition-metal oxide electrocatalysts have been pursued in the past for water splitting due to the high abundance of lanthanum and transition metals. 2 However, these catalysts suffer from low conductivity and inherent instability towards oxygen evolution. Binary sulfides, selenides, and phosphides have also been pursued, 3 but the compositional space of these materials is limited. Ternary sulfides offer a wider compositional space that allows fine-tuning for optimal performance. As a result, we have pursued the synthesis and application of the relatively unexplored ternary chalcogenides, LaMS 3 (M = Ni, Co, Fe, Mn) towards water electrolysis. The efficiency of water electrolysis and stability of the materials have been tested with linear sweep voltammetry, galvanostatic measurements, and electrochemical impedance spectroscopy. We found these materials to be active towards both the hydrogen and oxygen evolution reactions, in acidic and alkaline conditions, respectively, with the catalysts exhibiting an activity trend where LaNiS 3 > LaCoS 3 > LaFeS 3 > LaMnS 3 . For HER, LaNiS 3 exhibits an overpotential of 340 mV at 10 mAcm -2 with a Tafel slope of 79 mV/decade. For OER, an overpotential of 373 mV at 10 mAcm -2 and a low Tafel slope of 48 mV/decade are observed. In addition, LaNiS 3 proved to be highly stable with minimal change in potential over a period of 18 hours. The high activity and stability of LaNiS 3 towards both half reactions of water electrolysis make it a promising candidate for a bifunctional water splitting electrocatalyst. References Wang, S.; Lu, A.; Zhong, C.-J. Hydrogen Production from Water Electrolysis: Role of Catalysts. Nano Convergence 2021 , 8 (1), 4. Dias, J. A.; Andrade, M. A. S.; Santos, H. L. S.; Morelli, M. R.; Mascaro, L. H. Lanthanum‐Based Perovskites for Catalytic Oxygen Evolution Reaction. ChemElectroChem 2020 , 7 (15), 3173–3192. Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 2016 , 6 (12), 8069–8097.