Investigating Transition Metal Chalcogenides for Efficient Oxygen Evolution Electrocatalysis: The Effect of Covalency and Directionality

Water electrolysis leading to generation of oxygen and hydrogen, has been one of the most promising routes towards sustainable alternative energy generation and storage, with applications ranging from metal-air batteries, fuel cells, to solar-to-fuel energy conversion systems. Oxygen and Hydrogen evolution reaction (OER and HER respectively) are the two half reactions for water electrolysis, amongst which OER is the most challenging uphill process with a high electron count. Hence, designing efficient catalysts for OER process from earth-abundant resources has been one of the primary concerns for advancing this field. Recently transition metal chalcogenides has been identified as efficient OER electrocatalysts. We have synthesized a plethora of transition metal selenides including those based on Ni, Ni-Fe, Co, and Ni-Co, which show high catalytic efficiency characterized by low onset potential and overpotential at 10 mA/cm 2 [Ni 3 Se 2 - 200 - 290 mV; Co 7 Se 8 - 260 mV; FeNi 2 Se 4 -NrGO - 170 mV (NrGO - N-doped reduced graphene oxide); NiFe 2 Se 4 - 210 mV; NiCo 2 Se 4 - 190 mV]. We have proposed the idea that one of the primary reasons these selenides show a much better OER catalytic activity is due to increasing covalency in the metal-selenium bond compared to the oxides caused by decreasing electronegativity of the anion, which in turn leads to variation of chemical potential around the transition metal center, specifically, lowering the Ni 2+ --> Ni 3+ oxidation potential (Ni 3+ being the actually catalytically active species). In this presentation we will highlight the importance of this increasing covalency in enhancing the catalytic activity with the help of experimental evidence in selenide compositions ranging from binary Ni-selenides (Ni 3 Se 2 , NiSe 2 , NiSe), ternary mixed metal selenides (Ni-Co-Se, Ni-Fe-Se) as well as seleno-based molecular complex containing NiSe 4 tetrahedral core. We will illustrate how the Ni(II) --> Ni(III) oxidation potential is indeed lowered within the selenide coordination compared to the oxide, in pure single crystals of the seleno-based coordination complex which is devoid of any surface impurities and adsorbates. We will also emphasize the lattice-plane-dependent catalytic activity through the example of electrodeposited NiSe 2 which shows that if grown along the <311> direction exposing the Ni-rich terminating lattice plane, this catalyst can exhibit lowest overpotential at 10 mA/cm2 (140 mV) that has been reported so far for OER in alkaline medium. All of these selenide electrocatalysts has been characterized with pxrd, SEM, TEM, Raman, XPS, EDS and detailed electrochemical studies including LSV, CV, chronoameprometry, chronopotentiometry, determination of Faradaic efficiency, and ECSA. Through this presentation we will offer insight into possible reasons these selenides outperform most of the known OER electrocatalysts, which will hopefully initiate discussion and efforts to fully understand and utilize their full potential.