(Invited) Slurry of Transition Metal Chalcogenides for Multifaceted Electrochemical Applications: Energy Conversion, Storage, Sensing & Catalysis

Abstract

Transition metal chalcogenides has been at the center of attraction for materials chemistry for decades owing to their novel and interesting optoelectronic properties that can be tuned with anion composition, coordination and doping. Recently these transition metal chalcogenides have also been explored for energy-related applications with primary focus on sustainable energy conversion. Energy harvesting from solar and water has created ripples in sustainable energy research for the last several decades, complemented by the rise of Hydrogen as a clean fuel. It has become very apparent that hydrogen-on-demand technology needs to be developed to complement the growth of hydrogen fuel economy without adding on to the process cost by storing hydrogen in pressurized tanks or non-reactive framework. In this regard, water electrolysis leading to generation of oxygen and hydrogen, has been one of the most promising routes towards sustainable alternative energy generation and storage without depleting fossil-fuel based natural resources. However, the efficiency and practical feasibility of water electrolysis is limited by the anodic oxygen evolution reaction (OER), which is a kinetically sluggish, electron-intensive uphill reaction and designing efficient catalysts for OER process from earth-abundant resources has been one of the primary concerns for advancing solar water splitting. In the Nath group we have been successful in producing transition metal chalcogenide-based highly efficient OER electrocatalysts which surpasses efficiency of state-of-the-art precious metal oxides by several orders of magnitude. Our main design principle if based on the concept that chalcogenides, specifically, selenides and tellurides will show much better OER catalytic activity due to increasing covalency around the catalytically active transition metal site, 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, [e.g. lowering the Ni2+ --> Ni3+ oxidation potential in Ni-based catalysts where Ni3+ is the actually catalytically active species]. Based on such hypothesis, we have synthesized a plethora of transition metal selenides including those based on Ni, Ni-Fe, Co, Ni-Co, Cu, and Cu-Co which show high catalytic efficiency characterized by low onset potential and overpotential at 10 mA/cm2 [Ni3Se2 - 200 - 290 mV; Co7Se8 - 260 mV; FeNi2Se4-NrGO - 170 mV (NrGO - N-doped reduced graphene oxide); NiFe2Se4 - 210 mV; CoNi2Se4 - 190 mV; Ni3Te2 – 180 mV]. In this presentation we will highlight the importance of this increasing covalency in enhancing OER catalytic activity and illustrate with examples recently discovered examples of selenide and telluride compositions ranging from binary chalcogenides (Ni3Se2, NiSe2, NiSe, Cu2Se), to ternary and quaternary mixed metal selenides (Ni-Co-Se, Ni-Fe-Se) as well as seleno-based molecular complex containing NiSe4 tetrahedral core. We will illustrate how the Ni(II) --> Ni(III) oxidation potential is indeed lowered within the chalcogenide coordination compared to the oxide leading to the formation of mixed anionic (hydroxy)chalcogenide surface which enhances surface reactivity through preferential intermediate adsorption. Another aspect that has become more relevant is the electroreduction of atmospheric carbon dioxide into fuel or other value-added chemicals, thereby offering environmental remediation without the need to store large amounts of pressurized CO2. The electrochemical tunability of transition metal chalcogenide surfaces along with their increased lattice covalency has led to development of the concept that some of these compositions can enhance carbon dioxideCO2 reduction. Specifically, we have focused on Cu and Ni-based chalcogenides based on the hypothesis that higher d-electron occupancy of the transition metal within a covalent lattice will lead to better adsorption of intermediate CO on the surface through enhanced metal-to-ligand back-bonding, which in turn leads to longer dwell time and subsequent reduction of the CO to higher carbon content products. The Nath group has recently discovered copper and nickel chalcogenides as highly active electrocatalysts for CO2 reduction to C2 and C3 products with high selectivity. Interestingly it was observed that product composition could be controlled by tuning the applied potential with ethanol and acetic acid forming predominantly at low applied potential while formic acid was obtained exclusively at higher applied potential. Lastly, we will discuss the electrocatalytic activity of the transition metal chalcogenides towards biomolecule conversion enhancing their applicability as biosensors for detecting potentially life-threatening disorders. Detailed studies of the chemical reactivity, electrochemical activity, interfacial chemistry, and functional stability of the transition metal chalcogenides that makes all these applications feasible will be discussed in depth.