Designing Smart Materials for Efficient Electrosynthesis of Fuels and Environmental Remediation: The Story of Transition Metal Chalcogenides
Authors: Nath Manashi , Masud Jahangir , Swesi Abdurazag T , Nath Manashi , De Silva Umanga , Liyanage Wipula Priya Rasika , Umapathi Siddesh , Golrokh Amin Bahar 
Published: February 2020

Energy harvesting from solar and water has created ripples in materials energy research for the last several decades, complemented by the rise of Hydrogen as a clean fuel. 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. 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 regards, water electrolysis leading to generation of oxygen and hydrogen on demand, 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. A slow OER process also slows the other half-cell reaction, i.e. the hydrogen evolution reaction (HER) at the cathode. Hence, designing efficient catalysts for OER and HER process from earth-abundant resources has been one of the primary concerns for advancing solar water splitting. In the Nath group we have focused on transition metal chalcogenides as efficient electrocatalysts for several energy conversion processes. Recently we have discovered that these transition metal selenides are highly efficient electrocatalyst for electroreduction of CO2 to higher hydrocarbons such as ethanol, acetic acid, and acetaldehyde at reasonably low potential (-0.7 to -1.5 V) under ambient conditions. In this presentation we will first present that CO2 electroreduction with Ni, Cu and V-based selenides, and present a thorough investigation of the reaction products and their evolution as a function of applied potential and reaction time. We will also discuss about the selectivity of product formation by controlling catalyst composition and applied potential. For water splitting reactions, we had proposed the idea that these 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, Cu, and Ni-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]. We will also highlight the importance of this increasing covalency in enhancing OER catalytic activity with the help of experimental evidence in the chalcogenide series (Ni3S2, Ni3Se2, Ni3Te2). 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. To complement the experimental observations, we have also performed DFT calculation and will discuss how the OH adsorption energy varied between the oxide and chalcogenides surfaces and the implication of such variation in terms of catalytic activity. The later part of the talk will be focused on designing nanostructures for these catalysts and integrating them with nanostructured photoanode arrays to create an efficient solar-to-fuel energy conversion device as well as artificial photosynthetic units. We will illustrate growth of nanostructured (nanopillars and nanotubes) photoanode arrays through confined electrodeposition on lithographically patterned nanoelectrodes, and decorating them with nanostructured chalcogenide based electroctalaysts, and how this technique can be utilized for a hybrid energy conversion device. Results encompassing efficient photoelectrocatalytic performance of CdSe-NiSe, CdTe-Ni3Te2, CuInSe2-NiSe2 and Fe3Se4-NiSe hybrid nanocomposite will be presented.