Montmorillonite Clay-Based Electrocatalyst for Water Splitting Applications

Abstract

The goal of reaching net-zero emissions by the year 2050 requires a strategic overhaul to be realistically achievable. Hydrogen energy is considered an ideal substitute for fossil fuels, which leave carbon footprints and release hazardous chemicals into the atmosphere. Thus, large-scale commercialization of hydrogen energy could meet future energy demands with zero emissions. Electrochemical water splitting (EWS) is the most sustainable and efficient method to produce hydrogen for the next century. Thus, designing an electrocatalyst with an enhanced surface structure and abundant earth materials makes it a highly adoptable and affordable green energy. Smectite clay minerals are abundant on earth, non-hazardous, and easy to fabricate due to their clay nature. Among the smectites, montmorillonite is a promising candidate for electrocatalysts due to its unique physicochemical properties like expandable layered structure (TOT layers), high cation exchange, water adsorption, and swelling properties. Due to the high cationic exchange capacity of montmorillonite clays (70-110 meq/100g), the structural and interlayer cations can be tuned according to the water adsorption and desorption properties. When we look into the mineral structure, montmorillonite smectites belong to 2:1 phyllosilicate-layered sheets, composed of two tetrahedral silicate sheets (T) and one octahedral sheet (O). In each layer, an octahedral sheet is sandwiched between the two tetrahedral sheets (TOT), which is shown in Figure a. In the tetrahedral sheets, the dominant cation is Si4+, which is often substituted by the trivalent cation Al3+. The isomorphic substitution of Si4+ by Al3+ in the tetrahedral layer raises charge deficiency in the tetrahedral sheet, which is balanced by the absorbed interlayer cations like K+, Na+, and Ca2+ between the water molecules. In octahedral sheets, the common cations are Al3+, Mg2+, Fe3+, or Fe2+, and other cations like Ni2+, Li2+, and Cr2+. When the Mg2+ ions are substituted by Al3+ ions, it gives rise to a positive charge in the octahedral sheet. Depending upon the charges within the tetrahedral and octahedral sheets, the layered structure properties are altered. By considering their cation-rich and expandable properties, we have employed montmorillonite clay materials [Na,Ca)0.3 (Al,Mg)2 Si4O10(OH)2. nH2O] in electrocatalytic water splitting reactions for the first time. In our observation, annealing the montmorillonite at high temperatures such as 300 oC (Mont300/NF), 600 oC (Mont600/NF), and 900 oC (Mont900/NF), reconstruct the surface, and induce structural defects, which enables isomorphic substitution of bivalent (M2+) or trivalent cations (M3+). The annealed samples were prepared as slurry using binder additives (PVDF and NMP solvent) and coated on nickel foam (NF). The electrochemical performance was tested in a three-electrode potentiostat system in 1M KOH electrolyte, where the prepared sample was employed as the working electrode, graphite rod as the counter electrode, and Hg/HgO as the reference electrode. Figure b and c shows the electrochemical performances of the HER and OER reactions. Initially, the Mont600/NF catalysts show lower overpotential of 178 mV and 250 mV at 10 mA/cm2 for HER and OER reactions respectively. The pristine sample (Mont/NF) show much higher overpotential (240 mV for HER and 360 mV for OER). Thus, annealing promotes the formation of dangling bonds by generating cationic and anionic vacancies, which act as active sites for the adsorption of intermediates, which in turn reduces the requirement for higher overpotential. Furthermore, the pristine montmorillonite is mixed with functionalized CNT powder and annealed at the best performing temperature of 600oC (Mont+CNT600/NF). As a result, Mont+CNT600/NF sample shows much higher performance than Mont600/NF by requiring lower overpotential of 168 mV and 220 mV at 10 mA/cm2 for HER and OER reflecting the higher intrinsic kinetics owing to increased electrical conductivity due to CNT which enhances the electron transport properties and the water splitting kinetics. Interestingly, the used clay materials can be treated with organic solutions and can be reused due to their physicochemical retaining properties even after being treated at high temperatures. This would reduce the amount of solid catalyst waste going into landfills. Therefore, montmorillonite clay materials are the new era of catalyst’s material with recyclability due to their self-retaining physicochemical properties, water hydration, and dehydration properties. In this work, we have successfully optimized the montmorillonite properties by annealing at high temperatures, which helps to reconstruct the surface by breaking the OH lattice ions and creating dangling bonds. The formation of dangling bonds increases the rate of water splitting processes by acting as a surface-active site for the adsorption and desorption of H+ and OH- ions. Thus, tuning montmorillonite clays helps in achieving different water splitting properties. Figure 1