Molybdenum Disulfide and Carbon Nanotubes Composite Electrode for Electrochemical Conversion of Salinity Gradient Energy

Abstract

The ocean accounts for 97% of the total water resources on earth, covering over 70% of the map's surface area. With the continuous consumption of non-renewable energy sources such as fossil fuels and the rapid development of renewable energy, humans are increasingly paying attention to the utilization of ocean resources. Ocean energy includes tidal energy, wave energy, temperature difference energy, and salinity gradient energy. Salinity gradient energy is the energy generated by the interaction of seawater and fresh water, which is the ocean energy existing in the form of chemical energy. This energy is mostly generated in estuaries. The osmotic pressure generated by mixing water with different salinity can be converted into electrical energy driven by potential differences or ion gradients. Salinity gradient energy, as a new renewable energy source, has received widespread global attention and research in recent years, making rapid progress. The utilization of salinity gradient energy provides a renewable and sustainable alternative to the recent surge in global energy consumption.

At present, pressure delay osmosis technology, reverse electrodialysis technology and capacitive mixing technology are three main technologies for extracting salinity gradient energy. In this work, we built a new type of salt difference cell based on capacitive mixing technology, using molybdenum disulfide (MoS₂) and multiwalled carbon nanotubes (MoS₂/MWCNTs) composite electrode as the anode and an activated carbon (AC) as the cathode.

We composited two materials with different ion storage mechanisms together. MoS₂ has a layered structure like graphene, with an interlayer spacing of about twice that of graphene. It is a battery electrode material that can undergo intercalation reaction with Na⁺. MWCNTs have a typical double electric layer effect. When discharging, while adsorbing Na⁺ on its surface, it can help Na⁺ enter the interlayer of MoS₂ more quickly, accelerating the ion transport efficiency and the extraction efficiency of salt differential energy. We conducted physical and electrochemical characterizations of MoS₂/MWCNTs composite material, and tested its salt difference energy extraction ability on a salt difference battery composed of it and AC electrode. We found that the concentration response voltage reached 150 mV, and the energy density of the extracted salt difference energy after a complete four-step cycle reached up to 6.96 J·g^-1. The advantages of low raw material price of the device and without using ion membranes make it more environmentally friendly, providing a new approach for the study of extracting salinity gradient energy.

Keywords

Salinity gradient energy / Electrochemical conversion / Conversion efficiency / Molybdenum disulfide / Carbon nanotubes.

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