MoS2-Pt Nanoparticles Deposited on Carbon Nanostructures By a Microwave Assisted Method As Hydrogen Electrodes for H2 – Br2 Flow Batteries
Authors: Escalante García Ismailia Leilani , de la Rosa Gómez Alejandra , Perez Viramontes Nicté Julieta , Durón Torres Sergio Miguel , García Betancourt María Luisa 
Published: February 2020

The use and implementation of renewable energies has increased worldwide in recent years as a green and sustainable alternative to fossil fuels. Among the renewable energies available, wind and solar stand as the first choice to achieve this goal. Although the use of solar and wing have multiple advantages, their availability and efficient use are limited by their intermittent nature. For this reason, there has been an urgent need to design systems that store energy when is produced in excess for further use, thus, this will be always available. Redox flow batteries (RFBs) have been recognized as one of the most suitable options among the electrochemical technologies to store energy at large scale (kW - MW) and also to facilitate the integration of solar or eolic energy to the electricity grid. Over the last three decades, different RFB chemistries have been proposed and successfully demonstrated for practical applications, for instance, the all vanadium RFB. Recently, the hybrid H₂-Br₂ system has attracted attention as possible candidate for energy storage because its high power and efficiency, low reactant cost, fast kinetics and great reversibility. The redox reactions of Br₂ and H2 at the negative and positive electrode, respectively, are responsible for the energy storage with a theoretical Ecell=1.09 V. In general, the reactions of Br2 are reversible and exhibit fast kinetics on carbon felt electrodes. At the H2 electrode, the hydrogen reactions are performed at a Pt/C electrocatalyst in order to achieve low activation overpotential and fast kinetics. However, Pt electrocatalyst is poisoned by crossover of bromine and bromide species from the positive electrode to the negative electrode through the membrane during operation. Consequently, the kinetics of the H₂ reactions is affected which decreases the efficiency and energy storage capacity of the device. Some transition materials have been investigated as alternative electrocatalysts for hydrogen reactions. For example, it has been reported that MoS2 presents good catalytic activity and low overpotentials towards the hydrogen reactions. However, it is necessary to perform a fundamental study of MoS2 as electrocatalyst for the hydrogen reactions in presence of bromine and bromide species. Additionally, new synthesis methods should be explored in order to enhance the physical, chemical and electrocatalytic properties of MoS2 for its possible use in the design of H2 electrodes for the hybrid H2-Br2 RFB. In this research, MoS2 - Pt nanoparticles are synthesized directly on carbon nanostructures by a single step microwave assisted solvothermal or hydrothermal method. Carbon Nanosponge (CNS), N2 – doped Carbon Nanotubes (N2-CNT) or Multiwall Carbon Nanotubes (MWCNT) are considered in this study. The kinetics and electrocatalytic activity of the MoS2 – Pt nanoparticles are studied by cyclic voltammetry (CV), rotating disk electrode (EDR) and electrochemical impedance spectroscopy (EIS) towards the hydrogen reactions in acidic media with additions of HBr. Scanning electron microscopy (SEM) and X-Ray diffraction (XRD) are carried out to study the distribution and particle size, morphology and crystalline phases present in the MoS2-Pt/C nanomaterials. The synthesis time of the MoS2-Pt on carbon assisted by microwave is highly reduced as compared with the conventional solvothermal or hydrothermal methods, 10 min vs. 36 h. Preliminary electrochemical studies show that MoS₂-Pt based materials exhibit stability in the presence of HBr for further use as H₂ electrodes in a H₂-Br₂ RFB.
Keywords: energy storage, nanomaterials, hydrothermal synthesis, RFB