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Chemical Engineering Seminar
Dr. Shrihari “Shri” Sankarasubramanian
Abstract: The electrochemical reaction environment, typically consisting of ionic and molecular species and their supramolecular assemblies, has historically been chosen with transport property (ionic conductivity, transference number) considerations in mind. Herein, I examine the effect of the reactant and near–electrode environments on transport of reactants to the reaction site and the kinetics of the reaction. I show that the supramolecular structures formed in solution can be tailored to further lower the activation barrier for reactions (enhancing electrocatalysis). (1) A rational basis is presented for designing electrolytes with desirable supramolecular structures. Such designer systems yield a substitute for expensive catalysts. These principles are applied in aqueous and non-aqueous environments to develop cost-effective, electrode-decoupled redox flow batteries for large scale energy storage, and high-power alkali metal/oxygen batteries for transportation applications.
First, I will present a novel, electrode-decoupled redox flow battery chemistry that is estimated to cost ~1/3 of the cost of an all-vanadium system (wherein the cost of active materials alone would exceed the DOE system cost target of < $100/kWh) (2, 3) while offering comparable power densities. A key innovation in this system is the use of an anion exchange membrane (AEM) separator that ensures the long-term separation of the different cationic redox species in the anolyte and catholyte and allows us to demonstrate capacity- fade-free cycling over long time scales. This electrode-decoupled system satisfies the essential criteria for an RFB system of low cost, single-phase reactions, absence of parasitic side reactions, and >1-volt operating voltage.
Second, I will address the critical bottleneck in high-specific-energy alkali-metal/oxygen batteries for transportation applications, namely the oxygen reduction reaction (ORR). The ORR depends on the non-aqueous electrolyte that mediates ion and oxygen transport. (4) A multi-fold increase of the practical discharge power of these batteries is demonstrated by tuning the tripartite interactions between the salt cation, anion and the solvent in the electrolyte. (1) These phenomena were captured in an “electrochemical” Thiele Modulus incorporating Marcus-Hush kinetics. I present guidelines for the choice of electrolyte solvent and salt when designing systems for high discharge rates and demonstrate the rational, ab-initio design of electrolytes for high-power alkali-metal/oxygen batteries.
I conclude by presenting the advantages of sustaining locally disparate reaction (pH) environments in a given electrochemical system in the context of high-power direct borohydride fuel cells for marine transportation applications. (5)
Bio: Dr. Shrihari “Shri” Sankarasubramanian is a Research Scientist at the Department of Energy, Environmental and Chemical Engineering at Washington University in St. Louis, USA in the group of Prof. Vijay K. Ramani. He obtained his PhD in Chemical Engineering from the Illinois Institute of Technology with Prof. Jai Prakash, having been at various times the Dean’s Scholar and Energy Technology Fellow.
Shri’s main area of interest is in the development of electrochemical energy conversion and storage devices combining fundamental physical chemistry, materials development, device engineering and scale-up. His work has resulted in publications in venues such as Nature Energy and PNAS and has resulted in one issued and several pending US patents. His technology translation efforts as the tech-to-market lead on a $2M US Advanced Research Projects Agency-Energy (ARPA-E) project have yielded significant external funding and enabled the scale-up of redox flow battery separators and systems.
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