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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP Electrocatalytic Chemical Energy Storage: The redox flow battery architecture opens up possibilities for storing energy over very long periods of time, which is a challenging problem for current electrochemical storage systems. For instance, the redox flow architecture can be configured as an electrochemical conversion and storage system to carry out electrochemically catalyzed reactions of abundant and low cost materials using electricity generated from renewable sources (see sidebar on “Membrane Design, Solvation Mechanisms and Molecular Electrocatalysts for Energy Storage”). The energy would be stored in converted molecules dispersed in the electrolyte, and the chemical energy can be converted back to electricity when needed. In such an electrochemical system, H2O, CO2, N2, and other abundant chemicals are reduced by the electrons provided at the cathode, and oxidation happens on the anode to produce O2. The efficiency is determined by the charge transfer from the substrate to the molecules, the effectiveness of the catalysts for both the reduction and oxidation reactions, and transport of charged species in the vicinity of the electrodes and catalysts. This energy storage methodology involves a new set of cross-cutting challenges not emphasized in traditional battery systems, such as how to enhance the efficiency of desired electrochemical reactions, as well as how to control the solubility, stability, solvation, and transport of active species. These properties will dictate the cost, energy density, power capabilities, and lifetime of electrocatalytic energy storage systems. Solvation: Looking beyond the traditional paradigm of a single-atom solute in neat solvents for chemical storage in flow batteries or other molecular systems, it is important to understand how strong interactions (including solvent-solvent, ion-ion, and solvent-ion) influence molecular energy storage systems, and how these interactions can be strategically controlled for energy-intense technologies. Comprehensive understanding of complex solvent systems is needed, especially the interplay between the polyatomic solutes and solvent ions that often possess asymmetric structure and charge distributions, as well as that of competing counter ions. Clear understanding needs to be established regarding the interaction and exact role of location-specific charge allocation with respect to the size and steric effect of the ionic solute molecules. This knowledge will render new strategies to improve the overall functionalities of electrolytes. For example, by preferential solvation or formation of counter-ion pairs, the solubility of the redox active ionic materials can be significantly improved. Such understanding is required across length scales to enable control of solvation with the fundamental electrochemical processes that take place at electrode interfaces. Research opportunities in this area include the critical examination of solvent-mediated ion pair formation in ionic solutions, as well as the manipulation of solvation phenomena to favorably impact overall functionalities and electrochemical processes.31 Reaction Processes, Membranes, and Interfaces: Recent progress in nanoscience and nanotechnology has resulted in advances in flow battery design to enhance power density, as well as improved membrane technologies to mitigate crossover and improve stability.32,33 By decreasing the diffusion path of redox species and enlarging the contact area between the current collector and electrolyte, power performance can be improved. Dramatic advances in power density can also be achieved by better understanding of the reaction kinetics and charge-transfer processes of electroactive species via fundamental electrochemical characterization, which bridges the gap between actual battery performance and the theoretical capabilities of redox-active materials.34 Advanced redox flow batteries also rely on inhibited crossover of redox species between the anolyte and catholyte compartments; ion-selective membranes have traditionally been used for this function. Rational functionalization of the molecular structure of membranes provides an avenue to reduce the unwanted shuttling of redox species via size tuning35; pore size control and charge engineering of the membranes can also mitigate crossover issues.36 Moreover, to gain a better understanding of the stability of flow systems, analytical techniques coupled with computational transport and kinetics modeling represent promising avenues for study.37-39 Advances towards the next generation of scalable flow-based energy storage systems will thus be critically dependent on fine molecular tuning and fundamental understanding of both electroactive molecules and membranes.40 Such investigation will require an interdisciplinary effort integrating expertise in chemistry, materials science, and energy science. Electrochemical processes occur at the atomic to nanoscale. The transport of electrons and ions links these phenomena to the continuum, where properties are manifested through the overall architecture of the electrochemical cell. In regards to ion transport and charge transfer, research needs include the development of nanostructured electrode surfaces at the liquid/solid interface for fast electron and ion transport in membranes with high selectivity. Nature has the ability to selectively transport molecules through biological cells at rates orders of magnitude higher than any man-made membrane structures, which may provide inspiration to 60 PRIORITY RESEARCH DIRECTION – 4PDF Image | Next Generation Electrical Energy Storage
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