Exploring Biomass-Derived Electrode Precursors for Vanadium Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) are promising technologies for grid-scale energy storage capable of meeting increasing energy demands, seamlessly integrating intermittent renewable energy sources into the electric grid, and improving overall grid reliability and resilience. However, further cost reductions are needed for widespread adoption of RFBs, thus necessitating the advancement of critical system components to improve RFB performance. In particular, the porous carbon electrode plays a crucial role in RFBs by providing active sites for redox reactions, distributing liquid electrolyte, and governing pressure drop. RFB electrodes are typically repurposed gas diffusion layers (GDLs) developed for fuel cells. Though functional, these materials are suboptimal for flow battery applications in terms of surface chemistry and microstructural properties. Post-process modification of commercial GDLs is a common, but limited, approach to improving performance. For example, all-vanadium redox flow batteries (VRFBs), the current state-of-the-art technology, are plagued by sluggish redox reaction kinetics on carbon electrodes, resulting in large activation overpotential. To combat this issue, electrodes are typically oxidized through a variety of methods to alter the surface chemistry, thus improving reaction kinetics and wettability [1]. However, these methods are ultimately incremental as they target existent commercial materials with limited maxima, even when optimized [2]. Re-envisioning electrode materials derived from low-cost, sustainable precursors with inherently favorable properties (i.e., surface functionalities, high surface area, and hierarchical pore structure) is a promising pathway towards RFB performance enhancement and cost reduction. In this work, we demonstrate that biomass from refuse streams such as food waste and wood byproducts can be used to obtain electrode materials that enhance redox reaction kinetics through the introduction of surface functionalities presented in the selected precursor. Unlike fossil-derived carbon precursors (i.e., polyacrylonitrile), the biochemical constituents of biomass (e.g., proteins, carbohydrates, lipids) enable the manufacturing of electrodes with chemical functionalities uncommon in current-generation carbon electrodes. Activated carbon (AC) with hierarchical porosity and high surface area is produced by first stabilizing the biomass via hydrothermal processing at 200-300 °C for 6 h to obtain hydrochar, which is subsequently converted into AC by thermal activation at 850 °C in an inert environment. Evaluation of the electrochemical properties via ex-situ cyclic voltammetry and impedance spectroscopy of the biomass-derived ACs compared to carbon black (Vulcan XC-72) reveals that the ACs are not only conductive and electrochemically active, but also, in some cases, show higher activity for vanadium redox reactions. X-ray photoelectron spectroscopy suggests that increasing the amount of nitrogen heteroatoms reduces charge-transfer resistance and improves electrochemical activity, a hypothesis that is further supported by density functional theory calculations. Finally, we demonstrate that the combination of heat-treated electrodes and biomass electrocatalyst leads to synergistic benefits and increased power density in a full-cell VRFB. These results suggest a pathway to designing next-generation electrode materials with tailored properties for RFBs through the use of low-cost and sustainable feedstocks. [1] K.J. Kim, M.-S. Park, Y.-J. Kim, J.H. Kim, S.X. Dou, M. Skyllas-Kazacos, A technology review of electrodes and reaction mechanisms in vanadium redox flow batteries, J. Mater. Chem. A. 3 (2015) 16913–16933. doi:10.1039/C5TA02613J. [2] K.V. Greco, A. Forner-Cuenca, A. Mularczyk, J.J. Eller, F.R. Brushett, Elucidating the Nuanced Effects of Thermal Pretreatment on Carbon Paper Electrodes for Vanadium Redox Flow Batteries, ACS Appl. Mater. Interfaces. (2018). doi:10.1021/acsami.8b15793.