Material Design of Aqueous Redox Flow Batteries: Fundamental Challenges and Mitigation Strategies

Abstract

redox flow systems to date. [7] However, their high chemical cost (V 2 O 5 , $24 kg −1) and low energy density (E d < 50 Wh L −1 , normalized based on both side of the electrolytes, unless specified otherwise) limit their wide implementation for large-scale grid storage. To boost the energy density of RFBs, nonaqueous RFBs with a wider potential window were developed. However, nonaqueous RFBs face intrinsic challenges such as high cost, flammability, and low ionic conductivity of organic electrolytes. [8] Here, we review recent developments of advanced materials for electrolyte design in ARFBs. We further classified ARFBs based on the type of active material: inorganic ARFBs (i.e., AIRFBs, including vanadium-, zinc-, iron-, polysulfide-based, and two other emerging systems); and organic ARFBs (i.e., AORFBs, including viologen-, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)-, quinone-, and N-centered heteroaromatic-mole culebased systems). We first summarize the fundamental physicochemical properties of redox couples in each ARFB system and then discuss critical challenges and mitigation design strategies to shed light on the design guidelines for future RFBs. Finally, assessment methodologies and metrics for evaluating battery stability among different ARFBs are discussed. 2. Aqueous Inorganic Redox Flow Batteries (AIRFBs) 2.1. Vanadium-Based AIRFBs 2.1.1. Fundamental Physicochemical Properties All-vanadium redox flow batteries (VRFBs, Figure 1a) are the most developed and widely used RFB system to date. Applying a vanadium element with diverse valence states (i.e., +2, +3, +4, and +5) effectively minimizes cross contamination, facilitates electrolyte regeneration, and provides long-term durability (Figure 1b-e). For example, a 200 MW/800 MWh system has been under construction by Dalian Rongke Power since 2016. [9] The reversible cell voltage of a VRFB is 1.26 V. The nominal cell voltage of VRFBs ranges from 1.15 to 1.55 V in acidic electrolytes, since the catholyte potential is pH-dependent. [10] The redox reactions on both sides are shown in the cyclic voltammetry (CV) analysis in Figure 1f. [11] Redox flow batteries (RFBs) are critical enablers for next-generation grid-scale energy-storage systems, due to their scalability and flexibility in decoupling power and energy. Aqueous RFBs (ARFBs) using nonflammable electrolytes are intrinsically safe. However, their development has been limited by their low energy density and high cost. Developing ARFBs with higher energy density, lower cost, and longer lifespan than the current standard is of significant interest to academic and industrial research communities. Here, a critical review of the latest progress on advanced electrolyte material designs of ARFBs is presented, including a fundamental overview of their physicochemical properties, major challenges, and design strategies. Assessment methodologies and metrics for the evaluation of RFB stability are discussed. Finally, future directions for material design to realize practical applications and achieve the comme...

A Low-Crossover and Fast-Kinetics Thiolate Negolyte for Aqueous Redox Flow Batteries

“…The redox flow batteries (RFBs) are a promising energy storage technology owing to the decoupled power and energy, high scalability, and high safety compared to commercial Li-ion batteries [1][2][3][4][5][6][7][8]. Recently, sulfur-containing materials receive remarkable attention in both aqueous [9][10][11][12][13] and nonaqueous flow systems [14][15][16][17][18][19][20][21] owing to their low cost and high solubility compared with vanadium-based flow battery [22][23][24][25].…”

“…Recently, sulfur-containing materials receive remarkable attention in both aqueous [9][10][11][12][13] and nonaqueous flow systems [14][15][16][17][18][19][20][21] owing to their low cost and high solubility compared with vanadium-based flow battery [22][23][24][25]. However, polysulfides suffer from severe crossover [7,10,12], which results in low coulombic efficiency and limited lifespan.…”

The Economically Sustainable Hydrothermal Synthesis of Dextrosil-Viologen as a Robust Anolyte in Aqueous Redox Flow Batteries

“…1 An alternative and promising approach to grid energy storage technology are redox flow batteries (RFBs), which have decoupled energy and power units that allows for simplified scaling of long storage duration devices. [2][3][4][5][6][7][8][9][10][11] Despite this advantage, RFBs have only been commercially implemented in a handful of experimental grid applications. This is partially due to the high and volatile cost of active materials in the most commercially mature vanadium RFBs.…”

“…Aqueous Organic Redox Flow Batteries (AORFBs) are a promising approach to utilizing the scalability of liquid-state energy storage while reducing the materials cost using inexpensive organic redox molecules. [2][3][4][5][6][7][8][9][10][11] Furthermore, the extensive tunability of organic chemistry can be leveraged when there is a mechanistic understanding of the underlying physical, chemical, and electrochemical properties of the organic redox species. However, developing organic redox active species that are simultaneously aqueous soluble, chemically stable, high voltage, membrane compatible, and cost effective remains difficult.…”

Viologen-Decorated TEMPO for Neutral Aqueous Organic Redox Flow Batteries

“…Wide application of clean energy (e.g., solar and wind energy) in near future needs urgent development of electrochemical energy storage technologies [1]. Redox flow battery (ARFB) is one of the most important large-scale technologies, and all-vanadium redox flow battery (VRFB) has proved itself as one option [2]. The current challenge is the high cost of the electrolyte due to the resource scarcity of vanadium.…”