An Alkaline Flow Battery Based on the Coordination Chemistry of Iron and Cobalt

Abstract

We present the first alkaline redox flow battery (a-RFB) based on the coordination chemistry of cobalt with 1-[Bis(2-hydroxyethyl)amino]-2-propanol (mTEA) and iron with triethanolamine (TEA) in 5 M NaOH. The overall redox system has a cell voltage of 0.93 V in the charged state. Importantly, the coordination compounds are negatively charged and have limited transport through the cation exchange membrane (e.g., Nafion), minimizing the extent of redox species crossover during charge-discharge cycling. Fe-TEA is electrochemically reversible and soluble up to 0.8 M, whereas Co-mTEA presents quasireversible electron transfer kinetics and can be solubilized up to 0.7 M. Cyclability was tested with a flow cell at a concentration 0.5 M up to 30 cycles using a 50 μm thick Nafion membrane, at 30 mA/cm 2 , with minimal crossover (less than 4% of net concentration) or evolution of gases detected. The redox flow battery (RFB) has excellent potential for electrical grid energy storage. However, it has not yet been widely deployed often because of problems of stability and limited cycle life. In this report we describe the use of coordination compounds of cobalt with 1-[Bis(2-hydroxyethyl)amino]-2-propanol (mTEA, Figure 1A), and iron with triethanolamine (TEA, Figure 1B) in 5 M NaOH as a RFB. The flow battery was optimized to achieve stable cycling with 71% average energy efficiency in 30 cycles when passing 30 mA cm −2 , and using a 50 μm-thick Nafion membrane as the separator, at a concentration 0.5 M. Importantly, crossover of the redox species through the membrane was below 5% of the original concentration at the end of the 30 th cycle, with no evolution of gases detected during cycling. We developed this alkaline RFB as an alternative to state of the art RFBs, e.g., the all vanadium RFB, which are based on acidic electrolytes.1 Acidic RFBs often suffer capacity fading due to membrane crossover and the occurrence of undesired secondary reactions during battery cycling (e.g., precipitation, evolution of H 2 and Cl 2 gases).2 Acidic electrolytes have a high conductance, G max = 825 mS cm −1 for 3 M H 2 SO 4(aq), 3 but tend to be corrosive to the cell components, which translates into high operational and maintenance costs. 4 In contrast, alkaline electrolytes such as NaOH have lower conductance, G max = 410 mS cm −1 for 3.7 M NaOH (aq) 3 , but are less corrosive. Alkaline electrolytes employing transition metals generally require the use of coordination compounds as redox species to prevent precipitation of the hydroxides or hydrous oxides. The net charge on these ions can be tailored by ligand selection to minimize membrane crossover with a cation exchange membrane. Moreover, different ligands can be used to tune the electrode potential of the half-cells to optimize the voltage of the battery. 5The formation of chemically stable soluble coordination compounds of cobalt and iron in 5 M NaOH is challenging because of a thermodynamic tendency to form their insoluble hydroxides. For iron(III), with a solubility product, K sp , ...

Advanced Redox-Flow Batteries: A Perspective

“…The next-generation of RFB chemistries are likely to be engineered molecules or complexes, since this approach enables one to shift the SRP to a more desirable potential. One approach that has been utilized to do this is ligand-modified transition metals, which can be dissolved in aqueous 19 or non-aqueous 20 electrolyte solutions. Another promising approach is the use of relatively complex organic molecules, which is actually analogous to the first approach.…”

Hydrogen Production with a Simple and Scalable Membraneless Electrolyzer

“…In this work, we seek to substantially decrease both materials and assembly costs by exploring novel membraneless electrolyzer designs. In addition to eliminating the material costs of membranes and associated components, a membraneless electrolyzer can significantly relax design constraints associated with an MEA-based electrolyzer, opening up the possibility for a substantially simplified overall device that is amenable to low-cost, high volume assembly and manufacturing.Membraneless co-laminar flow-cells based on flow-by band electrodes have been demonstrated for fuel cell [20][21][22][23][24] and flow battery [24][25][26][27] applications. These studies revealed the potential for efficient membraneless device operation without significant crossover of species between the anode and cathode, but face significant challenges to scale-up beyond microfluidic applications.…”

“…Membraneless co-laminar flow-cells based on flow-by band electrodes have been demonstrated for fuel cell [20][21][22][23][24] and flow battery [24][25][26][27] applications. These studies revealed the potential for efficient membraneless device operation without significant crossover of species between the anode and cathode, but face significant challenges to scale-up beyond microfluidic applications.…”

Advances in Metal−Organic Ligand Systems for Polymer Electrolyte Membranes: A Review

“…There has been strong evidence that organic compounds can be combined with perovskite oxides via coordination chemistry without altering the crystalline structure. For example, Fe and Co were respectively functionalized with triethanol amine and 1‐[bis(2‐hydroxyethyl)amino]‐2‐propanol in 5M sodium hydroxide solutions .…”