Too Much of a Good Thing? Assessing Performance Tradeoffs of Two-Electron Compounds for Redox Flow Batteries

Abstract

Engineering redox-active compounds to support stable multi-electron transfer is an emerging strategy for enhancing the energy density and reducing the cost of redox flow batteries (RFBs). However, when sequential electron transfers occur at disparate redox potentials, increases in electrolyte capacity are accompanied by decreases in voltaic efficiency, restricting the viable design space. To understand these performance tradeoffs for two-electron compounds specifically, we apply theoretical models to investigate the influence of the electron transfer mechanism and redox-active species properties on galvanostatic processes. First, we model chronopotentiometry at a planar electrode to understand how the electrochemical response and associated concentration distributions depend on thermodynamic and mass transport factors. Second, using a zero-dimensional galvanostatic charge/discharge model, we assess the effects of these key descriptors on performance (i.e., electrode polarization and voltaic efficiency) for a single half-cell. Finally, we extend the galvanostatic model to include two-electron compounds in both half-cells, demonstrating compounding voltage losses for a full cell. These results fundamentally show why multi-electron compounds with disparate redox potentials are less attractive than those with concerted electron transfer. As such, we suggest new directions for molecular and systems engineering to improve the prospects of these materials for RFBs.

Extending and Automating Quantitative Microelectrode Voltammetry through an Oblate Spheroidal Coordinate Framework

“…28 We then use this framework to derive a general steady-state expression for two sequential electron transfers to demonstrate its ability to model more complex electrode processes. 5,21,24,34 Subsequently, we use oblate spheroidal coordinates to develop, code, and validate a numerical simulator capable of generating disk microelectrode voltammograms across conditions that cannot be accurately modeled using the steady-state approach. Finally, we regress the steady-state models to both numerically simulated and experimental voltammograms to assess the efficacy of estimating relevant parameters for an electrolyte solution containing redox-active compounds.…”

Ester-Substituted Bispyridinylidenes: Double Concerted Two-Electron Bipolar Molecules for Symmetric Organic Redox Flow Batteries

“…A few two-electron intrinsic bipolar systems are known, ,,,, but in all cases the redox events are a series of single electron couples (i.e., sequential first and second oxidations and reductions at four distinct potentials). While this theoretically improves energy density, it has been shown that such sequential electron transfers result in detrimental voltaic and energy efficiency, and therefore ROMs with concerted (or more nearly concerted) multielectron events are more desirable . Here we identify [2,2′]bispyridinylidene-4,4′-dialkyl esters as the first examples of intrinsic, two-electron bipolar molecules that exhibit 2-fold, basically concerted, two-electron redox activity at potentials suitable for a symmetric organic redox flow battery (ORFB), and demonstrate their energy storage behavior in static and flowing cells.…”

“…The separation between first-second and third-fourth redox pairs are 0.18 and 0.15 V, respectively. For battery applications, this would translate to a theoretical cell voltage of 1.44 V initially that drops off to 1.11 V, with the majority of the discharge occurring at the lower voltage owing to disproportionation . This kind of step-down in voltage is therefore inefficient and undesirable; however, this initial CV screening establishes that BPYME is indeed able to accept two electrons and form a dianion ( BPYME 2– ), which was not reported earlier.…”

“…Under similar conditions, the one-electron couple for ferrocene oxidation was observed to have Δ E p = 80 mV (Figure S1), suggesting that the smaller observed peak separation of 70 mV for the BPYME/BPYME 2– couple may represent an inverted potential redox process, while the 110 mV peak separation of the BPYME 2+ / BPYME couple suggests two single-electron processes that occur at nearly the same potential (Δ E approaching 0 V). RFB modeling has shown that an inverted potential has small efficiency advantages over two overlapping single-electron processes (Δ E = 0 V), but these CV results ultimately establish BPYME as the first basically concerted two-electron bipolar system. Though the theoretical cell voltage of 1.01 V leaves room for improvement, it has the Faradaic equivalency of a one-electron bipolar system with a cell voltage of 2.02 V, which has been rarely achieved .…”

A Minimal Information Set To Enable Verifiable Theoretical Battery Research

“…Once published, we suggest that the filled-out checklist be a part of the Supporting Information and provide the necessary background to the readers. To give examples of filled-out checklists for different theoretical studies, each author populated a checklist for at least one of their published articles. ,,,− We aimed to cover many different types of battery modeling paradigms in the author-populated checklists. It also serves as a good starting reference for battery modeling best practices and shortcomings.…”