“Innocent” Hexafluorophosphate Salts Induce Capacity Fade in Nonaqueous Redox Flow Batteries
Tang, Wenlong ; Steen, Jelte S. ; Hettinga, Jurjen W. ; Hjelm, Johan LU
and Otten, Edwin
(2025)
In ACS Applied Energy Materials
8(24).
p.18178-18186
- Abstract
The use of organic active materials in redox flow batteries (RFBs) presents a promising approach to sustainable large-scale energy storage. However, the stability of nonaqueous organic RFB electrolytes is generally limited by degradation reactions that cause capacity fade. These reactions are commonly thought to convert redox-active organics to products that are no longer electrochemically active. Here we uncover an additional pathway leading to capacity fade that involves the supporting electrolyte salt. Capacity fade in nonaqueous RFBs is studied in detail for the 1,2,4-benzotriazin-4-yl radical (1) as a model compound and extended to several other classes of representative redox-active organics. By using symmetrical batteries... (More)
The use of organic active materials in redox flow batteries (RFBs) presents a promising approach to sustainable large-scale energy storage. However, the stability of nonaqueous organic RFB electrolytes is generally limited by degradation reactions that cause capacity fade. These reactions are commonly thought to convert redox-active organics to products that are no longer electrochemically active. Here we uncover an additional pathway leading to capacity fade that involves the supporting electrolyte salt. Capacity fade in nonaqueous RFBs is studied in detail for the 1,2,4-benzotriazin-4-yl radical (1) as a model compound and extended to several other classes of representative redox-active organics. By using symmetrical batteries (10/–∥10/+), we delineate that capacity fade occurs in a nonlinear (autocatalytic) fashion via acid-induced decomposition of the supporting salt anion PF6– in the posolyte solution. This is shown to be a universal degradation reaction in the posolyte of nonaqueous RFBs. Although the acidic degradation products are not detrimental to the posolyte, the crossover of acid to the opposite compartment leads to capacity-limiting protonation of the negolyte active material. Replacement of PF6– with other anions substantially improves the stability of these nonaqueous electrolytes, as demonstrated with a symmetrical RFB based on 0.38 M active material 1 that can be cycled for >69 days with very high capacity retention (fade rate of ≤0.1% per day). The improved understanding of factors determining the lifetime of nonaqueous electrolytes unlocks rational strategies to develop more durable electrochemical energy storage systems.
(Less)
- author
- Tang, Wenlong
; Steen, Jelte S.
; Hettinga, Jurjen W.
; Hjelm, Johan
LU
and Otten, Edwin
- organization
- publishing date
- 2025-12-22
- type
- Contribution to journal
- publication status
- published
- subject
- keywords
- bipolar radicals, electrolyte lifetime, hexafluorophosphate degradation, nonaqueous electrolytes, redox flow batteries
- in
- ACS Applied Energy Materials
- volume
- 8
- issue
- 24
- pages
- 9 pages
- publisher
- The American Chemical Society (ACS)
- external identifiers
-
- pmid:41451307
- scopus:105025197744
- ISSN
- 2574-0962
- DOI
- 10.1021/acsaem.5c03070
- language
- English
- LU publication?
- yes
- additional info
- Publisher Copyright: © 2025 The Authors. Published by American Chemical Society
- id
- 5acbc148-8954-4499-8793-0b65924385e0
- date added to LUP
- 2026-02-10 16:37:43
- date last changed
- 2026-02-12 15:23:29
@article{5acbc148-8954-4499-8793-0b65924385e0,
abstract = {{<p>The use of organic active materials in redox flow batteries (RFBs) presents a promising approach to sustainable large-scale energy storage. However, the stability of nonaqueous organic RFB electrolytes is generally limited by degradation reactions that cause capacity fade. These reactions are commonly thought to convert redox-active organics to products that are no longer electrochemically active. Here we uncover an additional pathway leading to capacity fade that involves the supporting electrolyte salt. Capacity fade in nonaqueous RFBs is studied in detail for the 1,2,4-benzotriazin-4-yl radical (1) as a model compound and extended to several other classes of representative redox-active organics. By using symmetrical batteries (1<sup>0/–</sup>∥1<sup>0/+</sup>), we delineate that capacity fade occurs in a nonlinear (autocatalytic) fashion via acid-induced decomposition of the supporting salt anion PF<sub>6</sub><sup>–</sup> in the posolyte solution. This is shown to be a universal degradation reaction in the posolyte of nonaqueous RFBs. Although the acidic degradation products are not detrimental to the posolyte, the crossover of acid to the opposite compartment leads to capacity-limiting protonation of the negolyte active material. Replacement of PF<sub>6</sub><sup>–</sup> with other anions substantially improves the stability of these nonaqueous electrolytes, as demonstrated with a symmetrical RFB based on 0.38 M active material 1 that can be cycled for >69 days with very high capacity retention (fade rate of ≤0.1% per day). The improved understanding of factors determining the lifetime of nonaqueous electrolytes unlocks rational strategies to develop more durable electrochemical energy storage systems.</p>}},
author = {{Tang, Wenlong and Steen, Jelte S. and Hettinga, Jurjen W. and Hjelm, Johan and Otten, Edwin}},
issn = {{2574-0962}},
keywords = {{bipolar radicals; electrolyte lifetime; hexafluorophosphate degradation; nonaqueous electrolytes; redox flow batteries}},
language = {{eng}},
month = {{12}},
number = {{24}},
pages = {{18178--18186}},
publisher = {{The American Chemical Society (ACS)}},
series = {{ACS Applied Energy Materials}},
title = {{“Innocent” Hexafluorophosphate Salts Induce Capacity Fade in Nonaqueous Redox Flow Batteries}},
url = {{http://dx.doi.org/10.1021/acsaem.5c03070}},
doi = {{10.1021/acsaem.5c03070}},
volume = {{8}},
year = {{2025}},
}