A 2022 laboratory study built a non-aqueous redox flow cell using redox-active polypeptides: a TEMPO-based catholyte and a viologen-based anolyte. The cell produced a reported 1.1 V, but after 500 charge–discharge cycles it retained 60% of its initial capacity. That is an encouraging materials result, not evidence of a commercial or grid-scale battery.
What makes this a peptide-based flow battery?
In a redox flow battery, liquid electrolytes circulate from reservoirs through an electrochemical cell, where oxidation and reduction store or release energy. In this system, the active materials were not ordinary small molecules dissolved in liquid. The researchers attached redox-active pendant groups to α-helical polypeptide scaffolds: TEMPO-based polypeptide for the catholyte and viologen-based polypeptide for the anolyte. “Peptide” here means a synthesized polymer scaffold carrying electrochemically active groups, not a consumer peptide product.
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The larger polypeptide molecules were investigated partly because their size might reduce crossover through a separator—the unwanted movement of active material from one side of the cell to the other. In comparative tests with Daramic 175 and FAPQ 375 PP separators, the authors reported less crossover for the polypeptides than for small-molecule analogues; FAPQ 375 PP was the most effective of those two separators in the reported tests. This is a result for those materials and test conditions, not proof of a finished commercial separator.
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Liang and colleagues reported the experiment in Materials Advances in 2022. Their non-aqueous cell used acetonitrile containing 0.5 M tetraethylammonium bis(trifluoromethanesulfonyl)imide and an anion-exchange membrane. Each reservoir held 7.5 mL. The anolyte concentration was 50 mM and the catholyte concentration was 25 mM, expressed by repeat unit.
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These details matter when interpreting the performance: the results came from a small laboratory flow cell with a particular solvent, supporting electrolyte, membrane and electrolyte concentrations. They should not be treated as specifications for other flow batteries.
What performance did the cell show?
| Measure | Reported result | How to read it |
|---|---|---|
| Output voltage | 1.1 V | Reported for the experimental polypeptide cell. |
| Maximum capacity | 0.53 A h L−1, or 39% of theoretical capacity, at 10 mA cm−2 | The result is tied to the stated current density; it is not a general capacity rating. |
| Capacity utilization at a lower current | 59% of theoretical capacity at 5 mA cm−2 | Accessible capacity was higher at this lower discharge current than at 10 mA cm−2. |
| Coulombic efficiency | More than 99.5% | This measures charge returned relative to charge put in during cycling; it does not mean the cell preserved all of its capacity over time. |
| Variable-rate test | Complete capacity loss at 20 mA cm−2 | The authors observed capacity falling as current density increased in this test. |
The current dependence is important: a voltage and capacity number cannot be compared fairly with another battery study without matching current density and other test conditions. These figures alone do not establish that the chemistry outperforms other flow-battery technologies.
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How much capacity remained after 500 cycles?
The study’s authors reported: “After 500 charge–discharge cycles, 60% of the initial capacity was retained.” They also described capacity fade of about 0.1% per cycle over that 500-cycle test. The cell therefore continued to operate through the test but lost a substantial share of its starting capacity.
High coulombic efficiency and capacity retention describe different things. The reported efficiency above 99.5% concerns charge returned within cycling, while capacity retention tracks how much usable capacity remained over repeated cycles. A cell can have high coulombic efficiency and still lose capacity gradually.
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What caused the capacity fade, and what remains unproven?
Post-cycling analyses indicated stability of the polypeptide backbone and ester linkages. The authors said the noticeable capacity fade was likely due to degradation of the redox-active groups. That explanation is presented as the authors’ interpretation of their analyses, not as proof that every component remained unchanged.
- The work demonstrates a laboratory cell and comparative crossover tests, not a packaged battery or grid deployment.
- The study does not demonstrate commercial operation, cost competitiveness, or end-of-life recycling.
- The authors describe sustainability, degradation on demand and large-scale storage as future potential; these are goals rather than outcomes established by this experiment.
For a meaningful comparison with another flow-battery report, look for chemistry and solvent, voltage, capacity at a stated current density, coulombic efficiency, cycling retention, crossover conditions and cell scale. A single headline capacity or cycle count cannot settle the comparison.
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Study details
The paper, “Metal-free polypeptide redox flow batteries,” was authored by Z. Liang, T. P. Nguyen, N. H. Attanayake, A. D. Easley, J. L. Lutkenhaus, K. L. Wooley and S. A. Odom, and published in Materials Advances 3 (2022), 6558; it was first published on 12 July 2022. Read the paper at the Royal Society of Chemistry.
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