Symmetric Prussian Blue full cells use the same redox-active material family at both electrodes, but each electrode is driven into a different oxidation state during charging. The negative electrode is reduced to Everitt’s salt, or Prussian White, while the positive electrode is oxidized toward Berlin Green. This establishes the practically stable cell-voltage plateau at approximately 0.68 V; a higher plateau near 0.905 V may appear when the positive electrode is further oxidized to Prussian Yellow, but that state is unstable in aqueous systems.
The reliable operating plateau is approximately 0.68 V, corresponding to a Prussian White negative electrode and Berlin Green positive electrode. The ~0.905 V plateau associated with Prussian Yellow is generally transient because Prussian Yellow rapidly degrades in aqueous environments.
How the Symmetric Full-Cell Configuration Works
The same material family serves both electrodes
A symmetric full cell uses Prussian Blue family compounds on both the positive and negative electrodes. Although the electrode chemistry is related, the two electrodes undergo opposite redox reactions during cell operation.
This arrangement creates a voltage from the difference between the electrochemical potentials of the two electrodes rather than from using entirely different electrode material families.
Charging separates the electrode states
During charging, the negative electrode is reduced and moves toward Prussian White, also called Everitt’s salt. At the same time, the positive electrode is oxidized toward Berlin Green.
The resulting separation between the negative and positive electrode potentials produces the cell’s operating voltage.
Discharging reverses the redox processes
During discharge, the stored chemical and electrochemical gradients relax toward their original states. The reduced negative electrode is oxidized, while the oxidized positive electrode is reduced.
The cell voltage therefore reflects which Prussian Blue family oxidation states are present at each electrode.
What Voltage Plateaus Are Observed
The stable plateau: approximately 0.68 V
When the negative electrode is in the Prussian White state and the positive electrode is in the Berlin Green state, the full cell exhibits a stable open-circuit voltage plateau of approximately 0.68 V.
This is the most important plateau for practical aqueous-cell design because it corresponds to chemically sustainable electrode states under the conditions described in the reference.
The higher plateau: approximately 0.905 V
Further oxidation of the positive electrode toward Prussian Yellow can initially increase the cell voltage to approximately 0.905 V.
This higher value should not automatically be treated as the usable nominal voltage. It represents a higher-potential state that is not sufficiently stable in aqueous environments.
Why the higher plateau does not persist
Prussian Yellow is chemically unstable in water and quickly degrades back toward lower-potential states. As a result, the approximately 0.905 V plateau can decay, leaving the cell at the lower and more stable voltage region.
The distinction is important: electrochemical testing may reveal a high initial plateau even though the durable operating window is governed by the approximately 0.68 V plateau.
Why These Plateaus Matter in Cell Design
Voltage determines the practical operating window
The observed plateau identifies the potential range in which the cell can operate with relatively stable electrode chemistry. Designing around the stable 0.68 V region is generally more defensible than relying on the short-lived 0.905 V state.
A nominal voltage based only on the initial high-potential response could overstate the cell’s sustained performance.
Electrode-state control is central
The full-cell voltage depends on controlling the oxidation state of each electrode. The negative electrode must remain associated with the reduced Prussian White state, while the positive electrode should be operated in the stable Berlin Green region rather than being driven persistently into unstable Prussian Yellow.
This makes charge limits and electrochemical testing conditions important parts of cell optimization.
Separator selection must match the chemistry
Membrane separators influence ion transport, electrode isolation, and the practical stability of the full cell. Because the electrodes use related redox chemistry but operate at different states, separator selection should support the intended voltage window and prevent unwanted crossover or side reactions.
The voltage plateaus therefore provide a useful reference when comparing membrane materials and assembling cells.
Understanding the Trade-offs
Higher voltage versus chemical stability
The approximately 0.905 V plateau offers a higher initial voltage than the stable 0.68 V plateau. Its limitation is that it depends on Prussian Yellow, which is unstable in aqueous environments.
The trade-off is straightforward: higher initial voltage comes at the cost of poor state stability.
Symmetry simplifies material selection but not state management
Using Prussian Blue family compounds on both electrodes can simplify the active-material platform and create a well-defined redox system. However, the electrodes still require different oxidation states and therefore cannot be treated as chemically identical during operation.
A symmetric material family does not eliminate the need to manage electrode potentials independently.
Open-circuit voltage is not the same as durable operating voltage
A voltage measured immediately after charging may capture a transient high-potential state. Long-term cell behavior is better represented by the plateau that remains after unstable species relax or degrade.
Testing should therefore distinguish between initial voltage response, stable open-circuit voltage, and sustained operating voltage.
Making the Right Choice for Your Goal
The appropriate design target depends on whether the priority is stable operation, maximum initial voltage, or cell-component optimization.
- If your primary focus is stable aqueous operation: Design around the approximately 0.68 V plateau associated with Prussian White at the negative electrode and Berlin Green at the positive electrode.
- If your primary focus is maximum initial voltage: Recognize that the approximately 0.905 V Prussian Yellow plateau is transient and should not be treated as a durable operating voltage without stability evidence.
- If your primary focus is electrochemical characterization: Track the electrode oxidation states separately and distinguish the stable 0.68 V response from the short-lived 0.905 V response.
- If your primary focus is cell assembly: Use the expected voltage window to guide membrane-separator selection, charge limits, and evaluation of ion transport and chemical compatibility.
A reliable Prussian Blue symmetric full cell is built by optimizing for the stable redox-state pair, not merely the highest voltage observed during the first charge.
Summary Table:
| Configuration | Negative Electrode | Positive Electrode | Stable Voltage | Notes |
|---|---|---|---|---|
| Stable State | Prussian White (Everitt's salt) | Berlin Green | ~0.68 V | Durable operating window; recommended for practical cells |
| Higher State | Prussian White (Everitt's salt) | Prussian Yellow | ~0.905 V | Transient; degrades quickly in aqueous environments |
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