Two Electrode Systems and Voltage Measurements

[Teo]

(Copied over from email correspondence.)

I believe I remember a conversation in this group about two versus three electrode systems before winter break. It was in the back of my mind for a while so I decided to do some looking into it today to try and wrap my head around it. Using the following measurements we obtained during the Edinburgh training, and some extensive conversations with ChatGPT I was able to get some estimations on how off our voltage measurements may be due to using a two-electrode setup and some general takeaways. I’ve attached a heavily edited ChatGPT summary of what I looked into. I may be naïve in my understanding of all this and you guys already knew all of this but I’d love to hear your thoughts and for some double checking on it.

Measurements used:

6.48MS at 18.8C

6.55 MS at 19.1C

(I’m assuming the MS is mS/cm)

Percent Output (as shown on software)	Voltage
1%	2.510
2%	2.619
3%	2.720
4%	2.8
5%	2.913
6%	2.961
7%	2.97

Summary:

In a two-electrode electrolysis system, the voltage measured between the anode and cathode significantly overestimates the electrochemical potential that is directly relevant to electrode reactions (and that in the media). This is because the measured voltage includes several contributions that do not correspond to the electrode surface potential, which is the quantity that governs electrochemical reactions such as water splitting.

Voltage Contributions in a Two-Electrode System
The measured voltage can be approximated as:
V_“measured” =V_“anode” +IR_“solution” +V_“cathode”

Where:
V_“anode” : Overpotential at the working electrode (anode) interface
IR_“solution” : Ohmic voltage drop across the electrolyte between electrodes
V_“cathode” : Overpotential at the counter electrode (cathode) interface
Only the electrode surface potential (i.e., the potential of an electrode relative to a reference electrode) determines whether a given electrochemical reaction occurs. The solution IR drop and counter-electrode overpotential increase the voltage required from the power supply but do not directly increase the chemical driving force at the electrode surface.

Estimation of Voltage Contributions in the Present System
Solution conductivity: ~6.5 mS/cm
Electrode spacing: ~1.5 cm
Electrode dimensions: 6 mm diameter, 30–35 mm platinized length
Current: ~12 mA

1. Solution resistance and IR drop
   R_“solution” =ρd/A≈128" " Ω
   V_“IR” =IR_“solution” ≈1.5" V"

2. Electrode overpotentials (typical values)
   Platinum anode: ~50–150 mV
   Stainless steel cathode: ~300–800 mV
   Combined electrode overpotentials: ~0.4–0.9 V
   Total voltage not directly contributing to electrode surface potential
   V_“IR” +V_“overpotentials” ≈1.9"-" 2.4" V"

Thus, a measured voltage of 2.5V likely corresponds to an actual electrode surface potential of only ~0.1-0.6, depending on current. Importantly, this “true electrode potential” is the potential at the electrode–solution interface that governs electrochemical reactions.

Observations

High voltage is measured even at very low % output due to fixed IR drop and electrode overpotentials.

Voltage increases only modestly as output increases, indicating most of the measured voltage is not contributing to increased electrolysis. 

Significant electrochemical reactions are expected to begin only once the electrode surface potential crosses practical reaction thresholds, predicted to begin only at ~3–5% output (cannot remember if this was the case or not, will check when possible).

Relation to Water Splitting Thresholds

The commonly cited 1.6–2.0 V required for water splitting refers to the electrode surface potential relative to a reference electrode (e.g., SHE or Ag/AgCl), not the total voltage measured between two electrodes.

In a two-electrode system, achieving this surface potential typically requires several volts of total applied voltage, depending on solution resistance and counter-electrode kinetics.

Therefore, observing water electrolysis only at higher measured voltages is fully consistent with electrochemical theory.

Comparison to Three-Electrode Sydow Numbers

The voltage –2.0 V vs Ag/AgCl refer explicitly to the working electrode surface potential, measured independently of solution resistance and counter-electrode effects.

These values cannot be directly compared to two-electrode voltages (e.g., 2.5–3.5 V).

In practice, achieving such electrode surface potentials often requires similar or higher total applied voltages than those observed here.

Electric Field Experienced by Cells

Solution voltage drop: ~1.5 V across ~1.5 cm → ~100 V/m
Voltage across a bacterium (~1 µm): ~100 μV
Typical bacterial membrane potential: ~100–200 mV
Electroporation threshold: ~10⁶–10⁷ V/m
Cells are very likely safe from electrical harm

Electrophoretic Effects on Cells
Bacterial surface charge is small
Estimated drift velocity under ~100 V/m: ~1 µm/s
This is negligible compared to diffusion and convective mixing
No significant accumulation of cells at the anode or cathode is expected

Key Takeaways

Two-electrode voltage measurements substantially overestimate the electrochemical potential relevant to reactions.

Most measured voltage is expended overcoming solution resistance and electrode kinetics.

The chemically relevant quantity is the electrode surface potential.
The electrical environment experienced by cells is mild and unlikely to inhibit growth or cause damage.

[Martin]

So I think you’re concluding that if our lack of growth was due to voltage, it was too low voltage rather than too high..?

Would you agree that, if we are getting bubbles at the cathode and anode then we are splitting water hence producing hydrogen, hence we should have HOB growth?

If so, one benefit of moving to intermittent CO2 sparging is that it is easier to observe gas formation at the cathode and anode.

It should then be fairly easy to observe bubble formation. For AEP0.2 we may wish to consider either an easier opening observation port, or a camera installed in the sensor window to facilitate this.

If there is no bubble formation, two solutions would be to increase media conductivity or to move to a potentiometer that can provide sufficient voltage.

[Martin]

I thought it best to start a separate topic on Anode Discolouration but if my theory that it is iron phosphate discolouration, I think that will also be decreasing the voltage available for electrolysis. So a medium reformulation is probably top priority @Teo and @Amir .

[Teo]

No I don’t necessarily think our lack of growth was due to too low voltage, rather that I’m now less worried that our voltage is too high even at 1% output. Indeed the anode discoloration is perhaps the next big issue to tackle.