Electrolysis overpotential calculator
Applying 2 V to a cell whose reversible decomposition voltage is 1.23 V means an overpotential of 0.77 V, the extra push your power supply delivers beyond theory. Enter both voltages in the same unit and get the gap. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Applied voltage
- 2
- Thermodynamic
- 1.23
- Overpotential
- 0.77
- Unit hint
- same potential unit as inputs (typically volts)
In any real electrolysis cell the voltage you apply is larger than the reversible thermodynamic decomposition voltage for the reaction. That extra driving force is the overpotential—the practical gap between ideal electrochemistry and what your power supply must deliver. This capability takes the applied cell voltage and the thermodynamic (or reversible) decomposition voltage, subtracts them with a single deterministic formula, and returns the overpotential in the same unit you supplied. No tables, no network lookups, and no Monte Carlo noise: students, plant engineers, and lab automations get the same number every time for homework checks, stack design reviews, or pipeline validation of measured cell data against theory.
What overpotential means in an electrolysis cell
Thermodynamics fixes a minimum voltage at which a decomposition reaction is just possible under defined conditions: for water electrolysis near standard state that reversible figure is often quoted near 1.23 V, and other chemistries have their own E_thermo from free-energy change and Faraday relations. Kinetics, electrode materials, bubble coverage, mass-transport limits, and ohmic drop all force you to push harder, so the applied cell voltage E_applied sits above E_thermo.
The difference η = E_applied − E_thermo is the total cell overpotential in this simple accounting—an aggregate loss term rather than a split into activation, concentration, and IR contributions. Reporting that single gap is still the first step before you refine electrode kinetics or stack resistance, because it answers whether the cell is barely above theory or heavily overdriven. This tool implements exactly that subtraction and echoes both inputs so audit logs and lab notebooks stay aligned with the numbers that went in.
Where this fits in teaching, R&D, and automation
Chemistry and chemical-engineering courses use an overpotential calculator to turn textbook decomposition voltages and lab multimeter readings into a single loss figure without fragile spreadsheet formulas that quietly mix units. R&D teams screening electrolyzer electrodes compare measured cell voltages against a fixed E_thermo baseline to rank how much extra voltage each coating demands. Pair the result with Faraday efficiency and specific energy calculations in your own pipeline when you need full techno-economic context.
What you can do with it
Lab and homework checks
Convert a measured applied cell voltage and a known reversible decomposition voltage into overpotential for reports and graded problems.
Electrode screening
Compare how much extra voltage different anode or cathode materials require above the same thermodynamic baseline.
Plant efficiency logging
Automate η = E_applied − E_thermo from SCADA or LIMS voltages for stack health and energy-efficiency dashboards.
FAQ
What formula does the electrolysis overpotential calculator use?
η = E_applied − E_thermo, where E_applied is the applied cell voltage and E_thermo is the thermodynamic (reversible) decomposition voltage in the same unit.
Does this split activation, concentration, and ohmic overpotential?
No. It returns the single total gap between applied and thermodynamic voltage. Decompose kinetic and IR terms with separate models if you need them.
Can I use millivolts or other units?
Yes, as long as both voltages share the same unit. The overpotential is returned in that same unit without conversion.
What does a negative overpotential mean?
Usually that the two voltages were swapped or that the cell is being driven below its reversible voltage. The calculator still returns the negative value instead of hiding it, so you can spot the inverted inputs and correct them.
Which thermodynamic voltage should I use for water electrolysis?
Near standard conditions the reversible voltage for splitting water is usually quoted at about 1.23 V. Other reactions have their own value, derived from the change in Gibbs energy, so use the figure for the same reaction and reference conditions as your cell.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/chem/overpotential \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"applied_voltage":2,"thermodynamic_voltage":1.23}'const res = await fetch("https://api.kit.forhosting.com/chem/overpotential", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"applied_voltage": 2,
"thermodynamic_voltage": 1.23
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/overpotential",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"applied_voltage": 2,
"thermodynamic_voltage": 1.23
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/overpotential", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"applied_voltage":2,"thermodynamic_voltage":1.23}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"applied_voltage":2,"thermodynamic_voltage":1.23}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/overpotential", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"applied_voltage": 2,
"thermodynamic_voltage": 1.23
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.overpotential",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
How to choose inputs, units, and sign convention
Use any consistent potential unit—volts are conventional, millivolts are fine if both fields share the scale—because the math is unit-preserving subtraction. When E_applied exceeds E_thermo the overpotential is positive, which is the usual operating regime; if you accidentally swap fields or under-drive the cell you may get a negative η, which the calculator still returns honestly so you can catch inverted inputs. Pass applied_voltage as the total voltage actually imposed across the cell (or measured under load) and thermodynamic_voltage as the reversible or tabulated decomposition voltage for the same reaction and reference conditions. Aliases such as cell_voltage, E_applied, decomposition_voltage, E_thermo, and reversible_voltage are accepted so spreadsheet columns and API clients do not need a rename step. Non-numeric values, empty strings, NaN, and Infinity are rejected with invalid_input so a failed task is never charged.
Where this fits in teaching, R&D, and automation
Because the implementation is pure analytic JavaScript with no AI and no external APIs, browser preview and paid API results match byte-for-byte after stable rounding. Bill only successful API calls at $0.002 per request; missing fields or non-numeric voltages fail fast without consuming balance.
Technical questions
What happens if a voltage is not numeric?
The request fails with invalid_input stating that the field must be a finite number. Failed tasks are not charged.
How much does the API cost?
$0.002 per successful request. The on-page tool runs free in your browser with the same analytic core.