Hydrogen Production

Part of Electrochemistry

How to produce hydrogen gas through water electrolysis — splitting water into hydrogen and oxygen using electrical energy.

Why This Matters

Hydrogen is simultaneously a fuel, a chemical feedstock, and an energy storage medium. As a fuel, it burns cleanly — the only combustion product is water vapor. As a feedstock, it is essential for the Haber-Bosch process (ammonia → fertilizer) and for hydrogenating oils and reducing metal ores. As energy storage, it converts surplus electrical generation into a storable, transportable form that can later be burned in an engine or fuel cell.

Electrolysis of water is the most accessible route to hydrogen in a rebuilding context: the feedstock is water and electricity, both of which can be produced locally. The process requires no rare materials at small scale, and the product is a versatile, high-energy-density gas (energy density 33 kWh/kg — three times petroleum by mass).

The challenge is efficiency and safety: electrolysis converts electricity to chemical energy at 65–80% efficiency, and hydrogen is highly flammable. Understanding both the chemistry and the safety is essential.

The Electrolysis Reactions

In an alkaline (NaOH) electrolyzer:

Cathode (−): 4 H₂O + 4e⁻ → 2 H₂ + 4 OH⁻

Anode (+): 4 OH⁻ → 2 H₂O + O₂ + 4e⁻

Net: 2 H₂O → 2 H₂ + O₂

In an acid (H₂SO₄) electrolyzer:

Cathode: 2 H⁺ + 2e⁻ → H₂

Anode: H₂O → ½ O₂ + 2H⁺ + 2e⁻

Both configurations split water; the choice of electrolyte affects electrode material requirements and operating efficiency.

Electrolyte Choices

Electrolyte	Concentration	Temperature	Notes
KOH (potassium hydroxide)	20–30%	60–80°C	Best conductivity; preferred industrial choice
NaOH (sodium hydroxide)	20–30%	60–80°C	Good; cheaper than KOH
H₂SO₄ (sulfuric acid)	15–25%	20–40°C	Used in PEM (polymer membrane) cells; requires platinum catalysts
Distilled water only	—	Ambient	Very low conductivity; impractical efficiency

Most accessible for rebuilding context: KOH or NaOH alkaline solution. These allow use of nickel or stainless steel electrodes without platinum catalysts.

Cell Design

Simple Batch Cell

A glass or HDPE jar with two electrodes suspended in alkaline solution. Simplest to build, lowest efficiency.

• Electrodes: Nickel (best), stainless steel 316 (acceptable), mild steel (degrades).
• Electrode gap: 10–20 mm
• Electrolyte: 25% KOH in distilled water
• Power supply: 2–3 V per cell (more efficient at lower voltage; but lower production rate)
• Gas collection: Inverted graduated cylinders filled with water over each electrode to collect and measure H₂ and O₂ separately

Bipolar Stack Electrolyzer

Multiple cells connected in series within one module — each cell shares an electrode with its neighbors (the back of one cathode is the front of the next anode). Higher voltage, same current as a single cell.

Advantages: Better power utilization; more compact; standard industrial design.

Calculation: For a 10-cell bipolar stack at 2 V/cell: total voltage = 20 V, current = I. At 10 A: 10 × 2 × 10 = 200 W total power, same current density as a single cell at 2 V/10 A.

PEM (Polymer Electrolyte Membrane) Cell

Uses a solid polymer membrane (Nafion) as electrolyte. Allows very thin gaps, high current densities, and higher-purity hydrogen. Requires platinum or platinum-group metal catalysts — not suitable for bootstrapped production.

Operating Parameters and Efficiency

Thermodynamic minimum voltage for water splitting: 1.23 V at 25°C.

Actual operating voltage: 1.7–2.1 V for practical alkaline cells due to:

• Electrode overpotentials (kinetic barriers to H₂ and O₂ evolution)
• Ohmic drop in electrolyte

Current efficiency: 90–98% for alkaline cells. Most of the current deposits hydrogen; a small fraction drives side reactions or heats the electrolyte.

Energy efficiency: 60–80% (electrical energy in / hydrogen chemical energy out).

Production rate (from Faraday’s laws): At 100% current efficiency, 1 Faraday (96,485 C) produces 1 g H₂ (0.5 mol × 2 g/mol) = 11.2 L H₂ at STP.

At 100 A, 80% efficiency:

• H₂ per hour = (100 A × 3,600 s × 0.80) / 96,485 C/mol × 1 g/mol = 2.99 g/h ≈ 3 g/h
• Volume: 2.99 / 2 mol × 22.4 L/mol = 33.5 L/h

Gas Handling and Storage

Hydrogen Safety

Hydrogen is Extremely Flammable

Explosive range: 4–75% in air. Minimum ignition energy: 0.017 mJ (ten times more sensitive than gasoline-air). Flames are nearly invisible in daylight. Leaks in enclosed spaces accumulate to explosive concentration rapidly.

Safety rules:

• Electrolyzer must be in a well-ventilated area or outdoors — 6+ air changes per hour minimum
• No ignition sources within 3 m of hydrogen generation or storage
• Use explosion-proof electrical equipment (no standard switches or fans)
• All hydrogen piping must be leak-tested before use and inspected regularly
• Work with the minimum inventory necessary — do not store large quantities near habitation

Storage Options

Method	Pressure	Energy Density	Practicality
Low-pressure balloon/bag	<0.01 bar gauge	Low	Laboratory-scale only
Compressed gas cylinder	200–700 bar	High	Requires high-pressure equipment
Dissolved in metal hydrides	Solid-state	Medium	Safe, but materials are scarce
As ammonia (via Haber)	Liquid at 10 bar	High	Requires synthesis plant

For small-scale energy storage or immediate use (powering a lamp or engine), direct generation and immediate use avoids storage hazards entirely. For longer-term storage and transport, compressed cylinders are most practical.

Applications

Use	Notes
Fuel for internal combustion engines	Gasoline engines can be adapted; hydrogen burns faster — timing adjustment needed
Fuel for gas welding/cutting	Hydrogen-oxygen flame reaches 2,800°C — suitable for cutting and brazing
Ammonia synthesis (Haber-Bosch)	Critical for fertilizer production; requires nitrogen + hydrogen + iron catalyst
Metal reduction (ore smelting)	Reduces iron ore at high temperature; produces water vapor, not CO₂
Hydrogenation of fats/oils	Converts liquid oils to solid fats; extends shelf life