Batch-Type Alkaline Water Electrolysis Reactor

Alkaline Water Electrolysis Cell Diagram
Schematic diagram of an alkaline water electrolysis cell showing electrode configuration and gas separation

A comprehensive technical analysis detailing the thermodynamics, reaction kinetics, material degradation modes, efficiency calculations, and safety architecture involved in the stoichiometric production of oxyhydrogen gas via a unipolar AWE batch reactor.

Project Team: Infinity Explorers

مصطفى حاتم
هاني ايهاب
عمر محمد رضا
احمد سامي
ابانوب عماد
منه الرحمن محمد
مينا ناصر
سها مدحت
تسنيم عبد الرحمن
الاء ابراهيم

I. System Assembly and Construction

This section details the physical components and the step-by-step procedure for constructing the prototype. Interact with the assembly viewer to follow the sequential build process.

📋 Bill of Materials

  • Primary Reactor Vessel ~900ml airtight
  • Flashback Arrestor ~100ml airtight
  • Electrodes 2x Hex Bolt (2.5/6)
  • Hardware 24x Nut, 18x Washer
  • Pneumatics 1m Tubing, 15cm Pipe, Barb
  • Electrical 9V Battery, Switch, Wires
  • Electrolyte KOH or NaHCO3
  • Solvent Distilled Water

⚙️ Interactive Assembly Guide

Water Electrolysis Laboratory Setup
Typical laboratory water electrolysis setup showing gas collection and electrode arrangement
Step 1 of 9

Electrode Fabrication

Thread a washer and a nut onto a hex bolt, fastening tightly with a wrench.

II. Advanced Electrochemical Analysis

Industrial Water Electrolysis Process
Industrial-scale water electrolysis system showing the separation of hydrogen and oxygen gases

An engineering evaluation revealing thermodynamic inefficiencies and kinetic barriers. This section explores the fundamental governing equations and visualizes the energy requirements and actual performance of the DIY prototype.

📐 Governing Equations & Kinetics

Thermodynamics

Cell Potentials

Reversible potential based on Gibbs free energy ($\Delta G^\circ = 237.1 \text{ kJ/mol}$) vs. Thermoneutral voltage ($\Delta H^\circ = 285.8 \text{ kJ/mol}$).

$$ E^\circ_{rev} = \frac{\Delta G^\circ}{zF} \approx 1.23 \text{ V} $$ $$ E_{th} = \frac{\Delta H^\circ}{zF} \approx 1.48 \text{ V} $$
Alkaline Medium

Half-Cell Kinetics

Reduction at the cathode producing $H_2$ and oxidation at the anode producing $O_2$.

Cathode: $$ 2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^- $$ Anode: $$ 2OH^-(aq) \rightarrow \frac{1}{2}O_2(g) + H_2O(l) + 2e^- $$
System Losses

Operating Cell Voltage

Actual operating voltage required exceeds theoretical minimum due to activation, ohmic, and concentration overpotentials.

$$ V_{cell} = E^\circ_{rev} + \eta_{act,a} + |\eta_{act,c}| + \eta_{ohm} + \eta_{conc} $$ Tafel Equation (Activation): $$ \eta_{act} = \frac{RT}{\alpha z F} \ln\left(\frac{i}{i_0}\right) $$
Production Yield

Mass & Volume Transfer

Theoretical yield is governed by Faraday's First Law of Electrolysis based on current ($I$) and time ($t$).

$$ m = \frac{\int I dt \cdot M}{z \cdot F} $$ Volume at STP: $$ V_{H_2, theoretical} = \left( \frac{I \cdot t}{2 \cdot 96485} \right) \cdot 22.4 \text{ L} $$
Failure Mode

Anodic Dissolution (Steel Corrosion)

In the DIY setup, bare steel outcompetes the oxygen evolution reaction, dissolving the anode and reducing Faradaic efficiency ($\eta_F$).

Iron Oxidation: $$ Fe(s) \rightarrow Fe^{2+}(aq) + 2e^- $$ $$ Fe^{2+}(aq) + 2OH^- \rightarrow Fe(OH)_2(s) $$
Faradaic Efficiency: $$ \eta_F = \frac{V_{H_2, actual}}{V_{H_2, theoretical}} \times 100\% $$
Thermodynamics Data

Voltage Distribution & Ohmic Losses

Applying a 9V DC source across a single unipolar cell introduces severe inefficiencies. As mapped in the chart, over 80% of the applied energy is dissipated entirely as Joule heating (I²R losses), which simply heats the water.

Voltage Efficiency ($\varepsilon_V$) = 1.48 V / 9.0 V ≈ 16.4%
Yield Data

Practical Efficiency vs Parasitic Loss

The use of bare steel bolts leads to massive activation overpotentials. Low electrocatalytic activity means a severe voltage penalty is required.

Because a portion of the electrical current is wasted on side reactions (specifically the corrosion of the iron anode, as modeled in the equations above), the actual Faradaic efficiency of this DIY system falls significantly below theoretical limits.

III. System Failures & Industrial Scaling

Understanding the critical physical limitations of the prototype is essential for transitioning to safe, industrial-scale energy storage architectures.

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Fluidic Safety: Oxyhydrogen Hazard

Oxyhydrogen Flame Demonstration

The system generates mixed Oxyhydrogen (Brown's Gas), which has a detonation velocity exceeding 2800 m/s and a low ignition energy (0.02 mJ). The secondary container acts as a critical Hydraulic Flame Arrestor to quench backward flame propagation.

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Material Degradation

Iron Corrosion and Rust Formation

As established in the kinetic equations, unprotected steel rapidly dissolves into iron(II) ions in highly alkaline conditions, precipitating rust sludge, increasing ohmic resistance, and physically consuming the anode.

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Industrial Membrane Integration

PEM Electrolyzer Schematic

To scale safely, future iterations must incorporate a separator like a Proton Exchange Membrane (PEM) or Zirfon diaphragm. These physically isolate the $H_2$ and $O_2$ streams directly at evolution, ensuring 99.999% gas purity.