In this article you will learn how water can be split into hydrogen and oxygen by electrolysis, why the resulting hydrogen is called “green”, and how the process fits into the NCERT chemistry syllabus. We will start from the basic redox reactions, move through thermodynamic and kinetic considerations, compare the main types of electrolyzers, and finish with real‑world applications and recent scientific progress.
Fundamentals of Water Electrolysis
Redox picture of splitting water
Electrolysis is a classic example of a redox reaction driven by an external electric current. At the cathode, water gains electrons (reduction) to form hydrogen gas:
2H₂O + 2e⁻ → H₂ + 2OH⁻
At the anode, water loses electrons (oxidation) producing oxygen gas:
2H₂O → O₂ + 4H⁺ + 4e⁻
Overall, the net equation is:
2H₂O(l) → 2H₂(g) + O₂(g)
These half‑reactions are exactly the same as those that occur in a fuel cell, only the direction of electron flow is reversed.
Why “green” hydrogen?
Hydrogen itself is a clean energy carrier, but its environmental impact depends on how it is produced. When the electricity used for electrolysis comes from renewable sources—solar, wind, or hydro—the hydrogen is termed “green” because its carbon footprint is essentially zero. This contrasts with “grey” hydrogen from natural‑gas reforming, which releases CO₂.
Thermodynamics and Kinetics of Electrolysis
Standard potential and required voltage
The standard Gibbs free energy change (ΔG°) for water splitting at 25 °C is +237 kJ mol⁻¹. Converting this to an electrical potential using ΔG° = –nFE gives a theoretical minimum voltage of 1.23 V (where n = 2 mol e⁻, F = 96 485 C mol⁻¹, E = 1.23 V). This is the thermodynamic baseline; any real system needs a higher voltage because of kinetic barriers.
Overpotential and its sources
Two main sources of extra voltage—collectively called overpotential—are:
- Activation overpotential: energy needed to overcome the activation barrier of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).
- Ohmic overpotential: resistance of the electrolyte, electrodes, and connections.
Typical commercial electrolyzers operate at 1.8–2.2 V, meaning that about 0.6–1.0 V is lost to overpotential.
Temperature effects
Raising the temperature reduces the thermodynamic voltage (ΔG decreases) and can improve reaction kinetics, especially for the OER. However, high temperature also increases corrosion risk and may demand more robust materials.
Types of Electrolyzers: How They Differ
Three commercial technologies dominate the market. The table below highlights their key features.
| Electrolyzer Type | Electrolyte | Operating Temperature | Typical Cell Voltage | Key Advantages | Typical Applications |
|---|---|---|---|---|---|
| Alkaline Electrolyzer | KOH or NaOH (liquid) | 50–80 °C | 1.8–2.0 V | Simple design, mature technology, low cost | Large‑scale hydrogen plants, grid‑linked systems |
| Proton‑Exchange‑Membrane (PEM) Electrolyzer | Solid polymer membrane | 50–80 °C | 1.7–1.9 V | High current density, rapid response, compact | Renewable‑energy coupling, mobile hydrogen stations |
| Solid‑Oxide Electrolyzer (SOEC) | Oxide ceramic (Yttria‑stabilised zirconia) | 600–850 °C | 1.3–1.5 V | Very high efficiency at high temperature | Industrial heat‑integrated processes, CO₂ reduction |
Alkaline electrolyzers
These are the oldest commercial systems. They use a porous diaphragm to separate the cathode and anode compartments, preventing the mixing of H₂ and O₂. The alkaline environment favors inexpensive nickel‑based catalysts, which keeps the capital cost low. However, the relatively thick diaphragm limits current density, making the system bulkier.
PEM electrolyzers
PEM cells employ a thin polymer membrane that conducts protons while blocking electrons and gases. The membrane’s high proton conductivity enables operation at current densities >2 A cm⁻², which translates to compact stacks. Catalysts are usually platinum‑group metals (Pt for HER, Ir/Ru oxides for OER), raising material cost but delivering low overpotential.
For a detailed description, see the Britannica entry on proton‑exchange membranes.
Solid‑oxide electrolyzers
SOECs run at temperatures above 600 °C, where water dissociates thermally, reducing the electrical energy needed. They use ceramic electrolytes that conduct oxide ions (O²⁻). The high temperature also enables integration with waste heat from industrial processes, improving overall plant efficiency. The main challenges are material durability and start‑up time.
Catalysts and the Quest for Lower Overpotential
Hydrogen evolution reaction (HER) catalysts
In alkaline media, nickel‑based alloys (Ni‑Mo, Ni‑Fe) are widely used. In acidic PEM cells, platinum remains the benchmark because it binds hydrogen atoms just right—strong enough to split water but weak enough to release H₂ quickly.
Oxygen evolution reaction (OER) catalysts
OER is the kinetic bottleneck. Transition‑metal oxides such as ruthenium oxide (RuO₂) and iridium oxide (IrO₂) perform well in acidic PEM cells, while nickel‑iron layered double hydroxides excel in alkaline electrolyzers. Research is actively exploring earth‑abundant alternatives like cobalt‑phosphate or manganese oxides to cut costs.
Design strategies
Modern catalyst engineering focuses on:
- Increasing active surface area through nanostructuring.
- Optimising electronic structure via doping or alloying.
- Improving stability by supporting catalysts on conductive carbon or metal oxides.
Recent reviews in peer‑reviewed journals highlight that a synergistic combination of high surface area and tailored binding energy can lower the total cell voltage by 0.1–0.2 V, which is significant at industrial scale.
Green Hydrogen Production and Renewable Integration
Coupling with solar and wind
Because electrolysis can be turned on or off instantly, it pairs naturally with intermittent renewable sources. When solar panels generate excess electricity during midday, the surplus can feed a PEM electrolyzer, storing energy as hydrogen. Conversely, during low‑generation periods, the electrolyzer can be idled, avoiding curtailment.
Power‑to‑X concept
“Power‑to‑X” (P2X) describes converting electrical energy into other carriers—hydrogen (P2H), methane (P2M), or liquid fuels (P2L). Green hydrogen is the cornerstone of this concept, enabling sectors that are hard to electrify, such as steelmaking, long‑haul trucking, and aviation.
Scale of deployment in India
India’s National Hydrogen Mission envisions 5 GW of green hydrogen capacity by 2030, primarily using alkaline electrolyzers near coastal wind farms. While the exact numbers are evolving, the policy direction aligns with the push for renewable‑based hydrogen outlined in the renewable energy article.
Recent Scientific Context
Efficiency breakthroughs
In the past five years, laboratory‑scale PEM electrolyzers have achieved >80 % energy efficiency at current densities of 2 A cm⁻², thanks to ultra‑thin Nafion membranes and low‑loading Ir‑based OER catalysts. These numbers approach the theoretical limit set by thermodynamics, indicating that commercial systems may soon reach similar performance.
Integration with carbon‑capture
Researchers are exploring combined electrolysis‑CO₂ reduction reactors where the hydrogen produced feeds a downstream catalytic process that converts CO₂ into methanol or synthetic hydrocarbons. This “hydrogen‑plus‑CO₂” route could provide a carbon‑neutral fuel cycle, a topic highlighted in recent articles on the Nature Energy journal (access through institutional libraries).
Policy and market trends
Globally, green hydrogen investment exceeded $200 billion in 2023, with major projects in Europe, the Middle East, and Australia. India’s emphasis on renewable capacity expansion and the falling cost of solar PV (< $0.03 kWh⁻¹) are creating a favourable economics for green hydrogen, especially in regions with abundant sunlight.
Real‑World Applications of Green Hydrogen
Transportation
Fuel‑cell electric vehicles (FCEVs) use hydrogen directly in a fuel cell to generate electricity. The advantage over battery EVs is rapid refuelling (≈3 minutes) and longer range, making hydrogen attractive for buses, trucks, and trains.
Industrial processes
Refineries and ammonia plants traditionally rely on grey hydrogen. Switching to green hydrogen reduces CO₂ emissions dramatically. For instance, the Haber‑Bosch process for ammonia synthesis consumes about 150 kg of hydrogen per tonne of NH₃; replacing it with green hydrogen cuts the associated CO₂ by roughly 9 t CO₂ per tonne of ammonia.
Energy storage
Hydrogen can be stored in underground salt caverns or as metal hydrides for months or even years, providing seasonal storage that batteries cannot match. When demand spikes, hydrogen can be reconverted to electricity via fuel cells or combusted in turbines.
Important Questions for NEET and Boards
- Write the balanced half‑reactions for water electrolysis in acidic medium.
- Explain why the theoretical decomposition voltage of water is 1.23 V.
- Define activation overpotential and give two factors that influence it.
- Compare alkaline and PEM electrolyzers in terms of electrolyte, catalyst, and typical operating voltage.
- Why does increasing temperature lower the thermodynamic voltage for water splitting?
- List three advantages of using green hydrogen over fossil‑fuel‑derived hydrogen.
- Describe how a proton‑exchange‑membrane electrolyzer prevents mixing of H₂ and O₂ gases.
- Discuss the role of nickel‑iron layered double hydroxide in alkaline electrolysis.
- How does the “power‑to‑X” concept expand the utility of green hydrogen?
- Identify two industrial sectors that can benefit most from green hydrogen and justify your answer.
Frequently Asked Questions
Is electrolysis the only way to obtain green hydrogen?
Electrolysis powered by renewable electricity is the most widely accepted method, but emerging techniques like photoelectrochemical water splitting are also being researched for direct solar‑driven hydrogen production.
Can household solar panels be used for small‑scale electrolysis?
Yes, low‑power PEM or alkaline electrolyzers can be coupled to residential rooftop solar. The output is modest (a few litres of H₂ per day) but useful for fuel‑cell backup or small‑scale experiments.
Why is platinum still used despite its high cost?
Platinum’s unique ability to adsorb and release hydrogen atoms with minimal energy loss makes it the benchmark for HER in acidic media. Ongoing research aims to replace it with cheaper alloys without sacrificing performance.
What safety measures are needed when handling hydrogen?
Hydrogen is flammable and has a wide explosive limit (4–75 %). Proper ventilation, leak detection, and use of non‑spark‑producing tools are essential. Electrolyzer designs incorporate pressure relief valves and gas‑separation membranes to minimise risk.
Key Takeaways
- Electrolysis splits water into H₂ and O₂ via redox reactions; the minimum voltage required is 1.23 V.
- Overpotential (activation + ohmic) raises the practical cell voltage to 1.8–2.2 V.
- Alkaline, PEM, and solid‑oxide electrolyzers differ in electrolyte, temperature, and catalyst requirements.
- Low‑cost nickel‑based catalysts work well in alkaline media, while PEM cells rely on platinum‑group metals for high efficiency.
- When powered by renewable electricity, the hydrogen produced is termed “green” and has a negligible carbon footprint.
- Green hydrogen can be used in transport, industry, and long‑term energy storage, supporting India’s renewable‑energy goals.
- Recent laboratory advances have pushed PEM electrolyzer efficiencies above 80 %, bringing commercial green hydrogen closer to economic viability.