Hydrogen, Electrolysers & Fuel Cells

The hydrogen economy explained. Learn hydrogen properties and colours (grey, blue, green), production (steam methane reforming, electrolysis — alkaline, PEM, solid oxide), electrolyser efficiency and specific energy, storage (compressed, liquid, carriers like ammonia), fuel cells (PEMFC, SOFC), cell voltage and polarisation, round-trip efficiency, safety, end uses (steel, fertiliser, refining, mobility) and costs — with Python for electrolysis and fuel-cell sizing.

Hydrogen carries energy without CO₂ at the point of use and can decarbonise sectors that are hard to electrify: steelmaking, fertilisers, refining, shipping fuels and long-duration storage. India's National Green Hydrogen Mission targets 5 Mt/yr of green hydrogen by 2030.

But hydrogen is only as clean as the electricity used to make it, and every conversion loses energy — so engineering must focus on where hydrogen adds the most value.

An analogy

Hydrogen is like a rechargeable 'energy courier': electricity splits water to load it (electrolysis), it travels or waits in storage, then a fuel cell unloads it as electricity again — but the courier charges a hefty fee (losses) for each trip.

Hydrogen colours and uses

Grey hydrogen comes from natural gas by steam methane reforming (~10 kg CO₂ per kg H₂); blue adds carbon capture; green uses renewable electricity for electrolysis; others include pink (nuclear) and brown (coal). Today hydrogen is mostly used in refineries and ammonia plants; future uses include green steel (DRI), fertilisers, e-fuels, heavy transport and storage.

Electrolysers

Main technologies:

Storage and transport

Hydrogen is light but bulky: compressed (350–700 bar, compression costs ~5–10% of energy), liquid (−253 °C, liquefaction ~30%), or carried in molecules (ammonia, methanol, LOHC) or salt caverns. Pipelines can carry hydrogen (with embrittlement-resistant steels). Leaks and wide flammability range (4–75%) demand careful safety engineering.

Fuel cells and economics

Fuel cells convert hydrogen directly to electricity: PEMFC (vehicles, backup), SOFC (stationary, high efficiency, fuel-flexible). Cell voltage falls from 1.23 V due to activation, ohmic and mass-transport losses (polarisation curve); efficiency ≈ V/1.25. Levelised cost of hydrogen depends on electricity price, capacity factor and electrolyser capex; policy incentives (SIGHT in India, IRA in the US) aim for ≤ $2/kg.

Formulas

Worked example: Electricity for 1 t H₂

Given: SEC 52 kWh/kg

Find: Electricity.

Answer: 52 MWh

Worked example: Faraday

Given: 1000 A through one cell for 1 h

Find: H₂ produced.

Answer: 37.6 g

Worked example: Fuel-cell efficiency

Given: Cell at 0.70 V

Find: LHV efficiency.

Answer: ≈ 56%

Power-to-hydrogen-to-power chain

# Electrolysis and fuel cell energy chain
LHV_H2 = 33.3            # kWh/kg (lower heating value)
HHV_H2 = 39.4
elec_per_kg = 52.0       # kWh/kg for a PEM electrolyser system
print(f"electrolyser efficiency (HHV) {HHV_H2/elec_per_kg*100:.0f}%, (LHV) {LHV_H2/elec_per_kg*100:.0f}%")
fc_eff = 0.55
print(f"round trip power-to-H2-to-power: {LHV_H2/elec_per_kg*fc_eff*100:.0f}%")
# Fuel cell stack: 0.7 V per cell at 1 A/cm2, 300 cm2 cells, 100 kW
V_cell, j, area = 0.7, 1.0, 300
P_cell = V_cell * j * area
print(f"cells needed for 100 kW: {100000/P_cell:.0f}, H2 use {100/fc_eff/LHV_H2:.2f} kg/h")
print(f"green H2 cost at Rs 3/kWh electricity: Rs {elec_per_kg*3:.0f}/kg (electricity only)")
# electrolyser efficiency (HHV) 76%, (LHV) 64%
# round trip power-to-H2-to-power: 35%
# cells needed for 100 kW: 476, H2 use 5.46 kg/h
# green H2 cost at Rs 3/kWh electricity: Rs 156/kg (electricity only)

Power-to-hydrogen-to-power returns only ~35% of the electricity (batteries return ~90%), so hydrogen makes sense for seasonal storage or where molecules are needed (steel, ammonia). Electricity dominates green hydrogen cost: cheap renewables below ₹2/kWh and high electrolyser utilisation are essential.

Common mistakes

Applications

Key takeaways

Going further