Fuel Cells: How They Convert Chemical Energy Directly Into Electricity

#The hydrogen–oxygen fuel cell

The classic type, developed for the Apollo programme and still the standard textbook example.

Construction. Two porous carbon or nickel electrodes impregnated with a catalyst — typically platinum or palladium — separated by an electrolyte of concentrated aqueous potassium hydroxide (KOH). Hydrogen is supplied to the anode, oxygen to the cathode.

At the anode (oxidation):

2 H2 + 4 OH-  ->  4 H2O + 4 e-

At the cathode (reduction):

O2 + 2 H2O + 4 e-  ->  4 OH-

Overall:

2 H2 + O2  ->  2 H2O

Electrons released at the anode travel through the external circuit — the electric current — and return at the cathode. Hydroxide ions carry charge back through the electrolyte.

The only product is water. On spacecraft, that water is drinkable and was used as such.

Theoretical cell voltage is about 1.23 V; practical cells deliver 0.6–0.9 V under load, so they are stacked in series to reach useful voltages.

#Why efficiency is so high

A conventional power station makes electricity in three lossy steps:

chemical energy -> heat -> mechanical energy -> electrical energy

The middle conversion is limited by the Carnot efficiency, which depends on the temperature difference available. Real thermal power stations achieve roughly 35–45%.

A fuel cell skips the heat and mechanical stages entirely:

chemical energy -> electrical energy

Not being a heat engine, it is not bound by the Carnot limit. Practical efficiencies of 60–80% are achievable, and higher still in combined heat and power configurations that use the waste heat.

#Types of fuel cell

Type	Electrolyte	Temperature	Typical use
Alkaline (AFC)	KOH solution	60–90 °C	Spacecraft
Proton exchange membrane (PEMFC)	Solid polymer	50–100 °C	Vehicles, portable
Phosphoric acid (PAFC)	Phosphoric acid	150–200 °C	Stationary power
Molten carbonate (MCFC)	Molten carbonate salts	600–700 °C	Large stationary
Solid oxide (SOFC)	Ceramic oxide	700–1,000 °C	Stationary, CHP
Direct methanol (DMFC)	Polymer membrane	50–120 °C	Small portable

Alkaline cells are the most efficient but are poisoned by carbon dioxide, which reacts with the KOH — one reason they suit spacecraft, with pure oxygen supplies, better than terrestrial use with ordinary air.

#Other fuels

Hydrazine (N₂H₄). Reacts with oxygen to give nitrogen and water. It is a liquid, avoiding hydrogen storage problems, and offers a high theoretical voltage — but it is highly toxic and unstable, so it stays in specialised and military applications.

Methanol. Fed directly as a liquid in direct methanol fuel cells. Easy to handle, but lower power density and it produces carbon dioxide.

Natural gas. Reformed internally to hydrogen in high-temperature solid oxide cells, which is how stationary fuel cell generators typically run.

#Merits

• High efficiency, 60–80%, roughly double a thermal power station
• No combustion, so no smoke, soot, particulates or nitrogen oxides
• Water as the only product for hydrogen cells
• Silent, with no moving parts in the cell itself
• Little maintenance for the same reason
• Modular — stack more cells for more power, with no efficiency penalty at small scale
• Continuous operation while fuel is supplied, unlike a battery

#Demerits

• High cost, largely from platinum-group catalysts
• Hydrogen supply, which must be manufactured, compressed or liquefied, and distributed
• Catalyst poisoning by carbon monoxide and sulphur impurities, requiring very pure fuel
• Limited operating life as membranes and catalysts degrade
• Water management — too little dries the membrane, too much floods the electrodes
• Slow start-up for high-temperature types

#Where they are used

Apollo and Space Shuttle power systems; fuel cell buses, trucks and forklifts; stationary backup power for hospitals and data centres; combined heat and power units for buildings; submarines, where air-independent operation is the point.

The pattern is consistent: fuel cells win where efficiency, silence and zero local emissions justify the cost, and lose wherever a cheap engine or a battery will do.

The fuel it works best with brings its own difficulties — see why hydrogen is not widely used — and the limit fuel cells escape is the one that caps thermal power stations.

#The compact answer

In a fuel cell, the chemical energy of the fuel is converted directly into electrical energy by an electrochemical reaction, without combustion. In the hydrogen–oxygen fuel cell, hydrogen is oxidised at the anode and oxygen reduced at the cathode in an electrolyte of aqueous potassium hydroxide, producing water as the only product. Because it is not a heat engine, its efficiency is not limited by the Carnot cycle and can reach 60–80%.