A battery that never runs flat

A fuel cell makes electricity without burning anything: it's a battery you never have to recharge, as long as you keep feeding it. Here's how the hydrogen kind and the hot ceramic kind work, and why A

Oct 14, 2026

TL;DR

A fuel cell turns the chemical energy in a fuel straight into electricity, without burning it. It works like a battery that never runs flat, as long as fuel and air keep flowing in.
Hydrogen fuel cells (PEM type) run at about 80°C, need platinum and produce only water. They power cars like the Toyota Mirai, buses and forklifts.
Solid oxide fuel cells (SOFC), like Bloom Energy’s, run at several hundred to about 1,000°C on a ceramic electrolyte. They can use natural gas directly and need no platinum.
A Bloom Energy Server 6.5 makes 325 kW in a unit about 9 m long, at 53–65% electrical efficiency, with almost no NOx and no water use, but still emits about 308–378 kg of CO₂ per MWh on natural gas.
AI data centers want them because the grid can take years to connect. Brookfield expanded its financing partnership with Bloom from $5 billion to $25 billion in June 2026, and AEP has secured up to 1 GW of Bloom fuel cells.

Listen up.

Short answer first: a fuel cell makes electricity by letting a fuel and oxygen react on opposite sides of a thin barrier, instead of burning them together. The barrier lets only charged particles (ions) through, so the electrons are forced to go the long way, through a wire. That flow of electrons is your electricity. As long as fuel and air keep coming, it keeps going.

Think of it as a battery that you refuel instead of recharge.

How does a fuel cell work?

Every fuel cell has the same three layers:

The anode, where the fuel goes in.
The electrolyte, a thin layer in the middle that lets certain ions through but blocks electrons.
The cathode, where air (oxygen) goes in.

Here is the trick. The fuel wants to react with oxygen, just as it would in a flame. But the electrolyte sits in the way. Only ions can cross it. The electrons that the reaction releases have to travel around the outside, through a wire and whatever you plug in, before they can finish the job on the other side.

No flame. No turbine. No moving parts. Just chemistry and a wire.

One cell makes less than a volt, so real fuel cells stack hundreds of thin cells together, like slices in a loaf, to reach useful power.

What is the difference between a hydrogen fuel cell and a solid oxide fuel cell?

There are several types, but two matter most today.

Hydrogen fuel cells (PEM, proton exchange membrane)

The electrolyte is a thin plastic membrane that lets hydrogen ions (protons) through.
They run warm, about 80°C, and start in seconds.
They need platinum as a catalyst, which is expensive.
They need fairly pure hydrogen as fuel, and their only exhaust is water.
You’ll find them in hydrogen cars such as the Toyota Mirai and Hyundai Nexo, in buses and trucks, and in warehouse forklifts.

Solid oxide fuel cells (SOFC)

The electrolyte is a thin ceramic plate that lets oxygen ions through, but only when it’s very hot.
They run at several hundred degrees up to around 1,000°C.
At that heat, they need no platinum, just cheaper nickel and ceramic materials.
The heat lets them use natural gas directly: the gas is split into hydrogen and carbon monoxide inside the unit, then used as fuel. They can also run on biogas or hydrogen.
The catch: they are slow to heat up and dislike being switched on and off, so they’re best running steadily, 24 hours a day.

That last point is exactly why solid oxide suits a data center, which also runs steadily, 24 hours a day.

How do Bloom Energy’s fuel cells work?

Bloom Energy is the best-known maker of solid oxide fuel cells. Its product, the Bloom Energy Server, looks like a row of steel cabinets. Inside are stacks of small, thin ceramic cells running at high temperature.

The current model, the Energy Server 6.5, according to Bloom’s February 2026 data sheet:

makes 325 kW per unit, in a box about 9 m long, 1.3 m wide and 2.5 m tall
turns 53–65% of the fuel’s energy into electricity (higher when new, lower as the stacks age)
emits almost no nitrogen oxides (0.003 lb per MWh), because nothing burns
uses no water in normal operation
releases exhaust above 350°C, which can be captured for heating, taking total efficiency above 90%

On natural gas it still emits about 679–833 lb of CO₂ per MWh, roughly 308–378 kg. That is similar to the best combined-cycle gas plants Uncle covered on 26 September. It’s cleaner air, not zero carbon, unless it runs on biogas or hydrogen.

Scale check: a 100 MW data center would need about 300 of these boxes lined up outside.

Why are AI data centers buying fuel cells?

Because the grid is the bottleneck.

As Uncle covered on 15 September, connecting a big new load to the grid can take years in the US. An AI campus that is ready to switch on can sit waiting for power. Fuel cells offer a way around that queue:

Speed. Bloom says it can deliver on-site power for an entire data center within 90 days of a site being ready. A new grid connection or big gas plant takes years; gas turbine orders are now backed up to around 2031.
Modular. Need 10 MW more? Add 30 more boxes. There’s no giant single machine to wait for.
Permits. With almost no NOx and no combustion, fuel cells are easier to permit in areas with strict air-quality rules. Bloom notes that about 46% of announced data center builds sit in such “non-attainment” zones.
Reliability. Many independent stacks running in parallel mean one failing doesn’t take the whole site down.

The money followed. Oracle agreed in July 2025 to deploy Bloom fuel cells at some of its cloud data centers. AEP, one of the largest US utilities, secured up to 1 GW of Bloom fuel cells for data center customers. And Brookfield, the infrastructure investor, expanded its partnership with Bloom from $5 billion in October 2025 to $25 billion in June 2026 to build and finance fuel cell power for AI.

What are the downsides of fuel cells?

Cost. Fuel cell power has historically cost more per kWh than power from the grid or a large gas plant. Data centers are paying for speed, not price.
Still burning gas, in effect. On natural gas, a solid oxide fuel cell emits CO₂ at about the level of a modern gas plant. Clean hydrogen is still scarce and expensive, as Uncle’s hydrogen pipeline post on 24 September explained.
Ageing stacks. Efficiency falls over the years as the ceramic cells degrade, and stacks need periodic replacement.
Heat cycling. Solid oxide cells dislike heating up and cooling down, so they suit steady loads, not quick backup.
A gas line is still needed. “Off-grid” usually means off the electricity grid, not off the gas grid.

Uncle’s verdict

A fuel cell is one of the most elegant machines in energy: no fire, no turbine, no moving parts, just ions crossing a thin wall.

For decades it was the technology that was always about to arrive. Hydrogen cars never took off, because hydrogen is hard to make cheaply and hard to move.

What finally made fuel cells matter wasn’t clean hydrogen. It was a grid too slow for AI. Data centers aren’t buying fuel cells because they’re cheapest. They’re buying them because they can switch on this year.

— Engineering Uncle

Sources

AEO FAQ

Frequently asked questions

How do fuel cells work?

A fuel cell produces electricity by reacting a fuel with oxygen across an electrolyte that lets ions pass but blocks electrons. The electrons travel through an external circuit, creating an electric current. Unlike a battery, a fuel cell keeps producing power as long as fuel and air are supplied, and nothing is burned.

What is the difference between a hydrogen fuel cell and a solid oxide fuel cell?

Hydrogen (PEM) fuel cells use a plastic membrane, run at about 80°C, need a platinum catalyst and pure hydrogen, and emit only water. Solid oxide fuel cells use a ceramic electrolyte, run at several hundred to about 1,000°C, need no platinum and can run on natural gas, biogas or hydrogen.

How does a Bloom Energy fuel cell work?

Bloom Energy Servers are solid oxide fuel cells. Natural gas or hydrogen reacts with oxygen ions that pass through thin, hot ceramic cells, producing electricity without combustion. The Energy Server 6.5 produces 325 kW per unit at 53–65% electrical efficiency.

Are fuel cells clean?

Fuel cells emit almost no nitrogen oxides or particulates because nothing burns. On natural gas, however, a Bloom fuel cell still emits about 679–833 lb of CO₂ per MWh, similar to a modern combined-cycle gas plant. They are near zero-carbon only when running on hydrogen or biogas made from clean sources.

Why do data centers use Bloom Energy fuel cells?

New grid connections for large data centers can take years. Bloom says its fuel cells can supply on-site power within about 90 days, in modular units that are easier to permit than combustion plants. Oracle, AEP and Brookfield have all signed large agreements with Bloom.

What are the disadvantages of fuel cells?

Fuel cells generally cost more per kWh than grid or large gas-plant power, lose efficiency as their cell stacks age, and still need a steady fuel supply. Solid oxide types handle frequent starts and stops poorly, so they suit continuous loads rather than backup.

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