Ages 8–12 · about 9 minutes
The Pons–Fleischmann Palladium Cell
What did a palladium rod, heavy water, and a careful heat ledger show?
Do this first · Try this at home
Draw the spaces in a metal
Start with your hands. The reading makes far more sense afterwards.
You need: Paper and a pencil
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What should I notice?
Notice: This is a packing model. It does not show fusion, explain how atoms move, or reproduce the laboratory cell.
Now read on to find out why it happens.
The big idea
Pons and Fleischmann turned a tiny metal cell into a question with an energy ledger: where did the extra heat come from?
- fusion
- When two tiny nuclei join into one and let out a burst of energy.
- lattice
- The neat repeating grid that atoms sit in inside a metal.
- deuterium
- A heavy kind of hydrogen with one proton and one neutron in its nucleus.
- calorimetry
- Measuring heat carefully enough to keep an energy account.
In 1989, Martin Fleischmann, Stanley Pons and Marvin Hawkins studied a small palladium cell. Hawkins was added to the paper’s author list in its published correction. The researchers measured heat and looked for nuclear reaction products.
The cell held heavy water, whose hydrogen was deuterium. A palladium rod was the negative electrode, or cathode; platinum was the positive electrode. Electric current drove electrolysis, splitting water and allowing deuterium to enter the palladium.
Inside a metal, atoms form a repeating arrangement called a lattice. Deuterium can occupy spaces between the palladium atoms. Loading describes how much deuterium the metal contains. Packing deuterium into metal is not the same event as joining its nuclei in fusion.
Accounting for heat
Electrical power entering the cell is current × voltage: P_in = I × V. Current measures charge flowing each second; voltage measures energy per unit of charge. Some electrical energy becomes chemical energy carried away by the gases made during electrolysis, rather than heat in the cell.
The researchers calibrated a calorimeter, an instrument for measuring heat, using known heating. They recorded temperature and electrical input, and accounted for heat stored in the cell, heat lost to its surroundings, and energy carried with material entering or leaving. Stored heat belongs in the account once, not as two separate losses.
They reported heat above what their calibrated electrochemical account predicted and called the difference excess enthalpy. The calorimeter followed the whole cell. It did not directly show where an additional process occurred or how it worked.
A reported rate
The fuller 1990 calorimetry paper reported excess-heat rates around 100 W/cm³ of palladium at current densities near 1 A/cm². Watts measure energy each second; here the rate is divided by the palladium’s volume. Current density is current divided by electrode area.
This does not mean every cell produced 100 watts. The total excess power depends on the palladium volume and the result of that particular run. Nor is this number the efficiency of a complete power-generating machine.
Heat and nuclear products
The 1989 team also reported neutron and tritium measurements. A neutron is an electrically neutral nuclear particle; tritium is hydrogen with two neutrons in its nucleus. The reaction rates inferred from these measurements were too small to explain the reported heat through the usual deuterium–deuterium fusion reactions.
The authors therefore proposed an additional nuclear process, but did not identify it. The heat result and the nuclear-product measurements are separate pieces of evidence: measuring one does not supply the missing mechanism for the other. The cell described here was laboratory research, not a home activity.
Sources: Electrochemically induced nuclear fusion of deuterium, 10.1016/0022-0728(89)80006-3; published correction, 10.1016/0022-0728(89)80141-X; 1990 calorimetry paper, 10.1016/0022-0728(90)80009-U.